Fluoride-induced intermolecular excimer formation of bispyrenyl thioureas linked by polyethylene glycol chains

Article abstract of DOI:10.1016/j.tet.2011.08.060

New bispyrenyl thioureas linked by polyethylene glycol (PEG) chains, L1-L3, and methoxy benzene pyrene thiourea, L4, were synthesized. Upon binding with F^- in CHCl₃, L1-L3 exhibited strong excimer emission bands (I_E) and weak monomer emission bands (I_M), while L4 displayed the same intensity of both bands. However, little or no change was observed in fluorescence spectra of L1 upon adding OH^-, AcO ^-, BzO^-, H₂PO₄^-, Cl ^-, Br^-, and I^-. Therefore, only F^- induced the pyrene excimer formation. Job's plots showed 1/1 or 2/2 complexation of L1 with F^-. Ratios of I_E/I_M of L1·F^- complex were dependent on the concentration of L1, implying that the dimerization of L1 proceeded via the intermolecular excimer formation. Among L1-L4, L1 possessed the highest binding constant and sensitivity toward F^- implying the importance of the linking PEG chain. L1 was demonstrated to be an excellent probe for F^- in CHCl₃ with the detection limit as low as 46.2 μg/L.

Full text of DOI:10.1016/j.tet.2011.08.060

Tetrahedron 67 (2011) 8102e8109
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Fluoride-induced intermolecular excimer formation of bispyrenyl thioureas linked
by polyethylene glycol chains
*
Duangrat Thongkum, Thawatchai Tuntulani
Supramolecular Chemistry Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 April 2011
Received in revised form 27 July 2011
Accepted 22 August 2011
Available online 27 August 2011
New bispyrenyl thioureas linked by polyethylene glycol (PEG) chains, L1eL3, and methoxy benzene
pyrene thiourea, L4, were synthesized. Upon binding with Fꢀ in CHCl3, L1eL3 exhibited strong excimer
emission bands (IE) and weak monomer emission bands (IM), while L4 displayed the same intensity of
both bands. However, little or no change was observed in fluorescence spectra of L1 upon adding OHꢀ,
AcOꢀ, BzOꢀ, H2POꢀ4 , Clꢀ, Brꢀ, and Iꢀ. Therefore, only Fꢀ induced the pyrene excimer formation. Job’s plots
showed 1/1 or 2/2 complexation of L1 with Fꢀ. Ratios of IE/IM of L1$Fꢀ complex were dependent on the
concentration of L1, implying that the dimerization of L1 proceeded via the intermolecular excimer
formation. Among L1eL4, L1 possessed the highest binding constant and sensitivity toward Fꢀ implying
the importance of the linking PEG chain. L1 was demonstrated to be an excellent probe for Fꢀ in CHCl3
Keywords:
Pyrene
pep Stacking
Hydrogen bonding
Intermolecular excimer formation
Fluoride probe
with the detection limit as low as 46.2 mg/L.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
resulted in the enhancement of emission signals.7 We hypothesized
that if we linked two thiourea-modified pyrene units with a flexible
chain of polyethylene glycol or PEG, an inert polymer composed of
repeating units of CH2CH2O,8 we should obtain an anion sensor that
can give an emission of the pyrene excimer band resulting from
stacking of two pyrenes upon binding anions.
Herein, we reported the synthesis of compounds L1eL3 con-
taining various chain lengths of PEG linking with pyrene thioureas at
both ends, and a control pyrene thiourea L4 without the PEG chain
(Fig.1). The synthesized molecules were examined for their abilities
Fluoride plays an important role in human life, and the de-
ficiency or overexposure of the amount of fluoride causes osteo-
porosis and poor dental health.1 Many synthetic sensors for
detection of fluoride have been reported via various sensing
mechanisms, such as fluoride sensing by covalent bonding with
cationic borane,2 by hydrogen-bonding with thioureas,3 by fluo-
ride-p
interactions,4 and by fluoride-induced tautomerism of sen-
sors.5 Accordingly, there is still a need to develop a highly sensitive
and selective sensor for fluoride using new and simple mecha-
nisms. We are interested in the design and synthesis of receptors
for sensing fluoride where the recognition occurred through vari-
ous mechanisms in only one receptor: (i) hydrogen-bonding, (ii)
to form pep stacking interactions upon binding anions, especially
fluoride, using NMR spectroscopy and spectrofluorometry.
pe
p
stacking, and (iii) conformational change of a flexible link.
Pyrene is a polycyclic aromatic hydrocarbon consisting of four
fused benzene rings, resulting in a flat aromatic system. The number
of -electrons in pyrene can be increased by the intramolecular or
intermolecular overlapping of p-orbitals with the -conjugated
p
p
system. Therefore, pyreneandits derivatives haveoftenbeenused as
dyes and fluorescence probes because of their high sensitivity for
detection via excimer formation.6 Bispyrenyl compounds containing
two pyrene units linked by a short flexible alkyl chain were found to
give high local concentration of chromophores in solutions, and this
* Corresponding author. Tel.: þ66
2
2187643; fax: þ66
2 2187598; e-mail
address: thawatchai.t@chula.ac.th (T. Tuntulani).
Fig. 1. Structures of L1eL4.
