Photosensitization Behavior of Ir(III) Complexes in Selective Reduction of CO2 by Re(I)-Complex-Anchored TiO2 Hybrid Catalyst

Pitfalls in Photochemical and Photoelectrochemical Reduction of CO2 to Energy Products

The photochemical and photoelectrochemical reduction of CO2 is a promising approach for converting carbon dioxide into valuable chemicals (materials) and fuels. A key issue is ensuring the accuracy of experimental results in CO2 reduction reactions (CO2RRs) because of potential sources of false positives. This paper reports the results of investigations on various factors that may contribute to erroneous attribution of reduced-carbon species, including degradation of carbon species contained in photocatalysts, residual contaminants from synthetic procedures, laboratory glassware, environmental exposure, and the operator. The importance of rigorous experimental protocols, including the use of labeled 13CO2 and blank tests, to identify true CO2 reduction products (CO2RPs) accurately is highlighted. Our experimental data (eventually complemented with or compared to literature data) underline the possible sources of errors and, whenever possible, quantify the false positives with respect to the effective conversion of CO2 in clean conditions. This paper clarifies that the incidence of false positives is higher in the preliminary phase of photo-material development when CO2RPs are in the range of a few 10s of μg gcat-1 h-1, reducing its importance when significant conversions of CO2 are performed reaching 10s of mol gcat-1 h-1. This paper suggests procedures for improving the reliability and reproducibility of CO2RR experiments, thus validating such technologies.

Photocatalytic Conversion of CO2 to Formate/CO by an (η6-para-Cymene)Ru(II) Half-Metallocene Catalyst: Influence of Additives and TiO2 Immobilization on the Catalytic Mechanism and Product Selectivity

The catalytic efficacy of the monobipyridyl (η6-para-Cymene)Ru(II) half-metallocene, [(p-Cym)Ru(bpy)Cl]+ was evaluated in both mixed homogeneous (dye + catalyst) and heterogeneous hybrid systems (dye/TiO2/Catalyst) for photochemical CO2 reduction. A series of homogeneous photolysis experiments revealed that the (p-Cym)Ru(II) catalyst engages in two competitive routes for CO2 reduction (CO2 to formate conversion via RuII-hydride vs CO2 to CO conversion through a RuII-COOH intermediate). The conversion activity and product selectivity were notably impacted by the pKa value and the concentration of the proton source added. When a more acidic TEOA additive was introduced, the half-metallocene Ru(II) catalyst leaned toward producing formate through the RuII-H mechanism, with a formate selectivity of 86%. On the other hand, in homogeneous catalysis with TFE additive, the CO2-to-formate conversion through RuII-H was less effective, yielding a more efficient CO2-to-CO conversion with a selectivity of >80% (TONformate of 140 and TONCO of 626 over 48 h). The preference between the two pathways was elucidated through an electrochemical mechanistic study, monitoring the fate of the metal-hydride intermediate. Compared to the homogeneous system, the TiO2-heterogenized (p-Cym)Ru(II) catalyst demonstrated enhanced and enduring performance, attaining TONs of 1000 for CO2-to-CO and 665 for CO2-to-formate.

Electronic Tuning of Photoexcited Dynamics in Heteroleptic Cu(I) Complex Photosensitizers

Photoexcited dynamics of heteroleptic Cu(I) complexes as noble-metal-free photosensitizers are closely intertwined with the nature of their ligands. By utilizing ultrafast optical and X-ray transient absorption spectroscopies, we characterized a new set of heteroleptic Cu(I) complexes [Cu(PPh3)2(BPyR)]+ (R = CH3, H, Br to COOCH3), with an increase in the electron-withdrawing ability of the functional group (R). We found that after the transient photooxidation of Cu(I) to Cu(II), the increasing electron-withdrawing ability of R barely affects the internal conversion (IC) (e.g., Jahn-taller (JT) distortion) between singlet MLCT states. However, it does accelerate the dynamics of intersystem crossing (ISC) between singlet and triplet MLCT states and the subsequent decay from the triplet MLCT state to the ground state. The associated lifetime constants are reduced by up to 300%. Our understanding of the photoexcited dynamics in heteroleptic Cu(I) complexes through ligand electronic tuning provides valuable insight into the rational design of efficient Cu(I) complex photosensitizers.

