Preparation of New Electron-Accepting π-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 π-conjugated
polyquinoxalines with a molecular weight of 6 × 103
to
260 × 103. 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 (λmax 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.