Thermally Driven Polymorphic Transition Prompting a Naked‐Eye‐Detectable Bending and Straightening Motion of Single Crystals

The amplification of molecular motions so that they can be detected by the naked eye (10⁷‐fold amplification from the ångström to the millimeter scale) is a challenging issue in the development of mechanical molecular devices. In this context, the perfectly ordered molecular alignment of the crystalline phase has advantages, as demonstrated by the macroscale mechanical motions of single crystals upon the photochemical transformation of molecules. In the course of our studies on thermoresponsive amphiphiles containing tetra(ethylene glycol) (TEG) moieties, we serendipitously found that thermal conformational changes of TEG units trigger a single‐crystal‐to‐single‐crystal polymorphic phase transition. The single crystal of the amphiphile undergoes bending and straightening motion during both heating and cooling processes at the phase‐transition temperatures. Thus, the thermally triggered conformational change of PEG units may have the advantage of inducing mechanical motion in bulk materials.     

Self‐Healing Molecular Crystals

One of the most inevitable limitations of any material that is exposed to mechanical impact is that they are inexorably prone to mechanical damage, such as cracking, denting, gouging, or wearing. To confront this challenge, the field of polymers has developed materials that are capable of autonomous self‐healing and recover their macroscopic integrity similar to biological organisms. However, the study of this phenomenon has mostly remained within the soft materials community and has not been explored by solid‐state organic chemists. The first evidence of self‐healing in a molecular crystal is now presented using crystals of dipyrazolethiuram disulfide. The crystals were mildly compressed and the degree of healing was found to be 6.7 %. These findings show that the self‐healing properties can be extended beyond mesophasic materials and applied towards the realm of ordered solid‐state compounds.     

Thermally Twistable,Photobendable, Elastically Deformable,and Self‐Healable Soft Crystals

The first example of a smart crystalline material, the 2:1 cocrystal of probenecid and 4,4′‐azopyridine, which responds reversibly to multiple external stimuli (heat, UV light, and mechanical pressure) by twisting, bending, and elastic deformation without fracture is reported. This material is also able to self‐heal on heating and cooling, thereby overcoming the main setbacks of molecular crystals for future applications as crystal actuators. The photo‐ and thermomechanical effects and self‐healing capabilities of the material are rooted in reversible trans–cis isomerization of the azopyridine unit and crystal‐to‐crystal phase transition. Fairly isotropic intermolecular interactions and interlocked crisscrossed molecular packing secure high elasticity of the crystals.     

Thermally Twistable,Photobendable, Elastically Deformable,and Self‐Healable Soft Crystals

The first example of a smart crystalline material, the 2:1 cocrystal of probenecid and 4,4′‐azopyridine, which responds reversibly to multiple external stimuli (heat, UV light, and mechanical pressure) by twisting, bending, and elastic deformation without fracture is reported. This material is also able to self‐heal on heating and cooling, thereby overcoming the main setbacks of molecular crystals for future applications as crystal actuators. The photo‐ and thermomechanical effects and self‐healing capabilities of the material are rooted in reversible trans–cis isomerization of the azopyridine unit and crystal‐to‐crystal phase transition. Fairly isotropic intermolecular interactions and interlocked crisscrossed molecular packing secure high elasticity of the crystals.     

Perchlorination of Coronene Enhances its Propensity for Self‐Assembly on Graphene

Providing a quantitative understanding of the thermodynamics involved in molecular adsorption and self‐assembly at a nanostructured carbon material is of fundamental importance and finds outstanding applications in the graphene era. Here, we study the effect of edge perchlorination of coronene, which is a prototypical polyaromatic hydrocarbon, on the binding affinity for the basal planes of graphite. First, by comparing the desorption barrier of hydrogenated versus perchlorinated coronene measured by temperature‐programmed desorption, we quantify the enhancement of the strength of physisorption at the single‐molecule level though chlorine substitution. Then, by a thermodynamic analysis of the corresponding monolayers based on force‐field calculations and statistical mechanics, we show that perchlorination decreases the free energy of self‐assembly, not only enthalpically (by enhancing the strength of surface binding), but also entropically (by decreasing the surface concentration). The functional advantage of a chemically modulated 2D self‐assembly is demonstrated in the context of the molecule‐assisted liquid‐phase exfoliation of graphite into graphene.     

