Synthesis and electrochromic properties of polyamides having pendent carbazole groups

Materials Chemistry and Physics 141 (2013) 665e673

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Synthesis and electrochromic properties of polyamides having pendent carbazole groups Sheng-Huei Hsiao a, *, Hui-Min Wang a, Jun-Wen Lin a, Wenjeng Guo a, Yu-Ruei Kung b, Chyi-Ming Leu b, Tzong-Ming Lee b a b

Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan, ROC Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan, ROC

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


New polyamides with pendent carbazole groups have been successfully synthesized.
Solutions and thin films of these polyamides showed blue photoluminescence.
The polyamides displayed blue or green coloring upon electrochemical oxidation.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 January 2013 Received in revised form 16 May 2013 Accepted 17 May 2013

Two series of electroactive polyamides (5aed and 50 aed) with pendent carbazole groups were synthesized from N-(4-(3,6-di-tert-butylcarbazol-9-yl)phenyl)-3,5-diaminobenzamide (3) and N-(4-(carbazol-9-yl)phenyl)-3,5-diaminobenzamide (30 ), respectively, with four dicarboxylic acids via the phosphory lation polyamidation technique. These polyamides were readily soluble in many organic solvents and could afford strong and flexible films via solution casting. They showed useful levels of thermal stability, with Tg in the range of 271e322  C. The dilute solutions of these polyamides showed a blue photoluminescence with emission maxima around 375e438 nm. They showed well-defined and reversible redox couples upon electrochemical oxidation, together with a strong color change from a colorless neutral form to blue or green oxidized forms. The 5 series polyamides exhibited better redox-stability and electrochromic performance as compared to the corresponding analogs (the 50 series) without tert-butyl substituents on the active sites of the carbazole unit. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Polymers Chemical synthesis Electrochemical properties Thermal properties

1. Introduction Electrochromism refers to the reversible electromagnetic absorbance/transmittance and color change resulting from the oxidation or the reduction of the material in response to an externally applied potential by electrochemical means [1].

* Corresponding author. Tel.: þ886 2 27712171x2548; fax: þ886 2 27317117. E-mail address: [email protected] (S.-H. Hsiao). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.05.054

Electrochromic materials have been studied for different technological applications, such as smart windows for cars and buildings, antiglare rear-view mirrors, electrochromic displays, and earthtone chameleon materials [2e8]. Among the different types of electrochromic materials [9,10], conjugated polymers [11e16] such as polyanilines, polypyrroles, polyselenophenes, polythiophenes, and in particular, poly(3,4-ethylenedioxythiophene) (PEDOT) and its derivatives [17,18] attract much interest because of several advantages such as mechanical flexibility, ease in band-gap/colortuning via structural control, and the potential for low-cost

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processing for large-area devices. In recent years, triarylaminecontaining condensation polymers such as aromatic polyamides and polyimides have emerged as a new and attractive family of electrochromic materials because of high electroactivity of the triarylamine unit and high thermal stability of the polymer backbones [19]. Carbazole derivatives have been widely used as holetransporting and host materials in organic light-emitting diodes because of their good hole-transporting ability and sufficiently large triplet energy [20e22]. Carbazole can be easily functionalized at the (3,6)-, (2,7)-, or 9- (N-) positions and then covalently linked into polymeric systems, both in the main chain as building blocks and in the side chain as pendent groups [23e26]. Polymers-containing carbazole moieties in the main chain or side chain have attracted much attention because of their unique properties, which allow various optoelectronic applications such as electrophotography, light-emitting diodes, photorefractive materials, and photovoltaic devices [24e26]. As reported previously [27e29], carbazole group can be used as a fluorophore and electrochromophore onto the backbones of high-performance polymers such as aromatic polyamides and polyimides. However, the electrogenerated cation radical of carbazole is not reversible, possibly due to the electrochemical coupling of carbazoles through the active C-3 and C-6 sites [30]. This may lead to problems with electrochemical and electrochromic stability of this kind of electrochromic materials. It has been demonstrated that introduction

Fig. 1. (a) 1H NMR, (b)

