Redox-stable and visible/near-infrared electrochromic aramids with ...

Redox-stable and visible/near-infrared electrochromic aramids with main-chain triphenylamine and pendent 3,6-di-tert-butylcarbazole units Sheng-Huei Hsiao*, Hui-Min Wang, Shih-Ho Liao Article first published online: 16 DEC 2013 | DOI: 10.1039/c3py01239e Polymer Chemistry, Volume 5, Issue 7, pages 2473– 2483, 7 April 2014

Graphical Abstract The synthesis of novel electroactive aramids from 4,4’-diamino-4’’-(3,6-di-tert-butylcarbazol-9-yl)triphenylamine with aromatic dicarboxylic acids is described. The aramids showed reversible electrochemical oxidation with enhanced NIR contrast, fast switching time, and anodic green/blue electrochromic behaviors.

Polymer Chemistry PAPER

Cite this: Polym. Chem., 2014, 5, 2473

Redox-stable and visible/near-infrared electrochromic aramids with main-chain triphenylamine and pendent 3,6-di-tertbutylcarbazole units† Sheng-Huei Hsiao,*a Hui-Min Wanga and Shih-Ho Liaob A series of visible and near-infrared (NIR) electrochromic aramids with main-chain triphenylamine and pendent 3,6-bis(tert-butyl)carbazole units were prepared from the phosphorylation polyamidation reaction

of

4,40 -diamino-400 -(3,6-di-tert-butylcarbazol-9-yl)triphenylamine

with

various

aromatic

dicarboxylic acids. These aramids were readily soluble in many polar solvents and could be solution cast Received 6th September 2013 Accepted 16th December 2013

into flexible and strong films. They showed useful levels of thermal stability associated with relatively high glass-transition temperatures (Tg) (295–321

C) and high decomposition temperatures (no

DOI: 10.1039/c3py01239e

significant degradation occurred before 450 C in nitrogen or air). In addition, the polymer films showed reversible electrochemical oxidation with enhanced NIR contrast, fast switching time, and anodic green/

www.rsc.org/polymers

blue electrochromic behaviors.

Introduction Electrochromic materials exhibit a reversible optical change in absorption or transmittance upon being electrochemically oxidized or reduced; they are exemplied by thin lms of electroactive transition-metal oxides (e.g., WO3), organic dyes (e.g., viologens), inorganic coordination complexes, and p-conjugated polymers.1 These materials have received a tremendous amount of attention in recent years for many potential applications such as electronic papers, visual displays, smart windows, and camouage materials.2 Among several different types of electrochromic materials, conjugated polymers such as poly(3-alkylthiophene) and poly(3,4-alkylenedioxythiophene) derivatives have emerged with great potential in fabricating large area and exible electrochromic devices.3 In addition, conjugated polymers have demonstrated high coloration efficiency, short switching time, and easy color-tunability via structural control.4 In recent years, triarylamine-based condensation-type polymers such as aromatic polyamides and polyimides have been reported as a new and attractive family of electrochromic materials because of the high electroactivity of the triarylamine unit and high thermal stability of the polymer backbone.5,6 Most of the electrochromic materials usually exhibit distinctly different absorption bands in the visible

a

Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan. E-mail: [email protected]

b

Department of Chemical Engineering, Tatung University, Taipei 10451, Taiwan

† Electronic supplementary 10.1039/c3py01239e

information

(ESI)

This journal is © The Royal Society of Chemistry 2014

available.

See

DOI:

region (400–800 nm) at different redox states. However, materials that display electrochromism in the near-infrared (NIR) region (800–2000 nm) are much less well known,7 although they have important military and civilian applications in elds such as optical attenuation, day-to-night camouage, and thermal control for buildings or spacecras.8 Due to the fact that triarylamines play an important role as hole conducting species in optoelectronic devices9 and for electrochromic applications,10 numerous investigations have been devoted to their synthesis and the investigation of their electronic, optical and material properties.11 The widespread use of triarylamines has its origin in the high stability of the corresponding radical cations, i.e. triarylamines can reversibly be oxidized as long as the para-position of the phenyl rings is protected.12 The radical cationic species of N,N,N0 ,N0 -tetraphenyl-pphenylenediamine (TPPA), the smallest bis(triarylamine) derivative, was characterized by the Robin–Day class III (delocalized redox centers)13 mixed valence nature and showed a rather strong intervalence charge-transfer (IV-CT) band in the NIR region.14 It has also been reported that several aromatic polyamides and polyimides with the TPPA segments exhibit strong NIR electrochromism during the rst oxidation process.15 However, when the radical cationic species are further oxidized to dications, the NIR absorption gradually decreases and disappears. In recent years, carbazole derivatives have been widely used as effective host materials in phosphorescent light-emitting diodes because of their sufficiently high triplet energy and good hole-transporting ability.16 Carbazole can be easily functionalized at its (3,6-), (2,7-), or N-positions, and then covalently linked to polymeric systems, both in the main chain as the

