Selective photoinduced energy transfer from a thiophene rotaxane to acceptor

Article abstract of DOI:10.1021/ol102912g

An energy transfer process was investigated using cyclodextrin- oligothiophene rotaxanes (2T-[2]rotaxane). The excited energy of 2T-[2]rotaxane is transferred to the sexithiophene derivative which is included in the cavity of β-CD stoppers of 2T-[2]rotaxane.

Full text of DOI:10.1021/ol102912g

ORGANIC
LETTERS
2011
Vol. 13, No. 4
672–675
Selective Photoinduced Energy Transfer
from a Thiophene Rotaxane to Acceptor
Kazuya Sakamoto,† Yoshinori Takashima,† Norio Hamada,† Hideki Ichida,†,§
Hiroyasu Yamaguchi,† Hitoshi Yamamoto,† and Akira Harada*,†,‡
Department of Macromolecular Science, Graduate School of Science, Osaka University,
1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan, Core Research for
Evolutional Science and Technology (CREST), Japan Science and Technology Agency
(JST), 5 Sanban-cho, Chiyoda-ku, Tokyo 102-0075, Japan, and
Center for Advanced Science and Innovation, Osaka University,
2-1 Yamada-oka, Suita, Osaka, 565-0871, Japan
harada@chem.sci.osaka-u.ac.jp
Received December 1, 2010
ABSTRACT
An energy transfer process was investigated using cyclodextrin-oligothiophene rotaxanes (2T-[2]rotaxane). The excited energy of
2T-[2]rotaxane is transferred to the sexithiophene derivative which is included in the cavity of β-CD stoppers of 2T-[2]rotaxane.
Natural light harvesting systems are precisely designed
in photosynthetic proteins where light is converted into
chemical energy.1-3 Biologists and chemists are familiar
with molecular electronics based on electron transfer and
energy transfer processes. Recently, electron and energy
transfersystems, whichhavebeenusedineithercovalent or
noncovalent molecular devices, have attracted much atten-
tion.4-16 Covalent molecular devices, such as molecular
wires and artificial light-harvesting antennae, effectively
provide suitable orientations and distances between do-
nors and acceptors for efficient electron and energy trans-
fers. Supramolecular assemblies provide more complex and
sophisticated structures and functions. There have al-
ready been a variety of fluorescence molecular sensors for
cations, anions, and neutral molecules.4b Supramolecular
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† Department of Macromolecular Science, Graduate School of Science,
Osaka University.
§ Center for Advanced Science and Innovation, Osaka University.
‡ Japan Science and Technology Agency (JST).
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10.1021/ol102912g
Published on Web 01/06/2011
2011 American Chemical Society

