Management of Triplet States in Modified Mononuclear Ruthenium(II) Complexes for Enhanced Photocatalysis

Controlling the interplay between relaxation and charge/energy transfer processes in the excited states of photocatalysts is crucial for the performance of artificial photosynthesis. Metal-to-ligand charge-transfer triplet states (3MLCT*) of ruthenium(II) complexes are broadly implemented for photocatalysis, but an effective means of managing the triplets for enhanced photocatalysis has been lacking. Herein, We proposed a strategy to considerably prolong the triplet excited-state lifetime by decorating a ruthenium(II) phosphine complex (RuP-1) with pendent polyaromatic hydrocarbons (PAHs). Systematic studies demonstrate that in RuP-4 decorated with anthracene, sub-picosecond electron transfer from anthracene to 3MLCT* leads to a charge-separated state that can mediate the formation of the intra-ligand triplet state (3IL) of anthracene, resulting in an exceptionally long excited-state up to several milliseconds. This triplet management strategy enables impressive photocatalytic reduction of CO2 to CO with a turnover number (TON) of 404, an optimized quantum yield of 43 % and 100 % selectivity, which is the highest reported performance for mononuclear photocatalysts without additional photosensitizers. RuP-4 also catalyzes photochemical hydrogen generation under argon. This work opens up an avenue for regulating the excited-state charge/energy flow for the development of long-lived 3IL multi-functional mononuclear photocatalysts to boost artificial photosynthesis.

Factors that Impact Photochemical Cage Escape Yields

The utilization of visible light to mediate chemical reactions in fluid solutions has applications that range from solar fuel production to medicine and organic synthesis. These reactions are typically initiated by electron transfer between a photoexcited dye molecule (a photosensitizer) and a redox-active quencher to yield radical pairs that are intimately associated within a solvent cage. Many of these radicals undergo rapid thermodynamically favored "geminate" recombination and do not diffuse out of the solvent cage that surrounds them. Those that do escape the cage are useful reagents that may undergo subsequent reactions important to the above-mentioned applications. The cage escape process and the factors that determine the yields remain poorly understood despite decades of research motivated by their practical and fundamental importance. Herein, state-of-the-art research on light-induced electron transfer and cage escape that has appeared since the seminal 1972 review by J. P. Lorand entitled "The Cage Effect" is reviewed. This review also provides some background for those new to the field and discusses the cage escape process of both homolytic bond photodissociation and bimolecular light induced electron transfer reactions. The review concludes with some key goals and directions for future research that promise to elevate this very vibrant field to even greater heights.

Rational design and performance prediction of organic photosensitizer based on TATA+ dye for hydrogen production by photocatalytic decomposition of water

In comparison to metal complexes, organic photosensitive dyes employed in photocatalytic hydrogen production exhibit promising developmental prospects. Utilizing the organic dye molecule TA+0 as the foundational structure, a series of innovative organic dyes, denoted as TA1-1 to TA2-6, were systematically designed. Employing first-principles calculations, we methodically explored the modifying effects of diverse electron-donating groups on the R1 and R2 positions to assess their application potential. Our findings reveal that, relative to the experimentally synthesized TATA+03, the TA2-6 molecule boasts a spatial structure conducive to intramolecular electron transfer, showcasing the most negative reduction potential (Ered = -2.11 eV) and the maximum reaction driving force (△G0 2 = -1.26 eV). This configuration enhances its compatibility with the reduction catalyst, thereby facilitating efficient hydrogen evolution. The TA2-6 dye demonstrates outstanding photophysical properties and a robust solar energy capture capacity. Its maximum molar extinction coefficient (ε) stands at 2.616 × 104 M-1·cm-1, representing a remarkable 292.8% improvement over TATA+03. In conclusion, this research underscores the promising potential of the TA2-6 dye as an innovative organic photosensitizer, positioning it as an efficacious component in homogeneous photocatalytic systems.

