In operando visualization of redox flow battery in membrane-free microfluidic platform.

Redox flow batteries (RFBs) are attractive large-scale energy storage techniques, achieving remarkable progress in performance enhancement for the last decades. Nevertheless, an in-depth understanding of the reaction mechanism still remains challenging due to its unique operation mechanism, where electrochemistry and hydrodynamics simultaneously govern battery performance. Thus, to elucidate the precise reactions occurring in RFB systems, an appropriate analysis technique that enables the real-time observation of electrokinetic phenomena is indispensable. Herein, we report in operando visualization and analytical study of RFBs by employing a membrane-free microfluidic platform, that is, a membrane-free microfluidic RFB. Using this platform, the electrokinetic investigations were carried out for the 5,10-bis(2-methoxyethyl)-5,10-dihydrophenazine (BMEPZ) catholyte, which has been recently proposed as a high-performance multiredox organic molecule. Taking advantage of the inherent colorimetric property of BMEPZ, we unravel the intrinsic electrochemical properties in terms of charge and mass transfer kinetics during the multiredox reaction through in operando visualization, which enables theoretical study of physicochemical hydrodynamics in electrochemical systems. Based on insights on the electrokinetic limitations in RFBs, we verify the validity of electrode geometry design that can suppress the range of the depletion region, leading to enhanced cell performance.

Redox Potential Tuning of s-Tetrazine by Substitution of Electron-Withdrawing/Donating Groups for Organic Electrode Materials.

Herein, we tune the redox potential of 3,6-diphenyl-1,2,4,5-tetrazine (DPT) by introducing various electron-donating/withdrawing groups (methoxy, t-butyl, H, F, and trifluoromethyl) into its two peripheral benzene rings for use as electrode material in a Li-ion cell. By both the theoretical DFT calculations and the practical cyclic voltammetry (CV) measurements, it is shown that the redox potentials (E1/2) of the 1,2,4,5-tetrazines (s-tetrazines) have a strong correlation with the Hammett constant of the substituents. In Li-ion coin cells, the discharge voltages of the s-tetrazine electrodes are successfully tuned depending on the electron-donating/withdrawing capabilities of the substituents. Furthermore, it is found that the heterogeneous electron transfer rate (k0) of the s-tetrazine molecules and Li-ion diffusivity (DLi) in the s-tetrazine electrodes are much faster than conventional electrode active materials.

Anchored Mediator Enabling Shuttle-Free Redox Mediation in Lithium-Oxygen Batteries.

Redox mediators (RMs) are considered an effective countermeasure to reduce the large polarization in lithium-oxygen batteries. Nevertheless, achieving sufficient enhancement of the cyclability is limited by the trade-offs of freely mobile RMs, which are beneficial for charge transport but also trigger the shuttling phenomenon. Here, we successfully decoupled the charge-carrying redox property of RMs and shuttling phenomenon by anchoring the RMs in polymer form, where physical RM migration was replaced by charge transfer along polymer chains. Using PTMA (poly(2,2,6,6-tetramethyl-1-piperidinyloxy-4-yl methacrylate)) as a polymer model system based on the well-known RM tetramethylpiperidinyloxyl (TEMPO), it is demonstrated that PTMA can function as stationary RM, preserving the redox activity of TEMPO. The efficiency of RM-mediated Li2 O2 decomposition remains remarkably stable without the consumption of oxidized RMs or degradation of the lithium anode, resulting in an improved performance of the lithium-oxygen cell.

Stimuli-Responsive Reversible Fluorescence Switching in a Crystalline Donor-Acceptor Mixture Film: Mixed Stack Charge-Transfer Emission versus Segregated Stack Monomer Emission.

We report on a molecularly tailored 1:1 donor-acceptor (D-A) charge-transfer (CT) cocrystal that manifests strongly red-shifted CT luminescence characteristics, as well as noteworthy reconfigurable self-assembling behaviors. A loosely packed molecular organization is obtained as a consequence of the noncentrosymmetric chemical structure of molecule A1, which gives rise to considerable free volume and weak intermolecular interactions. The stacking features of the CT complex result in an external stimuli-responsive molecular stacking reorganization between the mixed and demixed phases of the D-A pair. Accordingly, high-contrast fluorescence switching (red↔blue) is realized on the basis of the strong alternation of the electronic properties between the mixed and demixed phases. A combination of structural, spectroscopic, and computational studies reveal the underlying mechanism of this stimuli-responsive behavior.

