Electrochromic films with high optical contrast prepared by oxidative electropolymerization of a novel multi-functionalized cyclometalating ligand and its neutral-charged Pt(II) complexes

Article abstract of DOI:10.1016/j.electacta.2011.11.083

A novel multi-functionalized cyclometalating ligand (HL) and its two neutral-charged Pt(II) complexes, i.e., [(L)PtCl] (C1) and [(L)Pt(CCC ₆H₅)] (C2) (HL = 4-{p-[N-(4-(9-carbazole))butyl-N-phenyl] anilino}-6-phenyl-2,2′-bipyridine),

Full text of DOI:10.1016/j.electacta.2011.11.083

Electrochimica Acta 60 (2012) 339–346
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Electrochimica Acta
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Electrochromic films with high optical contrast prepared by oxidative
electropolymerization of a novel multi-functionalized cyclometalating ligand and
its neutral-charged Pt(II) complexes
Dongfang Qiu, Xiaoyu Bao, Yuquan Feng, Kecheng Liu, Hongwei Wang, Hengzhen Shi∗,
Yingchen Guo, Qunzeng Huang, Junliang Zeng, Jing Zhou, Zuochao Xing
College of Chemistry and Pharmacy Engineering, Nanyang Normal University, Nanyang, 473061, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 August 2011
Received in revised form 9 November 2011
Accepted 16 November 2011
Available online 28 November 2011
A novel multi-functionalized cyclometalating ligand (HL) and its two neutral-charged Pt(II) complexes,
i.e., [(L)PtCl] (C1) and [(L)Pt(C CC6H5)] (C2) (HL = 4-{p-[N-(4-(9-carbazole))butyl-N-phenyl]anilino}-6-
phenyl-2,2ꢀ-bipyridine), have been successfully synthesized and verified by 1H NMR, 13C NMR, elemental
analysis and/or X-ray crystallography. The introduction of the electron-donating arylamino unit at the
4-position of the electron-withdrawing 6-phenyl-2,2ꢀ-bipyridine ligand forms the D–A system, resulting
in the enhanced 1MLCT bands in the absorption spectra and intense orange emissions in the PL spectra of
C1 and C2. The carbazole units grafted through alkyl chain in HL, C1 and C2 electrochemically trigger the
efficient cross-linking reaction to form polymer films, which have been characterized by AC impedance,
SEM and spectroelectrochemistry. Each of the poly-HL and poly-C1 films exhibits reversible oxidation
with significant color change having a high contrast ratio (81.8% for poly-HL film and 67.3% for poly-
C1 film), a reasonable coloration response time (6.3 s for poly-HL film and 3.1 s for poly-C1 film) and a
comparable coloration efficiency (158 C−1 cm2 for poly-HL film and 132 C−1 cm2 for poly-C1 film) that
can be switched by modulation of the applied potential.
Keywords:
Multi-functionalized
Cyclometalating ligand
Pt(II) complex
Electropolymerization
Anodic coloration
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
natures of the ligand-based and metal-based processes can be
switched with color changes in the regular potential window, these
Electrochromic materials with high optical contrast and col-
oration efficiency, long cycle life and appropriate response time
have attracted a great deal of attention due to their potential
applications in anti-glare mirrors, protective eyewear, smart win-
dows, re-usable price labels and controllable light-reflective or
light-transmissive display devices. A number of chemical species
including metal oxides, viologens, metal coordination complexes
and conducting polymers have been widely studied in solution
and/or as film [1].
Hybrid metallopolymers electrogenerated from transition
metal polypyridyl complexes, that consist of conjugated polymers
with transition metals linked to or directly in the ␲-conjugated
polymer backbone, is a promising group of electrochromic mate-
rials. Because their chromophoric properties, resulting from
low-energy metal-to-ligand charge transfer (MLCT), intervalence
charge transfer (IVCT), intraligand charge transfer (ILCT) and
related visible region electronic transitions, can be tuned via the
molecular design of ligands and complexes, and their multi-redox
polymeric systems have greatly expected to use in all-solid-state
electrochromic devices [2]. Iron(II), ruthenium(II) and osmium(II)
complexes containing bromomethyl- [3], vinyl- [4], amino- [5]
and pendant aniline- [6] substituted 2,2ꢀ-bipyridyl (NN) lig-
ˆ
and, and amino- and hydroxyl-substituted 2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridinyl
ˆ ˆ
(NNN) ligand [7] have been reductively or oxidatively elec-
tropolymerized to form electrochromic films. Among them, only
the Ru(II)-based polymer film containing hydroxyl-substituted
2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridinyl ligand shows the anodic coloration from
brown to dark yellow, the others exhibit the anodic de-coloration
behavior associated with the M(II)/M(III) interconversion.
