SPECTROSCOPIC, REDOX AND EMISSION PROPERTIES OF 2-NITRO-SUBSTITUTED FREE-BASE AND METALLO-TETRA-ARYL PORPHYRINS

Free-base, copper(II) and zinc(II) derivatives of 2-nitro-substituted 5,10,15,20-(tetra-aryl)porphyrins (aryl = phenyl, 2-naphthyl or diphen ylpyrazolyl) have been synthesized and investigated by optical absorpt ion and emission, H-1 NMR, ESR and cyclic voltammetric methods. Red sh ifts and broadening of the optical absorption bands, down-field shifts of both the beta-pyrrole and imino proton NMR peaks and decrements in the reduction potentials have been observed for these nitroporphyrins , in comparison with the corresponding spectral or electrochemical par ameters for the unsubstituted parent porphyrins. Solvent-dependent flu orescence spectral data of the free-base and zinc(II) derivatives of t he nitroporphyrins have been analysed in terms of twisted intramolecul ar charge transfer (TICT) in these complexes. However, the ESR spectra l data of copper(II) derivatives of the nitroporphyrins are indistingu ishable from the data for the unsubstituted analogues. All these featu res of the nitroporphyrins have been interpreted in terms of a perturb ation of the frontier orbitals of the porphyrin pi-ring system by the directly attached nitro group. Based on the results obtained, possible applications of the nitroporphyrins have been suggested.