Investigation of the relationship between dipolar properties and cis-trans configuration in retinal polyenes: A comparative study using stark spectroscopy and semiempirical calculations

Three biologically active conformers of retinal, all-trans, 9- and 13-cis, and their Sckiff bases (SB), are studied using a combination of electroabsorption (Stark) spectroscopy and semiempirical calculations. All of the retinal isomers studied show both a large change in dipole moment between the ground and excited states (\Delta mu\ greater than 8 D) and a large change in polarizability between the ground and excited states (<(Delta alpha)over bar> greater than 300 Angstrom(3)). The experimental \Delta mu\ values in the aldehydes are well predicted by semiempirical calculations. However, the calculated Delta mu values for the SB are more than 2 times smaller than experimental values. This discrepancy suggests mixing of the 1B(u) state with another state with a large dipole moment, most likely the 2A(g) state, which is not probed by our calculational method. For both the aldehydes and SB, Delta alpha(calc) values are 2-8 times lower than experimental values. Possible reasons for the deviation between theory and experiment are discussed.