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.08.060

D. Thongkum, T. Tuntulani / Tetrahedron 67 (2011) 8102e8109
8103
2. Results and discussion
pe
p*
transition of aromatic hydrocarbons in the range
250e400 nm (Fig. S9 in Supplementary data). These bands rep-
resent the characteristic shape of the pyrene group for S0/S1 and
S0/S2 transitions.9 Fluorescence emission spectra of L1eL4 in
CHCl3 displayed only intense characteristic monomer bands of
pyrene between 390 and 410 nm (Fig. S10 in Supplementary data).
Upon increasing concentrations of L1eL4 to 10ꢀ4 M, no excimer
bands were observed. The fluorescence emissions of L1eL3 were
observed in almost the same intensity at the concentration as low
as 5ꢁ10ꢀ6 M. However, the emission spectrum of L4 showed
a very weak monomer band at this concentration. Therefore, the
emission of L4 was measured at 5ꢁ10ꢀ5 M. The fluorescence
quantum yields of all synthesized receptors were calculated using
the integrated emission intensity of anthracene as a standard.10
The absorption and emission parameters for compounds L1eL4
in CHCl3 are summarized in Table 1. It should be noted that the
quantum yields increased upon increasing the chain lengths
of PEG.
2.1. Synthesis and photophysical properties of L1eL4
Compounds L1eL3 containing various chain lengths of PEG and
pyrene thioureas at both ends were synthesized by the procedure
shown in Scheme 1. Compound 1, 2-methoxyphenol, underwent
a nitration reaction using concentrated HNO3/CH3COOH in CH3CN
to give compound 2 in 16% yield. Substitution reaction of 2 with an
appropriate polyethylene glycol ditosylate in the presence of
K2CO3 in CH3CN resulted in compounds 3, 4, and 5 in 76%, 55%,
and 63% yields, respectively. Reduction of 3, 4, and 5 with Raney Ni
and hydrazine gave 6, 7, and 8, respectively, in quantitative yields.
Coupling reactions between 6, 7, and 8 and compound 9, 1-
isothiocyanatopyrene, in CHCl3 at room temperature gave L1, L2,
and L3 in 48%, 38%, and 52% yields, respectively. The control
compound, L4, was synthesized to compare the anion binding
ability in the absence of PEG. Compound 10 underwent a methyl-
ation reaction with CH3I in the presence of K2CO3 in CH3CN to give
compound 11 in 95% yield. Compound 11 was then reduced using
Raney Ni and hydrazine to give compound 12 in a quantitative
yield. The coupling reaction between 12 and 9 in CHCl3 gave L4 in
32% yield. All synthesized compounds were well soluble in non-
polar organic solvents, such as dichloromethane, chloroform and
in polar organic solvents, such as DMSO. 1H NMR and 13C NMR
spectroscopy, mass spectrometry, and elemental analysis were
used to confirm the structures of ligands L1eL4 (Fig. S1eS8 in
Supplementary data).
Table 1
The absorption and emission parameters for compounds L1eL4 in CHCl3
Compound
Absorption
Emission
a
labs (nm)
3
ꢁ104 (L molꢀ1 cmꢀ1
)
lem (nm)
FF (%)
L1
L2
L3
L4
347
347
347
346
6.58
6.60
6.29
3.04
402
407
401
393
19
12
11
6
a
UVevis absorption spectra of compounds L1eL4 were studied
in CHCl3 solution exhibiting two major absorption bands for the
Fluorescence quantum yields used anthracence as a standard (FST¼0.27 in
EtOH).
Scheme 1. Synthetic procedure for L1eL4.

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D. Thongkum, T. Tuntulani / Tetrahedron 67 (2011) 8102e8109
2.2. Anion binding studies
preference of fluoride to bind with eNH of thiourea groups through
H-bond was due to its higher basicity. Moreover, the smallest
atomic size of fluoride was suitable to encapsulate in the receptor
molecule. Interestingly, the complexation of L1 with OHꢀ displayed
the minimum IE/IM ratio, even the OHꢀ possessed the highest ba-
sicity. This suggested that not only the basicity of anions but also
the size of anions was an important factor that affected the binding
ability of L1.
The fluorescence titration spectra of L1 with fluoride (Fig. 4)
displayed the fluorescence intensity enhancement of pyrene exci-
mer band at 500 nm, which gradually increased until the addition
of fluoride anions into L1 solution reached 100 equiv. However,
there was a small change in monomer emission intensity at
398 nm. These suggested that the fluoride induced the formation of
The anion binding properties of L1 toward tetrabutylammonium
(TBA) salts of anions were investigated in CHCl3 solution using
spectrofluorometry. Upon adding 100 equiv of Fꢀ, OHꢀ, AcOꢀ, BzOꢀ,
H2POꢀ4 , Brꢀ, Clꢀ, and Iꢀ into a solution of L1 (5.0ꢁ10ꢀ6 M), only the
fluorescence spectrum of L1$Fꢀ displayed a pyrene excimer band at
500 nm when excited at 340 nm (Fig. 2). Little or no change was
observed in the fluorescence spectra of L1 in the presence of other
anions. Therefore, only Fꢀ can induce the excimer formation of
pyrene in CHCl3.
pep stacking of pyrene resulting in the enhancement of the exci-
mer band. From the fluorescence titration data, the binding con-
stants (Ks) for the formation of L1 with each anion were calculated
via BenesieHildebrand plots,9 and the ratio of the interception at
the origin to the slope yielded Ks. The binding constant of L1$Fꢀ was
found to be higher than that of H2POꢀ4 , BzOꢀ, and AcOꢀ as shown in
Table 2. Therefore, L1 was selective toward fluoride. This result
agreed with previous results reported by Gozen in which the un-
folded structure of pseudocyclic tristhiourea showed a common
preference for Fꢀ>H2PO4ꢀ>AcOꢀ.11
Fig. 2. Fluorescence emission of L1 (5.0ꢁ10ꢀ6 M) with 100 equiv TBA anion salts in
CHCl3.
