Design Strategy for Mechanochromic Materials and Investigating Properties of Water inside Lyotropic Liquid Crystalline Phases

Abstract

Stimuli-responsive smart material has rapidly grown from few obscure examples to one of the most vibrant domain in the modern material science. However, material that responds to multiple stimuli integrating with mechanochromic, vapochromic, solvatochromic, acidochromic and thermochromic behavior remains scarce for their unpredictable design principle. Introduction of these diverse stimuli in a single luminogen requires discrete criteria of respective stimulus, which ultimately makes them difficult to design. Particularly, in case of charge transfer luminogens, the biggest challenge to construct the stimuli-responsive mechanochromic material is the densely packed arrangement of luminogens in the solid state (mostly as head to tail driven by the oppositely charged character in donor–acceptor molecules) owing to their well-separated electron density. The densely packed arrangements in the solid state suppress the possibility of mechanochromism; as such packing is unable to produce metastable energy states under external mechanical force. Therefore, establishment of the structure-property relationship based on charge transfer luminogens along with an inherent molecular level understanding of mechanochromism undoubtedly paves a new way to design this type of novel material. On the other hand, in case of centro-symmetrically packed organic luminogens, one important issue in the field of mechanochromism needs to address or solve and the issue is “how to activate centrosymmetrically packed organic molecules? Generally, centrosymmetrically packed organic luminogens can’t be activated under external mechanical force due to their zero gross dipole moment and degenerate electronic energy states in the solid-state packing. This severely limits the application of large number of centrosymmetrically packed organic luminogens in the development of stimuliresponsive mechanochromic materials; thereby creating a barrier to apply the bulk amount of organic probes into the industrial or technological applications. Keeping mind regarding these two major limitations against the development of stimuli-responsive material, in beginning part of my thesis, we have planned to develop the new design strategy overcoming the above mentioned limiting issues. At first in chapter 2, a novel design strategy for development of new mechanochromic materials is depicted based on the donor-acceptor based charge transfer luminogens. Consequently in chapter 3, another design strategy has been depicted based on centrosymmetrically packed organic luminogens, which was a long standing problem in the mechanochromic field. Collectively, these two chapters (chapter 2 and chapter 3) provide a detailed design strategy for the future development of new mechanochromic materials based on charge transfer and non-charge transfer molecules.

In the next section of my thesis, I have investigated the nature of encapsulated water dynamics and excited state proton transfer dynamics (ESPT) inside other novel semi-solid lytropic liquid crystalline (LLC) materials, utilizing the various spectroscopic techniques. Investigations of water/ESPT dynamics or exploration of optical features into these unique materials have not yet done by any other research group. From application perspective, lipid LLC materials are considered as the potential carrier for drugs and other important biomolecules. Encapsulated water molecules are considered there to play a crucial role for the formation and stabilization of this LLC phases. Considering the importance of water, we have measured the dynamics of encapsulated water by time dependent Stokes shift method using Coumarin-343 as a solvation probe (Chapter 5). Moreover, the LLC materials are also found to hold unique features owing to their outstanding topology, excellent biocompatibility and wide range of practical applications. Hence, in the last chapter of my thesis (Chapter 6), we have investigated the topological influence of the various LLC phases (reverse hexagonal or HII, gyroid Ia3d and diamond Pn3m phases) on the dynamics of different steps of the reaction cycle of ESPT process. Each LLC phases are equally hydrated in order to complete idea of topological influence on the ESPT dynamics, which bears fundamental scientific importance towards the understanding of water network inside liquid crystalline phases.