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Macromolecules, Vol. 37, No. 13, 2004
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absorption spectroscopy (vide supra). In contrast, the
emission of 3d upon addition of Ca2+ in acetonitrile is
partly quenched as expected. In general, the emission
experiments independently confirm the results obtained
by UV/vis absorption, demonstrating the importance of
receptor length for effective binding of the desired cation
as well as the importance of binding affinities of cations
in general.
(6) Kasha, M. Radiat. Res. 1963, 20, 55-70. Chromophores that
can electronically interact with each other forming “pseudo-
excimers” exist in two possible states: When isolated from
each other they are referred to as unimers while aggregated
dimeric complexes displaying pseudo-exciton interactions are
labeled H-dimers. In contrast, the term monomer is used for
defining single molecule compounds, i.e., nonpolymeric chemi-
cal structures.
(7) (a) Treibs, A.; J acob, K. Angew. Chem., Int. Ed. Engl. 1965,
4, 694-695. For a review, see: (b) Schmidt, A. H. Synthesis
1980, 961-994.
Con clu sion
(8) For reviews, consult: (a) Das, S.; Thomas, K. G.; George, M.
V. Mol. Supramol. Photochem. 1997, 1, 467-517. (b) Law,
K.-Y. Mol. Supramol. Photochem. 1997, 1, 519-584. (c) Law,
K.-Y. Chem. Rev. 1993, 93, 449-486.
A new approach to the design of sensory polymers has
been demonstrated. Thereby, a set of polysquaraines
having alternating receptor and squaraine chromophore
units was synthesized. The novel modular and versatile
two-step one-pot procedure presented herein allows for
rapid and facile access to this new class of polymers.
These nonconjugated polysquaraines undergo signifi-
cant conformational changes that are controlled by
physical and chemical stimuli such as temperature,
solvent polarity, and addition of various cations. The
binding of analytes induces changes in the polymer
backbone conformation, leading to either preferential
folding to, or unfolding from, chromophore H-dimers.
Hence, the binding event is translated into a shift in
the unimer to H-dimer equilibrium of the squaraine
chromophores that can be conveniently visualized by
UV/vis and fluorescence spectroscopy and thus success-
fully be exploited for cation sensing. In the event of
analyte recognition, our polysquaraines respond with
an unusual dual signal pattern allowing for internal
self-calibration and display acceptable sensitivity and
specificity, rendering them suitable for future sensory
applications, in particular if receptors of higher selectiv-
ity and binding affinity would be incorporated into the
polymers.
(9) Terpetschnig, E.; Szmacinski, H.; Ozinskas, A.; Lakowicz, J .
R. Anal. Biochem. 1994, 217, 197-204.
(10) Law, K.-Y. J . Phys. Chem. 1988, 92, 4226-4231.
(11) Brightness is defined as product of molar absorption coef-
ficient ꢀ and fluorescense quantum yield Φf. See: Minta, A.;
Kao, J . P. Y.; Tsien, R. Y. J . Biol. Chem. 1989, 264, 8171-
8178.
(12) “Exciton interaction” is a common term used in squaraine
literature for describing the effect of electronic interactions
between squaraine pairs, although it is strictly speaking a
“pseudo-excimeric” interaction.
(13) For an example, see: Liang, K.; Farahat, M. S.; Perlstein,
J .; Law, K.-Y.; Whitten, D. G. J . Am. Chem. Soc. 1997, 119,
830-831.
(14) (a) Das, S.; Thomas, K. G.; Thomas, K. J .; Kamat, P. V.;
George, M. V. J . Phys. Chem. 1994, 98, 9291-9296. (b) Oguz,
U.; Akkaya, E. U. Tetrahedron Lett. 1997, 38, 4509-4512.
(c) Dilek, G.; Akkaya, E. U. Tetrahedron Lett. 2000, 41, 3721-
3724. During the course of our investigations, a calcium
sensor based on a bis(squaraine) molecule containing no
ortho-hydroxyl groups that exhibits enhanced dimer aggrega-
tion upon binding has been reported: (d) Ajayaghosh, A.;
Arunkumar, E.; Daub, J . Angew. Chem., Int. Ed. 2002, 41,
1766-1769. (e) Chenthamarakshan, C. R.; Ajayaghosh, A.
Tetrahedron Lett. 1998, 39, 1795-1798.
(15) For a comprehensive discussion of the squaraine condensation
mechanism, consult: Law, K.-Y.; Bailey, F. C. Can. J . Chem.
