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The preparation and electrochromic properties of the polyurethanes containing triphenylamine moiety
Journal of Electroanalytical Chemistry 717-718 (2014) 165–171
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The preparation and electrochromic properties of the polyurethanes containing triphenylamine moiety Chunyu Zhang a, Yanjun Hou a, Haijun Niu a,⇑, Yongfu Lian a, Xuduo Bai a, Cheng Wang a, Wen Wang b,⇑ a b
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Heilongjiang University, Harbin 150086, PR China School of Material Science and Engineering, Harbin Institute of Technology, Harbin 150080, PR China
a r t i c l e
i n f o
Article history: Received 18 August 2013 Received in revised form 7 January 2014 Accepted 16 January 2014 Available online 30 January 2014 Keywords: Polyurethane Triphenylamine Electrochromic Spectroelectrochemistry
a b s t r a c t A novel series of triphenylamine-containing aromatic polyurethanes (PUs) were synthesized via polymerization of N,N-bis (4-isocyanatophenyl) aniline monomer with different dihydroxy compounds. The structures of PUs were characterized by means of Fourier transform infrared (FTIR), 1H nuclear magnetic resonance (1H NMR) spectroscopy which showing an agreement with the proposed structure. All the PUs are soluble in a variety of organic solvents, and could be cast into transparent, tough films with good mechanical properties. The effects of molecular structure on thermal stability of the polymers were studied by thermo-gravimetric analyzer. The electrochemical behaviors were investigated by cycle voltammetry (CV) method, and the result revealed that there were a pair of reversible oxidative redox couples at potentials of 0.33 V and 1.15 V vs Ag/AgCl in acetonitrile solution. The electrochromic properties were examined by spectroelectrochemical methods. All the PUs exhibited excellent reversibility of electrochromic characteristics by continuous cyclic scans between 0 V and 1.6 V, with a color changing from yellow to blue. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Electrochromism is known as the reversible change of the color resulting from the oxidation or the reduction of the material via the electrochemical process [1]. Hitherto, a wide variety of electrochromic materials with high coloration efficiencies, low operating voltage, and fast switching capabilities have been developed, which can be classified into several distinct categories such as metal oxides (tungsten trioxide (WO3) or iridium dioxide (IrO2)), mixed-valence metal complexes (Prussian blue), small organic molecules (viologens, bipyridinium, and phthalocyanines), and conjugated polymers [2]. In the available electrochromic p-conjugated organic polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) in particular, has stimulated much interest as a cathodic electrochrome due to its fast switching between deep-blue and pale-blue hues, high coloration efficiency, optical contrast ratio, long-term redox stability, and wide range of colors [3,4]. Polyaniline (PANI) has also been extensively studied in the past due to its unique electronic, redox properties, stable and multicolor behaviors [5–7]. However, these polymers are normally insoluble in solvents due to the present of strong interchain interactions ⇑ Corresponding authors. Tel.: +86 13684501571; fax: +86 0451 86608616. E-mail addresses: [email protected] (H. Niu), [email protected] (W. Wang). http://dx.doi.org/10.1016/j.jelechem.2014.01.021 1572-6657/Ó 2014 Elsevier B.V. All rights reserved.
[8]. In order to expand the application of the polymers, the researchers used many methods to increase the solubility of electroactive polymers such as introducing long alkyl or side groups [9]. However, these would induce the Tg decreasing. One effective way to improve solubility without sacrificing thermostability is introducing propeller-like triphenylamine (TPA) group in the main chain or side chain [10]. Polymers containing TPA moieties have unique photochemical and electronic properties as well as high thermal stability, allowing them to be applied as various electrochromic materials [11–15]. On the other hand, PU chemistry paved the way to prepare a class of high performance materials such as coatings, adhesives, elastomers, fibers and foams. Especially, PUs have been used extensively in coating industry due to their excellent abrasion resistance, flexibility at low temperature, excellent chemical, physical, mechanical properties. And the durability of PU can significantly enhance the life of product. A series of PUs were also designed and prepared to investigate the electro-optical properties [16–21]. The urethane structure was designed to accomplish two aims: (1) to investigate the effect of the different monomers containing two hydroxyl groups on the spectra of the polyurethanes; (2) to improve the flexible properties of main chain and to increase the film-forming properties; (3) to improve the corrosion resistance to organic solvents. Furthermore, PU
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film can be formed directly by solution spinning whereas most conjugated polymers are not facile to be soluble. Thus, we tried to take strategic to attach polymer film to the In2O3SnO2 (ITO) well by introducing urethane structures to enhance the device stability. In the present article, a new series of PUs were synthesized and the characterization and investigation of properties were carried out by means of FT-IR, 1H NMR, ultraviolet visible (UV–vis) spectroscopy, thermo-gravimetric analyzer (TGA) and scanning electron microscope (SEM). The electrochemical and electrochromic properties of the PU films prepared by casting solution onto an ITO-coated glass substrate are also described herein.
2.3.1. Synthesis of N,N-bis(4-isocyanatophenyl)aniline (3) A solution of bis(tricholoromethyl) carbonate (0.3684 g, 1.24 mmol) in dry toluene was added dropwise to a stirred solution of compound 2 (0.500 g, 1.816 mmol), which prepared by reduction of the corresponding dinitro derivative, in dry toluene at room temperature under N2. The reaction mixture was stirred at 100 °C for 4 h, then filtered and the solvent was evaporated, resulting in 0.3450 g of a light blue solid (yield: 63.4%). FTIR spectrum (KBr pellet, cm1): 2262 (N@C@O stretching). 1 H NMR (400 MHz, DMSO-d6, ppm): 6.85 (d, 4H, ArH ortho to N), 6.95 (d, 4H, ArH ortho to NCO), 7.11 (m, 3H, ArH of Ph), 7.23 (m, 2H, ArH ortho to N in Ph).
