Crystal property engineering using molecular-supramolecular equivalence: mechanical property alteration in hydrogen bonded systems.

Most crystal engineering strategies exercised until now mainly rely on the alteration of weak non-covalent interactions to design structures and thus properties. Examples of mechanical property alteration for a given structure type are rare with only a few halogen bonded cases. The modular nature of halogen bonds with interaction strength tunability makes the task straightforward to obtain property differentiated crystals. However, the design of such crystals using hydrogen bond interactions has proven to be non-trivial, because of its relatively higher difference in bonding energies, and more importantly, disparate geometries of the functional groups. In the present crystal property engineering exercise, with the support of CSD analysis, we replaced a supramolecular precursor that leads to plastically bendable crystals, with a molecular equivalent, and obtained an equivalent crystal structure. As a result, the new structure, with comparable hydrogen bonding chains, produces elastically bendable single crystals (as opposed to plastically bendable crystals). In addition, the crystals show multidirectional (here two) elastic bending as well as rare elastic twisting. The occurrence of multiple isostructural examples, including a solid solution, with identical properties further demonstrates the general applicability of the proposed model. Crystals cannot display the concerned mechanical property in the absence of the desired structure type and fracture in a brittle manner on application of an external stress. Nanomechanical experiments and energy framework calculations also complement our results. To the best of our knowledge, this is the first example of a rational crystal engineering exercise using solely hydrogen bond interactions to obtain property differentiated crystals. This strategy namely molecular-supramolecular equivalence has been unexplored till now to tune mechanical properties, and hence is useful for crystal property engineering.

Microscopic Cracks Modulate Nucleation and Solid-State Crystallization Tendency of Amorphous Celecoxib.

Drugs have been classified as fast, moderate, and poor crystallizers based on their inherent solid-state crystallization tendency. Differential scanning calorimetry-based heat-cool-heat protocol serves as a valuable tool to define the solid-state crystallization tendency. This classification helps in the development of strategies for stabilizing amorphous drugs. However, microscopic characteristics of the samples were generally overlooked during these experiments. In the present study, we evaluated the influence of microscopic cracks on the crystallization tendency of a poorly water-soluble model drug, celecoxib. Cracks developed in the temperature range of 0-10 °C during the cooling cycle triggered the subsequent crystallization of the amorphous phase. Nanoindentation study suggested minimal differences in mechanical properties between samples, although the cracked sample showed relatively inhomogeneous mechanical properties. Nuclei nourishment experiments suggested crack-assisted nucleation, which was supported by Raman data that revealed subtle changes in intermolecular interactions between cracked and uncracked samples. Celecoxib has been generally classified as class II, i.e., a drug with moderate crystallization tendency. Interestingly, classification of amorphous celecoxib may change depending on the presence or absence of cracks in the amorphous sample. Hence, subtle events such as microscopic cracks should be given due consideration while defining the solid-state crystallization tendency of drugs.

Thermally activated delayed fluorescence in a mechanically soft charge-transfer complex: role of the locally excited state.

Molecular design for thermally activated delayed fluorescence (TADF) necessitates precise molecular geometric requirements along with definite electronic states to ensure high intersystem crossing (ISC) rate and photoluminescence quantum yield (PLQY). Achieving all these requirements synchronously while maintaining ease of synthesis and scalability is still challenging. To circumvent this, our strategy of combining a crystal engineering approach with basic molecular quantum mechanical principles appears promising. A holistic, non-covalent approach for achieving efficient TADF in crystalline materials with distinct mechanical properties is highlighted here. Charge transfer (CT) co-crystals of two carbazole-derived donors (ETC and DTBC) with an acceptor (TFDCNB) molecule are elaborated as a proof-of-concept. Using temperature-dependent steady-state and time-resolved photoluminescence techniques, we prove the need for a donor-centric triplet state (3LE) to ensure efficient TADF. Such intermediate states guarantee a naturally forbidden, energetically uphill reverse intersystem crossing (RISC) process, which is paramount for effective TADF. A unique single-crystal packing feature with isolated D-A-D trimeric units ensured minimal non-radiative exciton loss, leading to a high PLQY and displaying interesting mechanical plastic bending behaviour. Thus, a comprehensive approach involving a non-covalent strategy to circumvent the conflicting requirements of a small effective singlet-triplet energy offset and a high oscillator strength for efficient TADF emitters is achieved here.

