Biomimetic Approach toward Visible Light-Driven Hydrogen Generation Based on a Porphyrin-Based Coordination Polymer Gel

There has been a widespread interest in developing self-assembled porphyrin nanostructures to mimic nature's light-harvesting processes. Herein, porphyrin-based coordination polymer gel (CPG) has been developed as a "soft" photocatalyst material for hydrogen (H₂) production from water under visible light. The CPG offers a hierarchical nanofibrous network structure obtained through self-assembly of a terpyridine alkyl-amide appended porphyrin (TPY-POR)-based low molecular weight gelator with ruthenium ions (Ru^II) and produces H₂ with a rate of 5.7 mmol g^-1 h^-1 in the presence of triethylamine (TEA) as a sacrificial electron donor. Further, the [Fe₂(bdt)(CO)₆] (dbt = 1,2-benzenedithiol) cocatalyst, which can mimic the activity of iron hydrogenase, is coassembled in the CPG and shows remarkable improvement in H₂ evolution (catalytic activity; rate ∼10.6 mmol g^-1 h^-1 and turnover number ∼1287). The significant enhancement in catalytic activity was supported by several controlled experiments, including femtosecond transient absorption (TA) spectroscopy and also DFT calculation. The TA study supported the cascade electron transfer process from porphyrin core to [Ru(TPY)₂]²⁺ center, and subsequently, the electron transfers to the cocatalyst [Fe₂(bdt)(CO)₆] for H₂ production.