A systematic study of the optical properties of mononuclear hybrid organo–inorganic lanthanoid complexes

Matias Zapata-Lizamaab, Patricio Hermosilla-Ibáñezab, Diego Venegas-Yazigiab, Guillermo Mínguez Espallargasc, Lauro June Queiroz Maiad, Gisane Gasparottod, Ricardo Costa De Santana*d and Walter Cañón-Mancisidor*ab
aFacultad de Química y Biología, Depto. de Química de los Materiales, Universidad de Santiago de Chile, USACH, Chile. E-mail: walter.canon@usach.cl
bCentre for the Development of Nanoscience and Nanotechnology, CEDENNA, Chile
cInstituto de Ciencia Molecular (ICMol), Universidad de Valencia, Paterna, Spain
dInstituto de Física, Universidade Federal de Goiás, Goiânia, GO, Brazil. E-mail: santana@ufg.br; ricosta.santana@gmail.com

Received 26th February 2020 , Accepted 28th April 2020

First published on 30th April 2020


A series of hybrid organo–inorganic mononuclear lanthanoid complexes, [n-NBu4]3[LnH(PW11O39)(phen)2]·H2O, denoted as LM4-1-Ln (Ln = DyIII, TbIII, EuIII, NdIII, ErIII, HoIII and GdIII), were synthesized via hydrothermal synthesis and were structurally characterized by X-ray diffraction. The optical properties of all complexes have been investigated in the solid state. The temperature-dependent emission spectra of LM4-1-Dy, LM4-1-Tb and LM4-1-Eu complexes show intense lanthanoid emissions in the visible region, while LM4-1-Nd shows near-infrared (NIR) luminescence. The EuIII complex shows typical strong red emissions from the 5D07F0,1,2,3,4 transitions, with the CIE colour coordinates (0.631,0.364), the colour purity value of 83.9% and a quantum yield of up to 4.3%, suggesting that the organic fragment has an effect on the optical properties compared to fully inorganic systems, making this complex very attractive as a red component of light-emitting diodes. The luminescence decays of LM4-1-Dy, LM4-1-Tb and LM4-1-Eu exhibit a biexponential behaviour, with τAV = 4.1(7) μs, 0.35(2) ms and 0.94(3) ms, respectively. The values obtained for Judd–Ofelt intensity parameters Ω2 and Ω4 support the interaction between the EuIII and the ligands. Furthermore, those with ErIII and HoIII present weak emissions in the visible region. The T-dependent photoluminescence results show that the LM4-1-Dy, LM4-1-Tb and LM4-1-Nd complexes have good temperature sensitivity, demonstrating that the materials have the potential to be used as a sensing element for luminescent thermometers in different temperature ranges.


Introduction

Temperature-sensitive luminescent molecules have potential applications as optical thermo-sensors for electronic devices, and for medical and biological applications.1,2 Various thermo-sensing luminescent molecules like organic dyes,3 metal complexes, polymers and extended systems like metal organic frameworks (MOFs) have been reported.4

Among these types of molecules lanthanoid (Ln) complexes have been considered as ideal materials for these purposes since Ln ions have unique photophysical properties, showing sharp and characteristic transitions in the visible or near-infrared regions and also long excited-state lifetimes.5 The plasticity of the coordination chemistry of lanthanoid ions (Ln) allows the design of novel coordination compounds. In this sense Ln complexes based on aromatic organic ligands can work as “antennas” in order to harvest light. The energy is then transferred from the excited state of the ligand onto the metal ion which eventually gives off its characteristic light since the dipole strengths of f–f transitions are very small (Laporte forbidden) and the direct excitation into the 4f excited levels is very rare.5,6 Among trivalent lanthanoid ions, TbIII, EuIII and DyIII, generally, present intense green, red and blue emissions respectively, being used as emitters in white light-emitting diodes (WLED), as luminescent probes in the investigation of biochemical systems, and as red activators in X-ray detection materials among others.7–10

Moreover, inorganic ligands like polyoxometalates (POMs) have been also used in order to obtain Ln coordination compounds.11 Polyoxometalates (POMs) are metal–oxygen clusters of zero dimensionality, presenting characteristic architectures and various compositions as well as having potential applications in materials science.12 These molecules can be used as multidentate inorganic ligands, especially for tungsten lacunary POMs (LPOMs) with defined vacancies having also various sizes, shapes, solubility, flexible coordination sites and modes.13,14 These inorganic ligands have also shown properties of harvesting light like organic ones. In these systems, it is possible to observe that the luminescence intensity can be very low, suggesting that the excitation energy of the O → W charge transfer band of this compound can be quenched by non-radiative transitions within the LPOM.15

The combination of conjugated organic and inorganic ligands (LPOMs) with lanthanoid centres can generate hybrid organic–inorganic Ln complexes. This can afford species usually forming dinuclear and/or polynuclear complexes with two different routes for sensitizing the lanthanoid luminescence (the organic and inorganic ones).16–21 Many of the systems reported in the literature show that in general in polynuclear systems the coordination sphere of the Ln ion has low symmetry and its coordination sphere is completed by H2O molecules, quenching the emission properties.21 Moreover, this hybrid organic–inorganic molecule can have potential applications such as magneto-optical devices, optoelectronics, optical markers, laser materials, catalysts and others.5,6,22–26

So, high symmetry mononuclear systems with no water molecules are necessary in order to obtain good optical properties in Ln complexes. Despite the exquisite control of coordination chemistry, the preparation of mononuclear hybrid organo–inorganic Ln complexes has remained elusive. These hybrid materials could be benefitted from the combination of the flexibility of organic ligands with the robustness of the LPOM inorganic moieties.27

In this work the optical properties (UV-Vis-NIR absorption, excitation, emission and lifetime) in the solid state of the isostructural family of hybrid organo–inorganic Ln complexes (LM4-1-Ln), where Ln = DyIII, TbIII, EuIII, NdIII, ErIII, HoIII and GdIII, are presented).

Results and discussion

Structural characterization

The preparation of mononuclear hybrid organic–inorganic Ln complexes is not a simple task, dinuclear or polynuclear systems being the published ones.28–30

These compounds were obtained by hydrothermal synthesis at 160 °C for 48 h, giving single crystals of [n-NBu4]3[LnH(PW11O39)(phen)2]·H2O (LnIII = DyIII(LM4-1-Dy), TbIII(LM4-1-Tb), EuIII(LM4-1-Eu), NdIII(LM4-1-Nd), ErIII(LM4-1-Er), HoIII(LM4-1-Ho) and GdIII(LM4-1-Gd) suitable for X-ray diffraction (see section S1 to S2, Fig. S1 and Table S1). These compounds are all isostructural, crystallizing in the monoclinic P21/c space group (see section S3 and Table S2). Electroneutrality is achieved by three [n-NBu4]+ cations that exist in the crystal lattice, and by one proton that is delocalized over the oxygen atoms of the LPOM.31 A complex of the formula [LnH(PW11O39)(phen)2]3− is formed by two types of ligands, one inorganic and two organic, forming an octacoordinated complex (Fig. 1). The organic ligands correspond to two phenanthroline molecules and the inorganic one corresponds to the Keggin lacunary polyoxotungstate ([PW11O39]7−). The importance of having a lacunary POM is that it has a rigid crystal field around the lanthanoid cation, in contrast to the more flexible organic ligands being also softer than the POM from the HSAB point of view. The distances between the Ln and the nitrogen atoms of the phenanthrolines are in the range of 2.545(14) to 2.646(16) Å, whereas the distances between the Ln to oxygen atoms of the [PW11O39]7− are in the 2.225(14) to 2.358(11) Å range.


image file: d0qi00232a-f1.tif
Fig. 1 Ball-and-stick representation of hybrid organic–inorganic molecular complexes [LnH(PW11O39)(phen)2]3−. Water molecule and [n-NBu4]+ cations are omitted for clarity. Colour label: Ln (cyan), W (yellow), N (blue), C (grey) O (red) and P (green).

The compounds are arranged in the crystal packing by π–π stacking between the aromatic rings (N20 C17 C18 C19 C23 C24; N40 C37 C38 C39 C43 C44) of the phenanthroline ligands, forming pairs with distances ranging from 3.568 Å to 3.108 Å, for the studied complexes (Fig. S2). The distance between two molecules in each pair (Ln–Ln) is 9.714(1) Å, 9.783(1) Å, 9.784(1) Å, 9.791(2) Å, 9.783(2) Å, 9.781(1) Å and 9.778(1) Å, for LM4-1-Dy, LM4-1-Tb, LM4-1-Eu, LM4-1-Nd, LM4-1-Er, LM4-1-Ho and LM4-1-Gd complexes, respectively.

Continuous shape measurement (CShMs) calculations done using the SHAPE code32,33 reveal that the geometry of the Ln complexes can be described as a square antiprism (sa), thus implying that the Ln centres present a pseudo-D4d symmetry (Fig. S3). This type of geometry is very common since most LnIII complexes with octacoordinated geometries present square antiprism, or triangular dodecahedron or bicapped trigonal prism types of geometries.34–38 Reported data show that the metal centres of the fully inorganic ligand systems of [Ln(PW11O39)2]11− and [Ln(SiW11O39)2]13− present a square antiprism geometry,31 also observed for the systems presented in this work.

Finally, the XRPD pattern of LM4-1-Ln is quite similar for all the Ln complexes, but some slight differences in some intensities exist, which are due to the textural problems of the material. The crystalline products are isostructural and also are in agreement with their simulated patterns, demonstrating that the crystal structures of the compounds are truly representative of the bulk materials (Fig. S4 and S5).

