How Methylation Modifies the Photophysics of the Native All-trans-Retinal Protonated Schiff Base: A CASPT2/MD Study in Gas Phase and in Methanol

A comparison between the free-energy surfaces of the all-trans-retinal protonated Schiff base (RPSB) and its 10-methylated derivative in gas phase and methanol solution is performed at CASSCF//CASSCF and CASPT2//CASSCF levels. Solvent effects were included using the average solvent electrostatic potential from molecular dynamics method. This is a QM/MM (quantum mechanics/molecular mechanics) method that makes use of the mean field approximation. It is found that the methyl group bonded to C10 produces noticeable changes in the solution free-energy profile of the S₁ excited state, mainly in the relative stability of the minimum energy conical intersections (MECIs) with respect to the Franck–Condon (FC) point. The conical intersections yielding the 9-cis and 11-cis isomers are stabilized while that yielding the 13-cis isomer is destabilized; in fact, it becomes inaccessible by excitation to S₁. Furthermore, the planar S₁ minimum is not present in the methylated compound. The solvent notably stabilizes the S₂ excited state at the FC geometry. Therefore, if the S₂ state has an effect on the photoisomerization dynamics, it must be because it permits the RPSB population to branch around the FC point. All these changes combine to speed up the photoisomerization in the 10-methylated compound with respect to the native compound.