A new "turn-on" naphthalenedimide-based chemosensor for mercury ions with high selectivity: Successful utilization of the mechanism of twisted intramolecular charge transfer, near-ir fluorescence, and cell images

Article abstract of DOI:10.1021/ol300607m

For the first time, a new near-IR "turn-on" fluorescent chemosensor with high selectivity for Hg²⁺ ions was designed according to the twisted intramolecular charge transfer (TICT) mechanism. The selective fluorescence enhancement effect can be optimized by modulating the solvent systems. And this naphthalenedimide-based sensor with long wavelength absorption and emission can be used to image intracellular Hg²⁺ ions in living Hela cells.

Full text of DOI:10.1021/ol300607m

ORGANIC
LETTERS
2012
Vol. 14, No. 8
2094–2097
A New “Turn-on” Naphthalenedimide-Based
Chemosensor for Mercury Ions with High
Selectivity: Successful Utilization of the
Mechanism of Twisted Intramolecular
Charge Transfer, Near-IR Fluorescence,
and Cell Images
Qianqian Li,† Ming Peng,† Huiyang Li,† Cheng Zhong,† Liang Zhang,†
Xiaohong Cheng,† Xiaoniu Peng,‡ Ququan Wang,‡ Jingui Qin,† and Zhen Li*,†
Department of Chemistry and Department of Physics and Technology, Wuhan
University, Wuhan 430072, China
lizhen@whu.edu.cn; lichemlab@163.com
Received March 12, 2012
ABSTRACT
For the first time, a new near-IR “turn-on” fluorescent chemosensor with high selectivity for Hg2þ ions was designed according to the twisted
intramolecular charge transfer (TICT) mechanism. The selective fluorescence enhancement effect can be optimized by modulating the solvent
systems. And this naphthalenedimide-based sensor with long wavelength absorption and emission can be used to image intracellular Hg2þ ions
in living Hela cells.
Nowadays, it is very important to detect some analytes,
including ions, explosives, proteins, DNA, and RNA in
certain samples for medical diagnosis, antiterrorism, en-
vironmental monitoring, etc. Thus, chemosensors, espe-
cially fluorescent ones with high sensitivity and low back-
ground disturbance, have attracted increased interest.1
As required by biologists and medical experts, those with
near-IR emission are welcome since the signal could easily
penetrate the body tissue to facilitate detection and reduce
damage.2 Thanks to the enthusiastic efforts of scientists,
many good chemosensors were reported, and some design
rules have been summarized.3 In addition to the frequently
used fluorescence signaling mechanisms of photoinduced
electron transfer (PET) and intramolecular charge trans-
fer (ICT), the concept of twisted intramolecular charge
transfer (TICT), proposed by Lippert and co-workers,4
has seldom been utilized, possibly due to the difficulty of
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J. 2010, 16, 14424–14432. (c) Hung, C. H.; Chang, G. F.; Kumar, A.;
Lin, G. F.; Luo, L. Y.; Ching, W. M.; Diau, E. W. G. Chem. Commun.
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Duan, L. J. Am. Chem. Soc. 2008, 130, 16160–16161. (c) Qian, F.;
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† Department of Chemistry.
‡ Department of Physics and Technology.
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Pfeffer, F. M. Coord. Chem. Rev. 2006, 250, 3094–3117. (b) Chen, X.;
Zhou, Y.; Peng, X.; Yoon, J. Chem. Soc. Rev. 2010, 39, 2120ꢀ2135, 3781
(c) Braunstein, P.; Naud, F. Angew. Chem., Int. Ed. 2001, 40, 680–699.
(d) Dickinson, T. A.; White, J.; Kauer, J. S.; Walt, D. R. Nature 1996,
382, 697–700. (e) Maxwell, P. H.; Wiesener, M. S.; Chang, G. W.;
Clifford, S. C.; Vaux, E. C.; Cockman, M. E.; Wykoff, C. C.; Pugh,
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10.1021/ol300607m
Published on Web 04/11/2012
2012 American Chemical Society

