Fluorene and Triazine-Based Conjugated Polymer Networks with Tuned Frontier Orbital Energy Levels for Improving Organic Photocatalysis

Here, we describe a method for fine-tuning the frontier orbital energy levels or redox potential of a conjugated polymer network (CPN) consisting of triazine and fluorene by systematically introducing electron-withdrawing groups at the fluorene moiety through simple structural modification. The band gap of CPN decreases with an increase in the strength of the electron-withdrawing group, and the relative position of the frontier orbital energy becomes more favorable for reactions like photocatalytic aerobic oxidation, which has been further confirmed by theoretical and experimental studies. The CPN with a CN group at the 9 position of fluorene (CPN3) shows the maximum photocatalytic activity (32% higher) compared to CPN1 (hydrogen at the 9 position) in white light. The best-performing catalyst CPN3 was further employed for photocatalytic reactions under white light, like the oxidation of alcohols to carbonyl and boronic acid to the corresponding alcohol, which show noteworthy characteristics like low catalyst loading, high yield, and selectivity with broad substrate scope (aliphatic, aromatic, biphenyl, and heterocyclic). CPN3 exhibited turnover frequencies of 13.33 and 10.9 mmol g–1 h–1 in 3 W blue and white LED lights, respectively, which are much higher than those of state-of-art photocatalysts. The recyclability of the catalyst was tested for up to 5 cycles without much change in the catalytic activity. The practical usefulness of the suggested method was further demonstrated by the CPN3-photocatalyzed gram-scale synthesis of high-value chemicals such as acetophenone from 1-phenylethanol and phenol from phenylboronic acid.