Bohdan Schatschneider

Understanding the structure and electronic properties of molecular crystals under pressure: application of dispersion corrected DFT to oligoacenes.

Oligoacenes form a fundamental class of polycyclic aromatic hydrocarbons (PAH) which have been extensively explored for use as organic (semi) conductors in the bulk phase and thin films. For this reason it is important to understand their electronic properties in the condensed phase. In this investigation, we use density functional theory with Tkatchenko-Scheffler dispersion correction to explore several crystalline oligoacenes (naphthalene, anthracene, tetracene, and pentacene) under pressures up to 25 GPa in an effort to uncover unique electronic/optical properties. Excellent agreement with experiment is achieved for the pressure dependence of the crystal structure unit cell parameters, densities, and intermolecular close contacts. The pressure dependence of the band gaps is investigated as well as the pressure induced phase transition of tetracene using both generalized gradient approximated and hybrid functionals. It is concluded that none of the oligoacenes investigated become conducting under elevated pressures, assuming that the molecular identity of the system is maintained.

Fragment-based 13C nuclear magnetic resonance chemical shift predictions in molecular crystals: An alternative to planewave methods

We assess the quality of fragment-based ab initio isotropic (13)C chemical shift predictions for a collection of 25 molecular crystals with eight different density functionals. We explore the relative performance of cluster, two-body fragment, combined cluster/fragment, and the planewave gauge-including projector augmented wave (GIPAW) models relative to experiment. When electrostatic embedding is employed to capture many-body polarization effects, the simple and computationally inexpensive two-body fragment model predicts both isotropic (13)C chemical shifts and the chemical shielding tensors as well as both cluster models and the GIPAW approach. Unlike the GIPAW approach, hybrid density functionals can be used readily in a fragment model, and all four hybrid functionals tested here (PBE0, B3LYP, B3PW91, and B97-2) predict chemical shifts in noticeably better agreement with experiment than the four generalized gradient approximation (GGA) functionals considered (PBE, OPBE, BLYP, and BP86). A set of recommended linear regression parameters for mapping between calculated chemical shieldings and observed chemical shifts are provided based on these benchmark calculations. Statistical cross-validation procedures are used to demonstrate the robustness of these fits.

Electrodynamic response and stability of molecular crystals

We show that electrodynamic dipolar interactions, responsible for long-range fluctuations in matter, play a significant role in the stability of molecular crystals. Density functional theory calculations with van der Waals interactions determined from a semilocal “atom-in-a-molecule” model result in a large overestimation of the dielectric constants and sublimation enthalpies for polyacene crystals from naphthalene to pentacene, whereas an accurate treatment of nonlocal electrodynamic response leads to an agreement with the measured values for both quantities. Our findings suggest that collective response effects play a substantial role not only for optical excitations, but also for cohesive properties of noncovalently bound molecular crystals.

Effect of crystal packing on the excitonic properties of rubrene polymorphs

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.

Simulated pressure response of crystalline indole

The isostatic pressure response of crystalline indole up to 25 GPa was investigated through static geometry optimization using Tkatchenko-Scheffler dispersion-corrected density functional theory method. Different symmetries were identified in the structural evolution with increased pressure, but no motif transition was observed, owing to the stability of the herringbone (HB) motif for small polycyclic aromatic hydrocarbons. Hirshfeld surface analysis determined that there was an increase in the fraction of H···π and π···π contacts within the high pressure structures, while the fraction of H···H contacts was lowered via geometric rearrangements. It was found that isostatic pressure alone, up to 25 GPa, was not sufficient to induce a chemical reaction due to the poor π-orbital overlap existing within the HB motif. However, the applied pressure sets the stage for an activated chemical reaction when the molecules approach each other along the long molecular axis, with a reaction energy and reaction barrier of 1.05 eV and 1.80 eV per molecular unit, respectively.

A new parameter for classification of polycyclic aromatic hydrocarbon crystalline motifs: a Hirshfeld surface investigation

The classical crystalline motif categorizations of polycyclic aromatic hydrocarbons (PAHs) have gained interest as of late as it was discovered that the “defining” crystallographic axis was not always the “shortest crystallographic axis” as previously defined and that systems under pressure would exhibit a motifs typical axis length but not its typical interplanar angle (θ). Here we use Hirshfeld surfaces to investigate the relative percent of intermolecular close contact interactions existing within the four established crystalline PAH motifs under ambient and high pressures. It was discovered that the fraction of C⋯C interactions (C⋯C%) coupled with θ could ultimately define the structural motifs. A new parameter, the pi-degree parameter (π°), which relates the amount of π⋯π interaction to the interplanar angle, was developed as a function of θ and C⋯C% in order to unambiguously categorize PAH motifs. A new motif, beta-herringbone (β-HB), defining a subset of PAH structures with σ-bound aromatic groups is defined. The new motif parameter was then used to investigate several high pressure and low temperature PAH structures and provided unambiguous motif categorization regardless of the “shortest crystallographic axis”, temperature, or pressure.

