Synthesis and electroluminescence of novel copolymers containing crown ether spacers

Three novel copolymers containing alternating 1,4-bis(phenylethenyl) benzene, 1,4-bis(phenylethenyl)-2,5-dimethoxybenzene or 1,5-bis(phenylethenyl)naphthalene chromophores and crown ether (CE) spacers within the polymer backbone were synthesized by the Wittig polycondensation reaction. These copolymers exhibit good thermal stability (decomposition temperature around 380-411 degreesC). Photoluminescence (PL) and electroluminescence (EL) color can be easily tuned in these copolymers by simply changing the structure of the chromophores. Excimer emission is responsible for the changes in the PL spectra on going from solution to the thin films. Typical light-emitting electrochemical cell (LEC) behavior is observed in the EL devices with these copolymers as both electronic and ionic conductors. The turn-on voltage for emission is at +3.9 V, in forward bias, and at -4.4 V, under reverse bias. Efficient LECs with low turn-on voltages can be demonstrated upon addition of poly(ethylene oxide) (PEO) into the active layers. CE spacers in these copolymers contribute to the increasing ionic conductivity and the improvement in the morphology of the active layers. The d.c. response and a.c. impedance behaviors of the LECs with PEO were investigated, and the results indicate the operation of these LECs corresponds to the electrochemical doping mechanism.