Accounts of Chemical Research最新文献

High-precision neuromodulation plays a pivotal role in elucidating fundamental principles of neuroscience and treating specific neurological disorders. Optical neuromodulation, enabled by spatial resolution defined by the diffraction limit at the submicrometer scale, is a general strategy to achieve such precision. Optogenetics offers single-neuron spatial resolution with cellular specificity, whereas the requirement of genetic transfection hinders its clinical application. Direct photothermal modulation, an alternative nongenetic optical approach, often associates a large temperature increase with the risk of thermal damage to surrounding tissues.

Photoacoustic (also called optoacoustic) neural stimulation is an emerging technology for neural stimulation with the following key features demonstrated. First, the photoacoustic approach demonstrated high efficacy without the need for genetic modification. The generated pulsed ultrasound upon ns laser pulses with energy ranging from a few μJ to tens of μJ is sufficient to activate wild-type neurons. Second, the photoacoustic approach provides sub-100-μm spatial precision. It overcomes the fundamental wave diffraction limit of ultrasound by harnessing the localized ultrasound field generated through light absorption. A spatial precision of 400 μm has been achieved in rodent brains using a fiber-based photoacoustic emitter. Single-cell stimulation in neuronal cultures in vitro and in brain slices ex vivo is achieved using tapered fiber-based photoacoustic emitters. This precision is 10 to 100 times better than that for piezo-based low-frequency ultrasound and is essential to pinpoint a specific region or cell population in a living brain. Third, compared to direct photothermal stimulation via temperature increase, photoacoustic stimulation requires 40 times less laser energy dose to evoke neuron activities and is associated with a minimal temperature increase of less than 1 °C, preventing potential thermal damage to neurons. Fourth, photoacoustics is a versatile approach and can be designed in various platforms aiming at specific applications. Our team has shown the design of fiber-based photoacoustic emitters, photoacoustic nanotransducers, soft biocompatible photoacoustic films, and soft photoacoustic lenses. Since they interact with neurons through ultrasound without the need for direct contact, photoacoustic enables noninvasive transcranial and dura-penetrating brain stimulation without compromising high precision.

In this Account, we will first review the basic principles of photoacoustic and discuss the key design elements of PA transducers for neural modulation guided by the principle. We will also highlight how these design goals were achieved from a materials chemistry perspective. The design of different PA interfaces, their unique capability, and their applications in neural systems will be reviewed. In the end, we will discuss the remaining challenges and future perspectiv

The identification of neural networks for large areas and the regulation of neuronal activity at the single-neuron scale have garnered considerable attention in neuroscience. In addition, detecting biochemical molecules and electrically, optically, and chemically controlling neural functions are key research issues. However, conventional rigid and bulky bioelectronics face challenges for neural applications, including mechanical mismatch, unsatisfactory signal-to-noise ratio, and poor integration of multifunctional components, thereby degrading the sensing and modulation performance, long-term stability and biocompatibility, and diagnosis and therapy efficacy. Implantable bioelectronics have been developed to be mechanically compatible with the brain environment by adopting advanced geometric designs and utilizing intrinsically stretchable materials, but such advances have not been able to address all of the aforementioned challenges.

Recently, the exploration of nanomaterial synthesis and nanoscale fabrication strategies has facilitated the design of unconventional soft bioelectronics with mechanical properties similar to those of neural tissues and submicrometer-scale resolution comparable to typical neuron sizes. The introduction of nanotechnology has provided bioelectronics with improved spatial resolution, selectivity, single neuron targeting, and even multifunctionality. As a result, this state-of-the-art nanotechnology has been integrated with bioelectronics in two main types, i.e., bioelectronics integrated with synthesized nanomaterials and bioelectronics with nanoscale structures. The functional nanomaterials can be synthesized and assembled to compose bioelectronics, allowing easy customization of their functionality to meet specific requirements. The unique nanoscale structures implemented with the bioelectronics could maximize the performance in terms of sensing and stimulation. Such soft nanobioelectronics have demonstrated their applicability for neuronal recording and modulation over a long period at the intracellular level and incorporation of multiple functions, such as electrical, optical, and chemical sensing and stimulation functions.

