Novel stable blue-light-emitting oligofluorene networks immobilized by boronic acid anhydride linkages

Thermal dehydration of boronic acid groups to form six-membered boronic acid anhydride (boroxine) was employed as a means of immobilizing oligofluorenes. This approach appears to improve the photoluminescenct stability of the cross-linked films compared to polyfluorenes. The emergence of long-wavelength emission upon thermal treatment usually observed in polyfluorenes has been prevented in this system. Initially the fluorene dimer (F2BA), trimer (F3BA), and tetramer (F4BA) containing boronic acid groups were prepared. These compounds were found to be readily soluble in common solvents such as THF, acetone, and DMF. Transparent thin films of these materials could be easily prepared by casting their solutions in THF onto KBr disks or glass substrates. Using mild reaction conditions (60-130degreesC under vacuum for 2 h), these oligomers in the solid sate readily undergo cross-linking reactions by the dehydration of boronic acid groups as evidenced by FT-IR spectroscopy and DSC/TGA studies. The resulting cross-linked amorphous networks exhibit high thermal (T-d at 5% weight loss, 363-420degreesC) and morphological (T-g, 173-202degreesC) stability. Under UV irradiation, these compounds emit bright violet-blue (F2BA) and blue T3BA 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 150degreesC for 24 h, showing no long wavelength emission. The fabrication of LED devices using F3BA or F4BA as the light-emitting layer and a carbazole diboronic acid (CzBA) as the hole-transporting layer demonstrated that these thermally curable diboronic acids can be used to achieve double- (or multi-) layered configurations.