| Abstract
| - CASPT2//CASSCF/6-31G* photochemical reaction path computations for two 4-cis-nona-2,4,6,8-tetraeniminium cation derivatives, with the 4-cis double bond embedded in a seven- and eight-memberring, are carried out to model the reactivity of the corresponding ring-locked retinal chromophores. Thecomparison of the excited state branches of the two reaction paths with that of the native chromophore, isused to unveil the factors responsible for the remarkably short (60 fs) excited state (S1) lifetime observedwhen an artificial rhodopsin containing an eight member ring-locked retinal is photoexcited. Indeed, it isshown that the strain imposed by the eight-member ring on the chromophore backbone leads to a dramaticchange in the shape of the S1 energy surface. Our models are also used to investigate the nature of theprimary photoproducts observed in different artificial rhodopsins. It is seen that only the eight memberring-locked retinal model can access a shallow energy minimum on the ground state. This result impliesthat the primary, photorhodopsin-like, transient observed in artificial rhodopsins could correspond to a shallowexcited state minimum. Similarly, the second, bathorhodopsin-like, transient species could be assigned toa ground state structure displaying a nearly all-trans conformation.
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