Synthesis and nearly monochromatic photoluminescence properties of conjugated copolymers containing fluorene and rare earth complexes

A series of novel conjugated copolymers containing 9,9-dihexylfluorene and europium complex-chelated benzoate units in the main chain had been synthesized through a three-step process, involving Suzuki coupling copolymerization, hydrolysis of benzoate units, and postchelation. The chemical structures of the copolymers were characterized by FT-IR, ¹H NMR, ¹³C NMR, GPC, and ToF-SIMS. The copolymers exhibited relatively high glass transition temperatures after incorporation of the rare earth complexes. The photoluminescence (PL) properties of the copolymer complexes in solution and in film form were investigated. Intramolecular energy transfer from the fluorene groups to the europium complex occurred in diluted solutions of the copolymer. The efficiency of this process depended strongly on the structure of the ligands for the europium complex and the composition of the copolymers. The PL spectra of the copolymer complexes consisted of two emission bands, one in the 350-550 nm region and another at around 612 nm, corresponding to the π* → π transitions of the fluorene moieties and the f-f transitions of the europium ions, respectively. In the copolymer films cast from solutions, emission from the fluorene groups could be suppressed, and the absorbed excitation energy was transferred effectively to the europium complexes in the copolymer. Nearly monochromatic red emission (with a line width of about 4 nm) was detected under UV excitation at room temperature.