In this Account, we will discuss the technical pathways in soft bioelectronics integrated with nanomaterials and implementing nanostructures for application to neuroengineering. We traced the historical development of bioelectronics from rigid and bulky structures to soft and deformable devices to conform to neuroengineering requirements. Recent approaches that introduced nanotechnology into neural devices enhanced the spatiotemporal resolution and endowed various device functions. These soft nanobioelectronic technologies are discussed in two categories: bioelectronics with synthesized nanomaterials and bioelectronics with nanoscale structures. We describe nanomaterial-integrated soft bioelectronics exhibiting various functionalities and modalities depending on the integrated

Nanoporous frameworks are a large and diverse family of supramolecular materials, whose chemical building units (organic, inorganic, or both) are assembled into a 3D architecture with well-defined connectivity and topology, featuring intrinsic porosity. These materials play a key role in various industrial processes and applications, such as energy production and conversion, fluid separation, gas storage, water harvesting, and many more. The performance and suitability of nanoporous materials for each specific application are directly related to both their physical and chemical properties, and their determination is crucial for process engineering and optimization of performances. In this Account, we focus on some recent developments in the multiscale modeling of physical properties of nanoporous frameworks, highlighting the latest advances in three specific areas: mechanical properties, thermal properties, and adsorption.

In the study of the mechanical behavior of nanoporous materials, the past few years have seen a rapid acceleration of research. For example, computational resources have been pooled to create a public large-scale database of elastic constants as part of the Materials Project initiative to accelerate innovation in materials research: those can serve as a basis for data-based discovery of materials with targeted properties, as well as the training of machine learning predictor models.

The large-scale prediction of thermal behavior, in comparison, is not yet routinely performed at such a large scale. Tentative databases have been assembled at the DFT level on specific families of materials, such as zeolites, but prediction at larger scale currently requires the use of transferable classical force fields, whose accuracy can be limited.

Finally, adsorption is naturally one of the most studied physical properties of nanoporous frameworks, as fluid separation or storage is often the primary target for these materials. We highlight the recent achievements and open challenges for adsorption prediction at a large scale, focusing in particular on the accuracy of computational models and the reliability of comparisons with experimental data available. We detail some recent methodological improvements in the prediction of adsorption-related properties: in particular, we describe the recent research efforts to go beyond the study of thermodynamic quantities (uptake, adsorption enthalpy, and thermodynamic selectivity) and predict transport properties using data-based methods and high-throughput computational schemes. Finally, we stress the importance of data-based methods of addressing all sources of uncertainty.

The Account concludes with some perspectives about the latest developments and open questions in data-based approaches and the integration of computational and experimental data together in the materials discovery loop.

Lithium ion batteries (LIBs) with inorganic intercalation compounds as electrode active materials have become an indispensable part of human life. However, the rapid increase in their annual production raises concerns about limited mineral reserves and related environmental issues. Therefore, organic electrode materials (OEMs) for rechargeable batteries have once again come into the focus of researchers because of their design flexibility, sustainability, and environmental compatibility. Compared with conventional inorganic cathode materials for Li ion batteries, OEMs possess some unique characteristics including flexible molecular structure, weak intermolecular interaction, being highly soluble in electrolytes, and moderate electrochemical potentials. These unique characteristics make OEMs suitable for applications in multivalent ion batteries, low-temperature batteries, redox flow batteries, and decoupled water electrolysis. Specifically, the flexible molecular structure and weak intermolecular interaction of OEMs make multivalent ions easily accessible to the redox sites of OEMs and facilitate the desolvation process on the redox site, thus improving the low-temperature performance, while the highly soluble nature enables OEMs as redox couples for aqueous redox flow batteries. Finally, the moderate electrochemical potential and reversible proton storage and release of OEMs make them suitable as redox mediators for water electrolysis. Over the past ten years, although various new OEMs have been developed for Li-organic batteries, Na-organic batteries, Zn-organic batteries, and other battery systems, batteries with OEMs still face many challenges, such as poor cycle stability, inferior energy density, and limited rate capability. Therefore, previous reviews of OEMs mainly focused on organic molecular design for organic batteries or strategies to improve the electrochemical performance of OEMs. A comprehensive review to explore the characteristics of OEMs and establish the correlation between these characteristics and their specific application in energy storage and conversion is still lacking.

