Synthesis and characterization of polyfluorene copolymers containing trimetic thiophene derivatives

Abstract

A novel polyfluorene copolymers which contain mixed trimers composed of two thiophenes as donor units and other aryl units -EDOT, thiazole, thiophene, 2,5-pyridine, and 2,6-pyridine- between them are synthesized by Pd catalyzed Suzuki coupling method. All the polymers are soluble in organic solvents except for copolymer containing thiophene unit as aryl unit. Generally it is shown that five-membered rings, EDOT and thiazole, as aryl unit between two thiophene rings have long wavelength in UV-vis and PL than those of six membered-rings pyridine. It is due to the more planar conformation of the monomers in polymer chain and a smaller band gap in the case of thiophene rings. EDOT containing polymer showed the most red-shifted UV-vis (480 nm), PL spectra (587 nm) and the smallest band gap (2.19 eV, optical band gap)due to mainly long π-conjugation by unique properties of EDOT. Pyridine containing polymers, which are theoretically considered as low band gap polymer, exhibited larger band gaps (3.08 eV - 3.49 eV ). It can be explained that pyridine rings are not good electron acceptors. Cyclic voltammetry study of these polymers showed somewhat difference between optical band gap and electrochemical band gap because of the presence of space charge and interfacial dipoles. The EL properties of the copolymers were measured in the devices configuration of ITO/PEDOT/Polymers/Ca/Al. The polymers showed pale red emission except for 2,6-pyridine containing polymer which showed pale blue. 2,5-Pyridine containing polymer showed the highest maximum brightness of 2010cd/㎡of the polymers.