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 dualligand-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 (sigma m), in which -NH2 (sigma m = -0.16) and - Cl (sigma m = 0.37) were selected as the electron-pushing group and the electron-pulling group, respectively. Meanwhile, -CH3 (sigma m = - 0.07, electron-pushing) and -H (sigma 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.