Enhanced Contrast Ratios and Fast Switching Electrochromic

Polyamides Bearing 4-Piperidinotriphenylamine Units

Ying-Hsiu Hsiao, Yi-Chun Kung, Sheng-Huei Hsiao*

Department of Chemical Engineering, Tatung University

Taipei, Taiwan

E-mail: shhsiao@ttu.edu.tw

ABSTRACT

A series of electroactive polyamides with 4-piperidinotriphenylamine units in the backbone were prepared from a newly synthesized diamine monomer, 4,4’-diamino-4”-piperidinotriphenylamine, and various dicarboxylic acids via the

phophorylation polyamidation reaction. These polyamides are readily soluble in many organic solvents and can be solution-cast into tough and amorphous films. They had useful levels of thermal stability associated with relatively high glass-

transition temperatures (252-302 oC) and 10 % weight loss temperatures in excess of 500 oC. The polymer films showed reversible electrochemical oxidation accompanied by strong color changes with high coloration efficiency, high contrast

ratio, and rapid switching time. The polymers also displayed low ionization potentials as a result of their 4-piperidinotriphenylamine moieties. Cyclic voltammograms of the polyamide films on the indium-tin oxide (ITO)-coated glass substrate

exhibited a pair of reversible oxidation waves with very low onset potential (E1/2 = 0.44-0.49 V vs Ag/AgCl) in acetonitrile solution.

INTRODUCTION MONOMER SYNTHESIS

REFERENCES

Electrochromism is known as the reversible change in optical absorption or transmittance upon redox

switching.1 This interesting property led to the development of many technological applications such as

automatic anti-glazing mirror, smart windows, electrochromic displays, and chameleon materials.2 Many

different classes of electrochromic materials, such as organic systems, e.g., bipyridium salt (also known as

viologens),3 electroactive conducting polymers (e.g., polyanilines,4 polythiophenes,5 polypyrroles6), as well as

inorganic systems based on transition metal oxides (e.g., WO37) have been described. Conducting or

conjugated polymers have been found to be more promising as electrochromic materials because of their

better stability, faster switching speeds, and easy processing compared to the inorganic electrochromic

materials, but the most exciting properties are the display of multiple colors with the same material while

switching between their different redox states,8 and fine-tuning of the color transition through chemical

structure modification of the conjugated backbone.9,10 Considerable effort in the Reynolds group has been

made on the understanding and the tailoring of electrochromic properties in conducting polymers such as

poly(3,4-alkylenedioxythiophene)s5 and poly(3,4-alkylenedioxypyrrole)s6 and their derivatives.

Triarylamine derivatives are well known for photo- and electroactive properties that find optoelectronic

applications as photoconductors, hole-transporters, and light-emitters.11 Triarylamines can be easily oxidized

to form stable radical cations, and the oxidation process is always associated with a noticeable change of

coloration. Thus, many triarylamine-based electrochromic polymers have been reported in literature.12 In

recent years, we have developed a number of high-performance polymers (e.g., aromatic polyamides and

polyimides) carrying the triphenylamine (TPA) unit as an electrochromic functional moiety.13 Our strategy

was to synthesize the TPA-containing monomers such as diamines and dicarboxylic acids that were then

reacted with the corresponding comonomers through conventional polycondensation techniques. The obtained

polymers possessed characteristically high molecular weights and high thermal stability. Because of the

incorporation of packing-disruptive, propeller-shaped TPA units along the polymer backbone, most of these

polymers exhibited good solubility in polar organic solvents. They may form uniform, transparent amorphous

thin films by solution casting and spin-coating methods. This is advantageous for their ready fabrication of

large-area, thin-film devices.

In order to be useful for applications, electrochromic materials must exhibit long-term stability, rapid

redox switching, and large changes in transmittance (large Δ%T) between their bleached and colored states.14

As an electrochromic functional moiety, the TPA unit has two basic properties: (1) the easy oxidizability of

the nitrogen center and (2) its hole-transporting ability via the radical cation species. However, unsubstituted

TPA undergoes coupling-deprotonation to form tetraphenylbenzidine after the formation of the initial

monocation radical.15 The oxidation potential and the π-π* bandgap of the product, generally called

triaryldiamine, are different from that of the starting material. Therefore, the small concentration of the

product may cause an unstable color change of the electrochromic material during redox switching. The

formation of protons as by-products may deteriorate the coloration efficiency of the electrochromic devices

through undesirable side reactions. It has been well established that incorporation of electron-donating

substituents such as methoxy group at the para position of TPA prevents the coupling reactions and affords

stable radical cations.15,16 It has also been demonstrated that carbazole derivatives with 4,4’-diamino-4”-

piperidinotriphenylamine groups para to the carbazole nitrogen could afford quite stable radical cations in the

first one-electron oxidation process and reasonably stable dication quinonediimines could also be generated

by a second one-electron process.17 Therefore, we synthesized the diamine monomer, 4,4’-diamino-4”-

piperidinotriphenylamine, and its derived aromatic polyamides containing electroactive TPA units with

electron-donating piperidine para substituted on the pendent phenyl ring. The piperidine substituents are

expected to reduce the oxidation potential and increase the electrochemical stability and electric conductivity

of the polyamides. We anticipated that the electrochromic films prepared from the present polyamides would

be very stable to multiple redox switching and exhibit enhanced optical response times.

