Solvent-Molecule Interactions Govern Crystal-Habit Selection in Naphthalene Tetracarboxylic Diimides

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 core-chlorinated naphthalene diimide (NTCDI-1) and demonstrate the ability to tune its shape in thin films during post-deposition 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 post-deposition processing conditions a priori and to access prespecified crystal morphologies in thin films accordingly.