Immobilized polyazomethines containing triphenylamine groups on ITO: synthesis and acidochromic, electrochemical, electrochromic and photoelectronic properties

Abstract

A new series of polyazomethines (PAMs) containing triphenylamine (TPA) as easily-oxidized units were synthesized as highly soluble materials with electrochromism. 2,5-Bis(hexyloxy)terephthaldehyde and four different kinds of diamine were condensed to construct the PAMs under benign conditions with M̄_n values of 21 700–25 400 g mol⁻¹. The color of the PAM solution changed reversibly between light yellow and navy blue with the increase and decrease of pH. The PAMs with a high number of alkyl groups, which could dissolve facilely in many common organic solvents, were grafted onto ITO through siloxane linkages to be cast into tough films. During the electrochemical reoxidation process, the films showed color switching with high coloration efficiency. The HOMO and LUMO levels were determined via cyclic voltammograms combined with the UV-visible spectrum as −5.14 to −4.79 eV and −2.81 to −2.54 eV, respectively, which can be utilized in the hole transporting layer in organic light-emitting diodes (OLEDs) or solar cell (SC) devices.

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1. Introduction

Electrochromic polymers (ECPs), which can be modified with a low-redox potential to attain a varying color state,¹ have been extensively investigated for many potential applications such as smart windows,² electrochromic eyewear,³ electronic paper⁴ and displays.⁵ In addition to the synthetic accessibility of ECPs, the modification of the main architecture which is braced by multichromic monomers (e.g. pyrrole, thiophene and aniline) would pave the way to widening the use of electrochromes in the expansive electrochemical field.⁶ Recently, an amounts of noteworthy research has focused on tuning the color into even more colorful regions. Reynolds used three colored-to-transmissive switching electrochromic polymers to blend into multicomponent ECP mixtures which cooperated to estimate the color properties.⁷ Skene prepared a series of PAMs which were found to undergo an oxidation process with the multichromic process from the neutral to oxidized state.⁸

PAMs are interesting electrochromic materials due to their advantageous synthesis and straightforward electronic withdrawing under facile conditions. Furthermore, the internal sp² hybridization of the nitrogen atom in the main-chain makes the PAMs become an attractive material for electrochromic devices. This configuration has led to stronger absorption spectra in the visible origin with obvious color changes in the redox process. The other properties of PAMs such as well-documented excellent thermal, mechanical, electronic, optical, optoelectronic, and fiber forming properties also expand the additionally developing prospect.⁹

Owing to the strong electron-donating and hole-transporting/injecting properties of highly electron-rich and redox-active compounds, PAMs act as useful optic-electronic materials like in xerography, light-emitting diodes, and solar cells. Among many building blocks available for constructing electrochromic polymers, rotating TPAs are unique molecules possessing particular functions such as outstanding redox activity, fluorescence and ferromagnetism.¹⁰ These are likely due to their facile deionization, stable oxidizability of the nitrogen atom, and transportability of positive charge centers via radical cation species. Moreover, the propeller TPA is inclined to confer active solubility to polymers. In recent years, Hsiao has prepared new polyimides for manifesting stable electrochromes of TPA, with a light color neutral form, which switched color from red-violet/pale green for the reduced form to blue for the oxidized form.¹¹ Our groups have studied hole transportation and electrochromic properties of PAMs containing TPAs in OLEDs and electrochemical materials.¹²

However, the technological applications of most rigid PAMs are limited by processing difficulties because of the confined solubility in most organic solvents. Many approaches have been adopted to handle the problem, including introducing various substituted groups like bulk monomers containing certain heterocyclic units (cardo-structure, tetraphenylethene) in the main chain,¹³ and forming complexes with GaCl₃ or β-CD.¹⁴ Linking polymers to substrates with silicon oxide is an efficiently viable route to modulate polymer layers with a terminal chain combined onto the modified surface.¹⁵ The polymer films frequently fall off during the electrochemical process and can adopt this strong polymer/substrate conformation to improve the electrochemical and electrochromic properties.

