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 S 1 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 S 1 state, there are mutiple barriers to the isomerization and subsequent shuttling, due to steric interference from the cyclodextrin.