Observation of Photovoltaic Action from Photoacid-Modified Nafion Due to Light-Driven Ion Transport

Light-Driven Ionic and Molecular Transport through Atomically Thin Single Nanopores in MoS2/WS2 Heterobilayers

Nanofluidic ionic and molecular transport through atomically thin nanopore membranes attracts broad research interest from both scientific and industrial communities for environmental, healthcare, and energy-related technologies. To mimic the biological ion pumping functions, recently, light-induced and quantum effect-facilitated charge separation in heterogeneous 2D-material assemblies is proposed as the fourth type of driving force to achieve active and noninvasive transport of ionic species through synthetic membrane materials. However, to date, engineering versatile van der Waals heterostructures into 2D nanopore membranes remains largely unexplored. Herein, we fabricate single nanopores in heterobilayer transition metal dichalcogenide membranes with helium ion beam irradiation and demonstrate the light-driven ionic transport and molecular translocation phenomena through the atomically thin nanopores. Experimental and simulation results further elucidate the driving mechanism as the photoinduced near-pore electric potential difference due to type II band alignment of the semiconducting WS2 and MoS2 monolayers. The strength of the photoinduced localized electric field near the pore region can be approximately 1.5 times stronger than that of its counterpart under the conventional voltage-driven mode. Consequently, the light-driven mode offers better spatial resolution for single-molecule detection. Light-driven ionic and molecular transport through nanopores in van der Waals heterojunction membranes anticipates transformative working principles for next-generation biomolecular sequencing and gives rise to fascinating opportunities for light-to-chemical energy harvesting nanosystems.

Elementary Reaction Steps That Precede or Follow a Unimolecular Reaction Step Can Obfuscate Interpretation of the Driving-Force Dependence to Its Rate Constant

Assessing the validity of a driving-force-dependent kinetic theory for a unimolecular elementary reaction step is difficult when the observed reaction rate is strongly influenced by properties of the preceding or following elementary reaction step. A well-known example occurs for bimolecular reactions with weak orbital overlap, such as outer-sphere electron transfer, where bimolecular collisional encounters that precede a fast unimolecular electron-transfer step can limit the observed rate. A lesser-appreciated example occurs for bimolecular reactions with stronger orbital overlap, including many proton-transfer reactions, where equilibration of an endergonic unimolecular proton-transfer step results in a relatively small concentration of reaction products, thus slowing the rate of the following step such that it becomes rate limiting. Incomplete consideration of these points has led to discrepancies in interpretation of data from the literature. Our reanalysis of these data suggests that proton-transfer elementary reaction steps have a nonzero intrinsic free energy barrier, implying, in the parlance of Marcus theory, that there is non-negligible nuclear reorganization. Outcomes from our analyses are generalizable to inner-sphere electron-transfer reactions such as those involved in (photo)electrochemical fuel-forming reactions.

Bioinspired Light-Driven Proton Pump: Engineering Band Alignment of WS2 with PEDOT:PSS and PDINN

Bioinspired two-dimensional (2D) nanofluidic systems for photo-induced ion transport have attracted great attention, as they open a new pathway to enabling light-to-ionic energy conversion. However, there is still a great challenge in achieving a satisfactory performance. It is noticed that organic solar cells (OSCs, light-harvesting device based on photovoltaic effect) commonly require hole/electron transport layer materials (TLMs), PEDOT:PSS (PE) and PDINN (PD), respectively, to promote the energy conversion. Inspired by such a strategy, an artificial proton pump by coupling a nanofluidic system with TLMs is proposed, in which the PE- and PD-functionalized tungsten disulfide (WS2) multilayers construct a heterogeneous membrane, realizing an excellent output power of ≈1.13 nW. The proton transport is fine-regulated due to the TLMs-engineered band structure of WS2. Clearly, the incorporating TLMs of OSCs into 2D nanofluidic systems offers a feasible and promising approach for band edge engineering and promoting the light-to-ionic energy conversion.

Ionic Transistors

Biological voltage-gated ion channels, which behave as life's transistors, regulate ion transport precisely and selectively through atomic-scale selectivity filters to sustain important life activities. By this inspiration, voltage-adaptable ionic transistors that use ions as signal carriers may provide an alternative information processing unit beyond solid-state electronic devices. This review provides a comprehensive overview of the first generation of biomimetic ionic transistors, including their operating mechanisms, device architecture development, and property characterizations. Despite its infancy, significant progress has been made in the applications of ionic transistors in fields such as DNA detection, drug delivery, and ionic circuits. Challenges and prospects of full exploitation of ionic transistors for a broad spectrum of practical applications are also discussed.

Bioinspired light-driven chloride pump with helical porphyrin channels

Halorhodopsin, a light-driven chloride pump, utilizes photonic energy to drive chloride ions across biological membranes, regulating the ion balance and conveying biological information. In the light-driven chloride pump process, the chloride-binding chromophore (protonated Schiff base) is crucial, able to form the active center by absorbing light and triggering the transport cycle. Inspired by halorhodopsin, we demonstrate an artificial light-driven chloride pump using a helical porphyrin channel array with excellent photoactivity and specific chloride selectivity. The helical porphyrin channels are formed by a porphyrin-core star block copolymer, and the defects along the channels can be effectively repaired by doping a small number of porphyrins. The well-repaired porphyrin channel exhibits the light-driven Cl- migration against a 3-fold concentration gradient, showing the ion pumping behavior. The bio-inspired artificial light-driven chloride pump provides a prospect for designing bioinspired responsive ion channel systems and high-performance optogenetics.

