Singlet fission (SF), the conversion of one singlet exciton into two triplet excitons, may lead to the realization of high efficiency organic photovoltaics by generating two carriers from one photon. Recently, SF has been observed in molecular crystals of rubrene. While the orthorhombic form of rubrene is most often observed under ambient conditions, metastable monoclinic and triclinic polymorphs are known. Here, dispersion-inclusive density functional theory (DFT) is used to investigate the relative stability of all three phases. Many-body perturbation theory is then employed to study the effect of crystal structure on the electronic and excitonic properties. Band structures are calculated within the GW approximation and optical properties are calculated by solving the Bethe-Salpeter equation (BSE). We find that crystal packing significantly affects the electronic and excitonic properties of rubrene. Based on our calculations, the triclinic and especially the monoclinic forms of rubrene are expected to exhibit higher SF efficiencies than the orthorhombic form.

External pressure is known to alter the molecular and structural conformations of soft materials, leading to changes in the intermolecular interactions as well as the inherent physical properties. In part 1 of a two-part investigation, we introduce pressure within the dispersion-inclusive density functional theory framework (DFT + vdW) to perturb the structures and intermolecular interactions of 40 crystalline, herringbone polycyclic aromatic hydrocarbons. The applied pressure results in alterations of the crystalline unit cells, intermolecular interactions, and molecular conformations. In general, the unit cell lengths/volumes decrease monotonously with increasing pressure. Hirshfeld surface analysis typically reveals an increase in the C···H and C···C intermolecular close-contact fractions with increased pressure and a decrease in the H···H interactions. The increase in the C···H and C···C intermolecular interactions enhances the C–H···π and π···π interactions, decreasing intermolecular repulsion and increasing electron exchange interactions through increased molecular orbital overlap. Discontinuous pressure-dependent changes in the unit cell parameters and intermolecular close-contact fractions of several structures are observed, indicating the possibility of some phase transitions. In part 2 of this two-part investigation, the structural changes observed here are linked to changes in the electronic properties of these systems.

Understanding the effect intermolecular interactions have on the electronic properties of highly conjugated/aromatic organic networks is important for optimizing these materials for optoelectronic device applications. Here, dispersion inclusive density functional theory (DFT + vdW) is used to study the effect of pressure up to 20 GPa on the intermolecular interactions of 40 herringbone polycyclic aromatic hydrocarbons. In the first part of this twopart study (10.1021/acs.jpcc.8b07209), we reported the pressure-induced structural changes. Here, we elucidate the relation between those structural changes and the electronic properties, where it is shown that increased pressure leads to variations in the intermolecular interactions and molecular conformations, resulting in alterations of the band dispersion, band gap (magnitude as well as direct/indirect), and semiconductor polarity (n-type vs p-type). Specifically, increased pressure increases the C−H•••π and π•••π interactions, typically leading to increased intermolecular orbital overlap of the frontier molecular orbitals, resulting in increased intermolecular coupling and band dispersion. In general, the increased intermolecular coupling and band dispersion yields decreased band gaps and increased crystalline polarizabilities, although some variation in these trends occur. The majority of structures follow similar trends. However, some exhibit anomalous pressure responses, including switching between n-type and p-type polarity, transitions between direct/indirect gaps, and discontinuities in the pressure-dependent band gap curves.

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