Transitions In Molecular Crystals Research Articles

Incommensurate Phase in Λ-cobalt (III) Sepulchrate Trinitrate Governed by Highly Competitive N-H⋅⋅⋅O and C-H⋅⋅⋅O Hydrogen Bond Networks.

Phase transitions in molecular crystals are often determined by intermolecular interactions. The cage complex of [Co(C12H30N8)]3+ ⋅ 3 NO3 − is reported to undergo a disorder‐order phase transition at T c1 ≈133 K upon cooling. Temperature‐dependent neutron and synchrotron diffraction experiments revealed satellite reflections in addition to main reflections in the diffraction patterns below T c1. The modulation wave vector varies as function of temperature and locks in at T c3≈98 K. Here, we demonstrate that the crystal symmetry lowers from hexagonal to monoclinic in the incommensurately modulated phases in T c1<T<T c3. Distinctive levels of competitions: trade‐off between longer N−H⋅⋅⋅O and shorter C−H⋅⋅⋅O hydrogen bonds; steric constraints to dense C−H⋅⋅⋅O bonds give rise to pronounced modulation of the basic structure. Severely frustrated crystal packing in the incommensurate phase is precursor to optimal balance of intermolecular interactions in the lock‐in phase.

Cover Feature: Stepwise Stress‐Induced Transformations of Metal‐Organic Polyhedral Cluster‐Based Assemblies: Where Conformational and Supramolecular Features Meet (Chem. Eur. J. 55/2021)

The understanding of multiple polymorphic phase transitions in molecular crystals is still in its infancy. With this in mind, single-crystal high-pressure X-ray diffraction studies were targeted at a polyhedral oxo cluster featuring phenyl rings in the outer sphere. Pressure evolution leads to a gradual relaxation of induced structural strains through stepwise conformational twisting of the phenyl rings with the reorganization of cooperative noncovalent interactions; this propagates the conformational isomerization. More information can be found in the Full Paper by A. Katrusiak, J. Lewiński et al. (DOI: 10.1002/chem.202101732).

Activation Volume of the Elastic–Plastic Transition in Molecular Crystals

Nanoindentation experiments with a spherical indenter tip were performed on the major faces of l-alanine, p-nitroaniline, dl-tartaric acid, and sulfathiazole single crystals to measure the load at which an elastic-to-plastic deformation transition occurs, which is marked by a discrete displacement burst or pop-in. Large data sets (containing more than 100 points each) of the experimentally measured first pop-in loads, P1, were generated. By assuming that the statistical distribution in P1 is a consequence of the thermal fluctuations affecting the nucleation rate of incipient plastic events underneath the indenter, the activation volumes, Λ, for the elastic–plastic transition are determined. For the four molecular crystals examined, Λ values vary between 106 and 173 Å3, which are more than an order of magnitude than those in crystalline metals. However, they are similar to the molecular volume, Ψ. A linear relationship between Λ and Ψ suggests that the plastic deformation in molecular crystals is intimately linked to the size of the molecules. These results are discussed in the context of slip planes and their orientations to the indentation direction, interplanar spacings, intermolecular interactions, etc.

Formation of Nanonuclei during a Structural Phase Transition in Molecular Crystals of Normal Alkanes

Formation of Nanonuclei during a Structural Phase Transition in Molecular Crystals of Normal Alkanes

On the mechanism of solid-state phase transitions in molecular crystals - the role of cooperative motion in (quasi)racemic linear amino acids.

During single-crystal-to-single-crystal (SCSC) phase transitions, a polymorph of a compound can transform to a more stable form while remaining in the solid state. By understanding the mechanism of these transitions, strategies can be developed to control this phenomenon. This is particularly important in the pharmaceutical industry, but also relevant for other industries such as the food and agrochemical industries. Although extensive literature exists on SCSC phase transitions in inorganic crystals, it is unclear whether their classications and mechanisms translate to molecular crystals, with weaker interactions and more steric hindrance. A comparitive study of SCSC phase transitions in aliphatic linear-chain amino acid crystals, both racemates and quasi-racemates, is presented. A total of 34 transitions are considered and most are classified according to their structural change during the transition. Transitions without torsional changes show very different characteristics, such as transition temperature, enthalpy and free energy, compared with transitions that involve torsional changes. These differences can be rationalized using classical nucleation theory and in terms of a difference in mechanism; torsional changes occur in a molecule-by-molecule fashion, whereas transitions without torsional changes involve cooperative motion with multiple molecules at the same time.

