Synthesis of a novel fluorene-based conjugated polymer with pendent bulky caged adamantane moieties and its application in the detection of trace DNT explosives

Article abstract of DOI:10.1016/j.reactfunctpolym.2012.01.002

A novel fluorene-based conjugated polymer with phenylene spacers and steric bulky adamantane moieties in side chains has been synthesized by palladium-catalyzed Suzuki coupling reactions. This design strategy offers several advantages for the detection of trace 2,4-dinitrotoluene (DNT) vapor. The incorporation of the two groups into polymer side chains could retain an effective conjugation length and prevent the π-stacking of polymer chains. The detection of DNT vapor indicated that the polymer displayed higher fluorescence quenching sensitivity toward the explosives in films compared to reference polymers. The fluorescence quenching efficiency of the fluorescent polymer achieved 33.3% in 10 s and 71.1% in 60 s. The pathways or cavities generated by the two spacers are beneficial for the rapid diffusion of explosive vapor into the film interiors and increase the fluorescence quenching efficiency of the film.

Full text of DOI:10.1016/j.reactfunctpolym.2012.01.002

Reactive & Functional Polymers 72 (2012) 206–211
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Synthesis of a novel fluorene-based conjugated polymer with pendent bulky caged
adamantane moieties and its application in the detection of trace DNT explosives
⇑
HongFei Leng, WenHui Wu
School of Material Science and Engineering, Beijing Institute of Technology, Beijing 100081, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel fluorene-based conjugated polymer with phenylene spacers and steric bulky adamantane moie-
ties in side chains has been synthesized by palladium-catalyzed Suzuki coupling reactions. This design
strategy offers several advantages for the detection of trace 2,4-dinitrotoluene (DNT) vapor. The incorpo-
ration of the two groups into polymer side chains could retain an effective conjugation length and
Received 23 September 2011
Received in revised form 7 January 2012
Accepted 9 January 2012
Available online 18 January 2012
prevent the
p-stacking of polymer chains. The detection of DNT vapor indicated that the polymer dis-
played higher fluorescence quenching sensitivity toward the explosives in films compared to reference
polymers. The fluorescence quenching efficiency of the fluorescent polymer achieved 33.3% in 10 s and
71.1% in 60 s. The pathways or cavities generated by the two spacers are beneficial for the rapid diffusion
of explosive vapor into the film interiors and increase the fluorescence quenching efficiency of the film.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Conjugated polymers
Fluorescence quenching
Adamantane moieties
High sensitivity
Explosives detection
1. Introduction
with non-conjugated side groups are effective methods to prevent
chain aggregation [18–22]. Among these methods, introducing
The rise in security issues and threats across the globe has re-
quired that methods for trace explosives detection are sensitive,
inexpensive and selective. The most common methods for the
detection of explosives, such as gas chromatography, ion mobility
spectrometry, X-ray imaging and electrochemistry, have some dis-
advantages. For example, the instruments previously mentioned
are expensive, and their operating procedures are complicated. In
contrast, the design and fabrication of chemical sensors has at-
tracted considerable interest because of their easy operation, low
cost and rapid detection capabilities [1–5]. Furthermore, conju-
gated polymers based on fluorescence quenching are proven to
be one of the most sensitive and convenient methods for the
detection of nitroaromatic-based explosives [6–8]. Hence a series
of conjugated polymers, such as polysilanes, poly(p-phenylenee-
thynylenes), polymetalloles, polyacetylene and poly(p-phenylene-
vinylene), etc. were prepared as sensing materials for trace
explosives detection [9–14].
bulky spacers into polymer backbones or side chains is an effective
approach. For example, adamantane is a bulky hydrocarbon, con-
sisting of four cyclohexanes fused into a chair conformation. Li
et al. synthesized poly(fluorine-co-adamantane) as a blue light
emitting polymer [23]. However, the average molecular weight
(Mn) of the copolymer was only 2700. We modified the adaman-
tane units and synthesized 1,3-bis(4-bromo-benzylideneanimo)
adamantane with higher reactivity. We also prepared a series of
polyfluorenes with adamantane derivatives as inserted-backbone
units with higher molecular weights (the Mn was approximately
2.0 Â 104) [24]. However, non-conjugated adamantane groups in-
serted into the polymer backbone can lead to a shorter effective
conjugation length [25,26], and this could lower the sensitivity of
the polymer when used for sensing applications.
