Photofracking-Assisted Enhancement of Solid-State Photochemical Reactivity: α-Azido-5-phenyl-2,4-dienoate Derivatives

Solid-state photoreactions are unmatched by their liquid or gaseous counterparts for being spatially and temporally controllable and selective; however, these assets usually come with compromised yields-localized surface reactions are favored that generate a passivating layer of photoproducts and shield the crystal interior from reacting. Here, we demonstrate that this inherent limitation could be circumvented by capitalizing on the disintegrative effects of photodynamic crystals, such as cracking and fracturing, which can expose freshly cleaved surfaces and effectively increase the overall conversion. Upon irradiation with natural or artificial visible light, crystals of two 2-azido-5-phenyl-2,4-dienoate derivatives (1a and 1b) are converted into the corresponding 5-phenyl-1H-pyrrole-2-carboxylate derivatives (2a and 2b). Crystals were observed to crack and fracture throughout the reaction due to the release of dinitrogen, and the cracking occurs preferentially along planes defined by the weakest intermolecular interactions. Laser-flash photolysis in conjunction with density functional theory calculations confirmed that in both solution and in the solid, the triplet excited states of 1a and 1b rearrange to biradicals ³Br1a and ³Br1b, which then release dinitrogen to afford biradicals ³Br2a and ³Br2b; however, in the solid state, the crystal lattice rigidity extends the triplet lifetimes of both 1 and ³Br1. This work highlights the benefits of the "photofracking" that stands for photoinduced crystal disintegration, as a convenient strategy that naturally increases the efficiency of the solid-state reactions without external intervention.