|Muth, M.A.; Carrasco-Orozco, M.; Thelakkat, M.: Liquid-Crystalline Perylene Diester Polymers with Tunable Charge-Carrier Mobility, Advanced Functional Materials, 21(23), 4510-4518 (2011) -- DOI: 10.1002/adfm.201101694|
New classes of liquid-crystalline semiconductor polymers based on perylene diester benzimidazole and perylene diester imide mesogens are reported. Two highly soluble side-chain polymers, poly(perylene diester benzimidazole acrylate) (PPDB) and poly(perylene diester imide acrylate) (PPDI) are synthesized by nitroxide-mediated radical polymerization (NMRP). PPDB shows n-type semiconductor performance with electron mobilities of 3.2 x 10(-4) cm2 V(-1) s(-1) obtained in a diode configuration by fitting the space-charge-limited currents (SCLC) according to the MottGurney equation. Interestingly, PPDI performs preferentially as a p-type material with a hole mobility of 1.5 x 10(-4) cm2 V(-1) s(-1), which is attributed to the less electron-deficient perylene core of PPDI compared to PPDB. Optical properties are investigated by UV-vis and fluorescence spectroscopy. The extended p-conjugation system due to the benzimidazole unit of PPDB leads to a considerably broader absorption in the visible region compared to PPDI. HOMO and LUMO levels of the polymers are also determined by cyclic voltammetry; the resulting energy band-gaps are 1.86 eV for PPDB and 2.16 eV for PPDI. Thermal behavior and liquid crystallinity are studied by differential scanning calorimetry, polarized optical microscopy, and X-ray diffraction measurements. The results indicate liquid-crystalline order of the polymers over a broad temperature range. These thermal, electrical, and optical properties make the perylene side-chain polymers attractive materials for organic photovoltaics.