Solvent dependent femtosecond E-Z photoisomerization dynamics of hemiindigo derivatives

Hemiindigo is a photochromic dye constituted from half of indigo (3-oxindole) and half of stilbene (phenyl). The solvent dependence of the E-Z isomerization dynamics of two types of hemiindigos, 2-(4-dimethylamino)benzylidene)indolin-3-one (DABI) and its derivative with the amino moiety substituted by a 1-propyl group, was investigated by femtosecond transient absorption spectroscopy. The excited (S1) state lifetimes of the E- and Z- isomers of these compounds were all shorter than 100 ps, and they displayed dependence on polarity and hydrogen bonding (H-bonding) ability of the solvent. The S1 state lifetime of Z-DABI shortened from 76 ps in toluene to 9.3 ps in methanol. It was concluded that strengthening of the H-bond with the solvent molecule in the S1 state increases the rate of nonradiative decay to the original ground (S0) state and reduces the quantum yield of the photoisomerization. We think a similar deactivation process induced by the intermolecular H-bonding also takes place for photochromic indigo derivatives and, in the case of indigo carmine in aqueous solution, a water molecule trapped between NH and C=O moieties of indigo by H-bonds induces the "solvent mediated" proton transfer in the excited state [Roy et al., J. Phys. Chem. Lett. 11, 4156-4162 (2020)].