Novel polyimides containing flexible carbazole blocks with electrochromic and electrofluorescencechromic properties

A series of polyimides (PIs) were prepared by polycondensation of a diamine monomer with five anhydrides (1,2,4,5-benzenetetracarboxylic anhydride (BTA), 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTD), 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BTD), 4-[(1,3-dihydro-1,3-dioxo-5-isobenzofuranyl)oxy]-1,3-isobenzofurandione (DDII), and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BPTD)), which have anodic electrochromic (EC) properties. These PIs not only show good solubility and thermal stability, but also demonstrate stable electrochemical oxidation behavior and good EC properties, and the highest retained electroactivity reaches 99% after 600 cycles. In addition, the series of PIs exhibit excellent electrofluorescencechromic (EFC) properties. Therefore, the novel materials will contribute to the application of EC or EFC displays in the future.


Introduction
Aer electrochemical oxidation or reduction, the absorption or transmittance of EC materials show reversible optical changes. EC phenomena have been known for a long time, and studies of inorganic coordination compounds, transition metal oxides, and p-conjugated and organic molecule polymer lms have made great strides. [1][2][3][4][5][6][7][8][9][10] However, non-conjugated polymers such as PIs, polyamides (PAs), polyurethanes, polysilicone, dendrimers, epoxy, etc., have not been studied as EC materials extensively. [11][12][13][14] The change of color can be adjusted with the change of the redox state of polymers. Due to their excellent thermal stability and dielectric properties, PIs are a potential competitor in the EC eld. However, the main disadvantages of polymers are poor solubility and poor processability, and to a large extent, it is difficult to obtain lms with excellent lmforming properties and good stability. 15 This limits the application of the EC window because it requires a high degree of transparency and colorlessness. Liou 16 and Sun 17,18 proposed a strategy in that PAs prepared from alkylcycloadipic anhydride can prevent the neutral colorless electron cloud from owing effectively. Inspired by this concept, we designed and synthesized a carbazole monomer containing an alkyl branch. The alkyl group on the main chain increases the solubility of the polymer which enables realization of roll-to-roll printing lm, at the same time increasing the exibility of the lm electrode. What is more, it realizes a neutral colorless state. Functional carbazole groups are widely used in organic electronic elds such as organic eld-effect transistor, organic light-emitting diode, organic photovoltaic device, organic sensor and other commercial electronic devices. [19][20][21][22][23] It was demonstrated that aromatic PAs and PIs containing 4-(carbazole-9-yl) triphenylamine (CzTPA) segments exhibit attractive electrochemical and EC properties. [24][25][26] In other words, CzTPA introduces benzene into carbazole at the N atom position, which has good EC performance. In addition, the carbazole block with a planar structure has strong uorescence (PL), which endows the product with EFC properties. 27 CzTPA-based polymers exhibit high transparency in the neutral state, in contrast to EC p-conjugated polymers, which caters to the requirement of a smart window. According to the structure of carbazole, we prepared PIs by polycondensation of a diamine monomer with ve kinds of anhydrides (BTA, NTD, BPTD, DDII and BPTD). The reason is that the technical applications of most PIs are limited by their high melting points or glass transition temperatures (T g ), and their limited solubility in most organic solvents. In order to overcome these difficulties, we must modify the polymer structure and introduce ethyl groups in the main chain. But one of the common ways to increase the solubility and processability of PI without sacricing high thermal stability is the introduction of bulky phenyl, naphthyl, biphenyl, ether linkages and carbonyl groups into the polymer backbone. Furthermore, phenyl, naphthyl, biphenyl, ether linkage and carbonyl have different molecular structures, so they will play an important role in determining the electron cloud distribution in the PI backbone. In view of easy access and modulating the HOMO, LUMO and E g , different structures are selected. The solubility, thermal stability, electrochemical and EC stability and EFC properties of CzTPA-based PIs were investigated, the results indicating that PIs have prospective application in displays or smart windows.