Electronic spectra

The solid-state UV-Vis-NIR spectra of the LM4-1-Ln complexes collected at room temperature are depicted in Fig. S6. All LM4-1-Ln complexes do not have water molecules in the first coordination sphere of the LnIII ion.

The absorption spectrum of LM4-1-Dy shows typical bands for DyIII attributed to the transitions from the 6H15/2 ground state to the 6F3/2, 6F5/2, 6F7/2, 6H7/2, 6H9/2 and 6H11/2 excited states respectively, due to the electronic transitions of trivalent dysprosium with the 4f9 intra configuration (Fig. S6a).39

The UV-Vis-NIR spectrum of the LM4-1-Tb complex is depicted in Fig. S6b, where only one band could be observed in the whole spectral range and due to the 7F67F0 transition at 1694 nm.

The spectrum of LM4-1-Eu, Fig. S6c, taken from 350 to 850 nm, shows characteristic absorption lines assigned to the 7F05D0,1,2 and 7F05L6 transitions at 590, 540, 511 and 385 nm respectively.40 A small peak at 2075 nm is attributed to the 7F07F6 transition. Several absorptions associated with the overtones of OH of the POM and CH of the phenanthroline appear at 747 and 804, 1950, 2150 and 2280 nm.

Fig. S6d shows the absorption spectrum of LM4-1-Nd; several sharp bands can be observed in the 400–850 nm range, each one corresponding to the transitions from the 4I9/2 ground state to the excite states 4F3/2, (4F5/2, 2H9/2), 4F7/2, 4S3/2, 4F9/2, 4F3/2, 2H11/2, 4G5/2, 2G7/2, 4G7/2, 4G9/2, (2D3/2, 2G9/2), 4G11/2 and 2P1/2.41 The band positions are in good agreement with those of other NdIII polyoxometalate complexes.42

For the LM4-1-Er complex, the UV-Vis spectrum, Fig. S6e, shows a number of spectral bands corresponding to the transitions between the 4I15/2 ground state and the 4I9/2, 4F9/2, 4S3/2, 4F7/2, 4F5/2, (4G, 4F, 2H)9/2 and 4G11/2 excited states.43

The absorption spectrum for the LM4-1-Ho complex is presented in Fig. S6f, and the observed transitions from the 5I8 ground state to the 2S+1LJ excited states 5F5, (5F4, 5S2), 5F3, (5F2, 3K8), (5F1, 5G6) and 5G5.44

Fig. S7 shows the room temperature solid state absorption spectra of both free ligands, the inorganic ([PW11O39H3]4−) and the phenanthroline and also of the LM4-1-Gd complex. Fig. S7a and b correspond to the absorption spectra of the [PW11O39H3]4− ligand, taken in the UV-Vis region (200–500 nm), showing a broad band with a prominent shoulder at ∼250 nm, which can be associated with the oxygen to metal (W) charge transfer. The spectrum of the phenanthroline (Fig. S7c) ligand shows a broad absorption band in the 200–450 nm range, with a series of shoulders at 225, 275, 290 and 345 nm, as reported by Linnell and Kaczmarczyk.45

The absorption spectrum of LM4-1-Gd shown in Fig. S7d consists of a broad absorption, in which three intense bands are observed: 250, 290 and 345 nm. The first band can be associated with the [PW11O39]7− unit and the other two must correspond to the phenanthroline ligand, as observed in the spectra of free 1,10-phenanthroline. As the GdIII ion does not present any absorption below 311 nm, the LM4-1-Gd complex will be used to investigate any charge transfer processes between both ligands (1,10-phenanthroline and LPOM) to the emitting levels of DyIII, EuIII and TbIII ions. In the UV region (below 380 nm) no f–f transitions were observed for all studied compounds.

Photoluminescence results

Solid-state emission spectra for all LM4-1-Ln complexes have been collected at room temperature in the visible and IR regions. The results confirm that the antenna effect is observed for all compounds. The results for LM4-1-Gd will be used to determine the singlet and triplet state of the ligand for the analysis of the energy transfer (see ESI, Fig. S8). For compounds LM4-1-Dy, LM4-1-Tb, LM4-1-Eu and LM4-1-Nd the intensity profiles associated with each compound were observed with different excitation energies all of them in the range, in which both ligands absorb. Finally, the excitation energy used is the one that provides more intense emissions in each case (see Fig. S9).

The emission spectra of [NBu4]4[PW11O39H3] and 1,10-phenanthroline were obtained at room temperature, Fig. S10. The experimental measurements are in agreement with previously reported data, in which the observed emission of 1,10-phenantroline is in the range of 380 nm to 500 nm,46 and for the [PW11O39]7− anion the emission is in the same spectral range.47 The results show that the organic ligand has a more intense signal compared to the LPOM one, suggesting that the transition between the S0 and S1 energy levels of the organic ligand is more predominant compared to the 3T1u1A1g transition of the LPOM fragment. However, both ligands contribute to the antenna effect.

LM4-1-Dy

The excitation and emission spectra of the LM4-1-Dy compound are shown in Fig. 2. The excitation spectra, Fig. 2a, monitoring the emission corresponding the 4F9/26H13/2 transition at 573 nm, consist of an intense band centred at 352 nm attributed to the transition between the S0 and S1 energy levels of the phen ligand with the π–π* character; DyIII does not show absorption bands in this region. In Fig. 2b the temperature dependence of the emission spectra from 15 to 300 K is shown, in which the intensities of the bands are normalized to the 4F9/26H13/2 transition and obtained under excitation at 352 nm. All the spectra consist of three bands at 479, 571 and 658 nm assigned to the 4F9/26H15/2 (blue, B), 4F9/26H13/2 (yellow, Y) and 4F9/26H11/2 transitions, respectively. All the emission line energies are independent of the temperature; the 4F9/26H15/2 and 4F9/26H11/2 transitions are the highest and less intense bands, respectively. No change in the position of the DyIII transitions is observed in the whole temperature range. Also, one intense and broad band can be observed centred at ∼390 nm attributed to both ligands, as mentioned above (Fig. S10), which becomes less intense with the increasing temperature. The B transition has a magnetic dipole (MD) nature being less affected by the site symmetry, in contrast to the hypersensitive Y transition, which has an electric dipole (ED) nature that is strongly influenced by the local environment.48 The Y/B intensity ratio for DyIII can give information about the strength of the covalent/ionic bonding character between the DyIII and the atoms of the first coordination sphere, and the asymmetry of the site occupied by the lanthanoid ion.49 The temperature dependence of the Y/B intensity ratio, obtained from the experimental spectra, is nearly independent of temperature as shown in Fig. S11a, Y/B ≈ 0.63(6); this result is associated with the fact of the 4F9/26H15/2 transition is more intense than the 4F9/26H13/2 transition, suggesting a more covalent character of the bonding between the atoms of the first coordination sphere and the DyIII ion.50
image file: d0qi00232a-f2.tif
Fig. 2 Excitation spectra: (a) LM4-1-Dy by monitoring the 4F9/2 → 6H13/2 transition at 573 nm, (c) LM4-1-Tb by monitoring the 5D47F5 and 5D47F6 transitions at 543 and 485 nm and (e) excitation spectrum of the LM4-1-Eu complex by monitoring the 5D07F2 transition at 615 nm. Emission spectra: thermal dependency of the emission spectrum for (b) LM4-1-Dy (d) LM4-1-Tb and (f) LM4-1-Eu under excitation at 350, 325 and 394 nm, respectively. The emission spectra of LM4-1-Nd at room temperature under excitation at 330 and 350 nm are shown in (g) and the temperature dependence of the emission spectra under 804 nm excitation is shown in (h).

LM4-1-Tb

The excitation spectra of the LM4-1-Tb compound were acquired by monitoring the emissions at 543 and 485 nm, corresponding to the 5D47F5 and 5D47F6 transitions, respectively (Fig. 2c). The broad absorption band at ∼276 nm, by monitoring the emission at 485 nm, is assigned to the π–π* transition of the ligands, being much more intense than those attributed to the TbIII absorptions. In addition, the two peaks at 368 and 377 nm are due the 5L105D4 absorption. The presence of the broad band indicates the existence of the energy transfer from both ligands to the energy levels of the TbIII ion.

The emission spectra of the TbIII compound as a function of temperature, Fig. 2d, show typical emission bands corresponding to 5D47F6 (blue, B) at 486 nm, 5D47F5 (green, G) at 543 nm, 5D47F4 (586 nm) and 5D47F3 (618 nm) f–f transitions of TbIII ions as a function of the temperature from 15 to 300 K. The spectra also present a rather broad and intense emission in the 340–450 nm region attributed to the phen and LPOM ligand, as observed for LM4-1-Dy. No change in the position of the TbIII transitions is observed in the entire temperature range. Among all transitions, the 5D47F5 transition has a magnetic dipole (MD) character, being dominant over the others indicating that TbIII ions can emit green light when excited with UV light. The TbIII G/B ratio, as well as the Y/B of DyIII, plays the role describing the covalent/ionic bonding character between the TbIII and the first coordination sphere atoms. For LM4-1-Tb, the dependence of the G/B ratio as a function of temperature has a mean value of ∼1.9 in the whole temperature range and it is depicted in Fig. S11b.

