Jump to main content
Jump to site search

Issue 3, 1993
Previous Article Next Article

Steric and inductive effects on the basicity of porphyrins and on the site of protonation of porphyrin dianions: radiolytic reduction of porphyrins and metalloporphyrins to chlorins or phlorins

Abstract

A series of sulfonated, water-soluble, phenyl-substituted porphyrins has been prepared, containing halogen or alkyl groups in the ortho, di-ortho or para positions. While the para-substituted compounds exhibited monomer–dimer behaviour, all the ortho and di-ortho substituted porphyrins were monomeric in aqueous solution at 0.1 mol dm–3 ionic strength. The proton basicities varied over a 105 range along the series, from the strongly basic tetra(4-methoxyphenyl) species to the weakly basic and less deformable sulfonated tetrakis(2,6-dichlorophenyl) porphyrin. Certain of these porphyrins and related metalloporphyrins were reduced by radiolytic methods in aqueous solutions. Pulse radiolysis studies provided the spectra of the short-lived π-radical anions and γ-radiolysis led to formation of stable chlorins or phlorins, the products of two-electron reduction and protonation at the β-pyrrole or at the meso position, respectively. Whereas H2TPPS4[tetrakis(4-sulfonatophenyl)porphyrin] yields phlorin at all pH values, ZnII-, AlIII-, lnIII- and SnIV-TPPS4 form phlorins at high pH but mostly chlorins at lower pH. The ratio of phlorin to chlorin production is enhanced by increased pH and by increased metal electronegativity. 2,6-Disubstitution at the phenyl rings diminishes the likelihood of phlorin formation while N-methyl substitution at one of the central nitrogens of ZnTPPS4 enhances phlorin formation. These and other results indicate that electron withdrawal from the porphyrin π-system enhances the ratio of phlorin/chlorin production. On the other hand, steric crowding around the meso position retards protonation at this site by preventing the geometric reorientation necessary for phlorin formation.

Back to tab navigation

Article type: Paper
DOI: 10.1039/FT9938900495
J. Chem. Soc., Faraday Trans., 1993,89, 495-502

  •   Request permissions

    Steric and inductive effects on the basicity of porphyrins and on the site of protonation of porphyrin dianions: radiolytic reduction of porphyrins and metalloporphyrins to chlorins or phlorins

    T. P. G. Sutter, R. Rahimi, P. Hambright, J. C. Bommer, M. Kumar and P. Neta, J. Chem. Soc., Faraday Trans., 1993, 89, 495
    DOI: 10.1039/FT9938900495

Search articles by author

Spotlight

Advertisements