Resonance Raman intensity analysis of photoactive metal-organic frameworks.

Metal-organic frameworks (MOFs) are promising candidate materials for photo-driven processes. Their crystalline and tunable structure makes them well-suited for placing photoactive molecules at controlled distances and orientations that support processes such as light harvesting and photocatalysis. In order to optimize their performance, it is important to understand how these molecules evolve shortly after photoexcitation. Here, we use resonance Raman intensity analysis (RRIA) to quantify the excited state nuclear distortions of four modified UiO-68 MOFs. We find that stretching vibrations localized on the central ring within the terphenyl linker are most distorted upon interaction with light. We use a combined computational and experimental approach to create a picture of the early excited state structure of the MOFs upon photoactivation. Overall, we show that RRIA is an effective method to probe the excited state structure of photoactive MOFs and can guide the synthesis and optimization of photoactive designs.

Light control using vertically aligned dichroic dye films for windshield antireflection in automotive displays.

A vertically aligned dichroic dye (VA-Dye) film, whose absorption axis is perpendicular to that of its substrates, was laminated on a display panel in which the absorption axis of the top polarizer was set to 0°. In the vertical viewing angle of the display panel, the absorption axes of the top polarizer and dichroic dye are at right angles to each other, and, so, the light emitted from the display panel can be blocked. In the horizontal viewing angle of the display panel, the absorption axes of the top polarizer and dichroic dye are parallel to each other so that the light emitted from the display panel can be transmitted. Based on these polarization optics, we achieved complete elimination of light emitted in the upward or downward direction of the display panel, while the light emitted to the left and right is transmitted. We also added a designed optical compensation film using a positive biaxial (+B) retarder to the VA-Dye film so that the light emitted in the upward and downward directions of the display panel could be blocked in a wide viewing angle range (not only in the vertical direction, but also in the diagonal direction). The display panel using the VA-Dye film with the +B retarder showed excellent optical performance, such as significantly lower transmittance over a wide viewing angle range in the upward direction and relatively higher transmittance compared to that of a reference panel without the VA-Dye film. In addition, the VA-Dye film can be manufactured with a lower thickness, easier fabrication, and lower cost when compared with other technologies.

Mapping the temperature-dependent and network site-specific onset of spectral diffusion at the surface of a water cluster cage.

We explore the kinetic processes that sustain equilibrium in a microscopic, finite system. This is accomplished by monitoring the spontaneous, time-dependent frequency evolution (the frequency autocorrelation) of a single OH oscillator, embedded in a water cluster held in a temperature-controlled ion trap. The measurements are carried out by applying two-color, infrared-infrared photodissociation mass spectrometry to the D3O+·(HDO)(D2O)19 isotopologue of the "magic number" protonated water cluster, H+·(H2O)21 The OH group can occupy any one of the five spectroscopically distinct sites in the distorted pentagonal dodecahedron cage structure. The OH frequency is observed to evolve over tens of milliseconds in the temperature range (90 to 120 K). Starting at 100 K, large "jumps" are observed between two OH frequencies separated by ∼300 cm-1, indicating migration of the OH group from the bound OH site at 3,350 cm-1 to the free position at 3,686 cm-1 Increasing the temperature to 110 K leads to partial interconversion among many sites. All sites are observed to interconvert at 120 K such that the distribution of the unique OH group among them adopts the form one would expect for a canonical ensemble. The spectral dynamics displayed by the clusters thus offer an unprecedented view into the molecular-level processes that drive spectral diffusion in an extended network of water molecules.

Water Network Shape-Dependence of Local Interactions with the Microhydrated -NO2- and -CO2- Anionic Head Groups by Cold Ion Vibrational Spectroscopy.

