Mixed-ligand lanthanide complexes supported by ditopic bis(imino-methyl)-phenol/calix[4]arene macrocycles: synthesis, structures, and luminescence properties of [Ln₂(L²)(MeOH)₂] (Ln = La, Eu, Tb, Yb)

Abstract

The lanthanide binding ability of a macrocyclic ligand H₆L² comprising two bis(iminomethyl)phenol and two calix[4]arene units has been studied. H₆L² is a ditopic ligand which provides dinuclear neutral complexes of composition [Ln₂(L²)(MeOH)₂] (Ln = La (1), Eu (2), Tb (3), and Yb (4)) in very good yield. X-ray crystal structure analyses for 2 and 3 show that (L²)⁶⁻ accommodates two seven coordinated lanthanide ions in a distorted monocapped trigonal prismatic/octahedral coordination environment. UV-vis spectroscopic titrations performed with La³⁺, Eu³⁺, Tb³⁺ and Yb³⁺ ions in mixed MeOH/CH₂Cl₂ solution (I = 0.01 M NBu₄PF₆) reveal that a 2 : 1 (metal : ligand) stoichiometry is present in solution, with log K₁₁ and K₂₁ values ranging from 5.25 to 6.64. The ratio α = K₁₁/K₂₁ of the stepwise formation constants for the mononuclear (L² + M = ML², log K₁₁) and the dinuclear complexes (ML² + M = M₂L², log K₂₁) was found to be invariably smaller than unity indicating that the binding of the first Ln³⁺ ion augments the binding of the second Ln³⁺ ion. The present complexes are less luminescent than other seven-coordinated Eu and Tb complexes, which can be traced to vibrational relaxation of excited Eu^III and Tb^III states by the coligated MeOH and H₂O molecules and/or low-lying ligand-to-metal charge-transfer (LMCT) states.

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Introduction

The development of ligand systems that form both stable and luminescent lanthanide complexes is an active research area due to their many potential applications,¹ particularly the optical detection and sensing of biologically relevant or environmentally harmful molecules and ions.^2–4 The calix[4]arenes^5–8 with their hard O₄ donor set have turned out to be versatile supporting ligands that form stable complexes with a range of trivalent lanthanide ions.^9–17 Calixarenes bearing pendant chromophores can display an antenna effect, and several luminescent lanthanide calixarene complexes have already been investigated.^18–23

Calix[4]arene bearing pendant Schiff-base moieties have proved to be versatile ligands for transition metals.²⁴ We have investigated previously the lanthanide coordination chemistry of the hybrid calix[4]arene-Schiff-base ligand H₄L¹ (Fig. 1). This monotopic ligand supports dinuclear lanthanide complexes of composition [HNEt₃][Ln₂(HL¹)(L¹)] with Ln = Sm³⁺, Eu³⁺, Gd³⁺, and Tb³⁺,²⁵ where two monomeric [LnL¹] units dimerize via phenolato bridges from the calix[4]arenes. The coordinated salicylaldimines exhibit a triplet-state energy of 22 000 cm⁻¹,²⁵ and act as antennas enabling sensitization of the Eu^III and Tb^III emission when irradiated in the solid state (λ_ex = 311 nm). We have also reported a macrocyclic ligand H₆L² comprising of fluorescent bis(iminomethyl)phenol and calix[4]arene head units, which enabled the optical detection of Zn²⁺ ions among a series of biologically relevant metal ions in solution.²⁶ In the presence of Zn²⁺ ions the fluorescent intensity output increased, presumably due to coordination of the Zn²⁺ ions reducing the PET quenching mechanism of the imino functional groups. The observations made with H₄L¹ prompted us to investigate the lanthanide coordination chemistry of H₆L². Of particular interest to us was whether the macrocyclic structure would form stable Ln³⁺ complexes and whether the bis(iminomethyl)phenolate units could function as antennae and enable excitation of the lanthanide ion, noting that the ligand H₆L² displayed strong absorption bands at 346 nm and 447 nm.²⁶ To our knowledge, no lanthanide complexes of H₆L² have been reported so far only transition metal complexes have been investigated.^27–29 This paper demonstrates that H₆L² behaves as a ditopic ligand that supports dinuclear macrocyclic lanthanide complexes of the type [Eu₂(L²)(L′)₂] (Ln = La, Eu, Tb, Yb, L′ = solvent). Their structures and properties are compared with those of the previously published [HNEt₃][Ln₂(HL¹)(L¹)] complexes.

Fig. 1 Structures of the hybrid Schiff-base/calix[4]arene ligands H₄L¹ and H₆L².

Results and discussion

Synthesis and characterization of compounds

The macrocyclic ligand H₆L² was prepared by a [2 + 2] Schiff-base condensation reaction between 25,27-bis(aminoethoxy)-26,28-dihydroxy-calix[4]arene and 4-tert-butyl-2,6-diformylphenol according to literature procedures.^26,30 Reaction of H₆L² with La(NO₃)₃ and NEt₃ as a base, in a 1 : 2.2 : 5 molar ratio in MeOH/CH₂Cl₂, results in immediate formation of a yellow precipitate of composition [La₂(L²)(MeOH)₂] (1, where (L²)⁶⁻ represents the fully deprotonated form of H₆L²) in a very good (81%) yield (Scheme 1). The analogous Eu³⁺ (2), Tb³⁺ (3), and Yb³⁺ complexes (4) were also synthesized in this manner. Attempts to prepare mononuclear complexes (utilizing a 1 : 1 stoichiometric ratio) were unsuccessful, and only low yields of the dinuclear complexes could be isolated, most likely due to a lower solubility of the dinuclear complex species, but other reasons cannot be ruled out. All complexes are air-stable both in the solid and solution state. They are moderately soluble in polar aprotic solvents such as CHCl₃, CH₂Cl₂, MeCN, and THF, but do not dissolve in MeOH or EtOH. The guest solvate molecules hosted in the cavities of the calix[4]arenes are lost upon standing in air. The complexes gave satisfactory elemental analysis and were further characterized by ESI mass spectrometry, FTIR and UV-vis spectroscopy, and X-ray crystallography (for 2 and 3).

Scheme 1 Synthesis of the complexes 1–4.

Selected analytical data for H₆L² and the complexes 1–4 are listed in Table 1. FT-IR spectra of powdered samples were recorded in the ATR mode. As can be seen (Fig. S1, S3, S5 and S7†), the O^Ar–H stretches that were seen in the spectra of the free ligand at 3400 cm⁻¹ are absent in the spectra of complexes 1–4, indicative of Ln³⁺-bound phenolate functions. Coordination of the phenolate groups is further supported by the red shifted ν(C–O^phenol) stretches which occurs at 1340 cm⁻¹ for the free ligand (Fig. 2). The FT-IR spectra of the complexes feature also pronounced differences in the ν(ArO–C^ether) stretching region (1050–950 cm⁻¹), which imply the coordination of the phenol ether O donors as well.³¹ All complexes exhibit a band around 1635 cm⁻¹ of medium intensity that can be attributed to ν(C=N) (imine) stretching vibrations, a typical wavelength value for imine functional groups that are coordinated to Ln³⁺ ions.

Fig. 2 ATR-FT-IR spectra in the 1400–950 cm⁻¹ region of H₆L² and complexes 1–4.

