A selective fluorescent sensor for Zn2+ based on aggregation-induced emission (AIE) activity and metal chelating ability of bis(2-pyridyl)diphenylethylene

Article abstract of DOI:10.1039/c6dt02657e

Polypyridyl ethylenes have been prepared as heterocyclic analogues of tetraphenylethylene. In addition to conventional AIE effects, a derivative possessing a 1,1-bis(2-pyridyl)ethylene fragment displays Zn²⁺-selective enhanced fluorescence in aqueous solution, in contrast to the behavior of isomeric bis(3- and 4-pyridyl)ethylenes. The Zn²⁺-sensing capability of this material is attributed to formation of chelated Zn²⁺-bis(pyridyl) complexes and is supported by the X-ray crystal structure of a bis(2-pyridyl)ethylene-Zn(OAc)₂ complex.

Full text of DOI:10.1039/c6dt02657e

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A selective fluorescent sensor for Zn2+ based on aggregation-
induced emission (AIE) activity and metal chelating ability of
bis(2-pyridyl)diphenylethylene†
Received 00th January 20xx,
Accepted 00th January 20xx
Moustafa T. Gabr and F. Christopher Pigge*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Polypyridyl ethylenes have been prepared as heterocyclic ions.5 The TPEs employed in these studies generally feature
analogues of tetraphenylethylene. In addition to conventional AIE substituents that mediate aggregation events grafted onto the
effects,
a derivative possessing a 1,1-bis(2-pyridyl)ethylene periphery of an intact tetraphenylethylene core. In contrast,
fragment displays Zn2+-selective enhanced fluorescence in AIE-based compounds that possess functionality in the form of
aqeuous solution, in contrast to the behavior of isomeric bis(3- one or more heterocyclic rings in place of the core phenyl rings
and 4-pyridyl)ethylenes. The Zn2+-sensing capability of this are much less investigated.6
material is attributed to formation of chelated Zn2+-bis(pyridyl)
complexes and is supported by the X-ray crystal structure of a heteroaromatic tetraarylethylenes and examining the AIE
We are developing concise synthetic routes to
bis(2-pyridyl)ethylene-Zn(OAc)2 complex.
properties of these new small-molecule organic luminophores.
For example, we recently reported the synthesis and
The development of fluorescent sensors and/or detectors
capable of signalling the presence of specific analytes or
chemical events remains an important objective in both
materials and biological chemistry.1 Luminophores that exhibit
“turn-on” fluorescence are particularly desirable, and in this
context organic fluorophores that display aggregation-induced
emission (AIE) or aggregation-induced enhanced emission
(AIEE) are emerging as valuable building blocks for sensor
design across a range of disciplines.2
characterization of bis(pyridyl)-tetraarylethylenes
1 and 2
(Scheme 1).7 While both compounds displayed typical AIE
fluorescence profiles (weak emission in CH3CN that increased
significantly upon addition of H2O), neither compound showed
a fluorescence response when combined with various metal
ions, despite the presence of potential pyridine ligands.
Consequently, we have redesigned these bis(pyridyl) TPE
analogues to incorporate heteroarenes capable of acting as
chelating ligands toward a metal center with the aim of
eliciting a metal-selective AIE response. We report here the
synthesis and characterization of three new 2,2’-bis(pyridyl)
AIE active compounds are typically non-fluorescent in dilute
solution but become emissive upon aggregation in the
presence of poor solvents.3 Emissive properties of AIE active
compounds are often observed in the solid state as well. This
behaviour contrasts with that of many conventional organic
dyes that exhibit fluorescence quenching at high concentration
or when forced to aggregate in solution. The origin of
enhanced emission in many AIE-active compounds is believed
to stem from restriction of intramolecular motions (e.g., sp2-
sp2 bond rotations) when individual molecules are present as
nano-aggregates or in condensed phases.4
tetraarylethylenes (4-6) and the ability of these compounds to
function as fluorescent metal ion sensors selective for
detection of Zn2+ ions in aqueous solution.
Br
Br
N
Ar
Ar
N
Pd(OAc)2, PPh3
Na2CO2, ArB(OH)2
dioxane/H2O
N
N
3
X
X
4 Ar = Ph, 61%
Y
Y
5 Ar = 4-MeOC6H4, 75%
1 X = N, Y = CH
Tetraphenylethylene (TPE) is an iconic example of an AIE-
active compound whose fluorescence can be switched on as a
X = CH, Y = N
2
6 Ar =
67%
S
Scheme 1 Synthesis of 2,2’-bis(pyridyl) tetraarylethylenes 4-6.
function of molecular environment.
