Polyphenylcyclopentadienyl Ligands as an Effective Light-Harvesting

Article pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Polyphenylcyclopentadienyl Ligands as an Effective LightHarvesting π‑Bonded Antenna for Lanthanide +3 Ions Dmitrii M. Roitershtein,*,†,‡ Lada N. Puntus,†,§ Alexander A. Vinogradov,† Konstantin A. Lyssenko,∥ Mikhail E. Minyaev,† Mikhail D. Dobrokhodov,† Ilya V. Taidakov,∥,⊥ Evgenia A. Varaksina,†,⊥ Andrei V. Churakov,# and Ilya E. Nifant’ev†,△ †

A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 29 Leninsky Prospect, 119991, Moscow, Russia N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky Prospect, 119991, Moscow, Russia § V.A. Kotel’nikov Institute of Radioengineering and Electronics, Russian Academy of Sciences, 11-7 Mokhovaya Str., 125009, Moscow, Russia ∥ A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilova Str., 119991, Moscow, Russia ⊥ P.N. Lebedev Physical Institute, Russian Academy of Sciences, 53 Leninsky Prospect, Moscow 119991, Russia # N.S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 31 Leninsky Prospect, 119991, Moscow, Russia △ M.V. Lomonosov Moscow State University, Chemistry Department, 1 Leninskie Gory Str., Building 3, 119991, Moscow, Russia

Downloaded via UNIV OF SOUTH DAKOTA on July 27, 2018 at 14:20:08 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: A new approach to design “antenna-ligands” to enhance the photoluminescence of lanthanide coordination compounds has been developed based on a π-type ligand the polyphenyl-substituted cyclopentadienyl. The complexes of di-, tri-, and tetraphenyl cyclopentadienyl ligands with Tb and Gd have been synthesized and all the possible structural types from mononuclear to di- and tetranuclear complexes, as well as a coordination polymer were obtained. All types of the complexes have been studied by single-crystal X-ray diffraction and optical spectroscopy. All terbium complexes are luminescent at ambient temperature and two of them have relatively high quantum yields (50 and 60%). Analysis of energy transfer process has been performed and supported by quantum chemical calculations. The role of a low-lying intraligand charge transfer state formed by extra coordination with K+ in the Tb3+ ion luminescence sensitization is discussed. New aspects for design of lanthanide complexes containing π-type ligands with desired luminescence properties have been proposed.

■

transfer to the Ln3+ cation. At the same time various ligands can form the π-bond with lanthanides,7−9 but surprisingly, such ligands that one can call as “π-bonded antenna” have not been used for design of the homoleptic (only a π-bonded antenna) luminescent complexes. Such a problem has an evident interest from the practical point of view for the preparation of either complexes containing solely π-bonded antenna ligands or heteroleptic π-complexes with auxiliary σbonded antenna ligands. Furthermore, this problem is of interest from a theoretical point of view. The majority of empirical rules, for example, rules by Latva et al.,10 that interconnect the efficiency of luminescence sensitization with the energy of the lowest singlet and triplet states of the ligand were proposed basing on statistical analysis of experimental data for complexes with various bidentante σ-bonded ligands,

INTRODUCTION The lanthanide-based compounds are very attractive for the development of new materials for various domains from biology and medicine to catalyst, optics, and material science. Most of Ln3+ ions are luminescent with characteristic narrowline emission. This luminescence is caused by parity-forbidden 4f−4f transitions. Since molar absorption coefficients of these transitions are low, this leads to a low value of quantum yield of the Ln3+ ion luminescence. To overcome this limitation, an indirect excitation of the Ln3+ ion through the absorption bands of a coordinated ligand with subsequent energy transfer to the Ln3+ ion (“antenna” effect) is generally used.1 There are many examples of such an “antenna-ligand”, most of them are aromatic or unsaturated heterocyclic organic ligands coordinated to the lanthanide cation by a heteroatom (O, N).2−6 In this case, the π-system serves for light harvesting, while the electron lone pairs participate in the formation of a σ-bond between the Ln3+ ion and organic ligand, providing energy © XXXX American Chemical Society

Received: May 22, 2018

A

DOI: 10.1021/acs.inorgchem.8b01405 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry and thus the rules can be nonvalid in the case of π-systems. Therefore, one can propose that it may be possible to obtain bright luminescent complexes even in the case of ligands that are not “optimal” according to the known rules. Since the σbonded antenna is widely used for the Ln3+ ion luminescence sensitization, we have focused our attention on Ln3+ complexes with π-bonded ligands. Consequently, it is intriguing to clarify the possibility of using π-coordinated ligands as “antennas” for various applications, in particular, the cyclopentadienyl ligands (Cp ligands) being the most popular in the organolanthanide chemistry.7,11−14 The choice of Cp as a ligand is based on a few reports on photophysical characteristics of the lanthanide Cp derivatives (Eu2+ derivatives15,16 and several examples of (C5H5)3Ln complexes17−21) that have encouraged us to shed more light on their potential usage as the π-bonded antenna ligand. The lack of information on optical properties of lanthanide(III) Cp derivatives is not only due to the difficulties of handling these extremely air sensitive compounds but also due to dominance of traditional approach−usage of various σbonded antenna ligands. Assuming the lack of information on luminescent properties of lanthanide(III) Cp derivatives, we have proposed that the most appropriate series for such systematic analysis of πbonded antenna ligand for Ln luminescence sensitization would be phenyl-substituted Cp ligands, luminescent properties of which should depend on the number of the attached phenyl groups, The choice is justified by photophysical data for their synthetic precursorscyclopentadienes.22−24 Despite of synthetic availability of phenyl-substituted cyclopentadienes with different number of phenyl groups,25−27 polyphenylcyclopentadienyl complexes of lanthanides are nearly unexplored.15,26−32 Phenyl-substituted Cp ligands could be useful not only to improve the photophysical properties of organolanthanides but also to provide an effective steric control due to the introduction of bulky phenylsubstituents. Furthermore, in the case of ate complexes with alkali metals, their cations can additionally perturb πconjugation in the ligand system, which can be used as the other factor for the design of Ln complexes. The reactivity in organolanthanide chemistry is often affected by steric factors, and steric control can be manipulated by varying the ligand size.33−39 Therefore, phenyl substituted Cp-ligands have great promise in organolanthanide chemistry in this respect. In this work, we have used cyclopentadienyl (Cp), 1,3diphenylcyclopentadienyl (CpPh2), 1,2,4-triphenylcyclopentadienyl (CpPh3), and 1,2,3,4-tetraphenylcyclopentadienyl (CpPh4) ligands to design the different types of terbium and gadolinium complexes with different number of cyclopentadienyl ligands in the metal coordination sphere and different numbers of phenyl substituents at the Cp-rings, as well as to perform a comparative study of their structural peculiarities in combination with photophysical properties and DFT calculations. Pentaphenylcyclopentadienyl and monophenyl-substituted Cp ligands were not used because of the solubility issues31,32 and fast polymerization of CpPhH.40,41