In the present work, we synthesized four kinds of PAM from different TPA derivatives and discussed the effect of structure of the naphthalene and benzene group, biphenyl and phenyl group on the absorption and electrochemical properties, to gain insight into how to control the energy gap by regulating structures. The introduction of the naphthyl ring enhances the π-electron delocalization which significantly affects the electronic structure. And the replacement of the phenyl group with the biphenyl group results in the change of the space contracture and the reduction of steric hindrance. The prepared PAMs exhibited excellent solubility and were sensitive to pH. In order to prevent the PAM film peeling off from the ITO glass, 3-triethoxy silypropylamine (APTES) was used as a bridge to connect the PAM and ITO to demonstrate the stable electrochemical behavior and electrochromism of the TPA-based PAM film when stimulated by electrooxidation.

2. Experimental

2.1 Materials

N,N′-Di(1-naphthyl)benzidine, N,N′-di(2-naphthyl)-1,4-phenenylenediamine were bought from Qinhuangdao Bright Chemical Co. Ltd; p-toluidine, N,N′-diphenyl-1,4-phenylenediamine were purchased from TCI Co.; 2,5-bis(hexyloxy)terephthaldehyde was bought from Aldrich–Sigma Co.; Pd/C (10%) was purchased from Acros; tetrahydrofuran (THF), N,N′-diphenyl-1,4-phenylenediamine (DMAc), dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF) and toluene were supplied by Sinopharm Chemical Reagent Co.; Lithium perchlorate (LiClO₄) was dried under vacuum at 120 °C for 36 h. N,N′-Bis(4-aminophenyl)-N,N′-di(1-naphthyl)benzidine (diamine 1), N,N′-bis(4-aminophenyl)-N,N′-diphenylbenzidine (diamine 2), N,N′-bis(4-aminophenyl)-N,N′-di(2-naphthyl)-1,4-phenylenediamine (diamine 3) and N,N′-bis(4-aminophenyl)-N,N′-diphenyl-1,4-phenylenediamine (diamine 4) were prepared by Ullman reaction according to the method described in the literature.¹⁶

2.2 Synthesis of polymers

The synthesis of PAM1 is used as an example to illustrate the general synthetic procedure. To a three necked 50 mL glass reactor fitted with a magnetic stirrer, Dean–Stark trap and reflux condenser, 0.201 g (0.32 × 10⁻³ mol) of N,N′-bis(4-aminophenyl)-N,N′-di(1-naphthyl)benzidine was dissolved in DMSO following by adding 0.112 g (0.34 × 10⁻³ mol) of 2,5-bis(hexyloxy)terephthaldehyde under nitrogen. Then, the mixture solution of dialdehyde and diamine in 10 mL of DMSO and 2 mL of toluene was stirred at 120 °C for 10 h. After the reaction, the obtained polymer solution was poured slowly into 200 mL of ice water. The precipitate was collected by filtration, and washed thoroughly with hot methanol in Soxhlet apparatus for 48 h, then was dried under vacuum at 70 °C overnight. Yield: 0.225 g, 75.4%, yellow.

FTIR (KBr, cm⁻¹): 1683(–HC=O), 1619 (CH=N), ¹H NMR (400 MHz, CDCl₃, ppm): 10.55 (s, –CH=O end group), 8.98–9.02 (d, –CH=N–), 7.81–8.01 (m, aromatic ring of benzene –CH=N–), 7.04–7.56 (m, aromatic ring of triphenylamine), 4.06–4.21 (m, –OCH₂–), 0.82–1.91 (m, methylene and terminal methyl).

The other polymers were prepared by an analogous procedure shown in Scheme 1.

Scheme 1 Synthetic routes for PAM1–4 with different diamines.

Synthesis of PAM2. Yield: 0.286 g, 80.1%, yellow. FTIR (KBr, cm⁻¹): 1683 (–HC=O), 1618 (CH=N), ¹H NMR (400 MHz, CDCl₃, ppm): 10.56 (s, –HC=O end group), 9.02–9.04 (s, –CH=N–), 7.66–7.89 (m, aromatic ring of benzene –CH=N–), 7.13–7.57 (m, aromatic ring of triphenylamine), 4.03–4.23 (m, –OCH₂–), 0.80–1.97 (m, methylene and terminal methyl).

