Lanthanide complexes with aromatic o-phosphorylated ligands: synthesis, structure elucidation and photophysical properties.

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Cite this: Dalton Trans., 2014, 43, 3121

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Lanthanide complexes with aromatic o-phosphorylated ligands: synthesis, structure elucidation and photophysical properties† Sergey Shuvaev,*a Valentina Utochnikova,a Łukasz Marciniak,b Alexandra Freidzon,c Ilya Sinev,d Rik Van Deun,e Ricardo O. Freire,f Yan Zubavichus,g Wolfgang Grünertd and Natalia Kuzminaa

Received 20th September 2013, Accepted 18th October 2013 DOI: 10.1039/c3dt52600c www.rsc.org/dalton

Lanthanide complexes LnL3 (Ln = Sm, Eu, Tb, Dy, Tm, Yb, Lu) with aromatic o-phosphorylated ligands (HL1 and HL2) have been synthesized and identified. Their molecular structure was proposed on the basis of a new complex approach, including DFT calculations, Sparkle/PM3 modelling, EXAFS spectroscopy and luminescent probing. The photophysical properties of all of the complexes were investigated in detail to obtain a deeper insight into the energy transfer processes.

Introduction Up to now, a rich experience has been accumulated in the field of synthesis and design of luminescent lanthanide complexes. Meanwhile, the simultaneous adjustment of both the structural and photophysical properties still seems to be of a great desire. One of the possible candidates on the role of such dualistic ligands can be aromatic phosphine oxides. From the one hand, the tetrahedral co-ordination around a phosphorus atom provides the formation of three-dimensional structures, where PvO does not participate in the complexation, being just a structural junction.1,2 The PvO bond can directly take part in complexation, either as a neutral monodentate O-donor ligand or as a chelating one by introducing an additional O-donor group in the β-position relatively to the phosphorus atom such as a hydroxyl group3,4 or an ester

a Department of Materials Science, Lomonosov Moscow State University, 119991, Leninskie gory, 1/3, Moscow, Russia. E-mail: [email protected]; Tel: +7 495 9393836 b Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wroclaw, Poland c Photochemistry Center, Russian Academy of Sciences, ul. Novatorov 7a, Moscow 119421, Russia d Laboratory of Industrial Chemistry, Ruhr-University Bochum, Universitätsstraße 150, 44801 Bochum, Germany e 3 L – Luminescent Lanthanide Lab, f-Element Coordination Chemistry, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 – building S3, B-9000 Gent, Belgium f Pople Computational Chemistry Laboratory, Department of Chemistry, UFS, 49100000 São Cristóvão-SE, Brazil g National Research Center “Kurchatov Institute”, 123182 Moscow, Russia † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3dt52600c

This journal is © The Royal Society of Chemistry 2014

substituent.5 Moreover, bidentate imidodiphosphinates, containing two PvO groups, were also studied.6,7 Therefore, both anionic and neutral ligands can be introduced into the coordination sphere. In the case of the lanthanide complexes a prosperous photophysical performance is tightly connected with the crystal structure of a complex. One of the main obstacles is the quenching of luminescence via vibrations of the solvent molecules in the inner sphere, which can be overcome by the saturation of the co-ordination polyhedron by ancillary ligands or by the introduction of bulky substituents, which effective shield the lanthanide ions from an external impact. Both of the ligands described in the present paper are suggested to promote dimeric or oligomeric complex formation instead of the mononuclear one, and they can be considered as structural analogues of β-diketonates, forming binuclear complexes.8,9 From another point of view, contrary to β-diketonates, o-phosphorylated phenols can be co-ordinated either in the form of chelating ligands (similarly to β-diketonates) or in the form of bridging ligands,10 opening up wide possibilities in the construction of the co-ordination polyhedron. On the other hand, aromatic phosphine oxides are wellknown sensitizers of terbium luminescence due to an appropriate energy gap between their triplet state and the nearby resonance level 5D4 of terbium.11–13 Thereby, aromatic o-phosphorylated phenolates seem to be attractive and challenging anionic ligands from both photophysical and structural points of view. In present paper we describe the synthesis of several lanthanide complexes LnL3 (Ln = Sm, Eu, Gd, Tb, Dy, Tm, Yb, Lu) with o-phosphorylated phenols HL1 and HL2 (Fig. 1). We also

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Fig. 1 The molecular structures of HL1 and HL2. The ligating parts of the molecules are marked as cyan and the alkyl substituents are marked as blue.

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Fig. 3

The infrared spectra of HL1 (grey) and TbL13 (green) in KBr.

conduct thorough photophysical studies and a novel approach is proposed to estimate their co-ordination environment by means of DFT and semi-empirical Sparkle/PM3 calculations, coupled with EXAFS and luminescence spectroscopy. Synthesis and characterisation All of the lanthanide complexes were synthesized in accordance with the synthetic route previously reported.14 The LDI mass-spectra revealed the presence of both monomeric ([LnL2]+, [LnL3 + H+]+) and dimeric molecular ions ([Ln2L4 + OH−]+, [Ln2L5]+) as the only signals, suggesting that there is an oligomeric co-ordination polyhedron in the molecular structure (Fig. 2, S1–3†). The bidentate ligation of (L1)− and (L2)− is likely to provide only a one-dimensional oligomeric or polymer chain. The relatively high solubility (more than 10−2 mol L−1 in methanol) excludes a polymeric structure, whilst the monoexponential decay curves for all of the complexes suggest the presence of only one co-ordination environment, which can be observed only in the case of the mono- or binuclear complex, otherwise co-ordination environments for terminal and internal polyhedra would be different. Therefore, the dimeric polyhedron was considered as the most probable structure.

Fig. 4 The Raman spectra of a powder sample of HL1 (grey) and TbL13 (green).

The infrared and Raman spectra (Fig. 3 and 4) elucidated considerable changes in the position and the shape of several bands, ascribed to the ν(PvO) and ν(C–O) vibrations.14 First of all, a slight shift towards a lower energy is observed in case of the ν(PvO) vibrational mode, which is in line with an increase of the bond length under complexation in comparison with an initial protonated ligand.15 Secondly, the region that is believed to correspond to the ν(C–O) vibrations revealed a rise of an additional band on both the IR and Raman spectra, which can be ascribed to the occurrence of two types of C–O groups within the structure. This fact can be explained by a deviation in the lengths and denticity of C–O. It is worth mentioning that the occurrence of additional bands in the IR-spectrum of the dimeric complex in comparison with the monomeric species has been previously reported for β-diketonates,16 where they were ascribed to the C–O bonds. As an additional evidence of the complexation, the analysis of the Eu–O vibrational modes can be performed. However usually the precise analysis and ascription of these bands is hindered by an overlap with additional bands. However, in the case of the lanthanide complexes these vibrational modes can be revealed in the luminescence spectra as additional satellites of f–f transition bands. In our case, by juxtaposing the low-frequency IR spectrum with the corresponding luminescence spectrum several ν(Eu–O) bands centred at ca. 330 cm−1 have been revealed (Fig. S4†). XPS study

Fig. 2 The MS-LDI spectrum of EuL13. The magnified regions represent the comparison between the empirical spectra and the corresponding theoretically calculated ones.