Table 1 Selected analytical data for H₆L² and complexes 1–4

Compound	[small nu, Greek, tilde] /cm^−1	λ max/nm (ε/M^−1 cm^−1)^b
a ATR mode. b CH2Cl2/MeOH (1/1 v/v), 2 × 10^−5 M.
H6L^2	1640 (C=N)	226 (70 395) (π–π*, calixarene)
	1340 (C–O)	245 (sh, 55 435) (π–π*, calixarene)
		275 (sh, 25 651) (π–π*, calixarene)
		346 (10 735) (π–π*, PhCH=NR)
		446 (3721) (π –π*, PhCH=NR)
1 (La)	1632 (C=N)	231 (87 976) (π–π*, calixarene–OLn)
	1322 (C–O)	250 (59 730) (π–π*, calixarene–OLn)
		304 (13 306) (π–π*, PhCH=NR–Ln)
		410 (14 224) (π–π*, PhCH=NR–Ln)
2 (Eu)	1635 (C=N)	231 (87 092) (π–π*, calixarene–OLn)
	1317 (C–O)	254 (sh, 57 697) (π–π*, calixarene–OLn)
		302 (sh, 15 985) (π–π*, PhCH=NR–Ln)
		409 (15 339) (π–π*, PhCH=NR–Ln)
3 (Tb)	1636 (C=N)	227 (107 414) (π–π*, calixarene–OLn)
	1318 (C–O)	251 (69 553) (π–π*, calixarene–OLn)
		303 (18 343) (π–π*, PhCH=NR–Ln)
		409 (16 976) (π–π*, PhCH=NR–Ln)
4 (Yb)	1634 (C=N)	230 (72 147) (π–π*, calixarene–OLn)
	1330 (C–O)	257 (42 642) (π–π*, calixarene–OLn)
		307 (7994) (π–π*, PhCH=NR–Ln)
		417 (11 100) (π–π*, PhCH=NR–Ln)

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The ESI-MS data confirms the presence of dinuclear lanthanide complexes of the type [Ln₂(L²)(L′)₂] with two exchangeable coligands L′, in agreement with the X-ray crystallographic findings detailed below. However, the large and neutral charged dinuclear complexes are hard to vaporize leading to strong fragmentation and only weak intensity for the molecular ion peaks in the respective mass spectra (Fig. S2, S4, S6 and S8†).

Crystallographic characterization

In order to determine the structures of the complexes, single-crystals (for 2 and 3) were grown and subjected to X-ray crystallographic analyses. Single crystals of [Eu₂(L²)(MeOH)₂]·4CH₂Cl₂·MeOH (2·4CH₂Cl₂·MeOH), obtained from a mixed MeOH/CH₂Cl₂ system, were subjected to an X-ray crystallographic analysis. The crystal structure is composed of dinuclear [Eu₂(L²)(MeOH)₂] complexes (Fig. 3), and CH₂Cl₂ and MeOH solvate molecules. Some of the CH₂Cl₂ and MeOH solvate molecules were observed to occupy the voids in the structure. Each calixarene cavity is also occupied by one CH₂Cl₂ molecule, which is held in place by CH⋯π interactions with the aromatic rings of the calixarene units as illustrated in Fig. 4. Selected metrical data are listed in Table 2 and Fig. 3 and 4.

Fig. 3 Single-crystal X-ray diffraction structure of [Eu₂(L²)(MeOH)₂]·4CH₂Cl₂·MeOH (2·4CH₂Cl₂·MeOH) (ball-and-stick representation). The CH₂Cl₂ and MeOH solvate molecules are omitted for clarity. Dashed lines refer to intramolecular hydrogen bonding interactions (O11⋯N4 = 2.71, O12⋯N2 = 2.64).

Fig. 4 Left: Immediate coordination environment of the Eu³⁺ ions in 2·4CH₂Cl₂·MeOH. Right: Representation of the intermolecular CH⋯π interactions between the CH₂Cl₂ solvate molecules and the calix[4]arene units of the ligand. Parts of the complexes have been omitted for clarity. Symmetry code used to generate equivalent atoms: −x, 0.5 + y, 0.5 − z (′). Selected distances/Å: C92⋯X1a (center of ring) 3.566, C91⋯X1b 3.696, C94⋯X1b 3.450, C93⋯X1c 3.771.

Table 2 Selected bond lengths [Å] and angles in [Eu₂(L²)(MeOH)₂]·4CH₂Cl₂ (2·4CH₂Cl₂·MeOH) and [Tb₂(L²)(MeOH)(H₂O)]·(H₂O)_x (3·xH₂O)

M	2·4CH2Cl2·MeOH (M = Eu)	3·xH2O (M = Tb)
M1–O1	2.528(3)	2.482(2)
M1–O2	2.164(3)	2.095(3)
M1–O3	2.576(3)	2.505(3)
M1–O4	2.141(3)	2.089(3)
M1–O5	2.254(3)	2.189(2)
M1–O11	2.457(3)	2.322(3)
M1–N1	2.516(4)	2.442(3)
M2–O6	2.498(3)	2.487(2)
M2–O7	2.180(3)	2.110(2)
M2–O8	2.561(3)	2.439(3)
M2–O9	2.143(4)	2.068(3)
M2–O10	2.266(3)	2.205(3)
M2–O12	2.398(3)	2.324(2)
M2–N2	2.503(4)	2.402(3)
M1⋯M2	9.297(1)	9.291(1)

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The complex 2·4CH₂Cl₂·MeOH has pseudo-C₂ symmetry. The Eu³⁺ ions are seven-coordinate by four calix[4]arene O atoms and the imine N and O atoms of the appended Schiff base unit. A MeOH ligand completes the coordination polyhedron, which is best described as intermediate between distorted mono-capped trigonal prismatic or mono-capped octahedral, two of the most common coordination geometries for seven coordinate lanthanide complexes.^32,33 The Eu–O bond lengths range from 2.14 to 2.59 Å. The hard phenolate functions of the calixarenes form the shortest Eu–O bonds (mean value 2.15 Å), followed by slightly longer (weaker) Eu–O iminophenolate linkages at 2.26 Å. The weaker donor ability of the latter can be traced to mesomeric effects induced by the imine functions, which reduce the electron density on the O donors through delocalization into the aromatic π-system. Indeed, the stronger electron delocalization is also reflected in a decrease of the C–O bond length from 1.32 Å for the calixarene moiety to a value of 1.29 Å in the iminophenol (Schiff-base) units. The MeOH and phenol ether O atoms have much weaker donor ability, implicated by the very long Eu–O bond distances of 2.43 and 2.54 Å, respectively. The Eu–N bonds are also quite long at 2.51 Å. The structure is further stabilized by intramolecular hydrogen bonds between the MeOH ligands and the non-coordinating imine functions.^34–36 The intramolecular Eu⋯Eu distance in 2 is 9.29(3) Å, and interestingly the intermolecular Eu⋯Eu distance is shorter, 6.34(3) Å.

Crystals of [Tb₂(L²)(MeOH)(H₂O)]·(H₂O)_x (3·(H₂O)_x, x ∼ 10) obtained from a mixed CH₂Cl₂/MeOH solution are monoclinic, space group P2₁/n. The asymmetric unit contains a neutral, dinuclear [Tb₂(L²)(MeOH)(H₂O)] complex and several water solvate molecules. Most of the latter were heavily disordered and the corresponding electron density was removed with the SQUEEZE routine implemented in Platon.³⁷ The MeOH and H₂O molecules bonded to the Tb³⁺ atoms, on the other hand, were not disordered and were included in the structure refinement. Fig. 5 shows the structure of the dinuclear terbium complex 3. Table 2 lists selected bonds lengths. The atom labeling utilized for 2 was adapted for 3 to aid structural comparisons.

