VERTICAL AND ADIABATIC ELECTRONIC EXCITATIONS IN BIPHENYLENE - A THEORETICAL-STUDY

The low-lying singlet states of biphenylene have been studied using ab initio methods. Vertical excitation energies were calculated by multi configurational perturbation theory (CASPT2), starting from a complete active space self-consistent held (CASSCF) reference. The geometries of the most important low-lying excited states were individually optim ized at the CASSCF level to study the difference between vertical and adiabatic excitations. Extended atomic natural orbital (ANO)-type basi s sets were used to calculate state energies. Geometry optimizations w ere done with smaller ANO-type basis sets. Excitations from the ground state to the 1 B-1(3g) and 1 B-1(2u) excited singlet states lead to p ronounced geometry changes which alter the bond alternation pattern. T he theoretical results provide a solid basis for the assignment and in terpretation of experimental spectra. (C) 1997 American Institute of P hysics.