Synthesis of PAM3. Yield: 0.301 g, 81.6%, red. FTIR (KBr, cm⁻¹): 1682 (–HC=O), 1616 (CH=N), ¹H NMR (400 MHz, CDCl₃, ppm): 10.54 (s, –CH=O end group), 8.99–9.03 (d, –CH=N–), 7.66–7.89 (m, aromatic ring of benzene –CH=N–), 7.13–7.57 (m, aromatic ring of triphenylamine), 4.03–4.23, (m –OCH₂–), 0.82–1.93 (m, methylene and terminal methyl).

Synthesis of PAM4. Yield: 0.234 g, 74.6%, deep red. FTIR (KBr, cm⁻¹): 1682(–HC=O) 1618 (CH=N), ¹H NMR (400 MHz, CDCl₃, ppm): 10.56 (s, –HC=O end group), 8.96–9.01 (d, –CH=N–), 7.83–8.05 (m, aromatic ring of benzene –CH=N–), 6.95–7.49 (m, aromatic ring of triphenylamine), 4.03–4.16, (m –OCH₂–), 0.83–1.92 (m, methylene and terminal methyl).

2.3 Measurements

The obtained PAMs were characterized using the following techniques: ¹H NMR spectra were recorded on a Bruker AC-400 MHz spectrometer in CDCl₃, using tetramethylsilane as an internal reference. Infrared spectra were measured on a PerkinElmer Spectrum 100 Model FT-IR spectrometer. Ultraviolet-visible (UV-vis) spectra of the polymer films were determined on a UV-3600 (Shimadzu). Thermogravimetric analysis (TGA) was conducted with a PerkinElmer Pyris 6 TGA. Experiments were carried out on about 6–8 mg powder samples heated at a heating rate of 10 °C min⁻¹ in flowing nitrogen (flow rate = 10 cm³ min⁻¹). Number-average (M̄_n) and weight-average molecular weight (M̄_w) were obtained via gel permeation chromatography (GPC) analysis on a Malvern instrument connected with one refractive index detector (Viscotek-VE3580-RI-DETECTOR) by using a polymer in THF solution at a flow rate of 1.0 mL min⁻¹ at 35 °C and calibrated with polystyrene standards. Cyclic voltammetry (CV) measurements were conducted on a CH Instruments 660 A electrochemical analyzer at a scan rate of 50 mV s⁻¹ in 0.1 mol L⁻¹ LiClO₄/CH₃CN under a nitrogen atmosphere by using PAM films spun coated on an ITO-coated glass slide as the working electrode, a Pt flake and an Ag/AgCl electrode as the counter electrode and quasi-reference electrode respectively, and calibrated against the ferrocene/ferrocenium (Fc/Fc⁺) redox couple by assuming the absolute energy level of Fc/Fc⁺ as −4.80 eV to vacuum. The highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) energy levels of the PAMs were calculated by the onsets of oxidation potentials of the films and the onsets of the maximum peak in the UV-visible spectra.¹⁷ For the electrochromic investigation, PAM films were spun coated on ITO-coated glass slides modified by APTES and a homemade electrochemical cell was built from a commercial UV-vis cuvette. The cell was placed in the optical path of the light beam in a UV-vis spectrophotometer, which allowed the acquiring of the electronic absorption spectra under potential control under the same conditions as the CV determination. Scanning Electron Microscopy (SEM) measurements were carried out on an S-4800 instrument with an accelerating voltage of 2 kV, and the samples were sputtered with Pt prior to observation. The morphology observation of the samples was carried out on an atom force microscope (AFM, Nanoscope IIIa digital instrument, VECCO Co.) equipped with a silicon cantilever (typical spring constant = 40 N m⁻¹) in tapping mode under ambient conditions.

2.4 Preparation of the PAM films

The initially clean ITO glasses were preprocessed by dipping into dilute potassium hydroxide solution for 30 minutes under ultrasonic concussion, washing with ethanol water and drying under nitrogen. Then they were immersed in a 5.5% (volume ratio) of APTES solution in toluene for 40 min and washed with purified toluene. Subsequently, the monolayer substrates were formed by blowing with nitrogen gas and baking at 120 °C oven 30 min to form Si–O bonds. Next, the neat monolayers were soaked in PAM solution to adhere the polymer onto the substrates, and then the wet glass was settled under vacuum at 100 °C for 20 h (Scheme 2). According to the scheme, the structure of the PAM films were formed directly.