Herein, we report the synthesis of a novel polymer that displays
a valid isolation effect on the main chain, where the main chain
contains phenylene spacers in the C-9 carbon of fluorene units
and adamantane moieties in side chains. For the designed polymer,
the incorporation of adamantane units into the side chains was ex-
pected to maintain the effective conjugation length of the conju-
gated polymer and the amplified signal of fluorescence quenching
toward explosives. To demonstrate the chain-isolated effect and
the high response to DNT of the bulky adamantane groups as well
as the presence of the rigid phenylene spacer introduced in the side
chains, two reference polymers with the same backbone have also
been synthesized. The fluorescence properties of the conjugated
polymers and their sensing performances toward DNT vapor are
However,
p-stacking of the conjugated polymers in the
solid-state (thin film) form leads to significant fluorescence self-
quenching and is a major disadvantage encountered in the applica-
tion of chemical sensors [15–17]. Several methods have been
developed to prevent the
p–p stacking of the conjugated chains.
Designing star-like, hyper-branched and cross-linked architec-
tures, copolymerizing with bulky spacers and chemically grafting
⇑
Corresponding author. Tel.: +86 10 68912659.
E-mail address: wuwh@bit.edu.cn (W.H. Wu).
1381-5148/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.reactfunctpolym.2012.01.002

H.F. Leng, W.H. Wu / Reactive & Functional Polymers 72 (2012) 206–211
207
investigated. To the best of our knowledge, such polymers have not
been reported in the literature to date and are expected to improve
the sensing capabilities for explosives.
2. Experimental procedure
2.1. Materials
1-Adamantanol was purchased from OrgSynChem Technology
Co., Ltd. 2,7-dibromo-9-fluorenone and phenol were supplied by
Aladdin-Reagent Co., Ltd. 9,9-dihexyl-2,7-dibromofluorene, ben-
zen-1,4-bis(boronic acid)propane-1,3-diol and tertetrakis(triphen-
ylphosphine) palladium were obtained from Synwit Technology
Co., Ltd. Other reagents and solvents obtained from commercial
suppliers were used without further purification.
2.2. Instrumentation
All new compounds were identified by 1H NMR and FT-IR. 1H
spectra were recorded on a Bruker AVANCE DRX 400 NMR
spectrometer. FT-IR spectra were obtained using a Bruker TENSOR
27 with samples prepared as KBr disk pellets. Gel permeation
chromatography (GPC) was carried out on a Waters GPC515-
2410 System with THF as an eluant. UV-Vis absorption and photo-
luminescence (PL) spectra of polymers were recorded using a
Shimadzu UV-1700 Ultraviolet Spectrophotometer and a Hitachi
F-4500 Fluorescence Spectrophotometer, respectively. Thermo-
gravimetric analysis (TGA) and differential scanning calorimetry
(DSC) measurements were performed under a nitrogen atmo-
sphere at a heating rate of 10 °C/min using a TA TG2050 and a
DSC910S, respectively.
Scheme 1. Synthetic routes to monomers (3 and 4).
ppm): d 7.57 (d, 2 H, Ar–H), 7.45 (d, 2 H, Ar–H), 7.39 (d, 2 H, Ar–
H), 7.19 (d, 2 H, Ar–H), 6.58 (s, 2 H, Ar–H), 6.45 (d, 2 H, Ar–H),
4.76 (s, 2 H, –OH), 2.05 (m, 18 H, Ad–H), 1.75 (s, 12 H, Ad–H).
