Synthesis and utility of electronically diverse polycyclic aromatics

Abstract

This dissertation examines the synthesis of electronically diverse polycyclic aromatics within discrete molecular regimes and as incorporated into robust conjugated polymers. We have pursued two strategies for obtaining these large aromatics from rather elaborate yet readily available pendant aromatic precursors: electrophilic and oxidative cyclizations. Chapters One and Two describe research directed towards expansion of electrophilic cyclizations that utilize alkyne functionality to construct aromatized systems. Strongly-acidic conditions provide transient cationic intermediates that participate in annulations with adjacent arenes or amides to yield dibenzanthracenes or dioxanthracenes, respectively. In the latter case (Chapter Two), smaller model compounds confirmed this unique reactivity and helped to establish synthetic routes that yield isoquinolines in one synthetic step from aryl carbonyls appended with ortho-substituted alkynes. Chapters Three and Four describe efforts directed towards thiophene-based polycyclic aromatics. Tandem cyclization-polymerizations of pendant thiophenes provide robust electrochromic materials with readily tuneable optical bandgaps. Arenes placed adjacent to the highly-reactive oxidized thienyl moieties force this unique reactivity by biasing the system toward intramolecular cyclization. Chemical and electrochemical studies of related model systems further established the tandem reactivity. The generality of this controlled oxidative cyclization provided several readily-functionalized sulfur-based polycyclics. Using this chemistry, larger molecular and polymeric structures with tailored functionality may allow for a survey of electronic applications such as plastic organic field-effect transistors.

(cont.) After employing the new routes to large polycyclic aromatics, these scaffolds helped to study the optimization of exciton migration and the mechanical tuneability of polymer chromicity. The thiophene-based platforms allowed for an assessment of the photophysical effects of chromophore aromatization within conjugated arylene-ethynylene polymers. As detailed in Chapter Four, the incorporation of planar and aromatized moieties greatly reduces the rate of fluorescence decay relative to closely-related yet non-aromatized structures. These easily-functionalized substrates also led to an examination of a new type of mechanochromism as introduced in Chapter Five. Conformational alterations within the polymer could lead to attenuated excimer-like behavior, and the synthetic schemes presented allow for the incorporation of a variety of discrete chromophores that may provide clean and sharp optical properties when compared to conformationally-disordered conjugated polymers.