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building block and in a side chain as the subunit.17 Carbazolecontaining polymers are considered as a very important class of electroactive and photoactive materials.18 For instance, the excellent hole transporting properties of poly(2,7-carbazole)s make them highly promising materials for p-type transistors and solar cells.18c,d It has been demonstrated that poly(3,6carbazole)s exhibit interesting electrochromic properties because of the conjugation breaks that are present due to the inclusion of a 3,6-linkage. Reynolds et al. suggested that one of the interesting multicolor systems is that based upon the 3,6linked carbazole moiety.19 Interestingly, the intermolecular/ intramolecular electron transfer between bridged carbazole centers in the mixed-valence cation radical state leading to the absorption bands in the NIR region. Moreover, the absorbance of its dications also appeared in the NIR region due to the nearly coplanar structure of the carbazole redox center, indicating the great potential of anodic electrochromic systems for NIR applications. 4-(N-Carbazolyl)triphenylamine (CzTPA) is structurally similar to TPPA; therefore, polymers containing the CzTPA unit are expected to show IV-CT absorption bands upon electrooxidation. It has been demonstrated that the aramids bearing the CzTPA unit from the diamine monomer 4,40 -diamino-40 -(Ncarbazolyl)triphenylamine reveal good redox stability for the rst oxidation state, and the oxidation process is accompanied by a noticeable green and NIR electrochromism.20 However, the second oxidation process of these polymers is not reversible, possibly due to the electrochemical coupling of carbazoles through the active C-3 and C-6 sites. As described in the literature, the introduction of bulky groups on the electrochemically active sites (C-3 and C-6) of carbazole leads to enhanced electrochemical and morphological stability.16a Therefore, a molecular-design strategy to retain the useful properties of such polymers while enhancing the redox stability by substitution of the C-3 and C-6 positions of the carbazole unit with bulky t-butyl groups is reported in this work. Thus, a novel CzTPA-containing diamine monomer, 4,40 -diamino-400 -(3,6-di-tert-butylcarbazol-9yl)triphenylamine (compound 2; see Scheme 1), was synthesized, and several aramids were obtained from the polycondensation reactions of this diamino compound with aromatic dicarboxylic acids. The t-butyl groups are expected to increase the solubility and to give extra electrochemical stability to the resulting polymers. We anticipated that the prepared aramids should be stable for multiple electrochromic switching, together with enhanced NIR electrochromic performance.

Scheme 1

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For a comparative study, electrochemical and electrochromic properties of the present aramids were also compared with those of structurally related ones from N,N-bis(4-aminophenyl)N0 ,N0 -bis(4-tert-butylphenyl)-1,4-phenylenediamine (20 ).15e

Experimental part Materials 3,6-Di-tert-butyl-9-(4-aminophenyl)carbazole was prepared in good yields according to a published procedure.21 p-Fluoronitrobenzene (Acros), 10% palladium on charcoal (Pd/C, Lancaster), cesium uoride (CsF, Acros), hydrazine monohydrate (TCI), triphenyl phosphite (TPP, Acros), pyridine (Py, Wako), N,Ndimethylformamide (DMF, Tedia) and dimethyl sulfoxide (DMSO, Tedia) were used without further purication. NMethyl-2-pyrrolidone (NMP) was dried over calcium hydride for ˚ 24 h, distilled under reduced pressure, and stored over 4 A molecular sieves in a sealed bottle. Commercially available aromatic dicarboxylic acids that include terephthalic acid (3a, Wako), 4,40 -dicarboxydiphenyl ether (3b, TCI), bis(4-carboxyphenyl) sulfone (3c, New Japan Chemicals Co.), and 2,2-bis(4carboxyphenyl)hexauoropropane (3d, TCI) were used as received. Calcium chloride was dried under vacuum at 200 C for 6 h prior to use. Tetrabutylammonium perchlorate (TBAP, TCI) was recrystallized twice from ethyl acetate under a nitrogen atmosphere and then dried in vacuo before use. All other reagents were used as received from commercial sources. Monomer synthesis 4,40 -Dinitro-400 -(3,6-di-tert-butylcarbazol-9-yl)triphenylamine (1). To a solution of 15.0 g (0.04 mol) of 3,6-di-tert-butyl-9-(4aminophenyl)carbazole and 12.7 g (0.09 mol) of 4-uoronitrobenzene in 100 mL of DMSO, 13.7 g (0.09 mol) of cesium uoride was added with stirring all at once, and the mixture was heated at 130 C for 18 h. Aer cooling, the mixture was poured into 300 mL methanol and the precipitate was collected by ltration. The crude product was further puried by recrystallization from DMF affording orange crystals with a mp of 337– 338 C (by DSC) in 54% yield (13.4 g). IR (KBr): 2952 cm1 (tbutyl C–H stretch), 1579, 1309 cm1 (–NO2 stretch). 1H NMR (500 MHz, DMSO-d6, d, ppm): 1.43 (s, 18H, Hf), 7.35 (d, J ¼ 9.2 Hz, 4H, Hb), 7.42 (c, J ¼ 8.7 Hz, 2H, Ha), 7.51 (d, J ¼ 8.8 Hz, 2H, He), 7.52 (d, J ¼ 8.7 Hz, 2H, Hd), 7.74 (d, J ¼ 8.6 Hz, 2H, Hf), 8.25 (d, J ¼ 9.2 Hz, 4H, Ha), 8.29 (s, 2H, Hg).

