Acceleration of the Z to Ephotoisomerization of penta-2,4-dieniminium by hydrogen out-of-plane motion: theoretical study on a model system of retinal protonated Schiff base

We report the result of comparison between two reaction coordinates [on the potential energy surface of the first excited state (S1)] produced by CASSCF and these energies recalculated by MRMP2 in the Z to Ephotoisomerization of penta-2,4-dieniminium (PDI) as the minimal model of the retinal protonated Schiff base (RPSB). One coordinate is the S1 state minimum-energy-path (MEP) in mass-weighted coordinates from the S1 vertically excited point, where a strong hydrogen-out-of plane (HOOP) motion is not exhibited. The energy profile of the S1MEP at the MRMP2//CASSCF level shows a barrier for the rotation around the reactive C–C and hits the S1/S0 degeneracy space where the central C–C–C–C dihedral angle is distorted by 65°. The other coordinate is an S1 coordinate obtained by the relaxed scan strategy. The relaxed coordinate along the central C–C–C–C dihedral angle, which we call the HOOP coordinate, shows strong HOOP motion. According to the MRMP2//CASSCF calculation, there is no barrier on the HOOP coordinate. Furthermore, the S1 to S0 transition may be possible without the large skeletal deformation by HOOP motion because the HOOP coordinate encounters the S1/S0 degeneracy space where the central C–C–C–C dihedral angle is distorted by only 40°. Consequently, if PDI is a suitable model molecule for the RPSB as often assumed, the 11-cis to all-transphotoisomerization is predicted to be accelerated by the HOOP motion.