A mechanically interlocked [3]rotaxane as a light-harvesting antenna: Synthesis, characterization, and intramolecular energy transfer

Article abstract of DOI:10.1002/chem.200802228

A mechanically interlocked light-harvesting system [3]rotaxane A has been synthesized in high yield through Cu(I)-catalyzed azide-alkyne cycloaddition; the hexyl-substituted truxene units are introduced into the wheels as donors and an oligo(para-phenylen

Full text of DOI:10.1002/chem.200802228

FULL PAPER
DOI: 10.1002/chem.200802228
A Mechanically Interlocked [3]Rotaxane as a Light-Harvesting Antenna:
Synthesis, Characterization, and Intramolecular Energy Transfer
Jie-Yu Wang, Ji-Min Han, Jing Yan, Yuguo Ma,* and Jian Pei*[a]
Abstract: A mechanically interlocked
light-harvesting system [3]rotaxane A
has been synthesized in high yield
through Cu(I)-catalyzed azide–alkyne
cycloaddition; the hexyl-substituted
truxene units are introduced into the
wheels as donors and an oligo(para-
phenylenevinylene) (OPV) unit into
the axis as the acceptor. The structure
and the purity of [3]rotaxane A were
confirmed by 1H and 13C NMR spec-
troscopy and ESI HRMS. The azide–
alkyne cycloaddition is demonstrated
to be an efficient stoppering method in
the synthesis of the rotaxane contain-
ing dibenzo[24]crown-8 and dibenzyl
ammonium units. Detailed steady-state
UV/Vis absorption, photoluminescent,
and time-resolved fluorescence spec-
troscopy were performed to investigate
the photophysical properties of [3]ro-
taxane A and its reference compounds
in solution and as thin films. Even in
dilute solution, efficient energy transfer
from the truxene-functionalized wheels
to the OPV-based axis, through the di-
benzo[24]crown-8 and dibenzyl ammo-
nium interaction, is observed in [3]ro-
taxane A. The unique topology of
[3]rotaxane A not only efficiently pro-
motes the intramolecular energy-trans-
fer process, but also prevents intermo-
lecular aggregation in the solid state.
The new antenna system opens up the
possibility of controllable light-harvest-
ing molecular machines or other opto-
Keywords: aggregation
·
click
chemistry · cycloaddition · energy
transfer · molecular devices · oligo-
AHCTUNGTREG(NNUN phenylenevinylene)
AHCTUNGTRENGNeUN lectronic devices on the nanometer
scale.
Introduction
an outer antenna complex LH2 and an inner antenna com-
plex LH1 to collect solar energy and transfer it to the reac-
tion center.[1b] Mimicking such an antenna effects has at-
tracted considerable interest both for fundamental research
and practical applications.[2–4]
Several light-harvesting systems, mainly using shape-per-
sistent p-conjugated dendrimers, have been developed in
our previous contributions. In these systems the light ab-
sorbed by the outer chromophores is transferred efficiently
and unidirectionally toward the core.[5] However, in nature,
the donor and acceptor are held in close proximity by non-
covalent interactions rather than covalent bonds.[1b] Nonco-
valent interactions such as hydrogen bonding or p–p stack-
ing are more versatile and can be modulated easily by the
environment, thus offering a potential advantage over cova-
lent bonds.[6]
On the other hand, mechanically interlocked molecules
such as rotaxanes and catenanes have usually been used for
the construction of artificial molecular machines.[7] For ex-
ample, the controllable movement of individual components
in rotaxanes or catenanes has been employed widely to
mimic machine-like action on the molecular scale.[8] More-
over, special photophysical and electrical properties of some
interlocked molecules also provide the possibility of photo-
Most life on earth relies directly or indirectly on photosyn-
thesis, whereby solar energy is collected and funneled
through an energy gradient to the reaction center at which
charge separation occurs.[1] In this highly efficient process,
excitation energy transfer (ET) plays an important role. For
example, in the natural photosynthetic unit in plants, carot-
ACHTUNGTRENNUNGenoid molecules absorb in the region at which chlorophyll
molecules do not, and transfer the excitation energy to the
chlorophyll through singlet-singlet energy transfer, much
like antennas.[1c] Similarly, bacterial photosynthetic units use
[a] J.-Y. Wang, J.-M. Han, J. Yan, Prof. Y. Ma, Prof. J. Pei
Beijing National Laboratory for Molecular Sciences
The Key Laboratories of Bioorganic Chemistry and
Molecular Engineering, and
Polymer Chemistry and Physics of Ministry of Education
College of Chemistry, Peking University
Beijing 100871 (China)
Fax : (+86)10-6275-8145
E-mail: jianpei@pku.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802228.
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chemically or electrochemically active molecular devices.[9]
Some rotaxane/catenane structures constructed from por-
phyrin and fullerene derivatives have been developed to
mimic the charge-separation step in photosynthesis.[10] In
contrast, the photoinduced ET process has rarely been real-
ized in a mechanically linked
Results and Discussion
Molecular design and overall synthetic strategy: The struc-
tures of [3]rotaxane A, along with the reference compounds
B (axis) and C (wheel) are shown here. To demonstrate the
system,[11] possibly hampered by
the synthesis and purification of
the complex rotaxane/catenane
structures. Mechanical linkage
of chromophores has several
advantages over covalent bond-
ing. First, the ET between the
donor and the acceptor is not
complicated by through-bond
interactions, a situation favor-
ACHTUNGTRENNUNGable for Fçrster-type processes
in which the ET occurs through
space.[1c] Second, due to the re-
sponse of the mechanical link-
age to outside signals, a switch-
able energy-collecting system
can be constructed.
