Novel luminescent and electrochromic polyhydrazides and polyoxadiazoles bearing pyrenylamine moieties

Abstract

New aromatic polyhydrazides bearing redox-active pyrenylamine unit were prepared from the polycondensation reactions of N,N-di(4-carboxyphenyl)-1-aminopyrene with terephthalic dihydrazide (TPH) and isophthalic dihydrazide (IPH), respectively, via the phosphorylation reaction. These two hydrazide polymers could be further cyclodehydrated into the corresponding oxadiazole polymers in the range of 300–400 °C in the solid state. The resulting poly(1,3,4-oxdiazole)s had high glass-transition temperatures (298–304 °C) and high thermal stability (10% weight-loss temperature in excess of 520 °C). The dilute solutions of all the hydrazide and oxadiazole polymers showed a medium to high fluorescence with emission maxima around 457–545 nm in the blue to yellowish-green region. The polymer films revealed one redox couples upon electrochemical oxidation, together with interesting electrochromic behaviors: color changes from pale yellow neutral state to pale green oxidized states. Additionally, cyclic voltammetry studies of the oxadiazole polymers also showed reduction processes accompanied by strong color changes from pale yellow to orange, red or deep blue due to the formation of radical anions of the oxadiazole and pyrene units.

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Introduction

Aromatic polyoxadiazoles are known for their excellent thermal and hydrolytic stability.¹ The best known one is poly(p-phenylene-1,3,4-oxadiazole), which is used to produce heat-resistant Oxalon fiber in the Soviet Union.² However, aromatic polyoxadiazoles are generally difficult to process owing to their infusible and insoluble properties. Nonetheless, this kind of polymers can be synthesized via the preparation of a polyhydrazide as a soluble precursor polymer, by the reaction of dihydrazides of dicarboxylic acids with dicarboxylic acids or diacid chlorides followed by cyclization to the corresponding polyoxadiazoles by heating at elevated temperatures or by heating in dehydrating solvents.³ It has also been demonstrated that incorporation of flexible linkages like aryl ether and/or solubilizing groups such as hexafluoroisopropylidene and phthalide into the polyoxadiazole backbone or attachment of alkoxy pendent groups leads to an enhanced solubility in organic solvents while still retained desirable mechanical and thermal properties.⁴ On the other hand, oxadiazole derivatives are well-known electron-transporting materials used in organic light-emitting diodes (OLEDs).⁵ In recent years, the use of light-emitting conjugated polymers in electroluminescent devices has received a great deal of scientific and industrial attention because of several appealing advantages over other technologies.⁶ Electron-deficient oxadiazole units have been found to be efficient in promoting electron-transport properties when they are incorporated into conjugated polymer main chains or attached as side groups. Thus, 1,3,4-oxadiazole-containing conjugated polymers have been widely investigated and applied as electron-transport or emission layers in OLEDs.⁷

Pyrene is one of the most useful fluorogenic units for fluorescent sensors because it displays not only a well-defined monomer emission but also an efficient excimer emission.⁸ In close enough proximity, an excited-stated pyrene monomer and a ground-state monomer can form an excimer state that fluoresces at a substantially longer wavelength than the monomer emission. It has also been shown that the intensity of the excimer emission can be enhanced by both inter- and intramolecular aggregation of pyrene monomers due to a greater probability of dimerization. The long fluorescence lifetime, large Stokes shift, and tunable intensity of the excimer make the pyrene unit a useful fluorophore labeling in metal ion or nucleic acid probes.⁹ In recent years, pyrene derivatives,¹⁰polymers,¹¹ starbursts,¹² and dendrimers¹³ have been reported in the context of organic electronic applications such as organic light-emitting devices (OLEDs), due to their emissive properties combined with high charge carrier mobility.

