A new class of fluorescent dye for sensing water in organic solvents by photo-induced electron transfer - A (phenylamino)naphtho[1,2-d]oxazol-2-yl-type fluorophore with both proton-binding and proton-donating sites

Article abstract of DOI:10.1002/ejoc.200800606

A new class of fluorescent dye for sensing water in organic solvents by photo-induced electron transfer (PET), based on a (phenylamino)naphtho[1,2-d] oxazol-2-yl-type fluorophore with both proton binding and proton donating sites, has been designed developed. The fluorophore exhibits a weak emission in organic solvents but a drastic enhancement in the fluorescence intensity is observed with increasing water content in organic solvents. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800606

FULL PAPER
DOI: 10.1002/ejoc.200800606
A New Class of Fluorescent Dye for Sensing Water in Organic Solvents by
Photo-Induced Electron Transfer – A (Phenylamino)naphtho[1,2-d]oxazol-2-yl-
Type Fluorophore with both Proton-Binding and Proton-Donating Sites
Yousuke Ooyama,*[a] Haruka Egawa,[b] and Katsuhira Yoshida*[a,b]
Keywords: Photo-induced electron transfer / Fluorescence / Water / Sensors
A new class of fluorescent dye for sensing water in organic
solvents by photo-induced electron transfer (PET), based on
a (phenylamino)naphtho[1,2-d]oxazol-2-yl-type fluorophore
with both proton binding and proton donating sites, has been
designed developed. The fluorophore exhibits a weak emis-
sion in organic solvents but a drastic enhancement in the
fluorescence intensity is observed with increasing water con-
tent in organic solvents.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
fluorophore with both proton-binding and proton-donating
sites. The fluorophore exhibits a weak fluorescent emission
in organic solvents, but a drastic enhancement in the fluo-
rescence intensity is observed with an increase in water con-
tent in organic solvents.
Introduction
In recent years, many fluorescent dyes that recognize pro-
tons, cations, anions, and neutral organic species such as
saccharides have been developed, and their possible appli-
cations in optical, biochemical, and medical sensors and
optoelectronic devises have been investigated.[1,2] Most fluo-
rescent sensors are composed of a binding site for the target
species and a fluorescent moiety; good communication be-
tween the two sites is crucial for obtaining a guest-specific
fluorescent response. A fluorescent sensor for water in an
organic solvent is of interest to fundamental research in an-
alytical chemistry and important for industrial applications,
especially in electronics and fine chemicals.[3,4] Some fluo-
rescent water sensors have been developed that exhibit
changes in their fluorescence lifetime and intensity with
varying amounts of water in organic solvents.[5] However,
in almost all those cases, the fluorescence intensity de-
creases with an increase in water content in the organic sol-
vents.[6,7] The decrease in fluorescence intensity can be at-
tributed to a specific water–fluorophore interaction, and
also to the increase in the polarity of the solvent mixture.
Therefore, the exact determination of water content in a
solution can be difficult with these fluorescent dyes, because
the fluorescence intensity is strongly affected by the polarity
of the solution. Herein, we report a new class of fluorescent
dye that can be used to sense water in organic solvents by
photo-induced electron transfer (PET). The dye is based on
a substituted (phenylamino)naphtho[1,2-d]oxazol-2-yl-type
Results and Discussion
The synthesis of (phenylamino)naphtho[1,2-d]oxazol-2-
yl-type fluorophores 2a–2e is outlined in Scheme 1. We first
prepared the starting 4-aminated-1,2-naphthoquinones 1a
and 1b in 31% and 78% yields, respectively, by the reaction
of sodium 1,2-naphthoquinone-4-sulfonate with the corre-
sponding phenylamine in acetic acid in the presence of
nickel(II) chloride. Next, fluorophores 2a–2e were synthe-
sized in 14–72% yields by the reaction of p-substituted
benzaldehydes with the 4-aminated-1,2-naphthoquinones.