Synthetic route to diamine monomer 2.

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Polymer Chemistry

amide absorption bands at 3318 cm1 (N–H stretch) and 1688 cm1 (amide carbonyl). 1H NMR (500 MHz, DMSO-d6, d, ppm): 1.41 (s, 18H, Hh), 6.75 (d, 4H, Hb), 7.01 (d, 4H, Ha), 7.21 (d, 4H, Hj), 7.26 (d, 2H, He), 7.32 (d, 2H, Hc), 7.46 (d, 2H, Hd), 7.80 (d, 2H, Hf), 8.05 (d, 4H, Hi), 8.26 (s, 2H, Hg), 10.29 (s, 2H, amide). The other polyamides were prepared by an analogous procedure.

4,40 -Diamino-400 -(3,6-di-tert-butylcarbazol-9-yl)triphenylamine (2). In a 500 mL round-bottom ask equipped with a stirring bar, 3.5 g (5.71 mmol) of dinitro compound 1 and 0.1 g of 10% Pd/C were dissolved/suspended in 300 mL of ethanol. The suspension solution was heated to reux, and 3 mL of hydrazine monohydrate was added slowly to the mixture, then the solution was stirred at reux temperature. Aer a further 12 h of reux, the solution was ltered hot to remove Pd/C, and the ltrate was then cooled to precipitate white needles. The product was collected by ltration and dried in vacuo at 80 C to give 2.9 g (90% in yield) of white needles with a mp of 273–274 C (by DSC). IR (KBr): 3456, 3325 cm1 (N–H stretch). Anal. calcd for C38H40N4 (552.76): C, 82.57%; H, 7.29%; N, 10.14%. Found: C, 82.34%; H, 7.21%; N, 9.99%. 1H NMR (500 MHz, DMSO-d6, d, ppm): 1.41 (s, 18H, Hf), 5.03 (s, 4H, –NH2), 6.60 (d, J ¼ 8.6 Hz, 4H, Ha), 6.77 (d, J ¼ 8.9 Hz, 2H, Hc), 6.96 (d, J ¼ 8.6 Hz, 4H, Hb), 7.20–7.23 (m, 4H, Hd + He), 7.43 (d, J ¼ 8.6 Hz, 2H, Hf), 8.23 (s, 4H, Hg). 13C NMR (125 MHz, d, ppm, DMSO-d6): 31.8 (C14), 34.3 (C13), 108.9 (C10), 114.8 (C2), 116.2 (C6), 116.3 (C15), 122.2 (C16), 123.3 (C11), 126.5 (C8), 126.8 (C7), 127.7 (C3), 135.3 (C4), 139.0 (C12), 141.7 (C9), 146.0 (C1), 148.7 (C5).

Polymer synthesis The synthesis of polyamide 4b is used as an example to illustrate the general synthetic route. A mixture of 0.4422 g (0.8 mmol) of the diamine monomer 2, 0.2066 g (0.8 mmol) of 4,40 dicarboxydiphenyl ether (3b), 0.1 g of calcium chloride, 0.8 mL of triphenyl phosphite (TPP), 0.2 mL of pyridine, and 2.0 mL of NMP was heated with stirring at 120 C for 3 h. The resulting highly viscous polymer solution was poured slowly into 200 mL of stirring methanol giving rise to a stringy, ber-like precipitate that was collected by ltration, washed thoroughly with hot water and methanol, and dried under vacuum at 100 C. The inherent viscosity of the obtained polyamide 4b was 0.55 dL g1, measured at a concentration of 0.5 g dL1 in DMAc-5 wt% LiCl at 30 C. The IR spectrum of 4b (lm) exhibited characteristic


Preparation of the polyamide lms A solution of the polymer was made by dissolving about 0.7 g of the polyamide sample in 10 mL of DMAc. The homogeneous solution was poured into a 9 cm glass Petri dish, which was placed in a 90 C oven for 3 h to remove most of the solvent; then the semidried lm was further dried in vacuo at 170 C for 7 h. The obtained lms were about 50–60 mm thick and were used for X-ray diffraction measurements, solubility tests, and thermal analyses.

Instrumentation Infrared spectra were recorded on a Horiba FT-720 FT-IR spectrometer. Elemental analyses were carried out using a PerkinElmer 2400 CHN analyzer. 1H and 13C NMR spectra were measured on a Bruker AVANCE 500 FT-NMR system by using DMSO-d6 as solvent and TMS as internal standard. The inherent viscosities were determined with a Cannon-Fenske viscometer at 30 C. The number-average molecular weight (Mn), weight average molecular weight (Mw) and polydispersity index (PDI) values were obtained via gel permeation chromatography (GPC) on the basis of polystyrene calibration using a Waters 2410 as an apparatus and THF as the eluent. 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, ˚ The 30 mA), using nickel-ltered Cu-Ka radiation (l ¼ 1.5418 A). scanning rate was 2 min1 over a range of 2q ¼ 10–40 . Thermogravimetric analysis (TGA) was conducted with a PerkinElmer Pyris 1 TGA. Experiments were carried out on approximately 3–5 mg of samples heated in owing nitrogen or air (gas ow rate ¼ 40 cm3 min1) from 200 to 800 C at a heating rate of 20 C min1. DSC analyses were performed on a PerkinElmer Pyris 1 DSC at a scan rate of 20 C min1 under a nitrogen atmosphere. Thermomechanical analysis (TMA) was conducted with a PerkinElmer TMA 7 instrument. The TMA experiments were conducted from 50 to 350 C at a scan rate of 10 C min1 with a penetration probe 1.0 mm in diameter under an applied constant load of 10 mN. Soening temperatures (Ts)