A number of TPE
derivatives have been investigated for use as fluorescent
indicators, particularly in bioanalytical settings in which the AIE
response has been used to signal the presence of targeted
analytes, such as specific organic or biomolecules and metal
The synthesis of 2,2’-bis(pyridyl) tetraarylethylenes
4-6 is
illustrated in Scheme 1 and is similar to the straightforward
procedures we reported previously for the preparation of
and 2 7,8
Briefly, 2,2’-bis(pyridyl) ketone was converted to
1
3
.
upon treatment with CBr4/PPh3. Double Suzuki coupling
between and an aryl boronic acid then afforded in good
3
4-6
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yields. The aryl boronic acids employed in this study were plot showing the change in fluorescence intensity of
4 as a
selected to highlight the ease with which both simple and function of added Zn2+ is provided in Figure 2B. The emission
electronically distinct arenes (phenyl and substituted phenyl) rapidly increases with increasing amounts of Zn2+ until a
as well as more elaborate arenes (e.g., dibenzothiophene) can plateau is reached at ~ 4 equivalents added Zn(ClO4)2. This
be incorporated into the final products.
The fluorescence properties of were next examined in
CH3CN (a solvent in which all three compounds dissolve easily) presence of 2 equivalents Zn2+).11 The selectivity of
and CH3CN/H2O mixtures (
are insoluble in H2O). The Zn2+ ions is evident from the graph in Figure 2C. The
fluorescence spectra of in various combinations of fluorescence at 378 nm is normalized to
+ Zn2+, and only the
CH3CN/H2O are shown in Figure 1A. In 100% CH3CN
exhibits presence of Cd2+ was observed to produce any other notable
a fluorescence emission at 447 nm that decreases upon emission enhancement. Furthermore, most other metal ions
addition of H2O. Concomitantly, a new blue-shifted emission were found not to interfere with emission enhancement of
peak appears at 378 nm that increases in intensity as a in the presence of Zn2+ with the exception of Cu2+ and, to a
function of added H2O (Figure 1B). The shift to higher energy lesser extent, Fe3+. Compound
also displays high sensitivity
effect is also reflected in a threefold change in quantum yield
Φ378 = 9.1% in 9:1 H2O:CH3CN increasing to 31.9% in the
toward
4
-6
(
4
4-6
4
4
4
4
4
emission upon aggregation likely reflects a change in molecular toward Zn2+ under these conditions with the limit of detection
shape from a more planar conformation when highly solvated determined to be ~94 nM (ESI Figure S8).
to a more propeller-like arrangement of the aryl rings in multi-
molecular aggregates.9 In contrast,
in CH3CN at 451 nm that becomes red shifted upon addition of
H2O. The AIE effect in , however, is much less pronounced
compared to (see ESI, Figure S6). Finally, exhibits more or
5 exhibits weak emission
5
4
6
less conventional AIE behaviour with a significant increase in
fluorescence emission observed in 9:1 H2O:CH3CN (Figure S7).
Figure 1 (A) AIE profile of 4 in CH3CN/H2O mixtures. (B) Change in fluorescence
intensity at 378 nm as a function of added H2O. λex = 317 nm, [4] = 10 ꢀM
The effect of added metal ions on the fluorescence of
was then examined. Figure 2A shows a series of fluorescence
spectra of obtained in 9:1 H2O:CH3CN in the presence of 2
4-6
4
equivalents added metal ions (as ClO4 salts). The addition of
Zn2+ elicited a markedly enhanced emission at 378 nm that far
surpassed the effect of any other metal ion examined (Mg2+,
Ca2+, Ba2+, Co2+, Cd2+, Pb2+, Cu2+, Ni2+, Hg2+, Fe3+, Fe2+, Mn2+,
Na+, and K+). This is significant as Zn2+ is one of the most
abundant metal ions in the environment and is an essential
element in living organisms. As a result, new and selective
Figure 2 (A) Fluorescence spectra of 4 in the presence of various metal ions (2
equivalents). Inset: Picture taken under UV light of 4 in 9:1 H2O:CH3CN with no Zn2+
added (left) and 2 equivalents of Zn2+ (right). (B) Change in fluorescence intensity (I/Io)
at 378 nm of 4 as a function of added Zn2+. (C) Emission of 4 in the presence of various
metal ions and Zn2+-metal ion combinations (normalized to emission in the presence of
Zn2+). λex = 317 nm, [4] = 10 ꢀM
The effect of Zn2+ counterion on fluorescence was also
sensors
capable of detecting
and/or monitoring
probed. As shown in Figure 3A, emission enhancement of
4 in
Zn2+ concentration in aqueous solution are highly desired.10
A
the presence of Zn(ClO4)2, Zn(OAc)2, and ZnCl2 were similar,
2 | J. Name., 2012, 00, 1-3
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with Zn(NO3)2 exhibiting a diminished effect. To gauge – ethylene bond rotation and produce enhanced fluorescence.