Scheme 2 Process of preparing a PAM coated ITO electrode treated with APTES. Schematic diagram of the structure of PAM films.

3. Results and discussion

3.1 Synthesis and characterization of polymers

The PAMs were prepared from 2,5-bis(hexyloxy)terephthaldehyde and four kinds of diamine. The PAMs were obtained through a facile one-step polycondensation reaction under N₂ atmosphere conditions. The structures of the polymers were confirmed using FT-IR spectroscopy as shown in Fig. S1,† which indicated a strong absorption band at 1618 cm⁻¹ assigned to the azomethine (C=N) stretching. Meanwhile, the ratio of monomer concentration between dialdehyde and diamine is 1.06 to 1, resulting in the appearance of carbonyl C=O stretching vibration bands at 1682 cm⁻¹ that are attributed to end aldehyde group stretching on the main chain. In the ¹H NMR spectra, all the peaks could be readily assigned to hydrogen atoms in the repeating units (Fig. S2†). The signal at 5.0 ppm (attributed to NH₂–) disappears and a new peak at 9.0 ppm supports the formation of azomethine linkages. The proton of the aldehyde unit in terminal PAMs appear as a resonance at δ = 10.56 ppm.

Propeller-like features of the TPA group enhanced the PAM solubility behavior which is collected in Table S1.† Benefiting from the long alkyl side chains connecting the benzene unit, all the polymers can dissolve in common organic solvents, such as THF, toluene and m-cresol. Thus, the excellent solubility makes the PAMs capable of forming high performance thin films for optoelectronic devices. The typical micromorphologies of the resulting PAM films were characterized and analyzed using SEM and AFM, see Fig. 1. Through the figures, the APTES deposited on the ITO surface formed a dense and smooth thin layer on the surface of ITO (Fig. 1a and b). Additionally, many regular knots emerge on the surface of the APTES monolayer in the AFM image (Fig. 1e) compared to the bare ITO (Fig. 1d). PAM4 grafted on the surface (Fig. 1c and f) was smooth and regular, which showed that APTES, which was covalently bound to the ITO surface, could improve the adhesive ability of the polymer.

Fig. 1 SEM images of APTES in front (a) and in profile (b) of ITO, and PAM4 (c) on the ITO; AFM images of ITO (d), APTES in front of ITO (e) and PAM4 deposited on a silica surface (f).

3.2 Thermal properties and molecular weights

The PAMs are thermally stable up to approximately 400 °C, which the TG analysis under nitrogen depicted (Fig. 2). The PAMs exhibit good thermal stability with a 5 wt% loss at temperatures above 400 °C (Table 1), except for PAM4 which could result from the less conjugated phenyl group. And the higher the content of benzene in the main-chain, the higher the amount of carbonized residue (char yield) at 700 °C and the better the thermostability. The molecular weights of the PAMs were measured using GPC, with polystyrene as standard and THF as eluent. The weight-average molecular weight (M̄_w) and polydispersity (PDI) of the PAMs were determined, see Table 1, and the (M̄_w) of the PAMs was 3.62 × 10⁴ to 2.85 × 10⁴ (PAM1–4) with a PDI of 1.43–1.31, respectively. The replacement of the phenyl group with the biphenyl ring results in the reduction of steric hindrance, which significantly increases the molecular weight.

Fig. 2 TGA curves of PAM1 to PAM4.

Table 1 Thermal properties and molecular weights of the PAMs

	5%	10%	20%	Char yield^a wt%	Mw^b (g mol^−1)	Mn^b (g mol^−1)	PDI^c
a The amount of carbonized residue of the polymers in the nitrogen atmosphere at 700 °C.b Calibrated with polystyrene standards, using THF as the eluent at a constant flow rate of 0.8 mL min^−1 at 35 °C.c Polydispersity index (Mw/Mn).
PAM1	400	420	450	53	36 200	25 400	1.43
PAM2	405	425	440	45	33 800	24 200	1.39
PAM3	400	420	440	41	32 800	22 500	1.44
PAM4	340	400	430	35	28 500	21 700	1.31

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3.3 Geometry and electronic structure of PAMs

