Conversion of thioureas to fluorescent isothiouronium-based photoinduced electron transfer sensors for oxoanion sensing

Article abstract of DOI:10.1039/b201704k

A convenient conversion is described of thiourea-based receptors to fluorescent isothiouronium-based photoinduced electron transfer (PET) sensors for oxoanion sensing. Naphthalene- or anthracene-functionalized mono-isothiouroniums are synthesized from the corresponding thioureas, in which the fluorophore is connected to the sulfur atom of the thiourea moiety by a methylene or an ethylene spacer. Even though all of the isothiouroniums with a methylene spacer readily decompose in MeOH upon excitation of the fluorophore moiety, the isothiouronium with an ethylene spacer shows good stability under identical conditions. The naphthyl isothiouronium with an ethylene spacer shows a significant fluorescence enhancement upon formation of a 1 : 1 complex with oxoanions in MeOH, and the selectivity follows the order of hydrogen phosphate > acetate ? dihydrogen phosphate ? chloride. The results in the present work indicate that various types of fluorescent PET sensors might be readily obtainable from non-fluorescent thiourea-based receptors by the introduction of appropriate fluorophores at the sulfur atom of thiourea-binding sites.

Full text of DOI:10.1039/b201704k

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Conversion of thioureas to fluorescent isothiouronium-based
photoinduced electron transfer sensors for oxoanion sensing
Seiichi Nishizawa, Ying-Yu Cui, Masakazu Minagawa, Kotaro Morita, Yuichi Kato,
Shinichiro Taniguchi, Ryo Kato and Norio Teramae*
2
Department of Chemistry, Graduate School of Science, Tohoku University,
Aoba-ku, Sendai 980-8578, Japan. E-mail: tera@anal.chem.tohoku.ac.jp;
Fax: ϩ81 22 217 6552; Tel: ϩ81 22 217 6549
Received (in Cambridge, UK) 18th February 2002, Accepted 21st March 2002
First published as an Advance Article on the web 5th April 2002
A convenient conversion is described of thiourea-based receptors to fluorescent isothiouronium-based photoinduced
electron transfer (PET) sensors for oxoanion sensing. Naphthalene- or anthracene-functionalized mono-
isothiouroniums are synthesized from the corresponding thioureas, in which the fluorophore is connected to the
sulfur atom of the thiourea moiety by a methylene or an ethylene spacer. Even though all of the isothiouroniums
with a methylene spacer readily decompose in MeOH upon excitation of the fluorophore moiety, the isothiouronium
with an ethylene spacer shows good stability under identical conditions. The naphthyl isothiouronium with an
ethylene spacer shows a significant fluorescence enhancement upon formation of a 1 : 1 complex with oxoanions in
MeOH, and the selectivity follows the order of hydrogen phosphate > acetate ӷ dihydrogen phosphate ӷ chloride.
The results in the present work indicate that various types of fluorescent PET sensors might be readily obtainable
from non-fluorescent thiourea-based receptors by the introduction of appropriate fluorophores at the sulfur atom of
thiourea-binding sites.
The present paper discusses an alternative design of such
isothiouronium-based fluorescent anion receptors. In contrast
to the earlier system based on the conversion of fluorescent
Introduction
The design and synthesis of abiotic hydrogen-bonding recep-
tors for biologically and/or chemically important anions are
thioureas to isothiouroniums in which the fluorophore is linked
of current interest in host–guest chemistry.1,2 A wide variety of
to the nitrogen atom of the isothiouronium binding site,5 our
these receptors has been reported for selective anion detection,
transport, and catalysis. In these receptors, amides, (thio)ureas,
pyrroles, imidazoliums and guanidiniums are utilized as
strategy is based on the conversion of thioureas to isothio-
uroniums by attachment of the appropriate fluorophores
through the sulfur atom of the thiourea moiety (Scheme 1).
hydrogen-bond forming moieties to obtain selective binding
Since a number of thiourea compounds are now available as
affinities.
anion-binding reagents,9–12 our approach makes it possible to
Isothiouronium salts have been explored quite recently as a
new class of hydrogen-bonding subunit for the purpose of
anion recognition.3–5 The first use of isothiouronium com-
pounds was reported by Yeo and Hong3 for the binding of
5Ј-AMP or oxoanions such as phosphates and carboxylates.
