Influence of NH<sub>2</sub> Substitution and Position\non the Photocatalytic Activity of Ti-Based Metal–Organic Frameworks

Titanium-based metal–organic frameworks (MOFs)\nfeaturing\namino groups exhibit promising photocatalytic performance in the hydrogen\nevolution reaction (HER), owing to their reduced band gap and efficient\nspatial separation of charge carriers upon photoexcitation. However,\nit remains unclear how the position of the amino groups in the ligand\naffects their photocatalytic properties. Herein, we introduce two\nisoreticular Ti MOFs based on 4-(4-carboxyphenyl)benzoic acid, <b>Ti-BPDC-o-NH</b><sub><b>2</b></sub> and <b>Ti-BPDC-m-NH</b><sub><b>2</b></sub>, and explore how the spatial position of\namino groups within the framework affects photocatalytic performance.\nThe ortho-location of NH<sub>2</sub> groups in <b>Ti-BPDC-o-NH</b><sub><b>2</b></sub> exhibits up to a 5-fold increase in hydrogen\nproduction compared to <b>Ti</b>-<b>BPDC-m-NH</b><sub><b>2</b></sub>, where the amino groups are located in the meta-position.\nComprehensive characterizations, including time-correlated single\nphoton counting (TCSPC) and ab initio electronic structure calculations,\nsuggest that this difference likely arises from the presence of hydrogen\nbonding between the cluster and linker, alongside the amino group’s\nability for intramolecular charge transfer. Likewise, the influence\nof hydrogen bonding is confirmed by comparative analyses of activities\nand physicochemical properties, such as band gap, as well as by the\ndiminished lifetime and activity of <b>Ti-BPDC-o-NH</b><sub><b>2</b></sub> with increasing Lewis basicity of the solvent.\nAll these findings highlight the importance of secondary interactions\nin MOF photocatalysis.