Synthesis of dipicolylamino substituted quinazoline as chemosensor for cobalt(II) recognition based on excited-state intramolecular proton transfer

Article abstract of DOI:10.1016/j.saa.2007.09.012

A new fluorescent chemosensor for sensing Co(II) using di(2-picolyl)amino (DPA) as a recognition group and quinazoline as a reporting group has been synthesized and characterized. The quinazoline derivative contains an intramolecular hydrogen bond, which would undergo excited-state intramolecular proton transfer (ESIPT) at illumination. The fluorescence quenching is attributed to cation-induced inhibition of ESIPT, which constitutes the basis for the determination of Co(II) with the prepared chemosensor. The fluorophore forms 1:1 cobalt(II) complex with the logarithm of apparent dissociation constant log K_a = 6.8. The analytical performance characteristics of the proposed Co(II)-sensitive sensor were investigated. The chemosensor exhibits a linear response toward Co(II) in the concentration range 3.2 × 10^-8 to 1.4 × 10^-6 M, with a working pH range from 7.0 to 9.5 and high selectivity.

Full text of DOI:10.1016/j.saa.2007.09.012

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 70 (2008) 337–342
Synthesis of dipicolylamino substituted quinazoline as chemosensor
for cobalt(II) recognition based on excited-state
intramolecular proton transfer
Hong-Yuan Luoa,b, Xiao-Bing Zhanga,∗, Chun-Lian Hea, Guo-Li Shena, Ru-Qin Yua,∗
a State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry & Chemical Engineering,
Hunan University, Changsha 410082, PR China
b Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361003, PR China
Received 29 October 2006; received in revised form 30 August 2007; accepted 18 September 2007
Abstract
A new fluorescent chemosensor for sensing Co(II) using di(2-picolyl)amino (DPA) as a recognition group and quinazoline as a reporting group
has been synthesized and characterized. The quinazoline derivative contains an intramolecular hydrogen bond, which would undergo excited-state
intramolecular proton transfer (ESIPT) at illumination. The fluorescence quenching is attributed to cation-induced inhibition of ESIPT, which
constitutes the basis for the determination of Co(II) with the prepared chemosensor. The fluorophore forms 1:1 cobalt(II) complex with the
logarithm of apparent dissociation constant log Ka = 6.8. The analytical performance characteristics of the proposed Co(II)-sensitive sensor were
investigated. The chemosensor exhibits a linear response toward Co(II) in the concentration range 3.2 × 10−8 to 1.4 × 10−6 M, with a working pH
range from 7.0 to 9.5 and high selectivity.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Fluorophore; Quinazoline derivative; DPA; ESIPT; Cobalt ion
1. Introduction
ing for simple, sensitive and selective analytical techniques
that do not use expensive or complicated test equipment for a
cobalt assay attracted considerable interest to current analytical
research.
Pollution caused by heavy metals is a major environmental
problemintheworld. Miningandindustrialoperationsdischarge
large quantities of effluents into water bodies. Thus, rivers,
lakes and estuaries are polluted with heavy metals to different
degrees. Cobalt is one of such type of metals. It occurs in the
environment at concentrations ranging from 0.5 to 12 ␮g L−1
in seawater and up to 100 ␮g L−1 in wastewater [1,2]. On
the other hand, cobalt is an essential trace element in plants
and animals. It is widely distributed in the human body, with
high concentration in liver, bones and kidneys. It is a compo-
nent of vitamin B12. Cobalt deficiency in animals may lead to
retardedgrowth, lossofappetiteandanaemia, andrapidrecovery
from these symptoms occurs upon feeding them with a cobalt-
supplementary diet. The maximum dietary tolerable level of
cobalt for common livestock species is 10 ppm [3]. Thus, search-
The recent years have seen a growing interest in the devel-
opment of spectrophotometry for cobalt determination [4–7].
However, most of the chromogenic reagents are either not selec-
tive, or the products are hydrophobic and require extraction,
separation or even a computational step for the determination.
