Lanthanide Coordination Polymers with Soft-Base Ditopic Bisthiazolate Ligands.

In order to prepare the first lanthanide coordination polymers (CPs) based on ditopic sulfide ligands, benzo[1,2-d:4,5-d']bisthiazole-2,6(3H,7H)-dithione (H2L) was used as a linker. The reactions of lanthanide silylamides Ln[N(SiMe3)2]3 (Ln = Nd, Gd, Er, and Yb) with H2L result in the formation of soluble dimethyl sulfoxide (DMSO) ionic salts [Ln(DMSO)8][L]1.5 [Ln = Nd (1), Gd (2), Er (3), and Yb (4)]. Due to the lack of coordination of anionic ligands, compounds 1, 3, and 4 do not show sensitized metal-centered photoluminescence (PL), while Gd compound 2 shows weak phosphorescence at 77 K. It was found that the heating of 1 in a 1:9 DMSO/1,4-dioxane mixture leads to the formation of large crystals of 2D CP [Nd(DMSO)3L1.5·0.5diox]n (5), where deprotonated dithione H2L plays the role of a ditopic linker. This linker acts as an "antenna" in compound 5, providing an intense near-infrared (NIR) PL of Nd3+ ion upon near-UV and blue-light excitation. The application of a synthetic protocol similar to that of compounds 2-4 led to the formation of amorphous compounds [Ln(DMSO)3L1.5·0.5diox]n [Ln = Gd (6), Er (7), and Yb (8)], whose PL properties significantly differ from those of the parental ionic salts. In the case of Yb polymer 8, the PL excitation spectra are shifted to the red region due to a low-energy ligand-to-metal charge-transfer state. The synthesized compounds 5-8 are the first examples of lanthanide CPs using soft-base ditopic linkers in their structures.

Luminescence thermochromism in novel mixed Eu(II)-Cu(I) iodide.

A new mixed Eu(II)-Cu(I) iodide [Eu(DME)4][Cu2I4] (1) was synthesized by the reaction of an organosulphide salt of Eu(II) and CuI in DME media. X-ray analysis revealed that 1 is an ate-complex consisting of Eu(DME)4 dications and tetraiododicuprate dianions. Upon UV light excitation (λ = 365 nm), the compound exhibits intense double-peaked photoluminescence (PL) at 445 and 500 nm. The relative intensity of these peaks changes dramatically when the temperature changes in the range of 180-250 K. To understand the nature of the found PL thermochromism, the structure and time-resolved PL of 1 were studied at various temperatures. The time-resolved PL studies of 1 at various temperatures revealed the presence of two luminescent centers which are excited by the capture of an electron from the conduction band. The ratio of intensities at 445 and 500 nm (R = I445/I500) in the PL spectra of 1 changes by almost two orders of magnitude and the relative sensitivity S (S = (∂R/∂T)/R) exceeds 5% per K in the range of 190-245 K that makes this compound a promising luminescent thermometer for the range where ammonia exists in a liquid state.

Impact of n,γ-irradiation on organic complexes of rare earth metals.

The complexes of La, Ce, Nd, Sm, Eu, Tb and Yb with benzoxazolyl-phenolate, benzothiazolyl-phenolate, benzoxazolyl-naphtholate, benzothiazolyl-naphtholate and 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione ligands were treated with n,γ-irradiation upon a sustained (45 h, absorbed dose of 120 krad, flux of neutrons 5·1013 n/cm2) and a pulse mode (3 ms, absorbed dose of 130 krad, flux of neutrons 3.6·1013 n/cm2). It was found that main characteristics of the compounds (shape of substance, color, IR absorption and photoluminescent spectra) have not changed. With an example of cerium complex [Ce(OON)3]2 it was revealed that the molecular structure of compounds after strong pulse irradiation also does not changed. However, computer simulations of neutron exposure on the same complexes showed significant shift of metal atoms and ligands. Possible reasons for the detected discrepancy between experimental and calculated data are discussed.

X-Ray excited luminescence of organo-lanthanide complexes.

A comparative study of the photoluminescence (PL) and radioluminescence (RL) of lanthanide complexes with benzimidazolylphenolate (NON), 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionate (TTA) and 1,3-acetylacetonate (acac) ligands revealed significant differences in the total and relative intensity of emission. The PL spectra contain both bands of metal-centered and ligand centred emission while X-ray excited compounds display only the bands of Ln3+ ions, the relative intensity of which differs from that in UV excited analogues. The RL intensity of all the studied complexes is about 300 times lower than that of PL. Increasing the cathode current and tube voltage enhances the RL efficiency insignificantly. Simultaneous irradiation of the sample with X-ray and UV radiation leads to a corresponding increase in the intensity of emission. A possible mechanism of RL of organo-lanthanide complexes is discussed.

Features of the Molecular Structure and Luminescence of Rare-Earth Metal Complexes with Perfluorinated (Benzothiazolyl)phenolate Ligands.

A set of Sc, Nd, Sm, Eu, Ho, Gd, Er, Yb complexes with perfluorinated 2-(benzothiazol-2-yl)phenolate ligands Ln(SONF)3(DME) were synthesized by the reactions of silylamides Ln[N(SiMe3)2]3 with phenol H(SONF). The structure of the initial phenol, Sc, and Er complexes was established using X-ray analysis, which revealed that the obtained compounds are mononuclear, in contrast to the binuclear non-fluorinated analogues [Ln(SON)3]2 synthesized earlier. All the obtained complexes, both in solid state and in tetrahydrofuran (THF) solutions, upon excitation by light with λex 395 or 405 nm show intense luminance of the ligands at 440-470 nm. The Eu complex also exhibits weak metal-centered emission in the visible region, while the derivatives of Sm luminesces both in the visible and in the infrared region, and Nd, Er, and Yb complexes emit in the near IR (NIR) region of high intensity. DFT (density functional theory) calculation revealed that energy of frontier orbitals of the fluorinated complexes is lower than that of the non-fluorinated counterparts. The level of highest occupied molecular orbital (HOMO) decreases to a greater extent than the lowest occupied molecular orbital (LUMO) level.