Photoelectric responsive ionic channel for sustainable energy harvesting

Access to sustainable energy is paramount in today's world, with a significant emphasis on solar and water-based energy sources. Herein, we develop photo-responsive ionic dye-sensitized covalent organic framework membranes. These innovative membranes are designed to significantly enhance selective ion transport by exploiting the intricate interplay between photons, electrons, and ions. The nanofluidic devices engineered in our study showcase exceptional cation conductivity. Additionally, they can adeptly convert light into electrical signals due to photoexcitation-triggered ion movement. Combining the effects of salinity gradients with photo-induced ion movement, the efficiency of these devices is notably amplified. Specifically, under a salinity differential of 0.5/0.01 M NaCl and light exposure, the device reaches a peak power density of 129 W m-2, outperforming the current market standard by approximately 26-fold. Beyond introducing the idea of photoelectric activity in ionic membranes, our research highlights a potential pathway to cater to the escalating global energy needs.

Excited-State Proton Transfer in a Photoacid-Based Crystalline Coordination Compound: Reversible Photochromism, Near-Infrared Photothermal Conversion, and Conductivity

By harnessing the power of coordination self-assembly, crystalline materials can act as carriers for photoacids. Unlike their solution-based counterparts, these photoacids are capable of altering the properties of the crystalline material under light and can even generate proton transfer in a solid-state environment. Due to the photoinduced proton transfer and charge transfer processes within this functional material, this crystal exhibits powerful absorption spanning the visible to near-infrared spectrum upon light irradiation. This feature enables reproducible, significant chromatic variation, near-infrared photothermal conversion, and photocontrollable conductivity for this photoresponsive material. The findings suggest that the synthesis of pyranine photoacid-based crystalline materials via coordination self-assembly can not only enhance light-harvesting efficiency but also enable excited-state proton transfer processes within solid crystalline materials, thereby maintaining and even improving the properties of photoacids.

Dual-Functional Super-Assembled Mesoporous Carbon-Titania/AAO Hetero-Channels for Bidirectionally Photo-Regulated Ion Transport

Photo-regulated nanofluidic devices have attracted great attention in recent years due to their adjustable ion transport in real time. However, most of the photo-responsive nanofluidic devices can only adjust the ionic current unidirectionally, and cannot simultaneously increase or decrease the current signal intelligently by one device. Herein, a mesoporous carbon-titania/ anodized aluminum hetero-channels (MCT/AAO) is constructed by super-assembly strategy, which exhibits dual-function of cation selectivity and photo response. The polymer and TiO2 nanocrystals jointly build the MCT framework. Polymer framework with abundant negatively charged sites endows MCT/AAO with excellent cation selectivity, and TiO2 nanocrystals are responsible for the photo-regulated ion transport. High photo current densities of 1.8 mA m-2 (increase) and 1.2 mA m-2 (decrease) are realized by MCT/AAO benefiting from the ordered hetero-channels. Significantly, MCT/AAO can also achieve the bidirectionally adjustable osmotic energy by alternating the configurations of concentration gradient. Theoretical and experimental results reveal that the superior photo-generated potential is responsible for the bidirectionally adjustable ion transport. Consequently, MCT/AAO performs the function of harvesting ionic energy from the equilibrium electrolyte solution, which greatly expands its practical application field. This work provides a new strategy in constructing dual-functional hetero-channels toward bidirectionally photo-regulated ionic transport and energy harvesting.

Dyeing Non-Recyclable Polyethylene Plastic with Photoacid Phycocyanobilin from Spirulina Algae: Ultrafast Photoluminescence Studies

Despite the enormous environmental damage caused by plastic waste, it makes up over one-third of globally produced plastics. Polyethylene (PE) wastes have low recycling but high production rates. Towards the construction of ionic solar cells from PE, the present work describes the loading of a bioactive photoacid phycocyanobilin (PCB) dye from the pigment of Spirulina blue-green algae (as a natural resource) on low-density polyethylene (LDPE) plastic film. Dyeing was confirmed by X-ray photoelectron spectroscopy (XPS). Upon excitation of the Soret-band (400 nm), the photoluminescence (PL) spectra of PCB in neat solvents revealed two prominent emission peaks at 450-550 and 600-700 nm. The first band assigned to bilirubin-like (PCB_BR) species predominated the spectral profile in the highly rigid solvent glycerol and upon loading 0.45 % (w/w) of the dye on plastic. The photoluminescence excitation (PLE) spectra of PCB for the second region (Q-band) at 672 nm in the same solvents confirmed the ground state heterogenicity previously associated with the presence of PCB_A (neutral), PCB_B (cationic), and PCB_C (anionic) conformers. Time-resolved photoluminescence (TRPL) measurements induced via excitation of all PCB species at 510 nm in methanol revealed three-lifetime components with τ₁ = ~0.1 ns and τ₂ = ~2 ns associated with PCB_BR species and τ₃ = ~5 ns pertinent to the long-living photoproduct X*. Decay-associated spectra (DAS) analysis of the photoluminescence transient spectra of the final dyed films in the solid-state confirmed the improved generation of the long-living photoproduct as manifested in a significant increase in the PL intensity (~100-fold) and lifetime value (~90 ns) in the Q-region upon loading 6.92 % (w/w) of the dye on plastic. The photoproduct species were presumably assigned to the deprotonated PCB species, suggesting improved ionic mobility. The potential implementation of the PCB-sensitized PE solid wastes for the fabrication of ionic solar cells is discussed.

