polymers Article
Synthesis and Electrochromism of Highly Organosoluble Polyamides and Polyimides with Bulky Trityl-Substituted Triphenylamine Units Sheng-Huei Hsiao 1, * 1 2
*
ID
, Wei-Kai Liao 1,2 and Guey-Sheng Liou 2, *
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, No. 1, Sec. 3, Chunghsiao East Rd., Taipei 10608, Taiwan;
[email protected] Institute of Polymer Science and Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan Correspondence:
[email protected] (S.-H.H.);
[email protected] (G.-S.L.); Tel.: +886-2-2771-2171 (ext. 2548) (S.-H.H.); +886-2-3366-5315 (G.-S.L.); Fax: +886-2-2731-7117 (S.-H.H.); +886-2-3366-5237 (G.-S.L.)
Received: 5 September 2017; Accepted: 12 October 2017; Published: 14 October 2017
Abstract: Two series of polyamides and polyimides containing bulky trityl-substituted triphenylamine units were synthesized from condensation reactions of 4,40 -diamino-400 -trityltriphenylamine with various dicarboxylic acids and tetracarboxylic dianhydrides, respectively. The polymers showed good solubility and film-forming ability. Flexible or robust films could be readily obtained via solution-casting. The use of aliphatic diacid or dianhydride reduces interchain charge transfer complexing and leads to colorless polyamide and polyimide films. These polymers showed glass-transition temperatures in the range of 206–336 ◦ C. Cyclic voltammograms of the polyamide and polyimide films displayed reversible electrochemical oxidation processes in the range of 0–1.0 or 0–1.3 V. Upon oxidation, the color of polymer films changes from colorless to blue-green or blue. As compared to the polyimide counterparts, the polyamides showed lower oxidation potentials and thus a higher electrochromic stability and coloration efficiency. Simple electrochromic devices were also fabricated as a preliminary investigation for electrochromic applications of the prepared polymers. Keywords: trityl (triphenylmethyl); triphenylamine; polyamides; polyimides; redox-active polymers; electrochromic polymers
1. Introduction Aromatic polyamides and polyimides are classified as high performance polymers by their outstanding elevated thermal stability, high strength and stiffness, and broad chemical resistance [1–8]. They have been widely used in the advancement of technologies in many fields, such as automotive, aerospace, sport and safety equipment, and electronics. However, the fabrication of aromatic polyamides and polyimides, through the melt or the solution processing, is generally difficult because they have high softening or melting temperatures and are insoluble in most organic solvents. Fabrication of aramid fibers was generally carried out from anisotropic solutions in strong acids, such as concentrated sulfuric acid or in polar organic solvents containing dissolved inorganic salts. Although aromatic polyimides can be processed through a two-step process based on soluble poly(amic acid) precursors, this common technique still retains some inherent problems, such as instability of poly(amic acid)s and release of water during thermal curing. Solubility improvements of aromatic polyamides and polyimides could be achieved by macromolecular engineering, mainly by designing and synthesizing new monomers [3,9,10]. Moreover, most aromatic polyimide films usually exhibit deep color due to charge transfer complexing (CTC) [11,12]. Organosoluble and colorless
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aromatic polyimides could be obtained from the monomer combination of less electron-accepting bis(ether anhydride)s and less electron-donating trifluoromethyl-substituted diamines [13,14]. The use of aliphatic dianhydride and/or diamine monomers also could decrease the CTC effect and led to low color or colorless polyimides [15,16]. Triarylamine-based small molecules, dendrimers, star compounds, or polymers exhibit attractive hole-transporting properties and are promising candidates to be amenable for use in a variety of optoelectronic devices [17–22]. In addition, triarylamine-containing polymers have been reported to exhibit reversible electrochemical processes and stable electrochromic properties [23–25]. We disclosed in 2005 that aromatic polyimides bearing triphenylamine units show interesting redox-active and electrochromic behaviors in addition to their inherent properties, such as high thermal stability [26]. Since then, a lot of condensation-type triarylamine-based high performance polymers, typified by aromatic polyimides [27–32] and polyamides [33–38], have been synthesized and investigated for electrochromic applications. A comprehensive review on aromatic polyamides and polyimides containing triarylamine moieties for electrochromic applications has been reported by Yen and Liou [39]. Due to the presence of packing-disruptive, three-dimensional triarylamine moieties, these polymers, especially for polyamides, are generally organosoluble and can be easily fabricated into amorphous thin film via solution-casting process. This is helpful for their applications in optoelectronic devices. Incorporating bulky three-dimensional groups into the backbones of aromatic polyimides and polyamides might enhance the solubility without much compromising their thermal and mechanical properties. For example, Zhang et al. introduced a non-polar bulky triphenylmethyl (trityl) pendent group onto polyimides, which exhibited a high solubility and low dielectric constant [40]. Aromatic polyimides and polyamides with double propeller-shaped trityl-substituted triphenylamine units have been reported by Zhang’s group [41] and Banerjee’s group [42], respectively. The obtained polyimides and polyamides exhibited good solubility in many aprotic solvents and had moderate to high glass transition temperatures. Moreover, the incorporation of bulky trityl-substituted triphenylamine units increases the interchain spacing and reduces the packing efficiency, thereby increasing the intrinsic microporosity. Thus, they also exhibited a good gas separation performance. Although the gas transport properties of polyimides and polyamides bearing trityl-substituted triphenylamine units have been reported, little information was known for their electrochemical and electrochromic properties. As a continuation of our devotion to research and developments in easily processable high performance polymers with electrochromic function, herein we synthesized and characterized some aromatic and semi-aromatic polyamides and polyimides containing trityl-substituted triphenylamine units, and the electrochemistry, spectroscopy, and electrochromic cycling stability of the polymer films were also investigated. In addition to increased solubility caused by the incorporation of twisted nonplanar trityl substituent and triphenylamine group, the increased free volume was expected to allow better charge transfer at doping, and thus leading to fast switch speeds. 2. Experimental Section 2.1. Materials Triphenylmethyl chloride (Acros, Geel, Belgium), aniline (Acros), cesium fluoride (CsF, Acros), 10% palladium on activated carbon (Pd/C, Fluka, Milwaukee, WI, USA), triphenyl phosphite (TPP, Acros), and pyridine (Py, wako, San Francisco, CA, USA) were used as received. N-Methyl-2-pyrrolidine (NMP, TEDIA, Fairfield, OH, USA) and N,N-dimethylacetamide (DMAc, TEDIA) were distilled over calcium hydride. Terephthalic acid (4a) (Acros), 4,40 -dicarboxydiphenyl ether (4b) (TCI, Tokyo, Japan), 2,20 -bis(4-carboxyphenyl)hexafluoropropane (4c) (TCI), adipic acid (4d) (Acros), and 1,4-cyclohexanedicarboxylic acid (4e) (Acros) were thoroughly dried by heating at 150 ◦ C under vacuum for 10 h before use. 4,40 -Oxydiphthalic dianhydride (ODPA; 6a) (Oxychem, Dallas, TX, USA), 4,40 -hexafluoroisopropylidenediphthalic dianhydride (6FDA; 6b) (Hoechst Celanese, Irving, TX, USA), and 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA; 6c) (from Changzhou
