Fluorescence quenching of versatile fluorescent probes based on strongly electron-donating distyrylbenzenes responsive to aromatic chlorinated and nitro compounds, boronic acid and Ca2+

Article abstract of DOI:10.1016/j.saa.2007.03.024

The fluorescence quenching behavior of two distyrylbenzenes (DSBs) newly prepared bearing strongly electron-donating groups was investigated. Several chlorinated and nitro compounds quench the fluorescence of both DSBs with various efficiencies depending

Full text of DOI:10.1016/j.saa.2007.03.024

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 69 (2008) 167–173
Fluorescence quenching of versatile fluorescent probes based on strongly
electron-donating distyrylbenzenes responsive to aromatic chlorinated
and nitro compounds, boronic acid and Ca2+
Jiro Motoyoshiya∗, Zhu Fengqiang, Yoshinori Nishii, Hiromu Aoyama
Department of Chemistry, Faculty of Textile Science and Technology, Shinshu University, Ueda, Nagano 386-8567, Japan
Received 31 July 2006; accepted 15 March 2007
Abstract
Thefluorescencequenchingbehavioroftwodistyrylbenzenes(DSBs)newlypreparedbearingstronglyelectron-donatinggroupswasinvestigated.
Several chlorinated and nitro compounds quench the fluorescence of both DSBs with various efficiencies depending on the electron-withdrawing
properties; the strongly electron-withdrawing compound, such as 2,4,5-trichlorophenol and 2,4-dinitrofluorobenzene, effectively diminish the
fluorescence intensities of the DSBs. The fluorescence quenching was also detected in the interaction between the azacrown DSB and phenylboronic
acid, while the fluorescence recovers by adding triethylamine. These quenching phenomena are attributed to the photo-induced electron transfer
(PET) process. On the other hand, the azacrown DSB selectively interacts with Ca2+ to decrease its fluorescence intensity, but the DSB with the
dimethylamino groups did not. These results suggest a potential use of these types of compounds as sensors for strongly electron-withdrawing
substances and a suitable metal cation.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Distyrylbenzene; Fluorescence quenching; Azacrown ether; Photo-induced electron transfer; Metal cation recognition
1. Introduction
livesastherelicsofagriculturalchemicalsorexplosives[17–20].
Therefore, the development of a method for their accurate
and rapid detection is required. Some chemosensory devices
for this purpose utilize the fluorescence quenching phenomena
attributed to the electronic interaction between the strongly flu-
orescent compounds and the target molecules that are strongly
electron-withdrawing. On the other hand, some boron com-
pounds, such as boric acids and their esters, are also known
to act as fluorescence quenchers and interact with the ligands
such as anions, amines and alcohols, changing the orbital from
sp2 to sp3 [21,22], which led to the development of many boron
complexes bearing a fluorophore or chromophore as sensing
agents for inorganic anions and sugars [11,23,24,25]. The fluo-
rescent response of these sensing agents is based on the change
in the efficiency of the PET process between the fluorophores
and boron moieties or the energy transfer from the donor to the
acceptor.
Considerable attention has been paid to the fluorescent sen-
sors responsive to a wide range of substances including metal
ions, and organic and inorganic compounds [1–7], some of
which are based on the inhibition of a photo-induced electron
transfer (PET) process caused by the interaction of the electron-
donor sites and the target substances [8–10]. Various fused
aromatic compounds with a strong fluorescence such as pyrene,
anthracene, etc. have been frequently used for the fluorescent
sites [11,12]. In general, distyrylbenzenes are strongly fluores-
cent and very versatile compounds because of availability of a
numberofderivativeswithvariouspropertiessuchasabsorption,
fluorescence, oxidation potential, and solubility toward various
media, which can be controlled by the electronic nature of the
substituens on the distyrylbenzenes [13–16].
Detection of chlorinated or nitro compounds in the environ-
ments is a very important problem because they affect human
In this paper, we report the spectral properties of two strongly
electron-donating and fluorescent distyrylbenzenes (1 and 2)
bearing two alkoxy groups on the central benzene rings and
amino groups on each terminal benzene ring (Fig. 1). Based
on such properties, they will interact with electron-accepting
∗
Corresponding author. Tel.: +81 268 21 5402; fax: +81 268 21 5391.
