Cr-Ti Mixed Oxide Molecular Cages: Synthesis, Structure, Photoresponse, and Photocatalytic Properties

The solvothermal reaction of titanium isopropoxide and chromate in the presence of benzoate produced two novel host-guest clusters encapsulating Cs⁺ or H₃O⁺, (HO)@TiCr and Cs@TiCr. The most remarkable feature is that the Ti₇O₇ ring is concentrically embraced by a Cr₁₄O₁₄ ring to form a rigid TiCr host. ESI-MS and ¹³³Cs NMR revealed that the overall framework structures are preserved, whereas the benzoate ligands on the two clusters may be labile in solutions. Both (HO)@TiCr and Cs@TiCr exhibit good UV-vis light-responsive properties and photocatalytic activities, with absorption edges extending up to 780 nm. Cs@TiCr is an effective visible-light-responsive photocatalyst in both the heterogeneous methylene dye degradation and homogeneous CO₂ cycloaddition reaction under mild conditions like room temperature and 1 bar of CO₂. According to the mechanism studies, Cs⁺, as a rigid guest, can significantly improve the photogenerated charge separation efficiency of the TiCr host, thereby improving its interface charge separation properties, photocurrent, and photocatalytic activities. Our findings not only provide new members of heterometallic titanium oxide clusters to enrich the metal oxide cluster family but also open up new possibilities for their photoresponses, which may play an important role in solar energy harvesting for sustainable chemistry.