Click synthesized dianthryl-TTFV: An efficient fluorescent turn-on probe for transition metal ions

Article abstract of DOI:10.1039/c2ob06828a

Tetrathiafulvalene vinylogue (TTFV) was functionalized with two anthryl fluorophores via Cu(i)-catalyzed alkyne-azide [3 + 2] cycloaddition, forming a dianthryl-TTFV hybrid to show fluorescent turn-on sensing behaviour for Cu ²⁺, Fe²⁺, and Cd²⁺ ions in THF with remarkably low detection limit down to the sub-ppm level. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2ob06828a

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COMMUNICATION
Click synthesized dianthryl–TTFV: an efficient fluorescent turn-on probe for
transition metal ions†
Karimulla Mulla, Prateek Dongare, David W. Thompson and Yuming Zhao*
Received 31st October 2011, Accepted 31st January 2012
DOI: 10.1039/c2ob06828a
Tetrathiafulvalene vinylogue (TTFV) was functionalized with
two anthryl fluorophores via Cu(I)-catalyzed alkyne–azide
[
3 + 2] cycloaddition, forming a dianthryl–TTFV hybrid to
2+
2+
show fluorescent turn-on sensing behaviour for Cu , Fe ,
and Cd ions in THF with remarkably low detection limit
down to the sub-ppm level.
2+
1
Cu-catalyzed alkyne–azide cycloaddition (CuAAC) has been
widely used as a facile and efficient ligation tool for preparation
of molecular sensory systems, wherein the click product, 1,2,3-
triazole, acts as a structural (linkage) and/or functional (ligand/
2
receptor) component. Numerous recent reports have demon-
strated that 1,2,3-triazole can form stable complexes with various
3
4
5
metal cations, anions, and certain biomolecules, which
renders it an appealing synthon for ligand and receptor design.
In view of the versatile metal complexation behaviour, we have
recently set out to explore some new “click” chemosensors uti-
lizing click reaction-generated 1,2,3-triazole as an integral part to
6
Scheme 1 Synthesis and FMO properties of diathryl–TTFV 1.
achieve improved and modifed sensor properties.
In this communication, we report the facile click synthesis of
a dianthryl–tetrathiafulvalene vinylogue (TTFV) derivative (1,
Scheme 1) and its selective and sensitive fluorescent turn-on
sensing behaviour towards certain transition metal ions, includ-
The molecular structure and electronic properties of dianthryl–
TTFV 1 were investigated by density functional theory (DFT)
calculations at the B3LYP/6-31G* level. In the optimized geo-
9
2+
2+
2+
ing Cu , Fe , and Cd . It is worth noting that detection of
these metal ions is of great interest in respect to environmental
metry of 1 (bottom of Scheme 1), the TTFV unit shows a
pseudo-cis conformation. The HOMO is distributed at the
central TTFV core, while two degenerated LUMOs are solely
located at the two anthryl groups. Such spatially separated FMO
7
remediation, biological studies, and industrial applications. The
molecular structure of click sensor 1 consists of three essential
parts for a fluorescent chemosensor: (1) a central TTFV core as
10
characters together with the low oxidation potential of TTFV
8
electron donor, (2) two anthryl groups as fluorogenic units, and
suggest that photoinduced electron transfer (PET) quenching of
(3) two triazole rings as both linkages and receptors. The syn-
the fluorophore would provide a mechanistic basis for sensing
11
thesis of compound 1 was accomplished by a two-fold CuAAC
function. Indeed, compound 1 was found to show very weak
fluorescence (Φ = 0.009 in THF). In view of the good ligand
abilities of the triazole rings, compound 1 was predicted to bind
significantly to various transition metal ions. In principle, com-
plexation with metal ions would reduce the electron-donating
ability of the TTFV unit, which in turn attenuates the PET
process to give rise to fluorescence enhancement (turn-on).
To test the anticipated sensory performance, dilute solutions
of dianthryl–TTFV 1 in THF at μM concentrations were titrated
with eight different transition metal ions. Of the metal ions
8
a–c
reaction between dialkynyl–TTFV precursor 2
and azido-
anthracene 3 in the presence of CuI as catalyst and diisopropyl-
ethylamine (DIPEA) as base. The reaction was carried out in
THF at elevated temperature overnight to afford compound 1 in
a high yield.
