A-D-A sensors based on naphthoimidazoledione and boronic acid as turn-on cyanide probes in water

Article abstract of DOI:10.1021/jo900170r

(Chemical Equation Presented) Three fluorescence sensors based on naphthoquinoneimidazole and boronic acid (A-D-A system) have been developed with high selectivity for cyanide in water. The fluorescence band at 460 nm was switched on upon substitution of

Full text of DOI:10.1021/jo900170r

SCHEME 1. Synthesis Pathway of 3a, 3b, and 3c
A-D-A Sensors Based on Naphthoimidazoledione
and Boronic Acid as Turn-On Cyanide Probes in
Water
Matinee Jamkratoke,† Vithaya Ruangpornvisuti,†
Gamolwan Tumcharern,‡ Thawatchai Tuntulani,† and
Boosayarat Tomapatanaget†,*
Supramolecular Chemistry Research Unit, Department of
Chemistry, Faculty of Science, Chulalongkorn UniVersity,
Bangkok 10330, Thailand, and National Nanotechnology
Center, National Science and Technology DeVelopment
Agency, Patumthanee 12120, Thailand
tboosayarat@gmail.com
oxazine,4 squarine,5 dipyrrole carboxamide,6 acridinium salt,7
acyltriazenes,8 and coumarin9derivatives were used as sensors
for detecting cyanide employing the nucleophilic substitution
reaction.
ReceiVed January 26, 2009
From a variety of signal transductions, many researchers are
interested in taking the profit of the internal charge transfer (ICT)
process, which is well-known to be extremely sensitive to small
perturbations.10 Usually, common ICT organic dyes consist of
electron acceptor and electron donor sites.11 To be a chemosen-
sor, the interaction of a guest molecule (cation or anion) with
an electron donor or an acceptor site must induce the fluores-
cence or absorption changes. Recently, chemists have paid
attention to boronic acid based sensors for detecting the cyanide
ion. Although there were many reports regarding highly sensitive
chemosensors for the cyanide ion, a few sensors gave a large
wavelength shift and high intensity change in their emission
spectra.2 It is a challenging task to design turn-on cyanide probes
which can perform a fluorescence intensified signal with a large
shift. Therefore, we have developed high response fluorescence
probes using an acceptor-donor-acceptor (A-D-A) system. The
A-D-A sensors are composed of naphthoquinone connecting to
boronic acid with imidazole as a conjugative spacer. Naptho-
quinone and imidazole groups are a main A-D system that is
expected to have a large dipole moment change when excited
Three fluorescence sensors based on naphthoquinoneimida-
zole and boronic acid (A-D-A system) have been developed
with high selectivity for cyanide in water. The fluorescence
band at 460 nm was switched on upon substitution of cyanide
on sensors in the CTAB micelle.
Cyanide ion is one of the analytes of most concern in
environments due to its toxicity. The toxicity of the cyanide
ion is attributed to its ability to effectively bind with the active
site of cytochrome c oxidase resulting in the disruption of the
electron transport chain. Tissues that mainly depend on aerobic
respiration, such as the central nervous system and the heart,
are acutely affected.1 Fluorescent detection of CN- is one of
the best tools because of its excellent sensitivity. Several organic
and inorganic compounds such as boronic acid,2 cationic boran,3
(3) Hudnall, T. W.; Gabbai, F. P. J. Am. Chem. Soc. 2007, 129, 11978–
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71, 744–753. (c) Ren, J.; Zhu, W.; Tian, H. Talanta 2008, 75, 760–764.
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2249.
† Chulalongkorn University.
‡ National Science and Technology Development Agency.
(1) (a) Hathaway, G. J.; Proctor, N. H. Chemical Hazards of the Workplace,
5th ed.; John Wiley & Sons, Inc: Hoboken, NJ, 2004; pp 190-191. (b) Patnaik,
P. A ComprehensiVe Guide to the Hazardous Properties of Chemical Substance;
van Nostrand Reinhold: New York, 1992; pp 229-244. (c) Ishii, A.; Seno, H.;
Watanabe-Suzuki, K.; Suzuki, O.; Kumazawa, T. Anal. Chem. 1998, 70, 4873–
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1, 157–170. (b) Badugu, R.; Lakowicz, J. R.; Geddes, C. D. Dyes Pigments
2005, 64, 49–55. (c) Badugu, R.; Lakowicz, J. R.; Geddes, C. D. Anal. Biochem.
