Synthesis and optical and electrochemical properties of copolymers containing 9,9-dihexylfluorene and 9-dimethylaminopropylcarbazole chromophores

Soluble and well-defined 9,9-dihexylfluorene and 9-dimethylaminopropyl- carbazole based copolymers PFCN and 5PFCN have been prepared by Suzuki coupling polymerization. For comparison, alternate copolymer of 9,9-dihexylfluorene and 9-hexylcarbazole (PFC) was also prepared with the same method. Furthermore, alternate copolymer of 9,9-dihexylfluorene and 9-dimethylethylammoniumpropylcar- bazole (PFCNE) was prepared from PFCN by the ethylation of its dimethylaminopropyl groups with bromoethane. These copolymers were soluble in organic solvents and showed high glass-transition temperatures (75-160°C). The optimized architecture of PFCN from a simulation was a spiral, which was different from the linear structure of poly(9,9-dihexylfluorene) (PFO). Thermogravimetric analysis showed that the residual weights of 5PFCN, PFCN, PFC, and PFCNE at 800°C were all greater then 50%, whereas PFO showed complete thermal decomposition. Both the absorption and photoluminescence emission peaks of these copolymers showed blueshifts after the introduction of the carbazole units because of reduced conjugation. Moreover, the introduction of 9-hexylcarbazole and 9-dimethylamionpropylcarbazole moieties into copolymers PFC and PFCN, respectively, effectively prevented the excimer formation of PFO. According to cyclic voltammetry results, PFCNE containing quaternary amino pendant groups exhibited the most stable reduction-oxidation cycles. The turn-on electric fields of their electroluminescence devices decreased with increasing carbazole content because of the more balanced carrier injection.