The Dual Use of the Pyranine (HPTS) Fluorescent Probe: A Ground-State pH Indicator and an Excited-State Proton Transfer Probe

Molecular fluorescent probes are an essential experimental tool in many fields, ranging from biology to chemistry and materials science, to study the localization and other environmental properties surrounding the fluorescent probe. Thousands of different molecular fluorescent probes can be grouped into different families according to their photophysical properties. This Account focuses on a unique class of fluorescent probes that distinguishes itself from all other probes. This class is termed photoacids, which are molecules exhibiting a change in their acid-base transition between the ground and excited states, resulting in a large change in their pKa values between these two states, which is thermodynamically described using the Förster cycle. While there are many different photoacids, we focus only on pyranine, which is the most used photoacid, with pKa values of ∼7.4 and ∼0.4 for its ground and excited states, respectively. Such a difference between the pKa values is the basis for the dual use of the pyranine fluorescent probe. Furthermore, the protonated and deprotonated states of pyranine absorb and emit at different wavelengths, making it easy to focus on a specific state. Pyranine has been used for decades as a fluorescent pH indicator for physiological pH values, which is based on its acid-base equilibrium in the ground state. While the unique excited-state proton transfer (ESPT) properties of photoacids have been explored for more than a half-century, it is only recently that photoacids and especially pyranine have been used as fluorescent probes for the local environment of the probe, especially the hydration layer surrounding it and related proton diffusion properties. Such use of photoacids is based on their capability for ESPT from the photoacid to a nearby proton acceptor, which is usually, but not necessarily, water. In this Account, we detail the photophysical properties of pyranine, distinguishing between the processes in the ground state and the ones in the excited state. We further review the different utilization of pyranine for probing different properties of the environment. Our main perspective is on the emerging use of the ESPT process for deciphering the hydration layer around the probe and other parameters related to proton diffusion taking place while the molecule is in the excited state, focusing primarily on bio-related materials. Special attention is given to how to perform the experiments and, most importantly, how to interpret their results. We also briefly discuss the breadth of possibilities in making pyranine derivatives and the use of pyranine for controlling dynamic reactions.

Quantification of Excited-State Brønsted-Lowry Acidity of Weak Photoacids Using Steady-State Photoluminescence Spectroscopy and a Driving-Force-Dependent Kinetic Theory

Photoacids and photobases constitute a class of molecules that upon absorption of light undergoes a reversible change in acidity, i.e. pK_a. Knowledge of the excited-state pK_a value, pK_a*, is critical for predicting excited-state proton-transfer behavior. A reasonable approximation of pK_a* is possible using the Förster cycle analysis, but only when the ground-state pK_a is known. This poses a challenge for the study of weak photoacids (photobases) with less acidic (basic) excited states (pK_a* (pK_b*) > 7), because ground-state pK_a (pK_b) values are >14, making it difficult to quantify them accurately in water. Another method to determine pK_a* relies on acid-base titrations with photoluminescence detection and Henderson-Hasselbalch analysis. This method requires that the acid dissociation reaction involving the thermally equilibrated electronic excited state reaches chemical quasi-equilibrium, which does not occur for weak photoacids (photobases) due to slow rates of excited-state proton transfer. Herein, we report a method to overcome these limitations. We demonstrate that liquid water and aqueous hydroxide are unique proton-accepting quenchers of excited-state photoacids. We determine that Stern-Volmer quenching analysis is appropriate to extract rate constants for excited-state proton transfer in aqueous solutions from a weak photoacid, 5-aminonaphthalene-1-sulfonate, to a series of proton-accepting quenchers. Analysis of these data by Marcus-Cohen bond-energy-bond-order theory yields an accurate value for pK_a* of 5-aminonaphthalene-1-sulfonate. Our method is broadly accessible because it only requires readily available steady-state photoluminescence spectroscopy. Moreover, our results for weak photoacids are consistent with those from previous studies of strong photoacids, each showing the applicability of kinetic theories to interpret driving-force-dependent rate constants for proton-transfer reactions.

