Synthesis, complexation, and fluorescence behavior of 3,4-dimethylthieno[2, 3-b]thiophene carrying two monoaza-15-crown-5 ether groups

Article abstract of DOI:10.1016/j.saa.2011.07.058

A novel fluoroionophore 1 based on 3,4-dimethylthieno[2,3-b]thiophene bearing two monoaza-15-crown-5 ethers at 3- and 4-positions was prepared. UV-vis and fluorescence responses of compound 1 upon the addition of alkali and alkaline earth metal cations were evaluated in acetonitrile solution. Receptor 1 showed unique response for Ba²⁺ due to the formation of an intramolecular sandwich complex.

Full text of DOI:10.1016/j.saa.2011.07.058

Spectrochimica Acta Part A 82 (2011) 340–344
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, complexation, and fluorescence behavior of
3,4-dimethylthieno[2,3-b]thiophene carrying two
monoaza-15-crown-5 ether groups
Yong Xiang Wua, Jing Caoa,b,∗, Hai Yan Denga, Jun Xiang Fenga
a College of Chemistry, Xiangtan University, Xiangtan, Hunan 411105, People’s Republic of China
b Key Laboratory of Enviromentally Friendly Chemistry and Application of Ministry of Education, College of Chemistry,
Xiangtan University, Xiangtan, Hunan 411105, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel fluoroionophore 1 based on 3,4-dimethylthieno[2,3-b]thiophene bearing two monoaza-15-
crown-5 ethers at 3- and 4-positions was prepared. UV–vis and fluorescence responses of compound
1 upon the addition of alkali and alkaline earth metal cations were evaluated in acetonitrile solution.
Receptor 1 showed unique response for Ba2+ due to the formation of an intramolecular sandwich complex.
Received 28 March 2011
Received in revised form 15 July 2011
Accepted 18 July 2011
Keywords:
© 2011 Elsevier B.V. All rights reserved.
Fluorescent chemosensor
Monoaza15-crown-5 ether
Thieno[2,3-b]thiophene
Ba2+
Intramolecular sandwich complex
of 137Cs+ in the nuclear reactor wastes [3,4]. The detection of intra-
cellular Ca2+ is performed worldwide by employing the fluorescent
1. Introduction
Fluorescent chemosensors-molecules that change their fluo-
rescence in response to substrate representation an extremely
sensitive optical method for the real-time monitoring of molec-
ular interactions [1]. Such chemosensors are finding increased use
in fields as diverse as biology, medical analysis, and environmental
monitoring [1a]. The majority of fluorescent chemosensors oper-
ate by one of three mechanisms: (1) suppression of photoinduced
electron transfer or enhancement of heavy-atom quenching; (2)
variation of the distance between two fluorophores, modulating the
efficiency of interchromophore energy transfer; and (3) alteration
of the microenvironment of a solvatochromic fluorophore (e.g., by
displacement from a cyclodextrin cavity) [2].
The design of fluorescent chemosensors for the detection of spe-
cific analytes has attracted a growing interest during the last two
decades [1]. Considerable attention has been devoted to the devel-
opment of selective sensors for alkali and alkali-earth cations since
in situ detection of such analyte is needed in different research
areas (e.g., environmental and life sciences). For example, selective
fluorescent sensors for cesium were studied aimed at the detection
chemosensors developed by Tsien [5].
On the other hand, because of the increased industrial use of the
barium compounds as well as their enhanced discharge, toxic prop-
erties and other adverse effects [6], the problems should encourage
the development of new detection systems for this analyte. There-
fore, the design of novel and specific chemosensors for barium
seems to be particularly desirable.
Owing to the well-known ability of crown ethers to host s-block
metals [7], many fluorescent chemosensors designed in recent
years for alkali and alkali-earth ions contain at least one crown
ether moiety (or a related derivative) which behaves as a “recogni-
tion subunit” connected to one or more fluorophores [1b,8]. In these
systems, the output signal following the crown/cation interaction
involves a change in the emission properties of the fluorophore(s)
and is often related to a photoinduced electron transfer (PET)
process [9]. Other examples involving different mechanisms e.g.,
photoinduced energy transfer [10], excimer or exciplex formation
[11], conformational change [12], and proton transfer [13], have
also been reported.
