Unexpected highly selective fluorescence 'turn-on' and ratiometric detection of Hg2+ based on fluorescein platform

Article abstract of DOI:10.1016/j.tet.2011.09.028

Methylated fluorescein 1 was explored for fluorescence 'turn-on' and ratiometric detection of Hg²⁺ in THF and CH₂Cl ₂/MeOH (v/v=9:1), respectively, with unexpected high selectivity. In the presence of Hg²⁺, characteristic structured absorption band of 1 diminished and a new sharp band appeared at 445 nm. Meanwhile a blue shifted and enhanced emission was observed. The ratio of the fluorescence intensity at 559 and 478 nm increased linearly with [Hg²⁺], and solution color changing from yellow to cyan under irradiation at 365 nm in CH ₂Cl₂/MeOH. Job plot indicated a 1:1 stoichiometry for 1-Hg²⁺ complex in solution. ¹H NMR titration and IR spectra suggested the coordination of carbonyl group in xanthene moiety to Hg²⁺, affording its spectral behavior. Compound 2 bearing two triazolyl amino esters in place of methyl group showed quite similar behavior to Hg²⁺, which indicated that substituents did not interfere with the specific binding behavior of fluorescein platform. Our work presents a new way to explore xanthene dyes as new chemosensors by modulating electron density on the xanthene ring through non-covalent interactions with carbonyl group.

Full text of DOI:10.1016/j.tet.2011.09.028

Tetrahedron 67 (2011) 8717e8723
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Unexpected highly selective fluorescence ‘turn-on’ and ratiometric detection
of Hg2þ based on fluorescein platform
*
Yi-Bin Ruan, Juan Xie
PPSM, Institut d’Alembert, ENS Cachan, CNRS, 61 av President Wilson, F-94230 Cachan, France
a r t i c l e i n f o
a b s t r a c t
Methylated fluorescein 1 was explored for fluorescence ‘turn-on’ and ratiometric detection of Hg2þ in
Article history:
Received 11 July 2011
Received in revised form 30 August 2011
Accepted 9 September 2011
Available online 14 September 2011
THF and CH2Cl2/MeOH (v/v¼9:1), respectively, with unexpected high selectivity. In the presence of Hg2þ
,
characteristic structured absorption band of 1 diminished and a new sharp band appeared at 445 nm.
Meanwhile a blue shifted and enhanced emission was observed. The ratio of the fluorescence intensity at
559 and 478 nm increased linearly with [Hg2þ], and solution color changing from yellow to cyan under
irradiation at 365 nm in CH2Cl2/MeOH. Job plot indicated a 1:1 stoichiometry for 1eHg2þ complex in
solution. 1H NMR titration and IR spectra suggested the coordination of carbonyl group in xanthene
moiety to Hg2þ, affording its spectral behavior. Compound 2 bearing two triazolyl amino esters in place
of methyl group showed quite similar behavior to Hg2þ, which indicated that substituents did not in-
terfere with the specific binding behavior of fluorescein platform. Our work presents a new way to
explore xanthene dyes as new chemosensors by modulating electron density on the xanthene ring
through non-covalent interactions with carbonyl group.
Keywords:
Chemosensor
Methylated fluorescein
Hg2þ
Ratiometric detection
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
benzoic acid moiety can result in plenty of interesting properties for
the further applications.8 Many of them have been used as fluo-
Mercury as one of the most hazardous metal ions is a wide-
spread pollutant. Excessive exposure to mercury compounds cau-
ses serious and destructive effects on human health, such as heart
diseases and neurological problems.1 Fluorescent chemosensors
have attracted considerable interest for decades due to their in-
trinsic advantages in terms of simplicity, selectivity, sensitivity, and
biological applications. To date, important progress has been ach-
ieved by developing turn-on fluorescent analyses of mercury,2
which are mainly based on small organic fluorophores,3 fluores-
cent conjugated polymers4 and biomolecules.5 However, most of
them require sophisticated organic synthesis and/or high cost,
which limit their practical applications. Ratiometric methods rep-
resent an alternative approach by comparison of the fluorescence
intensity at two different wavelengths upon addition of analytes.