E-mail address: jmotoyo@giptc.shinshu-u.ac.jp (J. Motoyoshiya).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.03.024

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J. Motoyoshiya et al. / Spectrochimica Acta Part A 69 (2008) 167–173
(2.00 g, 3.15 mmol) in DMF (20 mL) and stirred for 30 min at
room temperature. To the slurry was added dropwise a solution
of 4-N-dimethylaminobenzaldehyde (0.78 g, 5.23 mmol) in
DMF (10 mL). After being stirred for 8 h at room temperature,
the solvent was removed by distillation under a reduced
pressure and then a saturated ammonium chloride solution was
added to the residue. The product was extracted with ether and
the organic phase was dried over anhydrous Na2SO4. After
removal of the solvent, the product was purified by column
chromatography on silica gel with benzene as an eluant to give
1 (0.62 g, 38%) as an oil. 1H NMR (400 MHz, CDCl3) δ 0.90t,
6H, J = 7.2 Hz, 0.99 (t, 6H, J = 7.2 Hz), 1.34–1.60 (m, 16H),
1.75–1.83 (m, 2H), 2.98 (s, 12H), 3.93 (d, 4H, J = 5.2 Hz),
6.72 (d, 4H, J = 8.4 Hz), 7.05 (d, 2H, J = 16 Hz), 7.09 (s, 4H),
7.29 (d, 2H, J = 16 Hz), 7.41 (d, 2H, J = 8.40 Hz). 13C NMR
(100 MHz, CDCl3) δ 11.82, 14.60, 23.43, 24.15, 30.33, 30.75,
38.05, 43.62, 77.12, 112.45, 113.08, 120.43, 124.42, 124.83,
126.21, 127.12, 143.76, 148.07.
Fig. 1. Structures of distyrylbenzene 1 and 2.
substances such as chloro, nitro and boronic compounds. Since
the azacrown moiety is known to act as a recognition site for a
suitable metal cation [26–28], the interaction of the distyrylben-
zenes with the azacrown moieties will form a chemosensor for a
suitable metal cation. We show that these types of distyrylben-
zenes have the possible potential of being used as chemosensors
for suitable substances.
2.3. Synthesis of (E,E)-2,5-bis(2ꢀ-ethylhexyloxy)-1,4-
bis[4ꢀꢀ-(N-monoaza-15-crown-5)-styryl]benzene (2)
This compound was prepared similarly to the procedure
described above using tert-BuOK (0.71 g, 6.33 mmol), 2,5-bis
(2ꢀ-ethylhexyloxy)-1,4-bis(diethylphosphonomethyl)benzene
(1.00 g, 1.57 mmol) and 4-(N-monoaza-15-crown-5)benzal-
2. Experimental
1
2.1. Reagents and apparatus
dehyde (0.75 g, 2.32 mmol) in 45% (0.51 g) yield. H NMR
(400 MHz, CDCl3) δ 0.88t, 6H, J = 7.3 Hz), 0.94 (t, 6H,
J = 7.3 Hz), 1.34–1.38 (m, 16H), 1.75–1.83 (m, 2H), 3.62–3.69
(m, 32H), 3.78 (t, 8H, J = 6.0 Hz), 3.93 (d, 4H, J = 5.2 Hz),
6.45 (d, 4H, J = 9.2 Hz), 7.02 (d, 2H, J = 16 Hz), 7.26 (s,
4H), 7.27 (d, 2H, J = 16 Hz), 7.38 (d, 2H, J = 8.40 Hz). 13C
NMR (100 MHz, CDCl3) δ 11.75, 14.54, 23.52, 24.66, 29.69,
31.33, 40.25, 52.98, 69.02, 70.59, 70.61, 71.73, 72.17, 110.24,
112.07, 119.51, 126.64, 127.17, 128.07, 128.53, 147.42,
151.29.