Recently, the cyclometalated Pt(II) complexes with triden-
ˆ ˆ
tate phenyl-substituted bipyridine (CNN) ligands [8–12] have
been found numerous applications in organic light-emitting
diodes (OLEDs) [13–16], optical limiting devices [17], DNA and
protein binding [18], sensor [19–21] and self-assembled nano-
structure [22]. However, to our best knowledge, using them as
electrochromic film materials has so far never been reported
yet. With the objective of obtaining a system having highly
anodic coloration, enhanced optoelectronic properties and facile
electropolymerization, we designed a novel multi-functionalized
cyclometalating ligand (HL) and its two neutral-charged Pt(II)
∗
Corresponding author. Tel.: +86 377 63513769; fax: +86 377 63513769.
E-mail address: hzshi2011@gmail.com (H. Shi).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.11.083

340
D. Qiu et al. / Electrochimica Acta 60 (2012) 339–346
and concentrated. The product mixture was purified by column
chromatography (silica gel, petroleum ether: CH2Cl2 = 2:1 (v/v)).
Compound 3 was obtained as a white needle crystal by recrystal-
lizing from MeOH/ethyl acetate solution: 9.5 g (yield: 78.5%). 1H
NMR (300 MHz, CDCl3): ı 8.10 (d, J = 7.8 Hz, 2H), 7.49–7.38 (m, 4H),
7.26–7.12 (m, 2H, Ar), 4.38 (t, J = 6.9 Hz, 2H), 3.38 (t, J = 6.4 Hz, 2H),
2.07 (m, 2H), 1.92 (m, 2H, –CH2–). 13C NMR (75 MHz, CDCl3): ı
140.2, 125.7, 122.9, 120.4, 118.9, 108.5 (Ar), 42.1, 33.1, 30.2, 27.6
(–CH2–). Anal. Calcd for C16H16BrN: C, 63.59; H, 5.34; N, 4.63.
Found: C, 63.38; H, 5.25; N, 4.43%.
2.2. Synthesis of 4-{p-[N-(4-(9-carbazole)butyl)-N-phenyl]
anilino}-6-phenyl-2,2ꢀ-bipyridine (HL)
4-(p-Bromophenyl)-6-phenyl-2,2ꢀ-bipyridine
(1)
(1.94 g,
5.0 mmol), palladium acetate (13.0 mg, 0.06 mmol) and DPEphos
(45.0 mg, 0.08 mmol) were charged into a flask and purged with
argon. Aniline (1.0 mL, 10.7 mmol) was added via syringe, followed
by toluene (20 mL). NaOBut (0.81 g, 8.5 mmol) was added in one
portion. The reaction mixture was heated to 80 ◦C under stirring for
12 h. The solvent was removed by rotary evaporation. The residue
was dissolved in dichloromethane, washed with distilled water,
and dried over anhydrous sodium sulfate and concentrated. The
crude product 2 was dissolved in THF (60 mL). NaH (2.5 g, >52%,
in mineral oil) was added and purged with argon. The reaction
mixture was stirred for 1 h at room temperature, and then 3 (6.04 g,
20.0 mmol) in THF (20 mL) was added dropwise via syringe. After
the solution was refluxed until the conversion was complete (as
monitored by thin layer chromatography), it was cooled to room
temperature and poured into ice-water mixture slowly, extracted
with CH2Cl2 until the water phase was colorless. The combined
organic solution was washed with distilled water, dried over
anhydrous sodium sulfate and concentrated. The crude product
was purified by column chromatography (silica gel, petroleum
ether:CH2Cl2 = 2:1 (v/v) containing 5% (v) triethylamine). Lig-
and HL was obtained as a pale yellow crystal by recrystallizing
from MeOH/ethyl acetate solution: 1.7 g (yield: 54.8%). 1H NMR
(300 MHz, CDCl3): ı 8.74–8.66 (m, 3H), 8.20 (d, J = 7.2 Hz, 2H),
8.10 (d, J = 7.8 Hz, 2H), 7.95 (s, 1H), 7.90 (t, J = 7.5 Hz, 1H), 7.69
(d, J = 8.7 Hz, 2H), 7.55–7.43 (m, 5H), 7.38–7.30 (m, 5H), 7.23 (t,
J = 6.9 Hz, 2H), 7.12–7.06 (m, 3H), 6.90 (d, J = 8.7 Hz, 2H, Ar), 4.33 (t,
J = 7.0 Hz, 2H), 3.75 (t, J = 7.3 Hz, 2H), 2.04–1.94 (m, 2H), 1.86–1.76
(m, 2H, –CH2–). Anal. Calcd for C44H36N4: C, 85.13; H, 5.85; N, 9.03.