Wholly π-conjugated low-molecular-weight organogelator that displays triple-channel responses to fluoride ions.

A novel salicylidene aniline-based wholly π-conjugated molecule that could be self-assembled into an organogel was synthesized. The rigid organogel collapsed into fluid solutions with significant changes in UV-vis absorption and fluorescence colors in response to fluoride ions. It was found that the hydroxyl group in the salicylidene aniline moiety played a key role not only in the gelation but also in fluoride ion responses.

A ferroelectric photocatalyst for enhancing hydrogen evolution: polarized particulate suspension.

A particle-based photocatalyst with a permanent internal field prepared by a corona poling method is presented as a novel approach to enhance the hydrogen evolution reaction in a particulate-suspension system. Photocatalytic activity of K0.5Na0.5NbO3 was significantly improved by 7.4 times after the polarization.

A nonconjugated radical polymer enables bimodal memory and in-sensor computing operation.

This study reports intrinsic multimodal memristivity of a nonconjugated radical polymer with ambient stability. Organic memristive devices represent powerful candidates for biorealistic data storage and processing. However, there exists a substantial knowledge gap in realizing the synthetic biorealistic systems capable of effectively emulating the cooperative and multimodal activation processes in biological systems. In addition, conventional organic memristive materials are centered on conjugated small and macromolecules, making them synthetically challenging or difficult to process. In this work, we first describe the intrinsic resistive switching of the radical polymer that resulted in an exceptional state retention of >105 s and on/off ratio of >106. Next, we demonstrate its bimodal cooperative switching, in response to the proton accumulation as a biological input. Last, we expand our system toward an advanced in-sensor computing system. Our research demonstrates a nonconjugated radical polymer with intrinsic memristivity, which is directly applicable to future electronics including data storage, neuromorphics, and in-sensor computing.

Realizing molecular pixel system for full-color fluorescence reproduction: RGB-emitting molecular mixture free from energy transfer crosstalk.

A full-color molecular pixel system is realized for the first time using simple mixtures composed of RGB-emitting excited-state intramolecular proton transfer (ESIPT) dyes, each of which has delicately tailored Stokes shift and independent emission capability completely free from energy transfer crosstalk between them. It is demonstrated that the whole range of emission colors enclosed within the RGB color triangle on the CIE 1931 diagram is predictable and conveniently reproducible from the RGB molecular pixels not only in the solution but also in the polymer film. It must be noted that mixing ratios to reproduce the desired color coordinates can be precisely calculated on the basis of additive color theory according to their molecular pixel behavior.

Fluorescent zinc sensor with minimized proton-induced interferences: photophysical mechanism for fluorescence turn-on response and detection of endogenous free zinc ions.

A new fluorescent zinc sensor (HNBO-DPA) consisting of 2-(2'-hydroxy-3'-naphthyl)benzoxazole (HNBO) chromophore and a di(2-picolyl)amine (DPA) metal chelator has been prepared and examined for zinc bioimaging. The probe exhibits zinc-induced fluorescence turn-on without any spectral shifts. Its crystal structure reveals that HNBO-DPA binds a zinc ion in a pentacoordinative fashion through the DPA and HNBO moieties. Steady-state photophysical studies establish zinc-induced deprotonation of the HNBO group. Nanosecond and femtosecond laser flash photolysis and electrochemical measurements provide evidence for zinc-induced modulation of photoinduced electron transfer (PeT) from DPA to HNBO. Thus, the zinc-responsive fluorescence turn-on is attributed to suppression of PeT exerted by deprotonation of HNBO and occupation of the electron pair of DPA, a conclusion that is further supported by density functional theory and time-dependent density functional theory (DFT/TD-DFT) calculations. Under physiological conditions (pH 7.0), the probe displays a 44-fold fluorescence turn-on in response to zinc ions with a K(d) value of 12 pM. The fluorescent response of the probe to zinc ions is conserved over a broad pH range with its excellent selectivity for zinc ions among biologically relevant metal ions. In particular, its sensing ability is not altered by divalent transition metal ions such as Fe(II), Cu(II), Cd(II), and Hg(II). Cell experiments using HNBO-DPA show its suitability for monitoring intracellular zinc ions. We have also demonstrated applicability of the probe to visualize intact zinc ions released from cells that undergo apoptosis. More interestingly, zinc-rich pools in zebrafish embryos are traced with HNBO-DPA during early developmental stages. The results obtained from the in vitro and in vivo imaging studies demonstrate the practical usefulness of the probe to detect zinc ions.

