Fluorescence turn on of coumarin derivatives by metal cations: A new signaling mechanism based on C=N isomerization

Article abstract of DOI:10.1021/ol062518z

(Chemical Equation Presented) A new sensing mechanism based on C=N isomerization, which shows a very significant fluorescence enhancement to the metal cations in a simple and efficient way, is demonstrated. A coumarin derivative (L) containing a C=N group was designed as an example for illustration. The free ligand L is almost nonfluorescent due to the isomerization of C=N double bond in the excited state. However, the solution of ligand shows about a 200-fold increase of fluorescence quantum yield (about 30%) upon addition of Zn(ClO₄)₂.

Full text of DOI:10.1021/ol062518z

ORGANIC
LETTERS
2007
Vol. 9, No. 1
33-36
Fluorescence Turn On of Coumarin
Derivatives by Metal Cations: A New
Signaling Mechanism Based on C
Isomerization
)N
Jia-Sheng Wu,†,‡ Wei-Min Liu,†,‡ Xiao-Qing Zhuang,†,‡ Fang Wang,†,‡
Peng-Fei Wang,*,† Si-Lu Tao,†,‡ Xiao-Hong Zhang,† Shi-Kang Wu,† and
Shuit-Tong Lee§
Nano-Organic Photoelectronic Laboratory, Laboratory of Organic Optoelectronic
Functional Materials and Molecular Engineering, Technical Institute of Physics and
Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China, Graduate
School of Chinese Academy of Sciences, Beijing, China, and Center of Super-Diamond
and AdVanced Films (COSDAF) and Department of Physics and Materials Science,
City UniVersity of Hong Kong, Hong Kong SAR, China
wangpf@mail.ipc.ac.cn
Received October 12, 2006
ABSTRACT
A new sensing mechanism based on C
a simple and efficient way, is demonstrated. A coumarin derivative (L) containing a C
The free ligand L is almost nonfluorescent due to the isomerization of C N double bond in the excited state. However, the solution of ligand
shows about a 200-fold increase of fluorescence quantum yield (about 30%) upon addition of Zn(ClO4)2.
d
N isomerization, which shows a very significant fluorescence enhancement to the metal cations in
d
N group was designed as an example for illustration.
d
Fluorescent probes/chemosensors capable of selectively
recognizing guest species are of particular interest in su-
pramolecular chemistry because of their high selectivity,
sensitivity, and simplicity.1,2 A fluorescent probe/chemosen-
sor is a molecular system for which the physicochemical
properties change upon interaction with a chemical species
in such a way as to produce a detectable fluorescent signal.3
Exploration of new interaction mechanisms between recogni-
tion and signal reporting units is of continuing interest for
design of new fluorescent probes/chemosensors. Indeed, until
now, a number of signaling mechanisms have been developed
and widely applied for the optical detection of different
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Am. Chem. Soc. 2006, 128, 6004. (b) Jisha, V. S.; Arun, K. T.; Hariharan,
M.; Ramaiah, D. J. Am. Chem. Soc. 2006, 128, 6024. (c) Fussl, A.;
Schleifenbaum, A.; Goritz, M.; Riddell, A.; Schultz, C.; Kramer, R. J. Am.
Chem. Soc. 2006, 128, 5986. (d) Kiyose, K.; Kojima, H.; Urano, Y.; Nagano,
T. J. Am. Chem. Soc. 2006, 128, 6548. (e) Suzuki, Y.; Yokoyama, K. J.
Am. Chem. Soc. 2005, 127, 17799. (f) Xie, H. Z.; Yi, S.; Yang, X.; Wu, S.
K. New J. Chem. 1999, 23, 1105. (g) Zhou, L. L.; Sun, H.; Li, H. P.; Wang,
H.; Zhang, X. H.; Wu, S. K.; Lee, S. T. Org. Lett. 2004, 6, 1071. (h) Mei,
M. H.; Wu, S. K. New J. Chem. 2001, 25, 471.
† Chinese Academy of Sciences.
‡ Graduate School of Chinese Academy of Sciences.
§ City University of Hong Kong.
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J. L.; Davis, J. M. Acc. Chem. Res. 2001, 34, 989. (d) Goldsmith, C. R.;
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10.1021/ol062518z CCC: $37.00
© 2007 American Chemical Society
Published on Web 12/13/2006

