The evolution of linear and cross-conjugated benzobisoxazole organic semiconductors designed for organic light-emitting diode application
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Abstract
Research efforts towards realizing electrochemically-stable organic semiconductors have been a focus of the organic light-emitting diode (OLED) industry for decades. This is especially true for the discovery of blue-light emitting materials and compounds with optical band gaps greater than 2.8 eV as these materials undergo rapid degradation resulting in poor operational lifetimes. Benzo[1,2-d:4,5-d']bis(oxazole)s and benzo[1,2-d:4,5-d']bis(oxazole)s (BBOs) are useful building blocks that generate highly fluorescent materials with robust thermal and photo-oxidative stabilities and optical band gaps >2.8 eV. Previously, the Jeffries-EL group has synthesized and studied polymeric BBO systems for OLED application, which achieved sky-blue electroluminescence (EL) with external quantum efficiencies (EQEs) of approximately 1.1 %. However, these systems are plagued with broad electroluminescence due to their polydispersity and have yet to achieve power efficiencies >10 lm/W. Small molecule BBO-based emitters (SM-BBOs) are advantageous due to their discrete size, uniform pi-electron delocalization, and high purity. To date, SM-BBOs have realized deep blue EL with good color purity and EQEs approaching 3%. However, the number of possible structural variants are vast, but known examples of these systems are limited, thus more work is required to increase our understanding of these systems to develop SM-BBOs-based OLEDs with higher efficiencies for commercial utility. Herein, several classes of SM-BBOs bearing various aryl substituents are computationally and experimentally studied to understand fundamental optical and electronic properties. This data is used to determine the structure-property relationships between SM-BBO systems and to design functional microelectronic OLEDs. The collected experimental data is used by our computational teams to improve our predictive strategies and refine synthetic efforts. As such, several SM-BBOs were achieved which produced near-UV and deep-blue EL while combinations of these products were used to obtain the first prototype white OLEDs with temperature tunability using SM-BBO emissive materials.