Strategic emission color tuning of highly fluorescent imidazole-based excited-state intramolecular proton transfer molecules.

Highly fluorescent molecules harnessing the excited state intramolecular proton transfer (ESIPT) process are promising for a new generation of displays and light sources because they can offer very unique and novel optoelectronic properties which are different from those of conventional fluorescent dyes. To realize innovative ESIPT devices comprising full emission colors over the whole visible region, a molecular design strategy for predictable emission color tuning should be established. Here, we have developed a general strategy for a wide-range spectral tuning of imidazole-based ESIPT materials based on three different strategies--introduction of a nodal plane model, extension of effective conjugation length, and modification of heterocyclic rings. A series of nine ESIPT molecules were designed, synthesized and comprehensively investigated for their characteristic emission properties. All these molecules commonly showed no clear and transparent visible range absorption with no absorption color, but showed different colors of intense photoluminescence over broad visible regions from 450 nm (HPI) to 630 nm (HPNO) depending on their molecular structure. With the aid of density functional theory and time-dependent DFT calculations using M06, wB97XD, and B3LYP parameters with the 6-31G(d,p) basis set, these tuned emission bands of nine emitters were assigned from the stabilized excited state conformations that were derived from modified molecular structures.

NO2-Affinitive Conjugated Polymer for Selective Sub-Parts-Per-Billion NO2 Detection in a Field-Effect Transistor Sensor.

Conjugated polymers (CPs) have provided versatile semiconducting implements for the development of soft electronic devices. When three CPs with the same conjugated framework but different side chains were adopted in the field-effect transistor (FET) sensor for NO2 detection, the response to NO2 showed an opposite tendency to the charge carrier mobility of each CP. Morphological and structural characterizations revealed that the flexible glycol side chain enhances NO2 affinity as well as prevents the formation of lamellar stacking of the CP chains, thereby providing routes for the facile diffusion of NO2. Additionally, theoretical calculations for CP-NO2 complex formation at the molecular level support the relatively low energy barrier for inter-chain transition of NO2 between the glycol-based conjugated frameworks, which implies the spontaneous internal diffusion of NO2 to the semiconductor-dielectric interface in the FET-based sensor. As a result, the CP with a NO2-affinitive morphology exhibited an exceptional sensitivity of 13.8%/ppb upon NO2 (100 ppb) exposure for 50 s and provided excellent selectivity to the FET-based sensor toward other environmentally abundant harmful gases, such as SO2, CO2, and NH3. In particular, the theoretic limit of detection reached down to 0.24 ppb, which is the lowest value ever reported for organic FET-based NO2 gas sensors.

Designing Nonfullerene Acceptors with Oligo(Ethylene Glycol) Side Chains: Unraveling the Origin of Increased Open-Circuit Voltage and Balanced Charge Carrier Mobilities.

Despite the recent rapid development of organic solar cells (OSCs), the low dielectric constant (ϵr =3-4) of organic semiconducting materials limits their performance lower than inorganic and perovskite solar cells. In this work, we introduce oligo(ethylene glycol) (OEG) side chains into the dicyanodistyrylbenzene-based non-fullerene acceptors (NIDCS) to increase its ϵr up to 5.4. In particular, a NIDCS acceptor bearing two triethylene glycol chains (NIDCS-EO3) shows VOC as high as 1.12 V in an OSC device with a polymer donor PTB7, which is attributed to reduced exciton binding energy of the blend film. Also, the larger size grain formation with well-ordered stacking structure of the NIDCS-EO3 blend film leads to the increased charge mobility and thus to the improved charge mobility balance, resulting in higher JSC , FF, and PCE in the OSC device compared to those of a device using the hexyl chain-based NIDCS acceptor (NIDCS-HO). Finally, we fabricate NIDCS-EO3 devices with various commercial donors including P3HT, DTS-F, and PCE11 to show higher photovoltaic performance than the NIDCS-HO devices, suggesting versatility of NIDCS-EO3.

