Understanding and Controlling Supramolecular Chirality in Self-assembled Structures

Pal, Triza (2024) Understanding and Controlling Supramolecular Chirality in Self-assembled Structures. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Triza Pal (18RS066)) 18RS066.pdf - Submitted Version Restricted to Repository staff only Download (18MB)

Abstract

This thesis comprises of five chapters that delve into the significant role of Supramolecular chirality in self-assembled systems. The exploration involves the design of various chiral and achiral systems to investigate the subtle impact of the assembly medium on the final anisotropy of the assembled structure. The primary focus of this thesis is on understanding the intricate role of solvation as well as solvent-solute interaction in the formation of supramolecular chiral structures. In addition, the crucial influence of several stimuli in controlling and enhancing the asymmetry of formed supramolecular structure. Finally, understanding the delicate balance between thermodynamics and kinetics is necessary for controlling diverse anisotropic structures and pathways in supramolecular systems. Chapter 1 discusses various important aspects of supramolecular chirality, which is an exciting area of research in supramolecular chemistry. It involves the formation of organized structures at both the molecular and supramolecular levels. Supramolecular chirality, which is the manifestation of chirality in these self-assembled systems, can arise from interactions between the constituent chiral as well as achiral molecules. The dynamic nature of self-assembly systems allows the control of supramolecular chirality through various factors such as Solvent, pH, temperature, and external additives. Unlike molecular chirality, supramolecular chirality exhibits several unique properties and has applications in various fields, which leads to intriguing phenomena like chiroptical switches and circularly polarized luminescence. Chapter 2 and Chapter 3, present the self-assembly of a series of chiral perylene bisimides in water-cosolvent mixtures, and our findings emphasize the intriguing impact of solvent-solute interactions on supramolecular anisotropy, both in terms of chirality and morphology. In Chapter 2, we found initially, the assembly process is independent of the composition of the solvent, resulting in weakly chiral, spherical nanostructures. However, within a very narrow range of solvent composition, these nanospheres transform into long, highly chiral supramolecular polymers. Furthermore, the chirality can be completely reversed by changing the achiral cosolvent. We explain how the solvent modulates specific noncovalent interactions and controls the kinetics and thermodynamics of crucial processes, such as spontaneous phase segregation, secondary nucleation, and cooperative growth. In the context of supramolecular polymerization, our findings suggest that it is important to shift the focus from the physical properties of a solvent, such as polarity and phase diagram, to the intricate nature of solvent-solute interactions. In Chapter 3, we witness a number of intriguing phenomena where chiral inversion occurs without altering the co-solvent, but with a mere 20% variation in the cosolvent concentration. Interestingly, distinctly different self-assembly behaviors are observed between the two enantiomers. Diverse temperature-dependent chiral inversion and asymmetry amplification phenomena have been identified for various enantiomers in different solvent mixtures. Furthermore, by combining varying ratios of enantiomers, we observed an abnormal "Majority Rules" effect, in which co-assembly occurs within a specific range of enantiomeric ratios. Outside of this range, self-sorting takes place. Additionally, we utilized one of our chiral monomers to induce chirality in the assembly of an achiral counterpart. This led to the formation of an amplified asymmetric structure through the co-assembly of the chiral and achiral components, demonstrating distinctive Sergeant-and-Soldier effects. In Chapter 4, we studied the self-assembly behavior of a mixture of multi-chromophoric stereoisomers that has a bright emission in its H-aggregated state. To introduce an asymmetry into the self-assembly of this racemic mixture, we employ a chiral additive. In the presence of a chiral additive, the mixture can form a homochiral structure having strong chirality. What's intriguing is that even after removing the chiral additive, the chirality of the formed assemblies remains unaffected, showcasing a remarkable chiral memory effect. This enables us to utilize the resulting chiral aggregate, formed from a racemic mixture, as a chiral seed to induce further asymmetry in the assembly of a new batch of free monomers. Additionally, we take advantage of the high-quantum yield and excitonic H-aggregated emission of our chiral assembly. The homochiral aggregate produces a strong red circularly polarized luminescence which shows a high dissymmetry factor (glum = 1.3 x 10⁻² at 670 nm) as well as a remarkable Bcpl value ⁓ 68 M⁻¹ cm⁻¹ in solution. In Chapter 5 we showed that successful Controlled Supramolecular Polymerization (CSP) relies on precise nucleation control, normally achieved by trapping free monomers in a kinetically controlled trap state. However, the presence of shallow traps is ineffective in preventing spontaneous nucleation, thus limiting the applicability of CSP. To overcome this, we use a molecular additive that can alter the energy landscape, and enhance kinetic stability. This additive catalyzes nucleation along the OFF-pathway while inhibiting aggregation along the ON-pathway, directing assembly towards the OFF-pathway. Mechanisms of catalysis and inhibition are explained. By specifically targeting the nucleation step, pathway selection is achieved at a low additive-to-monomer ratio of 1:100. This approach is versatile across different related molecular systems. Upon removal of the additive, a novel cross-nucleation phenomenon occurs, with the ON-pathway aggregate forming on the surface of the less stable OFF-pathway aggregate analog. The resulting supramolecular polymer not only displays a more uniform morphology but also exhibits significantly enhanced structural order, leading to a notable amplification of chiral asymmetry and a high absorption dissymmetry factor (gAbs) of 0.05.

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