The Influence of Pendent Anions on Electrochemical and Electrochromic Properties of Thiophene-Triphenylamine-Based Polymeric Ionic Liquids

A series of novel thiophene-triphenylamine-based ionic liquids, 1-(2-((4-(bis(4-(thiophene-2-yl)phenyl)amino)benzoyl)oxy)ethyl)-3-methyl-imidazolium tetrafluoroborate/trifluoromethane sulfonate/bis(trifluoromethanesulphonyl)imide/trifluoromethanesulfonate (TTPAC2IL-BF4, TTPAC2IL-PF6, TTPAC2IL-TFSI, TTPAC2IL-TOF) were designed and synthesized successfully. All the monomers exhibit similar electrochemical behaviors together with similar HOMO and LUMO energy levels due to the same conjugated triphenylamine and thiophene units while they present different hydrophilic/hydrophobic behaviors due to different anions. FT-IR and XPS spectrum indicate the anion exchange during the electropolymerization process of monomers. In order to prepare polymeric ionic liquid films with different anions, PTTPAC2IL-BF4 (TBAP), PTTPAC2IL-BF4 (LiClO4) and PTTPAC2IL-BF4 (LiBF4) were obtained by employing TTPAC2IL-BF4 as monomer while TBAP, LiClO4 and LiBF4 were used as electrolytes respectively. Their electrochemical and electrochromic properties were also further studied. The results imply that the polymeric ionic liquid films with pendent ClO4− anions exhibit faster switching time due to the better ion diffusion character. However, PTTPAC2IL-BF4 (LiBF4) film presents better cyclic stability due to the smaller anion size of BF4− anion during the doping/dedoping process. It is more intriguing that these thiophene-triphenylamine-based PILs could exhibit electrochromic behavior in free electrolyte. This work further broadens the application of PILs as electrochromic materials and provides great comprehension on the influence of pendent anions on PILs' properties.