Dynamics of Photoinduced Intramolecular and Intermolecular Electron Transfers in Ligand-Conjugated Ir(III)-Re(I) Photocatalysts

We report the electron transfer (ET) dynamics in a series of Ir(III)-Re(I) photocatalysts where two bipyridyl ligands of Ir and Re moieties are conjugated at the meta (m)- or para (p)-position of each side. Femtosecond transient absorption (TA) measurements identify the intramolecular ET (IET) dynamics from the Ir to Re moiety, followed by the formation of one-electron-reduced species (OERS) via the intermolecular ET with a sacrificial electron donor (SED). The IET rate depends on the bridging ligand (BL) structures (∼25 ps for BL_mm/mp vs ∼68 ps for BL_pm/pp), while the OERS formation happens on an even slower time scale (∼1.4 ns). Connecting the Re moiety at the meta-position of the bipyridyl of the Ir moiety can restrict the rotation around a covalent bond between two bipyridyl ligands by steric hindrances and facilitate the IET process. This highlights the importance of BL structures on the ET dynamics in photocatalysts.

Understanding Ir(III) Photocatalyst Structure-Activity Relationships: A Highly Parallelized Study of Light-Driven Metal Reduction Processes

High-throughput synthesis and screening methods were used to measure the photochemical activity of 1440 distinct heteroleptic [Ir(C^N)₂(N^N)]⁺ complexes for the photoreduction of Sn(II) and Zn(II) cations to their corresponding neutral metals. Kinetic data collection was carried out using home-built photoreactors and measured initial rates, obtained through an automated fitting algorithm, spanned between 0-120 μM/s for Sn(0) deposition and 0-90 μM/s for Zn(0) deposition. Photochemical reactivity was compared to photophysical properties previously measured such as deaerated excited state lifetime and emission spectral data for these same complexes; however, no clear correlations among these features were observed. A formal photochemical rate law was then developed to help elucidate the observed reactivity. Initial rates were found to be directly correlated to the product of incident photon flux with three reaction elementary efficiencies: (1) the fraction of light absorbed by the photocatalyst, (2) the fraction of excited state species that are quenched by the electron donor, and (3) the cage escape efficiency. The most active catalysts exhibit high efficiencies for all three steps, and catalyst engineering requirements to maximize these elementary efficiencies were postulated. The kinetic treatment provided the mechanistic information needed to decipher the observed structure/function trends in the high-throughput work.

