Vibrational behavior of molecular constituents of organic superconductors: TMTSF, its radical cation and the sulphur analogs TMTTF and TMTTF+

Abstract

The results of an extensive vibrational analysis of tetramethyltetrathiafulvalene (TMTTF) and tetramethyltetraselenafulvalene (TMTSF) and of their radical cations are presented. The polarized infrared absorption spectra of oriented crystalline samples of neutral TMTTF and TMTSF (4000–80 cm−1) are reported and compared with powder and solution spectra. The polarization data are used as a basis for the symmetry assignment of the infrared active fundamental modes. Powder and solution Raman spectra are presented and discussed considering the values of the depolarization ratios measured for some bands. Raman and infrared spectra of powder and solution samples of the (1:1) bromide and perchlorate salts of TMTTF and TMTSF are reported. Vibronic infrared absorptions originated by the coupling of the unpaired electrons to totally symmetric modes are identified and compared with those previously reported for unsubstituted tetrathiafulvalene (TTF) radical systems. The assignment of the normal modes of the radical cations is based on the measured Raman depolarization ratios and on the correlation with the neutral molecules and with neutral and ionized TTF. The results of a normal coordinate calculation using a modified valence force field for TMTTF, TMTSF, and their radical cations are presented and the calculated shape of the normal modes of the neutral molecules are reported. The frequency changes following ionization observed for the present case of TMTTF and TMTSF and previously reported for TTF are compared and discussed

Charge-ordering transition in (TMTTF)2X explored via dilatometry

“…They found that the intermolecular a g (ν 3 ) mode, which becomes infra-red active through electron-molecular-vibration coupling, splits below T CO . This feature is explained by the strong dependence of this vibronic mode with the degree of ionization of the TMTTF molecule 66 . From this study, it is estimated a charge disproportionation ratio of 5 : 4 and 2 : 1 for X = PF 6 and AsF 6 salts, respectively.…”

Comprehensive Optical Investigations of Charge Order in Organic Chain Compounds (TMTTF)2X

“…In the present paper, we relate our assignments of the vibrational modes to the nomenclature proposed by Meneghetti and coworkers [59] where the molecular structure of TMTTF is assumed to have D 2h symmetry. According to the character table for the D 2h point group (Table 2), there are eight different irreducible representations whereas only the antisymmetric b 1u , b 2u and b 3u modes are infrared active and the symmetric a g , b 1g , b 2g , b 3g are Raman active.…”

“…Sketch of the totally symmetric a g modes ν 3 and ν 4 and the asymmetric ν 28 (b 1u ) stretching modes of the TMTTF molecule that mainly involve the central and outer C=C bonds. In the right frame, we show the linear shift of the intramolecular vibrations ν 3 (a g ) and ν 28 (b 1u ) with the charge per TMTTF molecule obtained from optical spectroscopy by Meneghetti et al [59]. …”

Metal-insulator transition in the low-dimensional organic conductor (TMTSF)2FSO3 probed by infrared microspectroscopy

“…We suggest that both modes originate from the ν 51 (b 2u ) in-plane vibration of the TMTSF molecule. According to the normal-coordinate analysis [12,16] its frequency for a free TMTSF 0.5+ cation is 702 cm −1 . The corresponding atomic movements involve stretching of the Se-C side bond and rocking of the adjacent methyl group.…”

“…The TMTSF molecule with the point group symmetry D 2h has in total 72 local vibrational modes classified according to the following representations [12] …”