Theoretical Study on Controlling Factor of Conical Intersections

Conical intersections (CIs), which indicate the crossing of two or more adiabatic electronic states, are crucial in the mechanisms of photophysical, photochemical, and photobiological processes; including cis-trans photoisomerization of the primary events in vision and signals, excited-state proton transfer, and photodamage to deoxyribonucleic acid. In photo processes, information on the minimum-energy CIs (MECIs) between the ground state and the first singlet electronic excited state (S0/S1-MECI) is essential for investigating the energetically preferred internal conversion. Quntum chemical calculation is a powerful tool for atomic-level analysis in photo processes. Although various geometries and energy levels have been reported using quantum chemical calculations, systematic interpretation of the MECI geometries is unclear. It is because MECI geometries, unlike equilibrium geometries, are difficult to predict due to their complex structures with ring strain, ring opening, π-bond rotation, and σ-bond dissociation. We systematically investigated the S0/S1-MECI geometries of organic molecules using frozen orbital analysis (FZOA), which decomposes the energy difference of two electronic states into several excitation energy components for the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The investigation revealed two important controlling factors related to S0/S1-MECIs: the HOMO-LUMO exchange integral approximately vanishes and the HOMO-LUMO gap becomes close to the HOMO-LUMO Coulomb integral. This article describes the overview of FZOA and the controlling factors using several organic molecules. In addition, its application to the elucidation of internal conversion processes for bithiophene-fused isoquinoline is reported.