Supported thin flexible polymethylhydrosiloxane permeable films functionalised with silole groups: New approach for detection of nitroaromatics

Article abstract of DOI:10.1039/c0jm01165g

Novel re-useable luminescent sensors highly sensitive to quenching by nitroaromatic compounds in both solution and the vapour phase have been developed by covalently bonding a new allyl-substituted silole into substrate-supported crosslinked polymethylhyd

Full text of DOI:10.1039/c0jm01165g

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www.rsc.org/materials | Journal of Materials Chemistry
Supported thin flexible polymethylhydrosiloxane permeable films
functionalised with silole groups: new approach for detection of
nitroaromatics†
a
a
b
a
Kassem Amro, Sebastien Clement, Philippe Dejardin, William E. Douglas, Philippe Gerbier,
a
*
ꢀ
ꢀ
Jean-Marc Janot and Thierry Thami
ꢀ
b
b
*
Received 22nd April 2010, Accepted 4th July 2010
DOI: 10.1039/c0jm01165g
nitroaromatics present not only in the vapour phase but also in many
types of solvent because of the robust nature of the crosslinked
network and covalent bonding to the substrate. They can be made in
thicknesses ranging from 20 nm up to 1 mm. The silole groups are
readily accessible, and the sensors can be regenerated by washing with
solvents such as chloroform.
Novel re-useable luminescent sensors highly sensitive to quenching
by nitroaromatic compounds in both solution and the vapour phase
have been developed by covalently bonding a new allyl-substituted
silole into substrate-supported crosslinked polymethylhydrosiloxane
(PMHS) thin films by Pt-catalyzed hydrosilylation of the SiH
groups.
The new pale yellow fluorophore 1-allyl-1-methyl-2,3,4,5-tetra-
phenylsilole (1) was synthesized in 55% yield by using Curtis’s
method17 in two steps (ESI†): (a) lithiation of tolane, followed by (b)
reaction of the intermediate dilithiotetraphenylbutadiene with allyl-
methyldichlorosilane (Scheme 1).
Chemical sensors for detection of traces of explosives, in particular
nitroaromatic compounds, are of great current interest for purposes
of public security and environmental protection.1–3 A variety of
conjugated polymers have been used as sensors for nitroaromatic
explosives detection4 including metallole-containing polymers.5,6 The
first metallole to be used was a polysilole,7 polymers8,9 and nano-
particles10 containing silole groups being of particular interest. Silole
molecules and polymers exhibit aggregation-induced emission (AIE)
caused by the restricted intramolecular rotations of the peripheral
aromatic rings about the axes of the single bonds linked to the central
silole cores.11,12 The emission is quenched by nitroaromatic
compounds, the effect being enhanced by Lewis acid–base interac-
tions between the silacycle and the nitroaromatic compound.13
However, polymer-based sensors made from linear uncrosslinked
polymers usually suffer from the drawback of polymer leakage into
the medium and ensuing shortened lifetime and inefficient analyses.
We report here the development of a new family of fluorescent film
sensors for which these disadvantages have been overcome, made
from thin films of crosslinked resin covalently bonded to the substrate
and containing a very low concentration of silole groups. They
exhibit the AIE effect owing to restricted intramolecular rotation,12
and show enhanced sensitivity to nitroaromatic analytes because of
the very low concentration of silole groups. Thus, a new silole bearing
an allyl group at silicon has been incorporated into previously
reported novel reactive polysiloxane coatings made from poly-
methylhydrosiloxane (PMHS) polymers crosslinked by the sol–gel
process allowing subsequent functionalization by hydrosilylation of
the SiH reactive groups.14–16 The films can be used to test for
Crosslinked polymethylhydrosiloxane (PMHS) thin films of 5%
crosslinking density were prepared by room temperature sol–gel
polymerization of methyldiethoxysilane/triethoxysilane 95 : 5 (mol%)
sol mixtures deposited by spin-coating on silicon wafer Si(100) or
microscope glass slide substrates freshly activated with ‘‘piranha’’
solution (ESI†). The substrates bearing PMHS thin films were
then cured at 100 ꢀC in an oven for 10 min. The thickness of the
virgin PMHS films was found to be ca. 1 mm as measured by
infrared spectroscopy from the absorbance of the Si–H peak at ca.
