Synthesis of 2,2-bithiophene based dye sensor and optical properties toward metal cations

Article abstract of DOI:10.1080/15421406.2011.600640

We have designed and synthesized a 2,2-bithiophene-based new dye chemical sensor, and its selective and sensitive recognizing functions toward heavy metal cations were investigated. These find results were characterized using UV-Vis. spectrophotometer and spectrofluorometer. In addition, electrochemical properties were measured with cyclicvoltammetry test. Electron density distributions and HOMO/LUMO energy levels were computationally calculated and optimized. The energy levels of HOMO/LUMO were compared with computational calculation and cyclicvoltammetry measurement for each value. Finally, the formation type of metal ion bindings was determined by Job's plot measurements.

Full text of DOI:10.1080/15421406.2011.600640

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Synthesis of 2,2-bithiophene Based Dye
Sensor and Optical Properties Toward
Metal Cations
Young-Sung Kim a & Young-A Son a
a BK21 FTIT, Department of Advanced Organic Materials and Textile
System Engineering, Chungnam National University, Daejeon,
305-764, Korea
Version of record first published: 18 Oct 2011.
To cite this article: Young-Sung Kim & Young-A Son (2011): Synthesis of 2,2-bithiophene Based Dye
Sensor and Optical Properties Toward Metal Cations, Molecular Crystals and Liquid Crystals, 551:1,
163-171
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Mol. Cryst. Liq. Cryst., Vol. 551: pp. 163–171, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.600640
Synthesis of 2,2-bithiophene Based Dye Sensor
and Optical Properties Toward Metal Cations
YOUNG-SUNG KIM AND YOUNG-A SON∗
BK21 FTIT, Department of Advanced Organic Materials and Textile System
Engineering, Chungnam National University, Daejeon, 305-764, Korea
We have designed and synthesized a 2,2-bithiophene-based new dye chemical sensor,
and its selective and sensitive recognizing functions toward heavy metal cations were
investigated. These find results were characterized using UV-Vis. spectrophotometer
and spectrofluorometer. In addition, electrochemical properties were measured with
cyclicvoltammetry test. Electron density distributions and HOMO/LUMO energy levels
were computationally calculated and optimized. The energy levels of HOMO/LUMO
were compared with computational calculation and cyclicvoltammetry measurement for
each value. Finally, the formation type of metal ion bindings was determined by Job’s
plot measurements.
Keywords 2,2-bithiophene; cyclicvoltammetry; job’s plot; metal binding; HOMO/
LUMO; computational calculation; dye sensor
Introduction
Currently, various research groups have reported the colorimetric and fluorometric sensor
molecules for the utilization of harmful heavy metal cation detection. For this purpose,
structural characteristics with non-pair or dipolar molecular system such as cryptand,
spherand, and crown ether are commonly considered. This chemosensing effect is of
very importance in view point of analytical chemistry, which provides to meet the exact
measurement and calculation of the harmful metal cations in chemical, environmental and
biological conditions. Herein, we have focused the find the Cu2+ cation, Especially, Cu2+
detection has been considered as very important meanings due to its widespread use and
essential trace element in biological systems [1–2].
Most of studies on Cu2+ detection sensors are focused on the design of sensor molecules
providing higher sensing selectivity, which can be utilized with molecular charge transfer
system, π-conjugation and powerful donor moiety. Usually, thiophene types containing
fused aromatic rings, namely heteroarene, have much attention due to its various appli-
cation areas such as electro photographic photoreceptors, dye-sensitized solar cells, light
emitting materials, organic superconductors and so on [3]. The molecular structure con-
taining thiophene units is very promising electron donor material. This donor moiety is
important key factor in the metal complex detection because the metal binding reaction can
be occurred at the electron rich site being provided by donor moieties. [3–5]
In this study, we have synthesized dye chemical sensor having thiophene molecules as
a donor unit. The resulting detection properties of dye sensor were examined with UV-Vis
∗Corresponding author. E-mail: yason@cnu.ac.kr
[483]/163