Fig. 5 Single-crystal X-ray diffraction structure of the dinuclear [Tb₂(L²)(MeOH)(H₂O)] complex in crystals of 3·(H₂O)_x, x ∼ 10 (ball-and-stick representation). Dashed lines refer to intramolecular hydrogen bonding interactions (O11⋯N4 = 2.75 Å, O11⋯O13 2.78 Å, O12⋯N2 = 2.62 Å).

The structure of this complex is very similar to that of the europium complex 2. The macrocycle wraps around the two Tb³⁺ ions in the same fashion, but in contrast to 2, the coordination spheres of the Tb atoms are completed by either one methanol (Tb2) or one water ligand (Tb1). As in 2, the Tb-bound coligands are hydrogen bonded to the imine-nitrogen atoms of the supporting ligand, but the H₂O ligand hydrogen bonds also to a water solvate molecule (O11⋯O13). Likewise, three sets of Tb–O distances can be distinguished. However, the corresponding metal–ligand bond lengths in 2 and 3 differ by ca. 0.05 Å, as one might expect in view of the smaller ionic radius of Tb³⁺.³⁸ The intramolecular Tb⋯Tb distance in 3 is 9.290(1) Å, again about 3 Å longer than the shortest intermolecular Tb⋯Tb distance (7.65 Å). The packing of the [Ln₂(L²)(MeOH)₂] complexes in 2 and 3 is very similar, and appears not to be influenced by the type of co-crystallized solvate molecules.

The binding mode of H₆L² seen in the present lanthanide complexes differs significantly from that observed in the Zn and Ni complexes [Zn(L²)(py)₂] and [Ni(L²)], the only other complexes, whose crystal structures were reported so far with this large macrocyclic ligand system.²⁶ In both cases, H₆L² is a tetradentate ligand, binding the divalent 3d ions via two bidentate iminomethylphenolate functions in a square-planar (Ni²⁺) or octahedral (Zn²⁺) coordination geometry, respectively. The present lanthanide structures clearly demonstrate the potential of H₆L² in providing additional coordinating modes in which the calix[4]arene moieties act as additional binding sites.

Spectroscopic and photophysical properties

Electronic absorption spectra of the complexes 1–4 were recorded in a mixed CH₂Cl₂/MeOH (1 : 1 v/v) solution, as this was the solvent system in which they had the best solubility. In case of the free ligand, there are two intense absorption bands at 346 nm and 446 nm attributable to transitions into ¹(π–π*) states of the o,o′-bis(iminomethyl)phenol chromophore.³⁹ The intense band at 226 nm and the shoulders at 245 nm and 275 nm can be attributed to transitions into ¹(π–π*) states of the aromatic rings of the calix[4]arene moiety.⁴⁰

The UV-vis spectrum of the Eu complex 2 in CH₂Cl₂/MeOH differs from that of the free ligand in the same solvent system. Complex 2 exhibits two well-resolved absorption bands at 231 nm and 409 nm, both with higher intensity than the absorptions of H₆L². Two shoulders are observed at 254 nm and 302 nm. The two high-energy features below 300 nm region are associated with ¹(π–π*) states within the aromatic rings of the Eu-bound calix[4]arene units.⁴⁰ The broad band at 409 nm, on the other hand, is assigned to an intraligand transition to a ¹(π–π*) state involving the bis(iminomethyl)phenolate chromophore units. The electronic absorption spectral properties of the other complexes are very similar to those of the Eu complex. Again all spectra are dominated by the intraligand absorptions.

Calix[4]arenes^18,41,42 and iminomethyl-phenolate ligands⁴³ are known to act as antennas for sensitization of lanthanide luminescence. Also, lanthanide complexes with odd coordination numbers, in particular seven-coordinated structures, have the potential to be strong luminescent materials.^44–47 On these grounds, the Eu (2) and Tb (3) complexes were studied by luminescence spectroscopy. Although 2 is non-emissive in the solution (CH₃CN or CH₂Cl₂/MeOH), a red emission can be detected by the naked eye when solid 2 (neat powder) is irradiated at 77 K (see graphical abstract). Fig. 6 displays the photoluminescence spectrum (λ_ex = 361 nm) of a transparent poly-methylmethacrylate (PMMA) thin film containing 4 wt% of the Eu complex 2.⁴⁸ The luminescence spectrum of a PMMA film containing 4 wt% of the complex [HNEt₃][Eu₂(HL¹)(L¹)] is also shown in Fig. 6 for comparative purposes. Photoluminescence spectra of complex 2 were also recorded as a neat powder or in a frozen CH₂Cl₂/MeOH (1 : 1) matrix at 77 K and are provided in the ESI.†

Fig. 6 Photoluminescence spectra of [Eu₂(L²)(MeOH)₂] (2) and [HNEt₃][Eu₂(HL¹)(L¹)] at 77 K (PMMA thin films doped with 4 wt% 2 or [HNEt₃][Eu₂(HL¹)(L¹)], thickness ∼0.2 mm). The excitation wavelength is 361 nm. The transitions above 575 nm start from the ⁵D₀ state. The spectra are normalized with respect to the ⁵D₀ → ⁷F₁ transition.

The luminescence spectrum of 2 in PMMA matrix is characteristic and will be discussed. The spectrum displays several broad features attributed to the characteristic ⁵D₀ → ⁷F_J transitions (J = 0–6) from the excited ⁵D₀ state of the Eu³⁺ ion.⁴⁹ The ⁵D₀ → ⁷F₁ transition splits into three components (590 nm, 592 nm, 598 nm) and the ⁵D₀ → ⁷F₂ into two components (615 nm, 627 nm) as a consequence of the crystal-field splitting in the ground ⁷F₁ and ⁷F₂ manifolds, as observed in other seven-coordinate Eu(III) salicylaldimine complexes.^46,49 The features at 650 nm and 750 nm, attributed to the ⁵D₀ → ⁷F₃ and ⁵D₀ → ⁷F₄ transitions, respectively, are also split by the crystal field. The feature at 581 nm is very broad and can be assigned as the ⁵D₀ → ⁷F₀ transition, considering that immobilization in a PMMA matrix causes inhomogeneous line broadening.⁴⁹ In most Eu complexes, the ⁵D₀ → ⁷F₀ transition is weak; however, when the symmetry is low it may gain intensity, even higher intensity than the ⁵D₀ → ⁷F₁ band.⁵⁰ This observation is in agreement with the low local site symmetry of the coordination environment in 2 (shape factors = 1.60 (ESI) for the capped trigonal prismatic geometry and 1.62 for the capped octahedral coordination environment vide infra).⁵¹ Along with the corresponding photoluminescence profile, the excitation spectra of a thin PMMA film of complex 2 at 77 K monitored at 615 nm is shown in Fig. S17.† A broad absorption band from 300 to 400 nm attributable to the transition into the ¹(π–π*) state of the bis(iminomethyl)phenolato ligand is clearly discernable, showing that it can function as an antenna for sensitizing the Eu³⁺ ion.

