Intermolecular Interactions as Actors in Energy-Transfer Processes in

Publication Date (Web): June 12, 2009. Copyright © 2009 ... For instance, in addition to the classical ligand → Eu charge-transfer state (LMCT), an...
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J. Phys. Chem. B 2009, 113, 9265–9277

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Intermolecular Interactions as Actors in Energy-Transfer Processes in Lanthanide Complexes with 2,2′-Bipyridine Lada N. Puntus,*,†,‡ Konstantin A. Lyssenko,‡ Irina S. Pekareva,† and Jean-Claude G. Bu¨nzli§ Laboratory of Molecular Nanoelectronics, Institute of Radio Engineering & Electronics, Russian Academy of Sciences, 11-7 MokhoVaya, Moscow 125009, Russia, A. N. NesmeyanoV Institute of Organoelement Compounds, Russian Academy of Sciences, GSP-1, 28 VaViloV Street, Moscow 119991, Russia, and Laboratory of Lanthanide Supramolecular Chemistry, E´cole Polytechnique Fe´de´rale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland ReceiVed: March 17, 2009; ReVised Manuscript ReceiVed: May 8, 2009

A series of lanthanide complexes [LnClx(bpy)y(H2O)z]Cl3-x(H2O)n(EtOH)m (Ln ) Eu, Gd, Tb; x ) 1, 2; y ) 1, 2; z ) 2-4) with different numbers of 2,2′-bipyridine (bpy), chloride ions, and water molecules in the inner coordination sphere were synthesized and investigated with the aim of relating their molecular geometry and crystal packing to the efficiency of ligand-to-metal energy transfer. In conjunction to the rotation of the pyridine rings upon coordination to the Ln ion, the high flexibility displayed by bpy ligands leads to rather unexpected bending of these rings with respect to the central bond, owing to intermolecular interactions such as Cl · · · π and π-stacking ones. Deciphering the luminescence properties of the Eu and Tb complexes needs to take into account both the composition of the inner coordination sphere and the peculiarities of the crystal packing. For instance, in addition to the classical ligand f Eu charge-transfer state (LMCT), another chargetransfer state induced by π-stacking interactions (SICT) could be identified. These two states, located between the singlet and triplet states of the bpy ligand(s), provide relays facilitating the energy migration from the singlet to the triplet states and eventually to the excited Eu states, improving the overall ligand-to-Eu energy transfer. Another point is the involvement of the inner-sphere water molecules in H-bonding with chloride ions, which considerably lowers their luminescence quenching ability, so that the adducts remain highly luminescent. For instance, the terbium chloride with two bpy ligands is an efficient near-UV to green light converter, with an overall quantum yield equal to 37% despite the coordinated water molecules. The interpretations given are substantiated by DFT and TD-DFT theoretical calculations of the complexes and ligand assemblies. Introduction Owing to the growing interest in electroluminescent composite or nanomaterials and their associated devices,1 as well as in luminescent biosensors,2 the search for task-specific emissive compounds or activators is very actual. The unique ability of the lanthanides to emit well-defined narrow bands in different spectral ranges, from visible to near-infrared, with relatively long lifetimes and high quantum yields3 makes them perfect candidates for these multidisciplinary applications. Bidentate 2,2′-bipyridine (bpy) is one of the widest used ligand in the design of highly luminescent Ln-containing systems because of its intense absorption band in the near-UV and its ability to efficiently transfer energy onto the Ln excited states (antenna effect).4 A major challenge in the design of Lncontaining luminescent compounds is to avoid quenching through nonradiative deactivation by vibronic coupling with the vibrational states of ligand and/or solvent molecules (e.g., O-H, N-H, and C-H oscillators5). In addition, principally in the case of europium, owing to its low reduction potential, luminescence can be quenched by low-lying ligand f metal charge-transfer * To whom correspondence should be addressed. E-mail: lada_puntus@ mail.ru. † Institute of Radio Engineering & Electronics, Russian Academy of Sciences. ‡ A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences. § ´ Ecole Polytechnique Fe´de´rale de Lausanne (EPFL).

(LMCT) states.6 According to numerical estimates, the most pronounced luminescence quenching via the LMCT state occurs when the LMCT energy lies close to that of the lower ligand triplet level.7 This state is however difficult to detect because it often overlaps with other ligand states. On the other hand, this state may be shifted to higher energy by using ligands with higher optical electronegativity.8 In particular, the presence of coordinated anions such as chloride or fluoride should lead to higher energy of the LMCT state. Another interest of 2,2′-bipyridine and its complexes is their ability to form π-π stacking dimers or oligomers, as well as other supramolecular architectures, by weak ligand-ligand, cation-anion, and anion-anion interactions.9 These interactions can noticeably influence the photophysical properties of the resulting edifices by creating additional excited states.10,11 To the best of our knowledge, such situations have been rarely reported in the case of Ln-containing complexes. In a recent investigation devoted to lanthanide adducts with 1,10-phenantroline (phen), we have demonstrated that in addition to the intrinsic characteristics of the complexes, such as the number of coordinated ligands and water molecules as well as their mutual disposition, crystal packing effects may significantly affect the luminescence sensitization of the EuIII ion.11 Moreover a topological analysis of the electron density in [GdCl3 (phen)2(H2O)3], based on high-resolution X-ray diffraction data, revealed that the energy of Ln-ligand bonds and that of

10.1021/jp902390z CCC: $40.75  2009 American Chemical Society Published on Web 06/12/2009

empirical formula fw temp [K] cryst syst space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) Z F(000) Dcalc (g cm-1) µ (cm-1) 2θmax (°) reflns collected independent reflns (Rint) observed reflns [I > 2σ(I)] R1 (obs. refl.) wR2 (all data) GOF on F2 ∆Fmax/∆Fmin (e Å-3)

1Tb

C10H18Cl3N2O5Tb 511.53 100 triclinic P-1 6.9347(6) 10.998(1) 11.429(1) 89.275(5) 85.846(3) 75.644(3) 842.2(1) 2 496 2.017 46.93 58 24653 4677(0.0298) 4471

0.0153 0.036 1.023 0.986/-0.841

1Eu

C10H18Cl3EuN2O5 504.57 100 triclinic P-1 6.9602(3) 11.0460(5) 11.4320(5) 89.2875(8) 85.9659(8) 75.6017(8) 849.19(6) 2 492 1.97 41.83 59 10398 4672(0.0246) 4321

0.0215 0.0474 1.032 1.174/-0.788

1′Eu

0.026 0.0548 1.062 1.331/-1.389

C20H38.30Cl6N4O11.15Eu2 1029.87 100 triclinic P-1 8.077(4) 9.4719(4) 24.046(1) 86.2300(8) 87.6618(8) 75.2674(8) 1774.8(1) 2 1007 1.927 40.08 58 28850 9410(0.0317) 8444