LM4-1-Eu

The excitation spectrum of LM4-1-Eu (Fig. 2e) was acquired monitoring the hypersensitive 5D07F2 EuIII transition at 615 nm at room temperature, and it is possible to identify the characteristic bands related to EuIII excited states at 395 nm (7F05L6), 414 nm (7F05D3), 465 nm (7F05D2), 525 nm (7F05D1) and 535 nm (7F15D0), in good agreement with the absorption spectra. In addition, one broad band centred at 352 nm is observed and assigned as a MLCT from the EuIII to both ligands (phen and LPOM). The emission spectra under excitation at 394 nm at different temperatures are given in Fig. 2f, showing the characteristic emission bands of EuIII, being the first one centred at 579 nm corresponding to the 5D07F0 transition, and the second one at 593 (orange, O) nm is the 5D07F1 transition, which has a magnetic dipole (MD) nature and it is known to be less affected by the site symmetry. The third one is the most intense transition, centred at 614 nm (red, R, 5D07F2), being a hypersensitive electric dipole (ED) transition that is strongly influenced by the local environment.51,52 Weak EuIII emissions can also be observed at 652 and 697 nm, due to the 5D07F3 and 5D07F4 transitions, respectively. Also, a broad and less intense band can be observed to be centred around 440 nm due to the excitation of both ligands (Fig. S10). The presence of a single line of the 5D07F0 transition is an indication that the EuIII ions occupy one single site symmetry without an inversion centre,19,53 in agreement with the structural results. Note that all 5D07F1,2,3,4 split into doublets (Fig. 2f), Stark splitting, indicating that EuIII ions occupy a low site symmetry.51 The 5D07F6 transition does not appear in the spectrum due to the experimental conditions, being out of the range of the detector. No changes in the barycentre position of each EuIII transition are observed in the whole temperature range. The ratio of the integrated areas of the 5D07F2 (R) and 5D07F1 (O) transitions, R/O = I(5D07F2)/I(5D07F1), provides the same information as Y/B for DyIII. The R/O values are nearly independent of temperature and range from 4.3 (17 K) to 4.5 (300 K), Fig. S11c. This indicates a low degree of covalency between the EuIII and the donor atom of the first coordination sphere, with the EuIII occupying a low symmetry coordination centre.19,49 Furthermore, the obtained Y/B, G/B and R/O ratios can be correlated with those obtained in previous results for other Ln-POMs.20,21

To obtain more information about the spectral properties of the EuIII compound, the experimental intensity Ω2 and Ω4 parameters, the radiative emission rates (Arad), the radiative lifetime (τrad) and the branching ratios β02 and β04 were calculated. This was done by comparing the emission data associated with the 5D07F2 and 5D07F4 transitions to the MD transition 5D07F1, using a Judd–Ofelt (JO) theory.54,55

The JO theory states that the Ω2 intensity parameter is related to the LnIII site symmetry and could be interpreted as a consequence of a hypersensitive behaviour of the 5D07F2 transition, and that the spontaneous emission probabilities for EuIII transitions 5D07F0,1,2,3,4 could be expressed by the equation:52

 
image file: d0qi00232a-t1.tif(1)
where e is the electron charge, n0 is the index of refraction of the host, ω is the frequency of the transition, ħ is the Planck's constant, Ωλ are the Judd–Ofelt intensity parameters54,55 and 〈7FJ||U(λ)||5D02 is the reduced matrix element for λ = J = 0, 2 and 4, given by Carnall et al.56 The total radiative decay rate, Arad, for the particular case involving the EuIII ion is written in terms of the integrated area of their emission spectra I0J as:
 
image file: d0qi00232a-t2.tif(2)
where A01 is the spontaneous decay rate for the 5D07F1 transition given by A01 = A01n3 with A01 = 14.65 s−1 in a vacuum and I0J is the area of the emission curves. The intensity parameters Ωλ could be calculated with the relation:6
 
image file: d0qi00232a-t3.tif(3)
with λ = J = 0, 2 and 4. The reduced matrix elements: 〈5D0||U(λ)||7F22 = 0.0032 and 〈5D0||U(λ)||7F42 = 0.0023 were taken from a report by Carnall et al.56 The predicted radiative lifetime τrad is given by the inverse of the total area under the emission curves, τrad = 1/Arad, and the branching ratios are given by β0J = A0J/Arad, with J′ = 1, 2 or 4. As the 5D07F6 transition was not observed, Ω6 cannot be estimated from the experiment. The evolution of the temperature dependence of the obtained JO intensity parameters Ω2 and Ω4 is shown in Fig. 3. The radiative emission rates (Arad), the radiative lifetime (τrad) and the branching ratios β02 and β04 are presented in Fig. S12a and b. All these parameters are nearly independent of the temperature. In Table 1 some values of these parameters for selected temperatures are summarized, revealing relatively high values of the Ω2 parameter, which tends to Ω2 > Ω4 throughout the temperature range. The large Ω2 values could be associated with the distortion of the site symmetry of the EuIII compound (short range effect) and/or moderate covalence of the metal–ligand bonds,57 as shown by the crystallographic data. The Ω2 and Ω4 values can be compared with the [Eu(DPA)3]3− (DPA = dipicolinate) complex data: Ω2 = 10.5 × 10−20 cm2 and Ω4 = 5.31 × 10−20 cm2.57 The radiative lifetimes (τrad) of LM4-1-Eu are 2.9(3) at 17 K and 2.7(2) ms at 300 K, which can be compared with those of the fully organic Cs3[Eu(DPA)3] complex with a τrad = 2.6 ms and also with several other organic based EuIII complexes with benzothiazole-, benzoxazole-, and benzimidazole-pyridine ligands (2.7 to 6.8 ms).58,59 These results suggest that the hybrid organic–inorganic EuIII complex has a similar luminescence efficiency compared to the organic EuIII complexes. Analysing the β0J branching ratios (see Fig. S12b), β01 = 17%, β02 = 79% and β04 = 4% (300 K), follow the order of relative intensities 5D07F2 > 7F1 > 7F4, in the entire temperature range.


image file: d0qi00232a-f3.tif
Fig. 3 Judd–Ofelt Ω2 and Ω4 intensity parameters as a function of temperature for LM4-1-Eu.
Table 1 Radiative decay rates Arad, lifetime τrad, Ω2 and Ω4 Judd–Ofelt parameters and β0J branching ratios for selected temperatures
LM4-1-Eu
Temp. Arad τrad Ω2 Ω4 β01 β02 β04
[K] [s−1] [ms] [10−20 cm2] [%]
17 344.1 2.91 7.17 0.65 18 78 4
50 345.2 2.90 7.22 0.64 18 78 4
140 346.6 2.89 7.25 0.66 18 78 4
240 353.9 2.83 7.42 0.69 17 78 5
300 362.0 2.73 7.64 0.68 17 79 4


LM4-1-Nd

The photoluminescence spectrum in the UV-VIS and NIR regions for the LM4-1-Nd complex is displayed in Fig. 2. The emission spectra present a large and intense band with a maximum at 408 nm, when excited at wavelengths of 330 nm or 350 nm, which can be attributed to the emission of the 1,10-phenanthroline and LPOM ligands, whereas no emission of the NdIII ions in this region is observed (Fig. 2g). The NIR spectrum for LM4-1-Nd under an excitation at 804 nm (200 mW power from a diode laser), associated with 4F3/24I11/2, shows emissions at 890, 1066 and 1343 nm, attributed to the 4F3/24I9/2, 4F3/24I11/2 and 4F3/24I13/2 transitions (Fig. 2h), respectively.56 Similar results have been observed for other Nd-POM systems.42

Fig. 2h shows the NIR emission spectra for LM4-1-Nd as a function of temperature (20 to 300 K) under excitation at 804 nm. All the spectral data are normalized with respect to the 4F3/24I11/2 transition intensity taken at 20 K. The intensity of the transition 4F3/24I11/2 is nearly temperature independent, in contrast to the intensity of the 4F3/24I13/2 transition that presents a great temperature dependency. No change in the position of the barycentre of these transitions is observed in the whole temperature range.

LM4-1-Er and LM4-1-Ho complexes

The LM4-1-Er emission spectrum (Fig. S13a) presents two weak emission bands at 546 and 614 nm attributed to the ErIII 4S3/24I15/2 and 4S3/24I15/2 transitions, respectively, but does not exhibit emission in the IR region (800–1600 nm) under any excitation or temperature. The emission spectrum of LM4-1-Ho is depicted in Fig. S13b, in the range of 450–850 nm, exciting directly the HoIII 3K8(I1′) energy level,44 observing three intense broad bands centred at 510, 552 and 569 nm, and two other small bands at 649 and 660 nm. The high energy bands are attributed to the emission from the two types of ligands and the bands at lower energy are assigned to the 5F55I8 transition. As observed for LM4-1-Er no emission in the infrared region was also observed for LM4-1-Ho.

