Fluorescence Properties and Exciplex Formation of Emissive Naphthyridine Derivatives: Application as Sensors for Amines

Article abstract of DOI:10.1002/chem.201903643

Water-soluble donor–acceptor-type fluorophore 15Nap-Cl having two trifluoromethyl groups and a Cl group on a 1,5-aminonaphthyridine framework was prepared. Fluorophore 15Nap-Cl showed strong solvatochromic fluorescence, and, as the solvent polarity increased, a bathochromic shift was observed accompanied by an increase in the fluorescence quantum yield. In addition, in the presence of amines such as ethylamine, diethylamine, and aniline, further considerable bathochromic shifts in the fluorescence were observed. Density functional calculations identified the source of the fluorescence behavior as exciplex formation between 15-Nap-Cl and the corresponding amine. The fluorescence behavior was exploited to fabricate a sensor that can identify various primary, secondary, and tertiary amines.

Full text of DOI:10.1002/chem.201903643

A Journal of
Accepted Article
Title: Fluorescence Properties and Exciplex Formation of Emissive
Naphthyridine Derivatives: Applications as Sensors for Individual
Amine Species
Authors: Satoru Karasawa, Junko Hirota, Kazuteru Usui, Yasufumi
Fuchi, Masaomi Sakuma, Shota Matsumoto, and Ryusuke
Hagihara
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To be cited as: Chem. Eur. J. 10.1002/chem.201903643
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Fluorescence Properties and Exciplex Formation of Emissive
Naphthyridine Derivatives: Applications as Sensors for Amine
Species
Junko Hirota, Kazuteru Usui, Yasufumi Fuchi, [a] Masaomi Sakuma, [a] Shota Matsumoto, [a] Ryusuke
[
b]
[a]
Hagihara, [b] and Satoru Karasawa*[a]
Abstract: A water-soluble donor–acceptor type fluorophore (15Nap-
Cl) having two trifluoromethyl groups and a Cl group on a 1,5-
aminonaphthyridine framework was prepared. 15Nap-Cl showed
strong solvatochromic fluorescence, and, as the solvent polarity
increased, a bathochromic shift was observed accompanied by an
increase in the fluorescence quantum yield. In addition, in the
presence of amine species, such as ethylamine, diethylamine, and
aniline, further considerable bathochromic shifts in the fluorescence
were observed. Using density functional theory calculations, the
source of the fluorescence behavior was identified: exciplex formation
between 15-Nap-Cl and the corresponding amine. Taking advantage
of the fluorescence behavior, we created an amine sensor and
demonstrated that the sensor is able to identify various amine species
including primary, secondary, and tertiary amines.
18Nap framework, have been reported to exhibit fluorescence
changes in response to a proton source (Scheme 1) [14]
.
Scheme 1. Molecular Structures of aminoquinoline and aminonaphthyridine
families.
In this study, to develop an amine sensor using push–pull type
fluorophores, we prepared a novel naphthyridine derivative, 2-
chloro-6,8-bis(trifluoromethyl)-1,5-naphthyridin-3-amine (15Nap-
Cl), that shows a fluorescence response to amines. In the
presence of amine species, a considerable bathochromic shift in
the fluorescence peak occurred. The bathochromic shift was
caused by the formation of exciplexes. Taking advantage of this
feature, we created an amine sensor that is capable of identifying
various amine species. Herein, we describe the synthesis of
15Nap-Cl and the products of the reaction of amine species with
this compound, as well as X-ray diffractometry analyses,
absorption and fluorescence spectra, and density functional
theory (DFT) calculations of the aminonaphthylidine derivatives.
Finally, we describe the development of the amine sensor using
Introduction
Amines are essential biological compounds but high
concentrations of amines cause serious problem not only in
human health but also in the environment [1]. Thus, to date,
various amine sensors have been developed and reported [2]
.
Many amine sensors take advantage of changes in absorption,
electric current, pH, and fluorescence [3−8]. In particular, sensors
based on fluorescence have the potential to detect amines with
high sensitivity. In general, amine sensors are based on a
chemical reaction between the fluorophore and the amine, and
15Nap-Cl.
this interaction results in changes in fluorescence [9,10]
We have studied push–pull type fluorophores consisting of
.
Scheme 2. Preparation of 15Nap-Cl.
[
11]
aminoquinoline (TFMAQ)
derivatives[12], which show fluorescence changes in response to
various external stimuli such as solvent [11], heat [11–13], protons [14]
or aminonaphthyridine (18Nap)
,
light [15], scratching [16], grinding [17], and vapor [18]. Recently, the
tricyclic amidine derivatives, DHIm and DHPy, in which the
dihydroimidazole or the dihydropyrimidine ring is fused with the
Results and Discussion
[
[
a]
b]
Prof. Dr. S. Karasawa, Dr. K. Usui, Dr. Y. Fuchi, M. Sakuma, S.
Matsumoto
