Experimental Estimate of the Holstein Electron–Phonon Coupling Constants in Perylene

A careful analysis of the vibrational spectra of two isostructural charge transfer systems with potential applications in optoelectronics, (perylene)3-(TCNQF4)2, and (perylene)3-(TCNQF2)2, highlights the presence of three strong infrared bands, polarized along the (perylene-TCNQFx-perylene) trimeric units present in the crystal. These bands are recognized as due to the Holstein, or electron–molecular vibration (e–mv) interaction. Use of appropriate modeling, based on the analysis of the frequency shifts due to the e–mv coupling, allows to extract the relevant Holstein coupling constants for the HOMO of the perylene molecule. The so-obtained experimental values agree very well with density functional calculations, thus validating the results. The perylene relaxation energy, or small polaron binding energy, is about 70 meV, of the same order of magnitude as that of pentacene