An intramolecular charge transfer fluorescent probe: Synthesis and selective fluorescent sensing of Ag+

Article abstract of DOI:10.1016/j.saa.2007.10.006

An intramolecular charge transfer (ICT) fluorescent probe, in which the thiourea derivative moiety is linked to the fluorescent 4-(dimethylamino) benzamide, has been designed and synthesized. The ions-selective signaling behaviors of the probe were investigated. Upon the addition of Ag⁺, an overall emission enhancement of 14-fold was observed. Compound 1 displayed highly selective chelation enhanced fluorescence (CHEF) effect with Ag⁺ over alkali, alkali earth metal ions and some transition metal ions in aqueous methanol solutions. The prominent selective and efficient fluorescent enhancing behavior could be utilized as a new chemosensing probe for the analysis of Ag⁺ ion in aqueous environment.

Full text of DOI:10.1016/j.saa.2007.10.006

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Spectrochimica Acta Part A 70 (2008) 923–928
An intramolecular charge transfer fluorescent probe:
Synthesis and selective fluorescent sensing of Ag+
Honglei Mu, Rui Gong, Lin Ren, Cheng Zhong, Yimin Sun, Enqin Fu∗
Hubei Key Laboratory on Organic and Polymeric Optoelectronic Materials, Department of Chemistry,
Wuhan University, Wuhan 430072, PR China
Received 29 July 2007; received in revised form 26 September 2007; accepted 4 October 2007
Abstract
An intramolecular charge transfer (ICT) fluorescent probe, in which the thiourea derivative moiety is linked to the fluorescent 4-(dimethylamino)
benzamide, has been designed and synthesized. The ions-selective signaling behaviors of the probe were investigated. Upon the addition of Ag+,
an overall emission enhancement of 14-fold was observed. Compound 1 displayed highly selective chelation enhanced fluorescence (CHEF) effect
with Ag+ over alkali, alkali earth metal ions and some transition metal ions in aqueous methanol solutions. The prominent selective and efficient
fluorescent enhancing behavior could be utilized as a new chemosensing probe for the analysis of Ag+ ion in aqueous environment.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Molecular discrimination; Fluorescent probe; Fluorescent enhancing; Ag+
1. Introduction
The intramolecular charge transfer (ICT) mechanism has
been widely exploited for metal ions sensing [2a,b,5]. In
most cases, the ionophore is the electron-donor or electron-
deficient substituent of the fluorophore and coordination
of a metal ion leads to a pronounced blue or red shift
of the intramolecular charge transfer absorption band but
often to change in fluorescence intensity [6]. It is known
that the excited-state intramolecular charge transfer and the
accompanied dual fluorescence of the electron donor/acceptor
substituted benzenes such as 4-(dimethylamino)benzonitrile
(DMABN) and 4-(dimethylamino)benzamide (DMABA). The
generally favored concept to explain the anomalous, long wave-
length fluorescence is a TICT state [7]. The dual fluorescent
behavior depends sensitively on the nature of the electron
donor/acceptor [8], therefore it is possible to design fluores-
cent sensors for metal ions sensing. In this paper we report
an intramolecular charge transfer (ICT) fluorescent probe, in
which the thiourea derivative moiety is linked to the fluores-
cent 4-(dimethylamino)benzamide, and its Ag+ ion-selective
signaling behaviors. The fluorophore in this fluorescent probe
is a strong “push–pull” ␲-electron system, with an aniline
nitrogen atom as an electron donor and a carbonyl as an
electron acceptor. Based on the principle of hard and soft
acids and bases, the S atom in the thiourea moiety tends
to coordinate the Ag+ [9]. Therefore, we can expect that
the complexation of Ag+ ion will have a dramatic influence
Fluorescent probes capable of selectively recognizing guest
species are of particular interest in supramolecular chemistry
because of their high selectivity, sensitivity, and simplicity [1].
Especially, fluorescent sensing of heavy and transition metal
(HTM) ions have received increasing attention due to their
importance in many biological processes and environmental rel-
evancy [2]. In the previous literatures, a number of fluorescent
probes for HTM ions have been reported [3]. Most of them dis-
play fluorescence intensity changes, but relatively few of them
result in fluorescence enhancing with HTM analytes, such as Hg
(II), Pb (II), Ag (I), and Cu (II), since these ions generally act
as quenchers via the electron transfer or facilitated intersystem
crossing (isc) processes.
It is generally believed that sensors with a fluorescence
enhancement signal when interacting with analytes are much
more efficient. Whereas, the examples of Ag+-selective fluores-
cence enhancement probes have been still scarce [4]. Therefore
the development of new fluorescent Ag+ sensors, especially
those that exhibit selective Ag+-amplified emission, is still a
challenge.
∗
Corresponding author. Tel.: +86 27 87219044; fax: +86 27 68756757.
E-mail address: fueq@whu.edu.cn (E. Fu).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.10.006