Unusual Effects of the Metal Center Coordination Mode on the Photophysical Behavior of the Rhenium(I) and Rhenium(I)-Iridium(III) Complexes

Binuclear transition-metal complexes based on conjugated systems containing coordinating functions are potentially suitable for a wide range of applications, including light-emitting materials, sensors, light-harvesting systems, photocatalysts, etc., due to energy-transfer processes between chromophore centers. Herein we report on the synthesis, characterization, photophysical, and theoretical studies of relatively rare rhenium(I) and rhenium(I)-iridium(III) dyads prepared by using the nonsymmetrical polytopic ligands (NN2 and NN3) with the strongly conjugated phenanthroline and imidazole-quinoline/pyridine coordinating fragments. Availability of these different diimine chelating functions and targeted synthetic procedures allowed one to obtain a series of mononuclear (Re and Ir) and binuclear (Re-Re and Re-Ir) metal complexes with various modes of {Re(CO)3Cl} and {Ir(NC)2} metal fragment coordination. The obtained compounds were characterized by 1D 1H and 2D (COSY and NOESY) NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray diffraction crystallography. The photophysical study of the complexes (absorption, excitation and emission spectra, quantum yields, and excited-state lifetimes) showed that their emission parameters display strong dependence on the manner of metal center coordination to the diimine bidentate functions. The mononuclear complexes with an unoccupied imidazole-quinoline/pyridine fragment [Re(NN2), Re(NN3), and Ir(NC2)2(NN2)] or those containing a coordinated {Ir(NC)2} fragment in this position [Ir(NC2)2(NN1) and Re(NN2)Ir(NC1)2-Re(NN2)Ir(NC4)2] exhibit moderate-to-intense phosphorescence (quantum yields vary from 3% to 56% in a degassed solution), whereas the complexes containing a {Re(CO)3Cl} moiety in the imidazole-quinoline/pyridine position [Re2(NN2), Re2(NN3), and Ir(NC2)2(NN2)Re] demonstrate a strong reduction in the phosphorescence efficiency with a quantum yield of ≪0.1%. Quenching of the phosphorescence in the latter types of emitters is discussed in terms of a strong decrease in the radiative rate constants for these complexes compared to their analogues mentioned above, while the nonradiative constants remain nearly unchanged. Theoretical density functional theory (DFT) and time-dependent DFT (TD DFT) calculations, including evaluation of the radiative rate constants for the couple of structurally analogous complexes with and without a {Re(CO)3Cl} moiety coordinated to the imidazole-quinoline/pyridine chelating function, confirmed the observed trend in the variation of the emission intensity.

Electrochemical Properties of a Rhodium(III) Mono-Terpyridyl Complex and Use as a Catalyst for Light-Driven Hydrogen Evolution in Water

Molecular hydrogen (H₂) is considered one of the most promising fuels to decarbonize the industrial and transportation sectors, and its photocatalytic production from molecular catalysts is a research field that is still abounding. The search for new molecular catalysts for H₂ production with simple and easily synthesized ligands is still ongoing, and the terpyridine ligand with its particular electronic and coordination properties, is a good candidate to design new catalysts meeting these requirements. Herein, we have isolated the new mono-terpyridyl rhodium complex, [Rh^III(tpy)(CH₃CN)Cl₂](CF₃SO₃) (Rh-tpy), and shown that it can act as a catalyst for the light-induced proton reduction into H₂ in water in the presence of the [Ru(bpy)₃]Cl₂ (Ru) photosensitizer and ascorbate as sacrificial electron donor. Under photocatalytic conditions, in acetate buffer at pH 4.5 with 0.1 M of ascorbate and 530 μM of Ru, the Rh-tpy catalyst produces H₂ with turnover number versus catalyst (TON_Cat*) of 300 at a Rh concentration of 10 μM, and up to 1000 at a concentration of 1 μM. The photocatalytic performance of Ru/Rh-tpy/HA^-/H₂A has been also compared with that obtained with the bis-dimethyl-bipyridyl complex [Rh^III(dmbpy)₂Cl₂]⁺ (Rh2) as a catalyst in the same experimental conditions. The investigation of the electrochemical properties of Rh-tpy in DMF solvent reveals that the two-electrons reduced state of the complex, the square-planar [Rh^I(tpy)Cl] (Rh^I-tpy), is quantitatively electrogenerated by bulk electrolysis. This complex is stable for hours under an inert atmosphere owing to the π-acceptor property of the terpyridine ligand that stabilizes the low oxidation states of the rhodium, making this catalyst less prone to degrade during photocatalysis. The π-acceptor property of terpyridine also confers to the Rh-tpy catalyst a moderately negative reduction potential (Ep_c(Rh^III/Rh^I) = -0.83 V vs. SCE in DMF), making possible its reduction by the reduced state of Ru, [Ru^II(bpy)(bpy^•-)]⁺ (Ru^-) (E_1/2(Ru^II/Ru^-) = -1.50 V vs. SCE) generated by a reductive quenching of the Ru excited state (*Ru) by ascorbate during photocatalysis. A Stern-Volmer plot and transient absorption spectroscopy confirmed that the first step of the photocatalytic process is the reductive quenching of *Ru by ascorbate. The resulting reduced Ru species (Ru^-) were then able to activate the Rh^III-tpy H₂-evolving catalyst by reduction generating Rh^I-tpy, which can react with a proton on a sub-nanosecond time scale to form a Rh^III(H)-tpy hydride, the key intermediate for H₂ evolution.

