Novel dithieno-benzo-imidazole-based Pb2+ sensors: Substituent effects on sensitivity and reversibility

Article abstract of DOI:10.1039/c2cc31131c

Two novel dithieno-benzo-imidazole-based compounds (M2 and A2) showed remarkable sensitivities towards Pb²⁺ by 12-fold enhancement and 10-fold decay of fluorescence, respectively, in aqueous solutions. Substituent effects of different dithieno-benzo-imidazole-based moieties (M1, M2, A1 and A2) on the quantum yields, fluorescence lifetimes and sensitivities to Pb ²⁺ along with the reversibilities by S^2- were investigated.

Full text of DOI:10.1039/c2cc31131c

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Novel dithieno-benzo-imidazole-based Pb2+ sensors: substituent effects
on sensitivity and reversibilityw
Rudrakanta Satapathy, Yen-Hsing Wu and Hong-Cheu Lin*
Received 15th February 2012, Accepted 13th April 2012
DOI: 10.1039/c2cc31131c
Two novel dithieno-benzo-imidazole-based compounds (M2 and A2)
showed remarkable sensitivities towards Pb2+ by 12-fold enhance-
ment and 10-fold decay of fluorescence, respectively, in aqueous
solutions. Substituent effects of different dithieno-benzo-imidazole-
based moieties (M1, M2, A1 and A2) on the quantum yields,
fluorescence lifetimes and sensitivities to Pb2+ along with the
reversibilities by S2ꢀ were investigated.
Owing to the biological, medicinal, oceanographic and environ-
mental toxicity of cations, numerous analytical methods, such
as neutron activation analysis, ion sensitivity (to electrodes),
flame photometry, atomic absorption spectrometry, electron
microprobe analysis and inductively coupled plasma-mass
spectroscopy, were developed for their detection.1 However,
due to the drawbacks such as expensiveness, time consumption,
difficulties in continuous monitoring and requirement of large
size samples in these detection methods, the methods based on
fluorescent sensors are preferable as they offer diverse advan-
tages in terms of sensitivity, selectivity, response time and local
observation (e.g., fluorescence imaging spectroscopy).2 Lead,
being a poisonous neurotoxin substance, damages the nervous
system. Excessive lead also causes cardiovascular, reproductive
and developmental disorders in mammals and brain disorder.3
Thus, development of selective and sensitive methods for the
detection of lead ions is extensively challenging for chemists.
For instance, fluorescence turn-on sensing mechanisms could be
explained in terms of intramolecular charge transfer (ICT)2
and/or chelation5 effects, and the aggregation induced by
quenchers is one of the rudimentary reasons for fluorescence
turn-off.5
Fig. 1 Schematic representation of the fluorescence off–on–off and
on–off–off mechanisms of M2 and A2, respectively, towards Pb2+ and S2ꢀ
.
Fig. 1 shows the fluorescence off–on–off and on–off–off mecha-
nisms for M2 and A2, respectively, towards Pb2+ and S2ꢀ
.
Synthetic procedures for M1, M2, A1 and A2 are depicted in
Scheme 1. The easiest way to prepare 3,30-bithiophene was
from 3-bromothiophene by treating with n-BuLi and Cu
powder. 3,30-Bithiophene was acylated by oxalic acid without
the aid of any Lewis acid to yield compound 3, which was
coupled with 4-nitrobenzaldehyde in the presence of ammonium
acetate in acetic acid to get compound 4. N-Alkylation of
compound 4 produced compound 5. M1 was prepared by
reduction of compound 5 by Pd/C and hydrazine. Finally,
N-methylation of M1 acquired M2. Furthermore, compound 3
was coupled with 4-cyanobenzaldehyde to get compound 6.
Recently several probes such as cyclen,4a imidazoquinoxaline,4b
imidazopyrene,4c imidazophenazine,4d rhodamine4e and calixarene4f
have been successfully reported for selective sensing of Pb2+
.
To the best of our knowledge, thieno-imidazole-based sensors
have not been reported so far. Therefore, two novel dithieno-
benzo-imidazole-based compounds M2 and A2 were synthe-
sized to find the effects of chelation by ‘N’ and ‘S’ linkages
on selectivity and sensitivity towards target metal ions.
