Bifunctional maleimide dyes as selective anion sensors

Article abstract of DOI:10.1016/j.tet.2009.04.090

A class of disubstituted maleimide dyes with two symmetrical NH binding sites was found to exhibit distinct color change and fluorescence quenching effect for fluoride, cyanide, and dihydrogen phosphate anions. The intense red emission displayed apparent

Full text of DOI:10.1016/j.tet.2009.04.090

Tetrahedron 65 (2009) 5216–5221
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Bifunctional maleimide dyes as selective anion sensors
*
*
*
Zhenghuan Lin, Hung Cheng Chen, Shih-Sheng Sun , Chao-Ping Hsu , Tahsin J. Chow
Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
A class of disubstituted maleimide dyes with two symmetrical NH binding sites was found to exhibit
distinct color change and fluorescence quenching effect for fluoride, cyanide, and dihydrogen phosphate
anions. The intense red emission displayed apparent solvatochromic shift, indicating a strong charge-
transfer character. The interactions between the dyes and anions were variable depending on the amine
substituents at C(3,4) of the maleimides. For the dyes with two pyrrolyl receptor sites, the NH protons
were deprotonated by more basic anions such as fluoride or cyanide. For those with two indolyl receptor
sites, formation of a chelate with H2PO4ꢀ through hydrogen bonds played a major role.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 6 February 2009
Received in revised form 21 April 2009
Accepted 28 April 2009
Available online 6 May 2009
Keywords:
Anion sensor
Maleimide
Fluorescence quenching
Red dye
1. Introduction
highly stable compounds, yet display remarkable long wavelength
emissions both in solutions and in solid state. They have been
Anions play crucial roles in environmental, chemical, and bio-
logical systems in forms of drugs, food additives, agricultural fer-
tilizers, biologic metabolites, and waste liquid from chemical
plants, etc.1 Among them, cyanide, fluoride, and phosphate are of
particular concerns due to their usages as raw materials in the
production of nylon and resins; in the prevention of dental caries
and treatment of osteoporosis; and in the signal transduction and
energy storage in biological systems, respectively.2 Nevertheless,
these three kinds of anion have also caused huge damages on en-
vironments, such as toxic wastes, water eutrophication, and even in
the production of nerve gases.3 As a consequence there is in need of
sensitive and easy testing methods for detecting these anions.
Recently, fluorimetric sensors based on dipyrrolylquinoxaline
(DPQ) have been reported for detecting similar anions by hydrogen
bonding with pyrrole protons.4 However, DPQ and its simple de-
rivatives did not exhibit satisfactory sensitivity.5 By integrating DPQ
into a polymer backbone and coordinated with Ru(II) ion as de-
veloped by Sun et al.6 and Pavel et al.,7 the sensitivity of DPQ can be
improved substantially. These efforts have mostly been focused on
changing the electronic and optical properties of the quinoxaline
moieties, which have led to quite complicated molecular structures.
The intense red color and high luminescent efficiency of mal-
eimide-based fluorophores has attracted considerable attention
recently.8,9 These series of compounds are derivatives of natural
products, which can be synthesized easily in the lab. They are
utilized successfully on organic light-emitting diodes (OLEDs) as
emitting materials9 and on pharmaceutical investigations as po-
tential new drugs.10 The structure of derivatives with pyrrolyl
substituents at C(3,4) positions bears a good similarity to the
structural of DPQ, yet their unique red color renders them an
exceptional candidate beyond DPQ.
In this paper we describe an unprecedented design of maleimide
derivatives for use as effective anion sensors. The maleimide-based
derivatives were chosen as the fluorophore of anion sensors for three
reasons: (1) maleimide, as an electron-withdrawing group, is
expected to enhance the acidity and the H-bond donor property of
pyrrolyl groups. (2) The optical property of maleimide is sensitive to
receptor–anion interactions, thus suitable for both colorimetric and
fluorimetric detections. (3) There is an argument on the origin of
fluorescence quenching of these types of sensors, i.e., either it is
caused by the formation of hydrogen bonding,4,5,7 or simply due to
the result of deprotonation.6,11 In this report we tried to resolve these
arguments by analyzing this new type of maleimide sensors.