Solvents were purified and dried by the standard method. Two
distyrylbenzenes were synthesized as described bellow. Phenyl-
boronic acid was prepared according to the known method
[29]. Other chemicals were used as purchased. 1H and 13C
NMR spectra were recorded on a Bruker AVANCE-400 at
400 MHz and 100 MHz, respectively. The chemical shifts (δ)
are reported in ppm downfield from TMS as internal standard
or from the residual solvent peak. Coupling constants (J) are
reported in Hz. Analytical TLC was carried out on precoated
silica gel 60F-254 plates (E. Merck). Column chromatogra-
phy was performed on silica gel (E. Merck). Absorption and
fluorescence spectra were recorded on a U-3310 spectrom-
eter (Hitachi) and on a RF-5000 spectrometer (Shimadzu),
respectively. Fluorescence quantum yields were estimated using
9,10-diphenylanthracene (ΦF = 0.91 in benzene) as a standard.
Cyclic voltammetry was performed at room temperature with
a three-compartment cell in dry acetonitrile solution contain-
ing the substrate (ca. 10−4 M) and a supporting electrolyte
(0.1 M tetrabutylammonium perchlorate). Pt disk, Pt wire, and
saturated calomel electrode (SCE) were used as the working,
counter, andreferenceelectrodes, respectively. Thescanratewas
3. Result and discussion
3.1. Synthesis and spectra of distyrylbenzenes 1 and 2
The distyrylbenzenes 1 and 2 were prepared by a modi-
fication of the Wittig olefination reaction using a 1,4-
bis(diethylphosophonomethyl)benzene and the corresponding
aromatic aldehydes in the presence of tert-BuOK. Their geo-
metric structures were identified as all E-forms, and as shown
in Fig. 2a, they exhibit absorptions at 418 and 422 nm with
the large molecular extinction coefficients as log ε: 4.56 and
4.63 and intense fluorescence with the maximum emission
wavelengths located at 482 and 485 nm with the fluorescence
quantum yields of 0.48 and 0.51 for 1 and 2 in THF, respectively.
When comparing their spectra of the parent 4,4ꢀ-distyrylbenzene
without any substituent that has an absorption at 357 nm and
fluorescence at 414 nm (in dioxane) [30], those of 1 and 2
significantly shift to the long wavelength regions, which is
attributed to the presence of the terminal amino groups because
4,4ꢀ-bis(4ꢀꢀ-dimethylaminostyryl)benzene, lacking two alkoxy
100 mV s−1
.
2.2. Synthesis of (E,E)-2,5-bis(2ꢀ-ethylhexyloxy)-1,4-
bis(4ꢀꢀ-N-dimethylstyryl)benzene (1)
To a suspension of tert-BuOK (1.41 g, 12.60 mmol) in
DMF (10 mL) was added a solution (20 mL) of 2,5-bis
(2ꢀ-ethylhexyloxy)-1,4-bis(diethylphosphonomethyl)benzene

J. Motoyoshiya et al. / Spectrochimica Acta Part A 69 (2008) 167–173
169
Fig. 2. Absorption and fluorescence spectra. (a) Absorption and fluorescence spectra of 1 (dashed line) and 2 (solid line) in THF (1 × 10−5 M). (b) 1–5: the
fluorescence spectra of 2 (1 × 10−6 M) in THF, benzene, EtOH, MeOH, CF3CH2OH. (c) 1–4: the fluorescence spectra of 2 (1 × 10−6 M) in THF, THF:H2O = 4:1,
THF:H2O = 1:4, THF:H2O = 1:9.
groups, absorbs at 398 nm and emits at 463 nm [30,31], in almost
the same regions as 1 and 2. A spectral change depending on
the property of the solvents was also observed. For instance,
almost the same spectra for 2 were obtained in THF and in
benzene, while the alcoholic solvents affect the spectra to give
structureless ones with a decreasing intensity (Fig. 2b). Inter-
estingly, the spectra of 2 measured in aqueous THF change
in its structure with a decreasing intensity as the ratio of the
water increases (Fig. 2c), which indicates that the hydrogen-
bond formation at the amino group sites leads to the change
in the ratio of the emission for each transition. Such spectral
behavior was also observed for 1. Thus, these distyrylbenzenes
are sensitive to the environments around them and therefore
expected to respond to the change in the chemical situa-
tion.