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Celanese, Irving, TX, USA), and 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA; 6c) (from Sunlight Pharmaceutial Co., Changzhou, China) were purified by sublimation under vacuum. Changzhou Sunlight Pharmaceutial Co., Changzhou, China) weredianhydride purified by(HPMDA; sublimation Celanese, Irving, TX, USA), and 1,2,4,5-cyclohexanetetracarboxylic 6c) under (from Tetrabutylammonium perchlorate (TBAP; Bu4 NClO4 ) (TCI) was purified by recrystallization from ethyl vacuum. Tetrabutylammonium perchlorate (TBAP; Bu 4 NClO 4 ) (TCI) was purified by Changzhou Sunlight Pharmaceutial Co., Changzhou, China) were purified by sublimation under acetate. Heptyl viologen tetrafluoroborate (HV(BF4 )2 ) was synthesized according to the procedure recrystallization from ethyl acetate.perchlorate Heptyl viologen tetrafluoroborate 2) waspurified synthesized vacuum. (TBAP; Bu4NClO4) (HV(BF (TCI) 4)was by described inTetrabutylammonium literature [43]. according to the procedure in literature [43].tetrafluoroborate (HV(BF4)2) was synthesized recrystallization from ethyl described acetate. Heptyl viologen according to the procedure described in literature [43]. 2.2. Monomer Synthesis 2.2. Monomer Synthesis 4-Tritylaniline (1). In a 250 mL three-neck flask equipped with a stirring bar and nitrogen gas 2.2. Monomer Synthesis (1). In ammol) 250 mL flask equipped with12.5 a stirring and The nitrogen gas inlet 4-Tritylaniline were placed 14.4 g (50 ofthree-neck triphenylmethyl chloride and mL of bar aniline. reaction inlet 4-Tritylaniline were placed 14.4 (50 ofthree-neck triphenylmethyl chloride and mL ofatmosphere, aniline. The and reaction (1).g In ammol) 250 at mL flask for equipped with12.5 anitrogen stirring bar and nitrogen gas mixture was heated with stirring reflux temperature 10 min under then ◦ mixture was heated with stirring at reflux temperature for 10 min under nitrogen atmosphere, and inlet were placed 14.4 g (50 mmol) of triphenylmethyl chloride and 12.5 mL of aniline. The reaction the solution was slowly cooled to 95 C. Then, 240 mL of 2.0 M solution of hydrochloric acid and thenmL theof solution was slowly cooled to 95 °C. Then,refluxing 240for mL10for ofmin 2.0 M solution hydrochloric acid mixture was heated with stirring at reflux temperature under atmosphere, and 200 methanol were added into the flask. After another 30nitrogen min,of the reaction mixture and 200 mL of methanol were added into the flask. After refluxing for another 30 min, the reaction then the solution was slowly cooled to 95 °C. Then, 240 mL of 2.0 M solution of hydrochloric acid was cooled to the room temperature. The precipitated grey product was collected by filtration and mixture wasof15.1 cooled to the room temperature. The precipitated grey product collected and 200 methanol were added into the flask. After refluxing forpowder; another 30 was min, the−reaction dried to mL give g (90% yield) of the desired compound 1 as a grey m.p. = 257 258 ◦by C, ◦the − 1 (–NH 1 Hwas filtration was and dried g1 (90% yield)3474, of the desired 1 as a grey powder; m.p. = mixture cooled togive room The precipitated grey product collected by measured by DSC atto10 C 15.1 min . temperature. IR (KBr): 3380 cm−compound stretch). NMR (600 MHz, 2 −1. IR (KBr): 3474, 3380 cm−1 (–NH2 stretch). 1H NMR (600 257−258 °C, measured by DSC at 10 °C min filtration and dried to give 15.1 g (90% yield) of the desired compound 1 as a grey powder; m.p. = DMSO-d6 , δ, ppm) (for the peak assignments, see Figure 1): 4.99 (s, 2H, Hf ), 6.47 (d, J = 8.7 Hz, 2H, He ), −1. IR (KBr): −1 (s, MHz, 6, δ,2H, ppm) the peak Figure 4.99 2H,2 stretch). Haf),), 6.47 = 8.7 Hz, 257−258 measured by DSC at(d, 10 minHz, 3380 (–NH H 6.75 (d,DMSO-d J°C, = 8.7 Hz, Hd(for ), 7.12 J°C = assignments, 8.3 6H, Hsee 7.17 (t, J1): = cm 7.3 Hz, 3H, H 7.261(d, (t, NMR JJ = 7.7(600 Hz, c ), 3474, 2H, H e ), 6.75 (d, J = 8.7 Hz, 2H, H d ), 7.12 (d, J = 8.3 Hz, 6H, H c ), 7.17 (t, J = 7.3 Hz, 3H, H a ), 7.26 (t, J = 7.7 MHz, DMSO-d 6 , δ, ppm) (for the peak assignments, see Figure 1): 4.99 (s, 2H, H f ), 6.47 (d, J = 8.7 Hz, 6H, Hb ). Hz, H 6H, b). (d, J = 8.7 Hz, 2H, Hd), 7.12 (d, J = 8.3 Hz, 6H, Hc), 7.17 (t, J = 7.3 Hz, 3H, Ha), 7.26 (t, J = 7.7 2H, e), H 6.75 Hz, 6H, Hb).
Figure atoms of of 4-trylaniline. 4-trylaniline. Figure 1. 1. Structure Structure and and codes codes of of hydrogen hydrogen atoms Figure 1. Structure and codes of hydrogen atoms of 4-trylaniline.
4,4′-Dinitro-4′′-trityltriphenylamine (2). Into a 250-mL round-bottom flask equipped with a 4,40 -Dinitro-400 -trityltriphenylamine (2). Into a 250-mL round-bottom flask equipped with stirring bar were charged with 6.71 g (2). (20 Into mmol) of 4-tritylaniline (1), flask 6.77 equipped g (48 mmol) 4,4′-Dinitro-4′′-trityltriphenylamine a 250-mL round-bottom with of a a stirring bar were charged with 6.71 g (20 mmol) of 4-tritylaniline (1), 6.77 g (48 mmol) of p-fluoronitrobenzene, 7.29 gwith of cesium (CsF),ofand 50 mL of dimethyl (DMSO). stirring bar were charged 6.71 gfluoride (20 mmol) 4-tritylaniline (1), 6.77sulfoxide g (48 mmol) of p-fluoronitrobenzene, 7.29 g of cesium fluoride (CsF), and 50 mL of dimethyl sulfoxide (DMSO). °C for about 48 h. After cooling, the mixture was poured into 300 mL The mixture was heated7.29 at 140 p-fluoronitrobenzene, g of cesium fluoride (CsF), and 50 mL of dimethyl sulfoxide (DMSO). The mixture was heated at 140 ◦ C for about 48 h. After cooling, the mixture was poured into 300 mL of of methanol, and heated the precipitated product by filtration and dried to give into 8.66 300 g (75% aboutwas 48 h.collected After cooling, the mixture was poured mL The mixture was at 140 °C for methanol, and the precipitated product was collected by filtration and dried to give 8.66 g (75% yield) yield) of the desired compound 2 aswas yellow crystals; m.p. = 289−290 °C, measured by DSC at of methanol, and thedinitro precipitated product collected by filtration and dried to give 8.66 g (75% of the desired dinitro compound 2 as yellow crystals; m.p. = 289−290 ◦ C, measured by DSC at −1. IR (KBr): 1584, 1338 cm−1 (–NO2 stretch). 1H NMR (600 MHz, CDCl3, δ, ppm) (for the 10 ◦°C of min yield) the desired dinitro compound 2 as yellow crystals; m.p. = 289−290 °C, measured by DSC at 10 C min−−11 . IR (KBr): 1584, 1338 cm−1−1 (–NO2 stretch).1 1 H NMR (600 MHz, CDCl3 , δ, ppm) (for the peak see Figure 2): 7.18–7.25 (m,2 stretch). 17H, Ha + H HcNMR + Hd +(600 He +MHz, Hf), 7.33 (t, J3,=δ,7.7ppm) Hz, 6H, b), 10 °Cassignments, min . IR (KBr): 1584, 1338 cm (–NO CDCl (forH the peak assignments, see Figure 2): 7.18–7.25 (m, 17H, Ha + H+ c + Hd + He + Hf ), 7.33 (t, J = 7.7 Hz, 6H, peak assignments, see Figure 2): 7.18–7.25 17H, a +3O H4c) +: H d+ +577.2002; He + Hf), 7.33 (t, Jm/z = 7.7 Hz, 6H,[M Hb),+ calcd. (m, for (C 37HH 27N m/z found: 578.2076 8.20), (d, J = 9.2 Hz, 4H, Hg). ESI-MS: H b 8.20 (d, J = 9.2 Hz, 4H, Hg ). ESI-MS: calcd. for (C37 H27 N+3 O4 ) : m/z 577.2002; found: m/z 578.2076 + H] .+(d, Anal. calcd. for C37H Hg27 N 3O4 (577.64): 76.93%; N, 577.2002; 7.27%. Found: C,m/z 76.04%; H, 4.80%; ).C ESI-MS: calcd.C,for (C37H27H, N34.71%; O4) : m/z found: 578.2076 [M + 8.20 9.2 Hz, 4H, [M H]J+ =. Anal. calcd. for 37 H27 N3 O4 (577.64): C, 76.93%; H, 4.71%; N, 7.27%. Found: C, 76.04%; H, + N, 7.56%. H] . Anal. calcd. for C37H27N3O4 (577.64): C, 76.93%; H, 4.71%; N, 7.27%. Found: C, 76.04%; H, 4.80%; 4.80%; N, 7.56%. N, 7.56%.
