Long-lived Excited States Research Articles

The n–π* electronic transition induced by nitrogen vacancies enhances photocatalytic hydrogen production in carbon nitride

In semiconductor catalysts, long-lived excited states can effectually improve the utilization of photogenerated carriers to enhance photocatalytic performance. Herein, we used supramolecular engineering to synthesize a hollow tubular carbon nitride catalyst with N vacancies and an obvious n–π* transition. The unique hollow tubular structure provides abundant active sites, which are favorable for photocatalytic reaction. The presence of N vacancies expands the π-electron delocalization domains in the conjugated system, which excites the n–π* transition and thus triggers the red-shifted absorption edge at approximately 660 nm. Experiments and DFT calculations demonstrated that the N vacancies are beneficial for narrowing the bandgap and promoting the reduction of H+ by photogenerated electrons. Femtosecond transient absorption spectroscopy (fs-TAS) indicated that the n–π* electronic transition in the carbon nitride photocatalyst leads to slower exciton annihilation (lifetime: 38.64 ± 10.6 ps) and extended shallow electron trapping states (lifetime: 325.9 ± 19.3 ps). The appearance of these states adds more photogenerated electrons to the photocatalytic reaction process. The optimal hollow tubular carbon nitride catalyst exhibits a hydrogen production rate of 2664.47 μmol∙g−1∙h−1, which is 31.2 times higher than that of bulk carbon nitride (85.3325 μmol∙g−1∙h−1). This work highlights the ability of the n–π* transition induced by N vacancies to enhance the photocatalytic activity of carbon nitride.

Organic Room Temperature Phosphorescence from BN-Substituted Xanthene Derivatives.

Emissive organic materials are predominantly fluorescent and there is significant interest in realizing and understanding examples that defy this paradigm and exhibit phosphorescence under ambient conditions. Organic room temperature phosphorescence (ORTP) offers the long-lived excited states and bathochromically-shifted emission maxima of phosphorescence without the use of potentially toxic and expensive transition metals. Most ORTP materials rely on well-studied structural motifs that include aryl carbonyls, sulfones, and heavy main group elements. We report the unexpected ORTP of a series of heavy atom-free BN-substituted xanthene derivatives. The creation of heteroatom-rich scaffolds, combined with stabilizing C-H···F interactions in the solid-state, resulted in oxygen-tolerant heavy atom-free organic phosphorescence without relying on the use of cryogenic temperatures, polymer matrices, or host-guest interactions. The observation of ORTP in these simple systems sets a blueprint for the further development of heavy atom-free organic phosphors.

Current Context of Designing Phototheranostic Cyclometalated Iridium (III) Complexes to Open a New Avenue in Cancer Therapy.

Photo-induced chemotherapy offers the best option for the selective treatment of cancer among all the prevailing modalities. Iridium (III) complexes, flourished with excellent photophysical and photochemical properties, have been considered to be superior for undergoing photo-responsive cancer therapy. Large Stokes shift, long-lived triplet excited state, photostability, and tuneable emission have rendered its excellence as a phototheranostic agent. In particular, the cyclometalated Ir (III) complexes and their respective nanoparticles have made a strong niche in the arena of cancer therapy. In recent years, Ir (III) based complexes have shown promising utilities as both imaging and therapeutic agents as well. Therefore, this review summarises the recent advances in the strategic designing of cyclometalated Ir(III) complexes to augment their phototheranostic applications in precision medicine.

A Hybrid Strategy to Enhance Small-Sized Upconversion Nanocrystals

Upconversion nanoparticles (UCNPs) are characterized by high photostability, narrow spectral bands, excellent tuneability, and low biotoxicity, facilitating a broad range of biomedical applications. However, the small size required in many biological applications implies a lower luminescent brightness, as large surface-to-volume ratio is always accompanied with severe surface quenching. Herein, we introduce a strategy to overcome the surface quenching by incorporating an acceptor dye, sulforhodamine B (SRB) to surpass energy relaxation on long-lived lanthanide excited states. The surface modification of SRB led to up to 98.8% energy transfer efficiency, accompanied with the emergence of an intense SRB emission, with four orders of magnitude of change in the SRB/UCNPs emission ratio. The further structural optimisation led to an 8-fold upconversion emission enhancement. Moreover, the system exhibits excellent photostability, with only a 25% reduction over two hours under intense irradiation. By incorporating a pH responsive 5-carboxytetramethylrhodamine (5-TAMRA) to the UCNPs, we achieved a self-referencing protochromic sensor that are specific to protons and resistant to interference from various metal ions. This work provides a facile method for enhancing small-sized nanocrystals for biomedical sensing and imaging applications.

