Single-Crystal-to-Single-Crystal Synthesis of a Polymer in Two Distinct Topologies

A squaramide-based monomer, designed for topochemical azide-alkyne cycloaddition (TAAC) polymerization, crystallizes as two polymorphs, M1 and M2, both having crystal packing suitable for topochemical polymerization. The hydrogen-bonding between squaramide units bias the molecular organization in both the polymorphs. 3D packing of H-bonded stacks of monomer lead to juxtaposition of azide and alkyne units of adjacent molecules in a transition-state-like arrangement for their regiospecific cycloaddition reaction. The monomers are arranged as supramolecular sheets and supramolecular helices in polymorph M1 and M2 respectively. Both the polymorphs undergo slow and spontaneous regiospecific TAAC polymerization at room temperature, but react quickly at higher temperatures, resulting in 1,4-traizolyl-linked polymer, with distinct mechanical responses. Upon heating, single crystals of polymorph M1 show expansion followed by contraction without any permanent dimensional change, whereas crystals of polymorph M2 undergo splitting. At moderate temperatures, both the polymorphs undergo single-crystal-to-single-crystal (SCSC) polymerization, resulting in two polymer-polymorphs with distinct topologies that can be studied at atomic resolution by single-crystal X-ray crystallography. The polymorph M1 reacts to polymer P1 with β-sheet-like topology, and polymorph M2 reacts to polymer P2 having polymer chains of helical conformation. Nanoindentation experiments with crystals of these polymers revealed their distinct mechanical properties.

From Flexible Crystals to Piezoelectrics: The Advent of a New Class of Flexible Functional Molecular Materials

The recent discoveries of mechanically flexible molecular crystals have fuelled a resurgence of research interest in molecular piezoelectrics. This has raised the quest to explore structure-property relations in molecular piezoelectric crystals, which remain largely obscure. Here, the fundamental structural features associated with organic molecular piezoelectric crystals are explored in relation to their mechanical and supramolecular flexibility. Along with the electrostatic properties such as molecular dipole moments and spontaneous crystal polarization, possible correlations of piezoelectric coefficients with intermolecular interaction topologies and their anisotropy point toward their link with mechanical flexibility in molecular crystals. In addition, the possible roles of crystal packing efficiency, lattice cohesive energies, Young's moduli, and its anisotropy from elastic tensors have been examined. This quantitative overview suggests that piezoelectric response in molecular materials is a complex interplay of several structural and electrostatic factors. Based on these analyses and the fundamental aspects of electromechanical coupling, it becomes apparent that combining mechanical flexibility and supramolecular chirality/polarity can be a promising approach to discovering soft molecular piezoelectrics for novel actuators and energy-harvesting materials.

Structural Phase Transitions in Anthracene Crystals

Anthracene (C14 H10 ) and its derivatives, π-conjugated molecules in acenes, have been widely researched in terms of their reactions, physical properties, and self-assembly (or crystal engineering). These molecules can be functionalized to tune reactivities, optoelectronic properties, and self-assembling abilities. Structural changes in the molecular assemblies, solid states, and crystals have recently been discovered. Therefore, a systematic discussion of anthracene's molecular structure, packing, and optical properties based on its intermolecular structure and phase transitions is important for future chemical and structural design. In the present review, we discuss anthracene's molecular design, dimer packing, and crystal structure, focusing on the structural phase transitions of its crystals. We also provide examples of the phase transitions of anthracene crystals. Changes to edge-to-face of CH-π interaction and face-to-face packing of π-π interaction affect the thermodynamic stabilities of various crystal structures. These structures can inform the prediction of structural and physical properties.

Multi-Stimuli-Induced Mechanical Bending and Reversible Fluorescence Switching in a Single Organic Crystal

Fluorescent single crystals that respond to multiple external stimuli are of great interest in molecular machines, sensors, and displays. The integration of photo- or acid-induced fluorescence enhancement and bending in one organic crystal, however, has not been reported yet. Herein, we report the interesting plastic photomechanical bending and switching on of the fluorescence of an azine crystal in a single-crystal transformation, due to extended π-conjugation and molecular slippage. Moreover, the fluorescent plastic bending driven by multiple volatile acid vapors was firstly observed, and attributed to the synergistic effect of push-pull electronic structure and hydrogen bonding. The single crystal also shows high elasticity under external force. In addition, reversible fluorescence switching can be triggered by grinding and solvent fuming, as well as by the adsorption and desorption of HCl vapor. The integration of plastic, elastic bending and switch-on fluorescence into one single crystal provides a new strategy for next-generation smart materials.