Photo-generation of H2 by Heterometallic Complexes

Multiple and different metals in a complex can accomplish single and sequential multi-step reactions, providing valuable procedures to obtain chemicals in one-pot synthetic routes. Biology has shown how cooperative catalysis...

Efficient Convergent Energy Transfer in a Stereoisomerically Pure Heptanuclear Luminescent Terpyridine-Based Ru(II)-Os(II) Dendrimer

The stereoisomerically pure synthesis of a novel heptanuclear Ru(II)-Os(II) antenna bearing multitopic terpyridine ligands is reported. An unambiguous structural characterization was obtained by ¹H NMR spectroscopy and ion mobility spectrometry (IMS-MS). The heptanuclear complex exhibits large molar absorption coefficients (77900 M^-1 cm^-1 at 497 nm) and undergoes unitary, downhill, convergent energy transfer from the peripheral Ru(II) subunits to the central Os(II) that displays photoluminescence with a lifetime (τ = 161 ns) competent for diffusional excited-state electron transfer reactivity in solution.

Panchromatic dirhodium photocatalysts for dihydrogen generation with red light

A series of three dirhodium complexes cis-[Rh₂(DPhB)₂(bncn)₂](BF₄)₂ (1, DPhB = diphenylbenzamidine; bncn = benzocinnoline), cis-[Rh₂(DPhTA)₂(bncn)₂](BF₄)₂ (2, DPhTA = diphenyltriazenide), and cis-[Rh₂(DPhF)₂(bncn)₂](BF₄)₂ (3, DPhF = N,N′-diphenylformamidinate) shown to act as single-molecule photocatalysts for H₂ production was evaluated. Complexes 1–3 are able to generate H₂ in the absence of any other catalyst in homogenous acidic solution upon irradiation with red light in the presence of the sacrificial electron donor BNAH (1-benzyl-1,4-dihydronicotinamide). The excited state of each complex is reductively quenched by BNAH, producing the corresponding one-electron reduced complex. The latter is also able to absorb a photon and oxidize another BNAH molecule, producing the doubly-reduced, activated form of the catalyst that is able to generate H₂. The present work shows the effect of substitution on the bridging ligands on the driving force for reductive quenching and hydricity of the proposed active intermediate, both of which affect the efficiency of hydrogen production. Complexes 1–3 operate following a double reductive quenching mechanism and, importantly, are active with red light. This work lays the foundation for the design of single-molecule photocatalysts that operate from the ultraviolet to the near-infrared, such that solar photons throughout this entire range are harnessed and utilized for solar energy conversion.

Three dirhodium complexes cis-[Rh₂(DPhB)₂(bncn)₂](BF₄)₂, cis-[Rh₂(DPhTA)₂(bncn)₂](BF₄)₂ and cis-[Rh₂(DPhF)₂(bncn)₂](BF₄)₂ are shown to act as single-molecule photocatalysts for H₂ production.

