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In this work, we compare the dielectric properties of single-crystal and pelletized perovskite-like formamidinium-templated inorganic–organic hybrid [NH2CHNH2][Mn(HCOO)3] (FAMn) at ambient pressure. Furthermore, we report pressure-dependent dielectric measurements of pelletized FAMn that allow us to determine the equation of state, as well as changes in the dielectric properties and the rotational motion of formamidinium (FA) cations in the crystal lattice under mechanical stress. Our experiments show that the mechanism of FA cation motion is complex, encompassing six different reorientation paths around three two-fold symmetry axes. Multiple reorientation possibilities of FA cations inside the crystal lattice lead to their irregular molecular dynamics. High-pressure experiments allowed observing that formate anions together with manganese cations form a rigid skeleton, which is stable over a wide temperature and pressure range. Consequently, the hydrostatic pressure exerts a surprisingly small influence on the FA dynamics compared to the related compounds containing the same type of molecular cations. Finally, we reveal a different role of interactions between the A-site organic and the B-site metal cations with the X-site linkers in controlling the dynamics of the A-site moieties under various pressure–temperature conditions. Our results are supported by the molecular dynamics simulations.

In the past decades, para-terphenyls have been attracting tremendous attention due to their polymorphism and conformational diversity. In this work we report the synthesis, crystal structure, polymorphism and dielectric properties of two 2,2′′-bis-substituted para-terphenyls: 2,2′′-bis(hydroxymethyl)-para-terphenyl and 2,2′′-bis(acetyloxymethyl)-para-terphenyl. On the basis of calorimetric and X-ray studies, we showed that the latter compound occurs in at least four polymorphic forms with melting points equal to 364, 345, 341 and 326 K, differentiated also in terms of thermodynamic stability and crystal symmetry. The most stable polymorph I is characterized by the P21/n space group. 2,2′′-Bis(hydroxymethyl)-para-terphenyl crystallizes in the monoclinic P21/c space group. Both 2,2′′-bis-substituted para-terphenyls can undergo vitrification, which is a highly exceptional feature for this class of chemical compounds and has not been reported before. Consequently, the molecular dynamics and conformational changes in the glassy and supercooled liquid states were analyzed by means of IR and broadband dielectric spectroscopy. Two relaxation processes were observed for both compounds: structural α-relaxation, connected with reorientational motions of molecules in supercooled liquid, and intermolecular γ-relaxation, ascribed to rotational motions of substituents of the para-terphenyl skeleton. Taking into account the ongoing discussion about the conformational diversity of the para-terphenyl skeleton, we showed that although free rotation of benzene rings is suppressed, the molecules in the glassy and liquid states can adopt both twisted and helical conformations, which results in diversity of the polymorphic forms.

The rich dielectric properties of hybrid formate frameworks are tightly related to the dipolar dynamics and phase transitions of these compounds. A complete understanding of such phenomena requires experiments performed under non-standard conditions. Herein, we report high-pressure dielectric properties of anisotropic single crystals of a 1,4-diaminobutane zinc-formate framework [NH3(CH2)4NH3][Zn2(HCOO)6]. Experiments performed over wide temperature and pressure ranges allowed us to determine the equation of state of this compound. We reveal that a reversible structural phase transition, associated with a change of the crystal symmetry and partial ordering of the 1,4-diaminobutane cations, is characterized by a linear increase of the transition temperature with increasing pressure (dT0/dp = 5.8(2) K GPa−1). The X-ray diffraction and high-pressure dielectric experiments show that zinc cations together with formate anions form a rigid cage with crystal axis-dependent response to pressure, which affects the dynamics of 1,4-diaminobutane cations. We also demonstrate that the structural phase transition of this compound occurs at a constant value of the dielectric relaxation time (isochronic conditions) further revealing a tight relation between the framework and molecular cations. Our experimental results are supported by the molecular dynamics simulations.

In this work, we report the synthesis, unexpected glass-forming properties, molecular dynamics and conformational analysis of two thiacrown ethers: 6-methyl-2,3-dihydro-1,4-benzodithiine (1), with a six-membered heterocyclic ring, and macrocyclic 2,3-(4'-methylbenzo)-1,4-dithia-7-oxacyclononane (2). Based on the calorimetric studies, we showed that compound 1 is a viscous liquid at room temperature undergoing vitrification at 192 K. Compound 2 is a crystalline solid at room temperature characterized by a melting point at 331 K; however, it can be vitrified with ease after being melted by cooling down to 224 K. This gave us the unique possibility to analyze the dielectric response and to follow the molecular dynamics in supercooled liquid and glassy states for each thiacrown ether. Two relaxation processes were found for compound 1, which are structural α-relaxation, connected with the collective rotational motions of molecules in a liquid, and a low-temperature secondary γ-process, resulting from conformational changes in the heterocyclic ring. Beside these two relaxation processes, an additional intermolecular β-process of JG type was detected in the case of compound 2. Finally, based on the analysis of the thermal evolution of the Kirkwood-Fröhlich factor, it has also been shown that thiacrown ethers may be characterized by a local ordering between neighboring molecules in the supercooled liquid state.