whether these differences might be due to protonation of A Job plot constructed from NMR titration of and Zn(ClO4)2 in
4
4
under these conditions, the fluorescence spectrum was 1:1 D2O:CD3CN indicated a 1:1 binding stoichiometry (ESI
obtained in 1M aqueous HCl (ESI Figure S9).12 The emission of Figures S10-S11), consistent with the solid state structure. The
4
was observed to red-shift (λem ~430 nm) relative to absence of enhanced emission in the presence of metals other
fluorescence in 9:1 H2O:CH3CN, thus pyridine protonation does than Zn2+ may reflect an inability to form similar chelated
not appear to be contributing to the fluorescence response in adducts with these ions.
the presence of metal salts. The effect of pH on Zn2+ binding
by
4 was also probed and maximum emission enhancement
was observed between pH ~4 – 9 (Figure 3B). Thus
capable of Zn2+ sensing at physiologically relevant pH.
4 is
Figure 4 Molecular structure of 4·Zn(OAc)2.
The behaviour of
5
in the presence of Zn2+ qualitatively
obtained in
mirrors that of 4. The fluorescence spectra of 5
the presence of various metal ions is shown in Figure S12 (ESI).
A selective fluorescence enhancement was observed in the
presence of Zn2+ (and Cd2+), but other metals produced little to
no change in emission. Added Zn2+ continued to increase
fluorescence emission up to ~4 equivalents, after which
additional Zn2+ had little effect (Figure S13). Most other
metals (with the exception of paramagnetic Cu2+) were also
found not to interfere with fluorescence of
In contrast, addition of metal ions to in 9:1 H2O:CH3CN did
5
/Zn2+ (Figure S14).
6
not produce any fluorescence enhancement (Figures S15-S16).
We attribute this to steric effects from the dibenzothiophene
groups that impede pyridine-metal interaction.
Differences in metal ion-mediated enhanced fluorescence in
Figure
3
(A) Fluorescence spectra of 4
in the presence of Zn2+ with different
4
compared to the 4-pyridyl and 3-pyridyl isomers
highlights the ability to alter AIE sensing properties of
heteroaromatic tetraarylethylenes through synthetic
modification. Among these three compounds, the potential to
form chelated metal ion interactions in appears to be
important in governing fluorescence response compared to
and , which are both incapable of metal chelation. The
1 and 2
counterions. (B) Fluorescence intensity (378 nm) of 4 in the absence and presence of
Zn2+ (2 equiv) as a function of pH. λex = 317 nm, [4] = 10 ꢀM
To obtain insight into structural features that might
contribute to fluorescence enhancement in this system, a
4
1
single crystal X-ray structure of 4·Zn(OAc)2 was obtained. The
2
molecular structure of the coordination complex is illustrated
in Figure 4. The Zn2+ ion exhibits a distorted tetrahedral
coordination geometry with two N donors from the pyridine
rings and two acetate O donors. One acetate ligand exhibits a
weak bidentate coordination mode with one Zn – O distance
significantly shorter than the other (2.032 Å and 2.313 Å). The
Zn – O distance to the monodendate acetate and the Zn – N
distances are typical for Zn2+ complexes of this type (see ESI).
