Light driven formation of a supramolecular system with three CB[8]s locked between redox-active Ru(bpy)3 complexes

Article abstract of DOI:10.1039/b910112h

Three CB[8]s have been reversibly locked between two Ru(bpy) ₃-viologen complexes by light driven electron transfer reactions.

Full text of DOI:10.1039/b910112h

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Light driven formation of a supramolecular system with three CB[8]s locked
between redox-active Ru(bpy)3 complexes†
Samir Andersson,a Dapeng Zou,a Rong Zhang,b Shiguo Sunb and Licheng Sun*a
Received 21st May 2009, Accepted 12th June 2009
First published as an Advance Article on the web 13th July 2009
DOI: 10.1039/b910112h
Three CB[8]s have been reversibly locked between two Ru(bpy)3–viologen complexes by light driven
electron transfer reactions.
Introduction
The ability to construct and manipulate molecular systems on
the atomic level is the key to creating molecular machines and
switches. In recent years great effort has been made towards the
construction of supramolecular systems such as wires, motors and
switches.1 The cucurbit[n]uril host moiety has been explored due to
its excellent binding affinity between its cavity edges and positively
charged molecules.2 We have recently presented the interaction
between the dimethylviologen (DMV)–viologen (MV) system to
show movement of a CB[8] host with electrochemical methods,3 as
well as a light driven system composed of Ru–MV2+ÃCB[8].4 We
became interested to know whether the same type of interactions
can be performed on more complex systems. The Ru–DMV2+–
MV2+ type systems have previously been investigated as molecular
motors with crown ethers,5 but no research work has to our
knowledge so far been presented with cucurbituril hosts. Here,
we report on the first light driven system, composed of molecular
triad 1 and CB[8] (Chart 1), where a reversible [5]rotaxane
can be formed from two pseudorotaxanes consisting of two
ruthenium sensitizers as blockers and three CB[8]s. The triad
1 was synthesized by following the procedures described in
Fig. 1 1H NMR spectra (500 MHz, D2O) of the triad 1 at the
concentration of 2.9 mM with CB[8]. (a) 1 only; (b), (c) and (d) show
1 with 0.5, 1.0 and 2.0 equivalents of CB[8] respectively (see ESI for full
spectra).
the literature for the synthesis of similar compounds and fully
characterized by NMR spectroscopy and electrospray ionization
mass spectrometry (ESI MS) (see ESI†).
Interaction of triad 1 with CB[8]
to 1.68 ppm while the protons of the carbon chain a–d and
The inclusion complex between the guest 1 and host CB[8] was
e–g as well as protons 3, 5 and 6 exhibit a downfield shift (see ESI).
analysed by the 1H NMR spectral changes (see Fig. 1) and COSY
A downfield shift can also be distinguished on the a3 and b3
spectra (see ESI†). When up to one equivalent of CB[8] is added
to the solution of 1, the b1 and b2 protons of the DMV moiety
exhibit an upfield shift from 8.00 and 7.98 to 6.85 and 6.79 ppm
respectively, while the a1, a2, a1¢ and a2¢ shift from 8.91, 8.99,
protons of the MV moiety from 9.22 and 8.51 to 9.34 and 8.64 ppm
respectively.
These findings correlate well with previously reported data.3 In
the presence of two equivalents of CB[8] the 1H NMR of 1 (Fig. 1d)
9.00 and 9.09 to 8.74, 8.81, 8.47 and 8.48 ppm (Fig. 1a→c).
showed that the protons at carbon d were moved upfield from 4.88
The b-methyl groups move characteristically upfield from 2.37
to 4.85 ppm while no change could be seen at the e, f and g protons
of the carbon linker. The protons of the DMV moiety had a split
effect where the a2 and a2¢ shifted strongly downfield from 8.81
aDepartment of Chemistry, School of Chemical Science and Engineering,
Royal Institute of Technology (KTH), 100 44 Stockholm, Sweden. E-mail:
and 8.48 to 8.93 and 8.69 ppm, while no effect was seen at a1 and
a1¢. A much stronger downfield shift of the protons a, b, c of the
MV–DMV linker was seen, as well as strong upfield shifts of the
a3, a4, b3 and b4 protons of the MV moiety from 9.34, 9.06, 8.64
and 8.55 ppm to 9.05, 8.67, 7.65 and 7.54 ppm. These data indicate
that the CB host moved slightly away from the DMV moiety and
lichengs@kth.se; Fax: +46 8 7912333
bState Key Laboratory of Fine Chemicals, DUT–KTH Joint Education and
Research Center on Molecular Devices Dalian University of Technology
(DUT), ZhongShan Road 158-40, 116012, Dalian, China
† Electronic supplementary information (ESI) available: (1) Characteriza-
tion data for triad 1, (2) electrochemistry of the triad 1 and (3) its full 1H
NMR data. See DOI: 10.1039/b910112h
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Chart 1 Chemical structures of triad 1 and CB[8].
toward the Ru moiety as depicted in Scheme 1, most likely due
High resolution mass spectrometry (HRMS)
to steric effects between the two CB[8]s. This result fits well with
reported data where the CB host shows an increased ability to
move away from the center of an alkylated viologen compared to
methylviologen.6
The interactions between CB[8] and 1 have further been analyzed
by HRMS where the formation of 1 : 1 and 1 : 2 inclusion
complexes were confirmed. With equivalent amounts of 1 and
CB[8] in the solution, the ESMS gave a sextuply charged peak at
m/z 394.1364 (calculated for [1 + CB[8] - 6Cl-]6+, 394.1355), which
corresponds to the inclusion of the 1 into the cavity of a CB[8].