Figure 2. Structure and codes of hydrogen atoms of compound 2. Figure 2. Structure Structure and codes of hydrogen atoms of compound 2.
4,4’-Diamino-4”-trityltriphenylamine (3). A mixture of 1.5 g (2.6 mmol) of dinitro compound 2, 2 mL4,4’-Diamino-4”-trityltriphenylamine of hydrazine monohydrate, and 0.05 g(3). of A 10% Pd/C in ethanol heated to reflux mixture of 200 1.5 gmL (2.6ofmmol) of was dinitro compound 2, at the reflux temperatureand for 0.05 24 h.gThe solution filtered while hot to remove and 2and mLheld of hydrazine monohydrate, of 10% Pd/Cwas in 200 mL of ethanol was heatedPd/C, to reflux and held at the reflux temperature for 24 h. The solution was filtered while hot to remove Pd/C, and
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4,4’-Diamino-4”-trityltriphenylamine (3). A mixture of 1.5 g (2.6 mmol) of dinitro compound 2, 2Polymers mL of2017, hydrazine 9, 44 of Polymers 2017, 9, 511 511 monohydrate, and 0.05 g of 10% Pd/C in 200 mL of ethanol was heated to reflux of 19 19 and held at the reflux temperature for 24 h. The solution was filtered while hot to remove Pd/C, the filtrate waswas concentrated under nitrogen atmosphere. and the filtrate concentrated under nitrogen atmosphere.After Aftercooling, cooling,the the precipitated precipitated crystals of pure compound were collected by filtration and dried to give 1.09 g (81% in yield) diamino compound 3; were collected ◦ − 1 1 −1 1 244−246 3208 cm cm −1 (–NH m.p. −246 °CC (by (by DSC). DSC). IR (KBr): 3342, (–NH222stretch). stretch). 1H HNMR NMR (600 (600 MHz, MHz, DMSO-d666, m.p. == 244 3342, 3208 assignments, see Figure 3): 4.95 (s, 4H, H h ), 6.46 (d, J = 9.0 Hz, 2H, ee), 6.52 δ, ppm) (for the peak 4.95 (s, 4H, H ), 6.46 (d, J = 9.0 Hz, 2H,HH 6.52(d, (d,J δ, ppm) hh e ), HH gg),g6.81 (d,(d, J =J9.0 Hz,Hz, 2H,2H, Hdd), (d, (d, J = 8.7 4H, 4H, Hff), H 7.13 (d, J(d, = 7.4 cc), J= =8.7 8.7Hz, Hz,4H, 4H, ), 6.81 = 9.0 Hd6.82 ), 6.82 J = Hz, 8.7 Hz, J =Hz, 7.4 6H, Hz, H 6H, f ), 7.13 13 C NMR 13 13C).NMR 7.17 (t, J =(t,7.0 aa), H 7.27 (t, J =(t,7.7 6H, 6H, Hbb). H (150 (150 MHz,MHz, DMSO-d 66, δ, ppm): 63.6 H J =Hz, 7.0 3H, Hz, H 3H, J =Hz, 7.7 Hz, DMSO-d c ), 7.17 a ), 7.27 6 , δ, ppm): b 5 ), 114.6 8 ), 114.7 12 ), 125.6 1 ), 127.4 2 ), 127.6 11 ), 130.3 12),(C 11), (C (C55), (C 114.6 (C88), (C 114.7 (C12 125.6 (C11), (C 127.4 (C22), (C 127.6 (C11 130.3 (C33), 130.7 (C77), 134.8 (C66), 135.5 63.6 (C3 ), 130.7 (C7 ), 134.8 (C6 ), 10 ), 145.7 (C13 ), 146.8 (C4 ), 147.2 (C9 ). ESI–MS: calcd. for (C H N++ O )+ : m/z 517.2518; found: 135.5 37 27 3 4 10), (C 13), 146.8 (C44), 147.2 (C99). ESI–MS: calcd. for (C37 145.7 (C13 (C10 37H27 27N33O44) : m/z 517.2518; found: m/z + . Anal. calcd. for C H N (517.67): C, 85.85%; H, 6.04%; N, 8.12%. Found: C, m/z 518.2596 [M + H] + 37 31 3 518.2596 [M + H] +. Anal. calcd. for C37 37H31 31N33 (517.67): C, 85.85%; H, 6.04%; N, 8.12%. Found: C, 85.69%; 85.69%; H, H, 6.41%; 6.41%; N, N, 7.70%. 7.70%.
Figure 3. Structure and codes of hydrogen atoms of compound 3. Figure Figure 3. 3. Structure Structure and and codes codes of of hydrogen hydrogen atoms atoms of of compound compound 3. 3.
2.3. Synthesis of 2.3. Synthesis of Polyamides Polyamides The As aa The phosphorylation phosphorylation polyamidation polyamidation technique technique was was used used to to prepare prepare the the polyamides. polyamides. As representative example, the synthetic procedure of polyamide (PA) 5a is described as follows. representative example, the synthetic procedure of polyamide (PA) 5a is described as follows. A A solution of diamine monomer 3 (0.5176 1 mmol), of terephthalic acid(0.1661 (4a) (0.1661 g; 1 mmol), solution of diamine monomer 3 (0.5176 g; 1 g; mmol), of terephthalic acid (4a) g; 1 mmol), dried dried calcium chloride (0.1 g), and TPP (1.0 mL) in pyridine (0.3 mL) and NMP (1.0 mL) was heated at calcium chloride (0.1 g), and TPP (1.0 mL) in pyridine (0.3 mL) and NMP (1.0mL) was heated at 120 ◦ C for 3 h. A pale yellow fibrous precipitate was obtained when pouring the polymer solution into 120 °C for 3 h. A pale yellow fibrous precipitate was obtained when pouring the polymer solution into methanol. The yield yield of methanol. The of PA PA 5a 5a is is almost almost quantitative. quantitative. The The inherent inherent viscosity viscosity of of PA PA 5a 5awas was0.49 0.49dL/g, dL/g, −1 at 30 ◦ C. IR (film): 3300 cm−1 (N−H stretch), 1660 cm−1 (C=O stretch). 1 H NMR in DMAc on 0.5 g dL −1 at 30 °C. IR (film): 3300 cm−1 −1 (N−H stretch), 1660 cm−1 −1 (C=O stretch). 11H NMR in DMAc on 0.5 g dL−1 (600 7.03 (d, 2H, Hd ), 7.06 (d, 4H, Hf ), 7.17−7.20 (600 MHz, MHz, DMSO-d DMSO-d666,, δ, δ, ppm) ppm)(Figure (Figure4): 4): 6.88 6.88(d, (d,2H, 2H,HHe ), ee), 7.03 (d, 2H, H d d), 7.06 (d, 4H, Hff), 7.17−7.20 (m, Hcc),),7.30 7.30(t, (t,6H, 6H,H Hbb),),7.76 7.76(d, (d,4H, 4H,HHg), g ), 8.07 (s, 4H, Hi ), 10.37 (d, 2H, H). h ). (m, 9H, 9H, H Haaa ++ H c b g 8.07 (s, 4H, Hii), 10.37 (d, 2H, Hh h
Figure 4. Structure and codes of hydrogen atoms of polyamide 5a. Figure Figure 4. 4. Structure Structure and and codes codes of of hydrogen hydrogen atoms atoms of of polyamide polyamide 5a. 5a.