■

Cp3Gd(THF), (3b) Cp2TbCl(THF) (2a), Cp2GdCl(THF) (2b), and CpTbCl2(THF)3 (1a) were prepared according to the literature procedures.12−14,42 The lanthanide content was determined by direct complexometric titration with disodium salt of ethylenediaminetetraacetic acid, using the xylenol orange indicator. Elemental analyses were performed with a PerkinElmer 24000 Series II elemental CHNS/O analyzer. THF solutions of potassium arylcyclopentadienides were prepared as described earlier.30 1,3-diphenylcyclopentadiene,25 1,2,4-triphenylcyclopentadiene,26 and 1,2,3,4-tetraphenylcyclopentadiene27 were vacuum sublimed prior to use. General Synthetic Procedures. Synthesis of [{(1,2,4-Ph3C5H2)TbCl2(THF)}2KCl(THF)] (7a). A solution of PhCH2K (0.271 g, 2.08 mmol) in 10 mL of THF was added slowly to the 10 mL THF solution of 1,2,4-Ph3-C5H3 (0.601 g, 2.04 mmol). The reaction mixture was stirred for 15 min. The obtained solution of K[1,2,4-Ph3C5H2] was slowly added in small portions to a stirred suspension of TbCl3(thf)3 (0.963 g, 2 mmol) in THF (10 mL). Within a few minutes, almost all solid dissolved. The reaction mixture was stirred for 12 h. Then, reaction mixture was centrifuged (5000 rpm, 15 min) and obtained precipitate was removed. The solution was concentrated in a vacuum to a volume of ca. 5−10 mL and layered with hexane (15−20 mL) to initiate crystallization. Crystals were obtained after several days. The crystals of 7a were dried under dynamic vacuum during 3.5 h, yielded 1.196 g (1.61 mmol, 81%). Calcd for C31H33Cl3KO2Tb: Tb, 21.42%; C, 52.09%; H, 4.37%. Found Tb, 21.00%; C, 51.22%; H, 4.23%. Synthesis of 4a−10b. Complexes 4a−10b were prepared in a similar way to the synthesis of 7a. [(1,3-Ph2C5H3)TbCl2(THF)3] (4a). Following the procedure described above, [TbCl3(THF)3] (1 g, 2.07 mmol), [1,3-Ph2-C5H4] (0.462 g, 2.12 mmol), and PhCH2K (0.281 g, 2.16 mmol) yielded 0.680 g (1.02 mmol, 50%). Calcd for C29H37Cl2O3Tb: C, 52.50; H, 5.62; Tb, 23.95. Found: C, 51.78; H, 5.66; Tb, 23.62. [(1,3-Ph2C5H3)GdCl2(THF)3] (4b). Following the procedure described above, [GdCl3(THF)2.1] (0.415 g, 1.00 mmol), [1,3-Ph2C5H4] (0.223 g, 1.02 mmol), and PhCH2K (0.135 g, 1.04 mmol) yielded 0.505 g (0.76 mmol, 76%). Calcd for C29H37Cl2O3Gd: Gd, 23.76. Found: Gd, 23.93. [(1,3-Ph2C5H3)2TbCl][KCl(THF)] (5a). Following the procedure described above, [TbCl3(THF)3] (0.722 g, 1.5 mmol), [1,3-Ph2C5H4] (0.671 g, 3.07 mmol), and PhCH2K (0.408 g, 3.13 mmol) yielded 0.930 g (1.20 mmol, 80%). Calcd for C38H34Cl2KOTb: C, 58.85; H, 4.42; Tb, 20.49. Found: C, 58.50; H, 5.61; Tb, 20.29. [(1,3-Ph2C5H3)2GdCl][KCl(THF)] (5b). Following the procedure described above, [GdCl3(THF)2.1] (0.622 g, 1.5 mmol), [1,3-Ph2C5H4] (0.671 g, 3.07 mmol), and PhCH2K (0.408 g, 3.13 mmol) yielded 0.850 g (1.10 mmol, 73%). Calcd for C38H34Cl2GdKO: C, 58.97; H, 4.43; Gd, 20.32. Found C, 58.95 ; H, 4.31 ; Gd, 19.97. [(1,3-Ph2C5H3)3Tb] (6a). Following the procedure described above, [TbCl3(THF)3] (0.626 g, 1.3 mmol), [1,3-Ph2-C5H4] (0.865 g, 3.96 mmol), and PhCH2K (0.526 g, 4.04 mmol) yielded 0.640 g (0.80 mmol, 62%). Calcd for C51H39Tb: C, 75.55; H, 4.85; Tb, 19.60. Found: C, 75.57; H, 4.80; Tb, 19.25. Single-crystal X-ray analysis revealed that the unit cell of complex (6a) contains free THF molecule, this solvent molecule was lost after the compound was dried in vacuo. [{(1,2,4-Ph3C5H2)GdCl2(THF)}2KCl(THF)] (7b). Following the procedure described above, [GdCl3(THF)2.1] (0.830 g, 2 mmol), [1,2,4Ph3-C5H3] (0.606 g, 2.04 mmol), and PhCH2K (0.271 g, 2.08 mmol) yielded 0.847 g (1.23 mmol, 62%). Calcd for C58H58Cl5Gd2KO3: Gd, 23.58; C, 50.29; H, 4.49. Found: Gd, 22.93; C, 49.28; H, 4.05. [(1,2,4-Ph3C5H2)2TbCl(KCl)] (8a). Following the procedure described above, [TbCl3(THF)3] (0.963 g, 2 mmol), [1,2,4-Ph3-C5H3] (1.201 g, 4.08 mmol), and PhCH2K (0.542 g, 4.16 mmol) yielded 1.436 g (1.68 mmol, 84%). Calcd for C46H34Cl2KTb: Tb, 18.57; C, 64.57; H, 4.00. Found: Tb, 18.48%; C, 64.52%; H, 3.99%. [(1,2,4-Ph3C5H2)2GdCl(KCl)] (8b). Following the procedure described above, [GdCl3(THF)2.1] (0.830 g, 2 mmol), [1,2,4-Ph3-C5H3] (1.201 g, 4.08 mmol) and PhCH2K (0.542 g, 4.16 mmol), yielded

EXPERIMENTAL SECTION

General Experimental Remarks. All synthetic manipulations were carried out in prepurified argon atmosphere in absolute solvent media, using a glovebox. Tetrahydrofuran was predried over NaOH and distilled from potassium/benzophenone ketyl. Hexane was distilled from Na/K alloy. Toluene was distilled from sodium/ benzophenone ketyl. TbCl3(thf)3, GdCl3(thf)2.1, Cp3Tb(THF) (3a), B


Article

Inorganic Chemistry

(CpPh2)3Tb were optimized in C1, while latter in the C3 point group. Vibration frequencies were computed for the optimized complexes and were confirmed to be potential energy minima by the absence of imaginary modes. All calculations were performed by the G09 program package.51 For [(CpPh2)2TbCl2K(THF)] for TD-DFT calculation, we have used the experimental geometry but with normalization of C−H bonds to the ideal 1.08 Å value.

1.584 g (1.85 mmol, 93%). Calcd for C46H34Cl2KGd: Gd, 18.41; C, 64.69; H, 4.01. Found: Gd, 17.92; C, 64.62; H, 4.01. [(1,2,3,4-Ph4C5H)TbCl2(THF)]2[KCl(THF)2] (9a). Following the procedure described above, [TbCl3(THF)3] (0.722 g, 1.5 mmol), [1,2,3,4-Ph4-C5H2] (0.566 g, 1.53 mmol), and PhCH2K (0.203 g, 1.56 mmol) yielded 0.575 g (0.37 mmol, 49%). Calcd for C74H74Cl5KO4Tb2: C, 56.92; H, 4.78; Tb, 20.35. Found: C, 56.80; H, 5.10; Tb, 20.28. Single-crystal X-ray analysis revealed that the unit cell of complex (9a) contains 3 free THF molecules, these molecules were lost after drying in vacuo. [((1,2,3,4-Ph4C5H)GdCl)2(KCl)(THF)3] (9b). Following the procedure described above, [GdCl3(THF)2.1] (0.415 g, 1 mmol), [1,2,3,4Ph4-C5H2] (0.378 g, 1.02 mmol), and PhCH2K (0.136 g, 1.04 mmol) yielded 0.301 g (0.30 mmol, 60%). Calcd for C70H66Cl5KO3Tb2: Gd, 23.58; C, 56.57; H, 4.48. Found: Gd, 23.28; C, 56.08; H, 5.06. [(1,2,3,4-Ph4C5H)2TbCl(KCl)(THF)2] (10a). Following the procedure described above, [TbCl3(THF)3] (0.241 g, 0.5 mmol), [1,2,3,4-Ph4C5H2] (0.378 g, 1.02 mmol), and PhCH2K (0.136 g, 1.04 mmol) yielded 0.301 g (0.30 mmol, 60%). Calcd for C66H58Cl2KO2Tb: Tb, 13.79; C, 68.81; H, 5.07. Found: Tb, 13.40; C, 67.56; H, 4.99. Singlecrystal X-ray analysis revealed that the unit cell of (10a) contains free THF molecule, the molecule was lost after drying in vacuo. X-ray Diffraction Studies, Crystallographic Data, and Refinement Details. X-ray diffraction data for all studied complexes were collected on a SMART APEX II area-detector diffractometer (graphite monochromator, ω-scan technique), using Mo Kα radiation. The intensity data were integrated by the SAINT program and were corrected for absorption and decay.43 All structures were solved by direct methods and refined on F2 using SHELXL.44 Unresolved THF molecules in 6a and 9a were removed by the SQUEEZE method.45 Crystal data, data collection and structure refinement details are summarized in Table S1. Detailed structure descriptions can be found in sections 1.1−1.11 of the Supporting Information. Optical Measurements. Steady-state luminescence and excitation measurements in the visible region were performed with a Fluorolog FL 3−22 spectrometer from Horiba−Jobin−Yvon−Spex which has a 450 W xenon lamp as the excitation source and an R-928 photomultiplier. The special technique involved the usage of specially designed sealed quartz cuvettes for manipulating of air-sensitive compounds during luminescence measurements. Luminescence lanthanide lifetimes (τ) were measured at least three times, which were achieved by monitoring the decay at the maxima of the emission spectra. The single or biexponential decays were analyzed with Origin 8.1. The quantum yield measurements were carried out on solid samples with a Spectralone-covered G8 integration sphere (GMP SA, Switzerland) under ligand excitation, according to the absolute method of Wrighton.46−48 Each sample was measured several times under slightly different experimental conditions. The estimated error for quantum yields was ±10%. Assuming Forster’s dipole−dipole mechanism of energy transfer between metal-ion sites or between a chromophore and an ion, the efficiency of the latter is given by4

ÄÅ É 6 Ñ−1 ij ij τ yzyzz ÅÅÅÅ ij R yz ÑÑÑÑ j j z j z ηet = jjj1 − jj zzzzz = ÅÅÅ1 + jj zz ÑÑÑ j τ0 zz ÅÅ j R 0 z ÑÑ j k {{ ÅÅÇ k { ÑÑÖ k

■

RESULTS AND DISCUSSION Synthesis. The tetrahydrofuran adducts of lanthanide trichlorides LnCl3(THF)x, which have been shown to be convenient precursors to Ln(III) organometallic compounds, were used for the syntheses. Complexes with unsubstituted Cp ligand CpTbCl 2 (THF) 3 (1a), Cp 2 TbCl(THF) (2a), Cp2GdCl(THF) (2b), Cp3Tb(THF) (3a), and Cp3Gd(THF) (3b) were obtained according to the described procedures for synthesis of cyclopentadienyllanthanides11−14,42 (Scheme 1). Scheme 1. General Reaction Scheme of the Synthesis of Unsubstituted Cyclopentadienyl Complexes

The structures of 1a, 2a, and 2b, were established by X-ray diffraction method and appeared to be similar to the known structures of mono- and bis- Cp-complexes of Y and 4f-metals (see Structural Studies and Supporting Information for additional details). Complexes with phenyl-substituted Cp-ligands were obtained from LnCl3(THF)x and K(CpPh2), K(CpPh3) or K(CpPh4), which were generated in situ by the reaction of PhCH2K with corresponding phenyl-substituted cyclopentadienes. The introduction of phenyl substituents leads to increase in crystallizability of formed complexes. All the compounds were isolated by crystallization from THF/hexane mixtures as colorless to greenish crystals. Using diphenyl-substituted Cp ligands allowed us to prepare all the three possible type of cyclopentadienyl complexes, namely, mono(cyclopentadienyl) (CpPh2)LnCl2(THF)3 Ln = Tb (4a), Gd (4b), bis(cyclopentadienyl) [(CpPh2)2LnCl2K(THF)]n Ln = Tb (5a), Gd (5b), and tris(cyclopentadienyl) (CpPh2)3Tb (6a) derivatives. (Scheme 2). Complexes 4 and 6 form mononuclear complexes, while 5 forms an ate complex with a 1D coordination polymer structure (see Structural Studies and Supporting Information for additional details).