Excimers are dimers in the excited state. This term results from
the contraction of excited dimer. The pyrene excimer formed by the
pep stacking between two pyrene molecules. The fluorescence
band corresponding to an excimer is located at higher wavelength
than that of monomer and does not show vibronic bands. Two
important informations from an emission spectrum are fluores-
cence intensity ratio of excimer to monomer emission (IE/IM) and
maximum wavelength of excimer emission (lE). This ratio is sen-
sitive to the structure change while the lE is much less variable and
is usually located at 475e500 nm.6
The order of intensity ratio between excimer and monomer
bands (IE/IM) of L1 in the presence of anions varied
as Fꢀ>AcOꢀzBzOꢀ>H2PO4ꢀzClꢀ>BrꢀzIꢀ>OHꢀ (Fig. 3). The
Fig. 4. Fluorescence titration spectrum of L1 (5.0ꢁ10ꢀ6 M) with TBAF (0e100 equiv) in
CHCl3.
Table 2
Binding constant (Mꢀ1) of receptor L1 and TBA anion salts in CHCl3
L1$anion
Ks
R2
Ratio
lex/lem
Fꢀ
1.0ꢁ104
5.8ꢁ103
4.2ꢁ103
7.5ꢁ103
NA
NA
NA
NA
0.9969
0.9918
0.9936
0.9991
NA
NA
NA
NA
1/1
1/1
1/1
1/1
NA
NA
NA
NA
340/500
340/408
340/398
340/391
340/412
340/404
340/407
340/400
BzOꢀ
AcOꢀ
H2POꢀ4
Clꢀ
Brꢀ
Iꢀ
OHꢀ
NA¼values cannot be determined.
The Job’s plot of L1 with fluoride displayed a maximum com-
plexation at the mole fraction of 0.5 suggesting 1/1 or 2/2 com-
plexation of L1 with fluoride (Fig. S11 in Supplementary data).
Therefore, a possible structure of the L1$Fꢀ complex could be
a folded structure from the intramolecular pyreneepyrene stacking
Fig. 3. Plot of the intensity ratio of excimer to monomer emission (IE: 500 nm, IM
400 nm) of L1 (5.0ꢁ10ꢀ6 M) with 100 equiv. TBA anion salts in CHCl3.
:

D. Thongkum, T. Tuntulani / Tetrahedron 67 (2011) 8102e8109
8105
or an unfolded structure of the intermolecular pyreneepyrene
stacking as shown in Fig. 5.
Fig. 5. The possible structures of L1$Fꢀ complex.
Besides L1, the anion binding properties of L2eL4 with fluoride
were also studied by spectrofluorometry (Figs. S12eS14 in Sup-
plementary data). The fluorescence titration profiles of L2$Fꢀ and
L3$Fꢀ complexes displayed the excimer band emerged obviously
compared to the monomer band in the same manner as L1$Fꢀ
complex. However, in the case of L4$Fꢀ, both excimer and mono-
mer bands showed similar fluorescence intensity. The binding
constants of L1, L2, L3, and L4 toward Fꢀ can be calculated by
BenesieHildebrand plots,9 and are found to be 1.0ꢁ104, 1.0ꢁ103,
6.3ꢁ103, and 5.4ꢁ102 Mꢀ1 for L1, L2, L3, and L4, respectively.
Therefore, L1 is the most efficient probe for detecting fluoride be-
cause a longer flexible PEG chain may reduce the repulsion be-
tween the methoxy group and the oxygen of PEG to provide a more
stable pyrene-stacking complex. In addition, Fꢀ is the smallest
anion and provides the least steric hindrance guests for binding
with L1.
The role of the PEG linkage on the sensitivity of L1 and L4 to-
ward fluoride was investigated by monitoring the fluoride induced
an excimer emission at the minimum ligand concentrations in
CHCl3. In the presence of 100 equiv of fluoride, the minimum
concentrations of L1 and L4 for detection of the excimer emission
band were 5ꢁ10ꢀ6 M and 5ꢁ10ꢀ5 M in CHCl3, respectively. The
fluorescence intensity at 500 nm of L1$Fꢀ is two times as high as
the fluorescence intensity of L4$Fꢀ, even at lower concentration
(Fig. 6). Therefore, the excellent sensitivity of L1 and weaker sen-
sitivity of L4 indicated that the PEG linkage on L1 significantly
improves sensitivity and binding ability of L1 toward Fꢀ.
Fig. 6. Fluorescence emission spectra of various concentrations of (a) L1 and (b) L4
with 100 equiv of TBAF in CHCl3.