2.4. Synthesis of Polyimides Polyimides (PIs) 7a and 7c were synthesized by the one-step method. A 50-mL two-necked flask equipped with a magnetic stirrer, as well as a nitrogen inlet and outlet, was charged with equimolar amounts of each monomer (1.0 mmol of dianhydride ODPA or HPMDA and 1.0 mmol of diamine 3) and 10 mL of m-cresol. The reaction mixture was stirred at room temperature for 1 h
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2.4. Synthesis of Polyimides Polyimides (PIs) 7a and 7c were synthesized by the one-step method. A 50-mL two-necked flask equipped with a magnetic stirrer, as well as a nitrogen inlet and outlet, was charged with equimolar amounts of each monomer (1.0 mmol of dianhydride ODPA or HPMDA and 1.0 mmol of diamine 3) and 10 mL of m-cresol. The reaction mixture was stirred at room temperature for 1 h until complete dissolution of the monomers, and then heated to 80 ◦ C for 4 h. After a few drops of isoquinoline were added, Polymers the mixture was heated at 200 ◦ C for 12 h. Water vapor formed during the imidization was 2017, 9, 511 5 of 19 continuously removed by a steady flow of nitrogen. After cooling, the resulting solution was poured dissolution of the monomers, and then heated to 80 °C for 4 h. After a few drops of into 100until mLcomplete of methanol. The precipitated polymer was collected by filtration, washed thoroughly isoquinoline were added, the mixture was heated at 200 °C for 12 h. Water vapor formed during the with methanol, and dried. imidization was continuously removed by a steady flow of nitrogen. After cooling, the resulting PI solution 7b was was prepared 6FDA diamine via the two-step imidization poured from into 100 mL ofand methanol. The 3precipitated polymer chemical was collected by filtration,method. The diamine monomer (0.518 g 1.0 mmol) was dissolved in CaH2 -dried DMAc (4.1 mL) in a 50-mL washed thoroughly3 with methanol, and dried. PI 7b wasThen, prepared from(0.444 6FDA gand via the two-step chemical imidization round bottom flask. 6FDA 1.0diamine mmol) 3was added to the diamine solution method. in one portion. Thesolid diamine monomer 3 (0.518 g 1.0 mmol) was dissolved 20 in CaH 2-dried DMAc (4.1 mL) in a 50-mL Thus, the content of the solution is approximately wt %. The mixture was stirred at room round bottom flask. Then, 6FDA (0.444 g 1.0 mmol) was added to the diamine solution in one temperature for about 6 h to yield a viscous poly(amic acid) solution. Then, 0.5 mL of pyridine and portion. Thus, the solid content of the solution is approximately 20 wt %. The mixture was stirred at 0.5 mL of acetic anhydride the poly(amic acid) solution, and the Then, mixture heated at room temperature forwere aboutadded 6 h totoyield a viscous poly(amic acid) solution. 0.5 was mL of 120 ◦ C for 2 h to effect a complete imidization. The obtained polymer was precipitated into methanol, pyridine and 0.5 mL of acetic anhydride were added to the poly(amic acid) solution, and the mixture wasdried heatedunder at 120 vacuum °C for 2 hfor to effect complete imidization. The obtained polymer wasdL g−1 , then filtered and 24 h.a PI 7b exhibited inherent viscosity of 0.62 − 1 ◦ − 1 precipitated into methanol, then filtered dried under vacuum 24 cm h. PI 7b(imide exhibited inherent as measured in DMAc on 0.5 g dL at 30 and C. IR (film): 1780 and for 1717 ring C=O stretch). −1, as measured in DMAc on 0.5 g dL−1 at 30 °C. IR (film): 1780 and 1717 cm−1 viscosity of 0.62 dL g 1 H NMR (600 MHz, DMSO-d6 , δ, ppm) (for the peak assignments, see Figure 5): 7.00 (d, J = 8.8 Hz, 2H, (imide ring C=O stretch). 1H NMR (600 MHz, DMSO-d6, δ, ppm) (for the peak assignments, see He ), 7.10 (d, J = 8.8 Hz, 2H, Hd ), 7.13 (d, J = 8.4 Hz, 4H, Hf ), 7.15−7.20 (m, 15H, Ha + Hb + Hc ), 7.24 (d, Figure 5): 7.00 (d, J = 8.8 Hz, 2H, He), 7.10 (d, J = 8.8 Hz, 2H, Hd), 7.13 (d, J = 8.4 Hz, 4H, Hf), 7.15−7.20 J = 6.8 Hz, 7.78 (d, J = 7.9 Hz, 2H, Hj ), H 7.88 (s, 2H, Hh ),Hz, 7.95 (d,HJj),=7.88 8.1(s,Hz, i ). (d, (m, 4H, 15H, H Hga ), +H b + Hc), 7.24 (d, J = 6.8 Hz, 4H, g), 7.78 (d, J = 7.9 2H, 2H,2H, Hh),H 7.95 J = 8.1 Hz, 2H, Hi).
Figure 5. Structure and codes of hydrogen atoms of polyimide 7b.
Figure 5. Structure and codes of hydrogen atoms of polyimide 7b. 2.5 Fabrication of the Electrochromic Devices (ECDs)
2.5. Fabrication of the Electrochromic Devices (ECDs) 2.5.1. Electrodes
2.5.1. Electrodes EC polymer films were prepared by coating 300 μL solution of the polymer (2 mg/mL in ontofilms an ITO glass substrateby (25coating mm × 30300 mmµL × 0.7 mm, 5 Ω/square). The ITO(2glass used for ECDMAc) polymer were prepared solution of the polymer mg/mL in DMAc) EC device was cleaned by ultrasonication with water, acetone, and isopropanol each for 15 min. onto an ITO glass substrate (25 mm × 30 mm × 0.7 mm, 5 Ω/square). The ITO glass used for EC device The polymer was drop-coated onto the ITO glass with an active area (20 mm × 20 mm) then dried was cleaned ultrasonication with water, acetone, and isopropanol each for 15ofmin. The polymer under by vacuum. The thickness of dry polymer film is about 250 nm. The periphery the coated was drop-coated onto ITO glass with an active area (20 mm 20 mm) drieddispenser, under vacuum. polymer film wasthe surrounded with a thermally cured epoxy resin × adhesive bythen a full-auto The thickness dry polymer about 250 ITO nm. glass The was periphery polymer film was and oneof injection port wasfilm left. is Next, a blank covered of onthe the coated polymer-coated ITO glass. with The two electrodes cured were pressed using a subpress, leavingdispenser, a 120 μm gap surrounded a thermally epoxytogether resin adhesive by a full-auto anddistance one injection byabeaded glasses dispersed in the adhesive). After that, the electrodes were at port was(controlled left. Next, blank ITO glass was covered on the polymer-coated ITO glass. Theheated two electrodes 120 °C for 6 h. were pressed together using a subpress, leaving a 120 µm gap distance (controlled by beaded glasses dispersed inElectrolytes the adhesive). After that, the electrodes were heated at 120 ◦ C for 6 h. 2.5.2. A highly transparent and conductive semi-gelled electrolyte based on 10 wt % poly(methyl 2.5.2. Electrolytes
methacrylate) (PMMA) (Mw: 120,000), and LiBF4 was plasticized with propylene carbonate (PC).
PMMA (0.78 g) was dissolved in PC (5.5 mL) and LiBF4 (0.3 g) was added to the A highly transparent and conductive semi-gelled electrolyte based on polymer 10 wt %solution poly(methyl as supporting electrolyte. Moreover, the HV-containing electrolyte (0.02 M) in this study was (PC). methacrylate) (PMMA) (Mw: 120,000), and LiBF4 was plasticized with propylene carbonate prepared by following procedures: PMMA (0.78 g) was dissolved in PC (5.5 ml), then LiBF4 (0.3 g) and HV(BF4)2 (0.06 g) were added to the polymer solution, and then the mixture was thoroughly mixed to obtain a semi-gelled HV-electrolyte (a gel electrolyte containing HV(BF4)2).
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PMMA (0.78 g) was dissolved in PC (5.5 mL) and LiBF4 (0.3 g) was added to the polymer solution as supporting electrolyte. Moreover, the HV-containing electrolyte (0.02 M) in this study was prepared by following procedures: PMMA (0.78 g) was dissolved in PC (5.5 ml), then LiBF4 (0.3 g) and HV(BF4 )2 (0.06 g) were added to the polymer solution, and then the mixture was thoroughly mixed to obtain a semi-gelled HV-electrolyte (a gel electrolyte containing HV(BF4 )2 ). 2.5.3. Assembly The prepared electrolyte was injected by a subpress through the injection port into the gap between the two electrodes, using a 1 mL syringe. Finally, the injection port of the device was sealed using UV-curable sealant. 3. Results and Discussion 3.1. Monomer Synthesis By applying the literature method [41,42], 4,40 -diamino-400 -trityltriphenylamine (3) was synthesized by a three-step synthetic route (Scheme 1). First, para-tritylation of aniline with triphenylmethyl chloride gave 4-tritylaniline (1) in a good yield [40,44]. Then, 4,40 -dinitro-400 -trityltriphenylamine (2) was obtained by CsF-mediated fluoro-displacement of p-fluoronitrobenzene with 4-tritylaniline in DMSO. Finally, compound 2 was reduced by hydrazine and Pd catalyst in ethanol to 3. The structures of all the prepared compounds were confirmed by IR and NMR spectroscopies. Figure S1 (Supplementary Materials) illustrates FT-IR spectra of compounds 1−3. All of the nitro and amino functional groups could be evidenced by their characteristic absorptions. The 1 H NMR spectra of 4-tritylaniline (1) and dinitro compound 2 are summarized in Figure S2 (Supplementary Materials). The NMR spectra of the diamine monomer 3 are compiled in Figures 6 and 7. The 1 H NMR spectrum of 3 is consistent with those reported in literature [41,42]; however, exact assignments of all resonance signals are only shown here. The 13 C NMR spectrum of diamine monomer 3 and complete assignments of all peaks are first reported in this work (see Figure 7). All of the 1 H and 13 C NMR signals are well assigned to the structure of 3. Mass analysis results of Scheme 1. Synthesis of 4,4′-diamino-4′′-trityltriphenylamine (3). compounds 2 and 3 (Figure S3) are also in good agreement with their calculated values.
