Photochemistry of Acyclic 1,3-Dienes as Models for the Visual System

In the initial photochemical event in vision a photon of light is absorbed by retinal, a polyene attached to the protein opsin via a Schiff base. A very rapid cis - trans double bond photoisomerization occurs about only the 11- 12 double bond even though (as shown) there is a possibility for any of five double bonds to rotate. This geometry change eventually generates the optic nerve. We want to find a rationale for this huge photoregioselectivity by investigating the regioselectivity of model diene systems.

Trans, trans-1-fluoro-2, 4-hexadiene (EE-FHD) was initially used to explore the excited potential energy surfaces of acyclic 1,3-dienes. Our investigations show that the regioselectivity of double-bond photoisomerization is primarily controlled by the charge stabilizing characteristic of substituents on the double bonds of the diene. Further, changes in the photoregioselectivity with solvent polarity suggest that the excited unsymmetrical diene returns to the ground state by competing pathways which traverse polarized ¹B_u states and drop into two different conical intersections (see figure below).

The regiospecificities of the photoisomerization of other dienes are also being investigated. For instance 2,2-dimethyl-2,4-hexadiene shows almost no selectivity in the isomerization between the double bond bearing the methyl group as apposed to the t-butyl group, but amazingly the bond bearing the deuterium in E,Z-1-deuterio-1,3-pentadiene shows preferential photoisomerization by more than a factor of 20 approaching the selectivity of retinal itself!