Abstract
A series of π-conjugated polymers containing alternating benzoselenadiazole (BSe)-bi(thiophene derivative)-carbazole or benzoththiadiazole (BSe)-bi(thiophene derivative)-fluorene units were designed and synthesized. Thiophene derivatives, namely 3-hexylthiophene, 3,4-bihexyloxythiophene, and 3,4-bioctyloxythiophene, were used as the π-bridges of the polymers. The polymers were characterized in detail in terms of their thermal stabilities, cyclic voltammograms, UV-Vis absorption, spectroelectrochemistry, dynamic switching property and so forth. The alkoxy thiophene π-bridged polymers have lower onset oxidation potentials and bandgaps than that of their corresponding alkyl thiophene π-bridged polymers. The selection of the donor units between the carbazole and the fluorene units has nearly no effect on the bandgaps and colors as well as the onset oxidation potentials of the polymers. The increase in the length of the side alkyl chains on the thiophene ring caused a slight increase in the polymer bandgap, which may be caused by the space hindrance effect. The dynamic switching abilities of the polymers were obtained by the chronoabsorptometry method, and the results also suggested that the alkoxy thiophene-containing polymers (as π-bridges) have higher contrast ratios than the corresponding alkyl thiophene-containing polymers. Furthermore, the increase in the length of the side alkyl chain might have a detrimental effect on the optical contrast ratios of the polymers.
Highlights
Conjugated polymers (CPs) could be considered as a type of organic semiconductor material, which generally have expected electrical and optical properties based on the band gap theory [1].Fundamentally, the band gap is defined as the difference in the energy gap between the valence band and the conduction band of CPs, which is the energy difference between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) [2]
The molecular weight and polydispersity of polymers are the important parameters for electrochromic applications
A high molecular weight favors the performance of electrochromic devices in several aspects, including the improvement of the adhesion to the substrate, the resistance to dissolving in the supporting electrolyte, and the maintenance of a high optical contrast after repeated electrochromic switches [27,28,29]
Summary
Conjugated polymers (CPs) could be considered as a type of organic semiconductor material, which generally have expected electrical and optical properties based on the band gap theory [1].Fundamentally, the band gap is defined as the difference in the energy gap between the valence band and the conduction band of CPs, which is the energy difference between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) [2]. Conjugated polymers (CPs) could be considered as a type of organic semiconductor material, which generally have expected electrical and optical properties based on the band gap theory [1]. It is considered that bandgap values are the results of many factors, including planarity, resonance effects, interchain effects, and bond length alternation [3]. The alternating distribution of one donor unit and one acceptor unit along a polymer backbone is the basic feature of the donor-acceptor (D-A) theory, which is considered. The bandgaps of the D-A-type polymers can be modulated by structural modification through the selection of proper donor or acceptor units for stronger or weaker ones, or by changing the donor-to-acceptor ratios within the polymer backbone [13]
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