(1)

Computational Details. The optimization and electronic excitation energies for (CpPh4)TbCl2(THF)3, (CpPh3)TbCl2(THF)3, (CpPh2)TbCl2(THF)3, CpTbCl2(THF)3, Cp2TbCl(THF)2, and (CpPh2)3Tb were calculated with the hybrid PBE functional and large-core energy-adjusted RECPs for Tb, developed by the Stuttgart and Dresden groups, along with the accompanying basis set ECP54MWB to describe the valence electron density.49,50 Largecore energy-adjusted RECPs for lanthanides put 5s, 5p, 6d, and 6s shells in the valence space, whereas 4f electrons belonged to the core pseudopotentials. For other atoms, a 6-311+g* basis set was employed. Tight SCF convergence and standard optimization convergence criteria along with ultrafine grids were always used during the calculations. All structures with the exception of C


Article

Inorganic Chemistry

(10a) with tetraphenyl Cp-ligand were prepared in a similar way (Scheme 4). All the tetraphenylcyclopentadienyl complexes form ate complexes, similar to the triphenylcyclopentadienyl derivatives. Therefore, with exception of complexes 4a, 4b, and 6a, all the polyphenylcyclopentadienyl complexes were obtained in the form of ate complexes. All attempts to convert ate complexes to neutral mono or binuclear complexes and to isolate them as individual compounds were unsuccessful and led to the inseparable mixtures of compounds. Thus, the application of phenylcyclopentadienyl ligands allows to effectively control the Ln3+ ion coordination sphere saturation. Depending on the ratio of the Ln3+ ionic radius and bulkiness of the ligand (the number of introduced Ph-groups), we obtained all the possible structural types: from mononuclear complexes of the types (CpPh2)LnCl2(THF)3 and (CpPh2)3Ln to di- and tetranuclear complexes and coordination polymers.

Scheme 2. Synthesis of Diphenylcyclopentadienyl Complexes

■

STRUCTURAL STUDIES Mono(Cyclopentadienyl) Complexes. The mono(cyclopentadienyl) dichloride Tb complexes CpTbCl2(THF)3 (1a) (Figure 1, left) and (CpPh2)TbCl2(THF)3 (4a) (Figure 1 right) exhibit structures, where the Tb3+ cation is in a pseudooctahedral environment (CNTb = 8) with the chloride anions being in the trans position and the THF molecules being in the mer position. Complex 1a is isostructural with nine other (η5-C5H5)LnCl2(THF)3 complexes (see Supporting Information). Complex 4a is isostructural with previously reported compounds (CpPh2)LnCl2(THF)3 (Ln = Lu52 and Yb25). The phenyl rings in 4a are almost coplanar with the Cp ring (11.4(3) and 24.9(3)°). The introduction of Ph-rings leads to increase of the Tb−Cpcentroid distance from the 2.413(4) (1a) to 2.431(2) Å (4a). The detailed information on C−C and Ln−C bond distances, conformations and other parameters for the complexes 1a and 4a, as well as for all the other complexes, are summarized in the Tables S14−S17).

Triphenylcyclopentadienyl derivatives were synthesized as mono(cyclopentadienyl) [{(Cp Ph3 )Ln(THF)} 2 (μ-Cl) 5 K(THF)]2 Ln = Tb (7a), Gd (7b), and bis(cyclopentadienyl) [(CpPh3)2Ln(μ-Cl)2K]2 Ln = Tb (8a), Gd (8b) complexes (Scheme 3). The mono(cyclopentadienyl) complexes form tetranuclear ate complexes, while the bis(cyclopentadienyl) derivatives are isolated as binuclear ate complexes (see Structural Studies and Supporting Information for additional details). All attempts to obtain tris(cyclopentadienyl) complexes with the CpPh3 ligand failed. Obviously, the presence of three Ph groups makes the Cp ligand too bulky to accommodate three CpPh3 ligands in the coordination sphere of Ln. Mono(cyclopentadienyl) Tb and Gd complexes [{(CpPh4)Ln(THF)}2(μ-Cl)5K(THF)]2 Ln = Tb (9a), Gd (9b) and bis(cyclopentadienyl) complex [(CpPh4)2Tb(μ-Cl)2K(THF)2]2 Scheme 3. Synthesis of Triphenylcyclopentadienyl Complexes

D


Article

Inorganic Chemistry Scheme 4. Synthesis of Tetraphenylcyclopentadienyl Complexes

Figure 1. Crystal structures of CpTbCl2(THF)3 (1a) (left) and (CpPh2)TbCl2(THF)3 (4a) (right) .

Figure 2. Crystal structure of {[(CpPh3)Ln(THF)]2(μ2-Cl)2(μ3-Cl)3K(THF)}2, Ln= Tb (7a), Gd (7b). Here and after, only Cipso atoms (labeled as Ph) are shown for Ph groups that are not coordinated to K+.

The situation dramatically changes upon introduction of the third phenyl group into the Cp ligand. Isomorphous

mono(cyclopentadienyl) complexes of Tb and Gd, bearing the sterically more crowded CpPh3 anion, have been obtained E


Article

Inorganic Chemistry

Figure 3. Crystal structure of {[(CpPh4)Tb(THF)]2Cl5K(THF)2}2 (9a).

as tetranuclear ate complexes {[(CpPh3)Ln(THF)]2(μ2Cl)2(μ3-Cl)3K(THF)}2 (Ln = Tb, 7a, Figure 2; Gd, 7b) with the CNLn also equal to 8. Two [CpPh3Ln(THF)]2(μ2Cl)2(μ3-Cl)3K(THF) units are connected via two K−Cl bonds. The Cp-Ph rotation angles within the CpPh3 ligand for the Ph groups at the fourth position (17.5(2)−20.4(3)°) are substantially lower as compared to those for neighboring Ph substituents (positions 1 and 2) having extra coordination with K+ by two carbon atoms C34 and C35 (30.1(3)° for 7a and 29.9(3)° for 7b) or not having one (35.7(2)−38.7(2)°). The rotation of two Ph rings at positions 1 and 2 decreases their conjugation with the Cp ring and results in the similarity of the Ln−Cpcentroid distances for 4a and 7a (see Table S15). A similar tetranuclear structure was observed for {[(CpPh4)Tb(THF)]2Cl5K(THF)2}2(THF)3, 9a, (see Figure 3; CNTb = 8). The organolanthanide molecule is located at an inversion center and contains two dinuclear cores [Tb2KCl5] (see Figure S24). However, unlike in structures 7a/7b, two [Tb2KCl5] moieties are linked by two weak K···Ph interactions instead of K···Cl. Generally, the values of Ph−Cp rotation angles in the CpPh4 anion in 9a for the first and fourth positions are similar for those in the CpPh2 ligand in 4a while the rest are almost orthogonal (Table S16). It is worth mentioning that the Tb1− Cpcentroid distance for the CpPh3 anion interacting with K+ has increased by 0.040 Å, but the other Tb2−Cpcentroid distance has increased only by 0.012 Å in 9a as compared to 7a. Bis(Cyclopentadienyl) Complexes. Other structural motifs have been determined for bis(cyclopentadienyl) complexes. Gadolinium complex with nonsubstituted Cp ligands demonstrates a binuclear structure [Cp 2 Gd(THF)] 2 (μ-Cl) 2 (2b) (Figure 4) possessing a Gd 2 Cl 2 rhomboid core with μ2-bridging chloride anions (CNGd = 9). Atoms Gd1, Gd2, Cl1, Cl2, O1, and O2 form a nearly flat fragment. Structure 2a/2b possesses a regular structure of the [Cp′2Ln(thf)(μ-Hal)]2 type with the Tb−Cpcentroid 2.407(2)− 2.416(2) Å in 2a being almost equal to that in 1a. The structures of 2a/2b are similar to the previously reported structures of [Cp2Ln(THF)2(μ2-Cl)2] (Ln = Y,53 Nd,54 Dy,55 Er;56 see Supporting Information for details). The ate complexes [(CpPh2)2Ln(THF)Cl2K]∞ (Ln = Tb, 5a; Gd, 5b) represent a unique example of a 1D coordination polymer (Figure 5) within this series. Structures of 5a and 5b

Figure 4. Crystal structure of [Cp2Ln(THF)(μ-Cl)]2. Ln = Tb (2a), Gd (2b).