Synthesis of 4,4’-diamino-4”-piperidinotriphenylamine (4)

The new TPA-based diamine monomer 4 was prepared by a

four-step reaction sequence outlined in Scheme 1. In the first step,

1-(4-nitrophenyl)piperidine (1) was synthesized by nucleophilic

aromatic displacement of 4-fluoronitrobenzene with piperidine

using potassium carbonate as the base. In the second step, the

nitro compound 1 was reduced to give 4-piperidinoaniline (2) by

hydrazine monohydrate and Pd/C catalyst in refluxing ethanol. In

the third step, 4,4’-dinitro-4”-piperidinotriphenylamine (3) was

synthesized by the cesium fluoride (CsF)-promoted N,N-

diarylation reaction of 2 with two equivalent 4-

fluoronitrobenzene. In the final step, the nitro groups of

compound 3 were reduced by the same technique used in the

second step to give the targeted diamine monomer 4. The

molecular structure of 4 was confirmed by elemental analysis, IR,

and 1H and 13C NMR spectroscopy.

N

H

F NO2DMSO

K2CO3

N

NO2

1 2

N

NH2

EtOH

Pd/C hydrazine

EtOH

Pd/C hydrazine

N

N

NH2H2N

3

N

N

NO2O2N

DMSO

CsF2 F NO22

4Compound 4

yield：73 %

m.p.：172-174 oC

Compound 3

yield：65 %

m.p.：168-171 oC

Compound 2

yield：75 %

Compound 1

yield：97 %

m.p.：100-102 oC

Figure 1. IR spectra of compound 1 and 4.

Scheme 1. Synthetic route to the diamine monomer 4.

Figure 2. 1H NMR and 13C NMR spectra of the synthesized compounds 1- 4 in DMSO-d6.

N

NO2

N

NH2

N

N

NO2O2N

N

N

NH2H2N

a

b

c

d

e

f

N

N

NH2H2N

1

2

3

4

5

6

78

9

10

11

g

7

6

5

4

3

2

1

N

NO2

a

b

c

d

e

7

6

5

4

3

2

1

N

NH2

a

b

c

d

e

a

b

c

d

e

f

N

N

NO2O2N

1

2

3

4

5

6

78

9

10

11

g

1. Mortimer, R. J. Chem. Soc. Rev. 1997, 26, 1472.

2. Heuer, H. W.; Wehrmann, R.; Kirchmeyer, S. Adv. Funct. Mater. 2002, 12, 89.

3. Monk, P. M. S. The Viologens: Synthesis, Physicochemical Properties and Applications of the Salts of 4,4’-Bipyridine; Wiley: Chichester, 1998.

4. Manisankar, P.; Vedhi, C.; Selvanathan, G.; Somasundaram, R. M. Chem. Mater. 2005, 17, 1722.

5. Groenendaal, L.; Zotti, G.; Aubert, P.-H.; Waybright, S. M.; Reynolds, J. R. Adv. Mater. 2003, 15, 855.

6. Walczak, R. M.; Reynolds, J. R. Adv. Mater. 2006, 18, 1121.

7. Granqvist, G. V. Phys. Thin Films 1993, 17, 301.

8. Argun, A. A.; Aubert, P. H.; Thompson, B. C.; Schwendeman, I.; Gaupp, C. L.; Hwang, J.; Pinto, N. J.; Tanner, D B.; MacDiarmid, A. G.; Reynolds, J. R. Chem.

Mater. 2004, 16, 4401.

9. Thompson, B. C.; Kim, Y.-G.; McCarley, T. D.; Reynolds, J. R. J. Am. Chem. Soc. 2006, 128, 12714.

10. Sonmez, G.; Meng, H,; F. Wudl. Chem. Mater. 2004, 16, 574.

11. Shirota, Y. J. Mater. Chem. 2005, 15, 75.

12. Natera, J.; Otero, L.; Sereno, L.; Fungo, F.; Wang, N.-S.; Tsai, Y.-M.; Hwu, T.-Y.; Wong, K.-T. Macromolecules 2007, 40, 4456

13. (a) Liou, G.-S.; Hsiao, S.-H.; Huang, N.-K.; Yang, Y.-L. Macromolecules 2006, 39, 5337. (b) Liou, G.-S.; Hsiao, S.-H.; Chen, W.-C.; Yen, H.-J. Macromolecules 2006, 39, 6036. (c) Chang,

C.-W.; Liou, G.-S.; Hsiao, S.-H. J. Mater. Chem. 2007, 17, 1007.

