A THEORETICAL-STUDY OF THE ELECTRONIC-SPECTRUM OF TERTHIOPHENE

The electronic spectrum of planar 2,2':5',2 ''-terthiophene has been s tudied using multiconfigurational second-order perturbation theory. Fo ur valence states are located below the first dipole-allowed Rydberg s tate. The computed excitation energies (1(1)B(2): 2.86 eV; 2(1)A(1): 3 .71 eV; 2(1)B(2): 4.44 eV; 3(1)A(1): 4.96 eV) are in agreement with ex periment. The first transition results mainly from the HOMO-->LUMO sin gle electron promotion and represents the most prominent feature of th e spectrum. Comparisons are made with other theoretical calculations a nd between the electronic spectra of terthiophene and bithiophene.