9-methylanthracene (9MA) crystals show a strong photomechanical response, undergoing photoinduced bending and twisting motions depending on crystal shape. The rate and magnitude of the photomechanical motions will also depend on the reaction kinetics. However, the photochemical kinetics of 9MA in the solid-state are not well understood. Herein, we make a detailed study of the [4 + 4] photodimerization of 9MA using several methods, including UV-Vis spectroscopy, NMR and time-resolved fluorescence measurements. The evolution of the dimerization process can be described by using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The value of the Avrami parameter n is around 1.5, suggesting that the reaction follows heterogeneous nucleation and one-dimensional growth. The 9MA kinetics can also be analyzed in terms of the Finke-Watzky (F-W) model in which the reaction rate depends on the amount of product formed. These observations point to a clear difference between photomechanical molecular crystals and polymers, where homogeneous reaction kinetics are observed.

Heterogeneous reaction kinetics in a photomechanical molecular crystal

251st National Meeting (2016)

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