14. Schwendeman, I.; Hickman, R.; Sonmez, G.; Schottland, P.; Zong, K.; Welsh, D. M.; Reynolds, J. R. Chem. Mater. 2002, 14, 3118.

15. Nelson, R. F.; Adams, R. N. J. Am. Chem. Soc. 1968, 90, 3925.

16. Zhao, H.; Tanjutco, C.; Thayumanavan, S. Tetrahedron Lett. 2001, 42, 4421.

17 Ambrose, J. F.; Carpenter, L. L.; Nelson, R. F. J. Electrochem. Soc.: Electrochem. Sci. Technol. 1975, 122, 876.

18. Yamazaki, N.; Matsumoto, M.; Higashi, F. J. Polym. Sci. Polym. Chem. Ed. 1975, 13, 1373.

POLYMER SYNTHESIS AND

SOLUBILITY

THERMAL STABILITY OPTICAL AND ELECTROCHEMICAL

PROPERTIES

According to the phosphorylation technique described by Yamazaki and

co-workers ,18 a series of novel triphenylamine-based aromatic poly(amine

amide)s, 6a-6i, with piperidine molecular para-substituted on thependent

phenyl ring were prepared from the diamine 4 and various aromatic

dicarboxylic diacids (5a-5i) by the direct polycondensation reaction with

triphenyl phosphate (TPP) and pyridine as condensing agents (Scheme 2).

All the polymerizations proceeded homogeneously throughout the reaction

and afforded clear, highly viscous polymer solutions. These polymers

precipitated in a tough, fiber-like form when the resulting polymer solutions

were slowly poured with stirring into methanol. These polyamides were

obtained in almost quantitative yields, with ηinh values in the range of 0.42-

0.99 dL/g.

N

N

NH2H2N

4

C

O

Ar C

O

OHHO

5

N

N

NN

H

C

H O

Ar C

O

n

6

a)Ar = b) c) d) O

e)

SOO

f)

CF3F3C

g) h)

i)

Scheme 2. Synthesis of the poly(amine-amide)s 6a-6i.

Polymer Solubilityb

Code ηinh

a

(dL/g) NMP DMAc DMF DMSO m-Cresol THF

6a 0.63 ＋ ＋ ＋h ＋ ＋h －

6b 0.42 ＋h ＋h ＋h ＋h －

6c 0.90 ＋h ＋h ＋h ＋ ＋h －

6d 0.72 ＋ ＋ ＋h ＋ ＋h －

6e 0.55 ＋ ＋ ＋ ＋ ＋h －

6f 0.60 ＋ ＋ ＋ ＋ ＋h －

6g 0.61 ＋ ＋ ＋ ＋ ＋h －

6h 0.86 ＋h ＋h ＋h ＋h ＋h －

6i 0.99 －

a Inherent viscosity measured at a concentration of 0.5 dL/g in DMAc – 5 wt % LiCl at 30 oC. b The solubility was determined with a 10 mg sample in 1mL of a solvent.

Solubility: + : soluble at room temperature; : partially soluble ; +h: soluble on heating; －: insoluble even

on heating.

Solvent: NMP: N-methyl-2-pyrrolidone; DMAc: N,N-dimethylacetamide; DMF: N,N-dimethylformamide;

DMSO: dimethyl sulfoxide; THF: tetrahydrofuran.

Table 1. Inherent viscosity and Solubility behavior of poly(amine-amide)s

Polymer

code

Tgb

(°C)

Tsc

(°C)

Td at 5 wt %

lossd (°C)

Td at 10 wt %

lossd (°C)

Char

Yield

In N2 In Air In N2 In Air ( % )e

6a 287 (295)f 283 400 411 437 434 66

6b 281 (290) 278 435 432 476 478 72

6c 289 (302) 285 450 428 511 489 73

6d 280 (273) 274 439 448 486 511 69

6e 290 (296) 282 411 412 437 433 63

6f 302 (295) 297 474 465 535 512 63

6g 252 (288) 252 419 420 456 450 63

6h 295 (307) 297 422 435 455 489 71

6i 298 (256) 289 406 429 455 473 47

Table 2. Thermal properties of poly(amine-amide)sa

a The polymer film samples were heated at 300 °C for 30 min prior to all the thermal

analyses. b The sample were heated from 50 to 400 °C at a scan rate of 20 °C /min followed by

rapid cooling to 50 °C at – 200 °C /min in nitrogen. The midpoint temperature of

baseline shift on the subsequent DSC trace ( from 50 to 400 °C at heating rate 20 °C

/min ) was defined as Tg. c Softening temperature measured by TMA using a penetration method. d Decomposition temperature at which a 5 % or 10 % weight loss was recorded by TGA

at a heating rate of 20 °C /min. e Residual weight percentages at 800 °C under nitrogen flow. f Values in parentheses are data of analogous poly(amine-amide)s 6’ having the

corresponding diacid residue as in the 6 series.