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were taken as the onset temperatures of probe displacement on the TMA traces. Electrochemistry was performed with a CH Instruments 750A electrochemical analyzer. Cyclic voltammetry (CV) was conducted with the use of a three-electrode cell in which ITO (polymer lm area about 1 cm2, 0.8 cm 1.25 cm) was used as a working electrode. A platinum wire was used as a counter electrode. All cell potentials were taken with the use of a homemade Ag/AgCl, KCl (sat.) reference electrode. Ferrocene was used as an external reference for calibration (+0.44 V vs. Ag/ AgCl). Measurements were carried out in 0.1 M of TBAP as the supporting electrolyte in acetonitrile. Voltammograms are presented with the positive/negative potential pointing to the right/ le with increasing anodic/decreasing cathodic current pointing upward/downward. Ultraviolet-visible (UV-Vis) spectra of the polymer lms were measured using an Agilent 8453 UVVisible diode array spectrophotometer. The optical band gap (Eg) of the aramids was calculated from their low energy absorption edges (lonset), according to the Planck’s equation (Eg ¼ 1240/lonset).

Results and discussion Monomer synthesis As shown in Scheme 1, the new CzTPA-containing diamine monomer named 4,40 -diamino-400 -(3,6-di-tert-butylcarbazol-9-yl) triphenylamine (2) was successfully synthesized by hydrazine Pd/C-catalyzed reduction of the dinitro compound (1) resulting from the cesium uoride (CsF)-assisted N,N-diarylation reaction of 3,6-di-tert-butyl-9-(4-aminophenyl)carbazole with two equivalent amounts of p-uoronitrobenzene. The IR spectrum (see Fig. S1, ESI†) of dinitro compound 1 gave two characteristic bands at around 1579 and 1309 cm1 (–NO2 asymmetric and symmetric stretching). Aer reduction, the characteristic absorptions of the nitro group disappeared and the amino group showed the typical N–H stretching absorption pair in the region of 3300–3500 cm1. The 1H spectrum of dinitro compound 1 is illustrated in Fig. S2, ESI.† The 1H, 13C, H–H COSY, and C–H HMQC NMR spectra of the target diamine monomer 2 are compiled in Fig. 1. Assignments of each carbon and proton are also indicated in these spectra, and the spectra agree well with the proposed molecular structure of 2. The 1H NMR spectra conrm that the nitro groups have been completely transformed into amino groups by the high eld shi of the aromatic protons, especially for protons-a ortho to the amino group, and the resonance signal at around 5.0 ppm corresponding to the aryl primary amino protons. In addition, the elemental analysis result of 2 was in good agreement with the calculated values. Polymer synthesis According to the phosphorylation polyamidation technique reported by Yamazaki and co-workers,22 a series of novel aramids (4a–4d) with CzTPA units were synthesized from the diamine monomer 2 and various aromatic dicarboxylic acids 3a–3d via solution polycondensation using TPP and pyridine as condensing agents (Scheme 2). All the polymerization reactions

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proceeded homogeneously throughout the reaction and afforded clear and highly viscous polymer solutions, which precipitated in a tough, ber-like form when the resulting polymer solutions were slowly poured into stirring methanol. As shown in Table 1, the obtained polyamides had inherent viscosities in the range of 0.29–0.72 dL g1 and could be solution-cast into exible and tough lms, indicating high molecular weight polymers. The GPC measurement of the THF-soluble aramids 4c and 4d showed a weight-average molecular weight (Mw) of 42 500 and 50 500 with a polydispersity index (Mw/Mn) of 1.98 and 1.98, respectively. The formation of polyamides was also conrmed by IR and NMR spectroscopy. A typical IR spectrum of polyamide 4b is given in Fig. S3, ESI.† The characteristic IR absorption bands of the amide group appeared around 3300 (N– H stretching) and 1650 cm1 (amide carbonyl). Fig. S4† (ESI) shows the 1H NMR and COSY spectra of polyamide 4b in DMSOd6. All the peaks could be readily assigned to the hydrogen atoms in the repeating unit. The resonance peak appearing at 10.35 ppm in the 1H NMR spectrum also supports the formation of amide linkages. For comparison, a series of referenced aramids 40 a–40 d were synthesized from N,N-bis(4-aminophenyl)N0 ,N0 -bis(4-tert-butylphenyl)-1,4-phenylenediamine (20 )15e and dicarboxylic acids 3a–3d by the same synthetic method. The inherent viscosity data are also compiled in Table 1. Organo-solubility and lm properties The qualitative solubility properties of polyamides 4a–4d in several organic solvents at 10% (w/v) are summarized in Table 1. Similar to polyamides 40 a–40 d, these polyamides were highly soluble in polar aprotic organic solvents such as N-methyl-2pyrrolidinone (NMP), N,N-dimethylacetamide (DMAc), and N,Ndimethylformamide (DMF). Polyamides 4c and 4d were also easily soluble in less polar solvents such as m-cresol and tetrahydrofuran (THF). Thus, the excellent solubility makes these polymers potential candidates for practical applications by spin-coating or inkjet-printing processes to afford high performance thin lms for optoelectronic devices. The wide-angle X-ray diffraction (WAXD) patterns of the polyamide lms are shown in Fig. S5, ESI.† These polymers exhibited an amorphous nature due to the bulky, packing-disruptive t-butyl-substituted CzTPA unit which does not favor close chain packing. The amorphous nature of these polymers is important for their applications in exible optoelectronic devices. Thermal properties The thermal properties of the polyamides were examined by TGA, DSC and TMA techniques, and the relevant data are summarized in Table 2. Typical TGA curves of the representative polyamide 4d in both air and nitrogen atmospheres are shown in Fig. S6, ESI.† All the polymers exhibited good thermal stability with insignicant weight loss up to 450 C in both air and nitrogen atmospheres. The decomposition temperatures (Td) at a 10% weight-loss of the polyamides in nitrogen and air were recorded in the range of 470–498 and 469–497 C, respectively. The amount of carbonized residue (char yield) of these polymers in a nitrogen atmosphere is higher than 62% at


