Tubular conjugated polymer for chemosensory applications

Article abstract of DOI:10.1021/ja301918f

In this paper, we present the concept of a soluble tubular conjugated polymer (TCP). We report on a fluorescent 5,5′-Bicalixarene-based polymer where the calixarene units are seamlessly incorporated in the conjugated polymeric chain that can respond to a small molecule complexation inside the hydrophobic cavity. In particular, our system demonstrated a reversible rapid fluorescence quenching upon interaction of gaseous nitric oxide with the calixarene moiety.

Full text of DOI:10.1021/ja301918f

Communication
pubs.acs.org/JACS
Tubular Conjugated Polymer for Chemosensory Applications
Anat Molad, Israel Goldberg, and Arkadi Vigalok*
School of Chemistry, The Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel
S
* Supporting Information
polymeric backbone. Herein, we present the first polymer of
this family, utilizing the intrinsically tubular calixarene scaffolds
as building blocks for new conjugated materials.
ABSTRACT: In this paper, we present the concept of a
soluble tubular conjugated polymer (TCP). We report on
a fluorescent 5,5′-Bicalixarene-based polymer where the
calixarene units are seamlessly incorporated in the
conjugated polymeric chain that can respond to a small
molecule complexation inside the hydrophobic cavity. In
particular, our system demonstrated a reversible rapid
fluorescence quenching upon interaction of gaseous nitric
oxide with the calixarene moiety.
As the building block, we chose the 5,5′-Bicalixarene scaffold
that was first reported by Neri et al.13 This and similar
bicalixarene molecules are known to complex electron-deficient
organic compounds as well as fullerenes.14−16 Thus, 5,5′-
Bicalixarene 2 was prepared via the oxidation with FeCl3
(Scheme 1). The removal of the benzylic protecting groups
and the selective replacement of one of the free OH groups
with trifluoromethane sulfonate (triflate) at both calixarene
moieties provided monomer 3, which was tested in the direct
Sonogashira-type cross-coupling polymerization with 1,4-
diethynylbenzene. Unfortunately, no conjugated polymer
incorporating the 5,5′-Bicalixarene scaffold was obtained
under our improved conditions17 of the original cross-coupling
method,18 as mainly the homocoupling of the diethynylbenzene
spacer was observed. As the cross-coupling of the sterically
encumbered electron-rich triflate proved inefficient in the
condensation-polymerization, we chose to circumvent the
problem by replacing the triflate with the Et3Si−C≡C−
group.19 Under our standard cross-coupling conditions,20 the
desired 4 was isolated in a 48% yield (Scheme 1). The
replacement of the phenolic oxygens with alkyne did not
adversely affect the complexation properties of new bicalixar-
enes. For example, upon the addition of 1 equiv of N-
methylpyridinium triflate to 4,13,15 complete encapsulation of
igid cone-shaped calix[4]arene (calixarene) molecules
R
have long been recognized as valuable receptors for
various organic and inorganic analytes.1−3 Their lower,
phenolic rim can be functionalized to selectively coordinate
main-group and transition metal ions, while the hydrophobic
cavity of the calixarene scaffold can accommodate various
organic molecules and gases.4−7 This versatility of the
calixarene molecules as receptors was studied extensively in
the design of chemosensory materials.8 A particularly powerful
method to enhance the sensitivity of the calixarene-based
chemosensors is to attach them to a conjugated polymeric
network based on a “molecular wire” approach. In this method,
higher sensitivity is obtained due to the collective response of
the polymer bound receptors, involving both intra- and
intermolecular charge delocalization.9 There are only a few
reports of the incorporation of calixarene-based receptors into
“molecular wire” networks, mainly as pendant groups (Figure
1a).10,11 The direct attachment of the calixarene unit (at the
1
the cationic guest was observed in the H NMR spectrum
(Figure S1). The spectrum was in agreement with the organic
cation complexed within both calixarene cavities (closed
conformer). The removal of the Et3Si groups in 4 gave the
free acetylene 5 which, followed by the attachment of the
solubilizing hexanoyl groups at the free phenolic positions,
furnished diacetylene 6 in the overall 60% yield.
As 6 was prone to undergo oxidative coupling in the
presence of Pd(II) and Cu(I) catalysts, we decided to use this
Glaser-type reaction to prepare polymeric 7, which was
obtained in a 70% yield (Scheme 2). The extended conjugation
had a profound effect on the UV/vis and fluorescence
properties of the calixarene compounds. While compounds
4−6 showed only weak fluorescence, 7 was highly fluorescent
with about 30 nm Stokes shift (Figure 2).
Figure 1. Representative conjugated polymers with attached calixarene
scaffolds.10,12
.
