A highly selective Cd2+ sensor of naphthyridine: fluorescent enhancement and red-shift by the synergistic action of forming binuclear complex

Article abstract of DOI:10.1016/j.tetlet.2008.03.128

A new series of fluorophore derivatives from 1,8-naphthyridine have been developed. Compound D1 is the first naphthyridine PET sensor that can signal Cd²⁺ selectively with fluorescent enhancement and red-shift. A binuclear complex structure has

Full text of DOI:10.1016/j.tetlet.2008.03.128

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 3380–3384
A highly selective Cd2+ sensor of naphthyridine:
fluorescent enhancement and red-shift by the
synergistic action of forming binuclear complex
a
a,
b,
*
*
Ying Zhou , Yi Xiao , Xuhong Qian
a State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China
b State Key Laboratory of Bioreactor Engineering and Shanghai Key Laboratory of Chemical Biology,
East China University of Science and Technology, Shanghai 200237, China
Received 29 December 2007; revised 24 March 2008; accepted 26 March 2008
Available online 29 March 2008
Abstract
A new series of fluorophore derivatives from 1,8-naphthyridine have been developed. Compound D1 is the first naphthyridine PET
sensor that can signal Cd2+ selectively with fluorescent enhancement and red-shift. A binuclear complex structure has been demonstrated
in the D1–Cd2+ complex.
Ó 2008 Elsevier Ltd. All rights reserved.
In recent years, the fluorescent sensing of transition and
heavy metal ions has become an important research field
advancing rapidly.1 It is noticeable that fluorescent sensors
recognize metal ions mainly by forming single-nuclear
complex.2 In few literature, binuclear metal complex was
used as the report mechanism for the signal response.3,4
To form a stable binuclear metal complex, a receptor
should meet certain requirement of having well-organized
multidentate chelation sites and of providing a rigidity-
suitable cage with a favorable size. An appropriate
metal–metal synergistic effect is another integrant to ensure
the formation of a binuclear complex. All these factors
contribute to the high selectivity of the fluorescent sensors
of this kind.
cage-like receptor may enforce two metal centers so close
that a synergistic effect is expected. Lippard and coworkers
have explored a series of 1,8-naphthyridine derivatives as
potential multidentate ligands and reported a Cu-complex
as the first 1,8-naphthyridine binuclear complex.6
Secondly, superior to many ligands without fluores-
cence, 1,8-naphthyridine derivatives could possibly work
as a reporter to transfer the binding character by fluores-
cent changes.7 According to our experience in fluorophore
design,8 the introduction of different electron-donating
substituents at position 4 could result in intensive fluores-
cence by forming an intramolecular charge-transfer (ICT)
system; such a small fluorophore might be competitive to
many of the available large fluorescent markers in the field
of sensors.
Although considerable attention has been paid to
chemosensors for the cadmium ion,9 which is ubiquitous
in nature and toxic to a broad spectrum of organisms, there
is still room for further improvement. Moreover, even
some 1,8-naphthyridines are known as ligands, and the
derivatives designed as fluorescent sensors for the cadmium
ion have not been previously reported.
This study focused on 1,8-naphthyridine based on the
following considerations. Firstly, the bridging coordination
mode could form two metal centers by the two nitrogen
atoms on positions 1 and 8,5 and auxiliary chelating groups
could be incorporated at the 2 and 7 positions: so a special
*
Corresponding authors. Tel.: +86 4118899 3872; fax: +86 411 8367
3488 (Y.X.); tel.: +86 13501766476; fax: +86 21 64252603 (X.Q.).
E-mail addresses: xiaoyi@chem.dlut.edu.cn (Y. Xiao), xhqian@ecust.
edu.cn (X. Qian).
Therefore, in the present work, emphases are on the
development of 1,8-naphthyridine based fluorophores and
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.03.128

Y. Zhou et al. / Tetrahedron Letters 49 (2008) 3380–3384
3381
OH
O
O
º
300
C
PPA
+
N
N
1 h
100 ºC
OEt
N
NH2
N
N
O
A1 R= OCH3
A2 R= S-CH2-CH3
A5 R=
A6 R=
N
O
R
N
Cl
POCl3
N
N
HN
º
140 C, 1 h
N
N
N
A3 R=
A4 R=
HN
N
A7 R=
HN
A1-A7
N
Scheme 1. Synthesis of derivatives of 4-substituted 2,7-dimethyl-1,8-naphthyridine (yields: A1 90%; A2 45%; A3 39 %; A4 72%; A5 75%; A6 51%; A7
43%).
