Synthesis , structure and luminescent properties of lanthanide fl uoroalkoxides †

Alkoxides [Ln(OR)3(DME)]2 (R = CH(CF3)2, Ln = Sm (1), Yb (2)), [Ce(OR)3(Phen)]2 (3) (Phen = 1,10-phenanthroline), [Ce(OR’)3(DME)2]2 (R’ = C(CF3)3) (4), {Gd(OR’)3(DME)2} (5), {Ln2[O(CF3)2C–C(CF3)2O]3} (Ln = Ce (6), Gd (7)), {Ce2[O(CF3)2C–C(CF3)2O]3(Phen)2} (8), and {Ce[O(CF3)2C–C(CF3)2O][O(CF3)2–C(CF3)2OH] (Phen)2} (9) were synthesized by the reactions of silylamides Ln[N(SiMe3)2]3 with respective fluorinated alcohols. The heterovalent trinuclear complex {Sm2(μ2-OR)3(μ3-OR)2Sm(OR)2(THF)2.5(Et2O)0.5} (10) was obtained by treatment of SmI2(THF)2 with ROK. The reaction of europium(II) and yttrium(III) silylamides with ROH afforded the heterobimetallic alkoxide {Eu2(μ2-OR)3(μ3-OR)2Y(OR)2(DME)2} (11) containing divalent europium. The molecular structures of 1, 2, 3, 9, 10 and 11 were determined by X-ray analysis. All the prepared cerium derivatives as well as the europium–yttrium isopropoxide upon UV excitation exhibited photoluminescence in the regions of 370–425 (for Ce) and 485 nm (for Eu) which was assigned to 4d→5f transitions.


Introduction
Luminescent materials attract great attention because of their wide application in light sources and color displays, such as cell phones, computer and TV screens. In particular, organolanthanide emitters have inspired vigorous research activities owing to their long luminescence lifetimes and narrow characteristic emission bands originated from f-f transitions, which cover the entire visible and near-infrared region. 1 Besides, organic derivatives of cerium(III), gadolinium(III), europium(II), and ytterbium(II) can exhibit metal-centered emission in the UV-blue area and are attractive due to their potential application in the design of the excitation sources for chemical sensing devices, lithography, optical data recording and biomedicine. 2 The short-wave metal-centered luminescence of Ce 3+ , Eu 2+ and Yb 2+ occurs due to f-d transitions, which are parity-allowed, so the complexes of these metals have higher light outputs compared to f-f emitters. 3 The luminescence of f-d transitions can be observed as well for the Sm 2+ and Tm 2+ ions but in this case, the emission wavelength is above 650 nm. 4 Despite the attraction of these phosphors from the academic point of view and plausible application, the publications devoted to organolanthanide UV-blue emitters are scarce. Among the organocerium derivatives the UV luminescence was detected for the complexes of CeCl 3 with crown ethers 5 and bipyridine, 6 and for the pyrazinecarboxylate 7 and alkylamine trifluoromethanesulfonate. 8 The only known organogadolinium UV emitter is the diethylenetriaminepentaacetate, which revealed f-f photoluminescence (PL) at 312 nm. 9 Shortwavelength emission of Eu 2+ was found for the europium(II) chlorides and bromides with crown-ether, azacrown-ether and cryptand ligands. 10 The sole organoderivative of divalent lanthanide for which the electroluminescence (EL) was documented is europium(II) bis[tris(dimethylpyrazolyl)borate]. 11 The EL spectrum of this complex contains a weak band at 430 nm, which was assigned to f-d transition.
