Tuning Crystalline Solid-State Order and Charge Transport via Building-Block Modification of Oligothiophenes

Organic electronic materials have been the focus of intense research for more than twenty years. In addition to their oft-cited potential advantages of low-cost, compatibility with plastic substrates, and ease of processing, much of their appeal lies in the promise of functionality and performance by design. Ideally, electronic materials for applications ranging from flexible circuits and displays to radio frequency identification (RFID) tags, light-emitting diodes (LEDs), and chemical sensors will be synthesized on demand from a toolbox of organic components. While the field has yet to realize such lofty goals, the development of a library of organic electronic materials and new experimental techniques have offered structure–property relationships that will serve as the foundation for the development of the a priori prediction of optoelectronic properties. The highest performance organic semiconductors have the common feature of a delocalized, aromatic, electronically active core. Fused acenes, thiophenes, and oligothiophenes are prototypical systems that are comprised of only this component, and have demonstrated benchmark performance. Because the molecular subunits are relatively non-polar, however, intermolecular interactions are rather weak. The electronic homogeneity of the units results in crystal structures bound loosely by van der Waals forces that are only weakly favored thermodynamically. Experimentally, this is observed in the form of polymorphism and crystal disorder, as has been observed for linear acenes, as well as a plethora of oligothiophenes, even for some cases of substitutions designed to impart order (see reference [6] within Azumi et al.). The result of this phenomenon is a solid-state order and organization that is both difficult to predict and control. One approach to predictably imparting order involves functionalizing an aromatic core with bulky pendant groups, which strongly direct supramolecular organization through steric repulsion. While this provides for a wide variety of crystal packing motifs and often improves performance through increased order, it often has the side effect of spacing molecules more disparately or at disadvantageous orientations, which may actually impair transport. In particular, while functionalization at the center of the electronically active core very uniquely specifies a packing motif based on the relative sizes of bulky groups and aromatic moieties, it often has the effect of limiting longitudinal alignment of the cores. Here, we study the family of a-substituted oligothiophene derivatives shown in Figure 1 (see Supporting information, SI, for synthesis details) in order to determine the effects of terminal substituent density on the packing of the electronically active core units. The molecules consist of an aromatic oligothiophene core, functionalized at the terminal positions with trimethylsilane (TMS) groups. In addition to imparting solubility, the substituent TMS groups yield layer-by-layer ordering of the aromatic cores. By isolating the functionalization to the terminal positions, the TMS groups are isolated to between the bc-planes that dominate charge transport. Furthermore, this motif allows us to study specifically the steric effects of the TMS groups on the in-plane packing of the aromatic core. Our previous thin-film structure studies showed that end substitution on the aromatic molecule with less bulky linear alkyl chains did not change the molecular packing motif. To examine the effect of the terminal TMS substitutions on the molecular packing, single crystals were grown using the Physical Vapor Transport technique and characterized by X-Ray diffraction (see SI for details). All of the compounds in Figure 1 formed thin, nearly two-dimensional platelet crystals, ideal for single crystal transistor device fabrication. The TMS-substituted compounds were especially stable, leaving behind no starting material, even if melted prior to crystal growth. The structures of the single crystals of quaterthiophene (4T) and TMS-substituted C O M M U N IC A TI O N www.advmat.de