Flexible molecular crystals for optoelectronic applications

Unveiling Structure-Dynamic Processes in Crystals of 1D Cd(II) Coordination Polymers during the Elastic Flexible Events

Flexibility, an intriguing yet far from being a commonly observed property of single crystals, is highly sought after as it enables crystal application in innovative technologies. Here, we report on anisotropically elastic single crystals of two novel 1D coordination polymers (CPs) featuring bridging halides (Cl (1); Br (2)) and pyrazinamide ligands that enabled the determination of the bending mechanism and exploration of their mechanical properties. The mechanism identified distinctly differs from the mechanisms of elastically flexible molecular crystals subjected to mechanical stress and 1D plastically deformable crystals under quasi-hydrostatic pressure, the mechanisms of reconfigurable crystals uncovered thus far. By mapping the structural modification across the bent crystal 1 using microfocus synchrotron radiation, we found that both the 1D structural spine (controlling the distance between the organic ligands) and the ligands themselves adapt to the changes in external conditions. While the spine expands through the modifications of the bridging angles, causing a consequent enlargement of the metal···metal distances as going from the interior to the exterior of the bend, the ligands rotate toward linearity with the bending face. In addition, the extent of changes along the two other crystal axes is brought into a connection with the relative energies of the weakest links and crystal stiffness in those directions.

Elastic circular organic microcrystals prepared by photoinduced delamination

Circular organic crystals are essential as optically transducive components in flexible organic optoelectronics, yet this crystal habit is not easily obtained through traditional crystallization approaches. Here, we present a photoresponsive organic crystalline material that when exposed to ultraviolet or visible light, initially undergoes photoinduced bending, followed by photosalient effect and accompanied by delamination to elastic quasicircular microcrystals. Curvature analysis under different conditions confirms the controllability of this process. Light at 365 nm, 405 nm, and 445 nm generates microcrystals with high curvatures (11-12 mm-1), while 470 nm light produces lower curvature (5 mm-1), aligning with the absorption profile. Increasing the excitation power from 15 mW to 150 mW results in increase of the yield of microcrystals with high curvatures (10-20 mm-1) from 20% to 94%. This light-driven fabrication method provides a controlled and reproducible means of realizing rare crystal morphologies, highlighting the potential for exploring quantitative relationships between such morphologies and their unconventional optical properties.

Click To Stack: Shape-Assisted Self-Assembly of Unflattened Macrocycles into Waveguiding Elastic Crystals

Designing molecular crystals for practical applications requires precise control over intermolecular interactions. We report a click-to-stack strategy to enable facile construction and systematic structural modification of tetraarene-fused cyclooctatetraenes. These saddle-shaped molecules self-assemble into one-dimensional columns with exceptional precision, driven solely by weak van der Waals interactions. Single-crystal X-ray crystallographic and computational studies revealed fully eclipsed π-π stacking, reinforced by the shape complementarity of the negatively curved molecular contour. With alkoxy groups tethered, the antiparallel alignment of coaxial cable-like assemblies produces centimeter-long voidless crystals that exhibit remarkable structural durability under mechanical stress. These glow stick-like elastic crystalline rods display optical waveguiding properties, demonstrating the practical utility of highly anisotropic molecular assemblies with structural uniformity and resilience.

One-Dimensional Organic Lead Bromide Elastic Crystals with Strong Electron-Phonon Coupling

Two novel one-dimensional (1D) lead halide elastic crystals, [Pb2Br4(DMA)2]n (1) (DMA = N,N-dimethylacetamide) and [Pb2Br4(DMF)2]n (2) (DMF = N,N-dimethylformamide), were reported. Both compounds 1 and 2 feature a neutral 1D bimetallic axis chain structure. The bending strain of compound 1 is 1.36%, higher than those of all reported 1D single-metal axis chain coordinate polymers, indicating the superior elastic properties of the 1D bimetallic axis chain polymers. Compound 1 exhibits strong red-to-near-infrared (NIR) fluorescence emission below 200 K, with an emission peak at 700 nm and a full width at half maximum (fwhm) of 200 nm, indicating strong electron-phonon coupling in compound 1. The large Stokes shift and broad fwhm of compound 1 may be attributed to its excellent elasticity, as this elasticity allows the molecule's self-trapped exciton state to undergo greater structural distortion. It is suggested that 1D organic lead halide elastic crystals could be promising candidates for emerging applications in efficient NIR light-emitting diodes, supercontinuum sources, and flexible NIR optical waveguides.

