Oh-Hoon Kwon

Ulsan National Institute of Science and Technology | UNIST · Department of Chemistry

The coupled electronic and structural transitions in metal‐insulator transition (MIT) hinder ultrafast switching and ultimate endurance. Decoupling these transitions and achieving a zero‐strain electronic MIT can overcome the fundamental limitations of MIT in solid materials. Here, this study demonstrates that iso‐valent Ti dopants in supercooled V...

This study explores the optimal morphology of photochemical hydrogen evolution catalysts in a one‐dimensional system. Systematic engineering of metal tips on precisely defined CdSe@CdS dot‐in‐rods is conducted to exert control over morphology, composition, and both factors. The outcome yields an optimized configuration, a Au−Pt core‐shell structure...

Assessing the ergodicity of graphene liquid cell electron microscope measurements, we report that loop states of circular DNA interconvert reversibly and that loop numbers follow the Boltzmann distribution expected for this molecule in bulk solution, provided that the electron dose is low (80-keV electron energy and electron dose rate 1–20 e ⁻ Å ⁻²...

The Front Cover image is provided by Ye‐Jin Choi, Ulsan National Institute of Science and Technology (Republic of Korea). The image shows the excited‐state proton transfer and proposed influence of medium polarity on the hydrogen‐bond formation between a cationic photoacid and a base in the ground state. Read the full text of the Research Article a...

The identification and quantification of hydrogen (H)‐bonded complexes form the cornerstone of reaction‐mechanism analysis in ultrafast proton transfers. Traditionally, the Benesi‐Hildebrand method has been employed to obtain the formation constants of H‐bonded complexes, given that H‐bonding additives induce an alteration in spectral features excl...

In photocatalysis, metal-semiconductor hybrid structures have been proposed for ideal photocatalytic systems. In this study, we investigate the effect of morphology and surface nature of Pt cocatalysts on photocatalytic hydrogen evolution activity in Pt-tipped CdSe nanorods. Three distinct morphologies of Pt cocatalysts were synthesized and employe...

The shaping of matter into desired nanometric structures with on-demand functionalities can enhance the miniaturization of devices in nanotechnology. Herein, strong light-matter interaction was used as an optical lithographic tool to tailor two-dimensional (2D) matter into nanoscale architectures. We transformed 2D black phosphorus (BP) into ultraf...

Using an energy filter in transmission electron microscopy has enabled elemental mapping at the atomic scale and improved the precision of structural determination by gating inelastic and elastic imaging electrons, respectively. Here, we use an energy filter in ultrafast electron microscopy to enhance the temporal resolution toward the domain of at...

Low‐dimensional Cu(I)‐based metal halide materials are gaining attention due to their low toxicity, high stability and unique luminescence mechanism, which is mediated by self‐trapped excitons (STEs). Among them, Cs5Cu3Cl6I2, which emits blue light, is a promising candidate for applications as a next‐generation blue‐emitting material. In this artic...

A new benzodithiophene (BDT) building block, 4,8-bis(5-(2-ethylhexyl)-3-fluoro-4-hexylthiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (3-FBDT), was designed by tailoring the positions and numbers of alkyl and F substituents by adjusting the torsional energy barrier. The incorporation of...

Power conversion efficiencies (PCEs) of glove‐box (GB) processed, two‐component, single‐junction organic solar cells (OSCs) have recently exceeded 18%. However, their mass‐scale manufacture using roll‐to‐roll (R2R) coating techniques is impracticable if they must be fabricated in an air‐free environment. From a commercialization perspective, effici...

Two new Y6 derivatives of symmetrical YBO‐2O and asymmetrical YBO‐FO nonfullerene acceptors (NFAs) are prepared with a simplified synthetic procedure by incorporating octyl and fluorine substituents onto the terminal 2‐(3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene)malononitrile (INCN) moiety. By moving the alkyl substituents on the Y6 core to the terminal...

Base treatment and metal doping were evaluated as means of enhancing the photocatalytic activity of ZrO2 nanoparticles (NPs) via the generation of oxygen vacancies (OvS), and the sites responsible for this enhancement were identified and characterized by spectroscopic and microscopic techniques. We confirmed that OvS produced by base treatment enga...

Vectorial electron transfer is essential for developing ideal artificial photosystems. Conventional photosensitizers with only strong electronic coupling, as used in photovoltaics, cannot achieve vectorial electron transfer because of their rapid charge recombination. Herein, we present a design strategy for integrating strong (∼600 cm⁻¹) and relat...

The excited‐state proton transfer (ESPT) of the strong photoacid, N‐methyl‐7‐hydroxyquinolium (NM7HQ+), was studied in the presence of N‐methylbenzamide (NMB) as a base in the aprotic polar solvent, acetonitrile. According to the Benesi–Hildebrand relation, it was revealed that the hydrogen (H)‐bonded complex of NM7HQ+ and NMB exists in the ground...