Fig. 6 also displays the photoluminescence spectrum of the dinuclear europium(III) complex [HNEt₃][Eu₂(HL¹)(L¹)]. The spectrum is very similar to that of 2 in terms of the number and energies of the emission bands. The ratio R of the intensities of the ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ transitions is 10.3 in 2 and 10.1 in [Eu₂(HL¹)L¹]⁻ (R = 10.3 vs. 10.1). The similar spectral features can be traced to the similar coordination environments of the lanthanide ions in [Eu₂(L²)(MeOH)₂] and in [Eu₂(L¹)(HL¹)]⁻ as illustrated in Fig. 7. Thus, the changes in the coordination environments that occur upon going from [Eu(HL¹)(L¹)]⁻ to [Eu₂(L²)(MeOH)₂] concern only the organic residues on two of the seven donor atoms. That is, the phenolate donors (O^aH and O^b) in [Eu(HL¹)(L¹)]⁻ become phenolether O^c and methanol O^dH donors in 2. Notice that both coordination environments include also one O–H donor, each of which is involved in an intramolecular hydrogen bonding interaction. The similarity in the coordination environments is also illustrated by the shape factors, which provide a measure for the deviation from the ideal coordination environments. As already stated, the coordination geometry of the seven-coordinate Eu³⁺ ions in both 2 and [Eu₂(L²)(MeOH)₂] is intermediate between the capped octahedral and the capped trigonal prismatic coordination environments, with the calculated shape factors differing by only ΔS ∼ 0.9 (capped octahedron: S = 1.60 for 2, 2.30 for [Eu₂(L¹)(HL¹)]⁻, capped trigonal prism: S = 1.61 (2), S = 2.49 for ([Eu₂(L¹)(HL¹)]⁻)). The other possible coordination geometries reveal much larger deviations of the shape factor from the ideal values. The similarity of the luminescence spectra of the two seven-coordinate Eu complexes is not surprising in view of the structural similarities.

Fig. 7 Comparison of the immediate coordination environments in [HNEt₃][Eu₂(HL¹)(L¹)]²⁵ (left) and [Eu₂(L²)(MeOH)₂] (right) with major changes highlighted. The phenolate O atoms (O^aH, O^b) in the left structure become phenolether (O^c) and MeOH (O^d) donors in the right structure. Parts of the structures were omitted for clarity.

It was of interest to determine the quantum efficiencies for comparison with literature reporting that Eu³⁺ complexes with seven-coordinate coordination geometries exhibit high intrinsic quantum yields (Φ_ff) and large radiative rate constants (k_r).^52,53 Time resolved photoluminescence decay measurements on the PMMA thin film doped with complex 2 revealed upon excitation at 361 nm an emission lifetime τ_obs of 330 μs at 77 K for the ⁵D₀ level. The emission lifetime for [HNEt₃][Eu₂(HL¹)(L¹)] (τ_obs = 300 μs) under the same conditions is only slightly shorter than that for 2, again in good agreement with the similar coordination environments. Table 3 gathers selected photophysical parameters (k_r, k_nr, Φ_ff) for the two compounds along with the shape symmetry factors (average values for the two most ideal coordination environments). Complex 2 exhibits a slightly larger intrinsic quantum yield (Φ_ff = 28%) than [HNEt₃][Eu²(HL¹)(L¹)] (Φ_ff = 17%) as a consequence of a larger radiative rate constant (k_r = 0.857 × 10³ s⁻¹versus 0.560 × 10³ s⁻¹) and a smaller non-radiative rate constant k_nr (2.219 × 10³ s⁻¹). To our knowledge the photophysical parameters of only few seven-coordinated Eu³⁺ Schiff-base complexes have been reported. Thomas et al. reported a seven coordinated Eu³⁺ complex supported by TREN-(3,5-di-tert-butylsalicylidene)₃.³³ As for the present case the luminescence of this complex is only observed at low temperature upon enforcement of a delay time of 50 μs together with a residual emission from the ligand state, attributable to the presence of low-lying ligand-to-metal charge-transfer (LMCT) states.⁵⁴ The observations made for the present Eu³⁺ compounds are in good agreement with the reported trend.

Table 3 Lifetime (τ), intrinsic quantum yields (Φ_ff), radiative (k_r), and non-radiative decay constants (k_nr) for thin PMMA films of the lanthanide complexes [HNEt₃][Eu₂(HL¹)(L¹)], [HNEt₃][Tb₂(HL¹)(L¹)], [Eu₂(L²)(MeOH)₂] (2), and [Tb₂(L²)(MeOH)₂] (3)

Compound	T/K	Matrix	τ obs/μs	τ r ^c/μs	k r ^c/s^−1	k nr ^c/s^−1	Φ ff ^d/%	Symmetry factor for Ln^3+ ion^e
a λ ex = 310 nm (Perkin Elmer Fluorescence Spectrometer FL6500). b λ ex = 360 nm. c Radiative rate constant (kr) for Eu^III complexes (in PMMA matrix) were estimated as follows: kr = 1/τr = AMD,0·n^3 (Itot/IMD), where AMD,0 = 14.65 s^−1 (spontaneous emission probability for the ^5D0 → ^7F1 transition in vacuum), n = 1.5 (refractive index), and Itot/IMD being the ratio of the total area of the corrected Eu^3+ emission spectrum to the integrated intensity of the ^5D0 → ^7F1 transition; for the non radiative rate constant: knr = 1/τobs − 1/τr.^46 Radiative rate constant (kr) for Tb^III complexes (in PMMA matrix) were calculated assuming that the emission lifetime (τobs) at 77 K is the radiative emission lifetime (τr).^55 d Intrinsic emission quantum yield: Φff = kr/(kr + knr) = τobs/τr. e Mean value of all S factors in this structure.
[HNEt3][Eu2(HL^1)(L^1)]	77^a	PMMA thin film	304 ± 9^a	1790 ± 90	560 ± 30	2700 ± 200	17 ± 1	2.40
[Eu2(L^2)(MeOH)2] (2)	77	PMMA thin film	325 ± 9^b	1170 ± 60	860 ± 50	2200 ± 200	28 ± 2	1.62
[HNEt3][Tb2(HL^1)(L^1)]	298	PMMA thin film	175 ± 9^a	972 ± 3	1029 ± 3	4700 ± 300	18 ± 1	2.40
	77	PMMA thin film	972 ± 3^a	972 ± 3	1029 ± 3	<6	100 < Φff > 99
[Tb2(L^2)(MeOH)2] (3)	298	PMMA thin film	186 ± 2^b	1099 ± 2	910 ± 2	4480 ± 60	17 ± 1	1.42
	77	PMMA thin film	1099 ± 2^b	1099 ± 2	910 ± 2	<3	100 < Φff > 99

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Emission spectra of complex 2 (neat powder or CH₂Cl₂/MeOH 1 : 1 frozen glassy matrix) were also recorded at 77 K (utilizing a single photon counting photoluminescence spectrometer FluoTime 300 from PicoQuant). The spectra and photophysical parameters are similar to those recorded in PMMA matrix (Fig. S18 and S25†).

