Chromo-fluorogenic detection of nitroaromatic explosives by using silica mesoporous supports gated with tetrathiafulvalene derivatives

Article abstract of DOI:10.1002/chem.201302461

Three new hybrid gated mesoporous materials (SN₃-1, SNH ₂-2, and SN₃-3) loaded with the dye [Ru(bipy) ₃]²⁺ (bipy=bipyridine) and capped with different tetrathiafulvalene (TTF) derivatives (having different sizes and shapes and incorporating different numbers of sulfur atoms) have been prepared. The materials SN₃-1 and SN₃-3 are functionalized on their external surfaces with the TTF derivatives 1 and 3, respectively, which were attached by employing the "click" chemistry reaction, whereas SNH ₂-2 incorporates the TTF derivative 2, which was anchored to the solid through an amidation reaction. The final gated materials have been characterized by standard techniques. Suspensions of these solids in acetonitrile showed "zero release", most likely because of the formation of dense TTF networks around the pore outlets. The release of the entrapped [Ru(bipy)₃]²⁺ dye from SN₃-1, SNH₂-2, and SN₃-3 was studied in the presence of selected explosives (Tetryl, TNT, TNB, DNT, RDX, PETN, PA, and TATP). SNH₂-2 showed a fairly selective response to Tetryl, whereas for SN₃-1 and SN₃-3 dye release was found to occur with Tetryl, TNT, and TNB. The uncapping process in the three materials can be ascribed to the formation of charge-transfer complexes between the electron-donating TTF units and the electron-accepting nitroaromatic explosives. Finally, solids SNH₂-2 and SN₃-1 have been tested for Tetryl detection in soil with good results, pointing toward a possible use of these or similar hybrid capped materials as probes for the selective chromo-fluorogenic detection of nitroaromatic explosives. Copyright

Full text of DOI:10.1002/chem.201302461

DOI: 10.1002/chem.201302461
Full Paper
&
Sensors
Chromo-Fluorogenic Detection of Nitroaromatic Explosives by
Using Silica Mesoporous Supports Gated with Tetrathiafulvalene
Derivatives
Yolanda Salinas,[a, b] Marta V. Solano,[c] Rebecca E. Sørensen,[c] Karina R. Larsen,[c]
Jess Lycoops,[c] Jan O. Jeppesen,*[c] Ramꢀn Martꢁnez-MꢂÇez,*[a, b] Fꢃlix Sancenꢀn,[a, b]
M. Dolores Marcos,[a, b] Pedro Amorꢀs,[d] and Carmen Guillem[d]
Abstract: Three new hybrid gated mesoporous materials
(SN3-1, SNH2-2, and SN3-3) loaded with the dye [Ru(bipy)3]2+
(bipy=bipyridine) and capped with different tetrathiafulva-
lene (TTF) derivatives (having different sizes and shapes and
incorporating different numbers of sulfur atoms) have been
prepared. The materials SN3-1 and SN3-3 are functionalized
on their external surfaces with the TTF derivatives 1 and 3,
respectively, which were attached by employing the “click”
chemistry reaction, whereas SNH2-2 incorporates the TTF de-
rivative 2, which was anchored to the solid through an ami-
dation reaction. The final gated materials have been charac-
terized by standard techniques. Suspensions of these solids
in acetonitrile showed “zero release”, most likely because of
the formation of dense TTF networks around the pore out-
lets. The release of the entrapped [Ru(bipy)3]2+ dye from
SN3-1, SNH2-2, and SN3-3 was studied in the presence of se-
lected explosives (Tetryl, TNT, TNB, DNT, RDX, PETN, PA, and
TATP). SNH2-2 showed a fairly selective response to Tetryl,
whereas for SN3-1 and SN3-3 dye release was found to occur
with Tetryl, TNT, and TNB. The uncapping process in the
three materials can be ascribed to the formation of charge-
transfer complexes between the electron-donating TTF units
and the electron-accepting nitroaromatic explosives. Finally,
solids SNH2-2 and SN3-1 have been tested for Tetryl detec-
tion in soil with good results, pointing toward a possible use
of these or similar hybrid capped materials as probes for the
selective chromo-fluorogenic detection of nitroaromatic ex-
plosives.
Introduction
in the study of environmental problems associated with their
residues.[1]
The indiscriminate use of explosives in terrorist attacks in the
last few years has increased attention on the design of new
probes for the reliable and accurate detection of these lethal
chemicals. In this field, and taking into account the widespread
use of explosive formulations, the analysis of explosives is of
relevant interest in forensic research, landmine detection, and
Explosive compounds, depending on their chemical nature,
are classified into four classes, namely (i) nitroaromatics and ni-
troalkanes, (ii) nitramines, (iii) nitrate esters, and (iv) peroxides.[2]
Among these classes, nitroaromatics are perhaps the most
widely used in criminal acts, in landmines, and in cluster
bombs. Nitroaromatic explosives are molecules composed of
a benzene ring functionalized with several nitro groups. Nitro-
aromatic explosives are electron deficient in nature on account
of the presence of the electron-withdrawing nitro moieties,
which lower the energy of the empty p* orbital, thereby
making them good electron acceptors.[3]
[a] Dr. Y. Salinas, Prof. R. Martꢀnez-MꢁÇez, Dr. F. Sancenꢂn, Dr. M. D. Marcos
Centro de Reconocimiento Molecular y Desarrollo Tecnolꢂgico (IDM)
Unidad mixta Universidad Politꢃcnica de Valencia-Universidad de Valencia
Departamento de Quꢀmica, Universidad Politꢃcnica de Valencia
Camino de Vera s/n, 46022 Valencia (Spain)
E-mail: rmaez@qim.upv.es
Standard methodologies for the detection and quantification
of explosives involve, among others, the use of trained canine
teams,[4] gas chromatography coupled with mass spectrome-
try,[5] gas chromatography–electron capture detection,[6] sur-
face-enhanced Raman spectroscopy,[7] mass spectrometry,[8]
X-ray imaging,[9] thermal neutron analysis,[10] electrochemical
procedures,[11] and ion mobility spectroscopy (IMS).[12] The
above mentioned methods offer several advantages, but some
of them also have drawbacks related to a lack of portability
(for “in situ” detection), susceptibility to false positives owing
to environmental contaminants, or false negative readings on
account of interfering compounds. In this context, the use of
[b] Dr. Y. Salinas, Prof. R. Martꢀnez-MꢁÇez, Dr. F. Sancenꢂn, Dr. M. D. Marcos
CIBER de Bioingenierꢀa, Biomateriales y Nanomedicina (CIBER-BBN)
[c] Dr. M. V. Solano, R. E. Sørensen, Dr. K. R. Larsen, Dr. J. Lycoops,
Prof. J. O. Jeppesen
Department of Physics, Chemistry, and Pharmacy
University of Southern Denmark Campusvej 55, 5230 Odense M (Denmark)
E-mail: joj@sdu.dk
[d] Prof. P. Amorꢂs, Dr. C. Guillem
Institut de Ciꢄncia del Materials (ICMUV)
Universitat de Valꢄncia
P.O. Box 2085, 46071 Valencia (Spain)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201302461.
