Fluorescent gelators for detection of explosives

Article abstract of DOI:10.1246/bcsj.20160232

Carbazole-, quinoline-, benzothiazole-, and stilbene-containing fluorescent gelators are synthesized by connecting gelationdriving segments, and their gelation abilities are studied with 13 solvents. Fibrous thin-layer films are prepared on quartz plates

Full text of DOI:10.1246/bcsj.20160232

Advance Publication Cover Page
Fluorescent Gelators for Detection of Explosives
Kenji Hanabusa,* Shingo Takata, Masafumi Fujisaki, Yasushi Nomura, and Masahiro Suzuki
Advance Publication on the web July 29, 2016 by J-STAGE
doi:10.1246/bcsj.20160232
© 2016 The Chemical Society of Japan

Fluorescent gelators for detection of explosives
Kenji Hanabusa,*1,2 Shingo Takata,1 Masafumi Fujisaki,3 Yasushi Nomura,3 and Masahiro Suzuki1
1Interdisciplinary Graduate School of Science and Technology, Shinshu University, Ueda, 386-8567
2 Division of Frontier Fibers, Institute for Fiber Engineering, ICCER, Shinshu University, Ueda, 386-8567
3 Faculty of Textile Science & Technology, Shinshu University, Ueda, 386-8567
E-mail: hanaken@shinshu-u.ac.jp
Kenji Hanabusa
In 1979, Kenji Hanabusa started his academic carrier at Shinshu University as an Assistant Professor. He received
Ph.D. from Osaka University in 1981 in Japan. In 1987, he was promoted to an Associate Professor, and since 1999,
he has been a Full Professor in the Interdisciplinary Graduate School of Science and Technology at Shinshu University.
His research interest is the development of low-molecular weight gelators and thickeners on the basis of
supramolecular chemistry.
Abstract
Carbazole-, quinoline-, benzothiazole-, and stilbene-
oligomers, polymers, and dendrimers, respectively. Ajayaghosh
et al.8 examined oligo(p-phenylenevinylene)-based gelators for
TNT detection. Bhattacharya et al.9 reported the detection of
2,4,6-trinitrophenol in solutions by monitoring the fluorescence
quenching of a p-phenylenevinylene-based probe.
containing fluorescent gelators are synthesized by connecting
gelation-driving segments, and their gelation abilities are studied
with 13 solvents. Fibrous thin-layer films are prepared on quartz
plates from the solutions or gels, and they are studied as
chemosensors for explosives. Fluorescence quenching of the
films upon exposure to saturated TNT or RDX vapor is used to
evaluate the abilities of the films to detect explosives. The
relationship between the thickness of the thin-layer film and the
quenching efficiency upon exposure to TNT is studied. The
morphologies of the thin-layer films are observed by dynamic
force mode scanning probe microscopy and discussed with
regard to their fluorescence quenching. The interactions among
chromophores in the gels, thin-layer films, and solutions are
studied by variable-temperature spectroscopy. The mechanism
of TNT detection is discussed from the viewpoint of the HOMO
and LUMO energy levels.
In recent years, several researchers have shown considerable
interest in studying gelators that can physically gel water or
organic solvents. Such gelators are of interest because they can
immobilize substantial volumes of solvent. With advances in the
area of supramolecular chemistry, several reports on gelators
have been published in recent years.10-28 Gelators of low-
molecular-weight compounds have unique characteristics
including good solubility upon heating and the ability to induce
the smooth gelation of liquids at low concentration. Gelators are
also characterized by thermally reversible sol–gel transitions.
These features can be attributed to 3D network structures, which
are built up through noncovalent interactions such as hydrogen
bonding, electrostatic interaction, van der Waals interaction, and
 interaction. Although several gelators have already been
used as hardeners for spilled fluids and cooking oils, cosmetic
materials, and thickeners for paint, recent gelator research has
focused on applications such as drug delivery in
biomaterials,29,30 scaffold materials for cells in tissue
engineering,31,32 sensors33-36, templates for the synthesis of
inorganic nanostructures,37-41 auxiliary agents for producing
organic electronics,42,43 and electrolytes that prevent liquid
leakage.44
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
gelators.45
In this study, we report the synthesis of fluorescent gelators
using gelation-driving segments, and we employ fibrous thin-
layer films based on the gelators as chemosensors for the
detection of explosives. One of the most important features of
gelators is their formation of gels with self-assembled fibers
instead of crystals. Thin-layer films derived from gels have high
surface areas due to the formation of fibrous aggregates, which
is an advantage for sensor applications. We employed carbazole,
quinoline, benzothiazole, and stilbene as the fluorescent
segments. As the oxidative explosives, we focused on TNT and
1. Introduction
The recent rise of terrorism has become a matter of grave
concern owing to the immense damage that can be caused by
explosives. Scientists, chemists in particular, have developed
methods for the rapid detection of explosives. Several methods
have been practically applied to explosives detection, including
ion-mobility spectrometry, neutron backscattering, and
explosive-detecting dogs. However, these methods suffer from
many problems such as the need for non-portable heavy
instruments, technicians for operation (e.g., radiographers), and
high cost. A simple approach that utilizes reagents to detect
vapors from explosives is an attractive alternative that is small,
lightweight, and low-cost. Recently, Swager et al.1 successfully
detected nitro-aromatics using conjugated polymers containing
pentiptycene. Takeuchi et al.2 reported the detection of TNT
vapor using the quenching of donor–acceptor-substituted
stilbenes containing chiral binaphthyl groups. Liu et al.3 reported
a fluorescent sensor for TNT using electrospun nanofibrous
films of porphyrin. Zang et al.,4 Müllen et al.,5,6 and Han et al.7
successfully detected TNT using carbazole backbone-based
fluorescent compounds with amorphous morphologies owing to