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As soon as both ligating oxygen atoms have different chemical environments, the signals ascribed to these atoms can be easily distinguished in the O 1s XPS spectra (Fig. 5). To make a


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Fig. 5

The O 1s photoelectron spectra of TPPO, HL1 and TbL13.

plausible assignment of the signals a reference sample, triphenylphosphine oxide (TPPO), was chosen to attribute the phosphine oxygen peak. In both cases the band centred at ca. 529 eV (528.5 eV in TPPO and 528.9 eV in HL1) was observed. A second peak ascribed to the hydroxyl oxygen was observed at 530.7 eV in the case of HL1 and 530.9 eV in case of TbL13. In both cases the integral intensities of both signals in O 1s spectra are almost equal, which coincides with their chemical structures. An additional broad peak in the spectrum of TPPO was apparently attributed to the contamination from the solvent molecules. The analysis of the TbL13 O 1s XPS spectra revealed the high-energy shift of both signals (ca. 0.2 eV), which coincides with their ligation.17 The energy difference between both peaks are almost unchanged going from HL1 to TbL13, whilst the broadening of both peaks can serve as evidence for the existence of additional contributions, which are attributed to inequivalent ligand molecules in the structure (viz. different C–O and PvO environments). Meanwhile, the P 2p XPS spectra (Fig. S5†) revealed a substantially broadened spectrum of TbL13 in comparison with the initial ligand. Presumably, such a deviation can be also explained in terms of the inequivalent conformations of the ligated (L1)-molecules, which also affect the second co-ordination sphere, i.e. the phosphorus atoms. Instead of the typically observed doublet corresponding to the p sublevel multiplicity with an integral intensity ratio of 1 : 2, an overlap of apparently two doublets corresponding to two distinguishable conformers is observed. Quantum-chemical calculations Sparkle/PM3 modelling and Judd–Ofelt analysis. Due to the lack of samples appropriate for X-ray analysis (all of the samples turned out to be amorphous) only indirect methods were available to shed light onto the molecular structure of the lanthanide complexes. The results stated in the previous section led us to choose a dimeric structure for further optimization, which was further proved by semi-empirical Sparkle/


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PM3 modelling coupled with the fitting of the theoretically calculated Judd–Ofelt parameters with those that had been experimentally observed. The semi-empirical Sparkle model18–21 was proven to be a useful and reliable tool in the prediction of a co-ordination polyhedra around the lanthanide ions in the co-ordination compounds, and even can be comparable with substantially more time-consuming DFT calculations,22 though it possesses less accurate results in predicting the ligand structures. In this case, the proposed structure can be easily tuned in order to gain the best fitting between the calculated model and empirically observed properties, for instance, the luminescence spectra. Though the positions of the emission bands on the luminescence spectra of lanthanide ions are almost insensitive to the local environment, the relative intensities of the emission bands are affected by the odd components of the crystal field:23 λ1;λþ1ðoddÞ tðallÞ X X Bλtp 2 Ωλ ¼ ð2λ þ 1Þ ð2t þ 1Þ t p¼0 where Bλtp

2 tþ1 ðλ þ 1Þð2λ þ 3Þ λ t kr lθðt; pÞγ p kr lð1 σ λ Þ ¼ ΔE 2λ þ 1 ðλ Þ k f C f lΓ t δt;λþ1 p

are the crystal-field parameters. In this formula the term ρ(2β)2λ+1 comes from so-called simple overlap model:24 ρ represents an overlap between the 4f-orbitals of the lanthanide ion and the s,p-orbitals of the co-ordinating atoms and β characterises the centroid positioning of the overlap region relatively to the middle point of the line, connecting the lanthanide ion and ligand. The Odd-rank ligand field parameter 1=2 P 4π Y t;p γ tp ¼ e2 g j tþ1 , representing the electric dipole 2t þ 1 Rj j coupling mechanism, and the ligand polarizability dependent 1=2 P Y t;p 4π t term Γ p ¼ αj tþ1 , representing the dynamic 2t þ 1 Rj j coupling mechanism, contain charge factors gj and polarizabilities αj, which were adjusted in order to gain a minimum of the four-dimensional surface response. The response function calc Fresp is defined as Fresp = ∑|Ωcalc − Ωexp − Ωexp 2 2 | − |Ω4 4 |, calc calc where Ω2 and Ω4 are calculated Judd–Ofelt parameters, exp 25 while Ωexp 2 , Ω4 are empirically observed ones. We compared some luminescent properties such as the Judd–Ofelt intensity parameters Ωλ (λ = 2 and 4) and the radiative decay rates calculated from the monomeric and dimeric molecular structures by the Sparkle/PM3 model23 (Fig. 6 and ESI†) and revealed that the suggested dimeric structure provides a better match between the empirically and theoretically calculated Ω4 and Arad values (Tables 2 and 3). It is worth mentioning that the proposed monomeric structures can be easily co-ordinated by two water molecules, which only slightly affect the photophysical properties of the complex (see ESI†). At the same time, bulky substituents hamper an access

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Fig. 6 The calculated ground state geometries of the monomeric (left) and dimeric (right) molecular structures of EuL13 using the Sparkle/ PM3 model.

of water molecules to the inner co-ordination sphere in the case of the dimeric structure (vide supra). Therefore, both complexes EuL13 and EuL23 revealed a binuclear structure as the favourable one and this assumption has been used for further analysis. EXAFS refinement In order to verify the dimeric structure of the complexes suggested by the quantum mechanical calculations we applied EXAFS spectroscopy. The Fourier Transforms (FTs) of the experimental Tb L3-edge EXAFS spectra for the two representative complexes TbL23 and TbL13 are shown in Fig. 7. The two curves are reasonably similar to each other, which allowed us to use an approximate molecular model without alkyl substituents in the following DFT calculations, which is common for TbL23 and TbL13. These curves are dominated by the first-coordination-sphere peak corresponding to the Tb–O photoelectron scattering path. Most importantly, the contribution of the Tb⋯Tb contacts is apparent for both complexes, corroborating again the dimeric structure. The respective FT peaks, although rather weak, can be reliably assigned to a heavy atom (Tb) by comparing their relative intensities in differently weighted knχ(k) (with n = 1, 2, 3) curves. The experimental data were used to refine the local structure parameters of a model

Fig. 7 Fourier Transforms (FTs) of the Tb L3-edge EXAFS spectra for the TbL23 and TbL13 complexes: experimental data (solid lines) and best-fits (circles).

