Intriguing Electrochemical Behavior of Free Base Porphyrins: Effect of Porphyrin–<i>meso</i>-Phenyl Interaction Controlled by Position of Substituents on <i>meso</i>-Phenyls

Electrochemical properties of substituted free base <i>meso</i>-tetraphenylporphyrins (H₂T­(<i>o</i>,<i>o</i>′-X)­PP, H₂T­(<i>o</i>-X)­PP, and H₂T­(<i>p</i>-X)­PP, where X = OCH₃, CH₃, H, F, or Cl on the phenyl rings) are examined by cyclic voltammetry. When a substituent is located only at the para position of the <i>meso</i>-phenyl group, the difference between the first and second oxidation potentials (Δ<i>E</i>^ox, i.e., <i>E</i>₂^ox – <i>E</i>₁^ox), is generally significantly smaller than those of the H₂TPPs with bulky o,o′-substituents on the phenyl group. This trend is elucidated with density functional theory calculations and attributed mainly to the <i>sterically controlled</i> π-conjugation of the <i>meso</i>-phenyl groups to the central porphyrin ring, rather than the often discussed deformation of porphyrin