Effects of Core and/or Peripheral Protonation of meso-Tetra(2-, 3-, and 4-pyridyl)Porphyrin and meso-Tetra(3-methylpyridyl)Porphyrin on Their UV-vis Spectra

Abstract

Core and/or peripheral protonation of meso-tetra(pyridyl)porphyrins, H2(py)P, with weak and strong acids have been studied by UV-vis spectroscopy. The results support the proposal that the position of the Soret as well as the Q(0,0) bands of the dications is influenced by π donation from the meso substituents to the porphyrin core. The red shift of the Q(0,0) band of H2t(py)P dications with 2-, 3-, and 4-pyridyl substituents does not correlate with the order of π electron donor ability of the meso groups. The decreased electron donation from the central nitrogen atoms to the acids on going from 3-pyridyl group to 4- and 2-pyridyl ones has been used to explain this observation. In spite of the negative effect of N-methylation of pyridyl substituents on the basicity of the central nitrogen atoms of meso-tetra(3-methylpyridyl)porphyrin, H2t(3-Mepy)P, protonation of H2t(3-Mepy)P occurs with weak and strong acids. The blue shift of the Q(0,0) bands upon protonation of H2t(3-Mepy)P with weak and strong acids clearly shows the importance of resonance interactions between the π-systems of porphyrin and the aryl substituents on the position of the band.