Detection of amine vapors using luminescent xerogels from supramolecular metal-containing gelator

Article abstract of DOI:10.1246/bcsj.20170409

Supramolecular fluorescent gelators containing a tris(β- diketonato) complex are synthesized by using gelation-driving chelates, and their gelation abilities are studied with 15 solvents. Thin-layer films are prepared on quartz plates from the solutions and they are studied as chemosensors for amines. Fluorescence-quenching of the thin-layer films upon exposure to saturated primary and secondary amine vapors is monitored to evaluate the abilities of the chemosensors to detect amines. The morphologies of the thin-layer films are observed by transmission electron microscopy (TEM) and discussed in relation to their fluorescence-quenching. The fluorescence-quenching efficiencies upon exposure to saturated primary and secondary amines depend on the basicity and bulkiness of the amines rather than the vapor pressure. The fluorescence-quenching is caused by decomposition of a complex through nucleophilic addition of primary or secondary amines to its carbonyl group. The detection of tertiary amines is performed by monitoring the fluorescence emission from the thin-layer films, which are composed of a ligand and EuCl₃. The emergence of fluorescence originates from the formation of fluorescent Eu³⁺-containing gelator, in which dehydrochloric acid by tertiary amines is a trigger for the complexation.

Full text of DOI:10.1246/bcsj.20170409

Advance Publication Cover Page
Detection of Amine Vapors using Luminescent Xerogels from
Supramolecular Metal-Containing Gelator
Junpei Sasaki, Masahiro Suzuki, Kenji Hanabusa*
Advance Publication on the web January 17, 2018
doi:10.1246/bcsj.20170409
©
2018 The Chemical Society of Japan

Detection of Amine Vapors using Luminescent Xerogels from Supramolecular Metal-
Containing Gelator
1
2
2,3
Junpei Sasaki, Masahiro Suzuki, Kenji Hanabusa*
1
Faculty of Textile Science & Technology, Shinshu University, Ueda, 386-8567 Japan
2
3
Interdisciplinary Graduate School of Science & Technology, Shinshu University, Ueda, 386-8567 Japan
Division of Frontier Fibers, Institute for Fiber Engineering, ICCER, Shinshu University, Ueda, 386-8567 Japan
E-mail: hanaken@shinshu-u.ac.jp
Kenji Hanabusa
Abstract
Supramolecular fluorescent gelators containing a tris(-
crystallization. Calixarene derivatives,3-5 triphenylamine
6
,14
a a
bulky medium-sized molecule,8 and
derivatives,
1
2
diketonato) complex are synthesized by using gelation-driving
chelates, and their gelation abilities are studied with 15 solvents.
Thin-layer films are prepared on quartz plates from the solutions
and they are studied as chemosensors for amines. Fluorescence-
quenching of the thin-layer films upon exposure to saturated
primary and secondary amine vapors is monitored to evaluate
the abilities of the chemosensors to detect amines. The
morphologies of the thin-layer films are observed by
Transmission electron microscopy (TEM) and discussed in
relation to their fluorescence-quenching. The fluorescence-
quenching efficiencies upon exposure to saturated primary and
secondary amines depend on the basicity and bulkiness of the
amines rather than the vapor pressure. The fluorescence-
quenching is caused by decomposition of a complex through
nucleophilic addition of primary or secondary amines to its
carbonyl group. The detection of tertiary amines is performed by
monitoring the fluorescence emission from the thin-layer films,
conjugated polymer were reported as amorphous sensing
materials. It is also noteworthy that a fibrous xerogel film
prepared with gels formed using a gelator was firstly used as an
1
0
amine sensor. One of the noticeable features of gelators is their
formation of gels with self-assembled fibers rather than crystals.
As the xerogels derived from gels are composed of three
dimensional fibrous networks with high surface-to-volume
ratios, thin-layer films of xerogels are useful for sensor
applications.
Over the last two decades, we have developed many low-
molecular-weight gelators and proposed the concept of a
“gelation-driving segment.” This gelation-driving segment is
expected to be a useful tool for developing various types of
1
5
gelators. We prepared fluorescent gelators containing a
gelation-driving segment and reported thin-layer xerogel
chemosensors to detect explosives such as TNT and RDX by
1
6,17
fluorescence-quenching.
In this paper, we report the
which are composed of a ligand and EuCl
3
. The emergence of
synthesis of trifluoroacetylacetone-based metal-containing
gelators, and we employ fibrous thin-layer films derived from
the gelators as chemosensors for the detection of amines. Herein,
we demonstrate two different procedures to detect amines; one
is for detecting primary and secondary amines by monitoring
fluorescence-quenching, while the other is for exclusively
detecting tertiary amines by monitoring fluorescence emission
2. Experimental
3
+
fluorescence originates from the formation of fluorescent Eu -
containing gelator, in which dehydrochloric acid by tertiary
amines is a trigger for the complexation
1
. Introduction
The detection of organic amines in a trace vapor sample
has received and continues to receive considerable attention
because these are widely used in food industries, gas treatment
2.1. Instrumentation
Elemental analysis was performed with a Perkin-Elmer
2400 IICHNS/O analyzer. Infrared spectra were recorded on a
Jasco FS-420 spectrometer using KBr plate. Fluorescence
spectra were recorded on a Jasco FP-6300 spectrometer.
Transmission electron microscopy (TEM) and fluorescence
microscopy (FM) were performed with a JEOL JEM-SS and an
1
plants, and pharmaceutical production in spite of the fact that
low-molecular-weight amines are toxic and easily absorbed by
the body. Much research has been devoted to developing sensors
2
for amines in solutions; however, studies on solid-state sensors
for detecting amines have been reported only in a limited number
3
-14
1
of papers.