Scheme 1 Reagents and conditions: (a) n-BuLi, THF, ꢀ78 1C to ꢀ60 1C,
CuCl2, ꢀ60 1C to rt, 18 h, 77.9%; (b) CO2Cl2, 1,2-DCE, 90 1C, 4 days,
64.65%; (c) NH4OAc, AcOH, 100 1C, overnight; (d) 1-iodohexane,
K2CO3, DMF, 95 1C, overnight, (5 = 80.4%, A1 = 82.4%); (e) Pd/C,
NH2–NH2ꢁH2O, reflux, 4 h, 89.9%; (f) iodomethane, THF, reflux, 72 h,
61.3%; (g) aq. NaOH, EtOH, 2 h, 94%.
Department of Materials Science and Engineering,
National Chiao Tung University, Hsinchu 30049, Taiwan, ROC.
E-mail: linhc@mail.nctu.edu.tw
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc31131c
c
5668 Chem. Commun., 2012, 48, 5668–5670
This journal is The Royal Society of Chemistry 2012

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Table 1 Photophysical properties of M1, M2, A1 and A2
Fluorescence
responseb to Pb2+
Recovery
by S2ꢀ
Compound
Fa
tc/ns
M1
M2
A1
0.42
0.04
0.16
0.26
Turn off (1.8-fold)
Turn on (12-fold)
Turn on (5.2-fold)
Turn off (10-fold)
No
Yes
Yes
No
1.85
0.62
0.98
1.57
A2
a
Quantum yields of M1, M2, A2 (in DMSO) and A1 (in THF), 9–10 DPA
in THF as a standard (F = 0.9). b M1, M2, A2 (in DMSO/H2O = 1/1)
and A1 (in THF/H2O = 1/1). Fluorescence lifetimes.
Fig. 2 Fluorescence spectral changes of (a) M2 (1.4 ꢂ 10ꢀ5 M) in
DMSO/H2O (1 : 1) (lex = 240 nm) and (b) A2 (1.4 ꢂ 10ꢀ5 M) in
DMSO/H2O (1 : 1) (lex = 265 nm) upon titration of Pb2+ (0–1.5 ꢂ
10ꢀ3 M). Insets show PL spectral responses of (a) M2 and (b) A2 as a
c
N-Alkylation of compound 7 afforded A1. Further hydrolysis
of A1 by aqueous sodium hydroxide yielded A2. According to
Scheme S1 (ESIw), a phenanthrene-benzo-imidazole analogue
M without a ‘S’ linkage was synthesized to compare its
sensitivity with M2 (with a ‘S’ linkage).
function of Pb2+
.
As shown in Table 1, the quantum yields of M1, M2, A1 and
A2 are compared. A strong ICT occurred in M2 due to the
further electron transfer (ET) via the negative inductive effect
caused by the electron withdrawing quaternary ammonium
group, thus reducing the quantum yield of M2. Unlike M2,
backward ET took place in M1 due to the positive mesomeric
effect caused by the lone-pair of electrons on the electron-
donating NH2 group and thus the ICT effect was minimized.5c
This in turn increased the quantum yield of M1 (0.42) signifi-
cantly, compared with M2 (0.04). The electron cloud shifting
can be further explained by the computational analysis
illustrated in Fig. S1 (ESIw). Furthermore, the ICT effects
were moderate in the case of A1 and A2. The COONa group in
A2 made it a weaker electron-withdrawing group than the
cyanide group in A1 due to the delocalization of electrons in
the acetate group. Thus, ICT was more prominent in A1 than
A2. As a result, the quantum yield of A2 (0.26) was higher
than A1 (0.16). The values of fluorescence lifetime (t) for all
compounds obtained from time-resolved fluorescence spectro-
scopy followed the trend M1 > A2 > A1 > M2 (see Table 1),
which is similar to the trend of the quantum yields. Thus, both
fluorescence quantum yields and lifetimes are dependent on
the intramolecular ICT occurring in the compounds.
