A photosensitive metal–organic cage (MOC), denoted as MOC-Q2, consisting of two Pd2+ metal ions and four tridentate ligands, is synthesized, and then MOC-Q2 is linked with titania to obtain a hybrid material TiO2-MOC-Q2, which exhibits remarkable photocatalytic H2 evolution activities. After loading a molecular Re catalyst, the ReP/TiO2-MOC-Q2 (6.5 wt %) photocatalyst demonstrates a selective performance of CO2 reduction to CO in dry CO2-saturated dimethylformamide (DMF) solution. By adjusting only the water content in the reaction system, the ternary catalyst can produce syngas with controlled ratios of CO/H2 varying from 15:1 to 1:2. CO/H2 gas evolution rates of 638/604 and 499/964 μmol g–1 h–1 are gained with a CO/H2 ratio of 1:1 and 1:2 in 5 h experiments, respectively. And the corresponding TONCO/TONH2 in the 20 h persistence test is calculated to be 875/458. This system presents one of the best activities among the reported syngas production photocatalysts without using noble ruthenium complexes. This work provides a promising way for the development of multifunctional heterogeneous MOC-based photocatalysts.

Two metal-free calix[4]­arene dyes with pyridyl anchoring groups (C4BTP and C4STP) and a comparative single-chain dye (M-BTP) for comparison have been synthesized and applied in dye-sensitized photoelectrochemical cells in corporation with a pyridyl anchor-based molecular water oxidation catalyst (RuPy). All the devices based on the three dyes can conduct overall water splitting reaction under visible light with low or zero bias. It is confirmed that the cone structure and multiple light-harvesting units of calixarene dyes bring superior durability and photoelectroconversion property compared with chain dyes M-BTP, and the introduction of alkalinous pyridyl anchoring groups improves the adsorption firmness on TiO2 in aqueous solution through the Ti–N bond. The optimized TiO2|C4BTP+RuPy photoanode gives the highest photocurrent density of over 800 μA cm–2, while the TiO2|C4STP+RuPy photoanode exhibits the best Faradaic efficiency of 64.9% for O2 evolution. To meet the needs of practical applications, the TiO2|C4BTP+RuPy photoanode is utilized in a two-electrode system with 0 V bias, giving a steady photocurrent density of 42 μA cm–2 in 3600 s and a Faradaic efficiency of 45.9% for O2 evolution. This study offers an inspiration for developing efficient and durable organic photosensitizers in photocatalytic water-splitting devices.

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The demand for improving the activity, durability, and recyclability of metal-organic cages (MOCs) that work as photocatalytic molecular devices in a homogeneous system has promoted research to combine them with other solid materials. An M2L4 type photosensitive metal-organic cage MOC-Q2 with light-harvesting ligands and catalytic Pd2+ centers has been synthesized and further heterogenized with graphitic carbon nitride to prepare a robust direct Z-scheme heterojunction photocatalyst for visible-light-driven hydrogen generation. The optimized g-C3N4/MOC-Q2 (0.7 wt%) sample exhibits a high H2 evolution activity of 6423 μmol g−1 h−1 in 5 h, and a total turnover number of 39,695 after 10 h, significantly superior to the bare MOC-Q2 used in the homogeneous solution and the comparison sample Pd/g-C3N4/L-4. The enhanced performances of g-C3N4/MOC-Q2 can be ascribed to its direct Z-scheme heterostructure, which effectively improves the charge separation and transfer efficiency. This work presents a rational approach of designing a binary photocatalytic system through combing micromolecular MOCs with heterogeneous semiconductors for water splitting.