00 -trityltriphenylamine (3), measured in dimethyl sulfoxide Figure spectra of 4,4 Figure6.6.1 H1HNMR NMR spectra of0 -diamino-4 4,4′-diamino-4′′-trityltriphenylamine (3), measured in dimethyl (DMSO)-d . sulfoxide 6(DMSO)-d 6.
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Scheme 1. Synthesis of 4,40 -diamino-400 -trityltriphenylamine (3).
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Scheme 1. Synthesis of 4,4′-diamino-4′′-trityltriphenylamine (3).
Figure7.7.1313C C NMR NMR and (3) in 6. Figure and C-H C-HHMQC HMQCspectra spectraofof4,4′-diamino-4′′-trityltriphenylamine 4,40 -diamino-400 -trityltriphenylamine (3)DMSO-d in DMSO-d 6.
3.2. Synthesis of Polyamides 1H NMR spectra of 4,4′-diamino-4′′-trityltriphenylamine (3), measured in dimethyl Figure 6. 3.2. Synthesis of Polyamides sulfoxide (DMSO)-d 6. According to the phosphorylation technique described by Yamazaki and co-workers [45], a According to the phosphorylation technique described by Yamazaki and co-workers [45], a series series of polyamides 5a–5e were synthesized from the diamine monomer 3 with various aromatic or ofaliphatic polyamides 5a–5e were from the 3 with various aromatic or aliphatic dicarboxylic acidssynthesized (4a−4e) by means of diamine TPP and monomer pyridine (Scheme 2). As shown in Table 1, dicarboxylic acids (4a − 4e) by means of TPP and pyridine (Scheme 2). As shown in Table 1, the obtained −1 the obtained PAs had inherent viscosities in the range of 0.35−0.62 dL g . Polyamides 5a–5c can be solution-cast into flexible and transparent films (see photos shown in Scheme 2), indicating the formation of high molecular weight polymers. Although 5d and 5e have relatively lower inherent viscosities, they can still be casted into robust films on the glass substrates. Their cast films are highly transparent and colorless. The structures of the PAs were confirmed by IR and NMR spectroscopy. Figure S4 (Supplementary Materials) shows the IR spectra of PAs 5a−5e. All of the PAs −1
3.2. Synthesis of Polyamides According to the phosphorylation technique described by Yamazaki and co-workers [45], a series of polyamides 5a–5e were synthesized from the diamine monomer 3 with various aromatic or Polymers 2017, 9, 511 8 of 17 aliphatic dicarboxylic acids (4a−4e) by means of TPP and pyridine (Scheme 2). As shown in Table 1, −1 the obtained PAs had inherent viscosities in the range of 0.35−0.62 dL g . Polyamides 5a–5c can be solution-cast into viscosities flexible and transparent films (see dL photos shown in Scheme 2),be indicating the PAs had inherent in the range of 0.35 −0.62 g−1 . Polyamides 5a–5c can solution-cast formation of and hightransparent molecular weight polymers. Although 5d and2), 5eindicating have relatively lower inherent into flexible films (see photos shown in Scheme the formation of high viscosities, they can still be casted into robust films on the glass substrates. Their cast films molecular weight polymers. Although 5d and 5e have relatively lower inherent viscosities, they can are still highly transparent The structures of the confirmed by IRand and NMR be casted into robustand filmscolorless. on the glass substrates. Their cast PAs filmswere are highly transparent colorless. spectroscopy. (Supplementary Materials) shows IR spectra ofFigure PAs 5a−5e. All of the PAs The structuresFigure of theS4 PAs were confirmed by IR and NMRthe spectroscopy. S4 (Supplementary −1 (N−H stretching) showed the characteristic absorption bands of the amide group at around 3315 cm Materials) shows the IR spectra of PAs 5a−5e. All of the PAs showed the characteristic absorption −1 (amide C=O). As a typical example, −1 (amide and 1664 S5 showsand the 11664 H and COSYC=O). spectra bands of cm the amide group at around 3315 cm−1 (NFigure −H stretching) cmH−H Asofa polyamide 5a. All of the could be well assigned to the hydrogen in the typical example, Figure S5 resonance shows the 1signals H and H −H COSY spectra of polyamide 5a. All of atoms the resonance 1H NMR spectrum also verifies repeating unit. The resonance peak appearing at 10.37 ppm in the signals could be well assigned to the hydrogen atoms in the repeating unit. The resonance peak the formation of amide the polymer main appearing at 10.37 ppmlinkages in the 1 HinNMR spectrum alsochain. verifies the formation of amide linkages in the polymer main chain.
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Scheme 2. Synthesis of polyamides and their cast films (~25 µm in thickness). Scheme 2. Synthesis of polyamides and their cast films (~25 μm in thickness). Table 1. Inherent viscosity and organosolubility of the polymers. Table 1. Inherent viscosity and organosolubility of the polymers. Solubility in Various Solvents b,c −1 ) a Solubility in various solvents b,c Polymer Code η (dL g inh −1 a Polymer code ηinh (dL g ) DMSO m-CresolTHF THF NMP NMP DMAcDMAc DMFDMF DMSO m-Cresol 5a5a 0.49 0.49 ++ ++ ++ ++ ++ ++ ++ ++ ++++ +−+− 5b 0.62 ++ ++ ++ ++ ++ 5b 0.62 ++ ++ ++ ++ ++ +−+− 5c5c 0.43 ++ ++ ++ ++ ++ 0.43 ++ ++ ++ ++ ++ ++++ 5d 0.42 ++ ++ ++ ++ ++ 5d 0.42 ++ ++ ++ ++ ++ ++++ 5e 0.35 ++ ++ ++ ++ ++ ++ 5e 0.35 ++ ++ ++ ++ ++ ++ 7a 0.25 ++ ++ + + ++ ++ 7a 0.25 ++ ++ ++ ++ + ++ + + ++++ ++++ 7b 0.64 7b 0.64 ++ ++ ++ + ++ ++++ 7c 0.30 ++ ++ ++ ++ ++ 7c 0.30 ++ ++ ++ ++ ++ ++ −1
Inherent viscosity of the polymer solution at a concentration of 0.5 g dL in DMAc at 30 ◦ C; Inherent viscosity of the polymer solution at a concentration of 0.5 g dL−1 in DMAc 30 °C; NMP: N-methyl-2-pyrrolidone; DMAc: N,N-dimethylacetamide; DMF: N,N-dimethylformamide; DMSO:atdimethyl b NMP: N-methyl-2-pyrrolidone; sulfoxide; THF: tetrahydrofuran; c The DMAc: solubilityN,N-dimethylacetamide; was determined using 10 mgDMF: of the N,N-dimethylformamide; polymer sample and 1 mL of solvent. ++, soluble at room temperature; +, soluble on heating; +−, partially soluble on heating. c a
a b
DMSO: dimethyl sulfoxide; THF: tetrahydrofuran; The solubility was determined using 10 mg of the polymer sample and 1 mL of solvent. ++, soluble at room temperature; +, soluble on heating; +−, 3.3. Synthesis of Polyimides partially soluble on heating.
