Polytriazole bridged with 2,5-diphenyl-1,3,4-oxadiazole moieties: A highly sensitive and selective fluorescence chemosensor for Ag+

Article abstract of DOI:10.1039/c5ra07822a

Fluorescent conjugated polytriazoles (FCP 1-4) containing both 2,5-diphenyl-1,3,5-oxadiazole (OXD) and 1,2,3-triazole moieties in the main chain were synthesized from aromatic diazide (1) and dialkynes (2-5) via click polymerization, respectively. In the polymers, OXDs (fluorophores) and triazole rings (generated via CuAAC acting as metal ion ligands) comprise a fluorescent system. The polytriazoles displayed relatively strong emission with quantum yields in the range of 0.20-0.28 at room temperature in DMF. The study on their ion-responsive properties showed that, although all four FCPs have good selectivity for Ag⁺, the integration of alkoxy side groups (methoxy for FCP 2, hexyloxy for FCP 3 and 2-ethylhexyloxy for FCP 4) to the main chains of the polytriazoles decreased their sensitivity for Ag⁺via alteration of the polymer aggregation status and electron density of the main chains. Thus FCP 1 is highly sensitive for Ag⁺, where its K_sv is as high as 1.44 × 10⁵ M^-1 and its lowest detection limit is in the ppb range (4.22 × 10^-7 M). This study provides an efficient click approach to the synthesis of a novel fluorescence sensor for Ag⁺ detection, which could expand the application of click polymerization in designing fluorescence sensors based on the triazole unit. This journal is

Full text of DOI:10.1039/c5ra07822a

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Polymer Chemistry
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ARTICLE
Polytriazole
Bridged
with
2,5-Diphenyl-1,3,4-
Oxadiazole Moieties: A Highly Sensitive and Selective
Fluorescence Chemosensor for Ag
FCite
this:
DOI:
+
10.1039/x0xx00000x
*
Shoupeng Cao, Zhichao Pei, Yongqian Xu, Ruina Zhang, Yuxin Pei
Received 00th January 2012,
Accepted 00th January 2012
Fluorescent conjugated polytriazoles (FCP 1 ꢀ 4) containing both 2,5ꢀdiphenylꢀ1,3,5ꢀ
oxadiazole (OXD) and 1,2,3ꢀtriazole moieties in the main chain were synthesized from
aromatic diazide (1) and dialkynes (2 ꢀ 5) via click polymerization, respectively. In the
DOI: 10.1039/x0xx00000x
polymers, OXDs (fluorophores) and triazole rings (generated via CuAAC acting as metal ion
ligands) comprise a fluorescent system. The polytriazoles displayed relatively strong emission
with quantum yield in a range of 0.20 – 0.28 at room temperature in DMF. The study on their
www.rsc.org/
+
ionꢀresponsive properties showed that, although all four FCPs have good selectivity for Ag ,
the intergation of bulky alkoxy side groups (methoxy for FCP 2, hexyloxy for FCP 3 and 2ꢀ
ethylhexyloxy for FCP 4) to the main chain of the polytriazoles decreased their sensitivity for
+
Ag via alteration of polymer aggregation status and electron density of the main chain. Thus
+
5
ꢀ1
FCP 1 is highly sensitive for Ag , where its
K is as high as 1.44 × 10 M and its lowest
sv
ꢀ
7
detection limit is in the ppb range (4.22 × 10 M). This study provides an efficient click
approach to the synthesis of a novel fluorescence sensor for Ag detection, which could expand
+
the application of click polymerization in designing fluorescence sensor based on triazole unit.
Introduction
and feasibility to tune the electronic structure to change or
enhance the selectivity of the system.
1
2
The widespread applications of silver and its compounds in
electric, photographic, and pharmaceutical industries has led to
a large amount of silver release into the environment from
industrial wastes and emissions. As one of highly toxic heavy
metal ions, the excess silver ions causes bioaccumulation and
Click polymerization, which originates from click reaction
coined by Sharpless in 2001, has become a powerful
polymerization technique for new materials with tailored
functionalities, where most often is carried out between diꢀ or
13
multiazides and diꢀ or multialkynes with Cu(I) as catalyst
1
2
toxicity, inactivation of sulfhydryl enzymes, and irreversible
darkening of the skin. In the past decades, the upsurge of silver
nanoparticles (AgꢀNPs), extensively applied in medicine,
environmental remediation, and information technology, causes
14,15
(
copper catalyzed alkyneꢀazide cycloaddition, CuAAC).
Thus, polytriazoles (PAs, named after the formation of the
unique structural units of Nꢀcontaining fiveꢀmember 1,2,3ꢀ
triazole ring from azideꢀalkyne cycloaddition) have been
synthesized as highly efficient fluorescent polymer liquid
3
direct AgꢀNPs release into environmental systems. In the
presence of moisture, oxidation of AgꢀNPs results in the release
15
16
crystal,
semiconductors,
hyperꢀcrossꢀlinked fluorescent
+
of Ag ions. The relative environmental concerns are therefore
17
polymer nanoparticles for cell imaging, and highly sensitive
fluorescent chemosensor for nitroaromatic explosives,
more pressing than ever, thus the development of selective and
18,19
by
+
sensitive fluorescent chemosensors for Ag is highly desirable.
incorporating chosen functional groups. Most attractively,
numerous 1,2,3ꢀtriazole structural units along polymer
backbones can act as potentially versatile ligands for metal
+
To date, the majority of fluorescence sensors for Ag ions have
4
ꢀ6
been based on small or single molecule dyes. Few literatures
have reported fluorescent sensors based on conjugate polymers
20
21
coordination and produce the “molecular wire effect” for
sensing signal amplification when used as optical sensors for
metal ions. Moreover, by using a clickable monomer containing
a fluorophore, a fluorescent PA with high sensitivity can be
obtained. This extraordinary property, arising from the large
numbers of both binding sites and fluorophores, allows efficient
interaction with the target ions or molecules. Hence, a few PAꢀ
7
ꢀ9
for silverꢀion detection.
