Near-infrared and multicolored electrochromism of solution processable triphenylamine-anthraquinone imide hybrid systems

Article abstract of DOI:10.1016/j.electacta.2013.03.067

A series of novel donor-acceptor molecules (TPA-AQI, TPA-AQI2 and TPA-AQI3) with various number of electron-accepting anthraquinone imide (AQI) arms connected to the electron-donating triphenylamine (TPA) core were synthesized and characterized by UV-vis absorption spectroscopy, cyclic voltammetry and spectroelectrochemical measurements. The geometries of these molecules in ground-state as well as the electronic absorption properties on the basis of the optimized geometries were investigated by theoretical calculations. For all the three molecules, there are two main absorptions in the range of 300-450 nm and 450-700 nm, respectively. The former corresponds to ππ^* transitions and the latter a nature of intramolecular charge transfer (ICT). Hypsochromic shift of the ICT absorption was observed from the single armed molecule TPA-AQI to the tribranched molecule TPA-AQI3, which may result from the increased steric hindrance between the donor and acceptor segments. Electrochemical studies revealed the ambipolar properties of these molecules and three redox couples, with two quasi-reversible redox couples appearing in the cathodic regime and one redox couple in the anodic regime, on CV curves. CV results also indicated the stabilization of the HOMO levels by increasing AQI arms. Spectroelectrochemical measurements demonstrated the near-infrared (NIR) and multicolor electrochromism of these molecules. When reduced to radical anions or oxidized to radical cations, intense NIR absorptions were observed accompanied with color changes of the solutions. As for TPA-AQI, there existed four different colors at different redox states: Indian-red at the neutral state, bluish green at the radical cationic state, olive green at the radical anionic state, and dark blue at the dianion state. The detailed transitions of the observed NIR absorptions were also discussed.

Full text of DOI:10.1016/j.electacta.2013.03.067

Electrochimica Acta 99 (2013) 211–218
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Near-infrared and multicolored electrochromism of solution
processable triphenylamine-anthraquinone imide hybrid systems
Fengkun Chen, Xiangyu Fu, Jie Zhang, Xinhua Wan∗
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and
Molecular Engineering, Peking University, Beijing 100871, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel donor-acceptor molecules (TPA-AQI, TPA-AQI2 and TPA-AQI3) with various number of
electron-accepting anthraquinone imide (AQI) arms connected to the electron-donating triphenylamine
(TPA) core were synthesized and characterized by UV–vis absorption spectroscopy, cyclic voltammetry
and spectroelectrochemical measurements. The geometries of these molecules in ground-state as well
as the electronic absorption properties on the basis of the optimized geometries were investigated by
Received 8 February 2013
Received in revised form 8 March 2013
Accepted 10 March 2013
Available online xxx
theoretical calculations. For all the three molecules, there are two main absorptions in the range of
Keywords:
Anthraquinone imide
Triphenylamine
Near-infrared
Multicolor electrochromism
Theoretical calculation
*
300–450 nm and 450–700 nm, respectively. The former corresponds to ␲
␲
transitions and the latter
a nature of intramolecular charge transfer (ICT). Hypsochromic shift of the ICT absorption was observed
from the single armed molecule TPA-AQI to the tribranched molecule TPA-AQI3, which may result
from the increased steric hindrance between the donor and acceptor segments. Electrochemical studies
revealed the ambipolar properties of these molecules and three redox couples, with two quasi-reversible
redox couples appearing in the cathodic regime and one redox couple in the anodic regime, on CV curves.
CV results also indicated the stabilization of the HOMO levels by increasing AQI arms. Spectroelectro-
chemical measurements demonstrated the near-infrared (NIR) and multicolor electrochromism of these
molecules. When reduced to radical anions or oxidized to radical cations, intense NIR absorptions were
observed accompanied with color changes of the solutions. As for TPA-AQI, there existed four different
colors at different redox states: Indian-red at the neutral state, bluish green at the radical cationic state,
olive green at the radical anionic state, and dark blue at the dianion state. The detailed transitions of the
observed NIR absorptions were also discussed.
© 2013 Elsevier Ltd. All rights reserved.