Energy transfer between ligands and LnIII ions

Latva et al.60 have proposed an empirical rule that establishes the optimal conditions for an efficient energy transfer from a ligand to a metal centre based on the energy difference ΔE between the triplet excited state (T1) and the excited state of LnIII ions. This approach has been used by other authors, by defining regions depending on the LnIII ions, for EuIII from 2000 to 5000 cm−1, for DyIII from 2500 to 4500 cm−1 and for TbIII 2000–5000 cm−1.61–63 The emission spectrum of LM4-1-Gd was used to distinguish the emission bands of the ligands (both the organic and inorganic ones) (Fig. S8). It consists of a broad and intense band centred at 435 nm (22[thin space (1/6-em)]988 cm−1). Since the energy absorbed by the ligands could not be transfered to the 6P7/2 energy level at ∼312.5 nm (32[thin space (1/6-em)]000 cm−1) of the GdIII ions, the triplet state of the ligand is determined by the lower emission band observed in the GdIII spectrum.25

For these calculations it has been assumed that the energy transfer process occurs mainly through the phen ligand since it has the most intense emission band compared to the inorganic ligand. The energy transfer level schemes for LM4-1-Dy, LM4-1-Tb, LM4-1-Eu and LM4-1-Nd are shown in Fig. 4. The triplet energy level of the ligand is higher than that of the lowest EuIII excited state at 579 nm (17[thin space (1/6-em)]267 cm−1), ΔE[T15D0] = 5721 cm−1, the 4F9/2 state of DyIII at 479 nm (20[thin space (1/6-em)]877 cm−1), ΔE[T14F9/2] = 2111 cm−1, and to the 5D4 TbIII energy level at 486 nm (20[thin space (1/6-em)]576 cm−1), ΔE[T15D4] = 2412 cm−1. These results indicate that the emission mechanisms of EuIII, DyIII and TbIII compounds correspond to a ligand sensitized photoluminescence process, the so called antenna-effect being in agreement with the difference between the intensity of the ligand and the LnIII bands observed in the excitation spectra of LM4-1-Dy, LM4-1-Tb and LM4-1-Eu depicted in Fig. 2a–c–e.5 The large experimental ΔE values indicate that, for LM4-1-Dy, LM4-1-Tb and LM4-1-Eu, the phen ligand can sensitize more adequately the lanthanoid ions avoiding any energy back transfer in the process.64


image file: d0qi00232a-f4.tif
Fig. 4 The energy level diagram and energy transfer schemes for LM4-1-Dy, LM4-1-Tb, LM4-1-Eu and LM4-1-Nd. ISC and NRR refer to the intersystem crossing and non-radiative relaxation, respectively.

It is also possible to elucidate the partial energy level scheme for NdIII explaining the emission and excitation of LM4-1-Nd (Fig. 4). It can be inferred that the emission mechanism is a direct excitation of the 4F9/2 energy level followed by a non-radiative decay to the 4F3/2 level which leads to the three emissions observed, 4F3/24I13/2, 4I13/2 and 4I13/2. No NdIII emission was observed by exciting the ligand.

The relatively low intensity of the emissions observed (or the absence of these emissions) for LM4-1-Dy, LM4-1-Tb and LM4-1-Eu, or no emission in the case of M4-1-Nd, in the visible region taken at any temperature, is in accordance with the experimental data reported by Ritchie et al.65 for dinuclear and octanuclear TbIII and EuIII ternary lanthanoid-organic-polyoxometalate (Ln-org-POM) complexes, based on [As2W19O67(H2O)]14− and 2-picolinic acid (picH). This is attributed to different relaxation mechanisms such as non-radiative deactivation, and charge-transfer between the excited states of the ligands and the emitting Ln energy levels. Even the nature of the LPOM and the organic ligands play an important role in the photophysical properties of these materials and their applications as temperature sensing systems.

Luminescence lifetimes and colour coordinates

The luminescence lifetimes of the excited state of EuIII (5D0), DyIII (4F9/2) and TbIII (5D4) ions were estimated from the decay curves shown in Fig. 5. It was obtained by monitoring the emissions at 614, 571, and 543 nm corresponding to the 5D07F2, 4F9/26H13/2 and 5D47F5 transitions from EuIII, DyIII and TbIII ions, respectively. The decay curves were fitted with one- and two-exponential functions, obtaining the best results by using the bi-exponential expression I(t) = A1[thin space (1/6-em)]exp(−t/τ1) + A2[thin space (1/6-em)]exp(−t/τ2) + I0, where Ai represent the integrated areas, τ1 and τ2 are the decay components fast and slow, respectively, and I0 is the intensity at t = 0. The average lifetime τAV can be calculated by using the formula τAV = (A1τ12 + A2τ22)/(A1τ1 + A2τ2).66 The results of τAV, τi and Ai are given in Table 2 for LM4-1-Dy, LM4-1-Tb and LM4-1-Eu. The non mono-exponential decay results indicate that two types of transitions are involved in the observed emissions, as pointed by Priya et al.67 The short and long lifetimes correspond to two different mechanisms: the energy transfer from the ligands to the LnIII ions and to the sensitized emission of LnIII, respectively. At room temperature the average lifetime τAV of the multiplet 4F9/2, 5D0, and 5D4 for DyIII, EuIII and TbIII, respectively, was calculated to be 4.1(7) μs, 0.94(3) ms and 0.35(2) ms. These values are in good agreement with other reported Ln-POM systems.28,68,69
image file: d0qi00232a-f5.tif
Fig. 5 Luminescence decay curves for LM4-1-Dy (a), LM4-1-Tb (b) and LM4-1-Eu (c) complexes. Symbols and solid lines represent the experimental and theoretical data respectively.
Table 2 Photometric parameters: experimental lifetimes τ of the fitted decay curves, CIE (x,y) coordinates, CCT (K) and colour purity (CP) values for EuIII, TbIII and DyIII complexes
Sample T (K) τ1 (ms) τ2 (ms) τav (ms) CIE CCT (K) CP (%)
(A1) (A2)
LM4-1-Eu 300 1.05(1) 0.45(1) 0.94(3) (0.631, 0.364) 1897 84
  (81.6%) (18.4%)        
17       (0.629, 0.368) 1846 83
 
LM4-1-Tb 300 0.432(8) 0.123(2) 0.353(6) (0.185, 0.139) n.d. 48
  (75.1%) (24.9%)        
17       (0.201, 0.176) n.d. 42
 
LM4-1-Dy 300 0.011(2) 0.003(1) 0.0041(7) (0.218, 0.177) n.d. 52
  (18.8%) (81.2%)        
17       (0.225, 0.191) n.d. 45


For example, these values are practically the same as for [Eu(W5O18)2]9−, with a value of 2.8 ms at 300 K, and approximately 3 times higher than [Eu3(H2O)3(SbW9O33)(W5O18)3]18− compound, with a value of 1.1 ms in all the temperature range (4.2 to 300 K).70 Moreover, the obtained values for LM4-1-Tb and LM4-1-Eu are greater than those reported by Wang et al.24 for Lindqvist type POMs containing the 6-peroxoniobio-4-phosphate building block, [LnIII(H2O)6][H4(NbO2)6P4O24nH2O (Ln = Dy, Eu, Tb), where the LnIII ions are coordinated with eight oxygens, six of them being from water molecules, τTb ≈ 0.018 ms and τEu ≈ 0.148 ms.

The intrinsic quantum efficiency (η) for LM4-1-Eu was calculated through the ratio of τAV and τrad obtained from the Judd–Ofelt theory,71 and the value is ∼34.1%. The internal and external quantum yields (iQy and eQy) of the 5D07FJ emission of LM4-1-Eu under 394 nm excitation are 4.3 and 1.7%, respectively. These QY values are associated with the energy gap of EuIII 5D07F4 (712 nm, 14[thin space (1/6-em)]045 cm−1) that matches with the overtones of C–H and O–H vibration frequencies, 3ν ≈ 13[thin space (1/6-em)]953 cm−1, that promote a small emission quantum yield and a temperature-dependent luminescence, as observed for LM4-1-Eu, similar analysis can be made for both LM4-1-Dy and LM4-1-Tb compounds. The quantum yield values obtained for LM4-1-Eu are quite higher compared to those of fully inorganic systems like [Eu(W5O18)2]9− and [Eu3(H2O)3(SbW9O33)(W5O18)3]18− compounds, with a QY of ca. 1%.70

The CIE 1931 (Commission International d'Eclairage)72 diagram is a universal method for studying all the possible colours using the photoluminescence spectra and the changes in the intensity of the emission bands by determining the chromaticity coordinates (x,y). These coordinates are usually employed to distinguish the precision emission colours. For DyIII and TbIII complexes the (x,y) chromaticity coordinates lie in the blueish region, respectively, (0.215, 0.174) and (0.175, 0.125), while for the EuIII complex (0.631, 0.364) lies in the deep red region of the CIE diagram, near to the coordinates for ideal red phosphors (Fig. 6 and Table 2).73 These values can be compared with the {[Ln2(DMF)8(H2O)6][ZnW12O40]}·4DMF complex reported by Zhao et al.,74 whereas they report that for the TbIII analogue, the coordinates lie in the green region in contrast to that reported in this work that lies in the blue region. This could be because the emission spectrum for LM4-1-Dy and LM4-1-Tb complexes presents an important contribution coming from the organic and inorganic ligand emission, absent in the {[Tb2(DMF)8(H2O)6][ZnW12O40]}·4DMF compound.


image file: d0qi00232a-f6.tif
Fig. 6 CIE 1931 chromaticity coordinate diagram in LM4-1-Ln (LnIII = EuIII, DyIII and TbIII) complexes vs. temperature.

The colour purity of complexes LM4-1-Dy, LM4-1-Tb and LM4-1-Eu was calculated by using the equation:75

 
image file: d0qi00232a-t4.tif(4)
where CP is the colour purity, (x,y) for each sample given in Table 2, (xi,yi) = (0.333,0.333) and (xd,yd) are, respectively, the colour coordinates of the overall light emitted by each [LnH(PW11O39)(phen)2]3− complex, the standard white light and the dominant wavelength point, (0.688,0.331) for red, (0.15,0.06) for blue and (0.29,0.60) for green colour.

The calculated CP values are 47.6%, 52.0% and 84.6% for LM4-1-Dy, LM4-1-Tb and LM4-1-Eu, respectively. The CP value found for LM4-1-Dy is very close to the one reported by Wu et al.69 for [N(CH3)4]6K3H7[Dy(C4H2O6)(PW11O39)]2, with a CP value of ca. 43.35%. The correlated colour temperature (CCT) values were calculated, using the McCamy formula: CCT = −437n3 + 360n2 − 6861n + 5514.31, where n = (xxc)/(yyc), x and y being the chromaticity coordinates, and xc = 0.3320 and yc = 0.1858 being the coordinates of the chromaticity epicentre extracted from a report by McCamy et al.76 The found values for EuIII range between 1846 K at 17 K and 1897 K at 300 K. For DyIII and TbIII it was not possible to determine this parameter.

