Enantiotropic Phase Transition Research Articles

Alkylbenzoic and Alkyloxybenzoic Acid Blending for Expanding the Liquid Crystalline State and Improving Its Rheology.

Thermotropic mesogens typically exist as liquid crystals (LCs) in a narrow region of high temperatures, making lowering their melting point with the temperature expansion of the mesophase state an urgent task. Para-substituted benzoic acids can form LCs through noncovalent dimerization into homodimers via hydrogen bonds, whose strength and, consequently, the temperature region of the mesophase state can be potentially altered by creating asymmetric heterodimers from different acids. This work investigates equimolar blends of p-n-alkylbenzoic (kBA, where k is the number of carbon atoms in the alkyl radical) and p-n-alkyloxybenzoic (kOBA) acids by calorimetry and viscometry to establish their phase transitions and regions of mesophase existence. Non-symmetric dimerization of acids leads to the extension of the nematic state region towards low temperatures and the appearance of new monotropic and enantiotropic phase transitions in several cases. Moreover, the crystal-nematic and nematic-isotropic phase changes have a two-step character for some acid blends, suggesting the formation of symmetric and asymmetric associates from heterodimers. The mixing of 6BA and 8OBA most strongly extends the region of the nematic state towards low temperatures (from 95-114 °C and 108-147 °C for initial homodimers, respectively, to 57-133 °C for the resulting heterodimer), whereas the combination of 4OBA and 5OBA gives the most extended high-temperature nematic phase (up to 156 °C) and that of 6BA and 9OBA (or 12OBA) provides the existence of a smectic phase at the lowest temperatures (down to 51 °C).

Linear and Bent Cp*2 Si: Reversible Phase Transition of a Key Molecule.

The solid-state structure of decamethylsilicocene Cp*2 Si with a bent and a linear molecule in the same unit cell was so far considered an exception in relation to the structures of its all-bent heavier analogues Cp*2 E with E=Ge, Sn, Pb. Here, we present the solution to this conundrum by reporting a low-temperature phase, where all three symmetrically independent molecules are present in a bent formation. This reversible enantiotropic phase transition occurs in the temperature range between 80 K and 130 K and provides a rationale for the unexpected linear molecule based in entropy beyond hand-waving explanations such as electronic reasons or packing effects.

Engineering the Thermodynamic Stability and Metastability of Mesophases of Colloidal Bipyramids through Shape Entropy.

We report several types of entropy-driven phase transition behaviors in hard bipyramid systems using Monte Carlo simulations. Bipyramidal nanoparticle shapes are synthesizable from gold and silver, with sizes ranging from tens to hundreds of nanometers. We report numerous colloidal crystalline phases with varying symmetries and complexities as the bipyramid aspect ratio and base polygon are varied. Some bipyramids are mesogenic and undergo either monotropic or enantiotropic phase transitions. We show that such mesophase behavior can be modulated by tuning the bipyramid aspect ratio. In addition, we report stepwise kinetic crystallization and melting pathways that occur via an intermediate mesophase as the system gains or loses order in successive stages. Our results demonstrate that complex phase transition behavior involving mesophases can be driven by entropy alone. Importantly, our results can guide the synthesis of bipyramid shapes able to assemble target structures and can be used to engineer the kinetic pathways to and from those structures to involve or avoid mesophases.

Enantiotropy of Simvastatin as a Result of Weakened Interactions in the Crystal Lattice: Entropy-Driven Double Transitions and the Transient Modulated Phase as Seen by Solid-State NMR Spectroscopy.

