Microscopic calculation of dielectric tensor of quaterthiophene crystals

We present a microscopic calculation of the dielectric tensor for a quaterthiophene crystal near resonances of Frenkel excitons in a vibronic coupling regime. We show how the intermolecular interactions modify the intensity and energy of each vibronic level from the isolated molecule to the solid state after transverse excitation. In particular, a strong redistribution of oscillator strength is found between the vibronic levels of the upper Davydov state. On the basis of the calculated dielectric tensor, we reproduce the measured absorbance spectra as recorded in different experimental configurations taking into consideration the additional role of the longitudinal macroscopic field. Some replicas in the optical spectra are clearly attributable to transitions of Frenkel origin while the role of other types of excitons, such as those of charge-transfer parentage is also discussed.