Chemosensing of nitroaromatics with a new segmented conjugated quaterphenylene polymer

Article abstract of DOI:10.1016/j.tetlet.2011.07.055

A new approach for introducing porosity in fluorescent polymer films to increase sensing performance is presented. A novel segmented conjugated polymer composed of p-quaterphenylene segments tethered by their meta positions along the polymer main chain by 2,2-isopropylene spacers was synthesized. The bent nature of its microstructure generates amorphous morphologies that let analytes diffuse rapidly within the films. Fluorescence quenching studies with polymer films and methanol solutions of nitroaromatics showed high sensitivity, fast response and reversibility of the fluorescence quenching. Thus, half of the maximum quench (Q_50%) occurred with nitrobenzene at the micromolar range in less than 1 min in a reversible manner.

Full text of DOI:10.1016/j.tetlet.2011.07.055

Tetrahedron Letters 52 (2011) 4911–4915
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Chemosensing of nitroaromatics with a new segmented conjugated
quaterphenylene polymer
⇑
Pablo G. Del Rosso, Marcela F. Almassio, Raúl O. Garay
INQUISUR, Departamento de Química, Universidad Nacional del Sur, Alem 1253, CP 8000 Bahía Blanca, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
A new approach for introducing porosity in fluorescent polymer films to increase sensing performance is
presented. A novel segmented conjugated polymer composed of p-quaterphenylene segments tethered
by their meta positions along the polymer main chain by 2,2-isopropylene spacers was synthesized.
The bent nature of its microstructure generates amorphous morphologies that let analytes diffuse rapidly
within the films. Fluorescence quenching studies with polymer films and methanol solutions of nitroaro-
matics showed high sensitivity, fast response and reversibility of the fluorescence quenching. Thus, half
of the maximum quench (Q50%) occurred with nitrobenzene at the micromolar range in less than 1 min in
a reversible manner.
Received 1 June 2011
Accepted 12 July 2011
Available online 21 July 2011
Keywords:
Segmented conjugated polymer
Fluorescence quenching
Nitroaromatics
Ó 2011 Elsevier Ltd. All rights reserved.
Film sensor
Fluorescent polymers are functional materials with vast
applications in organic optoelectronic devices such as light-emit-
ting diodes,1 solar cells,2 electrochromic displays,3 and fluorescent
chemical sensors.4 In particular, conjugated polymers (CPs), which
ically laborious inclusion of three-dimensional ipticene moieties in
poly(phenyleneethynylene)8 main chains or [2,2,2] bicyclic ring
moieties in poly(phenylenevinylene)s9 or bulky [tris(alkyloxy-
phenyl)methyl]phenyl groups in polycarbazole10 keeps the poly-
mer backbones apart and generates the porosity of molecular
dimensions that let analytes diffuse rapidly within the films with
the subsequent enhancement of their sensing performance.
possess a conjugated p-electron system delocalized along the poly-
mer backbone, show inherent optoelectronic behavior differing
markedly from the properties of their isolated repeating units.
For example, CPs show increased sensitivity to fluorescence
quenchers compared to small chromophores due to the diffusion
of excitons throughout the individual polymer chains. This ampli-
fied response has led to an intense evaluation of CPs,4 conjugated
polyelectrolytes4,5 as well as nanomaterials containing CPs6 for
sensing applications. Thus, CPs have been assessed as chemosenso-
ry materials for the detection and quantification of nitroaromatic
compounds, a matter of importance in environmental pollution
control, industrial applications, munitions remediation sites, and
explosives detection.4,7
Most CPs have electron donor characteristics, therefore photo-
luminescence quenching by nitroaromatics, which follows the
optical excitation that produces an electron–hole pair, is attribut-
able to the electron-transfer quenching occurring from the excited
polymer to the LUMO of the electron deficient nitroaromatic mol-
ecule. In addition, these nonbonding electrostatic interactions be-
tween a polymer and an analyte should be weak because a large
association constant will otherwise result in irreversible responses
or very long reset times. However, the sensing response of CP films
does not only depend upon electronic factors but also on the way
the analyte can physically interact with the CPs. Thus, the synthet-
We have been interested in developing new regularly segmented
CPs, a subclass of CPs whose structures consist of non interacting11–
13 or weakly interacting14,15 conjugatedsegments tethered along the
main chain, usually by saturated spacers. Thus, we found that
oligo(biphenylmethylene)s, where the spacer is a single saturated
carbon atom, form amorphous glasses.16 We have further extended
our studies to a poly(terphenylenepropylene) segmented CP whose
highly rigid and contorted structure also forms very stable disor-
dered morphology with decreased segmental interactions.17 The
amorphous nature and high solubility of these materials suggested
to us that efficient packing in the solid state is frustrated thus origi-
nating void space which could eventually foster analyte exchange
with the media. Moreover, fluorescence depolarization measure-
ments in oligo(biphenylmethylene)s films indicated that exciton
migration by resonance energy transfer (RET) occurs in these disor-
dered assemblies.16 Therefore, it is conceivable that in the solid state
the array of short conjugated segments could retain, at least in part,
the amplified quenching effect of CPs. In this contribution we ex-
plore the quenching performance of quaterphenylene-based seg-
mented CP films against nitroaromatics. We present the rather
simple syntheses of a polymer composed of p-quaterphenylene seg-
ments tethered by their meta positions along the polymer main
chain by 2,2-isopropylene spacers and its corresponding model
compound (Scheme 1) along with their thermal and photophysical
⇑
Corresponding author.
