Synthesis and electrochemical properties of new main chain type polyquinones constituted of thiophene-fused benzoquinone and transformation of the polymers to a dicyanoquinonediimine type polymer

Abstract

New main chain type polyquinones, poly(3,7-dialkylbenzo[1,2-b:4,5-b′]dithiophene-4,8-dione-2,6-diyl), P(2,6-Th2Bq(diR)) (R = hexyl(Hex); octyl(Oct); dodecyl(Dod)), and poly(benzo[1,2-c:4,5-c′]dithiophene-4,8-dione-1,3-diyl), P(1,3-Th2Bq), were prepared by organometallic polycondensation. P(2,6-Th2Bq(diR)) was soluble in organic solvents such as chloroform and dichloromethane. P(1,3-Th2Bq) is soluble in CF3COOH, NMP, and DMI (1,3-dimethyl-2-imidazolidione). P(2,6-Th2Bq(diHex)) received electrochemical two step reduction with peak reduction potentials (Epc) of -1.18 V and -1.84 V vs. Ag+/Ag, respectively. This electrochemical reduction corresponds to Wurster type reduction characteristic of quinones. P(1,3-Th2Bq) was also electrochemically active and its reduction took place at Epc = -1.50 V and -1.74 V vs. Ag+/Ag. The film of P(1,3-Th2Bq) changed its color from brown to blue (electrochromism) on the electrochemical reduction. Powder XRD patterns of P(2,6-Th2Bq(diR))s indicated that they assumed an ordered structure; they showed an XRD peak in a low angle region (20 (Cu-Kα)≤ 7°). The d-value of the peak increased linearly with increase in the number of the carbon of the R groups, suggesting that the d-value corresponded to a distance between the polymer main chains separated by the R groups and P(2,6-Th2Bq(diR)) took a stacked structure in the solid. The quinone unit of P(2,6-Th2Bq(diR))s was transformed to a strong electron withdrawing dicyanoquinonediimine (DC-QNI) unit by reaction with bis(trimethylsilylcarbodiimide) (TMS-N = C = N-TMS); a high degree of transformation was observed by elemental analysis. A cast film of the obtained polymer underwent the electrochemical reduction with the first Epc1 at a very high potential of -0.56 V vs. Ag+/Ag, reflecting the high electron-withdrawing properties of the DCNQI unit.