Synthesis, crystal structure and photophysical properties of europium(III) and terbium(III) complexes with pyridine-2,6-dicarboxamide

Inorganica Chimica Acta 360 (2007) 102–108 www.elsevier.com/locate/ica

Synthesis, crystal structure and photophysical properties of europium(III) and terbium(III) complexes with pyridine-2,6-dicarboxamide Stefania Tanase a b

a,*

, Patricia Marques Gallego a, Rene´ de Gelder b, Wen Tian Fu

a

Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands Radboud University Nijmegen, Institute for Molecules and Materials, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands Received 14 June 2006; received in revised form 25 July 2006; accepted 25 July 2006 Available online 30 August 2006 Inorganic Chemistry – The Next Generation.

Abstract The reactions of pyridine-2,6-dicarboxamide with europium(III) and terbium(III) triflates led to the formation of mononuclear complexes of formula [Ln(pcam)3](CF3SO3)3 (Ln = Eu 1, Tb 2; pcam stands for pyridine-2,6-dicarboxamide). From single-crystal X-ray diffraction analysis, the complexes were found to be isomorphous and isostructural. The [Ln(pcam)3]3+ cations and triflate counterions are connected by intermolecular hydrogen bonds, resulting in a 3D network structure. Both the europium(III) and terbium(III) complexes exhibit efficient ligand sensitized luminescence in the visible region with lifetimes of 1.9 ms and 2.2 ms, respectively, in the solid state.  2006 Elsevier B.V. All rights reserved. Keywords: Lanthanide; Pyridine-2,6-dicarboxamide; Luminescence

1. Introduction The desire for tailored complexes for use as fluorescent probes in biological media and optical amplification in fiber-optic telecommunications systems is a driving interest in research on luminescent lanthanide complexes [1–8]. In particular, europium(III) and terbium(III) complexes have attracted attention due to their well-defined luminescence properties, including hypersensitivity to the coordination environment, narrow bandwidth and millisecond luminescence decay times [9,10]. The absorption coefficients of the optical transitions for these ions are, however, very low which limits their practical application considerably. This drawback can be overcome through the use of highly absorbent chelating ligands, which serve as efficient sensitizers. Although energy transfer to lanthanide(III) ions can occur from the excited singlet state of the ligand [11], *

Corresponding author. Tel.: +31 71 5274345; fax: +31 71 5274451. E-mail address: [email protected] (S. Tanase).

0020-1693/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.07.115

in the majority of the systems reported, the excited singlet state of the ligand undergoes fast intersystem crossing to the triplet state before engaging in intramolecular energy transfer. The intersystem crossing process is facilitated by the large spin–orbit coupling induced by the proximity of the lanthanide metal ion [12]. The luminescent ligand acts as an antenna chromophore, by analogy to the light harvesting center in photosynthetic systems. Typical organic ligands include (i) polycarboxylate ligands [13–16] due to their preference to form ionic bonds and hypersensitive luminescence under reduced site-symmetry; (ii) b-diketone ligands or the combination of b-diketones and a sensitizer [17,18], and (iii) carboxamide ligands [19–22] that allow the investigation of subtle electronic and steric effects induced by wrapped strands in triple-helicate lanthanide complexes. The ligand pyridine-2,6-dicarboxylic acid (H2dipic) has been shown to be an efficient sensitizer for europium(III) and terbium(III) luminescence in the solid state. Particularly, [Eu(dipic)3]3 is an efficient emitter through ligandto-metal energy transfer giving rise to an intense red

