Spectroscopic features of the low-lying singlet states of some N-alkyl retinylnitrone model systems and their involvement in oxaziridine formation

Abstract

The photo-excitation process and non-radiative decay channels of the model compounds of some N-alkyl retinylnitrones are studied at the CASSCF/6-31G*, CASMP2/6-31G* and PM3/CI level of theories. A relaxed planar geometry of the first excited singlet state is reached after the initial photo-excitation, which is followed by non-radiative decay processes through conical intersection (S₀/S₁) channels. Their first-excited singlet states (S₁) have mixed biradical–ionic nature, and are found to be dominated by configurations arising from HOMO² → LUMO², HOMO − 1 → LUMO and HOMO → LUMO excitations. Conical intersection geometries originating from the one-bond-flip and Hula-twist motions in the central part of these molecules are found at higher energies in comparison to their terminally twisted counter parts. In the N-methyl nitrone system, the lowest-energy intersection point arises due to a twist in the terminal part with an out-of-plane CNO-kink (R_C–O = 2.12 Å, R_N–O = 1.38 Å) or oxygen-bridge structure. Following the directions of its gradient difference vectors, the probable oxaziridine ground-state geometry (R_C–O = 1.38 Å, R_N–O = 1.44 Å, <OCN = 62.3^°, <ONC = 57.6^°) has been located as a saddle point, which is the only experimentally reported photoproduct of N-methyl retinylnitrone compound under room light. The radiative transition studies on the allowed S₀ → S₁ transitions at the ground state equilibrium geometry have given transition moment values between 4.5 and 5.0 Debye.