Figure 5. Crystal structure of [(CpPh2)2Ln(THF)Cl2K]∞. Ln = Tb (5a), Gd (5b).

are isomorphous. The structural unit of the polymer is a [(CpPh2)2Ln(THF)Cl2K] fragment (CNLn = 8) containing two Cl− ligands, which form bridges between Ln3+ and K+. In each fragment, six phenyl carbon atoms take part in the K···CPh interaction. Two [(CpPh2)2Ln(THF)Cl2K] fragments are connected via K···Cl interaction in the polymer with the unique [Ln(μ2-Cl)(μ3-Cl)K]∞ core (see SI). The angles between the Cp and noncoordinated Ph planes in 5a/5b (13.1(4)−40.2(3)°; see Table S16) are generally higher than F


Article

Inorganic Chemistry

Figure 6. Crystal structure of [(CpPh3)2Ln(μ2-Cl)(μ3-Cl)K]2. Ln = Tb (8a), Gd (8b).

bridging K+ cation, which is composed by two THF molecule and an η2 coordinated Ph ring. In contrast to 5a and 8a, the formation of bis(cyclopentadienyl) complexes bearing the CpPh4 ligand does not lead to the shortening of the Tb− Cpcentroid distance with respect to the mono(cyclopentadienyl) complexes. The variations of the Cp-Ph rotation angles in 10a and 9a have the same trend depending on Ph ring positions. Tris(Cyclopentadienyl) Complexes. From the series of used polyphenyl-substituted Cp ligands, only the least crowded CpPh2 anion has appeared to be suitable for the synthesis of a tris(cyclopentadienyl) complex. The unit cell of compound [(CpPh2)3Tb](thf)0.5 (6a) (Figure 8) contains three symmetrically independent [(CpPh2)3Tb] molecules (CNTb = 9) with each of them being located on 3-fold axes.

those in 4a due to steric hindrance, but the smallest Cp-Ph angles in 5a/5b correspond to the Ph rings coordinated with K+ (12.7(2)−12.8(2)°). Surprisingly, the Ln−Cpcentroid distances in 5a are even shorter than that in 1a and vary in the range of 2.380(2) to 2.394(2)Å. Two isomorphous bis(cyclopentadienyl) complexes with the more bulky 1,2,4-triphenylcyclopentadienyl anion have structures of a dimeric ate complex [(CpPh3)2LnCl2K]2 (Ln = Tb, 8a; Gd, 8b; Figure 6), in which the two units are interconnected by two K···Cl and four K···η6-Ph interactions. The Cp-Ph angles are like those in 7a with the same trend of the planarity increase for Ph rings coordinated by K (see SI). The Tb−Cpcentroid in 8a is also shortened by ca. 0.03 Å in comparison with monocyclopentadienyl analogue (Table S15). A similar structure has been determined for a dimeric ate complex [(CpPh4)2Tb(μ2-Cl)(μ3-Cl)K(THF)2]2(THF) (10a) (Figure 7) bearing the bulkier CpPh4 anion. The main difference between 10a and 8a is the coordination sphere of

Figure 8. Structure of one of three [(CpPh2)3Tb] molecules in 6a.

The Tb3+ atom in each of three molecules lies in a plane formed by Cp centroids with Cpcentroid−Tb−Cpcentroid angles being of 120.0(2)°. The Cp−Ph rotation angles lie in the range of 8.2(2)−23.7(1)° for all three molecules. The Tb−Cpcentroid distances vary in the range 2.411(2)−2.431(2) Å. The shortest Tb−Cpcentroid distance is observed in the molecule with the minimal Ph−Cp angles (see Tables S15 and S16).

Figure 7. Crystal structure of [(CpPh4)2Tb(μ2-Cl)(μ3-Cl)K(THF)2]2, 10a. G


Article

Inorganic Chemistry

Figure 9. Luminescence spectra of the 8a (a), 10a (b), 2a (c), 3a (d), 5a (e) (left) and 7a (a), 1a (b), 6a (c), 4a (d), 9a (e) (right) at 300 K.

is worth to mention that the luminescence of the Cp ligands is observed in the spectrum of complex 6a in the region of 400− 470 nm, indicating that intramolecular energy transfer from the Cp ligands to the Tb3+ ion is not completely efficient, and to less extent the same pattern is observed in the spectra of complexes 1a and 10a (see below). In contrast to Eu3+ ion, the full assignment of Stark components of the electronic transitions in the luminescence spectrum of the Tb3+ ion is impossible due to the large number of closely spaced energy levels, but several points can be indicated. Namely, the luminescence spectra of the complexes containing one Cp ligand (1a, 4a, 7a, 9a) can be combined in one group since they have similar grouping of Stark components of the 5D4 → 7FJ, J = 6−4 transitions and general splittings of these transitions are smaller than ones observed in the luminescence spectra of the second group. It is worth noticing that the luminescence spectra of 4a and 7a look very similar. This fact is in the line with X-ray crystal data, according to which the Cp ligand is situated at one side of the Tb3+ ion while the Cl− anions and the oxygen atoms of THF molecules are located from the other side with similar mutual disposition in both complexes. Such an arrangement of ligands can lead to similar charge distribution around the Tb ion in complexes 4a and 7a.57 The second group is formed by complexes 2a, 5a, 8a, and 10a. The tendency manifests in magnitude of general splittings that correlates well with molecular geometry and points out to a stronger crystal field formed by ligands in the bis(cyclopentadienyl) complexes. In the luminescence spectrum of complex 3a the general splittings of the 5D4 → 7FJ, J = 6−4 transitions are maximum that indicates the strongest crystal field (Figure 9 left, d). The radiative rates are important parameters for analysis of energy transfer process and are associated with geometric structures of the complexes. It is generally accepted that

Within the current work, we have extended the family of crystallographically characterized polyphenyl-substituted cyclopentadienyl rare-earth complexes, including ones with rare and rather uncommon structural motifs and cores. The structures are represented by neutral mononuclear complexes 1a, 4a, and 6a; neutral dinuclear complexes 2a and 2b possessing the [Ln(μ2-Cl)]2 rhomboid core; ate complexes 5a and 5b with the polymeric [Ln(μ2-Cl)(μ3-Cl)K]∞ core; dinuclear ate complexes 8a, 8b, and 10a possessing the [Ln(μ2-Cl)(μ3Cl)K]2 core; tetranuclear ate complexes 7a and 7b with the [Ln2(μ2-Cl)2(μ3-Cl)3K]2 core; dimeric tetranuclear ate complex 9a with two [Ln2(μ2-Cl)3(μ3-Cl)2K] cores. To discuss molecular geometry and especially the parameters that can affect the efficiency of sensibilization of lanthanide ion luminescence, we should point out that the Cp-Ph rotation angle is defined not only by the intrinsic steric characteristics of the ligand but also by the presence of additional coordination with potassium cation. Photophysical Properties. All structurally characterized Tb and Gd complexes containing Cp, CpPh2, CpPh3, and CpPh4 ligands have been studied by optical spectroscopy. The luminescence spectra of Tb complexes, recorded in the range of 470 to 720 nm under excitation at the Cp transitions, exhibit characteristic narrow emission bands that are assigned to the 4f8−4f8 transitions of the Tb3+ ion (Figure 9). The following electronic transitions were observed: 5D4 → 7F6 (480−500 nm), 5D4 → 7F5 (535−555 nm), 5D4 → 7F4 (575−595 nm), 5 D4 → 7F3 (610−630 nm), 5D4 → 7F2 (640−660 nm), 5D4 → 7 F1 (660−675 nm), 5D4 → 7F0 (675−685 nm). The latter three transitions expectedly have low intensity. The 5D4 → 7F5 transition is the most prominent and accounts for ∼40−65% of the total emitted intensity. It is important that the 5D3 → 7FJ transitions are not exhibited in the luminescence spectra of the Tb systems (the spectral interval between 400 and 475 nm). It H


Article

Inorganic Chemistry

Table 1. Energy of S1 and T1 States, Observed Lifetimes, Values E1 and E2, as Well as a Branching Ratio β and Absolute Quantum Yielda τobs (ms)

complex 1a 2a 3a 4a 5a 6a 7a 8a 9a 10a

0.92 0.42 0.54 0.68 0.35 0.65 0.50 0.32 1.01 0.35

± ± ± ± ± ± ± ± ± ±

0.03 0.02 0.02 0.03 0.01 0.02; 0.01 ± 0.01 0.01 0.01 0.03 0.01

β

S1 (nm)

T1 (nm)

E1 (cm−1)

E2 (cm−1)

b QLig Tb (%)

0.35 0.27 0.34 0.38 0.17 n/a 0.39 0.33 0.27 0.38

270 280 325 340 355 360 350 360 360 360

380 390 425 440 430 490 435 435 435 435

10721 10073 6617 6684 4913 7370 5583 4789 4789 4789

5816 5141 3029 2227 2756 8 2489 2489 2489 2489

4 7 32 14 50 10 11 25 60 25

All photophysical data were measured at ambient temperature. bEstimated relative error QLig Tb ± 10%.

a

Figure 10. Luminescence excitation spectra of the Tb complexes 6a (a), 3a (b), 2a (c), 1a (d), 4a (e) (left) and 5a (a), 9a (b), 10a (c), 7a (d), 8a (e) at 300 K. λreg = 454 nm.

asymmetry of the ligand field is reflected as increase in the branching ratio (β), resulting in increase of the radiative rate. The branching ratio is defined as that of the intensity of the electric-dipole transition (5D4 → 7F6, ΔJ = 2) to the magneticdipole transition (5D4 → 7F5, ΔJ = 1) and is presented in Table 1. Complex 5a exhibits the minimum β value (0.17) and 4a, 7a, and 10a are characterized by higher but almost similar ones (0.38, 0.39 and 0.38). Thus, the site symmetry of the Tb3+ ion is highest in 5a and lower in 4a, 7a, and 10a. In complex 9a the β value is low as well. The luminescence decay curves were measured for all Tb complexes, and they were adjusted with a single exponential function with one exception. The lifetime values (τ) of the 5D4 emitting level are presented in Table 1. The luminescence decay curves for complex 6a were better adjusted by a biexponential function (τ1 = 0.65 ms, τ2 = 0.01 ms) that indicates that both Cp ligands and Tb3+ ion are acting as emitting species with lifetimes of τ1 and τ2, respectively. The longest τ value was found for complexes 1a and 9a, while the shortest one was observed for the complex 5a (0.35 ± 0.01 ms). As lanthanide complexes potentially have a lot of optical application we consider excitation and luminescence processes

in detail. To simplify the discussion, we assume that sensitized luminescence in the Ln-containing systems results from energy migration from the singlet excited state of the ligand (S1) to its triplet state (T1), and finally to the excited levels of the lanthanide ion. In the case of the complexes containing Tb3+ ion, the probability for energy transfer from S1 to higher energy levels of the Tb3+ and for consequent decay to the 5D4 level by nonradiative relaxation is high. Back energy transfer (BET) process is often observed when the ligand triplet state has energy very close to the Ln3+ excited levels and so the probability of BET process to T1 level in the complexes with Tb3+ ion is also high. Therefore, the energies of S1 and T1 states are very important for the optimization of intraligand energy transfer while the introduction of two, three and more ligands has to decrease the probabilities of nonradiative processes. Moreover, according to generally accepted concept of energy gaps, the optimal energy difference between the S1 and T1 states (gap E1) for an efficient intersystem crossing is ca. 5000 cm−1, while the energy difference between T1 state and 5D4 level of the Tb3+ ion (gap E2) is optimal in the range 2500−4000 cm−1.10 Since the lowest-energy f−f* transition of the Gd3+ ion appears at 313 nm and the Ln3+ complexes have ligand states frequently at lower energies, the Gd complexes I