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derived from DFT optimization. The structural model contained in total 6 photoelectron scattering paths, including three Tb–O distances for the first co-ordination sphere with the total co-ordination number of 7 (1 + 5 + 1), Tb⋯C and Tb⋯P contributions for the second co-ordination sphere, and finally the Tb⋯Tb contact. The best-fit values for the localstructure parameters are summarized in Table 1. Good fits were obtained with meaningful sets of structural parameters that are very similar to the quantum mechanical predictions. This means that the experimental EXAFS data are fully compatible with the calculated dimeric configuration of the metal coordination site. The slight deviation observed in the case of TbL23 can be explained by a conformational change of aromatic substituents by introducing an ethyl substituent in the phenolate substituent. DFT calculations Previously, a multireference approach has been successfully used by one of the authors26 to simulate the electronic structure, excited states, and energy transfer pathways of lanthanide complexes. However, the size of the molecule in the present study (212 atoms) prevents us from using multireference methods. In this case, DFT is the only choice, and we focus on the ground-state properties that can be reliably simulated by DFT. Quantum chemical calculations were performed using the B3LYP functional with the 6-31G(d,p) basis set for light elements and scalar quasirelativistic 4f-in-core pseudopotentials for lanthanides (ECP52MWB for Eu3+ and ECP54MWB for Tb3+) with the corresponding basis sets27,28 and Grimme’s dispersion correction.29 All the calculations were performed using the Firefly program package.30 The calculations were performed for a model ligand L without alkyl substituents and its complexes with Tb3+ and Eu3+. For the free ligand HL we considered several conformers, two of them corresponding to open forms with intermolecular H bonds observed in the crystal structures of HL and its alkylated derivatives10,31 and one closed form with an intramolecular H bond. To interpret the absorption (excitation) spectra of HL and NaL in solution and in the solid phase, we also considered several dimers (Fig. 8), NaL, and its hydrated forms. The brutto formula LnL3 assumes the co-ordination number of Ln3+ to be 6. At the same time, typical co-ordination numbers for lanthanides are 7 or higher. Indeed, the geometry optimization of LnL3 showed that the co-ordination sphere of the central ion has enough space for at least one water molecule. At the same time, the LDI mass spectra as well as the EXAFS indicate the presence of the dimeric form of the complex. So, we considered both the LnL3(H2O) and binuclear Ln2L6 structures. The geometries of HL and its dimers, NaL and its hydrates, TbL3(H2O), Tb2L6, and Eu2L6 were fully optimized. Some characteristic geometrical parameters of the ligand are compared with the experimental data in Table 2. Our calculation shows that the calculated PvO bond length in the open form agrees better with the experiments (in which


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The parameters of the local environment of the Tb atoms in TbL23 and TbL13 according to EXAFSa


1st co-ordination sphere

2nd co-ordination sphere

3rd co-ordination sphere

Sample

Path

N

R (Å)

Path

N

R (Å)

Path

N

R (Å)

TbL23

Tb–O

4 3

3.42 3.51

Tb⋯Tb

1

3.98

Tb–O

2.26 2.39 2.51 2.28 2.38 2.59

Tb⋯C Tb⋯P

TbL13

1 5 1 1 5 1

Tb⋯C Tb⋯P

4 3

3.38 3.52

Tb⋯Tb

1

3.99

a

Fitting ranges: R = 1.4–4.1 Å; k = 3.0–13.0 Å−1.

Fig. 8 The molecular structures of the calculated dimeric structures of HL. (a) Structure 1 (no stacking). (b) Structure 2 (stacked phenyl rings). (c) Structure 3 (stacked phenolate rings, one hydrogen bond). (d) Structure 4 (stacked phenolate rings, two hydrogen bonds).

Table 2 A comparison of the bond lengths of the HL molecules in different conformations with the experimental data

HL closed

HL open 1

HL open 2

Exp.

PvO (Å)

1.521

1.502

1.505

C–O (Å)

1.342

1.360

1.366

1.502a 1.494–1.495b 1.348a 1.340–1.348b

a

Crystal structure of HL2. b Crystal structure of HL1.


only open forms were observed), while the C–O bond length in the experiment is closer to that in the calculated closed form. This can be explained by the fact that the open forms in the crystals are H-bonded with their neighbors, which is not the case in the calculated open monomeric form; at the same time, the PvO bond in the closed form can be slightly distorted. In the calculated structure of NaL, the PvO bond length increases to 1.535 Å and the C–O bond shortens to 1.293 Å.

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Table 4 The maxima of absorption bands of the different conformers of the protonated ligand HL

Species

λabs (nm)

Exp.

HL closed 2HL, structure 1 2HL, structure 2 2HL, structure 3 2HL, structure 4 HL open 1 HL 2MeOH open 1 HL open 2 HL 2MeOH open 2

289, 281, 262 257 254 266–269, 252 285, 283, 265, 260 253 272, 266 254 269

290a 260, 300b 280 (sh), 350 (s)c

a

Fig. 9 The molecular structure of the calculated dimeric complex Tb2L6.

Table 3 The bond lengths of the calculated structure and those determined by EXAFS

Tb–O (Å)

Tb–C (Å)

Tb–P (Å)

TbL3H2O

Tb2L6

Exp.

2.261 2.300 2.335 2.354 2.362 2.406 2.502 3.330 3.402 3.412

2.251 (2) 2.339 (2) 2.344 (2) 2.352 (2) 2.374 (2) 2.383 (2) 2.443 (2) 3.327 (2) 3.338 (2) 3.417 (2) 3.452 (2) 3.503 (2) 3.512 (2) 3.606 (2) 3.896

2.26–2.28 (1) 2.39–2.38 (5) 2.51–2.59 (1)

3.472 (2) 3.579

Tb–Tb (Å)

3.38–3.42 (4)

3.52–3.52 (3) 3.98–3.99

The calculated structure of Tb2L6 is shown in Fig. 9. Some characteristic geometric parameters of TbL3(H2O) and Tb2L6 are compared with the EXAFS data in Table 3. The agreement between the calculated and experimental data is very good for both TbL3(H2O) and Tb2L6, but the observed Tb–Tb contact at a distance virtually coinciding with the calculated one allows us to conclude that it is the dimeric form that is observed in EXAFS experiments. The calculated vibrational spectra of HL, NaL, and TbL3(H2O) bear little information. They indicate the presence of intense bands at ∼1200 and ∼1300 cm−1 which are typical for PvO and C–O bonds, respectively. However, it is not possible to isolate pure PvO or C–O stretch vibrations; instead, the vibrations at these frequencies are combinations with the C–C stretch and some other displacements. In NaL and TbL3(H2O), the PvO manifold shifts to ∼1100 cm−1 and the C–O manifold shifts to ∼1400 cm−1 which is in qualitative agreement with the experiment. The calculated IR spectrum of TbL3(H2O) exhibits some characteristic bands resulting from the presence of water in

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Toluene, Fig. S6, ESI.† b MeOH, ref. 10. c Solid phase, Fig. 10.

the coordination sphere: an intense band of the antisymmetric O–H stretch at ∼3500 cm−1, a less intense band of the symmetric O–H stretch at ∼3800 cm−1, and low-frequency bands corresponding to the libration of H2O in the coordination sphere of the lanthanide ion at 372 and 816 cm−1 that are more intense than their neighboring bands. The presence of these bands in the IR spectrum can indicate the presence of coordinated water in the complex. The absorption spectra were calculated using the TD-CAMB3LYP functional, which is less prone to artifact charge–transfer transitions,32 although slightly overestimates the transition energies. The calculations were performed using the GAMESS program package.33 The absorption spectra of the free ligand HL are summarized in Table 4. One can see that aggregation and H bonding (including intramolecular H bonds) shift the absorption band to longer wavelengths as compared to the forms without either H bonds or stacking. Therefore, the absorption pattern observed in the solid phase of HL can be attributed to the H-bonded molecular chains. At the same time, the band at 290 nm originates from the absorption of the closed HL form, which seems to be the most stable form in aprotic media. Deprotonation of the ligand (i.e., going from HL to NaL) results in the bathochromic shift of the absorption band. The calculated spectra of the NaL hydrates (NaL·4H2O and NaL·6H2O) show the best agreement with the experimental NaL spectrum, because these structures reproduce the environment of the chromophore most adequately (Table 5). In going

Table 5 The maxima of absorption bands of different conformations of the NaL1 and terbium complexes

Species

λabs (nm) (theoretical)

λabs (nm) (experimental)

NaL NaL·4H2O NaL·6H2O L− TbLCl2·3H2O TbL2Cl·2H2O TbL3 H2O Tb2L6

340, 286 278 273 330, 278 295, 270 281, 278 284, 280, 271 289, 273, 266

280a

a

300 (sh), 350 (s)b

MeOH, ref. 10. b Solid phase, Fig. 15.


