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Coherent quantum dynamics in donor–bridge–acceptor systems: beyond the hopping and super-exchange mechanisms
The population transfer dynamics of model donor–bridge–acceptor systems is studied by comparing a recently developed polaron-transformed quantum master equation (PQME) with the well-known Redfield and Förster theories of quantum transport. We show that the PQME approach reduces to these two theories in their respective limits of validity and naturally interpolates between them as a function of the system–bath coupling strength. By exploring the parameter space of our model problem, we identify novel regimes of transport dynamics in bridged systems like those encountered in biological and organic energy transfer problems. Furthermore, we demonstrate that three-level systems like the ones studied herein represent ideal minimal models for the identification of quantum coherent transport as embodied in super-exchange phenomena that cannot be captured by Förster-like hopping approaches.
Preparation of Core-Shell Coordination Molecular Assemblies via the Enrichment of Structure-Directing "Codes" of Bridging Ligands and Metathesis of Metal Units
A series of molybdenum- and copper-based MOPs were synthesized through coordination-driven process of a bridging ligand (3,3'-PDBAD, L-1) and dimetal paddlewheel clusters. Three conformers of the ligand exist with an ideal bridging angle between the two carboxylate groups of 0 degrees (H-2 zeta-L(1)), 120 degrees (H-2 beta-L-1), and of 90 degrees (H-2 beta-L-1), respectively. At ambient or lower temperature, (HL1)-L-2 and Mo-2(OAc)(4) or Cu-2(OAc)(4) were crystallized into a molecular square with ?-L-1 and Mo-2/Cu-2 units. With proper temperature elevation, not only the molecular square with ?-L-1 but also a lantern-shaped cage with a-L-1 formed simultaneously. Similar to how Watson-Crick pairs stabilize the helical structure of duplex DNA, the core-shell molecular assembly possesses favorable H-bonding interaction sites. This is dictated by the ligand conformation in the shell, coding for the formation and providing stabilization of the central lantern shaped core, which was not observed without this complementary interaction. On the basis of the crystallographic implications, a heterobimetallic cage was obtained through a postsynthetic metal ion metathesis, showing different reactivity of coordination bonds in the core and shell. As an innovative synthetic strategy, the site-selective metathesis broadens the structural diversity and properties of coordination assemblies.
LIGHT BRIDGE IN A DEVELOPING ACTIVE REGION. I. OBSERVATION OF LIGHT BRIDGE AND ITS DYNAMIC ACTIVITY PHENOMENA
Light bridges, the bright structures that divide the umbra of sunspots and pores into smaller pieces, are known to produce a wide variety of activity events in solar active regions (ARs). It is also known that the light bridges appear in the assembling process of nascent sunspots. The ultimate goal of this series of papers is to reveal the nature of light bridges in developing ARs and the occurrence of activity events associated with the light bridge structures from both observational and numerical approaches. In this first paper, exploiting the observational data obtained by Hinode, the Interface Region Imaging Spectrograph, and the Solar Dynamics Observatory, we investigate the detailed structure of the light bridge in NOAA AR 11974 and its dynamic activity phenomena. As a result, we find that the light bridge has a weak, horizontal magnetic field, which is transported from the interior by a large-scale convective upflow and is surrounded by strong, vertical fields of adjacent pores. In the chromosphere above the bridge, a transient brightening occurs repeatedly and intermittently, followed by a recurrent dark surge ejection into higher altitudes. Our analysis indicates that the brightening is the plasma heating due to magnetic reconnection at lower altitudes, while the dark surge is the cool, dense plasma ejected from the reconnection region. From the observational results, we conclude that the dynamic activity observed in a light bridge structure such as chromospheric brightenings and dark surge ejections are driven by magnetoconvective evolution within the light bridge and its interaction with the surrounding magnetic fields.
LIGHT BRIDGE IN A DEVELOPING ACTIVE REGION. II. NUMERICAL SIMULATION OF FLUX EMERGENCE AND LIGHT BRIDGE FORMATION
Light bridges, the bright structure dividing umbrae in sunspot regions, show various activity events. In Paper I, we reported on an analysis of multi-wavelength observations of a light bridge in a developing active region (AR) and concluded that the activity events are caused by magnetic reconnection driven by magnetconvective evolution. The aim of this second paper is to investigate the detailed magnetic and velocity structures and the formation mechanism of light bridges. For this purpose, we analyze numerical simulation data from a radiative magnetohydrodynamics model of an emerging AR. We find that a weakly magnetized plasma upflow in the near-surface layers of the convection zone is entrained between the emerging magnetic bundles that appear as pores at the solar surface. This convective upflow continuously transports horizontal fields to the surface layer and creates a light bridge structure. Due to the magnetic shear between the horizontal fields of the bridge and the vertical fields of the ambient pores, an elongated cusp-shaped current layer is formed above the bridge, which may be favorable for magnetic reconnection. The striking correspondence between the observational results of Paper I and the numerical results of this paper provides a consistent physical picture of light bridges. The dynamic activity phenomena occur as a natural result of the bridge formation and its convective nature, which has much in common with those of umbral dots and penumbral filaments.