controlling two crucial factors, the degree of the electron
transfer and the change of molecular geometry. Corre-
spondingly, the TICT-based fluorescent chemosensors are
still very scarce (Chart S1, Supporting Information),5
although in principle, the well-designed ones should show
very good performance.
NDI-1 nearly emitted nonluminescence in solution (“off”
state) (Figure 1), as a result of the presence of the TICT
state. Upon the addition of Hg2þ, the coordination be-
tween Hg2þ and the DPA group in NDI-1 could restrain
the formation of the TICT state, directly leading to strong
red emission (“on” state) (Figure 1). To the best of our
knowledge, this is the first time that a new near-IR
fluorescent chemosensor with high selectivity for mercury
ions was designed according to the TICT mechanism.
NDI-1 was easily prepared (Scheme S1, Supporting
Information) and well characterized. The fluorescent mea-
surements for NDI-1 were first performed in THF solu-
tion. As shown in Figure S1 (Supporting Information),
NDI-1 had a visible absorption maximum wavelength at
592 nm and emitted very weakly (λem = 640 nm). This
result was unexpected, since its analogue (Chart S3, Sup-
porting Information) consisting of core-substituted NDI
as the acceptor and butylamine unit as the donor exhibited
strong fluorescence,8 indicating that the DPA unit directly
bound to the NDI core quenched the emission for the
formation of the TICT state. Excitingly, upon the addition
of Hg2þ ions, the fluorescent intensity of NDI-1 increased
rapidly, and 1.0 equiv of Hg2þ ions triggered a 110-fold
enhancement. Correspondingly, its absorption maximum
wavelength blue-shifted to 566 nm, with a clean isosbestic
point of 574 nm. The apparent color change from blue to
purple could be distinguished by the naked eye as shown
in the inset of Figure S1 (Supporting Information). This
fluorescent behavior was almost totally different from
the recently reported NDI dyes (PND), which also contain
DPA moieties as the acceptor (Chart S4, Supporting
Information).9 The differences of their structures were
small; there was no spacer between the NDI core and the
DPA group in NDI-1, while there was in PND. This
seemingly ignorable difference directly led to the disparate
properties because of the totally different internal mecha-
nisms, PET for PND versus TICT of NDI-1. This phenom-
enon also partially confirmed that it was not easy to design
new chemosensors according to the TICT mechanism, as
mentioned above.
Figure 1. Hg2þ-ions suppress the TICT process of NDI-1 for the
detection of Hg2þ ions.
Mercuric ions (Hg2þ), one of the more severe
environmental pollutants, are very harmful to humans.
Specifically, methylmercury, yielded from the microbial
biomethylation of Hg2þ, is known to cause brain damage
and other chronic diseases. Hence, rapid and sensitive
analysis of Hg2þ is badly needed.6 Recently, according to
the ICT mechanism, based on the protection reaction
between ethanethiol and aldehyde, we have developed a
new approach for the design of ratiometric fluorescent
chemosensors for mercury ions (Chart S2, Supporting
Information).7 However, the emission ranges (450ꢀ
550 nm) were rather far from the near-IR range. Based
on our previous work, according to the TICT mechanism,
we have elaborately designed a new “turn-on” naphthale-
nedimide (NDI)-based chemosensor (NDI-1, Figure 1) for
Hg2þ, in which the core-substituted NDI acts as a fluor-
ophore, di-2-picolylamine (DPA) acts as a receptor for
Hg2þ, and an additional hexylamine unit was incorporated
as a strong electron-donor to the NDI core to extend the
pushꢀpull electronic system of the whole molecule for
the tuning of the emission to the desirable near-IR region.
The interaction and binding behavior between NDI-1
1
and Hg2þ ions were investigated with their H NMR
and ESI-MS spectra (Figures S2 and S3 and Table S1,
Supporting Information). Similar to typical examples,9 the
signal of the hydrogen atoms in pyridine rings showed a
significant downfield shift (up to Δδ = 0.66 ppm), indicat-
ing a charge transfer from the pyridine groups to the Hg2þ
ions; meanwhile, the slightly downfield shifts of the hydro-
gen atoms in the NDI core, disclosing that the nitrogen
atoms linked to the NDI core, have participated in the
binding process with Hg2þ ions. Thus, all three nitrogen
atoms in one DPA group interacted with the Hg2þ ions
(Figure 1). To further clarify their relationship, calcula-
tions based on a time-dependent density functional theory
(TDDFT) were conducted at the B3LYP/6-31G* level
(5) (a) Liu, Y.; Han, M.; Zhang, H.; Yang, L.; Jiang, W. Org. Lett.
2008, 10, 2873–2876. (b) Collins, G. E.; Choi, L. S.; Callahan, J. H. J.
Am. Chem. Soc. 1998, 120, 1474–1478. (c) Aoki, S.; Kagata, D.; Shiro,
M.; Takeda, K.; Kimura, E. J. Am. Chem. Soc. 2004, 126, 13377–13390.
(d) Kollmannsberger, M.; Rurack, K.; Resh-Genger, U.; Daub, J.
J. Phys. Chem. A 1998, 102, 10211–10220.
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Valeur, B.; Leray, I. Coord. Chem. Rev. 2000, 205, 3–40. (c) Nolan,
E. M.; Lippard, S. J. Chem. Rev. 2008, 108, 3343–3480. (d) Liu, L.;
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Commun. 2005, 2161–2163.
(7) (a) Cheng, X.; Li, S.; Jia, H.; Zhong, A.; Zhong, C.; Feng, J.; Qin,
J.; Li, Z. Chem.;Eur. J. 2012, 18, 1691–1699. (b) Cheng, X.; Li, Q.; Li,
C.; Qin, J.; Li, Z. Chem.;Eur. J. 2011, 17, 7276–7281. (c) Cheng, X.; Li,
S.; Zhong, A.; Qin, J.; Li, Z. Sensor. Actuat. B, Chem. 2011, 157, 57–63.
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1066–1072.
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Eur. J. 2010, 16, 8355–8364.
Org. Lett., Vol. 14, No. 8, 2012
2095

