3-(2-Pyridyl)-2-pyrazoline derivatives: Novel fluorescent probes for Zn2+ ion

Article abstract of DOI:10.1016/S0040-4039(01)01970-0

Spectroscopic studies revealed that 3-(2-pyridyl)-2-pyrazoline derivatives have rather strong affinity toward divalent transition metal ions, but have almost no interaction with alkali and alkaline-earth metal ions. In the case of the 5-(4-cyanophenyl) derivative, enhancement of the fluorescence intensity was observed upon addition of the Zn²⁺ ion, while most of other transition metal ions caused complete quenching.

Full text of DOI:10.1016/S0040-4039(01)01970-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 9199–9201
3-(2-Pyridyl)-2-pyrazoline derivatives: novel fluorescent
probes for Zn2+ ion
Pengfei Wang,† Nobuko Onozawa-Komatsuzaki, Yuichiro Himeda, Hideki Sugihara,
Hironori Arakawa and Kazuyuki Kasuga*
Photoreaction Control Research Center, National Institute of Advanced Industrial Science and Technology (AIST), AIST
Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
Received 20 August 2001; revised 18 October 2001; accepted 19 October 2001
Abstract—Spectroscopic studies revealed that 3-(2-pyridyl)-2-pyrazoline derivatives have rather strong affinity toward divalent
transition metal ions, but have almost no interaction with alkali and alkaline-earth metal ions. In the case of the 5-(4-cyanophenyl)
derivative, enhancement of the fluorescence intensity was observed upon addition of the Zn2+ ion, while most of other transition
metal ions caused complete quenching. © 2001 Elsevier Science Ltd. All rights reserved.
1,3,5-Triaryl-2-pyrazolines are well-known fluorescent
compounds with high quantum yields and are widely
used as whitening or brightening reagents.1 In addition,
these compounds have been utilized as fluorescence
probes in some elaborated chemosensors.2 In most, such
sensors, an acceptor (=ligand) and a fluorophore (=
pyrazoline) are covalently linked to form PET systems.
On the other hand, the fluorescent 3-(2-pyridyl)
analogues3 of triarylpyrazolines themselves can serve as
N,N%-type bidentate ligands for metal ions. In these
intrinsic fluorescent ligands, the metal ion-binding may
affect intramolecular charge transfer and consequently
induce spectral changes both in absorbance and emis-
sion. The foregoing may be also applicable to the sensing
of metal ions.4 However, to the best of our knowledge,
only a few examples5 have been reported on the interac-
tions between pyridylpyrazoline derivatives and metal
ions. In this study, we newly synthesized several 1,5-
diphenyl-3-(2-pyridyl)-2-pyrazoline derivatives 1a–d and
investigated their complexation properties with metal
ions. In these compounds, para-substituents on the
5-phenyl group were introduced in order to modulate the
spectroscopic properties of the resulting metal complexes
as well as those of ligands.
Pyridylpyrazoline derivatives 1a–d were prepared from
the corresponding chalcones according to the reported
method6 with a slight modification (Scheme 1). Pure
products were obtained in moderate yields after recrys-
tallization from ethanol.‡
X
phenyl
X
X
hydrazine
base
+
H
EtOH-AcOH
reflux
N
N
N
N
N
O
O
O
1a: X = H, 1b: X = OCH3
1c: X = CN, 1d: X = NO2
Scheme 1.
Keywords: chelation; fluorescence; pyrazolines; zinc.
* Corresponding author. Fax: +81 298 61 4687; e-mail: k.kasuga@aist.go.jp
† Current address: Department of Physics and Material Science, City University of Hong Kong, Tat Chee Ave., Kowloon, Hong Kong, China.
‡ All new compounds gave satisfactory 1H NMR, MS, ESI-MS, and elementary analysis data.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01970-0