The synthetic methods of the PIs 7a−7c are shown in Scheme 3. These PIs were prepared 3.3.condensation Synthesis of Polyimides by of diamine compound 3 with aromatic tetracarboxylic dianhydrides ODPA (6a), 6FDAThe (6b), and HPMDA (6c),ofrespectively. Theare resulting exhibited inherent in synthetic methods the PIs 7a−7c shown polyimides in Scheme 3. These PIs were viscosities prepared by − 1 the range of 0.25 0.64 dL gcompound , as listed 3inwith Tablearomatic 1. condensation of−diamine tetracarboxylic dianhydrides ODPA (6a),
6FDA (6b), and HPMDA (6c), respectively. The resulting polyimides exhibited inherent viscosities in the range of 0.25−0.64 dL g−1, as listed in Table 1. The structures of these polyimides were confirmed by IR and NMR analysis. Figure S6 (Supplementary Materials) shows the IR spectra of PIs 7a−7c. Characteristic absorptions at 1780 and 1717 cm−1 peculiar to imide groups could be observed in these spectra. Representative 1H NMR
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The structures of these polyimides were confirmed by IR and NMR analysis. Figure S6 (Supplementary Materials) shows the IR spectra of PIs 7a−7c. Characteristic absorptions at 1780 and 1717 cm−1 peculiar to imide groups could be observed in these spectra. Representative 1 H NMR spectra of polyimide 7b are presented in Figure S7. The resonance signals of the aromatic protons Polymers 2017, 9, 511 10 of 19 appear in the range of δ 6.9−8.0 ppm. The spectrum agrees well with the repeating unit of PI 7b.
7a
7b
7c
Scheme 3. Synthesis of polyimides and their cast films (~25 µm). Scheme 3. Synthesis of polyimides and their cast films (~25 μm).
3.4. Properties of Polymers 3.4. Properties of Polymers 3.4.1. Solubility 3.4.1. Solubility The solubility of the polymers was tested in some common organic solvents at 10% (w/v). solubility of the was tested common organicsoluble solvents 10%solvents (w/v). The TheThe results are shown in polymers Table 1. Almost all of in thesome polymers are readily in at polar that results are shown in Table 1. Almost all of the polymers are readily soluble in polar solvents that include NMP, DMAc, DMF, DMSO, and m-cresol. Except for PAs 5a and 5b, all other polymers, are also include DMF,All DMSO, m-cresol. Except for PAs 5asolubility and 5b, all other polymers, solubleNMP, in lessDMAc, polar THF. of the and polymers exhibit an enhanced in comparison withare the also soluble in less polar THF. All of the polymers exhibit an enhanced solubility in comparison analogous triphenylamine-based polymers without the trityl substituent [46]. The enhanced solubility with the polymers analogous polymers of without the trityl substituent [46]. The of these cantriphenylamine-based be explained by the introduction the trityl-substituted triphenylamine unit enhanced solubility which of these polymers can be explained by theand introduction the trityl-substituted in the backbones, hinders with close chain packing decreases of chain-chain interactions. triphenylamine unit in the backbones, whichapplications hinders with close chain packing and decreases Thus, the high solubility favors their practical via solution processing. chain-chain interactions. Thus, the high solubility favors their practical applications via solution 3.4.2. Thermal Stability processing. The thermal properties of all the polymers were studied by DSC and TGA. The relevant data are 3.4.2. Thermal Stability summarized in Table 2. Typical TGA thermograms of PAs 5c and 5d and PIs 7a−7c are reproduced The thermal properties of Materials). all the polymers were studied by DSC and TGA. relevant data in Figure S8 (Supplementary The glass-transition temperatures (Tg )The of these polymers ◦ are summarized in Table 2. Typical TGA thermograms of PAs 5c and 5d and PIs 7a−7c are by DSC were recorded between 206 and 336 C. Semi-aromatic PA 5d exhibited relatively lower ◦ ◦ reproduced in Figure S8 (Supplementary The glass-transition (Tthe g) offlexible these Tg values (206 C) when compared withMaterials). wholly aromatic 5c (Tg = 305 temperatures C) because of polymers bysegments. DSC were recorded between and 336 °C. Semi-aromatic exhibited relatively methylene PAs 5a−5c showed206 slightly higher Tg values thanPA the5dcorresponding parent lower Tg values (206 °C) compared wholly aromatic 5c (Tg of = segmental 305 °C) because the triphenylamine-based PAswhen [46]. This may bewith a result of decreased freedom motionof caused flexible segments. PAs 5a−5c higher Tg valuesweight than the corresponding by the methylene trityl substitution. Aromatic PAsshowed 5a to 5cslightly showed no significant loss below 400 ◦ C 10 parent triphenylamine-based PAs [46].temperatures This may be(T a dresult of decreased of segmental in air or in N2 . The 10% weight-loss ) of the PAs in N2freedom and air were recorded ◦ ◦ motion caused by the trityl substitution. Aromatic PAs 5a to 5c showed no significant weight loss in the range of 395–435 C and 390–460 C, respectively. As expected, PAs 5d and 5e reveal lower 10 below 400 °C in air or in N2. The weight-loss (Td ) of thetoPAs N2stable and air were decomposition temperatures and10% char yields thantemperatures PAs 5a−5c attributable the in less aliphatic recorded in the range of 395–435 °C and 390–460 °C, respectively. As expected, PAs 5d and 5e reveal lower decomposition temperatures and char yields than PAs 5a−5c attributable to the less stable aliphatic segments. Because of the rigid imide ring, the PIs exhibited higher Td and Tg values, as compared to the corresponding PAs. The good thermal stability of these PAs and PIs is beneficial to increase the morphological stability and life time of their cast films in device applications.
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Polymers 2017, 9, 511 of the rigid imide ring, the PIs exhibited higher T and Tg values, as compared 11 ofto 19 segments. Because d the corresponding PAs. The good thermal stability of these PAs and PIs is beneficial to increase the morphological stability and life time of their cast films in device applications.
Table 2. Thermal properties of the polymers.
Polymer Polymer 5a
5b
5a 5c 5b 5c5d 5d5e 5e7a 7a 7b7b 7c7c
Table 2. Thermal Td10 (°C)properties of the polymers.
Tg (°C) T g312 (◦ C)
288 312 305 288 206 305 206 295 295 292 292 314 314 336 336
in N2
in Air T d 10 (◦ C)
410 in N2 430 410 435 430 400 435 400 395 395 570 570 570 570 525 525
420 in Air 430 420 460 430 400 460 390 400 560 390 560 550 550 520 520
Char yield at 800 °C (wt %) Char Yield55 at 800 ◦ C (wt %)
55 55 55 55 2755 3227 6032 60 6767 3939
3.4.3. Electrochemical Electrochemical Property Property 3.4.3. The electrochemical electrochemicalproperty propertyofof PAs investigated by cyclic voltammetry (CV) The thethe PAs andand PIs PIs waswas investigated by cyclic voltammetry (CV) with with the cast films on ITO-glass in 0.1 M TBAP (Bu 4 NClO 4 )/acetonitrile (MeCN) solution. An the cast films on ITO-glass in 0.1 M TBAP (Bu4 NClO4 )/acetonitrile (MeCN) solution. An Ag/AgCl Ag/AgCl in saturated KCl solution was as a reference electrode (calibrated externally a5 in saturated KCl solution was used as used a reference electrode (calibrated externally using using a 5 mM Ox) of Ox mM solution of ferrocene/ferrocenium couple). The oxidation half-wave potential (E 1/2 solution of ferrocene/ferrocenium couple). The oxidation half-wave potential (E1/2 ) of ferrocene ferrocene found 0.44 V vs. Ag/AgCl (seeSupplementary Figure S9, Supplementary Materials). The typical was found was as 0.44 V vs.asAg/AgCl (see Figure S9, Materials). The typical CV diagrams CVrepresentative diagrams of PA representative PA depicted 5a and PI 7a are 8, depicted in of Figure 8, and those are of the other of 5a and PI 7a are in Figure and those the other polymers included polymers are included in the Supplementary Materials (Figures S10 and S11). The oxidation in the Supplementary Materials (Figures S10 and S11). The oxidation potentials of the polymers are potentials ofinthe polymers are summarized 3. The half-wave potentials (E1/2) of the summarized Table 3. The half-wave potentialsin(ETable 1/2 ) of the polymers were recorded in the ranges polymers were recorded in the ranges of1.11 0.81−0.86 (for the It PAs) and 1.09−1.11 (forPIs theshowed PIs). It is of 0.81−0.86 V (for the PAs) and 1.09− V (for Vthe PIs). is reasonable thatVthe a reasonable that the PIs showed a significant anodic shift of the potential in comparison to their PA significant anodic shift of the potential in comparison to their PA counterparts due to the presence of counterparts due to the presence electron-withdrawing imidefilms units. Upon oxidation, polymer electron-withdrawing imide units.of Upon oxidation, the polymer changed color fromthe colorless to films changed color from colorless to greenish blue or blue. The HOMO (highest occupied greenish blue or blue. The HOMO (highest occupied molecular orbital) energy levels of the polymers molecular orbital) energy of the polymers were calculated from cyclic and by were calculated from cycliclevels voltammetry and by comparison with ferrocene (5.1voltammetry eV) [47]. These data, comparison ferrocene (5.1 eV) together with absorption spectra, were then together withwith absorption spectra, were[47]. thenThese used data, to obtain the LUMO (lowest unoccupied molecular used to obtain the LUMO (lowest unoccupied molecular orbital) energy levels (Table 3). orbital) energy levels (Table 3).
