Photochemical Reactions Inside the Electron Microscope: Preferred Dimerization of Anthracene at Dislocations

Optical microscopic studies of UV-irradiated single crystals of anthracene, and of some of its derivatives, reveal(1,2) that nucleation of the dimer within the host matrix occurs heterogeneously; and that the cores of certain non-basal dislocations function as preferred regions for the appearance of the product. That other types of dislocations present in aromatic solids may also be involved in chemical transformations within the solid-state is indicated both by the facile photochemical decoration (with dimer nuclei) of several families of dislocations in acenaphthylene,(3) and by the unexpected stereochemical product of dimerization of 9-cyanoanthracene.(4.5) We have recently demonstrated(6) that, for 9-cyanoanthracene, stacking faults produced by partial dislocations gliding on (221) planes could account for the invariable production of trans dimer of 9-cyanoanthracene upon UV irradiation. In this communication we report the preliminary results of in situ investigations aimed at revealing directly, by transmission electron microscopy, basal dislocations and stacking faults in anthracene; and, in particular, the possible role of such imperfections in photo-stimulated reactions of the solid.