C O M M U N I C A T I O N S
tion-induced broadening of its absorption spectrum. Since it is
straightforward to synthesize squaraines with unsymmetrical struc-
tures and as oligomers, it should be possible to prepare a wide range
of squaraine-derived rotaxanes with useful photophysical, photo-
chemical, and mechanical properties.2
,14
Acknowledgment. This work was supported by the NIH and
the University of Notre Dame.
Supporting Information Available: Synthetic procedures and
X-ray crystal data (PDF, CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 2. Change in absorption (left) and emission (right) upon addition
of cysteine (5 mM) to 5 µM solutions of 1, 2, or 3 in THF-water (4:1) at
pH 6.0 and 22 °C.
References
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2) (a) Law, K.-Y. Chem. ReV. 1993, 93, 449. (b) Law, K.-Y. In Organic
Photochemistry; Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker:
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Organic Photochemistry; Ramamurthy, V., Schanze, K. S., Eds.; Marcel
Dekker: New York, 1997; p 467.
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3) (a) Ros-Lis, J. V.; Garc ´ı a, B.; Jim e´ nez, D.; Mart ´ı nez-M a´ n˜ ez, R.; Sancen o´ n,
F.; Soto, J.; Gonzalvo, F.; Valldecabres, M. C. J. Am. Chem. Soc. 2004,
1
26, 4064. (b) Ros-Lis, J. V.; Mart ´ı nez-M a´ n˜ ez, R.; Soto, J. Chem.
Commun. 2002, 2248.
Figure 3. Partial absorption spectra. Left: 6.5 µM solutions of (a) 1 in
DMSO, (b) 1 in DMSO-water (1:1), and (c) 3 in DMSO-water (1:9).
Right: solid-state films of (a) 1, (b) 2, and (c) 3.
(
4) (a) McKerrow, A. J.; Buncel, E.; Kazmaier, P. M. Can. J. Chem. 1995,
73, 1605. (b) Chen, H.; Farahat, M. S.; Law, K.-Y.; Whitten, D. G. J.
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M u¨ llen, K.; Jaiser, F.; Yang, X.; Neher, D. Macromolecules 2004, 37,
8297.
colorless within 48 h;3b a solution containing rotaxane 2 becomes
colorless within a week, whereas rotaxane 3 retains its blue color
for months. These reactivity results are attributed to the steric
protection provided by the macrocycle.12 The parallel xylylene units
in the macrocycle sit almost perfectly over both faces of the
electrophilic cyclobutene core of the squaraine thread and block
nucleophilic attack. The more tightly packed pyridyl-containing
macrocycle in rotaxane 3 offers the most steric protection. In
comparison, the more flexible isophthalamide-containing macro-
cycle in 2 is more likely to adopt transient conformations that expose
the squaraine thread to nucleophilic attack.
(5) Das, S.; Thomas, K. G.; George, M. V.; Kamat, P. V. J. Chem. Soc.,
Faraday Trans. 1992, 88, 3419.
(
6) (a) Anderson, S.; Anderson, H. L. Angew. Chem., Int. Ed. 1996, 35, 1956.
(b) Anderson, S.; Claridge, T. D. W.; Anderson, H. L. Angew. Chem.,
Int. Ed. 1997, 36, 1310. (c) Buston, J. E. H.; Young, J. R.; Anderson, H.
L. Chem. Commun. 2000, 905. (d) See also: Kwan, P. H.; MacLachlan,
M. J.; Swager, T. M. J. Am. Chem. Soc. 2004, 126, 8638.
(7) Seel, C.; V o¨ gtle, F. Chem.-Eur. J. 2000, 6, 1.
(
8) (a) Leigh, D. A.; Murphy, A.; Smart, J. P.; Slawain, A. M. Z. Angew.
Chem., Int. Ed. 1997, 36, 728. (b) Gatti, F. G.; Leigh, D. A.; Nepogodiev,
S. A.; Slawin, A. M. Z.; Teat, S. J.; Wong, J. K. Y. J. Am. Chem. Soc.
2
001, 123, 5983. (c) Leigh, D. A.; Venturini, A.; Wilson, A. J.; Wong, J.
