Lanthanide luminescence for functional materials and bio-sciences

Abstract

Recent startling interest for lanthanide luminescence is stimulated by the continuously expanding need for luminescent materials meeting the stringent requirements of telecommunication, lighting, electroluminescent devices, (bio-)analytical sensors and bio-imaging set-ups. This critical review describes the latest developments in (i) the sensitization of near-infrared luminescence, (ii) “soft” luminescent materials (liquid crystals, ionic liquids, ionogels), (iii) electroluminescent materials for organic light emitting diodes, with emphasis on white light generation, and (iv) applications in luminescent bio-sensing and bio-imaging based on time-resolved detection and multiphoton excitation (500 references).

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Svetlana V. Eliseeva

Svetlana V. Eliseeva graduated from Lomonosov Moscow State University (MSU), earning a degree in chemistry with honors in 2003 and a PhD degree in inorganic chemistry in 2006 under the supervision of Professor Natalia P. Kuzmina. After two years of a post-doctoral fellow under a joint program between the Department of Material Sciences at MSU and Saint-Gobain company (France), she joined the group of Professor Jean-Claude G. Bünzli at École Polytechnique Fédérale de Lausanne (EPFL). Her current research interests mainly focus on the development of luminescent lanthanide-containing coordination compounds and nanoparticles suitable for bio-analysis and lighting applications, as well as on multiphoton excitation measurements.

Jean-Claude Bünzli

Jean-Claude Bünzli is an active researcher in the field of coordination, supramolecular and biological chemistry of the lanthanide ions. He earned a degree in chemical engineering in 1968 and a PhD in 1971 from the École Polytechnique Fédérale de Lausanne (EPFL). He spent two years at the University of British Columbia (Canada) and one year at the Swiss Federal Institute of Technology in Zürich before being appointed at the University of Lausanne in 1974 and at EPFL in 2001 as a full professor of inorganic chemistry. Since September 2009 he also holds a professorship at Korea University. His research focuses on the self-assembly of building blocks for photoluminescent materials and of lanthanide luminescent bioprobes.

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1. Scope of the review

The present critical review aims at describing selected themes of current interest in science and technology and dealing with lanthanide luminescence. It can be considered as being a follow up and an extension of a 2005 review published in this journal.¹ To keep it self-consistent however, the general description of lanthanide luminescence and its applications is maintained and expanded. In view of the current trends in lanthanide luminescence, and reflecting the proportions of the data published recently, the topics discussed below refer to NIR luminescence, special “soft” luminescent materials with technological potentials, lighting devices such as light emitting diodes, and bio-analysis and imaging. The literature is selectively covered from late 2005 (late 2006 for NIR luminescence) to March 2009.

2. Lanthanide luminescence: why, how and what?

In 1937, J. H. van Vleck² wrote an article titled The Puzzle of Rare-Earth Spectra in Solids which perfectly reflects the fascination exerted by the intricate optical properties of the trivalent lanthanide ions, hereafter Ln^III. The electronic [Xe]4fⁿ configurations (n = 0–14) indeed generate a rich variety of electronic levels, the number of which is given by 14!/n!(14 −n)!, translating to 3432 for Gd^III, for instance (see Fig. 1). The energies of these levels are well defined due to the shielding of the 4f orbitals by the filled 5s²5p⁶ sub-shells and, in addition, they do not vary much with the chemical environments in which the lanthanide ions are inserted. As a corollary, inner-shell 4f–4f transitions are sharp and easily recognizable. This has been a definite advantage in the discovery of the lanthanide elements between 1803 and 1907 during which all of the lanthanides have been identified, barring artificial Pm (1947).

Fig. 1 Partial energy level diagram for Ln^III ions doped in a low-symmetry crystal (LaF₃). Redrawn from G. K. Liu.²¹

One of the landmarks in lanthanide luminescence is the discovery of the highly emissive Y₂O₃:Eu^III material,³ which is at the origin of the phosphors for cathode-ray tubes and fluorescent lamps, still in heavy use today.⁴ Other milestones for purely inorganic compounds are the findings of neodymium YAG (Yttrium Aluminium Garnet) lasers in 1964⁵ and Er-doped optical fibres for telecommunications in 1987.⁶ Attention on luminescent coordination compounds started in the mid-seventies when Finnish researchers proposed Eu^III and Tb^III (later also Sm^III and Dy^III) polyaminocarboxylates and β-diketonates as bioprobes in time-resolved luminescent (TRL) immunoassays.^7,8 This new technology generated a large interest and further developments, such as homogeneous TRL assays,⁹ optimization of bioconjugation methods for lanthanide luminescent chelates,¹⁰ and time-resolved luminescence microscopy (TRLM)¹¹ resulted in applications of lanthanide luminescent bioprobes (LLBs)¹² in many fields of biological and medicinal analyses, including tissue¹³ and cell imaging,¹⁴ as well as monitoring drug delivery.¹⁵ These bio-applications have been a major factor in the unprecedented expansion of lanthanide coordination chemistry during the past 20 years, together with the design of contrast agents for magnetic resonance imaging (MRI)¹⁶ and more efficient separation techniques.¹⁷ Other drives for lanthanide coordination chemistry arise from the design of lanthanide-doped OLEDs (Organic Light Emitting Diodes)¹⁸ emitting either in the NIR, such as Er^III tris(8-hydroxyquinolinate),¹⁹ or in the visible.²⁰

2.1 Lanthanide photophysics²²

Energy levels and f–f absorption spectra. The absorption of light by an electron moving around a nucleus occurs thanks to operators linked to the nature of light: the odd-parity electric dipole (ED) operator, the even-parity magnetic dipole (MD) and electric quadrupole (EQ) operators. Not all transitions are permitted since they have to obey selection rules. One of these is the so-called Laporte’s (or parity rule) requiring that for ED transitions, the sum of the angular momenta of the electrons in the initial and final states must change by an odd integer. The other selection rules applying to S, L and J quantum numbers for f–f transitions between spectroscopic states are listed in Table 1. They are derived assuming that the wavefunctions of the 4f electrons are described by fⁿ[SL]J functions (i.e. that Russell–Saunders spin–orbit coupling scheme is valid).

Table 1 Selection rules for f–f transitions between spectroscopic levels

Operator	Parity	ΔS	ΔL	ΔJ^a
a J = 0 to J′ = 0 transitions are always forbidden.
ED	Opposite	0	≤6	≤6 (2, 4, 6 if J or J′ = 0)
MD	Same	0	0	0, ±1
EQ	Same	0	0, ±1, ±2	0, ±1, ±2

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In addition, when the Ln^III ion is inserted into a chemical environment, the (2J + 1)-degenerate J-levels are split by ligand-field effects into so-called Stark sub-levels, the number of which depends on the site symmetry of the metal ion.²² Transitions between these sub-levels are governed by symmetry-related selection rules which can be obtained from:

Γ_op∈Γ(Ψ_i) ×Γ(Ψ_f) (1)

in which Γs are irreducible representations of the dipole operator (op) and of the initial (i) and final (f) wavefunctions. A full listing of these rules can be found in the review article by Görller-Walrand and Binnemans on the rationalization of crystal field parameters.²³ They allow the determination of site symmetry from either absorption or emission spectra (Table 2).

The selection rules are derived under several hypotheses which are not always completely fulfilled in reality (in particular 4f wavefunctions are not totally pure), so that the terms “forbidden” and “allowed” transitions cannot be taken too rigidly. A more correct wording would be that a forbidden transition has a low probability and an allowed transition a high probability of occurring. The intensities of the induced electric dipole f–f transitions can be derived from Judd–Ofelt (JO) theory, in which the dipole strength D_ed in 10³⁶ debye² is expressed by:

Here e is the electron charge (in esu), Ω_λ are three phenomenological parameters (in cm²) and the squared bracketed expressions are dimensionless doubly-reduced matrix elements which are insensitive to the metal environment and which are tabulated; see ref. 24 for a complete description. Some induced electric dipole transitions have proved experimentally to be particularly sensitive to changes in the metal-ion environment and are termed hypersensitive transitions or pseudo-quadrupolar transitions because they tend to obey EQ selection rules; see ref. 22 for a listing.

For Ln^III ions, intensities of ED transitions have the same order of magnitude than those of MD transitions, so that both are seen in the optical spectra, while EQ transitions are much weaker and have usually not been identified.

Table 2 Number of ligand-field sub-levels in function of the J quantum number

Symmetry	Site symmetry	Integer J
		0	1	2	3	4	5	6	7	8
Cubic	T, Td, Th, O, Oh	1	1	2	3	4	4	6	6	7
Hexagonal	C3h, D3h, C6, C6h, C6v, D6, D6h	1	2	3	5	6	7	9	10	11
Trigonal	C3, S6, C3v, D3, D3d	1	2	3	5	6	7	9	10	11
Tetragonal	C4, S4, C4h, C4v, D4, D2d, D4h	1	2	4	5	7	8	10	11	13
Low	C1, CS, C2, C2h, C2v, D2, D2h	1	3	5	7	9	11	13	15	17

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Symmetry	Site symmetry	Half-integer J
		1/2	3/2	5/2	7/2	9/2	11/2	13/2	15/2	17/2
a All ligand-field sub-levels are doubly degenerate (Kramer’s doublets).
Cubic	T, Td, Th, O, Oh	1	1	2	3	3	4	5	6	6
All others^a	See above	1	2	3	4	5	6	7	8	9

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4f–5d and charge-transfer transitions. The transfer of a 4f electron into the 5d sub-shell is parity allowed, therefore the corresponding transitions have sizeable intensities (ε in the range 200–1000 M⁻¹ cm⁻¹) and due to the much larger ligand-field effect on the d orbitals as compared to the f orbitals, their energies much depend upon the metal-ion surroundings. However, the spin-allowed 4f–5d transitions have large energies (>50 000 cm⁻¹, λ < 200 nm), except for Ce^III (>32 000 cm⁻¹, λ < 312 nm), Pr^III and Tb^III (>40 000 cm⁻¹, λ < 250 nm), so that they are rarely observed in coordination compounds.^25,26 Similarly, charge-transfer transitions (e.g. ligand-to-metal charge transfer, LMCT) are parity allowed, their energy is large, and they appear usually at wavelengths smaller than 200 nm. Exceptions are Eu^III and Yb^III (possibly Sm^III and Tm^III) which can be more easily reduced than the other Ln^III ions.¹

Emission spectra. With the exception of La^III and Lu^III, all of Ln^III ions are luminescent and their f–f emission lines cover the entire spectrum, from UV (Gd^III) to visible (e.g. Pr^III, Sm^III, Eu^III, Tb^III, Dy^III, Tm^III) and near-infrared (NIR, e.g. Pr^III, Nd^III, Ho^III, Er^III, Yb^III) ranges. Some ions are fluorescent (ΔS = 0), others are phosphorescent (ΔS≠ 0), and some are both. As for the absorption bands, the emission lines are sharp because the reorganization consecutive to the excitation of an electron into a 4f orbital of higher energy does not influence much the binding pattern of the molecules. As a consequence, the Stokes’ shift is very small when excitation is achieved directly into the 4f levels. On the other hand, the Ln^III excited state may also be populated by energy transfer from the surroundings of the metal ion and in this case, the apparent Stokes’ shift is large, a definitive advantage for luminescent probes (see below). The transition operators (ED, MD) and selection rules are the same as for the absorption spectra (see Table 1).

Important parameters characterizing the emission of light from a Ln^III ion are (i) the quantum yield Q, which is equal to the ratio between the number of emitted photons divided by the number of absorbed photons, and (ii) the lifetime of the excited state τ_obs = 1/k_obs with k_obs being the rate constant (in s⁻¹) of the de-population of the excited state. If the metal ion is directly excited into a 4f level, these two quantities are related by:

Here k^rad is the radiative rate constant. The value of Q^Ln_Ln, the intrinsic quantum yield, reflects the extent of non-radiative deactivation processes occurring both in the inner- and outer-coordination spheres of the metal ion. The observed rate constant k_obs is the sum of the rates of the various deactivation processes:where k^nr are the non-radiative rate constants; the superscript vibr points to vibration-induced processes while pet refers to photo-induced electron transfer deactivations, e.g. caused by LMCT states; the rate constants k′ are associated with the remaining deactivation paths. The intrinsic quantum yield essentially depends on the energy gap ΔE between the emissive state of the metal ion and the highest sub-level of its ground, or receiving, multiplet. The smaller this gap, the easier is its closing by non-radiative deactivation processes, for instance through vibrations of bound ligands, particularly those with high energy such as O–H, N–H or C–H. Second-sphere oscillators also participate in this quenching, but to a lesser extent.²⁷ As a rule of thumb, radiative de-excitation will compete efficiently with multiphonon processes if the energy gap is more than 6 quanta of the highest energy vibration present in the molecule.

In absence of non-radiative deactivation processes, k_obs = k^rad and the quantum yield is equal to 1, which seldom happens. Examples are, in solid state and under excitation at 254 nm, Y₂O₃:Eu^III (3%) with Q = 0.99 and terbium benzoate with Q = 1;²⁸ in solution, a terbium complex with a dipyrazolylpyridine bearing aminocarboxylate coordinating groups was reported having Q = 0.95.²⁹ To date, the largest quantum yield recorded for an Eu^III complex, [Eu(tta)₃DBSO] is 85% (tta is thenoyltrifluoroacetylacetonate and DBSO dibenzyl sulfoxide).³⁰ The latter quantum yields have been obtained by excitation into the metal–ion surroundings and are termed overall quantum yields, with η_sens, the sensitization efficiency, defined as the efficacy with which energy is transferred from the feeding levels of the metal-ion surroundings onto the Ln^III excited states:

Q^L_Ln = η_sensQ^Ln_Ln (5)

Experimental determination of the intrinsic quantum yield is difficult in view of the faint absorbance of f–f transitions. The use of eqn (3) requires evaluation of the radiative lifetime which can in principle be determined from the absorption spectrum ε([small nu, Greek, tilde]) using Einstein’s rates of spontaneous emission A from an initial state |Ψ_J〉, characterized by a quantum number J, to a final state |Ψ′_J′〉:where [small nu, Greek, tilde] is the mean energy of the transition, h Planck’s constant, and n the refractive index; D_ED is given by eqn (2) and D_MD by eqn (7):in which m_e is the mass of the electron and c the velocity of light in vacuo. The bracketed matrix elements are tabulated²⁴ and the radiative lifetime can therefore be calculated from the spectral intensities, that is, from eqn (2), (6) and (7). However, except in few cases, this calculation is not trivial and large errors may occur. On the other hand, if the absorption spectrum corresponding to an emission spectrum ε([small nu, Greek, tilde]) is known, which may be the case when the luminescence transitions terminate onto the ground level, the radiative lifetime can be extracted from the following equation with N_A being Avogadro’s number (6.023 × 10²³):Europium is a special case for which a simplified equation leads to the radiative lifetime:³¹with A_MD,0 being a constant equal to 14.65 s⁻¹ and (I_tot/I_MD) the ratio of the total integrated ⁵D₀→⁷F_J emission (J = 0–6) to that of the ⁵D₀→⁷F₁ transition.‡ The various means of calculating τ^rad have recently been tested on Ln^III dipicolinates (Ln^III = Eu, Tb, Yb).³² Please note that contrary to a widespread belief, the radiative lifetime is not a constant for a given Ln^III ion; it depends both on the emitting level and on the metal-ion surroundings (see eqn (6) in which n intervenes, for instance).

Energy transfer mechanisms and design of luminescent edifices. Population of Ln^III ion excited states by feeding level(s) of the bound ligands was discovered by Weissman in 1942³³ and is referred to as luminescence sensitization or antenna effect. This phenomenon can be modelled using Jablonsky’s diagram (Fig. 2) and is quite intricate since several mechanisms may be involved: (i) exchange or Dexter’s type, with a distance dependence in exp(–βR_DA), where β is a parameter determined by the strength of electron–lattice coupling; the selection rule is |ΔJ| = 0, 1 (but not J = J′ = 0), (ii) dipole–dipole or Förster’s type, with a distance dependence between donor and acceptor in (R_DA)⁻⁶, and (iii) dipole-multipolar with distance dependences in 1/[(R_DA)^λ+2]² (λ = 2, 4, 6); selection rules for the latter two mechanisms are |ΔJ| ≤λ≤ (J+J') with λ = 0, 2, 4, 6 (but J = J′ = 0 is forbidden).³⁴ Additionally, diverse types of feeding levels can funnel energy onto the Ln^III ions: singlet ¹S*, triplet ³T*, intra-ligand charger transfer (ILCT), ligand-to-metal charge transfer (LMCT), d-transition metal charge-transfer (³MLCT), 4f–5d, and, sometimes, 4f states. Occasionally a combination of such states is operating. Energy is usually transferred onto Ln^III levels with higher energy than the emissive level; otherwise, back energy transfer occurs, resulting in low quantum yields and short, temperature-dependent lifetimes.

Fig. 2 Schematic representation of energy absorption, migration, emission (plain arrows) and dissipation (dotted arrows) processes in a lanthanide complex. ¹S* or S = singlet state, ³T* or T = triplet state, A = absorption, F = fluorescence, P = phosphorescence, k = rate constant, r = radiative, nr = non-radiative, IC = internal conversion, ISC = intersystem crossing, ILCT (indices IL) = intra-ligand charge transfer, LMCT (indices LM) = ligand-to-metal charge transfer. Back transfer processes are not drawn for the sake of clarity.

One of the main energy migration path implies Laporte- and spin-allowed ligand-centred absorptions followed by intersystem crossing (¹S* →³T*, k_ISC), ³T* →Ln* (k^et) transfer, and metal-centred emission. It is noteworthy that although important, this energy transfer path is by far not the only operative one. For instance, Kleinerman reached the conclusion, based on a systematic study of over 600 lanthanide chelates, that excited singlet states contribute to the transfer and may even be the privileged donor states, depending on the relative values of the rate constants for the various intervening processes.³⁵ Such cases are documented for both Eu^III,³⁶ and Tb^III.³⁷

Photophysical requirements for highly emissive compounds are related to the two parameters intervening in eqn (5): energy transfer (η_sens) and minimization of non-radiative processes (Q^Ln_Ln). The first one is difficult to master in view of its intricate mechanisms. Some authors have nevertheless established phenomenological rules which ought to be used with caution: they rely on the simplified idea that the ¹S*–³T*–Ln* energy transfer path is the only one operative and on considering that the sole parameter of importance is the energy difference between ³T* and the emitting Ln^III level. The following lessons can be drawn from these data.^38–40

 

• The largest values of quantum yields occur when the triplet state energy is close to the energy of one of the higher excited states of the metal ion; if the energy of the feeding state becomes closer to the energy of the emitting state, back transfer operates. For Eu^III and Tb^III, a “safe” energy difference minimizing this process is around 2500–3500 cm⁻¹ and this probably applies to the other Ln^III ions too.

• For Eu^III, the energy of the triplet state corresponding to the largest quantum yields depends on the type of ligand: it is close to the energy of the ⁵D₀ level for Schiff-base complexes,⁴⁰⁵D₁ level for β-diketonates,³⁸ and ⁵D₂ level for polyaminocarboxylates.³⁹

• When comparing complexes with Eu^III and Tb^III with the same polyaminocarboxylate ligands, the maximum quantum yield values reached for Tb^III are larger than those for Eu^III: this reflects the smaller Eu(⁵D₀–⁷F₆) energy gap compared to Tb(⁵D₄–⁷F₀).