1. Introduction
region for their important military and civilian applications in fields
such as telecommunications, biomedicine and even thermal control
Electrochromism is well-known as the reversible color change
of a material upon electrochemical oxidation or reduction [1,2],
and electrochromic (EC) materials can be employed in numerous
practical applications such as optical memory devices [3–5], smart
windows [6–8], and color displays [9,10]. In spite of the fact that
the earliest reported EC materials are mainly inorganic oxides (i.e.,
WO3) [11], organic EC materials, such as bipyridiliums (viologens)
[12] and especially conducting polymers [13,14], demonstrate sev-
eral promising advantages over the former ones, such as large-scale
processability, fast response time, high coloration efficiency and
multiple colorations with the same material [15–21], as well as
fine-tuning of the colors through chemical structure modification
[22–25]. Recently, the interested optical changes of EC materials
have been extended into the near-infrared (NIR; e.g., 760–2500 nm)
for buildings or spacecrafts [26–29].
Up to now, several types of organic NIR electrochromic mate-
rials are developed, mainly including transition metal complexes
[30–32], aromatic quinones [33,34], and some conjugated poly-
mers [35–38]. On the other hand, however, the electrochromic
properties of these materials are mostly realized by electrochem-
ical oxidation. Only a limited number of materials have been
reported to achieve color changes relied on electrochemical reduc-
tion [33,34,39–41], due to the low environmental stability of the
reduced forms and lack of depth understanding of the structure-
property relationship for novel molecular design. We previously
explored the electrochromic properties of a series of electron-
deficient anthraquinone imide (AQI) and its derivatives, which
usually exhibited NIR absorptions in the range of 700–1100 nm
upon one-electron reduction [42–44]. It was demonstrated recently
that the NIR absorption of AQI radical anions could be feasibly elon-
gated to the range of 1100–1800 nm with a strong electron deficient
substitution [45].
∗
Corresponding author. Tel.: +86 10 62754187; fax: +86 10 62751708.
E-mail address: xhwan@pku.edu.cn (X. Wan).
0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2013.03.067

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F. Chen et al. / Electrochimica Acta 99 (2013) 211–218
Triphenylamine (TPA) derivatives and TPA-containing polymers
added dropwise. After addition, the solution was allowed to warm
up to room temperature for overnight reaction. After quenching
of the reaction with ammonium chloride solution, the water layer
was extracted with chloroform and the combined organic layer
was dried over MgSO4. After the remove of the solvent under
reduced pressure, the residue was purified by silica gel column
chromatography (petroleum ether/dichloromethane 1:1) to afford
the product as white solids.
are well-known electron-rich compounds with three dimensional
structures which are widely used as hole-transporting and pho-
tovoltaic materials [46–49]. In addition, triphenylamines can also
easily be oxidized to form TPA cationic radicals accompanying
with a noticeable change of coloration [50–53]. Liou et al. have
intensively explored EC materials based on TPA-containing poly-
imides which show interesting multichromism, good reversibility
and mechanical stability [54–57]. They also reported that the incor-
poration of electron-donating substituents such as methoxyl group
at the para-position of phenyl groups of TPA unit could not only
stabilize the TPA cationic radicals but also decrease the oxidation
potential [58,59]. In the meanwhile, additional interesting proper-
ties could be expected by combining the different electron-rich and
electron-deficient groups in one single molecule.
In this work, herein, we reported the synthesis and characteri-
zation of three star-shaped donor (D)-acceptor (A) molecules with
different electron-deficient AQI arms connected to the electron-
rich TPA core. The ambipolar character of these molecules was
revealed by electrochemical and spectroelectrochemical studies
and single layer EC device was fabricated. Upon oxidation to radi-
cal cations or reduction to radical anions, intense NIR absorptions
were observed. Multicolor could also be achieved in one single
material at different redox states, e.g., for TPA-AQI, Indian-red at
the neutral state, bluish green at the radical cationic state, olive
green at the radical anionic state, and dark blue at the dianion
state. Furthermore, detailed electronic transitions of the neutral
and radical absorptions were investigated by theoretical calcula-
tions.
2.2.2. 4-Methoxy-N-(4-methoxyphenyl)-N-[4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]benzenamine (2a)
Following general procedure A, 1a (1.67 g, 4.3 mmol), n-
BuLi (3 mL, 6.6 mmol) and 2-isopropoxy-4,4,5,5-tetramethyl-
[1,3,2]dioxaborolane (1.09 g, 5.9 mmol) were stirred in dry THF
(20 mL) overnight. Silica gel column chromatography afforded 2a
as white solids (0.62 g, 33%). 1H NMR (300 MHz, CDCl3): ı = 7.60 (br,
2H), 7.06 (br, 4H), 6.84 (d, J = 8.4 Hz, 6H), 3.80 (s, 6H), 1.32 ppm (s,
12H).