Metal–organic frameworks (MOFs), formed of metal centers coupled by organic molecules, exhibit inherent porosity and crystallinity. Although these systems have been examined for many potential applications, their multiferroic properties remain poorly understood. One of the approaches to get an insight into the ferroic features of these materials is the study of frameworks templated by protonated organic molecules possessing rotational and conformational freedom. The embedded organic cation with an internal electric dipole moment may be ordered on cooling, leading to the appearance of ferroelectric-like properties. Herein, we report a study of two structural phase transitions in one such material, the methylhydrazinium zinc formate [CH3NH2NH2][Zn(HCOO)3] framework (MHyZn), using various experimental techniques. High-temperature (HT) improper ferroelectric phase transition and low-temperature (LT) biferroic phase transition in MHyZn single crystals have been investigated in a broad temperature range using X-ray diffraction, longitudinal ultrasonic velocity and attenuation measurements, pyrocurrent measurements, and broadband dielectric spectroscopy. In addition, noncentrosymmetric to centrosymmetric structural transformation was confirmed for the HT phase transition with the use of temperature-resolved second harmonic generation on powder samples, while polarizing microscopy observation of a large deformation of ferroelastic-like domains attested to the noncontinuous character of the crystallographic system transformation (trigonal to triclinic) in the LT biferroic phase transition. Finally, we proved that the spontaneous electric polarization occurs due to freezing of motion around the 2- and 3-axes of the methylhydrazinium cation.

Metal–organic frameworks (MOFs), in which metal clusters are coupled by organic moieties, exhibit inherent porosity and crystallinity. Although these systems have been examined for vast potential applications, the elementary proton conduction in anhydrous MOFs still remains elusive. One of the approaches to deal with this problem is the utilization of protic organic molecules, to be accommodated in the porous framework. In this work we report the temperature-dependent crystal structure and proton conduction in [C2H5NH3][Na0.5Fe0.5(HCOO)3] metal–organic frameworks using X-ray diffraction and broadband dielectric spectroscopic techniques. The detailed analysis of the crystal structure reveals disorder of the terminal ethylene groups in the polar phase (space group Pn). The structural phase transition from Pn to P21/n at T ≈ 363 K involves the distortion of the metal formate framework and ordering of EtA+ cations due to the reduction of the cell volume. The dielectric data have been presented in the dynamic window of permittivity formalism to understand the ferroelectric phase transition. The relaxation times have been estimated from the Kramers–Kronig transformation of the dielectric permittivity. A Grotthuss type mechanism of the proton conduction is possible at low temperatures with the activation energy of 0.23 eV. This type of experimental observation is expected to provide new prospective on the fundamental aspect of elementary proton transfer in anhydrous MOFs.

The fundamental aspects of the relaxation dynamics in niccolite-type, mixed valence metal-organic framework, multiferroic [(CH3)2NH2][Fe(3+)Fe(2+)(HCOO)6] single crystals have been reported using dielectric relaxation spectroscopy covering eight decades in frequency (10(-2) ≤ f ≤ 10(6)) in the temperature range 120 K ≤ T ≤ 250 K. The compound shows antiferroelectric to paraelectric phase transition near T = 154 K with the relaxor nature of electric ordering. The temperature dependent dielectric response in modulus representation indicates three relaxation processes within the experimental window. The variable range hopping model of small polarons explains the bulk non-Debye type conductivity relaxation. The fastest relaxation with activation energy Ea = 0.17 eV is related to progressive freezing of the reorientation motions of DMA(+) cations. X-ray diffraction data revealed that complete freezing of orientational and translational motions of DMA(+) cations occurs well below phase transition temperature. These experimental observations are fundamentally important for the theoretical explanation of relaxation dynamics in niccolite-type metal-organic frameworks.

