The short-chain acroleiniminium and pentadieniminium cations: towards a model for retinal photoisomerization. A CASSCF/PT2 study

In this communication we report the results obtained in a computational study of the Minimum Energy Paths (MEP) found in the first excited state S1 and in the ground state S0 of two short-chain protonated Schiff bases (PSB): the s-cis 1-iminium-2-propene cation H2C=CH-CH=NH2 + and the tZt 1-iminium- 2,4-pentadiene cation H2C=CH-CH=CH-CH=NH2 +. This computational study has been performed at an high ab-initio level where the geometries of the relevant points have been optimized at the CAS-SCF level and the energetics have been refined via single-point computations at the CAS-PT2 level. This communication provides the important information that the photochemistry of the two studied PSB is driven by the spectroscopic 1B ionic state which remains the lowest excited state along all the optimized MEP. Both PSB show a S1/S0 Conical Intersection which is reached through a low barrier (barrierless) relaxation path for the shorter (longer) system and has an almost 90°twisted double bond (the CH2=CH-double bond for the shorter and the central double bond for the longer PSB) which provides a route for fully efficient non-adiabatic cis- > trans isomerization. In both PSB the crossing involves also a charge transfer between the two twisted fragments and the isomerization reaction coordinates on S1 are dominated by a stretching planar mode in the initial part of the MEP. Acroleiniminium; CASSCF/PT2; Pentadieniminium; Photoisomerization; Retinal