2.2.3. N-(4-methoxyphenyl)-4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-N-[4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl]benzenamine (2b)
Following general procedure A, 1b (2.17 g, 5.0 mmol), n-
BuLi (6 mL, 13.2 mmol) and 2-isopropoxy-4,4,5,5-tetramethyl-
[1,3,2]dioxaborolane (2.28 g, 12.2 mmol) were stirred in dry THF
(50 mL) overnight. Silica gel column chromatography afforded 2b
as white solids (1.60 g, 61%). 1H NMR (300 MHz, CDCl3); ı = 7.66 (d,
J = 8.1 Hz, 4H), 7.08 (d, J = 8.7 Hz, 2H), 7.03 (d, J = 8.4 Hz, 4H), 6.86 (d,
J = 8.7 Hz, 2H), 3.81 (s, 3H), 1.33 ppm (s, 24H).
2.2.4. Tris[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)phenyl]amine (2c)
2. Experimental
Following general procedure A, 1c (1.92 g, 4.0 mmol), n-
BuLi (6 mL, 13.2 mmol) and 2-isopropoxy-4,4,5,5-tetramethyl-
[1,3,2]dioxaborolane (2.46 g, 13.2 mmol) were stirred in dry THF
(50 mL) overnight. Silica gel column chromatography afforded 2c
as white solids (1.60 g, 64%). 1H NMR (300 MHz, CDCl3): ı = 7.69 (d,
J = 8.4 Hz, 6H), 7.09 (d, J = 8.4 Hz, 6H), 1.34 ppm (s, 36H).
2.1. Materials & methods
All reagents were obtained from J&K, Aldrich, Acros and
TCI Chemical Co., and used as received unless otherwise speci-
fied. The bromo substituted triphenyl amine derivatives, namely
p-bromo-N,N-dimethoxyphenylaniline (1a), 4-bromo-N,N-bis(4-
methoxyphenyl)aniline (1b) and tris(4-bromophenyl)amine (1c),
were prepared following the reported procedures [60–62].
Tetrakis(triphenylphosphine)palladium(0) ((PPh3)4Pd0) was syn-
thesized in our lab. 6-Bromo substituted anthraquinone imide 4
was prepared following the previously reported procedure with
slight modification of the N-alkyl chain [63].
2.2.5. General procedure B for synthesis of
triphenylamine-anthraquinone imide product
To a mixture of 6-bromo substituted anthraquinone imide 4,
Na2CO3 and Pd(PPh3)4, compound 2, toluene, ethanol and water
were added under nitrogen. The reaction mixture was heated
to reflux and kept for 6 h. After cooled to room temperature,
the water layer was extracted with dichloromethane and the
combined organic layer was dried over MgSO4. Volatile solvent
was removed under reduced pressure. Then, the raw product
was purified by silica gel column chromatography (petroleum
ether/dichloromethane 1:2) as solids.
1H NMR and 13C NMR spectra were measured on a Mercury
plus 300 (300 MHz) or Bruker ARX400 (400 MHz) spectrometer
at the ambient temperature with CDCl3 as the solvent. Chemical
shifts in 1H and 13C NMR were recorded in ppm with tetram-
ethylsilane (0 ppm) and CDCl3 (77 ppm) as standards, respectively.