We report the structural evolutions of water networks and solvatochromic response of the CH3NO2- radical anion in the OH and CH stretching regions by analysis of the vibrational spectra displayed by cryogenically cooled CH3NO2-·(H2O)n=1-6 clusters. The OH stretching bands evolve with a surprisingly large discontinuity at n = 6, which features the emergence of an intense, strongly red-shifted band along with a weaker feature that appears in the region assigned to a free OH fundamental. Very similar behavior is displayed by the perdeuterated carboxylate clusters, RCO2-·(H2O)n=5-7 (R = CD3CD2), indicating that this behavior is a general feature in the microhydration of the triatomic anionic domain and not associated with CH oscillators. Electronic structure calculations trace this behavior to the formation of a "book" isomer of the water hexamer that adopts a configuration in which one of the water molecules resides in an acceptor-acceptor-donor (AAD) (A = acceptor, D = donor) H-bonding site. Excitation of the bound OH in the AAD site explores the local network topology best suited to stabilize an incipient -XO2H-OH-(H2O)2 intracluster proton-transfer reaction. These systems thus provide particularly clear examples where the network shape controls the potential energy landscape that governs water network-mediated, intracluster proton transfer. The CH stretching bands of the CH3NO2-·(H2O)n=1-6 clusters also exhibit strong solvatochromic shifts, but in this case, they smoothly blue-shift with increasing hydration with no discontinuity at n = 6. This behavior is analyzed in the context of the solute-ion polarizability response and partial charge transfer to the water networks.

Vibrational Signatures of HNO3 Acidity When Complexed with Microhydrated Alkali Metal Ions, M+·(HNO3)(H2O)n=5 (M = Li, K, Na, Rb, Cs), at 20 K.

The speciation of strong acids like HNO3 under conditions of restricted hydration is an important factor in the rates of chemical reactions at the air-water interface. Here, we explore the trade-offs at play when HNO3 is attached to alkali ions (Li+-Cs+) with four water molecules in their primary hydration shells. This is achieved by analyzing the vibrational spectra of the M+·(HNO3)(H2O)5 clusters cooled to about 20 K in a cryogenic photofragmentation mass spectrometer. The local acidity of the acidic OH group is estimated by the extent of the red shift in its stretching frequency when attached to a single water molecule. The persistence of this local structural motif (HNO3-H2O) in all of these alkali metal clusters enables us to determine the competition between the effect of the direct complexation of the acid with the cation, which acts to enhance acidity, and the role of the water network in the first hydration shell around the ions, which acts to counter (screen) the intrinsic effect of the ion. Analysis of the vibrational features associated with the acid molecule, as well as those of the water network, reveals how cooperative interactions in the microhydration regime conspire to effectively offset the intrinsic enhancement of HNO3 acidity afforded by attachment to the smaller cations.

Molecular-level origin of the carboxylate head group response to divalent metal ion complexation at the air-water interface.

We exploit gas-phase cluster ion techniques to provide insight into the local interactions underlying divalent metal ion-driven changes in the spectra of carboxylic acids at the air-water interface. This information clarifies the experimental findings that the CO stretching bands of long-chain acids appear at very similar energies when the head group is deprotonated by high subphase pH or exposed to relatively high concentrations of Ca2+ metal ions. To this end, we report the evolution of the vibrational spectra of size-selected [Ca2+·RCO2-]+·(H2O) n=0to12 and RCO2-·(H2O) n=0to14 cluster ions toward the features observed at the air-water interface. Surprisingly, not only does stepwise hydration of the RCO2- anion and the [Ca2+·RCO2-]+ contact ion pair yield solvatochromic responses in opposite directions, but in both cases, the responses of the 2 (symmetric and asymmetric stretching) CO bands to hydration are opposite to each other. The result is that both CO bands evolve toward their interfacial asymptotes from opposite directions. Simulations of the [Ca2+·RCO2-]+·(H2O) n clusters indicate that the metal ion remains directly bound to the head group in a contact ion pair motif as the asymmetric CO stretch converges at the interfacial value by n = 12. This establishes that direct metal complexation or deprotonation can account for the interfacial behavior. We discuss these effects in the context of a model that invokes the water network-dependent local electric field along the C-C bond that connects the head group to the hydrocarbon tail as the key microscopic parameter that is correlated with the observed trends.

