Synthesis and properties of new fluorene-based polyquinoxalines with an ether linkage in the main chain for light-emitting diodes

Abstract

A new series of fluorene-based polyquinoxalines with an ether linkage in the main chain were prepared by the polycondensation reaction between a tetraketone monomer and 3,3,4,4'-tetraaminodiphenyl ether. The polycondensation was usually carried out in m-cresol. The resulting polymers (P1-P3) [P1 = poly(quinoxaline-co-9,9-dihexyl-2,7-dimethyl-9H-fluorene) P2 = poly(quioxahne-co-9,9-dihexyl-9pentyl-2,7-di-p-tolyl-9H-fluorene) P3 = poly(quioxaline-co-9,9-bis-(4-methoxy-phenyl)2,7-dimethyl-9H-fluorene)] showed good solubility in common organic solvents and high thermal stability with only a 5% weight loss up to 440 degrees C. P1 and P2 had very high glass-transition temperatures of 212 and 223 degrees C, respectively, whereas P3 did not show any phase-transition temperature in repeated scans up to 300 degrees C. All the polymers in photoluminescence showed blue emissions in the range of 432-465 nm, both in chloroform solutions and in thin films. Light-emitting diode devices of the configuration indium tin oxide/poly(3,4-ethylenedioxythiophene)/pol3,mer:poly(N-vinyl-carbazole) blend (2:8)/LiF/Al were fabricated with P1 or P2 and emitted blue light with electroluminescence peak wavelengths of 434 and 448 nm, respectively. The maximum brightness and the external quantum efficiency of P1 were 0.56 mu W/cm(2) at 29 V and 0.056%, whereas P2 showed 0.50 mu W/cm(2) at 34 V and a relatively low value of 0.015%, respectively. Cyclic voltammetry studies revealed that these polymers possessed low-lying ionization potential energy levels ranging from -5.49 to -5.86 eV and low-lying electron affinity energy levels ranging from -2.65 to -2.88 eV.