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Home > High-quality freestanding flexible poly(5-(2,3-dihydrothieno[3,4-b][1,4] dioxin-5-yl)-1H-indole) film: Electrosyntheses, characterization, and optical properties
High-quality freestanding flexible poly(5-(2,3-dihydrothieno[3,4-b][1,4] dioxin-5-yl)-1H-indole) film: Electrosyntheses, characterization, and optical properties
R. Wang,1 G. Ye,1 W. Zhou ,
1 F. Jiang,1 Y. Wu,1 J. Hou,2 D. Li,1 J. Wu,1 Y. Chang,1 A. Liang,1 J. Xu,1
Y. Du3
1Jiangxi Engineering Laboratory of Waterborne Coatings, Jiangxi Science and Technology Normal University, Nanchang, 330013,
China
2State Key Laboratory for Marine Corrosion and Protection, Luoyang Ship Material Research Institute, Qingdao, 266101, China
3College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, China
Correspondence to: W. Zhou (E-mail: zhouwqh@163.com) and J. Xu (E-mail: xujingkun@tsinghua.org.cn); Fax: +86-791-83823320,
Tel.: +86-791-88537967.
INTRODUCTION
The discovery of conducting polyacetylene has opened a new era of conducting polymers (CPs). Since then, much attention has been paid to linear π-conjugated polythiophenes because of their extensive potential applications in advanced technological fields such as sensors,1 light-emitting diodes,2 thin-film transistors,3 electrochromics,4 organic photovoltaics,5 etc. To pander to the fast development of various fields, highly urgent demand is to design and synthesize new molecules used as CP precursors according to the principle of adjustable band gap that affects optical and electrical properties.6–8 So far, the CPs based on thiophene units such as bithiophenes,9–12 2-(thiophen-2-yl)furan,13 dibenzothiophenes, thieno[3,2-b]thiophenes,17–20 benzodithiophenes,21–23 and cyclopentadithiophenes24,25 have been fabricated and constructed various organic electronic and photonic devices.
As an important polythiophene derivative, poly(3,4-ethylenedioxythiophene) (PEDOT) plays an overwhelming role in the field of CPs due to its small band gap, high conductivity, excellent environmental stability, rapid redox switch, and high transparency. However, PEDOT film encounters bottlenecks in the aspect of some advanced applications due to its insolubility, low biocompatibility, monochrome, nonfluorescence, etc. To overcome these inadequate characteristics, some corresponding strategies have been developed to elaborately modify the structure of PEDOT. Unique PEDOT derivatives can be obtained through grafting functional groups on the side chain of PEDOT. For example, Xiao et al. designed a hydrosoluble hydroxymethylated EDOT and electrosynthesized a better biocompatible poly(hydroxymethylated-3,-4-ethylenedioxythiophene) than PEDOT.26 Zhou et al. prepared water-soluble alkoxysulfonate-functionalized PEDOT by the enzyme-catalyzed polymerization of sodium 4-(2,3-dihydrothieno [3,4-b][1,4]dioxin-2-yl)-methoxybutane-1-sulfonate.27 Dong et al. prepared a pair of water-soluble chiral EDOT derivatives, (R)-20-hydroxymethyl-3,4-ethylenedioxythiophene and (S)-20-hydroxymethyl-3,4-ethylenedioxythiophene, and electrodeposited into corresponding polymers with excellent electrochemical enantiorecognition.28 Additionally, the incorporation of functional molecule moieties into the backbone of PEDOT has been also prevalently reported, which exhibited unique properties such as adjustable band gap, excellent photoluminescent properties, rich electrochromic properties, etc. The cases mainly employed the prefabricated comonomers to prepare the corresponding polymers, in which polymers can keep good alternate structure. For instance, Aubert et al. prepared 3,4-ethylenedioxythiophene-co-2,5-dioctyloxyphenylene and 3,4-ethylenedioxythiophene-co-9,90-dioctylfluorene comonomers and their copolymers with a band gap of 2.1~2.4 eV, emitting properties from blue-green to yellow and interesting electrochromic properties.29 Xu’s group synthesized 4,7-di(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine comonomer and electrodeposited a green electrochromic polymer with high coloration efficiency and fast switching ability.30 Moreover, several typical EDOT-based electrochromic copolymers such as poly(selenophene-co-EDOT) and poly(furan-co-EDOT) by the electropolymerization of comonomers have been also developed.31,32 Surely, the chemical and electrochemical copolymerization of two mixture monomers is also one of the efficient approaches to modify the properties of EDOT,33–35 but the structures and properties of as-obtained copolymers are very difficult to be controlled.36