Autonomous self-healing organic crystals for nonlinear optics.

Non-centrosymmetric molecular crystals have a plethora of applications, such as piezoelectric transducers, energy storage and nonlinear optical materials owing to their unique structural order which is absent in other synthetic materials. As most crystals are brittle, their efficiency declines upon prolonged usage due to fatigue or catastrophic failure, limiting their utilities. Some natural substances, like bone, enamel, leaf and skin, function efficiently, last a life-time, thanks to their inherent self-healing nature. Therefore, incorporating self-healing ability in crystalline materials will greatly broaden their scope. Here, we report single crystals of a dibenzoate derivative, capable of self-healing within milliseconds via autonomous actuation. Systematic quantitative experiments reveal the limit of mechanical forces that the self-healing crystals can withstand. As a proof-of-concept, we also demonstrate that our self-healed crystals can retain their second harmonic generation (SHG) with high efficiency. Kinematic analysis of the actuation in our system also revealed its impressive performance parameters, and shows actuation response times in the millisecond range.

Non-stoichiometric carbamazepine cocrystal hydrates of 3,4-/3,5-dihydroxybenzoic acids: coformer-water exchange.

The cocrystallisation of carbamazepine (CBZ) with 3,4-/3,5-dihydroxybenzoic acids (34/35DHBA) with different stoichiometries formed molecular alloys, exchanging a water molecule, in their isostructural CBZ dihydrate form. Furthermore, we show a correlation between the mechanical properties of the CBZ-DHBA cocrystals with the amount of coformer present.

Elastic organic semiconducting single crystals for durable all-flexible field-effect transistors: insights into the bending mechanism.

Although many examples of mechanically flexible crystals are currently known, their utility in all-flexible devices is not yet adequately demonstrated, despite their immense potential for fabricating high performance flexible devices. Here, we report two alkylated diketopyrrolopyrrole (DPP) semiconducting single crystals, one of which displays impressive elastic mechanical flexibility whilst the other is brittle. Using the single crystal structures and density functional theory (DFT) calculations, we show that the methylated diketopyrrolopyrrole (DPP-diMe) crystals, with dominant π-stacking interactions and large contributions from dispersive interactions, are superior in terms of their stress tolerance and field-effect mobility (µ FET) when compared to the brittle crystals of the ethylated diketopyrrolopyrrole derivative (DPP-diEt). Periodic dispersion-corrected DFT calculations revealed that upon the application of 3% uniaxial strain along the crystal growth (a)-axis, the elastically flexible DPP-diMe crystal displays a soft energy barrier of only 0.23 kJ mol-1 while the brittle DPP-diEt crystal displays a significantly larger energy barrier of 3.42 kJ mol-1, in both cases relative to the energy of the strain-free crystal. Such energy-structure-function correlations are currently lacking in the growing literature on mechanically compliant molecular crystals and have the potential to support a deeper understanding of the mechanism of mechanical bending. The field effect transistors (FETs) made of flexible substrates using elastic microcrystals of DPP-diMe retained µ FET (from 0.019 cm2 V-1 s-1 to 0.014 cm2 V-1 s-1) more efficiently even after 40 bending cycles when compared to the brittle microcrystals of DPP-diEt which showed a significant drop in µ FET just after 10 bending cycles. Our results not only provide valuable insights into the bending mechanism, but also demonstrate the untapped potential of mechanically flexible semiconducting crystals for designing all flexible durable field-effect transistor devices.

Viscoelastic Covalent Organic Nanotube Fabric via Macroscopic Entanglement.

