Fast Switching Properties and Ion Diffusion Behavior of Polytriphenylamine Derivative with Pendent Ionic Liquid Unit

A novel triphenylamine derivative-linked ionic liquid unit, 1-(6-((4-(bis(4-(thiophen-2-yl)phenyl)amino)benzoyl)oxy)hexyl)-3-methyl-imidazolium tetrafluoroborate (TTPAC₆IL-BF₄), was designed and synthesized successfully, and its corresponding polymer PTTPAC₆IL-BF₄ was obtained by the electropolymerization method. The highest occupied molecular orbital energy band of TTPAC₆IL-BF₄ is higher and the onset oxidative potential lower compared with that of 6-bromohexyl 4-(bis(4-(thiophen-2-yl)phenyl)amino) benzoate (TTPAC₆Br) without modifying the ionic liquid unit. Both PTTPAC₆IL-BF₄ and PTTPAC₆Br show similar color change and optical contrast under different redox states. However, PTTPAC₆IL-BF₄ presents a faster electrochromic switching time than PTTPAC₆Br owing to the improved ionic conductivity and ion diffusion coefficient with the introduction of a pendent ionic liquid unit. It is more intriguing that PTTPAC₆IL-BF₄ could show electrochromism under different potentials even without supplying any additional electrolyte. The particular behavior further proves that BF₄^- ions around imidazole cations at the side chain may participate in balancing the charge of the polymer backbone when redox reaction happens, resulting in faster movement of ions during the doping process. The results imply that introducing an ionic liquid unit to the side chain is an efficient method to improve the switching time of conjugated polymers and would be inspirational for the design and preparation of novel bifunctional electrochromic polymeric electrolytes.