Effects of Metal Centers and meso-substituents on Electronic and Catalytic Properties of Porphyrins: A Quantum Chemical Approach

Porphyrins constitute a versatile class of π-conjugated macrocycles with high structural stability and broad applicability in catalysis. However, the catalytic activity of free-base porphyrins is limited by the absence of metal-centers and limited electronic tunability. Metalation with meso-substitution introduces d-orbital interactions that regulate electronic structure, charge distribution, and geometry, enhancing electron-transfer and catalytic performance. In the present study, meso-substituted porphyrins, namely porphyrin salicylic acid (PSA) and porphyrin benzyl alcohol (PBA) were systematically investigated upon incorporation of Pt, Pd, Ni, and Cu centers using B3LYP/GENECP approach, employing 6-31G(d,p) for light atoms and LANL2DZ for transition metals. Metal incorporation produced HOMO–LUMO gaps between 2.76 (Cu-PBA) and 2.97 (Pt-PSA) compared with 2.58-2.60 eV for free ligands. Dipole moments increased from 4.19 D (PSA) and 2.00 D (PBA) to as high as 4.41 D (Pt-PSA) and 2.06 D (Pt-PBA, Ni-PBA) respectively. ESP maps revealed pronounced electron-rich regions around the oxygen-containing meso substituents and electron-deficient metal coordination centers. NSD analysis revealed reduced out-of-plane distortions for Pt-PSA (0.23 Å) and Pd-PSA (0.20 Å) relative to PSA (0.33 Å), whereas Ni-PBA exhibited pronounced ruffling with a distortion amplitude of 0.69 Å (pyrrole tilt = 7.6°). TD-DFT calculations revealed strong metal-dependent optical responses, with intense Soret transitions appearing at 372–388 nm (f = 0.856–0.973) for Pt and Pd derivatives, while Cu-PBA displayed a remarkable red-shifted excitation at 934 nm. Vibrational analysis further confirmed electronic redistribution, with the C=C stretching mode at 1531cm^-1 (PSA) shifts distinctively to 1515 cm^-1 in Pt-PSA and 1506 cm^-1 in Cu-PSA. NCI analysis confirms meso-salicylate groups engage in extensive intramolecular hydrogen bonding. These findings identify metal and meso-substituent character function as orthogonal handles for tuning macrocycle planarity, frontier orbital energetics, and optical response, offering a complete framework for the rational design of meso-substituted metallo-porphyrins with target electronic and photophysical profiles.