Among the known motifs,
the interaction between the di-
benzo[24]crown-8 and dibenzyl
ammonium (DCDA),[12] pio-
neered by Stoddart, has been
proved to be a reliable building
style for mechanical assemblies.
Based on hydrogen bonding as
well as p–p stacking interac-
tions, the binding motif is sensi-
tive to the property of the solvent and the temperature. Few
stoppering methods have been reported for rotaxanes incor-
porating DCDA interactions.[13] On the other hand, the well-
known Cu(I)-catalyzed azide–alkyne cycloaddition (“click”
reaction) has been successfully applied to various complex
structures due to its high efficiency and functional group tol-
erance.[14] Thus, we also wish to explore the possibility of ap-
plying the “click” reaction to the construction of complex
systems with DCDA interactions.
In this contribution we wish to report: 1) the design and
synthesis of [3]rotaxane A containing two hexyl-substituted
truxene units as the antenna groups and an oligo(para-phe-
nylenevinylene) (OPV) moiety unit as the energy-collecting
core and 2) a detailed investigation into its energy collecting
ability, especially the photoinduced ET process in this com-
plex system. We also wish to highlight the successful applica-
tion of Cu(I)-catalyzed azide–alkyne cycloaddition in the
construction of rotaxane using the DCDA interaction. Also,
we find that the special topology gives A desirable photo-
physical properties in the solid state, paving the way for its
application in light-harvesting materials.
idea of a mechanically interlocked antenna system, we care-
fully chose the donor, acceptor, mechanical linkage, and
stoppering group. We identified the truxene skeleton as an
ideal antenna unit, due to its easy functionalization, high
fluorescence quantum efficiency, and suitable photophysical
properties.[5,15] The alkyl chains introduced into the C-5, C-
10, and C-15 positions of the truxene core not only greatly
improve its solubility, but also prevent its intermolecular ag-
gregation in the solid state to some extent. As for the ac-
ceptor, OPV derivatives attracted our attention due to their
tunable photophysical and electrical properties, as well as
their self-assembly behavior through p–p stacking.[16] Again,
two dodecyloxy chains were introduced to the OPV skeleton
in consideration of its solubility in common organic solvents.
The DCDA interaction was chosen as the mechanical link-
age for its robustness, and Frꢁchet-type dendrons were
chosen as the stoppering units for synthetic simplicity. Sever-
al synthetic strategies have been developed for the rotaxane
structure, but we concentrate on the “threading-followed-
by-stoppering” approach,[12c] which is the most feasible one
in our case. Because polar solvents and high temperatures
decrease the binding constant of DCDA, both the threading
and the stoppering steps should be performed in a nonpro-
tonic solvent with low polarity (such as CH2Cl2) and under
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A [3]Rotaxane as a Light-Harvesting Antenna
FULL PAPER
Scheme 1. Synthetic route to C.
relatively low temperature. Therefore, for the key step of
the stoppering reaction, we employed azide–alkyne cycload-
dition due to its mild and efficient nature.[14]
A was essentially a one-pot synthesis in which the threading
and stoppering processes were performed in one step. The
1
validity of such a strategy was confirmed by H NMR meas-
urements before the stoppering step. Upon mixing 11 with
the donor unit C in CH2Cl2 at a concentration of 10ꢀ2 m, evi-
dent threading, though not complete, was observed. More-
over, such a process was very fast.[12a] Therefore, we em-
ployed reaction conditions similar to that for B, except that
a large excess of C was present to ensure complete thread-
ing. We reasoned that threading should proceed well before
the click reaction took place. After trying many catalytic
Synthesis and structural characterization: Scheme 1 outlines
the synthetic approach to the donor C. A Suzuki coupling
between monobromide 1 and boronic ester 2 catalyzed by
[Pd
with BBr3 in CH2Cl2 produced the diol 4. Cyclization of 4
with 1,2-bis[2-[2’-(2’’-tosyloxyethoxy)ethoxy]ethoxy]benzene
ACHTUNGTRENNUNG(PPh3)4] afforded 3 in 77% yield. Demethylation of 3
ACHTUNGTRENNUNG
(5) in a suspension of K2CO3 and DMF afforded C. The
overall yield for the above two steps was about 60%.
systems, [CuACTHNUTRGNEUNG(MeCN)4]PF6 turned out to be the most effec-
Scheme 2 illustrates the synthetic approach to reference
compound B. Condensation of dialdehyde-substituted OPV
derivative 6 and benzylamine 7 in refluxing toluene afforded
imine 8 in quantitative yield. Reduction of 8 with NaBH4 in
a mixed solvent of MeOH and CH2Cl2 gave 9 in 74% yield.
tive one for the click reaction. The success of the one-pot
synthesis could be inferred from the H NMR spectrum of
1
the crude product. However, purification of A still posed a
great practical difficulty, as often encountered in the synthe-
sis of rotaxane. Flash chromography, HPLC, and recrystalli-
zation all failed to separate A from excess C. Eventually,
preparative GPC was found to be an effective separation
method due to the large size difference between A and C.
The isolated yield of A was 80%, fairly acceptable for such
large and complex [3]rotaxane, which relies on the large
binding constant of the DCDA in CH2Cl2 as well as the effi-
cient cycloaddition reaction between azide and alkyne cata-
lyzed by CuI.
[CuACHTUNGTRENNUNG(MeCN)4]PF6 was chosen as the catalyst for the click re-
action between 9 and azide 10 in CH2Cl2, for reasons de-
tailed later. After reaction for two days at room tempera-
ture, through acidification with 6m HCl, and anion exchange
with NH4PF6, compound B was obtained in 62% yield.
