A light-stimulated molecular switch driven by radical-radical interactions in water

Article abstract of DOI:10.1002/anie.201102510

A redox-controllable bistable [2]rotaxane has been synthesized using templation and click chemistry. With assistance from a sacrificial electron donor, light-triggered switching through numerous cycles can be initiated by radical-pairing interactions between the reduced forms of the cyclophane and the bipyridinium unit in aqueous solution in the absence of air. In the presence of air (O₂), the interactions are reset by donor-acceptor charge transfer. Copyright

Full text of DOI:10.1002/anie.201102510

Communications
DOI: 10.1002/anie.201102510
Molecular Switches
A Light-Stimulated Molecular Switch Driven by Radical–Radical
Interactions in Water**
Hao Li, Albert C. Fahrenbach, Ali Coskun, Zhixue Zhu, Gokhan Barin, Yan-Li Zhao,
Youssry Y. Botros, Jean-Pierre Sauvage,* and J. Fraser Stoddart*
One of the unique properties of planet earth is its ability to
maintain a large amount of liquid water on its surface.
Although water in its liquid state is a relatively rare
occurrence in our solar system,[1] nature has taken advantage
of this aqueous medium to generate and support life, creating
biological molecular machines such as ATP synthase,[2]
kinesin,[3] myosin,[4] and bacterial flagella,[5] all of which
accomplish their important tasks in aqueous environments.
Inexorably, one of the challenges nowadays for the scientific
community is understanding conceptually how nature pres-
ents itself at the molecular level through the medium of
biology.
ability to exercise controllable intramolecular motion, involv-
ing their mechanically bonded components residing between
their different low energy states, mechanically interlocked
molecules[8] (MIMs), in the form of multi- and bistable
rotaxanes[9] and catenanes,[10] have attracted the attention of
many researchers as potential candidates for prototypical
artificial molecular machines. These MIMs have been shown
to participate in a wide range of applications in molecular
nanotechnology, including mechanical actuators,[11] molecular
memory,[12] and drug delivery vehicles.[13] Although MIMs
have been shown[14] to be capable of switching upon exposure
to light and so possess unique advantages in terms of their
addressability, most of this research has been conducted in
organic solvents. In our quest to better understand the
functions of biological molecular motors as well as to
orchestrate biocompatible applications, molecular switches
and machines that are soluble and operational in aqueous
media must be developed.[7b,14c,15] Only a few examples,
however, have been reported.[15–17] In particular, molecular
switches based on tetrathiafulvalene (TTF) units can be
switched[16] between two states by means of chemical or
electrochemical oxidation. On account of the poor reversi-
bility[17] of the redox processes of TTF in water, however, the
practical applications of these artificial systems are limited.
Our ability to fabricate artificial molecular machines, capable
of reversible switching, which can be stimulated by light in
aqueous environments as opposed to in organic solvents,
presents opportunities to develop nanobiomechanical sys-
tems for applications, in particular, molecular prosthetics.[7i,18]
Herein, we present a light-stimulated molecular switch in the
form of a bistable [2]rotaxane, which can undergo relative
mechanical movements of its components in water.
Synthetic analogues[6,7] of the biological molecular
motors—that is, artificially designed molecular switches[6]
and machines[7]—have undergone extensive development
during the past few decades. On account of their inherent
[*] H. Li, A. C. Fahrenbach, Dr. A. Coskun, Dr. Z. Zhu, G. Barin,
Dr. Y.-L. Zhao, Prof. J.-P. Sauvage, Prof. J. F. Stoddart
Department of Chemistry, Northwestern University
2145 Sheridan Road, Evanston, IL 60208 (USA)
Fax: (+1)847-491-1009
E-mail: stoddart@northwestern.edu
Homepage: http://stoddart.northwestern.edu
Prof. J. F. Stoddart
Graduate School of EEWS, Korea Advanced Institute of Science and
Technology (KAIST), Daejeon (Korea)
Y. Y. Botros
Department of Materials Science and Engineering, Northwestern
University, 2145 Sheridan Road, Evanston, IL 60208, (USA)
and
Intel Labs, Building RNB-6-61
2200 Mission College Blvd., Santa Clara, CA 95054-1549 (USA)
and
National Center for Nano Technology Research, King Abdulaziz City
for Science and Technology (KACST)
P.O. Box 6086, Riyadh, 11442 (Saudi Arabia)
Ruthenium(II)tris(2,2’-bipyridine) ([Ru(bpy)3]2+) is an
important inert metal complex well known[19] for its photo-
catalytic electron-transfer properties. In particular, its ability
to reduce 4,4’-dialkylbipyridinium (BIPY2+) units, upon
exposure to light in the presence of a sacrificial reducing
agent, has been exploited by us recently[20] in the templation
of mechanical bond formation by making use of radical-
pairing interactions between BIPY(C+) radical cations that
occur following photocatalytic reduction of the BIPY2+ units.
