| Abstract
| - Thermal dehydration of boronic acid groups to form six-membered boronic acid anhydride (boroxine) rings was employed as a means of immobilizing oligofluorenes. This approach appears to resolve the problems of the emergence of the long wavelength emission upon heating that has often been observed in polyfluorenes.
- Thermal dehydration of boronic acid groups to form six-membered boronic acid anhydride(boroxine) was employed as a means of immobilizing oligofluorenes. This approach appearsto improve the photoluminescenct stability of the cross-linked films compared to polyfluorenes. The emergence of long-wavelength emission upon thermal treatment usually observedin polyfluorenes has been prevented in this system. Initially the fluorene dimer (F2BA),trimer (F3BA), and tetramer (F4BA) containing boronic acid groups were prepared. Thesecompounds were found to be readily soluble in common solvents such as THF, acetone, andDMF. Transparent thin films of these materials could be easily prepared by casting theirsolutions in THF onto KBr disks or glass substrates. Using mild reaction conditions (60−130 °C under vacuum for 2 h), these oligomers in the solid sate readily undergo cross-linkingreactions by the dehydration of boronic acid groups as evidenced by FT-IR spectroscopy andDSC/TGA studies. The resulting cross-linked amorphous networks exhibit high thermal (Tdat 5% weight loss, 363−420 °C) and morphological (Tg, 173−202 °C) stability. Under UVirradiation, these compounds emit bright violet-blue (F2BA) and blue (F3BA and F4BA)light both in solution and in the solid state. The cured films exhibited almost identical UV−vis and fluorescence spectra even after heating at 150 °C for 24 h, showing no long wavelengthemission. The fabrication of LED devices using F3BA or F4BA as the light-emitting layerand a carbazole diboronic acid (CzBA) as the hole-transporting layer demonstrated thatthese thermally curable diboronic acids can be used to achieve double- (or multi-) layeredconfigurations.
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