Schiff bases containing triphenylamine and pyrrole units: Synthesis and electrochromic, acidochromic properties

Article abstract of DOI:10.1039/c6nj00321d

Five Schiff bases (SBs) containing pyrrole and different triphenylamine (TPA) moieties were synthesized and the structures were confirmed by infrared spectroscopy (IR) and the nuclear magnetic resonance (NMR) technique. SB1 exhibited intense fluorescence emission in contrast to SB2-SB5. The electrochemical behaviors of the SBs were investigated by the cyclic voltammetry (CV) technique. SB2-SB5 exhibited redox stability with one well-defined and reversible redox couple upon electrochemical oxidation. On the contrary, SB1 revealed two oxidation peaks and one reduction peak in the first CV curve, followed by four well-defined redox couples in the steady-state CV curves. Electrochromic behaviors of the SBs have been observed, accompanied with a new absorption peak appearing with obvious color change from pale yellow neutral form to red oxidation form. SB1-SB5 also displayed sensitive acidochromic behaviors, and a linear dependence of maximum absorption wavelength as a function of pH values has been observed which proved the SBs could be used as alternative pH sensors in the given range.

Full text of DOI:10.1039/c6nj00321d

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Schiffꢀbases Containing Triphenylamine and Pyrrole Units: Synthesis
and Electrochromic, Acidochromic Properties
Yu Lia, Yanhong Zhang a,*, Haijun Niu a,*, Cheng Wang a,*, Chuanli Qina, Xuduo Baia,
Wen Wangb
aKey Laboratory of Functional Inorganic Material Chemistry, Ministry of Education;
Department of Macromolecular Science and Engineering, School of Chemistry and
Chemical Engineering, Heilongjiang University, Harbin 150086, P R China
bSchool of Material Science and Engineering, Harbin Institute of Technology, Harbin
150080, P R China
Five Schiff bases (SBs) containing pyrrole and different triphenylamine (TPA) moieties were
synthesized and the structures were confirmed by Infrared spectroscopy (IR) and nuclear
magnetic resonant (NMR) technique. SB1 exhibited intense fluorescence emission in
contrast to SB2~SB5. The electrochemical behaviors of SBs were investigated by cyclic
voltammetry (CV) techniques. SB2~SB5 exhibited redoxꢀstability with one wellꢀdefined and
reversible redox couple upon electrochemical oxidation. On the contrary, SB1 revealed two
1

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oxidation peaks and one reduction peak in the first CV curve, followed by four wellꢀdefined
redox couples in the steadyꢀstate CV curves. Electrochromic behaviors of SBs have been
observed, accompanied with a new absorption peak appearing with obvious color changing
from pale yellow neutral form to red oxidation form. SB1~SB5 also displayed sensitive
acidochromic behaviors, and the linear dependence of maximum absorbance wavelength as a
function of pH values has been achieved which proved the SBs could be used as an
alternative pH sensor in the given range.
1. Introduction
SBs namely azomethines (ꢀC=Nꢀ), can be prepared easily by a simple condensation reaction
between an amine and aldehyde which are isoelectronic to their vinyl analogue and possess
similar optoelectronic properties.1,2 SBs have received a great deal of attention as conjugated
materials in organic electronics, such as organic fieldꢀeffect transistors (OFET),
electrochromic devices (ECD), organic photovoltaics (OPV),3,4 organic layer electronic
diode (OLED), chemical sensor, and optoelectronics and photonics.5 Su has reported two
chemical sensors based on polymethyl methacrylate films doped by SBs due to the reversible
and stable acidochromic properties, which could detect HCl gas in environment.6 Michiel L.
Petrus, used a smallꢀmolecule SB as an electron donor in a standard bulk heterojunction
solar cell configuration.7 It demonstrated that the smallꢀmolecule SBs were promising ideal
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candidates for the nextꢀgeneration lowꢀcost organic photovoltaic devices.
Pyrrole has widely application in bioꢀchemical filed, especially in derived drugs,8, 9
which provides not only wonderful biocompatibility, but also endows electric conductivity,
promising application in biological detection. Pyrrole has also been used as monomer to be
polymerized in opticꢀelectronic field, such as pseudocapacitive electrode material, solar cell
electrode material.
TPA derivatives and TPAꢀbased polymers are well known as their optical and electro
active properties, which have potentials applications for organic optoelectronic,
holeꢀtransporters, and memory devices.10 TPA can be easily oxidized to form stable radical
cation because of electronꢀdonating properties, and the oxidation process is always related to
a noticeable color change. Liou groups, have investigated TPAꢀbased Polybenzoxazines
(PBs) with good thermal properties and interesting multiꢀcolored behavior with highly
optical contrast,11 and introduced triarylamineꢀbased polybenzoxazines and
poly(triphenylamine)s as memory devices.10,12 They found that polymers based on TPA as
core exhibited stable, excellent, multiꢀcolor, and electrochromic characteristic.13ꢀ16 So
introducing TPA segment into the target compounds would greatly improve sensitivity and
opticalꢀelectro characteristic of target compounds.
