Solvent-Solute Interactions in Supramolecular Self-Assemblies

Datta, Saptarshi (2022) Solvent-Solute Interactions in Supramolecular Self-Assemblies. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Saptarshi Datta (15RS029)) 15RS029.pdf - Submitted Version Restricted to Repository staff only Download (10MB)

Abstract

This thesis consists of five chapters where the role of solvent-solute interactions in supramolecular self-assemblies has been investigated. Discussing the present situation of the related works, several systems were synthesised and studied to reveal the solvent role in self-assemblies and how they can be utilised to fabricate functional materials. Chapter 1 briefly discussed the importance of solvent-solute interactions in controlling the supramolecular self-assemblies with the aid of plausible thermodynamic and kinetic factors with suitable examples. We also discussed the role of solvent in supramolecular polymorphism, supramolecular gelation and photoinduced charge transfer. The effect of the pH of the medium on the outcome of self-assembly has also been discussed. We dedicated our work to investigate the solvent's role in controlling the assembly structures and pathways. We tried to utilise the uniqueness of such interactions to design reversible functional materials in our work, from solvent-induced supramolecular polymorphism to investigating the role of the medium's pH in the self-assembly outcome; from investigating the solvent effect on excited state charge transfer to a real-life application with a phase-selective supramolecular gelator. Reversible switching between supramolecular polymorphs offers a great way to introduce stimuli-responsiveness. Supramolecular polymorphism is usually achieved through pathway complexity or by exploiting solvent-solute interactions. However, steering self-assembly along a specific pathway to form a kinetically-stable aggregate is not easy. Also, changing solvent to switch between polymorphs is impractical. Chapter 2 presents a perylene bisimide molecule with a trans-azobenzene side group that assembles into three supramolecular polymorphs with distinct colours, morphologies, packing and aggregation mechanisms. Optical absorption and FTIR spectroscopy reveal the importance of hydrogen bonding interaction between protic solvent and azo N that controls the planarity of the azobenzene group and influences molecular packing. This solvent-induced interaction can be further modulated using temperature and solution pH to reversibly switch between the three polymorphs in solution as well as in a solid silica-gel matrix. In Chapter 3, we have focused on the photo-response of different self-assembled structures in various solvents and investigated how the pH of the medium can be crucial in controlling its photo-switchability. A reversible strain-driven disassembly on UV irradiation can be seen at higher pH in methanol that reassembles in visible light. Whereas, in water, aggregate undergoes aggregate photoswitching at high pH, with a distinct optical and morphological transition. No such aggregate switching has been seen at low pH. Finally, we have shown that this photo-switchable aggregate is efficient in mechanical work compared to the molecular switches, as it can efficiently break a strong gel network when added with only 0.05% respect to the gelator. This work explains how the smallest change in the self-assembly pattern influences the macroscopic outcome to design efficient photo-responsive systems with mechanical application. Also, understanding the electronic pictures of the molecule to self-assemblies in such systems will help us fabricate functional optoelectronic devices. This work offers dynamic control over optoelectronic properties and morphologies of supramolecular self-assembled systems using appropriate external stimuli which is crucial for fabricating dynamic soft-matter devices. In chapter 4, we report the photophysical properties of a molecular foldamer system PBI-AnEt₂-PBI, where the electron-donating N,N-diethyl aniline unit is covalently linked between two electron-accepting perylene diimide (PBI) chromophores. This small Accepter-Donor-Acceptor (A-D-A) triad serves as a model system to investigate the effect of self-assembly in PET to fabricate efficient OPVs. PBI-AnEt₂-PBI is conformationally flexible, adopting either an open or folded conformation, depending on the choice of solvent. We characterised the photoinduced electron transfer dynamics of both open and folded forms. Open PBI-AnEt₂-PBI undergoes charge separation to PBI●⁻-AnEt₂●⁺-PBI.In the folded form, two different charge-separated species are observed. In addition to the formation of PBI●⁻-AnEt₂●⁺-PBI, the dominant product in folded form is PBI●⁻-AnEt₂-PBI●⁺ via symmetry-breaking charge transfer. Both charge-separated products indicate varying degrees of interchromophoric coupling in the folded form. This work provides insight into how solvent-induced molecular conformation and interchromophoric coupling can mediate the balance between competitive photophysical pathways in structurally dynamic organic multi-chromophoric systems. Gelation is another unique outcome of solvent-solute interaction with several applications. Phase-selective organogelators (PSOGs) that congeal oil from water have emerged as a promising alternative to the existing methods of containing marine oil spills. However, all PSOGs reported to date suffer from one prohibitive limitation: poor dispersibility in the oil phase. Consequently, when used as a dry powder, these PSOGs take long hours to congeal oil and instead have to be applied after dissolving in or wetting with toxic and inflammable solvents. Chapter 5 reports a naphthalene diimide-based gelator that can solidify heavy crude oil from water in seconds when applied in powder form. Exceptional mechanical strength of the resultant gel, easy recoverability of oil, ability to congeal oil at 0°C and negligible aqueous solubility makes our compound by far the best-reported PSOG for oil-spill remediation.

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