Tuning π-Conjugated Polymers with Heteroatoms: Synthesis of Polytellurophenes and Soluble Polyacetylenes

Abstract

This Thesis describes the synthesis and investigation of π-conjugated oligomers and polymers with optoelectronic properties affected by the presence of a heteroatom. Two major classes of polymers are discussed: 1) polytellurophenes, where the backbone is formed by 5-membered heterocycles containing tellurium (Te), and 2) heteroatom (B, N, P)-appended polyacetylenes, where the polymer backbone consists of –(HC=CH)– repeat units. To begin, pinacolboronate (BPin)-functionalized tellurophenes with fused cycloalkane side group (5- or 6-membered rings) are transformed into new diiodinated tellurophene monomers to allow for homopolymerization through Yamamoto coupling. The size of the side group is found to drastically alter the HOMO-LUMO energy gap (Eg) of the resulting oligomers due to changes in backbone planarity. Density functional theory computations show that the effect is carried into copolymers with 3-alkylthiophenes, pointing at the tellurophene fused with the 5-membered ring as the best choice for planar backbones with extended π-conjugation. Additionally, a 3-borylated tellurophene was used in a Suzuki-Miyaura cross-coupling protocol to synthesize a new tellurophene monomer containing a cumenyl (-C6H4iPr) side chain. The corresponding poly(3-aryltellurophene) later obtained through Grignard metathesis (GRIM) polymerization shows a reduced HOMO-LUMO energy gap (Eg) when compared to the analogous polythiophene. The presence of aryl (cumenyl) side chains further reduces Eg below the values reported for its poly(3-alkyltellurophene) counterparts. Finally, a modified ring-opening metathesis polymerization (ROMP) protocol is used to prepare soluble functionalized polyacetylenes (PA) bearing: 1) electron-deficient boryl (-BR2) and phosphoryl (-P(O)R2) side chains, 2) electron-donating amino (-NR2) groups, and 3) ring-fused 1,2,3-triazolium units via strain-promoted Click chemistry. Not only are most of these new functional polyacetylenes soluble in conventional organic solvents, allowing film formation via ubiquitous spin coating methods, they retain similar intense light absorption across much of the visible spectral region as parent polyacetylene. Some of the resulting polyacetylenes also show greatly enhanced stability in air when compared to parent polyacetylene. Furthermore, placement of redox-active boryl and amino groups directly onto a polyacetylene backbone leads to switchable near-IR optical responses in the telecommunications range (ca. 1500 nm) upon chemical reduction or oxidation. A general route to cross-linked networks is also presented, and is based on the reduction of the polyacetylenes in solution.