Synthesis and photophysical investigation of squaraine rotaxanes by "clicked capping"

Article abstract of DOI:10.1021/ol801189a

(Chemical Equation Presented) Pseudorotaxane complexes of squaraine dyes and tetralactam macrocycles are converted into permanently interlocked rotaxane structures using copper-catalyzed and copper-free cycloaddition reactions with bulky stopper groups. T

Full text of DOI:10.1021/ol801189a

ORGANIC
LETTERS
2008
Vol. 10, No. 15
3343-3346
Synthesis and Photophysical
Investigation of Squaraine Rotaxanes by
“Clicked Capping”
Jeremiah J. Gassensmith, Lorna Barr, Jeffrey M. Baumes, Agelina Paek,
Anh Nguyen, and Bradley D. Smith*
Department of Chemistry and Biochemistry, UniVersity of Notre Dame,
Notre Dame, Indiana 46556
smith.115@nd.edu
Received June 4, 2008
ABSTRACT
Pseudorotaxane complexes of squaraine dyes and tetralactam macrocycles are converted into permanently interlocked rotaxane structures
using copper-catalyzed and copper-free cycloaddition reactions with bulky stopper groups. The photophysical properties of the encapsulated
squaraine depend on the structure of the macrocycle. In one case, squaraine rotaxanes are produced in near-quantitative yields and with
intense near-IR fluorescence. In another case, squaraine fluorescence is greatly diminished upon macrocyclic encapsulation but the signal
can be restored by dye displacement with anions.
Many of the new and emerging biophotonic technologies
require extremely bright and highly stable fluorescent dyes.1
One way to improve dye performance is molecular encap-
sulation inside a host molecule or nanoparticle.2 Our focus
is on squaraine rotaxanes, which are produced by encapsulat-
ing highly fluorescent near-IR squaraine dyes inside sur-
rounding macrocycles (Scheme 1).3 This sterically protects
the squaraine fluorophore such that squaraine rotaxanes
(2) (a) Arunkumar, E.; Forbes, C. C.; Smith, B. D. Eur. J. Org. Chem.
2005, 4051–4059. (b) Buston, J. E. H.; Young, J. R.; Anderson, H. L. Chem.
Commun. 2000, 905–906. (c) Park, J. S.; Wilson, J. N.; Hardcastle, K. I.;
Bunz, U. H. F.; Srinivasarao, M. J. Am. Chem. Soc. 2006, 128, 7714–7715.
(d) Mohanty, J.; Pal, H.; Ray, A. K.; Kumar, S.; Nau, W. M. ChemPhys-
Chem 2007, 8, 54–56. (e) Comes, M.; Marcos, M. D.; Mart´ınez-Ma´n˜ez,
R.; Milla´n, M. C.; Ros-Lis, J. V.; Sanceno´n, F.; Soto, J.; Villaescusa, L. A.
Chem. Eur. J. 2006, 12, 2162–2170. (f) Constantin, T. P.; Silva, G. L.;
Robertson, K. L.; Hamilton, T. P.; Fague, K.; Waggoner, A. S.; Armitage,
B. A. Org. Lett. 2008, 10, 1561–1564.
(1) (a) Biophotonics: Optical Science and Engineering for the 21st
Century; Shen, X., Van Wijk, R., Eds.; Springer: New York, 2005. (b)
Optical Imaging Techniques in Cell Biology; Cox, G. C., Ed.; CRC Press:
Boca Raton, 2007. (c) Wolfbeis, O. S. Fluorescence Methods and
Applications: Spectroscopy, Imaging, and Probes; Wiley-Blackwell: New
York, 2008.
(3) Arunkumar, E.; Forbes, C. C.; Noll, B. C.; Smith, B. D. J. Am. Chem.
Soc. 2005, 127, 3288–3289.
10.1021/ol801189a CCC: $40.75
Published on Web 06/27/2008
2008 American Chemical Society

