Molecular Dynamics,Phase Transition and Frequency‐Tuned Dielectric Switch of an Ionic Co‐Crystal

Dielectric switches that can be converted between high and low dielectric states by thermal stimuli have attracted much interest owing to their many potential applications. Currently one main drawback for practical application lies in the non‐tunability of their switch temperatures (T_S). We report here an ionic co‐crystal (Me₃NH)₄[Ni(NCS)₆] that contains a multiply rotatable Me₃NH⁺ ion and a solely rotatable one due to a more spacious supramolecular cage for the former one. This compound undergoes an isostructural order–disorder phase transition and it can function as a frequency‐tuned dielectric switch with highly adjustable T_S, which is further revealed by the variable‐temperature structure analyses and molecular dynamics simulations. In addition, the distinct arrangements and molecular dynamics of two coexisting Me₃NH⁺ ions confined in different lattice spaces as well as the notable offset effect on the promoting/hindering of dipolar reorientation after dielectric transition provide a rarely observed but fairly good model for understanding and modulating the dipole motion in crystalline environment.     

How to Switch Spin‐Crossover Metal Complexes at Constant Room Temperature

Spin‐crossover metal complexes represent a highly promising class of molecular switches, the diverse physicochemical properties of which can be reversibly changed by different physical and chemical stimuli. One of the most interesting and examined features of these materials is the change of magnetic properties by changing the temperature or by irradiation with light at low temperatures. However, most prospective applications of such complexes require functioning at room temperature. This Concept article provides an overview about how the switching of spin‐crossover metal complexes can be achieved at constant room temperature. The principles of switching by different physical and chemical methods in solution and in the solid state are presented in an easy‐to‐read form for nonspecialists. These are further supported and clarified by examples from the literature. The overview might also be interesting for experts that target spin‐crossover systems functioning at ambient conditions.     

A Multistate pH‐Triggered Nonlinear Optical Switch

By using hyper‐Rayleigh scattering experiments and quantum‐chemical calculations, we demonstrate that nonlinear optics can be used to probe unequivocally, within a non‐destructive process, the multiple electronic states that are activated upon pH‐ and light‐triggered transformations of the 4′‐hydroxyflavylium ion. These results open new perspectives in the design of molecular‐scale high‐density optical memory.     

Interfaces and ripple states in ferroelastic crystals—A simple model

A simple atomistic model with two harmonic potentials between layers of atoms describes surface relaxations equivalent to those found as kink solitary waves in ferroelastic crystals. The parameters of the model are expressed in terms of the entropy term in a Landau potential and the Ginzburg gradient energy. We discuss the possibility that metastable ripple states exist in which a modulation is superimposed to an underlying uniform order parameter. Such ripple states can occur at temperatures close to the transition point between a ferroelastic phase and an incommensurate phase. In the ripple state, domain walls consist of kinks with modulations on either side of the kink.