2.3.
pep Stacking formation of pyrenes induced by fluoride:
effects of solvents and metal ions
1H NMR spectra of L1 and L1 in the presence of 4 equiv of Fꢀ,
AcOꢀ, BzOꢀ, and H2PO4ꢀ in CDCl3 are illustrated in Fig. 7. Gener-
ally, the benzene protons (He, Hf, and Hg) of all spectra displayed
downfield shifts because of the inductive effect from anions. All
the PEG protons (Ha, Hb, and Hc) and the methoxy protons (Hd)
were almost unchanged indicating that the PEG chain and
methoxy oxygen did not involve in anion binding. Interestingly,
signals of pyrene protons of the complexes shifted downfield in
the presence of AcOꢀ, BzOꢀ, and H2POꢀ4 due to inductive effects
from the anions. However, in the case of Fꢀ the signal of pyrene
protons became broader and some protons shifted more upfield
as compared to pyrene signals in other spectra. The broadening
and shielding of these signals stemmed from the anisotropic ef-
fect of the stacking-pyrene ring current induced by Fꢀ.12,13 This
ring current caused some pyrene protons inside an anisotropic
region to be deshielded and other pyrene protons outside the
anisotropic region to be shielded. This NMR spectrum supported
Fig. 7. 1H NMR spectra of L1 in the presence of 4 equiv of tetrabutylammonium Fꢀ,
AcOꢀ, BzOꢀ, and H2PO4ꢀ in CDCl3 at 400 MHz.
Unfortunately, the protons of thiourea groups were overlapped
with the pyrene protons when CDCl3 was used as a solvent.
Therefore, we could not follow a shift of the NH protons, which
participated in hydrogen bonding interactions with fluoride. When
using DMSO-d6 as a solvent, NH signals were observed in the NMR
spectrum (Fig. S1 in Supplementary data), but disappeared com-
pletely upon addition of 4 equiv of fluoride probably due to the
deprotonation of the NH proton by Fꢀ.14
that Fꢀ induced
pep stacking of the pyrene rings in the complex
of L1$Fꢀ.

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D. Thongkum, T. Tuntulani / Tetrahedron 67 (2011) 8102e8109
In addition, NOESY of L1$Fꢀ in CDCl3 was carried out to assign
ions in CDCl3/CD3CN (9/1 v/v) showed small shifts in the methylene
region and almost unchanged in the aromatic region. Therefore, the
pep stacking interactions between two pyrene units could not be
the correlation between protons in the space. The NOESY connec-
tion of the PEG protons and the benzene protons was clearly ob-
served only for the signal of adjacent protons, Ha and Hb, Hb and Hc,
Hc and Hf as well as Hf and Hg. Therefore, these protons were in the
same space (Fig. S15 in Supplementary data).15 Unfortunately, the
signal of pyrene protons could not be assigned clearly due to
overlapping of the complicated proton signal. Therefore, this
technique cannot be used to differentiate clearly between the
intramolecular and intermolecular excimer formation.
Either intramolecular or intermolecular interactions can be
observed in a molecule with two reactive groups connected by
a flexible link.16 The intramolecular formation between these re-
active molecules produces a macrocyclic ring-closure product, and
the intermolecular formation results in a dimer and oligomer
structures. The IE/IM ratios of the former are independent of sub-
strate concentrations, while the latter are not. Therefore, in normal
solutions where diffusion is rapid, high substrate concentrations
favor polymerization while cyclizations occur in good chemical
yields only at low concentrations.17
In order to elucidate whether the pyrene stacking of L1 induced
by fluoride occurred in the intramolecular or intermolecular fash-
ion, the ratios of IE/IM of L1 in the presence of 100 equiv of Fꢀ in
various concentrations were monitored. Fig. 8 shows that the
fluorescence intensity ratios of excimer and monomer depended on
the concentration of ligand, which gradually decreased in the
concentration range of 5.0ꢁ10ꢀ4 M to 5.0ꢁ10ꢀ8 M. These results
suggested the pyrene stacking of L1$Fꢀ complex proceeded via the
intermolecular manner in which the representative structure was
shown in Fig. 5b.17 The intermolecular excimer structure of L1 did
not need to fold the podand chain to bind fluoride and could reduce
the steric hindrance on the receptor molecule. Therefore, the in-
termolecular excimer structure (Fig. 5b) should be more thermo-
dynamically favorable than the intramolecular excimer structure
(Fig. 5a).
induced by the alkali metals. Moreover, upon addition of sodium or
potassium to the CDCl3/CD3CN (9/1 v/v) solution of L1þ4 equiv of
Fꢀ resulted in the original spectrum of L1. This indicated decom-
plexation of L1$Fꢀ occurred in the presence of Naþ or Kþ (Fig. S17 in
Supplementary data). This behavior was similarly found in the
fluorescence titration. The excimer emission intensity at 500 nm of
L1þ100 equiv of Fꢀ was gradually quenched by adding aliquots of
sodium perchlorate (Fig. S18 in Supplementary data). These ex-
periments suggest that the
pep interactions induced by fluoride
are very weak and can be destroyed by ion-pairing.
It has been established that solvents can affect
pep in-
teractions18 and can, therefore, perturb the pyrene excimer for-
mation. Our studies found that the fluorescence spectra of L1$Fꢀ
showed the intensity enhancement of the excimer band upon in-
creasing the percentage of hexane in chloroform solution (Fig. S19
in Supplementary data). However, the excimer band intensity de-
creased upon adding 5% of methanol in chloroform solution of
L1$Fꢀ. Therefore, both metal ions and polar protic solvents can
disrupt the intermolecular pep stacking interactions of pyrene.
