Self-Doped Polyphenylenes Containing Electron-Accepting Viologen Side Group

Abstract

Polyphenylenes (PPs) with NH2 side groups, namely, PFluNH2 and PPhNH2, were synthesized by the Pd-complex-catalyzed reaction of 2,5-dibromoaniline with 9,9-dihexylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester and 2,6-dioctyloxybenzene-1,4-diboronic acid. The reaction of PFluNH2 and PPhNH2 with 1-alkyl-1′-(2,4-dinitrophenyl)-4,4′-bipyridinium dichloride (alkyl = ethyl and n-hexyl) eliminated 2,4-dinitroaniline to yield PPs with viologen (1,1′-disubstituted 4,4′-bipyridinium dications) side groups, namely, PFluBPyHex, PFluBPyEt, and PPhBPyEt. The UV−vis spectra of PFluBPyHex, PFluBPyEt, and PPhBPyEt showed absorptions due to the viologen radical cation that was formed under nitrogen by the electron transfer from the polymer backbone to the viologen moiety. In contrast to photoluminescent PFluNH2 and PPhNH2, PFluBPyHex, PFluBPyEt, and PPhBPyEt showed no photoluminescence because the viologen contained within them acted as a quencher. The ESR spectra of PFluBPyEt and PPhBPyEt confirmed the generation of the viologen radical. Cyclic voltammetry measurements suggested that an electrochemical reduction of the viologen moiety and oxidation of the polymer backbone within PFluBPyEt and PPhBPyEt. Furthermore, this reaction was accompanied by electrochromism. The electric conductivities (σ) of the pellets molded from PFluBPyEt and PPhBPyEt were 6.4 × 10−6 and 1.1 × 10−6 S cm−1, respectively; these σ values were higher than those of PFluNH2 and PPhNH2 (σ < 10−8 S cm−1, respectively) due to the self-doping in PFluBPyEt and PPhBPyEt.