On the other hand, triarylamine derivatives are widely used as hole-transporters, light-emitters, and host materials for electrophosphorescent OLEDs.^14,15 Oligomers and/or polymers containing triarylamine moieties in the main chain or side chain have attracted much attention because of their unique properties, which allow various optoelectronic applications such as photoconductive, electroluminescent, and photorefractive materials.¹⁶ Triarylamines can be easily oxidized to form stable radical cations, and the oxidation process is always associated with a noticeable change in coloration. Thus, many triarylamine-based polymers have been developed for potential electrochromic applications.¹⁷ In recent years, we have developed a number of high-performance polymers such as polyimides¹⁸ and polyamides¹⁹ carrying the triarylamine unit as a redox-chromophore. The triarylamine-containing monomers such as diamines and dicarboxylic acids could be easily prepared using well-established procedures,^20,21 and they could react with the corresponding co-monomers through conventional polycondensation techniques, producing the desired triarylamine-containing high-performance polymers. The polymers have high glass transition and decomposition temperature. Because of the incorporation of packing-disruptive, propeller-shaped triarylamine units along the polymer backbone, the resulting polymers generally exhibited good solubility in many organic solvents. They could afford tough and flexible films with good mechanical properties via simple solution processing, which is advantageous for their ready fabrication of large-area, thin-film devices. These polymers are transparent in the neutral state and can become highly colored in the oxidized states. This electrochromic behavior is different from that of the conjugated polymers extensively studied in recent years.²² Most of the reported conjugated polymers to be used as candidates for electrochromic applications are transparent at their oxidized state. In order to get the electrochromic device transmissive, it must continuously be kept under potential which may deteriorate the used polymers for fabrication in the time. Since our reported triarylamine-based polymers are highly transmissive at neutral state, it is safe to be used for this application and away from such possible deterioration.

In view of the attractive properties associated with the pyrene and triarylamine units, herein we report the synthesis of novel polyhydrazides from the direct polycondensation of dicarboxylic acid monomer 1, N,N-di(4-carboxyphenyl)-1-aminopyrene, with the corresponding dihydrazide monomers. These hydrazide polymers will be further converted into the 1,3,4-oxadiazole polymers by thermal cyclization. By incorporating the diphenylpyrenylamine unit into the backbone, the resultant hydrazide and 1,3,4-oxadiazole polymers are expected to exhibit the desired luminescent and electrochromic properties for optoelectronic applications. Some properties of the present polymers will be compared with those of structurally related ones derived from 4,4′-dicarboxytriphenylamine.

Experimental part

Materials

N,N-Di(4-carboxyphenyl)-1-aminopyrene (1) (mp = 312–314 °C) was synthesized according to the procedure reported previously.²¹ Synthetic details and characterization data of the pyrenylamine-containing diacid monomer 1 are described in the ESI†. Terephthalic dihydrazide (TPH) and isophthalic dihydrazide (IPH) were purchased from TCI and used without further purification. N,N-Dimethylacetamide (DMAc) (Tedia), N,N-dimethylformamide (DMF) (Tedia), dimethylsulfoxide (DMSO) (Tedia), N-methyl-2-pyrrolidone (NMP) (Tedia), pyridine (Py) (Wako) and diphenyl phosphite (DPP) (Acros) were also used as received. Commercially obtained calcium chloride (CaCl₂) (Wako) was dried under vacuum at 180 °C for 8 h prior to use. Tetrabutylammonium perchlorate, Bu₄NClO₄, was recrystallized from ethyl acetate under nitrogen atmosphere and then dried in vacuo prior to use. All other reagents and solvents were used as received from commercial sources.

Synthesis of polyhydrazides

The new polyhydrazides were synthesized from dicarboxylic acid monomer 1 and TPH and IPHvia the phosphorylation reaction.²³ A typical synthetic procedure for polyhydrazide I-IPH is described as follows. A 50 mL flask was charged with dicarboxylic acid monomer 1 (0.4575 g, 1.0 mmol), IPH (0.1942 g, 1.0 mmol), NMP (1.0 mL), CaCl₂ (0.1 g), DPP (1.0 mL), and pyridine (0.4 mL). The mixture was heated with stirring at 120 °C for 5 h. The resulting viscous solution was poured slowly with stirring into 200 mL of methanol, giving rise to a tough, fibrous precipitate. The precipitated product was collected by filtration, washed repeatedly with methanol and hot water, and dried to give a quantitative yield of polyhydrazide I-IPH. The inherent viscosity of the polymer was 0.31 dL g⁻¹, measured in NMP at a concentration of 0.5 g dL⁻¹ at 30 °C. The IR spectrum of polyhydrazide I-IPH (film) exhibited characteristic hydrazide absorption bands at 3261 (N–H str.) and 1655 cm⁻¹ (carbonyl str.). Another polyhydrazide I-TPH was prepared from diacid 1 and TPH by an analogous procedure.

