Tunable Light-Responsive Mechanical Properties of Supramolecular Assemblies of Photochromic Molecular Switches

Adak, Soumen (2026) Tunable Light-Responsive Mechanical Properties of Supramolecular Assemblies of Photochromic Molecular Switches. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Soumen Adak (18IP015)) 18IP015.pdf - Submitted Version Restricted to Repository staff only Download (21MB)

Abstract

This thesis focuses on the light-mediated control over mechanical properties of supramolecular architectures of the photochromic molecular switches. It explores the design, structure–property relationships, and functional implications of azobenzene-based switches. The later part of the thesis also contributes to the development of an emission-based probe for sequence-independent detection of low concentrations of nucleic acids. The research involved rational synthesis, thorough characterization, and in-depth studies of the synthesized systems. The introductory chapter, Chapter 1 presents a tutorial-style overview that traces the development of azobenzenes from prototypical photoswitches to structurally advanced azo-based architectures. By integrating fundamental concepts, it provides a didactic framework for understanding, selecting, and designing azobenzene-based photoswitches for next-generation light-controlled systems, with particular emphasis on the emergence of arylazopyrazoles AAPs). We extensively evaluate five major classes of azo-based photoswitches, highlighting their structure-property relationships, advantages, and limitations. A central focus of this thesis is placed on AAPs, an advanced class of azoheteroarenes that combine favorable electronic effects with distinctive Z-isomer conformations, enabling near-quantitative photoconversion and high thermal stability. Chapters 2 and 3 focus on the arylazopyrazole-based photoswitches. We demonstrate that suitably positioned aromatic units in the Z-isomer of AAPs adopt favorably twisted conformations that promote stabilizing π···π stacking interactions, providing a general molecular design principle for enhancing Z-isomer stability. In Chapter 2, crystallographic analysis reveals pronounced conformational and flexibility differences between the E and Z photo-isomers, which translate into distinct macroscopic mechanical behaviors. It highlights the intrinsic capability of molecular photoisomerization to modulate bulk mechanical responses. Building on these insights, in Chapter 3, we further systematically studied different substituents that contribute to the stabilization of the metastable isomer and revealed that halogen substituents can significantly enhance the thermal stability of the Z-isomer, likely via strengthened π···π stacking, C-X···π, C-H···X, and other favorable interactions. Further, in Chapter 4, this thesis extends the utility of an aggregation-induced emission (AIE)-active tetraphenylethene (TPE) scaffold, as a modular probe for the sequence-independent detection of nucleic acids under biologically relevant conditions. In the final chapter of my thesis, we have developed a light-responsive carbonic anhydrase mimic using tetra-ortho-substituted azobenzene enabling reversible control of catalytic activity.

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