Acidochromic property, that is, acidꢀinduced color modulation, which is importance in
3

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the design of pH sensors. For example, 1,4ꢀbis(paraꢀhydroxystyryl)benzene (BHSB) film can
undergo a clear color change in response to changes in the pH value, which could be applied
for biomimetic devices.17 There are many Schiff bases exhibiting acidochromic properties.
These structure provides extraordinary reversibility and the stability with changing pH
values, or even when changing the polarity and polarizability of the media.18ꢀ20 The
fabrication of a selfꢀindicating, photoresponsive and reusable acid vapor probe based on
spiropyran molecules with acidochromic behavior has been reported, and the acid gas
sensing platforms can be utilized on interfaces integrated to biological systems and
labꢀonꢀaꢀchip technologies as well as for monitoring in pHꢀsensitive environments.21 There
are also many other materials possess acidochromic behavior have been reported, such as
benzoxazole derivatives.22 Also, Schiff bases with π electrons in the C=N moiety and
nitrogen in the aromatic ring provided a good possibility for chelation with metal ions. So
these can be developed as a probe for the detection of metal ions.23
Recently, our group has reported the synthesis and acidochromic, electrochromic
properties of SBs bearing TPAꢀthiophene and TPAꢀfuran units through condensation of
different aldehydes and amines.24,25 Some researchers have investigated SBs containing the
carbazole and pyrrole. But few reported on SBs containing different TPA derivatives and
pyrrole and discussed the relationship between structure and properties of SBs. How the
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effect of C=N on synthesis and electrochemical properties is different from CꢀC, C=C bonds?
Based on above consideration we designed and synthesized a series of SBs containing
TPAꢀpyrrole segments based on TPA derivatives and pyrroleꢀ2ꢀaldehyde by condensation,
(Scheme 1). The structures of the compounds were confirmed by Fourier transform infrared
(FTꢀIR), hydrogen nuclear magnetic resonance (1HꢀNMR), carbon nuclear magnetic
resonance (13CꢀNMR) spectroscopies, and high resolution mass spectrometer (HRꢀMS), and
the electrochromic and acidochromic properties were studied by UVꢀvisible, fluorescence
spectroscopies, CV technique. The energy levels were also calculated. The results indicated
that the synthesized compounds have potential application in various fields, including H+
detection chemical sensor, hole transport materials and smart windows.26
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Scheme 1. Synthesis routes, A1~A4 and SB1~SB4.
2. Results and Discussion
2.1. Structural characterization
1
Structural features of these SBs were characterized by HꢀNMR, 13CꢀNMR, aromatic
1
region of the HꢀH COSY, IR and HRꢀMS techniques. HꢀNMR, 13CꢀNMR, aromatic region
of the HꢀH COSY spectra were recorded using a Bruker spectrometer at 400 MHz and 100
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MHz, using TMS as an internal standard, and DMSOꢀd6 as solvent. FTꢀIR spectra were
investigated on a PerkinꢀElmer Spectrum 100 Model FTꢀIR spectrometer. HRꢀMS were
investigated on a Bruker Microflex MALDIꢀTOF 337 nm no matrix. In 1HꢀNMR spectra of
SB5 (Fig. 1(a)), the characteristic peak appearing at 8.40 was assigned to the characteristic
groups of ꢀCH=Nꢀ, 11.6 to NꢀH vibration of pyrrole ring, 7.50ꢀ 6.00 to the aromatic ring,
3.89 and 1.33 to CꢀH of ꢀOCH2CH3. The assignments of aromatic region proton signals were
also assisted by the 2D NMR spectra. The spectra was in good agreement with the proposed
molecular structures of compound SB5, see Fig. 1(c). In 13CꢀNMR spectrum (Fig. 1(b)),
peak at 156 was attributed to C=N. There were 12 peaks between 110 and 150 which were
assigned to aromatic ring. The peaks at 64 and 15 were due to ꢀOCH2CH3. Carbon species
are accurate according to with standard calculation.
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Fig. 1. (a) 1HꢀNMR, (b) 13CꢀNMR and (c) aromatic region of the HꢀH COSY spectrum
of SB5 in DMSOꢀd6.