These BIPY(C+) radical cations can form[20] strong inclusion
[**] The research was supported by 1) the National Center for Nano
Technology Research at KACST in Saudi Arabia—we thank Dr.
Turki S. Al-Saud, Dr. Soliman H. Alkhowaiter, and Dr. Mohamed B.
Alfageeh for their support of this research—and 2) the Non-
Equilibrium Energy Research Center (NERC), which is an Energy
Frontier Research Center (EFRC) funded by the U.S. Department of
Energy (DoE), Office of Science, Office of Basic Energy Science,
under Award Number DE-SC0000989. G.B. would like to thank the
EFRC for a NERC Fellowship and the International Center for
Diffraction Data for the award of a 2011 Ludo Frevel Crystallography
Scholarship. J.F.S. was also supported by the WCU Program (R31-
2008-000-10055) at KAIST. A.C.F. would like to thank the National
Science Foundation (NSF) for a Graduate Research Fellowship.
complexes
with
cyclobis(paraquat-p-phenylene)[21]
(CBPQT4+) in its diradical dicationic redox state
(CBPQT2(C+)) as a result of a radical-pairing interaction
under redox control. We now demonstrate that the [Ru-
(bpy)3]2+ complex can be used as a photosensitizer to reduce
the BIPY2+ units to their radical cations and hence induce
switching in appropriate bistable [2]rotaxanes. To this end, we
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102510.
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Angew. Chem. Int. Ed. 2011, 50, 6782 –6788

have designed, synthesized, and characterized the [2]rotaxane
R·8Cl, as well as its organic soluble counterpart R·8PF6
(Scheme 1). The [2]rotaxane R·8Cl contains a CBPQT4+
ring and a dumbbell bearing the p-electron-rich 1,5-dioxy-
naphthalene (DNP) recognition site, and a p-electron-defi-
cient BIPY2+ unit, which serves as a binding site for the
CBPQT4+ ring when the [2]rotaxane is reduced to its
pentacationic trisradical state. In order to be able to activate
[Ru(bpy)3]2+ complex as one of its two stoppers. Upon
irradiation with visible light in the presence of the sacrificial
electron donor,[22] triethanolamine (TEOA), all three of the
BIPY2+ units, two in the CBPQT4+ ring and one in the
+
dumbbell, are reduced to BIPYC units by the [Ru(bpy)3]2+ in
its excited state, after which the RuIII state of the metal is
reduced back to RuII by an electron from TEOA. The
resulting diradical dicationic CBPQT2(C+) ring undergoes
+
+
switching of the ring between the DNP and BIPYC sites using
translation in order to encircle the BIPYC unit in the
light, the dumbbell component was designed to have a
dumbbell, on account of stabilizing radical-pairing interac-
tions,[20] in addition to the loss of recognition of the donor–
acceptor interactions between the DNP site and the reduced
+
CBPQT2(C+) ring. As soon as the BIPYC units are oxidized by
oxygen, the donor–acceptor interactions[8e,23] are reinstated
while the Coulombic repulsion between the CBPQT4+ ring
and the BIPY2+ unit in the dumbbell induces the ring to
shuttle back and encircle the DNP unit.
The template-directed strategy, which was employed in
the synthesis of R·8Cl, is outlined in Scheme 1 and described
in detail in the Supporting Information. The bipyridinium
derivative S8·2PF6 bearing an azide terminal function, and the
[Ru(bpy)3]2+ derivative S3·2PF6, incorporating a DNP unit
and a terminal alkyne function were obtained in relatively
high yields in four and three steps, respectively. The [2]rotax-
ane R·8PF6 was isolated in 41% yield, following the reaction
of S3·2PF6 with S8·2PF6 in Me2CO using a copper(I)-
catalyzed azide–alkyne cycloaddition[24] (CuAAC) in the
presence of CBPQT·4PF6 while relying upon a threading-
followed-by-stoppering approach to form the [2]rotaxane.
