A phenylbenzoxazole-amide-azacrown linkage as a selective fluorescent receptor for ratiometric sensing of Pb(ii) in aqueous media

Article abstract of DOI:10.1039/c3cc41151f

A phenylbenzoxazole-amide-azacrown linkage (L) acts as a fluorescent receptor for ratiometric Pb²⁺ sensing in aqueous media. This is promoted by the excited state intramolecular proton transfer (ESIPT) of the amide proton to benzoxazole nitrogen via coordination of Pb²⁺ with amide oxygen.

Full text of DOI:10.1039/c3cc41151f

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A phenylbenzoxazole–amide–azacrown linkage as a
selective fluorescent receptor for ratiometric sensing of
Pb(II) in aqueous media†
Cite this: Chem. Commun., 2013,
49, 3434
Received 12th February 2013,
Accepted 8th March 2013
Yasuhiro Shiraishi,* Yoshinao Matsunaga, Prannicha Hongpitakpong and
Takayuki Hirai
DOI: 10.1039/c3cc41151f
www.rsc.org/chemcomm
A phenylbenzoxazole–amide–azacrown linkage (L) acts as a fluorescent
receptor for ratiometric Pb2+ sensing in aqueous media. This is
promoted by the excited state intramolecular proton transfer (ESIPT) of
the amide proton to benzoxazole nitrogen via coordination of Pb2+ with
amide oxygen.
Pb2+ is one of the most toxic heavy metal cations. Intake of even very
small amounts of Pb2+ causes several health problems such as
memory loss, anemia, and slow nerve conduction velocity in
children.1 The Centers of Disease Control (CDC) reports that more
than 100 mg LÀ1 (0.48 mM) for Pb in the blood is very toxic to children.2
Accurate and quantitative detection of trace levels of Pb2+ in water with
neutral pH is therefore necessary. Design of fluorescent receptors for
Pb2+ has attracted much attention since they facilitate easy and rapid
detection by simple fluorescence analysis. Early-reported Pb2+ recep-
tors, however, suffer from several problems: (i) they exhibit poor
selectivity for Pb2+,3 (ii) they act only in organic media,4 and (iii) they
require long time for sensing (>10 min).5 Another desirable property
for accurate sensing is the ratiometric fluorescence response to Pb2+.6
This facilitates accurate quantification of Pb2+ just by monitoring the
intensity ratio of two fluorescence bands, without the effects of
instruments and receptor concentrations. However, to the best of
our knowledge, there is no report on fluorescent Pb2+ receptors
combining these functionalities.
Scheme 1 Emission mechanism for phenylbenzoxazole derivatives.
assigned to the emission from the H-transferred excited state
formed by ESIPT from the normal excited state.
We synthesized a phenylbenzoxazole-based receptor with an
azacrown moiety, which is often used for Pb2+ sensing.4a,b The
phenylbenzoxazole–amide–azacrown linkage (L, Scheme 2) acts as
a fluorescent receptor for selective, rapid, and ratiometric Pb2+
sensing in aqueous media. L exhibits two weak emissions from the
normal and H-transferred excited states. Addition of Pb2+ selectively
enhances the longer-wavelength emission within only 2 seconds.
This thus allows rapid and accurate quantification of Pb2+ based on
the intensity ratio of the longer- and shorter-wavelength emissions.
Receptor L was obtained with 52% yield as a yellow solid, by
condensation of chloroacetamide-modified phenylbenzoxazole with a
1-aza-18-crown 6-ether (see ESI†). The purity of Lwas confirmed by 1H,
13C NMR, and FAB-MS analysis (Fig. S1–S3, ESI†). Fig. 1 shows the
Our strategy for ratiometric Pb2+ sensing is based on the excited
state intramolecular proton transfer (ESIPT), occurring on the
phenylbenzoxazole dyes with an –OH or an –NH2 group at the ortho
position of the phenyl ring.7 As shown in Scheme 1, the proton of
these groups associates with benzoxazole nitrogen via a H-bonding
interaction in the ground state. These photoexcited dyes exhibit
fluorescence bands at 350–400 and 500–550 nm, respectively. The
shorter-wavelength fluorescence is assigned to the emission from
the normal excited state, and the longer-wavelength fluorescence is
Research Center for Solar Energy Chemistry, and Division of Chemical Engineering,
Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan.
