Optoeletronic Properties and Photochemical Reactivity of Organoboron Compounds

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Rao, Yingli
Photochemistry , Optoelectronics , Photochromism , Isomerization , Organoboron
This thesis focuses on the investigation of the optoelectronic, photo and thermal- responsive properties of organoboron compounds. In search of blue phosphors for phosphorescent organic light emitting diodes, new triarylboron functionalized phosphorescent Pt(II) complexes were synthesized and their device performances were evaluated. In Pt(II) complexes with the 2-phenylpyridyl N,C-chelate ligand and an acetylacetonato ancillary ligand, it was found that BMes2 substitution at the 4’ position of the phenyl ring can increase the phosphorescent emission energy, compared to the 5’-BMes2 substituted analogue. This occurred without substantial loss of luminescent quantum efficiency. At 100 cdm-2 luminance, electroluminescence devices with the newly synthesized Pt(II) complexes as emitters have achieved external quantum efficiency of ~ 4.7-13.4%. Tuning of the photo-responsive properties of biaryl N,C-chelate dimesitylboron compounds was achieved by functionalization with either a bisthienyl moiety or ferrocene unit. It was demonstrated that the bisthienyl unit has the ability to completely stabilize a N,C-chelate boryl chromophore toward photoisomerization. With the ferrocene unit being part of the chelation backbone of BMes2 moiety, the B−N bond of molecule B(2-ferrocenyl-N-Me-benzimidazolyl)Mes2 was found to undergo a dynamic dissociation/association process in solution, leading to its slow hydrolysis under ambient conditions. The oxidized ferrocenium species has a notable spin delocalization through space from the Fe(III) center to a flanking mesityl group. To further expand the photochromic family based on pyridyl N,C-chelate dimesityboron compounds, a systematic study was carried out with the pyridyl N donor replaced by N-heterocyclic carbene donor and azolyl, benzoazolyl N donors. These new classes of organoboranes all underwent transformation to their corresponding dark isomer in a similar fashion as the pyridyl N,C-chelate dimesitylboron. However, a second-step photoisomerization was observed in the NHC,C-chelate dark isomer via a “borylene”-like intermediate. The thiazolyl, benzoazolyl N,C-chelate dark isomers demonstrate multi-structural transformations, which include hydrogen atom transfer, 1,3-boryl shift and diastereomer interconversion via a spiropyran type ring-opening/closure process. The imidazolyl N,C-chelate dark isomer exhibits a consecutive photochromism phenomenon, namely an interconversion between azaboratabisnorcaradiene and azabenzotropilidene derivatives. The calculated mechanism for this bares an interesting resemblance to that of the “walk” rearrangement in norcaradiene.
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