The synthesis of [3]rotaxane A is shown in Scheme 3.
First, acidification of 9 with 6m HCl followed by exchange
of the anion produced dibenzyl ammonium 11, containing
two terminal alkyne groups. The key step in the synthesis of
Compounds A, B, and C were all readily soluble in
CH2Cl2, CHCl3, CH3CN, and
acetone. Their structures and
1
purity were verified by H and
13C NMR spectroscopy and
HRMS. Figure 1 shows the
1H NMR spectrum of [3]ro-
taxane A. The broad signals at
d=4.0–4.2 and 4.4–4.6 ppm
were assigned to protons H10
and H11, respectively, on the
axis. These typical signals dem-
onstrated the successful thread-
ing of the dibenzyl ammonium
into the dibenzo[24]crown-
8.[13a,17] Protons on the crown
ether with chemical shifts from
d=3.3 to 4.5 ppm were clearly
split into three groups, in which
part of Hr and Hs overlapped
Scheme 2. Synthetic route to B.
Scheme 3. Synthetic route to A.
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J. Pei, Y. Ma et al.
and the two reference compounds B and C in dilute solu-
tion. Compound C showed the absorption and photolumi-
nescence (PL) features of a phenyl-substituted truxene
unit.[15c] The absorption spectrum of B showed two distinct
absorption bands with lmax at 330 and 395 nm (Figure S1 in
the Supporting Information). Importantly, the absorption
spectrum of B had a large overlap with the emission spec-
trum of C, which was ideal for fluorescence resonance ET
through interaction between transition dipoles.[18] The emis-
sion maximum lmax of B peaked at 450 nm. To further illus-
trate the importance of mechanical linkage on the ET pro-
cess, we mixed B and C in a 1:2 molar ratio to give another
reference system H. The absorption features of A were
almost identical to that of H, which could be constructed by
addition of the absorption spectra of B and C. This result in-
dicated that no ground-state interaction existed for either H
or A. (Figure S2 in Supporting Information).
Figure 1. 1H NMR spectrum of [3]rotaxane A.
Figure 3 compares the emission spectra of compounds A,
B, C, and the mixed system H at the same concentration
(1ꢂ10ꢀ6 m) in CH2Cl2 at room temperature under the same
with H10 and H11.[13a,17] The simple and clear splitting pattern
reflects the overall C2 symmetry of the rotaxane structure.
The proof for the successful click reaction was the appear-
ance of protons on the triazole rings with chemical shift at
around d=8.0 ppm. Figure 2 shows the ESI MS spectrum of
Figure 3. The emission spectra of A, B, C, and H (the mixture of B and C
(1:2)) in CH2Cl2 (1ꢂ10ꢀ6 m) at room temperature. All emission spectra
were collected with an excitation wavelength at the absorption maximum
of C at 310 nm.
excitation wavelength (310 nm). The emission maximum
lmax of [3]rotaxane A peaked at 456 nm corresponding to
that of B, and the fluorescence of C from 330 to 390 nm
became very weak. This indicated a highly efficient intramo-
lecular ET process from the truxene unit to the OPV unit
on the axis. However, for mixed system H, its emission fea-
tures appeared as a weighted addition of B and C, and their
emission intensity was almost identical. Comparison be-
tween the emission behaviors of A and H clearly demon-
strated the importance of the mechanical linkage between
the donor and acceptor, which greatly shortened the dis-
tance between the chromophores and thus promoted Fçr-
ster-type ET processes through space. Therefore, in the
mixed system H, B, and C acted independently as non-inter-
acting chromophores, which rules out the possibility of trivi-
al ET in a simple mixture of B and C.[19]
Figure 2. ESI MS spectrum of [3]rotaxane A. Shown in the inset are the
ESI HRMS spectra of a) experimental results, b) calculated results.
[3]rotaxane A, in which the sharp peak at m/z=2138.4 cor-
ꢀ
responds to A2+ after loss of two PF6 units. Neither signal
for the side product B nor that of monowheel [2]rotaxane
was observed. The shape of the experimental result of the
ESI HRMS spectrum was almost identical to the theoreti-
cally predicted pattern, which again confirmed the identity
of A.
Figure 4 compares the emission spectra of A and H excit-
ed under different wavelengths. As shown in Figure 4 (left),
[3]rotaxane A showed excitation-wavelength-independent
Photophysical properties in solution: We first investigated
the steady-state photophysical properties of [3]rotaxane A
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A [3]Rotaxane as a Light-Harvesting Antenna
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similar OPV derivatives.[20]
Note that, due to the weak ab-
sorption in the short wave-
length region,
a lifetime of
1.51 ns could also be obtained
when exciting B at 310 nm. For
mixture H, we chose the excita-
tion wavelength as 310 nm and
monitored the fluorescence
decay at 370 and 441 nm, corre-
sponding to the emission
maxima of C and B, respective-
ly. The results were consistent
with the previous observation
that B and C acted as inde-
Figure 4. The emission spectra of A (left) and H (right; the mixture of B and C (1:2)) in CH2Cl2 (1ꢂ10ꢀ6 m))
recorded at different excitation wavelengths at room temperature.
emission behavior, meaning that all excitons generated in A
eventually go to one chromophore,[5b] the OPV unit in this
case. In addition, the emission intensity of A, when excited
at absorption lmax of C, was more than two times greater
than the intensity when excited at absorption lmax of B itself.