 

Since efficient ISC transfers take place when the energy difference between the singlet and triplet states is around 5000 cm⁻¹, ligand designers try to keep to the following rules: ΔE(¹S*–³T*) ≈ 5000 cm⁻¹ and ΔE(³T*–Ln* emissive level) in the bracket 2500–3500 cm⁻¹. However, minute energy differences in the ligand states sometimes lead to large differences in overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor, an essential parameter of the overall energy transfer process, thus resulting in large differences in quantum yield. When bound to soft-donor ligands, influence of the LMCT states becomes important: in complexes with diethyl-dithiocarbamate, [Ln(Et₂dtc)₃(bpy)] for instance, emission of Eu^III is quenched (E_CT = 19 300 cm⁻¹) but not of Yb^III (E_CT = 28 300 cm⁻¹).⁴¹

2.2 Information gained from lanthanide luminescence

Any luminescent lanthanide ion may act as a probe, but some ions bring more information (e.g. Eu^III), or are more luminescent (e.g. Tb^III) than others, which explains their preferential use. The following section will mainly deal with Eu^III. Generally speaking a lanthanide luminescent tag functions either as a structural probe deciphering the symmetry of the chemical environment and, partly, the composition of the inner-coordination sphere, or as simple analytical (bio)marker, the switching on (or off) or the modulation of its luminescence representing the analytical signal.

Structure probing. In view of its non-degenerate emissive state ⁵D₀, the Eu^III ion is most appropriate as luminescent structural probe for the determination of the number of metal ion sites in a compound, their symmetry, and their respective population. Additionally, solvation numbers may be determined for several Ln^III ions by the lifetime method.

 

•Number of metal-ion sites, N. High resolution of the Eu(⁵D₀→⁷F₀) transition, which is unique for a given chemical environment associated with spectral decomposition with Lorentzian–Gaussian shape functions^42,43 gives a direct access to N. Experimentally, laser-excited excitation spectra of the ⁵D₀←⁷F₀ transition yield more sensitive results.

•Composition of the first coordination sphere. The energy of the ⁵D₀←⁷F₀ transition at 298 K, [small nu, Greek, tilde]_calc in cm⁻¹, is correlated with the nephelauxetic effect δ_i generated by each coordinated group:⁴⁴

with C_CN being a constant depending on the coordination number and n_i the number of a given type of bound atom. This phenomenological equation is helpful but one has to realize that δ_i strongly depends on the Eu–L distance, so that the relationship does not always yield satisfying results if distances are not standard. Moreover only a limited number of δ_i parameters have been worked out.

•Population analysis. When several Ln^III sites are present, their relative populations P_i can be determined by analysis of the multi-exponential luminescence decay:

In these cases, an average lifetime can be defined:Alternatively, since the Eu(⁵D₀→⁷F₁) transition is purely MD and has therefore an intensity independent of the chemical environment, its decomposition into individual spectra determined by selective laser-excitation of the various sites and followed by integration gives access to P_i values.⁴⁵

•Site symmetry. This determination is best performed by analysing the Eu(⁵D₀) emission, deciphering the crystal field splitting of the ⁷F_J levels and comparing them to theoretical predictions.^23,45 To avoid wrong conclusions, vibronic contributions should be identified.⁴⁶

•Strength of the Ln–L bond. The intensity of vibronic satellites, which are particularly intense when associated with hypersensitive transitions, is proportional to the ligand-to-metal bond strength and constitutes a useful measure of the latter.⁴⁷

•Solution state of the Ln^III ion. Lanthanide luminescence is very sensitive to the quenching by high-energy vibrations, particularly O–H. This quenching can be turned into an advantage for calculating the number q of inner-sphere bonded water molecules by measuring the lifetime in both water and deuterated water. Provided that the O–H quenching is the main non-radiative process operating (i.e. in absence of other temperature-dependent processes such as photo-induced electron transfer or back transfer), phenomenological equations can be worked out (Table 3):

q = A(Δk_obs−B) −C

Δk_obs = k_H2O−k_D2O = 1/τ_H2O− 1/τ_D2O (13)

Simpler relationships for polyaminocarboxylates rely on the sole determination of k_H2O:

q = Ak_obs−C (14)

The latter equations are however more difficult to transpose to other classes of compounds. For Nd^III, several equations have been proposed, but their reliability is low.

Table 3 Parameters for the phenomenological equations (13) and (14)

Ln	Solvent	Ligands	A	B	C^a	Δkobs units	Ref.
a q^N is the number of N–H oscillators in the second coordination sphere.
Eu	H2O	Various	1.11	0.31	0	ms	48
Eu	H2O	Cyclen derivatives	1.2	0.25	0.075q^N	ms	27
Tb	H2O	Cyclen derivatives	5.0	0.06	0	ms	27
Yb	H2O	Cyclen derivatives	1.0	0.2	0	μs	27
Yb	MeOH	Cyclen derivatives	2.0	0.1	0	μs	27
Nd	H2O	Polyaminocarboxylates	0.36	0	2.0	μs	49
Sm	H2O	Polyaminocarboxylates	25.4	0	0.37	μs	50
Dy	H2O	Polyaminocarboxylates	21.1	0	0.60	μs	50

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•Donor–acceptor distances. Distances between two metal-ion sites or between a chromophore and an ion, R_DA, may be determined by measuring the lifetimes of the donor (D) in presence (τ_obs) and in absence (τ₀) of the acceptor (A):

Assuming Förster’s dipole–dipole mechanism of transfer, the efficiency of the latter is given by:R₀ being the D–A distance for which the yield is 50%; this parameter depends on the nature of the D–A pair involved.

Analytical probes. Lanthanide emission is either simply detected or modulated after suitable interaction with the analyte. There are several ways of modulating the emission signal, which have been described in our previous review (see Fig. 3 of ref. 1): interaction with the analyte induces either (i) a change in Ln^III solvation, (ii) a modification of the energy transfer ability of the bound ligand(s), or (iii) energy transfer from the analyte onto the reporter ion. Alternatively, quenching may be enabled instead of sensitization. The best way of carrying out an analysis is the ratiometric method.⁵¹ Lanthanide sensors for cations, anions, pH, pO₂, and aromatic molecules have been designed along these lines.^52,53

When it comes to bio-analysis, time-resolved detection enhances considerably the signal-to-noise ratio and additional selectivity can be obtained by using FRET (Förster resonant energy transfer) methodology,⁵⁴ a common practice in homoimmunoassays⁵⁵ or high-throughput screening.⁵⁶

3. Near-infrared luminescence

Compounds exhibiting lanthanide near-infrared (NIR) luminescence mainly fall into two categories: (i) purely inorganic substances, mostly oxides or doped semiconductor materials (e.g. GaN); they find use in optical fibres, lasers, and planar amplifiers for telecommunications, as well as in light-emitting diodes (LEDs), security inks, or bio-analysis; chalcogenide clusters with low-energy phonon density of states may be classified in this group as well; (ii) complexes with organic ligands, developed in the hope of producing electroluminescent materials for the same applications, but more economical and more versatile; this field has been tremendously stimulated by the discovery in 1990 of electroluminescence in conjugated polymers (namely poly(para-phenylene vinylene), PPV).⁵⁷ Since our last review on lanthanide NIR luminescence covering the literature until September 2006,⁵⁸ more than two hundred papers have appeared. In this section, we mainly focus on molecular compounds of Nd^III, Er^III, Yb^III, and to a lesser extent Pr^III, Tm^III, and classify them according to the sensitization mode. Furthermore attention is given to reports containing quantitative data (quantum yields, lifetimes) or describing innovative systems and/or energy transfer paths. Quantitative data remain scarce because few laboratories are equipped for quantum yield measurement in the NIR range; as an alternative, intrinsic quantum yields are often calculated from lifetimes, with a “literature value” of τ^rad, therefore they have to be considered with extreme care.

3.1 Sensitization by organic ligands

Despite the dramatic limitations inherent to the use of organic ligands for the sensitization of NIR luminescence,⁵⁸ namely the presence of high-energy vibrators such as C–H⁵⁹ or C=C, considerable efforts have been undertaken recently to test several classes of ligands.^60–100 A listing of the corresponding photophysical properties is provided in Table S1 (ESI†) while complexes with the best photophysical properties are listed in Table 4. Altogether, the top overall quantum yields obtained for Nd^III, Er^III and Yb^III lie in the ranges 0.1–0.4, 0.01–0.03 and 0.6–1.4%, respectively, for solid-state samples and 0.01–0.07, 0.01–0.02 and 0.5–3.8%, respectively, for solutions in non-deuterated organic solvents. Data for aqueous solutions are much scarcer and are around 0.03% for Nd^III and 0.14% for Yb^III. For the latter ion, the maximum overall quantum yield in water remains that reported by Korovin et al. for a dinuclear macrocyclic complex (0.53%).¹⁰¹

Table 4 Selected NIR-emitting systems published recently (2006–2009) and featuring luminescence sensitization by organic ligands (see Schemes 1–4)

Complex	Sample	Q^LLn (%)	τ/μs	Q^LnLn (%)	Ref.
a Average lifetimes: see eqn (17).b Recalculated from τav assuming the radiative lifetime found for Er-doped silica: 14 ms.
[Nd(2c)3]·MeOH	Solid	0.40	1.57	n.a.	62
[Nd(4h)3]	Solid	0.33	1.82	0.67	64
[Nd(8)3]	CD3CN	n.a.	44	n.a.	71
[Nd(21b)3(bpy)]	Toluene	0.072	n.a.	n.a.	83
[Nd(H36a)]	H2O (pH 7.4)	0.027	0.15	n.a.	67
[Er(7b)3]	Nanorods	n.a.	300^a	2.1^b	69
[Er0.5Yb0.5(7b)3]	Nanorods	n.a.	733^a	5.2^b	69
[Er(2c)3]	Solid	0.033	4.05	n.a.	62
[Er(8)3]	CD3CN	n.a.	741	n.a.	71
[Er(22)4]^−	MeCN	0.021	n.a	n.a.	98
[Yb(2c)3]·3H2O	Solid	1.40	20.6	n.a.	62
[KAlYb(5b)3](OTf)	Solid	1.17	22.6	n.a.	66
[Yb(8)3]	CD3CN	n.a.	1111	n.a.	71
DMSB[Yb(21f)4]	MeCN	n.a.	46.4	n.a.	99
[Yb(22)4]^−	MeCN	3.8	24.6	n.a.	98
[Yb(21b)3(phen)]	Toluene	1.28	n.a.	n.a.	83
[Yb(3)2](NO3)	MeCN	1.1	n.a.	n.a.	63
[Yb(6c)]	H2O (pH 7.4)	0.14	2.05	n.a.	100

---

Since erbium tris(8-hydroxyquinolinate) was shown to be brightly electroluminescent at 1.5 μm and compatible with silicon technology,¹⁰² 8-hydroxyquinolinate has been a chromophore commonly incorporated in ligands designed for sensitising Ln^III NIR luminescence (Scheme 1). As a matter of fact, the simple tridentate ligands H2, which lead to neutral, well defined tris(complexes), perform quite well in populating the Ln^III excited states, particularly when substituted by two bromine atoms in H2c. Indeed in going from H2a to H2c, the overall quantum yields increase by a factor 2.5–2.9 while the lifetimes are lengthened two- to four-fold. The main factor here is the removal of C–H vibrators on the ligand core, in addition to an increased heavy-atom effect.⁶² These building blocks have successfully been incorporated into ditopic ligands self-assembling into trimetallic helicates with overall formulae [KLn₂(5a)₃]OTf or [KAlLn(5b)₃]OTf (OTf is the triflate anion) in which the Yb^III ions retain most of the photophysical properties of the mononuclear precursors.⁶⁶

Scheme 1 Ligands incorporating 8-hydroxyquinoline moieties.^{60–64,66,67,100}

Another way of benefiting from the chromophoric properties of 8-hydroxyquinolinate together with a large chelate effect is to incorporate these units in tripodal⁶⁷ or tetrapodal¹⁰³ ligands fitted with sulfonate groups for ensuring water solubility. The tripod H₆6a performs less well than its tetrapodal counterpart, but the sensitization of the Yb^III luminescence remains noticeable in aerated water (Q^L_Ln = 0.13%). An additional advantage of the resulting podates is their stability which is comparable to that of edta chelates so that they may be envisaged as probes for in vitro analysis.⁶⁷ When the pivotal amine group is replaced by an 1,4,7-triazacyclononane core, similar results are obtained with quantum yields of 0.60 and 0.14% for the solid state sample [Yb(6b)] and an aqueous solution of [Yb(6c)], respectively.

Substituting the 2-position of 8-hydroxyquinoline by a benzoxazole moiety results in N,N,O-chelating units H4 bearing an extended chromophore, which can be modulated by grafting diverse substituents. Large absorption in the visible (508–527 nm, ε = 7.5–9.6 × 10³ M⁻¹ cm⁻¹ in the tris complexes) is another plus for these systems which proved to be particularly efficient for Nd^III luminescence with quantum yields up to 0.33%. Similarly to the chelates with H2, 5,7-dihalogenation of the 8-hydroxyquinoline core is beneficial, the Q^L_Nd increasing by a factor of two.⁶⁴ When benzoxazole is replaced by benzimidazole, yielding a tridentate N,N,N ligand, similar effects are observed for Nd^III, with [Nd(4n)₃]·3H₂O having Q^L_Nd = 0.34%.⁶⁵ Finally, an alternative approach to sensitising Yb^III luminescence has been recently proposed in which 8-hydroxyquinoline is decorated with a rhodamine chromophore (triplet state energy ≈17 000 cm⁻¹) via a carboxy-hydrazone linker (H3). Two tetradentate ligands provide a saturated coordination environment for Yb^III and the reported overall quantum yield amounts to 1.1% in acetonitrile.⁶³

In an effort to minimize C–H quenching in Er^III complexes meant for applications in telecommunications, bis(phenyl)phosphinic acid (H7a), has been fully fluorinated (H7b, Scheme 2).^68,69 The corresponding [Er(7)₃] salts simply obtained by reacting the metal chloride with the free acid in water, are thermally stable, can be easily dehydrated, and possess a polymeric structure. While the luminescence decay of the Er(⁴I_13/2→⁴I_15/2) transition at ≈1.5 μm is a single exponential function (τ = 4.9 μs) for the unfluorinated salt [Er(7a)₃], the perfluorinated species exhibit a more complicated behaviour and the decay had to be analyzed in terms of a “stretched exponential” function, β representing an empirical factor and 〈τ〉 an average lifetime:

Here Γ(1/β) is the gamma function.§ For [Er(7b)₃], 〈τ〉 = 300 μs,¶ a 60-fold improvement over the unfluorinated species.⁶⁸ Concentration quenching is another concern in inorganic optical fibres, severely limiting the useful Er^III concentration. Such quenching also occurs in [Y_xEr_1−x(7c)], as demonstrated by the lifetimes which increase from 〈τ〉 = 179 μs (x = 0; β = 0.73) to 714 μs (x = 0.995; β = 1) upon dilution. However, for x > 0.7, the lifetime is approximately constant, pointing to the (large) upper limit of Er^III concentration (up to 30 mol%) that could be utilized in optical devices based on this system.⁷⁰

Scheme 2 Phosphinic acid- and hydroxypyridine-containing ligands.^68–74

A slightly modified synthetic procedure of [Er(7b)₃] gives rise to nanorods with double-exponential luminescence decays. The average Er(⁴I_13/2) lifetime is equal to 300 μs, in perfect agreement with the above-mentioned value. It is considerably lengthened when the Er(⁴I_11/2) level is populated by the long-lived Yb(²F_5/2) level in [Er_0.5Yb_0.5(7b)₃] nanorods, reaching 733 μs.⁶⁹ In a similar attempt to eliminate the detrimental effect of C–H bonds, the imidodiphosphinate chelating unit was fitted with perfluorinated phenyl groups. Not only does the ligand H8 lacks C–H bonds, but it also features a low-lying πσ* (≈14 000 cm⁻¹) state mainly located on the perfluorobenzene units and very favourable to NIR sensitisation; as a matter of fact, the Nd^III, Er^III and Yb^III ions have among the longest lifetimes recorded for complexes with organic ligands, up to 1.1 ms for [Yb(8)₃] in deuterated acetonitrile for instance.⁷¹ After testing successfully the 1-hydroxypyridin-2-one chromophore (H₄9) for stimulating Eu^III emission, and considering the large stability of the 1:2 complexes (pEu = 18.6),¹⁰⁴ Raymond et al. have investigated the ability of the similar ligands H₄10 and H₄11 to sensitise NIR luminescence (Scheme 2). In Tris buffer (pH 7.4), the ≈1-μm lines of [Pr(H₂10)₂]⁻ (¹D₂→³F₄) and [Ho(H₂10)₂]⁻ (⁵F₅→⁵I₇) are indeed seen, in addition to visible emission; the corresponding lifetimes are τ(¹D₂) = 8.0(4) ns and τ(⁵F₅) = 6.5(3) ns, the latter being a very rare example of lifetime determination for a Ho^III complex in aqueous solution. Stability constants for the complexes with (H₂11)²⁻, log β_1i0 are sizeable, 11 (i = 1) and 19 (i = 2) and correspond to pLn = 14.7; the Nd^III and Yb^III bis(complexes) display NIR luminescence in MeOH and Tris buffer (pH 7.4).⁷⁴ The hexadentate tripodal ligand (H₆12) encapsulates the Yb^III ion into a coordinative environment completed by two water molecules, as determined at pH 7.4 from eqn (13) with τ_H2O = 0.37 and τ_D2O = 8.06 μs.⁷⁴

Reaction of sodium benzoate derivatives (Scheme 3) with LnCl₃ in THF and in presence of phenanthroline (phen) afforded well characterized dimeric compounds [Ln(13a)₃(phen)]₂ (Ln^III = Er, Yb) in which the metal ions lie at a short distance (Er–Er = 406 pm).⁷⁵ The Er^III emission is enhanced in co-crystals of Er^III and Yb^III in the ratio 7 : 3 due to Yb-to-Er energy transfer, the efficiency of which reaches 55% as calculated from the lifetimes of Yb(²F_7/2), 58.9 μs in the homodinuclear compound and 26.7 μs in the co-crystallized dimers, see eqn (15). The energy transfer process from a benzothiazole chromophore grafted onto a p-substituted benzoic acid to Ln^III ions in [Ln(H13x)(tpy)] (x = b, c, Ln^III = Nd, Er, Yb; tpy is terpyridine) depends both on the solvent (toluene or ethanol) and on the para substituent of the benzoic acid moiety: it is less efficient in ethanol (except for Yb^III and x = b) and more efficient for x = b compared to x = c because the hydroxyphenyl derivative exhibits excited state intramolecular proton transfer. The estimated rate of energy transfer in methanol amounts to 5.72 × 10⁹ s⁻¹ for Nd^III and 3.36 × 10⁹ s⁻¹ for Er^III but the sensitisation efficiencies remain very modest (0.2–0.9 × 10⁻⁴ for Er^III in toluene).⁷⁶ Naphthalene substitution in H14 leads to a noticeable improvement of the latter parameter (2.5–2.8 × 10⁻⁴ in chlorobenzene).⁷⁸ Among the other carboxylates tested,^{77,79–81,97} [Yb(15)₃] displays a modest 0.7% overall quantum yield in aerated acetonitrile⁷⁷ while Q^Yb_Yb = 0.15% for [Yb(17)]⁻ in water (pH 7.0);⁷⁹ on the other hand, [Eu(15)₃] is highly luminescent (Q^L_Eu = 60%).⁷⁷

