A pyridinyl-functionalized tetraphenylethylene fluorogen for specific sensing of trivalent cations

Article abstract of DOI:10.1039/c2cc38246f

A pyridinyl-functionalized tetraphenylethene (Py-TPE) was synthesized and it demonstrated colorimetric and ratiometric fluorescent responses to trivalent metal cations (M³⁺, M = Cr, Fe, Al) over a variety of mono- and divalent metal cations. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2cc38246f

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A pyridinyl-functionalized tetraphenylethylene
fluorogen for specific sensing of trivalent cations†
Xiujuan Chen,a Xiao Yuan Shen,a Erjia Guan,a Yi Liu,a Anjun Qin,a Jing Zhi Sun*a
and Ben Zhong Tang*ab
Cite this: Chem. Commun., 2013,
49, 1503
Received 15th November 2012,
Accepted 4th January 2013
DOI: 10.1039/c2cc38246f
www.rsc.org/chemcomm
A pyridinyl-functionalized tetraphenylethene (Py-TPE) was synthesized depends on the fluorogen’s structure and the situation in which
and it demonstrated colorimetric and ratiometric fluorescent responses the fluorogen is used. For instance, Lu et al. reported a highly
to trivalent metal cations (M3+, M = Cr, Fe, Al) over a variety of mono- selective and sensitive fluorescent turn-on chemosensor for Al3+
and divalent metal cations.
based on a photoinduced electron transfer (PET) approach.9 Li and
colleagues demonstrated that a dyad containing a rhodamine and a
Exploring fluorescent chemosensors for trivalent metal cations (M3+) naphthalimide moiety could be used as a Cr3+-selective fluorescent
is an important research subject. On the one hand, fluorescent probe for monitoring Cr3+ in living cells with a ratiometric fluor-
techniques offer the feasibility of fast, facile and highly sensitive escent method.5c The working mechanism is ascribed to fluores-
detection of target analytes. On the other hand, M3+ ions have their cent resonance energy transfer (FRET) from 1,8-naphthalimide to
own biological significance and environmental importance. For rhodamine. Dong and colleagues recently reported a fluorescent
example, Cr3+ is a necessary trace element in human nutrition. turn-on chemosensor for the selective detection of Al3+ based on an
Overdose of Cr3+ negatively affects cellular structure and function aggregation-induced emission (AIE) mechanism.10 AIE drives at a
and causes disturbance in glucose levels and lipid metabolism, while unique fluorescent behavior of some fluorogens that emit very
a deficiency of Cr3+ in humans can cause diabetes and cardio- weakly in dilute solution but become highly emissive when their
vascular disease.1 Meanwhile, industrial runoff of Cr3+ may lead to molecules form aggregates or they are frozen in solid matrices.11
detrimental pollution. As the third most abundant metal in the The complexation of carboxylic groups on the pentaphenylpyrrole
Earth’s crust, Al3+ is also one of the common species of metal moiety and Al3+ results in molecular aggregation. More recently,
cations. It is common sense that increasing free Al3+ due to acidic Barba-Bon et al. described a highly efficient fluorescent probe for
rain and human activities in the environment and surface water is trivalent cations. The fluorescence (FL) turn-on was realized via the
detrimental to growing plants.2 And in recent years, it has been formation of 1 : 1 or 2 : 1 ligand–metal complexes.6
recognized that excessive intake of Al3+ leads to a wide range of
Herein, we demonstrate a different method for the fluorescent
diseases, such as Alzheimer’s disease and osteoporosis.3 As to Fe3+, sensing of M3+ ions (M = Cr, Fe and Al) by using a pyridinyl-
its vital roles in a variety of cell functions have been summarized in functionalized tetraphenylethene (TPE) fluorogen. The chemical
tutorial reviews.4
structure of the target compound (Py-TPE) and its synthetic route
There have been research efforts to develop luminescence are displayed in Scheme 1, the experimental details are described
turn-on and/or ratiometric sensors based on available fluorogens in the ESI.† The resultant was characterized by 1H-NMR, 13C-NMR,
such as rhodamine,5 fluorescein,6 BODIPY,7 and cyclometalated high-resolution mass spectrometry, elemental analysis and
Ir(III) complex ([Ir(dfppy)2phen]+).8 The sensing mechanism satisfactory results were obtained (ESI† and Fig. S1, S2 and S3).
