Time-Resolved Interception of Multiple-Charge Accumulation in a Sensitizer-Acceptor Dyad

Time-Resolved Mechanistic Depiction of Photoinduced CO2 Reduction Catalysis on a Urea-Modified Iron Porphyrin

The development of functional artificial photosynthetic devices relies on the understanding of mechanistic aspects involved in specialized photocatalysts. Modified iron porphyrins have long been explored as efficient catalysts for the light-induced reduction of carbon dioxide (CO2) towards solar fuels. In spite of the advancements in homogeneous catalysis, the development of the next generation of catalysts requires a complete understanding of the fundamental photoinduced processes taking place prior to and after activation of the substrate by the catalyst. In this work, we employ a state-of-the-art nanosecond optical transient absorption spectroscopic setup with a double excitation capability to induce charge accumulation and trigger the reduction of CO2 to carbon monoxide (CO). Our biomimetic system is composed of a urea-modified iron(III) tetraphenylporphyrin (UrFeIII) catalyst, the prototypical [Ru(bpy)3]2+ (bpy=2,2'-bipyridine) used as a photosensitizer, and sodium ascorbate as an electron donor. Under inert atmosphere, we show that two electrons can be successively accumulated on the catalyst as the fates of the photogenerated UrFeII and UrFeI reduced species are tracked. In the presence of CO2, the catalytic cycle is kick-started providing further evidence on CO2 activation by the UrFe catalyst in its formal FeI oxidation state.

Better Covalent Connection in a Molecular Triad Enables More Efficient Photochemical Energy Storage

Numerous studies have explored the kinetics of light-induced charge separation and thermal charge recombination in donor-acceptor compounds, but quantum efficiencies have rarely been investigated. Here, we report on two essentially isomeric molecular triads, both comprising a π-extended tetrathiafulvalene (ExTTF) donor, a ruthenium(II)-based photosensitizer, and a naphthalene diimide (NDI) acceptor. The key difference between the two triads is how the NDI acceptor is connected. Linkage at the NDI core provides stronger electronic coupling to the other molecular components than connection via the nitrogen atoms of NDI. This change in molecular connectivity is expected to accelerate both energy-storing charge separation and energy-wasting charge recombination processes, but it is not a priori clear how this will affect the triad's ability to store photochemical energy; any gain resulting from faster charge separation could potentially be (over)compensated by losses through accelerated charge recombination. The new key insight emerging from our study is that the quantum yield for the formation of a long-lived charge-separated state increases by a factor of 5 when going from nitrogen- to core-connected NDI, providing the important proof of concept that better molecular connectivity indeed enables more efficient photochemical energy storage. The physical origin of this behavior seems to root in different orbital connectivity pathways for charge separation and charge recombination, as well as in differences in the relevant orbital interactions depending on NDI connection. Our work provides guidelines for how to discriminate between energy-storing and energy-wasting electron transfer reactions in order to improve the quantum yields for photochemical energy storage and solar energy conversion.

A Novel Pump-Pump-Probe Resonance Raman Approach Featuring Light-Induced Charge Accumulation on a Model Photosystem

Light-induced charge accumulation is at the heart of biomimetic systems aiming at solar fuel production in the realm of artificial photosynthesis. Understanding the mechanisms upon which these processes operate is a necessary condition to drive down the rational catalyst design road. We have built a nanosecond pump-pump-probe resonance Raman setup to witness the sequential charge accumulation process while probing vibrational features of different charge-separated states. By employing a reversible model system featuring methyl viologen (MV) as a dual electron acceptor, we have been able to watch the photosensitized production of its neutral form, MV⁰, resulting from two sequential electron transfer reactions. We have found that, upon double excitation, a fingerprint vibrational mode corresponding to the doubly reduced species appears at 992 cm^-1 and peaks at 30 μs after the second excitation. This has been further confirmed by simulated resonance Raman spectra which fully support our experimental findings in this unprecedented buildup of charge seen by a resonance Raman probe.