FT-IR (film, cmÀ1):
m 3550 (Ar–OH), 3035 (Ar H), 2905, 1358,
1305, (Ad H), 1118, 1061 (C–Br).
2.3. Synthesis of monomers
2.3.3. Adamantane substituted (2, 7-dibromo-9,9-bis(4-
octyloxyphenyl))fluorene (3)
2.3.1. (2,7-Dibromo-9,9-bis(4-hydroxyphenyl))fluorene(1)
2 (1.552 g, 2 mmol) and KOH (0.448 g, 8 mmol) were mixed in
25 mL of ethanol. The reaction mixture was heated to 80 °C for
0.5 h, and then bromooctane (1.2 g, 6 mmol) was added slowly
dropwise into the reaction mixture. The solution was kept at
80 °C for 20 h. After the reaction was complete, the mixture was
poured into water (300 mL), extracted with ethyl acetate and dried
over MgSO4. After removal of the organic solvent, the residue was
purified by column chromatography on silica gel using ethyl ace-
tate/petroleum ether (v/v 1/15) as an eluent to yield white solid
3 (1.70 g, 85%). 1H NMR (CDCl3, 400 M Hz, ppm): d 7.56 (d, 2 H,
Ar–H), 7.45 (d, 2 H, Ar–H), 7.39 (d, 2 H, Ar–H), 7.25 (d, 2 H, Ar–
H), 6.85 (s, 2 H, Ar–H), 6.62 (d, 2 H, Ar–H), 3.90 (s, 4 H, OCH2),
2.03 (m, 18 H, Ad–H), 1.83 (m, 12 H, Ad–H), 1.74 (m, 4 H, CH2),
1.52 (m, 4 H, CH2), 1.32 (m, 16 H, CH2), 0.87 (m, 6 H, CH3).
A three-neck 100 mL flask was filled with phenol (13.160 g,
140 mmol) and 2,7-dibromofluoren-9-one (4.732 g, 14 mmol).
Methanesulfonic (20 mL) and mercaptopropionic (0.075 g,
0.708 mmol) were added to the mixture and then reacted for
16 h at 50 °C. The reaction mixture was then poured into 500 mL
of cold water. The residue was washed with hot water several
times. Petroleum ether was added to the crude products dissolved
in ethyl acetate, and then the precipitate was collected and dried in
a vacuum oven to provide a light brown solid, monomer 1 (6.11 g,
86%). 1H NMR (CDCl3, 400 M Hz, ppm): d 9.41 (s, 2 H, –OH), 7.89 (d,
2H, Ar–H), 7.55 (d, 2 H, Ar–H), 7.47 (s, 2 H, Ar–H), 6.87 (d, 4 H,
Ar–H), 6.65 (d, 4 H, Ar–H). FT-IR (film, cm-1):
3060 (Ar H), 1175, 1053 (C–Br).
m 3296 (Ar–OH),
2.3.2. Bisubstituted (2, 7-dibromo-9,9-bis(4-hydroxyphenyl))fluorene
with adamantane moieties (2)
2.3.4. (2,7-Dibromo-9,9-bis(4-octyloxyphenyl))fluorene (4)
Monomer 1 (1.016 g, 2 mmol) and KOH (0.448 g, 8 mmol) were
mixed in 25 mL of ethanol. The reaction mixture was heated to
80 °C for 0.5 h, then bromooctane (1.2 g, 6 mmol) was added
slowly dropwise into the reaction mixture. The solution was kept
at 80 °C for 20 h. After the reaction was complete, the mixture
was poured into water (300 mL), and extracted with ethyl acetate.