Representative IR spectra for SBs were presented in the Fig. 2. Characteristic ꢀC=Nꢀ
group absorptions appeared around at 1618 cmꢀ1. The peaks from 3300 cmꢀ1 to 3600 cmꢀ1
were assigned to NꢀH stretching vibration of pyrrole ring, 2985 and 2850 cmꢀ1 to CꢀH
stretching vibration of –OCH2CH3, 1500 and 1471 cmꢀ1 to ꢀC=Cꢀ groups of aromatic ring,
The band at around 1237 cmꢀ1 was due to CꢀOꢀC stretching vibration of ꢀOCH2CH3. HRꢀMS:
m/z = calcd. SB1 [C23H19N3] 337.158, found 337.155. calcd. SB2 [C28H23N5] 429.195, found
429.200. calcd. SB3 [C33H27N7] 521.233, found 521.230. calcd. SB4 [C29H25N5] 443.211,
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1
found 443.214. calcd. SB5 [C30H27N5O] 473.222, found 473.225. HꢀNMR, 13CꢀNMR and
HꢀH COSY spectra of SB1~SB4 were illustrated in Fig. S1. According to IR, NMR and
HRꢀMS analysis, we can identify the structures of SBs.
Fig. 2. FTꢀIR spectra of SBs.
2.2. Strong absorption properties of SBs
UVꢀvis spectra were obtained on a UVꢀ3600 (Shimadzu), and were recorded in DMSO
solution at room temperature. UVꢀvis absorbance curves of the five compounds are presented
in Fig. 3, it can be observed that SB1~SB5 all exhibit two absorption peaks. The absorption
peaks at 310~330 nm and 390~400 nm are characteristic for the πꢀπ* transition of the
aromatic ring and charge transfer complex formation of compounds, respectively.15, 27 It is
obvious that the maximal absorption wavelengths of molecules are listed in the order: SB3>
SB5> SB4> SB2> SB1. SB3 needs the lowest energy to electron transition, which assigned
to increasing the conjugated length by incorporating three pyrroleꢀ2ꢀimine groups, and
makes the πꢀelectron is easy to flow. While, SB2, SB4 and SB5 are incorporated two
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pyrroleꢀ2ꢀimine moiety, and the same position are replaced by hydrogen, methyl and oxethyl
groups, respectively. Oxethyl and methyl groups are donor groups, which could make the
electron clouds density of the whole molecule distribute more uniform and easier to flow. So
the absorption of SB4, SB5 is larger than SB2. It revealed a red shift of SB5 and SB4. The
oxethyl is stronger than methyl in donating electrons. Therefore, SB5 is larger than SB4.
SB1 only owns single pyrroleꢀ2ꢀimine, thus it needs higher energy to produce electron
transition. This result is coordinated with the quantum chemical calculation. Furthermore,
the molar extinction coefficients (ε) at 400 nm have been calculated using the BeerꢀLambert
4
×
law, which can reflect the capacity of absorbing. The results are as following: εSB1=0.54 10
Mꢀ1cmꢀ1, εSB2=1.4 104 Mꢀ1cmꢀ1, εSB3=1.2 104 Mꢀ1cmꢀ1, εSB4=1.6 104 Mꢀ1cmꢀ1,
×
×
×
εSB5=1.8× 104 Mꢀ1cmꢀ1. It is obvious that these SBs have strong absorption properties, and
capacity of absorption of SB1 is the weakest among the five compounds.
Fig. 3. UV–vis spectra of SBs in DMSO.
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2.3. Acidochromic propertides and pH testing
The acidochromic characteristic of SB5 was investigated in a mixture solution (DMSO:
H2O = 3: 1, v/v) to meet the solubility requirements. The absorption spectra of SB5 solution
added with acidic solution are shown in Fig. 4. The spectra of SB1~SB4 are presented in Fig.
S2.
As seen from the spectra of Fig. 4, absorption peaks of mixture solutions changed with
different pH values. The pH value of the initially SB5 solution was 7.13. The color of the
solution was lightꢀyellow with the maximal absorption at 395 nm and a weak absorption at
305 nm. Upon continual being added the acidic solution, the color of solution changed from
pale yellow to orange with a new absorption band emerged at 500 nm, and the intensity
increased as the pH value decreased. Further the acidic solution was added into SB5 mixture
solution until the pH value approached to 5.17, the color of mixture solution presented
redꢀcolor with the absorption at 500 nm and the absorption at 395 nm disappeared. The
solution of SB5 was colorless immediately when pH value was less than 5.00. The
absorption changes with the pH values of SB5 only ranging from 7.13 to 5.17, pH values of
SB1(7.32~3.06), SB2(7.40~4.01), SB3(7.26~4.00) and SB4(7.38~3.56) were presented in
Fig. S2. The color variation photography was shown as inset in Fig. 4. This may be
attributed to molecular planarization resulted by the protonation of the nitrogen in the imine
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linkage by acid doping which leaded to an increased πꢀelectron delocalization. The
protonation process and the resonance form were showed in Scheme 2. The isoelectric point
of absorption spectra was at 438 nm that confirmed the coexistence of discrete
chromophores in the mixture solution at the time.
