Role of imperfections in the dimerization of substituted anthracenes. Part II. 1,8-Dichloro-10-methylanthracene

Abstract

Optical and electron microscopic examination of the (010)(ac) faces of the orthorhombic crystals of 1,8-dichloro-10-methylanthracene, together with supporting differential-enthalpic and dielectric measurements, have been employed with a view to explaining how the topochemically forbidden trans-photodimer is produced in the solid state upon u.v. irradiation. It is shown that dimer nuclei appear at emergent dislocations and tend to be aligned along the [001] and, to a lesser extent, the [100] directions. Two slip systems which would account for these observations are (210)[20] and (011)[121], both of which yield space-symmetries at the dislocation cores such that a trans-relationship exists between neighbouring molecules. The possible importance of stacking faults on (41) and (40) planes is discussed, and so also is the general significance of our results in relation to the recently evolved technique of crystal engineering.