Figure of the (PA) 5a and5a polyimide (PI) 7a films ITO-coated Figure 8.8.Cyclic Cyclicvoltammogram voltammogram of polyamide the polyamide (PA) and polyimide (PI)on7athe films on the − 1 −1 glass slide inglass 0.1 M TBAP/MeCN with a scan with rate of 50 mV s of. 50 mV s . ITO-coated slide in 0.1 M TBAP/MeCN a scan rate
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Table 3. Electrochemical properties of PAs 5a–5e and PIs 7a–7c. Polymer 5a 5b 5c 5d 5e 7a 7b 7c
UV-Vis Absorption (nm) a λmax
λonset
352 340 343 320 321 320 323 316
436 396 414 368 373 379 379 358
Oxidation Potential (V) b Eonset E1/2 Ox 0.70 0.74 0.72 0.70 0.71 1.02 1.00 0.99
0.85 0.85 0.86 0.83 0.81 1.11 1.10 1.09
Eg (eV) c
HOMO (eV) d
LUMO (eV) d
2.84 3.13 2.99 3.37 3.32 3.27 3.27 3.46
−5.51 −5.51 −5.52 −5.49 −5.47 −5.77 −5.76 −5.75
−2.67 −2.38 −2.53 −2.12 −2.15 −2.50 −2.49 −2.29
a
Measured as solid thin films; b Obtained from CV diagrams, relative to Ag/AgCl in 0.1 M TBAP/MeCN and the scan rate was 50 mV s−1 ; c Optical bandgap calculated from the UV–vis absorption edge of the polymer film: Eg = 1240/λonset ; d EHOMO = −[E1/2 Ox + 5.1 − 0.44] (eV); ELUMO = −(EHOMO + Eg ).
3.4.4. Electro-Optical Property In order to measure the optical properties in detail, the changes in the optical absorption profile of the polymer films coated on ITO electrode were recorded in situ to elucidate the electro-optical properties. The UV–vis–NIR absorption spectral change of PA 5b and PI 7a films at various applied potentials are illustrated in Figure 9. The spectroelectrographs of other polymers are given in the Supplementary Materials (Figures S12−S16). In the neutral state, the PA films exhibited a strong absorption centered at 320−352 nm, due to the π−π* transitions. Take 5b as an example. When the applied potential was increased to 1.0 V, the π−π* transition band at 340 nm gradually decreased while new absorption bands at 401, 655, and 828 nm gradually appeared, attributed to the formation of charge carriers. The spectral changes can be easily followed by color change from colorless to greenish blue during doping. Therein, semi-aromatic polymers show a slight blue shift, because the conjugate length is shorter than the wholly aromatic polymer, therefore the color change from greenish blue to blue upon oxidation. PI 7a showed a similar spectral change upon electro-oxidation; Polymers 2017, 9, 511 13 of 19 however, the absorption maxima at its doped state exhibited a slight blue shift as compared to PA 5b. (b)
(a)
0V
1.0 V
0V
1.2 V
9. Optical absorption spectra andcolor color changes changes ofof thethe castcast filmsfilms (thickness: 200 ± 30 200 nm) of Figure 9. Figure Optical absorption spectra and (thickness: ± 30 nm) of (a) PA 5b and (b) PI 7a on the ITO-coated glass slide in 0.1 M TBAP/MeCN at various applied (a) PA 5b and (b) PI 7a on the ITO-coated glass slide in 0.1 M TBAP/MeCN at various applied potentials. potentials.
3.4.5. Electrochromic Switching and Stability In order to determine the switching stability and response time, a square-wave potential method was used and then 0 V and 1.0 V, or 1.3 V was applied for a pulse width of 10 s. While the external applied potentials were switched, the absorbance at selected wavelengths was monitored. Figure 10 shows the percentage transmittance change of PA 5b film at 828 and PI 7a film at 753 nm
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3.4.5. Electrochromic Switching and Stability In order to determine the switching stability and response time, a square-wave potential method was used and then 0 V and 1.0 V, or 1.3 V was applied for a pulse width of 10 s. While the external applied potentials were switched, the absorbance at selected wavelengths was monitored. Figure 10 shows the percentage transmittance change of PA 5b film at 828 and PI 7a film at 753 nm upon potential switching. The decay in electrochromic coloration efficiency (CE = ∆OD/Qd ) after various switching steps for PA 5b is shown in Table 4. PA 5b exhibited good coloration efficiencies of up to 186 cm2 C−1 at 828 nm at the first switching cycle. It did not lose the electrochromic activity significantly and retained about 95% of its optical response after 100 switching cycles. Therefore, the electrochromic switching behavior of PAs appears to be a highly reversible process. In contrast, PI 7a exhibited a higher oxidation potential and significantly decreased the cycling stability than the corresponding PA 5b analog. The result can be explained by its lowered electrochemical stability arising from the electron-withdrawing effect of the imide units. The percentage transmittance change (∆%T) between neutral (at 0.0 V) and oxidized (at 1.05 V) states of the PA 5b film was up to 84% at 828 nm. PA 5b attained 90% of a complete blue-green coloring in 3.77 s, as shown in Figure S17 (Supplementary Materials). In contrast, PI 7a attained 90% oxidized blue states in 4.33 s. The electrochromic coloration efficiency (CE) value of PA 5b was found to be 2 C−12017, Polymers 9, 511nm by chronoamperometry. PI 7a showed a slightly lower CE value of 14 133 of 19 cm2 186 cm at 828 − 1 C at 754 nm. L*= 72 a*= 0 b*= 3
(a)
0V
(b)
L*= 75 a*= −1 b*= 9
0V
L*= 52 a*= −10 b*= 2
1.05 V
L*= 62 a*= −9 b*= −6
1.30 V
Figure 10. Cheonoabsorptometry experiments for polymer films on the ITO-glass slide (active area:
Figure 10.2 Cheonoabsorptometry experiments for polymer films on the ITO-glass slide (active area: 1 cm , thickness: 200 ± 30 nm): (a) PA 5b switched between 0.0 and 1.05 V with a pulse width of 10 s; 1 cm2 , thickness: 200 ± 30 nm): (a) PA 5b switched between 0.0 and 1.05 V with a pulse width of 10 s; (b) PI 7a switched between 0.0 and 1.30 V with a pulse width of 20 s. (b) PI 7a switched between 0.0 and 1.30 V with a pulse width of 20 s. Table 4. Coloration efficiency of polyamide 5b. Cycling times a ΔOD490 b Qd (mC cm−2) c CE (cm2 C−1) d Decay in CE (%) 1 10 20 30
0.99 0.99 0.98 0.98
5.33 5.33 5.31 5.31
186 186 185 185
0 0 0.5 0.5
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Table 4. Coloration efficiency of polyamide 5b. Cycling Times a 1 10 20 30 40 50 60 70 80 90 100
∆OD490 0.99 0.99 0.98 0.98 0.98 0.97 0.96 0.96 0.96 0.95 0.94
b
Qd (mC cm−2 ) c
CE (cm2 C−1 ) d
Decay in CE (%)
5.33 5.33 5.31 5.31 5.32 5.30 5.29 5.30 5.29 5.30 5.28
186 186 185 185 184 183 181 181 180 179 178
0 0 0.5 0.5 1 1.6 2.7 2.7 3.2 3.7 4.3
a Applied potential was switched between 0 and 1.05 V (vs. Ag/AgCl); b Optical Density (∆OD) = log[Tbleached /Tcolored ], where Tcolored and Tbleached are the maximum transmittance in the oxidized and neutral states, respectively; c Ejected charge, obtained from the in situ experiments; d Coloration efficiency calculated by CE = ∆OD/Qd .