the subsequent modifications for multidentate binuclear
chelator as a novel fluorescent sensor. Experiment indi-
cated that D1 could signal Cd2+ selectively with consider-
electron-rich benzylamino group, strong fluorescence was
observed in A3. The other two primary amine-substituted
compounds (A6 and A7) also emitted intensively. For the
compounds with relatively poor electron-donating substit-
uents, A1 (methoxy group) and A2 (ethylthio group) show
much weaker fluorescence than that of A3. Very weak fluo-
rescence was also observed in A4 and A5, although strong
electron-donating bisalkylamino was introduced. This
could be explained by the formation of twisted intramolec-
ular charge-transfer (TICT) state. It is safe to say that 4-
amino-1,8-naphthyridine is a new class of fluorophores
with small and simple molecular structure, and its fluores-
cent properties is considerably adjustable.
Based on this promising fluorophore, we developed a
new class of binuclear-type sensors. Scheme 2 summarizes
the synthetic routes of B1, C1, and D1. Noticeably, when
the 2,7-dimethyl-4-hydroxyl-1,8-naphthyridine was heated
in phosphorous oxybromide, bromination on both
hydroxy group and two methyl groups occurred, resulting
in the good yield of the key intermediated B1. In this reac-
tion, phosphorous oxybromide might form radicals, which
induced the bromination on side-methyl group. This is the
first report on multi-halogenated 1,8-naphthyridine deriva-
tives synthesized by phosphorous oxybromide, which takes
the advantages of the single product over the known proce-
dure utilizing NBS, which always results in a multi-prod-
ucts distribution.11
able fluorescent red-shift and enhancement, and
a
synergistic action was demonstrated in the D1–Cd2+
complexation.
First of all, the synthesis and the fluorescent properties
of fluorophores were studied. As illustrated in Scheme 1,
the key intermediate 4-chloro-2,7-dimethyl-1,8-naphthyr-
idine was easily prepared from 2-amine-6-methylpyridine;10
A1–A7 were facilely synthesized through the substitutions
of chlorine atom by O, S, and N types of nucleophilic
reagents. For comparison, the fundamental optical data
of A1–A7 are listed in Table 1.
Whereas precursor 4-chloro-2,7-dimethyl-1,8-naph-
thyridine was not fluorescent, an intramolecular charge-
transfer (ICT) system formed by the introduction of an
electron-donating group at the 4-positions resulted in fluo-
rescent emission. Furthermore, the characteristic of the
substituent greatly influenced the luminescence: With
Table 1
Spectral data for A1–A7 (1.0 Â 10À5 mol/L) in different solvents
Compd
Solvent
kabs (nm)
loge
kem (nm)
UF
A1
CH2Cl2
C2H5OH
299
298
3.97
3.84
304
340
0.04
0.05
A2
A3
A4
A5
A6
A7
CH2Cl2
C2H5OH
315
316
4.19
4.08
309
312
0.03
0.03
In D1, piperazine was chosen as a chelating group
because its nitrogen atoms were both ion receptors and
electron donors for PET.8a,b Incorporation of piperazine
units would induce a low basal fluorescence and would give
the potential for fluorescence enhancement when the sensor
bounds a suitable metal ion. The introduced partly rigid
arms contributed a favorable ring size and multi-nitrogen
atoms centers in D1’s structure. When this multidentate
ligand combined with suitable metal ions, it would hold
CH2Cl2
C2H5OH
332
336
3.88
4.00
398
416
0.23
0.27
CH2Cl2
C2H5OH
316
315
3.97
3.93
310
315
0.04
0.04
CH2Cl2
C2H5OH
316
315
4.07
4.12
420
449
0.03
0.04
CH2Cl2
C2H5OH
333
336
4.05
3.94
404
414
0.12
0.14
the metal ion close and provide
environment.
a
coordination
CH2Cl2
C2H5OH
334
337
3.90
4.12
403
414
0.12
0.18
For most of the PET sensors, both protonation and
metal ions chelation work on the sites of the electron
donors, which can inhibit PET and induce fluorescent
The fluorescence quantum yields (UF) are estimated with quinine-sulfate
(UF = 0.55 in 50 mM H2SO4 solution) as a standard.

