Highly selective colorimetric fluorescence sensor for Cu2+: Cation-induced 'switching on' of fluorescence due to excited state internal charge transfer in the red/near-infrared region of emission spectra

Article abstract of DOI:10.1016/j.tetlet.2010.08.048

A 1,8-naphthalimide-based colorimetric fluorescence perinone dye, receptor 1 is reported herein for the selective detection of Cu²⁺ over the other heavy and transition metal ions. Receptor 1 shows a strong colorimetric change from orange to purple and a dramatic enhancement of fluorescence intensity due to cation-induced excited state internal charge transfer during the sensing event, that is, a dual optical response that would facilitate naked-eye detection of Cu²⁺.

Full text of DOI:10.1016/j.tetlet.2010.08.048

Tetrahedron Letters 51 (2010) 5563–5566
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Highly selective colorimetric fluorescence sensor for Cu2+: cation-induced
‘switching on’ of fluorescence due to excited state internal charge transfer in
the red/near-infrared region of emission spectra
⇑
Shyamaprosad Goswami , Debabrata Sen, Nirmal Kumar Das, Giridhari Hazra
Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711 103, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A 1,8-naphthalimide-based colorimetric fluorescence perinone dye, receptor 1 is reported herein for the
selective detection of Cu2+ over the other heavy and transition metal ions. Receptor 1 shows a strong col-
orimetric change from orange to purple and a dramatic enhancement of fluorescence intensity due to cat-
ion-induced excited state internal charge transfer during the sensing event, that is, a dual optical
Received 19 May 2010
Revised 13 August 2010
Accepted 16 August 2010
Available online 20 August 2010
response that would facilitate naked-eye detection of Cu2+
.
Ó 2010 Elsevier Ltd. All rights reserved.
Development of molecular signaling systems especially for cat-
ions, rendering selective changes upon binding analytes either by
colorimetric or by fluorescence modulations has received much
attention as photonic molecular devices.1 For the past few decades,
fluorescent chemosensors for the detection of Cu2+ ion have ac-
tively been investigated as copper is a widely used industrial metal
and hence it is a significant environmental pollutant at high con-
centration2 but at the same time, it is an essential trace element
in biological systems. It is well known to all that Cu2+ is a notorious
fluorescence quencher3 because of its paramagnetic nature, so
most of the classic and early-reported Cu2+ sensors undergo fluo-
rescence quenching upon binding of Cu2+ either by electron or by
energy transfer mechanism.4 Therefore owing to sensitivity rea-
sons and high order of interference often caused by the chemically
closely related metal ions (e.g., Zn2+, Fe3+, and Hg2+) during spec-
troscopic studies, the development of new highly selective and
sensitive colorimetric Cu2+ fluoroionophores showing fluorescence
enhancement5 is a continued interesting and challenging task.
Fused heterocycles perinone and its derivatives are excellent chro-
mogenic and fluorogenic dyes which are widely used for industrial
plastics.6 Previously, we have mentioned that perimidine core at
the top of naphthalene moiety has excellent colorimetric response
upon Cu2+ complexation via internal charge transfer (ICT) mecha-
nism.7 Additionally, 1,8-naphthalimide-based fluorescent chemo-
sensors8 have excellent photophysical properties due to large
Stoke-shifted emission band, larger molar extinction coefficient,
and high quantum yield. In continuation of our interest in the
ICT concept as a promising metal ion-sensing strategy, we now re-
port an easy to access colorimetric ‘switching on’ ICT probe, a new
1,8-naphthalimide-based perinone dye for the selective detection
of Cu2+ ion. The chromophore shows a distinct color change from
orange to purple as a result of enhanced ICT and a dramatic
enhancement of emission intensity due to excited state charge
transfer (CT) in the red/near-infra-red (NIR) region9 during Cu2+
complexation. Successful reports of such chemosensors with cop-
per ion-induced enhancement of fluorescence at k >650 nm in
the literature are very scarce.10 Combination of dual optical re-
sponses via either colorimetric or fluorescence modulations raises
the sensitivity of the fluoroionophore and thus makes it a particu-
larly effective one for selective detection of Cu2+
.
