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 non-local 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 non-covalently bound molecular crystals

Phase transition between cubic and monoclinic polymorphs of the tetracyanoethylene crystal: the role of temperature and kinetics

Prediction of the relative stabilities and phase transition behavior of molecular crystalline polymorphs is highly coveted as distinct phases can possess different physical and chemical properties while having similar energies. Crystalline tetracyanoethylene (TCNE, C6N4) is known to exhibit rich solid state phase behavior under different thermodynamic conditions, as demonstrated by a wealth of experimental studies on this system. Despite this fact, the role of temperature and kinetics on the phase diagram of TCNE remains poorly understood. Here, first-principles calculations and high-resolution Fourier-transformed infrared (HR-FTIR) spectroscopy experiments are used to study the relative stabilities of the cubic and monoclinic phases of TCNE as a function of temperature. Specifically, density-functional theory with the van der Waals interactions method of Tkatchenko and Scheffler (DFT+vdW) is employed. The accuracy of this approach is demonstrated by the excellent agreement between the calculated and experimental structures. We find that the cubic phase is the most stable polymorph at 0 K, but becomes less favorable than the monoclinic phase at 160 K. This temperature-induced phase transition is explained on the basis of varying close contacts and vibrational entropies as a function of temperature. These findings are N vibrons

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 (PAR) 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

High-Throughput Investigation of the Geometry and Electronic Structures of Gas-Phase and Crystalline Polycyclic Aromatic Hydrocarbons

The quest for cheap, light, flexible materials for use in electronics applications has resulted in the exploration of soft organic materials as possible candidates, and several polycyclic aromatic hydrocarbons (PAHs) have been shown to be versatile (semi)conductors. In this investigation, dispersion inclusive density functional theory is used to explore all of the current crystalline PAHs within the Cambridge Structure Database (CSD) from both structural and electronic standpoints. Agreement is achieved between the experimental and calculated crystalline structures, as well as the electronic properties. Specifically, variation between the mass densities, unit cell parameters, and intermolecular close contact fractions were within +5\% +/-2\%, and +/-1 of experiment, respectively. It is found that a simple addition of a similar to 1 eV constant to the DFT-PBE gaps provides good agreement with the experimental optical gaps of both gas phase (within +/-2.6\%) and crystalline (within +/-3.5\%) PAHs. Structural and electronic analysis revealed several correlations/trends where ultimately limits in the band gaps as a function of structure are established. Finally, analysis of the difference between band gaps of the isolated molecules and crystals (Delta E-g(Xtal-Mols)) demonstrates that Delta E-g(Xtal-Mols) can be captured qualitatively by PBE and PBE0 functionals, yet significant quantitative deviations remain between these functionals and experiment

High-Throughput Investigation of the Geometry and Electronic Structures of Gas-Phase and Crystalline Polycyclic Aromatic Hydrocarbons

The quest for cheap, light, flexible materials for use in electronics applications has resulted in the exploration of soft organic materials as possible candidates, and several polycyclic aromatic hydrocarbons (PAHs) have been shown to be versatile (semi)­conductors. In this investigation, dispersion inclusive density functional theory is used to explore all of the current crystalline PAHs within the Cambridge Structure Database (CSD) from both structural and electronic standpoints. Agreement is achieved between the experimental and calculated crystalline structures, as well as the electronic properties. Specifically, variation between the mass densities, unit cell parameters, and intermolecular close contact fractions were within +5%, ±2%, and ±1% of experiment, respectively. It is found that a simple addition of a ∼1 eV constant to the DFT-PBE gaps provides good agreement with the experimental optical gaps of both gas phase (within ±2.6%) and crystalline (within ±3.5%) PAHs. Structural and electronic analysis revealed several correlations/trends, where ultimately limits in the band gaps as a function of structure are established. Finally, analysis of the difference between band gaps of the isolated molecules and crystals (Δ<i>E</i>_g^Xtal–Mol) demonstrates that Δ<i>E</i>_g^Xtal–Mol can be captured qualitatively by PBE and PBE0 functionals, yet significant quantitative deviations remain between these functionals and experiment

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