Article

Inorganic Chemistry

According to TD-DFT calculations the lowest energy transitions (T1−S0) and (S1−S0) are mainly HOMO to LUMO. As one can see using (CpPh2)TbCl2(THF)3 and (CpPh4)TbCl2(THF)3 as examples, the phenyl groups that are nearly orthogonal to Cp ring do not participate in both HOMO and LUMO (Figure 11). For CpPh4 and CpPh2 derivatives, the HOMO is located mainly at the Cp ring and in part at the coplanar to it phenyl rings. The main contribution to LUMO comes from the coplanar phenyl rings and the C−Ph bonds. Thus, these transitions correspond to transfer from the Cp system to phenyl rings. A rather surprising feature of LUMO for complexes with the CpPh4 ligands is some contribution of the Tb3+ ion that can be interpreted as the population of dx2−y2 orbital that is coplanar to the Cp ring. The low contribution of the orthogonal phenyl groups to the HOMO and LUMO well coincides with proximity of calculated and experimental (see above) S1 and T1 energies in the series of complexes with the CpPh2, CpPh3, and CpPh4 ligands. In the luminescence excitation spectra of complexes 5a, 7a, 8a, 9a, and 10a, the low lying excited state appears in the region of 410−430 nm. Since this band is very broad and, in some cases, weak, its maximum was considered as being approximately at 420 nm (23810 cm−1) in all complexes. Assuming that this band is also observed in the excitation spectra of the corresponding Gd complexes, it was tentatively assigned to intraligand charge transfer (ILCT) state.58−60 To understand the origin of this state, it is necessary to mention that all of these complexes are ate complexes, in which the K+ cation participates in additional coordination with the phenyl ring of the Cp ligands. Such K···π interaction should cause the additional charge redistribution from the Cp system to the phenyl group and should result in the ILCT state. Previously we have distinguished π-stacking and H-bond induced charge transfer states (SICT and HbCT) in Ln complexes by combined analysis of the experimental X-ray data, luminescence characteristic and quantum chemical calculations.61−63 Therefore, to clarify the origin of charge transfer state in the complexes studied, the title “K+CT” is introduced. The type of K···π interactions in 5a, 7a, 8a, 9a, and 10a can be divided into two types. Complexes 5a and 8a belong to the first group in which K+ forms the η6-interaction with a phenyl group, K···Cpcentroid distance equals to approximately 2.93 Å and the coordinated phenyl group is almost coplanar with cyclopentadienyl ring. In the rest of the complexes, the K···π interactions can be best described as η2 and the phenyl ring is turned by 30−90° out of the cyclopentadienyl ring plane. Thus, one can conclude that the most pronounced influence of K···π interactions might be observed for 5a and 8a. Indeed, the TD-DFT calculation of the ate complex[(CpPh2)2TbCl2K(THF)3] (without geometry optimization) has revealed the significant decrease of S1 and T1 energy (T1 = 416 nm, S1 = 339 nm) with respect to the neutral complexes (Table 2). The analysis of HOMO and LUMO in this complex has shown that in contrast to (CpPh2)TbCl2(THF)3 the LUMO is predominantly localized on the phenyl ring participating in the K···π interaction (Figure 12). Thus, in the considered series of the phenylcyclopentadienyl complexes, the energy of S1 and T1 states can be regulated by introduction and/or extra coordination of phenyl rings by potassium cation. The assignment of the bands in the excitation spectra along with experimentally determined energy of S1 and T1 states

can be used for the assignment of excited states for the coordinated ligand. The steady-state luminescence excitation spectra of the Tb complexes at room temperature, monitored around the peak of the intense 5D4→ 7F5 transition of the Tb3+ ion at 545 nm, are given in Figure 10. These spectra exhibit overlapped, intense, broaden bands corresponding to the π−π* transitions of the Cp ligands, extending from 250 to 320 nm in the complex 1a and up to 480 nm for complex 6a. Moreover, the excitation spectra display weak narrow absorption bands in the spectral range from 320 to 500 nm that are assigned to the 4f8intraconfigurational transitions: 7F6 → 5L6 (339 nm), 7F6 → 5L9 (350 nm), 7F6 → 5L10 (369 nm), 7F6 → 5G6 (376 nm), 7F6 → 5D3 (380 nm), and 7F6 → 5D4 (488 nm). These optical data indicate that the Cp ligand can be considered as an efficient antenna for sensitizing the Tb3+ ion luminescence in the complexes considered. On Figure 10 (right, b−e) the excitation luminescence spectra of the mono(cyclopentadienyl) complexes are presented. The comparison of these spectra, considering the peculiarities of the Cp ligand, allowed us to assign the broad bands observed. The energy of S1 state assigned to π−π* transitions of unsubstituted Cp (1a) is 270 nm (37037 cm−1). The introduction of two phenyl rings (4a) significantly shifts S1 state in long wavelength side by 70 nm (29416 cm−1). In the case of the introduction of the third and fourth phenyl rings (7a and 9a), the energy of S1 state is slightly decreased up to 350 and 355 nm (28570 and 28170 cm−1), respectively (Figure 10 (left, b, d)). The formation of complexes with a different number of Cp ligands also changes the S1 state energy in the range of 10−15 nm (Table 1). The assignment performed is well supported by quantum chemical calculations (see below). The energy of the excited triplet states of Cp ligands were determined by measuring the phosphorescence spectrum of the corresponding isostructural gadolinium complex. As energy of the T1 state the 0-phonon transitions of Cp ligands were taken (Figure S28−30, Table 1). In the phosphorescence spectra of Gd complexes 7b and 9b, the time-resolved measurement reveals a broad band with a maximum at 480 nm (0-phonon component at 435 nm)extended up to 600 nm, with a vibronic progression of about 1050 cm−1, which is typical for aromatic compounds (ring breathing mode).58 As one can see from data presented in Table 1 the energy gap E1 is quite optimal in the case of the complexes with CpPh3 and CpPh4 ligands (complexes 7a−10a), while E2 gap is equal to ∼2490 cm−1 in these complexes that is close to the limit value. Therefore, one can expect the influence of BET process on the energy transfer process in these complexes. The TD-DFT estimations of the energy of the S1 and T1 levels demonstrate the same trend for the S1 and T1 values as the function of phenyl ring numbers (Table 2). Table 2. Energy S1 and T1 States According to TD-DFT Calculation of the Optimized Structures compounds

S1 (nm)

T1 (nm)

(CpPh4)TbCl2(THF)3 (CpPh3)TbCl2(THF)3 (CpPh2)TbCl2(THF)3 CpTbCl2(THF)3 [Cp2TbCl(THF)]2 (CpPh2)3Tb

316.6 319.4 295.8 251.2 259.6 411.3

418.4 411.4 398.1 283.9 296.0 447.7 J


Article

Inorganic Chemistry

Figure 11. HOMO and LUMO in (CpPh4)TbCl2(THF)3 (left) and (CpPh2)TbCl2(THF)3 (right) complexes. The THF molecules are omitted for clarity.

energy gaps E1 (∼10700 and 10000 cm−1) and E2 (∼5800 and 5140 cm−1) (Table 1), the low QLig Tb values are reasonable for 1a and 2a, while in tris(cyclopentadienyl) complex 3a the remarkable increasing the QLig Tb up to 32% is observed. The S1 energy decreases in this complex by ∼2800 cm−1 (this is also supported by TD-DFT calculations) in comparison with one in 1a that optimizes the intersystem crossing process by more appropriate value for the E1 gap. The maxima of excitation broad bands were found by multipeak procedure for corresponding spectrum of the isostructural Gd complex (3b) (Figure S31). Moreover, strong green luminescence with maximum at 490 nm and short wavelength edge at 390 nm was detected at ambient temperature in Gd complex 3b. Previously a green luminescence of the Cp3Gd complex was assigned to an interligand triplet-singlet transition of the [Cp−]3 fragment.17 In such a way, the E2 value is equal to 3029 cm−1 that

Figure 12. LUMO in the [TbCpPh22Cl2K(THF)3]. Only CpPh2 ligand participating in the K···π interactions is shown.

