Photocatalytic oxidation of organic substrates has been of great interest in terms of utilization of photon energy to produce useful materials from abundant feedstock. As a photocatalyst that can mediate electron transfer (ET) from an oxidation catalyst to an oxidant upon photoexcitation, porphyrins can be useful in light of the strong visible-light absorption and clear redox behavior. Although porphyrins have been used as photocatalysts for reduction of organic substrates, photocatalytic oxidation of organic substrates has been seldom reported by using porphyrins as photocatalysts.

So far, we have established the utility of diprotonated porphyrins as electron acceptors in photoinduced ET.¹ On the basis of the concept, we prepared a water-soluble and saddle-distorted porphyrin, which can act as an electron acceptor in photoinduced ET upon diprotonation. The porphyrin (H₂P⁴⁺) possesses four N-methylpyridinium-4-yl substituents at the para positions of the four meso-phenyl groups of dodecaphenylporphyrin. We observed proton-coupled electron transfer processes of the diprotonated form of H₂P⁴⁺, H₄P⁶⁺, in water to establish the Pourbaix diagram of H₄P⁶⁺. We also determined the pKa value of the first protonation/deprotonation equilibrium to form the monoprotonated H₃P⁵⁺ (pKa1 = 4.04) and that of the second one to afford H₂P⁴⁺ (pKa2 = 7.83).

The highly positively charged H₄P⁶⁺ was revealed to form supramolecular assemblies with negatively charged heteropolyoxometalates (POMs) in the ratio of H₄P⁶⁺ : POM = 1 : 2 in water at pH 3 as indicated by Job's plots. Based on the redox properties of H₄P⁴⁺ and the formation of supramolecular assemblies, we examined intra-supramolecular photoinduced ET from heteropolyoxometalates as electron donors to H₄P⁶⁺ as an electron acceptor in water at pH 3 using femto-second laser flash photolysis upon photoexcitation at 393 nm. First-order rate constants of ET were analyzed on the basis of the Marcus theory of ET to determine the reorganization energy of ET to be 0.74 eV and the electronic coupling matrix element (V) to be 43 cm^-1.

We also examined its application to photocatalytic oxidation of organic substrates in water, using H₄P⁶⁺ as a photocatalyst, Na₂S₂O₆ as an oxidant, and a Ru-substituted POM (RuPOM) as an oxidation catalyst, under the visible light irradiation. Preliminary results on the photocatalytic oxidation of benzyl alcohol were 560 turnovers for 4h; blank tests clearly suggest that H₄P⁶⁺, RuPOM, the oxidant, and photoirradiation are all required to perform the photocatalytic reaction.

References

1. S. Fukuzumi, T. Honda, T. Kojima, Coord. Chem. Rev. 2012, 256, 2488-2508.