INSIGHTS INTO THE SOLVATO-/THERMO-PROMOTED INTRAMOLECULAR ELECTRON TRANSFER IN A TTF-sigma -TCNQ DYAD WITH AN EXTREMELY LOW HOMO-LUMO GAP

Abstract

The low-lying states of an organic donor-sigma-acceptor dyad, i.e. tetrathiafulvalene-sigma-tetracyano-p-quinodimethane (TTF-sigma-TCNQ), in gas phase and in various solvents have been investigated by means of hybrid DFT calculations in combination with the conductor-like polarizable continuum model to describe solvent effects. It has been shown that the dyad, though preferring a closed-shell singlet ground state with an eclipsed conformation in gas phase, adopts the charge-separated zwitterionic states with an extended conformation (TTF+-sigma-TCNQ(-)), i.e. open-shell singlet biradical ground state immediately followed by triplet biradical state, in polar solvent (CH3CN and CH2Cl2) as a result of the intramolecular electron transfer (ET) stimulated by solvent polarization. The degree of such intramolecular ET is so strongly dependent on the polarity (dielectric constant) of solvent that the zwitterionic biradical states become more stable with respect to the closed-shell singlet state with increasing polarity of the solvent. As such, the dyad should show a higher ratio of biradicals in more polar solvent and/or at higher temperature and, hence, is chameleonic in nature.