Some Electronic and Chemical Consequences of Non-basal Dislocations in Crystalline Anthracene

Abstract

Efforts have been made to detect and characterize individual dislocations, small-angle boundaries and extended (nonequilibrium) lattice vacancies, formed by the annihilation of dislocations, in single crystals of anthracene. Only those dislocations emergent at the basal (OOl) faces may be identified using the dislocation-etch-pit technique; but some information regarding dislocations which glide in the basal plane may be deduced from deformation and cleavage studies. Three distinct kinds of experiments have been carried out on a variety of crystals, the dislocation content of which ranges from 10² cm⁻² (for vapor-grown) to 10⁸ cm⁻² (for deformed melt-grown crystals). From studies of thermally stimulated currents, it is confirmed that the peak in the “glow” curve centered at around 273°K, corresponding to a positive hole trap of depth 0.7 eV, is an indigenous feature of highly purified crystalline anthracene. It is established that, somewhat surprisingly, the magnitude of this “glow” peak decreases with increasing density of non-basal dislocations. Studies of space-charge-limited-currents, using an injector electrode, yield a more complete picture of the trapping centers; and it is found that the total concentration of traps is directly proportional to the density of non-basal dislocations. By making reasonable estimates of the radius of “distortion” at dislocation cores, and by assuming that all molecules situated within this distorted region may function as trapping centers, it is possible to account for the observed trap densities, which range from 10¹³ to 10¹⁹ cm⁻³ depending on the origin and pretreatment of the particular crystal. A direct test of one chemical consequence of the existence of dislocations in anthracene was made by studying, using optical microscopy, the transformation of the solid into dipara-anthracene, by photolysis with UV light. The dimerization is favored at dislocation cores, a fact of some significance for the general study of organic solid-state reactions, particularly in the context of topochemistry.