Photoenergy Harvesting by Photoacid Solution

Solar energy has seen 180 years of development since the discovery of the photovoltaic effect, having achieved the most successful commercialization in the energy-harvesting fields. Despite its long history, even the most state-of-the-art photovoltaics remain confined to solid-state devices, limiting spatial and light utilization efficiencies. Herein, a liquid-state photoenergy harvester based on a photoacid (PA), a chemical that releases protons upon light irradiation and recombines with them in the dark through a fully reversible reaction, is demonstrated. Asymmetric light exposure on a PA solution contained in a transparent tube generates a pH gradient (ΔpH = 2) along the exposed and dark regions, which charges the Nernst potential up to 0.7 V across the two electrodes embedded at each end, as if a capacitor. Owing to the reversibility of PAs, a PA-driven liquid-state photoenergy harvester (PLPH) generates capacitive currents up to 0.72 mA m^-2  on an irradiation. Notably, the transparent nature of the PLPH enables vertical stacking up to 25 units, which multiplies the light-harvesting efficiencies by over 1000%. This unique approach provides a new route to harness solar energy with a form-factor-free design that maximizes spatial and light-use efficiencies.

Can Brønsted Photobases Act as Lewis Photobases?

Dative bonding or Lewis acid-base chemistry underpins a large number of chemical phenomena in a variety of fields, such as catalysis, metal-ligand interactions, and surface chemistry. Developing light-controlled Lewis acid-base interactions could offer a new way of controlling and understanding such phenomena. Photoinduced proton transfer, that is, excited-state Brønsted acidity and basicity, has been extensively studied and applied. Here, in direct analogy to excited-state Brønsted basicity, we show that exciting a photobasic molecule with light generates a thermodynamic drive for the transfer of a Lewis acid from a donor to a photobasic molecule. We have used the archetypal BF₃ as our Lewis acid and our photoactive Lewis bases are a family of quinolines, which are known Brønsted photobases as well. We have constructed the experimental Förster cycle for this system and have verified it computationally to demonstrate that a significant drive (0.2-0.7 eV) exists for the transfer of BF₃ to a photoexcited quinoline. The magnitude of this drive is similar to those reported for Brønsted photobasicity in quinolines. Computational results from TDDFT and energy decomposition analysis show that the origin of such an effect is similar to the Brønsted photoactivity of these molecules, in that they follow the Hammett parameter of substituent groups. These results suggest that photobases may be capable of controlling the chemical phenomena beyond proton transfer and may open opportunities for a new handle in photocatalysis.

Light-Powered Ion Pumping in a Cation-Selective Conducting Polymer Membrane

The simulation of the ion pumping against a proton gradient energized by light in photosynthesis is of significant importance for the energy conversion in a non-biological environment. Herein, we report light-powered ion pumping in a polystyrene sulfonate anion (PSS) doped polypyrrole (PPy) conducting polymer membrane (PSS-PPy) with a symmetric geometry. This PSS-PPy conducting polymer membrane exhibits a cationic selectivity and a light-responsive surface-charge-governed ion transport attributed to the negatively charged PSS groups. An asymmetric visible irradiation on one side of the PSS-PPy membrane induces a built-in electric field across the membrane due to the intrinsic photoelectronic property of PPy, which drives the cationic transport against the concentration gradient, demonstrating an ion-pumping effect. This work is a prototype that uses a geometry-symmetric conducting polymer membrane as a light-powered artificial ion pump for active ion transport, which exhibits potential applications in nanofluidic energy conversion.

Polarization-sensitive optoionic membranes from chiral plasmonic nanoparticles

Optoelectronic effects differentiating absorption of right and left circularly polarized photons in thin films of chiral materials are typically prohibitively small for their direct photocurrent observation. Chiral metasurfaces increase the electronic sensitivity to circular polarization, but their out-of-plane architecture entails manufacturing and performance trade-offs. Here, we show that nanoporous thin films of chiral nanoparticles enable high sensitivity to circular polarization due to light-induced polarization-dependent ion accumulation at nanoparticle interfaces. Self-assembled multilayers of gold nanoparticles modified with L-phenylalanine generate a photocurrent under right-handed circularly polarized light as high as 2.41 times higher than under left-handed circularly polarized light. The strong plasmonic coupling between the multiple nanoparticles producing planar chiroplasmonic modes facilitates the ejection of electrons, whose entrapment at the membrane-electrolyte interface is promoted by a thick layer of enantiopure phenylalanine. Demonstrated detection of light ellipticity with equal sensitivity at all incident angles mimics phenomenological aspects of polarization vision in marine animals. The simplicity of self-assembly and sensitivity of polarization detection found in optoionic membranes opens the door to a family of miniaturized fluidic devices for chiral photonics.

Light-Controlled Ionic/Molecular Transport through Solid-State Nanopores and Nanochannels

Biological nanochannels perfectly operate in organisms and exquisitely control mass transmembrane transport for complex life process. Inspired by biological nanochannels, plenty of intelligent artificial solid-state nanopores and nanochannels are constructed based on various materials and methods with the development of nanotechnology. Specially, the light-controlled nanopores/nanochannels have attracted much attention due to the unique advantages in terms of that ion and molecular transport can be regulated remotely, spatially and temporally. According to the structure and function of biological ion channels, light-controlled solid-state nanopores/nanochannels can be divided into light-regulated ion channels with ion gating and ion rectification functions, and light-driven ion pumps with active ion transport property. In this review, we present a systematic overview of light-controlled ion channels and ion pumps according to the photo-responsive components in the system. Then, the related applications of solid-state nanopores/nanochannels for molecular sensing, water purification and energy conversion are discussed. Finally, a brief conclusion and short outlook are offered for future development of the nanopore/nanochannel field.