Molecular simulation of the pressure-induced crystallographic phase transition of p-terphenyl.

The pressure- and temperature-induced polymorphic crystal phase transitions of p-terphenyl (PTP) have been modeled using a modified PCFF interaction force field. Modifications of the interaction potential were necessary to simultaneously model both the temperature-induced phase transition at ambient pressure and the pressure-induced phase transition at low temperature. Although the high-temperature and high-pressure phases are both characterized by flattening of the PTP molecule, the mechanisms of the temperature- and pressure-induced phase transitions are different. At high temperature thermal energy exceeds the torsional barrier, resulting in a bimodal phenyl ring twist angle distribution that averages to zero. In contrast, compression of PTP at high pressure results in a static planar structure. At high pressure the compression of the unit cell is also characterized by large compression of the a lattice parameter and weak compression of c, but some expansion of the b lattice parameter. The expansion of the b lattice parameter is likely associated with pressure-induced soft mode behavior of some lattice vibrations as well as soft mode behavior of pseudolocal phonons associated with impurities in PTP. The crystallographic angles α, β, and γ also indicate a triclinic crystal phase above the critical phase transition pressure of P(c) ~ 0.5 GPa at low temperature, suggesting a distinct phase separate from the monoclinic high-pressure phase at high temperature.

Crystal structure of the meta-stable intermediate in the photomechanical, crystal-to-crystal reaction of 9-tert-butyl anthracene ester

The photodimerization of 9-tert-butyl-anthracene ester in molecular crystal nanorods can cause expansions of up to 15%. This expansion results from the formation of a metastable crystalline intermediate, the solid-state reacted dimer (SSRD). In this paper, a combination of powder X-ray diffraction, solid-state nuclear magnetic resonance, and computational modeling is used to determine the crystal structure of the SSRD intermediate. An ensemble of six possible structures is obtained, which differ only in small packing details that lead to different crystal space groups. The structure with the best overall agreement with the experimental data belongs to the Pccn space group and, like the other members of the ensemble, retains a packing motif close to that of the monomer crystal. This crystal structure allows us to conclude that the SSRD crystal density is slightly greater than that of the monomer crystal and that gross changes in the volume or unit cell parameters of the SSRD are not responsible for the photoinduced expansion.

Molecular dynamics simulations of temperature- and pressure-induced solid–solid phase transitions in crystalline para-terphenyl

Molecular dynamics (MD) simulations of pressure- and temperature-induced solid–solid phase transitions in para-terphenyl have been investigated using Material Studio®. Initial simulations were performed using the COMPASS (condensed-phase optimised molecular potentials for atomistic simulation studies) force field to evaluate its ability to model the known temperature and pressure phase boundary between the triclinic and monoclinic crystal phases. Geometry optimisation using the universal COMPASS force field could not adequately model the experimental crystal structure at 113 K, and MD simulations could not adequately reproduce the known transition temperature at ambient pressure, nor yield a well-defined transition pressure at low temperature. However, a one-parameter optimisation of the torsion potential component of the polymer-consistent force field (PCFF) (incorporating COMPASS non-bond parameters) yielded MD simulations that accurately modelled the pressure–temperature boundary between the low-temperature low-pressure triclinic phase and the high-pressure high-temperature monoclinic phase of para-terphenyl.

On the possibility of singlet fission in crystalline quaterrylene

Singlet fission (SF), the spontaneous down-conversion of a singlet exciton into two triplet excitons residing on neighboring molecules, is a promising route to improve organic photovoltaic (OPV) device efficiencies by harvesting two charge carriers from one photon. However, only a few materials have been discovered that exhibit intermolecular SF in the solid state, most of which are acene derivatives. Recently, there has been a growing interest in rylenes as potential SF materials. We use many-body perturbation theory in the GW approximation and the Bethe-Salpeter equation to investigate the possibility of intermolecular SF in crystalline perylene and quaterrylene. A new method is presented for determining the percent charge transfer (%CT) character of an exciton wave-function from double-Bader analysis. This enables relating exciton probability distributions to crystal packing. Based on comparison to known and predicted SF materials with respect to the energy conservation criterion (ES-2ET) and %CT, crystalline quaterrylene is a promising candidate for intermolecular SF. Furthermore, quaterrylene is attractive for OPV applications, thanks to its high stability and narrow optical gap. Perylene is not expected to exhibit SF; however, it is a promising candidate for harvesting sub-gap photons by triplet-triplet annihilation.