Department of Chemistry, Memorial University of Newfoundland,
St. John’s, NL, Canada, A1B 3X7. E-mail: yuming@mun.ca;
Fax: +1-709-864-3702; Tel: +1-709-864-8747
2+
2+
2+
tested, Cu , Fe , and Cd give very significant fluorescence
enhancement. Fig. 1A illustrates the fluorescence spectral
†
Electronic supplementary information (ESI) available: Full experimen-
tal details and spectroscopic characterization data. See DOI: 10.1039/
c2ob06828a
2+
changes of 1 in titration with Cu . The titration results clearly
2542 | Org. Biomol. Chem., 2012, 10, 2542–2544
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Table 1 Binding constants of 1 with select transition metal ions and
1
4
TFA and standard electrode potentials (E°) of the cations
1
2
Cation
log(β11 × M )
log(β12 × M )
E° (298 K, V)
2
+
+
+
2+
0
Cu
Cd
Zn
7.12 ± 0.7
5.22 ± 0.6
4.78 ± 0.2
4.62 ± 0.4
4.58 ± 0.1
3.57 ± 0.1
2.72 ± 0.2
1.72 ± 0.2
1.63 ± 0.5
12.1 ± 0.8
9.75 ± 0.6
8.59 ± 0.3
10.8 ± 0.2
7.56 ± 0.1
6.67 ± 0.1
4.47 ± 0.2
2.46 ± 0.3
2.95 ± 0.6
+0.34 (Cu → Cu )
2+ 0
2
−0.40 (Cd → Cd )
2
2+
0
−0.76 (Zn → Zn )
2
+
2+ 0
Fe
−0.44 (Fe → Fe )
2+ 0
2
+
Mn
Ag
Hg
−1.19 (Mn → Mn )
+
+ 0
+0.80 (Ag → Ag )
2+ 0
2
+
+0.79 (Hg → Hg )
2
+
2+
0
Pb
−0.12 (Pb → Pb )
+
+
H
2
+0.00 (H → H )
Fig. 1 (A) Fluorescence titration of 1 (5.55 μM) with Cu(OTf)
THF at 298 ± 3 K, λex = 350 nm. Inset: Stern–Volmer plot calculated
from emission at λ = 415 nm. (B) Fluorescence enhancement (λ =
2
in
Fig. 2 Linear correlations of binding constants (log β) and standard
electrode potentials (E°) of transition metal and proton ions.
Plotting the binding constants (log β11 and log β ) of these
4
15 nm) at the endpoint of titration. F
o
and F refer to the fluorescence
12
intensities at the initial and ending points of titration. Inset: photographic
images of solutions of 1 with various transition metal ions and TFA in
THF under irradiation of a UV lamp.
cations against standard electrode potentials of the metal ions
(E°) turns out to be very informative. As shown in Fig. 2, the
cations fall in two distinct groups in terms of linear correlation
2+
2+
2+
between log β and E°. Group I consists of Cu , Cd , Zn ,
2
+
2+
2+
show that with increasing addition of Cu , an emission band of
anthracene at 395 nm and vibronic progressions at 415, 440, and
Fe , and Mn ions, which afford considerably larger binding
+
2+
+
constants than those of Group II, which includes Ag , Hg , H ,
and Pb ions. These observations are in line with expectations
2
+
2+
4
70 (sh) nm grow in as a function of [Cu ]. At the endpoint of
the titration, the fluorescence intensity is increased by 26-fold at
15 nm (detection limit: 0.26 μM). Note that the substantial flu-
orescence enhancement of 1 contrasts markedly the fluorescence
based on hard–soft acid–base (HSAB) theory. The three metal
2+
2+
2+
4
ions (Cu , Fe , and Cd ) that cause sizeable fluorescence
enhancement of sensor 1 share two common features: (1) they
belong to Group I, and (2) their relatively high E° values facili-
tate charge transfer from TTFV to metal ions, which efficiently
attenuates the PET process from TTFV to anthracene. Overall,
the relationships revealed in Fig. 2 are instructive for further
design and fine-tuning of the selectivity and sensitivity of che-
mosensors with analogous molecular architectures.
2
+
quenching effect of Cu on an anthryl–triazole sensor recently
reported by Varazo et al. Taken together, the TTFV donor must
play a pivotal role in the fluorescence turn-on mechanism.
Besides Cu ions, compound 1 also shows a considerable
degree of fluorescence enhancement in response to the titration
of Fe and Cd (see Fig. 1B). The detection limits are deter-
mined to be 0.48 μM for Fe and 0.86 μM for Cd respect-
ively. For comparison, the titration of 1 with a strong protic acid,
trifluoroacetic acid (TFA) was also performed. Fig. 1B clearly
shows that the sensitivity of 1 towards the tested cations exhibits
1
2
2+
2+
2+
2
+
2+
It was also noted that when the solution of compound 1 was
prepared at mM or higher concentrations in THF, significant pre-
cipitation took place upon addition of various metal ions. It is
rationalized that at higher concentrations, metal–sulfur inter-
actions become significant in addition to the metal–triazole inter-
actions. As such, the resulting complexes would stay in the form
2
+
2+
2+
+
a decreasing trend as follows: Cu > Fe > Cd > Ag
>
2
+
2+
2+
2+
+
Mn > Zn ∼ Hg > Pb > H . To further quantify the
binding properties of dianthryl–TTFV 1 with various metal ions,
the fluorescence titration data were subjected to SPECFIT global
analyses, using the Levenberg–Marquardt nonlinear regression
minimization procedure. The detailed binding stoichiometry and
binding constants are listed in Table 1.
