Viologen-based solution-processable ionic porous polymers for electrochromic applications

Electrochromic porous thin films are promising for applications in smart windows and energy-efficient optical displays. However, their generally poor processing ability and excessive processing times remain grand challenges. Herein, we report the design and convenient synthesis of core-altered N-arylated viologens with aldehyde groups (πV-CHO) as new building blocks to prepare soluble, viologen-embedded ionic porous polymers. We also demonstrate that these polymers can be easily solution-processed by drop-coating to fabricate high-quality electrochromic films with tunable optoelectronic properties in a cost-effective fashion. The prepared films exhibit excellent electrochromic performance, including a low driving voltage (1.2–1.4 V), fast switching times (0.8–1.7 s), great coloration efficiency (73–268 cm2 C−1), remarkably high optical contrast up to 95.6%, long cycling stability, and tunable oxidation and reduction colors. This work sheds important light on a new molecular engineering approach to produce redox-active polymers with combined properties of intrinsic porosity, reversible and tunable redox activity, and solution processability. This provides the materials with an inherently broad utility in a variety of electrochemical devices for energy storage, sensors, and electronic applications.


Introduction
2][3] Transparent EC thin lms with tunable colored and bleached states are a key requirement for achieving critical device performance.Most state-of-the-art electrochromic materials are based on inorganic transition metal oxides (e.g.WO 3 ). 4While these inorganic materials have been demonstrated to operate at low voltages with efficient electrical energy consumption and show high optical contrast, their limited color variation and the high cost of lm fabrication, however, pose great challenges to extend their application scope.Therefore, exploration of new materials for cost-effective electrochromic lm fabrication continues to be at the forefront of EC research.
Organic materials have gained considerable attention for EC applications owing to their structural versatility and optoelectronic tunability, as well as their outstanding processability.To date, various redox-active organic materials, such as smallmolecule viologens and linear conjugated polymers with tunable optoelectronic properties have successfully been developed. 5,6However, their practical application is hampered by long switching speeds (>3 s) and relatively poor stability, due to sluggish ion-and electron-transport during operation.][9][10][11][12] The presence of nanopores in the lms provides effective pathways for efficient ion diffusion and mass transport, making the redox sites more accessible to metal ions and analytes.For instance, Dincȃ et al. have developed lms based on a mesoporous electrochromic metal-organic framework (MOF) that can reversibly switch between transparent and colored states. 11Bein and coworkers reported a fully organic, porous covalent-organic framework (COF) lm with high coloration efficiency and short switching time. 9][9][10][11][12] These possessing steps add increased complexity and cost to the fabrication process, and cause difficulties with regard to reproduction or scale-up.Hence, development of solution-processible, redox-active porous polymers is highly desirable for ease and large-scale fabrication of electrochromic lms.
Towards this goal, the rational design of redox-active building blocks and development of efficient reaction conditions are very important.So far, most COFs and MOFs are constructed from neutral building blocks and the resulting materials are insoluble in most solvents due to the strong interlayer p-p interaction.This poses a signicant challenge for the design of suitable materials.Alleviating the strong interlayer interactions has been demonstrated as an effective strategy to improve the stability of two-dimensional porous polymers in solution by electrostatic charge repulsion.For instance, Jiang and coworkers report a versatile synthesis of highly soluble, charged COF nanosheets from a single solution by using dynamic covalent bonds. 13The obtained COF solutions enable facile casting of thin lms for proton exchange.Such emerging ionic porous polymers (IPPs) have attracted increasing attention and show promising utility in ion exchange membranes, energy-storage, and -conversion devices. 14However, the limited availability of suitable redox-active ionic building blocks restricts their broad exploration as functional electrochromic materials.
Viologens have attracted our attention, due to their cationic skeletons and promising electrochromic properties. 5,6This scaffold can typically undergo two reversible reduction processes at low potentials that are accompanied by intense color changes.6][17][18][19][20] One obvious weakness of molecular viologen-based EC devices is the necessity of a solution electrolyte, that potentially causes problematic leakage during longterm cycling.We anticipated that functionalization of viologens with suitable reactive groups would enable the construction of solution-processible IPPs for electrochromic lms with improved performance at low cost.
Herein, we report a convenient synthesis toward a new class of aldehyde-functionalized N-arylated viologen building blocks (pV-CHO, Scheme 1).It should be noted that it is inherently very challenging to incorporate aldehyde groups within electron-decient viologen building blocks, and only one, fairly complex multi-step synthesis has been reported to date.Our new procedure, on the other hand, is simple and versatile, allowing us to access to an entire library of core-extended viologens with tunable optoelectronic properties.Furthermore, we utilize dynamic covalent bonding to build a series of viologen-integrated redox-active polymers, that can be readily solution-processed into high-quality EC lms.These obtained lms exhibit promising electrochromic properties, such as low driving voltage, fast switching times, great coloration efficiency, remarkably high optical contrast, long cycling stability, and color controls.We believe that this solution-process approach toward viologen-embedded polymers opens a new pathway to functional lms for diverse purposes.

