Ultrafast internal conversion in neobetanin in comparison to betacyanins

S1 state deactivation in betacyanin-related dyes is controlled by: solvent viscosity, the strength of solute-solvent hydrogen bonds and the possibility of intramolecular hydrogen bonds formation. [Display omitted] •S1 state deactivation is mainly radiationless and involves intramolecular coordinates with large-amplitude motion.•S1 state lifetime depends on the solvent viscosity, the presence of intramolecular hydrogen bonds, and the strength of solute-solvent hydrogen bonds.•S1→S0 internal conversion produces the S0 ground state of the twisted species.•The S1 state does not give rise to photoproducts or to the triplet excited state. The photophysics of neobetanin (14,15-dehydro-betanin) is compared to that of the structurally similar betacyanins. The dye S1 state lifetime in solution depends on the solvent viscosity, the strength of solute-solvent hydrogen bonds and the possibility of intramolecular hydrogen bonds formation. The S1 state lifetime of neobetanin is surprisingly short (2.3ps in H2O), compared to betanin and isobetanin (6.4ps), despite the largest energy gap ΔE(S1−S0)=19,100cm−1 measured for neobetanin. The decay of S1 to S0 state proceeds through the internal conversion, preceded by a large-amplitude motion, probably involving twisting around the carbon-nitrogen and carbon–carbon bonds in the central part of the dye molecule. The internal conversion produces the nascent twisted neobetanin in its S0 state, with a transient band at λ=540nm in H2O, and 3.8ps lifetime determined in pump-probe experiments. The respective transients are clearly observed for betanin and isobetanin in the pump-dump-probe experiments.