X-ray liquidography decodes complex motions in azobenzene isomerization

Capturing ultrafast structural rearrangements of organic molecules in solution remains a central challenge, with isomerization being a key example. Yet, despite decades of studies, a detailed atomic-level structural understanding of how isomerization occurs remains elusive for most molecules. Even for azobenzene, a textbook case of trans – cis isomerization with its deceptively simple structure of two phenyl rings linked by an azo bridge, the mechanism remains contentious. Extensive experimental and theoretical studies 1–33 have proposed disparate pathways, including rotation 1–15 , inversion 16–19 , hula twist 20,21 and inversion-assisted rotation 22–30 , without reaching consensus owing to the lack of direct structural evidence. Here we apply femtosecond X-ray liquidography to trans -azobenzene in solution and resolve, with atomic-level precision, the molecular structures of two transient intermediates bridging the trans and cis forms. The unveiled structures reveal that the trans -to- cis conversion proceeds through a sequence of distinct motions, initiated by C–N torsion, a motion that has received little attention in previous studies, and subsequently dominated by N–N rotation. This work provides structural insights into the volume-conserving nature of azobenzene isomerization and establishes X-ray liquidography as a versatile tool for molecular filming of structural dynamics in solutes lacking heavy atoms, overcoming the limitations imposed by dominant solvent scattering.