2.4. Use of the synthesized compounds as fluoride sensors
To test the efficiency of all synthesized ligands for sensing
fluoride, the ratios of excimer to monomer intensity (IE/IM) were
compared between L1 and L4 with each anion and with mixed
anions as shown in Fig. 9. The IE/IM ratios of L1eL3 in the presence
of AcOꢀ, BzOꢀ, H2POꢀ4 , Clꢀ, Brꢀ, and Iꢀ showed very low emission
ratio as compared to that in the presence of Fꢀ. From the last col-
umn in Fig. 9, it can be concluded that other anions can interfere
with the detection of fluoride anion for all the synthesized ligands
by reducing the emission ratios (IE/IM). However, L1eL3 in anion
mixtures still retain the emission ratio in a similar manner to the
emission ratios found in the presence of only Fꢀ suggesting the
good selectivity for this anion, and L1 shows the best sensing re-
sponse to Fꢀ. In the case of L4, the emission ratio showed in-
significant difference from other columns suggesting the poor
selectivity of L4 toward Fꢀ.
Fig. 8. Plot of IE/IM of TBAF (100 equiv) with various concentration of L1.
Interestingly, our results agreed with the intermolecular pyrene
stacking in bichromophoric pyrene azine induced by Hg2þ reported
by Martinez et al.6c Upon addition of Hg2þ to the mentioned ligand,
there was a small change in monomer emission intensity and
a strong increase in excimer emission, similar to the addition of Fꢀ
to the solution of L1.
L1 contains a podand chain from the PEG linkage, which is able
to form complexes with alkali metal ions,16 the fluorescence titra-
tion of L1 with alkali metal, such as sodium or potassium has been
carried out and exhibited no excimer bands (Fig. S16 in Supple-
mentary data). 1H NMR spectra of L1 in the presence of alkali metal
Fig. 9. Selectivity of all the synthesized ligands toward various anions (100 equiv) and
the mixture of all anions.
To apply compound L1 as a molecular sensor for Fꢀ in CHCl3, the
sensing ability of L1 toward fluoride was studied. The detection
limit was calculated by using three times the standard deviation
(3SD) of the background noise to estimate the lowest concentration
of fluoride that can be measured from the ratio between 3SD of the
fluorescence intensity at 500 nm of free L1 and slope of the linear

D. Thongkum, T. Tuntulani / Tetrahedron 67 (2011) 8102e8109
8107
plot of fluorescence titration data of L1 with fluoride shown in
Fig. 10.19 The detection limit of L1 (5ꢁ10ꢀ6 M) toward Fꢀ was found
ethylacetate, were distilled before use. The progress of the re-
actions was monitored by TLC on silica gel and visualized by UV
light. The chromatographic separations were performed on silica
gel columns (0.063e0.200 mm) to purify the synthesized
compounds.
DMSO-d6 and CDCl3 were used as solvents for NMR experi-
ments. In complexation studies, anions were used in their tetra-
butylammonium salts. The binding constants between the ligand
and various ions were determined by the linear equation of Bene-
sieHildebrand plots.
to be 2.43ꢁ10ꢀ6 M or 46.2
mg/L in CHCl3. The fluoride sensing re-
sponse can even be detected with a naked eye for this concentra-
tion of fluoride by the appearance of the light blue fluorescence of
the pyrene chromophore upon exposure to UV irradiation at
365 nm (Fig. 10, inset).
4.2. Syntheses of L1eL4
4.2.1. Compound 2. The mixture of 1 (2.12 g, 0.017 mol) in CH3CN
(30 mL) and CH3COOH (15 mL) was stirred under nitrogen. A so-
lution of concentrated HNO3 1.20 mL in 15 mL of CH3CN was then
added dropwise and refluxed. The reaction was stirred at room
temperature for 12 h. After that, the reaction was cooled to 0 ꢂC, and
the pH was adjusted to 7e8 by saturated HCOꢀ3 solution. The sol-
vent was removed by a rotary evaporator. The resulting residue was
extracted with CH2Cl2 and water. The organic phase was isolated
and dried over anhydrous Na2SO4. The solvent was removed, and
the crude was purified by column chromatography with CH2Cl2 as
eluent. The product was collected and recrystallized with hexane/
CH2Cl2 to give compound 2 as yellþow solid (0.45 g, 16%). Mp:
121.0e122.0 ꢂC. MALDI-TOF (m/z) [M] : calcd 169.04, found 170.93.
IR (KBr): 1510 (nasym. NO2), 1341 (nsym. NO2) cmꢀ1. 1H NMR (400 MHz,
CDCl3)
d
7.88 (dd, J¼8.8, 2.4 Hz, 1H), 7.76 (d, J¼2.4 Hz, 1H), 6.98 (d,
Fig. 10. The linear plot of fluorescence titration data of L1 with Fꢀ and fluorescence
J¼8.8 Hz, 1H), 6.43 (s, 1H), 3.99 (s, 3H) ppm. 13C NMR (100 MHz,
appearance of L1 (5.0ꢁ10ꢀ6 M) in the presence of 46.2
m
g/L of Fꢀ.
CDCl3)
d 151.7, 146.1, 141.2, 118.6, 114.0, 106.4, 56.5 ppm.
4.2.2. Compounds 3e5. Generally, the mixture of a polyethylene
glycol, compound 2, K2CO3, and tetrabutylammonium bromide in
CH3CN (50 mL) was stirred and refluxed under nitrogen. After 2
days, the solvent was removed and pH of the mixture was ad-
justed to 1 with 3 M HCl. The residue was extracted with CH2Cl2
(25ꢁ3 mL) and H2O. The organic phase was collected and dried
over anhydrous Na2SO4. The solvent was removed, and the prod-
uct was recrystallized with CH3OH to yield compounds 3e5.
Compound 3 was obtained as a yellow solid (0.27 g, 76%). Mp:
114.0e116.0 ꢂC. MALDI-TOF (m/z) [M]þ: calcd 496.17, found 496.56.
