A key to the understanding of electron-molecular vibration coupling in organic charge-transfer salts: Application of the two-site Hubbard model

A two-site Hubbard model has been utilized for description of the electron-molecular vibration coupling (EMVC) in organic charge-transfer (CT) salts with dimerized structures. It was shown that the intermolecular charge oscillations in the anti-phase totally symmetric vibrations are accompanied by a decrease of energy of the occupied electronic state in the dimer. This effect is responsible for the frequency shift of i.r.-active electron-vibrational bands compared with relevant Raman modes, and can be described in terms of dimensionless coupling constants. The absorption coefficients for the bands were found to be proportional to the frequency shifts. The calculated values are in good quantitative agreement with experimental data for BEDT-TTF-based and TTF-based monovalent CT complexes.