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.

Redox tuning in Pt(bpy)-viologen catalyst-acceptor dyads enabling photocatalytic hydrogen evolution from water

A Pt(ii)-based photo-hydrogen-evolving molecular device tethered to dimethyl-substituted viologens (Pt(bpy)(dmMV²⁺)₂), providing higher driving force for hydrogen evolution reaction (HER) than the non-methylated analogue (Pt(bpy)(MV²⁺)₂), is found to exhibit improved photocatalytic performance. The observed behaviors are explained by the multiple HER pathways taken by evolving H₂ by the doubly and triply reduced species generated via consecutive photo-driven steps. Although the activity is still killed by the Dexter-type energy transfer quenching, our results provide new design strategies towards the development of more finely tuned molecular devices controlling the photocatalytic HER.

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.

Choosing sides: unusual ultrafast charge transfer pathways in an asymmetric electron-accepting cyclophane that binds an electron donor

Photo-driven electron transfer is faster from an electron donor guest to the harder to reduce acceptor in an asymmetric cyclophane host.

Constructing functional molecular systems for solar energy conversion and quantum information science requires a fundamental understanding of electron transfer in donor–bridge–acceptor (D–B–A) systems as well as competitive reaction pathways in acceptor–donor–acceptor (A–D–A) and acceptor–donor–acceptor′ (A–D–A′) systems. Herein we present a supramolecular complex comprising a tetracationic cyclophane having both phenyl-extended viologen (ExV²⁺) and dipyridylthiazolothiazole (TTz²⁺) electron acceptors doubly-linked by means of two p-xylylene linkers (TTzExVBox⁴⁺), which readily incorporates a perylene (Per) guest in its cavity (Per ⊂ TTzExVBox⁴⁺) to establish an A–D–A′ system, in which the ExV²⁺ and TTz²⁺ units serve as competing electron acceptors with different reduction potentials. Photoexcitation of the Per guest yields both TTz⁺˙–Per⁺˙–ExV²⁺ and TTz²⁺–Per⁺˙–ExV⁺˙ in <1 ps, while back electron transfer in TTz²⁺–Per⁺˙–ExV⁺˙ proceeds via the unusual sequence TTz²⁺–Per⁺˙–ExV⁺˙ → TTz⁺˙–Per⁺˙–ExV²⁺ → TTz²⁺–Per–ExV²⁺. In addition, selective chemical reduction of TTz²⁺ gives Per ⊂ TTzExVBox³⁺˙, turning the complex into a D–B–A system in which photoexcitation of TTz⁺˙ results in the reaction sequence ²*TTz⁺˙–Per–ExV²⁺ → TTz²⁺–Per–ExV⁺˙ → TTz⁺˙–Per–ExV²⁺. Both reactions TTz²⁺–Per⁺˙–ExV⁺˙ → TTz⁺˙–Per⁺˙–ExV²⁺ and TTz²⁺–Per–ExV⁺˙ → TTz⁺˙–Per–ExV²⁺ occur with a (16 ± 1 ps)^–1 rate constant irrespective of whether the bridge molecule is Per⁺˙ or Per. These results are explained using the superexchange mechanism in which the ionic states of the perylene guest serve as virtual states in each case and demonstrate a novel supramolecular platform for studying the effects of bridge energetics within D–B–A systems.

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.

An artificial photosynthetic system for photoaccumulation of two electrons on a fused dipyridophenazine (dppz)-pyridoquinolinone ligand

Increasing the efficiency of molecular artificial photosynthetic systems is mandatory for the construction of functional devices for solar fuel production. Decoupling the light-induced charge separation steps from the catalytic process is a promising strategy, which can be achieved thanks to the introduction of suitable electron relay units performing charge accumulation. We report here on a novel ruthenium tris-diimine complex able to temporarily store two electrons on a fused dipyridophenazine-pyridoquinolinone π-extended ligand upon visible-light irradiation in the presence of a sacrificial electron donor. Full characterization of this compound and of its singly and doubly reduced derivatives thanks to resonance Raman, EPR and (TD)DFT studies allowed us to localize the two electron-storage sites and to relate charge photoaccumulation with proton-coupled electron transfer processes.

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.

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.

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.