A red fluorescent 'turn-on' chemosensor for Hg2+ based on triphenylamine-triazines derivatives with aggregation-induced emission characteristics

Article abstract of DOI:10.1016/j.tetlet.2012.11.131

A new sensitive and selective red fluorescence 'turn on' chemosensor 1 for Hg²⁺ was developed by taking advantage of AIE feature of triphenylamine-triazines motif and the specific binding of thymine with Hg ²⁺. Moreover, chemosensor 1 exhibited large two-photon absorption cross-section (3328 GM).

Full text of DOI:10.1016/j.tetlet.2012.11.131

Tetrahedron Letters 54 (2013) 909–912
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A red fluorescent ‘turn-on’ chemosensor for Hg2+ based
on triphenylamine–triazines derivatives with aggregation-induced
emission characteristics
Hao Zhang a, Yi Qu a, Yuting Gao a, Jianli Hua a, , Jing Li a, Bo Li b
⇑
a Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China
b Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University, Shanghai 200241, PR China
a r t i c l e i n f o
a b s t r a c t
A new sensitive and selective red fluorescence ‘turn on’ chemosensor 1 for Hg2+ was developed by taking
Article history:
advantage of AIE feature of triphenylamine–triazines motif and the specific binding of thymine with Hg2+
.
Received 16 October 2012
Revised 21 November 2012
Accepted 27 November 2012
Available online 5 December 2012
Moreover, chemosensor 1 exhibited large two-photon absorption cross-section (3328 GM).
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Hg2+
AIE
Fluorescent sensor
Triphenylamine–triazines
Mercury cations are extremely toxic and cause significant dam-
age to environment and human health.1 Thus, the sensitive and
selective detection of Hg2+ is crucial. A variety of designing strate-
gies based on fluorescence techniques have been developed.2 How-
ever, most of the fluorescent sensors display one or more
drawbacks in terms of actual applicability. These include cross-
sensitivities from other metal ions (e.g., Ag+, Pb2+ ions), synthetic
difficulties, and fluorescence quenching in aqueous media. There-
fore, it is worthwhile to find a useful mercury recognition mecha-
nism for designing fluorescent sensor.
It is interesting to note that some organic molecules are practi-
cally non-luminescent in the solution state but become strongly
emissive when aggregated. This intriguing phenomenon named
aggregation-induced emission (AIE) is found by Tang and cowork-
ers.3 Recently, AIE materials have also been utilized for the develop-
ment of new sensing systems. By proper structural modifications to
the AIE molecules, their aggregation states can be influenced by the
analytes through electrostatic attraction, coordination binding, etc.,
In this way, Tang and Zhang’s groups reported many tetraphenyl-
ethylene and silole derivatives as fluorescence ‘turn on’ chemosen-
sors for metal ions,4 carbohydrate,5 protein,6 peparin,7 lactic,8 and
so on. Among them, most reported chemosensors usually emitted
blue fluorescence which can be strongly attenuated by a biological
specimen.9 It is well known that the wavelength range in the red
region corresponds to the transparency window of a biological
specimen, enabling optimal penetration of light into various biolog-
ical media.10 Therefore, the red-emitting probes are necessary for
biophotonic application.
Triarylamine has been widely used in opto- and electro-active
materials for its good electron donating and transporting capabil-
ity, as well as its special propeller starburst molecular structure.
And 1,3,5-triazine-based compounds show good optical and elec-
trical properties due to their high electron affinity and symmetrical
structure.11 Recently, our group has reported deep orange multi-
branched triphenylamine end-capped triazine derivatives that
exhibited AIE properties and large two-photon absorption cross
section.12 In this work, we have developed new red fluorescence
‘turn on’ chemosensor 1 for Hg2+ by making use of the unique
AIE feature of the triphenylamine–triazines motif and the selective
binding abilities of thymine (Scheme 1). As will be discussed be-
low, the sensing mechanism of 1 toward Hg2+ is attributed to
two facts: (1) triphenylamine–triazines derivative 1 shows weak
fluorescence in solution, but it becomes strong red fluorescence
after aggregation; (2) the intermolecular coordination of 1 with
Hg2+ will lead to aggregation and fluorescence enhancement.
The synthesis of 1 started from 2,4,6-tri(p-tolyl)-1,3,5-triazine
as shown in Scheme S1. Bromination of 2,4,6-tri(p-tolyl)-1,3,5-tri-
azine afforded 2,4,6-tris(4-(bromomethyl)phen-yl)-1,3,5-triazine,
followed by reaction with trimethyl phosphate, yielding triazine
derivative 2. The important intermediate aldehyde 4 was synthe-
sized by the Ullmann reaction between triphenylamine aldehyde
3 and N,N-diphenylamine. Then, compound 5 was obtained by
the Wittig–Horner reaction of compound 2 and 4. Compound 7
⇑
Corresponding author. Tel.: +86 21 64250940; fax: +86 21 64252758.
E-mail address: jlhua@ecust.edu.cn (J. Hua).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.11.131