(and as a consequence estimated values of E1 and E2 gaps) allows us to proceed decipher of the energy transfer process. In the 1a−3a complexes, the total quantum yield (QLig Tb ) equals to ∼4%, 7%, and 32%, respectively. Taking into account the large

Figure 13. HOMO and LUMO in (CpPh2)3Tb. K


Article

Inorganic Chemistry

and then a part of the energy is kept on the T1 level owing to BET process (E2 value is small). As a result, the QLig Tb value is relatively low for the Tb3+ ion (14%). For complexes in which the K+CT state is observed (5a, 7− 10a), the highest efficiency is found for 7a and 10a, while the lowest one is realized in the complexes 5a and 9a. The values of quantum yield demonstrate reverse tendency, the maximal QLig Tb values are observed in 5a and 9a (∼50 and 60%, respectively), while 7a has a lower value (11%). To explain the variation of the quantum yield the several points have to be taking into account. Since overall quantum yield QLig Tb is given Tb Tb by QLig Tb = ηsensQTb (where ηsens and QTb are sensitization efficiency and intrinsic quantum yield, respectively), the high QLig Tb found in 5a and 9a complexes can be a consequence of either the high QTb Tb value or the high value of ηsens. The latter can be achieved by lower loses in energy transfer process. Based only on the general accepted concept about the influence of the energy gaps E1 and E2 on efficiency of the Ln luminescence sensitization, one can expect the similar ηsens in complexes 7a−10a, since E1 and E2 have very similar values. The intrinsic quantum yield is defined by the following equation QTb Tb = τobs/τrad = krad/(krad + knrad), where τobs and τrad are observed and radiative lifetimes and krad and knrad are the radiative and nonradiative rate constants, respectively. As one of the longest lifetime of the 5D4 level is observed in 9a (τobs = 1.01 ms) it can be the reason for the high QTb Tb value in comparison with those in complexes 7a, 8a, and 10a. Other reasons, which are out of “energy gap discussion”, should be also disclosed. Concerning the angle of rotation of Ph rings, the complex 9a is similar to 4a, which has the highest efficiency of excitation through the ligand absorption bands. Thus, such an arrangement of two phenyl groups “works” well, while the introduction of two additional phenyl groups, which are nearly orthogonal to the Cp ring causes decreasing the energy of the S1 state, making it optimal. Furthermore, the most pronounced electron withdrawing effect of the CpPh4 ligand and extra K···Ph π-interaction in 9a allows proposing the significant change of its geometry upon the excitation that 62 also can change the QLig Tb value. Returning to effectiveness of the sensitization of the Tb ion luminescence in complex 5a, in addition to optimal E1 and E2 gaps this complex is characterized by the shortest Tb−Cpcentroid distance. Since in the frame of the Förster’s (dipole−dipole) mechanism of energy transfer its efficiency is proportional to 1/R6, one can expect a higher value of ηsens (more efficient energy transfer from Cp ligands to the Tb3+ ion). From the other side, it is necessary to mention the smallest β value (0.17) estimated for this complex, which indicates the lowest value of krad that by definition will also lead to a high QTb Tb value. The intrinsic quantum yield QTb Tb can be roughly estimated by the equation QTb Tb = τobs(298K)/τrad(77K) with the assumption that the decay process at 77 K in this complex is purely radiative. Therewith, the coordination sphere of the Tb3+ ion is formed only by two Cp ligands and two Cl− anions that along with a convenient value of the E2 gap made this assumption rightful. On the basis of these assumptions, QTb Tb equals to 80% and thus high value of the QLig Tb (50%) seems logical. There are other examples of Tb complexes with large quantum yields and short lifetimes like [Tb(hfa)3(dmtph)] (QLig Tb is 56% and tobs = 0.55 ms)66 and Tb-dipivaloylmethanato complex (QLig Tb is 40% and τobs = 0.46 ms).67,68 The authors66−68 explained such combination of QLig Tb and lifetimes by a short radiative lifetime due to orbital mixing. Indeed owing to electronic structure of

is also within the optimal range. Therefore, the formation of tris(cyclopentadienyl) complex with C3 symmetry (3a) causes the decreasing the energy of S1 and T1 states and as a consequence leads to more efficient sensitization of the Tb ion luminescence. Only one other example of tris-cyclopentadienyl complex was obtained, namely (CpPh2)3Tb (6a). This complex exhibits the low QLig Tb value (10%) and the widest excitation channel expanded from 250 to 550 nm. The maximum of the lowest band in the excitation spectrum is 475 nm (21050 cm−1) and long wavelength edge is expired up to 550 nm (18180 cm−1). As it was mentioned above, all phenyl rings in 6a are almost coplanar to Cp and the complex itself has the C3 symmetry even in the crystal structure, which probably causes the formation of additional generalized excited state with low energy. The energy of T1 state perfectly matches to the 5D4 level of the Tb3+ ion and as a result BET to the T1 level of CpPh2 ligands might occur. Surprisingly, although the probability of BET in 6a is high, the value QLig Tb is not very low. The luminescence decay of the 5D4 level (535−555 nm) for this complex could not be fitted by monoexponential equation as it was mentioned above but biexponential one with τ1 = 0.65 ± 0.02 and 0.99 ± 0.03 ms and short second component τ2 = 0.01 ± 0.01 and 0.02 ± 0.01 ms (for 300 and 77 K, respectively). This intense broad band presented in the luminescence spectrum of complex 6a is completely disappeared in the phosphorescence spectrum with decay ∼0.1 ms, thus it can be proposed that its appearance is related to the CpPh2 ligands. Taking into account all these data and the fact that the energy difference between S1 state and 5D4 level of the Tb3+ ion is 7400 cm−1, the direct transfer energy from S1 state to the excited levels of the Tb ion can be suggested. To shed more light on the nature of the additional states presented we performed quantum chemical modeling. According to TD-DFT calculation of (CpPh2)3Tb for experimental geometry with the C3 symmetry, the HOMO is almost totally localized on the equivalent cyclopentadienyl rings, while the LUMO is localized by 90% on Tb and corresponds to 5dz2 orbital (Figure 13). This result in a onedeterminant approximation and without the account of spin− orbital effects64,65 should be considered with caution. At the same time, it should be noted that this rather surprising result well coincides with the recent investigation of the complexes [(Me3SiC5H4)3Ln] and [(Me3SiC5H4)3Ln]−,33,34 where the HOMO of the anionic tris-cyclopentadienyl complexes of lanthanide(II) corresponds to 5dz2 orbital. Furthermore, it may serve as some justification of partial population of dx2−y2 orbital (see above) in the LUMO of (CpPh4)TbCl2(THF)3 and (CpPh2)2TbCl2K(THF)3 according to TD-DFT calculations. This result is of great interest for further developing of Cpbased ligands for highly luminescent lanthanide complexes for various applications. Normalizing the excitation spectra reported in Figure 10 with respect to the integrated intensity of the ligand transitions allows us to estimate the relative efficiency of the Ln3+ ion excitation through ligand absorption bands versus direct f−f excitation. The highest efficiency is observed for 4a. The energy transfer process in this complex can be described in the frame of typical energy gap discussion. Indeed, the excitation energy is effectively absorbed by the CpPh2 ligands (it has two phenyl rings, which are nearly coplanar to the Cp plane) but some part of this energy is nonradiatively lost due to nonoptimal intersystem crossing process (E1 value is larger) L


Article

Inorganic Chemistry the Tb3+ ion, the complex has many energy levels that can mix with appropriate ligand wave functions, including a relatively low lying 4f5d state. This may explain why the lifetime is relatively short (i.e., it implies that the phosphorescence character of the transition is partly lost). Previously, we have observed the f−d transition in the excitation spectra of terbium pyridine mono and dicarboxylates with maximum in the region of 300−320 nm.58,59 In such a way, complex 5a is extraordinary example of highly luminescent compound with very short lifetime (QLig Tb is 50% and τobs = 0.35 ms). Therefore, the fact that a lanthanide complex is highly luminescent yet possesses a short lifetime does not represent a contradiction.68 In summary, phenylsubstituted Cp ligands discussed in this work are relatively efficient “antenna” for the luminescence Tb3+ ion sensitization. As we have shown, many different factors (phenyl group rotation, geometry distortion in the excited state, an extra cationPh π-interaction, inter- and intraligand charge transfer states) should be taken into account in the case of these π-bonded antenna ligands upon the decoding of the energy transfer scheme. Thus, the presented study is the first step in the understanding of π-bonded antenna ligand peculiarities for further design of new lanthanide complexes for advanced optical materials.

contradiction but it can be a consequence of the participation of the low lying 4f5d state in energy transfer process. Additional experiments to analyze the photophysical process of energy back-transfer for these Tb complexes and contribution of f−d transitions in energy transfer process are underway. The main outcome of this work is a new strategy proposed for design of lanthanide luminescent complexes, the cornerstone of which is the usage of the previously underestimated in photochemistry of lanthanides but well-known in synthetic chemistry π-cyclopentadienyl ligands as the main or auxiliary light harvesting antenna. Assuming that proposed synthetic strategy can be expanded to the substituted phenyls and other aromatic rings, the study performed also provides novel aspects in the fields of inorganic, coordination, materials, and physical chemistry.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01405. Details of crystallographic data collection and structure solution, detailed description of the molecular and crystal structures of the complexes obtained, phosphorescence spectra of complexes 4b, 7b, and 9b, and luminescence excitation spectrum of complex 3b (PDF)

■

CONCLUSIONS A series of Tb and Gd polyphenylsubstituted mono-, bis-, and tris- cyclopentadienyl complexes has been obtained by using general synthetic methodology for different type of Cp-ligands. Various phenyl-substituted Cp-ligands provide an effective steric control of Ln coordination sphere, making possible to decrease or fully omit the solvate molecules, which are undesirable for the intense luminescence. Depending on the bulkiness of the ligand (the number of introduced Ph groups), all the possible structural types of lanthanide Cp complexes have been obtained and peculiarities of their self-assembly and geometries have been analyzed. The introduction of the phenyl rings and variation of their number in the Cp ligand can significantly alter the photophysical properties of the Tb complexes, in particular the luminescence efficiency. Indeed, the introduction of two phenyl rings into the Cp ligand in the case of monocyclopentadienyl Tb complex leads to 3.5-fold increase in quantum yield, compared to the Tb complex with unsubstituted Cp, while in the case of the bis(cyclopentadienyl) Tb complex 7-fold increase in quantum yield was obtained. Relatively high quantum yields were observed for coordination polymer with Cp Ph2 ligand (∼50%) and the mono(cyclopentadienyl) complex with CpPh4 ligand (∼60%) that discovers important aspects for design rare-earth complexes with luminescent properties desired, namely the interplay of symmetry of the complex and degree of conjugation of substituents as well as their number. Within the research presented, the low-lying intraligand charge transfer state formed owing to extra coordination with K cation was identified in ate complexes (K+CT). The formation of ate complexes with potassium can additionally perturb conjugation inside ligands and thus can be used as independent route for design of new luminescent materials. Moreover, the presence of such low energy CT states promotes the expansion of the excitation channel up to 550 nm. The complex 5a is extraordinary example of highly luminescent compound with very short lifetime. The fact that terbium complex having short lifetime and is highly luminescent does not represent a

Accession Codes

CCDC 1571590−1571599, 1819486−1819487, and 1841063 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dmitrii M. Roitershtein: 0000-0003-0320-1775 Mikhail E. Minyaev: 0000-0002-4089-3697 Andrei V. Churakov: 0000-0003-3336-4022 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by Russian Science Foundation (Grant 17-13-01357). Authors are thankful to Prof. JeanClaude G. Bünzli for the helpful discussions.