2004, 327, 82–90. (d) Badugu, R.; Lakowicz, J. R.; Geddes, C. D. J. Am. Chem.
Soc. 2005, 127, 3635–3641. (e) Badugu, R.; Lakowicz, J. R.; Geddes, C. D.
Anal. Chem. Acta 2004, 522, 9–17.
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5053–5056.
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Plenum: New York, 1994.
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10.1021/jo900170r CCC: $40.75
Published on Web 04/16/2009
2009 American Chemical Society
J. Org. Chem. 2009, 74, 3919–3922 3919

by light.11,12 These sensors utilize an alternation of electron
deficiency to electron rich of the boron center to give a distinct
change in their spectra. This process is stimulated by cyanide
substitution on the boron atom.
The synthetic pathway of new fluorescent sensors 3a, 3b,
and 3c is shown in Scheme 1. The oxidative condensation13 of
2,3-diamino-1,4-naphthoquinone14 and the protected corre-
sponding formylphenylboronate in nitrobenzene yielded the
precipitate of heterocyclic compounds 2a, 2b, or 2c in 29%,
45%, and 71% yield, respectively. These products were indicated
by the appearance of the NH proton at ca. 14.3 ppm and the
new series of aromatic protons at around 8.09 and 7.84 ppm in
the 1H NMR spectra. Methylation on the N-atom of the
heterocyclic ring was achieved by the reaction of the protected
derivative with CH3I in the presence of NaH in N,N-dimethy-
lacetamide at room temperature. Finally, the ester protecting
group was removed by refluxing in 30% H2O:CH3CN to give
3a, 3b, and 3c as yellow solids in 20%, 40%, and 45% yields,
respectively.
In the mixture of DMSO:H2O (50 µM of receptor in 0.1 mol/L
of NaCl in 50% HEPES pH 7.4:DMSO), sensors 3a, 3b, and
3c gave yellow solutions with absorption maxima at 338 and
387 nm. Moreover, all sensors showed the characteristic
luminescence with different emission maxima at 562, 565, and
572 nm for ortho, meta, and para isomers, respectively. Figure
1a shows fluorescence spectra of compound 3b with and without
500 equiv of anions. A high-intensity fluorescence band at 460
nm was observed upon addition of cyanide into solutions of
sensor 3b. The high response was also observed in sensor 3c,
whereas the ortho isomer, 3a, showed a slight response in the
presence of CN- (Figure 1b). Furthermore, upon exposure to
UV irradiation at 365 nm, the solution of 3b + CN- gave a
brighter luminescence than that of 3b + F- as shown in Figure
2.
FIGURE 1. (a) Fluorescence spectral changes of 3b after the addition
of 500 equiv of various anions (potassium salts). (b) Fluorescence
responses of 3a, 3b, and 3c with 500 equiv of various anions (50 µM
of receptor in 0.1 mol/L of NaCl in 50% HEPES pH 7.4:DMSO).
In the presence of a very high concentration of cyanide ion
(500 equiv), pH 7.4 HEPES solution was changed to pH 11. It
is possible that at high pH, OH- could be generated and
-
substituted on the boron center to form RB(OH)3 . This weak
point was averted by incorporating compounds 3b and 3c to a
CTAB micellar system to allow the detection of CN- at vary
low concentration (50 µM). In this system, the pH of the solution
remained pH 7.0 and the emission band at 460 nm appeared
intensely upon the addition of 50 µM CN- as shown in Figure
3 as well as Figure S9 in the Supporting Information.
Moreover, the probe 3b in the CTAB micellar system
exhibited promising selective binding with cyanide ion similar
to that in 50% HEPES pH 7.4:DMSO. This suggested that the
boron center was substituted with the cyanide anion. The
hybridization of the boron center changed from the electron
deficiency sp2 boron, R-B(OH)2, to the electron rich sp3 boron,
R-B(CN)3,15,16 resulting in the emission band at 460 nm.
Fluorescence responses of sensors 3b and 3c in the presence
of 50 µM of CN- showed a large response giving a high
FIGURE 2. Fluorescence responses of 50% HEPES pH 7.4:DMSO
solutions of 3b (left), 3b + CN- (middle), and 3b + F- (right).
intensity change (18-fold enhancement) and a large stoke shift
(∆λex-em ) 120 nm) as well as a large blue shift of ca. 100 nm.
