A comprehensive picture of the ultrafast excited-state dynamics of retinal

Abstract

All-trans retinal is the chromophore of microbial rhodopsins initiating energy conversion and cellular signalling by subpicosecond photoinduced switching. Here, we provide detailed UV-Vis transient absorption experiments to disentangle the complex photochemistry of this polyene, which is governed by its terminal aldehyde group. After photoexcitation to the S₂(¹B_u⁺) state, the system exhibits polarity-dependent branching, populating separate S₁(¹A_g⁻) and intramolecular charge transfer (ICT) species. In all solvents, population of a singlet nπ* state from S₁ is observed which represents the precursor of the T₁ triplet state. While triplet formation dominates in nonpolar solvents (67% quantum yield), it is dramatically reduced in polar solvents (4%). The channel closes completely upon replacing the aldehyde by a carboxyl group, due to an energetic up-shift of ¹nπ*. In that case, internal conversion via the ICT species becomes the main pathway, with preferential formation of the initially excited isomer.