In recent years, polyindole (PIn) and its derivatives have attracted a lot of attention due to their high thermal stability, good electroactivity, fast switchable electrochromic ability, and good blue photoluminescent properties.37 Studies indicated that 5-substituted indole by electron-withdrawing groups could be electrochemically polymerized into polymers, such as 5-cyanoindole, 5-nitroindole,40 indole-5-carboxylic acid,41,42 etc.; however, the electron-donating substituents on indole negatively affected its polymerization.43 In view of the advantages of PIns, it has instinctively promoted researchers to prepare and study copolymers based on EDOT and indoles. For example, Nie et al. prepared poly(EDOT-co-indole), poly(EDOT-co-5-cyanoindole), and poly(EDOT-co-6-cyanoindole) through electrochemical copolymerization of their mixture monomers and studied their electrochromic property.34,44,45 Gopi et al. electrochemically prepared poly(EDOT-co-indole-5-carboxylic acid) on stainless steel.46 The copolymer obtained at the feed ratio of monomers of 50:50 showed an effective corrosion protection. In addition, EDOT-bissubstituted indole precursor, 5,7-bis(2-[3,4-ethylenedioxy]thienyl)
indole (ETI), and its polymer (PETI) have been synthesized and electrochemically prepared by Nie’s group.47 The electrochromic device based on PETI and PEDOT exhibited fast response time, high coloration efficiency, and long-term stability. Similar to other EDOT-terminated precursors,30,48–51 the successful electropolymerization of ETI was mainly benefited from the structure of EDOTterminated indole. However, there are still no reports on the synthesis of EDOT-monosubstituted indole and its polymer. It may be a challenge for the electrochemical polymerization of EDOTmonosubstituted indole comonomer because of the effect of the electron-donating EDOT group on indole but is worthy of inquiry.
In view of the abovementioned problems, here, we designed and synthesized a novel fluorescent 5-(2,3-dihydrothieno[3,4-b][1,4] dioxin-5-yl)-1H-indole (EDTI) comonomer. It was wondrously found that the comonomer can be successfully electrodeposited into freestanding polymer film. The structure and optical properties of as-obtained polymer were characterized by FTIR, UV–vis, and fluorescence spectroscopy. Moreover, the electrochromics, morphology, and thermal stability of the polymer film were also investigated and compared with PIn and PEDOT.
EXPERIMENTAL
Materials
2,3-Dihydrothieno[3,4-b][1,4]dioxine (EDOT), 5-bromo-1H-indole (99%), tetrakis(triphenylphosphine) palladium(0) (Pd[(PPh3)4], 99.8%), 4-dimethylaminopyridine (DMAP; ≥90%), trifluoroacetic acid (TFA, 99%), and di-tert-butyl dicarbonate (Boc2O, 99%) were purchased from J&K Chemical Reagent Co., Ltd., (Shanghai, China), Tri-n-butyltin chloride (n-Bu3SnCl), n-butyllithium (nBuLi), and methylbenzene (PhMe; ≥99.5%, AR) were bought from Energy Chemical (Shanghai, China), Aladdin Chemical Reagent Co., Ltd., (Shanghai, China) Xilong Chemical Industry Incorporated Co., Ltd., (Guangzhou, China) respectively. Tetrahydrofuran (THF; 99%, AR) was purchased from Shanghai Titan Scientific Co., Ltd., (Shanghai, China) and was distilled over Na/benzophenone before use. Tetrabutylammonium tetrafluoroborate (Bu4NBF4; Acros Organics, 95%) was purchased from Shanghai Vita Chemical Reagent Co., Ltd. and was dried under vacuum at 60 C for 24 h before use. Dichloromethane (DCM), N,N-dimethylformamide (DMF; 99.5%, AR), and commercial HPLC-grade acetonitrile (ACN) were purchased from Tianjin Damao Chemical Reagent Plant (Tianjin, China).
