Indeno[1,2-b]fluorenes (IFs), while containing 4n π-electrons, are best described as two aromatic benzene rings fused to a weakly paratropic s-indacene core. In this study, we find that replacement of the outer benzene rings of an IF with benzothiophenes allows the antiaromaticity of the central s-indacene to strongly reassert itself. Herein we report a combined synthetic, computational, structural, and materials study of anti- and syn-indacenodibenzothiophenes (IDBTs). We have developed an efficient and scalable synthesis for preparation of a series of aryl- and ethynyl-substituted IDBTs. NICS-XY scans and ACID calculations reveal an increasingly antiaromatic core from [1,2-b]IF to anti-IDBT, with syn-IDBT being nearly as antiaromatic as the parent s-indacene. As an initial evaluation, the intermolecular electronic couplings and electronic band structure of a diethynyl anti-IDBT derivative reveal the potential for hole and / or electron transport. OFETs constructed using this molecule show the highest hole mobilities yet achieved for a fully conjugated IF derivative.

The functionalization of oligoacenes and similar π-conjugated chromophores with trialkylsilylethynyl groups has proven to be a versatile means to enhance solubility and solution processability and engineer solid-state packing arrangements to produce organic semiconductors that demonstrate outstanding charge-carrier transport characteristics. While a general, empirical-based geometric model has been developed and implemented to direct the solid-state packing arrangements of these molecular materials, there exist numerous examples where the model falters. Here, we employ electronic structure methods to probe the noncovalent, intermolecular interactions of two closely related systems that result in two very different crystal packing configurations: triisopropylsilylethynyl (TIPS) pentacene and its triethylsilylethynyl (TES) analog. The quantum-chemical evaluation details how the slightly larger electron density contained within the volume of the TIPS moiety with respect to TES is in part responsible for the solid-state packing variations. We also make use of periodic density functional theory (DFT) methods to develop in silico polymorphs of these systems and explore the electronic characteristics of varied packing arrangements. The results suggest that TES pentacene, if processed correctly, could be developed into a material with improved charge-carrier transport characteristics when compared to its native form. Overall, the theory-driven insight developed in this work lays an important foundation to build a more robust crystal engineering paradigm for these technologically relevant organic semiconductors.