2. Results and discussions
Two types of maleimide derivatives were synthesized by
replacing the bromide groups at C(3) and C(4) positions of N-benzyl-
2,3-dibromomaleimide with either indoles (1a–c) or pyrroles (2)
through Grignard reactions (Scheme 1). The bonding structure of
pyrrolyl moieties in compounds 1a,b is different from that in com-
pound 2, thus offers a chance for examining the structural effect on
anion sensing properties. Compound 1c does not have any free –NH
* Corresponding authors. Tel.: þ886 2 27898552; fax: þ886 2 27884179.
E-mail address: tjchow@chem.sinica.edu.tw (T.J. Chow).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.04.090

Z. Lin et al. / Tetrahedron 65 (2009) 5216–5221
5217
Table 1
Spectroscopic parameters of compounds 1a, 1b, and 2 in CH2Cl2 and their binding
constants toward anions K (Mꢀ
)
O
N
O
1a
1b
2
DPQa
O
N
O
pyrrole
b
labs (nm)
lemb (nm)
Fem (%)
456 (560)
610
459 (595)
616 (650)
2
517 (584)
653 (715)
7
d
or
or indole
O
N
O
d
EtMgCl
toluene
35
d
1.82ꢁ104
N
HN
NH
N
K (Fꢀ)
3.91ꢁ103
3.99ꢁ103
<50
6.19ꢁ102
1.53ꢁ103
<50
1.85ꢁ104
<100
R1 R1
R2
Br
R2
Br
K (H2POꢀ4 )
60
K (CNꢀ
)
3.63ꢁ104
<50
220
d
1a) R1 = R2 = H
b)
2
K (other anions)
<50
<50
R1 = CH3; R2 = H
c) R1 = R2 = CH3
Data in parentheses correspond to the anions.
a
Data obtained from Ref. 7.
The lowest energy band.
b
Scheme 1. Synthesis of compounds 1a–c and 2.
the same way by using 2; and dihydrogen phosphate ion by using
1a and 1b.
The visual colors of compounds 1a and 2 in the presence of
different anions exhibit clear distinctions (S4, S5). The yellow
moiety, therefore is not suitable for using as sensors. Nevertheless
its relatively lower pH dependency in the absorption and higher
quantum efficiency in the emission rendered it a more appropriate
model for the analysis of slovatochromic effect.
Compounds 1a–c and 2 exhibit three major absorption bands in
the range of 250–550 nm region (Fig.1 and Fig. S1). They are known
emission of dilute solutions (50 mM) of 1a in dichloromethane was
quenched by the addition of Fꢀ and H2POꢀ4 ions, yet persisted upon
the addition of other anions. The red emission of compound 2
exhibited similar quenching effect for Fꢀ and CNꢀ, but unaffected
by H2POꢀ4 ion. The relative intensities of fluorescence are shown as
bar charts in Figure 2 for a better comparison.
to derive from
p
–
p transitions, where the lower energy ones at
*
lmax 455–520 nm exhibit a significant charge-transfer character.8,12
Each molecule contains a pair of cross-intercepted dipoles, which
consist of an electron donor (pyrrole) and an electron acceptor
(maleimide carbonyl). Electron transition from HOMO to LUMO
thus generated a charge-separated state. An apparent solvent effect
can be seen in both the absorption and emission spectra of 1c (S1).
In its absorption spectrum, the lowest energy band is red-shifted
from 460 nm in cyclohexane to 480 nm in acetonitrile. The high
energy band at 283 nm, however, is not influenced by the solvents.
Upon photo-excitation, a broad featureless band appeared at lmax
575–610 nm, which changes both its intensity and wavelength in
different solvents. A substantial red shift and reduction of intensity
appeared along with the increase of solvent polarity. Similar
spectral patterns were also found in compounds 1a, 1b, and 2 (Fig. 1
and Figs. S2 and S3). The charge-transfer band in 2 is red-shifted to
a greater extend comparing to those of 1a–c. A list of their spec-
troscopic parameters is given in Table 1.