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chromo-fluorogenic chemosensors based on supramolecular
assemblies has been explored for the detection of explo-
sives.[13] In such molecular probes, interaction with the explo-
sive induces changes in color or in fluorescence that are easily
measurable. Based on this simple concept, chemosensors for
the optical detection of explosives based on various functional-
ized dye molecules have been reported.[14] Most of these
probes have been reliant on the electron-deficient character of
nitroaromatic explosives, as a result of which they form p-
stacking charge-transfer complexes with electron-donating
molecules.[15] However, a common drawback of these probes is
that fluorescence quenching is usually observed because of
the occurrence of electron-transfer processes between the cor-
responding excited fluorophore and the nitroaromatic explo-
sive. Probes that exhibit a turn-on response rather than turn-off
behavior would be preferable but, to the best of our knowl-
edge, only a few examples of probes showing a turn-on re-
sponse to nitroaromatic explosives have hitherto been repor-
ted in the literature.[16]
velopment of novel signaling systems through the selection of
different inorganic porous supports, diverse guest-selective
gate-like ensembles, and a wide range of dyes and fluoro-
phores. In addition, this approach separates the recognition
protocol (binding site–analyte interaction) from the signaling
event, making sensing independent of the stoichiometry of
the host–guest complex and sometimes giving rise to signal
amplification.[29]
Following this approach, in a seminal work, polyamine-func-
tionalized mesoporous materials were used for the selective
detection of ATP through interaction of this bulky anion with
the polyamines, which resulted in an inhibition of dye relea-
se.[27a] Inhibition of dye release by coordination of target ana-
lytes to binding sites on mesoporous supports has also been
used to prepare hybrid materials capable of selectively detect-
ing long-chain carboxylates[27b] (through electrostatic interac-
tions with grafted imidazolium cations) and borates[27c]
(through the formation of borate esters with grafted saccha-
rides). In another group of examples, the gated materials
remain closed until the interaction of a target analyte triggers
the release of the entrapped dye. Following this approach,
DNA-capped mesoporous materials have been used to detect
the presence of complementary DNA strands[27d] and for the
detection of Mycoplasma,[27e] mercury,[27f] and telomerase activi-
ty.[27g] Selective aptamers have also been used as caps for the
preparation of a hybrid material for the detection of a-throm-
bin in human plasma.[27h] Similar sensing systems have been
developed based on antibody-capped mesoporous supports
that can be selectively opened in the presence of the cor-
responding antigen. Using this approach, hybrid sensory mate-
rials for the detection of sulfathiazole,[27i] finasteride,[27j] and
TATP[27k] have been described. Moreover, small molecules, such
as methylmercury, have also been chromo-fluorogenically de-
tected through a pore-opening protocol based on hybrid ma-
terials capped with bulky thiol-containing organic moieties.[29]
Bearing in mind these independent concepts and following
our interest in the preparation of new probes for the detection
of explosives[16] and the design of new applications for capped
mesoporous materials,[30] we report herein the synthesis and
signaling behavior toward nitroaromatic explosives of three
gated mesoporous supports decorated on their external surfa-
ces with three different p-electron-donating TTF derivatives
(Scheme 1) and loaded with a dye (e.g., [Ru(bipy)3]2+). The pre-
pared nanoprobes were designed to show “zero release” be-
cause of the formation of a dense network of TTF units around
the pores. The signaling protocol (Scheme 1) relies on interac-
tion between electron-deficient nitroaromatic explosives and
the strongly p-electron-donating TTF moieties, which results in
rupture of the TTF–TTF interactions allowing release of the en-
trapped dye. It is demonstrated that this approach leads to re-
latively simple hybrid probes for nitroaromatic explosives cap-
able of showing a turn-on response, in contrast to the widely
observed turn-off behavior shown by probes based on single
molecules or fluorophore-containing polymers.[16]
Polycyclic aromatic hydrocarbons have been extensively
used as electron-rich molecules in the development of fluoro-
genic probes for the detection of nitroaromatic explosives.[17]
Other electron-rich molecules that merit attention are tetra-
thiafulvalenes (TTF). The huge potential of TTF derivatives was
first established by the discovery of their conductive behav-
ior.[18] Since then, other properties of the TTF unit, such as its
electro-active character and its strong p-donating nature, have
also been exploited.[19] Utilizing the latter property, TTF moie-
ties have recently been incorporated into calixarenes and
other supramolecular frameworks in order to prepare chemo-
sensors for nitroaromatic explosives.[20]
From another point of view, the blending of supramolecular
assemblies and inorganic materials has recently resulted in the
preparation of gated materials.[21] These are systems in which
the release of an entrapped cargo can be controlled on-com-
mand through the use of selected external stimuli that control
the state of the gate (open or closed).[22] These gated materials
usually consist of two components: i) a switchable gate-like en-
semble, and ii) a suitable inorganic support serving as a nano-
container, on which the gate-like system is grafted. Mesopo-
rous silica materials with different pore sizes and morphologies
have been selected for use as inorganic scaffolds in the prepa-
ration of gated ensembles. These supports have large surface
areas (up to 1200 m2 gÀ1), large load capacities, are chemically
inert, and are easy to functionalize through the use of estab-
lished alkoxysilane chemistry.[23] As regards the gate-like en-
semble, several molecular, supramolecular, and nanoscopic sys-
tems have been used. Moreover, opening of the gate and
cargo delivery has been accomplished by means of several
physical,[24] chemical,[25] and biochemical[26] triggers. Although
most of the reported gated materials have been designed for
drug-delivery applications, some recent examples have con-
cerned the use of capped mesoporous particles for sensing.[27]
The underlying concept here is that interaction of a target ana-
lyte with the gated ensemble controls the release of the cargo
(a dye), resulting in a chromo-fluorogenic signal.[28] This novel
approach is highly modular and displays potential for the de-
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the mesopores, which contained the [Ru(bipy)3]2+ dye. In the
second step (Scheme 2), the TTF derivatives 1 or 3, both bear-
ing an alkyne moiety, were attached to the outer surface of
the solid SN3 by a copper(I)-catalyzed Huisgen azide/alkyne
1,3-dipolar cycloaddition “click” reaction.[32] This led to the for-
mation of a 1,2,3-triazole heterocycle and yielded the final
hybrid materials SN3-1 and SN3-3, respectively. The resulting
solids were collected by filtration, extensively washed with
water and with organic solvents, before being dried overnight
at 368C. Water washings were carried out in order to remove
possible traces of copper from the surfaces of the final solids.
For the preparation of SNH2-2, the solid SNH2 was reacted
with the acyl chloride derivative 2 in acetonitrile/K2CO3 in
order to attach the TTF derivative through an amide linkage
(Scheme 2). After completion of the reaction, the final sensing
material was collected by filtration and extensively washed,
before being dried overnight at 368C.
Scheme 1. Top: Schematic representation of the sensory hybrid mesoporous
materials and the uncapping mechanism in the presence of nitroaromatic
explosives. Bottom: The structures of the TTF derivatives (1, 2, and 3) used
for surface functionalization.
