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Bottom-up Strategies towards Controlled Synthesis of Graphene Nanoribbons with Precise Edges and Structurally Aligned Semiconducting Polymers


Chapter 1 provides an overview on the synthesis of graphene nanoribbons. Various types of edge structures and bottom-up synthetic strategies, on-surface and in-solution, are covered before discussing our group’s third approach, solid-state. This is followed by a description of the effect of heteroatomic substitutions on electronic properties of GNRs. Lastly a brief discussion on other semiconducting polymer structure and charge transfer properties are introduced.

Chapter 2 details the synthesis of N = 8 armchair graphene nanoribbons (GNRs) using a two-step solid-state method. Four diarylbutadiyne precursors undergo topochemical polymerization to four distinct polydiacetylene (PDA) polymers, which subsequently cyclodehydrogenate and undergo side chain fragmentation to afford the same N =8 armchair GNR. Various spectroscopic and imaging techniques are used to characterize this transformation, in addition to calculations of the cyclization process on a model system used to verify the mechanism.

Chapter 3 describes the synthesis of GNRs with a fjord-edge structure and site-specific nitrogen substitutions using the two-step approach above. Two dipyridylbutadiyne precursors polymerize and cyclize to afford N = 8 fjord-edge N-GNRs, with side chains still intact. Spectroscopic characterization, imaging and mechanistic calculations of a pyridyl model system verify the transformation from butadiyne to GNR.

Lower the barrier of Hopf cyclizations, a step towards GNRs in our solid-state approach, through introduction of strained cycloalkenes could lead to room temperature GNR syntheses. Chapter 4 details the synthesis of two polydiacetylene synthons containing norbornadiene, a bis(norbornadienyl)1,3-butadiyne and trans-bis(norbornadienyl)enediyne. Challenges towards synthesizing both monomer units and future applications of other trans-enediynes towards GNRs are discussed.

Chapter 5 describes the synthesis of an amphiphilic semiconducting polymer, poly(cyclopentadithiophene-alt-thiophene) (PCT), according to a set of design rules aimed at straightening the polymer backbone in order to reduce polymer disorder and increase conductivity. The design rules are 1) hydrophobic polymer backbone and hydrophilic side chains, 2) alternating co-polymer such that all the side chains reside on one side, 3) side chains branched off an sp3 carbon to create a 3D wedge shape, and 4) complementary bond angles between monomer units to achieve a 180� dihedral angle. The solution phase of the polymer is characterized by small angle X-ray scattering (SAXS) and imaged using cryo-transition electron microscopy (TEM).

Applications of PCT towards controlling electron donor-acceptor complexes are explored in Chapter 6. PCT and poly(fluorene-alt-thiophene) (PFT), are complexed with two electron acceptors, a charged perylenediimide and a series of charged bis-pyrrolidium functionalized fullerenes. The structure of these co-assemblies are characterized by small angle X-ray scattering and photoluminescence quenching, concluding that complementary geometries between the polymer micelle and acceptor shapes result in increased amounts of photoluminescence quenching.

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