Film preparation and cyclodehydration of the hydrazide polymers

A solution of polymer was made by dissolving about 0.5 g of the polyhydrazide sample in 8 mL of DMAc, and the homogeneous solution was poured into a 7 cm glass Petri dish, which was placed in a 90 °C oven overnight to remove most of the solvent. The polymer film was released from the glass substrate and further dried at 150 °C in vacuum for 6 h. The obtained films with the thickness of 60–70 μm were used for X-ray diffraction measurements, solubility tests, and thermal analyses. The cyclodehydration of the hydrazide polymers to the corresponding 1,3,4-oxadiazole polymers was carried out by successive heating the above fabricated polymer films at 250 °C for 30 min, 300 °C for 1 h, and then 350 °C for 3 h under vacuum.

Measurements

Infrared spectra were recorded on a Horiba FT-720 FT-IR spectrometer. Elemental analyses were run in a Heraeus VarioEL-III CHNS elemental analyzer. ¹H and ¹³C NMR spectra were measured on a Bruker Avance-500 FT-NMR using tetramethylsilane as the internal standard. The inherent viscosities were determined at 0.5 g dL⁻¹ concentration using a Cannon-Fenske viscometer at 30 °C. Wide-angle X-ray diffraction (WAXD) measurements were performed at room temperature (ca. 25 °C) on a Shimadzu XRD-6000 X-ray diffractometer (40 kV, 30 mA), using graphite-monochromatized Cu-Kα radiation. Thermogravimetric analysis (TGA) was conducted with a PerkinElmer Pyris 1 TGA. Experiments were carried out on approximately 3–5 mg film samples heated in flowing nitrogen or air (flow rate = 20 cm³ min⁻¹) at a heating rate of 20 °C min⁻¹. DSC analyses were performed on a PerkinElmer Pyris 1 DSC at a scan rate of 20 °C min⁻¹ in flowing nitrogen (20 cm³ min⁻¹). Absorption spectra were measured with an Agilent 8453 UV-visible diode array spectrophotometer. Photoluminescence (PL) spectra were measured with a Varian Cary Eclipse fluorescence spectrophotometer. The fluorescent quantum yield was determined using solutions in NMP and was calculated by comparing emission with that of a standard solution of 9,10-diphenylanthracene in cyclohexane (Φ_PL = 90%) at room temperature. Electrochemistry was performed with a CH Instruments 611C electrochemical analyzer. Cyclic voltammetry was conducted with the use of a three-electrode cell in which ITO (polymer film area about 1 cm², 0.8 cm × 1.25 cm) was used as a working electrode. A platinum wire was used as an auxiliary electrode. All cell potentials were taken with the use of a home-made Ag/AgCl, KCl (sat.) reference electrode. Ferrocene was used as an external reference for calibration (+0.44 V vs.Ag/AgCl). Voltammograms are presented with the positive/negative potential pointing to the right/left with increasing anodic/decreasing cathodic current pointing upward/downward. Spectroelectrochemistry analyses were carried out with an electrolytic cell, which was composed of a 1 cm cuvette, ITO as a working electrode, a platinum wire as an auxiliary electrode, and a Ag/AgCl reference electrode. Absorption spectra in the spectroelectrochemical experiments were also measured with an Agilent 8453 UV-visible diode array spectrophotometer.