13

of bulky groups such as tert-butyl group on the electrochemically active sites (C-3 and C-6) of carbazole leads to enhanced electrochemical and morphological stability [31]. Continuing our effort in the synthesis of polyamides with potential electrochromic applications, in this work two m-phenylenediamines (3 and 30 ) with N-phenylcarbazole pendants via the amide linkage on the 5-position were synthesized and reacted with aliphatic or aromatic dicarboxylic acids leading to two series of polyamides with pendent carbazole units. Basic characterization and fluorescent, electrochemical and electrochromic properties of the polyamides were studied. The effects of the incorporation of tert-butyl groups on the C-3 and C-6 positions of pendent carbazole units on the properties of these polyamides were also investigated. 2. Experimental 2.1. Materials 3,6-Di-tert-butyl-N-(4-aminophenyl)carbazole (1) and N-(4aminophenyl)carbazole (10 ) were synthesized according to literature methods [27,31]. 3,5-Dinitrobenzoyl chloride (Arcos), 10% palladium on charcoal (Pd/C, Fluka), triethylamine (Acros), triphenyl phosphite (TPP, Acros), and hydrazine monohydrate (TCI) were used without further purification. Dimethyl sulfoxide (DMSO, Tedia), N,N-dimethylformamide (DMF, Acros), N,N-dimethylacetamide (DMAc, Fluka), pyridine (Py, Wako), and N-methyl-2-

C NMR, (c) HeH COSY, and (d) CeH HMQC spectra of diamine monomer 3 and in DMSO-d6.

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Scheme 1. Synthesis of diamine monomers 3 and 30 .

pyrrolidone (NMP, Tedia) were dried over calcium hydride for 24 h, distilled under reduced pressure, and stored over 4 Å molecular sieves in a sealed bottle. The dicarboxylic acid monomers such as 1,4-cyclohexanedicarboxylic acid (4a) (TCI), 4,40 -biphenydicarboxylic acid (4b) (TCI), 4,40 -dicarboxydiphenyl ether (4c) (TCI), and 2,2bis(4-carboxyphenyl)hexafluoropropane (4d) (TCI) were used as received. Calcium chloride was dried under vacuum at 150  C for 6 h prior to use. Tetrabutylammonium perchlorate (Bu4NClO4) (Arcos) was recrystallized twice by ethyl acetate under nitrogen atmosphere and then dried in vacuo prior to use. All other reagents were used as received from commercial sources. 2.2. Monomer synthesis 2.2.1. N-[4-(3,6-di-tert-butylcarbazol-9-yl)phenyl]-3,5dinitrobenzamide (2) In a 250 mL round-bottom flask equipped with a stirring bar and a nitrogen gas inlet tube, 2.96 g (0.008 mol) of 3,6-di-tert-butyl-N(4-aminophenyl)carbazole (1) was dissolved in 150 mL of NMP. After the reaction solution was cooled to 0e5  C, 3,5-dinitrobenzoyl chloride (1.84 g, 0.008 mol) was slowly added and the reaction mixture was stirred at room temperature for 3 h. The red precipitate was collected by filtration, washed with water, and dried in vacuum at 80  C to give 3.5 g of the dinitro compound (2) as red powder in 78% yield. Mp ¼ 312e313  C, measured by DSC at 10  C min1.

