Molecular Design of Organic Ionic Plastic Crystals Consisting of Tetracyanoborate with Ultralow Phase Transition Temperature

Organic ionic plastic crystals (OIPCs) are a ductile soft material where the composing ions are in isotropic free rotation, while their positions are aligned in order. The rotational motion in its plastic phase promotes ion conduction by decreasing the activation energy. Here, we report novel OIPCs comprised of tetracyanoborate ([TCB]-) and various organic cations. In particular, the OIPC composed of [TCB]- and spiro-(1,1')-bipyrrolidinium ([spiropyr]+) cations can transform into its plastic phase at ultralow temperature (Tp = -55 °C) while maintaining a high melting point (Tm = 242 °C). Replacement of the cation with either tetraalkylammonium or phosphonium and comparing their phase behavior, the high Tm was attributed to the relatively small interionic distance between [spiropyr]+ and [TCB]-. At the same time, the low Tp was realized by the restricted vibrational mode of the spirostructure, allowing the initiation of isotropic rotational motion with less thermal energy input.

Enhanced stability and complex phase behaviour of organic-inorganic green-emitting ionic manganese halides

Light-emitting materials based on earth-abundant metals, such as manganese hold great promise as emitters for organic lighting devices. In order to apply such emitter materials and, in particular, to overcome the problem of self-quenching due to cross-relaxation, we investigated a series of tetrabromidomanganate ([MnBr₄]^2-) salts with bulky tetraalkylphosphonium counter cations [P_nnn]⁺, namely [P_nnnn]₂[MnBr₄] (n = 4 (1), 6 (2) and 8 (3)), which can be obtained by a straightforward reaction of the respective phosphonium bromide and MnBr₂. Variation of the cation size allows control of the properties of the resulting ionic materials. 1 and 3 qualify as ionic liquids (ILs), where 1 features a melting point of 68 °C, and 3 is liquid at room temperature and even at very low temperatures. Furthermore, 1 and 2 show the formation of higher-ordered thermotropic mesophases. For 1 a transition to a thermodynamically metastable smectic liquid crystalline phase can be observed at room temperature upon reheating from the metastable glassy state; 2 appears to form a plastic crystalline phase at ∼63 °C, which persists up to the melting point of 235 °C. The photoemission is greatly affected by phase behaviour and ion dynamics. A photoluminescence quantum yield of 61% could be achieved, by balancing the increase in Mn²⁺-Mn²⁺ separation and reducing self-quenching through increasingly large organic cations which leads to adverse increased vibrational quenching. Compared to analogous ammonium compounds, which have been promoted as ̈inorganic hybrid perovskites̈, the phosphonium salts show superior performance, with respect to photoluminescent quantum yield and thermal and air/humidity stability. As the presented compounds are not sensitive to the atmosphere, in particular moisture, and show strong visible electroluminescence in the green region of light, they are important emitter materials for use in organic light-emitting devices.

Dipolar relaxation, conductivity, and polar order in AgCN

By using dielectric spectroscopy in a broad range of temperatures and frequencies, we have investigated dipolar relaxations, the dc conductivity, and the possible occurrence of polar order in AgCN. The conductivity contributions dominate the dielectric response at elevated temperatures and low frequencies, most likely arising from the mobility of the small silver ions. In addition, we observe the dipolar relaxation dynamics of the dumbbell-shaped CN- ions, whose temperature dependence follows the Arrhenius behavior with a hindering barrier of 0.59 eV (57 kJ/mol). It correlates well with a systematic development of the relaxation dynamics with the cation radius, previously observed in various alkali cyanides. By comparison with the latter, we conclude that AgCN does not exhibit a plastic high-temperature phase with free rotation of the cyanide ions. Instead, our results indicate that a phase with quadrupolar order, revealing dipolar head-to-tail disorder of the CN- ions, exists at elevated temperatures up to the decomposition temperature, which crosses over to long-range polar order of the CN dipole moments below about 475 K. Dipole ordering was also reported for NaCN and KCN, and a comparison with these systems suggests a critical relaxation rate of 105-107 Hz, marking the onset of dipolar order in the cyanides. The detected relaxation dynamics in this order-disorder type polar state points to glasslike freezing below about 195 K of a fraction of non-ordered CN dipoles.

