Understanding emergence of supramolecular chirality in lyotropic conjugated polymers

Xu, Zhuang

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https://hdl.handle.net/2142/129359 Copy

Description

Title

Understanding emergence of supramolecular chirality in lyotropic conjugated polymers

Author(s)

Xu, Zhuang

Issue Date

2024-12-18

Director of Research (if dissertation) or Advisor (if thesis)

Diao, Ying

Doctoral Committee Chair(s)

Diao, Ying

Committee Member(s)

• Chen, Qian
• Braun, Paul V.
• Jackson, Nicholas E.

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Semiconducting polymers
• Conjugated polymers
• Chiral electronics
• Chiral assembly
• self-assembly
• Supramolecular chirality
• Lyotropic liquid crystals
• Hierarchical chiral assembly
• Chirality emergence
• Molecular design
• Solvent selectivity
• Charge carrier mobility
• Chemical doping
• Thin-film morphology
• Organic field-effect transistors (OFETs)
• Machine learning in chiral systems
• Twist-bend nematic phase (Ntb)
• Supramolecular assembly pathways
• Spin selectivity
• Crystallinity and doping efficiency
• Entropy-driven helical structures.

Abstract

Hierarchical chiral assembly into sophisticated structures is vital for the structural complexity and functional versatility of biological systems. Inspired by the enigma of natural homochirality, the design and construction of artificial chiral hierarchical structures have garnered significant research attention. Recent advances in incorporating chirality into artificial systems have underscored its potential to elevate their performance to levels akin to those of natural systems. For conjugated polymers, understanding how to introduce and control chirality is critical to unlocking their full potential in future chiral electronics. This thesis focuses on elucidating the emergence of supramolecular chirality in conjugated polymers within lyotropic liquid crystals (LCs) and its dependence on molecular design and solvent selection. Furthermore, it explores the impact of this chirality on device-relevant properties, such as charge carrier mobility and conductivity upon chemical doping. In Chapter 2, I explore the LC-mediated assemblies of conjugated polymers, uncovering a ubiquitous yet previously overlooked phenomenon: the spontaneous chiral emergence during the multistep assembly of achiral conjugated polymers, occurring without the addition of any chiral source. Upon surpassing a specific concentration threshold, conjugated polymer nanofibers in solution transition into lyotropic liquid crystalline mesophases, exhibiting intricate supramolecular chiral morphologies. This spontaneous supramolecular chirality is observed in over twenty conjugated polymers, including well-known examples like N2200 and newly designed rigid polymers such as IDT-BT. I propose that this phenomenon is thermodynamically driven, with polymer fibers adopting helical conformations to maximize system entropy. Using molecular design and machine learning, I identify key molecular features that govern this widespread chiral emergence. Building on these insights, in Chapter 3, I investigate how varying LC-mediated assembly pathways influences thin-film morphology and electronic properties. I demonstrate that solution aggregate structures across length scales can be modulated by adjusting solvent selectivity toward the polymer backbone versus side chains and by controlling solution temperature. Using a combination of small-angle X-ray scattering (SAXS), electron diffraction (ED), optical and electron microscopy, and UV–vis absorption spectroscopy, I identify several distinct solution aggregate structures. These structures give rise to three drastically different assembly pathways, which define film morphology and charge-carrier mobility in organic field-effect transistors (OFETs). This work illustrates the power of controlling assembly pathways to tailor the properties of conjugated polymers. Inspired by the selective interactions of solvents, in Chapter 4, I examine how mixing two selective solvents can vary aggregate structures and LC assemblies in solutions. I find that lyotropic mesophases form only in mixtures where both the backbone and side chains are sufficiently soluble. By adjusting the solvent mixing ratio, I control the rigidity of nanofiber aggregates, which in turn affects the shape and structure of tactoids in the biphasic phase and modulates the nano- and micron-scale pitches in the final striped twist-bend nematic (Ntb) mesophase. Finally, in Chapter 5, I investigate how supramolecular chirality impacts the chemical doping of conjugated polymers. By manipulating solution aggregate structures through subtle variations in solvent nature, I tune the liquid crystal phase structures, achieving a spectrum of supramolecular chirality from achiral to weakly chiral and strongly chiral. Molecular dynamics simulations reveal that the differential solubilities of the side chains and backbones in various solvents drive these transitions. Upon sequential doping, strongly chiral films exhibit significantly higher charge carrier concentrations, resulting in the highest doping efficiency and electrical conductivity, followed by weakly chiral and achiral films. This trend is consistent across three polymer systems. I propose that enhanced crystallinity from chiral assembly facilitates doping, while chirality-induced spin selectivity may accelerate oxidation over reduction, collectively leading to increased doping efficiency in chiral structures. In summary, this thesis provides a fundamental understanding of the emergence of supramolecular chirality in conjugated polymers within lyotropic liquid crystals and its influence on device-relevant properties, including charge carrier mobility and conductivity. This insight offers valuable guidance for the future design and development of conjugated-polymer-based chiral electronics.

Graduation Semester

2025-05

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/129359

Copyright and License Information

Copyright © 2025 Zhuang Xu

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