Electrochromic polymers based on 2,5-di(thiophen-2-yl)thieno[3,2-b]thiophene and thiophene derivatives as potential anodic layers for high performance electrochromic devices
Graphical abstract
The dual-layer electrochromic devices were assembled by arranging PdTT, P(dTT-co-bTp), P(dTT-co-TF), P(dTT-co-CPdT), or P(dTT-co-CPdTK) as the anodically coloring electrochromic polymer and PEDOT-PSS as the cathodically coloring electrochromic polymer to face each other, and two electrodes were separated by a PMMA/PC/LiClO4 electrolyte. At 0.0 V, P(dTT-co-TF) was bright camel in its neutral state and PEDOT-PSS was transparent in its oxidized state. Therefore, the P(dTT-co-TF)/PEDOT-PSS ECD was transparent. As the potential increased from 0.0 V to +1.4 V, the color of P(dTT-co-TF) and PEDOT-PSS films became grayish-blue and light blue, respectively. As the potential increased further from +1.4 V to +2.0 V, the color of PEDOT-PSS film became dark blue in its neutral state. The color of P(dTT-co-TF)/PEDOT-PSS ECD was dark blue at +2.0 V. P(dTT-co-TF)/PEDOT-PSS ECD exhibits high △T (44.7% at 680 nm) and satisfactory long-term cycling stability.
Introduction
Electrochromic materials display reversible optical variation in transmittance (or absorption) when a redox process is executed. In the past three decades, organic, inorganic, and organic-inorganic hybrid electrochromic materials have gained more and more attention due to their promising applications in ski goggles, switchable architectural windows, vehicle's mirrors, and electronic tags, etc. [1], [2], [3], [4]. Metal oxides (e.g., WO3 and V2O5), Prussian blue (PB), 4,4′-bipyridinium salts (viologens) and conjugated polymers are famous electrochromic materials.
Compared to metal oxides, conjugated polymers display the vantages of fast switching time, ease of processability, high coloration efficiency and facile structural modifications. Generally, polycarbazole [5,6], polyamide [7,8], polythiophene [9,10], polydithienylpyrrole [11,12], poly(3,4-ethylenedioxyselenophene) [13], polyindole [14,15] and polyaniline [16,17] are the most frequently utilized conjugated polymers in electronic devices. Among these polymeric materials, polythiophenes are potential electrode materials due to their low band gap, synthetic versatility, good electrochemical reversibility and good charge carrier mobility [18]. PEDOT is one of the derivatives of polythiophenes, PEDOT:PSS is prepared by doping PEDOT cation with PSS anion, the polyelectrolyte is water soluble and shows satisfactory film-forming ability. Moreover, PEDOT:PSS can be used as a cathodic material of ECDs since PEDOT:PSS is transparent in its oxidized state and deep blue in its neutral state [19]. The thieno[3,2-b]thiophene (TT) unit is a fused heterocyclic ring, its coplanar structure and electron-donating ability can enhance carrier transporting properties of polymer chain [20]. Meng et al. reported the electrochromic performances of two di-EDOT substituted polythieno[3,2-b]thiophenes (P(2,5-BTE) and P(3,6-BTE)) and a tetri-EDOT substituted polythieno[3,2-b]thiophene (P(t-EDOT-TT)) [21,22]. P(2,5-BTE) was purple and gray-blue at –0.8 and +1.0 V, respectively, while P(3,6-BTE) was orange-red and cyan at –0.8 and +1.0 V, respectively. P(t-EDOT-TT) was pinkish-red, brown and light gray at –0.2, 0.6 and +1.0 V, respectively. Electrochromic switching studies displayed that △T of P(2,5-BTE) film was 19.2% at 570 nm and 48.4% at 1500 nm, whereas P(3,6-BTE) film achieved △T of 13.4% at 470 nm and 12.0% at 1500 nm. The △T of P(t-EDOT-TT) was 38% at 500 nm and 64% at 1500 nm. Xu et al. reported the synthesis of a TT end-capped heterocyclic monomer (TT-EDOT-TT) [23], its corresponding homopolymer (P(TT-EDOT-TT)) displayed high △T (69%), high η (255.3 cm2 C−1) and rapid switching times (≤ 1 s) at 1050 nm.