High-resolution mass spectra were recorded on a Bruker APEX
IV Fourier transform ion cyclotron resonance mass spectrome-
ter. Elemental analyses were performed on an Elementar Vario
EL instrument. UV/Vis absorption spectra were recorded on a
Perkin–Elmer lambda 35 spectrophotometer. Diffuse reflectance
measurements on powders were carried out on a Shimadzu UV-
3100 UV/Vis/NIR spectrophotometer.
2.2.6. N-(1-hexylheptyl)-6-[4-[bis(4-
methoxyphenyl)amino]phenyl]-anthraquinone-2,3-dicarboxylic
imide (TPA-AQI)
Following general procedure B, compound 4 (0.59 g, 1.1 mmol),
Na2CO3 (0.40 g, 3.8 mmol), Pd(PPh3)4 (60 mg) and compound 2a
(0.57 g, 1.4 mmol) were refluxed in a mixture of benzene (15 mL),
ethanol (3 mL) and water (6 mL) under nitrogen. Silica gel column
chromatography afforded the product TPA-AQI (0.47 g, 56%) as red
solids. 1H NMR (400 MHz, CDCl3): ı = 8.77 (s, 2H), 8.53 (d, J = 2.0 Hz,
1H), 8.38 (d, J = 8.4 Hz, 1H), 8.04 (q, J1 = 8.4, J2 = 2.0 Hz, 1H), 7.59 (d,
J = 8.8 Hz, 2H), 7.14 (d, J = 8.8 Hz, 4H), 7.02 (d, J = 8.4 Hz, 2H), 6.90 (d,
J = 9.2 Hz, 4H), 4.32–4.23 (m, 1H), 3.83 (s, 6H), 2.14–2.05 (m, 2H),
1.78–1.70 (m, 2H), 1.32–1.23 (m, 16H), 0.86 ppm (t, J = 6.8 Hz, 6H);
13C NMR (100 MHz, CDCl3): ı = 182.0, 181.1, 167.0, 156.5, 150.0,
147.3, 140.0, 138.4, 138.1, 135.6, 135.4, 133.5, 131.8, 130.8, 128.6,
2.2. Synthesis
2.2.1. General procedure A for synthesis of triphenylamine
substituted boronic ester (Scheme 1)
4-Bromo substituted triphenylamine was dissolved in dry
THF under nitrogen atmosphere and it was cooled to −78 ◦C.
n-BuLi was added with syringe and kept at −78 ◦C for one hour.
2-Isopropoxy-4,4,5,5-tetramethyl-[1,3,2]dioxaborolane
was

F. Chen et al. / Electrochimica Acta 99 (2013) 211–218
213
127.9, 127.3, 124.7, 122.4, 119.6, 55.5, 53.2, 32.3, 31.7, 28.9, 26.7,
2.4. Computational details
22.5, 14.0 ppm; MS: m/z calcd for C49H51N2O6 [M+H]+: 763.3747;
found: 763.3749; elemental analysis calcd (%) for C49H50N2O6: C
77.14, H 6.61, N 3.67; found: C 77.41, H 6.89, N 3.41.
Molecular geometries were fully optimized using Kohn–Sham
Density Functional Theory (DFT) based on the Becke’s three param-
eter hybrid functional B3LYP [65] with the 6–31 G (d) basis set. Each
optimization was verified by the frequency analysis. The frontier
molecular orbital (MO) shapes and energy levels were calculated
using the optimized structures. Time-dependent density functional
theory (TD-DFT) was employed to stimulate the vertical transitions
of the first 10 states using B3LYP/6-31G (d) basis set. All the calcula-
tions were performed using the Gaussian 09 program package [66]
and the orbital pictures were prepared using Gaussview [67]. To
simplify the calculations, methyl group was employed to replace
the N-1-hexylheptyl groups.
2.2.7. 4-Methoxy-N,N-bis[4-[N-(1-hexylheptyl)-6-
anthraquinoneyl-2,3-dicarboxylic imide]-phenyl]benzenamine
(TPA-AQI2)
Following general procedure B, compound 4 (0.64 g, 1.2 mmol),
Na2CO3 (0.21 g, 2.0 mmol), Pd(PPh3)4 (60 mg) and compound 2b
(0.20 g, 0.55 mmol) were refluxed in a mixture of benzene (15 mL),
ethanol (3 mL) and water (6 mL) under nitrogen. Silica gel col-
umn chromatography afforded the product TPA-AQI2 (0.36 g, 55%)
as red solids. 1H NMR (400 MHz, CDCl3): ı = 8.78 (s, 4H), 8.57 (d,
J = 2.0 Hz, 2H), 8.41 (d, J = 8.0 Hz, 2H), 8.08 (q, J1 = 8.0, J2 = 2.0 Hz,
2H), 7.68 (d, J = 8.4 Hz, 4H), 7.26–7.18 (m, 6H), 6.96 (d, J = 9.2 Hz,
2H), 4.31–4.23 (m, 2H), 3.87 (s, 3H), 2.15–2.05 (m, 4H), 1.79–1.71
(m, 4H), 1.33–1.23 (m, 32H), 0.86 ppm (t, J = 6.4 Hz, 12H); 13C
NMR (100 MHz, CDCl3): ı = 181.9, 181.1, 167.0, 146.9, 138.3, 138.1,
135.6, 135.5, 133.5, 132.2, 131.1, 128.6, 128.1, 125.2, 123.1, 123.0,
122.5, 122.4, 115.2, 115.1, 55.5, 53.2, 32.3, 31.7, 28.9, 26.7, 22.5,
14.0 ppm; MS: m/z calcd for C77H80N3O9 [M+H]+: 1190.5895;
found: 1190.5939; elemental analysis calcd (%) for C77H79N3O9:
C 77.69, H 6.69, N 3.53; found: C 77.41, H 6.65, N 3.34.