After removal of the organic solvent at reduced pressure, the resi-
due was purified by column chromatography on silica gel using
ethyl acetate/petroleum ether (v/v 1/6) as an eluent to yield white
solid 4 (1.30, 89%). 1H NMR (CDCl3, 400 M Hz, ppm): d 7.56 (d, 2 H,
Ar–H), 7.47 (d, 2 H, Ar–H), 7.44 (d, 4 H, Ar–H), 7.04 (d, 2 H, Ar–H),
6.71 (s, 4 H, Ar–H), 3.89 (s, 4 H, OCH2), 1.76 (m, 4 H, CH2), 1.44 (m, 4
H, CH2), 1.29 (m, 16 H, CH2), 0.87 (m, 6 H, CH3).
1-Adamantanol (2.736 g, 18 mmol) and monomer 1 (4.572 g,
9 mmol) were dissolved in CH2Cl2 (30 mL) at 0 °C, and then a mix-
ture of 98% H2SO4 (1.5 mL) and AcOH (7.5 mL) was added slowly
dropwise over 20 min. After 21 h of stirring at room temperature,
the resulting mixture was poured into water and extracted with
CH2Cl2, and the organic phase was dried over sodium sulfate. After
the removal of solvent at reduced pressure, the residue was puri-
fied by column chromatography on silica gel using ethyl acetate/
petroleum ether (v/v 1/6) as an eluent. White solid 2 was obtained
by evaporating of the eluent (4.54 g, 65%). In this reaction, 2 may
have three ortho-isomers, so 2 in Scheme 1 was representative of
one of the three isomer structures. 1H NMR (CDCl3, 400 M Hz,

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2.4. General procedure for synthesis of polymers PF1–PF3
isomers were used for the next synthesis without separation.
Monomers 3 and 4 were synthesized from the reaction of 2 and
3 with bromooctane in the presence of KOH. As shown in Scheme
2, PF1–PF3 were prepared by a palladium-catalyzed Suzuki cross-
coupling reaction.
The obtained polymers are completely soluble in common or-
ganic solvents, such as THF, DMF, toluene, and chloroform. The
GPC analysis indicated that the number-average molecular weight
of PF1 was 7900 with a 2.73 polydispersity index (PDI), that of PF2
was 6400 with a 2.11 polydispersity index (PDI) and that of PF3
was 7800 with a 2.15 polydispersity index (PDI).
The thermal stability of polymers PF1–PF3 were characterized
by thermogravimetric analysis (TGA). As shown in Fig. 1, the ther-
mal decomposition temperatures (Td, 95 wt.% residue) of PF1–PF3
were 387 °C, 381 °C and 416 °C in nitrogen, respectively. This dem-
onstrated that the three polymers were thermally stable enough
for use as sensing films at ambient temperatures. The glass transi-
tion temperatures (Tg) of polymers PF1–PF3 were characterized by
differential scanning calorimetry (DSC) (see Supplementary Fig S4).
The Tg of PF1 was not observed up to 152 °C, which was higher
than for the other two polymers. This suggests that the chain
General synthetic procedures for the polymers PF1-PF3 can be
described as follows. To a solution of fluorene derivatives (mono-
mers 3–5) and benzen-1,4-bis(boronic acid)propane-1,3-diol in
THF, aqueous potassium carbonate (2 M) was added. The mixture
was stirred and degassed with nitrogen for 15 min, and a catalytic
amount of tetrakis(triphenylphosphine) palladium (Pd(PPh3)4)
(2.0 mol%) was added quickly. The solution was then stirred at
69 °C for 48 h, and the mixture was cooled and poured into meth-
anol. The precipitates were collected by filtration and washed copi-
ously with methanol and water several times. After the organic
solvents and water were removed, the solid was washed with ace-
tone to remove oligomers and catalyst residues. Finally, the prod-
uct was obtained after being dried at 40 °C under vacuum for 1 day.
PF1: 3 (1.000 g, 1 mmol), benzen-1,4-bis(boronic acid)propane-
1,3-diol (0.246 g, 1 mmol), Pd(PPh3)4 (0.024 g, 0.018 mmol) and
2 M aqueous Na2CO3 solution (10 mL) were used for the synthesis
of PF1. The obtained solid was a light yellow powder (0.632 g, 69%).
