Light‐Harvesting Photocatalysis for Water Oxidation Using Mesoporous Organosilica

An organic‐based photocatalysis system for water oxidation, with visible‐light harvesting antennae, was constructed using periodic mesoporous organosilica (PMO). PMO containing acridone groups in the framework (Acd‐PMO), a visible‐light harvesting antenna, was supported with [Ru^II(bpy)₃²⁺] complex (bpy=2,2′‐bipyridyl) coupled with iridium oxide (IrO_x) particles in the mesochannels as photosensitizer and catalyst, respectively. Acd‐PMO absorbed visible light and funneled the light energy into the Ru complex in the mesochannels through excitation energy transfer. The excited state of Ru complex is oxidatively quenched by a sacrificial oxidant (Na₂S₂O₈) to form Ru³⁺ species. The Ru³⁺ species extracts an electron from IrO_x to oxidize water for oxygen production. The reaction quantum yield was 0.34 %, which was improved to 0.68 or 1.2 % by the modifications of PMO. A unique sequence of reactions mimicking natural photosystem II, 1) light‐harvesting, 2) charge separation, and 3) oxygen generation, were realized for the first time by using the light‐harvesting PMO.     

Model Reactions for Photosynthesis—Photoinduced Charge and Energy Transfer between Covalently Linked Porphyrin and Quinone Units

Photosynthesis is one of the fascinating fields of current interdisciplinary research. It seems miraculous that nature, in the process of evolution, has managed to bring about the process of photosynthesis. The first step involves a charge separation at the reaction centers, which proceeds with 100% quantum yield from the photoexcited singlet state of the bacteriochlorophyll donor, despite the fact that the wasteful deactivation of the electron into the ground state should be highly favored. Biomimetic model compounds (that is, those which resemble the pigments nature has developed) have been constructed from porphyrins and quinones. These model systems have allowed the study of the factors contributing to the highly efficient charge separation. This report focuses on recent developments in the study of electron transfer in porphyrinoquinones. Some of the results of these investigations may not be not fully understood and are often the subject of controversial discussions.     

A Calix[4]arene-Based Cyclic Dinuclear Ruthenium Complex for Light-Driven Catalytic Water Oxidation

A cyclic dinuclear ruthenium(bda) (bda: 2,2’-bipyridine-6,6’-dicarboxylate) complex equipped with oligo(ethylene glycol)-functionalized axial calix[4]arene ligands has been synthesized for homogenous catalytic water oxidation. This novel Ru(bda) macrocycle showed significantly increased catalytic activity in chemical and photocatalytic water oxidation compared to the archetype mononuclear reference [Ru(bda)(pic)₂]. Kinetic investigations, including kinetic isotope effect studies, disclosed a unimolecular water nucleophilic attack mechanism of this novel dinuclear water oxidation catalyst (WOC) under the involvement of the second coordination sphere. Photocatalytic water oxidation with this cyclic dinuclear Ru complex using [Ru(bpy)₃]Cl₂ as a standard photosensitizer revealed a turnover frequency of 15.5 s⁻¹ and a turnover number of 460. This so far highest photocatalytic performance reported for a Ru(bda) complex underlines the potential of this water-soluble WOC for artificial photosynthesis.     

Catalytic Water Oxidation by Ruthenium Complexes Containing Negatively Charged Ligand Frameworks

Artificial photosynthesis represents an attractive way of converting solar energy into storable chemical energy. The H₂O oxidation half‐reaction, which is essential for producing the necessary reduction equivalents, is an energy‐demanding transformation associated with a high kinetic barrier. Herein we present a couple of efficient Ru‐based catalysts capable of mediating this four‐proton‐four‐electron oxidation. We have focused on the incorporation of negatively charged ligands, such as carboxylate, phenol, and imidazole, into the catalysts to decrease the redox potentials. This account describes our work in designing Ru catalysts based on this idea. The presence of the negatively charged ligands is crucial for stabilizing the metal centers, allowing for light‐driven H₂O oxidation. Mechanistic details associated with the designed catalysts are also presented.

     

Photoinduced Electron and Energy Transfer in Dyads of Porphyrin Dimer and Perylene Tetracarboxylic Diimide

Two compounds containing a porphyrin dimer and a perylene tetracarboxylic diimide (PDI) linked by phenyl ( 1 ) or ethylene groups ( 2 ) are prepared. The photophysical properties of these two compounds are investigated by steady state electronic absorption and fluorescence spectra and lifetime measurements. The ground state absorption spectra reveal intense interactions between the porphyrin units within the porphyrin dimer, but no interactions between the porphyirn dimer and PDI. The fluorescence spectra suggest efficient energy transfer from PDI to porphyrin accompanied by less efficient electron transfer from porphyrin to PDI. The energy transfer is not affected by the dimeric structure of porphyrin or the linkage between the porphyrin dimer and PDI. However, the electron transfer from porphyrin to PDI is significantly affected by either the linkage between the donor and the acceptor or the polarity of the solvents. The dimeric structure of the porphyrin units in these compounds significantly promotes electron transfer in nonpolar, but not in polar solvents.     