Thermometric studies

The temperature dependency of the ratio between the LnIII emission intensity and the ligand emission intensity, related to the phen and LPOM, for LM4-1-Dy and LM4-1-Tb is analysed from 20 K to 300 K. For LM4-1-Dy (Fig. 7a), in the range of 120 to 300 K, the ratio between the intensity of the emission associated with the DyIII transition 4F9/26H13/2 at 479 nm and the intensity of the ligand emission image file: d0qi00232a-t5.tif shows a decrease in this temperature range, and between the 20 to 120 K range, the intensity ratio value becomes almost constant. For the temperature dependency of the ratio between the intensity of the 4F9/26H15/2 transition (at 571 nm) and the intensity of the ligand emission image file: d0qi00232a-t6.tif a similar trend is observed. The linear parts follow these equations: image file: d0qi00232a-t7.tif and image file: d0qi00232a-t8.tif. Thus, the ratios of both DyIII transitions decrease with, approximately, the same relative sensitivity ca. ∼ 0.27% per Kelvin.
image file: d0qi00232a-f7.tif
Fig. 7 Temperature dependent intensity ratios of (a) DyIII 4F9/26H15/2,13/2 transitions to ligands (b) TbIII 5D47F5,6 transitions to ligands, (c) 7F0,2,4 by 7F1 from EuIII 5D07F0,1,2,4 transitions and (d) NdIII: 4I11/2/4I13/2 emissions. Solid lines in (a), (b) and (d) are the best linear fits.

For the LM4-1-Tb complex, the ratios were calculated between the intensities of the TbIII 5D47F5,6 transitions (at 486 and 542 nm, respectively) and the intensity of the ligand emission image file: d0qi00232a-t9.tif, Fig. 7b. The results show a linear decrease over the entire temperature range, which follows these equations: image file: d0qi00232a-t10.tif and image file: d0qi00232a-t11.tif, indicating that for LM4-1-Tb the relative sensitivities are 0.03 and 0.07% per K. These results are in contrast to the obtained intensity ratios for LM4-1-Dy, where the rates for the two transitions are similar. One possible explanation for this difference is the relative sensitivity of LM4-1-Dy compared to the LM4-1-Tb sensitivity, since for LM4-1-Tb the emission band of both ligands is more intense, broad and closer to the first TbIII emission line. In the case of LM4-1-Dy, for the same emission bands of the ligands the intensity is less intense than the nearest DyIII emission (see Fig. 2b, d, 7a and b). The lower emission intensities for DyIII and TbIII emission with the increase of temperature are principally due to thermal nonradiative deactivation pathways involving the LnIII energy levels and the excited state from the two types of ligands of the complexes.

For the LM4-1-Dy compound the IDyIII/Iligand ratios indicate that the intensity of the 4F9/26H13/2 and 4F9/26H15/2 transitions decreases by ∼35% from 140 to 300 K. For LM4-1-Tb, the ITbIII/Iligand ratios, from 20 to 300 K, indicate that the transitions at 486 and 572 nm decrease by 32 and 10%, respectively. These results indicate an important role in the temperature response for both monitored DyIII transitions (4F9/26H15/2,13/2) and for the 5D47F5 transition at 486 nm of TbIII, in accordance with the CIE chromaticity diagram (Fig. 6), where it can be observed that LM4-1-Dy and LM4-1-Tb show small CIE coordinates temperature dependence.

For LM4-1-Nd, the NdIII transitions (4F3/24I11/2,13/2) do not present overlap and were used to evaluate the temperature response of NdIII emission. The ratio image file: d0qi00232a-t12.tif decreases linearly from 20 to 300 K, and can be fitted with the linear function: image file: d0qi00232a-t13.tif, i.e., the relative sensitivity of the image file: d0qi00232a-t14.tif intensity from 20 to 300 K is 0.95%, being the highest among the studied complexes (Fig. 7d).

The obtained relative sensitivities of LM4-1-Dy, LM4-1-Tb, and LM4-1-Nd can be compared with those of the [EuW10O36]9− polyoxometalate reported by Salomon et al.77 with a relative sensitivity of 0.26% K−1. As reported by Xu et al.,78 the thermometric mechanisms could be explained by the fact that as the temperature increases, the number of molecules in the excited state energy level will gradually increase in proportion, while the ratio of the number of molecules in the excited state of the low energy level will reduce. Thus, the intensity ratios of the LnIII bands with the ligand band undergo a remarkable decrease as the temperature increases.

Finally, the LM4-1-Dy, LM4-1-Tb, and LM4-1-Nd compounds have the necessary requirement to have potential applications as temperature sensors. Furthermore, LM4-1-Dy and LM4-1-Tb have an energy gap between the emitting LnIII levels and the excite sates of the ligands, in the range of 200 to 2000 cm−1, avoiding any overlap between them.2

Experimental

Materials and reagents

FTIR-ATR (Fourier Transform Infrared-Attenuated Total Reflectance) spectra (4000–400 cm−1) of the compounds were obtained using a Jasco FTIR-4600 spectrophotometer equipped with an ATR PRO ONE (Jasco, Easton, MD, USA), Fig. S1. Elemental analyses (C, N, H) of bulk samples were performed using a Thermo elemental analyser Flash 2000. The Ln[thin space (1/6-em)]:[thin space (1/6-em)]P[thin space (1/6-em)]:[thin space (1/6-em)]W ratios of the bulk samples were estimated by electron probe microanalysis (EPMA) performed with a Jeol, JSM 5410 equipped with an EDAX NORAN microprobe, Table S1. Amorphous, polycrystalline or crystalline samples of all compounds were lightly ground with a pestle in an agate mortar and filled into 0.5 mm borosilicate capillaries prior to being mounted and aligned on an Empyrean PANalytical powder diffractometer, using Cu Kα radiation (λ = 1.54056 Å). For each sample, two or three repeated measurements were collected at room temperature (2θ = 2–40°) and merged into a single diffractogram.

Synthesis of [n-NBu4]3[LnH(PW11O39)(phen)2]·[H2O]

All chemical reagents were directly used without further purification. [n-NBu4]4[PW11O39H3] was synthesized according to a previously reported method.79 Hydrothermal synthesis was done using a Parr reactor of 23 ml model 4749.

The corresponding hydrated LnIII acetates (0.1 mmol), LnAc3·XH2O where LnIII = DyIII (LM4-1-Dy), TbIII(LM4-1-Tb), EuIII(LM4-1-Eu), NdIII(LM4-1-Nd), ErIII(LM4-1-Er), HoIII(LM4-1-Ho) and GdIII(LM4-1-Gd) were mixed with [NBu4]4[PW11O39H3] (365 mg, 0.1 mmol) and phenanthroline (0.036 mg, 0.2 mmol) in 10 mL of water in a Parr reactor and heated under autogenous pressure at 160 °C for 48 hours. The reaction mixture was filtered off, and pale pink crystals of LM4-1-Ln, suitable for X-ray diffraction, were obtained by mechanical separation. Then, these crystals were washed with water and acetone. For more information see section S1 and S2 in the ESI.

Physical measurements

The single crystals obtained were mounted on the tip of a glass fibre. The intensities for LM4-1-Ln were recorded on a Bruker Smart Apex diffractometer, using separations of 0.3° between frames and 10 s by frame. Datasets were reduced by using SAINTPLUS,80 while the structure was solved by direct methods and completed by Difference Fourier Synthesis. Least-squares refinement was conducted by using SHELXL.81,82 All atoms were anisotropically refined. However, the N and C atoms of the tetra-n-butylammonium cations were isotropically refined. Hydrogen atom positions were calculated after each cycle of refinement with SHELXL using a riding model for each structure, with a C–H distance of 0.93 Å or 0.97 Å. Uiso(H) values were set equal to 1.2Ueq of the parent carbon atom. Additional crystallographic and refinement details are given in Table X. Structural drawings were carried out with DIAMOND-3.2k, supplied by Crystal Impact.83 Crystallographic data for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication number CCDC 1951517 for 1-Tb, 1962564 for 1-Eu, 1962563 for 1-Nd and 1951518 for 1-Ho. The crystallographic data of LM4-1-Dy, LM4-1-Er and LM4-1-Gd have been reported previously.27

Optical measurements

Solid-state absorption spectra were obtained on a PerkinElmer Lambda 1050 spectrometer, operating in the 350 to 2500 nm spectral range, making a KBr pellet with LM4-1-Ln complexes. Photoluminescence (PL) emission spectra of the solid samples were obtained using a Horiba-Jobin Yvon spectrofluorimeter, Model Fluorolog-3 (FL3-221), under excitation with a 450 W Xe lamp and Horiba PPD-850 picosecond photon detector in the UV-VIS region and an InGaAs detector in the infrared region. The excitation and emission slits used were of 1.15 nm. PL emission was corrected for the spectral response of the monochromators and the detector using a typical correction spectrum provided by the manufacturer. The luminescence decay curves were obtained by using a 50 W Xe-pulse lamp. Low-temperature spectra were obtained, using a closed cycle cryostat model CS202AI-X15 (ARS Cryo) monitoring the temperature with a Lake Shore model 332 controller. The quantum yield (QY) of the LnIII emission of all complexes was acquired using an integrating sphere (Quanta-φ equipment, F3029, Horiba Jobin Yvon) of Spectralon® coupled by means of optical fibers. The internal and external QYs were calculated following the method developed by Wrighton et al.84 The internal and external photoluminescence quantum yields (Φ) were determined with the FluorEssence V3.5 software that compares the number of photons emitted by the sample with the number of reflected photons from the reflection standard (Spectralon®).85 For IR emission measurements a diode laser (Crystal Laser LC) with a power of 200 mW was used.