Prediction and Validation of Mechanical Flexibility in Molecular Crystals: Dispersion Interactions Dictate Bending

Mechanically flexible crystals are a rapidly growing class of functional molecular materials. Typically, such flexible crystals are discovered by serendipity. Herein, we have predicted mechanical flexibility in a series of molecular crystals based on a structure screening approach that combines interaction topology and the presence of nitrile···nitrile interactions-a supramolecular motif hitherto not associated with bending property. Further, we have experimentally validated plastic/elastic bending properties in a series of crystal structures thus predicted. However, four out of five of these crystals showed the bending direction along π···π stacking despite the fact that the direction of strongest interaction anisotropy was rendered by the nitrile···nitrile interaction motifs. This is contrary to the commonly perceived anisotropy model and underscores the dominant role of dispersion forces over the electrostatically stabilized motifs in dictating the bending phenomena in molecular crystals. The interaction energies of these motifs have been evaluated using accurate structures from X-ray quantum crystallography. Analyses combining elastic tensors, interaction anisotropy indices, thermal expansion studies, and high-pressure simulations quantify the relative roles of the nitrile···nitrile motif and π···π stacking in mechanical flexibility. Our results point to the possibility of expanding the realm of flexible molecular materials to novel structural types aided by predictive computational models.

Mechanically deformable organic ferroelectric crystal with plasticity optimized by fluorination

The ability of plastic deformation exerts in bulk crystals would offer great promise for ferroelectrics to achieve emerging and exciting applications. However, conventional ferroelectric crystals generally suffer from inherent brittleness and are easy to fracture. Here, by implementing fluorination on anion, we successfully design a flexible organic ferroelectric phenylethylammonium trifluoromethanesulfonate (PEA-TFMS) with interesting plasticity in its bulk crystals. To our knowledge, it is the first observation since the discovery of organic ferroelectric crystal triglycine sulfate in 1956. Compared to parent PEA-MS (phenylethylammonium mesylate), fluorination subtly alters ionic orientation and interactions to reorganize dipole arrangement, which not only brings switchable spontaneous polarization but also endows PEA-TFMS crystal with macroscopical bending and spiral deformability, making it a competitive candidate for flexible and wearable devices. Our work will bring inspiration for obtaining mechanically deformable organic ferroelectric crystals toward flexible electronics.

Thermoelastic twisting-assisted crystal jumping based on a self-healing molecular crystal

Adaptive crystals have attracted significant attention from solid-state chemists and crystal engineers for their promising applications in memories, capacitors, sensors, and actuators. Among them, thermosalient crystals are particularly favored thanks to their efficient energy conversions and rapid responses. However, the mechanisms for the mechanical responses of thermosalient crystals remain largely unclear. Herein we demonstrate that thermosalient effects of molecular crystals could be driven by thermoelastic twisting behaviors. The crystal, based on a model compound with rigid dibenzothiophene sulfone planes and flexible ethoxy chains, can spontaneously self-heal from mechanical fractures. Upon heating, the crystal undergoes remarkable thermosalient behaviors driven by a distinctive left- or right-handed twisting. This thermoelastic twisting converts thermal energy into elastic potential energy, which is further released as kinetic energy upon untwisting to drive the crystal jump. Our demonstration on thermoelastic twisting-induced crystal jumping offers a different perspective on the origins of thermosalient crystals and could provide inspiration for future engineering and application of dynamic molecular crystals.