Synthesis of PIs
PIs (named as PI-6A, PI-6B, PI-6C, PI-6D and PI-6E) were prepared by a traditional two-step progress via reacting an equimolar amount of a diamine monomer (M1) 27 with various aromatic anhydrides (M2) to form poly(amic acid)s (PAAs) (named as PAA-6A, PAA-6B, PAA-6C, PAA-6D and PAA-6E), followed by heat ring dehydration. The synthesis of PI-6A is taken as an example to illustrate the general route of synthesis. 0.5360 g (0.34 mmol) diamine (M1) solution was added to 10.0 mL of a DMAc solution of BTA (0.2070 g, 0.34 mmol). Aer stirring at room temperature for about 24 h, a viscous PAA-6A solution was obtained. The solid content of the PAA-6A solution was about 8 wt%. The inherent viscosity of the resulting PAA-6A was 1.56 dL g À1 , measured in DMAc at a concentration of 0.5 dL g À1 at 30 C. The PAA-6A lm was prepared by drip-coating a reactive polymer solution on a glass plate which was then dried overnight in vacuum at 90 C. PAA-6A was puried from ice methanol by precipitation and being redissolved in DMAc twice. The PI was obtained by heating the PAA-6A lm at 100 C, 200 C and 280 C successively for 0.5 h under vacuum condition. The other PIs were prepared using similar methods.

Preparation of electrochromic device (ECD)
We fabricated a simple ECD to further study the EC properties of the PIs. A 3 mg sample of PIs (PI-6A-PI-6E) was dissolved in 1 mL of DMAc. The mixture was placed on a spin coater and spin-coated onto 1.0 cm Â 4.0 cm ITO at a speed of 500 rpm to obtain a uniform lm, and followed by drying in vacuum at 80 C for 12 h. UV curing adhesive was used to seal the devices. Finally electrolyte solution (0.1 M TBAP/ CH 3 CN) was injected into interlayers between two ITO glass layers. The device with PI used as electroactive material was assembled in a glovebox lled with Ar (concentration of O 2 < 90 ppm and H 2 O < 13 ppm) and the electrolyte free of water was bubbled with N 2 for 2 min. The testing process was carried out in sealed conditions.

Fundamental characteristics
A traditional two-step method was adopted to form PA intermediates by reacting equimolar diamine (M1) with various aromatic dianhydrides and conducting thermal dehydration or chemical ring dehydration to form PIs (PI-6A-PI-6E) (Scheme 1). 28 We used high-temperature stage heating to cast these PA acid intermediates into exible lms, which were then converted into tough polymer lms. As an alternative method, PA acid intermediates were dehydrated with a small amount of pyridine or triethylamine, and then the PIs were also formed by chemical ring cyclodehydration reaction. The complete imidization of the PIs is conrmed using the infrared spectroscopy technique. All of the PIs exhibit characteristic imide ring absorption bands near 1680, 1620 (anhydride C-O, stretching), 1180 (imide C-N, stretching) and 740 cm À1 (imide C-N, bending) (shown in Fig. 1S and 2S †). The absence of characteristic absorption bands for amido and carboxylic groups indicates that PIs have been fully imidized. For further research, we compared them with a series of previously reported PIs, PI-6A 0 -PI-6E 0 , which were based on 4,4 0 -diamino-4 00 -(carbazol-9-yl) triphenylamine and dianhydrides A-E (shown in Scheme 2). 29 Table 1 summarizes the solubility of PAAs and PIs in several organic solvents at a concentration of 10 mg mL À1 . It can be concluded that PI samples have excellent solubility, all of which are prepared by the thermal imidization method, and are insoluble in acetonitrile, but have good solubility in other common organic solvents. When compared with the analogous PI-6D 0.87 À À + + + + + + + + + + PI-6E 1.19 À À + À + À + + + + + + a Inherent viscosity of PAAs and PIs measured at a concentration of 0.5 dL g À1 in DMAc at 30 C. b The solubility was determined at a concentration of 10 mg mL À1 . Solubility: + +, soluble at room temperature; + À, partially soluble; À À, insoluble even on heating.
PI-6A 0 -PI-6E 0 , the solubility of the 6 series of PIs has been improved, which may be due to be increased conformational exibility or the introduction of free volume of a exible alkyl backbone in the repeating unit instead of a benzene ring. Tough lms can be obtained by thermal imination of PAA lms.
The thermal stability of PIs in nitrogen was evaluated by thermogravimetric analysis (TGA). Typical TGA curves of PIs (PI-6A-PI-6E) are shown in Fig. 1. All PIs showed good thermal stability. Under nitrogen atmosphere, the T d values of these PIs are recorded in the range of 348-473 C when the weight of these PIs is reduced by 10% (summarized in Table 2). From Table 2, it can be seen that the carbonization residue (carbon yield) of all PIs at 800 C in nitrogen is in the range of 38-47 wt%. The high char yields of these PIs can be due to the presence of a large number of thermally stable aromatic hydrocarbon groups in the polymer structure. As compared to the analogous PI-6A 0 -PI-6E 0 (640-652 C), the introduction of alkyl groups instead of benzene rings into the polymer main chains results in a slight decrease in thermal stability.

