Photoswitchable Organic Materials for Light-Mediated Control of Molecular Properties

Hossain, Munshi Sahid (2022) Photoswitchable Organic Materials for Light-Mediated Control of Molecular Properties. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Munshi Sahid Hossain (14IP029)) 14IP029.pdf - Submitted Version Restricted to Repository staff only Download (21MB)

Abstract

This thesis aims to address light-mediated control of molecular properties of azobenzene-based novel photoswitchable systems. The work involved the synthesis, characterization, and detailed studies of both acyclic and cyclic azobenzenes. The introductory Chapter 1 comprises of the evolution of azobenzene as a molecular machine. It also provides a comprehensive review of the existing literature on the modulation of molecular recognition using photoswitchable molecules. Chapter 2 deals with the molecular recognition and discrimination of the phosphorylated anions (ATP, ADP, AMP, GTP, and inorganic phosphate) using a single receptor based on a cyclic azobenzene. Interestingly, the macrocyclic receptor displays remarkable selectivity towards ATP in the E-isomer but in the macrocycle exhibited altered selectivity towards GTP in the photoisomerized Z-isomer. Chapter 3 includes an unusual Z → E isomerization of the azobenzene macrocycle in the presence of metal ions (Cu²⁺ and Ag⁺) without exposure to any light or any other external stimuli. The two fifteen-membered cyclic azobenzene systems having O- and S as coordination motifs display instant Z → E isomerization upon the addition of metal ions in the Z-isomer. Besides, both the macrocycles exhibited a very unique pull-back effect during photoisomerization in the presence of metal ions. Chapter 4 describes resonance energy transfer from a dendrimer that can enhance the rate of photoisomerization of azobenzene system in the dendrimer-azobenzene supramolecular assembly containing an anion- motif. A guest azobenzene system containing carboxylates can preferentially interact with the dendrimeric core having π-acidic triazine moiety and thus forms a host-guest assembly through the anion-π interaction. Therefore, the energy relay from the dendrimer can be used for trans → cis photoisomerization of the azobenzene system. Chapter 5 deals with azobenzene-based mechanically flexible organic crystals that display enhanced proton conductivity upon crystallization as compared to the as-synthesized compound owing to the flexible nature of the crystals that allows smooth reorganization of hydrogen bonds during proton conduction through the water channel present within the crystal. Furthermore, such proton conducting material was employed as an electrode in an aqueous proton battery (APBs) owing to the presence of a redox-active −N=N− center. The compound as the electrode material possesses the intrinsic proton-conducting property and thus facilitates the protoncoupled redox reactions.

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