Results and discussion

Polymer synthesis

A two-step procedure was employed to obtain the poly(1,3,4-oxadiazole)s from the dicarboxylic acid monomer 1 with TPH and IPH by using the phosphorylation technique described by Higashi and coworkers²³ (Scheme 1). The first stage consists of the synthesis of hydrazide prepolymers which are converted to the corresponding oxadiazole polymers in the second stage by the thermal cyclodehydration of the hydrazide group into the 1,3,4-oxadiazole ring. In the first stage, the polymerization proceeded homogeneously throughout the reaction and afforded viscous polymer solutions. All the hydrazide prepolymers precipitated in a fiber-like form when slowly pouring the resulting polymer solutions into methanol. The obtained hydrazide polymers had inherent viscosities in the range of 0.35–0.38 dL g⁻¹ (as shown in Table 1) and could be solution-cast into freestanding or flexible films, indicating the formation of moderate to high molecular weight polymers. IR and ¹H NMR spectroscopic techniques were used to identify the structures of the synthesized hydrazide polymers. The ¹H NMR and H–H COSY spectra included in Fig. 1 confirm the correct structure of polymer I-IPH. The representative IR spectrum for I-IPH (Fig. S3, ESI†) shows the characteristic absorption bands of the hydrazide group at around 1655 (carbonyl stretching) and 3261 cm⁻¹ (N–H stretching).

Scheme 1 Synthesis of the hydrazide and 1,3,4-oxadiazole polymers.

Table 1 Inherent viscosity and solubility behavior of polymers

Polymer code	η inh ^a/dL g^−1	Solubility in various solvents^b
		NMP	DMAc	DMF	DMSO	m-Cresol	THF
a Inherent viscosity measured at a concentration of 0.5 dL g^−1 in NMP at 30 °C. b ++: Soluble at room temperature; +−: partially soluble; +: soluble on heating; −: insoluble even on heating. NMP: N-methyl-2-pyrrolidone; DMAc: N,N-dimethylacetamide; DMF: N,N-dimethylformamide; DMSO: dimethylsulfoxide; THF: tetrahydrofuran.
I-TPH	0.38	++	++	++	++	++	+−
I-IPH	0.35	++	++	++	++	++	+−
II-TPH	0.22	++	+	+	+−	+−	−
II-IPH	0.24	++	+	+	+−	+−	−
I′-TPH	0.53	++	++	++	++	+	−
I′-IPH	0.46	++	++	++	++	+	−
II′-TPH	−	+−	−	−	−	−	−
II′-IPH	−	+−	−	−	−	−	−

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Fig. 1 (a) ¹H and (b) H–H COSY NMR spectra of polyhydrazide I-IPH in DMSO-d₆.

Thermal conversion of the hydrazide group to the 1,3,4-oxadiazole ring was confirmed by IR spectroscopy. The typical IR spectra for the representative polyhydrazide I-IPH and poly(1,3,4-oxadiazole) II-IPH are illustrated in Fig. S3, ESI†. After thermally curing at 350 °C for 3 h, the complete conversion from I-TPH to II-TPH could be confirmed by the disappearance of the absorption bands peculiar to the hydrazide group at 3261 and 1655 cm⁻¹, and the appearance of characteristic 1,3,4-oxadiazole ring vibration at around 1487, 1601 (C=N stretching), and 1071 cm⁻¹ (C–O–C stretching). The experiment conditions for the cyclization were selected on the basis of the weight loss behavior observed by dynamic TGA and the cyclodehydration endotherm on the DSC thermogram. A typical pair of DSC and TGA curves of polyhydrazide I-TPH and poly(1,3,4-oxadiazole) II-TPH are illustrated in Fig. S4 and S5 (ESI†), respectively. The main endothermic peak on the DSC curve and the weight loss at the first stage of the TGA thermogram reveal that the cyclodehydration reaction of hydrazide group with water evolution occurred in the range of 290–380 °C. The chemical compositions of the thermally converted oxadiazole polymers were also confirmed by the elemental analysis results listed in Table S1, ESI†.

Organo-solubility and film property

The solubility properties of all the polymers are also included in Table 1. All the hydrazide polymers were soluble in various solvents such as NMP, DMAc, DMF, DMSO, and m-cresol, and the good solubility could be attributed in part to the introduction of propeller-shaped triarylamine moiety and bulky pendent pyrene group in the repeat unit. Thus, the good solubility makes these polymers potential candidates for practical applications by common solution casting processes. The thermally cured 1,3,4-oxadiazole polymers showed a significantly decreased solubility as compared with their hydrazide precursors. All the hydrazide and oxadiazole polymer films were essentially amorphous as indicated by their X-ray diffraction patterns.