In this Account, we initially provide an overview of the sustainability and environmental friendliness of OEMs for energy storage and conversion. Subsequently, we summarize the charge storage mechanisms of the different types of OEMs. Thereafter, we explore the characteristics of OEMs in comparison with conventional inorganic intercalation compounds including their structural flexibility, high solubility in the electrolyte, and appropriate electrochemical potential in order to establish the correlations between their characteristics and potential applications. Unlike previous reviews that mainly introduce the electrochemical performance progress of different organic batteries, this Account specifically focuses on some exceptional applications of OEMs corresponding to the characteristics of organic electrode materials in energy storage and conv

Transition metal dichalcogenides (TMDCs) exhibit favorable properties for optical communication in the gigahertz (GHz) regime, such as large mobilities, high extinction coefficients, cheap fabrication, and silicon compatibility. While impressive improvements in their sensitivity have been realized over the past decade, the bandwidths of these devices have been mostly limited to a few megahertz. We argue that this shortcoming originates in the relatively large RC constants of TMDC-based photodetectors, which suffer from high surface defect densities, inefficient charge carrier injection at the electrode/TMDC interface, and long charging times. However, we show in a series of papers that rather simple adjustments in the device architecture afford TMDC-based photodetectors with bandwidths of several hundreds of megahertz. We rationalize the success of these adjustments in terms of the specific physical–chemical properties of TMDCs, namely their anisotropic in-plane/out-of-plane carrier behavior, large optical absorption, and chalcogenide-dependent surface chemistry. Just one surprisingly simple yet effective pathway to fast TMDC photodetection is the reduction of the photoresistance by using light-focusing optics, which enables bandwidths of 0.23 GHz with an energy consumption of only 27 fJ/bit.

By reflecting on the ultrafast intrinsic photoresponse times of a few picoseconds in TMDC heterostructures, we motivate the application of more demanding chemical strategies to exploit such ultrafast intrinsic properties for true GHz operation in real devices. A key aspect in this regard is the management of surface defects, which we discuss in terms of its dependence on the layer thickness, its tunability by molecular adlayers, and the prospects of replacing thermally evaporated metal contacts by laser-printed electrodes fabricated with inks of metalloid clusters. We highlight the benefits of combining TMDCs with graphene to heterostructures that exhibit the ultrafast photoresponse and large spectral range of Dirac materials with the low dark currents and high responsivities of semiconductors. We introduce the bulk photovoltaic effect in TMDC-based materials with broken inversion symmetry as well as a combination of TMDCs with plasmonic nanostructures as means for increasing the bandwidth and responsivity simultaneously. Finally, we describe the prospects of embedding TMDC photodetectors into optical cavities with the objective of tuning the lifetime of the photoexcited state and increasing the carrier mobility in the photoactive layer.

The findings and concepts detailed in this Account demonstrate that GHz photodetection with TMDCs is feasible, and we hope that these bright prospects for their application as next-generation optoelectronic materials motivate more chemists and material scientists to actively pursue the development of the more complicated material combinations outlined here.

