Preparation of New Electron-Accepting π-Conjugated Polyquinoxalines. Chemical and Electrochemical Reduction, Electrically Conducting Properties, and Use in Light-Emitting Diodes

Abstract

Dehalogenation polycondensation of 5,8-dibromoquinoxaline derivatives and 2,6-dibromoquinoxaline with zerovalent nickel complex affords a series of π-conjugated polyquinoxalines with a molecular weight of 6 × 10 3 to 260 × 10 3 . The polymers are electrochemically reduced (or n-doped) with an E ° value of −1.75 to −2.35 V vs Ag/Ag + and converted into electrically conducting materials with a conductivity of 1 × 10 - 4 to 7 × 10 - 3 S cm - 1 by chemical reduction. Poly(quinoxaline-5,8-diyl)s with aromatic substituents give strong fluorescence with emission peaks at 450−520 nm in solutions as well as in cast films. A light-emitting diode (LED), ITO/polymer/Mg(Ag) (polymer = poly(2,3-diphenylquinoxaline-5,8-diyl)), emits blue-green light (λ max at about 500 nm). Introduction of hole-transporting layers such as vacuum-deposited or spin-coated thin layers of poly(thiophene-2,5-diyl), poly( p -phenylene), and poly( N -vinylcarbazole) between ITO and the light-emitting layer enhances electroluminescence efficiency by about 2 orders of magnitude. Polyquinoxalines have an ionization potential of 5.83 ± 0.11 eV and a band gap of 2.56 ± 0.26 eV.