8-Hydroxyquinoline benzoates as highly sensitive fluorescent chemosensors for transition metal ions

Article abstract of DOI:10.1021/ol051614h

(Graph Presented) 8-Hydroxyquinoline benzoates were developed as a new set of 8-HQ derivatives for highly sensitive fluorescent chemosensors for transition metal ions. A prominent fluorescence enhancement was found in the presence of transition metal ions such as Hg²⁺ and Cu²⁺, and this was suggested to result from the suppression of radiationless transitions from the nπ* state in the chemosensors.

Full text of DOI:10.1021/ol051614h

ORGANIC
LETTERS
2005
Vol. 7, No. 19
4217-4220
8-Hydroxyquinoline Benzoates as Highly
Sensitive Fluorescent Chemosensors for
Transition Metal Ions
Han Zhang,† Li-Feng Han,† Klaas A. Zachariasse,‡ and Yun-Bao Jiang*,†
Department of Chemistry and the MOE Key Laboratory of Analytical Sciences,
Xiamen UniVersity, Xiamen 361005, China, and Max-Planck-Institute for Biophysical
Chemistry, 37070 Go¨ttingen, Germany
ybjiang@xmu.edu.cn
Received July 10, 2005
ABSTRACT
8-Hydroxyquinoline benzoates were developed as a new set of 8-HQ derivatives for highly sensitive fluorescent chemosensors for transition
+
+
metal ions. A prominent fluorescence enhancement was found in the presence of transition metal ions such as Hg2 and Cu2 , and this was
suggested to result from the suppression of radiationless transitions from the n * state in the chemosensors.
π
8-Hydroxyquinoline (8-HQ, 1, Figure 1) is one of the most
important chelators for metal ions and has found significant
applications in a variety of investigations involving metal
complexes.1 An important property that makes 8-HQ even
more attractive as a chelator is the appreciable change in its
fluorescence upon metal binding.2 Therefore, 8-HQ has been
used extensively to construct highly sensitive fluorescent
chemosensors for sensing and imaging of metal ions of
important biological and/or environmental significance,3-6
and its chelates, in particular those with Al3+, are major
components for organic light-emitting diodes (OLEDs).7 It
is known that the excited-state intramolecular proton transfer
(ESIPT), from 8-OH to the quinolino N atom, makes 8-HQ
† Xiamen University.
‡ Max-Planck-Institute for Biophysical Chemistry.
(1) (a) Zhang L.; Meggers, E. J. Am. Chem. Soc. 2005, 127, 74-75. (b)
Jiang, P.; Guo, Z. Coord. Chem. ReV. 2004, 248, 205-229. (c) Zelder, F.
Z.; Brunner, J.; Kra¨mer, R. Chem. Commun. 2004, 902-903. (d) Valeur,
B.; Leray, I. Coord. Chem. ReV. 2000, 205, 3-40. (e) Soroka, K.; Vithanage,
R. S.; Phillips, D. A.; Walker, B.; Dagupta, P. K. Anal. Chem. 1987, 59,
629-636.
Figure 1. Chemical structure of 8-HQ 1 and its derivatives 2-4.
(2) Bardez, E.; Devol, I.; Larrey, B.; Valeur, B. J. Phys. Chem. B 1997,
101, 7786-7793.