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Fig. 10 A comparison between the co-ordination polyhedra in the calculated Tb2L16 structure (left) and the experimentally observed Tb2(thd)6 (right). The bond lengths are in Angstroms.

from Na+ to Eu3+, the bathochromic shift is even larger. However, the calculated absorption spectra of the Tb complexes considered below only qualitatively agree with the experiment. This can be explained by the existence of intermolecular aggregates in the solid phase. In all of the HL forms and complexes, the transition is from the HOMO (or HOMOs) localized on the phenolate ring to the unoccupied orbitals localized at least partially on the same phenolate ring. Weak transitions, however, have a lower contribution from the phenolate-localized unoccupied orbital. At the same time, the method is not completely free from spurious charge-transfer transitions, such as the first transition in NaL (340 nm, HOMO→3s Na). Both the TG-DSC curves and infrared spectra revealed the absence of water molecules within the vacuum-dried samples, and the proposed dimeric molecular structure with two bridging molecules seems to be plausible and appropriate for further discussion. The obtained molecular structure is very similar to those observed in the case of the Ln2(thd)6 complexes (Ln = Ce–Gd),9,34 while complexes of thd− with ions of the yttrium subgroup form monomeric species or mixtures, reflecting the decline of the ionic radii in the lanthanide row. The main distinction between these structures (viz. Tb2L6 and Tb2(thd)6) is an absence of an inversion centre in the Tb2O12 fragment (Fig. 10), leading to two crystallographically non-equivalent positions, occupied by terbium ions. The coordination number of 7 is untypical for terbium ions in complexes and is mainly observed in the case of bulky substituents. In such a way, the substitution of one thd− anion by a less voluminous pivalate one results in an increase of the coordination number up to 8 by introducing two methanol molecules into the co-ordination sphere.35 In our case, it is reasonable to suppose that the distinctions in the ionic radii do not affect the molecular structure due to the substantial voluminosity of substituents within the ligands and complexes LnL13 with all lanthanide ions have the same molecular structure, which is in line with the aforementioned LDI massspectra. Thereby, the proposed complex approach, embracing the quantum-chemical calculation, luminescence spectra analysis and EXAFS spectroscopy has gained a plausible molecular structure.


Fig. 11

The luminescence excitation and emission spectra of HL1.

Photophysical properties Luminescence properties of HL, LuL3 and GdL3. The luminescence excitation and emission spectra of both the HL1 and HL2 molecules (Fig. 11) feature two broad bands ascribed to an aggregation and an intramolecular electron transfer S1→S0 between the singlet states (433 nm and 594 nm for HL1 and 440 nm and 589 nm for HL2, respectively, both in solids). It is noteworthy that the band at lower energies disappears in the methanol solution. As could be easily suggested in both cases only a minor distinction is observed in the position, shape and energy of the molecular orbitals. Complexes of gadolinium and lutetium are usually used for NMR analysis and triplet level estimation.36 The determination of the triplet energy level is essential for the elucidation of the energy transfer mechanism between the ligand and lanthanide ion, as within the ligand itself. The gadolinium ion features a partially filled sub-shell and has a closer ion radii to both the europium and terbium ions and hence a more relevant triplet energy value. Furthermore, the excited energy level (6P7/2) within Gd3+ is too high to be pumped via a direct transfer from the ligand triplet state, whilst a paramagnetic central ion is promoting mainly the phosphorescence band by a partial mixing of the singlet and triplet levels at low temperatures. Actually, the analysis of the phosphorescence spectra taken at low temperatures is usually hindered by an absence of a fine vibrational structure, required to ascribe the 0–0 phonon transition. Thus, a deconvolution procedure on the Gaussian curves was carried out with the difference of about 1000–1100 cm−1 corresponding to the most intensive vibrations, which can be ascribed to the ‘breathing’ of the aromatic rings (Fig. 12). To prove the phosphorescent nature of the observed bands, lifetime measurements were carried out and they turned out to be typical for a phosphorescence (2.81 ms for GdL13 and 2.54 ms for GdL23). Surprisingly, both the band maxima and the 0–0 phonon transition of the (L1)− and (L2)− triplet energy level turned out to be quite different (25 000 cm−1 in the case of GdL13 and 22 800 cm−1 in the case of GdL23). The possible explanation is that the two methyl substituents increase the energy gap between the HOMO and LUMO in comparison with an ethyl

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Fig. 12

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The deconvoluted phosphorescence bands of GdL13 and GdL23 at 77 K.

Fig. 13 The energy diagram of LnL23 representing the mutual positions of the singlet and triplet energy levels in the ligand and the energy levels within the luminescent lanthanide complexes. Fig. 14 The energy level diagram for dimeric EuL13, showing the most probable channels for the intramolecular energy transfer process.

one, which also coincides with forthcoming calculations. Therefore, (L2)− seems to be the better choice for the sensitization of the europium and terbium complexes, as the energy gap between T1((L2)−) and the nearby resonance levels of the lanthanide ions is considerably less (Fig. 13). The same should be valid in the case of the dysprosium and samarium complexes, but the 1G4 level of Tm3+ lies too close to the 0–0 phonon transition that can result in an effective back energy transfer. Luminescence of europium complexes. To calculate the singlet and triplet state energies in EuL3, the configuration interaction single (CIS) based on the intermediate neglect of the differential overlap/spectroscopic (INDO/S) method37,38 was implemented in the ZINDO program.39 Though the singlet energy level turned out to be largely overestimated, which is typical for ZINDO calculations, the triplet energy values are closer to the empirically obtained values (23 760 cm−1 and 25 000 cm−1 for EuL13 and 23 380 cm−1 and 22 800 cm−1 for EuL23). The most probable channels for the intramolecular energy transfer process are represented in Fig. 14. Since the energy

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transfer from the ligand to the central ion predominantly occurs between the triplet level and the nearby resonance levels of the lanthanide ion, the energy gap between T1 and the corresponding atomic levels mainly foreordains the efficiency of sensitization. Taking into account the empirical relationship initially stated by Latva et al.,40 the suitable energy gap (Δ) is observed in both the europium and terbium complexes. However, an additional elucidation should be done at this point. In the case of the terbium complexes only one 5D4 level is suitable for energy transfer as the other components of the multiplet 5DJ are lying at substantially higher energies. In the case of the Eu3+ ion the components of the 5DJ multiplet are relatively close in energy, and therefore the energy transfer can occur on either 5D2 or 5D1 or 5D0. In fact, it was affirmed in several papers that the type of energy level participating in the energy transfer process is highly influenced not only by the triplet energy level, but also by the nature of substituents.41 In our case, the energy gap between T1 and 5D0 (ca. 7500 cm−1 for EuL13 and 5700 cm−1 for EuL23) is too large for sensitization,


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Fig. 15 The luminescence spectra of EuL13 (red) and EuL23 (violet), both taken at 77 K.