Recently we have reported the preparation, structures and luminescent properties of fluorinated isopropoxides of Ce(III), Eu(II)(III), Gd(III), Tm(III) and Yb(II)(III). 12 The PL (λ em = 330 nm) was found only for europium complex Eu 3 (OR F ) 7 (DME) 2 . In continuation of these studies and in search of new UV-blue phosphors, we have synthesized a set of new fluorinated isopropoxides, tert-butoxides and 2,3-butanediolates of Ce(III), Sm(II)(III), Eu(II), Gd(III) and Yb(III). Fluorinated alkoxide ligands were chosen because: (i) eliminating of C-H groups (which are well known luminescence quenchers) facilitates emitting efficiency; 13 (ii) fluorine substituents improve the hydrolytic and thermal stability, and enhance the volatility of the compounds 14 which are important in the preparation of OLED devices by the vacuum deposition method; (iii) the polyfluorinated ligands can provide high-energy metal-ligand charge transfer state (MLCT) and short-wavelength emission. 12

Experimental section
All experiments were performed in evacuated tubes using standard Schlenk techniques, thus excluding traces of air and water. The solvents were purified by distillation from sodium/ benzophenone ketyl (THF, DME, diethyl ether) and sodium (hexane, toluene). MeCN for electron spectroscopy, ROH, R′OH, HO(CF 3 ) 2 C-C(CF 3 ) 2 OH, 1,10-phenanthroline, and Ln[N(SiMe 3 ) 2 ] 3 (Ln = Ce, Gd, Sm, Yb, Dy, Y), were purchased from commercial suppliers. Iodide SmI 2 (THF) 2 and silylamide complex Eu[N(SiMe 3 ) 2 ] 2 (DME) 2 were prepared according to the published procedures. 15,16 IR spectra were recorded on a Specord M-75 instrument in the region of 4000-450 cm −1 . The C, H, N elemental analyses were performed by using the Vario El cube CHNS elemental analyzer (Nizhny Novgorod State University). Yttrium and lanthanide contents were analysed by complexometric titration. Magnetic susceptibility measurements were carried out according to the procedure. 17 Absorption spectra were recorded on a UV/VIS instrument "Perkin-Elmer Lambda-25" from 200 to 800 nm. Emission spectra were registered from 220 to 700 nm on a fluorescent spectrometer "Perkin Elmer LS-55". Registration of absorption and emission spectra were performed in a 1 cm fluorescent quartz cuvette.

(Phen)] 2 (3)
A solution of ROH (0.209 g, 1.24 mmol) and 1,10-phenanthroline (0.071 g, 0.39 mmol) in 10 ml of PhCH 3 was added to a solution of Ce[N(SiMe 3 ) 2 ] 3 (0.238 g, 0.38 mmol) in 10 ml of PhCH 3 . The reaction mixture was stirred at 70°C for 4 h. The precipitated yellow crystals were filtered off and washed with cold PhCH 3 (2 × 10 ml). Drying of the crystals gave 0.219 g (70%) of complex 3. Mp: 147-148°C (dec.      The resulting pale-brown solid was washed with hexane (3 × 15 ml) and dried. Recrystallization of the residue from the mixture DME-PhCH 3 gave orange crystals of complex 9 (0.022 g, 5%) which were isolated via decantation of the supernatant solution. After evaporation of the solvent from the mother liquor in vacuo and washing of the residue with hexane (2 × 10 ml), alkoxide 8 was obtained as a pale-brown solid. Yield of the product was 0.266 g (83%). Mp: 135°C (dec.). Anal. Calcd (%) for C 42 H 16 A solution of Ce[N(SiMe 3 ) 2 ] 3 (0.100 g, 0.16 mmol) and 1,10phenanthroline (0.058 g, 0.32 mmol) in 10 ml of THF was added to a solution of HO(CF 3 ) 2 C-C(CF 3 ) 2 OH (0.108 g, 0.32 mmol) in 5 ml of THF. The reaction mixture was stirred for 3 h. The solvent was removed by condensation in vacuo. The resulting pale-brown solid was washed with hexane (3 × 15 ml) and dried. Recrystallization of the product from the mixture DME-PhCH 3 gave complex 9 as orange crystals  A solution of SmI 2 (THF) 2 (0.351 g, 0.64 mmol) in 10 ml of THF was added to a solution of ROK ( prepared from 0.050 g (1.25 mmol) of KH and 0.236 g (1.40 mmol) of ROH according to the known procedure) in 10 ml of THF. The reaction mixture was stirred for 5 h. The precipitate of KI was filtered off and the solvent from the resulting solution was removed by condensation in vacuo. The residue was extracted with diethyl ether (4 × 10 ml). After concentrating and cooling of the extract, dark red crystals of 10 (0.155 g, 40%) were obtained. μ eff = 2.91μ B . Anal. calcd (%) for C 33 A solution of ROH (0.158 g, 0.94 mmol) in 5 ml of Et 2 O was added slowly to a solution of Y[N(SiMe 3 ) 2 ] 3 (0.072 g, 0.13 mmol) and Eu[N(SiMe 3 ) 2 ] 2 (DME) 2 (0.157 g, 0.24 mmol) in 10 ml of Et 2 O. The reaction mixture was stirred for 3 h. The product was obtained as pale greenish-yellow crystals after concentrating and cooling of the resulting solution (0.115 g, 55%). Mp: 208-210°C (dec.). Anal. calcd (%) for C 29

Device fabrication
The three-layer device of structure ITO/TPD (30 nm)/complex 3 (50 nm)/BATH (20 nm)/Yb, consisting of triphenyldiamine derivative (TPD) as a hole transport layer, 4,7-diphenyl-1,10-phenanthroline (BATH) as an electron-transporting and hole-blocking layer and the lanthanide complex as an emission layer, was fabricated in a vacuum chamber (10 −6 mbar) with different resistive heaters for organic and metal layers. A commercial ITO on a glass substrate with 5 Ω sq. −1 was used as the anode material (Luminescence Technology Corp.) and commercial Yb, 99.9% trace metal basis (Sigma-Aldrich) as the cathode material. The deposition rate for the organic compounds and metallocomplex was 1 nm s −1 . The active area of the device was 5 × 5 mm. The EL spectra and current-voltage characteristics were measured using an Ocean Optics USB-2000 fluorimeter, the computer controlled GW Instek PPE 3323 power supply and a GW Instek GDM 8246 digital multimeter under ambient conditions.