Design and synthesis of building blocks
2][23][24][25][26][27] However, this synthesis involves the utilization of the classic Zincke reaction and tedious multistep protection-deprotection sequence (Scheme 1a).To the best of our knowledge, CHO-functionalized, N-arylated viologens with core-extended skeletons have not been reported.The lack of examples could be attributed to the fact that N-arylated electron-decient "p-extended viologens" are difficult to synthesize by the Zincke route. 159][30] This approach provides an accessible means to prepare p-extended viologens with electron-decient N-aryl substitutions.Based on the previous work, we rst attempted the quaternization of 4,4 0 -bipyridine with symmetric bis(4-formylphenyl)iodonium tetrauoroborate (M1) in the presence of Cu(OAc) 2 as the catalyst in DMF at 100 °C for 24 h (Scheme 1b).However, the desired product was not obtained.We ascribe this observation to the reduced reactivity of the electron-efficient iodine(III) reagent and its premature decomposition in solution. 15o overcome the decomposition challenge, we then focused on utilizing the asymmetric diaryliodonium salt [Mes-I-PhCHO] BF 4 (M2) (Scheme 1b). 31The introduction of an electrondonating mesityl group was expected to increase the stability of diaryliodonium reagent.Moreover, previous literature suggests that the bulky mesityl group could serve as a "dummy ligand" toward a chemoselective reaction. 29,32To our satisfaction, the reaction of 4,4 0 -bipyridine with the asymmetric M2 in the presence of Cu(OAc) 2 immediately provided the desired product V-CHO in quantitative yield and high purity.Conveniently, due to the high reaction efficiency, no tedious column chromatography is needed, and the product can be isolated by direct precipitation into diethyl ether.Fig. S1 † illustrates the reaction mechanism, involving the typical mechanism for a Cucatalyzed N-arylation by diaryliodonium salts through Cu III species, according to Gao. 29 The side product 2,4,6-trimethyliodobenzene (MesI) was identied by single-crystal X-ray diffractometry (Fig. S1 †) and also characterized by 1 H NMR spectroscopy (Fig. S2 †), supporting the proposed mechanism.Subsequently, we tested the scope of the N-arylation with a range of p-extended pyridines bridged with different linkers, such as anthracene (An), the electron-withdrawing thiazolothiazole (TTz) and the electron-donating bithiophene (Bt).The reactivities are unaffected by the aromatic linker.The Cu(II)catalyzed process transfers the PhCHO functional groups to the N-atoms with excellent chemoselectivity and in excellent yields (95-100%).Even for the synthesis of TTzV-CHO containing the electron-decient TTz linker, the reaction is completed within 24 hours, indicating its fast reaction kinetics.All compounds were characterized by 1 H and 13 C NMR spectroscopy, as well as high-resolution mass spectrometry (details in the ESI †).Overall, we provide an unprecedented, straightforward and valuable approach for synthesis of a family of CHO-functionalized Narylated p-extended viologens in very high yield.