Compared to small molecule sensors,
1
0,11
fluorescent
conjugated polymer chemosensors for metal ions are
increasingly attractive to researchers due to their advantages
from enhancements associated with electronic communication
between receptors along polymer backbones, processability,
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DOI: 10.1039Jo/Curnal Na2Ame
2+
based fluorescent sensors selective for Hg were synthesized
2
2ꢀ24
by
Zhu
and
his
coworkers.
Incorporating
Results and Discussion
benzo[2,1,3]thiadiazole moieties to PA, the same researchers
2+
successfully synthesized a fluorescent polymer for Ni with
To synthesize polytriazoles bridged with 2,5ꢀdiphenylꢀ1,3,4ꢀ
detection limit as low as 1.1 × 10 M. However, to the best of oxadiazole (OXD) moieties via click polymerization, diazide
ꢀ
7
25
our knowledge, there is no polytriazole fluorescent monomer 2,5ꢀbis(4ꢀazidophenyl)ꢀ1,3,4ꢀoxadiazole
(
1
)
and
), 1,4ꢀdiethynylꢀ
1,4ꢀdiethynylꢀ2,5ꢀ
), and 1,4ꢀdiethynylꢀ2,5ꢀbis(2ꢀ
) were obtained first according to the
+
chemosensor for Ag reported.
dialkyne monomers 1,4ꢀdiethynylbenzene (
2
On the other hand, 1,3,4ꢀoxadiazole unit with electronꢀ 2,5ꢀbis(methoxyl)benzene
(3
),
deficient nature is well known for their high electron affinity bis(hexyloxy)benzene
values and excellent electronꢀtransporting properties, and is a ethylhexyloxy)benzene (
very popular chromophore in polymers for photonic devices, published procedures (Scheme 2a ꢀ c).
(4
5
3
1ꢀ37,55
Monomer
1 was
2
6
27
such as OLEDs,
polymer lightꢀemitting diodes,
and designed to contain a 1,3,4ꢀoxadiazole unit for incorporating it
2
8
photovoltaic cells. Polymers containing 1,3,4ꢀoxadiazole units to the polymer backbone later via click polymerization.
endow the final polymer not only high thermoꢀstability and Clickable functional groups of azide and alkyne were chosen
chemical stability, but also high photoluminescence specifically for performing CuAAC reaction to generate 1,2,3ꢀ
2
9
efficiency. For instance, Cheng and his coworkers synthesized triazole unit. Methoxyl,
linear chiral polymers based on optically active polybinaphthyls groups were introduced to monomer
and 1,3,4ꢀoxadiazole, which showed strong greenꢀblue obtain polytriazoles with side chains in order to study their
n
ꢀhexyloxy and 2ꢀethylhexyloxy
3
,
4
and , respectively, to
5
3
0
fluorescence.
sensitivities when used as fluorescent chemosensor for metal Thus, via click reaction between azide and alkyne groups, four
ions, no selectivity was observed: the polymers showed similar FCPs were synthesized: FCP 1 was from and , FCP 2 from
, FCP 4 was from and
Although the polymers showed excellent influence on solubility, aggregation, and fluorescent properties.
1
2
1
2+
2+
2+
2+
2+
+
quenching efficiencies for Ni , Cu , Pb , Zn , Hg , and Ag . and
3
, FCP 3 from
1
and
4
1
5
,
+
15
Moderate coordination ability of Ag and the strong interferes respectively, in DMF in the presence of Cu(PPh
of Hg and other heavy metal ions make Ag difficult to be units of oxadiazole and triazole were incorporated into the
discriminated from other chemically similar heavy metal polymer backbones. After precipitation, washing, and drying
ions.
The design and synthesis of highly sensitive and steps, FCP 1, FCP 2, FCP 3, and FCP 4 were obtained with a
selective system for the determination of trace amount of silver yield of 37 %, 49 %, 61 %, and 67 %, respectively, as yellowish
)
3
Br. Both
3
2+
+
6
,7
ions remains a challenge.
In this work, by using 2,5ꢀbis(4ꢀazidophenyl)ꢀ1,3,4ꢀ
solids.
It’s worthy to mention that the residual copper in the final
Br used in the
oxadiazole (
1
) as diazide monomer, and 1,4ꢀdiethynylbenzene polymers from the catalyst Cu(PPh
), 1,4ꢀdiethynylꢀ2,5ꢀbis(methoxyl)benzene (
), 1,4ꢀdiethynylꢀ polymerization is rather low (roughly equal to 0.1equiv.), the
,5ꢀbis(hexyloxy)benzene
), and 1,4ꢀdiethynylꢀ2,5ꢀbis(2ꢀ quenching caused by it is little and can be ignored (see Fig.
