A fast electrochromic polymer based on TEMPO substituted polytriphenylamine

A novel strategy to obtain rapid electrochromic switching response by introducing 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) moiety into polytriphenylamine backbone has been developed. The electrochromic properties of the integrated polymer film are investigated and a possible mechanism is proposed with TEMPO as a counterion-reservoir group to rapidly balance the charges during electrochromic switching, which leads to significantly improved electrochromism performance.

Instrumentation. FT− IR spectra were obtained on a Nicolet 6700 spectrometer (Thermo Fisher Nicolet, USA) with KBr pellets. Scanning electron microscopy (SEM) measurements were performed using a Hitachi S-4800 scanning electron microscope (Hitachi, Japan). The electrochemical properties and long-term stability test were measured in a three-electrode cell (ITO/glass electrode as the working electrode, platinum wire electrode as the counter electrode and Ag/AgCl electrode as the reference electrode) with a CHI 660E electrochemical workstation in a 0.1 M LiClO 4 /CH 3 CN solution with iR compensation. UV-Vis spectra, optical contrast and switching time measurements were carried out on a Shimadzu UV-1800 UV-Vis spectrophotometer (Shimadzu, Japan). The contact angle measurement was recorded on a Datahpysics OCA30 (Germany).
Synthesis of compound 2 (4-carboxy-N,N-diphenylaniline) 4-Cyano-N,N-diphenylaniline (1.0 g) and KOH (2.1 g) were firstly dissolved in a mixture of deionized water (30 mL) and glacial acetic acid (20 mL) in a pre-dried three-necked flask and heated at reflux (85 °C) for 48 h. After cooling, hydrochloric acid (1 M) was added dropwise to the reaction solution until the pH value of the reaction solution was adjusted to about 1. Meanwhile, a lot of white precipitate appeared, then it was isolated by filtration and washed with a large amount of deionized water. The obtained white powder of 4-carboxy-N,N-diphenylaniline (2) was dried in vacuum at 60 °C for 24 h to give an 82% (0.82 g) yield. 1  Synthesis of compound 3 (4-carboxy-N,N-diphenylaniline-2,2,6,6-tetramethylpiperidin-1-yloxy) 4-Hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl free radical (1.6 g) and 4-carboxy-N,N-diphenylaniline (2.0 g) were dissolved in CH 2 Cl 2 (50 mL) in a pre-dried three-necked flask, then 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (0.4 g) as the dehydrating agent and 4-dimethylaminopyridine (1.6 g) as the acylating catalyst were added, and the mixture was stirred for 24 h at room temperature. The reaction mixture was then separated by vacuum filtration. The filtrate was washed with saturated brine three times and the organic phase was dried using anhydrous Na 2 SO 4 . The ester was purified column chromatography using silica gel and petroleum ether/ethyl acetate to afford the title compound (3) as a pink powder with 61.8% (1.9 g) yield. MS: m/z (EI): 443. The synthesis of TPAT monomer via 1-3 is summarized in Fig. 1.

Results and Discussion
Preparation and characterization of the PTPAT film. The PTPAT film was prepared via electropolymerization of the TPAT monomer onto the ITO substrate, Fig. 2a. As shown in Fig. 2b, electropolymerization was performed in a solution of 0.1 M LiClO 4 /CH 3 CN containing 1 mM TPAT monomers via repetitive cycling at a scan rate of 100 mV s −1 . In the first cycle, TPAT exhibits a quasi-reversible redox couple with an oxidative peak at 1.07 V (onset at 0.90 V) and a reductive peak at 0.85 V (Fig. S1). During successive CV scans, both the oxidation and reduction currents gradually increased, indicating successful deposition of an electroactive PTPAT polymer film on the ITO surface. In this process, a new oxidation peak appeared at around 0.9 V which is consistent with the behavior of the as-formed PTPAT film on the ITO electrode surface (note below). With an increase in the deposited film thickness, the potential separation between the anodic and cathodic peaks (∆ E = E p,a − E p,c ) increases slightly, consistent with an increase in the internal resistance