Studies in crystal engineering. Photochemical and crystallographic investigations of bromocoumarins and (±)-7-(p-bromobenzylidene)piperitone

Photodimerisations of bromo-substituted coumarins 1 and 2 and (±)-7-(bromobenzylidene)piperitone 3 have been investigated in the solid state. 6-Bromocoumarin and 7-bromocoumarin crystals yield syn head-to-head dimers upon irradiation, whereas 3 is photostable. The photobehaviour of these systems is rationalised in terms of their crystal packing. The β steering ability of the bromo group has been examined for crystal engineering and the effect of the topological features of the organic moiety on the molecular packing is emphasised