Synthesis And Properties Investigation Of Fluorene-Based Bluelight-Emitting Materials

Fluorene and its derivatives were broadly investigated due to their blue luminance and application in full-color display. However, on account of the occurrence of molecular aggregation and excimer by fluorenone defects, green band emission happened during the fabrication of fluorene-based organic light-emitting devices; thus, the luminous purity was highly influenced. In this paper, a series of copolymers were synthesized by using fluorene as raw material in different approaches and their properties were analyzed.(1) 9,9'-dioctyl-2,7-diaminofluorene was synthesized by the alkylation, nitration and reduction of fluorene as a monomer. Afterwards, we got two kinds of copolymers by Schiff Base reaction with N-octyl-3,6-dialdehydecarbazole or dialdehydetriphenylamine as another monomer independently:PF-alter-Cz and PF-alter-TPA. Thermogravimetric analysis and differential scanning calorimetry showed that they were good thermal-resistant with their onset decomposition temperature:PF-alter-Cz,330℃and PF-alter-TPA,420℃. Cyclic voltammagrams indicated that the band gap of PF-alter-Cz was lower because the carbazole moiety acts as a role of decreasing the turn-on voltage. Mixation of POSS in copolymers made little influence on change of band gap, but could apparently result in decline of electrochemical activity. Ultraviolet-visible absorption and photoluminescence spectra exposited that the optical stability of PF-alter-TPA was better than PF-alter-Cz because of its steric configuration as well as more benzene rigid structure. Mixation of POSS into these two copolymers could effectively suppress the aggregation and improve the luminous purity.(2) Two monomers:9,9'-dioctyl-2,7-dibromofluorene and N-octyl-3,6-divinylcarbazole were synthesized; then, a copolymer (PFC) based on them was obtained by Heck coupling reaction. It had good thermal properties with an onset decomposition temperature of 370℃and no obvious glass transition temperature. The band gap of PFC was 3.6eV. Elctrochemical reaction of PFC was reversible and stable after scanning three times. PFC could emit blue fluorescence both in dilute chloroform solution and in solid thin film, but it still needs deeper modification to improve its luminescent property owing to the green band emission resulted from the inevitable molecular aggregation in thin film.