TABLE 1: Crystal Data and Structure Refinement Parameters

0.025 0.0501 1.068 0.915/-1.041

C20H38.30Cl6N4O11.15Tb2 1043.79 100 triclinic P-1 8.0501 (5) 9.4525(5) 23.955(1) 86.419(1) 87.878(1) 75.301(1) 1759.3(2) 2 1015 1.97 44.97 58 21258 9312(0.0224) 8315

1′Tb

0.0316 0.0634 1.024 1.025/-0.923

C20H20Cl3EuN4O2 606.71 100 monoclinic C2/c 15.660(1) 11.3228(8) 13.900(2) 90 117.066(1) 90 2194.8(3) 4 1192 1.836 32.48 57 13030 2891(0.0711) 2525

2Eu

0.0391 0.0957 1.03 1.979/-1.346

C20H26Cl3N4O5Tb 667.72 100 monoclinic C2/c 17.097(1) 9.2782(5) 17.131(1) 90 116.493(1) 90 2432.1(2) 4 1320 1.824 32.76 58 14581 6463(0.0434) 5075

5Tb

0.0567 0.0884 0.928 1.646/-1.572

C40H44Cl6Eu2 N8O6 1249.45 100 triclinic P-1 7.4596(8) 17.774(2) 19.394(2) 66.106(3) 88.232(3) 83.116(3) 2333.6(4) 2 1232 1.778 30.6 56 18243 11030(0.0771) 6134

3Eu

0.0282 0.0696 1.069 0.686/-0.534

C22.50H28.24Cl3EuN4O3.62 670.96 100 cubic I23 25.847(2) 25.847(2) 25.847(2) 90 90 90 17268(2) 24 8021 1.549 24.89 57 96377 7670(0.0850) 7057

4Eu

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TABLE 2: Selected Bond Lengths in the Europium Complexes 1Eu

1′Eu(A)

1′Eu(B)

Eu(1)-O(1) Eu(1)-O(2) Eu(1)-O(3) Eu(1)-O(4)

2.4060(17) 2.4252(17) 2.4704(17) 2.3737(17)

Eu(1)-O(4A) Eu(1-O(1A) Eu(1)-O(2A) Eu(1)-O(3A)

2.4035(19) 2.405(2) 2.418(2) 2.463(2)

Eu(1)-N(2) Eu(1)-N(2′) Eu(1)-Cl(1) Eu(1)-Cl(2)

2.557(2) 2.560(2) 2.7216(6) 2.7823(6)

Eu(1)-N(2A′) Eu(1)-N(2A) Eu(1)-Cl(1A) Eu(1)-Cl(2A)

2.566(2) 2.575(2) 2.7486(7) 2.7490(7)

Eu(2)-O(1B) Eu(2)-O(2B) Eu(2)-O(5B) Eu(2)-O(3B) Eu(2)-O(4B) Eu(2)-N(2B) Eu(2)-N(2′B) Eu(2)-Cl(1B)

2.418(2) 2.407(2) 2.422(2) 2.422(2) 2.425(2) 2.549(2) 2.579(2) 2.6806(7)

Average Bond Lengths (Å) Eu-Cl Eu-O Eu-N

2.752 2.419 2.559 2Eu

2.7488 2.422 2.5705 3Eu(A)

Eu(1)-O(1)

2.398(2)

Eu(1)-N(2) Eu(1)-N(2′)

2.613(3) 2.586(2)

Eu(1)-Cl(1)

2.7097(8)

Eu(1)-O(2) Eu(1)-O(1) Eu(1)-N(2A′) Eu(1)-N(2B) Eu(1)-N(2B′) Eu(1)-N(2A) Eu(1)-Cl(2) Eu(1)-Cl(1)

2.681 2.425 2.564 3Eu(B)

2.405(6) 2.419(5) 2.558(7) 2.563(7) 2.571(7) 2.579(6) 2.706(2) 2.742(2)

Eu(1′)-O(1′) Eu(1′)-O(2′) Eu(1′)-N(2D′) Eu(1′)-N(2D) Eu(1′)-N(2C) Eu(1′)-N(2C′) Eu(1′)-Cl(1′) Eu(1′)-Cl(2′)

4Eu 2.333(6) 2.363(6) 2.587(7) 2.596(7) 2.596(6) 2.596(7) 2.679(2) 2.718(2)

Eu(1)-O(2) Eu(1)-O(1) Eu(1)-N(2A) Eu(1)-N(2′) Eu(1)-N(2A′) Eu(1)-N(2) Eu(1)-Cl(2) Eu(1)-Cl(1)

2.405(3) 2.417(3) 2.551(3) 2.586(3) 2.602(3) 2.608(3) 2.7074(9) 2.7208(9)

Average Bond Lengths (Å) Eu-Cl Eu-O Eu-N

2.710 2.398 2.599

2.724 2.412 2.568

noncovalent interactions (H-bonding and π-stacking) are of the same order of magnitude. Compared to 1,10-phenanthroline, 2,2′-bipyridine has less steric volume and a higher flexibility due to the rotation of aromatic rings, so that it was intriguing to verify if this ligand also generates intermolecular interactions influencing the energytransfer process. Therefore, a series of isomeric lanthanide chlorides with one and two coordinated bpy molecules were synthesized to study the influence of the molecular geometry, distortion of coordination polyhedron, and crystal packing (including π-stacking) on the efficiency of the energy-transfer process to Eu and Tb. The presence of charge-transfer states was probed, and to complement the experiments, theoretical approaches were used to ascertain the assignments performed and rationalize the photophysical data. Finally, a qualitative description of the energy transfer scheme was proposed and confirmed by measurements of the absolute quantum yields. Experimental Section Materials and Methods. All reagents were purchased from Aldrich and used as received. All solvents were reagent grade and purified by standard techniques whenever required. Low-resolution luminescence measurements (spectra and lifetimes) were recorded on a Fluorolog FL 3-22 spectrometer from Horiba-Jobin-Yvon-Spex at 293 and 77 K. Phosphorescence lifetimes (τ) were measured on samples put into quartz capillaries; they are averages of at least three independent measurements, which were achieved by monitoring the decay at the maxima of the emission spectra. The single or biexponential decays were analyzed with Origin 7.0. Quantum yields L ) of EuIII-centered luminescence were determined by an (QLn absolute method12 with a specially designed integration sphere.13 High-resolution, laser-excited luminescence spectra and lifetimes have been measured at variable temperature using published

