Solvent Response and Protonation Effects of Novel Aramides

Sep 6, 2013 - R&D Center for Membrane Technology, Department of Chemical Engineering, Chung Yuan University, 32023 Chung-Li, Taiwan...
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Solvent Response and Protonation Effects of Novel Aramides Containing Pyridine and Unsymmetrical Carbazole Moieties Ying-Chi Huang,† Kun-Li Wang,‡ Cheng-Hung Chang,† Yi-An Liao,§ Der-Jang Liaw,*,† Kueir-Rarn Lee,∥ and Juin-Yih Lai∥ †

Department of Chemical Engineering, National Taiwan University of Science and Technology, 10607 Taipei, Taiwan Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 10608 Taipei, Taiwan § School of Medicine, Faculty of Medicine, National Yang-Ming University, Taipei, Taiwan ∥ R&D Center for Membrane Technology, Department of Chemical Engineering, Chung Yuan University, 32023 Chung-Li, Taiwan ‡

S Supporting Information *

ABSTRACT: A new diamine containing pyridine and carbazole groups was synthesized via a Chichibabin reaction and subsequent reduction; this compound was then used in the preparation of organo-soluble aramides (PA-a−i). The resultant polymers had high glass transition temperatures, which fell within the ranges of 271−287 °C, and Td10 at 510−535 °C in a nitrogen atmosphere. The pristine PA-a exhibited the maximum value in the UV− vis absorption spectrum at 307 nm, and this value shifted to 394 nm after protonation by HCl. When the emission of the polymer in THF solution was observed, the intensity of the emission peak at 420 nm decreased and the intensity of a new emission peak at 552 nm increased as the acid concentration increased. Additionally, the color of the polymer solution changed from blue to yellow after protonation. The intensity of the emission at 552 nm based on excimers increased as MeOH content increased. The color of polymer films also changed irreversibly from yellow, which was indicative of the neutral form, to a dark brown oxidized form during the application of bias voltages ranging from 0 to 1.6 V.

1. INTRODUCTION Aramides received considerable attention as high performance materials because of their outstanding thermal stability and chemical resistance as well as their electrical and mechanical properties. However, the application of these materials is restricted due to their poor solubility in organic solvents and prohibitively high glass transition temperatures that makes them very difficult to process when using spin-coating or thermoforming techniques.1 Many attempts have been made to create structurally modified aromatic polymers with increased solubility and ease of processing while maintaining high thermal stability. It is well established that the solubility of polymers often increases with the introduction of flexible bonds, large pendent groups, and unsymmetrical or polar constituents into the polymer chains.2−4 If the pendent groups are carefully chosen, it should be possible to modulate solubility without sacrificing any thermal or mechanical properties in a deleterious manner.4,5 Furthermore, the introduction of various heterocyclic aromatic rings into the main chain of polyamides has led to the development of polymers with superior thermal stability. Pyridine, which is a nitrogen-containing heterocyclic compound, has been a key molecule used in the construction of functional materials due to its high thermal and chemical stability.6−11 In addition, polymers that contain a pyridine moiety display electron-transporting abilities and interesting optical properties that can be modulated via protonation. The © 2013 American Chemical Society

optical tuning effects attributed to protonation are the result of changes to the localized lone pair, which are contained in an sp2 orbital on the nitrogen atom.6−11 In our previous studies,6−11 we described the synthesis of different polyamides, which contained heterocyclic moieties in the main chain, with improved thermal properties. Additionally, Koleva’s group has observed a relationship between the structures and spectroscopic properties of some pyridinium compounds.12 Recently, Liu et al. showed that a conjugated polymer containing pyridine in the main chain exhibited potential as a polymeric memory material.13 Carbazole is a conjugated group that has demonstrated characteristics which are promising for optical and electronic applications, such as photoconductivity, photorefractivity, and high charge mobility.14,15 The carbazole substituent is an interesting functional group for incorporation into polymer backbones due to its high thermal stability, good solubility, extended glassy state and moderately high oxidation potential.16 Accordingly, a novel diamine containing pyridine and unsymmetrical carbazole moieties was synthesized via a Chichibabin reaction.6−11 This compound was subsequently reacted with a variety of commercially available acid chlorides to prepare a series of novel aramides, which are expected to be Received: July 5, 2013 Revised: August 1, 2013 Published: September 6, 2013 7443

