Absolute determination of the orientational order quality in a columnar discotic liquid crystal
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Absolute determination of the orientational order quality in a columnar discotic liquid crystal M. Ndao1, R. Lefort,1 P. Huber2, J.-M.Zanotti4, B. Frick5, A. Schoenhals6, D. Morineau1 IPR, UMR CNRS 6251, Université de Rennes 1, France [email protected] 2 Experimental Physics, Saarland University, D-66041 Saarbücken, Germany 4 LLB, CEA Saclay, France 5 Institut Laue Langevin, 6 rue Jules Horowitz, 38000 GRENOBLE, France 6 BAM Federal Institute for Materials Research and Testing; Unter den Eichen 87, 12205 Berlin, Germany
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ABSTRACT Simple NMR techniques can provide an absolute quantification of the quality of the orientational order of discotic columnar phases, provided the anisotropic local magnetic interactions are thoroughly characterized. For the prototypical discotic liquid crystal hexapentoxy-triphenylene, we measure the 13C chemical shift anisotropy of the triphenylene carbons, and use this result to analyze the orientational order, that occurs through a first order phase transition from the high temperature liquid phase, and is almost saturated (order parameter close to 0.85). INTRODUCTION Organic electronics is recognized throughout the world as one of the most promising industrial routes towards effective low-cost, portable and disposable elements such as tunable light emitting diodes (OLEDs), thin film field-effect transistors (OTFTs) or photovoltaic chips (OPVs) [1-4]. Although typical photovoltaic conversion efficiencies or the minimum typical integration dimensions of most organic-based or hybrid electronic devices still remain limited compared to the ones achieved by the silicon industry, the performance of organic devices has been considerably improved in the last decade, mainly due to a rapidly growing fundamental research on the chemistry of photoconductive polymers. Further developments with strong economic impacts are expected from hybrid materials such as Graetzel cells [5]. A considerable effort of research has been maintained in the last few years, leading to the emergence of new hybrid organic/inorganic thin film structures, taking advantage of self-organizing π-conjugated molecular species whose photoconductive properties can be enhanced by anisotropic coupling with highly ordered nanoporous matrices [6]. Among these systems, discotic columnar liquid crystals have attracted considerable attention, due to their peculiar ability to self-organize in close-packed molecular stackings, favouring strong orbital overlap in one dimension (columnar arrangement). Due to this highly anisotropic supramolecular arrangement, DCLCs have proven to outstand the performance of many photoconductive polymers, for instance in terms of charge carrier transport or short-lived excitonic response [7]. However, the design of DCLC based efficient electronic or photonic devices requires to achieve the formation of columnar phases over large length scales (monodomains), at room temperature, and also to control and tune the anisotropic parameters
(planar/homeotropic alignment) when realizing the coupling wit
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