Scheme 3 Carboxylic and polyaminocarboxylic acids.^{75,77–81,91,97}

β-Diketonates remain much studied luminescent complexes. Ligand-to-metal energy transfer in [Er(tta)₃(tpy)] has been proved to involve the excited triplet state of the ligand and since oxygen does not quench the NIR luminescence, its rate is faster than 10⁷ s⁻¹.¹⁰⁵ In toluene solution, β-diketonate ternary complexes with phen^82,83 and bpy⁸³ have sizeable overall quantum yields, up to 1.28% for ligand H21b and Yb^III (Scheme 4),⁸³ but fail to yield highly luminescent complexes with Nd^III and Er^III. Noteworthy features are the visible excitation of [Ln(20a)₃(phen)] via an ILCT state⁸² and the observation of an appreciable quantum yield for the bis(hydrate) [Yb(21b)₃(H₂O)₂]⁸³ as also pointed out for [Nd(18)(H₂O)₂]⁻.⁸⁰ Moreover, fluorination of the diketonate core allied with complexation by fluorinated phosphoryl ternary ligands in [Er(21e)₃(OP(C₆F₅)₃)₂] results in a complex with a lifetime of 16.8 μs and a quantum yield larger by one order of magnitude with respect to [Er(21d)₃(OP(C₆F₅)₃)₂], 0.17%. The radiative lifetime is up to 4 ms and the threshold intensity as low as 3 W cm⁻², an encouraging result for designing planar waveguides pumped by LEDs.¹⁰⁶

Scheme 4 β-Diketones and miscellaneous ligands.^{82,83,85,86,98,99}

Intense NIR luminescence from Nd^III, Er^III and Yb^III is detected when anionic tetrakis(diketonates) [Ln(21f)₄]⁻ are associated with the hemicyanine cationic chromophore DMSB (Scheme 4). Lifetimes of the excited states in acetonitrile are as long as 1.0, 3.2 and 46.4 μs for ⁴F_3/2, ⁴I_13/2 and ²F_5/2, respectively.⁹⁹

The azulene derivative H22 (Scheme 4) is a versatile ligand with a low-energy triplet state (14 300 cm⁻¹); it transfers energy onto several NIR-emitting ions, Nd^III, Er^III, Tm^III and Yb^III, the quantum yield for the latter being up to 3.8% in acetonitrile.⁹⁸ The CH-devoid ligand H23 forms crystalline Cs₂[Ln(23)₅·nEt₂O] complexes which have particularly long lifetimes: τ = 22.9 (Er^III, n = 1) and 〈τ〉 = 159 μs (Yb^III, n = 0.5).⁸⁵ In a similar attempt, H24a was used as chelating unit with low-energy vibrations in conjunction with phen and DIP (dipyridophenazine) as antennae and the intrinsic quantum yields of [Yb(24a)₃(phen)₂] and [Yb(24a)₃(DIP)] in dmso-d₆, measured upon excitation at 940 nm, amount to 7.4 and 9.1%, respectively.⁸⁶ Inserting lanthanide ions into nanoparticles is a way of decreasing radiationless deactivation: the Yb(²F_5/2) lifetime increases from 12.4 μs in [Yb(24b)₄]⁻ (Scheme 4) to 68 μs for the ions inserted into the core of tropolonate-decorated NaY_0.8Ln_0.2F₄ (Ln^III = Nd, Yb) nanocrystals; a similar effect is observed for Nd(⁴F_3/2).^84,107

Unusual sensitisation of NIR luminescence has been demonstrated in [Ln(hfa)₃(APB)] (Ln^III = Nd, Er, Yb),¹⁰⁸ in which the anthracene antenna (A) proceeds by electron transfer: [¹A*–PB–Ln] → [A˙⁺–PB˙⁻–Ln] → [³A*–PB–Ln] → [A–PB–Ln*] (PB is a chelating unit of the APB, 2-(2-pyridyl)benzimidazole, Scheme 4).

Finally, a dendrimeric Yb^III complex with ethylenediamine core and quinoline chromophores proved to be a useful NIR sensor of thiocyanate.¹⁰⁹

3.2 Sensitisation by macrocyclic ligands

A neutral bioprobe combining visible-light excitation (488 nm), strong coordinating units, and NIR emission was obtained by grafting fluorescein on the cyclen (1,4,7,10-tetrazacyclododecane) framework (Scheme 5); as a result, luminescence of [Nd(25)] can be detected in time-resolved mode in water at pH 8 (τ = 2.3 μs).⁸⁷ Quinoxaline is an alternative chromophore which has the advantage of sensitising both Nd^III, Eu^III and Yb^III, but the lifetimes recorded for [Ln(26)]³⁺ in methanol remain short, the less coordinating amide groups allowing interaction with the solvent (q = 0.4 and 0.7 for Nd^III and Yb^III, respectively).⁸⁸

Scheme 5 Cyclen derivatives.^87,88

Most of the work with macrocyclic ligands has concentrated on porphyrinate derivatives. Indeed porphyrins have well defined absorptions (Q and Soret bands at ≈420 and 550–590 nm) and emission bands (650, 710 nm) and their low excited states are convenient for populating excited states of NIR-emitting Ln^III ions. Interest in lanthanide porphyrinates is due to (i) their ability to serve both as charge-transport, electron hole recombinant, and NIR emitter materials in electroluminescent devices, (ii) their potentiality in photodynamic therapy of cancer, and (iii) their optical limiting properties. Optical limiters are transparent at normal light intensities and opaque to very bright light, henceforth they help avoiding damages caused to human eyes or optical components by sudden and intense laser pulses. Their characteristics are low ground state absorption and strong excited state absorption, which is the case for porphyrinates (see ref. 110 for a summary on lanthanide porphyrinates as optical limiters). For efficient NIR sensitisation, the coordination of the Ln^III ion in monoporphyrinates has to be completed by a capping ligand to avoid solvent interaction; these ligands include β-diketonates and tripodal anions such as L_OR⁻ (cyclopentadienyl-tris(dialkylphosphito)cobaltate) or Tp^R− (hydridotris(pyrazolyl)borate). Tetraphenylporphyrins are the most used ligands and the influence of substitution on the phenyl or on the five-membered cycles has been investigated (Scheme 6).

Scheme 6 Tri- and tetra-phenylporphyrins.^89–91

The monoporphyrinates [Ln(27)(H₂O)₃] react with cyanometallates K₂M(CN)₄ (M^II = Ni, Pt) and K₃Fe(CN)₆ to form trimetallic species with a linear Ln–CN–M–CN–Ln core in which the NIR luminescence is partially quenched.⁸⁹ Preparation of monoporphyrinate is often difficult and a new method has been proposed, starting from the free base and Ln^III acetate in 1,2,4-trichlorobenzene. The obtained [Yb(27a)(O₂CCH₃)(MeOH)₂] complex is a starting material for the preparation of highly functionalized porphyrinates. For instance, methanol can easily be replaced by a 4-methylphenanthroline chromophore and the corresponding eight-coordinated complex has Q^Yb_Yb = 0.86%.⁹⁰

Metalloporphyrinates are also viable chromophores for Ln^III NIR luminescence. For instance, three [Pt(27c)]⁻ and one terpyridine molecule assemble around an Er^III ion to form fully saturated nine-coordinate complex {Er[Pt(27c)]₃(tpy)} with noticeable NIR emission. Modelling of the energy transfer process pointed to the Er(⁴F_9/2) state receiving 83% of the energy transferred from the ligand triplet state, the other receiving level being Er(⁴I_9/2).⁹¹ The quantum yield of the Yb^III luminescence in [Yb(28)(acac)] and [Yb(29)(acac)] can be altered by varying the substituents on the porphyrins and [Yb(28b)(acac)] was found to be the best emissive complex, with Q^L_Yb = 0.47%.⁹² No quantum yields have been measured for Yb(L_OR⁻) complexes with derivatised tetraphenylporphyrins (Scheme S1, ESI†), but Yb(²F_5/2) lifetimes in the range 30–40 μs are reported in toluene⁹³ and around 10 μs in water.⁹⁴ On the other hand, Yb^III chelates with N-confused porphyrins display short lifetimes in toluene (0.2–0.4 μs).⁹⁵

3.3 Sensitisation by metal-containing Schiff bases

In addition to metalloporphyrins, authors have made use of Schiff-base complexes with zinc¹¹¹ and cadmium¹¹² to sensitise NIR luminescence. Quantitative data are very rare, except for [Ln(NO₃)₃Zn(32)] (Ln^III = Nd, Yb, Scheme 7) in acetonitrile for which lifetimes are available: 1.23–1.27 and 13.40–15.89 μs for Nd^III and Yb^III derivatives, respectively.⁹⁶

Scheme 7 Zinc complexes with Schiff bases.

3.4 Sensitisation by d–f energy transfer

The intricate energy transfer processes leading to population of 4fⁿ excited states in which broad excited states from the ligands interact with narrow, long-lived metal-ion states render the design of suitable molecular systems very experimental. A more controlled approach is to take advantage of intermetallic communication between two (or more) metal ions inserted into polymetallic edifices so that directional energy transfer becomes feasible.

Such a strategy has been mostly used for sensitising NIR emitting Ln^III ions;¹ long-lived ³MLCT states of d-transition metals (e.g. Ru^II, Re^I, Os^II, Au^I, Pt^II, Ir^III) can be excited by visible light and transfer efficiently their energy onto the 4fⁿ manifolds, thus providing an effective pathway for energy migration within heterometallic complexes. The structures of the latter are shown on Scheme 8 and their photophysical properties are listed in Table S2 (ESI†).

Scheme 8 Pt–Ln complexes with bpy and tpy.^113–117

The most studied systems are those containing Pt^II and their design is described in Fig. 3: Pt^II is chelated by a bidentate ligand and linked to two Ln^III complexes, usually [Ln(hfa)₃] (hfa is hexafluoroacetylacetonate) or [Ln(tta)₃], via an electronic relay featuring alkyl and aromatic units.^113–118 Alternatively, both Pt^II and Ln^III ions can be electronically connected by a bis(bidentate) ligand such as bppz (2,3-bis(2-pyridyl)pyrazine).¹¹⁸

Fig. 3 Design of a PtLn₂ edifice for Pt–Ln transfer (top) and practical example of transfer rates in PtLn₂(33f)¹¹⁵ (bottom).

In the complexes Pt_xLn_2x(33a)_x (x = 1, 2; Scheme 8) the bidentate ligand is 1,2-bis(diphenylphosphino)methane (dppm), which is bridging in the complexes with x = 2. Excitation into the ³MLCT (x = 1) or ³MMLCT (x = 2) states of the Pt^II alkynyl chromophores occurs in a spectral range (360–500 nm) in which the model complex [Ln(hfa)₃(alkynyl-tpy)] does not absorb. The Pt-to-Ln energy transfer is incomplete, as ascertained by the observation of an ³ILCT luminescence in solution, as well as a weak ³MMLCT emission. The rates of transfer in the hexanuclear complexes could be estimated from the lifetimes of the residual Pt–ligand phosphorescence: k_ET(Nd^III), 6.07 × 10⁷ s⁻¹, is 286-fold larger than k_ET(Yb^III), a fact attributed to the numerous Nd^III acceptor levels in the energy range of the ³MMLCT state.¹¹³

Similar energy transfers from mixed ³MMLCT and ³ILCT states are operative in PtLn₂(33b,c) (Ln^III = Nd, Yb), in which dppm has been replaced by the longer molecules dppe (1,2-bis(diphenylphosphino)ethane) and dppp (1,2-bis(diphenylphosphino)propane).¹¹⁴ The rate of the Pt-to-Ln energy transfer is modulated by the nature and length of the electronic relay as deciphered in comparing PtLn₂(33d,e,f) for which the bridge is (i) an acetylide and the coordinating unit bpy (33e) leading to a Pt–Ln distance of ≈8.6 Å, (ii) an acetylide–phenylene–tpy unit (33d, Pt–Ln ≈ 14.1 Å), or (iii) an acetylide–phenylene–acetylide–bpy moiety (33f, Pt–Ln ≈ 14.9 Å). As expected, the longer bridge results in a slower energy transfer, 1.4 × 10⁷ s⁻¹ for Yb^III as compared to >10⁸ s⁻¹ for the two other complexes. Here again, the rate of transfer in the Nd^III edifice with 33f is faster (1.24 × 10⁸ s⁻¹) than in the Yb^III chelate and this is also true for Er^III (>10⁸ s⁻¹) which possesses several electronic levels in the range of the ³MLCT state.¹¹⁵

Analogous data are extracted from a comparison between PtLn₂(33g) with Pt–Ln ≈ 8.4 Å and PtLn₃(33h) with Pt–Ln ≈ 13.3 Å: energy migration across the longer Pt–bpyC≡C–C≡Cbpy–Ln array is less efficient, 2.82 × 10⁶ s⁻¹ versus >10⁸ s⁻¹, and k_ET(Nd^III) ≈20 k_ET(Yb^III) in the tetranuclear species.¹¹⁶ The corresponding transfer rates are somewhat reduced in the higher nuclearity complexes Pt₆Ln₆(33j): in contrast to a seemingly complete transfer from the Pt(bpy)(acetylide)₂ chromophore evidenced in the PtLn₂(33i) building blocks, the transfer from the Pt(bpy)(C≡CR)₂ unit is incomplete and rate constants amount to 1.02 × 10⁷ and 1.83 × 10⁵ s⁻¹ for Nd^III and Yb^III, respectively.¹¹⁷ Following a similar strategy as for the Pt–Ln assemblies, Chen et al. have come up with Au–Ln polymetallic complexes (Scheme S2, ESI†) which display ILCT, ³MMLCT and f–f emission (Ln^III = Nd, Er, Yb).¹¹⁹

The bppz ditopic ligand provides two advantages over other bridging ligands: the association constants between the 5d-transition chromophore and the Ln^IIIβ-diketonates are usually large and the 5d-Ln^III separation remains substantial (≈7.4 Å in ReGd(35b) for instance). It has been used to compare Pt^II- and Re^I-chromophores (see Scheme 9). In the PtLn(33l) complexes, the Pt-to-Ln rate of energy transfer decreases from 10⁹ s⁻¹ (Nd^III) to 1.4 × 10⁸ s⁻¹ (Pr^III). Additionally, the ³MLCT luminescence is also quenched in the PtGd and PtLu adducts in which none of the Ln^III ions has suitable accepting level. The effect is ascribed to an increase in vibrational quenching upon complex formation. This effect is very large in Re^I adducts, so that the additional quenching by the NIR-luminescent Ln^III ion does not affect further the lifetime of the ³MLCT level, thus no 5d–4f rate constant could be estimated.¹¹⁸ A Re(CO)₃(bpy)(py) chromophore has also been appended to a cyclen framework to produce a dual imaging agent and Yb^III luminescence was observed in ReYb(35c) upon energy transfer from the ³MLCT state.¹²⁰

Scheme 9 Bppz-, bpy- and cyclen-based complexes for studying Pt–Ln and Re–Ln energy transfers.^118,120,121

The Ir^III chromophores in the tetranuclear neutral complex Ir₃Yb(36a) (Scheme 10) transfer energy onto Yb(²F_5/2) with an efficiency of 65% relative to the efficacy of the energy transfer in the reference Yb^III chelate (i.e. without the appended Ir^III moieties) and the overall quantum yield is up to 0.7% in methylene chloride while the Yb(²F_5/2) lifetime amounts to 17.7 μs.¹²² An analogous lifetime (22.1 μs) has been recorded for the Ir₂Yb(36b) array and sensitised Nd^III and Er^III luminescence could be observed with this ligand system.¹²³

Scheme 10 Ir–Ln complexes.^122,123

Ruthenium and osmium [M(tBut₂bpy)(bpym)]X₂ (M^II = Ru, Os; bpym = 2,2′-bipyrimidine; X = Cl, NCS, PF₆) chromophores have low-energy MLCT states because of the presence of bpym and have consequently been tested as antennae for f-metal excitation. In fact, only RuLn(37a) and RuLn(37b) (see Scheme 11) with PF₆⁻ as counterion (³MLCT at ≈13 500 cm⁻¹) display Ln^III-sensitised luminescence for Nd^III and Yb^III, respectively.¹²⁴ An important issue is to determine which mechanism is operating in d–f energy transfer. The detailed study of MLn(37c–e) (M^II = Ru, Os; Ln^III = Nd, Er) by Ward et al. sheds light on this aspect.¹²⁵ Rate constants for the energy transfer have been calculated from

k_et = τ_q⁻¹−τ_u⁻¹ (18)

in which τ_q and τ_u are the lifetimes of the quenched (i.e. the dinuclear complex) and unquenched (i.e. the antenna alone) species; the yield of transfer is then obtained from eqn (15).

Scheme 11 Ru–Ln and Os–Ln complexes.^124,125

Analyzing the data in terms of Förster’s and Dexter’s mechanisms, the authors came to the conclusion that the first type of transfer is precluded because the critical distances extracted from the data modelled with this mechanism are small (≈8.5 Å in RuNd(37c)) and would correspond to an improbable, highly folded conformation of the chromophore. Therefore they concluded that energy is transferred via a Dexter’s super-exchange interaction mediated by the conjugated bridging ligands. Dexter’s mechanism requires direct orbital overlap and therefore is typically operating at short ranges. But when the donor and the acceptor are linked by conjugated bridging ligands, coupling occurs via the orbitals of the latter and Dexter-type energy transfer operates at longer distances, up to 20 Å in MLn(37d,e).¹²⁵

The increase in transfer yield from 46% for RuNd(37a) to 91% for RuNd(37d) despite an increase in Ru–Nd separation illustrates the role of the electronic relay provided by the phenyl group. On the other hand, the efficiency decreases to 44% in RuNd(37e) because of the increased Ru–Nd distance. Ru–Er (63%) and Ru–Yb (52%) transfers are smaller in RuLn(37d) due to reduced availability of f-states. Os–Nd energy transfer is much smaller, 3 and 36% for OsNd(37c) and OsNd(37d), respectively.

Finally, large efficiency for Ru–Nd energy transfer (90%) has been reported for a cyclam-based dendrimer. (Scheme S3, ESI†).¹²⁶

Polynuclear d–f coordination polymers. Upon coordination to [Ln(diket)₃] (diket = hfa, tta), the platinum-bipyridine–diacetylide chromophores Pt(33m), Pt(33n) and Pt(33o) (Scheme 9) produce covalently-linked polynuclear assemblies: [Pt(33m)Ln(tta)₃]_∞ and [Pt(33n)Ln(hfa)₃]_∞ are one-dimensional coordination polymers, while {Pt(33o)[Ln(tta)₃]₂} is a molecular trinuclear species. In concentrated CH₂Cl₂ solutions and in the solid state, Pt⋯Pt interactions generate ³MMLCT states which emit around 630 nm (0-phonon transition at ≈19 000 cm⁻¹) and which transfer energy onto Ln^III (Ln^III = Pr, Nd, Er and Yb). In solid state, the transfer rate constants for the two coordination polymers decrease from 10⁸ s⁻¹ (Nd^III) to 2–4 × 10⁷ s⁻¹ (Pr^III, Er^III), and 1–2 × 10⁶ s⁻¹ (Yb^III).¹²¹

To promote Ru-to-Ln energy transfer, heterometallic polynuclear assemblies and coordination networks have been prepared from d-block cyanometallates and lanthanide salts. A rich variety of structures have been characterized, discrete {[Ru(CN)₄(phen)]₄[Ln(phen)₂(H₂O)]₂}²⁻, honeycomb sheets, ladders, single-stranded chains, and chains of squares.¹²⁷ Rates of Ru-to-Ln energy transfer in the honeycomb sheets range from 2.5 × 10⁷ s⁻¹ for Yb^III to 5 × 10⁸ s⁻¹ for Nd^III.