RDX, which are the main components of plastic explosives.
7.49%. Calcd for C52H68N4O2: C 79.96, H 8.77, N 7.17%.
Quino-1: The preparation of this compound was reported in our
previous paper. 46
2. Experimental
2.1. Instrumentation
Quino-2: A mixture of 0.64 g (4.4 mmol) of 8-hydroxyquinoline,
0.88 g (6.6 mmol) of potassium carbonate, 0.66 g (4.0 mmol) of
potassium iodide, 1.70 g (4 mmol) of 6-bromohexyloxy-
cyclo(L-asparaginyl-L-phenylalanyl),46 and 20 mL of dry DMF
was stirred at 40°C overnight under an argon atmosphere. The
mixture was poured to iced water and the precipitated matter was
filtered off, washed with water, and dried. A crude product
obtained by recrystallization from ethyl acetate was purified by
column chromatography on silica gel (elution with
chloroform/methanol = 9:1; Rf = 0.7). Yield 0.56 g (29%). IR
(KBr, cm-1): 1740 (νC=O ester), 1671(νC=O amide I), 1108
(quinoline); Found: C 67.36, H 6.41, N 8.44%. Calcd for
C28H31N3O5: C 68.69, H 6.38, N 8.58%.
Elemental analysis was performed with a Perkin-Elmer
240B analyzer. Infrared spectra were recorded on a Jasco FTIR-
7300 spectrometer using KBr plate. UV-vis and fluorescence
spectra were recorded on a Jasco V-570UV/VIS/NIR and a Jasco
FP-6300. Dynamic force mode (DFM) of scanning probe
microscope was done with a SII SPA-4001. Thickness of films
was measured by a Yamato Scientific Alpha-Step D-600.
Ionization potential was measured by
photoemission spectrometer AC-2.
2.2. Synthesis of fluorescent gelators
a Riken Keiki
Carbz-1: The preparation of this compound was reported in our
previous paper. 46
Carbz-2: A mixture of 7.03 g (20 mmol) of 11-(9H-Carbzol-9-
Quino-3: This compound was obtained from a mixture of 0.64
g (4.4 mmol) of 8-hydroxyquinoline, 0.88 g (6.6 mmol) of
potassium carbonate, 0.66 g (4.0 mmol) of potassium iodide,
1.98 g (4 mmol) of 11-bromoundecyloxycyclo(L-asparaginyl-L-
phenylalanyl), and 20 mL of dry DMF by the similar procedure
described in Quino-2. Recrystallization from ethanol gave 1.46
g (65%) of Quino-3. Rf = 0.9 (chloroform/methanol = 9:1, silica
gel); IR (KBr, cm-1): 1741 (νC=O ester), 1671(νC=O amide I),
1101 (quinoline); Found: C 71.67, H 7.98, N 6.87%. Calcd for
C33H41N3O5: C 70.82, H 7.38, N 7.51%.
47
yl)undecanoic acid,
octadecane, 2.28
7.37 g (20 mmol) of L-valylamino-
(22 mmol) of DIPC (N,N’-
g
diisopropylcarbodiimide), 2.44 g (20 mmol) of DMAP (4-
dimethylaminopyridine), and 100 mL of dichloromethane was
refluxed overnight. Chloroform (100 mL) was added to the
reaction mixture and an insoluble matter was removed. The
resulting filtrate was washed with 100 mL of 1 M hydrochloric
acid, followed by 100 mL of water and dried. After evaporating,
recrystallization from methanol/ethanol (7:3) gave 8.02 g (57%)
of Carbz-2. Rf = 0.9 (chloroform, alumina); IR (KBr, cm-1):
3289 (νNH amide A), 1631 (νC=O amide I), 748, 719
(Carbazole); Found: C 78.72, H 11.26, N 6.13%. Calcd for
C46H75N3O2: C 78.69, H 10.77, N 5.98%.
Quino-4: This compound was obtained from a mixture of 0.64
g (4.4 mmol) of 8-hydroxyquinoline, 0.88 g (6.6 mmol) of
potassium carbonate, 0.66 g (4.0 mmol) of potassium iodide,
1.47
g
(2.0 mmol) of trans-(1R,2R)-bis(11-bromo-
Carbz-3: This compound was obtained from a mixture of 0.88
g (2.5 mmol) of 11-(9H-Carbzol-9-yl)undecanoic acid, 1.24 g
(2.5 mmol) of L-isoleucyl-L-isoleucylaminooctadecane, 46 0.35
g (2.75 mmol) of DIPC, 0.31 g (2.5 mmol) of DMAP, and 20 mL
of dichloromethane by the similar procedure described in
Carbz-2. Recrystallization from methanol gave 0.92 g (44%) of
Carbz-3. Rf = 0.82 (chloroform, alumina); IR (KBr, cm-1): 3283
(νNH amide A), 1633 (νC=O amide I), 747, 720 (Carbazole);
Found: C 76.36, H 11.05, N 6.54%. Calcd for C53H88N4O3: C
76.76, H 10.70, N 6.76%.
Carbz-4: To a solution of 8.36 g (50 mmol) of carbazole and
2.87 g (50 mmol) of potassium hydroxide in 500 mLof dry DMF,
was added 9.05 g (50 mmol) of 6-bromohexanol and 0.83 g (5
mmol) of potassium iodide. The resulting mixture was stirred
overnight under an argon atmosphere. The mixture was poured
to iced water and the precipitated matter was filtered off, washed
with water. Recrystallization from 400 mL of methanol gave 11-
(9H-Carbzol-9-yl)hexanol in a yield of 10.22 g (76%). Rf = 0.25
(chloroform, silica gel). A mixture of 2.62 g (10 mmol) of cyclo-
(L-asparaginyl-L-phenylalanyl), 47 2.67 g (10 mmol) of 11-(9H-
undecanoylamino)cyclohexane,46 and 20 mL of dry DMF by the
similar procedure described in Quino-2. A crude product
obtained by recrystallization from 70 mLof acetone was purified
by column chromatography on alumina (elution with
chloroform; Rf
=
0.7). Quino-4 was obtained by
recrystallization from 40 mL of acetone in a yield of 1.25 g
(85%). IR (KBr, cm-1): 1638 (νC=O amide I), 1108 (quinoline);
Found: C 73.25, H 8.96, N 7.35%. Calcd for C46H64N4O4: C
74.96, H 8.75, N 7.60%.
Benth-1, Benth-2, and Benth-3: These compounds were
prepared according to our previous paper.46
Stilb-1: Amixture of 2.16 g (10 mmol) of 4-nitrobenzyl bromide
and 2.62 g (10 mmol) of triphenylphosphine in 50 mL of toluene
was refluxed for 6 h. After evaporating toluene, the residue was
dried in vacuo. 4-Dimethylaminobenzaldehyde (1.49 g, 10
mmol), 3.69 g (10 mmol) of tetrabutylammonium iodide, and 70
mL of CH2Cl2 were added to the residue. The resulting solution
was added dropwise to a saturated solution of potassium
carbonate in 100 mL of water, and then refluxed overnight. The
separated organic layer after standing was washed with water
twice, and dried with MgSO4, followed by evaporated. A crude
product was purified by column chromatography on silica gel
(elution with chloroform; Rf = 0.70). Recrystallization from a
mixture of 40 mL of THF and 20 mL of hexane gave 1.18 g
(44%) of 4’-dimethylamino-4-nitrostilbene.
A mixture of 0.54 g (2.0 mmol) of 4’-dimethylamino-4-
nitrostilbene and 1.13 g (10 mmol) of SnCl2/2H2O in 40 mL of
ethanol was refluxed for 24 h under an argon atmosphere. The
cooled mixture was poured to ~30 g of ice and then 2 g of
NaHCO3 was added by portions. The matter was extracted with
ethyl acetate and washed with water twice. The obtained crude
product after evaporation was purified by column
chromatography on alumina (elution with chloroform; Rf = 0.65).
Recrystallization from ethanol/hexane gave 4-amino-4’-
dimethylaminostilbene in a yield of 0.28 g (59%). Found: C
80.44, H 7.80, N 11.42%. Calcd for C16H18N2: C 80.63, H 7.61,
Carbzol-9-yl)hexanol, 3.24
g (11 mmol) of DPTS (4-
dimethylamino pyridinium p-toluenesulfonate), 1.38 g (11
mmol) of DIPC, and 30 mL of dry DMF was stirred overnight at
35°C. After evaporating DMF, the residue was added to ice-
water. Recrystallization from 400 mL of 1-propanol gave 2.76 g
(54%) of Carbz-4. IR (KBr, cm-1): 1740 (νC=O ester),
1671(νC=O amide I), 750, 731 (Carbazole); Found: C 71.47, H
6.43, N 8.37%. Calcd for C31H33N3O4: C 72.78, H 6.50, N 8.21%.
Carbz-5: A mixture of 4.03 g (11.5 mmol) of 11-(9H-Carbzol-
9-yl)undecanoic acid, 0.66 g (5.74 mmol) of trans-(1R,2R)-
diaminocyclohexane, 1.59 g (12.6 mmol) of DIPC, 1.40 g (11.5
mmol) of DMAP, and 20 mL of dichloromethane was refluxed
overnight. After evaporation, the resulting product was
recrystallized from 200 mL of 1-propanol. Yield 3.36 g (75%);
Rf = 0.65 (chloroform, alumina); IR (KBr, cm-1): 1634 (νC=O
amide I), 751, 720 (Carbazole); Found: C 79.61, H 8.99, N

