N-Amidothiourea based PET chemosensors for anions

Article abstract of DOI:10.1039/b703122j

Neutral N-amidothiourea based PET anion sensors bearing a pyrene fluorophore, 1-3, were synthesized and their fluorescent response toward anions was assessed. The anion quenching and binding constants were found to be much higher than those of the corresponding PET sensors bearing a simple thiourea receptor despite a higher oxidation potential of the electron donor and a relatively longer spacer (CH₂)₃ between the signal reporter and binding receptor in 1-3. This was explained in terms of a much more substantial increase in the electron donating ability of amidothiourea upon anion binding. The Royal Society of Chemistry.

Full text of DOI:10.1039/b703122j

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
N-Amidothiourea based PET chemosensors for anions†
Wen-Xia Liu and Yun-Bao Jiang*
Received 1st March 2007, Accepted 18th April 2007
First published as an Advance Article on the web 3rd May 2007
DOI: 10.1039/b703122j
Neutral N-amidothiourea based PET anion sensors bearing a pyrene fluorophore, 1–3, were synthesized
and their fluorescent response toward anions was assessed. The anion quenching and binding constants
were found to be much higher than those of the corresponding PET sensors bearing a simple thiourea
receptor despite a higher oxidation potential of the electron donor and a relatively longer spacer (CH2)3
between the signal reporter and binding receptor in 1–3. This was explained in terms of a much more
substantial increase in the electron donating ability of amidothiourea upon anion binding.
a much more substantial increase in the electron-donating ability
Introduction
of amidothiourea upon anion binding compared to that of the
traditional thiourea itself. It was therefore envisaged that with N-
amidothiourea, instead of simply thiourea being introduced into
PET anion sensors, Ksv would be much higher thus likely allowing
a more sensitive anion sensing.
Herein we report the synthesis and evaluation of three neu-
tral PET chemosensors for anions, N-(1-pyrenebutanamide)-
Nꢀ-(substituted-phenyl)thioureas (1–3, Fig. 1), in which N-
amidothiourea is the binding receptor that is linked to the pyrene
fluorophore via a relatively longer (CH2)3 spacer. Indeed, the
Ksv’s in acetonitrile (MeCN) were found at 105 mol−1 L orders
of magnitude, 1–2 orders higher than those of the corresponding
thiourea-based PET sensors with a shorter CH2 spacer.
A photo-induced electron transfer (PET) signalling mechanism,
developed for reporting the presence of metal cations and protons,
was first proposed by Weller1 and developed further by de
Silva2 and others.3 The first example of anion sensing under
a PET mechanism was described by the Czarnik group who
utilized anthrylpolyamines for the detection of phosphate and
pyrophosphate in aqueous solutions.4 Recent PET anion sensors
mostly bore charged receptors.5,6 With neutral PET anion sensors,
thiourea is one of the important binding receptors via hydrogen
bonding.7 Linking thiourea with pyrene through a CH2 spacer,
Teramae et al.7k synthesized a neutral PET anion sensor, the fluo-
rescence of which was quenched while a long-wavelength emission
developed upon anion binding. This lower energy emission was
assigned to exciplex formed from pyrene and an anion–thiourea
binding unit. This is the first direct evidence of electron transfer
in PET anion sensors. Employing anthracene as a fluorophore
and thiourea as the anion binding receptor, Gunnlaugsson et al.7i,e
synthesized a series of PET sensors whose fluorescence was found
to be quenched in the presence of anion whereas absorption
remained unchanged. This was ascribed to the increased electron-
donating ability of the anion–thiourea unit compared to the
original thiourea moiety that leads to enhanced PET quenching of
fluorescence. In the reported thiourea-based neutral PET sensors,
however, the fluorescence quenching constants (Ksv)8 were at
103 mol−1 L orders of magnitude or lower. This could be due
to low anion affinity of the sensor and/or less favorable PET
thermodynamics (DG) in the anion binding complex. We previ-
ously reported a new kind of thiourea-based receptor for anions,
N-benzamidothioureas,9 that showed a substantially enhanced
anion binding affinity despite the lower acidity of the thioureido –
NHs. This was attributed to the occurrence of ground-state charge
transfer in the anion-N-benzamidothiourea binding complexes in
which the thiourea moiety is the electron donor. This pointed to
Fig. 1 Chemical structure of sensors 1–3.
