Vibrational spectroscopy of mixed stack organic semiconductors: Neutral and ionic phases of tetrathiafulvalene–chloranil (TTF–CA) charge transfer complex

Abstract

The infrared and Raman spectra for the room temperature, quasineutral, and the low temperature, quasi-ionic, phases of the mixed stack charge transfer complex tetrathiafulvalene–chloranil (TTF–CA) are reported. The analysis of the analogous data for a newly synthesized room temperature phase point to a dimerized segregated stack structure. All the vibrational data are interpreted and exploited through a clear identification of the differences, for the two types of stacks, in the spectroscopic effects due to the vibronic interaction, i.e., the coupling between electron and molecular vibration (e-mv). It is shown that for distorted mixed stack complexes both Raman and infrared spectra can be substantially influenced by the vibronic interaction, whereas the dimerized segregated stack complexes, as already known, display striking vibronic effects only in infrared. The theoretical model which explains the origin of these effects is briefly summarized and its extension to mixed stack structures successfully used to reproduce the infrared vibronic absorption spectrum of the TTF–CA low temperature distorted phase. By this procedure the experimental values of the e-mv coupling constants of TTF and CA units are extracted. A degree of ionicity ρ practically equal to one is determined for the segregated stack phase, whereas the fractional charge for the two mixed stack phases results to be: ρ=0.24±0.1 at 300 K and ρ=0.64±0.1 at 15 K. Some aspects of the neutral to ionic phase transition are discussed in terms of the temperature dependence of the vibrational spectra.