Crystal design for organic semicondutors: The effects of substitution on crystal packing

Electronic devices based on organic materials have been an area of intense research for the past several years. Pentacene, an oligoacene, is regarded as a benchmark due to its high charge carrier mobility. However it has poor solubilty, un-optimized pi-overlap and is highly unstable. We have successfully demonstrated that peri-functionalization with trialkylsilyl ethynyl groups enhances the pi-stacking, solubility and stability of pentacene and related compounds. TIPS pentacene exhibits both two dimensional (2D) pi-stacking and good solubilty in common organic solvents. However changing the solubilizing groups or functionalizing the acene core results in a disruption of the desired 2D pi-stacking. Molecules with one dimensional (ID) pi-stacking have poorer mobility than those with 2D pi-stacking and hence larger solubilizing groups are required to change the a-stacking to 2D. The emphasis of this paper is to show how small changes in substitution can disrupt a-stacking in these molecules and how one can achieve the desired pi-stacking by careful selection of solubilzing groups.