New insights into the influence of weak and strong acids on the oxidative stability and photocatalytic activity of porphyrins

Abstract

A series of electron-rich and electron-demanding meso-tetra(aryl)porphyrins (aryl = phenyl, 4-methoxyphenyl, 4-methylphenyl, 4-chlorophenyl, 2-methylphenyl and 2-chlorophenyl) and their dications with weak and strong acids (CF₃COOH, CHCl₂COOH, CH₂ClCOOH, HCOOH, HCl and HClO₄) were used as photosensitizers for the aerobic photooxidation of cyclooctene under homogeneous conditions. Mechanistic studies in the presence of a singlet oxygen quencher (1,3-diphenylisobenzofuran) and 1,4-benzoquinone as the superoxide anion radical scavenger confirmed the involvement of a singlet-oxygen mechanism in the oxidation reactions. Also, the solvent effects on the catalytic activity of the porphyrin diacids were explained by the relative lifetimes of singlet oxygen in different solvents. The formation of dications was found to either increase or decrease the photocatalytic activity, photostability and singlet oxygen quantum yield (ϕ_Δ) of porphyrins. The non-radiative rate constants (k_nr) for the decay of the S₁ excited state of the photosensitizers were often greater in the case of porphyrin diacids. The ϕ_Δ values of the porphyrins and dications were in the range of 0.48–0.89 and 0.06–0.98, respectively. Interestingly, despite the large ϕ_Δ value (0.98) found for H₄T(4-OMe)PP(CF₃COO)₂ and H₄T(4-OMe)PP(HCOO)₂, H₄T(4-Cl)PP(HCOO)₂ (ϕ_Δ = 0.29) showed the highest catalytic activity of the series. This observation was attributed to extensive oxidative degradation of the former. It is noteworthy that the ϕ_Δ parameters were evaluated in a short reaction time (ca. 20 min) and therefore no detectable catalyst degradation was observed in this time interval. Furthermore, H₄TPP(HCOO)₂ was the most stable photocatalyst under the reaction conditions.