approaches provide an effective and selective energy trans-
fer system by the suitably organized space and an affinity
between host and guest molecules. Modified cyclodextrins
are of major interest as molecular fluorescence sensors,
which have selective binding for guest molecules, as a
typical example of chromophores bound to CD for energy
harvesting purposes.17
meet requirements for the absorption overlap for 2T-[2]-
rotaxane and solubility in water. As an electron acceptor
molecule, an adamantane bipyridinium guest dimer (Ad2Bpy)
was also chosen. These guest molecules showed relatively
high affinities to β-CD, such as 6TCA2Na2 (K1 =9.2 ꢀ
3.2 ꢀ 104 M-1).20 These results indicate that inclusion
complexes are formed between the guest molecules and
β-CD stopper groups of [2]rotaxanes in aqueous solutions.
103 M-1 and K2 = 1.8 ꢀ 105 M-1 19
)
and Ad2Bpy (K =
Figure 1. Chemical structures of [2]rotaxanes as donor and guest
acceptor molecules.
Previously, we have prepared dimethyl-β-cyclodextrin
(DM-β-CD);rotaxanes with oligothiophenes as an axis
molecule and β-cyclodextrin (β-CD) as stoppers, such as
bithiophene-[2]rotaxane (2T-[2]rotaxane) and terthiophe-
ne-[2]rotaxane (3T-[2]rotaxane). β-CD stoppers have the
ability to bind guest molecules selectively in aqueous
solutions.18 The findings led us to hypothesize that there
is energy transfer between [2]rotaxanes as a donor mole-
cule and some acceptor molecules. Thus two guest mole-
cules were added onto [2]rotaxanes to see whether selective
energy transfer would take place between hosts and guests.
Figure 1 shows the chemical structures of [2]rotaxanes and
guest molecules. We chose 2,20:50,200:500,2000:5000,20000:50000,200000
-
sexithiophene-300,4000-dicarboxylic acid disodium salt
(6TCA2Na2), which has six thiophene rings as energy
acceptor guest molecules. 6TCA2Na2 has been designed to
Figure 2. Absorption (a) and fluorescence (b) spectra of
2T-[2]rotaxane, 6TCA2Na2, and the mixture of 2T-[2]rotaxane
and 6TCA2Na2 in aqueous solutions. Concentrations were
adjusted to 15 μM, and samples were excited at λex = 378 nm.
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Figure 2 shows the absorption and fluorescence spectra
of 2T-[2]rotaxane, 6TCA2Na2, and a mixture of 2T-[2]-
rotaxane and 6TCA2Na2 in aqueous solutions. Excitation
spectra of these compounds are shown in Figure S18. The
absorption maximum of 2T-[2]rotaxane and 6TCA2Na2
appears at 378 and 424 nm, respectively. When the solutions
were irradiated with light at a wavelength of 378 nm, the
emission spectra of 2T-[2]rotaxaneand 6TCA2Na2 showed
the maximum at 443 and 534 nm, respectively. When
2T-[2]rotaxane was excited by irradiation at 378 nm in
the presence of 6TCA2Na2, 99% of emission from
2T-[2]rotaxane at 443 nm was quenched. Simultaneously,
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Org. Lett., Vol. 13, No. 4, 2011
673

