Artificial Photosynthetic Devices Research Articles

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.

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

AbstractThe 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.

Solar light-driven hydrogen evolution by co-catalyst-free subphthalocyanine-sensitized photocatalysts

Subphthalocyanines (SubPcs) have emerged as promising organic dyes for the development of efficient artificial photosynthetic devices, offering intense absorption in the visible region along with excellent charge transport properties.

Design guides for artificial photosynthetic devices consisting of voltage-matched perovskite/silicon tandem solar-cell modules and electrochemical reactor modules

We clarified the design guides for H2- and CO-producing artificial photosynthetic devices. The combination of a voltage-matched (VM) tandem solar-cell (SC) module and an electrochemical (EC) module was adopted. The parallel-connected top and bottom SC modules, in which multiple organic–inorganic hybrid perovskite (PVK) SCs with a bandgap of 1.7 eV and crystalline-silicon SCs were connected in series, respectively, powered the EC module consisting of series-connected multiple EC reactors. It was found that the design parameters of the series connection numbers must be optimized under slightly greater solar intensity and higher temperature than the average values to minimize the mismatch between the device operating voltage and SC maximal power voltage. This is in contrast to that the annual electricity production of the VM SC module coupled with a power conditioner is not sensitive to the optimization conditions. Increases in the bandgaps of the PVK SCs do not affect the annual production significantly.

Theoretical FRET Efficiency of an Antenna Material Containing Natural Dyes and Zeolite L.

We calculated the Förster resonance energy-transfer (FRET) efficiency of a theoretical host-guest composite formed by all-trans β-cryptoxanthin (BCRY), all-trans zeaxanthin (ZEA), and a zeolite-LTL (Linde Type L) nanochannel with the help of computational chemistry tools. Climate change demands urgently the development of novel renewable energies, and in such a context, artificial photosynthesis arises as a promising technology capable of contributing to satisfying humankind's energy needs. All artificial photosynthetic devices need antennas to harvest and transfer energy to a reaction center efficiently. Antenna materials integrated by highly fluorescent synthetic pigments embedded onto the nanochannels of a zeolite-LTL have already been shown experimentally to be very efficient supramolecular assemblies. However, research work computing the efficiency of an antenna made of nonfluorescent natural pigments and a zeolite-LTL nanochannel has not been undertaken yet, at least to our knowledge. Fortunately, natural dyes possess outstanding features to study them dynamically; they are environmentally friendly, inexpensive, ubiquitous, and abundant. Density functional theory (DFT) methods were chiefly employed along with the CAM-B3LYP functional and the 3-21G*/6-311+G(d,p) basis sets. The ONIOM method enabled geometry and energy calculations of dyes inside the zeolite-LTL (ZL) nanochannel. The Förster resonance energy-transfer (FRET) efficiency and the Förster radius of the composite were 40.9% and 24.9 Å, respectively. Theoretical findings suggested that this composite might contribute to diminishing costs and improving the environmental friendliness of an antenna system.

Flow cell for operando X-ray photon-in-photon-out studies on photo-electrochemical thin film devices.

