Fine Tuning of Retinal Photoinduced Decay in Solution.

Single methylation at position C of the all-trans retinal protonated Schiff base switches its excited-state decay in methanol from a slower picosecond into an ultrafast, protein-like subpicosecond process. QM/MM modeling in conjunction with on-the-fly excited-state dynamics provides fundamental understanding of the fine-tuning mechanics that "catalyzes" the photoinduced decay of solvated retinals. Methylation alters the interplay between the ionic S and covalent S states, reducing the excited-state lifetime by favoring the formation of a S transient fluorescent state with fully inverted bond lengths that accounts for the recorded transient spectroscopy and from which a space-saving conical intersection seam is quickly (<1 ps) reached. Minimal and apparently innocent chemical modifications thus affect the characteristic intramolecular charge-transfer of the S state as well as the interaction with the covalent S excited state, eventually providing the high tunability of retinal photophysics and photochemistry and delivering a new concept for the rational design of retinal-based photoactive molecular devices.