Table 1. Photophysical Properties of 2T-[2]Rotaxane, 6TCA2Na2, 2T-[2]Rotaxane-6TCA2Na2, Respectively, Excited at 390 nma
Rate constant
Lifetime/ps
Compound
λ
em/nmb
λ
obs/nmc
Φem
kr/ps-1e
τ1, τ2
χ2
d
2T-[2]rotaxane
443
495
0.10
2.9 ꢀ 10-4
1200 (5.4%),
339 (95%)f
641g
1.30
6TCA2Na2
534
534
540
540
0.11
0.13
1.7 ꢀ 10-4
1.0 ꢀ 10-4
1.06
1.21
2T-[2]rotaxane þ
6TCA2Na2
1300g
β-CD þ 6TCA2Na2
534
540
0.10
5.9 ꢀ 10-3
1120g
1.18
a Measured in a degassed aqueous solution at 25 °C. b Emission maximum. c Fluorescence spectra were analyzed by a streak-camera system attached
to a 15-cm spectrometer. d Absolute fluorescence quantum yield of emission. e Rate constant kr can be calculated as the following equation: kr = Φem/τ.
f Fluorescence decay of 2T-[2]rotaxane was fitted by two-component model. g Fluorescence decay was fitted by single-component model.
the emission intensity from 6TCA2Na2 with 2T-[2]-
rotaxane at 534 nm was twice that of 6TCA2Na2 without
2T-[2]rotaxane. On the other hand, when 3T-[2]rotaxane
was excited by irradiation at 407 nm in the presence of
6TCA2Na2, the emission of 6TCA2Na2 with 3T-[2]-
rotaxane caused a slightly incease (20%) at 534 nm
(Figure S14). The reason that the emission from 2T-[2]-
rotaxane effectively quenched rather than that from 3T-[2]-
rotaxane is related to the fact that the absorption band of
6TCA2Na2 overlaps the fluorescence band of 2T-[2]rotaxane,
whereas the fluorescence band of 3T-[2]rotaxane slightly over-
laps the absorption band of 6TCA2Na2 (Figure S15). This
poor overlap may be the reason for the weak energy transfer in
the 3T-[2]rotaxane-6TCA2Na2 system. These results provide
depends on distance, orientation factor, and overlap integra-
tion between an emission band of the donor and an absorp-
tion band of the acceptor. It is important to note that donor
€
molecules are within close proximity, i.e., the Forster critical
radius (R0) (15-60 A)21-24 of acceptor molecules. R0 values
betweenthecenterof2T-[2]rotaxaneand6TCA2Na2 were ob-
tained as 31.8 A for 2T-[2]rotaxane-6TCA2Na2 and 23.5 A
for 3T-[2]rotaxane-6TCA2Na2, respectively. These results
indicate that the energy transfer of the 2T-[2]rotaxane-
6TCA2Na2 complex effectively performed rather than the
energy transfer of the 3T-[2]rotaxane-6TCA2Na2 complex
even with the long distance between the donor and acceptor.
The fluorescence decays of 2T-[2]rotaxane, 6TCA2Na2,
and 2T-[2]rotaxane-6TCA2Na2 were recorded in water at
room temperature by using a streakcamerasystem, respec-
tively. These compounds at 15 μM were excited by a
picoseconds laser pulse at 390 nm; the pulse duration is
clear evidence that the 2T-[2]rotaxane-6TCA2Na2 system
21-24
€
effectively demonstrates Forster energy transfer.
The addition of an excess amount of adamantane car-
boxylic acid sodium salt (AdCANa)25-27 into an aqueous
solution of 2T-[2]rotaxane in the presence of 6TCA2Na2
caused a marked decrease (54%) in the fluorescence in-
tensity at 534 nm from 6TCA2Na2 (Figure S16). However,
the emission intensity from 2T-[2]rotaxane recovered 50%
based on the emission of 2T-[2]rotaxane alone. These results
prove that the formation of inclusion complexes of 6TCA2-
Na2 in the cavity of the β-CD stoppers is important to
€
perform the Forster energy transfer.
€
We considered the Forster mechanism of energy transfer
from [2]rotaxane to 6TCA2Na2. The Forster energy transfer
€
€
€
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(27) The absorbance intensity of 2T-[2]rotaxane was not subject to the
addition of AdCANa. The fluorescence intensity of 2T-[2]rotaxane was
slightly increased when the excess amount of AdCANa was added to
2T-[2]rotaxane, indicating that AdCANa does not function as a quencher
for 2T-[2]rotaxane. See Figure S17.
Figure 3. Emission properties of 2T-[2]rotaxane, 6TCA2Na2,
and 2T-[2]rotaxane-6TCA2Na2 in the solid state on glass
plates, respectively. (a) Under visible light and (b) under UV
light (λex = 365 nm).
674
Org. Lett., Vol. 13, No. 4, 2011