Background: Photo-electro-chemical (PEC) water splitting represents a promising technology towards an artificial photosynthetic device but many fundamental electronic processes, which govern long-term stability and energetics, are not yet fully understood. X-ray absorption spectroscopy (XAS), and particularly its high energy resolution fluorescence-detected (HERFD) mode, emerges as a powerful tool to study photo-excited charge carrier behavior under operating conditions. The established thin film device architecture of PEC cells provides a well-defined measurement geometry, but it puts many constraints on conducting operando XAS experiments. It remains a challenge to establish a standardized thin film exchange procedure and concurrently record high-quality photoelectrochemical and X‑ray absorption spectroscopy data that is unperturbed by bubble formation. Here we address and overcome these instrumental limitations for photoelectrochemical operando HERFD-XAS. Methods: We constructed a novel operando photo-electro-chemical cell by computer numerical control milling, guided by the materials' X‑ray and visible light absorption properties to optimize signal detection. To test the cell's functionality, semiconducting thin film photoelectrodes have been fabricated via solution deposition and their photoelectrochemical responses under simulated solar light were studied using a commercial potentiostat in a three-electrode configuration during HERFD-XAS experiments at a synchrotron. Results: We demonstrate the cell's capabilities to measure and control potentiostatically and in open‑circuit, to detect X‑ray signals unperturbed by bubbles and to fluently exchange different thin film samples by collecting high-resolution Fe K-edge spectra of hematite ( α -Fe 2O 3) and ferrite thin film ( MFe 2O 4, M= Zn, Ni) photoelectrodes during water oxidation. Conclusions: Our cell establishes a measurement routine that will provide experimental access of photo-electro-chemical operando HERFD-XAS experiments to a broader scientific community, particularly due to the ease of sample exchange. We believe to enable a broad range of experiments which acquired fundamental insights will spur further photoelectrochemical research and commercialization of water splitting technologies.

Optical cavity-mediated exciton dynamics in photosynthetic light harvesting 2 complexes

Strong light-matter interaction leads to the formation of hybrid polariton states and alters the photophysical dynamics of organic materials and biological systems without modifying their chemical structure. Here, we experimentally investigated a well-known photosynthetic protein, light harvesting 2 complexes (LH2) from purple bacteria under strong coupling with the light mode of a Fabry-Perot optical microcavity. Using femtosecond pump probe spectroscopy, we analyzed the polariton dynamics of the strongly coupled system and observed a significant prolongation of the excited state lifetime compared with the bare exciton, which can be explained in terms of the exciton reservoir model. Our findings indicate the potential of tuning the dynamic of the whole photosynthetic unit, which contains several light harvesting complexes and reaction centers, with the help of strong exciton-photon coupling, and opening the discussion about possible design strategies of artificial photosynthetic devices.

Optimization of Z-Scheme Photocatalytic Reactors for Solar Water Splitting

The U.S. Department of Energy recently announced its first Energy Earthshot on Clean Hydrogen, with a cost target of $1/kg-H2 by 2031. Assuming future utility-scale grid electricity prices from photovoltaics ($0.02/kWh), 80% of the cost of H2 would come from performing low-temperature water electrolysis at its thermoneutral voltage, with zero additional overpotential. This fact motivates alternative, less-expensive means of using light to generate mobile charge carriers than photovoltaics, and reactor designs with exceedingly low capital costs, like those we recently invented. Systems using low capital cost reactors benefit from low-voltage operation, which represents a paradigm shift from current state-of-the-art electrolyzers that aim to operate at high current densities. Analytical models predict that solar photocatalytic water splitting inherently operates at low voltages through use of an ensemble of optically thin photoabsorbers each operating at a low rate. Collectively the ensemble exhibits larger overall solar-to-hydrogen conversion efficiencies in comparison to optically thick designs. In efforts to attain these predicted higher efficiencies, we are performing detailed studies on the properties of state-of-the-art doped SrTiO3 and BiVO4 photocatalyst particles. During my talk, I will share our recent efforts in atomic-layer deposited ultrathin oxide coatings to impart redox selectivity and materials stability, single-photocatalyst-particle current–potential behavior and mobile charge carrier properties, and atomic-level information on dopant distributions and materials interfaces obtained from electron microscopies and X-ray spectroscopies. Collectively, our discoveries provide new design guidelines and additional research pathways for the development of effective composite materials to serve as active components in techno-economically viable artificial photosynthetic devices.