Conclusions

In summary, this article describes the synthesis, crystal structure and photophysical properties of a family of mononuclear hybrid organic–inorganic lanthanoid complexes (Ln = DyIII, TbIII, EuIII, NdIII, ErIII, HoIII and GdIII). All LnIII ions are eight-coordinated with 4 O atoms from the lacunary Keggin POM [PW11O39]7− and 4 N atoms from the phen molecules, with a square-antiprism geometry (pseudo D4d symmetry). As the minor distance between LnIII ions is ∼10 Å the energy transfer process by the exchange mechanism is almost absent, so the principal mechanism occurs through the ligands and the emitting levels of LnIII ions. The thermal-dependence of the luminescence intensity ratios Y/B, R/O and G/B of LM4-1-Eu, LM4-1-Eu and LM4-1-Tb, respectively, indicates that these compounds present a thermal structural stability and lanthanoid emissions within the studied temperature range. On the other hand, the ratios between lanthanoid and ligand emissions change as a function of temperature giving the possibility of being used as thermosensors with good sensitivity (from 140 K to 300 K for the LM4-1-Dy complex, and from 20 K to 300 K for the LM4-1-Tb and LM4-1-Nd complexes). The CIE coordinates and the high colour purity values for LM4-1-Tb and LM4-1-Eu show that these compounds are good candidates to be applied as red and blue components of WLEDs. Also, for the LM4-1-Eu complex the Judd–Ofelt intensity parameters were determined from the emission spectra. The obtained value for the Ω2 parameter suggests a moderate covalent degree of the metal–ligand bonds. The temperature luminescence studies of LM4-1-Eu show that the intensity, CIE coordinates and CCT values do not change in the temperature range of 20 to 300 K, and a quantum yield of 4.3% is obtained, which is four times larger than that of the fully inorganic analogue, conferring to this material interesting characteristics for applications as active media in red OLEDs or even in the catalytic and biological imaging fields.

Abbreviations

phen1,10-Phenanthroline
POMPolyoxometalate
MOFMetal organic framework
LPOMLacunary polyoxometalate
DMFDimethylformamide
picH2-Picolinic acid
iQYInternal quantum yield
eQYExternal quantum yield
CIECommission International d'Eclairage
CCTCorrelated colour temperature
OLEDOrganic light-emitting diode
WLEDWhite light-emitting diode
dpaDipicolinate

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

W. C.-M. thanks FONDECYT 11160830 for financial support. The authors thank the REDI170277 project for financial support. The authors also acknowledge the partial support from Financiamiento Basal AFB180001 (CEDENNA). This work was done under the LIA-M3-1027 CNRS Collaborative Program. The present work has been also funded by the Spanish MICINN (Unidad de Excelencia “María de Maeztu” MDM-2015-0538 and project CTQ2017-89528-P), and the Generalitat Valenciana (PROMETEU/2019/066). G. M. E acknowledges MICINN for a Ramón y Cajal contract. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) – Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo à Pesquisa do Estado de Goiás (FAPEG) and Financiadora de Estudos e Projetos (FINEP) Brazilian agencies.