Monovalent nickel ion as ionization probe in matrix-assisted laser desorption/ionization mass spectrometry

We have developed NiO/HM20, a matrix composed of NiO nanoparticles (NiO) loaded on zeolite surface, for matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS). By photoexcitation, monovalent nickel ion (Ni+) was dominantly observed with high intensity. The mechanism of Ni+ production was studied by picosecond time-resolved emission spectroscopy. By loading NiO on the zeolite surface, a new relaxation pathway to other electronic excited states was observed, which increased the lifetime of electron–hole pairs and promoted the reduction of nickel. The long-lived charge-separated electronic excited state corresponding to the large polarization of NiO was confirmed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction spectroscopy (XRD). The correlation between the Auger parameter related to the polarization energy and the Ni+ peak intensity in the mass spectrum was confirmed. By applying the developed NiO/HM20 matrix to the MS measurement of small molecules, we found that molecules were ionized through complex formation with Ni+ using the coordination of lone electron pairs or π-electrons in the analyte molecules.

Origin of Intersystem Crossing in Red-Light Absorbing Bodipy Derivatives: Time-Resolved Transient Optical and Electron Paramagnetic Resonance Spectral Studies with Twisted and Planar Compounds.

We studied the intersystem crossing (ISC) property of red-light absorbing heavy atom-free dihydronaphtho[b]-fused Bodipy derivatives (with phenyl group attached at the lower rim via ethylene bridge, taking constrained geometry, i.e., BDP-1 and the half-oxidized product BDP-2) and dispiroflourene[b]-fused Bodipy (BDP-3) that have a twisted π-conjugated framework. BDP-1 and BDP-3 show strong and sharp absorption bands (i.e., ε = 2.0 × 105 M-1 cm-1 at 639 nm, fwhm ∼491 cm-1 for BDP-3). BDP-1 is significantly twisted (φ = 21.6°), while upon mono-oxidation, BDP-2 becomes nearly planar on the oxidized side (φ = 3.5°). Interestingly, BDP-2 showed efficient ISC (triplet state quantum yield, ΦT = 40%) due to S1/T2 state energy matching. Long-lived triplet excited state was observed (τT = 212 μs in solution and 2.4 ms in polymer matrix), and ISC takes 4.0 ns. Differently, twisted BDP-1 gives weak ISC only 5%, ISC takes 7.7 ns, and the triplet state is populated only with addition of ethyl iodide. Time-resolved electron paramagnetic resonance spectra of BDP-1 revealed the coexistence of two triplet states, with drastically different zero-field splitting D parameters of -2047 MHz and -1370 MHz, respectively, along with varying sublevel population ratios. We demonstrate that the ISC is not necessarily enhanced by torsion of the π-conjugation framework; instead, S1/Tn state energy matching is more efficient to induce ISC even in compounds that have planar molecular structures.

Stable Meisenheimer Complexes as Powerful Photoreductants Readily Obtained from Aza-Hetero Aromatic Compounds.

Excited states of radical anions derived from the photoreduction of stable organic molecules are suggested to serve as potent reductants. However, excited states of these species are too short-lived to allow bimolecular quenching processes. Recently, the singlet excited state of Meisenheimer complexes, which possess a long-lived excited state, was identified as the competent species for the reduction of challenging organic substrates (-2.63 V vs. SCE, saturated calomel electrode). To produce reasonably stable and simply accessible different Meisenheimer complexes, the addition of nBuLi to readily available aromatic heterocycles was investigated, and the photoreactivity of the generated species was studied. In this paper, we present the straightforward preparation of a family of powerful photoreductants (*Eox<-3 V vs. SCE in their excited states, determined by DFT and time-dependent TD-DFT calculations; DFT, density functional theory) that can induce dehalogenation of electron-rich aryl chlorides and to form C-C bond through radical cyclization. Photophysical analyses and computational studies in combination with experimental mechanistic investigations demonstrate the ability of the adduct to act as a strong electron donor under visible light irradiation.

Harnessing Solar Power for Oxidation of Organic Compounds by Re(I)Dipyrrinato Complexes.