3382
Y. Zhou et al. / Tetrahedron Letters 49 (2008) 3380–3384
HN
N
Br
N
Br
N
OH
N
(a) butanamine
(b) K2CO3
(a) methyipierazine
POBr3
(b) K2CO3
º
140 C,1 h
N
N
CH3CN
DMSO
N
N
120 ºC, 4 h
reflux, 4 h
N
N
N
N
N
N
N
Br
Br
N
B1
C1
D1
Scheme 2. Synthesis of derivatives of 2,7-dimethyl-1,8-naphthyridine (yields: B1 29%; C1 37%; D1 30%).
enhancement in a similar way. Protons frequently interfere
with metal-ion fluorescence analysis, and thus the pH
response experiment is particular important in a research
work on PET metal ions sensors.
D1 did not exhibit any detectable fluorescent change
between pH 13 and 10.4. When pH was further decreased,
fluorescent maxima gradually decreased in intensity. A
marked fluorescence decrease response with a 7 nm red-
shift characterized the lower pH region between pH 5.5
and 4.0, but below pH 2.0 no obvious fluorescent change
was observed (Fig. 1). Based on the fluorescent titration
curve, the pKa of D1 is about 5.18.
The pH-dependent spectra properties of D1 cannot be
simply explained by piperazine protonation, but rather
results from other processes associated with naphthyridine.
The protonated nitrogen atoms of naphthyridine caused
absorption increase and disrupted the whole conjugated
electronic system of the ICT moiety, resulting in a fluores-
cence quenching of the latter.
lem of interference from protonation of piperazine group
could be ruled out to some extent in D1.
Fluorescence titrations with various transition-metal
ions were performed to shed light on the binding properties
of D1 with metal ions. These titration studies were con-
ducted in a mixed solution containing Tris–HCl (0.01 M,
pH = 7.0). D1 displayed a fluorescence quenching effect
with Ag+, Co2+, Cr3+, Cu2+, Fe3+, Hg2+, Mg2+, Ni2+
,
and Zn2+ (Supplementary data); Only with Cd2+, a differ-
ent responding signal of considerable fluorescent enhance-
ment and red-shift was observed (Fig. 2).
To examine the signal response toward Cd2+, the
absorption and emission responses of D1 were tested in dif-
ferent concentration of Cd2+. As shown in Figure S2 (Sup-
plementary data), upon the addition of Cd2+ to 1 equiv,
there was an obvious increase in the absorption intensity.
However, when another equivalence Cd2+ was added, the
absorption intensity increased just slightly, with a final
red-shift of 5 nm.
A similar reduction in fluorescence was also found in A3
under an acidic condition (Supplementary data). For A3
without methylpiperazine, the protonation of the naph-
thyridine primarily caused the reduction of the fluores-
cence, which could test the proposed explanation of D1’s
fluorescent changes. These results demonstrated that D1
worked in a manner which was different from that of the
usual PET sensor: it responded to proton mainly by fluoro-
phore moiety instead of electron donor part, so the prob-
Cd2+
18
none
15
Co2+
Zn2+
Cr3+
16
Mg2+
10
Ni2+
4.0
3.5
Fe3+
14
Hg2+
Ag+
5
Cu2+
12
40
3.0
10.4
35
2.5
10
0
5.0
4.0
30
25
20
400
450
500
550
2.0
8
6
4
2
0
wavelength (nm)
2.0
15
10
1.5
5
0
1.0
400 420 440 460 480 500 520
0.5
Wavelengh (nm)
Ag Co Cr Cu Fe Hg Mg Ni Zn Cd
none
0.0
2
4
6
8
pH
10
12
14
Fig. 2. The fluorescence intensity change profile of D1 (1.0 Â 10À5 M) in a
mixture (7:3, v/v) of ethanol and an aqueous Tris–HCl (0.01 M,
pH = 7.00) buffer solution in the presence of selected metal ions
(5.0 Â 10À5 M). Inset: Corresponding fluorescent spectra of D1 in the
presence of different metal ions. (The photo was taken under a handheld
UV (365 nm) lamp after the addition of metal ions.)
Fig. 1. Fluorescence intensity of D1 (1 Â 10À5 M) as a function of pH in a
mixed solution of ethanol and water (4:6, v/v) (pH adjusted by 75%
HClO4 or 10% (CH3)4N+OHÀ). Inset, dependence of the fluorescence
intensity of D1 on pH.

Y. Zhou et al. / Tetrahedron Letters 49 (2008) 3380–3384
3383
The addition of Cd2+ dramatically changed the fluores-
cence spectra. When less than 0.4 equiv Cd2+ was added,
the fluorescent intensity was gradually decreased with a
red-shift. After the addition of more than 0.5 equiv of
Cd2+, a band at 450 nm appeared with a fluorescence
increase. 1 equiv of Cd2+ ions increased the fluorescence
intensity back to almost the same level as that of the back-
ground and shifted the fluorescence maximum to 458 nm.