Concerning the different structures, the optimized geometries of the PAMs are shown in Fig. 3. The average C–C bond length between the C=N linkage and O-phenylene in PAM1, PAM2, PAM3 and PAM4 are 1.462, 1.461, 1.461 and 1.461 Å respectively, while that between the C=N and N-phenylene ring is 1.371 Å for all. The C=N bond lengths of the PAMs are about 1.317 Å. The azomethine bond distances are shorter than its carbon homologue resulting in part in the observed high degree of planarity. However, an obvious difference is shown in the torsional angles (Φ₁) between the C=N linkage and the adjacent N-phenylene ring in Table 2, where PAM1 has a larger Φ₁ of 13.8° as compared with that (9.3°) of PAM3. The torsional angles (Φ₂) between the C=N linkage and the adjacent O-phenylene ring in PAM1 and PAM2 are 14.3° and 15.3°, respectively. The introduction of a biphenyl group alters the π-stacking structure that results in a large electrostatic repulsion force between the adjacent hydrogen atoms on the C=N linkage and the N-substituted benzene (H2–H3 repulsion).¹⁸ In addition, the torsional angles (Φ₁ and Φ₂) of PAM4 are 6.7° and 11.2° which are all smaller than those of PAM3. This indicates that the naphthalene substitution significantly enhances the nonplanarity of the PAM3 backbone.¹⁹

Fig. 3 Optimized geometries of the PAMs.

Table 2 Calculated bond lengths, dihedral angles (Φ₁, Φ₂), and electronic structure (E_HOMO, E_LUMO, and E_g) of the PAMs

	RC1–C2^a Å	RC2=N^a Å	RN–C3^a Å	Φ1^b deg	Φ2^b deg	EHOMO eV	ELUMO eV	Eg eV
a RC1–C2, RC2=N, and RN–C3 are defined in Fig. 2.b Average value.
PAM1	1.462	1.317	1.371	13.8	14.3	−4.78	−1.81	2.94
PAM2	1.461	1.318	1.371	5.6	15.7	−4.99	−1.90	3.09
PAM3	1.461	1.317	1.371	9.3	15.3	−4.73	−2.18	2.55
PAM4	1.462	1.316	1.371	6.7	11.2	−4.74	−2.15	2.59

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The variation of the dihedral angles (Φ₁ and Φ₂) by the biphenyl or naphthalene substitution might result in the variant HOMO and LUMO energy levels of PAMs in Fig. 4. For the LUMO, π-conjugation of the orbital could be effectively extended across the terephthaldehyde to azomethine-phenylene cores. In contrast, the HOMO orbital shows extension of π-conjugation over azomethine and triphenylamine cores except for PAM1 which the charge separation is localized only at the terminal triphenylamine and one of the phenyl groups. The difference of the electron cloud distribution in the HOMO orbital of PAM1 may be attributed to the interruption of π-electron conjugation resulting from the introduction of the deep kink disorder of the biphenyl group.²⁰ It is known that the decrease of planarity lowers the HOMO level but raises the LUMO level and thus leads to a larger band gap. The calculated electronic structures (E_g) of PAM1 to PAM4 are 2.94, 3.09, 2.55 and 2.59 eV, respectively. The E_g of PAM1 and PAM3 are smaller than that of PAM2 and PAM4, respectively. This is probably accounted to the naphthalene electron-withdrawing substitution. Meanwhile, the replacement of the phenyl group with the biphenyl groups aggravates the non-planarity which lowers the degree of conjugation of PAM1 and PAM2. Thus the two conformations together change the E_g of the PAMs into the order with PAM2 < PAM1 < PAM4 < PAM3.

Fig. 4 Pictorial representations of the electron density in the frontier molecular orbitals of repetition units.