In crystalline molecular solids, in the absence of strong intermolecular interactions, entropy-driven processes play a key role in the formation of dynamically modulated transient phases. Specifically, in crystalline simvastatin, the observed fully reversible enantiotropic behavior is associated with multiple order–disorder transitions: upon cooling, the dynamically disordered high-temperature polymorphic Form I is transformed to the completely ordered low-temperature polymorphic Form III via the intermediate (transient) modulated phase II. This behavior is associated with a significant reduction in the kinetic energy of the rotating and flipping ester substituents, as well as a decrease in structural ordering into two distinct positions. In transient phase II, the conventional three-dimensional structure is modulated by periodic distortions caused by cooperative conformation exchange of the ester substituent between the two states, which is enabled by weakened hydrogen bonding. Based on solid-state NMR data analysis, the mechanism of the enantiotropic phase transition and the presence of the transient modulated phase are documented.

Syntheses and Properties of New Smectic Reactive Mesogens and Their Application in Guest-Host Polarizer

We designed and synthesized new smectic liquid crystalline monomers containing photopolymerizable acrylate groups and a mesogenic core unit composed of two benzene rings, one pyrimidine-based heteroaromatic ring and an acetylene linking group. The structures of monomers were characterized by 1H nuclear magnetic resonance spectroscopy, and their mesomorphic behaviors were investigated by differential scanning calorimetry and polarized optical microscopy. Both of monomers showed an enantiotropic phase transition behavior with exhibiting both nematic phase and highly ordered smectic phase. Both of monomers showed smectic phases during the cooling process in the temperature range of 144.2–94.0 °C and 154.0–85.8 °C, respectively. Neither of these compounds showed a homogeneous alignment even at various temperatures, and therefore, their 50:50 mixture was chosen for the preparation of guest-host mixture because of its lowest crystallization point, wide smectic phase temperature range and superior aligning property. Polymerizable “host-guest” mixture formulated from the host liquid crystalline monomer mixture, dichroic dye and additives was injected into the sandwiched glass substrates having rubbed alignment layers at the isotropic temperature. Subsequent in-situ photopolymerization by UV irradiation at room temperature successfully resulted in a thin film polarizer with good polarizing properties. The fabricated guest-host polarizer showed a dichroic ratio (DR) of 13.1 and a degree of polarization (DOP) of 95.5% with the thickness of 5 µm.

Polymorphism in Simvastatin: Twinning, Disorder, and Enantiotropic Phase Transitions.

Simvastatin is one of the most widely used active pharmaceutical ingredients for the treatment of hyperlipidemias. Because the compound is employed as a solid in drug formulations, particular attention should be given to the characterization of different polymorphs, their stability domains, and the nature of the phase transitions that relate them. In this work, the phase transitions delimiting the stability domains of three previously reported simvastatin forms were investigated from structural, energetics, and dynamical points of view based on single crystal X-ray diffraction (SCXRD), hot stage microscopy (HSM), and differential scanning calorimetry (DSC) experiments (conventional scans and heat capacity measurements), complemented with molecular dynamics (MD) simulations. Previous assignments of the crystal forms were confirmed by SCXRD: forms I and II were found to be orthorhombic ( P212121, Z'/ Z = 1/4) and form III was monoclinic ( P21, Z'/ Z = 2/4). The obtained results further indicated that (i) the transitions between different forms are observed at 235.9 ± 0.1 K (form III → form II) and at 275.2 ± 0.2 K (form II → form I) in DSC runs carried out at 10 K min-1 and close to these values when other types of techniques are used (e.g., HSM). (ii) They are enantiotropic (i.e., there is a transition temperature relating the two phases before fusion at which the stability order is reversed), fast, reversible, with very little hysteresis between heating and cooling modes, and occur under single crystal to single crystal conditions. (iii) A nucleation and growth mechanism seems to be followed since HSM experiments on single crystals evidenced the propagation of an interface, accompanied by a change of birefringence and crystal contraction or expansion (more subtle in the case of form III → form II), when the phase transitions are triggered. (iv) Consistent with the reversible and small hysteresis nature of the phase transitions, the SCXRD results indicated that the molecular packing is very similar in all forms and the main structural differences are associated with conformational changes of the "ester tail". (v) The MD simulations further suggested that the tail is essentially "frozen" in two conformations below the III → II transition temperature, becomes progressively less hindered throughout the stability domain of form II, and acquires a large conformational freedom above the II → I transition. Finally, the fact that these transitions were found to be fast and reversible suggests that polymorphism is unlikely to be a problem for pharmaceutical formulations employing crystalline simvastatin because, if present, the III and II forms will readily convert to form I at ambient temperature.