Found: C, 84.92; H, 5.67; N, 8.62%.
Scheme 1. Synthetic route of HL, C1 and C2.
complexes (C1 and C2) (Scheme 1). Incorporating an electron-
donating diphenylamine (DPA) moiety at the 4-position of
ꢀ
ˆ ˆ
the electron-withdrawing 6-phenyl-2,2 -bipyridine (CNN) ligand
aimed to form a donor–acceptor (D–A) system with the appearance
of an intense n–␲* transition in UV region and a bathochromically
shifted and much increased 1MLCT transition of its corresponding
Pt(II) complex in visible region [23]. A carbazole (CBZ) unit was
introduced through alkyl chain to independently serve as an elec-
tropolymerizable site [24–27], which was expected to induce the
efficient cross-linkage reaction to form CBZ polymers with high
electrical conductivity and good optical quality.
2.3. Synthesis of [(L)PtCl] (C1)
A mixture of HL (0.32 g, 0.5 mmol), K2PtCl4 (0.21 g, 0.5 mmol)
and glacial acetic acid (30 mL) was refluxed for 24 h under a nitro-
gen atmosphere in the absence of light. The reaction mixture was
then cooled to room temperature and filtered. The obtained solid
was recrystallized from MeOH/CH2Cl2 solution twice to form the
desired product as a red crystal: 0.35 g (yield: 82.4%). 1H NMR
(300 MHz, CDCl3): ı 8.89 (d, J = 5.1 Hz, 1H), 8.68 (d, J = 8.1 Hz, 1H),
8.38 (s, 1H), 8.34 (t, J = 7.8 Hz, 1H), 8.14 (d, J = 7.8 Hz, 2H), 8.12 (d,
J = 5.4 Hz, 1H), 7.92 (d, J = 8.7 Hz, 2H), 7.88 (t, J = 6.6 Hz, 1H), 7.75 (d,
J = 7.5 Hz, 1H), 7.59 (d, J = 8.1 Hz, 1H), 7.50–7.36 (m, 5H), 7.21–7.04
(m, 8H), 6.85 (d, J = 8.7 Hz, 2H, Ar), 4.42 (t, J = 6.9 Hz, 2H), 3.80 (t,
J = 6.9 Hz, 2H), 1.88 (m, 2H), 1.72 (m, 2H, –CH2–). Anal. Calcd for
C44H35ClN4Pt: C, 62.15; H, 4.15; N, 6.59. Found: C, 61.67; H, 4.11;
N, 6.28%.
2. Experimental
9-Butyl-carbazole [28], 4-[p-(N-butyl-N-phenyl)anilino]-6-
phenyl-2,2ꢀ-bipyridine (HL2) [23] were synthesized according to
the literature methods for parallel studies.
