Photoisomerization Path for a Realistic Retinal Chromophore Model:  The Nonatetraeniminium Cation

In this paper, ab initio CASSCF computations are used to investigate the photoisomerization path of the protonated Schiff base (PSB) 4-cis-γ-methylnona-2,4,6,8-tetraeniminium cation: a five conjugated double bond model of the retinal chromophore of rhodopsin (the human retina visual pigment).We show that, after initial skeletal relaxation from the Franck−Condon region (which involves a large increase in the central CC bond length), the system is “trapped” in an energy plateau on the S1 energy surface which may be the origin of the “slow” cis → trans isomerization dynamics observed in retinal PSBs in solution. The energy plateau is absent in shorter retinal chromophore models which have a steeper S1 isomerization path. The rhodopsin cavity (where the native chromophore is embedded) may have the effect of removing the energy plateau from the S1 potential thus dramatically increasing the photoisomerization rate from picoseconds to femtoseconds.