In Situ Generation of Microwire Heterojunctions with Flexible Optical Waveguide and Hydration-Mediated Photochromism

Flexible heterojunctions based on molecular systems are in high demand for applications in photonics, electronics, and smart materials, but fabrication challenges have hindered progress. Herein, we present an in situ approach to creating optical heterojunctions using hydration-mediated flexible molecular crystals. These hydrated multi-component molecular solids display strong blue emitting optical waveguides with minimal optical loss (0.005 dB/μm) and excellent flexibility (elastic modulus: 3.87 GPa). The water-mediated process enables the molecular microwires with tunable elastic and plastic deformation, as well as reversible uptake and release of lattice water, facilitating the formation of flexible heterojunctions. Spectral analysis and theoretical modeling reveal that these microwires exhibit both photochromism and color-tunable dual emission (fluorescence and phosphorescence), expanding their utility in photonic information encoding. Therefore, this work introduces a hydration-mediated molecular engineering strategy for fabricating crystalline heterojunctions with on-demand processability and controllable emission sequences, enabling optical signal manipulation at the micro/nanoscale.

Precise Photochemical Post-Processing of Molecular Crystals

Molecular crystals carry a great potential as new soft smart materials, with a plethora of recent examples overcoming the major obstacle of mechanical flexibility, and this research direction holds enormous potential to revolutionize optics, electronics, medicine, and space exploration. However, shaping organic crystals into desired shapes and sizes remains a major practical challenge due to the lack of control over the crystallization process, and the difficulties in mechanical post-processing without introduction of defects that are usually imparted by their soft nature. Here we present an innovative approach that employs photochemical processing for precise and nondestructive cutting of a molecular crystal. Our proposed method uses light to post-process crystals of the desired size and shape, similar to using light to cut other materials. This reaction induces strain, ensuring sharp cleavage without the need for melting or other processes. We further demonstrate the potential of this approach by producing crystals of arbitrary size, which can be used as controllable optical waveguides. Among other potential applications, this method can be used to prepare dynamic crystals, particularly those with aspect ratios crucial for mechanical deformation, such as flexible electronics, soft robotics, and sensing.

Ultra-Wide Modulation and Reversible Reconfiguration of a Flexible Organic Crystalline Optical Waveguide Between 645 and 731 nm

Flexible organic crystalline optical waveguides, which deliver input or self-emit light through various dynamic organic crystals, have attracted increasing attention in the past decade. However, the modulation of the waveguide output relies on chemical design and substituent modification, being time-consuming and laborious. Here we report an elastic organic crystal that displays long-distance light transduction up to 2.0 cm and an ultra-wide modulation of crystalline optical waveguides between red (645 nm) and near infrared (731 nm) in both the pristine and the elastically bent states based on a pre-designed self-absorption effect. The flexible organic crystalline optical waveguides can be precisely and reversibly reconfigured by controlling the irradiation point. In addition, deep-red amplified spontaneous emission (ASE) that is able to transduce through a 5.0 mm bent crystal with an ultra-low optical loss coefficient of 0.093 dB/mm has been attained. To the best of our knowledge, this is the first report of flexible organic ASE waveguides. The present study not only provides a simple yet effective strategy to remarkably modulate flexible organic crystalline optical waveguides but also demonstrates the superiority of lasing over normal emission as flexible optical communication elements.

Mechanical Twisting-Induced Enhancement of Second-Order Optical Nonlinearity in a Flexible Molecular Crystal

Flexible molecular crystals are essential for advancing smart materials, providing unique functionality and adaptability for applications in next-generation electronics, pharmaceuticals, and energy storage. However, the optical applications of flexible molecular crystals have been largely restricted to linear optics, with nonlinear optical (NLO) properties rarely explored. Herein, we report on the application of mechanical twisting of flexible molecular crystals for second-order nonlinear optics. The crystal formed through the self-assembly of the model compound 9-anthraldehyde (AA) features an intrinsic chiral and noncentrosymmetric structure, demonstrating high efficiency second harmonic generation (SHG) and NLO circular dichroism, which could be greatly enhanced by macroscopic mechanical twisting. The anisotropic molecular stacking imparts the AA crystal with mechanical flexibility of combined elastic bending and plastic twisting. The isochiral mechanical twisting could greatly enhance the SHG intensities by an order of magnitude depending on their M- or P-configuration. Meanwhile, the SHG circular dichroism factor gSHG-CD of the isochiral twisted crystal is greatly increased, achieving the highest reported NLO anisotropy factor among organic NLO materials. These boosted NLO performances of SHG intensity and nonlinear chiroptical response are expected to greatly expand the photonic applications of flexible molecular crystals.

