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Effect of Chain Length on Thermal Conversion of Alkoxy-Substituted Copper Phthalocyanine Precursors

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journal contribution
posted on 21.11.2011, 00:00 by Takamitsu Fukuda, Yuu Kikukawa, Ryota Tsuruya, Akira Fuyuhiro, Naoto Ishikawa, Nagao Kobayashi
A series of dialkoxy-substituted copper phthalocyanine (CuPc) precursors (4a4d) have been prepared by treating phthalonitrile with the corresponding lithium alkoxide under mild conditions. The precursors exhibited high solubilities in common organic solvents, including acetone, toluene, tetrahydrofuran (THF), CH2Cl2, and CHCl3. Elongation of the alkoxy chains improved the solubilities of the precursors effectively, and accordingly, the butoxy-substituted derivative (4d) showed the highest solubility among 4a4d. X-ray crystallography clarified that the conjugated skeletons of 4a4d are all isostructural, and have two alkoxy groups in a syn-conformation fashion, leading to highly bent structures. Thermal conversions of the precursors examined by thermogravimetry (TG) and differential thermal analysis (DTA) demonstrate that 4a was converted into CuPc via two distinct exothermic processes in the 200–250 °C temperature range, while 4d exhibits only one exothermic signal in the DTA. In the field emission scanning electron microscopy (FESEM) images of 4a, the presence of two types of distinct crystal morphology (prismatic and plate-like crystals) can be recognized, implying that the two observed exothermic processes in the DTA can be attributed to the different crystal morphologies of the samples rather than the step-by-step elimination of the alkoxy groups. The thermal formation of CuPc from the precursors has been unambiguously confirmed by X-ray powder diffraction, UV–vis spectroscopy, and elemental analysis. The precursors were converted into CuPc at lower temperature with increasing chain length, presumably because of the increased void volume in the crystals. Thermal conversion performed in the solution phase results in a bright blue-colored solution with prominent absorption bands in the 650–700 nm region, strongly supporting the formation of CuPc.