2. Experimental
2.3.2. Synthesis of PUs A 40 ml heavy walled glass vial was charged with dihydroxy compound (a–e) and compound 3 in dry DMF under nitrogen atmosphere. The mixture was stirred at 70 °C for 48 h, yielding a dark viscous solution, and the polyurethanes (denoted as PU-a–e) were prepared as shown in Scheme 2. PU molecular weights were summarized in Table S1. Structures of the PUs were confirmed by 1 H NMR results shown in Fig. S1. PU-a: FTIR spectrum (KBr pellet, cm1): 3321 (NAH stretching), 1669 (C@O stretching), 1014 (NACOAO stretching band), 1538 (CAN stretching and NAH bending), 1269 (CANAH combination), 1593, 1501, 753 (aromatic ring of benzene). 1H NMR (400 MHz, DMSO-d6, ppm): 7.97 (d, NHACOO), 6.85 (m, aromatic ring of benzene N), 7.42–7.23 (m, aromatic ring of triphenylamine). [g]: 1.58 dL/g. PU-b: FTIR spectrum (KBr pellet, cm1): 3341 (NAH stretching), 1658 (C@O stretching), 1022 (NACOAO stretching band), 1544 (CAN stretching and NAH bending), 1253 (CANAH combination), 1601, 1501, 755 (aromatic ring of benzene). 1H NMR (400 MHz, DMSO-d6, ppm): 7.95 (d, NHACOO), 6.80 (m, aromatic ring of benzene N), 7.34–7.37 (m, aromatic ring of triphenylamine). [g]: 1.53 dL/g. PU-c: FTIR spectrum (KBr pellet, cm1): 3339 (NAH stretching), 1663 (C@O stretching), 1028 (NACOAO stretching band), 1536 (CAN stretching and NAH bending), 1242 (CANAH combination), 1600, 1504, 771 (aromatic ring of benzene). 1H NMR (400 MHz, DMSO-d6, ppm): 7.95 (d, NHACOO), 6.80 (m, aromatic ring of benzene N), 7.61–7.58 (m, aromatic ring of triphenylamine). [g]: 0.978 dL/g. PU-d: FTIR spectrum (KBr pellet, cm1): 3327 (NAH stretching), 1655 (C@O stretching), 1063 (NACOAO stretching band), 1536 (CAN stretching and NAH bending), 1264 (CANAH combination), 1592, 1512, 737 (aromatic ring of benzene). 1H NMR (400 MHz, DMSO-d6, ppm): 7.92 (d, NHACOO), 6.89 (m, aromatic ring of benzene N), 7.41–7.26 (m, aromatic ring of triphenylamine). [g]: 0.955 dL/g. PU-e: FTIR spectrum (KBr pellet, cm1): 3328 (NAH stretching), 1658 (C@O stretching), 1012 (NACOAO stretching band), 1543 (CAN stretching and NAH bending), 1218 (CANAH combination), 1612, 1510, 757 (aromatic ring of benzene). 1H NMR(400 MHz, DMSO-d6, ppm): 7.97 (d, NH-COO), 6.85 (m, aromatic ring of benzene N), 7.42–7.23 (m, aromatic ring of triphenylamine). [g]: 0.957 dL/g.
2.1. Materials All the chemicals were purchased from TCI and Shanghai Sinopharm Co. N,N-dimethylformamide (DMF) was dried over CaH2 and distilled under reduced pressure. Aniline and 1-fluoro-4-nitrobenzene were distilled before used. Reagent grade aromatic dihydroxy compounds such as phenolphthalein, bisphenol A, 4,40 -dihydroxybiphenyl, 4,40 -bishydroxybenzophenone, bisphenolfluorene were used as received. Methylbenzene were distilled before used. Lithium perchlorate (LiClO4) was dried under vacuum at 100 °C for 96 h. 2.2. Measurements FT-IR spectra were recorded on a PerkinElmer Spectrum 100 Model FT-IR spectrometer. 1H NMR spectra were measured on a Bruker AC-400 MHz spectrometer using dimethyl sulfoxide (DMSO-d6) as solvent. CV measurements (the oxidation and reduction potentials) were conducted on a CH Instruments 660A electrochemical analyzer at a scan rate of 50 mV/s with a 0.1 M solution of LiClO4 as an electrolyte under nitrogen atmosphere in dried acetonitrile (CH3CN). The polymer films coated on an ITO disk, a Pt wire and an Ag/AgCl electrode were used as a working electrode, counter electrode and a quasi reference electrode, respectively, and calibrated against the ferrocene/ferrocenium (Fc/Fc+) redox couple. UV–vis spectra were determined on a SHIMADZU UV-3600. Thermo-gravimetric analysis (TGA) was conducted with approximately 6–8 mg powder samples heated in flowing nitrogen (flow rate = 20 cm3/min) at a heating rate of 10 °C/min using a PerkinElmer Pyris 6. SEM measurement was carried out on an S4800 instrument with an accelerating voltage of 20 kV, and the samples were sputtered with Pt prior to observation. Density-functional theory (DFT) calculations were performed on a computer. Geometry optimizations were carried out using the B3LYP functional as implemented in Gaussian 98 program [22]. The viscosities of polymers in DMSO were determined with Ubbelohde viscometer in 35 °C. The thickness of PU film was determined on profilometer XP-100 (KLA-Tencor), by the probe testing the different height of blank ITO-coated glass and filmed ITO-coated glass. Gel permeation chromatographic (GPC) analysis was performed on a Malvern instrument connected with one refractive index detector (ViscotekVE3580-RI-DETECTOR) by using a polymer/DMF solution at a flow rate of 0.8 ml/min at 35 °C and calibrated with polystyrene standards. 2.3. Synthesis of monomers 4,40 -Dinitrotriphenylamine (1) and 4,40 -diaminotriphenylamine (2) were synthesized according to the literature [23,24] outlined in Scheme 1.
2.4. Preparation of film electrode A solution of the PUs in DMF was cast on an ITO-coated glass. Then the thermally cured films were obtained from the PUs by programmed increasing temperature to 100 °C at heating rate of 10 °C/ min and keeping the temperature for 2 h. The thickness of polymer film was about 100 nm determined by profilometer XP-100 (KLATencor). To keep the same thickness of different PUs, we control
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Scheme 1. Synthesis route of isocyanate monomer.
