Theoretical insights of ionization potential and electron affinity modulations on planar vs. ruffled iron(III) porphyrins

Heme-type porphyrins can become distorted depending on their environment, altering their chemical and electronic properties. While trends have been established for several ruffle-distorted porphyrins, their research on synthetic models of heme-type porphyrins is limited. We identified a dynamic process on the ruffled porphyrin bis(2-methylpyridinato)iron(III)-Protoporphyrin-IX (2) that does not exist in bis(pyridinato)iron(III)-Protoporphyrin-IX (1). We modeled these molecules as {OMP(2-CH 3 Py) 2 Fe}[Formula: see text] and {[OMP](Py) 2 Fe}[Formula: see text] respectively. Geometry optimization using density functional theory (DFT) suggests the ruffled conformation of 2 (angle [Formula: see text] of -29.7°) and the planar conformation of 1. The calculated electron affinity and ionization potential vary depending upon ruffling. The calculated 0.253 eV difference in electron affinity and the 0.223 eV in ionization potential indicate that the reduction of the planar structure has a higher energy requirement than the ruffled-distorted structure but has a lower energy requirement for its oxidation. We found that these energy differences are not solely attributed to the distortion of the macrocycle. The difference is linked to the interaction between the d xy orbital and the HOMO-1 3a 2u , allowed by symmetry when a porphyrin ring has a ruffled conformation stabilizing the frontier orbitals, thus making the oxidation process have a higher energy barrier. On the other hand, the reduction process is facilitated by the interaction of metal d[Formula: see text] orbitals with the porphyrin 4e. These highlight the distinct differences between the ruffled and the planar conformations.