Accumulation of Four Electrons on a Terphenyl (Bis)disulfide

The activation of N₂ , CO₂ or H₂ O to energy-rich products relies on multi-electron transfer reactions, and consequently it seems desirable to understand the basics of light-driven accumulation of multiple redox equivalents. Most of the previously reported molecular acceptors merely allow the storage of up to two electrons. We report on a terphenyl compound including two disulfide bridges, which undergoes four-electron reduction in two separate electrochemical steps, aided by a combination of potential compression and inversion. Under visible-light irradiation using the organic super-electron donor tetrakis(dimethylamino)ethylene, a cascade of light-induced reaction steps is observed, leading to the cleavage of both disulfide bonds. Whereas one of them undergoes extrusion of sulfur to result in a thiophene, the other disulfide is converted to a dithiolate. These insights seem relevant to enhance the current fundamental understanding of photochemical energy storage.

Influence of the Linker Chemistry on the Photoinduced Charge‐Transfer Dynamics of Hetero‐dinuclear Photocatalysts

To optimize light‐driven catalytic processes, light‐mediated multi‐electron transfer dynamics in molecular dyads need to be studied and correlated with structural changes focusing on the catalytically active metastable intermediates. Here, spectro‐electrochemistry has been employed to investigate the structure‐dependent photoelectron transfer kinetics in catalytically active intermediates of two Ru−Rh catalysts for light‐driven NAD⁺ reduction. The excited‐state reactivity of short‐lived intermediates was studied along different photoreaction pathways by resonance Raman and time‐resolved transient absorption spectro‐electrochemistry with sub‐picosecond time resolution under operando conditions. The results demonstrate, for the first time, how the bridging ligand serves as a (multi‐)electron storage structure, mediates the strength of the electronic coupling of catalytic and photocenter and impacts the targeted electron transfer as well as parasitic electron‐transfer kinetics.

A Combined Spectroscopic and Theoretical Study on a Ruthenium Complex Featuring a π-Extended dppz Ligand for Light-Driven Accumulation of Multiple Reducing Equivalents

The design of photoactive systems capable of storing and relaying multiple electrons is highly demanded in the field of artificial photosynthesis, where transformations of interest rely on multielectronic redox processes. The photophysical properties of the ruthenium photosensitizer [(bpy)₂ Ru(oxim-dppqp)]²⁺ (Ru), storing two electrons coupled to two protons on the π-extended oxim-dppqp ligand under light-driven conditions, are investigated by means of excitation wavelength-dependent resonance Raman and transient absorption spectroscopies, in combination with time-dependent density functional theory; the results are discussed in comparison to the parent [(bpy)₂ Ru(dppz)]²⁺ and [(bpy)₂ Ru(oxo-dppqp)]²⁺ complexes. In addition, this study provides in-depth insights on the impact of protonation or of accumulation of multiple reducing equivalents on the reactive excited states.

Recent Advances and Perspectives in Photodriven Charge Accumulation in Molecular Compounds: A Mini Review

The formation of so-called solar fuels from abundant low-energetic compounds, such as carbon dioxide or water, relies on the chemical elementary steps of photoinduced electron transfer and accumulation of multiple redox equivalents. The majority of molecular systems explored to date require sacrificial electron donors to accumulate multiple electrons on a single acceptor unit, but the use of high-energetic sacrificial redox reagents is unsustainable. In recent years, an increasing number of molecular compounds for reversible light-driven accumulation of redox equivalents that do not need sacrificial electron donors has been reported. Those compounds are the focus of this mini review. Different concepts, such as redox potential compression (achieved by proton-coupled electron transfer, Lewis acid-base interactions, or structural rearrangements), hybrids with inorganic nanoparticles, and diffusion-controlled multi-component systems, will be discussed. Newly developed strategies to outcompete unproductive reaction pathways in favor of desired photoproduct formation will be compared, and the importance of identifying reaction intermediates in the course of multiphotonic excitation by different time-resolved spectroscopic techniques will be discussed. The mechanistic insights gained from molecular donor-photosensitizer-acceptor compounds inform the design of next-generation charge accumulation systems for solar energy conversion.

