Structure, spectroscopy, and spectral tuning of the gas-phase retinal chromophore: The β-ionone "Handle" and alkyl group effect

The low-lying singlet states (i.e. S ₀, S ₁, and S ₂) of the chromophore of rhodopsin, the protonated Schiff base of 11-cis-retinal (PSB11), and of its all-trans photoproduct have been 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. On the basis of the S ₀→S ₁ vertical excitation, the gas-phase absorption maximum of PSB11 is predicted to be 545 nm (2.28 eV). Thus, the predicted absorption maximum appears to be closer to that of the rhodopsin pigment (2.48 eV) and considerably red-shifted with respect to that measured in solution (2.82 eV in methanol). In addition, the absorption maxima associated with the blue, green, and red cone visual pigments are tentatively rationalized in terms of the spectral changes computed for PSB11 structures featuring differently twisted β-ionone rings. More specifically, a blue-shifted absorption maximum is explained in terms of a large twisting of the β-ionone ring (with respect to the main conjugated chain) in the visual S-cone (blue) pigment chromophore. In contrast, the chromophore of the visual L-cone (red) pigment is expected to have a nearly coplanar β-ionone ring yielding a six double bond fully conjugated framework. Finally, the M-cone (green) chromophore is expected to feature a twisting angle between 10 and 60°. The spectroscopic effects of the alkyl substituents on the PSB11 spectroscopic properties have also been investigated. It is found that they have a not negligible stabilizing effect on the S ₁→S ₀ energy gap (and, thus, cause a red shift of the absorption maximum) only when the double bond of the β-ionone ring conjugates significantly with the rest of the conjugated chain.