Synthesis and properties of fluorenyl-pyridinyl alternatingcopolymers for light-emitting diodes

Abstract

A novel series of well-defined alternating poly[2,7-(9,9-di(2-ethylhexyl)fluorenyl)-alt-pyridinyl] (PDEHFP) copolymers were synthesized using palladium(0)-catalyzed Suzuki coupling reaction in high yields. These polymers were characterized using 1H NMR, UV-visible and fluorescence spectroscopies, gel permeation chromatography, thermal analysis and cyclic voltammetry. The optical properties of the copolymers, including photoluminescence (PL) and electroluminescence (EL), were studied. The difference in linkage position of pyridinyl units in the polymer backbone has significant effects on the electronic and optical properties of polymers in solution and in film state. Meta-linkage (3,5- and 2,6-linkage) of pyridinyl units in the polymer backbone is more favorable for pure blue emission and prevention of aggregation of polymer chains. PDEHFPs with 2,6- and 3,5-linkage of pyridinyl units have relatively high PL efficiency of 37 and 44% in the film state. In comparison with homopolymer PDEHF, the copolymers with pyridinyl units possess low lowest unoccupied molecular orbital energy levels for easy electron injection from a cathode. Strong EL is observed and light-emitting diodes (LEDs) exhibit typical rectifying characteristics. The emission intensity starts to increase at around 12 V. The emission peak wavelengths of the polymers roughly coincide with those of PL. This series of fluorene–pyridine-based alternating copolymers seem to be candidates for polymeric LEDs. © 2013 Society of Chemical Industry