Synthesis
The synthetic procedure of EDTI comonomer was embodied in detail in Figure 1. The preparation of compound (1), tributyl(2,-3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)stannane, was referred to the previous synthetic route.52 For the synthesis of compound (2), Boc2O was utilized to protect the amidogen (N–H) of 5-bromoindole.53 For the synthesis of compound (3), the Stille coupling reaction of tert-butyl 5-bromo-1H-indole-1-carboxylate and tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)stannane was executed in PhMe under nitrogen atmosphere in the amount of Pd[(PPh3)4] used as a catalyst.53 For the synthesis of compound (4), the target molecule was obtained by way of deprotection of the last production, and this regular method was achieved with TFA in CH2Cl2.
Tributyl(2,3-Dihydrothieno[3,4-b][1,4]dioxin-5-yl)stannane. 3,4-Ethylenedioxythiophene (EDOT) (4.25 g, 30 mmol) was dissolved in 150 mL of THF. After the solution was cooled to−78 C, n-BuLi (18.8 mL, 1.6 M, 30 mmol) was added dropwise and stirred for 2 h. Then, n-Bu3SnCl (36 mL, 1 M, 36 mmol) was added and stirred at room temperature for 8 h. After the solvent was removed with a rotary evaporator, the residue was dissolved in hexane and filtered. The filtrate was evaporated to produce a yellow liquid of 12.9 g and used in the next step without further purification.
tert-Butyl 5-Bromo-1H-indole-1-carboxylate (2). THF (180 mL) was placed into a dry 250 mL round-bottom three-neck flask, and then 5-bromoindole (5.0 g, 25.5 mmol) and DMAP were added into the flask under ice bath. Boc2O (11.13 g, 51 mmol) was slowly added to the flask by a clean syringe and stirred at room temperature for 4 h. After that, deionized water was added into the reaction suspension, and the mixture was extracted with CH2Cl2 for three times and then washed the organic layer with saturated sodium chloride solution. The combined organic phases were dried over anhydrous Na2SO4, filtered, and evaporated with a rotary evaporator to acquire a concentrated liquid. The crude product was then purified through column chromatography (eluent: PE/EA, 10:1) to obtain a colorless transparent liquid (6.77 g, 22.8 mmol, yield 89%). 1 H-NMR (Figure S1) [400 MHz, (CD3)2SO), δ]: 7.88 (d, J = 8.8 Hz, 1H), 7.68 (s, 1H) 7.57 (d, J = 3.6 Hz, 1H), 7.32 (d, J = 8.8 Hz, 1H), 6.55 (d, J = 3.6 Hz, 1H), 1.49 (s, 9H).
tert-Butyl 5-(2,3-Dihydrothieno[3,4-b][1,4]dioxin-5-yl)-1Hindole-1-carboxylate (3). tert-Butyl 5-bromo-1H-indole-1-carboxylate (5.0 g, 17 mmol) and Pd[(PPh3)4] (0.99 g, 0.85 mmol) were dissolved in 150 mL of anhydrous PhMe. Tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)stannane was added dropwise under ice bath, and the mixture was intensely purged with nitrogen (N2) for 10 min. The resulting solution was stirred and refluxed overnight at 115 C. After cooling, deionized water was added, and then the resulting mixture was extracted with CH2Cl2 for three times. After washing with saturated sodium chloride solution, the organic layers were combined, dried, and evaporated. The crude product was subjected to column chromatography using PE/EA (20:1) mixture as eluent to give a white sticky liquid (3.40 g, 9.52 mmol, yield 56%).