species. These include photoinduced electron/energy transfer
(PET),4 metal-ligand charge transfer (MLCT),5 intramo-
lecular charge transfer (ICT),6 excimer/exciplex formation,7
and excited-state intra-/intermolecular proton transfer (ESPT),8
etc. However, it should be noted that CdN isomerization
has not been used as a signaling mechanism for fluorescent
probes/chemosensors, although it has been documented
previously.9
coumarin derivativescompound L increases about 200-fold
upon complexation with Zn2+.
In the present work, a typically fluorescent dye, coumarin,
was chosen as a fluorophore, and an antipyrine derivative
acts as an additional chelating moiety. Both parts are linked
by a CdN bond to form a potential fluorescent-sensing
molecule (L) for metal cations. Compound L was synthesized
according to the methodology as shown in Figure 2 and
Figure 2. Synthetic method of compound L.
1
characterized by H NMR, 13C NMR, MS, and elemental
Figure 1. Molecular structures of unbridged and bridged CdN
compounds.
analysis.
In our previous work, as shown in Figure 1, we found
that CdN isomerization is the predominant decay process
of the excited states for compounds with an unbridged Cd
N structure so that those compounds are often nonfluorescent.
In contrast, the fluorescence of its analogues with a co-
valently bridged CdN structure increases dramatically due
to the suppression of CdN isomerization in the excited
states.10 With this in mind, we can reasonably expect that
the CdN isomerization may also be inhibited by guest
species through complexation in a sophisticatedly designed
fluorescent-sensing molecule rather than covalent bridging
of CdN bond. Herein, we report a novel signaling system
via CdN isomerization as part of our continuing efforts for
exploration of new sensing mechanisms for fluorescent
probes/chemosensors.7e Fluorescence quantum yield of the
Figure 3. (A) UV-vis spectra of L (10 µM) and L (10 µM) upon
addition of 10 equiv of Zn(ClO4)2 in CH3CN. (B) Fluorescence
spectra of L (10 µM) in CH3CN upon addition of increasing
concentrations of Zn(ClO4)2 (0, 2.5, 5, 7.5, 10, 15, 20, 30, 40, 50,
60, 70, 80, 90, and 100 µM) with an excitation wavelength of
450 nm. Inset: Photos of L in CH3CN with and without addition
of Zn(ClO4)2.
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McAllister, M. A. J. Org. Chem. 2001, 66, 7979.
Figure 3 shows variations of UV-vis and fluorescence
spectra of L upon addition of zinc cation in CH3CN. It was
found that the maximum absorption wavelength of L in CH3-
CN shifts from 453 to 490 nm upon addition of 10 equiv of
Zn(ClO4)2 (Figure 3A). On the other hand, the fluorescence
of the solution increases dramatically with increasing con-
centrations of Zn2+; concomitantly, the maximum emissive
wavelength shifts from 500 to 522 nm (Figure 3B). Actually,
an obviously green emission of the solution can be easily
observed by the naked eye, as shown in Figure 3B.
Fluorescence quantum yield (Φf) of free L in CH3CN is
0.15%, whereas it reaches 30% when L binds with Zn2+
(10) (a) Yang, G. Q.; Morlet-Savary, F.; Peng, Z. K.; Wu, S. K.;
Fouassier, J. P. Chem. Phys. Lett. 1996, 256, 536. (b) Li, Z. M.; Wu, S. K.
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Photobiol. A: Chem. 1995, 86, 109.
34
Org. Lett., Vol. 9, No. 1, 2007