2.698 2.350 2.594

2.714 2.411 2.587

procedures.14 Attenuated total reflectance IR spectra were obtained from powdered samples with a Perkin-Elmer Spectrum One FT-IR spectrometer. Complexes. Lanthanide chlorides were reacted with 2,2′bipyridine in 1:1 or 1:2 (Ln/bpy) stoichiometric ratios (Ln ) Eu, Gd, Tb) to give the following complexes, according to a procedure similar to the one used for the complexes with phen:11 [LnCl2(bpy)1(H2O)4]Cl1(H2O), Ln ) Eu (1Eu), Tb (1Tb); {[LnCl2 (bpy)1(H2O)4]Cl1(H2O)1.15}2, Ln ) Eu (1′Eu), Tb (1′Tb); [EuCl2(bpy)2(H2O)2]Cl1 (2Eu); [GdCl2(bpy)2(H2O)2]Cl1 (2Gd); {[EuCl2(bpy)2(H2O)2]Cl1(H2O)}2 (3Eu); [EuCl2(bpy)2(H2O)2]Cl1(C2H6O)1.25(H2O)0.37 (4Eu); and [TbCl1(bpy)2(H2O)3]Cl2(H2O) (5Tb). Compounds 1Ln and 1′Ln were obtained by adding 3-4 drops of bidistilled water to the final solutions. Complexes 1′Ln, 3Eu, and 4Eu were obtained during the preparation of single crystals and were not subjected to elemental analysis, but all corresponding spectroscopic measurements were performed on the crystals used for X-ray analysis. Complex 2Gd was obtained in analogue with 2Eu and doped by 2% Eu for the control of the structure of the complex by luminescence spectroscopy. 1Eu. Yield 64%. Anal. Calcd. for C10H18N2O5EuCl3 (504.58): C, 23.80%; H, 3.60%; N, 5.55%. Found: C, 23.94%; H, 3.65%; N, 5.71%. 1Tb. Yield 76%. Anal. Calcd. for C10H18N2O5TbCl3 (511.55): C, 23.48%; H, 3.55%; N, 5.48%. Found: C, 23.54%; H, 3.61%; N, 5.55%. 2Eu. Yield 97%. Anal. Calcd. for C20H22N4O3EuCl3 (624.74): C, 38.45%; H, 3.55%; N, 8.97%. Found: C, 38.57%; H, 3.67%; N, 8.91%. 2Gd. Yield 70%. Anal. Calcd. for C20H22N4O3GdCl3 (630.03): C, 38.13%; H, 3.52%; N, 8.89%. Found: C, 38.25%; H, 3.96%; N, 8.86%.

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Figure 1. ORTEP view of the complex cations in the investigated structures, with thermal ellipsoids at 50% probability.

5Tb. Yield 90%. Anal. Calcd. for C20H24N4O4TbCl3 (649.72): C, 36.97%; H, 3.72%; N, 8.62%. Found: C, 37.04%; H, 3.82%; N, 8.71%. X-ray Crystallography. Single crystals were grown by slow evaporation from ethanol for 2-3 days. All diffraction data were collected on a Bruker SMART APEX II CCD diffractometer [λ(Mo KR) ) 0.71072 Å, ω-scans] at 100 K. The substantial

redundancy in data allows empirical absorption correction to be performed with SADABS,15 using multiple measurements of equivalent reflections. The structures were solved by direct methods and refined by the full-matrix least-squares technique against F2 in the anisotropic-isotropic approximation. The hydrogen atoms of ordered water molecules were located from the Fourier density synthesis and refined with the riding model.

Energy-Transfer Processes in Lanthanide Complexes with bpy The hydrogen atoms of disordered water molecules with occupancies of less than 0.5 in complexes 1′Eu, 1′Eu(A), and 4Eu, as well as an ethanol molecule in 4Eu with an occupancy of 0.25, were not located. All calculations were performed with the SHELXTL software package.16 Crystal data and structure refinement parameters are listed in Table 1; selected bond lengths for Eu complexes are listed in Table 2, while those for terbium complexes are given in Tables S9-S11 (Supporting Information). Computational Details. Quantum chemistry calculations were performed using density functional theory (DFT) with Gaussian03.17 Relativistic effects were considered through the use of relativistic effective core potentials (RECP).18 Spin-orbit effects were not taken into account. The structures of the complexes were fully optimized in their electronic ground state and without symmetry constraints. The electronic excitation energies were calculated via the time-dependent density functional theory (TD-DFT). The hybrid B3LYP functional19 was employed. Large-core energy-adjusted RECPs for Eu developed in the Stuttgart and Dresden groups were used together with the accompanying basis sets to describe the valence electron density. Large-core RECPs for lanthanides put 5s, 5p, 6d, and 6s shells in the valence space, whereas 4f electrons belong to the core pseudopotentials. The corresponding valence basis sets associated with the pseudopotentials are (6s6p5d) contracted to [4s4p4d].20 For the C, H, N, and O atoms, a 6-31G* basis was employed, while a 6-311G* one was used for Cl. The ∆SCF approach was also utilized to determine the triplet state energies of 1Eu, 2Eu, and 4Eu. In this approach, the excitation energy is given as the energy difference between the ground state and the excited state determined from two independent calculations; in this way, the geometry of the compound can be optimized in its lowest triplet state, whereas TD-DFT excitation energies are calculated for the ground state geometry (“vertical energy”). Results and Discussion Peculiarities of Geometry and Crystal Packing. According to XRD data, the metal ion is eight-coordinated in all of the studied complexes, and the coordination polyhedron is a distorted square antiprism (Figure 1). Crystals of 1′Eu and 3Eu contain two independent molecules denoted by A and B and correspond to different types of complexes; therefore, they must be considered as cocrystals. The investigated structures reveal four types of complex cations, the monocations [EuCl2(bpy)2(H2O)2]+ (I) and [EuCl2(bpy)(H2O)4]+ (II) and the dications [TbCl(bpy)2(H2O)3]2+ (III) and [LnCl(bpy)(H2O)5]2+ (IV) (Scheme 1). Three isomers are observed for type I and two for type II. Considering electrostatic and/or steric requirements, one would predict chloride ions and bpy ligands preferring positions with a maximum separation from each other. However, almost all possible isomers were obtained. In particular, in 1Eu, 1Tb, and 3Eu(A), the chloride ions belong to the same square base, and in the latter, the Cl · · · Cl distance is minimum. On the other hand, the donor atoms of each bpy ligand in structures 2Eu-5Tb are located in different square bases, but in 4Eu, the two aromatic ligands are situated directly under each other, with a short intramolecular N(2A) · · · C(1′) contact equal to 3.572(2) Å (Figure 1). The different isomers are characterized by different degrees of distortion of their coordination polyhedra. The latter has been characterized by the mean displacement d of the atoms out of the mean base planes, the dihedral angles δ between the base planes, and the distance dLn between the metal ion and the center