Figure 3. Fluorescence spectra profiles of NDI-1 in acetoneꢀ
water (1:1, v/v, 20 μM) with various metal ions. Strip bars
represent the addition of the analytes (20 μM): (0) Free com-
pound NDI-1, (1) Agþ, (2) Al3þ, (3) Ba2þ, (4) Cd2þ, (5) Co2þ, (6)
Figure 2. Fluorescence spectra profiles of NDI-1 in THF (λem
=
610 nm, 20 μM) with various metal ions (20 μM). Inset:
fluorescence photograph of compound NDI-1 with various
metal ions (1.0 equiv), from left to right, Hg2þ, Agþ, Al3þ
Ba2þ, Cd2þ, Co2þ, Cr3þ, Cu2þ, Fe2þ, Fe3þ, Kþ, Liþ, Mg2þ
,
,
Cr3þ, (7) Cu2þ, (8) Fe2þ, (9) Fe3þ, (10) Kþ, (11) Liþ, (12) Mg2þ
,
(13) Mn2þ, (14) Naþ, (15) Ni2þ, (16) Pb2þ, (17) Zn2þ. Black gray
bars represent the subsequent addition of Hg2þ (20 μM) to the
solution.
Mn2þ, Naþ, Ni2þ, Pb2þ, Zn2þ
.
with the Gaussian 09 program10 to explain the electronic
structural properties of the ground and excited states
behavior of NDI-1 and NDI-1-Hg2þ. As shown in Figure
S4 (Supporting Information), in the ground state, the
optimized structure of NDI-1 has a little dihedral angle
about 35° between the NDI core and the DPA unit, and
the hexylamine unit was nearly coplanar to the NDI core.
The HOMO was delocalized through the whole π system,
while the LUMO located in the NDI core resulted in the
“D(donor)ꢀA(acceptor)ꢀD” system. However, in the first
excited state, for the large degree of electron transfer
between the donor (the DPA unit) and acceptor (the NDI
core), the TICT state, also the nonradiative excited state,
was formed. In this case, the DPA unit was nearly perpen-
dicular to the NDI core, and the HOMO was restricted on
the DPA unit, while the LUMO was located in the NDI
core, leading to the charge separation state between the
donor and acceptor. Thus, the TICT states of the rotors
caused nearly no fluorescence emission of NDI-1.
the blue-shifted absorption peak. Also, the formation of
the NDI-1ꢀHg2þ complex induced the enlargement of the
dihedral angle between the NDI core and the DPA unit,
further decreasing their electron transfer. As shown in
Figure S5 (Supporting Information), in the complex of
NDI-1 and Hg2þ ions, both of the LUMO and HOMO
almost focused on the NDI and hexylamine moieties and
the optimized structure nearly remained the same either in
the ground state or the first excited state. Thus, the binding
of Hg2þ prohibited the TICT formation of NDI-1, largely
promoting the strong red fluorescence emission, as ob-
served in Figure S1 (Supporting Information).
To further understand the TICT mechanism and study
the effect of the molecular motion on charge transfer at
an excited state, the fluorescence intensity dependence
on solvent and the effect of the solvent viscosity and
temperature were assessed (Figures S6ꢀS8, Supporting
Information). The obtained experimental results con-
firmed that the excited state of NDI-1 was decayed by
the TICT process and the cooperative binding of Hg2þ
through a DPA moiety could restrain the formation of the
TICT state efficiently, leading to the “turn-on” fluores-
cence response of NDI-1.
Upon the addition of Hg2þ, the binding of Hg2þ withthe
DPA unit decreasedits electron donor ability, as proven by
(10) Gaussian 09, Revision E.1: Frisch, M. J.; Trucks, G. W.;
Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.;
Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam,
J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.;
Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.;
Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.;
Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.;
Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck,
A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul,
A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko,
A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; M. A.
Al-Laham, Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill,
P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople,
J. A. Gaussian, Inc., Pittsburgh, PA, 2009.
To evaluate the specificity of NDI-1 toward Hg2þ
,
various ions were examined in parallel under the same
conditions. As shown in Figure 2, the binding of NDI-1
with Hg2þ ions resulted in a 110-fold increase of the
fluorescence intensity, whereas Agþ, Al3þ, Ba2þ, Cd2þ
Co2þ, Cr3þ, Cu2þ, Fe2þ, Fe3þ, Kþ, Liþ, Mg2þ, Mn2þ
,
,
Naþ, Ni2þ, Pb2þ, and Zn2þ had negligible influence.
However, when 1.0 equiv of Hg2þ was added into the
THF solution of NDI-1 in the presence of one of the other
metal ions (1.0 equiv), the selectivity was interfered with by
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Org. Lett., Vol. 14, No. 8, 2012