Water Oxidation by In Situ Generated [RuII(OH2)(NCNHCO)(pic)2]+

A dinuclear ruthenium complex [Ru^II(NC^NHC O)(pic)₂]₂²⁺ ( 2 ) was firstly prepared and characterized spectroscopically and electrochemically. Instead of the conventional ligand exchange, complex 2 dissociates in situ to afford two single‐site Ru aqua complexes, [Ru^II(OH₂)(NC^NHC O)(pic)₂]⁺, which mediates water oxidation through proton‐coupled electron transfer events. In electrokinetic studies, complex 2 demonstrated a TOF of 150.3 s⁻¹ comparable to those state‐of‐the‐art catalysts at neutral conditions. TONs of 2173 and 217 were attained in chemical and photochemical water oxidation when 2 was used as a catalyst, exhibiting good stability. Notably, a TOF of 1.3 s⁻¹ was achieved at CAN‐driven water oxidation, which outperformed most of the reported single‐site Ru complexes, indicating that complex 2 is one of most active water oxidation catalysts (WOCs) to date. The unique coordination configuration and outstanding catalytic performance of complex 2 might shed light on the design of novel molecular WOCs.     

Photochemical,Electrochemical, and Photoelectrochemical Water Oxidation Catalyzed by Water‐Soluble Mononuclear Ruthenium Complexes

Two mononuclear ruthenium complexes [Ru(H₂tcbp)(isoq)₂] ( 1 ) and [Ru(H₂tcbp)(pic)₂] ( 2 ) (H₄tcbp=4,4′,6,6′‐tetracarboxy‐2,2′‐bipyridine, isoq=isoquinoline, pic=4‐picoline) are synthesized and fully characterized. Two spare carboxyl groups on the 4,4′‐positions are introduced to enhance the solubility of 1 and 2 in water and to simultaneously allow them to tether to the electrode surface by an ester linkage. The photochemical, electrochemical, and photoelectrochemical water oxidation performance of 1 in neutral aqueous solution is investigated. Under electrochemical conditions, water oxidation is conducted on the deposited indium‐tin‐oxide anode, and a turnover number higher than 15,000 per water oxidation catalyst (WOC) 1 is obtained during 10 h of electrolysis under 1.42 V vs. NHE, corresponding to a turnover frequency of 0.41 s^?1. The low overpotential (0.17 V) of electrochemical water oxidation for 1 in the homogeneous solution enables water oxidation under visible light by using [Ru(bpy)₃]²⁺ ( P1 ) (bpy=2,2′‐bipyridine) or [Ru(bpy)₂(4,4′‐(COOEt)₂‐bpy)]²⁺ ( P2 ) as a photosensitizer. In a three‐component system containing 1 or 2 as a light‐driven WOC, P1 or P2 as a photosensitizer, and Na₂S₂O₈ or [CoCl(NH₃)₅]Cl₂ as a sacrificial electron acceptor, a high turnover frequency of 0.81 s^?1 and a turnover number of up to 600 for 1 under different catalytic conditions are achieved. In a photoelectrochemical system, the WOC 1 and photosensitizer are immobilized together on the photoanode. The electrons efficiently transfer from the WOC to the photogenerated oxidizing photosensitizer, and a high photocurrent density of 85 μA cm^?2 is obtained by applying 0.3 V bias vs. NHE.     

Design of Mononuclear Ruthenium Catalysts for Low‐Overpotential Water Oxidation

Water oxidation is a key reaction in natural photosynthesis and in many schemes for artificial photosynthesis. Inspired by energy challenges and the emerging understanding of photosystem II, the development of artificial molecular catalysts for water oxidation has become a highly active area of research in recent years. In this Focus Review, we describe recent achievements in the development of single‐site ruthenium catalysts for water oxidation with a particular focus on the overpotential of water oxidation. First, we introduce the general scheme to access the high‐valent ruthenium–oxo species, the key species of the water‐oxidation reaction. Next, the mechanisms of the O? O bond formation from the active ruthenium–oxo species are described. We then discuss strategies to decrease the onset potentials of the water‐oxidation reaction. We hope this Focus Review will contribute to the further development of efficient catalysts toward sustainable energy‐conversion systems.     

Excitation‐Wavelength‐Dependent,Ultrafast Photoinduced Electron Transfer in Bisferrocene/BF2‐Chelated‐Azadipyrromethene/Fullerene Tetrads

Donor–acceptor distance, orientation, and photoexcitation wavelength are key factors in governing the efficiency and mechanism of electron‐transfer reactions both in natural and synthetic systems. Although distance and orientation effects have been successfully demonstrated in simple donor–acceptor dyads, revealing excitation‐wavelength‐dependent photochemical properties demands multimodular, photosynthetic‐reaction‐center model compounds. Here, we successfully demonstrate donor– acceptor excitation‐wavelength‐dependent, ultrafast charge separation and charge recombination in newly synthesized, novel tetrads featuring bisferrocene, BF₂‐chelated azadipyrromethene, and fullerene entities. The tetrads synthesized using multistep synthetic procedure revealed characteristic optical, redox, and photo reactivities of the individual components and featured “closely” and “distantly” positioned donor–acceptor systems. The near‐IR‐emitting BF₂‐chelated azadipyrromethene acted as a photosensitizing electron acceptor along with fullerene, while the ferrocene entities acted as electron donors. Both tetrads revealed excitation‐wavelength‐dependent, photoinduced, electron‐transfer events as probed by femtosecond transient absorption spectroscopy. That is, formation of the Fc⁺–ADP–C₆₀^.? charge‐separated state upon C₆₀ excitation, and Fc⁺–ADP^.?–C₆₀ formation upon ADP excitation is demonstrated.     