Metal dipyrrinato complexes of 4d and 5d metals have distinctive features such as high absorption coefficients in the visible section and room temperature phosphorescence in the red region. This work demonstrates the light-assisted oxidation of organic compounds employing rhenium(I)dipyrrinato complexes as catalysts. The heavy atom effect in rhenium(I)dipyrrinato complexes leads to the formation of long-lived triplet excited states, and these complexes can generate singlet oxygen in excellent yields (up to 84 %). A method was developed for photocatalytic aerobic oxidation of sulfides and amines using only 0.05 mol % and 0.025 mol % of the rhenium(I)dipyrrinato complexes, respectively. The method is efficient, and within 2 h, a variety of substrates were oxidized to produce sulfoxides and imines in high yields (up to 97 %). Rhenium(I)dipyrrinato complexes work very well both in visible light and sunlight, making them promising candidates for photocatalytic applications.

Maximizing Nanoscale Downshifting Energy Transfer in a Metallosupramolecular Cr(III)-Er(III) Assembly.

Pseudo-octahedral CrIIIN6 chromophores hold a unique appeal for low-energy sensitization of NIR lanthanide luminescence due to their exceptionally long-lived spin-flip excited states. This allure persists despite the obstacles and complexities involved in integrating both elements into a metallosupramolecular assembly. In this work, we have designed a structurally optimized heteroleptic CrIII building block capable of binding rare earths. Following a complex-as-ligand synthetic strategy, two heterometallic supramolecular assemblies, in which three peripherical CrIII sensitizers coordinated through a molecular wire to a central ErIII or YIII, have been prepared. Upon excitation of the CrIII spin-flip states, the downshifted Er(4I13/2 → 4I15/2) emission at 1550 nm was induced through intramolecular energy transfer. Time-resolved experiments at room temperature reveal a CrIII → ErIII energy transfer of 62-73% efficiencies with rate constants of about 8.5 × 105 s-1 despite the long donor-acceptor distance (circa 14 Å). This efficient directional intermetallic energy transfer can be rationalized using the Dexter formalism, which is promoted by a rigid linear electron-rich alkyne bridge that acts as a molecular wire connecting the CrIII and ErIII ions.

Excited-State Dynamics of a Bright Fluorescent Dye with Precise Control of Emission Color Using Acid-Base Equilibrium, Intramolecular Charge Transfer, and Host-Guest Chemistry.

A fluorescent dye, a dithiophene-conjugated benzothiazole derivative (DTBz), was prepared to have high fluorescence emission quantum yields (ΦF) across various organic solvents. Its emission color modulation, from bright blue to deep red, was achieved through intramolecular charge transfer (ICT), acid-base equilibrium, and host-guest chemistry. Although it exhibits a weak solvatochromic effect, DTBz exhibited a bright fluorescence emission around 480 nm upon excitation at 390 nm in most solvents. In polar solvents, such as MeOH (methanol), EtOH (ethanol), DMF (N,N-dimethylforamide), and DMSO (dimethyl sulfoxide), an additional ICT emission band emerged around 640 nm, notably intense in DMSO, resulting in a bright greenish-white emission (ΦF = 0.67). The addition of 1,8-diazabicyclo[5,4.0]undec-7-ene (DBU) altered emission characteristics, reducing emission from the local excited (LE) state and enhancing ICT state emission. The degree of emission spectral change saturation with DBU addition varied with the solvent nature. Polar solvents with high dielectric constants, like DMSO and DMF, saw a complete disappearance of LE state emission with 5 equiv of DBU, resulting in a deep red emission (ΦFs of 0.53 and 0.48, respectively). Femtosecond transient absorption spectroscopy and time-resolved photoluminescence measurements elucidated the excited-state dynamics, revealing a long-lived excited state (τ-H = 10.3 ns) at a lower energy emission (640 nm), identified as DTBz-*, supported by transient absorption spectra analysis. Further analysis, including time-resolved fluorescence decay measurements and time-dependent density-functional theory (TD-DFT) calculations, underscored the role of deprotonation of DTBz's hydroxyl group in promoting the ICT process. The CIE coordination plot demonstrated wide linear emission color changes upon successive DBU additions in all solvents, while emission color precision was achieved through host-guest chemistry. Emission changes induced by DBU were reverted to the original state upon beta-cyclodextrin (β-CD) addition, with the 1H NMR study revealing the competition between acid-base equilibrium and host-guest complex formation as the cause of emission color change.

Elucidating the Mechanism of Efficient Eu(III) and Yb(III) Sensitisation from a Re(I) Tetrazolato Triangular Assembly.