Martensitic transition in molecular crystals for dynamic functional materials.

Molecular martensitic materials are an emerging class of smart materials with enormous tunability in physicochemical properties, attributed to the tailored molecular and crystal structures through molecular design. This class of materials exhibits ultrafast and reversible structural transitions in response to thermal and mechanical stimuli, which underlies fascinating properties such as thermoelasticity, superelasticity, ferroelasticity, and shape memory effect. These dynamic properties are not widely explored in molecular crystals and therefore molecular martensitic materials represent a new frontier in the field of solid-state chemistry. In martensitic transitions, the materials not only exhibit substantial shape changes but also remember the functions in the associated polymorphic phases. This suggests promising applicability towards light-weight actuators, lifts, dampers, sensors, shape-/function-memory and ultraflexible optoelectronic devices. In this article, we review characteristics, detailed transition mechanisms, and potential applications of molecular martensitic materials. In particular, we aim to describe transition characteristics by collecting cases with similar transition principles in order to glean insights into further advancement of molecular martensitic materials. Overall, we believe that molecular martensitic materials are emerging as the next generation smart materials that have shown promise in advancing a wide range of domains of applications.

Single Atom Substitution Alters the Polymorphic Transition Mechanism in Organic Electronic Crystals

Understanding the molecular mechanism of polymorphic transition is essential for controlling molecular packing for high-performance organic electronics. Polymorphic transition in molecular crystals...

High-Temperature Reversible Martensitic Transition in an Excited-State Intramolecular Proton Transfer Fluorophore

Reversible martensitic transitions in molecular crystals are an unusual phenomenon, and most of the cases reported in the literature occur at relatively low temperatures. The title compound, 2-(5-(...

Martensitic organic crystals as soft actuators.

Being capable of rapid and complete structure switching, the martensitic phase transitions in molecular crystals are thought to hold a tremendous potential as thermally driven organic actuators. However, the mechanical engineering parlance in the assessment of their performance is not immediately legible to the chemistry research community that starts to explore these materials, and the unavailability of performance indices has precluded molecular crystals from being considered in the device design process. Here, we demonstrate that an organic martensite, hexamethylbenzene, can be used to perform work that is comparable to that of most actuator classes. Millimeter-size single crystals of this material undergo a transition between two forms by uniaxial expansion at a rate of 6.36(2) mm s-1, exerting force in the range 10-100 mN. The force-to-weight ratio of the crystals is on the order of 104 and is superior to that of some living creatures. An actuator performance chart reveals that the performance of this material is close to that of nanomuscles, electrostatic actuators and voice coils, with a strain higher than that of electro/magnetostrictive actuators and ceramic piezoelectrics and stress higher than that of the electroactive polymers, MEMS devices, nanomuscles, voice coils, and some solenoids. Moreover, the crystals of this material are mechanically compliant and can be reversibly bent and shaped to fit the desired application. Altogether, the results point to the untapped potential of molecular crystals as rapid and efficient soft, organic actuators.

Crystalline transformers: more within than meets the eye.

Studies of phase transitions in molecular crystals are becoming more commonplace as improvements in instrumentation and technique enable more efficient exploration of the behavior of samples with varying external conditions, usually temperature. This scientific commentary provides contextual background on this type of study, with reference to an article on transformations in a ferrocenyl-acetylide-gold(I) complex [Makal (2018). Acta Cryst. B74, 427-435].

Translating Microscopic Molecular Motion into Macroscopic Body Motion: Reversible Self-Reshaping in the Solid State Transition of an Organic Crystal

The amplification of microscopic molecular motions so as to produce a controlled macroscopic body effect is the main challenge in the development of molecular mechanical devices. That amplification requires the coherent and ordered movement of each molecule of a whole macroscopic set, such as that taking place in a single-crystal-to-single-crystal transition. Actually, single-crystal-to-single-crystal transitions in molecular crystals can produce a variety of mechanical effects potentially useful in the development of smart materials. A challenging issue in these dynamic crystals, propedeutic to many possible applications in devices, is the gaining of a strict control over the mechanical effects associated with the transition. Here we report an example in which the control of the mechanical effects was successfully obtained. The compound studied undergoes a reversible single-crystal-to-single-crystal transition at 71 °C, from a planar stacked to a herringbone type packing. To this transition, a reversible...

