Water-soluble benzo- and naphtho-thiadiazole-based bistriazoles and their metal-binding properties

Article abstract of DOI:10.1039/b920484a

A click reaction furnishes water-soluble acenothiadiazole-based bistriazoles with red-shifted absorption and emission characteristics. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b920484a

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Water-soluble benzo- and naphtho-thiadiazole-based bistriazoles
and their metal-binding propertiesw
Scott M. Brombosz, Anthony Lucas Appleton, Andrew J. Zappas II and Uwe H. F. Bunz*
Received (in Berkeley, CA, USA) 2nd October 2009, Accepted 12th January 2010
First published as an Advance Article on the web 30th January 2010
DOI: 10.1039/b920484a
A click reaction furnishes water-soluble acenothiadiazole-based
bistriazoles with red-shifted absorption and emission characteristics.
The 1,3-dipolar cycloaddition of alkynes to azides was first
investigated by Szeimies, Huisgen et al.,1 and was later
retooled as a copper catalyzed process for the easy access
towards 1,4-disubstituted triazoles.2 While intended by Kolb
and Sharpless3 for the construction of biologically active
Scheme 1 Synthesis of compounds 4 and 5.
molecules, the ‘‘click’’ reaction is now also popular for the
construction of polymers and materials.4
1,3-dipolar azide adducts. As the precursor alkynes do not
dissolve in water, only the quantum yields of 4 and 5 were
obtained for aqueous solutions. While 4 displays quite a robust
quantum yield of 13% in water, in the case of 5 the quantum
yield in water drops to 1.3%. With its emission wavelength of
519 nm and a quantum yield of 13%, 4 is potentially a useful
fluorophore core as it is stable towards photobleaching. In
addition, the emissive lifetimes of 4 in DCM (14 ns) and in water
(7 ns) solutions are unusually high. The larger congener 5
displays similarly large fluorescence lifetimes of 18 ns and 5 ns
in DCM and water, respectively. This is testimony to the
photophysical properties of the benzothiadiazole core, as the
precursor alkynes also display relatively long emissive lifetimes,
although not as long as those observed for the 1,3-cycloadducts.
The long fluorescence lifetime of the cycloadduct 4 is promising
for potential applications as a bio-fluorophore. As complex
biological matrices such as cells, serum, etc. are fraught with
background fluorescence10 with an emissive half-life of around
3 ns, fluorophores such as 4 should shine if time-gated detection is
used. Quite unusual is also that the emission and absorption
wavelengths of 4 and 5 are not very solvent dependent, and that
we actually see a slight blue-shift in the absorption features for
both 4 and 5 when going from DCM into water as a result of the
change of the refractive index (Table 1).
A generally attractive but less explored aspect of the triazole
formation is their incorporation, as functional modules, into
fluorophores and/or chromophores; the triazole group could either
work as an auxochromic group or a conjugative bridge between
two chromophores or p-systems.5 Herein we report that triazoles
are quite powerful auxochromes, i.e., functional groups that
red-shift the absorption and emission of acenothiadiazole-types.
We6,7 and others8 are interested in metallo-responsive
fluorophore systems, and we have recently disclosed that a
dipolar 1,3-cycloadduct containing a 2-pyridyl residue leads to
turn-on fluorescence when exposed towards metal cations. In
this communication we present attractive, novel water-soluble
and fluorescent bis-cycloadducts 4 and 5 that display binding
pockets for metal cations.
The TMS-protected diethynylbenzothiadiazole (1)9 is depro-
tected in situ by KF; the azide 3, CuSO4 (2.5 eq.) and sodium
ascorbate (2.5 eq.) are added to give 4 in 55% yield (Scheme 1).
The same approach works for 5 which is formed analogously in
60% yield by reaction of 2 with 3 under identical conditions.
The use of a large excess of copper sulfate was necessary to
catalyze this reaction, as the products 4 and 5 bind copper salts
quite efficiently. We selected the oligoethylene azide 3 as sub-
stituent, as it (a) confers water solubility and (b) is known for its
‘‘biophobic’’ properties, i.e., it suppresses non-specific inter-
actions with proteins, etc. Table 1 summarizes the relevant
photophysical properties of 1, 2, 4 and 5, while Fig. 1 shows the
absorption and emission spectra of 1, 2, 4 and 5.