) as dialkynyl monomers (Scheme 1), S14).
novel fluorescent polytriazoles containing both units of 1,2,3ꢀ
The polymers were characterized by GPC, FTIR, TGA, and DSC,
)
3 3
3
8
(
2
3
2
(4
ethylhexyloxy)benzene (
5
triazole and 1,3,4ꢀoxadiazole were first synthesized via click respectively.
polymerization (CuAAC). Thereafter, the polytriazoles were
3
3
o
FCPs
M (×10 )
M (×10 ) PDI T ( C)
n
w
d
characterized by FTIR, GPC, TGA, and DSC, respectively.
Their fluorescence property, selectivity, and sensitivity for
heavy metal ions were investigated. To our delight, we found
that FCP 1 (fluorescent conjugated polytriazole formed with
FCP 1
FCP 2
FCP 3
FCP 4
3.78
5.24
11.6
12.0
5.7
6.9
1.51
1.32
1.24
1.50
289
204
273
239
14.5
18.0
monomer
for Ag .
1 and 2) showed excellent selectivity and sensitivity
Table 1 The characteristic data of FCP 1 – 4 from GPC and TGA.
+
The numberꢀaverage molecular weights (Mn), the weightꢀ
average molecular weights (Mw), and polydispersity indexes
(
PDI) of FCPs were determined by GPC using polystyrene
standards in DMF (Table 1). It can be seen that the polymers
synthesized in this work have moderate molecular weights and
acceptable PDIs: 3780 and 1.51 for FCP 1, 5240 and 1.32 for
FCP 2, 11600 and 1.24 for FCP 3, and 12000 and 1.50 for FCP
4
. A noteworthy observation is the molecular weights of FCP 3
and 4, which are much higher than FCP 1 under the same
polymerization conditions, potentially as a result of the
synergetic effect of higher monomer reactivity of
4 and 5 due to
Scheme 1 The structures of monomers used for click polymerization.
the electron–donating bulky alkoxy groups at orthoꢀposition of
benzene ring and better solubility of comb oligomers formed
during the polymerization processes.
2
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ART22ICLE
DOI: 10.1039/C5RA078 A
The click polymerization between azide and alkynyl groups (see Fig. 2), it is thus reasonable to assign the second peak at
1
o
(
monomer 1 and 2) was followed by NMR for 2 h. The H NMR ∼320 C to the thermal decomposition of the polymer skeletons.
spectra of the starting materials and the reaction mixture after 2 h The results showed that the polymers possess desirable thermal
was recorded and shown in Fig. S1. It can be seen that a new peak properties, which most likely can be attributed to the multiꢀring
appeared at 9.56 ppm, attributed to the proton in the newꢀgenerated structure on the main chains of the polymers, although the side
triazole ring. In the meantime, the integration of the peak at 4.38 groups (methoxyl for FCP 2, hexyloxy for FCP 3, and 2ꢀ
ppm (HC Cꢀ) decreased with time, indicating a consumption of ethylhexyloxy for FCP 4) impaired the thermostability of the
≡
alkynyl groups with reaction and a successful click reaction between polymers to some extent, as shown in Fig. 2 and Fig. S5.
monomer 1 and 2.
The FTIR spectrum of FCP 1 is shown in Fig. 1. Compared to the
ꢀ
1
IR spectra of monomer 1 and 2, a new absorption peak at 3106 cm
was observed, which is assigned to the stretching vibration of CꢀH
3
9
on triazole ring formed in click polymerization. The sharp
4
0
ꢀ1
absorption band of C=NꢀN=C in monomer 1 at 1490 cm
deformed slightly due to the stretching vibrations of C=C, N=N, and
ꢀ
1 40ꢀ43
triazole ring at 1508 cm .
Although the stretching vibrations of
ꢀ
1
ꢀ1
CꢀH (3296 cm ) in alkyne and azide (2092 cm ) in the polymer
were still observed, the relative intensities of azide to C=N in OXD
ꢀ
1 40, 41
unit (1610 cm )
and C≡CH decreased significantly. A similar
phenomenon was also observed in the IR spectra of FCP 2, 3, and 4,
though the intensity decrease was even more significant (Fig. S2, S3,
and S4) due to the lower percentages of the residual end groups of
azide and alkyne and higher DPs (degrees of polymerization) in FCP
Fig. 2 TGA curves of FCP 1 ꢀ 4.
2
, 3, and 4 compared to FCP 1 (see Table 1). All these results
indicated a successful click reaction had taken place between
monomer 1 and 2 – 5, respectively.
FCPs are nonfluorescent in solid state. However, under UV light
(λex = 365 nm), FCPs showed blueꢀgreen fluorescence in common
aprotic organic solvents such as dichloromethane, THF, chloroform,
acetonitrile, DMF, and DMSO. While in protic solvent methanol, no
fluorescence was observed. It is wellꢀknown that alcohols can form
intermolecular hydrogenꢀbonded complex with small fluorescent
4
4
molecules to produce profound effect on their fluorescent behavior.
However, few reports were found to discuss the effect of methanol
on emission of conjugated fluorescent polytriazoles (CPAs): Bunz
and his coꢀworkers reported that methanol had no alteration of the
emission of poly(arylenetriazolyene)s, while addition of acid to the
polymers caused a red shift, which was attributed by the authors to
1
6
conformation changes by protonation of triazole rings. In the other
study, a mixture of methanol/water (1:1, v/v) was used as solvent to
2
+ 24
dissolve CPA to investigate the quenching efficiency of Hg .