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Fig. 1

Polymer Chemistry

(a) 1H, (b) 13C, (c) H–H COSY, and (d) C–H HMQC spectra of the diamine monomer 2 in DMSO-d6.

Scheme 2

Syntheses of aramids 4a–4d.

800 C. The high char yields of these polymers can be ascribed to their high aromatic content. The glass-transition temperatures (Tgs) of all the polymers were observed in the range of 295– 321 C by DSC and decreased with decreasing rigidity of the aromatic dicarboxylic acid residue. The lowest Tg value of 4b can be explained in terms of the exible ether linkage in its diacid


component. When compared with the structurally related polyamides 40 a–40 d, the 4 series polyamides showed an increased Tg and an enhanced thermal stability because of the decreased conformational exibility caused by the introduction of planar carbazole groups in the repeat unit. All the polymers indicated no clear melting endotherms up to the decomposition

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Inherent viscosity and solubility of polyamides Solubility in various solventsb

Polymer code

hinha (dL g1)

NMP

DMAc

DMF

DMSO

m-Cresol

THF

4a 4b 4c 4d 40 a 40 b 40 c 40 d

0.72 0.55 0.39 0.39 0.71 0.63 0.48 0.51

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

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

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

++ + ++

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

++ ++ ++ ++

a Measured at a polymer concentration of 0.5 g dL1 in DMAc-5 wt% LiCl at 30 C. b The qualitative solubility was tested with 10 mg of a sample in 1 mL of stirred solvent. Notation: + +, soluble at room temperature; +, soluble on heating at 100 C; , partially soluble. NMP: N-methyl-2-pyrrolidone; DMAc: N,N-dimethylacetamide; DMF: N,N-dimethylformamide; DMSO: dimethyl sulfoxide; THF: tetrahydrofuran.

Table 2

Thermal properties of polyamides

thermal analysis results revealed that these polyamides exhibited excellent thermal stability, which in turn is benecial to increase the service time in device application and enhance the morphological stability of the spin-coated lm. Optical properties

Td at 10% weight lossd ( C) Polymer codea

Tgb ( C)

Tsc ( C)

In N2

In air

Char yielde (wt%)

4a 4b 4c 4d

310 (287)f 295 (269) 321 (296) 310 (287)

312 (273) 298 (264) 330 (280) 304 (272)

507 509 487 522

497 500 479 524

67 67 62 64

a The polymer lm samples were heated at 300 C for 1 h before all the thermal analyses. b Midpoint temperature of the baseline shi on the second DSC heating trace (rate ¼ 20 C min1) of the sample aer quenching from 400 to 50 C (rate ¼ 200 C min1) in nitrogen. c Soening temperature measured by TMA with a constant applied load of 10 mN at a heating rate of 10 C min1. d 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 ow rate of 40 cm3 min1. e Residual weight percentage at 800 C in nitrogen. f Data in parentheses are those of structurally similar polyamides 40 a–40 d having the corresponding diacid residue as in the 4 series.

temperatures on the DSC thermograms. This result also supports the amorphous nature associated with these polymers. The soening temperatures (Ts) (may be referred to as apparent Tg) of the polymer lm samples were determined by the TMA method with a loaded penetration probe. They were obtained from the onset temperature of the probe displacement on the TMA trace. As a representative example, the TMA trace of polyamide 4b is illustrated in Fig. S7, ESI.† The Ts values obtained by TMA were recorded in the 298–330 C range. The

