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

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 S 2 ( 1 B u + ) state, the system exhibits polarity-dependent branching, populating separate S 1 ( 1 A g − ) and intramolecular charge transfer (ICT) species. In all solvents, population of a singlet nπ* state from S 1 is observed which represents the precursor of the T 1 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 1 nπ*. In that case, internal conversion via the ICT species becomes the main pathway, with preferential formation of the initially excited isomer. UV-Vis transient absorption follows kinetics of 1 nπ* and photoisomerisation of all- trans retinal.