[a] Department of Material Science, Faculty of Science, Kochi Uni-
versity,
Akebono-cho, Kochi 780-8520, Japan
Fax: +81-88-844-8359
E-mail: yooyama@hiroshima-u.ac.jp
[b] JST (Japan Science and Technology Agency) Satellite Kochi,
2-17-47, Asakurahommachi, Kochi 780-8073, Japan
E-mail: Kyoshida@cc.kochi-u.ac.jp
Scheme 1. Syntheses of fluorophores 2a–2e.
Eur. J. Org. Chem. 2008, 5239–5243
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Y. Ooyama, H. Egawa, K. Yoshida
FULL PAPER
Table 1. Spectroscopic properties of 2a–2e in solution.
abs
fl
Entry
Compound
Solvent
λmax [nm][a] (εmax/dm3 mol–1 cm–1)
λmax [nm][b]
Φ[c]
1
2
3
4
5
6
7
8
2a
benzene
406 (24300), 335 (9000)
407 (24200), 335 (8700)
408 (25400), 336 (8500)
410 (26900), 335 (9200)
416 (24000), 334 (8500)
408 (25200), 334 (9200)
400 (20700), 336 (8600)
401 (26100), 332 (9900)
406 (25000), 335 (9800)
420 (31700), 350 (18600)
420 (25300), 350 (15500)
420 (23700), 349 (14200)
420 (24200), 350 (15000)
431 (25300), 350 (15300)
387 (23300), 311 (11300)
413 (23800), 338 (15400)
413 (21100), 338 (13600)
400 (20700), 328 (13400)
385 (24500), 312 (12800)
415 (25800), 337 (15800)
417 (26000), 339 (16100)
384 (24000), 312 (10000)
477
495
517
545
542
524
491
534
535
485
483
498
527
538
504
490
508
493
520
500
526
509
0.86
0.92
0.83
0.04
0.03
0.04
0.95
0.66
1,4-dioxane
dichloromethane
acetone
ethanol
acetic acid
1,4-dioxane
acetone
acetic acid
benzene
1,4-dioxane
dichloromethane
acetone
2b
2c
9
0.03
10
11
12
13
14
15
16
17
18
19
20
21
22
0.001
0.001
0.002
0.001
0.001
0.18
0.001
0.0003
0.00006
0.20
ethanol
acetic acid
1,4-dioxane
acetone
2d
2e
ethanol
acetic acid
1,4-dioxane
acetone
0.002
0.00004
0.20
acetic acid
[a] c = 2.5ϫ10–5 . [b] c = 2.5ϫ10–6 . [c] The Φ value was determined with 9,10-diphenylanthracene (Φ = 0.67, λex = 357 nm) in
benzene as the standard.
Absorption and fluorescence spectroscopic data of 2a–2e
in solution are summarized in Table 1. Fluorophores 2a and
2b exhibited a weak absorption band at around 335 nm, a
single intense absorption band at around 400 nm, and a sin-
gle intense fluorescence band at around 495 nm (Φ = 0.92–
0.95) in 1,4-dioxane (Figure 1, a). The absorption maxima
of 2a and 2b were affected slightly by changing the solvent
from 1,4-dioxane to acetone, while the fluorescence maxima
showed a large bathochromic shift. Therefore, the Stokes
shift value in polar solvents became greater than that in
nonpolar solvents. A significant dependence of the fluores-
cence quantum yield on the solvent polarity was also ob-
served; the Φ value of 2a was reduced to about 4% with an
increase in polarity on going from 1,4-dioxane to acetone.
Similar spectral changes are generally observed for most
fluorescent dyes with dipole moments in the excited state
that are larger than those in the ground state.[8]
On the other hand, fluorophores 2c–2e exhibited two ab-
sorption bands at around 420 nm and 350 nm and a weak
fluorescence band at around 490 nm in 1,4-dioxane (Fig-
ure 1, b). As with fluorophores 2a and 2b, a dependence of
the fluorescence maxima on the solvent polarity was ob-
served. However, the dependence was smaller for 2d and 2e.
