Clarifying the Active Site Role of meso‐Carboxyphenyl Group for Free Base Porphyrins in Photocatalytic H₂ Evolution Reaction

Abstract

Three kinds of free base meso‐carboxyphenylporphyrins were compared about their photocatalytic activity to elucidate the role of −COOH group playing in H2 evolution from water splitting. Meso‐tetra(3,5‐dicarboxyphenyl)porphyrin (H2TDCPP) gives a much larger H2 evolution rate than other two porphyrins, attributable to more −COOH groups involved that may interact with H2O. This interaction, as revealed by DFT calculation, initiates H2 evolution from the splitting of associated H2O under ultraviolet light irradiation resulted by effective transfer of photogenerated electron from central porphine ring to meso‐substituents and then further to the interacted water. Improving the accessibility of −COOH groups to water molecule by optimizing H2TDCPP dosage induces an H2 evolution rate around 31.21 mmol ⋅ h−1 ⋅ g−1 (pH=7, full‐spectrum light), which is unprecedented for free based porphyrin photocatalysts without metal‐cocatalysts. This work clarifies the active site role of −COOH groups and paves a new way to design high‐performed organic photocatalysis by introducing polar groups‐based active sites.

Improving the photocatalytic activity of porphyrins for hydrogen evolution from water splitting through combined effects inspired by meso-carboxyfuryl substitution

“…It has been mentioned in our previous work that –COOH attached to the meso -phenyl group of porphyrins acts as a photocatalytic active site for H 2 evolution. 39 In accordance with this point, the DFT calculation of the interaction of water with porphyrins was analyzed. Two possible associated modes and their corresponding association energies are shown in Fig.…”

Advances in organic semiconductors for photocatalytic hydrogen evolution reaction

“…Photocatalytic water splitting efficiency and hydrogen production selectivity are contingent on catalyst active sites. [70][71][72] Active sites/regions for hydrogen production include adsorption of *H sites, -H-containing functional groups, and photogenerated electron-rich areas. The diiron [Fe 2 S 2 ] subunit operates as the proton reduction catalytic center, with the [Fe 4 S 4 ] cluster facilitating electron transfer to and from the H-cluster active site.…”

“…Photocatalytic water splitting efficiency and hydrogen production selectivity are contingent on catalyst active sites[70][71][72] . Active sites/regions for hydrogen production include adsorption of *H sites, -H-containing functional groups, and photogenerated electron-rich areas.…”