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H. Mu et al. / Spectrochimica Acta Part A 70 (2008) 923–928
upon the emission of fluorophore and transmit the signal of
recognition.
stirring. The solution was allowed to reflux for 6 h and cooled
to room temperature. Saturated aqueous sodium carbonate was
added with stirring until pH was 7–8. The precipitated prod-
uct was filtered and recrystallized from ethanol and water (1:1)
2. Experimental
1
to give white crystals 4 in 71% yields: H NMR (300 MHz,
CDCl3): δ = 7.93(d, J = 8.7 Hz, 2H), 6.66(d, J = 8.7 Hz, 2H),
4.33(q, J = 7.2 Hz, 2H), 3.03(s, 6H), 1.36(t, J = 7.2 Hz, 3H).
2.1. Reagents and apparatus
4-(dimethylamino)benzoic acid hydrazide (3). In
a
Absorption spectra were determined on a UV-2550
UV–visible spectrophotometer. Fluorescence spectra were
determined on a F-4500 fluorescence spectrophotometer. Melt-
ing points were determined on a X-6 micro-melting point
apparatus and were uncorrected. IR spectra were obtained on
a Nicolet 170SX FT-IR or a Shimadzu FT-IR 8000 spectropho-
tometer. NMR spectra were recorded on Varian Mercury VX300
FT-NMR spectrometer with (CH3)4Si as internal reference.
Elemental analyses were determined on a Perkin-Elmer204B
elemental auto analysis apparatus. Twice-distilled water was
used throughout the experiments. All the materials for synthesis
and test were purchased from Shanghai Chemicals (Shanghai,
China) and used as received. Except specified, other chemicals
were analytical reagent grade and used without further purifi-
cation. The solutions of metal ions were prepared from their
acetate salts, except for Pb2+, Ag + from nitrate and Fe2+ from
sulfate, respectively.
100 mL round-bottomed flask was placed 1.0 g(0.005 mol)
4-(dimethylamino)benzoic acid ethyl ester with 50 mL alcohol.
Then 5 g hydrazine hydrate (85%) was dropped into under
stirring. The solution was allowed to reflux for 2 days. The
reaction mixture was allowed to cool to room temperature and
evaporated under reduced pressure to give a solid. The solid
recrystallized from ethanol to give white product 3 in 75%
yields: 1H NMR (300 MHz, CDCl3): δ = 7.68(d, J = 8.7 Hz,
2H), 7.29(s, 1H), 6.68(d, J = 8.7 Hz, 2H), 4.06(s, 2H), 3.02(s,
6H).
N-4-(dimethylamino)benzamido-N’-1-naphthylthiourea
(1).A mixture of 0.72 g (0.004 mol) 3 and 0.74 g(0.004 mol)
1-naphthyl isothiocyanate were stirred to dissolve in 50 mL
ethylene glycol monomethyl ether. The mixture was then stirred
at room temperature for 8 h and filtered. The solid was washed
with ethanol (3 × 50 mL) and further dried under a vacuum
pump to afford 0.88 g white powder product 1 in 67% yield:
1H NMR (300 MHz, DMSO-d6): δ = 10.31(s, 1H), 9.96 (s, 1H),
9.65 (s, 1H), 7.97–7.90 (m, 2H), 7.85–7.81(m, 3H), 7.50–7.46
(m, 3H), 7.32 (d, J = 9.0 Hz, 1H), 6.71 (d, J = 9.0 Hz, 2H),
2.95 (s, 6H). 13C NMR (75 MHz, DMSO-d6): δ = 183.4, 166.8,
153.1, 136.6, 134.3, 131.5, 130.1, 128.4, 127.4, 127.1, 126.6,
126.4, 126.0, 124.6, 119.6, 111.2; IR (KBr): ␯/cm−1 3336.6,
3298.0, 3261.0, 1649.9, 1603.3, 1501.0, 1464.1, 1295.1,
1203.0, 769.1; EI MS found: m/z = 364.2 (M+); Anal. Calcd for
C20H20N4OS: C 65.91, H 5.53, N 15.37; found: C 65.76, H
5.55, N 15.34.
1H NMR studies were recorded after adding two equivalents
Ag+ into probe (20 mM) in DMSO-d6. The effect of the metal
ions upon the absorption and fluorescence intensity was exam-
ined by adding a few microliters of stock solution of the metal
cations to a known volume of the solution (2 mL). The addition
was limited to 0.1 mL, so that dilution remained insignificant.
Association constants (1:2) of 1 with Ag+ are calculated by
followed equation [10] and linearly fitted in origin 7.0:
ꢀ
ꢁ
(X − X0)
log
= n log[G] + log Kd
(X∞ − X)
N-4-(dimethylamino) benzamido-Nꢀ-1-naphthylurea (2). A
mixtureof0.72 g(0.004 mol)3and0.70 g(0.004 mol)1-naphthyl
isothiocyanate were stirred to dissolve in 50 mL ethylene glycol
monomethyl ether. The mixture was then stirred at room tem-
perature for 12 h and filtered. The solid was washed with ethanol
(3 × 50 mL) and further dried under a vacuum pump to afford
0.94 gwhitepowderproduct2in55%yield:1HNMR(300 MHz,
DMSO-d6): δ = 10.07(s, 1H), 8.89 (s, 1H), 8.34 (s, 1H), 8.1(d,
J = 9.0 Hz, 1H), 7.92 (d, J = 9.0 Hz, 1H), 7.82 (d, J = 9.0 Hz, 3H),
7.65(d, J = 9.0 Hz, 1H), 7.59–7.43 (m, 3H), 6.73 (d, J = 9.0 Hz,
2H), 2.98 (s, 6H); 13C NMR (75 MHz, DMSO-d6): δ = 167.1,
157.2, 153.1, 135.1, 134.4, 129.6, 128.9, 126.5, 126.4, 126.3,
124.1, 122.6, 119.6, 111.4; IR(KBr): ␯/cm−1 3277.7, 1650.1,
1598.7, 1552.1, 1485.5, 1205.6, 765.8;EIMSfound:m/z = 347.9
(M+); Anal. Calcd for C20H20N4O2: C 68.95, H 5.79, N 16.08;
found: C 69.03, H 5.80, N 16.03.
X0: fluorescent intensity of host without guest; X : fluorescent
∞
intensity reaching a limitation by adding excessive guest; n: the
stoichiometric ratio of host and guest; [G]: concentrations of
guest.
2.2. Synthesis of 1 and 2
The synthetic route of 1 and 2 is shown in Scheme 1. 4-
(dimethylamino)benzoic acid [11](5). 3.2 g(0.019 mol) AgNO3
was dissolved into a 60 mL potassium hydroxide aqueous solu-
tion (7%) and 1.5 g(0.01 mol) 4-(dimethylamino)-benzaldehyde
was added into this solution. The mixture was stirred at 60 ◦C
for 24 h, then cooled to room temperature, and filtered. After
the solution was acidified with concentration hydrochloric acid,
the product precipitated gradually. The solid was filtrated and
then recrystallized from ethanol to give white needle product 5 in
89% yields: 1H NMR (300 MHz, CDCl3): δ = 7.98(d, J = 8.1 Hz,
2H), 6.68(d, J = 8.1 Hz, 2H), 3.06(s, 6H).
3. Results and discussion
4-(dimethylamino)benzoic acid ethyl ester [12] (4). In a
100 mL round-bottomed flask was placed 3.0 g(0.018 mol) 4-
(dimethylamino)benzoic acid with 50 mL alcohol. Then 2 mL
concentrated sulfuric acid was added dropwise under vigorous
The metal ion binding properties of compound 1 were inves-
tigated by UV–vis absorption and fluorescence spectroscopy.
The titration experiments were carried out in H2O–CH3OH sys-

