Electronic spectra of 2,2′-bithiophene and 2,2′:5′,2"-terthiophene radical cations:: A theoretical analysis

Analysis of the electronic spectra of 2,2'-bithiophene and 2,2':5',2 " -terthiophene radical cations has been performed by using multiconfigurational second-order perturbation theory (CASPT2). Atomic natural orbital (ANO) type basis sets of split valence quality, including polarization functions on all heavy atoms, have been used. In agreement with experimental data, in the energy range below the lowest optically allowed transition of the respective neutral system, theoretical results predict two main absorption bands for both cations. The 2(2)A(u) and 3(2)A(u) states computed at 1.94 and 2.80 W, respectively, are related to the corresponding band maxima recorded for bithiophene cation. The 1(2)B(1) state of terthiophene cation placed at 1.31 eV is assigned to the observed lowest-energy band. Nevertheless, according to the present results, the highest-energy band observed in terthiophene cation stems from two electronic transitions. They involve the 2(2)B(1)(1.94 eV) and 3(2)B(1)(2.12 eV) excited states.