The emission of 4 displays a slight red-shift when going
from DCM to water, while 5 displays a slight blue-shift upon
the same solvent change (Table 1). The similarity of the
spectral properties in water and in DCM suggests that these
fluorophores do not exhibit a large degree of charge transfer
character in the ground or excited states; 4 and 5 are electron
poor as both of their constituent modules (triazoles and
benzothiadiazole) are electron accepting.
There are several noteworthy trends. In dichloromethane
(DCM) the emissive quantum yields of all investigated species
(1, 2, 4, and 5) are high. Generally, the quantum yields of
the precursor alkynes are somewhat higher than those of their
Surprising are the quite significant red-shifts in the absorption
onset and particularly emission wavelengths of the 1,3-diploar
cycloadducts 4 and 5 in comparison to their respective diyne
precursors 1 and 2.11 Quantum chemical calculations (B3LYP
6-31G**//6-31G** as implemented by SPARTAN for
Windows) support this trend (Table 1) and show that the
HOMO and the LUMO positions are both raised, however
the destabilization of the HOMO is more pronounced than the
School of Chemistry and Biochemistry, Georgia Institute of
Technology, 901 Atlantic Drive, Atlanta, GA 30332, USA.
E-mail: uwe.bunz@chemistry.gatech.edu; Fax: +1 404 385 1795;
Tel: +1 404 385 1795
w Electronic supplementary information (ESI) available: Synthetic
procedures, characterization data, spectral data of cation addition,
Stern–Volmer plots, and titration data of compounds 4 and 5. See
DOI: 10.1039/b920484a
ꢀc
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Table 1 Photophysical properties of compounds 1, 2, 4, and 5 in dichloromethane and water
Emission
lmax
Fluorescence
lifetime (ns, t)
Stokes
shift/cmꢁ1
Gap calcdb
eV (nm)
FMO positions
calculatedb/eV
Cpd
Abs lmax
Fa
DCM
H2O
DCM
H2O
DCM
H2O
NA
0.13
NA
0.013
DCM
H2O
DCM
H2O
HOMO
LUMO
1c
4
386
406
528
505
NA
385
NA
475
443
508
534
610
NA
519
NA
602
0.85
0.72
0.28
0.09
7.9
14.0
15.2
18.4
NA
7.0
NA
5.3
3334
4946
212
NA
6706
NA
3.41 (364)
3.10 (399)
2.50 (496)
2.35 (527)
ꢁ6.15
ꢁ5.64
ꢁ5.55
ꢁ5.13
ꢁ2.74
ꢁ2.54
ꢁ3.05
ꢁ2.78
2c
5
3409
4442
a
b
c
Quinine sulfate in 0.1 M H2SO4 (aq.) used as a reference standard. B3LYP 6-31G**//6-31G** SPARTAN. In the case of 1 and 2, the TIPS
protected analogue was utilized due to its greater stability than the corresponding TMS or TES analogues.
Fig. 2 Representative absorption titration of 4 (blue trace; 151.5 mM)
with CuSO4 in water. [Cu] ranges from 0–6.26 ꢂ 10ꢁ1 M (red trace
4–Cu2+ complex).
Fig. 1 Normalized absorption and emission spectra of 1 (blue), and 2
(green, both top) and 4 (blue) and 5 (green, both bottom) in dichloro-
methane. Solid lines depict absorption spectra while broken lines
depict emission spectra. In the case of 1 and 2, the TIPS protected
analogue was utilized due to its greater stability than the corresponding
TMS or TES analogues.
destabilization of the LUMO when going from alkyne to
triazole. The absorption and emission profiles for 6a (DCM;
Fig. 3 Emission data (black dots) of the titration of 4 (151.5 mM)
with CuSO4 in water. Red line indicates the fitted equation used to
lmax abs = 409 nm, lmax emission = 507 nm) are very similar to
those obtained for 4 and Spartan (B3LYP 6-31G**//B3LYP
determine binding constant. [Cu] ranges from 0–6.26 ꢂ 10ꢁ1 M. Inset:
6-31G**) computes HOMO and LUMO of the model
compound 6b at ꢁ5.69 eV and ꢁ2.66 eV. Consequently, while
representative emission titration of 4 with CuSO4 in water.
atomic radius as previously demonstrated by us and by Xie
et al. for similar types of 1,3-dipolar cycloadducts, but only in
non-aqueous solutions.12
the triazole unit has a strong auxochromic effect, it is poor at
transmitting electronic communication between two p-systems.
