Metal-Free Catalysis: A Redox-Active Donor-Acceptor Conjugated Microporous Polymer for Selective Visible-Light-Driven CO2 Reduction to CH4.

Achieving more than a two-electron photochemical CO2 reduction process using a metal-free system is quite exciting and challenging, as it needs proper channeling of electrons. In the present study, we report the rational design and synthesis of a redox-active conjugated microporous polymer (CMP), TPA-PQ, by assimilating an electron donor, tris(4-ethynylphenyl)amine (TPA), with an acceptor, phenanthraquinone (PQ). The TPA-PQ shows intramolecular charge-transfer (ICT)-assisted catalytic activity for visible-light-driven photoreduction of CO2 to CH4 (yield = 32.2 mmol g-1) with an impressive rate (2.15 mmol h-1 g-1) and high selectivity (>97%). Mechanistic analysis based on experimental results, in situ DRIFTS, and computational studies reveals that the potential of TPA-PQ for catalyzing photoreduction of CO2 to CH4 was energetically driven by photoactivated ICT upon surface adsorption of CO2, wherein adjacent keto groups of PQ unit play a pivotal role. The critical role of ICT for stimulating photocatalysis is further illustrated by synthesizing another redox-active CMP (TEB-PQ), bearing triethynylbenzene (TEB) and PQ, that shows 8-fold lesser activity for photoreduction toward CO2 to CH4 (yield = 4.4 mmol g-1) as compared to TPA-PQ. The results demonstrate a novel concept for CO2 photoreduction to CH4 using an efficient, sustainable, and recyclable metal-free robust organic photocatalyst.