Photoconductivity of Discotic Liquid Crystals Derived from A Long-Chain Tetraphenylporphyrin
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Department of Organic Materials, Osaka National Research Institute (ONRI), AIST-MITI Midorigaoka 1-8-31, Ikeda, Osaka 563, Japan ABSTRACT Photoconductivity of a mesogenic tetraphenylporphyrin (TPP) is shown in relation to the mesomorphic phase transition behavior. The TPP mesogen shows two types of mesophases having smectic-like order (lamellar mesophases : DL and DLC phases named in this work) and the photocurrent changes stepwise at the phase transitions with non-Arrhenius dependence whilst the darkcurrent tends to be Arrhenius. The hole mobility was measured for the random domain cell and was found to be faster than 10-4 cm 2 V-1 sec-l in the mesophases. The cell used for the photocurrent measurements was a sandwich-type one using two ITO glass slides as electrtodes and show rectification behavior for the crystal and the low-temperature mesophase (DLc) in contrast to non-rectification of photocurrent in the high-temperature mesophase (DL). These properties of charge carrier migration as an electronic process are profoundly related to molecular order as well as molecular motions in mesophase. INTRODUCTION Columnar mesophases have a very attractive feature in their molecular arrangements for photonic and electronic applications involving charge and/or energy migrations because of molecular stacks in columns [1,2] which has been well-known to be suitable for quasi-onedimensional electronic conduction [3]. In addition, mesophases are expected to provide new technologies for obtaining molecularly homogeneous films with high performance utilizing relatively easy control of the molecular orientation. In fact, photoconductivity and electronic conductivity of mesomorphic materials have been extensively studied for several years [4-71. Especially, fast hole mobility of a columnar mesophase (0.1 cm 2 V- Is- 1 ) was reported for a plastic crystalline phase of a triphenylene discotic mesogen [8]. On the other hand, tetraphenylporphyrin is an interesting class of disc-shaped compounds as a candidate for materials applicable to thin film photonic devices because of its electronic structure posessing a large extended it-electronic conjugation system as well as metallation-derived functional properties [9]. The first discotic liquid crystal derived from porphyrin was an Uro-porphyrin substituted with eight long chains showing a monotropic columnar mesophase with a very narrow temperature range (0.1 'C) [10]. Another type of octasubstituted porphyrins and their metal complexes were synthesized and their columnar mesomorphism was revealed [I I]. The photovoltaic effect of a sandwich-type cell using ITO glasses as electrodes was also reported for crystalline solid of the mesogenic Zn complex and it was found the interaction between the electrode and excited porphyrins leads to generation of holes at the interaface [ 12]. However, the photocurrent drastically decreases at the crystal to mesophase transition [13]. Also tetraphenylporphyrin-based liquid crystals I In undergraduate study course joined from Department of Material Chemis
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