A typical titration curve of fluoride by 2 is shown in Figure 3. The
long wavelength absorption at lmax 516 nm decreased in the
presence of fluoride along with the increasing of 580 nm band. A
distinct isosbestic point can be detected at 542 nm, indicating the
formation of a stable new species. Another isosbestic point at
414 nm was observed simultaneously as a result of increasing the
362 nm band. In the emission spectrum of 2, the intensity of
646 nm band decreased gradually along with the increased con-
centration of fluoride ion. For instance, in the presence of 25 molar
equivalence of fluoride ion, the emission intensity of 2 reduced to
<10% of the original level. Compounds 1a and 1b behave similarly
toward fluoride ion, i.e., a red-shift in the absorption spectra and
a quenching of fluorescence (S6, S9).
The sensory properties of 1a and 2 were selective between cy-
anide and dihydrogen phosphate ions. In the presence of cyanide
ion, compound 2 showed a color change from red to blue, quite
similar to what observed for fluoride ion. The absorption band at
lmax 580 nm increased along with a decrease of the 516 nm band
(S11). The red fluorescence at 646 nm was effectively quenched by
cyanide ion. Compounds 1a,b did not exhibit any interaction with
cyanide ion, but showed significant fluorescence quenching with
dihydrogen phosphate (Fig. 4 and Fig. S7). It is clear that the anion
sensing interactions of compounds 1a,b are not quite the same as
that of 2. Both the distance and relative orientation between the
two NH groups must have played an important role.13
The qualitative evaluation of the anion sensing capability of 1a,
1b, and 2 was performed in dichloromethane solutions containing
a series of anions, namely, Fꢀ, Clꢀ, Brꢀ, Iꢀ, H2POꢀ4 , HSO4ꢀ, NOꢀ3 , OAcꢀ,
and CNꢀ. Among them highly selective sensory properties were
found for Fꢀ, CNꢀ, and H2POꢀ4 anions. In the presence of fluoride
ion, the color of solutions 1a, 1b, and 2 changed along with sig-
nificant fluorescence quenching. The cyanide ion can be detected in
1.0
0.5
0.0
1a
1b
2
In the presence of fluoride ion the emission of 2 was quenched
(Fig. 3), yet along with an apparent red shift of the spectrum. A
weak new band centered at ca. 705 nm cannot be ignored, and is
likely to have derived from the emission of the anion of 2. The
relatively stronger basicity of fluoride and cyanide anions can ab-
stract a proton from the pyrrole of 2 to yield a mono-anion with
stable cyclic structure (Eq. 1). The emissive property of this anion is
further examined by DFT analysis.
O
N
O
O
N
H
O
+ HA
+ A
ð1Þ
300
400
500
Wavelength (nm)
600
700
800
HN
NH
N
N
Figure 1. Optical absorption (solid lines) and luminescence (dashed lines) spectra of
1a, 1b, and 2.

5218
Z. Lin et al. / Tetrahedron 65 (2009) 5216–5221
1
1
0.5
0
0.5
0
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
9
10
Figure 2. (Left) Bar chart indicating the relative intensity of fluorescence of 1a (10
of 2 (10
CNꢀ
m
M) at 604 nm in the presence of different anions (2 mM). (Right) Relative fluorescence intensity
m
M) at 646 nm in the presence of different anions (2 mM). Anions arranged from left to right: 1-none, 2-Fꢀ, 3-Clꢀ, 4-Brꢀ, 5-Iꢀ, 6-H2PO4ꢀ, 7-HSO4ꢀ, 8-NO3ꢀ, 9-OAcꢀ, and 10-
.