Results and Discussion
The gated material
The prepared hybrid solids are schematically illustrated in
Scheme 2. The MCM-41 mesoporous support was prepared ac-
cording to a well-known procedure from tetraethyl orthosili-
cate (TEOS) as an inorganic precursor and n-cetyltrimethylam-
monium bromide (CTAB) as a porogenic species.[31] The surfac-
tant was removed by calcination to obtain the final MCM-41
starting material. The solid was then loaded with [Ru(bipy)3]2+
as a suitable fluorophore, and the outer surface was functiona-
lized with 3-(azidopropyl)triethoxysilane groups (giving SN3, for
the further preparation of the sensing materials SN3-1 and
SN3-3) or with 3-(aminopropyl)triethoxysilane groups (giving
SNH2, for the preparation of the sensing support SNH2-2). By
this grafting procedure, the silane groups were preferentially
attached to the external surface rather than to the inside of
Scheme 3. Synthetic route for the preparation of compound 3. Ts=tosyl,
TBAF=tetra-n-butylammonium fluoride.
The TTF derivatives 1[33] and
2[34] were prepared according to
literature procedures, whereas
the novel TTF derivative 3 was
synthesized by a four-step pro-
cedure as depicted in Scheme 3.
The synthesis of 3 involved an
initial cross-coupling of 4,5-bis-
(butylthio)-1,3-dithiole-2-thione
(3a)[35] with N-tosyl-1,3-dithiolo-
[4,5-c]pyrrole-2-one (3b)[36] in re-
fluxing triethyl phosphite, which
afforded the TTF derivative 3c in
85% yield. The tosyl protecting
group in 3c was then removed
Scheme 2. Synthetic routes for the preparation of solids SN3-1, SNH2-2, and SN3-3.
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by treatment with sodium methoxide in THF/methanol to
afford compound 3d (89% yield). The third step was a micro-
wave-assisted coupling between 3d and (4-iodophenylethy-
nyl)trimethylsilane (3e) catalyzed by CuI, which afforded the
TTF derivative 3 f in 68% yield. Finally, the alkynyl-TTF deriva-
tive 3 was obtained in almost quantitative yield by deprotec-
tion[37] of the TMS group using tetra-n-butylammonium fluo-
ride (TBAF).
pore voids by the ruthenium complex. Nevertheless, the clear
presence of the (100) peak in these patterns indicated that the
loading and functionalization process did not substantially
modify the mesoporous structure of the final supports.
The presence of the mesoporous structure in the final func-
tionalized solids was also confirmed by TEM analysis, in which
the typical channels of the MCM-41 matrix were seen as black
and white stripes (Figure 2). TEM images also showed that the
Characterization of the materials
The starting MCM-41 and the final solid sensing materials
SN3-1, SNH2-2, and SN3-3 were characterized by conventional
solid-state procedures. The mesoporous structure of the MCM-
41 material was confirmed by powder X-ray diffraction (PXRD)
and transmission electron microscopy (TEM) techniques.
Figure 1 shows the PXRD patterns of the as-synthesized MCM-
Figure 2. TEM images of a) calcined MCM-41 and b) the final solid SNH2-2
showing the typical porosity of the MCM-41 matrix.
final solids were obtained as microparticles. SEM analysis by
energy-dispersive X-ray spectroscopy (EDX, 20 kV) did not
show any characteristic signal for Cu, indicating that no residu-
al Cu from the “click” reaction was present on the final isolated
SN3-1 and SN3-3 materials. Moreover, the EDX analysis clearly
indicated the presence of sulfur atoms (due to grafted TTF
moieties) in SN3-1, SN3-3, and SNH2-2.
N2 adsorption–desorption studies of the calcined MCM-41,
SN3-1, SNH2-2, and SN3-3 materials were also carried out.
Figure 3 shows the nitrogen adsorption–desorption isotherms
obtained for calcined MCM-41, SNH2, and SNH2-2. The calcined
MCM-41 showed a typical curve for a mesoporous support,
consisting of an adsorption step at an intermediate P/P0 values
(0.1–0.3). These isotherms could be classified as type IV, charac-
teristic of mesoporous materials in which the observed step re-
lates to nitrogen condensation inside the mesopores. The pore
Figure 1. Small-angle PXRD patterns of as-synthesized MCM-41, calcined
MCM-41, and the final solids SN3-1, SNH2-2, and SN3-3.
41, calcined MCM-41, and the final supports SN3-1, SNH2-2,
and SN3-3. The PXRD pattern of siliceous as-synthesized MCM-
41 shows four low-angle reflections typical of a hexagonal or-
dered array, which can be indexed as (100), (110), (200), and
(210) Bragg peaks. A shift of the (100) reflection and a remark-
able broadening of the (110) and (200) peaks was observed in
the PXRD pattern of the calcined MCM-41. This corresponds to
an approximate cell contraction of 6–8 ꢅ during the calcination
step. Despite this partial loss of order, the fact that the over-
lapped (100), (110), and (200) reflections were still observed in-
dicated that the mesopore symmetry was preserved after calci-
nation. The PXRD patterns of the final capped solids (SN3-1,
SNH2-2, and SN3-3) only displayed the (100) reflection. For
these solids, peaks (110) and (200) were lost, which was most
probably due to reduced contrast arising from the filling of the
Figure 3. Nitrogen adsorption–desorption isotherms for calcined MCM-41,
SNH2, and SNH2-2. STP=standard temperature and pressure.
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size distributions (PSD) of these samples were calculated by
employing the Barrett–Joyner–Halenda (BJH) method.[38] The
pore diameters estimated from analysis of the respective TEM
images were in agreement with these values. The narrow BJH
pore size distribution and the absence of a hysteresis loop in
this interval pointed to the existence of uniform cylindrical
mesopores (pore diameter 2.25 nm and pore volume
0.45 cm3 gÀ1, calculated by using the BJH model on the adsorp-
tion branch of the isotherm). Application of the Brunauer–
tion of grafted organic groups and of the ruthenium complex
(11.66%, 31.06%, and 12.13% for SN3-1, SNH2-2, and SN3-3, re-
spectively) and, finally, condensation of silanol groups on the
siliceous surface (0.93%, 1.18%, and 1.17% for SN3-1, SNH2-2,
and SN3-3, respectively).
From the data in Tables 1 and 2, the specific surface areas of
the final solids were clearly correlated to the dye contents in
the porous networks. For instance, solid SN3-1 had a higher
specific surface area (816.2 m2 gÀ1) than that of SNH2-2
(111.4 m2 gÀ1), which correlates well with the lower rutheniu-
m(II) content found for the former.
Emmett–Teller (BET) model[39] resulted in
a
value of
1036.2 m2 gÀ1 for the total specific surface area. From the
PXRD pattern, porosimetry, and TEM studies, the a0 cell para-
meter (3.65 nm) and the wall thickness (1.4 nm) were obtained.