Faculty of Pharmaceutical Sciences, Showa Pharmaceutical
University
Syntheses
2-Chloro-6,8-bis(trifluoromethyl)-1,5-naphthyridin-3-amine
3-3165 Higashi-Tamagawagakuen, Machida, Tokyo 194-8543
(
15Nap-Cl) was prepared from 2-hydroxy-5-nitropyridine as the
(Japan)
E-mail: karasawa@ac.shoyaku.ac.jp
J. Hirota, R. Hagihara
Graduate School of Pharmaceutical Sciences, Kyushu Universty
starting material; the reaction was carried out in four steps. The
preparation route is shown in Scheme 2. Via a nitration reaction
in the presence of fuming nitric acid, fuming sulfuric acid, and
3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582 (Japan)
conc. sulfuric acid, 2-hydroxy-3,5-dinitropyridine (1) was prepared
as a yellowish solid [19]. Subsequently, the chlorination of 1 was
carried out in the presence of phosphorus oxychloride, affording
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2
-chloro-3,5-dinitropyridine (2) as a yellowish solid [20]. Using 2, a
reduction reaction using tin was carried out, affording 2-chloro-
,5-diaminopyridine (3) as a white solid. Finally, by Combes
groups of 15Nap-DEA and –AN were pyramidal. 15Nap-Cl
and -OH adopted a planar structure between the naphthyridine
3
ring and amino group. The bond lengths between the C
atoms (b in Figure 1) were 1.376 and 1.373 Å for 15Nap-DEA
and –AN, respectively. In contrast, C –N bond lengths of 1.351
3 3
and N
synthesis in the presence of hexafluoroacetylacetone and
montmorillonite K10 as a catalyst [11–18], 15Nap-Cl was obtained
as a yellowish solid.
3
3
and 1.350 Å were observed for 15Nap-Cl and –OH, respectively,
1
5Nap-Cl readily reacted with various nucleophiles, such as
which are shorter than those of the DEA and AN derivatives. The
C N Hamine angles (in Figure 1) of 15Nap-DEA and –AN are
3 3
−
amines and OH, at the Cl group, giving rise to the substitution
products. In the presence of excess ethylamine (EA),
diethylamine (DEA), aniline (AN), or OH in protic solvents,
103−126° and 115−123º, respectively, indicating the large
−
2
deviation from the typical 120° angle observed for sp -hybridized
reaction products containing the corresponding substituents
atoms. In contrast, the corresponding angles of 15Nap-Cl
and -OH were 119−123° and 118−124°, similar to the 120° angle
(
15Nap-EA, - DEA, -AN, and -OH) were obtained in high yields.
2
The preparation routes are shown in Scheme 3.
of sp hybrids. The bond lengths and angles indicate that the N
2
atoms in 15Nap-Cl and –OH are sp hybridized, whereas the N
3
Scheme 3. Preparation of 15Nap-X and its tautomer 15Nap-OH.
atoms in 15Nap-DEA and –AN have sp character. Because of
the through-space intramolecular interaction between Cl and H
2
atoms in 15Nap-Cl and the tautomerization in –OH, the sp -
hybridized character might be preferred in the solid state.
The crystal packings of all of 15Nap-X derivatives are shown in
Figure S1. All of the 15Nap-X derivatives show hydrogen-
1
bonding interactions between the N and Hamine atoms in the
amine, thus forming zig-zag hydrogen-bonded chains in 15Nap-
Cl, -DEA, and –AN. In addition to the zig-zag chains, 15Nap-OH
contains further hydrogen-bonding interactions between the O
X-ray Analyses
From solutions containing 15Nap-X, four single crystals suitable
for X-ray diffraction were obtained. Crystals of 15Nap-EA were
not obtained. The single crystals of 15Nap-Cl, -AN, and -OH
atom and HN5 atoms on N , thus forming a dimer and a 3D
5
hydrogen-bonded network. The bond lengths of the hydrogen
bonds into the zig-zag chains were 3.09–3.16 Å, and the
corresponding length of inter-zig-zag chains in 15Nap-OH was
2.83 Å, which is considerably shorter than those of the zig-zag
chains. The top and side views of the zig-zag chains and 3D
hydrogen network are shown in Figure S1.
crystallized in the monoclinic space group P2
5Nap-DEA crystallized in the orthorhombic space group Fdd2
#43). ORTEP drawings of the four 15-Nap-X compounds are
1
/c (#14). However,
1
(
shown in Figure 1, and the crystallographic parameters are
summarized in Table S1.
Absorption and Fluorescence Spectra
In the solution state
The 15Nap-X derivatives were soluble in various solvents, even
water, except for 15Nap-AN. The absorption and fluorescence
spectra were measured in six polar and non-polar solvents: n-
2 3 2
hexane, Bu O, CHCl , AcOEt, MeOH, and H O. The absorption
and fluorescence spectra are shown in Figure 2 and Figure S3.
The photophysical values obtained from these measurements are
summarized in Table 1. In the absorption spectra of 15Nap-Cl,
the absorption maxima (maxab) ranged from 371 to 391 nm, and
there was subtle solvatochromism. In contrast, the fluorescence
spectra of 15Nap-Cl showed a strong solvent dependence, and
fl
the obtained fluorescence maxima (max ) ranged from 395 to 500
fl
nm. Interestingly, the fluorescence quantum yields ( ) were
independent of the polarity of the solvents, and a relatively high
fl

