Synthesis And Property Of Fluorene-based Blue Light-Emitting Polymers

As a novel displaying technique with promising application in flat-panel displays, organic light-emitting diodes (OLEDs) have received particular attention by virtue of the following properties:direct current driving, self-luminous, small volume, fast response, full color emission, wide view angle and simple manufacture techniques. Among the three primary colors of OLEDs, the green light-emitting materials have achieved a practical level while the red and blue ones still lag behind them especially the blue materials with high stability and efficiency. Nowadays to develop blue emitters with high performance has been the focus of the research. The polyfluorene (PF) with rigid planar biphenyl structure is just enough to supply the demand due to its wide energy gap, high fluorescent quantum yield and ease to be modified.However, the blue light-emitting material of polyfluorene have a drawback that will cause long-wave emission if annealed in air or operation in air for a long time, which seriously affect the comprehensive performance of the electroluminescence. Currently, it is widely accepted that the 9-C position of fluorine ring was oxidized to fluorenone, which give rise to the long-wave emission. In this dissertation, in order to improve the spectra stability of polyfluorenes, we introduce the strong electron-withdrawing 2,5-difluorobenzene to the backbone of polyfluorene, with the expectation to reduce the electron density on fluorene ring and hence to greatly reduce the possibility of fluorine oxidation. It is generally accepted that large amount of electron-withdrawing substituent will decrease the luminescent efficiency and charge mobility. In order to optimize the fluorescent quantum yield, solubility and charge-transporting ability of the resulted polyfluorenes at the same time, therefore, we tuned the contents of the difluorobenzene group and prepared a series of copolymer with different ratio of 2,5-difluorophenyl. The products were purified by the method of dissolution-deposition and soxhlet extraction. The distribution of molecular weight and molecular structure was identified by GPC,1HNMR spectroscopy and elemental analysis.This series of polymers have good solubility in common organic solvents. So it is possible to prepare films and devices by solution methods such as spin coating or ink jet printing. The spectroscopy study exhibited that the UV absorption and fluorescence spectra of this series of polymers had a blue shift in comparison with those of the parent polyfluorene, and the fluorescence quantum yields had a small decrease.The films of the obtained polymers were annealed in air at 100℃for 2 hours to detect the emission spectra stability. No green emission was observed after the annealing, suggesting that these novel polyfluorenes with difluorobenzene units on backbone are not prone to air oxidation.