On the elasticity of Copper(II)acetylacetonate crystals

Structure-property correlations in a wide variety of elastic molecular crystals indicate that (a) corrugated structures that can interlock readily (prevent plastic deformation either through geometrical or energetic considerations) and (b) a multitude of weak and dispersive intermolecular interactions that would act as structural buffers through easy rupture and reformation during deformation, are structural features that impart a crystal the ability to accommodate large elastic strains.1-4 Based on their studies on copper(II)acetylacetonate crystals (1, hereafter), Worthy et al. claimed that these criteria are “incorrect”.5 In order to examine their claim critically, we conducted detailed experiments on crystals of 1. Our flexure experiments show that they indeed readily undergo plastic deformation when bent on the (101) face, which contradicts Worthy et al.‘s assertion that the (101) face is highly elastic. Therefore, the crystal 1 is not an ideal model system for rejecting the prevailing and widely accepted-molecular mechanisms for exceptional elastic flexibility of organic crystals

The Relationship of Solid-State Plasticity to Mechanochromic Luminescence in Difluoroboron Avobenzone Polymorphs

In solid-state mechanochromic luminescence (ML) materials, it remains a challenge to establish the origin of fluorescence color changes upon mechanical action and to determine why only some fluorophores exhibit ML behavior. The study of mechanical properties by nanoindentation, followed by ML experiments on green- and cyan-emitting polymorphs of difluoroboron avobenzone reveals that upon smearing, the plastically deformable cyan form shows a prominent color change to yellow, while in the harder green form the redshifted emission is barely detectable. Crystal structure analysis reveals the presence of slip planes in the softer cyan form that can facilitate the formation of recoverable and low energy defects in the structure. Hence, the cyan form exhibits prominent and reversible ML behavior. This suggests a potential design strategy for efficient ML materials

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

Ductility, which is a common phenomenon in most metals and metal-based alloys, is hard to achieve in molecular crystals. Organic crystals have been recently shown to deform plastically, but only on one or two faces, and fracture when stressed in any other arbitrary direction. Here, we report an exceptional metal-like ductility in crystals of two globular molecules, BH3NMe3 and BF3NMe3, with characteristic stretching, necking and thinning with deformations as large as ~ 500%. Surprisingly, the mechanically deformed samples not only retained good long range order, but also allowed structure determination by single crystal X-ray diffraction. Molecules in these high symmetry crystals interact predominantly via electrostatic forces (B––N+) and form columnar structures, thus forming multiple slip planes with weak dispersive forces among columns. While the former interactions hold molecules together, the latter facilitate exceptional malleability. On the other hand, the limited number of facile slip planes and strong dihydrogen bonding in BH3NHMe2 negates ductility. We show the possibility to simultaneously achieve both exceptional ductility and crystallinity in solids of certain globular molecules, which may enable designing highly modular, easy-to-cast crystalline functional organics, for applications in barocalorimetry, ferroelectrics and soft-robotics

Mechanoelectronics: Flexible Organic Semiconducting Single Crystals for Durable All-Flexible Field-Effect Transistors

Although many examples of mechanically flexible crystals are currently known, their utility in all-flexible devices is not yet demonstrated, despite their immense potential for fabricating high-performance flexible devices. 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 (µFET) when compared to the brittle crystals of ethylated diketopyrrolopyrrole derivative (DPP-diEt). The field effect transistors (FETs) made of flexible substrates using elastic microcrystals of DPPdiMe retained µFET (from 0.019 cm2/Vs to 0.014 cm2/Vs ) more efficiently even after 40 bending cycles when compared to the brittle microcrystals of DPP-diEt which showed a significant drop in µFET just after 10 bending cycles. Our results not only provide valuable insights into bending mechanism but also demonstrate the untapped potential of mechanically flexible semiconducting crystals for designing all flexible durable devices

CCDC 808345: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

Structure–mechanical property correlations in mechanochromic luminescent crystals of boron difluoride dibenzoylmethane derivatives

The structure and mechanical properties of crystalline materials of three boron difluoride dibenzoylmethane (BF2dbm) derivatives were investigated to examine the correlation, if any, among mechanochromic luminescence (ML) behaviour, solid-state structure, and the mechanical behaviour of single crystals. Qualitative mechanical deformation tests show that the crystals of BF2dbm(tBu)2 can be bent permanently, whereas those of BF2dbm(OMe)2 exhibit an inhomogeneous shearing mode of deformation, and finally BF2dbmOMe crystals are brittle. Quantitative mechanical analysis by nanoindentation on the major facets of the crystals shows that BF2dbm(tBu)2 is soft and compliant with low values of elastic modulus, E, and hardness, H, confirming its superior suceptibility for plastic deformation, which is attributed to the presence of a multitude of slip systems in the crystal structure. In contrast, both BF2dbm(OMe)2 and BF2dbmOMe are considerably stiffer and harder with comparable E and H, which are rationalized through analysis of the structural attributes such as the intermolecular interactions, slip systems and their relative orientation with respect to the indentation direction. As expected from the qualitative mechanical behaviour, prominent ML was observed in BF2dbm(tBu)2, whereas BF2dbm(OMe)2 exhibits only a moderate ML and BF2dbmOMe shows no detectable ML, all examined under identical conditions. These results confirm that the extent of ML in crystalline organic solid-state fluorophore materials can be correlated positively with the extent of plasticity (low recovery). In turn, they offer opportunities to design new and improved efficient ML materials using crystal engineering principles

CCDC 808348: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

CCDC 808347: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

CCDC 808346: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures