Abstract
A total of six conjugated polymers, namely PDBT-Th, PDBT-Th:Th, PDBT-2Th, PDBT-Th:2Th, PDBT-2Th:Th, and PDBT-2Th:2Th, consisting of dibenzothiophene, thiophene, and bithiophene were electrochemically synthesized. Their electrochemical and electrochromic properties were investigated in relation to the conjugation chain lengths of the thiophene units in the conjugated backbones. Density functional theory (DFT) calculations showed that longer conjugation lengths resulted in decreased HOMO-LUMO gaps in the polymers. The optical band gaps (Eg,opt) and electrochemical band gaps (Eg,cv) were decreased from PDBT-Th to PDBT-Th:Th, however, PDBT-Th:2Th, PDBT-2Th, PDBT-2Th:Th and PDBT-2Th:2Th displayed the similar band gaps. The conjugation length increments significantly improved the electrochemical stability of the conjugated polymers and exhibited reversible color changes due to the formation of polarons and bipolarons. The results suggest that the conjugated polymers prepared herein are promising candidates for fabricating flexible organic electrochromic devices.
Highlights
It is indispensable for the preparation of high-performance conjugated polymers and development of state-of-the-art applications to explore the structure-performance relationship of conjugated polymers
Thiophene was used as a fundamental unit to progressively construct conjugated polymers of various lengths by electrochemical polymerization from polymeric precursor monomers (DBT-Th and DBT-2Th) and comonomers
All conjugated polymers exhibited decreased highest occupied molecular orbitals (HOMOs)-LUMO gaps, significantly improved electrochemical stability, and noteworthy color changes when transitioning from the oxidized to neutral state with increasing conjugation length
Summary
It is indispensable for the preparation of high-performance conjugated polymers and development of state-of-the-art applications to explore the structure-performance relationship of conjugated polymers. Breakthroughs in organic optoelectronics, including organic solar cells, dye-sensitized solar cells, organic field effect transistors, and electrochromism (Zhang et al, 2013; Jin et al, 2014; Zhou et al, 2015; Li et al, 2019a), has been performed by altering the main conjugation length. Strategies for altering the main conjugation length of organic optoelectronic materials include chemical and electrochemical polymerization methods (Jin et al, 2014; Zhou et al, 2015). The relationship between the main chain conjugation length and electrochromic properties, as well as the electrochemical redox activity and stability of the conjugated polymer, were studied in detail. This study provides theoretical guidance for the development of related fields
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