X-Ray observations of orientations of photodimer in single crystals of anthracene

By use of X-ray diffraction, the photodimer, obtained when single crystals of anthracene are irradiated with Pyrex-filtered u.v. light, has been examined and the geometric relationship of the dimer to the original anthracene crystal has been established. Although the crystal structure of the dimer is apparently not modified by the presence of the surrounding anthracene crystal, the orientation of the dimer crystals is distinctly not random and appears to be dictated by the periodicity of the surrounding unreacted anthracene. Parallel matching repeat distances between the monomer and dimer are compared. Each dimer crystal forms is one of several (at least six) orientations permitted by the anthracene crystal, each orientation being associated with a specific well-developed face of the anthracene crystal.