Results and discussion
Sensors 1–3 in MeCN show characteristic absorption and flu-
orescence of pyrene (Fig. 2), with quantum yields (UF)10 of
0.0532, 0.0537 and 0.0515, respectively. Compared with that of
1-methylpyrene (UF = 0.77 in MeCN)11 bearing no receptor, it
is obvious that PET is active in 1–3 prior to anion binding.
With the addition of a series of anions (F−, AcO−, H2PO4
,
−
HSO4−, Cl−, Br−, I−, NO3−, ClO4−) as their tetrabutylammonium
salts, fluorescence of 1–3 in MeCN was quenched whereas the
absorption of pyrene, the fluorophore remained unaffected. Fig. 2a
and 2b show absorption and fluorescence spectra of 2 in the
Department of Chemistry, College of Chemistry and Chemical Engineering,
and the MOE Key Laboratory of Analytical Sciences, Xiamen University,
Xiamen, 361005, China. E-mail: ybjiang@xmu.edu.cn; Fax: +86 592 218
5662; Tel: +86 592 218 5662
† Electronic supplementary information (ESI) available: Job plot of 2 with
presence of AcO−. F− and H2PO4 produced similar effects. UF’s
−
AcO−; Stern–Volmer plots for 1–3–AcO−(F−, H2PO4−) systems; 2D COSY
1
spectrum of N-acetamidothiourea in CD3CN; H and 13C NMR of 1–3.
of anion-2 complexes were 0.0112, 0.0084 and 0.0109, respectively,
See DOI: 10.1039/b703122j
for AcO−, F−, and H2PO4−. Obviously PET in 2 becomes more
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Fig. 2 Absorption (a) and fluorescence (b) spectra of 2 in MeCN in the
presence of AcO− and plots of I/I0 versus anion concentration (c). [2] =
8.55 × 10−6 mol L−1, [anion] = 0–2.5 × 10−4 mol L−1. Excitation wavelength
was 340 nm.
efficient after anion binding. The quenching of other anions is
much less. The quantum yields of anion complexes are listed in
Fig. 3 Trace of NMR titration by F− of 2 in CD3CN. [2] = 6.0 ×
10−3 mol L−1. The spectra were recorded after addition of 0 (a), 0.25
(b), 0.50 (c), 0.75 (d), 1.0 (e), 1.5 (f), 2.0 (g), 4.0 (h) and 8.0 (i) equiv. of F−,
respectively.
Table 1. The order of the quenching for 1–3 is F− > AcO−
>
H2PO4 ꢁ HSO4 > Cl−, Br−, I−, NO3−, ClO4
.
−
−
−
The nature of the interactions of 1–3 with anions was investi-
gated by H NMR titrations in CD3CN. As seen in Fig. 3, two
1
of the three –NH resonances of 2 were at 8.60 and 7.87 ppm,
respectively. Another one was at ca. 8.22 ppm which was wrapped
in the pyrene CH resonances (signals of –NH protons were
assigned also by referring to the 2D COSY spectrum of N-
acetamidothiourea, Fig. S1 in the ESI†). The signals of three
NH protons were initially broadened in the presence of up to
1.0 equivalent F−, after which they were sharpened again. The
signals of NH protons moved downfield to 9.89 and 9.44 ppm,
respectively. This suggested the hydrogen bonding nature of the
interaction between N-amidothiourea and F−. Meanwhile, the
Nꢀ-phenyl CHd signal moved downfield because of a deshielding
effect, whereas signals of CHe and CHf moved upfield, again
supporting the hydrogen bonding interaction between 2 and F−.
binding constant is higher for F− or AcO− than for H2PO4− (Fig. 2c
and Table 1) are in line with the hydrogen bonding nature of 1–3
with these anions.7,9
The corresponding quenching constants (KSV)8 in MeCN are
at 105 mol−1 L orders of magnitude (Table 1 and Stern–Volmer
plots in Fig. S3†). Compared with KSV’s of PET sensors bearing
simple thiourea receptors of 102–103 mol−1 L orders of magnitude,
the KSV’s of 1–3 with the N-amidothiourea receptor are much
higher despite a relatively longer (CH2)3 spacer in 1–3 whereas
CH2 is normally an optimal spacer in classic PET sensors.2 This
is surprising since the oxidation potential of N-amidothiourea
estimated from CV is higher than that of thiourea (Fig. 4).