In cyanide-bridged 3d–4f discrete [Ln(dmf)₄(H₂O)₃(μ-CN)Co(CN)₅] species and {[Ln(dmf)₄(H₂O)₂(μ-CN)₂Cr(CN)₄]}_∞ infinite chains, the red emission of the [M(CN)₆]³⁻ moieties is completely quenched when Ln^III = Nd, Yb, the quenching rate constants being >10⁸ s⁻¹.¹²⁸

Rare infinite triple-stranded helical motifs {[ErAg₃(dpa)₃(H₂O)]}_∞ or 3D porous sandwich-type frameworks {[ErAg₃(L)₂(H₂O)]}_∞ (L = 4-hydroxypyridine-2,6-dicarboxylate) display Er^III emission in the NIR.¹²⁹

3.6 Chalcogen-bound clusters and aryloxide complexes

As evidenced in the above description, non-radiative processes remain the main operative de-excitation paths in complexes with organic ligands, the intrinsic quantum yields being rarely larger than 1% for Yb^III while values for Nd^III and Er^III are orders of magnitude lower. Improving this situation requires designing edifices with much smaller vibrational energies and inorganic chalcogen-containing clusters with phonon frequencies in the range 450–700 cm⁻¹ are good candidates. In 2005, Brennan et al. reported an amazing chalcogenide-bound Er^III cluster, (thf)₁₄Er₁₀S₆Se₁₂I₆, with a record intrinsic quantum yield in the range 75–78%.¹³⁰ Recent investigations of related Nd^III and Tm^III chalcogen-bound clusters and their mononuclear precursors,^131–134 as well as mononuclear complexes with fluorinated phenoxide ligands OC₆F₅ (Table 5),¹³⁵ point to intrinsic quantum yields up to 41% for [(py)₂₄Nd₂₈F₆₈(SePh)₁₆], that is only a factor two or so smaller than in Nd-doped silica materials,¹³³ 16% for [(dme)₂Er(OC₆F₅)₃],¹³⁵ and 11% for [(dme)₄Tm₄(SPh)₁₂].¹³⁴ The intrinsic yields are calculated from eqn (3) with τ^rad estimated from eqn (2), (6), (7) and Judd–Ofelt parameters extracted from the absorption spectra; they can therefore be considered as being “experimental” data. In order to take into account energy migration among Ln^III ions, back transfer and potential up-conversion processes, luminescence decays were modelled by Monte Carlo simulations, from which averaged τ_obs data were calculated.¹³¹ The [(py)₂₄Nd₂₈F₆₈(SePh)₁₆] cluster, prepared by reaction of Nd(SePh)₃ with NH₄F in pyridine and subsequently crystallized upon layering with hexane or slow cooling, is shown on Fig. 4. It contains a central set of four 12-coordinate Nd^III ions and twenty four 8- or 9-coordinate Nd^III ions located on the surface.

Fig. 4 Fluoride cluster [(py)₂₄Ln₂₈F₆₈(SePh)₁₆] with C and H atoms removed for clarity. Reproduced with permission from ref. 133, © Wiley-VCH Verlag GmbH & Co 2008.

Table 5 Selected properties of Ln-containing NIR-emitting clusters (2007–2009)^a

Compound	Transition	λem/nm	τobs/μs	τ^rad/ms^b	Q^LnLn (%)	Ref.
a Excitation at 800 nm for Nd and Tm and at 980 nm for Er samples; dme = 1,2-dimethoxyethane, thf = tetrahydrofuran, py = pyridine.b Calculated from Judd–Ofelt theory (two decimal digits).c Calculated from the reported values of τobs and Q^LnLn.
(dme)2Nd(SC6F5)3	^4F3/2→^4I11/2	1071	110	1.40	7.9	131
	^4F3/2→^4I13/2	1347	130	1.40	9.3	131
(dme)2Nd(OC6F5)3	^4F3/2→^4I11/2	1059	170	8.60	2.0	135
(thf)8Nd8O2Se2(SePh)16	^4F3/2→^4I11/2	1078	186	1.19	15.7	131
	^4F3/2→^4I9/2	927	78	1.19	6.6	131
[(py)18Nd12O6Se4(Se2)4(SePh)4(Se2Ph)2Hg2(SePh)4][(Hg(SePh)3]2	^4F3/2→^4I11/2	1082	141	1.16	12.2	132
[(py)24Nd28F68(SePh)16]	^4F3/2→^4I11/2	1076	303	12.4^c	41	133
(dme)2Tm(SC6F5)3	^3H4→^3F4	1470	70	3.20	2.2	134
(dme)2Tm(OC6F5)3	^3H4→^3F4	1458	145	7.52	1.9	135
	^3F4→^3H6	1759	127	2.78	4.6	135
(dme)4Tm4(SPh)12	^3H4→^3F4	1470	111	1.00	11.1	134
	^3F4→^3H6	1772	120	18.73	0.64	134
(thf)6Tm4Se9(SeC6F5)2	^3H4→^3F4	1470	80	1.66	4.8	134
	^3F4→^3H6	1772	110	35.71	0.31	134
(dme)2Er(OC6F5)3	^4I13/2→^4I15/2	1550	1000	6.16	16.2	135

---

On the other hand, the self-assembled H₁₀[Yb₉(hesa)₁₆(μ-O)₁₀(NO₃)] cluster which features an organic sensitiser (hesa is hexylsalicylate) is a less efficient light-converter compared to the inorganic clusters, with τ(²F_5/2) = 0.6 and 2 μs for the two species present in methanol, that is an order of magnitude smaller than those listed for molecular compounds (Table 5).¹³⁶

3.7 Extended structures

In an effort toward the design of practical applications, NIR-emitting molecules and compounds have been inserted into extended structures such as polymers, silica matrices,¹³⁷ sol–gel glasses or mesoporous materials; the latter two classes of compounds are often referred to as inorganic-organic hybrid materials or metal–organic frameworks (MOFs).^138,139 In view of the limitations discussed above, no frontier-breaking achievements have been recorded recently and only selected examples are presented below.

Polymethylmethacrylate (PMMA) is a widespread polymer which easily forms thin films, thus it is often used for designing various optical materials. For instance, the DIP (dipyridophenazine) adduct of Yb^III bis(perfluoromethanesulfonyl)imide salt, Yb(24a)₃(H₂O)₈(DIP) (Scheme 4) displays overall quantum yields of 0.18 and 0.26% when inserted into PMMA or PMMA/DMSO thin films respectively; these figures are enhanced to 0.72–0.73% upon annealing the films at 80 °C during 1 h,¹⁴⁰ but they remain modest (see Table 4 for comparison). Similarly, the TPPO (triphenylphosphine oxide) adduct of Er^III pentafluorobenzoate co-polymerized with PMMA displays a modest intrinsic yield of 0.12% (τ^rad from JO parameters = 12.66 ms).¹⁴¹

Tetraethoxysilane (TEOS) is a common starting material for the synthesis of doped glasses by sol–gel procedures,¹⁴² despite of the presence of O–H groups potentially detrimental for the Ln-centred NIR luminescence. Several complexes have been inserted in such xerogels. In [Er_1.4Yb_0.6(benzoate)₆(phen)₂]-doped glass⁷⁵ for instance, τ_obs(⁴I_13/2) of Er^III populated through intramolecular transfer from Yb^III decreases to 12.7 μs from 19.3 μs in the bulk complex and this has been ascribed to O–H quenching; with τ^rad = 8.95 ms, calculated from JO parameters, the intrinsic quantum yield amounts to only 0.14%.¹⁴³ An example of Ho^III and Tm^III luminescence in the same xerogel is provided by the β-diketonate complexes [Ln(tta)₃(L)] with L = phen, bpy or TPPO. Similarly to the previous example, the Ho(⁵F₅) and Tm(³H₄) lifetimes are substantially reduced in the xerogel in comparison with the bulk complexes: for the TPPO adducts, τ(⁵F₅) decreases from 24.3 to 18.3 ns and τ(³H₄) from 43.3 to 25.4 ns. The radiative lifetime of Ho(⁵F₅) calculated from the JO parameters amounts to 5.39 ms, therefore Q^Ho_Ho = 3.4 × 10⁻⁴% in the xerogel.¹⁴⁴ Complexes can also be covalently attached to the xerogel for instance through an adequately derivatised phen molecule (Fig. 5); in this way, NIR emission bands ⁴G_5/2→⁶F_J (J = 5/2, 7/2, 9/2) of a Sm^III fluorinated β-diketonate ternary complex were observed at 0.95, 1.03 and 1.18 μm. The bi-exponential decay corresponds to lifetimes of 4.6 (37%) and 20 (63%) μs.¹⁴⁵ Luminescence from Nd^III, Er^III and Yb^III is sensitised as well¹⁴⁶ and similar complexes were covalently attached to the mesoporous material MCM-41; in the latter τ(⁴G_5/2) is substantially longer than in the covalently bound xerogel: 12 (26%) and 57 (74%) μs.¹⁴⁷ On the other hand, doping simple 8-hydroxyquinolinates (Ln^III = Nd, Er, Yb) into mesoporous SBA-15 results in shorter lifetimes than in the corresponding bulk complexes.

Fig. 5 β-Diketonate ternary complex covalently linked to a xerogel. Reproduced with permission from ref. 145, © Royal Society of Chemistry 2009.

Coordination polymers. Many inorganic-organic frameworks have been prepared which exhibit lanthanide NIR luminescence, but quantitative data are almost inexistent. We note however that fluorination of benzenedicarboxylate leads to a four-fold increase in the Er^III emission at 1.5 μm compared to the hydrogenated coordination polymer.¹⁴⁸ Some of these frameworks sensitise the luminescence of several lanthanide ions.¹⁴⁹ Among the structural diversity in 2,2′-bipyrimidine (bpm) adducts with Nd^III nitrate or β-diketonates, a one-dimensional coordination polymer, [Nd(tta)₃(μ-bpm)(MeOH)]_∞ is luminescent,¹⁵⁰ and it seems that bpm is also able to sensitise the luminescence of several lanthanide ions.

4. Luminescent materials

In this section we present selected examples of various classes of materials with the emphasis on “soft” and electroluminescent materials. Polyoxometallates, which feature O → W or O → Mo LMCT states able to sensitise lanthanide luminescence have been reviewed recently and are not dealt with here.^151,152

4.1 Nanomaterials

The fascination for nanosciences is reflected in the continuously expanding number of papers dealing with one aspect or another of this new field of investigation. As far as rare earths are concerned, two major applications are attracting most of the attention, namely (i) visible- and/or NIR-emitting phosphors for lighting devices and solar cells, as well as (ii) up-converting nanoparticles for bio-analysis. The former are the subject of intense research most of it concentrating on microparticles. But recently, authors have doped a combination of Er^III, Ho^III, or Tm^III_, together with Yb^III as activator, in Y₂O₃,^153,154 YAlO₃,¹⁵⁵ Gd₂(MoO₄)₃,¹⁵⁶ LuGaO,¹⁵⁷ LaF₃,^158,159 nanoparticles or nanocrystals, as well as in SiO₂–Al₂O₃–NaF–YF₃ nanocomposites¹⁶⁰ or in oxyfluoride nano-glasses.¹⁶¹ Upon photodiode excitation into the Yb(²F_5/2) level, a combination of red, green and blue luminescence results in white light emission. Alternatively, red and green emission from Eu^III and Tb^III combined with blue emission of Ga₂O₃ nanoparticles also leads to white light emission.¹⁶² The same result is obtained by mixing the blue emission from the substrate of GdGaO nanoparticles¹⁶³ or GdAl₃(BO₃)₄¹⁶⁴ nanorods with the yellow Dy^III emission (activated by Ce^III in the latter material).

Harsh synthetic conditions such as the non-aqueous route proposed by Karmaoui et al.¹⁶⁵ starting from lanthanide sesquioxides reacted at 275 °C in presence of 4-biphenylmethane result in lamellar nanostructures into which doped luminescent Ln^III ions present an intense emission. In particular, the Eu^III nanohybrid in Gd₂O₃ has a luminance value larger than the conventional Y₂O₃:Eu^III phosphor.¹⁶⁶ Colour-tuning has been obtained with LaF₃ nanoparticles coated with a benzoic acid derivative and combining the emission of both the ligand and a Ln^III ion (e.g. Eu^III).¹⁶⁷

Other evolving fields are (i) the production of luminescent security coatings^168–170 and inks for which nanoparticles seem to be promising,¹⁷¹ as well as (ii) coatings for optical applications such as white light emitting diodes, low refractive index layers for antireflective devices, or silica binders in photocatalysis.¹⁷²

Nanomaterials for bio-applications have been reviewed very recently,^173,174 both from their design, synthesis, and application points of view, so that we do not discuss them here.

4.2 Ionic liquids and ionogels

Ionic liquids are salts with a low melting point (<100 °C) and several of them are liquid at room temperature (RTIL). The cation is generally an organic moiety (e.g. 1-alkyl-3-methylimidazolium, see Scheme 12) and the anion modulates the hydrophilicity of the liquid.¹⁷⁵ RTILs find uses in catalysis,¹⁷⁶ in the design of inorganic materials,¹⁷⁷ in extraction processes,¹⁷⁸ particularly within the context of nuclear fuel reprocessing,¹⁷⁹ in electrolytes for batteries and photovoltaic devices,¹⁸⁰ and in the electro-deposition of zero-valent lanthanide metals.¹⁸¹ Investigations of spectroscopic properties of Ln^III ions in RTILs, described in Binnemans’ recent review,¹⁷⁵ revealed that (i) the ionic liquid may participate in the sensitisation process, (ii) non-radiative deactivation processes are minimized compared to solid state or organic solutions, at least as long as the (otherwise highly hygroscopic) RTIL is reasonably anhydrous, (iii) therefore ionic liquids are attractive media for NIR luminescence, and (iv) lanthanide luminescence is helpful for investigating structural aspects of these solvents.

Scheme 12 Cations of main RTILs used in spectroscopic studies.

When compared to Eu^III, the visible emission of Sm^III is usually much less intense, at least in aqueous solvents. However, luminescence of this ion also occurs in the NIR portion of the electromagnetic spectrum, a duality that may be helpful for analytical sensors. The photophysics of three Sm^III anionic tetrakis(β-diketonates) as well as of [Sm(dpa)₃]³⁻ has been elucidated in [C₆mim]Tf₂N. The imidazolium cation has been chosen as the cationic counterpart of the complexes, an elegant way of circumventing solubility problems.¹⁸² Moreover, while β-diketonates are known to be photo-degraded under UV excitation, their photostability is enhanced in ionic liquids.¹⁸³ This is for instance the case of [C₆mim][Sm(tta)₄] the absorbance of which decreases by 2–6% in acetonitrile, ethanol or chloroform after 1h of irradiation at 340 nm, and to more than 10% in N,N-dimethylformamide after 10 min, while it remains practically constant in the RTIL or in methylene chloride. However, both lifetimes and quantum yields are smaller by ∼25–35% in the RTIL compared to acetonitrile; this is ascribed to quenching by O⋯H bonding between the imidazolium cations and the O atoms of the diketonate.¹⁸²

Most of the latest investigations on photophysical properties in RTIL deal with Eu^III. For instance, while the interaction of Eu^III and Cm^III with Cu^II leads to the quenching of the luminescence of both f ions in water, only Eu^III emission is quenched in [C₄mim]Tf₂N (k_SV = 1.54 × 10⁶ M⁻¹ s⁻¹ compared to 1.20 × 10⁴ M⁻¹ s⁻¹ in water), this points to different types of chemistry between the 4f and 5f elements in RTIL and opens perspectives for their separation.¹⁸⁴ This conclusion is supported by the complexation of azide ion to Eu^III in the same RTIL which results in both static and dynamic fast quenching of the ⁵D₀ emission while the interaction kinetics is much slower for Cm^III and Am^III; additionally, azide complexation seems to be stronger with Eu^III triflate compared to perchlorate, an effect which may be traced back to a stronger electrostatic repulsion of N₃⁻ by the perchlorate anion.¹⁸⁵ When water is added to [C₄mim]Cl containing europium chloride, a whole range of mixed aqua–chloro complexes form, including the full aquated species when the molar ratio of water reaches 5, as shown by ⁵D₀←⁷F₀ excitation spectra and corresponding site-selectively excited emission spectra.¹⁸⁶ Derivatization of the 3-position of the imidazolium cation by a carboxylic acid in Carb-mim gives rise to a so-called “task-specific” RTIL which can dissolve lanthanide oxides. Complexes such as [Eu(tta)₃(phen)] are directly obtained by reacting Eu₂O₃, the β-diketone and phenanthroline in [Carb-mim]Br;¹⁸⁷ such a procedure is easily extendable to polynuclear d-transition metal complexes.¹⁸⁸ A series of low-melting europium-containing ionic liquids of composition (IL)_x[Eu(Tf₂N)_3+x] (IL = C₃mim, C₄mim and x = 1; or IL = C₄mpyr and x = 2) represent the first Ln-based ILs obtained without the use of any co-ligand. Melting points are between 10 and 53 °C and the crystal structures of the solids show the Eu^III ion lying in a distorted tricapped trigonal prismatic, nine-coordinate environment. ⁵D₀ lifetimes are around 1.7–1.9 ms for the C_3,4mim compounds, somewhat shorter than in the solid state, due to enhanced deactivation by collisions.¹⁸⁹

A facile procedure for the synthesis of LnF₃ and Ln-doped YF₃ (Ln^III = Eu, Tb) rhombic-shaped nanoparticles involves precipitation from [Ln(acac)₃] or Ln(OAc)₃ solutions in ethanol containing [C₄mim]BF₄ as the source of fluoride. Varying the nature and concentration of the Ln^III precursor allows one to tune the size of the particles from 130 to 340 nm (long axis) and addition of Ln nitrate (Ln^III = Eu, Tb) results in doped luminescent particles.¹⁹⁰ A large-scale synthesis of similar nanoparticles (Ln^III = La, Ce, Pr, Nd, Sm, Eu and Er), although with different shapes, starts from lanthanide nitrates in ethanol, to which are added adequate amounts of the RTIL, [C_xmim]PF₆ (x = 4, 8) or [C₈mim]BF₄. Eu-containing nanocrystals are quite luminescent and Tb-doped CeF₃ nanocrystals display both Ce^III and Tb^III emission in the RTIL.¹⁹¹

Hybrid materials consisting in an ionic liquid confined inside nano-sized pores of a silica matrix, termed ionogels, feature the advantages of both the optical transparency of silica and the ionic conductivity of ionic liquids.¹⁹² Ionogels doped with [C₆mim][Ln(tta)₄] (Ln^III = Nd, Sm, Eu, Tb, Ho, Er, Yb) and [choline]₃[Tb(dpa)₃] are highly luminescent inorganic-organic hybrids. This is ascertained by the lifetimes in the ionogel which are very comparable to those in the ionic liquid, for instance, 1.2 vs. 1.3 μs for Nd^III, 86 vs. 81 μs for Sm^III, 1.80 vs. 1.65 ms for Tb^III, 1.6 vs. 1.9 μs for Er^III, and 16 versus 17 μs for Yb^III. Thus the emission colour of these gels can easily be tuned by changing the emissive ion and their mechanical properties make them ideal luminophores.¹⁹³

Some ionic liquids behave as liquid crystals as well, for instance [C₁₂mim]Cl.¹⁹⁴ The potential of the Eu^III structural probe has been exploited to evidence the crystal to smectic A, Cr → SmA (0 °C) and SmA → I (100 °C) transitions of this RTIL doped with 5 mol% of europium nitrate by monitoring the intensity ratio of two components of the hypersensitive transition ⁵D₀→⁷F₂.¹⁹⁵ The luminescent properties of adducts of lanthanide chlorides (Ln^III = Eu, Tb) with phen and bpy have also been analyzed in this IL, particularly with respect to energy transfer mechanisms.¹⁹⁶ Finally, highly luminescent ionic liquid crystalline phases can be engineered by coupling one or two mesogenic units (cholesterol or cyanobiphenyl) to an imidazolium cation itself associated with an anionic tetrakis(β-diketonate) such as [Eu(tta)₄]⁻.¹⁹⁷

4.3 Liquid crystals: lanthanidomesogens

The predictive electronic, optical and magnetic metal-centred properties of lanthanide ions make them particularly attractive for being inserted into switchable macroscopic materials responding to external electric and magnetic stimuli such as thermotropic liquid crystals (LC) which are then termed lanthanidomesogens.¹⁹⁸ Interest for these materials dates back in the mid 1980s and an amazing variety of structures with specific properties have been synthesized,¹⁹⁹ especially with respect to luminescent compounds,²⁰⁰ but rational synthesis only developed recently.^201,202 Lately, efforts have essentially been focused on developing new ligands for d- or f-metallomesogens, e.g. imidazo[4,5-f]1,10-phenanthrolines,²⁰³ and for unravelling the thermodynamics of LC phase formation,^204,205 their alignment under magnetic field,²⁰⁶ or their intimate structure. The field has been regularly reviewed.^198–201 Only new developments involving luminescence data are therefore presented here.