3.2. Fluorescence quenching by chlorinated aromatic
compounds
When distyrylbenzenes 1 and 2 were dissolved in a mix-
ture of chlorobenzene and THF, their fluorescence intensity
decreased depending on the amount of chlorobenzene (Fig. 3a
and b). The fluorescence quenching effect of dichlorobenzene
was greater than chlorobenzene, and in this case, the azacrown
2 was quenched much more effectively than 1 (Fig. 3c and
d). No structural change of the fluorophores in the solution
of the mixtures was confirmed by 1H NMR spectrum. The
measurements with other chlorinated compounds are summa-
rized in Fig. 4. The stronger the electron-withdrawing character
of the coexisting chlorinated compounds, the weaker the flu-
orescence; 2,4,6-trichlorophenol was the strongest quencher
Fig. 3. Fluorescence spectra of 1 (a, c) and 2 (b, d) in the presence of chlorobenzene and o-dichlorobenzene, [1] = [2] = 1 × 10−6 M in THF. (a) and (b) 1–5: The
concentrations of chlorobenzene are in the order of 0 M, 5 × 10−3 M, 1 × 10−2 M, 2 × 10−2 M, 3 × 10−2 M. (c) and (d) 1–5: The concentrations of o-dichlorobenzene
are in the order of 0 M, 5 × 10−3 M, 1 × 10−2 M, 2 × 10−2 M, 3 × 10−2 M, respectively.

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J. Motoyoshiya et al. / Spectrochimica Acta Part A 69 (2008) 167–173
Fig. 4. Quenching efficiency of 1 and 2 in THF (1 × 10−6 M) vs. the concen-
tration of chlorobenzene, o-dichlorobenzene and 2,4,6-trichlorophenol.
Fig. 6. Quenching efficiency of 1 and 2 in THF (1 × 10−5 M) vs. the concentra-
tion of nitroethane, p-nitrophenol, nitrobenzene, and 2,4-dinitrofluorobenzene.
of all those used in this study (Fig. 4). On the other hand,
carbon tetrachloride and chloroform also reduced the fluo-
rescence as a result of the photochemical conversion of 1
and 2 into the non-fluorescent adducts. Such the fluores-
cence quenching by carbon tetrachloride has been documented
in the aromatic fluorescent materials such as anthracene, in
which the radical species generated from carbon tetrachlo-
ride by the photochemical process reacted with anthracene
[32].
nitroethane also interacts with the excited state of 1. Other
nitro compounds, such as nitrobenzene, 4-hydroxynitrobenzene,
and 2,4-dinitrofluorobenzene, act as quenchers (Fig. 6) and the
quenching efficiency increases with the increasing electron-
accepting property of the nitro compounds, among which
2,4-dinitrofluorobenzene is the most effective. A similar, but
more effective fluorescence quenching was observed for 2 as
can be seen in Fig. 6. As the previously reported chemosen-
sory effect for nitro compound that consisted of the fluorescent
poly(phenylene vinylene) with the alkoxy groups is known to be
highly sensitive, application of the present DSBs as the fluores-
cent units would be expected to provide a more effective sensory
effect.
3.3. Fluorescence quenching by nitro compounds
The fluorescence intensity of 2 in THF also gradually
decreased with the increasing concentration of nitroethane.
The concentration dependence of the quenching efficiency was
described as a result of the decrease in both the excitation and
fluorescence intensities (Fig. 5a). The fluorescence is almost
completely quenched when the concentration of nitroethane
exceeds 80 mg/mL in a 1 × 10−6 M solution of 1 in THF.