E-mail address: rgaray@criba.edu.ar (R.O. Garay).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.07.055

4912
P. G. Del Rosso et al. / Tetrahedron Letters 52 (2011) 4911–4915
O
Br
B
O
(a), (b)
O
1
Br
B
O
H3C CH3
t-Bu
OH
(c),(d)
Br
OCH3
HO
OH
(c),(d)
CH3
H3C
Br
Br
t-Bu
OCH3
H3CO
2
3
(e)
(f)
MeO
1
1
MeO
t-Bu
n
H3C CH3
t-Bu
5
4
OMe
OMe
Scheme 1. Reagents and conditions: (a) BuLi/THF then B(OPri)3 then H2SO4/H2O, ꢁ78 °C; (b) 1,3-propanodiol/toluene, reflux; (c) K2CO3/acetone then CH3I, 35 °C; (d) Br2/
CH2Cl2, 4 °C; (e) Pd2(dba)3/P(o-tolyl)3/CO3Na2 (aq)/THF, 80 °C (f) Pd(PPh4)3/CO3Na2 (aq)/THF, 80 °C.
properties. We then perform fluorescence quenching studies with
amorphous films of the polymer and methanol solutions of some
typical nitroaromatics.
The absorption and emission spectra of 4 and 5 are shown in
Figure 1 for both samples in dilute solution and neat films. The cor-
responding photophysical data are reported in Table 1. Altogether,
the methoxy and alkyl substituents cause a ꢀ14 nm red-shift in
their absorption and emission peaks compared to those of p-quat-
erphenyl15 and the photophysical properties of the model com-
pound are mirrored in the polymer. Little distinction can be
made in the optical and fluorescence spectra in dilute chloroform
solutions (Fig. 1a) between 4 and 5, indicating that the isopropyl-
ene spacer indeed isolates the conjugated segments electronically
along the backbone. In addition, both 4 and 5 have comparable mo-
lar (per repeating unit) absorption coefficients and fluorescence
quantum yields (see Table 1), whose considerable magnitudes
indicate that they retain the excellent photophysical properties
of the p-quaterphenyl chromophore. Thus, from the observed sim-
ilarity in the optical and photophysical properties a regular micro-
The model compound 4 and polymer 5 were synthesized from
readily available starting materials such as bisphenol A, their syn-
thetic routes are illustrated in Scheme 1. The treatment of dibro-
mobiphenyl with butyl lithium followed by triisopropylborate
addition and acidification at a low temperature gave diboronic acid
which was transformed into the diboronate 1 in an overall yield of
40%. The synthesis of 2 by dimethylation followed by dibromina-
tion was accomplished in 81% yield over the two steps. We have
previously reported the preparation of 3 in 68% yield by the same
synthetic procedure.17 The bromides 2 and 3 were then used in the
preparation of the model compound 418 and polymer 519 via palla-
dium-catalyzed Suzuki condensations with 1. Gel permeation
chromatography (GPC, PS standards, THF) showed that 5 had a
monomodal distribution (Mw = 7500 Da, PDI = 1.70) after repre-
cipitation from methanol. As expected, polymer 5 was readily sol-
uble in common organic solvents such as acetonitrile, THF and
DMF despite the absence of solubilizing chains. In particular, this
polymer is highly soluble in CHCl3 (50 wt %). Model compound 4
gives, after melting in a broad range around 150 °C, a kinetically
stable amorphous phase (Tg = 51 °C) while polymer 5 is amor-
phous in nature; its DSC traces registered at a temperature range
between 50 and 300 °C showed only a distinct glass transition at
161 °C and no melting transitions were found upon heating beyond
the glass transition temperature. Besides, no birefringence was de-
tected for 5 by polarized optical microscopy POM observations car-
ried out in the same temperature range. Films cast on quartz plates
and dried under vacuum gave smooth films suitable for optical
measurements.