Studies on the nature and pressure evolution of phase transitions in 1-adamantylamine and 1-adamantanol

In this paper, several experimental techniques, i.e., differential scanning calorimetry, X-ray diffraction, Fourier transform infrared, Raman, and broadband dielectric spectroscopy were applied to study the nature of the phase transitions in 1-adamantylamine (1-NH₂-ADM, C₁₀H₁₇N) and 1-adamantanol (1-OH-ADM, C₁₀H₁₆O). Calorimetric measurements showed one and three endothermic peaks in thermograms for the latter and the former substance, respectively. Indeed, results of spectroscopic investigations indicated that the observed thermal events in 1-NH₂-ADM correspond to transitions between various plastic crystal (PC) phases (I, II, III, IV), while the endothermic process in 1-OH-ADM can be assigned to a phase transition between the PC and the ordinary crystal (OC). Especially interesting were the outcomes of dielectric studies carried out both at ambient and high-pressure conditions, during heating and cooling cycles. They showed: i) noticeable changes in the frequency dependencies of the imaginary (ε^'') and real (ε^') parts of the complex dielectric permittivity that occurred around temperatures of the characteristic endothermic events detected by the calorimetry, and ii) significant fluctuations of ε^'' and ε^' at pressures attributed to the respective phase transitions. Moreover, the pressure coefficients of the phase transition temperatures were estimated to be approximately equal to 0.2 K/MPa for both compounds. In turn, volume variation (ΔV) at the PC (II)-PC (III) and PC (III)-PC (IV) transition temperatures for 1-NH₂-ADM was essentially different than ΔV for the PC-OC transition in 1-OH-ADM.

Harnessing Plasticity in an Amine-Borane as a Piezoelectric and Pyroelectric Flexible Film

We demonstrate that trimethylamine borane can exhibit desirable piezoelectric and pyroelectric properties. The material was shown to be able operate as a flexible film for both thermal sensing, thermal energy conversion and mechanical sensing with high open circuit voltages (>10 V). A piezoelectric coefficient of d₃₃ ≈10-16 pC N^-1 , and pyroelectric coefficient of p≈25.8 μC m^-2  K^-1 were achieved after poling, with high pyroelectric figure of merits for sensing and harvesting, along with a relative permittivity of ϵ 33 σ ≈ 6.3.

The dynamics of the plastically crystalline phase of cyanoadamantane revisited by NMR line shape analysis and field-cycling relaxometry

The dynamics of cyanoadamantane (CN-ADA) in its plastically crystalline phase encompasses three processes: overall tumbling of the rigid molecule, rotation around the molecular symmetry axis, and vacancy diffusion. This makes CN-ADA a prototypical case to be studied by field-cycling as well as by conventional NMR relaxometry. Data are collected from 430 K down to about 4 K and frequencies in the range of 10 kHz-56 MHz are covered. The overall tumbling is interpreted as a cooperative jump process preceding along the orthogonal axis of the cubic lattice and exhibiting a temperature independent non-Lorentzian spectral density. Consequently, a master curve is constructed, which yields model-independent correlation times, which agree well with those reported in the literature. It can be interpolated by a Cole-Davidson function with a width parameter β_CD = 0.83. The uniaxial rotation persisting in the glassy crystal (T < T_g = 170 K) is governed by a broad distribution of activation energies, g(E). In this case, the standard master curve construction applied for the overall tumbling, for example, fails, as the actually probed distribution of correlation times G(ln τ) strongly changes with temperature. We suggest a scaling method that generally applies for the case that a relaxation process is determined by a distribution of thermally activated processes. Frequency as well as temperature dependence of the relaxation rate can be used to reconstruct g(E). In addition, g(E) is extracted from the proton line-shape, which was measured down to 4 K. Vacancy diffusion governs the relaxation dispersion at highest temperatures; yet, a quantitative analysis is not possible due to instrumental limitations.