Scheme 1
exhibit increased stability and decreased intermolecular
yield after column chromatography. We also find that
different macrocycles alter squaraine photophysical properties
in different ways; thus, the encapsulating macrocycle is a
structural parameter that can be altered to control squaraine
rotaxane function.
Capping strategies with phenylene-containing tetralactam
macrocycles such as M1 are not effective because of very poor
macrocycle solubility.8 To circumvent this problem, we inves-
tigated other macrocycles that have a similar isophthalamide
tetralactam motif but exhibit more favorable solubility in organic
solvents. We initially examined the well-known M2, which is
soluble in methylene chloride and has been extensively studied
as a macrocyclic component in various interlocked mol-
ecules.9,10 NMR titration experiments showed that M2 can
encapsulate squaraine dye D1 with an association constant
quenching.4 Furthermore, squaraine rotaxanes are well suited
for structural conversion into molecular probes for bio-
imaging applications.5 The first-generation squaraine rotax-
anes were prepared in yields of 10-30% using a Leigh-type
clipping method;6 that is, a macrocyclization reaction that
traps a squaraine template inside a tetralactam macrocycle.
This synthetic strategy allows rapid production of prototype
designs; however, the low yields limit the capacity to prepare
more complex later generation structures. We have explored
alternative methods of preparing squaraine rotaxanes, and
we report here a series of related synthetic capping strate-
gies.7 In each case, a reversible pseudorotaxane complex is
converted into a permanently interlocked rotaxane by
covalent capping with bulky stopper groups (Scheme 2). In
the best cases, squaraine rotaxanes are produced in >90%
(4) (a) Arunkumar, E.; Fu, N.; Smith, B. D. Chem. Eur. J. 2006, 12,
4684–4690. (b) Arunkumar, E.; Sudeep, P. K.; Kamat, K. V.; Noll, B.;
Smith, B. D. New J. Chem. 2007, 31, 677–683.
(5) Johnson, J. R.; Fu, N.; Arunkumar, E.; Leevy, W. M.; Gammon,
S. T.; Piwnica-Worms, D.; Smith, B. D. Angew. Chem., Int. Ed. 2007, 46,
5528–5531.
Scheme 2
.
Synthesis of Squaraine Rotaxanes via Capping (Top)
and Clipping (Bottom)
(6) (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.
2001, 123, 5983–5989. (c) Leigh, D. A.; Wong, J. K. Y.; Dehez, F.; Zerbetto,
F. Nature 2003, 424, 174–179.
(7) (a) Klotz, E. J. F.; Claridge, T. D. W.; Anderson, H. L. J. Am. Chem.
Soc. 2006, 128, 15374–15375. (b) Hu¨bner, G. M.; Reuter, C.; Seel, C.;
Vo¨gtle, F. Synthesis 2000, 1, 103–108. (c) Dunnwald, T.; Ja¨ger, R.; Vo¨gtle,
F. Chem. Eur. J. 1997, 3, 2043–2051.
(8) Inoue, Y.; Kanbara, T.; Yamamoto, T. Tetrahedron Lett. 2003, 44,
5167–5169.
(9) (a) Schalley, C. A.; Weilandt, T.; Bruggemann, J.; Vo¨gtle, F. Top.
Curr. Chem. 2004, 248, 141–200. (b) Affeld, A.; Hu¨bner, G. M.; Seel, C.;
Schalley, C. A. Eur. J. Org. Chem. 2001, 2877–2890. (c) Fischer, C.; Nieger,
M.; Mogck, O.; Bo¨ehmer, V.; Ungaro, R.; Vo¨gtle, F. J. Eur. J. Org. Chem.
1998, 155–161. (d) Handel, M.; Plevoets, M.; Gestermann, S.; Vo¨gtle, F. J.
Angew. Chem., Int. Ed. 1997, 109, 1248–1250
.
3344
Org. Lett., Vol. 10, No. 15, 2008

