Elastic Crystals Research Articles

Schiff base flexible organic crystals toward multifunctional applications

AbstractThe emergence of flexible organic crystals changed the perception of molecular crystals that were regarded as brittle entities over a long period of time, and sparked a great interest in exploring mechanically compliant organic crystalline materials toward next‐generation smart materials during the past decade. Schiff base compounds are considered to be one of the most promising candidates for flexible organic crystals owing to their easy synthesis, high yield, stimuli responsiveness and good mechanical properties. This paper gives an overview of the recent development of Schiff base flexible organic crystals (including elastic organic crystals, plastic organic crystals, and flexible organic crystals integrating elasticity and plasticity) from serendipitous discovery to design strategies and versatile applications such as stimuli responses, optical waveguides, optoelectronic devices, biomimetic soft robots, and organic photonic integrated circuits. Notably, atomic force microscopy‐micromanipulation technique has been utilized to bring the multifunctional applications of flexible organic crystals from the macroscopic level to the microscopic world. Since understanding mechanical flexibility at the molecular level through crystal engineering can assist us to trace down the structural origin of mechanical properties, we focus on the packing structures of various Schiff base flexible organic crystals driven by non‐covalent intermolecular interactions and their close correlation with mechanical behaviors. We hope that the information given here will help in the design of novel flexible organic crystals combined with other unique properties, and promote further research into the area of mechanically compliant organic crystalline materials toward multifunctional applications.

Elastic Organic Crystals Exhibiting Amplified Spontaneous Emission Waveguides with Standard Red Chromaticity of the Rec.2020 Gamut.

The use of mechanically flexible molecular crystals as optical transuding media is demonstrated for a plethora of applications; however, the spectral peaks of optical outputs located mainly in the range of 400-600nm are insufficient for practical telecommunication and full-color display applications. Herein, two elastically bendable organic crystals are reported that show red emission of the rec.709 gamut under 365nm UV light irradiation yet generate rec.2020 gamut red optical waveguides and amplified spontaneous emissions when irradiated by a 355nm laser. Capitalizing on the extended π-conjugation and donor-acceptor character, as well as mechanical elasticity, these organic crystals exhibit flexible optical waveguides with Commission Internationale de L'Eclairage (CIE) coordinates of (0.70, 0.29), nearly identical to the red chromaticity of the rec.2020 gamut required for ultrahigh-definition (UHD) displays. Notably, one of the elastic crystals functions as a soft resonance cavity, resulting in amplified spontaneous emission waveguides with CIE coordinates of (0.71, 0.29) and the standard red chromaticity of the rec.2020 gamut, both in straight and bent states. This study presents a new avenue for the development of high-purity red-emissive crystalline materials to create all-organic, lightweight, and mechanically compliant optical telecommunication and UHD display devices.

Mechanoresponsive Flexible Crystals.

Flexible crystals have gained significant attention owing to their remarkable pliability, plasticity, and adaptability, making them highly popular in various research and application fields. The main challenges in developing flexible crystals lie in the rational design, preparation, and performance optimization of such crystals. Therefore, a comprehensive understanding of the fundamental origins of crystal flexibility is crucial for establishing evaluation criteria and design principles. This Perspective offers a retrospective analysis of the development of flexible crystals over the past two decades. It summarizes the elastic standards and possible plastic bending mechanisms tailored to diverse flexible crystals and analyzes the assessment of their theoretical basis and applicability. Meanwhile, the compatibility between crystal elasticity and plasticity has been discussed, unveiling the immense prospects of elastic/plastic crystals for applications in biomedicine, flexible electronic devices, and flexible optics. Furthermore, this Perspective presents state-of-the-art experimental avenues and analysis methods for investigating molecular interactions in molecular crystals, which is vital for the future exploration of the mechanisms of crystal flexibility.

