How much background in chemistry do material science and engineering students require

The discovery of molecular single crystals that display interesting elastic behaviour has generated excitement regarding their potential applications as it has upended the common perception of crystals as brittle objects. In order to design new functional materials based on molecular crystals, a comprehensive understanding of how these materials respond to deformation on a molecular-level is required. An introduction to the underlying mechanical theory and how it may be applied to single crystals is provided, along with a comprehensive discussion on how these mechanical properties can be characterised. While this field has already presented a large number of elastically flexible crystals, there is a lack of detailed mechanical characterisation data and some contention regarding the atomic-scale mechanism of elasticity. Due to the discrepancies and contradictions between theories proposed in the literature, it is not yet understood why some crystals are elastic while others shatter under applied force. To dispel ambiguity and guide future research, a set of criteria are proposed to define an elastically flexible crystal, so that these materials may find applications among future technologies.

Elastically flexible molecular crystals.

This highlight gives an overview of recent advances in mechanically flexible molecular crystals, with qualitative and quantitative studies performed on different molecular systems. The diverse methods for the tuning of mechanical properties via crystal engineering have gained immense attention in the field of materials science. Such studies render support in establishing the structure to properties to function parallels. The understanding of intermolecular interactions helps in a systematic placement of different molecular crystals in the elastic-plastic spectrum. This overview helps in emphasizing the potential as well as challenges faced in predicting/designing mechanically compliant crystalline compounds.

Potential and challenges of engineering mechanically flexible molecular crystals

Elastic organic crystals are the materials foundation of future lightweight flexible electronic, optical and sensing devices, yet precise control over their deformation has not been accomplished. Here, we report a general non-destructive approach to remote bending of organic crystals. Flexible organic crystals are coupled to magnetic nanoparticles to prepare hybrid actuating elements whose shape can be arbitrarily and precisely controlled simply by using magnetic field. The crystals are mechanically and chemically robust, and can be flexed precisely to a predetermined curvature with complete retention of their macroscopic integrity at least several thousand times in contactless mode, in air or in a liquid medium. These crystals are used as optical waveguides whose light output can be precisely and remotely controlled by using a permanent magnet. This approach expands the range of applications of flexible organic crystals beyond the known limitations with other methods for control of their shape, and opens prospects for their direct implementation in flexible devices such as sensors, emitters, and other (opto)electronics. 

/pdf/remote-and-precise-control-over-morphology-and-motion-of-3d42qmax.pdf

Remote and precise control over morphology and motion of organic crystals by using magnetic field

Mechanically compliant organic crystals are the foundation of the development of future flexible, light-weight single-crystal electronics, and this requires reversibly deformable crystalline organic materials with permanent magnetism. Here, we report and characterize the first instance of a plastically bendable single crystal of a permanent organic radical, 4-(4′-cyano-2′,3′,4′,5′-tetrafluorophenyl)-1,2,3,5-dithiadiazolyl. The weak interactions between the radicals render single crystals of the β phase of this material exceedingly soft, and the S–N interactions facilitate plastic bending. EPR imaging of a bent single crystal reveals the effect of deformation on the three-dimensional spin density of the crystal. The unusual mechanical compliance of this material opens prospects for exploration into flexible crystals of other stable organic radicals towards the development of flexible light-weight organic magnetoresistance devices based on weak, non-hydrogen-bonded interactions in molecular crystals.

/pdf/mechanically-compliant-single-crystals-of-a-stable-organic-523xk5av9c.pdf

Mechanically compliant single crystals of a stable organic radical

The mechanism of bending in a plastically flexible crystal.

While the first report of molecular crystals that could bend without breaking was well over a decade ago, the development of suitable characterisation tools remains a priority. Due to the broad reaching applications of these materials in advanced technologies, it is important to develop both mechanical and mechanistic understanding. Micro-focused mapping experiments were designed with the intent of bridging the gap between the measurement of mechanical properties and molecular-scale structural understanding. Herein, we describe a methodology for determining the mechanisms of deformation in flexible crystals with atomic precision and provide examples where it has been implemented. Although micro-focused mapping experiments have potential for application in the determination of mechanisms of flexibility, great care must be taken during both experimental design and data analysis.

Determining the mechanisms of deformation in flexible crystals using micro-focus X-ray diffraction

In a recent study, Dey et al.1
propose a mechanism of elastic bending in co-crystals of caffeine,
4-chloro-3-nitrobenzoic acid and methanol (1) in which mechanical interlocking
is proposed to allow for the reversible flexibility observed. We have now determined the mechanism to atomic
resolution using micro-focused synchrotron radiation,2 which is
different to that previously reported. When subjected to strain the
inter-molecular distances change and hydrogen-bonded dimers rotate over two orthogonal
directions to allow the compression and expansion producing flexibility.

Amy J. Thompson

Papers

Elastically flexible molecular crystals.

The mechanism of bending in a plastically flexible crystal.

Determining the mechanisms of deformation in flexible crystals using micro-focus X-ray diffraction

The mechanism of bending in co-crystals of caffeine and 4-chloro-3-nitrobenzoic acid.

1,001 crystal structures: understanding the mechanisms of deformation in elastically flexible crystals using automated analysis