Inorganometallic Photocatalyst for CO2 Reduction

ConspectusDuring the last few decades, the design of catalytic systems for CO₂ reduction has been extensively researched and generally involves (1) traditional approaches using molecular organic/organometallic materials and heterogeneous inorganic semiconductors and (2) combinatory approaches wherein these materials are combined as needed. Recently, we have devised a number of new TiO₂-mediated multicomponent hybrid systems that synergistically integrate the intrinsic merits of various materials, namely, molecular photosensitizers/catalysts and n-type TiO₂ semiconductors, and lower the energetic and kinetic barriers between components. We have termed such multicomponent hybrid systems assembled from the hybridization of various organic/inorganic/organometallic units in a single platform inorganometallic photocatalysts. The multicomponent inorganometallic (MIOM) hybrid system onto which the photosensitizer and catalyst are coadsorbed efficiently eliminates the need for bulk-phase diffusion of the components and avoids the accumulation of radical intermediates that invokes a degradation pathway, in contrast to the homogeneous system, in which the free reactive species are concentrated in a confined reaction space. In particular, in energetic terms, we discovered that in nonaqueous media, the conduction band (CB) levels of reduced TiO₂ (TiO₂(e^-)) are positioned at a higher level (in the range -1.5 to -1.9 V vs SCE). This energetic benefit of reduced TiO₂ allows smooth electron transfer (ET) from injected electrons (TiO₂(e^-)) to the coadsorbed CO₂ reduction catalyst, which requires relatively high reducing power (at least more than -1.1 V vs SCE). On the other hand, the existence of various shallow surface trapping sites and surface bands, which are 0.3-1.0 eV below the CB of TiO₂, efficiently facilitates electron injection from any photosensitizer (including dyes having low excited energy levels) to TiO₂ without energetic limitation. This is contrasted with most photocatalytic systems, wherein successive absorption of single high-energy photons is required to produce excited states with enough energy to fulfill photocatalytic reaction, which may allow unwanted side reactions during photocatalysis. In this Account, we present our recent research efforts toward advancing these MIOM hybrid systems for photochemical CO₂ reduction and discuss their working mechanisms in detail. Basic ET processes within the MIOM system, including intervalence ET in organic/organometallic redox systems, metal-to-ligand charge transfer of organometallic complexes, and interfacial/outer-sphere charge transfer between components, were investigated by conducting serial photophysical and electrochemical analyses. Because such ET events occur primarily at the interface between the components, the efficiency of interfacial ET between the molecular components (organic/organometallic photosensitizers and molecular reduction catalysts) and the bulk inorganic solid (mainly n-type TiO₂ semiconductors) has a significant influence on the overall photochemical reaction kinetics and mechanism. In some TiO₂-mediated MIOM hybrids, the chemical attachment of organic or organometallic photosensitizing units onto TiO₂ semiconductors efficiently eliminates the step of diffusion/collision-controlled ET between components and prevents the accumulation of reactive species (oxidatively quenched cations or reductively quenched anions) in the reaction solution, ensuring steady photosensitization over an extended reaction period. The site isolation of a single-site organometallic catalyst employing TiO₂ immobilization promotes the monomeric catalytic pathway during the CO₂ reduction process, resulting in enhanced product selectivity and catalytic performance, including lifetime extension. In addition, as an alternative inorganic solid scaffold, the introduction of a host porphyrin matrix (interlinked in a metal-organic framework (MOF) material) led to efficient and durable photocatalytic CO₂ conversion by the new MOF-Re(I) hybrid as a result of efficient light harvesting/exciton migration in the porphyrinic MOF and rapid quenching of the photogenerated electrons by the doped Re(I) catalytic sites. Overall, the case studies presented herein provide valuable insights for the rational design of advanced multicomponent hybrid systems for artificial photosynthesis involving CO₂ reduction.

Secondary Coordination Effect on Monobipyridyl Ru(II) Catalysts in Photochemical CO2 Reduction: Effective Proton Shuttle of Pendant Brønsted Acid/Base Sites (OH and N(CH3)2) and Its Mechanistic Investigation

While the incorporation of pendant Brønsted acid/base sites in the secondary coordination sphere is a promising and effective strategy to increase the catalytic performance and product selectivity in organometallic catalysis for CO₂ reduction, the control of product selectivity still faces a great challenge. Herein, we report two new trans(Cl)-[Ru(6-X-bpy)(CO)₂Cl₂] complexes functionalized with a saturated ethylene-linked functional group (bpy = 2,2'-bipyridine; X = -(CH₂)₂-OH or -(CH₂)₂-N(CH₃)₂) at the ortho(6)-position of bpy ligand, which are named Ru-bpy^OH and Ru-bpy^diMeN, respectively. In the series of photolysis experiments, compared to nontethered case, the asymmetric attachment of tethering ligand to the bpy ligand led to less efficient but more selective formate production with inactivation of CO₂-to-CO conversion route during photoreaction. From a series of in situ FTIR analyses, it was found that the Ru-formate intermediates are stabilized by a highly probable hydrogen bonding between pendent proton donors (-diMeN⁺H or -OH) and the oxygen atom of metal-bound formate (Ru^I-OCHO···H-E-(CH₂)₂-, E = O or diMeN⁺). Under such conformation, the liberation of formate from the stabilized Ru^I-formate becomes less efficient compared to the nontethered case, consequently lowering the CO₂-to-formate conversion activities during photoreaction. At the same time, such stabilization of Ru-formate species prevents the dehydration reaction route (η¹-OCHO → η¹-COOH on Ru metal) which leads toward the generation of Ru-CO species (key intermediate for CO production), eventually leading to the reduction of CO₂-to-CO conversion activity.

