External Stimuli Driven Transformation of BN-Heterocycles
BN-heterocycles have important applications in synthetic methodologies, chemical reactivity and optoelectronics. This thesis focuses on the investigation of the thermal and photo-responsive properties of BN-heterocycles as well as applications in optoelectronic devices. In search of more efficient photoelimination of BN-heterocycles, we found that BN-heterocycles could have different and unique transformations upon external stimuli. The preparation of the BN-polycyclic aromatic hydrocarbons (PAHs) is also described. Boranes with the general formula of HBR2 have been found to undergo a facile 1,1-hydroboration reaction with pyrido[1,2-a]isoindole, resulting in insertion of a BR2 unit into a C-N bond and the formation of a variety of BN-heterocycles. Investigation on the thermal reactivity of the BN-heterocycles revealed that these molecules have two distinct and competitive thermal elimination pathways: HBR2 elimination (or retro-hydroboration) versus R-H elimination, depending on the R group on the B atom and the chelate backbone. Given the 1,3-dipolar nature pyrido[1,2-a]isoindole, a “one-pot” cycloaddition procedure of pyrido[1,2-a]isoindole and its analogues with dimesitylboron functionalized alkynes has been successfully developed for the synthesis of a series of BMes2-functionalized N-heterocycles. To further expand the photoelimination reaction to multiple BN-doped PAHs, new BN-heterocyclic compounds have been found to undergo double photoelimination, forming rare yellow fluorescent (BN)2-arenes that contain two B-N units. Most significant is the discovery that the double elimination can also be driven by excitons generated electrically within electroluminescent (EL) devices, enabling the in situ solid-state conversion of BN-heterocycles to BN-arenes and the use of BN-arenes as emitters for EL devices. The in situ exciton-driven elimination (EDE) phenomenon has also been observed for other BN-heterocycles. Besides changing external stimuli to improve the efficiency of elimination of BN-heterocycles, the internal factor - the effect of substituents on boron atom was also investigated. We found that both electronic and steric effects have a significant and distinct impact on the photoreaction pathways of BN-heterocycles. With bulky, electron-donating aryl rings on the B atom, the photoelimination efficiency is enhanced. In contrast, with sufficiently electron-withdrawing aryl groups on the B atom, bulky or not, photoisomerization occurs exclusively with a high stereoselectivity.
URI for this recordhttp://hdl.handle.net/1974/22968
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