H. Mu et al. / Spectrochimica Acta Part A 70 (2008) 923–928
925
Scheme 1. Synthesis of 1 and 2.
tem (1:3, v/v) by adding of Ag+. Fig. 1 shows the changes in the
absorption spectra of probe 1 in a mixture (1:3) of H2O–CH3OH
containing HEPES (20 mM) with the increasing of Ag+. As the
concentration of Ag+ increased, we observed a regular decreas-
ing in the absorption at 313 nm and a new absorption band at
331 nm occurred and increased prominently to their limiting val-
ues. The changes of the absorption suggest the coordination of
Ag+ to the probe and the effect of the ICT state had been dis-
turbed. The interaction of Ag+ with probe 1 might enhance the
electron-withdrawing character of the amide group in probe 1
(acceptor group), which causes the clear red shift.
solution (H2O–CH3OH, 1:3, v/v, 20 mM HEPES, pH = 7.0)
(λex = 353 nm). 1 showed a very weak fluorescence (LE state)
at ca. 380 nm and the long wavelength CT emission was not
found in this condition. The fluorescence intensity enhancement
was observed in the LE emission region in the presence of Ag+
ions. Fig. 2a shows the fluorescence titration of 1 with Ag+.
The fluorescence of the solution increased dramatically with
increasing concentrations of Ag+. Addition of two equiv of Ag+
resulted in a 14-fold enhancement of fluorescence intensity
of probe 1, the maximum emissive wavelength red-shifted
from 378 to 385 nm slightly. The fluorescence intensity
increased linearly on the concentrations of Ag+ between 0 and
2.0 × 10−5 M (inset of Fig. 2b). The regression equation is:
The fluorescent spectral properties of probe
1
(c = 1 × 10−5 M) were also determined in aqueous methanol
IF = 82.02 + 5.36 × 107 CAg+
.
Then we carried out Job’s plot experiment by varying the con-
centration of both 1 and Ag+ (Fig. 3). The maximum point at the
mole fraction of 0.33, which indicated that a 1:2 (ligand:metal)
complex was formed [13].
The complexation of 1 with Ag+ ion was further evidenced
by the 1H NMR measurement (Fig. 4). The binding of silver ion
to 1 was monitored by the changes in the 1H NMR spectrum of
1 in DMSO-d6 in the presence of Ag+ ion.
When two equiv of Ag+ ions were added, the peaks, assigned
to the thiourea protons of 1 and the amide N–H proton appeared
at 9.66, 9.96 and 10.31 ppm, experience clear downfield shifted
to 10.47, 10.69 and 10.82 ppm, respectively. This could be
attributed to a deshielding effect, arising from the decrease of
the electron density of nitrogen in the fluorophore caused by
1-Ag+ complexation. After addition of Ag+ ions, the aromatic
protons display almost no change of chemical shift and only
small change in the shape of the peak. This proved that the S
atom of the thiourea and the O atom of the amide possibly were
the coordinating sites for Ag+. Based on the above information,
we calculated the possible interaction model between Ag+ and
Fig. 1. Changes in the UV–vis spectra of 1 (10−5 M) on addition of Ag+
(0–5 × 10−5 M) in aqueous methanol solution (H2O–CH3OH, 1:3, v/v, 20 mM
HEPES, pH = 7.0).