Polymer 7 has an intrinsically hollow tubular structure
containing only carbon and hydrogen atoms at its core. With
the pinched cone opening at the upper rim of ca. 1 nm, it can
be viewed as a conjugated “hydrocarbon nanotube” that can be
filled by small molecule guests. Both, open (shown in the
upper rim) to a conjugated polymer has also been reported by
Swager and co-workers and has resulted in rigid zigzag
structures (Figure 1b).12 However, short conjugated fragments
combined with the nonlinear geometry gave rather moderate
sensory responses with selected analytes. To enhance the
sensitivity, we decided to prepare conjugated materials where
calixarene scaffolds would be the part of uninterrupted linear
Received: February 27, 2012
Published: April 23, 2012
© 2012 American Chemical Society
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Scheme 1
Scheme 2
Schemes) and closed (capsule-type) conformations have been
established for 5,5′-Bicalixarene molecules,13,16 with the latter
being proposed to significantly enhance the complexation
properties of the calixarene scaffold. Thus, we were eager to
explore the fluorescence response of 7 to the presence of
important analytes. One such analyte is nitric oxide (NO), a
small gaseous molecule with an enormous importance in
human physiology.21,22 Many fluorescent probes for NO
sensing have been reported, most of them relying either on
the formation of an “NO+” equivalent in the presence of an
oxidant or on the complexation to a transition metal
center.23−25 NO entrapment within the nonfluorescent
calixarene systems has also been documented, the reaction
being based on the oxidation of the calixarene moiety.26 The
removal of the guest molecule could only be achieved by
adding a reducing reagent allowing for the release of free NO.27
We were delighted to find that the fluorescence of 7 rapidly
decreases in the presence of NO. Although no color change or
effect on the NMR signals set was observed upon brief
exposure of a chloroform solution of 7 to NO vapor, the
fluorescence was quenched. Importantly, the NO could easily
be removed in a vacuum, with a few seconds of low vacuum
restoring the fluorescence of 7 (Figure 3). The signal could not
Figure 2. Absorption and emission (dashed line) spectra of polymer 7
(a, CHCl3, excitation at 400 nm) and Bicalixarene 4 (b, CHCl3,
excitation at 280 nm).
be recovered in full, likely as the result of the irreversible
reaction with small amounts of higher oxides of nitrogen. The
exposure of 7 to NO2 or NO+ results in the irreversible
fluorescence loss. Importantly, the fluorescence was not
affected by the presence of the atmospheric oxygen or by the
addition of carbon monoxide.
To verify that the calixarene moiety is indeed responsible for
the interactions with NO, we prepared 8 and its linear analogue
9 which bears the same chromophore (Figure 4). The
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Communication
for technical assistance, and Prof. Doron Shabat for the access
to the fluorimeter.
REFERENCES
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(1) Cadogan, F.; Nolan, K.; Diamond, D. Sensor Applications, in
Calixarenes 2001; Asfari, Z., Bohmer, V., Harrowfield, J., Vicens, J.,
Eds.; Kluwer: Dordrecht, 2001; pp 627−641.
(2) Kim, S. K.; Sessler, J. L. Chem. Soc. Rev. 2010, 39, 3784−3809.
(3) Joseph, R.; Rao, C. P. Chem. Rev. 2011, 111, 4658−4702.
(4) Floriani, C.; Floriani-Moro, R. Adv. Organomet. Chem. 2001, 47,
167−233.
(5) Kotzen, N.; Vigalok, A. J. Supramol. Chem. 2008, 20, 129−139.
(6) Rebek, J., Jr. Chem. Commun. 2000, 637−643.
(7) Rudkevich, D. M. Eur. J. Org. Chem. 2007, 20, 3255−3270.
(8) (a) Sansone, F.; Baldini, L.; Casnati, A.; Ungaro, R. New J. Chem.
2010, 34, 2715−2728. (b) Kim, J. S.; Quang, D. T. Chem. Rev. 2007,
107, 3780−3799.
Figure 3. Fluorescence change of polymer 7 before the addition of
NO (a), after the addition of NO (b) and after 30 s at ca. 200 mmHg
(c).
(9) (a) Thomas, S. W., III; Joly, G. D.; Swager, T. M. Chem. Rev.
2007, 107, 1339−1386. (b) Swager, T. M. Acc. Chem. Res. 1998, 31,
201−207.
(10) Marsella, M. J.; Swager, T. M. J. Am. Chem. Soc. 1993, 115,
12214−12215.
(11) (a) Wosnick, J. H.; Swager, T. M. Chem. Commun. 2004, 2744−
2745. (b) Costa, A. I.; Ferreira, L. F. V.; Prata, J. V. J. Polym. Sci., Part
A: Polym. Chem. 2008, 46, 6477−6488.
