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.