A theoretical study on the molecular structure of biphenylene in its first excited singlet and triplet states: quantum chemical calculations on the structural changes of an antiaromatic molecule upon excitation

Abstract

The structural changes accompanying the electronic excitation of the antiaromatic biphenylene molecule were studied by semiempirical quantum chemical calculations, using the AM1 hamiltonian with configuration interaction (S 1, S 2, T 1, T 2, X̃ + states). The largest bond length differences of the molecular states investigated were observed within the central four-membered ring. Potential curves of the CC bonds which connect the two benzene rings have been calculated for all molecular states investigated. For the S 0 and S 1 states, potential hypersurfaces, using the two different bond lengths of the central four-membered “cyclobutadiene” subsystem as variables, were presented. Furthermore, the geometry of 1,3-cyclobutadiene in different states (S 0, S 1, T 1, X̃ + states) was investigated with the AM1-CI hamiltonian and compared with recent 6-31G ∗ MC-SCF ab initio results.