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Benzotriazole-based donor-acceptor type low band gap polymers with a siloxane-terminated side-chain for electrochromic applications
Polymer 116 (2017) 226e232
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Benzotriazole-based donor-acceptor type low band gap polymers with a siloxane-terminated side-chain for electrochromic applications Sivalingam Suganya, Namhyeon Kim, Jae Yun Jeong, Jong S. Park* Organic Photo Funtional Material Laboratory, Department of Organic Material Science and Engineering, Pusan National University, Busan 46241, North Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 February 2017 Received in revised form 16 March 2017 Accepted 28 March 2017 Available online 30 March 2017
A series of donoreacceptor (D-A) type polymers (P1-P4) bearing a benzotriazole group with a siloxane terminated side-chain and a long alkyl chain were synthesized via Stille coupling, and the electrochromic (EC) properties were characterized by various analytical techniques. The incorporation of hybrid siloxane-solubilizing groups in P1 and P2 influenced the intermolecular interactions among the adjacent polymer chains and increased the solubility of the resulting polymers in common organic solvents. The EC behavior of the polymers revealed a color transition from red to blue upon oxidation and reduction. In particular, P1, containing a short siloxane-terminated side-chain, showed superior optical performances, with transmittance change of 42.7%, the change in optical density of 0.767, and coloration efficiency of up to 327 cm2/C, and exhibited superior operational stability, with little transmittance changes after 100 potential steps. Overall, these properties highlight the potential applications of current polymers in displays and smart windows. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Benzotriazole Electrochromic polymer Siloxane side chain
1. Introduction Over the past few decades, conjugated polymers have attracted considerable attention because of their usefulness as electroactive materials in multiple device applications, including organic light emitting diodes, organic field effect transistors, and organic solar cells [1e8]. On the other hand, the interest in conjugated polymers towards electrochromic applications [9e13] has attracted only the interest of researchers. Compared to inorganic electrochromes [14e16], organic electrochromic materials [17e23] are used widely in many applications because of their low cost, solution processability, high optical contrast ratio, multi-colors display, high stability, and long cycle life. The attractive properties of CPs are based mostly on the ability to manipulate the electronic and spectral properties through chemical and structural modifications. In particular, the optical transitions, which depend on the redox potentials from the neutral to oxidized states, are critically important for their use as EC materials. Most EC polymers, which are insulating in the neutral state, undergo a change in band structure when modified electrochemically, resulting in noticeable energy
* Corresponding author. E-mail address: [email protected] (J.S. Park). http://dx.doi.org/10.1016/j.polymer.2017.03.075 0032-3861/© 2017 Elsevier Ltd. All rights reserved.
transitions and strong absorption bands in the visible region. Donoreacceptor (D-A) type conjugated polymers have attracted wide attention, where electron rich and electron deficient groups are present alternatively on the polymer chain, which facilitate a low band gap and solution processability [24e27]. Conjugated polyheterocyclic polymers, such as polyaniline, poly(3,4ethylenedioxythiophene) (PEDOT), poly(3,4propylenedioxythiophene) (PProDOT), and their derivatives, have been studied widely due mainly to their high conductivity originating from their low band gap and facile optical modulation. Recently, the electrochromic performance of many conjugated polymers containing electron accepting units, such as benzotriazole [28e30], benzothiadiazole [31e33], and quinoxaline [34,35], have been reported. Among them, the authors have been intrigued by the benzotriazole moiety due to the presence of two electronwithdrawing imine (C]N) functionalities, which enhance the electron transporting ability significantly. The electrochromic performance of conjugated polymers is closely associated with the structure-property relationship, which influences the commercialization of organic electrochromic devices. In addition, the solubility of conjugated polymers in common organic solvents is a significant factor that needs to be considered. A literature survey on the solubility of conjugated polymers showed that a linear or branched alkyl chain-attached polymer backbone exhibited
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enhanced solubility. Compared to the numerous attempts to introduce new conjugated backbone structures for EC applications, there are few reports on the side-chain engineering of EC polymers. Interestingly, Bao et al. [36] and Yang et al. [37] introduced a new hybrid siloxane solubilizing group on the polymer backbone. The incorporation of hybrid siloxane-solubilizing groups is expected to influence the intermolecular interactions between the adjacent polymer chains, to make enhanced solubility and act as a cross linking site. Moreover, the siloxane side-chain exhibits better adhesion of the EC polymer to the ITO glass surface leading to the uniform formation of a thin film compared to the siloxane free alkyl chain. Only alkylated benzotriazole has been reported to be an electrochromic material. Inspired by the work by Xu and coworkers [38] on diketopyrrolopyrrole containing siloxane side-chain as an EC polymer, it has been intended to develop new EC polymers with siloxane terminated side-chain anchored on the benzotriazole backbone. This paper reports the synthesis and characterization of a new benzotriazole and thiophene/bithiophene containing donoracceptor conjugated polymers. The benzotriazole unit was alkylated with two different side-chains, such as the octadecyl chain and siloxane unit anchored pentyl and undecyl chain. To the best of the authors' knowledge, this is the first report on benzotriazole containing siloxane side chain as EC polymers. The photophysical properties of the synthesized polymers were examined by ultraviolet-visible (UV-vis) spectroscopy, photoluminescence (PL), and cyclic voltammetry (CV). The electrochromic performance of the EC devices was investigated, in which the red color (neutral state) of the polymers was transformed to a transmissive blue color upon oxidation and reduction.
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(m, 3H), 2.17e2.12 (m, 3H). 13C NMR (CDCl3, 125 MHz):143.71, 130.21, 128.80, 116.24, 114.96, 110.04, 56.69. Mass (TOF-MS) (Mþ1): 343.93 (calcd.), 343.87 (found). Elemental Analysis (C11H11Br2N3): C, 40.09; H, 5.34; N, 8.25 (calcd), C, 40.38; H, 5.41; N, 8.72 (found). 2.2.2. Synthesis of 4,7-dibromo-2-undec-10-enyl-2H-benzotriazole (5) A solution of 11-bromo-1-undecene (4.63 g, 19.86 mmol) was added drop wise to a mixture of 4,7-dibromo-2H-benzotriazole (5.00 g, 18.05 mmol), and K2CO3 (5.00 g, 36.17 mmol) in anhydrous DMF (150 mL). The mixture was heated to 100 C overnight under reflux, and then isolated according to the procedures used for 4 to give a purified product (yield 70.6%). 1H NMR (CDCl3, 500 MHz): 7.43 (s, 2H), 5.84e5.74 (m, 1H), 4.98e4.89 (dd, 2H), 4.79e4.74 (t, 2H), 2.16e2.10 (m, 2H), 2.04e1.99 (m, 2H), 1.40e1.23 (br, 12H). 13C NMR (CDCl3, 125 MHz): 143.68, 130.14, 128.81, 109.97, 31.39, 31.16, 30.17, 18.36. Mass (TOF-MS) (Mþ1): 429.19 (calcd.), 429.99 (found).