Suzuki and co-workers4 described an application of iso-
thiouronium compounds for solvent polymeric membrane
ion-selective electrodes with iodide selectivity. Of particular
interest is a sophisticated combination of the isothiouronium
binding unit and a suitable chromophore as reported by Kubo
et al.5 Their anion chemosensor was based on a naphthalene-
isothiouronium dyad that was synthesized from the corre-
sponding naphthalene-thiourea, i.e., 2-(1,3-dimethylthio-
ureylene)naphthalene, by reaction with benzyl bromide. This
receptor showed a significant increase in fluorescence intensity
upon binding with acetate in MeCN, while only a low response
was observed in the presence of chloride anion. Although
the details of the response mechanisms are not clear for
their system, the complexation-induced enhancement in the
fluorescence intensity is quite attractive from a practical
viewpoint. To date, only a few fluorescent anion sensors exist
that allow the detection of anionic species based on chelation-
enhanced fluorescence or luminescence.6–8 Furthermore, due to
their high affinity for oxoanions over chloride,5 isothiouronium-
based receptors may be promising candidates for the recog-
nition of biologically relevant substrates such as phosphates
and their derivatives under physiological conditions.
Scheme 1
866
J. Chem. Soc., Perkin Trans. 2, 2002, 866–870
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b201704k

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easily derive various types of fluorescent receptors from the
corresponding thioureas without the need for changes to
the receptor skeleton. In order to assess our strategy, very
simple naphthalene- or anthracene-functionalized mono-iso-
thiouroniums 1 to 5 were synthesized from the corresponding
thioureas in the present study. The photostabilities and binding
properties of these isothiouronium-based compounds were
examined in MeOH, and the response mechanism was dis-
cussed on the basis of photoinduced electron transfer (PET)
reactions as a possible operating principle.
Fig. 2 Fluorescence spectra of (A) receptor 1 and (B) receptor 2 under
repeated scan conditions. [1] = [2] = 20 µM in MeOH. Excitation
wavelength: 270 nm. Scan speed: 100 nm minϪ1. Scan range: 300–450
nm. (a): 1st; (b) 2nd; (c) 3rd scans.
Results and discussion
As shown in Scheme 1, the syntheses of the isothiouronium
compounds were carried out by nucleophilic substitution of
halogenides with the appropriate thioureas. 2-(2-Bromo-
ethyl)naphthalene was prepared from naphthalene-2-acetic acid
by reduction using LiAlH4 to give the corresponding alcohol,
followed by bromination with CBr4. All of the isothiouronium
compounds could be easily obtained in high yields (∼80%)
although the purification process lowered final yields to ∼50%.
Fig. 1 shows the absorption and fluorescence spectra of 1 and
Me-Naph. Even though the observed quenching may be partly
ascribable to a heavy-atom effect from the bromide ion, it is
most likely that, as expected from the results of electrochemical
measurements (vide infra), the fluorescence of the naphthalene
moiety in 1 and 2 is quenched by a PET mechanism,13–15
in which the naphthalene and isothiouronium moieties act as
electron donor and acceptor, respectively.
The feasibility of PET between the naphthalene (Naph) and
isothiouronium (TU) moieties can be assessed according to the
Rehm–Weller equation:14,15
∆G/kcal molϪ1 = 23.06[Eox(D) Ϫ Ered(A)] Ϫ ωp Ϫ G00(D)
where ∆G, Eox(D), Ered(A), ωp, and G00(D) are the free energy
change of electron transfer, the oxidation potential of the
donor (D), the reduction potential of the acceptor (A), the
ion-pairing energy, and the excitation energy of the donor
(D), respectively. The oxidation potential of naphthalene,
Eox(Naph), is 1.6 V vs. SCE, and G00(Naph) is 92 kcal molϪ1.15
The value of ωp is usually estimated to be Ϫ1.3 kcal molϪ1
in MeCN.15 Ered(TU) is ca. Ϫ1.83 V vs. Ag/Agϩ, which was
measured in MeCN solution by cyclic voltammetry. Although
accurate determination of the free energy change of electron
transfer was impossible due to the irreversibility of the TU
reduction, the ∆G within the present system calculated from the
above values is indeed negative. This is clear evidence that the
naphthalene and isothiouronium moieties act as electron donor
and acceptor, respectively.