K. Y.; Zerbotto, F. Chem.-Eur. J. 2004, 10, 4960.
(9) Distances (Å) and angles (°) for the four unequal NH‚‚‚O hydrogen bonds
in 2: 3.05, 165; 2.80, 163; 3.02, 163; 2.76, 164, and the four equal NH‚
‚
‚O hydrogen bonds in 3: 2.84, 157. Note that the benzylic carbons in
Squaraine dyes are known to form aggregates in mixed DMSO-
the macrocycle of 2 are slightly disordered.
4
(10) Affeld, A.; H u¨ bner, G. M.; Seel, C.; Schalley, C. A. Eur. J. Org. Chem.
water solutions, which broadens their absorption spectra. As shown
2001, 2877 and references therein.
in Figure 3, a DMSO solution of squaraine 1 has a sharp aborption
at 649 nm, whereas a 1:1 DMSO-water mixture has blue-shifted
and red-shifted bands that are attributed to H- and J-aggregates,
respectively. In contrast, addition of water to DMSO solutions
containing rotaxanes 2 or 3 leads only to minor spectral broadening.
Even a solution of rotaxane 3 in 1:9 DMSO-water exhibits a
relatively sharp absorption band at 637 nm. Very similar results
are obtained when the compounds are in the solid state. Broadening
of squaraine absorption in the solid state has been attributed to
intermolecular charge-transfer interactions between the dye mol-
(
11) Unlike the usual Leigh-type rotaxanes, the NMR spectra of rotaxanes 2
and 3 in THF-d do not exhibit dynamic NMR behavior over the
8
temperature range of 298-203 K. It is likely that the macrocycle pirouettes
around the squaraine thread, but the molecular symmetry does not allow
this motion to be detected by NMR.
(
12) (a) Parham, A. H.; Windisch, B.; V o¨ gtle, F. Eur. J. Org. Chem. 1999,
1
233, 3. (b) Reuter, C.; V o¨ gtle, F. Org. Lett. 2000, 2, 593. (c) Ghosh, P.;
Mermagenaand, O.; Schalley, C. A. Chem. Commun. 2002, 2628. (d) Oku,
T.; Furusho, Y.; Takata, T. Org. Lett. 2003, 5, 4923.
(
13) The unit cell for 2 contains four chloroform molecules, whereas 3 contains
two water molecules. See Supporting Information for pictures of crystal
packing and additional structural views.
(
14) Squaraine dyes have been considered for applications such as bioimaging
probes, photosensitizers for photodynamic therapy, chemosensors, xero-
graphic photoreceptors, photovoltaic cells, conducting polymers, and
materials for nonlinear optics; see ref 2c. For more recent references,
see: (a) Oswald, B.; Patsenker, L.; Duschl, J.; Szmacinski, H.; Wolfbeis,
O. S.; Terpetschnig, E. Bioconjugate Chem. 1999, 10, 925. (b) Ajayaghosh,
A.; Eldo, J. Org. Lett. 2001, 3, 2595. (c) Arunkumar, E.; Chithra, P.;
Ajayaghosh, A. J. Am. Chem. Soc. 2004, 126, 6590. (d) Ramaiah, D.;
Eckert, I.; Arun, K. T.; Weidenfeller, L.; Epe, B. Photochem. Photobiol.
2
ecules which have an interplanar distance of ∼3.5 Å. In the case
of rotaxanes 2 and 3, the solid-state interplanar distances between
1
3
squaraine threads are ∼10.4 and ∼7.0 Å, respectively, and their
absorption bands are narrower (Figure 3).
In summary, we report a simple, two-step synthesis of squaraine-
derived rotaxanes. The encapsulating macrocycle greatly increases
the chemical stability of the squaraine thread and inhibits aggrega-
2004, 79, 99.
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