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.

Graphical abstractA series of thioether-bridged polyphenylquinoxalines (PPQs) were synthesized from two newly developed aromatic tetraketones, 4,4′-thiobis[(p-phenyleneoxy)benzil] (STK, 1) and 4,4′-thiobis[(p-phenylenesulfanyl)benzil] (3STK, 2) and three commercially available aromatic tetraamines, respectively. The prepared PPQs exhibited good combined properties, including enhanced solubility in the aprotic solvent - N-methyl-2-pyrrolidinone (NMP), good thermal and thermo-oxidative stability (decomposition temperatures >500 °C in nitrogen; glass transition temperatures >215 °C), and acceptable tensile properties (tensile strength >70 MPa). PPQ films derived from 3,3′,4,4′-tetraaminodiphenylsulfone and STK (PPQ-1c) or 3STK (PPQ-2c) exhibited good optical transparency with the transmittances around 80% at 450 nm wavelength. The current thioether-bridged PPQs exhibited ultra-high refractive index values up to 1.7953 at 632.8 nm and birefringence close to zero.Download full-size image