J. Phys. Chem. B, Vol. 113, No. 27, 2009 9269 SCHEME 1: Schematic Representation of the Square Antiprism Polyhedra in the Complexes Studied Using Newman-Type Projectiona

a

The bpy ligands are represented by an arc. The bottom square base of the antiprism is shown by dashed lines.

of the polyhedron (Table 3). This distortion is more pronounced for 2Eu and 3Eu(B) and is almost negligible for 4Eu. Thus, the Cl · · · bpy repulsion affects the polyhedron to a greater extent than the Cl · · · Cl and bpy · · · bpy repulsions. Although the degree of polyhedron distortion can be viewed as an intrinsic feature of the complex, the comparison of data for the isomers 2Eu and 3Eu(B) clearly shows that this distortion can also be influenced by the crystal packing. The crystals investigated display a supramolecular organization arising from stacking interactions and a rich network of the O-H · · · Cl and O-H · · · O H-bonds which assemble the complex cations into extended 3D frameworks. Depending on the number of water molecules and/or bpy ligands, the O-H · · · Cl and/or O-H · · · O bonds link the cations into chains (2Eu), double chains (3Eu), layers (1Eu, 1Tb, 1′Eu, and 1′Tb), or even a 3D framework (5Tb). The only exception is complex 4Eu, in which H-bonds lead to the formation of discrete trimers connected by numerous C-H · · · Cl contacts and stacking interactions into “supramolecular balls” encapsulating a chloride and solvation molecules (water and ethanol). Detailed analysis of the H-bond networks (Tables S1-S8, Supporting Information) reveals that they considerably differ in strength for the different complex types I-IV and significantly affect the Eu-ligand bond lengths (Table 2). The widest distribution of bond lengths is observed for the Eu-O and Eu-Cl bonds, with differences with up to 0.13 and 0.1 Å, respectively, while such variations are smaller (0.06 Å) for the Eu-N bonds. The influence of the H-bonding strength on the Eu-O bond length is clearly illustrated using 3Eu(B) as an example; this cation possesses the shortest Eu-O(water) bond (Eu(1′)-O(1′) ) 2.333(6) Å) of all studied europium compounds, and the corresponding water molecule is involved in the shortest O-H · · · Cl hydrogen bonds, with O · · · Cl separations equal to 2.915(4) Å (Table S6, Supporting Information). It is noteworthy that this H-bond is one of the shortest among all known crystal structures of lanthanide and transition-metal complexes according to the Cambridge Structural Database (CSD). In the two complexes 2Eu and 4Eu, the coordinated chloride ions participate in H-bonding only with coordinated water

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TABLE 3: Distortion of the LnIII Coordination Polyhedra in the Investigated Compounds and Conformational Parameters for bpya compound [EuCl2bpy1(H2O)4] [TbCl2bpy1(H2O)4] [EuCl2bpy1(H2O)4] [EuCl1bpy1(H2O)5] [TbCl2bpy1(H2O)4] [TbCl1bpy1(H2O)5] [EuCl2bpy2(H2O)2] [EuCl2bpy2(H2O)2] [EuCl2bpy2(H2O)2] [EuCl2bpy2(H2O)2] [TbCl1bpy2(H2O)3]

1Eu 1Tb 1′Eu(A) 1′Eu(B) 1′Tb(A) 1′Tb(B) 2Eu 3Eu(A) 3Eu(B) 4Eu 5Tb

d, Å

δ, °

dLn, Å

φ, °

DB, °

0.05; 0.17 0.05; 0.17 0.11; 0.24 0.24; 0.11 0.10; 0.23 0.23; 0.11 0.18; 0.18 0.06; 0.06 0.06; 0.11 0.01; 0.06 0.02; 0.06

5.7 5.6 9.5 7.8 8.9 7.7 9.8 3.8 9.0 0.3 1.0

0.07 0.06 0.08 0.05 0.08 0.05 0 0.03 0.07 0.11 0.04

6.0 6.1 4.0 3.2 3.3 3.9 0.1 8.3; 4.3 9.7; 9.8 19.9; 9.6 1.0; 2.5

3.0 7.1 4.5 1.2 5.4 1.4 13.7 1.8; 6.0 1.2; 3.2 1.2; 4.5 4.5; 8.1

a Key: d ) displacement of the atoms from the square planes; δ ) dihedral angles between the square planes; dLn ) distance of the Ln atoms from the center of the polyhedron; φ ) torsion angle between pyridine rings; DB ) degree of bending of the pyridine rings (see text and Scheme 3).

Figure 2. Fragments of the H-bonded chain in 2Eu and the H-bonded trimer in 4Eu.

molecules (Figure 2). Taking into account that coordinated water molecules are characterized by a negative total charge,11 we may expect that such H-bonding will reduce the charge transfer from the chloride ion to water. This assumption is in line with the decrease in the Eu-Cl bond lengths (Table 2). At the same time, however, we cannot exclude the possible role of noncoordinated anions, which participate in H-bonds only with coordinated water molecules in the case of 2Eu and therefore can decrease the positive charge of the cationic species. Although bonds are undoubtedly affected by crystal packing, some features are governed by intramolecular electronic and steric effects. The most interesting one is the elongation of the Eu(1)-Cl(2) bond in comparison to the Eu(1)-Cl(2) one observed for 1Eu (2.7823(6), 2.7216(6) Å). Since the Eu(1)-Cl(2) bond is almost coplanar to the bpy plane (dihedral angles ) 8.7°), it is reasonable to propose that such lengthening is the consequence of an effect analogous to the trans effect of the bpy ligands. Another feature is the elongation of the Eu(1)-N(2) and Eu(1)-N(2A) bonds compared to Eu(1)-N(2A) and Eu(1)-N(2A′) observed for complex 4Eu. This may be the consequence of intramolecular interactions between two bpy ligands, in line with significant stabilization of complex 4Eu with respect to 2Eu (see above). The diversity of the geometry, isomers, and types of complexes observed for the complexes with bpy is, to a great extent, the consequence of the ligand conformational lability. Although the planar conformation is optimum from the points

SCHEME 2: Two Possible Way of Distortion of Planar bpy Rings

of view of pyridine ring conjugation and coordination to metal ions, the steric repulsion of the hydrogen atoms causes a rotation of the pyridine rings around the C(1)-C(1′) bond. The torsion angle N(2)C(1)C(1′)N(2′) (hereafter φ angle) is a measure of such a rotation (Scheme 2). Examination of the bpy geometry shows that bpy ligands are nonplanar in all complexes and that φ varies in the range of 0.1-11.9°. In fact, complexes with φ close to 0° are characterized by significant bending along the C(1)-C(1′) bond. The degree of bending (DB) was calculated as the sum of two angles between the plane of the pyridine ring and the C(1)-C(1′) bond (Scheme 2). The most pronounced bending is observed for 2Eu, for which φ ) 0.1° and DB ) 13.7°. Another high DB (8.1°) is observed for one of the ligands in 5Tb. Although such distortion of bpy is surprising, analysis of CSD revealed similar high DB values in nine crystal structures of lanthanide complexes with bpy ligands. Similar distortions are observed for numerous transition-metal complexes as well. Since DB and φ values for complexes belonging to the same type, for example, 2Eu and 3Eu(B) (type I), are different (see

Energy-Transfer Processes in Lanthanide Complexes with bpy

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Figure 3. Stacking interaction in crystals of 1Eu, 2Eu, and 5Tb.

