Tetra- and Heptametallic Ru(II),Rh(III) Supramolecular Hydrogen Production Photocatalysts

Abstract

Supramolecular mixed metal complexes combining the trimetallic chromophore [{(bpy)Ru(dpp)}Ru(dpp)] (Ru) with [Rh(bpy)Cl] or [RhCl] catalytic fragments to form [{(bpy)Ru(dpp)}Ru(dpp)RhCl(bpy)](PF) (RuRh) or [{(bpy)Ru(dpp)}Ru(dpp)]RhCl(PF) (RuRhRu) (bpy = 2,2'-bipyridine and dpp = 2,3-bis(2-pyridyl)pyrazine) catalyze the photochemical reduction of protons to H. This first example of a heptametallic Ru,Rh photocatalyst produces over 300 turnovers of H upon photolysis of a solution of acetonitrile, water, triflic acid, and N,N-dimethylaniline as an electron donor. In contrast, the tetrametallic RuRh produces only 40 turnovers of H due to differences in the excited state properties and nature of the catalysts upon reduction as ascertained from electrochemical data, transient absorption spectroscopy, and flash-quench experiments. While the lowest unoccupied molecular orbital of RuRh is localized on a bridging ligand, it is Rh-centered in RuRhRu facilitating electron collection at Rh in the excited state and reductively quenched state. The Ru → Rh charge separated state of RuRhRu is endergonic with respect to the emissive Ru → dpp MLCT excited and cannot be formed by static electron transfer quenching of theMLCT state. Instead, a mechanism of subnanosecond charge separation from high lying states is proposed. Multiple reductions of Ru and RuRh using sodium amalgam were carried out to compare UV-vis absorption spectra of reduced species and to evaluate the stability of highly reduced complexes. The Ru and RuRh can be reduced by 10 and 13 electrons, respectively, to final states with all bridging ligands doubly reduced and all bpy ligands singly reduced.

Robust Heterogeneous Photocatalyst for Visible-Light-Driven Hydrogen Evolution Promotion: Immobilization of a Fluorescein Dye-Encapsulated Metal–Organic Cage on TiO2

“…Recently, more and more explorations have been made on intramolecular photochemical molecular devices (PMDs) by combining the catalytic centers, chromophoric photosensitizer, and electron relay elements at the molecular level for efficient energy and electron transfer in photocatalysis. , Versatile active polymetallic PMDs, such as Ru 3 RhRu 3 , bridged Ru­(II) and Pt­(II)-based complexes, and RuPt, have been explored for homogeneous catalysis. Moreover, some efforts in PMDs based on MOCs, for example, Pd-Ru MOC-16, − have been made to promote the effective and directional electron transfer from the light-absorbing centers to catalytic metal active sites, owing to the multiple electron transfer required for H 2 production.…”

“…Moreover, some efforts in PMDs based on MOCs, for example, Pd-Ru MOC-16, − have been made to promote the effective and directional electron transfer from the light-absorbing centers to catalytic metal active sites, owing to the multiple electron transfer required for H 2 production. The conventional route to fabricating multiple photosensitized ligands at the photocentre or catalytic center is to provide a variety of independent energy transfer and electron collection channels. − Another promising method has been applied for encapsulating organic sensitizers inside the MOCs by increasing the proximity between the sensitizer substrate and the active metal center, and thus improving the effective photocatalytic reaction. This strategy could construct an enzyme-mimicking photochemical molecular device at a supramolecular level, also named as a supramolecular photochemical molecular device (SPMD), which is conducive to efficient energy and/or electron transfers in photocatalysis. , …”

Water‐Soluble Conjugated Molecule for Solar‐Driven Hydrogen Evolution from Salt Water

“…To eliminate this obstacle, we learned from natural photosynthesis systems in which many light‐harvesting molecules are noncovalently surrounded and simultaneously transport photogenerated electrons to one reaction center . We report here an electrostatically induced supramolecular approach in which small molecules are constructed into self‐assembled superstructures to provide a platform for loading of Pt nanoparticle catalyst and to facilitate charge transfer to reach a high photocatalysis efficiency.…”

Efficient Photogeneration of Hydrogen Boosted by Long-Lived Dye-Modified Ir(III) Photosensitizers and Polyoxometalate Catalyst

“…4,[9][10][11] Notwithstanding, key issues have been identified regarding the exploration of viable, efficient, and inexpensive water reduction catalysts (WRCs), as well as the design of tunable, stable, and strong visible-light-responsive photosensitizers. [12][13][14][15][16] As an emerging type of multielectron-transfer WRCs, transition-metal-substituted polyoxometalates (TMSPOMs) with tunable structural compositions, physicochemical, and photochemical properties, could combine the advantages of both heterogeneous metal oxide catalysts (e.g., stability) and homogeneous molecular catalysts (e.g., high activity, selectivity, and tunability). 2,[17][18][19][20][21] After years of numerous efforts, a series of Mn, Ni, Co, and Cu-substituted POMs have been synthesized and studied for catalytic H 2 evolution when coupled with visible-light-absorbing photosensitizers and sacrificial reagents.…”