Axio Imager M1 microscope, respectively. H NMR spectra
High surface-to-volume ratios, which facilitate the
sensing of analytes adsorbed on their surfaces, are indispensable
in developing solid-state sensors. The adsorption of sensing
molecules on supports with a high surface area requires a high
surface-to-volume ratio; for instance, microcrystalline
cellulose, filter paper, porous polymers, and TLC were used
as supports. Amorphous sensing materials are also useful for
developing solid-state sensors, because such compounds can
form amorphous structures with high surface areas without
were recorded on a Bruker AVANCE 400 spectrometer. Dynamic
force mode (DFM) of scanning probe microscope was done with
a SII SPA-4001. Wide angle X-ray diffraction (WAXD) was
performed with a Rigaku Rotaflex RU-200B.
2.2. Gelation test
7
11
9
13
Gelation test was carried out by an upside-down test tube
method. Atypical procedure is as follows: Aweighed sample and
1 mL of solvent in a septum-capped test tube with internal
diameter of 14 mm was heated until the solid dissolved. The

resulting solution was cooled at 25°C for 2 h and then the
gelation was checked visually. When no fluid ran down the wall
of the test tube upon inversion of the test tube, we judged it to be
gel. The gelation ability was evaluated by the minimum gel
concentration of a gelator being necessary for gelation at 25°C.
The unit is g L (gelator/solvent). The solvents used for gelation
test were hexane, dodecane, cyclohexane, methanol, ethanol, 1-
propanol, acetone, ethyl acetate, THF, 1,4-dioxane, DMF,
DMSO, γ-BL (-butyrolactone), toluene, and chloroform.
DMF at 40ºC. To the solution was added 0.386 g (2.8 mmol) of
CO and 0.332 g (2.0 mmol) of KI, followed by stirred at 40℃
K
2
3
overnight under an argon atmosphere. The mixture was poured
to iced water and acidified by conc. HCl. A precipitated matter
was filtered off, washed with water, and dried. Recrystallization
from ethanol gave the compound 4 in a yield of 1.11 g (71%).
-
1
-
1
-1
FT-IR (KBr); 1633 cm (νC=O, amide I), 1603 cm (νC=O,
diketone). Found: C 69.19, H 10.29, N 3.69%. Calcd for
1
C
45
H
75
N
2
O
5
: C 69.20, H 9.68, N 3.59%. H NMR (400 MHz,
2
.3. Synthesis
CDCl
2H, J = 9.0 Hz, Phe), 6.50 (s, 1H, enol-OH), 6.07 (d, 1H, J = 8.1
Hz, CONHCH), 5.87 (t, 1H, J = 6.0 Hz, CONHCH ), 4.19 (t, 1H,
J = 8.1 Hz, CONHCHCONH), 4.04 (t, 2H, J = 6.5 Hz,
OCH CH ), 3.72 (q, 1H, J = 7.0 Hz, CH(CH )(CH CH )), 3.14-
3.35 (m, 2H, CONHCH ), 2.20 (t, 2H, J = 7.56 Hz, CH CONH),
1.76-1.88 (m, 1H, COCHCOH), 1.41-1.57 (m, 2H,
CONHCH CH ), 1.20-1.39 (m, 48H, alkyl and
CH(CH )(CH CH )), 0.85-0.95 (m, 9H, CH ).
3
, TMS, 25 ºC) δ = 7.92 (d, 2H, J = 9.0 Hz, Phe), 6.97 (d,
Compound 1: A mixture of 21 mL (176 mmol) of ethyl
trifluoroacetate, 18.4 g (135 mmol) of 4-hydroxyacetophenone,
and 1.26 mL of ethanol was dissolved in 77 mL of dry DMF. To
the reaction mixture, 12.7 g (317 mmol) of NaH (60 wt% oil)
was added little by little at 0 ºC. The reaction mixture was stirred
for 2 h at room temperature, and then for 3 h at 35~40 ºC. After
adding crushed ice and 37 mL of 12 M HCl, the reaction mixture
was extracted with ether and washed with water. The ether layer
2
2
2
3
2
3
2
2
2
2
3
2
3
3
was dried with MgSO
4
and evaporated. The residue was
Complex 5: A solution of 2.00 g (2.87 mmol) of
compound 3 in 50 mLof ethanol was added to a solution of 0.350
g (0.960 mmol) of europium (III) chloride hexahydrate and 440
μL (3.16 mmol) of triethylamine in 10 mL of ethanol. The
mixture was stirred for 2 h at 50ºC, and then the resulting mixture
was poured on a crushed ice. A precipitated matter was collected
and recrystallized from a mixture of THF/hexane (60 mL/60 mL).
The complex 5 was obtained in a yield of 1.76 g (77%). FT-
recrystallized from a mixture of ligroin/THF (50 mL/80 mL).
The sublimation at 110ºC in vacuo (1 mmHg) of a crude product
followed by recrystallization from a mixture of toluene/ligroin
(80 mL/50 mL) provided the pure compound 1 in a yield of 15.7
-
1
-1
g (50%). FT-IR (KBr); 3377 cm (ν -OH, phenol), 1660 cm ,
-
1
-1
1
600 cm , 1576 cm (ν C=O, diketone), 1161 cm-1 (ν C-O, δ -
OH, phenol). Found: C 51.91, H 2.92%. Calcd for C10 : C
, TMS, 25 ºC) δ =
.90 (d, 2H, J = 8.8, Phe), 6.94 (d, 2H, J = 8.8, Phe), 6.50 (s, 1H,
7 3 3
H F O
1
-1
-1
5
7
1.74, H 3.04%. H NMR (400 MHz, CDCl
3
IR(KBr); 1735 cm (ν C=O, ester), 1636 cm (νC=O, amide I),
-
1
1542 cm (νC=O, diketone). Found: C 61.41, H 8.00, N 4.13%.