S. Tanase et al. / Inorganica Chimica Acta 360 (2007) 102–108

emission upon excitation with UV light at room temperature [23]. The luminescence spectra of Na3[Eu(dipic)3] Æ nH2O have been studied in detail with respect to the variation in crystal field splitting, the linewidths and the relative intensities induced by changes in crystal hydration [24]. Different arrangements of the hydration sphere have resulted in distinct and resolvable features of the luminescence emission spectra. A decrease in the lifetime and lower efficiency is observed as a result of efficient vibrational deactivation of the excited state (5D0) of europium(III) by O–H vibrational modes of the coordinated water molecules [23,25]. To overcome this deactivation pathway, the functionalization of pyridine-2, 6-dicarboxylic acid with alkyl amines has been employed with success in the synthesis of lanthanide mono- and dimetallic 4f–4f and d–4f triple-helical assemblies with mono- and di-topic ligands [3,26–30]. The theoretical and rational modeling of the self-assembly of these molecular edifices would enable a more rational approach for the design of multimetallic lanthanide complexes with optimized structural and photophysical properties [3,26–30]. In the present contribution, the photophysical properties of cationic lanthanide complexes of pyridine-2, 6-dicarboxamide (pcam) are examined to gain a better understanding of photoinduced energy transfer processes in non-covalently linked donor–acceptor systems in the solid state. Herein, europium(III) and terbium(III) complexes of pyridine-2,6-dicarboxamide have been investigated systematically and their synthesis, crystal structure and photophysical properties are reported. Particular attention is focused on their solid state photophysical properties and they are compared to their pyridine-2,6-dicarboxylate analogues. Furthermore, the solution stability and their electrochemical properties of the europium(III) and terbium(III) complexes of pyridine-2,6-dicarboxamide are discussed. 2. Experimental

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[Eu(pcam)3](CF3SO3)3 (1). Yield: 117 mg (59%). Anal. Calc. for C24H21EuF9N9O15S3 (1094.67): C, 26.34; H, 1.93; N, 11.52; S, 8.79. Found: C, 26.23; H, 1.55; N, 11.62; S, 8.36%. IR (mmax/cm1): 3362 (w), 3195 (br), 1668 (s), 1573 (m), 1422 (w), 1274 (m), 1243 (m), 1228 (s), 1182 (w), 1158 (m), 1123 (w), 1029 (s), 1016 (s), 883 (m), 752 (m), 652 (m), 626 (s), 573 (w), 514 (s), 412 (w). [Tb(pcam)3](CF3SO3)3 (2). Yield: 132 mg (67%). Anal. Calc. for C24H21TbF9N9O15S3 (1101.56): C, 26.17; H, 1.92; N, 11.44; S, 8.73. Found: C, 26.61; H, 1.81; N, 11.73; S, 8.32%. IR (mmax/cm1): 3372 (w), 3194 (br), 1668 (s), 1575 (m), 1423 (w), 1276 (m), 1243 (m), 1229 (s), 1184 (w), 1158 (m), 1122 (w), 1029 (s), 1016 (s), 884 (m), 754 (m), 653 (m), 627 (s), 581 (w), 514 (s), 413 (w). 2.1.2. Physical measurements C, H, N and S analyses were performed with a Perkin– Elmer 2400 series II analyzer. Infrared spectra (4000– 300 cm1, resol. 4 cm1) were recorded on a Perkin–Elmer Paragon 1000 FTIR spectrometer equipped with a Golden Gate ATR device, using the reflectance technique. The electrochemical measurements were performed with an Autolab PGstat10 potentiostat controlled by GPES4 software. A three-electrode system was used, consisting of a platinum working electrode, a platinum auxiliary electrode, and an Ag/AgCl reference electrode. The experiments were carried out at room temperature under argon with tetrabutylammonium hexafluorophosphate as the supporting electrolyte. All potentials are reported relative to Ag/AgCl. Electrospray mass spectra (ESI-MS) in CH3CN or CH3OH were recorded on a Thermo Finnigan AQA apparatus. The excitation and emission spectra were recorded with a Perkin Elmer LS50B luminescence spectrometer. The intensity of the emission spectra was corrected for the sensitivity of the detector. Luminescent lifetime was measured using an excimer laser pumped dye laser (Lambda Physik) for excitation (pulse width 20 ns) and 0.2 m Acton Research monochromator with a RCA C31034 photomultiplier tube for detection of the emission. Luminescence decay curves were recorded using a Tektronix 200 MHz digital oscilloscope.