The fluorescence band at 460 nm possibly assigned to the ICT
state was switched on by the cyanide ion. This ICT state was
predominant with a poor acceptability on the boron center after
cyanide addition. A large fluorescence response toward CN-
recognition of these receptors occurred since the boronic center
behaved as a complementary acceptor of a standard electron
donor-acceptor system (D-A or A-D).11 The presence of the
electron donor (imidazole group) and the electron acceptor
(naphthoquinone group) as a main A-D site caused a prerequisite
dipole moment change resulting in a large change in spectral
properties.11
(12) de Silva, A. P.; Gunaratne, H. Q. N.; Habib-Jiwan, J. L.; McCoy, C. P.;
Rice, T. E.; Soumillion, J. P. Angew. Chem., Int. Ed. Engl. 1995, 34, 1728–
1731.
(13) Kim, S. J.; Kool, E. T. J. Am. Chem. Soc. 2006, 128, 6164–6171.
(14) Wunkelmann, E. Tetrahedron 1969, 25, 2427–2454.
(15) (a) Lorand, J. P.; Edward, J. O. J. Org. Chem. 1959, 24, 769–774. (b)
Katz, H. E J. Am. Chem. Soc. 1986, 108, 7640–7645.
(16) (a) Cooper, C. R.; Spencer, N.; James, T. D. Chem. Commun. 1998,
1365–1366. (b) DiCesare, N.; Lakowizc, J. R. Anal. Biochem. 2002, 301, 111–
116. (c) Kubo, Y.; Kobayashi, A.; Ishida, T.; Misawa, Y.; James, T. D. Chem.
Commun. 1998, 1365–1366. (d) Badugu, R.; Lakowizc, J. R.; Geddes, C. D.
Sens. Actuators. 2005, 104, 103–110.
For better understanding of the fluorescence behaviors,
structures of the sensors 3a-c and their cyanide-substituted
products have been calculated by using density function
theory (DFT) at the B3LYP/6-31+G(d) level. Natural bond
orbital (NBO) charges of donor-acceptor segments of sensors
3920 J. Org. Chem. Vol. 74, No. 10, 2009

3b and 3c to provide powerful probes for detecting a very low
concentration of CN- in water. These A-D-A probes offer
considerable promises as cyanide selective fluorescence probes
with a large response emission band at 460 nm and a large blue
shift (ca. 100 nm).
Experimental Section
Compounds 2a, 2b, and 2c. The protected formylphenylbor-
onate was synthesized according to literature procedure.17 To a
solution of 2,3-diamino-1,4-naphthaquinone (0.940 g, 5 mmol) in
nitrobenzene (75 mL) was added the protected corresponding
formylphenylboronate (5 mmol) in nitrobenzene (25 mL) dropwise.
ꢀ
The reaction mixture was heated at 150 C and stirred for 12 h
under nitrogen. The solution was then cooled to room temperature
and the precipitate was slowly formed. The precipitate was filtered
and washed with diethyl ether to give a yellow solid of the
protecting products (2a 29%, 2b 45%, and 2c 71%).
FIGURE 3. Fluorescence spectra of 3b (50 µM) with 50 µM of various
anions in the CTAB micellar system (5 mM of CTAB in 10% ethanol:
water).
Compound 2a: 1H NMR (400 MHz, DMSO-d6) δ 14.40 (broad,
1H), 8.12 (m, 3H), 7.84 (m, 2H), 7.47 (m, 3H), 3.74 (s, 4H), 1.06
(s, 6H). Anal. Calcd for C22H19BN2O4: C, 68.42; H, 4.96; N, 7.25.
Found: C, 67.99; H, 4.82; N, 7.55. EI-MS m/z 387.15 (M + 2H)+.
Compound 2b: 1H NMR (400 MHz, DMSO-d6) δ 14.42 (s, 1H),
8.57 (s, 1H), 8.26 (d, J ) 7.6 Hz, 1H), 8.083 (m, 2H), 7.83 (m,
2H), 7.77 (d, J ) 7.6 Hz, 1H), 7.50 (t, J ) 7.2 Hz, 1H), 4.14 (t, J
) 4.8 Hz, 4H), 2.02 (t, J ) 5.2 Hz, 2H). Anal. Calcd for
C22H19BN2O4: C, 67.07; H, 4.22; N, 7.82. Found: C, 66.96; H, 4.23;
N, 7.88. MALDI-TOF m/z 359.64 (M + 2H)+.