whilst the Δ(T1 − 5D1) = 5900 and 3700 cm−1 and Δ(T1 − 5D2) = ca. 3400 and 1200 cm−1, respectively, which obeys the aforementioned empirical criteria. Moreover, the direct energy transfer from the triplet state of the ligand to the 5D0 state of the lanthanide ion is parity forbidden in both the exchange and coulomb interaction models,42 and therefore T1→5D1 seems to be the most appropriate energy transfer process from the ligand to the europium ion. Meanwhile, the luminescence spectra of EuL13 revealed the presence of the 5D1→7F1 transition band, whilst such a transition is absent in the spectrum of EuL23 (Fig. 15). Therefore, we suggested the T1→5D1 path for EuL13 and T1→5D0 route for EuL23. However, the quantum yield observed at room temperature was too low (0.5%). This fact cannot be explained in terms of back-energy transfer to the triplet state. The theoretical assumptions, considering the energy transfer between the triplet level of the ligand and resonance lanthanide levels, also predict a quantum yield of about 20% (Tables 2 and 3). In this case the quantum yield was calculated in accordance with the following expression: q ¼ ηsens η ¼ ηsens

Arad τobs ¼ ηsens Arad þ Anrad τrad

where ηsens is the sensitization efficiency. The predicted sensitization efficiency of about 100% is largely overestimated, and thus an introduction of the additional intermediate energy levels into consideration seems to be essential, which can introduce additional energy migration channels. Analyzing the luminescence excitation spectra of EuL13 taken at 298 K and at 77 K (Fig. 16), one can suggest that the intensity of the band corresponding to S0→S1 is comparable with that of the ionic transitions. Such an observance can be a clear evidence of an ineffective energy transfer between the ligand and lanthanide ion. Actually, the luminescence intensity of both the europium complexes at room temperature is extremely low, featuring an additional broad band ascribed to either the ligand-centered luminescence or emission from the LMCT (ligand-to-metal charge transfer state). At 77 K any additional bands in the low-wavelength region almost disappear, indicating a weakening of the back-energy transfer. Nonetheless, an improvement of the T1→Ln3+ energy transfer does not considerably affect both the intensity and radiative


Fig. 16

The luminescence excitation spectra of EuL23 and TbL23.

lifetimes, assuming an existence of an additional low-lying intermediate energy level, which effectively depopulates the excited states of Eu3+. Up to now, there has not been suggested any reliable direct approach to confidently identify an intermediate state taking part in an energy transfer process as a charge transfer state, thus only indirect methods can be used. Generally, an absorption or excitation spectra of europium and another lanthanide complex (i.e. terbium or gadolinium) are compared in order to reveal additional bands (‘shoulders’) which can be attributed to a charge transfer state,43 since a reduction potential of Eu3+ is relatively low (EuIII/EuII = −0.35 V44). However, a precise determination of the LMCT state seems to be rather difficult (or even impossible), while additional important parameters such as the activation energy of a back-transfer process on the LMCT from resonance levels of the europium ion could be calculated by the analysis of the temperature dependence of excitation lifetimes.45 To derive the influence of the position of the LMCT state on the luminescence quenching Malta carried out theoretical calculations46 and revealed a broad valley in the quantum yield from 5000 cm−1 until T1 (it was disclosed that quenching by the LMCT state is dominantly determined by its position relatively to the singlet and triplet states of the ligand rather than to the resonance levels of Eu3+). Therefore, in our case it would be reasonable to assume that the 0–0 phonon component of the LMCT level is lower than 25 000 cm−1 and 22 800 cm−1 for EuL13 and EuL23, respectively (Fig. 17). Comparing the luminescence spectra of the terbium and europium complexes one can deduce that an additional broad band red shifted relative to the gadolinium fluorescence band can be ascribed either to the ligand-centered emission or to the emission from the LMCT state. At the same time, the ytterbium complexes possess a relatively low reduction potential (YbIII/YbII = −1.1 V44), resulting in a possible emission from the LMCT state in YbL3. Unfortunately, the sensitivity of our equipment has not allowed us to detect an emission from the LMCT state in YbL3, however by comparing the luminescence excitation spectra of the ytterbium and europium complexes, one can observe the disappearance of an additional band in the spectra of EuL3, which

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Dalton Transactions

and υ0J is the energy barycenter of the corresponding transitions. Substituting the obtained values in the following equation the JO parameters then could be easily obtained:


Ωλ ¼

Fig. 17 The energy level diagram elucidating the role of the LMCT in the energy transfer process in the case of EuL23.

was ascribed to an excitation of the LMCT state, which is presumably explained by its shift in the case of the ytterbium species with a subsequent overlap with the S1←S0 transition.43 In fact, the energy gap between the main band and the shoulder in case of EuL23 is about 2700 cm−1, which is in line with the energy difference LMCT in the case of the europium and ytterbium complexes reported previously (ca. 3200 cm−1), providing an effective overlap of two bands in the case of the ytterbium complex. Europium complex as a luminescent probe The ionic luminescence spectrum of lanthanides consists of both allowed magnetic dipole (MD) transitions and forbidden induced electric dipole (ED) bands, and their shape and relative intensities are highly influenced by the nature of ligands. The europium ion is the most fruitful one for luminescent probing, since pure magnetic and electric dipole transitions are presented within a spectrum, and therefore the MD can be used as an intrinsic reference (MD transitions are believed to be insensitive to the local environment) to elucidate the impact of the crystal field, which can be expressed in terms of the Judd–Ofelt (JO) parameters (Ω2, Ω4 and Ω6). Since the intensity of the 5D0→7F6 transition is extremely low, the reliable determination of Ω6 is impossible. The interpretation of the JO parameters is relatively complicated and allows only qualitative analysis by comparison with reference samples. Thus, Ω2 is believed to be sensitive to a site symmetry of the lanthanide ion and the covalence degree of the metal–ligand bond,47 whilst Ω4 mainly reflects long range effects.48 Parameter Ω2 is tightly connected with a radiation rate, the larger the Ω2 value (and hence larger asymmetry of the co-ordination environment), the shorter the lifetime of an excited state.49 The emission probability A0J of both transitions (5D0→7F2 and 5 D0→7F4) is extracted in accordance with the following relationship: A0J ¼ A01

I 0J υ01 ; I 01 υ0J

where A01 is a coefficient, which is assumed to be equal 50 s−1, while I0J is an integrated intensity of the 5D0→7FJ transition

3130 | Dalton Trans., 2014, 43, 3121–3136

27hð2J þ 1ÞA0J ; þ 2Þ2 j, J jjU ðλÞ jj J′ >j2

64e2 π4 ˜υ3 nðn2

where h is Planck’s constant (6.63 × 10−27 erg s), e is an electronic charge (4.803 × 10−10 esu), n is a refraction index of the solvent (in our case isopropyl alcohol is used with n = 1.38) ˜υ3 is the average transition energy (cm−1), (2J + 1) is a degeneracy of the initial state and ||2 is a corresponding reduced matrix element independent of the local chemical environment around the lanthanide ion described elsewhere.49 Nearly the same high values are obtained in both cases, indicating the similar low-symmetry local environment in the case of EuL13 and EuL23 (Tables 6 and 7). To determine a purely radiative lifetime of an excited state of europium(III) different approaches are suggested. The most widely used experimental technique is based on the measurements of lifetimes at a low temperature, assuming that all the non-radiative paths are frozen. Meanwhile, there are no sufficient grounds to assert such a behaviour and therefore several authors strive to use more theoretically substantiated techniques.49 One of the possible solutions is the theoretical calculation of a radiative lifetime from the emission spectra. Assuming the energy and the dipole strength of the MD transition (5D0→7F1) to be constant, a simple and fairly good matching experimental data relationship can be used to derive τR: 1 ; τR ¼ I tot AMD;0 n3 I MD where AMD,0 is the spontaneous emission probability of the 5 D0→7F1 transition in a vacuum (assumed to be equal to 14.65 I tot is the s−1), n is the refractive index of the medium and I MD ratio of the total area of the emission spectrum and area of the 5 D0→7F1 band. Due to the lack of the molecular structure, some structural data can be also obtained by an examination of Stark splittings (Fig. 18). An occurrence of the forbidden non-degenerate 5 D0→7F0 transition centered at 579 nm and the monoexponential decay curves indicate an absence of an inversion center on the europium ion and limits the site symmetry to Cs, Cn and CnV while the existence of only a single 0–0 transition indicates the presence of only one crystallographic site occupied by the europium ion. The 5D0→7F1 band is split into three sub-bands, and the possible site symmetries are restricted to C2v, C2, Ci and Cs. At the same time, the 5D0→7F4 is split into 7 components finally suggesting a C2v symmetry. Such a point group with a co-ordination number of 7, observed in the calculated molecular structure, can correspond, for instance, to a capped trigonal prism polyhedron, which is slightly distorted in the case of the predicted structure (see above). The aforementioned 5D0→7F0 transition, being forbidden according to the selection rules and observed due to the