Enantiotropic phase transition in a molecular solid involving Z′ = 12

Enantiotropic phase transition in a molecular solid involving <i>Z</i>′ = 12

Anisotropic Thermal Expansion of p-Terphenyl: a Self-Assembled Supramolecular Array of Poly-p-phenyl Nanoribbons

The recent discovery of superconductivity in a metallic aromatic hydrocarbon, alkali-doped p-terphenyl, has attracted considerable interest. The critical temperature T c ranges from few to 123 K, the record for organic superconductors, due to uncontrolled competition of multiple phases and dopants concentrations. In the proposed mechanism of Fano resonance in a superlattice of quantum wires with coexisting polarons and Fermi particles, the lattice properties play a key role. Here, we report a study of the temperature evolution of the parent compound p-terphenyl crystal structure proposed to be made of a self-assembled supramolecular network of nanoscale nanoribbons. Using temperature-dependent synchrotron X-ray diffraction, we report the anisotropic thermal expansion in the ab plane, which supports the presence of a nanoscale network of one-dimensional nanoribbons running in the b-axis direction in the P21/a structure. Below the enantiotropic phase transition at 193 K, the order parameter of the C-1 structure follows a power law behavior with the critical exponent α = 0.34 ± 0.02 and the thermal expansion of the a-axis and b-axis show major changes supporting the formation of a two-dimensional bonds network. The large temperature range of the orientation fluctuations in a double well potential of the central phenyl has been determined.

The role of dispersion type metalπ interaction in the enantiotropic phase transition of two polymorphs of tris-(thienyl)bismuthine.

Two polymorphs of tris(thienyl)bismuthine Bi(2-C4H3S)3 (1) were isolated upon crystallization from n-hexane at different temperatures. The high temperature form 1-HT crystallized at 269 K in the trigonal space group R3[combining macron], whereas the low temperature form 1-LT crystallized at 245 K in the triclinic space group P1[combining macron]. An enantiotropic phase transition was observed at 250 K showing a transition energy of 1.4 kJ mol-1. Both polymorphs reveal the formation of centrosymmetric dimers that are based on London dispersion type bismuthπ heteroarene interactions. These primary building units show additional dispersion type interactions between neighbouring dimers and as a result 2D networks are formed. DFT calculations on the model systems BiX3π thiophene (X = Cl, Me) verify the hypothesis of a soft and shallow binding potential of the London dispersion type bismuthπ heteroarene interaction, providing an explanation for the reversibility of the phase transition.

Two reversible enantiotropic phase transitions in a pentacoordinate silicon complex with an O,N,O'-tridentate valinate ligand.

Dimethyl[N-(4-oxidopent-3-en-2-ylidene)valinato-κ(3)O,N,O']silicon(IV), C12H21NO3Si, (II), crystallizes in the orthorhombic space group P2₁2₁2₁. The chiral compound undergoes two sharp enantiotropic phase transitions upon cooling. The first transformation occurs at 163 K to yield a unit cell with one axis having double length. This intermediate-temperature form has the monoclinic space group P2₁. The second transition takes place at 142 K and converts the single crystal into the low-temperature form in the orthorhombic space group P2₁2₁2₁. This transition proceeds under tripling of the a axis of the high-temperature form. Both phase transitions are fully reversible and correspond to order-disorder transitions of the isopropyl group of the valine unit in the ligand backbone. The phase transitions presented here raise questions, since they do not fit into the rules of group-subgroup relationships.

Polymorphism of NaVO2F2: a P2₁/c superstructure with pseudosymmetry of P2₁/m in the subcell.