1H NMR (CDCl3, 400 M Hz, ppm): d 7.83 (d, 2 H, Ar–H), 7.50–7.61
(m, 6 H, Ar–H), 7.31–7.46 (m, 4 H, Ar–H), 6.85 (d, 2 H, Ar–H),
6.64 (d, 2 H, Ar–H), 3.90 (s, 4 H, OCH2), 2.07 (m, 18 H, Ad–H),
1.87 (m, 12 H, Ad–H), 1.75 (m, 4 H, CH2), 1.53 (s, 4 H, CH2),
1.41–1.22 (m, 16 H, CH2), 0.87 (m, 6 H, CH3).
PF2: 4 (0.732 g, 1 mmol), benzen-1, 4-bis(boronic acid)pro-
pane-1,3-diol (0.246 g, 1 mmol), Pd(PPh3)4 (0.024 g, 0.018 mmol)
and 2 M aqueous Na2CO3 solution (10 mL) were used for the syn-
thesis of PF2. The obtained solid was a yellow powder (0.505 g,
78%). 1H NMR (CDCl3, 400 M Hz, ppm): d 7.86 (d, 2 H, Ar–H),
7.39–7.68 (m, 8 H, Ar–H), 7.26 (d, 4 H, Ar–H), 6.82 (d, 4 H, Ar–H),
3.93 (s, 4 H, OCH2), 1.78 (m, 4 H, CH2), 1.46 (m, 4 H, CH2), 1.31–
1.20 (m, 16 H, CH2), 0.91 (m, 6 H, CH3).
PF3: Monomer 5 (0.492 g, 1 mmol), benzen-1,4-bis(boronic
acid)propane-1,3-diol (0.246 g, 1 mmol), Pd(PPh3)4 (0.024 g,
0.018 mmol) and 2 M aqueous Na2CO3 solution (10 mL) were used
for the synthesis of PF3. The obtained solid was a light gray powder
(0.330 g, 81%). 1H NMR (CDCl3, 400 M Hz, ppm): d 7.83 (d, 4 H,
Ar–H), 7.67 (d, 2 H, Ar–H), 7.31 (d, 4 H, Ar–H), 2.11 (m, 4 H,
CH2), 1.27–1.11 (m, 16 H, CH2), 0.81 (m, 6 H, CH3).
2.5. Fabrication of sensing films
The films PF1–PF3 were prepared by a conventional spin-casting
technique. In brief, the polymer solution (0.5 mg in 1.0 mL THF) was
spin-coated onto a glass plate (microscope slide, length: 24 mm,
width: 24 mm and thickness: 1 mm) at a spin rate of 1000 rpm.
Then, the films were dried at 30 °C under vacuum for 1 day to re-
move any residue in the films.
Scheme 2. Synthetic routes to polymers (PF1–PF3).
3. Results and discussion
3.1. Synthesis and characterization
Our strategy for the synthesis of the monomers is outlined in
Scheme 1, where 7-dibromofluoren-9-one in phenol and methane-
sulfonic acid in the presence of a catalytic amount of mercaptopro-
pionic acid afforded compound 1. The acid-catalyzed Friedel–Crafts
reaction of 1 and 1-Adamantanol gave bisubstituted 2 which was
confirmed by 1H NMR (see Supplementary Fig S1). The eighteen
hydrogen atoms at 2.05 ppm and the twelve hydrogen atoms at
1.75 ppm indicated that two adamantane units were incorporated
into monomer 1. As described in the literature [27], the hydrogen
atom at ortho-position of the phenolic hydroxyl group is substi-
tuted with adamantane units by Friedel–Crafts alkylation. In this
reaction, ortho-isomers of monomer 2 may exist, and these
Fig. 1. TG curves of polymers PF1–PF3.