Fig. 3 (a) Time-resolved fluorescence spectra of M2 (empty circle),
and M2 + Pb2+ (solid circle); (b) A2 (empty square) and A2 + Pb2+
(solid square).
of Pb2+ showed significant upfield shifts of peaks corre-
sponding to M2 (Fig. S5, ESIw). A Job’s plot for M2 by
taking the variation of the absorption at 406 nm as a function
of [Pb2+]/[M2] showed 1 : 1 stoichiometry and the detection
limit was obtained as 9.48 mM (Fig. S7, ESIw). Again, based on
the time-resolved fluorescence spectra (Fig. 3a), the fluores-
cence lifetime of M2 (0.62 ns) was elevated to 1.86 ns upon the
addition of Pb2+, which supports the turn-on mechanism. For
model compound M (where the binding site is only the ‘N’
1
atom on the imidazole ring), the H NMR and PL titrations
did not show any significant changes even at higher concen-
trations of Pb2+ (Fig. S8a and b, ESIw). Thus, it may be
concluded that the coordination of Pb2+ with both ‘S’ and ‘N’
atoms occurred in the dithieno-benzo-imidazole moiety of M2,
which was the key binding site causing chelation-induced
fluorescence enhancement.
Aqueous solutions of various metal ions such as Na+, K+,
Mg2+, Ca2+, Ba2+, Ag+, Co2+, Ni2+, Zn2+, Cu2+, Fe2+
,
Pb2+ and Hg2+ were added to the stock solutions of M2 and
A2. Their effects on fluorescence signals of M2 and A2
(for both single- and dual-metal systems) are depicted in
Fig. S3 and S4 (ESIw), respectively. Trivial fluorescent changes
However, the CHEF values could be amended by the
effective ICT in molecules due to their various substituents.
For instance, the ICT effect on A2 was enhanced owing to the
auxiliary electron withdrawing by the COONa group. Thus,
upon complexation of Pb2+ with A2, the fluorescence was
expected to be enhanced due to the obstruction of original
ICT. Surprisingly, the fluorescence intensity was quenched
upon the addition of Pb2+ (Fig. 4), which might be attributed
to the binding of the metal ions to the COOꢀ groups. Thus the
quencher induced aggregation plays a major role while com-
peting with obstruction of ICT causing quenching of fluores-
cence. A Stern–Volmer plot for the fluorescence quenching of
A2 indicated that the binding constant of Pb2+ for A2 would
decrease upon increasing the temperature which indicated the
static quenching mechanism (Fig. S9, ESIw).6 Furthermore,
from Fig. 3b, fluorescence lifetime of A2 (1.57 ns) decayed to
0.81 ns upon the addition of Pb2+, which was a further
evidence for the above-mentioned quenching mechanism.
were observed in M2 as other metal ions (Na+, K+, Mg2+
,
Ca2+, Ba2+, Co2+, Ni2+, Zn2+, Cu2+, Fe2+ and Hg2+) were
added, while a noteworthy enhancement of fluorescence intensity
(ca. 12-fold) in M2 was observed in the presence of Pb2+
(Fig. 2a). Whereas, A2 showed a quenching of fluorescence
(ca. 10-fold) in the presence of Pb2+ (Fig. 2b). In the case of
M2, the ICT effect was further enhanced by the ET effect due
to a strong electron-withdrawing quaternary ammonium salt
group which in turn minimized the quantum yield of M2.
Furthermore, upon the addition of Pb2+, both obstruction of
ICT and chelation-induced fluorescence enhanced the fluores-
cence intensity of M2 (chelation enhanced fluorescence factor,
CHEF = 12),4d and two new peaks at 403 and 451 nm
appeared consequently. 1H NMR titrations with 0–1 equiv.
c
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Chem. Commun., 2012, 48, 5668–5670 5669

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the color of the solution from light yellow to dark pink and
shifting of absorption maxima up to 85 nm (from 410 to 495 nm)
upon addition of 1 equiv. of Fꢀ ions to 4 (Fig. S15, ESIw).
