Photochemical Reaction Front Propagation and Control in Molecular Crystals

4-Fluoro-9-anthracenecarboxylic acid (4F-9AC) undergoes a reversible [4 + 4] photodimerization reaction that can generate bending and twisting in microcrystals. Larger crystals resist photomechanical deformation, permitting direct visualization of the [4 + 4] photodimerization reaction dynamics under a variety of conditions. Both birefringence and fluorescence imaging show that the photodimerization reaction starts preferentially at a crystal edge and then sweeps across the crystal as a propagating reaction front. To explain these results, a theory is developed that postulates an exponentially decaying mechanical reaction field that extends from product domain into reactant domains to catalyze the photochemical reaction. Analyzing the data with this theory, we estimate a reaction field penetration distance of ∼20 nm for the 4F-9AC crystal. The propagation velocity can be controlled by varying the excitation intensity or by embedding amorphous barrier regions into the crystal using electron beam lithography. These barriers can channel the reaction flow to select areas of the crystal while other regions remain unreacted.