With increasing polarity of the solvent, the Φ value of 2c,
2d, and 2e were also reduced. Interestingly, in acetic acid,
fluorophores 2c–2e exhibited a medium fluorescence inten-
sity (Φ = 0.18–0.20), while fluorophores 2a and 2b exhibited
a low fluorescence intensity (Φ = 0.03–0.04). The absorp-
tion bands of 2c–2e appeared at around 385 nm, which was
blue-shifted by about 30 nm, and the absorption bands at
around 350 nm became broader than those observed in 1,4-
dioxane. The corresponding fluorescence maxima were ob-
served at around 500–520 nm, which were red-shifted by
10–30 nm compared with those in 1,4-dioxane.
Figure 1. Absorption and fluorescence spectra in 1,4-dioxane (dot-
ted line) and acetic acid (solid line): a) 2a (black), 2b (red); b) 2c
(blue), 2d (green), and 2e (orange).
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Eur. J. Org. Chem. 2008, 5239–5243

Fluorescent Dye for Sensing Water in Organic Solvents
The photophysical properties of 2d in a mixture of or- tonation of the diethylamino group by intramolecular pro-
ganic solvent and water were particularly interesting. The ton transfer from the carboxyl group, because the addition
absorption and fluorescence spectra underwent only small of water to organic solvents promoted the dissociation of
changes on changing from 1,4-dioxane to 1,4-dioxane con- the carboxyl proton of 2d. Consequently, we concluded that
taining 15 vol.-% water (Figure 2). Upon increasing the
water content from 30 vol.-% to 45 vol.-%, the absorption
and fluorescence spectra exhibited significant changes. The
absorption band at around 410 nm was blue-shifted by
20 nm, and the absorption band at around 340 nm broad-
ened. In the corresponding fluorescence spectra, the fluo-
rescence band at around 490 nm was slightly blue-shifted
with the enhancement in its intensity. In 1,4-dioxane con-
taining 45 vol.-% water, a fluorescence quantum yield of
0.18 was achieved (Table 2). In acetone or ethanol contain-
ing 45 vol.-% water, similar absorption and fluorescence
changes were observed. However, such absorption and fluo-
rescence properties were not observed in the case of 2a, 2b,
2c, and 2e. With increasing concentrations of water in 1,4-
dioxane, the fluorescence intensities of 2a and 2b decreased
dramatically, and the fluorescence maxima underwent a red
shift, due to the increase in the polarity of the solution. On
the other hand, there are the fluorescent dyes such as the
1,8-naphthalimide derivatives, whose fluorescence efficiency
in water is higher than that in organic solvents because of
a change in the microscopic polarity around the fluoro-
phore.[9] Although the compounds 2c–2e belong to the
same category because they all contain a diethylamino
group as a proton acceptor and similar dipole moments,
only 2d, with a carboxyl group as a proton donor, exhibited
enhancements in the fluorescence intensity with increasing
concentrations of water in 1,4-dioxane. Therefore, the en-
hancement of fluorescence intensity of 2d would not be at-
tributable to a change in the microscopic polarity around
the fluorophore with increasing concentrations of water in
1,4-dioxane.
On the bases of the above results, we considered that the
Figure 2. a) Absorption and b) fluorescence spectra of 2d in water/
absorption and fluorescence properties of 2d in a mixture
1,4-dioxane with different volume fractions of water ([2d] =
of organic solvent and water were associated with the pro- 2.5ϫ10–5 ).
Table 2. Spectroscopic properties of 2a–2e in solvent/water mixtures.