E-mail: shiraish@cheng.es.osaka-u.ac.jp; Fax: +81668506273; Tel: +81668506271
† Electronic supplementary information (ESI) available: Experimental and sup-
plementary data (Tables S1 and S2 and Fig. S1–S12), and cartesian coordinates for
compounds. See DOI: 10.1039/c3cc41151f
Scheme 2 Structure of receptor L.
c
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Table 1 Interfacial plots of molecular orbitals for L and the L–Pb2+ complex, and
optimized structures for their phenylbenzoxazole-amide moietya,b
L
L–Pb2+ complex
Fig. 1 (a) Fluorescence spectra (lex = 311 nm) of L (20 mM) measured in a
buffered water–MeCN mixture (1/1 v/v; HEPES 100 mM; pH 7.0) with each
respective metal cation (1 equiv.) at 298 K, after stirring the solution for 3 min. (b)
Fluorescence intensity monitored at 508 nm. Absorption spectra for respective
solutions are summarized in Fig. S4 (ESI†).
LUMO (À1.55 eV)
LUMO (À2.01 eV)
fluorescence spectra (lex = 311 nm) of L (20 mM) measured in a
buffered water–MeCN mixture (1/1 v/v; pH 7.0) at 298 K. L itself shows
two weak fluorescence bands at 378 and 508 nm, assigned to the
emissions from normal and H-transferred excited states, respectively.
The emission quantum yield is relatively low (FF = 0.011) because the
photoinduced electron transfer (PET)8 from azacrown nitrogen to the
excited state phenylbenzoxazole moiety quenches fluorescence. This
is confirmed by ab initio calculation based on the time-dependent
density functional theory (TDDFT) within the Gaussian 03 program.
As shown in Table S1 (ESI†), the singlet electronic transition of L is
mainly contributed by HOMO À 1 - LUMO (S0 - S2) transition. Its
energy is 3.83 eV (324 nm), which is very close to the observed
absorption maximum (303 nm; Fig. S4, ESI†). As shown in Table 1,
electrons of HOMO À 1 and LUMO are located on the phenylbenzox-
azole moiety, but electrons of HOMO lie on the azacrown nitrogen.
This clearly suggests that PET from azacrown nitrogen to the excited
state phenylbenzoxazole moiety indeed quenches fluorescence.
As shown in Fig. 1, addition of Pb2+ (1 equiv.) to the solution
containing L increases both emissions (FF = 0.053). In contrast,
other metal cations do not promote any spectral change. This
indicates that L shows Pb2+-selective fluorescence enhancement. It
is noted that the fluorescence enhancement upon addition of Pb2+
terminates within only 2 seconds (Fig. S5, ESI†), which is the shortest
among the Pb2+ receptors reported earlier.3–6 It is also noted that, as
shown in Fig. S6 (ESI†), the emission enhanced upon addition of
HOMO (À5.22 eV)
HOMO (À6.34 eV)
HOMO À 1 (À5.88 eV)
HOMO À 1 (À6.91 eV)
a
Gray, white, blue, red, and pale blue atoms indicate C, H, N, O, and Pb
atoms. Green and deep red parts on the molecular orbitals refer to the
different phases of molecular wave functions (isovalue: 0.02 a.u.). b Azacrown
moieties of molecular frameworks in the final column are omitted for clarity.
Pb2+ is scarcely affected by many other metal cations. This indicates [L + Pb2+ À H+]+ ion. Adding an excess amount of EDTA (20 equiv.) to
that L coordinates with Pb2+ more strongly and enables selective the resulting solution shows a fluorescence spectrum similar to that
Pb2+ sensing even in the presence of these cations. Addition of Cu2+ obtained without the metal cation (Fig. S9, ESI†), indicating that L
or Hg2+, however, decreases the emission significantly. This is reversibly coordinates with Pb2+ in a 1 : 1 stoichiometry. As shown in
probably because, as observed in related systems,9 Cu2+ or Hg2+ Fig. S10 (ESI†), the emission enhancement by Pb2+ takes place at pH
coordinates with the azacrown moiety more strongly than Pb2+.
