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Organic Solids (organic + solid)
Selected AbstractsPhotoconduction in Amorphous Organic SolidsCHEMPHYSCHEM, Issue 5 2008Dirk Hertel Dr. Abstract Herein, we focus on the principles of photoconduction in random semiconductors,the key processes being optical generation of charge carriers and their subsequent transport. This is not an overview of the current work in this area, but rather a highlight of elementary processes, their involvement in modern devices and a summary of recent developments and achievements. Experimental results and models are discussed briefly to visualize the mechanism of optical charge generation in pure and doped organic solids. We show current limits of models based on the Onsager theory of charge generation. After the introduction of experimental techniques to characterize charge transport, the hopping concept for transport in organic semiconductors is outlined. The peculiarities of the transport of excitons and charges in disorderd organic semiconductors are highlighted. Finally, a short discussion of ultrafast transport and single chain transport completes the review. [source] Anisotropic surface chemistry of aspirin crystalsJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 8 2007Jerry Y.Y. Heng Abstract The wettability of the (001), (100), and (011) crystallographic facets of macroscopic aspirin crystals has been experimentally investigated using a sessile drop contact angle (,) method. , for a nonpolar liquid was very similar for all three facets, though significant , differences were observed for three polar probe liquids. The observed hydrophobicity of the (001) and (100) facets is ascribed to a reduced hydrogen bonding potential at these surfaces, whilst the observed hydrophilicity of facet (011) may be attributed to presence of surface carboxylic functionalities as confirmed by X-ray photoelectron spectroscopy (XPS). The dispersive component of the surface free energy (,) was similar for all three facets (35,±,2 mJ/m2). The total surface energy, ,s varied between 46 and 60 mJ/m2 due to significant variations in the polar/acid,base components of , for all facets. Surface polarity as determined by , measurements and XPS data were in good agreement, linking the variations in wettability to the concentration of oxygen containing surface functional groups. In conclusion, the wettability and the surface energy of a crystalline organic solid, such as aspirin, was found to be anisotropic and facet dependant, and in this case, related to the presence of surface carboxylic functionalities. © 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 96:2134,2144, 2007 [source] The effect of disorder on the chemical reactivity of an organic solid, tetraglycine methyl ester: Change of the reaction mechanismJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 2 2002Evgenyi Shalaev Abstract Many drugs undergo chemical changes in the solid state, and understanding chemical reactivity of organic crystals is a critical factor in the drug development process. In this report, the impact of milling on the thermal chemical reactivity of an organic solid, tetraglycine methyl ester, was studied using DSC, isothermal calorimetry, chemical analysis (HPLC and insoluble residue determination), and powder X-ray diffraction. Significant changes in both X-ray diffraction patterns and DSC curves were detected after very brief milling (5 s). The changes were interpreted as the formation of a disordered phase. The disordered phase was tentatively identified as a crystal mesophase that combines properties of both crystalline (i.e., long-range order) and amorphous (i.e., glass transition) states. In the disordered material, the reaction mechanism changed from the methyl transfer reaction, which was observed in the intact crystal, to a polycondensation reaction when the reaction was performed at 165°C. Such changes in the reaction mechanism occurred in materials milled for >,30 s. © 2002 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 91:584,593, 2002 [source] PSSP, a computer program for the crystal structure solution of molecular materials from X-ray powder diffraction dataJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2010Silvina Pagola This work describes the computer program PSSP (powder structure solution program) for the crystal structure solution of molecular solids from X-ray powder diffraction data. This direct-space structure solution program uses the simulated annealing global optimization algorithm to minimize the difference between integrated intensities calculated from trial models and those extracted in a Le Bail fit of the experimental pattern, using a cost function for dealing with peak overlap through defined intensity correlation coefficients, computationally faster to calculate than Rwp. The methodology outlined is applicable to organic solids composed of moderately complex rigid and flexible molecules, using diffraction data up to relatively low resolution. PSSP performance tests using 11 molecular solids with six to 20 degrees of freedom are analyzed. [source] Informatic calibration of a materials properties database for predictive assessment of mechanically activated disordering potential for small molecule organic solidsJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 8 2009Yannan Lin Abstract The potential for small molecule organic crystalline materials to become disordered as a result of high shear mechanical processing was investigated. A data-driven model was generated from a database of critical materials properties, which were expected to correlate with the potential of a small molecule organic crystalline solid to become fully disordered by the application of mechanical energy. The model was compared with a previously published disordering model based on fundamental thermodynamic relationships. Samples of 23 crystalline solids were subjected to extensive comminution under controlled temperature conditions; powder X-ray diffraction (PXRD) and differential scanning calorimetry (DSC) were used to confirm disordering. Logistic regression was used to investigate the significance of each materials property with respect to the prediction of disordering potential. Seven materials properties (glass transition temperature, melting temperature, heat of fusion, crystallographic density, Young's modulus, molar volume and attachment energy) were identified as having a significant correlation with the potential for material disordering. Stepwise multivariate logistic regression was used to further assess the correlation between disordering potential and each of the seven properties. A linear probability model based on two materials properties (glass transition temperature and molar volume) was developed for the prediction of disordering potential. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 98:2696,2708, 2009 [source] Hopping transport in 1D chains (DNA vs.PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2004Abstract We discuss charge transport in one-dimensional organic solids (DNA and discotic liquid-crystalline glass (DLC)), focusing on the effects of static and dynamic disorder. In the presence of static disorder it can be shown that the temperature dependence of the low-field mobility is , , exp [,(T0/T)2], with characteristic temperature T0 depending on the scale of the energy distribution of localized states responsible for transport. In the case of both static and dynamic disorder the situation is different. We obtain a temperature independent mobility in our molecular dynamics calculations in the case of large static and dynamic disorder compared to the energy overlap integral between the neighbouring sites. The theoretical results are in good agreement with experimental data. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Concentration dependence of the hopping mobility in disordered organic solidsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2004O. Rubel Abstract Traditionally the dependance of the drift mobility, ,, on the concentration of localized states, N, in disordered organic solids is plotted in the form , , exp[,C(N,3),p] with p = 1/3 and constant C. This representation cannot be correct, because transport in disordered organic solids is essentially a variablerange-hopping process with a weaker dependence ,(N). We study this dependence theoretically and show that both parameters p and C strongly depend on temperature and hence they are not universal. Only at very high temperatures the formula with p = 1/3 is valid. The result is significant in particular for a correct diagnostics of the localization length , from the measured dependence ,(N). (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Photoconduction in Amorphous Organic SolidsCHEMPHYSCHEM, Issue 5 2008Dirk Hertel Dr. Abstract Herein, we focus on the principles of photoconduction in random semiconductors,the key processes being optical generation of charge carriers and their subsequent transport. This is not an overview of the current work in this area, but rather a highlight of elementary processes, their involvement in modern devices and a summary of recent developments and achievements. Experimental results and models are discussed briefly to visualize the mechanism of optical charge generation in pure and doped organic solids. We show current limits of models based on the Onsager theory of charge generation. After the introduction of experimental techniques to characterize charge transport, the hopping concept for transport in organic semiconductors is outlined. The peculiarities of the transport of excitons and charges in disorderd organic semiconductors are highlighted. Finally, a short discussion of ultrafast transport and single chain transport completes the review. [source] | |||||||||||||