Mechanism of photodimerization in single crystals of anthracene

The anthracene photodimer, di-p-anthracene, crystallizes in at least six orientations within the anthracene single crystal. The orientations are classified, according to mosaic spread, into two sets of three orientations each. The relative spatial arrangements of the three orientations in each set are identical and each form a simulated repeated twin. The photodimerization begins as a surface reaction. The particular mechanism depends upon the geometry of the surface where the dimerization begins. Two mechanisms are described in this paper. One, associated with the (001) face of anthracene and low mosaic spread reflections, is the uniting of symmetry-related monomer units and the other, associated with the (20) face and the high mosaic spread reflections, is the uniting of translation-related monomer units. More molecular movement is needed for the uniting of the translation-related monomer units as inferred from the greater mosaic spread of their reflections.