2.1. General remarks Starting materials were purchased from Aldrich and all manipulations were performed using materials as received. The complexes Na3[Ln(dipic)3] Æ nH2O (Ln = Eu, Tb) were synthesised by literature methods [31]. 2.1.1. General Synthesis of [Ln(pcam)3](CF3SO3)3 A solution of Ln(CF3SO3)3 Æ xH2O (0.18 mmol) in acetonitrile (5 ml) was added to a suspension of pyridine-2,6dicarboxamide (0.45 mmol) in acetonitrile (5 ml). The reaction mixture was stirred for 5 min at room temperature and then the solution was filtered to remove any insoluble material. Slow solvent evaporation of the filtrate leads to the formation of crystalline materials in all cases. Crystals suitable for X-ray diffraction were obtained by slow diffusion of diethylether into acetonitrile solution.

2.1.3. X-ray crystallographic analysis and data collection Intensity data for single crystals of 1 and 2 were col˚ ) on a Nonius lected using Mo Ka radiation (k = 0.71073 A Kappa CCD diffractometer. The intensity data were corrected for Lorentz and polarization effects, for absorption (w-scan absorption correction) and extinction. The structures were solved by Patterson methods. The programs EvalCCD, DIRDIF96 and SHELXL-97 were used for data reduction, structure solution and structure refinement, respectively [32–34]. Refinement of F2 was done against all reflections. The weighted R factor, wR, and goodness of fit S are based on F2. Conventional R factors are based on F, with F set to zero for negative F2. All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogens were placed at calculated positions and were refined riding on the parent atoms. There are two

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˚ 3 and types of voids in the structure with volumes of 85 A 3 ˚ 150 A , containing, according to the SOLVE procedure of PLATON [35], 40 and 55 electrons, respectively. Based on the synthetic route and the crystallization procedure, these voids can contain disordered water, acetonitrile and/or diethylether. We used the SQUEEZE procedure of PLATON to correct for these electron densities. The presence of the solvent molecules was not taken into account while calculating the physical properties of the compound, since an unambiguous interpretation of the contents of the voids is not feasible. Geometric calculations and molecular graphics were performed with the PLATON package [35]. 3. Results and discussions 3.1. Synthesis and description of crystal structure The ligand pyridine-2,6-dicarboxamide (pcam) is of particular interest because (i) it provides hard oxygen and nitrogen donors which favour the coordination to lanthanide(III) ions and (ii) it provides for none-coordination of the lanthanide(III) ion upon appropriate stoichiometric reaction with the lanthanide salts examined. The reaction of lanthanide salts with the ligand pcam in certain solvents, such as acetonitrile, prevents crystal hydration thus blocking a key pathway for non-radiative relaxation of the excited states of lanthanide(III) ion through coupling with O–H vibrations. The [Ln(pcam)3](CF3SO3)3 complexes (1 and 2) were obtained by the reaction of Ln(CF3SO3)3 Æ xH2O with the neutral donor ligand pyridine-2,6-dicarboxamide in acetonitrile and at room temperature. It was found that the use of a lanthanide to ligand molar ratio of 1:2.5 provides the highest yields (59–67%). Analytically pure compounds were obtained in all cases as indicated by elemental analysis. Crystals suitable for X-ray diffraction studies were obtained by slow diffusion of diethylether into an acetonitrile solution. The infrared spectra of 1 and 2 display sharp absorptions in the range 3190–3670 cm1 assigned to mN–H stretching vibrations. The carbonyl stretching frequency is blue shifted relative to that of the free ligand from 1668 cm1 to 1695 cm1 in complexes 1 and 2, which indicates that coordination of the carbonyl oxygen to the lanthanide(III) ions takes place. The absorption bands at 1243 cm1 in 1 and at 1246 cm1 in 2 are characteristic to the mS@O stretching vibrations of the non-coordinate triflate anion; the splitting of this absorption into three components indicates decreased symmetry due to the hydrogen bonding interactions, in agreement with the crystal structure described below. Complexes 1 and 2 are isostructural and crystallize in the space group R-3. As expected, the asymmetric unit contains one third of the lanthanide cation complex [Ln(pcam)3]3+ and one CF3 SO3  counterion. Crystallographic data for 1 and 2 are collected in Table 1 and relevant bond distances and angles are given in Table 2. A Pluton drawing with the atom numbering scheme for the europium(III) complex 1 is shown in Fig. 1. The coordina-

