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.     

Spin Changes Accompany Ultrafast Structural Interconversion in the Ground State of a Cobalt Nitrosyl Complex

Ultrafast, reversible intersystem crossing (ISC) is reported under ambient conditions for the electronic ground state of the pentacoordinate cobalt nitrosyl complexes, [CoX₂(NO)(PMePh₂)₂] (X=Cl, Br), in solution. ISCs on such short timescales are more typically observed in electronically excited states reached by absorption of ultraviolet or visible light. Singlet and triplet electron spin states of the complex, corresponding to two different isomers, are populated at room temperature, and the two isomers exchange on a timescale of a few picoseconds. Ultrafast two‐dimensional infrared spectroscopy observes the change in wavenumber of the NO ligand band accompanying the isomerization and associated ISC on the (spin) adiabatic ground potential energy surface. Comparison of the dynamics of the chloro‐ and bromo‐complexes shows that inertial effects of the ligand motion have a greater effect than spin–orbit coupling on determining the forward and reverse isomerization and ISC rates.     

Detecting Counterion Dynamics in DNA–Protein Association

Due to a high density of negative charges on its surface, DNA condenses cations as counterions, forming the so-called “ion atmosphere”. Although the release of counterions upon DNA–protein association has been postulated to have a major contribution to the binding thermodynamics, this release remains to be confirmed through a direct observation of the ions. Herein, we report the characterization of the ion atmosphere around DNA using NMR spectroscopy and directly detect the release of counterions upon DNA–protein association. NMR-based diffusion data reveal the highly dynamic nature of counterions within the ion atmosphere around DNA. Counterion release is observed as an increase in the apparent ionic diffusion coefficient, which directly provides the number of counterions released upon DNA–protein association.     

The Barrier to Proton Transfer in the Dimer of Formic Acid: A Pure Rotational Study

The rotational spectra of three C‐deuterated isotopologues of the dimer of formic acid have been measured, thanks to the small dipole moment induced by asymmetric H→D substitution(s). For the DCOOH–HCOOH species, the concerted double proton transfer of the two hydroxy hydrogen atoms takes place between two equivalent minima and generates a tunneling splitting of 331.2(6) MHz. This splitting can be reproduced by a 3D model with a barrier of 2559 cm^?1 (30.6 kJ mol^?1) as obtained from theoretical calculations.     

Phase Regulation Strategy of Perovskite Nanocrystals from 1D Orthomorphic NH4PbI3 to 3D Cubic (NH4)0.5Cs0.5Pb(I0.5Br0.5)3 Phase Enhances Photoluminescence

This work reports this first synthesis of 1D orthomorphic NH₄PbI₃ perovskite nanocrystals (NCs) considering the role of inorganic ammonium ions at the nanoscale. The addition of bromide ions at the halogen site did not improve the photoluminescence properties. Furthermore, the 3D cubic phase of (NH₄)_0.5Cs_0.5Pb(I_0.5Br_0.5)₃ NCs with bright photoluminescence was synthesized by adding Cs ions into the crystal lattice of (NH₄)Pb(I_0.5Br_0.5)₃. Moreover, the photophysical properties of different phase structures were studied using femtosecond transient absorption (FTA) spectroscopy. The ultrafast trap state capture process is a key factor in the change of photoluminescence properties and the cubic phase may be the best structure for photoluminescence. These results suggest that the ammonium ion perovskite (AIP) nanocrystals could be potential materials for optoelectronic applications through A‐site cation substitution.