■

REFERENCES

(1) Weissman, S. I. Intramolecular Energy Transfer. The Fluorescence of Complexes of Europium. J. Chem. Phys. 1942, 10, 214−217. (2) Bunzli, J.-C. G. Benefiting from the unique properties of lanthanide ions. Acc. Chem. Res. 2006, 39, 53−61. (3) Eliseeva, S. V.; Bunzli, J.-C. G. Rare earths: jewels for functional materials of the future. New J. Chem. 2011, 35, 1165−1176. (4) Eliseeva, S. V.; Bunzli, J.-C. G. Lanthanide luminescence for functional materials and bio-sciences. Chem. Soc. Rev. 2010, 39, 189− 227. M


Article

Inorganic Chemistry (5) Bunzli, J.-C. G.; Eliseeva, S. V. Intriguing aspects of lanthanide luminescence. Chem. Sci. 2013, 4, 1939−1949. (6) de Bettencourt-Dias, A. In Luminescence of Lanthanide Ions in Coordination Compounds and Nanomaterials; de Bettencourt-Dias, A., Ed.; Wiley: Hoboken, NJ, 2014; Chapter 1, pp 1−48. DOI: DOI: 10.1002/9781118682760 (7) Schumann, H.; Meese-Marktscheffel, J. A.; Esser, L. Synthesis, Structure, and Reactivity of Organometallic π-Complexes of the Rare Earths in the Oxidation State Ln3+ with Aromatic Ligands. Chem. Rev. 1995, 95, 865−986. (8) Arndt, S.; Okuda, J. Mono(cyclopentadienyl) Complexes of the Rare-Earth Metals. Chem. Rev. 2002, 102, 1953−1976. (9) Evans, W. J.; Davis, B. L. Chemistry of Tris(pentamethylcyclopentadienyl) f-Element Complexes, (C5Me5)3M. Chem. Rev. 2002, 102, 2119−2136. (10) Latva, M.; Takalo, H.; Mukkala, V.-M.; Matachescu, C.; Rodriguez-Ubis, J. C.; Kankare, J. Correlation between the lowest triplet state energy level of the ligand and lanthanide(III) luminescence quantum yield. J. Lumin. 1997, 75, 149−169. (11) Wilkinson, G.; Birmingham, J. M. Cyclopentadienyl compounds of Sc, Y, La, Ce and some lanthanide elements. J. Am. Chem. Soc. 1954, 76, 6210−6210. (12) Birmingham, J. M.; Wilkinson, G. The Cyclopentadienides of Scandium, Yttrium and Some Rare Earth Elements. J. Am. Chem. Soc. 1956, 78, 42−44. (13) Maginn, R. E.; Manastyrskyj, S.; Dubeck, M. The Dicyclopentadienyllanthanide Chlorides. J. Am. Chem. Soc. 1963, 85, 672−676. (14) Manastyrskyj, S.; Maginn, R. E.; Dubeck, M. The Preparation of Cyclopentadienyllanthanide Dichlorides. Inorg. Chem. 1963, 2, 904−905. (15) Harder, S.; Naglav, D.; Ruspic, C.; Wickleder, C.; Adlung, M.; Hermes, W.; Eul, M.; Poettgen, R.; Rego, D. B.; Poineau, F.; Czerwinski, K.; Herber, R. H.; Nowik, I. Physical Properties of Superbulky Lanthanide Metallocenes: Synthesis and Extraordinary Luminescence of [Eu(II)(CpBIG)2] (CpBIG = (4-nBu-C6H4)5Cyclopentadienyl. Chem. - Eur. J. 2013, 19, 12272−12280. (16) Kelly, R. P.; Bell, T. D. M.; Cox, R. P.; Daniels, D. P.; Deacon, G. B.; Jaroschik, F.; Junk, P. C.; Le Goff, X. F.; Lemercier, G.; Martinez, A.; Wang, J.; Werner, D. Divalent Tetra- and Pentaphenylcyclopentadienyl Europium and Samarium Sandwich and HalfSandwich Complexes: Synthesis, Characterization, and Remarkable Luminescence Properties. Organometallics 2015, 34, 5624−5636. (17) Strasser, A.; Vogler, A. Optical properties of tris(cyclopentadienyl)gadolinium. Luminescence from an interligand triplet under ambient conditions. Chem. Phys. Lett. 2003, 379, 287− 290. (18) Pappalardo, R.; Losi, S. Band emission of gadolinium tricyclopentadienide and lanthanum tricyclopentadienide. J. Inorg. Nucl. Chem. 1965, 27, 733−735. (19) Schlesener, C. J.; Ellis, A. B. Ground-state and excited-state properties of adducts derived from tris(η5‑cyclopentadienyi)ytterbium (III). Organometallics 1983, 2, 529−534. (20) Brittain, H. G.; Meadows, J. H.; Evans, W. J. Luminescence of organometallic lanthanide compounds. Tetrahydrofuran adducts of tricyclopentadienylterbium(III) and tris(methylcyclopentadienyl)terbium(III). Organometallics 1983, 2, 1661−1665. (21) Liu, G.; Beitz, J. V.; Kanellakopulos, B. Laser-induced fluorescence studies of terbium triscyclopentadienide. J. Alloys Compd. 1992, 180, 157−163. (22) Yang, L.; Ye, J.; Xu, L.; Yang, X.; Gong, W.; Lin, Y.; Ning, G. L. Synthesis and properties of aggregation-induced emission enhancement compounds derived from triarylcyclopentadiene. RSC Adv. 2012, 2, 11529−11535. (23) Ye, J.; Deng, D.; Gao, Y.; Wang, X.; Yang, L.; Lin, Y.; Ning, G. Synthesis, molecular structure and photoluminescence properties of 1,2-diphenyl-4-(3-methoxyphenyl)-1,3-cyclopentadiene. Spectrochim. Acta, Part A 2015, 134, 22−27.

(24) Zhang, X.; Ye, J.; Xu, L.; Yang, L.; Deng, D.; Ning, G. Synthesis, crystal structures and aggregation-induced emission enhancement of aryl-substituted cyclopentadiene derivatives. J. Lumin. 2013, 139, 28− 34. (25) Roitershtein, D. M.; Minyaev, M. E.; Mikhailyuk, A. A.; Lyssenko, K. A.; Belyakov, P. A.; Antipin, M. Y. Lutetium complexes with the 1,3-diphenylcyclopentadienyl ligand. Syntheses and molecular structures of the complexes {(Ph2C5H3)Lu(C2Ph4)(THF)} and {(Ph2C5H3)LuCl2(THF)3}. Russ. Chem. Bull. 2007, 56, 1978−1985 and references therein. (26) Hirsch, S. S.; Bailey, W. J. Base-catalyzed alkylation of cyclopentadiene rings with alcohols and amines. J. Org. Chem. 1978, 43, 4090−4094. (27) Castellani, M. P.; Wright, J. M.; Geib, S. J.; Rheingold, A. L.; Trogler, W. C. Synthesis, molecular structure, and 2-D NMR analysis of bis(tetraphenylcyclopentadienyl)iron(II). Organometallics 1986, 5, 1116−1122. (28) Bonnet, F.; Visseaux, M.; Barbier-Baudry, D. New divalent samarocenes for butadiene polymerisation: influence of the steric effect and the electron density on the catalytic activity. J. Organomet. Chem. 2004, 689, 264−269. (29) Barbier-Baudry, D.; Bouyer, F.; Madureira Bruno, A. S.; Visseaux, M. Lanthanide borohydrido complexes for MMA polymerization: syndio- vs iso-stereocontrol. Appl. Organomet. Chem. 2006, 20, 24−31. (30) Minyaev, M. E.; Vinogradov, A. A.; Roitershtein, D. M.; Borisov, R. S.; Ananyev, I. V.; Churakov, A. V.; Nifant’ev, I. E. Catalytic activity of phenyl substituted cyclopentadienyl neodymium complexes in the ethylene oligomerization process. J. Organomet. Chem. 2016, 818, 128−136. (31) Field, L. D.; Lindall, C. M.; Masters, A. F.; Clentsmith, G. K. B. Penta-arylcyclopentadienyl complexes. Coord. Chem. Rev. 2011, 255, 1733−1790. (32) Deacon, G. B.; Jaroschik, F.; Junk, P. C.; Kelly, R. P. Bulky Group 2 Octaphenylmetallocenes and Direct Access to Calcium and Ytterbium Pseudo-Grignard Complexes. Organometallics 2015, 34, 2369−2377. (33) Evans, W. J. Tutorial on the Role of Cyclopentadienyl Ligands in the Discovery of Molecular Complexes of the Rare-Earth and Actinide Metals in New Oxidation States. Organometallics 2016, 35, 3088−3100. (34) Fieser, M. E.; Ferrier, M. G.; Su, J.; Batista, E.; Cary, S. K.; Engle, J. W.; Evans, W. J.; Lezama Pacheco, J. S. L.; Kozimor, S. A.; Olson, A. C.; Ryan, A. J.; Stein, B. W.; Wagner, G. L.; Woen, D. H.; Vitova, T.; Yang, P. Evaluating the electronic structure of formal LnII ions in LnII(C5H4SiMe3)31‑ using XANES spectroscopy and DFT calculations. Chem. Sci. 2017, 8, 6076−6091. (35) Evans, W. J. The organometallic chemistry of the lanthanide elements in low oxidation states. Polyhedron 1987, 6, 803−805. (36) Evans, W. J. Perspectives in reductive lanthanide chemistry. Coord. Chem. Rev. 2000, 206−207, 263−283. (37) Edelmann, F. T. Complexes of group 3 and lanthanide elements. In Comprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: New York, 2007; Vol. 4, pp 1− 190. (38) Anwander, R. In Lanthanides: Chemistry and Use in Organic Synthesis. Topics in Organometallic Chemistry; Kobayashi, S., Eds.; Springer: Berlin, 1999; Vol. 2, pp 1−61. (39) Evans, W. J. The Importance of Questioning Scientific Assumptions: Some Lessons from f-Element Chemistry. Inorg. Chem. 2007, 46, 3435−3439. (40) Bernard-Henriet, C.; Chanon, M. The Diels-Alder additions of phenylcyclopentadienes: Tests for the PMO theory and the diradicaloid model. Tetrahedron Lett. 1996, 37, 2417−2420. (41) Singh, P.; Rausch, M. D.; Bitterwolf, T. E. Formation and synthetic utility of benzyl-cyclopentadienylthallium and phenylcyclopentadienylthallium. J. Organomet. Chem. 1988, 352, 273−282. (42) Edelmann, F. T.; Poremba, P. In Synthetic Methods of Organometallic and Inorganic Chemistry (Herrman/Brauer); N