Scheme 2. Synthesis route of PUs.
the concentration of the solution and casting speed at the same level. 3. Results and discussion 3.1. FTIR analysis and 1H NMR In Fig. 1, the distinct absorption peak of N,N-bis(4-isocyanatophenyl)aniline at 2262 cm1 confirms the existence of ANCO group. After polymerization, there is not absorption peak around 2262 cm1 representing ANCO, which proves the reaction was complete. The characteristic peaks of urethane group of PU-a– PU-e are observed at 3321 cm1 and 1669 cm1, corresponding
to stretching vibrations of NAH and C@O groups, respectively. There are peaks at 1269 cm1 owing to vibrations of CANAH groups. There is no absorption peak around 1647 cm1 due to urea hydroxy in the ureido ester, showing that the reaction is complete. The 1H NMR spectra (as shown in Fig. S1) of PU-a–PU-e show that the peaks at 7.97, 7.95, 7.95, 7.92 and 7.97 are representing the hydrogens in NHACOO group. All the detected peaks and ratios between the peak areas are consistent with the proposed structures. 3.2. UV–vis absorption and photoluminescence (PL) The PUs (PU-a–PU-e) exhibit strong UV–vis absorption bands in the range of 283–374 nm in DMSO solution and in the range of 390–394 nm for solid films, assignable to the p–p transitions resulting from the conjugated TPA segment (shown in Fig. 2(a)). The PL spectra of PUs measured in DMSO (conc.: 108 mol/L) indicated that the PL peaks located in the range of 367–383 nm showing little Stocks shift (shown in Fig. 2(b)). The quantum yields of these polymers can be calculated according to equation [25].
/unk ¼ /std
Fig. 1. IR spectra of the compound 3 and PU-a–PU-e.
Iunk Astd gunk 2 Istd Aunk gstd
where /unk, /std, Iunk, Istd, Aunk, Astd, gunk, and gstd, are the fluorescent quantum yield, integration of the emission intensity, absorbance at the excitation wavelength, and the refractive indices of the corresponding solutions for the samples and the standard, respectively. Here, we use the refractive indices of the pure solvents as those of the solutions. The PL quantum yields in DMSO solution are calculated to be: 3.64% (PU-a), 17.1% (PU-b), 9.84% (PU-c), 4.43% (PU-d), and 15.8% (PU-e), respectively. This photoluminescence phenomenon can possibly be attributed to the forming of conjugated
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switching rate. In order to achieve sufficient speed, the morphology should be controlled to minimize the diffusion distance in PU film. SEM images of PU show that the surface is rough or nanofibrous which suggest the presence of channels or pinholes in the film would be favor to transport counter ions. TGA curves of the PUs in Fig. 4 show that PUs (PU-a–PU-e) have good thermal stability with a 10 wt.% loss temperature of about 210 °C. 60% weight-loss temperatures of PU-a, PU-c and PU-d in nitrogen are recorded at about 450 °C. The high char yields of the PUs can be ascribed to their high aromatic content. Hence, the PUs could fulfill the requirements of high-temperature stability as required for optoelectronic thin-layer devices. The number-average molecular weight (Mn) and polydispersity (PDI) were determined to be in the range of 10,781–20,398 and 1.065–1.708, respectively (shown in Table S1). The Mn suggests that the polymerization has come into being. 3.4. Electrochemical properties
Fig. 2. (a) UV–vis absorption. (b) PL spectra of PUs in DMSO.
structure in the PU molecule. Especially, bisphenol A and bisphenolfluorene groups which are more planar contribute more to the emission of PUs than the other groups [26]. 3.3. Polymer film morphologies, thermal stability and molecular weights The morphologies of PU-a and PU-d were investigated by SEM in Fig. 3 and Fig. S2. In common, the rate limiting step in the electrochromic switching process involved diffusion of dopant counter ions into the polymer structure. Consequently, the polymer morphology plays an important role in affecting the electrochromic
The electronic states including highest occupied molecular orbital /lower unoccupied molecular orbital (HOMO/LUMO) levels of the polymers were investigated by CV. The results of polymers are shown in Fig. 5 and summarized in Table 1. The redox peaks have been associated with reduction–oxidation process accompa nying the double injection/extraction of ClO4 ions and electrons. All the PUs (PU-a–PU-e) exhibit one pair of obvious reversible oxidation peaks which correspond to the oxidation of the TPA moiety [24]. During the electrochemical oxidation of the thin PUs films, the color of PUs films changed from red to blue. In order to obtain accurate oxidation reduction potential, the reference electrode was calibrated by the Fc/Fc+, whose redox potential is assumed to be energy level of 4.80 eV to vacuum. The energies of the HOMO and LUMO levels of the investigated PUs can be determined from the onset oxidation potentials and the onset absorption wavelength which are listed in Table 1. The HOMO energy level values were calculated using the equation [27]:
EHOMO ¼ eðEOX onset vs Ag=AgCl þ 4:36Þ eV where EOX onset is the onset oxidation potential. The HOMO energy level values of PU-c and PU-d were calculated to be at in the range from4.882 to 5.046 eV. The LUMO energy levels of the polymers are estimated from the HOMO energy levels and Eg curves which could be obtained as following:
ELUMO ¼ EHOMO þ Eg Eg ¼ 1240=konset
Fig. 3. SEM images of PU: (a) PU-a and (b) PU-d.
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Fig. 4. TGA traces of the PU-a–PU-e. Fig. 6. Pictorial representations of the electron density in the frontier molecular orbitals of repetition units for PUs (PU-a–PU-e).
Fig. 5. CVs for PU-a–PU-e in 0.1 M LiClO4/CH3CN with Fc/Fc+ as an internal standard at the scanning rate of 50 mV/s.
