A theoretical study of the electronic spectrum of biphenyl

The electronic spectrum of biphenyl in the energy range up to 6.0 eV has been studied using multiconfigurational second-order perturbation theory (CASPT2) and a basis set of ANO type, including polarization functions on all carbon atoms. The calculated spectrum gives conclusive assignments to all valence excited singlet states and the low-lying triplet states. The change of the torsional angle between the two benzene rings in the different excited states is shown to be of considerable importance and explains the different excitation energies observed in the gas phase as compared to solution or crystalline biphenyl at low temperatures. The intense transition to the 1 1B1u state is mostly affected. The first Rydberg transition (3s) is found at 5.6 eV. The appearance of a state of 1Ag symmetry at energies around 5.0 eV is ruled out. The first state of this symmetry appears at 5.9 eV.