X. Wu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 140 (2015) 398–406
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or heterocycle), which have the conjugated moiety, imine groups
(C@N) in the backbone which are similar to AC@CA group with
isoelectronic characteristics, planar configurations and optical
behaviors [1]. SBs exhibit the ability of capturing proton, excellent
thermal stability, good mechanical properties and metal-chelating
ability. SBs would be the suitable alterative for electronic applica-
tions, pH and ion sensors [2], molecular wires [3], organic light-
emitting diodes (OLED) [4], non-linear optical devices [5] and
organic photovoltaics [6].
The TPA unit is easy to be oxidized in the nitrogen center and
has the excellent ability to transport carriers via the radical cation
species [7,8]. In the year of 2008, Iwan group investigated deeply
on a series of aromatic polyazomethines with attractive optic-elec-
tronic properties bearing TPA [9]. TPA derivatives have showed
excellent thermal and electrochemical stability, electron-donating
ability and optoelectronic properties which have been used as
advanced materials for memories, electroluminescence and photo-
electric devices [10–14]. Thiophene derivatives have attracted
great attention owing to the outstanding stability and optical prop-
erties. Silvia Destri et al. designed and studied a great amount of
electrochromic materials made from thiophene derivatives [15].
Skene et al. also synthesized a series of azomethines containing
thiophene with low optical band gaps (1.3–2.0 eV), and all poly-
mers were claimed to be chemically and photochemically stable
[16]. The introduction of thiophene increased the overall conjuga-
tion and improved the opto-electronic properties. SBs were suit-
able for electronic materials due to reversible acid doping,
reoxidation and electrochromic properties. The variation of the
backbone structure distinctly affected the dihedral angles and
changed the electronic properties of SBs.
trum 100 Model FT-IR spectrometer. Using tetramethylsilane as
an internal reference, 1H NMR spectra were recorded on a Bruker
AC-400 MHz spectrometer in CDCl3. UV–vis spectra were con-
ducted on a UV-3600 (Shimadzu). The electrochemical and electro-
chromic properties of the SBs were determined by CV techniques
and UV–vis spectroscopy. For the electrochromic investigations, a
homemade electrochemical cell was built from a commercial
UV–vis cuvette. The cell was placed in the optical path of the sam-
ple light beam in a UV–vis spectrophotometer to acquire electronic
absorption spectra under potential control in a 0.1 M solution of
LiClO4 as an electrolyte under nitrogen atmosphere in dry CH3CN.
CV was conducted on a CH Instruments 660 A electrochemical ana-
lyzer at a scan rate of 50 mV/s in 0.1 M LiClO4/CH3CN with the use
of a three-electrode cell in which ITO-coated was used as a work-
ing electrode. The oxidation and reduction potentials of SBs were
taken with the use of an Ag/AgCl reference electrode, a platinum
wire being used as an auxiliary electrode, and were calibrated
against the ferrocene/ferrocenium (Fc/Fc+) redox couple. The pH
values were determined by using the SARTORIUS PB-10 pH meter,
which were calibrated by standard buffer solutions of pH 4.01 and
6.86. The pH value of solution was adjusted by addition of HCl
vapor to 0.1 M DMSO/H2O solution.
Synthesis of monomer
4-methyl-40,400-diamino-triphenylamine (A4), 4-ethoxy-40,400-
diamino-triphenylamine (A5), 2-thiophenecarboxaldehyde, have
been synthesized, and corresponding spectra are presented in Sup-
porting Information.
In this article, we reported a series of novel SBs compounds,
which synthesized from thiophene aldehyde with five different
amines containing TPA. The structures of the compounds were
characterized by 1H NMR and FT-IR. The optical, acidochromic
properties were tested by UV–visible spectroscopy and electro-
chemical properties were tested by CV techniques. By being doped
HCl vapor, the color of most SBs changed from yellow to red. The
change trend of Eg measured experimentally is consistent with
the theoretical value, which is calculated through quantum chem-
ical simulation. The designed SBs provide indispensable materials
for the electrochromic and acidochromic field.