[28]. The HOMO and LUMO energies and HOMO–LUMO energy gaps are given in Table 1. For PUs, the electronic wave-function of the HOMO is distributed entirely over TPA moieties, which is beneficial for obtaining higher hole mobility. The electron lone pair of the nitrogen atoms has strong coupling with p electrons in TPA which lead to an intramolecular charge transfer (ICT) with significant distribution of the electronic density over the entire molecule [28]. The electron wave-function of LUMO is mainly localized on the dihydroxy compound part. Thus, the TPA segment and dihydroxy compound effectively regular the bandgap together. The trend of Eg for the oligomer series follows in the order: PU-b < PU-c < PU-d < PU-e < PU-a in Table 1. Actual measured data have a slight deviation compared with the theoretical data due to the affection of the solvation process, molecular structure, the electron of the electron-withdrawing group. 3.6. Electrochromic properties
ELUMOs are at between 2.089 (PU-c) and 2.228 (PU-d) eV. The results disclosed that the different dihydrophenyl derivative structures played the key role in the electron structure and regulated the gaps of PU. Thus, our novel PUs can be employed as potential candidates in the development of dynamic electrochromic screen devices due to their suitable HOMO values, thermal stabilities and reversible electrochemical behaviors. 3.5. Quantum chemical calculation The ground-state geometries of the compounds were optimized by hybrid DFT (B3LYP) with 6-31G basis set in the Gaussian 98 program package [22] and were presented in Fig. 6. EHOMO is often associated with the electron donating ability of the molecule and ELUMO indicates the ability of the molecules to accept electrons
All these PUs exhibit similar electrochromic properties, and the typical electrochromic transmittance spectrum of PU-a is shown in Fig. 7. When the applied potentials increased positively from 0.0 to 1.6 V, the peak of absorption at 394 nm, characteristic for neutral form for PU-a increased gradually, while two new bands grew up at 610 and 830 nm, respectively. For electrochromic switching studies, polymer films were cast on ITO-coated glass slides in the same manner as described above, and chronoamperometric and absorbance measurements were performed at the same time. While the voltage was switched, the absorbance at the given wavelength was monitored as a function of time with UV–vis–NIR spectroscopy (Fig. 8). The switching time was defined as the time that required for reach 90% of the full change in absorbance after applying potential. Thin films of PU-e would require 3.0 s at 0.8 V for switching absorbance at 600 nm
Table 1 Optical and electrochemical properties of the PUs.
PU-a PU-b PU-c PU-d PU-e
kfilm (nm)
konset (nm)
Eox onset vs Ag=AgCl (V)
EHOMO (eV)
ELUMO (eV)
Eg (eV)
Equntum HOMO (eV)
Equntum LUMO (eV)
Equntum (eV) g
394 392 390 392 394
435 446 444 440 438
0.604 0.566 0.522 0.686 0.636
4.964 4.926 4.882 5.046 4.996
2.114 2.146 2.089 2.228 2.165
2.850 2.780 2.793 2.818 2.831
5.339 5.114 5.382 5.459 5.223
1.686 0.751 0.912 1.622 0.940
3.653 4.263 4.470 3.837 4.283
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proportional to the amount of created color centers; g denotes the coloration efficiency (CE) and Q (mC/cm2) is the amount of injected/ejected charge per unit sample area. The CEs of PU films are calculated to be: 26.06 cm2/C (PU-a), 11.08 cm2/C (PU-b), 49.13 cm2/C (PU-c), 38.30 cm2/C (PU-d), and 43.78 cm2/C (PU-e), respectively, and are summarized in Table S2. PUs electrochromic coloration efficiencies are listed in the order: PU-b < PU-a < PUd < PU-e < PU-c. PU containing dihydroxy biphenyl groups has the highest CE contrast to the PU contain bisphenol A having the lowest CE. The reason may be the more facile electron transfer in PU containing dihydroxy biphenyl groups that play the key role in the coloration efficiencies. 4. Conclusions
Fig. 7. Electrochromic behaviors of PU-a thin film (in CH3CN with 0.1 M LiCO4 as the supporting electrolyte).
In summary, a new series of TPA-based PUs have been successfully synthesized, which have excellent film-forming quality and good solubility in low-boiling-point solvents. The thermally cured PUs exhibit thermal stabilities and reversible electrochemical behaviors. The calculated HOMO and LUMO energy levels of the PUs by experimental method are in the range of 5.14 eV to 5.46 eV and 0.75 eV to 1.68 eV vs the vacuum level, respectively. All obtained PUs revealed excellent stability of electrochromic characteristics, with changing color from original yellowish to blue. Acknowledgements The authors are grateful to the support of the National Science Foundation of China (Grant Nos. 51373049, 51372055, 51273056, and 21372067), Doctoral Fund of Ministry of Education of China (20132301120004 and 20132301110001), and Foundation of Heilongjiang Education Bureau (12531504). Appendix A. Supplementary material
Fig. 8. Optical switching procedures: (a) potential step absorptiometry of PU-e at 600 nm (0.1 M LiClO4 as the supporting electrolyte) by applying a potential step (0.0–0.8 V). (b) Current consumption of PU-e.