In order to confirm the spectral nature of the anion, compound 2
recognition of 2, however, was disturbed by the presence of certain
anions, e.g., HSOꢀ4 or OAcꢀ (S14, S15), yet not by others all. The
disturbance may be caused by a competition for proton among all
the anions.
was treated with a strong base N(Et)4OH in dichloromethane, and
its absorption and emission spectra were recorded (S12). In the
presence of excess amount of base, indeed a new absorption band
appeared at 584 nm and a new emission band at 715 nm. The
wavelength of these transitions complies well with the new bands
observed while treated with fluoride and cyanide ions (Fig. 3 and
Fig. S11). It is therefore concluded that the reaction between 2 and
anions was mainly acid–base in nature, and not necessarily in-
volved a direct association of the two.14
The bonding nature of 1a,b with anions was different from that
of 2. Their active response to H2POꢀ4 , but not to CNꢀ as a stronger
base, cannot be ascribed solely to acidity. The spectra of anionic
form of 1a,b can be obtained by the addition of N(Et)4OH to their
solutions. In the presence of excess base, the absorption spectra of
1a and 1b display distinctive low energy bands at 560 and 595 nm
(Fig. 4 inset, S8 and S10), respectively. These spectral features were
totally different from those obtained when 1a,b were treated with
either Fꢀ or H2POꢀ4 . Even for the same dye, the spectral responses to
Fꢀ and H2POꢀ4 were also different. For example, when 1a was
treated with H2POꢀ4 , the long wavelength absorption at 455 nm
diminished along with the increase of peak at 474 nm (S7). Yet
when it was treated with Fꢀ, the new peak appeared at 504 nm
(S6). Moreover, the anion of 1b exhibited a weak emission centered
at 650 nm (inset in Fig. 4), which did not appear upon reacting with
either Fꢀ or H2POꢀ4 . All these information revealed that the in-
teractions of 1a,b with the anions must involve a direct association
with each other.
The binding stoichiometry of 1a,b with both Fꢀ and H2PO4ꢀ was
determined by Job plots and was found to be 1:1. However, their
binding geometries cannot be resolved unambiguously, as in-
dicated by the fuzzy isosbestic points in their titration spectra
(Fig. 4 inset, S6, S7, and S9). Earlier reports on diindolylquinoxaline
(DIQ) have shown that extended polymeric structures may exist in
the association reaction with dihydrogen phosphate.13 A more
flexible bonding conformation of these complexes rendered them
difficult for crystallization.
The association constants were estimated by using the emission
quenching data and the results are given in Table 1. The binding
strength of 1a,b with Fꢀ (K¼0.6–3.9ꢁ103) is slightly lower than that
with H2POꢀ4 (K¼1.5–4.0ꢁ103). The slightly lower affinity of H2POꢀ4
with 1b than that with 1a may be ascribed to a steric hindrance
imposed by the methyl substituents of indoles. The K values of 1a,b
with H2POꢀ4 are about two orders of magnitude than that with DPQ
(1.8ꢁ104).
The physical nature of molecule 2 was further confirmed by
computational models based on the density functional theory (DFT)
with the three-parameter Becke-style hybrid functional (B3LYP)
and the 6-31G(d) basis set for geometry optimizations. In the
neutral form of 2 at ground state, the two pyrrolyl rings are twisted
from each other forming a significant dihedral angle of 42ꢂ. When
deprotonated it formed a mono-anion, with the two pyrrolyl rings
coplanar with the central ring of maleimide (Fig. 5). Such a planar
conformation is favored by the formation of a cyclic 7-member ring
through H-bonding as depicted in Eq. 1. The high stability of the
ring structure rendered 2 more acidic than both 1a and 1b.
The selectivity of anion recognition in the presence of other
anions was also examined by a series of competitive tests. The re-
sults indicated that the sensing effect of either Fꢀ or H2PO4ꢀ by 1a
was not influenced by the presence of other anions (S13). The anion
1.0
0.5
0.0
1.0
0.5
0.0
600
700
Wavelength (nm)
800
300
400 500
Wavelength (nm)
600
700
Figure 3. Absorption (left) and fluorescence (right) spectra of 2 (10
542 and 414 nm. Fluorescence spectra were taken by irradiating at 542 nm.
m
M) upon the addition of Fꢀ (0–0.25 mM in CH2Cl2). In the absorption spectra isosbestic points were observed at

Z. Lin et al. / Tetrahedron 65 (2009) 5216–5221
5219
1.0
0.5
0.0
1b
60 eq base
80 eq base
1b
1.0
60 eq base
80 eq base
500
600
700
800
nm
300
400
500
600
700
0.5
0.0
300
400
500
Wavelength (nm)
600
550
600
650
700
Wavelength (nm)
750
800
Figure 4. Absorption (left) and fluorescence (right) spectra of 1b (10
486 nm. Inset in the absorption spectrum indicates the changes upon the addition of N(Et)4OH.