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Scheme 1. Synthesis of Dinitro (CBNPP) and Diamine (CBAPP) Monomers with Pyridine and Carbazole Groups

Scheme 2. Reaction Mechanism of the Chichibabin Reaction

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Scheme 3. (a) Pyridine Aramides Could Not Be Synthesized by Dicarboxylic Acid (Pyridine Protonation and NH3+ Insoluble (Ionic Cross-Linking)); (b) Synthesis of Pyridine Aramide by Diacid Chloride

obtained on a TA Instruments Dynamic TGA 2950 under nitrogen flowing at a rate of 30 cm3 min−1 and a heating rate of 20 °C min−1. Differential scanning calorimetric analysis was performed on a differential scanning calorimeter (TA Instruments TA 910) under nitrogen flow at a rate of 30 cm3 min−1 and a heating rate of 10 °C min−1. Tensile properties were determined from stress−strain curves obtained with an Instron mode 5544 based on the ASTM D412 method. UV−vis spectra of polymer films or solutions were recorded on a JASCO V-550 spectrophotometer at room temperature. Weight-average (Mw) and number-average (Mn) molecular weights were determined by gel permeation chromatography (GPC). Four Waters (Ultrastyragel) columns (300 mm × 7.7 mm, guard, 105, 104,

thermally stable and organosoluble. The basic characteristics and optical and electrical properties of these aramides were investigated in this study

2. RESULTS AND DISCUSSION 2.1. Measurements. IR spectra were recorded in the range of 4000−400 cm−1 on a JASCO IR-700 spectrometer. Elemental analyses were conducted on a PerkinElmer 2400 instrument. The 1H and 13C NMR spectra were obtained by BRUKER AVANCE 500 NMR operated at 500 MHz for proton experiments and 125 MHz for carbon experiments. The inherent viscosities of all polymers were measured using a capillary Ubbelohde viscometer. Thermogravimetric data were 7445

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103, and 500 Å in series) were used for GPC analysis with tetrahydrofuran (THF; 1 mL min−1) as the eluent. The eluent was monitored with a UV detector (JMST Systems, VUV-24, USA) at 254 nm. Polystyrene was used as the standard. Cyclic voltammetry (CV; CHI model 619A) was conducted with the use of a three-electrode cell. The electrochemical cell was composed of a 1 cm cuvette, ITO as a working electrode, a platinum wire as an auxiliary electrode, and an Ag/Ag+ as a reference electrode. The spectroelectrochemical cell was composed of a 1 cm cuvette, ITO as a working electrode, a platinum wire as an auxiliary electrode, and an Ag/Ag+ reference electrode. Absorption spectra in spectroelectrochemical analysis were measured with a JASCO V-550 spectrophotometer. 2.2. Monomer Synthesis. The dinitro compound (CBNPP), which contained pyridine and unsymmetrical carbazole as pendant groups, was synthesized with a modified Chichibabin reaction (Scheme 1), which is a facile method for the preparation of substituted pyridine structures. The condensation of 9-ethyl-3-carbazolecarboxaldehyde with 4′nitroacetophenone was conducted in the presence of ammonium acetate to afford dinitro (CBNPP) in one step with a low yield (41%). Under these conditions, it was found that an aldehyde appears as a substituent in the 4-position with methylene groups at positions 3 and 5. The low yield could be explained by the postulated mechanism shown in Scheme 2. An aldol condensation occurs between benzaldehyde and acetophenone to form chalcone, which is followed by a Michael-type reaction with acetophenone to form a 1,5-diketone. Next, the ring closure of the 1,5-diketone with ammonia, which is formed in situ from ammonium acetate, forms a dihydropyridine, which is dehydrogenated via transfer of hydrogen to benzalacetophenone to form the desired pyridine. For each mole of pyridine formed, a mole of benzalacetophenone should be reduced. It was difficult to confirm the structure of the compound by NMR due to its poor solubility at room temperature; the structure was confirmed roughly by examining the weak peaks. The poor solubility may result from its coplanar conformation and the polar nitro groups from CBNPP. However, the results of FT-IR and elemental analysis support the proposed structure. Reduction of the dinitro derivative (CBNPP), which was performed at 90 °C in ethanol using hydrazine monohydrate in the presence of catalytic amount of palladium on activated carbon, produced a new diamine compound (CBAPP). Elemental analysis, as well as IR and NMR spectra, confirmed the structure of this compound. As shown in the Experimental Section, the characteristic absorbances, which were correlated with the presence of nitro groups, were absent, and new absorptions at 3471 and 3379 (N−H stretching) and 1620 cm−1 (N−H deformation) appeared. In the NMR spectra of the diamine (CBAPP), a singlet peak at 5.45 ppm was assigned for its amino protons in the 1H NMR spectrum, and three carbon singlets (156.5, 149.4, and 112.7 ppm) in the 13C NMR spectrum confirmed the formation of pyridine ring. The NMR, FT-IR, and elemental analysis results clearly confirm that the diamine (CBAPP) prepared herein is consistent with the proposed structure. 2.3. Preparation of Poly(pyridine amide). We tried to synthesize polyamides with a dicarboxylic acid and the pyridinecontaining diamine by phosphorylation polycondensation (Scheme 3a) using triphenyl phosphite as the condensation agent and LiCl or CaCl2 as promoter. However, only a salt-like organic-insoluble solid was obtained.4