Such an antenna effect was attributed to the larger absorp-
tion coefficient of the wheel at shorter wavelength as well as
its larger number. In contrast, as shown in Figure 4 (right),
the mixed system H showed different emission features
when excited at different wavelengths. Under excitation at
the absorption maximum wavelength of B (398 nm), we did
not observe that the emission from the B moiety was dra-
matically enhanced. This was readily explained by the inde-
pendence of the donor and acceptor chromophores in the
mixed solution.
pendent fluorescent chromophores in solution: the decay
profile monitored at 370 nm was almost identical to that of
C alone, whereas the decay profile at 441 nm contained two
components. The shorter lifetime (1.37 ns), which dominated
the first 20 ns, corresponds to B, while the longer one
(11.55 ns) resulted from the tailing of the emission of C into
the long wavelength region. The contribution of these two
components was similar (54.1% for the short life time and
45.9% for the longer one).
Next, we turned to [3]rotaxane A. Due to the very weak
residual emission near 370 nm, we could not obtain the life-
time information for A at this wavelength. When excited at
310 nm and monitored at 456 nm, a decay curve very similar
to that of B was obtained. Note that the contribution from
the wheel, which could be observed clearly in the mixture
H, disappeared completely. This meant that, before the ex-
cited states of the wheel underwent radiative decay, another
faster process occurred, for example, ET to the axis. Ac-
cording to simple kinetic models, the fluorescence signal of
the acceptor should follow a biexponential function, show-
ing one growth term and one decay term.[19] However, in
our case, we did not obtain kinetic information for the
growth. This is because the ET process is much faster than
the fluorescent decays of both the donor and the acceptor,
so the contribution of the first term in the function is mani-
fested mainly in the very beginning of the decay profile, for
which the response of the instrument has significant interfer-
ence.[19b,21]
To give more insight into such ET process, time-resolved
fluorescence spectroscopy was performed to obtain the fluo-
rescence lifetime of [3]rotaxane A, B, and C, as well as the
mixture H in CH2Cl2, by using a time-correlated photon-
counting instrument. The fluorescence lifetimes of [3]ro-
taxane A and references B, C, and H are shown in Table 1.
Table 1. Excitation wavelength (lex), monitoring wavelength (lem) and
fluorescence lifetime (t) of A, B, C, and H obtained from the time-re-
solved fluorescence measurement in CH2Cl2 solution (10ꢀ7 m).
lex [nm]
lem [nm]
t [ns][a]
c2
A
B
310
310
395
310
310
310
456
450
450
370
370
441
1.53
1.51
1.53
11.09
1.279
0.937
1.003
1.104
1.193
1.093
C
H
Discussion of the ET process: To quantify the energy trans-
fer efficiency, we compared the fluorescence quantum yields
of the donor in the presence and absence of acceptor using
Equation (1), in which Qdonor and Qacceptor correspond to the
fluorescence quantum efficiency of the donor in the absence
and presence of the acceptor, respectively.
12.14
1.37 (54.1%), 11.55 (45.9%)
[a] Time-resolved fluorescence of all compounds except H shows mono-
exponential decay. The percentage in parentheses indicates the contribu-
tion from each lifetime component.
QdonorðacceptorÞ
When excited at 310 nm and monitored at their emission
maximum, the decay profiles of C and H were single expo-
nential with a lifetime of 11.09 ns, typical of truxene deriva-
tives.[5b] For reference B, when excited at its absorption lmax
and monitored at its emission maximum, it showed a single
exponential decay with a shorter lifetime of 1.53 ns, close to
ð1Þ
ꢀET ¼ 1ꢀ
Qdonor
The ET quantum yield of [3]rotaxane A was calculated to
be 97% on the basis of the measurement shown in Figure 3.
Moreover, the ET rate could be deduced from Equa-
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J. Pei, Y. Ma et al.
tion (2).[1c]
1
tdonor
pounds exhibited good film-forming properties. For wheel
C, the absorption spectra in dilute solution and in thin film
were almost identical, except for a tail into 600 nm, consis-
tent with previous results. For B, a slight red-shift of lmax
and considerable spectrum broadening were observed, indi-
cating intermolecular aggregation to some extent.[25] Axis B
in the mixture H showed similar evidence of aggregation.
However, for [3]rotaxane A, such aggregation was com-
pletely suppressed, as demonstrated by the same absorption
spectra in dilute solution and in film. This clearly showed
the ability of the two wheels to protect the central OPV
unit, which usually has a strong tendency to p–p stack. Such
a phenomenon is reminiscent of molecular wires with large
dendrons, in which the conjugated polymer is shielded from
the environment by the dendrons.[2a,22,26] In our previous
contributions, we employed a shape-persistent dendrimer
scaffold or three-dimensional rigid structure to solve the ag-
gregation problem.[5,15,24] Here, we have provided another
solution. The strategy of threading a conjugated dumbbell
through one or more macrocycles has been used previously
to prevent such aggregation;[27] however, the macrocycles
used were usually photophysically inert compounds, such as
cyclodextrin. In our case, the protection is afforded by me-
chanically connected chromophores. In other words, wheel
C has dual functions, namely an optic function (to collect
light energy and deliver it to the center) and a protective
function.
ꢀ
ꢁ
1
kET
¼
ð2Þ
1=ꢀETÞꢀ1
In this way, the ET rate was calculated to be 2.92ꢂ109 sꢀ1.
The high ET efficiency and the fast rate can be ascribed to
the large overlap between the emission spectrum of the
donor and the absorption spectrum of the acceptor, as well
as the relatively short distance between the donor and the
acceptor in A as compared with those in H.