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H. Mu et al. / Spectrochimica Acta Part A 70 (2008) 923–928
Fig. 2. (a) Fluorescence titration of 1 (10−5 M) in the presence of different concentration of Ag+ in aqueous methanol solution (H2O–CH3OH, 1:3, v/v, 20 mM
HEPES, pH = 7.0). (λex = 353 nm). (b) The plot of fluorescence intensity at 385 nm vs. equivalents of Ag+.
1 (Fig. 5). The O, N atoms of amide and the S atom of thiourea
coordinated two silver ions in the calculated model. This model
could well explain the experimental results.
The detection limit of 1 as a fluorescent probe for the analysis
of Ag+ was also determined by the equation [14]:
3sB
cL =
b
where sB was the blank signal standard deviation and b
was the slope of the calibration graph. The detection limit was
approximately 8 × 10−7 M, which was sufficiently low for the
detection of Ag+ in many chemical and biological systems. Then
the apparent association constant, Kd, was determined. The asso-
ciation constant between the host and metal ion was 1.8 × 108
M
−2. As a result, the probe 1 displayed a highly selective chela-
tion enhanced fluorescence (CHEF) effect [15] with Ag+. The
large CHEF effects with Ag+ could be explained possibly when
the probe 1 had caught a silver ion, the electron- withdrawing
abilityoftheamidegroupinprobe1wouldbeenhanced, andthus
Fig. 3. Job plot for 1 and Ag+.
Fig. 4. Evolution of the 1H NMR spectra of (a) 1, (b) 1 + 2equiv Ag+ in DMSO-d6.