Thermal properties

The thermal data of polyhydrazides and polyoxadiazoles are summarized in Table 2. The hydrazide polymers I-TPH and I-IPH showed a well-defined T_g by DSC (254 and 220 °C, respectively), and they were converted to the corresponding oxadiazole polymers when heated to 400 °C at a scanning rate of 20 °C min⁻¹ in nitrogen. Because of the increased chain rigidity, all the oxadiazole polymers showed increased T_g values (304 °C for II-TPH and 298 °C for II-IPH) in comparison with the corresponding hydrazide polymers. As compared to the I′ and II′ series analogs, the present series polymers exhibit a remarkably increased T_g as a result of the presence of rigid pyrene segments. All of the polyoxadiazoles exhibited a similar TGA pattern with no significant weight loss below 460 °C in air or nitrogen atmosphere. The 10% weight-loss temperatures (T_d¹⁰) of the polyoxadiazoles in nitrogen and air were recorded in the range of 517–526 °C and 527–531 °C, respectively. The amount of carbonized residue (char yield) of these polymers was more than 64% at 800 °C in nitrogen. The high char yields of these polymers could be ascribed to their high aromatic content. Due to the incorporation of thermally stable pyrene unit, all the polymers exhibited higher T_d values compared to their corresponding I′ and II′ series counterparts derived from 4,4′-dicarboxytriphenylamine. The thermal analysis results revealed that these oxadiazole polymers exhibited excellent thermal stability, which in turn is beneficial to increase the service time in device application and enhance the morphological stability to the cast film.

Table 2 Thermal properties of the polymers^a

Code	Polyhydrazides^b			Code	Poly(1,3,4-oxadiazole)s
	T g/°C	T o/°C	T p/°C		T g/°C	T d ^10/°C^c		Rw ^800 (%)^d
						N2	Air
a The polymer film samples were vacuum-dried at 200 °C for 1 h prior to all the thermal analyses. b The sample was heated from 50 to 400 °C at a scan rate of 20 °C min^−1 followed by rapid cooling to 50 °C at −200 °C min^−1 in nitrogen. The midpoint temperature of baseline shift on the subsequent DSC trace (from 50 to 400 °C at a heating rate of 20 °C min^−1) was defined as Tg. To: extrapolated onset temperature of the endothermic peak; Tp: endothermic peak temperature. c Decomposition temperature at which a 10% weight loss was recorded by TGA at a heating rate of 20 °C min^−1. d Residual weight (Rw) percentages at 800 °C under nitrogen flow.
I-TPH	254	320	340	II-TPH	304	526	531	64
I-IPH	220	312	335	II-IPH	298	517	527	65
I′-TPH	197	298	324	II′-TPH	267	491	518	60
I′-IPH	182	304	338	II′-IPH	263	490	508	53

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Optical properties

The optical properties of the polyhydrazides and poly(1,3,4-oxadiazole)s were investigated by absorption and photoluminescence (PL) spectroscopy. The relevant data are summarized in Table 3. Fig. 2 shows the absorption and PL spectra of polyhydrazides and poly(1,3,4-oxadiazole)s in NMP solutions, together with their PL images on exposure to a standard laboratory UV lamp. These polymers exhibited strong UV-vis absorption bands at 331–397 nm in NMP solutions, assignable to the π–π* transitions of the pyrene and other π-conjugated moieties in the polymer backbone. In the solid state, the polyhydrazides and poly(1,3,4-oxadiazole)s showed absorption characteristics similar to those observed in solutions, with low-energy absorption λ_max centered at 359–399 nm with absorption onsets at 434–454 nm corresponding to optical band gaps of 2.73–2.86 eV. Their PL spectra in NMP solutions showed maximum bands around 457–545 nm in the blue or greenish-yellow region with fluorescence quantum yield (Φ_PL) ranging from 4.7% for II-TPH to 50.9% for I-IPH. The relatively lower Φ_PL values (4.7–15.3%) for the polymers I-TPH, II-TPH and II-IPH may be caused by self-quenching due to intermolecular electronic interactions between diphenylpyrenylamine donors and benzoyl/oxadiazole acceptors. Less fluorescence efficiency of the polymers derived from TPH as compared to those from IPH might be attributed to the quenching effect arising from interchain charge transfer. Larger Stokes shift for the polyoxadiazole II-TPH might be explained by its longer conjugation length in the backbone. The decreased Φ_PL of the polyoxadiazoles II with respect to polyhydrazides I also could be attributable to the quenching effect caused by the intermolecular charge transfer between the diphenylpyrenylamine donor and the oxadiazole acceptor. Although these polymers exhibited a low Φ_PL, their thin solid films showed an obvious green or blue PL when irradiated by a standard laboratory UV lamp (see the images shown in the top of Fig. 2). The highest Φ_PL (50.9%) of polyhydrazide I-IPH can be rationalized by the decreasing charge transfer effect caused by the meta-catenation of IPH. Furthermore, I series polyhydrazides exhibited higher Φ_PL compared with I′ series. This could be attributed to the incorporation of highly fluorescent pyrene chromophores. In addition, most of the polymers exhibited a prominent red-shifted emission in the solid state as compared to that measured in NMP solutions, possibly due to an increased interchain interaction in the film form.