IR (KBr) [Figure S1; Supplemental Information (SI)]: 3045 cm1 (amide NeH stretch), 2956 cm1 (t-butyl CeH stretch), 1519, 1342 (eNO2 stretch). 1H NMR (500 MHz, DMSO-d6, d, ppm) (for the peak assignments, see SI Figure S2): 1.43 (s, 18H, Ha, t-butyl), 7.33 (d, J ¼ 8.6 Hz, 2H, Hd), 7.50 (d, J ¼ 8.6 Hz, 2H, Hc), 7.66 (d, J ¼ 8.8 Hz, 2H, He), 8.08 (d, J ¼ 8.8 Hz, 2H, Hf), 8.29 (s, 2H, Hb), 9.04 (s, 1H, Hh), 9.23 (s, 2H, Hg), 11.09 (s, 1H, amide). 2.2.2. N-[4-(3,6-di-tert-butylcarbazol-9-yl)phenyl]-3,5-diamino benzamide (3) In a 500 mL three-neck round-bottom flask equipped with a stirring bar, 3.38 g (0.006 mol) of dinitro compound 2 and 0.15 g of 10% Pd/C were dissolved/suspended in 200 mL of ethanol under nitrogen atmosphere. The suspension solution was heated to reflux, and 4 mL of hydrazine monohydrate was added slowly to the mixture. After a further 24 h of reflux, the solution was filtered hot to remove Pd/C, and the filtrate was then cooled to precipitate white product. The product was collected by filtration and dried in vacuum at 80  C to give 3.5 g of the desired diamino compound 3 as colorless crystals in 82% yield. Mp ¼ 282e283  C, measured by DSC at 10  C min1. IR (KBr): 3200e3500 cm1 (amide and amine NeH stretch). 1H NMR (500 MHz, DMSO-d6, d, ppm) [for the peak assignments, see Fig. 1(a)]: 1.41 (s, 18H, Ha, t-butyl), 4.96 (s, 4H, eNH2), 6.04 (s, 1H, Hh), 6.36 (s, 2H, Hg), 7.28 (d, J ¼ 8.7 Hz, 2H, Hd), 7.47 (d, J ¼ 8.7 Hz, 2H, Hc), 7.53 (d, J ¼ 8.8 Hz, 2H, He), 8.03 (d, J ¼ 8.8 Hz, 2H, Hf), 8.28

Scheme 2. Synthesis of polyamides 5aed and 50 aed.

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2.3. Polymer synthesis The synthesis of polyamide 5c was used as an example to illustrate the general synthetic route used to produce the polyamides. A mixture of 0.404 g (0.80 mmol) of diamine monomer 3, 0.207 g (0.80 mmol) of 4,40 -dicarboxydiphenyl ether (4c), 0.15 g of anhydrous calcium chloride, 0.80 mL of triphenyl phosphite (TPP), 0.20 mL of pyridine, and 0.60 mL of NMP was heated with stirring at 120  C for 3 h. The polymerization proceeded homogeneously throughout the reaction and afforded clear, highly viscous polymer solution. The resulting viscous solution was poured slowly with stirring into 150 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 polyamide 5c. The inherent viscosity of the polymer was 0.51 dL g1, measured in DMAc (containing 5 wt% LiCl) at a concentration of 0.5 g dL1 at 30  C. IR spectrum of 5c (film): 3300 cm1 (amide NeH stretch), 2960 cm1 (t-butyl CeH stretch), 1658 cm1 (amide carbonyl). Fig. 2. Typical IR spectra of polyamides 5c and 50 c.

2.4. Preparation of the polyamide films (s, 2H, Hb), 10.20 (s, 1H, amide protons). 13C NMR (125 Hz, DMSO-d6, d, ppm) [for the peak assignments, see Fig. 1(b)]: 31.82 (C16), 34.45 (C15), 102.32 (C12, C14), 108.99 (C5), 116.53 (C2), 121.17 (C9), 122.60 (C1), 123.60 (C4), 126.59 (C8), 132.03 (C10), 136.72 (C11), 138.62 (C7), 138.77 (C3), 142.25 (C6), 149.16 (C13), 167.45 (amide carbons).

Fig. 3. (a) 1H NMR, (b)

13

A solution of the polymer was made by dissolving about 0.50 g of the polyamide sample in 8 mL of DMAc. The homogeneous solution was poured into a 7-cm glass Petri dish, which was placed in a 90  C oven for 3 h to remove most of the solvent; then the semidried film was further dried in vacuo at 150  C for 8 h. The

C NMR, (c) HeH COSY, and (d) CeH HMQC spectra of polyamide 5c and in DMSO-d6.

