Primary optical excitations and excited-state interaction energies in sexithiophene

Abstract

Based on a unique combination of angle-resolved transmission spectroscopy and transmission data at high pressure, we identify the primary photoexcitations and the relevant excited-state interaction energies in a sexithiophene crystal. Optical excitations include charge-transfer excitons and Davydov polaritons. By extrapo- lation, we predict that in sexithiophene at hydrostatic pressures above 180 kbar, intermolecular excitations are lower in energy than intramolecular ones. The results are representative for a wide class of p-conjugated molecular semiconductors because ~1! the pertinent interaction energies and lengths scales are nearly identical and ~2! published data on different molecules are consistent with our interpretation.