Deep-blue Electroluminescence of 9,9-Dialkyl-substituted Polyfluorene Copolymers Incorporating Diphenylsilane Units

Abstract

Bis(7-bromo-9,9-dialkylfluorenyl)diphenylsilanes with different alkyl chain lengths ( Cn = 4 and 8) were synthesized in this work. Conventional 9,9-dialkyl-substituted polyfluorenes (PFs) were modified through copolymerization of the diphenylsilane-containing comonomers by the Suzuki coupling method to improve electroluminescence (EL) color purity. The PF copolymers possessed high molecular weights that were enough for film-forming ability and good solubility in various common organic solvents such as toluene. The incorporation of the diphenylsilane units suppressed effectively the PF-inherent green PL, which is undesirable for the present purpose, with a slight blue-shift of the interesting blue emission band. The results are closely related to a bent and bulky structure at the diphenylsilane units which disturb interchain stacking and a controlled conjugation length along the PF main chains. This effect was observed much more prominently in the EL spectra. Even in the di- n-butyl-substituted PF copolymer which tends to show a strong green excimer emission, the incorporation of the diphenylsilane units completely erased the green EL band. The results suggest that the PF copolymers can be a good candidate as a novel deep-blue emission layer material in EL devices.