Fig. 8 displays photoluminescence spectra of PMMA thin films doped with the Tb³⁺ complexes [HNEt₃][Tb₂(HL¹)(L¹)] and [Tb₂(L²)(MeOH)₂] (3) at 298 K and 77 K. The 77 K spectrum of 3 shows four transitions centered at 480 nm, 550 nm, 580 nm, and 625 nm, which correspond to the ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) transitions from the excited ⁵D₄ state of the Tb³⁺ ion. Of these, the “green” ⁵D₄ → ⁷F₅ transition has the highest intensity. All signals are split by the crystal field. The spectrum of [HNEt₃][Tb₂(HL¹)(L¹)] is very similar to that of 3 with four transitions split by the crystal field at similar wavelengths. The similar spectroscopic features can again be traced to the similar coordination environments of the seven-coordinate Tb³⁺ ions. Thus, the symmetry shape factors differ again only slightly (capped octahedron: S = 1.63 for 3, 2.30 for [Tb₂(L¹)(HL¹)]⁻, capped trigonal prism: S = 1.22 (3), S = 2.49 for ([Tb₂(L¹)(HL¹)]⁻)), as observed for the above pair of Eu³⁺ complexes. The excitation spectrum of a thin PMMA film of complex 3 at 298 K monitored at 545 nm is shown in Fig. S17.† A broad absorption band occurs at 380 nm attributable to the transition into the ¹(π–π*) state of the bis(iminomethyl)phenolato ligand, showing that it can function as an antenna for sensitizing the Tb³⁺ ion.

Fig. 8 Photoluminescence spectra of [HNEt₃][Tb₂(HL¹)(L¹)] and [Tb₂(L²)(MeOH)₂] (3) immobilized in a PMMA thin film (4 wt% 3, thickness ∼0.2 mm) at 77 K and 298 K. The excitation wavelength is 361 nm. The spectra are normalized with respect to the ⁵D₄ → ⁷F₆ transition.

Time resolved photoluminescence decay measurements on the PMMA thin film doped with Tb³⁺ complex 3 revealed upon excitation at 361 nm an emission lifetime τ_obs of 1100 μs at 77 K for the emitting ⁵D₄ level. The emission lifetime for [HNEt₃][Tb₂(HL¹)(L¹)] under the same conditions was found to be similar (907 μs), again in good agreement with the similar coordination environments. The photophysical parameters (k_r, k_nr, Φ_ff) for the two Tb³⁺ complexes (in PMMA matrix) are listed in Table 3. At room temperature, the value of Φ_ff is 18% for complex 3 and 17% for [HNEt₃][Tb₂(HL¹)(L¹)]. There are not many seven-coordinated Tb³⁺ Schiff-base complexes whose photophysical parameters have been reported. Hasegawa and co-workers have reported the photophysical properties of [Tb(tmh)₃(tppo)], where tmh = tetramethylheptanedionato and tppo = triphenylphosphine oxide.⁴⁶ This mono-capped octahedral Tb³⁺ complex has a similar emission lifetime (840 ms) and k_r value (1040 s⁻¹) but a much smaller k_nr value (149 s⁻¹) such that the value of Φ_ff is much higher (88% vs. 18% for 3). It should be mentioned that Φ_ff decreases significantly with increasing temperature. This decrease in intensity is attributed to O–H-overtone-mediated vibrational relaxation of the excited state of the metal ion by the coligated MeOH and H₂O, although quenching due to low-lying ligand-to-metal charge-transfer (LMCT) states cannot be ruled out.³³ Emission spectra of complex 2 (neat powder or CH₂Cl₂/MeOH 1 : 1 frozen glassy matrix) were also recorded at 77 K (utilizing the spectrometer from PicoQuant mentioned above). The spectra and photophysical parameters are similar to those recorded in PMMA matrix (Fig. S22 and S28,†Table 4).

Table 4 Lifetime (τ), photoluminescence quantum yield (Φ_L), radiative (k_r) and non-radiative decay constants (k_nr) for the lanthanide complexes 2 and 3 under various conditions

Compound	T/K	λ exc/nm	Form	τ av ^b/μs	Φ L/%	k r ^c/s^−1	k nr ^d/10^3 s^−1
		λ em/nm
a Data recorded on a PicoQuant FluoTime300 photoluminescence spectrometer and an integrating sphere from Hamamatsu. This experimental setup cannot measure these quantities for samples embedded in a PMMA matrix. b Amplitude-weighted average lifetime. The decay components and their relative amplitudes can be found along with the fitted decays in the ESI.† c Average radiative rate constant calculated using the equation kr = ΦL/τav. d Average radiationless deactivation rate constant calculated using the equation knr = (1 − ΦL)/τav. e The upper limit of the ΦL is used for the calculation. f The upper and the lower limits of ΦL are used to calculate knr. g Data recorded on a Varian Cary-Eclipse fluorescence spectrometer.
2	77	360	CH2Cl2/MeOH^a	402 ± 6	3 ± 2	80 ± 50	2.4 ± 0.9
		610	(1 : 1) glassy matrix
	77	360	Powder^a	143 ± 4	<2	<140^e	6.98 > knr > 6.84^f
		607
3	77	360	CH2Cl2/MeOH^a	900 ± 30	19 ± 2	280 ± 40	1.19 ± 0.07
		543	(1 : 1) glassy matrix
	77	360	Powder^a	30 ± 2	<2	<670^e	3.49 > knr > 3.42^f
		550
	298	292	DMSO/HEPES^g	526
		550

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Unlike 2, complex 3 was found to be emissive in the solution state, albeit very weakly. Fig. 9 displays the photoluminescence spectrum of a 50 μM solution of complex 3 in H₂O/DMSO (95/5 v/v) solution containing 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer. As can be seen, the spectrum is similar to that recorded for the neat powder or the PMMA thin film, exhibiting four well-separated transitions centered at 490 nm, 545 nm, 582 nm, and 620 nm. The lifetime of 3 was found to be significantly longer (τ = 526 μs) under these conditions. This increase in luminescence lifetime presumably relates to substitution of the Tb-bound MeOH co-ligands by the anionic sulfonate groups from the HEPES buffer, as replacement of MeOH by RSO₃⁻ (or DMSO) ligands is expected to prevent vibrational quenching through the loss of the O–H oscillators.^56,57 It is not clear whether the Tb³⁺ complexes retain their seven-coordinate structures. Nevertheless, this finding suggests that these complexes may be interesting to investigate as receptors for anionic sulfate ions.

Fig. 9 Emission spectrum of a 5 × 10⁻⁵ M solution of complex 3 in H₂O/DMSO (95/5 v/v, HEPES-buffer, 1 × 10^–2 M concentration, pH 7.4). Excitation wavelength λ_ex 292 nm.

Spectrophotometric titrations

UV-vis spectrophotometric titrations were performed in order to determine the speciation in solution and the binding affinity of H₃L² for the La³⁺, Eu³⁺, Tb³⁺ and Yb³⁺ ions. The titrations were carried out in a batch mode in CH₂Cl₂/MeOH (1 : 1 v/v) solution where all compounds had the best solubility. Measurements were performed at constant ionic strength (10^–2 M nBu₄NPF₆) and pH value (2 × 10^–4 M NEt₃ buffer) as done in previous titrations.

Fig. 10 shows the spectral changes that occur upon addition of a solution of Eu(NO₃)₃·6H₂O to a solution of H₆L², which is assumed to be deprotonated under our experimental conditions, in the 190 nm–700 nm spectral region. The other titration data are shown in the ESI (Fig. S9, S12 and S14†). As can be seen, the intensity of the bands/shoulder (sh) at 231 nm, 271 (sh) nm, 345 nm, and 420 nm for the ligand decreases steadily with increasing Eu³⁺ concentration, and new bands for [Eu₂(L²)(MeOH)₂] develop with maxima at 231 nm and 409 nm. The spectrophotometric titration of H₆L² with the other lanthanide salts displayed a similar evolution of absorbance values (ESI†).

