Naphthalene Diimide-Based Conjugated Polymers as Promising Organocatalysts for Photocatalytic CO₂ Reaction

Nowadays, the photochemical conversion of CO₂ to high-value products is attracting numerous research interest. Developing artificial photocatalysts with excellent catalytic activity and long-term stability is still a challenge. This work demonstrates that solution-processable naphthalenedimide (NDI)-based conjugated polymers, PNDI-BT, PNDI-DTBT and PNDI-BP, which are copolymerization products of NDI with bithiophene (BT), 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT) and biphenyl (BP), respectively, can catalyze the photochemical reduction of CO₂ to produce CO in the presence of trace amounts of water without the need for metal-containing co-catalysts or sacrificial agents. In particular, the PNDI-BP-catalyzed reaction generated CH₄ as well as CO. Results from time-resolved photoluminescence, photovoltage decay, electrochemical impedance spectroscopy and transient photocurrent response experiments indicate that PNDI-BP, which had the largest dihedral angles along the conjugated backbone, possessed the longest electron lifetime, the lowest charge-carrier recombination rate, and the smallest interfacial charge transfer resistance. Consequently, it had the best catalytic performance. Notably, PNDI-BP exhibited excellent recyclability, robust structural stability, and extremely steady catalytic activity for more than 330 hours during a photocatalytic CO₂ reaction. Furthermore, the solution-processability of the linear polymer allows the incorporation of porous substrates, which improve the reaction interface. The catalyst system of PNDI-BP@molecular sieves with H₂O/triethylamine doubled the CO yield to 214.8 μmol/g and enhanced the CH₄ yield by ~36 times to 61.4 μmol/g in an 18 hr reaction.