Dimerization stereochemistry and local order in organic crystal imperfections. Part 1. Photodimerization of some dichloroanthracenes in the solid state and in fluid solution

Abstract

A study of solid state and fluid solution photodimerization of three dichloroanthracenes (I)–(III) shows that the ‘head-to-head’ photodimers are predominant in crystals whereas the ‘head-to-tail’ photodimers are favoured in fluid solutions; in these media, where solvating molecules leave the monomers free to take up all possible mutual orientations, the ‘head-to-tail’ dimer is preferred, because, we believe, of the repulsion between the chlorine atoms in the ‘head-to-head’ activated complex; in the solid state, this repulsion can be compensated by crystal forces in the reaction cavity. It is shown that, in that state, the reaction occurs in disorder sites, and it is argued that these regions maintain, however, a high degree of ‘head-to-head’ mutual orientation of the monomers, reflecting the ideal β type crystal structure. This might be of general significance for crystals of aromatic molecules where facile exciton migration occurs.