λmax [nm][a] (εmax/dm3 mol–1 cm–1)
λmax [nm][b]
Φ[c]
abs
fl
Entry
Compound Solvent
1
2
3
4
5
6
7
8
2a
2b
2c
2d
1,4-dioxane (water-free)
1,4-dioxane/water (45 vol.-%)
1,4-dioxane (water-free)
1,4-dioxane/water (45 vol.-%)
1,4-dioxane (water-free)
1,4-dioxane/water (45 vol.-%)
1,4-dioxane (water-free)
1,4-dioxane/water (15 vol.-%)
1,4-dioxane/water (30 vol.-%)
1,4-dioxane/water (40 vol.-%)
1,4-dioxane/water (45 vol.-%)
acetone (water-free)
acetone/water (45 vol.-%)
ethanol (water-free)
ethanol/water (45 vol.-%)
1,4-dioxane (water-free)
1,4-dioxane/water (45 vol.-%)
407 (24200), 335 (8700)
414 (21900), 334 (8300)
400 (20700), 336 (8600)
396 (22700), 335 (10800)
420 (25300), 350 (15500)
426 (21200), 340 (14000)
413 (23800), 338 (15400)
413 (23300), 338 (15400)
411 (21900), 338 (15600)
404 (20100), 338 (15800)
392 (19800), 338 (15900)
413 (21100), 338 (13600)
396 (–)[e], 337 (–)[e]
495
548
491
508
483
534
490
490
490
487
486
508
495
493
495
500
541
0.92
0.02
0.95
0.04
[d]
–
0.02
[d]
–
[d]
–
9
0.02
0.03
0.18
10
11
12
13
14
15
16
17
[d]
–
0.08
[d]
400 (20700), 328 (13400)
–
390 (–)[e], 325 (–)[e]
0.17
[d]
2e
415 (25800), 337 (15800)
419 (–)[e], 338 (–)[e]
–
[d]
–
[a] c = 2.5ϫ10–5 . [b] c = 2.5ϫ10–6 . [c] The Φ values were determined with a calibrated integrating sphere system (λex = 325 nm). [d]
Too weak (Φрр0.01) to be measured. [e] Poor solubility.
Eur. J. Org. Chem. 2008, 5239–5243
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5241

Y. Ooyama, H. Egawa, K. Yoshida
FULL PAPER
this fluorescent dye exhibits PET,[1] which in this case takes
place from the diethylamino group to the naphtho[1,2-d]-
oxazole fluorophore skeleton, and can not exhibit fluores-
cent emission. When the diethylamino group was proton-
ated, the structure became the zwitterionic form, electron
transfer was prevented, and a drastic fluorescence enhance-
ment was observed (Figure 3). Thus, fluorophores 2a (with-
out both the carboxyl and diethylamino groups), 2b (with-
out a diethylamino group), and 2c and 2e (without a car-
boxyl group) cannot function as fluorescent dye by PET.
Instead, the fluorescence properties of 2c–2e in acetic acid
are assumed to be caused by protonation of the di-
ethylamino group by acetic acid.
Experimental Section
General: IR spectra were recorded with a JASCO FT/IR-5300 spec-
trophotometer for samples in KBr pellet form. Absorption spectra
were observed with a JASCO U-best30 spectrophotometer. For the
measurement of the emission spectra, a JASCO FP-777 spectrome-
ter was used. In Table 1, the fluorescence quantum yields (Φ) were
determined with 9,10-diphenylanthracene (Φ = 0.67, λex = 357 nm)
in benzene as the standard. In Table 2, the fluorescence quantum
yields (Φ) were determined with a calibrated integrating sphere sys-
tem (λex = 325 nm). Elemental analyses were recorded with a Per-
kin–Elmer 2400 II CHN analyzer. 1H NMR spectra were recorded
with a JNM-LA-400 (400 MHz) FT NMR spectrometer with tet-
ramethylsilane (TMS) as an internal standard. Column chromatog-
raphy was performed with silica gel (KANTO CHEMICAL, 60 N,
spherical, neutral).
