Organic semiconductor co-crystals: reactivity, mobility, and spectroscopy

Organic semiconductors are of interest because low processing temperatures and cost which make such materials candidates for flexible electronics. The charge transport properties of the material are largely dependent on solid-state arrangement of the molecules. This thesis focuses on co-crystallization as a means to achieve [2+2] photodimerization with organic semiconductors, the impact the co-crystal former has on mobility, the use of a co-crystal former to obtain different conformations of a flexible system, and the ability to detect the change in conformation by infrared spectroscopy.

[2+2] photodimerization is studied as a way to alter the orientation of the π-systems in the solid state. To align a semiconductor building block into an orientation suitable for [2+2] photodimerization a co-crystallization method was used. The result of the photoreaction is the formation of a dimer in which the π-systems of the semiconductor building block are in a different orientation then before. Changes in the physical properties of the material through photodimerization are explored as a method for patterning thin films.

The impact the second component has on the overall mobility in our system is examined. The second component is not expected to act as a semiconductor and the impact on mobility the by its inclusion in the solid is unknown. The impact of a second component on mobility is studied by observing the mobility of multiple co-crystals along with the mobility of the single component. It was found that the mobility could be increased by a factor of approximately 200 with addition of a second component. The mobility change seen in the two-component crystals is equated to the changes observed in the crystal packing.

The conformation a molecule adopts in a solid can vary. It was discovered that the addition of a second component can be used to select the major conformation a bithiophene adopts in a solid. The change in conformation changes the orientation of the π-systems between molecules within the solid. The ability for a second component to alter the conformation of a bithiophene is explored. Infrared spectroscopy is used as a facial method to detect the change in the bithiophene conformation.