Because pyridine is more basic than an arylamine (pyridine pKa = 5.14, aniline pKa = 4.19), indicating that pyridine is more basic than the amino group in the monomer, it was possible to form protonated-pyridine-containing monomeric species with diacids, the formation of which successfully outcompeted the polymerization reaction Therefore, a low-temperature solution polycondensation technique, which used propylene oxide as an acid scavenger, was used for the synthesis of the pyridinecontaining polyamides from the diamine; additionally, more reactive diacid chlorides were used, as shown in Scheme 3b. All polymerization reactions remained homogeneous throughout the reaction and afforded clear, highly viscous polymer solutions. All the polymers precipitated in a tough fiber-like form when the reaction solutions were slowly poured into methanol with stirring. The polyamides had inherent viscosities in the range of 0.33−0.89 dL/g, which was measured in DMAc at a concentration of 0.5 g/dL at 30 °C. Number-average (Mn) and weight-average (Mw) molecular weights of PA-a were found to be 5.7 × 104 and 9.0 × 104, respectively. Figure 1

Figure 1. 1H NMR spectrum of PA-g.

shows a typical 1H NMR spectrum of PA-g in DMSO-d6, in which all the peaks have been assigned to the hydrogen atoms in the repeating unit. Structural features of these polyamides were also confirmed by FTIR spectroscopy. They exhibited characteristic absorptions from the amide group of approximately 3317 cm−1 (N−H stretching) and 1662 cm−1 (CO stretching). These results are consistent with the proposed structure. 2.4. Basic Characterization. The solubility behavior of polyamides was tested qualitatively, and the results are summarized in Table 1. All polyamides were highly soluble in polar solvents, such as NMP, DMAc, and DMF. The enhanced solubility could be attributed to the introduction of the unsymmetrical carbazole moiety into the repeating unit. These films were subjected to the testing tensile properties, and the 7446

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Table 1. Solubility Behavior of Various Aramidesa

Table 3. Optical UV and PL Properties of Various Aramides

PA

a

b

c

d

e

f

g

h

i

NMP DMAc DMF DMSO THF m-cresol

++ ++ ++ ++ ++ ++

++ ++ ++ +− +− ++

++ ++ ++ ++ +− ++

++ ++ ++ ++ −− +−

++ ++ ++ +− +− +−

++ ++ ++ ++ −− +−

++ ++ ++ + −− +−

++ ++ ++ ++ +− ++

++ ++ ++ +− −− ++

solutiona λ (nm)

a

The solubility was determined with a 1 mg of sample in 1 mL of a solvent. ++: soluble at room temperature; +: soluble on heating to 70 °C; +−: partially soluble on heating 70 °C; −−: insoluble.

tensile strengths, elongations until breakage, and initial moduli of these films were in the ranges of 40−62 MPa, 2.9−12.8%, and 2.1−2.4 GPa, respectively. 2.5. Thermal Properties. None of the polymers showed clear melting endotherms up to decomposition temperatures in the DSC thermograms. The Tgs of these polyamides were measured to be in the ranges of 271−287 °C (by DSC). All of the polyamides exhibited high thermal stability with insignificant weight loss up to 510 °C under nitrogen. Their 10% weight-loss temperatures in nitrogen and air were recorded at 510−535 and 442−510 °C, respectively. Char yield was in the range of 33−89% when the temperature was 800 °C.

