Development of strategy in energy conversion and storage is indispensable for a sustainable society. So far, redox-active transition metal complexes have been used as efficient catalysts and photosensitizers in energy-converting reactions such as hydrogen evolution through thermal and photoinduced electron transfer (ET).

We have been working on development of redox and photochemical functionality of diprotonated saddle-distorted porphyrins based on ET reactions. Herein we would like to highlight recent advance in our study of catalytic dioxygen reduction and hydrogen evolution using diprotonated forms of dodecaphenylporphyrin (H₂DPP) derivatives as organocatalysts and photosensitizers.

A diprotonated form of a water-soluble H₂DPP derivative can act as a photosensitizer (PS) in photocatalytic oxidation of organic substrates in water using Na₂S₂O₈ as an oxidant under visible-light irradiation.¹ On the contrary, diprotonated H₂DPP has been used as a PS in photocatalytic H2 evolution using platinum nanoparticles as catalysts under near-infrared (710 nm) irradiation in the quantum yield of 17%.²

Diprotonated H₂DPP (H₄DPP²⁺) can serve as an organo-photocatalyst in highly selective and efficient O₂ reduction to afford H₂O₂ under photoirradiation at 480 nm; the quantum yield of H2O2 generation was 12% and turnover frequency was 500 h^–1.³

Interconversion of O2 and H2O2 has been achieved using an N21,N23-dimethylated H₂DPP derivative.⁴ The porphyrin is reduced by H₂O₂ to form the corresponding isophlorin derivative as a two-electron-reduced product, which is in turn oxidized by O2 to afford the porphyrin and H2O2. We also developed efficient O2 reduction using N21,N22- and N21,N23-dimethylated H₂DPP derivatives as catalysts; in the catalysis, catalytic mechanisms and performance depend on the structures of catalysts used in terms of interaction between O2 and the corresponding isophlorins.⁵

References

1. T. Ishizuka et al., Green Chem. 2018, 20, 1975.
2. H. Kotani et al., ACS Appl. Energy Mater. 2020, 3, 3193.
3. E. Aoki et al., Chem. Commun. 2019, 55, 4925.
4. W. Suzuki et al., J. Am. Chem. Soc. 2019, 141, 5987.
5. W. Suzuki et al., Chem. Eur. J. 2020, 26, 10480.