1H-NMR (Figure S2) [400 MHz, (CD3)2SO), δ]: 7.96 (d, J = 8.8 Hz, 1H), 7.82 (s, 1H), 7.57 (d, J = 3.2 Hz, 1H), 7.53 (d, J = 8.8 Hz, 1H), 6.64 (d, J = 3.2 Hz, 1H), 6.47 (s, 1H), 4.10–4.24 (m, 4H), 1.53 (s, 9H). EDTI Comonomer (4). For the preparation of EDTI, tert-butyl 5-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-1H-indole- 1-carboxylate (0.5 g, 1.4 mmol) was dissolved in 15 mL of CH2Cl2. After cooling, 2 mL of TFA was added under ice bath.
The mixture was stirred for 2 h. After extracting with water and CH2Cl2, the resulting organic phase was dried over Na2SO4 and evaporated to integrate. The product was loaded on column chromatography and eluted (eluent: PE/EA, 10:1) to obtain a yellow solid (0.19 g, 0.47 mmol, yield 53%). 1H-NMR (Figure S3) (400 MHz, CDCl3, δ): 8.22 (s, 1H), 8.07 (d, J = 7.2 Hz, 1H), 7.64 (d, J = 8.4 Hz, 1H), 7.44 (d, J = 8.8 Hz, 1H), 7.28 (s, 1H), 6.68 (d, J = 13.6 Hz, 1H), 6.33 (s, 1H), 4.38–4.32 (m, 4H).
Electrochemical Preparation of Poly(5-(2,3-Dihydrothieno[3,4-b][1,4]dioxin-5-yl)-1H-indole) (PEDTI)
Electropolymerization and examination were performed in a onecompartment cell using a CHI660E potentiostat/galvanostat (Shanghai Chenhua Instrumental Co., Ltd., China). An indiumtin oxide (ITO)-coated glass and a platinum wire (1.0 mm diameter) were used as the working electrode and the counter electrode, respectively, and Ag/AgCl electrode was used as the reference electrode. Working and counter electrodes were placed at a distance of 5 mm. The electrolyte solution was 5 mL of CH2Cl2 containing 0.01 M EDTI comonomer and 0.1 M Bu4NBF4. Asobtained polymer films were washed with CH2Cl2 and dried in vacuum at 60 C for 24 h.
Characterization
The surface resistance of polymer film was measured by a Keithley 2700 multimeter (Cleveland, OH, USA). Ambienttemperature 1H-NMR spectra were recorded in CDCl3 used as solvent on a Bruker AV-400 NMR spectrometer, and chemical shifts (in ppm) were given using tetramethylsilane (Me4Si) as internal standard. To observe surface morphology of polymer, the scanning electron microscopy (SEM) of polymer film was measured by using a JEOL JSM-6700F scanning electron microscope. The fluorescence spectra were performed on an F-4500 fluorescence spectrophotometer (Hitachi). Infrared spectra were recorded using Bruker Vertex 70 FTIR spectrometer with KBr pellets. Ultraviolet–visible spectra (UV–vis) were investigated with a Cary 50 UV–vis–NIR spectrophotometer. The thermogravimetric analysis (TGA) was conducted with a NETZSCH STA 449F3 thermal analyzer.
