Di Zhangab,
Min Wangc,
Xiaoli Liua and
Jinsheng Zhao*a
aShandong Key Laboratory of Chemical Energy Storage and Novel Cell Technology, Liaocheng University, Liaocheng, 252059, P. R. China. E-mail: j.s.zhao@163.com; Fax: +86-635-8539607; Tel: +86-635-8539607
bCollege of Chemical Engineering, China University of Petroleum (East China), QingDao, 266580, P. R. China
cLiaocheng People's Hospital, Liaocheng, 252000, P.R.China
First published on 20th September 2016
Two novel electrochromic copolymers, poly(benzodithiophene-benzotriazole) (PBDBT) and poly(s-indacenodithiophene-benzotriazole) (PIDBT) were synthesized successfully through Stille coupling reactions. Both polymers were characterized by cyclic voltammetry (CV), UV-Vis spectroscopy, density functional theory (DFT), colorimetry and thermal gravimetric analysis. The band gaps of the two polymers calculated according to spectroelectrochemistry data are 1.90 eV for PBDBT and 1.93 eV for PIDBT, respectively. The PBDBT film turns rosybrown to transparent grey when it is oxidized and the PIDBT film turns pale violet-red in the neutral state to transparent light grey in the oxidized state. PBDBT has an 55.0% optical contrast at 1500 nm, PIDBT has an 60.8% optical contrast at 1560 nm, and their corresponding response times are 1.49 s and 1.85 s, respectively. In addition, the coloration efficiency of PBDBT at 1500 nm is 144.70 cm2 C−1, and the coloration efficiency of PIDBT at 1560 nm is 222.00 cm2 C−1. Both copolymers demonstrate good properties, and can be promising electrochromic materials for commercial applications.
The band gap values are regarded as the vital parameters resolving the electrochromic characteristics of polymers,2,10 which could be tuned by some feasible strategies including bond-length alternation, inducing order, creating an planar structure and adopting donor–acceptor (D–A) approach.7 D–A approach is the most common and useful method among them. It functions by the push–pull interaction in the molecular backbone, which can facilitate intramolecular charge transfer.11 The electron donor group can raise highest occupied molecular orbital (HOMO) energy, making oxidation easy to achieve, and the acceptor group can lower the lowest unoccupied molecular orbital (LUMO) energy, which can promote the reduction process.5,12 Vinylene, thiophene, 3,4-ethylenedioxythiophene (EDOT) are electron-rich units, while quinoxaline, 2,1,3-benzothiadiazole, 2,1,3-benzoselenadiazole, thienopyrazine13 are usually electron-deficient units.
Taking benzotriazole as acceptor units, some D–A type polymers have been prepared and be proven to be promising candidates as electrochromic or solar energy generation materials, although the electron affinity ability of benzotriazole is weaker than that of the majority of electron withdrawing units mentioned above.14 Abidin Balan and his colleagues synthesized one polymer taking EDOT as donor unit and benzotriazole as acceptor unit, and the resulting copolymer showed superior properties in many aspects.15 When changing the donor unit from EDOT to thiophene, Abidin Balan synthesized another copolymer monomer, 2-dodecyl-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole (TBT), which could be polymerized to a polymer by the electrodeposition method, the obtained polymer could display three primary colours, changing from red to green and final blue when it is oxidized, while when it is n-doped, its colour could change to light blue, and eventually transparent.16 When the alkyl group is substituted on the thiophene unit, the steric repulsion effects could be anticipated between the repeating units along the backbone of the polymer chains, which could be avoided while placing the alkyl group on the nitrogen atom of benzotriazole unit, and then making the resultant copolymer more planar.17
Benzodithiophene has a high hole mobility and can form π–π stacking easily due to its large planar structure,13,18 which was proved by the X-ray diffraction (XRD) analysis.18 Jianhui Hou and his colleagues investigated the copolymers based on benzodithiophene in details, and their band gaps can be tuned between 1.0 eV to 2.49 eV by different combined manner.19 Although the steric hindrance could affect the band gap, in case of the benzodithiophene unit, the alkoxyl chains exist on the side positions including 4 and 9 positions, which reduce the steric hindrance effects to a very low degree.19 So, benzodithiophene and its derivatives are most popular donor units for the fabrication of D–A type conjugated polymers.20 When benzodithiophene unit is taken as the donor core, the introduction of the alternate electron deficient units in the polymer chain could generate intra-molecular charge transfer, and consequently the copolymer has a small bad gap.21
Polymers incorporating 4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene (IDT) as the donor units typically have excellent solubilities.22 Planarity plays an important role in molecular packing, and influencing not only the conjugation length but also the charge transfer speeds within the polymer films stacked by individual polymer molecules.23 As known to us, IDT possesses ladder planar structure, which can extend the π electron delocalization of the polymer backbone containing it, making it a good electron donor.22 Taking IDT as the donor unit, D–A type conjugated polymer could be achieved through a copolymerization reaction with appropriate acceptor unit, and the resultant polymer has been applied to field effect transistors24 and solar cells.23 Wei Teng Neo and his colleagues firstly applied IDT-containing polymers to electrochromic area.22 The reports about copolymers containing the above mentioned units for electrochromic applications are still very limited, further efforts in this area should be strengthened.