2478 | Polym. Chem., 2014, 5, 2473–2483

The absorption and photoluminescence (PL) spectra of compounds 1 and 2 were measured in NMP, and the pertinent data are presented in Table 3. Fig. S8† (ESI) shows the UV-vis absorption and PL spectra of compounds 1 and 2 and a representative polyamide 4a in NMP (concentration: 1 105 mol L1), together with their PL images on exposure to a standard laboratory UV lamp. Two absorption peaks at 297 (aromatic ring) and 421 nm (a typical charge-transfer band in this donor– acceptor system) were observed for the dinitro compound 1, whereas only one peak at 299 nm with an adjoining shoulder around 325 nm was observed for the diamino compound 2. Compound 2 showed moderate emission intensity at 477 nm in the blue region, whereas compound 1 showed a very weak PL intensity due to the charge-transfer quenching effect. These polyamides exhibited strong UV-Vis absorption bands at 298 and 324–353 nm in NMP solutions, assignable to the p–p* and n–p* transitions of the CzTPA 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 lmax centered at 336–347 nm with absorption onsets at 386–469 nm corresponding to optical band gaps of 2.64–3.21 eV. Their PL spectra in dilute NMP solution showed maximum bands around 426–496 nm in the violet-blue region with extremely low uorescence quantum yield (FF). The low uorescence efficiency can be attributed to the intramolecular charge transfer quenching effect between the TPA/carbazole donors and the amide acceptors.23 Electrochemical properties The electrochemical characteristics of 1, 2 and the polyamides were studied by cyclic voltammetry (CV), and their redox potentials are listed in Table 3. The CV diagrams of compounds 1 and 2 are depicted in Fig. 2. The diamine monomer 2 exhibits


Paper Table 3

Polymer Chemistry Optical and electrochemical properties of 1, 2 and polyamides Absorption and PL spectra

Cyclic voltammetry (vs. Ag/AgCl) Solid lmd

Solutiona

Oxidation potential (V)

Code

labs max (nm)

b lPL max (nm)

FPLc(%)

lmax (nm)

ledge (nm)

e Eopt (eV) g

1 2 4a 4b 4c 4d 40 a 40 b 40 c 40 d

297, 421 299, 325 299, 353 299, 343 299, 324 299, 350 309 349 325 355

432 477 434 496 426 438 362 439 390 448

0.06 2.98 0.32 0.17 0.13 0.27 0.50 0.16 0.24 0.14

— — 299, 347 300, 337 299, 336 299, 336 311 338 311 310

— — 425 386 469 395 448 412 428 429

— — 2.92 3.21 2.64 3.14 2.77 3.01 2.90 2.76

Energy levels (eV)

Eonset

Eox1 1/2

Eox2 1/2

Eox3 1/2

HOMOf

LUMOg

0.67 0.66 0.70 0.69 0.44 0.47 0.45 0.47

1.14 0.33 0.82 0.82 0.85 0.86 0.58 0.59 0.59 0.57

1.50 0.68 1.19 1.17 1.20 1.17 0.96 0.96 0.97 0.95

— 1.33 — — — — — — — —

5.50 5.04 5.18 5.18 5.21 5.22 4.94 4.95 4.95 4.93

— — 2.29 1.97 2.57 2.08 2.17 1.94 2.05 2.17

a Measured in dilute solutions in NMP at a concentration of about 1 105 mol L1. b Excited at the absorption maximum. c The uorescence quantum yield was calculated in an integrating sphere with quinine sulfate as the standard (FPL ¼ 54.6%). d Drop-coated from N,Ndimethylacetamide solution. e Optical band gap obtained from Eg ¼ 1240/ledge. f The HOMO energy levels were calculated from E1/2 and were referenced to ferrocene (4.8 eV). g LUMO ¼ HOMO Eg.

three reversible one-electron redox processes, indicating that the primary amino group, the TPA center and the carbazolyl nitrogen atom are electronically coupled. The dinitro compound 1 shows two quasi-reversible redox processes. The oxidation should start from the TPA center, followed by the carbazolyl group. The presence of the electron-withdrawing nitro group in compound 1 results in a signicant anodic shi of the potential and decreased stability of the cationic species. Fig. 3 compares the CV diagrams of polyamides 4a and 40 a. Polyamide 4a shows two reversible oxidation redox couples at half-wave potentials (E1/2) of 0.82 V (Eonset ¼ 0.67 V) and 1.19 V, corresponding to successive one electron removal from the TPA

Fig. 2

and carbazolyl moieties. During the electrochemical oxidation, the color of the polymer lm changed from colorless to green and then to deep blue. In comparison, the CzTPA-based polyamide 4a (Ep,a ¼ 0.92 V) exhibited a higher rst oxidation potential than the corresponding TPPA-based 40 a (Ep,a ¼ 0.69 V). This can be attributed to the fact that the lone pair of electrons of the nitrogen atom of carbazole prefer to localize within the coplanar ring, making the carbazolyl group a weaker electrondonor as compared to the diphenylamino group. When compared with the corresponding polyamides without the tbutyl groups on the carbazole unit,20 the present polyamides reveal a much higher stability in the second oxidation process.

CV diagrams of the solutions of (a) dinitro compound 1 and diamine monomer 2 in 0.1 M TBAP/CH3CN at a scan rate of 50 mV s1.


