Monitoring structural transformations in crystals. 6. The [4 + 4] photodimerization of 9-methylanthracene

The structural changes in a crystal of 9-methylanthracene (1) during the [4 + 4] photodimerization were monitored by means of X-ray diffraction. This is the first example in the literature of such a study of a [4 + 4] photodimerization. The results obtained were compared with data for the [2 + 2] photodimerization. The shape of the product molecules and their preferred packing can explain the crystal disintegration. This was the reason that the reaction was monitored only to 28% completion. As far as could be determined the reaction proceeds with a constant rate. The cell volume increases at the beginning of the transformation and decreases afterwards. The product molecules do not assume a fixed position in the crystal during the photo-reaction, but move in a smooth way that includes a rotational component. The movements of the reactant are much smaller. Movements of molecules characterized by a rotational component were also observed in the case of the [2 + 2] photodimerization of 5-benzylidene-2-benzylcyclopentanone and 5-benzylidene-2-(4-chlorobenzyl)-cyclopentanone. The distance between the reacting atoms of the adjacent monomer molecules of (1) decreases with the degree of reaction completion, but more slowly than in the case of the [2 + 2] photodimerizations cited above. The orientation of the neighbouring monomer molecules changes during the phototransformation so that the monomer pair resembles the dimer product.