Based on these reports, it is interesting to further investigate the
fluorescent behaviour of FCPs upon the addition of methanol and
acid. The results are displayed in Fig. 3. It can be seen that the
fluorescence intensity of FCP 1 in DMF decreased gradually with
Fig. 1 FTIR spectra of monomer 1, 2, and FCP 1.
Thermal properties of FCPs were evaluated by TGA and DSC increasing percentage of methanol. On the other hand, a slight
o
ꢀ1
under N atmosphere at a heating rate of 10 C min . According to increase of fluorescence intensity (4 % upon 25 equiv.) was
2
the thermograms from TGA (Fig. 2), the four polymers had similar observed when FCP 1 in DMF was titrated with trifluoroacetic acid
thermal stabilities: 5 % weight loss was observed at higher than 200 (TFA). This suggested that the increase of methanol in the solution
o
C, and an apparently degradation was observed at temperature caused stronger aggregation of the macromolecules and πꢀstacking
ranging from 240 °C to 650 °C. The temperatures for 5 % weight interaction between neighbouring polymer chains, which in the end
loss (T ) of FCP 1 ꢀ 4 is 289, 204, 273 and 239 °C, respectively led to the fluorescence quenching of FCPs. On the other hand,
d
(
Table 1). While two endothermic peaks centered at ∼180 and ∼320 protonation of triazole rings by TFA afforded the polymer chains to
o
C were recorded on the DSC thermograms of the FCPs (Fig. S5). be positively charged, which kept them from aggregation and the
o
We speculated that the peak at 180 C could be attributed to the homogeneity of the solution was enhanced. This is why a slight
deterioration of small segments or terminal groups, corresponding to increase in emission intensity was observed.
the temperature for less than 5 % weight loss on TGA thermograms.
o
Since the obvious weight loss of the FCPs occurred after 300 C
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Fig. 3 Fluorescence intensity of FCP 1 (46 ꢁM, corresponding to the triazole
unit) in DMF upon the addition of CH OH and TFA (inset), λex = 323 nm.
Fig. 4 Absorption and fluorescence emission spectra of FCPs in DMF (con.
of FCP 1, 2, 3, and 4 were 46, 41, 32, and 29 ꢁM corresponding to the
triazole unit, respectively).
3
The quantum yield of emission (Ф ) of FCPs in DMF was 0.20,
f
0
.24, 0.22, and 0.28 for FCP 1, 2, 3, and 4, respectively, with
Bearing several donor sites, 1,2,3ꢀtriazoles, known since the end
reference to quinine sulfate in 0.5 mol/L H SO solution, which was
calculated according to the literature method, as shown in Eqn. S1.
th
2
4
of the 19 century, are now versatile ligands for metal
45
20
coordination. FCPs synthesized via CuAAC in the present work,
The UVꢀVis absorption and fluorescent properties of FCPs in DMF
were studied by UVꢀVis and fluorescence spectroscopies,
respectively. The results are shown in Fig. 4. UVꢀVis spectra of
FCPs in DMF disclosed the absorption maximum was 323 nm for
FCP 1, 347 nm for FCP 2, 365 nm for FCP 3, and 340 nm for FCP 4,
respectively. The analysis by fluorescence spectroscopy showed that
the emission maximum of FCP 1 in DMF appeared at 373 nm with a
shoulder at 359 nm under an excitation of 323 nm, while for FCP 2,
containing multi oxadiazole and triazole moieties, are expected to
have distinctive metal ion responsive behaviors. The selectivity of
FCPs for metal ions was examined by monitoring the change of
fluorescence intensity upon the addition of various metal ions,
2+
2+
3+
2+
2+
2+
2+
2+
2+
2+
including Hg , Cu , Cr , Cd , Co , Mn , Ni , Pb , Zn , Ba ,
2+
2+
+
+
+
Mn , Ca , K , Na , and Ag . The results shown in Fig. 5 were
obtained by using the solution of FCP 1 in DMF (46 ꢁM,
corresponding to the triazole unit). It was found that the fluorescence
intensity of the PA changed little in the presence of most metal ions
examined (10 equiv., quantity related to corresponding triazole unit).
3
, and 4, the emission maxima appeared at 498 nm, 489 nm, and 493
nm, respectively. Compared to the Stoke shift of FCP 1 (50 nm),
FCP 2, 3 and 4 gave a much larger Stoke shift, 151 nm, 124 nm, and
2+
Under the same conditions, the addition of Cu resulted in a
1
53 nm, respectively. Besides, the vibrational structure observed in
moderate decrease in fluorescence intensity. In contrast, FCP 1
the emission spectrum of FCP 1 disappeared in the emission spectra
of FCP 2 – 4.
+
responded to Ag significantly: its fluorescence intensity decreased
+
by 95 % upon the addition of 1.0 equiv. Ag , and the quantum yield
Considering the shoulder in the emission spectrum of FCP 1
disappeared with the addition of methanol (Fig 3), we surmise that
the shoulder corresponds to the isomers resulted from proton transfer
due to the intramolecular hydrogen bonds in FCP 1, which were
broken by the intermolecular hydrogen bonds formed between FCP
decreased to 0.03. We also studied the kinetics of the fluorescence
behaviour by adding 1.0 equiv. Ag into FCP 1 solution. As shown
in Fig. S6, the quenching of 90 % fluorescence intensity took about
+
2
00 seconds, indicating a fast response process.