Although fluorescence signaling offers the advantage of high
sensitivityoverabsorptionorreflectancesignaling, onlyfewsen-
sors based on fluorescence are reported for cobalt determination
[8–11]. Most of the fluorophores respond toward cobalt only in
the presence of oxidizing agents such as hydrogen peroxide and
in a basic medium [8,9]. Moreover, some of the fluorophores
possess poor selectivity [11] and a relatively high background
[8–10]. Searching for new fluorophores which would response
toward Co(II) with sufficient high selectivity is still an active
field as well as a challenge for the analytical chemistry
research.
∗
Corresponding author. Tel.: +86 731 8822577.
The majority of fluorescent sensors are functioned as cation-
responsive optical switches that translate the binding event into
E-mail addresses: xbzhang@hnu.cn (X.-B. Zhang),
rqyu@hnu.cn (R.-Q. Yu).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.09.012

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H.-Y. Luo et al. / Spectrochimica Acta Part A 70 (2008) 337–342
an increase or decrease of the emission intensity [12]. They are
commonly composed of two structural subunits: a fluorophore
(for signal transduction) and an ionophore (for selective recog-
nition of metal ion). The two subunits are connected through
a linking bridge. Along this line we designed and synthesized
a new fluorescent sensor for Co(II). The new sensor contains
two subunits: di(2-picolyl)amino (DPA) moiety for selective
recognition of Co(II) and quinazoline moiety for fluorescence
signal transduction. The quinazoline derivative containing an
intramolecular hydrogen bond would undergo excited-state
intramolecular proton transfer (ESIPT) at illumination. When
the three nitrogen-donor atoms of DPA chelate with the cation,
the chelation process would inhibit the ESIPT process. Thus, the
fluorescence is quenched. Di(2-picolyl)amine (DPA) function-
ing as recognition group for zinc has been extensively researched
[13–16]. But its function as recognition group for cobalt has
not been reported so far. In this paper, we reported that DPA
could also recognize cobalt for the first time. Based on cation-
induced inhibition of ESIPT, a new fluorescent chemosensor
for cobalt determination was designed and characterized. The
sensor exhibits linear response over the concentration range
from 3.21 × 10−8 M to 1.41 × 10−6 M Co(II) with a working
pH range from pH 7.0 to 9.5, and high selectivity.
surements were made on MultiSpec 1501 (SHIMADZU). The
pH measurements were carried out on a Mettler-Toledo Delta
320 pH meter.
2.3. Synthesis of fluorophore
The fluorophore was synthesized in three steps (see
Scheme 1).
2.3.1. 2-(2ꢀ-Nitrophenyl)-4(3H)-quinazolinone [17]
Anthranilamide
(544.3 mg,
4 mmol)
and
2-
nitrobenzaldehyde (604.4 mg, 4 mmol) were dissolved in
anhydrous ethanol and stirred at room temperature for 1 h and
then p-toluenesulfonic acid (10.7 mg, 0.05 mmol) was added.
The reaction mixture was refluxed for another 3 h. The resulting
solution was cooled to 0 ◦C and 2,3-dichloro-4,5-dicyano-1,4-
benzoquinone (911.4 mg, 4 mmol) was added in 3 portions
during 30 min. The mixture was stirred at 0 ◦C for 2 h. A white
crystalloid was precipitated in ethanol. The solid was washed
with ethanol and recrystallized from ethanol to give colorless
solid (545 mg, yield: 51%). 1H NMR (400 MHz, CDCl3), δ
(ppm), 11.24 (br s, 1 H), 8.25 (d, 1 H, J = 7.6 Hz), 8.21 (d, 1
H, J = 8 Hz), 7.84–7.74 (m, 5 H), 7.55 (t, 1 H, J = 7.6, 7.2 Hz);
EI-MS: m/z 267 ([M]+, 100), 221 (13), 135 (29), 119 (48), 76
(16).
