Influence of NH₂ Substitution and Position on the Photocatalytic Activity of Ti-Based Metal–Organic Frameworks

Titanium-based metal–organic frameworks (MOFs) featuring amino groups exhibit promising photocatalytic performance in the hydrogen evolution reaction (HER), owing to their reduced band gap and efficient spatial separation of charge carriers upon photoexcitation. However, it remains unclear how the position of the amino groups in the ligand affects their photocatalytic properties. Herein, we introduce two isoreticular Ti MOFs based on 4-(4-carboxyphenyl)benzoic acid, Ti-BPDC-o-NH2 and Ti-BPDC-m-NH2, and explore how the spatial position of amino groups within the framework affects photocatalytic performance. The ortho-location of NH2 groups in Ti-BPDC-o-NH2 exhibits up to a 5-fold increase in hydrogen production compared to Ti-BPDC-m-NH2, where the amino groups are located in the meta-position. Comprehensive characterizations, including time-correlated single photon counting (TCSPC) and ab initio electronic structure calculations, suggest that this difference likely arises from the presence of hydrogen bonding between the cluster and linker, alongside the amino group’s ability for intramolecular charge transfer. Likewise, the influence of hydrogen bonding is confirmed by comparative analyses of activities and physicochemical properties, such as band gap, as well as by the diminished lifetime and activity of Ti-BPDC-o-NH2 with increasing Lewis basicity of the solvent. All these findings highlight the importance of secondary interactions in MOF photocatalysis.