film λ (nm)

polymer

λabs,maxa (nm)

PLmaxb (nm)

λabs,max (nm)

PLmax (nm)

λabs,onset (nm)

PA-a PA-b PA-c PA-d PA-e PA-f PA-g PA-h PA-i

307 302 309 336 301 307 310 307 334

420 418 405 442 419 423 428 423 443

315 311 318 344 310 314 318 314 343

434 445 428 461 439 440 442 440 464

378 373 382 405 375 377 387 377 404

a Measured at a concentration of 10−5 mol/L in THF based on structure units. bThey were excited at the λabs,max for the solution state.

Table 2. Thermal Properties of Various Aramides Td10%b (°C) polymer

Tga (°C)

in N2

in air

char yieldc (%)

PA-3a PA-3b PA-3c PA-3d PA-3e PA-3f PA-3g PA-3h PA-3i

271 287 275 279 281 277 275 283 277

510 521 520 522 520 519 522 523 535

455 480 475 489 456 487 442 495 510

0.76 0.41 0.33 0.44 0.71 0.43 0.45 0.33 0.89

From DSC measurements conducted at a heating rate of 10 °C/min. Temperature at 10% weight loss (Td10%) was determined by TGA at a heating rate of 20 °C/min. cResidual weight percentage at 800 °C in nitrogen.

a b

Figure 2. (a) UV−vis and (b) PL spectra of protonated PA-a solution by HCl.

2.6. Optical Properties of Polyamides. The optical properties of the polyamides were investigated by UV−vis and photoluminescence spectroscopy. The results are summarized in Table 3. UV−vis absorption spectra of these polymers in THF solutions exhibited a strong absorption at 307−336 nm, which can be assigned to a π−π* transition resulting from the conjugated structure. The UV−vis absorption of polyamide films also showed similar single absorbance at 310−344 nm. Their PL spectra of THF solutions and films showed maximum bands approximately at 434−464 nm and 428−464 nm, respectively. 2.7. Protonation Properties. Pyridine is an electrondeficient aromatic heterocyclic substituent, with localized lone pair electrons in sp 2 orbital on the nitrogen atom. Consequently, the derived polymer offers the possibility of protonation or alkylation of the lone pair as a way of modifying their properties. The optical properties of the polymers after protonation with hydrochloric acid (HCl) were also investigated. The absorption spectra of PA-a are shown in Figure 2a

as a function of HCl concentration. At low protonated concentration (lower than 0.1 M), the absorption bands are almost the same as the pristine original sample of polymer. As the acid concentration increased (higher than 0.1 M), it is apparent that the absorption decreases at around 307 nm and a new absorption band at 394 nm formed gradually after the HCl addition, which means these polyamides can be used as proton sensors via a protonation mechanism. The new band at 394 nm could be attributed to the protonated form structure of the pyridine moiety. At high degree levels of protonation, the isosbestic point is lost, which is due to repulsion between the charged pyridinium fragments.6 The emission spectra of the polymer protonated with HCl in different concentrations are shown in Figure 2b. The intensity of the fluorescent peak at 420 nm observed in the neutral polymer solution decreases, and a new fluorescent peak at 552 nm arises after protonation by 7447

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HCl. The intensity of the new peak increases as the HCl concentration becomes higher. It means the protonated polymer structure absorbs longer visual wavelength and emits longer wavelength than the unprotonated polymer structure. The intensity of emission peak at 552 nm is stronger than the peak at 420 nm when HCl concentration is higher than 0.1 M. This phenomenon also means these polyamides can be used as proton sensors. As shown in Figure S1, the 1H NMR spectrum of the protonated polyamide has an additional peak around 4.5 ppm as a proof of the protonation. As shown in Figure 3, the

Figure 4. (a) Oxidation and (b) reduction voltammograms of PA-a in CH3CN containing 0.1 M TBAP. The scan rate was 0.1 V/s.