Kinetics and Mechanism of Intramolecular Electron Transfer in Ru(II)-Re(I) Supramolecular CO2-Reduction Photocatalysts: Effects of Bridging Ligands

The supramolecular photocatalysts in which a Ru(II) complex as a molecular redox photosensitizer unit and a Re(I) complex as a molecular catalyst unit are connected with a various alkyl or ether chain have attracted attention because they can efficiently photocatalyze CO₂ reduction with high durability and high selectivity of CO formation, especially on various solid materials such as semiconductor electrodes and mesoporous organosilica. The intramolecular electron transfer from the one-electron reduced photosensitizer unit to the catalyst unit, which follows excitation of the photosensitizer unit and subsequent reductive quenching of the excited photosensitizer unit by a reductant, is one of the most important processes in the photocatalytic reduction of CO₂. We succeeded in determining the rate constants of this intramolecular electron transfer process by using subnanosecond time-resolved IR spectroscopy. The logarithm of rate constants shows a linear relationship with the lengths of the bridging chain in the supramolecular photocatalysts with one bridging alkyl or ether chain. In conformity with the exponential decay of the wave function and the coupling element in the long-distance electron transfer, the apparent decay coefficient factor (β) in the supramolecular photocatalysts with one bridging chain was determined to be 0.74 Å^-1. In the supramolecular photocatalyst with two ethylene chains connecting between the photosensitizer and catalyst units, on the other hand, the intramolecular electron transfer rate is much faster than that with only one ethylene chain. These results strongly indicate that the intramolecular electron transfer from the one-electron reduced species of the Ru photosensitizer unit to the Re catalyst unit proceeds by the through-bond mechanism.

Design and photophysical studies of iridium(iii)–cobalt(iii) dyads and their application for dihydrogen photo-evolution

We report several new dyads constituted of cationic iridium(iii) photosensitizers and cobalt(iii) catalyst connected via free pendant pyridine on the photosensitizers.

Mini-review on an engineering approach towards the selection of transition metal complex-based catalysts for photocatalytic H2 production

Advances in transition-metal (Ru, Co, Cu, and Fe) complex-based catalysts since 2000 are briefly summarized in terms of catalyst selection and application for photocatalytic H₂ evolution.

Metal-complex chromophores for solar hydrogen generation

Solar H₂ generation from water has been intensively investigated as a clean method to convert solar energy into hydrogen fuel. During the past few decades, many studies have demonstrated that metal complexes can act as efficient photoactive materials for photocatalytic H₂ production. Here, we review the recent progress in the application of metal-complex chromophores to solar-to-H₂ conversion, including metal-complex photosensitizers and supramolecular photocatalysts. A brief overview of the fundamental principles of photocatalytic H₂ production is given. Then, different metal-complex photosensitizers and supramolecular photocatalysts are introduced in detail, and the most important factors that strictly determine their photocatalytic performance are also discussed. Finally, we illustrate some challenges and opportunities for future research in this promising area.

A Bisamide Ruthenium Polypyridyl Complex as a Robust and Efficient Photosensitizer for Hydrogen Production

A photosensitizer based on a ruthenium complex of a bisamide-polypyridyl ligand gives rise to a large improvement in photocatalytic stability, rate of activity, and efficiency in photocatalytic H₂ production compared to [Ru(bpy)₃ ]²⁺ (bpy=2,2'-bpyridine). The bisamide ruthenium polypyridyl complex combined with a cobaltoxime-based photocatalyst was found to be highly efficient under blue-light (turnover number (TON)=7800) and green-light irradiation (TON=7200) whereas [Ru(bpy)₃ ]²⁺ was significantly less effective with a TON of 2600 and 1100, respectively. The greatest improvement was under red-light-emitting diodes, with bisamide ruthenium polypyridyl complex and cobaltoxime exhibiting a TON of 4200 compared to [Ru(bpy)₃ ]²⁺ and cobaltoxime at a TON of only 71.

Robust Cooperative Photo-oxidation of Sulfides without Sacrificial Reagent under Air Using a Dinuclear RuII -CuII Assembly

A molecular chromophore-catalyst assembly containing a chromophore ruthenium(II) center (RuIIchro ) and a catalytic copper(II) center (CuIIcat ) has been prepared easily. The assembly was employed for photocatalytic oxidation of sulfides without sacrificial reagent in the presence of dioxygen under blue light irradiation. Unprecedented turnover number (TON) up to 32 000 was achieved. It was elucidated that an electron transferred from excited state of chromophore RuII*chro to CuIIcat along with generation of CuIcat that was further activated by O₂ . These results demonstrate a promising strategy for efficient cooperative photocatalytic reactions under air using the chromophore-catalyst assembly.