Nature has established a sustainable way to maintain aerobic life on earth by inventing one of the most sophisticated biological processes, namely, natural photosynthesis, which delivers us with organic matter and molecular oxygen derived from the two abundant resources sunlight and water. The thermodynamically demanding photosynthetic water splitting is catalyzed by the oxygen-evolving complex in photosystem II (OEC-PSII), which comprises a distorted tetramanganese–calcium cluster (CaMn₄O₅) as catalytic core. As an ubiquitous concept for fine-tuning and regulating the reactivity of the active site of metalloenzymes, the surrounding protein domain creates a sophisticated environment that promotes substrate preorganization through secondary, noncovalent interactions such as hydrogen bonding or electrostatic interactions. Based on the high-resolution X-ray structure of PSII, several water channels were identified near the active site, which are filled with extensive hydrogen-bonding networks of preorganized water molecules, connecting the OEC with the protein surface. As an integral part of the outer coordination sphere of natural metalloenzymes, these channels control the substrate and product delivery, carefully regulate the proton flow by promoting pivotal proton-coupled electron transfer processes, and simultaneously stabilize short-lived oxidized intermediates, thus highlighting the importance of an ordered water network for the remarkable efficiency of the natural OEC.

Transferring this concept from nature to the engineering of artificial metal catalysts for fuel production has fostered the fascinating field of metallosupramolecular chemistry by generating defined cavities that conceptually mimic enzymatic pockets. However, the application of supramolecular approaches to generate artificial water oxidation catalysts remained scarce prior to our initial reports, since such molecular design strategies for efficient activation of substrate water molecules in confined nanoenvironments were lacking. In this Account, we describe our research efforts on combining the state-of-the art Ru(bda) catalytic framework with structurally programmed ditopic ligands to guide the water oxidation process in defined metallosupramolecular assemblies in spatial proximity. We will elucidate the governing factors that control the quality of hydrogen-bonding water networks in multinuclear cavities of varying sizes and geometries to obtain high-performance, state-of-the-art water oxidation catalysts. Pushing the boundaries of artificial catalyst design, embedding a single catalytic Ru center into a well-defined molecular pocket enabled sophisticated water preorganization in front of the active site through an encoded basic recognition site, resulting in high catalytic rates comparable to those of the natural counterpart OEC-PSII.

To fully explore their potential for solar fuel devices, the suitability of our metallosupramolecul

Boron heterocycles represent an important subset of heteroatom-incorporated rings, attracting attention from organic, inorganic, and materials chemists. The empty p_z orbital at the boron center makes them stand out as quintessential Lewis acidic molecules, also serving as a means to modulate electronic structure and photophysical properties in a facile manner. As boracycles are ripe for extensive functionalization, they are used in catalysis, chemical biology, materials science, and continue to be explored as chemical synthons for conjugated materials and reagents. Neutral boron(III)-incorporated polycyclic molecules are some of the most studied types of boracycles, and understanding their redox transformations is important for applications relying on electron transfer and charge transport. While relevant redox species can often be electrochemically observed, it remains challenging to isolate and characterize boracycles where the boron center and/or polycyclic skeleton have been chemically reduced.

We describe our recent work isolating 5-, 6-, and 7-membered boracyclic radicals, anions, and cations, focusing on stabilization strategies, ligand-mediated bonding situations, and reactivity. We present a versatile neutral ligand coordination chemistry approach that permits the transformation of boracycles from potent electrophiles to powerful nucleophilic heterocycles that facilitate diverse electron transfer and bond activation chemistry. Although there are a wide range of suitable stabilizing ligands, we have employed both diamino-N-heterocyclic carbenes (NHCs) and cyclic(alkyl)(amino) carbenes (CAACs), which led to boracycles with tunable electronic structures and aromaticity trends. We highlight successful isolation of borafluorene radicals and demonstrate their reversible redox behavior, undergoing oxidation to the cation or reduction to the anion. The borafluorene anion is a chemical synthon that has been used to prepare boryl main-group and transition-metal bonds, luminescent oxabora-spirocycles, borafluorenate-crown ethers, and CO-releasing molecules via carbon dioxide activation. We expanded to 6-membered boracycles and characterized neutral bis(NHC-supported 9-boraphenanthrene)s and the corresponding bis(CAAC-stabilized 9-boraphenanthrene) biradical. We detail the interconvertible multiredox states of boraphenalene, where the boraphenalenyl radical, anion, and cation mimic the charge-states of the all-hydrocarbon analogue. Reactivity studies of the boraphenalenyl anion displayed unusual nucleophilic reactivity at multiple sites on the periphery of the boraphenalenyl tricyclic scaffold. Reduced borepins, 7-membered boron containing heterocycles, have also been isolated. We used a stepwise one-pot synthesis combining the halo-borepin precursor, CAAC, and KC₈ to afford the monomeric borepin radicals and anions. The π-system was extended to contain two borepin rings fused in a pentacyclic scaffold, which