Supplementary material associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.jelechem.2014.01.021. References
and 3.0 s for bleaching. The color switching times of PU-e are also shown in Fig. 8. After over continuous 300 cyclic switches between 0.0 and 0.8 V, the polymer films still exhibited good stability of electrochromic characteristics. As shown in Figs. S3–S6 when the applied potentials increased positively from 0.0 to 2.0 V, the peaks of absorption at 392 nm, 390 nm, 392 nm and 394 nm, characteristic for neutral form for PU-b, PU-c, PU-d and PU-e increased gradually, while two new bands grew up at about 600 and 894 nm, respectively. Thin films of PU-a, PU-b, PU-c and PU-d require 2.0 s, 2.0 s, 3.0 s and 3.0 s at 0.8 V for switching absorbance at 600 nm and 2.0 s, 2.0 s, 3.0 s and 3.0 s for bleaching respectively. The color switching time of PUs is also shown in Figs. S7–S10. After over continuous 300 cyclic switches between 0.0 and 0.8 V, the polymer films still exhibited good stability of electrochromic characteristics. The electrochromic coloration efficiency (CE; g) is also an important characteristic for the electrochromic materials. CE can be calculated using the equations and given below [29]:
dOD ¼ lgðT b =T c Þ
g ¼ dOD =Q where Tb and Tc are the transmittances before and after coloration, respectively; dOD is the change of the optical density which is
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The preparation and electrochromic properties of the polyurethanes containing triphenylamine moiety Chunyu Zhang a, Yanjun Hou a, Haijun Niu a,⇑, Yongfu Lian a, Xuduo Bai a, Cheng Wang a, Wen Wang b,⇑ a b
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Heilongjiang University, Harbin 150086, PR China School of Material Science and Engineering, Harbin Institute of Technology, Harbin 150080, PR China
a r t i c l e
i n f o
Article history: Received 18 August 2013 Received in revised form 7 January 2014 Accepted 16 January 2014 Available online 30 January 2014 Keywords: Polyurethane Triphenylamine Electrochromic Spectroelectrochemistry
a b s t r a c t A novel series of triphenylamine-containing aromatic polyurethanes (PUs) were synthesized via polymerization of N,N-bis (4-isocyanatophenyl) aniline monomer with different dihydroxy compounds. The structures of PUs were characterized by means of Fourier transform infrared (FTIR), 1H nuclear magnetic resonance (1H NMR) spectroscopy which showing an agreement with the proposed structure. All the PUs are soluble in a variety of organic solvents, and could be cast into transparent, tough films with good mechanical properties. The effects of molecular structure on thermal stability of the polymers were studied by thermo-gravimetric analyzer. The electrochemical behaviors were investigated by cycle voltammetry (CV) method, and the result revealed that there were a pair of reversible oxidative redox couples at potentials of 0.33 V and 1.15 V vs Ag/AgCl in acetonitrile solution. The electrochromic properties were examined by spectroelectrochemical methods. All the PUs exhibited excellent reversibility of electrochromic characteristics by continuous cyclic scans between 0 V and 1.6 V, with a color changing from yellow to blue. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Electrochromism is known as the reversible change of the color resulting from the oxidation or the reduction of the material via the electrochemical process [1]. Hitherto, a wide variety of electrochromic materials with high coloration efficiencies, low operating voltage, and fast switching capabilities have been developed, which can be classified into several distinct categories such as metal oxides (tungsten trioxide (WO3) or iridium dioxide (IrO2)), mixed-valence metal complexes (Prussian blue), small organic molecules (viologens, bipyridinium, and phthalocyanines), and conjugated polymers [2]. In the available electrochromic p-conjugated organic polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) in particular, has stimulated much interest as a cathodic electrochrome due to its fast switching between deep-blue and pale-blue hues, high coloration efficiency, optical contrast ratio, long-term redox stability, and wide range of colors [3,4]. Polyaniline (PANI) has also been extensively studied in the past due to its unique electronic, redox properties, stable and multicolor behaviors [5–7]. However, these polymers are normally insoluble in solvents due to the present of strong interchain interactions ⇑ Corresponding authors. Tel.: +86 13684501571; fax: +86 0451 86608616. E-mail addresses: [email protected] (H. Niu), [email protected] (W. Wang). http://dx.doi.org/10.1016/j.jelechem.2014.01.021 1572-6657/Ó 2014 Elsevier B.V. All rights reserved.
[8]. In order to expand the application of the polymers, the researchers used many methods to increase the solubility of electroactive polymers such as introducing long alkyl or side groups [9]. However, these would induce the Tg decreasing. One effective way to improve solubility without sacrificing thermostability is introducing propeller-like triphenylamine (TPA) group in the main chain or side chain [10]. Polymers containing TPA moieties have unique photochemical and electronic properties as well as high thermal stability, allowing them to be applied as various electrochromic materials [11–15]. On the other hand, PU chemistry paved the way to prepare a class of high performance materials such as coatings, adhesives, elastomers, fibers and foams. Especially, PUs have been used extensively in coating industry due to their excellent abrasion resistance, flexibility at low temperature, excellent chemical, physical, mechanical properties. And the durability of PU can significantly enhance the life of product. A series of PUs were also designed and prepared to investigate the electro-optical properties [16–21]. The urethane structure was designed to accomplish two aims: (1) to investigate the effect of the different monomers containing two hydroxyl groups on the spectra of the polyurethanes; (2) to improve the flexible properties of main chain and to increase the film-forming properties; (3) to improve the corrosion resistance to organic solvents. Furthermore, PU
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film can be formed directly by solution spinning whereas most conjugated polymers are not facile to be soluble. Thus, we tried to take strategic to attach polymer film to the In2O3SnO2 (ITO) well by introducing urethane structures to enhance the device stability. In the present article, a new series of PUs were synthesized and the characterization and investigation of properties were carried out by means of FT-IR, 1H NMR, ultraviolet visible (UV–vis) spectroscopy, thermo-gravimetric analyzer (TGA) and scanning electron microscope (SEM). The electrochemical and electrochromic properties of the PU films prepared by casting solution onto an ITO-coated glass substrate are also described herein.
2.3.1. Synthesis of N,N-bis(4-isocyanatophenyl)aniline (3) A solution of bis(tricholoromethyl) carbonate (0.3684 g, 1.24 mmol) in dry toluene was added dropwise to a stirred solution of compound 2 (0.500 g, 1.816 mmol), which prepared by reduction of the corresponding dinitro derivative, in dry toluene at room temperature under N2. The reaction mixture was stirred at 100 °C for 4 h, then filtered and the solvent was evaporated, resulting in 0.3450 g of a light blue solid (yield: 63.4%). FTIR spectrum (KBr pellet, cm1): 2262 (N@C@O stretching). 1 H NMR (400 MHz, DMSO-d6, ppm): 6.85 (d, 4H, ArH ortho to N), 6.95 (d, 4H, ArH ortho to NCO), 7.11 (m, 3H, ArH of Ph), 7.23 (m, 2H, ArH ortho to N in Ph).