On the other hand, the absorption of 1 at 418 nm does not
change in the presence of nitroethane (Fig. 5b), indicating that
3.4. Fluorescence quenching by the phenylboronic acid
Analogous to the nitro compounds, phenylboronic acid also
interacts with 2, but not with 1. Fig. 7a shows the fluores-
cence quenching of 2 by phenylboronic acid that depends on
the concentration of the quencher. However, the fluorescence
was not completely quenched even when excess phenylboronic
Fig. 5. Absorption and fluorescence spectra of 1 in the presence of nitroethane, [1] = 1 × 10−5 M in THF. (a) 1ꢀ–6ꢀ: The excitation spectra, 1–6: the fluorescence
spectra. (b) The absorption spectra. (a) and (b) The concentrations of nitroethane are in the order of 0 M, 2 × 10−2 M, 5 × 10−2 M, 0.1 M, 0.2 M, 1 M.

J. Motoyoshiya et al. / Spectrochimica Acta Part A 69 (2008) 167–173
171
Fig. 7. Fluorescence spectra of 2 in the presence of phenylboronic acid, [2] = 1 × 10−5 M in CH3CN. (a) 1–6: The concentrations of phenylboronic acid are in the
order of 0 M, 2 × 10−2 M, 5 × 10−2 M, 0.1 M, 0.2 M, 0.5 M. (b) 1–6: The concentration of phenylboronic acid is 0.5 M and the concentrations of triethylamine are
in the order of 0 M, 2 × 10−2 M, 5 × 10−2 M, 0.1 M, 0.2 M, 0.5 M.
acid was added. When triethylamine was added to the quenched
solution of 2 by phenylboronic acid, the fluorescence recov-
ered (Fig. 7b). In this case, triethylamine would acts as an
electron-donor rather than a base as mentioned below. This flu-
orescence property, quenching and recovery, might provide a
possibility of 2 to be used as a sensor for substances that interact
with boronic acid derivatives to set off their electron-accepting
abilities.
3.6. Fluorescence quenching mechanism
The possible mechanism of the fluorescence quenching
observed in the present study is also regarded to involve a PET
process. Considering that there is no change in the absorption
spectra of both solutions of 1 and 2 containing the chlorinated or
nitro compounds, the quenching phenomenon is due to the elec-
tronic interaction between the fluorophores in the excited states
as the donors and the chlorinated or nitro compounds as the
acceptors. To explore this hypothesis, convenient tests were car-
ried out employing p-benzoquinone, a strong electron-acceptor
and p-dimethoxybenzene, a strong electron-donor. As expected,
p-benzoquinone also effectively quenched the fluorescence of
1, while the fluorescence recovered when p-dimethoxybenzene
was added to the non-fluorescent solution containing 1 and
nitroethane, supporting the involvement of a PET process. We
also investigated the electrochemical properties in order to dis-
tinguish the difference in the electron-donating abilities between
1 and 2 by cyclic voltammetry (CV) measurements. The fluores-
cence of the former was not quenched by phenylboronic acid, but
that of the latter decreased in the presence of the quencher. The
half-wave oxidation potentials (E1/2) of 1 and 2 are 0.41 V and
0.49 V (vs. SCE), respectively, which suggests that the azacrown
moiety is inclined to release an electron more easily than the
3.5. Recognition of Ca2+ by the fluorescence quenching
The change in the electron density of the terminal nitrogen
atom affects the fluorescence because a significant decrease in
the intensity was observed in the acidic medium; for instance, the
fluorescence completely disappeared when the nitrogen atoms
were protonated by hydrochloric acid. Accordingly, there is
the possibility of metal cation recognition using 2; namely, if
the electron-donating character of the terminal azacrown moi-
eties modulates the fluorescence intensity of 2, chelation by
a suitable metal cation that fits the 15-azacrown-5 can serve
as a signal of metal cation recognition. Among several metal
cations, such as Li+, Na+, K+, Rb+, Ca2+ and Pd2+, only Ca2+
was found to effectively reduce the fluorescence intensity of
2 in acetonitrile. Absence of the spectral change by Ca2+ in 1
supports the fact that the azacrown moieties work as the recog-
nition sites. The cavity size of the 15-azacrown-5 fits Na+ in
general, but it is known that the 15-azacrown-5 moiety forms
a complex with Ca2+ [33,34]. Fig. 8 shows the fluorescence
spectra of 2 in the presence of calcium perchlorite. The fluores-
cence intensity decreased with the increasing amount of Ca2+,
but did not completely disappear even when excess Ca2+ was
added. A decrease in the fluorescence intensity can be detected
when more than 15 times the amount of Ca2+ versus 2 was
present. A slight red-shift of the remaining fluorescence spec-
trum indicated that the complex of 2 and Ca2+ also has a weaker
fluorescence than the free 2 in almost the same region and the
spectral change of 2 observed in the presence of Ca2+ is the
result of the formation of the weakly fluorescent complex of 2
with Ca2+.