In terms of PET thermodynamics,13 the enhanced fluorescence
quenching by anions observed with 1–3 means that a much
stronger lowering in the oxidation potential of N-amidothiourea
should occur upon anion binding, so as to promote PET in
the binding complexes to a much higher extent than that with
the corresponding sensors bearing a simple thiourea receptor.
The binding constants of AcO− and F− of 1–3 in MeCN are
too high to allow for a credible correlation with the Hammett
−
1H NMR titrations by AcO− and H2PO4 showed similar profiles
to that of F−.
A 1 : 1 stoichiometry of anion binding to 1–3 was made evident
from Job plots (Fig. S2†). Anion binding constants of 1–3 in
MeCN were evaluated by nonlinearly fitting12 the fluorescence
intensity versus anion concentration (Fig. 2c) and are listed in
Table 1. They are at 105–107 mol−1 L orders of magnitude. The
observations that the binding constant decreases in the order of
1 < 2 < 3 for H2PO4 in MeCN (binding constants for F− and
constant of substituent R, those of H2PO4 in MeCN (Table 1)
−
−
AcO− in MeCN are too high thus with high fitting uncertainty)
and for AcO− in water containing MeCN (Table 2), and that the
and of AcO− in water containing MeCN (Table 2) show a
stronger substituent dependence than that of the corresponding
Table 1 Fluorescence quantum yield UF, fluorescence quenching constant KSV and anion binding constant K of 1–3 in MeCN
1
2
3
UF
K
SV/105 mol−1
L
K/106 mol−1
L
UF
K
SV/105 mol−1
L
K/106 mol−1
L
UF
K
SV/105 mol−1
L
K/106 mol−1
Free
AcO−
F−
0.0532
0.0537
0.0515
0.0099 5.87 0.22
0.0065 5.05 0.12
0.0091 1.41 0.06
25.9 9.6
27.4 7.9
0.429 0.026
0.0112 5.81 0.15
0.0084 5.80 0.09
0.0109 2.60 0.04
7.89 1.93
14.5 1.80
1.63 0.22
0.0036 3.81 0.21
0.0034 9.28 0.52
0.0036 1.74 0.08
32.9 20.6
58.6 22.3
8.86 4.23
−
H2PO4
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Fig. 4 Cyclic voltammograms of receptors vs. SCE in MeCN containing
0.1 mol L−1 (n-C4H9)4NClO4. The working electrode was a glassy carbon
electrode. The scan rate was 50 mV s−1
.
N-(substitued-phenyl)thioureas.9a This actually points to a bind-
ing signal amplification in N-amidothiourea receptors in their
binding to anions.
Anion sensing by neutral sensors following hydrogen bonding
interactions has, in most cases, been carried out in aprotic organic
solvents such as MeCN, CHCl3 and DMSO.14 In protic solvents,
multiple hydrogen bonding would be needed to guarantee a
noticeable binding.15 Neutral sensors 1–3, however, are able to
sense anions in up to 8% H2O–MeCN binary solvent with a
binding constant as high as 105 mol−1 L orders of magnitude.
Detailed K, KSV and UF values of 1–3 with AcO− in H2O–MeCN
binary solvents are listed in Table 2. With these promising results
in hand, we are currently modifying the structure of the sensor
molecule in order to enhance its performance in protic and highly
competitive solvents such as water.