N 11.75%.
A mixture of 0.48
0.53 g (2.0 mmol) of triphenylphosphine, and 20 mL of toluene
was refluxed for 4 h. After evaporating, the matter was dissolved
in 40 mL of DMF, and then 0.12 g (3.0 mmol) of NaH (assay
60%) was added. The mixture was stirred at 40°C for 1 h under
an argon atmosphere. 11-(4-Formylphenoxy)undecanoyl-L-
isoleucylaminooctadecane (1.0 g, 1.5 mmol) was added to the
mixture and stirred at 40°C for 24 h under an argon atmosphere.
The mixture was poured to iced water and the precipitated matter
was filtered off, washed with water. Recrystallization from 40
mL of ethanol gave Stilb-4 in a yield of 0.92 g (80%). Rf = 0.78
(chloroform, alumina); IR (KBr, cm-1): 2225 (CN);
disappearance of 1687 (C=O, aldehyde); Found: C 78.16, H
10.60, N 5.24%. Calcd for C50H79N3O3: C 77.97, H 10.34, N
5.46%.
g
(2.0 mmol) of 4-amino-4’-
dimethylaminostilbene and 1.19 g (2.0 mmol) of N-(6-
isocyanato-trimethylhexylaminocarbonyl)-L-isoleucylamino-
octadecane48 in 30 mL of dry toluene was stirred at 85°C
overnight under an argon atmosphere. After evaporating toluene,
the purification from 60 mL of ethanol gave 1.18 g (71%) of
Stilb-1. Rf = 0.10 (chloroform, silica gel); IR(KBr, cm-1): 1640
(C=O, amide I); Found: C 73.12, H 11.06, N 9.82%. Calcd for
C51H86N6O3: C 7369, H 10.43, N 10.11%.
Stilb-2: A solution of 0.98 g (3.3 mmol) of triphosgene in 15 mL
of CH2Cl2 was cooled in ice-water bath, and then equimolecular
amounts of 2.42 g (10 mmol) of n-hexadecyl alcohol and 0.79 g
(10 mmol) of pyridine were added by small portions. The
mixture was stirred for 48 h at room temperature, evaporated,
and dried. The resulting matter was treated by 20 mL of hot THF
and NEt3/HCl was removed. After evaporating, the oily product
was purified by vacuum distillation (~200°C /3 mmHg). n-
Hexadecyl chloroformate was obtained in a yield of 2.26 g
(74%).
Amixture of 1.40 g (7.4 mmol) of L-isoleucine methyl ester
hydrochloride, 1.62 g (16 mmol) of NEt3, and 35 mL of dry THF
was cooled in ice-water bath, and then 2.26 g (7.4 mmol) of n-
hexadecyl chloroformate was added by small portions. After
stirred for 4 h at ambient temperature, NEt3/HCl was removed.
Recrystallization from 30 mL of ligroin gave 2.39 g (78%) of N-
n-hexadecyloxycarbonyl-L-isoleucine methyl ester. The ester of
2.39 g was hydrolyzed in 1.5 mL of 4M NaOH and 50 mL of
methanol. The precipitate after neutralization was recrystallized
from 30 mL of hexane to afford 2.52 g (85%) of N-n-
hexadecyloxycarbonyl-L-isoleucine. IR(KBr, cm-1): 1717 (C=O,
carboxylic acid), 1687 (C=O, urethane); Found: C 69.24, H
12.03, N 3.68%. Calcd for C23H45NO4: C 69.13, H 11.35, N
3.39%.
TNT and RDX: These explosives were prepared carefully
according to the literature50-52. TNT was obtained as pale yellow
plate crystal by recrystallization from
a
mixture of
tetrachloromethane and ethanol. RDX was obtained as colorless
plate crystal by recrystallization from a mixture of acetone and
ethanol. 1H-NMR for TNT (400 MHz, CDCl3, TMS, 25°C): δ =
8.84 (s, 2H), 2.72 (s, 3H). 1H-NMR for RDX (400 MHz, DMSO-
d6, TMS, 25°C): δ = 6.10 (s).
2.3. Gelation test
Gelation test was carried out by upside-down test tube method.
The concentration for gelation test was fixed to be 40 mg mL-1
(gelator/solvent). A typical procedure is as follows: Forty
milligrams of a sample and 1 mLof a 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 1
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.
2.4. Preparation of thin-layer films as sensors and exposure
to saturated explosive vapor
A
mixture of 0.31
g
(0.8 mmol) of N-n-
Thin-layer films as chemosensor were prepared by directly drop-
casting method. A typical procedure is as follows: The warm
ethanol solution of Carbz-1 (3 L of 1.0 x 10-3 M) was drop-
casted on a quartz plate of 15W x 2D x 50H, followed by spin-
coated, and then dried in vacuo. Exposure to saturated explosive
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 explosive was
placed.
hexadecyloxycarbonyl-L-isoleucine, 0.21 g (0.8 mmol) of trans-
4-(4-aminostyryl)benzonitrile,49 0.11 g (0.88 mmol) of DIPC,
0.10 g (0.8 mmol) of DMAP, and 25 mL of dichloromethane was
stirred at 35°C overnight. After evaporating, the purification
from 30 mL of ethanol gave 0.63 g (52%) of Stilb-2. Rf = 0.12
(chloroform, silica gel); IR(KBr, cm-1): 1693 (C=O, urethane);
1659 (C=O, amide); Found: C 75.74, H 9.28, N 7.11%. Calcd for
C38H55N3O3: C 75.83, H 9.21, N 6.98%.
2.5. Theoretical calculations
Stilb-3: This compound was obtained from a mixture of 0.80 g
(2.0 mmol) of N-n-hexadecyloxycarbonyl-L-isoleucine, 0.21 g
(1.0 mmol) of 4-amino-4’-aminostilbene, 0.28 g (2.2 mmol) of
DIPC, 0.25 g (2.0 mmol) of DMAP, and 25 mL of
dichloromethane by the similar procedure described in Stilb-2.
Recrystallization from 30 mL of 1-propanol gave 0.63 g (65%)
of Stilb-3. Rf = 0.62 (chloroform, alumina); IR (KBr, cm-1): 1691
(C=O, urethane); 1662 (C=O, amide); Found: C 74.16, H 10.67,
N 6.07%. Calcd for C60H100N4O6: C 74.03, H 10.35, N 5.76%.
The TNT was geometry-optimized in gas phase using the
B3LYP/6-31G* method, and the HOMO and LUMO levels were
calculated.
3. Results and Discussion
3.1. Fluorescent compounds and gelation abilities
The structures of the carbazole-containing gelators
(Carbz-1, Carbz-2, Carbz-3, Carbz-4, and Carbz-5) and the
quinoline-containing gelators (Quino-1, Quino-2, Quino-3, and
Quino-4) are shown in Scheme 1, and those of the
benzothiazole-containing gelators (Benth-1, Benth-2, and
Benth-3) and the stilbene-containing gelators (Stilb-1, Stilb-2,
Stilb-3, and Stilb-4) are shown in Scheme 2. These compounds
were prepared by Williamson synthesis or coupling reactions
with gelation-driving segments developed by us.45 The
compounds include an amino acid residue, trans-(1R,2R)-
diaminocyclohexane, and cyclo(L-asparaginyl-L-phenylalanyl)
as gelation-driving segments.
Gelation tests were carried out using the upside-down test-
tube method. The concentrations were fixed at 40 mg mL-1
(gelator/solvent). Because this concentration is generally enough
to cause physical gelation for good gelators. The results of the
gelation tests with 13 solvents are summarized in Table 1 and
Stilb-4:
hydroxybenzaldehyde, 0.84
carbonate, 0.33 g (2.0 mmol) of potassium iodide, 1.26 g (2.0
mmol) of N-11-bromoundecanoyl-L-isoleucylamino-
A
mixture of 0.25
g
(2.0 mmol) of p-
g
(3.2 mmol) of potassium
octadecane,49 and 40 mL of dry DMF was stirred at 60°C
overnight under an argon atmosphere. The mixture was poured
to iced water and the precipitated matter was filtered off, washed
with water, and dried. A crude product was recrystallized from
40 mL of methanol. 11-(4-Formylphenoxy)undecanoyl-L-
isoleucylaminooctadecane was obtained in a yield of 1.18 g
(88%). Rf = 0.78 (chloroform, alumina); IR (KBr, cm-1): 1691
(C=O, aldehyde); 1633 (C=O, amide); Found: C 75.31, H 11.01,
N 4.08%. Calcd for C42H74N2O4: C 75.17, H 11.12, N 4.17%.
A solution of 0.39 g (2.0 mmol) of p-cyanobenzyl bromide,

