Polymorphism and structure–property relations

The design of materials requires knowledge of the relationship between the structure of a substance and its properties. Polymorphic systems provide ideal, perhaps even unique, systems for studying structure-property relations since the molecular entity is a constant and differences in properties must be due to differences in structure. This chapter expands on this strategy of studying structure-property relations by investigating polymorphic systems. Systems studied may be categorized either in terms of properties due essentially to the individual molecules or in terms of bulk properties resulting from interactions between the molecules in the structure. The latter category includes electrical conductivity, magnetism, photovoltaicity and photoconductivity, nonlinear optical activity, second harmonic generation, photochromism, thermochromism, mechanochromism, and thermosalient effects. Among molecular properties considered are infrared and Raman spectra, UV/visible spectra, excimer emission, and excited state diffraction studies. Additional topics include solid state photochemical reactions, thermal reactions, and high pressure studies.