Calcd for C114 18Eu: C 61.05, H 7.96, N 3.75%.
enol-OH), 2.59 (s, 1H, COCHCOH).
174 9 6
H F N O
Compound 2: A mixture of 1.00 g (10 mmol) of succinic
anhydride and 3.84 g (10 mmol) of L-isoleucylamino-
octadecane was added in 300 mL of dry CH Cl and stirred
2 2
Complex 6: This compound was prepared from 0.150 g
(0.645 mmol) of 1, 0.0825 g (0.218 mmol) of europium (III)
chloride hexahydrate, and 100 μL (0.713 mmol) of triethylamine
according to the same procedure described above. The
recrystallization from a mixture of THF/hexane (10 mL/10 mL)
gave 0.15 g (86%) of complex 6.
1
6
overnight at room temperature. The reaction mixture was
evaporated and recrystallized from 450 mL of ethyl acetate to
-
1
give 4.59 g (95%) of the compound 2. FT-IR(KBr); 1707 cm (ν
C=O, carboxylic acid). Found: C 70.31, H 11.69, N 5.62%.
Complex 7-Eu: A solution of 1.00 g (1.28 mmol) of
compound 4 in 50 mL of ethanol was added to a solution of 0.17
g (0.47 mmol) of europium (III) chloride hexahydrate and 193
μL (1.41 mmol) of triethylamine in 10 mL of ethanol. The
mixture was stirred for 5 h at 50ºC, and then the resulting mixture
was poured on a crushed ice. A precipitated matter was collected
and recrystallized from a mixture of THF/hexane (20 mL/20 mL).
The complex 7-Eu was obtained in a yield of 0.59 g (58%).
1
Calcd for C28
400 MHz, CDCl
CONHCH), 6.23 (t, 1H, J = 5.6, CONHCH
CONHCHCONH), 3.09-3.37 (m, 2H, CONHCH
H
54
N
3
2
O
4
: C 69.66, H 11.27, N 5.80%. H NMR
(
, TMS, 25ºC) δ = 7.10 (d, 1H, J = 8.9,
), 4.21 (t, 1H, J = 8.4,
), 2.64-2.74 (m,
), 1.77-1.89 (m,
)), 1.44-1.58 (m, 2H, CONHCH CH ),
.21-1.34 (m, 32H, alkyl, CH(CH )(CH CH )), 0.84-0.94 (m,
H, CH ).
Compound 3: A mixture of 1.00 g (4.31 mmol) of
2
2
2
1
1
9
H, CH
H, CH(CH
2
CONH), 2.49-2.59 (m, 2H, HOOCCH
)(CH CH
2
3
2
3
2
2
3
2
3
3
Found: C 65.20, H 9.21, N 3.93%. Calcd for C135
222 9 6
H F N O15Eu:
-
1
C 65.06, H 8.98, N 3.54%. FT-IR(KBr); 1635 cm (νC=O,
-
1
compound 1, 2.08 g (4.31 mmol) of compound 2, 0.544 g (4.31
mmol) of DiPC (N,N’-diisopropylcarbodiimide), 1.27 g (4.31
mmol) of DPTS (4-dimethylamino pyridinium p-
amide I), 1599 cm (νC=O, diketone).
Complex 7-Tb: A solution of 0.438 g (0.561 mmol) of
compound 4 in 30 mL of methanol was added to a solution of
0.140 g (0.374 mmol) of terbium (III) chloride hexahydrate and
156 μL (1.41 mmol) of triethylamine in 10 mL of methanol. The
mixture was stirred for 3 h at 50ºC, and then the resulting mixture
was poured on a crushed ice. A precipitated matter was collected
and recrystallized from a mixture of THF/hexane (15 mL/25 mL).
The complex 7-Tb was obtained in a yield of 0.369 g (79%). FT-
2 2
toluenesulfonate) in 40 mL of dry CH Cl was stirred overnight
at 40ºC. The matter after evaporation was purified by
recrystallization from 50 mL of ethanol followed by from a
mixture of methanol/toluene (125 mL/25 mL). The compound 3
-
was obtained in a yield of 2.31 g (77%). FT-IR(KBr); 1761 cm
1
(
C
ν C=O, ester). Found: C 65.96, H 9.04, N 4.44%. Calcd for
1
-1
-1
38
H
59
F
3
N
2
O
6
: C 65.49, H 8.53, N 4.02%. H NMR (400 MHz,
, TMS, 25ºC) δ = 7.97 (d, 2H, J = 8.8, Phe), 7.26 (d, 2H,
J = 8.8, Phe), 6.54 (s, 1H, enol-OH), 6.34 (d, 1H, J = 8.7,
CONHCH), 5.94 (t, 1H, J = 5.7, CONHCH ), 4.23 (dd, 1H, J =
.0, 8.7, CONHCHCONH), 3.10-3.34 (m, 2H, CONHCH ), 2.95
t, 2H, J = 6.6, CH CONH), 2.66 (s, 1H, COCHCOH), 2.50-2.73
m, 2H, OCOCH ), 1.81-1.99 (m, 1H, CH(CH )(CH CH )),
.39-1.57 (m, 2H, CONHCH CH ), 1.17-1.35 (m, 32H, alkyl,
CH(CH )(CH CH )), 0.83-0.96 (m, 9H, CH ).
Compound 4: A mixture of 1.12 g (2.0 mmol) of N-11-
IR(KBr); 1635 cm (νC=O, amide I), 1600 cm (νC=O,
diketone). Found: C 64.69, H 9.40, N 3.81%. Calcd for
CDCl
3
C
135
H
222
F
9
N
6
O15Tb: C 64.88, H 8.95, N 3.36%.