Synthesis
Isopropoxides 1 and 2 were prepared by reactions of respective silylamides Ln[N(SiMe 3 ) 2 ] 3 (Ln = Sm, Yb) and hexafluoroisopropanol in DME solution. To synthesize cerium isopropoxide 3 in the reaction mixture phenanthroline was added. Note that complexes 1 and 2 have been prepared earlier, 18 but their structures and luminescent properties have not been studied.
The products were isolated from the ether-toluene mixture as microcrystalline powders soluble in THF, diethyl ether, and DME. Unlike hydrolyzed and nonsublimable 1 and 2, cerium isopropoxide 3, due to the presence of the shielding phenanthroline at the metal center, is relatively stable in air and can be sublimed in vacuo without decomposition which made it possible to design the OLED device on its base and study the EL properties (vide infra).
X-ray analysis revealed that complexes 1 and 2 are centrosymmetrical, and isostructural dimeric compounds (Fig. 1) and have the arrangement quite analogous to that of the Ce and Tm isopropoxides reported earlier. 12 In alkoxides 1 and 2 two lanthanide ions are linked via two bridging μ 2 -OR groups, each ion is bonded with two terminal isopropoxide groups and one DME molecule. The terminal Ln-O distances (2.130 (5) It is interesting to note that there are close intramolecular contacts Sm(1)⋯F(12A) (Sm(1A)⋯F(12)) in 1 whereas the analogous interactions in 2 are absent. Really, the Yb(1)⋯F(12A) distance in 2 is 3.056(4) Å and notably exceeds the analogous one in 1 (2.852(4) Å). According to the literature data, [19][20][21][22] typical interval values for intramolecular Yb⋯F and Sm⋯F interactions are 2.48(1)-2.726(9) Å and 2.537(2)-2.813(3) Å respectively. Besides, the intramolecular Sm(1A)⋯F(12) interaction leads to elongation of the C(6A)-F(12A) (1.364(7) Å) bond length compared to other distances in this CF 3 group (1.317(8)-1.329(8) Å). In order to understand why the intramolecular Yb(1A)⋯F (12) interactions are absent in 2, we have analyzed the saturation of the metal coordination sphere (G-parameter) 23 in these complexes. According to our calculations the saturation of the metal coordination sphere by ligands in 2 is 97.3 (2)% that markedly exceeds the analogous one in 1 (94.9 (2)%). Thus, there is insufficient room around the metal in 2 to realize an additional Yb⋯F interaction. In other words, steric factors inhibit realization of the intramolecular Yb⋯F contact in 2.
According to X-ray data complex 3 has a centrosymmetrical, dimeric structure (Fig. 2a) which is close to that of 1 and 2.
The main difference of complex 3 from 1 and 2 lies in coordination phenanthroline molecules instead of the DME ones. Cerium ions are linked by two μ 2 -bridged OR ligands. Each metal ion is bound to two terminal isopropoxide groups and one phenanthroline molecule. The distance Ce⋯Ce(1A) (4.1305(2) Å) is slightly longer than that in the complex Ce 2 (OR) 6 (DME) 2 (4.064 Å). 12 As in 1 the intramolecular Ce⋯F interactions in 3 are realized. The Ce(1)⋯F(16) distance (2.9182(14) Å) exceeds the analogous distances in 1 and lies within the interval of values for published data (2.6248(16)-2.9206(13) Å). 24,25 As one should expect that such an interaction leads to elongation of the C(21)-F(16) bond length (1.362(2) Å) compared to other distances in this CF 3 -group (1.330(3)-1.339(3) Å). The steric saturation of the metal coordination sphere by ligands in 3 is 90.5 (2)% that is less than in 2. Thus, the steric factors do not prevent the realization of the intramolecular Ce⋯F interaction in 3 as it is distinct from 1. Due to the presence of phenanthroline containing extensive π-systems, in a crystal of 3, intermolecular π⋯π interactions are realized which combine the molecules in couples (Fig. 2b). The distances between centers of six-membered rings are 3.665 Å and satisfy the geometric criterion for the existence of π-π interactions (3.8 Å) 26 between phenanthroline molecules in neighboring complexes.
