Visible absorption and luminescence spectra of N-methyldeuteroporphyrin complexes and the question of “sitting-atop” complexes

Visible spectra of manganese(II), cobalt(II), and zinc(II) complexes of N-methyldeuteroporphyrin IX dimethyl ester are similar to each other in both energies and intensities of absorption bands. These spectra are also quite similar to the spectrum of the monoprotonated ligand, as found earlier for complexes of N-methyltetraphenylporphyrin. Luminescence intensities of N-methyldeuteroporphyrin complexes, however, are dependent on identity of the metal ion and axial ligand, varying in the order: free ligand /gt Cl-Zn(II) complex /gt Br-Zn(II) complex ⪢ Mn(II) complex, Co(II) complex. Complexation of metal ions to N-methyltetraphenylporphyrin results in reduction in emission, with intensities of the complexes varying in the order: Zn(II) /gt Cd(II) ⪢ Fe(II), Mn(II), Co(II). Formation of ion pairs consisting of a porphyrin cation and a metal-containing anion results in a fluorescence spectrum similar to that of the porphyrin ion pair formed by addition of HCl, without substantial quenching. Luminescence spectra provide conclusive evidence for the formation of ion pairs rather than “sitting-atop” complexes under conditions where the nature of such species has been controversial.