2
Complex 7-Al: This compound was prepared from 0.400
7
2
g (0.513 mmol) of compound 4 and 0.0455 g (0.342 mmol) of
aluminum (III) chloride according to the same procedure
described in complex 7-Tb. Ayield; 0.295 g (73%). FT-IR(KBr);
(
(
2
2
3
2
3
-
1
-1
1
2
2
1635 cm (νC=O, amide I), 1600 cm (νC=O, diketone). Found:
3
2
3
3
C 68.09, H 9.61, N 3.81%. Calcd for C135
68.50, H 9.45, N 3.55%.
222 9 6
H F N O15Al: C
1
6
bromoundecanoylL-isoleucylaminooctadecane and 0.464 g
2.0 mmol) of the compound 1 was dissolved in 30 mL of dry
2.4. Preparation of thin-layer films as sensors
Thin-layer films as chemosensor were prepared by
(

directly drop-casting method. A typical procedure is as follows:
observed at 344, 370, 339, 342, and 348 nm, respectively.
-
3
The warm DMSO solution of complex 5 (3 µL of 1.0×10 M)
was drop-casted on a quartz plate of 15W×2D×50H and dried in
vacuum. Exposure to saturated amine vapor was done in a glass
tube vessel with a dimeter of 37 mm and a height of 110 mm, in
the bottom of which an amine source was placed (Figure S1).
Forty percent of methylamine solution in methanol and 25% of
ammonia solution were used as their vapor sources. Aniline,
butylamine, hexylamine, and 2-naphthylamine were used as they
were. Thin-layer films for detecting tertiary amines were
prepared as follows: The ligand 3 (0.696 mg, 1.0 mmol) and
0
.366 mg (1.0 mmol) of europium (III) chloride hexahydrate was
dissolved in 1 mL of DMSO and then 3 µL of the resulting
solution was drop-casted on a quartz plate of 15W×2D×50H and
dried in vacuum.
3
. Results and Discussion
.1. Gelation behavior of fluorescent metal-containing
gelators
Recently, several researchers have shown considerable
3
interest in studying gelators that can physically gel fluids. Such
gelators are of interest because they can immobilize substantial
1
8
volumes of solvent. Many gelators have so far been reported
and information regarding their gelation-driving segments has
been accumulated. Gelators of low-molecular-weight
compounds are characterized by thermally reversible sol–gel
transitions, because three-dimensional network structures are
built up through noncovalent interactions such as hydrogen
bonding, electrostatic interactions, van der Waals interactions,
and  interactions. To prepare gelators as ligands, we
1
6
employed a gelation-driving segment based on L-isoleucine.
Chelate gelators (3 and 4) and fluorescent metal-containing
gelators (complexes 5, 7-Eu, 7-Tb, and 7-Al) were prepared
according to Scheme 1. Complex 6, which had no gelation ability,
was also prepared for comparison. Complexes 5, 6, and 7-Eu are
3
+
3+
Eu -containing complexes, while 7-Tb and 7-Al contain Tb
3
+
and Al , respectively. Gelation tests were performed using the
upside-down test-tube method and their results for 15 solvents
are summarized in Table 1, where solvents are placed in order of
their dielectric constants. All the compounds were too soluble in
chloroform and THF to act as gelators. Complex 6, which is not
included in Table 1, was miscible in solvents having a high
polarity more than chloroform and almost insoluble in solvents
that have low polarity than 1,4-dioxane. Chelates 3 and 4, which
were constructed from a gelation-driving segment of L-
isoleucine, could gel 13 solvents but not chloroform and THF. In
3
+
spite of the strong gelation behavior of 3 and 4, their Eu -
containing complexes (5 and 7-Eu) gelled only DMSO and
resulted in “I” or “P” towards most solvents. The insolubility and
precipitation of 3 and 4 could be due to the strong solvophobic
3
+
character of their Eu complexes. Complex 7-Tb, which was
prepared from chelate 4 and terbium (III) chloride, could gel
polar solvents, such as 1,4-dioxane, ethyl acetate, acetone,
ethanol, methanol, DMF, DMSO, and γ-BL. Complex 7-Al was
an excellent gelator in terms of the number of solvents that could
be gelled. Considering that the gelation ability of 7-Al was equal
3
+
to that of chelate 4, its complexation with Al had a significant
influence on the formation of three dimensional networks for
gelation.
3
.2. UV and fluorescence spectroscopy
Fluorescence excitation spectra and emission spectra of
thin-layer films of 5, 6, 7-Eu, 7-Tb, and 7-Al are shown in
Figure S2, where the thin-layer films were prepared by drop-
casting 1 mM DMSO solutions onto quartz plates. The
maximum excitations (ex) of 5, 6, 7-Eu, 7-Tb, and 7-Al were
Scheme 1. Synthetic routes for the complexes.

Table 1. Gelation test of chelates and metal-containing gelators at 25°C
Solvent
Hexane
Dodecane
Cyclohexane
Toluene
3
4
5
I
I
P
S
PG
S
S
7-Eu
I
P
I
S
P
S
I
7-Tb
P
P
P
S
GO (40)
S
GO (40)
S
7-Al
GTL (10)
GTL (10)
GT (20)
GT (20)
GTL (40)
S
GTL (10)
GT (4)
GT (8)
GT (10)
GO (20)
S
GO (10)
GO (10)
GTL (10)
GTL (40)
GTL (40)
S
1
,4-Dioxane
Chloroform
Ethyl acetate
THF
GO (8)
S
GTL (10)
S
GO (20)
S
S
S
Acetone
PG
GTL (8)
GO (40)
GO (40)
GO (10)
GTL (10)
GTL (10)
GTL (8)
P
P
P
I
I
P
I
I
GO (20)
PG
GO (20)
S
1
-Propanol
GTL (40)
GO (40)
GO (40)
GTL (40)
GTL (20)
GTL (20)
Ethanol
Methanol
DMF
GO (40)
GTL (20)
GTL (10)
GT (4)
GT (10)
GO (20)
GO (20)
GT (20)
GT (4)
GTL (8)
I
GTL (20)
GTL (20)
P
DMSO
GTL (20)
I

-BL
GT: Transparent gel, GTL: Translucent gel, GO: Opaque gel, PG: Partial gel, I: Almost insoluble, P: Precipitation, S:
Soluble, -BL: -Butyrolactone.
