Electropolymerization of high quality electrochromic poly(3-alkyl-thiophene)s via a room temperature ionic liquid
Introduction
Over the past two decades the field of conjugated polymers, which were prepared either by chemical or electrochemical polymerization, has received significant attention due to the wide range of electrical, electrochemical, and optical properties they displayed [1], [2], [3]. The electrochemical synthesis offers several advantages, including rapidity, simplicity and generation of the polymer directly on the electrode in the doped or undoped states and easy controlled synthesis of these compounds. The preparation, characterization and application of electrochemically active, electronically conjugated polymeric systems are still in the foreground of research activity in electrochemistry [4]. Polythiophene and its derivatives are the most challenging class of conjugated polymers. With their environmental and thermal stabilities, they can be used as electrical conductors [5], non-linear optical devices [6], polymer light emitting diodes [7], sensors [8], batteries [9], organic transistors [10] and electrochromic devices [11], [12], [13]. These and the variety of other potential applications have precipitated extensive research into the development of these polymers, focusing on improving their electrochemical and physical properties. While considerable effort has been expended toward the development of new high-performance derivatives, less attention has been paid to the importance of the electrolyte in determining material performance [14], [15].
Ionic liquids are room temperature molten salts and have been put in a wide range of synthetic application as green solvents during the past decade for their good chemical and physical properties [16], [17]. They are nonvolatile, nonflammable, have low toxicity and high thermal stability and are good solvents for a wide range of organic and inorganic materials [18]. Furthermore, ionic liquids often have wide electrochemical windows (typically over 4 V wide), good electrical conductivity and solvent transport properties, which make them desirable as a promising medium for electrochemical synthesis and application [19].
An electrochromic material is one that changes color in a persistent but reversible manner by an electrochemical reaction, and the phenomenon is called electrochromism. Conjugated polymers can be repeatedly driven from insulating to conductive states electrochemically with high contrast in color and are promising electrochromic materials for electrochromic device technology.
This work based on our previous studies of ionic liquid electrochemical polymerization of 3-chlorothiophene(ClT), whose thiophene monomer was substituted with strong electro-withdrawing group [20]. We found that ClT can be successfully electropolymerized in ionic liquid [BMIM]PF6, which is difficult to be synthesized in ordinarily organic solvent. But owing to high oxidation potential, the electrochromic properties of PClT were not perfect. Considering substitution of thiophene by electron-donating group to decrease oxidation potential, we synthesized poly(3-alkylthiophene)s via a room temperature ionic liquid by electrochemical methods, in order to find more promising materials for constructing electrochromic devices. Although PMeT exhibits distinct electrochromism, the insolubility of this material is a disadvantage for its technological application [21]. Thus, increasing attention is currently devoted to poly(3-alkylthiophene)s bearing long and flexible alkyl side chains, which render the materials soluble and, in consequence, processable. Moreover, it is known that increasing the length of the alkyl chain of 3-substituted thiophenes produces an increase in the electrochemical reversibility of the undoped-doped process [22]. However, in traditional electrochemical solvents, high quality films of poly(3-alkylthiophene)s are difficult to synthesize. In the present study, electrochemical polymerization of 3-methylthiophene, 3-hexylthiophene and 3-octylthiophene (MeT, HexT and OcT) were carried out via potentiodynamic and galvanostat methods by using ionic liquid [BMIM]PF6 as the growth medium and the supporting electrolyte. Addition of small amounts of polar organic solvents into ionic liquids could not only improve the stability and electrochemical capacitance of the conjugated polymers but also lower the viscosity of the solution and increase the ion disassociation, and thus enhance the conductivity of electrolytes [23]. Therefore, spectroelectrochemical and electrochromic properties of the resultant polymers were investigated in [BMIM]PF6/CH3CN (1:1, v/v) solution. Through this work, the use of commercially available alkyl-substituted thiophene monomers to provide electrochromic polymers in ionic liquid was examined. These polymers exhibit high chromatic contrast, comparative switching time, great electrochromic efficiency and long-term switching stability. The findings showed that these polymers are potential materials for constructing electrochromic devices.
Section snippets
Materials
MeT, HexT and OcT were purchased from Aldrich and were not distilled before use. The ionic liquid [BMIM]PF6 was synthesized in our lab following a procedure described in the literature [24] and thoroughly dried and degassed before use. All chemicals were of analytical grade.
Instrumentation
All electrochemical experiments were performed on CHI660a electrochemical working station (CHI, Chenhua, Shanghai, China). Spectroelectrochemical studies were carried out on a Cary 50 scan UV–Vis spectrophotometer (Varian,
Electropolymerization and electrochemical characterization
The anodic electrochemical polymerization of MeT, HexT and OcT, along with the accessibility of the redox sites of these polymers, were achieved by cyclic voltammograms at 50 mV s−1. As shown in Fig. 1, the monomers were all electroactivity. Upon sequential cycles, there were gradual increases in the current intensity, which indicated that the films were formed on the surface of the electrode. Repeated cycling resulted in a gradual decrease in the onset of oxidation potential. It was because more
Conclusions
High quality electrochromic poly(3-alkylthiophene)s (PMeT, PHexT and POcT) films were electrochemically synthesized by potentiodynamic and galvanostat methods in colorless, odorless and non-hazardous ionic liquid [BMIM]PF6. Good-quality, homogenous and smooth electroactive and electrochromic films were obtained. The results showed that polymerization with this method may be proved highly applicable in situations where more costly new synthesized monomers are not available. Via electrochromic
Acknowledgements
We acknowledge support of this project from the National Science Foundation of China (no. 20575021 and no. 20675032) and the Shanghai Rising-Star Program (06QH14004).
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