Light-Modulated Cationic and Anionic Transport across Protein Biopolymers*

Light is a convenient source of energy and the heart of light-harvesting natural systems and devices. Here, we show light-modulation of both the chemical nature and ionic charge carrier concentration within a protein-based biopolymer that was covalently functionalized with photoacids or photobases. We explore the capability of the biopolymer-tethered photoacids and photobases to undergo excited-state proton transfer and capture, respectively. Electrical measurements show that both the photoacid- and photobase-functionalized biopolymers exhibit an impressive light-modulated increase in ionic conductivity. Whereas cationic protons are the charge carriers for the photoacid-functionalized biopolymer, water-derived anionic hydroxides are the suggested charge carriers for the photobase-functionalized biopolymer. Our work introduces a versatile toolbox to photomodulate both protons and hydroxides as charge carriers in polymers, which can be of interest for a variety of applications.

Harnessing Ionic Power from Equilibrium Electrolyte Solution via Photoinduced Active Ion Transport through van-der-Waals-Like Heterostructures

Nanofluidic ion transport through van der Waals heterostructures, composed of two or more types of reconstructed 2D nanomaterials, gives rise to fascinating opportunities for light-energy harvesting, due to coupling between the optoelectronic properties of the layered constituents and ion transport in between the atomic layers. Here, a photoinduced active ion transport phenomenon through transition metal dichalcogenides (TMDs)-based van-der-Waals-like multilayer heterostructures is reported for harnessing ionic power from equilibrium electrolyte solution. The binary heterostructure comprises sequentially stacked 2D-WS₂ and 2D-MoS₂ multilayers with sub-1 nm interlayer spacing. Upon visible-light illumination, a net ionic flow is initiated through the Janus membrane, suggesting a directional cationic transport from WS₂ to MoS₂ part. The transport mechanism is explained in terms of a photovoltaic effect due to type II band alignment of WS₂ /MoS₂ heterostructures. The driving mechanism can be generally applied to a variety of heterogeneous TMD membranes with type II semiconductor heterojunctions. In equilibrium ionic solutions, the maximum ionic photoresponse approaches ≈21 µA cm^-2 and ≈45 mV under one sun equivalent excitation. Under optimized conditions, the harvested power density reaches 2 mW m^-2 . The proof-of-concept demonstration of photonic-to-ionic power generation within angstrom-scale confinement anticipates potential for light-controlled ionic circuits, artificial photosynthesis, and biomimetic energy conversion.

Steady-State Spectroscopy to Single Out the Contact Ion Pair in Excited-State Proton Transfer

Despite the outstanding relevance of proton transfer reactions, investigations of the solvent dependence on the elementary step are scarce. We present here a probe system of a pyrene-based photoacid and a phosphine oxide, which forms stable hydrogen-bonded complexes in aprotic solvents of a broad polarity range. By using a photoacid, an excited-state proton transfer (ESPT) along the hydrogen bond can be triggered by a photon and observed via fluorescence spectroscopy. Two emission bands could be identified and assigned to the complexed photoacid (CPX) and the hydrogen-bonded ion pair (HBIP) by a solvatochromism analysis based on the Lippert-Mataga model. The latter indicates that the difference in the change of the permanent dipole moment of the two species upon excitation is ∼3 D. This implies a displacement of the acidic hydrogen by ∼65 pm, which is in quantitative agreement with a change of the hydrogen bond configuration from O-H···O to ^-O···H-O⁺.

Quinoline Photobasicity: Investigation within Water-Soluble Light-Responsive Copolymers

Quinoline photobases exhibit a distinctly higher pKa in their electronically excited state than in the ground state, thereby enabling light-controlled proton transfer reactions, for example, in molecular catalysis. The absorption of UV light translates to a pKa jump of approximately 10 units, as established for small-molecule photobases. This contribution presents the first synthesis of quinoline-based polymeric photobases prepared by reversible addition-fragmentation chain-transfer (RAFT) polymerization. The integration of quinolines as photobase chromophores within copolymers offers new possibilities for light-triggered proton transfer in nanostructured materials, that is, in nanoparticles, at surfaces, membranes and interfaces. To exploit the light-triggered reactivity of photobases within such materials, we first investigated how the ground- and excited-state properties of the quinoline unit changes upon polymer integration. To address this matter, we combined absorption and emission spectroscopy with time-resolved transient-absorption studies to reveal photoinduced proton-transfer dynamics in various solvents. The results yield important insights into the thermodynamic and kinetic properties of these polymeric quinoline photobases.