Phase behavior and design rules for plastic colloidal crystals of hard polyhedra via consideration of directional entropic forces

Plastic crystals - like liquid crystals - are mesophases that can exist between liquids and crystals and possess some of the characteristic traits of each of these states of matter. Plastic crystals exhibit translational order but orientational disorder. Here, we characterize the phase behavior in systems of hard polyhedra that self-assemble plastic face-centered cubic (pFCC) colloidal crystals. We report a first-order transition from a pFCC to a body-centered tetragonal (BCT) crystal, a smooth crossover from pFCC to an orientationally-ordered FCC crystal, and an apparent orientational glass transition wherein long-range order fails to develop from a plastic crystal upon an increase in density. Using global order parameters and local environment descriptors, we describe how particle shape influences the development of orientational order with increasing density, and we provide design rules based on the arrangement of facets for engineering plastic crystal behavior in colloidal systems.

Lead halide perovskites: Crystal-liquid duality, phonon glass electron crystals, and large polaron formation

Lead halide perovskites have been demonstrated as high performance materials in solar cells and light-emitting devices. These materials are characterized by coherent band transport expected from crystalline semiconductors, but dielectric responses and phonon dynamics typical of liquids. This "crystal-liquid" duality implies that lead halide perovskites belong to phonon glass electron crystals, a class of materials believed to make the most efficient thermoelectrics. We show that the crystal-liquid duality and the resulting dielectric response are responsible for large polaron formation and screening of charge carriers, leading to defect tolerance, moderate charge carrier mobility, and radiative recombination properties. Large polaron formation, along with the phonon glass character, may also explain the marked reduction in hot carrier cooling rates in these materials.

Development of structural complexity by liquid-crystal self-assembly

Since the discovery of the liquid-crystalline state of matter 125 years ago, this field has developed into a scientific area with many facets. This Review presents recent developments in the molecular design and self-assembly of liquid crystals. The focus is on new exciting soft-matter structures distinct from the usually observed nematic, smectic, and columnar phases. These new structures have enhanced complexity, including multicompartment and cellular structures, periodic and quasiperiodic arrays of spheres, and new emergent properties, such as ferroelctricity and spontaneous achiral symmetry-breaking. Comparisons are made with developments in related fields, such as self-assembled monolayers, multiblock copolymers, and nanoparticle arrays. Measures of structural complexity used herein are the size of the lattice, the number of distinct compartments, the dimensionality, and the logic depth of the resulting supramolecular structures.

Crystal structures of R-thiocamphor

The crystal structure of the globular organic molecule R-thiocamphor (C10H16S) has been studied at ambient and low temperatures by high resolution powder X-ray diffraction using synchrotron radiation. At around 260 K thiocamphor transforms on cooling to an ordered orthorhombic structure from the orientationally-disordered body-centered cubic phase with a = 8.1477(1)Å at room temperature. The structure of the low-temperature phase was solved by auto-indexing, then by intensity extraction and direct methods using the EXPO package and refined by the Rietveld Method. The low temperature structure has a = 9.0490(1)Å, b = 14.5887(3)Å, c = 7.2931(1)Å, space group P212121, Z = 4, ρ calc = 1.161 g cm-3.

Orientational order and rotational relaxation in the plastic crystal phase of tetrahedral molecules

A methodology recently introduced to describe orientational order in liquid carbon tetrachloride is extended to the plastic crystal phase of XY4 molecules. The notion that liquid and plastic crystal phases are germane regarding orientational order is confirmed for short intermolecular distances but is seen to fail beyond, as long range orientational correlations are found for the simulated solid phase. It is argued that, if real, such a phenomenon may not to be accessible with direct (diffraction) methods due to the high molecular symmetry. This behavior is linked to the existence of preferential orientation with respect to the fcc crystalline network defined by the centers of mass. It is found that the dominant class accounts, at most, for one-third of all configurations, with a feeble dependence on temperature. Finally, the issue of rotational relaxation is also addressed, with an excellent agreement with experimental measures. It is shown that relaxation is nonhomogeneous in the picosecond range, with a slight dispersion of decay times depending on the initial orientational class. The results reported mainly correspond to neopentane over a wide temperature range, although results for carbon tetrachloride are included, as well.