3. Results and discussion
3.1. Synthesis
The synthetic route of the three star-shaped D-A molecules
is illustrated in Scheme 1. The bromine substituted tripheny-
lamine 1 were transferred to corresponding boronic ester 2
firstly with n-BuLi and then 2-isopropoxy-4,4,5,5-tetramethyl-
[1,3,2]dioxaborolane. Classic palladium-catalyzed Suzuki-coupling
reactions between corresponding boronic ester 2 and 6-bromo
substituted anthraquinone imide 4 afforded the three target star-
shaped molecules (namely, TPA-AQI, TPA-AQI2 and TPA-AQI3).
The long hexylheptyl group as well as the propeller-like structures
of the molecules ensure the solubility of these molecules in com-
mon organic solvents such as dichloromethane, chloroform and
toluene. 4-Methoxyl group was incorporated at the para-position
of phenyl groups of TPA unit in TPA-AQI and TPA-AQI2 to reduce
the oxidation potential and also stabilize the corresponding radical
cations as mentioned above.
The chemical structures of these molecules were fully confirmed
by 1H NMR, high resolution mass spectrometry and elemental anal-
ysis. As revealed in the 1H NMR spectra (Fig. S1), chemical shifts of
the aromatic protons in TPA segment were moved gradually toward
the low field with increasing AQI arms connected to it owing to
the electron-withdrawing nature of AQI group, while the chemi-
cal shifts of the aromatic protons in AQI segment were much less
influenced.
2.2.8. Tri[4-[N-(1-hexylheptyl)-6-anthraquinoneyl-
2,3-dicarboxylic imide]-phenyl]amine (TPA-AQI3)
Following general procedure B, compound 4 (0.85 g, 1.6 mmol),
Na2CO3 (0.85 g, 8.0 mmol), Pd(PPh3)4 (100 mg) and compound
2c (0.30 g, 0.48 mmol) were refluxed in a mixture of benzene
(15 mL), ethanol (3 mL) and water (6 mL) under nitrogen. Sil-
ica gel column chromatography afforded the product TPA-AQI3
(0.48 g, 62%) as black solids. 1H NMR (400 MHz, CDCl3): ı = 8.79
(d, J = 1.2 Hz, 6H), 8.60 (d, J = 1.2 Hz, 3H), 8.44 (d, J = 8.4 Hz, 3H),
8.12 (q, J1 = 8.4 Hz, J2 = 1.6 Hz, 3H), 7.76 (d, J = 8.4 Hz, 6H), 7.38 (d,
J = 8.4 Hz, 6H), 4.32–4.24 (m, 3H), 2.15–2.06 (m, 6H), 1.78–1.72 (m,
6H), 1.29–1.24 (m, 48H), 0.86 ppm (t, J = 6.8 Hz, 18H); 13C NMR
(100 MHz, CDCl3): ı = 181.8, 181.2, 167.0, 147.8, 146.7, 138.2, 138.0,
135.7, 135.5, 133.5, 132.4, 131.4, 128.7, 128.6, 125.4, 124.9, 124.8,
122.5, 111.5, 53.2, 32.3, 31.7, 28.9, 26.7, 22.5, 14.0 ppm; MS: m/z
calcd for C105H109N4O12 [M+H]+: 1617.8042; found: 1617.8084;
elemental analysis calcd (%) for C105H108N4O12: C 77.94, H 6.73,
N 3.46; found: C 78.08, H 6.54, N 3.29.
3.2. Photophysical properties
Absorption spectra of compounds TPA-AQI, TPA-AQI2 and TPA-
AQI3 in dilute CH2Cl2 solutions are shown in Fig. 1. There are
two groups of absorption bands between 300 and 700 nm for the
molecules solution. The intense and weakly structured absorption
2.3. Electrochemical characterizations
*
Cyclic voltammetry was carried out on a CHI 840B electrochem-
ical workstation. The solutions were made in CH2Cl2 containing
0.1 M tetra-n-butylammonium perchlorate (TBAP) as supporting
electrolyte and were degassed with nitrogen prior to measurement.
