Squaraine-based [2] rotaxanes that function as visibly active molecular switches

Article abstract of DOI:10.1002/chem.200903304

(Figure Presented) In a flash! A squaraine-based optical molecular switch functions with striking fluorescence signal changes that are visible to the naked eye (see figure).

Full text of DOI:10.1002/chem.200903304

COMMUNICATION
DOI: 10.1002/chem.200903304
Squaraine-Based [2]Rotaxanes that Function as
Visibly Active Molecular Switches
Sheng-Yao Hsueh,[a] Chien-Chen Lai,[b] and Sheng-Hsien Chiu*[a]
Molecular switches undergo reversible (co)conformational
shifts or molecular transformations, under the influence of
pH,[1] photons,[2] cations,[3] anions,[4] heat,[5] and/or elec-
trons,[6] between two or more different stable states. Al-
though many external stimuli (inputs) are available to oper-
ate molecular switches, the most important outputs are gen-
erally mechanical forces and optical signals: the former
allows the construction of artificial molecular machines[7]
and the latter provides the possibility for molecular sens-
ing.[8] Thanks to burgeoning growth in the synthesis of inter-
locked molecules, several rotaxane- and catenane-based mo-
lecular switches have been developed in which detectable
optical signals accompany the migration of interlocked mac-
rocycles between different recognition sites; such materials
are potentially useful for constructing complicated molecu-
lar logic[9] and optical devices.[10] Light-active molecular
switches providing fluorescence outputs at long wavelengths,
approaching the near-infrared (NIR) region, have potential
practical importance in biomedicine and photodynamic ther-
apy for sensing and signaling in living tissues and cells.[11]
Squaraine-based fluorescence dyes generally exhibit low
quantum yields in polar solvents; in their host-encapsulated
forms, however, they can experience less-polar local envi-
ronments and, therefore, exhibit significantly increased
quantum yields, even in polar solvents.[12] This behavior sug-
gests a straightforward approach toward the design of squar-
aine-based optical switches: if the solvent-exposed and en-
capsulated forms of a squaraine unit can be generated as
two distinguishable stable states of a switchable rotaxane in
a polar solvent, variations in the optical signals in the long-
wavelength-fluorescence region should be produced within
each switching cycle. Herein, we report the synthesis and
operation of squaraine-based optical molecular switches,
which display striking changes in their fluorescence signals
that are visible to the naked eye.
Previously, we reported that the molecular cage 1 forms
complexes with the squaraine derivatives 2 under the assis-
tance of Na+ ions as templates.[13] Thus, we anticipated that
the [2]rotaxane[4-long·Na2]ACHTNUGRTENUNG[4ClO4] (Scheme 1) would func-
tion as a molecular switch in CD3CN if we could remove the
templating Na+ ions and, thereby, move the molecular cage
from the squaraine station to the bipyridinium one; this pro-
cess should have the effect of decreasing the intensity of the
long-wavelength-fluorescence emission of the [2]rotaxane.
We synthesized the molecular switches [4-long·Na2]-
ACHUNTGRENUN[G 4ClO4] and [4-short·Na2]ACHUTNGTREN[NUGN 4ClO4], each featuring one squar-
aine and one bipyridinium station, but differing in the
length of the alkyl spacer (C16 and C8 units, respectively) be-
tween them, through the reactions of the squaraine-contain-
ing pyridine derivatives 6-long·Br and 6-short·Br, respective-
ly, with the benzyl bromide 7 in the presence of the molecu-
lar cage 1 and NaClO4 (42:42:63:105 mm) in CH3CN. Our
reasons for choosing to install bipyridinium moieties as com-
peting recognition units in these [2]rotaxanes were twofold:
1) their potentially strong binding to the crown ether like
[a] S.-Y. Hsueh, Prof. S.-H. Chiu
Department of Chemistry, National Taiwan University
No. 1, Sec. 4, Roosevelt Road, Taipei, Taiwan (ROC)
Fax : (+886)2-3366-1677
E-mail: shchiu@ntu.edu.tw
[b] Prof. C.-C. Lai
Institute of Molecular Biology
National Chung Hsing University
and Department of Medical Genetics
China Medical University Hospital, Taichung, Taiwan (ROC)
À
(C H···O hydrogen bonds) and catechol (p stacking) motifs
of the molecular cage moiety would allow the macrocycle to
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903304.
reside at these stations after removing the Na+ template
Chem. Eur. J. 2010, 16, 2997 – 3000
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2997