The ADDSYM routine in the program PLATON [Spek (2015). Acta Cryst. C71, 9-18] has helped researchers to avoid structures of (metal-)organic compounds being reported in an unnecessarily low symmetry space group. However, determination of the correct space group may get more complicated in cases of pseudosymmetric inorganic compounds. One example is NaVO2F2, which was reported [Crosnier-Lopez et al. (1994). Eur. J. Solid State Inorg. Chem. 31, 957-965] in the acentric space group P2₁ based on properties but flagged by ADDSYM as (pseudo)centrosymmetric P2₁/m within default distance tolerances. Herein a systematic investigation reveals that NaVO2F2 exists in at least four polymorphs: P2₁, (I), P2₁/m, (II), P2₁/c, (III), and one or more low-temperature ones. The new centrosymmetric modification, (III), with the space group P2₁/c has a similar atomic packing geometry to phase (I), except for having a doubled c axis. The double-cell of phase (III) arises from atomic shifts from the glide plane c at (x, ¼, z). With increasing temperature, the number of observed reflections decreases. The odd l reflections gradually become weaker and, correspondingly, all atoms shift towards the glide plane, resulting in a gradual second-order transformation of (III) into high-temperature phase (II) (P2₁/m) at below 493 K. At least one first-order enantiotropic phase transition was observed below 139 K from both the single-crystal X-ray diffraction and the differential scanning calorimetry analyses. Periodic first-principles calculations within density functional theory show that both P2₁/c superstructure (III) and P2₁ substructure (I) are more stable than P2₁/m structure (II), and that P2₁/c superstructure (III) is more stable that P2₁ substructure (I).

Synthesis and some features of phase behavior of chiral p-alkoxyphenyl glycerol ethers

A series of (S)-3-(4-alkoxyphenoxy)propane-1,2-diols was prepared with the enantiomeric excess 86–92% by Sharpless asymmetric dihydroxylation of 1-alkoxy-4-allyloxybenzenes with an AD-mix-β mixture. R-Enantiomers with enantiomeric excess 97–99% and racemic samples were obtained by reaction of sodium p-substituted phenolates with (R)- and rac-3-chloropropane-1,2-diol. The phase behavior of racemates and scalemates in the produced homolog series of glycerol aryl ethers with hydrocarbon substituents of various length was examined by means of thermomicroscopy. The higher memberes of the homolog series of both racemic and scalemic diols undergo at heating an enantiotropic phase transition to a smectic liquid crystal phase.

Do solid-to-solid polymorphic transitions in DL-norleucine proceed through nucleation?

DL-Norleucine is a molecular crystal exhibiting two enantiotropic phase transitions. The high temperature α ↔ γ transition has been shown to proceed through nucleation and growth [Mnyukh et al., J. Phys. Chem. Solids, 1975, 36, 127]. We focus on the low temperature β ↔ α transition in a combined computational and experimental study. The temperature dependence of the structural and energetic properties of both polymorphic forms is nearly identical. Molecular dynamics simulations and nudged elastic band calculations of the transition process itself, suggest that the transition is governed by cooperative movements of bilayers over relatively large energy barriers.

Polymorphs Related by an Enantiotropic Phase Transition but Having Only Order–Disorder-Level Conformational Differences in a Spiro-Conjugated Imidazolidine Ring Compound

