Enhanced dipole moments in photo-excited TTF-TCNQ dimers

We have studied the dynamics of electron transfer between the molecules of an organic donor-acceptor pair upon absorption of light. Specifically, we considered the tetrathiafulvalene (TTF)-7,7,8,8-tetracyanoquinodimethane (TCNQ) donor-acceptor pair using time-dependent density functional theory with local-density approximation. The molecular planes of the two components are parallel to each other, and the optical transition probability is found to be highest when the optical electric field is parallel to these planes. Under these conditions, absorption induces additional electron transfer from TTF to TCNQ in the π-orbitals perpendicular to the molecular plane and, consequently, we found that the dimer's dipole moment perpendicular to the molecular axes is enhanced with the increase rate of 1% in 15 fs. This enhancement reflects the fact that photo-excited electron-hole pairs tend to dissociate, i.e. electrons and holes move away from each other. We thus suggest potential photovoltaic devices employing these molecules as building blocks.