Elastic foundation-introduced defective phononic crystals for tunable energy harvesting

Defective phononic crystals offer the advantage of concentrating elastic waves, thereby enhancing the potential for piezoelectric energy harvesting (PEH). However, a key limitation is their reliance on a fixed operating frequency, rendering them susceptible to the prevailing vibration environment. To surmount this constraint, this study introduces a novel approach involving a tunable elastic foundation system for defective phononic crystal structures. The newly developed phononic crystal is fashioned by integrating a periodically elastic foundation beneath a uniform beam. A defect is induced by selectively removing specific elastic foundations and integrating piezoelectric components. Explicit analytical solutions are established through the transfer matrix method and the spectral element method, which are subsequently corroborated via comparison with finite element results. The findings underscore that the periodic elastic foundations impart bandgaps in the elastic wave band structure. The absence of specific elastic foundations results in the emergence of distinct defect modes. Additionally, frequency response analysis exposes the potential for energy enhancement, albeit with inherent variations. Noteworthy is the revelation that manipulating the stiffness of the elastic foundation triggers shifts in the resonant frequency of the output voltage. Therefore, the proposed tunable elastic foundation system exhibits promising potential to engender versatile and adaptive phononic crystal configurations, thereby advancing the domain of PEH.

Dual control of passive light output direction by light and mechanical forces in elastic crystals

An organic crystal with photomechanical and elastic properties has been prepared.

Coordination of hydrogen-bonding and π–π stacking induced elasticity and efficient optical-waveguiding in 4,7-bis(phenyl ethynyl)benzo[c][1,2,5] thiadiazole-based crystals

Hydrogen bonding and π–π stacking were combined in a single component, inducing elasticity and efficient optical waveguiding properties in crystals.

An elastic single crystal composed of one-dimensional chiral coordination polymers.

A single crystal composed of one-dimensional coordinated polymers, [CdCl2(1-methyl-2-pyridone)]n, has been synthesized and characterized. This compound exhibits outstanding elastic bending due to the molecular spring nature of the CdCl2 coordination framework and weak intermolecular interactions between the coordination chains. Owing to the helical arrangement of organic ligands surrounding the coordination structure, the compound crystallizes in a chiral space group. As a result, it displays compelling circular dichroism spectra and second harmonic generation properties.

Low-Temperature-Resistant Elastic Organic Crystals with Variable Amplified Spontaneous Emissions

Low-Temperature-Resistant Elastic Organic Crystals with Variable Amplified Spontaneous Emissions

Auxetic Photonic Patterns with Ultrasensitive Mechanochromism.

Photonic crystals with mechanochromic properties are currently under intensive study to provide intuitive colorimetric detection of strains for various applications. However, the sensitivity of color change to strain is intrinsically limited, as the degree of deformation determines the wavelength shift. To overcome this limitation, auxetic photonic patterns that exhibit ultra-sensitive mechanochromism are designed. These patterns have a regular arrangement of cuts that expand to accommodate the strain, while the skeletal framework undergoes torsional deformation. Elastic photonic crystals composed of a non-close-packed array of colloidal particles are embedded in the cut area of the auxetic patterns. As the cut area amplifies the strains, the elastic photonic crystals show significant color change even for small total strains. The degree of local-strain amplification, or sensitivity of color change, is controllable by adjusting the width of cuts in the auxetic framework. In this work, a maximum sensitivity of up to 60nm/% is achieved, which is 20 times higher than bulk films. It is believed that the auxetic photonic patterns with ultra-sensitive mechanochromism will provide new opportunities for the pragmatic use of mechanochromic materials in various fields, including structural health monitoring.

Mechanical Effects of Elastic Crystals Driven by Natural Sunlight and Force

AbstractFlexible crystals that can capture solar energy and convert it into mechanical energy are promising for a wide range of applications such as information storage and actuators, but obtaining them remains a challenge. Herein, an elastic crystal of a barbiturate derivative was found to be an excellent candidate, demonstrating plastic bending behavior under natural sunlight irradiation. 1H NMR and high‐resolution mass spectrum data of microcrystals before and after light irradiation demonstrated that light‐induced [2+2] cycloaddition was the driving force for the photomechanical effects. Interestingly, the crystals retained elastic bending even after light irradiation. This is the first report of flexible crystals that can be driven by natural sunlight and that have both photomechanical properties and elasticity. Furthermore, regulation of the passive light output direction of the crystals and transport of objects by applying mechanical forces and light was demonstrated.

