Discotic Liquid Crystalline Porphyrins: Photophysical and Photoelectrical Properties of Large-Area Crystalline Films
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DISCOTIC LIQUID CRYSTALLINE PORPHYRINS: PHOTOPHYSICAL AND PHOTOELECTRICAL PROPERTIES OF LARGE-AREA CRYSTALLINE FILMS BRIAN A. GREGG*, MARYE ANNE FOX** AND ALLEN J. BARD" *University of Texas at Austin, Dept. of Chemical Engineering, Austin, TX 78712. "**Universityof Texas at Austin, Dept. of Chemistry, Austin, TX 78712. ABSTRACT Porphyrins exhibiting discotic liquid crystalline phases have been developed in order to prepare thin, large-area, crystalline films of molecular conductors. A series of octaalkylporphyrins bearing different side chains have been synthesized including some with electron-withdrawing substituents at the meso positions. The photophysical properties of thin films of these compounds are a strong function of the film order (crystallinity). A substantial and persistent photovoltaic effect was achieved (e.g., Voc - 0.3 V, jis - 0.4 mA/cm 2 under white light, 150 mW/cm 2 ) in capillary-filled symmetrical cells with indium-tin oxide electrodes. A model based on kineticallycontrolled asymmetric exciton dissociation leading to photoinjection at the illuminated interface is presented to explain these results. This appears to be the first unambiguous example of a photovoltaic cell controlled entirely by interfacial kinetics. The predominance of the photoinjection process in these cells is attributed to the single-crystal-like character of the porphyrin films. INTRODUCTION The self-ordering properties of liquid crystals (LCs) can be used to obtain large-area arrays of ordered materials. For example, single (liquid) crystal devices covering hundreds of square centimeters are easily obtained by capillary-filling the isotropic liquid followed by slow cooling into the LC phase. If liquid crystals can be made electroactive, these inexpensive, thin, large-area devices could find a number of applications in such areas as molecular electronics, the matrix addressing of flat panel displays and the conversion of solar energy. We describe here the photophysical and photoelectrical properties of some of the recently synthesized liquid crystal porphyrins (LCPs) [ 1,21. Their properties are a strong function of the degree of crystallinity of the films [31; this report will primarily be concerned with the most highly ordered films. The chemical structure of the LCPs is shown in Figure Ia while a schematic diagram of the columnar discotic liquid crystal phase is given in Figure lb. Two families of LCPs have been synthesized: most of the octaesters (R - COOCnH 2n+I in Figure la) exhibited two columnar discotic mesophases before melting to isotropic liquids [1]; in contrast, the metallo
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Materials Research Society
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