Tuning Linkers in Azo-Linked Porphyrin-Based Porous Organic Polymers for Enhanced CO₂ Capture

The increased concentration of atmospheric CO₂ and its impact on the environment drive the search for new materials that can capture and store this greenhouse gas effectively. Porous organic polymers (POPs) are very promising materials for this task because of their stability and the tunability of their pore structure and chemistry. In this study, we synthesized a series of azo-linked porphyrin-based porous organic polymers (APPs) using either heteroatom-containing linkers (hydroxylated biphenyl in APP-BP-OH and carbonyl-bearing anthraquinone in APP-AQ) or sterically hindered linkers (methylated biphenyl and phenyl in APP-BP-Me and APP-Ph-Me, respectively). Structural characterization confirmed the formation of azo linkages and the amorphous nature of the frameworks, while thermal analysis showed that APPs are stable up to at least 200 °C. Gas sorption studies revealed notable differences in porosity and CO₂ uptake. APP-Ph-Me exhibited the largest surface area (673 m² g^-1), whereas APP-BP-OH had a smaller surface area (488 m² g^-1) but adsorbed more CO₂ (49 mg g^-1) compared to APP-Ph-Me (41 mg g^-1). These results illustrate that CO₂ adsorption in APPs is governed by not only the surface area but also the chemical environment. Nitrogen-rich porphyrin and azo moieties play a dominant role, while polar hydroxyl groups and biphenyl linkers provide additional contributions. Computational results supported these findings, indicating that hydroxyl and carbonyl groups create favorable binding sites, while methyl groups limit accessibility to porphyrin and azo regions. Overall, our results highlight how linker design and functionalization directly influence porosity and adsorption performance and offer useful guidelines for the development of new POPs for CO₂ capture.