Conjugated and partially conjugated 2,5-diphenylthiophene-containing light-emitting copolymers in polymeric light-emitting diode (PLED) device fabrications

To study the effect of nonconjugation on polymeric and photophysical properties of thiophene-containing polymers, new light-emitting copolymers comprising either alternate 2,5-diphenylthiophene and vinylene or alternate 2,5-diphenylthiophene and aliphatic ether segments were synthesized. Both copolymers contained 2,5-diphenylthiophene as the major chromophore and emitted a sky bluish fluorescence in dilute solution (10(-2) mg/mL). With a rigid and planarity structure and the concomitant crystallinity, the former copolymer (fully conjugated) possessed a higher quantum efficiency, a higher glass-transition temperature, and a better thermal stability. In contrast, the latter copolymer (conjugated-nonconjugated) had better solubility and provided enhanced photophysical properties for the fabricated polymeric light-emitting diode (PLED) device: at 15 V, the maximum current and brightness were 110 mA/cm(2) and 4289 cd/m(2), respectively, and the electroluminescence efficiency remained constant at approximately 4.9 cd/A in a voltage range of 8 to 14 V. The existence of intramolecular/intermolecular aggregates in the latter copolymer was corroborated from the the UV-vis and photoluminescence spectra of its solutions. With an increase in solution concentration, the shape and lambda(max_) of the photoluminescence spectrum were redshifted. In a solution with a concentration as high as 10 mg/mL, the redshift was so drastic that the photoluminescence spectrum was nearly identical to that of a solid-film. (C) 2004 Wiley Periodicals, Inc.