Mechanical Effects of Elastic Crystals Driven by Natural Sunlight and Force.

Flexible crystals that can capture solar energy and convert it into mechanical energy are promising for a wide range of applications such as information storage and actuators, but obtaining them remains a challenge. Herein, an elastic crystal of a barbiturate derivative was found to be an excellent candidate, demonstrating plastic bending behavior under natural sunlight irradiation. 1 H NMR and high-resolution mass spectrum data of microcrystals before and after light irradiation demonstrated that light-induced [2+2] cycloaddition was the driving force for the photomechanical effects. Interestingly, the crystals retained elastic bending even after light irradiation. This is the first report of flexible crystals that can be driven by natural sunlight and that have both photomechanical properties and elasticity. Furthermore, regulation of the passive light output direction of the crystals and transport of objects by applying mechanical forces and light was demonstrated.

Observation of triple-branch valley topological edge states and abnormal refraction in elastic phononic crystal plates

Topological edge states (TES) have attracted wide interest in elastic waves, due to its backscattering immunity and directional wave transportation. In this work, the triple-branch TES and their rich refraction behaviors are investigated in one single valley chiral phononic crystal (PC) plate with double-sided symmetric pillars. The triple-branch TES emerge simultaneously in one structure, and their robust transportation against disorder, cavity, and sharp bends are demonstrated in both simulation and experiment. Furthermore, the refraction behaviors for triple-branch TES from PC plate into bare plate are unveiled both theoretically and experimentally, leading to the normal refraction for first branch TES while the abnormal refraction for the second or third branch TES. These results offer opportunity for designing mechanical device or controlling the wave propagation along phonon circuits in multiple bandwidths and with robustness.

Laser Injection Technique for Liquid Crystal Droplets with Arbitrary Shape

We propose a simple and precise method for processing three-dimensional droplet structures of liquid crystal with arbitrary shapes. By irradiating liquid crystal emulsions with a laser beam, a photothermally induced Marangoni effect is introduced. This leads to the injection of water solution into the liquid crystal droplets. With the help of the elasticity of liquid crystal, the injected water solution spontaneously forms into monodispersed guest droplets which latterly self-assemble into branched droplet chains. This method enjoys great flexibility due to its non-contact and in-situ processing manner, allows the simultaneous and multiple processing of all liquid crystal droplets under the irradiation region, and offers excellent compatibility with technologies such as microfluidics. The proposed method is applicable in material science, photonics, and sensors.

An elastic luminescent organic single crystal with linear and nonlinear optical waveguide properties

In recent years, organic single crystals combining elastic and photoluminescent (PL) properties have attracted extensive attention due to their promising applications in many fields. However, elastic organic crystals with two-photon luminescence (2 PL) waveguiding properties are still rarely found, which is imperative for various applications, including near-infrared (NIR) bioimaging and nonlinear optical (NLO) photonic devices. This work reports an elastic single crystal with a long needle shape, anthracene-9,10-dicarbaldehyde (An-DC). Its crystal (100) face exhibits highly reversible bending by applying the mechanical force, which is facilitated by the van der Waals (C– H···π), H-bond (C–H⋯O), and π···π interactions among molecules. The crystal emits a strong orange-red fluorescence when excited by blue-violet light. Interestingly, it also exhibits excellent two-photon fluorescence emission when stimulated by near-infrared light. Based on these linear and nonlinear optical characteristics, the waveguide applications of the An-DC crystal were implemented. Notably, in the bending state, the luminescence performance and the crystal structure of the An-DC crystal are maintained well, which can be used as a low-loss material for linear and nonlinear optical waveguide devices, thus expanding the potential use of flexible organic crystals.