Fig. 10 Spectrophotometric titration of H₆L² with Eu(NO₃)₃·6H₂O in a MeOH/CH₂Cl₂ (1 : 1 v/v) solvent mixture (2 × 10^–5 M concentration) at constant ionic strength (1 × 10^–2 M N(n-Bu₄)PF₆, T = 298 K) in the presence of 2 × 10^–4 M NEt₃. The green curve refers to a final molar ratio of Eu³⁺/(L²)⁶⁻ = 5.0. The inset shows the evolution of selected absorbance values versus the [Eu³⁺]/[(L²)⁶⁻] molar ratio. The solid lines are drawn to illustrate the consecutive complex formations from 1 : 1 to a 2 : 1 molar ratio.

The mole ratio method was applied to determine the metal to ligand ratio.⁵⁸ The inset of Fig. 10 shows a plot of two selected absorbance values versus molar ratio [Eu³⁺]/[(L²)⁶⁻]. This ratio increases steadily up to a molar ratio of about 2 and then remains constant. This behavior clearly confirms that the 2 : 1 (metal : ligand) stoichiometry found for the solid state is also maintained in the solution phase.

The titration data were analyzed by nonlinear least-squares refinements. The data could only be fitted by assuming a speciation model that involved the deprotonated ligand, a mononuclear 1 : 1 (M : L²) complex and a dinuclear 2 : 1 (M : L²) complex. Apparent stability constants of log K₁₁ = 6.01(7) for the mononuclear and log K₂₁ = 6.04(6) for the dinuclear complex were determined. The data for the other lanthanide ions are listed in Table 5. Overall, the stability constants show a strong affinity of (L²)⁶⁻ towards lanthanide ions.

Table 5 Equilibrium constants for lanthanide complexation by H₆L² as determined by spectrophotometric titrations^a

	La^3+	Eu^3+	Tb^3+	Yb^3+
a Solvent: CH2Cl2/MeOH (1 : 1 v/v), cBase = 2 × 10^–4 M NEt3, I = 0.01 M N(n-Bu)4PF6, T = 298 K.
log K11	5.42(5)	6.01(7)	6.56(6)	5.25(8)
log K21	6.07(6)	6.04(6)	6.64(5)	6.33(8)
α	0.06	0.23	0.21	0.02

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The interaction parameter (α)⁵⁹ for the stepwise formation of a homotropic 2 : 1 M₂ : L² complex according to eqn (1) provides an insight for the cooperativity effect between the formation of 1 : 1 and 2 : 1 complexes.⁶⁰ If α = 1 (i.e. K₁₁ = 4K₂₁), the stepwise 2 : 1 binding is noncooperative; if α < 1 (i.e. K₁₁ < 4K₂₁), the stepwise 1 : 2 binding is positively cooperative; if α > 1 (i.e. K₁₁ > 4K₂₁) the stepwise 1 : 2 binding is negatively cooperative.⁶¹ For the present complexes the parameter α was found invariably smaller than 1, indicative of positive cooperativity, that is the binding of the first Ln³⁺ ion augments the binding of the second Ln³⁺ ion. This may be traced to mechanical coupling of the two binding sites by the rather stiff bis(iminomethyl)phenol functions. Cooperative binding of lanthanide ions by multitopic receptors is rare, but is of importance for the design of selective receptors and also the targeted construction of supramolecular structures.^62,63

Few studies have reported solution thermodynamic data for f-element calixarene complexes.^64–69 We have investigated the Ln³⁺ binding ability of the related salicylaldimine functionalized calix[4]arene ligand (L¹)⁴⁻ in MeOH. The log K₁₁ value for this complex of 6.0 compares well with that determined for the present complexes.

Conclusion

The coordination behaviour of the macrocyclic ligand H₆L² comprising two calix[4]arene head units linked by two bis(iminomethyl)phenol-chromophores towards selected lanthanide ions (La³⁺, Eu³⁺, Tb³⁺, Yb³⁺) has been investigated both in solution and solid state. H₆L² was found to be an effective dinucleating ligand for the Ln³⁺ ions forming mixed-ligand complexes of composition [Ln₂(L²)(MeOH)₂] with one exchangeable coligand per seven-coordinated Ln site. A 2 : 1 complex stoichiometry is also present in solution state as suggested by ESI MS and spectroscopic titration. The two binding pockets of H₆L² have similar affinities for the Ln³⁺ ions, with the stepwise binding constants log K₁₁ and log K₂₁ in a narrow 5.2 to 6.6 range. The interaction parameter α = K₁₁/4K₂₁ is invariably smaller than unity indicating that the binding of the first Ln³⁺ ion augments the binding of the second Ln³⁺ ion. The investigation of the photophysical properties clearly show that the bis(iminomethyl)phenolate units in H₆L² can sensitize luminescence of the coordinated Eu³⁺ and Tb³⁺ ions. However, the external MeOH coligands in 2 and 3 provide non-emissive relaxation pathways, which render the complexes less luminescent than other 7-coordinate Eu³⁺ and Tb³⁺ compounds supported by enolate ligands. On the other hand, replacement of the MeOH by RSO₃⁻ ligands appears to prevent vibrational quenching through the loss of the O–H oscillators, and it could be interesting to investigate them as displacement receptors for anionic sulfonate or other S-based ions.

Experimental section

Materials and methods

The ligand H₆L² was prepared as described in the literature.^26,30 All reagents and solvents were commercial grade and used without further purification. Melting points were determined with an Electrothermal IA9000 series instrument using open glass capillaries and are uncorrected. Elemental analyses were carried out on a VARIO EL elemental analyzer (Elementar Analysensysteme GmbH, Hanau). Mass spectra were obtained using the positive ion electrospray ionization modus (ESI) on a Bruker Daltronics ESQUIRE 3000 Plus ITMS or Impact II UHR Qq-TOF instrument. Infrared spectra (4000–640 cm⁻¹) were recorded at 2 cm⁻¹ resolution on a Bruker TENSOR 27 FT-IR spectrometer using a MIRacle ZnSe ATR accessory from PIKE Technologies. Solution absorption spectra were collected on a Jasco V-670 UV–vis–NIR device using 1 cm quartz cells (Hellma).

Synthesis and analysis of compounds

[La₂(L²)(MeOH)₂] (1). This compound was prepared from H₆L² (75 mg, 55 μmol), NEt₃ (38 μL, 275 μmol) and La(NO₃)₃·5H₂O (52 mg, 121 μmol) in analogy to the europium complex 2 yielding 73 mg (81%) of a green powder. M.p. >220 °C (decomp.); ATR-IR (ZnSe): [small nu, Greek, tilde] = 670 (w), 714 (w), 758 (m), 810 (w), 838 (w), 863 (w), 913 (w), 950 (w), 1011 (w), 1039 (w), 1084 (m), 1109 (w), 1126 (w), 1156 (w), 1190 (w), 1222 (w), 1244 (w), 1299 (m), 1322 (w), 1364 (w), 1394 (w), 1425 (w), 1452 (s), 1530 (w), 1591 (w), 1632 (m), 1650 (m), 2874 (w), 2914 (w), 2954 (w), 3019 (w), 3057 (w), 3450 cm⁻¹ (br); UV-vis (CH₂Cl₂/MeOH, 1/1 v/v): λ_max (ε) = 231 (87 976), 250 (sh) (59 730), 304 (sh) (13 306), 410 (14 224) nm (M⁻¹ cm⁻¹); ESI-MS(+): m/z: 1633.386 ([M + H⁺]⁺, calc. 1633.425), 1696.387 ([M + 2MeOH]⁺, calc. 1696.469); elemental analysis calcd (%) for C₈₈H₈₂La₂N₄O₁₀·2CH₃OH: C 63.68, H 5.34, N 3.30; found: C 63.83, H 5.49, N 3.33.