Jumping Crystal of a Hydrogen‐Bonded Organic Framework Induced by the Collective Molecular Motion of a Twisted π System

There is a limited number of reports on mechanically responsive molecular crystals, including thermo‐responsive and light‐responsive crystals. Rigid ordered molecular crystals with a close‐packing structure are less able to accept distortion, which hampers the development of such molecular crystals. The thermosalient effect, or “crystal jumping”, refers to a thermo‐responsive system that converts heat into mechanical force by thermally induced phase transition. While they have recently attracted attention as potential highly efficient molecular actuators, less than two dozens of thermosalient molecular crystals have been reported to date, and the design of such molecules as well as how they assemble to express a thermosalient effect are unknown. Herein, we demonstrate how the cooperative molecular motion of twisted π units could serve to develop a thermo‐responsive jumping molecular crystal with a hydrogen‐bonded organic framework (HOF) of tetra[2,3]thienylene tetracarboxylic acid ( 1 ). The cooperative change in the molecular structure triggered by the desolvation of THF in the channel of the HOF structure induced not only a change in the structure of HOF but also mechanical force. Hydrogen bonding interactions contributed significant thermal stability to maintain the HOF assembly even with a dynamic structural change.     

Metastable Self‐Assembly of Theta‐Shaped Colloids and Twinning of Their Crystal Phases

Anisotropic colloids self‐assemble into different crystal structures compared to spherical colloids. Exploring and understanding their self‐assembly behavior could lead to creation of new materials with hierarchical structures through a bottom‐up process. Herein, we report metastable self‐assembly of theta‐shaped SiO₂ colloids interacting with a depletion force in a quasi‐two‐dimensional space and we demonstrate that both a metastable “prone” crystal phase and a stable “standing” crystal phase can be formed, depending on the self‐assembly path. Path selection stems from an interplay between particle–particle interactions and particle–wall interactions. In particular, a twinning of the metastable crystals was observed and two twinning mirror axes were found. A variety of complex twinned crystals were formed by each individual mirror axis or their combinations.     

Light‐Driven Self‐Healing of Nanoparticle‐Based Metamolecules

Metamolecules and crystals consisting of nanoscale building blocks offer rich models to study colloidal chemistry, materials science, and photonics. Herein we demonstrate the self‐assembly of colloidal Ag nanoparticles into quasi‐one‐dimensional metamolecules with an intriguing self‐healing ability in a linearly polarized optical field. By investigating the spatial stability of the metamolecules, we found that the origin of self‐healing is the inhomogeneous interparticle electrodynamic interactions enhanced by the formation of unusual nanoparticle dimers, which minimize the free energy of the whole structure. The equilibrium configuration and self‐healing behavior can be further tuned by modifying the electrical double layers surrounding the nanoparticles. Our results reveal a unique route to build self‐healing colloidal structures assembled from simple metal nanoparticles. This approach could potentially lead to reconfigurable plasmonic devices for photonic and sensing applications.     

Photochemically and Thermally Driven Full‐Color Reflection in a Self‐Organized Helical Superstructure Enabled by a Halogen‐Bonded Chiral Molecular Switch

Supramolecular approaches toward the fabrication of functional materials and systems have been an enabling endeavor. Recently, halogen bonding has been harnessed as a promising supramolecular tool. Herein we report the synthesis and characterization of a novel halogen‐bonded light‐driven axially chiral molecular switch. The photoactive halogen‐bonded chiral switch is able to induce a self‐organized, tunable helical superstructure, that is, cholesteric liquid crystal (CLC), when doped into an achiral liquid crystal (LC) host. The halogen‐bonded switch as a chiral dopant has a high helical twisting power (HTP) and shows a large change of its HTP upon photoisomerization. This light‐driven dynamic modulation enables reversible selective reflection color tuning across the entire visible spectrum. The chiral switch also displays a temperature‐dependent HTP change that enables thermally driven red, green, and blue (RGB) reflection colors in the self‐organized helical superstructure.     