Scheme 2. The proposed simplified protonation process and resonance form of SBs.
Fig. 4. UVꢀvis absorption spectra of SB5 protonated by HCl solution in DMSO.
The different proton concentrations of solvents have a great effect on the absorption
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spectral properties. The investigation revealed that the SBs were quite sensitive to pH value
and the sensors based on SB may be applied for detection environmental proton. The
relationship between intensity of absorbance peak at 500 nm and pH values was shown as
inset in Fig. 5. Absorbance was plotted as a linear increasing function vs. pH values with a
very good regression coefficient as R=0.988. This linear response could be used for
detection of pH values ranged from 7.13 to 5.17 according to the followed equation (1):
(IꢀI0)/I0 = ꢀ0.118 [pH] + 0.884
(1)
Where I is the absorbance intensity with different pH values at 500 nm, while I0 is the
absorbance intensity with pH is 7.13 at 500 nm, (IꢀI0)/I0 is relative absorption intensity. The
other four SBs had the similar behavior, see Fig. S3.
Fig. 5. Relationship between (IꢀI0)/I0 and pH value at 500 nm.
It indicated that the absorbance followed a linear relationship with acidic concentration.
So, we can test H+ in accordance with this relationship. Depending on Fig. 4, we calculated
the pKa of SB5 using the equation (2): 28
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A = AA− + [((AHA−A) [H+])/Ka]
(2)
−
Curve is plotted in Fig. 6, where A is the solution absorbance under different pH value, AA
and AHA are the absorbance of conjugated base and conjugated acid in their maximal
concentrations at 500 nm. From the slope, we calculated the pKa = 5.82. According to the Eq
(2), we can get the pKa of SB2, SB3, SB4 being 4.77, 4.96, and 4.89, as shown in Fig. S4.
The values are listed in the order: pKa(SB5) > pKa(SB3) > pKa(SB4) > pKa(SB2). Aromatic amine
nitrogen has a pair of isolated electrons. The isolated electrons of nitrogen and πꢀelectron of
phenyl can form a uniform conjugated system. Both oxethyl and methyl groups are
donorꢀelectron groups could increase the electron clouds density, and could make the
electrons on nitrogen easy to lose. The pyrroleꢀ2ꢀimine a conjugated moiety, which can
increase the conjugated degree of the whole molecular results in the isolated electrons tend
to flow to nitrogen and average. Therefore, SB2 is the minimal alkaline among the
compounds, and pKa(SB2) value is minimal. The donorꢀelectron ability of oxethyl is stronger
than methyl group, thus, pKa(SB5) > pKa(SB4). The pyrroleꢀ2ꢀimine which is weak alkaline of
these compounds results pKa(SB5) > pKa(SB3) > pKa(SB4). The study of the pH effect on
absorption spectra can provide a good tool for researching molecular interactions, including
hydrogen transfer processes under different condition.
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Fig. 6. The linear relationship between A and (AHAꢀA)/[H+] at 500 nm.
2.4. Solvatochromic properties of SB1
To examine the fluorescence and solvatochromic properties, the SB1 dissolved in
DMSO, CH3OH and CH2Cl2. The spectra were recorded by JASCO FPꢀ6200 fluorescence
φ
spectrometer. PL quantum yields ( s ) of the samples in DMSO, CH3OH and CH2Cl2 were
measured by using quinine sulfate dissolved in 0.1 mol Lꢀ1 sulfuric acids as a reference
φ
φunk
standard ( std = 0.546). The
of SB1 in the different solvents after refractive index
correction can be calculated according to the equation (3):



2
Iunk
A
ηunk
std








ϕunk
= ϕstd
Istd Aunk ηstd




(3)
φstd Iunk Istd Aunk
A
η
ηstd
φunk
std
where, the
,
,
,
,
,
,
unk , and
are the fluorescent quantum
yield, integration of the emission intensity, absorbance at the excitation wavelength, and the
refractive indices of the corresponding solutions for the samples and the standard,
respectively. Here, we use the refractive indices of the pure solvents as the solutions. We got
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ϕ
ϕ
ϕDMSO
CH3OH
CH2Cl2
the results as following:
=1.49%,
=0.49%,
=0.52%. Most of
φ
previously reported polyazomethines have low fluorescence quantum yields ( s ) < 2%,
φs
including polyazomethines containing Triphenylamine performed
< 1% reported by
us.29 Skene demonstrated the unparalleled high fluorescence of a novel fluorescent monomer
consisting of complementary amino and aldehyde functional groups,30 and another ones are
conjugated copolyazomethines consisting of alternating fluorene and thiophenes31 with the
φs
> 10%. However, most imines exhibited weakly fluorescence usually. The PL spectra of
SB1 were shown in Fig. 7, and the corresponding images were inserted. The SB1 presents
strong solventꢀpolarity dependence, displaying that a dominant emission band underwent
bathochromic shifts with an decrease of the solvent polarity. The emission color changed
from blue (λmax = 450 nm) in DMSO to cyan (λmax = 480 nm) in methanol, and to orange
(λmax = 500 nm) in CH2Cl2. The solvatochromism could be ascribed to large change of dipole
moment in the exciteꢀstate, which was associated with the polar emissive excited states
stabilized by the solvents with different polarity.