A Hybrid Ru(II)/TiO2 Catalyst for Steadfast Photocatalytic CO2 to CO/Formate Conversion Following a Molecular Catalytic Route

Herein, we employed a molecular Ru(II) catalyst immobilized onto TiO₂ particulates of (4,4'-Y₂-bpy)Ru^II(CO)₂Cl₂ (RuP; Y = CH₂PO(OH)₂), as a hybrid catalyst system to secure the efficient and steady catalytic activity of a molecular bipyridyl Ru(II)-complex-based photocatalytic system for CO₂ reduction. From a series of operando FTIR spectrochemical analyses, it was found that the TiO₂-fixed molecular Ru(II) complex leads to efficient stabilization of the key monomeric intermediate, Ru^II-hydride (LRu^II(H)(CO)₂Cl), and suppresses the formation of polymeric Ru(II) complex (-(L(CO)₂Ru-Ru(CO)₂L)_n-), which is a major deactivation product produced during photoreaction via the Ru-Ru dimeric route. Active promotion of the monomeric catalytic route in a hetero-binary system (IrPS + TiO₂/RuP) that uses TiO₂-bound Ru(II) complex as reduction catalyst led to highly increased activity as well as durability of photocatalytic behavior with respect to the homogeneous catalysis of free Ru(II) catalyst (IrPS + Ru(II) catalyst). This catalytic strategy produced maximal turnover numbers (TONs) of >4816 and >2228, respectively, for CO and HCOO^- production in CO₂-saturated N,N-dimethylformamide (DMF)/TEOA (16.7 vol % TEOA) solution containing a 0.1 M sacrificial electron donor.

Two Excited State Collaboration of Heteroleptic Ir(III)-Coumarin Complexes for H2 Evolution Dye-Sensitized Photocatalysts

Interfacial electron injection from a photoexcited surface-immobilized dye to a semiconductor substrate is a key reaction for dye-sensitized photocatalysts. We previously reported that the molecular orientation of heteroleptic Ir(III) photosensitizer on the TiO2 nanoparticle surface was important for efficient interfacial electron injection. In this work, to overcome the weak light absorption ability of heteroleptic Ir(III) photosensitizer and to improve the photoinduced charge-separation efficiency at the dye–semiconductor interface, we synthesized two heteroleptic Ir(III) complexes with different coumarin dyes, [Ir(C6)2(H4CPbpy)]Cl and [Ir(C30)2(H4CPbpy)]Cl [Ir-CX; X = 6 or 30; HC6 = 3-(2-enzothiazolyl)-7-(diethylamino)coumarin, HC30 = 3-(2-N-methylbenzimidazolyl)-7-N,N-diethylaminocoumarin, H4CPbpy = 4,4′-bis(methylphosphonic acid)-2,2′-bipyridine], as the cyclometalated ligands and immobilized them on the surface of Pt-cocatalyst-loaded TiO2 nanoparticles. Ultraviolet-visible absorption and emission spectroscopy revealed that the singlet ligand-centered (1LC) absorption and triplet 3LC emission bands of Ir-C30 occurred at shorter wavelengths than those of Ir-C6, while time-dependent density-functional-theory data suggested that the ligand-to-ligand charge transfer (LLCT) excited states of the two complexes were comparable. The photocatalytic H2 evolution activity of the Ir-C6-sensitized Pt-TiO2 nanoparticles (Ir-C6@Pt-TiO2) under visible light irradiation (λ > 420 nm) was higher than that of Ir-C30@Pt-TiO2. In contrast, their activities were comparable under irradiation with monochromatic light (λ = 450 ± 10 nm), which is absorbed comparably by both Ir-CX complexes. These results suggest that the internal conversion from the higher-lying LC state to the LLCT state effectively occurs in both Ir-CX complexes to trigger electron injection to TiO2.

Triplet Excited States Modulated by Push-Pull Substituents in Monocyclometalated Iridium(III) Photosensitizers