The N2 adsorption–desorption isotherms of solids SN3, SNH2,
SN3-1, SNH2-2, and SN3-3 were typical of mesoporous systems
with partially filled mesopores, and decreases in the N2 volume
adsorbed and specific surface area were observed (see
Table 1). This was especially pronounced for the solids SN3,
In addition, the data in Table 2 also indicate that functionali-
zation with the TTF derivatives on the azide- and amine-func-
tionalized solids SN3 and SNH2 was incomplete. We calculated
that around 44% and 15% of the anchored azide groups on
SN3 had been functionalized with the TTF derivatives 1 and 3
in solids SN3-1 and SN3-3, respectively, and that around 57%
of the anchored amine groups on SNH2 had reacted with the
TTF derivative 2.
29Si NMR spectroscopic studies on the solids SNH2 and
SNH2-2 as well as the starting MCM-41 confirmed the anchor-
ing of the organosilane groups (see Figure 4 and Table 3). A
relative decrease in the amount of Q2 +Q3 (silanol) sites in
Table 1. BET specific surface areas, pore volumes, and pore sizes calculat-
ed from the N2 adsorption–desorption isotherms.
Sample
SBET
[m2 gÀ1
BJH Pore Size[a,b]
[nm]
Total Pore Volume[a]
[cm3 gÀ1
]
]
MCM-41
SN3
SNH2
SN3-1
SN3-3
SNH2-2
1036.2
77.9
50.2
816.2
558.0
111.4
2.25
<2
<2
2.27
2.61
<2
0.45
0.03
0.01
0.37
0.27
0.15
[a] Pore volumes and pore sizes relate only to intraparticle mesopores.
[b] Pore size estimated by the BJH model applied to the adsorption
branch of the isotherm.
SNH2, SNH2-2, and SN3-3, whereas for solid SN3-1 a relatively
high specific surface area was still observed. The reduction in
the BET surface area compared with that of the MCM-41 start-
ing material can be attributed to the loading of the pores with
the ruthenium(II) complex and the functionalization of the sur-
face silanol groups with bulky TTF moieties.[40]
The contents of the [Ru(bipy)3]2+ complex and the TTF deri-
vatives in the solids were determined by thermogravimetric
analysis, EDX, and elemental analysis (see Table 2). The ther-
mogravimetric traces for SN3-1, SNH2-2, and SN3-3 showed
three weight loss steps (T < 2108C, 2108C < T < 8008C, T >
8008C) related to solvent elimination (11.77%, 8.23%, and
15.49% for SN3-1, SNH2-2, and SN3-3, respectively), decomposi-
Table 2. Content (a) of anchored molecules and dye in mmolgÀ1 of SiO2.
Solid
aamine
aazide
adye
aTTF
SN3
–
–
–
0.73
–
0.65
0.53
0.04
0.14
0.55
0.52
–
SN3-1
SN3-3
SNH2
SNH2-2
–
–
–
–
0.29
0.10
–
Figure 4. 29Si NMR spectra (400 MHz) recorded at 298 K of the solids
a) MCM-4, b) SNH2, and c) SNH2-2.
0.42
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Table 3. 29Si NMR spectroscopic data for the solids MCM-41, SNH2, and
SNH2-2 in [%].
Q2
7.3
5.2
12.7
Q3
Q4
T1
T2
T3
MCM-41
SNH2
SNH2-2
41.7
35.2
46.2
51.0
48.5
32.9
–
1.6
–
–
4.6
3.8
–
4.9
4.4
favor of Q4 (fully condensed) sites was observed in SNH2 com-
pared with the MCM-41 material. This variation was in accord-
ance with the appearance of the T1, T2, and T3 peaks cor-
responding to the anchored silane groups. However, taking
into account that each 3-(aminopropyl)triethoxysilane moiety
is anchored through at least one silanol group, if not two or
three, to guarantee a certain anchoring effect, we could rough-
ly estimate that the decrease in the number of silanol groups
was somewhat lower than the incorporation of organosilane
groups. Hence, a certain degree of condensation among the
incorporated organosilane groups cannot be excluded in addi-
tion to their simultaneous anchoring on the silica surface. If
there is condensation of silane groups, these species will be lo-
cated predominantly at the mesopore entrances. Finally, after
the second preparative step leading to SNH2-2, there was ap-
preciable hydroxylation of the silica surface, affecting both the
Q and T sites. Hence, while the T3 and T2 sites were largely pre-
served, the preparative conditions appear to favor the leaching
of T1-type organosilane groups.
Figure 5. Kinetics of the release of the [Ru(bipy)3]2+ dye from solid SN3-3 in
the absence (blank) and presence of the nitroaromatic explosives Tetryl, TNT,
and TNB (2.0ꢆ10À3 molLÀ1) in acetonitrile.
TNB induced opening of the pores and subsequent release of
the dye. After around 60 min, the release reached a plateau. In
contrast, DNT, RDX, PETN, PA, TATP, NM, NT, and NB were
unable to induce release of the ruthenium complex (data not
shown) when added to suspensions of probe SN3-3.
Dye release studies on solids SN3-1 and SNH2-2 gave similar
overall results, that is, a tight capping of the pores in the solid
alone and release of the dye in the presence of certain explo-
sives. As a summary of the release behavior, Figure 6 shows
the relative releases of ruthenium complex from suspensions
of solids SN3-1, SNH2-2, and SN3-3 in acetonitrile after around
60 min following the addition of different species. As can be
seen, Tetryl, TNT, and TNB were able to induce some degree of
opening of the “molecular gate” in the three solids with subse-
quent dye release and turn-on of the emission, whereas the
other substances tested were unable to induce payload re-
lease. SN3-3 proved to be a less selective material, with Tetryl,
TNT, and TNB inducing almost the same behavior. On the
other hand, solid SNH2-2 showed a fairly selective response to
Tetryl. The studies showed that changes in the nature of the
appended TTF derivatives (different sizes and numbers of
sulfur atoms) ultimately gave rise to subtle differences in the
responses of the three sensing materials.
Dye release studies in the presence of explosives
In order to test the proposed sensing mechanism (Scheme 1),
the interactions of SN3-1, SNH2-2, and SN3-3 with selected ex-
plosives were studied on the basis of release of the ruthenium
complex entrapped in the porous network of the inorganic
supports. Release studies were carried out in acetonitrile solu-
tions, in which tight pore closure was observed and in which
both the ruthenium dye and the studied explosives are freely
soluble. The following molecules were selected for this study:
2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), 2,4,6-trini-
trophenylmethylnitramine (Tetryl), trinitrobenzene (TNB), hexa-
hydro-1,3,5-trinitro-1,3,5-triazine (RDX), pentaerythritol tetrani-
trate (PETN), picric acid (PA), triacetone triperoxide (TATP),
N-methylaniline (NM), 2-nitrotoluene (NT), and nitrobenzene
(NB).