Our original intention of acquiring Py-TPE was to make use of the
a MoE Key Laboratory of Macromolecule Synthesis and Functionalization,
AIE properties of the TPE and the complexation ability of the
pyridine moieties, because TPE is a representative AIE-active
fluorogen and widely used as a converter to turn classical
Department of Polymer Science and Engineering, Zhejiang University,
Hangzhou 310027, China. E-mail: sunjz@zju.edu.cn; Fax: +86 (571)87953734;
Tel: +86 571-87953734
b Department of Chemistry, Institute for Advanced Study, State Key Laboratory of
Molecular NeuroScience, and Division of Biomedical Engineering, The Hong Kong
University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China.
E-mail: tangbenz@ust.hk; Fax: +86 852-23581594; Tel: +86 852-23587375
† Electronic supplementary information (ESI) available: Detailed synthesis and
characterization of Py-TPE; UV and FL spectra for AIE and titration; frontier
molecular orbital plots and energy diagrams of Py-TPE and protonated Py-TPE.
Scheme 1
See DOI: 10.1039/c2cc38246f
c
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chromophores into AIE-active ones.12 In our expectation, com- can be observed. Though Hg2+ can also induce a change in emission
plexation of metal cations and the pyridinyl moiety of Py-TPE feature, it is easy to recognize Hg2+ from M3+ ions by the difference in
would change the solubility and trigger molecular aggregation, emission colour (orange for Hg2+ and red for M3+ ions).
thereby inducing emission from the TPE moiety. Consequently,
metal cations could be detected in an FL turn-on strategy.
The first characteristic suggests that fluorescent response relates
to the ionic valence rather than to the kind of cation. Some M2+ ions
In dilute ethanol solution, Py-TPE emits very weakly. But it that have higher capacity for coordinating with the pyridine moiety
becomes highly emissive when a suspension is formed in ethanol– (i.e. Zn2+, Co2+, and Cu2+) are regardless to the existence of Py-TPE
water (solvent–non-solvent) mixtures (Fig. S4, ESI†). This observation and show no FL change. While the M3+ ions demonstrate evident
indicates that Py-TPE is an AIE-active compound. Moreover, the non- fluorescent responses though it is well-known that pyridine is not a
aggregation in ethanol–water solutions with low water fraction allows proper ligand to Cr3+, Fe3+ or Al3+. The second characteristic implies
us to use Py-TPE as a fluorescent probe to detect metal cations that that the red emission may come from new fluorescent species other
have acceptable solubility in aqueous media. We firstly tried the than Py-TPE aggregates, otherwise blue emission should be
detection of Zn2+ in Py-TPE ethanol solution. As shown in Fig. 1, in observed (see Fig. S4, ESI†). The fact that Py-TPE has not formed
comparison with the FL free from metal cations, there are no evident aggregates is also confirmed by the photographs of the mixtures
changes in the fluorescent features before and after the addition of taken in daylight, which show no precipitates in the solutions.
Zn2+ into the solution. This result is out of our expectation. Because
To gain a better understanding of the impact of M3+ on the
we have reckoned that Zn2+ has a strong affinity to the pyridinyl fluorescent behaviour of Py-TPE, Cr3+ was used as a representative
group and the derived complex has low solubility and readily (named as Cr3+@Py-TPE system) and the FL changes of Py-TPE with
precipitates in the solution, then the aggregates give rise to strong different concentrations of Cr3+ ([Cr3+]) were monitored (Fig. 2).
