Reaction Pathway and Free Energy Profile for Conversion of π-Conjugation Modes in Porphyrin Isomer

Porphycene is a structural isomer of porphyrin with 18π-conjugated aromatic character. Porphycene modified with trifluoromethyl (CF(3)) groups in the periphery of the framework readily affords the isolable 20π-conjugated antiaromatic form through a reaction with a proton-donating reductant. The 20π-conjugated form can be characterized by not only a variety of spectroscopies in solutions but also X-ray crystallography. This paper focuses on the free energy profile in the conversion of the 18π-conjugated porphycene into the 20π-conjugated form. From the results of kinetics, electrochemical measurements, and acid/base titrations, the 20π-conjugated CF(3) porphycene is formed by a concerted proton-electron transfer (CPET) from a hydroquinone reagent to the 18π-conjugated form. The hydrogen-atom affinity of the 18π-conjugated CF(3) porphycene (for two hydrogen atoms) was calculated to be -490 kJ mol(-1), indicating that the N-H bonds in the 20π-conjugated form are rather easily cleaved. This reflects the antiaromatic characteristics of the 20π-conjugated porphycene. We propose that the kinetic and thermochemical analysis using redox potentials and pK(a) data is applicable for determining the reaction pathway in conversion of aromatic/antiaromatic mode of π-conjugated macrocycles as well as popular investigations for oxidations of organic molecules.