Although this method shows great advantages in its dynamic range
and has a built-in correction for environmental effects, there are
still few reports of detection of Hg2þ based on fluorescent ratio-
metric signaling transduction.6
rescent chemosensors for monitoring pH,9 metal ions,10 anion,11
and other biologically important targets.12 At present there are
mainly three approaches proposed for the sensory mechanisms: (a)
spirolactone ring opening; (b) photoinduced electron transfer
(PET); (c) modulation on hydroxyl/carbonyl group.10i,14 Lippard’s
group has pioneered in the field of developing NO, Zn2þ, and Hg2þ
‘turn-on’ fluorescent chemosensors through suppressing the PET
process from electron donating nitrogen atom to various fluores-
cein cores.10aee,12 As a continuing interest in fluorescent chemo-
sensors for metal ions,10j,13 we turn our attention to modulation of
hydroxyl/carbonyl group of xanthene moiety for sensory applica-
tions as its photophysical properties can be easily modulated by
changing chemical environment based on covalent and/or non-
covalent interactions (Scheme 1). For example, alkylation of hy-
droxyl group significantly reduced the fluorescence efficiency.14 In
addition, fluorescein derivatives display quite different spectral
behavior in solutions with different hydrogen bonding ability and/
or polarity.14ꢀ16 Based on the cleavage of CeO ether bond, many
‘turn-on’ fluorescent chemosensors for different analytes have been
developed.15 To date, little attention has been paid to metal co-
ordination interaction with carbonyl group of xanthene moiety,
which is stronger than hydrogen bond while weaker than covalent
bond. According to the literatures, hydrogen bonding interaction
usually increases fluorescence quantum yield of fluorescein de-
rivatives;16 while formation of CeO ether bond usually induces
Fluorescein derivatives have been extensively utilized in bi-
ological applications due to their excellent photophysical proper-
ties.7 Appropriate modification of fluorescein either on xanthene or
* Corresponding author. Tel.: þ33 1 47405586; fax: þ33 1 47402454; e-mail
address: joanne.xie@ens-cachan.fr (J. Xie).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.09.028

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Y.-B. Ruan, J. Xie / Tetrahedron 67 (2011) 8717e8723
Scheme 1. Schematic illustration of our sensing approach.
Fig. 1. Absorption (a) and fluorescence (b) spectra of 1 in various solvents; excitation
wavelengths corresponding to the maximum absorption band, [1]¼10
mM.
fluorescence quenching.14 When the carbonyl group in xanthene
moiety is a binding site for metal ions, how metal coordination
interaction will affect spectral properties of the xanthene moiety
attracts our interest. For this proposal, we used compounds 1 and 2
to investigate metal coordination effect on spectral properties of
xanthene fluorophores (Scheme 1). Out of expectation, we found
that compound 1 with carbonyl group as the single binding site
could selectively response to Hg2þ in THF in ‘turn-on’ fluorescence
manner. Furthermore, we also developed ratiometric detection of
Hg2þ in CH2Cl2/CH3OH (v/v¼9:1) through utilization of both in-
termolecular hydrogen bonding and metal coordination
interactions.
Table 1
Photophysical data of 1 and 2 in different organic solvents
Compounds Solvents lmA ax
3
lFmax
[nm]
FF
a
00
[nm]
[104 L molꢀ1 cmꢀ1
]
%
1
Dioxane 432, 456, 487 1.7, 1.8, 1.0
518, 562
527, 562
529, 561
520, 553
518, 562
0.6
4.0
8.5
3.6
0.5
DMSO
CH2Cl2
MeCN
THF
433, 458, 488 1.7, 2.1, 1.3
434, 458, 488 1.8, 2.1, 1.3
432, 454, 485 1.7, 1.9, 1.2
431, 455, 486 1.7, 1.8, 1.0
433, 458, 487 1.7, 2.5, 1.9
432, 456, 484 1.7, 2.5, 2.0
EtOH
523, 549 27.3
523, 549 27.2
MeOH
2
Dioxane 433, 456, 488 1.8, 2.2, 1.4
526, 565
524, 559
528, 561
522, 557
526, 565
1.8
4.2
8.1
4.6
1.1
DMSO
CH2Cl2
MeCN
THF
438, 460, 489 1.9, 2.2, 1.4
434, 458, 490 1.9, 2.2, 1.3
432, 456, 486 1.8, 2.1, 1.3
433, 456, 488 1.8, 1.9, 1.1
434, 459, 489 1.8, 2.5, 1.9
433, 457, 486 1.8, 2.6, 2.0
2. Results and discussion
2.1. Synthesis
EtOH
523, 553 24.1
523, 553 24.8
MeOH
a
Coumarin 153 was used as the standard in EtOH20
.
