Emergent properties in locally ordered molecular materials

The two classic structural classifications of solid matter, crystalline and amorphous, are quite useful for inorganic solids, from antimony trichloride to zinc dibromide and beyond. However, for many molecular materials, especially those based on components containing ten or more non-hydrogen atoms, ordered single crystals are less common, and intermediate structures that are neither amorphous nor crystalline frequently dominate. Herein, we discuss several situations in which there is local (or partial) ordering in such molecular materials. These materials go beyond the standard concepts concerning polymers and must account for imperfect but substantial local ordering, such as that caused by weak intermolecular interactions (e.g., π stacking, H-bonding, and/or dispersion forces). This local ordering has important implications for, and applications in, materials properties: we specifically consider dielectric response, electron transport, and exciton transfer. Finally, we discuss the fundamental importance of the effective dimensionality of partially ordered domains in molecular materials.