Boron-Based Conjugated Materials: Structural Analysis, Photophysical Properties, and Redox Chemistry

Artificial lighting serves as a fundamental necessity to the daily lives of the global population, and consumes around 20% of universal energy production. Although significant progress has been made to the design, fabrication, and synthesis of blue light emitting diodes (LEDs) since their first discovery in the 1990s, the efficiency and quality of these light sources has yet to reach a limit. As an advancement to inorganic LEDs, organic light emitting diodes (OLEDs) produce thinner and more flexible display technologies. Therefore, increasing attention in both the synthetic and materials chemistry communities has been dedicated towards the synthesis of new molecular scaffolds for application in OLED technologies. Boron-based molecules are some of the most promising and highlight the importance of heteroatom-doping in new OLED structures. Due to its inherent electrophilicity, the inclusion of neutral boron moieties into carbon-based materials entirely perturbs the overall electronics of the system and transforms the molecules into much better electron acceptors. In chapter one, an overview of strategies for developing novel main-group materials is highlighted, and relevant seminal discoveries are detailed.
The work highlighted in this dissertation spans a five-year effort to further understand and expand the known chemistry of p-block element-based polycyclic aromatic hydrocarbons. Chapter two of this dissertation focuses on the synthesis and characterization of pyrene-fused N-heterocyclic germylenes (NHGe). These extremely twisted and bent NHGes laid the preliminary synthetic foundation for our subsequent studies on luminescent N-heterocyclic boranes featured in chapter three. The tunability of their fluorescence color designates these fused NHBs as promising candidates for functional main-group materials.
While electron deficient boron-based compounds are quite common, nucleophilic species are exceedingly rare, and the majority are highly reactive. However, with the optimum conditions, anionic species can be stabilized and their reactivity can be harnessed as powerful nucleophiles in synthetic chemistry. Chapter four of this dissertation details the challenging synthesis and structural characterization of 9-carbene-9-borafluorene monoanions. Notably, we prepared six novel carbene-coordinated borafluorene monoanions via two electron chemical reduction of their corresponding neutral tetracoordinate starting materials. Furthermore, the 9-carbene-9-borafluorenes were demonstrated to react as nucleophiles with metal halide substituents. Alternatively, when the borafluorene anions are introduced to diketones, the carbene ligands are displaced and new boron-based spirocycles are accessed. In chapter five, the redox chemistry of the borafluorene monoanions was probed with elemental selenium which resulted in the formation of new boryl-substituted selenides and diselenides, many of which resulted from 2,6-diisopropylphenyl migration.
Chapter six of this dissertation focuses on expanding the structural and electronic tuning of the 9-carbene-9-borafluorene monoanions by modulation of the cation-boron distances. When encapsulating the potassium cations with chelating ligands, the electron rich-ness of the boron center is increased. Therefore, unique optical properties of novel spirocycles can be accessed by introducing the charge-separated 9-carbene-9-borafluorene monoanions to diketones. Reaction modelling via density functional theory supports the formation of the spirocycles to proceed via a single-electron transformation from the boron atom to the diketone, thus demonstrating the ambiphilic nature of the 9-carbene-9-borafluorene monoanions. Furthermore, chapter seven details the activation of carbon dioxide by a 9-carbene-9-borafluorene monoanion which produces the novel trioxaborinanone. Remarkably, photolysis or thermolysis of the trioxaborinanone releases carbon monoxide and a fluorescence response is turned on.