Guest modulating the photoactivity of a titanium-oxide cage

Two host-guest Ti-oxide clusters, Ti₁₄(NH₄)₂ and Ti₁₄Cs₂, were synthesized and thoroughly characterized. They possess a rarely seen biloculate structure that encapsulates two NH₄ ⁺ and Cs⁺ guests, respectively. Interestingly, alkali metal cations can exchange places with NH₄ ⁺. The ability of the host to capture the guest cations follows the order Cs⁺ > NH₄ ⁺ > Rb⁺ > K⁺. The guests heavily influence the physiochemical properties and photocatalytic activities of the complexes. Ti₁₄Cs₂ exhibits a redshifted visible-light absorption edge, increased charge-separation properties, and enhanced interfacial charge-transfer ability compared to Ti₁₄(NH₄)₂. It also demonstrates excellent performance in photocatalytic CO₂/epoxide cycloaddition reactions regarding the reaction rate, scalability, sunlight usage, catalyst recyclability, and stability. This study presents a novel Ti-oxide-based cage cluster with exchangeable guests and provides insights for enhancing the solar harvesting applications of Ti-oxide cages.