10.1021/ol051614h CCC: $30.25
© 2005 American Chemical Society
Published on Web 08/13/2005

Table 1. Absorption and Fluorescence Spectral Data of 3a-e and Their Hg2+, Cu2+ and Pb2+ Chelates in ACN
λabs, nm
ꢀ,104 M-1 cm-1
λflu, nm
FEFb
Ka, 104 M-1c
Φd
τ, nse
kr, s-1
krl, s-1
3a
226/260
239/265
226/260
226/260
228/276
238/313
228/313
228/276
226/276
237/312
226/312
226/276
228/276
238/313
228/276
228/276
225/262
238/259
225/262
225/262
3.43/2.17
3.78/2.08
3.43/1.04
3.43/2.17
4.17/0.68
4.76/0.58
4.17/0.19
4.17/0.68
4.70/0.58
4.58/0.58
4.70/0.32
4.70/0.58
4.17/0.68
4.63/0.55
4.17/0.68
4.17/0.68
3.53/1.61
3.60/1.62
3.53/1.61
3.53/1.61
385a
450
450
450
385a
462
462
462
385a
460
460
460
385a
457
457
457
385a
450
450
450
0.00051
0.00092
f
f
3a + Hg2+
3a + Cu2+
3a + Pb2+
3b
2.7
1.3
1.2
4.49 ( 0.69
0.00078
0.60
0.14
0.045
0.00076
0.57
0.178
29.38
0.44 × 107
2.04 × 107
5.61 × 109
1.36 × 107
3b + Hg2+
3b + Cu2+
3b + Pb2+
3c
3c + Hg2+
3c + Cu2+
3c + Pb2+
3d
3d + Hg2+
3d + Cu2+
3d + Pb2+
3e
3e + Hg2+
3e + Cu2+
3e + Pb2+
647
243
37
>103
4.96 ( 0.84
1.88 ( 0.33
0.204
31.23
0.37 × 107
1.83 × 107
4.90 × 109
1.38 × 107
1204
312
79
>103
3.63 ( 0.47
1.15 ( 0.17
0.13
0.038
0.00074
0.63
0.115
31.73
0.64 × 107
1.99 × 107
8.69 × 109
1.17 × 107
855
341
50
>103
4.94 ( 0.92
1.70 ( 0.26
0.13
0.035
0.00040
0.0096
0.0038
0.00074
0.148
0.964
0.27 × 107
1.00 × 107
6.75 × 109
1.03 × 109
53
8.0
2.5
>103
2.20 ( 0.32
a 3a-e are extremely weakly fluorescent, and their emission maxima were therefore located at ca. 385 nm with uncertainty. b Fluorescence enhancement
factor, ratio of the intensity of 3 in the presence of 50 equiv of metal ion to that in the absence of metal ion. c Metal binding constant was obtained via
nonlinear fitting of the fluorescence titration data assuming a 1:1 binding stoichiometry; see: Bourson, J.; Pouget, J.; Valeur, B. J. Phys. Chem. 1993, 97,
4552-4557. Nice fitting supported the assumed 1:1 stioichiometry, which was also confirmed by Job plots (Figure S4). d The uncertainties in the fluorescence
quantum yields were within 10% for 3-metal chelates and within 30% for 3a-e. e Lifetimes were measured on a nanosecond TCSPC setup with a pulsed
N2 lamp; for details, see: Leinhos, U.; Ku¨hnle, W.; Zachariasse, K. A. J. Phys. Chem. 1991, 95, 2013-2021. f The lifetimes are too short to be measured
on this nanosecond setup.
weakly fluorescent and metal binding to 8-HQ blocks the
ESIPT channel, thereby restoring the fluorescence.2,5a 8-HQ
could hence be employed to build fluorescent chemosensors
for metal ions via ESIPT suppression, with increased
fluorescence signal upon metal binding. To enhance metal
binding selectivity of 8-HQ, introduction of additional
binding sites at the C-2 and/or C-7 positions, adjacent to
the original binding sites in 8-HQ (Figure 1), has been
extensively examined.3-6 In the case where 8-HQ was
substituted with methyleneamine at the C-2 or C-7 position,
a photoinduced electron transfer (PET) process leading to
fluorescence quenching was recently identified,4a and metal
binding resulting in PET suppression was also demonstrated,
which mechanism is currently intensively employed in
constructing fluorescent chemosensors for transition metal
ions operating under fluorescence enhancement mode.8 It is
noted, however, that these investigations hitherto reported
are limited mainly to 8-HQ derivatives with substituents on
the aryl ring. Derivation at the 8-OH group has not been
explored in detail. A possible reason could be that the
intensively investigated ether derivatives of 8-HQ, such as
2 (Figure 1), are highly fluorescent,9 as the ESIPT channel
is blocked, and hence not much room remains for further
fluorescence enhancement resulting from metal binding. We
report here on a new set of 8-HQ derivatives modified at
the 8-OH group, the readily synthesized benzoates of 8-HQ
(3) (a) Shults, M. D.; Imperiali, B. J. Am. Chem. Soc. 2003, 125, 14248-
14249. (b) Shults, M. D.; Pearce, D. A.; Imperiali, B. J. Am. Chem. Soc.
