Synthesis and characterization of organo-soluble thioether-bridged polyphenylquinoxalines with ultra-high refractive indices and low birefringences

Two aromatic tetraketones, 4,4′-thiobis[(p-phenyleneoxy)benzil] (STK, 1) and 4,4′-thiobis[(p-phenylenesulfanyl)benzil] (3STK, 2) were synthesized by the nitro nucleophilic substituent reactions of 4-nitrobenzil and corresponding diol compounds. The two tetraketones were polymerized with three aromatic tetraamines, including 3,3′-diaminobenzidine (a), 3,3′,4,4′-tetraaminodiphenylether (b) and 3,3′,4,4′-tetraaminodiphenylsulfone (c), respectively to afford six thioether-bridged polyphenylquinoxalines (PPQs) – PPQ-1a–1c and PPQ-2a–2c. The obtained PPQs exhibited good solubility not only in conventional m-cresol and chloroform, but in the aprotic solvent – N-methyl-2-pyrrolidinone (NMP). PPQ-1c and 2c containing sulfone units were even soluble in tetrahydrofuran at room temperature with a solid content of 15 wt%. Flexible and tough PPQ films cast from their NMP solution showed good thermal stabilities, including glass transition temperatures in the range of 215–248 °C and 5% weight loss temperatures exceeding 500 °C in nitrogen. The PPQ films at a thickness of ∼10 μm exhibited moderate optical transparency at 450 nm. The best optical transmittance around 80% was achieved by PPQ-1c and 2c containing electron-withdrawing sulfone moieties. The synergic effects of flexible thioether linkages and highly conjugated quinoxaline rings in the present PPQs endowed them with ultra-high refractive indices up to 1.7953 at 632.8 nm and birefringences close to zero.