Bandgap engineering in benzotrithiophene-based conjugated microporous polymers: a strategy for screening metal-free heterogeneous photocatalysts

Abstract

Metal-free conjugated microporous polymers (CMPs) as visible-light active and recyclable photocatalysts offer a green and sustainable alternative to classical metal-based photosensitizers. However, the strategy for screening CMP-based heterogeneous photocatalysts has not been interpreted up to now. Herein, we present a general strategy for obtaining excellent solid photocatalysts, which is to implement bandgap engineering in the same series of materials. As a proof of concept, three conjugated porous materials containing benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene building blocks (BTT-CMP1, BTT-CMP2 and BTT-CMP3) were successfully constructed. They possess permanent porosity with a large specific surface area and excellent stability. By changing the linker between benzotrithiophene units, the bandgaps, energy levels and photoelectric performances including the absorption, transient photocurrent responses and photocatalytic performances of BTT-CMPs could be handily modulated. Indeed, BTT-CMP2 displayed the best catalytic activity for visible-light-induced synthesis of benzimidazoles among the three CMP materials, even higher than that of small molecule photocatalysts. As a metal-free photocatalyst, interestingly, the screened BTT-CMP2 also showed extensive substrate applicability and outstanding recyclability. Additionally, we have the opinion that this strategy will prove to be a guiding principle for screening superior CMP-based photocatalysts and broaden their application fields.