Photoinduced Selective B-H Activation of nido-Carboranes.

The development of new synthetic methods for B-H bond activation has been an important research area in boron cluster chemistry, which may provide opportunities to broaden the application scope of boron clusters. Herein, we present a new reaction strategy for the direct site-selective B-H functionalization of nido-carboranes initiated by photoinduced cage activation via a noncovalent cage···π interaction. As a result, the nido-carborane cage radical is generated through a single electron transfer from the 3D nido-carborane cage to a 2D photocatalyst upon irradiation with green light. The resulting transient nido-carborane cage radical could be directly probed by an advanced time-resolved EPR technique. In air, the subsequent transformations of the active nido-carborane cage radical have led to efficient and selective B-N, B-S, and B-Se couplings in the presence of N-heterocycles, imines, thioethers, thioamides, and selenium ethers. This protocol also facilitates both the late-stage modification of drugs and the synthesis of nido-carborane-based drug candidates for boron neutron capture therapy (BNCT).

Direct Observation of All Open-Shell Intermediates in a Photocatalytic Cycle.

Molecular photocatalysis has shown tremendous success in sustainable energy and chemical synthesis. However, visualizing the transient open-shell intermediates in photocatalysis is a significant and long-standing challenge. By employing our recently developed innovative time-resolved electron paramagnetic resonance technique, we directly observed all radicals and radical ions involved in the photocatalytic addition of pempidine to tert-butyl acrylate. The full picture of the photocatalytic cycle is vividly illustrated by the fine structures, chemical kinetics, and dynamic spin polarization of all open-shell intermediates directly observed in this prototypical system. Given the universality of this methodology, we believe it greatly empowers the research paradigm of direct observation in both photocatalysis and radical chemistry.

Time-resolved electron paramagnetic resonance spectrometer based on ultrawide single-sideband phase-sensitive detection.

A time-resolved electron paramagnetic resonance (TREPR) method with 40 ns time resolution and a high sensitivity suitable for the detection of short-lived radicals under thermal equilibrium is developed. The key is the introduction of a new detection technique named ultrawide single sideband phase sensitive detection (U-PSD) to the conventional continuous-wave EPR, which remarkably enhanced the sensitivity for the detection of broadband transient signals compared with the direct detection protocol. By repeatedly triggering a transient kinetic event f(t) (e.g., by laser flash photolysis) under a 100 kHz magnetic field modulation with precise phase control, this technique can build an ultrawide single sideband modulated signal. After single sideband demodulation, the flicker noise-suppressed signal f(t) with wide bandwidth is recovered. A U-PSD TREPR spectrometer prototype has been built, which integrated timing sequence control, laser flash excitation, data acquisition systems, and the U-PSD algorithm with a conventional continuous-wave EPR. It exhibited excellent performance in monitoring a model transient radical system, laser flash photolysis of benzophenone in isopropanol. Both the intense chemically induced dynamic electron polarization signals and the much weaker thermal equilibrium EPR signals of the generated acetone ketyl radical and benzophenone ketyl radical were clearly observed within a wide timescale ranging from sub-microsecond to milliseconds. This prototype validated the feasibility of the U-PSD technique and demonstrated its superior performance in studying complex photochemical systems containing various transient radicals, which complements the established TREPR techniques and provides a powerful tool for deep mechanistic understandings, such as in photoredox catalysis and artificial photosynthesis.

Longevity gene responsible for robust blue organic materials employing thermally activated delayed fluorescence.

The 3rd-Gen OLED materials employing thermally-activated delayed fluorescence (TADF) combine advantages of first two for high-efficiency and low-cost devices. Though urgently needed, blue TADF emitters have not met stability requirement for applications. It is essential to elucidate the degradation mechanism and identify the tailored descriptor for material stability and device lifetime. Here, via in-material chemistry, we demonstrate chemical degradation of TADF materials involves critical role of bond cleavage at triplet state rather than singlet, and disclose the difference between bond dissociation energy of fragile bonds and first triplet state energy (BDE-ET1) is linearly correlated with logarithm of reported device lifetime for various blue TADF emitters. This significant quantitative correlation strongly reveals the degradation mechanism of TADF materials have general characteristic in essence and BDE-ET1 could be the shared "longevity gene". Our findings provide a critical molecular descriptor for high-throughput-virtual-screening and rational design to unlock the full potential of TADF materials and devices.

Iron-modulated Ni3S2 derived from a Ni-MOF-based Prussian blue analogue for a highly efficient oxygen evolution reaction.

