Tailoring the Exciton Binding Energy of 2D Conjugated Polymers for Powering Metal-Free CO₂ Photoreduction

Metal-free conjugated polymer photocatalysts have provided promising environmentally friendly options in photoreduction of CO2 to high value-added chemicals. However, due to the strong Coulombic interaction of excited electron-hole pairs, most polymer photocatalysts show moderate photocatalytic performances compared with inorganic semiconductor photocatalysts. Herein, we prepared a series of 2D porphyrin-based conjugated polymers. By simply altering the aromatic acetylene linkers, we found that the exciton binding energy (Eb) of the 2D conjugated polymers could be fine-tuned. With an increasing number of acetylene units in linkers, their Eb values were reduced by 27% accompanied by promoted charge-carrier generation (about a 4-fold increase in photocurrent) and obviously enhanced catalytic activity of CO2 photoreduction. The optimized polymer, PTPP-DA, linked with diacetylene groups, affords CO and CH4 as the carbonaceous products with rates of 465 and 237 μmol g-1 h-1, respectively, without any metal or photosensitizer. This work reveals the important implications of conjugated linkers on mediating the Eb values of 2D conjugated polymers and provides new insight for designing efficient metal-free polymeric photocatalysts for CO2 reduction.