The values indicate the minimum gel concentrations at 25°C; the units are g L (gelator/solvent).
-
1
The wavelengths of the maximum emissions (em) of 5, 6, 7-Eu,
-Tb, and 7-Al were found at 613, 613, 614, 545, and 412 nm,
aggregates will be fixed in thin-layer films prepared from
gelators. The high surface area resulting from the formation of
fibrous aggregates is an advantage for sensor applications.
7
according to their Stokes’ shifts. The detection of amines was
evaluated by monitoring the fluorescence-quenching of the
aforementioned fluorescence em peaks. We also studied the
concentration dependence of the absorption spectra of solutions
of 5 and 7. Figure 1 shows the absorption spectra of 5, 7-Eu, and
(a) Complex 5
20 mg/mL
10 mg/mL
7
-Tb in DMSO solutions at various concentrations. The
wavelengths of the absorption maxima (max) of 5 were
approximately constant at 380 nm regardless of the
concentrations (Figure 1a), where the decrease in absorbance as
the concentration increased was due to the formation of opaque
gels. The max of 7-Eu exhibited a red-shift from 376 to 387 nm
along with an increase in the concentration from 2.5 to 20 mg
5
.0 mg/mL
.5 mg/mL
2
340
370
400
−1
Wavelength (nm)
mL (Figure 1b). The max of 7-Tb also exhibited a red-shift
from 364 to 376 nm accompanied by an increase in the
−
1
concentration from 0.5 to 4.0 mg mL (Figure 1c). It should be
(b) Complex 7-Eu
mentioned that the minimum gel concentrations of 7-Eu and 7-
2
0 mg/mL
0 mg/mL
.0 mg/mL
−1
Tb in DMSO are 20 and 4 mg mL , respectively. The slight
decrease in absorbance in Figure 1b and Figure 1c is probably
due to the light scattering of gels of 7-Eu and 7-Tb. The
observed red-shift accompanied by an increase in the
concentration suggests that there are interactions, such as J-
aggregate formation, within complexes 7-Eu and 7-Tb. Since
the decamethylene segments in 7-Eu and 7-Tb insulate the
complex regions from the hydrogen bonding regions adjacent to
the L-isoleucine residue, the complex regions may form J-
aggregates without hindrance from the rigid intermolecular
hydrogen bonding regions. In the case of 5, the tris(-
diketonato)europium region was fixed close to the hydrogen
bonding region with a short ethylene segment; consequently, the
rigidity of the hydrogen bonding regions will prevent J-
aggregate formation.
1
5
2.5 mg/mL
350
380
410
Wavelength (nm)
(
c) Complex 7-Tb
4.0 mg/mL
2.0 mg/mL
1.0 mg/mL
0.5 mg/mL
3
.3. Fluorescent sensors for detecting amines
Thin-layer sensors were prepared by directly drop-casting
warm solutions onto quartz plates, followed by spin-coating and
drying in vacuo. Physical gelation by gelators occurs via the self-
aggregation of molecules driven by noncovalent bonding,
resulting in the formation of fibrous aggregates. The fibrous
aggregates ultimately form a 3D network structure; namely,
nanosized fibrous aggregates predictably form during the initial
stage of gelation. Therefore, it can be assumed that the fibrous
3
20
350
380
410
Wavelength (nm)
Figure 1. Absorption spectra of 5, 7-Eu, and 7-Tb in DMSO
solutions at various concentrations.

butylamine vapor (Figure 3b). The superiority of the sensing
abilities of complex 5 is attributed to the fact mentioned above;
namely, fibrous aggregates of juxtaposed and interlocked fibers
were formed in thin-layer films of 5.
Complex 5, DMSO (2 mM)
Complex 6, DMSO (2 mM) (c)
(a)
(a) Upon exposure to ammonia
1
00
Compound 5
Compound 6
8
6
4
0
0
0
5
5 %
74 %
46 %
0
.5 μm
0.5 μm
1
5 %
Compound 2ꢀDMSO (0.1 M)
20
0
Complex 5, DMSO (0.01 M) (b)
Complex 6, DMSO (0.01 M) (d)
0
10
20
30
Time (min)
b) Upon exposure to butylamine
(
1
00
8
2 %
93 %
86 %
8
0
0
6
Compound 5
Compound 6
5
0 µm
1
0 μm
50 μm
40
48 %
20
Figure 2. TEM and FM images.
0
(
a) TEM image of a xerogel prepared from DMSO gel of 5 (2
0
10
20
30
mM).
Time (min)
(
b) FM image of DMSO gel of 5 (10 mM).
(
c) TEM image of dried sample prepared from DMSO solution
Figure 3. Fluorescence-quenching efficiencies of thin-layer
films prepared from complexes 5 and 6 upon exposure to
saturated vapors of ammonia and butylamine.
of 6 (2 mM).
(d) FM image of DMSO solution of 6 (10 mM).