Glassy carbon was employed as the working electrode, platinum
wire as the counter electrode and Ag/AgCl as the reference elec-
trode. The pseudo-reference was calibrated externally using a
2–5 mM solution of ferrocene (Fc/Fc+). The energy level of Fc/Fc+
is assumed to be −4.8 eV below the vacuum level [64] and the
LUMO levels of TPA-AQI, TPA-AQI2 and TPA-AQI3 were estimated
from the half-wave potentials of the reduction peaks. Spectroelec-
trochemical measurements were carried out on a Hitachi U-4100
spectrophotometer connected to a computer in an optical transpar-
ent thin layer (OTTLE) cell. The solutions were as the same as for
CV, and the redox potentials were chosen according to the results
of CV.
bands in the range of 300–450 nm correspond to the ␲–␲ tran-
sitions. And, the visible absorption bands can be assigned to an
intramolecular charge transfer (ICT) from the donor (TPA) to the
acceptor (AQI) [68].
Comparison of the spectra of TPA-AQI, TPA-AQI2 and TPA-AQI3
shows that the progressive incorporation of AQI arm onto TPA core
induces a concomitant increase of the intensity of the ␲–␲* transi-
tion and ICT band. Another noteworthy is the hypsochromic shift of
ICT bands. As for TPA-AQI3, the ICT based absorption band occurs at
about 504 nm, while that of TPA-AQI and TPA-AQI2 at 542 nm and
522 nm, respectively. This phenomenon could be ascribed to the
strong molecular polarity of TPA-AQI and TPA-AQI2 which favors
the ICT electron delocalization [69], and also the increased steric
hindrance in TPA-AQI3 as indicated in the optimized geometries
obtained by density-functional theory (DFT) at the B3LYP/6-31 G
(d) level (Fig. 2). With the increasing number of AQI arms, dihedral

214
F. Chen et al. / Electrochimica Acta 99 (2013) 211–218
Scheme 1. Synthetic route of star-shaped molecules TPA-AQI, TPA-AQI2 and TPA-AQI3.
angles between the donor and acceptor segments are enlarged,
which reduces the conjugation degree and thereby increases the
HOMO (highest occupied molecular orbital)-LUMO (lowest unoc-
cupied molecular orbital) gap as revealed in Fig. 2.
transitions well explains the linearly increasing intensity of the
charge-transfer bands from TPA-AQI to TPA-AQI3.
3.3. Electrochemical properties
The intrinsic characters of these absorptions are also revealed
by the TD-DFT calculations. The calculated absorptions and cor-
responding transitions are illustrated in Fig. S2 and Table S1,
respectively. It is calculated that the lowest-energy absorptions
have a nature of charge transfer. For the single branched molecule
TPA-AQI, the S0 → S1 excitation is pure HOMO → LUMO transition,
with the HOMO and LUMO orbitals concentrated on the donor
and acceptor moieties, respectively (Fig. 2). With an increase of
AQI arms as in TPA-AQI2 and TPA-AQI3, a second CT excitation
(S0 → S2) is present in the CT band. This S0 → S2 excitation is due to
the HOMO → LUMO + 1 transition. As shown in Fig. 2, it is meaning-
ful to point out that the LUMO and LUMO + 1 orbitals of TPA-AQI2
are the in-phase and out-of-phase combinations of the local LUMO
orbital of each AQI arm [70]. The superposition of these two CT
Cyclic voltammetry (CV) is a preliminary measurement to deter-
mine the redox properties of organic and polymeric materials. The
redox properties of these three molecules were investigated by CV
measurement in CH2Cl2 solutions with tetra-n-butylammonium
perchlorate (TBAP) as supporting electrolyte. As shown in Fig. 3,
the CV curves of all compounds are similar and there are three
reduction/oxidation processes. In the cathodic regime, there are
two quasi-reversible characteristic reduction peaks of AQI moi-
ety, corresponding to the formation of radical anions and dianions,
respectively. And in the anodic side, the oxidation process is related
to the oxidation of the TPA group to form radical cations.