Light-Driven Active Ion Transport

Solar energy can be harvested by biological systems to regulate the directional transport of protons and ions across cells and organelles. Structural and functional bio-mimic photo-active ion nanofluidic conductors, usually in the forms of ion channels and ion pumps, have been increasingly applied to realize active ion transport. However, progress in attaining effective light-driven active transport of ions (protons) has been constrained by the inherent limitations of membrane materials and their chemical and topological structures. Recent advances in the construction of photo-responsive physical ion pump in all-solid-state membranes could potentially lead to new classes of membrane-based materials for active ion transport. In this concept, the development of the state-of-the-art technologies for manufacturing artificial light-driven active ion transport systems are presented and discussed, which mainly involves the utilization of solar energy to realize two types of active ion transport, chemically and physically active ion transport. Afterward, we summarize the key factors towards culminating highly effective and selective membranes for active ion transport. To conclude, we highlight the promising application perspectives of this light-driven active ion transport technique in the field of energy conversion, bio-interfaces and water treatment.

Broadband fluorescence reveals mechanistic differences in excited-state proton transfer to protic and aprotic solvents

Excited-state proton transfer (ESPT) to solvent is often explained according to the two-step Eigen-Weller model including a contact ion pair (CIP*) as an intermediate, but general applicability of the model has not been thoroughly examined. Furthermore, examples of the spectral identification of CIP* are scarce. Here, we report on a detailed investigation of ESPT to protic (H₂O, D₂O, MeOH and EtOH) and aprotic (DMSO) solvents utilizing a broadband fluorescence technique with sub-200 fs time resolution. The time-resolved spectra are decomposed into contributions from the protonated and deprotonated species and a clear signature of CIP* is identified in DMSO and MeOH. Interestingly, the CIP* intermediate is not observable in aqueous environment although the dynamics in all solvents are multi-exponential. Global analysis based on the Eigen-Weller model is satisfactory in all solvents, but the marked mechanistic differences between aqueous and organic solvents cast doubt on the physical validity of the rate constants obtained.

Block Copolymers Featuring Highly Photostable Photoacids Based on Vinylnaphthol: Synthesis and Self-Assembly

The synthesis of a photoresponsive amphiphilic diblock quarterpolymer containing 5-vinyl-1-naphthol (VN) as a photostable photoacidic comonomer is presented. The preparation is realized via a sequential reversible addition fragmentation chain transfer (RAFT) polymerization starting from a nona(ethylene glycol) methyl ether methacrylate (MEO₉ MA/"O") hydrophilic block, which is then used as a macro-RAFT agent in the terpolymerization of styrene (S), 2-vinylpyridine (2VP), and TBS-protected VN (tVN). The terpolymerization proceeds in a controlled fashion and two diblock quarterpolymers, P(O_m )-b-P(S_x -co-2VP_y -co-VN_z ), with varying functional comonomer compositions are prepared. These diblock quarterpolymers form spherical core-corona micelles in aqueous media according to dynamic light scattering (DLS) and cryogenic transmission electron microscopy (cryo-TEM). Upon irradiation, the photoacids within the micellar core experience a drastic increase in acidity causing a proton transfer from the photoacid to neighboring 2VP units. As a result, the hydrophilic/hydrophobic balance of the entire assembly is shifted, and the encapsulated cargo is released.

Light-Powered Directional Nanofluidic Ion Transport in Kirigami-Made Asymmetric Photonic-Ionic Devices

Nacre-mimetic 2D nanofluidic materials with densely packed sub-nanometer-height lamellar channels find widespread applications in water-, energy-, and environment-related aspects by virtue of their scalable fabrication methods and exceptional transport properties. Recently, light-powered nanofluidic ion transport in synthetic materials gained considerable attention for its remote, noninvasive, and active control of the membrane transport property using the energy of light. Toward practical application, a critical challenge is to overcome the dependence on inhomogeneous or site-specific light illumination. Here, asymmetric photonic-ionic devices based on kirigami-tailored graphene oxide paper are fabricated, and directional nanofluidic ion transport properties therein powered by full-area light illumination are demonstrated. The in-plane asymmetry of the graphene oxide paper is essential to the generation of photoelectric driving force under homogeneous illumination. This light-powered ion transport phenomenon is explained based on a modified carrier diffusion model. In asymmetric nanofluidic structures, enhanced recombination of photoexcited charge carriers at the membrane boundary breaks the electric potential balance in the horizontal direction, and thus drives the ion transport in that direction under symmetric illumination. The kirigami-based strategy provides a facile and scalable way to fabricate paper-like photonic-ionic devices with arbitrary shapes, working as fundamental elements for large-scale light-harvesting nanofluidic circuits.

Engineering Smart Nanofluidic Systems for Artificial Ion Channels and Ion Pumps: From Single-Pore to Multichannel Membranes

Biological ion channels and ion pumps with intricate ion transport functions widely exist in living organisms and play irreplaceable roles in almost all physiological functions. Nanofluidics provides exciting opportunities to mimic these working processes, which not only helps understand ion transport in biological systems but also paves the way for the applications of artificial devices in many valuable areas. Recent progress in the engineering of smart nanofluidic systems for artificial ion channels and ion pumps is summarized. The artificial systems range from chemically and structurally diverse lipid-membrane-based nanopores to robust and scalable solid-state nanopores. A generic strategy of gate location design is proposed. The single-pore-based platform concept can be rationally extended into multichannel membrane systems and shows unprecedented potential in many application areas, such as single-molecule analysis, smart mass delivery, and energy conversion. Finally, some present underpinning issues that need to be addressed are discussed.