Photophysics of Perylene Diimide Dianions and Their Application in Photoredox Catalysis

The two‐electron reduced forms of perylene diimides (PDIs) are luminescent closed‐shell species whose photochemical properties seem underexplored. Our proof‐of‐concept study demonstrates that straightforward (single) excitation of PDI dianions with green photons provides an excited state that is similarly or more reducing than the much shorter‐lived excited states of PDI radical monoanions, which are typically accessible after biphotonic excitation with blue photons. Thermodynamically demanding photocatalytic reductive dehalogenations and reductive C−O bond cleavage reactions of lignin model compounds have been performed using sodium dithionite acts as a reductant, either in aqueous solution or in biphasic water–acetonitrile mixtures in the presence of a phase transfer reagent. Our work illustrates the concept of multi‐electron reduction of a photocatalyst by a sacrificial reagent prior to irradiation with low‐energy photons as a means of generating very reactive excited states.

Naphthalene diimides: perspectives and promise

In this review, we describe the developments in the field of naphthalene diimides (NDIs) from 2016 to the presentday. NDIs are shown to be an increasingly interesting class of molecules due to their electronic properties, large electron deficient aromatic cores and tendency to self-assemble into functional structures. Almost all NDIs possess high electron affinity, good charge carrier mobility, and excellent thermal and oxidative stability, making them promising candidates for applications in organic electronics, photovoltaic devices, and flexible displays. NDIs have also been extensively studied due to their potential real-world uses across a wide variety of applications including supramolecular chemistry, sensing, host-guest complexes for molecular switching devices, such as catenanes and rotaxanes, ion-channels, catalysis, and medicine and as non-fullerene accepters in solar cells. In recent years, NDI research with respect to supramolecular assemblies and mechanoluminescent properties has also gained considerable traction. Thus, this review will assist a wide range of readers and researchers including chemists, physicists, biologists, medicinal chemists and materials scientists in understanding the scope for development and applicability of NDI dyes in their respective fields through a discussion of the main properties of NDI derivatives and of the status of emerging applications.

Characterizing photocatalysts for water splitting: from atoms to bulk and from slow to ultrafast processes

This review provides a comprehensive overview on characterisation techniques for light-driven redox-catalysts highlighting spectroscopic, microscopic, electrochemical and spectroelectrochemical approaches.

Photoinduced Electron Transfer and Energy Transfer Processes in a Flexible BODIPY-C60 Dyad

A new donor-acceptor dyad composed of a BODIPY (4,4'-difluoro-4-bora-3a,4a-diaza-s-indacene) donor and a fullerene C₆₀ acceptor has been synthesized and characterized. This derivative has been prepared using a clickable fullerene building block that bears an alkyne moiety and a maleimide unit. The post-functionalization of the maleimide group by a BODIPY thiol leads to a BODIPY-C₆₀ dyad, leaving the alkyne moiety for further functional arrangement. On the basis of the combination of semi-empirical and density functional theory (DFT) calculations, spectroelectrochemical experiments, and steady-state and time-resolved spectroscopies, the photophysical properties of this new BODIPY-C₆₀ dyad were thoroughly studied. By using semi-empirical calculations, the equilibrium of three conformations of the BODIPY-C₆₀ dyad has been deduced, and their molecular orbital structures have been analyzed using DFT calculations. Two short fluorescence lifetimes were attributed to two extended conformers displaying variable donor-acceptor distances (17.5 and 20.0 Å). Additionally, the driving force for photoinduced electron transfer from the singlet excited state of BODIPY to the C₆₀ moiety was calculated using redox potentials determined with electrochemical studies. Spectroelectrochemical measurements were also carried out to investigate the absorption profiles of radicals in the BODIPY-C₆₀ dyad in order to assign the transient species in pump-probe experiments. Under selective photoexcitation of the BODIPY moiety, occurrences of both energy and electron transfers were demonstrated for the dyad by femtosecond and nanosecond transient absorption spectroscopies. Photoinduced electron transfer occurs in the folded conformer, while energy transfer is observed in extended conformers.