(fluorescein in 0.1 N NaOH as a reference, Φf ) 0.85).11 In
other words, the free ligand L is non-emissive, when it
encounters zinc cation in CH3CN, its fluorescence can be
turned from “off” to “on”, resulting in a 200-fold enhance-
ment of the fluorescence quantum yield.12
observed an obvious enhancement of fluorescence quantum
yields even in the presence of Cd2+, Cu2+, Pb2+, and Hg2+.
The reason can be explained by the competition between
the fluorescence enhancement due to the inhibition of CdN
isomerization and quenching of fluorescence from the metal
cation-induced electron or energy transfer processes. This
result indicates that the CdN isomerization plays a pre-
dominant role, although the restriction of s-cis versus s-trans
conformation in the fluorophore may also be a little partial
cause of the observed fluorescence turn-on (vide infra) and
gives rise to more significantly spectral and photophysical
response to transition metal cations.
In order to get insight into the decay process, the lifetime
of L in CH3CN was measured by single photon counting,
and it shows a good single-exponential decay. The lifetime
of free L is quite short (0.15 ns). The radiative decay rate
constant (kr) and nonradiative decay rate constant (knr) are
calculated to be 1.0 × 107 and 6.6 × 109 s-1, respectively,
indicating that the nonradiative decay is the predominant
process in the excited states.14 When 10 equiv of Zn2+ was
added, the lifetime of compound L increased to 0.72 ns,
which is longer than that of free L, and the radiative and
nonradiative decay rate constants changed to 4.2 × 108 and
9.7 × 108 s-1, respectively; both the radiative and nonra-
diative decay processes became comparative so that a strong
fluorescence was observed.
It is generally understood that chelating groups CdN and
CdO exhibit a high affinity to transition and post-transition
metal cations, but less binding affinity toward alkali metal
and alkaline earth metal cations due to the difference of
electronic structures. Variation of fluorescence spectra of
ligand L upon addition of different metal cations including
Fe2+, Co2+, Ni2+, Zn2+, Cd2+, Cu2+, Hg2+, Pb2+, alkali metal
and alkaline earth metal cations, is shown in Supporting
Information (Figure S1). Addition of 10 equiv Fe2+, Co2+
and Ni2+ can only induce very little changes of fluorescence
spectra, whereas 10 equiv Zn2+, Cd2+, Cu2+, Hg2+ and Pb2+
can induce an obvious fluorescence enhancement. Photo-
physical data of L upon addition of different metal cations
are also shown in Table 1. It can be found that Zn2+ gives
Table 1. Photophysical Data of L upon Addition of Different
Metal Cationsa
L + M2+ none Fe2+ Co2+ Ni2+ Cu2+ Zn2+ Pb2+ Cd2+ Hg2+
λmax (nm) 500 513 516 512 500 522 530 524 521
The stability constant between L and Zn2+ in CH3CN was
calculated to be (1.09 ( 0.08) × 106 M-1 by the nonlinear
least-squares fit according to the fluorescent titration data
in Figure 3B with a good relationship (R ) 0.991), implying
that the formation of the complex with a 1:1 stoichiometry.3,15
Since the free ligand L is relatively flexible, Zn2+ should be
easy to fit into the pseudocavity formed between the CdN
structure and two carbonyl groups of coumarin and antipyrine
moieties. The fourth coordination site of Zn2+ may be
occupied by the counteranion ClO4- or acetonitrile molecule.
The proposed interaction mode between L and Zn2+ was
Im/I0
Φf (%)
1.0 1.7 1.2 1.0 26
0.15 0.24 0.16 0.18 2.2
339 9.4 43
30 1.2 4.1 2.0
21
a λmax
: maximum emissive wavelength. Im/I0: ratio of maximum
fluorescence intensity with and without metal cation. Φf: fluorescence
quantum yield (fluorescein in 0.1 N NaOH as a reference, Φf ) 0.85).11
rise to the largest fluorescence enhancement among these
metal cations, although Cd2+, Cu2+, Pb2+, and Hg2+ can also
induce about 27-, 14-, 8-, and 13-fold increases of fluores-
cence quantum yields, respectively. Such a variation in
fluorescence quantum yields is relatively small compared
with Zn2+ (200-fold), indicating that L shows the strongest
response in fluorescence spectrum to Zn2+ among these metal
cations. In general, transition and post-transition cations with
open shell d-orbitals often quench the fluorescence of
fluorophores due to the electron or energy transfer between
the metal cations and fluorophores, providing a very fast and
efficient nonradiative decay of the excited states. In contrast,
the transition cations with close shell d-orbitals, such as Zn2+,
do not introduce low-energy metal-centered or charge-
separated excited states so energy and electron-transfer
processes cannot take place.13 In the present work, we
-
shown in Figure 4 (X ) ClO4 or CH3CN).
Figure 4. Proposed interaction mode between L and Zn2+
.
(11) (a) Paeker, C. A.; Rees, W. T. Analyst 1960, 85, 587. (b) Gabe, T.;
Urano, Y.; Kikuchi, K.; Kojima, H.; Nagano, T. J. Am. Chem. Soc. 2004,
126, 3357.
To further support the proposed complexation model, and
also evaluate the role of oxygen atom of carbonyl group in
coumarin it plays in the interaction process with metal
cations, compound M (inset of Figure 5) was also synthe-
sized16 and characterized by 1H NMR, 13C NMR, and TOF-
(12) It is generally understood that the variation of fluorescence quantum
yield should be in the same order as that in fluorescence intensity if the
fluorescence spectra do not shift or change the shape too much. However,
in this case, the emission spectra shift from 500 to 522 nm in the presence
of Zn2+; more importantly, the shape of spectrum is also changed, and a
new shoulder appears at about 560 nm, which should be the main reason
for the difference of variations between the fluorescence quantum yield
and fluorescence intensity.
(14) Turro, N. J. Modern Molecular Photochemistry; The Benjamin/
Cummings Publishing Co., Inc.: Menlo Park, 1978; Chapter 5.
(15) Connors, K. A. Binding Constants, the Measurement of Molecu-
larComplex Stability; John Wiley & Sons: New York, 1987; p 24.
(13) Gunnlaugsson, T.; Lee, T. C.; Parkesh, R. Tetrahedron 2004, 60,
11239.
Org. Lett., Vol. 9, No. 1, 2007
35