Synthesis and Catalytic Water Oxidation Activities of Ruthenium Complexes Containing Neutral Ligands

Two dinuclear and one mononuclear ruthenium complexes containing neutral polypyridyl ligands have been synthesised as pre‐water oxidation catalysts and characterised by ¹H and ¹³C NMR spectroscopy and ESI‐MS. Their catalytic water oxidation properties in the presence of [Ce(NH₄)₂(NO₃)₆] (Ce^IV) as oxidant at pH 1.0 have been investigated. At low concentrations of Ce^IV (5 mM ), high turnover numbers of up to 4500 have been achieved. An ¹⁸O‐labelling experiment established that both O atoms in the evolved O₂ originate from water. Combined electrochemical study and electrospray ionisation mass spectrometric analysis suggest that ligand exchange between coordinated 4‐picoline and free water produces Ru aquo species as the real water oxidation catalysts.     

Synthesis,Characterization, and Reactivity of Dyad Ruthenium‐Based Molecules for Light‐Driven Oxidation Catalysis

Dyad molecules containing the 2,3,5,6‐tetrakis(2‐pyridyl)pyrazine (tppz) ligand with general formula [(tpy)Ru(μ‐tppz)Ru(X)(L‐L)]ⁿ⁺ (X=Cl, CF₃COO, or H₂O; L‐L=2,2′‐bipyridine (bpy) or 3,5‐bis(2‐pyridyl)pyrazole (Hbpp); tpy=2,2′:6′,2“‐terpyridine) have been prepared, purified, and isolated. The complexes have been characterized by analytical and spectroscopic techniques and by X‐ray diffraction analysis for two of them. Additionally, full electrochemical characterization based on cyclic voltammetry, differential pulse voltammetry, and square wave voltammetry has been also performed. The pH dependence of the redox couples for the aqua complexes have also been studied and their corresponding Pourbaix diagrams drawn. Furthermore, their capacity to catalytically oxidize organic substrates, such as alcohols, alkenes, and sulfides, has been carried out chemically, electrochemically, and photochemically. Finally, their capacity to behave as water oxidation catalysts has also been tested.     

An Electron Dynamics Mechanism of Charge Separation in the Initial‐Stage Dynamics of Photoinduced Water Splitting in XMnWater (X=OH,OCaH) and Electron–Proton Acceptors

An electron dynamics mechanism of charge separation in the initial stage of excited‐state reactions of the class of X?Mn?OH₂???A${ \to }$ X?Mn?OH???HA (X=OH or OCaH; A=N‐methylformamidine, guanidine, imidazole, or ammonia cluster) is reported. The dynamic effect of calcium doping is also revealed. This study provides a novel factor to be considered in designing efficient systems for photoinduced water splitting.     

Symmetry Effects in Photoinduced Electron Transfer in Chlorin-Quinone Dyads: Adiabatic Suppression in the Marcus Inverted Region

In donor–acceptor dyads undergoing photoinduced electron transfer (PET), a direction or pathway for electron movement is usually dictated by the redox properties and the separation distance between the donor and acceptor subunits, while the effect of symmetry is less recognized. We have designed and synthesized two isomeric donor–acceptor assemblies in which electronic coupling between donor and acceptor is altered by the orbital symmetry control with the reorganization energy and charge transfer exothermicity being kept unchanged. Analysis of the optical absorption and luminescence spectra, supported by the DFT and TD-DFT calculations, showed that PET in these assemblies corresponds to the Marcus inverted region (MIR) and has larger rate for isomer with weaker electronic coupling. This surprising observation provides the first experimental evidence for theoretically predicted adiabatic suppression of PET in MIR, which unambiguously controlled solely by symmetry.     

Heterogeneous Molecular Systems for Photocatalytic CO2 Reduction with Water Oxidation

Artificial photosynthesis—reduction of CO₂ into chemicals and fuels with water oxidation in the presence of sunlight as the energy source—mimics natural photosynthesis in green plants, and is considered to have a significant part to play in future energy supply and protection of our environment. The high quantum efficiency and easy manipulation of heterogeneous molecular photosystems based on metal complexes enables them to act as promising platforms to achieve efficient conversion of solar energy. This Review describes recent developments in the heterogenization of such photocatalysts. The latest state‐of‐the‐art approaches to overcome the drawbacks of low durability and inconvenient practical application in homogeneous molecular systems are presented. The coupling of photocatalytic CO₂ reduction with water oxidation through molecular devices to mimic natural photosynthesis is also discussed.