V.N. Semkin

Steric conditions for donor–acceptor interactions in C60 complexes with planar organic donors

Abstract Charge transfer complexes of fullerene C 60 with planar donors of tetrathiafulvalene, dithiadiazafulvalene and pyranylidene family were investigated by IR- and UV-VIS-NIR spectroscopy. The analysis of IR and X-ray data shows that the charge transfer complexes of fullerene with the planar donors are involved in polarization interactions of van der Waals type. Charge transfer is very weak in these compounds and is hindered by unfavourable steric factors. As a result, the CT rate does not correlate with the ionization potential of the donor; the charge transfer absorption bands are very weak too.

Optical study of the electronic structure and electron-molecular vibrational coupling in a novel organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Cl0.5Br0.5

Abstract Reflectance spectra of a new ambient-pressure superconductor, κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl 0.5 Br 0.5 ( T c = 11.3 K), where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene, are measured in the range from 750 to 5500 cm −1 and from 9000 to 40 000 cm −1 for different polarizations. The detailed analysis of the infrared conductivity, obtained after a Kramers-Kronig transformation, is performed in the framework of the approach, based on the ‘phase phonons’ theory of M.J. Rice, with the help of special transformation of the experimental data. Parameters of the electronic subsystem and electron-molecular vibration coupling are determined. The determined ‘total’ dimensionless coupling constant (λ) is compared with those of the organic superconductors κ-(BEDT-TTF) 2 Cu(NCS) 2 ( T c = 10.4 K) and (BEDT-TTF) 4 Hg 2.89 Br 8 ( T c = 4.3 K). It is found that a larger λ produces a larger T c in accordance with the known Bardeen-Cooper-Schriffer formula for T c .

Electronic structure and electron-molecular vibrational coupling in organic superconductors ET4Hg2.89Br8 and ET2Cu(NCS)2: comparative studies by optical spectroscopy

Abstract Reflectance spectra of organic superconductors ET 4 Hg 2.89 Br 8 and ET 2 Cu(NCS) 2 are measured in the range from 650 to 40 000 cm −1 for different polarizations. The reflectance is analysed in the framework of the ‘phase phonons’ theory of M. J. Rice with the help of special transformation of the experimental data. Parameters of the electronic subsystem and of electron-molecular vibration coupling are determined. The role of anion-cation interaction is shown to be important for the onset of superconductivity.

Infrared and visible spectra of the complexes of fullerene-70 and its salt

Abstract Electronic and vibrational spectroscopies were used to investigate the electron processes in neutral C 70 fullerene and its complexes with crown-shaped sulfur and 9,9′-bis- trans -(telluraxanthenyl) donors; spectral properties of the salt [(C 6 H 5 ) 4 P] 2 C 70 Br are discussed also. It is concluded that the complexes are weak, molecular complexes of the van der Waals type. The activation of silent modes of C 70 as well as distinct changes of the donor IR spectra have been discovered in the [(C 6 H 5 ) 4 P] 2 C 70 Br salt.

Electronic and electron-phonon phenomena in low-dimensional BEDT-TTF-based organic conductors and superconductors

Two related groups of k-phase ion-radical salts (BEDT-TTF) with different electrical properties, namely, superconductors with different transition temperatures and conductors, which transfer to insulating state with decreasing temperature, have been studied by micro-optic spectroscopy. Polarized reflectance spectra of microcrystals have been measured for the three principal crystallographic directions within the 700–40000 cm−1 region, and the corresponding spectra of the optical functions obtained. The anisotropy of the electronic system in the crystals has been established as two-dimensional. The spectra obtained were quantitatively analyzed, the key parameters of the electronic structure and the vibronic coupling constants determined. It is concluded that the conductors have smaller vibronic coupling constants, more narrow allowed electronic bands, and stronger electron-electron interaction compared to those of the superconductors, and that vibronic coupling is the necessary condition for the onset of superconductivity in the superconductors studied.

Optical Properties of Deuterated Organic Conductor (BEDT-TTF)2[Hg(SCN)2Br]

Reflectance spectra of 2:1 charge-transfer salt single crystals of deuterated bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) cation with Hg(SCN)2Br are reported for two polarizations and compared with the data for the hydrogenated compound. The Hubbard-type model for two orthogonal dimers in the case of infinite on-site Coulomb repulsion U is developed and emlpoyed for the interpretation of the observed charge-transfer bands and vibrational peaks resulting from electron-molecular vibration coupling.

Comparative spectroscopic study of three ET-based charge-transfer salts

Abstract Polarized reflectance spectra of κ-phase ET 2 [Hg(SCN)Cl 2 ], ET 2 [Hg(SCN) 2 Br], and (D 8 -ET) 2 [Hg(SCN) 2 Br] single crystals are measured in the range from 600 to 40000 cm −1 . The Hubbard model for orthogonal dimers is employed for interpretation of the observed charge-transfer bands and vibrational peaks resulting from electron-molecular vibration coupling.

Effect of deuteration on the electronic and electron-vibrational properties of the organic conductor k-(D8-BEDT-TTF)2[Hg(SCN)2Br]

We present here for the first time polarized reflection spectra and optical conductivity spectra of single crystals of the newly deuterated organic conductor k-(D8-BEDT-TTF)2[Hg(SCN)2Br] at room temperature. The spectral region investigated is 700–40 000 cm−1. We examined the effect of deuteration on electronic and electron-vibrational transitions observed in the spectra. The observed shift of the electron “dimer” transition in the infrared toward lower frequencies upon deuteration is linked with an increase in the interaction between neighboring, mutually perpendicular dimers in the structure of the deuterated crystal. A lowering of the symmetry of the BEDT-TTF molecule is demonstrated in crystals similar to k-(BEDT-TTF)2[Hg(SCN)2Br], relative to the symmetry D2h of the free molecule. We refine the assignment of the spectral features determined by the interaction of electrons with the fully-symmetric intramolecular vibrations of the C=C, C-S, and C-C-H bonds of the BEDT-TTF molecule.