A series of novel monocyclometalated [Ir(tpy)(btp)Cl]⁺ complexes (Ir2-Ir5) were synthesized using 2,2':6',2″-terpyridine (tpy) and 2-(2-pyridyl)benzo[b]thiophene (btp) ligands, as well as their derivatives bearing electron-donating tert-butyl (t-Bu) and electron-withdrawing trifluoromethyl (CF₃) groups. Ir2-Ir5 exhibited visible-light absorption stronger than that of the known complex [Ir(tpy)(ppy)Cl]⁺ (Ir1; ppy = 2-phenylpyridine). Spectroscopic and computational studies revealed that two triplet states were involved in the excited-state dynamics. One is a weakly emissive and short-lived ligand to ligand charge-transfer (LLCT) state originating from the charge transfer from the btp to the tpy ligand. The other is a highly emissive and long-lived ligand-centered (LC) state localized on the btp ligand. Interestingly, the excited state dominant with ³LLCT was completely changed to the ³LC state upon the introduction of substituents on both the tpy and btp ligands. For instance, the excited state of the parent complex Ir2 was weakly emissive (Φ = 2%) and short-lived (τ = 110 ns) in CH₂Cl₂; conversely, Ir5, fully furnished with t-Bu and CF₃ groups, displayed intense phosphorescence with a prolonged lifetime (τ = 14 μs). This difference became increasingly prominent when the solvent was changed to aqueous CH₃CN, most probably due to the ³LLCT stabilization. The predominant excited-state nature was switchable between the ³LLCT and ³LC states depending on the substituents employed; this was demonstrated through investigations of Ir3 and Ir4, bearing either the t-Bu or the CF₃ group, where the complexes exhibited properties intermediate between those of Ir2 and Ir5. All of the Ir(III) complexes were tested as photosensitizers in photocatalytic H₂ evolution over a Co molecular catalyst, and Ir5 outperformed the others, including Ir1, due to improvement in the following key properties: visible-light-absorption ability, excited-state lifetime, and reductive power of the one-electron-reduced species against the catalyst.

Competitive Desulfonylative Reduction and Oxidation of α-Sulfonylketones Promoted by Photoinduced Electron Transfer with 2-Hydroxyaryl-1,3-dimethylbenzimidazolines under Air

Desulfonylation reactions of α-sulfonylketones promoted by photoinduced electron transfer with 2-hydroxyarylbenzimidazolines (BIH-ArOH) were investigated. Under aerobic conditions, photoexcited 2-hydroxynaphthylbenzimidazoline (BIH-NapOH) promotes competitive reduction (forming alkylketones) and oxidation (producing α-hydroxyketones) of sulfonylketones through pathways involving the intermediacy of α-ketoalkyl radicals. The results of an examination of the effects of solvents, radical trapping reagents, substituents of sulfonylketones, and a variety of hydroxyaryl- and aryl-benzimidazolines (BIH-ArOH and BIH-Ar) suggest that the oxidation products are produced by dissociation of α-ketoalkyl radicals from the initially formed solvent-caged radical ion pairs followed by reaction with molecular oxygen. In addition, the observations indicate that the reduction products are generated by proton or hydrogen atom transfer in solvent-caged radical ion pairs derived from benzimidazolines and sulfonylketones. The results also suggest that arylsulfinate anions arising by carbon-sulfur bond cleavage of sulfonylketone radical anions act as reductants in the oxidation pathway to convert initially formed α-hydroperoxyketones to α-hydroxyketones. Finally, density functional theory calculations were performed to explore the structures and properties of radical ions of sulfonylketones as well as BIH-NapOH.

Rhodium catalysts with cofactor mimics for the biomimetic reduction of CN bonds

Bio-inspired reduction of CN bonds was successfully performed using rhodium catalysts containing cofactor mimics. The intramolecular cooperation between rhodium and cofactor mimics enabled the transformation with good selectivity. A plausible mechanism was also proposed.

Sterically Regulated α-Oxygenation of α-Bromocarbonyl Compounds Promoted Using 2-Aryl-1,3-dimethylbenzimidazolines and Air

A debrominative oxygenation protocol has been developed for the conversion of α-bromo-α,α-dialkyl-substituted carbonyl compounds to their corresponding α-hydroxy analogues. For example, stirring a solution of α-bromoisobutyrophenone and 2-aryl-1,3-dimethylbenzimidazoline (BIH-Ar) at room temperature under an air atmosphere leads to the efficient formation of α-hydroperoxyisobutyrophenone, which can be converted to α-hydroxyisobutyrophenone using Me₂S reduction. In contrast, reaction of α-bromoacetophenone under the same conditions produces the α-hydrogenated product acetophenone. α-Keto-alkyl and benzimidazolyl radicals (BI^•-Ar), generated via dissociative electron transfer from BIH-Ar to α-bromoketone substrates, serve as key intermediates in the oxidation and reduction processes. The dramatic switch from hydrogenation to oxygenation is attributed to a steric effect of α-alkyl substituents, which causes hydrogen atom abstraction from sterically crowded BIH-Ar to α-keto-alkyl radicals to be slow and enable preferential reaction with molecular oxygen. Generation of the α-keto-alkyl radical and BI^•-Ar intermediates in these process and their sterically governed hydrogen atom transfer reactions are supported by results arising from DFT calculations. Moreover, an electron spin resonance study showed that visible light irradiation of phenyl benzimidazoline (BIH-Ph) in the presence of molecular oxygen produces the benzimidazolyl radical (BI^•-Ph). The addition of thiophenol into the reaction of α-bromoisobutyrophenone and BIH-Ph predominantly produced α-phenylthiolated isobutyrophenone even if a high concentration of molecular oxygen exists. Furthermore, the developed protocol was applied to other α-bromo-α,α-dialkylated carbonyl compounds.