The chemical structure of cyclopentadithiophene (CPdT) is analogous to fluorene, two thiophene rings of a CPdT unit and two phenyl rings of a fluorene unit are bridged by a methylene group. CPdT is more rigid than that of bTp, the planar geometry of CPdT induced by the methylene group connecting the two thiophene rings gives rise to a rigid geometric structure. The incorporation of CPdT unit in copolymer chain increases the polymer backbone planarity. CPdTK is similar to CPdT while a ketone group is attached at the bridge-head position of cyclopentadithiophene core. The ketone group of CPdTK facilitates to lower the LUMO energy level of electrode materials. Moreover, the 2-(2-thienyl)furan group is a combination of structural motifs found in furan and thiophene, the five-membered thiophene and furan rings exhibit impressive charge transport and optical properties [24]. In the present work, a thiophene derivative (dTT) is synthesized, five dTT-based anodic materials (PdTT, P(dTT-co-bTp), P(dTT-co-TF), P(dTT-co-CPdT), P(dTT-co-CPdTK)) are electrodeposited on the ITO surfaces. The spectral and electrochromic switching properties of five dTT-based anodic materials are explored. Moreover, dual-layer organic ECDs are assembled using PdTT, P(dTT-co-bTp), P(dTT-co-TF), P(dTT-co-CPdT) or P(dTT-co-CPdTK) as the anodically coloring electrochromic polymer and PEDOT-PSS as the cathodically coloring electrochromic polymer. The spectral properties, kinetic characterizations, open-circuit memory, and electrochemical redox stability of PdTT/PEDOT-PSS, P(dTT-co-bTp)/PEDOT-PSS, P(dTT-co-TF)/PEDOT-PSS, P(dTT-co-CPdT)/PEDOT-PSS and P(dTT-co-CPdTK)/PEDOT-PSS ECDs are systematically studied.
Section snippets
Materials and electrode preparations
2,2′-bithiophene, 2-(2-thienyl)furan, CPdT, CPdTK, 4,4,5,5-tetramethyl-2-(thiophen-2-yl)−1,3,2-dioxaborolane, 2,5-dibromothieno[3,2-b]thiophene and PdCl2(PPh3)2 were purchased from Alfa Aesar, TCI and Sigma-Aldrich. The electrosynthesis of PdTT, P(dTT-co-bTp), P(dTT-co-TF), P(dTT-co-CPdT), P(dTT-co-CPdTK) membranes was carried out in a 0.2 M LiClO4/ACN/DCM solution (ACN/DCM = 3:2 (vol./vol.)), and the feed monomers of anodes were summarized in Table 1. The synthetic scheme of
Electrochemical characterization of electrodes
Fig. 2 shows the cyclic voltammograms of dTT, bTp, TF, CPdT and CPdTK electrooxidation in 0.2 M LiClO4/ACN/DCM solution. The onset potentials (Eonset) of dTT, bTp, TF, CPdT and CPdTK are 0.63, 0.82, 0.68, 0.54 and 0.80 V, respectively. dTT shows lower Eonset than that of bTp, implying dTT displays more extended conjugated degree than that of bTp. CPdT shows lower Eonset than that of CPdTK, this can be ascribed to the ketone group of CPdTK is an electron withdrawing unit. Fig. 3 displays the
Conclusions
dTT was synthesized by Suzuki-Miyaura coupling reaction and its homopolymer (PdTT) and four copolymers (P(dTT-co-bTp), P(dTT-co-TF), P(dTT-co-CPdT) and P(dTT-co-CPdTK)) were prepared electrochemically. P(dTT-co-CPdT) film shows three kinds of distinct color variations from neutral to oxidized state (yellow ocher, bluish-green, and grayish-blue at 0.0 V, 0.8 V and 0.9 V, respectively), whereas P(dTT-co-CPdTK) film is brownish-red, grayish-green, and grayish-blue at 0.0 V, 0.8 V and 1.0 V,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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