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Fig. 7. Fluorescence spectra of SB1 (the picture was taken using a UV lamp excited at
365 nm) of SB1 in DMSO (a), CH3OH (b), CH2Cl2 (c).
2.5. Electrochemical properties of SBs
CV was performed by CHIꢀ660E electrochemical workstation. The electrochemical
three ꢀelectrode includes a Pt wire used as counter electrode, Ag/AgCl electrode as the
quasiꢀreference electrode, and ITOꢀcoated (9 mm × 45 mm) as the working electrode. The
electrochemical behavior was determined under nitrogen atmosphere in 0.1 M
LiClO4/acetonitrile (CH3CN) was used as electrolyte solution with the scanning rate being
50 mV/s. All of the compounds exhibited similar electrochemical properties except SB1.
The representative CV diagrams for SB1 and SB2 are illustrated in Fig. 8. The CV diagram
of SB2 displays a reversible electrochemical behavior by several continuous cyclic scan
between 0 V and 2.0 V, which indicats that the intermediates are produced in the oxidation
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process and are stable during the time of CV scan. The SB3ꢀSB5 analogs show a similar CV
behavior to that of SB2, which are shown in Fig. S5. In contrast, the electrochemical
property of SB1 is completely different from them. From the CV diagram of SB1 in Fig. 8,
we find two oxidation peaks at 0.59 V and 1.14 V, and one deoxidation peak at about 0.69V
from the first curve. So we assume a coupling reaction occurred, and a steady structure was
produced.32 The coupling reaction is facile to take place in 1 position prior to in 2 site based
on theoretical knowledge, but it actually occurred in 2 position, in Scheme 2. After several
CV scans, fourꢀcouple reversible redox peaks appeared at 0.52 V, 0.80 V, 1.02 V and 1.24 V.
The four potentials can be ascribed to oxidation of the electronꢀrich nitrogen atoms in the
TPA core to form radical cations, as shown in Scheme 3. SB2~SB5 only exhibite one couple
of redox process. This can be explained by the second group being introduced into TPA
moiety greatly prevented the coupling reaction. While, SB1exhibited particular CV property
compared with SB2~SB5, because there are two spaces C on the end of TPA in SB1, andin
only one space position in SB2, and SB3~SB5 have no space position. So, SB1 could have
coupling reaction, but others can not react.
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Fig. 8. CV curves of SB1 and SB2 in CH3CN containing 0.1 M LiClO4 at a scan rate of
50 mV·s−1.
The peaks can be assigned to reoxidation of the electronꢀrich nitrogen atom in the TPA
moiety. These compounds showed the different values of potential peaks and current
intensities of peaks. It can be understood that the different substituted groups have different
pulling and pushing electron abilities, in which the ability of pushing electron raised from
SB2 to SB5, so that the redox potentials are listed in the order: ESB2> ESB3> ESB4> ESB5.
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Scheme 3. Proposed simplified redox process and the resonance form of SBs.
In the CV curves, the redox peaks have been associated with redoxidation process
accompanied by the injection/extraction of ClOꢀ4 ions and electrons. To obtain accurate
redox potentials, the reference electrode was calibrated by the ferrocene/ferrocenium
(Fc/Fc+), which is assumed to have an absolute energy level of ꢀ4.80 eV compared to
vacuum. We got the E1/2 (Fc/Fc+) vs Ag/AgCl of 0.37 V by CV test. As a result, the highest
occupied molecular orbital (HOMO) energy values were calculated using the equation as
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follows: 33
(4)
OX
onset
E
Where
is the onset oxidation potential vs Ag/AgCl. The HOMO energy values of
SB1~SB5 were calculated to be in the range from ꢀ5.18 to ꢀ4.76 eV. Because the reduction
curves could be hardly obtained, the lowest unoccupied molecular orbital (LUMO) energy
levels of the SBs were estimated from the HOMO energy levels and Eg by using the
following equation:
ELUMO
=
EHOMO
+
Eg
(5)
The optical band gap (Eg) was determined as follows:
Eg = 1240/λonset
(6)
ELUMO s were calculated to be between ꢀ2.55 and ꢀ1.45 eV.
