MOLECULAR-CRYSTALS - PAST AND PRESENT

Five different types of binding in crystals are generally recognized. These are (with common examples chosen from organic materials given, in some cases, in parentheses): molecular (anthracene, C60), ionic (K3C60, perhaps some charge-transfer complexes), covalent (diamond?), metallic (the choice of organic here depends on the definition of a metal) and amorphous (polymers, for example, polyacetylene or polythiophene might be of current interest). This paper will focus on molecular crystals, in particular describing the developments over the past 40 years in linear and nonlinear optical properties and in electrical behaviour. Studies in two areas of molecular crystals were given a great impetus just before 1950. Davydov (Zh. Eksp. Theor. Fiz., 18 (1948) 210) extended the earlier work of Frenkel and Peierls, presenting a clear and quantitative model for exciton behaviour, while Akamatu and Inokuchi (J. Chem. Phys., 18 (1950) 810) as well as Vartanian (Zh. Fiz. Khim., 22 (1948) 769) began an investigation of the electrical conductance and photoconductance in these samples. Where these early ideas have led will be the substance of this paper.