Gelation-induced enhanced fluorescence emission from organogels of salicylanilide-containing compounds exhibiting excited-state intramolecular proton transfer: synthesis and self-assembly.

Self-assembly structure, stability, hydrogen-bonding interaction, and optical properties of a new class of low molecular weight organogelators (LMOGs) formed by salicylanilides 3 and 4 have been investigated by field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), UV/Vis absorption and photoluminescence, as well as theoretical studies by DFT and semiempirical calculations with CI (AM1/PECI=8) methods. It was found that salicylanilides form gels in nonpolar solvents due to pi-stacking interaction complemented by the presence of both inter- and intramolecular hydrogen bonding. The supramolecular arrangement in these organogels predicted by XRD shows lamellar and hexagonal columnar structures for gelators 3 and 4, respectively. Of particular interest is the observation of significant fluorescence enhancement accompanying gelation, which was ascribed to the formation of J-aggregates and inhibition of intramolecular rotation in the gel state.

Mellitic Triimides Showing Three One-Electron Redox Reactions with Increased Redox Potential as New Electrode Materials for Li-Ion Batteries.

The mellitic triimide (MTI) bearing three imide groups on a benzene core with C3 symmetry is proposed as a new building block for organic electrode materials in lithium-ion batteries. MTI was anticipated to deliver a higher theoretical specific capacity of up to 282 mAh g-1 with increased reduction potentials compared with the well-known pyromellitic diimide building block bearing two imide groups because the additional imide group can accept one more electron and provide an electron-withdrawing effect. A model compound, ethyl-substituted mellitic triimide (ETTI), shows three well distinguished and reversible one-electron redox reactions at -0.97, -1.62, and -2.34 V versus Ag/Ag+ in 0.1 m tetrabutylammonium hexafluorophosphate electrolyte, but the redox potentials were increased in 2 m lithium bis(trifluoromethanesulfonyl)imide electrolyte: -0.60 V, -0.86 V, and -1.42 V vs. Ag/Ag+ . The DFT calculations revealed that the unique C3 symmetric structural design leads to the higher reduction potential of MTI in the Li-based electrolyte by formation of a stable 7-membered ring with a Li ion and the two carbonyl oxygen atoms from the adjacent imide groups. In a Li-ion coin cell, the ETTI electrode delivered a specific capacity of 176 mAh g-1 , corresponding to 81 % of capacity utilization, with three clear voltage plateaus. The higher average discharge voltage (2.41 V vs. Li/Li+ ) of ETTI allows it to deliver one of the highest specific energies (421 Wh kg-1 ) among reported diimide-based electrode materials. Finally, its redox mechanism was investigated by ex situ FTIR measurements and DFT calculations.

Improvement of Electrical Conductivity in Conjugated Polymers through Cascade Doping with Small-Molecular Dopants.

Doping capability is primitively governed by the energy level offset between the highest occupied molecular orbital (HOMO) of conjugated polymers (CPs) and the lowest unoccupied molecular orbital (LUMO) of dopants. A poor doping efficiency is obtained when doping directly using NOBF4 forming a large energy offset with the CP, while the devised doping strategy is found to significantly improve the doping efficiency (electrical conductivity) by sequentially treating the NOBF4 to the pre-doped CP with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino-dimethane (F4TCNQ), establishing a relatively small energy level offset. It is verified that the cascade doping strategy requires receptive sites for each dopant to further improve the doping efficiency, and provides fast reaction kinetics energetically. An outstanding electrical conductivity (>610 S cm-1 ) is achieved through the optimization of the devised doping strategy, and spectroscopy analysis, including Hall effect measurement, supports more efficient charge carrier generation via the devised cascade doping.

A white-light-emitting molecule: frustrated energy transfer between constituent emitting centers.