The fibrous aggregates fixed in thin-layer films; that is to
say, the morphology of fibrous aggregates in xerogels, can be
characterized by TEM. We also used FM, which enables direct
observation of a wet gel containing solvent. Figure 2a shows a
TEM image of a xerogel prepared from a warm DMSO solution
(a)
1
00
(b) Ethyl acetate
8
2 %
9
0 %
4 %
8
6
4
2
0
0
0
0
0
7
5
9 %
DMSO thin-layer
Ethyl acetate thin-layer
-
1
of complex 5 (2 mM; 4.48 g L ), in which fibrous aggregates of
juxtaposed and interlocked fibers with widths ranging from 27
to 104 nm were observed. Figure 2b shows an FM image of a
0
10
20
30
0
.5 μm
-
1
Time (min)
DMSO gel of 5 (10 mM; 22.4 g L ). The FM image was
constructed by monitoring the emission at 605~670 nm under
excitation at 365 nm. Fibers with widths ranging from 0.83 to
Figure 4. (a) Fluorescence quenching efficiencies of thin-layers
prepared from DMSO and ethyl acetate, when exposed to
saturated butylamine vapor, (b) TEM image of sample prepared
from ethyl acetate solution (2 mM) of 5.
2
.5 µm were observed in the DMSO gel of 5 (Figure 2b). On the
other hand, a TEM image of a sample prepared from a DMSO
-
1
solution (2 mM; 1.69 g L ) of complex 6, which had no gelation
ability, showed a gathering of nanoparticles, whose diameters
ranged from 18 to 54 nm (Figure 2c). An FM image of a DMSO
Thin-layer films were prepared by dropping DMSO and
ethyl acetate solutions of 5, and their fluorescence-quenching
efficiencies with butylamine were studied (Figure 4a). Complex
-
1
solution of 6 (10 mM; 8.45 g L ) also confirmed microstructures
with diameters of 5 to 20 µm (Figure 2d). The TEM and FM
images clearly show that fibrous aggregates responsible for
physical gelation were formed in a DMSO gel of complex 5; on
the contrary, nano- to micro-sized particles were observed in a
DMSO solution of complex 6. It can be expected that xerogel
films prepared by drop-casting a warm DMSO solution of 5 will
have a large surface area that can successfully make contact with
amine vapors, resulting in effective quenching.
We compared the sensing abilities of thin-layer films
prepared from 5 and 6. Figure 3 shows the fluorescence-
quenching efficiencies of the thin-layer films upon exposure to
saturated vapors of ammonia and butylamine. These thin-layer
films were prepared by dropping 3 µL of 1.0 mM DMSO
solutions of each compound. The excitation wavelength was 344
nm and the fluorescence-monitoring wavelength was 613 nm.
The fluorescence-quenching efficiencies of the thin-layer film of
complex 5 were ~55% and 74% after exposure to ammonia for
5
could gel DMSO but not ethyl acetate (see Table 1). The
fluorescence-quenching efficiencies of the thin-layer film
prepared from a warm DMSO solution were 82% and 93% after
1
0 and 30 min, respectively; whereas those of the thin-layer film
prepared from the ethyl acetate solution were limited to 59% and
2%. The efficiency of the quenching of the thin-layer film
8
prepared from the ethyl acetate solution of 5 was as low as that
of 6. Figure 4b shows a TEM image of a dry sample prepared
-
1
from an ethyl acetate solution (2 mM; 4.48 g L ) of complex 5,
which shows short and wide plate-like fibers with widths of ~50
nm. The short and wide plate-like fibers contrasted with the
juxtaposed and interlocked fibers observed in a xerogel prepared
from a warm DMSO solution (see Figure 2a).
The sensing abilities of thin-layer films were studied with
saturated vapors of various amines. When a thin layer (thickness
~
165 nm) prepared from a DMSO solution of complex 5 was
used as a fluorescent sensor for detecting hexylamine, the
quenching efficiencies were 94% and 96% after exposure for 20
and 30 min, respectively (Figure S3). The fluorescence-
quenching of thin layers of complex 5 upon exposure to
saturated vapors of ammonia, methylamine, butylamine,
1
0 and 30 min, respectively. On the contrary, the efficiencies of
complex 6 were limited to ~15% and 46%, respectively. A
similar tendency was observed with respect to the sensing
abilities when the complexes were exposed to saturated

hexylamine, aniline, and 1-naphthylamine for different time
periods is shown in Figure 5a, where the thin-layer films were
prepared by dropping 3 µL of 1.0 mM DMSO solution. Note the
vapor pressures for 25% ammonia solution (48.3 kPa), 40%
methylamine solution (31 kPa), butylamine (10.9 kPa),
hexylamine (0.866 kPa), aniline (0.04 kPa), and 1-
naphthylamine (0.00053 kPa). The fluorescence-quenching
efficiencies of the thin-layer films of 5 after exposure for 60 min
were ~97% (hexylamine), ~93% (butylamine), ~86%
to that to primary amines with respect to the fluorescence-
quenching efficiency, and (iii) tertiary amines fail to quench the
fluorescence of complex 5. From these results, it was deduced
that the fluorescence-quenching is caused by the decomposition
of complex 5 on the surface of the thin-layer film; namely,
nucleophilic addition of an unshared electron pair of the primary
or secondary amines to the carbonyl of the -diketonato group
3
+
in 5 results in decomposition of the Eu complex. Wang et al.
reported a similar mechanism of fluorescence-quenching via
7
(methylamine), ~74% (ammonia), ~59% (aniline), and ~40% (1-
nucleophilic addition of primary or secondary amines.
naphthylamine). These results suggest that there is no
relationship between the fluorescence-quenching efficiency and
the vapor pressure. The order of fluorescence-quenching
efficiencies shown in Figure 5a can be explained by the basicity
of amines rather than the vapor pressure. The pKa values of
hexylamine, butylamine, methylamine, ammonia, aniline, and 1-
naphthylamine are 10.87, 10.77, 10.51, 9.25, 4.61, and 3.92,
respectively. This order is coincident with that of the
fluorescence-quenching efficiency in Figure 5a.