Fig. 8. Fluorescence spectra of 2 in the presence of Ca2+, [2] = 1 × 10−6 M in
CH3CN. (a) 1–7: The concentrations of Ca(ClO4)2 are in the order of 0 M,
5 × 10−6 M, 1 × 10−5 M, 2 × 10−5 M, 3 × 10−5 M, 4 × 10−5 M, 5 × 10−5 M.

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J. Motoyoshiya et al. / Spectrochimica Acta Part A 69 (2008) 167–173
Fig. 11. The relationship between K(Imax − I0)/I0 and reduction potentials (E)
Fig. 9. Theoretical fit of the equation to I0/(I − I0) versus [A]−1. A is the con-
of the nitro compounds.
centration of the nitro compounds.
acceptor. The relative value of K can be estimated without
knowing Imax, because the first term on the right side of the equa-
tion must be constant. Using the data of Figs. 4 and 6, a plot of
I0/(I − I0) versus [A]−1 gave a good linear relationship as shown
in Figs. 9 and 10. The good fit to the equation reveals that 1 or 2
forms a 1:1 stoichiometric complex with the electron-acceptors,
which indicates that electron-acceptors interact at one group
of the excited 1 or 2 undergoes fluorescence quenching. The
relationship between the reduction potentials of the electron-
acceptors, the standard of the electron-withdrawing ability, and
the reciprocal values of the slope estimated from Figs. 9 and 10,
K(Imax − I0)/I0, is shown in Figs. 11 and 12. The linear relation-
ship shows that the fluorescence quenching constants are closely
rationalized to the electron-accepting ability of the quenchers
and strongly suggests the PET process is concerned to fluores-
cence quenching observed in the present system [36].
dimethylamino group and explains the difference in the spectral
behavior in the presence of the quenchers.
When the relation between the fluorescence intensity and the
concentration of the electron-acceptors is supported by the fol-
lowing equations for the complex with a 1:1 stoichiometry, the
association constant, K, for the equilibrium can be related to the
concentration of the electron-acceptor [A] as expressed by the
following equation [35].
ꢀ
ꢁ
I0
I0
1
=
+ 1
I − I0
Imax − I0 K[A]
where I0 and I are the fluorescence intensity monitored in the
absence and presence of the electron-acceptor, respectively, and
[A] is the total concentration of the electron-acceptor. Imax is the
fluorescence intensity of the fully interaction with the electron-
Fig. 10. Theoretical fit of the equation to I0/(I − I0) versus [A]−1. A is the
Fig. 12. The relationship between K(Imax − I0)/I0 and reduction potentials (E)
concentration of the chlorinated compounds.
of the chlorinated compounds.

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173
4. Conclusion
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We have successfully evaluated the interesting spectral
behavior of two strongly electron-donating distyrylbenzenes, 1
and 2, whose fluorescence is quenched by electron-withdrawing
compounds, such as chlorinated and nitro compounds, as well as
phenylboronic acid. Recognition of Ca2+ by the 15-azacrown-5
moiety was also conducted by the selective complex formation,
which results in a reduced the fluorescence intensity. Con-
sidering the relationship between the fluorescence quenching
constants and the reduction potentials of the quenchers, the
present quenching phenomena are due to the PET process.
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This work was partially supported by the CLUSTER of the
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