Electrochemical and photophysical properties of pV-CHO
Cyclic voltammetry (CV) experiments were carried out to investigate the electrochemical properties of the pV-CHO building blocks.The CV proles, reduction potentials and reversibility of the redox events were found to depend strongly on the central viologen cores (Fig. 1a).The results are summarized in Table S1.† V-CHO shows two successive reversible reduction events in DMF (E red,1 = −0.59V and E red,2 = −0.76V, vs. Fc/Fc + ).By introducing bridging p-linkers between the two terminal pyridinium groups, the other three linear pV-CHO species exhibit single-step two-electron reduction processes, due to the loss of electronic communication between the two electroactive sites. 32,33The reduction potentials follow the trend BtV-CHO (E red = −0.92V vs. Fc/Fc + ) < AnV-CHO (E red = −0.85V vs. Fc/Fc + ) < TTzV-CHO (E red = −0.60V vs. Fc/Fc + ).This trend is consistent with the electron-accepting properties of the plinkers.The redox signals of pV-CHO are retained very well, when using various scanning speeds from 100 mV s −1 to 1000 mV s −1 , indicating excellent electrochemical reversibility (Fig. S3 †).When compared to their N,N 0 -dimethylated counterparts (pV-Me: V-Me, AnV-Me, BtV-Me and TTzV-Me, structures shown in Fig. S4 †), the electron injection in pV-CHO is facilitated by 0.3-0.4V, as evidenced by higher reduction potentials (Fig. 1a and Table S1 †).These results demonstrate that the new pV-CHO building blocks display strong electronacceptor character, which will be highly benecial for electrochromic applications with low-driving voltages.
The photophysical properties of pV-CHO can also effectively be tuned by the central p linkers (Fig. 1b).Compared to V-CHO (l max = 300 nm), the extended conjugated species show lowerenergy absorption bands (AnV-CHO: l max = 427 nm; BtV-CHO: l max = 463 nm; TTzV-CHO: l max = 416 nm).The electrondonating bithiophene linker has the strongest inuence on the optical properties with a red shi of Dl max = 163 nm, and its shallow absorption band was attributed to intramolecular charge transfer.The HOMO-LUMO energy gaps (E g ) were estimated from the absorption onset of UV-vis absorption spectra (Fig. S4 †) to be 3.44 eV (V-CHO) > 2.66 eV (TTzV-CHO) > 2.51 eV (AnV-CHO) > 2.34 eV (BtV-CHO).Meanwhile, the N-arylated species display an obvious red shi in their absorption maxima by 20-50 nm and smaller HOMO-LUMO energy gap than pV-Me (Fig. 1c and S5 †), as a result of the peripheral extension of conjugation.

