| Electrochromic materials have great potential for applications in displays,windows,camouflage,etc.Among them,transition metal oxides have been widely investigated and commercialized.However,they can hardly fulfill the requirements for a number of applications due to their inherent brittle nature and poor electrochromic performances,namely slow switching,and low optical contrasts and color efficiencies.Conjugated polymers exhibit more promising characteristics,such as intrinsic flexibility,adjustable performances and low cost,which draw extraordinary research interest in novel material.Recently,conjugated polymers have been often employed in electrochromic field as attentive materials due to fast switching speed,easy solution processability,facile colortuning,and high optical contrasts.These attractive performances were attributed to the ability to lower the band gap energies and control interchain charge transfers by adjusting the HOMO and LUMO levels of polymers and rendering the intermolecular packing and backbone conformation.For polymeric electrochromism,the relationship between the polymer structures(including molecular and aggregated structures)and their electrochromic properties(including electrochromic colors,switching time and cyclic stability)was still a key issue to be concerned.In this regard,the side-chain engineering is as important as the conjugated lengths in designing polymer structures.The flexible side-chains provides the necessary solubility of rigid ?-conjugated planes in organic solvents.While extensive researches have been carried out concerning the effect of ?-conjugated planes on the electrical and electro-chemical properties of polymers,the study about the conjugated lengths has been scarce.Therefore,We designed and synthesized branched and linear polymers with different conjugation lengths to study their electrochromic properties,should also be one fundamental aspect to considered for the deep understanding of structure-property relationship in the cases of electrochromic conjugated polymer.In the first part,we introduced the electrochemical polymerization,UV absorption spectrum and electrochemical stability of pPHT and pPHBT.The structural difference between pPHT and pPHBT is that the polymerization unit changes from bithiophene to quadruple thiophene,and the electrochromic property is from nondiscoloring to multicolor.Therefore,we designed and synthesized three branched thiophene polymers with different conjugation lengths of thiophene groups,pTPBT,pTPTT,pTPQT mainly based on bithiophene,trithiophene and quadruple thiophene respectively.Electrochromic properties demonstrated that pTBTT displayed none electrochromism,pTPTT exhibited an unstable one,while pTPQT showed an obvious stable electrochromism.These results indicated that thiophene polymers with effective electrochromism should be of the smallest conjugation length of quadruple thiophene at least.In the second part,we introduced thiophene,bithiophene,and trithiophene units into tetraphenylsilicon structure to form new tetraphenylsilicon derivatives(SiTPTP,SiTPBTP,SiTPTTP),which also possess two thiophene sites for polymerization in the peripheral part in the molecular structure.It is beneficial to form linear polymer structure.By controlling the length of thiophene unit,we further control polymer with different conjugation lengths.Therefore,we investigated the electrochemical and electrochromic properties of the definite conjugation lengths linear polymers.The electrochemical curves and theoretical calculation results reflected that the UV absorption of them showed red-shift and the oxidation potential of polymers decreased gradually as the conjugation lengths of thiophene groups increased,which was favorable to the appearance of electrochromism.According to the SEM of the thin films,pSiTPTTP has a looser structure,which may be the reason for the good electrochromism.The conjugation lengths of pSiTPTTP is higher than that of pSiTPBTP,which may also lead to the better electrochromism of the former. |
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