The fused fluoroionophore11 receptor 1 having six fused rings is
synthesized (Scheme 1) in a single step simply by cyclization of
1,8-diaminonaphthalene with 1,8-naphthalic anhydride in dry tet-
rahydrofuran under refluxing condition and the product was ob-
tained in an excellent yield. The rigid receptor constitutes a
perimidine core as an ion recognition unit, which is fused together
by fluorescence naphthalene moieties which are certainly
O
O
O
O
N
H2N
H2N
THF
+
reflux, 2 h
N
Receptor 1
O
N
O
O
O
H2N
H2N
(i) THF
+
(ii)
DMF, DCC
N
Receptor 2
⇑
Corresponding author. Tel.: +91 33 2668 2961–3; fax: +91 33 2668 2916.
E-mail address: spgoswamical@yahoo.com (S. Goswami).
Scheme 1. Synthesis of the receptors.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.08.048

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S. Goswami et al. / Tetrahedron Letters 51 (2010) 5563–5566
responsible for signal transduction during spectral investigations.
The known phthalimide analog perinone, receptor 2 was also syn-
thesized by the reported procedure12 for comparison of binding
and fluorescence sensitivity with Cu2+ which thus demonstrated
that our designed naphthalimide analog is the most effective fluo-
0.4
0.3
0.2
0.1
0.0
Free Cu2+
(a)
(b)
rescence sensor for Cu2+
.
The plausible ICT mechanism for our system is illustrated in
Scheme 2. The metal ion chelation at the three potential binding
sites, comprising of the perimidine nitrogens and the carbonyl oxy-
gen functions could trigger the enhanced ICT and thus we may
anticipate significant photophysical perturbations of the ionophore
in the presence of Cu2+. The pronounced spectral variations, partic-
ularly the largely red shifted ICT absorption by Cu2+ are consistent
with the expected increase in the ICT interaction mediated by the
electron donation from the probable donor perimidine nitrogens
along with amide carbonyl of the naphthalimide subunit, acting
as an acceptor. This mode of binding is supported by the IR data.
In the IR spectrum of receptor 1 (see Supplementary data), the
peaks at 1680 and 1600 cmÀ1 ascribed to the C@O and C@N
stretching, respectively, are observed to shift upon Cu2+ complexa-
320 400 480 560 640
Wavelength (nm)
Figure 1. (a) UV–vis absorption spectra of receptor 1 upon addition of 50 l
M Cu2+
in CH3CN/H2O (90:10, v/v). (b) Color changes from orange to purple upon Cu2+
complexation.
3
0.16
(a)
(b)
0.12
0.08
0.04
0.00
2
1
0
tion toward lower wave numbers of 1660 and 1585 cmÀ1
respectively.13
,
The photophysical behavior of the chemosensor with several
guest cations (Li+, Na+, K+, Mg2+, Ca2+, Cu2+, Zn2+, Cd2+, Hg2+, Pb2+
,
Ni2+, Co2+, Fe3+, and Mn2+) in CH3CN/H2O (90:10, v/v) is investi-
gated by means of UV–vis and fluorescence measurements. All
the cations are employed using their perchlorate salts and the
titration experiments are carried out at pH 7.1 (35 mM HEPES buf-
fer). The UV–vis absorption spectrum of the metal-free form of
receptor 1 (c = 2.0 Â 10À5 M) in CH3CN/H2O (90:10, v/v) exhibits
two kmax peaks at 343 and 359 nm and a low energy broad band
at around 470 nm which is possibly attributed to the weak ICT type
transition from pyrimidine core to naphthalimide subunit in the
ground state level of the dye. Spectroscopic investigations are car-
ried out by using the above-mentioned set of metal cations
(c = 2.0 Â 10À4 M). From the instrumental read-out, it can easily
be concluded that Cu2+ promotes remarkable response toward
the receptor (Fig. 1a). Upon gradual addition of Cu2+ solution to
the receptor 1, the bands at 343 and 359 nm progressively de-
crease and get slightly blue shifted while the band at 470 nm dis-
appears gradually with simultaneous generation of a new broad
band appearing at 557 nm whose intensity increases progressively
0.2 0.4 0.6 0.8 1.0
Xh
10
20
30
40
[Cu2+]
Figure 2. (a) Intensity ratio (A557/A470) as a function of Cu2+ concentration (in
(b) Job plot diagram of receptor 1 for Cu2+ determined by UV–vis method (where Xh
is the mole fraction of the host and I indicates the change of absorbance).
lM).
