Ab initio study on the low-lying excited states of retinal

Ab initio results for the electronic spectrum of all-trans-retinal and its truncated model 3-methyl-all-trans (10-s-cis)-2,4,6,8,10-undecapentaen-1-al are presented. The study includes geometry determination of the ground state. Vertical excitation energies have been computed using multiconfigurational second-order perturbation theory through the CASPT2 formalism. The lowest singlet excited state in gas phase is predicted to be of nπ∗ character. The lowest triplet state corresponds, however, to a ππ∗ state. The most intense feature of the spectrum is due to the strongly dipole-allowed ππ∗ transition, in accordance with the observed maximum in the one-photon spectra. The vertical excitation energies of the Bu- and Ag-like states are found close, the latter ≈ 1 eV higher than the maximum in the two-photon spectra. Solvent effects and nonvertical nature of the observed maximum in the two-photon spectra are invoked in rationalizing the deviation with respect to the best present estimate for the Ag-like state. In addition, qualitative aspects of the one-bond photoisomerization about the C11 = C12 double bond of retinal are considered. The overall isomerization picture from 11-cis into all-trans-retinal, as taking place mainly along the triplet manifold, agrees with experimental evidence.