Perfluorinated tert-butoxides 4, 5 and diolates 6, 7, 8 were prepared similarly to 1-3. In the reactions with the diol, the molar ratio 2 : 3 was used. The butoxides contain two molecules of DME whereas diolates have no coordinated solvent at all. LDI-TOF analysis revealed that cerium tert-butoxide is a  Change of the cerium amide : diol ratio to 1 : 2 resulted in an increase of the yield of 9 up to 59%. Probably, the reason for formation of such a product is due to a slight excess of alcohol and phenanthroline in the reaction mixture.
According to X-ray analysis complex 9 (Fig. 3a) has a monomeric structure in which the cerium ion is linked to one butanediolate, one hydroxybutanediolate and two phenanthroline ligands. The cerium-oxygen distances of the butanediolate ligand (2.311(4) and 2.344(4) Å) are significantly longer than the length of the cerium-oxygen bond of the hydroxybutane-diolate group (2.258(5) Å). Such differences in the bond lengths can be caused by steric effects in the coordination sphere of cerium. In complex 9 as for 3 there is an intramolecular Ce(1)⋯F(1) (2.950 (4) Å) interaction which leads to the elongation of the bond C(3)-F(1) (1.353(9) Å) length compared to analogous ones (1.337(9)-1.341(9) Å). The steric saturation of the metal coordination sphere by ligands in 3 is 87.7(2)%. It should be noted that phenanthroline molecules neighboring complexes 9 in the crystal have offset disposition to each other but the distance between the centers of the sixmembered rings of these ligands (3.875 Å) slightly exceeds the geometric criterion for the existence of the π-π interaction (Fig. 3b). 26 The general properties of alkoxides 4-9 are similar to those of isopropoxides 1 and 3: they are slowly hydrolyzed in air, soluble in common organic solvents and do not sublime in vacuo which prevents their application as an emitting material in OLED devices.
As it has been mentioned, f-d emission of lanthanide complexes is of considerable interest because of their wide application potential. In an effort to obtain samarium(II) the f-d emitter, interaction of SmI 2 with potassium isopropoxide KOR was carried out. However, the reaction afforded the complex {Sm 2 (μ 2 -OR) 3 (μ 3 -OR) 2 Sm(OR) 2 (THF) 2.5 (Et 2 O) 0.5 } (Fig. 4) containing one Sm 3+ and two Sm 2+ cations.  Formation of heterovalent product can be stipulated by the reaction of the formed at initial stage samarium(II) isopropoxide with ROH which remains after the synthesis of potassium salt and its excess cannot be removed from the reaction solution without decomposition of this salt.
According to the X-ray analysis, the metal atoms in 10 are linked to each other via μ 2 -and μ 3 -isopropoxide ligands. Two terminal OR groups are bonded to the Sm 3+ ion. The μ 3 -oxygen atoms (O(1) and O(2)) are disposed over and under the plain Sm(1)Sm(2)Sm(3) forming the trigonal-bipyramidal skeleton. Note that there are two independent molecules of 10 in the asymmetric unit cell. The Sm 2+ cations in one of the independent molecules of 10 are coordinated by three THF molecules whereas in other molecules they are coordinated by one Et 2 O and two THF molecules. Additionally, there are four close contacts between samarium(II) ions and fluorine atoms of μ 3 -OR ligands in the complex.
Another potential f-d emitterheterobimetallic alkoxide of divalent europium {Eu 2 (μ 2 -OR) 3 (μ 3 -OR) 2 Y(OR) 2 (DME) 2 } (11) was synthesized by the reaction of Y[N(SiMe 3 ) 2 ] 3 and Eu[N(SiMe 3 ) 2 ] 2 with isopropanol in Et 2 O solution. Yttrium was chosen as a sensitizer of europium emission because it can form high-lying ligand-to-metal charge transfer state (LMCT) from which transfer of absorbing energy to resonance levels of Eu(II) may occur. 12 It should be noted that compound 11 is the first structurally characterized heterobimetallic complex, which contains yttrium and europium ions (Fig. 5).
