Role of imperfections in the dimerization of substituted anthracenes. Part I. 1,8-Dichloro-9-methylanthracene

Abstract

Both the separation distance and the relative orientation of neighbouring molecules in the solid 1,8-dichloro-9-methylanthracene monomer are inconducive for the photostimulated production of trans-dimer molecules, yet these are relatively readily formed upon u.v. irradiation. However, the presence of structural imperfections, chiefly dislocations, offers an explanation of this fact. Optical microscopic studies of the distribution of dimer nuclei, dislocation-etch-pits, and slip traces, as well as of the preferred orientation of dimer nuclei on as-grown and cleavage faces of orthorhombic 1,8-dichloro-9-methylanthracene crystals are described. The most probable operative slip systems are identified; in particular, it is possible to formulate the nature of the imperfections which lead to the almost exclusive orientation of dimer along [01], [201], and [001] directions in the monomer matrix. It is demonstrated that slip along [010], particularly when the Burgers vector in this direction is of half strength, is conducive for the photoproduction of the trans-dimer.