Excited-state singlet manifold and oscillatory features of a nonatetraeniminium retinal chromophore model

In this paper we use ab initio multireference Møller-Plesset second-order perturbation theory computations to map the first five singlet states (S₀, S₁, S₂, S₃, and S ₄) along the initial part of the photoisomerization coordinate for the isolated rhodopsin chromophore model 4-cis-γ -methylnona-2,4,6,8-tetraeniminium cation. We show that this information not only provides an explanation for the spectral features associated to the chromophore in solution but also, subject to a tentative hypothesis on the effect of the protein cavity, may be employed to explain/assign the ultrafast near-IR excited-state absorption, stimulated emission, and transient excited-state absorption bands observed in rhodopsin proteins (e.g. rhodopsin and bacteriorhodopsin). We also show that the results of vibrational frequency computations reveal a general structure for the first (S₁) excited-state energy surface of PSBs that is consistent with the existence of the coherent oscillatory motions observed both in solution and in bacteriorhodopsin.