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 triisopropyl­silylethynyl­pentacene (TIPS-Pn) to produce triisopropyl­germanyl­ethynyl­pentacene (TIPGe-Pn), with the TIPGe side group comparable in size to TIPS, but higher in electron density. We find TIPGe-Pn single crystals exhibit slip-stack, herringbone, and brickwork packing motifs depending on growth conditions, a stark contrast to TIPS-Pn, which accesses only 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 postdeposition annealing. Our experiments suggest that access to the structural phase space is not guided solely by the size of the side group; the electronic character of the side group in functionalized compounds also plays a significant role. As such, simple atomistic substitutions can cause significant differences in the accessible solid structures.

Polymorphism is pervasive in molecular solids. While computational predictions of the molecular polymorphic landscape have improved significantly, identifying which polymorphs are preferentially accessed and experimentally stable remains a challenge. We report a framework that correlates short intermolecular contacts with polymorphic stability. The presence of short contacts between neighboring molecules prevents structural rearrangement and stabilizes the packing arrangement, even when the stabilized polymorph is not enthalpically favored. In the absence of such intermolecular short contacts, the molecules have added degrees of freedom for structural rearrangement, and solid-solid polymorphic transformations occur readily. Starting with a series of core-halogenated naphthalene tetracarboxylic diimides, we establish this framework with the packing polymorphs of more than 20 compounds, ranging from molecular semiconductors to pharmaceutics and biological building blocks. This framework, widely applicable across molecular solids, can help refine computational predictions by identifying the polymorphs that are kinetically stable.

Controlling hierarchical structural development in organic semiconductors, and across synthetic materials more broadly, is critical to the performance of the material in device applications. Such regulation across multiple scales, from the atomic level to the macroscale, however, is a challenging task given the often-heterogeneous nature of interactions in the processing environment that determines the kinetics and thermodynamics of material growth. Here, we elucidate factors that govern the crystal habit of a corechlorinated naphthalene diimide (NTCDI-1) and demonstrate the ability to tune its shape in thin films during postdeposition solvent−vapor annealing. Judicious selection of solvent choice and solvent−vapor concentration controls the growth kinetics along different crystallographic axes, and, thus, the resulting habit; we can access isotropic plates that span hundreds of microns to highly anisotropic needles whose long axis can be many millimeters of crystals adopting the same packing polymorph. We find the growth rate along the π-stacking direction of NTCDI-1 during solvent−vapor annealing to scale with its solubility in the solvent and the solvent's viscosity and dielectric constant, with the two former facilitating plasticization. The dielectric constant of the solvent matters because it captures NTCDI-1−solvent interactions. Polar solvents promote π-interactions between neighboring NTCDI-1 molecules, whereas aromatic solvents disrupt these same interactions. Our quantitative understanding of the factors governing crystal-habit selection affords the ability to determine proper postdeposition processing conditions a priori and to access prespecified crystal morphologies in thin films accordingly.

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