When two equivalents of CB[8] were added to a solution of 1, the
ESMS gave a sextuply charged peak at m/z 616.7490 (calculated
for [1 + 2 CB[8] - 6Cl-]6+, 615.5342), which corresponds to the
inclusion of the 1 into the cavities of two CB[8]s. Both 1H NMR
and HRMS results thus provide strong evidence for the formation
of the 1 : 1 and 1 : 2 host–guest complexes between 1 and CB[8].
Electrochemistry
∑
Electrochemical study is used as a strong indicator for MV+
dimer formation.2 The cyclic voltammogram of 1 in acetonitrile
shows typical oxidation peaks of Ru(II) and reduction peaks of
DMV and MV (Fig. S5†). Differential pulse voltammetry (DPV)
measurements in water performed on 1 showed four reduction
∑
∑
peaks which are assigned to MV2+ → MV+ , DMV2+ → DMV+ ,
∑
∑
MV+ → MV0, DMV+ → DMV0 (see Fig. 2).
The process with zero and two equivalents of CB[8] was
investigated in aqueous solvent and scanned at 0.05 V s-1. With two
equivalents CB[8] added to the solution of 1, the first reduction
∑
peak, corresponding to the reduction of MV2+ → MV+ , shifts
to a more positive value at -0.40 V vs. Ag/AgCl. The second
reduction peak of the MV moiety shifted to a more negative value
at ca. -1.20 V vs. Ag/AgCl (see Fig. 2). These types of shifts are in
accordance with how the reduction potentials of methylviologen
shift when forming the radical dimer inside a CB[8] cavity.2
It is worth noting that the positive shift of the reduction
potential is smaller compared to a similar reaction with free
Scheme 1 Schematic presentation of the interactions between 1 and CB[8]
based on 1H NMR spectra (500 MHz, D2O). The arrows indicate the slight
movement of the CB[8].
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Fig. 3 UV/Vis spectra of 1 (4 ¥ 10-5) in water solution, with TEOA as sac-
rificial electron donor: (a) before irradiation (solid); (b) after 50 min light
irradiation (dashed); (c) with 2 equiv CB[8] before irradiation (dotted);
(d) after 50 min light irradiation with 2 equiv CB[8] (dotted–dashed).
Fig. 2 Differential pulse voltammograms of 1 (solid line) and 1 with two
equivalents CB[8] (dotted–dashed) in pH 7 buffer solution at 1 mM, scan
∑
rate 0.05 V s-1. The peaks are assigned as: (a) MV2+→ MV+ ; (b) DMV2+
→
∑
∑
∑
DMV+ ; (c) MV+ → MV0 and (d) DMV+ → DMV0. With two equivalents
∑
∑
∑
CB[8]: (a¢) MV2+→ MV+ ; (b¢) DMV2+→DMV+ ; (c¢) MV+ → MV0.
MV2+Ã[CB[8].3–4,6 The second reduction peak, corresponding to
∑
the reduction of DMV2+→DMV+ , has a remarkable shift of
-250 mV compared to its potential in the free triad. At this time
the shift is assumed to be due to the synergism of two interactions:
(1) the DMV2+ moiety inside the cavity CB[8] and (2) the proximity
∑
to a formed MV+ dimer. The second reduction peak of the DMV
∑
moiety, DMV+ → DMV0, is no longer detectable within the
solvent window.
Photochemistry
∑
To confirm the formation of the MV+ radical dimer inside
CB[8], photochemistry was performed on 1 in the presence of
triethanolamine (TEOA) as sacrificial electron donor. For the
aqueous solution of 1 (4 ¥ 10-5 M) and TEOA (4 ¥ 10-2 M),
two absorption peaks at 398 and 600 nm could be observed after
50 min light irradiation. These peaks are known and attributed to
the formation of the free Ru2+–DMV2+–MV+∑ radical (see Fig. 3b).
With the addition of two equivalents of CB[8] to the solution of
Fig. 4 1H NMR spectra (500 MHz, D2 O) of triad 1 and TEOA before
irradiation (a) and after 5 min irradiation (b).
measurements, the reduction of both the DMV and the MV
moiety can be seen from the disappearance of the a and b peaks
corresponding to either viologen moiety from the normal NMR
range of these compounds, indicating radical formation (Fig. 4b).
