Effects of an axial ligand on the reduction potential, proton dissociation, and fluorescence quantum yield of hydroxo(porphyrinato)antimony(V) complexes

The substituent effects on the reduction potentials (E-1/2(red)) the proton dissociation constants of a hydroxo ligand (K-a), and the fluorescence quantum yields (Phi) in aryloxo(hydroxo)tetraarylporphyrinatoantimony(V) complexes were investigated. The E-1/2(red) and K-a values were affected by substituents in the axial aryloxo ligand but, little affected by substituents in the porphyrin ring. Since E-1/2(red) of tetraarylporphyrinatoantimony(V) complexes were higher than those of other metal-porphyrin complexes, it was suggested that the metal orbital greatly contributed to the LUMO level of the complex which can be related to the E-1/2(red) and Ka values. Therefore, the substituent effects of the axial aryloxo ligand on the E-1/2(red) and K-a values were attributed to the electron density of the antimony ion, which affected the LUMO level of the complexes. Moreover, the Phi of the porphyrin moiety depended on both the oxidation potential of the axial aryloxo ligands and the polarity of the solvent used. The fluorescences of the porphyrin moiety were quenched by the axial aryloxo ligands at rate constants of 10(8) - 10(11) s(-1).