Optical properties
The optical properties of the PIs in NMP solution (concentration: 5 Â 10 À5 M) were studied by UV-visible spectrophotometry and PL spectroscopy (Fig. 2). The PIs show maximum absorption at 324-327 nm in NMP solution, while the absorption peaks of the PI lms were at 259-284 nm, which can be mainly due to the p-p* transition of the carbon-carbon double bond in the Cz unit. However, the absorption spectra of PI lms are blue-shied compared to solution spectra. This suggests that the PIs exhibit reduced molecular planarity in the thin-lm state compared to solution. In solid lm, the molecules get closer than in solution due to the D-A charge effect. So we speculate that the solvent will interact with the molecules which would induce a HOMO decrease or LUMO increase resulting in the absorption spectra of PI lm being blue-shied compared to    Fig. 3. PI-6C has the highest uorescence intensity and F PL due to less quenching of the charge transfer between the CzTPA donor and the imide acceptor. The optical properties of the PIs (PI-6A-PI-6E) are summarized in Table 3.

EC properties
The electrochemical behaviors of PIs (PI-6A-PI-6E) were investigated using the cyclic voltammetry (CV) technique. Under an atmosphere of nitrogen, a PI lm electrode was used as the working electrode. The oxidation and reduction values of polymer lms were measured in CH 3 CN, as shown in Fig. 4. Based on the potential reported by Hsiao for oxidation of these compounds, 29 we propose a possible oxidation sequence of redox centers for PIs (PI-6A-PI-6E) (see Fig. 4). Interestingly, the TPA core may be oxidized earlier than the carbazole unit due to the electron donation of the substituent ethyl group. For example, there are double quasi-reversible oxidation redox couples at half-wave potential (E 1/2 ) which is the average potential of the redox couple peaks, with values of 0.72 V and 0.92 V (E onset ¼ 0.59 V) for PI-6A (Fig. 4a). In the oxidation scan of CV of PI-6A, two spikes are observed at E pa ¼ 0.82 V and 1.07 V, respectively. Compared with its parent analog PI-6A 0 (E onset ¼ 0.89 V; E 1/2 ¼ 1.05 V), PI-6A has a lower initial oxidation potential (E onset ¼ 0.59 V; E 1/2 ¼ 0.72 V). Earlier literature 29 had reported that the model compound 9-phenylcarbazole exhibited a quasi-reversible redox wave at E pa ¼ 1.50 V. In the continuous scans, an oxidation wave at E pa ¼ 1.10 V gradually grows, indicative of new species formation. It can be further proved that the redox pair in potential scanning means that CzTPA + participates in a very fast electrochemical reaction, resulting in a new structure that is more easily oxidized than the parent Cz. This theory was rst proposed and proved by Ambrose and colleagues. In the anodization of Cz and various N-substituted Czs, ring-to-ring coupling is the main route. 32 Coincidentally, similar results have been reported in recent publications. 33 Other PIs show the same trend of CV behavior, and the relevant oxidation potentials are summarized in Table 4.
The redox potentials of the various PIs (PI-6A-PI-6E) are summarized in Table 4, and HOMO and LUMO potentials of the PIs are shown in Fig. 5. The HOMO levels of the PIs are evaluated as À4.98 to À5.12 eV and À4.99 to À5.19 eV, calculated   36 the results show that the calculated values agree well with the optical experimental data. These data are used to calculate the LUMO energy level. The low ionization potential indicates that it is easier for ITO electrodes to inject holes into active lms in electronic devices.
To further investigate the redox stability of PI material, CV tests were conducted of the PI lms deposited on ITO electrodes using potential scan between neutral and oxidation states in 0.1 M TBAP/CH 3 CN without monomer at a potential scan rate of  Fig. S3 †). The retained electroactivities were observed to be 94% (PI-6B), 91% (PI-6C), 88% (PI-6D) and 99% (PI-6E) aer 600 cycles. Since the PIs are in direct contact with the electrolyte, degradation reaction occurs under the catalysis of trace oxygen and water. If the electrodes are assembled into an ECD being sealed off from the air, the switch stability will be greatly improved. Overall, the PIs are both very robust and redox stable, making them good candidates for EC applications.