The above studies demonstrated that SN3-1, SNH2-2, and
SN3-3 were firmly capped when suspended in acetonitrile. This
could be attributed to the formation of dense TTF networks
through p-stacking face-to-face and S···S interactions around
the pores, which inhibited cargo release. Semiempirical molec-
ular calculations using HyperChem software showed that the
anchored TTF derivatives had significant dipole moments of
around 1.8, 3.9, and 6.4 D for 1, 2, and 3, respectively. There-
fore, it is indeed likely that there will be strong dipole–dipole
interactions between anchored molecules around the pore
outlets, creating a barrier that inhibits dye release.
In a typical assay, 4 mg of the corresponding solid (SN3-1,
SNH2-2, or SN3-3) were suspended in acetonitrile (10 mL) in
the presence of DNT, TNT, Tetryl, TNB, RDX, PETN, PA, TATP, NM,
NT, or NB (2.0ꢆ10À3 molLÀ1) at 258C. Release of the [Ru-
(bipy)3]2+ complex to the bulk solution was easily detected by
monitoring its d-p metal-to-ligand charge-transfer (MLCT) tran-
sition band at 453 nm or its luminescence at 615 nm (lex =
453 nm).[41] As an illustrative example, Figure 5 shows the re-
lease kinetics of the ruthenium dye from SN3-3 in the presence
of selected explosives. As can be seen, solid SN3-3 was tightly
capped in solution and showed negligible release of the en-
trapped [Ru(bipy)3]2+ complex. The presence of Tetryl, TNT, or
When certain nitroaromatic explosives are added to suspen-
sions of the sensing solids, however, the cargo is clearly re-
leased. This behavior can most likely be attributed to the for-
mation of TTF–nitroaromatic complexes, which results in rup-
ture of the TTF–TTF interactions (including p-stacking and
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spectrum of the final solid SN3-1 showed the same two well-
defined zones, with the signals of the propyl carbon atoms in
the region d=20–60 ppm and those of the aromatic carbon
atoms in the region d=120–170 ppm. The formation of the
1,2,3-triazole ring (due to the “click” reaction) with concomi-
tant immobilization of the TTF moiety resulted in the appear-
ance of new carbon resonances at d=122.2, 122.9, 129.4,
137.4, 155.7, 161.6, and 162.8 ppm. In a further experiment,
the solid SN3-1 was suspended in acetonitrile and an excess of
Tetryl was added. The mixture was stirred for 24 h in order to
achieve the maximum TTF–Tetryl interaction and concomitant
release of the [Ru(bipy)3]2+ complex. The most remarkable fea-
ture of the 13C MAS NMR spectrum recorded for this solid was
a broadening and small downfield shifts (Dd=0.1–0.2 ppm) of
the TTF signals (d=120–170 ppm), which we tentatively attri-
bute to the formation of TTF–Tetryl charge-transfer complexes.
In addition, the UV/Vis diffuse-reflectance spectrum (see the
Supporting Information) of a mixture of SN3-1 and Tetryl in
acetonitrile, recorded after 60 min, revealed a slight increase in
the reflectance at around 650 nm, which we again tentatively
attribute to charge-transfer interactions between the TTF moie-
ties in SN3-1 and Tetryl.[20b]
To complete the characterization of the sensing behavior of
the hybrid solids SN3-1, SNH2-2, and SN3-3, the limits of detec-
tion (LODs) in the presence of TNT, TNB, and Tetryl were deter-
mined from the corresponding calibration curves using fluores-
cence as the detection method (see Table 4). As an example,
Figure 7 shows the calibration curve obtained for SNH2-2 upon
addition of increasing quantities of TNB. LODs for TNT, TNB,
Table 4. Limits of detection in [ppm] for Tetryl, TNT, and TNB using solids
SN3-1, SNH2-2, and SN3-3.
Solid
Tetryl
TNT
TNB
SN3-1
SNH2-2
SN3-3
1
1
2
6
2
7
10
2
6
Figure 6. Release of the ruthenium complex from suspensions in acetonitrile
of the solids a) SN3-1, b) SNH2-2, and c) SN3-3 in the presence of different
explosives (2ꢆ10À3 molLÀ1). The emission at 615 nm (excitation at 453 nm)
was measured 60 min after addition of the explosive.
dipole interactions), allowing the gate to open. Moreover, this
behavior was only observed for molecules displaying strong
electron-accepting character, that is, Tetryl, TNT, and TNB, an
observation that suggests that it is their interaction with the
electron-donating TTF units that is responsible for the uncap-
ping process. Furthermore, 13C MAS NMR spectroscopic investi-
gations carried out on the solids SN3 and SN3-1 in the pre-
sence of Tetryl supported the proposed pore-opening mecha-
nism. Solid SN3 showed two well-defined zones in its 13C spec-
trum, one in the region d=20–60 ppm, corresponding to the
resonances of the carbon atoms of the propyl chain linking the
azido moiety to the inorganic scaffold, and another in the
region d=120–160 ppm, corresponding to the signals of the
bipyridine moieties of the entrapped dye. A 13C MAS NMR
Figure 7. Calibration curve obtained from suspensions in acetonitrile of
SNH2-2 upon addition of increasing quantities of TNB (lex =453 nm,
lem =615 nm), measuring at 60 min after the addition of TNB. The inset
shows the turn-on emission spectra of the [Ru(bipy)3]2+ dye released from
SNH2-2 upon addition of increasing quantities of TNB.
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and Tetryl were found to be in the range 1–10 ppm. These
values are similar to those found for molecular-based
probes,[20] but are higher than those obtained with gold nano-
particles,[42] organic polymers,[43] and tin oxide printed electro-
des.[44] A noteworthy distinction, however, is that solids SN3-1,
SNH2-2, and SN3-3 display a turn-on response to nitroaromatic
explosives, as opposed to the more common turn-off behavior
seen with other sensing systems.
and 3. The capping TTF units (1 and 3) used for the prepara-
tion of the SN3-1 and SN3-3 sensory materials were grafted
through a “click” chemistry reaction, whereas SNH2-2 was func-
tionalized through an amidation reaction with TTF derivative 2.
The three novel materials showed “zero release” when sus-
pended in acetonitrile. Subtle differences in the chemical struc-
tures of the TTF capping moieties resulted in different respons-
es of the solids. The explosives Tetryl, TNT, and TNB were
found to be capable of inducing, to some extent, dye release
from SN3-1 and SN3-3, whereas solid SNH2-2 showed a fairly
selective response to Tetryl. Most notably, a turn-on response
was observed. All other substances tested were unable to
induce payload release. Moreover, color or emission measure-
ment revealed that detection of Tetryl, TNB, and TNT at the
ppm level could be achieved. Finally, SN3-1 and SNH2-2 have
been used for the chromogenic detection of Tetryl in soil sam-
ples with good results. The principle of this method is amena-
ble to various modifications. For instance, the support, the p-
donor units, and the dye may be easily changed without the
use of complex chemistry in order to tune the selectivity and
the optical properties of the signaling dye.