m
M) upon the addition of H2POꢀ4 (0–2.5 mM in CH2Cl2). Fluorescence spectra were taken by irradiating at
The spectra of 2 and its mono-anion were simulated by TDDFT in
both the gas phase and under an external electric field to mimic the
polarity of solvent. The wavelengths, worked out byan image charge
approximation (ICA), correlated remarkably well with the experi-
mental data (Table 2 and Tables T1–T4). For example, a simulated
wavelength for the absorption of 2 at 496 nm fits well to the ex-
perimental value of 517 nm, and that of 2ꢀ at 592 nm fits to the
experimental value of 584 nm. For the emission spectra, the calcu-
lated value of wavelength for 2ꢀ at 718 nm not only matches nicely
with the fluorescence observed at 715 nm (Fig. 3), but also explains
its weak intensity by a small value of oscillator strength (0.0187).
Thus the computational results provided a strong support for an
acid–base type interaction of 2 with both Fꢀ and CNꢀ.
The charge-transfer nature of the electronic transitions of 2 and
its mono-anion are better illustrated by the drawings of molecular
orbitals. The electron density distributions of HOMO and LUMO can
be depicted in Figure 6. As expected the electron density in the
HOMO of 2 is mainly populated on the two symmetrically equiva-
lent pyrrolyl moieties, while the LUMO is localized on the central
maleimide moiety. Such an electronic configuration does not
change much for the anion 2ꢀ when 2 is deprotonated. The planar
conformation of the anion should have an effect of enhancing
charge delocalization across the separated rings in the ground state
as well as in the excited state. As a result a narrower band gap is
predicted, that is indeed verified by the longer wavelengths in both
the absorption and emission spectra of the anion.
types of binding geometries, such as the extended structures in DIQ
in more concentrated solutions or condensed phases.
In summary, bifunctional maleimide derivatives with indolyl
groups (1a and 1b) and pyrrolyl groups (2) were found colorimetric
sensitive to certain anions. However, the color changing mecha-
nisms of 1a,b and 2 are not the same. The sensing property of 2 was
based on an acid–base reaction, while a stable mono-anion of 2
formed in the presence of a stronger base. However, the spectro-
scopic characteristics of 1a,b did not comply with the formation of
anions while reacting with either Fꢀ or H2PO4ꢀ. Theoretical esti-
mation by B3LYP/6-31G supported the association of 1:1 complexes
in dilute solutions. The association constants of 1a,b with dihy-
drogen phosphate are measured in the proximity of w103, which is
better than literature reported values of DPQ and DIQ.
3. Experimental
3.1. General
1H and 13C NMR spectra were recorded using Bruker AVA300
spectrometer. The chemical shifts (d, ppm) are referenced to tetra-
methylsilane (TMS) as an internal standard. Mass spectra were taken
on a Joel JMS 700 double-focusing spectrometer, facilitated with fast
atom bombardment (FAB) for sample ionization. Microanalyses were
completed on a Perkin–Elmer 2400 elemental analyzer. UV–vis ab-
sorbance spectra were recorded using a Hewlett–Packard 8453
spectrophotometer. Fluorescence spectra were recorded using
a Hitachi F-4500 fluorescence spectrometer. All starting materials
and solvents were obtained from commercial suppliers, e.g., N-
benzyl-2,3-dibromomaleimide was obtained from Aldrich Chem-
icals. Dilute solutions used for all photophysical experiments were
prepared in spectroscopic grade solvents. Thin layer chromatography
A final check on the feasibility of forming a 1:1 complex between
1a and H2POꢀ4 was explored by B3LYP with 6-31G basis set. From the
optimized structure in Figure 7, that a 1:1 association of the two is
plausible in dilute solutions. The two indolyl N–H groups chelate the
phosphate forming a comfortable geometry at a potential mini-
mum. It certainly does not rule out the possibility of forming other
Table 2
Calculated parameters for the lowest transitions of compound 2 and its anion, using
TDDFT/B3LYP with 6-31G(d) basis set. Simulations are performed in the gas phase
and under an external electric field
TDDFT
Compound 2
Mono-anion of 2
Absorption
Emission
Absorption
Emission
Gas phase
ICAc
eV (nm)a
f1
2.54 (488)
0.1922
2.25 (551)
0.1837
2.11 (587)
0.173
1.95 (637)
0.1638
b
eV (nm)a
2.50 (496)
0.1617
2.28 (543)
0.1223
2.09 (592)d
0.1444
1.73 (718)
0.0187
b
f1
All other transitions are between S0 and S1.
a
Transition energy.