Utility of Squaraine Dyes for Dye-Sensitized Photocatalysis on Water or Carbon Dioxide Reduction

Red light-sensitized squaraine (SQ) dyes were developed and incorporated into dye-sensitized catalysts (DSCs) with the formula of SQ/TiO₂/Cat, and their efficacies were evaluated in terms of performance on either water or carbon dioxide reduction. Pt nanoparticles or fac-[Re(4,4'-bis-(diethoxyphosphorylmethyl)-2,2'-bipyridine)(CO)₃Cl] were used as each catalytic center within the DSC frame of SQ/TiO₂/Pt (Type I) or SQ/TiO₂/Re(I) (Type II). In order to convey the potential utility of SQ in low energy sensitization, the following catalytic reductions were carried out under selective lower energy irradiation (>500 nm). Type I and II showed different catalytic performances, primarily due to the choice of solvent for each catalytic condition: hydrogenation was carried out in H₂O, but CO₂ reduction in dimethylformamide (DMF), and SQ was more stable in aqueous acid conditions for hydrogen generation than CO₂ reduction in DMF. A suspension of Type I in 3 mL water containing 0.1 M ascorbic acid (pH = 2.66) resulted in efficient photocatalytic hydrogen evolution, producing 37 μmol of H₂ for 4 h. However, in photocatalysis of Type II (SQ/TiO₂/Re(I)) in 3 mL DMF containing 0.1 M 1,3-dimethyl-2-phenyl-1,3-dihydrobenzimidazole, the TiO₂-bound SQ dyes were not capable of working as a low energy sensitizer because SQ was susceptible to dye decomposition in nucleophilic DMF conditions, resulting in DSC deactivation for the CO₂ reduction. Even with the limitation of solvent, the DSC conditions for the utility of SQ have been established: the anchoring group effect of SQ with either phosphonic acid or carboxylic acid onto the TiO₂ surface; energy alignment of SQ with the flat band potentials (E _fb) of TiO₂ semiconductors and the reduction power of electron donors; and the wavelength range of the light source used, particularly when >500 nm.

Metal-Complex/Semiconductor Hybrid Photocatalysts and Photoelectrodes for CO2 Reduction Driven by Visible Light

CO₂ reduction to carbon feedstocks using heterogeneous photocatalysts is an attractive means of addressing both climate change and the depletion of fossil fuels. Of particular importance is the development of a photosystem capable of functioning in response to visible light, which accounts for the majority of the solar spectrum, representing a kind of artificial photosynthesis. Hybrid systems comprising a metal complex and a semiconductor are promising because of the excellent electrochemical (and/or photocatalytic) activity of metal complexes during CO₂ reduction and the ability of semiconductors to efficiently oxidize water to molecular O₂ . Here, the development of hybrid photocatalysts and photoelectrodes for CO₂ reduction in combination with water oxidation is described.

Core–shell g-C3N4/Pt/TiO2 nanowires for simultaneous photocatalytic H2 evolution and RhB degradation under visible light irradiation

Core–shell structural diagram (a) and proposed photocatalytic mechanism (b) for the CN/Pt/TiO₂ composite.

Photofunctions of iridium(iii) complexes in vesicles: long-lived excited states and visible-light sensitization for hydrogen evolution in aqueous solution

Utilization of DPPC vesicles allows water-insoluble photoactive Ir(iii) complexes to be dispersed in bulk aqueous solution.

Long-lived charge separation in dye–semiconductor assemblies: a pathway to multi-electron transfer reactions

Key approaches to achieve long-lived charge separation and promote conduction band mediated electron transfer in dye-sensitized semiconductor assemblies.