2.1. Synthesis of 9-(4-bromobutane)carbazole (3)
To a mixture of KOH (11.2 g) and nBu4NBr in H2O (20 mL) was
added dropwise a solution of carbazole (6.68 g, 40.0 mmol) and 1,4-
dibromobutane (20 mL, 160 mmol) in THF (80 mL), and the mixture
was refluxed for 18 h. After the mixture was cooled to room temper-
ature, the product was extracted with dichloromethane, washed
with distilled water, and dried over anhydrous sodium sulfate
2.4. Synthesis of [(L)Pt(C CC6H5)] (C2)
A mixture of C1 (0.30 mmol), C6H5C CH (1 mmol), Et3N (5 mL)
and CuI (5 mg) in degassed CH2Cl2 (50 mL) was stirred for 12 h
under a nitrogen atmosphere at room temperature in the absence of

D. Qiu et al. / Electrochimica Acta 60 (2012) 339–346
341
Table 1
light. The reaction mixture was then evaporated to dryness under
Crystal data and crystal structure parameters.
reduced pressure. The crude product was purified by flash chro-
matography (neutral Al2O3, CH2Cl2 as eluent) and recrystallized
from CH2Cl2/MeOH solution. Complex C2 was obtained as a red
crystal in a yield of 87%. 1H NMR (300 MHz, DMSO-d6): ı 9.04 (d,
J = 4.8 Hz, 1H), 8.69 (d, J = 8.1 Hz, 1H), 8.44 (s, 1H), 8.35 (t, J = 7.8 Hz,
1H), 8.18 (s, 1H), 8.14 (d, J = 7.8 Hz, 2H), 7.93 (d, J = 8.7 Hz, 2H),
7.86–7.75 (m, 3H), 7.60 (d, J = 8.1 Hz, 2H), 7.47–7.38 (m, 6H), 7.28
(t, J = 7.8 Hz, 2H), 7.22–7.04 (m, 8H), 6.87 (d, J = 9.0 Hz, 2H, Ar), 4.42
(t, J = 6.9 Hz, 2H), 3.81 (t, J = 6.9 Hz, 2H), 1.88 (m, 2H), 1.72 (m, 2H,
–CH2).
Complex
C2
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C52H40N4Pt
915.96
293(2)
0.71073
Triclinic
P-1
9.994(6)
10.566(7)
23.245(13)
96.802(12)
96.313(11)
115.686(11)
2160(2)
2
b (Å)
c (Å)
˛ (◦)
ˇ (◦)
ꢀ (◦)
3. Results and discussion
Volume (Å3)
Z
3.1. Synthesis of monomers
Dc (mg m−3
)
1.408
3.286
916
Absorption coefficient (mm−1
F(0 0 0)
)
Scheme
1 illustrates the synthesis of ligand and com-
plexes explored in this work. 4-(p-bromophenyl)-6-phenyl-2,2ꢀ-
bipyridine (1) was obtained using Kröhnke’s method [29]. The
powerful palladium-catalyzed Buchwald method and the efficient
Pd(OAc)2/DPEphos catalyst/ligand system [30] were adopted to
facilitate the condensation of aniline with 4-(p-bromophenyl)-
6-phenyl-2,2ꢀ-bipyridine (1). Then the arylamino-substituted
intermediate (2) reacted with 9-(4-bromobutane)carbazole (3)
to afford ligand HL. The cyclometalated Pt(II) chloride [(L)PtCl]
(C1) was synthesized by refluxing the ligand HL and K2PtCl4
in glacial acetic acid for 12 h. The Pt(II) acetylide complex
[(L)Pt(C CC6H5)] (C2) was obtained by employing Sonogashira’s
conditions (CuI/Et3N/CH2Cl2) and chromatography separation. All
the compounds have good solubility in CH2Cl2 and were verified by
1H NMR, 13C NMR and/or elementary analysis. Structure of complex
C2 was revealed by X-ray crystallography.
Crystal size (mm)
0.15 × 0.13 × 0.08
ꢁ range for data collection (◦)
Limiting indices
2.51–25.00
−11 ≤ h ≤ 11, −10 ≤ k ≤ 12, −22 ≤ l ≤ 27
98.9%
7526/248/514
Completeness to ꢁ = 25.00◦
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2ꢂ(I)]
R indices (all data)
1.029
R1 = 0.0781, wR2 = 0.1765
R1 = 0.1249, wR2 = 0.1979
1.929 and −1.641
−3
˚
Largest diff. peak and hole (e A
)
ˆ ˆ
of the secondary ligand and the plane of the [(CNN)Pt] moiety is
71.5◦.
