Crystallization-induced properties from morphology-controlled organic crystals.

During the past two decades, many materials chemists have focused on the development of organic molecules that can serve as the basis of cost-effective and flexible electronic, optical, and energy conversion devices. Among the potential candidate molecules, metal-free or metal-containing conjugated organic molecules offer high-order electronic conjugation levels that can directly support fast charge carrier transport, rapid optoelectric responses, and reliable exciton manipulation. Early studies of these molecules focused on the design and synthesis of organic unit molecules that exhibit active electrical and optical properties when produced in the form of thin film devices. Since then, researchers have worked to enhance the properties upon crystallization of the unit molecules as single crystals provide higher carrier mobilities and exciton recombination yields. Most recently, researchers have conducted in-depth studies to understand how crystallization induces property changes, especially those that depend on specific crystal surfaces. The different properties that depend on the crystal facets have been of particular interest. Most unit molecules have anisotropic structures, and therefore produce crystals with several unique crystal facets with dissimilar molecular arrangements. These structural differences would also lead to diverse electrical conductance, optical absorption/emission, and even chemical interaction properties depending on the crystal facet investigated. To study the effects of crystallization and crystal facet-dependent property changes, researchers must grow or synthesize crystals of highly conjugated molecules that have both a variety of morphologies and high crystallinity. Morphologically well-defined organic crystals, that form structures such as wires, rods, disks, and cubes, provide objects that researchers can use to evaluate these material properties. Such structures typically occur as single crystals with well-developed facets with dissimilar molecular arrangements. Recently, researchers have proposed several approaches for the vapor and solution phase synthesis of high quality organic crystals with various morphologies. In this Account, we focus on methodologies for the synthesis of various organic- and metal-containing highly conjugated molecular crystals. We also examine the new optical and chemical properties of these materials. In addition, we introduce recent experimental results demonstrating that high crystallinity and specific molecular arrangements lead to crystallization-induced property changes. We believe that the understanding of the crystallization-induced property changes in organic crystals will provide both fundamental knowledge of the chemical processes occurring at various interfaces and opportunities for researchers to take advantage of crystallization-induced property changes in the development of high-performance organic devices.