COUPLING OF ELECTRONS TO INTERMOLECULAR PHONONS IN MOLECULAR CHARGE-TRANSFER DIMERS - A RESONANCE RAMAN-STUDY

We report resonance Raman scattering RRS spectra and Raman excitation profiles REP of a system containing dimers of identical molecular radical ions measured with laser excitation in resonance with the charge transfer CT transition. A Peierls–Hubbard PH Hamiltonian has been used to model the investigated system and to calculate its optical and RRS properties. Results are reported for two polyoxometallate salts of tetrathiafulvalene TTF, namely TTF2W6O19) and TTF2 Mo6 O19) whose structures contain almost isolated TTF ) 2 dimers. The RRS spectra of TTF2W6O19), measured in resonance with the CT absorption band centered at 832 nm, show 1 three phonon modes located at 55, 90, and 116 cm modes have been associated to the out-of-phase combinations of the translational motions of the two molecules composing the dimer. Such modes are effective in modulating the intradimer transfer integral, thus providing an efficient mechanism for coupling with the electronic system and for enhancement of the scattering intensity at resonance with the CT transition. The REP for the three strongly coupled modes of TTF2W6O19) have been measured with laser excitation wavelengths ranging from 740 to 930 nm. Quantitative analysis of the REP data has been performed based on a perturbative solution of the PH model to second order in the electron-molecular-vibration EMV and electron-intermolecular-phonon EIP interactions. The CT absorption profile and the REP’s have been calculated using a time correlator technique and the model parameters have been optimized in order to fit the experimental REP data. Infrared vibronic absorptions of TTF2W6O19), originated by the EMV coupling, have been measured and independent information on the electronic parameters of the PH model have been derived. This has made the choice of the fitting parameters used for the REP calculations rather unambiguous and has allowed us to obtain, for the first time, reliable experimental estimates of the EIP coupling constants.