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Table 6 The theoretical intensity parameters Ω2, Ω4 and Ω6, radiative (Arad) and non-radiative (Anrad) decay rates, sensitization efficiency (η) and quantum yield (q) values in case of EuL13 derived from the optimized Sparkle/PM3 model

Structure

Ω2 (10−20 cm2)

Ω4 (10−20 cm2)

Ω6 (10−20 cm2)

ARAD (s−1)

ANRAD (s−1)

η (%)

q (%)

Experimental data Sparkle/PM3 monomer structure Sparkle/PM3 dimer structure

12.70 12.99 13.09

5.71 0.76 4.01

— 0.16 0.16

515.8 453.12 505.74

1611.86 1674.53 1621.92

24.2 21.3 23.8

0.55 21.1 23.5

Table 7 The theoretical intensity parameters Ω2, Ω4 and Ω6, radiative (Arad) and non-radiative (Anrad) decay rates quantum efficiency (η) and quantum yield (q) values in case of EuL23 derived from the optimized Sparkle/PM3 model

Structure

Ω2

Ω4

Ω6

ARAD (s−1)

ANRAD (s−1)

η (%)

q (%)

Experimental data Sparkle/PM3 monomer structure Sparkle/PM3 dimer structure

13.90 14.23 14.37

6.03 0.85 4.05

— 0.19 0.18

553.7 492.2 545.0

1946.3 2007.8 1955.0

22.0 19.7 21.8

0.5 19.5 21.6

J-mixing and nephelauxetic effect and previous studies of Horrocks52,53 and Choppin,54 where differences in the J-mixing degree are neglected, can be handled with a great care. Thus, the total formal negative charge can be calculated according to the following relationship proposed by Horrocks:52 υ ¼ 0:76p 2 þ 2:29p þ 17273;

Fig. 18

The Stark splitting of the emission bands in EuL23 at 77 K.

J-admixing of the other energy terms to the 7F0 level (7F1, 7F2, etc.), can be considered as a linear combination of these states: j7 F 0 >¼

X

CJM J 7 FJM J l

JM J

To estimate the degree of J-mixing the special term R02 can be introduced,50 and is equal to the ratio between the integral intensities between 5D0→7F0 and 5D0→7F2, the larger the R02 value, the larger degree of J-mixing. In our case typical R02 values (0.0094 for EuL13 and 0.0098 for EuL23) indicate a moderate J-mixing, which can explain an occurrence of a parity forbidden energy transfer T1→5D0 in the case of EuL23. Besides the site symmetry group and J-mixing degree the 5 D0→7F0 transition can provide useful information concerning the covalence degree of the metal–ligand bond. Though the metal–ligand bonds in lanthanide(III) complexes are presumably considered as almost purely ionic, a minor covalent degree (up to 5%) is also present51 and influences the position of the 0–0 transition. Recent investigations revealed that the position of the 5D0→7F0 transition is influenced by both the


where υ is the frequency of the 5D0→7F0 transition (in cm−1) and p is the total formal charge. In our case this value is equal to −3.3, which is in good agreement with the theoretically expected value of −3. Though the accuracy of this equation is pretty low and depends on the type of ligands in hand, it can be used to distinguish between the ligands on the inner- and outer-spheres, as only the inner-sphere ligands contribute to this equation. However, Choppin54 revealed that this equation is violated in the case of complexes with neutral donor groups and with large unbound anionic ligands. In the present case we deal with a quite simple bidentate ligand, where a pronounced conjugation between the donor and anionic ligating groups is not expected and therefore this equation is obeyed. Moreover, the co-ordination number around Eu3+ was estimated in accordance with the following expression:54 CNL ¼ 0:237Δν þ 0:628; and the value of 7.5 ± 1.0 was obtained, which is in line with our previously discussed modeling. This empirical equation was obtained by fitting an experimental plot for a series of compounds, and mirrors a decrease of an effective charge on the europium ion with an increasing number of ligated atoms within a coordination sphere around the central ion. However, a considerable drawback of this expression is the comparison between different types of ligands, disregarding the type of coordinated atom (both neutral and charged, as well as oxygenand nitrogen-donor atoms are considered). Therefore, a more thorough study of this correlation seems to be of a great desire and hence substantially less error could be observed. The first step towards considering the nature of coordinated atoms was made by Horrocks. In his later paper abundant data

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concerning the 5D0→7F0 band position in different complexes53 was gathered. Analyzing these observations Malta revealed that the covalency tends to be slightly higher for the β-diketonate oxygen than for the carboxylate oxygen,55 comparing Eu(btfa)3·2H2O and Eu(tta)3·2H2O (btfa = benzoyltrifluoroacetonate, tta = thenoyltrifluoroacetone) with Eu( picNO)3terpy ( picNO = picolinate-N-oxide, terpy = terpyridine) (a gaseous Eu3+ was chosen as a reference56). This affirmation becomes obvious if one compares the nephelauxetic parameters for β-diketonate oxygen (−17.0) and charged carboxylic oxygen (−10.5 on conversion to a single oxygen). Horrocks suggested that the difference between the observed 5D0→7F0 transition value and the reference sample (17 374 cm−1 for gaseous Eu3+) can be considered to be the sum of the impacts ascribed to each of the co-ordinated groups with the corresponding coefficients and calculated these values for complexes co-ordinated by several abundant functional groups for different co-ordination numbers. In our case, a similar analysis was carried out in order to reveal the coefficients for both the hydroxyl and phosphoryl groups, assuming the co-ordination number is equal to 7. To the best of our knowledge, there have not been coefficients δi calculated for the europium complexes co-ordinated by PvO groups, therefore we appeal to the work of Bünzli, where the 5D0→7F0 transition value in europium complexes with calix[4]arenes fitted with phosphinoyl pendant arms (L) was analyzed simultaneously with the co-ordination environment around Eu3+.57 Assuming an average transition value between two single bands ascribed to two conformers (17 263 cm−1), the calculated value of −16.0 was utilized to obtain the coefficient for hydroxyl group in EuL3. The obtained (−19.3) value matches fairly well with the value assigned to a β-diketonate oxygen (−21.0), rather than to an aliphatic hydroxyl oxygen (−14.2) or a charged carboxyl one (−10.5). Actually, Horrocks considered both oxygens in the carboxylate group as one single charged oxygen, therefore such deviation is easily explained, making the Ln–O bonds in the case of both the β-diketonate and the ortho-phosphorylated phenolate lanthanide complexes more covalent than those in the case of the aromatic carboxylates. Samarium, dysprosium and thulium complexes The luminescent properties of the samarium(III), dysprosium(III) and thulium(III) complexes, which also emit in the visible region (Fig. 19), attract considerably less attention in comparison with the terbium(III) and europium(III) species. The main reason for being under the shadow is their low quantum yields due to an occurrence of a large number of intermediate states and therefore a high probability of non-radiative deactivation via coupling with lattice vibrations. Nevertheless, several relatively brightly luminescent Sm3+,58 Dy3+ 59 and Tm3+ 60 complexes have been reported and even tested as active layers in organic LED structures. In the present work, the highest quantum yield (Table 8) was obtained in the case of the terbium and dysprosium complexes, and to the best of our knowledge it is the highest

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Fig. 19 The luminescence spectra of EuL13, TbL13, DyL13, SmL13 and TmL13 at 298 K.

