Packing of Planar Organic Molecules: Interplay of van der Waals and Electrostatic Interaction

Abstract

The molecular packing motifs occurring in the crystal structures of pentacene and its two oxo-derivatives (6,13pentacenequinone and 5,7,12,14-pentacenetetrone) have been analyzed. Both oxygen containing species exhibit an almost coplanar stacking while pentacene adopts a face-on-edge herringbone packing. The different packing motifs are well explained by quantum chemical ab initio calculations of the electronic structure of the molecular entities exhibiting a pronounced charge localization at the oxygen atoms of both oxo-derivatives which causes an additional electrostatic O-π interaction favoring a planar stacking. Moreover, the polarizability of the π-system is reduced and the molecular quadrupole moment is altered, both resulting in a decrease of the lattice energy of the oxo-species as evidenced by the sublimation energy obtained for all three species from thermal desorption measurements. This emphasizes the importance of the balance between electrostatic and van der Waals interactions for the packing in molecular crystals.

Rubrene: The Interplay between Intramolecular and Intermolecular Interactions Determines the Planarization of Its Tetracene Core in the Solid State

“…[1][2][3] Importantly, polymorphs can display markedly different electronic properties 3 simply due to the variations in molecular packing. 4,5 While the solid-state conformation and packing of organic molecules intimately depends on the growth conditions, the chemical structure plays an obvious, defining role. [6][7][8][9] A classic example is tetracene functionalized with phenyl rings at the 5-, 6-, 11-, and 12-positions, a compound referred to as rubrene, 1 (see Figure 1).…”

“…12,13,14,15,16 However, electronic-structure calculations on isolated rubrene molecules show that the presence of the side phenyl rings makes the tetracene backbone of rubrene preferentially twist (~40°), [17][18][19][20] which is confirmed by experimental evidence of twisted conformations both in solution 18,21 and thin films. 17,22 Interestingly, examination of rubrene thin films grown on Au(111) surfaces reveals that the twisted conformation is present in the initial layers and then transitions solely to the planar 4 conformation, an indication of the decisive influence that surrounding molecules play in leading to a planar conformation in the bulk molecular structure. 17 Figure 1.…”

Redox-controlled potassium intercalation into two polyaromatic hydrocarbon solids

“…14 The largest voids in the structure are located between the picene layers, adjacent to the saturated C-H bonds and far from the electron density of the PAH π-systems ( Figure 1a). This contrasts with C60, where the octahedral and tetrahedral voids in the fcc lattice are adjacent to the conjugated π-electron system (Figure 1b) 15 .…”

Lattice Matching as the Determining Factor for Molecular Tilt and Multilayer Growth Mode of the Nanographene Hexa-peri-hexabenzocoronene

“…A planar stacking arrangement in the crystalline lattice is frequently found for substituted PAHs (e.g., PTCDA, pentacenetetrone) which exhibit notable polarity within the molecular π-system and thus lead to additional electrostatic interactions. 50 In contrast, at present there is no comprehensive theory which explains decisively why some compounds nucleate in a new polymorph with planar configuration, which remains stable even in multilayer films while others relax into their bulk structure. It may be expected that in the near future crystal prediction theory will allow for such projections.…”