Figure 3. PL spectra of protonated PA-a solution.

intensity of the fluorescent peak at 552 nm in polyamide solution protonated by 1 M HCl increases as more MeOH is added. Because the intramolecular and intermolecular hydrogen bonds are cleaved by adding polar additives like methanol, the intensity of the emission at 552 nm based on excimers increased as MeOH content increased. This means that the structure of polyamide transforms from being more packed, congested, and overlapping with each other to become less packed and form more excimers. When MeOH is added, the chains become loose as a solvent response and become less crowded.17 2.8. Electrochemical behavior. The electrochemical behavior of the polyamides was investigated by cyclic voltammetry, which was conducted on film cast on an ITOcoated glass substrate as the working electrode in dry acetonitrile (CH3CN) containing 0.1 M of TBAP as an electrolyte under a nitrogen atmosphere. Figure 4 exhibits the electrochemical behavior for PA-a. Only oxidation was observed and no reduction occurs in the cyclic voltammogram of PA-a. 2.9. Computational Study for Oxidation Mechanism. All theoretical calculations in this study were carried out using the quantum mechanical package Gaussian 03.19,20 Equilibrium structures of the basic units of the studied polyamides, as shown in Figure 5, were determined using CAM using the B3LYP method and the 6-31G(d) basis set. The atomic charge was determined by Mulliken population analysis. The sketch map of PA-a and optimized structure, which was determined by CAM(B3LYP/6-31G(d)), are plotted in Figure 5. A portion of the calculated electronic properties are summarized in Table 4. The main atomic charge difference were located on 31C, 32C, 42C, 43C, and 49N atoms. The pendant carbazole moieties have almost all of the oxidation potential. Therefore, the irreversible oxidation can be explained by a possible coupling reaction between carbazoleium radical cations and biscarbazo-

Figure 5. Sketch map of the studied polyamide structure.

Table 4. Atomic Charge Distribution of Selected Atoms with the First Oxidation State Polyamide Model OX1

31C

32C

42C

43C

49N

0.2334

0.2337

0.2211

0.2322

0.4292

leium dications, which was previously reported by Ambrose and co-workers in their pioneering work17 on the anodic oxidation of carbazole and other various N-substituted derivatives; their studies indicate that ring−ring coupling is the predominant decay pathway. It means the new polyamides have potential to use in a WORM polymer memory. The energy of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for the investigated polyamides could be determined from the oxidation onset potentials and the onset absorption wavelength of the polymer films. The oxidation half-wave potential for PAa was determined to be 1.46 V vs Ag/Ag+ in CH3CN. The external ferrocene/ferrocenium (Fc/Fc+) redox standard E1/2 was 0.62 V vs Ag/Ag+ in CH3CN. Assuming that the HOMO energy for the ferrocene standard was −4.8 eV with respect to the zero vacuum level, the HOMO energy for PA-a was determined to be −5.64 eV. The LUMO of PA-a was calculated to be −2.36 eV based on the energy gap obtained from UV−vis spectrum. The HOMOs, LUMOs, and energy gaps of the polyamides are summarized in Table 5. 2.10. Electrochromic Characteristics. Electrochromism of PA-a (thin film) was examined by an optically transparent thin-layer electrode coupled with UV−vis spectroscopy. The electrode preparations and solution conditions were identical to 7448