We report the synthesis, crystal structure, dielectric, vibrational and emission spectra of heterometallic MOFs, [C2H5NH3][Na0.5Cr0.5(HCOO)3] (EtANaCr), [C2H5NH3][Na0.5Al0.5(HCOO)3] (EtANaAl) and [C2H5NH3][Na0.5Al0.475Cr0.025(HCOO)3] (EtANaAlCr). These compounds crystallize in non-centrosymmetric monoclinic polar structures (space group Pn) and undergo order–disorder phase transitions upon heating to the monoclinic centrosymmetric structure (space group P21/n) at 369 (EtANaAl) and 373 K (EtANaCr). In principle, they are ferroelectric below these temperatures. In the high-temperature phase, ethylammonium (EtA+) cations are dynamically disordered over two symmetrically independent positions while upon cooling they begin to order. The ordering is accompanied by distortion of the metal formate framework. The hydrogen bonds (HBs) between the NH3+ group and NaO6 octahedral units are more robust than between the NH3+ group and CrO6 (AlO6) octahedral units and this feature explains a much stronger distortion of the former units and a weak effect of a trivalent cation type on the phase transition temperature. The dielectric studies have confirmed the occurrence of phase transitions of dipolar character and dipole relaxation processes. The optical studies show that EtANaCr and EtANaAlCr exhibit efficient Cr(III)-based emission characteristics for intermediate-ligand field strength.

We report temperature-dependent electric, IR and Raman studies of niccolite-type formate frameworks templated by protonated 1,4-diaminobutane. Our results show that the zinc-analogue exhibits a first-order phase transition close to 240 K. Single-crystal dielectric data show a much stronger anomaly at the phase transition for e′ along the a-direction compared to the c-direction. They also reveal that dipole relaxation exists in bnZn. Pronounced temperature-dependence observed for bending and torsion modes of the NH3+ groups proves that ordering of protonated amine plays a major role in the phase transition mechanism. The ordering is associated with distortion of the zinc formate framework but the number of observed vibrational modes is much smaller than expected assuming 36-fold multiplication of the unit cell below TC. It is also much smaller than reported for the Mn-analogue, which exhibits only a two-fold increase of the unit cell below TC. We discuss the origin of this behavior. Our results also show that the Co-analogue exhibits a similar phase transition to its Zn-counterpart. However, the observed narrowing and splitting of the corresponding bands is significantly smaller, suggesting weaker distortion of the framework and the presence of some disorder for this compound even at 5 K. The Raman and IR spectra of the Fe-analogue show weak narrowing of bands upon cooling, indicative of statistical freezing of the protonated amine at low temperatures. We also report high-pressure Raman scattering studies of the zinc-analogue. This study revealed a pressure-induced reversible phase transition between 3.4 and 4.1 GPa. Large shifts and splitting of modes corresponding to the vibrations of HCOO− ions associated with weak changes of the protonated amine prove that the major contribution to the phase transition mechanism comes from distortion of the zinc formate framework.

In this paper, we combine Broadband Dielectric Spectroscopy (BDS) at ambient and high pressure, and positron annihilation lifetime spectroscopy (PALS) data of 2-ethylhexanol in the bulk state and when infiltrated in native silica nanopores to elucidate the relative role of surface effects on the Debye and structural relaxation processes under 2D spatial constraints. We show that the two processes have different sensitivities to (i) the changes in density as quantified by the EV/Hp ratio and (ii) the degree of confinement. Significant enhancement of the dynamics of the confined molecules at low temperatures is related to the vitrification of the interfacial molecules (Tg,int) affecting the packing density of the core molecules. This is corroborated by the PALS measurements, which demonstrated that the effective volume for the confined samples is slightly higher and seems to be temperature invariant below Tg,int. Consequently, negative pressure systematically develops with lowering temperature reaching values of -100 and -110 MPa (depending on the pore size) at the glass transition temperature. This result offers a better understanding of the counterbalance between surface and finite size effects as well as the role of negative pressure in controlling the dynamics and the glass transition of liquids under 2D spatial restrictions.

Kinetics and dynamics of the curing of bisphenol-A diglycidyl ether (DGEBA) with various agents i.e. cyclohexylamine, aniline and 2-ethylhexylamine, were investigated by means of Broadband Dielectric Spectroscopy (BDS) over a wide range of thermodynamic conditions. We proposed a novel method of dielectric data analysis to extract information on the progress of the curing reaction. This approach was validated by complementary calorimetry measurements. High pressure studies indicated that compression significantly reduces the time of the reaction. At the same time, a decrease in monomer conversion and the glass transition temperature of the recovered product was observed. This was due to vitrification of the system that occurs in a relatively short time. We found a linear correspondence between the time of the polymerization after which the investigated system undergoes glass transition and the degree of monomer conversion. Additionally, activation volumes for the investigated reactions were determined and found to lie in the range −18 and −38 cm3 mol−1. The calculated values are lower than ΔV estimated for the ring opening reaction provided in the literature. However our data unquestionably showed that this quantity depends strongly on both the chemical structure of the substrates as well as thermodynamic conditions.