Table 8 The lifetimes (298 K and 77 K) of an excited state and quantum yield (298 K) in the solid state

Sample

τ (μs) 298 K

τ (μs) 77 K

Quantum yield (%)

SmL13 SmL23 DyL13 DyL23 TmL13 TmL23 EuL13 EuL23 TbL13 TbL23 YbL13 YbL23

55.8 52.3 44.9 48.1 25.53 14.16 470 400 870 550 15 22

63.1 60.3 61.8 64.7 7.08 7.40 580 520 1450a 1490a n.a. n.a.

2.50 1.90 3.75 5.15 0.30 0.50 0.5 0.55 39.15 37.80 n.a. n.a.

a

Measurements were carried out in i-propanol.

quantum yield reported up to now for dysprosium complexes. Apparently, the relatively high quantum yield was promoted by both an appropriate energy gap between the resonance level of the lanthanide ion (4F9/2) and the 0–0 phonon transition component of the triplet level of the ligand and by an effective overlap between the overlying 4F7/2 state and the triplet ‘band’, promoting an effective energy transfer path T1→4F7/2→4F9/2 and an absence of solvent molecules within the inner co-ordination sphere. Though this energy gap is very close to that of the europium species, an absence of the LMCT state, which effectively quenches ionic photoluminescence in the case of the europium complexes, led to considerably higher quantum yields. The energy gap Δ in the samarium complexes in considerably larger, leading to a relatively low quantum yield. Contrary to the dysprosium complex, firstly, energy is promoted on one of the overlying levels in the samarium ion. It then follows a non-radiative path to either 4F3/2 or 4G5/2, after which it undergoes a radiative transition to the ground state. Though additional energy levels apparently quench the luminescent


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intensity by an efficient coupling with the vibrational modes of the crystal lattice, values obtained for the samarium complexes (2.50% for SmL13) are also one of the largest among the reported until now (5.0% in acetone-d6).61 Thulium(III) complexes usually possess considerably lower quantum yields, which are explained by the occurrence of additional intermediate states at ca. 15 000 cm−1, providing narrow energy gaps, which are effectively quenched by the lattice vibrations. At the same time, the resonance level of Tm3+ (21 200 cm−1) is almost equal to that of the terbium ion (20 400 cm−1), therefore the ligands sensitizing the terbium emission, should also be appropriate for the sensitization of the thulium samples. In the present paper the quantum yields of TmL3 are larger than the typical values in the case of the thulium(III) complexes,62 which is apparently explained by an absence of water molecules in the inner co-ordination sphere63 and a suitable triplet energy level. Surprisingly, the thulium complexes demonstrated considerably longer lifetimes (in microsecond region) in comparison with the previously described compounds (tens of nanoseconds), which is reflected in the relatively high quantum yields.64 At a low temperature (77 K) the band, corresponding to the intraligand transition (centered at ca. 395 nm for TmL13 and ca. 371 nm for TmL23), becomes bathochromically shifted in comparison with the spectra taken at room temperature (centered at ca. 346 nm for TmL13 and ca. 324 nm for TmL23) simultaneously with a disappearance of the 1 G4→3H6 band in the case of TmL13 (Fig. 20), a substantial reduce of intensity in the case of TmL23 and a decline of the lifetimes under lowering temperature, which is untypical for the lanthanide luminescent complexes. Furthermore, a substantial increase of the high energy shoulder is observed at 77 K in the excitation spectra (Fig. S12†) and is ascribed to a S2←S0 transition, suggesting S2←S0 as a preferable path at lower temperatures and S1←S0 at higher temperatures. At the same time, in the case of TmL23 an emission from the 1D2 level is observed at 77 K, which is not populated at higher

temperatures, whilst in the case of TmL13 a broad band ascribed to the T1→S0 transition is observed. The latter is in line with a difference in the triplet energy levels, a larger energy gap in the case of TmL13 provides a less efficient energy transfer on the 1G4 level of the thulium ion. Meanwhile, an occurrence of the 1D2→3F4 transition cannot be explained by a direct excitation to 1D2 or by the T1→1D2 energy transfer route and hence additional intermediate energy levels should be introduced, but this issue goes beyond the scope of the present paper. Previously, Serra faced a similar issue,65 where the emitting energy level of thulium was lying under the triplet level of the ligand. In that case an observed ionic luminescence was explained in terms of an additional charge–transfer state transferring energy to the 1G4 state however without any further thorough study.

Fig. 20 The emission spectra taken at 298 K (solid line) and 77 K (dotted line).

Fig. 21 The luminescence excitation (left) and luminescence spectra (right) of YbL23 at room temperature.


Ytterbium complexes The luminescent properties of the lanthanide complexes emitting in the infrared region has been acquiring an increasing attention in the recent years, and ytterbium complexes are the most promising ones, mainly due to the fact that they have the largest energy gap between the excited and initial states and hence have a lower probability of non-radiative quenching in comparison with other lanthanide ions emitting in the infrared region. A special interest is attracted to phosphoryl-containing ligands,66 mainly due to the lower vibrational frequencies of the PvO bonds in comparison with the C–O bonds and hence to a lower probability to cause non-radiative quenching of an excited state of the lanthanide ion. The luminescence spectra of both complexes YbL13 and YbL23 revealed a typical ionic luminescence (Fig. 21), corresponding to the transitions between the Stark sub-levels of 2F7/2 and 2F5/2 with the maximum at 975.0 nm. However, the monoexponential lifetimes of an excited state turned out to be of the same order of magnitude as the results obtained by group of Hasegawa. Such a result indicates the comparable efficiency of the non-radiative quenching without fluorination. The ytterbium ion possess only one transition, namely the magnetic-dipole 2F7/2→2F5/2, which as was previously

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mentioned is considered to be insensitive to a local co-ordination environment. The pure radiative lifetime should vary only insufficiently in case of different complexes and is believed to be equal to 2 ms.67 Hence, the calculated quantum yields are about 1% (0.0075 and 0.011 for YbL13 and YbL23, respectively), which is a typical value for ytterbium complexes.

