Photochemical Reaction\nFront Propagation and Control\nin Molecular Crystals

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