Structural and functional impact of troponin C-mediated Ca 2+ sensitization on myofilament lattice spacing and cross-bridge mechanics in mouse cardiac muscle
Acto-myosin cross-bridge kinetics are important for beat-to-beat regulation of cardiac contractility; however, physiological and pathophysiological mechanisms for regulation of contractile kinetics are incompletely understood. Here we explored whether thin filament-mediated Ca 2+ sensitization influences cross-bridge kinetics in permeabilized, osmotically compressed cardiac muscle preparations. We used a murine model of hypertrophic cardiomyopathy (HCM) harboring a cardiac troponin C (cTnC) Ca 2+ -sensitizing mutation, Ala8Val in the regulatory N-domain. We also treated wild-type murine muscle with bepridil, a cTnC-targeting Ca 2+ sensitizer. Our findings suggest that both methods of increasing myofilament Ca 2+ sensitivity increase cross-bridge cycling rate measured by the rate of tension redevelopment (k TR ); force per cross-bridge was also enhanced as measured by sinusoidal stiffness and I 1,1 -1/I 1,0 ratio from X-ray diffraction. Computational modeling suggests that Ca 2+ sensitization through this cTnC mutation or bepridil accelerates k TR primarily by promoting faster cross-bridge detachment. To elucidate if myofilament structural rearrangements are associated with changes in k(TR), we used small angle X-ray diffraction to simultaneously measure myofilament lattice spacing and isometric force during steady-state Ca 2+ activations. Within in vivo lattice dimensions, lattice spacing and steady-state isometric force increased significantly at submaximal activation. We conclude that the cTnC N-domain controls force by modulating both the number and rate of cycling cross-bridges, and that the both methods of Ca 2+ sensitization may act through stabilization of cTnC's D-helix. Furthermore, we propose that the transient expansion of the myofilament lattice during Ca 2+ activation may be an additional factor that could increase the rate of cross-bridge cycling in cardiac muscle. These findings may have implications for the pathophysiology of HCM.
Bridge-bursting and the action-integral in exploding bridge-wire (EBW) detonators
Methods of simulating the electrical characteristics of exploding bridge-wire detonators using only inert materials are reported. Specifically, it is found that both pressed finely powdered sucrose (confectioners sugar) and 3 µm aluminum oxide powder match the signals measured in a real EBW detonator with PETN fill. The exact pressing density of the inert powder is found not to affect to the measured electrical signals. Additionally, the action-integral (the time integral of bridge current squared) and energy-to-burst are measured for the same gold bridge design with several capacitor discharge units at different charge voltages and inductances as well as pseudo-constant current fireset made from charged coaxial-cables. It is found that in contrast to some reports, the action-integral to burst is not a constant for a specific bridge geometry and material, but instead is a function of the rate of current change in the bridge around the time of burst. This is explained by the confining effects of bridge-wire inertia affecting the rapid expansion of the bridge around burst time and hence the dynamic resistance.
Cation templating and electronic structure effects in uranyl cage clusters probed by the isolation of peroxide-bridged uranyl dimers
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A PWM Strategy for Cascaded H-bridges to Reduce the Loss Caused by Parasitic Capacitances of Medium Voltage Dual Active Bridge Transformers
In this paper, to reduce the power loss caused by the parasitic capacitances of medium voltage medium frequency transformers in a two-stage dc/ac converter, including a dual active bridge (DAB)-based dc/dc stage and a cascaded H-bridge (CHB)-based dc/ac stage, a PWM strategy is proposed for the dc/ac stage. For each H-bridge, by shifting the high switching frequency to one half-bridge and having the other half-bridge switching at the fundamental frequency of the ac voltage, equivalent frequencies of transformer terminal-to-ground voltages are reduced. As a result, the frequencies of charging and discharging current to the parasitic capacitance are reduced, and therefore the loss caused by the parasitic capacitances is reduced. Compared to the conventional phase-shift PWM, the proposed PWM strategy can reduce the parasitic capacitance loss by about 75%. Experimental test results of one phase in the 13.8 kV/ 100 kW converter prototype are provided to validate the approach.
