The influence of side chains on the structures and properties of functionalized pentacenes

We investigated substituent-induced variations in microstructure and physical properties of a family of functionalized pentacenes, materials currently of intensive interest for making organic electronic devices such as thin film transistors, to shed light on the complex relationships between functionalization, film formation, stability, and microstructure. In this study, the pentacenes were modified with alkyl acetylene or alkylsilylethynyl groups with systematic variations in the alkyl chain length. With a proper side chain, this modification can effectively disrupt the herringbone packing seen in neat pentacene, promoting face-to-face arrangements between the acene rings and providing solubility in a variety of convenient solvents. Thin films can be readily formed by solution casting from THF, bromobenzene, toluene and other organic solvents. We have investigated the structure and properties of the functionalized pentacenes using UV-vis spectroscopy, hot stage optical microscopy, differential scanning calorimetry, transmission electron microscopy, X-ray and electron diffraction. The materials show regular variations in their thermal behavior, crystal packing and macroscopic properties as the chemistry of the side-group substituent changes.