Hybrid Strategies for Enhancing the Multifunctionality of Smart Dynamic Molecular Crystal Materials

Dynamic molecular crystals are an emerging class of smart engineering materials that possess unique ability to convert external energy into mechanical motion. Moreover, they have being considered as strong candidates for dynamic elements in applications such as flexible electronic devices, artificial muscles, sensors, and soft robots. However, the inherent defects of molecular crystals like brittleness, short-life and fatigue, have significantly impeded their practical applications. Inspired by the concept of "the whole is greater than the sum of its parts" in the field of biology, building stimuli-response composites materials can be regarded as one of the ways to break through the current limitations of dynamic molecular crystals. Moreover, the hybrid materials can exhibit new functionalities that cannot be achieved by a single object. In this review, the focus was placed on the analysis and discussion of various hybrid strategies and options, as well as the functionalities of hybrid dynamic molecular crystal materials and the important practical applications of composite materials, with the introduction of photomechanical molecular crystals and flexible molecular crystals as a starting point. Moreover, the efficiency, limitations, and advantages of different hybrid methods were compared and discussed. Furthermore, the promising perspectives of smart dynamic molecular crystal materials were also discussed and the potential directions for future work were suggested.

Charge-transfer mediated J-aggregation in red emitting ultra-small-single-benzenic meta-fluorophore crystals

Red emission in crystals has been observed with an ultra-small-single-benzenic meta-fluorophore (MF) with a molecular weight (MW) of only 197 Da, bettering the literature report of fluorophores with the lowest MW = 252 Da. Supramolecular extensive hydrogen-bonding and J-aggregate type centrosymmetric discrete-dimers or a 1D chain of MFs led to red emission (λ max em = 610-636 nm) in MF crystals. Unlike in the solution phase showing one absorption band, in thin films and in crystals the transition from the S0 state to both the S1 - state and S1 + state becomes feasible. The angle between the transition dipole moments has been obtained to be 66.99° and the exciton splitting energy has been obtained to be (-) 55.7 meV. Significant overlap have been observed and the extent of overlaps integrals between the HOMOs and the LUMOs were assessed to be 0.0068 and (-) 0.00024, respectively. Planar molecules are shown to be involved in anti-parallel stacking with a slip-angle of 44.05° and an inter-planar longitudinal distance of 3.40 Å. A large magnitude of ΔE ES (energy difference between the S1 - state and S1 + state) (0.83 eV) has been obtained. A much higher magnitude of the CT coupling constant (-0.708 for MF2) has been noted in comparison to the coulombic coupling constant (0.016 for MF2). The excited-state-lifetime has been shown to increase from 5.98 ns (in hexane) to 30.90 ns in the crystal. All these extra-ordinary optical properties point to the existence of a charge-transfer mediated J-aggregation phenomenon in these MF crystals. Based on these fascinating observations, highly stable, bright and colour pure white LEDs could be generated.

The interplay between hydrogen bonds and stacking/T-type interactions in molecular cocrystals

Supramolecular synthon and hydrogen bond pairing approaches have influenced the understanding of cocrystal formation for decades, but are hydrogen bonds really the dominant interaction in cocrystals? To investigate this, an extensive analysis of 1:1 two-component cocrystals in the Cambridge Structural Database was undertaken, revealing that stacking and T-type interactions are just as, if not more important than hydrogen bonds in molecular cocrystals. A total of 84% of the most common coformers in the dataset are aromatic. When analysing cocrystal dimers, only 20% consist of solely strong hydrogen bonds, with over 50% of contacts involving stacking and T-type interactions. Combining interaction strength and frequency, both hydrogen bond and stacking/T-type interactions contribute equally to the stabilisation of cocrystal lattices. Therefore, we state that crystal engineering and cocrystal design concepts of the future should not solely revolve around supramolecular synthon pairing via hydrogen bonds, but instead consider optimising both hydrogen bonding and stacking/T-type interactions.