(12) Yu, H; Xu, B.; Swager, T. M. J. Am. Chem. Soc. 2003, 125,
1142−1143.
(13) (a) Neri, P.; Bottino, A.; Cunsolo, F.; Piattelli, M.; Gavuzzo, E.
Angew. Chem., Int. Ed. 1998, 37, 166−169. (b) Consolia, G. M. L.;
Cunsoloa, F.; Geracia, C.; Neri, P. Lett. Org. Chem. 2005, 2, 252−257.
(14) Bottino, A.; Cunsolo, F.; Piattelli, M.; Gavuzzo, E.; Neri, P.
Tetrahedron Lett. 2000, 41, 10065−10069.
Figure 4. Fluorescent 5,5′-Bicalixarene 8 and linear chromophores, 9
and 10. Only 8 undergoes fluorescence quenching in the presence of
NO.
(15) Araki, K.; Hisaichi, K.; Kanai, T.; Shinkai, S. Chem. Lett. 1995,
569−570.
compounds show similar UV spectra and are fluorescent in
their CHCl3 solutions. While the fluorescence of 8 was fully
quenched upon the exposure to NO, no effect on the
fluorescence of 9 was observed, indicating the importance of
the calixarene fragment for NO complexation. Fluorescent 10
(Figure 4) that has the diacetylene motif in its structure (in
analogy to 7) also showed no response in the presence of NO
gas. Thus, the observed effect is unique as it shows a reversible
supramolecular entrapment of gaseous NO by a soluble well-
defined conjugated polymer.
In conclusion, we prepared the first tubular conjugated
fluorescent polymer with fused calixarene cavities. The presence
of the calixarenes allows for the detection of small molecules,
such as NO, based on the reversible complexation. The detailed
analysis of the NO fluorescence quenching in 7 and other
applications of these new materials are currently under
investigation in our laboratories.
(16) Iglesias-Sanchez, J. C.; Fragoso, A.; de Mendoza, J.; Prados, P.
Org. Lett. 2006, 8, 2571−2574.
(17) Bukhaltsev, E.; Goldberg, I.; Vigalok, A. Organometallics 2007,
26, 4015−4020.
(18) Al-Saraierh, H.; Miller, D. O.; Georghiou, P. E. J. Org. Chem.
2005, 70, 8273−8280.
(19) For detailed description of the synthesis optimizations,
including model compounds, see Supporting Information.
(20) Tzadka, E.; Goldberg, I.; Vigalok, A. Chem. Commun. 2009,
2041−2043.
(21) Nitric Oxide Biology and Pathbiology, 2nd ed.; Ignarro, L. J., Ed.;
Academic Press: San Diego, CA, 2009.
(22) Mariotto, S.; Menegazzi, M.; Suzuki, H. Curr. Pharm. Design
2004, 10, 1627−1645.
(23) (a) Chen, X.; Tian, X.; Shin, I.; Yoon, J. Chem. Soc. Rev. 2011,
40, 4783−4804. (b) McQuade, L. E.; Lippard, S. J. Curr. Opin. Chem.
Biol. 2010, 14, 43−49.
(24) (a) Lim, M. H.; Wong, B. A.; Pitcock, W. H.; Mokshagundam,
D.; Baik, M. H.; Lippard, S. J. J. Am. Chem. Soc. 2006, 128, 14364−
14373. (b) Shioya, T.; Swager, T. M. Chem. Commun. 2002, 1364−
1365.
(25) Yang, Y.; Seidlits, S. K.; Adams, M. M.; Lynch, V. M.; Schmidt,
C. E.; Anslyn, E. V.; Shear, J. B. J. Am. Chem. Soc. 2010, 132, 13114−
13116.
ASSOCIATED CONTENT
* Supporting Information
Complete experimental details for all new compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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S
(26) (a) Rathore, R.; Lindeman, S. V.; Rao, K. S. S. P.; Sun, D.;
Kochi, J. K. Angew. Chem., Int. Ed. 2000, 39, 2123−2127. (b) Rathore,
R.; Abdelwahed, S. H.; Guzei, I. A. J. Am. Chem. Soc. 2004, 126,
13582−13583. (c) Zyryanov, G. V.; Kang, Y.; Rudkevich, D. M. J. Am.
Chem. Soc. 2003, 125, 2997−3007.
(27) Wanigasekara, E.; Gaeta, C.; Neri, P.; Rudkevich, D. M. Org.
Lett. 2008, 10, 1263−1266.
AUTHOR INFORMATION
Corresponding Author
avigal@post.tau.ac.il
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the Israel Science Foundation for supporting this
research. We also thank Naama Karton and Konstantin Press
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