To further clarify the photophysical process inside [3]ro-
taxane A, we employed the classical ET theory to analyze
the experimental results. The mechanical linkage in A pre-
cludes the possibility of a Dexter-type ET process, in which
spatial overlap between the orbital of the donor and the ac-
ceptor is required.[22] Also, the absence of ET in H rules out
the possibility of a trivial process. Therefore, we identify the
photophysical process in A as a Fçrster-type ET process.
Basically, this mechanism operates through the coulombic
interactions between the donor and the acceptor transition
dipoles.[1c,18] In Fçrster-type processes, the ET rate can be
predicted by Equation (3), in which fdonor and tdonor are the
fluorescence quantum yield and the excited singlet state life-
time of the donor in the absence of the acceptor, respective-
ly, n is the index of refraction of the solvent (CH2Cl2), and
N is Avogadroꢃs number.
The emission spectra, collectively shown in Figure 5, dem-
onstrate such a shielding effect more clearly. The emission
features of C in thin film became broad with lmax red-shifted
9000ðln 10Þk2ꢀdonor
¼
128p5n4NtdonorR6
J
ð3Þ
kET
The value of fdonor of A was calculated to be 74% with
9,10-dibenzoanthracene in hexane used as the standard. The
overlap integral J was calculated to be 7.24ꢂ10ꢀ14 cm6 molꢀ1.
The parameter k2 is a function of the relative orientation of
the transition dipole moments, and was assigned a value of
2/3.[1c] The only unknown parameter in Equation (3) is R,
the distance between the centers of the two dipoles. By sub-
stituting all known parameters into Equation (3), we ob-
tained a value of 24.8 ꢄ for R (the experimental value of
kET was used for the calculation). This value was very close
to the distance between the centers of the transition dipoles
of the donor and acceptor, if the truxene plane were parallel
to the OPV plane. This possibility was supported by the
DCDA crystal structure, in which one of the dibenzyl am-
monium benzene rings lies nearly parallel to the dibenzo
crown-8 unit.[11b,12a,23,24]
Figure 5. The emission spectra of A, B, C, and H (the mixture of B and C
(1:2)) in film at room temperature. All emission spectra were collected
with an excitation wavelength at the absorption maximum of C at
310 nm.
Photophysical properties in film: Solid formation is general-
ly required for application in optoelectronic devices. A long-
standing problem for aromatic chromophores is their strong
aggregation tendency in films,[24] which greatly affects their
photophysical properties. To test whether the topology of
our molecular antenna would overcome this problem, we in-
vestigated the absorption and emission spectra of A, B, C,
and H in films. Thin films used for UV/Vis and PL measure-
ments were obtained by spin-coating the solution of the
sample in CH2Cl2 onto quartz plates at 1000 rpm. All com-
by 14 nm, indicating the formation of excimers. For B in
film, an emission with quite low intensity and large red-shift
(ꢁ33 nm) was observed, consistent with the observation in
its absorption spectrum. Similarly, for H, the emission maxi-
mum lmax at 489 nm, also with very low intensity, was red-
shifted by approximately 40 nm compared with that in
dilute solution. However, such a large shift was not evident
in the film of A, due to the shielding effect of the wheel, as
mentioned above. An additional observation in A was the
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A [3]Rotaxane as a Light-Harvesting Antenna
FULL PAPER
complete disappearance of the residual emission from the
truxene unit, suggesting increased ET efficiency. Such a phe-
nomenon was more pronounced in H, in which the intensity
of the emission band of the wheel from 340 to 400 nm de-
creased significantly. This is explained by additional inter-
molecular ET due to the shortening of the distance between
the donor and the acceptor in film.[5] The emission features
of [3]rotaxane A in film recorded at different excitation
wavelengths at room temperature are presented in Figure 6.
Similarly to A in solution, A in the film showed wavelength-
action is compatible with the DCDA interaction, adding this
valuable tool to the synthesis of functional rotaxane/cate-
nane. Our work will benefit future research in the fields of
both light-harvesting materials and mechanically interlocked
molecules.
Experimental Section
General methods: Commercial chemicals were purchased and used as re-
ceived. All air- and water-sensitive reactions were performed under a ni-
trogen atmosphere. 1H and 13C NMR spectra were recorded on a Varian
Mercury 200 MHz, Mercury plus 300 MHz or ARX400 400 MHz at room
temperature in appropriate deuterated solvents. All chemical shifts are
reported in parts per million (ppm). Absorption spectra were recorded
on a Perkin–Elmer Lambda 35 UV/Vis Spectrometer. PL spectra were
recorded on
a
Perkin–Elmer LS55 Luminescence Spectrometer.
MALDI-TOF mass spectra were recorded on a Bruker BIFLEX III or
AUTOFLEX III time-of-flight (TOF) mass spectrometer (Bruker Dal-
tonics, Billerica, MA, USA) using a 337 nm nitrogen laser with dithranol
as matrix. ESI HRMS spectra were recorded on a Bruker Apex IV
FTMS. The time-resolved fluorescence measurements were performed by
using the time-correlated single-photon counting technique following ex-
citation by nanosecond flash lamp (Edinburgh Instruments FL900).
Compound 3: A mixture of 1 (0.720 g, 0.780 mmol), boronic ester 2
(0.411 g, 1.56 mmol), 2m NaOH (93 mg, 2.33 mmol), and [PdACHTUNGTRENNUNG(PPh3)4] in
Figure 6. The emission spectra of [3]rotaxane A in film recorded at differ-
ent excitation wavelengths at room temperature.