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Table 1 Inherent viscosity and solubility of polyamides.

hinha (dL g1)

Polymer code

Solubility in various solventsb NMP

0

5a (5 a) 5b (50 b) 5c (50 c) 5d (50 d)

0.30 0.43 0.51 0.36

(0.55) (0.84) (0.63) (0.51)

c

þþ þþ þþ þþ

c

(þþ) (þþ) (þþ) (þþ)

DMAc

DMF

DMSO

m-cresol

THF

þþ þþ þþ þþ

þþ (þþ) þ () þþ (þþ) þþ (þþ)

þþ þþ þþ þþ

þ  þ þ

þþ ()  () þ () þþ (þþ)

(þþ) (þþ) (þþ) (þþ)

(þþ) (þþ) (þþ) (þþ)

(þ) () (þ) (þ)

þþ, soluble at room temperature; þ, soluble on heating; , partially soluble; , insoluble even on heating. a Measured at a polymer concentration of 0.5 g dL1 in DMAce5 wt% LiCl at 30  C. b The qualitative solubility was tested with 10 mg of a sample in 1 mL of stirred solvent. c Data shown in parentheses are those of the 50 series polyamides.

obtained films were about 40e60 mm thick and were used for X-ray diffraction measurements, solubility tests, and thermal analyses. 2.5. Measurements Infrared (IR) spectra were recorded on a Horiba FT-720 FT-IR spectrometer. 1H and 13C NMR spectra were measured on a Bruker AVANCE 500 FT-NMR system with tetramethylsilane as an internal standard. The inherent viscosities were determined with a CannoneFenske 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 with a graphite monochromator (operating at 40 kV and 30 mA), using nickel-filtered Cu-Ka radiation (l ¼ 1.5418 Å). The scanning rate was 2 min1 over a range of 2q ¼ 10e40 . Thermogravimetric analysis (TGA) was performed with a PerkineElmer Pyris 1 TGA. Experiments were carried out on approximately 4e6 mg of samples heated in flowing nitrogen or air (flow rate ¼ 40 cm3 min1) at a heating rate of 20  C min1. DSC analyses were performed on a PerkineElmer Pyris 1 DSC at a scan rate of 20  C min1 in flowing nitrogen. Ultravioletevisible (UVevis) spectra of the polymer films were recorded on an Agilent 8453 UVevisible spectrometer. Electrochemistry was performed with a CHI 750A electrochemical analyzer. Voltammograms are presented with the positive potential pointing to the left and with increasing anodic currents pointing downwards. Cyclic voltammetry (CV) was conducted with the use of a three-electrode cell in which ITO (polymer films area about 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 (þ0.48 V vs. Ag/AgCl). 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 measured with an Agilent 8453 UVevisible spectrophotometer. Photoluminescence (PL) spectra were measured with a Varian Cary Eclipse fluorescence spectrophotometer. Fluorescence quantum yields (FF) values of the samples in NMP were measured by using quinine sulfate in 1 N H2SO4 as a reference standard (FF ¼ 0.546). All corrected fluorescence excitation spectra were found to be equivalent to their respective absorption spectra. 3. Results and discussion 3.1. Monomer synthesis The diamine monomers-containing N-phenylcarbazole pendent group, 3 and 30 , were prepared by the synthetic route outlined in Scheme 1. The intermediate dinitro compounds 2 and 20 were prepared by condensation of 3,6-di-tert-butyl-N-(4-aminophenyl) carbazole (1) and N-(4-aminophenyl)carbazole (10 ), respectively, with 3,5-dinitrobenzoyl chloride. Diamines 3 and 30 were prepared in good yields by the hydrazine Pd/C-catalyzed reduction of the respective dinitro compounds in refluxing ethanol. IR, 1H NMR and 13 C NMR spectroscopic techniques were used to identify structures of the intermediate dinitro compounds and the target diamine monomers. Figure S1 (Supplemental Information; SI) illustrates FT-IR spectra of all the synthesized compounds. The IR spectra of compounds 2 and 20 gave characteristic bands of nitro groups at around 1537 and 1340 cm1 (eNO2 asymmetric and symmetric stretching),

Table 2 Thermal properties of polyamides.a Polymer code

Tg ( C)b

5a (50 a) 5b (50 b) 5c (50 c) 5d (50 d)

315 e 306 322

(275)e (296) (271) (301)

Td at 5% weight loss ( C)c

Td at 10% weight loss ( C)c

N2

N2

433 412 448 457

Air (418) (491) (450) (477)

449 420 451 491

(411) (504) (458) (493)

