Topochemistry. Part XXXIII. The Solid‐State Photochemistry of Some Anthracene Derivatives

The solid‐state photochemistry of several substituted anthracenes has been investigated with particular reference to the behaviour of polymorphic forms of the monomers. A class of 9‐substituted anthracenes (methyl, methoxycarbonyl, chloro, bromo) crystallising in anti‐parallel, sandwich packing arrangements give centrosymmetric dianthracenes in accordance with the topochemical principle, which also operates on light‐stable monomers packing in non‐centrosymmetric space groups having axes > 8 Å (e. g. 9‐methoxy, 9‐ethoxy‐, 1‐cyano‐) and showing no short parallel interplanar contacts. Some but not all monomers, crystallising in the parallel‐stack type with a 4 Å axis, dimerise to the non‐topochemical head‐to‐tail dimer. Two monomers substituted elsewhere than in the 9‐position give the theoretical number of solution dimers but yield only one dimer in the solid state.