Polymeric Halogen‐Bond‐Based Donor Systems Showing Self‐Healing Behavior in Thin Films

The synthesis and comprehensive characterization of a systematic series of cleft‐type anion receptors imbedded into a polymeric architecture is presented. For the first time, isothermal calorimetric titrations on polymeric halogen‐bond‐based donors were exploited to evaluate the dependence of the anion affinity on different key parameters (i.e. monomeric versus polymeric receptor, halogen versus hydrogen bonding, charge assistance). The combination of these donor systems with a copolymer bearing accepting carboxylate groups led to supramolecular cross‐linked polymer networks showing excellent intrinsic self‐healing behavior. FT‐Raman spectroscopy and nano‐indentation measurements were utilized to clarify the thermally induced self‐healing mechanism based on the formation of halogen bonds. These first self‐healing materials based on halogen bonds pave the way for new applications of halogen‐bond donors in polymer and material science.     

Atomic Force Microscopy Investigation of Poly[(R)‐3‐hydroxybutyrate] Lamellar Single Crystals: Relationship between Molecular Weight and Enzymatic Degradation Behavior

Enzymatic degradation behavior of a lamella of single crystals of poly(R)‐3‐hydroxybutyrate (P(3HB)) with an extracellular polyhydroxybutyrate (PHB) depolymerase purified from Alcaligenes faecalis T1 has been investigated by atomic force microscopy (AFM) in order to obtain further information for the chain packing state of P(3HB) in a lamellar single crystal. Two kinds of P(3HB) single crystals with different molecular weights, denoted respectively as H‐ and L‐P(3HB) for high and low molecular weights, respectively, were prepared. The enzymatic treatment was conducted for P(3HB) single crystals adsorbed on a surface of highly ordered pyrolytic graphite. The enzymatic degradation of both P(3HB) single crystals generates several crevices crosswise across the crystal at an early stage. Subsequently, the enzymatic degradation yields numbers of cracks lengthwise along the crystal. In addition to these common features, the interval between cracks crosswise across a lamella in H‐P(3HB) single crystal is longer than that in L‐P(3HB) single crystal, and each crack has V‐shaped and rectangular shaped morphology for H‐ and L‐P(3HB) single crystals, respectively. Based on these results, it is concluded that a lamella of P(3HB) single crystal has straight degradation pathways, that may correspond to a switchboard region, along the long axis of the crystal, independent of molecular weight of P(3HB) samples, and that a H‐P(3HB) single crystal has broader degradation pathways with longer intervals crosswise across the crystal than a L‐P(3HB) single crystal.     

A combined crystallographic,thermal, Raman and computational study on polymorphism and phase transition in 1‐(4‐hexyloxy‐3‐hydroxyphenyl)ethanone

The crystal structure of the triclinic polymorph of 1‐(4‐hexyloxy‐3‐hydroxyphenyl)ethanone, C₁₄H₂₀O₃, differs markedly from that of the orthorhombic polymorph [Manzano et al. (2015). Acta Cryst. C 71 , 1022–1027]. The two molecular structures are alike with respect to their bond lengths and angles, but differ in their spatial arrangement. This gives rise to quite different packing schemes, even if built up by similar chains having the hydroxy–ethanone O—H…O hydrogen‐bond synthon in common. Both phases were found to be related by a first‐order thermally driven phase transformation at 338–340 K, which is discussed in detail. The relative stabilities of both polymorphs are explained on the basis of both the noncovalent interactions operating in each structure and quantum chemical calculations. The polymorphic phase transition has also been studied experimentally by means of differential scanning calorimetry (DSC) experiments, conducted on individual single crystals, Raman spectroscopy and controlled heating under a microscope of individual single crystals, which were further characterized by powder and single‐crystal X‐ray diffraction.     