Figure 4. Stacking bonded layers in 2Eu (top) and Cl · · · π interactions in 5Tb (bottom).

Table 3), it is reasonable to propose that the distortion of bpy ligands is the manifestation of crystal packing effects. As a consequence of the high lability of bpy in some complexes, the pyridine rings participate independently in stacking interactions. In other words, the interplanar distances for two ring pairs can be different upon preservation of the parallel arrangement of interacting rings. The degree and type of overlap between interacting bpy ligands are different, with a minimum area

observed for 1Eu and comparable for each pyridine ring in other complexes (Figure 3). Angles between planes of pyridine rings involved in stacking interactions vary in the narrow range of 0-8°. The C · · · C and N · · · C contacts vary in the ranges of 3.334(3)-3.580(3) Å and 3.502(4)-3.534(4) Å, respectively, with the shortest ones observed in the crystals 2Eu and 5Tb. Although the latter type of overlap for both complexes is almost identical, there are distinct differences. In 2Eu, two types of stacking interactions assemble cations into infinite layers in such a manner that each bpy participates in two interactions (Figure 4, top), while in 5Tb, stacking dimers are significantly separated from each other (Figure 4, bottom). This separation is the consequence of an uncommon Cl · · · π interaction with a Cl · · · C separation equal to 3.425(2) Å. This contact is characterized by a pronounced directionality (the C(1′B)C(1B)Cl(2) angle is 97.6°) and is shorter than the one found in the analogous complexes with phen (3.555(1)Å).11 Thus, the large bending of bpy in 2Eu and 5Tb is the consequence of two concurrent types of stacking and strong Cl · · · π interactions, respectively. Metal-Centered Luminescence. All of the europium complexes exhibit the characteristic line-like luminescence from 5D1 and 5D0 levels of the EuIII ion under broad-band excitation into the bpy levels (Figure 5). The crystal field splittings of the 7FJ levels (J ) 0-4) extracted from these spectra can be interpreted in terms of a low point group of symmetry, in line with the crystal structures. In the spectra of 1′Eu and 3Eu, the number of components observed for the 5D0 f 7F1 transition (four) is larger than theoretically allowed (2J + 1 ) 3), indicating the presence of at least two nonequivalent surroundings for the europium ion, consistent with the existence of cocrystals, as discussed above. Several features can be outlined from the analysis of the highresolution luminescence spectrum of 2Eu (inserts in Figure 5). (i) The 5D0 f 7F0 transition is unique and very weak; (ii) the overall splittings of the 7F1,2,4 levels are the smallest of the series (60, 155, and 370 cm-1, respectively); (iii) the number of Stark components of the 7FJ levels is 1, 2 (3), 3, 2, and 5 for J ) 0-4, respectively, which suggests a higher symmetry than the one obtained from X-ray analysis (C2); indeed the splitting of the 7F1 levels bears the trace of the higher tetragonal symmetry

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Figure 7. Excitation spectra of 2Eu (a), 2Gd (b), and 5Tb (c) at 77 K.

Figure 5. Luminescence spectra of 1Eu (a), 1′Eu (b), 2Eu (c), 3Eu (d), and 4Eu (e) at 77 K. The parts of the high-resolution luminescence spectrum of 2Eu are given in the insets.

Figure 8. Excitation spectra of 2Eu (a), 3Eu (b), and 1Eu (c) at 77 K.

Figure 6. Luminescence spectra of 1Tb (a) and 5Tb (b) at 77 K. The region of the 5D4 f 7FJ (J ) 0-2) transitions is presented in the insets.

of the idealized coordination polyhedron in that its low-energy Stark sublevel at ∼360 cm-1 is in fact a doublet with components separated by about 10 cm-1; and (iv) the large intensity of the first component of the 5D0 f 7F2 transition (613 nm, ∼20% of the total emitted light, full width at half-height ) 20 cm-1) is potentially interesting for designing a high color purity emitter. Vibronic sidebands associated with electronic transitions and which could be analyzed in terms of metal-toligand bond strength were identified in the luminescence spectrum as well. Faint satellites observed on the vibronic wing of the 5D0 f 7F0 transition (17400-16850 cm-1) mostly arise from the Eu-Cl vibrations. The latter have lower frequencies than the ones observed for the corresponding complex with phen11,21 (130, 215 versus 140, 220 cm-1, respectively), pointing to the weaker bonding of chloride anions in 2Eu reflected in

Figure 9. Simplified diagram of energy migration processes in the europium complexes. Key: S1 ) singlet state, T ) triplet state, SICT ) ligand charge-transfer state induced by a stacking interaction, LMCT ) ligand-to-metal charge-transfer state, and dotted and solid lines represent nonradiative and radiative processes, respectively.

the longer bond length (2.710 versus 2.703 Å). Other vibronic sidebands are also observed, and their detailed scrutiny, along with crystal field splitting analysis, will be presented elsewhere. The luminescence spectrum of 4Eu is very similar to the one of 2Eu, and therefore, it is not further discussed here. In the luminescence spectra of the TbIII-containing complexes 1Tb and 5Tb, the following transitions could be detected (Figure 6): 5D4 f 7F6 (480-500 nm), 5D4 f 7F5 (535-555 nm), 5 D4 f 7F4 (575-595 nm), 5D4 f 7F3 (610-630 nm), 5D4 f 7 F2 (640-660 nm), 5D4 f 7F1 (660-675 nm), and 5D4 f 7F0 (675-685 nm). The latter three transitions have expectedly low intensity and a splitting pattern similar to that of the corre-

