Fig. 4 Time dependent emission intensity of the thin film of 1 upon
exposure to the saturated vapour of nitrobenzene (left) and quenching
efficiency of 1 and 2 as a function of exposure time (right).
(ESIw) and such level of sensitivity falls below the permissible
level of TNT in drinking water established by the US EPA.8
Solution state fluorescence quenching of 1 was a good
enough indication to explore the possibility of its use in the
solid state. Solid state sensing of explosives vapour is most
appealing for infield practical use. To further explore the
quenching efficiency of supramolecular polymer 1 in the
solid-state, we monitored the emission intensity of a freshly
made thin film of 1 as a function of time before and after
exposure to the saturated nitroaromatic vapour. The initial
emission intensity of the film decreased significantly upon
exposing the film to the saturated nitrobenzene vapour at
room temperature for a certain period of time. The emission
was quenched by about 77% within only 35 s exposure. Upon
continuous exposure for longer time, the quenching reached its
equilibrium value at 96% after just 110 s (Fig. 4), which is very
high compared to just B8% quenching in the case of 2 upon
exposure for 300 s. The efficient quenching of fluorescence
intensity in the case of 1 might be due to the presence of a
supramolecular polymeric structure and also due to the easy
passage of analytes into the intrinsic internal pores in the solid
state. IR and STM images of the film indicated that it has a
similar H-bonded structural pattern (ESIw). Supramolecular
interaction (H-bonding) between the adjacent molecular
entities in 1 provided great amplification of optical signals
because of the long-range exciton communication along the
entire supramolecular chain and thus results in very high
sensitivity of 1 for the electron deficient quencher.9
Fig. 5 Reversibility of the thin film of 1 to the saturated vapour of
nitrobenzene (NB). The quenching was performed by exposing the film
to NB vapour for 120 s and the fluorescence of the film was recovered
by washing with methanol.
to traditional covalent polymers, the present system (1) repre-
sents the first example of a supramolecular polymer as an
efficient chemical sensor for selective sensing of nitroaromatic
explosives. The present study also reveals that the polymeric
arrangement via H-bonding in 1 facilitates the long-range
excitons migration and thus increases the overall quenching
efficiency/sensitivity compared to analogous discrete systems
(2 and 3) in the solid state. This new methodology of amplifica-
tion of a fluorescence chemosensory event using supramolecular
interaction may enable future development of much improved
sensors for chemical explosives. Research is under progress to
establish the full scope of this unique methodology.
The authors are grateful to the Department of Science and
Technology (DST), India, for financial support and S.S. thanks
the CSIR, India, for research fellowship. The authors sincerely
thank the reviewers for fruitful suggestions and comments.
Notes and references
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The thin film of 1 also showed highly reversible sensing of
nitroaromatic explosives. After exposing the film to a saturated
nitrobenzene vapour at room temperature for certain time,
initial emission intensity of the film was recovered by washing
with methanol and drying in hot air. By these treatments, the
thin-film quickly regains the emission intensity and thus can be
re-used for a significant number of cycles (Fig. 5).
Over several numbers of cycles, the quenching efficiency of the
film towards nitrobenzene decreased gradually presumably due to
the occupation of the free volume of the film by solvent molecules.
It is noteworthy that the efficient regaining of initial fluorescence
intensity over repeated cycles implies the high photostability of the
film for their long time infield explosive screening application.
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nitroaromatic explosives. Sensor 1 shows highly selective
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c
10048 Chem. Commun., 2011, 47, 10046–10048
This journal is The Royal Society of Chemistry 2011