Preparation, electrochemical and spectral properties of N-methylated pyridylethynyl porphyrins

A new series of electron-deficient porphyrins were prepared by attaching one or two N-methylated 2-, 3- or 4-pyridylethynyl groups to the 10,20-meso positions of ( 5,15-biphenylporphinato) zinc(II). Electrochemical studies showed significant changes in the reduction potentials of these porphyrins, and N-methyl-2-pyridylethyne is the strongest electron-withdrawing substituent in the series. UV-visible spectra demonstrated largely red-shifted absorptions, and N-methyl-4-pyridylethyne has the greatest impact to the porphyrin absorptions. Electrochemical, UV-visible and EPR results concluded that porphyrins Zn2 and Zn6 reversibly undergo two one-electron porphyrinring reductions to their anion radicals then dianions. The first reductions of porphyrins Zn1, Zn3, Zn4 and Zn5 were irreversible one-electron transfer processes. The unstability of these reduction products was suggested to result from the eletrophilic attacks at the substituents.