Tuning of the neutral state color of the π-conjugated donor–acceptor–donor type polymer from blue to green via changing the donor strength on the polymer
Graphical abstract
Introduction
Research in the area of conducting polymers increased dramatically with the doping of polyacetylene which was the simplest organic polymer with high conductivity [1]. Conjugated polymeric organic materials have attracted considerable attention over the past decades for their potential applications in organic electronic devices, due to their tunable band gaps, redox properties, processability, flexibility and low cost [2], [3], [4].
Electron-rich heterocycle based polymers such as polythiophene and its derivatives are the most promising and best studied conducting polymers because of their flexibility towards synthetic modifications. The most useful fundamental property that can be controlled by the structural modification is the polymer band gap, Eg, whose magnitude defines the color of conducting polymer [5]. The different colors observed with these compounds while switching between their different redox states is one of the most important advantages of organic electrochromic materials. Applications that use this property include architectural smart windows, rear-view mirrors for cars, sensors and electrochromic displays [6]. In the history of electrochromic materials, the discovery of third additive primary color green was probably one of the most important steps for the commercialization of full-color electrochromic displays [7]. Recently, conjugated polymers that are green at neutral state were the derivatives of quinoxaline [8], [9]. The latest contributions have come from alternation of electron-rich (donor) and electron-poor (acceptor) units in the polyconjugated backbone. A regular alternation of conjugated donor and acceptor moieties in a conjugated polymer that increases double bond character leads to broadening of valence and conduction bands and induces small band gaps [10], [11], [12], [13]. Reports on alternating quinoxaline/oligothiophene copolymers show interesting absorption properties revealing independence of the absorption maxima on the length of the oligothiophene [14], [15].
In this study, the contributions of donor heterocycles of varying strength on the electrochromic properties of new donor–acceptor–donor polymers were examined. Thiophene and EDOT have been used as the donor moieties. To complete alternation on the monomers, acceptor unit with benzo-1,4-dioxane was inserted in the molecule.
Section snippets
Materials
All chemicals were purchased from Aldrich except anhydrous tetrahydrofuran (THF) and n-butyl lithium which were purchased from Acros. They were used as received without purification. 4,7-Dibromo-2,1,3-benzothiadiazole (2) [16], 3,6-dibromo-1,2-phenylene-diamine (3) [17], 5,8-dibromo-2-(2,3-dihydrobenzo[b][1,4]dioxin-5-yl)-3-(2,3-dihydrobenzo[b][1,4]dioxin-8-yl)quinoxaline (6) [18], tributyl(thiophen-2-yl)stannane (8) [19] and tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)stannane (10) [20]
Synthesis
Bromination of benzothiadiazole was performed in a mixture of HBr/Br2 to give the dibrominated compound in very high yields [16]. Subsequent reduction of the compound was achieved using excess amount of NaBH4 [17]. A simple condensation reaction was performed with the dibromo diamino and 1,2-dione to give the corresponding dibromoquinoxaline [18]. Stannylation of EDOT and thiophene was achieved by addition of equimolar strong base, n-BuLi followed by addition of Bu3SnCl [19], [20]. Lastly a
Conclusion
Target monomers based on 2-(2,3-dihydrobenzo[b][1,4]dioxin-5-yl)-3-(2,3-dihydrobenzo[b][1,4]dioxin-8-yl)quinoxaline as the common acceptor unit were synthesized to understand the effects of donor strength on the optoelectronic and redox properties of the resulting electropolymerized materials. The electrochemical cyclic voltammograms of the polymers showed a distinct reversible redox peaks. The band gap of the resulting polymers was found from the investigation of optical absorption properties
Acknowledgments
Authors gratefully thank TUBITAK-Department of Science Fellowships and Grant Programmes.
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