In this study, two D–A type conducting polymers, poly(benzodithiophene-benzotriazole) (PBDBT) and poly(s-indacenodithiophene-benzotriazole) (PIDBT) were synthesized by Stille coupling reaction, and then spray casting method was used to pattern a copolymer film on ITO conducting glass. Finally, a series of characterization methods, including electrochemistry, spectroelectrochemistry, density functional theory (DFT) calculation, colorimetry, kinetics and thermogravimetry, were carried out for the detailed acquaintance of polymers, and the results witnessed the excellent electrochromic properties of the polymers. In this case, both of the polymers could be potential candidates for the fabrication of electrochromic device.
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Fig. 1 CV of spray coated PBDBT (a) film from −2.0 V to 2.0 V and PIDBT (b) film from −2.0 V to 2.5 V. |
The initial redox peaks at the positive potentials suggested the existence of the p-type doping, which might be related with the electron-donating property of benzodithiophene or indacenodithiophene.26 In D–A type polymer, the HOMO values of the donor unit is approximately equal to the values of the polymer, which is then determined by the initial oxidative potential (Eonset) of the polymer. The stronger electron donating ability of the donor unit is beneficial to reduce the Eonset of the polymer, and finally lead to the reduced Eg of the polymer. With the potential increased, copolymers lose electrons, the first peak appearing. The Eonset value of PBDBT (0.64 V) is lower than that of PIDBT (1.01 V), demonstrating the stronger electron-donating ability of benzodithiophene than that of indacenodithiophene unit (Fig. 1). And, when the potentials decrease from the positive potentials to 0 V, the polymers restored to their neutral states. Both copolymers have n-type doping abilities and the initial negative potentials denote n-type doping. The n-type doping voltage of PBDBT is larger (−1.43 V) than PIDBT (−1.56 V), signifying the PBDBT has the stronger electron-withdrawing ability (Fig. 1).25
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Fig. 2 UV-Vis absorption spectra of PBDBT and PIDBT dissolved in CHCl3 (a) and those of their films coated on ITO electrode in TBAPF6/ACN (b) in neutral state. |
The absorption peaks located at 532 nm in solution and 582 nm in film solid state for PBDBT was observed, and 531 nm in solution and 536 nm in film state were also found for PIDBT, respectively. Compared with the absorption in the solution, there is an apparent red shift for the polymers existing in the form of film, which is ascribed to the better interchain interaction stemmed from π–π stacking of the copolymers.28 The colour of PBDBT film and PIDBT film are rosybrown and pale violet red, respectively, at the neutral state. There is an obvious bathochromic shift for PBDBT films (582 nm) compared with PIDBT films (536 nm) in neutral state, manifesting that PBDBT has a smaller energy absorbance. In other words, PBDBT need a small energy to run π–π* transition and it has a more conjugated structure, which is anticipated by researchers.
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Fig. 4 Spectroelectrochemistry of PBDBT (a) and PIDBT (b) and their photos in neutral states and oxidized states. |
For PBDBT, with potential increasing, the initial peak located at 578 nm decreases, while another two new peaks appear in near infrared region at 781 nm and 1388 nm, respectively. The peak at 781 nm is because of the formation of polaron (singly charged species), while the peak at 1388 nm which located in a low energy region can be ascribed to the bipolaron.29 The polaron peak reaches largest when the potential is up to 1.42 V, while the bipolaron peak is rising with the increase of voltage, which is assigned to the repression of inter-band transition.28,30 Same with PBDBT, the three peaks of PIDBT are at 532 nm, 841 nm, 1390 nm corresponding to π–π* transition, formation of polaron and bipolaron, respectively. The polaron peak reaches a maximum when the applied potential is 1.22 V, and the bipolaron peak is increasing when the potential is turning large.
As shown in Fig. 4(a), with the applied potential increasing, the colours of PBDBT film change from rosybrown in neutral states to transparent grey in oxidized states. White light is a mixture of light that is formed by the superposition of various wavelengths of light. When the potential changes, the absorption of light of different frequencies varied, and generated different colours. In neutral states, there is an absorption peak located mainly in green light region, so the film demonstrates rosybrown. With the potential increasing, the absorption of green light deceases, while the absorption of red light increases. Finally, the absorption of two peaks in visible region is nearly equal, therefore, the film is transparent grey in oxidized states. Similarly, PIDBT also experiences an palpable colour change from pale violet red to transparent light grey after it is oxidized.