15
of clusters or agglomerates, which lead to substantial fluor-
escence quenching. These complexes are soluble in polar
organic solvents such as acetonitrile, and the solutions of certain
13
2+
2+
2+
2+
metal complexes (Cu , Fe , Cd , and Pb ) in acetonitrile
display visually distinct colours to enable straightforward colori-
metric detection (see Fig. 3A). UV-Vis spectral analyses
The SPECFIT analyses indicate that compound 1 can bind
with transition metal and proton ions to form both 1 : 1 and 1 : 2
complexes, while the binding constants vary substantially.
2+
2+
2+
(Fig. 3B) clearly show that Cu , Fe , and Cd ions when com-
plexing with 1 at relatively high concentrations give pronounced
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compound 1 serve as effective ligands (receptors) to coordinate
with metal ions. To the best of our knowledge, compound 1 is
the first example of TTFV-based “click sensors”, and we believe
it can be a useful design motif for the development of fluoro-
phore-based molecular sensory and optoelectronic devices.
Acknowledgements
This work was supported by a Discovery Grant of Natural
Sciences and Engineering Research Council of Canada (NSERC).
Notes and references
1
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2
3
J. E. Moses and A. D. Moorhouse, Chem. Soc. Rev., 2007, 36, 1249–1262.
(a) K. J. Kilpin, E. L. Gavey, C. J. McAdam, C. B. Anderson, S. J. Lind,
C. C. Keep, K. C. Gordon and J. D. Crowley, Inorg. Chem., 2011, 50,
Fig. 3 (A) Photographic images of solutions of compound 1 and
various transition metal ions in acetonitrile. (B) UV-Vis absorption
spectra of compound 1 with various transition metal ions (ca. 11 mol
equiv.) measured in acetonitrile at room temperature.
6334–6346; (b) D. Schweinfurth, F. Weisser, D. Bubrin, L. Bogani and
B. Sarkar, Inorg. Chem., 2011, 50, 6114–6121; (c) M. L. Gower and J.
D. Crowley, Dalton Trans., 2010, 39, 2371–2378; (d) O. Fleischel,
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absorptions in the Vis-NIR region of the spectrum. Of particular
(f) E. M. Schuster, M. Botoshansky and M. Gandelman, Angew. Chem.,
Int. Ed., 2008, 47, 4555–4558; (g) H. Struthers, B. Spingler, T. L. Mindt
and R. Schibli, Chem.–Eur. J., 2008, 14, 6173–6183; (h) T. L. Mindt,
H. Struthers, L. Brans, T. Anguelov, C. Schweinsberg, V. Maes,
D. Tourwé and R. Schibli, J. Am. Chem. Soc., 2006, 128, 15096–15097.
note is the rise of a new band at ca. 500–800 nm, which is likely
+
2+
due to the formation of [TTFV] and [TTFV] via TTFV-to-
metal charge transfer.
8,16
In addition, a broad weak band ranging
from ca. 800 to 1100 nm can be clearly seen in the Vis-NIR
4 (a) Y. Hua and A. H. Flood, Chem. Soc. Rev., 2010, 39, 1262–1271;
(b) S. Lee, Y. Hua, H. Park and A. H. Flood, Org. Lett., 2010, 12, 2100–
2+
2+
profiles of Cu and Fe complexes, which is tentatively
2102; (c) Y. Li and A. H. Flood, Angew. Chem., Int. Ed., 2008, 47, 2649–
652; (d) A. Kumar and P. S. Pandey, Org. Lett., 2008, 10, 165–168.
+
assigned to [TTFV] . The UV-Vis data substantiate that at high
2
concentrations the sulfur groups of 1 can also act as ligands to
interact with oxidative metal ions, such as Cu , Fe , and Cd ,
to form colourful charge-transfer complexes.
5 (a) S. Sparapani, S. M. Haider, F. Doria, M. Gunaratnam and S. Neidle,
J. Am. Chem. Soc., 2010, 132, 12263–12272; (b) A. D. Moorhouse, A.
M. Santos, M. Gunaratnam, M. Moore, S. Neidle and J. E. Moses, J. Am.
Chem. Soc., 2006, 128, 15972–15973.