Developing efficient, environmentally friendly and cost-effective non-precious metal electrocatalysts for the oxygen evolution reaction (OER) is essential to alleviate the energy crisis and environmental pollution. Herein, we report a simple and practical method to prepare non-precious metal catalysts, namely iron-modulated Ni3S2 (Fe-Ni3S2/NF) on nickel foam, by growing a Ni-MOF directly on 3D porous conductive nickel foam, followed by the formation of Ni-MOF-based Prussian blue analogs (Ni-MOF@PBA) via in situ cation exchange reactions, which are further sulfidated to iron-modulated Ni3S2. Based on a series of characterization results, it is confirmed that iron acts as a modulator at the Ni active site, leading to electron depletion, thereby modulating the electron spin state and optimizing the binding energy of key reaction intermediates, resulting in highly exposed active sites and acceleration of OER reaction kinetics. The synthesized Fe-Ni3S2/NF exhibits excellent activity in alkaline media, which needs overpotentials of only 232 mV and 287 mV to drive current densities of 10 mA cm-2 and 50 mA cm-2, respectively. Additionally, Fe-Ni3S2/NF exhibits excellent stability for at least 24 h during the OER process. This work presents a rational design and synthesis of transition metal-based catalysts with nanocone structures, providing a new strategy for assembling advanced materials and insights for exploring various energy storage and conversion systems.

Prussian blue analogue assisted formation of iron doped CoNiSe2 nanosheet arrays for efficient oxygen evolution reaction.

Electrochemical water splitting is a promising approach to produce hydrogen gas, but sluggish four-electron transfer of the oxygen evolution reaction (OER) severely limits the overall energy conversion efficiency of water splitting. Herein, as an excellent OER electrocatalyst, a technique of synthesizing Fe doped CoNiSe2 nanosheet (Fe-CoNiSe2) whole series using CoFe prussian blue analog produced by Co-ZIF-L reaction as a template is proposed here. The introduction of iron ions promotes the redistribution of the cobalt-nickel charge density, which enhances the OER kinetics. In view of the abovementioned points, Fe-CoNiSe2/NF has excellent activity, electrocatalytic properties and excellent stability in alkaline media, which only demands a lower overpotential of 244 mV and 271 mV to deliver a current density of 10 mA cm-2 and 50 mA cm-2, respectively. The material also exhibits excellent stability for at least 24 h during the OER process. This work may provide some new insights into the assembly of advanced and highly-active materials for a variety of other energy conversion applications.

Water-Soluble Tris(cyclometalated) Iridium(III) Complexes for Aqueous Electron and Energy Transfer Photochemistry.

Cyclometalated iridium(III) complexes are frequently employed in organic light emitting diodes, and they are popular photocatalysts for solar energy conversion and synthetic organic chemistry. They luminesce from redox-active excited states that can have high triplet energies and long lifetimes, making them well suited for energy transfer and photoredox catalysis. Homoleptic tris(cyclometalated) iridium(III) complexes are typically very hydrophobic and do not dissolve well in polar solvents, somewhat limiting their application scope. We developed a family of water-soluble sulfonate-decorated variants with tailored redox potentials and excited-state energies to address several key challenges in aqueous photochemistry.First, we aimed at combining enzyme with photoredox catalysis to synthesize enantioenriched products in a cyclic reaction network. Since the employed biocatalyst operates best in aqueous solution, a water-soluble photocatalyst was needed. A new tris(cyclometalated) iridium(III) complex provided enough reducing power for the photochemical reduction of imines to racemic mixtures of amines and furthermore was compatible with monoamine oxidase (MAO-N-9), which deracemized this mixture through a kinetic resolution of the racemic amine via oxidation to the corresponding imine. This process led to the accumulation of the unreactive amine enantiomer over time. In subsequent studies, we discovered that the same iridium(III) complex photoionizes under intense irradiation to give hydrated electrons as a result of consecutive two-photon excitation. With visible light as energy input, hydrated electrons become available in a catalytic fashion, thereby allowing the comparatively mild reduction of substrates that would typically only be reactive under harsher conditions. Finally, we became interested in photochemical upconversion in aqueous solution, for which it was desirable to obtain water-soluble iridium(III) compounds with very high triplet excited-state energies. This goal was achieved through improved ligand design and ultimately enabled sensitized triplet-triplet annihilation upconversion unusually far into the ultraviolet spectral range.Studies of photoredox catalysis, energy transfer catalysis, and photochemical upconversion typically rely on the use of organic solvents. Water could potentially be an attractive alternative in many cases, but photocatalyst development lags somewhat behind for aqueous solution compared to organic solvent. The purpose of this Account is to provide an overview of the breadth of new research perspectives that emerged from the development of water-soluble fac-[Ir(ppy)]3 complexes (ppy = 2-phenylpyridine) with sulfonated ligands. We hope to inspire the use of some of these or related coordination compounds in aqueous photochemistry and to stimulate further conceptual developments at the interfaces of coordination chemistry, photophysics, biocatalysis, and sustainable chemistry.