480 nm decreased, and concomitantly, a slight increase of
fluorescence was found probably due to the restriction of
s-cis versus s-trans conformation in this fluorophore. How-
ever, no significant enhancement of fluorescence, as found
for compound L, was observed because CdN bond is not
included in the formed chelating complex as in compound
L, and the isomerization of CdN is not suppressed dramati-
cally. This is completely different from the behaviors for L,
implying that the carbonyl group of coumarin moiety of L
plays a very important role in the interaction process with
metal cations.
In summary, we have demonstrated a new sensing mech-
anism based on CdN isomerization, which shows a very sig-
nificant fluorescence enhancement of the compoundsa
coumarin derivative to the metal cations in a simple and
efficient way. We believe that it can be extended to other
sensing systems for recognition of different species. The pres-
ent work also provides a novel concept for design of fluor-
escent probes/chemosensors with remarkable changes in
fluorescence. Investigations along these lines for the devel-
opment of more sophisticated systems based on the same
mechanism for recognition of cations or anions, especially
in aqueous solution, are in progress in our laboratory.
Figure 5. Fluorescence spectra of M (10 µM) in CH3CN upon
addition of different metal cations including Zn2+, Cu2+, Hg2+, Pb2+
,
Fe2+, and Co2+ (each concentration was 100 µM) with an excitation
wavelength of 400 nm. Inset: molecular structure of M.
HRMS (Supporting Information). It is closely similar to L
in structure except for lack of carbonyl group of coumarin
group. Figure 5 shows the fluorescence spectra of compound
M in CH3CN in the presence of different metal cations such
as Zn2+, Cu2+, Hg2+, Pb2+, Fe2+, Co2+, etc. The emission of
M peaked at 480 nm was very weak (the fluorescence
quantum yield is 0.69%)11 and quenched dramatically in the
presence of Cu2+, Hg2+, Pb2+, whereas only a small change
in fluorescence was observed upon addition of 10 equiv of
Fe2+, Co2+. Interestingly, upon addition of 10 equiv of Zn2+,
a new peak at about 565 nm appeared while the peak at
Acknowledgment. This work was supported by NNSF
of China (No.20673132), the Croucher Foundation of Hong
Kong, and the Chinese Academy of Sciences “hundred
talents program”.
Supporting Information Available: Synthesis of com-
pound L and M, experimental details, and additional
spectrum. This material is available free of charge via the
Internet at http://pubs.acs.org.
(16) Klymchenko, A. S.; Ozturk, T.; Pivovarenko, V. G.; Demchenko,
A. P. Tetrahedron Lett. 2001, 42, 7967.
OL062518Z
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