Monitoring structural transformations in crystals. 5. A topotactic [2 + 2]-photodimerization reaction.

The structural changes in a crystal of 5-benzylidene-2-(4-chlorobenzyl)cyclopentanone during [2 + 2]-photodimerization were monitored by means of X-ray diffraction. It was observed that the monomers moved gradually from the position occupied in the crystal at the initial stage of the photoreaction and the dimers moved towards the position assumed at the final step. The movements of the molecules possess a rotational component. Moreover, with the progress of the phototransformation the monomers in the reacting pair gradually move closer and change their relative orientation to resemble more the product molecule. The behaviour of the molecules and also the variation of the cell constants for the studied compound were compared with data for 5-benzylidene-2-benzylcyclopentanone.