Structure Solution of the 6,13-Pentacenequinone Surface-Induced Polymorph by Combining X-ray Diffraction Reciprocal-Space Mapping and Theoretical Structure Modeling

Abstract

Thin films of 6,13-pentacenequinone (PQ) on native silicon oxide (SiO x ) grown in a yet unsolved surface-induced thin-film polymorph were investigated by X-ray diffraction reciprocal-space mapping yielding triclinic unit-cell parameters of a = 4.69 A ˚, b = 5.99 A ˚, c = 13.45 A ˚, R = 77.8°, β = 84.1°, and γ = 81.1°. Most notably, this polymorph exhibits only one molecule per unit cell in contrast to the single crystal phase with two molecules per unit cell. Therefore, in contrast to the single crystal phase, PQ does not exhibit a herringbone arrangement in the substrate-induced thin-film polymorph. The orientation of the molecule within the unit cell was modeled via force field calculations yielding a reliable full structure solution. Being corroborated by a direct derivation from the experimental intensities, this procedure represents a reliable pathway for full structure solutions of thin-film polymorphs. The results allowed for comparative electronic structure calculations of the two PQ polymorphs, which demonstrate a significantly wider dispersion of the lowest unoccupied molecular orbital-derived band (conduction band) in the surface-induced phase, which might be beneficial for electron transport.

Toward a comprehensive understanding of molecular doping organic semiconductors (review)

“…Importantly, charge transport in a working OFET is confined to percolation pathways in the vicinity to the gate dielectric, e.g., in the first two monolayers for PEN based OFETs [70], where both structure, molecular orientation, and the overall structural quality of the film (e.g., the grain-boundary density [71,72]) are crucial for the charge carrier mobility and, consequently, the overall OFET performance [73]. In particular, OSCs tend to adopt molecular arrangements in the vicinity of a substrate that can significantly differ from their bulk crystal structures, that is, substrate-mediated polymorphs can occur in thin-films of application relevant OSCs [74][75][76][77][78][79][80][81]. Apart from structural order, also device geometry and the energy levels of the materials employed are decisive for the device functionality.…”

Multiple scattering in grazing-incidence X-ray diffraction: impact on lattice-constant determination in thin films

“…Particular interest lies in investigating new polymorphs, which frequently occurs when crystallization processes take place in the presence of a substrate (Wedl et al, 2012). The solution of such polymorphs, however, still remains challenging, as no universal procedure has been developed so far (Salzmann et al, 2011;Mannsfeld et al, 2011;Krauss et al, 2008;Nabok et al, 2007). Apart from fully theoretical approaches like attempts to predict crystal structures directly from the chemical formulae of the constituents (Della Valle et al, 2008), the first step towards crystal structure determination of (ultra) thin films is deriving the unit-cell parameters on the basis of the experimental Bragg peak positions of GIXD data.…”

“…Apart from fully theoretical approaches like attempts to predict crystal structures directly from the chemical formulae of the constituents (Della Valle et al, 2008), the first step towards crystal structure determination of (ultra) thin films is deriving the unit-cell parameters on the basis of the experimental Bragg peak positions of GIXD data. Subsequently, the molecular packing is either determined from the experimental peak intensities (Schiefer et al, 2007;Mannsfeld et al, 2011;Salzmann et al, 2012;Pichler et al, 2014) or by theoretical modelling (Nabok et al, 2007;Salzmann et al, 2011) with the experimental data used to evaluate the quality of the result.…”

Substrate-Induced Phase of a [1]Benzothieno[3,2-b]benzothiophene Derivative and Phase Evolution by Aging and Solvent Vapor Annealing

“…6,7 These polymorphic phases, which are typically not found as single crystal structures, were first observed in films of the prototypical organic semiconductor pentacene, [8][9][10][11][12][13] and have since been found in other systems. [14][15][16][17][18][19][20] In films of pentacene, the SIP is found to be less energetically favorable than the bulk phase in isolation, but is stabilized close to the substrate due to improved compatibility of the structure with the flat surface of the substrate such that in the presence of the substrate it is the most stable form. 21,22 The structural difference between the SIPs and bulk phases of π conjugated molecules is most often related to a small change in the tilt angle between the approximately upright standing molecules and the substrate, resulting in more favorable molecule substrate interactions and a decrease in the out of plane lattice spacing such that the two phases are similar but distinct from one another.…”