The reaction of Re(CO)5Br with deprotonated 1H-(5-(2,2':6',2''-terpyridine)pyrid-2-yl)tetrazole yields a triangular assembly formed by tricarbonyl Re(I) vertices. Photophysical measurements reveal blue-green emission with a maximum at 520 nm, 32 % quantum yield, and 2430 ns long-lived excited state decay lifetime in deaerated dichloromethane solution. Coordination of lanthanoid ions to the terpyridine units red-shifts the emission to 570 nm and also reveals efficient (90 %) and fast sensitisation of both Eu(III) and Yb(III) at room temperature, with a similar rate constant kET on the order of 107 s-1. Efficient sensitisation of Eu(III) from Re(I) is unprecedented, especially when considering the close proximity in energy between the donor and acceptor excited states. On the other hand, comparative measurements at 77 K reveal that energy transfer to Yb(III) is two orders of magnitude slower than that to Eu(III). A two-step mechanism of sensitisation is therefore proposed, whereby the rate-determining step is a thermally activated energy transfer step between the Re(I) centre and the terpyridine functionality, followed by rapid energy transfer to the respective Ln(III) excited states. At 77 K, the direct Re(I) to Eu(III) energy transfer seems to proceed via a ligand-mediated superexchange Dexter-type mechanism.

Enter MnIV-NHC: A Dark Photooxidant with a Long-Lived Charge-Transfer Excited State.

Detailed photophysical investigation of a Mn(IV)-carbene complex has revealed that excitation into its lowest-energy absorption band (∼500 nm) results in the formation of an energetic ligand-to-metal charge-transfer (LMCT) state with a lifetime of 15 ns. To the best of our knowledge, this is the longest lifetime reported for charge-transfer states of first-row-based transition metal complexes in solution, barring those based on Cu, with a d10 configuration. A so-called superoxidant, Mn(IV)-carbene exhibits an excited state potential typically only harnessed via excited states of reactive organic radical species. Furthermore, the long-lived excited state in this case is found to be a dark doublet, with its transition to the quartet ground state being spin-forbidden, a contrast to most first-row literature examples, and a possible cause of the long lifetime. Showcasing excited state properties which in some cases exceed those of complexes based on precious metals, these findings not only advance the library of earth-abundant photosensitizers but also shed general insight into the photophysics of d3 and related Mn complexes.

In situ copper photocatalysts triggering halide atom transfer of unactivated alkyl halides for general C(sp3)-N couplings

Direct reduction of unactivated alkyl halides for C(sp3)-N couplings under mild conditions presents a significant challenge in organic synthesis due to their low reduction potential. Herein, we introduce an in situ formed pyridyl-carbene-ligated copper (I) catalyst that is capable of abstracting halide atom and generating alkyl radicals for general C(sp3)-N couplings under visible light. Control experiments confirmed that the mono-pyridyl-carbene-ligated copper complex is the active species responsible for catalysis. Mechanistic investigations using transient absorption spectroscopy across multiple decades of timescales revealed ultrafast intersystem crossing (260 ps) of the photoexcited copper (I) complexes into their long-lived triplet excited states (>2 μs). The non-Stern-Volmer quenching dynamics of the triplets by unactivated alkyl halides suggests an association between copper (I) complexes and alkyl halides, thereby facilitating the abstraction of halide atoms via inner-sphere single electron transfer (SET), rather than outer-sphere SET, for the formation of alkyl radicals for subsequent cross couplings.

Using Iron L-Edge and Nitrogen K-Edge X-ray Absorption Spectroscopy to Improve the Understanding of the Electronic Structure of Iron Carbene Complexes.

Iron-centered N-heterocyclic carbene compounds have attracted much attention in recent years due to their long-lived excited states with charge transfer (CT) character. Understanding the orbital interactions between the metal and ligand orbitals is of great importance for the rational tuning of the transition metal compound properties, e.g., for future photovoltaic and photocatalytic applications. Here, we investigate a series of iron-centered N-heterocyclic carbene complexes with +2, + 3, and +4 oxidation states of the central iron ion using iron L-edge and nitrogen K-edge X-ray absorption spectroscopy (XAS). The experimental Fe L-edge XAS data were simulated and interpreted through restricted-active space (RAS) and multiplet calculations. The experimental N K-edge XAS is simulated and compared with time-dependent density functional theory (TDDFT) calculations. Through the combination of the complementary Fe L-edge and N K-edge XAS, direct probing of the complex interplay of the metal and ligand character orbitals was possible. The σ-donating and π-accepting capabilities of different ligands are compared, evaluated, and discussed. The results show how X-ray spectroscopy, together with advanced modeling, can be a powerful tool for understanding the complex interplay of metal and ligand.

