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: [email protected]
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)