Microfluidic system for extraterrestrial artificial photosynthetic device

Microfluidic system for extraterrestrial artificial photosynthetic device

(Invited) Optimization of Z-Scheme Photocatalytic Reactors for Solar Water Splitting

The U.S. Department of Energy recently announced its first Energy Earthshot on Clean Hydrogen, with a cost target of $1/kg-H2 by 2031. Assuming future utility-scale grid electricity prices from photovoltaics ($0.02/kWh), 80% of the cost of H2 would come from performing low-temperature water electrolysis at its thermoneutral voltage, with zero additional overpotential. This fact motivates alternative, less-expensive means of using light to generate mobile charge carriers than photovoltaics, and reactor designs with exceedingly low capital costs, which we recently invented. Systems that use low capital cost reactors benefit from low-voltage operation, which represents a paradigm shift from current state-of-the-art electrolyzers that aim to operate at high current densities. Analytical models predict that solar photocatalytic water splitting inherently exhibits such low-voltage operation through use of an ensemble of optically thin photoabsorbers, which results in larger overall solar-to-hydrogen conversion efficiencies in comparison to optically thick designs. In efforts to attain these predicted higher efficiencies, we are performing detailed studies on the properties of state-of-the-art doped SrTiO3 photocatalyst particles. During my talk, I will share our recent efforts in atomic-layer deposited ultrathin oxide coatings to impart redox selectivity and materials stability, single-photocatalyst-particle current–potential behavior and mobile charge carrier properties, and atomic-level information on dopant distributions and materials interfaces obtained from electron microscopies and X-ray spectroscopies. Collectively our discoveries provide new design guidelines and additional research pathways for the development of effective composite materials to serve as active components in techno-economically viable artificial photosynthetic devices.

Flow cell for operando X-ray photon-in-photon-out studies on photo-electrochemical thin film devices

Background: Photo-electro-chemical (PEC) water splitting represents a promising technology towards an artificial photosynthetic device but many fundamental electronic processes, which govern long-term stability and energetics, are not yet fully understood. X-ray absorption spectroscopy (XAS), and particularly its high energy resolution fluorescence-detected (HERFD) mode, emerges as a powerful tool to study photo-excited charge carrier behavior under operating conditions. The established thin film device architecture of PEC cells provides a well-defined measurement geometry, but it puts many constraints on conducting operando XAS experiments. It remains a challenge to establish a standardized thin film exchange procedure and concurrently record high-quality photoelectrochemical and X‑ray absorption spectroscopy data that is unperturbed by bubble formation. Here we address and overcome these instrumental limitations for photoelectrochemical operando HERFD-XAS. Methods: We constructed a novel operando photo-electro-chemical cell by computer numerical control milling, guided by the materials’ X‑ray and visible light absorption properties to optimize signal detection. To test the cell’s functionality, semiconducting thin film photoelectrodes have been fabricated via solution deposition and their photoelectrochemical responses under simulated solar light were studied using a commercial potentiostat in a three-electrode configuration during HERFD-XAS experiments at a synchrotron. Results: We demonstrate the cell’s capabilities to measure and control potentiostatically and in open‑circuit, to detect X‑ray signals unperturbed by bubbles and to fluently exchange different thin film samples by collecting high-resolution Fe K-edge spectra of hematite (α -Fe2O3) and ferrite thin film (MFe2O4, M= Zn, Ni) photoelectrodes during water oxidation. Conclusions: Our cell establishes a measurement routine that will provide experimental access of photo-electro-chemical operando HERFD-XAS experiments to a broader scientific community, particularly due to the ease of sample exchange. We believe to enable a broad range of experiments which acquired fundamental insights will spur further photoelectrochemical research and commercialization of water splitting technologies

Heterogenization of Molecular Water Oxidation Catalysts in Electrodes for (Photo)Electrochemical Water Oxidation

Water oxidation is still one of the most important challenges to develop efficient artificial photosynthetic devices. In recent decades, the development and study of molecular complexes for water oxidation have allowed insight into the principles governing catalytic activity and the mechanism as well as establish ligand design guidelines to improve performance. However, their durability and long-term stability compromise the performance of molecular-based artificial photosynthetic devices. In this context, heterogenization of molecular water oxidation catalysts on electrode surfaces has emerged as a promising approach for efficient long-lasting water oxidation for artificial photosynthetic devices. This review covers the state of the art of strategies for the heterogenization of molecular water oxidation catalysts onto electrodes for (photo)electrochemical water oxidation. An overview and description of the main binding strategies are provided explaining the advantages of each strategy and their scope. Moreover, selected examples are discussed together with the the differences in activity and stability between the homogeneous and the heterogenized system when reported. Finally, the common design principles for efficient (photo)electrocatalytic performance summarized.

