Unveiling the excited-state cycloaddition mechanism of crystalline 9-anthracenecarboxylic acid via transient absorption spectroscopy and theoretical calculations.

Abstract

Anthracene-based compounds that undergo photocycloaddition have garnered extensive attention for the fabrication of light-responsive crystalline materials. Nevertheless, their solid-state photocycloaddition mechanism remains elusive. Herein, transient absorption spectroscopy combined with theoretical calculations was employed to elucidate the cycloaddition mechanism of a monoclinic 9-anthracenecarboxylic acid (9AC) crystal, which possesses head-to-head cofacially stacked anthracene dimers. It is found that the photodimerization of 9AC proceeds via a stepwise mechanism in the triplet state. This is supported by the first-observed rapid population and decay of the excited state absorption signal from the triplet state. During excited-state dynamical evolution, the locally excited S1 state first rapidly relaxes to a delocalized S1 state. Subsequently, an efficient intersystem crossing process occurs at the S1/T2 crossing point, followed by internal conversion to the T1 state. Next, the crossing of a transition state facilitates the first C-C bond formation and generates a diradical intermediate. Finally, the second C-C bond is formed through the transition of the diradical intermediate to the cycloaddition product via the T1/S0 crossing point. Notably, the final step is promoted by the intermolecular hydrogen bonds that constrain the rotational variations between two anthracene units. Besides, the transition state acts as a singlet-triplet crossing point and forms an unfavorable energy dissipation channel, accounting for the slow cycloaddition rate of 9AC. This finding provides an unambiguous insight into the solid-state [4+4] photocycloaddition mechanism, which is essential for the rational design and development of anthracene-based smart materials.