Large Local Internal Stress in an Elastically Bent Molecular Crystal Revealed by Raman Shifts

The structural dynamics involved in the mechanical flexibility of molecular crystals and the internal stress in such flexible materials remain obscure. Here, the study reports an elastically bending lipidated molecular crystal that shows systematic shifts in characteristic vibrational frequencies across the bent crystal region - revealing the nature of structural changes during bending and the local internal stress distribution. The blueshifts in the bond stretching modes (such as C═O and C-H modes) in the inner arc region and redshifts in the outer arc region of the bent crystals observed via micro-Raman mapping are counterintuitive to the bending models based on intermolecular hydrogen bonds. Correlating these shifts with the trends observed from high-pressure Raman studies on the crystal reveals the local stress difference between the inner arc and outer arc regions of the bent crystal to be ≈2 GPa, more than an order of magnitude higher than the previously proposed value in elastically bending crystals. High local internal stress can have direct ramifications on the properties of molecular piezoelectric energy harvesters, actuators, semiconductors, and flexible optoelectronic materials.

Macroscopic Twisting of Chiral Metal Halide Single Crystals Driven by Thermo-Induced Topochemical Dehydration

Dynamic twisting crystals, combining the features of dynamic crystals and twisting crystals, promise advanced applications in targeted drug delivery, biosensors, microrobots, and spiral optoelectronics. However, the determination of dynamic twisting crystals with specific directions remains a formidable challenge in practical applications. Herein, based on organic-inorganic hybrid metal halide (OIHMH) single crystals, we have realized the chirality-induced macroscopic twisting of single crystals driven by a thermo-induced topochemical dehydration reaction. These crystals exhibit molecular-chirality-induced twisting upon heating, along with reversals in their linear chiroptical circular dichroism and nonlinear chiroptical second harmonic generation circular dichroism. Such an induced twisting has been attributed to the alteration of the helical arrangement of chiral cation post-topochemical dehydration. The feasibility of tuning the macroscopic twisting of OIHMH single crystals and the switching in their linear and nonlinear chiroptical properties might open up new avenues for developing dynamic crystals for microactuating and optoelectronic applications.

Preparation of intrinsically fragile bent crystals

Although molecular crystals have long been considered to be intrinsically brittle, a study of molecular crystals that are capable of plastic or elastic bending upon applying mechanical stress recently attracted significant attention. Malleable molecular crystals often need to meet specific criteria regarding the intermolecular interaction patterns within the crystal structure. Accordingly, examples have been reported where one polymorph shows bending, while other polymorphs of the same compound exhibit fracturing upon exposure to mechanical force. Here, we have succeeded in preparing bent crystals of an intrinsically fragile polymorph. Methylated flufenamic acid (1) can form three different polymorphs, i.e., 1α, 1β, and 1γ, of which 1β is difficult to isolate. Under mechanical force, the crystals of 1α exhibit remarkable plastic deformation, while those of 1γ are readily broken. Similar to the mechanical properties, the emission properties of 1 differ depending on the polymorph, i.e., 1γ exhibits a shorter-wavelength emission maximum and much higher emission quantum yield than 1α. Remarkably, both the unbent and bent forms of the 1α crystals can undergo a phase transition to the 1γ phase upon exposure to ethyl acetate. In this manner, phase transitions of the mechanically bent crystals of polymorph 1α afforded bent crystals of the intrinsically fragile polymorph 1γ. These findings may lead to a potential post-modification method for the preparation of functional flexible materials with enhanced emission properties in order to expand their applications.