2.1. Materials
2.2.3. Synthesis of 4,7-dibromo-(triethoxy silane 2-yl) pent-4-yl2H-benzotriazole (6) Triethoxysilane (2.03 g, 12.3 mmol) was slowly added to 4 (3.34 g, 9.68 mmol) dissolved in 60 mL of anhydrous toluene followed by the addition of a drop of Karstedt's catalyst under argon. The reaction mixture was stirred overnight at 70 C, and the completion of the reaction was monitored by TLC. The target compound was purified by chromatography on silica with in hexane and CHCl3 as the eluent, and isolated as a colorless liquid (yield 70%). 1H NMR (CDCl3, 500 MHz): 7.43 (s, 2H), 4.78e4.75 (t, 2H), 3.80e3.76 (q, 6H), 2.17e2.10 (m, 2H), 1.50e1.36 (m, 4H), 1.23e1.17 (t, 9H), 0.64e0.59 (t, 2H). 13C NMR (CDCl3, 125 MHz):143.68, 130.14, 128.82, 109.97, 59.38, 58.34, 18.72, 17.82. Mass (ESI-MS) (Mþ1): 510.39 (calcd.) 510.03 (found). Elemental Analysis (C17H27Br2N3O3Si): C, 38.29; H, 3.21; N, 12.18 (calcd), C, 38.53; H, 3.10; N, 12.61 (found).
Materials, such as 2,1,3-benzothiadiazole, 2-bromo-1-pentene, 11-bromo-1-undecene, 1-bromooctadecane, triethoxysilane, Karstedt's catalyst (Platinum(0)-1,3-divinyl-1,1,3,3tetramethyldisiloxane complex solution in xylene, Pt ~2%), and 3,4-ethylenedioxythiophene, were purchased commercially and used as received. Dry solvents, such as toluene, dimethyl formamide (DMF), and chloroform (CHCl3), were obtained from sigma Aldrich (Korea) and used without further purification. Indium tin oxide (ITO, 15 U) coated glass slides were purchased and used as the substrates.
2.2.4. Synthesis of 4,7-dibromo-(triethoxy silane 2-yl) undec-10-yl2H-benzotriazole (7) Using a procedure similar to that used for 5 (4.7 g, 10.95 mmol) in place of 4, compound 7 was isolated as a colorless liquid (yield 72%). 1H NMR (CDCl3, 500 MHz): 7.43 (s, 2H), 4.78e4.73 (t, 2H), 3.83e3.77 (q, 6H), 2.16e2.11 (m, 2H), 1.32e1.19 (br, 24H), 0.63e0.58 (t, 2H). 13C NMR (CDCl3, 125 MHz): 143.93, 130.14, 128.81, 109.92, 59.35, 58.25, 17.85, 10.36. Mass (ESI-MS) (Mþ1): 594.47 (calcd), 594.12 (found). Elemental Analysis (C23H39Br2N3O3Si): C, 46.55; H, 6.62; N, 7.08 (calcd), C, 41.54; H, 6.08; N, 6.00 (found).
2.2. Synthesis
2.2.5. Synthesis of 4,7-dibromo-2-octadecyl-2H-benzo[d][1,2,3] triazole (8) 1-Bromooctadecane (1.66 g, 5 mmol) was added to a reaction mixture containing 4,7-dibromo-2H-benzo[d] [1e3]triazole (1 g, 3.6 mmol) and potassium carbonate (1 g, 7.2 mmol) in 25 mL of degassed DMF under nitrogen. The reaction mixture was heated to 100 C overnight. The product was extracted in CHCl3, washed with brine, and dried over MgSO4. The product was isolated by column chromatography and isolated as a white solid (yield 60%). 1H NMR (CDCl3, 400 MHz d ppm): 7.44 (s, 2H), 4.74e4.79 (t, 2H), 2.11e2.16 (m, 2H), 1.24e1.34 (m, 30 H), 0.84e0.89 (m, 3H)$13C NMR (CDCl3, 125 MHz):143.74, 129.52, 110.00, 57.50, 31.96, 30.24, 29.72, 29.61, 29.51, 29.37, 29.00, 26.53, 22.72, 14.15. The general procedures for polymerization are as follows. Dibrominated benzotriazole (0.5 mmol), distannyl thiophene/ bithiophene (0.5 mmol), tris(dibenzylidenacetone)dipalladium (8 mg), tri(o-tolyl)phosphine (4 mg) were placed in a pressure vial followed by the addition of anhydrous toluene (8 mL) and purged with argon for 1 h. The reaction mixture was heated to 95 C with
2. Experimental section
Monomers 1 [39], 2 [40], 3 [41] and 9 [42] were synthesized using the reported procedures. Their spectroscopic data were well matched with the reported data. The detailed synthesis of the monomers is as follows: 2.2.1. Synthesis of 4,7-dibromo-2-pent-4-enyl-2H-benzotriazole (4) A solution of 5-bromo-1-pentene (3 g, 19.86 mmol) was added drop wise to a mixture of 4,7-dibromo-2H-benzotriazole (5.00 g, 18.05 mmol), and K2CO3 (5.00 g, 36.17 mmol) in anhydrous DMF (150 mL). The mixture was heated to 100 C overnight under reflux. The reaction was cooled to room temperature and the solvent was removed under reduced pressure. The product was extracted in CHCl3, washed with brine, and dried over MgSO4. The desired product was isolated by column chromatography, eluting with a hexane and CHCl3 mixture. A white solid was isolated as the product (yield 64%). 1H NMR (CDCl3, 500 MHz): 7.43 (s, 2H), 5.86e5.74 (m, 1H), 5.10e5.00 (dd, 2H), 4.80e4.76 (t, 3H), 2.28e2.21
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Scheme 1. Synthetic route to the monomers and conjugated polymers P1-P4.
Table 1 Molecular weight distribution and decomposition temperature of P1-P4. Polymers
Color of polymer
Mn (kDa)
Mw (kDa)
PDI
Td in N2 ( C)
P1 P2 P3 P4
Dark Dark Dark Dark
25.1 41.7 19.3 4.2
58.8 79.1 23.7 6.7
2.34 1.89 1.22 1.59
387 356 387 328
red red purple red
Mn: number average molecular weight, Mw: weight average molecular weight, PDI: poly dispersity index, Td: decomposition temperature at 5% weight loss.
Fig. 1. (a) Absorption spectra of P1-P4 in the THF and film state. (b) Emission spectra of P1-P4 in THF.
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229
Fig. 2. Cyclic voltammograms of thin films of (a) P1, (b) P2, (c) P3, and (d) P4 in a 0.1 M acetonitrile solution of TBAPF6 at a scan rate of 100 mV/s.