Covalent organic nanotubes (CONTs) are one-dimensional porous frameworks constructed from organic building blocks via dynamic covalent chemistry. CONTs are synthesized as insoluble powder that restricts their potential applications. The judicious selection of 2,2'-bipyridine-5,5'-dicarbaldehyde and tetraaminotriptycene as building blocks for TAT-BPy CONTs has led to constructing flexible yet robust and self-standing fabric up to 3 µm thickness. The TAT-BPy CONTs and TAT-BPy CONT fabric have been characterized by solid-state one-dimensional (1D) 13C CP-MAS, two-dimensional (2D) 13C-1H correlation NMR, 2D 1H-1H DQ-SQ NMR, and 2D 14N-1H correlation NMR spectroscopy. The mechanism of fabric formation has been established by using high-resolution transmission electron microscopy and scanning electron microscopy techniques. The as-synthesized viscoelastic TAT-BPy CONT fabric exhibits high mechanical strength with a reduced modulus (Er) of 8 (±3) GPa and hardness (H) of 0.6 (±0.3) GPa. Interestingly, the viscoelastic fabric shows time-dependent elastic depth recovery up to 50-70%.

De novo synthesis of hybrid d-f block metal complex salts for electronic charge transport applications.

The advent of d-d type complex salts for designing smart functional materials with versatile utility inspired us to develop a novel type of M(II)-Ce(IV) complex salts [M(II) = Cu and Zn ions]. In this study, we present for the first time a holistic approach to design and prepare metal complex salts of the novel hybrid d-f block type, [Cu(bpy)2]2[Ce(NO3)6]2 (1), [Cu(phen)2(NO3)]2[Ce(NO3)6](HNO3) (2), [Zn(bpy)2(NO3)][ClO4] (3), and [Zn(phen)2(NO3)]2 [Ce(NO3)6] (4); [bpy = 2,2'-bipyridine; phen = 1,10-phenanthroline]. The intrinsic structural and morphological properties of the compounds have been revealed by employing a suite of analytical and spectroscopic methods. X-ray structural analysis reveals that the copper(II) centres in the cationic complex units of 1 and 2 adopt a highly distorted tetrahedral and a rare bicapped square pyramidal coordination geometry, respectively. The zinc(II) ions in both 3 and 4 adopt the rare bicapped square pyramidal geometry while the cerium(IV) ions in 1, 2 and 4 exist in a dodecahedral geometry. Investigation of supramolecular interactions reveals that intermolecular O⋯H and O⋯π short contacts bind the complex units in 1, while predominant π⋯π interactions, along with O⋯H and O⋯π short contacts, produce the binding force among the complex units in 2. We further employed the complex salts (1-4) to construct Schottky devices to reveal the role of these new complex salts in the charge-transport phenomenon. The carrier mobilities (µ) for salts 1-4 were determined to be 1.76 × 10-6, 9.02 × 10-6, 1.86 × 10-8, and 4.31 × 10-8 m2 V-1 s-1, with respective transit times (τ) of 439, 85, 4.17 × 103, and 1.79 × 103 ns, which suggest that complex salt 2 is the best candidate with the highest transport properties among all the complex salts. A crystal engineering perspective sheds light on the charge-transport properties of the complex salts, emphasizing the attribution of the best performance of 2 to its predominant π⋯π interactions. The synthesis of this new type of complex salts, their physicochemical properties and their charge-transport applications envisage great promise for the development of novel crystalline materials with smart functionalities.

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.

Dual Nanomechanics in Anisotropic Porous Covalent Organic Framework Janus-Type Thin Films.

