Mapping the Excited-State Potential Energy Surface of a Photomolecular Motor

A detailed understanding of the operation and efficiency of unidirectional photomolecular rotary motors is essential for their effective exploitation in molecular nanomachines. Unidirectional motion relies on light-driven conversion from a stable (1 a) to a metastable (1 b) conformation, which then relaxes through a thermally driven helix inversion in the ground state. The excited-state surface has thus far only been experimentally characterised for 1 a. Here we probe the metastable, 1 b, excited state, utilising ultrafast transient absorption and femtosecond stimulated Raman spectroscopy. These reveal that the "dark" excited-state intermediate between 1 a and 1 b has a different lifetime and structure depending on the initial ground-state conformation excited. This suggests that the reaction coordinate connecting 1 a to 1 b differs to that for the reverse photochemical process. The result is contrasted with earlier calculations.