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acetate, and then vacuum-dried before use. All other reagents were used as received from commercial sources. 3.2. Synthesis of 4-(9-Ethyl-3-carbazole)-2,6-bis(4nitrophenyl)pyridine (CBNPP) (Scheme 1). In a 500 mL, roundbottomed flask, a mixture of 16.67 g (75 mmol) of 9-ethyl-3carbazolecarboxaldehyde, 24.66 g (150 mmol) of 4′-nitroacetophenone, 115.10 g (1.5 mol) of ammonium acetate, and 360 mL of glacial acetic acid was refluxed for 72 h. Upon cooling, the precipitated light yellow solid was collected by filtration and washed with cold acetic acid. The product was recrystallized from DMF five times to afford 8.0 g (41%) of light yellow powder; mp 280 °C (by DSC). 1H NMR (500 MHz, DMSO-d6): δ 8.99 (s, 1H), 8.68−8.66 (d, 4H), 8.59 (s, 2H), 8.41−8.39 (d, 4H), 8.35−8.33 (d, 1H), 8.25−8.23 (d, 1H), 7.80−7.78 (d, 1H), 7.68−7.78 (d, 1H), 7.54−7.51 (t, 1H), 7.31−7.28 (t, 1H), 4.55−4.51 (m, 2H), 1.39−1.36 (t, 3H). 13C NMR (500 MHz, DMSOd6): δ 154.46, 151.08, 147.88, 144.66, 140.41, 140.14, 128.23, 127.25, 126.24, 125.20, 123.84, 122.93, 122.43, 120.76, 119.78, 119.21, 118.65, 109.62, 109.49, 37.15, 13.73. IR (KBr) 1591, 1341 cm−1 (NO2 stretch). Anal. Calcd for C31H22N4O4: C, 72.36; H,4.31; N,10.89; O, 12.44; Found: C, 72.29; H, 4.32; N, 10.58. 3.3. Synthesis of 4-(9-Ethyl-3-carbazole)-2,6-bis(4aminophenyl)pyridine (CBAPP). A mixture of 4.42 g (8.60 mmol) CBNPP, 0.15 g of 10% Pd/C, 20 mL of hydrazine monohydrate, and 35 mL of ethanol was placed in a flask. The reaction was heated to 90 °C for 24 h and then subsequently filtered to remove the Pd/C. After cooling, the precipitated crystals were isolated by filtration, recrystallized from ethanol twice, ground into powder, and vacuum-dried to afford 1.57 g (44%) of white powder; mp 125 °C. 1 H NMR (500 MHz, DMSO-d6): δ 8.89 (s, 1H), 8.40−8.38 (d, 1H), 8.17−8.38 (d, 4H), 8.10−8.08 (s, 2H), 8.03 (s, 1H), 7.71−7.70 (d, 1H), 7.60−7.58 (d, 1H), 7.49−7.46 (t, 1H), 7.28−7.25 (t, 1H), 6.81− 6.79 (d, 4H), 5.44 (s, 4H), 4.46−4.42 (m, 2H), 1.33−1.31 (t, 3H). 13C NMR (500 MHz, DMSO-d6): δ 156.52, 149.76, 149.38, 140.93, 139.93, 129.10, 127.78, 126.95, 126.03, 124.81, 122.88, 122.47, 120.86, 119.13, 118.95, 113.72, 112.65, 109.41, 109.24, 37.06, 13.66. IR (KBr) 3471 and 3379 (N−H stretching) and 1620 cm−1 (N−H deformation) Anal. Calcd for C31H26N4: C, 81.91; H, 5.77; N, 12.33; Found: C, 81.84; H, 5.86; N, 12.07. 3.4. Synthesis of Polyamides. All aramides were prepared using similar procedures. The preparation of PA-a is described below as an example. A solution of 0.345 g (0.760 mmol) of CBAPP in 3.8 mL of NMP was cooled in an ice bath. Next, 0.8 mL of propylene oxide was added to the mixture, which was followed by the addition of 0.326 g (0.760 mmol) of 2,2-bis(4-carboxyphenyl)hexafluoropropane acid chloride. The mixture was stirred at room temperature for 6 h. The polymer solution was poured slowly into 100 mL of methanol, which resulted in the formation of a stringy, fiber-like precipitate that was collected and washed by Soxhlet. Finally, the polymer was vacuum-dried and received in a quantitative yield. The IR spectrum of PA-a (film) exhibited characteristic amide absorption bands at approximately 3317 cm−1 (N−H stretching) and 1662 cm−1 (CO stretching). 1H NMR (500 MHz, DMSO-d6): δ 10.1 (amide NH), 8.92 (s, 1H), 8.35−8.37 (d, 5H), 8.25 (s, 1H), 8.14−8.16 (s, 1H), 7.83−7.85 (d, 2H), 7.74−7.75 (d, 1H), 7.61−7.62 (d, 1H), 7.48 (s, 1H), 7.25 (s, 1H), 4.49 (s, 2H), 2.45 (s, 2H), 1.71− 1.76 (m, 2H), 1.34 (s, 3H) 13C NMR (500 MHz, DMSO-d6): δ 171.3, 155.8, 150.0, 140.3, 140.2, 140.1, 133.96, 128.8, 128.3, 128.1, 127.3, 126.0, 124.9, 122.9, 122.5, 120.8, 119.4, 118.9, 115.0, 109.5, 109.3, 37.1, 36.4, 25.1, 13.7. Anal. Calcd for C31H26N4: C, 71.0%; H, 4.1%; N, 6.9%. Found: C, 69.3%; H, 3.8%; N, 7.1%.