2. Experimental
2.1. Reagents
2.3.2. 2-(2ꢀ-Aminophenyl)-4(3H)-quinazolinone
2-(2ꢀ-Nitrophenyl)-4(3H)-quinazolinone
(23.9 mg,
Tin dichloride (SnCl2), p-toluenesulfonic acid (TsOH) and
cobalt acetate (Co(OAc)2) were supplied by Shanghai Chemi-
cal Reagents (Shanghai) and used as received. Anthranilamide
(Acros), 2-chloromethylpyridine hydrochloride (Lancaster),
2,3-dichloro-4,5-dicyano-1,4-benzoquinone (Lancaster), and
2-nitrobenzaldehyde (Aldrich) were used as received. Twice-
distilled water was used throughout all experiments.
0.09 mmol) and tin dichloride (3 g, 19.4 mmol) were sus-
pended in the mixed solution of 30 mL ethanol and 30 mL
concentrated hydrochloric acid and then stirred vigorously at
60 ◦C for 5 h. The pH of the resulting solution was adjusted
to 7–8 with concentrated ammonia. The solid was extracted
with CH2Cl2 (3 × 100 mL). The organic layer was dried over
anhydrous Na2SO4, and evaporated to give a colorless solid
1
2.2. Apparatus
(18.9 mg, yield: 89%). H NMR (400 MHz, CDCl3), δ (ppm),
9.52 (br s, 1 H), 8.30 (d, 1 H, J = 8 Hz), 7.78–7.71 (m, 2 H),
7.53–7.46 (m, 2 H), 7.30–7.27 (m, 1 H), 6.83–6.8 (m, 2 H),
6.25 (br s, 2 H); EI-MS: m/z 237 ([M]+, 83), 119 (100), 77
(8).
All fluorescence measurements were carried out on an F4500
luminescence spectrometer (HITACHI) with excitation slit set at
5.0 nm and emission at 10.0 nm. The UV–vis absorbance mea-
Scheme 1. Scheme for synthetic route.

H.-Y. Luo et al. / Spectrochimica Acta Part A 70 (2008) 337–342
339
2.3.3. 2-(2ꢀ-(Bispyridin-2ꢀꢀ-ylmethylaminophenyl)-4(3H)-
quinazolinone
(BPyPQ)
2-(2ꢀ-Aminophenyl)-4(3H)-quinazolinone
(24.1 mg,
0.1 mmol), 2-chloromethylpyridine hydrochloride (50.2 mg,
0.3 mmol), anhydrous K2CO3 (140.2 mg, 1.5 mmol) and KI
(49.8 mg, 0.3 mmol) were dissolved in the mixed solution
of 10 mL THF and 5 mL water and then stirred at room
temperature for 7 days. The solution was evaporated to give red
solid. The solid was dissolved in 50 mL water and extracted
with CH2Cl2 (3 × 100 mL) and the organic layer was washed
with water (3 × 100 mL). The organic layer was dried over
anhydrous Na2SO4, and evaporated. The crude product was
purified on silica gel (CH2Cl2/CH3OH 50:1) to give a colorless
solid (8.1 mg, yield: 19%). 1H NMR (400 MHz, CDCl3), δ
(ppm), 8.62 (d, 2 H, J = 4.8 Hz), 8.38 (d, 1 H, 1.2 Hz), 7.99 (dd,
1 H, J = 7.2, 1.2 Hz), 7.82–7.74 (m, 2 H), 7.56–7.47 (m, 4 H),
7.20–7.09 (m, 7 H), 4.49 (s, 4 H), ACPI-MS Positive, m/z 420.1
([MH+]+).