Interplay between Terminal and Bridging Diiron Hydrides in Neutral and Oxidized States
This study describes the structural, spectroscopic, and electrochemical properties of electronically unsymmetrical diiron hydrides. The terminal hydride Cp*Fe(pdt)Fe(dppe)(CO)H ([1(t-H)]0, Cp*– = Me5C5 –, pdt2– = CH2(CH2S–)2, dppe = Ph2PC2H4PPh2) was prepared by hydride reduction of [Cp*Fe(pdt)Fe(dppe)(CO)(NCMe)]+. As established by X-ray crystallography, [1(t- H)]0 features a terminal hydride ligand. Unlike previous examples of terminal diiron hydrides, [1(t-H)]0 does not isomerize to the bridging hydride [1(µ-H)]0. Oxidation of [1(t-H)]0 gives [1(t- H)]+, which was also characterized crystallographically as its BF4 – salt. Density functional theory (DFT) calculations indicate that [1(t-H)]+ is best described as containing an Cp*Fe(III) center. In solution, [1(t-H)]+ isomerizes to [1(µ-H)]+ as anticipated by DFT. Reduction of [1(µ- H)]+ by Cp2Co affords the diferrous bridging hydride [1(µ-H)]0. Electrochemical measurements and DFT calculations indicate that the couples [1(t-H)]+/0 and [1(µ-H)]+/0 differ by 210 mV. Qualitative measurements indicate that [1(t-H)]0 and [1(µ-H)]0 are close in free energy. Protonation of [1(t-H)]0 in MeCN solution affords H2 even with weak acids via hydride transfer.
A Simple Unified Model for Generic Operation of Dual Active Bridge Converter
Not provided.
Electronic influences of bridging and chelating diimine ligand coordination in formamidinate-bridged Rh 2 (II,II) dimers
We report two new formamidinate-bridged Rh 2 II,II complexes, cis-[Rh 2 II,II (μ-DTolF) 2 (μ-np) 2 ] 2+ (3; DTolF = N,N'-di-p-tolylformamidinate; np = 1,8-naphthyridine) and cis-[Rh 2 II,II (μ-DTolF) 2 (κ 2 -dap) 2 ] 2+ (4; dap = 1,12-diazaperylene), were synthesized from cis-[Rh 2 II,II (μ-DTolF) 2 (CH 3 CN) 6 ](BF 4 ) 2 (1), and their properties were compared to those of cis-[Rh 2 II,II (μ-DTolF) 2 (phen) 2 ](BF 4 ) 2 (2). Density functional theory (DFT) and electrochemical analyses support the description of the highest occupied molecular orbitals (HOMOs) of 3 and 4 as possessing contributions from the metals and formamidinate bridging ligands, with Rh 2 /form character, and lowest unoccupied molecular orbitals (LUMOs) localized on the respective diimine ligand np and dap π* orbitals. Both 3 and 4 display strong, low energy Rh 2 /form → diimine(π*) metal/ligand-to-ligand charger transfer ( 1 ML–LCT) transitions with maxima at 566 nm (ε = 3600 M -1 cm -1 ) for 3 and at 630 nm (ε = 2900 M -1 cm -1 ) for 4 in CH 3 CN. Time dependent-DFT (TD-DFT) calculations support these assignments. Finally, the ability of both the bridging np and chelating dap diimine ligands to produce strong absorption of these Rh 2 II,II complexes throughout the visible region is potentially useful for the development of new photocatalysts for H 2 production and photochemotherapeutics.
Analytical Expression for DC Link Capacitor Current in a Cascaded H-Bridge Multi-Level Active Front-End Converter
Medium-voltage grid-tied systems often use a cascaded H-bridge multi-level active front-end. In this converter, dc link bus capacitors play an important role in stabilizing the converter and enabling both active and reactive power injections. The present work provides analytical expressions for the capacitor current, which are essential for optimizing system design (especially capacitor size vs. lifetime). Then, the expression is incorporated into the grid connected bidirectional power system model. Consequently, this work contributes to the guiding principles to choose accurate dc link capacitor ratings against grid-side power delivery requirements. The analytical results have been validated with detailed simulations and hardware results.
Equivalent Circuit Models of Voltage-Controlled Dual Active Bridge Converters: Preprint
In this paper, we introduce a new method of modeling voltage-controlled dual active bridge converters as equivalent circuits. What makes the proposed model unique is that the entire closed loop system converter and control are represented together in one equivalent circuit. Since the full system can be recast as a circuit, this allows for deeper insights on how the closed-loop system performs and for the direct application of circuit analysis techniques. In particular, we reveal how classical control notions can be understood as circuit laws.
Bridging-Controlled Network Microstructure and Long-Wavelength Fluctuations in Silica-Poly(2-vinylpyridine) Nanocomposites: Experimental Results and Theoretical Analysis
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