Regioregular head-to-tail poly(4-alkylquinoline)s: synthesis, characterization, self-organization, photophysics, and electroluminescence of new n-type conjugated polymers

Five poly(4-alkylquinoline-2,6-diyl)s (P4AQs), having 100% head-to-tail regioregularity, were synthesized, characterized, and explored as emissive and electron transport materials in polymer light-emitting diodes. The organic solvent (chloroform, tetrahydrofuran, methylene chloride, etc.) soluble poly(4-alkylquinoline)s had reversible electrochemical reduction with an electron affinity (2.6 eV) that was independent of alkyl chain length. X-ray diffraction studies showed that solution-cast films of regioregular P4AQs have self-organized lamellar structures with an interchain pi-stacking distance of 3.9-4.1 Angstrom and an interlayer d-spacing that varied linearly from 13.4 Angstrom for butyl to 22.3 Angstrom for tridecyl side chains. The new poly(4-alkylquinoline)s emit blue light at 423-426 nm in dilute chloroform solution and yellow light at 542-557 nm in the solid state. Yellow electroluminescence with a luminance of up to 86 cd/m(2) and an external quantum efficiency of up to 0.6% was observed for the P4AQs as emissive materials. A large improvement in performance of poly(2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylenevinylene)-based light-emitting diodes (a luminance of up to 700 cd/m(2) and an external quantum efficiency of up to 3.0%) was observed when the P4AQs were used as the electron transport materials. The new regioregular poly(4-alkylquinoline)s are promising n-type conjugated polymer semiconductors for electronic and optoelectronic devices.