463 437 478 483

Air (460) (565) (511) (543)

497 453 493 535

(476) (567) (532) (544)

Char yield (wt%)d 59 56 54 52

(47) (76) (69) (65)

a The polymer film samples were heated at 300  C for 30 min before all the thermal analyses. b Midpoint temperature of the baseline shift on the second DSC heating trace (rate ¼ 20  C min1) of the sample after quenching from 400 to 50  C (rate ¼ 200  C min1) in nitrogen. c Decomposition temperature at which a 5% or 10% weight loss was recorded by TGA at a heating rate of 20  C min1 and a gas flow rate of 20 cm3 min1. d Residual weight percentage at 800  C in nitrogen. e Data shown in parentheses are those of the 50 series polyamides.

Fig. 4. TGA curves of polyamides 5c and 50 c with a heating rate 20  C min1.

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Table 3 Optical properties of polyamides.a Polymer In solutionb code abs

lmax

5a (5 a) 5b (50 b) 5c (50 c) 5d (50 d)

298 299 298 298

(294) (295) (294) (294)

FFc (%)

(nm)b

(nm) 0

As film

lPL max

d

375 372 375 375

(375) (411) (435) (384)

0.22 0.07 0.10 0.09

(0.12) (0.10) (0.06) (0.04)

labs onset

labs max

lPL max

(nm)

(nm)

(nm)

378 363 366 377

299 300 297 300

465 500 473 477

(371) (353) (344) (358)

(296) (297) (296) (296)

(371) (353) (344) (358)

a

Excited at the absorption maximum for both solution and the solid film states. Measured in dilute solutions in NMP at a concentration of about 1  105 mol L1. c Fluorescence quantum yield calculated in an integrating sphere with quinine sulfate as the standard (FF ¼ 54.6%). d Data shown in parentheses are those of the 50 series polyamides. b

which disappeared after reduction. Diamines 3 and 30 showed a typical eNH2 stretching absorption pair in the region of 3200e 3500 cm1. Compounds 2 and 3 showed an additional aliphatic Ce H stretching absorption around 2956 cm1 due to the presence of tert-butyl groups. The 1H and 13C NMR spectra of dinitro compound 2 are compiled in Figure S2 (SI). Fig. 1 shows the 1H and 13C NMR spectra of the diamine monomer 3. The 1H NMR spectra confirm that the nitro groups have been completely transformed into amino groups by the high field shift of the protons Hg and Hh and the resonance signals at around 5.0 ppm corresponding to the amino protons. The presence of amide linkage in diamine monomers 3 and 30 can be evidenced by the resonance peaks at 10.2 ppm in their 1H NMR spectra and at 167 ppm in their 13C NMR spectra. Assignments of all proton and carbon signals were assisted by the twodimensional (2-D) NMR spectra. These spectra are in good agreement with their proposed molecular structures. The NMR spectra of compounds 20 and 30 shown in Figures S3 and S4 (SI) are essentially identical to those reported in literature [32]; however, we believe that a more correct assignment of the resonance signals is made here through the 2-D NMR spectra. Fig. 5. Cyclic voltammograms of the cast films of polyamides (a) 5d and (b) 50 d on the ITO-coated glass slide in CH3CN-containing 0.1 M Bu4NClO4 at a scan rate of 100 mV s1.

3.2. Polymer synthesis The direct polycondensation of diamine 3 or 30 with the dicarboxylic acids (4aed) was undertaken successfully according to the phosphorylation method reported by Higashi for the synthesis of polyamides [33]. The schematic synthesis route and the structure of resulting polyamides are shown in Scheme 2. All the polymerizations proceeded homogeneously throughout the reaction and afforded clear, highly viscous polymer solutions. The polymers precipitated in a tough, fiber-like form when the resulting polymer solutions were slowly poured into stirred methanol. These polyamides were obtained in almost quantitative yields, with inherent viscosities in the range of 0.30e0.84 dL g1. All the polymers can be solution cast into flexible and tough films,

and this is indicative of the formation of high-molecular-weight polymers. The formation of polyamides was also confirmed by IR, 1H and 13 C NMR spectra. Fig. 2 shows typical IR spectra of polyamides 5c and 50 c. The characteristic IR absorption bands of the amide group appeared at around 3300 cm1 (NeH stretching) and 1657 cm1 (amide carbonyl). Fig. 3 and Figure S5 show the 1H NMR, 13C NMR, and COSY spectra of polyamides 5c and 50 c, respectively. All the peaks could be readily assigned to the hydrogen and carbon atoms in the repeating unit. The resonance peaks appearing at 10.6 ppm in