D
culated at m/z 382.0722 and 381.0714 which are found at m/z
382.0720 and 381.0708, respectively.
The addition of other metal ions even in excess such as Li+, Na+,
K+, Mg2+, Ca2+, Zn2+, Cd2+, Hg2+, Pb2+, Ni2+, Co2+, Fe3+, Mn2+, and Ag+
produces insignificant changes in the absorption spectra. In all the
cases, a nominal decrease in absorbance intensities is observed.
The UV–vis absorption spectra of receptor 2 and its subsequent
color change during complexation with Cu2+ as a result of ICT are
depicted in Figure 3.
The cation-binding properties of the receptor 1 with Cu2+ in the
excited state are also studied by fluorescence spectroscopic meth-
od. Chemosensor in the absence of Cu2+ upon excitation at 470 nm
shows a very weak fluorescence at 650 nm. This weakly emissive
band probably has its origin in the ICT-type excited state. Upon
gradual addition of Cu2+ solution to the receptor solution, a
remarkable enhancement of fluorescence intensity (almost 25-fold
increase) is observed and the emission intensity gets slightly red
shifted at 675 nm assigning Cu2+-induced enhancement of ICT,
which is clearly observed (Fig. 4a).
reaching the maxima when 50
lM of this metal ion is added. The
new red shifted low-energy absorption band (
D
k = 87 nm) is ob-
served confirming the efficient charge transfer phenomenon which
results in a naked-eye color change from orange to purple (Fig. 1b).
Two well-defined isosbestic points at 404 and 514 nm are observed
during the spectral titrations, indicating the presence of a unique
complex in equilibrium with the receptor.
The resulting titration isotherm fits nicely with a 1:1 binding
model as suggested by the Job plot diagram (Fig. 2b). The associa-
tion constant (Ka) determined by UV–vis method14 is found to be
2.16 Â 104 MÀ1 (error <10%). The HRMS mass spectrum of the
Cu2+ complex of receptor 1 (see Supplementary data) also supports
a single mononuclear complex between the metal ion and receptor
unit. The [MÀH]+ and [MÀ2H]+ peaks for the Cu2+ complex are cal-
Free Cu2+
0.4
(a)
0.3
(b)
0.2
O
0.1
0.0
O
Cu2+
ICT
N
N
Cu2+
N
300
400
500
600
Wavelength (nm)
N
Figure 3. (a) UV–vis absorption spectra of receptor 2 upon addition of 60 l .
M Cu2+
Scheme 2. Proposed mode of binding with Cu2+
.
(b) Color changes from yellow to purple upon Cu2+ complexation.

S. Goswami et al. / Tetrahedron Letters 51 (2010) 5563–5566
5565
440
330
220
110
0
640
Free
Cu2+
(a)
480
320
160
0
(b)
540 600 660 720 780
Wavelength (nm)
600
675
750
825
Wavelength (nm)
Figure 6. Fluorescence emission spectra of receptor 2 upon gradual addition of Cu2+
Figure 4. (a) Fluorescence emission spectra of receptor 1 upon gradual addition of
in CH3CN/H2O (90:10, v/v).
