Synthesis of the 3,3'-(Ethane-1,2-diylidene)bis(indolin-2-one) Unit Promoted by Thermally-activated Electron Transfer and Photoreduction of CO2 to CH4 and CO.

A Sonogashira coupling reaction leads to the formation of a serendipitous product C with the 3,3'-(ethane-1,2-diylidene)bis(indolin-2-one) unit. To our knowledge, our study provides the first example demonstrating that electron transfer between isoindigo and triethylamine can be thermally activated and be employed in synthesis. The physical properties of C suggest that C possesses decent photo-induced electron-transfer capabilities. Under the illumination of 136 mW cm-2 intensity, C furnishes ~2.4 mmol gcat-1 (per gram of catalyst) of CH4 and ~0.5 mmol gcat-1 of CO in 20 hours in the absence of additional metal, co-catalyst, and amine sacrificial agent. The primary kinetic isotope effect suggests that the bond cleavage of water is a rate-determining step in the reduction. Moreover, the CH4 and CO production can be boosted as the illuminance increases. This study demonstrates that organic donor-acceptor conjugated molecules are potential photocatalysts for CO2 reduction.