H. Mu et al. / Spectrochimica Acta Part A 70 (2008) 923–928
927
Fig. 5. The calculated result for the interaction between Ag+ and 1 based on
PM3 of MOPAC 2007 software.
Fig. 8. Fluorescent ratiometric responses of 1 (1 × 10−5 M) containing Ag+
(2 × 10−5) and the background metal ions (10−4 M).
addition of Ag+, and even when the total amount of Ag+ ion
exceeded 30 equiv of the probe 2, equilibrium had still not
achieved (Fig. 6). The 1H NMR experiment was also carried out
in order to investigate the interaction of Ag+ with 2. The addition
of 1 equiv of Ag+ did not cause the change of the proton signals
of 2. These results indicated the association constant of 2 with
Ag+ was small. The result showed the S atom of thiourea in 1
played an important role as the recognition site, which caused
the CHEF effect upon the addition of Ag+.
Toevaluatetheselectivityofprobe1, theeffectsofothermetal
ions on the fluorescent spectral properties of probe 1 were also
investigated. The results were shown in Fig. 7. Some transition
metal ions, for example, Co2+, Ni2+, Cu2+, Zn2+, Mn2+, Fe2+,
and the alkali, alkali earth metal ions had only a little effect
on the fluorescence intensity (λem = 385 nm) of probe 1 under
the same condition, except the Hg2+ caused 6-fold enhancement
of fluorescence intensity. These results indicated 1 as a highly
selective fluorescent probe for Ag+ ion.
Fig. 6. Changes in the fluorescence spectra of 2 (10−5 M) on addition of Ag+
(0–3 × 10−4 M) in aqueous methanol solution (H2O–CH3OH, 1:3, v/v, 20mM
HEPES, pH = 7.0).
a red-shift in emission spectra and enhancement of fluorescence
intensity should be observed [16].
For comparison, the fluorescent spectral properties of con-
trol molecule 2 in the presence of Ag+ were also determined
in order to prove the role of S atom in the fluorescent probe.
The fluorescence intensity of 2 decreased continually upon the
To explore practical applicability of 1 as an Ag+-selective flu-
orescent probe, competition experiments were also performed.
The solution of 1 in the presence of Ag+(2 × 10−5 M) was mixed
Fig. 7. (a) Fluorescence spectra of 1 (1 × 10−5 M) in the presence of 2 equiv of different metal ions in aqueous methanol solution (H2O-CH3OH, 1:3, v/v, 20 mM
HEPES, pH = 7.0). (b) fluorescence intensity at 383 nm in the presence of equiv of different metal ions in aqueous methanol solution (H2O–CH3OH, 1:3, v/v, 20 mM
HEPES, pH = 7.0).

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H. Mu et al. / Spectrochimica Acta Part A 70 (2008) 923–928
with background metal ions (10−4 M), including Hg2+, Co2+,
Ni2+, Cu2+, Zn2+, Mn2+, Fe2+ the alkali and alkali earth metal
ions, respectively. The fluorescence intensity of all above the
solutions showed no obvious variation comparing with that only
containing Ag+ (Fig. 8).
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In summary, we have synthesized a new fluorescent probe 1
for Ag+ based on the ICT mechanism with high sensitivity and
selectivity, using the amide and thiourea group as the recognition
moiety and 4-(dimethylamino)benzamide as the signal group.
Compound 1 showed chelation enhanced fluorescence (CHEF)
effect with Ag+ over other competing metal ions in aqueous
methanol solutions. Therefore, probe 1 could be utilized as a new
fluorescent probe for the analysis of micromolar concentration
range of Ag+ ion in aqueous environment.
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Acknowledgement
We gratefully acknowledge the financial support from the
Natural Science Foundation of China (No. 20272045 and No.
20672085).
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