Cu2+ in CH3CN/H2O (90:10, v/v). (b) Red emission as observed during titration
experiment with Cu2+
.
induced enhanced ICT (Fig. 3) but due to its poor stabilizing capac-
ity relative to 1,8-naphthalimide subunit such excited state ICT is
possibly prevented and the emission intensity quenched steadily
during Cu2+ complexation (Fig. 6). Therefore receptor 2 behaves
It is noteworthy to mention that addition of other metal ions
produces insignificant change in emission intensity and only a
slight decrease of fluorescence intensity is observed at 650 nm un-
der identical experimental conditions (Fig. 5). Though the nature of
the excited states involved in this phenomenon is presently unde-
fined, we assume that two different but distinct emissive channels
are involved within the Cu2+-bound state of the fluorophore. Fluo-
rescence enhancement at 675 nm in the present case is probably
due to the engagement of different donors and acceptor groups
within the proximity during complexation and thereby a tightly
bound metal complex is formed due to strong chelation of Cu2+
with the receptor unit.15 Hence, the responsible mechanism of
fluorescence quenching via non-radiative pathways prevailing in
the free receptor unit might be suppressed that leads to an
enhancement of fluorescence in the presence of the Cu2+ ion. An-
other factor for the fluorescence enhancement is that coordination
of the amide oxygen with the target metal ion probably increases
the electron-withdrawing character of the 1,8-naphthalimide sub-
unit and therefore some degree of efficient excited state ICT occurs
between the Cu2+-bound substrate and the conjugated electronic
system of the 1,8-naphthalimide acceptor subunit within such a
microenvironment of the fluorophore. Hence, a substantial change
in the geometry followed by a change in the dipole moment of the
probe in the excited state is expected that possibly modulates the
as a ‘switched off’ fluorescence probe toward Cu2+
.
Participation of the 1,8-naphthalimide unit in most effective
sensor 1 in the excited state probably induces a highly emissive
stable CT excited state that strongly amplifies the fluorescence sig-
nal emitted by the system upon binding with Cu2+. Since the broad
and large Stokes shifted emission bands are in visible region, this
visible CT emission allows the sensor to detect Cu2+ through naked
eye experiment readily and a colorless to promising red fluores-
cence is observed (Fig. 4b). The association constant as determined
by fluorescence titration method following Benesi–Hildebrand
equation14 is found to be 2.54 Â 104 MÀ1 (error <10%).
In summary, we have reported here a highly selective and sen-
sitive colorimetric fluoroionophore for Cu2+ based on metal trig-
gered enhanced ICT in a six rings annulated structurally rigid
perinone dye with efficient cation-induced ‘switching on’ of fluores-
cence in the red/near-infrared spectral region. An important fea-
ture of this charge transfer fluoroionophore is that its highly
selective fluorescence response toward Cu2+ is not only due to
the binding preference of the metal ion with perimidine donor sub-
unit (like receptor 2) but also due to the combined response of the
electron donor in the perimidine core, and the conjugated naph-
thalimide acceptor unit in the excited state provides a new strat-
fluorescence signal and a slight red shift (Dk = 25 nm) in fluores-
egy for constructing ‘switching on’ ICT fluorophore for Cu2+
.
cence is observed during Cu2+ sensing. Such metal ion-induced
fluorescence activation of the donor subunit followed by excited
state ICT16 between the donor and the attached acceptor fluoro-
phore is supported when we compare receptor 2 (single fluoro-
phore system) with receptor 1 (dual fluorophore system) for Cu2+
sensing. The phthalimide analog perinone exhibits similar cation-
Acknowledgments
We thank CSIR [No. 01(2292)/09/EMR-II], Government of India
for financial support. D.S. and G.H. acknowledge CSIR and N.K.D.
acknowledges UGC Government of India for providing research fel-
lowships. We also acknowledge DST, AICTE, UGC and MHRD for
funding instrumental facilities in our department.
415
Cu2+
Supplementary data
410
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.08.048.
405
400
References and notes
1. (a) James, T. D.; Linnane, P.; Shinkai, S. Chem. Commun. 1996, 281; (b) Beer, P. D.
Chem. Commun. 1996, 689; (c) de Silva, A. P.; Gunaratne, H. Q. N.; McVeigh, C.;
Maguire, G. E. M.; Maxwell, P. R. S.; O’Hanlon, E. Chem. Commun. 1996, 2191.
