Multigrain crystallography / 3DXRD microscopy / Crystal structure determination / Polycrystal diffraction / Crystal indexing Abstract. We summarize exploratory work on multigrain crystallography. The experimental arrangement comprises a monochromatic beam, a fully illuminated sample with up to several hundred grains in transmission geometry on a rotary table and a 2D detector. Novel algorithms are presented for indexing, integration and filtering with emphasis on handling the complications of spot overlap and the need for on-line analysis. The structure solution and refinement steps are performed by conventional single crystal programs. Simulations are used to verify the algorithms and to probe the overall limitations of the methodology in terms of number of grains, size of unit cell and direct space resolution. First experimental results in the fields of chemistry, structural biology and time-resolved studies in photochemistry are presented. As an outlook, the concept of TotalCrystallography is introduced, defined as the simultaneous characterization of the 3D atomic, and 3D grain-scale structure of polycrystalline specimens with phases of unknown composition and structure.
The ultrafast structural dynamics of the [2+2] photocycloaddition of alpha-styrylpyrylium trifluoromethanesulfonate (TFMS) has been studied in great detail. During the photoreaction, optical and infrared spectroscopy confirms that crystals of alpha-styrylpyrylium change color. Since the reaction is reversible, it has been suggested to be used as an organic holographic storage device. The present photocrystallographic studies (with high spatial resolution) allow for an electron density analysis of the overall reaction kinetics, revealing the mechanism of bond-breaking and bond-formation. It could furthermore be proved how the reaction is influenced by the rearrangement of the surrounding moieties. Picosecond time-resolved X-ray diffraction studies allow for the monitoring the photoreaction in crystalline thin films under experimental conditions where the transformation times are greatly enhanced. These investigations are discussed in the context of the photocrystallographic results. It has been found that alpha-styrylpyrylium TFMS undergoes an ultrafast photoreaction to the dimer product state and back-reaction to the monomer reactant state which is temperature driven. The present experiments indicate that TFMS reacts on time scales which are the fundamental limiting ones of two-quantum systems and therefore has the potential to be used as an ultrafast organic molecular switcher.
Photoinduced structural variations in single crystals of 2,4-dichloro-trans-cinnamic acid (C9H6Cl2O2, DiClCA) have been investigated using X-ray diffraction (photocrystallography) and optical spectroscopic methods. During UV irradiation, which initiates the irreversible dimerization reaction, a loss of the long-range order of the reactant single crystal was found, i.e., that the dimerization is a heterogeneous one. This unexpected result emphasizes the still-existing problem of predicting changes or of remaining periodicity during chemical reactions in the solid state. On the basis of the experimental results, we propose a qualitative kinetic reaction scheme for DiClCA heterogeneous dimerization reaction.
A general outline of how to perform a light-excited time-resolved diffraction experiment by applying the optical pump/X-ray probe technique is given. Owing to the difference in penetration depths between the optical light (laser) pump and the X-ray probe, only specific or specially designed crystalline systems can be investigated, so special requirements have to be fulfilled concerning the sample and its compartments. A summary of the experimental conditions of optical pump/X-ray probe experiments is presented, emphasizing why the use of powder diffraction is a useful and necessary X-ray technique for this kind of experiment. The possibilities and bottlenecks of time-resolved X-ray diffraction on the picosecond time scale will be demonstrated in the powder diffraction studies of N,N-dimethylaminobenzonitrile and N,N-diisopropylaminobenzonitrile, where the photo-induced structural changes of these molecular organic systems have been studied as a function of time.
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