Understanding the molecular mechanism of polymorphic transition is essential for controlling molecular packing for high-performance organic electronics. Polymorphic transition in molecular crystals mostly follows the nucleation and growth mechanism. We recently discovered a cooperative polymorphic transition in organic semiconductor single crystals driven by bulky side-chain rotation. In this work, we demonstrate that a single atom substitution in the side-chains from carbon to silicon can completely alter the transition pathway from a cooperative transition to nucleation and growth. We reveal that bulkier side-chains become interlocked to inhibit side-chain rotation and thereby hinder molecular cooperativity to lead to the nucleation and growth mechanism. We report the utilities of both types of transitions in organic electronic devices. Nucleation and growth allows kinetic access to metastable polymorphs at ambient conditions for structure− property study. On the other hand, cooperative transition enables in situ and reversible access to polymorphs for rapid modulation of electronic properties while maintaining structural integrity. Using this simple molecular design rule, we can access both polymorphic transition pathways and selectively utilize their advantages in organic electronic applications.

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.

In this study, we use a nonplanar aromatic molecule, contorted octabenzo­circumbiphenyl (c-OBCB), to sort semiconducting single-walled carbon nanotubes (SWNTs) by their chiral angles. From absorption spectroscopy, photoluminescence excitation spectroscopy, and Raman spectroscopy studies, we find that c-OBCB preferentially binds and sorts for a number of semiconducting carbon nanotubes with chiral angles greater than 12°. Molecular dynamics simulations reveal that the contorted aromatic core of c-OBCB binds strongly to only certain SWNTs, especially those with matching curvature, and that this discriminatory binding interaction is reinforced by preferences of the side chains on the c-OBCB to stick to SWNT surface rather than interact with the solvent. This opens the door to side chain/solvent engineering to bias the selection of certain (m,n) SWNT variants. We also investigate the temperature dependence of hole mobility in field-effect transistors comprising c-OBCB-sorted semiconducting carbon-nanotube networks and find hole transport in these networks to be thermally activated.

Two woven covalent organic framework materials (COF-505 and COF-506) have been synthesized since 2016, and the latter demonstrated the ability to take up dyes and other small molecules. This opens the door to applications such as separations, sensing, and catalysis. However, accelerating the design of future woven materials by changing the chemistry of the “threads” will require a computational model for these materials. Since no such atomic-scale model exists, we have developed a protocol for optimizing a force field for woven materials which can be used as the input to molecular dynamics simulations. Their high density and elasticity made these COFs challenging to model at a semiempirical level. Our modeling approach required simultaneous optimization of lattice parameters and elasticity using density functional theory-derived energy barriers and available experimental results. We used this force field, parameterized to fit COF-505, without change, to predict the structure of COF-506. This model allowed us to predict an anisotropy in 505’s elasticity and preferred directions for diffusion which cannot be seen experimentally. The pore size distribution for 506 is dominated by small pores (80% <10 Å dia.), though 5% of the pores are up to 20 Å in diameter. We confirmed the experimental result that gases (barring helium) do not diffuse appreciably in COF-505. We validated our (unaltered) force field model to accurately predict experimental uptake data for tetrahydrofuran and methyl orange dye in COF-506. We proposed an atomic-scale mechanism by which COF-505 becomes metallated and demetallated. In addition, in advance of experimental studies, we determined the ability of 505 to incorporate other metals, such as Zn and Fe, which might be considered artificial photosynthesis agents. These predictions validate that Cu was a particularly appropriate choice of metal center for the synthesis, showcasing the ability of this model to play a role in designing woven materials a priori.