Synthesis And Properties Of Novel Viologens Electrochromic Materials

Electrochromic?EC?materials have been extensively used in diverse electrochemical applications such as anti-glare car rear-view mirrors,electronic sensors,military equipment camouflage,information displays,protective eyewear,electrochromic smart windows for buildings or cars and so on.Viologens,N,N-substituted bipyridiniums,have received wide attention owing to their exceptional electrochromic performances of vivid color change,dramatic optical contrast,fast response time,low voltage operation,low cost and designability.Howerer,the high concentration of the colored viologen radical cations causes the generation of agglomerates on the surface of electrode during electrochemical reaction,resulting in incomplete bleaching and poor cycling stability of the viologen-based electrochromic devices?ECDs?.The slow coloration rate and poor cycle stability are majaor limitation to the commercialization of ECDs as display materials.To avoid the dimerization of viologen,symmetric viologen,alkyl-aryl asymmetric viologens and polyviologens were successfully synthesized through menschutkin or radical polymerization in this work.Viologen-based ECDs were fabricated by introducing polyionic liquid?PIL?,where PIL was employed as polymer matrix and ion conductor.The influence of viologen substituents on ECDs properties was studied.The main research work is as follows:Firstly,synthesis and characterization of viologen compounds were studied.The symmetric or alkyl-aryl asymmetric viologens such as symmetric diheptyl viologen?DHV?,1-heptyl-1'-?1-heptyl-1'-hexyl-4,4'-bipyridyl?-4,4'-bipyridinium salt?HHV?,symmetric vinyl benzyl viologen?VBV?,1-heptyl-1'-?4-vinyl benzyl?asymmetric viologen?HBV?were synthesized through menschutkin of4,4'-bipyridine and corresponding halogenated hydrocarbon,while the polyviologen such as poly?heptyl vinyl benzyl viologen??PHBV?was synthesized through radical polymerization of HBV.Additionally,the poly?VBIm-co-VBV?copolymer was synthesized through radical polymerization of VBIm Br and VBV.The chemical structures of viologen compounds synthesized in this work were confirmed with ¹H NMR and FT-IR.These results showed that viologen compounds and polymers were successfully synthesized.Secondly,viologen-based ECDs were prepared and studied.EC ion gel electrolytes were prepared by blending poly?1-vinyl-3-butylimidazolium bromide??poly?VBIm Br??,various viologens and ferrocene?Fc?.Then,the viologen-based ECDs were assembled by blade coating.The influence of viologen substituents on ECDs properties was studied and compared in terms of cyclic stability,spectroelectrochemistry,and cyclic voltammetry as well.Various colors of viologens could be achieved.It was found that the DHV/Fc ECD revealed a excellent cyclic stability over 96%of initial transmittance change after 4000 cycles of switching,and a high coloration efficiency of 164.1 cm²/C at 606 nm.Thirdly,symmetric viologen-based ECDs with utilizing polymeric ionic liquids were assembled and investigated.Poly?VBIm Br?were successfully synthesized.We incorporated polyionic liquid gel electrolyte into electrochromic mixtures consisting of symmetric diethyl viologen?Et V?and Fc to fabricate viologen-based ECDs by blade coating.This work was proposed also to investigate the influence of of PIL,viologens and Fc on the performance of viologen-based ECDs in terms of cyclic stability,spectroelectrochemistry,and cyclic voltammetry as well.It was found that the Et V/Fc ECD presented a better transmittance change of 52%at 604 nm,and a high coloration efficiency of 134.8 cm²/C.Especially,the resulting Et V²⁺showed good electrochromic properties,and a better cyclic stability over 81%of initial transmittance change after 4000 cycles of switching.