3.4 Optical properties

The optical properties of the PAM solutions were investigated using UV-visible spectroscopy (Fig. 5), and all the UV-vis absorption spectra of the PAMs exhibited strong absorption at 306–461 nm. The maximum absorption around 460 nm is assigned to the π–π* transition resulting from the conjugation between the aromatic rings and TPA units combined by C=N double bonds in the backbone. It is observed that the PAMs containing biphenyl groups have maximum peaks at shorter sites than those containing 1,4-phenyl groups, as it is: λ_PAM1 = λ_PAM2 < λ_PAM3 = λ_PAM4. This is due to the tiny difference of dihedral angle between the biphenyl and phenyl groups substituted, where Φ_PAM1 > Φ_PAM3 and Φ_PAM2 > Φ_PAM4. Whereas, the absorption maxima of PAM3 are same as that of PAM4 due to the replacement of the benzene ring with the naphthalene ring enhancing the degree of the conjugation and the π-electron delocalization. From Fig. S3,† there are small increases in the absorption maxima from solution to thin films for all PAMs. The band gaps of the polymer films obtained from the optical absorption edge are in the range of 2.25–2.33 eV, which are lower than the theoretical results. This can be regarded as due to the increase in absorption maximum which is almost derived from the increased planarity of the chain in the solid state.

Fig. 5 UV-visible absorption spectra of PAMs in CH₂Cl₂ at room temperature.

3.5 Acidochromic properties

As shown in the spectra (Fig. 6), after doping HCl vapor new absorption bands emerged at 591 nm. At the moment, the absorption maxima (451 nm) of PAM2 are going steadily downhill. With the acidity strengthening, the value of pH is decreasing from 6.60 to 4.50, the color of the solution deepens from the original yellow to protonated blue. The present isosbestic point in absorption spectra of PAM2 confirms the coexistence of different forms of compound. While after neutralizing the acidic solution with NH₃ vapor, the color of the solution changed back. The similar color changes of the other PAMs are shown in Fig. S4.†

Fig. 6 UV-vis spectra changes of (a) PAM2 and (b) PAM3 in CH₂Cl₂ as it was protonated by HCl vapor in steps, showing a decrease with the value of pH decreasing. The concentration of pure PAM was 0.03 mg L⁻¹. Inset photographs indicate the color evolved in the process of doping with HCl vapor. The undoped molecular structure and protonated extreme resonance forms of the PAMs are shown on the right.

Because of the conjugation of the three phenyl rings in TPA, the lone pair electrons of the central nitrogen would delocalize on the whole TPA which will weaken the activity of the lone pair electrons towards acid. The color change after being protonated by hydrochloric acid vapor should be attributed to the nitrogen atom in the imine linkage (–HC=N–) being doped with H⁺ rather than the nitrogen in the TPA core; triphenylamine (TPA) is used as an illustration shown in the Fig. S5† (single TPA solution was doped with concentrated sulfuric acid with a color change from colorless to blue, and concentrated hydrochloric acid without switching color). The nitrogen atom in C=N doped by acid resulted in the creation of a positive charge and a stable conformation with the chloride anion. The new-form contracture increased π-electron delocalization and confirmed a much stronger electron-accepting center. Because of this, the charge separation of π-electrons in the PAMs with a donor–acceptor structure accelerated electron cloud overlap and reduced the band gap. Moreover, after NH₃ vapor undoping, the changed one restores the former constitution. To sum up, PAMs reveal noteworthy acidochromic properties and are especially sensitive to the pH value.

3.6 Electrochemical properties

The electrochemical properties of these PAM films, which were cast on an ITO-coated glass substrate, as a working electrode were investigated using CV (Fig. 7), and the whole test procedures were performed in dry acetonitrile under a nitrogen atmosphere. The CV diagrams of PAM1 and PAM2 demonstrate oxidative and reductive peaks at (1.06, 0.97 V) and (0.62, 0.82 V), respectively. The difference between PAM1 and PAM2 can be attributed to the stronger conjugated structure of the substituent naphthalene group. The one pair of redox peaks of PAM1 and PAM2 are attributed to the lack or absence of electrochemical splitting, rather than to electrostatic effects or electron coupling with oxidation of the electron-rich nitrogen atom in the TPA core. On the contrary, PAM3 and PAM4 exhibited similar two couples of symmetrical redox waves, in which the oxidative peaks at 0.79 and 1.13 V (PAM3), 0.75 and 1.05 V (PAM4) vs. Ag/AgCl, respectively; the reductive peaks at 0.67, 0.96 V (PAM3) and 0.60, 0.85 V (PAM4) vs. Ag/AgCl, respectively. The first oxidative peaks of PAM3 and PAM4 are attributed to the oxidation of one of the N atoms in the TPA cores; the second ones resulted from a dication structure formed from radical recombination.²¹

Fig. 7 Cyclic voltammetry curves for PAMs in CH₃CN containing 0.1 mol L⁻¹ LiClO₄, at a scan rate of 50 mV s⁻¹.