Tetryl detection in soils
The obtained LODs (see Table 4) were relatively low and sug-
gested that it might be possible to use the capped solids
SN3-1, SNH2-2, and SN3-3 to detect explosives within the maxi-
mum permissible limits in certain samples. In this context, for
instance, the EPA (U.S. Environmental Protection Agency) has
stipulated values of 19 mg TNTgÀ1 for residential soil and 79 mg
TNTgÀ1 for industrial soil as safety guidelines.[45] After assessing
the selective responses of solids SN3-1, SNH2-2, and SN3-3 to
nitroaromatic explosives in solution, we focused our interest
on the possible use of these materials for the detection of ex-
plosives in soil samples. For this purpose, we selected solids
SN3-1 and SNH2-2 as sensing systems and Tetryl as a represen-
tative explosive. In a typical experiment, 10 g of a finely
ground homogeneous soil sample was doped with a known
amount of Tetryl (10 mL of a 1.0ꢆ10À4 moldmÀ3 Tetryl solution
in acetonitrile). The prepared Tetryl-spiked soil sample was
then extracted with acetonitrile (3ꢆ15 mL). The obtained ex-
tracts were combined and concentrated, and the concentrate
was redissolved in acetonitrile (10 mL). Five different samples
(each 2 mL of the crude extract) were prepared with the addi-
tion of increasing volumes of a standard Tetryl solution in ace-
tonitrile, and the final volume of each sample was adjusted to
10 mL by adding the requisite volumes of acetonitrile. Finally,
SN3-1 or SNH2-2 (4 mg) was added to each 10 mL sample solu-
tion and the mixtures were stirred for 60 min. Each mixture
was then filtered and the absorbance at 453 nm was mea-
sured. The well-known standard addition method was followed
to calculate the Tetryl concentration. Linear regression analysis
was performed, and the slope and y-intercept of the calibra-
tion curve were used to calculate the concentration of Tetryl in
the sample. From the linear regression A=m·Cadded + b, values
of b=0.04408 and m=46.78 were obtained, with R2 =0.96 for
solid SN3-1. For SNH2-2, values of b=0.1279, m=59.11, and
R2 =0.91 were obtained. Concentrations of 25.2 mg Tetryl gÀ1
soil using SN3-1 and 27.4 mg Tetryl gÀ1 soil using SNH2-2 were
obtained. These values were in good agreement with the
doped concentration (28.7 mg Tetryl gÀ1 soil).
Experimental Section
Caution! Explosives should be used with extreme caution and
handled only in small quantities.
General methods
Powder X-ray diffraction, thermogravimetric (TG) analysis, and N2
adsorption–desorption techniques were used to characterize the
prepared materials. X-ray measurements were carried out on a Phi-
lips D8 Advance diffractometer using CuKa radiation. TG analyses
were carried out on a TGA/SDTA 851e Mettler Toledo balance
under an oxidizing atmosphere (air, 80 mLminÀ1) with a heating
program consisting of a heating ramp of 10 K per minute from
293 K to 1273 K and an isothermal heating step at this temperature
of duration 30 min. N2 adsorption–desorption isotherms were re-
corded with a Micromeritics ASAP2010 automated sorption ana-
lyzer. Samples were degassed at 393 K in vacuum overnight. Spe-
cific surface areas were calculated from the adsorption data in the
low-pressure range using the Brunauer–Emmett–Teller (BET) model.
Pore size was determined according to the Barrett–Joyner–Halenda
(BJH) method. TEM images were obtained with a Philips CM10 mi-
croscope operating at 100 kV. UV/Vis spectra were measured at
298 K on a Perkin–Elmer Lambda 35 UV/Vis spectrophotometer.
Fluorescence spectroscopy studies were carried out at 298 K with
JASCO FP-8500 and Felix 32 Analysis, version 1.2 (Build 56) PTI
(Photon Technology International) spectrofluorimeters. 1D and 2D
NMR spectra were obtained on a Bruker AVANCE III 400 MHz spec-
1
trometer; H NMR spectra were recorded at 400 MHz at 298 K and
13C NMR spectra were recorded at 100 MHz at 298 K. Samples for
NMR analysis were dissolved in deuterated solvents purchased
from Cambridge Isotope Laboratories or Sigma–Aldrich, and TMS
or the residual solvent was used as an internal standard. 29Si and
13C MAS NMR spectra were recorded on a Bruker WB AVANCE III
400 MHz spectrometer operating at 79.5 and 128.3 MHz, respec-
tively, with a magic-angle spinning speed of 10 kHz. Electron
impact ionization mass spectrometry (EI-MS) was performed on
a Thermo Finnigan MAT SSQ710 single-stage quadrupole instru-
ment. Matrix-assisted laser-desorption/ionization mass spectrome-
Conclusion
We have reported herein the preparation of three new meso-
porous silica supports capped with different electron-donating
TTF moieties (SN3-1, SNH2-2, and SN3-3) for a simple chromo-
fluorogenic sensing of nitroaromatic explosives. These materi-
als consist of a mesoporous MCM-41-like structure loaded with
a [Ru(bipy)3]2+ complex and capped with TTF derivatives 1, 2,
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try was performed on a Bruker Autoflex III Smartbeam mass spec-
trometer, utilizing a 2,5-dihydroxybenzoic acid (DHB) matrix. In
order to estimate the dipole moments of the grafted TTF deriva-
tives (products 1, 2, and 3), quantum chemical calculations were
carried out at the semi-empirical level with HyperChem 6.03 soft-
ware (HyperChem 6.03 Molecular Modeling System, Hypercube
Inc., Gainesville, Florida, USA, 2000).
load the pores of the MCM-41 scaffold. An excess of 3-(azidopro-
pyl)triethoxysilane (0.49 mL, 2 mmol) was then added to the reac-
tion mixture, and the suspension was stirred for 5.5 h. Thereafter,
the orange-yellow solid was collected by filtration to afford SN3.
Meanwhile, anhydrous DMF (50 mL) was purged with N2 for 1 h
and the TTF derivative 1 (121.1 mg, 0.531 mmol) was placed in an
empty flask. Using a needle, an aliquot of the N2-purged DMF
(10 mL) was transferred into the flask containing 1. A similar proce-
dure was used for a mixture of ascorbic acid (56.3 mg, 0.32 mmol)
and CuSO4·5H2O (39.9 mg, 0.16 mmol). Thereafter, solid SN3 was
suspended in DMF/water (1:2, v/v) containing [Ru(bipy)3]Cl2·6H2O
(0.6 g, 0.8 mmol). The ascorbic acid/CuSO4·5H2O solution in anhy-
drous DMF was then added to the reaction mixture by means of
a needle. Finally, a similar operation was carried out with the solu-
tion of the TTF derivative 1 in DMF. After the additions were com-
pleted, the reaction mixture was stirred at room temperature for
5 days. Thereafter, the resulting microparticles were collected by fil-
tration, washed with CH2Cl2 (100 mL), water (100 mL), acetonitrile
(100 mL), and THF (100 mL), and dried at 358C for 12 h to afford
SN3-1 (872 mg) as a green-brown solid.