The difference in the emission intensities between 1 and 2
seems to be another piece of evidence for quenching due to the
PET mechanism. It is reasonable that the fluorescence intensity
of 1 with the CH2 spacer is weaker than that of 2 with the
CH2CH2 spacer because of the smaller distance separating
the terminal groups, i.e., the naphthalene and isothiouronium
moieties. Use of the CH2 spacer is therefore desirable in terms
of obtaining fast PET rates, since it allows efficient fluorescence
signalling, i.e. ‘switched off–switching on’.13,16 The CH2 spacer
may also contribute to the suppression of exciplex emissions,
thus simplifying the fluorescence behaviour,13 although this
type of emission benefits the ratiometric analysis in well-
designed systems.17 Additionally the CH2 spacer is more con-
venient for synthesis via benzylic functionalities.13 However,
as shown in Fig. 2A, the fluorescence intensity of 1, with the
CH2 spacer, increased uniformly after repeated scans, indicating
the instability of receptor 1 under the conditions of photo-
irradiation (excitation wavelength λex: 270 nm). Similar results
were obtained for all the isothiouronium-based compounds
with the CH2 spacer. Again, the fluorescence intensities of
receptors 3 to 5 increased uniformly upon continuous excitation
of the fluorophore moiety (λex: 270 nm for 3; 349 nm for 4 and
5). It seems likely that, under the photo-irradiation conditions,
photodissociation of the CH2–S bond occurs in all the com-
pounds with the CH2 spacer when considering the photo-
chemistry of arylmethyl compounds.18 It is well recognized that
Fig. 1 (A) Absorption and (B) fluorescence spectra of receptors 1, 2
and 2-methylnaphthalene (Me-Naph) in non-degassed MeOH. The
emission intensities of the fluorescence spectra of 1 and 2 are magnified
by a factor of 10. [Compound] = 20 µM. Excitation wavelength: 270
nm.
2 in MeOH. For comparison, the absorption and fluorescence
spectra of 2-methylnaphthalene (Me-Naph) are also shown. As
can be seen from Fig. 1A, compounds 1 and 2 show absorption
maxima at 225 nm, accompanied by vibronic absorption bands
above 250 nm. These features of the absorption spectra are
characteristic of alkyl-substituted naphthalene molecules,
and no marked differences are observed as compared with the
spectrum of Me-Naph (λmax: 224 nm). This result indicates that
there is no significant interaction between naphthalene and the
isothiouronium moieties in the ground state. By contrast, as
shown in Fig. 1B, the fluorescence spectra of isothiouronium
compounds 1 and 2 are clearly different from the spectrum of
Me-Naph. While both the shape and position of the emission
spectra peaks of 1 and 2 almost coincide with those of Me-
Naph (cf. Fig. 2), the intensity at 333 nm is reduced to 1/260
and 1/23 for 1 and 2, respectively, as compared with that of
J. Chem. Soc., Perkin Trans. 2, 2002, 866–870
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the photochemical reactions of arylmethyl compounds with
leaving groups (ArCH2–LG) give various products from both
ؒ ؒ
the intermediate arylmethyl radical (ArCH2 LG) as well as ion
pairs (ArCH2ϩ :LGϪ).18 One of possible processes in protic and
nucleophilic solvents like alcohols (ROH) is photosolvolysis
where the ion pair is trapped by the solvent to give substitution
products (ArCH2–OR and HLG).18 On the other hand, as
shown in Fig. 2B, no changes in the fluorescence spectra of 2
with the CH2CH2 spacer appeared after repeated scans, indi-
cating the good photostability of 2 under identical conditions
(λex: 270 nm). On the basis of these results, we conclude that
the CH2 group is not suitable as a spacer for the design of
isothiouronium-based PET systems when the fluorophores are
attached through the sulfur atom of the isothiouronium moiety.
Thus, all subsequent binding studies were performed with the
isothiouronium-naphthalene with a CH2CH2 spacer, receptor 2.
The effect of HPO42Ϫ as [Kϩ–18-crown-6] salts on the absorp-
tion and fluorescence spectra of receptor 2 was examined in
MeOH. While the absorption spectra above 260 nm did not
change at all in the presence or absence of this anion, significant
changes occurred in the fluorescence spectra. Fig. 3 shows the
Fig. 4 Dependence of the intensity ratio, F/F0, at 333 nm of 2 (20 µM)
on the concentrations of (᭹) HPO42Ϫ, (᭜) CH3COOϪ, (ꢀ) H2PO4Ϫ and
(᭺) ClϪ in non-degassed MeOH. Anions are added as the [Kϩ–18-
crown-6] salts. (—): Non-linear fitting based on the equation for 1 : 1
complexation.
only other anion to be bound substantially under these con-
ditions. When 2 forms a 1 : 1 complex with acetate (K11 = 1.7
0.2 × 103 MϪ1), the fluorescence intensity at 333 nm increases by
a factor of 5.0, which corresponds to half the value observed
2Ϫ
Ϫ
for HPO4 binding. In the presence of H2PO4 , only a slight
change in the fluorescence spectrum is observed so that
determination of the association constant between 2 and
H2PO4 is impossible. In the case of ClϪ, 2 does not show any
Ϫ
obvious spectral changes even with a 30-fold excess. Thus, 2
Ϫ
shows selectivity in the order of HPO42Ϫ > MeCO2Ϫ ӷ H2PO4
ӷ ClϪ. As has been found previously,5 the lack of binding affin-
ity for ClϪ seems quite desirable for the sensing of oxoanions.