The water-soluble [2]rotaxane R·8Cl is obtained in quantita-
tive yield from R·8PF6 after counterion exchange with a
saturated solution of tetrabutylammonium chloride (TBACl)
in MeCN. The resulting [2]rotaxane was fully characterized by
NMR spectroscopy and mass spectrometry (see the Support-
ing Information). The 1D and 2D 1H NMR spectra of R·8Cl in
D2O revealed (Figure S2) that the CBPQT4+ ring encircles
the DNP unit in the ground-state co-conformation (GSCC) of
the [2]rotaxane.
The light-induced switching of R·8Cl (for similar inves-
tigations carried out on R·8PF6 in MeCN, see the Supporting
Information) has been monitored using UV/Vis spectroscopy.
The absorption spectrum of R·8Cl was recorded (Figure 1a)
at room temperature in degassed H2O. The [Ru(bpy)3]2+ unit
in R8+ has a characteristic absorption band with a maximum at
lmax = 456 nm. Irradiation of the aqueous solution (in the
presence of TEOA) with a household, visible-light lamp gave
rise to another characteristic absorption (lmax = 534 nm)
band, accompanied by the appearance of a broad band
centered on 1083 nm and consistent with the absorption
+
spectra of the BIPYC radical cations in their pimerized
Scheme 1. a) The template-directed synthesis of the [2]rotaxane R·8Cl
relying upon a threading-followed-by-stoppering approach based on
click chemistry, as does the synthesis of its corresponding dumbbell
D·4Cl. b) The graphical representation of R8+ and the process of light-
stimulated switching. Upon exposure to light, the ruthenium stopper
transfers a total of three electrons photocatalytically, to all three BIPY2+
units situated in the ring and dumbbell components, generating the
reduced trisradical species of R8+, wherein the CBPQT2(C+) ring encir-
cles the BIPYC+ unit in the dumbbell component. Exposure to O2
forms.[20] On exposing the R·8Cl solution to air, the absorption
band at 534 nm and that centered on 1080 nm were no longer
observed, indicating that the CBPQT4+ ring is once again
encircling the DNP unit.[25] We have also investigated (Fig-
ure 1b) how the photoinduced reduction takes place over
time and discovered that the photosensitized electron transfer
takes aproximately 270 min to reach completion. Control
experiments were also carried out on the dumbbell D·4Cl
(inset in Figure 1a & Figure S8 in the Supporting Informa-
reoxidizes R(2+)(3 +)
and leads to the ground-state co-conformation,
C
wherein the CBPQT4+ ring encircles the DNP unit.
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In order to evaluate the ability of this integrated
molecular system to serve as a prototypical artificial molec-
ular machine, a solution of R·8Cl in degassed H2O was
irradiated for 30 min[26] per cycle, followed by flushing with
oxygen for 1 min. No evidence of significant decomposition
was observed[27] over five consecutive cycles while monitoring
by UV/Vis spectroscopy.
Cyclic voltammetry (CV) of R·8Cl in degassed H2O (0.1m
TBACl) reveals (Figure 2a) a single reduction process occur-
ring[28] at a voltage of À0.45 V (redox potential) versus the
Ag/AgCl reference electrode. A combination of chronocoul-
ometry (Figure 2b) and 1H NMR diffusion-ordered spectros-
copy (DOSY) experiments (see the Supporting Information)
indicate the occurrence of a three-electron process at this
potential. This observation confirms our hypothesis that,
upon irradiation with visible light, R8+ undergoes a three-
Figure 1. a) The UV/Vis absorption spectra of the [2]rotaxane R·8Cl
(0.2 mm) in the presence of TEOA with no exposure to visible light
(red trace), and after irradiation with visible light for 240 min (purple
trace). Shown in the inset is the UV/Vis absorption spectra of the
dumbbell D·4Cl (0.03 mm) in the presence of TEOA with no exposure
to visible light (black trace), and after irradiation with visible light for
240 min (blue trace). All UV/Vis spectra were recorded in a quartz cell
with a 2 mm cell path length under the same conditions of temper-
ature (298 K) and solvent (argon-purged H2O). b) A plot of absorbance
at 550 nm versus irradiation time for an aqeous solution of R·8Cl
(0.2 mm) in the presence of TEOA. All UV/Vis spectra were recorded
in a quartz cell with a 2 mm cell path length under the same
conditions of temperature (298 K). c) A plot of the absorbance at
550 nm versus the number of cycles as an aqueous solution of R·8Cl is
cycled through photocatalytic reduction upon exposure to light in the
presence of TEOA (purple dots) and subsequent oxidation with aerial
oxygen (red dots). All UV/Vis spectra were recorded in a quartz cell
with a 10 mm cell path length under the same conditions of temper-
ature (298 K), solvent (argon-purged H2O), and concentration
(0.03 mm).