structure of
5
constituted of well-defined noninteracting
chromophores can be inferred. Although there are no distinctive
differences between the absorption spectra of neat films of 4 and
5, the corresponding fluorescence spectra show discrepancies at
the low energy edge (Fig. 1b). The spectrum of 4 presents a broader
full width at half-maximum, fwhm, and has a noticeable red tail
that is absent in the spectrum of 5. Thus, it is likely that spectral
broadening is a result of interchromophoric contacts which are
not completely impeded by tert-butyl disubstitution in the elon-
gated quaterphenylene model compound. Apparently, the geminal
substitution in the polymer backbone, which forces the chromoph-
ores into an angular disposition producing a contorted polymer
microstructure, succeeds in removing segmental interactions so
that the solution and the film fluorescence spectra of 5 are quite
similar. Monoexponential decay curves of the luminescence were

P. G. Del Rosso et al. / Tetrahedron Letters 52 (2011) 4911–4915
4913
thermal annealing (6 h, N2, 170 °C) the same fluorescence spectra
and anisotropy values (0.271 vs 0.268), corroborating that the
amorphous morphology of 5 is quite stable.
a
4
5
The steady-state fluorescence response of polymer 5 films to
increasing concentrations of various analytes was observed for
methanol solutions of nitrobenzene (NB), 4-nitroaniline (NA),
1,3-dinitrobenzene (DNB), 2,4-dinitrofluorobenzene (DNFB), trini-
trotoluene (TNT), picric acid (PA) and nitropropane (NP). Contam-
ination of the analysis systems by polymer leaching is always a
concern for sensing configurations consisting of films immersed
in a liquid medium.20 In our case, control experiments confirmed
the structural stability of polymer 5 films; no fluorescence was de-
tected in methanol that soaked films for 12 h and the fluorescence
intensity of pristine films did not decrease after such immersion
period. We observe instead that wet films increase 25–30% of the
fluorescence intensity reaching a maximum steady value in few
minutes. Figure 2a shows the fluorescence intensity of 5 as a func-
tion of added NB. The fluorescence intensity was recorded first in
neat methanol, Io, and then after the quencher solution was added,
300
350
400
450
500
550
Wavelength (nm)
I. The film response was rapid, that is, steady fluorescence intensity
was always reached in ca. 40–50 s after the addition of an aliquot
of the quencher solution, indicating that fast diffusion of the ana-
lyte within the film must occur in order to readily attain its film/
b
4
5
a
2
R = 0.998
1
0
40µ
80µ
[Q]1/M20µ
160µ
300
350
400
450
500
550
Wavelength (nm)
Figure 1. (a) Normalized absorption and emission spectra for 4 (dashed curves) and
5 (solid curves) measured as (a) dilute solutions in CHCl3 or (b) neat films on quartz.
350
400
450
500
Table 1
Wavelength (nm)
Photophysical properties of model compound 4 and polymer 5 measured from CHCl3
solutions and thin neat films
SSd UF
sff
‹r›g
e
Media
Absorption
Fluorescence
b
a
b
c
kmax(emax)
kmax
fwhm
300
4
5
CHCl3
Film
CHCl3
Film
306 (29500)
310
309 (28700)
306
366, 378
360, 384
379
48
86
50
65
72
74
70
81
0.91
0.72
0.13
0.25
0.192
0.271
387
200
100
0
a
Absorption maxima measured in dilute CHCl3 solutions and on films and
extinction coefficients (based upon the molar repeating unit for 5) in Mꢁ1cmꢁ1
.
b
Emission maxima measured in dilute CHCl3 solutions and on films
(kexc = 310 nm). Bold data indicate the major peaks.
c
Full width at half-maximum of the fluorescence bands (in nm).
Stokes shifts in dilute CHCl3 solutions and on films (kmax,em - kmax,abs; in nm).
Fluorescence quantum yields measured in CHCl3 (kexc = 310 nm).
d
e
f
Observed lifetime at kmax em
,
= 400 nm on thin films (in ns).
Average anisotropy measured in thin films (kexc = 335 nm) in an emission range
of 60 nm around the kmax,em
g
.