Empowered by crystalline ordered structures and homogeneous fabrication techniques, covalent organic frameworks (COFs) have been realized with uniform morphologies and isotropic properties. However, such homogeneity often hinders various surface-dependent properties observed in asymmetric nanostructures. The challenge remains to induce heterogeneity in COFs by creating an asymmetric superstructure such as a Janus thin film. In this regard, we propose a versatile yet straightforward interfacial layer-grafting strategy to fabricate free-standing Janus-type COF-graphene thin films. Herein, two-dimensional graphene sheets were utilized as the suitable grafter due to the possibility of noncovalent interactions between the layers. The versatility of the approach was demonstrated by fabricating two distinct Janus-type films, with the COF surface interwoven with nanofibers and nanospheres. The Janus-type films showcase opposing surface morphologies originating from graphene sheets and COF nanofibers or nanospheres, preserving the porosity (552-600 m2 g-1). The unique surface chemistries of the constituent layers further endow the films with orthogonal mechanical properties, as confirmed by the nanoindentation technique. Interestingly, the graphene sheets favor the Janus-type assembly of COF nanofibers over the nanospheres. This is reflected in the better nanomechanical properties of COFfiber-graphene films (Egraphene = 300-1200 MPa; ECOF = 15-60 MPa) compared to the COFsphere-graphene films (Egraphene = 11-14 MPa; ECOF = 2-5 MPa). These results indicate a direct relationship between the mechanical properties and homo/heterogeneity of Janus-type COF films.

Autonomous self-repair in piezoelectric molecular crystals.

Living tissue uses stress-accumulated electrical charge to close wounds. Self-repairing synthetic materials, which are typically soft and amorphous, usually require external stimuli, prolonged physical contact, and long healing times. We overcome many of these limitations in piezoelectric bipyrazole organic crystals, which recombine following mechanical fracture without any external direction, autonomously self-healing in milliseconds with crystallographic precision. Kelvin probe force microscopy, birefringence experiments, and atomic-resolution structural studies reveal that these noncentrosymmetric crystals, with a combination of hydrogen bonds and dispersive interactions, develop large stress-induced opposite electrical charges on fracture surfaces, prompting an electrostatically driven precise recombination of the pieces via diffusionless self-healing.

Investigation of Poor Solubility of a Salt-Cocrystal Hydrate: A Case Study of the Common-Ion Effect in Betrixaban, an Anticoagulant Drug.

Achieving the desired solubility and dissolution of active pharmaceutical ingredients (APIs) continues to be a big challenge in the pharmaceutical industry. In this regard, multicomponent solids of APIs such as salts and cocrystals have shown significant promise in resolving such solubility/dissolution issues. However, very little is known on how the APIs' solubility or dissolution is affected by the drug to coformer ratio in multicomponent solids. Betrixaban, is an anticoagulant drug approved in 2017 for the prevention of venous thromboembolism. During the alternate solid form development studies of the known betrixaban maleate, a rare multicomponent solid form, salt-cocrystal hydrate of betrixaban, was discovered and characterized thoroughly by spectroscopic, thermal, and X-ray crystallographic methods. Significantly, the new betrixaban maleate maleic acid hydrate (1:1:2:1) form has shown lower melting point (80 °C) as compared to its parent salt (197.5 °C). From such a large melting difference (117 °C) between the salt and salt-cocrystal hydrate of API, we anticipated substantially better solubility for the salt-cocrystal hydrate (low enthalpy). Furthermore, the predicted solubility also supported our anticipation. However, the powder dissolution tests at different pH conditions provided contrary results, that is, the salt-cocrystal hydrate showed 10 times lower solubility as compared to its salt. A detailed investigation, considering all the potential factors, revealed that "common-ion effect" could be a critical factor for the low solubility of the salt-cocrystal hydrate in which the API to coformer ratio is 1:3. To the best of our knowledge, this is the first case study on the solubility of pharmaceutical salt-cocrystal hydrates with an emphasis on "common-ion effect" or drug to coformer ratio.

Self-Assembly-Driven Nanomechanics in Porous Covalent Organic Framework Thin Films.