References

  1. Y. Hasegawa, Y. Kitagawa and T. Nakanishi, Effective Photosensitized, Electrosensitized, and Mechanosensitized Luminescence of Lanthanide Complexes, NPG Asia Mater., 2018, 10, 52–70 CrossRef CAS.
  2. Y. Hasegawa and Y. Kitagawa, Thermo-Sensitive Luminescence of Lanthanide Complexes, Clusters, Coordination Polymers and Metal–Organic Frameworks with Organic Photosensitizers, J. Mater. Chem. C, 2019, 7, 7494–7511 RSC.
  3. J. H. Kim, Y. Jung, D. Lee and W. D. Jang, Thermoresponsive Polymer and Fluorescent Dye Hybrids for Tunable Multicolor Emission, Adv. Mater., 2016, 28, 3499–3503 CrossRef CAS PubMed.
  4. M. S. Deshmukh, A. Yadav, R. Pant and R. Boomishankar, Thermochromic and Mechanochromic Luminescence Umpolung in Isostructural Metal–Organic Frameworks Based on Cu6I6 Clusters, Inorg. Chem., 2015, 54, 1337–1345 CrossRef CAS PubMed.
  5. K. Binnemans, Lanthanide-Based Luminescent Hybrid Materials, Chem. Rev., 2009, 109, 4283–4374 CrossRef CAS PubMed.
  6. L. D. Carlos, R. A. S. Ferreira, V. de Z. Bermudez and S. J. L. Ribeiro, Lanthanide-Containing Light-Emitting Organic-Inorganic Hybrids: A Bet on the Future, Adv. Mater., 2009, 21, 509–534 CrossRef CAS PubMed.
  7. D. F. Parra, A. Mucciolo and H. F. Brito, Green Luminescence System Containing a Tb3+ β-Diketonate Complex Doped in the Epoxy Resin as Sensitizer, J. Appl. Polym. Sci., 2004, 94, 865–870 CrossRef CAS.
  8. A. O. Ribeiro and O. A. Serra, Spectroscopic Study of [Tb3+(β-Diketonate)3]: Alpha-Cyclodextrin Inclusion Compounds in Aqueous Solution, J. Braz. Chem. Soc., 2007, 18, 273–278 CrossRef CAS.
  9. M. Elbanowski and B. Mąkowska, The Lanthanides as Luminescent Probes in Investigations of Biochemical Systems, J. Photochem. Photobiol., A, 1996, 99, 85–92 CrossRef CAS.
  10. J. Zhao, L. Huang, T. Liang, S. Zhao and S. Xu, Luminescent Properties of Eu3+ Doped Heavy Tellurite Scintillating Glasses, J. Lumin., 2019, 205, 342–345 CrossRef CAS.
  11. C. Boskovic, Rare Earth Polyoxometalates, Acc. Chem. Res., 2017, 50, 2205–2214 CrossRef CAS PubMed.
  12. X. Ma, W. Yang, L. Chen and J. Zhao, Significant Developments in Rare-Earth-Containing Polyoxometalate Chemistry: Synthetic Strategies, Structural Diversities and Correlative Properties, CrystEngComm, 2015, 17, 8175–8197 RSC.
  13. L.-L. Li, H.-Y. Han, Y.-H. Wang, H.-Q. Tan, H.-Y. Zang and Y.-G. Li, Construction of Polyoxometalates from Dynamic Lacunary Polyoxotungstate Building Blocks and Lanthanide Linkers, Dalton Trans., 2015, 44, 11429–11436 RSC.
  14. M. A. AlDamen, S. Cardona-Serra, J. M. Clemente-Juan, E. Coronado, A. Gaita-Ariño, C. Martí-Gastaldo, F. Luis and O. Montero, Mononuclear Lanthanide Single Molecule Magnets Based on the Polyoxometalates [Ln(W5O18)2]9− and [Ln(β2-SiW11O39)2]13− LnIII=Tb, Dy, Ho, Er, Tm, and Yb), Inorg. Chem., 2009, 48, 3467–3479 CrossRef CAS PubMed.
  15. T. Yamase, Photo- and Electrochromism of Polyoxometalates and Related Materials, Chem. Rev., 1998, 98, 307–326 CrossRef CAS PubMed.
  16. P. Ma, F. Hu, R. Wan, Y. Huo, D. Zhang, J. Niu and J. Wang, Magnetic Double-Tartaric Bridging Mono-Lanthanide Substituted Phosphotungstates with Photochromic and Switchable Luminescence Properties, J. Mater. Chem. C, 2016, 4, 5424–5433 RSC.
  17. S. Zhang, Y. Wang, J. Zhao, P. Ma, J. Wang and J. Niu, Two Types of Oxalate-Bridging Rare-Earth-Substituted Keggin-Type Phosphotungstates {[(α-PW11O39)RE(H2O)]2(C2O4)}10− and {(α-x-PW10O38)RE2(C2O4)(H2O)2}3−, Dalton Trans., 2012, 41, 3764–3772 RSC.
  18. P. Ma, R. Wan, Y. Si, F. Hu, Y. Wang, J. Niu and J. Wang, Double-Malate Bridging Tri-Lanthanoid Cluster Encapsulated Arsenotungstates: Syntheses, Structures, Luminescence and Magnetic Properties, Dalton Trans., 2015, 44, 11514–11523 RSC.
  19. R. Gupta, F. Hussain, J. N. Behera, A. M. Bossoh, I. M. Mbomekalle and P. de Oliveira, Structure, Electrochemistry and Luminescence Properties of Lanthano–Germanotungstates, RSC Adv., 2015, 5, 99754–99765 RSC.
  20. Y. Lu, Y. Li, E. Wang, X. Xu and Y. Ma, A New Family of Polyoxometalate Compounds Built up of Preyssler Anions and Trivalent Lanthanide Cations, Inorg. Chim. Acta, 2007, 360, 2063–2070 CrossRef CAS.
  21. L. Ni, F. Hussain, B. Spingler, S. Weyeneth and G. R. Patzke, Lanthanoid-Containing Open Wells–Dawson Silicotungstates: Synthesis, Crystal Structures, and Properties, Inorg. Chem., 2011, 50, 4944–4955 CrossRef CAS PubMed.
  22. J. C. G. Bünzli and S. V. Eliseeva, Photophysics of Lanthanoid Coordination Compounds, Compr. Inorg. Chem. II, 2013, 8, 339–398 Search PubMed.
  23. L. Armelao, S. Quici, F. Barigelletti, G. Accorsi, G. Bottaro, M. Cavazzini and E. Tondello, Design of Luminescent Lanthanide Complexes: From Molecules to Highly Efficient Photo-Emitting Materials, Coord. Chem. Rev., 2010, 254, 487–505 CrossRef CAS.
  24. H. Wang, J. Li, J. Sun, Y. Wang, Z. Liang, P. Ma, D. Zhang, J. Wang and J. Niu, Synthesis, Structure, and Luminescent Properties of a Family of Lanthanide-Functionalized Peroxoniobiophosphates, Sci. Rep., 2017, 7, 10653 CrossRef PubMed.
  25. G. Shao, Y. Li, K. Feng, F. Gan and M. Gong, Diphenylethyne Based β-Diketonate Europium(III) Complexes as Red Phosphors Applied in LED, Sens. Actuators, B, 2012, 173, 692–697 CrossRef CAS.
  26. S.-S. Wang and G.-Y. Yang, Recent Advances in Polyoxometalate-Catalyzed Reactions, Chem. Rev., 2015, 115, 4893–4962 CrossRef CAS PubMed.
  27. W. Cañón-Mancisidor, M. Zapata-Lizama, P. Hermosilla-Ibáñez, C. Cruz, D. Venegas-Yazigi and G. Mínguez Espallargas, Hybrid Organic–Inorganic Mononuclear Lanthanoid Single Ion Magnets, Chem. Commun., 2019, 55, 14992–14995 RSC.
  28. D. Zhang, C. Zhang, H. Chen, P. Ma, J. Wang and J. Niu, Structures and Properties of Dimeric Rare Earth Derivatives Based on Monovacant Keggin-Type Polyoxotungstates, Inorg. Chim. Acta, 2012, 391, 218–223 CrossRef CAS.
  29. J. Niu, K. Wang, H. Chen, J. Zhao, M. Pengtao, J. Wang, M. Li, Y. Bai and D. Dang, Assembly Chemistry between Lanthanide Cations and Monovacant Keggin Polyoxotungstates: Two Types of Lanthanide Substituted Phosphotungstates [{(α-PW11O39H)Ln(H2O)3}2]6− and [{(α-PW11O39)Ln(H2O)(η2,μ-1,1)-CH3COO}2]10−, Cryst. Growth Des., 2009, 9, 4362–4372 CrossRef CAS.
  30. L. Xiao, T.-T. Zhang, Z. Liu, X. Shi, H. Zhang, L. Yin, L.-Y. Yao, C.-C. Xing and X.-B. Cui, Syntheses and Characterizations of the First N-Containing Organic Ligand Functionalized Mono-Lanthanide-Substituted Polyoxometalates, Inorg. Chem. Commun., 2018, 95, 86–89 CrossRef CAS.
  31. P. Ma, F. Hu, Y. Huo, D. Zhang, C. Zhang, J. Niu and J. Wang, Magnetoluminescent Bifunctional Dysprosium-Based Phosphotungstates with Synthesis and Correlations between Structures and Properties, Cryst. Growth Des., 2017, 17, 1947–1956 CrossRef CAS.
  32. D. Casanova, J. Cirera, M. Llunell, P. Alemany, D. Avnir and S. Alvarez, Minimal Distortion Pathways in Polyhedral Rearrangements, J. Am. Chem. Soc., 2004, 126, 1755–1763 CrossRef CAS PubMed.
  33. D. Casanova, M. Llunell, P. Alemany and S. Álvarez, The Rich Stereochemistry of Eight-Vertex Polyhedra: A Continuous Shape Measures Study, Chem. – Eur. J., 2005, 11, 1479–1494 CrossRef CAS PubMed.
  34. F. J. Kettles, V. A. Milway, F. Tuna, R. Valiente, L. H. Thomas, W. Wernsdorfer, S. T. Ochsenbein and M. Murrie, Exchange Interactions at the Origin of Slow Relaxation of the Magnetization in {TbCu3} and {DyCu3} Single-Molecule Magnets, Inorg. Chem., 2014, 53, 8970–8978 CrossRef CAS PubMed.
  35. J. Zhu, C. Wang, F. Luan, T. Liu, P. Yan and G. Li, Local Coordination Geometry Perturbed β-Diketone Dysprosium Single-Ion Magnets, Inorg. Chem., 2014, 53, 8895–8901 CrossRef CAS PubMed.
  36. M. M. Hänninen, A. J. Mota, D. Aravena, E. Ruiz, R. Sillanpää, A. Camón, M. Evangelisti and E. Colacio, Two C3-Symmetric Dy3III Complexes with Triple Di-μ-Methoxo-μ-Phenoxo Bridges, Magnetic Ground State, and Single-Molecule Magnetic Behavior, Chem. – Eur. J., 2014, 20, 8410–8420 CrossRef PubMed.
  37. X. Li, D. Y. Wei, S. J. Huang and Y. Q. Zheng, Syntheses and Characterization of Novel Lanthanide Adamantine-Dicarboxylate Coordination Complexes, J. Solid State Chem., 2009, 182, 95–101 CrossRef CAS.
  38. X. Q. Zhang, M. S. Lin, B. Hu, W. Q. Chen, L. N. Zheng, J. Wu, Y. M. Chen, F. Y. Zhou, Y. H. Li and W. Li, Anionic Lanthanide Complexes Supported by a Pyrrole-Based Tetradentate Schiff Base Ligand: Synthesis, Structures and Catalytic Activity toward the Polymerization of ε-caprolactone, Polyhedron, 2012, 33, 273–279 CrossRef CAS.
  39. R. Faoro, F. Moglia, M. Tonelli, N. Magnani and E. Cavalli, Energy Levels and Emission Parameters of the Dy3+ Ion Doped into the YPO4 Host Lattice, J. Phys.: Condens. Matter, 2009, 21, 275501 CrossRef CAS PubMed.
  40. W. T. Carnall, G. L. Goodman, K. Rajnak and R. S. Rana, Systematic Analysis of the Spectra of the Lanthanides Doped into Single Crystal LaF3, J. Chem. Phys., 1989, 90, 3443–3457 CrossRef CAS.
  41. A. A. S. da Gama, G. F. de Sá, P. Porcher and P. Caro, Energy Levels of Nd3+ in LiYF4, J. Chem. Phys., 1981, 75, 2583–2587 CrossRef CAS.
  42. S. But, S. Lis, R. Van Deun, T. N. Parac-Vogt, C. Görller-Walrand and K. Binnemans, Spectroscopic Properties of Neodymium(III)-Containing Polyoxometalates in Aqueous Solution, Spectrochim. Acta, Part A, 2005, 62, 478–482 CrossRef PubMed.