THF (15 mL) was heated under reflux overnight under a nitrogen atmos-
phere. The mixture was extracted with ethyl acetate. The combined or-
ganic extracts were dried over MgSO4. After removal of solvents under
reduced pressure, the residue was purified by column chromatography
(petroleum ether/ethyl acetate=10:1) to afford 3 as a yellow oil in 77%
yield. 1H NMR (300 MHz, CDCl3, TMS): d=8.40–8.43 (m, 3H; Ar-H),
7.64–7.67 (m, 2H; Ar-H), 7.30–7.48 (m, 8H; Ar-H), 7.03 (d, J=7.5 Hz,
1H; Ar-H), 4.03 (s, 3H; OCH3), 3.96 (s, 3H; OCH3), 2.95–3.12 (m, 6H;
CH2), 2.07–2.21 (m, 6H; CH2), 0.80–0.99 (m, 36H; CH2), 0.45–0.62 ppm
(m, 30H; CH2 and CH3); 13C NMR (75 MHz, CDCl3, TMS): d=154.5,
153.8, 153.7, 149.4, 148.8, 145.1, 145.0, 144.9, 140.49, 140.46, 139.5, 139.0,
138.57, 138.59, 138.2, 134.6, 126.6, 126.2, 125.0, 124.9, 124.8, 124.7, 122.3,
120.4, 119.6, 111.7, 110.6, 56.1, 55.9, 55.8, 37.2, 31.71, 31.68, 31.65, 29.73,
29.66, 24.1, 22.5, 14.1 ppm; MALDI-TOF MS: m/z: 983 [M]+, 898
[MꢀC6H13]+.
independent emission. Also, the emission intensity excited
at 310 nm showed a twofold increase compared to that di-
rectly excited at the absorption lmax of B, meaning that the
antenna effect of the wheels was not significantly perturbed
in film.
Conclusion
In conclusion, we have successfully demonstrated the con-
cept of a mechanically interlocked light-harvesting system
through the synthesis and characterization of A. Our results
not only add one new member to the light-harvesting sys-
tems, but also open the possibility of intelligent or controlla-
ble energy-collecting machines. For example, by using out-
side signals such as acid or electrons, the antenna effect
could be switched on and off. Also, by coupling this special
optical property to the well-known movement in rotaxanes,
novel molecular machines can be imagined. Another direc-
tion for such a mechanically interlocked light-harvesting
system is to incorporate the charge-transfer process to
mimic the whole natural photosynthetic process. Moreover,
by increasing the number of wheels, the antenna effect
could be further amplified. Synthesis of larger [n]rotaxanes
is underway in our laboratory and will be reported in due
course.
Compound 4:
A solution of BBr3 (0.370 g, 1.48 mmol) in CH2Cl2
(1.4 mL) was added to a mixture of 3 (0.363 g, 0.369 mmol) in CH2Cl2
(10 mL) at 08C. The mixture was stirred at 08C for 1 h and then at room
temperature overnight. NaHCO3 (1m) was added and the reaction mix-
ture was partitioned between water and CH2Cl2. The aqueous layer was
extracted with CH2Cl2. The combined organic extracts were dried over
MgSO4. After removal of solvents under reduced pressure, the residue
was purified by column chromatography (petroleum ether/ethyl acetate=
10:1) to afford 4 as a white solid in 90% yield. 1H NMR (300 MHz,
CDCl3, TMS): d=8.37–8.40 (m, 3H; Ar-H), 7.25–7.61 (m, 10H; Ar-H),
7.02 (d, J=8.1 Hz, 1H; Ar-H), 5.31 (s, 1H; OH), 5.26 (s, 1H; OH), 2.93–
3.02 (m, 6H; CH2), 2.07–2.13 (m, 6H; CH2), 0.81–0.99 (m, 36H; CH2),
0.51–0.62 ppm (m, 30H; CH2 and CH3); 13C NMR (75 MHz, CDCl3,
TMS): d=154.5, 153.83, 153.79, 145.2, 145.0, 144.0, 143.1, 140.55, 140.51,
139.5, 138.6, 138.2, 135.2, 126.6, 126.2, 125.0, 124.9, 122.4, 120.3, 120.0,
116.0, 114.4, 55.9, 55.8, 37.2, 31.7, 29.7, 24.1, 22.5, 14.1 ppm; MALDI-
TOF MS: m/z: 955 [M]+, 870 [MꢀC6H13]+.
Compound 7: LiAlH4 (0.730 g, 19.1 mmol) was added slowly to a solution
of 4-prop-2-ynyloxy-benzonitrile (0.300 g, 1.91 mmol) in anhydrous THF
at 08C. After being at room temperature overnight, the mixture was
quenched with aqueous Na2SO4. After filtration, the solution was concen-
In addition, we have also demonstrated that the topology
of rotaxane can effectively prevent intermolecular aggrega-
tion in the solid state, offering a general solution for the
long-standing problem in functional organic materials.
Meanwhile, we demonstrate that the CuI-catalyzed click re-
trated under reduced pressure to afford
7
in 64% yield. 1H NMR
(200 MHz, CDCl3, TMS): d=7.28 (d, J=8.6 Hz, 2H; Ar-H), 6.95 (d, J=
8.6 Hz, 2H; Ar-H), 4.69 (d, J=2.2 Hz, 2H; CH2), 3.74 (s, 2H; CH2),
2.52 ppm (t, J=2.2 Hz, 1H; CH); 13C NMR (75 MHz, CDCl3, TMS): d=
Chem. Eur. J. 2009, 15, 3585 – 3594
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www.chemeurj.org
3591

J. Pei, Y. Ma et al.
156.7, 135.2, 128.7, 115.1, 78.8, 75.7, 56.0, 45.4 ppm; MS (EI): m/z: 161
[M]+.