9200
P. Wang et al. / Tetrahedron Letters 42 (2001) 9199–9201
Binding affinities of pyridylpyrazoline derivatives 1a–d
toward divalent transition metal ions, Co2+, Ni2+, Cu2+
and Zn2+ were evaluated by UV–vis spectroscopy mea-
surements. Upon addition of these metal ions, the
absorption spectrum of each derivative changes in a
similar manner as shown in Fig. 1 (1c). The addition of
metal ions causes a decrease in optical density of the
absorption maximum (360 nm for 1c), which may
correspond to the p–p* transitions, with a peak broad-
ening except for the Cu2+ ion. In this case, a new
absorption band appears. These results indicate that
derivatives 1a–d have high binding affinity toward these
metal ions. And these distinct changes in the spectra
allow us to estimate the stability constants and stoi-
chiometries of the metal complexes of 1a–d by means of
spectrophotometric titration. By contrast, the addition
of alkali metal ions, Li+, Na+, K+ and alkaline earth
metal ions, Mg2+, Ca2+ causes almost no change in the
absorption spectra of 1a–d. This observation implies
rather weak interactions of 1a–d with alkali and alka-
line earth metal ions.
formation of ML2 complexes for Co2+, Ni2+, Zn2+ ions
as well as for the Cu2+ ion.§ The obtained stability
constants for the complexes of 1a and 1c with metal
ions are depicted in Table 1. Generally, the Cu2+ ion
makes complexes with N,N%-type ligands more tightly
than other metal ions (Irving–Williams series).8 In the
present case, however, overall stability constants for
Cu2+ complexes (ML2) are smaller than those for the
corresponding Zn2+ complexes, respectively. This may
be due to the inhibition of the formation of a square
planar Cu2+ complex, which is energetically more favor-
able, by the 1-phenyl group of the pyrazoline ring of
the ligands. The same phenomenon has been observed
for Zinquin, a Zn2+ specific fluorophore having a steri-
cally-hindered methyl group.9
The fluorescence spectra of 1a–c change significantly
upon addition of divalent transition metal ions, while
1d is not emissive itself by the presence of a strong
electron-withdrawing group (NO2), which induces
intramolecular electron transfer.10 In the cases of Co2+,
Cu2+, Ni2+ ions, typical metal-induced fluorescence
quenching4 is observed for 1a–c. In addition, for 1a and
1b, the fluorescence is also quenched by the Zn2+ ion
though it is not complete. On the other hand, for 1c,
upon addition of the Zn2+ ion, the emission maximum
at 479 nm decreases in intensity with the concomitant
appearance of a new emission band at around 568 nm.
The new band can be assigned to the emission of the
Stability constants for metal complexes of 1 were deter-
mined by nonlinear least-squares analysis of the spec-
trophotometric titration data assuming the formation
of 1:1 (ML) and/or 1:2 (ML2) complexes.7 As a result,
for Co2+, Ni2+ and Zn2+ ions, the existence of only one
species, ML2 describes the change in optical density. In
the case of the Cu2+ ion, the coexistence of ML and
ML2 gives good agreement with the titration results.
The results of ESI-MS measurements also support the
Figure 1. Absorption spectra of 1c (20 mM) in the presence of
various divalent transition metal ions ([M]/[L]=50) in aceto-
nitrile.
Figure 2. Fluorescence spectra of 1c (20 mM) in the presence
of increasing Zn2+ concentrations in acetonitrile. Excitation
wavelength was 360 nm.
Table 1. Stability constants (Ks) for the metal complexes of 1a,c in acetonitrilea
Zn2+
Ni2+
Co2+
Cu2+
1a
1c
3.4×1011
2.9×1011
5.1×109
4.3×1010
1.3×109
2.9×104b
3.8×104b
7.9×104c
4.1×105c
4.4×1010
K
s
a M+2L X ML2.
K
s
b M+L X ML.
K
s
c ML+L X ML2.
§ For example, ZnL2 (L=1c) complex: ESI-MS (m/z): [M-ClO4]+ calcd. for C42H32N8ZnClO4, 813.6; found 813.1.