The NMR peaks corresponding to the carbon linker and the
N-methyl group also disappeared while the peaks of the ruthenium
moiety were clearly visible, see Fig. S3 for full spectra.6† When two
equivalents of the host CB[8] were added (Fig. 5a) and irradiated
with light (Fig. 5b) no NMR shift could be observed on the peaks
corresponding to the DMV moieties, clearly indicating that the
DMV moieties are still inside and stabilized by a CB[8] host.
The peaks corresponding to the a and b protons of the MV
moiety become broadened and slightly shifted in position. This
is typical behavior of the MV moiety after forming a stabilized
radical dimer inside a CB[8] host.6
∑
1 and TEOA, no peaks for the free Ru2+–DMV2+–MV+ radical
could be observed after irradiation (Fig. 3d). In this case, however,
a very strong peak at 370 nm and a broad peak around 540 nm as
well as a broad peak at 870 nm were observed. The blue-shifted
∑
peaks are attributed to the formation of a MV+ dimer. The two
MV moieties are now “locked” inside the cavity of one CB[8],
∑
forming Ru2+–DMV2+(CB[8])–(MV+ )2(CB[8])–DMV2+ (CB[8])–
Ru2+ (see Scheme 2). This is in agreement with previously reported
data.4 Molecular oxygen can quench the radical dimer leading to
the original form of Ru2+–DMV2+(CB[8])–MV2+(CB[8]).
The whole process can be repeated several times without
significant photo degradation.
To establish that no losses of cucurbituril occurred when
the rotaxane was formed, i.e. giving [3] or [4]rotaxanes, 1H
NMR measurements were performed. First a reference test was
performed with compound 1. After addition of TEOA as a
sacrificial donor and degassing with nitrogen (Fig. 4a) the NMR
The NMR spectrum clearly shows that both viologen moieties
∑
are inside CB[8] hosts when the MV+ radical dimer is formed.
This can, in combination with the electrochemical shift of the
MV moiety, only be due to the formation of a [5]rotaxane with
∑
the DMV2+ moieties as well as the MV+ radical dimer inside the
1
tube was irradiated as previously described. From the H NMR
cavity of CB[8] host, see Fig. S4 for the entire spectrum.†
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Scheme 2 Schematic representation of the light driven interaction between 1 and CB[8], forming the [5]rotaxane.
reduction via intramolecular electron transfer, the formation of
∑
the MV+ radical dimer inside a CB[8] cavity can be achieved,
∑
forming a [5]rotaxane. Molecular oxygen can quench the MV+
radical and the original structure can be reformed. This result
shows that cucurbituril can be used and easily controlled by light
even in more complex systems.
Experimental procedures
General
NMR spectra were obtained on a Bruker AVANCE 500 MHz
spectrometer, using TMS as internal standard. High Resolution
Mass Spectrometry (HRMS) was conducted on a Q-Tof mass
spectrometer (Micromass, Manchester, England) equipped with
z-spray ionisation source.
Fig. 5 1H NMR spectra (500 MHz, D2O) of triad 1 with two equiv CB[8]
and TEOA before irradiation (a) and after 10 min irradiation (b).
Conclusions
Electrochemistry
The interactions of CB[8] with the molecular triad 1, where the
two viologens are covalently linked via different carbon chains
to a ruthenium moiety acting both as a stopper and a photo-
sensitizer, has been studied by 1H NMR, ESI-MS, electrochemistry
and photochemistry. Our results demonstrate that 1 equiv CB[8]
can selectively bind to the DMV2+ moiety on triad 1. A second
CB[8] can be positioned on the MV moiety. By photochemical
Cyclic voltammetry was carried out with a three-compartment cell
connected to an Autolab potentiostat with a GPES electrochem-
ical interface (Eco Chemie). The working electrode was a freshly
polished glassy carbon disk (diameter 3 mm), the counter electrode
was a platinum wire. Potentials were measured vs. a non-aqueous
Ag/Ag+ reference electrode (CH instruments, 10 mM AgNO3 in
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acetonitrile) and can be converted to SCE by adding 300 mV to
the measured value. Solutions were prepared from dry acetonitrile
containing ca. 0.5 or 1 mM of the analyte, as defined in the
text and 0.1 M tetrabutylammonium hexafluorophosphate (Fluka,
electrochemical grade, dried at 373 K for 48 h) as supporting
electrolyte. Before all measurements, oxygen was removed by
bubbling the solutions for 20 min with solvent saturated argon
and the samples were kept under argon atmosphere during the
measurements. In water solution the counter ions used were Cl-
and the reference electrode was Ag/AgCl in KCl. The supporting
electrolyte was a pH 7 phosphate buffer solution and can be
converted to SCE by subtracting 45 mV to the measured value.
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Acknowledgements
We acknowledge the Swedish Research Council, Swedish Energy
Agency, and the K & A Wallenberg foundation for financial
support of this work.
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The Royal Society of Chemistry 2009
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