4-[(4-Butylphenyl)amino]-1,2-naphthoquinone (1a): A solution of
1,2-naphthoquinone-4-sulfonic acid sodium salt (25 g, 96.1 mmol),
p-butylaniline (14.3 g, 96.1 mmol), sodium acetate (7.88 g,
96.1 mmol), and NiCl2 (12.45 g, 96.1 mmol) in acetic acid (150 mL)
was stirred at room temperature for 5 d. The solution was neutral-
ized with aqueous Na2CO3, and the resulting precipitate was fil-
tered. The resulting precipitate was extracted with CH2Cl2. The
organic extract was washed with water. The CH2Cl2 extract was
concentrated, and the residue was chromatographed on silica gel
(CH2Cl2/ethyl acetate, 5:1 as the eluent) to give 1a (8.91 g, 31%) as
1
a red powder; m.p. 234–236 °C. H NMR (400 MHz, [D6]acetone,
Figure 3. Proposed mechanism for the fluorescent water-sensor 2d
by PET.
TMS): δ = 0.95 (t, 3 H), 1.34–1.44 (m, 2 H), 1.61–1.68 (m, 2 H),
2.67 (t, 2 H), 6.14 (s, 1 H), 7.14 (br., 2 H), 7.33 (d, J = 8.28 Hz, 2
H), 7.73–7.77 (m, 1 H), 7.84–7.88 (m, 1 H), 8.02 (dd, J = 1.44,
In order to investigate its possibility as a pH sensor, the
absorption and fluorescence spectra of 2c were investigated
in solutions of varying acidity, prepared by the addition of
acetic acid or trifluoroacetic acid to a 1,4-dioxane solution.
When the acid/2c ratio was 20, the fluorescence was weak
in acetic acid, but a strong fluorescence band appeared at
around 490 nm in trifluoroacetic acid. These results demon-
strated that the (phenylamino)naphtho[1,2-d]oxazol-2-yl-
type fluorophores are promising for use as pH sensors.
7.89 Hz, 1 H), 8.34 (br., 1 H) ppm. IR (KBr): ν = 3222, 1606, 1579
˜
cm–1. C20H19NO2 (305.37): calcd. C 78.66, H 6.27, N 4.59; found
C 78.94, H 6.25, N 4.56.
4-{[4-(Diethylamino)phenyl]amino}-1,2-naphthoquinone (1b): To a
solution of 1,2-naphthoquinone-4-sulfonic acid sodium salt (9.48 g,
36.4 mmol), sodium acetate (9.91 g, 72.8 mmol), and NiCl2 (4.72 g,
36.4 mmol) in acetic acid (25 mL) was added a solution of N,N-
diethyl-1,4-phenylenediamine dihydrochloride (8.64 g, 36.4 mmol)
in acetic acid (25 mL) dropwise with stirring at room temperature.
After additional stirring for 3 h, the reaction mixture was poured
into water. The solution was neutralized with aqueous Na2CO3,
and the resulting precipitate was filtered, washed with water, and
dried. The residue was chromatographed on silica gel (CH2Cl2/ethyl
acetate, 3:1 as the eluent) to give 1b (9.10 g, 78%) as a blue solid;
Conclusions
We have designed and developed a new class of fluores-
cent dye for sensing water, a (phenylamino)naphtho[1,2-d]-
oxazol-2-yl-type fluorophore with both proton donor and
acceptor sites. The photophysical properties were investi-
gated in organic solvents as a function of water content.
The fluorophore with both a carboxyl and dialkylamino
group exhibited a weak fluorescent emission in organic sol-
1
m.p. 229–231 °C (dec.). H NMR (400 MHz, [D6]DMSO, TMS): δ
= 1.11 (t, 6 H), 3.42 (q, 4 H), 5.63 (s, 1 H), 6.75 (d, J = 8.05 Hz, 2
H), 7.14 (d, J = 8.05 Hz, 2 H), 7.70–7.72 (m, 1 H), 7.83–7.85 (m,
1 H), 8.02 (d, 1 H), 8.29 (d, 1 H), 9.77 (s, 1 H) ppm. IR (KBr): ν
˜
= 3219, 1593, 1568 cm–1. C20H20N2O2 (320.39): calcd. C 74.98, H
6.29, N 8.74; found C 74.77, H 6.11, N 8.87.