Figure 4. Schematic illustration of the photophysical property of 2T-[2]rotaxane with 6TCA2Na2 or Ad2Bpy in aqueous solutions.
∼0.2 ps. Table 1 shows photophysical properties of 2T-
[2]rotaxane, 6TCA2Na2, and 2T-[2]rotaxane-6TCA2Na2,
respectively. At 495 nm, the fluorescence decay of
2T-[2]rotaxane showed that the major component of 339
ps (95%) and a longer component of 1200 ps (5.4%) were
observed, where analysis of the data using a nonlinear
least-squares deconvolution method shows that a sum of
double exponentials is sufficient to gain a satisfactory fit.
The fluorescence decay of 6TCA2Na2 showed a single
component of 641 ps at 540 nm, which was fitted by a
single exponential. The same experiment was carried out
with 2T-[2]rotaxane-6TCA2Na2 upon excitation at 390
nm and observation at 495 and 540 nm, respectively. When
the fluorescence decay was observed at 540 nm, which is
close to the emission maximum, a single component of
1300 ps was observed. The fact that the fluorescence decay
derived from 6TCA2Na2 in 2T-[2]rotaxane-6TCA2Na2
was longer than that of stand-alone 6TCA2Na2 is related
to the formation of the inclusion complex between the β-
CD unit and 6TCA2Na2. The fluorescence decay of
6TCA2Na2 in the presence of β-CD showed 1120 ps, which
is close to the long lifetime component in the 2T-
[2]rotaxan-6TCA2Na2 system. Unfortunately, we could
not observe a fast initial rise component less than 50 ps as it
would be too fast to detect with our equipment.
Bipyridinium compounds are well-known as electron
acceptor molecules.28 When Ad2Bpy was used as an electron
acceptor molecule, the emission from 2T-[2]rotaxane was
significantly quenched by Ad2BPy, although the absorption
maximum of 2T-[2]rotaxane was largely unresponsive.
These results suggest that the excited state of 2T-[2]rotaxane
deactivates nonradiatively by energy transfer to Ad2Bpy.
Figure 3 shows emission properties of 2T-[2]rotaxane,
6TCA2Na2, and 2T-[2]rotaxane-6TCA2Na2 in the solid
state on glass plates, respectively. Aqueous solutions
(1 mM, 45 μL) of these compounds were dropped onto the
glass plates, respectively, and then dried in air. Under
visible light, 2T-[2]rotaxane, 6TCA2Na2, and the 2T-[2]-
rotaxane-6TCA2Na2 complex appeared pale yellow, or-
ange, and light orange, respectively. Under UV irradiation
(λex = 365 nm), 2T-[2]rotaxane and the 2T-[2]rotaxane-
6TCA2Na2 complex showed blue and bright yellow emis-
sion, whereas 6TCA2Na2 emitted a significantly weak
orange light (Figure 3b). 6TCA2Na2 self-quenched in the
solid state due to self-aggregation through a π-π stacking
interaction. These results indicate that Forster energy
transfer of the 2T-[2]rotaxane-6TCA2Na2 complex was
clearly observed in the solid state.
€
€
We successfully observed Forster energy transfer be-
tween a bithiophene axis of the rotaxane (2T-[2]rotaxane)
and an oligothiophene guest (6TCA2Na2) through the
host-guest interaction. Figure 4 shows the emission be-
havior of 6TCA2Na2 or Ad2Bpy in the presence of 2T-[2]-
rotaxane. There was a weak energy transfer in the 3T-[2]-
rotaxane-6TCA2Na2 system even though the absorption
band of 6TCA2Na2 overlapped the emission band of 3T-[2]-
rotaxane. When Ad2Bpy was used as a guest, the electron
transfer took place and marked fluorescence quench-
ing was observed. The energy transfer of the 2T-[2]rotaxane-
6TCA2Na2 complex was clearly observed in the solid
state. The excitation energy of the bithiophene axle of
2T-[2]rotaxane was selectively transferred to an accep-
tor guest included in the β-CD cavity. Currently, we
are investigating the combination of rotaxanes and
fluorescent materials as effective triggers for energy and
electron transfer.
Acknowledgment. The authors thank Mr. S. Adachi
(Osaka University) for his support with 2D-NMR
experiments. This work was supported by the “Core
Research for Evolutional Science and Technology”
program of the Japan Science and Technology Agency,
Japan.
Supporting Information Available. Experimental pro-
cedures and spectral data. This material is available free
of charge via the Internet at http://pubs.acs.org.
(28) Monk, P. M. S., Ed. The Viologens: Physicochemical Properties,
Synthesis and Applications of the Salts of 4,40-Bipyridine; Wiley-VCH
Verlag GmbH: Weinheim, 1999.
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