of 33% was observed in water. As the polarity of the solvent
fl
increased,  increased, which is the opposite behavior to that of
the typical push–pull type fluorophores such as TFMAQ
8Nap derivatives [12]. The 15Nap-EA, and -DEA derivatives have
similar absorption spectra and similar max values. However, the
values of the molar extinction coefficient () of 15Nap-EA in all
solvents were approximately twice as large as those of 15Nap-Cl.
In the fluorescence spectra, both –EA and -DEA derivatives
showed less solvatochromic behavior than 15Nap-Cl. The values
Figure 1. ORTEP drawings of 15Nap-X. Figures (a), (b), (c), and (d) show
[11]
and
15Nap-Cl, -DEA, -AN, and –OH, respectively. Characters “b” and “” denote
1
the bond length between C and N amine angle (see text),
3
7
and the C
3
N
3
H
ab
respectively. Atoms colored gray, blue, red, green, light green, and white are C,
N, O, Cl, and H, respectively.
All 15Nap-X compounds have similar molecular structures,
except for 15Nap-OH, which dominantly adopted a pyridone
tautomer rather than the hydroxyl pyridine. The primary amino
fl
of  for 15Nap-EA were similar to those of 15Nap-Cl, even in
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Table 1. Photophysical Values (maxab, max Stokes Shift (),
fl,
#[22]
and  ) for
fl
fl
water, in which the value of  increased with increasing solvent
fl
15Nap-X in Various Solvents.
polarity. However, the value of  of 15Nap-DEA decreased in
1
5Nap-X
Cl
n-
Bu
2
O
CHCl
3
AcOEt
MeOH
2
H O
the polar solvents, and no fluorescence could be detected in the
MeOH solution. The absorption spectra of 15Nap-AN was similar
to that of 15Nap-DEA, but fluorescence red shifts of
approximately 50 nm were observed compared to that of 15Nap-
DEA because of the strong donor effect in non-polar solvents
such as n-hexane and Bu O. In the case of 15Nap-OH, no
2
solvatochromic behavior in the absorption spectra was observed,
but H NMR spectra suggested that the tautomerism based on the
Hexane
374
ab


max /nm
382
439
57
371
456
85
384
460
76
391
494
103
55.4
384
448
64
379
500
121
34.2
378
461
83
fl
max /nm
395
21

fl

/%
2.0
368
397
29
22.7
378
415
37
21.4
374
416
42
48.0
382
425
43
ab
EA
DEA
AN



max /nm
fl

max /nm
1


isomers was present; hydroxypyridine form (HP) in CDCl
3
fl
(
(
medium polarity solvent) and pyridone form (PD) in DMSO-d6
polarity solvent) (Scheme 3 and Figures S4). Whereas, the
 /%
8.1
370
426
56
11.3
383
438
55
13.8
385
447
62
14.9
389
450
61
22.4
396
458
62
33.3
ab
*
max /nm
i.s.
i.s.
fluorescence spectra showed a strong solvent dependence,
approximately a 100 nm deviation (maxfl) between non- and
polar solvents. Furthermore, in TDDFT calculations (Table S2),
the corresponding fluorescence of both PD and HP forms showed
fl
*

