Nonlinear optical properties of bacteriorhodopsin, retinal, and related molecules

The first hyperpolarizabilities, β, of bacteriorhodopsin, retinal, and related molecules were determined experimentally by using the hyper-Rayleigh scattering technique and compared to the calculated values obtained with the semiempirical intermediate neglect of differential overlap/configuration interaction/sum-over-states method. The experimental and theoretical results are in excellent mutual agreeement. The hyper-Rayleigh scattering technique is shown to be very sensitive to the degree of solubilization of bacteriorhodopsin. Theoretical and experimental data confirm the expected dependence of β on the first transition energy as well as an exponential increase of β with the number of double bonds. It was found that, upon trans to 13-cis or 9-cis isomerization of a retinal double bond, a constant fraction of the β value is lost, regardless of the nature of the electron withdrawing group or the solvent of choice.