4-{5-[(4-Butylphenyl)amino]naphtho[1,2-d]oxazol-2-yl}benzonitrile
vents because of PET, which takes place from the dialk- (2a): A solution of compound 1a (2.00 g, 6.55 mmol), p-cyanobenz-
aldehyde (0.86 g, 6.55 mmol), and ammonium acetate (10.1 g,
0.13 mol) in acetic acid (150 mL) was stirred for 2 h at 90 °C. After
concentrating the mixture under reduced pressure, the resulting res-
idue was neutralized with aqueous Na2CO3, and the product was
extracted with CH2Cl2. The organic extract was washed with water,
and the solvents were evaporated. The residue was then chromato-
graphed on silica gel (CH2Cl2 as the eluent) to give 2a (1.80 g, 66%)
ylamino group to the naphtho[1,2-d]oxazole fluorophore
skeleton. A dramatic enhancement in the fluorescence was
observed with increasing concentrations of water in organic
solvents, which is attributable to the suppression of PET by
the intramolecular proton transfer of the carboxyl proton
to the dialkylamino group. Thus, a new concept for the mo-
lecular design of a fluorescent dye for sensing water in or-
ganic solvents by PET has been demonstrated. Further
studies on fluorescent dyes with high water solubility are
now in progress and will be reported in the next paper.
1
as an yellow powder; m.p. 189–191 °C. H NMR (400 MHz, [D6]-
acetone, TMS): δ = 0.96 (t, 3 H), 1.36–1.43 (m, 2 H), 1.60–1.68 (m,
2 H), 2.63 (t, 2 H), 7.21–7.27 (m, 4 H), 7.53 (s, 1 H), 7.59–7.64 (m,
1 H), 7.74–7.79 (m, 1 H), 7.83 (br., -NH), 8.01 (d, J = 8.8 Hz, 1
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Eur. J. Org. Chem. 2008, 5239–5243

Fluorescent Dye for Sensing Water in Organic Solvents
H), 8.40–8.44 (m, 3 H), 8.52–8.54 (m, 1 H) ppm. IR (KBr): ν = 1.57 (m, 2 H), 1.76–1.82 (m, 2 H), 3.40–3.46 (m, 4 H), 4.36 (t, 2
˜
3377, 2230 cm–1. C28H23N3O (417.50): calcd. C 80.55, H 5.55, N
10.06; found C 80.52, H 5.53, N 10.03.
H), 6.82 (d, J = 9.04 Hz, 2 H), 7.12 (s, 1 H), 7.21 (d, J = 9.04 Hz,
2 H), 7.55–7.61 (m, 2 H), 7.72–7.76 (m, 1 H), 8.19 (d, J = 8.76 Hz,
2 H), 8.33 (d, J = 8.8 Hz, 2 H), 8.44 (d, J = 8.52 Hz, 1 H), 8.50 (d, J
4-{5-(4-Butylphenyl)amino]naphtho[1,2-d]oxazol-2-yl}benzoic Acid
(2b): Compound 1a (3.00 g, 9.83 mmol), p-carboxybenzaldehyde
(1.48 g, 9.83 mmol), and ammonium acetate (15.16 g, 0.20 mol) in
acetic acid (250 mL) were stirred for 2 h at 90 °C. After concentrat-
ing the mixture under reduced pressure, the resulting residue was
dissolved in CH2Cl2 and washed with water. The organic extract
was concentrated. The residue was then chromatographed on silica
gel (CH2Cl2/ethyl acetate, 3:1 as the eluent) to give 2b (0.62 g, 14%)
as a yellow powder; m.p. 251–252 °C. 1H NMR (400 MHz, [D6]-
acetone, TMS): δ = 0.96 (t, 3 H), 1.36–1.43 (m, 2 H), 1.60–1.68 (m,
2 H), 2.62 (t, 2 H), 7.21–7.25 (m, 4 H), 7.56 (s, 1 H), 7.59–7.63 (m,
1 H), 7.75–7.79 (m, 1 H), 8.25 (d, J = 8.8 Hz, 1 H), 8.37–8.41 (m,
= 9.04 Hz, 1 H) ppm. IR (KBr): ν = 3359, 1592 cm–1. C H N O
˜
32 33
3
3
(507.62): calcd. C 75.71, H 6.55, N 8.28; found C 75.43, H 6.49, N
8.17.
