Autonomous Self-Healing and Novel Electromechanical Properties of Organic Crystals: Role of Crystal Symmetry

Ghosh, Ishita (2025) Autonomous Self-Healing and Novel Electromechanical Properties of Organic Crystals: Role of Crystal Symmetry. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

The functionality and properties of ordered materials are directly related to their molecular structure and crystal packing. As such, an understanding of internal structure and symmetry is crucial for design and development of organic crystals for practical applications like sensors, waveguides, piezoelectrics, ferroelectrics or even pharmaceuticals. Crystal engineering approaches have been utilized in many ways to tune and design various properties in crystals, including mechanical properties. My research work is focussed on exploring fundamental relationships between structural organization at different hierarchical levels – from molecular to supramolecular to macroscopic – and their mechanical as well as electromechanical properties. In my second Chapter, autonomous self-healing is shown to occur in centrosymmetric molecular crystals with layered crystal packing via a symmetry-breaking mediated pathway. We have studied the fracture mechanics of the system and correlated with its internal crystal structure. Spectroscopic techniques like Raman spectroscopy and second harmonic generation shed light on the mechanism behind such ultrafast self-repair process. To further extend the realm of self-healing crystals, in my third Chapter, I have designed a self-healing photochromic sensor based on salicylidene moiety which has light induced photoswitching ability and applications in inkless printing. To evaluate the mechanism of self-healing, it was found that the mechanical fracture gives rise to complementary surface charges on oppositely cleaved surfaces, which is responsible for bringing the newly formed surfaces close to each other to facilitate self-repair. A targeted synthon approach covering a wide spectrum of organic crystals, in my fourth Chapter, reveals that the surface charge phenomenon, although unclear till now, appears to be widespread in molecular crystals. Mechanical impact induced charging can have profound effects in our understanding of various electrical and mechanical effects of organic materials around us. The role of crystal polarity and optimum plasticity for favourable charge-based attraction behaviour is evident from this study. Finally, in my fifth Chapter, I carried out the systematic investigation of surface potentials in a select set of molecular crystals possessing centrosymmetric as well as non- centrosymmetric packing. The obtained data was compared with known standard inorganic perovskite SrTiO3 to quantitatively assess the output generated from organic crystals via stress- induced symmetry breaking by conducting electrical nanoindentation termed as nano-ECR.

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