Low Temperature and High-Pressure Study of Bending L-Leucinium Hydrogen Maleate Crystals

The polymorphism of molecular crystals is a well-known phenomenon, resulting in modifications of physicochemical properties of solid phases. Low temperatures and high pressures are widely used to find phase transitions and quench new solid forms. In this study, L-Leucinium hydrogen maleate (LLHM), the first molecular crystal that preserves its anomalous plasticity at cryogenic temperatures, is studied at extreme conditions using Raman spectroscopy and optical microscopy. LLHM was cooled down to 11 K without any phase transition, while high pressure impact leads to perceptible changes in crystal structure in the interval of 0.0–1.35 GPa using pentane-isopentane media. Surprisingly, pressure transmitting media (PTM) play a significant role in the behavior of the LLHM system at extreme conditions—we did not find any phase change up to 3.05 GPa using paraffin as PTM. A phase transition of LLHM to amorphous form or solid–solid phase transition(s) that results in crystal fracture is reported at high pressures. LLHM stability at low temperatures suggests an alluring idea to prove LLHM preserves plasticity below 77 K.

Temperature-Reliant Dynamic Properties and Elasto-Plastic to Plastic Crystal (Rotator) Phase Transition in a Metal Oxyacid Salt

Although, dynamic crystals are attractive for use in many technologies, molecular level mechanisms of various solid-state dynamic processes and their interdependence, remain poorly understood. Here, we report a rare example of a dynamic crystal (1), involving a heavy transition metal, rhenium, with an initial two-face elasticity (within ≈1 % strain), followed by elasto-plastic deformation, at room temperature. Further, these crystals transform to a rotator (plastic) crystal phase at ≈105 °C, displaying exceptional malleability. Qualitative and quantitative mechanical tests, X-ray diffraction, μ-Raman and polarized light microscopy experiments reveal that the elasto-plastic deformation involves both partial molecular rotations and slip, while malleability in the rotator phase is facilitated by reorientational motions and increased symmetry (slip planes). Our work, connecting the plastically bendable (1D or 2D) crystals with the rotator phases (3D), is important for designing multi-functional dynamic crystals.

Elastic Flexibility in an Optically Active Naphthalidenimine-Based Single Crystal

Organic single crystals that combine mechanical flexibility and optical properties are important for developing flexible optical devices, but examples of such crystals remain scarce. Both mechanical flexibility and optical activity depend on the underlying crystal packing and the nature of the intermolecular interactions present in the solid state. Hence, both properties can be expected to be tunable by small chemical modifications to the organic molecule. By incorporating a chlorine atom, a reportedly mechanically flexible crystal of (E)-1-(4-bromo-phenyl)iminomethyl-2-hydroxyl-naphthalene (BPIN) produces (E)-1-(4-bromo-2-chloro-phenyl)iminomethyl-2-hydroxyl-naphthalene (BCPIN). BCPIN crystals show elastic bending similar to BPIN upon mechanical stress, but exhibit a remarkable difference in their optical properties as a result of the chemical modification to the backbone of the organic molecule. This work thus demonstrates that the optical properties and mechanical flexibility of molecular materials can, in principle, be tuned independently.

Bending for Better: Flexible Organic Single Crystals with Controllable Curvature and Curvature-Related Conductivity for Customized Electronic Devices

Electronic microdevices of self-bending coronene crystals are developed to reveal an unexplored link between mechanical deformation and crystal function. First, a facile approach towards length/width/curvature-controllable micro-crystals through bottom-up solution crystallization was proposed for high processability and stability. The bending crystal devices show a significant increase beyond seven orders of magnitude in conductivity than the straight ones, providing the first example of deformation-induced function enhancement in crystal materials. Besides, double effects caused by bending, including the change of π electron level as well as the enhancement of carrier mobility, were determined, respectively by the X-ray photoelectric spectroscopy and X-ray crystallography to coexist, contributing to the conductivity improvement. Our findings will promote future creation of flexible organic crystal systems with deformation-enhanced functional features towards customized smart devices.