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A combined experimental and theoretical approach is used to solve the unknown crystal structure of para-cyano-quaterphenylene. Thin films are prepared by physical vapor deposition on isotropic surfaces, whereby a two-dimensional powder is formed. Using specular and grazing incidence X-ray diffraction, the crystallographic unit cell of the dominant polymorph phase is obtained. A monoclinic lattice with unit cell parameters ${\rm a}=5.56\,{\rm \AA}$, ${\rm b}=7.67\,{\rm \AA}$, ${\rm c}=20.85\,{\rm \AA}$, and β=97.4○is determined. These experimental parameters are used as input for ab initiocalculations within the framework of density-functional theory to determine the molecular packing within the crystal structure. Three types of molecular arrangements are found, exhibiting very close energies of formation. From the comparison of calculated structure factors with the experimentally observed intensities two solutions could be excluded. The molecule is found to pack in a herringbone pattern with layers of upright standing molecules.

The layer growth, thermal stability, and desorption kinetics of ultrahigh vacuum (UHV)-grown organic discoid molecules (hexaaza-triphenylene-hexacarbonitrile (HATCN)) on Ag(111) have been studied by atomic force microscopy (AFM), thermal desorption spectroscopy (TDS), low-energy electron diffraction (LEED), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). In the initial growth stage (≤0.24 nm mean film thickness) a strongly bonded monolayer of face-on oriented molecules is formed which does not desorb upon heating to 900 K, probably due to the formation of paracyanogen. With increasing film thickness the monolayer of face-on oriented molecules transforms at room temperature into a monolayer built up of edge-on oriented molecules, which saturates at a mean film thickness of around 0.8 nm. This layer partially decomposes during heating, and C2N2fragments desorb between 650 and 900 K. On the monolayer of standing molecules a second, metastable, layer forms, which saturates at 1.36 nm. With further increase of the coverage, this metastable layer dewets abruptly upon heating at about 350 K, and all molecules, besides those in the first monolayer, get incorporated into the island-like bulk phase. The formed three-dimensional (3D) islands, which are thermally stable up to 400 K, possess bulk crystal structure, as verified by XRD measurements, but the crystallites are randomly oriented.

A combined experimental and theoretical approach is used to solve the unknown crystal structure of para-cyano-quaterphenylene. Thin films are prepared by physical vapor deposition on isotropic surfaces, whereby a two-dimensional powder is formed. Using specular and grazing incidence X-ray diffraction, the crystallographic unit cell of the dominant polymorph phase is obtained. A monoclinic lattice with unit cell parameters a=5.56?, b=7.67?, c=20.85?, and ?=97.4?is determined. These experimental parameters are used as input for ab initio calculations within the framework of density-functional theory to determine the molecular packing within the crystal structure. Three types of molecular arrangements are found, exhibiting very close energies of formation. From the comparison of calculated structure factors with the experimentally observed intensities two solutions could be excluded. The molecule is found to pack in a herringbone pattern with layers of upright standing molecules.

Abstract: A combined experimental and theoretical approach is used to solve the unknown crystal structure of para-cyano-quaterphenylene. Thin films are prepared by physical vapor deposition on isotropic surfaces, whereby a two-dimensional powder is formed. Using specular and grazing incidence X-ray diffraction, the crystallographic unit cell of the dominant polymorph phase is obtained. A monoclinic lattice with unit cell parameters ${\rm a}=5.56\,{\rm \AA}$, ${\rm b}=7.67\,{\rm \AA}$, ${\rm c}=20.85\,{\rm \AA}$, and β=97.4○ is determined. These experimental parameters are used as input for ab initio calculations within the framework of density-functional theory to determine the molecular packing within the crystal structure. Three types of molecular arrangements are found, exhibiting very close energies of formation. From the comparison of calculated structure factors with the experimentally observed intensities two solutions could be excluded. The molecule is found to pack in a herringbone pattern with layers of upright standing molecules.