Caging ultrafast proton transfer and twisting motion of 1-hydroxy-2-acetonaphthone

We report on emission studies of 1-hydroxy-2-acetonaphthone (HAN) in solution nanocavities. In neutral water solutions, intramolecular proton transfer in the enol (E*) structure of HAN and a subsequent internal rotation in the formed keto-type tautomer (K*) occur in few tens of ps, to produce a ns-living rotamers (KR*). When caging HAN in cyclodextrins, the dynamics of twisting is greatly affected. Depending on the size of the cage, the isomerization channel leading to KR* can be closed, and emission occurs from K*. In a larger caging cavity, the fast component of the anisotropy decay becomes shorter while the slow one becomes longer.