Abstract
Porphyrin isomers have been received much attention in recent years because of their diverse applications in biology, medicine, catalysis and material science. Particularly, N-confused porphyrin (NCP), is a porphyrin isomer in which one of the pyrrole rings is confused. The confused pyrrole affords the unique properties for this ligand, which are not observed in normal porphyrins. In this lecture, I would like to focus on one of the most important properties of NCP, the NH tautomerism. Previously, we have reported that the NH tautomerism can be controlled by solvent polarity. In non-polar solvents such as dichloromethane, an inner 3H tautomer [P(3H)], having both CH and two NHs inside the porphyrin ring, could be observed, whereas in polar and hydrogen-accepting solvents such as DMF, an inner 2H tautomer [P(2H)], having an outer NH, was predominant. These two tautomers differ largely in the electronic state and aromaticity, and this causes the remarkable change in the optical absorption and solution color. Recently, various factors like, solvent, substituent, concentration and isotope effect, affecting the NH tautomerism, have been confirmed by NMR measurements. Based on these studies, a possible reaction mechanism of the tautomerism will be discussed.