H.F. Leng, W.H. Wu / Reactive & Functional Polymers 72 (2012) 206–211
209
mobility of this polymer was effectively suppressed by the incor-
poration of the rigid adamantane component. In addition, the
absorption and fluorescence spectra of the three polymers were
not changed in solution (such as THF, CHCl3, DMF and toluene)
for 1 month, thereby indicating that they were stable in solvents.
3.2. Photophysical properties
The photophysical properties of PF1–PF3 were examined in
both THF solution and the solid films. The absorption and PL spec-
tra in films are shown in Fig. 2. The absorption and fluorescence
spectra of the three polymers in THF solution are similar (see Sup-
plementary Fig S3). This is because the conjugated polymers can
disperse well in the THF solution, and chain aggregation among
the three polymers was either very weak or did not exist.
The absorption peaks for PF1–PF3 in the solid state were shifted
by approximately 5 nm, 21 nm and 24 nm to longer wavelengths
compared with that in THF, respectively. In addition, the obvious
green emission peak at 520 nm for PF2 and PF3 as a film were
much stronger than that of PF1, and this correlated to aggregation
of polymer chains in the solid state [18,19]. The red shift in the
absorption spectra of PF2 was shorter than that of PF3, and the
green emission peak of PF2 at 520 nm was also weaker than that
of PF3. This revealed the phenylene spacer can prevent interchain
interactions in the solid state, and the bulky adamantane moieties
could further effectively suppress the aggregation of polymer
chains in the film.
The fluorescence quantum efficiency of PF1–PF3 in THF solution
was similar (UPL was estimated using quinine sulfate in 0.05 M
H2SO4, UPL = 0.546). However, as we expected, the UPL of the three
polymers is ordered as follows: PF1 (0.58) > PF2 (0.45) > PF3 (0.42)
in the film state. The higher UPL of PF2 versus PF3 was attributed to
the phenylene units in the C-9 carbon of fluorene groups, as these
have steric hindrance effects on the main chains of the polymers
[28]. The improvement UPL of PF1 can be attributed to the adaman-
tane moieties, thereby sterically isolating the polymer backbones
and reducing self-quenching that usually accompanies the solid
state form. The adamantane moieties have both positive and neg-
ative effects on the UPL of PF1, as it is a non-conjugated group.
However, the higher UPL of PF1 indicated that the positive effect
was the leading factor, so the UPL of PF1 was increased. Both the
phenylene and adamantane moieties in the side chains benefit
the higher UPL of PF1 in the film state.
Fig. 3. The fluorescence quenching efficiencies of PF1–PF3 in THF solution toward
DNT.
fluorescence quenching efficiencies (FQEs) of PF1–PF3 in THF are
shown in Fig. 3. The fluorescence quenching efficiency (FQE) is de-
fined as (I0 À I)/I0 Â 100%, where I0 and I are the fluorescence inten-
sity in the absence and presence of DNT, respectively. Compared
with PF2 and PF3, PF1 exhibited a higher FQE to DNT. The sensitiv-
ity of PF1 to DNT in THF may be related to the more structurally
restricted environment of PF1, as PF1 may cause the DNT to be
effectively ‘‘bound’’ to the polymer backbones rather than simply
diffuse through, and this can effectively increase the probability
of charge-transfer [29]. As indicated in early literature [30], the
fluorescence quenching of polymers is affected by the binding
strength between polymers and analytes, so the higher FQE of
PF1 may be related to the higher binding strength between PF1
and DNT molecules.
3.4. Fluorescence quenching characteristics of PF1–PF3 to DNT vapor
in films
Fluorescence response experiments for the three polymers to
the DNT vapor were performed in the following manner [30].
Briefly, the film was inserted into a glass bottle containing a small
amount of DNT powder and cotton gauze (to help maintain a con-
stant saturated vapor pressure at room temperature and prevent
the film from directly contacting the DNT powder). By measuring
the fluorescence intensity of the film after exposure to the DNT
vapor for different time periods, a time-dependence of the fluores-
cence quenching was obtained.