Supramolecular strategies in artificial photosynthesis.

Artificial photosynthesis is a major scientific endeavor aimed at converting solar power into a chemical fuel as a viable approach to sustainable energy production and storage. Photosynthesis requires three fundamental actions performed in order; light harvesting, charge-separation and redox catalysis. These actions span different timescales and require the integration of functional architectures developed in different fields of study. The development of artificial photosynthetic devices is therefore inherently complex and requires an interdisciplinary approach. Supramolecular chemistry has evolved to a mature scientific field in which programmed molecular components form larger functional structures by self-assembly processes. Supramolecular chemistry could provide important tools in preparing, integrating and optimizing artificial photosynthetic devices as it allows precise control over molecular components within such a device. This is illustrated in this perspective by discussing state-of-the-art devices and the current limiting factors – such as recombination and low stability of reactive intermediates – and providing exemplary supramolecular approaches to alleviate some of those problems. Inspiring supramolecular solutions such as those discussed herein will incite expansion of the supramolecular toolbox, which eventually may be needed for the development of applied artificial photosynthesis.

A high-pressure artificial photosynthetic device: pumping carbon dioxide as well as achieving selectivity

By leveraging a distinct operational strategy, a high-pressure artificial-photosynthetic device achieved CO2R selectivity over 80% and solar-to-chemical efficiency exceeding 0.9%.

Peter Stang wins AIC Gold Medal

Peter Stang, Distinguished Professor of Chemistry at the University of Utah and editor of the Journal of the American Chemical Society, is the recipient of the 2020 American Institute of Chemists Gold Medal. The award recognizes excellence, contributions to society through the application of scientific research, and contributions to the chemical and biochemical professions. Stang’s research focuses on molecular architecture and supramolecular chemistry via self-assembly, with applications in areas such as information storage and artificial photosynthetic devices. He will receive the award during a celebration in Philadelphia May 5.

Biomimetic Approach to CO2 Reduction.

The development of artificial photosynthetic technologies able to produce solar-fuels from CO2 reduction is a fundamental task that requires the employment of specific catalysts being accomplished. Besides, effective catalysts are also demanded to capture atmospheric CO2, mitigating the effects of its constantly increasing emission. Biomimetic transition metal complexes are considered ideal platforms to develop efficient and selective catalysts to be implemented in electrocatalytic and photocatalytic devices. These catalysts, designed according to the inspiration provided by nature, are simple synthetic molecular systems capable of mimic features of the enzymatic activity. The present review aims to focus the attention on the mechanistic and structural aspects highlighted to be necessary to promote a proper catalytic activity. The determination of these characteristics is of interest both for clarifying aspects of the catalytic cycle of natural enzymes that are still unknown and for developing synthetic molecular catalysts that can readily be applied to artificial photosynthetic devices.

Layer-By-Layer Construction of Three-Dimensional Metal Organic Frameworks with Porphyrin on the Solid Surfaces