2. (a) Krämer, R. Angew. Chem., Int. Ed. 1998, 37, 772; (b) Linder, M. C.; Hazegh-
Azam, M. Am. J. Clin. Nutr. 1996, 63, 797S–811S; (c) Uauy, R.; Olivares, M.;
Gonzalez, M. Am. J. Clin. Nutr. 1998, 67, 952S–959S; (d) Harris, Z. L.; Gitlin, J. D.
Am. J. Clin. Nutr. 1996, 63, 836S–841S.
0
3. (a) Kim, H. J.; Park, S. Y.; Yoon, S.; Kim, J. S. Tetrahedron 2008, 64, 1294; (b) Kim,
H. J.; Hong, J.; Hong, A.; Ham, S.; Lee, J. H.; Kim, J. S. Org. Lett. 2008, 10, 1963; (c)
Figure 5. Change of fluorescence intensity of receptor 1 after adding 50 lM of each
of the guest cation in CH3CN/H2O (90:10, v/v).

5566
S. Goswami et al. / Tetrahedron Letters 51 (2010) 5563–5566
Lin, W.; Yuan, L.; Tan, W.; Feng, J.; Long, L. Chem. Eur. J. 2009, 15, 1030; (d)
Zheng, Y.; Gattás-Asfura, K. M.; Konka, V.; Leblanc, R. M. Chem. Commun. 2002,
2350.
(d) Chovelon, J.-M.; Grabchev, I. Spectrochim. Acta, Part A 2007, 67, 87; (e)
Wang, J.; Xiao, Y.; Zhang, Z.; Qian, X.; Yang, Y.; Xu, Q. J. Mater. Chem. 2005, 15,
2836; (f) Wen, G.-T.; Zhu, M.-Z.; Wang, Z.; Meng, X.-M.; Hu, H.-Y.; Guo, Q.-X.
Chin. J. Chem. 2006, 24, 1230.
4. (a) Xie, J.; Ménand, M.; Maisonneuve, S.; Métivier, R. J. Org. Chem. 2007, 72,
5980; (b) Mu, H.; Gong, R.; Ma, Q.; Sun, Y.; Fu, E. Tetrahedron Lett. 2007, 48,
5525; (c) Goswami, S.; Chakraborty, R. Tetrahedron Lett. 2009, 50, 2911; (d)
Choi, J. K.; Kim, S. H.; Yoon, J.; Lee, K.-H.; Bartscha, R. A.; Kim, J. S. J. Org. Chem.
2006, 71, 8011; (e) Park, S. M.; Kim, M. H.; Choe, J.-I.; No, K. T.; Chang, S.-K. J.
Org. Chem. 2007, 72, 3550; (f) Wen, Y.-Q.; Yue, F.; Zhong, Y.-R.; Ye, B.-H. Inorg.
Chem. 2007, 46, 7749.
5. (a) Ghosh, P.; Bharadwaj, P. K. J. Am. Chem. Soc. 1996, 118, 1553; (b) Li, G.-K.; Xu,
Z.-X.; Chen, C.-F.; Huang, Z.-T. Chem. Commun. 2008, 1774; (c) Jung, S.-H.; Park,
M.; Han, Y.-D.; Kim, E.; Lee, C.; Ham, S.; Kim, S.-J. Org. Lett. 2009, 11, 3378; (d)
Weng, Z.-C.; Yang, R.; He, H.; Jiang, Y.-B. Chem. Commun. 2006, 106; (e) Xu, Z.;
Xiao, Y.; Qian, X.; Cui, J.; Cui, D. Org. Lett. 2005, 7, 3029; (f) Zhou, Y.; Wang, F.;
Kim, Y.; Kim, S.-J.; Yoon, J. Org. Lett. 2009, 11, 4442; (g) YuFang, X.; Feng, L.;
ZhaoChao, X.; TanYu, C.; XuHong, Q. Sci. China, Ser. B Chem. 2009, 52, 771; (h)
Kaur, S.; Kumar, S. Chem. Commun. 2002, 2840; (i) Bhattacharya, S.; Gulyani, A.
Chem. Commun. 2003, 1158; (j) Bhattacharya, S.; Thomas, M. Tetrahedron Lett.
2000, 41, 10313; (k) Zheng, Y.; Huo, Q.; Kele, P.; Andreopoulos, F. M.; Pham, S.