Fig. 1. Fluorescence emission of BPyPQ equilibrium with different concentra-
tion of Co(II): (1) 0 M; (2) 3.2 × 10−8 M; (3) 6.4 × 10−8 M; (4) 1.0 × 10−7 M;
(5) 1.6 × 10−7 M; (6) 3.2 × 10−7 M; (7) 6.4 × 10−7 M; (8) 1.0 × 10−6 M; (9)
1.1 × 10−6 M; (10) 1.2 × 10−6 M; (11) 1.3 × 10−6 M; (12) 1.4 × 10−6 M; (13)
1.7 × 10−6 M; (14) 2.0 × 10−6 M; (15) 3.2 × 10−6 M.
2.4. Preparation of solutions
3. Results and discussion
A 1 × 10−6 M stock solution of BPyPQ was prepared by
dissolving BPyPQ in absolute ethanol.
3.1. Spectral properties
A stock standard solution of Co(II) (0.01 M) was prepared by
dissolving 1.2455 mg Co(OAc)2·4H2O in water and adjusting
the volume to 500 mL in a volumetric flask. The other stan-
dard solution of Co(II) was obtained by serial dilution of 0.01 M
Co(OAc)2 solution.
The wide pH range buffered solution was obtained by adjust-
ment of 0.05 M Tris–HCl solution with a HCl or NaOH solution.
The complex solution of Co(II)/BPyPQ was prepared by
adding 5.0 mL of the stock solution of BPyPQ and 5.0 mL of
the stock solution of Co(II) in a 10 mL volumetric flask. In the
solution thus obtained, the concentrations were 0.5 × 10−6 M
BPyPQ and 1 × 10−3 to 1 × 10−8 M of Co(II). The solution was
protected from light and kept at 4 ◦C for further use. Blank solu-
tion of BPyPQ was prepared under the same conditions without
Co(II).
The fluorescence spectra changes of BPyPQ when excited
at 309.0 nm under various Co(II) concentrations are shown in
Fig. 1, which are recorded at λex = 309.0 nm, λem = 320–580 nm.
This compound exhibits fluorescence emission at 467.0 nm. As
can be seen from Fig. 1, the fluorescence intensities decrease
with increasing concentration of Co(II), which constitutes the
basis for the determination of Co(II) with the proposed sensor.
3.2. Response mechanism
Quinazoline derivatives (TA) containing an intramolecular
hydrogen bond are good fluorophores [13,18]. They possess a
highly Stokes shifted emission. Given appropriate energy, the
molecules of derivatives are excited from the ground state (TA)
to the excited-state (TA∗). The molecules in the excited-state
(TA∗) are instantaneously converted into the proton-transfer tau-
tomer (TB∗) through exited-stated intramolecular proton transfer
(ESIPT). The proton-transfer tautomer (TB∗) emits the fluores-
cence and turns to the ground-stated tautomer (TB). Commonly,
the ground-stated tautomer (TB) is unstable and converted
into the stable form (TA). Simplified diagram illustrating the
excited-state intramolecular proton transfer process is shown in
Scheme 2. Thus, quinazoline derivatives possess a highly Stokes
shifted emission. In this paper the BPyPQ exhibits fluorescence
emission maximum at 467.0 nm when excited by the radiation
of 309.0 nm. Cobalt is a paramagnetic metal cation. The DPA
moiety binds cobalt tightly with three nitrogen-donor atoms. So
the tertiary amine would not accept proton anymore as its lone-
pair electrons participated coordination with cobalt. In this way
cobalt inhibits the excited-state intramolecular proton transfer-
ring process, which results in the fluorescence quenching.
2.5. Measurement procedure
The fluorescence intensity was measured with the maximal
excitation wavelength of 309.0 nm and at the maximal emission
wavelength of 471.0 nm. Before each measurement, the solution
was allowed to stand for 5 min to allow complete formation of
metal–ligand complex.
2.6. UV–vis spectra
For the purpose of studying the response mechanism, an
ethanol solution of BPyPQ (2.0 × 10−4 M) and equivalent molar
aqueous Co(OAc)2 solution (pH 8.42) were mixed at room tem-
perature. Then its UV–vis spectrum was recorded and compared
with that of an ethanol solution of BPyPQ (2.0 × 10−4 M) mixed
with blank buffer solution (pH 8.42).