Co2þ, Cu2þ, and Ni2þ (Figure S9, Supporting Informa-
tion). As mentioned above, in the TICT process, the
positive and negative charges were located in two different
and separated functional parts of NDI-1, with an increase
in the molecular dipole moment, and the high polarity
solvent could induce this process and stabilize the charge
separatedstate.5b Thus, in principle, different solvents with
different polarities should affect the emission spectra of
NDI-1, especially the intensity, to some degree (Figure S6,
Supporting Information). In other words, the change of
the solvent might be a method to improve the selectivity
for Hg2þ. After the relatively systematic tests in various
solvents, we chose a mixture of acetone and water as the
solvent system for NDI-1 to detect Hg2þ, with the best
ratio of 1:1 (v/v). As shown in Figure S10 (Supporting
Information), only a small change appeared upon the
addition of other metal ions, but a strong fluorescence
response was observed with the subsequent addition of
Hg2þ, exhibiting good selectivity for Hg2þ ions in the
complicated systems.
The fluorescence emission of NDI-1 over a wide range
of pH values was studied (Figure S11, Supporting
Information). No apparent changes of the fluorescence
spectra were observed when the pH values ranged from
4.0 to 13.0, and the detection of Hg2þ can work well in in
the range of pH 5.0ꢀ9.0 (Figure S11, Supporting
Information). The selectivity of NDI-1 to Hg2þ was also
examined in the presence of various metal ions in these pH
values. As shown in Figure S12 (Supporting Information),
NDI-1 still worked well and exhibited good selectivity.
It should be pointed out that the stable fluorescence
and good selectivity of NDI-1 at a pH value of ∼7.0 was
favorable for the in vivo application. Thus, once more, the
fluorescencetitrationsofNDI-1withHg2þ wereconducted
under the optimized conditions (acetone/water = 1/1 (v/v),
pH = 7.0), with the results summarized in Figure S13
(Supporting Information). Accompanying the increase of
the concentration of the added Hg2þ ions, an obvious
“turn-on” process of the fluorescence intensity at 610 nm
was observed, and the maximum intensity was achieved: a
100-fold enhancement. As shown in Figure 3, the emission
profile of the NDI-1ꢀHg2þ complex was unperturbed in
the presence of 1.0 equiv of other metal ions. Additionally,
the counteranions did not have obvious influences on the
fluorescence spectra (Figure S14, Supporting Information).
The good performance of NDI-1 prompted us to utilize
it in the image of the HL cells by a confocal laser scanning
microscopy. Bright-field measurements confirmed that
the cells after treatment with Hg2þ and NDI-1 were viable
throughout the imaging experiments (Figure 4DꢀF).
Under selective excitation of NDI-1, staining HL cells with
a 2.5 μM solution of NDI-1 for 10 min led to very weak
Figure 4. Confocal fluorescence images of living HL cells: (A) HL
cells incubated with Hg2þ (5 μM) for 10 min; (B) HL cells cells
incubated with compound NDI-1 (2.5 μM) for 10 min; (C) HL
cells incubated with NDI-1 (2.5 μM) for 10 min after preincuba-
tion with Hg2þ (2.5 μM) for 10 min. (D), (E), and F) represent the
bright-field images of (A), (B), and (C), respectively.
intracellular fluorescence (Figure 4B), followed by the
addition of Hg2þ (2.5 μM) for another 10 min; the
apparent enhancement of fluorescence was easily observed
(Figure 4C), suggesting that NDI-1 can penetrate the cell
membrane and be used for imaging of Hg2þ
.
In conclusion, NDI-1 has been designed according to the
TICT mechanism, which exhibited “turn-on” fluorescence
response toward Hg2þ ions. Coupled with the long wave-
length fluorescence, cell-permeability, and its function in
the wide range of pH, NDI-1 can be applied to image Hg2þ
ions in living cells and in vivo potentiality. And actually,
for the even better performance, the structure of NDI-1
should be further modified to improve the solubility in
aqueous solutions. Thus, this good example might stimu-
late wide interest of scientists for further development of
new chemosensors possessing excellent performance with
the guidance of the TICT mechanism.
Acknowledgment. We are grateful to the National
Science Foundation of China (No. 21002075) and the Na-
tional Fundamental Key Research Program (2011CB932702)
for financial support.
Supporting Information Available. Experimental sec-
tion, fluorescence spectra, MS spectra, and 1H NMR and
13C NMR spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
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