3.3. Fluorescence quenching characteristics of PF1–PF3 to DNT in
solution
Fig. 4 shows the time-dependent fluorescence intensities for
PF1 films upon exposure to DNT vapor. The fluorescence intensity
of PF1 gradually decreased over time after exposure to the DNT
vapor. The FQE at different time periods of exposure to the DNT
vapor is shown in Fig. 5. Rapid PL quenching was observed for
PF1 upon exposure to the DNT vapor. For example, the FQE of
PF1 was 33.3% in 10 s. However, the FQE of PF2 and PF3 were only
8.2% and 6.1% in 10 s, respectively. Compared with PF2 and PF3,
PF1 was four-fold more sensitive toward the DNT vapor. After
exposure to the DNT vapor for 300 s, the FQE of PF1 was 91.6%,
and this was similar to that of poly(iptycenebutadiynylene) deriv-
atives reported by Swager’s group (which was 90% around in 300 s)
[29]. As we expected, the PF1 film showed higher sensitivity than
the other two polymers after exposure to DNT vapor.
The fluorescence quenching of PF1–PF3 to DNT as a representa-
tive nitroaromatic-based explosive was studied in THF. The
Clearly, the presence of the phenylene and adamantane moie-
ties in PF1, which maintain a high fluorescence quantum efficiency,
is a reasonable explanation for the observed enhanced fluorescence
quenching efficiency. Furthermore, the incorporation of bulky
three-dimensional adamantane groups and phenylene spacers
introduce pathways or cavities in the solid-state material. These
Fig. 2. Absorption and PL spectra of PF1–PF3 in films.

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H.F. Leng, W.H. Wu / Reactive & Functional Polymers 72 (2012) 206–211
the backbones into several isolated shorter conjugated chain
segments. Therefore, the conjugated polymers’ superiority of
amplified fluorescence quenching will disappear [30]. So, the intro-
duction of adamantane moieties into side chains can maintain the
long conjugated length of the polymer chain and the high sensitiv-
ity towards explosives.
The fluorescence quenching-recovery of the PF1 thin film has
also been tested. Fig. 6 shows the results of fluorescence quench-
ing-recovery cycles. After exposure to DNT explosive vapor, the
film’s fluorescence intensity can be slowly recovered by exposure
to air for a few days or quickly recovered by immersing in metha-
nol for 20 min followed by drying in air. The recovered film showed
similar quenching efficiency when re-exposed to the vapor of the
DNT explosive. This indicates that the film could be used repeat-
edly. Efficient quenching and recovery were obtained for repeated
use, and this implies a good reversibility of the material and a high
fluorescence stability for the film against photobleaching.
Fig. 4. Time-dependent fluorescence intensities of PF1 upon exposure to DNT vapor
in different time intervals.
4. Conclusion
In this work, we have synthesized a novel fluorescent polymer
containing phenylene and adamantane moieties as side groups
through a typical Pd catalyzed Suzuki coupling reaction. The incor-
poration of adamantane units into the side chain (versus insertion
into the backbone) maintains the effective conjugation length of
conjugated polymer and the higher FQE towards DNT. It is shown
that the incorporation of the rigid three-dimensional structure of
adamantane moieties and phenylene spacers could effectively sup-
press interchain interactions and generate pathways or cavities
that can improve the diffusion of explosive molecules into films,
and thus increase the fluorescence quenching efficiency toward
DNT in films.
Appendix A. Supplementary material
Fig. 5. Time-dependence of FQE of synthesized polymers in thin films upon
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.reactfunctpolym.2012.01.002.
exposure to DNT vapor at different times.
were suitable for accommodating DNT molecules and made the
diffusion of the DNT molecules into the films easier. These also
enhanced charge-transfer from the excited polymer backbone to
the DNT molecules and have a rapid response. Although the poly-
mers with adamantane moieties inserted into the backbone can
generate a larger free volume in the solid state, as suggested in
the literature of Li [23], the adamantane units in the backbone
interrupt the conjugated structure of the polymer and separate
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