(Ka) of 5 × 103 M-1 in methylene chloride at 25 °C.11 We
then covalently capped the protruding alkyne ends of the
pseudorotaxane complex by conducting a copper mediated
alkyne azide cycloaddition reaction.12 After some experi-
mentation, we found that mixing macrocycle M2 with excess
dye D1 and stopper S1 followed by treatment with
Cu(Ph3P)3Br catalyst generated the “clicked rotaxane”
M2⊃T1 with an isolated yield of 40%. The rotaxane
structure was verified by standard spectroscopic methods
1
including H NMR (Figure 1). The downfield changes in
Figure 2. Fluorescence emissions of separate CH2Cl2 solutions of
(a) D1, (b) M2⊃D1, and M2⊃D1 in the presence of (c) TBA+·Cl-,
(d) TBA+·H2PO4-, (e) TBA+·CH3COO-, and (f) TBA+·C6H5COO-
at 5 mM for each anion. The concentrations of D1 and M2 were 1
µM and 3 mM, respectively.
of excess M2 lowers the emission intensity of D1 by about
a factor of 4 (Figure 2 a vs b). Additional evidence that the
quenching is due to inclusion of D1 inside M2 was gained
by conducting anion displacement experiments.13 As shown
in Figure 2, treatment of the M2⊃D1 psuedorotaxane system
with the tetrabutylammonium salts of chloride, acetate, or
benzoate leads to displacement of squaraine D1 from the
macrocyclic cavity and nearly complete restoration of its
fluorescence intensity. These anions are known to bind
strongly to the NH residues in M2 and form hydrogen-
bonded complexes (Ka > 105 M-1).14 Interestingly, tetra-
butylammonium dihydrogenphosphate was less effective at
squaraine displacement, suggesting that the M2 cavity does
not readily accommodate this larger anion. The fluorescence
quenching induced by M2 means that this system is unlikely
to have utility as a bioimaging probe; however, the pseudo-
rotaxane system M2⊃D1 is an effective and selective anion
sensor with near-IR fluorescence.15
1
Figure 1
.
Partial H NMR spectra of M2 (top), T1 (bottom), and
the resulting clicked rotaxane M2⊃T1 (middle).
chemical shift for the macrocycle amide and aryl protons
are indicative of deshielding by the encapsulated squaraine
thread T1 and are consistent with previously observed
1
changes in H NMR spectra for squaraine rotaxanes.
The yield of squaraine rotaxane was sufficient to allow
photophysical measurements. A direct comparison of squaraine
thread T1 and rotaxane M2⊃T1 revealed a modest red shift
in both absorption (+10 nm) and fluorescence emission (+7
nm) and approximately a 3-fold decrease in fluorescence
quantum yield. It is also noteworthy that addition of two
trizaole rings did not significantly alter the quantum yield
in thread T1 as compared to dye D1 (Table 1). The
macrocycle-induced quenching effect was verified by fluo-
rescence titration experiments that added aliquots of M2 to
a solution of squaraine D1 in methylene chloride. Addition
Compared to M2, the anthrylene macrocycle M3 is a more
promising building block for squaraine rotaxane fabrication and
probe development. In a prior study, we showed that an
admixture of M3 and D1 self-assembles quantitatively at
millimolar concentration in chloroform solution (Ka of 1.8 ×
105 M-1) to produce an inclusion complex whose absorption
(10) (a) Hunter, C. A. Chem. Commun. 1991, 749–751. (b) Hunter, C. A.
J. Am. Chem. Soc. 1992, 114, 5303–5311.
(11) Macrocycle M2 binds uncharged carbonyl-containing guests with
association constants <103 M-1 in CH2Cl2. For further information, see:
(a) Seel, C.; Parham, A. H.; Safarowsky, O.; Hu¨bner, G. M.; Vo¨gtle, F. J.
Org. Chem. 1999, 64, 7236–7242.
(12) (a) Aucagne, V.; Hanni, K. D.; Leigh, D. A.; Lusby, P. J.; Walker,
D. B. J. Am. Chem. Soc. 2006, 128, 2186–2187. (b) Dichtel, W. R.; Miljanic´,
O. S.; Spruell, J. M.; Heath, J. R.; Stoddart, J. F. J. Am. Chem. Soc. 2006,
128, 10388–10390. (c) Miljanic´, O. S.; Dichtel, W. R.; Khan, S. I.;
Mortezaei, S.; Heath, J. R.; Stoddart, J. F. J. Am. Chem. Soc. 2007, 129,
8236–8246.
Table 1. Absorption, Emission, Extinction Coefficients, and
Quantum Yields of Select Squaraine Derivatives in CHCl3
compd
λabs (nm)
log ε
λem (nm)
Φfa
(13) (a) Montalti, M.; Prodi, L. Chem. Commun. 1998, 1461–1462. (b)
Wiskur, S. L.; Ait-Haddou, H.; Lavigne, J. J.; Anslyn, E. V. Acc. Chem.
Res. 2001, 34, 963–972.
D1
T1
631
635
645
661
5.65
4.91
5.23
5.24
651
649
656
704
0.68
0.65
0.24
0.47
M2⊃[T1]
(14) Hu¨bner, G. M.; Gla¨ser, J.; Seel, C.; Vo¨gtle, F. Angew. Chem., Int.
Ed. 1999, 38, 383–386.
M3⊃[T2]
(15) Preliminary anion titration experiments with the rotaxane M2⊃T2
show that it acts as a fluorescent anion sensor. Full details will be reported
when the studies are complete.
a Error ( 5%.
Org. Lett., Vol. 10, No. 15, 2008
3345