Article

Inorganic Chemistry Edelmann, F. T., Herrmann, W. A., Eds.; Verlag: Stuttgart, Germany, 1997; Vol. 6, pp 34−35. (43) Bruker. APEXII; Bruker AXS Inc.: Madison, Wisconsin, USA, 2008. (44) Sheldrick, G. M. Crystal structure refinement with SHELXL. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8. (45) Spek, A. L. PLATON SQUEEZE: a tool for the calculation of the disordered solvent contribution to the calculated structure factors. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 9−18. (46) Wrighton, M. S.; Ginley, D. S.; Morse, D. L. Technique for the determination of absolute emission quantum yields of powdered samples. J. Phys. Chem. 1974, 78, 2229−2233. (47) de Mello, J. C.; Wittmann, H. F.; Friend, R. H. An improved experimental determination of external photoluminescence quantum efficiency. Adv. Mater. 1997, 9, 230−232. (48) Greenham, N. C.; Samuel, I. D. W.; Hayes, G. R.; Phillips, R. T.; Kessener, Y. A. R. R.; Moratti, S. C.; Holmes, A. B.; Friend, R. H. Measurement of absolute photoluminescence quantum efficiencies in conjugated polymers. Chem. Phys. Lett. 1995, 241, 89−96. (49) Dolg, M.; Stoll, H.; Savin, A.; Preuss, H. Energy-adjusted pseudopotentials for the rare earth elements. Theor. Chim. Acta 1989, 75, 173−194. (50) Dolg, M.; Stoll, H.; Preuss, H. A combination of quasirelativistic pseudopotential and ligand field calculations for lanthanoid compounds. Theor. Chim. Acta 1993, 85, 441−450. (51) Frisch, M.; Trucks, G.; Schlegel, H.; Scuseria, G.; Robb, M.; Cheeseman, J.; Scalmani, G.; Barone, V.; Petersson, G.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A.; Bloino, J.; Janesko, B.; Gomperts, R.; Mennucci, B.; Hratchian, H.; Ortiz, J.; Izmaylov, A.; Sonnenberg, J.; Williams-Young, D.; Ding, F.; Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson, T.; Ranasinghe, D.; Zakrzewski, V.; Gao, J.; Rega, N.; Zheng, G.; Liang, W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Throssell, K.; Montgomery, J.; Peralta, J.; Ogliaro, F.; Bearpark, M.; Heyd, J.; Brothers, E.; Kudin, K.; Staroverov, V.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J.; Iyengar, S.; Tomasi, J.; Cossi, M.; Millam, J.; Klene, M.; Adamo, C.; Cammi, R.; Ochterski, J.; Martin, R.; Morokuma, K.; Farkas, O.; Foresman, J.; Fox, D. Gaussian 09; Gaussian, Inc., Wallingford CT, 2016. (52) Roitershtein, D. M.; Minyaev, M. E.; Mikhaylyuk, A. A.; Lyssenko, K. A.; Glukhov, I. V.; Belyakov, P. A. Polyphenylcyclopentadienyl complexes of rare earth elements. Russ. Chem. Bull. 2012, 61, 1726−1732. (53) Sizov, A. I.; Zvukova, T. M.; Starikova, Z. A.; Bulychev, B. M. Molecular and crystal structures of adducts (Cp2YCl)2L (L = 2THF, DME, 1,4-dioxane). Moscow Univ. Chem. Bull. (Engl. Transl.) 2007, 62, 109−111. (54) Jin, Z.; Liu, Y.; Cheng, W. Synthesis and crystal structure of [(η -C5H5)2NdCl*OC4H8]2. Sci.Sin.,Ser.B(Engl.Ed.) 1987, 30, 1136− 1143. (55) Sulway, S. A.; Layfield, R. A.; Tuna, F.; Wernsdorfer, W.; Winpenny, R. E. P. Single-molecule magnetism in cyclopentadienyldysprosium chlorides. Chem. Commun. 2012, 48, 1508−1510. (56) Jin, J.-Z.; Jin, Z.-S.; Chen, W.-Q. Crystal Structure of [Cp2ErCl(THF)]2. Jiegou Huaxue (Chin.) (Chin. J. Struct. Chem.) 1992, 11, 204−207. (57) Puntus, L.; Lyssenko, K.; Pekareva, I.; Antipin, M. Yu. Characterisation of geometric isomers of europium chlorides with 2,2 ′-bipyridine based on X-ray diffraction, luminescence and quantum chemical data. Mol. Phys. 2010, 108, 557−572. (58) Zolin, V. F.; Puntus, L. N.; Tsaryuk, V. I.; Kudryashova, V. A.; Legendziewicz, J.; Gawryszewska, P.; Szostak, R. Spectroscopy of europium and terbium pyridine-carboxylates. J. Alloys Compd. 2004, 380, 279−284. (59) Puntus, L. N.; Zolin, V. F.; Babushkina, T. A.; Kutuza, I. B. Luminescence properties of isomeric and tautomeric lanthanide pyridinedicarboxylates. J. Alloys Compd. 2004, 380, 310−314.

(60) Puntus, L. N.; Chauvin, A.-S.; Varbanov, S.; Bünzli, J.-C. G. Lanthanide complexes with a calix[8]arene bearing phosphinoyl pendant arms. Eur. J. Inorg. Chem. 2007, 2007, 2315−2326. (61) Puntus, L.; Lyssenko, K.; Antipin, M.; Bünzli, J.-C. G. Role of Inner- and Outer-Sphere Bonding in the Sensitization of Eu(III)Luminescence Deciphered by Combined Analysis of Experimental Electron Density Distribution Function and Photophysical Data. Inorg. Chem. 2008, 47, 11095−11107. (62) Puntus, L.; Lyssenko, K.; Pekareva, I. S.; Bünzli, J.-C. G. Intermolecular Interactions as Actors in Energy-Transfer Processes in Lanthanide Complexes with 2,2′-Bipyridine. J. Phys. Chem. B 2009, 113, 9265−9277. (63) Nelyubina, Y. V.; Puntus, L. N.; Lyssenko, K. A. The Dark Side of Hydrogen Bonds in the Design of Optical Materials: A ChargeDensity Perspective. Chem. - Eur. J. 2014, 20, 2860−2865. (64) Beltrán-Leiva, M. J.; Páez-Hernández, D.; Arratia-Pérez, R. Theoretical Determination of Energy Transfer Processes and Influence of Symmetry in Lanthanide(III) Complexes: Methodological Considerations. Inorg. Chem. 2018, 57, 5120−5132. (65) Beltrán-Leiva, M. J.; Cantero-López, P.; Zúñiga, C.; BulhõesFigueira, A.; Páez-Hernández, D.; Arratia-Pérez, R. Theoretical Method for an Accurate Elucidation of Energy Transfer Pathways in Europium(III) Complexes with Dipyridophenazine (dppz) Ligand: One More Step in the Study of the Molecular Antenna Effect. Inorg. Chem. 2017, 56, 9200−9208. (66) Eliseeva, S. V.; Kotova, O. V.; Gumy, F.; Semenov, S. N.; Kessler, V. G.; Lepnev, L. S.; Bünzli, J.-C. G.; Kuzmina, N. P. Role of the ancillary ligand N,N-dimethylaminoethanol in the sensitization of Eu-III and Tb-III luminescence in dimeric beta-diketonates. J. Phys. Chem. A 2008, 112, 3614−3626. (67) Ramya, A. R.; Reddy, M. L. P.; Cowley, A. H.; Vasudevan, V. Synthesis, Crystal Structure, and Photoluminescence of Homodinuclear Lanthanide 4-(Dibenzylamino)benzoate Complexes. Inorg. Chem. 2010, 49, 2407−2415. (68) Eliseeva, S. V.; Pleshkov, D. N.; Lyssenko, K. A.; Lepnev, L. S.; Bü nzli, J.-C. G.; Kuzmina, N. P. Highly Luminescent and Triboluminescent Coordination Polymers Assembled from Lanthanide beta-Diketonates and Aromatic Bidentate O-Donor Ligands. Inorg. Chem. 2010, 49, 9300−9311.

O


Polyphenylcyclopentadienyl Ligands as an Effective Light-Harvesting

Recommend Documents