Two-dimensional and Photo-responsive Organic Functional Materials

Paul, Subhankar (2026) Two-dimensional and Photo-responsive Organic Functional Materials. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Subhankar Paul (20RS007)) 20RS007.pdf - Submitted Version Restricted to Repository staff only Download (13MB)

Abstract

Two-dimensional materials, characterised by high surface-to-volume ratio, generate widespread interest with cross disciplinary implications, spanning fields such as catalysis, energy storage, sensing, to biomedical applications. In this context, inorganic or organic-inorganic hybrid 2D materials have generated much interest but the scope of structural and functional tunability in these materials is limited. This is an area where bottom-up approaches like molecular assembly can be consequential. The development of supramolecular architecture with high dimensional order and precision is the ultimate goal of molecular assembly to mimic the nature or various biological functions. Synthetic supramolecular chemistry has made significant advancement into that direction by designing molecular building blocks for molecular assembly into various nanostructures/dimensions like spheres, vesicles, fibers, rod etc. However, designing of monomers for 2D assembly (sheets) remains highly challenging. The first two working chapter of this thesis covers two examples of small organic molecule based 2D assembly and their molecular level understanding of 2D growth. The additional advantage of molecular assembly is the dynamic and reversible nature of noncovalent interactions which can pave the way for structural reconfiguration in response to external stimuli and endow new properties and functionality to the system. Owing to the possibility of the remote application, high spatiotemporal precision and zero waste, light is advantageous over other external stimuli to control material’s property. The second part of this thesis involves photoisomerization of hydrazone-based photoswitches in self-assembled and crystal state disclosing some complex phenomena of aggregate-to-aggregate photoconversion and polymorph-to-polymorph variation in photoresponses respectively. Chapter 1 briefly discusses the various aspects of designing molecular building blocks for two dimensional self-assembly and stabilizing it into the solution state. It involves discussions related to the different types of suitable building blocks for 2D assembly, the role of solvent in stabilising the 2D structure, the thermodynamic and kinetic aspects of the aggregation and finally the applications. It highlights the challenges as well as the necessity of designing new building blocks for 2D assembly. Finally, it introduces photo-responsive materials, focusing on hydrazone as a versatile photo-switch capable of undergoing configuration changes when exposed to light, which can trigger morphological transformations in molecular crystals and supramolecular polymers. Chapter 2 discloses molecular self-assembly as a powerful strategy for fabricating two-dimensional material. A core cyano substituted naphthalene bisimide derivative with fluoroalkyl chain imide substituent can form two 2D supramolecular polymorphs with distinct surface characteristics, caused by the differences in the conformation and organization of the fluoroalkyl chains at the 2D surface. Changing the solvent composition or temperature prompts conformational changes into the fluoroalkyl chain resulting into polymorph-to-polymorph reorganisation. Along with this, how changes in dioxane−water microheterogeneity with solution composition and temperature may affect the 2D structures through solvent−solute interactions is also discussed. However, both of the 2D surfaces of these have exposed fluoroalkyl chain, that is, identical. Obtaining 2D material with non-identical surfaces that is ‘Janus’, is the next advancing step in this direction of research. In chapter 3, we report an unusual self-assembly of an asymmetric naphthalene bisimide in water into thin two-dimensional nanosheet with uniform thickness with two non-identical surfaces. Moreover, the building block we choose has a chiral centre that renders a well-defined chirality to the 2D-nanosheets. The chiroptical properties in solution and film state also discussed. In the film state it shows an unusual complete inversion of circular transmission on sample flipping, which is known as apparent circular dichroism (APR). In the second part of this thesis, we focused on photoresponsive organic materials. We choose hydrazone derivatives as the building blocks for molecular self-assembly and molecular single crystal. The aggregate-to-aggregate photoconversion pathway, via monomers or not (indirect vs. direct), depends on a suitable balance between intermolecular interactions and accessible free space for isomerisation within the aggregate. Chapter 4 demonstrates introducing fluorophilic interactions by side chain engineering in a hydrazone-based system can alter the photoconversion mechanism from indirect to direct, accompanied by distinct photophysical, morphological and mechanical changes. Interestingly, hydrazone derivative containing a butyl chain forms two type of crystals with nearly identical morphology and single crystal structure, that is isostructural polymorphs. But chapter 5 shows despite having such similarities they exhibit different mechanical and photomechanical actuation. Where one of the polymorphs show elastic deformation and photomechanical bending, the other one shows plastic deformation and photo-induced disintegration into layers. These findings remark the attentive handling of crystal polymorphs and also deepen our understanding of the relation between structure and dynamic behaviours of crystals, and finally are of paramount importance for further designing hydrazone-based multifunctional dynamic crystal materials.

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