Preparation of new electron-accepting pi-conjugated polyquinoxalines. Chemical and electrochemical reduction, electrically conducting properties, and use in light-emitting diodes

Dehalogenation polycondensation of 5,8-dibromoquinoxaline derivatives and 2,6-dibromoquinoxaline with zerovalent nickel complex affords a series of n-conjugated polyquinoxalines with a molecular weight of 6 x 10(3) to 260 x 10(3). The polymers are electrochemically reduced (or n-doped) with an E degrees value of -1.75 to -2.35 V vs Ag/Ag+ and converted into electrically conducting materials with a conductivity of 1 x 10(-4) to 7 x 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 (lambda(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.