Table 2. All compounds were too soluble in chloroform to act as
gelators; in contrast, they were sparingly soluble in hexane. They
failed to cause gelation in THF. Carbz-1, Carbz-2, Carbz-3,
Quino-1, Benth-1, and Stilb-4 were excellent gelators in terms
of the number of solvents that could be gelled. These results
indicate that the amino acid-containing segment, especially
alkanoyl-L-isoleucylaminooctadecane, is very useful for driving
gelation. Considering that Carbz-4, Carbz-5, Quino-2, Quino-
3, Quino-4, Benth-2, and Benth-3 are not really good gelators,
we can conclude that trans-(1R,2R)-diaminocyclohexane and
cyclo(L-asparaginyl-L-phenylalanyl) are not suitable as gelation-
driving segments for the development of fluorescent gelators.
Carbz-4 and Carbz-5 were sparingly soluble or precipitated as
crystals in most solvents studied. This can be explained by taking
into account the strong intermolecular interactions of trans-
(1R,2R)-diaminocyclohexane or cyclo(L-asparaginyl-L-phenyl-
alanyl). Regarding fluorescent chromophores, the order of
solubility was quinoline-containing gelators = benzothiazole-
containing gelators > carbazole-containing gelators (compare
Carbz-4, Carbz-5, Quino-2, Quino-3, and Benth-2 in Table 1
and Table 2). Since both 8-hydroxyquinoline and 2-(2-
hydroxyphenyl)benzothiazole were miscible in ordinary
solvents, unlike carbazole, the characteristics of the starting
fluorescent materials were probably reflected in the solubility of
their gelators. As for stilbene-containing gelators, Stilb-4 was a
modestly good gelator, but Stilb-1, Stilb-2, and Stilb-3 were
almost insoluble or precipitated as crystals in most of the
solvents. The lack of gelating abilities for Stilb-1, Stilb-2, and
Stilb-3 may arise from the absence of an appropriate
hydrophile–lipophile balance.
exposing them to TNT-saturated vapor. Figure 1 shows the
fluorescence quenching of thin layers (thickness ~70 nm) of
Carbz-1, Carbz-3, Carbz-4, Carbz-5, and carbazole upon
exposure to saturated TNT vapor (vapor pressure ~5.0 ppb) for
different time periods. The thin-layer films in Figure 1 were
prepared from the corresponding chloroform solutions. The
excitation wavelength was 346 nm and the fluorescence-
monitoring wavelength was 355 nm. The fluorescence-
quenching efficiencies of the thin-layer film of Carbz-4 were
~60% and 100% after exposure to TNT for 2 and 40 min,
respectively. Nearly the same behavior was observed for a thin
layer (thickness ~70 nm) formed from the ethanol gel of Carbz-
4; the fluorescence-quenching efficiencies were ~76% and 96%
for 2 and 30 min exposures, respectively. When the thin-layer
film of Carbz-1 was exposed to the saturated vapor of TNT, a
~73% reduction in fluorescence intensity was observed after
exposure for 60 min. Note that the TNT fluorescence-quenching
efficiency of films prepared by drop-casting from the chloroform
solution of carbazole was limited to ~41%, even after 60 min. As
mentioned later, the thin layer formed from the chloroform
solution of carbazole was of a microcrystalline nature and was
not a xerogel. Figure 1 indicates that the fluorescence-quenching
efficiency decreases in the order Carbz-4 > Carbz-1 > Carbz-
3 > Carbz-5 > carbazole.
We used DFM to characterize the morphologies of the thin
layers prepared by direct drop-casting. Figure 2 shows the DFM
images of the thin layers prepared from the chloroform solutions
of Carbz-1, Carbz-3, Carbz-4, Carbz-5, and carbazole. The
thin layer of carbazole, unsurprisingly, exhibited punctate
crystalline nanostructures. The thin layers of carbazole-
containing gelators, which exhibited quenching when exposed to
TNT, exhibited 3D networks of juxtaposed and interlocked
fibers with widths of 50–300 nm. Although Carbz-4 could not
gel chloroform (see Table 1), the DFM images exhibited dense
entangled fibrous aggregates with very uneven surfaces, but not
microcrystals. The formation of dense 3D fibrous networks with
average widths of 30 nm was observed in the DFM image; the
fiber diameters were nearly homogeneous. The height profiles of
selected regions exhibit fibers with widths of several tens of
nanometers, and heights in the range 10–20 nm, with the fibers
rising on the wafer. The effective quenching of the films
obtained from chloroform solutions of carbazole-containing
gelators and the formation of dense 3D fibrous networks suggest
that these fluorescent gelators formed fibrous aggregates in
chloroform, despite failing to physically gelate. The thin-layer
films of carbazole-containing gelators have high surface areas
compared with that of carbazole. The high surface areas can
successfully come into contact with the explosive vapors,
resulting in effective quenching.
3.2. UV and fluorescence spectroscopy
UV and fluorescence spectra of Carbz-1, Quino-1, Benth-1,
Stilb-1, Stilb-2, and Stilb-3 measured in chloroform solutions at
1.0 mM are shown in Figure S1. The absorption peaks of Carbz-
1 are at 332 and 346 nm (Figure S1a). The fluorescence peaks
were observed at 355 and 370 nm, according to the Stokes shift.
The absorbance and fluorescence peaks for Carbz-2, Carbz-3,
Carbz-4, and Carbz-5 were almost the same as for Carbz-1.
The wavelengths of the absorbance and fluorescence spectra
maxima, max, of Quino-1, Quino-2, Quino-3, and Quino-4 are
found at 306 and 392 nm, respectively (Figure S1b). As shown
in Figure S1c, benzothiazole-containing gelators (Benth-1,
Benth-2, and Benth-3) reveal three absorption peaks at 300, 326,
and 338 nm, and three fluorescence peaks at 356, 368, and 386
nm. With respect to stilbene-containing gelators (Stilb-1, Stilb-
2, Stilb-3, and Stilb-4), their absorbance and fluorescence
spectra are quite different because of the dissimilar substituent
groups attached to the stilbene unit. The spectra of Stilb-1 and
Stilb-3 are shown in Figure S1d and S1e. TNT or RDX was
detected by monitoring the fluorescence quenching of the
aforementioned fluorescence max peaks.
The relationship between the thickness of the thin-layer
films and the fluorescence-quenching efficiency was measured.
The fluorescence-quenching efficiencies of thin-layer films
obtained from the ethanol gel of Carbz-1 with different
thicknesses (70–2700 nm) after 2 min of exposure to TNT were
studied (Figure S2). The fluorescence-quenching efficiency
markedly decreased with increasing film thickness. The
quenching efficiency decreased from 71% for a film thickness of
70 nm to ~10% for a thickness of 200 nm. This result can be
explained by considering the residual molecules deep within the
thick films, which are not quenched by exposure to TNT. Thus,
we can reasonably assume that thinner films exhibit more
effective fluorescence quenching. It should be mentioned that
when the thin-layer film thickness is less than 20 nm, the
fluorescence is too weak to detect. Choosing fluorescent
materials having a large emission intensity may be important for
sensors to detect TNT, because the thickness of the layers could
3.3. Fluorescent sensors of thin-layer films for detecting TNT
Thin-layer sensors were prepared by directly drop-casting the
gels or solutions onto quartz plates, followed by spin-coating and
drying in vacuo. Gelation by low-molecular-weight compounds
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 in which solvent molecules are trapped. Thus,
nanosized fibrous aggregates inevitably form during the initial
stage of gelation. Hence, it is reasonable to assume that the
fibrous aggregates will be fixed in thin-layer films prepared from
low-molecular-weight gelators. The high surface area resulting
from the formation of fibrous aggregates is an advantage for
sensor applications.
The sensing abilities of the thin layers were studied by

