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.

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.

A Bioinspired Disulfide/Dithiol Redox Switch in a Rhenium Complex as Proton, H Atom, and Hydride Transfer Reagent

The transfer of multiple electrons and protons is of crucial importance in many reactions relevant in biology and chemistry. Natural redox-active cofactors are capable of storing and releasing electrons and protons under relatively mild conditions and thus serve as blueprints for synthetic proton-coupled electron transfer (PCET) reagents. Inspired by the prominence of the 2e^-/2H⁺ disulfide/dithiol couple in biology, we investigate herein the diverse PCET reactivity of a Re complex equipped with a bipyridine ligand featuring a unique SH···^-S moiety in the backbone. The disulfide bond in fac-[Re(^S-Sbpy)(CO)₃Cl] (1, ^S-Sbpy = [1,2]dithiino[4,3-b:5,6-b']dipyridine) undergoes two successive reductions at equal potentials of -1.16 V vs Fc^+|0 at room temperature forming [Re(^S2bpy)(CO)₃Cl]^2- (1^2-, ^S2bpy = [2,2'-bipyridine]-3,3'-bis(thiolate)). 1^2- has two adjacent thiolate functions at the bpy periphery, which can be protonated forming the S-H···^-S unit, 1H^-. The disulfide/dithiol switch exhibits a rich PCET reactivity and can release a proton (ΔG°_H⁺ = 34 kcal mol^-1, pK_a = 24.7), an H atom (ΔG°_H^• = 59 kcal mol^-1), or a hydride ion (ΔG°_H^- = 60 kcal mol^-1) as demonstrated in the reactivity with various organic test substrates.

Oligophenyls with Multiple Disulfide Bridges as Higher Homologues of Dibenzo[c,e][1,2]dithiin: Synthesis and Application in Lithium-Ion Batteries

Higher homologues of dibenzo[c,e][1,2]dithiin were synthesized from oligophenyls bearing multiple methylthio groups. Single-crystal X-ray analyses revealed their nonplanar structures and helical enantiomers of higher meta-congener 6. Such dibenzo[1,2]dithiin homologues are demonstrated to be applicable to lithium-ion batteries as cathode, displaying a high capacity of 118 mAh g-1 at a current density of 50 mA g-1 .

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.

Robust Binding of Disulfide-Substituted Rhenium Bipyridyl Complexes for CO2 Reduction on Gold Electrodes

Heterogenization of homogenous catalysts on electrode surfaces provides a valuable approach for characterization of catalytic processes in operando conditions using surface selective spectroelectrochemistry methods. Ligand design plays a central role in the attachment mode and the resulting functionality of the heterogenized catalyst as determined by the orientation of the catalyst relative to the surface and the nature of specific interactions that modulate the redox properties under the heterogeneous electrode conditions. Here, we introduce new [Re(L)(CO)₃Cl] catalysts for CO₂ reduction with sulfur-based anchoring groups on a bipyridyl ligand, where L = 3,3′-disulfide-2,2′-bipyridine (SSbpy) and 3,3′-thio-2,2′-bipyridine (Sbpy). Spectroscopic and electrochemical analysis complemented by computational modeling at the density functional theory level identify the complex [Re(SSbpy)(CO)₃Cl] as a multi-electron acceptor that combines the redox properties of both the rhenium tricarbonyl core and the disulfide functional group on the bipyridyl ligand. The first reduction at −0.85 V (vs. SCE) involves a two-electron process that breaks the disulfide bond, activating it for surface attachment. The heterogenized complex exhibits robust anchoring on gold surfaces, as probed by vibrational sum-frequency generation (SFG) spectroscopy. The binding configuration is normal to the surface, exposing the active site to the CO₂ substrate in solution. The attachment mode is thus particularly suitable for electrocatalytic CO₂ reduction.

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.

Computational electrochemistry: prediction of liquid-phase reduction potentials

This article reviews recent developments and applications in the area of computational electrochemistry. Our focus is on predicting the reduction potentials of electron transfer and other electrochemical reactions and half-reactions in both aqueous and nonaqueous solutions. Topics covered include various computational protocols that combine quantum mechanical electronic structure methods (such as density functional theory) with implicit-solvent models, explicit-solvent protocols that employ Monte Carlo or molecular dynamics simulations (for example, Car-Parrinello molecular dynamics using the grand canonical ensemble formalism), and the Marcus theory of electronic charge transfer. We also review computational approaches based on empirical relationships between molecular and electronic structure and electron transfer reactivity. The scope of the implicit-solvent protocols is emphasized, and the present status of the theory and future directions are outlined.