Scheme 1.
ions and 2) their fluorescence quenching ability would allow
the tuning of the fluorescence intensity of the squaraine
units by controlling the distance between the two guest sta-
tions.[14]
spectively (Table 1); the former value is higher and the
latter lower than that of the free squaraine dye 3 (F=0.15)
under the same conditions, suggesting that the fluorescence
enhancement caused by encapsulating the squaraine dye
within 1 and the fluorescence quenching caused by installing
2D COSY and NOSY experiments allowed us to identify
1
most of the signals in the H NMR spectrum of the [2]ro-
taxane [4-long·Na2]ACHTUNGTRENNUNG[4ClO4] in CD3CN at 298 K (Figure 1a);
we confirmed that, under these conditions, the molecular
cage resided around the squaraine station (see the Support-
ing Information). The addition of [2.2.2]cryptand to a solu-
tion of the [2]rotaxane[4-long·Na2]ACTHNUTRGEN[UNG 4ClO4] in CD3CN, there-
by sequestering the Na+ ions, resulted in significant shifts to
1
several of the signals in the H NMR spectrum (Figure 1b),
suggesting the migration of the molecular cage moiety from
the squaraine unit to the bipyridinium station. The
2D NOSY spectrum of this mixture displayed (see the Sup-
porting Information) cross peaks between the signals of the
aromatic protons (HAr) of the molecular cage moiety and
the bipyridinium protons (Ha and Hb), confirming the encir-
cling of the macrocyclic component around the bipyridinium
station under these conditions. Subsequent addition of
NaClO4 (2 equiv) returned the macrocycle back to its origi-
nal position around the squaraine station (Figure 1c), there-
by demonstrating the reversible translocation of the molecu-
Figure 1. Partial 1H NMR spectra (400 MHz, CD3CN, 298 K) of a) the
[2]rotaxane [4-long·Na2]ACTHNUTRGNE[NUG 4ClO4] (3 mm), b) the mixture obtained after
adding [2.2.2]cryptand (2 equiv) to the solution in a), and c) the mixture
obtained after adding NaClO4 (2 equiv) to the solution in b).
lar cage unit within the [2]rotaxane [4-long·Na2]ACTHNUTRGEN[NUG 4ClO4].
We determined the quantum yields of the [2]rotaxanes [4-
long]2+ and [4-long·Na2]4+ in CH3CN to be 0.11 and 0.22, re-
2998
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Chem. Eur. J. 2010, 16, 2997 – 3000