The electrical resistances between the PIs and the electrolyte were further evaluated by electrochemical impedance spectroscopy (EIS) and tted by Zview soware. 37 Fig. 7 shows impendence spectra of PI-6A-PI-6E lms. CPE is the capacitance between the substrate and the PI lm, and R s is the series resistance. Warburg diffusion element (W d ) is a commonly used diffusion circuit element that simulates semi-innite linear diffusion, that is, unlimited diffusion of large planar electrodes. 38-41 W d is difficult to identify because it is always associated with double layer capacitance and charge-transfer resistance. The charge-transfer resistance R ct is the resistance when a current passes through one electron or ion in the system and causes a voltage difference between the other electrons and ions in the system, and the charge transfer can be known by the transfer impedance (it is difficult for electrons and ions to undergo electrochemical reactions at the electrode/electrolyte   interface to transfer to the surface of the active material). 42 Among the above parameters, R ct is the most critical parameter affecting EC performance. PI-6B has the lowest R ct value among the ve polymers, indicating that ions migrate faster between the PI lm and electrolyte. However, the R ct of PI-6D with the highest charge and ion transfer resistance is 20.85 U. The reason for the poor ion transfer capacity may be the poor charge and ion transfer capacity of the PI-6D lm. Table 5 summarizes the data obtained by tting the equivalent circuit. The R ct for conjugated polymers reported in the literature is in the range 15-35 U. The R ct values of PIs in this article are similar to those reported in the literature. 43,47 Optical properties changed aer oxidation by EC experiments. The absorption values are similar to those for the analog PI-6A 0 reported in the literature. PI-6A 0 showed two strong absorption peaks at 296 and 330 nm in the neutral form. Under oxidation (the applied voltage increased from 0.0 V to 1.1 V), there was a new peak at 412 nm, and the near-infrared region expanded from 800 nm to 1100 nm, and the intensity gradually increased. In comparison, the spectral changes of PI-6A in different oxidation states are shown in Fig. 8. In neutral form, at 0 V (vs. Ag/AgCl), PI-6A exhibits a strong absorption peak at approximately 266 nm, representing a characteristic p-p* transition, which is almost transparent. When the PI-6A lm is oxidized with the applied voltage being increased from 0.00 V to 0.80 V, the absorption peak intensity increases gradually at 340 nm. At the same time, a new absorption peak appears at 573 nm, the intensity of which gradually increases. We attribute these spectral changes to the formation of localized CzTPA electronic stability. As the applied potential increases toward 1.10 V, the number of cationic radicals increases gradually. A new strong absorption band is formed in the near-infrared region, and its center position is around 840 nm which is attributed to the formation of cations in the CzTPA segment. The UV-visible absorption changes of PI-6A lms at different potentials are completely reversible which are easily seen by the naked eye. We can observe from Fig. 8 that the PI-6A lm changes from a light yellow transparent neutral state to a brown yellow highly absorbing semi-oxidized state and a deep yellow fully oxidized state. Although the color is basically in the ultraviolet absorption region, but because there is no conjugate channel, there is no strong electron cloud shi, so the color of the lm is almost colorless. Other PIs have similar trends (as shown in Fig. S4 †). Meanwhile, the typical spectral electrochemical and transmission-wavelength application potential correlations of PI-6A-PI-6E lms are shown in Fig. S5. † The square wave potential method was used to study the optical response characteristics of PI (PI-6A-PI-6E) lms. The changes in transmittance of the lms were measured and recorded at the maximum absorption wavelength by periodically converting the voltages in the neutral and oxidation states at certain time intervals. From Fig. 9, it can be concluded that the transmittance of the PI-6A lm at a maximum incident wavelength time interval of 20 s at 573 nm is 55%. In addition, compared with some reported polymers with similar backbones, 44 the PI-6A lm in this paper has higher optical contrast. However, when the time interval is set to 20 s, the fading time of PI-6A from the oxidized state to the reduced state is longer, lasting 10.0 s. The corresponding coloring time from the neutral state to the oxidized state is 7.7 s. Table 6 summarizes the optical contrast, response time, and coloring efficiency of the PI Fig. 7 Impedance spectra of PI-6A-PI-6E films on an ITO-coated glass substrate.  lms at the maximum absorption wavelength in a time interval of 20 s. The response speed of PI-6A lm is similar to that of some other soluble EC PI lms reported in the literature, 45,46 which is enough to be comparable to that of EC conductive polymers.
However, the advantages of absorption contrast and response time make it possible to develop and apply in non-lightemitting display devices. Other PIs (PI-6A-PI-6E) have similar trends (shown in Fig. S6 †).