Flexible Förster resonance energy transfer‐assisted optical waveguide based on elastic mixed molecular crystals

AbstractFlexible molecular crystal waveguides based on elastic molecular crystals (EMCs) are essential in flexible and compact optical materials. An increased loss coefficient α due to self‐absorption is often a problem in optical waveguides (OWGs) of fluorescent chemical materials waveguiding photons in active mode. Herein, the development of anthracene‐based elastic mixed molecular crystals (EMMCs) is reported for Förster Resonance Energy Transfer (FRET)‐assisted OWG. To yield a FRET crystal system based on elastic molecular crystals, 1%–5% acceptor doping for fluorescent molecular crystals of 9,10‐dibromoanthracene 1 was successful by selecting the same regioisomer having electron‐withdrawing group, 9,10‐diformylanthracene 2, as a dopant. In addition to conversion to the mixed system, there is a difference in the elastic modulus and hardness in EMC C1 and EMMC C2@1. However, the elastic behaviour was also shown in a few percent doping of the acceptor. The α value of this EMMC, composed of 1 including 1% of 2 (0.0077 dB/μm), is much lower than that of EMC composed of 1 (0.1258 dB/μm) because of reducing self‐absorption in the FRET system. An efficient and flexible OWG was successfully developed by selecting an appropriate acceptor molecule and its low doping rate for mixed crystal construction. This method is a practical approach in various functional and flexible crystal systems.

Biomimetic Multi-Color Separating Photonic Skin Using Electrically Stretchable Chiral Photonic Elastomers.

Structural colors can be produced by nano-periodic dielectric structures using soft materials. Chiral photonic elastomers (CPEs) produced from elastic chiral liquid crystal molecules can self-organize into a helical nanostructure, and the chiral nano-structural color can be tuned by stretching. However, the ability to control the separation of biomimetic multi-colors for practical applications beyond simple uniaxial stretching of single-colored structures has been limited until now. Here, stretchable CPEs with simultaneous multi-color control, including electrical control, are presented. By engineering the heterogeneous elastic modulus of the CPEs, stretchable and simultaneous multi-color separations from an initially homogeneous color are realized. Electrically stretchable multi-color separation is investigated using a hybrid CPE structure on dielectric elastomer actuators, and multi-arrayed color binning and chameleon-like photonic e-skin are further developed for device applications. Moreover, multi-color concealed camouflage switching and control of invisible photonic e-skin are demonstrated. This multi-color control of stretchable photonic systems improves the functionality of various potential photonic applications. This article is protected by copyright. All rights reserved.

Phononic Band Structure by Calculating Effective Parameters of One-Dimensional Metamaterials

Using a theory of homogenization that consists in the discretization of the inclusion of a binary phononic crystal in small volumes, in which the material parameters can be expanded in Fourier series, we have determined the dependence of the effective elastic parameters as a function of the frequency. In particular, the frequency dependence of all the elements that constitute the effective tensors of stiffness (moduli of elasticity) and density was analyzed for a 1D phononic crystal conformed of materials whose main characteristic is the high contrast between their elastic properties. In this dynamic case of homogenization, it was found that the effective parameters can reproduce the exact dispersion relations for the acoustic modes that propagate along the periodicity direction of the crystal. Particularly, in the second pass band (high-frequency branch) corresponding to the transverse vibrational modes, the homogenized elastic phononic crystal exhibits a metamaterial behavior because the effective C44-component (shear modulus) and dynamic mass density were found to be both negative. It is noteworthy that the study derived from this homogenization technique can lead to design of double negative metamaterial systems for potential applications.