Closed-Loop Control of Electroadhesion Using Current Regulation.

Electroadhesion displays provide controllable friction between the fingertip and screen. However, the change of contact condition causes variability in the produced friction. In this paper, we demonstrate a novel method for closed-loop control using current regulation to improve the precision of the electroadhesion force regardless of contact conditions. The current sensor obtains static current (when the finger is stationary) and dynamic current (when the finger is sliding). The static current is used to estimate the apparent contact area. The estimated contact area modulates the driving voltage along with the dynamic current. To verify the proposed method, we measured electroadhesion forces under open-loop control and closed-loop control. The benefit of using this closed-loop control is shown by comparing the relative static error of open-loop control and closed-loop control. The relative error reductions achieved over 34 % (max 112 %) for four changing contact conditions.

Effect of Temperature on the Absolute and Discrimination Thresholds of Voltage on Electrovibration Tactile Display.

Multi-dimensions tactile displays, such as thermal and texture display, are desirable for enhancing perception while users experience virtual shopping such as touching a garment in virtual reality. Understanding the effect of one dimension on the other is fundamental for design of multi-dimensions tactile display. In this article, we report the effect of temperature on thresholds of voltage applied on an electrovibration tactile display. Three temperatures of the electrovibration tactile display at 18°C (cold), 30°C (neutral) and 38°C (warm) were considered in two experiments. In Experiment I, we measured the absolute thresholds of square wave voltage with 25 Hz, 140 Hz and 485 Hz. In Experiment II, we measured the amplitude discrimination thresholds of same voltage signals as in Experiment I. The results show that the absolute thresholds differed significantly between 18°C and 38°C for all the three frequencies. No significant difference in the absolute threshold was found between 18°C and 30°C, except for the 485 Hz voltage. The amplitude discrimination thresholds were essentially constant except for that of the 485 Hz voltage at 18°C, which were 17.11 Vpp and 16.86 Vpp larger than those at 30°C and 38°C, respectively.

Photo-triggered hydrogen atom transfer from an iridium hydride complex to unactivated olefins.

Many photoactive metal complexes can act as electron donors or acceptors upon photoexcitation, but hydrogen atom transfer (HAT) reactivity is rare. We discovered that a typical representative of a widely used class of iridium hydride complexes acts as an H-atom donor to unactivated olefins upon irradiation at 470 nm in the presence of tertiary alkyl amines as sacrificial electron and proton sources. The catalytic hydrogenation of simple olefins served as a test ground to establish this new photo-reactivity of iridium hydrides. Substrates that are very difficult to activate by photoinduced electron transfer were readily hydrogenated, and structure-reactivity relationships established with 12 different olefins are in line with typical HAT reactivity, reflecting the relative stabilities of radical intermediates formed by HAT. Radical clock, H/D isotope labeling, and transient absorption experiments provide further mechanistic insight and corroborate the interpretation of the overall reactivity in terms of photo-triggered hydrogen atom transfer (photo-HAT). The catalytically active species is identified as an Ir(ii) hydride with an IrII-H bond dissociation free energy around 44 kcal mol-1, which is formed after reductive 3MLCT excited-state quenching of the corresponding Ir(iii) hydride, i.e. the actual HAT step occurs on the ground-state potential energy surface. The photo-HAT reactivity presented here represents a conceptually novel approach to photocatalysis with metal complexes, which is fundamentally different from the many prior studies relying on photoinduced electron transfer.

Shortcuts for Electron-Transfer through the Secondary Structure of Helical Oligo-1,2-Naphthylenes.

Atropisomeric 1,2-naphthylene scaffolds provide access to donor-acceptor compounds with helical oligomer-based bridges, and transient absorption studies revealed a highly unusual dependence of the electron-transfer rate on oligomer length, which is due to their well-defined secondary structure. Close noncovalent intramolecular contacts enable shortcuts for electron transfer that would otherwise have to occur over longer distances along covalent pathways, reminiscent of the behavior seen for certain proteins. The simplistic picture of tube-like electron transfer can describe this superposition of different pathways including both the covalent helical backbone, as well as noncovalent contacts, contrasting the wire-like behavior reported many times before for more conventional molecular bridges. The exquisite control over the molecular architecture, achievable with the configurationally stable and topologically defined 1,2-naphthylene-based scaffolds, is of key importance for the tube-like electron transfer behavior. Our insights are relevant for the emerging field of multidimensional electron transfer and for possible future applications in molecular electronics.

