Experimental and computational study on the electrochemistry of meso-tetrasubstituted porphyrins: Effects of resonance and inductive substituents

Abstract

Meso-tetrasubstituted free base porphyrins with six-membered rings, which exhibit inductive or resonance effects between the substituents and the porphyrin rings, were synthesized and their electrochemical behavior was examined by employing cyclic voltammetry (CV) and density functional theory (DFT). The experimental results show that the free-base porphyrin with 4-hexoxy-biphenyl substituents (H2TBoC6P) has the smallest half-wave oxidation potential splitting (∆E = 2nd E1/2 − 1st E1/2), compared to that with tetra-naphthalene (TNP) or tetra-phenyl (TPP) substituents. The ∆E values follow the trend of H2TBoC6P < H2T(2 − N)P < H2T(1 − N)P < H2TPP. The DFT results show that sterically controlled inter π-conjugation between the porphyrin ring and meso-substituents, defined by dihedral angles (ψ), mainly contributes to the half-wave oxidation potentials of free-base porphyrins rather than the saddle deformation of the porphyrin rings. Furthermore, the half-wave oxidation potential splitting ∆E is related to the highest spin density (HSD) distribution on porphyrin rings and HOMO-LUMO energy gaps of free-base porphyrin cationic radicals. A good agreement between calculated and experimental results indicates that steric hindrance between the porphyrin ring and meso-substituents is more important than substituent effects in determining the electrochemistry of free-base porphyrins.