Table 4 Electrochemical properties of polyamides. Polymer code

Oxidation potential (V)a Eonset

0

5a (5 a) 5b (50 b) 5c (50 c) 5d (50 d) a b c d e

1.05 0.99 0.99 0.99

(1.12) (1.11) (1.11) (1.11)

Bandgap (eV)b

E1/2 e

1.24 1.21 1.21 1.21

(1.24) (1.26) (1.24) (1.24)

3.31 3.42 3.39 3.29

(3.34) (3.51) (3.61) (3.46)

HOMO (eV)c

LUMO (eV)d

Eonset

E1/2

5.38 5.35 5.35 5.35

5.60 5.57 5.57 5.44

Oxidation potentials from cyclic voltammograms (vs. Ag/AgCl in CH3CN). Energy gap ¼ 1240/lonset of the polymer film. The HOMO energy levels were calculated from E1/2 or Eonset, referenced to ferrocene (4.8 eV). LUMO ¼ HOMO  Bandgap. Data shown in parentheses are those of the 50 series polyamides.

(5.47) (5.46) (5.46) (5.46)

(5.60) (5.62) (5.60) (5.60)

Eonset

E1/2

2.14 2.00 2.03 2.13

2.29 2.15 2.18 2.15

(2.14) (1.95) (1.85) (2.00)

(2.25) (2.11) (1.99) (2.14)

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the 1H NMR spectra and at 170 ppm in the 13C NMR spectra also support the formation of amide linkages in the main chain. 3.3. Polymer properties 3.3.1. Basic characterization The wide-angle X-ray diffraction (WAXD) patterns of the polyamide films are shown in Figure S6 (SI). These polyamides exhibited an amorphous nature because of the bulky, packing-disruptive pendent carbazole and tert-butyl groups in the polymer side chain, which does not favor their close chain packing. The solubility behavior of polyamides was tested qualitatively, and the results are summarized in Table 1. All the polyamides were highly soluble in polar solvents such as NMP, DMAc, DMF, and DMSO, and the high solubility could be attributed to the introduction of bulky, packing-disruptive bulky pendent substituents in the repeat unit, which decrease interchain interactions and increase the free volume. The 5 series polyamides showed a better solubility as compared to the 50 series due to the effect of the large volume of tert-butyl groups. Thus, the excellent solubility makes these polymers potential candidates for practical applications by spin-coating or inkjet-printing processes to afford high-performance thin films for optoelectronic devices. 3.3.2. Thermal properties The thermal properties of the polyamides were investigated by TGA and DSC techniques. The thermal behavior data are summarized in Table 2. Typical TGA thermograms for polyamides 5c and 50 c are shown in Fig. 4. They showed no significant weight loss before 400  C. Their decomposition temperatures (Td) at a 10% weight loss in nitrogen were recorded at 437e483  C for the 5 series and 460e565  C for the 50 series. These polyamides left high carbonized residue (char yield) in excess of 47% at 800  C in an inert atmosphere, due to their high aromatic content. Almost all the 5 series polyamides exhibited a lower Td value as compared with their corresponding 50 series counterparts without tert-butyl group. This is reasonable when considering the less stable tert-butyl

