Understanding the Structure and Electronic Properties of Molecular Crystals Under Pressure: Application of Dispersion Corrected DFT to Oligoacenes

Abstract

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.

Structural and excited-state properties of oligoacene crystals from first principles

“…2(a) with volumes calculated using the LDA, PBE, PBE-D2, PBE-TS (from Refs. [37,49]), DF1, DF2, and DF-cx approaches. A similar comparison for cohesive energies is given in Fig.…”

“…This low-temperature polymorph has been successfully described within the TS method in Ref. [49]. For pentacene, three well-known polymorphs are considered, using experimental structures available in the CSD [108].…”

“…In each case, we compare the computed geometry, electronic structure, and optical excitations with experiment, for both the gas phase and solid state. To account for long-range van der Waals (vdW) dispersive interactions, we use primarily nonlocal vdW density functionals (vdW-DFs), but also employ Grimme "D2" pairwise corrections [47] and compare our results where possible with previously reported data computed with the Tkatchenko-Scheffler (TS) [48] pairwise correction approach [37,49]. We find that the new consistent-exchange (cx) vdW density functional (vdW-DF-cx) [50,51] can predict acene lattice parameters within 1% of low-temperature measurements, as can the TS method.…”

A first-principles study of the vibrational properties of crystalline tetracene under pressure

“…A recent DFT study performed on tetracene and other molecular crystals, and containing a correction for the vdW interactions, correctly predicts the structural, electronic and optical properties of the crystals [19,20]. However extensions of these methods to phonon properties…”

Ab initiothermodynamic properties and their uncertainties for crystalline α-methanol

“…However, thermodynamic data is much scarcer at high pressures due to the complexity of the experiments and the associated uncertainties. Therefore, a reliable computational methodology capable of generating thermodynamic data for molecular crystals, even at extreme conditions, 1 would help to generate potentially useful data or to explain experimentally observed phenomena from the structural or molecular point of view. Most computational studies of molecular crystals neglect thermal contributions to thermochemical properties at finite temperatures and pressures, since calculating static cohesive electronic energies is much simpler than rigorously accounting for all relevant vibrational and thermal terms.…”