Conjugated Polymers as Photoredox Catalysts: Visible-Light-Driven Reduction of Aryl Aldehydes by Poly(p-phenylene)

The use of visible light in photocatalysis has been intensively studied because of its natural abundance, ease of use, and promising potential for industrial applications. However, there are several challenges to utilizing visible light for organic functional group transformations. These challenges include the low absorptivity of most organic compounds in the visible spectrum, and side reactions are often prevalent in photochemical reactions. Visible-light-sensitive catalysts offer a means to overcome these obstacles. Conjugated polymers are semiconductors that offer a large range of redox potentials, they are stable, and they often absorb visible light. Despite these desirable properties for photocatalysis, only a limited number of organic reactions utilizing conjugated polymers as photocatalysts have been reported. In one such example, poly(p-phenylene) was used to induce the pinacol coupling reaction of benzaldehyde upon irradiation with visible light. In this work, visible light, thiols, and poly(p-phenylene) were employed to reduce aryl aldehydes to their respective alcohols to better characterize the reaction mechanisms of this system. The effects of varying reaction conditions on the rate of photocatalysis indicated interfacial electron transfer from the poly(p-phenylene) surface to the substrate as the initial productive step. Additionally, the chemoselective reduction of aryl aldehydes over aryl ketones and alkyl aldehydes was achieved with this system.