2167 cmꢁ1 and the calibrated absorption coefficient (ESI†).
The hydrosilylation reaction between the allyl group of 1 and the
PMHS Si–H function was performed in air by placing the virgin
PMHS thin film/substrate samples in a toluene solution of 1 in the
ꢀ
presence of Karstedt’s catalyst for 18 h at 60 C with stirring. The
samples were then removed from the reaction mixture, rinsed with
toluene and chloroform to remove any physisorbed material, and
dried in a stream of dinitrogen (ESI†). Dioxygen acts as a co-catalyst
in hydrosilylation reactions catalyzed by Karstedt’s catalyst.18
Hydrosilylation of 1 with triethoxysilane showed that the reaction is
regiospecific at the allyl group.
Previous studies on hydrosilylation by PMHS have shown that the
reaction goes essentially to completion.14,15 Indeed, in this case too, no
IR spectral evidence for any remaining unreacted silole allyl groups
aLabo. CMOS, CNRS UMR 5253, Institut Gerhardt, Universiteꢀ
Montpellier II, Place Egeꢁne Bataillon, 34095 Montpellier cedex 5,
France. E-mail: gerbier@univ-montp2.fr; Fax: +33 467143852; Tel: +33
467143972
bInstitut Europeꢀen des Membranes, ENSCM, Universiteꢀ Montpellier 2,
ꢁ
CNRS, Place Eugene Bataillon, 34095 Montpellier cedex 5, France.
Scheme 1 Synthesis of 1-allyl-1-methyl-2,3,4,5-tetraphenylsilole (1).
Reagents and conditions: (i) Li shavings, THF, 12 h and room tempera-
ture; (ii) allylmethyldichlorosilane, THF, 2 h at room temperature and 5
h under reflux.
E-mail: thierry.thami@iemm.univ-montp2.fr; Fax: +33 467149119; Tel:
+33 467149184
† Electronic supplementary information (ESI) available: Preparation and
characterization of 1 and PMHS films. See DOI: 10.1039/c0jm01165g
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Fig. 1 IR absorption spectra: PMHS film on glass slide (A) after
(magnified ꢂ10) and (B) before incorporation of 1 by hydrosilylation; (C)
1 in KBr.
present in the thin films was observed. The degree of silole incorpo-
ration into the thin films was found to be 7–8% from the normalized
IR absorbance ratio of the methylene group nas(CH2) antisymmetric
stretch at ca. 2922 cmꢁ1 (Fig. 1) and the n(SiH) band at ca. 2167 cmꢁ1
for the unreacted sample (virgin PMHS), all the methylene groups
originating from the silole allyl groups (ESI†). As expected, the total
intensity of the methylene absorbance corresponded to three meth-
ylene groups per silole following complete hydrosilylation of 1 and
covalent bonding into the PMHS network. The residual SiH function
was determined to be ca. 15–17% from the IR spectra by measuring
the total integrated absorbances of the n(SiH) bands in the 2100–2300
cmꢁ1 region for the reacted and unreacted samples (Fig. 1).
Fig. 2 (a) UV/visible absorption, and (b) emission spectra (lexcit. ¼ 375
nm) of PMHS film containing 1.
To summarize, 15–17% of the SiH groups remained unreacted, 7–
8% underwent hydrosilylation, and the remainder (ca. 75%) under-
went reaction with water (present from air moisture) catalyzed by
Karstedt’s catalyst18 (Scheme 2) as confirmed by the increase in the
IR Si–O–Si stretch band at ca. 1100 cmꢁ1 and the Si–OH band at ca.
3500 cmꢁ1 observed after hydrosilylation of 1 with the PMHS films.
The formation of additional Si–O–Si crosslinks serves to strengthen
the film resistance.