Kinetic Evidence for the Necessity of Two Proton Donor Molecules for Successful Excited State Proton Transfer by a Photobase

Photobases are molecules that convert light to proton transfer drive and therefore have potential applications in many areas of chemistry. Previously, we studied the photobasicity of quinolines and explored their applications. While it is possible to tether a photobase near a target proton donor, for the sake of versatility it is desirable to explore their capability to deprotonate molecules dispersed in a solution. Previous evidence suggested that in this scenario at least two proton donors were necessary for successful excited state proton transfer: one to donate a proton and the second to stabilize the photogenerated donor anion. Here we report kinetic evidence from transient absorption (TA) and time-correlated single photon counting (TCSPC) in support of this hypothesis. We used 5-methoxyquinoline as the photobase and 2,2,2-trifluoroethanol (TFE), a low pK_a alcohol, as the proton donor. A constant concentration of the photobase was used for a range of proton-donor dilutions spanning several orders of magnitude in an aprotic background solvent. Absorption spectra confirm that over most of the studied range the majority of the photobase population is hydrogen bonded to at least one donor. Short-pulse TA was used to measure the faster (2-500 ps) dynamics, while TSCPC was used to measure the slower (>500 ps) dynamics. The measured proton transfer time constants varied as a function of donor concentration over a wide range. A log-log plot of the proton transfer rate constant as a function of proton-donor concentration shows two regimes: nondiffusive at high donor concentrations where multiple proton donors are near the photobase and diffusive at low donor concentrations where proton donors are more dilute. The nondiffusive regime has a slope of approximately one, suggesting that the proton transfer process is dependent on one donor molecule in addition to the donor molecule already hydrogen bonded with the photobase. The diffusive regime reasonably follows diffusion kinetics. We propose a model for how the second proton-donor molecule may interact with the photogenerated alkoxide to stabilize it. This work highlights the importance of inducing irreversible changes, in this case solvation of the alkoxide, after proton transfer. Understanding of such details is likely to be important in applications of photobases.

Highly Efficient Ionic Photocurrent Generation through WS2 -Based 2D Nanofluidic Channels

The unique feature of nacre-like 2D layered materials provides a facile, yet highly efficient way to modulate the transmembrane ion transport from two orthogonal transport directions, either vertical or horizontal. Recently, light-driven active transport of ionic species in synthetic nanofluidic systems attracts broad research interest. Herein, taking advantage of the photoelectric semiconducting properties of 2D transition metal dichalcogenides, the generation of a directional and greatly enhanced cationic flow through WS₂ -based 2D nanofluidic membranes upon asymmetric visible light illumination is reported. Compared with graphene-based materials, the magnitude of the ionic photocurrent can be enhanced by tens of times, and its photo-responsiveness can be 2-3.5 times faster. This enhancement is explained by the coexistence of semiconducting and metallic WS₂ nanosheets in the hybrid membrane that facilitates the asymmetric diffusion of photoexcited charge carriers on the channel wall, and the high ionic conductance due to the neat membrane structure. To further demonstrate its application, photonic ion switches, photonic ion diodes, and photonic ion transistors as the fundamental elements for light-controlled nanofluidic circuits are further developed. Exploring new possibilities in the family of liquid processable colloidal 2D materials provides a way toward high-performance light-harvesting nanofluidic systems for artificial photosynthesis and sunlight-driven desalination.

Photoacid-Modified Nafion Membrane Morphology Determined by Resonant X-ray Scattering and Spectroscopy

Covalent attachment of photoacid dye molecules to perfluorinated sulfonic acid membranes is a promising route to enable active light-driven ion pumps, but the complex relationship between chemical modification and morphology is not well understood in this class of functional materials. In this study we demonstrate the effect of bound photoacid dyes on phase-segregated membrane morphology. Resonant X-ray scattering near the sulfur K-edge reveals that introduction of photoacid dyes to the end of the ionomer side chains enhances phase segregation among ionomer domains, and the ionomer domain spacing increases with increasing amount of bound dye. Furthermore, relative crystallinity is marginally enhanced within semicrystalline domains composed of the perfluorinated backbone. X-ray absorption spectroscopy coupled with first-principles density functional theory calculations suggest that above a critical concentration, the multiple hydrophilic groups on the attached photoacid dye may help increase residual water content and promote hydration of adjacent sulfonic acid side chains under dry or ambient conditions.

