The Zr₈O₆ Secondary Building Unit and Porphyrin Linker Catalyze Light-Driven H₂ Evolution in Porphyrin-Based Metal...

The four Zr₈O₆-based metal-organic frameworks (MOFs), porous coordination network (PCN) 221 M, comprising earth-abundant metalloporphyrin tetracarboxylate (M-TCPP, M: 2 H, Zn, Ni, and a mixture of 1:1 Zn and Ni), are investigated for light-driven H₂ evolution reaction (HER) in water. Under the irradiation of a 405 nm light emitting diode source and in the presence of triethanolamine (TEOA) as a sacrificial electron donor, the photocatalytic HER activity of PCN 221 varies with the metal center in the porphyrin linker. Among the tested MOFs, the Zn-porphyrin derivative (PCN 221 Zn) produces H₂ at a TON = 4, which is about seven times greater than that of homogeneous Zn-TCPP (0.6) and superior to its 2 H (2.7), Ni (0.40), and ZnNi (1.6) analogs. Detailed photochemical studies via time-resolved and steady-state spectroscopy reveal two distinct charge transfer pathways: Direct H₂ evolution from Zn-TCPP itself, and electron transfer from the Zn-TCPP photosensitizer to the Zr₈O₆ SBU catalytic sites. The improved HER performance of PCN 221 Zn is attributed to its favorable features, such as optical absorption, excited-state properties, and charge separation dynamics, as well as the coordination of TEOA. This study provides fundamental insights into the design of MOF-based heterogeneous photocatalysts exploiting earth-abundant metal-based porphyrin for solar fuel generation.