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Fig. 3

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CV diagrams of the cast films of polyamides 4a and 40 a on the ITO-coated glass slide in 0.1 M TBAP/CH3CN at a scan rate of 50 mV s1.

Thus, the electrochemical stability of these polyamides can be attributed to the blocking of the active sites of carbazole by the bulky t-butyl group. The HOMO (highest occupied molecular orbital) energy levels of the investigated polyamides were calculated from the half-wave potentials of the rst oxidation wave (Eox1 1/2 ) and by comparing with the ionization potential of ferrocene (4.8 eV), and the LUMO (lowest unoccupied molecular orbital) values were deduced from the band gap calculated from the absorption edge. According to the HOMO and LUMO energy

levels obtained (Table 3), the present polyamides appear to be appropriate as hole injection and transport materials.

Spectroelectrochemical and electrochromic properties Spectroelectrochemical measurements were performed on lms of the aramids drop-coated onto ITO-coated glass slides immersed in electrolyte solution. The electrode preparations and solution conditions were identical to those used in the CV

Fig. 4 (a) Spectroelectrochemistry of the polyamide 4a thin film on the ITO-coated glass substrate in 0.1 M TBAP/CH3CN at various applied potentials; (b) optical change in %T as a function of applied potential for the three absorption bands at 421, 783 and 900 nm (inset, the photo shows the color change of the film on an ITO electrode at indicated potentials); (c) 3-D spectroelectrochemical behavior from 0.0 to 1.3 V (vs. Ag/ AgCl) of the polyamide 4a thin film on the ITO-coated glass substrate in 0.1 M TBAP/CH3CN; and (d) the oxidation pathway of polyamide 4a.

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experiments. Fig. 4a and c respectively illustrate the UV-Vis-NIR absorption proles correlated with applied potentials and the three-dimensional absorbance–wavelength–potential correlation of polyamide 4a. In the neutral form, polyamide 4a exhibited strong absorptions at 298 and 350 nm, assignable to the p–p* and n–p* transitions of the CzTPA and other p-conjugated moieties in the polymer backbone, but it was almost transparent in the visible and NIR regions. The band gap of polymer 4a was calculated to be 2.92 eV from the onset of the p–p* transition at 425 nm. Upon electro-oxidation of the 4a lm (increasing the applied voltage from 0 to 0.9 V), the absorption of the p–p* transition at 350 nm gradually decreased while two new absorption peaks at 421 and 783 nm and a broad band from 900 nm extended to the NIR region grew up. Since the potentials examined are similar to those of the rst anodic process, the spectral changes are assigned to the radical cation 4a+ (polaron) formation arising from the oxidation of the TPA unit. The absorption band in the NIR region may be attributed to an intervalence charge transfer (IV-CT) between states in which the positive charge is centered at different amino centers (TPA and carbazole). Further increase of the applied electrical

Polymer Chemistry

potential to 1.30 V led to the drop of the absorption intensity at 421 nm, with concurrent increase of the absorption intensity at 783 nm and in the NIR region. The spectral change is assigned to the formation of dicationic species 4a2+. The observed UVVis-NIR absorption changes of the 4a lm were fully reversible upon varying the applied potential. In addition, the polymer lm was associated with signicant color changes (from transparent to green to blue) that were homogeneous across the ITO electrode surface and easy to detect with the naked eye (inset in Fig. 4b). Thus, the electrochemical and spectroelectrochemical results support the proposed mechanism in the oxidative process of polyamide 4a (Fig. 4d). Dynamic changes in % transmittance of 421, 783, and 900 nm at varying applied potentials are illustrated in Fig. 4b. The electrochromic polymer 4a shows a high optical contrast both in the visible and NIR regions with a transmittance change (DT%) of 60% at 421 nm and 67% at 783 nm for green coloring at the rst oxidation stage, and 87% at 783 nm and 80% at 900 nm for blue coloring at the second oxidation stage. In addition, it is interesting to note that the CzTPA-based polyamide 4a revealed an enhanced NIR absorption at its fully oxidized state as compared to the

Fig. 5 UV-vis-NIR absorption spectra of the cast films of polyamides 4a and 40 a on the ITO-coated glass substrate in 0.1 M TBAP/CH3CN at their neutral, cation, and dication forms.

Fig. 6 Potential step absorptometry of the cast film (120 10 nm in thickness) of polyamide 4a on the ITO-glass slide (coated area: 1 cm2) (in CH3CN with 0.1 M TBAP as the supporting electrolyte) at (a) lmax ¼ 783 nm as the applied voltage was stepped between 0 and 0.9 V (b) lmax ¼ 900 nm as the applied voltage was stepped between 0 and 1.3 V with a cycle time of 20 s.