1
and methanol. The lack of the intramolecular hydrogen bonds in
FCP 2 – 4, therefore no isomers formed in the solutions, could be the
reason why the vibrational structure observed in the emission
spectrum of FCP 1 disappeared in the emission spectra of FCP 2 –
46
4
.
The absorption maxima, emission maxima, Stoke shifts, and
quantum yields of FCP 1, FCP 2, FCP 3, and FCP 4 are summarized
in Table 2.
Polymer λabsꢀmax λemꢀmax Stoke shift (nm) Φf
FCP 1
FCP 2
FCP 3
FCP 4
323
347
365
340
373
498
489
493
50
151
124
153
0.20
0.24
0.22
0.28
Table 2 The absorption maxima, emission maxima, Stoke shifts, and
Fig. 5 The selectivity of FCP 1 (46 ꢁM, corresponding to the triazole unit)
towards Ag and other metal ions. The fluorescence measurement was taken
quantum yields of FCP 1 ꢀ 4.
+
4
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at λex = 323 nm in DMF at room temperature in the presence of metal ions (10
equiv. each, except Ag , which was 1.0 equiv.).
process, plotting relative fluorescence intensity (I /I) against the
0
+
concentration of [Q] should produce a linear line, and the slope of
51
+
the line equals to Ksv constant. However, static and dynamic
quenching very often occur simultaneously in the quenching
processes of fluorescent conjugated polymerꢀmetal ion system,
resulting in a deviation of the quenching plot from SternꢀVolmer
The high selectivity of FCP 1 towards Ag was based on the
+
following features of Ag and the polytriazole: 1) The weak Lewis
+
acidity of Ag endows a better affinity to the Lewis basic triazole
47
rings; 2) Silver ions has a strong coordination ability to electronꢀ
1
2,24,52
2
0,48
linearity.
Thus, the steadyꢀstate emission SternꢀVolmer
rich triazole moieties in the polymer backbone,
and the
+
analysis for FCP 1ꢀAg interaction in DMF solution led to a positive
deviation from the linear Stern–Volmer line (inset in Fig. 7). The
averaged K value of FCP 1 was 1.44 × 10 M , which is equally
complexation induced the aggregation of the polymer which resulted
7
in the fluorescence quenching. Thus, combination of OXDs as
5
ꢀ1
sv
fluorophores and triazole rings as metal ligands made FCP 1 an
+
high as previous reported data for other excellent fluorescence
excellent fluorescent conjugate polymer sensor selectively for Ag .
7
,15,24,30,53
conjugated polymer sensors,
and indicated that the polymer
The quenching mechanism (Fig. 6) is believed to relate to the
quenching behavior of energy transfer along the polymer
+
is highly sensitive to Ag . Calibration curve of FCP 1 was obtained
from the plot of fluorescence intensity in the presence of Ag from 1 :
+
1
2
+
7
backbones and Ag induced aggregation effect.
0
– 1 : 1.0 molar ratios (Fig. S7). Based on the curve, the lowest
+
detection limit of the polymer FCP 1 to Ag was decided to be 4.22
×
concentration of Ag encountered practically.
ꢀ
7
10 M, which is low enough for sensing subꢀmillimole
+
Fig. 6 The illustration of the quenching mechanism exemplified with FCP
+
1
in the presence of Ag in DMF.
Fig. 8 Metal specificity of FCP 1 (46 ꢁM, corresponding to the triazole) in
+
+
DMF with a mixture of Ag and other metal ions (Ag : other metals = 1 : 5 in
+
equiv.). The equivalent of Ag was kept for 0.6 equiv., λex = 323 nm.
In addition to the prerequisites of high selectivity and sensitivity
for an excellent fluorescence sensor, one more crucial requirement is
minor or no disturbance from other metal ions. Thus, competition
experiments were conducted by subjecting FCP 1 to appropriate
+
mixtures of Ag and five folds of other various metal ions.
Competition events were monitored by fluorescence spectroscopy.
As shown in Fig. 8, less than 5 % deviations were observed from
other metals interference, which proved that the polymer could show
Fig. 7 Fluorescence quenching of FCP 1 (46 ꢁM, corresponding to triazole
unit) in DMF upon the addition of Ag with increasing concentrations (0 ꢀ 1
equiv.), λex = 323 nm.
+
+
remarkable fluorescence quenching response to Ag without much
As is evident from Fig. 7, obvious fluorescence quenching was
observed upon the increasing addition of Ag . Coincidentally, green
light from FCP 1 in DMF gradually disappeared, which was easily
visible by the naked eye under UV light (λex = 365 nm).
interference from other metal ions. On the whole, the polytriazole
bridged with 2,5ꢀdiphenylꢀ1,3,4ꢀoxadiazole moieties in the main
chain displayed fantastic response to Ag with high selectivity, and
+
+
sensitivity, therefore can be used as a fluorescent chemosensor for
+
The quenching process was analyzed by SternꢀVolmer analysis
detection of Ag .
4
9,50
according to the SternꢀVolmer equation shown below:
To illustrate the potential application of FCPs, real water samples,
such as tap water and water from Wutai reservoir, were used. Three
I /I = 1 + K [Q]
0
sv
+
concentrations of Ag were prepared and analysed using a DMF
Where I and I are the fluorescence intensities observed in the
0
solution of FCP 1 (46 ꢁM, corresponding to the triazole unit). The
data were shown in Table 3 and Fig. S8. Both water samples
exhibited nice fluorescence response, the recovery yield is higher
absence and presence of metal ions, respectively. Q is the quencher
concentration. Ksv is the SternꢀVolmer quenching constant and
related to quenching efficiency. For a purely static quenching
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than 91% and the deviation is in the acceptable range (1.1 % to
8
.7 %).