Ultra-wideband valley transmission on elastic topological phononic crystals

The Valley Hall effect and topological chiral boundary states play a crucial role in investigating elastic wave transmission properties. In this study, to construct the phononic crystal plate structure, periodic scatterer structures are arranged on the bottom plate at a certain thickness. Simulation analysis reveal that modulating the edge states of the designed elastic phononic crystal plate achieves greater degrees of freedom and enhanced backscattering suppression capabilities. Straight boundary, right-angle turning boundary, and valley topological transport with defect and disordered boundary are achieved by utilizing edge states. Experimental validation confirm the robustness against immunodeficiency and holes. The relative width of the topological band gap in the elastic wave system examined in this study exceeds 60%, offering significant advantages and potential for practical applications. This research contributes new insights for engineering applications of ultra-wideband acoustic antennas, acoustic logic control, and other devices.

An elastic organic crystal with multilevel stimuli-responsive room temperature phosphorescence

•Elastic organic RTP crystal with multilevel stimuli responsiveness was first reported •Highest RTP quantum yield of 25.1% was recorded in elastic organic RTP crystal •Modulation of RTP was realized via the transition of excited state conformation •RTP single to double emission under external stimuli in single-component system In this work, we report a series of single-component molecules crystals named dialkyl 4,4′-oxalyldibenzoate (n-BZ) with both efficient room temperature phosphorescence (RTP) emission, multilevel stimuli responsiveness, and elasticity or flexibility. The crystalline n-BZ will change from original RTP single emission to RTP dual emission after grinding or thermal annealing (TA). Interestingly, the emission wavelength difference between the RTP dual-emission peaks after grinding or TA reaches 82 nm with emission color changes from cyan blue to orange. In addition, to our knowledge, the largest RTP quantum yield of 25.1% (6-BZ) has been achieved as the highest recorded for organic RTP elastic crystals. Through a series of experiments and theoretical calculations, we demonstrate that the multilevel stimuli responsiveness of n-BZ originates from the transition of excited state conformation and its elasticity from many weak intermolecular interactions, which will provide a molecular design and construction strategy for organic stimuli-responsive RTP materials. In this work, we report a series of single-component molecules crystals named dialkyl 4,4′-oxalyldibenzoate (n-BZ) with both efficient room temperature phosphorescence (RTP) emission, multilevel stimuli responsiveness, and elasticity or flexibility. The crystalline n-BZ will change from original RTP single emission to RTP dual emission after grinding or thermal annealing (TA). Interestingly, the emission wavelength difference between the RTP dual-emission peaks after grinding or TA reaches 82 nm with emission color changes from cyan blue to orange. In addition, to our knowledge, the largest RTP quantum yield of 25.1% (6-BZ) has been achieved as the highest recorded for organic RTP elastic crystals. Through a series of experiments and theoretical calculations, we demonstrate that the multilevel stimuli responsiveness of n-BZ originates from the transition of excited state conformation and its elasticity from many weak intermolecular interactions, which will provide a molecular design and construction strategy for organic stimuli-responsive RTP materials.

Manipulation of elastic wave by reconfigurable elastic topological waveguide

We report a numerical study of a reconfigurable topological waveguide based on honeycomb-lattice elastic phononic crystals (EPCs) which consist of two kinds of cavities filled with water. We can realize the EPCs with different symmetries by adjusting the water depth of the cavities, and obtain a Dirac cone for the EPCs composed of the cavities with the same water depth, in which the Dirac frequency can be modulated by adjusting the water depth. When the water depths of the cavities are different, the inversion symmetry of the EPC is broken, destroying the two-fold degeneracy of the Dirac point, and opening an omnidirectional bandgap. Based on EPC-I and EPC-II with opposite valley Hall phases, we design a valley topological waveguide of elastic wave, and obtain valley edge states in the domain wall (DW). Importantly, by adjusting the water depths, we can achieve the conversion between EPC-I and EPC-II, and realize arbitrary DWs for the propagations of elastic waves in the topological waveguide. Finally, we discuss an interesting application of a path-selective waveguide based on a linear interference mechanism. The designed reconfigurable topological waveguide provides an effective method to manipulate valley topological transports of elastic waves, and a theoretical basis for designing advanced topological devices.