tion polyhedron around the europium(III) ion is best described by a distorted tricapped trigonal prismatic geometry where the six oxygen atoms of the carboxamide groups occupy the vertices of the trigonal prism and the three nitrogen atoms of the pyridine rings occupy the equatorial plane. The twist angle between the two faces of the trigonal prism is 0; the centroids of these planes are separated by ˚ . The europium(III) ion lies on a threefold rotation 3.249 A axis and it is located in the plane formed by the nitrogen atoms of the pyridine rings. The average Eu–N(py) of ˚ and the average Eu–O(amide) of 2.406 A ˚ bond 2.544 A lengths are similar to those found in the related lanthanide complexes with carboxamide ligands and pyridine-2,6dicarboxylate [23,26–30]. The analysis of the packing mode of 1 is of special interest because of hydrogen-bonding interactions present. The cationic complex [Eu(pcam)3]3+, and the anionic Table 1 Crystal data and structure refinement for 1 and 2

Formula Formula weight Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) V (A Z Dcalc (Mg m3) l (Mo Ka) (mm1) Crystal size (mm) T (K) Data collected Unique data Rint R(F) [I > 2r(I)] Rw(F2) S ˚ 3) Dqmin, Dqmax (e A

1

2

C24H21EuF9N9O15S3 1094.64 trigonal R-3 15.5167(9) 15.5167(9) 29.9753(18) 90 90 120 6250.2(6) 6 1.745 1.768 0.08 · 0.06 · 0.04 208 23 194 2451 0.128 1841 0.0746 1.127 1.657, 0.790

C24H21TbF9N9O15S3 1101.63 trigonal R-3 15.4410(10) 15.4410(10) 30.152(3) 90 90 120 6225.9(9) 6 1.763 1.967 0.10 · 0.10 · 0.10 208 23 889 3192 0.109 2399 0.0748 1.109 1.007, 1.292

Table 2 ˚ ) and angles () for 1 and 2 Selected bond lengths (A Bond lengths

Bond angles

1 Eu–O1 Eu–O2 Eu–N3

2.415(8) 2.397(6) 2.544(8)

O1–Eu–O2 O1–Eu–N3 O1–Eu–O1a O1–Eu–O2a O1–Eu–N3a O2–Eu–O2a

125.9(2) 63.0(3) 79.5(3) 148.1(2) 135.7(3) 79.3(2)

2 Tb–O1 Tb–O2 Tb–N3

2.385(7) 2.396(6) 2.520(7)

O1–Tb–O2 O1–Tb–N3 O1–Tb–O1a O1–Tb–O2a O1–Tb–N3a O2–Tb–O2a

127.1(2) 63.6(2) 78.5(2) 87.3(2) 71.4(2) 78.4(2)

S. Tanase et al. / Inorganica Chimica Acta 360 (2007) 102–108

counterions, CF3 SO3  , are held together by both electrostatic forces and hydrogen bonding interactions. The NH2 donor groups of the three pcam ligands are strongly linked to the triflate acceptors by N–H  O ˚ and av. N– hydrogen bonds (av. N  O = 2.919(7) A H  O = 161.2(5)). Each pcam ligand uses its NH2 groups to hydrogen bond with four different CF3 SO3  groups; concomitantly, each CF3 SO3  anion accepts four hydrogen bonds from four NH2 groups of four different [Eu(pcam)3]3+ cations to form a three-dimensional network structure (Fig. 2). The overall structure is further stabilized by weak C–H  O contacts established between

the aromatic rings and the CF3 SO3  anions (av. ˚ and av. C–H  O = 156.5(5)). The C  O = 3.345(5) A ˚ . This value is shortest Eu  Eu separation is 8.963 A smaller as compared to the corresponding distance (ca. ˚ ) in the pyridine-2,6-dicarboxylate analogues with 10 A various counter cations [23]. 3.2. Photophysical properties in solid state Emission lifetimes and emission spectra of complexes 1 and 2 were measured at room temperature on solid samples. Fig. 3 shows the emission spectrum of complex 1 obtained with kexc = 254 nm. The spectrum is composed of the well-known europium(III) red emission lines which

