Structure, spectroscopy, and spectral tuning of the gas-phase retinal chromophore: The beta-ionone 'handle' and alkyl group effect

The low-lying singlet states of the chromophore of rhodopsin, the protonated Schiff base of 11-cis-retinal (PSB11), and of its all-trans photoproduct are studied in isolated conditions by using ab initio multiconfigurational second-order perturbation theory. The computed spectroscopic features include the vertical excitation, the band origin, and the fluorescence maximum of both isomers.