Nanomechanics signifies a key tool to interpret the macroscopic mechanical properties of a porous solid in the context of molecular-level structure. However, establishing such a correlation has proved to be significantly challenging in porous covalent organic frameworks (COFs). Structural defects or packing faults within the porous matrix, poor understanding of the crystalline assembly, and surface roughness are critical factors that contribute to this difficulty. In this regard, we have fabricated two distinct types of COF thin films by controlling the internal order and self-assembly of the same building blocks. Interestingly, the defect density and the nature of supramolecular interactions played a significant role in determining the corresponding thin films' stress-strain behavior. Thin films assembled from nanofibers (∼1-2 µm) underwent large deformation on the application of small external stress (Tp-Azofiber film: E ≈ 1.46 GPa; H ≈ 23 MPa) due to weak internal forces. On the other hand, thin films threaded with nanospheres (∼600 nm) exhibit a much stiffer and harder mechanical response (Tp-Azosphere film: E ≈ 15.3 GPa; H ≈ 66 MPa) due to strong covalent interactions and higher crystallinity. These porous COF films further exhibited a significant elastic recovery (∼80%), ideal for applications dealing with shock-resistant materials. This work provides in-depth insight into the fabrication of industrially relevant crystalline porous thin films and membranes by addressing the previously unanswered questions about the mechanical constraints in COFs.

Harnessing molecular rotations in plastic crystals: a holistic view for crystal engineering of adaptive soft materials.

Plastic crystals (PCs), formed by certain types of molecules or ions with reorientational freedom, offer both exceptional mechanical plasticity and long range order, hence they are attractive for many mechano-adaptable technologies. While most classic PCs belong to simple globular molecular systems, a vast number of examples in the literature with diverse geometrical (cylindrical, bent, disk, etc.) and chemical (neutral, ionic, etc.) natures have proven their wide scope and opportunities. All the recent reviews on PCs aim to provide insights into a particular application, for instance, organic plastic crystal electrolytes or ferroelectrics. This tutorial review presents a holistic view of PCs by unifying the recent excellent progress in fundamental concepts from diverse areas as well as comparing them with liquid crystals, amphidynamic crystals, ordered crystals, etc. We cover the molecular and structural origins of the unique characteristics of PCs, such as exceptional plasticity, facile reversible switching of order-to-disorder states and associated colossal heat changes, and diffusion of ions/molecules, and their attractive applications in solid electrolytes, opto-electronics, ferroeletrics, piezoelectrics, pyroelectrics, barocalorics, magnetics, nonlinear optics, and so on. The recent progress not only demonstrates the diversity of scientific areas in which PCs are gaining attention but also the opportunities one can exploit using a crystal engineering approach, for example, the design of novel dynamic functional soft materials for future use in flexible devices or soft-robotic machines.

Mechanical-Bending-Induced Fluorescence Enhancement in Plastically Flexible Crystals of a GFP Chromophore Analogue.

Single crystals of optoelectronic materials that respond to external stimuli, such as mechanical, light, or heat, are immensely attractive for next generation smart materials. Here we report single crystals of a green fluorescent protein (GFP) chromophore analogue with irreversible mechanical bending and associated unusual enhancement of the fluorescence, which is attributed to the strained molecular packing in the perturbed region. Soft crystalline materials with such fluorescence intensity modulations occurring in response to mechanical stimuli under ambient pressure conditions will have potential implications for the design of technologically relevant tunable fluorescent materials.

Mechanophotonics: Flexible Single-Crystal Organic Waveguides and Circuits.

We present the one-dimensional optical-waveguiding crystal dithieno[3,2-a:2',3'-c]phenazine with a high aspect ratio, high mechanical flexibility, and selective self-absorbance of the blue part of its fluorescence (FL). While macrocrystals exhibit elasticity, microcrystals deposited at a glass surface behave more like plastic crystals due to significant surface adherence, making them suitable for constructing photonic circuits via micromechanical operation with an atomic-force-microscopy cantilever tip. The flexible crystalline waveguides display optical-path-dependent FL signals at the output termini in both straight and bent configurations, making them appropriate for wavelength-division multiplexing technologies. A reconfigurable 2×2-directional coupler fabricated via micromanipulation by combining two arc-shaped crystals splits the optical signal via evanescent coupling and delivers the signals at two output terminals with different splitting ratios. The presented mechanical micromanipulation technique could also be effectively extended to other flexible crystals.

Isomorphism: 'molecular similarity to crystal structure similarity' in multicomponent forms of analgesic drugs tolfenamic and mefenamic acid.