[Eu₂(L²)(MeOH)₂] (2). To a suspension of H₆L² (75 mg, 55 μmol) and NEt₃ (38 μL, 275 μmol) in CH₂Cl₂/MeOH (1/1 v/v, 25 mL) was added a solution of Eu(NO₃)₃·6H₂O (54 mg, 121 μmol) in MeOH (2 mL) at room temperature. The reaction mixture was stirred for 24 h and evaporated to ∼1/3 of its original volume. The resulting yellow precipitate was isolated by filtration, washed with MeOH and Et₂O and dried at 60 °C to constant weight to yield 79 mg (86%) of 2 as a bright yellow powder. M.p. >225 °C (decomp.); ATR-IR (ZnSe): [small nu, Greek, tilde] = 673 (w), 695 (w), 714 (w), 746 (m), 754 (m), 784 (w), 805 (w), 832 (w), 867 (w), 899 (w), 931 (w), 955 (w), 980 (w), 1006 (w), 1033 (w), 1075 (w), 1085 (w), 1124 (w), 1152 (w), 1187 (w), 1224 (w), 1246 (m), 1266 (w), 1289 (s), 1317 (w), 1402 (w), 1426 (w), 1449 (s), 1547 (w), 1590 (w), 1635 (m), 2839 (w), 2879 (w), 2915 (w), 2998 (w), 3020 (w), 3053 (w), 3508 (br), 3586 cm⁻¹ (w); UV-vis (CH₂Cl₂/MeOH, 1/1 v/v): λ_max (ε) = 231 (87 092), 254 (sh) (57 697), 302 (sh) (15 985), 409 (15 339) nm (M⁻¹ cm⁻¹); ESI-MS(+): m/z: 1677.469 ([M + H₂O + H⁺]⁺, calc. 1677.461), 1722.452 ([M + 2MeOH]⁺, calc. 1722.495); elemental analysis calcd (%) for C₈₈H₈₂Eu₂N₄O₁₀·2CH₃OH: C 62.72, H 5.26, N 3.25; found: C 62.84, H 5.44, N 3.26.

[Tb₂(L²)(MeOH)₂] (3). H₆L² (75 mg, 55 μmol), NEt₃ (38 μL, 275 μmol) and Tb(NO₃)₃·6H₂O (55 mg, 121 μmol) were reacted in analogy to the procedure detailed above for the europium complex 2 to give 78 mg (85%) of 3 as a pale-yellow powder. M.p. >225 °C (decomp.); ATR-IR (ZnSe): [small nu, Greek, tilde] = 676 (w), 695 (w), 716 (w), 745 (m), 806 (w), 833 (w), 867 (w), 900 (w), 934 (w), 960 (w), 984 (w), 1033 (w), 1076 (w), 1085 (w), 1123 (w), 1152 (w), 1188 (w), 1224 (w), 1246 (w), 1268 (w), 1289 (m), 1319 (m), 1401 (w), 1427 (m), 1449 (s), 1548 (w), 1591 (w), 1636 (m), 2841 (w), 2882 (w), 2913 (w), 2998 (w), 3018 (w), 3054 (w), 3601 cm⁻¹ (w); UV-vis (CH₂Cl₂/MeOH, 1/1: v/v): λ_max (ε) = 227 (107 414), 251 (69 553), 303 (sh) (18 343), 409 (16 976) nm (M⁻¹ cm⁻¹); ESI-MS(+): m/z: 1657.521 ([M − CH₃]⁺, calc. 1657.430), 1705.500 ([M + MeOH + H⁺]⁺, calc. 1705.488), 1737.440 ([M + 2MeOH + H⁺]⁺, calc. 1737.506); elemental analysis calcd (%) for C₈₈H₈₂N₄O₁₀Tb₂·2CH₃OH: C 62.21, H 5.22, N 3.22; found: C 62.36, H 5.55, N 3.18.

[Yb₂(L²)] (4). Complex 4 was prepared from H₆L² (75 mg, 55 μmol), NEt₃ (38 μL, 275 μmol) and Yb(NO₃)₃·5H₂O (54 mg, 121 μmol) in accordance with the procedure described for the europium complex 2 yielding 75 mg (80%) of an off-yellow powder. M.p. >220 °C (decomp.); ATR-IR (ZnSe): [small nu, Greek, tilde] = 668 (w), 718 (w), 756 (m), 800 (w), 812 (w), 834 (w), 871 (w), 911 (w), 999 (w), 1036 (w), 1082 (m), 1108 (w), 1126 (w), 1155 (w), 1188 (w), 1222 (w), 1245 (w), 1304 (m), 1330 (m), 1363 (w), 1396 (w), 1428 (w), 1456 (s), 1536 (m), 1589 (w), 1634 (m), 1653 (m), 2839 (w), 2872 (w), 2913 (w), 2956 (w), 3021 (w), 3055 cm⁻¹ (w); UV-vis (CH₂Cl₂/MeOH, 1/1 v/v): λ_max (ε) = 230 (72 147), 257 (sh) (42 642), 307 (sh) (7994), 417 (11 100), 456 (sh) (7020) nm (M⁻¹ cm⁻¹); ESI-MS(+): m/z: 1703.445 ([M + H⁺]⁺, calc. 1703.476), 1764.440 ([M + 2MeOH]⁺, calc. 1764.521); elemental analysis calcd (%) for ₈₈H₈₂N₄O₁₀Yb₂·2CH₃OH: C 61.22, H 5.14, N 3.17; found: C 61.42, H 5.27, N 3.19.

Crystallography

Single crystals of [Eu₂(L²)(MeOH)₂]·4CH₂Cl₂·MeOH (2·4CH₂Cl₂·MeOH) and [Tb₂(L²)(MeOH)(H₂O)]·(H₂O)_x (3·(H₂O)_x, x ∼ 10) suitable for X-ray crystallographic analyses were selected and mounted on the tip of a glass fibre using perfluoropolyether oil. The data sets were collected at 180(2) K using a STOE STADIVARI diffractometer using graphite monochromated Cu-K_α radiation (λ = 1.54186 Å). The data were processed with the programs XAREA.⁷⁰ Structures were solved by direct methods⁷¹ and refined by full-matrix least-squares techniques on the basis of all data against F² using SHELXL-2018/3.⁷² PLATON was used to search for higher symmetry.³⁷ All non-hydrogen atoms were refined anisotropically. Graphics were produced with Ortep3 for Windows and PovRAY.

Crystallographic data for [Eu₂(L²)(MeOH)₂]·4CH₂Cl₂·MeOH (2·4CH₂Cl₂·MeOH). C₉₅H₁₀₂Cl₈Eu₂N₄O₁₃, M_r = 2095.33 g mol⁻¹, monoclinic space group P2₁/c, a = 14.6466(3) Å, b = 33.1024(7) Å, c = 20.4799(4) Å, β = 110.101(2)°, V = 9324.6(3) ų, Z = 4, ρ_calcd = 1.493 g cm⁻³, T = 180(2) K, μ(Cu K_α) = 12.147 mm⁻¹ (λ = 1.54186 Å), 117 925 reflections measured, 17 529 unique, 10 619 with I > 2σ(I). Final R₁ = 0.0431, wR₂ = 0.1074 (I > 2σ(I)), 1109 parameters/0 restraints, min./max. residual electron density = −1.422 /0.895 e Å⁻³. The hydrogen atoms of the Eu-bound MeOH ligands were located from final Fourier maps and were refined isotropically.