The values of ELUMO, the onset oxidation potentials and the onset absorption wavelength are
listed in Table 1.
2.6. Quantum chemical calculation
To better understand the processes of the oxidation and reduction potentials, the
electronic structures of the compounds series were evaluated by using DFT at the
B3LYP/6ꢀ31G basis set in the Gaussian 03 program package, and pictorial representations of
the frontier molecular orbital of SBs are presented in Fig. 9. The values of HOMO energy,
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LUMO energy and HOMOꢀLUMO energy gaps ( Egquntum ) are summarized in Table 1. Figures
illustrate that the electron density distribution of the HOMO state is mainly located on TPA
group. While, the electron cloud of the LUMO state is distributed over the whole molecule.
The whole molecule effectively reduces the Eg due to the lower LUMO energy level of the
SBs. The lone pair of electrons on the nitrogen atoms have strong coupling with πꢀelectrons
in TPA moiety, which lead to intramolecular charge transfer (ICT) with significant
distribution of the electronic density over the entire molecule.
Fig. 9. Pictorial representations of the electron density in the frontier molecular orbitals
of SBs.
The theoretical calculation values of HOMO energy level of SBs basically are
corresponded with the experimental results. The trend of actual measured Eg for the SBs is
listed in the order: E(SB1)> E(SB2)> E(SB4)> E(SB5)> E(SB3). It indicates that SB1 has the
strongest antioxidation property among the five SBs. The different result can be attributed to
the different TPA structures. The different groups linked to TPA moiety played the key role
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in the electron structure and regulated the gaps of SBs. Actually, the experimental results are
also influenced by the electrolyte and other complexity. In contrast to our group’s previously
reported SBs which have furan24 and thiophene25 moieties, the energy gaps of SB1~SB5
were listed in the order: E(furan)> E(thiophene)>E( pyrrole).
Table 1. Electrochemical properties and energy levels of SB1 to SB5.
experimental results
theoretical calculation
Egquntum
ox
quntum
HOMO
quntum
E
LUMO
Eg
EHOMO
λonset
E
ELUMO
E
onset
nm
eV
eV
eV
eV
eV
eV
eV
SB1
SB2
SB3
SB4
SB5
375
450
475
465
475
0.33
0.75
0.73
0.65
0.55
ꢀ4.76
ꢀ5.18
ꢀ5.16
ꢀ5.08
ꢀ4.98
ꢀ1.45
ꢀ2.42
ꢀ2.55
ꢀ2.41
ꢀ2.37
3.31
2.76
2.61
2.67
2.61
ꢀ4.74
ꢀ4.66
ꢀ4.61
ꢀ4.67
ꢀ4.64
ꢀ1.34
ꢀ1.43
ꢀ1.48
ꢀ1.47
ꢀ1.45
3.40
3.23
3.13
3.20
3.19
a UVꢀvis absorption spectra measured in DMSO.
b λonset: the absorption starting wavelength.
c Eox/onset (Fc/Fc+ vs Ag/AgCl) = 0.37eV.
d based on formula (4).
e based on formula (5).
f based on formula (6).
g Equntum : Values of theoretical calculation.
2.7. Electrochromic properties of SBs
The electrochromic properties of the SBs were determined by UVꢀvis spectroscopy
combined with CV techniques. For the electrochromic investigations, a homemade
electrochemical cell was built from a commercial UVꢀvis cuvette. The cell was placed in the
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optical path of the sample light beam in a UVꢀvis spectrophotometer to acquire electronic
absorption spectra under potential controlled in the same way to determine electrochemical
behavior. All SBs exhibited similar electrochromic properties, and the typical spectra of SB5
are presented in Fig. 10. Two absorptions of neutral form appeared, with a strong band at
408 nm and a weak band at 310 nm. When the applied voltages were increased from 0 V to
1.2 V, the peak of the characteristic absorbance at 310 nm increased and that at 408 nm
decreased gradually in intensity, respectively, whereas one new band grew at 600 nm. The
color of the SB changed from original yellow to red. We attribute the spectral changes to the
formation of the radical cationic of TPA+ moiety. A description of the redox process and the
resonance form of SBs is presented in Scheme 3. In order to neutralize the charge, the anions
would insert into the molecule. The first lost electron is assumed to occur at the N atom on
the core of TPA segment, which is more electronꢀrich than that in the substituent ꢀCH=Nꢀ
group. The other four spectra are shown in Fig. S6.