2. Experimental
2.3.2. Synthesis of PUs A 40 ml heavy walled glass vial was charged with dihydroxy compound (a–e) and compound 3 in dry DMF under nitrogen atmosphere. The mixture was stirred at 70 °C for 48 h, yielding a dark viscous solution, and the polyurethanes (denoted as PU-a–e) were prepared as shown in Scheme 2. PU molecular weights were summarized in Table S1. Structures of the PUs were confirmed by 1 H NMR results shown in Fig. S1. PU-a: FTIR spectrum (KBr pellet, cm1): 3321 (NAH stretching), 1669 (C@O stretching), 1014 (NACOAO stretching band), 1538 (CAN stretching and NAH bending), 1269 (CANAH combination), 1593, 1501, 753 (aromatic ring of benzene). 1H NMR (400 MHz, DMSO-d6, ppm): 7.97 (d, NHACOO), 6.85 (m, aromatic ring of benzene N), 7.42–7.23 (m, aromatic ring of triphenylamine). [g]: 1.58 dL/g. PU-b: FTIR spectrum (KBr pellet, cm1): 3341 (NAH stretching), 1658 (C@O stretching), 1022 (NACOAO stretching band), 1544 (CAN stretching and NAH bending), 1253 (CANAH combination), 1601, 1501, 755 (aromatic ring of benzene). 1H NMR (400 MHz, DMSO-d6, ppm): 7.95 (d, NHACOO), 6.80 (m, aromatic ring of benzene N), 7.34–7.37 (m, aromatic ring of triphenylamine). [g]: 1.53 dL/g. PU-c: FTIR spectrum (KBr pellet, cm1): 3339 (NAH stretching), 1663 (C@O stretching), 1028 (NACOAO stretching band), 1536 (CAN stretching and NAH bending), 1242 (CANAH combination), 1600, 1504, 771 (aromatic ring of benzene). 1H NMR (400 MHz, DMSO-d6, ppm): 7.95 (d, NHACOO), 6.80 (m, aromatic ring of benzene N), 7.61–7.58 (m, aromatic ring of triphenylamine). [g]: 0.978 dL/g. PU-d: FTIR spectrum (KBr pellet, cm1): 3327 (NAH stretching), 1655 (C@O stretching), 1063 (NACOAO stretching band), 1536 (CAN stretching and NAH bending), 1264 (CANAH combination), 1592, 1512, 737 (aromatic ring of benzene). 1H NMR (400 MHz, DMSO-d6, ppm): 7.92 (d, NHACOO), 6.89 (m, aromatic ring of benzene N), 7.41–7.26 (m, aromatic ring of triphenylamine). [g]: 0.955 dL/g. PU-e: FTIR spectrum (KBr pellet, cm1): 3328 (NAH stretching), 1658 (C@O stretching), 1012 (NACOAO stretching band), 1543 (CAN stretching and NAH bending), 1218 (CANAH combination), 1612, 1510, 757 (aromatic ring of benzene). 1H NMR(400 MHz, DMSO-d6, ppm): 7.97 (d, NH-COO), 6.85 (m, aromatic ring of benzene N), 7.42–7.23 (m, aromatic ring of triphenylamine). [g]: 0.957 dL/g.
2.1. Materials All the chemicals were purchased from TCI and Shanghai Sinopharm Co. N,N-dimethylformamide (DMF) was dried over CaH2 and distilled under reduced pressure. Aniline and 1-fluoro-4-nitrobenzene were distilled before used. Reagent grade aromatic dihydroxy compounds such as phenolphthalein, bisphenol A, 4,40 -dihydroxybiphenyl, 4,40 -bishydroxybenzophenone, bisphenolfluorene were used as received. Methylbenzene were distilled before used. Lithium perchlorate (LiClO4) was dried under vacuum at 100 °C for 96 h. 2.2. Measurements FT-IR spectra were recorded on a PerkinElmer Spectrum 100 Model FT-IR spectrometer. 1H NMR spectra were measured on a Bruker AC-400 MHz spectrometer using dimethyl sulfoxide (DMSO-d6) as solvent. CV measurements (the oxidation and reduction potentials) were conducted on a CH Instruments 660A electrochemical analyzer at a scan rate of 50 mV/s with a 0.1 M solution of LiClO4 as an electrolyte under nitrogen atmosphere in dried acetonitrile (CH3CN). The polymer films coated on an ITO disk, a Pt wire and an Ag/AgCl electrode were used as a working electrode, counter electrode and a quasi reference electrode, respectively, and calibrated against the ferrocene/ferrocenium (Fc/Fc+) redox couple. UV–vis spectra were determined on a SHIMADZU UV-3600. Thermo-gravimetric analysis (TGA) was conducted with approximately 6–8 mg powder samples heated in flowing nitrogen (flow rate = 20 cm3/min) at a heating rate of 10 °C/min using a PerkinElmer Pyris 6. SEM measurement was carried out on an S4800 instrument with an accelerating voltage of 20 kV, and the samples were sputtered with Pt prior to observation. Density-functional theory (DFT) calculations were performed on a computer. Geometry optimizations were carried out using the B3LYP functional as implemented in Gaussian 98 program [22]. The viscosities of polymers in DMSO were determined with Ubbelohde viscometer in 35 °C. The thickness of PU film was determined on profilometer XP-100 (KLA-Tencor), by the probe testing the different height of blank ITO-coated glass and filmed ITO-coated glass. Gel permeation chromatographic (GPC) analysis was performed on a Malvern instrument connected with one refractive index detector (ViscotekVE3580-RI-DETECTOR) by using a polymer/DMF solution at a flow rate of 0.8 ml/min at 35 °C and calibrated with polystyrene standards. 2.3. Synthesis of monomers 4,40 -Dinitrotriphenylamine (1) and 4,40 -diaminotriphenylamine (2) were synthesized according to the literature [23,24] outlined in Scheme 1.
2.4. Preparation of film electrode A solution of the PUs in DMF was cast on an ITO-coated glass. Then the thermally cured films were obtained from the PUs by programmed increasing temperature to 100 °C at heating rate of 10 °C/ min and keeping the temperature for 2 h. The thickness of polymer film was about 100 nm determined by profilometer XP-100 (KLATencor). To keep the same thickness of different PUs, we control
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Scheme 1. Synthesis route of isocyanate monomer.