5–9. Mole fraction distribution of the species determined by poten-
Fig. 2a shows the change in fluorescence spectra of L with an tiometric titration (Table S2, ESI†) revealed that the emission
amount of Pb2+ added. Stepwise addition of Pb2+ increases the enhancement is consistent with the formation of the L–Pb2+
fluorescence intensity, and the increase terminates upon addition complex (pH 5–9). These data clearly indicate that the formation
of 1 equiv. of Pb2+. This implies that L coordinates with Pb2+ in a 1 : 1 of the L–Pb2+ complex triggers fluorescence enhancement.
stoichiometry. This is confirmed by Job’s plot analysis (Fig. S7, ESI†).
A very important feature of L is the ratiometric response to Pb2+.
The fluorescence intensity shows a maximum at the mole fraction As shown in Fig. 2a, addition of Pb2+ selectively enhances the longer-
0.5, clearly indicative of 1 : 1 coordination. ESI-MS analysis further wavelength emission, whereas the increase in shorter-wavelength
confirms this (Fig. S8, ESI†); a water–MeCN (1/1 v/v) mixture contain- emission is much lower. As shown in Fig. 2b, the ratiometric plot of
ing L with 1 equiv. of Pb2+ shows a peak at m/z 720.1, assigned to the the intensity ratio of the two emission bands versus the Pb2+
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for L is À0.638, but that for the L–Pb2+ complex is more positive
(À0.586), indicating that coordination of amide oxygen with Pb2+
indeed decreases the electron density of amide nitrogen. As shown
in Table 1 (bottom), the bond length of Namide–H for L is 1.02 Å, and
the distance between benzoxazole N and amide H (NbenzoxazoleÁÁÁH)
is 1.93 Å. In contrast, the bond length of Namide–H for L–Pb2+
(1.04 Å) is longer than that for L (1.02 Å), and the NbenzoxazoleÁÁÁH
distance (1.75 Å) is shorter than that for L (1.93 Å). This indicates
that weakened Namide–H interaction by the coordination with Pb2+
results in strong NbenzoxazoleÁÁÁH association. This enhances ESIPT
and efficiently produces the H-transferred excited state, resulting in
selective enhancement of longer-wavelength emission.
In conclusion, we found that a phenylbenzoxazole–amide–
azacrown linkage (L) acts as a selective fluorescent receptor for
ratiometric sensing of Pb2+ in aqueous media.11 L detects Pb2+
rapidly and enables accurate quantification of very low levels of
Pb2+. The molecular design presented here, which involves the
coordination between amide oxygen and Pb2+ controlling the
ESIPT efficiency, may contribute to the design of more efficient
and useful fluorescent receptors for Pb2+ in aqueous media.
This work was supported by the Grant-in-Aid for Scientific
Research (No. 23656504) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan (MEXT).
Fig. 2 (a) Fluorescence spectra (lex = 311 nm) of L (20 mM) in a buffered water–
MeCN mixture (1/1 v/v; HEPES 100 mM; pH 7.0) with Pb2+. (b) Ratiometric plot of
the intensity ratio of the two emission bands versus the Pb2+ concentration.
Notes and references
concentration exhibits a linear relationship in the range of
0.16–20 mM. This indicates that L enables quantification of Pb2+
in this concentration range by ratiometric analysis. The detection
limit, 0.16 mM, is sufficiently below the dangerous level of Pb2+ in
children’s blood (0.48 mM) as reported by CDC,2 suggesting that L
enables sensitive Pb2+ detection.
1 J. S. Lin-Fu, Lead Poisoning, A Century of Discovery and Rediscovery, in
Human Lead Exposure, ed. H. L. Needleman, Lewis Publishing, Boca
Raton, FL, 1992.
2 Centers for Disease, Control, and Prevention, Preventing lead poison-
ing in young children, Atlanta, GA, 2005.
3 (a) M. Arduini, F. Mancin, P. Tecilla and U. Tonellato, Langmuir,
2007, 23, 8632–8636; (b) R. Zhou, B. Li, N. Wu, G. Gao, J. You and
J. Lan, Chem. Commun., 2011, 47, 6668–6670; (c) L.-J. Ma, Y.-F. Liu
and Y. Wu, Chem. Commun., 2006, 2702–2704.
4 (a) C.-T. Chen and W.-P. Huang, J. Am. Chem. Soc., 2002, 124, 6246–6247;
(b) M. Schmittel and H. Lin, Bull. Chem. Soc. Jpn., 2008, 81, 1595–1598;
(c) J. Y. Kwon, Y. J. Jang, Y. J. Lee, K. M. Kim, M. S. Seo, W. Nam and
J. Yoon, J. Am. Chem. Soc., 2005, 127, 10107–10111; (d) 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, 3765–3767.