Ultrafast excitonic dynamics in DNA: Bridging correlated quantum dynamics and sequence dependence.

After photoexcitation of DNA, the excited electron (in the LUMO) and the remaining hole (in the HOMO) localized on the same DNA base form a bound pair, called the Frenkel exciton, due to their mutual Coulomb interaction. In this study, we demonstrate that a tight-binding (TB) approach, using TB parameters for electrons and holes available in the literature, allows us to correlate relaxation properties, average charge separation, and dipole moments to a large ensemble of double-stranded DNA sequences (all 16384 possible sequences with 14 nucleobases). This way, we are able to identify a relatively small subensemble of sequences responsible for long-lived excited states, high average charge separation, and high dipole moment. Further analysis shows that these sequences are particularly T rich. By systematically screening the impact of electron-hole interaction (Coulomb forces), we verify that these correlations are relatively robust against finite-size variations of the interaction parameter, not directly accessible experimentally. This methodology combines simulation methods from quantum physics and physical chemistry with statistical analysis known from genetics and epigenetics, thus representing a powerful bridge to combine information from both fields.

Harnessing synergistic effects in GQD@Pt(II) nanocomposites for enhanced photovoltaic performance: a computational study.

The development of efficient solar energy conversion technologies is crucial for addressing global energy challenges and reducing reliance on fossil fuels. Platinum(II) complexes are promising materials for photovoltaic applications due to their strong light absorption and long-lived excited states. However, their narrow absorption in the visible spectrum and stability issues limit their performance. Combining platinum(II) complexes with graphene quantum dots (GQDs) can enhance photovoltaic performance by leveraging the complementary light harvesting and charge transfer characteristics of the two components. This study utilizes density functional theory (DFT) calculations to explore their electronic structures, charge transfer dynamics, and photoelectric performance. Specifically, it investigates the effects of incorporating different substituents, either electron-donating or electron-withdrawing, onto the fluorene motif of the Pt(II) complex. The findings reveal that combining GQDs with Pt(II) complexes extends light absorption into the UV range, enabling comprehensive solar utilization. Upon photoexcitation, electrons migrate between the GQD conduction band and the Pt(II) complex, stabilizing charges and enhancing extraction. Substituents significantly influence charge transfer dynamics: electron-withdrawing groups promote transfer to the GQD, while electron-donating groups encourage charge separation and delocalization. Nanocomposites featuring electron-donating substituents achieve the highest energy conversion efficiencies, with GQD@Pt(II)-NPh2 reaching 24.6%. This is attributed to improved light harvesting, efficient charge injection, and reduced recombination. These insights guide the rational design of GQD-Pt(II) nanocomposites, optimizing charge separation and transfer processes for enhanced photovoltaic performance. The computational approach employed here provides a robust tool for developing advanced materials in renewable energy technologies. The computational studies reported in this work were performed using the DFT approach, specifically employing the hybrid functional PBE0. The PBE0 functional's accuracy in describing electronic structures and excited-state properties is essential for understanding charge transfer processes, photoabsorption, and emission characteristics in metal-organic complexes. Geometry optimizations and time-dependent DFT (TD-DFT) calculations were carried out to investigate the properties of the nanocomposites. The effects of solvents were replicated using the conductor-like polarizable continuum model (CPCM). The charge transfer length (ΔL) and interfragment charge transfer (ΔQ) were calculated using the Multiwfn software package, and all calculations were performed using the BDF software package.

Time-Resolved X-ray Emission Spectroscopy and Synthetic High-Spin Model Complexes Resolve Ambiguities in Excited-State Assignments of Transition-Metal Chromophores: A Case Study of Fe-Amido Complexes.

