Involvement of triplet state in isomerization of retinal analogs

The aim of this work is to study the involvement of a triplet state in the photoisomerization process of the retinal protonated Schiff-base (RPSB) and its analogs. The isomerization is the initial process in the photocycle of the rhodopsin. Here we use quantum chemical calculations to understand this light-induced process since the involvement of the triplet state in the initial step of the RPSB isomerization is unclear.

Three molecules were studied: protonated Schiff base (PSB), Schiff base (i.e. non-protonated SB), and aldehydes. For each molecule three models were investigated with three, four and five double bonds. All calculations in this work were performed at the extended multi-state complete active space second order perturbation (XMS-CASPT2) level of theory using cc-pVDZ basis set. The XMS-CASPT2 method is employed to obtain the optimized ground state geometry, the minimum energy conical intersection (MECI) and the relaxed scan in the excited state.

To estimate the reactivity in the excited state, we have interpolated the electronic state energies at the XMS-CASPT2 level between the optimized Franck-Condon point and the MECI. We found that the SB and aldehyde molecules with three double bonds are the most probable to facilitate a singlet-triplet crossing.