Acid-base equilibria of axial ligand and peripheral pyridyl group with stepwise formation of nine species of aluminum (III) tera(4-pyridyl) porphyrin

Abstract As one of the promising molecular catalysts for the two-electron water oxidation initiated through one-electron oxidation of the catalyst, trans-diaqua-coordinated 5, 10, 15, 20-tetra(4-pyridyl)porphyrinatealuminum (III) (AlTPyP(OH2)2) was studied on the protolytic behavior of the axial ligating water molecules and the redox properties in aqueous methanol solution. The reversible stepwise protolytic reactions of the two axially ligated water molecules, as well as four peripheral pyridyl groups at the meso-position of AlTPyP, were characterized using UV–vis absorption spectroscopy, 1H NMR, fluorescence lifetime measurements, and cyclic voltammetry. The result showed that four-step protolytic equilibrium reactions of the axial ligands proceeded under the basic pH condition and those of meso-substituted pyridyl groups did also under the acidic pH condition. All the nine species are in equilibria in the ground states as characterized with each pKa value. On the other hand, they exist as individual species in the excited states having each single exponential decay time constant. Here we will report the first detailed study on the formation of nine species from a single molecule of AlTPyP through protolytic reactions in the pH range between 0.54 and 12.3.