Impact of Photosensitizing Multilayered Structure on Ruthenium(II)-Dye-Sensitized TiO2-Nanoparticle Photocatalysts

To improve the efficiency of photoinduced charge separation on the surface of dye-sensitized TiO₂ nanoparticles, we synthesized the Ru(II)-photosensitizer-immobilized, Pt-cocatalyst-loaded TiO₂ nanoparticles RuCP ² @Pt-TiO₂, RuCP ² -Zr-RuP ⁶ @Pt-TiO₂, and RuCP ² -Zr-RuP ⁴ -Zr-RuP ⁶ @Pt-TiO₂ (RuCP ² = [Ru(bpy)₂(mpbpy)]^2-, RuP ⁴ = [Ru(bpy)(pbpy)₂]^6-, RuP ⁶ = [Ru(pbpy)₃]^10-, H₄mpbpy = 2,2'-bipyridine-4,4'-bis(methanephosphonic acid), and H₄pbpy = 2,2'-bipyridine-4,4'-bis(phosphonic acid)) using phosphonate linkers with bridging Zr⁴⁺ ions. X-ray fluorescence and ultraviolet-visible absorption spectra revealed that a layered molecular structure composed of Ru(II) photosensitizers and Zr⁴⁺ ions (i.e., RuCP ² -Zr-RuP ⁶ and RuCP ² -Zr-RuP ⁴ -Zr-RuP ⁶ ) was successfully formed on the surface of Pt-TiO₂ nanoparticles, which increased the surface coverage from 0.113 nmol/cm² for singly layered RuCP ² @Pt-TiO₂ to 0.330 nmol/cm² for triply layered RuCP ² -Zr-RuP ⁴ -Zr-RuP ⁶ @Pt-TiO₂. The photocatalytic H₂ evolution activity of the doubly layered RuCP ² -Zr-RuP ⁶ @Pt-TiO₂ was three times higher than that of the singly layered RuCP ² @Pt-TiO₂, whereas the activity of triply layered RuCP ² -Zr-RuP ⁴ -Zr-RuP ⁶ @Pt-TiO₂ was less than half of that for RuCP ² @Pt-TiO₂. The photosensitizing efficiencies of these Ru(II)-photosensitizer-immobilized nanoparticles for the O₂ evolution reaction catalyzed by the Co(II)-containing Prussian blue analogue [Co^II(H₂O)₂]_1.31[{Co^III(CN)₆}_0.63{Pt^II(CN)₄}_0.37] decreased as the number of Ru(II)-photosensitizing layers increased. Thus, crucial aspects of the energy- and electron-transfer mechanism for the photocatalytic H₂ and O₂ evolution reactions involve not only the Ru(II)-complex-TiO₂ interface but also the multilayered structure of the Ru(II)-photosensitizers on the Pt-TiO₂ surface.

Selectivity control between Mizoroki-Heck and homo-coupling reactions for synthesising multinuclear metal complexes: unique addition effects of tertiary phosphines and O2

The addition of a tertiary phosphine and O₂ to reaction solutions strongly affected the reactivity and selectivity of coupling reactions between transition metal complexes. The Mizoroki-Heck reaction between metal complexes with bromo and those with vinyl groups in the diimine ligand did not proceed using Pd(OAc)₂ in the presence of 2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl (Sphos) under Ar but proceeded selectively after injection of air into the reaction vessel. In the absence of the phosphine ligand, on the other hand, not only the Mizoroki-Heck reaction but also a homo-coupling reaction between the metal complexes with the bromo groups proceeded at the same time. Mechanistic investigation showed that nanoparticles of Pd species were produced in the absence of the phosphine ligand and worked as catalysts for both the Mizoroki-Heck and homo-coupling reactions. On the other hand, larger Pd particles, which were produced in the presence of Sphos but after addition of air for oxidising Sphos, selectively catalysed the Mizoroki-Heck reaction. 'Molecular' Pd species that were stabilised in the presence of non-oxidised Sphos could not catalyse both coupling reactions under the reaction conditions. Based on these results, reaction conditions were established for the selective progress of the Mizoroki-Heck and the homo-coupling reactions.