max /nm
*


n.d.
fl
*

/%
8.2
376
448
72
10.9
386
481
95
4.1
2.3
< 1.0
398
i.s.
394
fl
ab
a similar value (PD: max = 414−418 nm and HP: 414−418 nm).
max /nm
maxfl/nm
384
514
130
389
*
*
**
*
Those results suggested that the remarkable fluorescence
dependence among various solvents was caused due to a simple
polarity dependence, not but excited-state intramolecular proton
n.d.
n.d.
n.*d.
n.d.
*
*
*
*


n.d.
n.d.
fl
**
**

/%
24.1
359
377
18
5.0
< 1.0
364
422
58
n.d.
n.d.
n.*d.
transfer (ESIPT) [21]
.
ab
OH

max /nm
361
416
55
365
433
68
366
459
93
360
466
106
45.8
fl

max /nm


fl

/%
12.7
16.9
23.1
22.1
39.7
*
insoluble, ** not detected. max - max

ab
fl
To understand the quantitative photophysical properties for the
5Nap-X derivatives, a Lippert–Mataga plot[23] (Figure 3) was
1
drawn using the Stokes shift () and the solvent parameter (f),
which was estimated using equation 1, where  and n are the
dielectric constant and reflective index of the individual solvents,
respectively.
ꢁ
휀−1
푛 −1
Δ푓 = 2
ꢀ
(1)
ꢁ
휀+1
2푛 +1
Figure 2. Absorption (dotted lines for left axis) and fluorescence (solid lines for
right axis) spectra of 15Nap-Cl (a), -EA (b), -DEA (c), and –OH (d). Lines
The difference in the dipole moment between the ground state
and the excited-state () was estimated using equation 2,
colored purple, blue, green, orange, red, and pink indicate n-hexane, Bu
2
O,
CHCl , AcOEt, MeOH, and H O, respectively.
3
2
２
ℎ푐푎3
∆µ２
∆
 =
∆푓 ꢂ ꢃ표ꢄ푠푡ꢅꢄ푡,
(2)
where a denotes the Onsager radius, which was calculated from
the DFT calculations. The resulting  values were 13.0, 10.0,
15.4, and 12.9 D for 15Nap-Cl, -EA, -AN, and –OH, respectively.
These values are considerable higher than those of TFMAQ (8.1
D) and 18Nap (4.6 D) analogs, indicating that 15Nap-X preferably
undergoes charge separation in the excited-state. However, even
3
in non-polar solvents such as n-hexane and CHCl , 15Nap-DEA
showed large  values that were independent of the solvent
polarity, causing a small  (4.3 D). The reason for the small 
could be the difficulty in forming the twisted intramolecular charge
transfer (TICT) state [24], and the reasons for the difficulty in
forming the TICT could be (i) the steric repulsion between the
6 7
diethyl amino group at C and the amino group at C and (ii) the
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intramolecular CHn interactions (2.295 Å) between the amino
group as a proton-acceptor and ethyl group as a proton donor
Figure 4. Absorption (dotted lines for left axis) and fluorescence (solid lines for
right axis) spectra of 15Nap-Cl (a), -EA (b), -DEA (c), and –OH (d).
(
Figure S2).
Table 2. Photophysical Values (maxab, max , and  ) for 15Nap-X in the Solid
fl,
fl
State.
fl
1
5Nap

maxab/nm

maxfl/nm
Stokes
shift
54
 /%
derivative
Cl
409
387
401
397
387
463
440
465
490
442
1.6
1.0
1.9
4.6
1.2
EA
DEA
AN
53
64
93
55
OH
Addition of triethylamine (TEA) to 15Nap-Cl solution
The absorption and fluorescence spectral changes of a 0.1 mM
5Nap-Cl solution in the absence and presence of amines
including TEA, DEA, and EA, which have dielectric constants of
.4, 3.8, and 6.9, respectively, were measured in tetrahydrofuran
THF), 1,4-dioxane, and MeOH, respectively. Figures 5a and b
1
2
(
Figure 3. Lippert–Mataga plot of 15Nap-X. Marks and lines colored blue, green,
orange, red, and blue indicate 15Nap-Cl, -EA, -DEA, -AN, and –OH,
respectively. The solid lines are the least-square fittings.
show the spectral changes upon the addition of TEA in THF. In
ab
the absence of TEA, max was 370 nm. However, the addition of
ab
up to 70 vol% TEA resulted in a shift in max to lower wavelengths
with a subtle increase in . Specifically, on the addition of 40 vol%
TEA, the fluorescence intensity abruptly decreased with a subtle
blue shift. On the further addition of TEA to 70 vol%, a
fluorescence peak at long wavelength (approximately 600 nm)
appeared, accompanied by the complete quenching of the
fluorescence at 460 nm. The fluorescence maximum gradually
shifted to shorter wavelength with increasing addition of TEA. The
resulting blue shifts related to the absorption at 370 nm and the
fluorescence at 460 nm indicate solvatochromic behavior induced
by the polarity of TEA ( = 2.4), which is much lower than that of
THF ( = 7.5). In contrast, the abrupt decrease in the fluorescence
emission at 460 nm and the appearance of a fluorescence
emission at 600 nm suggest that TEA caused fluorescence
quenching accompanied by the formation of an excited-state
complex, that is, an exciplex (15Nap-Cl-TEA). The fluorescence
lifetime () of the exciplex was estimated to be about 10 ns, which
is twice that of 15Nap-Cl (ca. 5 ns). In 1,4-dioxane solution ( =
In the solid state
The absorption and fluorescence spectra in the solid state were
obtained. The absorption values were estimated by a conversion
from a diffuse reflection spectrum using a Kubelka–Munk (KM)
function. The values of max were 409, 387, 401, 397, and 387
nm for 15Nap-Cl, -EA, -DEA, -AN, and –OH, respectively, which
ab
are comparable to those in polar solutions such as MeOH and
ab
H O. In the fluorescence spectra, the values of max were 463,
2
4
40, 465, 490, and 442 nm for 15Nap-Cl, -EA, -DEA, -AN,
and -OH, respectively, which are comparable to those in the
moderately polar solutions such as CHCl and AcOEt. Therefore,
3
the Stokes shift values were 54, 53, 64, 93, and 55 nm. Because
of the donor effect of the phenyl group, the largest value was
obtained for 15Nap-AN. All of the 15Nap-X derivatives showed
weak fluorescence intensities because of the quenching effect of
the intermolecular close contacts in the crystals. The absorption
and fluorescence spectra and the photophysical values are
shown in Figure 4 and listed in Table 2, respectively.
2.0), in addition, similar solvatochromic behavior for both
absorption and fluorescence was observed. Furthermore, the
abrupt decrease in fluorescence at approximately 460 nm
induced by quenching and the appearance of a new fluorescence
peak at approximately 600 nm (as for THF) were observed
(
Figure S5-1).
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Figure 5. Absorption (a) and fluorescence (b) spectral changes in THF for
15Nap-Cl in the absence and the presence of TEA. In (b), the inset shows an
enlargement of the dotted region. Percentages indicate the volume ratio of TEA
with respect to THF. The absorption (c) and fluorescence (d) spectral changes
in MeOH for 15Nap-Cl in the absence and presence of TEA. Percentages
indicate the volume ratio of TEA to MeOH.
Figure 6. Schematic of the PET mechanism and the formation of the exciplex
based on 15Nap-Cl in the presence of TEA.
However, in the MeOH solution, although an abrupt decrease in
the fluorescence intensity at approximately 460 nm was observed,
as for the above-mentioned solvents, no red shifts, increase in the
absorbance intensity, nor fluorescence at longer wavelengths
was observed on the addition of TEA (Figures 5c and d).
It is well known that the locally excited (LE) states of an electron
donor (D) and acceptor (A) in a van der Waals (vdW) complex can
lead to the formation of an excited state charge transfer (CT) state
D A ), namely, an exciplex [27]. In many cases, the close
+
-
(
*
interaction between A and D results in the disappearance of the
emission of A* and emission from the exciplex. In our case,
2
Furthermore, on the addition of H O to the 1,4-dioxane solution,
the resulting long-wavelength fluorescence obtained upon the
addition of 40 vol% TEA readily disappeared, accompanied by the
recovery of the fluorescence at approximately 460 nm (Figure S5-
1
5Nap-Cl and TEA correspond to D and A, respectively, and the
CT is the [15Nap-Cl−TEA] complex. Tramer et al. reported [ that
CT complexes can be classified in three cases having different
values of (equation 3),
28]
2
).
Thus,
a
common photoinduced electron transfer (PET)[25]
between 15Nap-Cl and TEA caused the fluorescence quenching
at short wavelengths, approximately 460 nm in a pure solvent. In

