Spectroscopic, redox and emission properties of 2-nitro-substituted free base- and metallo-tetra-aryl porphyrins

Abstract

Free-base, copper(II) and zinc(II) derivatives of 2-nitro-substituted 5,10,15,20-(tetra-aryl)porphyrins (aryl = phenyl, 2-naphthyl or diphenylpyrazolyl) have been synthesized and investigated by optical absorption and emission, ¹H NMR, ESR and cyclic voltammetric methods. Red shifts and broadening of the optical absorption bands, downfield shifts of both the β-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 parameters for the unsubstituted parent porphyrins. Solvent-dependent fluorescence spectral data of the free-base and zinc(II) derivatives of the nitroporphyrins have been analysed in terms of twisted intramolecular charge transfer (TICT) in these complexes. However, the ESR spectral data of copper(II) derivatives of the nitroporphyrins are indistinguishable from the data for the unsubstituted analogues. All these features of the nitroporphyrins have been interpreted in terms of a perturbation of the frontier orbitals of the porphyrin p-ring system by the directly attached nitro group. Based on the results obtained, possible applications of the nitroporphyrins have been suggested.