with brine and diethyl ether to afford a red solid. NH4PF6 (0.326 g,
2.00 mmol) and H2O (20 mL) were added to a solution of the red solid in
hot acetone (20 mL). The mixture was warmed for 10 min, then acetone
was evaporated off. The mixture was filtered and washed with brine, and
dried in air to afford B in 62% yield. 1H NMR (400 MHz, CD3CN,
TMS): d=7.90 (s, 2H; Ar-H), 7.10–7.70 (m, 42H; Ar-H), 6.63 (t, J=
2.0 Hz, 2H; Ar-H), 6.57 (d, J=2.1 Hz, 4H; Ar-H), 5.51 (s, 4H; CH2),
5.23 (s, 4H; CH2), 5.08 (s, 8H; CH2), 4.19–4.25 (m, 8H; CH2), 4.14 (m,
4H; CH2), 1.85–1.95 (m, 4H; CH2), 1.55–1.65 (m, 4H; CH2), 1.20–1.50
(m, 32H; CH2), 0.89–0.94 ppm (m, 6H; CH3); 13C NMR (100 MHz,
CD3CN, TMS): d=160.2, 159.3, 151.1, 143.3, 139.3, 137.9, 136.9, 131.9,
130.6, 129.3, 128.51, 128.48, 128.1, 127.9, 127.71, 127.67, 127.6, 126.8,
126.7, 124.5, 124.0, 122.7, 115.1, 110.9, 107.1, 101.7, 69.8, 69.2, 61.4, 53.4,
50.9, 50.8, 31.6, 29.4, 29.35, 29.33, 29.28, 29.09, 29.05, 29.0, 25.9, 22.4,
13.4 ppm; HRMS (ESI): m/z calcd: 844.9961; found: 844.9935
Compound 9: A solution of dialdehyde 6 (0.155 g, 0.219 mmol) and 7
(0.0850 g, 0.526 mmol) in toluene (50 mL) was heated under reflux for
12 h. After the reaction mixture had been cooled to room temperature,
removal of solvent under reduced pressure afforded 8 as a yellow solid,
which was used directly for the next step without purification. NaBH4
(0.166 g, 4.38 mmol) was added to the solution of 8 in the mixture of
MeOH and CH2Cl2 (100 mL ca. 1:1) at 08C. The reaction mixture was
stirred for 10 h, then concentrated HCl was added (pH<3). The mixture
was partitioned between aqueous NaOH (1m, 30 mL) and CH2Cl2. The
organic layer was washed with aqueous NaHCO3 and brine, and then
dried over MgSO4. Removal of the solvent under vacuum followed by re-
crystallization from petroleum ether afforded 9 in 74% yield from 6.
1H NMR (300 MHz, CDCl3, TMS): d=7.50 (d, J=8.4 Hz, 4H; Ar-H),
7.44 (s, 2H; Ar-H), 7.32 (d, J=8.4 Hz, 4H; Ar-H), 7.28 (d, J=9.0 Hz,
4H; Ar-H), 7.14 (d, J=10.2 Hz, 2H; CH), 7.11 (d, J=6.3 Hz, 2H; CH),
6.95 (d, J=9.0 Hz, 4H; Ar-H), 4.69 (d, J=2.4 Hz, 4H; CH2), 4.05 (t, J=
6.6 Hz, 4H; CH2), 3.80 (s, 4H; CH2), 3.76 (s, 4H; CH2), 2.51 (t, J=
2.4 Hz, 2H; CH), 1.20–1.90 (m, 32H; CH2), 0.84–0.89 ppm (m, 6H;
CH3); 13C NMR (75 MHz, CDCl3, TMS): d=156.9, 151.3, 139.6, 137.0,
133.4, 129.6, 128.7, 127.1, 126.8, 123.4, 115.0, 110.8, 78.8, 75.6, 69.7, 56.0,
52.9, 52.5, 52.4, 32.0, 29.80, 29.75, 29.6, 29.5, 26.4, 22.8, 14.2 ppm; HRMS
(ESI): m/z: calcd: 997.6817; found: 997.6828 [M+H]+.
[Mꢀ2PF6+2H]2+
.
Compound C: A mixture of 4 (1.00 g, 1.05 mmol) and bistosylate 5
(0.715 g, 1.05 mmol) in anhydrous DMF (20 mL) was added dropwise to
a suspension of K2CO3 (0.579 g, 4.19 mmol) in anhydrous DMF (25 mL)
under a nitrogen atmosphere at 1008C over 12 h. The mixture was stirred
at 1008C for another three days. After cooling down to ambient tempera-
ture, the mixture was filtered and extracted with ethyl acetate. The fil-
trate was concentrated under reduced pressure. The residue was dis-
solved in CH2Cl2 and washed with dilute HCl and water. Organic extracts
were dried over anhydrous MgSO4. After removal of solvent under re-
duced pressure, the residue was purified by column chromatography over
silica gel (petroleum ether/ethyl acetate=3:1) to afford C in 67% yield.