Indeed, the structure resembles the asymmetric unit found in
the crystal structure of Zn(OAc)2 with tetra(2-pyridyl)ethane
and tetra(2-pyridyl)ethylene ligands.13 As envisioned, the
addition of Zn2+ to these heteroarylethylenes was performed
under conditions in which the organic substrates were already
aggregated (9:1 H2O:CH3CN solution). Thus, formation of a
chelated adduct in
4 would serve to further rigidify the
molecular framework by placing additional restraints on
intramolecular bond rotation, thereby resulting in enhanced
fluorescence. In the case of
1 and 2, intercalation of metal
ions such as Zn2+ into aggregated assemblies may afford
intermolecular coordination complexes without concomitant
increases in molecular rigidity, resulting in an attenuated
fluorescence response.
tetraarylethylene
4 functions as a chelating bidendate ligand
toward the metal center with the Zn2+ ion coordinated below
the plane of the ethylene unit (torsion angles between the
pyridine rings and the alkene are 53.80° and 57.25°).
Formation of similar solution-phase chelated complexes in
To explore this issue further and to expand the range of
heteroaromatic tetraarylethylenes, we prepared the
symmetrical tetrapyridylethylenes
9
and 10 via Suzuki coupling
and
(Scheme 2). We envisioned that the additional pyridine rings
of pyridine boronic acids and geminal dibromoethylenes
7
8
aggregates of
4 would further restrict intramolecular pyridine
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in and 10 would contribute to formation of more highly
Journal Name
1
(a) J. Chan, S. C. Dodani and C. J. Chang, Nat. Chem., 2012,
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4487-4493.
4
,
,
,
9
“cross-linked” coordination aggregates in the presence of
metal ions, resulting in altered (increased) fluorescence.
Hetero-TPE analogues
luminescence in CH3CN/H2O mixtures, similar to the behaviour
of and . Addition of metal ions to aqueous solution phase
aggregates of
9 and 10 exhibited conventional AIE
2
3
J. Mei, N. L. C. Leung, R. T. K. Kwok, J. W. Y. Lam and B. Z.
Tang, Chem. Rev., 2015, 115, 11718-11940.
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1
2
9
, however, elicited only a small fluorescence
enhancement in the case of Zn2+. Similar experiments
4
5
(a) N. L. C. Leung, N. Xie, W. Yuan, Y. Liu, Q. Wu, Q. Peng, Q.
Miao, J. W. Y. Lam and B. Z. Tang, Chem. Eur. J., 2014, 20
performed with 10 showed little to no fluorescence
,
enhancement (Figures S19-S22). Thus, the results with
9 and
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Hu, T. Chen, Z. Huang, X. Meng and M.-Q. Zhu, Chem.
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Tang, Chem. Soc. Rev., 2015, 44, 4228-4238.
10 seem to further implicate the importance of chelated
coordination complexes for enhanced emission under aqueous
conditions. Despite the absence of effective metal ion sensing,
the potential tetracoordinate ligand framework embodied in
AIE-active
9 and 10 renders these compounds attractive
building blocks for other applications in supramolecular
chemistry.
6
(a) D. Jana and B. K. Ghorai, Tetrahedron, 2012, 68, 7309-
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,
Scheme 2 Synthesis of tetrapyridylethylenes 9 and 10.
In conclusion, a convenient synthetic approach to AIE active
poly(pyridyl) tetraarylethylenes is reported. A subset of these
compounds possessing a 1,1-bis(2-pyridyl) ethylene fragment
(i.e.,
4 and 5)
exhibit Zn2+-selective aggregation-induced
enhanced emission in 9:1 H2O:CH3CN solution that is not
exhibited in isomeric 3- and 4-pyridyl tetraarylethylenes. The
and B. Z. Tang, ACS Appl. Mater. Interfaces, 2011, 3, 3411-
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with Zn2+ ions, as confirmed through X-ray crystallography, is
believed to be important in governing the fluorescence
response. “Turn-on” luminescent metal ion sensors capable of
operating in aqueous environments have many potential
applications in environmental and biological chemistry, and
examples of Zn2+-selective AIE-active sensors are rare.14
Tetraarylethylene 4 shows excellent selectivity and sub-
micromolar sensitivity in its fluorescence response toward Zn2+
ions in predominantly aqueous solution. The molecular
framework of
4
can be easily modified, and we are currently
designed to display
investigating structural analogues of
4
enhanced metal ion sensing abilities that further exploit the
inherent AIE properties of these materials.
We thank the Department of Chemistry and the Graduate
College of the University of Iowa for support. We thank Mr.
Gonzalo Campillo-Alvarado and Dr. Dale Swenson for
assistance with X-ray crystallography.
Notes and references
4 | J. Name., 2012, 00, 1-3
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