Synthesis, structure and long-lived NIR luminescence of lanthanide ate complexes with perfluorinated 2-mercaptobenzothiazole.

To obtain new efficient lanthanide-based NIR luminophores perfluorinated 2-mercaptobenzothiazole was used as a ligand. The ate-complexes [(Ln(mbtF)4)-(Na(DME)3)+] of Nd (1), Sm (2), Tb (3), Er (4), Yb (5) and [(Y(mbtF)4)-(Li(DME)3)+] (6) were synthesized in high yields by the reactions of the respective silylamide compounds Ln[N(SiMe3)2]3 and M[N(SiMe3)2] (M = Li, Na) with 4,5,6,7-tetrafluoro-1,3-benzothiazol-2(3H)-thione (HmbtF) in DME media. The complexes 1-3 and 6 were structurally characterized by X-ray diffraction analysis. It has been shown that the mbtF ligands sensitize the luminescence of Nd, Sm, Tb, Er and Yb ions upon mild UV or blue light excitation. The NIR luminescence of crystalline compounds 1, 2, 4 and 6 has been studied by time-resolved techniques. As expected, the compounds exhibit prolonged NIR luminescence due to the removal of C-H groups from lanthanide centers and the absence of C-O bonds in the coordination sphere of the lanthanides. The synthesized compounds are promising materials for NIR laser applications.

Cerium(iii) complexes with azolyl-substituted thiophenolate ligands: synthesis, structure and red luminescence.

In order to obtain molecular Ce(iii) complexes which emit red light by f-d transitions the azolyl-substituted thiophenolates were used as the ligands. The thiophenolate Ce(iii) complexes were synthesized by the reaction of Ce[N(SiMe3)2]3 with respective thiophenols 2-(2'-mercaptophenyl)benzimidazole (H(NSN)), 2-(2'-mercaptophenyl)benzoxazole (H(OSN)) and 2-(2'-mercaptophenyl)benzothiazole (H(SSN)) in DME media. The structures of the benzimidazolate (Ce(NSN)3(DME)) and benzothiazolate (Ce(SSN)3(DME)) derivatives were determined by X-ray analysis which revealed that the cerium ion in the molecules is coordinated by one DME and three anionic thiophenolate ligands. The lanthanum complex La(OSN)3(DME) has been synthesized similarly and structurally characterized. It was found that the solids of Ce(SSN)3(DME) and Ce(OSN)3(DME) exhibit a broad band photoluminescence peaking at 620 nm which disappears upon solvatation. With an example of OSN derivatives it was proposed that this behaviour is caused by the blue shift of the f-d transition of Ce3+ ions.

LMCT facilitated room temperature phosphorescence and energy transfer in substituted thiophenolates of Gd and Yb.

To obtain luminescent lanthanide complexes with a low energy LMCT state the 2-(2'-mercaptophenyl)benzothiazolates, Ln(SSN)3, and 2-(2'-mercaptophenyl)benzoxazolates, Ln2(OSN)6 (Ln = Gd, Yb), were synthesized by the reaction of amides Ln[N(SiMe3)2]3 with respective thiophenols. Ytterbium complexes were structurally characterized by X-ray diffraction analysis. Cyclic voltammetry revealed that the deprotonated mercaptophenyl ligands have significantly lower oxidation potentials than their phenoxy analogues and some ß-diketones. The photophysical properties of Gd and Yb compounds were studied both in solution and in the solid state. The fluorescence spectra of the compounds in solution display the bands of the keto and enol forms of the ligands. No energy transfer from the organic part to Yb3+ has been detected in solutions of both Yb complexes, whereas in solids an intense metal-centered emission in the near infrared region was observed. The solid Gd compounds exhibited room temperature phosphorescence caused by unusually efficient intersystem crossing facilitated by the essentially reducing properties of OSN and SSN ligands. To explain the sensitization process occurring in solids Yb2(OSN)6 and Yb(SSN)3 a specific non-resonant energy transfer mechanism via a ligand to metal charge transfer state has been proposed. Based on the Yb derivatives, NIR-emitting OLEDs with 860 µW cm-2 maximal irradiance were obtained. Their Gd counterparts showed bright electrophosphorescence (up to 1350 cd m-2) in the devices containing doped emission layers.

Luminescent properties of 2-mercaptobenzothiazolates of trivalent lanthanides.

A series of lanthanide complexes (Ln = Nd, Sm, Eu, Gd and Yb) with anionic 2-mercaptobenothiazolate (mbt) ligands were synthesized. Depending on the solvents chosen for the synthesis, Ln(mbt)3(THF)2 and Ln(mbt)3(Et2O) complexes were precipitated from THF and Et2O solutions respectively. The structure of Yb(mbt)3(Et2O) was determined by X-ray analysis. Photophysical properties of the complexes were studied. It was found that under photoexcitation Nd and Yb derivatives exhibit bright metal-centered luminescence in the NIR region while Sm(mbt)3(THF)2 demonstrates intensive visible emission corresponding to (4)G5/2 → (6)HJ (J = 5/2, 7/2, 9/2, 11/2) f-f transitions of Sm(3+) along with NIR emission of moderate intensity. In the case of europium compounds as well as Sm(mbt)3(Et2O) no luminescence was detected. It is assumed that the difference in photoluminescence of Yb and Eu complexes can be explained by an intramolecular electron transfer process, which efficiently proceeds in these compounds.