FL. To check whether the unexpected result is observed by chance or Without Cr3+, Py-TPE itself emits weak blue FL in ethanol solution
is a general phenomenon, we tried different metal cations in three with a maximum at 488 nm. When Cr3+ is introduced into the
categories. (i) Metal cations in the same period as Zn2+ in the solution, the blue emission band is decreased. Meanwhile, a new red
periodic table of elements, including Cr3+, Mn2+, Fe2+/Fe3+, Co2+, emission band with a peak at 630 nm grows as [Cr3+] increases. This
Ni2+, and Cu2+. (ii) Some hazardous heavy metal ions such as Pb2+, red band begins to be visually observed at a [Cr3+] of 6 mM and
Cd2+, Hg2+ and Ag+. (iii) A few metal cations in the IA, IIA and IIIA becomes dominant when [Cr3+] is higher than 20 mM. These results
groups such as Na+, K+, Ca2+, Mg2+, and Al3+. The results are indicate that Py-TPE has a higher Cr3+ cation sensitivity (as low as
displayed in Fig. 1. Surveying these data, two characteristics can be 6 mM) over the reported Cr3+ fluorescent probes based on chelating,
found. The first is that all of the trivalent cations showed positive PET and FRET mechanisms.5–9 The plot of I630/I488 vs. [Cr3+] and the
responses to the fluorescent probe, as quantitatively and visually FL images of the mixtures shown in Fig. 2B and the inset provide
revealed by Fig. 1A and B, respectively. And the second is that the visual and clear information of the colorimetric fluorescent detection
emission colour changes from blue of Py-TPE to red when enough of Cr3+ using Py-TPE as a fluorescent probe. Similar results were
M3+ aqueous solution is added into the Py-TPE ethanol solution monitored in the cases of Fe3+ and Al3+ (Fig. S6 and S7, ESI†).
(Fig. 1C). In all of the tested M+ and M2+ ions, only Hg2+ shows a
It is noticeable that there is an isosbestic point at 535 nm in
positive fluorescent response to Py-TPE. As revealed by the spectra Fig. 2A and similar points are also observed for the systems of
and image in Fig. S5 (ESI†), orange fluorescence (peak at 597 nm) Fe3+@Py-TPE and Al3+@Py-TPE (Fig. S6 and S7, ESI†). The existence
of an isosbestic point further verifies that the red emission originates
from distinct species from Py-TPE. The red-emission species cannot
be complexes of M3+(Py-TPE)x. On the one hand, Cr3+(Py-TPE)x,
Fe3+(Py-TPE)x and Al3+(Py-TPE)x should give different FL features
due to the different electronic structures of the central cations. On
the other hand, M3+(Py-TPE)x would precipitate, because the coordi-
nation of the pyridine moiety with M3+ bestows the complex with a
Fig. 1 (A) Fluorescence (FL) spectra of Py-TPE in ethanol and in ethanol–water
mixture solutions in the presence of 10 equivalents of different metal cations. (B)
Diagram of I630/I488 corresponding to Py-TPE in ethanol and each metal cation in
ethanol–water mixture solution. I630 and I488 are the maximal intensities of the
blue and red emission bands, respectively. The data are extracted from (A). (C), (D)
Photographs of the above solutions taken under UV light (lex = 365 nm) and
daylight, respectively. Concentration of Py-TPE ([Py-TPE]):10 mM.
Fig. 2 (A) FL spectra of Py-TPE in ethanol and in ethanol–water mixture solutions with
different concentrations of Cr3+ ([Cr3+]). (B) Plot of I630/I488 versus [Cr3+]. The data are
extracted from (A). The inset of (B) shows fluorescent images taken under UV light (lex
365 nm) for the solutions containing different [Cr3+], [Py-TPE] = 10 mM.
=
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1504 Chem. Commun., 2013, 49, 1503--1505
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hydrophobic aromatic shell that makes the molecule hardly soluble of the fluorescent behaviour observed for H+@Py-TPE and
in water–ethanol mixtures. However, the experimental results show M3+@Py-TPE (M = Cr, Fe, Al) indicates that the colorimetric and
that Cr3+@Py-TPE, Fe3+@Py-TPE and Al3+@Py-TPE exhibit identical ratiometric responses of Py-TPE to M3+ come intrinsically from a
fluorescent behaviour and no precipitates have been observed for proton-binding-induced fluorescence change.