then be reduced.
Next, we studied quinoline-containing gelators as
decomposition of the fluorescent gelators may be the cause of
the fluorescence quenching. The cause for quenching by RDX
requires further investigation considering the HOMO-LUMO
energy levels, excitation energy, and energy of ionization.
chemosensors. Figure 3 shows the fluorescence quenching of the
thin-layer films (thickness ~70 nm) obtained from chloroform
solutions of Quino-1, Quino-2, Quino-3, and Quino-4 upon
exposure to saturated TNT vapor. Here the excitation
wavelength and the fluorescence wavelength for monitoring
were 306 nm and 392 nm, respectively. Quino-1 showed the best
quenching efficiency among the quinoline-containing gelators;
however, the fluorescence quenching was limited to ~89% after
exposure for 30 min. The quenching efficiencies of films
comprising quinoline-containing gelators were low compared
with carbazole-containing gelators. The fluorescence quenching
of the thin-layer films obtained from chloroform solutions of
Benth-1, Benth-2, and Benth-3 was also studied (Figure S3).
The excitation wavelength was 326 nm and the fluorescence
wavelength for monitoring was 365 nm. It is noteworthy that the
thin-layer film obtained from the chloroform solution of Benth-
1 showed 100% quenching after 60 min; however, the quenching
efficiencies of benzothiazole-containing gelators were
unremarkable when compared with carbazole-containing
gelators.
Figure 4 shows the fluorescence quenching of the thin-layer
films of stilbene-containing gelators. The excitation wavelengths
for Stilb-1, Stilb-2, Stilb-3, and Stilb-4 were 350, 344, 334, and
340 nm, respectively. Their fluorescence wavelengths for
monitoring were 430, 425, 398, and 425 nm, respectively. The
fluorescence-quenching efficiencies of the thin-layer film
(thickness ~70 nm) obtained from the toluene gel of Stilb-1 upon
exposure to saturated TNT vapor were ~37% and 96% for 2 and
40 min, respectively. The fluorescence-quenching efficiency
decreased in the order Stilb-1 > Stilb-4 > Stilb-2 > Stilb-3. The
quenching efficiency of Stilb-1 was better than that of the other
stilbene-containing gelators.
3.5. Fluorescence quenching upon exposure to nonexplosive
aromatic compounds
Fluorescence quenching of the thin-layer film prepared from the
ethanol gel of Carbz-1 was studied in the presence of
nonexplosive aromatic compounds. Figure
6 shows the
fluorescence quenching when exposed to saturated DNT (2,4-
dinitrotoluene), nitrobenzene, terephthalonitrile, and p-
dimethoxybenzene. The vapor pressures of DNT, nitrobenzene,
terephthalonitrile, and p-dimethoxybenzene are ~380 ppb, 198
ppm, 7.5 ppm, and 100 ppm, respectively. When the thin-layer
film was exposed to saturated vapors of DNT, the fluorescence-
quenching efficiencies were ~92% and 100% for 10 and 20 min
exposures, respectively. The fluorescence was also quenched by
exposure to nitrobenzene. Despite the low vapor pressure (~380
ppb) of DNT compared with nitrobenzene (~198 ppm), the
fluorescence-quenching rate by DNT was comparatively fast.
These results indicate that the number of nitro groups is more
important than the vapor pressure. On the other hand, the
fluorescence-quenching rate by terephthalonitrile (7.5 ppm) with
two electron-withdrawing cyano groups was slow, and
quenching by p-dimethoxybenzene with two electron-donating
methoxy groups was almost undetectable. The compounds with
oxidative electron-withdrawing groups such as nitro groups
seem to be effectively detected.
3.6. Variable-temperature UV-vis spectroscopy
To confirm that the fluorescence chromophore was situated in
the gels, solutions, and thin-layer films, we measured their UV-
vis spectra. The spectrum of the ethanol gel (10 mM) of Carbz-
1 at 25°C was characterized by absorption peaks at wavelengths
of 350 and 336 nm. The absorption peaks of a thin-layer film of
xerogel prepared from the ethanol gel (10 mM) of Carbz-1 were
observed at 352 and 336 nm. The spectrum of the ethanol
solution (10 mM) at 60°C was characterized by absorption peaks
at 344 and 330 nm. The slight red-shift of the gel and xerogel
states compared with the solution spectrum at 60°C suggests that
there is an interaction, such as J-aggregate formation, between
the carbazole units in Carbz-1 in the gel and xerogel states.
Nearly the same behavior was observed in the case of Carbz-2.
We studied the interaction of the stilbene chromophore in
Stilb-1, Stilb-2, Stilb-3, and Stilb-4 by variable-temperature
UV-vis spectroscopy (Figure S4). Stilb-1, Stilb-2, Stilb-3, and
Stilb-4 could form translucent gels in -butyrolactone, dodecane,
toluene, and DMF at 5 mM, respectively. Gels that formed at
25°C transformed into the corresponding isotropic solutions at
85°C. The peak absorption wavelength of Stilb-4 was almost
unchanged when gels formed upon cooling the solutions. The
peak absorption wavelength of 364 nm in the solution of Stilb-1
at 85°C was slightly red-shifted to 362 nm in the gel state at 25°C.
On the contrary, for Stilb-2, the peak absorption wavelength of
322 nm in the solution at 85°C was significantly blue-shifted to
296 nm in the gel state at 25°C. A large blue-shift was also
observed in Stilb-3; the peak at 324 nm in the solution at 85°C
was blue-shifted to 308 nm in the gel state at 25°C. The large
blue-shifts of Stilb-2 and Stilb-3 accompanied by gelation
suggest the existence of – stacking among the stilbene units
in the gels, which is similar to that in an H-aggregate. The lack
of observation of significant shifts in the peak absorption
wavelengths of Stilb-1 and Stilb-4 when gels were formed
indicates disordered stilbene units in the gels. Since the
fluorescence-quenching rate decreased in the order Stilb-1 >
Regarding the fluorescence quenching after exposure to
saturated TNT vapor for 20 min, the order of quenching
efficiencies is: Carbz-2 (100%, ethanol) > Carbz-4 (96%,
chloroform) > Carbz-1 (94%, ethanol) > Stilb-1 (91%, toluene)
> Carbz-3 (88%, ethanol) > Benth-1 (79%, chloroform) >
Carbz-5 (75%, toluene) > Quino-1 (69%, chloroform) > Benth-
2 (65%, chloroform) > Stilb-4 (62%, toluene) > Stilb-2 (54%,
chloroform) > Stilb-3 (48%, chloroform) > Benth-3 (46%,
chloroform) > Quino-3 (37%, chloroform) > Quino-2 (19%,
chloroform), where the fluorescence-quenching efficiency and
drop-casting solvents are shown in parentheses.
3.4. Fluorescent chemosensors of thin-layer films for
detecting RDX
We studied the detection of the nonaromatic explosive RDX
(hexahydro-1,3,5-trinitro-1,3,5-triazine). Figure 5 shows the
fluorescence-quenching efficiencies of the thin-layer films
(thickness ~70 nm) of Carbz-4, Quino-1, and Benth-1 upon
exposure to saturated RDX vapor for different time periods. The
thin-layer films were prepared by drop-casting from the
chloroform solutions. The fluorescence-quenching efficiencies
of the thin-layer film of Quino-1 were ~46% and 50% for 20 and
60 min exposures, respectively. Despite the very low vapor
pressure (~5 × 10-3 ppb) of saturated RDX, significant decreases
in quenching efficiency were observed for the thin-layer films of
the Quino-1 and Benth-1 gelators; their quenching efficiencies
were both ~50% after 60 min. Carbazole, compared with
quinoline and benzothiazole, may be unsuitable as
a
chromophore for sensing RDX. The fluorescence-quenching
efficiency upon exposure to RDX is surprising because RDX has
no absorption in the visible light region. The contact of RDX
vapor with the thin-layer films and the subsequent oxidative