Introduction Porphyrins have planar structure and effectively absorb visible-light normal to the ring plane. Thus, they can be applied to artificial photosynthetic devices and photovoltaic cells when the porphyrin ring is flatly adsorbed on the solid substrate [1]. On the other hand, metal-organic frameworks (MOFs), which have a high surface area, flexibility, and designability, are one of the highly promissing materials to construct the highly oriented three-dimensional structures [2]. Thus, it is expected to construct three-dimentional MOFs with porphyrin, in which porphyrin ring is highly oriented, on the solid surfaces, in order to construct artifical photosynthetic devices. We have succeeded to construct the three-dimensional MOFs on the solid surfaces by a layer-by-layer method [3,4] and also to construct self-assembled monolayers (SAMs) with porphyrin [5-10]. Thus, in this study, we tried to construct the three-dimensional MOFs with porphyrin group, in which porphyrin ring is oriented parallel to the solid surfaces, using various SAM modified solid surfaces, such as iso-nicotinic acid (IA) SAM on TiO2(110), mercapto-propionic acid (MPA) SAM on Au(111), acrylic acid (AA) SAM on Si(111), and amino-propyl tri-methoxy silane (APTMS) SAM on glass substrate. At each step to construct these MOFs with tetra-carboxyphenyl porphyrin (TCPP), Zn tetra-phenyl porphyrin (Zn-TPP), and bipyridine (Bipy) by a layer-by-layer fasion, the adsorbed amounts and orientation were evaluated by x-ray photoelectron spectroscopy (XPS), polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS), attenuated total refection infrared spectroscopy (ATR-IRS), and ultraviolet/visible absorption spectroscopy (UV-vis). Experiment IA SAM on TiO2(110), MPA SAM on Au(111), AA SAM on Si(111), and APTMS SAM on glass were prepared by dipping each substrate into DMF solution with saturated IA at 110 °C for 10 h, ethanol solution with 1 mM MBA, ethanol solution with saturated methoxy acrylate and then hydrolysis of methoxy group to carboxylate, and ethanol solution with saturated ATPMS at 40 °C for 10h, respectively. After these SAM preparations, two-dimentional TCPP MOF sheets, which is prepared on the water surface [11], was transferred onto IA SAM on TiO2(110) and glass substrate. On the MPA SAM on Au(111) and AA SAM on Si(111), the substrates were dipped instead of the ethanol solutions with Zn-TPP and Bipy. At each step, layered structures and orientation were evaluated XPS, PM-IRRAS, ATR-IRS, and UV-vis. Results and discussion As an example, it is explained that IA SAM was confirmed to construct the TiO2(110) surface. When the pre-treated TiO2(110) substrate was dipped into DMF solution with saturated IA at 110 °C for 10 h, XP peaks around 401 eV in the N1s region and 532 eV in the O1s region due to the aniline ring and calboxylate, respectively, were observed. From these XP peaks, we confirmed that the IA SAM is formed on the TiO2(110) surface. After that, two-dimensional porphyrin sheet was transferred onto this SAM modified TiO2(110) surface by a layer-by-layer fasion.

(Invited) Synthetic Catalysts for Water Oxidation Using Metal Complexes and Nanoparticles

With solar cells, sunlight can be efficiently converted to electric power, however the electric power is instant. As soon as the Sun goes down, the electric power goes dim. On the other hand, conversion of sunlight into fuels provides a viable way of storing the sunlight for its permanent utilization. Green plants, algae, and cyanobacteria provide an efficient way of storing the sunlight in the form of sugars and starches using water and carbon dioxide (CO2) as feedstock materials by enzyme systems of high complexity in photosystems I and II in a process known as photosynthesis. The sunlight is utilized to get out electrons and protons from water via its oxidation at photosystem II with extremely high catalytic activity with turnover frequencies (TOFs) of 100-400 s-1 at overpotential (η) of only 160 mV. The electrons and protons are then siphoned off for the reduction of CO2 to sugars and starches at photosystem I. Inspired from photosynthesis, researchers worldwide are trying to design an artificial photosynthetic device that could efficiently convert the sunlight to fuels (H2 or CO2 reduction products such as CH4, CH3OH, CO, HCOOH, …etc.) using a combination of inexpensive materials and water or water and CO2 as sustainable raw resources. Two artificial photosynthetic devices have been designed, photoelectrochemical (PEC, artificial-leaf) and photovoltaic (PV) driven electrolyzer. In both electrolyzers, water via its oxidation at the photoanode/anode is the source of protons and electrons required for the fuel production at the photocathode/cathode. Water oxidation, however, is a kinetically sluggish process because of the complexity of the reaction mechanism that involves the removal of four protons and four electrons from two water molecules to form O=O double bond It consequently requires a potential substantially higher than the thermodynamic value [E 0(H2O/O2) = 1.23 V, pH = 0], that is η, likely due to the generation of energetic intermediates in the multiple electron and proton coupling steps. The high η value complicates the design of efficient artificial photosynthetic device for fuel production. Consequently, it is crucial to develop highly efficient anodes for water oxidation to decrease η and thus realize efficient artificial photosynthetic device. Herein we report our recent progress on water oxidation catalyst of metal complexes and nanoparticles.