As the Cd2+ amount increased from 1 to 2 equiv, the fluo-
rescent intensity increased linearly with an unobvious red-
shift. No further spectra change was observed after Cd2+
was over 2 equiv, and a final twofold fluorescence enhance-
ment and a 35 nm red-shift were achieved at the emission
maximum (Fig. 3). Cd2+ titration experiments were
repeated with several different concentrations (1 Â
10À4 M, 1 Â 10À5 M, 5 Â 10À6 M) of D1, and all indicated
that the 2 equiv Cd2+ is the necessary requirement for the
constant complexation.
Spectra study clearly indicated that D1 can form binu-
clear complex with Cd2+, and the two Cd2+ ions were
caught by two steps. The calculation (Supplementary data)
by Hyperchem 7.0 demonstrated two different binding
modes in D1–Cd2+ complexation (Fig. 4), and the N–
Cd2+ lengthes were close to the reported ones.12
D1 may coordinate the first Cd2+ ion by two nitrogen
atoms on positions 1 and 8 of naphthyridine, resulting in
a quenching fluorescence by reducing the electron density
of the fluorophore. Then, the two arms of piperazines
may get closer to provide a rigidity-suitable cage, which
would favor the complexation of another Cd2+ ion. The
complexation of the first Cd2+ would be rearranged as
the second Cd2+ behaved as another metal center, which
could give rise to the binuclear structure as shown in Figure
4. This kind of synergistic action may lead to the formation
of a stable complex consisting of two metal ions. As the
receptors reached a steady conformation by the coopera-
tion of two accommodating Cd2+ ions, fluorescence
enhancement was easily observed through a reduced PET
process. This proposed mechanism could well explain the
experimental phenomena of slight shift in emission spectra
and little increase in the absorption maximum intensity,
after the first coordination with Cd2+ ion.
50
45
40
35
2.0 equ.
50
1.5 equ.
1.0 equ.
0.5 equ.
30
25
20
15
40
30
20
10
0
0
equ.
0.0 0.5 1.0 1.5 2.0 2.5
2+
-5
[Cd ]/10
0.4 equ.
Job’s plot (Supplementary data) suggested that D1
finally formed a 1:2 complex with Cd2+ ion. The associa-
tion constant was K1 = 0.75 Â 105 MÀ1and K2 =
2.19 Â 105 MÀ1 determined by a nonlinear least-square
(Supplementary data), which corresponded to the first
and second phase of the related line between the fluores-
cence intensity and concentration of Cd2+ shown in Figure
400
440
480
520
560
600
Wavelength (nm)
Fig. 3. Fluorescent spectra of D1 (1.0 Â 10À5 M) in a Tris–HCl (0.01 M)
solution (ethanol/water = 7:3, v/v, pH = 7.00) in the presence of different
equiv of Cd2+. Inset: Fluorescent intensity changes of D1 in the presence
of different equiv of Cd2+
.
Fig. 4. Proposed D1–Cd2+ complex structure and the energy-minimized conformation by using Hyperchem software with the molecular mechanics
subroutine.

3384
Y. Zhou et al. / Tetrahedron Letters 49 (2008) 3380–3384
3, respectively. We recently reported a new fluorescent
Hg2+ sensor that disclosed an unusual positively coopera-
tive complexation.4 The fluorescence change led us to
assume that D1–Cd2+ was another example of a positively
cooperative complex.
A competitive metal-ion exchange experiment permitted
further comparison of the binding properties of D1 (Sup-
plementary data). The sensor D1 exhibited better selectiv-
ity to Cd2+ against most other metal ions, such as Ag+,
Co2+, Mg2+, Ni2+, Zn2+, Cr3+, and Fe3+. The emission
of the sensor D1–Cd2+ complex was not remarkably
affected by them. Only Hg2+ and Cu2+ induced fluores-
cence quench.
References and notes
1. (a) Coskun, A.; Akkaya, E. U. J. Am. Chem. Soc. 2006, 128, 14474;
(b) Lee, H. N.; Xu, Z. C.; Kim, S. K.; Swamy, K. M. K.; Kim, Y.;
Kim, S. J.; Yoon, J. Y. J. Am. Chem. Soc. 2007, 129, 3828; (c)
Komatsu, K.; Urano, Y.; Kojima, H.; Nagano, T. J. Am. Chem. Soc.
2007, 129, 13447; (d) Wang, J. B.; Qian, X. H. Chem. Commun. 2006,
109.
