Topochemistry. Part XV. The solid-state photochemistry of p-quinones

Abstract

The known solid-state photochemistry of p-benzoquinone (I), 2,3-, 2,5-, and 2,6-dimethyl-p-benzoquinones (II, III, and IV), duroquinone (V), and thymoquinone is discussed in terms of the packing arrangements of these compounds.

The structure of the dimers (cyclobutanes and oxetans) of (III) and (IV) can be related to the packing geometry, in the monomer crystals, of parallel double bonds (CC, CO) with centre-to-centre distances up to 4·3 Å. This correlation is unsatisfactory for (II).

The absence of short parallel contacts in (I) and (V) may account for the inability of these two quinones to yield dimers. We postulate that the formation of polymeric material is related to the presence of short contacts between carbon atoms of nearest-neighbour non-parallel [graphic omitted] groups whose interaction leads to open-chain dimer diradicals; these cannot terminate by ring-closure but may instead initiate polymerisation.