Energy-Transfer Processes in Lanthanide Complexes with bpy sponding Eu complexes. The 5D4 f 7F5 transition is the most prominent and accounts for ∼42 and 47% of the total emitted intensity for 1Tb and 5Tb, respectively. Energy Transfer Process. To simplify the discussion, we assume that the sensitized luminescence in the systems studied results from energy migration from the singlet excited state of the ligand (S1, 330 nm/30300 cm-1) to the triplet state (T, 435 nm/23000 cm-1) and finally to the excited levels of the lanthanide ion. The excited singlet state displays a fine structure with vibronic progression of ∼1010 cm-1, probably attributable to the “breathing” symmetric vibrational mode of the bpy rings (see the excitation spectrum of 2Gd in Figure 7). According to phenomenological rules, two energy gaps play a key role in the sensitization efficiency, namely, ∆E1 ) E(S1) - E(T) and ∆E2 ) E(T) - E(5DJ). In principal, direct transfer from the singlet S1 state is also possible but seems to be less important for Eu than that for Tb.22 The excitation spectra of the europium complexes measured at 77 K display, in addition to the narrow f-f transitions (5L6 r 7F0, 5D4,2,1,0 r 7F0; see Figure 7a), a broad band extending from 250 to 420 nm with one intense component at 275 nm (36 360 cm-1), two smaller ones at ∼330 (30 300 cm-1) and 340 nm (29 415 cm-1), and a shoulder at ∼365 nm (27 400 cm-1). In order to distinguish these bands, a multipeak fitting procedure was performed, and four states were found (Figure S1, Supporting Information). The first band corresponds to n f π* and π f π* transitions of bpy ligands (Figure S2, Supporting Information), while the second one is attributed to the S1 state. Clarification of the origin of the two other bands comes from the analysis of the excitation spectrum of GdCl3(bpy)2(H2O)3 (Figure 7b). The latter contains a broad band with a maximum at ∼370 nm (27030 cm-1) corresponding to one of the components of the multipeaks fit of the Eu excitation spectrum. Therefore, this low-energy state cannot be assigned to a ligand f Eu charge-transfer (LMCT) state but, rather, to an intraligand charge-transfer state. A similar state has been identified in the analogous series of complexes with phen.11 In fact, the analysis of charge density distribution performed for [GdCl2(phen)2(H2O)2]Cl1(H2O) revealed that the two phen ligands bear different positive charge (0.33 and 0.13 e) due to the C-H · · · Cl bonding and π-stacking interactions. Hence, it seems reasonable to assign the broad 365 nm band to an intraligand charge-transfer state caused by ligand charge redistribution as the result of strong π-stacking interactions (denoted below as the stacking-induced charge transfer (SICT) state). It is noteworthy that the SICT band is better resolved in the bpy complexes compared to that in phen ones, owing to a blue shift of the S1 state bpy (∼1700 cm-1). The proposed assignment is also in line with data published on several EuIII complexes containing coordinated bpy ligands in stacking interaction with noncoordinated ones. In these complexes, an increase of the intensity of the bpy band in the excitation spectra was observed and explained by the participation of the noncoordinated bpy in the energy transfer to the EuIII ion by widening of the excitation channel.23,24 These data clearly demonstrate that crystal packing as well as molecular geometry influence the energy-transfer processes and, as a consequence, the intensity of the EuIII luminescence. It should be also noted that a similar SICT band was observed in the excitation spectrum of 5Tb (Figure 7c), slightly blue shifted (≈ 400 cm-1) with respect to 2Eu (Figure S3, Supporting Information). The latter seems to be logical, taking into account that Eu and Tb complexes have differences in stacking interactions. Finally, this assignment has been confirmed by TD-DFT calculations (see below).

J. Phys. Chem. B, Vol. 113, No. 27, 2009 9273 TABLE 4: Integrated Intensity Ratios Itot/IMD,0, Lifetimes, and Intrinsic EuIII Quantum Yields for Some Complexes complex

Itot/IMD,0

τobs/ms, 300 K

Eu QEu ( 2%

τobs/ms, 77 K

1Eu 2Eu 2phena 3Eu

8.2 11.9 9.1 6.3

0.26 ( 0.02 0.37 ( 0.03 0.38 ( 0.03 0.31 ( 0.02

11 22 17 10

0.27 ( 0.02 0.37 ( 0.03 0.35 ( 0.01 0.35 ( 0.03

a

Reference 11.

The second low-energy component at 340 nm is absent in the terbium excitation spectrum (Figure 7c) and can therefore be assigned to a LMCT state. Taking into account the electronegativity of EuIII, uncorrected for spin correlation, χuncorr(Eu) ) 1.9925 and ECTS = 30 000 · [χopt(X) - χuncorr(Eu)] cm-1,8 the optical electronegativity of the ligand χopt amounts to 2.97. The latter is similar to Pauling’s electronegativity for chlorine (3.0), and we infer therefore that charge transfer from Cl- to EuIII has the main contribution to the LMCT state at 370 nm. Additional confirmation for this assignment can be gained from the structural data. First, as seen above during the analysis of H-bonding, the presence of H-bonds between the coordinated chloride ions and coordinated water molecules in 2Eu leads to short Eu-Cl bonds (∼2.71 Å). Second, the noncoordinated chlorides participate in H-bonds only with coordinated water molecules, which results in a decrease in the positive charge of the metal ion. This charge redistribution seems to be a reasonable base for the proposed Cl- f Eu3+ charge-transfer state. A similar energy of this state was observed for polychloro species in solution, for instance, 31250 cm-1 (320 nm) for EuCl3 in [Bu-mim]Tf2N26 and 33300 cm-1 (300 nm) for [EuCl6]3- in acetonitrile saturated with Et4NCl.27 Finally, the energy of the LMCT state in 2Eu also correlates with the reported rule that the shorter the Eu-ligand bond length, the shorter the wavelength of the charge-transfer band.28 Normalizing the excitation spectra reported in Figure 8 with respect to the integrated intensity of the magnetic dipole 5D1 r 7F0 transition allows one to estimate the relative efficiency of excitation through ligands versus direct f-f excitation. The highest ligand efficiency is observed for 1Eu, which correlates well with (i) the shortest Eu-N bond length (2.56 Å) providing a high efficiency of the ligand f metal energy transfer, (ii) the longest Eu-Cl bond (2.75 Å) promoting lower deactivation via the LMCT state, and (iii) the rather weak stacking interaction decreasing the contribution of the SICT state in the dissipation of the excitation energy. These features are illustrated in the simplified diagram given in Figure 9. The overall efficiency of the ligand-to-europium energy transfer has been estimated for 2Eu since this complex has only one luminescent center and is analogous to [EuCl2(phen)2 (H2O)2]Cl.H2O (2phen).21 The intrinsic quantum yields of the Eu , calculated by means of europium-centered emission, QEu 29,30 lifetimes of the excited 5D0 level, as well Werts’s formula, as integrated intensity ratios Itot/IMD,0 are given in Table 4. L for 2Eu amounts to The overall absolute quantum yield QLn 19.2 ( 0.4%, which represents a substantial value, taking into consideration the multiphonon radiationless relaxation by four O-H oscillators from the two bonded water molecules. The reported overall quantum yield for 2phen (12.7%) is lower by ∼34%, while the intrinsic quantum yield is smaller by ∼23%. The latter point is somewhat surprising since the 5D0 lifetimes are equal, within experimental errors, while the Eu-O(water) bond lengths in 2Eu are even slightly shorter than those in 2phen (2.40 versus 2.41 Å), which would rather favor radiationless deactivation in 2Eu. Considering QLLn ) ηsens · QLLn, where