RESULTS AND DISCUSSION
Electrochemical Polymerization of EDTI Comonomer CPs can be electrodeposited from corresponding monomers in acetonitrile, CH2Cl2, and BFEE.55 It was found that EDTI was difficultly soluble in acetonitrile and unstable in BFEE similar to EDOT; however, it was well soluble in CH2Cl2. Therefore, CH2Cl2 was selected as solvent to electropolymerize EDTI. Figure 2 displays the anodic polymerization and successive cyclic voltammetry (CV) curves of 0.01 M EDTI, indole, and EDOT in CH2Cl2 containing 0.1 M Bu4NBF4 at 100 mV s−1. It can be seen from Figure 2A that the oxidation onset of EDTI was initiated at 0.8 V vs Ag/AgCl, which was lower than those of indole (0.96 V vs Ag/AgCl) and EDOT (1.35 V vs Ag/AgCl). This indicated that the incorporation of indole on the 2-position of EDOT well
increased the conjugated chain, which led to a lower oxidation onset potential.13 When the potential exceeded the oxidation onset of corresponding monomers, the polymers could be observed from the surface of working electrode. From Figure 2(B–D), indole and EDOT all showed characteristic features of typical CV during potentiodynamic syntheses, namely, the current densities increased when the cycling number increased. Unfortunately, the CV of EDTI were ill-defined, in which the current densities decreased with increasing cycling number. This is possibly ascribed to the following factors: The
electron-donating groups such as methoxy and hydroxy and methyl 5-substituents on indole ring made against the polymerization of indole.43 For EDTI, EDOT unit can also negatively affect the polymerization of indole due to EDOT moiety is an electron-donating group.29,56 Nevertheless, a freestanding PEDTI film was still fabricated possibly due to the existence of multiple polymerization sites on EDTI comonomer, which is beneficial to form crosslinking polymer film (Figure 3). The electrochemical polymerization sites of EDTI have been demonstrated by below FTIR results.
Electrochemical and Electrochromic Behaviors of PEDTI
Figure 4 shows the CV of PEDTI film and bare ITO in monomer-free CH2Cl2 containing 0.1 M Bu4NBF4. In comparison with the CV of bare ITO, it can be clearly seen that the PEDTI film had an irreversible oxidation process between 0.5 and 1.3 V without corresponding reduction. It can be irreversibly reduced only between −0.5 and − 1.3 V without corresponding oxidation. This situation meant that the generated negative charges on the chains by reduction (n-doping) from −0.5 to−1.3 V were changed into positive charges by oxidation (p-doping) only at 0.5 V above.57 It was experimentally observed that the purple color of PEDTI film was unchanged during p-doping/p-dedoping, and the brown color was also invariable during ndoping/n-dedoping, yet the purple and brown colors of PEDTI film could switch reversibly during the n- and p-doping. Note that CV of PEDTI film in CH2Cl2 containing 0.1 M Bu4NBF4 at different scan rates was not provided due to the electrochemical activity of PEDTI film was very much unstable with the change of scan rates. Additionally, the colors (Table I and Figure S5) and electrochemical behaviors (Figure S4) of PEDTI film were distinctly different from those of PIn, PEDOT, and PETI.
Structural, Morphological Characterization, and Thermal Analysis
As shown in Figure 5, FTIR spectra of EDTI and PEDTI were
investigated. The characteristic absorption of N–H appears at 3391 and 3442 cm−1 for EDTI and PEDTI, respectively, indicating that the N place of EDTI was not the polymerization sites. The bands from 1630 to 1280 cm−1 can be ascribed to C=C and C–C of EDTI and PEDTI. The vibrations at about 674 cm−1 that appeared in EDTI and PEDTI were assigned to the modes of the C–S band. The band at 1068 cm−1 is assigned to the stretching mode of C–O–C in the EDTI, which shifts to 1086 cm−1 for PEDTI. It can be easily found that the peak at 3104 cm−1 ascribed to the vibration of C–H in the thiophene in the spectrum of monomer is nearly absent in the spectrum of polymer. It indicates that the 5-position of the thiophene ring should be classified as the polymerization site of EDTI. As can be seen from Figure 5, the vibrations at 807 cm−1 (two adjacent hydrogens) and 874 cm−1 (isolated hydrogen) were assigned to C–H deformation vibration of benzene ring, which suggested 1,2,4-trisubstituted benzene ring on EDTI and PEDTI. The bands at 769 and 737 cm−1 were assigned to the out-of-plane deformation of the C2–H and C3–H bonds on indole ring,58 respectively, which appeared in the spectrum of EDTI but nearly disappeared in the spectrum of PEDTI. These very weak bands of the C2–H and C3–H on indole ring were probably due to the nonbound ends of polymer chains. This indicated that the 2,3-positions on indole ring also became polymerization sites. To summarize, the polymerization of EDTI mainly occurred at the 5-position on thiophene ring and 2,3-positions on indole ring, as shown in Figure 3.