TABLE 1. NBO Charges of Segments of 3a, 3b, and 3c and of
-
3a(CN)3-, 3b(CN)3-, and 3b(CN)3 Derived from the B3LYP/
6-31+G(d) Computations
Compound 2c: 1H NMR (400 MHz, DMSO-d6) δ 14.39 (s, 1H),
8.19 (d, J ) 8.5 Hz, 2H), 8.09 (m, 2H), 7.49 (m, 2H), 7.78 (d, J )
8.0 Hz, 2H), 4.12 (t, J ) 4.1 Hz, 4H), 2.01 (t, J ) 5.2 Hz, 2H).
Anal. Calcd for C20H15BN2O4: C, 67.07; H, 4.22; B, 3.02; N, 7.82.
Found: C, 67.06; H, 4.59; N, 7.76. MALDI-TOF m/z 358.63 (M +
H)+.
chargea
left
segment
right
segment
total
species
free hosts
chargea
Compounds 3a, 3b, and 3c. Compounds 2a, 2b, or 2c (5 mmol)
and NaH (0.132 g, 5.5 mmol) were charged with 25 mL of N,N-
dimethylacetamide under nitrogen. Methyl iodide (343 µL, 5.5
mmol) was added to the reaction mixture. The reaction mixture
was stirred at room temperature for 2 days. The solvent was
removed under vacuum to give the solid of methylated products.
The protecting group was removed by refluxing in 30% H2O:
CH3CN. The solution was filtered and washed with diethyl ether
to provide a yellow solid of compound 3 (3a 20%, 3b 40%, and
3c 35%).
3a
3b
3c
-0.02685
-0.03985
-0.02358
0.02685
0.03985
0.02358
0.00
0.00
0.00
full CN-substituted hosts
-
3a(CN)3-
-0.04235
-0.08330
-0.01968
-0.95762
-0.91670
-0.97892
-1.00
-1.00
-1.00
3b(CN)3-
3c(CN)3
a The elementary charge unit ) 1.602 × 10-19 coulombs.
Compound 3a: 1H NMR (400 MHz, DMSO-d6) δ 8.11 (m, 2H),
8.01 (s, 2H), 7.87 (m, 2H), 7.77 (m, 1H), 7.54 (m, 3H), 3.84 (s,
3H). 13C NMR (100.6 MHz, DMSO-d6) δ 178.9, 174.4, 157.0,
142.6, 134.6, 134.5, 134.4, 133.1, 132.7, 129.8, 129.6, 126.8, 126.6,
34.3. Anal. Calcd for C18H23BN2O4: 65.10; H, 3.95; N, 8.43. Found:
C, 65.27; H, 3.98; N, 8.48. MALDI-TOF m/z 334.66 (M + 3H)+.
Compound 3b: 1H NMR (400 MHz, DMSO-d6) δ 8.29 (s, 2H),
8.21 (s, 1H), 8.09 (t, J ) 8.1 Hz, 2H), 7.99 (d, J ) 8.0 Hz, 1H),
7.85 (d, J ) 7.8 Hz, 1H), 7.56 (t, J ) 7.5 Hz, 1H), 4.06 (s, 3H).
13C NMR (100.6 MHz, DMSO-d6) δ 178.9, 176.4, 154.9, 143.0,
136.5, 135.5, 134.5, 134.3, 133.8, 133.3,132.9, 131.3, 128.3, 127.8,
126.8, 126.6, 34.8. Anal. Calcd for C18H23BN2O4: 65.10; H, 3.95;
N, 8.43. Found: C, 65.28; H, 3.90; N, 8.48. MALDI-TOF m/z
331.60 (M)+.
3a-c suggested that napthoquinoneimidazole possessing
negative charges acted as a donor site and boronic acid
possessing positive charges acted as an acceptor site (Table
1). Upon the addition of CN-, the right segment charge
became negative corresponding to the reduction in accept-
ability of boronic acid and subsequent appearance of a
fluorescence band at 460 nm.