Figure 2. a) A cyclic voltammogram (CV) of R·8Cl which shows a
single three-electron process that occurs at approximately À0.4 V vs.
Ag/AgCl. The CV was recorded at 298 K in argon-purged H2O;
concentration: 0.6 mm and electrolyte: 0.1m (TBACl). The scan rate
was set at 50 mVsÀ1. b) Integrated Cottrell chronocoulometry plot of
the rotaxane R·8Cl. Application of the Anson equation, using a value
for the diffusion coefficient obtained (see the Supporting Information)
from 1H DOSY NMR allowed us to determine that the reduction peak
shown above is a three-electron process.
tion), which provides additional evidence for the hypothesis
that the absorption bands (Figure 1) of R·8Cl centered on
534 nm and on 1083 nm can be attributed to the mechanical
+
movement of the CBPQT2(C+) ring to the BIPYC unit of the
dumbbell component in R8+ in an aqueous environment.
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Angew. Chem. Int. Ed. 2011, 50, 6782 –6788

electron reduction process, resulting in the formation of the
In order to demonstrate that, in its fully oxidized ground
state, the CBPQT4+ ring component of R8+ resides around the
DNP unit, and that the ring returns after oxidation of the
trisradical form of R8+, 1H NMR spectroscopy was carried out
on R·8Cl at 298 K. The light-induced switching of R8+, after
exposure to light, (for investigations of R·8PF6 in MeCN, see
the Supporting Information) was monitored by 1H NMR
spectroscopy at 298 K in degassed D2O. The 1H NMR
spectroscopic data (Figure 4) support the hypothesis that
R8+ has the CBPQT4+ ring encircling the DNP unit in the
ground state prior to undergoing light-stimulated switching in
the presence of TEOA. The resonances associated with the
H2/6 and H3/7 protons of the DNP unit, the upfield shifts of
which compared to those in the dumbbell (see the Supporting
Information), provide direct evidence for the encirclement of
the CBPQT4+ ring around the DNP unit (DNPꢀCBPQT4+) in
the GSCC. On irradiation with visible light for 240 min[26] in
the presence of excess of TEOA (Figure 4), the resonances
associated with the protons of the encircled DNP unit are
observed to broaden out gradually into the baseline,[30] on
+
BIPYC ꢀCBPQT2(C+) co-conformation.
Square-wave differential pulse voltammetry (SQWDPV)
experiments have been performed (Figure 3) in aqueous
solutions[29] on the supramolecular components and the
+
account of the formation of the single unpaired BIPYC and
CBPQT2(C+) radicals, as suggested by UV/Vis spectroscopy.
After aeration of the sample for only 1 min, the H2/6 and H3/7
resonances associated with the protons of the encircled DNP
are restored, indicating that the radicals become fully
oxidized back to the CBPQT4+ ring and BIPY2+ unit. When
the switching sequence was repeated a second time in D2O,
the process was found to be reproducible. This repeatable
reversible switching of R8+ points to its potential to serve as a
molecular machine that can be powered over and over again
in an aqueous environment by light and oxygen in an
alternating sequence.
Figure 3. Square-wave differential pulse voltammograms (SQWDPV) of
the viologen derivative V2+ (0.5 mm, black trace), the CBPQT4+ ring
(0.06 mm, blue trace), a 1:1 mixture of V2+ and CBPQT4+ (0.5 mm,
pink trace), which leads to formation of the trisradical complex upon
reduction, and the [2]rotaxane R8+ (0.6 mm, purple trace). All voltam-
mograms were recorded in H2O at 298 K, with 0.1m TBACl supporting
electrolyte. Ref.: Ag/AgCl.