The HOMO and LUMO energy levels of the polymer can be determined from the oxidation onset potentials of CV, and the electronic structure parameters of the PAMs are given in Table 3. When the benzene ring (PAM2 and PAM4) in the side chain was replaced by a naphthalene (PAM1 and PAM3) unit, the E_g decreased only slightly. The E_g of PAM1 and PAM2 is larger than those of PAM3 and PAM4, which is attributed to the interruption of the linearity of the conjugated backbone.

Table 3 Optical and electrochemical properties of the PAMs

	λ^abssolution^a (nm)	λ^absfilm^b (nm)	λ^absonset^b (nm)	E1/2 vs. Ag/AgCl^c	E^peakonset vs. Ag/AgCl	E^electroHOMO^e (eV)	E^electroLUMO^e (eV)	E^filmg^d (eV)
a UV-vis absorption and PL spectra measurements in CH2Cl2 at room temperature.b λonset of the polymer film.c E1/2: average potential of the redox couple peaks.d E = 1240/λonset.e The HOMO energy levels were calculated from cyclic voltammetry and were referenced to ferrocene (4.8 eV). ELUMO = EHOMO + Eg. Eox/onset (Fc/Fc vs. Ag/AgCl) = 0.45 V.
PAM1	453	470	530	0.84	0.79	−5.14	−2.81	2.33
PAM2	453	456	534	0.90	0.60	−4.95	−2.63	2.32
PAM3	461	470	546	0.73	0.50	−4.85	−2.58	2.27
PAM4	461	466	552	0.68	0.44	−4.79	−2.54	2.25

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3.7 Electrochromic properties

The excellent solubility could enhance the processibility of the PAMs, but it also can be a threat to the stability of PAM films in the electrochemical redox process. Thereby, in the electrochemical treatments, the polymer films were tailored to graft onto the ITO surface by siloxane linkages.

The electrochromic absorption spectra were monitored with a UV-vis spectrometer at different applied potentials. The typical electrochromic absorption spectra of PAM1 and PAM3 are shown in Fig. 8, and the electrochromic absorption spectra of the other PAMs are shown in Fig. S6.† The absorption of PAM1 started to change at 470 nm, which is characteristic of a π–π* transition happening in the TPA group at 1.2 V. When the applied potentials increased positively from 1.2 to 1.9 V, the characteristic absorbance peak at 470 nm decreased gradually, and one new band grew at 602 nm with the color change from yellow to blue due to the electron oxidation. However the absorption of PAM3 showed varying processes resulting from the different oxidation processes. When the applied potential was increased to 1.0 V, new bands grew up at 986 nm gradually with the color change to green. And with the potential increasing to 1.7, the absorption at 470 nm of PAM3 faded away and the other new peaks climbed at 1144 nm with a color change to blue. This alternative absorption can be attributed to the step-by-step formation of dications in the PAM segments by the further oxidation of monocation radical species.

Fig. 8 Electronic absorption spectra of films of PAM1 (a) and PAM3 (b) and their 3D spectra, in the process of electrochemical doping with 0.1 V potential intervals in CH₃CN containing 0.1 mol L⁻¹ LiClO₄ as the supporting electrolyte (vs. Ag/AgCl).

Since the PAMs showed color changes during oxidation processes, PAM2 was chosen to fabricate an electrochromic device which showed remarkable electrochromic properties (shown in Fig. 9).

Fig. 9 Electrochromic device of PAM2 containing 0.5 mol L⁻¹ LiClO₄ as the supporting electrolyte in the neutral (left) and oxidated (right) states.