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ηsens is the efficacy with which electromagnetic energy is transferred from the ligands to the EuIII ion and amounts to ∼87 and 73% for 2Eu and 2phen, respectively, it appears that to the bipyridine ligand is better suited for sensitizing EuIII luminescence. Interestingly, the increased overall quantum yield L QLn for 2Eu is therefore due to more effective energy transfer despite the presence of the lower-lying LMCT state. Since there is only a marginal difference in the Eu-N bond lengths (2.60 in 2Eu versus 2.61 Å in 2phen), modeling by TD-DFT calculations has been performed to shed light on this effect (see below). The information gained from the above analysis of the energytransfer processes may be summarized as follows. The energy differences between the S1 state and the LMCT, SICT, and T states amount to 900, 2900, and 7300 cm-1, respectively. The latter difference remarkably exceeds the optimum value, which should be ∼5000 cm-1 for effective intersystem conversion,31 but the presence of intermediate “stairs” (the LMCT and SICT states) situated at 6400 and 4400 cm-1 above the triplet state, respectively, probably promotes the high efficiency of the energy-transfer process; this means that the rate constant of the LMCT-to-SICT transfer is comparable or larger than the deactivation rate constant of the LMCT state, and the same holds true for the next transfer steps, from SICT to T or Ln*. Moreover the temperatureindependent lifetime of the excited 5D0 level in 2Eu (Table 4) indicates the absence of a thermally activated quenching mechanism.6 In other words, such a small difference in energies of S1 and LMCT states (10 nm/900 cm-1 only) can be a reason for the noneffective deactivation process via the LMCT state, especially at room temperature. We consequently suggest these closely situated excited states serves as successive relays for the excitation energy. The excitation spectrum of 2Eu recorded at 300 K, at which the above broad excitation band expands up to 450 nm, clearly demonstrates the latter conclusion (Figure 10a). Furthermore, 3D representation of luminescence spectra measured at different excitation wavelengths (from 320-400 nm) are given on Figure S4 (Supporting Information). The unique properties of the europium system described here provide an unusual opportunity to detect experimentally the LMCT state. While the proposed description of the energy-migration process is qualitative, it can probably be used as an initial model for further quantitative analysis. Finally, the overall quantum yield was measured for the most L luminescent terbium complex, 5Tb; it amounts to QLn ) 37.3 ( 0.4%. This value is unexpectedly high, taking into consideration that three water molecules are coordinated. It points to a very effective ligand-to-metal energy-transfer process resulting from a more adequate energy gap between the lowest triplet state and the 5D4 level of TbIII (∆E2 ) 2500 cm-1, well in the optimum range reported).31 The luminescence decay is monoexponential, and the lifetime of the 5D4 level amounts 0.78 ( 0.03 and 0.74 ( 0.02 ms at 77 and 300 K, respectively. The longer lifetime of TbIII in comparison with that of EuIII (in Eu2) is reasonable since the energy gap for Tb(5D4-7F0) is larger than the corresponding one for Eu(5D0-7F6). Moreover, lifetime of ∼1 ms for TbIII and QLTb ) 43% have been reported, despite the coordination of four water molecules in a benzenedicarboxylate coordination polymer, is known.32 Large quantum yields for TbIII complexes are not uncommon, and many systems have QLTb values in the range of 40-60%,33 for instance, with 2-hydroxyisophthalate34 and 2-hydroxyisophthalamide chelating units.35 It is noteworthy that large quantum yields can also be obtained with such simple ligands as bpy even if several water molecules are coordinated by the lanthanide ion.

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Figure 10. Excitation spectra of 2Eu (a) and 5Tb (b) at 300 K. The * denote the second order of λreg.

Figure 11. Schematic diagram of selected frontier orbitals for stacking dimers in 2Eu.

Theoretical Modeling. In order to support some of the hypotheses made in the interpretation of the luminescence data, as well as to clarify some intriguing points, TD-DFT calculations and the so-called SCF approach which relies on the difference between the geometry of ground state and the relaxed triplet state have been performed on 2Eu and 2phen. According to these calculations, the ∆E1 energy differences are large for 2Eu and 2phen, 8623 (exp. 7300 cm-1) and 9414 cm-1 (exp. 6570 cm-1), respectively (Table S12, Supporting Information), while ∆E2 is very similar for the two complexes, 5460 (exp. 5765 cm-1) and 5502 cm-1 (exp. 4765 cm-1), respectively. Thus, theoretical calculations are in reasonable agreement with experimental data and indicate that the increase of the luminescence intensity of 2Eu in comparison with that of 2phen is partly related to slightly more favorable energy gaps ∆E1. It should be noted that the calculated energy of the blue-shifted S1 state of bpy with respect to the one of phen qualitatively agrees with experimental data. To shed light on the SICT band found in the excitation spectra (see Figure 7), we have performed TD-DFT calculations of dimers involved in stacking interactions. Considering that the correct description of the stacking interactions within the DFT technique is still problematic,36 we have taken the experimental geometry of 2Eu as a model for the stacking bpy pairs. This geometry bears information both about inner- and outer-sphere bonding of the ligands. In particular, the geometry adopted by the bpy ligand participating in stacking interactions is the consequence of (i) a bending along the C(1)-C(1A) line, (ii) interaction with the metal, and (iii) other types of stacking interactions in which this ligand is involved (Figure 4, top). TD-DFT calculations revealed the presence of an additional singlet state which is absent for the isolated bpy molecule. Its energy equals 27056 cm-1 (369.6 nm), and it mainly arises from

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SCHEME 3: Bond Lengths in bpy and phen in 2Eu and 2phen Calculated for the Ground State (Underlined Numbers) and the Excited Triplet State

electron transfer from the HOMO - 1 to LUMO (51%) and from the HOMO - 2 to HOMO + 2 (35%). Therefore, according to molecular orbital analysis, this state is governed by the stacking interaction between the two aromatic rings (Figure 11). The energy found for this state is reasonably close to the one reported for corresponding singlet states caused by π-stacking interactions in [2.2]paracyclophane and substituted paracyclophanes.37 For 5Tb, which displays differences in intermolecular stacking interactions with respect to 2Eu, the SICT transition is blue shifted to 27800 cm-1 (360 nm) and appears as a faint band in the excitation spectrum. The TD-DFT calculated energy of the SICT state in 5Tb amounts to 27863 cm-1 (358.9 nm), and therefore, the estimated energy difference of SICT states between 5Tb and 2Eu (∼800 cm-1/∼10 nm) perfectly matches the experimental data. This blue shift cannot be explained by variations in the ligand geometry because both the bending (DB) and twist (φ) angles of the bpy ligands are comparable in 2Eu and 5Tb. The same is true for the overlap area and angle between aromatic rings in the centrosymmetric stacking dimers. The main difference is the C · · · C separation, which is shorter by 0.069(3) Å in 2Eu, which indicates a higher degree of p-orbital overlap. Therefore, the C · · · C separation affects the SICT state energy to a greater degree than the overlap area of the aromatic rings. We can also mention that the procedure used for the analysis of organic assemblies based on experimental geometry may be a very efficient way for identification of the SICT state nature. In particular, it may be the best method for the investigation of such labile systems as bpy in which ligand distortions play a major role in addition to the classical parameters, namely, the overlap area, interplanar distance, and angle between aromatic systems. As far as the above-discussed LMCT state can play the main role in the efficiency of sensitization of EuIII luminescence depending on its energy, we have attempted to calculate its energy by TD-DFT calculations. While calculations based on