2003, 125, 10591-10597. (c) Pearce, D. A.; Jotterand, N.; Carrico, I. S.;
Imperiali, B. J. Am. Chem. Soc. 2001, 123, 5160-5161. (d) Jotterand, N.;
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R. D.; Dalley, N. K.; Izatt, R. M.; Bradshaw, J. S.; Savage, P. B. Tetrahedron
2004, 60, 11139-11144. (b) Bronson, R. T.; Bradshaw, J. S.; Savage, P.
B.; Fuangswasdi, S.; Lee, S. C.; Krakowiak, K. E.; Izatt, R. M. J. Org.
Chem. 2001, 66, 4752-4758. (c) Prodi, L.; Montalti, M.; Zaccheroni, N.;
Bradshaw, J. S.; Izatt, R. M.; Savage, P. B. Tetrahedron Lett. 2001, 42,
2941-2944. (d) Prodi, L.; Bargossi, C.; Montalti, M.; Zaccheroni, N.; Su,
N.; Bradshaw, J. S.; Izatt, R. M.; Savage, P. B. J. Am. Chem. Soc. 2000,
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N. K.; Kou, X.; Izatt, R. M. Inorg. Chem. 1996, 35, 7229-7240. (h)
Bronson, R. T.; Michaelis, D. J.; Lamb, R. D.; Husseini, G. A.; Fransworth,
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K.; Bowen, C. M.; Michelet, V. J. Am. Chem. Soc. 1998, 120, 3237-3242.
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Org. Lett., Vol. 7, No. 19, 2005

Figure 3. Plots against metal concentration of the fluorescence
enhancement factors of 3a-e in ACN. [3a-e] ) 1.0 × 10-5 M.
The curves through the data points are nonlinearly fitted ones
assuming a 1:1 binding stoichiometry following a reported proce-
dure: Bourson, J.; Pouget, J.; Valeur, B. J. Phys. Chem. 1993, 97,
4552-4557.
Figure 2. Fluorescence spectra of 3c in ACN in the presence of
increasing concentration of Hg(ClO4)2. Excitation wavelength was
300 nm. [3c] ) 1.0 × 10-5 M.
(3, Figure 1), as sensitive fluorescent chemsensors for
transition metal ions that emit enhanced fluorescence upon
metal binding.
of 2, an ether derivative of 1, was found indeed to be highly
efficiently quenched by the tested transition metal ions,
especially Hg2+ and Cu2+ (Figure S1, Supporting Informa-
tion). The fact that the emission band positions of the 3a-e
chelates at ca. 460 nm were almost independent of X suggests
that metal ion binding blocks the radiationless decay channel
in 3a-e. The radiation (kr) and radiationless (krl) rate
constants of 3 and 3-Hg2+ in ACN were calculated (Table
1). It was found that with 3b-d the substantial increase in
the quantum yield in the presence of Hg2+ is due to a
dramatic decrease in krl, whereas kr remains unchanged within
experimental error. This means that the radiationless decay
in 3 is indeed blocked upon metal binding and that the
emission of Hg2+-3 and 3 too originates from a ππ* state.10a
Of particular significance is that the presence of 10 µM of
Hg2+ results in an extraordinarily large enhancement (1204-
fold) of the fluorescence of 3c in ACN, whereas the other
metal ions tested showed much smaller enhancements (Figure
3), 3c being hence a highly sensitive and selective “turn-
on” fluorescent chemosensor for Hg2+.11 In 5% H2O-ACN
solution, a highly selective, although less sensitive, response
in the fluorescence of 3d (for example) toward Hg2+ was
also found (Figure S2, Ka ) 8.5 × 104 M-1), indicating the
potential of 3 as fluorescent chemsensors for practical
applications.
Benzoates 3a-e were synthesized via a simple one-step
reaction of 8-HQ with benzoyl chlorides. They were designed
on the basis of the consideration that in the esters the
carbonyl oxygen lone pair is brought into immediate proxim-
ity to the 8-HQ fluorophore, making them weakly fluorescent
as a result of a radiationless process via the nπ* state.10 This
interpretation is supported by the fact that 3a-e in aceto-
nitrile (ACN) are extremely weakly fluorescent, with quan-
tum yields (Φ) around 10-4 and lifetimes (τ) of 0.1-0.2 ns,
irrespective of the substituent X, ranging from electron
donating to highly withdrawing (Table 1). Further, the
observation that Φ of 3c in the protic solvent MeOH is 1.5
times that in ACN also supports this assumption. These
parameters of 3a-e clearly differ from those of their highly
fluorescent ether derivatives, for example, Φ ) 0.5 and τ )
9.7 ns for 2 in ACN. Therefore, 3a-e appear to be promising
candidates for fluorescent chemosensors for metal ions that
show enhanced fluorescence emission upon binding metal
ions, if their radiationless channel could be blocked by metal
binding.