Table 3 Absorption and fluorescence properties of polymers

Code	In solution^a			As solid film/nm
	λ max ^abs/nm	λ max ^PL /nm^b	Φ PL (%)^c	λ 0	λ max ^abs	λ onset ^abs	λ max ^PL ^b
a The polymer concentration was 10^−5 mol L^−1 in NMP. b Excited at the absorption maximum for both the solid and solution states. c The fluorescent quantum yield was calculated in an integrating sphere with 9,10-diphenylanthracene as the standard (ΦPL = 90%).
I-TPH	332	457	15.3	441	372	446	486
I-IPH	331	457	50.9	425	359	434	472
II-TPH	397	545	4.7	450	399	454	520
II-IPH	381	470	28.5	432	385	437	480
I′-TPH	354	502	10.9	412	359	428	482
I′-IPH	361	462	25.5	407	357	415	460
II′-TPH	391	518	9.8	428	393	444	505
II′-IPH	382	462	35.6	419	384	432	465

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Fig. 2 UV-vis absorption and PL spectra of the dilute solutions of the prepared polymers in NMP (10⁻⁵ M). A solution of 9,10-diphenylanthracene (DPA) dissolved in cyclohexane (10⁻⁵ M) was used as the standard (Φ_PL = 90%). Images show the appearance of these polymer solutions and solid films before and after exposure to a standard laboratory UV lamp (excited at 365 nm).

Electrochemical properties

The electrochemical properties of the hydrazide and oxadiazole polymers were investigated by CV conducted for the cast films on an ITO-coated glass substrate as working electrode in dry acetonitrile (CH₃CN) (for the anodic scan) or DMF (for the cathodic scan) containing 0.1 M of Bu₄NClO₄ as an electrolyte under nitrogen atmosphere. The onset oxidation and reduction potentials are summarized in Table 4. The typical CV diagrams for the representative polyhydrazide I-TPH and poly(1,3,4-oxadiazole) II-TPH are illustrated in Fig. 3. An irreversible oxidation process (Fig. 3a), corresponding to the removal of an electron from the diphenylpyrenylamine unit, was detected at E_onset = 1.05 V (E_pa = 1.50 V) in the CV of I-TPH. This polymer also had a quasi-reversible reduction wave at E_onset = −1.62 V (E_pc = −1.94 V) due to the pyrene unit. The oxidation wave for II-TPH shifted to a higher potential (E_onset = 1.08 V, E_pa = 1.60 V) attributable to the electron-deficient 1,3,4-oxadiazole. In addition, well-defined reduction waves for the formation of radical anion and dianion of the oxadiazole units (Fig. 3b) were observed in the negative side of the voltammogram at E_1/2 = −1.56 and −2.05 V, respectively. Another reduction couple (at E_1/2 = −1.88 V) observed in the CV curve of II-TPH can be attributable to the reduction of pyrene moiety. As shown in Fig. S6 (ESI†), the dihydrazide monomers IPH and TPH show no electrochemical activity in the applied voltage range. Thus, the possible mechanisms of electro-oxidation and reduction of the diphenylpyrenylamine and oxadiazole moieties are proposed in Scheme 2. The band gaps of the polyhydrazides and poly(1,3,4-oxadiazole)s can be estimated from the difference in the onset potentials of reduction and oxidation processes, ranging from 2.56 to 2.75 eV. The electrochemical data were used to estimate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels (Table 4). The HOMO levels for these poly(1,3,4-oxadiazole)s were calculated to be between 5.44 and 5.46 eV (relative to the vacuum energy level), whereas the values for the LUMO levels lay between 2.71 and 2.88 eV.