Computational predictions of metal-macrocycle stability constants require accurate treatments of local solvent and pH effects.

Rational design of molecular chelating agents requires a detailed understanding of physicochemical ligand-metal interactions in solvent phase. Computational quantum chemistry methods should be able to provide this, but computational reports have shown poor accuracy when determining absolute binding constants for many chelating molecules. To understand why, we compare and benchmark static- and dynamics-based computational procedures for a range of monovalent and divalent cations binding to a conventional cryptand molecule: 2.2.2-cryptand ([2.2.2]). The benchmarking comparison shows that dynamics simulations using standard OPLS-AA classical potentials can reasonably predict binding constants for monovalent cations, but these procedures fail for divalent cations. We also consider computationally efficient static procedure using Kohn-Sham density functional theory (DFT) and cluster-continuum modeling that accounts for local microsolvation and pH effects. This approach accurately predicts binding energies for monovalent and divalent cations with an average error of 3.2 kcal mol-1 compared to experiment. This static procedure thus should be useful for future molecular screening efforts, and high absolute errors in the literature may be due to inadequate modeling of local solvent and pH effects.

Quantifying Uncertainties in Solvation Procedures for Modeling Aqueous Phase Reaction Mechanisms.

Computational quantum chemistry provides fundamental chemical and physical insights into solvated reaction mechanisms across many areas of chemistry, especially in homogeneous and heterogeneous renewable energy catalysis. Such reactions may depend on explicit interactions with ions and solvent molecules that are nontrivial to characterize. Rigorously modeling explicit solvent effects with molecular dynamics usually brings steep computational costs while the performance of continuum solvent models such as polarizable continuum model (PCM), charge-asymmetric nonlocally determined local-electric (CANDLE), conductor-like screening model for real solvents (COSMO-RS), and effective screening medium method with the reference interaction site model (ESM-RISM) are less well understood for reaction mechanisms. Here, we revisit a fundamental aqueous hydride transfer reaction-carbon dioxide (CO2) reduction by sodium borohydride (NaBH4)-as a test case to evaluate how different solvent models perform in aqueous phase charge migrations that would be relevant to renewable energy catalysis mechanisms. For this system, quantum mechanics/molecular mechanics (QM/MM) molecular dynamics simulations almost exactly reproduced energy profiles from QM simulations, and the Na+ counterion in the QM/MM simulations plays an insignificant role over ensemble averaged trajectories that describe the reaction pathway. However, solvent models used on static calculations gave much more variability in data depending on whether the system was modeled using explicit solvent shells and/or the counterion. We pinpoint this variability due to unphysical descriptions of charge-separated states in the gas phase (i.e., self-interaction errors), and we show that using more accurate hybrid functionals and/or explicit solvent shells lessens these errors. This work closes with recommended procedures for treating solvation in future computational efforts in studying renewable energy catalysis mechanisms.

Liquid crystal cell asymmetrically anchored for high transmittance and triggered with a vertical field for fast switching.

The optical performance of an asymmetrically surface-anchored liquid crystal (LC) cell driven with three-terminal electrodes is demonstrated. The transmittance of an asymmetrically anchored cell is considerably higher than that of a symmetrically anchored cell. However, the slow response of an asymmetrically anchored cell makes its practical application difficult. In this work, we demonstrate that the slowest GTG response time from a high to low grey level in an asymmetrically anchored cell can be reduced to less than 0.7 ms by applying a vertical trigger pulse with three-terminal electrodes while maintaining the high transmittance of an asymmetrically anchored cell.

Low-power control of haze using a liquid-crystal phase-grating device with two-dimensional polymer walls.

