Electrochemical Preparation And Electrochromic Properties Of Conjugated Polymers Based On Star-shaped Thiophene Derivatives

As one of the energy-saving environmental protection materials,electrochromic(EC)materials have been attracting a great deal of interest due to their potential applications in the fields of smart windows,displays and sensors.Most recent studies on EC materials are devoted to conjugated polymers because of their superior EC properties,such as better processability,faster switching speed and color tunability.Conjugated polymers based on thiophene derivatives have attracted the attention of many researchers because of their excellent electrochromic properties.Recently,organic molecular monomers with star-shaped structure can obtain stable crosslinked network polymer films by electrochemical polymerization,which are widely used in the structural design of new electrochromic materials.In this paper,by introducing thiophenes into the star-shaped monomers with different central cores,we synthesized the homopolymers of the monomers and their copolymers with EDOT by electrochemical polymerization and thus obtained electrochromic polymers with multi-color display property and investigated the relationship between the molecular structure and the electrochromic properties of the polymer.By introducing different central cores with different electron donating/withdrawing abilities,we synthesized three star-shaped monomers TPAT,PHT and TPTT,and the corresponding homopolymers were further successfully prepared via electrochemical polymerization,namely pTPAT,pPHT and pTPTT.The cyclic voltammetry curves showed that pPHT and pTPTT displayed the relatively higher onset oxidative potentials than p TPAT.After being applied positive voltages in an electrochemical cell,pTPAT exhibited obvious electrochromic behaviors,while such phenomenons were not observed in the case of pPHT and pTPTT.The theory calculation results together with the electrochemistry properties revealed that the electrochromism of pTPAT may mainly depend on its relatively lower oxidative potential because of the introduction of triphenylamine with low ionization energy.By further copolymering with EDOT which has lower ionization energy,the obtained copolymers pPHT-EDOT and pTPTT-EDOT both showed good electrochromic properties.The results above indicated that the oxidative potential of polymer is one of the key factors determining the electrochromic properties.At the same time,it is an effective method to improve the electrochromic properties of conjugated polymers by copolymerizing with the monomers which have excellent electrochromic properties and low ionization energy.Increasing the conjugation length of monomers can effectively reduce the corresponding polymers' oxidation potential and improve their electrochromic performance.We synthesized two couple star-shaped monomers with same central cores and different conjugation length,namely PHT/BPHT and TPAT/BTPAT,and prepared corresponding homopolymers via electrochemical polymerization.The cyclic voltammetry curves showed that both pBPHT and pBTPAT displayed the relatively lower onset oxidative potentials than pTPAT and pPHT.It indicated that increasing the conjugation length of monomers which have the same central cores,can effectively reduce the oxidation potential of the corresponding polymers.pBPHT and pBTPAT showed better electrochemical activities and electrochromic properties than pPHT and pTPAT,it indicated again that the oxidative potential of polymer is one of the key factors determining the electrochromic properties.And the spectroelecchemistry analysis revealed that the molecular structure of the electrochromic polymers determining their spectroelectrochemical properties in a large part.