Figure 12. Schematic diagram of selected frontier orbitals for 2Eu according to DFT calculations based on the experimental geometry.

the optimized structure of 2Eu did not lead to any additional singlet state level, using the experimental geometry revealed the appearance of an additional state with energy equal to 29156 cm-1 (342 nm), which is rather close to the experimental value (29412 cm-1/340 nm). This level is due to electron transfer from the HOMO to the LUMO, that is, it corresponds to charge transfer from the chloride anions to a molecular orbital with both metal and ligand contributions (Figure 12). It should be noted, however, that the composition of the LUMO, particularly the contribution of metal orbitals, can be systematically biased by the use of large-core RECPs in which 4f electrons belong to the core pseudopotential.38 One of the reasons why the overall quantum yield for 2Eu is larger compared to that for 2phen, despite similar energy gaps, is the larger ηsens value. Analysis of the triplet-state geometry clearly shows a remarkable difference in ligand distortions and the Eu-ligand bond lengths between 2Eu and 2phen (Scheme 3). The most striking feature in geometrical parameters for these complexes is an alternate exchange of bond lengths in the phen and bpy ligands upon the transition from S1 to the lowest T state. The appearance of a symmetric “breathing” vibration in the vibronic wing of the excited S1 state is in line with variation of the bpy geometry upon excitation. The most significant variation is observed for the central C-C bond. The shortening of the latter consequently leads to the alternation of the C-N and C-C bonds in the pyridine cycle. However, the degree of central bond shortening as well as the degree of alternation of bond lengths in the ring is much more pronounced in bpy than that in phen. Furthermore, the DB value increases upon transition to the triplet-state geometry in all bpy complexes. Indeed, in complex 1Eu, with one coordinated bpy ligand, it increases only slightly from 2.9 to 4.4°, whereas it changes more remarkably in 4Eu from 3.4 up to 13.0°. For 2Eu, the variation is maximum since DB reaches the value of 28.3° in the T state. On the contrary, the degree of planarity of the phenanthroline ligand in both the ground and triplet states remains almost the same due to the rigidity of the molecule. The comparison of the Eu-X bond lengths in the complexes 1Eu, 2Eu, 4Eu, and 2phen in the lowest triplet state with those in the corresponding ground state clearly shows that the Eu-Cl and Eu-O bonds have almost the same length, the averaged differences being ∼0.01 and 0.005 Å, respectively. On the other hand, the Eu-N bond lengths are substantially different in the triplet state. In the case of 2Eu, the Eu-N distances are shorter by 0.03-0.034 Å with respect to the ground state, while in 2phen, they are the same as those in the ground state; the Eu-N bonds in the ground state are almost the same for 2Eu (2.62 Å) and 2phen (2.63 Å). We therefore think that the shortening of the Ln-N bonds in the triplet state of 2Eu promotes a more efficient energy transfer compared to that for 2phen, henceforth the larger quantum yield.

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Conclusions The detailed analysis of the structural and photophysical properties of a series of lanthanide adducts with different numbers of 2,2′-bipyridine, chloride ions and water molecules in the inner coordination sphere, backed by TD-DFT theoretical calculations, allowed us to decipher the influence of crystal packing effects on these properties. From the structural point of view, both the bond length distribution and the coordination polyhedra are influenced by the presence of an extended network of strong H-bonds, which can significantly reduce the Eu-O bond lengths. Despite the similarity in the bond lengths evidenced for bpy and phen ligands and comparable values of the Ln-N bond lengths, bpy is more flexible, so that in addition to the rotation of the pyridine rings, this generates a rather unexpected bending of these rings with respect to the central bond. This bending is mainly governed by intermolecular contacts such as Cl · · · π and π-stacking interactions. The photophysical properties of the Eu and Tb complexes correlate well with their structure and with the peculiarities of the chemical bonding pattern both in the inner and outer coordination spheres. In particular, the pronounced π-stacking interaction observed in 2Eu causes a redistribution of charges on the bpy ligands and leads to the appearance of an additional excited state (SICT), the energy of which correlates with the strength of π-staking interactions for all of the complexes studied. In addition, the presence of a Cl- f EuIII charge-transfer state (LMCT) is ascertained. The LMCT and SICT states provide a practical relay channel between the excited singlet and triplet states, which are separated by a large and not optimum energy difference of 7300 cm-1 and therefore facilitate the sensitization of the EuIII luminescence. Another contribution to the better sensitization of the EuIII luminescence by bpy (87%) compared to that by phen (73%) unravelled by the calculations lies in the shortening of the Eu-N bonds by ∼0.03 Å in the triplet (donor) state of bpy, contrary to phen, for which these distances are the same as those in the ground state. This is caused by the significant bending of the bpy ligands although the π-bond redistribution in the triplet state in bpy and phen is similar. As a consequence, 2Eu is more luminescent than the corresponding complex with phen. It is noteworthy that a good agreement has been found between experimental energies for the S1, T, LMCT, and SICT states and theoretical estimates performed with moderately sized basis sets and without bulk solvent effects. Furthermore, the use of the nonoptimized experimental geometry led to better results, and this methodological aspect is rather important, considering the problems of the correct description of weak interactions within DFT, particularly when it comes to flexible ligands such as bpy. Finally, the unexpected large value of the overall quantum yield of 5Tb (37.3 ( 0.4%), despite the presence of three water molecules in the inner coordination sphere, opens good perspectives for the usage of this simple terbium complex as an effective converter of near-UV light to strong TbIII green emission. Taking into account the role of the LMCT and SICT in the energy migration leading to visible-light excitation of the metal ion in 2Eu, the simple and cheap sensitization systems described here for both Eu and Tb may be envisaged as the basis for highly luminescent sensors and electroluminescent devices, especially if they are incorporated into microsized particles and liquid-crystal composites.39,40

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