The non-steroidal anti-inflammatory drugs mefenamic acid (MFA) and tolfenamic acid (TFA) have a close resemblance in their molecular scaffold, whereby a methyl group in MFA is substituted by a chloro group in TFA. The present study demonstrates the isomorphous nature of these compounds in a series of their multicomponent solids. Furthermore, the unique nature of MFA and TFA has been demonstrated while excavating their alternate solid forms in that, by varying the drug (MFA or TFA) to coformer [4-di-methyl-amino-pyridine (DMAP)] stoichiometric ratio, both drugs have produced three different types of multicomponent crystals, viz. salt (1:1; API to coformer ratio), salt hydrate (1:1:1) and cocrystal salt (2:1). Interestingly, as anticipated from the close similarity of TFA and MFA structures, these multicomponent solids have shown an isomorphous relation. A thorough characterization and structural investigation of the new multicomponent forms of MFA and TFA revealed their similarity in terms of space group and structural packing with isomorphic nature among the pairs. Herein, the experimental results are generalized in a broader perspective for predictably identifying any possible new forms of comparable compounds by mapping their crystal structure landscapes. The utility of such an approach is evident from the identification of polymorph VI of TFA from hetero-seeding with isomorphous MFA form I from acetone-methanol (1:1) solution. That aside, a pseudopolymorph of TFA with di-methyl-formamide (DMF) was obtained, which also has some structural similarity to that of the solvate MFA:DMF. These new isostructural pairs are discussed in the context of solid form screening using structural landscape similarity.

Mechanical Actuation and Patterning of Rewritable Crystalline Monomer-Polymer Heterostructures via Topochemical Polymerization in a Dual-Responsive Photochromic Organic Material.

The dark-orange monomer single crystals of 1,1'-dioxo-1H-2,2'-biindene-3,3'-diyldidodecanoate (BIT-dodeca2) convert to a transparent single-crystalline polymer (PBIT-dodeca2) material via a single-crystal-to-single-crystal (SCSC) polymerization reaction under sunlight, which then undergoes reverse thermal transformation into BIT-dodeca2 single crystals, leading to reversible photo-/thermochromism, coupled with mechanical actuation. We exploit the properties of this unique material to demonstrate the formation of monomer-polymer heterostructures in selected regions of single crystals with micrometer-scale precision using a laser. This is the first example of heterostructure patterning involving monomer-polymer domains in single crystals. We reveal that the speed of photomechanical bending induced by the polymerization reaction in this example is comparable to those of the well-known diarylethene derivatives, in which electrocyclic ring-closing-ring-opening reactions operate. Furthermore, we characterize the distinct mechanical properties of the monomer and polymer using a quantitative nanoindentation technique as well as demonstrate photopatterning on a monomer-coated paper for potential use in security devices. These crystals with several advantages, such as photomechanical bending (weight lifting) even when the crystal size is large, responsiveness to both UV and visible light, distinct solubilities (the polymer is insoluble, whereas the monomer is soluble in most organic solvents) and colors, provide unique opportunities for their use at different length scales of the sample (µm to mm) for various purposes.

Metal-like Ductility in Organic Plastic Crystals: Role of Molecular Shape and Dihydrogen Bonding Interactions in Aminoboranes.

Ductility is a common phenomenon in many metals but is difficult to achieve in molecular crystals. Organic crystals bend plastically on one or two face-specific directions but fracture when stressed in any other arbitrary directions. An exceptional metal-like ductility and malleability in the isomorphous crystals of two globular molecules, BH3 NMe3 and BF3 NMe3 , is reported, with characteristic tensile stretching, compression, twisting, and thinning. The mechanically deformed samples, which transition to lower symmetry phases, retain good long-range order amenable to structure determination by single-crystal X-ray diffraction. Molecules in these high-symmetry crystals interact through electrostatic forces (B- -N+ ) to form columnar structures with multiple slip planes and weak dispersive forces between columns. On the other hand, the limited number of facile slip planes and strong dihydrogen bonding in BH3 NHMe2 negates ductility. Our study has implications for the design of soft ferroelectrics, solid electrolytes, barocalorics, and soft robotics.