Monochromatic light-emitting copolymers of N-vinylcarbazole and Eu-complexed 4-vinylbenzoate and their single layer high luminance PLEDs

A series of bifunctional copolymers containing hole transporting units and light-emitting units in the main chain had been synthesized via radical copolymerization of N-vinylcarbazole and Eu-complexed 4-vinylbenzoate. The chemical structure and composition of the copolymers were characterized by FT-IR and UV-visible absorption spectroscopy, ¹H and ¹³C NMR spectroscopy, GPC, ICP, mass spectroscopy, and elemental analysis. The copolymers exhibited good solubility, as well as good thermal stability and high glass transition temperatures. The photoluminescence (PL) properties of the copolymers in solution and in film form were investigated. Intramolecular energy transfer from the carbazole groups to the europium complex occurred even in diluted solutions. The efficiency of this process depended also on the composition of the copolymers. The PL spectra of the copolymers consisted of two emission bands, one in the 340-550 nm region and another at around 612 nm, corresponding to the π* → transitions of the carbazole moieties and the f-f transitions of the europium ions, respectively. In the solid state, emission from the carbazole groups was suppressed and the absorbed excitation energy was transferred effectively to the europium complexes in the copolymer. Nearly monochromatic red emission with a turn-on voltage of 8 V was detected from the single layer copolymer light-emitting diode (PLED). This single layer PLED exhibited a maximum luminance of 126 cd m^-2 at 22 V and 328 mA cm^-2, and a luminance efficiency of 0.56 cd A^-1.