Considering the sensitivity of each isomer, calculated struc-
tures (Figure S10, Supporting Information) showed that the
dihedral angles of the donor and acceptor planes of 3a, 3b, and
3c were 56.5°, 37.5°, and 37.7°, respectively. After cyanide
substitution, the dihedral angles of CN-substituted 3b and 3c
changed slightly (∼2°) while that of substituted 3a changed
significantly (nearly 12°). This probably inhibited the substitu-
tion of cyanide in the boronic center of 3a. The calculated
structures thus agreed very well with the experimental results,
which showed that the emission spectra of 3a before and after
cyanide substitution were insignificantly different.
Compound 3c: 1H NMR (400 MHz, DMSO-d6) δ 8.26 (s, 2H),
8.09 (t, J ) 8.0 Hz, 2H), 7.98 (d, J ) 8.0 Hz, 2H), 7.86 (t, J ) 7.8
Hz, 2H), 7.78 (d, J ) 7.8 Hz, 2H), 4.08 (s, 3H). 13C NMR (100.6
MHz, DMSO-d6) δ 178.8, 176.4, 154.5, 143.0, 134.7, 134.5, 134.3,
133.8, 133.3, 132.8, 129.9, 128.7, 126.8, 126.6, 34.8. Anal. Calcd
for C18H23BN2O4: 65.10; H, 3.95; N, 8.43. Found: C, 65.07; H,
3.90; N, 8.56. MALDI-TOF m/z 332.39 (M + H)+.
In summary, we have successfully synthesized A-D-A
fluorescence sensors containing naphthoquinoneimidazole and
boronic acid, 3a, 3b, and 3c, as turn-on cyanide probes in the
CTAB micellar system. In this approach, CTAB can incorporate
(17) James, T. D.; Sandanayake, K. R. S.; Iguchi, R.; Shinkai, S. J. Am.
Chem. Soc. 1995, 117, 8982–8987.
J. Org. Chem. Vol. 74, No. 10, 2009 3921

Fluorescence Measurement Procedure of Sensors 3a-3c
with Excess Anions in 50% HEPES pH 7.4:DMSO. Receptors
3a, 3b, and 3c were prepared as stock solution in spectroscopic
grade DMSO at concentrations of 0.1 mM. Stock solutions of
sensors (1.5 mL) and 500 equiv of anions (KCN, KF, KAcO, KBzO,
KH2PO4, KNO3, KClO4, KCl, KBr, KSCN, and KI) which were
dissolved in 1.5 mL of 0.2 mol/L of NaCl in HEPES buffer pH 7.4
were mixed to give a final concentration of sensors (50 µM) with
500 equiv of anions added in 0.1 mol/L of NaCl in 50% HEPES
pH 7.4:DMSO. Fluorescence spectra were recorded with the
excitation wavelength of 344 nm. In the case of the addition of
KF, stock solution sensors and KF were mixed to give a final
concentration of sensors (50 µM) with 500 equiv of anions added
in 0.1 mol/L of NaCl in 60% HEPES pH 7.4:DMSO because of
KF solubility.
KAcO, KBzO, KH2PO4, KNO3, KClO4, KCl, KBr, KSCN, and KI)
was added to the probe. Fluorescence spectra were recorded with
the excitation wavelength of 344 nm.
Acknowledgment. M.J. is a Ph.D student supported by the
Royal Golden Jubilee Program (PHD/0049/2550). We thank the
National Nanotechnology Center (NN-B-22-b15-94-49-55),
the Thailand Research Fund (RTA5080006), and the National
Center of Excellence for Petroleum, Petrochemical, and Ad-
vanced Materials (NCE-PPAM) for financial support.
Supporting Information Available: Details of the procedure
for the synthesis of protected formylphenylboronate; 1H and 13
C
NMR spectra of 3a, 3b, and 3c; fluorescence spectra changes
of 3a, 3b, and 3c after addition of various anions (potassium
salts); and other data. This material is available free of charge
via the Internet at http://pubs.acs.org.
Fluorescence Measurement Procedure of Sensors 3b with
Excess Anion in the CTAB Micellar System. In a 5 mL
volumetric flask, 1.0 mL of an ethanol solution of fluorescence
probes (0.025 mM) was mixed with 2.0 mL of 12.5 mM CTAB in
water. Then, 10 µL of 2.5 mM potassium salt solution (KCN, KF,
JO900170R
3922 J. Org. Chem. Vol. 74, No. 10, 2009