A4: FT-IR: (KBr, v/cmꢁ1): 3407 (NAH stretching), 1592, 1509,
1490 (aromatic ring of benzene), 1314, 1280 (CAN stretching).
1H NMR: (CDCl3, 400 M); 6.54, 6.87, 7.32 (aromatic ring of
triphenylamine), 1.67 (CH3), 4.26 (NH2).
A5: FT-IR: (KBr, v/cmꢁ1): 3398 (NAH stretching), 1585, 1505,
1493 (aromatic ring of benzene), 1314, 1280 (CAN stretching).
1H NMR: (CDCl3, 400 M): 6.54, 6.63, 7.03 (aromatic ring of
triphenylamine), 4.07 (CH2), 1.47 (CH3), 4.26 (NH2).
2-thiophenecarboxaldehyde: FT-IR: (KBr, v/cmꢁ1): 2753 (CAH
stretching), 1708 (C@O stretching).
1H NMR: (CDCl3, 400 M): 10.15 (CH@O), 7.39, 8.07, 8.22 (thi-
ophene ring-H).
Synthesis and characterization of SBs
Experimental
Materials
Take preparation of SB1 for example: 4-amino-triphenylamine
(A1) (0.5217 g, 0.0020 mol) was dissolved in 100 ml THF and the
mixture was poured into a 100 ml, three-neck, round-bottom flask.
2-thiophenecarboxaldehyde (0.3017 g, 0.0027 mol) was added
slowly into the mixture, then a drop of acetic acid was added. After
that the solution was stirred at 60 °C reflux temperature for 4 h
under nitrogen atmosphere, it became transparent and orange.
The solution was evaporated by rotary evaporation to powders in
brown with the yield of 73.1% (0.5174 g). The other SBs were pre-
pared in the same procedure, and the synthesis routes of SBs are
showed Scheme 1.
5% palladium on activated carbon (Pd-C) was purchased from
Acros. 4-fluoronitrobenzene, N,N-dimethylformamide (DMF),
dimethylsulfoxide (DMSO, A.R.) were supplied from Sinopharm
Chemical Reagent, and used as received. DMF and DMSO were
dried over CaH2 and distilled under reduced pressure. Lithium per-
chlorate (LiClO4) was dried under vacuum at 120 °C for 36 h. 4-
amino-triphenylamine (A1), 4,40-diamino-triphenylamine (A2)
and 4,40,400-triamino-triphenylamine (A3) were synthesized by
Pd/C-catalyzed reduction of the corresponding nitro-compounds
obtained from the reaction of diphenylamine derivatives with 4-
fluoronitrobenzene in the presence of sodium hydride. 4-methyl-
40,400-diamino-triphenylamine (A4), 4-ethoxy-40,400-diamino-tri-
phenylamine (A5) and thiophene formaldehyde were synthesized
according to the references [17–22].
SB1: 1H NMR: (CDCl3, 400 M): 8.628 (s, 1), 7.47–7.56 (m, 1),
7.15–7.21 (m, 6), 7.10–7.14 (m, 2), 7.02–7.06 (d, 6); FT-IR:
(KBr, v/cmꢁ1): 2925, 2869, 1613, 1597, 1581, 1480, 833, 719;
yield 73.81% (0.5174 g), melt point (mp) 158–159 °C.
SB2: 1H NMR: (CDCl3, 400 M): 8.638 (s, 2), 7.46–7.52 (m, 2),
7.15–7.23 (d, 6), 7.03–7.10 (m, 1), 6.92–7.02 (d, 6), 6.65–6.70
(d, 4); FT-IR: (KBr, v/cmꢁ1): 3031, 2922, 2868, 1611, 1510,
1491, 826, 715; yield 66.3% (0.6759 g), melt point (mp) 174–
175 °C.
Measurements
The obtained compounds were characterized by the following
techniques: FT-IR spectra were measured on a PerkinElmer Spec-