Thermally Induced Intra‐Carboxyl Proton Shuttle in a Molecular Rack‐and‐Pinion Cascade Achieving Macroscopic Crystal Deformation

Proton transport via dynamic molecules is ubiquitous in chemistry and biology. However, its use as a switching mechanism for properties in functional molecular assemblies is far less common. In this study, we demonstrate how an intra‐carboxyl proton shuttle can be generated in a molecular assembly akin to a rack‐and‐pinion cascade via a thermally induced single‐crystal‐to‐single‐crystal phase transition. In a triply interpenetrated supramolecular organic framework (SOF), a 4,4′‐azopyridine (azpy) molecule connects to two biphenyl‐3,3′,5,5′‐tetracarboxylic acid (H₄BPTC) molecules to form a functional molecular system with switchable mechanical properties. A temperature change reversibly triggers a molecular movement akin to a rack‐and‐pinion cascade, which mainly involves 1) an intra‐carboxyl proton shuttle coupled with tilting of the azo molecules and azo pedal motion and 2) H₄BPTC translation. Moreover, both the molecular motions are collective, and being propagated across the entire framework, leading to a macroscopic crystal expansion and contraction.     

A Columnar Liquid‐Crystal Phase Formed by Hydrogen‐Bonded Perylene Bisimide J‐Aggregates

A new perylene bisimide (PBI) dye self‐assembles through hydrogen bonds and π–π interactions into J‐aggregates that in turn self‐organize into liquid‐crystalline (LC) columnar hexagonal domains. The PBI cores are organized with the transition dipole moments parallel to the columnar axis, which is an unprecedented structural organization in π‐conjugated columnar liquid crystals. Middle and wide‐angle X‐ray analyses reveal a helical structure consisting of three self‐assembled hydrogen‐bonded PBI strands that constitute a single column of the columnar hexagonal phase. This remarkable assembly mode for columnar liquid crystals may afford new anisotropic LC materials for applications in photonics.     

Systematic Control of the Excited‐State Dynamics and Carrier‐Transport Properties of Functionalized Benzo[ghi]perylene and Coronene Derivatives

A series of benzo[ghi]perylene (Bp) and coronene (Cor) derivatives substituted with electron‐withdrawing methoxycarbonyl (COOMe) or electron‐donating methoxyl (MeO) groups was synthesized. The electrochemical, spectroscopic, and photophysical properties of these compounds were investigated by cyclic voltammetry, steady‐state and time‐resolved spectroscopy, and quantum‐yield measurements. Introduction of suitable substituents onto the aromatic rings enabled control of electrochemical and spectroscopic behavior. Examination of excited‐state dynamics revealed that fluorescence quantum yields increased with increasing number of COOMe groups in both Bp and Cor derivatives, consistent with the findings of DFT calculations. Single‐crystal analysis allowed the performance of field‐effect transistors containing single crystals of the derivatives to be rationalized.     

Detailed Investigation of the Structural,Thermal, and Electronic Properties of Gold Isocyanide Complexes with Mechano‐Triggered Single‐Crystal‐to‐Single‐Crystal Phase Transitions