Time-Resolved Resonant Raman Spectroscopy of the Photoinduced Electron Transfer from Ruthenium(II) Trisbipyridine to Methyl Viologen

We report the first time-resolved resonant Raman (TR3) spectra of photoinduced charge transfer from [Ru(bpy)₃]²⁺ to methyl viologen, with observations of vibrational structure. The presence of singly charged methyl viologen in solution is noted by the appearance of several spectroscopic lines, which are visible in the spectra following subtraction of reagent molecules. Assignments are confirmed using both density functional theory (DFT) calculations and literature values and are shown to be consistent with transient absorption spectroscopy data. This presents proof-of-concept for the application of TR3 in mechanistic studies of photocatalytic systems.

Multi-Photon Excitation in Photoredox Catalysis: Concepts, Applications, Methods

The energy of visible photons and the accessible redox potentials of common photocatalysts set thermodynamic limits to photochemical reactions that can be driven by traditional visible-light irradiation. UV excitation can be damaging and induce side reactions, hence visible or even near-IR light is usually preferable. Thus, photochemistry currently faces two divergent challenges, namely the desire to perform ever more thermodynamically demanding reactions with increasingly lower photon energies. The pooling of two low-energy photons can address both challenges simultaneously, and whilst multi-photon spectroscopy is well established, synthetic photoredox chemistry has only recently started to exploit multi-photon processes on the preparative scale. Herein, we have a critical look at currently developed reactions and mechanistic concepts, discuss pertinent experimental methods, and provide an outlook into possible future developments of this rapidly emerging area.

Electrochemical and Photophysical Properties of Ruthenium(II) Complexes Equipped with Sulfurated Bipyridine Ligands

The development of new solar-to-fuel scenarios is of great importance, but the construction of molecular systems that convert sunlight into chemical energy represents a challenge. One specific issue is that the molecular systems have to be able to accumulate redox equivalents to mediate the photodriven transformation of relevant small molecules, which mostly involves the orchestrated transfer of multiple electrons and protons. Disulfide/dithiol interconversions are prominent 2e^-/2H⁺ couples and can play an important role for redox control and charge storage. With this background in mind, a new photosensitizer [Ru(^S-Sbpy)(bpy)₂]²⁺ (1²⁺) equipped with a disulfide functionalized bpy ligand (^S-Sbpy, bpy = 2,2'-bipyridine) was synthesized and has been comprehensively studied, including structural characterization by X-ray diffraction. In-depth electrochemical studies show that the ^S-Sbpy ligand in 1²⁺ can be reduced twice at moderate potentials (around -1.1 V vs Fc^+/0), and simulation of the cyclic voltammetry (CV) traces revealed potential inversion (E₂ > E₁) and allowed to derive kinetic parameters for the sequential electron-transfer processes. However, reduction at room temperature also triggers the ejection of one sulfur atom from 1²⁺, leading to the formation of [Ru(^Sbpy)(bpy)₂]²⁺(2²⁺). This chemical reaction can be suppressed by decreasing the temperature from 298 to 248 K. Compared to the archetypical photosensitizer [Ru(bpy)₃]²⁺, 1²⁺ features an additional low energy optical excitation in the MLCT region, originating from charge transfer from the metal center to the ^S-Sbpy ligand (aka MSCT) according to time-dependent density functional theory (TD-DFT) calculations. Analysis of the excited states of 1²⁺ on the basis of ground-state Wigner sampling and using charge-transfer descriptors has shown that bpy modification with a peripheral disulfide moiety leads to an energy splitting between charge-transfer excitations to the ^S-Sbpy and the bpy ligands, offering the possibility of selective charge transfer from the metal to either type of ligands. Compound 1²⁺ is photostable and shows an emission from a ³MLCT state in deoxygenated acetonitrile with a lifetime of 109 ns. This work demonstrates a rationally designed system that enables future studies of photoinduced multielectron, multiproton PCET chemistry.