The Effect of Applied Normal Force on the Electrovibration.

Electrovibration has become one of the promising approaches for adding tactile feedback on touchscreen. Previous studies revealed that the normal force applied on the touchscreen by the finger affects significantly the electrostatic force. It is obvious that the normal force affects the electrostatic force if it changes the contact area between the finger and the touchscreen. However, it is unclear whether the normal force affects the electrostatic force when the apparent contact area is constant. In this paper, we estimated the electrostatic force via measuring the tangential force of the finger sliding on a 3M touchscreen at different normal forces under the constant apparent contact area. We found that the electrostatic force increases significantly as the normal force increases from 0.5 to 4.5N. We explained the experimental results using the most recently proposed electrostatic force model, which considers the effect of air gap. We estimated the averaged air gap thickness using the electrostatic force model. The results showed that the relationship between the air gap thickness and the normal force follows a power function. Our experiment suggests that the normal force has a significant effect on the air gap thickness, thus require consideration in the design of tactile feedback.

Unexpected Hydrated Electron Source for Preparative Visible-Light Driven Photoredox Catalysis.

The hydrated electron is experiencing a renaissance as a superreductant in lab-scale reductions driven by light, both for the degradation of recalcitrant pollutants and for challenging chemical reactions. However, examples for its sustainable generation under mild conditions are scarce. By combining a water-soluble Ir catalyst with unique photochemical properties and an inexpensive diode laser as light source, we produce hydrated electrons through a two-photon mechanism previously thought to be unimportant for laboratory applications. Adding cheap sacrificial donors turns our new hydrated electron source into a catalytic cycle operating in pure water over a wide pH range. Not only is that catalytic system capable of detoxifying a chlorinated model compound with turnover numbers of up to 200, but it can also be employed for two novel hydrated electron reactions, namely, the decomposition of quaternary ammonium compounds and the conversion of trifluoromethyl to difluoromethyl groups.

Detection and Discrimination Thresholds for Haptic Gratings on Electrostatic Tactile Displays.

Designing algorithmsfor rendering haptic texture on electrostatic tactile displays requires a quantitative understanding of human perception. In this paper, we report detection and discrimination thresholds for haptic gratings rendered on such displays based on the waveform and amplitude of the applied voltage. The haptic gratings consist of functions that describe the variation in voltage amplitude as a function of the position of finger on the display. Four types of virtual haptic gratings are considered in two experiments. In Experiment I, we estimate the absolute detection thresholds of haptic gratings for four different voltage amplitude functions, consisting of spatial waveforms with sinusoidal, square, triangle, or sawtooth shape. In Experiment II, we report discrimination thresholds for haptic gratings at five reference values of the voltage amplitude (80, 120, 160, 200, and 240 Vpp) for each of the voltage amplitude functions used in Experiment I. The results indicate that the detection thresholds for the four virtual haptic gratings are between 30 and 36 Vpp, and that the JND increases with the increase of voltage amplitudes. In addition, the JNDs of the four virtual gratings differ significantly, with the lowest and highest values being given by the triangle and sawtooth waveform, respectively.

Electron Transfer across o-Phenylene Wires.

Photoinduced electron transfer across rigid rod-like oligo- p-phenylenes has been thoroughly investigated in the past, but their o-connected counterparts are yet entirely unexplored in this regard. We report on three molecular dyads comprised of a triarylamine donor and a Ru(bpy)32+ (bpy =2,2'-bipyridine) acceptor connected covalently by 2 to 6 o-phenylene units. Pulsed excitation of the Ru(II) sensitizer at 532 nm leads to the rapid formation of oxidized triarylamine and reduced ruthenium complex via intramolecular electron transfer. The subsequent thermal reverse charge-shift reaction to reinstate the electronic ground-state occurs on a time scale of 120-220 ns in deaerated CH3CN at 25 °C. The conformational flexibility of the o-phenylene bridges causes multiexponential transient absorption kinetics for the photoinduced forward process, but the thermal reverse reaction produces single-exponential transient absorption decays. The key finding is that the flexible o-phenylene bridges permit rapid formation of photoproducts storing ca. 1.7 eV of energy with lifetimes on the order of hundreds of nanoseconds, similar to what is possible with rigid rod-like donor-acceptor compounds. Thus, the conformational flexibility of the o-phenylenes represents no disadvantage with regard to the photoproduct lifetimes, and this is relevant in the greater context of light-to-chemical energy conversion.

