Analysis of reaction kinetics in the photomechanical molecular crystal 9-methylanthracene using an extended Finke-Watzky model

In order to develop an improved description of the photomechanical response of 9-methylanthracene (9MA) microcrystals, a detailed study of its solid-state photochemical reaction kinetics is performed. The reaction progress is monitored through the decrease in absorption of an optically microcrystalline thin film. The evolution of the time-dependent photoluminescence during the reaction is also measured. Both the photochemical reaction and nonradiative relaxation rates increase as more photoproduct is formed. In order to analyze the data, an extended version of the Finke-Watzky kinetic model for photochemical reactions is derived, denoted the FW-P model. This extended version enables a systematic analysis of photochemical reaction kinetics in solid-state molecular systems at varying levels of approximation. The FW-P model describes the non-exponential decrease in reactant and also correctly predicts the magnitude of the observed decrease in photoluminescence lifetime over the course of the reaction. The lifetime analysis is complicated by the fact that the microcrystalline 9MA sample contains multiple emitting species, and the extended FW-P model fails to capture the exact dependence of the photoluminescence on product formation. Analysis of the 9MA data indicates that both the photodimerization and the nonradiative relaxation rates can be accelerated by a factor of 10 over the course of the reaction. The results in this paper demonstrate that autocatalytic photodimerization kinetics are present in crystalline 9MA and may influence its photomechanical response.