Conjugation-Broken Thiophene-Based Electropolymerized Polymers with Well-Defined Structure: Effect of Conjugation Lengths on Electrochromic Properties
A series of monomers containing tetraphenylsilane connected the different number of thiophenes such as thiophene, bithiophene and terthiophene are designed and synthesized, which are further prepared into the corresponding polymers via electrochemical polymerization(pSiTPTP, pSiTPBTP and pSiTPTTP). From the polymers, the effective conjugate elements can be well defined as bithiophene, quaterthiophene and sexithiophene because the sp3 Si-atomcan blocks the conjugation between thiophene units in the polymer backbone. The spectroelectrochemistry results indicate that pSiTPTP is incapable of electrochromism, which may attribute to insufficient conjugation length of independent bithiophene. In contrast, the pSiTPBTP and pSiTPTTP both exhibit the obvious electrochromic properties, and furthermore the pSiTPTTP displays the shorter switching time and better stability. Such different electrochemical behaviors can be ascribed to the more loose stacking structure and lower oxidation potential of the pSiTPTTP with the independent sexithiophene unit. The EIS measurements also confirm the lower charge-transfer resistance and higher ion-diffusion rate of the pSiTPTTP with independent sexithiophene unit. Hence, we can conclude that the effect of electrochromic behavior of conjugation-broken polythiophene derivatives depends on the increased conjugation length of thiophenes repeating unit, in which the incapable electrochromism with bithiophene units would change to the superior electrochromic properties with increased sexithiophene units.