Electroluminescence efficiencies of deep-blue quasi-two-dimensional (2D) perovskite are limited by a lack of post-treatment strategies that can both construct an ideal energy-transfer tunnel structure minimizing the exciton losses and passivate...

By taking advantage of bulk-heterojunction structures formed by blending conjugated donor polymers and non-fullerene acceptors, organic photovoltaic devices have recently attained promising power conversion efficiencies of above 18%. For optimizing organic photovoltaic devices, it is essential to understand the elementary processes that constitute...

Semiconductor hybrid structures containing multiple components have been considered an ideal photocatalyst design to generate long-lived charge-separated states. Particularly for the reactions requiring high activation energies, such as a CO 2 reduction reaction (CO 2 RR), the reaction activity is highly susceptible to the catalyst component and mo...

A major difficulty in the polymer solar cell (PSC) community is discovering a methodology capable of accessing polymeric photovoltaic materials with controllable/predictable molecular weights. Effective mathematical equations that enable the reproduction of polymer batches with precisely controlled molecular weight, by mixing as‐synthesized polymer...

We demonstrate planar heterojunctions comprised of a nonfullerene small molecular acceptor (NFA) and a polymer donor by transferring polymer films on a water surface to on top of the NFA layers. So far, most of solution processed layer-by-layer architectures have been reported as sequentially-deposited bulk heterojunctions or pseudo-bilayers, becau...

The excited-state proton transfer (ESPT) of a cationic super-photoacid, N-methyl-7-hydroxyquinolium, has been studied within the water pool of an aerosol-OT reverse micelle (RM). Time-resolved emission spectra were obtained and analyzed to reveal the effect of environmental heterogeneity on the ESPT process. The ESPT was found to involve two main e...

Water molecules in the immediate vicinity of biomacromolecules and biomimetic organized assemblies often exhibit a markedly distinct behavior from that of their bulk counterparts. The overall sluggish behavior of biological water substantially affects the stability and integrity of biomolecules, as well as the successful execution of various crucia...

We investigated the hydration dynamics in the nanosized water pool of an Aerosol-OT (AOT) reverse micelle (RM) using a prototropic (proton-donating) fluorescent dye, N-methyl-7-hydroxyquinolinium (NM7HQ⁺), as the probe. NM7HQ⁺ is a photoacid, and the deprotonation of its excited-state cationic form (C*) gives rise to a neutral keto species (K*). In...

Black phosphorus (BP) is an elemental layered material with a strong in-plane anisotropic structure. This anisotropic structure is accompanied by anisotropic optical, electrical, thermal, and mechanical properties. Despite interest in BP from both fundamental and technical aspects, investigation into the structural dynamics of BP caused by strain f...

Solution processed bilayer organic solar cells (OSCs) with high performance are demonstrated for non-fullerene small molecular acceptors (NFAs). Unlike fullerene acceptors, NFAs show significant spectral overlap between their absorption and the photoluminescence (PL) of a polymer donor, which makes the design of efficiently exciton harvesting bilay...

Here, we propose an experimental methodology based on femtosecond‐resolved fluorescence spectroscopy to measure the hydrogen (H)‐bond free energy of water at protein surfaces under isothermal conditions. A demonstration was conducted by installing a non‐canonical isostere of tryptophan (7‐azatryptophan) at the surface of a coiled‐coil protein to ex...

Biological water regulates the structure, dynamics, and function of the building blocks for life, which evolved in a hydrated environment. In their Research Article (DOI: 10.1002/anie.202002025), S. K. Kwak, T. H. Yoo, O.‐H. Kwon, and co‐workers present the development of an experimental method for measuring the hydrogen‐bond free energy of biologi...

Biologisches Wasser reguliert die Struktur, Dynamiken und Funktion der Bausteine des Lebens, die in einer hydratisierten Umgebung evolvierten. In ihrem Forschungsartikel (DOI: 10.1002/ange.202002025) präsentieren S. K. Kwak, T. H. Yoo, O.‐H. Kwon und Mitarbeiter die Entwicklung einer experimentellen Methode für die Messung der Freien Wasserstoffbrü...

Set me free: A new experimental approach for estimating the H-bond free energy of local biological water has been proposed. This was made possible by combining time-resolved spectroscopy and installation of a fluorescent, prototropic hydration probe at protein surfaces. This study promises access to energetics and dynamics of local biological water...

Significance Graphene liquid-cell electron microscopy reveals intermediate states of self-assembly—in this example, DNA when single strands form double helices. Molecules are observed for up to minutes at a time without apparent beam damage when electron energy and electron dose are low. Simultaneous in situ single-molecule imaging of conformationa...

