Ah-Young Jee

Unilamellar Nanosheet of Layered Manganese Cobalt Nickel Oxide and Its Heterolayered Film with Polycations

The exfoliation of layered Li[Mn(1/3)Co(1/3)Ni(1/3)]O(2) into individual monolayers could be achieved through the intercalation of quaternary tetramethylammonium (TMA(+)) ions into protonated metal oxide. An effective exfoliation occurred when the TMA(+)/H(+) ratio was 0.5-50. Reactions outside this range produced no colloidal suspension, but all the manganese cobalt nickel oxides precipitated. Atomic force microscopy and transmission electron microscopy clearly demonstrated that exfoliated manganese cobalt nickel oxide nanosheets have a nanometer-level thickness, underscoring the formation of unilamellar nanosheets. The maintenance of the hexagonal atomic arrangement of the manganese cobalt nickel oxide layer upon the exfoliation was confirmed by selected area electron diffraction analysis. According to diffuse reflectance ultraviolet--visible spectroscopy, the exfoliated manganese cobalt nickel oxides displayed distinct absorption peaks at approximately 354 and approximately 480 nm corresponding to the d-d transitions of octahedral metal ions, which contrasted with the featureless spectrum of the pristine metal oxide. In the light of zeta potential data showing the negative surface charge of manganese cobalt nickel oxide nanosheets, a heterolayered film of manganese cobalt nickel oxide and conductive polymers could be prepared through the successive coating process with colloidal suspension and polycations. The UV--vis and X-ray diffraction studies verified the layer-by-layer ordered structure of the obtained heterolayered film, respectively.

Soft-chemical exfoliation route to layered cobalt oxide monolayers and its application for film deposition and nanoparticle synthesis.

A colloidal suspension of exfoliated, layered cobalt oxide nanosheets has been synthesized through the intercalation of quaternary tetramethylammonium ions into protonated lithium cobalt oxide. According to atomic force microscopy, exfoliated nanosheets of layered cobalt oxide show a plateau-like height profile with nanometer-level height, underscoring the formation of unilamellar 2D nanosheets. The exfoliation of layered cobalt oxide was cross-confirmed by X-ray diffraction, UV/Vis spectroscopy, and transmission electron microscopy. The maintenance of the hexagonal in-plane structure of the cobalt oxide lattice after the exfoliation process was evidenced by selected-area electron diffraction and Co K-edge X-ray absorption near-edge structure analysis. The zeta-potential measurements clearly demonstrated the negative surface charge of cobalt oxide nanosheets. Adopting the nanosheets of layered cobalt oxide as a precursor, we were able to prepare the monodisperse CoO nanocrystals with a particle size of approximately 10 nm as well as the heterolayered film composed of cobalt oxide monolayer and polycation.

Internal Twisting Dynamics of Dicyanovinyljulolidine in Polymers

The fluorescence quantum yield of 9-dicyanovinyljulolidine (DCVJ) is very low in fluid solutions but increases markedly in solids because the medium rigidity slows down the internal motion, which acts as a major nonradiative decay channel. In this work, the excited-state twisting motion of DCVJ in polymers was investigated by time-resolved fluorescence spectroscopy, and it was observed that the fluorescence lifetime of DCVJ in polymers depends on the mechanical properties of the medium. Therefore, our results indicate that the elastic modulus is a determining factor for molecular rotor dynamics in soft matter, and its description requires a comprehensive visco-elasto-plastic theory.

Internal motion of an electronically excited molecule in viscoelastic media

The twisting motion of trans-4-[4-(dimethylamino)-styryl]-1-methylpyridinium iodide (4-DASPI) in the excited state was investigated in solutions and various polymers in order to understand dependence of molecular rotor dynamics on viscoelasticity. It was observed that the internal motion of electronically excited 4-DASPI correlates strongly with dynamic viscosity and elastic modulus. Our results also showed that condensed phase dynamics of 4-DASPI are governed by the explicit mode coupling between the rotamerizing coordinate and mechanical properties of viscoelastic media.

Excited‐State Dynamics of a Hemicyanine Dye in Polymer Blends

The molecular rotor dynamics of electronically excited molecules is influenced by the medium viscosity in liquids and by the stiffness (rigidity) in solids. In the liquid state, the rotational process can be described by the stochastic Langevin equation, in which friction plays a key role. The friction coefficient can be converted into the solvent viscosity by applying the Stokes–Einstein equation. In the solid state, the motion also experiences friction, but the conversion of friction to stiffness is not a trivial matter. Therefore, most of the work has been carried out in solution and studies in rigid media are rare. A goal of this work is to understand how the stiffness of the medium affects the internal rotation of an electronically excited molecule. For this purpose, we have chosen trans-4-[4-(dimethylamino)-styryl]-1-methylpyridinium iodide (4-DASPI) as a molecular probe. To cover a broad range of medium stiffness, polymer blends of polystyrene (PS) and ethylene and 1-octene copolymer (EOC) were prepared at various weight ratios. Figure 1 depicts the molecular structure of 4-DASPI. The photophyiscs of 4-DASPI has been extensively investigated in the condensed phase, and medium effects on its excited-state

Elasticity-controlled molecular dynamics of 9,9′-bifluorenyldene as a function of temperature and force

Medium elasticity is an important coupling parameter that influences the molecular motions that occur in solids. In this work, the light-induced twisting motion of 9,9′-bifluorenyldene (BF) was investigated in an elastic polymer, ultra low density polyethylene (ULDPE), as a function of temperature and applied force. Analysis of the BF fluorescence lifetime in accordance with the modulus change of the ULDPE showed that temperature and force significantly affected the excited-state molecular dynamics in the elasticity-controlled regime.