Quantitative insights into charge-separated states from one- and two-pulse laser experiments relevant for artificial photosynthesis

Charge-separated states (CSSs) are key intermediates in photosynthesis and solar energy conversion. However, the factors governing the formation efficiencies of CSSs are still poorly understood, and light-induced electron-hole recombinations as deactivation pathways competing with desired charge accumulations are largely unexplored. This greatly limits the possibility to perform efficient multi-electron transfer, which is essential for artificial photosynthesis. We present a systematic investigation of two donor-sensitizer-acceptor triads (with different donor-acceptor distances) capable of storing as much as 2.0 eV in their CSSs upon the absorption of a visible photon. Using quantitative one- and two-pulse laser flash photolysis, we provide deep insights into both the CSS formation quantum yield, which can reach up to 80%, and the fate of the CSS upon further (secondary) excitation with green photons. The triad with shorter intramolecular distances shows a remarkable excitation wavelength dependence of the CSS formation quantum yield, and the CSS of this triad undergoes more efficient light-induced charge recombination than the longer equivalent by about one order of magnitude, whilst thermal charge recombination shows the exact opposite behavior. The unexpected results of our detailed photophysical study can be rationalized by detrimental singlet charge transfer states or structural considerations, and could significantly contribute to the future design of CSS precursors for accumulative multi-electron transfer and artificial photosynthesis.

Excited-state dynamics of heteroleptic copper(i) photosensitizers and their electrochemically reduced forms containing a dipyridophenazine moiety – a spectroelectrochemical transient absorption study

Heteroleptic copper(i) dipyridophenazine complexes were investigated by transient absorption spectroelectrochemistry to examine their multi-electron photoaccumulation properties.

Proton-coupled multi-electron transfer and its relevance for artificial photosynthesis and photoredox catalysis

Photoinduced PCET meets catalysis, and the accumulation of multiple redox equivalents is of key importance.

Electron Transfer across o-Phenylene Wires

Photoinduced electron transfer across rigid rod-like oligo- p-phenylenes has been thoroughly investigated in the past, but their o-connected counterparts are yet entirely unexplored in this regard. We report on three molecular dyads comprised of a triarylamine donor and a Ru(bpy)₃²⁺ (bpy =2,2'-bipyridine) acceptor connected covalently by 2 to 6 o-phenylene units. Pulsed excitation of the Ru(II) sensitizer at 532 nm leads to the rapid formation of oxidized triarylamine and reduced ruthenium complex via intramolecular electron transfer. The subsequent thermal reverse charge-shift reaction to reinstate the electronic ground-state occurs on a time scale of 120-220 ns in deaerated CH₃CN at 25 °C. The conformational flexibility of the o-phenylene bridges causes multiexponential transient absorption kinetics for the photoinduced forward process, but the thermal reverse reaction produces single-exponential transient absorption decays. The key finding is that the flexible o-phenylene bridges permit rapid formation of photoproducts storing ca. 1.7 eV of energy with lifetimes on the order of hundreds of nanoseconds, similar to what is possible with rigid rod-like donor-acceptor compounds. Thus, the conformational flexibility of the o-phenylenes represents no disadvantage with regard to the photoproduct lifetimes, and this is relevant in the greater context of light-to-chemical energy conversion.

Photoinduced electron transfer in a molecular dyad by nanosecond pump-pump-probe spectroscopy

The design of robust and inexpensive molecular photocatalysts for the conversion of abundant stable molecules like H2O and CO2 into an energetic carrier is one of the major fundamental questions for scientists nowadays. The outstanding challenge is to couple single photoinduced charge separation events with the sequential accumulation of redox equivalents at the catalytic unit for performing multielectronic catalytic reactions. Herein, double excitation by nanosecond pump-pump-probe experiments was used to interrogate the photoinduced charge transfer and charge accumulation on a molecular dyad composed of a porphyrin chromophore and a ruthenium-based catalyst in the presence of a reversible electron acceptor. An accumulative charge transfer state is unattainable because of rapid reverse electron transfer to the photosensitizer upon the second excitation and the low driving force of the forward photodriven electron transfer reaction. Such a method allows the fundamental understanding of the relaxation mechanism after two sequential photon absorptions, deciphering the undesired electron transfer reactions that limit the charge accumulation efficiency. This study is a step toward the improvement of synthetic strategies of molecular photocatalysts for light-induced charge accumulation and more generally, for solar energy conversion.