N3

O2

N1

O1

Eu1

Intensity/arbitr. units

400

N2

105

J=2

300

5D 0

200

7F J

100

J=1 J=4 0 600 Fig. 1. Pluton projection of the complex cation [Eu(pcam)3]3+ in 1. Hydrogens are omitted for clarity.

650 λ /nm

700

Fig. 3. The emission spectrum (kexc = 245 nm) of the complex [Eu(pcam)3](CF3SO3)3 (1) in the solid state.

Fig. 2. View of the hydrogen bonding network in 1.

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are assigned to transitions between the first excited state (5D0) and the ground multiplet 7FJ (J = 0–6). Emission bands were observed at 594, 617 and 696 nm and were attributed to the f–f transitions 5D0 ! 7F1 (magnetic dipole transition), 5D0 ! 7F2 and 5D0 ! 7F4 (electric dipole transitions), respectively. The hypersensitive 5D0 ! 7F2 transition is the most intense, pointing to a highly polarizable chemical environment around the europium(III) center. Changes in the excitation wavelength did not produce significant alterations either in the number of emission components or in the spectral energy distribution. The excitation spectrum of 1 was obtained by monitoring the emission wavelength of europium(III) at 617 nm; it exhibits broad excitation features in the range 200–300 nm (kexc = 248) that can be assigned to the p ! p* transition centered onto the pyridine-2,6-dicarboxamide ligand. The strong absorption centered at 395 nm is attributed to 7 F0 ! 5L6, 5G2, 5L7 and 5G3 transition of europium(III) [36]. The emission decay kinetics of the europium(III) complex for the most intense emission (617 nm) are monoexponential, suggesting the presence of a single luminescent site in complex 1 and a homogenous sample in the solid state in agreement with the results from the crystal structure determination which show a single crystallographic site for Eu. The decay curves are fitted into a single-exponential function as I = I0 exp(t/s), where I and I0 are the luminescent intensities at times t and 0 and s is defined as the luminescence lifetime. The experimentally fitted value of s for 1 is 1.9 ms. This value is very similar to the value found for the complexes Na3[Eu(dipic)3] Æ nH2O (s = 1.3 ms) [37] and [N(C2H5)4]3[Eu(dipic)3] Æ nH2O (s = 2.02 ms) [23]. The emission spectrum of 2 (Fig. 4) excited into the lowest energy ligand centered absorption (kexc = 254 nm) exhibits typical narrow band features, originating from the transition from the 5D4 ground state to the 7FJ (J = 6–0) multiplet. The bands at 490, 584 and 623 nm are attributed to the transitions from the 5D4 state to the 500

7

F6, 7F4 and 7F3, respectively; these transitions have medium intensity and show moderate sensitivity to the ligand environment. The most intense band is observed at 545 nm and it is due to the 5D4 ! 7F5 transition. The excitation spectrum of the 545 nm emission shows two strong absorptions at 248 nm due to the p ! p* transition of the ligand and at 326 nm due to the ligand to metal energy transfer. The observed multiple bands at 353 nm, 370 nm and 380 nm are assigned to the 7F6 ! 5G5, 5D2, 5G4 and 5 L9, and 7F6 ! 5D3, 5G6 and 5L10 transitions, respectively [36]. The luminescence decay curve of the 545 nm emission for complex 2 in the crystalline state was measured at room temperature. The decay curve was fitted with single-exponential decay time in the millisecond range (s = 2.2 ms). The excitation spectra show that europium(III) and terbium(III) emission is observed for excitation at short wavelengths in bands that are attributed to ligand absorption. The location of the ligand states at high energy prevents both a mixing of these states with the 4f states and a back-transfer from the excited lanthanide ion to the ligand. The vibronic coupling involving the C@O groups and the lanthanide(III) levels plays a key role in the energy transfer from the donor energy level of the ligand to the emitting level of the lanthanide(III) [23]. Therefore, the lifetimes measured for Eu(5D0) excited level in 1 and Tb(5D4) in 2 are comparable to those found for the analogues containing the ligand pyridine-2,6-dicarboxylate. The relative long lifetimes for 1 and 2 are diagnostic for the absence of highfrequency oscillators in the first coordination sphere. Although there are high-energy N–H vibrations in the complexes, these are not in the first coordination sphere and thus are less effective in causing radiationless deactivation. For the particular case of [Ln(pcam)3](CF3SO3)3, it appears that the N–H  O hydrogen bonds involving the lanthanide cationic species and the triflate counterions play an important role in the energy transfer process. Replacement of CF3 SO3  anions with chloride anions results in less efficient energy transfer and we are currently exploring this critical aspect. 3.3. Solution stability and electrochemical properties