Synthesis and characterization of IPPs
With the new series of pV-CHO in hand, the reactivity of CHOgroups with primary amine and acylhydrazine were then tested (details in the ESI †).Reaction with p-toluidine or benzohydrazide leads to the corresponding imine or acylhydrazone species in high yield, respectively.Notably, condensation reactions between linear pV-CHO and benzohydrazide with a molar ratio of 1 : 2 is completed within several hours, even at room temperature, supporting the high efficiency of the process (Fig. S6-S8 †).
Encouraged by the facile formation of stable hydrazones, we exploited the preparation of IPPs by combination of pV-CHO with a trigonal benzene-1,3,5-tricarbohydrazide (BTH), as shown in Fig. 2a.Upon mixing the linear pV-CHO and BTH precursors in DMSO at a molar ratio of 3 : 2, the solution color immediately turned darker, indicating a rapid reaction between these monomers.The uidity of the solution was found to depend on the overall concentration of the two precursors pV-CHO and BTH.At low concentration (2 mg mL −1 based on pV-CHO), the obtained solutions of P pV-BTH retain homogeneity (Fig. 2b) even over several weeks.At higher concentrations (10 mg mL −1 based on pV-CHO), transparent organogels formed aer several hours, even at room temperature (Fig. 2c).Scanning Electron Microscopy (SEM) analysis of the four P pV- BTH xerogels clearly reveals 2D morphologies (Fig. 2c).We found that the formed organogels do not revert back to the solution state even upon heating or diluting the mixture.Importantly, gelation was not observed at low concentration (2 mg mL −1 based on pV-CHO) even aer one month.The above concentration-dependent sol/gel states with different uidity and solubility can be attributed to the formation of oligomers (or molecular cages) at low concentration and polymers with higher degree of polymerization (DP) at high concentration.We tentatively attribute the transformation from sol to gel to the reconstruction of the dynamic acylhydrazone bond that is facilitated by increased concentration.
To verify the formation of the above species, the reactivity was monitored using 1 H NMR by combining the pV-CHO and BTH precursors in d6-DMSO at a molar ratio of 3 : 2 aer heating at 50 °C for 24 h.At low concentration of pV-CHO (2 mg mL −1 ), a new set of sharp resonances was observed for P pV-BTH .As shown in Fig. 2d, the characteristic 1 H NMR peaks of CHO at 10.22 ppm and NH 2 at 4.56 ppm from the two precursors (V-CHO and BTH) completely disappear.Instead, a new peak at 8.65 ppm resulting from the of CH]N group of P V-BTH appears, along with a pronounced downeld shi of the NH group from 9.84 ppm (for BTH) to 12.4 ppm (for P V-BTH ).Similar studies on the other three P pV-BTH polymers (Fig. S9-S12 †) support the high efficiency of the polymerization process.Moreover, the 1 H NMR spectra of P pV-BTH are consistent with their small molecular models (Fig. S6-S8 †).The above results suggest a complete reaction between two precursors and the well-resolved NMR signals suggest the formation of oligomers or molecular cage, based on previous research. 33,34In contrast, the 1 H NMR spectrum of the organogel in d6-DMSO exhibits broad and nondiscernable peaks, probably as a result of formation of a polymer with higher DP and stronger p-p interactions (Fig. S13 †).The identity of the P pV-BTH polymers was also conrmed by FT-IR.As shown in Fig. 2e, the FT-IR spectra of the four P V-BTH show the disappearance of the N-H stretching band of BTH at 3290 cm −1 and stretching vibration of aldehydes of V-CHO at ca. 1700 cm −1 , supporting the completeness of the reaction.New peaks at ca. 1630 cm −1 (C]N) and 1660 cm −1 (C]O) belonging to the acylhydrazone bonds were observed instead.Similar changes were also observed in the IR spectra for other three P pV- BTH polymers (Fig. S14 †).
Overall, the pV-CHO species are highly valuable building blocks.The presence of the aldehyde groups allows reaction with BTH to efficiently form dynamic acylhydrazone bonds under mild conditions.4][35][36] In contrast, at high concentration, the reversible cleavage/ formation of dynamic bonds forms polymers with high DP.This supports the above concentration-dependent sol and gel states.][9][10][11][12] Since no aliphatic chains commonly used to increase the solubility are present, we attribute the high solubility of the material to the presence of cationic charges that provide the colloidal stability through electrostatic repulsion.

Fabrication of electrochromic lms
As shown in Fig. 3a, the EC lms were prepared by drop-casting a solution of oligomeric P pV-BTH (2 mg mL −1 in DMSO) onto a conductive uorine-doped tin oxide (FTO) electrode, followed by solvent evaporation under heating.Upon slow evaporation, a viscous gel layer was observed on the FTO surface.This phenomenon could be attributed to the formation of crosslinked polymer with high DP, similar to the above concentration-dependent sol and gel states in solution.This strategy also allows easy fabrication of thin-lms at large scale (10 cm × 10 cm, Fig. S15 †).Although such solution-based fabrication of a high-quality lm by the surface sol-gel process has been reported on inorganic metal oxide, 37 it is unprecedented for organic polymer systems.
To our satisfaction, the obtained lms were found to be transparent, strongly adhere to the FTO surface, and are resistant to delamination in the presence of solvents (Fig. 3b).As conrmed by SEM (Fig. 3c-f), the prepared lms are highly uniform and without any defects.Their thickness was measured to be 400-500 nm, when the areal mass weights were 0.1 mg cm −2 (Fig. S16 †).Water contact angles for the four P pV-BTH lms were measured to be 54-76°, supporting their hydrophilic character (Fig. S17 †).X-ray diffraction (XRD) patterns of all the lms show broad peaks at 20-50°, indicating some degree of amorphous morphology (Fig. S18 †), and energy dispersive X-ray (EDX) elemental mapping showed uniform distribution for the element composition over the entire lms (Fig. S19-S21 †).Compared to pV-CHO, the UV-vis spectra of the P pV-BTH lms generally exhibit obvious red shis, consistent with an extended network structure (Fig. S22 †).High-resolution TEM (HRTEM) analysis of four P pV-BTH lms reveals the presence of nanopores (Fig. S23 †).Brunauer-Emmett-Teller (BET) surface areas of P V-BTH lm CONs and COTs were found to be less than 1 m 2 g −1 (Fig. S24 †).[43]