Stilb-4 > Stilb-2 > Stilb-3 (Figure 6), the dispersal of
chromophores in a disordered manner seems to be important for
effective fluorescence quenching.
quenching efficiency were observed for thin-layer films of
Quino-1 and Benth-1. The observed red-shift of the gel and
xerogel states of Carbz-1 in UV-vis spectroscopy suggests an
interaction similar to J-aggregation between carbazole units. The
peak absorption wavelengths of Stilb-1 and Stilb-4 were almost
unchanged when gels were formed upon cooling their heated
solutions. This indicates disorder among the stilbene units in the
gels. The peak absorption wavelengths of Stilb-2 and Stilb-3 in
solutions at 85°C were significantly blue-shifted in the gel state
at 25°C, which suggests the existence of – stacking similar to
H-aggregation among the stilbene units in the gels. The
estimated HOMO and LUMO energy levels of carbazole-
containing gelators and TNT indicate that photoinduced electron
transfer from fluorescent gelators to TNT is possible.
3.7. Mechanism of TNT detection
HOMO and LUMO energies were measured to study the
mechanism of TNT detection. The HOMO and LUMO energies
of carbazole, Carbz-1, Carbz-3, Carbz-4, and Carbz-5 are
summarized in Table 3. The HOMO levels were determined by
measuring the ionization potential (IP). The LUMO levels are
calculated from the IP and band-gap energies estimated from the
UV-vis spectra. The HOMO and LUMO levels of carbazole were
−5.80 and −2.18 eV, respectively, whereas those of Carbz-1,
Carbz-3, Carbz-4, and Carbz-5 were calculated to be ~−5.8 eV
and ~−2.2 eV, respectively. These results suggest that the HOMO
and LUMO levels of the carbazole unit are mostly uninfluenced
by the connection of gelation-driving segments. Energy
calculations with optimized geometries were carried out using
density functional theory at the B3LYP/6-31G* level for the
TNT molecule. The HOMO and LUMO energies of TNT were
−8.7 and −3.8 eV, respectively. The HOMO and LUMO energy
levels of carbazole, Carbz-1, Carbz-3, Carbz-4, Carbz-5, and
TNT are shown in Figure 7. Photoinduced electron transfer from
carbazole and the fluorescent gelators to TNT is clearly possible,
resulting in fluorescence quenching. Despite the almost identical
HOMO and LUMO energies of carbazole, Carbz-1, Carbz-3,
Carbz-4, and Carbz-5, an obvious difference in the
fluorescence-quenching efficiency was observed: Carbz-4 >
Carbz-1 > Carbz-3 > Carbz-5 > carbazole (Figure 1). These
results indicate that the morphology of the thin-layer films plays
an important role. The high surface areas can successfully come
into contact with TNT vapor, resulting in effective quenching.
Acknowledgement
The present research was supported in part by JSPS
KAKENHI Grant Number JP15K05623.
References
1. A. Narayanan, O. P. Varnavski, T. M. Swager, J. Phys.
Chem. C 2008, 112, 881.
2. C. Vijayakumar, G. Tobin, W. Schmitt, M.-J. Kim, M.
Takeuchi, Chem. Commun. 2010, 46, 874.
3. Y. Yang, H. Wang, K. Su, Y. Long, Z. Peng, N. Li, F. Liu, J.
Mater. Chem. 2011, 21, 11895.
4. C. Zhang, Y. Che, X. Yang, B. R. Bunes, L. Zang, Chem.
Commun. 2010, 46, 5560.
5. H. Nie, Y. Zhao, M. Zhang, Y. Ma, M. Baumgarten, K.
Müllen, Chem. Commun. 2011, 47, 1234.
6. H. Nie, G. Sun, M. Zhang, M. Baumgarten, K. Müllen, J.
Mater. Chem. 2012, 22, 2129.
7. Z. Ding, Q. Zhao, R. Xing, X. Wang, J. Ding, L. Wang, Y.
Han, J. Mater. Chem. C 2013, 1, 786.
4. Conclusion
New compounds containing carbazole, quinoline,
benzothiazole, and stilbene were prepared as fluorescent gelators.
The obtained compounds could form physical gels in some
solvents. Thin-layer films were prepared on quartz plates by
drop-casting, and their abilities to detect explosives such as
saturated TNT or RDX vapor were evaluated by fluorescence
quenching. The fluorescence-quenching efficiencies of the thin-
layer films of Carbz-4 were ~60% and 100% after exposure to
TNT for 2 and 40 min, respectively. The fluorescence-quenching
rate decreased in the order Carbz-4 > Carbz-1 > Carbz-3 >
Carbz-5 > carbazole. The morphology, studied by DFM, of the
thin-layer films obtained from the chloroform solution of
carbazole was microcrystalline and was not a xerogel. The
morphologies of the thin-layer films of Carbz-1, Carbz-3,
Carbz-4, and Carbz-5 were 3D networks of juxtaposed and
interlocked fibers with very uneven surfaces. Quino-1 showed
the best quenching efficiency among the quinoline-containing
gelators. Although the thin-layer film of Benth-1 showed 100%
quenching after 60 min, the quenching efficiencies of
benzothiazole-containing gelators were unremarkable when
compared with carbazole-containing gelators. The fluorescence-
quenching efficiencies of the thin-layer film obtained from the
toluene gel of Stilb-1 by TNT vapor were ~37% and 96% for 2
and 40 min exposures, respectively. The fluorescence-quenching
rate decreased in the order Stilb-1 > Stilb-4 > Stilb-2 > Stilb-3.
The fluorescence-quenching efficiency decreased distinctly with
increasing film thickness. The fluorescence-quenching
efficiency of Carbz-1 decreased from 71% for a film thickness
of 70 nm to ~10% for a thickness of 200 nm. When the thin-layer
film thickness was less than 20 nm, the fluorescence was too
weak to detect TNT. Despite the very low vapor pressure of
saturated RDX, moderately good decreases (~50%) in
8. K. K. Kartha, S. S. Babu, S. Srinivasan, A. Ajayaghosh, J.
Am. Chem. Soc. 2012, 134, 4834.
9. N. Dey, S. K. Samanta, S. Bhattacharya, ACS Appl. Mater.
Interfaces 2013, 5, 8394.
10. P. Terech, R. G. Weiss, Chem. Rev. 1997, 97, 3133.
11. J. H. Esch, B. L. Feringa, Angew. Chem., Int. Ed. 2000, 39,
2263.
12. L. A. Estroff, A. D. Hamilton, Chem. Rev. 2004, 104, 1201.
13. P. Dastidar, Chem. Soc. Rev. 2008, 37, 2699.
14. S. Banerjee, R. K. Das, U. Maitra, J. Mater. Chem. 2009,
19, 6649.
15. M. Suzuki, K. Hanabusa, Chem. Soc. Rev. 2009, 38, 967.
16. M. Suzuki, K. Hanabusa, Chem. Soc. Rev. 2010, 39, 455.
17. J.-L. Li, X.-Y. Liu, Adv. Funct. Mater. 2010, 20, 3196.
18. G. John, B. V. Shankar, S. R. Jadhav, P. K. Vemula,
Langmuir 2010, 26, 17843.
19. H. Svobodová, V. Noponen, E. Kolehmainen, E. Sievänen,
RSC Adv. 2012, 2, 4985.
20. S. S. Babu, S. Prasanthkumar, A. Ajayaghosh, Angew.
Chem., Int. Ed. 2012, 51, 1766.
21. A. Y.-Y. Tam, V. W.-W. Yam, Chem. Soc. Rev. 2013, 42,
1540.
22. J. Raeburn, A. Z. Cardoso, D. J. Adams, Chem. Soc. Rev.
2013, 42, 1543.
23. G. Yu, X. Yan, C. Han, F. Huang, Chem. Soc. Rev. 2013, 42,
6697.
24. M. D, Segarra-Maset, V. J. Nebot, J. F. Miravet, B. Escuder,
Chem. Soc. Rev. 2013, 42, 7086.
25. S. S. Babu, V. K. Praveen, A. Ajayaghosh, Chem. Rev. 2014,
114, 1973.
26. D. K. Kumar, J. W. Steed, Chem. Soc. Rev. 2014, 43, 2080.
27. V. K. Praveen, C. Ranjith, N. Armaroli, Angew. Chem., Int.