One of hallmark features of glass forming ultraviscous liquids is the decoupling between translational and orientational dynamics. This report presents studies of this phenomenon in glycerol, a canonical molecular glass former, heading for the impact of two exogenic factors: high pressures up to extreme 1.5 GPa and silver (Ag) nanoparticles (NP). The analysis is focused on the fractional Debye-Stokes-Einstein (FDSE) relation $\sigma(T,P)*(\tau(T,P))^S = const$, linking DC electric conductivity $(\sigma)$ and primary $(\alpha)$ relaxation time $(\tau_\alpha)$. In glycerol and its nanocolloid (glycerol with Ag-NP) under atmospheric pressure only the negligible decoupling $(S = 1)$ was detected. However, in the compressed nanocolloid a well-defined transformation (at P = 1.2 GPa) from $S \thickapprox 1$ to the very strongly decoupled dynamics $(S \thickapprox 0.5)$ occurred. For comparison, in pressurized 'pure' glycerol the stretched shift from $S \thickapprox 1$ to $S \thickapprox 0.7$ took place. This report presents also the general discussion of FDSE behavior in ultraviscous liquids, including the new link between FDSE exponent, fragility and the apparent activation enthalpy and volume.

Metabolic fingerprinting was performed on a set of botanical extracts to compare the extraction efficiency of different solvents to inform the construction of phytochemical libraries. We compared the extraction efficiency, examining both yield and chemical diversity, of eight single-solvent extractions prepared in parallel and using solvent–solvent partitioning. Three-dimensional data were reduced into features, which were used as unbiased metrics to identify solvents that would produce botanical extracts with the greatest chemical diversity. Chemical diversity and extract yield did not necessarily increase together. For each species and tissue, the total number of observable chemical features closely approached maximum values when three different single-solvent extractions were performed in parallel. The dynamic range of detectable compounds in plant extracts was increased significantly by performing solvent partitioning. Overall, maximum chemical diversity in a plant extract was most efficiently approached if solvent partitioning was performed on an extract made with 70% ethanol. We have shown that using metabolic fingerprinting is useful for assessing compound diversity in complex plant extracts.

The nematic order parameter S = 〈P2(cosθ)〉 was calculated from the dielectric relaxation time characterizing the molecular rotation around the short axes using formulae derived by Coffey et al. (W. T. Coffey, Yu. P. Kalmykov and J. P. Waldron, Liq. Cryst., 1995, 18, 677; W. T. Coffey, D. S. F. Crothers, Yu. P. Kalmykov and J. P. Waldron, Physica A, 1995, 213, 551; Yu. P. Kalmykov and W. T. Coffey, Liq. Cryst., 1998, 25, 329). In order to test the approximations on which the theory is based (rod-like shape of molecules, the diffusional model of reorientational motion in the presence of an ordering Maier–Saupe potential) several nematogens were chosen for which the S(T) behaviour is known from other experimental techniques as well. These molecules (1OCB, 5OCB, 7CB, 5PCH, 3CCH, 5CCH and 7CCH) have rigid or flexible cores, strongly polar –CN group at one side and the alkyl or alkoxy tail at the other side. A common analysis of the results coming from different experimental methods show large differences between data. Using the Haller-type formula a re-scaling of the S-values was proposed which reduced the scatter of experimental sets of points to a reasonable range. It was established that the Coffey et al. approach leads to the order parameter being consistent with other data if the molecular cores are sufficiently rigid.

Fourier Transform Infra Red (FTIR) and Broadband Dielectric Spectroscopy (BDS) are combined to study both the intra- and inter-molecular dynamics of two isomers of glass forming fucose, far below and above the calorimetric glass transition temperature, T(g). It is shown that the various IR-active vibrations exhibit in their spectral position and oscillator strength quite different temperature dependencies, proving their specific signature in the course of densification and glass formation. The coupling between intra- and inter molecular dynamics is exemplified by distinct changes in IR active ring vibrations far above the calorimetric glass transition temperature at about 1.16T(g), where the dynamic glass transition (α relaxation) and the secondary β relaxation merge. For physically annealed samples it is demonstrated that upon aging the different moieties show characteristic features as well, proving the necessity of atomistic descriptions beyond coarse-grained models.