 I
D
+ E
A
A
  ,
(3)
2
contrast, in protic solvents such as alcohols and H O, no long-
where I
of species A, and 
D
is the ionization potential of D, E
A
is the electron affinity
wavelength fluorescence was observed. In contrast, in aprotic
solvents such as THF and 1,4-dioxane, long-wavelength
fluorescence was detected. The spectral changes observed in all
the solvents strongly suggest the formation of an exciplex[26]
between 15Nap-Cl and TEA. A schematic drawing of the PET and
the formation of the exciplex is shown in Figure 6.
A
is the energy of the lowest excited singlet
state of A. (i) Large values of  > 4.5 eV correspond to the case
in which the LE state is lowest in energy for excitation from the
ground-state vdW complex and a weak interaction between D and
A is present. (ii) In the case when  < 3.5 eV, the CT state is
lowest in energy. (iii) In the third, intermediate between (i) and (ii),
both energy states are similar and, consequently, nearly resonant,
and A*D and A D states strongly interact [29]. 15Nap-Cl possess
a relative high E
A
(1.71 eV)[ compared to typical aromatic
-
+
30]
hydrocarbons such as anthracene (0.8 eV) and perylene (0.55
eV). Using equation 3,  for [15Nap-Cl−TEA] was calculated to
be 2.16 eV [ , classing it as type (ii), which is the same category
30]
as
[
[perylene−N,N-dimethylaniline
(DMA)],
and
anthracene−DMA] [31]. In addition, because of the stability effect,
-
+
protic solvents can lead to the formation of an ion pair (A D ),
giving rise to the disappearance of the long-wavelength
fluorescence . Therefore, the 600 nm fluorescence caused by
the [15Nap-Cl−TEA] exciplex (Figure 5d) was not observed in
MeOH solution, and the fluorescence was quenched by the
[32]
2
addition of H O (Figure S5-2).
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Addition of DEA to 15Nap-Cl
Cl−DEA] (2.16 eV) [28,30]. The absorption and fluorescence
spectral changes in the absence and presence of EA in THF, 1,4-
dioxane, and MeOH are shown in Figures 7 and S7.
In the case of the addition of DEA to 5Nap-Cl solutions, similar
fluorescence behavior was observed; that is, with increasing
addition of DEA, the short-wavelength fluorescence at
approximately 470 nm was reduced, accompanied by the
appearance of the long-wavelength fluorescence at
approximately 600 nm in aprotic solvents). This fluorescence
behavior suggests the formation of an exciplex, i.e., [15Nap-
Cl−DEA]. Furthermore, the appearance of the long-wavelength
fluorescence in protic solvents was not observed, as also
observed for TEA. Notably, even secondary amines, such as DEA,
resulted in exciplex fluorescence because an excited-state proton
transfer (ESPT) often occurs in the presence of secondary and
primary amines, accompanied by the quenching of the long-
wavelength fluorescence. In the absorption spectra, subtle
hypochromic and bathochromic effects were observed. Using
equation 3, was calculated to be 6.13 eV,[3] which is slightly
larger than that of [15Nap-Cl−DEA]. The absorption and
fluorescence spectral changes are shown in Figures S6.
Figure 7. Absorption (left) and fluorescence (right) spectral changes in THF for
15Nap-Cl in the absence and the presence of EA. The dashed outline in the
right-hand figure represents the enlarged part (inset). Percentages are the
volume ratio of EA to THF.
Addition of TEA to TFMAQ, TFMAQ-diPh, and 18-Nap
To reveal the specificity of the excited-state complex of 15Nap-X
with amines, complexation of amines with aminoquinoline
derivatives (TFMAQ and TFMAQ-diPh)[11] and 18-Nap[12] was
carried out, and the absorption and fluorescence spectral
changes upon the addition of the amine were measured. With
increasing addition of TEA, the fluorescence of the vdW complex
abruptly decreased without the appearance of the long-
wavelength fluorescence emission (Figure S7). These results
indicate that the reduction in the fluorescence via PET occurred
even for TFMAQ,TFMAQ-diPh, and 18Nap derivatives, and the
appearance of long-wavelength fluorescence is a feature of
15Nap-X derivatives.
Addition of EA to 15Nap-Cl
Figure 7 shows the absorption and fluorescence spectral
changes in the absence and presence of EA (70 vol% aqueous
solution) in THF solution. On the addition of EA to solvents such
ab
as THF, 1,4-dioxane, and MeOH, the max at about 380 nm
showed drastic changes from that in the absence of EA because
of the strong hyperchromic effect. These intense increases in the
absorbance suggested that a quantitative chemical reaction took
place to produce 15Nap-EA. 15Nap-EA has an  value twice that
of 15Nap-Cl. The addition of 10 vol% EA resulted in a reduction
in fluorescence together with a subtle blue shift, and the further
addition of 20 vol% EA resulted in the recovery of fluorescence
with increased intensity and an increase in the fluorescence
emission at approximately 610 nm. Surprisingly, even in MeOH,
the long-wavelength fluorescence at approximately 610 nm was
clearly observed. In addition, the spectral changes in MeOH
exhibited time dependence, in which only the short-wavelength
fluorescence increased on the addition of 10 vol% EA, and not
only the short-wavelength but also the long-wavelength
fluorescence increased on the addition of more than 40 vol% EA,
suggesting that the chemical reaction took place slowly in MeOH.
Those results suggested the following phenomena might take
place in the solution; (i) until 10 vol%, the substitution reaction
from 15Nap-Cl to 15Nap-EA occurred, and this was
accompanied by an increase in the absorption and the
fluorescence quenching by EA. (ii) Over 20 vol%, the
fluorescence intensity at short (ca. 470 nm) and long (ca. 610 nm)
wavelengths increased, indicating that the chemical reactions
were further proceeding, and, in the excited-state, the resulting
DFT Calculations
To determine whether the long-wavelength fluorescence involves
the exciplex, time-dependent density functional theory (TDDFT)
calculations using the wB97XD functional, which includes a
dispersion correction term, were performed [ . In the most stable
structure, TEA is located at the center of 15Nap-Cl, and the two
molecules are separated by 3.125 Å (Figure 8a). However, the
fluorescence wavelength was estimated to be 658 nm, which is
significantly longer than that of the experimental result. The
second most stable structure was 0.35 kcal/mol higher in energy
than the most stable structure, and, in this structure, TEA is
located near the center of 15Nap-Cl with a slight shift toward N5
position. The shortest distance between 15Nap-Cl and TEA was
3.153 Å, identical to the above-mentioned distance (Figure 8b),
and the estimated fluorescence was 630 nm, which is a smaller
deviation from the experimental value than that of the most stable
structure. Thus, the resulting long-wavelength fluorescence was
assigned as arising from exciplex-based fluorescence, and the
structure of [15Nap-Cl−TEA] corresponds to the second-most-
stable structure or a mixture of these species.
33]
15Nap-EA formed an exciplex, [15Nap-EA−EA]. Compared to
the [15Nap-Cl−TEA] and [15Nap-Cl−DEA] excimers,
interestingly, the [15Nap-EA−EA] excimer showed no quenching,
not only by EA but also by the protic solvents, indicating its
different properties compared to the 15Nap-Cl−based excimers.
In fact, for 15Nap-EA, E
A
= 1.10 eV,[30] which is smaller than that
of 15Nap-Cl (1.71 eV). Furthermore, using equation 3,  was
estimated to be 2.52 eV, which is larger than that of [15Nap-
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Chemistry - A European Journal
10.1002/chem.201903643
FULL PAPER
by visual observation. Neat samples, except for EA (70 vol%),
were added dropwise onto the films, and the obtained
fluorescence was observed using 345 nm UV light produced by a
UV lamp. The photographs taken under UV light in the absence
and presence the amines are shown in Figure 10. In the absence
of the amines, all of the glass plates displayed a similar bluish
color (spectra are shown in Figure 9 and photographs are shown
in the upper parts of Figure 10). In contrast, the photographs after
the addition of the amines show a change from the bluish color to
that corresponding to the added amine. In the case of short-chain
alkylamines such as EA, DEA, and TEA (Figure 10a), the color