IR (KBr): 1518 (nasym. NO2), 1338 (nsym. NO2) cmꢀ1. 1H NMR (400 MHz,
3. Conclusion
We have synthesized new bisthiourea pyrenyl compounds
linked by various PEG chains, L1eL3 and the monomeric pyrene
thiourea, L4 with no PEG chain. 1H NMR spectroscopy and spec-
trofluorometry showed that only Fꢀ could induce the excimer for-
mation from intermolecular pyrene stacking. The results also
revealed the importance role of the PEG chain in Fꢀ binding and
sensing abilities. L1 possessing the longest podand chain showed
the best binding ability and sensitivity toward Fꢀ while L4 showed
the worst sensing and binding ability. Therefore, L1 could be an
excellent sensor for Fꢀ. It was demonstrated to detect as low as
CDCl3)
4.26 (s, 4H), 3.92 (s, 10H), 3.72 (s, 4H), 3.66 (s, 4H) ppm. 13C NMR
(100 MHz, CDCl3) 153.8, 149.0, 141.5, 117.6, 111.3, 106.6, 70.9, 70.6,
d
7.86 (d, J¼8.4 Hz, 2H), 7.71 (s, 2H), 6.94 (d, J¼8.8 Hz, 2H),
46.2 m
g/L of Fꢀ in CHCl3.
d
69.3, 68.7, 56.2 ppm.
4. Experimental
4.1. General
Compound 4 was obtained as a yellow solid (0.16 g, 55%).
MALDI-TOF (m/z) [M]þ: calcd 452.14, found 452.65. IR (KBr): 1518
(
nasym. NO2), 1346 (nsym. NO2) cmꢀ1. 1H NMR (400 MHz, CDCl3)
d 7.87
(dd, J¼8.8, 2.8 Hz, 2H), 7.73 (d, J¼2.8 Hz, 2H), 6.94 (d, J¼8.8 Hz, 2H),
1H NMR and 13C NMR spectra were recorded with a Varian
Mercury Plus 400 NMR spectrometer and a Bruker UltrashieldÔ
Plus 400 NMR spectrometer. 2D NOESY spectrum was recorded on
Varian Mercury Plus 400 NMR spectrometer. Fluorescence spectra
were recorded on a Varian Cary Eclipse fluorescence spectropho-
tometer. UVevis spectra were recorded on a Varian Cary 50 UV–vis
spectrometer.
All compounds were synthesized under nitrogen atmosphere.
Compounds 1, 2-methoxyphenol, and 10, 4-nitrophenol, were
obtained from Merck and Aldrich, respectively. PEG ditosylates
were synthesized from PEGs and p-toluenesulfonyl chloride by
adapting methods from the previously published procedure.20
Compound 9, 1-isothiocyanatopyrene, was synthesized using the
procedure reported previously.21 All materials and reagents were
standard analytical grade, and used without further purification.
Commercial grade solvents, methanol, dichloromethane, hexane,
4.27 (t, J¼4.8, 5.2 Hz 4H), 3.93 (t, J¼5.2, 4.4 Hz, 10H), 3.75 (s, 4H)
ppm. 13C NMR (100 MHz, CDCl3)
106.8, 71.0, 69.4, 68.9, 56.3 ppm.
d 153.9, 149.2, 141.7, 117.6, 111.4,
Compound 5 was obtained as a yellow solid (0.14 g, 63%).
MALDI-TOF (m/z) [M]þ: calcd 408.12, found 408.73. IR (KBr): 1508
(
nasym. NO2), 1339 (nsym. NO2) cmꢀ1. 1H NMR (400 MHz, CDCl3)
d 7.87
(dd, J¼8.8, 2.8 Hz, 2H), 7.73 (d, J¼2.8 Hz, 2H), 6.94 (d, J¼8.8 Hz, 2H),
4.30 (t, J¼4.4, 5.2 Hz, 4H), 4.01 (t, J¼4.4, 4.8 Hz, 4H) 3.93 (s, 6H)
ppm. 13C NMR (100 MHz, CDCl3)
106.6, 69.6, 68.8, 56.2 ppm.
d 153.7, 149.1, 141.6, 117.5, 111.3,
4.2.3. Compounds 6e8. A mixture of 3e5 (0.201 mmol) in CH3OH
(5 mL) and EtOAc (10 mL) was stirred with molecular sieve under
nitrogen. After 15 min, Raney Ni (1/4 spoon) and 2 mL of hydra-
zine hydrate were added and refluxed for 1 h. The color of the
reaction was changed from yellow to colorless solution. The

8108
D. Thongkum, T. Tuntulani / Tetrahedron 67 (2011) 8102e8109
molecular sieve and Raney Ni were removed by filtration. The
residue was evaporated to dryness and extracted with CH2Cl2 and
H2O. The organic phase was collected and dried over anhydrous
Na2SO4. Solvent was evaporated to give a yellow oil product in
a quantitative yield. The product was used in the next step
without further purification.
125.1, 124.9, 124.4, 123.8, 122.7, 116.8, 113.0, 109.7, 69.9, 68.9, 68.0,
55.4 ppm.
4.2.8. Compound L3. A similar procedure to the preparation of L1,
L3 was synthesized from the coupling reaction of compound 8 and
compound 9. The compound L3 was obtained as yellow powders
(0.045 g, 52%). MALDI-TOF (m/z) [M]þ: calcd 866.26, found 865.07.