White-light-emitting single molecules are promising materials for use in a new generation of displays and light sources because they offer the possibility of simple fabrication with perfect color reproducibility and stability. To realize white-light emission at the molecular scale, thereby eliminating the detrimental concentration- or environment-dependent energy transfer problem in conventional fluorescent or phosphorescent systems, energy transfer between a larger band-gap donor and a smaller band-gap acceptor must be fundamentally blocked. Here, we present the first example of a concentration-independent ultimate white-light-emitting molecule based on excited-state intramolecular proton transfer materials. Our molecule is composed of covalently linked blue- and orange-light-emitting moieties between which energy transfer is entirely frustrated, leading to the production of reproducible, stable white photo- and electroluminescence.

s-Tetrazines as a New Electrode-Active Material for Secondary Batteries.

Because of the limitations of conventional metal-oxide-based electrodes, studies on organic redox-active materials as alternative electrodes for secondary batteries are emerging. However, reported organic electrode materials are still limited to a few kinds of organic redox groups. Therefore, the development of new redox-active groups for high-performance electrode materials is indispensable. Here, we evaluate s-tetrazine derivatives as a new electrode material in Li-ion batteries and study their charge/discharge mechanisms by ex situ XPS measurements. The porous carbon CMK-3 was introduced to encapsulate the s-tetrazines, which allowed 100 % utilization of the theoretical capacity and stable cycle performance of the s-tetrazines by preventing dissolution of the molecules into the electrolytes. This new class of redox-active group can pave the way for the next-generation of energy storage systems.

Characterization of food waste-driven carbon dot focusing on chemical structural, electron relaxation behavior and Fe3+ selective sensing.

In the study, carbon dot (CD) with high fluorescence properties was obtained via one-step hydrothermal carbonization of food model and sandwich leftover, respectively. The data in the article represent the change of the chemical structure and PL properties of the food waste-driven carbon dot (FWCDs). In higher carbonization temperature, pyridinic N and graphitic N were increased while amino N and pyrrolic N was decreased. The lifetime was increased with the increase of temperature. The CD prepared from sandwich leftover showed the dependency of the emission on the exciting wavelength and excellent Fe3+ sensitivity without significant change of lifetime. It also had a pH-sensitive fluorescence feature and good stability in NaCl solutions. For more insight, please see Food waste-driven N-doped carbon dots: Applications for Fe3+ sensing and cell imaging Ahn et al., 2019.

Dual-color fluorescent nanoparticles showing perfect color-specific photoswitching for bioimaging and super-resolution microscopy.

Dual-emissive systems showing color-specific photoswitching are promising in bioimaging and super-resolution microscopy. However, their switching efficiency has been limited because a delicate manipulation of all the energy transfer crosstalks in the systems is unfeasible. Here, we report a perfect color-specific photoswitching, which is rationally designed by combining the complete off-to-on fluorescence switching capability of a fluorescent photochromic diarylethene and the frustrated energy transfer to the other fluorescent dye based on the excited-state intramolecular proton transfer (ESIPT) process. Upon alternation of UV and visible light irradiations, the system achieves 100% switching on/off of blue emission from the diarylethene while orange emission from the ESIPT dye is unchanged in the polymer film. By fabricating this system into biocompatible polymer nanoparticles, we demonstrate microscopic imaging of RAW264.7 macrophage cells with reversible blue-color specific fluorescence switching that enables super-resolution imaging with a resolution of 70 nm.

Food waste-driven N-doped carbon dots: Applications for Fe3+ sensing and cell imaging.

We report highly fluorescent N-doped carbon dots (CDs) synthesized from food waste via one-step hydrothermal carbonization. To study the chemical transition of carbon dots from food wastes, the cat feed stocks driven from food waste were used as the waste model. In the model study, the core of the CDs was successfully self N-doped without extra pre- or post-treatments. The experimental results reveal that the nitrogen in the waste model played an important role in the formation of graphitic N and pyridinic N in the core and functional groups on the surface. Especially, high process temperature (≥180 °C) resulted in high quantum yield as 23% of the CDs from the waste model. To demonstrate the conversion of real food waste into CDs, the hamburger sandwich leftover was used as a precursor for CDs. The food waste driven CDs had similar chemical and fluorescent properties to that of the waste model, having quantum yield of 28%. This study exhibits the food waste driven carbon dots are excellent candidates for fluorescence probe to Fe3+ with high selectivity even under the interference of other metal, and for bio-imaging material with good cell viability over 80%.