became bluish purple, red, and dark-red, respectively, as the
exciplex formed on the surfaces. In contrast, for alkylamines with
a long chain such as HA and DHA (Figure 10b), the color became
purple and deep-red, respectively, because of the formation of the
exciplex. In the case of a cyclic structure with an amine such as
CHDA, a greenish color and no indication of exciplex formation
was observed (Figure 10b). For the aromatic amines such as
aniline and p-HA, a very dark-red color compared to that of DEA,
together with fluorescence quenching, was observed because of
the formation of the exciplex. Finally, the addition of 30%
ammonia solution resulted in a pale greenish color. Thus, the
15Nap-Cl-embedded film on a glass plate turned different colors
on the addition of amines, and the color corresponded to
individual amines. Thus, these results indicate that our sensor
allows the identification of individual amines by eye. The
reversibilities toward amine analogues were examined (Figure
S9). The recovery of the fluorescence intensity and wavelength
based on 15Nap-Cl was insufficient due to durability of PMMA
films toward amine analogues.
1
Figure 8. Calculated molecular structures of the S excited states of [15Nap-
Cl−TEA]
(TD-wB97XD/def2-TZVP−IEFPCM(THF)//TD-wB97XD/def2-SVP).
The most stable (a) and second-most-stable (b) structures. The numbers
indicate the shortest distance between 15Nap-Cl and TEA within the exciplex.
Manufacturing an Amine Sensor
Inspired by the reactivities and the fluorescence spectral changes
of 15Nap-Cl in the absence and presence of amines, we planned
to make a sensor containing 15Nap-Cl to identify amines such as
TEA, DEA, EA, and aniline. To enable analysis by the naked eye
in addition to spectral analysis, solutions in cuvettes were
compared (Figure 9a). After the addition of 40 vol% EA, DEA, or
TEA in 1,4-dioxane containing 20 M 15Nap-Cl, the solutions
turned pale blue, deep-red, and red, respectively, and this change
was clearly visible by the naked eye. To create a sensor suitable
for practical use, a poly(methyl methacrylate) (PMMA) film[34]
containing 10 wt% 15Nap-Cl was prepared on a glass plate.
These films were sufficiently transparent films to allow the
measurement of fluorescence spectra (Figures 9b and c; blueish
solid lines). In addition, when the amine was added, the films
retained sufficient transparency for measurement, yielding flmax
at 602, 665, and 592 nm, respectively (Figures 9b and c); red
solid lines).
Figure 9. (a): Photographs of 20 M 15Nap-Cl in 1,4-dioxane under 345-nm
Figure 10. The photographs of glass plates coated with 15Nap-Cl in the
absence (upper) and presence (lower) of the corresponding amines: (a) shows,
from left to right, ethylamine (EA), diethylamine (DEA), trimethylamine (TEA),
UV irradiation. Those photographs were taken after the addition of 40 vol% EA
(left), DEA (center), and TEA (right) solutions in 1,2-dioxane for 1.0, 1.5, and 1.0
min, respectively. (b)−(d): Fluorescence spectral change (ex. 400 nm) of the
PMMA films containing 10% 15Nap-Cl in the absence of the amine (blue solid
lines) and in the presence of TEA (b), DEA (c), and EA (d). The red solid lines
indicate the films in the presence of amine.
1
,2-trans-cyclohexyldiamine (CHDA), and aniline, and (b) shows, from left to
right, ammonia, hexylamine (HA), dihexylamine (DHA), and p-hexylaniline (p-
HA). The photographs were taken under 345-nm UV irradiation.
Using the glass coated with the 15Nap-Cl films, identification of
amine species (TEA, DEA, EA, aniline, 1,2-trans-
cyclohexanediamine (CHDA), hexylamine (HA), dihexylamine
Conclusions
We prepared a water-soluble push–pull type fluorophore, 15Nap-
Cl, that can produce a fluorescence response in the presence of
(
DHA), and p-hexylaniline (p-HA)) including tertiary, secondary,
primary, long-chain, cyclic, and aromatic amines was carried out
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Chemistry - A European Journal
10.1002/chem.201903643
FULL PAPER
amines. Furthermore, 15Nap-X derivatives bearing different
substituents instead of the Cl group in 15Nap-Cl were prepared.
All 15Nap-X species showed solvatochromic fluorescence
carrying out frequency calculations at the same level of theory. No
imaginary frequencies were found. Excitation energies and fluorescence
emission spectra were also obtained at the TD-wB97XD/def2-TZVP–
IEFPCM (THF) level using the optimized lowest excited singlet state
structures.
fl
behavior together with relatively high  values. In the presence
of an amine in an aprotic solvent, the fluorescence of 15Nap-Cl
changed drastically, yielding long-wavelength fluorescence. The
resulting fluorescence was attributed to the formation of an
exciplex between 15Nap-Cl and the amine. Taking advantage of
the fluorescence variation upon an addition of the amines, we
created a sensor capable of identifying the individual amines.
Using 15Nap-Cl dispersed in a PMMA film on a glass plate, we
found that (i) the fluorescence color changes in the presence of
the amine, except for some specific amines, and (ii) the colors
were dependent on the amine species. Thus, our results show
promise for the development of sensors for the visual
identification of amines. To increase the sensor sensitivity, more
significant color changes and improvement of durability in films
are required; thus, the design of new compounds and films is
underway.
Materials: Unless otherwise stated, all the reagents and solvents were
used as received without further purification. Specifically, 2-hydroxy-5-
nitropyridine, fuming H
2
SO
4
, conc.H
2 4
SO , fuming HNO
3
, NaOH, POCl
3
Na CO , MgSO4, 35%
2
3
HCl
solution,
Sn
powder, and
hexafluoroacetylacetone, were purchased from Nakalai Tesque Co. Ltd.,
Fujifilm Wako Pure Chemical Co. Ltd., Sigma Aldrich, or from the Tokyo
Chemical Industry Co., Ltd. 1 and 2 were prepared according to the
previously reported procedures [11–18,19,20]. Silica gel for column
chromatography was purchased from Kanto Chemical Ltd., Japan. Thin
layer chromatography (TLC) was performed on silica gel plates, using a
60 F254 (Merck).
-Hydroxy-3,5-dinitropyridine (1): Following the published procedure[19], to
2
2
-hydroxy-5-nitropyridine (5.37 g, 0.038 mol), 25% fuming H
SO (4.3 mL) were added dropwise, and the resulting
solution was heated to 80 °C. After the further addition of 60% fuming
HNO (4.3 mL) to the solution, the solution was stirred for 6 h. The solution
2 4
SO (37.6
mL) and conc. H
2
4
3
was cooled to rt and was neutralized with a 10% NaOH solution. The
precipitate was vacuum filtered, and the resulting solid was recrystallized
Experimental Section
from hot water by cooling to 4 °C to afford 5.57 g of 1 as a yellowish solid
General information: Infrared spectra were recorded using a JASCO
1
in 78.5% yield. H NMR (500 MHz, [D
4
] MeOD
,
23 °C, TMS): δ 9.00 (d, J =
4
600 FT–IR spectrometer. UV-vis spectra were recorded using JASCO
V570 and V760 spectrometers. The 1H and 13C NMR spectra were
measured by a Bruker Biospin AVANCE III 500 spectrometer using CDCl
or CD OD as solvents and tetramethylsilane (TMS) as a reference. High-
3.0 Hz. 1H), 8.86 (d, J = 3.0 Hz 1H) ppm.
3
2
-Chloro-3,5-dinitropyridine (2): Following the published procedure [20], to
3
a dimethylformamide (DMF) solution (16 mL) of 1 (5 g, 27.0 mmol), POCl
9.8 mL) was added dropwise in an ice bath and, then, stirred at 100 °C
for 4 h. The resulting solution was neutralized with a Na CO solution at rt
and extracted with CHCl three times. The combined organic layer was
dried over MgSO and evaporated. The crude product was purified by a
3
resolution electrospray ionization mass spectra were recorded using a
Bruker micrOTOF spectrometer. The melting points were obtained using a
Büchi melting point apparatus M–560 (uncalibrated).
(
2
3
3
4
Single crystal X-ray diffraction (SXRD): The crystallographic data are
summarized in Table S1. Suitable single crystals were glued onto a glass
fiber using epoxy resin. All of the X-ray data were collected on Rigaku R-
AXIS rapid or Bruker APEX-II diffractometers with graphite
silica gel column chromatography eluted with n-hexane and AcOEt (10:1)
to afford 3.5 g of 2 as a yellowish solid in 63.9% yield. H NMR (500 MHz,
CDCl3, 23 °C, TMS) δ 9.12 (d, J = 2.5 Hz. 1H), 8.90 (d, J = 2.0 Hz 1H) ppm.
1
 