Conclusions The detailed structural and photophysical analysis performed for the lanthanide complexes with two related aromatic o-phosphorylated phenolate ligands has shown a tight connection between the molecular structure and the optical properties of the lanthanide complexes. On the one hand, theoretical modelling carried out in the framework of DFT and PM3 calculations was corroborated with the analysis of the luminescent properties and EXAFS measurements. On the other hand, only a minor change in the ligand structure reflected in a change of energy transfer path. It is noteworthy that special attention was dedicated to the analysis of dysprosium, samarium and thulium complexes, which still arouse only a limited interest in comparison with the terbium and europium species. Eventually, this new class of anionic aromatic o-phosphorylated phenolate ligands can pave the way towards the controlled synthesis and design of new classes of highly effective lanthanide complexes due to the wide possibilities in the introduction of substituents and the adjustment of the denticity.

Experimental Materials and methods The following commercially available chemicals were used without further purification: Gd(NO3)3·6H2O (Aldrich), sodium hydroxide (Aldrich), methanol (Aldrich), ethanol (Aldrich). Sm(NO3)3·6H2O, Eu(NO3)3·6H2O, Tb(NO3)3·6H2O, Dy(NO3)3·6H2O, Tm(NO3)3·6H2O, Yb(NO3)3·6H2O and Lu(NO3)3·4H2O were synthesised by treating the respective lanthanide oxides (99.998% grade) with concentrated nitric acid, followed by the evaporation of excess acid. HL1 and HL2 were synthesized according to the procedure described elsewhere.14,68 All of the LnL3 complexes were synthesised in accordance with a previously described synthetic route.14 All of the complexes were dried in a vacuum (10−2 bar) to eliminate solvent residues. Complex SmL13. Anal. calcd for C60H54SmP3O6 (%): C 64.67, H 4.88. Found: C 64.75, H 4.82. Complex SmL23. Anal. calcd for C60H54SmP3O6 (%): C 64.67, H 4.88. Found: C 64.71, H 4.95. Complex EuL13. Anal. calcd for C60H54EuP3O6 (%): C 64.58, H 4.88. Found: C 64.22, H 4.90. Complex EuL23. Anal. calcd for C60H54EuP3O6 (%): C 64.58, H 4.88. Found: C 64.30, H 4.95.

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Complex GdL13. Anal. calcd for C60H54GdP3O6 (%): C 64.27, H 4.85. Found: C 64.34, H 4.92. Complex GdL23. Anal. calcd for C60H54GdP3O6 (%): C 64.27, H 4.85. Found: C 64.36, H 4.90. Complex TbL13. Anal. calcd for C60H54TbP3O6 (%): C 64.18, H 4.85. Found: C 64.47, H 4.87. Complex TbL23. Anal. calcd for C60H54TbP3O6 (%): C 64.18, H 4.85. Found: C 63.93, H 4.88. Complex DyL13. Anal. calcd for C60H54DyP3O6 (%): C 63.97, H 4.83. Found: C 63.90, H 5.00. Complex DyL13. Anal. calcd for C60H54DyP3O6 (%): C 63.97, H 4.83. Found: C 64.02, H 4.93. Complex TmL13. Anal. calcd for C60H54TmP3O6 (%): C 63.61, H 4.80. Found: C 63.70, H 4.87. Complex TmL13. Anal. calcd for C60H54TmP3O6 (%): C 63.61, H 4.80. Found: C 63.64, H 4.95. Complex YbL13. Anal. calcd for C60H54YbP3O6 (%): C 63.38, H 4.79. Found: C 63.45, H 4.87. Complex YbL13. Anal. calcd for C60H54YbP3O6 (%): C 63.38, H 4.79. Found: C 63.42, H 4.72. Complex LuL13. Anal. calcd for C60H54LuP3O6 (%): C 63.27, H 4.78. Found: C 63.19, H 5.03. Complex LuL13. Anal. calcd for C60H54LuP3O6 (%): C 63.27, H 4.78. Found: C 63.35, H 4.95. Elemental analyses (C, H, N) were performed on a Vario Micro Cube (Elementar, Germany) by the Microanalytical Service of the Lomonosov Moscow State University. IR spectra were recorded on bulk samples in the range of 4000–600 cm−1 with a Perkin-Elmer Spectrum One spectrometer equipped with a universal attenuated total reflection sampler. The lowfrequency IR spectra were recorded on a Perkin–Elmer 1720 in the range of 50–800 cm−1. Raman spectra were recorded in the range of 400–2000 cm−1with a Renishaw InVia spectrometer . LDI-TOF mass spectra were run on an Autoflex II (Bruker Daltonics, Germany) using the electron-impact positive mode (accelerating voltage 19 kV) and a nitrogen laser (337 nm, impulse duration 1 ns). The local structure of the two terbium complexes, viz., TbL13 and TbL13, has been elucidated using Tb L3-edge EXAFS spectroscopy. The X-ray absorption spectra were measured at the Structural Materials Science beamline69 of the Kurchatov Centre for Synchrotron Radiation (National Research Centre “Kurchatov Institute”, Moscow, Russia) in the transmission mode using two ionization chambers filled with appropriate N2–Ar mixtures. The experimental data reductions and analyses were performed using the IFEFFIT software suite70 with FEFF71 ab initio photoelectron phase and amplitude functions. X-ray photoelectron spectroscopy (XPS) measurements were carried out in an ultra-high vacuum (UHV) set-up equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV), operated at 14.5 kV and 35 mA, and a high resolution Gammadata-Scienta SES 2002 analyzer. The base pressure in the measurement chamber was maintained at about 7 × 10−10 bar. The measurements were carried out in the fixed transmission mode with a pass energy of 200 eV resulting in an overall energy resolution of 0.25 eV. A flood gun was applied to compensate the charging effects. The high-


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resolution spectra for the C 1s, O 1s and P 2p photoelectron lines were recorded. The binding energy scales were corrected to the charge shift by referring the most intensive sp2 hybridized C 1s contribution to 284.5 eV. The Casa XPS software with a Gaussian–Lorentzian product function and Shirley background subtraction was used for peak deconvolution. Emission and excitation spectra were measured with a Fluorolog FL3–22 spectrofluoremeter from Horiba-Jobin-Yvon Ltd. and corrected for instrumental functions. The ytterbium complexes were measured with an Edinburgh Instruments FLSP920 UV−vis-NIR spectrofluorimeter, using a 450W xenon lamp as the steady-state excitation source and a Hamamatsu R928P PMT detector, which has a response curve between 200 and 900 nm. Lifetimes were determined under ligand excitation and by monitoring the 5D0→7F2 (EuIII), 5D4→7F5 (TbIII), 1 G4→3H6 (TmIII), 5G5/2→5H9/2 (SmIII) and 7F9/2→6H13/2 (DyIII) using either a Fluorolog FL3–22 spectrofluoremeter or a homebuilt system with a pulsed laser, utilizing three harmonics of Nd-YAG (266, 355, 532 nm) detected by an intensified ANDOR iStar CCD camera synchronized to the laser pulses. In the case of the ytterbium complexes the 2F7/2→2F5/2 transition was monitored using an Edinburgh Instruments FLSP920 UV-visNIR spectrofluoremeter with a 60 W pulsed xenon lamp operating at a pulse frequency of 100 Hz. Luminescence decays were analyzed with Origin and proved to be single-exponential functions in all cases. Quantum yields were determined with the Fluorolog FL3–22 spectrofluoremeter at room temperature under excitation into ligand states according to an absolute method using an integration sphere. Each sample was measured three times to get an average value. The estimated error for the quantum yields is ±10%.

Acknowledgements The corresponding author is utterly thankful to Prof. Yulia Gorbunova of N.S. Kurnakov Institute of General and Inorganic Chemistry RAS for fruitful discussion of the manuscript.

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