Charge Accumulation and Multi-Electron Photoredox Chemistry with a Sensitizer-Catalyst-Sensitizer Triad.

Photoinduced electron transfer in donor-sensitizer-acceptor compounds usually leads to simple electron-hole pairs, and photoredox catalysis typically relies on single-electron transfer (SET) events. This work reports on a molecular triad able to accumulate two electrons on a central dibenzo[1,2]dithiin moiety flanked by two peripheral RuII photosensitizers. Under continuous illumination, the doubly reduced form of the dibenzo[1,2]dithiin undergoes thiolate-disulfide exchange with an aliphatic disulfide substrate, thereby acting as a two-electron catalyst after two initial SET events with triethylamine at the RuII sensitizers. The use of a relatively simple triad for coupling two separate SET processes to a subsequent two-electron reduction is an important conceptual advance from photoinduced SET and light-driven charge accumulation towards multi-electron photoredox catalysis. This is relevant for artificial photosynthesis and light-driven multi-electron chemistry in general.

Mixed-Valent Molecular Triple Deckers.

Two phenothiazine (PTZ) moieties were connected via naphthalene spacers to a central arene to result in stacked PTZ-arene-PTZ structure elements. Benzene and tetramethoxybenzene units served as central arenes mediating electronic communication between the two PTZ units. Based on cyclic voltammetry, UV/Vis-NIR absorption, EPR spectroscopy, and computational studies, the one-electron oxidized forms of the resulting compounds behave as class II organic mixed-valence species in which the unpaired electron is partially delocalized over both PTZ units. The barrier for intramolecular electron transfer depends on the nature of the central arene sandwiched between the two PTZ moieties. These are the first examples of rigid organic mixed-valent triple-decker compounds with possible electron-transfer pathways directly across a stacked structure, and they illustrate the potential of oligo-naphthalene building blocks for long-range electron transfer and a future molecular electronics technology.

Enantioselective synthesis of amines by combining photoredox and enzymatic catalysis in a cyclic reaction network.

Visible light-driven reduction of imines to enantioenriched amines in aqueous solution is demonstrated for the first time. Excitation of a new water-soluble variant of the widely used [Ir(ppy)3] (ppy = 2-phenylpyridine) photosensitizer in the presence of a cyclic imine affords a highly reactive α-amino alkyl radical that is intercepted by hydrogen atom transfer (HAT) from ascorbate or thiol donors to afford the corresponding amine. The enzyme monoamine oxidase (MAO-N-9) selectively catalyzes the oxidation of one of the enantiomers to the corresponding imine. Upon combining the photoredox and biocatalytic processes under continuous photo-irradiation, enantioenriched amines are obtained in excellent yields. To the best of our knowledge, this is the first demonstration of a concurrent photoredox- and enzymatic catalysis leading to a light-driven asymmetric synthesis of amines.

Reductive Amination by Photoredox Catalysis and Polarity-Matched Hydrogen Atom Transfer.

The excitation of a RuII photosensitizer in the presence of ascorbic acid leads to the reduction of iminium ions to electron-rich α-aminoalkyl radical intermediates, which are rapidly converted into reductive amination products by thiol-mediated hydrogen atom transfer (HAT). As a result, the reductive amination of carbonyl compounds with amines by photoredox catalysis proceeds in good to excellent yields and with broad substrate scope and good functional group tolerance. The three key features of this work are 1) the rapid interception of electron-rich α-aminoalkyl radical intermediates by polarity-matched HAT in a photoredox reaction, 2) the method of reductive amination by photoredox catalysis itself, and 3) the application of this new method for temporally and spatially controlled reactions on a solid support, as demonstrated by the attachment of a fluorescent dye on an activated cellulose support by photoredox-catalyzed reductive amination.

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

In a molecular triad comprised of a central naphthalene diimide (NDI) unit flanked by two [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) sensitizers, NDI2- is formed after irradiation with visible light in deaerated CH3CN in the presence of excess triethylamine. The mechanism for this electron accumulation involves a combination of photoinduced and thermal elementary steps. In a structurally related molecular pentad with two peripheral triarylamine (TAA) electron donors attached covalently to a central [Ru(bpy)3]2+-NDI-[Ru(bpy)3]2+ core but no sacrificial reagents present, photoexcitation only leads to NDI- (and TAA+), whereas NDI2- is unattainable due to rapid electron transfer events counteracting charge accumulation. For solar energy conversion, this finding means that fully integrated systems with covalently linked photosensitizers and catalysts are not necessarily superior to multicomponent systems, because the fully integrated systems can suffer from rapid undesired electron transfer events that impede multielectron reactions on the catalyst.