Sample
T1
T2
T3
R1
R2
R3
ꢀ
5
C (10 M)
1.166 2.000 2.833 1.166 2.000 2.833
1.179 2.086 2.873 1.065 1.893 2.672
0
ꢀ
5
C (10 M)
1
Recovery (%) 101.1 104.3 101.4 91.3
94.7
94.3
ꢀ
7
SD (10 M)
Table 3 The application of FCP 1 used to detect the concentration of Ag in
real water samples. T: tap water; R: water from Wutai reservoir; C : Real
Detected
2.666 2.913 5.666 1.041 2.585 5.229
+
0
+
concentrations of Ag added to the water samples;
1
C :
+
concentrations of Ag by a DMF solution of FCP 1; SD: standard error.
Optoelectronic properties of fluorescent polymers would be
influenced and governed bulky side groups tailed to the polymer
7
backbone. In addition, the electronic nature of side group changes
the energy transfer of the polymer backbones, which is more Fig. 10 Fluorescence quenching of FCP 3 (32 ꢁM, corresponding to triazole
+
unit) in DMF upon the addition of Ag with increasing concentrations (0 ꢀ 55
equiv.), λex = 365 nm.
relevant to the optoelectronic properties of the resultant polymers.
To gain insight into the effects of side groups on the fluorescence
behaviour of the polytriazoles synthesized in this work, methoxyl, nꢀ
The selectivity and sensitivity of FCP 2, 3, and 4 were studied
hexyloxy, and 2ꢀethylhexyloxy groups tailed polymers FCP 2, FCP 3,
with similar conditions as used for FCP 1. Surprisingly, although
and FCP 4 were investigated for comparison with FCP 1. The
FCP 2, 3, and 4 showed similar selectivity (Fig. S9, S10, and S11) as
methoxyl, nꢀhexyloxy, and 2ꢀethylhexyloxy groups as not formally
part of the conjugated network were expected to alter the energy
transfer of the polymer backbone and minimize the aggregation of
the macromolecules by interrupting the stacking of the polymer in
solution, which affects the fluorescent properties of the polymers.
+
2+
FCP 1, their sensitivity for Ag and Cu decreased considerably: the
+
addition of 10 equiv. of Ag could only quench 59 % fluorescence of
FCP 2 (Fig. 9), 52 % fluorescence of FCP 3 (Fig. 10), and 43 %
fluorescence of FCP 4 (Fig. 11), respectively; the addition of 10
2+
equiv. of Cu could only quench 5 % fluorescence of FCP 2 (Fig.
S9), 15 % fluorescence of FCP 3 (Fig. S10), and 17 % fluorescence
of FCP 4 (Fig. S11), respectively. In addition, the quenching plots of
the three polymers are downward curvatures (insets in Fig. 9, 10, and
1
1), indicating that the quenching processes involved heterogeneous
population, i.e. only a fraction of triazole moities was accessible to
5
4
the quencher.
It has been proved that interchain interactions, which are less
hindered by the less bulky alkoxyl side groups, promote energy
3
3
transfer to lower energy chromophores; and the ability of
metal ions to completely quench the fluorescence polymer
resulted from the electronꢀtransfer interactions between the
5
2
conjugated polymer backbone and metal ion. Therefore, it is
logical to speculate that the decreased sensitivity of FCP 2, 3,
and 4 to the analytes and the difference of their quenching
processes from that of FCP 1 are due to the side groups in the
main chains of FCP 2 (methoxyl), FCP 3 (nꢀhexyloxy), and 4
Fig. 9 Fluorescence quenching of FCP 2 (41 ꢁM, corresponding to triazole (2ꢀethylhexyloxy), which altered the polymer aggregation
+
7
unit) in DMF upon the addition of Ag with increasing concentrations (0 ꢀ24
status and energy transfer in the backbone, and further
equiv.), λex = 347 nm.
influenced the fluorescence behavior of the resultant
polymers.
3
3
6
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ART22ICLE
DOI: 10.1039/C5RA078 A
pellets. Thermogravimetric analysis (TGA) thermograms were
recorded on an autoꢀsimultaneous measurement of thermogravimetry
and differential thermal analysis (Shimazu DTGꢀ60A). Differential
Scanning Calorimetry analysis (DSC) thermograms were recorded
on TA DSC Q2000 instrument. Molecular weight was determined by
gel permeation chromatography (GPC) with Watersꢀ515 HPLC
pump using DMF as solvent and compared to polystyrene standards.
Metal ions such as nitrate salts or perchlorate salts were used to
prepare metal ion stock solutions. 2,5ꢀbis(pꢀaminophenyl)ꢀ1,3,4ꢀ
oxadiazole (6), 1,4ꢀDi(2ꢀmethylꢀ2ꢀhydroxyꢀ3ꢀbutynyl)ꢀbenzene (7),
1
,4ꢀdi(2ꢀmethylꢀ2ꢀhydroxyꢀ3ꢀbutynyl)ꢀbenzene
methylbutꢀ3ꢀynꢀ2ꢀol)ꢀ2,5ꢀbis(hexyloxy)benzene
bis(trimethylsilylethynyl)ꢀ2,5ꢀbis(2ꢀethylhexyloxy)benzene
(8),
(11a),
1,4ꢀbis(2ꢀ
1,4ꢀ
(11b)
(details
32,33,36,37,55
were prepared according to the publish procedures
can be found in ESI).
