Vibronic coupling in Frenkel and charge-transfer states of oligothiophene crystals



A novel approach, recently proposed to describe exciton–phonon coupling in Frenkel states of an infinite molecular crystal, is generalized to incorporate charge-transfer (CT) excitons. Both types of electronic excitations are treated on the same footing. The corresponding Hamiltonian in the Lang–Firsov representation is subdivided into subspaces representing one-particle, two-particle, etc., excitations, and (after truncating the basis set to manageable size) numerically diagonalized to yield the absorption as well as the electroabsorption (EA) spectrum. The emphasis is on the latter, previously reproduced only in the limiting cases of weak or strong vibronic coupling. The calculations are presented for the sexithiophene crystal, with parameterization based on independent experimental and theoretical estimates.