New Soluble n-Type Conjugated Polymers for Use as Electron Transport Materials in Light-Emitting Diodes

Abstract

We report the synthesis of a series of six new soluble n-type conjugated copolymers that incorporate bis(phenylquinoline) and regioregular dialkylbithiophene in the backbone, the investigation of their electrochemical, photophysical, and electroluminescent properties, and their use as electron transport materials in light-emitting devices. Variation of the alkyl chain length of the head-to-head dialkylbithiophene linkage from butyl (C4) to dodecyl (C12) had little or no effect on the photophysical or redox properties of the new polyquinolines. However, the glass transition temperature decreased from 256 °C for the butyl derivative to 76 °C for the dodecyl derivative. A single-crystal X-ray structure of a model compound allowed quantification of the intramolecular torsion angles which explain the significant difference in optical band gap and redox properties between the head-to-tail and head-to-head polymers. The high electron affinity (2.88−2.97 eV), robust thermal stability, suitable solution processability, and weak-to-moderate intrinsic electroluminescence of the polymers make them attractive for use as electron transport materials in organic LEDs. Initial use of the polymers as the electron transport materials in bilayer poly(2-methoxy-5-(2‘-ethylhexyloxy)-1,4-phenylenevinylene) (MEH−PPV) LEDs showed substantial enhancement in device performance under ambient air conditions (1.4% external quantum efficiency, 2170 cd/m2) compared to most current electron transport polymers for LEDs.