Unusual Spectral Properties of meso-(N,N-Dialkylaminophenyl)Porphyrins

Abstract

The behaviour of the porphyrins meso-tetrakis(4′-dialkylaminophenyl)porphyrin and 5,15-bis(4′-dimethylaminophenyl)-2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin in acidic solution is examined by ultraviolet-visible and ¹H n.m.r. spectroscopy. The unusual nature of the spectra may be explained by the formation of N,N-dialkyliminoquinomethene moieties in which the porphyrin chromophore is modified by interruption of the π-system. Furthermore, acidification renders the macrocycle more susceptible to oxidation so that facile aerial oxidation can occur. The spectral properties of the oxidized species are discussed, and compared to those of analogous quinomethene macrocycles prepared from meso-tetrakis(4′-hydroxyphenyl)porphyrins.