Table 5. Electrochemical Properties of Various Aramides polymer code

oxidationa (V)

band gapb (eV)

HOMOc (eV)

LUMOc (eV)

PA-a PA-b PA-c PA-d PA-e PA-f PA-g PA-h PA-i

1.46 1.38 1.54 1.42 1.51 1.41 1.38 1.53 1.40

3.28 2.75 2.68 2.53 2.73 2.72 2.65 2.72 2.53

−5.64 −5.56 −5.72 −5.60 −5.69 −5.59 −5.56 −5.71 −5.58

−2.36 −2.81 −3.04 −3.07 −2.96 −2.87 −2.91 −2.99 −3.05

a

From cyclic voltammograms vs Ag/Ag+ in CH3CN. bBand gap = 1240/λabs,onset of the polymer film. cCalculated from the equations HOMO= −(Eox onset − EFconset) − 4.8 and LUMO = HOMO + band gap. EFconset = 0.62.

those used in CV. All of the polymers exhibited similar colorchanging behaviors, and the typical behavior of PA-a is shown in Figure 6. When the applied potentials increased in the

Figure 6. Electrochromic behavior of a PA-a film (in CH3CN with 0.1 M TBAP as the supporting electrolyte) at (1) 0.00, (2) 1.35, (3) 1.40, (4) 1.45, (5) 1.50, (6) 1.55, and (7) 1.60 V.

positive direction, from 0 to 1.35 V, the absorbance at 315 nm, which is a characteristic peak for the neutral form of polyamide PA-a, decreased gradually. One new band appeared at 387 nm, which was due to the first stage of oxidation. However, the change is irreversible due to ring−ring coupling of oxidized carbazole, as described above.

3. EXPERIMENTAL SECTION 3.1. Materials. 9-Ethyl-3-carbazolecarboxaldehyde was purchased from Aldrich. 4′-Nitroacetopeheone, ammonium acetate, hydrazine monohydrate, and 10% Pd/C (Merck) were used as received. N,NDimethylacetamide (DMAc; MERCK), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidinone (NMP) (MERCK), and pyridine (MERCK) were dried over calcium hydride overnight, distilled under reduced pressure, and stored over 4 Å molecular sieves in a sealed bottle. Commercially available aromatic dicarboxylic acids, which included terephthalic acid (Aldrich), isophthalic acid (Aldrich), 5-tert-butylisophthalic acid (Aldrich), 4,4′-oxidibenzoic acid (Aldrich), 4,4′-sulfonyldibenzoic acid (Aldrich), 2,6-naphthalenedicarboxylic acid (TCI), 1,4-naphthalenedicarboxylic acid (TCI), 4,4′-diphenyldicarboxylic acid (TCI), and 2,2-bis(4-carboxyphenyl)hexafluoropropane (TCI), were used as received. Tetra-n-butylammonium perchlorate (TBAP) was obtained from ACROS, recrystallized twice from ethyl

4. CONCLUSIONS A series of polyamides were successfully prepared from commercially available diacid chlorides and a novel diamine monomer containing pyridine and unsymmetric carbazole moieties. The structures of the polyamides that had unsymmetric pendant carbazole were identified. The new polyamides had excellent solubility in common organic 7449

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solvents, high molecular weight, good thermal stability, and high glass transition temperatures; these properties meet the basic requirements for the development of optoelectronics and photonics. When the acid concentration increased (higher than 0.1 M), it is apparent a new absorption band formed gradually after protonation by addition of HCl; that means these polyamides can be used as proton sensors via a protonation mechanism. The fluorescent intensity of protonated polymer solution can be enhanced by solvent response (adding methanol).

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone 886-2-27376638 or 886-2-27335050; Fax 886-223781441 or 886-2-27376644; e-mail [email protected]. tw, [email protected], [email protected], or liawdj@yahoo. com.tw (D.-J.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Science Council of the Republic of China for the financial support of this work.

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REFERENCES

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dx.doi.org/10.1021/ma4013972 | Macromolecules 2013, 46, 7443−7450