Charge Sensitive Vibrations and Electron-Molecular Vibration Coupling in Bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF)

A complete analysis of the vibrational dynamics of the most important molecule in the field of organic superconductors, bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF), has been performed through first-principles calculations. Different ionization states have been investigated, both as isolated BEDT-TTF unit, and as symmetric self-dimer. The ionization process affects the frequencies of some vibrations, the "charge sensitive modes", but also their intensities. In particular, a 100-fold increase in the infrared intensity of the antisymmetric C=C stretching mode is predicted to occur upon removal of one electron. The discovery of this dramatic difference will help to interpret the spectral phenomena observed in correspondence to the charge-order processes undergone by some BEDT-TTF salts. The electron-molecular vibration (e-mv) coupling and its effects on the infrared spectra is also fully reanalysed by adopting the proper molecular symmetry of the BEDT-TTF+ monomer, and by investigating (BEDT-TTF)(2)(2+) and (BEDT-TTF)(2)(+) dimers. A new approach to estimate the relative values of the e-mv coupling constants is proposed.