Dual-Ligand Ti-MOFs with Push–Pull Effect for Photocatalytic H2 Production

Enhancing the photogenerated electrons transfer efficiency is crucial for photocatalytic reactions. Herein, a dual-ligand-induced push-pull effect was manipulated to intensify the transfer of photogenerated electrons between organic ligands and metal clusters using NH₂-MIL-125(Ti), a kind of Ti-based metal-organic framework (MOF), as the model system. The dual-ligand MOF, NH₂/Cl-MIL-125, was designed and synthesized based on the Hammett constant (σ_m), in which -NH₂ (σ_m = -0.16) and -Cl (σ_m = 0.37) were selected as the electron-pushing group and the electron-pulling group, respectively. Meanwhile, -CH₃ (σ_m = -0.07, electron-pushing) and -H (σ_m = 0, neither electron-pushing nor electron-pulling) were selected as the reference groups to prepare NH₂/CH₃-MIL-125 and NH₂/H-MIL-125, respectively, to validate the electron push-pull effect. NH₂/Cl-MIL-125 (5.32 mmol g^-1 h^-1) exhibits a higher photocatalytic H₂ evolution activity than single-ligand NH₂-MIL-125 (1.93 mmol g^-1 h^-1), NH₂/CH₃-MIL-125 (4.45 mmol g^-1 h^-1), and NH₂/H-MIL-125 (4.73 mmol g^-1 h^-1) under full-spectrum irradiation. The result can be attributed to the electron push-pull effect between -NH₂ and -Cl, which boosts the electron transfer along the ligand-metal-ligand direction. Our dual-ligand-induced push-pull strategy for enhancing the electron transfer may offer some novel insights into the rational design and synthesis of photocatalysts for many other reactions.