Table 2 Summary of the optical and electrochemical properties of P1-P4. Polymer
Solution
Film
labs
lemi
labs
lonset
P1 P2 P3 P4
523 523 532 523
592 588 569 594
558 558 554 574
718 693 697 765
Eopt g (eV)
Ffl (%)
Eox (V)
Eox, onset
HOMO (ev)
Ered (V)
Ered, onset
LUMO (eV)
Eec g (eV)
1.72 1.78 1.77 1.62
3.91 3.52 4.10 3.92
0.81 0.86 1.15 1.19
0.50 0.66 0.82 0.69
5.30 5.46 5.62 5.49
1.36 1.53 1.47 1.18
0.90 1.00 1.07 0.90
3.90 3.80 3.73 3.90
1.40 1.66 1.89 1.59
Eopt ¼ 1240/lonset; HOMO ¼ (Eox,onset vs. Eferrocene) 4.8; LUMO ¼ (Ered,onset vs. Eferrocene) 4.8; Eopt ¼ optical band gap; Eec g g g ¼ electrochemical band gap.
vigorous stirring for 5 days. After cooling to room temperature, the reaction mixture was poured into 250 mL of methanol. The resulting solid was filtered and dissolved in chloroform. The chloroform fraction was passed through celite to remove the palladium particle. After chloroform was concentrated to few milliliters, methanol was added to re-precipitate the polymers, yielding dark red/purple colored products, which were dried in vacuo. Polymer 4 was prepared using a slight modification of the procedures reported elsewhere [43]. Polymer 1 (P1): 1H NMR (CDCl3, 500 MHz d ppm): 8.19 (s, benzotriazole), 7.69 (s thiophene), 4.80 (br), 3.74 (br), 2.16 (br), 1.51 (br), 1.14 (br), 0.74 (br), 0.59 (br). GPC analysis (PS standard in THF): Mn 25185, Mw 58832, PDI 2.33. Elemental analysis (C25H33N3O3S2Si): C 58.22, H 6.45, N 8.15 (calcd.), C 57.70, H6.42, N 9.71 (found). Polymer 2 (P2):1H NMR (CDCl3, 500 MHz d ppm): 8.18 (s benzotriazole), 7.69 (s thiophene), 4.81 (br), 3.73 (br), 2.19 (br), 1.50 (br), 1.14 (br), 0.74 (br), 0.54 (br). GPC analysis (PS standard in THF):
Mn 41799, Mw79169, PDI 1.89. Elemental analysis (C31H45N3O3S2Si): C 62.06, H 7.56, N7.00 (calcd.), C 61.54, H 8.18, N 7.05 (found). Polymer 3 (P3): 1H NMR (CDCl3, 500 MHz d ppm): 7.47 (s benzotriazole), 7.29 (s thiophene), 7.06 (s thiophene), 4.03e4.06 (t), 2.26e2.28 (t), 2.20e2.23 (t), 1.25 (br). GPC analysis (PS standard in THF): Mn 19361, Mw 23618, PDI 1.22. Elemental analysis (C36H45N3S4): C 66.72, H 7.00, N 6.48 (calcd.), C 64.20, H 8.12, N 6.37 (found). Polymer 4 (P4):1H NMR (CDCl3, 500 MHz d ppm): 8.23 (s benzotriazole), 8.10 (s thiophene), 4.88 (br), 4.48 (br), 3.52 (s), 1.26e1.60 (br), 0.89 (br). GPC analysis (PS standard in THF): Mn 4274, Mw 6775, PDI 1.58. 2.3. Characterization The molecular weight of the polymers was analyzed by gel permeation chromatography (GPC) using a YL9100 HPLC system (Young-Lin) calibrated at 35 C with polystyrene (PS) standards and
S. Suganya et al. / Polymer 116 (2017) 226e232 Fig. 3. Spectroelectrochemistry of spin coated ITO films of (a) P1, (b) P2, and (c) P3, with applied potentials between 0 V and 1.0 V. The insets show n-doping spectroelectrochemistry of corresponding films at 0 V, 1.5 V, and 1.7 V.
230
tetrahydro furan (THF) as the eluent. Thermogravimetric analysis (TGA, STA6000, Perkin-Elmer) was performed under nitrogen at a heating rate of 10 C/min. The absorption and spectroelectrochemical measurements were taken using a UV-1800 UV spectrophotometer (Shimadzu). The emission of the polymers was obtained from a LS-45 fluorescence spectrometer (Perkin-Elmer). The electrochemical and spectroelectrochemical properties of the polymers were measured in a three-electrode cell, consisting of a polymer-coated ITO glass slide, Pt wire, and Ag wire as the working, counter, and reference electrodes, respectively. A 0.1 M tetrabutyl ammonium hexafluorophosphate (TBAPF6) solution in acetonitrile was used as the supporting electrolyte solution for all measurements. P1 and P2 (10 mg/mL) were spin coated on ITO coated glass using a THF solution of the polymers, and, in the case of P3 and P4, a chloroform solution was used to obtain the thin films. The change in optical density (DOD) and coloration efficiency (h) are calculated based on the following relationships, as defined by Ref. [44].
h¼
DOD Qd
¼
log Tb =Tc Qd
where Tb and Tc are the bleached and colored transmittance values, and Qd is the injected/ejected charge per unit area. 3. Results and discussion Scheme 1 presents the synthetic route of the polymers. 4,7Dibromo-2H benzo[d] [1e3]triazole (3) was synthesized using the reported procedures [41]. Dibromo benzotriazole was alkylated with 1-bromopentene, 11-bromo-1-undecene, and 1bromooctadecane in the presence of K2CO3 in DMF. Two different alkylated benzotriazole was obtained in the reactions, and the resulting monomers 4, 5, and 8 were isolated by column chromatography. After the siloxane side chain was anchored to the alkene chains of 4 and 5 using Karsted's catalyst, monomers 6 and 7 were obtained. Bromination of EDOT with NBS resulted in monomer 9 in satisfactory yield. Furthermore, dibrominated benzotriaole and distanylated thiophene/bithiophene were reacted via Stille coupling in the presence of Pd2(dba)3 and P(o-tolyl) as a catalyst in toluene to give conjugated polymers P1-P3 in reasonably good yield. P4 was synthesized with a combination of three different monomers, 8, 9, and 10, via Stille coupling. The structures of all the monomers and copolymers were characterized by 1H NMR spectroscopy and GPC. P4 showed comparatively less molecular weight than other polymers P1-P3, which may be the usage of excess amount of catalyst for the polymer synthesis. TGA showed that all four newly prepared EC polymers exhibited excellent thermal stability with 5% weight loss observed at 320e380 C in a N2 atmosphere. Table 1 lists the molecular weights, poly dispersity index, and thermal decomposition temperature of the polymers. Herein, it is worthwhile to notice that the incorporation of hybrid siloxanesolubilizing groups in P1 and P2 influences the intermolecular interactions among the adjacent polymer chains, increasing the solubility, thus leading to high molecular weight of the resulting polymers. Fig. 1a shows the absorption spectra of all polymers, P1-P4, in a THF solution and film state. In the solution state, all four polymers showed an absorption maximum at approximately 520 nm corresponding to a p-p* transition, and the absorption maxima was shifted significantly into the red region with spectral broadening in the film state. With the increasing alkyl chain lengths in P1 and P2, there were no significant changes in the absorption maxima. P1-P3 displays the absorption maximum around 550 nm, whereas P4 shows absorption at 574 nm in the film state, which was attributed
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Fig. 4. Square-wave potential step absorptiometry of the (a)P1and (b) P2films at 540 nm on an ITO coated polymers and 0.1 M of TBAPF6 in ACN between 0 V and 1.5 V.
