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

Abstract

Core and/or peripheral protonation ofmeso-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 byπdonation from themesosubstituents 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 ofπelectron donor ability of themesogroups. 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 ofmeso-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 theπ-systems of porphyrin and the aryl substituents on the position of the band.