N

N

N

H H

C

O

Ar C

O

n

6'

Figure 3. TMA curves of poly(amine-amide) 6g with a heating rate 10 ℃/min.

Figure 4. TGA curves of poly(amine-amide) 6f with a heating rate 20 ℃/min.

Table 3. Optical and electrochemical properties of the poly(amine-amide)s

Index

Solution (nm)a

Film (nm)

E1/2(V)c

(vs. Ag/AgCl)

Eg

(eV)d

HOMO

(eV)e

LUMO

(eV)f

ΦF (%)b First Second Eonset E1/2 Eonset E1/2 Eonset

6a 300, 366 449 0.11 363 460 0.44 (0.87)g 0.82 0.27 2.70 4.80 4.70 2.10 2.00

6b 331 424 0.98 330 434 0.47 (0.85) 0.84 0.32 2.86 4.83 4.75 1.97 1.89

6c 300, 361 424 0.07 361 449 0.47 (0.86) 0.86 0.29 2.76 4.83 4.72 2.07 1.96

6d 343 432 0.13 344 423 0.46 (0.86) 0.84 0.35 2.93 4.82 4.78 1.89 1.85

6e 303, 361 446 0.15 365 473 0.48 (0.88) 0.85 0.30 2.62 4.84 4.73 2.22 2.11

6f 296, 353 424 0.13 358 445 0.46 (0.88) 0.83 0.33 2.79 4.82 4.76 2.03 1.97

6g 316 418 0.09 313 425 0.49 (0.85) 0.84 0.34 2.92 4.85 4.77 1.93 1.85

6h 303 423 0.10 307 414 0.47 (0.85) 0.88 0.28 3.00 4.83 4.71 1.83 1.71

6i 322 429 3.57 325 370 0.45 (0.82) 0.80 0.28 3.35 4.81 4.71 1.46 1.36

a Spectra in NMP (1 × 10-5 mol/L). b The quantum yield in dilute solution was calculated in an integrating sphere with quinine sulfate as the standard

(ΦF = 54.6 % ). c Oxidation half-wave potentias from cyclic votammograms. d The data were calculated with the following equation: Eg = 1240/λabs,onset. e The HOMO energy levels were calculated from CV and were referenced to ferrocene (4.8 eV). f LUMO = HOMO- Eg. g.Values in parentheses are data of analogous poly(amine-amide)s 6’ having the corresponding diacid residue as in

the 6 series.

N

N

N

H H

C

O

Ar C

O

n

6'

Figure 5. UV-Vis absorption and photoluminescence (PL)

spectra of poly(amine-amide)s 6b, 6d, 6i in NMP (1 × 10-5

M). Quinine sulfate dissolved ) in 1 N H2SO4 (aq.) with a

concentration of 1 × 10-5 M as the standard (ΦF = 54.6 %).

Figure 6. Cyclic voltammograms of (a)

ferrocene (b) poly(amine-amide) 6d (c)

poly(amine-amide) 6d’ film onto an indium-

tin oxide (ITO) coated glass substrate in

CH3CN containing 0.1 M TBAP. Scan rate =

0.1 V/s.

absλmax

PLλmax

absλmax

abs

onsetλ

Figure 7. Spectral change of 6d thin film on the ITO-coated

glass substrate (in CH3CN with 0.1 M TBAP as the

supporting electrolyte) along with increasing of the applied

voltage: 0 (■), 0.50 (●), 0.60 (▲), 0.65 (▼), 0.75 (◆), 0.95

(□), 1.00 (○), 1.05 (△), 1.15 (▽), and 1.20 (◇) vs

Ag/AgCl couple as reference. The inset shows the

photographic images of the film at indicated applied voltages.

Figure 8. (a) Potential step absorptometry and (b) current

consumption of the polyamide 6d film on to the ITO-coated

glass substrate (coated area: 1 cm2) during the continuous

cycling test by switching potentials between 0 and 0.50 V (vs.

Ag/AgCl).

6d'

n

N

NN

H

C

H O

O C

O

6d

n

N

N

NN

H

C

H O

O C

O

N

N

NN

H

C

H O

Ar C

O

n

6

Ar =b) d)

O

i)