The Photocycloaddition Reaction and Excited-State Dynamics of Benzo[g]quinoxaline Derivatives Featuring X-Aggregated Dimers

Reversible photocycloaddition reactions of anthracene-based compounds have attracted considerable attention. However, the solid-state cycloaddition mechanism is unclear, and the cycloaddition behavior of X-aggregated anthracene dimers remains unexplored. Herein, we found that 2,3-diethylbenzo[g]quinoxaline (EBQ), which assembles into crossing-stacked anthracene dimers with an -8.8° rotation angle, can undergo single-crystal to single-crystal photocycloaddition, accompanied by photochromism and fluorescence modulation. Detailed analyses via transient absorption spectroscopy and theoretical calculation reveal that the solid-state cycloaddition of EBQ follows a stepwise mechanism: (1) fast structural arrangement from the localized S1 state to the delocalized excimer state upon photoexcitation; (2) intersystem crossing via the S1/T2 crossing point; (3) formation of the first C-C bond via a rate-determining transition state, dominated by intermolecular steric hindrance; and (4) formation of the second C-C bond via the T1/S0 crossing point. Therefore, this work not only confirms the feasibility of the solid-state photocycloaddition of X-aggregated anthracene dimers but also elucidates the solid-state cycloaddition mechanism.