EFC properties
It is worth noting that PI lms have not only EC behavior but also EFC behavior. As shown in Fig. 10, upon application of a series of positive potentials, PI-6A shows its EFC switch characteristics. Upon being excited at 365 nm, dynamic response behavior was tested through an oxidation step of 0.0 to 1.8 V. When the voltage increases, the PL intensity of PI-6A decreases which is attributed to the increase of the effective PL quencher (CzTPA + ) number in the polymer. At the same time, the intensity of the maximum PL peak of neutral PI-6A at 448 nm decreases signicantly and continuously with the increase of applied potential. When the voltage is as high as 1.8 V, the PL almost disappears and the PL spectrum is close to the baseline. In comparison to PI-6A, uorescence of other PIs is quenched by increasing the voltage above 2.0 V or 1.8 V (shown in Fig. S7 †). The PL of PIs indicates that the uorescence quenching originated from the electrochemical oxidization of the PIs. 47,48  a The specied wavelength of measurement data. b Voltage application time required to reach 90% of maximum transmittance at specied wavelength. c DOD ¼ log(T b /T c ), where T c is the maximum transmittance in colored state and T b the maximum transmittance in faded state. d Q d is the number of charges in or drawn out per unit area, determined from the in situ experiments. e Coloration efficiency is the ratio of the change in the optical absorption of a material to the charge and loss per unit area at a specied wavelength, derived from the equation CE ¼ DOD/Q d .  This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 6992-7003 | 7001

Conclusion
A series of novel PIs with EC and EFC properties containing exible carbazole blocks were synthesized by the traditional two-step method. All of the prepared PI lms have excellent thermal stability. The oxidation potentials of the polymers are signicantly reduced. These PIs also show good electrochemical stability, and retention of electroactivities is observed to be 92% (PI-6A), 94% (PI-6B), 91% (PI-6C), 88% (PI-6D) and 99% (PI-6E) aer 600 cycles. Aer two-stage oxidation, the color changes from yellowish to neutral brown yellow and dark yellow in the oxidized state. In addition, PIs show excellent and unique optical behavior in both solution and lm states, and have different emission colors and PL quantum yields. Therefore, the application and development prospects of prefabricated ECDs can be greatly improved.

Conflicts of interest
There are no conicts to declare.