Phthalocyanine-based organic semiconductors catalyzed C−H activation for heteroarenes functional materials in the sunlight

Abstract

Organic conjugated materials have attracted much attention in organic optoelectronics due to their excellent optical and electrical properties. In effort of continuous work, material system is continuously enriched, and the performance is rapidly improved. Currently, devices based on organic conjugate functional materials have achieved remarkable development, such as OLED, OFET, OPV, OLET, OFEW. Meanwhile, developing simple, efficient and green organic synthesis methods for these materials is a significant topic. And the most efficient synthesis method is the direct arylation of heteroarenes by C−H bond activation. Synthetic methods based on transition metal, for example, Ru and Ir complexes, catalyzed C−H activation are widely used to construct organic conjugated functional materials. Compared with the transition metal catalyzed reaction, photocatalytic reaction conditions are mild, low temperature and more environmentally friendly. Recently, the direct C−H activation using semiconductor photo-catalysts, owing to good light absorption, is found to be a simple and effective method for organic functional materials. Lots of works based on inorganic semiconductors, such as CdS, ZnO, TiO2, have been developed. Few works based on organic semiconductors, such as Eosin Y, porphyrin have also been reported. But, it is still not systematic enough, and necessary to further expand the kinds of organic semiconductor photocatalysis and develop a photocatalytic system with high efficiency and stability. In this paper, a green method for synthesizing heteroarenes functional materials via direct C−H activation has been developed. Phthalocyanine-based semiconductors are used as photocatalysts, and sunlight, the most common clean energy, is used as a light source at room temperature. Firstly, the reaction conditions were optimized for the direct arylation of thiophene ( 2a ) with diazonium salt ( 1a ), 2 h sunlight irradiation, and 1 mol% phthalocyanine catalyst at room temperature. It’s found that the desired product was obtained in all cases and titanium-phthalocyanine (TiOPc) semiconductor material exhibites the highest catalytic efficiency. The energy band structure of a photocatalytic material plays a key role in determining the performances of photocatalyst. According to the reported works, energy gaps of Pc, CuPc, TiOPc are respectively 1.40, 1.60−1.70, 1.10−1.20 eV. Narrower gaps response to better and wider light absorb. Having optimized the reaction conditions, we examined the scope of the reaction of thiophene with different aryl diazonium salts. Among the aryl diazonium salts used for direct arylation of thiophene, electron-acceptor (−NO2) diazonium salt is found to be more efficient for product formation than electron donor substituted ones (−OCH3, Br, Cl). Notably, halogen-substituted aryl diazonium salts successfully underwent C−H bond arylation leaving the C−halogen bond intact, which is useful for further synthetic elaboration. The C−H arylation of heteroarenes with aryl diazonium salts using TiOPc was expected to proceed through a radical mechanism. A mechanism for the photocatalytic, kinds of direct C–H arylation of heteroarenes with aryl diazonium salts catalyzed by TiO2, TiCl3, eosin Y, porphyrin were proposed. The crucial step involves generation of an aryl radical through single-electron transfer from the excited state of listed photocatalysts, to the aryl diazonium salt and its subsequent addition to a heteroarene to furnish an aryl radical intermediate. We reason that in our case a similar mechanism should operate. Under light irradiation, exited TiOPc proved to be efficient electron donors and acceptors. Furthermore, semiconductor functional material with common anthracene-based structure is synthesized with TiOPc direct catalyzing C−H activation. For further optimized the catalytic efficiency, applying an external electric field or fabricating a semiconductor heterojunction should be considered. In summary, this green method could be further extended to the synthesis of other organic semiconductor functional materials, which is of great significance in organic optoelectronics.