The photoisomerization of 11-cis-retinal protonated Schiff base in gas phase: insight from spin-flip density functional theory.

This extensive theoretical study employed the spin-flip density functional theory (SFDFT) method to investigate the photoisomerization of 11-cis-retinal protonated Schiff base (PSB11) and its minimal model tZt-penta-3,5-dieniminium cation (PSB3). Our calculated results indicate that SFDFT can perform very well in describing the ground- and excited-state geometries of PSB3 and PSB11. We located the conical intersection (CI) point and constructed the photoisomerization reaction path of PSB3 and PSB11 by using the SFDFT method. To further verify the SFDFT results, we computed the energy profiles along the constructed linearly interpolated internal coordinate (LIIC) pathways by using high-level theoretical methods, such as the EOM-CCSD, CR-EOM-CCSD(T), CASPT2, NEVPT2, and XMCQDPT2 methods. The SFDFT method predicts that the photoisomerization of PSB3 is barrierless, in accordance with previous complete-active-space self-consistent-field (CASSCF) results. However, an energy barrier is predicted along the LIIC pathways of PSB11. This finding is different from previous CASSCF results and may indicate that the photoisomerization of PSB11 in gas phase is similar to that in solution. However, the higher spin contamination of the SFDFT method in the vicinity of the CI point caused the located CI geometry to deviate from that of the real CI. In addition, the LIIC pathways are only approximations to the minimum energy path (MEP). Thus, further experimental and theoretical studies are needed to verify the existence of an energy barrier along the photoisomerization reaction path of PSB11 in gas phase.