In the quest for dynamic multimodal probing of a material's structure and functionality, it is critical to be able to quantify the chemical state on the atomic and nanoscale using element specific electronic and structurally sensitive tools such as electron energy loss spectroscopy (EELS). Ultrafast EELF, with combined energy, time, and spatial res...

Anisotropic Structural Dynamics of Few-Layer Black Phosphorus Revealed by Ultrafast Electron Microscopy - Volume 25 Supplement - Ye-Jin Kim, Oh-Hoon Kwon

By integrating a direct electron detector with ultrafast electron microscopy and controlling the quality of the pulsed electron beam, unprecedented spatiotemporal resolution in real-space imaging was achieved under practical pump-probe measurements. As a proof-of-principle demonstration of this new technique, the acoustic vibrations of plasmonic na...

Herein, we report chemoselective trifluoroethylation routes of unmasked 2-arylquinazolin-4(3H)-ones using mesityl(2,2,2-trifluoroethyl)iodonium triflate at room temperature. Homologous C-, O-, and N-functionalized subclasses are accessed in a straightforward manner with a wide substrate scope. These chemoselective branching events are driven by Pd-...

There has been extensive research on the sensing of explosive nitroaromatic compounds (NACs) using fluorescent metal-organic frameworks (MOFs). However, ambiguity in the sensing mechanism has hampered the development of efficient explosive sensors. Here we report the synthesis of a hydroxyl-functionalized MOF for rapid and efficient sensing of NACs...

In this issue of Chem, El-Zohry et al. employ a novel time-resolved electron microscopy technique to report that photoinduced charge-carrier diffusion on a semiconducting cadmium telluride single-crystal surface exhibits unusual features that are not only distinct from those of the bulk crystal but also considerably dependent on the crystal facet o...

Use of a Direct Electron-Detection Camera in Ultrafast Electron Microscopy for Low Dose Rate Time-Resolved Imaging - Volume 24 Supplement - Ye-Jin Kim, Oh-Hoon Kwon

Use of electron-based microscopy in aqueous media has been held back because aqueous samples tend to suffer from water radiolysis and other chemical degradation caused by the high energy of incident electrons. Here we show that aqueous liquid pockets in graphene liquid cells at room temperature display significantly improved stability when using de...

In the quest for dynamic multimodal probing of a material's structure and functionality, it is critical to be able to quantify the chemical state on the atomic-/nanoscale using element-specific electronic and structurally sensitive tools such as electron energy-loss spectroscopy (EELS). Ultrafast EELS, with combined energy, time, and spatial resolu...

Carbon dots (CDs) have potential applications in various fields such as energy, catalysis, and bioimaging due to their strong and tuneable photoluminescence (PL), low toxicity, and robust chemical inertness. Although several PL mechanisms have been proposed, the origin of PL in CDs is still in debate because of the ensembled nature of the heterogen...

The development of cost-effective and environmentally friendly photocatalysts and photosensitizers has received tremendous attention because of their potential utilization in solar harvesting applications. In this respect, carbon dots (CDs) prepared by bottom-up methods have been considered to be promising light-harvesting materials. Through prepar...

A single diol molecule posing a well-configured intramolecular hydrogen (H) bond between the internal two hydroxyl groups is revealed to serve as a Brønsted base for a model photoacid as strong as sulfuric acid. By means of spectroscopy and computation, our observation offers mechanistic insight at the molecular level into the understanding of diol...

Materials exhibiting excitation-wavelength-dependent photoluminescence, PL, are useful in a range of biomedical and optoelectronic applications. This paper describes a nanoparticulate material whose PL is tunable across the entire visible range and is achieved without adjusting particle size, any post-synthetic doping, or surface modification. A st...

As viscous hydroxylic organic compounds, diols are of interest for their functional molecular conformation, which is based on inter- and intramolecular hydrogen (H)-bonds. By utilising steady-state electronic and vibrational spectroscopy, time-resolved fluorescence spectroscopy, and computational analyses, we report the association of the hydroxyl...

In this work, a simple but effective molecular design strategy is developed for the generation of intense blue emission in the solid state including single crystals through the multiple secondary intermolecular interactions such as CF…HC hydrogen bonding. The synthesized novel imidazole-based excited-state intramolecular proton transfer (ESIPT) mol...

In the past decade, we have witnessed the rapid growth of the field of ultrafast electron microscopy (UEM), which provides intuitive means to watch atomic and molecular motions of matter. Yet, because of the limited current of the pulsed electron beam resulting from space-charge effects, observations have been mainly made to periodic motions of the...

The tunable photoluminescence of carbon-based nanomaterials has received much attention for a wide range of applications. Herein, a unique, broad-solvatochromic hybrid carbon nanosheet (CNS) synthesized through the hydrothermal carbonization of molecular precursors exploiting graphene oxide as a template is reported, resulting in the formation of c...