Bis(crown ether) derivatives, in which two crown ether moieties
are linked by a flexible aliphatic chain, in some cases form more
stable complexes with metal cations and show a higher cation-
binding selectivity compared to the monocyclic analogues [14].
This is due to the ability of bis(crown ethers) to form intramolecular
∗
Corresponding author at: College of Chemistry, Xiangtan University, Xiangtan,
Hunan 411105, China. Tel.: +86 73158292251; fax: +86 73158292251.
E-mail address: caojing8088@xtu.edu.cn (J. Cao).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.07.058

Y.X. Wu et al. / Spectrochimica Acta Part A 82 (2011) 340–344
341
Scheme 1. Synthetic route to the fluorescence chemosensor 1.
sandwich-type complexes with particular cations whose size is
2.2.2. Synthesis of 3,4-bis(bromomethyl)thieno[2,3-b]thiophene
(2) [16]
larger than the cavity size of the crown ether ring.
In this paper, we report the behavior of
a
novel 3,4-
To a solution of 3,4-dimethylthieno[2,3-b]thiophene (3) (25.2 g,
0.15 mol) and NBS (52 g, 0.3 mol) in CCl4 (100 mL), AIBN (0.60 g)
was added. The mixture was refluxed for 5 h. After the reaction was
complete, the crude mixture was filtered at a room temperature to
remove succinimide, and the filtrate was evaporated. Compound 2
were purified by recrystallization from petroleum ether.
dimethylthieno[2,3-b] thiophene bearing two monoaza15-crown-
5 moieties at 3- and 4-positions (1) (Scheme 1), which displays
a strong fluorescence enhancement upon interaction with Ba2+
.
This selective enhancement is ascribed to the formation of the
intramolecular sandwich complex.
Yield: 86%. 1H NMR (CDCl3, 400 MHz): ı 7.43(s, 2H), 4.96(s,
4H) ppm. 13C NMR (CDCl3, 400 MHz): ı 142.1, 140.1, 130.6, 129.2,
27.1 ppm. GC–MS: m/z 325[M+]. Anal. Calcd for C8H6Br2S2: C,
29.47; H, 1.85; S, 19.67. Found C, 29.55; H, 1.88; S, 19.69.
2. Experimental
2.1. Materials and general methods
2.2.3. Synthesis of 3,4-bis((1,4,7,10-tetraoxa-13-azacyclo-
pentadecan-13-yl)methyl)thieno[2,3-b]thiophene (1)
All chemicals were purchased from Aladdin Reagent Company
(Shanghai, China). All the solvents except the chromatographically
pure acetonitrile were of analytic grade. All other chemicals were
purchased from aladdin and used as received. 1H NMR and 13C
NMR were recorded on a Bruker AVANCE 400 MHz spectrometer
with chemical shifts reported in ppm (in CDCl3; TMS as internal
standard). GC–MS analyses were obtained from an Agilent7890A
GC/5975C MSD(EI) instrument, and MALDI-TOF MS analyses were
performed on an Bruker autoflex III smartbeam instrument using
2,5-dihydroxy-benzoic acid (DHB) as the matrix. The microanalyses
were obtained with an elementar vario EL III analyzer. UV–vis spec-
tra was recorded on a Lambda 25 spectrophotometer. Fluorescence
spectra obtained on a photoluminescent spectra was taken on a
Perkin-Elmer LS55 luminescence spectrometer using a xenon lamp
as the light source, with 10 nm excitation slit widths, 20 nm emis-
sion slit widths and excitation wavelength at 300 nm. All solutions
of fluorescent chemosensor 1 and were prepared in chromato-
graphically pure acetonitrile without special efforts to exclude
water or air. Both of absorption and fluorescence titrations were
carried out with a concomitant addition of small volumes of metal
ion aqueous solutions to acetonitrile solution of compound 1 (metal
salts: NaClO4, KCl, CaCl2, CdCl2, MgSO4, MnSO4, Ni(ClO4)2·6H2O,
SrCl2·6H2O, BaCl2·2H2O, CoCl2·6H2O, HgCl2, LiClO4).