Crystallographic data for [Tb₂(L²)(MeOH)(OH)]·(H₂O)_x (3·xH₂O, x ∼ 10). C₈₉H₈₈N₄O₁₃Tb₂, M_r = 1739.47 g mol⁻¹, monoclinic space group P2₁/n, a = 14.5534(3) Å, b = 33.1629(6) Å, c = 20.4530(4) Å, β = 108.277(2)°, V = 9373.3(3) ų, Z = 4, ρ_calcd = 1.233 g cm⁻³, T = 180(2) K, μ(Cu K_α) = 7.771 mm⁻¹ (λ = 1.54186 Å), 117 989 reflections measured, 17 592 unique, 14 074 with I > 2σ(I). Final R₁ = 0.0405, wR₂ = 0.1125 (I > 2σ(I)), 979 parameters/0 restraints, min./max. residual electron density = −1.159 /−0.891 e Å⁻³. The O–H hydrogen atom of the Tb-bound H₂O and MeOH ligands were located from final Fourier maps and were refined isotropically. The H₂O solvate molecules in this structure were found to be heavily disordered and all attempts to model this disorder failed. The corresponding electron density was removed from the structure (and the corresponding F₀) with the SQUEEZE algorithm implemented in the PLATON program suite. The SQUEEZE routine revealed total potential solvent area of 2500 ų (400e⁻) per unit cell corresponding to approximately ten H₂O molecules per formula unit.

CCDC 2005666 (2) and 2005669 (3)† contain the supplementary crystallographic data for this paper.

Synthesis of PMMA thin films

PMMA (0.30 g) was dissolved in CH₂Cl₂ (1.5 mL) and stirred for 10 min. A solution of the lanthanide complex (V = 1 mL, 8 × 10^–3 M) in CH₂Cl₂ was added to the PMMA solution. The resulting mixture was spread on a Petri dish (d = 5 cm) and the solvent was allowed to evaporate in open air over night.

Photophysical characterization

Steady state fluorescence excitation and emission spectra and luminescence lifetime measurements for PMMA thin films of [HNEt₃][Eu₂(HL¹)(L¹)], [HNEt₃][Tb₂(HL¹)(L¹)], 2, and 3 were recorded on a PerkinElmer FL 6500 fluorescence spectrophotometer using precision cells for powder samples. Lifetime analysis was performed using the commercial PerkinElmer software. The luminescence emission spectrum and lifetime measurement of an aqueous solution of complex 3 was recorded at 23 °C using a Varian Cary-Eclipse fluorescence spectrophotometer set to phosphorescence mode. A quartz cell with a path length of 10 mm and a volume of 400 μL was used, the instrument excitation and emission slit widths were both set at 5 nm.

Steady-state excitation and emission spectra of compounds 2 and 3 (neat powder or frozen CH₂Cl₂/MeOH (1 : 1) matrix at 77 K) were recorded on a FluoTime300 spectrometer from PicoQuant equipped with a 300 W ozone-free Xe lamp (250–900 nm), a 10 W Xe flash-lamp (250–900 nm, pulse width <10 μs) with repetition rates of 0.1–300 Hz, an excitation monochromator (Czerny-Turner 2.7 nm mm⁻¹ dispersion, 1200 grooves per mm, blazed at 300 nm), diode lasers (pulse width <80 ps) operated by a computer-controlled laser driver PDL-820 (repetition rate up to 80 MHz, burst mode for slow and weak decays), two emission monochromators (Czerny-Turner, selectable gratings blazed at 500 nm with 2.7 nm mm⁻¹ dispersion and 1200 grooves per mm, or blazed at 1250 nm with 5.4 nm mm⁻¹ dispersion and 600 grooves per mm), Glan–Thompson polarizers for excitation (Xe-lamps) and emission, a Peltier-thermostatized sample holder from Quantum Northwest (−40 °C–105 °C), and two detectors, namely a PMA Hybrid 40 (transit time spread FWHM < 120 ps, 300–720 nm) and a R5509-42 NIR-photomultiplier tube (transit time spread FWHM 1.5 ns, 300–1400 nm) with external cooling (−80 °C) from Hamamatsu. Steady-state and fluorescence lifetimes were recorded in TCSPC or MCS mode by a PicoHarp 300 (minimum base resolution 4 ps). Emission and excitation spectra were corrected for source intensity (lamp and grating) by standard correction curves. Lifetime analysis was performed using the commercial FluoFit software. The quality of the fit was assessed by minimizing the reduced chi squared function (χ²) and visual inspection of the weighted residuals and their autocorrelation.

Luminescence quantum yields (Φ_L) for 2 and 3 (neat powder or frozen CH₂Cl₂/MeOH (1 : 1) matrix at 77 K) were measured with a Hamamatsu Photonics absolute PL quantum yield measurement system (C9920-02) equipped with a L9799-01 CW Xenon light source (150 W), monochromator, C7473 photonic multi-channel analyzer, integrating sphere and employing U6039-05 PLQY measurement software (Hamamatsu Photonics, Ltd, Shizuoka, Japan). All solvents used were of spectrometric grade (Uvasol®).

Spectrophotometric titrations

Stock solutions of ligands and metal salts were obtained by dissolving carefully weighed samples in HPLC-grade solvent mixtures. Batch titrations were carried out with quartz cells (Hellma 110-QS, 1 cm optical path length) containing a stock solution of H₆L² (1 mL) with (n-Bu)₄NPF₆ as supporting electrolyte (0.01 M) and constant concentrations (2 × 10⁻⁵ M) at 298 K. For each experiment, 19 solutions were prepared by adding aliquots of 0–200 μL of the metals salt solutions (Ln(NO₃)₃·6H₂O, Ln = La, Eu, Yb, 10⁻³ M) with an Eppendorf micropipette (volume range 10–100 μL and 100–1000 μL; 0.71–0.10% error). After stirring for 12 h, UV–vis absorption spectra were collected in the 190–700 nm range at uniform data point intervals of 1 nm with a double-beam V-670 (Jasco) spectrophotometer. The multiwavelength data sets were analyzed by a nonlinear least-squares procedure implemented in the Hyperquad2008 v1.1.33 software.^73,74 Goodness-of-fit parameters were assessed by the overall standard deviation (σ), the visual inspection of the residuals, and by the physical meaning of the calculated electronic absorption spectra. Unless otherwise noted, the reported uncertainties correspond to the standard deviation of the refined parameters that were returned by the software.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

We are thankful to Prof. Dr H. Krautscheid for providing facilities for X-ray crystallographic measurements. This work was supported by the Deutsche Forschungsgemeinschaft (DFG Priority Program, “Light Controlled Reactivity of Metal Complexes”) and the DAAD “Towards Luminescent Hydrosulfide Probes”. Matias E. Gutierrez Suburu acknowledges the DAAD for a doctoral fellowship. Cristian A. Strassert gratefully acknowledges the DFG (Cluster of Excellence Cells in Motion EXC 1003, Priority Program, “Light Controlled Reactivity of Metal Complexes”) for financial support.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2005666 and 2005669. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0dt02303e

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