Thermally- and photo-stimulated precipitation from single-crystal organic matrices

Abstract

A structural approach to precipitation within a range of organic solids is presented. Emphasis is placed on interrelating ostensibly disparate organic structures and also on possible novel solid-state syntheses in which a single-crystal parent may be smoothly converted, with retention of crystalline integrity, to a new daughter phase. For thermally-induced processes, four categories, each illustrated by specific examples, are identified: (i) co-operative movement, such as recurrent shear or recurrent twinning at the unit cell level, of molecules on or at well-defined crystallographic planes; (ii) change of molecular conformation leading to new unit cells and superlattices within the parent matrix; (iii) topochemical reactions, in which dimer or polymer products may be formed within a monomeric solid provided certain geometric and symmetry relations vis à vis parent and daughter structures are met; and (iv) stress-induced reactions in which either a new polymorph of the same chemical composition as the parent or a chemically distinct daughter may be formed.

For photo-induced processes, two specific examples are considered: the acridine-rich mixed crystal of acridine/anthracene in which precipitation may be readily induced by the absorption of light below a certain threshold wavelength; and the conversion of anthracene to one or more of four distinct products, depending upon the environmental conditions.