Instead of forming a continuous chain with Pt–Pt linkage in
ˆ ˆ
[(CNN)PtCl] [33,34], the complex molecules are stacked in a head-
˚
to-tail fashion (Fig. 1) with a Pt–Pt distance of 5.62 A, suggesting no
ˆ ˆ
metal–metal interaction. The neighboring [(CNN)Pt] moieties are
parallel with the least dihedral angle of 0.000◦, and their interpla-
˚
3.2. Crystal structure of complex C2
nar separation of 3.64 A indicates that they are likely to be held
together by a weak ␲–␲ interaction between the CNN ligands.
ˆ ˆ
The single crystal of [(L)Pt(C CC6H5)] (C2) was grown by slowly
evaporating the solvent from its concentrated CH2Cl2/MeOH solu-
tion, whose crystal data and crystal structure parameters are listed
in Table 1. Fig. 1 shows its perspective view and the dimensional
packing diagram. The coordinate geometry of the Pt atom is a
distorted square planar configuration with the C(1)–Pt(1)–N(1)
and N(1)–Pt(1)–N(2) angles of 78.7(3)◦ and 80.5(3)◦. The bond
distances of Pt–C(1), Pt–N(1) and Pt–N(2) are 2.097(10), 1.984
3.3. Photophysical properties of monomers
Table 2 summarizes the absorption maxima and extinction
coefficients of their absorption bands. The intense absorption
bands in the high-energy region are dominated by 1IL tran-
sitions, while the low-energy band at ꢃmax = 448 nm in the
absorption spectrum of C1 is assigned to the spin-allowed sin-
glet d␲(Pt) → ␲*(L) metal-to-ligand charge transfer (1MLCT)
transition, which is bathochromically shifted and more intense
when the Pt(II) halide (C1) changes to the Pt(II) acetylide (C2).
Owing to the strong D–A interaction between the electron-
˚
(7) and 2.016 (8) A, respectively, which are comparable to those
of previously reported analogous [31–34]. The configuration of
ˆ ˆ
CNN ligand is basically a plane with the C(1)–C(6)–C(7)–N(1) and
N(2)–C(12)–C(11)–N(1) torsion angles of 0.03◦ and 4.81◦. The dihe-
ˆ ˆ
ˆ ˆ
dral angle between the phenyl ring at 4-position of the CNN ligand
donating aniline moieties and the electron-deficient (CNN)
cyclometalating ligand [23], the molar extinction coefficients of
◦
ˆ ˆ
and the plane of the [(CNN)Pt] moiety is 23.22 . The geometry
of the N(3) atom is a distorted tetrahedral configuration with
the C(20)–N(3)–N(23), C(20)–N(3)–N(29) and C(23)–N(3)–N(29)
angles of 120.6(9)◦, 121.4(9)◦ and 115.9(8)◦. The peripheral CBZ
unit is approximately a plane with the C(37)–C(38)–C(39)–C(40)
torsion angle of 0.62◦. The dihedral angle between the phenyl ring
the MLCT bands of complexes C1 and C2 are 2–3 times larger
ˆ ˆ
than those of the reported (CNN) cyclometalated Pt(II) com-
plexes without the arylamine group [8–14]. Compared to those
of the reported complexes [(L2)PtCl] and [(L2)Pt(C CC6H5)]
(HL2 = 4-[p-(N-butyl-N-phenyl)anilino]-6-phenyl-2,2ꢀ-bipyridine)
Table 2
Photophysical properties of the ligand and Pt(II) complexes.
Compound
UV, in fluid solutiona, ꢃmax,abs (nm) (ε × 103 dm3 mol−1 cm−1
)
PL, ꢃmax,em(nm)b (˚c)
In fluid solutiona
In solid state
HL
C1
C2
346 (28.5), 294 (37.0), 262 (50.9)
448 (19.8), 376 (20.6), 328 (27.2), 294 (55.3), 264 (75.3)
462 (sh,20.5), 448 (20.9), 378 (22.2), 332 (26.2), 286 (58.3), 264 (62.4)
462
592(0.10)
598(0.24)
446
612
598, 623(sh)
a
Measured in degassed CH2Cl2 solutions at 298 K (concentration ∼ 1 × 10−5 mol dm−3).