Methylated fluorescein 1 was synthesized by a two-step re-
action according to the literature (Scheme 2).17 Compound 2, where
the two methyl groups were replaced by two triazolyl amino esters
was prepared previously by Cu(I)-catalyzed Huisgen cycloaddition
reaction.18
to characteristics of the quinoid form of fluorescein. In aprotic
solvents, only slight differences in the absorptivity and fluorescence
quantum yields could be observed (Fig. 1 and Table 1), showing no
direct connection between spectral properties and solvent polarity.
It is worthy of notice that stronger absorptivity at 487 nm and
higher fluorescence intensity in protic solvents like EtOH and
MeOH were observed. Moreover, fluorescence quantum yields of 1
in protic solvents are higher than in non-protic ones, which are
consistent with the results mentioned above (Table 1). This can be
explained by intermolecular hydrogen bonding interaction be-
tween
1 and protic solvents, which could reduce the in-
terconversion in the excited states and consequently improved the
emission efficiency. Evidences from Martin’s group demonstrated
that higher the solvent hydrogen bond donating power was, larger
the blue shift of the absorption and fluorescence spectra and en-
hancement of fluorescence quantum yields.16
Scheme 2. Synthesis of compounds 1 and 2.
2.3. Spectral behavior of 1 toward metal ions
2.2. Photophysical properties
To investigate metal binding ability of 1 and effect of metal
coordination interaction on its photophysical properties, we tested
We first investigated solvatochromic effect of 1 in selected or-
ganic solvents. As presented in Fig. 1 and Table 1, compound 1
exhibited structured absorption bands at ca. 432, 458, 488 nm and
emission bands at 518e562 nm without remarkable wavelength
shift (less than 13 nm) in different organic solvents, corresponding
spectral response of 1 to various metal ions such as Hg2þ (40
m
M),
Liþ, Naþ, Kþ, Agþ, Ba2þ, Cd2þ, Co2þ, Cu2þ, Fe2þ, Mg2þ, Mn2þ, Ni2þ
,
Pb2þ, and Zn2þ at 80
mM in THF. To our surprise, only the addition of
Hg2þ resulted in blue shift of the absorption and emission bands,
concomitantly with remarkable fluorescence enhancement (Fig. 2).

Y.-B. Ruan, J. Xie / Tetrahedron 67 (2011) 8717e8723
8719
Other tested metal ions showed almost no influence. The selectivity
of 1 to Hg2þ was further confirmed by the presence of interfering
metal ions. As shown in Fig. 3, except Agþ, the coexisted metal ions
(1 equiv) produced no influence on the spectral response of 1 to
Hg2þ in THF. These results demonstrated that compound 1,
containing carbonyl group as the single binding site, exhibited se-
lective response to Hg2þ in THF.
Spectral titrations were then carried out to get further insight
into the binding process of 1 to Hg2þ in THF. As shown in Fig. 4
(top), upon addition of increasing [Hg2þ], absorption band of 1 at
490 nm decreased, concomitantly with appearance of a new nar-
row band at 445 nm. Well-defined isosbestic points were observed
at 462 and 403 nm, indicating equilibrium of the complexation
process. Job’s plot, which showed the maximum at 0.5 of the
fraction demonstrated that the binding stoichiometry between 1
and Hg2þ is 1:1 (Fig. 5). The binding constant in the ground state
was determined to be 3.55ꢁ105 Mꢀ1 by using non-linear regression
analysis based on 1:1 binding ratio (Fig. 4 (top)). Meanwhile, ad-
dition of Hg2þ induced a blue shift of the emission band from 560 to
489 nm, with remarkably enhanced fluorescence intensity (ca. 13
folds) (Fig. 4 (bottom)) and solution color changing from dark to
cyan under UV irradiation at 365 nm. The association constant in
the excited state was determined to be 3.14ꢁ105 Mꢀ1. The binding
constant of 104e105 Mꢀ1 for a carbonyl-Hg2þ interaction in aqueous
or acetone solution has been reported by Avirah et al.19 The de-
tection limit of the probe in THF was 39 nM (3s .
/k) for Hg2þ
However, when it was performed in aqueous solution
(VH O=VTHF ¼ 5 : 95), quite little influence was observed on the
2
spectral properties of 1 even in the presence of 4.0 equiv of Hg2þ
,
which could be attributed to strong hydration of Hg2þ inhibiting its
complexation with the receptor (Fig. S1).