Elemental analysis for C52H42N4O5S2: calcd C, 72.03; H, 4.88; N,
4.2.4. Compound 11. A mixture of 10 (3.48 g, 0.025 mol), anhy-
drous K2CO3 (1.73 g, 0.0125 mol) in CH3CN (25 mL) was refluxed
under nitrogen. CH3I (3 mL, 0.0482 mol) in CH3CN (25 mL) was
added dropwise to the mixture. After 5 h, the solvent was re-
moved under reduced pressure. The residue was dissolved in
CH2Cl2 and 3 M HCl was added until the pH of the solution be-
came 1. The solvent was removed, and the resulting residue was
extracted with CH2Cl2 (25 mLꢁ3) and water. The organic phase
was dried over anhydrous Na2SO4, filtered, and the solvent was
removed. The crude residue was purified by column chromato-
graph using CH2Cl2 as eluent to yield compound 11 as a yellowish
green solid (3.65 g, 95%). MALDI-TOF (m/z) [M]þ: calcd 153.04,
found 153.99. IR (KBr): 1500 (nasym. NO2), 1333 (nsym. NO2) cmꢀ1. 1H
6.46. Found C, 72.25; H, 4.86; N, 6.42. IR (KBr): 1509 (
NMR (400 MHz, DMSO-d6) 10.02 (s, 2H), 9.69 (s, 2H), 8.28e8.02
(m, 18H), 7.16 (d, J¼2 Hz, 2H), 6.92 (m, 4H), 4.03 (t, J¼4 Hz, 4H), 3.71
(m, 10H) ppm. 13C NMR (100 MHz, DMSO-d6)
181.0, 148.4, 145.1,
n
C]S) cmꢀ1. 1H
d
d
133.0, 132.5, 130.4, 130.2, 129.0, 127.2, 127.0, 126.8, 126.5, 126.3,
126.2, 125.1, 124.9, 124.7, 124.2, 123.6, 122.5, 116.6, 113.0, 109.5, 68.8,
67.9, 55.2 ppm.
4.2.9. Compound L4. Compound 12 (0.300 mmol) and triethyl-
amine (0.20 mL, 1.5 mmol) in CHCl3 (10 mL) was stirred under ni-
trogen at room temperature for 30 min and then compound 9
(0.093 g, 0.360 mmol) in CHCl3 (20 mL) was added to the reaction.
The reaction mixture was stirred for 3 days, and the solvent was
evaporated to dryness. The residue was extracted with CH2Cl2 and
H2O. The organic phase was collected, evaporated to dryness, and
purified by recrystallization with CH2Cl2/EtOAc to give a white solid
of L4 (0.037 g, 32%). MALDI-TOF (m/z) [M]þ: calcd 382.11, found
382.70. Elemental analysis: for C24H18N2OS: calcd C, 75.37; H, 4.74;
NMR (400 MHz, CDCl3)
d
8.19 (dd, J¼7.2, 2.0 Hz, 2H), 6.95 (dd,
J¼7.2, 2.0 Hz, 2H), 3.9 (s, 3H) ppm. 13C NMR (100 MHz, CDCl3)
d
164.6, 141.6, 125.9, 114.0, 56.0 ppm.
4.2.5. Compound 12. A mixture of 11 (0.05 g, 0.3 mmol) in CH3OH
(1 mL) and EtOAc (20 mL) was stirred with molecular sieve under
nitrogen. After 15 min, Raney Ni (1/4 spoon) and 2 mL of hydra-
zine hydrate were added and refluxed for 1 h. The color of the
reaction was changed from yellow to colorless. The molecular
sieve and Raney Ni were removed by filtration. The residue was
evaporated to dryness and extracted with CH2Cl2 and H2O. The
organic phase was collected and dried over anhydrous Na2SO4.
Solvent was evaporated to give a yellow oil. The product was used
in the next step without purification.
N, 7.32. Found C, 75.40; H, 4.83; N, 7.41. IR (KBr): 1512 (n .
C]S) cmꢀ1
1H NMR (400 MHz, CDCl3)
d
8.05 (m, 11H), 7.32 (d, J¼8 Hz, 2H), 6.90
(d, J¼8 Hz, 2H), 3.79 (s, 3H) ppm. 13C NMR (100 MHz, DMSO-d6)
181.6, 156.6, 133.0, 132.2, 130.6, 130.4, 129.2, 127.3, 127.1, 126.9,
126.6, 126.4, 126.3, 125.3, 125.1, 124.8, 124.4, 123.8, 122.6, 113.5,
55.1 ppm.
4.3. Spectrofluorometry
4.3.1. Anion titrations. Solutions of compounds L1eL3 (5.0ꢁ10ꢀ6 M)
and L4 (5.0ꢁ10ꢀ5 M) were prepared in a 10 mL volumetric flask and
2 mL of each ligand was pipetted into a 1 cm path length quartz
cuvette. The emission spectra were recorded in the range
360e650 nm at room temperature by using the excitation wave-
length at 340 nm. The solution of an anion (1.0ꢁ10ꢀ3 or 1.0ꢁ10ꢀ2 M
up to concentration of ligand) was prepared in a 10 mL volumetric
flask and transferred to a 2 mL microburette. The anion solution was
introduced in portions to the cuvette and stirred for 60 s prior to
measurement.