In spite of the fact that complexes 10 and 11 were prepared by different reactions, the molecular structures of these compounds are very similar. Moreover, arrangement of the prepared earlier complexes Eu 3 (OR) 7 (DME) 2 and Yb 3 (OR) 7 (THF)(Et 2 O) 12 appeared to be analogous to that of complexes 10 and 11. In both complexes one trivalent and two divalent metal cations are linked to each other via three μ 2 -and two μ 3 -isopropoxide ligands. Two terminal OR groups are bonded to Sm 3+ (in 10) and Y 3+ (in 11) ions. Two coordination sites around each of divalent ions are occupied by Et 2 O and THF molecules in samarium isopropoxide and by DME in europium-yttrium complex. The  It is interesting to note that among the prepared alkoxides 1, 3, and 9-11 where intramolecular Ln⋯F interactions are observed, the steric saturation of the metal coordination sphere varies in the range of 87.7(2)-94.9(2)%. In turn, in complex 2 where such interactions are absent, the G-parameter (97.3 (2)%) significantly exceeds the analogous values for the complexes with close Ln-F contacts.

Luminescent properties
Among the prepared complexes, PL was observed for the europium-yttrium isopropoxide and all the cerium derivatives. Acetonitrile solutions of cerium compounds 4 and 6 showed emission in the region of 370-400 nm (Fig. 6). These bands exhibit multimodal character and each of them can be decomposed into two Gaussian peaks with maxima at 343, 373 and 392, 425 nm respectively. The energy difference between these peaks is close to 2000 cm −1 , in good agreement with the characteristic splitting of the two Ce 3+ 4f ground levels induced by the spin-orbit interaction. 28 Therefore, the PL of 4 and 6 is attributed to the electric-dipole 4d→5f transitions in the cerium ion from the lowest excited state 2 D 3/2 to the ground states 2 F 5/2 and 2 F 7/2 .
Besides, the f-d emission of the Ce 3+ ion was observed for complexes 3, 8 and 9 (Fig. 7). Comparison of the PL spectra of 6 and 8 shows that insertion of the phenanthroline ligand in the cerium butanediolate caused slight blue shifting of the emission maximum from 410 to 405 nm and decreasing PL intensity.
Heterobimetallic complex 11 revealed blue PL with a maximum at 485 nm (Fig. 8). As the compound does not contain chromophore ligands and the band of emission lies in the range of Eu 2+ luminescence, 10 so the observed peak was attributed to 4f 6 5d 1 →4f 7 transition in the europium(II) ion.
Interestingly, complexes 3 and 11 did not reveal luminescence in MeCN solution in contrast to THF medium probably because of differences in the symmetry of the crystal field at cerium atoms in these solvents.
As noted above, due to low volatility and thermal stability of the majority of the prepared complexes, the only OLED device based on isopropoxide 3 was fabricated. A simple three-layered device of structure ITO/TPD/complex 3/BATH/Yb was prepared by the vacuum evaporation method. The diode displayed weak orange luminescence, the spectrum of which contained a single broad band with the maximum at 620 nm (Fig. 9). The EL efficiency did not exceed 2 cd m −2 at 30 V.   The observed EL can be ascribed to the emission of the electroplex formed at the TPD/3 interface. Confirmation of the supposition is the absence of PL of the double layer and blend TPD-3 samples. Similar EL of electroplex was registered previously for the OLED devices based on the lanthanide pentafluorophenolates. 29 Conclusions A set of Ce(III), Sm(II)(III), Eu(II), Gd(III) and Yb(III) complexes with fluorinated isopropoxide, tert-butoxide and butanediolate ligands was prepared. X-ray and LDI-TOF analysis revealed that Ln(III) isopropoxides as well as Ce tert-butoxide have binuclear arrangement whereas gadolinium butoxide is mononuclear probably due to a smaller radius of the Gd 3+ ion. Isostructural alkoxides 10 and 11 are trinuclear clusters in which two Ln 2+ cations are bonded via bridging RO ligands to the Sm 3+ cation (in 10) or the Y 3+ cation (in 11). All the prepared cerium complexes upon UV excitation showed short-wavelength emission at the region of 370-425 nm as broadened bands, which is characteristic for 5d→4f transitions in the cerium ion. 30 Heterobimetallic alkoxide 11 revealed blue PL with a maximum at 485 nm which was assigned to f-d emission of the europium(II) ion. Attempts to prepare OLED devices using the synthesized alkoxides as emitter layers for investigation of their EL properties failed because of low thermal stability of the compounds. The only complex, on the basis of which we were able to design a diode, was cerium compound 3 but this device displayed the electroplex luminescence and not the metal-centered emission.