We investigated the dynamics of excited-state proton transfer (ESPT) of photo-amphiprotic 7-hydroxyquinoline (7HQ) in the presence of a hydrogen (H)-bond bridging diol in a polar aprotic medium. The formation of 1 : 1 H-bonded complexes of 7HQ with various diols of different alkane chain lengths was revealed using steady-state electronic spectrosco...

The pursuit of exceptionally high photoluminescence (PL) and stability is critical in the development of novel fluorophores for use in challenging bioimaging and optoelectronic devices. Carbon nanodots (CDs) doped with heteroatoms provide a particularly attractive means of effectively tailoring their intrinsic properties and exploiting new phenomen...

Protein inactivation by reactive oxygen species (ROS) such as singlet oxygen ((1)O2) and superoxide radical (O2(•-)) is considered to trigger cell death pathways associated with protein dysfunction; however, the detailed mechanisms and direct involvement in photodynamic therapy (PDT) have not been revealed. Thereby, we report herein Ir(III) complex...

The alkyl oxonium ion, which is a protonated alcohol, has long been proposed as a key reaction intermediate in alcohol dehydration. Nonetheless, the dynamics and structure of this simple but important intermediate species have not been adequately examined due to the transient nature of the oxonium ion. Here, we devised a model system for the key st...

The excited-state proton transfer (ESPT) of the strong photoacid, N-methyl-7-hydroxyquinolinium, was studied in the presence of different weak bases such as methanol, ethanol, and dimethyl sulfoxide in an aprotic solvent of acetonitrile. Here, we present chemical kinetics analysis of the ESPT mechanism to explain biphasic fluorescence decay of the...

8-Hydroxyquinoline (8-HQ), commonly known as oxine, is the most popular among the hydroxyquinolines due to its excellence in complex formation with metal ions and with a wide spectrum of pharmacological applications. 8-HQ and many of its derivatives are fluorogenic ligands when complexed with metal ions. Thus they are regularly used for detection,...

Alcohol, the simplest amphiprotic organic compound, can exhibit either acidic or basic behavior by donating or accepting a proton. In this study, exploring proton dissociation of a model photoacid in solution with using time-resolved spectroscopy, we reveal quantitatively for the first time that alcohol acts as a Brønsted base because of H-bonded c...

The phase transition of crystalline ordering is a general phenomenon, but its evolution in space and time requires microscopic probes for visualization. Here we report direct imaging of the transformation of amorphous titanium dioxide nanofilm, from the liquid state, passing through the nucleation step and finally to the ordered crystal phase. Sing...

In this Letter, we introduce conical-scanning dark-field imaging in fourdimensional (4D) ultrafast electron microscopy to visualize single-particle dynamics of a polycrystalline ensemble undergoing phase transitions. Specifically, the ultrafast metalinsulator phase transition of vanadium dioxide is induced using laser excitation and followed by tak...

We use ultrafast electron microscopy (UEM) to visualize the spin-crossover dynamics of single, isolated metal-organic framework nanocrystals. The approach reported here has potential applications in other nanosystems undergoing (bio)chemical transformations.

The advancement of techniques that can probe the behaviour of individual nanoscopic objects is of paramount importance in various disciplines, including photonics and electronics. As it provides images with a spatiotemporal resolution, four-dimensional electron microscopy, in principle, should enable the visualization of single-nanoparticle structu...

4D electron microscopy enables simultaneous real- and reciprocal-space imaging of nanostructured materials with unprecedented spatio-temporal resolns. Here we apply it to visualize the photoswitching dynamics of single spin-crossover nanocrystals. The nanoparticles are largely influenced by the frictional and thermal characteristics of the interfac...

The energy and time correlation, i.e. the chirp, of imaging electron pulses in dispersive propagation is measured by time-slicing (temporal hole burning) using photon-induced near-field electron microscopy. The chirp coefficient and the degree of correlation are obtained in addition to the duration of the electron pulse and its energy spread. Impro...

The pronounced ability of water to form a hyperdense hydrogen (H)-bond network among itself is at the heart of its exceptional properties. Due to the unique H-bonding capability and amphoteric nature, water is not only a passive medium, but also behaves as an active participant in many chemical and biological reactions. Here, we reveal the catalyti...

Macromolecular conformation dynamics, which span a wide range of time scales, are fundamental to the understanding of properties and functions of their structures. Here, we report direct imaging of structural dynamics of helical macromolecules over the time scales of conformational dynamics (ns to subsecond) by means of four-dimensional (4D) electr...

Water-protein interactions dictate many processes crucial to protein function including folding, dynamics, interactions with other biomolecules, and enzymatic catalysis. Here we examine the effect of surface fluorination on water-protein interactions. Modification of designed coiled-coil proteins by incorporation of 5,5,5-trifluoroleucine or (4S)-2...