36951-71-0Relevant academic research and scientific papers
Steric and Inductive Effects on the Basicity of Porphyrins and on the Site of Protonation of Porphyrin Dianions. Radiolytic Reproduction of Porphyrins and Metalloporphyrins to Chlorins and Phlorins
Sutter, Thimothy P. G.,Rahimi, Rahmatollah,Hambright, Peter,Bommer, Jerry C.,Kumar, Manmohan,Neta, Pedatsur
, p. 495 - 502 (1993)
A series of sulfonated, water-soluble, phenyl-substituted porphyrins has been prepared, containing halogen or alkyl groups in the ortho, di-ortho or para position.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 fo the short-lived ?-radical anions and γ-radiolysis led to formation of stable chlorins and phlorins, the products of two-electron reduction and protonation at the β-pyrrole or at the meso postion, respectively.Whereas H2TPPS4 yields phlorin at all pH values, Zn(II)-, Al(III)-, In(III)- and Sn(IV)-TPPS4 form phlorins at high pH and by increased metal electronegativity. 2,6-Disubstitution at the phenyl rings diminishes the likehood of phlorin formation.These and other results indicate that electron withdrawal from the porphyrin ?-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.