to the presence of EDOT in the polymer backbone. The optical band gap was calculated from the lonset values of P1-P4 in film state acquired from UV-vis spectroscopy. The estimated optical band gaps of all polymers (P1-P4) were found to be 1.72, 1.78, 1.77, and 1.62 eV, respectively. The calculated band gaps of polymers P1 and P2 were comparable to the previously reported alkyl chain linked benzotriazole polymer [28]. Similarly, the fluorescence intensity of all polymers in the THF solution was recorded with excitation of 520 nm (Fig. 1b). Benzotriazole-derived conjugated polymers are well-known as orange emissive materials; hence, polymers P1-P4 display emission maximum in the range of 570e600 nm. The quantum efficiency of all polymers in THF was calculated using a 0.1 M H2SO4 solution of quinine sulphate as a standard. P1-P4 exhibited large overlap of the emission maxima, which is related to the absorption maxima with a Stokes shift around 90 nm. Consequently, the quantum efficiency of the polymers was found to be 3.5e4.0%. which are estimated to be slightly higher than the values of previous reports [30]. To investigate the electrochemical properties, a THF solution of polymers (10 mg/mL) was spin coated onto ITO glass slides. A three electrode system was applied with Pt wire, Ag wire, and polymer film as the counter, reference, and working electrode, respectively. A 0.1 M TBAPF6 solution in ACN as the supporting electrolyte was used to measure the stable cyclic voltammogram of the P1-P4 at a scan rate of 100 mV/s. All polymers exhibited reversible redox peaks and dual doping (Fig. 2). As the alkyl length chain was increased from P1 to P2, the oxidation and reduction potentials also increased. Similarly, the incorporation of a long octadecyl chain in P4 leads to a higher oxidation potential at 1.19 eV due mainly to the generation of additional torsional strain [28]. From the oxidation onset and reduction E onset onset potentials obtained from CV Eox red measurement, HOMO and LUMO values were calculated. The HOMO and LUMO energy levels of P1-P4 were in the range, 5.30 to 5.62 eV and 3.73 to 3.90 eV, respectively. Therefore, the electrochemical band gaps of the polymers were estimated to be low with ranges of 1.40e1.89 eV. The electrochemical band gap of P1 and P2 were found to be lower than their optical band gap which is attributed to the incomplete oxidation of the polymers during spectroelectrochemistry studies. Table 2 lists the data obtained from optical and electrochemical studies of the polymers. To investigate the spectral response of the polymers under external potential, the spin-coated polymer films were subjected to oxidation and reduction. Fig. 3 presents the spectral changes of the
polymer films upon p- and n-doping. In the neutral state, P1 and P2 appeared red in color, and exhibited absorption maxima around 545e550 nm. With the stepwise oxidation of the P1 and P2 films from 0 to 1 V, the absorption maximum around 545 nm decreased with the concomitant appearance of a broad polaronic and bipolaronic bands at around 650e1100 nm. The transition of color from red to blue was observed at around 0.80e0.85 V. Consistently, the red color of the polymer films changed to a transmissive blue color at the fully oxidized state. P3 exhibited a purple color due to the presence of a bithiophene unit. Similar spectral results were obtained for P3; the absorption intensity around 550 nm was reduced with the appearance of a new polaronic band in the visible region as oxidation was increased from 0 to 1 V. The purple colored P3 film transformed to blue at around 0.5 V. The spectroelectrochemical curves of negatively doped states are also represented in the insets. In the reduced state, all the polymers became completely transparent; thus, the absorption around 545e550 nm was weakened at 1.7 V, confirming that the introduction of charge carriers to the current conjugated systems induced the optical changes during the n-doping process. Kinetic studies were carried out to study the electrochromic performance of polymers P1 and P2, where the transmittance changes at a particular wavelength were recorded as a function of time (Fig. 4). The polymer films was prepared by spin coating the polymer on an ITO glass substrate, air dried at RT for 2 h. A squarewave potential was switched between its fully oxidized states for the P1and P2 film in the visible region around wavelength of 540 nm. When the potential was repeatedly sweep between 0 and 1.5 V by electrochemical oxidation, P1 and P2 exhibited a transmittance change of 42.7% and 25.3%, respectively, and the change in optical density (DOD) of 0.767 and 0.285, respectively, in the visible region. These polymers were found to exhibit superior operational stability, only with less than 3% of the transmittance changes after 100 potential steps. P1, containing a short siloxane-terminated side-chain, showed a high optical density, which is originated from small oxidation and reduction potentials. It should be noted that these value are estimated to be higher than that previously reported for the benzotriazole polymer with a normal alkyl chain [28]. This suggests that the siloxane unit influences the intermolecular interactions and exhibits strongly anchored and forms well organized cross-linkage on the ITO substrate. On the other hand, obvious asymmetry in transmittance was observed during the oxidation and reduction of the polymer. Such changes are mainly
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because the semiconducting nature of a neutral state polymer is difficult to attain during the oxidation process, and, upon p-doping, the polymer becomes highly conducting, resulting in faster charge movement. The coloration efficiency (CE) of P1 and P2 was calculated using the percentage transmittance of the bleached and colored states, and the injected/ejected charge per unit area (C/ cm2). The coloration efficiency for P1 and P2 was calculated to be 327 cm2/C and 138 cm2/C, respectively. The value of P1 is twice higher than the commercially available neutral state polymer poly(3,4-ethylenedioxythiophene) (CE ¼ 183 cm2/C) [45]. The colored-to-transmissive property and electrochromic performance, such as the optical density and coloration efficiency, suggest that the polymer can be applied in displays and smart windows. 4. Conclusion A series of benzotriazole bearing two component and three component donor-acceptor type low band gap polymers were developed by Stille coupling. The siloxane terminated side-chain made the polymer P1 and P2 more soluble in common organic solvents and acted as a good anchoring group in film formation. The prepared polymers displayed a red and purple color in their neutral state with the absorption maximum around 550 nm. The optical, electrochemical, and spectroelectrochemical properties of all the polymers were thoroughly studied. The optical and electrochemical band gaps of the polymers were calculated from the absorption and cyclovoltametric measurements. Upon both oxidation and reduction, the polymers exhibited red to blue transition with the concomitant decrease in the absorption band at around 550 nm and the appearance of a new band in the longer wavelength range. In particular, P1, containing a short siloxane-terminated side-chain, showed superior electrochromic performances, with transmittance change of 42.7%, the change in optical density of 0.767, and coloration efficiency of up to 327 cm2/C, and exhibited superior operational stability, with little transmittance changes after 100 potential steps. Overall, these properties highlight the potential applications of current polymers in displays and smart windows. Acknowledgments This work was supported by the Technology Innovation Program and Industrial Strategic Technology Development Program (10062383, Electrochromic fibers exhibiting RGB primary colors in low voltage level) funded by the Ministry of Trade, Industry and Energy(MI, Korea). References [1] A.C. Grimsdale, K.L. Chan, R.E. Martin, P.G. Jokisz, A.B. Holmes, Chem. Rev. 109 (2009) 897e1091. [2] M.H. Song, D. Kabra, B. Wenger, R.H. Friend, H.J. Snaith, Adv. Funct. Mater 19 (2009) 2130e2136.
