Experimental determination of excitonic levels in α-oligothiophenes

The effects of intermolecular interactions on the optical spectra of oligothiophenes are examined. Absorption spectra of isolated molecules are calculated and experimentally recorded in a rigid host matrix whereby molecules are distributed randomly or organized unidirectionally. Absorption spectra of thin films in an ordered and disordered state are given. Ordered films have three principal spectral regions which are discussed in terms of classical exciton theory. Absorption spectra in transmission of single crystals of quinquethiophene and sexithiophene are analyzed. The lowest optically allowed transition in the crystal corresponds to the lowest Davydov component. It consists of a sharp peak that is observed for even-numbered oligothiophenes in b polarization, and absent for odd-numbered rings due to the perfect alignment of the transition dipole moment with the long molecular axis. The upper Davydov component is viewed in both thin film and single crystal spectra. In between the two principal Davydov components lies a broadband that is tentatively attributed to charge-transfer states or to a noninteracting molecular transition reminiscent of isolated matrix spectra. The Davydov splitting of the first optically allowed transition is determined to be about 10 000 cm(-1) and increases slightly with chain length. The Herzberg-Teller region in 6T reveals a dominant coupling mode of 340 cm(-1). Vibronic structure in excitation and emission spectra shows peaks of 1460 cm(-1) spacing. (C) 2000 American Institute of Physics. [S0021-9606(00)70325-4].