TTF-TCNE a charge transfer p-molecular crystal with partial ionic ground state: Optical properties and electron-molecular vibration interaction

The electronic ground state structure of the charge transfer molecular complex TTF-TCNE is investigated on the basis of its electronic and vibrational spectra. Highly oriented polycrystalline films and the spectra of the fully deuterated complex, TTF-d(4)-TCNE, allow one to obtain a full exploitation of the spectra. Using the vibrational frequencies as local probes of the electronic structure one finds a value of 0.5+/-0.1 for the degree of charge transfer of this molecular solid. This partial degree of charge transfer and the alternation of self-dimers of TTF and TCNE along the one-dimensional electronic pi-structure reveal themselves in the vibrational spectra and particularly in the charge transfer vibronic resonances present in the infrared and Raman spectra. These resonances and the electronic spectrum related to the charge transfer excitations are understood on the basis of a Holstein-Hubbard model which allows the determination of the electron-intramolecular vibration coupling constants of TTF and TCNE.