Electrochromic properties of P V-BTH lms
The electrochromic properties of the FTO-supported P V-BTH lm were then evaluated in a typical three-electrode system for the electrochemical and spectroelectrochemical experiments (Fig. 4a).Cyclic voltammetry (CV) of the P V-BTH lm was performed in 0.1 M aqueous LiCl electrolyte with Pt mesh as counter electrode and Ag/AgCl as reference electrode (Fig. 4b).The P V-BTH lm showed two reversible reduction peaks at −0.09 and −0.50 V (vs.Ag/AgCl) over a potential range of 0.3 to −1.0 V, leading to the formation of the radial cation (Vc + ) and neutral species (V), respectively.
It is important to note that the LiClO 4 /propylene carbonate (PC) electrolyte solution, commonly used for 2D COF lms with neutral skeleton, is not suitable for our materials.As shown in Fig. S25, † the CV of the P V-BTH lm exhibits a broad redox pair at −1.11/0.35V with a large peak-to-peak separation (1.45 V), indicating high polarization, probably due to the slow diffusion of Li + through the lm pores in the organic solvent.We surmise that in an aqueous electrolyte, Li + ions could efficiently pass through the hydrophilic pores of the lm to access the cationic viologen moieties for the redox reactions more effectively.
To better understand the electrochemical kinetics, the CVs of the P V-BTH lm at different scan rates were recorded.Upon increasing the scan rates from 10 to 100 mV s −1 , the current response also increases, and all multiple redox proles retain very well, supporting desirable electrochemical stability.According to the power law (i = av b ), where i represents the current of the CV prole, v is the sweep rate, and a and b are adjustable parameters. 44It has been suggested that the chargestorage process is mainly dependent on the b value.If b equals 0.5, the process can be considered faradaic, while if b has a value of 1, the process is capacitive.As shown in Fig. 4c, the linear t of log(v) and log(i) reveals slopes in the range of 0.8-1.0  for b of both the anodic and cathodic peaks that indicates capacitance-dominant kinetics for the electrochromic chemistry.This result supports fast ion transport that benets from the porous structure of the lm.
Electrochemical reduction of the P V-BTH lm causes a gradual color change from initially pale yellow (V 2+ ) to brown (Vc + ), and then to dark grey/black (V), as shown in Fig. 4d.The corresponding change in the UV-vis spectra upon stepwise reduction were studied by in situ spectroelectrochemistry.As shown in Fig. 4e and f, the pristine P V-BTH lm is pale yellow and transparent, exhibiting an intense absorption peak at 333 nm.Applying potentials from 0 V to −0.4 V leads to the gradual formation of intense peaks at 483, 685 and 773 nm.During the process, a clear isosbestic point was observed at 410 nm, indicating the clean and gradual generation of a single new species.This process corresponds to the rst reduction of the viologen moiety, leading to the formation of radical cation (Vc + ).When the applied potential was further adjusted from −0.4 V to −0.8 V, an intense new band emerged at around 610 nm, while the characteristic peaks for the radical cation signicantly decrease.These spectral changes can be attributed to the second reduction of viologen, forming the neutral species (V).Importantly, the electrochromic reduction is reversible, and the initial P V-BTH spectrum can be recovered by reversing the applied voltage back to +0.4 V.
These changes are also clearly observed in the corresponding transmission spectra (Fig. 4g and h).By switching the voltages between +0.4 V and −0.8 V (vs.Ag/AgCl), the initial contrast ratios (DT%) between yellow and dark states was determined to be 70.0%at 600 nm.This value only dropped only by ca.14% aer 1000 cycles, suggesting excellent stability of the EC P V-BTH lm.The response time for the switching is determined by DT% experiments when the contrast ratio reaches over 90% of its maximum between bleached and colored states.The coloration (t c ) and bleaching (t b ) time of P V-BTH lm at 600 nm were calculated to be 1.4 s and 1.7 s, respectively (Fig. 5j).The coloration efficiency (CE) at 600 nm was calculated to be 73 cm 2 C −1 , respectively, according to the equation.
where Q is the charge density.