Multiple crossed isopolymorphism: two-component systems CCl4 +CBr2Cl2 and CBrCl3 + CBr2Cl2; inference of a metastable rhombohedral phase of CBr2Cl2

The phases diagrams of the two-component systems CCl4 +CBr2Cl2 and CBrCl3 + CBr2Cl2 have been determined by means of X-ray powder diffraction and thermal analysis techniques from the low-temperature ordered phase to the liquid state. The isomorphism relationship between the stable orientationally disordered (OD) face-centered cubic (FCC) phases of CBrCl3 and CBr2Cl2 and the metastable OD FCC phase (monotropic behavior with respect to the OD rhombohedral stable phase) of CCl4 has been put into evidence throughout the continuous evolution of the lattice parameters and the existence of the two-phase equilibrium [FCC + L] for the whole range of composition in both two-component systems. This equilibrium interferes, for the CCl4 +CBr2Cl2 system, with a rhombohedral (R) plus liquid ([R + L]) equilibrium giving rise to a peritectic invariant. In addition, whatever the system, [R + FCC] equilibrium also interferes with the low-temperature equilibria between the low-temperature monoclinic (C2/c) phase and the OD R and FCC phases. In regards to the low-temperature monoclinic phases, isomorphism is evidenced, and by means of Rietveld profile refinement, any ordering of the molecules by varying the fractional occupancy of the halogen sites has been detected. The thermodynamic assessment, conducted by means of the concept of crossed isopolymorphism, coherently reproduces all the involved equilibria and provides a coherent set of data for the thermodynamic properties of nonexperimentally available phase transitions of pure compound CBr2Cl2 which enables us to obtain the topological properties of its pressure-temperature phase diagram and to infer the existence of a high-pressure R phase for such a compound.

Nuclear magnetic resonance and dielectric investigations of molecular motions in a glassy crystal: The mixed compound (CN-adm)0.75(Cl-adm)0.25

The dynamic properties of plastic crystalline mixed adamantane's derivatives namely cyanoadamantane (75%) and chloroadamantane (25%) were investigated by dielectric and nuclear magnetic resonance (NMR) spectroscopy, covering a spectral range of 12 decades in the temperature range 110-420 K. Phase transformations were studied and dynamical parameters of the plastic (I), glassy (Ig), and ordered (III) phases were determined and compared with those of pure compounds. The dynamics of the supercooled plastic phase is characterized by an alpha-process exhibiting an Arrhenius behavior which classified the mixed compound as a strong glass former. In the plastic phase, NMR relaxation times were interpreted by using a Frenkel model, which takes into account structural equilibrium positions. This model explains adequately the experimental results by considering two molecular motions. In both the glassy state and plastic phase the motional parameters agree with those of 1-cyanoadamantane. On the contrary, in the ordered phase, the motional parameters related to the uniaxial rotation of chloroadamantane molecules indicate an accelerated motion.

Olfactory perceptual learning: the critical role of memory in odor discrimination

The major problem in olfactory neuroscience is to determine how the brain discriminates one odorant from another. The traditional approach involves identifying how particular features of a chemical stimulus are represented in the olfactory system. However, this perspective is at odds with a growing body of evidence, from both neurobiology and psychology, which places primary emphasis on synthetic processing and experiential factors--perceptual learning--rather than on the structural features of the stimulus as critical for odor discrimination. In the present review of both psychological and sensory physiological data, we argue that the initial odorant feature extraction/analytical processing is not behaviorally/consciously accessible, but rather is a first necessary stage for subsequent cortical synthetic processing which in turn drives olfactory behavior. Cortical synthetic coding reflects an experience-dependent process that allows synthesis of novel co-occurring features, similar to processes used for visual object coding. Thus, we propose that experience and cortical plasticity are not only important for traditional associative olfactory memory (e.g. fear conditioning, maze learning, and delayed-match-to-sample paradigms), but also play a critical, defining role in odor discrimination.