Development and comparative investigation of structural and functional properties of Ni-based metal-organic frameworks designed for photocatalytic hydrogen production.

Designing efficient, economical and stable photocatalysts utilizing the vast potential of sunlight remains a challenge, but at the same time it presents a valuable pursuit in developing alternative renewable energy solutions. Guided by this objective we synthesized and characterized two new metal-organic frameworks (MOFs), namely 1 (Ni-DIA-TBAPY, DIA = 9,10-di(1H-imidazol-1-yl)anthracene, TBAPY = 1,3,6,8-tetrakis(p-benzoic acid)pyrene) and 2 (Ni-DIP-TBAPY, DIP = 1,6-di(1H-imidazol-1-yl)pyrene) designed to act as photocatalysts. Structural analyses using single-crystal X-ray diffraction (SCXRD) revealed their practically identical secondary building units. However, their 3D-frameworks differ significantly, with one exhibiting a flexible structure and the other a rigid one. Both structures demonstrate chemical and thermal stability by maintaining framework integrity even in boiling water, but the flexible structure of 1 exhibits unusual solvent-induced reversible crystalline-to-amorphous transition. Based on their similar optical properties which proved their ability to utilize visible light, their photocatalytic performance in the hydrogen evolution reaction (HER) was investigated. Under simulated AM 1.5G solar irradiation both MOFs proved to be active photocatalysts without any cocatalysts. However, their performance differed significantly in their long-term stability, with 1 partially degrading over consecutive reaction cycles, while 2 remained completely stable without significant loss in photocatalytic activity, proving its better applicability.