Fig. 2. Absorption (top) and fluorescence (bottom) spectra of 1 in the presence of
various metal ions in THF; [Mnþ]¼80
m
M, [Hg2þ]¼40
m
M, [1]¼10 M, lex¼445 nm.
m
Fig. 3. Fluorescence spectra of 1 in the coexistence of various metal ions and Hg2þ in
Fig. 4. Absorption (top) and fluorescence (bottom) spectra of 1 in the presence of
THF; [Mnþ]¼40
m
M, [Hg2þ]¼40
m
M, [1]¼10
mM, lex¼446 nm.
varying [Hg2þ] in THF; [1]¼10
M, lex¼460 nm.
m

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Y.-B. Ruan, J. Xie / Tetrahedron 67 (2011) 8717e8723
It reached an equilibrium more quickly at 2.0 equiv level and the
association constant (4.49ꢁ106
M
ꢀ1) is improved about one order
of magnitude when compared with that in THF. However, the
fluorescence behavior was quite different. With increasing [Hg2þ],
fluorescence intensity at 559 nm decreased, meanwhile a new band
at 478 nm enhanced progressively (Fig. 6 bottom) and solution
color changed from yellow to cyan under irradiation at 365 nm. An
isosbestic emission point was observed at 524 nm. The ratio of the
fluorescence intensity at 559 and 478 nm increased linearly with
[Hg2þ] (0e1.0 equiv). The high association constant (8.76ꢁ105 Mꢀ1
)
indicated that 1 had a strong binding ability to Hg2þ in CH2Cl2/
MeOH (v/v¼9:1).
Reversibility of 1 to Hg2þ was examined by the addition of
2 equiv of Brꢀ/Clꢀ to the complex solution in THF (Fig. 7, top and
Fig. S2). Immediate fluorescence quenching and solution color
change from cyan to dark indicated decomplexation of 1eHg2þ. The
same reversibility was observed in CH2Cl2/MeOH (v/v¼9:1) (Fig. 7,
bottom) where recovery of the fluorescence spectra and solution
color was observed after the addition of 2 equiv Iꢀ.
Fig. 5. Job plot for 1eHg2þ in THF. The total concentration of [Hg2þ] and [1] is 20
mM.
Intermolecular hydrogen bonding-induced fluorescence en-
hancement of 1 with quite slight wavelength shift (Fig. 1) and
specific Hg2þ chelation-promoted fluorescence enhancement with
remarkable blue shift (Fig. 2) led us to explore the possibility of
ratiometric detection of Hg2þ by changing solvent media. We then
performed the spectral titration in CH2Cl2/MeOH (v/v¼9:1). As
shown in Fig. 6 (top), the absorption spectra exhibited almost the
same behavior as that in THF. With increasing [Hg2þ], absorbance of
1 at 489 nm decreased and a new sharp band at 446 nm appeared.
Fig. 7. Fluorescence spectra of 1 in the presence of Hg2þ, Hg2þ and Brꢀ in THF (top) or
in the presence of Hg2þ, Hg2þ, and Iꢀ in CH2Cl2/MeOH (v/v¼9:1) (bottom). [1]¼10
mM,
[Hg2þ]¼40
m
M, [Brꢀ]¼[Iꢀ]¼80
M, lex¼446 nm.
m
1H NMR titration in CD3CN was carried out to examine the
binding sites of 1 to Hg2þ. By comparing the NMR spectra of 1
(Figs. S8e10) with a known fluorescein methyl ester derivative,14b
we could assign the signal at 3.6 ppm to methyl group of benzoic
moiety while the one at 3.9 ppm to methyl group of xanthene
moiety, as confirmed by 2D NMR (HMQC, Fig. S10). The chemical
Fig. 6. Absorption (top) and fluorescence (bottom) spectra of 1 in the presence of
varying [Hg2þ] in CH2Cl2/MeOH (v/v¼9:1); [1]¼10
M, lex¼446 nm.
m

Y.-B. Ruan, J. Xie / Tetrahedron 67 (2011) 8717e8723
8721
shifts of protons of the xanthene ring ranged from 6.3 to 7.0 ppm
while protons of the benzoic acid moiety from 7.3 to 8.2 ppm. As
shown in Fig. 8, upon complexation with Hg2þ, all the protons of
xanthene moiety shifted downfield dramatically (ca. 0.6e1.0 ppm)
while protons of benzoic acid moiety shifted downfield slightly
(less than 0.1 ppm), suggesting that the complexation should oc-
Fig. 10. Time-dependent absorbance of 1 at 445 nm in the presence of 2 equiv of Hg2þ
in THF.