P. Wang et al. / Tetrahedron Letters 42 (2001) 9199–9201
9201
Table 2. Relative fluorescence emission intensities of 1c at 568 nm in the presence of various metal ions in acetonitrilea
Metal ion
Intensity
None
15
Zn2+
100
Ni2+
2
Co2+
2
Cu2+
2
Cd2+
39
Li+
16
Na+
15
K+
15
Mg2+
16
Ca2+
15
a The concentration of 1c was 20 mM. The concentrations of metal ions were 400 mM. The excitation wavelength was 360 nm.
Zn2+ complex because its intensity increases with the
increase of Zn2+ concentration (Fig. 2). Fluorescence
quantum yields of free ligands 1a and 1c and their Zn2+
complexes were calculated according to the known
method11 using the value of 1,3,5-triphenyl-2-
pyrazoline10 as a standard. In the case of 1a, the
corresponding Zn2+ complex shows considerably low
value (Ff=0.027) compared with that of the ligand
(Ff=0.830). Therefore, it is rather difficult to observe
the emission originated from the Zn2+ complex. On the
other hand, for 1c, the electron-withdrawing group
(CN) on the 5-phenyl group decreases the quantum
yield of 1c as in the case of 1d. On complexation with
Zn2+, this electron transfer effect may be relieved by the
increased charge transfer from the 1-phenyl to the
3-pyridyl group. Consequently, quantum yields of the
ligand (Ff=0.037) and the complex (Ff=0.057) become
comparable. This makes it possible to detect the emis-
sion from the Zn2+ complex without interference from
that of the free ligand 1c.
which are of current interest.4,12 In order to improve the
Zn2+-selectivity and provide good water-solubility to 1,
further studies are underway.
References
1. (a) Wagner, A.; Schellhammer, C. W.; Petersen, S.
Angew. Chem., Int. Ed. Engl. 1966, 5, 699–704; (b) Dor-
lars, H.; Schellhammer, C. W.; Schroeder, J. Angew.
Chem., Int. Ed. Engl. 1975, 14, 665–679.
2. (a) Bissell, R. A.; de Silva, A. P.; Gunaratne, H. Q. N.;
Lynch, P. L. M.; Maguire, G. E. M.; McCoy, C. P.;
Sandanayake, K. R. A. S. Top. Curr. Chem. 1993, 168,
223–264; (b) de Silva, A. P.; Gunaratne, H. Q. N.;
Gunnlaugsson, T.; Huxley, A. J.; McCoy, C. P.;
Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997, 97,
1515–1566.
3. (a) Toi, Y.; Kawai, M.; Isagawa, K.; Maruyama, T.;
Fushizaki, Y. Nippon Kagaku Zasshi 1965, 86, 1322–
1327; (b) Toi, Y.; Kawai, M.; Isagawa, K.; Fushizaki, Y.
Nippon Kagaku Zasshi 1967, 88, 1095–1099; (c) Szu¨cs, L.
Chem. Zvesti 1969, 23, 677–686.
4. (a) Czarnik, A. W. Acc. Chem. Res. 1994, 27, 302–308;
(b) Czarnik, A. W. In Fluorescent Chemosensors for Ion
and Molecule Recognition. ACS Symposium series 538;
Czarnik, A. W., Ed.; American Chemical Society: Wash-
ington DC, 1992; pp. 1–9 and pp. 104–129. See also Ref.
2b.
5. de Silva et al. utilized the fluorescence quenching of
tridentate 1,3-di(2-pyridyl) analogue with Hg2+ for con-
structing molecular logic gates: de Silva, A. P.; Dixon, I.
M.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Maxwell, P.
R. S.; Rice, T. E. J. Am. Chem. Soc. 1999, 121, 1393–
1394.
6. Buryakovskaya, E. G.; Tsukerman, S. V.; Lavrushkin, V.
F. Russ. J. Phys. Chem. 1969, 43, 477–480.
7. Cazaux, L.; Faher, M.; Lopez, A.; Picard, C.; Tisnes, P.
J. Photochem. Photobiol. A: Chem. 1994, 77, 217–225.
8. Irving, H.; Mellor, D. H. J. Chem. Soc. 1962, 5222–5237.
9. Fahrni, C. J.; O’Halloran, T. V. J. Am. Chem. Soc. 1999,
121, 11448–11458.
Although the results of UV–vis measurements indicate
that the interactions of 1a–d with alkali and alkaline
earth metal ions are weak, the effects of these ions on
the fluorescence of 1 were investigated for 1c. The
fluorescence spectrum of 1c (20 mM) does not change
by the presence of 4 mM of Li+, Ca2+, Mg2+ although
higher concentrations (>50 mM) induce an increase in
the fluorescent intensity (5–10%) with the red shift
(10–20 nm) of the emission maximum. In addition, high
concentrations (0.1 M) of Na+ and K+ ions have no
effect on the fluorescence of 1c. These results suggest
that the fluorescences of 1c and its Zn2+ complex are
unaffected by the presence of large excess amounts of
biologically important metal ions, Li+, Na+, K+, Mg2+
and Ca2+. Furthermore, the addition of Cd2+, which
often behaves like Zn2+,4 increases the fluorescence
intensity of 1c somewhat, showing good selectivity for
Zn2+ over Cd2+. The effects of added metal ions on the
fluorescent intensity of 1c are summarized in Table 2.
In summary, pyridylpyrazoline derivatives, especially
1c, show specific fluorescent behavior toward the Zn2+
ion among divalent transition metal ions, while no
interactions exist between 1a–d and alkali and alkaline
earth metal ions. These findings indicate that
pyridylpyrazolines 1a–d are potential compounds for
developing efficient fluorescent Zn2+ chemosensors,
10. Sahyun, M. R. V.; Crooks, G. P.; Sharma, D. K. Proc.
SPIE-Int. Soc. Opt. Eng. 1991, 1436, 125–133.
11. Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 5,
991–1024.
12. Kimura, E.; Koike, T. Chem. Soc. Rev. 1998, 27, 179–184
and references cited therein.