Scheme 2. Synthesis route of PUs.
the concentration of the solution and casting speed at the same level. 3. Results and discussion 3.1. FTIR analysis and 1H NMR In Fig. 1, the distinct absorption peak of N,N-bis(4-isocyanatophenyl)aniline at 2262 cm1 confirms the existence of ANCO group. After polymerization, there is not absorption peak around 2262 cm1 representing ANCO, which proves the reaction was complete. The characteristic peaks of urethane group of PU-a– PU-e are observed at 3321 cm1 and 1669 cm1, corresponding
to stretching vibrations of NAH and C@O groups, respectively. There are peaks at 1269 cm1 owing to vibrations of CANAH groups. There is no absorption peak around 1647 cm1 due to urea hydroxy in the ureido ester, showing that the reaction is complete. The 1H NMR spectra (as shown in Fig. S1) of PU-a–PU-e show that the peaks at 7.97, 7.95, 7.95, 7.92 and 7.97 are representing the hydrogens in NHACOO group. All the detected peaks and ratios between the peak areas are consistent with the proposed structures. 3.2. UV–vis absorption and photoluminescence (PL) The PUs (PU-a–PU-e) exhibit strong UV–vis absorption bands in the range of 283–374 nm in DMSO solution and in the range of 390–394 nm for solid films, assignable to the p–p transitions resulting from the conjugated TPA segment (shown in Fig. 2(a)). The PL spectra of PUs measured in DMSO (conc.: 108 mol/L) indicated that the PL peaks located in the range of 367–383 nm showing little Stocks shift (shown in Fig. 2(b)). The quantum yields of these polymers can be calculated according to equation [25].
/unk ¼ /std
Fig. 1. IR spectra of the compound 3 and PU-a–PU-e.
Iunk Astd gunk 2 Istd Aunk gstd
where /unk, /std, Iunk, Istd, Aunk, Astd, gunk, and gstd, are the fluorescent quantum yield, integration of the emission intensity, absorbance at the excitation wavelength, and the refractive indices of the corresponding solutions for the samples and the standard, respectively. Here, we use the refractive indices of the pure solvents as those of the solutions. The PL quantum yields in DMSO solution are calculated to be: 3.64% (PU-a), 17.1% (PU-b), 9.84% (PU-c), 4.43% (PU-d), and 15.8% (PU-e), respectively. This photoluminescence phenomenon can possibly be attributed to the forming of conjugated
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switching rate. In order to achieve sufficient speed, the morphology should be controlled to minimize the diffusion distance in PU film. SEM images of PU show that the surface is rough or nanofibrous which suggest the presence of channels or pinholes in the film would be favor to transport counter ions. TGA curves of the PUs in Fig. 4 show that PUs (PU-a–PU-e) have good thermal stability with a 10 wt.% loss temperature of about 210 °C. 60% weight-loss temperatures of PU-a, PU-c and PU-d in nitrogen are recorded at about 450 °C. The high char yields of the PUs can be ascribed to their high aromatic content. Hence, the PUs could fulfill the requirements of high-temperature stability as required for optoelectronic thin-layer devices. The number-average molecular weight (Mn) and polydispersity (PDI) were determined to be in the range of 10,781–20,398 and 1.065–1.708, respectively (shown in Table S1). The Mn suggests that the polymerization has come into being. 3.4. Electrochemical properties
Fig. 2. (a) UV–vis absorption. (b) PL spectra of PUs in DMSO.
structure in the PU molecule. Especially, bisphenol A and bisphenolfluorene groups which are more planar contribute more to the emission of PUs than the other groups [26]. 3.3. Polymer film morphologies, thermal stability and molecular weights The morphologies of PU-a and PU-d were investigated by SEM in Fig. 3 and Fig. S2. In common, the rate limiting step in the electrochromic switching process involved diffusion of dopant counter ions into the polymer structure. Consequently, the polymer morphology plays an important role in affecting the electrochromic
The electronic states including highest occupied molecular orbital /lower unoccupied molecular orbital (HOMO/LUMO) levels of the polymers were investigated by CV. The results of polymers are shown in Fig. 5 and summarized in Table 1. The redox peaks have been associated with reduction–oxidation process accompa nying the double injection/extraction of ClO4 ions and electrons. All the PUs (PU-a–PU-e) exhibit one pair of obvious reversible oxidation peaks which correspond to the oxidation of the TPA moiety [24]. During the electrochemical oxidation of the thin PUs films, the color of PUs films changed from red to blue. In order to obtain accurate oxidation reduction potential, the reference electrode was calibrated by the Fc/Fc+, whose redox potential is assumed to be energy level of 4.80 eV to vacuum. The energies of the HOMO and LUMO levels of the investigated PUs can be determined from the onset oxidation potentials and the onset absorption wavelength which are listed in Table 1. The HOMO energy level values were calculated using the equation [27]:
EHOMO ¼ eðEOX onset vs Ag=AgCl þ 4:36Þ eV where EOX onset is the onset oxidation potential. The HOMO energy level values of PU-c and PU-d were calculated to be at in the range from4.882 to 5.046 eV. The LUMO energy levels of the polymers are estimated from the HOMO energy levels and Eg curves which could be obtained as following:
ELUMO ¼ EHOMO þ Eg Eg ¼ 1240=konset
Fig. 3. SEM images of PU: (a) PU-a and (b) PU-d.
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Fig. 4. TGA traces of the PU-a–PU-e. Fig. 6. Pictorial representations of the electron density in the frontier molecular orbitals of repetition units for PUs (PU-a–PU-e).
Fig. 5. CVs for PU-a–PU-e in 0.1 M LiClO4/CH3CN with Fc/Fc+ as an internal standard at the scanning rate of 50 mV/s.