Excited wavelength: 450 nm.
[Ru(bpy)3](PF6)2 in degassed acetonitrile at 298 K (Фr = 0.062) as reference.
b
c

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D. Qiu et al. / Electrochimica Acta 60 (2012) 339–346
Fig. 1. Perspective view (top) and crystal packing (bottom) of C2 with the numbering scheme adopted (hydrogen atoms have been omitted for clarity).
[23], the long-wavelength absorption spectra of C1 and C2 have no
apparent change, suggesting the introduction of CBZ unit through
alkyl chain can not influence on the energy level of the [(L2)Pt]
moiety.
The PL data of monomers are also listed in Table 2. Ligand HL
shows a blue emission band at 462 nm in CH2Cl2 solution, which
is blue-shifted to 446 nm in solid state due to the decrease of ther-
mal vibration in fluid solution. Excited at 450 nm, which is into
the Pt(II)-based MLCT absorption band, complex C1 and C2 dis-
play strong emissions at ꢃmax = 592 and 598 nm with relatively high
quantum yield of 0.10 and 0.24 in CH2Cl2 solution at 298 K, respec-
tively, originated from the spin-forbidden triplet excited states. In
solid state, red-shifted emission bands at ꢃmax = 612 and 623 nm
were observed for C1 and C2, respectively, which are likely arise
from the ␲–␲ interactions between neighboring [(CNN)Pt] moi-
eties.
ˆ ˆ
Fig. 2. First and second CV curves of HL (a), HL2 (b), 9-butyl-carbazole (c), C1 (d)
and C2 (e) on a Pt electrode.
Fig. 3. Repetitive cyclic voltammograms of HL (a), C1 (b) and C2 (c) on a Pt electrode.

D. Qiu et al. / Electrochimica Acta 60 (2012) 339–346
343
Fig. 5. Impedance spectra (−Zꢀꢀ vs. Zꢀ) measured on poly-HL (top), poly-C1 (middle)
and poly-C2 (bottom) modified Pt disk electrodes after potential sweeping cycles of
10, 20 and 30 in 0.1 mol L−1 Bu4NClO4/CH3CN solution containing 1.0 × 10−3 mol L−1
ferrocene as a redox probe.
Fig. 4. Cyclic voltammograms of poly-HL (top), poly-C1 (middle) and poly-C2 (bot-
tom) after 10 cycles on a Pt disk electrode in 0.1 mol L−1 Bu4NClO4/CH2Cl2 solution
at the potential scan rates of 20 mV s−1 (1), 50 mV s−1 (2), 100 mV s−1 (3), 150 mV s−1
(4), 200 mV s−1 (5) and 250 mV s−1 (6). (Inset: dependence of the anodic and cathodic
currents on the potential scan rate.)
redox behaviors of HL2 (Fig. 2(b)) and 9-butyl-carbazole (Fig. 2(c)),
we propose that the DPA moiety of HL is involved in the first oxi-
dation process and the CBZ group of HL is involved in the second
oxidation process. In the second anodic scan, a broad wave at more
negative potential clearly appeared, suggesting that new electroac-
tive species were generated after the first CV scan. As the cyclic
scan proceeded, intensity of anodic and cathodic currents gradu-
ally increased with the formation of a broad oxidation wave from
overlapping of the three anodic waves and apparently negative
shift of the cathodic wave (Fig. 3(a)), indicating the presence of a
3.4. Electrochemical polymerization (EP)
Two anodic waves at Ep = +1.13 and +1.34 V (vs. SCE) and a com-
bined cathodic wave at Ep = +0.65 V (vs. SCE) were observed in the
first cycling scan of HL (Fig. 2(a)). It is known that arylamine and
CBZ moieties can be oxidized to the respective mono radical cations
and that CBZ is oxidized at a potential higher than that of the struc-
turally related arylamine analogues [35]. And with reference to the

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D. Qiu et al. / Electrochimica Acta 60 (2012) 339–346
Table 3
Solution resistance (Rs), electron transfer resistance (Ret) and capacity (C) of the
various modified Pt electrodes.