The fact that the [2]rotaxane R8+ can have its components
driven back and forth relative to each other through many
results compared to those obtained for the rotaxane. The
first reduction potential of the BIPY2+ derivative V2+, leading
to its radical cation form, was found to come at À0.64 V, while
the CBPQT4+ ring alone in solution is much easier to reduce,
its first reduction potential at À0.46 V, generating the
diradical dicationic, CBPQT2(C+). SQWDPV of an equimolar
mixture of V2+ and CBPQT4+ reveals that only one redox
process is observed, at À0.44 V and is similar to that of
CBPQT4+ alone in aqueous solution. The fact that the
reduction of V2+ alone in water is not detected in this case
is an observation which is consistent with the formation of the
+
trisradical complex VC ꢀCBPQT2(C+). (See the Supporting
Information for a CV titration study.) The large thermody-
namic driving force for the formation of the trisradical
complex acts to shift the reduction of V2+ to a more positive
potential, such that it occurs at the same potential as does the
first reduction of the CBPQT4+ ring. These results are
consistent with what we have observed[20a] previously for the
formation of a trisradical complex in MeCN. SQWDPVof the
rotaxane R8+ in water shows that the first reduction occurs at
a potential À0.45 V, which is nearly identical to that of the 1:1
mixture of CBPQT4+ and V2+, both of which can be assigned
to a three-electron reduction of each of the BIPY2+ units—
two involving the CBPQT4+ ring and one involving the thread
or dumbbell components.
Figure 4. Partial 500 MHz 1H NMR spectra recorded in D2O at 298 K
of 1) R·8Cl, 2) after 240 min irradiation with visible light in the
presence of TEOA (20 equiv), 3) after opening the NMR tube for 1 min
to the air, 4) after another 240 min irradiation with visible light in the
presence of TEOA (20 equiv), and 5) after opening the NMR tube for
1 min yet again to the air.
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Communications
7.0 Hz, 12H). 13C NMR (125 MHz, CD3CN): d = 156.6, 156.5, 156.0,
153.9, 152.5, 151.3, 151.3, 151.1, 151.1, 150.8, 150.7, 150.4, 150.1, 149.8,
149.2, 145.8, 145.1, 144.1, 141.4, 137.3, 128.0, 127.1, 126.8, 126.4, 126.1,
126.0, 125.1, 125.0, 124.7, 124.7, 124.6, 124.4, 123.8, 123.8, 122.9, 121.2,
113.7, 113.6, 105.6, 105.4, 73.2, 70.1, 69.7, 69.1, 69.0, 69.0, 68.4, 67.7,
63.5, 61.5, 61.4, 49.2, 30.2, 29.1, 25.7, 25.0, 24.4, 23.0, 19.9 ppm. ESI-
HRMS calcd for m/z = 1936.5490 [MÀPF6] +, found m/z = 1936.5540.
Counterion exchange from D·4PF6 to D·4Cl: Excess of tetrabutyl-
ammonium chloride (TBACl) was added to a solution of D·4PF6 in
MeCN. Some orange solid was precipitated, which was collected by
filtration and washed by MeCN to afford R·8Cl in quantitative yield
as an orange powder.
cycles without decomposition in aqueous solution represents
a step forward in the design and operation of an artificial
molecular machine that can function in water. This demon-
stration opens up opportunities for developing integrated
nanobiomechanical systems in the direction of applications
such as molecular prosthetics.[18]
Experimental Section
R·8PF6: A solution of S3·2PF6 (77 mg, 0.061 mmol), S8·2PF6 (50 mg,
0.061 mmol), CBPQT·4PF6 (67 mg, 0.061 mmol), TBTA (9 mg,
0.017 mmol), and [Cu(MeCN)4]PF6 (6 mg, 0.017 mmol) in anhydrous
Me2CO (5 mL) was stirred for 24 h at room temperature. The solvent
was then evaporated off and the resulting orange solid was purified by
column chromatography (SiO2: 2m NH4Cl/MeOH/MeNO2 12:7:1),
then MeOH, Me2CO, and 2% NH4PF6/Me2CO, respectively. The
dark orange fraction in Me2CO was collected, and concentrated to a
minimum volume, before the crude product was precipitated by the
addition of H2O. The resulting solid was collected by filtration to
afford the [2]rotaxane R·8PF6 (80 mg, 41%) as an orange powder.
