Synthesis And Characterization Of IDT-based Soluble Electrochromic Conjugated Polymers And Their Application In Electrochromic Device

Electrochromism is a phenomenon that exhibits a reversible change in absorbance or transmittance upon electrochemical oxidation or reduction.Electrochromic materials mainly contain transition metallic oxide as WO3,sandwich materials as prussian blue and conjugated polymers.In recent decades,conjugated polymers have been studied quite much for their abilities to tune conveniently physical properties such optical,photovoltaic properties and so on.But as well as inorganic electrochromic materials,most eletrochromic polymer thin films can only be prepared via eletropolymerization and chemical oxidation for their insolubilities in common organic solvents.Insolubility is really a problem that increases the difficulty of the production of large-area electrochromic thin films and limits the development of electrochromic materials.IDT(indancenodithiophene)is one of the building blocks used for conjugated polymers.With its own ?-conjugated “ladder structure” that makes it a good donor and enhances its hole mobility,IDT has been widely applied in senors,solar cells,light emitting diodes(LED),field effect and transistor[1-5].And furthermore,due to sp3-hybridized carbon atoms on IDT unit,solubilizing groups like long alkyl chain,ester groups can be easily leaded in so that IDT-based polymers usually possesses good solubilities in common organic solvents such as THF,toluene,chloroform and so on.Though IDT-based conjugated polymers have been widely uesd,their electrochromic properties have been reported by few papers.It is quite interesting and useful to explore IDT-based soluble conjugated polymers' electrochromic properties with high possibilities of being used for large-area electrochromic thin films.In this paper,six IDT-based conjugated polymers which can be soluble in chloroform have been synthesized through Stille coupling reaction with IDT unit as one co-monomer and one of copolymerizing units of phenyl(PH),naphthyl(NAPH),anthryl(AN),benzo[c][1,2,5]thiadiazole(TZ),4,7-bis(thiophen-2-yl)benzo[c][1,2,5]thiadiazole(TH-TZ-TH),benzo[c][1,2,5]thiadiazole(TZ),4,7-bis(thiophen-2-yl)benzo[c][1,2,5]thiadiazole(TT-TZ-TT),respectively,as the other co-monomer.These polymers,namely PIDT-PH,PIDT-NAPH,PIDT-AN,PIDT-TZ,PIDT-TH-TZ-TH,PIDT-TT-TZ-TT,respectively display strawberry red,sulfur yellow,deep yellow,sky blue,purple red,wine red in their neural states,respectively and except PIDT-AN for its light mint green-colored oxidation state,the other polymers all exhibit transparent-colorless in their oxidation states.These polymers except PIDT-AN are all required for application in smart windows and green displays for their redox switches from vibrantly colored neural states to transparent-colorless oxidation states.Moreover,with the goal of obtaining green-to-transparent-colorless electrochrom,PIDT-NAPH(yellow in its neural state)and PIDT-TZ(blue in its neural state)have beenmixed together to yield solution-proccessable five ECP mixtures,namely Mix-1,Mix-2,Mix-3,Mix-4,Mix-5,in the mass ratio of 3:1,2:1,1:1,1:2,1:3,respectively.It is The Mix-3 that exhibits a reversible color change from green to transparent-colorless while the other mixtures are respectively green yellow,wine green,mint turquoise and pastel turquoise.Due to green being hard to obtain for the existence of at least two absorptive band in the red and blue region,a flexible electrochromic device with the Mix-3 as the electrochromic layer has been assembled to explore the IDT-based ECP's application in electrochromic device.The flexible device exhibits a redox color change of green-to-colorless-transparent,a fast switching time of 2.9s at 641 nm and coloration efficiencies of more than 300 cm2/C both at 641 nm and 1100 nm.