Fig. 11 Fluorescence quenching of FCP 4 (29 ꢁM, corresponding to triazole
unit) in DMF upon the addition of Ag with increasing concentrations (0 ꢀ 17
+
Synthesis of Monomer 1 - 5 and FCPs
equiv.), λex = 340 nm.
Conclusions
In summary, fluorescent conjugated polytriazoles (FCP 1 ꢀ 4)
containing both 2,5ꢀdiphenylꢀ1,3,5ꢀoxadiazole and 1,2,3ꢀtriazole
moieties in the main chain were synthesized from aromatic diazide
(
1) and dialkynes (2 ꢀ 5) via click polymerization, respectively. In
the particularly designed polymers, OXD moieties (acting as
31
fluorophores ) and triazole rings generated via CuAAC (acting as
metal ion receptors) comprise a fluorescent system. The study on
their ionꢀresponsive properties shows that, although all four FCPs
+
have good selectivity on Ag , the integration of alkoxy side groups
as not formally part of the conjugated network to the main chain of
the polytriazoles can have significant effect on the optical properties
+
of conjugated systems, which decreased their sensitivity for Ag via
alteration of polymer aggregation status and energy transfer of the
main chain. Moreover, FCP 1 as a highly sensitive and selective
+
fluorescent chemosensor for Ag was obtained via rational design,
5
ꢀ1
where its Ksv is as high as 1.44 × 10 M and the lowest detection
ꢀ
7
limit is in ppb range (4.22 × 10 M). This study provides an efficient
click approach to the synthesis of a novel fluorescence sensor for
+
Ag detection, which could expand the application of the click
polymerization in designing fluorescent sensor based on triazole unit.
Scheme 2 The synthesis of monomer 1 – 5 and FCPs. Reagents and
Experimental Section
o
conditions: (i) N
0~5 C, 2 h; (iii) 2ꢀmethylꢀ3ꢀbutynꢀ2ꢀol, Pd(Ph
2
H
4
ꢀH
2
O, PPA, N
2
, 130 C, 13 h; (ii) NaNO
P) , CuI, NEt
iv) NaOH, toluene, reflux, 6 h; (v) TMSA, Pd(Ph P) , CuI, NEt
vi) K CO , THF, 6 h; (vii) NaOH, 1ꢀbromoalkane, NaI, reflux, 48 h; (viii)
Cl /CH COOH, reflux, 24 h; (ix) Cu(PPh Br, DMF, N , 12 h, 50
2 3
, HCl, NaN ,
o
o
Instruments and Materials
3
4
3
, 110 C, 8 h;
o
(
(
3
4
3
,70 C, 12 h;
All the reagents were purchased from commercial suppliers and
used without further purification. THF and Et N were purified by
distillation in the presence of CaH before use. Other reagents and
2
3
3
NBS, CH
2
2
3
3
)
3
2
o
2
C.
solvents were analytical grade and used as received. Flash
chromatography was performed using silica gel with a grain size of
2
,5ꢀbis(4ꢀazidophenyl)ꢀ1,3,4ꢀoxadiazole (1)
Following the literature method (Scheme 2a), a solution of
NaNO (151 mg, 2.2 mmol ) in water (2 mL) was added dropwise to
32
1
4
0ꢀ63 ꢁm (Qingdao Haiyang Co., Ltd) and hexane as eluent. H and
1
3
2
C NMR spectra were recorded on a Bruker Advance 500
instrument in CDCl , using the residual signals from CHCl ( H: δ
.26 ppm; C: δ 77.0 ppm) as internal standard. UVꢀVis spectra
1
a solution of 2,5ꢀBis(pꢀaminophenyl)ꢀ1,3,4ꢀoxadiazole (252 mg, 1
mmol) in 2 N HCl (3 mL) at 0 ꢀ 5 ºC with vigorous stirring. The
mixture was kept below 5ºC for 30 min, the diazonium salt solution
was neutralized with CaCO , and then a solution of NaN (143 mg,
3
3
13
7
were obtained from Shimadzu UVꢀ1750 and fluorescence spectra
were obtained from Shimadzu RFꢀ5301. Infrared spectra were
recorded on an FTIRꢀinstrument (BRUKER TENSOR 27) with KBr
3
3
2
.2 mmol) in water (5 mL) was added dropwise while the
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temperature was kept below 5ºC. The solid precipitate was filtered added to the residue, and the two phases were separated. The
and washed twice with water. Recrystallization from ethanol aqueous phase was extracted with hexane three times. The combined
1
provided 1 as a pale yellow solid (224 mg, 74 %). The H NMR and organic phases were dried over MgSO , and the solvent was
4
13
C NMR spectroscopic data agreed with those published for the evaporated. The residue was purified by flash chromatography on a
same compound. H NMR (CDCl , 500 MHz, δ ppm): 8.12 (d, J = very short silica gel column to get 5 as light yellow oil (187 mg, 49
1
3
1
3
1
13
8
.6 Hz, 4H), 7.17 (d, J = 8.6 Hz, 4H). C NMR (CDCl , 126 MHz, δ %). The H NMR and C NMR spectroscopic data agreed with
3
1
ppm): 163.92, 143.65, 128.58, 120.40, 119.71.