Luminescence is a highly sensitive analytical technique able to sense small differences generated by phase transitions, not only in the inner-coordination sphere but also in the outer environment of the emissive Ln^III ion. Following the initial demonstration that indeed phase transitions in LC can be detected by variations in luminescence intensity and lifetime,^204,207 or in relative band intensity of transitions to specific ligand-field (Stark) sub-levels,¹⁹⁵ photoacoustic spectroscopy (PA) was tested for the same purpose in LC phases doped with Eu^III, Tb^III and Ho^IIIβ-diketonates.²⁰⁸ Although interpretation of data requires a relatively involved theoretical treatment, this technique proved to be helpful, principally when it comes to evaluating energy transfer efficiencies in luminescent lanthanidomesogens.²⁰⁹ Pressure-induced phase transitions between (i) the SmA phase of [Eu(bta)₃(38)₂] and its lamellar structure and (ii) the latter and an amorphous state (Scheme 13) are clearly evidenced by changes in the Raman spectrum and in the photophysical properties. For instance, the energy of the Eu(⁵D₀→⁷F₀) transition first decreases until the transition to the amorphous state is completed and then increases again. This blue shift is indicative of a smaller nephelauxetic effect, due to a weaker Eu–O(phenol) bond compared to Eu–O(phenolate), a consequence of the proton transfer.²¹⁰

Scheme 13 Pressure-induced phase transition from lamellar (left) to amorphous (right) states in [Eu(bta)₃(38)₂].²¹⁰

One of the rationale for synthesizing luminescent LC phases is the development of luminescent displays. For this purpose, luminescent lanthanide complexes are usually dissolved in nematic phases such as MBBA (N-(methoxybenzylidene)-4-butylaniline), 5CB (4-n-pentyl-4′-cyanobiphenyl), or 6CHBT (Scheme 14). When doped into the latter, the luminescent complexes [Ln(39)₃(bpy)], with quantum yields of 5 (Eu^III) and 19% (Tb^III) in CHCl₃, retain their characteristics. For instance the Eu(⁵D₀) observed and radiative lifetimes amounts to 1.69 and 5.3 ms, as compared to 1.37 and 4.1 ms in chloroform. In addition, the intensities of the hypersensitive transition Eu(⁵D₀→⁷F₂) and of the Tb(⁵D₄→⁷F₅) transition sustain a ≈2-fold increase when the electric field is varied from 0 to ±30 V, reflecting the development of asymmetric environments in the LC phase. A sizeable hysteresis of this phenomenon is associated with the ion transport through the LC layer and to adsorption of residual charged molecules at the surface.²¹¹ Similarly, introduction of a small amount (0.1 wt%) of [Nd(tta)₃(phen)] into a commercially available chiral nematic LC phase to which cholesteryl nonanoate is added results in a decrease in the NIR emission of Nd^III (contrast ratio 3 : 1) and Yb^III (1.5 : 1) when an AC voltage between 50 and 220 V is applied to the cell for inducing the cholesterol-to-nematic phase transition. This effect is due to the better scattering of the excitation light by the chiral nematic (cholesteric) phase.²¹²

Scheme 14 Ligands for synthesising luminescent liquid crystalline phases.^211,213

Ligand H40 bears two mesogenic C₁₂ and C₁₆ chains grafted in the para positions of diphenylacetylacetone (Scheme 14) and the [Ln(40)₃(phen)] complexes with Ln^III = Eu, Er and Yb are luminescent at room temperature; all of the investigated complexes with the heavier lanthanides (Ln^III = Eu–Dy, Er, Yb) exhibit a monotropic smectic A phase with I→SmA transition in the range 115–135 °C.²¹³

4.4 Up-converting materials

Up-conversion is a non-linear, sequential absorption phenomenon of two or more photons via long-lived excited states, followed by emission of light with a shorter wavelength than the pump light source (red-to-blue conversion). Three types of absorption mechanisms have been recognised: excited state absorption (ESA) by a single ion, energy transfer up-conversion between two neighbouring ions (ETU), and photon avalanche (PA), a more complex process also involving two neighbouring ions. ETU is by far the most efficient process and is, in addition, independent of the pump power.²¹⁴ These excitation mechanisms are different from so-called multiphoton excitation in which photons are absorbed simultaneously, and not sequentially, by the chromophore and which necessitate the use of powerful lasers operating in the femtosecond range (see section 5.5). Inorganic, usually lanthanide-containing, phosphors with up-conversion characteristics²¹⁵ are used in light-emitting diodes, solar cells (see section 4.6), lasers, optical communications, data storage, security inks, and flat-panel displays.²¹⁶ More recently, they have found applications in immunoassays⁵⁸ and other biomedical analyses¹⁷³ (see section 5.3). The materials are habitually used under the form of doped glasses,²¹⁷ single crystals, or nanoparticles.²¹⁸ White-light generation is an often sought-for property, displayed by tri-doped up-converting phosphors (Tm/Er/Yb)²¹⁹ or glasses (Tm/Ho/Yb,^220,221 Pr/Er/Yb²²²). The subject is vast and a bit too technical to fit into this overview; more information can be found in recent review articles.^223,224

An exciting new development is the preparation of coordination polymers from mixed carboxylic acids (oxalic acid, H₂ox and 4,4′-oxybis(benzoic acid), H₂oba) under hydrothermal conditions, [Ln(oba)(ox)_0.5(H₂O)₂]_∞, Ln^III = Y, Er, Yb or co-doped Er, Yb into Y. Typical visible emission of Er^III in the latter is seen upon excitation by a continuous diode laser at 975 nm and both two- and three-photon processes have been evidenced.²²⁵ The high intensity of up-conversion, compared to the previously reported coordination polymer with pyrazine dicarboxylic acid²²⁶ or 1,4-benzenedicarboxylic acid²²⁷ is attributed to the presence of oxalate, which is devoid of high-energy vibrations. Some simple complexes, e.g. dipicolinates (Ln^III = Nd, Er, Tm) also display up-conversion properties and have been characterized in an effort to develop luminescent biolabels.²²⁸

4.5 Electroluminescent materials

Organic light emitting diodes (OLEDs). A typical OLED consists of a number of layers deposited on either solid (glass) or flexible (polymer) substrates (Fig. 6). Thin films can be obtained by different techniques, however the most commonly used are vacuum deposition and solution-processable methods such as spin-, deep-coating and ink-jet printing. When a voltage bias is applied to the diode, holes are injected from an anode and electrons from a cathode. After adequate transport they recombine in the emission layer with formation of excitons which then deactivate with release of light through transmissive anode and substrate.²²⁹ Thus, the design of OLEDs requires: (i) efficient injection of charge carriers from electrodes, (ii) balanced electron-hole transport, (iii) effective charge recombination (limited by the multiplicity of the excited states: 25% for fluorescent and 100% for phosphorescent materials), (iv) strong emission from excited states and (v) efficient light extraction.

Fig. 6 Typical set-up of an OLED and simplified scheme of injection, transport and recombination of charge carriers.

To meet the first requirement the use of low work-function metals adjusted to the lowest unoccupied molecular orbital (LUMO) level of the next layer is necessary, as well as appropriate surface treatment of anodes (usually indium tin oxide, ITO). Charge recombination and emission are mostly determined by the nature of the emission layer material and can be reasonably optimized by a proper design while a balance of electrons and holes is difficult to achieve in a single layer. That is why usually additional layers are introduced into the OLED structure with high hole (HTL) or electron (ETL) mobility, their energy levels being tuned to the highest occupied molecular orbital (HOMO) or LUMO of the emissive layer, respectively (chemical formulae of some typical layers are drawn on Scheme 15). It is worth noting that encapsulation of OLEDs plays an important role in device stability. More detailed information about possible ways of OLED structure optimization, mechanisms of injection, transport, and recombination can be found in refs. 215 and 229–231.

Scheme 15 Examples of hosts and/or hole and electron transporting or blocking materials used in OLEDs.

For describing precisely OLED operation, current–voltage and luminance (or brightness)–voltage characteristics have to be measured. The so called turn-on voltage (U_i), defined as the voltage at which brightness (L) exceeds 1 candela per square metre (cd m⁻²) can then be extracted from these data. Several quantitative parameters characterize OLED efficiency.

The luminous efficacy (η_v [lm W⁻¹]) is the ratio of luminous flux (lumen) emitted by the source to the input electrical power (watt) and is determined by two factors:

η_v = η_eK (19)

where η_e is the radiant efficiency of the source (radiant flux to input electrical power). K is the luminous efficacy of radiation (luminous flux per radiant flux) and is determined by the spectral distribution S(λ) of the source and spectral sensitivity of the human eye V(λ):where K_m = 683 lm W⁻¹.²³²

The external quantum efficiency (η_ext) is the ratio between the number of photons emitted and the number of electrons injected to the device; it can be decomposed into the following terms:

η_ext = η_recη_ELη_extr (21)

where η_rec, η_EL and η_extr represent efficiencies of recombination, emission and extraction processes, respectively.²¹⁵

When it comes to practical lighting applications of OLEDs, such an important characteristic as the colour rendering index (CRI) has to be estimated, because CRI shows how natural colours of the objects look under given illumination. Other parameters such as CIE (Commission Internationale de l’éclairage) coordinates and correlated colour temperature (CCT) should be determined as well.²³²

From the above discussion it is quite obvious that designing efficient OLEDs is a complex problem. Here, we concentrate mostly on the elaboration of emission layer materials which should meet the following criteria: (i) high photoluminescent efficiency; (ii) good carrier injection and transportability; (iii) large thermal and electrochemical stability; (iv) adequate solubility or volatility in order to obtain high-quality thin films. Different classes of compounds have been tested as electroluminescent materials for OLEDs, from conjugated polymers,⁵⁷ small molecular compounds, either fluorescent²²⁹ or phosphorescent,^233,234 to inorganic–organic hybrids (MOFs).^230,235 Lanthanide complexes with organic ligands present two major advantages in view of their application in OLEDs: (i) quasi monochromatic emission and (ii) an efficiency η_rec (see eqn (21)) significantly improved in comparison with materials based on pure fluorescent compounds because both singlet (25%) and triplet (75%) excitons formed as a result of electron-hole recombination can be utilized for emission.²³³ A starting point for tailoring lanthanide-based materials for OLEDs was the demonstration in 1990 by Kido et al. of a characteristic green electroluminescence of the ternary complex terbium tris(acetylacetonate) with o-phenanthroline.²³⁶ The corresponding OLED with a simple structure ITO|TPD|Tb(acac)₃(phen)|Al had a brightness of 7 cd m⁻² at a current density of 0.4 mA cm⁻². After this pioneer work many efforts have been devoted towards both the synthesis of new lanthanide-containing compounds with improved characteristics and the optimization of OLED architectures.^{18,58,215,233,234,237–240}

Three main classes of anionic organic ligands have been under test for potential applications to visible-emitting OLEDs: β-diketonates and their derivatives,^241–266 pyrazolonates,^267–270 (Scheme 16 and 17) and carboxylates (Scheme 18).^271–279 In addition, recent reports describe OLEDs made of a Tb^III cluster with a benzamide derivative²⁸⁰ or of a Ce^III complex with a polybenzimidazole-based tripodal ligand.²⁸¹ Selected examples of visible-emitting monochromatic OLEDs reported recently are presented in Table 6 (those with the best performances have been selected).

Scheme 16 β-Diketones, pyrazolones, and their derivatives as ligands for lanthanide-based emissive layers in OLEDs.

Scheme 17 Ancillary ligands used for improving the transport properties of lanthanide-containing edifices in OLEDs.

Scheme 18 Carboxylic acids and polybenzimidazole-based ligand for emissive layers in OLEDs.

Table 6 Selected examples of monochromatic OLEDs and their characteristics

OLED structure^a	Ui/V	Lmax/cd m^−2 (U/V)	ηv/lm W^−1	ηext (%)	Ref.
a CuPc = copper phthalocyanine; MADN = 2-methyl-9,10-di(2-naphthyl)anthracene; PEDOT:PSS = poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate).b Brightness and luminous efficacy at 9.1 V, not maximum value.
Eu^III-emitting OLEDs
ITO|TPD|Eu(dbm)2(41e)(Bath)–PBD (1 : 1 mol ratio)|BCP|Alq3|Mg0.9Ag0.1|Ag	n.a.	2797 (14.0)	0.27	n.a.	233
ITO|TAPC|Eu(dpm)3–BCP (1 : 1)|BCP|Alq3|Mg0.9Ag0.1|Ag	n.a.	2123 (10.0)	n.a.	∼1.0	241
ITO|TPD|Eu(dbm)3(Tmphen) (5.5%)–DCJTB (0.2%)–CBP|BCP|Alq3|LiF|Al	4.6	2450 (20.4)	n.a.	n.a.	242
ITO|TPD|Eu(dbm)3(Tmphen) (5.5%)–CBP|BCP|Alq3|LiF|Al	4.8	492 (18.6)	n.a.	n.a.	242
ITO|TPD|Eu(dbm)3(Tmphen)|BCP|Alq3|LiF|Al	8.4	160 (20.2)	n.a.	n.a.	242
ITO|NPB|Eu(tta)3(CPPO)–CBP|BCP|Alq3|Mg0.9Ag0.1|Ag	7.4	1271 (19.6)	2.35	3.60	243
ITO|NPB|Eu(tta)3(CPPO)|BCP|Alq3|Mg0.9Ag0.1|Ag	8.0	852 (22.2)	1.23	2.08	244
ITO|NPB|Eu(tta)3(TAPO)|BCP|Alq3|Mg0.9Ag0.1|Ag	4.4	1195 (19.6)	3.62	3.2	244
ITO|NPB|Eu(tta)3(NADAPO)|BCP|Alq3|Mg0.9Ag0.1|Ag	4.8	1158 (18.0)	3.69	3.71	244
ITO|NPB|Eu(tta)3(CPO)|BCP|Alq3|Mg0.9Ag0.1|Ag	8.4	399 (20.0)	0.99	1.68	243
ITO|PEDOT:PSS|Eu(43c)3(phen) (10 wt%)–DCJTB (3 wt%)–PVK|ZnS|LiF|Al	n.a.	650 (20)	n.a.	n.a.	272
ITO|PEDOT:PSS|Eu(43c)3(phen) (10 wt%)–DCJTB (3 wt%)–PVK|BCP|Alq3|LiF|Al	n.a.	420 (20)	n.a.	n.a.	272
ITO|NPB|Eu(42c)3(TPPO)(H2O) (15%)–CPB|BCP|Alq3|Mg0.9Ag0.1	∼12	247 (18.5)	n.a.	n.a.	270
Tb^III-emitting OLEDs
ITO|NPB|Tb(42a)3(TPPO)|BCP|Alq3|Mg0.9Ag0.1|Ag	7.0	12 000 (18)	11.3	n.a.	268
ITO|Tb(41a)3(H2O)2 (5 wt%)–PVK–PBD|Alpop|CsF|Al	8.0	550 (20.0)	n.a.	1.1	247
ITO|Tb(43c)3(phen)–PVK (1 : 3 wt ratio)|ZnS|Al	n.a.	326 (18.0)	n.a.	n.a.	271
ITO|Tb(43b)3(phen)–PVK (1 : 5 wt ratio)|Alq3|LiF|Al	7.0	180 (15.0)	n.a.	n.a.	273
ITO|TbY(43a)6(phen)2–PVK (1 : 5 wt ratio)|Alq3|LiF|Al	∼12	334 (23.0)	n.a.	n.a.	274
Sm^III-emitting OLEDs
ITO|PVK|Sm(bfa)3(phen)|PBD|Al	10.0	135 (21.0)	n.a.	n.a.	245
ITO|TPD|Sm(41d)3(phen)|BCP|Alq3|LiF|Al	10.4	82 (19.4)	0.007	n.a.	246
Dy^III-emitting OLEDs
ITO|CuPc|Dy(42b)3(TPPO)2|BCP|Alq3|LiF|Al^a	∼10	524 (19.0)	0.16	n.a.	269
Blue-emitting OLEDs
ITO|PVK|Tm(acac)3(phen)|Al	n.a.	6.0 (16.0)	n.a.	n.a.	298
ITO|NPB|[Ce(44)2](OTf)3 (5%)–MADN|Alq3|LiF|Al^b	n.a.	1.5 (9.1)^b	0.52^c	n.a.	281

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For NIR-emitting lanthanide ions, mainly Nd^III, Er^III and Yb^III, the most efficient OLEDs are built from complexes with 8-hydroxyquinolinates,^{19,102,282–287}β-diketonates^288–293 or porphyrin-containing ligands.^293–297 The maximum achieved values of external quantum efficiency are 7 × 10⁻³% for Nd^III,²⁹⁰ 10⁻⁴% for Er^III,²⁹⁰ and 0.1% for Yb^III.²⁹⁴ However, to our knowledge, since the latest reviews appeared,^18,58,233 no report on this subject has been published in periodical journals, so that they are not discussed further.