1H NMR (500 MHz, CD3CN, 233 K): d = 9.02–9.00 (m, 4H),8.93 (d,
J = 6.0 Hz, 2H), 8.83 (d, J = 6.0 Hz, 2H), 8.62–8.60 (m, 4H), 8.50 (d,
J = 7.0 Hz, 2H), 8.47 (d, J = 7.0 Hz, 2H), 8.41 (s, 1H), 8.37 (d, J =
5.5 Hz, 2H), 8.35 (d, J = 5.5 Hz, 2H), 8.30 (s, 1H), 8.10 (s, 1H), 8.05–
8.03 (m, 4H), 7.95 (br, 2H), 7.89 (br, 2H), 7.87 (br, 2H), 7.71 (s, 1H),
7.70–7.68 (m, 4H), 7.59 (d, J = 5.0 Hz, 1H), 7.53 (d, J = 5.0 Hz, 1H),
7.41–7.34 (m, 8H), 7.30 (d, J = 4.5 Hz, 1H), 7.23 (d, J = 5.0 Hz, 1H),
7.18 (br, 2H), 7.14–708 (m, 5H), 6.23 (d, J = 6.5 Hz, 1H), 6.20 (d, J =
7.0 Hz, 1H), 5.95–5.91 (m, 2H), 5.63–5.52 (m, 8H), 4.84 (br, 4H),
4.61(s, 2H), 4.51 (t, J = 6.5 Hz, 2H), 4.28–3.78 (m, 24H), 3.27 (septet,
J = 7.0 Hz, 2H), 2.47 (s, 3H), 2.29 (d, J = 7.0 Hz, 1H), 2.27 (d, J =
7.0 Hz, 1H), 1.87–1.15 (m, 8H), 1.15 ppm (d, J = 7.0 Hz, 12H).
13C NMR (125 MHz, CD3CN): d = 156.7, 156.6, 155.8, 152.4, 151.3,
151.3, 151.1, 151.0, 150.7, 150.6, 150.5, 150.2, 149.8, 149.4, 145.8, 145.2,
144.9, 144.3, 143.6, 143.2, 141.4, 137.4, 136.2, 131.0, 128.1, 127.7, 127.2,
126.8, 126.4, 125.9, 124.9, 124.7, 124.5, 124.0, 123.9, 123.8, 123.8, 123.5,
121.6, 107.9, 107.8, 104.0, 103.9, 98.4, 73.2, 70.5, 70.2, 70.2, 70.1, 69.8,
69.4, 69.3, 68.7, 68.3, 68.0, 68.0, 67.0, 64.7, 63.2, 61.5, 61.4, 54.0, 53.8,
49.2, 31.2, 30.9, 30.3, 30.0, 29.6, 29.2, 28.4, 25.7, 25.0, 24.6, 23.0, 19.9,
18.9 ppm. ESI-HRMS calcd for m/z = 915.5811 [MÀ3PF6]3+, found m/
z = 915.5855. Counterion exchange from R·8PF6 to R·8Cl: Excess of
tetrabutylammonium chloride (TBACl) was added to a solution of
R·8PF6 in MeCN. Some orange solid was precipitated, which was
collected by filtration and washed by MeCN to afford R·8Cl in
quantitative yield as an orange powder.
Received: April 11, 2011
Published online: June 29, 2011
Keywords: biomimicry · electrochemistry ·
.