Fig. 10. Electronic absorption change of SB5 during the process of electrochemical
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doping occurred in CH3CN containing 0.1 M LiClO4 as the supporting electrolyte.
3. Conclusions
A new series of Schiff bases compounds consisted of pyrrole and triphenylamine (TPA)
moieties with good opticalꢀelectro properties have been synthesized and confirmed by IR,
1HꢀNMR, and 13CꢀNMR. The compounds exhibite reversible electrochromic, acidochromic
and continuous cycle stability CV characteristics. The color of Schiff bases solutions could
be varied from initially pale yellow to red via electrooxidation. In addition, all obtained
compounds displayed multiple color states with different pH values and the results showed a
perfect linear relationship between the absorption and H+ concentration. Thus, these new
Schiff base compounds can be used as H+ detection materials due to their sensitive
acidochromic properties.
4. Experimental
4.1. Chemicals and reagents
All commercially available chemicals were A.R. grade. All solvents used were purified
by standard methods. N,Nꢀdimethylformamide (DMF) was dried over CaH2 and distilled
under reduced pressure. Phosphorus chloride (POCl3), dichloroethane and pyrrole were
purified by distilling under reduced pressure. Lithium perchlorate (LiClO4) was dried under
vacuum at 160 oC for 48 h. 4ꢀaminoꢀtriphenylamine (A1), 4,4'ꢀdiaminoꢀtriphenylamine (A2),
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4,4',4''ꢀtriaminoꢀtriphenylamine
(A3),
4ꢀmethylꢀ4,4'ꢀdiaminoꢀtriphenylamine
(A4),
4ꢀethoxyꢀ4',4''ꢀdiaminoꢀtriphenylamine (A5) and pyrroleꢀ2ꢀaldehyde were synthesized in our
laboratory, which have been characterization.
4.2. Synthesis
4.2.1 Synthesis of A1~A5
The raw compounds A1~A5 have been synthesized as reference published before.24, 25
Structures were described in Scheme 1.
4.2.2. Synthesis of pyrroleꢀ2ꢀaldehyde
Pyrroleꢀ2ꢀaldehyde was synthesized according to the published method.34 The product
was purified by recrystallization to be white needle crystal. Yield: 75%; M.P.: 42~44oC;
1HꢀNMR, FTꢀIR spectra were showed in Fig. S7, Fig. S8, and the synthetic route described
in Scheme S1 in supporting information. 1HꢀNMR (400MHz, DMSOꢀd6, δ, ppm): 12 (NꢀH),
9.75 (OCHꢀ), 6.24, 6.97, 7.23 (aromatic ring of pyrrole). FTꢀIR (KBr,
700ꢀ1500 ( aromatic ring ).
v
/cmꢀ1): 1650 (ꢀC=O ) ,
4.2.3. Synthesis SBs
SBs are easily to be prepared by condensation between amine and aldehyde with water
being the sole byproduct.6 The synthesis routes and structures of SB1~SB5 were showed in
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Scheme 1. Take preparation of SB5 for example. A5 (0.8004 g, 0.0025 mol) in 30 ml of
ethanol was added in a threeꢀneck roundꢀbottomed flask equipped with a DeanꢀStark trap
o
and a reflux condenser at room temperature. Then, the suspension was heated up to 80 C
with stirring under nitrogen. A5 dissolved and the color of the solution turned from black to
burgundy when the ethanol refluxed. Pyrroleꢀ2ꢀaldehyde (0.4998 g, 0.0053 mol) dissolved in
10 ml ethanol was added dropwise in the flask. The reaction mixture refluxed for 24 h. After
cooling to room temperature, the precipitate was obtained in saturated ice brine and collected
by filtration immediately. Followed by being purified with column chromatography (Al2O3
column, elution with Vethanol: Vethyl acetate: Vpetroleum ether =1:0.5:2.5 in volume). The desired
product was obtained as pale yellow solid. The compound was characterized by NMR and
FTꢀIR techniques. The other four SBs were synthesized through the same procedure.
Acknowledgements
The authors were grateful to the support of the National Science Foundation of China
(Grant No. 51373049, 51372055, 21372067, 21206034, 51303045, 51473046), Doctoral
Fund of Ministry of Education of China (20132301120004, 20132301110001), Reserve
Talented Person of Harbin (2015RAXXJ015).
Reference
1. A. Bolduc, S. Dufresne and W. G. Skene, Acta Crystallogr., Sect. C: Cryst. Struct.
27

New Journal of Chemistry
Page 28 of 33
View Article Online
DOI: 10.1039/C6NJ00321D
Commun., 2013, 69, 1196–1199.