Dependence of the electrochromism on the viologen skeleton
The electrochromic properties can be readily tailored by the viologen skeleton.In contrast to the P V-BTH lm, the other three P pV-BTH lms incorporating TTzV, BtV and AnV exhibit one-step two-electron reduction peaks under aqueous conditions.8][19] As shown in Fig. 5, S26 and S27, † the redox couples for P TTzV-BTH and P BtV-BTH were determined to be −0.68/0.24V and −0.64/−0.43V (vs.Ag/AgCl) at 20 mV s −1 .The b values of both anodic and cathodic peaks were determined to be in the range of 0.83-1.0,indicating capacitance-dominant kinetics of the electrochromic chemistry here as well.Upon reduction, P TTzV-BTH and P BtV-BTH lms also exhibit dramatic color changes.However, due to the large steric effects from bulky anthracene linker, the radical species cannot be properly delocalized throughout the anthracene moiety, and hence, the no obvious electrochromic behavior was observed for the P AnV-BTH lm (Fig. S28 †).
For P TTzV-BTH (Fig. 5a-d), three reversible color states, yellow (TTzV 2+ ), green (TTzVc + ) and blue (TTzV), are observed during electrochemical cycling and by the spectroelectrochemistry.The rst reduction from TTzV 2+ to TTzVc + by applying potentials from +0.6 V to −0.2 V, leads to the formation of new absorption bands at 609 and 677 nm.Upon further reduction (−0.2 V to −0.7 V), a strong absorption band appears at 820 nm, characteristic of the TTzV neutral species.It should be noted that the characteristic peaks of TTzVc + also continue to increase during further reduction, due to the closely spaced two-electron reduction processes that essentially occur in parallel during the spectroelectrochemistry experiments.
For the P BtV-BTH lm (Fig. 5e-h), formation of the reduced species is accompanied by a color change from the initial red color, to deep green and then to dark grey/black.Reduction to the radical cation gives rise to a new absorption band in the 600-950 nm range that continues to grow with increasing negative voltages, supporting the continued presence of the radical cation that is likely re-generated during the process via comproportionation.
The EC properties of the three P pV-BTH lms are summarized in Table 1.All lms display fast response times (coloration and bleaching) of less than 2 s.The two lms of coreextended viologen-based IPPs display improved DT%, cycling stability, and high color efficiency.The P TTzV-BTH and P BtV-BTH lms have initial contrast ratios (DT%) of 89.8% and 95.6%.These values were reduced by only 6% and 7% aer 1000 consecutive on-off switching cycles, demonstrating their excellent long-term cycling stability.Their outstanding cycling performance is attributed to the non-uidity of viologen in rigid lm that reduces the possibility of dimerization.The color efficiencies of the EC P TTzV-BTH and P BtV-BTH lms were 268 and 235 cm 2 C −1 , a three-fold increase over that of P V-BTH (73 cm 2 C −1 ).The improved color efficiencies of P TTzV- BTH and P BtV-BTH lms can be partially attributed to the reduction-induced large spectra modulation, as a result of large conjugated viologen centers.We also found that P TTzV- BTH and P BtV-BTH lms had improved electronic conductivity over that of the P V-BTH lm with shorter conjugated backbone.As shown in the electrochemical impedance spectroscopy (EIS), the P TTzV-BTH and P BtV-BTH lms have smaller charge transfer resistance (Fig. S29 †).Overall, we demonstrate a class of easily processible lms that exhibit excellent EC performances that are superior to those of the state-of-the-art 2D COFs and comparable with many reported viologen-based EC materials (Table S2 †). 5 These foundational results indicate that there is plenty of room to further improve the EC performance, by tuning the central viologen cores and the amine linker.