The majority of the investigations dealing with visible-emitting OLEDs utilize Eu^III- or Tb^III-containing materials as emission layers. However, some examples of OLEDs based on Sm^IIIβ-diketonates with o-phenanthroline, [Sm(bfa)₃(phen)]²⁴⁵ and [Sm(41d)₃(phen)],²⁴⁶ or Dy^III pyrazolonate with triphenylphosphine oxide, [Dy(42b)₃(TPPO)₂]²⁶⁹ are documented, but their efficiencies are usually much lower. It is worth noting that the Dy^III complex could be a candidate for white-emitting OLEDs because its CIE coordinates (0.35; 0.40) lie at the edge of the white region.²⁶⁹ There is one example of an OLED made of a ternary complex of Tm^III acetylacetonate with phen and displaying a characteristic blue electroluminescence centred at 482 nm but in addition to low brightness, 6 cd m⁻², the device exhibits an irreversible change of emission colour to orange when voltage bias values higher than 18 V are applied.²⁹⁸ The Ce^III 5d → 4f emission in [Ce(44)₂](OTf)₃ is also promising in the design of blue-emitting OLEDs.²⁸¹

The best red-emitting OLEDs contain Eu^IIIβ-diketonates: the maximum brightness (2797 cd m⁻²) and the highest external quantum yield (7.5%) with corresponding luminous efficacy of 10 lm W⁻¹ are reached for [Eu(dbm)₂(41e)(Bath)]²³³ and [Eu(dbm)₃(Bath)],²⁵¹ respectively (Bath is 4,7-diphenylphenanthroline). OLEDs with Eu^III carboxylates as emissive layers have lower brightness values²⁷² and devices made of Eu^III pyrazolonate, e.g. [Eu(42c)₃(TPPO)(H₂O)],²⁷⁰ are even less efficient. On the other hand, L_max as high as 12 000 cd m⁻² and luminous efficacy of 11.3 lm W⁻¹ were recorded for a green-emitting OLED incorporating a Tb^III pyrazolonate ternary complex with triphenylphosphine oxide.²⁶⁸ Conversely, the maximum brightness of an OLED containing the Tb^III oxadiazole-functionalized β-diketonate [Tb(41a)₃(H₂O)₂]²⁴⁷ is only 550 cd m⁻² and the corresponding values for Tb^III carboxylates do not exceed 350 cd m⁻². ^271,273,274

These results reflect, firstly, the suitability of the corresponding classes of ligands for sensitising either Eu^III or Tb^III photoluminescence: the triplet level of pyrazolonates is usually too energetic for an efficient energy transfer to Eu^III,²⁷⁰ while that of β-diketonates with conjugated substituents has too low energy for Tb^III.²⁹⁹ In addition, home-built solution processable OLEDs generally exhibit lower efficiencies than the ones obtained by vacuum deposition;²⁵⁸ this is for instance the case for lanthanide carboxylates because of their polymeric structure and thus low volatility.³⁰⁰

Several approaches towards the improvement of the emissive layer properties can be sketched:

 

• Doping of lanthanide compounds into polymers or small molecular compounds with high hole and/or electron mobility. This approach, along with an enhancement in the transportability of the emission layer, improves film-forming properties and leads to high-quality thin films. Moreover, doping Eu^IIIβ-diketonates into blue-emitting polymers such as poly(9,9-dioctylfluorene) and their derivatives is promising in the design of colour-tuneable OLEDs.^301,302 The most efficient host material for the enhancement of Eu^III electroluminescence appears to be 4,4′-bis(carbazol-9-yl)biphenyl (CBP, Scheme 15). For example, OLEDs relying on [Eu(dbm)₃(Tmphen)]²⁴² or [Eu(tta)₃(CPPO)]^243,244 doped into CBP have ∼3- and 1.5-fold increases in values of L_max, respectively, in addition for the former device to a ∼2-fold decrease in turn-on voltage. Doping [Eu(dbm)₃(DIP)] (Scheme 4) into Bath is beneficial as well: the ITO|TPD|Eu(dbm)₃(DIP) (25–30%)–Bath|Bath|LiF|Al OLED has a brightness of 1320 cd m⁻² and η_ext∼2.5%.²⁵⁰ Co-doping the emission layer with assistant compounds such as the organic dye DCJTB^242,272 or the Ir^III complex FIrpic²⁵⁵ (Scheme 15) also helps optimizing OLED performances. However, one major drawback of the doping approach is the danger of phase separation during device operation.²³³

• Grafting hole and/or electron-transporting groups on anionic and/or ancillary ligands. Examples of modification of phen, bpy and anionic β-diketonates, mainly dibenzoylmethanates, can be found in ref. 233. Covalent attachment of lanthanide complexes to polymers is reviewed in ref. 18. According to recent publications, derivatives of triphenylphosphine oxide such as TAPO, NADAPO, CPPO, CPO (Scheme 17) appear to be promising neutral ligands for lanthanide-based electroluminescent materials.^234,244 Grafting phen or bpy with calix[4]arene derivatives is also helpful.²⁴⁹ However, these strategies usually result in an increased molecular weight and thus in a loss of volatility. To overcome this disadvantage Adachi et al. have suggested a vacuum co-deposition method of volatile Ln^III compound (e.g. [Eu(dpm)₃]) and BCP, a material with good transport properties.²⁴¹ The obtained OLED exhibits a brightness of 2123 cd m⁻² and η_ext∼1%, which positions it in the best three devices reported so far. The problem of low volatility and solubility is also real for polymeric lanthanide aromatic carboxylates which feature high photoluminescence quantum yields (Tb^III benzoate has 100% efficiency).²⁸ A possible way out could be a vacuum deposition method based on an exchange reaction between a volatile Ln^IIIβ-diketonate and benzoic acid.³⁰³

• Another way to boost OLED efficiency is the introduction of inorganic layers such as TiO₂²⁶⁴ or ZnS^{271,272,278,279} which act as electron-transporting and hole-blocking compounds. Thus, at least 1.5-fold improvement in brightness was observed for OLEDs containing [Ln(43c)₃(phen)] when BCP and Alq₃ (Eu^III) or Alq₃ and LiF (Tb^III) layers were substituted with ZnS.^271,272 Enhancement of Tb^III or Eu^III electroluminescence also occurs upon partial substitution of Ln^III with Y^III in devices incorporating [Tb(43a)₃(phen)₂]²⁷⁴ or [Eu(tta)₃(bpy)].²⁶⁵ For the latter complex, a similar effect happens when Gd^III 264 or Tb^III 266 are introduced.

White-light generation. The interest in generation of white light from either OLEDs or inorganic LEDs stems from their potential use for general lighting applications and display backlighting. High-quality white-light illumination requires a source with CIE coordinates (0.333; 0.333), with CCT in the range 2500–6500 K, and CRI above 80%.³⁰⁴ Emission from organic or inorganic phosphors covers only part of the visible spectrum. To overcome this limitation, various architectures of diodes combining monochromatic emission from different compounds have been suggested. For OLEDs the four most common schemes are exemplified on Fig. 7.^304–306

Fig. 7 Simplified scheme of four general approaches for construction of white OLEDs.^304–306

In the case of inorganic LEDs, two approaches have been privileged: (i) combination of individual red, green and blue LEDs and (ii) conversion of blue or near UV light to lower energy emission by means of yellow-emitting phosphors, or by combinations of blue, green and red phosphors coated on the semiconductor chip (Fig. 8).^215,307

Fig. 8 Principle of white-light generation in phosphor-coated LEDs.²¹⁵

The number of publications dealing with white-emitting OLEDs relying on lanthanide coordination compounds remains quite scarce.^308–312 Early works describe near-white OLEDs built by stacking green- and red-emitting layers containing Tb^III or Eu^III ternary β-diketonates, respectively.^308,310 Alternatively, a layer doped with mixed [Tb_xEu_1−x(acac)₃(phen)] complexes is also adequate.³⁰⁹ Use of a dendritic Eu^III complex grafted with carbazole units as a single dopant in a device with composition ITO|NPB|EuL–CBP|BCP|Mg_0.9Ag_0.1 resulted in white light emission with CIE coordinates (0.333; 0.348) at 16.2 V.³¹¹ The maximum brightness of this OLED reached 229 cd m⁻² at 20.5 V with η_ext = 1.1%. Another very efficient white-emitting OLED was built by combining red emission from a [Eu(tta)₃(Tmphen)] (Scheme 17) layer co-doped with an Ir^III phosphorescent complex in CBP host and blue emission from a layer consisting in the dye p-bis(p-N,N-diphenylaminostyryl) benzene.³¹² Once optimized, the device had a maximum brightness of 19 000 cd m⁻² at 17 V and luminous efficacy of 4.5 lm W⁻¹, while CIE coordinates changed from (0.39; 0.32) at 6 V to (0.33; 0.30) at 14 V.

White light generation in a single layer is feasible with polymers incorporating Eu^III, Tb^III and Dy^III complexes in appropriate ratios,^313,314 by combining a blue-emitting Dy^III polymer with a red-emitting Ru^II complex,³¹⁵ or by relying on heterobimetallic Eu^III–Ir^III complexes.³¹⁶ The design of OLEDs based on the latter complexes was patented in 2006.³¹⁷

A combination of a blue-emitting OLED, containing either a fluorescent polymer or a phosphorescent Ir^III complex, with down-conversion layers is efficient in generating white light too (Fig. 7).^215,306 When the down-conversion layer consists in a nitridosilicate phosphor ([Sr,Ba,Ca]₂Si₅N₈:Eu²⁺), cool white light with CIE coordinates (0.26; 0.40) and luminous efficacy of 25 lm W⁻¹ is emitted.³⁰⁶ Following the same idea, a white OLED with CCT = 5000 K and CRI = 88% was built from a combination of perylene orange, red dyes and the Y(Gd)Al₅O₁₂:Ce³⁺ phosphor.²¹⁵ This is one of several examples of white-light generation relying on inorganic phosphors. Garnets, silicates, sulfides, oxynitrides or nitrides doped with an intricate mix of lanthanides are commonly found in so-called phosphor-converted LEDs (pc-LEDs, Fig. 8).^{307,318–324} According to the working principle of these devices, the phosphors should possess strong absorption in the blue- or near UV and have high photochemical stability. In fact, pc-LEDs are already on the market, and further developments in this area, not taking into account technological aspects such as light out-coupling efficiency and LED package gain, should be targeted at improving the colour quality, i.e. CTT in the range 2500–6500 K.²¹⁵ Recent developments deal with M₂Si₅N₈:Eu²⁺ (M^II = Sr, Ba)³²⁵ or SrAlSi₄N₇:Eu²⁺.³²⁶ Another approach is the use of near-UV semiconductor chips instead of blue-emitting ones, with a combination of red, green and blue phosphors.²¹⁵ One of the challenging doping ions is Eu³⁺, however: weak absorption is a real problem. To overcome this, some recent achievements use ternary Eu^IIIβ-diketonates with phen,^20,327 or CdO-Al₂O₃-3SiO₂:Eu³⁺ which displays an emission ∼3-fold more intense than the well-known commercial Y₂O₃:Eu³⁺ phosphor,³²⁸ or Ca₃Ln₂W₂O₁₂:Eu³⁺ (Ln = La, Gd, Y).³²⁹ In parallel, generation of white light from single-emitting inorganic phosphors is developing, for instance, Eu²⁺-activated (Ba, Sr)_13−xAl_22−2xSi_10+2xO₆₆³³⁰ or incorporation of Si⁴⁺–N³⁻ into Ce³⁺-doped garnets.³³¹

Optical amplifiers and waveguides. The rapid development of telecommunication networks requires high speed data transmission and thus stimulates interest in optical integrated components such as amplifiers, splitters, couplers, multiplexers and de-multiplexers. Optical amplifiers are basic elements of photonic integrated circuits and consist usually of a guiding layer with a high refractive index positioned between low-refractive cladding layers. When the guiding layer is doped with active elements, optical gain arises upon appropriate pumping. Lanthanide ions present a special interest as doping elements, especially Er^III (1530 nm) and Nd^III (1060 nm), as well as the less studied Pr^III (1300 nm) and Tm^III (1500 nm), since they exhibit transitions in the range of telecommunication windows (1000–1600 nm).^332,333 Yb^III which emits at 980 nm is less interesting from this point of view, but acts as an efficient sensitiser of these ions,²¹⁴ thus it is usually introduced as a co-doping element. Host materials for guiding layers should have low absorption at pump and emission wavelengths, and different classes of inorganic compounds have been developed for this purpose: silicate,³³⁴ phosphate^335,336 or oxyfluoride³³⁷ glasses, nanocomposites SiO₂–HfO₂,³³⁸ sesquioxides,^339–341 and LiNdO₃,³⁴² in addition to polymers such as polymethylmethacrylate (PMMA),^343–345 ethylene glycol dimethacrylate (EGDMA)³⁴⁶ or fluorinated ZPU12-470.³⁴⁷ A large variety of techniques have been applied to produce lanthanide-doped waveguide amplifiers in optical materials, ranging from ion exchange,^334,336 to femtosecond laser inscription³³⁷ and embossing,³⁴⁶ to name a few. The use of ion beam implantation for the fabrication of 2-D waveguides is reviewed in ref. 348. Different techniques tested for producing guiding layers based on LiNdO₃:Ln^III along with activation methods of LiNdO₃ by Ln^III, and the main problems limiting luminescence efficiency such as quenching and optical damage in these systems can be found in ref. 342. An investigation of energy transfer processes in highly-doped (up to 10%) Y₃Al₅O₁₂:Ln (Ln^III = Nd, Yb, Pr) single crystals obtained by liquid phase epitaxy has been performed.³⁴⁹ It confirmed that different concentration-dependent mechanisms operate in these systems including cross-relaxation, up- and down-conversion, as well as in the case of Pr^III, excited-state absorption to the 4f–5d levels resulting in UV emission. Moreover, liquid phase epitaxy is a promising method leading to high-quality, highly-doped samples without structural defects.³⁴⁹ Finally, fundamental conditions have been developed for the design of flat gain Er-doped glass optical amplifiers.³⁵⁰

The growing interest in Ln-doped polymeric optical waveguides can be traced back from their better flexibility and a large core diameter which enable efficient coupling.^332,333 However, insolubility of lanthanide inorganic salts in organic systems creates problems. One of the feasible solutions is the use of lanthanide complexes with organic ligands, for example, ternary Er^III dibenzoylmethanate with phen,^346,347 Er^III nitrate with derivatives of triphenylphosphine oxide³⁴³ or Er^III/Yb^III co-doped tris(4-pentylbenzoate) with phen.³⁴⁵ An embedded 15-mm long waveguide based on the latter complex with polymer SU-8 as a cladding layer showed an optical gain of 5.2 dB at 1550 nm for a signal power of 0.3 mW. When PMMA-glycidylmethacrylate was used, the optical gain reached 6.5 dB in a 12-mm long waveguide with an input power of 1 mW. Another way out takes advantage of nanosystems such as Er^III/Yb^III co-doped lanthanum fluoride^351,352 or phosphate³⁵³ nanoparticles modified with organic ligands or dispersed in sol–gel. In addition, the feasibility of using Y₂O₃:Er^III nanotubes for the same purpose has been established.³⁵⁴ A 22-mm long waveguide based on a hybrid inorganic–organic system with LaF₃:Er^III,Yb^III nanoparticles demonstrated a relative optical gain of 5 dB at 1535 nm.³⁵² Detailed description about problems dealing with NIR polymer optical fibres and their possible solutions, as well as earlier achievements in this area are summarized in ref. 58.

Recent examples describing improvements in waveguide performance are to be mentioned. For instance, (i) a new pumping scheme at 477 nm for Si-nanocluster sensitised Er^III-doped waveguide amplifiers has been suggested³⁵⁵ and (ii) metal-catalyzed growth via self-organized Er^III or Au^I:Er^III implantation was exploited to produce SiO_x nanowires on silicon surfaces.³⁵⁶ These nanostructures exhibit characteristic Er^III photoluminescence at 1530 nm with a remarkably long lifetime of 24 ms and could find wide application in optical systems.

Some researchers have also demonstrated the feasibility of optical amplification in the visible range in polymer waveguides doped with ternary Eu^IIIβ-diketonates with phen^346,347 or Ln^III–Al nanoclusters (Ln = Eu, Tb).^357–359 For the latter system the highest optical gain coefficients were estimated to be 0.56 and 1.28 mm⁻¹ for Tb^III–Al and Eu^III–Al nanoclusters, respectively.

4.6 Solar energy conversion

To meet the steadily rising energy requirements of modern civilization and take into account the ongoing energy crisis and global warming problems the quest for harvesting renewable energy sources such as sunlight is becoming a vital issue. A theory based on quantum electrodynamics has been developed which establishes correlations between structures and fundamental mechanisms operating in energy harvesting systems, with emphasis on dendrimers and lanthanide-doped solid compounds.³⁶⁰

Sunlight can be directly converted into electricity in so-called solar (SC) or photovoltaic (PV) cells. The most widely used inorganic semiconductor for SC is silicon (Si, E_g = 1.12 eV).³⁶¹ The two major loss mechanisms originating from the discrete band-gap structure of semiconductors have to be overcome for any single-junction SC, to enhance device efficiency: (i) lattice thermalisation and (ii) transparency to sub-bandgap losses. One of possible ways out is photon conversion through: (i) down-shifting, (ii) down-conversion (known also as quantum cutting), or (iii) up-conversion mechanisms. The main principle of this concept is the conversion of the solar spectrum via luminescence for matching the absorption properties of the semiconductor device. The up-conversion layer is usually located on the rear reflector to capture sub-bandgap light, while the down-conversion layer is deposited on top of the solar cell to convert each high-energy photon into multiple low-energy photons (Fig. 9). Once optimised both up- and down-conversion layers can be combined in one solar cell leading to increased efficiency.^362–366

Fig. 9 Simplified schemes of bifacial and monofacial solar cells with up- and down-converting layers, respectively.

Down-shifting is similar to down-conversion, except that the luminescence quantum efficiency of this process is less than unity. However, it can still result in significant SC efficiency improvement.³⁶⁷ Interest in lanthanide-based materials for photon converting layers in SC again stems from their unique luminescent properties with emission from UV to NIR spectral ranges (Section 2), while the main drawback to overcome, i.e. low absorption coefficients, remains. In earlier studies materials incorporating Eu^III or Tb^III nitrates with phen or bpy were tested as down-shifting layers;^368–369 presently, however, more attention is paid to lanthanide-containing inorganic phosphors.^{217,370–379} Recent examples are down-shifting layers consisting in Yb^III-doped glass ceramics containing Ba₂TiSi₂O₈ nanocrystals.³⁷⁰ In this material the conversion of near-UV radiation (250–350 nm) into NIR emission of Yb^III (970–1100 nm) occurs through energy transfer from the silicon–oxygen-related defects in SiO₄ units to the ²F_5/2 level, thus overcoming the problem of low absorptivity.

Down-conversion in phosphors can be achieved via cooperative energy transfer when the inorganic matrix is co-doped with appropriate lanthanide ions.³⁶³ In transparent glass ceramics with embedded Pr^III/Yb^III:β-YF₃ nanocrystals an efficient Yb^III NIR luminescence with optimum quantum efficiency close to 200% is achieved.³⁷¹ Following the same principle, combination of Yb^III with Tb^III in polyborate La_0.99−xYb_xBaB₉O₁₆:Tb_0.01³⁷² or doped into Zn₂SiO₄ thin films,³⁷⁷ as well as Tm^III/Yb^III co-doping into YPO₄³⁷³ or oxyfluoride glass ceramics containing LaF₃ nanocrystals³⁷⁶ also lead to conversion of UV-blue emission into NIR with quantum efficiency higher than 150%. Benefiting from the strong 5d ← 4f absorption of Ce^III in the range 250–380 nm, high cooperative energy transfer to Yb^III with efficiency of 74% and corresponding quantum efficiency of 174% was measured in Ce^III/Yb^III co-doped borate glasses.³⁷⁴ An interesting example of three-photon infrared quantum-cutting material is Er_0.3Gd_0.7VO₄: when the ²H_11/2 level is excited at 523.5 nm, NIR emission at 1532.5 nm (⁴I_13/2→⁴I_15/2 transition) is stimulated, the estimated efficiency being 178.6%.³⁷⁵

Up-converting materials for solar cells often contain Er^III-doped compounds^364,365 which convert absorbed light in the range 1480–1580 nm to emission at ∼545, 665, 800 and 980 nm, as recently demonstrated with the Gd₂(MoO₄)₃:Er^III nanophosphor.³⁷⁹ Moreover, up-conversion of 1170 nm excitation into 650- and 910-nm emission in Ho^III-doped oxyfluoride nanophase glass ceramics has been recently achieved.³⁷⁸ This material, in combination with Er-doped up-converting phosphors, could increase absorption of the solar spectrum and the efficiency of silicon SCs.

Luminescent solar concentrators which are able to concentrate sunlight onto a small area represent an alternative for boosting the efficiency of silicon-based SCs. An overview of potential compounds for this purpose, including lanthanide-based systems is given in ref. 380.

Apart from energy problems, optimization of sunlight conversion efficiency is also welcome in the photosynthesis of plants, in order to improve crop yield. Usually, plant pigments such as chlorophyll, carotenes and xanthophylls, harvest primarily the blue and red parts of sunlight. Thus, the efficiency of photosynthesis can be optimized if the unused portions of the solar spectrum are converted into blue and red light. Following this, the concept of dual-excitation dual-emission phosphors (DE²) has been suggested.³⁸¹ For instance, Ca_0.6Sr_0.4S:0.005Cu⁺, 0.001Eu²⁺ has an optimal efficiency for converting ultraviolet into blue and green into red. In this way, more than 20% increase in cabbage and pimento production was achieved by using plastic films covered with the suggested phosphor.