J=5 Intensity/arbitr. unit.

400

300

200

5D 4

J=6

100

0 450

J=4 500

550

600

7F J

J=3 650

700

λ /nm Fig. 4. The emission spectrum (kexc = 245 nm) of the complex [Tb(pcam)3](CF3SO3)3 (2) in the solid state.

Evidence for the existence of the cationic species [Ln(pcam)3]3+ in solution was obtained by ESI-MS. The ESI-MS analysis of the acetonitrile solutions of lanthanide to ligand molar ratio of 1:2.5 after 30 min of reaction displayed a major peak that corresponds to the fragment {[Ln(pcam)3](CF3SO3)2}+ (m/z = 945 for 1 and m/z = 951 for 2). Other minor peaks could be assigned to the fragments formed by the loss of a second triflate anion or a pcam ligand. These results show that the complexation reactions are extremely fast as compared with substituted pyridine-2,6-dicarboxamide ligands that show considerable kinetic inertness as a result of intramolecular hydrogen bonds [26]. The dissolution of 1 and 2 in methanol gives ESI-MS spectra similar to those obtained upon direct reaction of

S. Tanase et al. / Inorganica Chimica Acta 360 (2007) 102–108

E2=-0.38 V

107

energy transfer between [Ln(pcam)3]3+ cationic species and anionic species containing transition metal ions that have high-lying energy levels is currently under investigation.

E3=-0.22 V

5. Supplementary data Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC 609690 and 609691. Acknowledgements E1=-0.78 V

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

E/V vs Ag/AgCl Fig. 5. Cyclic voltammogram of a solution of [Eu(pcam)3](CF3SO3)3 103 M in methanol at 0.1 V s1 scan rate.

the lanthanide salts and pcam ligand. Therefore, the redox behaviour of [Ln(pcam)3]3+ species in solution was further studied by cyclic voltammetry in methanol, in the presence of 0.1 M Bu4NPF6 with a Pt working electrode and an Ag reference electrode. The cyclic voltammogram of [Eu(pcam)3](CF3SO3)3 between 1.2 and 1.5 V at a scan rate of 0.1 V s1 (Fig. 5) shows one irreversible reduction at 0.78 V which is ascribed to the Eu3+/Eu2+ couple; the peak at ca. 0.22 V is likely to be due to the dynamic exchange of pyridine-2,6-dicarboxamide with methanol solvent molecules. Notably, the voltammogram of the complex Na3[Eu(dipic)3] Æ nH2O does not show any redox behaviour in the range 1.2 to 1.5 V. These results suggest the destabilization of europium(III) and stabilization of europium(II) by the cavity formed by the three ligands pcam with respect to the Na3[Eu(dipic)3] Æ nH2O. For [Tb(pcam)3](CF3SO3)3, the cyclic voltammogram shows only one peak at 1.27 V that is assigned to the ligand reduction by comparison with the voltammogram of the pure ligand. 4. Conclusions Two new europium(III) and terbium(III) complexes, [Ln(pcam)3](CF3SO3)3 (Ln = Eu, Tb), with pyridine-2,6dicarboxamide (pcam) were synthesized and characterized by various techniques. The molecular structures of europium(III) (1) and terbium(III) (2) complexes show that the three ligands are each tridentate coordinated around the lanthanide(III) ion to form a tricapped trigonal prismatic arrangement. The photophysical properties in the solid state reveal the presence of a single luminescent site in complexes 1 and 2 and efficient ligand-to-metal energy transfer. The relatively long lifetimes are indicative of the absence of water molecules in the inner coordination sphere. Further work aiming at the study of the electronic

