title: Reaction pathway and free energy profile for conversion of π-conjugation modes in porphyrin isomer.
abstract: 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.
authors.affiliation: Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara 630-0192, Japan. tmatsuo@ms.naist.jp
authors.forename: Takashi
authors.initials: T
authors.lastname: Matsuo
authors.forename: Yuji
authors.initials: Y
authors.lastname: Tohi
authors.forename: Takashi
authors.initials: T
authors.lastname: Hayashi
mesh_terms: D018360:Crystallography, X-Ray; D008958:Models, Molecular; D015394:Molecular Structure; D011166:Porphyrins / Q000737:chemistry*; D011789:Quantum Theory; D013816:Thermodynamics
chemical_list: D011166:Porphyrins