across the film. Figure 2c shows the FT− IR spectrum for an as-prepared PTPAT film. The peaks at 1593.2, 1497.5 and 1322.3 cm −1 can be ascribed to the fundamental vibrations of the triphenylamine moieties, corresponding to the C= C ring stretching, the C− C stretching and the C− H bending, respectively. The peaks at 1292.1 and 828.9 cm −1 are attributed to the C− N stretching of the tertiary amine and the C− H out-of-plane vibration from 1, 4− disubstituted benzene rings. While the stretching of C= O group of ester carbonyl is found at 1692.5 cm −1 , the peaks at 1112.2, 1164.9 and 1221.1 cm −1 are attributable to the stretching of γ c−o−c exsiting in the ester linkage. TEMPO moieties are evidenced by peaks at 1434.7 and 1463.0 cm −1 for the C− H stretching of the − CH 3 and − CH 2 − . The results of FT− IR spectroscopy indicate that TPAT monomers were successfully synthesized and upon electropolymerization, both the triphenylamine and TEMPO moieties were well-retained and incorporated into the PTPAT polymer film 31 . Figure 2d shows CV of the as-prepared PTPAT recorded in 0.1 M LiClO 4 /CH 3 CN solution at a scan rate of 10 mV s −1 . The dominant peaks are located at E p,a = 1.02 and E p,c = 0.93 V which can be attributed to the redox reaction of the triphenylamine units of PTPAT. Additional weak redox peaks are observed at 0.80 and 0.78 V arising from the p− type doping of nitroxide radical and its reciprocal conversion to the oxoammonium cation (A magnified view of CV in Fig. 2d is shown in Fig. S2). Similarly, another couple of redox peaks appearing at around − 0.2 V is related with the n− type doping of nitroxide radical, corresponding to the redox process  between the aminoxy anions and the nitroxyl radicals 31 . The CV profile is consistent with earlier reports on PTPAT electrochemistry 31 , indicating further the successful fabrication of the PTPAT film.
Scan rate alternation experiments were conducted to probe the charge transfer within the polymer film. As shown in Fig. 3a, the redox peak of the triphenylamine unit of PTPAT is enhanced with increasing the scan rate within the potential range from 0.4 to 1.2 V. As expected for a surface-bound couple, the anodic and cathodic peak currents are linear with the scan rate (Fig. 3b), indicating that the PTPAT film adheres firmly onto the ITO surface and the charge transfer across the polymer film is not restricted by the diffusion of balance charges (i.e. counterions ClO 4 − ). Figure 3c,d show the top-down and cross-sectional SEM images of the PTPAT film with images of low magnification shown in Fig. S3. As can be seen from Fig. 3c, the surface of the PTPAT film consists of some spherical nanoparticles with diameters less than 250 nm. The image in Fig. 3d reveals that PTPAT part consists of a thin layer of approximately 50 nm (marked by red dash lines) and globules with diameters in the tens to hundreds of nanometers range grown on the thin layer (a schematic diagram of the PTPAT electrode was illustrated in Fig. S4). The presence of these globules could result in a higher surface contact area between the PTPAT film and the electrolyte solution, which thus favors more rapid doping of counterions.
Electrochromism performance. Figure 4 shows UV-Vis absorption spectra of a PTPAT film under different applied potentials. At an applied potential of 0 V, a well-defined absorption band centered at 409 nm is observed, which can be ascribed to the π -π * transition of the neutral state polymer backbone. With increasing the applied potential, the intensity of this peak decreases, indicating that oxidation of TPA units in the polymer backbone alters the band gap of PTPAT by the doping of counterions. The attenuation in the main band at 409 nm is accompanied with the appearance of charge carrier bands at round 700 and 1100 nm that arises from the evolution of polaron and bipolaron bands [18][19][20][21][22] . With increase in applied potentials leading to the change of the film from its neutral state (0 V) to oxidized state (1.2 V), the color of the film turns from yellow to dark green (Fig. 4 inset).
