The impact of solid solution composition on kinetics and mechanism of [2 + 2] photodimerization of cinnamic acid derivatives

We propose the use of solid solutions for the modulation of the kinetics in [2 + 2] photocycloaddition. Solid solutions with general formula [1H]Br_xCl_1−x (1 = 4-amino-cinnamic acid; 0 < x < 1) were prepared via mechanochemical methods, structurally characterized, and found to undergo a single-crystal-to-single-crystal [2 + 2] photodimerization. The effect of the solid solution composition on the kinetics was successfully investigated via a combination of FTIR and principal component analysis (PCA), and the results were compared with those obtained for the parent materials [1H]Cl and [1H]Br. All systems follow the same nearly first-order law, and the overall reaction kinetics is influenced by a subtle interplay of structural and electronic factors as well as other processes regulating the formation and lifetime of the reacting excimer.