Amplified fluorescence quenching and electroluminescence of a cationic poly(p-phenylene-co-thiophene) polyelectrolyte

A water-soluble conjugated polymer was developed that displays significant fluorescence quenching (25% quenching of polymer's fluorescence at quencher concentrations of 500 nM) and forms well-defined multilayer structures via electrostatic deposition. The polymer contains alternating 2,5-thienyl- and 2,5-bis[2-(N,N-diethylamine)-1-oxapropyl]-substituted 1,4-phenylene units formed by a Stille reaction protocol. Poly({2,5-bis[2-(N,N-diethylamino)-1-oxapropyl]-1,4-phenylene}-alt-2,5-thienylene) (PPT-NEt2) was synthesized in 80% yield from 2,5-bis(trimethylstannyl)thiophene and the reactive diiodinated species 2,5-bis(3-[N,N-diethylaminol-1-oxapropyl]-1,4-diiodobenzene. The neutral polymer was soluble in THF, chloroform, and 1 M HCl (aq). A number-average molecular weight of 5300 g mol(-1) (ca. 40 rings) was measured by GPC (relative to polystyrene standards). PPT-NEt2 is easily converted to the water-soluble polymer, poly{2,5-bis[2-(N,N,N-triethylammonium)-1-oxapropyl]-1,4-phenylene-alt-2,5-thienylene} dibromide (PPT-NEt3+), by quaternization with bromoethane. The polymer's electronic absorption is dependent on quaternization with absorption/emission wavelength maxima (lambda(max)) corresponding to 460/519 nm for the neutral polymer and 411/495 nm for the quaternized version in the appropriate solvent. Well-defined, thin multilayer films of PPT-NEt3+, alternating with poly(acrylic acid) [PAA] or poly(styrene sulfonie acid) [PSS], were deposited via electrostatic deposition onto indium tin oxide coated glass. Polymer light-emitting diodes prepared from PPT-NEt3+/PSS exhibited the best results with a lambda(max) of absorption and emission equal to 450 and 558 nm, respectively, external quantum efficiency of 0.017%, and light output of 110 nW.