Electrode name
Rs (ꢄ)
Ret (ꢄ)
Cdl (F cm−2
)
Poly-HL-10
Poly-HL-20
Poly-HL-30
Poly-C1-10
Poly-C1-20
Poly-C1-30
Poly-C2-10
Poly-C2-20
Poly-C2-30
178
120
178
160
170
300
160
130
30
4250
3550
2870
3180
2820
2310
1590
1450
1320
2.26 × 10−9
5.50 × 10−8
4.30 × 10−8
5.50 × 10−8
5.50 × 10−8
8.80 × 10−8
6.50 × 10−9
8.50 × 10−9
8.50 × 10−9
Fig. 6. The equivalent circuit proposed for polymer films modified ITO electrodes.
conductive polymer on the electrode. With the fact that HL2 can not
be electropolymerized under the same condition, these results indi-
cate that introduction of a CBZ unit in HL triggers the cross-linking
reaction following an E(CE)n-type mechanism [36], and that the
generated 3,6-polycarbazole moiety is more easily oxidized than
the parent compound [37]. However, because the CBZ radical cation
has coupling rate constants that are 4–5 orders of magnitude higher
than those of the arylamine analogues [35], we cannot exclude
the oxidative coupling of arylamine radical cations simultaneously
involving in the electropolymerization process.
Similarly, orange poly-1C and poly-2C films were obtained by
the oxidative electropolymerization of 1C and 2C, respectively.
Compared with that of HL, the CV spectra of 1C (Fig. 2(d)) and
2C (Fig. 2(e)) also show the Pt(II)-based anodic waves at +0.99 V
(vs. SCE). The single peak at +1.28 V in the first anodic scan of 1C
is attributed to the combination of the aniline-based wave and
the negative-shifted CBZ-based wave, while the smaller negative-
shifted potential of the CBZ-based wave was found and the
DPA-based wave was observed as a shoulder peak in the first anodic
scan of 2C. The oxidation wave of generated 3,6-polycarbazole moi-
ety at the range of 0.6–0.8 V [37] also appeared in the second scan
of 1C and 2C. Due to the overlapping effect of multi-redox waves,
the poor-defined CV spectra of 1C (Fig. 3(b)) and 2C (Fig. 3(c)) were
finally obtained after 10 cycling scans.
Fully washed with CH2Cl2 and ethanol to remove the unre-
acted monomer and oligomeric species, each of the poly-HL and
Fig. 8. Spectroelectrochemical spectra of poly-HL (top), ploy-C1 (middle) and poly-
C2 (bottom) films coated ITO.
Fig. 7. SEM images of poly-HL film modified ITO after potential sweeping cycles of 10 (left), 20 (middle) and 30 (right), respectively.

D. Qiu et al. / Electrochimica Acta 60 (2012) 339–346
345
Fig. 9. Dynamic changes of the transmittance and current upon switching the potential between 0 and +1.8 V with a pulse width of 200 s applied to the poly-HL (left) and
poly-C1 (right) films coated ITO in 0.1 mol L−1 Bu4NClO4/CH2Cl2 solution.
poly-1C films showed the linear increase of anodic and cathodic
currents with the scan rate in monomer-free electrolytic solution
(Fig. 4(a) and (b)) and a good stability upon over hundreds of poten-
tial sweeping cycle, indicating that polymer films adhere well to Pt
electrode. While poly-C2 film displayed an ill-linear dependence of
anodic and cathodic currents upon the scan rate (Fig. 4(c)).
Meanwhile, comparison of the Ret values of poly-C1/ITO and
poly-C2/ITO electrodes to poly-HL/ITO electrodes demonstrates a
clear drop in impedance, which means that the introduction of Pt(II)
complexes in organic poly-HL film improves the electron trans-
fer to the electrolyte by reducing the impedance of the electrode.
Such enhanced conductance is likely attributed to the conductiv-
ity of the metal centers, the reduced electron-localization effect of
Poly-HL, poly-C1 and poly-C2 films on Pt and ITO electrodes
with different coverage were obtained by controlling the potential
scan cycle, respectively. Fig. 5 shows the AC impedance spectra of
polymer films modified Pt electrodes after 10, 20 and 30 potential
sweeping cycles. The resulting circuit is shown in Fig. 6. Table 3
summarizes the values obtained for the typical elements of the
equivalent circuit, including the solution resistance (Rs), the elec-
tron transfer resistance (Ret) and the double layer capacitance (Cdl).