2. S. Dufresne and W. G. Skene, J. Phys. Org. Chem., 2012, 25, 211−221.
3. M. E. Mullholland, D. Navarathne, M. L. Petrus, T. J. Dingemans and W. G. Skene, J.
Mater. Chem. C, 2014, 2, 9099−9108.
4. A. Iwan and D. Sek, Prog. Polym. Sci., 2008, 33, 289−345.
5. Y. Z. Xie, G. G. Shan, Z. Y. Zhou and Z. M. Su, Sens. Actuators, B, 2013, 177, 41–49.
6. M. L. Petrus, F. S. F. Morgenstern, A. Sadhanala, R. H. Friend, N. C. Greenham and T.
J. Dingemans, Chem. Mater., 2015, 27, 2990−2997.
7. S. F. Wang, C. L. Guo, K. K. Cui, Y. T. Zhu, J. X. Ding, X. Y. Zou and Y. H. Li,
Ultrason. Sonochem., 2015, 26, 81–86.
8. T. Y. Liu, L. Finn, M. H. Yu, H. Y. Wang, T. Zhai, X. H. Lu, Y. X. Tong and Y. Li,
Nano Lett., 2014, 14, 2522−2527.
9. Mukundan and Thelakkat, Macromol. Mater. Eng., 2002, 287, 442–461.
10. J. H. Wu and G. S. Liou, ACS Appl. Mater. Interfaces, 2015, 7, 15988–15994.
11. L. C. Lin, H. J. Yen, Y. R. Kung, C. M. Leu, T. M. Lee and G. S. Liou, J. Mater. Chem. C
,
2014, 2, 7796–7803.
12. L. C. Lin, H. J. Yen, C. J. Chen, C. L. Tsai and G. S. Liou, Chem. Commun., 2014, 50,
13917–13920.
13. Y. C. Kung, W. F. Lee, S. H. Hsiao and G. S. Liou, J. Polym. Sci., Part A: Polym. Chem.,
2011, 49, 2210–2221.
14. H. J. Yen, S. M. Gou, J. M. Yen and G. S. Liou, J. Polym. Sci., Part A: Polym. Chem.,
2011, 49, 3637–3646.
15. L. T. Huang, H. J. Yen and G.S. Liou, Macromolecules, 2011, 44, 9595–9610.
16. H. J. Yen and G. S. Liou, Polym. Chem., 2012, 3, 255–264.
28

Page 29 of 33
New Journal of Chemistry
View Article Online
DOI: 10.1039/C6NJ00321D
17. L. D. Zhang and P. Naumov, Angew. Chem. Int. Ed., 2015, 54, 8642–8647.
18. A. JiménezꢀSánchez, N. Farfán, and R. Santillan, J. Phys. Chem. C, 2015, 119,
13814–13826.
19. L. N. Ma, H. J. Niu, J. W. Cai, Y. F. Lian, W. J. Wu, X. D. Bai, W. Wang, J Nanopart
Res., 2013, 15, 2135–2143.
20. A. JimenezꢀSanchez, M. Rodrıguez, R. Metivier, G. RamosꢀOrtız, J. L. Maldonado, N.
Reboles, N. Farfan, K. Nakatani and R. Santillan, New J. Chem., 2014, 38, 730–738
21. M. E. Genovese, A. Athanassiou and D. Fragouli, J. Mater. Chem. A, 2015, 3,
22441–22447.
22. K. C. Naeem, A. Subhakumari, S. Varughesec and V. C. Nair, J. Mater. Chem. C, 2015,
3, 10225–10231.
23. R. Rani, K. Paul and V. Luxami, New J. Chem., 2016, 40, 2418–2422.
24. X. T. Wu, W. Wang, B. Li, Y. J. Hou, H. J. Niu, Y. H. Zhang, S. H. Wang and X. D. Bai,
Acta, Part A, 2015, 140, 398–406.
25. H. L. Wen, H. J. Niu, B. Li, X. C. Ma, X. D. Bai, Y. H. Zhang and W. Wang, Synth.
Met., 2015, 202, 89–97.
26. J. L. Zhou, G. Luo, Y. X. Wei, J. M. Zheng, C. Y. Xu, Electrochimica Acta, 2015, 186,
182–191.
27. L. N. Ma, P. Zhao, W. J. Wu, H. J. Niu, J. W. Cai, Y. F. Lian, X. D. Bai and W. Wang,
Polym. Chem., 2013, 4, 4746–4754.
28. S. H. Hsiao, H. M. Wang, J. W. Lin, W. J. Guo, Y. R. Kung, C. M. Leu and T. M. Lee,
Mater. Chem. Phys., 2013, 141, 665−673.
29. J. W. Cai, H. J. Niu, C. Wang, L. N. Ma and X. D. Bai, W. Wang, Electrochimica Acta
,
2012, 76, 229−241.
29