100
8
2 %
0
.1 mM
.5 mM
80
80 %
76 %
0
60
40
20
0
65
%
1.0 mM
48%
5
.0 mM
0 mM
1
0
10
20
30
Time (min)
Figure 6. Fluorescence quenching by thin-layer films obtained
from various concentrations of DMSO solutions of 5 upon
exposure to saturated aniline vapor.
The relationship between the thickness of the thin-layer
films and the fluorescence-quenching efficiency was
investigated. The thickness of the films was determined from
DFM images and the thickness was controlled by varying the
concentrations of DMSO solutions of complex 5 with the drop
volume fixed at 3 µL. Although the thickness of the thin-layer
-
1
film prepared from the 0.1 mM (0.22 g L ) solution was too thin
to determine accurately, those prepared by direct drop-casting of
-
1
0
.5, 1.0, 5.0, and 10 mM (1.12, 2.24, 5.60, and 22.4 g L )
solutions were 50–60, 160–170, 200–230, and 380–400 nm,
respectively. Figure 6 shows the fluorescence-quenching of thin-
layer films with various thicknesses upon exposure to saturated
aniline vapor. The thin-layer film prepared from the 1.0 mM
solution (thickness; ~55 nm) showed the best quenching
efficiency; however, the fluorescence-quenching of the thin-
layer film prepared from the 10 mM solution was limited to
Figure 5. Fluorescence-quenching efficiencies by thin-layer
films prepared from complex 5 upon exposure to saturated
vapors of ammonia, methylamine, butylamine, hexylamine,
aniline,1-naphthylamine, methylamine, dimethylamine, and
dicyclohexylamine.
~
76% after exposure for 30 min. The lower quenching efficiency
of the film prepared from the 10 mM solution can be explained
by considering the residual molecules deep within the thin-layer
films, which are not quenched by exposure to aniline. Thus, we
can reasonably assume that thinner films exhibit more effective
fluorescence-quenching. However, it should be mentioned that
when the thin-layer film is extremely thin, the fluorescence is too
weak to detect. Choosing fluorescent materials having a large
emission intensity may be important for sensors, because the
thickness of the layers could then be reduced.
Next, we studied the fluorescence-quenching after
exposure to secondary amines. Figure 5b shows the
fluorescence-quenching of the thin-layer films upon exposure to
saturated dicyclohexylamine and dimethylamine. The
fluorescence-quenching efficiencies after exposure to
dicyclohexylamine and dimethylamine for 60 min were ~20%
and ~68%, respectively. Considering the high basicity of
dicyclohexylamine (pKa = 11.43) and dimethylamine (pKa =
1
0.73), the efficiencies of fluorescence-quenching by
dicyclohexylamine and dimethylamine were rather low. This can
be explained by taking into account the steric hindrance of
secondary amines. In particular, the bulky cyclohexyl groups
prevent dicyclohexylamine from coming into contact with
complex 5 on the surface of the thin-layer film. It is also
noteworthy that no fluorescence-quenching was observed when
the film was exposed to the saturated vapor of triethylamine
Figure 7. TEM images of xerogels prepared from DMSO loose
gels of complex 5 of 0.1, 1.0, and 5.0 mM.
(
=
pKa = 10.76, vapor pressure = 7.2 kPa) and tributylamine (pKa
12.5, vapor pressure = 0.018 kPa). As a summary of the
To clarify the importance of the thickness of the thin-layer
films, we measured the morphology of the aggregates by TEM.
Figure 7 shows TEM images of xerogels prepared from DMSO
loose gels of complex 5 at concentrations of 0.1, 1.0, and 5.0 mM.
foregoing results, we concluded that (i) the efficiency of
fluorescence-quenching by primary amines can be explained by
their basicity; (ii) the sensitivity to secondary amines is inferior

In the image of the film prepared from the 0.1 mM solution,
sparse rod-like aggregates with widths of 37–93 nm were
observed. The films prepared from 1.0 and 5.0 mM solutions,
which were the most appropriate concentrations for effective
fluorescence-quenching, exhibited long and slender fibers that
were juxtaposed and interlocked. Their widths were 19–30 nm
for 1.0 mM and 37–84 nm for 5.0 mM. The wide rod-like
aggregates are responsible for the lower quenching efficiency.
aniline.
4.0 Å
7
-Al
3
.8 Å
1
5 Å
7-Tb
1
0 Å
3.8 Å
7
.5 Å
3
.4. Fluorescent sensors based on 7-Eu, 7-Tb, and 7-Al
The sensing abilities of different kinds of complexes, i.e.,
-Eu, 7-Tb, and 7-Al, were studied as previously stated. The
7
-Eu
7
gelation abilities of 7-Tb and 7-Al are far superior to those of 5
and 7-Eu (Table 1). Figure 8 shows the efficiencies of
fluorescence-quenching of thin-layer films prepared from 1.0
Figure 9. WAXD patters for xerogels prepared from DMSO
gels of 7-Eu complex, 7-Tb, and 7-Al.
