Electron Accumulation on Naphthalene Diimide Photosensitized by [Ru(2,2′-Bipyridine)₃]²⁺

Abstract

In a molecular triad comprised of a central naphthalene diimide (NDI) unit flanked by two [Ru(bpy)] (bpy = 2,2'-bipyridine) sensitizers, NDI is formed after irradiation with visible light in deaerated CHCN in the presence of excess triethylamine. The mechanism for this electron accumulation involves a combination of photoinduced and thermal elementary steps. In a structurally related molecular pentad with two peripheral triarylamine (TAA) electron donors attached covalently to a central [Ru(bpy)]-NDI-[Ru(bpy)] core but no sacrificial reagents present, photoexcitation only leads to NDI (and TAA), whereas NDI is unattainable due to rapid electron transfer events counteracting charge accumulation. For solar energy conversion, this finding means that fully integrated systems with covalently linked photosensitizers and catalysts are not necessarily superior to multicomponent systems, because the fully integrated systems can suffer from rapid undesired electron transfer events that impede multielectron reactions on the catalyst.

Tuning the Electron Storage Potential of a Charge‐Photoaccumulating RuII Complex by a DFT‐Guided Approach

“…Ta king inspiration from these systems, more sophisticated light-harvesting units, capable of storing multiple electrons upon visible light irradiation,h ave been synthesized and studied. [20][21][22] Twoc onceptually differentm olecular designs have proven successful in storing multiple reducing equivalents in artificial photosynthetics ystems: either the system assembles multiple chromophores each of whichi ndependently transfers electrons to the acceptors ite (such as ab ridging ligand betweent wo chromophore units), [24][25][26][27][28][29][30][31][32][33][34][35][36][37] or it containsasingle protonated species [1H 2 ] 2 + .T he occurrence of PCET processes strongly lowers the reducing power of the system, precluding the use of charge photoaccumulation in [1H 2 ] 2 + to drive catalytic H 2 production.T his motivated us to search for structurally modified derivatives to store electrons at more negative potentials,e ven after protonation.U sing density functional theory( DFT) calculations, we screened electron-rich substituents ands elected the introduction of an oxime group in place of the carbonyl moiety in [1](PF 6 ) 2 .H erein we describe the process of this DFT-guidedd esign and the synthesis of the selected oxime-modified PS [2](PF 6 ) 2, together with its spectroscopic and (spectro)electrochemical characterization. Under irradiation in the presence of ap roton source, [2] 2 + transforms into the doubly protonated species [2H 2 ] 2 + ,w hichi sastronger reductant than [1H 2 ] 2 + .…”

“…Moreover,t he ability of this novel artificial photosynthetic system to store two photogenerated electrons at a more reducing potential, via ap roton-coupled electrontransfer mechanism,was demonstrated.Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.chromophore and relies on as acrificiale lectron donor to perform multiple excitation-accumulation cycles. [38][39][40][41][42][43][44][45] In the latter case, either as ingle or multiple electron storage sites can be assembled adjacentt ot he chromophore.T ypical light-harvesting units are porphyrins, [24,36] Ir III[37] or Ru II polypyridine [25][26][27][28][29][30][31][32][33][34][35][38][39][40][41][42][43][44][45] complexes,c ovalently linked to aw ide variety of electron storage sites, such as am etal centre, [25] polyoxometalates, [13,36] naphthalene diimide, [35,45] perylene diimide, [24] anthraquinone, [30,33,34] or p-extended phenanthroline-based [26-29, 44, 46, 47] ligands.R ecently,W enger and co-workers produced one of the first examples of the beneficial role played by electron photoaccumulation in photoredox catalysis. They used assemblies of Ru II chromophores with ac entral dibenzo[1,2]dithiin electron storageu nit to accumulate two photogenerated electrons, further used to perform dithiolate/disulfide int...…”

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

“…Another possible competitive pathway that can be considered is the transfer of excitation energy by a FRET (Forster resonance energy transfer) process between the excited state of the Ru chromophore, which emits at 610 nm, and the NDI .− unit, which presents an absorption band in this region (see the Supporting Information). In a recent article by Wenger and co‐workers, the authors succeeded to accumulate two negative charges on the NDI moiety within a ruthenium triad, under continuous illumination and in the presence of an irreversible electron donor. However, in the presence of ascorbate as a reversible donor, the authors could not observed the generation of NDI 2− owing to the short lifetime of the Ru‐NDI .− state (20 ns) generated from the ruthenium chromophore excited state.…”

“…In a recent article by Wenger and co‐workers, the authors succeeded to accumulate two negative charges on the NDI moiety within a ruthenium triad, under continuous illumination and in the presence of an irreversible electron donor. However, in the presence of ascorbate as a reversible donor, the authors could not observed the generation of NDI 2− owing to the short lifetime of the Ru‐NDI .− state (20 ns) generated from the ruthenium chromophore excited state. Thus, it is noteworthy that even if a lower 2 nd CSS formation yield is found, the much slower dynamics (two orders of magnitude difference for the apparent rates of 1 st and 2 nd formation) outline that the deleterious competitive reaction pathways are either slow or inefficient, enabling a second charge transfer to occur and to be observed in our pump–pump–probe experiments.…”

From Photodriven Charge Accumulation to Fueling Enzyme Cascades in Molecular Factories

“…2). [10] Selective excitation of the ruthenium(ii) complexes in neat CH 3 CN rapidly (< 10 ns) leads to intramolecular photoinduced electron transfer to result in NDI -, but the resulting charge-separated state is unstable and decays back to the ground state almost equally rapidly. Charge accumulation on NDI in neat CH 3 CN is not possible in this triad.…”