Conclusions
In summary, we report a novel design strategy to effectively fabricate redox-active 2D porous polymers, involving a convenient synthesis of aldehyde-functionalized N-arylated viologens (pV-CHO) with tunable chemical structures, and the use of dynamic covalent acylhydrazone bonds that offer fast reaction times and provide self-healing.Benetting from a "chargeinduced dispersion", these 2D ionic porous polymers have excellent stability in DMSO, and importantly, can be readily solution-processed into high-quality EC lms.The lms exhibit low driving voltages (1.2-1.4V), fast response times (0.8-1.7 s), excellent coloration efficiencies of 73-268 cm 2 C −1 , remarkably high optical contrasts up to 95.6%, and tunable oxidation and reduction colors dependent on their respective viologen cores.We believe that the solution processability, tunable redox and photophysical property as well as intrinsic porosity offered by these IPPs now effectively addresses some of most challenging problems, such as cost factor, large-scale, and simple fabrication in electrochromic applications.This research may also inspire the development of advanced materials for energy storage and photonic devices.

Fig. 1
Fig. 1 Electrochemical and photophysical properties of pV-CHO and pV-Me.(a) CV and (b), (c) normalized UV-vis spectra of the pV-CHO series and their N-methylated congeners pV-Me.CVs were conducted in DMF solution (c = 1 mM) with 0.1 M TBAPF 6 as the electrolyte.UV-vis spectra were measured in CH 3 CN.

Fig. 2
Fig. 2 Characterization of P pV-BTH .(a) Synthesis and structure of P pV-BTH .(b) Pictures of four P pV-BTH solutions in DMSO (2 mg mL −1 based on pV-CHO).(c) SEM images of xerogels based on P V-BTH , P BtV-BTH , P TTzV-BTH and P AnV-BTH formed in DMSO (10 mg mL −1 based on pV-CHO).Insets show the photographs of the corresponding P pV-BTH organogels.(d) 1 H NMR and (e) IR spectra of V-CHO, BTH and P V-BTH .

Fig. 3
Fig. 3 Fabrication of P pV-BTH films.(a) Schematic illustration of the preparation of the P pV-BTH electrochromic thin film.(b) Pictures and (c)-(f) SEM images of the four prepared thin films.

Fig. 4
Fig. 4 Electrochromic characterization of P V-BTH film.(a) Schematic illustration of the electrochemical and in situ spectroelectrochemical measurements.(b) CV curves of the P V-BTH film measured in 0.1 M LiCl aqueous electrolyte at scan rates of 10, 20, 40, 60, 80, and 100 mV s −1 .(c) Log(i) versus log(v) plots to determine the b values of different peaks.(d) Pictures showing the reversible color changes of P V-BTH EC film.(e), (f) UV-vis and (g), (h) transmittance spectra changes of the P V-BTH film in 0.1 M LiCl aqueous electrolyte recorded during the spectroelectrochemistry.The applied potentials are referenced to Ag/AgCl.(i) Plots of transmittance at 600 nm and current vs. time with alternating voltage of −0.8 V and +0.4 V. (j) Coloration/bleaching transmission spectrum of the P V-BTH film at 600 nm with an applied voltage of −0.8 V and +0.4 V. (k) Plots of the optical density vs. charge density and the slope as coloration efficiency.(l) Cycling stability of the P V-BTH film between −0.8 V and +0.4 V.

Fig. 5
Fig. 5 Electrochromic characterization of P TTzV-BTH and P BtV-BTH films.(a) CV curves of the P TTzV-BTH films measured in 0.1 M aqueous LiCl electrolyte at scan rates of 10, 20, 40, 60, 80, and 100 mV s −1 .(b) UV-vis spectra changes of an P TTzV-BTH film in 0.1 M aqueous LiCl electrolyte recorded during spectroelectrochemistry. (c) Pictures showing the reversible color changes of an P TTzV-BTH EC film.(d) Cycling stability of an P TTzV-BTH film between −0.7 V and +0.6 V. (e) CVs of an P BtV-BTH film measured in 0.1 M aqueous LiCl electrolyte at scan rates of 10, 20, 40, 60, 80, and 100 mV s −1 .(f) UV-vis spectral changes of an P BtV-BTH film in 0.1 M aqueous LiCl electrolyte recorded during spectroelectrochemistry.(g) Pictures showing the reversible color changes of the EC P BtV-BTH film.(h) Cycling stability of an P BtV-BTH film between −1.0 V and +0.4 V.

Table 1
ECD performance of P pV-BTH films a Color at oxidized state.b Color at reduced state.c Bleaching (V b ) and coloration potential (V c ). d Bleaching (t b ) and coloration time (t c ). e Transmittance change during coloration and bleaching process.f Coloration efficiency.