Variable-temperature single-crystal X-ray diffraction analyses of 1,3-diethyl-spiro[imidazolidine-2,2′-[2H]indene]-1′,3′-dione (1) were performed at low temperatures. The temperature dependence of the cell parameters exhibited a discontinuous change between 158 and 153 K, indicating the existence of a phase transition and hence the occurrence of two polymorphs, I and II, which was also confirmed by DSC analysis: reversible exothermic and endothermic peaks with ΔH = 2.1 kJ mol–1 were observed in the cooling and heating processes, respectively. The molecular packing in I and II at 200 and 90 K, respectively, remained essentially unchanged, retaining the space group Pnma. I and II are conformationally isomorphic crystals, including two crystallographically independent molecules, A and B, with different conformations in a 1:2 ratio. Upon the phase transition from I to II with decreasing temperature, the dynamic disorder of the imidazolidine ring of A in I, which interconverts between quasi-half-chair conformations, is frozen to settle in an envelope conformation, whereas B is retained in a half-chair conformation in both I and II. The close intermolecular contacts in I and II were compared on the basis of Hirshfeld surface analysis. DFT calculations at the B3LYP/6-311G(d,p) level show that the half-chair structure is more stable than the envelope conformer by 2.6 kJ mol–1 in the gas phase. The quasi-half-chair conformation was not obtained as an energy-minimum geometry on the potential energy surface. Temperature-dependent IR spectroscopy along with theoretical calculation using the QST method gave assignments of the carbonyl stretching bands for each conformation. Compound 1 provides a unique example of the isostructural polymorphs in which the structural differences are small, being only at the order–disorder level in the ring conformation; nevertheless, the existence of the enantiotropic phase transition clearly indicates that they are polymorphs.

Low-Temperature Phase Transition in Crystalline Aripiprazole Leads to an Eighth Polymorph

A new crystalline polymorph (Form VIII) of the antidepressant drug aripiprazole has been discovered, resulting from a previously unknown enantiotropic phase transition from Form II at 225 K. This finding makes aripiprazole one of the most polymorphic flexible organic solids currently known, equaling flufenamic acid in number of solved forms (eight polymorphs). Enantiotropic solid–solid phase transitions are relatively uncommon for pharmaceuticals; however, for the aripiprazole system, such phase transitions play a central role in several of its polymorphic transformations. A combination of solid-state density functional theory and single-crystal X-ray diffraction has been used to investigate the energies involved in the formation of Form VIII. This work reveals that Form VIII is stable despite containing molecules with unfavorable conformations. The stability of this polymorph originates from improved intermolecular binding energy due to enhanced London dispersion forces at low temperatures.

Side-Chain Liquid Crystalline Polyacetylenes with Increasing Length of Alkyl Tails: From Highly Ordered Smectic to Smectic C Phase

Phase structures and transitions of a series of side-chain liquid crystalline polyacetylene (SCLCPA) with a short spacer of three methylene units and different lengths of alkyl tails, namely, poly(5-{[(4′-alkyl-4-biphenylyl)carbonyl]oxy}-1-pentyne) (P-3,m, m is the number of the carbon atoms in the alkyl tail, m = 5, 7, 9, 11), were investigated using differential scanning calorimeter, polarized light microscopy, and one- and two-dimensional wide-angle X-ray diffraction. With the short spacer, P-3,m possesses the mesogenic groups on the side chain and polyacetylene backbone coupled together and thus renders sheetlike shape with the width nearly twice of the extended side-chain length. Experimental results reveal that the liquid crystalline (LC) structure of P-3,m is strongly dependent on the side-chain tail length, different from that of other SCLCPAs. For m < 11, several layers of the sheetlike P-3,m molecules stack together to form a smectic A (SmA) block, and the number of molecular layers increases with increasing m. The adjacent SmA blocks slide halfway to each other, leading to a highly ordered smectic phase with frustrated molecular packing at low temperatures. The enantiotropic phase transition sequence of P-3,m (m < 11) follows: highly ordered smectic with additional ordering on the subnanometer scale ↔ highly ordered smectic ↔ semctic C (SmC) ↔ isotropic. However, when m is increased to 11, the packing of sheetlike P-3,11 gives the SmC phase, with the transition sequence of SmC with additional ordering on the subnanometer scale ↔ SmC ↔ isotropic. The phase structures of P-3,m were studied by computer modeling. The phase changing from the highly ordered one to SmC with increasing m may be ascribed to that the P-3,11 molecules intend to maximize the interaction between the biphenyl moieties in neighboring chains. We also investigated the orientation behavior of the highly ordered smectic phase under electric field, wherein a unique striplike texture was well developed, with the side chain and main chain parallel and perpendicular to the electric field, respectively.

