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next the quencher solution was withdrawn, the film was washed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
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PA
NB
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[Q] / M
Figure 3. Stern–Volmer plots of 5 in response to nitro compounds.