Conclusions
In summary, we have designed neutral PET anion sensors 1–3
employing N-amidothiourea as the receptor that is linked to a
pyrene fluorophore by a relatively longer (CH2)3 spacer. They have
a higher Eox than those of the corresponding sensors bearing a
simple thiourea receptor, yet show much higher anion quenching
constants at 105 mol−1 L orders of magnitude in MeCN and
therefore much higher sensitivity. This suggests a greater drop
in the oxidation potential of the electron donors in 1–3 upon
anion binding. The present results thus demonstrate that N-
amidothiourea as an electron donor is much better than thiourea
itself in constructing electron transfer type anion sensors and
it shows signal amplification in anion binding. Because of the
extremely high binding constants in pure MeCN, simple neutral
sensor 3 is already able to show a sensitive response toward AcO−
in MeCN containing up to 8% by volume of water.
Experimental
General procedures and materials
UV–Vis spectra were recorded on a Varian Cary-300 spectropho-
1
tometer using a 1 cm quartz cell. H NMR (400 MHz) and 13C
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NMR (100 MHz) spectra in DMSO-d6 or CD3CN were obtained
on a Bruker AV400 NMR spectrometer using TMS as an internal
standard. The HRMS were recorded with Micromass-LCT high
resolution mass spectrometer by injection of a methanol solution
of the sample. Cyclic voltammograms were obtained on a LabNet
VA2000 electrochemical analyzer. Absorption spectral titrations
for anion binding were carried out by adding an aliquot of anion
solution into bulk sensor solution at a given concentration.
Solvents used for sensor syntheses were commercially available
at AR grade. Solvents for spectral titrations were purified by
re-distillation until no fluorescent impurity could be detected.
Tetrabutylammonium salts of the anions were prepared by neu-
tralization of the corresponding acids with tetrabutylammonium
hydroxide.
135.4, 129.9, 129.4, 128.3, 128.1, 127.2, 126.5, 126.4, 126.2, 125.5,
125.1, 124.4, 123.9, 123.8, 123.2, 123.2, 122.5, 121.7, 120.8, 120.2,
61.8, 32.0, 31.2, 25.9. HRMS exact mass calcd for [C28H22F3N3OS
+ H]+ 506.1514, found 506.1516.
Acknowledgements
This project has been supported by the Natural Science Foun-
dation of China (20425518 and 20675069) and by the Ministry of
Education (MOE) of China. VolkswagenStiftung is acknowledged
for supporting the purchase of the Varian Cary-300 absorption
spectrophotometer.
Syntheses of 1–3
Notes and references
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N-(1-Pyrenebutanamido)-Nꢀ-(p-tolyl)thiourea (1). 1H NMR
(400 MHz, DMSO-d6) (ppm): 9.88 (s, 1H), 9.50 (s, 2H), 8.43 (d,
J = 9.2 Hz, 1H), 8.29–8.21 (m, 4H), 8.16–8.11 (m, 2H), 8.06 (t,
J = 7.6 Hz, 1H), 7.98 (d, J = 8.0 Hz, 1H), 7.28 (d, J = 6.4 Hz,
2H), 7.12 (d, J = 8.4 Hz, 2H), 3.39 (t, J = 7.6 Hz, 2H), 2.36 (t,
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DMSO-d6) (ppm): 181.3, 172.0, 136.6, 136.4, 134.2, 130.8, 130.4,
129.3, 128.5, 128.2, 127.4, 127.4, 127.2, 126.4, 126.0, 124.9, 124.7,
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(400 MHz, DMSO-d6) (ppm): 9.89 (s, 1H), 9.56 (s, 2H), 8.43 (d,
J = 9.2 Hz, 1H), 8.29–8.22 (m, 4H), 8.16–8.11 (m, 2H), 8.06 (t,
J = 7.6 Hz, 1H), 7.98 (d, J = 8.0 Hz, 1H), 7.42 (s, 2H), 7.32 (t, J =
8.0 Hz, 2H), 7.15 (t, J = 7.2 Hz, 1H), 3.39 (t, J = 8.0 Hz, 2H), 2.36
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found 460.1457.
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(m, 2H), 8.06 (t, J = 7.6 Hz, 1H), 7.98 (d, J = 7.6 Hz, 1H), 7.81
(d, J = 7.6 Hz, 2H), 7.56 (t, J = 8.0 Hz, 1H), 7.49 (d, J = 8.0 Hz,
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