5. Applications in bio-sciences

The biological chemistry of rare earths has roots in the 19th century when cerium oxalate was widely prescribed as an antiemetic drug to cure sickness due to pregnancy. Moreover, the in vitro antimicrobial properties of several lanthanide complexes stimulated clinical trials in the treatment of tuberculosis and leprosy although their impact was minimal.³⁸² Lanthanide ions have also anticoagulant properties but their main therapeutic applications lie presently in the radioactive treatment of cancers. The toxicity of free lanthanide ions vary considerably, IC₅₀ being reported between 1 and 1000 μM for Gd^III for instance,^383–385 while under given experimental conditions they even enhance cell proliferation.³⁸⁶ Gadolinium chelates are ubiquitous contrast agents for magnetic resonance imaging^387–389 and despite some recently reported problems with one class of compounds³⁹⁰ they are well tolerated and considered as being harmless.

Since the mid-1970s, time-resolved capability of lanthanide luminescent bioprobes (LLBs)¹² and the availability of adequate bioconjugation protocols¹⁰ allowed the development of highly sensitive immunoassays,^391,392 protein staining assays,³⁹³ nucleic acid analyses,⁵⁵ or the determination of enzyme activities.⁵⁶ More recently, time-resolved microscopy^11,394 has also benefited from the intrinsic advantages of LLBs and cell-penetrating optical probes allow not only the visualisation of live cells¹⁴ but also carrying out targeted analyses of key biochemical metabolites.^395–397 Extension to immunocytochemical analyses and immunohistochemical detection of cancerous tissues are now at hand.^13,398

The field is measureless and hundreds of articles are published yearly in photophysical, chemical, biochemical, pharmaceutical, and medicinal journals. In the following we therefore restrict ourselves to highlighting only a few characteristics of these analyses, particularly those related to fundamental aspects of photophysics and to the latest development in time-resolved imaging; the reader is referred to the abundant reviews published recently on these subjects for further information.^{55,56,104,392,393,395,396,399–402}

5.1 General considerations

A LLB may be used in various ways taking advantage of different properties of the Ln^III ion. Initial applications have dealt with (i) simple substitution of Ca^II or Zn^II by Ln^III in proteins to gain information on the composition of metal-binding sites (e.g. hydration number) or metal-to-metal and/or metal-to-chromophore distances by Förster resonant energy transfer experiments (FRET),⁴⁰³ and (ii) titration of a bio-compound with Ln^III salts to determine the number of metal-binding sites.^45,404 Nowadays more astute techniques are at hand based on carefully tailored chelates. The latter are either used directly, e.g. in analytical responsive probes and simple cellular imaging experiments without specific targeting, or conjugated to a protein or to an antibody which specifically recognizes a targeted biomolecule.

Requirements for efficient LLBs are challenging: (i) water solubility, (ii) large thermodynamic stability, (iii) kinetic inertness, (iv) intense absorption above 330 nm, (v) efficient energy transfer onto the Ln^III ion, (vi) coordination cavity minimizing non-radiative deactivations, (vii) long excited-state lifetime, (viii) when relevant, ability to conjugate with bioactive molecules while retaining its photophysical properties and not altering the bio-affinity of the host.

Generally speaking a LLB can be used in a direct way or indirect way. In the first case, the LLB luminescence (or its enhancement, or its quenching) is simply detected, preferably in time-resolved (TR) mode, after specific interaction with the analyte. In turn, these responsive systems can be applied directly, the chelate interacting itself with the analyte, such as in the cases of e.g. oxygen, pH, ATP, or anion sensors, or indirectly, after bioconjugation to a suitable antibody or peptide⁴⁰⁵ which performs the recognition of the analyte via a specific biochemical reaction (Fig. 10, top). Interaction of the probe with the analyte often relies on the strong and specific avidin (resp. streptavidin)—biotin interaction (K_as≈10¹⁵ M⁻¹); to increase the number of LLB per analyte and thus to boost the sensitivity, the LLBs are sometimes additionally attached to bovine or human serum albumin (BSA, or HSA); alternatively, two Ln^III ions are grafted onto the same peptide, yielding double lanthanide binding tags (dlbt).⁴⁰⁶

Fig. 10 Direct (top) and indirect (bottom) use of a LLB.

Bioconjugation is most commonly performed in solution by allowing a chelate with an activated group to react with a functional group of a bioactive molecule. Derivatization of synthetic biomolecules can be conveniently achieved in solid phase with standard peptide and nucleotide synthesizers, but solution-phase labelling of proteins is also common. In all cases, the labelling reaction must be as selective and effective as possible. The various protocols for efficient covalent bioconjugation of luminescent chelates have been reviewed recently.¹⁰

In the second indirect method, the excitation energy is transferred via the Ln^III ion onto an organic acceptor by FRET (Fig. 10, bottom). FRET occurs when the donor (D) and the acceptor (A) lie at distances larger than 40 pm, the corresponding mechanism being dipole–dipolar (through space) and its yield is defined by eqn (15) and (16). FRET eliminates the need for washing unreacted reagents since the transfer only occurs when the two entities, the LLB donor and the organic acceptor, are linked together via some kind of conjugation. The essential point is that the organic fluorophore emits light with an apparent lifetime equal to the lifetime of the donor, its excited state being populated via the Ln^III ion. LLBs perform better than organic dyes in that the distance at which the transfer may be detected is larger (typically up to 800–1000 pm, as compared to 500–600 pm for organic dyes). Applications of FRET are far reaching, ranging from homogeneous immunoassays to DNA hybridization assays, or analyte imaging in cells (such as calcium or phosphorylated molecules).⁴⁰⁷

5.2 Responsive luminescent probes

One emerging area of investigation is the determination of enzyme activity such as glucose oxidase, catalase and peroxidase. Enzymes catalyzing phosphorylation or de-phosphorylation reactions are also under investigation in view of their high relevance to drug targets: protein kinases, adenyl cyclases, phosphodiesterases, phosphatases, ATPases, to name a few.

As an example, tetracycline (Scheme 19) complexes with 1 : 1 or 3 : 1 Eu : L stoichiometry are among the most convenient Eu^III probes for quantifying phosphate, ATP and hydrogen peroxide. For instance, [Eu(45a)] luminescence increases 15-fold upon interaction with H₂O₂ which is the product of the activity of almost all oxidases and therefore this enhancement may be used to quantify the latter. On the other hand the luminescence of [Eu(45a)] is quenched by the presence of phosphate, leading to an assay of this anion on microliter plates with a detection limit of 3 μmol L⁻¹.⁴⁰⁸ Citrate and oxytetracycline (45b) can by conveniently probed by Eu^III in complex media such as drug tablets or serum with detection limits in the range 0.1–0.2 μg mL⁻¹.⁴⁰⁹ Alkaline phosphatase enhances the luminescence of [Eu₃(45a)] and the corresponding time-resolved luminescence assay has a detection limit of 4 μmol L⁻¹.⁴¹⁰ Applications of europium tetracycline complexes in assays for phosphate, malate, lecithin, heparin, HSA, citrate, low density lipoproteins (LDL), and so on are detailed in the review by Courrol and Samad,⁴⁰⁰ while more Eu^III probes are described and compared in Spangler et al.⁵⁶ Nucleotides and phosphate probes often rely on Tb^III luminescence,⁴¹¹ for instance the 4.5 : 1 complex of Tb^III with norfloxacin (46) exhibits a five-fold increase in luminescence upon interaction with ATP.

Scheme 19 Ligands used for Ln luminescent probes. From top to bottom: tetracycline (left) and norflaxin (right); cyclen core structures (S = sensitising unit); cascade energy transfer in a coumarin-rhodamine cyclen complex; ligands for reporters of redox metabolism.

Many probes for DNA analysis have been proposed,^55,412,413 some of them exploiting the time-resolved capability of the Ln^III ions leading to detection limits up to 50-fold larger than those reached with the organic dye FITC,^414–416 their propensity to be easily entrapped into nanoparticles,⁴⁰² their up-conversion ability,⁴¹⁷ or their NIR luminescence.⁴¹⁸ Examples of the latter are the cyclen complexes [Ln(48)] (Ln^III = Nd, Yb; Scheme 19): the cascade energy transfers which populates the Ln^III excited states is interrupted when the coumarin substituent interacts with ds-DNA, making the complexes DNA probes; however, no detection limit is given.⁴¹⁸

A series of successful lanthanide responsive probes for quantifying analytes of biological relevance are based on the cyclen framework (Scheme 19).^{52,395,396,419} The latter usually features three coordinating units (carboxylates, substituted amides, phosphinates) and one sensitising pendant which often simultaneously binds the central metal ion. Signalling is achieved by enhancing or quenching the metal-centred luminescence. Typical analytes which can be quantified are pH, p(O₂) and various anions (halides, HPO₄²⁻, SO₄²⁻, acetate, oxalate, malonate, succinate, lactate, citrate)^420–422 or amino acids.⁵² One of the most prominent results reported recently is the ratiometric method for the determination of lactate or citrate in samples of human serum, urine or prostate fluids for diagnosis of prostate cancer.⁴²³ Luminescent reporter substrates for redox metabolism, such as [Nd(49(CO))], see their metal-centred NIR luminescence switched on upon reduction of the ketone group into alcohol, leading to [Nd(49(OH))]; these sensors are able to detect human aldo-ketoreductases, which are involved in steroid hormone metabolism and stress response mechanisms.⁴²⁴

Alternate substrates have been used for similar purposes, for instance chiral tripodal ligands for the analysis of anions,⁴²⁵ polyaminocarboxylates immobilized on sensory chips for detection and separation of histidine-tagged ubiquitin proteins,⁴²⁶ transferrin and lactoferrin complexes of Tb^III for pH sensing,⁴²⁷ or dendrimeric complexes for NIR detection of anions.¹⁰⁹

5.3 Up-converting phosphors

Among recent developments in lanthanide-aided bio-analysis, nanoparticles,¹⁷⁴ quantum dots,⁵⁴ and up-converting phosphors (UCPs),^417,428,429 attract much attention. UCPs are rare-earth doped ceramic-type materials (oxides, oxysulfides, fluorides, oxyfluorides for instance) which convert red light into visible light. They appear as microspheres or nanospheres, were proposed for bioassays at the beginning of the 1990s and represent now a well established class of bio-probes. They bear many advantages, such as low sensitivity to photobleaching, high optical sensitivity due to the presence of many emitting centres per particle, capability for multiplex analyses in view of the possibility of doping different Ln^III ions, as well as cheap diode laser excitation (e.g. at 980 nm). Most of the UCPs feature Er^III as the emitter since it has two emission lines in the green (540 nm) and in the red (654 nm), along with Yb^III as activator, but other pairs such as Tm^III/Ho^III can also be envisaged.

The synthesis of the microspheres involves several steps. For instance for (Y_0.86Yb_0.08Er_0.06)₂OS,⁴³⁰ the lanthanide salts are first mixed in stoichiometric ratios and the particles are precipitated by added urea before being thermally converted to oxides and treated with H₂S. Activation of the particles follows by fluidization under argon at elevated temperature. Aggregation is minimized and monodispersed spheres with average diameter of 0.4 μm are obtained. Homogeneity of the sample is important to ensure constancy in the emission intensity.⁴³¹ Secondly, the hydrophilicity of the microspheres is increased by coating them with a 20–30 nm thick layer of silica. Finally the surface is functionalized by covalent linkage of the adequate antibody (Fig. 11, top) using known bioconjugation methods. Another important experimental point is to ensure that the excitation light from the diode laser is focused tightly. A weakly focused beam may lead to 10-fold smaller emission intensities.⁴³²

Fig. 11 (Top) Coated and derivatised up-converting phosphor (UCP) microsphere for targeting a specific antigen. (Bottom) Scheme of a multiplex assay.

The microspheres are excellent reporters for the detection of antigens in tissue sections or cell membranes; presently they are often obtained as nano-objects, with diameters of 2–20 nm. They have been initially tested on the prostate-specific antigen in tissues and the CD4 membrane antigen on human lymphocytes⁴³³ and then used in several homogeneous immunoassays, e.g. for estradiol.⁴³⁴ Up-converting phosphors are also able to transfer energy onto “tandem dyes” comprised of an absorber (B-phycoerythrin) and an emitter (an Alexa Fluor 680 dye); in this way, low-power NIR excitation results in specific dye-centred visible emission devoid of autofluorescence, even without time-resolved detection, thus allowing very sensitive homogeneous immunoassays.⁴³⁵ In view of these successes, various types of up-converting nanophosphors have been synthesized and studied for their spectroscopic properties.^{428,436–440} For example, NaYF₄ nanoparticles (diameter ≈11 nm) doped with Yb^III, Tm^III or Yb^III, Ho^III have been derivatised with carboxylic acid functions; after classical treatment with EDC (1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride) and sulfo-NHS (N-hydroxysuccinimide)¹² conjugation with streptavidin, and coupling with a biotinylated peptide functioning as capture-DNA they have proved to be adequate FRET sensors for reporter DNA.⁴⁴¹ The performances of UCPs for bio-assays have been compared to those of Eu^III-based labels⁴⁴² while their usefulness for these assays is summarized in two recent review articles.^417,429

5.4 Cell imaging and sensing

Given that many lanthanide complexes are cell-permeable and that techniques of time-resolved detection in microscopy are well mastered,^11,443 scientists have used the unique spectroscopic properties of Ln^III ions to get images of cells, for instance in the context of the follow-up of cancer therapy. Additionally, LLBs are also able to signal the presence of many important analytes (e.g. Ca^II, bicarbonate, ascorbate, urate) so that their fluctuations are tractable with high spatial resolution and yield information of utmost importance on cellular metabolism. Optical probes are unique in doing this type of job since acquisition of signals is fast. Specific modification of the LLBs, or their bioconjugation, furthermore opens the door to deciphering where analytes accumulate in live cells and what are their concentrations in the various cell organelles, mitochondria, for example. Furthermore, analysis of cancerous tissues is crucial to both medical diagnosis and monitoring of a therapeutic treatment^13,398 and here again the time-resolved (TR) capability of LLBs decreases the detection limits of tumour cell biomarkers.⁴⁴⁴ Curiously, despite that TR detection has made the success of lanthanide-based immunoassays for decades, few studies on cell imaging took advantage of it until recently.^445–448

In the course of an extensive and seminal investigation of over 60 different cyclen-based LLBs fitted with various chromophores (Scheme 19), the group of Parker has established that the nature of the chromophore and its attachment mode to the macrocycle primarily determines the cell uptake and localization and not the charge of the complex or its lipophilicity. The cell lines studied were mouse skin fibroblasts (NIH-T3 cells), Chinese hamster ovarian (CHO), or cervix carcinoma HeLa cells. The reason invoked is that polycyclic sensitiser units are recognized by protein association. About 80% of the macrocyclic complexes have endosomal/lysosomal localization; in these cases the rates of uptake and egress are fast. Half a dozen of these complexes are also found to display fast uptake, but slower egress, and are found to shuttle between mitochondria and endosomal/lysosomal compartments. Complexes staying in the mitochondria have low IC₅₀ values and cause cell apoptosis while those internalized in lysosomes are non-toxic and therefore can act as responsive probes. Finally a few complexes display slow uptake and egress and enter the ribosomes and the nucleoli. Their IC₅₀ values are in the range 40–90 μM.³⁹⁶ The nature of the substituent on the sensitiser unit of the macrocyclic ligand is a key factor with respect to the sensitivity of the LLB; it also strongly affects protein affinity.⁴⁴⁹

Another class of cell-imaging LLBs are binuclear helicates [Ln₂L₃] which self-assemble in water at pH 7.4 and room temperature. The polyoxyethylene pendants ensure water solubility and higher hydrophilicity of the helicates (Scheme 20). The latter are thermodynamically stable and kinetically inert.^14,450 Several ligands have been tested^451–455 and the best ones turned out to be (50a)²⁻ which sensitises the luminescence of several lanthanides, primarily Eu^III (Q^L_Ln = 21%) and Tb^III (11%), but also Sm^III (0.4%) and Yb^III (0.15%), thus allowing multiplex analyses, as well as (50b)²⁻ amenable to excitation at the beginning of the visible range. A sensitive and versatile method of DNA and polymerase chain reaction (PCR) product analysis has been devised with [Eu₂(50a)₃].⁴¹⁴ All the chelates are non-cytotoxic for HeLa, MCF-7 (human breast cancer), Jurkat (human T leukaemia), 5D10 (mouse hybridoma) and HaCat (non-malignant epithelial human cells) cell lines, with IC₅₀ > 500 μM. The uptake in the cells (Fig. 12) is slow and proceeds by endocytosis, with a localization in the endoplasmatic reticulum, irrespective of the substituent or nature of the ligand core,^454,455 and an average number of helicates per cell in the range 2–5 × 10⁸, much as for the cyclen-based complexes. Egress is also slow and almost no leakage is seen after 24 h. Microscopy images recorded in TR mode have a sensitivity two- to three-fold higher than those measured conventionally.⁴⁵⁶

Scheme 20 Left: examples of ditopic, hexadentate ligands for the self-assembly of binuclear helicates. Right: cryptand and bis(β-diketone) for luminescent Eu^III complexes.

Fig. 12 Merged bright-field and time-resolved luminescence microscopy of HeLa cells incubated 0.5 and 3 h with 200 mM {Eu₂(50b)₃}. Adapted from ref. 455.

In view of their advantages, nano-materials are also being tested for cell imaging. For instance, Eu^III-doped nanorods (20 × 500 nm) coated with chromophore-grafted silica stain the cytoplasm of living human lung carcinoma cells. They are internalized and maximum emission is seen after 2 h.⁴⁵⁷ Nanoparticles entrapping the Eu^III complex with H₂52 and with diameter < 50 nm can distinguish the pathogen Giardia lamblia in environmental water when detected under TR mode.⁴⁵⁸

The homogeneous time-resolved luminescence method designed for immunoassays⁴⁵⁹ and relying on FRET can be extended to in vivo cellular imaging by labelling specific molecules within the cells. A proof of concept has been given with an assay on transfected living HEK cells expressing a tagged membrane receptor (GABA_B2) which was detected by a specific monoclonal antibody labelled with the chelate [Eu(51)]⁻.⁴⁶⁰

One of the first reports on the suitability of lanthanide NIR luminescence for imaging tumour cells dates back to 1989^461,462 when Russian workers detected tumours in mice with tetraphenylporphyrinate derivatives of Yb^III with contrast ratios up to 45, the porphyrinate binding specifically to cancer cells. The field has then been silent until a recent revival when these macrocyclic complexes were coupled to BSA for enhanced emission.⁴⁶³ A more elaborate probe was proposed lately for both NIR tumour imaging and photodynamic treatment of cancer: an amphiphilic Yb^III bis(porphyrinate) complex which is up-taken by rat Sarcoma 180 cells, localizes in their lysosome and displays a large cytotoxicity (light dose required for 50% death: 6.2 J cm⁻²).⁹⁴ Nd^III emission from core/shell NdF₃/SiO₂ nanoparticles has been recorded after their injection into thigh and abdominal cavities of mice at a depth of 0.3 and 1 cm, respectively, confirming the interest of NIR luminescence for imaging in view of its penetration depth in tissues.⁴⁶⁴

5.5 The advent of multiphoton excitation

Several of the probe systems experimented so far either for analyte sensing in vitro or in vivo or for cell and tissue imaging have the disadvantage of necessitating a relatively short wavelength of excitation (typically 300–360 nm). Since UV light damages cells and tissues, it is desirable to extend this wavelength into the visible. However, there are intrinsic limits to the design of Ln-containing luminescent molecules which severely limit this approach, at least as far as visible-emitting probes are concerned; for instance, Eu^III can rarely be excited above 400–420 nm because of back-transfer^455,465 problems. For instance, when dipicolinic acid is substituted to give H₂53e–g (Scheme 21) with tuneable charge-transfer states (push–pull ligands), the corresponding Eu^III complexes have absorption maxima around 320 nm, compared to 280 nm, and they display sizeable quantum yields, up to 43%. However, when the excitation wavelength is shifted to 427 nm in [Eu(53h)₃]³⁻, no Eu^III emission is detected at room temperature.⁹⁷ A way out are the up-converting phosphors described above. Another solution, similar and sometimes confused with up-conversion, is multiphoton absorption (MPA), principally two- or three-photon absorption (2PA or 3PA). Multiphoton absorption allows non-invasive 3D imaging of biological tissues without creating collateral damages. For MPA to occur with reasonable probability, high-power femto lasers with output in the range 700–800 nm are required. Microscopes fitted with such an excitation source are available,⁴⁶⁶ so that efforts are being devoted toward the design of systems displaying suitable MPA cross sections. An extensive and comprehensive review on multiphoton absorbing materials, their design, characterization and applications has been published recently in what has to be considered as a reference work.⁴⁶⁷

Scheme 21 Chemical structures of organic ligands for MPA derived from dipicolinic acid.