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Benzotriazole-based donor-acceptor type low band gap polymers with a siloxane-terminated side-chain for electrochromic applications Sivalingam Suganya, Namhyeon Kim, Jae Yun Jeong, Jong S. Park* Organic Photo Funtional Material Laboratory, Department of Organic Material Science and Engineering, Pusan National University, Busan 46241, North Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 February 2017 Received in revised form 16 March 2017 Accepted 28 March 2017 Available online 30 March 2017
A series of donoreacceptor (D-A) type polymers (P1-P4) bearing a benzotriazole group with a siloxane terminated side-chain and a long alkyl chain were synthesized via Stille coupling, and the electrochromic (EC) properties were characterized by various analytical techniques. The incorporation of hybrid siloxane-solubilizing groups in P1 and P2 influenced the intermolecular interactions among the adjacent polymer chains and increased the solubility of the resulting polymers in common organic solvents. The EC behavior of the polymers revealed a color transition from red to blue upon oxidation and reduction. In particular, P1, containing a short siloxane-terminated side-chain, showed superior optical performances, with transmittance change of 42.7%, the change in optical density of 0.767, and coloration efficiency of up to 327 cm2/C, and exhibited superior operational stability, with little transmittance changes after 100 potential steps. Overall, these properties highlight the potential applications of current polymers in displays and smart windows. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Benzotriazole Electrochromic polymer Siloxane side chain
1. Introduction Over the past few decades, conjugated polymers have attracted considerable attention because of their usefulness as electroactive materials in multiple device applications, including organic light emitting diodes, organic field effect transistors, and organic solar cells [1e8]. On the other hand, the interest in conjugated polymers towards electrochromic applications [9e13] has attracted only the interest of researchers. Compared to inorganic electrochromes [14e16], organic electrochromic materials [17e23] are used widely in many applications because of their low cost, solution processability, high optical contrast ratio, multi-colors display, high stability, and long cycle life. The attractive properties of CPs are based mostly on the ability to manipulate the electronic and spectral properties through chemical and structural modifications. In particular, the optical transitions, which depend on the redox potentials from the neutral to oxidized states, are critically important for their use as EC materials. Most EC polymers, which are insulating in the neutral state, undergo a change in band structure when modified electrochemically, resulting in noticeable energy
* Corresponding author. E-mail address: [email protected] (J.S. Park). http://dx.doi.org/10.1016/j.polymer.2017.03.075 0032-3861/© 2017 Elsevier Ltd. All rights reserved.
transitions and strong absorption bands in the visible region. Donoreacceptor (D-A) type conjugated polymers have attracted wide attention, where electron rich and electron deficient groups are present alternatively on the polymer chain, which facilitate a low band gap and solution processability [24e27]. Conjugated polyheterocyclic polymers, such as polyaniline, poly(3,4ethylenedioxythiophene) (PEDOT), poly(3,4propylenedioxythiophene) (PProDOT), and their derivatives, have been studied widely due mainly to their high conductivity originating from their low band gap and facile optical modulation. Recently, the electrochromic performance of many conjugated polymers containing electron accepting units, such as benzotriazole [28e30], benzothiadiazole [31e33], and quinoxaline [34,35], have been reported. Among them, the authors have been intrigued by the benzotriazole moiety due to the presence of two electronwithdrawing imine (C]N) functionalities, which enhance the electron transporting ability significantly. The electrochromic performance of conjugated polymers is closely associated with the structure-property relationship, which influences the commercialization of organic electrochromic devices. In addition, the solubility of conjugated polymers in common organic solvents is a significant factor that needs to be considered. A literature survey on the solubility of conjugated polymers showed that a linear or branched alkyl chain-attached polymer backbone exhibited
S. Suganya et al. / Polymer 116 (2017) 226e232
enhanced solubility. Compared to the numerous attempts to introduce new conjugated backbone structures for EC applications, there are few reports on the side-chain engineering of EC polymers. Interestingly, Bao et al. [36] and Yang et al. [37] introduced a new hybrid siloxane solubilizing group on the polymer backbone. The incorporation of hybrid siloxane-solubilizing groups is expected to influence the intermolecular interactions between the adjacent polymer chains, to make enhanced solubility and act as a cross linking site. Moreover, the siloxane side-chain exhibits better adhesion of the EC polymer to the ITO glass surface leading to the uniform formation of a thin film compared to the siloxane free alkyl chain. Only alkylated benzotriazole has been reported to be an electrochromic material. Inspired by the work by Xu and coworkers [38] on diketopyrrolopyrrole containing siloxane side-chain as an EC polymer, it has been intended to develop new EC polymers with siloxane terminated side-chain anchored on the benzotriazole backbone. This paper reports the synthesis and characterization of a new benzotriazole and thiophene/bithiophene containing donoracceptor conjugated polymers. The benzotriazole unit was alkylated with two different side-chains, such as the octadecyl chain and siloxane unit anchored pentyl and undecyl chain. To the best of the authors' knowledge, this is the first report on benzotriazole containing siloxane side chain as EC polymers. The photophysical properties of the synthesized polymers were examined by ultraviolet-visible (UV-vis) spectroscopy, photoluminescence (PL), and cyclic voltammetry (CV). The electrochromic performance of the EC devices was investigated, in which the red color (neutral state) of the polymers was transformed to a transmissive blue color upon oxidation and reduction.
227
(m, 3H), 2.17e2.12 (m, 3H). 13C NMR (CDCl3, 125 MHz):143.71, 130.21, 128.80, 116.24, 114.96, 110.04, 56.69. Mass (TOF-MS) (Mþ1): 343.93 (calcd.), 343.87 (found). Elemental Analysis (C11H11Br2N3): C, 40.09; H, 5.34; N, 8.25 (calcd), C, 40.38; H, 5.41; N, 8.72 (found). 2.2.2. Synthesis of 4,7-dibromo-2-undec-10-enyl-2H-benzotriazole (5) A solution of 11-bromo-1-undecene (4.63 g, 19.86 mmol) was added drop wise to a mixture of 4,7-dibromo-2H-benzotriazole (5.00 g, 18.05 mmol), and K2CO3 (5.00 g, 36.17 mmol) in anhydrous DMF (150 mL). The mixture was heated to 100 C overnight under reflux, and then isolated according to the procedures used for 4 to give a purified product (yield 70.6%). 1H NMR (CDCl3, 500 MHz): 7.43 (s, 2H), 5.84e5.74 (m, 1H), 4.98e4.89 (dd, 2H), 4.79e4.74 (t, 2H), 2.16e2.10 (m, 2H), 2.04e1.99 (m, 2H), 1.40e1.23 (br, 12H). 13C NMR (CDCl3, 125 MHz): 143.68, 130.14, 128.81, 109.97, 31.39, 31.16, 30.17, 18.36. Mass (TOF-MS) (Mþ1): 429.19 (calcd.), 429.99 (found).
