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 (1a) to a metastable (1b) 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 1a. Here we probe the metastable, 1b, excited state, utilising ultrafast transient absorption and femtosecond stimulated Raman spectroscopy. These reveal that the dark excited-state intermediate between 1a and 1b has a different lifetime and structure depending on the initial ground-state conformation excited. This suggests that the reaction coordinate connecting 1a to 1b differs to that for the reverse photochemical process. The result is contrasted with earlier calculations.