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H.-Y. Luo et al. / Spectrochimica Acta Part A 70 (2008) 337–342
Fig. 3. Job plot for BPyPQ and Co(II): [BPyPQ] + [Co(II)] = 2.0 × 10−6 M.
was used (Fig. 3). As expected, the result obtained from the Job
plot unambiguously indicates the formation of a 1:1 complex
between BPyPQ and Co(II) [20,21].
Taking into account that the concentration of BPyPQ is
unchanged in the complex formation, the complexation equilib-
rium of BPyPQ (A) with Co(II) (B) with an apparent association
constant Ka can be expressed by the modified Stern–Volmer
equation [22,23].
Scheme 2. Simplified diagram illustrating the excited-state intramolecular pro-
ton transfer.
ꢀ
ꢁ
For the purpose of studying the response mechanism, the
absorption spectra of BPyPQ with and without equivalent Co(II)
were recorded (Fig. 2). Upon addition of Co(II), the spectrum
is almost not changed. Evidently, the fluorophore chelats Co(II)
with three nitrogen of the DPA moiety. The nitrogen of quinazo-
line does not participate in coordination to Co(II) as the spectrum
of BPyPQ mixed with Co(II) does not show a new redshifted
absorption band [19]. A slight shift (293 → 295 nm) might be
attributed to the Co(II)-coordination of the lone-pair electrons
of the nitrogen of tertiary amine.
ꢀF
F
log
= log Ka + (n − 1)log [A] + m log[B]
(1)
Here ꢀF is the change of fluorescence intensity; F denote
the fluorescence intensity of BPyPQ in presence of Co(II);
m and n are the complexing ratio of the coordination com-
plex. The response curve is shown in Fig. 4. In the Co(II)
concentration range of 3.2 × 10−8 to 1.4 × 10−6 M, a linear
relationship between log(ꢀF/F) and log[Co(II)] was obtained,
with a correlation coefficient of 0.9979 (n = 8) according
to an equation: log(ꢀF/F) = 6.811 + 1.131 log[Co(II)]. The
logarithm of apparent association constant (log Ka) of the
In order to determine the stoichimetry of the coordination
complex, the method of continuous variations (Job’s method)
Fig. 4. The plot of log((F0 − F)/F) as a function of the log[Co(II)]. (F and F0
denote the fluorescence intensities of BPyPQ with and without Co(II), respec-
tively).
Fig. 2. Changes in the UV–vis spectra of BPyPQ (2.0 × 10−4 M) upon the
addition of equivalent molar Co(II).

H.-Y. Luo et al. / Spectrochimica Acta Part A 70 (2008) 337–342
341
Table 1
Interference of different species to the fluorescence determination of Co(II) with the proposed fluorophore
a
Interferant
Concentration (mol l−1
)
Fluorescence (ꢀF = F0 − F)b
Relative error% (ꢀF/F0) × 100
Li+
1.0 × 10−2
1.0 × 10−2
1.0 × 10−2
1.0 × 10−2
1.0 × 10−2
2.0 × 10−2
1.0 × 10−3
5.0 × 10−4
5.0 × 10−4
1.0 × 10−4
1.0 × 10−4
1.0 × 10−4
1.0 × 10−4
1.0 × 10−4
1.0 × 10−5
8.9
5.5
12.5
3.3
15.6
5.5
Precipitated
12.3
12.1
15.4
5.5
15.6
6.9
7.5
1.9
1.2
2.7
0.7
3.4
1.2
Na+
K+
Mg2+
Ca2+
Al3+
Ag+
Pb2+
Mn2+
Ni2+
Hg2+
Cu2+
Fe3+
Fe2+
Zn2+
2.6
2.6
3.3
1.2
3.4
1.5
1.6
3.6
16.5
a
The concentration of Co(II) is fixed at 1.0 × 10−6M (pH 8.42).
