Electronic excited states and electronic spectra of biphenyl: a study using many-body wavefunction methods and density functional theories

Abstract

The low-lying electronic excited states of biphenyl were studied using the symmetry-adapted cluster-configuration interaction (SAC-CI), complete active space self-consistent field (CASSCF), complete active space perturbation theory of the second-order (CASPT2), and the time-dependent density functional theory (TDDFT). The molecular geometries in the ground and excited states were optimized using the SAC-CI and TDDFT for singlet and triplet states. The energies of vertical excitations, emissions, and adiabatic transitions were calculated. The TDDFT calculations significantly underestimated the excitation energy of the 1¹B₁ state, while the SAC-CI and CASPT2 provided essentially similar results. The present SAC-CI and CASPT2 calculations concluded that the lowest singlet state of isolated biphenyl is the 1¹B₃ state that takes a planar geometry and the second lowest state is the 1¹B₂ state with a twisted geometry. The present results were consistent with the previous experimental findings. The 1¹B₁ state that has a charge-separated biracial character in the vertical excitation relaxed into a planar quinoid structure in which bond alternations were emphasized. The other states took a benzenoid structure. The ultraviolet (UV) absorption and circular dichroism (CD) spectra below 7 eV were calculated with the SAC-CI method. The valence–Rydberg mixings were found to be significant in the second and higher series of excited states.