-
1
-1
mM DMSO solutions of 7-Eu (2.49 g L ), 7-Tb (2.50 g L ),
-
1
and 7-Al (2.37 g L ) by hexylamine and aniline. The efficiencies
of fluorescence-quenching of 7-Tb and 7-Al reached 95 and
Complexes 7-Tb and 7-Al exhibited high gelation abilities
and could gel almost all the solvents, including DMSO, DMF,
and γ-BL (Table 1). The effect of the solvents used to prepare the
thin-layer films on the fluorescence-quenching efficiencies was
studied. Figure 10 shows the efficiencies of fluorescence-
quenching of thin-layer films prepared from DMSO, DMF, and
γ-BL solutions of 7-Tb and 7-Al. The fluorescence of the thin-
layer films prepared from 7-Al was effectively quenched upon
exposure to saturated hexylamine vapor for 15 min (Figure 10a).
When the thin-layer films prepared from 7-Tb were exposed to
saturated aniline vapor with low vapor pressure and low basicity,
the fluorescence-quenching efficiencies were over 90%
regardless of the preparation solvents; in particular, the
fluorescence of the thin-layer film prepared from a γ-BL solution
was quantitatively quenched even after only 5 min (Figure 10b).
TEM images of xerogels prepared from DMSO, DMF, and γ-BL
solutions of 7-Tb and 7-Al revealed fibrous aggregates of
juxtaposed and interlocked fibers with widths of several tens of
nanometers (Figure S4).
9
7% after exposure to hexylamine for 30 min, respectively
(Figure 8a). The sensing abilities of 7-Tb and 7-Al are almost
comparable to that of complex 5. In spite of aniline being weakly
basic (pKa = 4.61), 7-Tb and 7-Al were so sensitive to aniline
that the fluorescence-quenching efficiencies were 91 and 94%,
respectively, after 30 min (Figure 8b). Considering that the
efficiency of fluorescence-quenching of 5 by aniline was limited
to 59% even after 60 min, the high sensitivity of 7-Tb and 7-Al
was very surprising to us. Moreover, the efficiency of
fluorescence-quenching of 7-Eu was substantially low, although
7
-Eu was prepared from the same ligand as 7-Tb and 7-Al. To
determine the reason for this, we studied the molecular packing
in xerogels by WAXD. Figure 9 shows WAXD patterns for
xerogels prepared from DMSO gels of 7-Eu, 7-Tb, and 7-Al.
The pattern of the xerogel from 7-Eu showed four diffraction
peaks corresponding to d-spacings of 15, 10, 7.5, and 3.8 Å, and
the xerogels from 7-Tb and 7-Al exhibited broad peaks at 3.8
and 4.0 Å, respectively, indicating that the three complexes had
different structures. Since the d-spacings of 15, 10, and 7.5 Å
observed in 7-Eu fully matched the ratio of 1:2/3:1/2, it appears
that a tetragonal-like aggregate was formed in the xerogel. A
broad diffraction peak with a 2θ value of ~24, commonly
observed in the three complexes, was ascribed to the short
distance between gelator molecules within the aggregates. The
low efficiency of fluorescence-quenching of the thin-layer films
of 7-Eu can be explained by taking into account the low surface
area of crystalline precipitates resulting from the high
crystallinity.
(a)
9
9 %
97 %
7 %
1
00
9
80
from γ-BL
from DMSO
from DMF
60
40
20
0
0
10
20
30
(
a)
Time (min)
9
9
7 %
5 %
1
00
(b)
9
9 %
91 %
0 %
81 %
1
00
8
6
4
2
0
0
0
0
0
9
8
0
0
7-Tb
7-Al
6
from γ-BL
from DMSO
from DMF
7-Eu
40
20
0
0
10
20
30
Time (min)
0
10
20
30
(
b)
Time (min)
1
00
94 %
1 %
9
80
60
40
20
0
40 %
Figure 10. Fluorescence-quenching efficiencies by thin-layer
films prepared from DMSO, DMF, and γ-butyrolactone
solutions of 7-Tb and 7-Al.
7
-Tb
7-Al
-Eu
7
(a) 7-Al upon exposure to saturated vapor of hexylamine.
(b) 7-Tb upon exposure to saturated vapor of aniline.
0
10
20
30
Time (min)
3
.5. Detection of tertiary amines by stimulating fluorescence
emission
It is difficult to detect tertiary amines because these have
Figure 8. Fluorescence-quenching efficiencies by thin-layer
films prepared from DMSO solutions of 7-Eu, 7-Tb, and 7-Al
upon exposure to saturated vapors of (a) hexylamine and (b)

no H-atom, which is required for the nucleophilic addition and a
subsequent H-elimination, and they also create steric hindrance.
To the best of our knowledge, studies on solid-state sensors for
pressure of the amines and could be explained by their basicity
and bulkiness. The fluorescence-quenching by primary and
secondary amines was caused by decomposition of the complex
through nucleophilic addition of primary or secondary amines to
6
,12
detecting tertiary amines are extremely limited.
the fluorescence of the thin-layer films prepared from complexes
, 7-Eu, 7-Tb, and 7-Al was not quenched when they were
In practice,
3
its carbonyl group. Thin-layer films composed of MCl and a
5
ligand emitted fluorescence when they were exposed to tertiary
amines. The increase in the fluorescence intensity was explained
by the formation of complex 5, which was produced by reaction
with tertiary amines. These films were characterized as unique
sensors to exclusively detect tertiary amines.
exposed to the vapors of tertiary amines that did not contain the
H-atom required for a nucleophilic addition and a subsequent H-
elimination. In order to detect tertiary amines, we conceived the
idea that if the vapor of a tertiary amine reacts with MCl
3
on a
thin-layer film composed of MCl and ligand, the
3
a
corresponding complex is formed by dehydrochloric acid and
then the complex generates fluorescence. With this idea in mind,
thin-layer films composed of ligand 3 and europium (III)
chloride hexahydrate were exposed to saturated vapors of
trimethylamine, triethylamine, and tributylamine. Although the
Acknowledgement
The present research was supported in part by JSPS
KAKENHI Grant Number JP15K05623.
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