Acknowledgments
This work was partially supported by a Grant-in-Aid for Science
and Research from the Ministry of Education, Science, Sport and
Culture of Japan (Grant 18350100) and by a Special Research
Grant for Green Science from Kochi University.
3 H), 8.54–8.56 (m, 1 H) ppm. IR (KBr): ν = 3387, 1686 cm–1.
˜
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C28H24N2O3 (436.50): calcd. C 77.04, H 5.54, N 6.42; found C
77.34, H 5.54, N 6.37.
4-{5-[4-(Diethylamino)phenylamino]naphtho[1,2-d]oxazol-2-yl}-
benzonitrile (2c): To a solution of compound 1b (5.00 g, 15.6 mmol)
and ammonium acetate (24.05 g, 0.31 mol) in acetic acid (170 mL)
was added dropwise a solution of p-cyanobenzaldehyde (2.05 g,
15.6 mmol) in acetic acid (100 mL) with stirring at 90 °C. After
additional stirring for 3 h, the reaction mixture was neutralized
with aqueous Na2CO3, and the product was extracted with
CH2Cl2. The organic extract was washed with water, and the sol-
vents were evaporated. The residue was then chromatographed on
silica gel (CH2Cl2/ethyl acetate, 10:1 as the eluent) to give 2c
(4.86 g, 72 %) as a yellow powder; m.p. 205–206 °C. 1H NMR
(400 MHz, [D6]DMSO, TMS): δ = 1.11 (t, 6 H), 3.35 (q, 4 H), 6.76
(d, J = 9.03 Hz, 2 H), 6.99 (s, 1 H), 7.16 (d, 2 H), 7.76 (t, 1 H),
7.74 (t, 1 H), 8.02 (d, 2 H), 8.26–8.28 (m, 2 H), 8.31 (s, 1 H), 8.39
(d, J = 8.29 Hz, 1 H), 8.48 (d, J = 8.29 Hz, 1 H) ppm. IR (KBr):
ν = 3404, 2224 cm–1. C H N O (432.52): calcd. C 77.75, H 5.59,
[3] a) M. A. Kessler, J. G. Gailer, O. S. Wolfbeis, Sens. Actuators B
1991, 3, 267–272; b) M. Bai, W. R. Seiz, Talanta 1994, 41, 993–
999.
˜
28 24
4
N 12.95; found C 77.74, H 5.59, N 12.98.
4-{5-[4-(Diethylamino)phenylamino]naphtho[1,2-d]oxazol-2-yl}-
benzoic Acid (2d): To a solution of compound 1b (4.00 g,
12.5 mmol) and ammonium acetate (15.4 g, 0.20 mol) in acetic acid
(150 mL) was added dropwise a solution of 4-formylbenzoic acid
(1.88 g, 12.5 mmol) in acetic acid (100 mL) with stirring at 90 °C
for 8 h. After concentrating the mixture under reduced pressure,
the resulting residue was dissolved in CH2Cl2 and washed with
water. The organic extract was concentrated. The residue was then
chromatographed on silica gel (ethyl acetate as the eluent) to give
2d (1.80 g, 32%) as a red powder; m.p. 268–271 °C (dec.). 1H NMR
(400 MHz, [D6]DMSO, TMS): δ = 1.11 (t, 6 H), 3.33–3.36 (m, 4
H), 6.57 (d, J = 9.00 Hz, 2 H), 7.01 (s, 1 H), 7.15 (d, J = 9.00 Hz,
2 H), 7.58 (t, 1 H), 7.74 (t, 1 H), 8.10 (d, J = 8.54 Hz, 2 H), 8.22
(s, 1 H), 8.28 (d, J = 8.54 Hz, 2 H), 8.39 (d, J = 7.56 Hz, 1 H) ppm.