2
+
2+
2+
Experiments to elucidate the metal ion binding properties
6
(a) K. Mulla, P. Dongare, N. Zhou, G. Chen, D. W. Thompson and
Y. Zhao, Org. Biomol. Chem., 2011, 9, 1332–1336; (b) M. Shao and
Y. Zhao, Tetrahedron Lett., 2010, 51, 2508–2511.
1
were undertaken by H NMR titration between dianthryl–TTFV
2
+
2+
+
and select transition metal ions, Cu , Cd , and Ag in THF-d .
8
2+
7 A. W. Czarnik and J. P. Desvergne, Chemosensors of Ion and Molecule
Recognition, Kluwer Academic, Dordrecht, 1997.
For the Cu ion titration, pronounced line-broadening of all the
NMR signals (see ESI†) and significant precipitation were
8 (a) G. Chen, I. Mahmud, L. N. Dawe, L. M. Daniels and Y. Zhao, J. Org.
Chem., 2011, 76, 2701–2715; (b) G. Chen, I. Mahmud, L. N. Dawe and
Y. Zhao, Org. Lett., 2010, 12, 704–707; (c) G. Chen, S. Bouzan and
Y. Zhao, Tetrahedron Lett., 2010, 51, 6552–6556; (d) D. Lorcy,
R. Carlier, A. Robert, A. Tallec, P. Le Maguerès and L. Ouahab, J. Org.
Chem., 1995, 60, 2443–2447.
2
+
observed. The results suggest strong interactions of Cu with 1
and a considerable degree of aggregation for the resulting com-
2
+
+
plexes in THF-d . For the titration of Cd and Ag ions, the for-
8
2+
mation of precipitate was not as notable as observed in the Cu
titration. In general, the H NMR spectral responses of 1 to Cd
and Ag ions were similar. In the case of Cd titration, most of
the aromatic protons do not shift with increasing [Cd ], except
1
2+
9
Calculations were done by using the Spartan’10 software package (Wave-
+
2+
function Inc.).
10 Compound 1 shows a quasi-reversible redox wave pair at Epc = +0.48 V
2
+
and Epa = +0.61 V (vs. Ag/AgCl) in its cyclic voltammogram measured
for the triazolyl proton which shows a downfield shift from 7.60
to 7.73 ppm after ca. 7.0 molar equivalents of Cd was added.
The observation substantiates that the triazole units in compound
in THF (electrolyte: 0.1 M Bu
Pt, reference: Ag/AgCl, scan rate: 20 mV s ).
1 The triazole linker is also known to play a key role in mediating electron
transfer. See: G. de Miguel, M. Wielopolski, D. I. Schuster, M. A. Fazio,
O. P. Lee, C. K. Haley, A. L. Ortiz, L. Echegoyen, T. Clark and D.
M. Guldi, J. Am. Chem. Soc., 2011, 133, 13036–13054.
4 4
NBF , working: glassy carbon, counter:
−
1
2
+
1
1
act as active ligands to coordinate with transition metals. It is
also noted that the line-broadening of certain phenyl protons and
the SMe protons (see ESI†) appear to be far more pronounced
than that of the other protons. This unusual phenomenon
suggests that ligand exchange is rapid on the NMR time scale,
when specific metals are bound to sensor 1.
12 K. Varazo, F. Xie, D. Gulledge and Q. Wang, Tetrahedron Lett., 2008, 49,
5293–5296.
13 H. Gampp, M. Maeder, C. J. Meyer and A. D. Zuberbühler, Talanta,
1985, 32, 251–264.
1
1
4 R. D. Harrison, Book of Data, Longman Group Ltd, London, 1972.
5 (a) M. Yamamoto, X. Gan, T. Kuroda-Sowa, M. Maekawa, Y. Suenaga
and M. Munakata, Inorg. Chim. Acta, 1997, 261, 169–174; (b) X. Gan,
M. Munakata, T. Kuroda-Sowa, M. Maekawa and Y. Misaki, Polyhedron,
In summary, click-generated dianthryl–TTFV 1 exhibits fluor-
2
+
2+
2+
escence turn-on sensing function towards Cu , Fe , and Cd
1
995, 14, 1343–1350; (c) X. Gan, M. Munakata, T. Kuroda-Sowa,
ions at μM concentrations in THF with excellent sensitivity,
while at mM concentrations in acetonitrile it allows for colori-
metric detection of these three metal ions. The investigation on
metal-ion binding properties has shown that triazole linkers in
M. Maekawa and M. Yamamoto, Polyhedron, 1995, 14, 1647–1651.
6 The spectral assignments are evidenced by UV-Vis titration studies of a
TTFV precursor with transition metal ions and TFA. See Fig. S-27 to
S-30 in ESI† for details.
1
2544 | Org. Biomol. Chem., 2012, 10, 2542–2544
This journal is © The Royal Society of Chemistry 2012