Insights into the mechanism and aging of a noble-metal free H2-evolving dye-sensitized photocathode

Co-grafting of a cobalt diimine–dioxime catalyst and push–pull organic dye on NiO yields a photocathode evolving hydrogen from aqueous solution under sunlight, with equivalent performances compared to a dyad-based architecture using similar components.

Dye-sensitized photo-electrochemical cells (DS-PECs) form an emerging technology for the large-scale storage of solar energy in the form of (solar) fuels because of the low cost and ease of processing of their constitutive photoelectrode materials. Preparing such molecular photocathodes requires a well-controlled co-immobilization of molecular dyes and catalysts onto transparent semiconducting materials. Here we used a series of surface analysis techniques to describe the molecular assembly of a push–pull organic dye and a cobalt diimine–dioxime catalyst co-grafted on a p-type NiO electrode substrate. (Photo)electrochemical measurements allowed characterization of electron transfer processes within such an assembly and to demonstrate for the first time that a Co^I species is formed as the entry into the light-driven H₂ evolution mechanism of a dye-sensitized photocathode. This co-grafted noble-metal free H₂-evolving photocathode architecture displays similar performances to its covalent dye–catalyst counterpart based on the same catalytic moiety. Post-operando time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis of these photoelectrodes after extensive photoelectrochemical operation suggested decomposition pathways of the dye and triazole linkage used to graft the catalyst onto NiO, providing grounds for the design of optimized molecular DS-PEC components with increased robustness upon turnover.

Exploiting Potential Inversion for Photoinduced Multielectron Transfer and Accumulation of Redox Equivalents in a Molecular Heptad

Photoinduced multielectron transfer and reversible accumulation of redox equivalents is accomplished in a fully integrated molecular heptad composed of four donors, two photosensitizers, and one acceptor. The second reduction of the dibenzo[1,2]dithiin acceptor occurs more easily than the first by 1.3 V, and this potential inversion facilitates the light-driven formation of a two-electron reduced state with a lifetime of 66 ns in deaerated CH₃CN. The quantum yield for formation of this doubly charge-separated photoproduct is 0.5%. In acidic oxygen-free solution, the reduction product is a stable dithiol. Under steady-state photoirradiation, our heptad catalyzes the two-electron reduction of an aliphatic disulfide via thiolate-disulfide interchange. Exploitation of potential inversion for the reversible light-driven accumulation of redox equivalents in artificial systems is unprecedented and the use of such a charge-accumulated state for multielectron photoredox catalysis represents an important proof-of-concept.

Snapshots of Light Induced Accumulation of Two Charges on Methylviologen using a Sequential Nanosecond Pump-Pump Photoexcitation

Methylviologen (MV²⁺) is perhaps the most used component as a reversible electron acceptor in photophysical studies. While MV²⁺ is most commonly implicated as a reversible one-electron mediator, its electrochemical properties clearly evidence two successive one-electron reduction processes. In this report, we have investigated on the light driven two-charge accumulation on MV²⁺ using a multicomponent system composed of the prototypical molecular photosensitizer [Ru(bpy)₃]²⁺ and MV²⁺ in the presence of ascorbate as reversible electron donor. The sequential addition of two electrons on the methylviologen was tracked upon sequential excitation of the [Ru(bpy)₃]²⁺ at optimized concentration of the electron acceptor. The charge accumulated state carries an energy of 0.9 eV above the ground state and has a lifetime of ca. 50 μs. We have reached a fairly good global yield of approximately 9% for the two-charge accumulation. This result clearly demonstrates the potential of this simple approach for applications in artificial photosynthesis.

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.