For each of polymer films, every spectrum was fitted by a semicircle
at high frequencies and the diameter of fitted semicircle decreased
with the scan number. Because the diameter of the semicircle
corresponds to the electron transfer resistance (Ret), the results
suggested that the more potential scan cycle, the easier charge
transfer from the electrolyte to the polymer/ITO electrode surface.
The inverse dependence of AC impedance on the potential scan
cycle is probably related to the morphology of polymer film at
different growth stage. In order to verify this proposal, poly-HL,
poly-C1 and poly-C2 films on ITO electrodes prepared after 10, 20
and 30 potential sweeping cycles were captured by SEM, respec-
tively. The same changes of the film morphology (Fig. 7), from the
discontinuous island (left) to the dense layer (middle) and then to
the island-on-layer morphology (right), were observed for each of
polymer films during its growth process. The continuous and dense
coverage likely benefits the electron transfer in the polymer film.
ˆ ˆ
the electron-deficient (CNN) cyclometalating ligand and the planar
ˆ ˆ
configuration of [(CNN)Pt] moieties in the obtained hybrid metal-
lopolymers.
3.5. Spectroelectrochemical characterization of the
electrogenerated films in solution
The spectroelectrochemical properties of poly-HL, poly-C1 and
poly-C2 films on ITO electrodes were performed at various applied
potentials, respectively (Fig. 8). In the neutral form, at 0 V, poly-
HL film exhibited a red-shifted 1IL transition at ꢃmax = 360 nm
and a new wave at ꢃmax = 468 nm, which indicate that the for-
mation of a large ␲-conjugated D–A system [38]. A broad wave
having its maximum absorption wavelength at 620 nm was gradu-
ally formed and blue-shifted with positive increase of the applied
potential (Fig. 8 (a)). This result is in agreement with that of the
oxidized biscarbazole within the range of 500–800 nm [39,40],
reconfirming that the dimerization of the CBZ moiety of HL is
involved in the electropolymerization process. We note, however,
that the oxidation of oligomeric arylamine analogues, such as
N,N,Nꢀ,Nꢀ-tetraphenyl-4,4ꢀ-diaminobiphenyl (TPB), produces radi-
cal dications that also exhibit a broad absorption band in this range
[41,42]. Thus, the contribution from the absorption of oxidized

346
D. Qiu et al. / Electrochimica Acta 60 (2012) 339–346
arylamine oligomer, generated from simultaneously coupling of
arylamine radical cations in the electropolymerization process,
cannot be excluded.
ZX2010012) and the Project supported by the Young Core Instruc-
tor from the Education Commission of Henan Province.
The UV–vis spectrum of poly-C1 (Fig. 8(b)) at 0.8 V shows an
enhanced 1IL transition in the high-energy region and a red-shifted
1MLCT wave at ꢃmax = 460 nm. As the applied potential became
more anodic, the 1MLCT wave blue-shifted with decrease of its
intensity, and a broad band at ꢃmax = 602 nm gradually increased
in intensity. The clear isosbestic point at 502 nm suggested that
the conversion between the neutral and oxidized states of poly-C1
film. Poly-C2 film showed similar changes of the UV–vis spectrum
with the presence of an isosbestic point at 507 nm, but no apparent
absorption peak at the low-energy region was observed (Fig. 8(c)).
All of the polymer films underwent a clear color change from orange
to bluish black after their full oxidation.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.electacta.2011.11.083.
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ˆ ˆ
into the electron-deficient (CNN) moiety can strengthen the
donor–acceptor (D–A) interaction. The electrochemical studies
suggested that the introduction of carbazole unit through alkyl
chain in ligand and complexes can trigger the efficient cross-linking
reaction to form polymer films, which were confirmed by the spec-
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and poly-C1 films with good stability and significantly anodic col-
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This work was financially supported by the Natural Science
Foundation of Henan Province (No: 102300410221), the Nat-
ural Science Foundation of Nanyang Normal University (No: