Unveiling Controlling Factors of the S0/S1 Minimum Energy Conical Intersection: A Theoretical Study.

The minimum energy conical intersection (MECI) geometries play an important role in photophysics and photochemistry. Although a number of MECI geometries can be identified using quantum chemical methods, their chemical interpretation remains unclear. In this study, a systematic analysis was performed on the MECIs between the singlet (S0) and lowest singlet excited (S1) states of organic molecules. The frozen orbital analysis (FZOA), which approximates the excited states with minimal main configurations, was adopted to analyze the excitation energy components at the S0/S1 MECI geometries as well as the S0 and S1 equilibrium geometries. At the S0/S1 MECI geometries, the HOMO-LUMO gaps decreased as expected but did not disappear. The remaining gaps were balanced with the HOMO-LUMO Coulomb integrals. Furthermore, we discovered that the HOMO-LUMO exchange integrals became approximately zero. On the basis of this fact, a systematic interpretation of the S0/S1 MECI geometries has been described.