The 1 : 1 L–Pb2+ complex involves coordination with the azacrown
cavity9b and amide oxygen (Table 1). The coordination of Pb2+ with
amide oxygen is confirmed by IR analysis (Fig. S11, ESI†). L dissolved
in a D2O–MeCN (1/1 v/v, pH 7.0) mixture shows an amide CQO
stretching band at 1667 cmÀ1. Addition of Pb2+ shifts this band to
lower frequency (D = 20 cmÀ1), indicating that the L–Pb2+ complex
possesses 7-coordinated structure (Table 1). The fluorescence
enhancement of L upon addition of Pb2+ is because the PET process
is suppressed by the coordination of azacrown nitrogen with Pb2+.
This is confirmed by ab initio calculation. As shown in Table S1 (ESI†),
singlet electronic excitation of the L–Pb2+ complex is contributed by
HOMO - LUMO (S0 - S1) transition. Its energy is 3.85 eV (322 nm)
and is similar to the observed absorption maximum (303 nm; Fig. S4,
ESI†). As shown in Table 1, the electrons of HOMO and LUMO are
located on the phenylbenzoxazole moiety, where no electron density
lies on the azacrown nitrogen. This suggests that the energy level of
lone pair azacrown nitrogen becomes lower upon coordination with
Pb2+. This thus suppresses the PET quenching process and allows
fluorescence enhancement.
5 (a) M. Sun, D. Shangguan, H. Ma, L. Nie, X. Li, S. Xiong, G. Liu and
W. Thiemann, Biopolymers, 2003, 72, 413–420; (b) A. K. Singh,
R. Kanchanapally, Z. Fan, D. Senapati and P. C. Ray, Chem.
Commun., 2012, 48, 9047–9049; (c) L. Guo, S. Hong, X. Lin, Z. Xie
and G. Chen, Sens. Actuators, B, 2008, 130, 789–794.
6 (a) S. Deo and H. A. Godwin, J. Am. Chem. Soc., 2000, 122, 174–175; (b) R.
´
Metivier, I. Leray and B. Valeur, Chem. Commun., 2003, 996–997; (c) X.-L.
Ni, S. Wang, X. Zeng, Z. Tao and T. Yamato, Org. Lett., 2011, 13, 552–555.
7 (a) Y. Wu, X. Peng, J. Fan, S. Gao, M. Tian, J. Zhao and S. Sun, J. Org.
Chem., 2007, 72, 62–70; (b) O. K. Abou-Zied, R. Jimenez, E. H. Z.
Thompson, D. P. Millar and F. E. Romesberg, J. Phys. Chem. A, 2002,
106, 3665–3672; (c) M. M. Henary and C. J. Fahrni, J. Phys. Chem. A,
2002, 106, 5210–5220.
8 Y. Kohno, Y. Shiraishi and T. Hirai, J. Photochem. Photobiol., A, 2008,
195, 267–276.
9 (a) K. A. Byriel, K. R. Dunster, L. R. Gahan, C. H. L. Kennard,
J. L. Latten and I. L. Swann, Inorg. Chim. Acta, 1993, 205, 191–198;
(b) K. Byriel, K. R. Dunster, L. R. Gahan, C. H. L. Kennard,
J. L. Latten and I. L. Swann, Polyhedron, 1992, 11, 1205–1212.
10 E. D. Glendening and J. A. Hrabal II, J. Am. Chem. Soc., 1997, 119,
12940–12946.
11 The water content of solution strongly affects the emission property of L
(Fig. S12, ESI†): the increase in water content decreases the intensity of
longer-wavelength fluorescence because the H-bonding interaction of the
benzoxazole moiety with water suppresses ESIPT (ref. 7b). It is also noted
that the emission enhanced by Pb2+ is quenched by the addition of Cu2+
or Hg2+ regardless of the water content.
The selective enhancement of longer-wavelength emission by
Pb2+ is because the coordination of amide oxygen with Pb2+
decreases the electron density of amide nitrogen and weakens the
amide N–H (Namide–H) bond. This enhances ESIPT and produces
the H-transferred excited state efficiently (Scheme 1). The decrease
in electron density of amide nitrogen is confirmed by natural
population analysis (NPA).10 The NPA charge of amide nitrogen
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3436 Chem. Commun., 2013, 49, 3434--3436
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