We propose a two-dimensional (2D) polymer-walled liquid-crystal (LC) phase-grating device, which can be used to control the haze with a very low power. 2D polymer walls can be formed in an LC cell through ultraviolet light irradiation while applying an in-plane electric field through phase separation induced by the spatial elastic energy difference. The transparent and translucent states can be realized by applying vertical and in-plane electric fields to the 2D polymer-walled LC cell, respectively. The cell can be operated with a very low power as the transparent [translucent] state is maintained even after the applied vertical [in-plane] electric field is removed. It consumes power only during state switching. The fabricated device exhibits outstanding performances, such as a very low operating voltage (< 10 V), low haze (< 2%) in the transparent state, high haze (> 90%) in the translucent state, and short switching time (< 2 ms), compared to those of other bistable LC devices, which can be used to control the haze.

Model potential study of non-valence correlation-bound anions of (C60)n clusters: the role of electric field-induced charge transfer.

A polarization model which accounts for electric field-induced charge transfer between fullerene molecules is introduced. Application of this model to the C60 dimer and trimer shows that intermolecular charge transfer makes a significant contribution to the polarizabilities of these clusters. This polarization model is incorporated into a one-electron Hamiltonian for describing non-valence correlation-bound anions, allowing us to further demonstrate that intermolecular charge transfer also results in increased stability of these anion states.

Independent control of haze and total transmittance with a dye-doped liquid crystal phase-grating device.

This paper presents a dye-doped liquid crystal (LC) phase-grating cell that is switchable between transparent, dark, and opaque states. The device can control haze and transmittance independently. Initially, LC and dye molecules are twist-aligned to make the cell opaque but haze-free due to the absorption of incident light without scattering. Switching to the transparent state could be achieved by applying a vertical electric field, whereas switching to the opaque state could be achieved by applying an in-plane electric field. It exhibited several advantages, such as a low switching voltage (<18 V) and fast response time (<30 ms).

Switching between transparent and translucent states of a two-dimensional liquid crystal phase grating device with crossed interdigitated electrodes.

We report an electrically-switchable two-dimensional liquid crystal (LC) phase grating device for window display applications. The device consists of the top and bottom substrates with crossed interdigitated electrodes and vertically-aligned LCs sandwiched between the two substrates. The device, switchable between the transparent and translucent states by applying an electric field, can provide high haze by the strong diffraction effect thanks to a large spatial phase difference with little dependence on the azimuth angle. We found that the device has outstanding features, such as a low operating voltage, high transparency, and wide viewing angle characteristics in the transparent state and high haze in the translucent state. Moreover, we achieved submillisecond switching between transparent and translucent states by employing the overdrive scheme and a vertical trigger pulse.

Implementation of analytical gradients and of a mixed real and momentum space DVR method for excess electron systems described by a self-consistent polarization model.

This work presents two extensions of our self-consistent polarization model for treating non-valence excess electron systems. The first extension is the implementation of analytical gradients, and the second extension is the implementation of a mixed real space plus momentum space approach combined with fast Fourier transforms to reduce the computational time compared to a purely real space discrete variable representation approach. The performance of the new algorithms is assessed in calculations of the excess electron states of various size water clusters and of the non-valence correlation-bound anion of the C240 fullerene.

Electro-optical characteristics of an in-plane-switching liquid crystal cell with zero rubbing angle: dependence on the electrode structure.

When an electric field is applied to in-plane switching (IPS) and fringe-field switching (FFS) cells with zero rubbing angle, virtual walls are built such that the switching speed can be increased several-fold. In this study, we investigate the dependence on the interdigitated electrode structure of the electro-optical characteristics of IPS and FFS cells with zero rubbing angle. We found that when the rubbing angle is zero, the single-layered IPS electrode structure provides a higher transmittance than the double-layered FFS electrode structure because of the reduced width of dead zones at domain boundaries between interdigitated electrodes. Single-layered IPS electrodes not only minimize the transmittance decrease but also provide a shorter response time than double-layered FFS electrodes, although the operating voltage is higher and fabrication requires a more precise rubbing process. The transmittance decrease due to the zero rubbing angle in an IPS cell can be minimized using optimization of the electrode structure while retaining a short response time.

Interdigitated pixel electrodes with alternating tilts for fast fringe-field switching of liquid crystals.