1986, 64, 2267-2273.
(16) Dirk, C. W.; Herndon, W. C.; Cervantes-Lee, F.; Selnau, H.;
Martinez, S.; Kalamegham, P.; Tan, A.; Campos, G.; Velez,
M.; Zyss, J .; Ledoux, I.; Cheng, L.-T. J . Am. Chem. Soc. 1995,
117, 2214-2225.
(17) Tertiary amines are necessary to prevent squaramide forma-
tion. For the only exception, see: Bello, K. A.; Corns, S. N.;
Griffiths, J . J . Chem. Soc., Chem. Commun. 1993, 452-454.
(18) See Supporting Information.
(19) See for example: (a) Havinga, E. E.; Pomp, A.; Ten Hoeve,
W.; Wynberg, H. Synth. Met. 1995, 69, 581-582. For a
review, see: (b) Ajayaghosh, A. Chem. Soc. Rev. 2003, 32,
181-191 and references therein.
(20) Law, K.-Y. J . Photochem. Photobiol. A 1994, 84, 123-132.
(21) Chenthamarakshan, C. R.; Eldo, J .; Ajayaghosh, A. Macro-
molecules 1999, 32, 251-257.
Ack n ow led gm en t. Generous support by the Sofja
Kovalevskaja Award of the Alexander von Humboldt
Foundation, endowed by the Federal Ministry of Educa-
tion and Research (BMBF) within the Program for
Investment in the Future (ZIP) of the German Govern-
ment, is gratefully acknowledged.
Su p p or tin g In for m a tion Ava ila ble: Syntheses and char-
acterizations of monomers, model compound, and polymers.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(22) See, for example: (a) Liang, K.; Law, K.-Y.; Whitten, D. G.
J . Phys. Chem. 1994, 98, 13379-13384. (b) Chen, H.; Fara-
hat, M. S.; Law, K.-Y.; Whitten, D. G. J . Am. Chem. Soc.
1996, 118, 2584-2594.
Refer en ces a n d Notes
(23) The quantitative determination of the degree of chromophore
dimerization within the polymer is not possible. There are
two unknown variables, i.e., the molar extinction coefficient
of the H-dimer and the equilibrium constant Keq, which
cannot be determined independently from each other because
the pure H-dimer state is not accessible.
(1) (a) Czarnik, A. W., Ed. Fluorescent Chemosensors for Ion and
Molecular Recognition; American Chemical Society: Wash-
ington, DC, 1993. (b) de Silva, A. P.; Gunaratne, H. Q. N.;
Gunnlaugsson, T.; Huxley, A. J . M.; McCoy, C. P.; Radema-
cher, J . T.; Rice, T. E. Chem. Rev. 1997, 97, 1515-1566 and
references therein.
(2) For a review, see: McQuade, D. T.; Pullen, A. E.; Swager, T.
M. Chem. Rev. 2000, 100, 2537-2574 and references therein.
(3) Swager, T. M. Acc. Chem. Res. 1998, 31, 201-207.
(4) See for example: Chen, L.; McBranch, D. W.; Wang, H.-L.;
Helgeson, R.; Wudl, F.; Whitten, D. G. Proc. Natl. Acad. Sci.
U.S.A. 1999, 96, 12287-12292.
(5) For an early example of cooperative cation binding, see: (a)
Kopolow, S.; Hogen Esch, T. E.; Smid, J . Macromolecules
1973, 6, 133-142. An example involving energy migration
and hence signal amplification in grafted, nonconjugated
systems has very recently been reported in: (b) Broadwater,
(24) Kamlet, M. J .; Abboud, J . L. M.; Abraham, M. H.; Taft, R.
W. J . Org. Chem. 1983, 48, 2877-2887.
(25) Ouchi, M.; Inoue, Y.; Kanzaki, T.; Hakushi, T. J . Org. Chem.
1984, 49, 1408-1412.
(26) Vo¨gtle, F.; Weber, E. Angew. Chem., Int. Ed. Engl. 1979, 18,
753-777.
(27) Langley, G. J .; Hecquet, E.; Morris, I. P.; Hamilton, D. G.
Rapid Commun. Mass Spectrom. 1997, 11, 165-170.
(28) As the H-dimer bands in the monomer spectra are rather
small, the absorbance ratios IU/ID for model monomer 4 might
be associated with a larger error.
(29) In Handbook of Chemistry and Physics, 81st ed.; Lide, D. R.,
Ed.; CRC Press: Washington, DC, 2000; pp 12-14.