The response time and coloration efficiency upon electrochromic switching of the polymer film from its neutral to oxidized form was monitored (Fig. 10). For reaching 90% of the full change in absorbance, the required switching time is calculated. PAM1 would require 3.0 s at 1.0 V for switching transmittance at 602 nm and 3.0 s for bleaching. After continuous cyclic scans between 0.0 V and 1.0 V for 100 cycles (Fig. 10B), the polymer films still exhibited good stability of electrochromic characteristics, indicating that the film was stable and had good adhesion on the ITO substrate. From Table 4, we can conclude that the coloration efficiency (η) of the PAMs are in the order of PAM4 (332) > PAM1 (208) > PAM2 (172) > PAM3 (78). The difference of coloration efficiency may be attributed to the combination of the electron transportation along the PAM main chain and the ion transferability of the PAM’s supermolecular assembly on the ITO. The replacement of the phenyl group with the biphenyl and naphthyl ring improved the conjugation of the PAM chain which may enhance the electron or hole transferability from PAM4 to PAM1. Meanwhile the ion transfer can also be affected by the aggregation of PAM chains. The larger twisting biphenyl and higher steric hindrance of the naphthyl may impel the PAM chain to depart with a longer distance from the other which poses an aggravated effect for ion transfer between chains. The combination of the two aspects of effect contributed to the highest coloration efficiency of PAM4 and the lowest one of PAM3. The coloration efficiency of the PAM4 film is up to 332 cm² C⁻¹ at 602 nm and the film shows highly stable electrochromism.

Fig. 10 Potential step absorptiometry (A) and current consumption (B) of PAM1 at 602 nm (in 0.1 mol L⁻¹ LiClO₄/CH₃CN) by applying a potential step (0.0–1.0 V) with a cycle time of 10 s.

Table 4 Electrochemical data collected for coloration efficiency measurements of the PAMs

Polymer code^a	λ^b (nm)	δOD^c	Q^d (mC cm^−2)	η^e (cm^2 C^−1)
a Switching between 0.0 and 0.8 V for the PAMs (V vs. Ag/AgCl).b The given wavelength where the data were determined.c Optical density change at the given wavelength.d Injected charge, determined from the in situ experiments.e Coloration efficiency is derived from the equation η = δOD/Q.
PAM1	602	0.0092	0.044	208
PAM2	620	0.0014	0.008	172
PAM3	470	0.0034	0.042	78
PAM4	466	0.016	0.048	332

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3.8 Photoelectrical properties

Besides the stable and notable electrochemical performance, a reversible rise/decay of the photocurrent and photovoltaics were observed when the light was switched on and off many times, as shown in Fig. 11. The measurement was tested in 0.1 mol L⁻¹ LiClO₄ electrolyte solution via a simple method which included illumination by a 500 W Xe arc lamp (white light of 150 mW cm⁻² intensity). Upon irradiation, the photocurrent increased and rapidly reached an approximate steady-state value of 2.4 × 10⁻⁶ A. When the light was off, the current dropped dramatically to the original state. Otherwise, the excited current density decreased slightly after several on/off cycles. This indicated that PAM1 has a stable response to light and reversibly changes under photoinduction; a similar trend was observed for the other PAMs (Fig. S7†). It is known that the photo-generated electrons were transported from the lowest unoccupied molecular orbital to the conduction band of the ITO surface, and then moved to the external circuit. The photocurrent and open-circuit photovoltaic response were generated by the hopping of electrons. Similar characteristics were observed for the other PAMs of this series. Thus, these PAMs could be used as the photoelectric conversion material for optoelectronic applications.

Fig. 11 A typical photocurrent (lower) and photovoltaic (upper) response for PAM1 film immobilized on ITO glass upon exposure to light with switching at room temperature.

4. Conclusions

A series of highly soluble electrochromic polymers with excellent optical transmittance change have been synthesized from dialdehyde and four TPA derivatives. The good solubility in many organic solvents such as THF, CHCl₃, DMF and DMAc is beneficial for polymer film formation. An obvious color change was observed from a yellow neutral form to blue after doping with acid vapor. The PAM was immobilized on ITO via siloxane linkages (APTES) and exhibited excellent reversible electrochemical behavior and continuous cycling stability of the electrochromic characteristics. The color of the PAMs could be varied from initial yellow to blue via electrooxidation. Thus, these PAMs can be used in electrochromic and optoelectronic devices and sensors for pH.

Acknowledgements

The authors are grateful for the support of the National Science Foundation of China (Grant No. 51373049, 51372055, 51303045, 51273056, 21372067), Doctoral Fund of Ministry of Education of China (20132301120004, 20132301110001).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra22918a

‡ These authors contributed equally to this work.

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