In conclusion, two novel dithieno-benzo-imidazole-based
compounds M2 and A2 showed remarkable sensitivity towards
Pb2+ over the other metal ions. In the case of M2, the fluorescence
was almost 12-fold enhanced. However, the fluorescence of A2
was quenched almost 10-fold upon titration with Pb2+. Model
compound phenanthrene-benzo-imidazole-based M has almost no
momentous effects during sensing of Pb2+, which indicated the
unique sensitivity of dithieno-benzo-imidazole-based M2 and A2
towards Pb2+ via chelation with ‘S’ and ‘N’ atoms. The quantum
yields and fluorescence lifetime values followed the trend
M1 > A2 > A1 > M2, which was in consistence with their
intramolecular ICT fashion. In the case of M2 and A1, the
obstruction of ICT induced the enhancements of fluorescence
Fig. 4 Fluorescence recovery responses of M2–Pb and A2–Pb upon
titration with 0–0.1 equiv. of S2ꢀ (i.e., 0–1.4 ꢂ 10ꢀ6 M) w.r.t. the
concentration of M2/A2 (1.4 ꢂ 10ꢀ5 M) (M2: lex = 240 nm; A2: lex
=
265 nm).
As shown in Table 1 and Fig. S10(a) (ESIw), A1 showed a
turn-on response during sensing of Pb2+ similar to M2.
However, the sensitivity of A1 (CHEF = 5.2) was lower than
that of M2 (CHEF = 12). This is due to a stronger electron
withdrawing effect of the N+Me3 group than CNꢀ causing less
favourable ICT in A1 than M2.
owing to the binding of a thieno-imidazole unit to Pb2+
.
However, the quencher-induced aggregation played a major
role in the fluorescence quenching for A2 and M1 due to the
auxiliary binding of NH2 and COONa with Pb2+. Compared
with other anions, trace amounts of S2ꢀ induced reversible
binding effects of Pb2+ with both M2 and A1. Nevertheless,
the reversible binding effects of Pb2+ by adding S2ꢀ were not
observed for M1 and A2 due to stronger binding of Pb2+ with
NH2 and COONa groups, respectively.
Upon complexation of Pb2+ with M1, the fluorescence intensity
was quenched. This can be attributed to the amine group coordi-
nated with the Pb2+ ions to cause further electron-withdrawal and
thus to quench the fluorescence. Binding of the amine group with
metal ions was confirmed by proton NMR titrations (Fig. S11,
ESIw). Upon the addition of 10 equiv. of Pb2+, the amine peak at
5.6 ppm completely disappeared. Peaks at 6.72, 7.48, 7.75 and
7.85 ppm in M1 were shifted to 6.78, 7.55, 7.73 and 7.91 ppm,
respectively. Similar to A2, upon binding to Pb2+, although
ICT was obstructed for M1, the quencher-induced aggregation
played a major role in causing the fluorescence quenching.
The reversibility of PL for M2 upon binding to Pb2+ was
investigated by further addition of different anions, for example
The financial support of this project is provided by the
National Science Council of Taiwan (ROC) through NSC
99-2113-M-009-006-MY2 and National Chiao Tung University
through 97W807.
Notes and references
Clꢀ, HCO3ꢀ, HSO3ꢀ, HSO4ꢀ, Iꢀ, NO3ꢀ, OHꢀ, PO43ꢀ, S2O3
,
2ꢀ
1 (a) F. Teixidor, M. A. Flores, L. Escriche, C. Vinas and J. Casabo,
´
SCNꢀ and S2ꢀ (Fig. S12, ESIw). Among all these anions, the
enhanced PL of the M2–Pb complex was mainly annihilated
upon the addition of a diminutive amount of S2ꢀ (0.1 equiv. w.r.t.
the concentration of M2). A similar result has been obtained for
A1–Pb complex upon the addition of S2ꢀ. Fluorescence titrations,
by the addition of successive aliquots of S2ꢀ to the solutions of
M2–Pb and A1–Pb, are illustrated in Fig. 4a and Fig. S13a
(ESIw), respectively. To confirm the sensitivity of S2ꢀ towards
M2–Pb and A1–Pb complexes, fluorescence signal responses of
solo M2 and A1 towards S2ꢀ in the absence of Pb2+ were
obtained. As shown in Fig. S14a and b (ESIw), very irrelevant
changes in the fluorescence of M2 and A1 were observed upon the
addition of even higher concentrations of S2ꢀ (10 equiv. w.r.t.