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segments. The glass-transition temperatures (Tg) were observed in the range of 306e322  C for the 5 series polyamides and 271e 301  C for the 50 series polyamides. The higher Tg values for the 5 series polymers than the corresponding 50 analogs can be attributed to the increased mobility hindrance of the tert-butyl substituents. All the polymers indicated no clear melting endotherms up to the decomposition temperatures on the DSC thermograms. The thermal analysis results revealed that these polyamides exhibited good thermal stability, which in turn is beneficial to increase the service time in device application and enhance the morphological stability to the spin-coated film. 3.3.3. Optical properties All the polyamides were examined both by UVevis absorption and PL spectroscopy in solution and in solid state, and the relevant data are presented in Table 3. The UVevis absorption and PL spectra of polyamides 5ae5d in NMP are shown in Figure S7 (SI). These polyamides in dilute NMP solution exhibited strong UVevis absorption bands around 294e299 nm, assignable to that arising from the carbazole based pep* transitions and other p-conjugated moieties in the polymer backbone. In the solid state, the polyamides showed absorption characteristics similar to those observed in solutions, with low-energy absorption labs max centered at 294e300 nm and absorption onsets at 344e378 nm corresponding to optical band gaps of 3.29e3.61 eV. Their PL spectra in dilute NMP solution showed maximum bands around 372e435 nm in the blue region with fluorescence quantum yields (FF) ranging from 0.06% to 0.22%. Although these polyamides exhibited a low FF, their solutions and solid films showed an obvious blue PL when irradiated by a standard laboratory UV lamp (see Figure S8, SI). 3.3.4. Electrochemical properties The electrochemical behavior of the polyamides was investigated by cyclic voltammetry (CV) conducted for the cast film on an ITO-coated glass substrate as working electrode in dry acetonitrile (CH3CN) containing 0.1 M of Bu4NClO4 as an electrolyte under nitrogen atmosphere. The derived oxidation potentials are

Scheme 3. The anodic oxidation pathways of polyamides 5d and 50 d.

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summarized in Table 4. For comparison, the representative cyclic voltammograms for polyamides 5d (with tert-butyl substituents on the active sites of the carbazole unit) and 50 d (without the tert-butyl substituents on its carbazole unit) are illustrated in Fig. 5. Polyamide 5d displayed a reversible electrochemical behavior (E1/ 2 ¼ 1.24 V), which indicates that the intermediates produced in the oxidation process are stable on the time scale of the CV scan. Repetitive scans between 0 and 1.5 V provided the similar patterns as those observed in the first scan, and no new peaks were detected under these experimental conditions. In contrast, the electrochemical properties of the analogous 50 d are completely different. For the first positive potential scan, we observed an oxidation peak at about 1.13 V. From the first reverse negative potential scan, we detected two cathodic peaks at 1.17 and 0.97 V. After the second scan, a new oxidation peak appeared at about 1.10 V, which was the complementary anodic process of the cathodic peak at 0.97 V. In the meantime, the original oxidation peak moved to a more anodic potential at 1.48 V. The observation of a new oxidation couple in the second potential scan indicates that the carbazole radical cations were involved in very fast electrochemical reactions that produced a new structure that was easier to oxidize than was the parent carbazole. As reported by Ambrose and Nelson in their pioneering work [30] devoted to anodic oxidation of carbazole and other Nsubstituted derivatives, ringering coupling is the predominant decay pathway. One possible coupling reaction of carbazolium radical cations to biscarbazole shown in Scheme 3 can be used to explain the irreversible oxidation process occurring in polyamide 50 d. Thus, in the second CV curve of 50 d, the first anodic peak corresponds to one-electron oxidation of the biscarbazole units to form radical cations, followed subsequently by the oxidation to dicationic species. Thus, the introduction of electron-donating tertbutyl group not only greatly prevents the coupling reaction but also slightly lowers half-wave potentials of the present polyamides. The HOMO and LUMO energy levels of the corresponding polyamides can be determined from the oxidation onset (Eonset) or half-wave potentials (E1/2) and the onset absorption wavelength of the UVevis spectra by comparison with ferrocene (4.8 eV), and the results are listed in Table 4. The high-lying HOMO energy level and reversible electrochemical oxidation of these polymers suggest that they have potential for use as hole-transporting materials in OLED devices. 3.3.5. Spectroelectrochemical and electrochromic properties Spectroelectrochemical measurements were performed on films of polymers drop-coated onto ITO-coated glass slides immerged in an electrolyte solution. The electrode preparations and solution conditions were identical to those used in the CV experiments. Fig. 6(a) presents the UVeviseNIR absorption spectra of polyamide 5d film at various applied potentials. In the neutral form, polyamide 5d exhibited strong absorptions at 298 nm, characteristic for pep* transitions, but it was almost transparent in the visible and NIR regions. When the applied voltage was stepped to 1.4 V, a shoulder at 380 nm and a broadband around 910 nm appeared. We attribute these spectral changes to the formation of a stable cation radical of the side-chain carbazole moiety. The observed electronic absorption changes in the film of 5d at the potentials are fully reversible and associated with strong color changes; indeed, it can be seen by the naked eye that the film switches from a transmissive neutral state (nearly colorless) to a highly absorbing oxidized state (blue). For a comparative study, the spectroelectrochemical series of polyamide 50 d without the tert-butyl substituents on the carbazole unit are presented in Fig. 6(b). In the neutral form, polyimide 50 d exhibited strong absorption at wavelength around 294 nm, characteristic for carbazole, but it almost transmissive in the visible

