Substituent effects on CO₂ adsorption in functionalized porphyrins: Mechanistic insights into electronic structure...

Understanding how peripheral substituents control adsorption and electronic response is essential for the molecular design of porphyrin-based CO₂-responsive materials. In this work, pristine porphyrin and eight substituted derivatives bearing CH₃, CHO, CN, COOH, F, NH₂, NO₂, and OH groups were systematically examined toward CO₂ adsorption. The optimized geometries show that CO₂ adopts either surface-oriented configurations above the porphyrin core or edge-oriented arrangements near the substituents, depending on the local chemical environment. Among the studied systems, P-COOH and P-F exhibit the strongest adsorption energies (-0.15 eV), whereas P-CN shows the weakest interaction (-0.10 eV), indicating the most weakly bound and readily reversible adsorption. Although substituent identity substantially modifies the intrinsic frontier-orbital energies of the porphyrin frameworks, CO₂ adsorption induces only minor HOMO-LUMO gap changes (-0.57 to 2.14%), consistent with limited electronic perturbation. Charge analysis reveals negligible charge transfer to CO₂, while reduced density gradient analysis confirms that adsorption is dominated by weak noncovalent interactions. Importantly, the results reveal a clear substituent-dependent trade-off among binding strength, electronic signal modulation, and recovery behavior: P-COOH and P-F favor relatively stronger CO₂ retention, P-CN is most suitable for rapid and reversible response, and P-CH₃ and P-OH show the largest predicted sensing responses. These findings establish a practical structure-property framework for tuning CO₂ adsorption and interfacial selectivity in functionalized porphyrins.