In summary, we have examined the conversion of thiourea-
based receptors to fluorescent isothiouroniums by introduction
of appropriate fluorophores at the sulfur atom of the thiourea
moiety. Whereas all the isothiouroniums with a methylene
spacer readily decomposed in MeOH upon excitation of
the fluorophore moiety, receptor 2 with an ethylene spacer,
exhibited good stability and could function as a fluorescent
2Ϫ
Ϫ
PET sensor for oxoanions such as HPO4 and MeCO2 .
In addition to the strong anion-binding affinity, another
promising feature of the isothiouronium was its electron-
acceptor ability [Ered(TU): ca. Ϫ1.83 V], which makes anion
detection possible on the basis of fluorescence enhancement.
Although both the isothiouroniums reported here and by Kubo
et al.5 are naphthalene-functionalized receptors, the Rehm–
Weller equation predicted that various fluorophores such as
pyrene [Eox(Py): 1.20 V; G00(Py): 77 kcal molϪ1]15 and anthra-
cene [Eox(An): 1.16 V; G00(An): 76.3 kcal molϪ1]15 were suitable
for the design of isothiouronium-based PET sensors. Our
approach would thus allow easy derivation of various types of
fluorescent receptors from the corresponding thioureas; such
work is now in progress in our laboratory.
Fig. 3 Changes in the fluorescence spectra of 2 (20 µM) upon addition
of HPO4 (0, 20, 100, 400 µM) as the [Kϩ–18-crown-6] salt in non-
2Ϫ
degassed MeOH. Excitation wavelength: 270 nm.
2Ϫ
fluorescence spectra of 2 as a function of HPO4 concentra-
2Ϫ
tion, when excited at 270 nm. Increasing the HPO4 concen-
tration results in a marked enhancement of the fluorescence
intensity while the spectral shape and position of fluorescence
2Ϫ
are independent of the HPO4 concentration. This signalling
action, in which only one parameter (fluorescence quantum
yield) is anion-controlled, is typical of fluorescent PET-based
molecular sensors. In spite of the use of the ethylene group as a
spacer, the fluorescence intensity at 333 nm in the HPO42Ϫ com-
plex is indeed higher by a factor of 10 than that in the free
receptor. This indicates that use of the CH2CH2 spacer does not
cause any serious problems for fluorescent signalling. A non-
linear fitting shows that the changes in the fluorescence spectra
can be explained by the formation of a 1 : 1 complex (cf.
Experimental
Reagents and apparatus
2Ϫ
Fig. 4),19 and the association constant between 2 and HPO4
is calculated to be 1.1 0.1 × 104 MϪ1. This corresponds to
approximately half the value obtained with a bis-guanidinium
receptor in the same solvent (1.83 × 104 MϪ1),20 indicating that
the binding affinity of isothiouronium is indeed comparable
to that of biologically important guanidinium groups,21 which
have been extensively utilized as anion-binding subunits in
artificial systems.1
All the guest anions were commercially available as Kϩ salts
and used as-received. 18-Crown-6 from Wako Pure Chemical
Industries (Tokyo, Japan) was used to dissolve the Kϩ
salts. Tetrabutylammonium perchlorate from Tokyo Chemical
Industry Co., Ltd. (Tokyo, Japan) was purified four times
by recrystallization from ethyl acetate. Spectrometric grade
methanol from Nacalai Tesque, Inc. (Kyoto, Japan) was used
as-received. Dry acetonitrile (for organic synthesis, Wako) was
used without further purification for electrochemical measure-
ments. Dimethyl sulfoxide-d6 (DMSO-d6) and chloroform-d1
were purchased from E. Merck (>99.8%, Darmstadt,
Germany).
The selectivity of receptor 2 was also examined in MeOH.
Similar, but less pronounced, responses were observed upon
Ϫ
Ϫ
addition of other monovalent anions (H2PO4 , MeCO2 , ClϪ).