monochromated CuK or MoK radiation ( = 1.54187 or 0.71075 Å),
respectively. Reflections were collected at (90 ± 1) or (93 ± 1) K. The
molecular structures were solved using direct methods (SIR 92 or SHELXS
2
-Chloro-3,5-diaminopyridine (3): 2 (2.0 g, 9.82 mmol) was dissolved in a
35% HCl solution (12 mL) in an ice bath. Then, Sn powder (5.07 g) was
added to the HCl solution, and the resulting solution was stirred at 70 °C
for 1.5 h. The solution was cooled to rt and neutralized with a NaOH
solution. The solution was extracted with CHCl
combined organic layer was dried over MgSO
vacuum. The crude product was purified by elution with AcOEt in a silica
gel column chromatography to afford 3 (405.2 mg) as a white solid in
9
7) [35, 36]. The refinements were performed using the full-matrix least
squares method in the Crystal Structure software package [37], yielding
structures in P2 /c (no. 14) for 15Nap-Cl, 15Nap-AN, and 15Nap-OH, and
3
three times and the
and evaporated in a
1
4
Fdd2 (no. 43) for 15Nap-DEA. All of the non-hydrogen atoms were refined
anisotropically, and the hydrogen atoms were refined isotropically. The
crystallographic data for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data Center as
supplementary publications no. CCDC 1945404−1945406, and 1945418
for 15Nap-Cl, -DEA, -OH, and -AN, respectively.
1
2
1
8.7% yield. H NMR (500 MHz, CDCl3, 23 °C, TMS) δ7.31 (d, J = 2.6 Hz,
H), 6.38 (d, J = 2.6 Hz, 1H), 3.95 (br, 2H), 3.58 (br, 2H) ppm; 13C NMR
(
126 MHz, [D
ppm; Elemental analysis calcd (%) for C
found C 41.95, H 4.17, N 29.13. m.p. 75−77 °C.
4
] MeOD, 23 °C, TMS) δ 146.5, 142.6, 126.0, 125.8, 109.6
5 7
H
ClN : C 41.83, H 4.21, N 29.27;
2
Manufacturing the amine sensor: To an acetonitrile solution containing
PMMA (ca. 1 mg), 10% w/w 15Nap-Cl with respect to PMMA was added
and stirred at rt until a transparent solution was obtained. The resulting
solution was coated on a glass plate using a pustule pipette and dried at rt
for several hours. Then, the corresponding amine species were added
dropwise to the films on the glass plates, and the fluorescence changes
were monitored by eye.
2
a
-Cholo-6,8-bis(trifluoromethyl)-1,5-naphthyridin-3-amine (15Nap-Cl) To
1,4-dioxane solution (10 mL) of (1.35 g, 9.40 mmol) and
3
montmorillonite K10 (2.7 g), hexafluoroacetylacetone (1.46 mL, 10.34
mmol) was added dropwise, and the mixture was refluxed for 9.5 h. After
the further addition of hexafluoroacetylacetone (0.7 mL), the resulting
solution was refluxed for a further 1 h. The solution was cooled to rt and
filtered through celite. The residue was carefully washed with AcOEt. The
filtrate was evaporated, and the crude product was purified by a silica gel
Computational details: The excited geometries of 15Nap-Cl−TEA were
optimized at the TD-wB97XD level in the gas phase using the def2-SVP
basis set as implemented in the 16A.03 revision of the Gaussian 16
program package. All optimized geometries were confirmed as minima by
column chromatography eluted with a mixture of n-hexane and AcOEt
1
(10:1) to afford 4 (2.72 g) as a yellowish solid in 91.5% yield. H NMR (500
MHz, CDCl3, 23 °C, TMS): δ 7.93 (s, 1H), 7.64 (s, 1H), 4.83 (br, 2H) ppm;
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Chemistry - A European Journal
10.1002/chem.201903643
FULL PAPER
13C-NMR (126 MHz, [D
45.3, 144.7, 144.6, 134.3 (q, Jc-c-f = 31.6 Hz), 129.4, 122.2 (q, Jc-f = 275.7
23 °C, TMS) δ147.1 (q, Jc-c-f = 35.2 Hz),
[D6] DMSO, 23 °C, TMS): δ 157.9, 143.2, 143.1, 140.1 (q, Jc-c-f = 34.9 Hz),
125.5, 122.2 (q, Jc-f = 274.7 Hz), 121.4 (q, Jc-f = 274.3 Hz), 110.5, 104.9
6
] DMSO
,
1
Hz), 120.8 (q, Jc-f = 276.0 Hz), 113.4, 112.6 ppm; MS (ESI, MeOH): m/z:
ppm; MS (ESI, MeOH): m/z: calcd (%) for C10
H] ); found 296.0278; Elemental analysis calcd (%) for C10
H
5
F
6
N
3
O: 296.0264 ( [M -
-
-
calcd (%) for C10
4 6 3
H ClF N : 313.9925 ([M - H] ); found 313.9947; IR: 3489,
H F
5
6
N
3
O: calc.
351 cm-1 (-NH
C 40.42, H 1.7, N 14.14; found C 40.83, H 1.93, N 13.56; IR: 3485 cm (-
-1
3
2
); m.p. 160−165 °C.
NH-), 3339, 3194 cm-1 (-NH ), 1681 cm-1 (-C=O); m. p. 264°C (DSC).
2
N-ethyl-6,8-bis(trifluoromethyl)-1,5-naphthyridine-2,3-diamine: (15Nap-
EA): To a 10 mL water solution of 15Nap-Cl (100 mg, 0.36 mmol), a 70%
ethylamine (2.64 mL, 31.6 mmol) was added dropwise and stirred rt for 4
day. The mixture was neutralized by 10% HCl and evaporated in a vacuum.
The curude product was purified by a silica gel column chromatography
eluted with n-hexane : AcOEt (10:1), to afford 15Nap-EA as a white color
solid (93.5 mg) in 91% yield. 1H NMR (500 MHz, [D4] MeOD, 23 °C,
TMS):δ 7.70 (s, 1H), 7.15 (s, 1H), 3.69 (q, J = 7.5Hz, 2H), 1.33 (t, J = 7.0Hz,
Acknowledgements
This work was partially supported by the JST PRESTO (grant
numbers JPMJPR14K4 for S.K) and a Grant-in-Aid for Scientific
Research (C) (19K07016 for S.K.).
3
1
H) ppm; 13C NMR (126 MHz, [D4] MeOD, 23 °C, TMS): δ 151.8, 145.9,
41.4 (q, Jc-c-f = 34.4 Hz), 139.1, 135.6, 131.1 (q, Jc-c-f = 30.7 Hz), 124.8 (q,
Keywords: Naphthyridine •Exciplex • Amine sensor •
Fluorescence change • Film
J
c-f = 273.8 Hz), 123.3 (q, Jc-f = 271.7 Hz), 112.5, 110.4, 37.4, 14.6 ppm;
-
MS (ESI, MeOH) : m/z: calcd (%) for C12
10 6
H F N
4
: 323.0737( [M - H] ); found
3
23.0732; Elemental analysis calcd (%) for C12
10 6 4
H F N : calc. C 44.45, H
-
1
,
3.11, N 17.28; found C 44.76, H 2.94, N 16.92; IR : 3458 cm (-NH
2
); m.