The authors thank Prof. Jan Reedijk (Leiden University) and Dr. Wesley R. Browne (Groningen University) for valuable discussions. Prof. Andries Meijerink (Utrecht University) is acknowledged for assistance with the lifetime measurements and critically reading the manuscript. This research was supported by a Veni grant from The Netherlands Organization for Scientific Research (NWO) to S.T. References [1] J.C.G. Bunzli, Acc. Chem. Res. 39 (2006) 53. [2] P. Gawryszewska, J. Sokolnicki, J. Legendziewicz, Coord. Chem. Rev. 249 (2005) 2489. [3] J.C.G. Bunzli, C. Piguet, Chem. Soc. Rev. 34 (2005) 1048. [4] I. Hemmila, V. Laitala, J. Fluoresc. 15 (2005) 529. [5] Y. Hasegawa, Y. Wada, S. Yanagida, J. Photochem. Photobiol. CPhotochem. Rev. 5 (2004) 183. [6] S. Faulkner, S.J.A. Pope, B.P. Burton-Pye, Appl. Spectrosc. Rev. 40 (2005) 1. [7] K. Kuriki, Y. Koike, Y. Okamoto, Chem. Rev. 102 (2002) 2347. [8] N. Sabbatini, M. Guardigli, I. Manet, in: C.N. Douglas, D.H. Volman, G. von Bunau (Eds.), Advances in Photochemistry, John Wiley & Sons, Inc., 1997. [9] J.C.G. Bunzli, in: J.C.G. Bunzli, G.R. Choppin (Eds.), Lanthanide Probes in Life, Chemical and Earths Sciences: Theory and Practice, Elsevier, Amsterdam, 1989. [10] W.T. Carnall, in: K.A. Gschneidner, L.R. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, North-Holland, Amsterdam, 1979. [11] G.A. Hebbink, S.I. Klink, L. Grave, P. Alink, F. van Veggel, Chem. Phys. Chem. 3 (2002) 1014. [12] G.A. Hebbink, L. Grave, L.A. Woldering, D.N. Reinhoudt, F. van Veggel, J. Phys. Chem. A 107 (2003) 2483. [13] M. Kawa, J.M.J. Frechet, Chem. Mater. 10 (1998) 286. [14] S.L. Wu, W.D. Horrocks, J. Chem. Soc., Dalton Trans. (1997) 1497. [15] S.L. Wu, S.J. Franklin, K.N. Raymond, W.D. Horrocks, Inorg. Chem. 35 (1996) 162. [16] G.R. Choppin, P.J. Wong (Eds.), in: Coordination Chemistry, 1994. [17] T.C. Morrill, Lanthanide Shift Reagents in Stereochemical Analysis, VCH, New York, 1986. [18] R.E. Sievers, Nuclear Magnetic Resonance Shift Reagents, Academic Press, New York, 1973. [19] C. Piguet, M. Borkovec, J. Hamacek, K. Zeckert, Coord. Chem. Rev. 249 (2005) 705. [20] N. Andre, T.B. Jensen, R. Scopelliti, D. Imbert, M. Elhabiri, G. Hopfgartner, C. Piguet, J.C.G. Bunzli, Inorg. Chem. 43 (2004) 515. [21] S. Floquet, N. Ouali, B. Bocquet, G. Bernardinelli, D. Imbert, J.C.G. Bunzli, G. Hopfgartner, C. Piguet, Chem. Eur. J. 9 (2003) 1860. [22] J.C.G. Bunzli, C. Piguet, Chem. Rev. 102 (2002) 1897.

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