Crystal Engineering Study of Stimuli Responsive Organic Crystals

Devarapalli, Ramesh (2016) Crystal Engineering Study of Stimuli Responsive Organic Crystals. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Crystal engineering is a multi-faceted field of research in the context of design of organic and metal-organic functional materials with desired physical and chemical properties. From the recent advances in this field, I learned that the mechanical properties shall play a crucial role for determination and creation of new functional materials with desired properties. It is worth mentioning that the mechanical properties require precise control over the weak intermolecular interactions in the structure because their strength and directionality play a crucial role in the deformation mechanism. Thus, I aimed at appreciating the intermolecular interactions to understand the structure - mechanical property correlations in different classes of molecules including naphalenediimide derivatives, boron difluoride complexes, and schiff bases. Chapter 1 gives a brief overview of different topics in the crystal engineering including mechanical properties, nanoindentation, useful glossary for crystal structure analyses and a survey of stimuli responsive single crystals. Chapter 2 demonstrates the crystal engineering approach to design mechanically reconfigurable, plastically flexible single crystals of three unrelated types of compounds by introducing active slip planes in structures via different noninterfering supramolecular weak interactions, namely van der Waals, π-stacking and hydrogen bonding groups. This study establishes the potential of soft interactions for tuning mechanical behaviour of ordered molecular materials, including those from π- conjugated systems. In Chapter 3, the mechanical flexibility of all the three types of crystals (plastic, elastic, brittle) was probed by three-point bending tests using nanoindenter as well as molecular dynamics simulations. The close resemblance of the three π- conjugated molecules in this study allowed me to compare the three cases and link the differences in mechanical properties largely to the structural differences. This study provides insights to design highly elastic molecular crystals (with high elastic modulus) by using crystal engineering approach. In Chapter 4, 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) behavior, solid-state structure, and the mechanical behavior of single crystals. The results of this study confirm that the extent of ML in crystalline organic solid-state fluorophore materials can be correlated positively with the extent of plasticity. In Chapter 5, a single-crystal-to-single-crystal phase transformation by the heat as an external stimulus for two isomorphous Schiff bases is studied by both experiments and molecular modelling simulations. Upon heating, the compounds viz. o-vanilidene p-chloroaniline and o-vanilidene p-bromoaniline were irreversibly converted to new polymorphs, interestingly which are also isomorphous. In addition, the parent crystals show plastic bending under mechanical stress but as a consequence of phase transition they convert to brittle polymorphs. The plasticity and hardness of polymorphs were quantified with the nanoindentation technique. The reasons for such gratuitous mechanical properties were deeply investigated.

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