[1] V. P. Aneja, P. A. Roelle, G. C. Murray, J. Southerland, J. W. Erisman , W. A.
p. 172−173 °C.
H. Asman, N. Patni Atmos. Environ.2001, 35, 1903−1911.
[
[
[
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J. 2018, 24, 7369−7373.
N,N-diethyl-6,8-bis(trifluoromethyl)-1,5-naphthyridine-2,3-diamine
(
15Nap-DEA): To a 10 mL water solution of 15Nap-Cl (100 mg, 0.32
mmol), diethylamine (3.2 mL, 31.6 mmol) was added and stirred at 50°C
for 9.5 h. The mixture was cooled to rt and extracted with Et O three times.
The combined organic layer was dried over MgSO and evaporated. The
[
5] S. Bagchi, R. Achla, S. K. Mondal. IEEE Sens. Lett. 2018, 2, 1−4.
6] Y. Zhang, B. R. Bunes, N. Wu, A. Ansari, S. Rajabali, L. Zang Sensor Actuat
B-Chem 2018, 255, 1814−1818.
2
[
4
crude product was purified by a silica gel column chromatography eluted
[
[
[
[
7] Upender R. G., Anila, H. A., F. Ali, N. Tate, S. Chattopadhyay, and A. Das,
with n-hexane and AcOEt (10:1), to afford 15Nap-DEA (93.9 mg) as a
yellowish solid in 84% yield. 1H NMR (500 MHz, CDCl
Org, Lett. 2015, 17, 5532−5535.
8] S.W. Hong, C.-H. Ahn, J. Huh, W. H. Jo, Macromolecules 2006, 39,
7694−7700.
3
, 23 °C, TMS):δ
7
=
1
1
1
-
.83 (s, 1H), 7.40 (s, 1H), 4.24 (br, 2H), 3.50 (q, J = 7.0Hz, 4H), 1.23 (t, J
7.0 Hz, 6H) ppm; 13C NMR (126 MHz, [D4] MeOD, 23°C, TMS):δ 154.7,
9] M. Gao, S. Li, Y. Lin, Y. Geng, X. Ling, L. Wang, A. Qin, B. Z. Tang, ACS
44.6, 144.2 (q, Jc-c-f = 34.0 Hz), 139.6, 133.2, 133.1 (q, Jc-c-f = 31.5 Hz),
23.1 (q, Jc-f = 275.1 Hz), 121.6 (q, Jc-f = 275.1 Hz), 115.3, 113.2, 44.0,
Sens. 2016, 1, 179−184.
10] G. Niu, P. Zhang, W. Liu, M. Wang, H. Zhang, J. Wu, L. Zhang, P. Wang,
Anal. Chem. 2017, 89, 1922−1929.
3.3 ppm; MS (ESI, MeOH) : m/z: calcd (%) for C14
H
14
F
6
N
4
: 351.1050 ( [M
-
-1
2
H] ); found 351.1079; IR: 3453, 3323 cm (-NH ); m. p. 153−155 °C.
[
[
[
11] Y. Abe, S. Karasawa, N. Koga Chem. Eur. J. 2012, 18, 15038−15048.
12] N. Harada, Y. Abe, S. Karasawa, N. Koga Org. Lett. 2012,14, 6282−6285.
13] N. Harada, S. Karasawa, T. Matsumoto, N. Koga Cryst.Growth Des. 2013,
N-phenyl-6,8-bis(trifluoromethyl)-1,5-naphthyridine-2,3-diamine (15Nap-
AN): To a 10 mL EtOH solution of 15Nap-Cl (100 mg, 0.32 mmol), aniline
1
3, 4705−4713.
14] S. Matsumoto, K. Usui, Y. Fuchi, S. Karasawa, J. Org. Chem. 2019, 84,
612−6622.
15] S. Karasawa, J. Todo, K. Usui, N. Harada, K. Yoza, N. Koga Chem. Eur. J.
016 16, 7771−7781.
(3 mL, 31.6 mmol) was added dropwise and stirred at 70°C for 37 h. The
[
[
[
mixture was cooled to rt and was neutralized with a 10% HCl solution. The
resulting solution was extracted with AcOEt three times. The combined
organic layer was dried over MgSO4 and evaporated. The crude product
was purified by a silica gel column chromatography eluted with n-hexane
6
2
16] S. Karasawa, R. Hagihara, Y. Abe, N. Harada, J. Todo, N. Koga Cryst.
and AcOEt (10:1), to afford 15Nap-AN (77.2 mg) as a yellowidh solid in
Growth Des. 2014, 14, 2468−2478.
17] R. Hagihara, K. Usui, S. Karasawa Dyes Pigm. 2017, 143, 401− 408.
1
6
5% yield. H NMR (500 MHz, CDCl
3
, 23 °C, TMS):δ 7.91 (s, 1H), 7.89 (d,
[
[
J = 8.0 Hz 2H), 7.58 (s, 1H), 7.41 (t, J = 8.0 Hz 2H), 7.26 (s, 1H), 7.23 (br,
18]
R.
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1
2
1
H), 7.15 (t, J = 7.5 Hz 1H), 3.91(br, 2H) ppm; 13C NMR (126 MHz, CDCl
,
3
3°C, TMS): δ 149.2, 143.5, 143.5 (q, Jc-c-f = 35.5 Hz), 138.7, 135.2, 135.5,
32.6 (q, Jc-c-f = 31.6Hz), 129.1, 124.0, 123.0 (q, Jc-f = 275.1 Hz), 121.6 (q,
[
[
[
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c-f = 275.1 Hz), 120.1, 119.2, 114.8 ppm; MS (ESI, MeOH): m/z: calcd (%)
-
-1
10 6 4 2
for C16H F N : 371.0731 ([M - H] ); found 371.0779; IR: 3400 cm (-NH );
2019, 123, 11224−11232.
m. p. 118−119 °C.
[
[
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2
-amino-6,8-bis(trifluoromethyl)-1,5-naphthyridin-2-ol (15Nap-OH): To a
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[
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cooled to rt and was neutralized with a 10% HCl. The resulting solution
[
[
was extracted with 10% HCl solution and extracted with Et
2
O three times.
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[
with n-hexane and AcOEt (6:1), to afford 15Nap-OH (86 mg) as a white
7785−7792.
1
solid in 91% yield. H NMR (500 MHz, [D6] DMSO, 23°C, TMS): δ 11.78
(
br, 1H), 7.71 (s, 1H), 6.89 (s, 1H), 6.62 (br, 2H) ppm; 13C NMR (126 MHz,
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Chemistry - A European Journal
10.1002/chem.201903643
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Chemistry - A European Journal
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FULL PAPER
Entry for the Table of Contents
FULL PAPER
A
push–pull-type 1,5-naphthyridine-
J. Hirota, K. Usui, Y. Fuchi, M. Sakuma,
S. Matsumoto, R. Hagihara, and S.
Karasawa*
based fluorophore (1) capable of
recognizing various amine species was
prepared. Adding various amines to a
solution of
1
resulted in long-
Page No. – Page No.
wavelength fluorescence in aprotic
solvents because of the formation of an
exciplex. For the development of an
amine sensor, 1 was dispersed in a
polymer to yield a sensor. Adding the
amine species dropwise to the sensor
film resulted in different colored
emissions.
Fluorescence Properties and Exciplex
Formation of Emissive Naphthyridine
Derivatives: Applications as Sensors for
Amine Species
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