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Y.-S. Kim and Y.-A Son
absorption and fluorescent emission measurements. In addition, computational calculation
was also carried out to determine the potential energy levels in the ground and excited
states. Findings on the electron density distributions provided the estimation of complex
characteristic functions between free dye-senor and metal complexed dye-sensor. All theo-
retically computational calculations for the energy potential and the electron distributions of
HOMO/LUMO states were performed with Material Studio 4.3 package program [6–10].
Furthermore, we have also considered cyclicvoltammetric (CV) experiments to determine
the energy potentials for the electrochemical redox potential of dye-sensor. We have used
the CV oxidation onset potential of HOMO energy levels and the UV absorption fit curve
calculation of band gap energy. According to this calculation we have determined the cor-
responding LUMO energy levels [11–14]. Finally, the complex formation types of metal
detection were examined by Job’s plot measurements.
Experimental
2.1 Synthesis
2,2-bihtiophene based dye sensor was synthesized. As presented in Scheme 1, mono α-
aldehyde (a) was prepared by the formylation reaction at room temperature, where 12 mmol
of 2,2-bithiophene and DMF were dissolved in 20 ml of 1,2-dichloroethane. After that, the
reaction mixture was cooled to 0◦C and 12 mmol of POCl3 was then added dropwise
while stirring. The reaction mixture was refluxed at 60◦C for 4h. After the reaction, the
mixture was poured into 100 ml of saturated sodium acetate solution and diluted with
ether. The diluted solution was washed with water, saturated sodium bicarbonate solution
and dried with excessive anhydrous MgSO4 for overnight. The solvent was removed under
CHO
S
S
S
S
POCl3, DMF, ClCH2CH2Cl
60oC, 4h
(a)
O
CHO
S
S
H2N
N
OH
O
N
S
S
N
OH
(b)
Scheme 1.

Optical Properties Toward Metal Cations
[485]/165
reduced pressure. The general procedures were followed by the indicated references [3].
For the next step, the dye chemosensor was obtained from the prepared intermediates of
2,2-bithiophene-5-carboxaldehyde and benzoxazole moiety. 2 mmol of 2,2-bithiophene-5-
carboxaldehyde and benzoxazole moiety were dissolved in 15 ml of benzene. 5∼6 drops of
piperidine was added dropwise and refluxed for 48 h. The reaction products were filtered
with benzene and dried in vacuum.
(yield: 0.38g, 45%). 1H-NMR (400MHz, acetone-d6) δ 8.82 (s, 1H), δ 8.08 (d, 1H), δ
7.79 (m, 2H), δ 7.68 (d, 1H), δ 7.57 (m, 1H), δ 7.50 (m, 2H), δ 7.40(d, 1H), δ 7.35(s, 3H), δ
7.17(m, 1H), δ 7.00(m, 1H). MS: 402(M+). Anal. Calculated for C22H14N2O2S2: C, 65.65;
H, 3.51; N, 6.96; S, 15.93: found; C, 66.07; H, 3.83; N, 6.82; S, 14.74.
2.2 Measurements
The spectroscopic characteristics and the fluorescence properties of the prepared dye
chemosensor were examined and determined using Agilent 8453 UV-Vis spectrophotome-
1
ter and Shimadzu RF-5301 spectrofluorophotometer, respectively. H NMR spectra and
elemental analyses were recorded with a JNM-AL400 spectrometer operated at 400 MHz
NMR and a Carlo Elba Model 1106 analyzer, respectively. Electron distributions and energy
potentials were calculated with Material Studio 4.3 [15]. Cyclic voltammograms were ex-
amined with a Versa STAT3 using three-electrode conventional electro chemical cell. Cyclic
voltammetry test was conducted in an acetonitrile solution containing tetrabutlyammonium
hexafluorophosphate electrolyte. The reference electrode, Ag/Ag+ was directly immersed
in the reaction cell. The working electrode was a glassy carbon. The counter electrode was
a platinum wire. The scan rate was commonly 100mV/s.
2.3 Job’s plot measurements
Using job’s method determination, the stoichiometrical characteristics of metal binding
ratio with dye sensor were examined. Equimolar solutions of dye sensor and various metal
cations (Cd2+, Cu2+, Hg2+, Ni2+, Zn2+, Al2+ and Fe2+) were mixed in different volume
ratios (1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, 9:1, 10:0). The maximum absorption of these
mixtures was characterized.
Results and Discussion
In this work, the designed dye chemosensor was synthesized using 2,2-bithiophene as
a donor unit and benzoxazole dye intermediate as a signal unit. UV-Vis absorption and
fluorescence emission spectra were investigated to monitor the optical changes and sensing
properties of the dye sensor with various different metal cations. The UV-Vis absorption
and fluorescence emission spectra of dye sensor (1 × 10−5 M) in MeOH : Water (9:1)
solution with various metal cations (Cd2+, Cu2+, Hg2+, Ni2+, Zn2+, Al2+ and Fe2+, 1 ×
10−4 M) are shown in Figure 1 and 2, respectively.
From Figure 1 and Figure 2, the dye sensor showed the higher selective detection
toward Cu2+ cations. As Figure 1 and Figure 2 show, upon the addition of Cu2+ to the
solution of the prepared dye sensor, the absorption band at 390 nm progressively decreased
in intensity and a weak new peak at 470 nm appeared; an isobestic point at 430 nm
also developed. The appearance of this isobestic point suggests that at least one stable
dye sensor-Cu2+ metal cation species is present in solution and is indicative that a stable
complex formed between dye sensor and Cu2+. In the case of fluorescence emission band at

