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

Core and/or peripheral protonation of meso-tetra(pyridyl)porphyrins,h 2(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. © 2013 saeed zakavi et al.