Heat-Triggered Crystallization of Liquid Crystalline Macrocycles Allowing for Conductance Switching through Hysteretic Thermal Phase Transitions

A polymesomorphic thermal phase-transition of a macrocyclic amphiphile consisting of aromatic groups and oligoethylene glycol (OEG) chains is reported. The macrocyclic amphiphile exists in a highly-ordered liquid crystal (LC) phase at room temperature. Upon heating, this macrocycle shows phase-transition from columnar-lamellar to nematic LC phases followed by crystallization before melting. Spectroscopic studies suggest that the thermally induced crystallization is triggered by a conformational change at the OEG chains. Interestingly, while the macrocycle returns to the columnar-lamellar phase after cooling from the isotropic liquid, it retains the crystallinity after cooling from the thermally-induced crystal. Thanks to this bistability, conductance switching was successfully demonstrated. A different macrocyclic amphiphile also shows an analogous phase-transition behavior, suggesting that this molecular design is universal for developing switchable and memorizable materials, by means of hysteretic phase-transition processes.

Joint Experimental and Computational Investigation of the Flexibility of a Diacetylene-Based Mixed-Linker MOF: Revealing the Existence of Two Low-Temperature Phase Transitions and the Presence of Colossal Positive and Giant Negative Thermal Expansions

Solvothermal reaction in N,N-dimethylformamide (DMF) between 1,6-bis(1-imidazolyl)-2,4-hexadiyne monohydrate (L1⋅H₂ O), isophthalic acid (H₂ L2), and Zn(NO₃ )₂ ⋅6 H₂ O gives the diacetylene-based mixed-ligand coordination polymer {[Zn(L1)(L2)](DMF)₂ }_n (UMON-44) in 38 % yield. Combination of DSC with variable-temperature single-crystal X-ray diffraction revealed the occurrence of two phase transitions spanning the ranges 129-144 K and 158-188 K. Furthermore, the three structurally similar phases of UMON-44 show giant negative and/or colossal positive thermal expansions. These unusual phenomena exist without any change in the contents of the unit cell. DFT calculations using the PBE+D3 dispersion scheme were able to distinguish between these polymorphs by accurately reproducing their salient structural features, although corrections in the size of the unit cell turned out to be necessary for the high-temperature phase to account for its large thermal expansion. In addition, the infrared spectra (vibration frequencies and peak intensities) of these theoretical models were calculated, allowing for univocal identification of the corresponding polymorphs. Last, the limits of our computational method were tested by calculating the phase transition temperatures and their associated enthalpies, and the derived figures compare favorably with the values determined experimentally.

The hydrochloride and hydrobromide salt forms of (S)-amphetamine

Despite the high profile of amphetamine, there have been relatively few structural studies of its salt forms. The lack of any halide salt forms is surprising as the typical synthetic route for amphetamine initially produces the chloride salt. (S)-Amphetamine hydrochloride [systematic name: (2S)-1-phenylpropan-2-aminium chloride], C9H14N(+)·Cl(-), has a Z' = 6 structure with six independent cation-anion pairs. That these are indeed crystallographically independent is supported by different packing orientations of the cations and by the observation of a wide range of cation conformations generated by rotation about the phenyl-CH2 bond. The supramolecular contacts about the anions also differ, such that both a wide variation in the geometry of the three N-H...Cl hydrogen bonds formed by each chloride anion and differences in C-H...Cl contacts are apparent. (S)-Amphetamine hydrobromide [systematic name: (2S)-1-phenylpropan-2-aminium bromide], C9H14N(+)·Br(-), is broadly similar to the hydrochloride in terms of cation conformation, the existence of three N-H...X hydrogen-bond contacts per anion and the overall two-dimensional hydrogen-bonded sheet motif. However, only the chloride structure features organic bilayers and Z' > 1.

Experimental and computational studies on the formation of cyanate from early metal terminal nitrido ligands and carbon monoxide

DFT and experimental studies are used to elucidate key aspects in the design of a transition metal complex that mediates the reduction of dinitrogen by carbon monoxide and an electron source through a terminal metal nitride complex.

Lattice dynamics through the structural phase transition in D-amphetamine sulfate

The polarized infrared and Raman spectra of the single-crystalline D-amphetamine sulfate have been measured as a function of temperature in the vicinity of the structural phase transition. Infrared and Raman-active modes are identified and assigned. Significant signatures of the structural phase transition are observed in the temperature dependence of infrared modes both of the D-amphetamine unit and the sulfate anion. The changes reflect differences in the unit cell between low- and high-temperature phases of the D-amphetamine sulfate. Temperature dependence of the vibrational mode parameters displays pronounced hysteresis between 333 and 338 K that is extended over a smaller temperature range than 325-345 K found in the earlier DSC study.