Extending the π-Conjugated System in Conjugated Microporous Polymers to Modulate Excitonic Effects for Metal-Free Selective CO2 Photoreduction to CH4

Photocatalytic CO2 reduction into CH4 is an appealing approach to alleviate the current energy and environmental crisis; however, achieving high selectivity and conversion efficiency still remains challenging. The rational design of photocatalysts for CO2 adsorption and activation is thus crucial. Here, we designed and synthesized two redox-active truxene-based conjugated microporous polymers linked by thiazolo[5,4-d]thiazole for metal-free photocatalytic reduction of CO2 to CH4. Significantly, the optimized polymer with the extended π-conjugated system, denoted Tx-TzTz-CMP-2, presented a higher CH4 production rate of 300.6 μmol g–1 h–1 with a selectivity of 71.2% without any metal cocatalyst and photosensitizer, which outperformed most of the previously reported photocatalysts. Experimental and theoretical investigations revealed that introducing phenyl as a π-bridge extending electron delocalization of conjugated system might effectively minimize exciton binding energy, thereby boosting the intramolecular charge transfer and separation abilities. Meanwhile, the N-site of the thiazole unit acted as an electron reservoir and a catalytic center for activating the adsorbed CO2 and forming CH4 by subsequent hydrogenation. This work highlights the significance of the extended π-conjugated system in metal-free conjugated microporous polymers to improve the activity and selectivity of CO2 photoreduction to CH4.