Our adducts (4 and 5) allow the screening of metal binding
in water. Upon addition of the triflate salts of Li+, Na+, K+,
Ca2+, Zn2+, Mg2+, and Sn2+ to an aqueous solution of 4, no
change in the absorption or emission spectra is observed.
Upon addition of Hg2+ and trifluoroacetic acid (TFA) no
change in the absorption spectra is observed, however the
emission of 4 becomes quenched (see ESIw). Upon the addition
Adducts 4 and 5 display a binding pocket that should
readily bind to metal analytes of appropriate charge and
ꢀc
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Table 2 Binding data of 4 and 5 with copper(II) and nickel(II) in water
with the values obtained from the Stern–Volmer plots of the
emission spectra. Assuming a 1 : 1 complex agrees very well
with the obtained data. Apparently upon coordination to one
copper ion, the second binding pocket becomes too electron
poor to effectively bind another copper ion in aqueous
solution; it is not clear in how far the oligoethylene glycol
units partake in the binding of the copper.
Compound
Metala
4
4
5
Cu(II)
Ni(II)
Cu(II)
log K from absorptionb
log K from emissionc
2.83
3.18
3.17 ꢄ 0.01
2.96
2.71 ꢄ 0.03
2.70 ꢄ 0.01
The sulfate salt was used in all cases. Values obtained from the
a
b
deconvolution of the absorption spectra utilizing Datan software.14
In conclusion, we have prepared water-soluble bistriazoles 4
and 5 and the model compound 6a. From the combination of
spectroscopic and computational data, we can conclude that
the triazole-ring has a strong auxochromic effect and leads to
red-shifted spectroscopic features for the connected arene in
the 4-position, but at the same time is a poor electronic
conduit, as the spectroscopic properties of 6a are almost
identical to that of 4. The adducts 4 and 5 do not show large
solvent dependencies of their spectroscopic properties, and 4 is
surprisingly fluorescent in water and binds both copper and
nickel in aqueous solution. Overall, the 1,3-dipolar cycloaddition
of alkynes to azides is a superb tool to prepare functional,
metallo-responsive fluorophores.
c
Values obtained from fitting the quenching of the emission spectra
with eqn (1).
of Cu2+ or Ni2+ the charge transfer band in the absorption
spectra red-shifts (B20–30 nm) and the emission also
quenches (Fig. 2 and 3). Titrations of 4 and 5 with copper
sulfate and 4 with nickel sulfate in water were performed to
determine the strength of the binding.
Attempting to plot the fluorescence quenching spectra
according to the standard Stern–Volmer equation resulted in
significant deviation from linearity (see ESIw). However, the
data were well correlated when eqn (1) is employed,12,13
Ifinal ꢁ Io
We thank the Petroleum Research Funds for generous
funding. We would also like to thank A. J. Zucchero for
lifetime measurements and insightful discussions.
Iq ¼ Io þ
2
8
9
>
1
"
#
>
<
ꢀ
ꢁ
2
2=
½Qꢃ
1
½Qꢃ
1
½Qꢃ
½Fꢃ
ꢂ
1 þ
þ
ꢁ
1 þ
þ
ꢁ4
>
:
>
;
½Fꢃ KSV½Fꢃ
½Fꢃ KSV½Fꢃ
Notes and references
ð1Þ
1 R. Huisgen, G. Szeimies and L. Moebius, Chem. Ber., 1967, 100,
2494; R. Huisgen, R. Knorr, L. Moebius and G. Szeimies, Chem.