Oscillator strength.
Image charge approximation (ICA) as an external electric reaction field.
S0/S3 as the most probable transition.
b
c
Figure 5. Optimized geometry of the mono-anion of 2. Side view shows the coplanar
conformation of the two pyrrole rings and the maleimide ring.
d

5220
Z. Lin et al. / Tetrahedron 65 (2009) 5216–5221
Figure 6. Frontier MOs of 2 and its anion. (a) HOMO and (b) LUMO of 2; (c) HOMO and (d) LUMO of 2ꢀ
.
was performed on MERCK Silica Gel 60 thick layer plates. Column
chromatography was performed on Sorbent Technologies brand
silica gel (40–63 mm, Standard grade). The association constants, K,
to simulate an external electric reaction field, was performed
according to our previous published procedure.15 All analyses were
performed by using the Q-Chem 3.0 package.
were determined by using the software SPECFIT 3.0 from Spectrum
Software Associates, which employs a global system with expanded
factor analysis and Marquardt least-squares minimization to obtain
globally optimized parameters.
Geometry optimizations were completed according to the
density functional theory (DFT) using B3LYP/6-31G(d) settings. For
approximated structures of vibrationally relaxed LE state (emission
state), they were optimized on their S1 surface at the CIS/6-31G(d)
level. For the excited state parameters, time-dependent density
functional theory (TDDFT) with the B3LYP/6-31G(d) settings was
employed. The image charge approximation (ICA), which was used
3.2. 2,3-Bis(3-indolyl)-N-benzylmaleimide (1a)
A similar procedure to the preparation of 1b was employed from
indole and N-benzyl-2,3-dibromomaleimde. Compound 1a was
obtained as orange powders in 70% yield. Anal. Calcd for
C27H19N3O2: C, 77.68; H, 4.59; N, 10.07. Found: C, 77.70; H, 4.92; N,
9.75. 1H NMR (CD3COCD3, 300 MHz):
d
4.85 (s, 2H), 6.62 (t, J¼7.6 Hz,
2H), 6.93 (d, J¼8.1 Hz, 2H), 7.02 (t, J¼7.6 Hz, 2H), 7.28–7.47 (m, 7H),
7.90 (s, 2H), 10.87 (s, 2H). 13C NMR (CD3COCD3, 75 MHz):
41.27,
d
106.62, 111.52, 119.55, 121.46, 121.89, 125.91, 127.33, 127.41, 127.89,
128.50, 129.16, 136.38, 137.78, 171.65. HRMS (FAB) m/z [M]þ calcd
417.1477, found 417.1478.
3.3. 2,3-Bis(20-methyl-3-indolyl)-N-benzylmaleimide (1b)
To
a toluene (10 mL) solution of 2-methylindole (0.85 g,
6.5 mmol) was added EtMgCl (3 M in THF, 2.16 mL) at room tem-
perature under a nitrogen atmosphere, and the solution was heated
to 45 ꢂC for 45 min. A solution of N-benzyl-2,3-dibromomaleimde
(0.50 g, 1.45 mmol) in toluene (10 mL) was then added to the above
solution, which was heated to reflux for 12 h. The solution was
cooled to room temperature and diluted with ethyl acetate (20 mL).
The solution was successively washed with 1 N aqueous HCl
(30 mL), water (30 mL), and brine (30 mL). It was then dehydrated
over anhydrous MgSO4 and dried in vacuo, forming a dark red res-
idue. The crude product was purified by column chromatography
with EA/hexane (1:3) as the eluant, affording compound 1b in 75%
yield (0.48 g). Anal. Calcd for C29H23N3O2: C, 78.18; H, 5.20; N, 9.43.
