Origin of the anomalous X-ray diffraction in phthalocyanine films

The impact of the submolecular electron density on the X-ray diffraction profile of a layer-stacked thin film is studied experimentally and compared with numerical simulations based on the molecular structure and angular arrangement. Important structural information is contained in the X-ray diffraction profile of highly anisotropic molecular thin films, such as phthalocyanines. The results show that the intensity distribution of the diffraction peaks belonging to the same series of lattice planes provides important structural information including the molecular tilt angle and the center electron density of the molecule. Copyright c © EPLA, 2008 Introduction. – The electronic, optical, mechanical and other properties of solids depend critically and delicately on their physical and chemical arrangement of atoms [1]. Diffraction (X-ray, neutron, electrons) is a well-established method for the determination of the structure of solids and thin films. While the absolute X-ray diffraction (XRD) intensity is proportional to the square of the atomic index Z, generally the peak ratio of intensities of higher-order diffraction lines is independent of the Z number for a fixed structure; i.e. it is only determined by form factors, arising from the unit cell structure and the lattice configuration. Due to the complex molecular structure of metallo-organic solids it is customary in first approximation to model the structure of thin films based on the heavier metal ions neglecting the lighter organic atoms of the structure, or by introducing simple line shapes to simulate the contribution from the organic part [2–4]. We demonstrate here that including the organic ring is essential in diffraction studies of certain classes of organic solids, and of phthalocyanines in particular. Indeed, the diffraction by the organic portion of the molecules gives rise to unusual dependencies as a function of the Z-number of the metal ion. The diffraction peak ratios depend on the orientation of the molecules with respect to the substrates. This allows discrimination between various polymorphs of a molecule, which, in turn, has important consequences for the electronic and optical properties of thin films. (a)E-mail: [email protected] The macromolecular structure is routinely reconstructed from single crystal or powder X-ray diffraction data using a refinement procedure [5,6]. Usually, the important phase information is lost in the ordinary diffraction experiment, although there are exceptions for specific experimental conditions [7,8]. Classical techniques to retrieve phase information include molecular replacement [9], isomorphous replacement [10], and anomalous scattering [11], which perturb the diffraction pattern physically or chemically in order to partially deduce phase information. For monolayers, it is also feasible to measure the molecular tilt angle using Brewster angle microscopy [12,13]. However, for multilayered thin films, the experimental data obtained from such methods is incomplete and cannot recover the full molecular structure. Furthermore, off-specular diffraction profiles yield only very limited information, such as in-plane coherence length due to the polycrystalline nature of most organic thin films. In this case, the precise submolecular structure of anisotropic molecules in thin films is commonly ignored [3,4,14]. Even so the destructive interference of large, anisotropic molecules profoundly impacts the XRD intensities of higher-order diffraction peaks. This allows inferring molecular orientations (tilt angles) and discriminating between different polymorphs using the high-angle X-ray diffraction data shown here. A key class of organic solids, such as quaterthiophene (4T), 3, 4, 9, 10 perylenetetracarboxylic dianhydride (PTCDA), 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST), and others [15–17], are constructed