therefore demonstrated that chelation of carbonyl group to Hg2þ
reduced the electronic density on the xanthene moiety and con-
sequently caused remarkable blue shift of both absorption and
fluorescence spectra. Similar to hydrogen bonding interaction,
coordination of Hg2þ to carbonyl group of xanthene fluorophore
might reduce radiationless deactivation of the excited states in
organic medium.16
Compound 2, where the two methyl groups were replaced by
two triazolyl amino esters, was used to investigate how substituent
on the xanthene ring affected the binding process of carbonyl to
metal ions. Compound 2 showed almost the same photophysical
properties as those of 1 including solvatochromism, spectral
wavelength and fluorescence quantum yield (Fig. 11, Table 1), in-
dicating that the triazolyl moiety was just a linker and produced no
electronic effects on the fluorescent core. Further investigation of
its metal ion binding properties showed that introduction of tri-
azolyl group did not affect the selective binding interaction be-
tween carbonyl group and Hg2þ in THF (Fig. 12 and Fig. S3).
Interestingly, spectral titrations of 2 to Hg2þ in THF and CH2Cl2/
MeOH (v/v¼9:1) exhibited very similar spectral behavior as that of
1, such as associate constants (2.94ꢁ105 Mꢀ1 in THF (Fig. 13) and
2.26ꢁ106 Mꢀ1 in CH2Cl2/MeOH (v/v¼9:1)) (Fig. 14), binding stoi-
chiometry and reversibility (Figs. S4 to S7). These results together
Fig. 8. Partial 1H NMR spectra of 1 in the presence of varying concentrations of Hg2þ in
CD3CN.
cur on the xanthene moiety rather than on the benzoic unit.
Meanwhile, protons of methyl ether at 3.9 ppm also shifted
downfield by 0.2 ppm and spited into two groups while protons of
methyl ester at 3.6 ppm were almost not shifted. This fact further
supported our assumption that xanthene moiety participated in
the complexation process. By analysis of molecular structure of 1,
it is quite obvious that carbonyl group is the binding site for Hg2þ
.
This assumption was further confirmed by investigation of IR
spectra of 1 in the presence and absence of Hg2þ (Fig. 9). Char-
acteristic carbonyl stretching frequency of the xanthene moiety
appeared at 1723 cmꢀ1 and bending vibration at 1212 cmꢀ1 (the
ester carbonyl appeared at 1712 cmꢀ1). In 1eHg2þ complex, they
both downshifted to 1653 and 1184 cmꢀ1, respectively. Time-de-
pendent absorbance of 1 in the presence of Hg2þ spectra was also
carried out, which showed that the recognition process was im-
mediate (Fig. 10), suggesting a complexation process rather than
a chemical reaction since no Hg2þ-sensitive chemical function
(alkene or alkyne) existed in the structure of 1. Our results
Fig. 11. Absorption (left) and fluorescence (right) spectra of 2 in various solvents;
excitation wavelengths correspond to the peaks of absorption band in different sol-
Fig. 9. Partial FTIR of 1 and 1eHg2þ
.
vents, [2]¼10
mM.

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Y.-B. Ruan, J. Xie / Tetrahedron 67 (2011) 8717e8723
Fig. 14. Fluorescence spectra of 2 in the presence of varying concentrations of Hg2þ in
CH2Cl2/MeOH (v/v¼9:1), [2]¼10 M, lex¼447 nm.
m
3. Conclusions
In summary, we have found that easily synthesized fluorescein
derivatives 1 and 2 could be used as highly selective chemosensors
for Hg2þ over a range of metal ions in THF. In the absence and
presence of intermolecular hydrogen bonding interaction, we have
developed ‘turn-on’ fluorescence response to Hg2þ in THF and
ratiometric detection of Hg2þ in CH2Cl2/MeOH (v/v¼9:1), re-
spectively. 1H NMR titration and IR spectra suggested that specific
spectral behavior was due to the complexation between carbonyl
group of xanthene moiety and Hg2þ. We thus demonstrated how
metal coordination, one of most important non-covalent in-
teractions, affected the spectral properties of xanthene fluo-
rophore through direct interaction with its carbonyl group.