stock solutions of M2 and A1). This experiment confirmed the
sensitivity of S2ꢀ towards Pb2+ only in their metal–ligand
complex state. Similarly, the effects of S2ꢀ on M1–Pb and
A2–Pb were tested as depicted in Fig. S13 (b) (ESIw), and
Fig. 4(b), respectively. It was found that the fluorescence of the
mixtures was further quenched upon the addition of S2ꢀ. This
can be attributed to the stronger binding of the NH2 (in M1) and
COOꢀ groups (in A2) to metal ions, which could not be cleaved
by S2ꢀ. The ICT was restored (in M1/A2) due to the breakage of
imidazole ‘N’ and ‘S’ linkages with Pb2+. Moreover, the addi-
tional enhancement of ICT, due to the auxiliary binding of metal
ions to amine/COONa, caused sheer quenching of fluorescence.
Again we observed that compound 4 showed the best
colorimetric and ratiometric sensing ability with Fꢀ via changing
Chem. Commun., 1994, 963; (b) S. Chung, W. Kim, S. B. Park,
I. Yoon, S. S. Lee and D. D. Sung, Chem. Commun., 1997, 965;
(c) K. Kimura, S. Yajima, K. Tatsumi, M. Yokoyama and M. Oue,
Anal. Chem., 2000, 72, 5290; (d) A. Ceresa, A. Radu, S. Peper,
E. Bakker and E. Pretsch, Anal. Chem., 2002, 74, 4027.
2 (a) A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. M.
Huxley, C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem.
Rev., 1997, 97, 1515; (b) V. Amendola, L. Fabbrizzi, M. Licchelli,
C. Mangano, P. Pallavicini, L. Parodi and A. Poggi, Coord. Chem.
Rev., 1999, 190–192, 649; (c) L. Prodi, F. Bolletta, M. Montalti and
N. Zaccheroni, Coord. Chem. Rev., 2000, 205, 59; (d) Q. Zhao,
F. Y. Li and C. H. Huang, Chem. Soc. Rev., 2010, 39, 3007;
(e) B. Valeur and I. Leray, Coord. Chem. Rev., 2000, 205, 3;
(f) A. K. Dwivedi, G. Saikia and P. K. Iyer, J. Mater. Chem.,
2011, 21, 2502; (g) B. Chetia and P. K. Iyer, Tetrahedron Lett., 2008,
49, 94.
3 J. S. Liu-Fu, Lead Poisoning:
A
Century of Discovery
and Rediscovery, in Human Lead Exposure, ed. H. L. Needleman,
Lewis Publishing, Boca Raton, FL, 1992.
4 (a) R. Zhou, B. Li, N. Wu, G. Gao, J. You and J. Lan, Chem.
Commun., 2011, 47, 6668; (b) M. Alfonso, A. T’arraga and
P. Molina, Dalton Trans., 2010, 39, 8637; (c) M. Alfonso,
A. Espinosa, A. T’arraga and P. Molina, Org. Lett., 2011,
13, 2078; (d) M. Alfonso, A. T’arraga and P. Molina, J. Org. Chem.,
2011, 76, 939; (e) Z.-Q. Hu, C.-S. Lin, X.-M. Wang, L. Ding,
C.-L. Cui, S.-F. Liu and H. Y. Lu, Chem. Commun., 2010,
46, 3765; (f) X.-L. Ni, S. Wang, X. Zeng, Z. Tao and T. Yamato,
Org. Lett., 2011, 13, 552.
5 (a) C. Qin, W. Y. Wong and L. Wang, Macromolecules, 2011,
44, 483; (b) C. Qin, Y. Cheng, L. Wang, X. Jing and F. Wang,
Macromolecules, 2008, 41, 7798; (c) M. Dekhtyar, W. Rettig and
W. Weigel, Chem. Phys., 2008, 344, 237.
6 S. Sirilaksanapong, M. Sukwattanasinitt and P. Rashatasakhon,
Chem. Commun., 2012, 48, 293.
c
5670 Chem. Commun., 2012, 48, 5668–5670
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