Ed. 2014, 53, 365.
28. Y. Lan, M. G. Corradini, R. G. Weiss, S. R. Raghavanc, M.
A. Rogers, Chem. Soc. Rev. 2015, 44, 6035.
29. X. Li, J. Li, Y. Gao, Y. Kuang, J. Shi, B. Xu, J. Am. Chem.
Soc. 2010, 132, 17707.
30. J. Li, Y.Gao, Y. Kuang, J. Shi, X. Du, H. Wang, Z. Yang, B.
Xu, J. Am. Chem. Soc. 2013, 135, 9907.
31. L. Chronopoulou, A. R. Togna, G. Guarguaglini, G. Masci,
F. Giammarco, G. I. Togna, C. Palocci, Soft Matter. 2012, 8,
5784.
32. B. F. Lin, K. A. Megley, N. Viswanathan, D. V. Krogstad,
L. B. Drews, M. J. Kade, Y. Qian, M. V. Tirrell, J. Mater.
Chem. 2012, 22, 19447.
33. K. Murata, M. Aoki, T. Nishi, A. Ikeda, S. Shinkai, Chem.
Commun. 1991, 1715.
34. J. J. D. de Jong, L. N. Lucas, R. M. Kellogg, J. H. van Esch,
B. L. Feringa, Science 2004, 304, 278.
35. J. Chen, W. Wu, A. J. McNeil, Chem. Commun. 2012, 48,
7310.
36. P. Xue, Q. Xu, P. Gong, C. Qian, A. Ren, Y. Zhang, R. Lu,
Chem. Commun. 2013, 49, 5838.
37. Y. Ono, K. Nakashima, M. Sano, Y. Kanekiyo, K. Inoue, J.
Hojo, S. Shinkai, Chem. Commun. 1998, 1477.
38. H. Basit, A. Paul, S. Sen, S. Bhattacharya, Chem.-Eur. J.
2008, 14, 6534.
39. S. Kobayashi, K. Hanabusa, N. Hamasaki, M. Kimura, H.
Shirai, S. Shinkai, Chem. Mater. 2000, 12, 1523.
40. S. Kobayashi, N. Hamasaki, M. Suzuki, M. Kimura, H.
Shirai, K. Hanabusa, J. Am. Chem. Soc. 2002, 124, 6550.
41. Y. Yang, M. Suzuki, H. Fukui, H. Shirai, K. Hanabusa,
Chem. Mater. 2006, 18, 1324.
42. W. Kubo, S. Kambe, S. Nakade, T. Kitamura, K. Hanabusa,
Y. Wada, S. Yanagida, J. Phys. Chem. B 2003, 107, 4374.
43. S. S. Babu, S. Prasanthkumar, A. Ajayaghosh, Angew.
Chem., Int. Ed. 2012, 51, 1766.
44. K. Hanabusa, K. Hiratsuka, M. Kimura, H. Shirai, Chem.
Mater. 1999, 11, 649.
45. K. Hanabusa, M. Suzuki, Bull. Chem. Soc. Jpn. 2016, 89,
174.
46. K. Hanabusa, T. Ueda, S.Takata, M. Suzuki, Chem. Eur. J.
in contribution.
47. K. Hanabusa, H. Fukui, M. Suzuki, H. Shirai, Langmuir
2005, 21, 10383.
48. M. Suzuki, R. Yanagida, C. Setoguchi, H. Shirai, K.
Hanabusa, J. Polym. Sci., Part A: Polym. Chem. 2008, 46,
353.
49. K. Hanabusa, K. Harano, M. Fujisaki, Y. Nomura, M.
Suzuki, Macromolecular Sympo. 2016, 46, 353.
50. R. C. Dorey, W. R. Carper, J. Chem. Eng. Data 1984, 29,
93.
51. G. C. Hale, J. Am. Chem. Soc. 1925, 364, 7.
52. W. E. Bachmann, J. Sheehan, J. Am. Chem. Soc. 1949, 71,
1842.