As also exhibited in the CV curves, these compounds show
almost the same reduction potentials, indicating the negligible
influence of TPA group on AQI moiety, which is consistent with
the chemical shifts of the aromatic protons in TPA segment. How-
ever, the cathodic current increases linearly with AQI arms, which
is understandable considering the simultaneous electrochemical
process in different branches. In contrast, the oxidation peaks in
the positive regime are dramatically influenced and shift toward
more positive region with the increase of AQI arms, indicating the
progressive stabilization of the HOMO level due to the electron-
withdrawing characteristic of AQI group.
The redox potentials (vs Ag/AgCl) of these compounds as well as
the unsubstituted AQI are summarized in Table 1. As demonstrated
in Table 1, all these compounds share almost the same reduction
potential, while the oxidation potentials shift to more and more
positive. The frontier MO energies and band-gaps calculated from
the electrochemical results are also listed in Table 1. The HOMO
level of TPA-AQI3 is largely stabilized as compared to TPA-AQI. The
band-gap of each these molecules is in the range of 1.43–1.75 eV,
which agrees with the broad absorption of these compounds and
is beneficial for photovoltaic applications [71]. These experimental
results show considerable consistency with the theoretical ones
as illustrated in Fig. 2. And the differences between experimental
and theoretical values are a well-known consequence of the lack of
medium-related factors in calculations.
Fig. 1. UV–vis spectra of the three compounds in dilute CH2Cl2.

F. Chen et al. / Electrochimica Acta 99 (2013) 211–218
215
Fig. 2. Optimized geometries and frontier MOs with corresponding energy levels of the compounds obtained at the B3LYP/6-31 G (d) level.
3.4. Spectroelectrochemical characterization
TPA-AQI
80
60
TPA-AQI2
Spectroelectrochemical measurements are carried out using the
OTTLE cell to investigate the electrochromic properties and the
typical electrochromic behaviors of TPA-AQI are demonstrated
in Fig. 4. In the neutral state, besides the absorption in the UV
region, there is another structureless ICT based absorption cen-
tered at 542 nm. When applied a positive potential of 0.8 V, this ICT
absorption disappeared gradually and three new peaks in the visi-
ble region centered at 500 nm, 626 nm and 764 nm, corresponding
to the formation of TPA radical cations [72], appeared. Meanwhile,
the solution color changes from Indian-red to bluish green due
the disruption of the ICT absorption (Fig. 4a). When a low nega-
tive potential was applied, the radical cations were reduced to the
neutral state and the ICT absorption re-appeared with the solution
color returning to Indian-red (Fig. 4b). A similar effect is observed
during reduction of TPA-AQI to its radical anions when the applied
negative potential was increased: the ICT absorption disappeared
again and a broad NIR absorption ranging from 700 to 1100 nm
turned up together with the color changing to olive green (Fig. 4c).
TPA-AQI3
40
+
Fc/Fc
20
0
-20
-40
-60
1.0
0.5
0.0
-0.5
Potential /V
-1.0
-1.5
Fig. 3. Cyclic voltammograms of the compounds (1.0 mM) recorded in a CH2Cl2
solution of Bu4NClO4 (0.10 M) at a scanning rate of 100 mV/s (vs Ag/AgCl).
Table 1
Redox potentialsa and related MO energies of the compounds.
Comp.
Eox/V
Ered1/V
Ered2/V
EHOMO/eV b
ELUMO/eV b
Eg/eV c
TPA-AQI
TPA-AQI2
TPA-AQI3
AQI d
0.53
0.69
0.89
–
−0.90
−0.87
−0.86
−0.86
−1.38
−1.37
−1.36
−1.34
−5.10
−5.26
−5.46
–
−3.67
−3.70
−3.71
−3.71
1.43
1.56
1.75
–
a
c = 1 × 10−3 mol/L in CH2Cl2/TBAP (0.1 M) vs Ag/AgCl at 100 mV/s.
1
b
c
LUMO (eV) = − (Ered − EFc/Fc+ + 4.8) (eV), HOMO (eV) = − (Eox − EFc/Fc+ + 4.8) (eV).
Eg = LUMO-HOMO.
d
CV results for unsubstituted AQI.

216
F. Chen et al. / Electrochimica Acta 99 (2013) 211–218
Fig. 4. UV–vis–NIR spectra of TPA-AQI (1.0 mM containing 0.1 M Bu4NClO4) at different redox states in CH2Cl2 (inset: photos at different states).