2.1. Materials
2.2.3. Synthesis of 4,7-dibromo-(triethoxy silane 2-yl) pent-4-yl2H-benzotriazole (6) Triethoxysilane (2.03 g, 12.3 mmol) was slowly added to 4 (3.34 g, 9.68 mmol) dissolved in 60 mL of anhydrous toluene followed by the addition of a drop of Karstedt's catalyst under argon. The reaction mixture was stirred overnight at 70 C, and the completion of the reaction was monitored by TLC. The target compound was purified by chromatography on silica with in hexane and CHCl3 as the eluent, and isolated as a colorless liquid (yield 70%). 1H NMR (CDCl3, 500 MHz): 7.43 (s, 2H), 4.78e4.75 (t, 2H), 3.80e3.76 (q, 6H), 2.17e2.10 (m, 2H), 1.50e1.36 (m, 4H), 1.23e1.17 (t, 9H), 0.64e0.59 (t, 2H). 13C NMR (CDCl3, 125 MHz):143.68, 130.14, 128.82, 109.97, 59.38, 58.34, 18.72, 17.82. Mass (ESI-MS) (Mþ1): 510.39 (calcd.) 510.03 (found). Elemental Analysis (C17H27Br2N3O3Si): C, 38.29; H, 3.21; N, 12.18 (calcd), C, 38.53; H, 3.10; N, 12.61 (found).
Materials, such as 2,1,3-benzothiadiazole, 2-bromo-1-pentene, 11-bromo-1-undecene, 1-bromooctadecane, triethoxysilane, Karstedt's catalyst (Platinum(0)-1,3-divinyl-1,1,3,3tetramethyldisiloxane complex solution in xylene, Pt ~2%), and 3,4-ethylenedioxythiophene, were purchased commercially and used as received. Dry solvents, such as toluene, dimethyl formamide (DMF), and chloroform (CHCl3), were obtained from sigma Aldrich (Korea) and used without further purification. Indium tin oxide (ITO, 15 U) coated glass slides were purchased and used as the substrates.
2.2.4. Synthesis of 4,7-dibromo-(triethoxy silane 2-yl) undec-10-yl2H-benzotriazole (7) Using a procedure similar to that used for 5 (4.7 g, 10.95 mmol) in place of 4, compound 7 was isolated as a colorless liquid (yield 72%). 1H NMR (CDCl3, 500 MHz): 7.43 (s, 2H), 4.78e4.73 (t, 2H), 3.83e3.77 (q, 6H), 2.16e2.11 (m, 2H), 1.32e1.19 (br, 24H), 0.63e0.58 (t, 2H). 13C NMR (CDCl3, 125 MHz): 143.93, 130.14, 128.81, 109.92, 59.35, 58.25, 17.85, 10.36. Mass (ESI-MS) (Mþ1): 594.47 (calcd), 594.12 (found). Elemental Analysis (C23H39Br2N3O3Si): C, 46.55; H, 6.62; N, 7.08 (calcd), C, 41.54; H, 6.08; N, 6.00 (found).
2.2. Synthesis
2.2.5. Synthesis of 4,7-dibromo-2-octadecyl-2H-benzo[d][1,2,3] triazole (8) 1-Bromooctadecane (1.66 g, 5 mmol) was added to a reaction mixture containing 4,7-dibromo-2H-benzo[d] [1e3]triazole (1 g, 3.6 mmol) and potassium carbonate (1 g, 7.2 mmol) in 25 mL of degassed DMF under nitrogen. The reaction mixture was heated to 100 C overnight. The product was extracted in CHCl3, washed with brine, and dried over MgSO4. The product was isolated by column chromatography and isolated as a white solid (yield 60%). 1H NMR (CDCl3, 400 MHz d ppm): 7.44 (s, 2H), 4.74e4.79 (t, 2H), 2.11e2.16 (m, 2H), 1.24e1.34 (m, 30 H), 0.84e0.89 (m, 3H)$13C NMR (CDCl3, 125 MHz):143.74, 129.52, 110.00, 57.50, 31.96, 30.24, 29.72, 29.61, 29.51, 29.37, 29.00, 26.53, 22.72, 14.15. The general procedures for polymerization are as follows. Dibrominated benzotriazole (0.5 mmol), distannyl thiophene/ bithiophene (0.5 mmol), tris(dibenzylidenacetone)dipalladium (8 mg), tri(o-tolyl)phosphine (4 mg) were placed in a pressure vial followed by the addition of anhydrous toluene (8 mL) and purged with argon for 1 h. The reaction mixture was heated to 95 C with
2. Experimental section
Monomers 1 [39], 2 [40], 3 [41] and 9 [42] were synthesized using the reported procedures. Their spectroscopic data were well matched with the reported data. The detailed synthesis of the monomers is as follows: 2.2.1. Synthesis of 4,7-dibromo-2-pent-4-enyl-2H-benzotriazole (4) A solution of 5-bromo-1-pentene (3 g, 19.86 mmol) was added drop wise to a mixture of 4,7-dibromo-2H-benzotriazole (5.00 g, 18.05 mmol), and K2CO3 (5.00 g, 36.17 mmol) in anhydrous DMF (150 mL). The mixture was heated to 100 C overnight under reflux. The reaction was cooled to room temperature and the solvent was removed under reduced pressure. The product was extracted in CHCl3, washed with brine, and dried over MgSO4. The desired product was isolated by column chromatography, eluting with a hexane and CHCl3 mixture. A white solid was isolated as the product (yield 64%). 1H NMR (CDCl3, 500 MHz): 7.43 (s, 2H), 5.86e5.74 (m, 1H), 5.10e5.00 (dd, 2H), 4.80e4.76 (t, 3H), 2.28e2.21
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S. Suganya et al. / Polymer 116 (2017) 226e232
Scheme 1. Synthetic route to the monomers and conjugated polymers P1-P4.
Table 1 Molecular weight distribution and decomposition temperature of P1-P4. Polymers
Color of polymer
Mn (kDa)
Mw (kDa)
PDI
Td in N2 ( C)
P1 P2 P3 P4
Dark Dark Dark Dark
25.1 41.7 19.3 4.2
58.8 79.1 23.7 6.7
2.34 1.89 1.22 1.59
387 356 387 328
red red purple red
Mn: number average molecular weight, Mw: weight average molecular weight, PDI: poly dispersity index, Td: decomposition temperature at 5% weight loss.
Fig. 1. (a) Absorption spectra of P1-P4 in the THF and film state. (b) Emission spectra of P1-P4 in THF.
S. Suganya et al. / Polymer 116 (2017) 226e232
229
Fig. 2. Cyclic voltammograms of thin films of (a) P1, (b) P2, (c) P3, and (d) P4 in a 0.1 M acetonitrile solution of TBAPF6 at a scan rate of 100 mV/s.