4.2.6. Compound L1. A solution of compound 6 (0.100 mmol) and
triethylamine (0.10 mL, 0.717 mmol) in CHCl3 (10 mL) was stirred
under nitrogen at room temperature for 30 min and then com-
pound 9 (0.066 g, 0.255 mmol) in CHCl3 (20 mL) was added to the
reaction. The reaction mixture was stirred for 3 days and then
evaporated to dryness. The residue was extracted with CH2Cl2 and
H2O. The organic phase was collected and evaporated to dryness
and purified by column chromatography using EtOAc as eluent.
The final product L1 was recrystallized with CH2Cl2/CH3OH to give
a yellow solid (0.046 g, 48%). Mp: 140.0e142.8 ꢂC. MALDI-TOF (m/
z) [Mþ]: calcd 954.31, found 953.0. Elemental analysis for
C56H50N4O7S2: calcd C, 70.42; H, 5.28; N, 5.87. Found C, 70.40; H,
4.3.2. Metal ion titrations. The preparations of the ligands were
similar to the titration of anions. However, the sodium solution
(1.0ꢁ10ꢀ3 M or 1.0ꢁ10ꢀ2 M) was prepared in CH3CN and was in-
troduced in portions to the cuvette by a microburette. The mixture
between the ligand and sodium was stirred for 2 min prior to
measurement.
5.23; N, 5.82. IR (KBr): 1513 (n .
C]S) cmꢀ1 1H NMR (400 MHz,
CDCl3)
d 8.0 (m, 22H), 7.0 (s, 2H), 6.8 (s, 4H), 4.1 (s, 4H), 3.8 (s,
10H), 3.6 (d, J¼7.6 Hz, 8H) ppm. 13C NMR (100 MHz, DMSO-d6)
d
181.2, 148.5, 145.3, 133.2, 132.6, 130.6, 130.4, 129.2, 127.3, 127.1,
127.0, 126.7, 126.4, 126.3, 125.3, 125.1, 124.8, 124.4, 123.8, 122.7,
116.7, 113.0, 109.7, 69.8, 69.7, 68.9, 68.0, 55.4 ppm.
4.4. 1H NMR spectroscopy
4.4.1. Anion titrations. The solution of a ligand (1.0ꢁ10ꢀ3 M) in
CDCl3 (0.5 mL) was prepared in a 5 mm of an NMR tube. The 1H
NMR spectrum of the free ligand was recorded. The solution of
fluoride ion (1.0ꢁ10ꢀ2 M) in CDCl3 (1.0 mL) was prepared in a vial.
Then, fluoride solution was added in portions to the NMR tube via
4.2.7. Compound L2. A similar procedure to the preparation of L1,
L2 was synthesized from the coupling reaction of compound 7 and
compound 9. The compound L2 was obtained as yellow powders
(0.035 g, 38%). MALDI-TOF (m/z) [M]þ: calcd 910.290, found
909.407. Elemental analysis for C54H46N4O6S2: calcd C, 71.19; H,
5.09; N, 6.15. Found C, 71.05; H, 4.92; N, 6.20. IR (KBr): 1509
a microsyringe (10 and 50
mixture was recorded.
m
L portions). 1H NMR spectrum of the
(n d 10.07 (s, 2H), 9.73 (s,
C]S) cmꢀ1. 1H NMR (400 MHz, DMSO-d6)
2H), 8.31e8.06 (m, 18H), 7.20 (d, J¼2 Hz, 2H), 6.97 (dd, J¼27,
4.4.2. Metal ion titrations. The solution of ligand L1 (1.0ꢁ10ꢀ3 M) in
CDCl3/CD3CN (9/1 v/v) (0.5 mL) was prepared in a 5 mm NMR tube.
The 1H NMR spectrum of free ligand was recorded. The solution of
8.8 Hz, 4H), 4.04 (t, J¼4.8 Hz, 4H), 3.74 (m, 10H), 3.60 (s, 4H) ppm.
13C NMR (100 MHz, DMSO-d6)
d 181.2, 148.5, 145.3, 133.2, 132.6,
130.6, 130.4, 129.2, 127.3, 127.2, 127.0, 126.7, 126.4, 126.4, 125.3,

D. Thongkum, T. Tuntulani / Tetrahedron 67 (2011) 8102e8109
8109
sodium ion (1.0ꢁ10ꢀ2 M) in CDCl3/CD3CN (1.0 mL) was prepared in
a vial and was added via a microsyringe to the free ligand solution.
1H NMR spectra were recorded after each addition.
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of compound L1 was recorded in the presence of mixed anions.
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Acknowledgements
11. Dahan, A.; Ashkenazi, T.; Kuznetsov, V.; Makievski, S.; Drug, E.; Fadeev, L.;
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We would like to thank Commission on Higher Education for
supporting the grant to D.T. to pursue her Ph.D. under the program
of Strategic Scholarships for Frontier Research Network. The Thai-
land Research Fund (RTA5380003) and the National Research
University of CHE and the Ratchadaphiseksomphot Endowment
Fund (AM1006A) are acknowledged for financial support.
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Supplementary data
1H and 13C NMR spectra of L1eL4; UVevis spectra of L1eL4;
NOESY of L1 with TBAF; 1H NMR and fluorescence titration spectra
of L1eL4 with various ions. Supplementary data associated with
this article can be found, in the online version, at doi:10.1016/
j.tet.2011.08.060.
ꢀ
A. Acc. Chem. Res. 2006, 39, 465e475; (c) Amendola, V.; Esteban-Gomez, D.;
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References and notes
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F. M. Coord. Chem. Rev. 2006, 250, 3094e3117; (b) Batista, R. M. F.; Oliveir, E.;