,4ꢀdiethynylbenzene (2)
Following the literature method (Scheme 2b), powdered NaOH
800 mg, 20 mmol) was added to a solution of 6 (1.2 g, 5 mmol) in
toluene (20 mL). The mixture was refluxed for 4 ꢀ 6 h until no
those published for the same compound. H NMR (CDCl , 500
3
MHz, δ ppm): 6.95 (s, 2H), 3.85 (d, J = 5.8 Hz, 4H), 3.30 (s, 2H),
1
1
3
1
.75 (m, 2H), 1.59 ꢀ 1.24 (m, 16H), 0.97 ꢀ 0.86 (m, 12H). C NMR
3
7
(
7
126 MHz, CDCl , δ ppm): 154.28, 117.58, 113.27, 82.31, 79.82,
2.15, 39.39, 30.52, 29.07, 23.91, 23.06, 14.09, 11.17.
3
(
acetone escaped from the reaction mixture, cooled to room Synthesis of FCPs via Copper-Catalyzed Click
Polymerization
temperature, and filtered. The solvent was removed under vacuum.
Purification of the residue by flash chromatography provided 2 as a
To a 5 mL flask were added 1 (0.1 mmol), 2 (0.1 mmol), and
1
13
colourless solid (500 mg, 80 %). The H NMR and C NMR Cu(PPh Br (0.005 mmol) under nitrogen. The mixture was
)
3 3
spectroscopic data agreed with those published for the same degassed by several cycles. Then dry DMF (1.5 mL) was injected
1
compound. H NMR (CDCl , 500 MHz, δ ppm): 3.17 (s, 2H), 7.42 (s into the flask to dissolve the monomers and catalyst. The reaction
3
13
4
7
H). C NMR (CDCl , 126 MHz, δ ppm): 132.04, 122.59, 83.06, mixture was degassed one more time and the reaction flask was
3
9.11.
evacuated and flushed with argon. The reaction mixture was stirred
at 50 °C for 12 h. Then, the mixture was diluted with chloroform and
added drop wise to a mixture of a hexane/chloroform (10:1 by
volume). The precipitate was allowed to stand overnight, then
collected and dried to get FCP 1 with a yield of 37 % (Scheme 2e).
By replacing monomer 2 with 3, 4, or 5, FCP 2, FCP 3, and FCP 4
1
,4ꢀdiethynylꢀ2,5ꢀbis(methoxy)benzene (3)
Following the literature method (Scheme 2c), K CO (345 mg,
.5 mmol) was added to a solution of 8 (330 mg, 1 mmol) in 5 mL
THF. The reaction mixture was stirred at room temperature for 6 h
and filtered to remove the solid. The solvent was removed under
vacuum. Water was added to the residue, and the two phases were
separated. The aqueous phase was extracted with hexane three times.
The combined organic phases were dried over MgSO , and the
solvent was evaporated. The residue was purified by flash
5
5
2
3
2
were synthesized by following the same procedure with a yield of
–1
4
3
2
2
2
9 %, 61 %, or 67 %, respectively. FTIR (KBr) ν (cm ): FCP 1
296, 3106, 2127, 2092, 1610, 1508, 1490; FCP 2 3292, 3096, 2940,
840, 2120, 2091, 1603, 1504, 1495; FCP 3 3310, 3061, 2924, 2854,
125, 2091, 1671, 1502, 1492; FCP 4 3309, 3059, 2954, 2922, 2855,
129, 2091, 1611, 1502, 1499.
4
1
chromatography to get 3 as a white solid (135 mg, 73 %). The H
1
3
NMR and C NMR spectroscopic data agreed with those published
1
for the same compound. H NMR (CDCl , 500 MHz, δ ppm): 6.97 Acknowledgements
s, 2H), 3.85 (s, 6H), 3.39(s, 2H). C NMR (126 MHz, CDCl , δ
ppm): 154.41, 116.15, 112.63, 82.83, 79.67, 54.43.
3
1
3
(
3
We thank the National Natural Science Foundation of China for
financial support (NSFC21174113). The authors also thank Mr.
Yihan Pei from Clare College, University of Cambridge for the help
1
,4ꢀdiethynylꢀ2,5ꢀbis(hexyloxy)benzene (4)
Following the literature method (Scheme 2c), K CO (345 mg, with language.
3
6
2
3
2
.5 mmol) was added to a solution of 11a (442 mg, 1 mmol) in 5 mL
THF. The reaction mixture was stirred at room temperature for 6 h Notes and references
and filtered to remove the solid. The solvent was removed under
Address, College of Science, Northwest A&F University, Yangling, Shaanxi
vacuum. Water was added to the residue, and the two phases were 712100, People’s Republic of China, Fax: +86 2987092769; Tel: +86
2
987091196; Eꢀmail: peiyx@nwafu.edu.cn
separated. The aqueous phase was extracted with hexane three times.
The combined organic phases were dried over MgSO , and the
solvent was evaporated. The residue was purified by flash
4
†
Electronic Supplementary Information (ESI) available: Experimental
procedures and additional results including fluorescence curves, NMR
data and FTIR spectra. See DOI: 10.1039/b000000x/
chromatography to get 4 as a light yellow solid (211 mg, 65 %). The
1
13
H NMR and C NMR spectroscopic data agreed with those
1
published for the same compound. H NMR (CDCl , 500 MHz, δ
3
1
2
3
H. T. Ratte, Environ. Toxicol. Chem, 1999, 18, 89ꢀ108.
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