mechanically interlocked molecules · molecular devices ·
template synthesis
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D·4PF6: A solution of S3·2PF6 (77 mg, 0.061 mmol), S8·2PF6
(50 mg, 0.061 mmol), TBTA (9 mg, 0.017 mmol), and [Cu-
(MeCN)4]PF6 (6 mg, 0.017 mmol) in anhydrous Me2CO (5 mL)
were stirred for 24 h at room temperature. The solvent was then
evaporated off and the resulting orange solid was purified by column
chromatography (SiO2: 2m NH4Cl/MeOH/MeNO2 12:7:1), then
MeOH, Me2CO, and 2% NH4PF6 in Me2CO, respectively. The
orange fractions were collected, and concentrated to a minimum
volume, before being precipitated by the addition of H2O. The
resulting solid was collected by filtration to afford D·4PF6 (30 mg,
24%) as an orange powder. 1H NMR (500 MHz, CD3CN): d = 8.96 (d,
J = 7.0 Hz, 2H), 8.84 (d, J = 6.5 Hz, 2H), 8.51(d, J=8.0 Hz, 2H), 8.49
(d, J = 3.5 Hz, 1H), 8.47 (d, J = 3.0 Hz, 1H), 8.43 (s, 1H), 8.39 (d, J =
7.0 Hz, 2H), 8.34 (d, J = 6.5 Hz, 2H), 8.32 (s, 1H), 8.08–8.03 (m, 4H),
7.75–7.71 (m, 7H), 7.61 (d, J = 5.5 Hz, 1H), 7.54 (d, J = 6.0 Hz, 1H),
7.41–7.37 (m, 4H), 7.35–7.32 (m, 2H), 7.26–87.23 (m, 2H), 7.16–7.09
(m, 3H), 6.94 (d, J = 7.5 Hz, 1H), 6.84 (d, J = 7.5 Hz, 1H), 4.87 (t, J =
5.0 Hz, 2H), 4.77 (s, 2H), 4.58 (s, 2H), 4.57 (t, J = 7.5 Hz, 2H), 4.32 (t,
J = 7.0 Hz, 2H), 4.29 (t, J = 4.5 Hz, 2H), 4.32–4.22 (m, 2H), 4.11 (t,
J = 4.5 Hz, 2H), 3.98–3.69 (m, 16H), 3.32 (septet, J = 7.0 Hz, 2H),
2.49 (s, 3H), 1.88–1.83 (m, 2H), 1.37–1.31 (m, 4H), 1.19 ppm (d, J =
6786 www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 6782 –6788

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[25] The absorption spectrum of R·8PF6, as well as that of its
dumbbell D·4PF6 (see Figure S7 and Figure S8 respectively),
recorded in MeCN show that this light-driven mechanical
movement can also take place in organic solvents.
[26] The timescale of the sacrificial-assisted light-stimulated reduc-
tion reaction and the consequent mechanical movement of the
ring component is relatively long as shown by both UV/Vis and
1H NMR spectroscopies. This observation can be attributed to
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generated by the reduction of the RuII stopper in its excited state.
For an example that demonstrates that a trace amount of oxygen
+
slows down the BIPYC generation by the reduction of excited
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[27] For comparison, we have performed a similar experiment but
instead of using light, we employed the reducing agent Na2S2O4.
After five cycles, almost no sign of the formation of a radical
cation could be observed in the absorption spectrum, an
observation which could possibly be explained by the decom-
position of R·8Cl.
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À0.3 V, while in water it is À0.44 V. In a similar fashion, the first
reduction potentials of both V2+ and CBPQT4+ as well as the 1:1
mixture are all shifted to more negative voltages in water
compared to those observed in MeCN. We hypothesize that
these observations are a result of the poorer ability of water to
Angew. Chem. Int. Ed. 2011, 50, 6782 –6788
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 6787

Communications
solvate less charged species (specifically the radical cationic and
occur at 1 mm in H2O, but not in the case of MeCN, a solvent in
which no precipitation is observed at that concentration. At the
slightly lower concentration of 0.6 mm in H2O, however, the
rotaxane remains soluble enough such that precipitation on the
neutral forms of the components) and, as a consequence, shift
the redox potentials consistently to more negative potentials in
the case of all the species.
[29] At 1 mm concentration, the rotaxane R8+ was observed to
precipitate on the electrode after the first three-electron
reduction process. This observation relates to our hypothesis
that water is not as able to support less charged species
compared to the situation in MeCN, causing precipitation to
electrode can be avoided.
+
[30] The trisradical complex BIPYC ꢀCBPQT2(C+) has
a single
unpaired radical, which renders it EPR active. The single
unpaired radical broadens all the NMR signals. For details, see
Ref. [20a].
6788 www.angewandte.org
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Angew. Chem. Int. Ed. 2011, 50, 6782 –6788