Graphical Abstract
<Title>
Fluorescent gelators for detection of explosives
<Authors' names>
Kenji Hanabusa, Shingo Takata, Masafumi Fujisaki, Yasushi Nomura, and Masahiro Suzuki
<Summary>
Fluorescent gelators are synthesized and fibrous thin layer films are prepared on quartz plates. Fluorescence quenching of the films
upon exposure to saturated TNT or RDX vapor is used to evaluate the abilities of the films to detect explosives.
<Diagram>
; Carbz-4
100
; Carbz-1
80
; Carbz-3
60
; Carbz-5
40
20
0
; carbazole
0
20
40
60
Exposure time of TNT vapor (min)

Table 1. Gelation test of carbazole- and quinoline-containing gelators at 25°C.
Solvent
Carbz-1 Carbz-2 Carbz-3 Carbz-4 Carbz-5 Quino-1 Quino-2 Quino-3 Quino-4
Hexane
I
I
I
I
I
I
P
I
I
I
I
I
I
I
Dodecane
Toluene
Gel
Gel
S
Gel
Gel
S
Gel
Gel
S
Gel
Gel
S
Gel
S
I
Gel
S
Gel
S
Gel
S
Gel
S
Chloroform
Ethyl acetate Gel
Gel
S
Gel
P
I
Gel
S
Gel
S
P
Gel
S
THF
S
I
P
S
Acetone
1-Propanol
Ethanol
Methanol
DMF
Gel
Gel
Gel
Gel
Gel
Gel
Gel
P
P
I
I
Gel
Gel
Gel
Gel
Gel
Gel
Gel
P
S
P
Gel
Gel
Gel
Gel
Gel
Gel
Gel
Gel
Gel
Gel
Gel
Gel
P
I
Gel
P
Gel
Gel
P
Gel
P
Gel
Gel
P
I
I
P
P
S
P
P
S
S
S
-BL
S
S
S
Gel
P
DMSO
P
S
S
P: Precipitation, S: Soluble, I: Almost insoluble, -BL: -Butyrolactone.
The concentrations were fixed at 40 mg mL-1 (gelator/solvent).

Table 2. Gelation test of benzothiazole- and stilbene-containing gelators at 25°C.
Solvent
Benth-1 Benth-2 Benth-3
Stilb-1 Stilb-2
Stilb-3 Stilb-4
Hexane
I
I
I
I
I
I
P
I
Gel
P
I
I
I
Dodecane
Toluene
Gel
Gel
S
P
Gel
S
Gel
S
Gel
S
Gel
S
Gel
S
Chloroform
S
Ethyl acetate Gel
Gel
S
Gel
S
P
P
P
PG
S
THF
S
P
S
P
Acetone
Gel
P
P
P
P
I
Gel
Gel
P
1-Propanol Gel
Gel
Gel
P
Gel
Gel
P
P
P
I
Ethanol
Methanol
DMF
Gel
Gel
Gel
Gel
Gel
P
P
I
P
P
I
P
S
S
P
S
P
Gel
Gel
Gel
-BL
S
Gel
P
Gel
P
P
Gel
P
DMSO
S
S
PG: Partially gel, P: Precipitation, S: Soluble, I: Almost insoluble, -BL: -Butyrolactone.
The concentrations were fixed at 40 mg mL-1 (gelator/solvent).

Table 3. HOMO and LUMO energy levels of carbazole and fluorescent gelators.
Compounds
carbazole
Carbz-1
Carbz-3
Carbz-4
Carbz-5
HOMO
-5.80
-5.82
-5.75
-5.88
-5.62
LUMO
-2.18
-2.14
-2.15
-2.31
-2.10
HOMO energy levels were determined by measuring ionization potentials (IPs). LUMO energy levels
were calculated from the IPs and band-gap energies estimated from UV-vis spectra.

O
O
H
H
N
N
N
N
C18H37
N
N
C18H37
H
H
O
O
Carbz-2
Carbz-1
O
O
N
O
O
H
H
O
N
C18H37
HN
N
N
N
H
H
NH
O
O
Carbz-3
H
Carbz-4
O
O
N
N
NH
NH
Carbz-5
O
O
O
H
N
O
HN
O
H
NH
N
Quino-2
O
O
N
C18H37
H
H
N
O
Quino-1
N
O
O
O
O
O
O
O
H
O
N
NH
NH
HN
NH
O
Quino-3
H
N
Quino-4
Scheme 1. Structures of Carbazole- and Quinoline-Containing Gelators

Scheme 2. Structures of Benzothiazole- and Stilbene-Containing Gelators

100
80
60
40
20
0
; Carbz-4
; Carbz-1
; Carbz-3
; Carbz-5
; carbazole
0
20
40
60
Exposure time of TNT vapor (min)
Figure 1. Fluorescence quenching of thin layers of Carbz-1, Carbz-3, Carbz-4, Carbz-5, and
carbazole prepared by chloroform solution, when exposed to saturated TNT (ca.5.0 ppb) vapor.

carbazole
Carbz-1
Carbz-3
Carbz-4
Carbz-5
Figure 2. DFM images of thin layers prepared from chloroform solution (1mM).

100
80
60
40
20
0
Quino-1
Quino-2
Quino-3
Quino-4
0
10
20
30
40
50
60
Exposure time of TNT vapor (min)
Figure 3. Fluorescence quenching of thin layers of Quino-1, Quino-2, Quino-3, and Quino-4, when
exposed to saturated TNT (ca.5.0 ppb) vapor.

100
80
60
40
20
0
Stilb-1
Stilb-2
Stilb-3
Stilb-4
0
10
20
30
40
Exposure time of TNT vapor (min)
Figure 4. Fluorescence quenching of thin-layers of Stilb-1, Stilb-2, Stilb-3, and Stilb-4, when
exposed to saturated TNT(ca.5.0ppb) vapor.

60
50
40
30
20
10
0
Carbz-4
Quino-1
Benth-1
0
10
20
30
40
50
60
Exposure time of RDX vapor (min)
Figure 5. Fluorescence quenching of thin layers of Carbz-4, Quino-1, and Benth-1, when exposed
to saturated RDX (ca.0.005 ppb) vapor.

100
80
60
40
20
0
DNT
Nitrobenzene
Terephthalonitrile
p-Dimethoxybenzene
0
10
20
30
40
Exposure time of various aromatic compounds (min)
Figure 6. Fluorescence quenching of thin layers of Carbz-1, when exposed to saturated aromatic
compounds’ vapors.

Figure 7. HOMO and LUMO energy levels of carbazole, Carbz-1, Carbz-3, Carbz-4, Carbz-5, and
TNT.