1
2
3
4
5
6
7
8
9
10
Quenching Cycles
Figure 2. (a). Fluorescence spectra change of 5 film as a function of an added NB in
MeOH; [NB] = 6.0 10ꢁ6–3.8 10ꢁ4
(top to bottom). The corresponding Stern–
Volmer plot is shown in the inset including the correlation coefficient for the linear
fit. (b) Ten continuous cycles of quenching-recovery test of 5 film. The quenching
was measured after exposing the film to [NB] = 4.3 10ꢁ3 M for less than 1 min.
observed in the films of 4 and 5 which showed lifetimes of the sin-
gle-chromophore excited states in the same range. The steady-
state anisotropies which indicate energy migration were likewise
comparable. Finally, a film sample of 5 showed before and after
M

4914
P. G. Del Rosso et al. / Tetrahedron Letters 52 (2011) 4911–4915
Table 2
solution distribution ratio. The plot of the Stern–Volmer (S–V)
equation, (Io/I) ꢁ 1 = KSV [Q], for fluorescence quenching shown in
Figure 2a (inset) indicates that 5 has a considerable sensitivity to-
wards NB. Thus, half of the maximum quench, Q50%, that is, the
quencher concentration needed to reach (Io/I) ꢁ 1 = 1, of 5 with
Comparison of quenching efficiencies
Quencher (Q)
KSV, Mꢁ1
.
Q50%, l
Ma
NA
PA
NB
DNFB
DNB
64100
32000
20100
4000
18
37
64
240
860
NB occurred at the micromolar range ([NB] = 64
lM) while almost
complete quenching (Q10%) was achieved with a 300
lM solution. A
610
linear relationship was observed between 10% and 70% of fluores-
cence quenching with a S–V constant (KSV) of 2.01 104 Mꢁ1; a value
higher than those observed for conjugated polymers in solution,21,7
where diffusion of the analyte is not hindered. However, at higher
quencher concentrations a nonlinear S–V relationship with upward
curvature occurred. Similar upward deviations from linearity have
been observed in other solid/solution sensing configurations.22 The
reversibility of the sensing response was also examined. Figure 2b
shows ten continuous cycles of fluorescence quenching-recovery
using NB as an example nitro aromatic compound which indicate
that quenching is intrinsically reversible. In these experiments, a
quencher solution was employed to quench the film fluorescence,
next the quencher solution was withdrawn, the film was washed
with methanol and its fluorescence was recorded, then measure-
ments in the presence and absence of the analyte were repeated
several times (each cycle took nearly 6 min). Thus, fast fluores-
cence response equilibration and reversible quenching evidenced
that the amorphous morphology of the segmented conjugated
polymer 5 has porosity of molecular dimensions and therefore ana-
lytes can rapidly diffuse in and out of the films.
Figure 3 shows representative S–V plots for 5 with the different
analytes tested. TNT was not included because no fluorescence
quenching was observed for this analyte. Overall, the data appear
linear up to 50–70% of the fluorescence quench, and then upward
curving of the S–V plots take place for all nitroaromatics employed
in this study. The data from the linear range could be fitted with
correlation coefficients above 0.994 for all fittings and the values
of KSV determined from the slopes are listed in Table 2.
It can be observed that efficiency in fluorescence quenching fol-
lows the order of NA > PA > NB ꢂ DNFB > DNB o TNT. Although
the trend in S–V constants usually reflects the electron acceptor
ability of the quencher, for example, TNT > DNT > NT,8 the films
of polymer 5 exhibit on the whole a quenching efficiency which
seem to reflect a combination of the steric and electronic charac-
teristics of the analyte as well as the occurrence of noncovalent
interactions between polymer and analyte. We noted that larger
a
[Q] for (Io/I) ꢁ 1 = 1.
responses were observed with NA and PA that can interact by
hydrogen bonding with the methoxy groups of the polymer. An
alternate explanation for the relatively large response of NA and
PA is that they are nonfluorescent molecules which absorb near
the polymer emission range and act as acceptors for the RET from
the excited polymer, thus opening another channel for fluores-
cence quenching. In addition, it would seem that the access of lar-
ger compounds into the film is difficult or obstructed as in the case
of TNT. These results highlight the difficulties already encountered
in establishing definitive trends for quenching responses in con-
densed state.8,20,23 In summary, a new approach for introducing
porosity in fluorescent polymer films to increase sensing perfor-
mance was presented. A quaterphenylene-based segmented conju-
gated polymer was synthesized using an unsophisticated synthetic
route. The chromophores are tethered by their meta positions
along the polymer main chain by 2,2-isopropylene spacers which
force them into an angular arrangement and produce a contorted
polymer microstructure. We attribute the high sensitivity and fast
reversible response of the fluorescence quenching by nitroaromat-
ics to the amorphous morphology of the polymer as well as to the
exciton migration by RET that occurs in these disordered assem-
blies of the chromophores.