Furthermore, combination of different non-covalent and/or co-
valent interactions with carbonyl group may further promote the
extensive applications of xanthene dyes in biological and envi-
ronmental systems.
Fig. 12. Absorption (top) and fluorescence (bottom) spectra of 2 in the presence of
various metal ions in THF; [Mnþ]¼80
m
M, [Hg2þ]¼40
m
M, [2]¼10 M, lex¼456 nm.
m
4. Experimental
4.1. Materials and methods
Absorption spectra were recorded on a Uvikon-940 KON-TRON
spectrophotometer and corrected emission spectra were per-
formed on a Jobin-Yvon Spex Fluorolog 1681 spectrofluorometer
(1 cm quartz cell was used).
All the solvents were spectroscopic grade and used as received.
The tested metal salts included LiClO4, NaClO4, KClO4, AgNO3,
Ba(ClO4)2, Co(ClO4)2, Cd(ClO4)2, Cu(ClO4)2, Fe(ClO4)2, Mg(ClO4)2,
Mn(ClO4)2, Ni(ClO4)2, Pb(ClO4)2, Zn(ClO4)2, and Hg(ClO4)2.
4.2. Spectral measurement
Stock solution of compounds 1 and 2 was prepared in DMSO
while metal perchlorate salts in MeCN. For all the spectral tests,
Fig. 13. Fluorescence spectra of 2 in the presence of varying concentrations of Hg2þ in
THF, [2]¼10 M, lex¼468 nm.
concentrations of receptors were fixed at 10 m
M. 1H NMR titration
m
was tested in CD3CN.
demonstrated that carbonyl group of the xanthenic moiety actually
could be a selective binding site for Hg2þ and easy accessibility of
click reaction might facilitate versatile functionalization on the
triazolyl moiety to investigate other sensing approaches, such as
FRET.
4.3. Quantum yield measurement
Fluorescence quantum yield was determined using Coumarin
153 as a standard reference20 and the quantum yield is calculated
using the following equation.

Y.-B. Ruan, J. Xie / Tetrahedron 67 (2011) 8717e8723
8723
ꢀ
ꢁ
2
5. (a) Liu, J.; Lu, Y. Angew. Chem., Int. Ed. 2007, 46, 7587; (b) Zhang, L.; Li, T.; Li, B.;
Li, J.; Wang, E. Chem. Commun. 2010, 1476.
ðIunk=Aunk
fstd
Þ
hunk
hstd
funk
¼
ðIstd=Astd
Þ
6. (a) Li, C.-Y.; Zhang, X.-B.; Qiao, L.; Zhao, Y.; He, C.-M.; Huan, S.-Y.; Lu, L.-M.; Jian,
L.-X.; Shen, G.-L.; Yu, R.-Q. Anal. Chem. 2009, 81, 9993; (b) Tian, M.; Ihmels, H.
Chem. Commun. 2009, 3175; (c) Suresh, M.; Mishra, S.; Mishra, S. K.; Suresh, E.;
Mandal, A. K.; Shrivastav, A.; Das, A. Org. Lett. 2009, 11, 2740; (d) Joshi, B. P.;
Lohani, C. R.; Lee, K.-H. Org. Biomol. Chem. 2010, 8, 3220; (e) Santra, M.; Roy, B.;
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6, 35.
Where funk and fstd are the quantum yield of the sample and
standard, Iunk and Istd are the integrated emission intensity of the
corrected spectra of the sample and standard, Aunk and Astd are the
absorbance of the sample and standard at the excitation wave-
length (lower than 0.05).
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4.4. Determination of association constant
Ylim ꢀ Y0
Y ¼ Y0 þ
2
8
<
9
=
"
#
1=2
ꢀ
ꢁ
2
CM
CL KCL
1
CM
1
CM
CL
ꢁ
1 þ
þ
ꢀ
1 þ
þ
CL KCL
ꢀ4
:
;
Where Y donates the fluorescence intensityorabsorbance; CM and CL
are the concentration of metal ions and ligand; K is the association
constant of 1:1 complex. Y0 and Ylim are the fluorescence intensity or
absorbance value when CM¼0 and for full complexation.
ꢀ
ꢀ
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Acknowledgements
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Y.B.R. thanks the China Scholarship Council (CSC) for support
through a scholarship to work at PPSM, ENS Cachan.
ꢁ
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Supplementary data
Supplementary data including related spectra data and NMR
spectra. Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tet.2011.09.028.
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