To fully harness the potential of abundant metal coordination complex photosensitizers, a detailed understanding of the molecular properties that dictate and control the electronic excited-state population dynamics initiated by light absorption is critical. In the absence of detectable luminescence, optical transient absorption (TA) spectroscopy is the most widely employed method for interpreting electron redistribution in such excited states, particularly for those with a charge-transfer character. The assignment of excited-state TA spectral features often relies on spectroelectrochemical measurements, where the transient absorption spectrum generated by a metal-to-ligand charge-transfer (MLCT) electronic excited state, for instance, can be approximated using steady-state spectra generated by electrochemical ligand reduction and metal oxidation and accounting for the loss of absorptions by the electronic ground state. However, the reliability of this approach can be clouded when multiple electronic configurations have similar optical signatures. Using a case study of Fe(II) complexes supported by benzannulated diarylamido ligands, we highlight an example of such an ambiguity and show how time-resolved X-ray emission spectroscopy (XES) measurements can reliably assign excited states from the perspective of the metal, particularly in conjunction with accurate synthetic models of ligand-field electronic excited states, leading to a reinterpretation of the long-lived excited state as a ligand-field metal-centered quintet state. A detailed analysis of the XES data on the long-lived excited state is presented, along with a discussion of the ultrafast dynamics following the photoexcitation of low-spin Fe(II)-Namido complexes using a high-spin ground-state analogue as a spectral model for the 5T2 excited state.

Nanoarchitectonics of fluorescent gold nanoclusters: A platform for image guided photodynamic therapy of hypoxic tumor

Metal nanoclusters are atomically precise materials comprising metal core of few atoms exhibiting unique photoluminescence properties, unlike their bigger counterparts. Some metal nanocluster with ligand-to-metal charge transfer, long-lived excited state and excited triplet state contribute to inherent photosensitizing (PS) property. However, the therapeutic efficacy of PDT is hindered by the insufficient oxygen supply (O2) in tumor microenvironment. In the present work, cysteine-capped gold nanocluster (AuC) are studied for their unique molecular architecture for PS efficiency. The co-existence of monodispersed and self-assembled structures contribute to the photoluminescence from the quantum confinement of electronic states and aggregation-induced emission (AIE) based PS property, respectively. In-silico model was performed to study the interaction of cysteine to gold cluster, its ground and excited-state properties and the charge transfer mechanism. The AuC as PS generates cytotoxic radicals in both Type I and Type II photodynamic pathways and the dominant radical species involved were elucidated by EPR spectroscopy. In vitro analysis in HeLa cells showed excellent biocompatibility and bioimaging properties. The intracellular ROS production and Live/Dead assay confirmed the generation of ROS in HeLa cells upon laser irradiation. The image-guided photodynamic property with synergistic Type I and Type II PDT reactions of AuC promises its potential application in cancer therapy in both hypoxic and normoxic conditions

Excited-State Identification of a Nickel-Bipyridine Photocatalyst by Time-Resolved X-ray Absorption Spectroscopy.

Photoassisted catalysis using Ni complexes is an emerging field for cross-coupling reactions in organic synthesis. However, the mechanism by which light enables and enhances the reactivity of these complexes often remains elusive. Although optical techniques have been widely used to study the ground and excited states of photocatalysts, they lack the specificity to interrogate the electronic and structural changes at specific atoms. Herein, we report metal-specific studies using transient Ni L- and K-edge X-ray absorption spectroscopy of a prototypical Ni photocatalyst, (dtbbpy)Ni(o-tol)Cl (dtb = 4,4'-di-tert-butyl, bpy = bipyridine, o-tol = ortho-tolyl), in solution. We unambiguously confirm via direct experimental evidence that the long-lived (∼5 ns) excited state is a tetrahedral metal-centered triplet state. These results demonstrate the power of ultrafast X-ray spectroscopies to unambiguously elucidate the nature of excited states in important transition-metal-based photocatalytic systems.

Intermolecular Proton-Coupled Electron Transfer Reactivity from a Persistent Charge-Transfer State for Reductive Photoelectrocatalysis.

Interest in applying proton-coupled electron transfer (PCET) reagents in reductive electro- and photocatalysis requires strategies that mitigate the competing hydrogen evolution reaction. Photoexcitation of a PCET donor to a charge-separated state (CSS) can produce a powerful H-atom donor capable of being electrochemically recycled at a comparatively anodic potential corresponding to its ground state. However, the challenge is designing a mediator with a sufficiently long-lived excited state for bimolecular reactivity. Here, we describe a powerful ferrocene-derived photoelectrochemical PCET mediator exhibiting an unusually long-lived CSS (τ ∼ 0.9 μs). In addition to detailed photophysical studies, proof-of-concept stoichiometric and catalytic proton-coupled reductive transformations are presented, which illustrate the promise of this approach.