Some parameters of spectroelectrochemistry of PBDBT and PIDBT were read from their graphs or calculated according to previous studies27,31 and were summarized in Table 1, including onset of the optical absorption spectra in neutral states (λonset), maximum absorption wavelength of PBDBT and PIDBT in solution (λmax, solution) and film (λmax, film) morphology, onset oxidation potential (Eonset), optical band gap (Eg) and HOMO/LUMO energy levels of both copolymers. Eg, EHOMO and EHOMO were calculated with the formula Eg = 1241/λonset, EHOMO = −e(Eonset + 4.4) and ELUMO = EHOMO + Eg, respectively.27,31
The Eg values of the two polymers are 1.90 eV and 1.93 eV, respectively, and the HOMO values exist a large diversity. The electron donor group can raise HOMO energy, while the acceptor group can lower LUMO energy.5,12 Since both polymers have same acceptor unit, the difference of HOMO can be assigned to the donor unit. In case of benzodithiophene unit, benzene links with two thiophene directly, while in case of s-indacenodithiophene unit, benzene is connected with thiophene through cyclopentene, so benzodithiophene has a better planarity and more conjugated structure than s-indacenodithiophene. Consequently, PBDBT has a smaller band gap and a larger HOMO value than that of PIDBT. As a kind of homopolymer of benzodithiophenes, poly(4,8-bis(dodecyloxy)benzo[1,2-b:4,5-b′]dithiophene) (H2, shown in Table 1) has a band gap of 2.13 eV,19 which was higher than that of PBDBT (1.90 eV). The introduction of TBT unit is beneficial to lower the band gap of the resultant copolymers, and also demonstrates the advantages of donor–acceptor approach for this purpose. Compared with H2, the maximum absorption wavelength of PBDBT has a red shift no matter for polymer solution or polymer film, which could be ascribed to the better conjugated structure of PBDBT backbone than that of H2.
Similarly, a recent reported copolymer of substituted s-indacenodithiophene poly(2-([2,2′-bithiophen]-5-yl)-4,4,9,9-tetrakis(4-(octyloxy)phenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene) (P3, Table 1), has a slightly higher band gap than that of PIDBT due to the introduction of the bithiophene and other than the TBT unit into the copolymer.22 Because benzo[d][1,2,3]triazole is a good acceptor unit, when it is introduced to the polymer backbone, the push–pull interaction will promote the carrier transfer and enhance the delocalized structure, and then lower the band gaps of the copolymers.11 More specifically, the introduction of the benzo[d][1,2,3]triazole unit has a pronounced impact on the LUMO energies of the copolymers, as a resultant, the LUMO value of PIDBT is much lower than that of P3.
In addition, we should notice that the dihedral angles in both monomers have an obvious diversity. θ1 and θ2 are the dihedral angles in benzotriazole unit, and θ3 is the dihedral angle between benzotriazole unit and benzodithiophene unit or s-indacenodithiophene unit. θ1, θ2, θ3 are 0.26°, 0.53°, 3.61° for BDBT, and 0.92°, 3.15°, 5.51° for IDBT. The dihedral angle of BDBT monomer is smaller than IDIT, indicating that BDBT had a more obvious planar structure. Since planar structure can extend π electron delocalization, which is beneficial to reducing the band gap, so the band gap of PBDBT is smaller than PIDBT, which is consistent with the experiment.
The optical contrast (ΔT%) is the change in transmission rate of light between the neutral and doped state, which is important to evaluate the property of polymers.32 As shown in Fig. 6, the ΔT% of PBDBT are 16.8% at 580 nm, 15.7% at 850 nm and 55.0% at 1500 nm, and the ΔT% of PIDBT are 32.0% at 570 nm, 26.2% at 850 nm and 60.8% at 1560 nm. Both polymers had large ΔT%, demonstrating that the colour change is obvious. Besides, the range of transmittance in near infrared region is larger than that in visible region, indicating good potential NIR applications.
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Fig. 6 Electrochromic switching of PBDBT (a) between 0 V and 1.45 V and PIDBT (b) between −0.3 V and 1.4 V. |
Response time (t95%), reflecting the speed switching between neutral state and oxidized state, is defined as the time needed for obtaining 95% of the entire optical contrast, because the naked eye isn't very sensitive to the remaining 5% changes.35 As shown in Table 2, the response time of PBDBT are 1.01 s at 580 nm, 0.75 s at 850 nm and 1.49 s at 1500 nm, and the response time of PIDBT are 1.84 s at 570 nm, 0.66 s at 850 nm and 1.85 s at 1560 nm, respectively. Both copolymers exhibited outgoing switching times.