A Ruthenium Complex-Porphyrin-Fullerene-Linked Molecular Pentad as an Integrative Photosynthetic Model

A ruthenium complex, porphyrin sensitizer, fullerene acceptor molecular pentad has been synthesized and a long-lived hole-electron pair was achieved in aqueous solution by photoinduced multistep electron transfer: Upon irradiation by visible light, the excited-state of a zinc porphyrin (¹ ZnP*) was quenched by fullerene (C₆₀ ) to afford a radical ion pair, ^1,3 (ZnP^.+ -C₆₀^.- ). This was followed by the subsequent electron transfer from a water oxidation catalyst unit (Ru^II ) to ZnP^.+ to give the long-lived charge-separated state, Ru^III -ZnP-C₆₀^.- , with a lifetime of 14 μs. The ZnP worked as a visible-light-harvesting antenna, while the C₆₀ acted as an excellent electron acceptor. As a consequence, visible-light-driven water oxidation by this integrated photosynthetic model compound was achieved in the presence of sacrificial oxidant and redox mediator.

Synthesis, Characterization, and Photocatalytic H2-Evolving Activity of a Family of [Co(N4Py)(X)](n+) Complexes in Aqueous Solution

A series of [Co(III)(N4Py)(X)](ClO4)n (X = Cl(-), Br(-), OH(-), N3(-), NCS(-)-κN, n = 2: X = OH2, NCMe, DMSO-κO, n = 3) complexes containing the tetrapyridyl N5 ligand N4Py (N4Py = 1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine) has been prepared and fully characterized by infrared (IR), UV-visible, and NMR spectroscopies, high-resolution electrospray ionization mass spectrometry (HRESI-MS), elemental analysis, X-ray crystallography, and electrochemistry. The reduced Co(II) and Co(I) species of these complexes have been also generated by bulk electrolyses in MeCN and characterized by UV-visible and EPR spectroscopies. All tested complexes are catalysts for the photocatalytic production of H2 from water at pH 4.0 in the presence of ascorbic acid/ascorbate, using [Ru(bpy)3](2+) as a photosensitizer, and all display similar H2-evolving activities. Detailed mechanistic studies show that while the complexes retain the monodentate X ligand upon electrochemical reduction to Co(II) species in MeCN solution, in aqueous solution, upon reduction by ascorbate (photocatalytic conditions), [Co(II)(N4Py)(HA)](+) is formed in all cases and is the precursor to the Co(I) species which presumably reacts with a proton. These results are in accordance with the fact that the H2-evolving activity does not depend on the chemical nature of the monodentate ligand and differ from those previously reported for similar complexes. The catalytic activity of this series of complexes in terms of turnover number versus catalyst (TONCat) was also found to be dependent on the catalyst concentration, with the highest value of 230 TONCat at 5 × 10(-6) M. As revealed by nanosecond transient absorption spectroscopy measurements, the first electron-transfer steps of the photocatalytic mechanism involve a reductive quenching of the excited state of [Ru(bpy)3](2+) by ascorbate followed by an electron transfer from [Ru(II)(bpy)2(bpy(•-))](+) to the [Co(II)(N4Py)(HA)](+) catalyst. The reduced catalyst then enters into the H2-evolution cycle.

A new hydrophilic supramolecular photocatalyst for the production of H2 in aerobic aqueous solutions

Addition of sulfonated ligands into a Ru,Rh,Ru photocatalyst increases solubility and H₂ production in aqueous aerobic solutions.

Generation of a stable supramolecular hydrogen evolving photocatalyst by alteration of the catalytic center

The change of the catalytic center from MX₂ to RhCp*Cl leads to a stability boost in [(tbbpy)₂Ru(tpphz)] based supramolecular photocatalysts.

A new Ru(II)Rh(III) bimetallic with a single Rh-Cl bond as a supramolecular photocatalyst for proton reduction

A new Ru(II)Rh(III) structural motif [(bpy)2Ru(dpp)RhCl(tpy)](4+) with one halide on the Rh(III) center demonstrates light-driven proton reduction ability, establishing that two halide ligands are not mandatory despite all prior systems containing a cis-RhCl2 catalytic site. This new design provides a novel approach to modulate Rh(III) redox behavior and catalytic activity with insight into catalytic intermediates.