Fig. 6. Spectroelectrochemistry of the cast films of polyamides (a) 5d and (b) 50 d on the ITO-coated glass slide in CH3CN-containing 0.1 M Bu4NClO4 at a scan rate of 50 mV s1.

region. When the applied voltage was stepped from 0 to 1.4 V, a new absorption peak at 422 nm and a broadband centered at about 876 nm gradually increased in intensity. We believe that these spectral changes are attributed to the formation of both radical cations of carbazole moiety and dicationic species of biscarbazole moiety. As can be seen in the inset of Fig. 6(b), the film switched from a nearly colorless neutral state to a bluish green oxidized state. When the applied voltage returned from 1.4 to 1.0 V, the intensity of absorption band at longer wavelengths decreased dramatically and the absorption at 422 nm slightly intensified. Meanwhile, the film changed color from green to yellow. This phenomenon persisted for several subsequent scans. The new yellow oxidized state may be attributed to the formation of radical cations of biscarbazole units. Electrochromic switching studies of the polyamide films were performed to monitor the changes in percentage transmittance as a function of time by stepping potential repeatedly between the neutral and oxidized states. The active area of the polymer film on ITO-glass is ca 1 cm2. As a typical example, Fig. 7 depicts the transmittance changes of polyamide 5d as a function of time at 384 and 910 nm by applying potential steps between 0 and 1.4 V with a

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polyamide films have potential for use in optoelectronics applications. Acknowledgements We acknowledge the financial supports from National Science Council of Taiwan and Industrial Technology Research Institute. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.matchemphys.2013.05.054. References

Fig. 7. Optical transmittance change for polyamide 5d film (with an optical density of 0.287 a.u.) monitored at 384 nm and 910 nm in 0.1 M Bu4NClO4/CH3CN by applying a potential step 0.0 to þ1.4 V (vs. Ag/AgCl) and cycle time 10 s. The response time was calculated by 90% of the full-transmittance change between neutral and oxidized states.

residence time of 10 s. In the beginning, the polyamide film exhibited a high optical contrast of the change in percentage transmittance (6%T) between the neutral and oxidized states up to 57% at 910 nm. The color switching time, defined as the time required for reaching 90% of the full change in absorbance after switching potential, could be estimated from the absorbance profiles, being 5.7 s for coloration and only 1.4 s for bleaching. However, polyamide 5d showed a moderate optical contrast loss (decreased to about 30% at 910 nm) after 10 full switches, indicating not very high stability of the polymer film upon electro-oxidation. Apparently, there are some problems and challenges that still need to be solved for electrochromic applications of these polyamides. 4. Conclusions Two series of novel aromatic polyamides with pendent carbazole units were readily prepared from the aromatic diamine monomers 3 and 30 with alicyclic or aromatic dicarboxylic acids via the phosphorylation polyamidation reaction. By substitution of the electrochemically active C-3 and C-6 sites of the carbazole unit with bulky tert-butyl groups, the 5 series polyamides exhibited an enhanced redox-stability and electrochromic performance than their analogs without tert-butyl substituents. These polyamides also showed good film-forming ability and high thermal stability. These polymers exhibited blue fluorescence emission both in film and in solution. Thus, these characteristics suggest that these

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