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H. Zhang et al. / Tetrahedron Letters 54 (2013) 909–912
O
N
O
O
NH
N
N
N
N
N
N
O
N
NH
O
O
1
Scheme 1. Molecular structure of compound 1.
was easily synthesized by two nucleophilic substitution reaction.
Finally, the target compound 1 was obtained by the Horner–Wads-
worth–Emmons reaction between compound 5 and 7. Their struc-
tures were characterized by spectroscopic methods. The synthetic
details and characterization data are provided in the Supplemen-
tary data.
Figure 1 showed the fluorescence spectrum of compound 1 with
different amounts of Hg(CH3COO)2 in DMSO/H2O (9:1, v/v). Com-
pound 1 showed a very weak emission at this concentration (see
cence intensity upon addition of Hg2+ was attributed to the coordi-
nation of thymine moieties of 1 with Hg2+ ions leading to the
aggregation of 1 as schematically shown in Scheme 2. The corre-
sponding absorption, 1H NMR spectral studies, and scanning elec-
tron microscopy (SEM) analysis can prove the above assumption.
(1) Figure S2 showed the absorption spectra of 1 (2.5 Â 10À5 M in
DMSO/H2O, 9:1 v/v) in the presence of increasing amounts of
Hg(CH3COO)2 (from 0 to 30 lM). The absorption band around
373 nm became weak and the absorption intensity in the range
of 490–600 nm increased gradually in the presence of Hg2+. This
implied that the compound 1 molecule aggregated into nanoparti-
cles in the presence of Hg2+, as it is well known that the Mie effect
of nanoparticles causes such level-off tails in the absorption spec-
tra.13 (2) Variation of 1H NMR spectra of 1 in the presence of Hg2+
also indicated the aggregation of 1. As shown in Figure S3, the sig-
nals around 6.9–9.0 ppm due to aromatic protons of 1 were upfield
shifted and became weak after the gradual addition of Hg2+. In
addition, the signal around 11.55 ppm belonging to imide protons
on thymine also became weak and that around 12.00 ppm belong-
ing to the protons on CH3COOH (O–H) became strong in the pres-
ence of Hg2+. (3) SEM image analysis revealed that a nanoscopic
aggregate was formed in the solution of 1 containing 1.0 equiv of
Hg2+ (Figure S4 (a,b)). Evidently, the emission of 1 is induced by
aggregation formation of the coordination of thymine moieties of
1 with Hg2+ ions.
Fig. S1), with a quantum yield (uF) of 0.012 (relative to Rhodamine
B as a standard). However, after addition of Hg(CH3COO)2, the
emission band at 610 nm emerged and the fluorescence intensity
started to increase gradually as displayed in the inset of Figure 1.
Moreover, the fluorescence intensity of 1 at 610 nm increased al-
most linearly with the concentration of Hg2+ in the range of 0–
50
approximately 20-fold increase relative to the original value
F = 0.24) after addition of 1 equiv of Hg2+ (kex = 400 nm). As can
lM as shown in the inset of Figure 1, at which it showed an
(u
be seen in the inset of Figure 1, the fluorescence difference for
the solution of 1 before and after addition of Hg2+ can be distin-
guished visually and the resulting mixture displayed an intense
red emission. The detection limit of Hg2+ was estimated to be ca.
6.6 Â 10À8 M (K = 18.18).The dramatic enhancement in the fluores-
The selectivity of this assay for Hg2+ ions was investigated by
measuring fluorescent responses in DMSO/H2O (9:1, v/v) solution
upon excitation at 400 nm. Figure 2a indicated that 1 detected
Hg2+ with high selectivity. No remarkable changes were detected
after addition of other metal ions (Pb2+, Cu2+, Ba2+, Ni2+, Zn2+
,
Mg2+, Co2+, Na+, K+, and Ag+). And competition experiments were
conducted to confirm a potential applicability of 1 for the selective
detection of Hg2+ ion in the presence of 5 equiv of various metal
ions (Fig. 2b, red bars). Different fluorescence changes were also
obtained by photograph (Fig. 2a). The result shows that compound
1 has high selectivity toward Hg2+, which is attributed to the coor-
dination of thymine moieties of 1 with Hg2+ ions.
Compared to single-photon fluorescence imaging, two-photon
fluorescence imaging has shown promising perspectives due to
its lower phototoxicity, reduced substrate autofluorescence, and
deeper tissue penetration.14 Most of two-photon absorption
(TPA) dyes’ fluorescence is often quenched on aggregation,
Figure 1. Fluorescence spectra of compound 1 (5 Â 10À5 M) in DMSO/H2O (9:1, v/v)
in the presence of increasing amounts of Hg(CH3COO)2 (From
0 to 70 lM);
kex = 400 nm. Inset: (1) the plot of the fluorescence intensity (I610) versus the
concentration of Hg(CH3COO)2. (2) The photos of the solution of compound 1 before
and after addition of 1 equiv of Hg(CH3COO)2 under UV light illumination (365 nm).