  43. W. F. Krupke and J. B. Gruber, Absorption and Fluorescence Spectra of Er3+ (4f11) in LaF3, J. Chem. Phys., 1963, 39, 1024–1030 CrossRef CAS.
  44. H. H. Caspers, H. E. Rast and J. L. Fry, Absorption, Fluorescence, and Energy Levels of Ho3+ in LaF3, J. Chem. Phys., 1970, 53, 3208–3216 CrossRef CAS.
  45. R. H. Linnell and A. Kaczmarczyk, Ultraviolet Spectra of [ILL] Compounds, J. Phys. Chem., 1961, 65, 1196–1200 CrossRef CAS.
  46. G. Accorsi, A. Listorti, K. Yoosaf and N. Armaroli, 1,10-Phenanthrolines: versatile building blocks for luminescent molecules, materials and metal complexes, Chem. Soc. Rev., 2009, 38, 1690–1700 RSC.
  47. Z. Li, L.-D. Lin, D. Zhao, Y.-Q. Sun and S.-T. Zheng, A Series of Unprecedented Linear Mixed-Metal-Substituted Polyoxometalate Trimers: Syntheses, Structures, Luminescence, and Proton Conductivity Properties, Eur. J. Inorg. Chem., 2019, 2019, 437–441 CrossRef CAS.
  48. S. Cotton, Lanthanide and Actinide Chemistry, John Wiley & Sons, Ltd, Chichester, UK, 2006 Search PubMed.
  49. R. T. Moura, A. N. Carneiro Neto, R. L. Longo and O. L. Malta, On the Calculation and Interpretation of Covalency in the Intensity Parameters of 4f–4f Transitions in Eu3+ Complexes Based on the Chemical Bond Overlap Polarizability, J. Lumin., 2016, 170, 420–430 CrossRef CAS.
  50. N. Vijaya, K. Upendra Kumar and C. K. Jayasankar, Spectrochim., Dy3+-Doped Zinc Fluorophosphate Glasses for White Luminescence Applications, Spectrochim. Acta, Part A, 2013, 113, 145–153 CrossRef CAS PubMed.
  51. K. Binnemans, Interpretation of Europium(III) Spectra, Coord. Chem. Rev., 2015, 295, 1–45 CrossRef CAS.
  52. L. D. Carlos, R. A. Sá Ferreira, V. de Zea Bermudez, C. Molina, L. A. Bueno and S. J. L. Ribeiro, White Light Emission of Eu3+ Based Hybrid Xerogels, Phys. Rev. B: Condens. Matter Mater. Phys., 1999, 60, 10042–10053 CrossRef CAS.
  53. F. A. de Jesus, S. T. S. Santos, J. M. A. Caiut and V. H. V. Sarmento, Effects of Thermal Treatment on the Structure and Luminescent Properties of Eu3+ Doped SiO2−PMMA Hybrid Nanocomposites Prepared by a Sol–Gel Process, J. Lumin., 2016, 170, 588–593 CrossRef CAS.
  54. B. R. Judd, Optical Absorption Intensities of Rare-Earth Ions, Phys. Rev., 1962, 127, 750–761 CrossRef CAS.
  55. G. S. Ofelt, Intensities of Crystal Spectra of Rare-Earth Ions, J. Chem. Phys., 1962, 37, 511–520 CrossRef CAS.
  56. W. T. Carnall, P. R. Fields and B. G. Wybourne, Spectral Intensities of the Trivalent Lanthanides and Actinides in Solution. I. Pr3+, Nd3+, Er3+, Tm3+, and Yb3+, J. Chem. Phys., 1965, 42, 3797–3806 CrossRef CAS.
  57. K. Binnemans, K. Van Herck and C. Görller-Walrand, Influence of Dipicolinate Ligands on the Spectroscopic Properties of Europium(III) in Solution, Chem. Phys. Lett., 1997, 266, 297–302 CrossRef CAS.
  58. A. Aebischer, F. Gumy and J.-C. G. Bünzli, Intrinsic Quantum Yields and Radiative Lifetimes of Lanthanide Tris(Dipicolinates), Phys. Chem. Chem. Phys., 2009, 11, 1346–1353 RSC.
  59. J. C. G. Bünzli, A. S. Chauvin, H. K. Kim, E. Deiters and S. V. Eliseeva, Lanthanide Luminescence Efficiency in Eight- and Nine-Coordinate Complexes: Role of the Radiative Lifetime, Coord. Chem. Rev., 2010, 254, 2623–2633 CrossRef.
  60. M. Latva, H. Takalo, V.-M. Mukkala, C. Matachescu, J. C. Rodríguez-Ubis and J. Kankare, Correlation between the Lowest Triplet State Energy Level of the Ligand and Lanthanide(III) Luminescence Quantum Yield, J. Lumin., 1997, 75, 149–169 CrossRef CAS.
  61. X.-P. Yang, B.-S. Kang, W.-K. Wong, C.-Y. Su and H.-Q. Liu, Syntheses, Crystal Structures, and Luminescent Properties of Lanthanide Complexes with Tripodal Ligands Bearing Benzimidazole and Pyridine Groups, Inorg. Chem., 2003, 42, 169–179 CrossRef CAS PubMed.
  62. P. Xiao, P. Wang, R. Q. Fan, X. Du, W. Chen, H. J. Zhang, Y. Song and Y. L. Yang, Lanthanide MOFs Constructed Based on a Difunctional Ligand with Bimodal Emission and Eu3+ Doped Dy3+ Materials: White Emission and Color Tuning, RSC Adv., 2016, 6, 83091–83100 RSC.
  63. F. J. Steemers, W. Verboom, D. N. Reinhoudt, E. B. van der Tol and J. W. Verhoeven, New Sensitizer-Modified Calix[4]Arenes Enabling Near-UV Excitation of Complexed Luminescent Lanthanide Ions, J. Am. Chem. Soc., 1995, 117, 9408–9414 CrossRef CAS.
  64. H. Wu, B. Yan, R. Liang, V. Singh, P. Ma, J. Wang and J. Niu, An organic chromophore -modified samarium- containing polyoxometalate: excitation-dependent color tunable behavior from the organic chromophores to the lanthanide ion, Dalton Trans., 2020, 49, 388–394 RSC.
  65. C. Ritchie, V. Baslon, E. G. Moore, C. Reber and C. Boskovic, Sensitization of Lanthanoid Luminescence by Organic and Inorganic Ligands in Lanthanoid-Organic-Polyoxometalates, Inorg. Chem., 2012, 51, 1142–1151 CrossRef CAS PubMed.
  66. J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer US, Boston, MA, 2006 Search PubMed.
  67. J. Priya, N. K. Gondia, A. K. Kunti and S. K. Sharma, Pure White Light Emitting Tetrakis β-Diketonate Dysprosium Complexes for OLED Applications, ECS J. Solid State Sci. Technol., 2016, 5, R166–R171 CrossRef CAS.
  68. T. Yamase, T. Kobayashi, M. Sugeta and H. Naruke, Europium(III) Luminescence and Intramolecular Energy Transfer Studies of Polyoxometalloeuropates, J. Phys. Chem. A, 1997, 101, 5046–5053 CrossRef CAS.
  69. H. Wu, B. Yan, H. Li, V. Singh, P. Ma, J. Niu and J. Wang, Enhanced Photostability Luminescent Properties of Er3+-Doped Near-White-Emitting DyxEr(1−x)-POM Derivatives, Inorg. Chem., 2018, 57, 7665–7675 CrossRef CAS PubMed.
  70. T. Yamase, in Handbook on the Physics and Chemistry of Rare Earths, Elesvier B.V., 1st edn, 2009, vol. 39, pp. 297–356 Search PubMed.
  71. S. Comby, D. Imbert, A.-S. Chauvin, J.-C. G. Bünzli, L. J. Charbonnière and R. F. Ziessel, Influence of Anionic Functions on the Coordination and Photophysical Properties of Lanthanide(III) Complexes with Tridentate Bipyridines, Inorg. Chem., 2004, 43, 7369–7379 CrossRef CAS PubMed.
  72. C. Wyman, P.-P. Sloan and P. Shirley, Simple Analytic Approximations to the CIE XYZ Color Matching Functions, J. Comput. Graph. Tech., 2013, 2, 1–11 Search PubMed.
  73. P. D. Bhoyar, G. B. Nair and S. J. Dhoble, Photoluminescence Properties of Different Luminescent Ions (Ce3+, Eu3+, Eu2+, Dy3+, Cu+) in K2LaCl5 Host Matrix, Optik, 2017, 134, 33–44 CrossRef CAS.
  74. W.-F. Zhao, C. Zou, L.-X. Shi, J.-C. Yu, G.-D. Qian and C.-D. Wu, Synthesis of Diamondoid Lanthanide–Polyoxometalate Solids as Tunable Photoluminescent Materials, Dalton Trans., 2012, 41, 10091–10096 RSC.
  75. H. L. Li, Z. L. Wang, S. J. Xu and J. H. Hao, Improved Performance of Spherical BaWO4:Tb3+ Phosphors for Field-Emission Displays, J. Electrochem. Soc., 2009, 156, J112–J116 CrossRef CAS.
  76. C. S. McCamy, Correlated Color Temperature as an Explicit Function of Chromaticity Coordinates, Color Res. Appl., 1992, 17, 142–144 CrossRef.
  77. W. Salomon, A. Dolbecq, C. Roch-Marchal, G. Paille, R. Dessapt, P. Mialane and H. Serier-Brault, Multifunctional Dual-Luminescent Polyoxometalate@Metal-Organic Framework EuW10@UiO-67 Composite as Chemical Probe and Temperature Sensor, Front. Chem., 2018, 6, 425 CrossRef CAS PubMed.
  78. Q. Xu, Z. Li, Y. Wang and H. Li, Temperature-dependent luminescence properties of lanthanide(III) β-diketonate complex-doped LAPONITE®, Photochem. Photobiol. Sci., 2016, 15, 405–411 RSC.
  79. E. Radkov and R. H. Beer, High Yield Synthesis of Mixed-Metal Keggin Polyoxoanions in Non-Aqueous Solvents: Preparation of (n-Bu4N)4[PMW11O40] (M = V, Nb, Ta), Polyhedron, 1995, 14, 2139–2143 CrossRef CAS.
  80. SAINTPLUS, Version 6.02, Brucker AXS, Madison, WI, USA, 1999 Search PubMed.
  81. G. M. Sheldrick, Crystal Structure Refinement with SHELXL, Acta Crystallogr., Sect. C: Struct. Chem., 2015, 71, 3–8 Search PubMed.
  82. G. M. Sheldrick, SHELXT – Integrated Space-Group and Crystal-Structure Determination, Acta Crystallogr., Sect. A: Found. Adv., 2015, 71, 3–8 CrossRef PubMed.
  83. K. Brandenburg, Diamond, Version 3.2k, Cryst. Impact GbR, Bonn, Ger, 2014 Search PubMed.
  84. M. S. Wrighton, Technique for the Determination of Absolute Emission Quantum Yields of Powdered Samples. D. S. Ginley and D. L. Morse, J. Phys. Chem., 1974, 78, 2229–2233 CrossRef CAS.
  85. J. A. Do Nascimento Neto, A. K. S. M. Valdo, C. C. da Silva, F. F. Guimarães, L. H. K. Queiroz Júnior, L. J. Q. Maia, R. C. de Santana and F. T. Martins, Blue Light Emitting Cadmium Coordination Polymer with 75.4% External Quantum Efficiency, J. Am. Chem. Soc., 2019, 141, 3400–3403 CrossRef PubMed.

Footnotes

Electronic supplementary information (ESI) available: Additional figures, schemes and tables and X-ray crystallographic file in CIF format for compounds LM4-1-Tb, LM4-1-Eu, LM4-1-Nd and LM4-1-Ho. The X-Ray crystallographic data for LM4-1-Dy, LM4-1-Er and LM4-1-Gd have been previously reported (see ref. 27). CCDC 1951517 for LM4-1-Tb, 1962564 for LM4-1-Eu, 1962563 for LM4-1-Nd and 1951518 for 1-Ho. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0qi00232a
LM4 = Laboratory of molecular magnetism and molecular materials.

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