Ultrafast dynamics of an azobenzene-containing molecular shuttle based on a rotaxane

An ultrafast spectroscopic study was carried out for a photoisomerizable, rotaxane-based molecular shuttle, in which photoisomerization of the azobenzene moiety of the thread-like guest drives a shuttling motion of a cyclodextrin host. Femtosecond upconversion and time-resolved absorption measurements revealed distinct S1 dynamics with time constants of 1.2 and 17 ps. Both time constants are smaller when the cyclodextrin host is absent, implying that, within the S1 state, there are mutiple barriers to the isomerization and subsequent shuttling, due to steric interference from the cyclodextrin.