Squaraine-Based [2]Rotaxanes
COMMUNICATION
Table 1. Absorption and emission properties of [2]rotaxane-based molec-
ular switches in CH3CN.
taxane [4-long·Na2]ACTHNUTRGNEUNG[4ClO4] slowed the rate of shuttling of
the molecular cage unit between the two competing binding
sites in the ground state,[15] thereby providing more-effective
macrocycle encapsulation of the squaraine station and, con-
sequently, enhanced fluorescence.
We also prepared the [2]rotaxane [9·Na2]ACHTUNGTRENNUNG[3ClO4]
(Scheme 2) to examine the effect of replacing the strongly
[a]
Compound
labs [nm]
loge
lem [nm]
Ff[b]
[4-long·Na2]
G
629
634
630
634
630
635
5.33
5.32
5.39
5.38
5.12
5.10
641
645
643
653
635
648
0.22
0.11
0.09
0.05
0.22
0.18
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
fluorescence-quenching bipyridinium ion in [4-short·Na2]-
ACHTUNGTRENNUNG
AHCTUNGRTEG[NNUN 4ClO4] with a (mono)pyridinium unit (i.e., a much weaker
[a] Solutions of [4-long·Na2]
ACHUTGTNRNE[NUG 4ClO4], [4-long]ACTHUNGTRENNNUG
fluorescence quencher and a much more weakly interacting
station for the molecular cage unit). Although 1H NMR
spectroscopy revealed the successful reversible migration of
the molecular cage moiety between the squaraine and pyri-
dinium stations in this [2]rotaxane when adding and remov-
ing Na+ ions, disappointingly the quantum yields of the
[2]rotaxanes [9·Na2]3+ and [9]+ did not decrease significant-
ly (0.22 and 0.18, respectively). Figure 2 reveals that no
striking fluorescence changes occurred after adding-
G
E
ACHTUNGTRENNUNG
639, 649, 637, 644, 639, and 645 nm, respectively. [b] Fluorescence quan-
tum yields were determined by using the squaraine dye 3 as the standard
(Ff =0.15 in CH3CN; see ref. [12b]); error: Æ10%.
the bipyridinium station were well balanced in this molecu-
lar switch. Figure 2 reveals that, when irradiated with
365 nm UV light, the fluorescence intensity of [4-
AHCTUNGTREG[NNNU 2.2.2]cryptand (2 equiv) to a solution of the [2]rotaxane
[9·Na2]ACHTNUGRTNEUNG[3ClO4] (2 mm) in CH3CN irradiated with UV light
(365 nm). Compared with the behavior of [4-short·Na2]4+
(F=0.09), the absence of a strong fluorescence quencher in
[9·Na2]3+ (F=0.22) appears to be the key factor affecting
its higher quantum yield.
The molecular cage moiety has, however, a relatively
weak binding affinity for the (mono)pyridinium station in
Figure 2. Photographic images of solutions of molecular switches (2 mm)
in CH3CN in the presence and absence of Na+ ions (excited at 365 nm):
a) [4-long·Na2]
ACHTUNGTRENNUNG
in a); c) NaClO4 (2 equiv) added to the solution in b); d) [9·Na2]AHCTUNGTRENNNUG
e) [2.2.2]cryptand (2 equiv) added to the solution in d); f) NaClO4
(2 equiv) added to the solution in e).
long·Na2]4+ in CH3CN decreased significantly after
[2.2.2]cryptand had been added, that is, after translocation
of the molecular cage from the squaraine unit to the bipyri-
dinium station. Again, addition of NaClO4 to this solution
led to anchoring of the molecular cage around the squaraine
station, restoring the fluorescence signal back to that of the
original solution. Thus, long-wavelength-fluorescence signals
detectable to the naked eye were produced reversibly upon
switching the molecular cage moiety between the squaraine
and bipyridinium stations.
The [2]rotaxane [4-short·Na2]ACHTUNRTGNEUNG[4ClO4], which contains a
relatively short linker between the stations, exhibited switch-
ing of the interlocked molecular cage unit between its squar-
aine and bipyridinium moieties, mediated by the removal
and addition of Na+ ions, that was similar to that of the
[2]rotaxane [4-long·Na2]ACHTNUTRGENNUG[4ClO4], as confirmed by using
1H NMR spectroscopy. The quantum yields of the [4-
short]2+ and [4-short·Na2]4+ species (F=0.05 and 0.09, re-
spectively) were, however, both significantly lower than
those of [4-long]2+ and [4-long·Na2]4+
.
Thus, the length of the spacer unit influenced the fluores-
cence quenching effect of the bipyridinium unit dramatical-
ly, consistent with previous findings.[14] In addition, we sus-
pect that the longer spacer between the stations in the [2]ro-
Scheme 2.
Chem. Eur. J. 2010, 16, 2997 – 3000
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2999

S.-H. Chiu et al.
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shielding the squaraine unit to some extent; this behavior
leads to a slightly higher quantum yield than that of the
nonencapsulated squaraine dye 3 (F=0.15).
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In conclusion, we have developed a visibly active molecu-
lar switch, [2]rotaxane [4-long·Na2]ACTHNUTRGNE[UNG 4ClO4], that provides
outputs in the long-wavelength-fluorescence region that can
be discriminated by the naked eye. This molecular switch
functions through the selective encapsulation and exposure
of the squaraine station of a two-station [2]rotaxane upon
the addition and removal of Na+ ions, respectively. The
lengthy spacer unit between the squaraine and bipyridinium
moieties allows the bipyridinium unit to function as a fluo-
rescence quencher. We are currently investigating the possi-
bility of extending this system into molecular switches capa-
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Acknowledgements
We thank the National Science Council (Taiwan) for financial support
(NSC-98-2113M-002-004-MY3).
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Keywords: bipyridinium ions · molecular cages · molecular
switches · rotaxanes · squaraines
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[15] Although Scheme 1 depicts the molecular cage unit in [4-long·Na2]-
AHCTUNGRTEG[NNNU 4ClO4] as surrounding the squaraine station, in actuality this mac-
rocyclic unit shuttles between the two stations, but with a preference
for the squaraine moiety because of stronger noncovalent interac-
tions. A longer alkyl spacer provides a greater energy barrier to
shuttling, thereby reducing the rate of translocation.
Received: December 2, 2009
Published online: February 15, 2010
3000
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Chem. Eur. J. 2010, 16, 2997 – 3000