The possibility of a chromophore to simultaneously absorb two (or more) photons has been mentioned for the first time by Maria Göppert-Mayer in 1931. To describe this optical phenomenon in quantum mechanical terms, let us consider a laser beam of frequency ν passing through a non-linear absorbing medium. The occurrence of 2PA can be visualized as one photon being absorbed first to promote the molecule into an “intermediate state” from which absorption of a second photon props it up into the excited state. Contrary to up-conversion, the “intermediate state” needs not to be a real state; in this situation, the molecule may be in any of its eigenstates (except the ground and final excited state) with a distribution between them. Because the uncertainty of the distribution is large, the lifetime of the “intermediate state” is extremely short according to the uncertainty principle, so that the two absorption steps occur quasi-simultaneously. While most of the experiments are conducted with single-frequency laser light (degenerate MPA), simultaneous absorption of several photons of different energies is also feasible. MPA is related to non-linear optical (NLO) properties of the chromophore under consideration and the cross section is controlled by its molecular structure. There is an evident correlation between intramolecular charge-transfer processes and MPA: the permanent ground-state dipole moment and the transition dipole moments are essential factors in these phenomena. Regarding 2PA, the extent of conjugation between the π-donor and the π-acceptor is critical as is co-planarity between the two poles of the molecule. Increasing the number of conjugation paths to form two- or three-dimensional structures greatly increases the 2PA cross-section.⁴⁶⁷

The 2PA (σ⁽²⁾) and 3PA (σ⁽³⁾) cross sections are expressed in GM (Göppert-Mayer) units: 1 GM = 10⁻⁵⁰ cm⁴ s photon⁻¹ (2PA) or 10⁻⁸⁵ cm⁶ s² photon⁻² (3PA). They are difficult to determine in an absolute way and experimental uncertainties are often on the order of ±25%. Measurements of 2PA are usually made by comparison with a standard:

Here the subscripts S and R stand for sample and reference, respectively, c is the concentration, n the refractive index, F(λ) the integrated emission spectrum, and Q the (one-photon) quantum yield. For 2PA, Rhodamine B (Q = 0.45, σ⁽²⁾ = 120 GM at 800 nm) or Coumarin 307 (0.56 and 34 GM at 776 nm) are among usual standards.⁴⁶⁸ In some instances, the “action cross section” is reported, that is the product of the cross section by the quantum yield: σ⁽²⁾Q, sometimes written as δQ.

Determination of the level of MPA is simply made by plotting the logarithm of the luminescence intensity versus the logarithm of incident power. The slope of the straight line (1, 2 or 3) reflects 1PA, 2PA or 3PA processes. This experiment is important to carry out when lanthanide complexes are measured in view of the high power laser excitation needed for MPA. For instance, [Eu(dpa)₃]³⁻ emission can be excited at 532 nm, but the intensity/power plot has slope 1: excitation goes through the faint ⁵D₁←⁷F₁ transition (⁷F₁ population at room temperature: ≈35%; estimated ε at 532 nm: 0.015 M⁻¹ cm⁻¹) and corresponds to 1PA.⁴⁶⁹ We note that 2PA is allowed for f–f transitions but is rarely seen in complexes with organic ligands while it is well documented for inorganic compounds.^24,470

Quantitative data for 2PA in lanthanide complexes are still scarce and most of them deal with Eu^III. They are listed in Table 7 along with other photophysical data. Several classes of ligands have been tested for 2PA, the more extensive series being derivatives of dipicolinic acid (Scheme 21) since the conjugation criterion can easily be met and since the complexes have idealized octupolar D₃ symmetry, two favourable assets for MPA. A parameter to be carefully tuned is the energy of the (intraligand) charge transfer state; for dipicolinic acid derivatives the ideal energy appears to be around 23 000 cm⁻¹, that is slightly higher than the Eu(⁵D₁) level.⁹⁷ The best chromophore is ligand (53h)²⁻ bearing a stronger amino donor group compared with alkoxy groups in (53e)²⁻ with, to our knowledge, the largest σ⁽²⁾ reported to date for a lanthanide complex, 775 GM at 740 nm. This is close to the maximum that one may hope to reach for such molecules. Reducing the conjugation either by twisting or by shortening the π-skeleton in (53g)²⁻ and (53i)²⁻, respectively, leads to substantial decreases in σ⁽²⁾ (96 GM at 740 nm and 193 GM at 730 nm, respectively).⁴⁷¹

Table 7 Photophysical and 2PA data for lanthanide complexes (Schemes 21–23)

Compound	State	λ^maxabs/nm^a	10^3ε^max/M^−1 cm^−1 ^a	τobs/ms	Q^LLn (1PA) (%)	λ^max2PA/nm^b	σ^(2)max/GM^c	Ref.
a Value for the lowest-energy absorption band.b Wavelength at which maximum value of multiphoton absorption cross-section was detected.c 1 GM = 10^−50 cm^4 s photon^−1.d Emission from charge transfer state at 565 nm.e Wavelength at which cross-section was determined; maximum value of σ^(2) not yet reached.f In cetyltrimethyl ammonium bromide (CTAB) micelles (dav = 33.1 nm).g Maximum value under excitation at 402–420 nm, measured at 283 K.h Action cross section σ^(2)Q per nanoparticle; the same value per [Eu(tta)3(dpbt)] molecule is 37.4 GM.i Values for solution in H2O, pH = 7.4.j Action cross section σ^(2)Q.
[NBu4]3[Eu(53a)3]	CH2Cl2	335	86.0	0.90	9.0	700	110	471
[NBu4]3[Eu(53b)3]	CH2Cl2	335	131.0	0.45	3.6	700	218	471
Na3[Eu(53c)3]	H2O	332	78.7	1.06	15.7	700	92	490
[NBu4]3[Eu(53d)3]	CH2Cl2	427	94.0	n.a.	7.0^d	840	173	471
[NBu4]3[Eu(53e)3]	CH2Cl2	321	92.0	1.90	15.0	700	14	471
[NBu4]3[Eu(53f)3]	CH2Cl2	322	79.0	1.42	43.0	700	53	471
[NBu4]3[Eu(53g)3]	CH2Cl2	318	58.5	1.81	27.0	740	96	471
[NBu4]3[Eu(53h)3]	CH2Cl2	370	89.2	0.85	7.0	740	775	471
[NBu4]3[Eu(53i)3]	CH2Cl2	364	63.6	1.75	28.0	730	193	471
[Tb(53j)3]^3^−	H2O (pH 8.3)	314	23.0	n.a.	31.0	705^e	>26^e	473
[Eu(54)3][OTf]3	MeCN	360	75.0	0.27	5.6	720	96	500
Na2[Eu(55)]	H2O (pH 7.0)	334	22.8	1.1	15	705	28.6	472
[Eu(tta)3(dpbt)]	Toluene	406	63.0	n.a.	58.0	808	141	475
	Toluene	406	55.0	0.48	52.0	808	157	36, 477
[Eu(tta)3(dpbt)]^f	H2O-MeOH/CTAB	420	43.7	0.42, 0.22, 0.07	27.0^g	832	3.2 × 10^5 ^h	474
[Eu(tta)3(dmpbt)]	Toluene	409	11.0	n.a.	59.0	812	149	475
[Eu(fod)3(Mk)]	Toluene	413	31.0	n.a.	17.0	810	253	473
[Tb(57a)(NO3)3]	MeOH	∼295		n.a.	11.0	690	3.1	487
[Eu(60a)(H2O)]^3+	D2O (pD 7.8)	384^i	4.07^i	0.90	8.0	760^e	1.7^e	486
[Eu(60b)(H2O)]^3+	D2O (pD 7.8)	n.a.	n.a.	n.a.	1.6	760	0.4	486
[Eu(60b)(H2O)]^3+ + 20 mM HCO3^−	D2O (pD 7.8)	n.a.	n.a.	n.a.	5.0	760	0.4	486
Tb2-Transferrin	H2O (pH 9.0)	n.a.	n.a.	n.a.	n.a.	500	7.4^j	489

---

Carboxylates and aminocarboxylates complexes (Scheme 22) perform less well as seen for Na₂[Eu(55)] for instance (Table 7).⁴⁷² On the other hand, β-diketonates have larger σ⁽²⁾ values, the fod ternary complex with Michler’s ketone reaching σ⁽²⁾ = 253 GM at 810 nm.⁴⁷³ Interestingly, while [Eu(tta)₃(dpbt)] has an action cross section σ⁽²⁾Q = 37.4 GM at 832 nm, its insertion into colloidal nanoparticles (cetyltrimethyl ammonium bromide micelles) results in σ⁽²⁾Q = 3.2 × 10⁵ GM per particle,⁴⁷⁴ a value comparable to the largest ones reported for organic molecules.⁴⁶⁷ Other complexes, for instance with benzamide-containing chelating ligands (Scheme 23), have much lower values of σ⁽²⁾.

Scheme 22 Carboxylate-containing ligands and complexes (top) and Eu^IIIβ-diketonate ternary complexes (bottom); Mk is Michler’s ketone.^36,472–478

Scheme 23 Benzamide-containing chelating agents.^{480,481,486–488}

Less ligand optimization has been done for Tb^III complexes, and the cross sections determined to date are only between 3 and 26 GM.

Some reports describe 3PA Ln^III complexes, but no absolute cross-section is reported to our present knowledge. For instance, polymeric [Ln(BTC)(DMF)(phen)]_n carboxylate ternary complexes (Scheme 22) with phen acting as the antenna (Ln^III = Nd, Eu, Tb) are luminescent under excitation at 800 nm, the Nd^III complex representing a rare example of 3PA-induced NIR luminescence in Ln^III complexes with organic ligands.⁴⁷⁶ Power dependence plots for the polymeric tripodal complexes [Ln(NO₃)₃(57b)]_n (Ln^III = Eu, Tb, see Scheme 23) clearly point to 3PA mechanism when excited at 845 nm.⁴⁷⁹ The [Tb(NO₃)₃(H₂O)₃](59)₂ complex is particular in that the azido ligand lies in the second coordination sphere, being only linked by H-bonds to the bound water molecules; both ligand and Tb^III emission are generated by 3PA.⁴⁸⁰ To close this short overview of MPA phenomena in lanthanide complexes, we would like to mention the polymeric Tb^III complex [Tb(NO₃)₃(57c)]_n, which upon laser excitation at 1.26 μm exhibits green Tb^III emission from a 4PA process as well as blue and red emission resulting from single and third harmonic generation, the complex playing the role of the gain medium.⁴⁸¹

Two-photon excitation luminescence microscopy is a relatively new technique which started in the 1990s.⁴⁸² Besides using a “cell-friendly” NIR excitation wavelength, it allows the imaging of difficult samples with good resolution. The laser beam is spatially focused and signals from the out-of-focused zone are reduced to practically zero, enhancing the contrast. The technique is suited for micro-volume bioaffinity assays and microarray platforms;⁴⁸³ probes are available for the determination of intracellular free metal ions, acidic vesicles, and lipid rafts in live tissues.⁴⁸⁴ 3PA is also feasible, as demonstrated by the measurement of serotonin distribution in live cells.⁴⁸⁵

Applications involving LLBs are still in their infancy despite their potential. For instance, the 2PA action cross section for Tb^III bound to transferrin at 500 nm, Tb₂Trf, (Table 7) is comparable to those of the strongly DNA-binding fluorescent dye DAPI (4′,6-diamidino-2-phenylindole) or of Coumarin 307.⁴⁸⁹ And cross sections for Eu^III complexes are much larger.

The first 2PA microscopy images involving an Ln^III tag relied on [Tb(dpa)₃]³⁻ as LLB and were those of single crystals of a lysozyme derivative of the tris(dipicolinate) complex.⁴⁶⁹ The same authors subsequently obtained luminescence images of T24 cancer cells, fixed in ethanol and loaded with [Eu(53c)₃]³⁻, upon 2PA excitation at 760 nm; the cross section for the LLB in the cell is σ⁽²⁾ = 19 GM.⁴⁹⁰ After demonstrating 2PA phenomena in three Eu^III complexes with derivatised cyclens, [Eu(60a,b)(H₂O)]³⁺ (Scheme 24) having cross sections up to 2 GM,⁴⁸⁶ Parker et al. obtained 2PA images of NIH-3T3 and HeLa cells upon excitation at 720 nm after loading the cells with the macrocyclic complexes [Tb(61a)]³⁺ bioconjugated to arginine ([Tb(61b)]³⁺) or guanidinium ([Tb(61c)]³⁺). The guanidinium conjugate localises in mitochondria and is cytotoxic (IC₅₀ = 12 μM).⁴⁹¹ Another Tb^III probe, [Tb(NO₃)₃(57a)] was loaded into human lung carcinoma (A549), HeLa and HONE1 cells which could be imaged under 800-nm irradiation, that is upon 3PA excitation.⁴⁸⁷

Scheme 24 Cyclen derivatives and tripodal ligand for MPA experiments.^486,491

Near-infrared MPA excitation of NIR-emitting Ln^III ions has also been demonstrated, for instance for [Ln(56)]²⁻ (Scheme 22) with Ln^III = Nd, Yb in D₂O,⁴⁹² but no practical imaging experiment has been tempted to date. However, although based on a slightly different excitation mechanism, we would like to mention here the use of up-converting nanophosphors (see section 5.3) for imaging purposes. For instance, the digestive system of live C. elegans has been visualized with UC Y₂O₃ nanoparticles doped with Er^III and Yb^III,⁴⁹³ while polyethyleneimine-coated NaYF₄:Yb^III,Er^III nanoparticles have been injected into the body of rats and their luminescence could be detected upon 980-nm excitation.⁴⁹⁴ However the wide-field images recorded were of moderate quality and in order to improve the situation, Yu et al. have proposed a new method of laser scanning up-conversion luminescence microscopy eliminating interferences. In this way, sharp images of HeLa cells dual-labelled with the organic dye DiIC18 (1,1′-dioctadecyl-3,3,3′,3′-tetramethylindodicarbocyanine perchlorate salt) and the same fluoride nanophosphor as above could be recorded. The technique is little sensitive to photobleaching.⁴⁹⁵

6. Hopes for a bright future

The diversity of the subjects dealt with in this review, despite its limited scope, allied to the wealth of original papers and review articles published monthly on these subjects speak for themselves: lanthanide luminescence has still an incommensurable, untouched space to expand and yield exciting results, both at the fundamental and applied levels.

One striking example of contribution to fundamental aspects of photophysics is the d–f edifices tailored for sensitising NIR luminescence. The rich variety of polynuclear structures which can be generated by mixing the sterically demanding d-block elements with the versatile spherical lanthanides opens a vast playground for investigating and quantifying energy transfer mechanisms in which broad ligand or d-metal centred states populate energetically well defined f states. Similarly the advances in organic synthesis facilitate studying the influence of electronic effects from substituents grafted onto basic ligand cores, henceforth allowing to understand the fine-tuning of lanthanide luminescence. In this respect, one starts to discover the role played by non-covalent intermolecular interactions (particularly H-bonds, but also π-stacking) in funnelling energy from the metal-ion surrounding into the f states, thanks to electron density maps;^47,496 the latter becoming more accessible thanks to improvements in the precision of X-ray diffraction measurements. Following the same line of thinking, reports are now published in which sizeable luminescence is found for Ln^III ions bound to highly quenching water molecules;^47,497 the key to this apparent mystery is again tractable to non-covalent H-bonding involving these molecules and which considerably weaken the Ln–O(H₂) bonds, henceforth decreasing the quenching effect. All these outcomes will have to be incorporated in the design of new, highly luminescent lanthanide edifices.

Sensitisation of NIR luminescence remains a real challenge when organic ligands are used and the numerous studies described in section 3 clearly point to an unavoidable solution: decreasing the energy of the vibrational oscillators located in the surrounding of the emitting ion, not only in the first coordination sphere, but also much further away.⁵⁹ Fluorination is beneficial, but involves more complicated syntheses of the ligands. Another way out would be to find how to decrease the radiative lifetime (see eqn (3)) but although one example is now documented,⁴⁹⁸ we have little clue as how to reach this goal.

Electroluminescent materials, particularly for OLEDs and solar energy conversion, are a bonanza for lanthanide chemists. A lot remains to be done in order to fully optimize these devices which have to feature good optical quality, easy tuning of the refractive index and the emitted colour, facile processing and adaptability to various surfaces, for instance. Replacement of the highly luminescent β-diketonates by complexes less sensitive to UV degradation is an important goal. The field has also to take full advantage of nanomaterials in which luminescent lanthanide centres can be introduced at will, often without loss of optical properties, on the contrary.

Life sciences are bound to highly benefit from lanthanide luminescent bioprobes. Although unavoidable since more than 20 years in immunoassays, a field boosted by the advent of up-converting phosphors, they have still to gain ground in the sensing of metabolites and deciphering of signalling processes in live cells. The definitive advantage of time-resolved detection should help rendering these probes more ubiquitous both for in vitro and in vivo sensing and imaging. From this point of view, structure–activity relationships are still missing for understanding how metal complexes penetrate into live cells and why they locate into given organelles. More efficient NIR emitting probes are also needed to improve the penetration depth of imaging,⁴⁹⁹ a very important aspect when monitoring, for instance a chirurgical ablation of tumour. Bifunctional agents acting both as cell-imaging probes and therapeutic agents, such as Yb^III porphyrinates aimed at photodynamic treatment of cancer⁹⁴ should also be the focus of much attention, as well as dual imaging (MRI/optical) probes.

Almost every field of our present technological society needs the help of lanthanide luminescence. More particularly it seems that this luminescence will be of invaluable help in solving the two major technological problems the world is facing, namely supplying enough sustainable energy and food for a rapidly expanding population, in addition to providing its assistance in medicine. This should incite more researchers to join the club!

Acknowledgements

This research is supported through grants from the Swiss National Science Foundation and the Swiss Office for Science and Education (within the frame of COST Action D38 from the European Science Foundation). JCB also thanks the World Class University program from the National Research Foundation of Korea (Ministry of Education, Science and Technology) for support (grant R31-1003r).

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Footnotes

† Electronic supplementary information (ESI) available: Tables S1 and S2, and Schemes S1–S3 (photophysical properties of NIR emitting Nd^III, Er^III and Yb^III molecular compounds). See DOI: 10.1039/b905604c

‡ For integration, spectra must be corrected for the instrumental function and expressed in terms of photons/s versus wavelength.

§ A gamma function calculator can be found at: http://functions.wolfram.com/webMathematica/FunctionEvaluation.jsp?name=Gamma

¶ The value of 441 μs reported in ref. 68 is erroneous (see J. Mater. Sci: Mater. Electron., 2009, 20, 788).

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