3.5. Electrochromic Devices The semi-aromatic PA 5e and PI 7c films were chosen as active layers in the fabication of ECDs due to the fact that they are almost colorless in neutral state and are expected to exhibit high electrochromic contrast upon oxidation. The CV diagrams and electrospectrographs of 5e and 5e/HV devices are illustrated in Figure 11. As shown in Figure 11a, the 5e device revealed one oxidation peak at 1.7 V. The high working potential might result in waste of energy and less stability of the devices. In order to improve these drawbacks, the cathodically electrochromic material HV was introduced into the semi-gelled electrolyte layer of ECDs. The CV diagram of 5e/HV is depicted in Figure 11b, demonstrating reversible redox steps with a decreased oxidative potential (Epa at 1.2 V). This effect could be ascribed to HV2+ having the capability to accept electrons from TPA moieties during the oxidation process, and then the resulted HV+ also could donate electrons back to TPA radical cations during the reducing process. The spectroelectrochemical graphs of 5e ECD are presented in Figure 11c. The spectra and color changes are similar to those observed in the electrochemical cell. Spectroelectrochemical behavior of ECDs 5e/HV was also studied and depicted in Figure 8d. New absorption peaks could be observed at 366 nm and 627 nm due to the reductive process of HV+2 to HV+ . Because of the combination of TPA oxidation and HV reduction at the same time, the dual ECD revealed a deeper blue coloring as the potential was applied. Similar results were also observed for the 7c and 7c/HV ECDs as shown in Figure 12. Thus, by adding HV as an efficient charge trapping agent in electrolyte layer, not only the working voltage could be reduced but also the performance of overall system was enhanced.
electrochemical cell. Spectroelectrochemical behavior of ECDs 5e/HV was also studied and depicted in Figure 8d. New absorption peaks could be observed at 366 nm and 627 nm due to the reductive process of HV+2 to HV+. Because of the combination of TPA oxidation and HV reduction at the same time, the dual ECD revealed a deeper blue coloring as the potential was applied. Similar results were also observed for the 7c and 7c/HV ECDs as shown in Figure 12. Thus, by adding HV as an efficient charge Polymers 2017, 9, 511trapping agent in electrolyte layer, not only the working voltage could be reduced but also the performance of overall system was enhanced.
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Figure 11. Figure (a) Cyclic (CV)(CV) diagram Electrochromic Devices 5e 11. (a) voltammetry Cyclic voltammetry diagramof ofthe the Electrochromic Devices (ECD) (ECD) (ITO/PA (ITO/PA 5e film/gel electrolyte/ITO) based castfilm film (thickness: (thickness: 250250 ± 30±nm) PA of 5e on film/gel electrolyte/ITO) based on on thethe cast 30 of nm) PAthe 5eITO-coated on the ITO-coated glass substrate; (b) CV diagram of the ECD based on PA 5e/HV; (c) Spectra and color change of the glass substrate; (b) CV diagram of the ECD based on PA 5e/HV; (c) Spectra and color change of the ECD based on PA 5e at indicated applied voltages; (d) Spectra and color change of the PA 5e/HV ECD based on PA 5e at indicated applied voltages; (d) Spectra and color change of the PA 5e/HV ECD at indicated applied voltages. The semi-gelled electrolyte consists of 10-wt % PMMA (average ECD at indicated applied The semi-gelled (average carbonate. electrolyte consists of 10-wt % PMMA Mw = 120,000) in 0.6voltages. M LiBF4/propylene Polymers 2017, 9, 511 16 of 19 Mw = 120,000) in 0.6 M LiBF4 /propylene carbonate. (a)
(c) PI 7c ECD 0V
2.0 V
7c
(b)
(d)
0V
1.6 V
PI 7c/HV ECD
12. (a) CV diagram of the ECD (ITO/PI 7c film/gel electrolyte/ITO) based on the cast film Figure 12.Figure (a) CV diagram of the ECD (ITO/PI 7c film/gel electrolyte/ITO) based on the cast film (thickness: 250 ± 30 nm) of PI 7c on the ITO-coated glass substrate; (b) CV diagram of the ECD based (thickness:on250 ± 30 nm) of PIand 7c on the ITO-coated glass substrate; CV diagram of the ECD based PI 7c/HV; (c) Spectra color change of the ECD based on PI 7c at(b) indicated applied voltages; on PI 7c/HV; (c) Spectra and color of the ECD based on PI 7cvoltages. at indicated applied voltages; (d) Spectra and color change of change the PI 7c/HV ECD at indicated applied The semi-gelled carbonate. electrolyte consists of 10-wt PMMA (average ECD Mw = 120,000) in 0.6 Mapplied LiBF4/propylene (d) Spectra and color change of%the PI 7c/HV at indicated voltages. The semi-gelled electrolyte consists of 10-wt % PMMA (average Mw = 120,000) in 0.6 M LiBF4 /propylene carbonate.
4. Conclusions
Electroactive polyamides and polyimides containing trityl-triphenylamine units were successfully synthesized and characterized. Due to the bulky, double propeller-shaped structure of the diamine component, all of the prepared polymers exhibit high solubility in various organic solvents and could afford flexible or robust films. The polymers derived from aliphatic dicarboxylic acid or dianhydride could afford almost colorless films. The wholly aromatic polyamides and polyimides exhibited good thermal stability and could afford flexible and strong films. All of the cast films on the ITO electrode revealed reversible redox behavior upon electro-oxidation,
Polymers 2017, 9, 511
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4. Conclusions Electroactive polyamides and polyimides containing trityl-triphenylamine units were successfully synthesized and characterized. Due to the bulky, double propeller-shaped structure of the diamine component, all of the prepared polymers exhibit high solubility in various organic solvents and could afford flexible or robust films. The polymers derived from aliphatic dicarboxylic acid or dianhydride could afford almost colorless films. The wholly aromatic polyamides and polyimides exhibited good thermal stability and could afford flexible and strong films. All of the cast films on the ITO electrode revealed reversible redox behavior upon electro-oxidation, accompanying with a green-blue or blue coloring change. The polyimides exhibited a slightly higher oxidation potential and less stable cycling stability than the corresponding polyamide analogs. The electrochromic devices using the prepared polymers as active layers showed promising performance. Additionally, incorporating HV into the electrolyte as charge balance material led to a decreased working potential and an increased contrast in electrochromic coloring. Supplementary Materials: The following are available online at www.mdpi.com/2073-4360/9/10/511/s1, Figure S1: IR spectra of compounds 1 to 3; Figure S2: 1 H NMR spectra of compounds 1 and 2 in DMSO-d6 ; Figure S3: Mass analysis of compounds 2 and 3; Figure S4: IR spectra of PAs 5a−5e; Figure S5: 1 H H-H COSY NMR spectra of polyamide 5a in DMSO-d6 ; Figure S6: IR spectra of PIs 7a−7c; Figure S7: 1 H H-H COSY NMR spectra of polyimide 7b in DMSO-d6 ; Figure S8: TGA curves of PAs 5c and 5d and PIs 7a−7c; Figure S9: Cyclic voltammogram of 1 mM ferrocene in 0.1 M Bu4 NClO4 /MeCN at a scan rate of 50 mV·s−1 ; Figure S10: Cyclic voltammograms of PAs 5b–5e films on the ITO-coated glass substrate in 0.1 M Bu4 NClO4 /MeCN at a scan rate of 50 mV·s−1 ; Figure S11: Cyclic voltammograms of PIs 7b and 7c films on the ITO-coated glass substrate in 0.1 M Bu4 NClO4 /MeCN at a scan rate of 50 mV·s−1 ; Figures S12–S16: Optical absorption spectra of PAs and PIs. Figure S17: Current monitored and optical transmittance changes of the PA 5b and PI 7a films while the potential was switched. Acknowledgments: This work was financially supported by the Ministry of Science and Technology, Taiwan (Grant MOST 105-2221-E-027-131-MY3). Author Contributions: Sheng-Huei Hsiao and Guey-Sheng Liou planned this project. Wei-Kai Liao performed the experiments. Wei-Kai Liao, Sheng-Huei Hsiao and Guey-Sheng Liou analyzed the data and co-wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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