To a solution of 3,4-bis(bromomethyl)thieno[2,3-b]thiophene
(2) (0.1 g, 0.3 mmol) and 1,4,7,10-tetraoxa-13-azacyclopenta-
decane (0.134 g, 0.6 mmol) in DMF (6 mL), dried potassium car-
bonate (0.086 g) and tetrabutylammonium bromide (0.01 g) was
added. The reaction mixture was stirred at 30 ◦C for 24 h. After the
reaction was complete, the crude mixture was filtered at a room
temperature. The mixture solution was extracted with ethyl acetate
(20 mL), and the combined organic layer was washed with water
(10 mL). The residue was purified by flash column chromatography
(neutrality alumina, ethyl acetate) to give yellow oily liquid.
Yield: 76%. 1H NMR (CDCl3, 400 MHz): ı 7.24(s, 2H), 3.93(s,
4H), 3.66–3.59(m, 32H), 2.86–2.83(t, 8H) ppm.13C NMR (CDCl3,
400 MHz): ı 144.7, 138.5, 133.9, 125.4, 71.1, 70.6, 70.2, 70.0,
56.0, 54.8 ppm. MALDI-TOF-MS: m/z 625 [M+Na]+. Anal. Calcd for
C28H46N2O8S2: C, 55.79; H, 7.69; N, 4.65; S, 10.64; Found C, 55.84;
H, 7.82; N, 4.68; S, 10.72.
3. Results and discussion
The working principle of sensor 1 is schematically depicted in
the graphical Fig. 1. In this mechanism, uncomplexed 1 is supposed
to show quenched fluorescence (turn off) as a result of photoin-
duced electron transfer (PET) from the two aza-15-crown-5 donors
to the thieno[2,3-b]thiophene moieties. After complexation with
metal cations, the electrondonating ability of the two monoaza-15-
crown-5 moieties is attenuated because of cation-␲ attraction [17].
This effect will in turn suppress the PET process to result in fluores-
cence enhancement (turn on). Fig. 1 shows that sensor 1 consists of
one conjugated group, but it exhibits almost no fluorescence emis-
sion in acetonitrile (concentration: 2.0 × 10−6 M, excited at 300 nm,
‘off-state’), which is resulting from PET process.
It was known that crown ethers could bind certain metal ions,
which inspired us to test the presence of special metal ions by
inspecting the changes of fluorescence intensity of complex 1 in
the presence of metal ions. The alkali metal cations (Li+, Na+, K+),
alkaline-earth metal cations (Mg2+, Ca2+, Sr2+, Ba2+) and heavy and
2.2. Synthesis routes and the characterization data
Fluorescent chemosensor 1 was synthesized according to the
similar methods reported previously [15]. The structures of these
compounds are shown in Scheme 1 together with their abbrevia-
tions.
2.2.1. Synthesis of diethyl
3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylate (4) and
3,4-dimethylthieno[2,3-b]thiophene (3)
Diethyl 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylate
(4) [15] and 3,4-dimethyl thieno[2,3-b]thiophene (3) [15] were pre-
pared following the already published procedures.

342
Y.X. Wu et al. / Spectrochimica Acta Part A 82 (2011) 340–344
1000
800
600
400
200
Ba2+
1
0.1e
0.2e
0.4e
1e
5e
10e
20e
50e
100e
200e
0
340
360
380
400
420
440
Fig. 1. The compound 1.
