Electrochemistry of tri-N-methylpyridyl porphyrins

Trisubstituted porphyrin derivatives [(TriMPyP)M](3)+(Cl-) (3) (where M = 2H, Zn-II or Pd-II) containing three N-methyl-4-pyridyl (N-MePy+) groups were synthesized and characterized electrochemically and spectroscopically in DMSO containing 0.1 M tetrabutylammonium salts. The effect of specific counter-anions, type of metal ion and number of meso-N-MePy+ groups on the redox behavior and spectroscopic properties of the neutral and reduced species are examined before and after controlled potential reduction in a thin-layer cell. Each tri-cationic derivative initially undergoes a global two-electron reduction to give a two-banded absorption spectrum with a broad near-IR band ranging from 750-807 nm depending on the type of metal ion. The position of the near-IR band for the doubly reduced porphyrin was also found to correlate with the number of meso-N-MePy+ groups on the macrocycle which varied from 1 to 4 while the first reduction potential remained essentially unchanged upon going from macrocycles with two to three to four meso-N-MePy+ groups.