2. (a) de Silva, A. P.; Fox, D. B.; Huxley, A. J. M.; McClenaghan, N. D.;
Roiron, J. Coord. Chem. Rev. 1999, 297; (b) Royzen, M.; Durandin,
A.; Young, V. G.; Geacintoy, N. E.; Canary, J. W. J. Am. Chem. Soc.
2006, 128, 3854; (c) Lee, S. H.; Kim, S. H.; Kim, S. K.; Jung, J. H.;
Kim, J. S. J. Org. Chem. 2005, 70, 9288.
3. Barrios, A. M.; Lippard, S. J. J. Am. Chem. Soc. 1999, 121,
11751.
4. Wang, J. B.; Qian, X. H. Org. Lett. 2006, 8, 3721.
In conclusion, a series of new fluorophores derivative
from 1,8-naphthyridine with tuneable optical properties
have been developed. Based on our design of novel binu-
clear-complex utilizing synergistic action, we synthesized
a fluorescent sensor with cage-like multidentate receptors,
which are composed of naphthyridine and piperazines.
D1 was the first 1,8-naphthyridine-based sensor that could
exhibit a high selectivity and sensitivity for Cd2+ with fluo-
rescent enhancement and red-shift by forming 1:2 complex
with Cd2+ ions.
5. (a) Suzuki, T. Inorg. Chim. Acta 2006, 359, 2431; (b) Basato, M.;
Biffis, A.; Martinati, G.; Tubaro, C.; Graiff, C.; Tiripicchio, A.;
Aronica, L. A.; Caporusso, A. M. J. Org. Chem. 2006, 691, 3464.
6. (a) He, C.; Lippard, S. J. Tetrahedron 2000, 56, 8245; (b) He, C.;
Lippard, S. J. J. Am. Chem. Soc. 2000, 122, 184.
7. (a) Huang, J. H.; Wen, W. H.; Sun, Y. Y.; Chou, P. T.; Fang, J. M. J.
Org. Chem. 2005, 70, 5827; (b) Sun, Y. Y.; Liao, J. H.; Fang, J. M.;
Chou, P. T.; Shen, C. H.; Hsu, C. W.; Chen, L. C. Org. Lett. 2006, 8,
3713.
8. (a) Xiao, Y.; Qian, X. H. Tetrahedron Lett. 2003, 44, 2087; (b) Guo,
X. F.; Qian, X. H.; Jia, L. H. J. Am. Chem. Soc. 2004, 126, 2272; (c)
Xu, Z. C.; Qian, X. H.; Cui, J. N. Org. Lett. 2005, 7, 3029; (d) Wang,
J. B.; Qian, X. H.; Cui, J. N. J. Org. Chem. 2006, 71, 4308.
9. (a) Peng, X. J.; Du, J. J.; Fan, J. L.; Wang, J. Y.; Wu, Y. K.; Zhao, J.
Z.; Sun, S. G.; Xu, T. J. Am. Chem. Soc. 2007, 129, 1500; (b) Lu, C.
L.; Xu, Z. C.; Cui, J. N.; Zhang, R.; Qian, X. H. J. Org. Chem. 2007,
72, 3554; (c) Royzen, M.; Dai, Z.; Canary, J. W. J. Am. Chem. Soc.
2005, 127, 1612; (d) Bronson, R. T.; Michaelis, D. J.; Lamb, R. D.;
Husseini, G. A.; Farnsworth, P. B.; Linford, M. R.; Izatt, R. M.;
Bradshaw, J. S.; Savage, P. B. Org. Lett. 2005, 7, 1105.
10. Chandler, C. J.; Deady, L. W.; Reiss, J. A. J. Heterocycl. Chem. 1982,
19, 1017.
Acknowledgments
The authors thank the National Natural Science Foun-
dation of China (Nos. 20406004, 20572012, 20536010) and
Program for Changjiang Scholars and Innovative Research
Team in University (IRT0711) for the financial support.
Supplementary data
11. Goswami, S.; Dey, S.; Jana, S.; Adak, A. K. Chem. Lett. 2004, 7, 916.
12. (a) Allred, R. A.; McAlexander, L. H.; Arif, A. M.; Berreau, L. M.
Inorg. Chem. 2002, 41, 6790; (b) Huang, K. L.; He, Y. T.; Niu, X. L.;
Chen, X. Y.; Chi, Y. N.; Gong, Y.; Hu, C. W. Inorg. Chem. Commun.
2006, 9, 1243.
Supplementary data (synthesis and characterization of
compounds, MR, NMR spectra, and spectroscopic data)
associated with this article can be found, in the online
version, at doi:10.1016/j.tetlet.2008.03.128.