Figure 6 shows the SEM images of PIn, PEDOT, and PEDTI films, along with the photograph of freestanding PEDTI film. In Figure 6A, PIn film exhibited a rough and compact surface, but it would be hard to be peeled into freestanding film. PEDOT film showed porous structure formed by small-particle stacking (Figure 6B), and it was also very difficult to peel into freestanding film. However, PEDTI film had a smooth and compact surface (Figure 6C) and was peeled into flawless freestanding film (Figure 6D). The micromorphology of PEDTI film was different from that of PETI consisting of small particles.47 The flawless freestanding property of PEDTI film stemmed from crosslinking polymer film formation. Additionally, the resistance value of PEDTI film was measured to be 60 MΩ/cm at room temperature,
which was larger than those of PEDOT (106 Ω/cm) and PIn (28 MΩ/cm) films. The high-resistance value of PEDTI film was possibly attributed to the large distortion of the conjugated backbone from planarity.
TGA is an important dynamic method to measure the thermal stability of various materials. Figure 7 shows the thermogravimetry (TG) and differential thermogravimetry (DTG) curves of PIn, PEDOT, and PEDTI films. In Figure 7A, there were several degradation stages at 660 K below for PIn and PEDOT, which indicated that PIn and PEDOT films mainly included some oligomers with different chain lengths or moisture trapped in the polymer,59 whereas PEDTI film exhibited one degradation stage and started to lose weight at higher temperatures (465 K). Between 660 and 1272 K, the weight loss was mainly ascribed to the degradation of the polymer backbone chain; the weight loss was 29.1% for PIn, 27% for PEDOT, and 25.7% for PEDTI. At the same time, the total weight loss reached up to 67.7% for PIn,
78.3% for PEDOT, and 62.5% for PEDTI. It can be seen from Figure 7B that the weight loss rate of PEDTI film was fastest at 621 and 703 K, which is different from those of PIn (456, 623, and 726 K) and PEDOT (380, 482, and 640 K). These results indicated that PEDTI film had better thermal stability than PIn and PEDOT films. The good thermal stability of PEDTI film was mainly attributed to the crosslinking polymer structure.
Optical Properties
Figure 8 displays the UV–vis spectra of EDTI, indole, and EDOT monomers and PEDTI, PIn, and PEDOT films at different potentials. It can be seen from Figure 8A that the UV–vis spectrum of EDTI was different from those of indole and EDOT monomers. The absorption of EDTI at longer wavelengths indicated that EDTI monomer had a higher conjugation length that was derived from the linkage of indole and EDOT. In Figure 8B, PEDTI, PIn, and PEDOT films showed different UV–vis absorptions. At 1.3 V, PEDTI film exhibited a strong absorption peak at 554 nm and a very weak absorption peak at 960 nm, which were attributed to the absorption of conductive species such as polaron or bipolaron. At 0 V, the absorption intensity at 554 nm decreased, and the absorption peak at 456 nm shifted to 430 nm. At −1.3 V, the absorption intensity at 554 nm further decreased, and an absorption peak at 404 nm appeared, which corresponds to the π–π* electronic transition. When the potential was changed from 1.3 to 0 V, the absorption at 695 nm for PIn film disappeared, along with an increase in the absorption peak at 385 nm derived from the π–π* electronic transition. For PEDOT film at 1.3 to−1.3 V, the strong absorption of conductive species after 600 nm decreased, and a well-defined π–π* absorption peak appeared at 595 nm. The optical band gap (Eg) was calculated from the formula Eg = 1240/λonset, in which the λonset is the onset value of the π–π* absorption spectrum in a long wave direction; for PEDTI, PIn, and PEDOT, the λonset values were 532, 456, and 760 nm, respectively. The calculated Eg values of PEDTI, PIn, and PEDOT films were ~2.33, ~2.72, and ~1.63 eV, respectively.