Figure 7. Optimized structure (B3LYP/6-31G) of 1a/H2POꢀ4 complex.

Z. Lin et al. / Tetrahedron 65 (2009) 5216–5221
5221
Found: C, 78.17; H, 5.09; N, 9.43. 1H NMR (CD3COCD3, 300 MHz):
Economic Aspects, The Minerals, Metals & Materials Society Annual Meeting,
New Orleans, Louisiana, February 12–15, 2001; (b) Wiseman, A. Handbook of
Experimental Pharmacology XX/2, Part 2; Springer: Berlin, 1970; (c) Kirk, K. L.
Biochemistry of the Halogens and Inorganic Halides; Plenum: New York, NY,
1991; (d) Stryer, L. Biochemistry, 4th ed.; W.H. Freeman: New York, NY, 1995.
3. (a) Dreisbuch, R. H. Handbook of Poisoning; Lange Medical: Los Altos, CA, 1980;
(b) Kulig, K. W. Cyanide Toxicity; U.S. Department of Health and Human Services:
Atlanta, GA, 1991; (c) Baird, C.; Cann, M. Environmental Chemistry; Freeman: New
York, NY, 2005.
d
2.10 (s, 6H), 4.86 (s, 2H), 6.77 (t, J¼7.3 Hz, 2H), 6.96 (t, J¼7.3 Hz, 2H),
7.14 (d, J¼7.9 Hz, 2H), 7.26 (d, J¼7.9 Hz, 2H), 7.35 (m, 1H), 7.40 (t,
J¼7.3 Hz, 2H), 7.49 (d, J¼7.3 Hz, 2H), 10.59 (s, 2H). 13C NMR
(CD3COCD3, 75 MHz):
d 13.52, 42.15, 105.02, 111.33, 120.28, 120.70,
121.97, 128.01, 128.15, 128.70, 129.38, 132.59, 136.84, 138.26, 138.70,
171.88. HRMS (FAB) m/z [M]þ calcd 445.1790, found 445.1782.
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3.4. 2,3-Bis(2-pyrrolyl)-N-benzylmaleimide (2)
A similar procedure to the preparation of 1b was employed from
pyrrole and N-benzyl-2,3-dibromomaleimde. Compound 3 was
obtained as dark purple solids in 10% yield. Anal. Calcd for
C19H15N3O2: C, 71.91; H, 4.76; N, 13.24. Found: C, 72.31; H, 4.92; N,
12.76. 1H NMR (CD3COCD3, 300 MHz):
d
4.75 (s, 2H), 6.35 (dt, J¼2.6,
4.1 Hz, 2H), 7.01 (td, J¼2.6, 1.2 Hz, 2H), 7.29–7.40 (m, 7H), 10.40 (s,
2H). 13C NMR (CD3COCD3, 75 MHz):
41.06, 109.85, 113.57, 118.86,
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d
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122.31,122.92,127.42,127.72,128.51,137.19,171.85. HRMS (FAB) m/z
[M]þ calcd 317.1164, found 317.1158.
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Acknowledgements
This work was supported by both the National Science Council
and Academia Sinica of The Republic of China.
Supplementary data
Spectra of solvent shifts of 1a, 1c, and 2 (S1–S3). Absorption and
fluorescence spectra of 1a with Fꢀ, H2PO4ꢀ, and OHꢀ (S6–S8), 1b
with Fꢀ and OHꢀ (S9, S10), 2 with CNꢀ and OHꢀ (S11, S12). Pho-
tographs of solutions of 1a and 2 with a series of anions (S4, S5).
Competitive tests of 1a with Fꢀ in the presence of other anions
(S13), and those of 2 with Fꢀ and CNꢀ in the presence of other
anions (S14, S15). Computational results of photophysics of dye 2
using TDDFT B3LYP/6-31G(d) basis set with image charge approx-
imation (ICA) as external electric reaction field (Tables T1–T4).
Supplementary data associated with this article can be found in the
online version, at doi:10.1016/j.tet.2009.04.090.
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