Twist–Bend Stage in the Relaxation of Sheared Chiral Nematic Suspensions of Cellulose Nanocrystals

Aqueous suspensions of cellulose nanocrystals (CNCs), prepared from natural cellulose by sulfuric acid hydrolysis, form stable chiral nematic suspensions above a critical CNC concentration. The chiral nematic organization may be preserved in films prepared from the suspensions by evaporation. However, shrinkage, gelation, and shear during film formation impair the optical properties of the dry film. In this article, we report an unusual behavior for a sample in which gelation occurred before the sample reached iridescent pitch values. In attempting to decouple changes in texture due to evaporation from those due to shear relaxation effects for this sample, we observed a transitory nematic-like texture that was induced by shear during the preparation of the sample for polarized light microscopy. We propose that the transition between chiral nematic and nematic structures involves a twist–bend-like intermediate and not an untwisting of the chiral nematic phase

Formation of Chiral Nematic Films from Cellulose Nanocrystal Suspensions Is a Two-Stage Process

The evaporation of aqueous suspensions of cellulose nanocrystals (CNCs) gives iridescent chiral nematic films with reflection colors at visible wavelengths. A key problem is controlling the chiral nematic pitch, <i>P</i>, and hence the reflection colors of CNC films. By adding d-(+)-glucose to the suspension, we show that the change in <i>P</i> during evaporation occurs in two distinct stages. The first stage is the decrease in <i>P</i> as the concentration of CNC in the chiral nematic suspension increases due to evaporation; the addition of glucose causes a decrease in <i>P</i> at this stage. In a second stage, a concentration of CNC is reached where the formation of ordered gels and glasses prevents further major changes in <i>P</i>. The addition of glucose lowers the CNC concentration at which this occurs, leading to an increase in <i>P</i> and hence an overall shift to the red end of the spectrum in the final film

Simple and Efficient System for Combined Solar Energy Harvesting and Reversible Hydrogen Storage

Solar energy harvesting and hydrogen economy are the two most important green energy endeavors for the future. However, a critical hurdle to the latter is how to safely and densely store and transfer hydrogen. Herein, we developed a reversible hydrogen storage system based on low-cost liquid organic cyclic hydrocarbons at room temperature and atmospheric pressure. A facile switch of hydrogen addition (>97% conversion) and release (>99% conversion) with superior capacity of 7.1 H₂ wt % can be quickly achieved over a rationally optimized platinum catalyst with high electron density, simply regulated by dark/light conditions. Furthermore, the photodriven dehydrogenation of cyclic alkanes gave an excellent apparent quantum efficiency of 6.0% under visible light illumination (420–600 nm) without any other energy input, which provides an alternative route to artificial photosynthesis for directly harvesting and storing solar energy in the form of chemical fuel

Photoinduced Conversion of Methane into Benzene over GaN Nanowires

As a class of key building blocks in the chemical industry, aromatic compounds are mainly derived from the catalytic reforming of petroleum-based long chain hydrocarbons. The dehydroaromatization of methane can also be achieved by using zeolitic catalysts under relatively high temperature. Herein we demonstrate that Si-doped GaN nanowires (NWs) with a 97% rationally constructed <i>m</i>-plane can directly convert methane into benzene and molecular hydrogen under ultraviolet (UV) illumination at rt. Mechanistic studies suggest that the exposed <i>m</i>-plane of GaN exhibited particularly high activity toward methane C–H bond activation and the quantum efficiency increased linearly as a function of light intensity. The incorporation of a Si-donor or Mg-acceptor dopants into GaN also has a large influence on the photocatalytic performance

Photo-induced Metal-Catalyst-Free Aromatic Finkelstein Reaction

The facile iodination of aromatic compounds under mild conditions is a great challenge for both organic and medicinal chemistry. Particularly, the synthesis of functionalized aryl iodides by light has long been considered impossible due to their photo-lability, which actually makes aryl iodides popular starting materials in many photo-substitution reactions. Herein, a photo-induced halogen exchange in aryl or vinyl halides has been discovered for the first time. A broad scope of aryl iodides can be prepared in high yields at room temperature under exceptionally mild conditions without any metal or photo-redox catalysts. The presence of a catalytic amount of elemental iodine could promote the reaction significantly