H. Zhang et al. / Tetrahedron Letters 54 (2013) 909–912
911
Scheme 2. Schematic illustration of the coordination mode between 1 and Hg2+
.
Figure 2. (a) The photo of the solution of 1 after addition of different metal ions; And the fluorescence intensity (I610nm) of compound 1 (5 Â 10À5 M) in DMSO/H2O (9:1, v/v)
in the presence of 1.0 equiv of the Hg2+, K+, and Na+ was 50 equiv, other metal ions are 5.0 equiv respectively; (b) The result of competition test Hg2+ and selection ions; a
2.0 mL solution of compound 1 (5 Â 10À5 M) in DMSO/H2O (9:1, v/v) was used, K+ and Na+ were 50 equiv, other metal ions are 5.0 equiv, respectively. The excitation
wavelength was 400 nm and emission intensity was measured at 610 nm.
Figure 3. (a) Open-aperture Z-scan traces of compound 1 (scattered circles are the experimental data, and the straight line is the theoretical fitted data); (b) Two-photon
fluorescence emission spectra for compound 1 (5 Â 10À5 M) in DMSO/H2O (9:1, v/v) before and after addition of 1 equiv of Hg(CH3COO)2. The photos of the solution of
compound 1 before and after addition of 1 equiv of Hg(CH3COO)2 under the laser illumination (800 nm).
although they may show high fluorescence efficiency in solutions,
which greatly limits their biophotonic applications. In this regard,
chromophore with AIE offers a promising platform with large TPA
cross section (r) data. The r value of 1 was determined by the fem-
tosecond open aperture Z-scan technique according to the previ-
ously described method.15 The open-aperture Z-scan traces of

912
H. Zhang et al. / Tetrahedron Letters 54 (2013) 909–912
compound 1 are shown in Figure 3a, and
r
value of 1 is 3328 GM
ence of Hg2+) associated with this article can be found, in the on-
by fitting the experimental data with theoretical expression given
line version, at http://dx.doi.org/10.1016/j.tetlet.2012.11.131.
in Ref.15. The high TPA properties of 1 are attributed to the ex-
tended
p
-system and enhanced intramolecular charge transfer
References and notes
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fluorescence in the DMSO/H2O (9:1, v/v) after addition of 1.0 equiv
of Hg2+. As shown in Figure 3b, the two-photon fluorescence (TPF)
was remarkably intensified by nanoaggregation.
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In summary, a new red fluorescent sensor 1 for Hg2+ was devel-
oped by making use of the specific binding of thymine with Hg2+ as
well as the AIE feature of triphenylamine–triazines motif. The
aggregation was demonstrated by absorption, 1H NMR spectral
and TEM analyses of 1 in the presence of Hg2+. This assay showed
high selectivity and sensitivity for Hg2+ ions even in the presence of
other metal ions in a mixture. Moreover, 1 exhibited large two-
photon absorption cross-section (3328 GM), suggesting that the
targeted two-photon imaging is highly desirable for biological
applications.
Acknowledgements
This work was supported by NSFC/China (2116110444 and
21172073), National Basic Research 973 Program (2013CB733
700), the Fundamental Research Funds for the Central Universities
(WJ0913001and WJ1114050), and Ph. D. Programs Foundation of
Ministry of Education of China (20090074110004).
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Supplementary data(synthetic procedures and characterization
data of compound 1; the absorption and fluorescent titration
study; 1H NMR spectra and SEM images of compound 1 in the pres-