Synergistic catalysis is a powerful tool that involves two or more distinctive catalytic systems to activate reaction partners simultaneously, thereby expanding the reactivity space of individual catalysis. As an established catalytic strategy, organocatalysis has found numerous applications in enantioselective transformations under rather mild conditions. Recently, the introduction of other catalytic systems has significantly expanded the reaction space of typical organocatalysis. In this regard, aminocatalysis is a prototypical example of synergistic catalysis. The combination of aminocatalyst and transition metal could be traced back to the early days of organocatalysis and has now been well explored as an enabling catalytic strategy. Particularly, the acid–base properties of aminocatalysis can be significantly expanded to include usually electrophiles generated in situ via metal-catalyzed cycles. Later on, aminocatalyst has also been exploited in synergistically combining with photochemical and electrochemical processes to facilitate redox transformations. However, synergistically combining one type of aminocatalyst with many different catalytic systems remains a great challenge. One of the most daunting challenges is the compatibility of aminocatalysts in coexistence with other catalytic species. As nucleophilic species, aminocatalysts may also bind with metal, which leads to mutual inhibition or even quenching of the individual catalytic activity. In addition, oxidative stability of aminocatalyst is also a non-neglectable issue, which causes difficulties in exploring oxidative enamine transformations.

In 2007, we developed a vicinal diamine type of chiral primary aminocatalysts. This class of primary aminocatalysts was developed and evolved as functional and mechanistic mimics to the natural aldolase and has been widely applied in a number of enamine/iminium ion-based transformations. By following a “1 + x” synergistic strategy, the chiral primary amine catalysts were found to work synergistically or cooperatively with a number of transition metal catalysts, such as Pd, Rh, Ag, Co, and Cu, or other organocatalysts, such as B(C₆F₅)₃, ketone, selenium, and iodide. Photocatalysis and electrochemical processes can also be incorporated to work together with the chiral primary amine catalysts. The 1 + x catalytic strategy enabled us to execute unexploited transformations by fine-tuning the acid–base and redox properties of the enamine intermediates and to achieve effective reaction and stereocontrol beyond the reach individually. During these efforts, an unprecedented excited-state chemistry of enamine was uncovered to make possible an effective deracemization process. In this Account, we describe our recent efforts since 2015 in exploring synergistic chiral primary amine catalysis, and the content is categorized according to the type of synergistic partner such that in each section

Magnetic resonance techniques represent a fundamental class of spectroscopic methods used in physics, chemistry, biology, and medicine. Electron paramagnetic resonance (EPR) is an extremely powerful technique for characterizing systems with an open-shell electronic nature, whereas nuclear magnetic resonance (NMR) has traditionally been used to investigate diamagnetic (closed-shell) systems. However, these two techniques are tightly connected by the electron–nucleus hyperfine interaction operating in paramagnetic (open-shell) systems. Hyperfine interaction of the nuclear spin with unpaired electron(s) induces large temperature-dependent shifts of nuclear resonance frequencies that are designated as hyperfine NMR shifts (δ^HF).

Three fundamental physical mechanisms shape the total hyperfine interaction: Fermi-contact, paramagnetic spin–orbit, and spin–dipolar. The corresponding hyperfine NMR contributions can be interpreted in terms of through-bond and through-space effects. In this Account, we provide an elemental theory behind the hyperfine interaction and NMR shifts and describe recent progress in understanding the structural and electronic principles underlying individual hyperfine terms.

