Confinement effects on the photorelaxation of a proton-transfer phototautomer

We report on emission studies of excited phototautomers of 1′- hydroxy-2′-acetonaphthone in aqueous solutions of α-cyclodextrin (α-CD). Caging by α-CD provokes a ∼40 nm blue shift of the emission, and increases the emission lifetime from 90 ps to 1 ns. The rotational time of the caged phototautomers varies from 50 to 180 ps, while that of global rotational time of 1:2 complex (HAN:(α-CD)2) is ∼950 ps. These results are explained on the basis of confinement effects on the twisting motion of the formed phototautomers within the cavities provided by one or two CD.