the three systems (Fig. 2B, Fig. S6 and S7, ESI†). Furthermore, the
In summary, pyridinyl-functionalized TPE (Py-TPE) has been
absorption spectra of Cr3+@Py-TPE, Fe3+@Py-TPE and Al3+@Py-TPE synthesized and displays weak blue FL in solution. When various
systems all show evidently red-shifted absorption bands as mono-, di- and trivalent metal cations were added into the Py-TPE
compared to Py-TPE (Fig. S8 to S11, ESI†). These results indicate ethanol solution, only Fe3+, Cr3+ and Al3+ can alter the emission
that the addition of M3+ to Py-TPE ethanol solution must have feature from weak blue to strong red. By analyzing the similarity
triggered the formation of new species in the mixture.
between the fluorescent responses of Py-TPE to Fe3+, Cr3+ and Al3+
We next turned our attention to the pyridinyl moiety on Py-TPE and comparing the Ksp values of M(OH)n for all the measured metal
which is a typical proton acceptor. Since water–ethanol mixture was cations, we deduced that the red emission band originates from
used as the detection medium, it is possible to capture protons. protonated Py-TPE. This deduction has been validated by both a
Moreover, checking the database of Ksp (the constant of solubility comparative study of the fluorescent responses of Py-TPE to TFA in
product) of M(OH)n (n = 1, 2, and 3 for M+, M2+, and M3+ different concentrations and theoretical predictions. It is concluded
respectively), we found that Fe(OH)3, Al(OH)3 and Cr(OH)3 are the that the differentiated FL response of Py-TPE to M3+ over other tested
smallest three in all of our tested metal cations.13 Meanwhile, the M2+ and M+ ions depends on the higher hydrolyzing ability of the
order of Ksp for Fe(OH)3, Al(OH)3 and Cr(OH)3 is completely M3+, which releases more protons in the water–ethanol mixture. This
consistent with the order of I630/I488 shown in Fig. 1B. It implies fluorescent detection of trivalent cations is achieved by a mechanism
that M3+ is intensively hydrolyzed in water according to the master distinct from literature reported ones. The colorimetric (blue to red)
3Àx
equation M3+ + xH2O 2 M(OH)x
+ xH+. Thus the addition of and ratiometric (I630 vs. I488) characteristics of this kind of fluorescent
M3+ aqueous solution into Py-TPE ethanol solution is equivalent to probe have their own advantages. In addition, the emission maxima
the addition of acid. at 630 and 488 nm perfectly match with red and blue laser sources.
Following this concept, we investigated the influence of protons This colorimetric and ratiometric FL probe may find promising
on the FL behavior of Py-TPE. As shown in Fig. 3, on adding applications as an active element of chemo- and biosensors.
trifluoroacetic acid (TFA) into Py-TPE ethanol solution, the FL
This work was supported by the National Science Founda-
features gradually change. The red emission peak at 630 nm tion of China (51273175); the key project of the Ministry of
manifests in the spectra at a TFA concentration ([TFA]) of 4 mM. Science and Technology of China (2013CB834704), the
This red band becomes predominant when [TFA] is equal to and/or Research Grants Council of Hong Kong (603509, HKUST2/
higher than 8 mM. The blue emission of Py-TPE (peak at 488 nm) CRF/10, and 604711), and the University Grants Committee of
vanishes when [TFA] is equal to or higher than 40 mM. The proton- Hong Kong (AoE/P-03/08).
induced changes in FL intensity and colour are demonstrated by
the plot of I630/I488 vs. [TFA] and the images in the inset of Fig. 3B.
In addition, there exists an isosbestic point at 535 nm in Fig. 3A,
which indicates that the blue and red emission bands are ascribed
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Fig. 3 (A) FL spectra of Py-TPE in ethanol solutions with different concentrations
of trifluoroacetic acid or TFA ([TFA]). (B) Plot of I630/I488 versus [TFA]. The inset of
(B) displays FL images taken under UV light (lex = 365 nm) for the solutions
containing different amounts of TFA. [Py-TPE] = 10 mM. The inset chemical
equation shows the formation of protonated Py-TPE upon acceptance of a
proton by the pyridinyl moiety.
13 At 25 1C, for Al(OH)3, Cr(OH)3, and Fe(OH)3, Ksp = 4.6 Â 10À33
,
1.0 Â 10À33, and 3.0 Â10À39, respectively.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 1503--1505 1505