Relationship between spatially heterogeneous reaction dynamics and photochemical kinetics in single crystals of...

Understanding physicochemical property changes based on reaction kinetics is required to design materials exhibiting desired functions at arbitrary timings. In this work, we investigated the photodimerization of anthracene derivatives in single crystals. Single crystals of 9-cyanoanthracene (9CA) and 9-anthraldehyde (9AA) exhibited reaction front propagation on the optical length scale, while 9-methylanthracene and 9-acetylanthracene crystals underwent spatially homogeneous conversion. Moreover, the sigmoidal behavior in the absorbance change associated with the reaction was much pronounced in the case of 9CA and 9AA and correlated with the observation of heterogeneous reaction progress. A kinetic analysis based on the Finke-Watzky model showed that the effective quantum yield of the photochemical reaction changes by more than an order of magnitude during the course of the reaction in 9CA and 9AA. Both the reaction front propagation and nonlinear kinetic behavior could be rationalized in terms of the difference in the cooperativity of the reactions. We propose a plausible mechanism for the heterogeneous reaction progress in single crystals that depends on the magnitude of the conformational change required for reaction. Our results provide useful information to understand the connection between photochemical reaction progress in the crystalline phase and the dynamic changes in the physicochemical properties.