The electrochromic switching performance of the PTPAT film was examined with a residence time of 5 s at the visible (409 nm) and near-IR light region (1100 nm) between 0 and 1.2 V. Figure 5a displays optical contrasts of the PTPAT film which exhibits a 29.9% contrast at 409 nm and 47.3% at 1100 nm between its neutral and oxidized states. Figure 5b exhibits the switching response of a PTPAT film at 409 and 1100 nm, respectively. The switching time is defined as the time required to reaching 95% of the full change in absorbance after switching the potential. At 409 nm, the switching time is 0.37 s for coloring and 0.72 s for bleaching. At 1100 nm, the switching for bleaching only needs 0.38 s, while the time slightly extends to 1.34 s for coloring. The high optical contrast and rapid switching time make the PTPAT film among the best electrochromic materials based on polytriphenlylamine derivatives [18][19][20][21][22] . It is worth to compare, in particular, to the most structurally related polymers, poly(4-cyanotriphenylamine) with pendent -CN or poly(4-nitrotriphenylamine) with pendent -NO 2 22 . As listed in Table 1, the switching time of PTPAT decreases by almost one order of magnitude as compared to the above two, which confirms the essential role played by the TEMPO moieties. It is also important to note that unsubstituted PTPA cannot be prepared by electropolymerization 16 and thus TEMPO moieties are also crucial in achieving the good film-forming property of PTPAT in addition to acting as a counterion-reservoir group for rapid switching response.  Electrochromic mechanism. It is known that the switching response is dependent on the counterion transport into the polymer layer. An effective way to improve the switching rate is thus to reduce the counterion transporting resistance in the polymer. The usual approach to realize this is to design and prepare nanostructured EC materials that could provide intrinsically high porosity and high surface area by template method [32][33][34] . However, the report on molecular design to achieve prompt switching response has been rare. Based on our experimental observation, we propose here a new chemical approach to realize rapid switching response by introduction of TEMPO moieties into polyphenylamine backbone. Although TEMPO moieties are not electrochromically active, we speculate that they can function as counterion-reservoir groups to obtain more rapid switching response. A schematic representation of the electrochromic mechanism of the PTPAT film is illustrated in Fig. 6. The electrochromism is presumed to proceed through three steps from neutral state to oxidized state: (1) With the applied potential maintained at 0 V, counterions (ClO 4 − ) dispersed uniformly in the electrolyte solution; (2) By increasing the potential beyond 0.80 V, the pendent TEMPO moieties at the interface and/or in the interior of the polymer film are oxidized to the oxoammonium cations. Upon oxidation, counterions doping occurs simultaneously to balance the charge. At this stage, electrochromic phenomenon is not observed because TEMPO moiety is an inactive component toward electrochromism. However, it might increase the counterion density near or within the film. (3) By further increasing the potential to 1.02 V, the electroactive triphenylamine units in the skeleton are beginning to oxidize. With the counterions accumulated within or near the PTPAT film, they are ready to balance the oxidized triphenylamine cationic units, thus resulting in rapid color switching. In the reserve process for reduction of the oxidized triphenylamine units, a similar process might occur with the subsequent reduction of the oxidized TEMPO moiety promoting the counterions to move out of the film.
As an additional support for the proposed mechanism, we tested the surface wettability of PTPAT film by measuring the CH 3 CN contact angle (Fig. S5). It was found that the contact angle between CH 3 CN and PTPAT was around 9.1°, which indicates a quite well surface wettability of PTPAT film in CH 3 CN solution. The favorable wettability makes it possible for counterions in the CH 3 CN solution to rapidly dope the PTPAT film, leading to the prompt switching response.
Stability. The electrochemical stability of EC materials during long-term switching between the neutral and oxidized states is one of the most important parameters for application in EC devices. Stability test for the PTPAT film was conducted by successive CV cycles between 0.4 and 1.2 V in 0.1 M LiClO 4 /CH 3 CN solution. As shown in Fig. 7, the PTPAT film remained 86.5% of its original electroactivity after 300 cycles and 74.1% after 500 cycles, which confirms a decent stability of the PTPAT film and provides a promising material candidate for EC device.

Conclusions
In conclusion, a PTPAT film with TEMPO moieties has been synthesized and firstly applied as an EC material, which shows an excellent film-forming property and electrochromism with reversible color changes between yellow (neutral state) and dark green (oxidized state). The electrochromic performance test on the PTPAT film exhibits a quite high optical contrast of 47.3% at the near-IR region (1100 nm), and a significantly rapid switching time of 0.37 s for coloring and 0.72 s for bleaching in the visible light region (409 nm). The stability of the PTPAT film is also impressive with 74.1% of its original electroactivity retained after 500 repeated cycles. The proposed mechanism suggests that the redox process of pendent TEMPO moieties could greatly improve the electrochromic response of the triphenylamine units with TEMPO serving as a counterion (ClO 4 − )-reservoir. The remarkable electrochromic performance of the PTPAT film, especially its blink switching response indicates that this material may serve as a promising candidate for the design and fabrication of an excellent EC device.