The assignment of this NIR absorption was indicated by the TD-DFT
calculation results (Table 2).
absorption around 800 nm appeared while a broad NIR absorption
appeared when the potential changed to a negative one, which is
the same as that in the solution. The optical switching properties
at 800 nm and 900 nm were also measured and an optical contrast
of 23% was achieved at 800 nm (Fig. S6).
As shown in Table 2, the first two lowest-energy transitions both
have a combined contribution from the SOMO → LUMO transition
and SOMO → LUMO + 2 transition. Due to the smaller calculated
oscillator strength, the calculated first lowest energy absorption at
953 nm was observed as a side-band in the experimental spectra as
indicated Fig. 4c. The observed NIR absorption of TPA-AQI radical
cations was simulated to result from transitions from the inter-
nal orbitals to LUMO (HOMO-1 → LUMO and HOMO-3 → LUMO,
Table 2). And the calculated NIR absorptions are presented in Fig.
S3. The NIR absorption of the AQI radical anions could be trans-
ferred to the absorption of dianions by application of even higher
negative potentials, which showed intensive absorption from 450
to 700 nm, corresponding to the generated dark-blue color (Fig. 4d).
These results show that the ICT interactions can be modulated
by both the oxidation of the TPA core as well as the reduction of
the AQI arms, and multicolor switching also can be achieved elec-
trochemically. The other two molecules demonstrate the similar
electrochromic behaviors (Fig. S4). Due to the ambipolar charac-
teristics of these compounds, an all-solid state EC device based
on molecule TPA-AQI was fabricated as a typical example (Fig.
S5). When a positive potential was applied to the device, a new
4. Conclusion
In summary, three D-A molecules with different AQI arms con-
nected to TPA core were synthesized and characterized. Their
optical, electrochemical, spectroelectrochemical properties were
studied and discussed in terms of correlation with their chemical
structures. Due to the propeller-like structures, these compounds
show good solubility in common organic solvents. In the absorption
spectra of dilute CH2Cl2 solutions, there are two main absorp-
tion bands with the lower energy absorption corresponding to the
intramolecular charge transfer from the electron-donating TPA unit
to the electron-accepting AQI unit and the higher energy absorption
*
assigned to the ␲–␲ transition. With increasing the AQI arms, an
enhancement of the absorption intensity and hypsochromic shift of
ICT band were observed, which may result from the increased steric
hindrance. The experimental optical properties agree well with
the theoretical calculated neutral absorptions. All these molecules
show broad absorptions and narrow band-gaps, which may find
applications in optoelectronic fields. CV results reveal that all these
compounds exhibit three redox peaks, two in the cathodic regime
which corresponds to the formation of AQI radical anions and
dianions, respectively, one in the positive regime corresponding
to the TPA radical cations. Spectroelectrochemical measurements
demonstrate that the ICT based absorption could be tuned electro-
chemically and multi-color electrochromism can be achieved in one
single molecule. When oxidized to radical cations or reduced to rad-
ical anions, intensive NIR absorptions were measured. According
to the simulation results, the origin of NIR absorption for TPA-AQI
radical cations has a nature of transitions from internal orbitals
(HOMO-1 and HOMO-3) to LUMO, while from SOMO to LUMO and
LUMO + 2 for radical anions.
Table 2
Calculated excitation energy (ꢀE), oscillator strength (f) and dominant transitions
of the first 10 main predicted transitions of TPA-AQI radical cations and anions (H:
HOMO; L: LUMO; S: SOMO, single occupied molecular orbital).*
Sn
ꢀE(eV, nm)
f
Dominant transitions (%)
Radical cations
S2
S3
S4
S5
1.56, 795
0.136
0.112
0.081
0.231
H-1->L ␤ (82%), H-3->L ␤ (15%)
H-2->L ␤ (57%), H-3->L ␤ (28%)
H-3->L ␤ (52%), H-2->L ␤ (38%)
H-6->L ␤ (97%)
1.66, 746
1.68, 738
1.79, 693
Radical anions
S1
S2
1.30, 953
1.50, 824
0.127
0.186
S->L ␣ (73%), S->L + 2 ␣ (18%)
S->L + 2 ␣ (63%), H->L ␣ (25%)
*
Only those transitions with f > 0.01 are listed.

F. Chen et al. / Electrochimica Acta 99 (2013) 211–218
217
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Appendix A. Supplementary data
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