1H NMR (300 MHz, CDCl3, TMS): d=8.38–8.40 (m, 3H; Ar-H), 7.58–
7.62 (m, 2H; Ar-H), 7.29–7.49 (m, 8H; Ar-H), 7.01 (d, J=8.1 Hz, 1H;
Ar-H), 6.91 (s, 4H; Ar-H), 4.31–4.33 (m, 2H; CH2), 4.24–4.26 (m, 2H;
CH2), 4.18–4.20 (m, 4H; CH2), 3.95–4.02 (m, 8H; CH2), 3.89 (s, 8H;
CH2), 2.92–3.05 (m, 6H; CH2), 2.01–2.16 (m, 6H; CH2), 0.78–0.99 (m,
36H; CH2), 0.41–0.62 ppm (m, 30H; CH2 and CH3); 13C NMR (75 MHz,
CDCl3, TMS): d=154.5, 153.8, 149.2, 149.1, 148.7, 145.2, 145.0, 144.9,
140.51, 140.47, 139.5, 138.8, 138.5, 138.1, 135.0, 126.8, 126.5, 126.1, 125.0,
124.8, 122.3, 121.6, 120.3, 114.3, 114.1, 113.5, 71.5, 70.2, 70.1, 70.0, 69.7,
69.6, 55.9, 55.8, 37.2, 31.71, 31.68, 29.73, 29.69, 29.67, 24.1, 22.5, 14.1 ppm;
MALDI-TOF MS: m/z: 1332 [M+K]+, 1316 [M+Na]+, 1293 [M]+, 1208
[MꢀC6H13]+.
Compound 11: 9 (50 mg, 0.0500 mmol) was dissolved in 6m HCl (50 mL).
After a few minutes, water was evaporated off. The residue was filtered
and washed with brine and ether to afford a red solid. NH4PF6 (0.163 g,
1.00 mmol) and H2O (20 mL) were added to a solution of the red solid in
hot acetone (30 mL). The mixture was warmed for 10 min, then the ace-
tone was evaporated off. The mixture was filtered, washed with brine,
and dried in air to afford 11 in quantitative yield. 1H NMR (300 MHz,
CD3COCD3, TMS): d=7.57–7.67 (m, 14H; Ar-H), 7.37–7.43 (m, 4H;
CH), 7.08 (d, J=9.0 Hz, 4H; Ar-H), 4.84 (d, J=2.4 Hz, 4H; CH2), 4.61
(s, 4H; CH2), 4.57 (s, 4H; CH2), 4.14 (t, J=6.3 Hz, 4H; CH2), 3.14 (t, J=
2.4 Hz, 2H; CH), 1.22–1.94 (m, 32H; CH2), 0.82–0.90 ppm (m, 6H;
CH3); 13C NMR (100 MHz, CD3CN, TMS): d=158.4, 151.1, 139.2, 131.8,
130.6, 129.7, 128.1, 126.8, 126.7, 124.4, 123.5, 115.2, 110.9, 78.4, 76.1, 69.2,
55.6, 51.0, 50.9, 31.6, 29.31, 29.26, 29.1, 29.04, 29.00, 25.9, 22.4, 13.4 ppm;
HRMS (ESI): m/z calcd: 499.3450; found: 499.3464 [Mꢀ2PF6]2+
.
Compound A:
0.217 mmol),
A
mixture of 11 (70 mg, 0.0540 mmol), 10 (75 mg,
C
(0.281 g, 0.217 mmol), and Cu(MeCN)4PF6 (40 mg,
ACHTUNGTRENNUNG
0.108 mmol) was stirred in dry CH2Cl2 (1 mL) at room temperature for
two days. After removal of the solvent, the residue was first purified by
column chromatography (eluent: CH2Cl2/MeOH=20:1) to afford the
crude product. Preparative GPC (eluent: CHCl3) was used to obtain the
pure product A as a yellow solid in 80% yield. 1H NMR (300 MHz,
CDCl3, TMS): d=8.30–8.42 (m, 6H; Ar-H), 7.96 (s, 2H; Ar-H), 6.72–7.68
(m, 72H; Ar-H), 6.62 (d, J=2.1 Hz, 4H; Ar-H), 6.56 (t, J=2.1 Hz, 2H;
Ar-H), 5.52 (s, 4H; CH2), 5.18 (s, 4H; CH2), 5.01 (s, 8H; CH2), 4.46–4.62
(m, 8H; CH2), 4.20–4.42 (m, 8H; CH2), 4.00–4.18 (m, 12H; CH2), 3.64–
3.98 (m, 16H; CH2), 3.30–3.56 (m, 16H; CH2), 2.84–3.04 (m, 12H; CH2),
1.98–2.18 (m, 12H; CH2), 1.84–1.96 (m, 4H; CH2), 1.46–1.60 (m, 4H;
CH2), 0.42–1.46 ppm (m, 170H; CH2 and CH3); 13C NMR (100 MHz,
CDCl3, TMS): d=160.3, 158.9, 154.5, 153.6, 153.5, 151.2, 148.0, 147.3,
147.23, 147.20, 144.9, 144.8, 143.6, 140.3, 140.2, 139.6, 138.8, 138.45,
138.40, 138.2, 137.8, 137.1, 136.6, 135.4, 131.0, 130.8, 129.5, 128.5, 128.0,
127.9, 127.5, 126.7, 126.6, 126.4, 126.0, 124.9, 124.8, 124.6, 124.5, 123.9,
123.8, 122.2, 121.9, 121.8, 120.4, 120.2, 115.1, 113.4, 112.9, 112.7, 111.9,
110.7, 107.3, 102.4, 70.6, 70.2, 70.1, 70.0, 69.4, 68.7, 68.5, 68.4, 61.4, 55.7,
55.59, 55.57, 54.0, 52.3, 52.2, 36.9, 31.9, 31.5, 31.4, 29.64, 29.59, 29.5, 29.44,
29.42, 29.3, 26.2, 23.90, 23.85, 22.6, 22.23, 22.21, 14.1, 13.8 ppm; HRMS
Acknowledgements
This work was supported by the Major State Basic Research Develop-
ment Program (Nos. 2006CB921602 and 2007CB808000) from the Minis-
try of Science and Technology and National Natural Science Foundation
of China.
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Chem. Eur. J. 2009, 15, 3585 – 3594