Force field prediction of a reversal transition in molecular crystals

Force field prediction of a reversal transition in molecular crystals

Influence of semiempirical long-range dispersion corrections of the density functional in the study of phase transitions in molecular crystals

The influence of empirical dispersion corrections of the density functional has been investigated when calculating the electronic structure of molecular crystals undergoing structural phase transitions. A phonon spectrum of calomel and benzyl crystals has been calculated, dispersion dependences of optical and acoustic phonons have been constructed, and the phase transition model in calomel crystals has been studied.

Phase transitions in molecular crystals as an example of coupled systems studies

Phase transitions in molecular crystals as an example of coupled systems studies

Kinetic model of polymorphic transitions in molecular crystals

A polychromous model in the form of an integral equation of first kind is suggested for autocatalytic polymorphic transitions of arbitrary order in molecular crystals. The model relates the observed heat absorption to the distribution of the volume of the crystalline substance over the concentrations of the initial and final modifications. The activation energy of the polymorphic transition has been calculated. The inverse problem of reconstructing the distribution function from experimental data has been solved. The model is in good quantitative and qualitative agreement with calorimetric data.

Phase transitions in molecular crystals of dicarboxylic acids

A comparative analysis of phase transitions in molecular crystals of saturated dicarboxylic acids with {COOH(CH2) n COOH} chains of different lengths has been performed by differential scanning calorimetry, which revealed a number of new effects related with specific features of the phase transitions. A quantitative analysis of temperature dependence of the heat capacity has been carried out in the context of the theory of smeared (Λ-shaped) first-order phase transitions.

Phase transitions in molecular crystals of carboxylic acids

A comparative analysis of phase transitions in molecular crystals of carboxylic acids (n-alkano acids) with various chain lengths, (CH3(CH2) n COOH) is performed with the use of DSC. A number of new effects related to first-order phase transitions are discovered. The temperature dependence of heat capacity is quantitatively analyzed in terms of the theory of blurred (Λ-shaped) phase transitions of the first order.

High-pressure phases of hydrogen cyanide: formation of hydrogenated carbon nitride polymers and layers and their electronic properties

We have performed a set of first-principles simulations to consider the possible phase transitions in molecular crystals of HCN under high pressure. Our calculations reveal several transition paths from the orthorhombic phase to tetragonal and then to triclinic phases. The transitions from the orthorhombic to the tetragonal phases are of the second order, whereas those from the tetragonal to the triclinic phases turn out to be of the first-order type and characterized by an abrupt decrease in volume. Our calculations show that, by adjustment of the temperature and pressure of the HCN molecular crystal, novel layered and polymeric crystals with insulating, semiconducting or metallic properties can be found. Based on our simulation results, two different crystal formation mechanisms are deduced. The stabilities of the predicted structures at ambient pressure are further assessed by performing phonon or MD simulations. In addition, the electron transport properties of the predicted polymers are obtained using the non-equilibrium Green’s function technique combined with density functional theory. The results show that the polymers have metallic-like I–V characteristics.

Specific features of phase transitions in molecular crystals of diols

A comparative analysis of the phase transitions in molecular crystals of diols with different chain lengths [1,15-pentadecanediol (CH2)14(OH)2 and 1,16-hexadecanediol (CH2)15(OH)2] has been performed using differential scanning calorimetry. A quantitative analysis of the temperature dependence of the heat capacity within the theory of diffuse (Λ-shaped) first-order phase transitions and the use of the temperature hysteresis have revealed a number of new effects associated with the specific features of the phase transitions in molecular crystals of diols.

Phase transitions in molecular crystals of n-alkane alcohols

A comparative analysis of the phase transitions in molecular crystals of n-alkane alcohols with different lengths of the (CnH2n + 2OH) chain was performed using differential scanning calorimetry (DSC). Investigation of the temperature hysteresis made it possible to reveal a number of new effects associated with the specific features of the first-order phase transitions. A quantitative analysis of the temperature dependence of the heat capacity was carried out in the framework of the theory of diffuse (Λ-shaped) first-order phase transitions.