Light-Driven Active Proton Transport through Photoacid- and Photobase-Doped Janus Graphene Oxide Membranes

Biological electrogenic systems use protein-based ionic pumps to move salt ions uphill across a cell membrane to accumulate an ion concentration gradient from the equilibrium physiological environment. Toward high-performance and robust artificial electric organs, attaining an antigradient ion transport mode by fully abiotic materials remains a great challenge. Herein, a light-driven proton pump transport phenomenon through a Janus graphene oxide membrane (JGOM) is reported. The JGOM is fabricated by sequential deposition of graphene oxide (GO) nanosheets modified with photobase (BOH) and photoacid (HA) molecules. Upon ultraviolet light illumination, the generation of a net protonic photocurrent through the JGOM, from the HA-GO to the BOH-GO side, is observed. The directional proton flow can thus establish a transmembrane proton concentration gradient of up to 0.8 pH units mm^-2 membrane area at a proton transport rate of 3.0 mol h^-1 m^-2 . Against a concentration gradient, antigradient proton transport can be achieved. The working principle is explained in terms of asymmetric surface charge polarization on HA-GO and BOH-GO multilayers triggered by photoisomerization reactions, and the consequent intramembrane proton concentration gradient. The implementation of membrane-scale light-harvesting 2D nanofluidic system that mimics the charge process of the bioelectric organs makes a straightforward step toward artificial electrogenic and photosynthetic applications.

A redox-generated biomimetic membrane potential across polypyrrole films

Inspired by the formation mechanism of a biological membrane potential, we described the generation of an artificial membrane potential through redox-regulating anion distribution on the two sides of a polypyrrole film. The polarity of the membrane potential could be regulated by the redox reaction.

Donor-acceptor preassociation, excited state solvation threshold, and optical energy cost as challenges in chemical applications of photobases

Photobases are molecules with increased pKa in the excited state that can serve to transduce light energy into proton removal capability. They can be used to control chemical reactions using light, such as removing protons from a catalytic site in reactions that are rate-limited by proton transfer. We identify and explore several major challenges toward their practical applications. Two important challenges are the need for pre-association (or ground state hydrogen bonding) between the proton donor and the photobase, and the need for excited state solvation of the photogenerated products. We investigate these two challenges with the photobase 5-methoxyquinoline as the proton acceptor and a low-pKa alcohol, 2,2,2-trifluoroethanol, as the proton donor. We vary the concentration of the donor in a background non-hydrogen-bonding solvent. Using absorption spectroscopy, we have identified that the donor-acceptor concentration ratio must exceed 100 : 1 to achieve appreciable ground state hydrogen bonding. Interestingly, emission spectroscopy reveals that the onset of ground state hydrogen bonding does not guarantee successful excited state proton transfer. It takes an additional order of magnitude increase in donor-acceptor ratio to achieve that goal, revealing that it is necessary to have excess donor molecules to reach the solvation threshold for the photogenerated products. The next challenge is reducing the large ground-excited state energy gap, which often requires UV photons to drive proton transfer. We show experimental and computational data comparing the photobasicity and optical energy gap for a few N-aromatic heterocyclic photobases. In general, we find that reducing the energy gap by increasing the conjugation size necessarily reduces photobasicity, while adding substituents of varying electron-withdrawing strength allows some fine-tuning of this effect. The combination of these two factors provide a preliminary design space for creating new photobasic molecules.

Photosynthesis-inspired bifunctional energy-harvesting devices that convert light and salinity gradients into electricity

An energy-harvesting device that is capable of converting light and a salinity gradient into electricity simultaneously was demonstrated conceptually.

Syntheses of Exceptionally Stable Aluminum(III) Metal-Organic Frameworks: How to Grow High-Quality, Large, Single Crystals

The difficulty of obtaining large single crystals of aluminum carboxylate metal-organic frameworks (MOFs) for structure determinations has limited the development of these water and thermally stable MOFs. Herein, how large single crystals of known MIL-53(Al) and the first two tetrahedral ligand-based, visible-light-absorbing 3D Al-MOFs, [Al₃ (OH)₃ (HTCS)₂ ] (AlTCS-1) and [Al₅ O₂ (OH)₃ (TCS)₂ (H₂ O)₂ ] (AlTCS-2; TCS=tetrakis(4-oxycarbonylphenyl)silane), are obtained in the presence of hydrofluoric or formic acid for conventional single-crystal diffraction measurements is presented. The technique of obtaining those single crystals has potential to be a general method for obtaining large and good-quality single crystals of Al-MOFs. AlTCS-1 and -2 are stable over a wide pH range (1-11), and AlTCS-1 is even stable in aqua regia solution for at least 24 h. The BET specific surface areas of AlTCS-1 and -2 are 11 and 1506 m²  g^-1 , respectively. AlTCS-2 takes up 51 cm³  (STP) g^-1 CO₂ and 15 cm³  (STP) g^-1 CH₄ at 298 K and 1 bar, which is relatively high among MOF materials. AlTCS-1 takes up 30 cm³  g^-1 CO₂ and 4.2 cm³  g^-1 CH₄ at 298 K and 1 bar. The rapid and stable photocurrent responses of AlTCS-1 and -2 under UV and visible-light illumination are observed. Moreover, AlTCS-1 photocatalyzes the water-splitting reaction under visible light with an average hydrogen evolution efficiency of 50 μmol g^-1  h^-1 for the first 10 h in a mixture of water and triethanolamine.