Primary optical excitations and excited-state interaction energies in sexithiophene

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 extrapolation, 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 pi-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.