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corresponding TPPA-based 40 a (Fig. 5). The bathochromic shi in the NIR region may indicate that a more stable charged moiety is present in the lm structure of 4a2+ due to the planar carbazole unit. Similar CV and spectroelectrochemistry results for the 4b/40 b pair are included in ESI Fig. S9 and S10.† Switching data for the representative cast lm of polyamide 4a are shown in Fig. 6. The switching time was calculated at 90% of the full switch because it is difficult to perceive any further color change with the naked eye beyond this point. The polyamide switches rapidly between the highly transmissive neutral state and the colored oxidized state. The thin lm from polyamide 4a required 4.1 s at 0.9 V for switching absorbance at 783 nm and 1.7 s for bleaching. When the potential was set at 1.3 V, polyamide 4a would require 3.0 s for the coloring process at 900 nm and 2.4 s for the bleaching process. The 4a lm also shows reproducible optical contrast upon switching between its neutral and oxidation states. When potential square-wave cycles (0 4 0.9 V and 0 4 1.3 V) were applied to the polymer lm as shown in Fig. 6, the lm exhibited a high contrast of the optical transmittance change (DT%) up to 67% and 80% at 783 and 900 nm. Most of the reported electroactive p-conjugated polymers used in electrochromic applications are transparent at their oxidized state.24 In order to make the electrochromic device transparent, it must be continuously kept under a potential and this may deteriorate the electrochromic material during working. This is the main disadvantage of such materials. Since our green/blue electrochromic TPPA- or CzTPA-based aramids are transparent in their neutral state, they are safer to be used for this application avoiding such inevitable deterioration as compared to neutral blue and green p-conjugated polymers that are able to switch to a transmissive state upon electrochemical oxidation of the conjugated main chain.

Conclusions A series of visible/NIR electrochromic aramids have been readily prepared from the newly synthesized diamine monomer, namely 4,40 -diamino-400 -(3,6-di-tert-butylcarbazol-9-yl)triphenylamine, and various aromatic dicarboxylic acids via the phosphorylation polyamidation reaction. Introduction of the (3,6-ditert-butylcarbazol-9-yl)triphenylamine segment into the aramid backbone not only affords high Tg and good thermal stability but also leads to good solubility and electrochemical stability of the aramids. All the aramids reveal valuable electrochromic characteristics such as enhanced absorption in the NIR region upon oxidation, green and blue electrochromic behaviors, fast switching time, and high electrochemical/electrochromic reversibility. Thus, these aramids can be candidates for the design of organic visible/NIR electrochromic devices.

Acknowledgements We thank the National Science Council of Taiwan for nancial support.

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Supporting Information for Redox-stable and visible/near-infrared electrochromic aramids with main-chain triphenylamine and pendent 3,6-di-tert-butylcarbazole units by Sheng-Huei Hsiao,a* Hui-Min Wang,a Shih-Ho Liaob

a

Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan. E-mail: [email protected] b Department of Chemical Engineering, Tatung University, Taipei 10451, Taiwan

List of Contents for Supplementary Material: Fig. S1 IR spectra of dinitro compound 1 and target diamine monomer 2. Fig. S2 (a) 1H NMR spectrum of dinitro compound 1 in DMSO-d6. Fig. S3. Typical IR spectrum of polyamide 4b. Fig. S4 (a) 1H NMR spectrum and (b) aromatic portion of the 1H-1H COSY spectrum of polyamide 5b in DMSO-d6. Fig. S5 WAXD patterns of the polyamide films. Fig. S6 TGA curves of polyamide 4d with a heating rate of 20 oC/min. Fig. S7 TMA of polyamide 4b with a heating rate of 10 oC/min. Fig. S8 UV-Vis absorption and PL spectra of the dilute solutions of compounds 1 and 2 and polyamide 4a in NMP (10-5 M). Photographs show the PL images of their solutions on exposure to a standard laboratory UV lamp (Excited at 365 nm). Fig. S9 CV diagrams of the cast films of polyamides 4b and 4’b on the ITO-coated glass slide in 0.1 M TBAP/CH3CN at a scan rate of 50 mV/s. Fig. S10 UV-Vis-NIR absorption spectra of the cast films of polyamides 4b and 4’b on the ITO-coated glass substrate in 0.1 M TBAP/CH3CN at their neutral, cation, and dication forms.

~1~

Fig. S1. IR spectra of dinitro compound 1 and target diamine monomer 2.

Fig. S2 (a) 1H NMR spectrum of dinitro compound 1 in DMSO-d6.

~2~

Fig. S3. Typical IR spectrum of polyamide 4b.

Fig. S4 (a) 1H NMR spectrum and (b) aromatic portion of the 1H-1H COSY spectrum of polyamide 5b in DMSO-d6.

~3~

Fig. S5 WAXD patterns of the polyamide films.

Fig. S6 TGA curves of polyamide 4d with a heating rate of 20 oC/min.

~4~

Fig. S7 TMA of polyamide 4b with a heating rate of 10 oC/min.

Fig. S8 UV-Vis absorption and PL spectra of the dilute solutions of compounds 1 and 2 and polyamide 4a in NMP (10-5 M). Photographs show the PL images of their solutions on exposure to a standard laboratory UV lamp (Excited at 365 nm).

~5~

Fig. S9 CV diagrams of the cast films of polyamides 4b and 4’b on the ITO-coated glass slide in 0.1 M TBAP/CH3CN at a scan rate of 50 mV/s.

Fig. S10 UV-Vis-NIR absorption spectra of the cast films of polyamides 4b and 4’b on the ITO-coated glass substrate in 0.1 M TBAP/CH3CN at their neutral, cation, and dication forms.

~6~