[28]. The HOMO and LUMO energies and HOMO–LUMO energy gaps are given in Table 1. For PUs, the electronic wave-function of the HOMO is distributed entirely over TPA moieties, which is beneficial for obtaining higher hole mobility. The electron lone pair of the nitrogen atoms has strong coupling with p electrons in TPA which lead to an intramolecular charge transfer (ICT) with significant distribution of the electronic density over the entire molecule [28]. The electron wave-function of LUMO is mainly localized on the dihydroxy compound part. Thus, the TPA segment and dihydroxy compound effectively regular the bandgap together. The trend of Eg for the oligomer series follows in the order: PU-b < PU-c < PU-d < PU-e < PU-a in Table 1. Actual measured data have a slight deviation compared with the theoretical data due to the affection of the solvation process, molecular structure, the electron of the electron-withdrawing group. 3.6. Electrochromic properties
ELUMOs are at between 2.089 (PU-c) and 2.228 (PU-d) eV. The results disclosed that the different dihydrophenyl derivative structures played the key role in the electron structure and regulated the gaps of PU. Thus, our novel PUs can be employed as potential candidates in the development of dynamic electrochromic screen devices due to their suitable HOMO values, thermal stabilities and reversible electrochemical behaviors. 3.5. Quantum chemical calculation The ground-state geometries of the compounds were optimized by hybrid DFT (B3LYP) with 6-31G basis set in the Gaussian 98 program package [22] and were presented in Fig. 6. EHOMO is often associated with the electron donating ability of the molecule and ELUMO indicates the ability of the molecules to accept electrons
All these PUs exhibit similar electrochromic properties, and the typical electrochromic transmittance spectrum of PU-a is shown in Fig. 7. When the applied potentials increased positively from 0.0 to 1.6 V, the peak of absorption at 394 nm, characteristic for neutral form for PU-a increased gradually, while two new bands grew up at 610 and 830 nm, respectively. For electrochromic switching studies, polymer films were cast on ITO-coated glass slides in the same manner as described above, and chronoamperometric and absorbance measurements were performed at the same time. While the voltage was switched, the absorbance at the given wavelength was monitored as a function of time with UV–vis–NIR spectroscopy (Fig. 8). The switching time was defined as the time that required for reach 90% of the full change in absorbance after applying potential. Thin films of PU-e would require 3.0 s at 0.8 V for switching absorbance at 600 nm
Table 1 Optical and electrochemical properties of the PUs.
PU-a PU-b PU-c PU-d PU-e
kfilm (nm)
konset (nm)
Eox onset vs Ag=AgCl (V)
EHOMO (eV)
ELUMO (eV)
Eg (eV)
Equntum HOMO (eV)
Equntum LUMO (eV)
Equntum (eV) g
394 392 390 392 394
435 446 444 440 438
0.604 0.566 0.522 0.686 0.636
4.964 4.926 4.882 5.046 4.996
2.114 2.146 2.089 2.228 2.165
2.850 2.780 2.793 2.818 2.831
5.339 5.114 5.382 5.459 5.223
1.686 0.751 0.912 1.622 0.940
3.653 4.263 4.470 3.837 4.283
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proportional to the amount of created color centers; g denotes the coloration efficiency (CE) and Q (mC/cm2) is the amount of injected/ejected charge per unit sample area. The CEs of PU films are calculated to be: 26.06 cm2/C (PU-a), 11.08 cm2/C (PU-b), 49.13 cm2/C (PU-c), 38.30 cm2/C (PU-d), and 43.78 cm2/C (PU-e), respectively, and are summarized in Table S2. PUs electrochromic coloration efficiencies are listed in the order: PU-b < PU-a < PUd < PU-e < PU-c. PU containing dihydroxy biphenyl groups has the highest CE contrast to the PU contain bisphenol A having the lowest CE. The reason may be the more facile electron transfer in PU containing dihydroxy biphenyl groups that play the key role in the coloration efficiencies. 4. Conclusions
Fig. 7. Electrochromic behaviors of PU-a thin film (in CH3CN with 0.1 M LiCO4 as the supporting electrolyte).
In summary, a new series of TPA-based PUs have been successfully synthesized, which have excellent film-forming quality and good solubility in low-boiling-point solvents. The thermally cured PUs exhibit thermal stabilities and reversible electrochemical behaviors. The calculated HOMO and LUMO energy levels of the PUs by experimental method are in the range of 5.14 eV to 5.46 eV and 0.75 eV to 1.68 eV vs the vacuum level, respectively. All obtained PUs revealed excellent stability of electrochromic characteristics, with changing color from original yellowish to blue. Acknowledgements The authors are grateful to the support of the National Science Foundation of China (Grant Nos. 51373049, 51372055, 51273056, and 21372067), Doctoral Fund of Ministry of Education of China (20132301120004 and 20132301110001), and Foundation of Heilongjiang Education Bureau (12531504). Appendix A. Supplementary material
Fig. 8. Optical switching procedures: (a) potential step absorptiometry of PU-e at 600 nm (0.1 M LiClO4 as the supporting electrolyte) by applying a potential step (0.0–0.8 V). (b) Current consumption of PU-e.
Supplementary material associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.jelechem.2014.01.021. References
and 3.0 s for bleaching. The color switching times of PU-e are also shown in Fig. 8. After over continuous 300 cyclic switches between 0.0 and 0.8 V, the polymer films still exhibited good stability of electrochromic characteristics. As shown in Figs. S3–S6 when the applied potentials increased positively from 0.0 to 2.0 V, the peaks of absorption at 392 nm, 390 nm, 392 nm and 394 nm, characteristic for neutral form for PU-b, PU-c, PU-d and PU-e increased gradually, while two new bands grew up at about 600 and 894 nm, respectively. Thin films of PU-a, PU-b, PU-c and PU-d require 2.0 s, 2.0 s, 3.0 s and 3.0 s at 0.8 V for switching absorbance at 600 nm and 2.0 s, 2.0 s, 3.0 s and 3.0 s for bleaching respectively. The color switching time of PUs is also shown in Figs. S7–S10. After over continuous 300 cyclic switches between 0.0 and 0.8 V, the polymer films still exhibited good stability of electrochromic characteristics. The electrochromic coloration efficiency (CE; g) is also an important characteristic for the electrochromic materials. CE can be calculated using the equations and given below [29]:
dOD ¼ lgðT b =T c Þ
g ¼ dOD =Q where Tb and Tc are the transmittances before and after coloration, respectively; dOD is the change of the optical density which is
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