In the presence of transition metal ions such as Cu2+, Pb2+,
Zn2+, Cd2+, Hg2+, and Ni2+, an enhancement of 3a-e
fluorescence was observed to a different extent for the
various substituent (X) in 3 and metal species (Table 1, Table
S1 in Supporting Information). A prominent enhancement
of the fluorescence of 3 bearing appropriate X (Me, H, Cl)
was observed in the case of Hg2+ and Cu2+, in particular
Hg2+, of otherwise highly efficient fluorescence quenchers,
Figures 2 and 3. Under the same conditions, fluorescence
The observed selectivity of the fluorescence response of
3 for metal ions was found to result from the differences
both in the metal binding affinity, as seen from absorption
titrations (Figure S3), and in the fluorescence quantum yields
of the metal chelates (Tables 1 and S1). Job plots obtained
for 3c- and 3d-Hg2+ systems in ACN suggest a 1:1 binding
(10) (a) Zhou, Z.; Fahrni, C. J. J. Am. Chem. Soc. 2004, 126, 8862-
8863. (b) Leray, I.; O’Reilly, F.; Habib Jiwan, J.-L.; Soumillion, J.-Ph.;
Valeur, B. Chem. Commun. 1999, 795-796. (c) Young, V. G., Jr.; Quiring,
H. L.; Sykes, A. K. J. Am. Chem. Soc. 1997, 119, 12477-12480.
(11) For excellent recent examples of “turn on” fluorescent chemosensors
for Hg2+, see: (a) Ono, A.; Togashi, H. Angew. Chem., Int. Ed. 2004, 43,
4300-4302. (b) Rurack, K.; Kollmannsberger, M.; Resch-Genger, U.; Daub,
J. J. Am. Chem. Soc. 2000, 122, 968-969. (c) Reference 8c,d.
Org. Lett., Vol. 7, No. 19, 2005
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mode (Figure S4). This 1:1 binding stoichiometry was also
supported by the nice nonlinear fitting of the fluorescence
intensity against metal concentration assuming a 1:1 binding
ratio (Figure 3). The fact that the fluorescence in ACN of 4
(Figure 1), a control molecule for 3c, does not show a
response toward Hg2+ indicates that the quinolino N atom
in 3 is involved in the metal binding. It was also found that
the emission of the 1- and 2-Hg2+ chelates in ACN was at
500 nm, whereas that of the 3-Hg2+ chelates was at 460 nm
(Figure S5). It is hence clear that, in 3-Hg2+, the carbonyl O
atom together with the quinolino N atom is invloved in the
binding of metal ion, thereby blocking radiationless decays
via the nπ* state. The metal center is hence located further
away from the fluorophore in the chelates of 3 than in those
of 1 and 2, making 3 more efficient fluorescent chemo-
sensors.
that 3 operates under the mechanism that metal binding
blocks the radiationless nπ* transition in 3. Future work will
be focused on tuning the selectivity for given metal ions,
based on the known chemistry of 8-HQ and/or by modifying
the benzoyl moiety (Figure 3) or replacing it with aliphatic
counterparts. Metal chelates of 3 may also be a significant
subject in OLED investigations, in view of their large
fluorescence quantum yields (>0.5) and long lifetimes of
ca. 30 ns (Table 1).
Acknowledgment. This work was supported by the
Ministry of Education of China, NSFC (20425518), and the
VolkswagenStiftung (I/77 072).
Supporting Information Available: Characterization
data, absorption and fluorescence spectra, and reaction
parameters of 3a-e in the presence of metal ions. This
material is available free of charge via the Internet at
http://pubs.acs.org.
In summary, we developed 8-HQ benzoates as a new set
of fluorescent chemsensors for transition metal ions such as
Hg2+ and Cu2+. Especially 3c shows a highly sensitive and
selective response in its emission toward Hg2+. We suggest
OL051614H
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Org. Lett., Vol. 7, No. 19, 2005