Unconventional configuration of excimer in luminescent aggregates uncovered by in situ photo-difference electron density

Excimers host an important role in nature and modern industrial applications; however, a precise understanding regarding their configuration remains elusive owing to the difficulty in observing their formation in situ. Here we present an experimental scheme enabling in-situ visualization of structural evolution of molecules through photo-difference electron density. Application to pyrene crystals revealed an unanticipated excimer formation pathway. By observing the photo-induced molecular shift and rotation, we discovered the unconventional T-shaped configuration of pyrene excimers, which consists of more closely adjacent nonparallel molecules tending towards a perpendicular orientation, rather than the conventional sandwich configuration formed by adjacent parallel molecules. The results help demystify the molecular orientation and configuration associated with excimer discovered decades ago, and shed light on revealing fundamental mechanism of excimers in the fields of biomolecule detection, photovoltaics, and organic electronics.