Exploring the impact of atomistic substitution on thin-film structure in a germanyl-ethynyl functionalized pentacene

Functionalization of organic semiconductors through the attachment of bulky side groups to the conjugated core has imparted solution processability to this class of otherwise insoluble materials. A consequence of this functionalization is that the bulky side groups impact the solid-state packing of these materials. To examine the importance of side group electronic character on accessing the structural phase space of functionalized materials, germanium was substituted for silicon in triisopropylsilylethynyl-pentacene (TIPS-Pn) to produce triisopropylgermanylethynyl-pentacene (TIPGe-Pn), with the TIPGe side group comparable in size to TIPS, but higher in electron density. We find TIPGe-Pn single crystals to exhibit slip-stack, herringbone and brickwork packing depending on growth conditions, a stark contrast to TIPS-Pn, which only accesses the brickwork packing motif in both single crystals and thin films. Polycrystalline thin films of TIPGe-Pn exhibit two new, unidentified polymorphs from spin-coating and post-deposition annealing. Our experiments suggest that access to the structural phase space is not solely guided by the size of the side group. Its electronic character appears to play a significant role in dictating the accessible solid structures.