Table 4 Redox potentials and energy levels of polymers

Index	E onset ^Ox /V^a	E onset ^Red/V^b	E g ^el/eV^c	E g ^opt/eV^d	E HOMO/LUMO/eV^e
a vs. Ag/AgCl in CH3CN. b vs. Ag/AgCl in DMF. c E g ^el (electrochemical band gap): difference between ELUMO and EHOMO. d The data were calculated from polymer films by the equation: Eg^opt = 1240/λonset. e The HOMO and LUMO energy levels were calculated from Eonset values of redox potentials and were referenced to ferrocene (4.8 eV). f No discernible signal was observed.
I-TPH	1.05	−1.62	2.67	2.78	5.41/2.74
I-IPH	1.06	−1.64	2.70	2.86	5.42/2.72
II-TPH	1.08	−1.48	2.56	2.73	5.44/2.88
II-IPH	1.10	−1.65	2.75	2.84	5.46/2.71
I′-TPH	1.08	—^f	—	2.90	5.44/2.54
I′-IPH	1.09	—	—	2.99	5.45/2.46
II′-TPH	1.10	—1.41	2.51	2.79	5.46/2.95
II′-IPH	1.11	—1.52	2.63	2.87	5.47/2.84

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Fig. 3 Cyclic voltammograms of the cast films of (a) polyhydrazide I-TPH and (b) poly(1,3,4-oxadiazole) II-TPH on an ITO-coated glass substrate in 0.1 M Bu₄NClO₄ acetonitrile (for the oxidation process) and DMF (for the reduction process) solutions at scan rates of 50 and 100 mV s⁻¹, respectively.

Scheme 2 Proposed redox chemistry of polyhydrazides and poly(1,3,4-oxadiazole)s.

Spectroelectrochemical and electrochromic properties

Spectroelectrochemical measurements were performed on films of polymers drop-coated onto ITO-coated glass slides immersed in the electrolyte solution. The electrode preparations and solution conditions are identical to those used in the CV experiments. The electrochromic absorbance spectra of polyhydrazide I-TPH upon electro-oxidation are illustrated in Fig. 4. In the neutral state, the polymer exhibited strong absorption at wavelengths around 333 and 372 nm, characteristic of triarylamine π–π* transitions, and its films exhibited a very light yellow color. When the potential was increased to 1.5 V, the intensity of the peaks in the UV region decreased, and a new double peak emerged in the visible region with maxima at 546 and 590 nm. The spectral changes are assigned to the radical cation formation arising from the oxidation of diphenylpyrenylamine unit. Meanwhile, the color of the film turned into light green (as shown in Fig. 4, inset). Upon electro-oxidation, polyhydrazide I-IPH showed similar spectral changes as those observed for I-TPH. Moreover, coloration changes were also observed in these polyhydrazides upon reduction. However, the color change is not strong as that observed in the anodic scanning.

Fig. 4 Spectroelectrochemistry of the polyhydrazide I-TPH thin film on the ITO-coated glass substrate in 0.1 M Bu₄NClO₄/CH₃CN at various applied potentials. The insets show the color changes in the polymer films at indicated electrode potentials.