Chemicals
Tetraethyl orthosilicate (TEOS; 98%), n-cetyltrimethylammonium
bromide (CTAB; ꢀ 99%), sodium hydroxide (ꢀ 98%), triethanola-
mine (TEAH3; ꢀ 99%), tris(2,2’-bipyridyl)dichlororuthenium(II) hexa-
hydrate ([Ru(bipy)3]Cl2·6H2O; 100%), 2,4-dinitrotoluene (DNT),
N-methylaniline (NM; 98%), 2-nitrotoluene (NT), nitrobenzene (NB;
99%), picric acid (PA), tetrathiafulvalene (TTF), perfluorohexyl
iodide (PFHI), lithium diisopropylamide (LDA), potassium fluoride
(KF), ethynyltrimethylsilane, tetrakis(palladium)triphenylphosphine
(Pd(PPh3)4), mercury(II) acetate, acetic acid, Celite 545, CH2Cl2,
NaHCO3, MgSO4, triethyl phosphate, toluene, diethyl ether, petrole-
um ether, LiBr, dimethyl sulfoxide (DMSO), MeOH, sodium methox-
ide, CuI, K3PO4, LiOH·H2O, oxalyl chloride, tetra-n-butylammonium
fluoride, (Æ)-trans-1,2-diaminocyclohexane, pyridine, 3-(aminopro-
pyl)triethoxysilane, and tetrahydrofuran (THF) were provided by
Sigma–Aldrich and were used as received. The nitroaromatic explo-
sives 2,4,6-trinitrophenylmethylnitramine (Tetryl), 2,4,6-trinitroto-
luene (TNT), 1,3,5-trinitrobenzene (TNB), hexahydro-1,3,5-trinitro-
1,3,5-triazine (RDX), pentaerythritol tetranitrate (PETN), and 3-(azi-
dopropyl)triethoxysilane were purchased as 3% solutions in aceto-
nitrile from SelectLab Chemicals. Triacetone triperoxide (TATP),[46]
4,5-bis(butylthio)-1,3-dithiole-2-thione[35] (3a), and N-tosyl-1,3-
dithiolo[4,5-c]pyrrole-2-one[36] (3b) were synthesized according to
literature procedures. l-Ascorbic acid sodium salt (99%), potassium
carbonate, sodium sulfate, and copper(II) sulfate pentahydrate
(CuSO4·5H2O) were acquired from Scharlab (Barcelona, Spain). Di-
isopropylamine (iPr2NH) was acquired from Fluka. Analytical grade
solvents were purchased from Scharlab (Barcelona, Spain). All re-
agents were used as received, except for THF, which was distilled
from sodium/benzophenone prior to use.
Solid SNH2-2
In
a typical synthesis, template-free MCM-41 (1 g) and [Ru-
(bipy)3]Cl2·6H2O (0.6 g, 0.8 mmol) were suspended in acetonitrile
(40 mL) in a round-bottomed flask. To remove the adsorbed water,
10 mL of acetonitrile was distilled off using a Dean–Stark set-up.
The mixture was then stirred for 24 h at room temperature to fully
load the pores of the MCM-41 scaffold. An excess of 3-(aminopro-
pyl)triethoxysilane (0.47 mL, 2 mmol) was then added, and the sus-
pension was stirred for 5.5 h. The resulting orange-yellow solid was
collected by filtration to afford SNH2. Subsequently, SNH2 (1 g) was
suspended in acetonitrile (40 mL) and compound 2 (378.3 mg,
1.06 mmol), the ruthenium dye (600 mg, 0.8 mmol), and K2CO3
(2 g, 14.5 mmol) were added. The resulting suspension was heated
under reflux for 16 h under an inert atmosphere of argon. Finally,
the solid SNH2 was collected by filtration, extensively washed with
water and thereafter with a mixture of acetonitrile and water
(100 mL of acetonitrile and 100 mL of milli-Q water per 100 mg of
final solid, stirring for 24 h) before being dried overnight at 368C
to afford SNH2-2 (1.98 g) as a brown solid.
Mesoporous MCM-41 microparticles
The molar ratio of the reagents in the mother liquor was fixed at
7.0 TEAH3 : 2.0 TEOS : 0:52 CTAB : 0.50 NaOH : 8.89 H2O. The me-
soporous MCM-41 support was first synthesized according to the
so-called “atrane route”, in which CTAB (4.68 g) was added to a so-
lution of TEAH3 (25.79 g) containing a silatrane derivative (TEOS;
11 mL, 0.049 mol) at 1188C. Next, water (80 mL) was slowly added
with vigorous stirring at 708C. After a few minutes, a white suspen-
sion was formed. This mixture was aged at room temperature
overnight. The resulting powder was collected by filtration and
washed with water and ethanol until the washings were of pH 6–7.
Finally, the white solid was dried at 708C (as-synthesized MCM-41).
To prepare the final porous material, the as-synthesized solid was
calcined at 5508C under an oxidizing atmosphere for 5 h in order
to remove the template phase (MCM-41).
5-Tosyl-2-[4,5-bis(butylthio)-1,3-dithiol-2-ylidene]-
[1,3]dithiolo[4,5-c]pyrrole (3c)
The thione 3a (4.00 g, 12.8 mmol) and the ketone 3b (3.36 g,
10.79 mmol) were dissolved in freshly distilled triethyl phosphite
(60 mL) and the mixture was degassed under an argon atmosphere
for 15 min. The reaction mixture was quickly heated to 1308C.
After 16 min at 1308C, another portion of the thione 3a (4.00 g,
12.8 mmol) was added and the reaction mixture was stirred at
1308C for 1.2 h, then cooled to room temperature. Cold methanol
(75 mL) was added and the resulting orange solid was collected by
filtration. The filtrate was left in a freezer overnight to produce
a second crop of the product, which was collected by filtration.
The combined solids were washed with small portions of methanol
(5ꢆ15 mL) and dried in vacuo before being purified by flash chro-
matography (SiO2; CH2Cl2/petroleum ether, 1:1, as eluent). The
yellow band (Rf =0.29, CH2Cl2/petroleum ether, 1:1) was collected
and concentrated to give the title compound 3c (5.25 g, 85%) as
yellow crystals.[47] 1H NMR (500 MHz, (CD3)2SO): d=0.86 (t, J=
7.3 Hz, 6H), 1.31–1.43 (m, 4H), 1.47–1.57 (m, 4H), 2.38 (s, 3H), 2.84
(t, J=7.3 Hz, 4H), 6.94 (s, 2H), 7.30 (d, J=8.6 Hz, 2H), 7.73 ppm (d,
J=8.6 Hz, 2H); 13C NMR (125 MHz, (CD3)2SO): d=13.5, 20.9, 21.1,
Solid SN3-1
In
a typical synthesis, template-free MCM-41 (1 g) and [Ru-
(bipy)3]Cl2·6H2O (0.6 g, 0.8 mmol) were suspended in acetonitrile
(40 mL) in a round-bottomed flask. To remove the adsorbed water,
10 mL of acetonitrile was distilled off using a Dean–Stark set-up.
The mixture was then stirred for 24 h at room temperature to fully
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31.3, 35.1, 112.4, 112.7, 117.4, 125.9, 126.7, 126.8, 130.4, 134.5,
145.8 ppm; MS (MALDI): m/z: calcd for C23H27NO2S7+: 573.01;
found: 572.98.