Wavelength(nm)
Fig. 3. Fluorescence spectra of compound 1 (2.0 × 10−6 M) upon the titration of
Ba2+ (0–200 equiv.) in acetonitrile solution. Fluorescence intensity was regarded as
a function of Ba2+ concentration.
transition metal (HTM) cations (Ni2+, Mn2+, Co2+, Cd2+, Hg2+) were
used to evaluate the binding ability. Consequently, when different
metal ions (50 equiv.) were added in the acetonitrile solution of the
sensor 1, the different degree fluorescent enhancements of sensor
1 have happened in some alkaline metal ions, alkaline-earth metal
ions and heavy and transition metal (HTM) cations (Fig. 2). Sur-
prisingly, 1 displayed a large chelation-enhancement fluorescence
(CHEF) [18] with only Ba2+ among these metal ions. The experimen-
tal data (Icomplex/Ifree1) demonstrated that 1 is a highly sensitive and
selective sensor for Ba2+ in acetonitrile solution. However, under
the same conditions, no obvious changes were observed for other
widely tested metal ions. Because of its strong complex properties
among its relatives [19–21], it was important to note that molec-
ular recognition can result from a selective binding or selective
response. Metal ions of the same family show quite similar bind-
ing properties, and the alkaline-earth metal ions may bind the O
and N [22]. But the large distinctions are still extant among the
alkaline-metal ions, such as the relative fluorescence enhancement
intensity of Ba2+ reaches to (22.67), which is three times than the
Sr2+ (7.43) and much more than the Ca2+ (1.59). Among the diva-
lent alkaline-metal ions, with the increasing of the ion diameter,
the fluorescence yields of their complexes increased. These results
suggested that Ba2+ ions might bind much stronger with host 1
than other tested metal ions. Thus, complex 1 could acts as a Ba2+
selective supramolecular fluorescence probe.
To explore the recognition property of complex 1 to Ba2+ ions,
we have also carried out fluorescence titration experiments. As
shown in Fig. 3, upon the addition of Ba2+ into the solution of com-
plex 1 in acetonitrile solution, the emission band in the region
of 320–410 nm gradually increased. When Ba2+ was added, the
nitrogen atom was involved in complexing with Ba2+ and lost its
ability to donate an electron to the excited state of the thieno[2,3-
b]thiophene ring. Thus, the addition of Ba2+ caused the recovery
of the fluorescence which meant that the complexation for guest
cations inhibit the PET occurring from the nitrogen atom in the two
aza-15-crown-5 ethers to thieno[2,3-b]thiophene.
On the basis of the fluorescence titrations of 1, we have deter-
mined the association constant Ka for Ba2+ to be 1.80 × 105,
following the Benesi–Hildebrand-type analysis [23] (Fig. 4:
Y = A + BX, Ka = A/B, X = 1/[Mn+], Y = I0/(IF − Io); Ka: the association
constant, I0: the fluorescent intensity of sensor 1, IF: the flu-
orescent intensity of 1-complex; A = 0.27888, B = 1.55263 × 10−6
,
R2 = 0.9312, Ka = A/B = 1.7962 × 105). The stoichiometry of the
1–Ba2+ complex was also estimated to be 1:1 using Job’s method
[24] of continuous variation for the fluorescence data (Fig. 5). In
the case of the complexation with Ba2+, the complex approached a
maximum when the mole fraction of guest ca. 0.5, suggesting 1:1
host–guest complexation.
8
7
6
5
4
3
2
1
0
20
15
10
5
0
Ca2+ K+ Ba2+ Ni2+ Na+ Mn2+ Co2+ Cd2+ Mg2+ Li+ Hg2+ Sr2+
Free1
-1
0
1x106
2x106
3x106
4x106
5x106
1/[Ba2+]
Fig. 2. The relative fluorescence enhancement intensity of compound
1
(2.0 × 10−6 M) in acetonitrile solution at room temperature upon addition of various
metal cations. The excitation wavelength was at 300 nm, and the concentration of
metal cations was 50 equiv. of compound 1.
Fig. 4. The Benesi–Hildebrand analysis of compound 1 in different Ba2+ concentra-
tions.

Y.X. Wu et al. / Spectrochimica Acta Part A 82 (2011) 340–344
343
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1
Ba2+
0.25e
0.75e
1e
5e
10e
100e
300
250
0.0
0.2
0.4
0.6
0.8
1.0
Wavelength(nm)
[1]/([1]+[Ba2+])
Fig. 6. The UV–vis spectra of compound 1 (2.0 × 10−5 M) upon the titration of
Ba2+ (0–100 equiv.) in acetonitrile. The concentration of compound 1 was regard
as 1 equiv.
