Isoreticular Squaraine-Linked Titanium-Organic Frameworks for Photocatalytic Water Splitting to Hydrogen Under Visible Light.

Inspired by the excellent photocatalytic activity of TiO₂, titanium metal-organic frameworks (Ti-MOFs) with broad absorption of visible light are regarded as promising photocatalysts, but carboxylate-linkers used in them are mainly limited to the large extended π-electron systems. Developing Ti-MOFs using organic linkers with a donor-acceptor-donor (D-A-D) structure is expected to improve their charge separation but is still challenging. Herein the design of two new isoreticular Ti-MOFs, Ti₆-SQ1 and Ti₆-SQ2 are reported, by using squaraines bearing different electron donors as organic linkers. Discrete fourier transform (DFT) calculations demonstrate that ligand-to-metal charge transfer (LMCT) from the acceptor units of squaraines to the Ti₆-oxo secondary building units (SBUs) drives the photocatalytic water splitting to hydrogen reaction. Compared with Ti₆-SQ2, the shorter distance between the squaraine centers and the Ti₆-oxo SBUs in Ti₆-SQ1 makes stronger LMCT, showing higher photocatalytic hydrogen evolution efficiency of 11.5 mmol g^-1 h^-1 under visible light (λ > 420 nm), which is ≈8 times that of Ti-based MOF photocatalysts reported so far. This work provides a new strategy to design Ti-MOF photocatalysts and understand their structure-property relationship.