We propose an interdigitated pixel electrode structure with alternating tilts for fast fringe-field switching of liquid crystals (LCs). In contrast to an LC cell, where the pixel electrodes are parallel to the LC alignment direction, this device does not require a non-zero pretilt angle, owing to an obliquely applied electric field; thus, it can retain a much wider viewing angle by aligning the LCs without a pretilt. In addition to a short response time and wide viewing angle, the proposed device allows a much larger deviation of the LC alignment direction, which is essential for mass production. Moreover, LCs with negative dielectric anisotropy can be used to minimize the transmittance decrease.

Effect of two-dimensional confinement on switching of vertically aligned liquid crystals by an in-plane electric field.

We investigated the two-dimensional (2-D) confinement effect of liquid crystals (LCs) on the switching of vertically aligned LCs by an in-plane electric field. When an in-plane field is applied to a vertical alignment (VA) cell, virtual walls are built at the center of the interdigitated electrodes and at the middle of the gaps between them. The LC molecules are confined not only by the two substrates but also by the virtual walls so that the turn-off time of a VA cell driven by an in-plane field is dependent on the pitch of the interdigitated electrodes as well as the cell gap. Therefore, the turn-off time of a VA cell driven by an in-plane field can be reduced simply by decreasing the pitch of the interdigitated electrodes as a result of the enhanced anchoring provided by the virtual walls. The experimental results showed good agreement with a simple model based on the 2-D confinement effect of LCs.

Origin of Acid-Base Catalytic Effects on Formaldehyde Hydration.

The mechanisms of hydronium- and hydroxide-catalyzed formaldehyde hydrations were investigated by quantum mechanical/molecular mechanical molecular dynamics in combination with flexible coordinates. A stepwise bimolecular and a concerted termolecular mechanism were found with a hydronium catalyst. The latter is more favorable and better consistent with experiment. Structurally, a dipole-bound species initially arranges the nucleophile in a favorable configuration for both routes, significantly enhancing the reactive collisions. On the one hand, the hydronium catalyst also plays a role of a reactant in the bimolecular path. On the other hand, only a stepwise mechanism was found with a hydroxide catalyst. Overall, hydroxide is a stronger catalyst than a hydronium when it is in contact distance with formaldehyde.

Like-charge ion pairs of hydronium and hydroxide in aqueous solution?

The like-charge ion pairings of hydronium and hydroxide were investigated using both ab initio cluster calculations and QM/MM-MD aqueous simulations. While only a two-water-bridged H3O(+)(H2O)2H3O(+) is found in hydronium cluster calculations, three clusters of HO(-)(H2O)2HO(-), HO(-)(H2O)3HO(-) and HO(-)(H2O)4HO(-) are stable dihydroxide aggregates. In addition, an interesting yet very stable parallelogram structure of [O-H···H-O](2-) without any bridging water was also discovered using QM/MM-MD simulations. According to our analysis, its unique structure reduces the electrostatic repulsion and allows stable coordination with solvents at the same time. In conclusion, hydroxide can form stronger like-ion pairs than hydronium in aqueous solution mostly due to its versatile coordination ability with solvents.

Conformational studies of dammarane-type triterpenoids using computational and NMR spectroscopic methods.

Natural triterpenoids are of great interest to researchers of various fields as they possess diverse physicochemical and biological properties. In medicinal chemistry, detailed information about the chemical structures of bioactive triterpenoids often helps find new lead compounds. Herein, the low-energy structures of (20S)-protopanaxadiol and (20S)-protopanaxatriol, the aglycones of various triterpenoid saponins found in Panax ginseng, and their (20R)-epimers have been predicted by the geometry optimization of the conformers extracted from molecular dynamics simulations with the self-consistent-charge density functional tight-binding method. By performing quantum mechanical calculations on the low-energy conformers, we have estimated the NMR chemical shifts of the compounds, which display good agreement with the most recently reported experimental values within an expected range of errors. Our results indicate that theoretical estimation of the NMR parameters of a relatively large molecule with a molecular mass of 500 is feasible.