Fig. 4 shows the dependence of the intensity ratio F/F0 (F and
F0 are the fluorescence intensities of the receptor in the presence
and absence of guest anions, respectively) at 333 nm on the
concentration of anion in MeOH. We can see that acetate is the
Fluorescence spectra were measured at 25 ЊC with a JASCO
FP-777W spectrophotometer (Japan Spectroscopic Co. Ltd.,
868
J. Chem. Soc., Perkin Trans. 2, 2002, 866–870

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Tokyo, Japan); the slits for the excitation and emission mono-
chromators were 3.0 and 1.5 nm, respectively. All emission
spectra were uncorrected. Absorption spectra were recorded at
25 ЊC with a HITACHI U-3000 UV-VIS spectrophotometer
(Hitachi, Ltd., Tokyo, Japan) by using a quartz cell of 0.1 cm
path length. 1H NMR spectra were obtained on a JEOL α-270
spectrometer (270 MHz; JEOL Datum, Tokyo, Japan). All
chemical shift values (δ) are reported in parts per million (ppm),
using the residual solvent signal of DMSO (δ 2.49) or CHCl3
(δ 7.24) as a reference.
solid, yield: 48%). 1H NMR (270 MHz, CDCl3): δ 10.09 (br, 1H,
NH), 9.72 (br, 1H, NH), 8.34 (s, 1H, Naph), 8.19 (d, J = 8.1 Hz,
2H, Naph), 7.92 (d, J = 8.1 Hz, 2H, Naph), 7.50 (t, J = 8.1 Hz,
2H, Naph), 7.40 (t, J = 8.1 Hz, 2H, Naph), 5.68 (s, 2H, -CH2-),
3.48 (s, 3H, -CH3), 2.95 (s, 3H, -CH3).
S-(9-Anthrylmethyl)-N,NЈ-diphenylisothiouronium chloride
(5). This compound was synthesized analogously to 1 from
9-(chloromethyl)anthracence and 1,3-diphenylthiourea (yellow
solid). 1H NMR (270 MHz, DMSO-d6): δ 10.09 (s, 2H,
NH), 8.64 (s, 1H, Naph), 8.41 (d, J = 8.1 Hz, 2H, Naph), 8.11
(d, J = 8.1 Hz, 2H, Naph), 7.63–7.08 (14H, Ar), 5.56 (s, 2H,
-CH2-).
Synthesis
S-(2-Naphthylmethyl)-N,NЈ-dimethylisothiouronium bromide
(1). 2-(Bromomethyl)naphthalene was added to a stirred solu-
tion of 1,3-dimethylthiourea (0.24 g, 2.26 mmol) in dry ethanol
(30 ml) under a nitrogen atmosphere. The reaction solution was
stirred for 1 h at 80 ЊC, and then the solvent was evaporated.
Purification by silica gel column chromatography with CHCl3–
CH3OH (10 : 1) as eluent gave the product as a white solid
(0.34 g, 46%). 1H NMR (270 MHz, DMSO-d6): δ 9.24 (br, 2H,
NH), 7.96–7.89 (4H, Naph), 7.57–7.52 (3H, Naph), 4.70 (s, 2H,
-CH2-), 2.92 (s, 6H, -CH3). Anal. Calcd for C14H17N2SBr
(325.27): C, 51.7; H, 5.27; N, 8.61; S, 9.86; Br, 24.57%. Found:
C, 51.49; H, 5.37; N, 8.53; S, 10.13; Br, 24.61%.
Determination of association constants by fluorescence
spectroscopy
To determine the 1 : 1 association constant K11, a series of
methanol solutions was prepared that contained the guest
anions (as their [Kϩ–18-crown-6] salts) and receptor 2. The
concentration of 2 was fixed at 20 µM, and the concentration
of guest anions ranged from 0 to 2.0 mM. The changes in
fluorescence intensity at 333 nm were monitored as a function
of the guest concentration. The resulting titration curves were
analyzed by a non-linear regression based on a 1 : 1 binding
isotherm model:19
2-(2-Naphthyl)ethanol. Naphthalene-2-acetic acid (1.0 g,
5.4 mmol) was added to a stirred solution of LiAlH4 (0.25 g,
6.6 mmol) in freshly distilled THF (50 ml), keeping the reaction
mixture at 0 ЊC under a nitrogen atmosphere. After stirring at
0 ЊC for 30 min, ethyl acetate, diethyl ether saturated with water,
and water were added successively. The organic phase was dried
over Na2SO4, and then the solvent was evaporated. Purification
by silica gel column chromatography with CH2Cl2–CH3OH
(10 : 1) as eluent gave the product as a white solid (0.4 g, 43%).