Copolymers | λ, nm | ΔT%, % | Response time (t95%), s | Coloration efficiency (η), cm2 C−1 |
---|---|---|---|---|
PBDBT | 580 | 16.8 | 1.01 | 69.16 |
850 | 15.7 | 0.75 | 66.93 | |
1500 | 55.0 | 1.49 | 144.70 | |
PIDBT | 570 | 32.0 | 1.84 | 143.22 |
850 | 26.2 | 0.66 | 85.75 | |
1560 | 60.8 | 1.85 | 222.00 |
Coloration efficiency (η) is taken for evaluating the utilization efficiency of the electronic power during switching, and defined as the ratio of optical density change (ΔOD) and electric charge per unit area (ΔQ) and illustrated by the following equations.32
η = ΔOD/ΔQ |
ΔOD = lg(Tb/Tc) |
ΔQ = Q/A |
Changing the interval in multi-potential steps from 10 s to 4 s, 2 s, and finally to 1 s, we can compare the decline degree of optical contrast. As shown in Fig. 7, when the interval decreases from 10 s to 1 s, PBDBT maintains an optical contrast of 16.5% at 580 nm with a 2.6% decrease, an optical contrast of 15.5% at 850 nm with only a 1.1% drop and an optical contrast of 45.4% at 1500 nm with a 8.2% decrease, while PIDBT maintains an optical contrast of 24.0% at 570 nm with a 12.6% decrease, an optical contrast of 21.7% at 850 nm with a 6.0% drop and an optical contrast of 47.2% at 1560 nm with a 14.7% decrease. Compared with PIDBT, PBDBT decreases inconspicuously, which may be ascribed to its good planarity, promoting the transfer speed of electron.
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Fig. 7 Electrochromic switching of PBDBT (a1–a3) and PIDBT (b1–b3) with an interval of 10 s, 4 s, 2 s and 1 s. |
Fig. 8 demonstrates that the lightness (L*) varies with the increase of voltage applied to the film. Three films with different absorption values are measured for both polymers, and the a* values and b* values in neutral states and oxidized states are also marked in the graph. For PBDBT, when the voltage rises, lightness (L*) firstly stays steady, then climbs to a stable stage, meaning that the colour of film turns to be light and transparent. a* turns from a big positive value to a negative value, in other words, red turns to be green, while b* shows a contrary trend, increasing from a big negative value to a small negative value, that means the color changes from blue to yellow. As shown in Fig. 8(a2) and (b2), it can be observed the trend how a* and b* change with the voltage. When the voltage increases, a* and b* go from the fourth quadrant to third quadrant. As a result, the film changes from rosybrown to transparent grey.
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Fig. 8 Plot of lightness as a function of applied voltage of PBDBT (a1) and PIDBT (b1) and the a*–b* graphs of PBDBT (a2) and PIDBT (b2). |
In addition, we can get the result that the thinner the film is, the lighter it is. When the film thickness of PBDBT changes from 0.23 a.u. (532 nm) to 0.45 a.u. and to 0.73 a.u., the lightness is 83.7, 69.9, 54.4 in neutral state and 91.8, 83.2, 77.0 in oxidized state, respectively (Fig. 8(a1)). There exist a significant increase, indicating that film thickness is an important parameter and should be considered carefully when people want to tune the film lightness.
Similar with PBDBT, the lightness of PIDBT film firstly keeps stable, and then has a dramatic increase when the voltage is nearly 1.0 V. a* value gets smaller, while b* value gets bigger (Fig. 8(b1)). When the film thickness is 0.20 a.u. (531 nm), the L*, a*, b* changes from 87.1, 12.2, −4.9 to 96.3, −3.2, 0.6, respectively. When the film turns thicker, the lightness turns darker (Fig. 8(b1)), with the same tendency as that of PBDBT.
To evaluate the thermal stability of the two copolymers, thermal gravimetric (TG) analysis experiment was carried out. The slight mass loss found in both polymers between 50 °C and 100 °C might be caused by the evaporation of the trace amounts of water or organic solvents contained in the polymers (Fig. 9). As shown in Fig. 9(a), PBDBT begins to decompose when the temperature reaches to 320 °C, and PIDBT begins to decompose when the temperature is nearly 460 °C. They both have a high initial decomposition temperature, and PIDBT is more stable than PBDBT with temperature rising. In order to more conveniently compare the thermal stabilities of two polymers, differential thermal gravity (DTG) curves were also shown in Fig. 9, from which the temperature points with the maximum degradation rates are found to be 356.6 °C and 489.6 °C, for PBDBT and PTDBT, respectively. The results also showed that PTDBT has better stability than that of PBDBT, which might due to the multi benzene structure of PTDBT backbone. The data revealed that both polymers had enough thermal stability for electrochromic applications.35
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