Table 2 Summary of the optical and electrochemical properties of P1-P4. Polymer
Solution
Film
labs
lemi
labs
lonset
P1 P2 P3 P4
523 523 532 523
592 588 569 594
558 558 554 574
718 693 697 765
Eopt g (eV)
Ffl (%)
Eox (V)
Eox, onset
HOMO (ev)
Ered (V)
Ered, onset
LUMO (eV)
Eec g (eV)
1.72 1.78 1.77 1.62
3.91 3.52 4.10 3.92
0.81 0.86 1.15 1.19
0.50 0.66 0.82 0.69
5.30 5.46 5.62 5.49
1.36 1.53 1.47 1.18
0.90 1.00 1.07 0.90
3.90 3.80 3.73 3.90
1.40 1.66 1.89 1.59
Eopt ¼ 1240/lonset; HOMO ¼ (Eox,onset vs. Eferrocene) 4.8; LUMO ¼ (Ered,onset vs. Eferrocene) 4.8; Eopt ¼ optical band gap; Eec g g g ¼ electrochemical band gap.
vigorous stirring for 5 days. After cooling to room temperature, the reaction mixture was poured into 250 mL of methanol. The resulting solid was filtered and dissolved in chloroform. The chloroform fraction was passed through celite to remove the palladium particle. After chloroform was concentrated to few milliliters, methanol was added to re-precipitate the polymers, yielding dark red/purple colored products, which were dried in vacuo. Polymer 4 was prepared using a slight modification of the procedures reported elsewhere [43]. Polymer 1 (P1): 1H NMR (CDCl3, 500 MHz d ppm): 8.19 (s, benzotriazole), 7.69 (s thiophene), 4.80 (br), 3.74 (br), 2.16 (br), 1.51 (br), 1.14 (br), 0.74 (br), 0.59 (br). GPC analysis (PS standard in THF): Mn 25185, Mw 58832, PDI 2.33. Elemental analysis (C25H33N3O3S2Si): C 58.22, H 6.45, N 8.15 (calcd.), C 57.70, H6.42, N 9.71 (found). Polymer 2 (P2):1H NMR (CDCl3, 500 MHz d ppm): 8.18 (s benzotriazole), 7.69 (s thiophene), 4.81 (br), 3.73 (br), 2.19 (br), 1.50 (br), 1.14 (br), 0.74 (br), 0.54 (br). GPC analysis (PS standard in THF):
Mn 41799, Mw79169, PDI 1.89. Elemental analysis (C31H45N3O3S2Si): C 62.06, H 7.56, N7.00 (calcd.), C 61.54, H 8.18, N 7.05 (found). Polymer 3 (P3): 1H NMR (CDCl3, 500 MHz d ppm): 7.47 (s benzotriazole), 7.29 (s thiophene), 7.06 (s thiophene), 4.03e4.06 (t), 2.26e2.28 (t), 2.20e2.23 (t), 1.25 (br). GPC analysis (PS standard in THF): Mn 19361, Mw 23618, PDI 1.22. Elemental analysis (C36H45N3S4): C 66.72, H 7.00, N 6.48 (calcd.), C 64.20, H 8.12, N 6.37 (found). Polymer 4 (P4):1H NMR (CDCl3, 500 MHz d ppm): 8.23 (s benzotriazole), 8.10 (s thiophene), 4.88 (br), 4.48 (br), 3.52 (s), 1.26e1.60 (br), 0.89 (br). GPC analysis (PS standard in THF): Mn 4274, Mw 6775, PDI 1.58. 2.3. Characterization The molecular weight of the polymers was analyzed by gel permeation chromatography (GPC) using a YL9100 HPLC system (Young-Lin) calibrated at 35 C with polystyrene (PS) standards and
S. Suganya et al. / Polymer 116 (2017) 226e232 Fig. 3. Spectroelectrochemistry of spin coated ITO films of (a) P1, (b) P2, and (c) P3, with applied potentials between 0 V and 1.0 V. The insets show n-doping spectroelectrochemistry of corresponding films at 0 V, 1.5 V, and 1.7 V.
230
tetrahydro furan (THF) as the eluent. Thermogravimetric analysis (TGA, STA6000, Perkin-Elmer) was performed under nitrogen at a heating rate of 10 C/min. The absorption and spectroelectrochemical measurements were taken using a UV-1800 UV spectrophotometer (Shimadzu). The emission of the polymers was obtained from a LS-45 fluorescence spectrometer (Perkin-Elmer). The electrochemical and spectroelectrochemical properties of the polymers were measured in a three-electrode cell, consisting of a polymer-coated ITO glass slide, Pt wire, and Ag wire as the working, counter, and reference electrodes, respectively. A 0.1 M tetrabutyl ammonium hexafluorophosphate (TBAPF6) solution in acetonitrile was used as the supporting electrolyte solution for all measurements. P1 and P2 (10 mg/mL) were spin coated on ITO coated glass using a THF solution of the polymers, and, in the case of P3 and P4, a chloroform solution was used to obtain the thin films. The change in optical density (DOD) and coloration efficiency (h) are calculated based on the following relationships, as defined by Ref. [44].
h¼
DOD Qd
¼
log Tb =Tc Qd
where Tb and Tc are the bleached and colored transmittance values, and Qd is the injected/ejected charge per unit area. 3. Results and discussion Scheme 1 presents the synthetic route of the polymers. 4,7Dibromo-2H benzo[d] [1e3]triazole (3) was synthesized using the reported procedures [41]. Dibromo benzotriazole was alkylated with 1-bromopentene, 11-bromo-1-undecene, and 1bromooctadecane in the presence of K2CO3 in DMF. Two different alkylated benzotriazole was obtained in the reactions, and the resulting monomers 4, 5, and 8 were isolated by column chromatography. After the siloxane side chain was anchored to the alkene chains of 4 and 5 using Karsted's catalyst, monomers 6 and 7 were obtained. Bromination of EDOT with NBS resulted in monomer 9 in satisfactory yield. Furthermore, dibrominated benzotriaole and distanylated thiophene/bithiophene were reacted via Stille coupling in the presence of Pd2(dba)3 and P(o-tolyl) as a catalyst in toluene to give conjugated polymers P1-P3 in reasonably good yield. P4 was synthesized with a combination of three different monomers, 8, 9, and 10, via Stille coupling. The structures of all the monomers and copolymers were characterized by 1H NMR spectroscopy and GPC. P4 showed comparatively less molecular weight than other polymers P1-P3, which may be the usage of excess amount of catalyst for the polymer synthesis. TGA showed that all four newly prepared EC polymers exhibited excellent thermal stability with 5% weight loss observed at 320e380 C in a N2 atmosphere. Table 1 lists the molecular weights, poly dispersity index, and thermal decomposition temperature of the polymers. Herein, it is worthwhile to notice that the incorporation of hybrid siloxanesolubilizing groups in P1 and P2 influences the intermolecular interactions among the adjacent polymer chains, increasing the solubility, thus leading to high molecular weight of the resulting polymers. Fig. 1a shows the absorption spectra of all polymers, P1-P4, in a THF solution and film state. In the solution state, all four polymers showed an absorption maximum at approximately 520 nm corresponding to a p-p* transition, and the absorption maxima was shifted significantly into the red region with spectral broadening in the film state. With the increasing alkyl chain lengths in P1 and P2, there were no significant changes in the absorption maxima. P1-P3 displays the absorption maximum around 550 nm, whereas P4 shows absorption at 574 nm in the film state, which was attributed
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Fig. 4. Square-wave potential step absorptiometry of the (a)P1and (b) P2films at 540 nm on an ITO coated polymers and 0.1 M of TBAPF6 in ACN between 0 V and 1.5 V.