The different Eg values may stem from their different conjugated chain lengths.60 The fluorescence spectra of the EDTI and dissolvable PEDTI have been measured in DMSO and compared with those of indole, PIn, and EDOT in DMSO, as shown in Figure 9. Note that small dissolvable PEDTI belongs to the short conjugated oligomer, whereas most parts of PEDTI films are insoluble due to the formation of long conjugated chains. In comparison, EDOT dissolved in DMSO displayed a very weak emission peak at about 450 nm, which was in well accordance with the description in the literature.61 At the same time, because PEDOT has a small band gap of ~1.6 eV, the rapid electron–hole recombination results in a nonfluorescent PEDOT.
For indole, an obvious emission was found at 330 nm, and its polymer (PIn) showed an emission peak at 440 nm and a shoulder peak at 421 nm, which indicated that PIn was a typical blue light emitter. However, EDTI comonomer, different from the fluorescence properties of indole and PIn, showed a strong emission peak at 459 nm and several weak shoulder peaks at 430, 480, and 535 nm. Compared to EDOT, the emission wavelength of EDTI comonomer showed a red shift of about 9 nm. These phenomena about the increased intensity and red shift, on the one hand, stemmed from the fluorescence enhancement of electron-donating group EDOT to indole, but on the other, the enlarged conjugate plane of comonomer contributed to the shift of emission peak to longer wavelengths. In comparison
with the fluorescence spectra of EDTI and EDOT, two welldefined bands at 461 and 488 nm and a weak shoulder peak at 535 nm appeared for PEDTI, and the quantum yield reached ca. 2.4%. This implied that PEDTI was a good blue-green light emitter. In addition, the difference of the emission spectra between PEDTI and EDTI was mainly caused by the elongated conjugation length and the structural change in local conformations of dissolvable PEDTI.
CONCLUSIONS
Novel fluorescent EDTI was synthesized using Stille coupling reaction and electrodeposited into freestanding PEDTI film in CH2Cl2 containing 0.1 M Bu4NBF4. The FTIR results indicated that the polymerization of EDTI mainly occurred at the 5-position of thiophene ring and 2,3-positions of indole ring, thus forming the crosslinking polymer film. The colors of asprepared PEDTI could switch reversibly from purple to brown under applied potentials of 1.3 and −1.3 V. The TGA results indicated that PEDTI film had better thermal stability than PIn and PEDOT films. In addition, the maximum emission peaks were observed at 459 nm for EDTI and 461 and 488 nm for PEDTI, which were indicative of good blue-green light emitters. The work also inspires us to design and prepare other new materials through the functionalization of parent PEDTI containing fluorescent-enhanced groups such as cyano groups. Further research is currently in progress.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China (grant nos. 51662012 and 51463008), Jiangxi Outstanding Young Talent Fund Projects (20171BCB23076), the Natural Science Foundation of Jiangxi Province (20171BAB206013), Jiangxi Provincial Department of Education (GJJ160762), Innovation Driven “5511” Project of Jiangxi Province (20165BCB18016), and Scientific Research Projects (2016QNBJRC001, 2015CXTD001 and YC2018-X41) of Jiangxi Science and Technology Normal University.
AUTHOR CONTRIBUTIONS
R.W., G.Y., W.Z., and J.X. conceived the idea and designed the experiments. R.W. and G.Y. performed the experiments, and R.W. wrote the paper. W.Z. supervised the research. J.X. contributed the materials, tools, and laboratory. F.J. analyzed the electrochemical data. Y.W. offered help in the discussion about optical properties. J.H. gave a hand for TGA and SEM measurements. D.L. and J.W. tested the fluorescence spectra. Y.C. tested the FTIR spectra. A.L. offered help in the preparation of figures. Y.D. provided help in the paper writing. Additionally, each author actively participated in the polishing of the paper.
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