Dye-sensitized photoelectrochemical water oxidation through a buried junction

Water oxidation has long been a challenge in artificial photosynthetic devices that convert solar energy into fuels. Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) provide a modular approach for integrating light-harvesting molecules with water-oxidation catalysts on metal-oxide electrodes. Despite recent progress in improving the efficiency of these devices by introducing good molecular water-oxidation catalysts, WS-DSPECs have poor stability, owing to the oxidation of molecular components at very positive electrode potentials. Here we demonstrate that a solid-state dye-sensitized solar cell (ss-DSSC) can be used as a buried junction for stable photoelectrochemical water splitting. A thin protecting layer of TiO2 grown by atomic layer deposition (ALD) stabilizes the operation of the photoanode in aqueous solution, although as a solar cell there is a performance loss due to increased series resistance after the coating. With an electrodeposited iridium oxide layer, a photocurrent density of 1.43 mA cm-2 was observed in 0.1 M pH 6.7 phosphate solution at 1.23 V versus reversible hydrogen electrode, with good stability over 1 h. We measured an incident photon-to-current efficiency of 22% at 540 nm and a Faradaic efficiency of 43% for oxygen evolution. While the potential profile of the catalyst layer suggested otherwise, we confirmed the formation of a buried junction in the as-prepared photoelectrode. The buried junction design of ss-DSSs adds to our understanding of semiconductor-electrocatalyst junction behaviors in the presence of a poor semiconducting material.

Anion-Mediated Photophysical Behavior in a C60 Fullerene [3]Rotaxane Shuttle.

By addressing the challenge of controlling molecular motion, mechanically interlocked molecular machines are primed for a variety of applications in the field of nanotechnology. Specifically, the designed manipulation of communication pathways between electron donor and acceptor moieties that are strategically integrated into dynamic photoactive rotaxanes and catenanes may lead to efficient artificial photosynthetic devices. In this pursuit, a novel [3]rotaxane molecular shuttle consisting of a four-station bis-naphthalene diimide (NDI) and central C60 fullerene bis-triazolium axle component and two mechanically bonded ferrocenyl-functionalized isophthalamide anion binding site-containing macrocycles is constructed using an anion template synthetic methodology. Dynamic coconformational anion recognition-mediated shuttling, which alters the relative positions of the electron donor and acceptor motifs of the [3]rotaxane's macrocycle and axle components, is demonstrated initially by 1H NMR spectroscopy. Detailed steady-state and time-resolved UV-vis-IR absorption and emission spectroscopies as well as electrochemical studies are employed to further probe the anion-dependent positional macrocycle-axle station state of the molecular shuttle, revealing a striking on/off switchable emission response induced by anion binding. Specifically, the [3]rotaxane chloride coconformation, where the ferrocenyl-functionalized macrocycles reside at the center of the axle component, precludes electron transfer to NDI, resulting in the switching-on of emission from the NDI fluorophore and concomitant formation of a C60 fullerene-based charge-separated state. By stark contrast, in the absence of chloride as the hexafluorophosphate salt, the ferrocenyl-functionalized macrocycles shuttle to the peripheral NDI axle stations, quenching the NDI emission via formation of a NDI-containing charge-separated state. Such anion-mediated control of the photophysical behavior of a rotaxane through molecular motion is unprecedented.