Mechano‐induced phase transitions in organic crystalline materials, which can alter their properties, have received much attention. However, most mechano‐responsive molecular crystals exhibit crystal‐to‐amorphous phase transitions, and the intermolecular interaction patterns in the daughter phase are difficult to characterize. We have investigated phenyl(phenylisocyanide)gold(I) ( 1 ) and phenyl(3,5‐dimethylphenylisocyanide)gold(I) ( 2 ) complexes, which exhibit a mechano‐triggered single‐crystal‐to‐single‐crystal phase transition. Previous reports of complexes 1 and 2 have focused on the relationships between the crystalline structures and photoluminescence properties; in this work we have focused on other aspects. The face index measurements of complexes 1 and 2 before and after the mechano‐induced phase transitions have indicated that they undergo non‐epitaxial phase transitions without a rigorous orientational relationship between the mother and daughter phases. Differential scanning calorimetry analyses revealed the phase transition of complex 1 to be enthalpically driven by the formation of new aurophilic interactions. In contrast, the phase transition of complex 2 was found to be entropically driven, with the closure of an empty void in the mother phase. Scanning electron microscopy observation showed that the degree of the charging effect of both complexes 1 and 2 was changed by the phase transitions, which suggests that the formation of the aurophilic interactions affords more effective conductive pathways. Moreover, flash‐photolysis time‐resolved microwave conductivity measurements revealed that complex 1 increased in conductivity after the phase change, whereas the conductivity of complex 2 decreased. These contrasting results were explained by the different patterns in the aurophilic interactions. Finally, an intriguing disappearing polymorphism of complex 2 has been reported, in which a polymorph form could not be obtained again after some period of time, even with repeated trials. The present studies provide us with a variety of hitherto unknown insights into mechano‐responsive molecular crystals, which help us to understand the phase transition behaviors upon mechanical stimulation and establish rational design principles.     

Investigation of the deformation of homogeneous poly(ethylene‐co‐1‐octene) by wide‐ and small‐angle X‐ray scattering using synchrotron radiation

The deformation behavior of homogeneous ethylene‐1‐octene copolymers was investigated as a function of the crystallinity and the crystal size and perfection, respectively, by wide‐ and small‐angle X‐ray scattering using synchrotron radiation. The crystallinity and the crystal size and perfection, respectively, are controlled by the copolymer composition and the condition of melt crystallization. The deformation includes rotation of crystals, followed by plastic deformation and complete melting of the initial crystal population, and final formation of microfibrils. The process of rotation, plastic deformation, and melting of crystals of the initial structure is completed at lower strain if the size and perfection of the crystals, respectively, decrease, that is, if crystals thermally melt at lower temperature. The kinetics of the fibrillation of the initial structure seems independent of the crystal symmetry, that is, rotation and melting of pseudohexagonal and orthorhombic polyethylene crystals (as evident in low‐crystalline specimens) are similar. The structure of the microfibrils, before and after stress release, is almost independent of the condition of prior melt crystallization, which supports the notion of complete melting of the initial crystal population. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1919–1930, 2002     

Water‐Binding‐Mediated Gelation/Crystallization and Thermosensitive Superchirality

Determination of molecular structural parameters of hydrophobic cholesterol–naphthalimide conjugates for water binding capabilities as well as their moisture‐sensitive supramolecular self‐assembly were revealed. Water binding was a key factor in leading trace water‐induced crystallization against gelation in apolar solvent. Ordered water molecules entrapped in self‐assembly arrays revealed by crystal structures behave as hydrogen‐bonding linkers to facilitate three‐dimensional growth into crystals rather than one‐dimensional gel nanofibers. Water binding was also reflected on the supramolecular chirality inversion of vesicle self‐assembly in aqueous media via heating‐induced dehydration. Structural parameters that favor water binding were evaluated in detail, which could help rationally design organic building units for advancing soft materials, crystal engineering, and chiral recognition.     

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.     

Single‐Crystal‐to‐Single‐Crystal Transition in an Enantiopure [7]Helquat Salt: The First Observation of a Reversible Phase Transition in a Helicene‐Like Compound

Here it is reported that crystals of an enantiopure [7]helquat salt undergo reversible thermal solid–solid phase transition at 404 K. Differential scanning calorimetry (DSC), capillary electrophoresis (CE), and X‐ray diffraction analysis were used to unravel the mechanistic details of this process. The single‐crystal‐to‐single‐crystal course enabled direct monitoring of the structural changes by in situ variable‐temperature X‐ray diffraction, thus providing the first direct evidence of a solid phase transition in a helicene‐like compound.