[4] a) W. Liu, Y. Wang, W. Jin, G. Shen, R. Yu, Anal. Chim. Acta
1999, 383, 299–307; b) H. Mishra, V. Misra, M. S. Mehata,
T. C. Pant, H. B. Tripathi, J. Phys. Chem. A 2004, 108, 2346–
2352.
[5] a) Q. Chang, Z. Murtaza, J. R. Lakowicz, G. Rao, Anal. Chim.
Acta 1997, 350, 97–104; b) S. J. Glenn, B. M. Gullum, R. B.
Nair, D. A. Nivens, C. J. Murphy, S. M. Angel, Anal. Chim.
Acta 2001, 448, 1–8.
[6] a) X. Yang, C.-G. Niu, Z.-J. Shang, G.-L. Shen, R.-Q. Yu, Sens.
Actuators B 2001, 75, 43–47; b) D. Citterio, K. Minamihashi,
Y. Kuniyoshi, H. Hisamoto, S. Sasaki, K. Suzuki, Anal. Chem.
2001, 73, 5339–5345; c) C.-G. Niu, A.-L. Guan, G.-M. Zeng,
Y.-G. Liu, Z.-W. Li, Anal. Chim. Acta 2006, 577, 264–270; d)
C.-G. Niu, P.-Z. Qin, G.-M. Zeng, X.-Q. Gui, A.-L. Guan,
Anal. Bioanal. Chem. 2007, 387, 1067–1074.
[7] a) G. Orellana, A. M. Gomez-Carneros, C. de Dios, A. A. Gar-
cia-Martinez, M. C. Moreno-Bondi, Anal. Chem. 1995, 67,
2231–2238; b) M. M. F. Choi, O. L. Tse, Anal. Chim. Acta
1999, 378, 127–134.
[8] a) F. Würthner, S. Ahmed, C. Thalacker, T. Debaerdemaeker,
Chem. Eur. J. 2002, 8, 4742–4750; b) Z. Shen, H. Röhr, K.
Rurack, H. Uno, M. Spieles, B. Schuls, G. Reck, N. Ono,
Chem. Eur. J. 2004, 10, 4853–4871.
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A. Samanta, Tetrahedron 2000, 56, 7041–7044; b) B. Ramach-
andram, G. Saroja, N. B. Sankaran, A. Samanta, J. Phys.
Chem. B 2000, 104, 11824–11832.
IR (KBr): ν = 3410, 1589 cm–1. C H N O (451.52): calcd. C
˜
28 25
3
3
74.48, H 5.58, N 9.31; found C 74.44, H 5.55, N 9.24.
Butyl 4-{5-[4-(Diethylamino)phenylamino]naphtho[1,2-d]oxazol-2-
yl}benzoate (2e): Compound 1b (1.00 g, 3.12 mmol), butyl 4-for-
mylbenzoate (0.64 g, 3.12 mmol), and ammonium acetate (4.8 g,
0.06 mol) in acetic acid (100 mL) were stirred for 5 h at 90 °C. After
concentrating the mixture under reduced pressure, the resulting res-
idue was dissolved in CH2Cl2, and washed with water. The organic
extract was concentrated. The residue was then chromatographed
on silica gel (CH2Cl2/ethyl acetate, 10: 1 as the eluent) to give 2e
(0.59 g, 37%) as a red powder; m.p. 170–172 °C. 1H NMR
(400 MHz, [D6]acetone, TMS): δ = 1.00 (t, 3 H), 1.11 (t, 6 H), 1.47–
Received: June 21, 2008
Published Online: September 22, 2008
Eur. J. Org. Chem. 2008, 5239–5243
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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