A theoretical study of the electronic spectrum of terthiophene

The electronic spectrum of planar 2,2′:5′,2″-terthiophene has been studied using multiconfigurational second-order perturbation theory. Four valence states are located below the first dipole-allowed Rydberg state. The computed excitation energies (1 1B2: 2.86 eV; 21A1; 3.71 eV; 21B2: 4.44 eV; 31A1: 4.96 eV) are in agreement with experiment. The first transition results mainly from the HOMO → LUMO single electron promotion and represents the most prominent feature of the spectrum. Comparisons are made with other theoretical calculations and between the electronic spectra of terthiophene and bithiophene.