to the presence of EDOT in the polymer backbone. The optical band gap was calculated from the lonset values of P1-P4 in film state acquired from UV-vis spectroscopy. The estimated optical band gaps of all polymers (P1-P4) were found to be 1.72, 1.78, 1.77, and 1.62 eV, respectively. The calculated band gaps of polymers P1 and P2 were comparable to the previously reported alkyl chain linked benzotriazole polymer [28]. Similarly, the fluorescence intensity of all polymers in the THF solution was recorded with excitation of 520 nm (Fig. 1b). Benzotriazole-derived conjugated polymers are well-known as orange emissive materials; hence, polymers P1-P4 display emission maximum in the range of 570e600 nm. The quantum efficiency of all polymers in THF was calculated using a 0.1 M H2SO4 solution of quinine sulphate as a standard. P1-P4 exhibited large overlap of the emission maxima, which is related to the absorption maxima with a Stokes shift around 90 nm. Consequently, the quantum efficiency of the polymers was found to be 3.5e4.0%. which are estimated to be slightly higher than the values of previous reports [30]. To investigate the electrochemical properties, a THF solution of polymers (10 mg/mL) was spin coated onto ITO glass slides. A three electrode system was applied with Pt wire, Ag wire, and polymer film as the counter, reference, and working electrode, respectively. A 0.1 M TBAPF6 solution in ACN as the supporting electrolyte was used to measure the stable cyclic voltammogram of the P1-P4 at a scan rate of 100 mV/s. All polymers exhibited reversible redox peaks and dual doping (Fig. 2). As the alkyl length chain was increased from P1 to P2, the oxidation and reduction potentials also increased. Similarly, the incorporation of a long octadecyl chain in P4 leads to a higher oxidation potential at 1.19 eV due mainly to the generation of additional torsional strain [28]. From the oxidation onset and reduction E onset onset potentials obtained from CV Eox red measurement, HOMO and LUMO values were calculated. The HOMO and LUMO energy levels of P1-P4 were in the range, 5.30 to 5.62 eV and 3.73 to 3.90 eV, respectively. Therefore, the electrochemical band gaps of the polymers were estimated to be low with ranges of 1.40e1.89 eV. The electrochemical band gap of P1 and P2 were found to be lower than their optical band gap which is attributed to the incomplete oxidation of the polymers during spectroelectrochemistry studies. Table 2 lists the data obtained from optical and electrochemical studies of the polymers. To investigate the spectral response of the polymers under external potential, the spin-coated polymer films were subjected to oxidation and reduction. Fig. 3 presents the spectral changes of the
polymer films upon p- and n-doping. In the neutral state, P1 and P2 appeared red in color, and exhibited absorption maxima around 545e550 nm. With the stepwise oxidation of the P1 and P2 films from 0 to 1 V, the absorption maximum around 545 nm decreased with the concomitant appearance of a broad polaronic and bipolaronic bands at around 650e1100 nm. The transition of color from red to blue was observed at around 0.80e0.85 V. Consistently, the red color of the polymer films changed to a transmissive blue color at the fully oxidized state. P3 exhibited a purple color due to the presence of a bithiophene unit. Similar spectral results were obtained for P3; the absorption intensity around 550 nm was reduced with the appearance of a new polaronic band in the visible region as oxidation was increased from 0 to 1 V. The purple colored P3 film transformed to blue at around 0.5 V. The spectroelectrochemical curves of negatively doped states are also represented in the insets. In the reduced state, all the polymers became completely transparent; thus, the absorption around 545e550 nm was weakened at 1.7 V, confirming that the introduction of charge carriers to the current conjugated systems induced the optical changes during the n-doping process. Kinetic studies were carried out to study the electrochromic performance of polymers P1 and P2, where the transmittance changes at a particular wavelength were recorded as a function of time (Fig. 4). The polymer films was prepared by spin coating the polymer on an ITO glass substrate, air dried at RT for 2 h. A squarewave potential was switched between its fully oxidized states for the P1and P2 film in the visible region around wavelength of 540 nm. When the potential was repeatedly sweep between 0 and 1.5 V by electrochemical oxidation, P1 and P2 exhibited a transmittance change of 42.7% and 25.3%, respectively, and the change in optical density (DOD) of 0.767 and 0.285, respectively, in the visible region. These polymers were found to exhibit superior operational stability, only with less than 3% of the transmittance changes after 100 potential steps. P1, containing a short siloxane-terminated side-chain, showed a high optical density, which is originated from small oxidation and reduction potentials. It should be noted that these value are estimated to be higher than that previously reported for the benzotriazole polymer with a normal alkyl chain [28]. This suggests that the siloxane unit influences the intermolecular interactions and exhibits strongly anchored and forms well organized cross-linkage on the ITO substrate. On the other hand, obvious asymmetry in transmittance was observed during the oxidation and reduction of the polymer. Such changes are mainly
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because the semiconducting nature of a neutral state polymer is difficult to attain during the oxidation process, and, upon p-doping, the polymer becomes highly conducting, resulting in faster charge movement. The coloration efficiency (CE) of P1 and P2 was calculated using the percentage transmittance of the bleached and colored states, and the injected/ejected charge per unit area (C/ cm2). The coloration efficiency for P1 and P2 was calculated to be 327 cm2/C and 138 cm2/C, respectively. The value of P1 is twice higher than the commercially available neutral state polymer poly(3,4-ethylenedioxythiophene) (CE ¼ 183 cm2/C) [45]. The colored-to-transmissive property and electrochromic performance, such as the optical density and coloration efficiency, suggest that the polymer can be applied in displays and smart windows. 4. Conclusion A series of benzotriazole bearing two component and three component donor-acceptor type low band gap polymers were developed by Stille coupling. The siloxane terminated side-chain made the polymer P1 and P2 more soluble in common organic solvents and acted as a good anchoring group in film formation. The prepared polymers displayed a red and purple color in their neutral state with the absorption maximum around 550 nm. The optical, electrochemical, and spectroelectrochemical properties of all the polymers were thoroughly studied. The optical and electrochemical band gaps of the polymers were calculated from the absorption and cyclovoltametric measurements. Upon both oxidation and reduction, the polymers exhibited red to blue transition with the concomitant decrease in the absorption band at around 550 nm and the appearance of a new band in the longer wavelength range. In particular, P1, containing a short siloxane-terminated side-chain, showed superior electrochromic performances, with transmittance change of 42.7%, the change in optical density of 0.767, and coloration efficiency of up to 327 cm2/C, and exhibited superior operational stability, with little transmittance changes after 100 potential steps. Overall, these properties highlight the potential applications of current polymers in displays and smart windows. Acknowledgments This work was supported by the Technology Innovation Program and Industrial Strategic Technology Development Program (10062383, Electrochromic fibers exhibiting RGB primary colors in low voltage level) funded by the Ministry of Trade, Industry and Energy(MI, Korea). References [1] A.C. Grimsdale, K.L. Chan, R.E. Martin, P.G. Jokisz, A.B. Holmes, Chem. Rev. 109 (2009) 897e1091. [2] M.H. Song, D. Kabra, B. Wenger, R.H. Friend, H.J. Snaith, Adv. Funct. Mater 19 (2009) 2130e2136.
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