166/[486]
Y.-S. Kim and Y.-A Son
Figure 1. UV-Vis. absorption of dye sensor (1.0 × 10−5 M) in MeOH: water (9:1) solution with
Cu2+
.
445 nm gradually increased with the addition of Cu2+ cations. Usually, this metal binding
reaction occurs, the fluorescence intensity is enhanced due to the effect of MLCT (metal
to ligand charge transfer) system. However, none of the other metal cations investigated,
namely Cd2+, Hg2+, Ni2+, Zn2+, Al2+ and Fe2+ had any noticeable effect on absorption
and emission, as shown in Figure 3 and Figure 4.
Figure 2. Fluorescence spectra of dye sensor (1.0 × 10−5 M) in MeOH : water (9:1) solution with
Cu2+
.

Optical Properties Toward Metal Cations
[487]/167
0.05
0.00
-0.05
-0.10
-0.15
Cd2+
Zn2+
Al2+
Ni2+
Fe2+ Cu2+ Hg2+
Figure 3. Effects of metal cations on UV-Vis absorption of dye sensor (1.0 × 10−5 M) in MeOH:
water (9:1) solution (A and A0 are the absorbance in the presence and the absence of metal ions,
respectively at 390 nm).
Metal complexing behavior can be considered as electrophile attack reaction to the dye
sensor molecules. This result is related with electrostatic properties and electron distribu-
tions through the dye sensor molecules. Thus, it is possible to predict the binding position
with dye sensor and Cu2+ cations. The complex position is an electron rich binding group in
the dye sensor molecule, namely nitrogen and hydroxyl group. Thus, electron energy levels
and distributions of the dye sensor were further examined by molecular orbital calculations,
70
60
50
40
30
20
10
0
-10
Cd2+ Zn2+
Al2+
Ni 2+
Fe 2+ Cu2+ Hg2+
Figure 4. Effects of metal cations on fluorescence intensity of dye sensor (1.0 × 10−5 M) in MeOH:
water (9:1) solution (F and F0 are the emission in the presence and the absence of metal ions,
respectively at 445 nm).