M.; Leblanc, R. M. Org. Lett. 2001, 3, 3277; (l) Zheng, Y.; Cao, X.; Orbulescu, J.;
Konka, V.; Andreopoulos, F. M.; Pham, S. M.; Leblanc, R. M. Anal. Chem. 2001, 75,
1706.
9. (a) Miller, J. N. Long-wavelength and Near-infrared Fluorescence. State of the
Art, Future Applications, and Standards. In Standardization and Quality
Assurance in Fluorescence Measurement II. Springer Ser. Fluoresc.; Resch-
Genger, U., Ed.; Springer: Berlin, 2008; Vol. 5, pp 147–162; (b) Li, X.; Shi, W.;
Chen, S.; Jia, J.; Ma, H.; Wolfbeis, O. S. Chem. Commun. 2009, 5000.
10. Rurack, K.; Resch-Genger, U.; Bricks, J. L.; Spieles, M. Chem. Commun. 2000,
2103.
11. Thomas, P. Eur. Pat. Appl. EP 780444, Jun 25, 1997. PCT Int. Appl., 2009, 21pp.
Chemical Indexing Equivalent to 152:120631(DE). CODEN: PIXXD2 WO
2009127182 A1 20091022 CAN 151:471181 AN 2009: 1289891.
12. Alfredo, N. K.; Likhatchev, D.; Ramirez, S. B.; Vazquez, J. R.; Valverde, G. C.;
Alexandrova, L. Polymer 2008, 49, 3654–3662.
13. Bardez, E.; Devol, I.; Larrey, B.; Valeur, B. J. Phys. Chem. B 1997, 101, 7786.
14. Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71, 2703.
15. (a) Mcfarland, S. A.; Finney, N. S. J. Am. Chem. Soc. 2002, 124, 1178; (b) Zhang, L.;
Clark, R. J.; Zhu, L. Chem. Eur. J. 2008, 14, 2894; (c) Badugu, R.; Lakowicz, J. R.;
Geddes, C. D. J. Am. Chem. Soc. 2005, 127, 3635.
16. (a) Wen, Z.-C.; Yang, R.; Hea, H.; Jiang, Y.-B. Chem. Commun. 2006, 106; (b) Lee,
S. Y.; Kim, H. J.; Wua, J.-S.; No, K.; Kim, J. S. Tetrahedron Lett. 2008, 49, 6141; (c)
Jung, J. H.; Lee, M. H.; Kim, H. J.; Jung, H. S.; Lee, S. Y.; Shin, N. R.; No, K.; Kim, J.
S. Tetrahedron Lett. 2009, 50, 2013; (d) Sumner, J. P.; Westerberg, N. M.;
Stoddard, A. K.; Fierke, C. A.; Kopelman, R. Sens. Actuators, B: Chem. 2006, 13,
760; (e) Rurack, K.; Bricks, J. L.; Schulz, B.; Maus, M.; Reck, G.; Resch-Genger, U.
J. Phys. Chem. A 2000, 104, 6171; (f) Leray, I.; Lefevre, J.-P.; Delouis, J.-F.; Delaire,
J.; Valeur, B. Chem. Eur. J. 2001, 7, 4590.
6. (a) Herbst, W.; Hunger, K. Industrial Organic Pigments, 3rd ed.; Wiley, 2004.; (b)
Hays, B. G.; Bindra, A. P. In Polymeric Materials Encyclopedia; Salamone, J. C., Ed.;
CRC Press: Boca Raton, 1996; pp 1321–1332; (c) Nagao, Y.; Tsuda, K.; Kozawa,
K.; Uchida, T. Heterocycles 2001, 54, 757.
7. Goswami, S.; Sen, D.; Das, N. K. Org. Lett. 2010, 12, 856.
8. (a) Guo, X.; Qian, X.; Jia, L. J. Am. Chem. Soc. 2004, 126, 2272; (b) Xu, Z.; Qian, X.;
Cui, J. Org. Lett. 2005, 7, 3029; (c) Wang, J.; Qian, X. Chem. Commun. 2006, 109;