The UV/visible absorption and emission spectra of the thin films
confirm the presence of silole groups showing a characteristic
absorption band at ca. 370 nm assigned to the silole p–p* transition
(Fig. 2(a)) and an intense emission band at 485 nm (Fig. 2(b)). The
spectra remain unchanged after repeated washing of the samples with
chloroform, thus confirming the covalent bonding of the silole groups
Fig. 3 Decrease in emission intensity (lexcit. ¼ 375 nm) of chloroform
solution of 1 (10ꢁ5 M) with increasing concentration of (a) m-dinitro-
benzene, and (b) picric acid.
Scheme 2 Reactions of PMHS (represented by [Si]–H) in the presence of
Karstedt’s catalyst and air: hydrosilylation of 1 and reaction with H2O.
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Fig. 5 (a) Emission spectra (lexcit. ¼ 375 nm) and (b) Stern–Volmer plot
for sample of PMHS containing 1 immersed in chloroform solution with
increasing concentration of picric acid (a period of 1 h passed between the
two measurements in the presence of 0.025 mM picric acid).
Fig. 4 (a) Emission spectra (lexcit. ¼ 375 nm) and (b) Stern–Volmer plot
for sample of PMHS containing 1 immersed in chloroform solution with
increasing concentration of m-dinitrobenzene.
to the PMHS matrix and ruling out physisorption. The silole-con-
taining PMHS films retain their fluorescent properties for at least 3
months in air, and the Si–H groups in PMHS gels are stable.16
The ability of the new silole to act as a nitroaromatic sensor was
first studied in solution. Fluorimetric analysis of a solution of 1 in
chloroform (10ꢁ5 M) containing increasing amounts of m-dinitro-
benzene (Fig. 3(a)) and picric acid (Fig. 3(b)) showed the expected
fluorescence quenching attributed to Lewis acid–base interactions
between the silole silicon centres and the lone pairs of the nitro groups
of the nitroaromatic analyte, providing a bridging bond through
which electron transfer occurs from the excited state of the silole to
the analyte.13
In the case of the m-dinitrobenzene analyte the quenching effect is
found to be reversible. Simple washing of the detector film with
chloroform regenerates the initial fluorescence peak with the same
intensity and form (Fig. 6(B)) as for the pristine film (Fig. 6(A)). The
same procedure of washing with chloroform the sensor film treated
with picric acid gives rise to an emission spectrum of lower intensity
and showing two peaks at 408 and 455 nm (Fig. 6(C)). This behav-
iour is consistent with the formation of a new fluorescent product by
reaction of the silole groups and picric acid. Indeed, with picric acid
the quenching effect has been found previously to be partly irre-
versible in some cases.7,19
Studies on the new sensor films are continuing, in particular con-
cerning the effect of nitroaromatic compounds in the vapour phase.
Similar results were observed for a thin film of PMHS containing
1. The sample was immersed in a chloroform solution and increasing
amounts of m-dinitrobenzene (Fig. 4) and picric acid (Fig. 5) were
added. The emission spectra show that the quenching effect increases
with the concentration of m-dinitrobenzene (Fig. 4) or picric acid
(Fig. 5).
No corrections for inner filter effects or analyte absorption in the
emission spectra were made since, firstly, re-absorption of emitted
photons in the ultra-thin PMHS films can be neglected and, secondly,
a front-face setup with an optical path of ca. 1 cm and extremely low
analyte concentrations were used.
In the case of the m-dinitrobenzene solution, the emission
spectra did not change with time of immersion. However, after
the sample had been immersed for 1 h in the picric acid solution,
the emission intensity decreased markedly (cf. Fig. 5(a), the two
measurements in the presence of 0.025 mM picric acid) consistent
with some secondary reaction occurring between the silole
groups and picric acid. This is reflected in the hiatus exhibited by
the Stern–Volmer plot (Fig. 5(b)).
Fig. 6 Emission spectra (lexcit. ¼ 375 nm) of sample of PMHS con-
taining 1: (A) pristine film, (B) after immersion in chloroform solution
containing m-dinitrobenzene followed by rinsing with chloroform, and
(C) after immersion in chloroform solution containing picric acid fol-
lowed by rinsing with chloroform.
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Preliminary results indicate that the films are very sensitive exhibiting
almost instantaneous visible quenching, the effect being reversible
after ca. 1 h in ambient air.
Financial support was received from the Agence Nationale de la
Recherche and the CNRS.
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