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 Schiff 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 (|Δμ| greater than 8 D) and a large change in polarizability between the ground and excited states ( greater than 300 Å3). The experimental |Δμ| values in the aldehydes are well predicted by semiempirical calculations. However, the calculated Δμ values for the SB are more than 2 times smaller than experimental values. This discrepancy suggests mixing of the 1Bu state with another state with a large dipole moment, most likely the 2Ag state, which is not probed by our calculational method. For both the aldehydes and SB, values are 2−8 times lower than experimental values. Possible reasons for the deviation between theory and experiment are discussed.