Directing the Solid-State Properties of Phenylene Ethynylenes through Programmable Interactions of Side Chains
Sharber, Seth.
2019
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Although π-conjugated organic materials enable the pursuit of
attractive new technologies due to their unique properties, programming material
function from chemical structure is hindered by the challenge of controlling
spatial arrangement of conjugated materials (CMs) in solids, which is integral to
bulk properties but driven by weak non-covalent interactions. Rational design
requires strategies ... read moreto direct molecular packing through non-covalent interactions.
By taking advantage of underutilized chemical side chains, we direct the
solid-state properties in three-ring phenylene ethynylenes (PEs) through stacking
interactions between fluoroarenes in side chains and terminal arenes in the
chromophore (ArF-ArH interactions). These interactions program a twist into PE
backbones, which dictates their fluorescence in solids. Chapter 2 describes how
electronic substituents in PEs modulate the dominance of ArF-ArH interactions and
thus switch molecular packing, resulting in large fluorescent shifts.
Electron-donating substituents strengthen the interaction, and crystal structures
reveal twisted, non-aggregated assemblies of PEs. In contrast,
electron-withdrawing substituents weaken the interaction, and sufficiently
electron-withdrawing groups result in bathochromically shifted solids with planar
and/or aggregated PEs in crystal structures. Chapters 3 and 4 investigate how
lengths of terminal alkyl chains in PEs tune phase behavior and stimuli-responsive
properties. In chapter 3, alkyl chains in aniline-terminated PEs can tune aspects
of mechanically induced changes in fluorescence—mechanofluorochromism (MFC). The
lengths of terminal aliphatic substituents tune the thermal recovery process in
MFC as a result of modifying molecular packing of PEs. In chapter 4, competition
between non-covalent interactions enables crystal polymorphism. Weakening ArF-ArH
interactions via ester substituents yields polymorphs featuring either programmed
twisting or PE aggregation, and alkyl chains can bias competition between these
phases, affecting polymorphism, MFC, and crystal-to-crystal transitions of
metastable phases. Chapter 5 focuses on fluorinated side chains, where minimal
structural changes can broadly switch properties. Regiochemical variations in the
fluorine substitution pattern of side chains would not normally be expected to
direct packing, but regioisomeric PEs that differ only in the position of a single
fluorine in the side chains can show large shifts in fluorescence as well as
inversion of the direction of MFC shifts due to switches in the presence or
absence of programmed twisting.
Thesis (Ph.D.)--Tufts University, 2019.
Submitted to the Dept. of Chemistry.
Advisor: Samuel Thomas.
Committee: Clay Bennett, Charlie Sykes, and Mu-Ping Nieh.
Keywords: Chemistry, and Organic chemistry.read less - ID:
- xd07h6953
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