Fig. 5 presents the absorbance spectral changes in the film of polyoxadiazole II-TPH upon electrochemical oxidation and reduction. In the neutral form the film exhibits strong bands at 320 and 399 nm due to the π–π* transitions, but it is almost transparent at longer than 450 nm. Upon oxidation of the II-TPH film (increasing electrode potential from 0 to 1.6 V), the intensity of the absorption bands in the UV region gradually decreased while a new small peak in the visible region at 602 nm together with a broad band from 500 nm extending out into the NIR region gradually increased in intensity (Fig. 5a). Meanwhile, the film changed from pale yellow to pale green. We can suggest that these spectral changes are attributed to the electron removal from the diphenylpyrenylamine unit. Fig. 5b shows the spectral changes in the II-TPH film upon reduction. As the film was charged with electrons at −1.00 to −1.75 V, the intensity of the absorbance in the UV region decreased significantly, and a new strong band grew up in the visible region with maximum at 490 nm together with a broad band from 650 nm extending out into the NIR region beyond 1100 nm gradually increased in intensity. As shown in the inset of Fig. 5b, the film at this reduced form appeared orange in color. As the potential was continuously decreased to −2.15 V, the intensity of the absorbance in the UV region further dropped, at the same time the peak at 490 nm slightly shifted to longer wavelength (509 nm) with an increasing intensity. At this fully reduced state, the polymer film turned into orange-red.

Fig. 5 Spectroelectrochemistry of poly(1,3,4-oxadiazole) II-TPH thin film on an ITO-coated glass substrate (in CH₃CN or DMF with 0.1 M Bu₄NClO₄ as the supporting electrolyte vs.Ag/AgCl couple as reference) along with increasing the applied voltage to +1.6 V (for the oxidation process in acetonitrile) and decreasing to −2.15 V (for the reduction process in DMF). The potential was varied in 50 to 100 mV intervals. The inset shows the color change of the polymer film at indicated potentials.

Upon oxidation, the film of poly(1,3,4-oxadiazole) II-IPH showed a similar spectroelectrochemical behavior to that of II-TPH. However, its electro-optical property is quite different from that of II-TPH during the cathodic scanning. Fig. 6 shows the spectral changes in the II-IPH film when it was switched for n-doping. When the potential is 0.0 V, the polymer was at the neutral state, where the film had strong absorption in the UV region with a peak centered at 385 nm. It was almost transparent in the visible region. As the film was charged with electrons at potentials between −1.40 and −1.80 V, the intensity of the peak in the UV region decreased, and a new peak appeared in the visible region at 502 nm together with a broad band from 550 to 750 nm gradually increasing in intensity. As shown in the inset of Fig. 6b, the film of this electron charged state is light gray in color. With the potential continuously reduced to −2.05 V, the intensity of the peak at 385 nm further decreased, and the peak at 502 nm increased in intensity significantly together with an intensified shoulder around 612 nm. As the potential was switched from −1.80 to −2.05 V, the polymer film changed from light gray to blue. Referring to the CV data in Fig. 6a, we can suggest that the peak at 502 nm is associated with the polaron on the oxadiazole segment, and the peak at 612 nm is related to the polaron on the pyrene segment.

Fig. 6 (a) Cyclic voltammetry and (b) spectroelectrochemistry of the cast film of poly(1,3,4-oxadiazole) II-IPH on an ITO-coated glass substrate in 0.1 M Bu₄NClO₄/DMF solution at a scan rate of 100 mV s⁻¹. The inset shows the color change of the polymer film at indicated potentials.

Conclusions

Two new aromatic polyhydrazides were prepared from a newly synthesized dicarboxylic acid 1 with terephthalic dihydrazide and isophthalic dihydrazide, respectively, via the Yamazaki–Higashi phosphorylation reaction. Subsequent thermal cyclodehydration of these hydrazide polymers gave the corresponding poly(1,3,4-oxadiazole)s. These polymers exhibited blue or yellowish green fluorescence emission maximum between 457 and 545 nm in NMP solution with a quantum yield of up to 50.9%. The poly(1,3,4-oxadiazole)s exhibited ambipolar electrochemical and electrochromic properties. Thus, these novel polyhydrazides and poly(1,3,4-oxadiazole)s may find applications in OLED and electrochromic devices due to their excellent thermal stability, medium to strong fluorescence, stable electrochemical redox behavior, and multicolored electrochromic properties.

Acknowledgements

We thank the National Science Council of Taiwan for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c1py00115a

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