produces five membered triazoline ring structures.18 As a
biocompatible method for capping pseudorotaxanes, the
reaction is appealing because it is perfectly atom economical
and does not require copper catalysis. As a proof of concept,
we heated a mixture of bis-azide dye D2, macrocycle M3,
and stopper S3 (1:1:2 molar ratio) in chloroform for 3 days
and produced the squaraine rotaxane M3⊃T3 in near-
quantitative yield (Scheme 3). The compound is stable
Scheme 3
.
Capping of Pseudorotaxane M3⊃D2 To Produce
M3⊃T3
1
Figure 3. Partial H NMR spectra of M3 (top), D1 (bottom), and
the resulting clicked rotaxane M3⊃T2 (middle).
and emission maxima are red-shifted by +40 nm.16 We now
report that “clicking” both ends of this pseudorotaxane with 2
molar equiv of stopper S2 produces the squaraine rotaxane
enough for immediate characterization but slowly decom-
poses if left standing under laboratory lights.19 Thus, the
capping reaction is quite efficient but the product instability
may possibly limit applications.
1
M3⊃T2 in near-quantitative yield. The H NMR spectra in
Figure 3 exhibit the expected changes in chemical shift for a
squaraine rotaxane, and the fluorescence spectra in Figure 4
In summary, we report that squaraine rotaxanes with the
anthrylene-containing macrocycle M3 can be prepared in
high yield by capping the alkyne groups that protrude from
pseudorotaxane precursors using either copper catalyzed
azide-alkyne cycloaddition or uncatalyzed azide-alkene
cycloaddition reactions. The resulting squaraine rotaxanes
exhibit intense near-IR absorption/emission maxima, and it
should be possible to develop them into molecular probes
for many types of photonic and bioimaging applications. In
contrast, squaraine fluorescence intensity is greatly dimin-
ished when the dye is encapsulated by macrocycle M2. The
fluorescence is restored when a suitable anionic guest is used
to displace the squaraine dye from a pseudorotaxane
complex; thus, the multicomponent system can be employed
as a fluorescent anion sensor.
Figure 4. Fluorescence spectra in chloroform (ex: 590 nm); (a)
pseudorotaxane M3⊃D1 (6 µM) yields free D1 (638 nm) and
M3⊃D1 (694 nm); (b) M3⊃T2 (6 µM).
Acknowledgment. We are grateful for useful discussions
with Professor Marvin J. Miller and his co-workers. This
work was funded by the NSF.
Supporting Information Available: Experimental pro-
cedures and spectral data for all new products This material
is available free of charge via the Internet at http://pubs.acs.org.
demonstrate that it is a permanently interlocked structure.
Pseudorotaxane M3⊃D1 partially dissociates at micromolar
concentrations in chloroform and produces two emission peaks,
one at 638 nm which corresponds to free squaraine D1 and
one at 694 nm, corresponding to pseudorotaxane. In contrast,
the squaraine rotaxane M3⊃T2 does not dissociate under these
conditions or in more polar solvents such as pure methanol (see
the Supporting Information).
OL801189A
(16) Gassensmith, J. J.; Arunkumar, E.; Barr, L.; Baumes, J. M.;
DiVittorio, K. M.; Johnson, J. R.; Noll, B. C.; Smith, B. D. J. Am. Chem.
Soc. 2007, 129, 15054–15059.
One of our long-term research goals is to assemble highly
stable squaraine rotaxane probes in complex biological
environments such as the interior of cells. This requires the
development of covalent capping methods that are biocom-
patible; that is, they do not use cytotoxic copper(I). One
possible approach is to employ strain-promoted cycloaddi-
tions with cyclic alkynes that undergo uncatalyzed reactions
at practically useful rates.17 We were attracted to the related
idea of a strained bicyclic alkene azide cycloaddition which
(17) (a) Agard, N. J.; Prescher, J. A.; Bertozzi, C. R. J. Am. Chem. Soc.
2004, 126, 15046–15047. (b) Baskin, J. M.; Prescher, J. A.; Laughlin, S. T.;
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C. R. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 16793–16797. (c) Ning,
X. H.; Guo, J.; Wolfert, M. A.; Boons, G. J. Angew. Chem., Int. Ed. 2008,
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(c) Stout, D. M.; Takaya, T.; Meyers, A. I. J. Org. Chem. 1974, 40, 563–
569.
(19) Triazolines can liberate N2 producing acid-sensitive aziridines. See
ref 18c.
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