A Temperature Dependent X-ray Study of the Order–Disorder Enantiotropic Phase Transition of p-Terphenyl

A temperature dependent (100–296 K) X-ray diffraction study of the structural changes associated with the order–disorder enantiotropic phase transition of crystalline para-terphenyl (PTP) is presented. Reflections were collected every degree through the phase transition region 183–196 K. The temperature dependent X-ray results characterize a transition from a disordered monoclinic phase (P21/a, non-standard monoclinic space group) at room temperature to an ordered triclinic phase (C-1, non-standard space group) with reticular, pseudo-merohedral twinning below 193 K. The present study confirms general conclusions of prior temperature dependent neutron scattering and low temperature X-ray studies, and it makes the following new contributions: (a) quantitative assessment of the temperature dependent residual order present in the high-temperature disordered-monoclinic phase (e.g. weak temperature dependent superlattice reflections in the long-range disordered-monoclinic phase up to 296 K), (b) quantitative measure of the temperature dependent disorder present in the low-temperature ordered-triclinic phase (e.g. down to 188 K), (c) identification of the twin symmetry law (i.e. twin components related to each other by a mirror-plane perpendicular to the b-axis or a twofold rotation parallel to the b-axis of the triclinic cell), (d) the minor/major twin components are found to be temperature dependent in the low temperature ordered-triclinic phase, (e) a revised low temperature triclinic structure (C-1 non-standard space group) with reticular, pseudo-merohedral twinning is reported, with an improved R1 value of 0.039 at 100 K. The low temperature structure retains four unique PTP enantiomers, with improved torsional angles. Temperature dependent X-ray study of the structural changes associated with the order–disorder phase transition of para-terphenyl (e.g. torsion angles, order, disorder, and twinning ratio).

Thermoanalytical Evidence of Metastable Molecular Defects in Form I of Benzamide

Friedrich Wohler and Justus von Liebig discovered the first case of molecular polymorphism when crystallizing benzamide in 1832.(1) Thus far only microcrystalline mixtures of metastable phases with the thermodynamically stable form I were available. Optimizing the conditions for crystal growth now gave access to single crystals of the metastable form III which allowed for a detailed comparison of form I and form III by Raman and IR spectroscopy and thermoanalysis. Surprisingly, in the course of detailed differential scanning calorimetry (DSC) measurements for form I an endothermic peak prior to melting was observed. As proven by temperature dependent powder diffraction, thermal microscopy, molecular dynamic (MD) simulations, and solid-state NMR, this premelting thermosignal is not related to an enantiotropic phase transition of form I into form III with a transition temperature close to the melting point. Rather the thermosignal is related to a significant population of metastable defects in form I. While...

Vapor Multifold Nucleation Rate Surfaces for Case of the Polimorfous Enantiotropic Phase Transitions of a Condenced Phase

Examples of the nucleation rate topologies for systems which are having the enantiotropy polymorphic phase transitions, using the idea of the nucleation rate surface semiempirical design over the phase equilibria diagrams, are presented. It was shown that multichannel vapor nucleation in according the independent set of the nucleation rate surfaces are realized in these cases. The traditional using for these systems one channel approximations of the nucleation rate theories is wrongful. The nucleation rate surfaces designed can be used as illustration of the phase sequence rule, the first version of which was formulated earlier by Ostwald

Concomitant Twinning and Polymorphism of Ti(C5H4tBu)2Cl2

The title compound (Ti(C5H4tBu)2Cl2, 1) exists in three polymorphs and undergoes two enantiotropic phase transitions between them. The low-temperature, non-merohedrally twinned monoclinic phase III...