1H), 7.53 (s, 2H), 7.55 ppm (s, 4H); 13C NMR (125 MHz, (CD3)2SO):
d=13.3, 20.8, 31.1, 34.9, 81.1, 82.7, 109.6, 111.3, 118.5, 121.4, 126.6,
133.1, 139.2 ppm (two signals are missing/overlapping); HRMS-EI:
m/z: calcd for C24H25NS6+: 519.0306; found: 518.9600; elemental
analysis calcd (%) for C24H25NS6 (519.9): C 55.45, H 4.85, N 2.69;
found: C 55.38, H 4.89, N 2.80.
2-[4,5-Bis(butylthio)-1,3-dithiol-2-ylidene]-[1,3]dithiolo[4,5-
c]pyrrole (3d)
Compound 3c (4.46 g, 7.77 mmol) was dissolved in a mixture of
anhydrous THF (250 mL) and methanol (100 mL) and the mixture
was degassed under an argon atmosphere for 60 min. Sodium
methoxide (25% w/w in MeOH, 4.323 g, 80.03 mmol) was added in
one portion and the reaction mixture was heated under reflux for
30 min. After allowing the mixture to cool to room temperature,
water (400 mL) was added, and the resulting mixture was extracted
with CH2Cl2 (2ꢆ200 mL). The combined organic phases were dried
(MgSO4) and concentrated to give a yellow oil, which was purified
by flash chromatography (SiO2; CH2Cl2/cyclohexane, 2:1, gradient
elution). The broad yellow band was collected and concentrated to
afford the title compound 3d (2.90 g, 89%) as a yellow powder.[47]
1H NMR (500 MHz, (CD3)2SO): d=0.87 (t, J=7.3 Hz, 6H), 1.35–1.42
(m, 4H), 1.49–1.59 (m, 4H), 2.85 (t, J=7.3 Hz, 4H), 6.81 (s, 2H),
11.14 ppm (brs, 1H); 13C NMR (125 MHz, (CD3)2SO): d=13.5, 20.9,
31.31, 35.0, 107.4, 110.8, 117.1, 121.7, 126.7 ppm; MS (MALDI): m/z:
calcd for C16H21NS6+: 419.00; found: 418.98.
Solid SN3-3
In
a typical synthesis, template-free MCM-41 (1 g) and [Ru-
(bipy)3]Cl2·6H2O (0.6 g, 0.8 mmol) were suspended in acetonitrile
(40 mL) in a round-bottomed flask. To remove the adsorbed water,
10 mL of acetonitrile was distilled off using a Dean–Stark set-up.
The mixture was then stirred for 24 h at room temperature to fully
load the pores of the MCM-41 scaffold. An excess of 3-(azidopro-
pyl)triethoxysilane (1.20 mL, 5 mmol) was then added, and the sus-
pension was stirred for 5.5 h. The resulting orange-yellow solid was
collected by filtration and washed with CH2Cl2 (50 mL) to afford
SN3. Subsequently a mixture of SN3 (0.5 g) and compound 3
(303.2 mg, 0.98 mmol) was suspended in DMF/water (1:1, v/v,
40 mL) in the presence of an excess of the ruthenium(II) dye (0.3 g,
0.4 mmol) (in order to avoid release of the dye from the pores to
the bulk solution during the synthesis of this solid). Finally, a solu-
tion of CuSO4·5H2O (1.248 mg, 0.005 mmol) and sodium ascorbate
(9.905 mg, 0.05 mmol) was added and the reaction mixture was
stirred at room temperature for 5 days. The microparticles were
collected by filtration and extensively washed with water and then
with acetonitrile (100 mL of acetonitrile per 100 mg of final solid,
stirring for 24 h), and finally dried at 368C for 12 h to afford SN3-3
as a yellow solid (0.55 g).
2-[4,5-Bis(butylthio)-1,3-dithiol-2-ylidene]-5-{4-[(trimethyl-
silyl)ethynyl]phenyl}-[1,3]dithiolo[4,5-c]pyrrole (3 f)
Compound 3d (1.05 g, 2.502 mmol), CuI (0.93 g, 4.878 mmol),
K3PO4 (1.78 g, 8.388 mmol), and (4-iodophenylethynyl)trimethylsi-
lane (3e) (3.60 g, 12.0 mmol) were dissolved in freshly distilled THF
(50 mL) and the mixture was degassed under an argon atmosphere
for 20 min, and then (Æ)-trans-1,2-diaminocyclohexane (1.06 g,
1.2 mL, 9.33 mmol) was added in one portion. The mixture was
heated at 1158C in a microwave oven for 3 h, then allowed to cool
to room temperature and extracted with CH2Cl2 (100 mL). The or-
ganic phase was washed with aqueous NaOH solution (5% in
100 mL H2O; 3ꢆ20 mL) and water (3ꢆ150 mL), and then dried
(MgSO4). Concentration gave a yellow residue, which was purified
by column chromatography (SiO2; CH2Cl2/petroleum ether, 1:3, gra-
dient elution). The yellow band (Rf =0.74, CH2Cl2/petroleum ether,
1:1) was collected and concentrated to provide the title compound
3 f (0.99 g, 68%) as yellow crystals. M.p. 116.0–118.08C; 1H NMR
(500 MHz, CD2Cl2): d=0.25 (s, 9H), 0.92 (t, J=7.3 Hz, 6H), 1.40–1.48
(m, 4H), 1.59–1.65 (m, 4H), 2.84 (t, J=7.3 Hz, 4H), 6.94 (s, 2H), 7.28
(d, J=8.8 Hz, 2H), 7.50 ppm (d, J=8.8 Hz, 2H); 13C NMR (125 MHz,
CD2Cl2): d=0.1, 13.9, 22.2, 32.4, 36.5, 95.6, 104.4, 111.2, 119.9, 121.2,
123.3, 128.2, 133.9, 140.3 ppm (two signals are missing/overlap-
ping); HRMS-EI: m/z: calcd for C27H33NS6Si+: 591.0701; found:
591.1000; elemental analysis calcd (%) for C27H33NS6Si (592.0): C
54.78, H 5.62, N 2.37; found: C 54.97, H 5.80, N 2.38.
Acknowledgements
Financial support from the Spanish Government (project
MAT2012–38429-C04–01) and the Generalitat Valencia (project
PROMETEO/2009/016) is gratefully acknowledged. J. O. Jeppe-
sen gratefully acknowledges financial support provided by the
EC FP7 ITN “FUNMOLS” Project No. 212942), the Villum Foun-
dation, the Carlsberg Foundation, and the Danish Natural Sci-
ence Research Council (#11–106744). Y. Salinas thanks the
Spanish Ministry of Science and Innovation for her grant. We
also thank Stephen Boyer, School of Human Sciences, Science
Centre, London Metropolitan University, for carrying out the el-
emental analyses.
Keywords: gated materials
nitroaromatic explosives
tetrathiafulvalene
·
mesoporous materials
·
·
·
optical detection sensors
·
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Received: June 26, 2013
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