F/F0 = {1 ϩ ∆FK11[G]}/{1 ϩ K11[G]}
(1)
where F and F0 are the fluorescence intensities of the receptor
in the presence and absence of guest anions, respectively,
and ∆F is the complexation-induced maximum change in the
fluorescence intensity ratio. The free guest concentration, [G],
can be related to known initial concentrations of guest (G0) and
receptor (R0), by the following equation:
G0 = [G] ϩ {R0K11[G]}/{1 ϩ K11[G]}
Together, eqns. (1) and (2) describe the system.
Electrochemical measurements
(2)
2-(2-Bromoethyl)naphthalene. Naphthalene-2-ethanol (0.2 g,
1.16 mmol) and CBr4 (0.58 g, 1.74 mmol) were dissolved
in CH2Cl2 (purified by passage through a column of basic
alumina, 20 ml), and triphenylphosphine (0.37 g, 1.39 mmol)
was gradually added. The reaction solution was stirred for 1 h
at room temperature under a nitrogen atmosphere, followed by
washing with water several times. The organic phase was dried
over Na2SO4, and then the solvent was evaporated. Purification
by column chromatography (silica gel, CHCl3) gave the product
as a white solid (0.2 g, 73%).
Cyclic voltammograms of receptor
2 (1.0 mM) were
measured at 25 ЊC in MeCN containing 0.1 M N(C4H9)4ClO4 as
a supporting electrolyte. A sweep rate of 100 mV sϪ1 was used.
A potentiostat (Solatron 1286, Schlumberger Technologies,
Hampshire, England) equipped with an X-Y recorder was used
with a three-electrode configuration: 1 mm ꢀ glassy carbon
electrode as the working electrode, an Ag/Agϩ electrode as
the reference electrode, and a platinum wire as the auxiliary
electrode. Sample solutions were degassed by Ar gas bubbling
before measurements.
S-(2-Naphthylethyl)-N,NЈ-dimethylisothiouronium bromide
(2). This compound was synthesized analogously to 1 from 1,3-
dimethylthiourea and 2-(2-bromoethyl)naphthalene. It was
purified by silica gel column chromatography with CHCl3–
CH3OH (10 : 1) as eluent, giving the product as a pale yellow
solid (yield: 55% as calculated for S-alkylation of 1,3-dimethyl-
1
Acknowledgements
2-thiourea). H NMR (270 MHz, DMSO-d6): δ 9.03 (s, 2H,
NH), 7.90–7.79 (4H, Naph), 7.53–7.44 (3H, Naph), 3.61 (t,
J = 7.3 Hz, 2H, -CH2-), 3.13 (t, J = 7.3 Hz, 2H, -CH2-), 2.90
and 2.82 (6H, -CH3). Anal. Calcd for C15H19N2SBr (339.30):
C, 53.1; H, 5.64; N, 8.26; S, 9.45; Br, 23.55%. Found: C, 53.16;
H, 5.63; N, 7.84; S, 9.48; Br, 23.75%.
We thank Dr Hinako Ando, Instrumental Analysis Center for
Chemistry, Faculty of Science, Tohoku University, for per-
forming the microanalyses. This work was partially supported
by Grants-in-Aid for Scientific Research (A), No. 11304054,
and for Scientific Research on Priority Research Areas (B),
No. 13129201, from the Ministry of Education, Culture,
Sports, Science and Technology, Japan, RFTF, and Japan–
China Cooperative Research Program from the Japan Society
for the Promotion of Science.
S-(2-Naphthylmethyl)-N,NЈ-diphenylisothiouronium bromide
(3). This compound was synthesized analogously to 1 from
2-(bromomethyl)naphthalene and 1,3-diphenylthiourea (pale
yellow solid). 1H NMR (270 MHz, DMSO-d6): δ 7.93–7.89 (2H,
Ar), 7.80 (d, J = 6.5 Hz, 2H, Ar), 7.54–7.44 (3H, Ar), 7.32–7.13
(10H, Ar), 4.58 (s, 2H, -CH2-).
References
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(4). This compound was synthesized analogously to 1 from
9-(chloromethyl)anthracene and 1,3-dimethylthiourea (yellow
J. Chem. Soc., Perkin Trans. 2, 2002, 866–870
869

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