Direct observation of coherent azobenzene photochemistry

Molecular photoswitches are versatile natural or synthetic molecules that undergo reversible conformational changes in response to light. In chemistry azobenzenes act as ubiquitous synthetic photoswitches ¹ with applications ranging from opto-electronics ² , over molecular machines ³ to photopharmacology ⁴ . Their isomerization mechanism defines their molecular properties and yet is controversially debated, as the underlying ultrafast photochemistry is challenging to resolve in time and space. In this study, we have used an X-ray Free Electron Laser to observe coherent structural transitions in the prototypical photoswitch azo-combretastatin A4 bound to its protein target tubulin. A molecular movie assembled from a crystallographic femtosecond scan and snapshots of kinetic intermediates in the femto-to-nanosecond range show how the cis isomer overcomes energy barriers in the excited state, traverses the conical intersection into a twisted ground state conformation, cools, stretches and finally relaxes into a planarized trans conformer. Our crystallographic data at near-atomic resolution contrasted with femtosecond transient absorption spectroscopic data and quantum chemical trajectories provides an experimental and theoretical description of the ultrafast azobenzene photoreaction. These fundamental insights reveal surprising parallels between natural and synthetic photoswitches opening a route for the design of artificial photoswitching proteins.