b
F and F0 are the fluorescence intensities of BPyPQ contacting with 1.0 × 10−6 M Co(II) solution with and without adding the interferant (F0 = 460.5).
proton-transfer tautomer (TB∗) is not formed. So the fluorescence
intensity declines. At the same time it is also observed that the
fluorescence quenching effect is strongest within the range of pH
7–9.5. At thisrangeof pH cobalt isproneto chelate theDPAmoi-
ety, thus fluorescence quenching effect is strongest. Therefore,
a pH 8.48 Tris–HCl buffer (25 mL 0.2 M Tris + 12.5 mL 0.1 M
HCl 6 + 2.5 mL H2O) was used in subsequent experiments.
1:1 complex formation was calculated to be 6.8 by this
equation.
3.3. Effect of pH
The effects of pH on the fluorescence intensity of BPyPQ
in the absence and in the presence of Co(II) were carried out
by adjusting in Tris–HCl buffer with hydrochloric acid and
sodium hydroxide and fixing the Co(II) concentration at 1 ␮M.
The results are shown in Fig. 5. One may observe that the flu-
orescence intensity of BPyPQ increases with the pH increasing
at pH 2–9.5, the intensity increases greatly at pH 7–9.5, and
at pH > 9.5 the intensity decreases. It is suggested that the pH
values affect the ESIPT process. According to the research of
Romary [20] and Gruenwedel [21], we know that pKa of the
tertiary amine is 7.3. At pH < 7.3 the tertiary amine is protoned,
which will not accept the transferring proton at this range of
pH. So the fluorescence intensity is limited. At pH > 9.5 the pro-
ton might transfer to the hydroxy ion enriched solution, thus the
3.4. Selectivity
A study of the effect of interfering ions was made by adding
different amounts of other ions including Li(I), Na(I), K(I),
Mg(II), Ca(II), Al(III), Pb(II), Mn(II), Ni(II), Ag(I), Hg(II),
Cu(II), Fe(III), Zn(II) to solutions containing 1 ␮M Co(II).
For this purpose samples containing a fixed concentration of
Co(II) (1 ␮mol L−1) and different concentrations of other metal
ions were analyzed by the method. A species was considered as
an interferent if it causes a variation more than 5% in analytical
signal when compared to the analytical signal obtained in the
absence of the interfering species. Ions such as Li(I), Na(I), K(I),
Mg(II), Ca(II), Al(III) show no interference on the determination
of Co(II) (Table 1). Most of transition metal ions such as Pb(II),
Mn(II), Ni(II), Ag(I), Hg(II), Cu(II), Fe(III), Fe(II), Zn(II) show
a little interference on the determination of Co(II) in a 100-
fold excess. The main interfering species in the determination
of Co(II) was Zn(II). The interference is negligible in 10-fold
excess.
4. Conclusion
In this paper a new fluorophore (BPyPQ) has been syn-
thesized and characterized. We have developed a fluorescent
method for Co(II) by using it. The fluorescence quenching is
attributed to cation-induced inhibition of ESIPT. In the absence
of Co(II) at pH 7–9.5, the fluorophore undergoes ESIPT to yield
a highly Stokes shifted emission from the proton-transfer tau-
tomer. Coordination of Co(II) inhibits the ESIPT process and
Fig. 5. Changes of fluorescence intensity of BPyPQ (1 ␮M) upon the addition
of 1 ␮M Co(II) as pH varied.

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H.-Y. Luo et al. / Spectrochimica Acta Part A 70 (2008) 337–342
quenches the emission from the proton-transfer tautomer. Com-
pared with some other fluorescence methods for determination
of Co(II), this method has advantages such as large Stokes shift,
high selectivity and not requiring separation. BPyPQ has good
solubility in polar solvent such as ethanol. Accordingly, the
proposed method may be used for a simple and sensitive deter-
mination of Co(II) in pharmaceutical preparations such as drinks
containing of vitamin B12.
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