Acknowledgments
Financial support from SGCyT-UNS and CONICET is acknowl-
edged. P.G.D.R. and M.F.A. are members of the research staff of
CIC-PBA. R.O.G. is member of the research staff of CONICET.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.07.055.
References and notes
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PA
NB
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250µ
500µ
750µ
13. Song, C.; Swager, T. M. Macromolecules 2009, 42, 1472.
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[Q] / M
Figure 3. Stern–Volmer plots of 5 in response to nitro compounds.

P. G. Del Rosso et al. / Tetrahedron Letters 52 (2011) 4911–4915
16. Del Rosso, P. G.; Almassio, M. F.; Antollini, S. S.; Garay, R. O. Opt. Mater. 2007, 0.38 mmol), diboronate ester
4915
1
(0.540 g, 1.67 mmol), dibromo
3
(0.70 g,
30, 478.
1.67 mmol), Na2CO3 (2.82 g, 26.6 mmol) and and the mixture was kept under
Ar atmosphere. Dry THF (13.3 mL) and water (13.3 mL) were added via a
syringe. The mixture was heated at 80 °C for 7 days. The reaction mixture was
then poured into methanol (30 mL). The precipitate was dissolved in CHCl3
(5 mL) and poured into methanol (30 mL). The procedure was repeated twice.
17. Del Rosso, P. G.; Almassio, M. F.; Aramendia, P.; Antollini, S. S.; Garay, R. O. Eur.
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18. Synthesis of 2,2000-dimethoxy-50,5000-diterbutyl-4,10:40,100:400,1000-quaterphenyl
(4). A Schlenk tube was charged with diboronate ester 1 (0.20 g, 0.62 mmol),
bromide 2 (0.35 g, 1.36 mmol), Pd2(dba)3 (2 mg, 1.2
l
mol), P(o-tolil)3 (2 mg,
The precipitate was collected and dried under vacuum to yield 5 as a yellowish
´
7
l
mol), Na2CO3 (0.89 g, 8.25 mmol) and the mixture was kept under Ar
powder (0.63 g, 92%). 1H NMR (CDCl3): = 7.63 (m, 4H), 7.37 (d, 1H,
atmosphere. Dry THF (3.3 mL) and water (3.3 mL) were added via a syringe.
The mixture was heated at 80 °C for 48 h, then poured into MeOH (15 mL),
extracted with CHCl3 (3 ꢃ 15 mL) and dried with SO4Na2.The solvent was
removed and the resulting solid was recrystallized from ethanol.(0.11 g, 38%).
Mp = 148–151 °C. 1H NMR – (CDCl3): = 7.69 (d, 2H, Jo = 8.58 Hz), 7.63 (d, 2H,
Jo = 8.58 Hz), 7.40 (d, 1H, Jm = 2.67 Hz), 7.34 (dd, 2H, Jo = 8.58 Hz, Jm = 2.67 Hz),
6.94 (d, 1H, Jo = 8.58 Hz), 3.83 (s, 3H), 1.36 (s, 9H). 13C NMR: 31.6, 34.2, 55.7,
110.9, 125.2, 126.8, 128.1, 129.7, 129.9, 138.0, 139.6, 143.6, 154.4. C34H38O2
(478.66): C, 85.31; H, 8.00. Found: C, 85.08; H, 7.93.
Jm = 2.48 Hz), 7.26 (dd, 1H, Jo = 8.70 Hz, Jm = 2.48 Hz), 6.91 (d, 1H,
Jo = 8.70 Hz), 3.84 (s, 3H), 1.72 (s, 3H). 13C NMR (CDCl3): = 154.5, 143.2,
139.5, 137.8, 132.0, 130.0, 129.4, 127.3, 110.8, 55.6, 41.9, 31.2. Anal. Calcd for
(C29H26O2)n (406.5)n: C, 85.68; H, 6.45. Found: C, 85.19; H, 6.14.
20. He, G.; Zhang, G.; Lu, F.; Fang, Y. Chem. Mater. 2009, 21, 1494.
21. Sohn, H.; Sailor, M. J.; Magde, D.; Trogler, W. C. J. Am. Chem. Soc. 2003, 125,
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22. (a) Qin, A.; Lam, J. W. Y.; Tang, L.; Jim, C. K. W.; Zhao, H.; Sun, J.; Tang, B. Z
Macromolecules 2009, 42, 1421; (b) Feng, J.; Li, Y.; Yang, M. Sens. Actuators B:
Chem 2010, 145, 438.
19. Synthesis
of
Poly(2,2000-dimethoxy-4,10:40,100:400,1000-quaterphenyl-5,5000
Schlenk tube was charged with Pd(PPh3)4 (0.44 g,
-
ylene)propylene (5).
A
23. Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864.