Investigation of the Dynamics of Two Chiral Smectogens by Means of 2H NMR

Z-Scheme Water Splitting under Near-Ambient Pressure using a Zirconium Oxide Coating on Printable Photocatalyst Sheets

Sunlight-driven water splitting systems operating under ambient pressure are essential for practical renewable hydrogen production. Printable photocatalyst sheets, composed of a hydrogen evolution photocatalyst (HEP), an oxygen evolution photocatalyst (OEP), and conductive metal nanoparticles, are cost-effective and scalable systems. However, the decrease in water splitting activity under ambient pressure due to reverse reactions hampers their practical application. In this study, coating zirconium oxide (ZrO_x ) by facile drop-casting onto a printed photocatalyst sheet, which consists of SrTiO₃  : Rh, BiVO₄  : Mo, and Au nanocolloids as the HEP, OEP, and electron mediator, respectively, effectively maintains the water splitting activity at elevated pressure. The ZrO_x -coated photocatalyst sheet retains 90 % and 84 % of the base performance (the pristine sheet at 10 kPa) at 50 and 90 kPa, respectively. Achieving efficient water splitting at the ambient pressure by inexpensive and extensible processes is an important step toward solar hydrogen production.

Comparison of the dielectric and NMR results for liquid crystals: dynamic aspects

Critical analysis of the results of studies of molecular rotational dynamics in liquid crystalline substances with the aid of the dielectric spectroscopy (DS) and nuclear magnetic resonance (NMR) is given. Both methods are known to be sensitive to different aspects of molecular rotations: the polarization vector and the relaxation time τ(DS) in the case of DS, a tensor describing a nuclear interaction and the correlation time τ(NMR) for NMR method. Furthermore, both methods provide correlation functions with different rank values. A common basis for the comparison between τ(DS) and τ(NMR) is postulated. Several examples of the temperature dependence of the correlation times coming from the two spectroscopic methods are presented. Qualitative agreements of the correlation times were achieved in most cases.

Structural and orientational properties of the ferro, antiferroelectric, and re-entrant smectic C phases of ZLL7/ by deuterium NMR and other experimental techniques

In this work, two selectively deuterium-labeled isotopomers of the (S)-2-methylbutyl- [4'-(4' '-heptyloxyphenyl)-benzoyl-4-oxy-(S)-2-((S)-2')-benzoyl)-propionyl)]-propionate (ZLL 7/), one labeled on the phenyl ring (ZLL 7/-phe-D2) and the other one on the biphenyl fragment (ZLL 7/-biphe-D2), have been investigated by deuterium NMR (DNMR) spectroscopy and other experimental techniques. These compounds possess the paraelectric SmA, the ferroelectric SmC, the antiferroelectric SmC(A), the re-entrant ferroelectric SmC(re), and the ferroelectric hexatic phases down to room temperature. The orientational ordering properties of the two labeled fragments have been determined by means of DNMR, and the mesophase behavior at two magnetic fields is discussed. In particular, the effect of the magnetic field on the supramolecular structure of the SmC and SmC(re) phases is commented. This study revealed to be useful to understand the structural and conformational properties of the ferroelectric/antiferroelectric/re-entrant/hexatic smectic phases. Mesomorphic properties, spontaneous tilt angle, polarization, and layer spacing have been studied for the labeled materials and compared with those obtained for the nonlabeled compound. The two self-consistent set of data, from optical and DNMR measurements and X-ray results, allow us to associate at the transition from the SmC to the SmC(A) phase a change of the molecular conformation.

Modeling a Liquid Crystal Dynamics by Atomistic Simulation with an Ab Initio Derived Force Field

Atomistic molecular dynamics (MD) simulations of 4-n-pentyl 4'-cyano-biphenyl (5CB) have been performed, adopting a specific ab initio derived force field. Two state points in the nematic phase and three in the isotropic phase, as determined in a previous work, have been considered. At each state point, at least 10 ns have been produced, allowing us to accurately calculate single-molecule properties. In the isotropic phase, the values of the translational diffusion coefficient, and even more so the activation energy for the process, agree well with experimental data. Qualitatively, also the dynamic anisotropy of the nematic phase is correctly accounted for. Rotational diffusion coefficients, which describe spinning and tumbling motions, fall well within the range of experimental values. The reorientational dynamics of our model 5CB covers diverse time regimes. The longest one is strongly temperature dependent and characterized by a relaxation time in accord with experimental dielectric relaxation data. Shear viscosity and Landau-de Gennes relaxation times, typically collective variables, reproduce the experimental results very well in the isotropic phase. In the nematic phase, despite a large statistical uncertainty due to the extremely slow relaxation of the correlation functions involved, our simulation yields the correct relative order of the three experimental Miesowicz viscosities.

Interlayer Self-Diffusion and Structure of Chiral Smectic Phases Studied by 2H NMR Exchange Experiment

A deuterium two-dimensional exchange NMR technique is used to study the chiral smectic C (SmC*) and chiral subphases (SmC*(Fi1), SmC*(Fi2)) of a smectogen (s)-1-methylheptyl 4'-(4-n-decyloxy-benzoyloxy)biphenyl-4-carboxylate (10B1M7). The aim is to demonstrate how this technique can be combined with sample rotation in the magnetic field to obtain the fast translational self-diffusion constant along the helical pitch in the SmC* phase and to shed light on the structure of helicoidal superlattice in the three-layer SmC*(Fi1) and four-layer SmC*(Fi2) phases. The cross-peak intensities in the 2D exchange spectrum are sensitive to the disposition of molecules in the three- and four-layer base unit. The results support the "asymmetric Clock model" as an appropriate description of the ferrielectric phases in this compound.

NMR study of orientational ordering in the chiral- phase of partially deuterated smectic liquid crystals

Carbon-13 NMR spectroscopy is used to probe the orientational ordering in the tilted chiral smectic phase of two partially deuterated liquid crystals. Quantitative analyses of the chemical shifts of the aromatic carbons in the smectic- and - phases enable us to obtain the location of the long molecular axis of the core and the off-diagonal order parameter of the molecular core in the smectic- phase. The advantage of knowing the ordering information of these molecules in their mesophases from deuteron studies is demonstrated in this work, as they can be used to guide the assignments. The temperature behaviors of the chemical shifts from the aromatic sites in the helical structure of the smectic- phase is caused by both an increase of the tilt angle and a change in the orientation of the principal frame that describes the molecular ordering.

Deuterium NMR relaxation in the smectic-A phase of a chiral smectogen

Correlated internal rotations are studied for a deuterated decyloxy chain in the chiral liquid crystal, 1-methylheptyl 4(')-(4-n-decyloxy-benzoyloxy)biphenyl-4-carboxyate (10B1M7) using deuteron spin-lattice relaxation and quadrupolar splitting measurements. The present study is limited to the uniaxial smectic-A phase of 10B1M7, in which the observed deuterium nuclear magnetic resonance spectra show well-resolved peaks. The deuteron Zeeman (T(1Z)) and quadrupolar (T(1Q)) spin-lattice relaxation times are simultaneously determined for the methylene deuterons using the Wimperis broadband pulse sequence at two different Larmor frequencies. The quadrupolar splittings at each temperature are modeled by the additive potential method to obtain the orienting potential of mean torque. A decoupled model using 683 conformations for the decyloxy chain is used to explain the observed relaxation rates. In this model, the overall reorientation is described by a small step rotational diffusion model, while both gauche migration and gauche pair production as well as the so called one- and two-bond motions are allowed to occur in the chain. Transition rates for the chain dynamics and the overall rotational diffusion constants are obtained using a global target analysis of the relaxation data of 10B1M7.