168/[488]
Y.-S. Kim and Y.-A Son
Figure 5. Electron density distributions of dye sensor.
which were based on the density functional theory (DFT) level using the exchange correc-
tion functional of the generalized gradient approximation (GGA), employing the Materials
Studio 4.3 suite of programs.
The resulting computational calculation indicated that the energy levels of free dye
sensor were HOMO (−5.170 eV) and LUMO (−3.113 eV) and the energy levels of metal
complexed dye sensor were HOMO (−4.811 eV) and LUMO (−3.131 eV), respectively.
HOMO energy of free dye sensor showed the delocalized electron distribution. In
contrast, the electron distribution of HOMO energy in metal complexed dye sensor was
localized in the area of Cu2+ cation biding legand (Figure 5).
Further electrochemical properties were calculated with cyclicvoltammetry (CV) mea-
surement. Using CV measurement, we have found the oxidation peak at 0.845 V(onset,
free dye sensor) and 0.908 V(onset, metal complexed dye sensor). The resulting cyclic-
voltammogram was summarized in Figure 6.
In addition, the energy potentials of oxidation onset peak and absorption band gap
were used to calculate empirical HOMO/LUMO energy levels. The following equation was
used [11].
HOMO (or LUMO) (eV) = −4.8 − (Epeak or onset − E1/2 (Ferrocene)
)
The resulting cyclicvoltammetry calculation showed that the energy levels of free dye
sensor were HOMO (−5.225 eV) and LUMO (−2.538 eV) and the energy levels of metal
complexed dye sensor were HOMO (−5.288 eV) and LUMO (−2.773 eV), respectively.
The comparison of computational calculation and experimental determination showed
the similar values of HOMO and LUMO energy levels (Table 1 and Figure 7). Thus, the
computational calculation results can be used as good supporting data for the analysis of
electrochemical properties.
Finally, we have determined the binding ratio between dye sensor and Cu2+ metal
cations using the Job’s method [16]. For Job’s plot measurements, 1 × 10−5 M of dye

Optical Properties Toward Metal Cations
[489]/169
Table 1. HOMO and LUMO values of dye sensor
Computationally
calculated values
Cyclicvoltammetry
values
Dye sensor
HOMO LUMO ꢀE HOMO LUMO ꢀE
−5.170 −3.113 2.057 −5.225 −2.538 2.687
Free dye sensor
Metal complexed dye sensor −4.811 −3.131 1.680 −5.288 −2.773 2.515
Figure 6. Cyclicvoltammogram of dye sensor in acetonitrile
Figure 7. HOMO and LUMO energy levels of dye sensor

170/[490]
Y.-S. Kim and Y.-A Son
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0.0
0.2
0.4
0.6
0.8
1.0
Wavelength (nm)
Figure 8. Job’s plots for the binding ratio of dye sensor and Cu2+
.
sensor and 1 × 10−4 M Cu2+ cations were prepared using various molar ratios of dye
sensor and Cu2+ cations (1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, 9:1, 10:0). The relationship
between maximum absorption peak and mole fraction of Cu2+ cations is displayed in Figure
8. From the findings, dye sensor exhibited that the molecular fraction was close to 50%,
which indicated 1:1 complex composition between dye sensor and Cu2+ cations.
Conclusions
In this work, the designed dye chemosensor was synthesized using 2,2-bithiophene as a
donor unit and benzoxazole intermediate as a signal unit. UV-Vis absorption and fluo-
rescence emission spectra were investigated to monitor the optical changes and sensing
properties of the dye sensor with various different metal cations (Cd2+, Zn2+, Al2+, Ni2+
,
Fe2+, Hg2+). The dye sensor showed the higher selective detection toward Cu2+ cations.
The comparison of computational calculation and experimental determination showed the
similar values of HOMO and LUMO energy levels. Thus, the computational calculation
results can be used as good supporting data for the analysis of electrochemical properties.
From the Job’s method, dye sensor exhibited that the molecular fraction was close to 50%,
which indicated 1:1 complex composition between dye sensor and Cu2+ cations.
Acknowledgment
This research was supported by a grant from the Fundamental R&D Program for Core
Technology of Materials funded by the Ministry of Knowledge Economy, Republic of
Korea.
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