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
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Abstract
Electrochemical properties of substituted free base <i>meso</i>-tetraphenylporphyrins (H<sub>2</sub>T(<i>o</i>,<i>o</i>′-X)PP, H<sub>2</sub>T(<i>o</i>-X)PP,
and H<sub>2</sub>T(<i>p</i>-X)PP, where X = OCH<sub>3</sub>, CH<sub>3</sub>, 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><sup>ox</sup>, i.e., <i>E</i><sub>2</sub><sup>ox</sup> – <i>E</i><sub>1</sub><sup>ox</sup>), is
generally significantly smaller than those of the H<sub>2</sub>TPPs
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