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.