Fig. 5. Job plot for determining the stoichiometry of compound 1 and Ba2+. The total
concentration of compound 1 and Ba2+ was 2.0 × 10−6 M. Molar fraction was given
by [1]/([1] + [Ba2+]).
UV–vis absorption spectra except the intensity slight differences
(Fig. 6). In general, when a crown ether and a chromophore which
has no interaction site with guest cation are connected with a
methylene spacer, there are only minor spectral changes upon the
complexation with a guest. Furthermore when Ba2+ was added, the
absorbance derived from fluorophore moiety was more obviously
increased as their shapes do no shifted, which was proved the PET
process.
The electronic properties of the thieno[2,3-b]thiophene deriva-
tive 1 were investigated by UV–vis absorption. In the high-energy
region, an intense sharp band at 229 nm is absorptions due to
the thieno[2,3-b]thiophene moieties, which are ascribed to the
␲ → ␲* transitions at the central thieno[2,3-b]thiophene core by
comparison to the UV–vis data of other thieno[2,3-b]thiophene
derivatives [17].
Owing to the known ability of monoaza15-crown-5 ether to
interact with Ba2+ according to a sandwich-like mode [25], the
results obtained from both spectrophotometric and spectrofluo-
rimetric titration experiments can be explained by the formation
of an adduct according to a formal 1:1 stoichiometry, in which the
Ba2+ ions coordinate to two aza-15-crown-5 subunits belonging to
different molecules of 1. Spectrofluorimetric titration experiments
performed on acetonitrile solution of the bis-crown compound 1
(10−6 M) with different cations showed that sensor 1 has selectivity
for alkaline-earth metal ions with a relatively large diameter such as
Ba2+. The discriminative sensing behavior is likely originated from
a complementarity of metal ions and the rigid binding pocket of 1
in terms of size matching and noncovalent (e.g., cation–dipole and
cation–␲) interactions.
UV–vis spectral changes of 1 upon the addition of Ba2+ ions
in acetonitrile solution were showed that they have the similar
Plausible equilibria are illustrated in Scheme 2. For a 1:1 com-
plex of 1, two possible structures are formed, that is, a complex in
Scheme 2. The possible mechanism complex of 1 with metal ion.

344
Y.X. Wu et al. / Spectrochimica Acta Part A 82 (2011) 340–344
which one azacrown is occupied with a metal ion and the other is
unoccupied (1M) and an intramolecular sandwich complex (1Mꢀ).
Meanwhile, in a certian extent, the structure of 1M2 will be formed,
but it is just a small proportion of the complex. The fluorescence
intensities of 1M and 1Mꢀ are expected to be larger than that of
free 1, because the PET process is blocked by coordination of the
nitrogen atoms to the metal ions. The UV–vis and fluorescence
titrations of 1 with Ba2+ showed the pronounced spectral changes.
A Job plot analysis of the fluorescence responses indicates the
formation of 1:1 complex with Ba2+. The ionic diameter for Ba2+
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˚
(2.70 A) [26] is larger than the effective cavity diameter for aza-
˚
15-crown-5 (1.7–2.2 A). Thus 1 forms the intramolecular sandwich
complex with Ba2+ (1–Ba2+ in Scheme 2) as observed in the crystal
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We have synthesized
a novel receptor based on 3,4-
dimethylthieno[2,3-b]thiophene moiety for cation recognition. The
receptor 1 shows distinctive UV–vis and fluorescence responses for
Ba2+ due to restriction of the conformational change through the
formation of the intramolecular sandwich complex. Selectivity of
receptors would be tuned by substitution of aza-15-crown-5 moi-
eties to appropriate functional groups. Further studies on this line
are in progress. We believe that a variety of fluorophores can be
effectively used in different media (for example in water/organic
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We thank for financial support by the open project program of
Key Laboratory of Environmentally Friendly Chemistry and Appli-
cations of Ministry of Education China (No. 09HJYH04) and Hunan
Science & Technology Department Project (No. 2009FJ3166).
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[2] Other mechanistic bases for fluorescent chemosensors, are known (induced
charge transfer, excited-state proton transfer, e.g.). See Ref. [1].