Ber., 1965, 98, 4014.
where Iq is the intensity of the fluorescence at a given quencher
concentration, Io is the initial fluorescence intensity of the
fluorophore, Ifinal is the final intensity of the fluorescence of the
quenched fluorophore, [Q] is the concentration of the quencher
added, [F] is the concentration of the fluorophore and Ksv is
the apparent Stern–Volmer constant.
2 J. F. Lutz, Angew. Chem., 2007, 46, 1018; W. H. Binder and
R. Sachsenhofer, Macromol. Rapid Commun., 2007, 28, 15;
M. Meldal and C. W. Tornoe, Chem. Rev., 2008, 108, 2952.
3 H. C. Kolb and K. B. Sharpless, Drug Discovery Today, 2003, 8,
1128; R. L. Phillips, I. B. Kim, L. M. Tolbert and U. H. F. Bunz,
J. Am. Chem. Soc., 2008, 130, 6952; R. L. Phillips, I. B. Kim,
B. E. Carson, B. Tidbeck, Y. Bai, T. L. Lowary, L. M. Tolbert and
U. H. F. Bunz, Macromolecules, 2008, 41, 7316.
4 B. C. Engert, S. Bakbak and U. H. F. Bunz, Macromolecules, 2005,
38, 5868; S. Bakbak, P. J. Leech, B. E. Carson, S. Saxena,
W. P. King and U. H. F. Bunz, Macromolecules, 2006, 39, 6793;
B. Erdogan, L. L. Song, J. N. Wilson, J. O. Park, M. Srinivasarao
and U. H. F. Bunz, J. Am. Chem. Soc., 2004, 126, 3678.
5 J. A. Opsteen and J. C. M. van Hest, Chem. Commun., 2005, 57.
6 D. Schweinfurth, K. I. Hardcastle and U. H. F. Bunz, Chem.
Commun., 2008, 2203.
7 J. N. Wilson and U. H. F. Bunz, J. Am. Chem. Soc., 2005, 127,
4124; A. J. Zucchero, J. N. Wilson and U. H. F. Bunz, J. Am.
Chem. Soc., 2006, 128, 11872; J. Tolosa, A. J. Zucchero and U. H.
F. Bunz, J. Am. Chem. Soc., 2008, 132, 6498.
8 X. Y. Wang, A. Kimyonok and M. Weck, Chem. Commun., 2006,
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The results of the titration are summarized in Table 2. The
binding of 4 to Ni(II) in water resulted in a binding constant of
log K = 3.17 ꢄ 0.01. The lower binding constant in com-
parison to Xie’s (log K = 4.48 ꢄ 0.03) is expected as the
titration was performed in water, which is a more competitive
ligating solvent than acetonitrile. The binding constant for
Cu(II) in water was slightly smaller in magnitude than that of
nickel with log K = 2.70 ꢄ 0.01.
The binding constant for the binding of copper to 5 was
determined to be log K = 2.71 ꢄ 0.03. This binding constant
was nearly identical to that of 4 which demonstrated the
independence of the binding upon the size of the aceno portion
of the core. Interestingly, the necessity of a stoichiometric
amount of copper in the synthesis of 4 and 5 can be attributed
to this high binding constant as once the triazole group is
formed, the effective concentration of free copper available to
catalyze the reaction is drastically reduced.
10 J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer,
New York, NY, 3rd edn, 2006.
11 J. A. Gonzlez-Vera, E. Lukovi and B. Imperiali, J. Org. Chem.,
2009, 74, DOI: 10.1021/jo901369k ASAP.
12 S. Maisonneuve, Q. Fang and J. Xie, Tetrahedron, 2008, 64, 8716.
13 R. L. Phillips, O. R. Miranda, D. E. Mortenson, C. Subramani,
V. M. Rotello and U. H. F. Bunz, Soft Matter, 2009, 5, 5042.
14 http://www.iss.com/products/components/datan/.
Binding constants were also obtained from the deconvolution
of the absorption spectra from the titration of the metal
utilizing Datan software.14 In all cases, the constants obtained
from the absorption spectra were in good to excellent agreement
ꢀc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 1419–1421 | 1421