The Fermi-contact (FC) mechanism reflects the propagation of electron-spin density throughout the molecule and is proportional to the spin density at the nuclear position. As the imbalance in spin density can be thought of as originating at the paramagnetic metal center and being propagated to the observed nucleus via chemical bonds, FC is an excellent indicator of the bond character. The paramagnetic spin–orbit (PSO) mechanism originates in the orbital current density generated by the spin–orbit coupling interaction at the metal center. The PSO mechanism of the ligand NMR shift then reflects the transmission of the spin polarization through bonds, similar to the FC mechanism, but it also makes a substantial through-space contribution in long-range situations. In contrast, the spin–dipolar (SD) mechanism is relatively unimportant at short-range with significant spin polarization on the spectator atom. The PSO and SD mechanisms combine at long-range to form the so-called pseudocontact shift, traditionally used as a structural and dynamics probe in paramagnetic NMR (pNMR). Note that the PSO and SD terms both contribute to the isotropic NMR shift only at the relativistic spin–orbit level of theory.

We demonstrate the advantages of calculating and analyzing the NMR shifts at relativistic two- and four-component levels of theory and present analytical tools and approaches based on perturbation theory. We show that paramagnetic NMR effects can be interpreted by spin-delocalization and spin-polarization mechanisms related to chemical bond concepts of electron conjugation in π-space and hyperconjugation in σ-space in the framework of the molecular orbital (MO) theory. Further, we discuss the effects of environment (supramolecular int

Magnetism is an area of immense fundamental and technological importance. At the atomic level, magnetism originates from electron “spin”. The field of nanospintronics (or nanoscale spin-based electronics) aims to control spins in nanoscale systems, which has resulted in astronomical improvement in data storage and magnetic field sensing technologies over the past few decades, recognized by the 2007 Nobel Prize in Physics. Spins in nanoscale solid-state devices can also act as quantum bits or qubits for emerging quantum technologies, such as quantum computing and quantum sensing.

Due to the fundamental connection between magnetism and spins, ferromagnets play a key role in many solid-state spintronic devices. This is because at the Fermi level, electron density of states is spin-polarized, which permits ferromagnets to act as electrical injectors and detectors of spins. Ferromagnets, however, have limitations in terms of low spin polarization at the Fermi level, stray magnetic fields, crosstalk, and thermal instability at the nanoscale. Therefore, new physics and new materials are needed to propel spintronic and quantum device technologies to the true atomic limit. Emerging new phenomena such as chirality induced spin selectivity or CISS, in which an intriguing correlation between carrier spin and medium chirality is observed, could therefore be instrumental in nanospintronics. This effect could allow molecular-scale, chirality controlled spin injection and detection without the need for any ferromagnet, thus opening a fundamentally new direction for device spintronics.

While CISS finds a myriad of applications in diverse areas such as chiral separation, recognition, detection, and asymmetric catalysis, in this focused Account, we exclusively review spintronic device results of this effect due to its immense potential for future spintronics. The first generation of CISS-based spintronic devices have primarily used chiral bioorganic molecules; however, many practical limitations of these materials have also been identified. Therefore, our discussion revolves around the family of chiral composite materials, which may emerge as an ideal platform for CISS due to their ability to assimilate various desirable material properties on a single platform. This class of materials has been extensively studied by the organic chemistry community in the past decades, and we discuss the various chirality transfer mechanisms that have been identified, which play a central role in CISS. Next, we discuss CISS device studies performed on some of these chiral composite materials. Emphasis is given to the family of chiral organic-carbon allotrope composites, which have been extensively studied by the authors of this Account over the past several years. Interestingly, due to the presence of multiple materials, CISS signals from hybrid chiral systems sometimes differ from those observed in purely chiral systems. Given the sheer diversity of