Photo-Responsive Signatures in a Porous Organic Polymer Enable Visible Light-Driven CO2 Photofixation

Porous organic polymers (POPs) continue to garner immense attention for CO2 capture and sequestration (CCS) as well as CO2 fixation to generate useful chemicals for alleviating global warming. Functionally engineered, visible light responsive organic photopolymers with extended pi-conjugation and abundant heteroatoms enable photogenerated charge carriers, enhancement in visible light absorption, higher charge separation, and reduction in charge recombination during photocatalysis. In this work, we have explored the construction of a chemically stable, pyridine-equipped, and imine-linked porous organic polymer (Py-POP) by template-free Schiff base condensation of 1,3,5-tris(4-aminophenyl) benzene (APB) and 2,6-pyridinedicarboxaldehyde (PDC). This donor-acceptor Py-POP with extensive pi-conjugations enables photocatalytic fixation of CO2 with styrene epoxide (STE) under visible light illumination. We have achieved an impressive conversion of STE to styrene carbonate (STC) (similar to 99%) under optimized reaction conditions using tert-butyl ammonium bromide (TBAB) as a promoter. Both the efficient CO2 adsorption and activation for photocatalytic fixation reaction are enabled by the existence of both imine and pyridine moieties in Py-POP. The interaction between Py-POP and CO2 is further illustrated by density functional theory (DFT) calculations that show that all the POP-CO2 interactions are favorable and exergonic. Using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) characterization techniques, we elucidate the mechanistic pathways of active key surface species in CO2 photofixation with Py-POP. Our results provide mechanistic insight into the effectiveness of efficient, sustainable porous organic photocatalysts in visible light-driven CO2 conversion for various energy applications.