Communication: Time-resolved fluorescence of highly single crystalline molecular wires of azobenzene.

We report the enhanced fluorescence with the remarkably long lifetime (1.17 ns) in the first excited state (S(1)) of highly crystalline molecular wires of azobenzene at the excitation wavelength of 467 nm for the first time. This observation suggests that trans-cis photoisomerization through the rotation or inversion mechanism may not be a favorable pathway after excitation to the S(1) state in highly single crystalline molecular wires of azobenzene due to the hindered motion within densely packed crystal structure. We also measured the fluorescence lifetime image of a single crystalline molecular wire of azobenzene, indicating that the lifetime was remarkably uniform and that there was only a very minor variation within the crystal.

Enzyme leaps fuel antichemotaxis

Significance Challenging the traditional view that enzyme kinetics are only a matter of catalyzing chemical reactions, there is mounting evidence that the enzyme catalysis enhances enzyme mobility. This is significant to programming spatio-temporal patterns of molecular response to chemical stimulus, which is common to living matter as well as to significant chemical technology. This paper shows that the enhanced diffusivity of enzymes is a “run-and-tumble” process analogous to that performed by swimming microorganisms, executed in this situation by molecules that lack the decision-making machinery of microorganisms. The result is that enzymes display “antichemotaxis” when they turn over substrate; they migrate in the direction of lesser reactant concentration. There is mounting evidence that enzyme diffusivity is enhanced when the enzyme is catalytically active. Here, using superresolution microscopy [stimulated emission-depletion fluorescence correlation spectroscopy (STED-FCS)], we show that active enzymes migrate spontaneously in the direction of lower substrate concentration (“antichemotaxis”) by a process analogous to the run-and-tumble foraging strategy of swimming microorganisms and our theory quantifies the mechanism. The two enzymes studied, urease and acetylcholinesterase, display two families of transit times through subdiffraction-sized focus spots, a diffusive mode and a ballistic mode, and the latter transit time is close to the inverse rate of catalytic turnover. This biochemical information-processing algorithm may be useful to design synthetic self-propelled swimmers and nanoparticles relevant to active materials. Executed by molecules lacking the decision-making circuitry of microorganisms, antichemotaxis by this run-and-tumble process offers the biological function to homogenize product concentration, which could be significant in situations when the reactant concentration varies from spot to spot.

Comparing Geometry and Chemistry When Confined Molecules Diffuse in Monodisperse Metal–Organic Framework Pores

The monodisperse pore structure of MOFs (metal-organic frameworks) is advantageous for investigating how porosity influences diffusion. Here we report translational and rotational diffusion using fluorescence correlation spectroscopy and time-correlated single-photon counting, using the three-dimensional pores of the zeolitic-like metal-organic framework family. We compare the influence of size and electric charge as well as dependence on pore size that we controlled through postsynthetic cation-exchange modifications. Charge-charge interactions with the MOF appeared to produce transient adsorption, manifested as a relatively fast and a slower diffusion process, but diffusants without net electric charge displayed a single diffusion process. Obtained from this family of guest molecules selected to be fluorescent, these findings suggest potentially useful general design rules to predict how pore size, guest size, and host-guest interaction control guest mobility within nanopores. With striking fidelity, diffusion coefficient scales with the ratio of cross-sectional areas of diffusant and host pores when charge is taken into account.

DNA molecules deviate from shortest trajectory when driven through hydrogel

Dynamic fluorescence-based single-molecule imaging of λ-DNA molecules driven through agarose hydrogels by DC electric fields reveals that passage through the hydrogel (98.5% water content) induces mobility orthogonal to the external field. Tortuous paths followed by the DNA molecules, which are heavily entangled in the hydrogel mesh as their contour length is nearly 100 times the hydrogel mesh size of 200 nm, cause them to appear to diffuse orthogonal to the driving force. The higher the driving field, from 2 to 16 V/cm, the higher the off-axis dispersion is, over the same time interval. We measure the off-axis displacement distribution over 3 orders of magnitude of probability density and find a master curve after normalizing for time (t) elapsed, but the power of time for normalizing increases with the external field, from t0.25 to t0.6 with increasing field. Comparing trajectories over the same distance traveled in the electric field direction, we observe whereas for the highest field strengths DNA molecules come closest to taking the shortest trajectory between two points in space, deviations from the shortest trajectory grow larger and larger (up to 40% larger) as one approaches the case of small yet finite external field strength.