Fluorescence control of boron enaminoketonate using a rotaxane shuttle

Article abstract of DOI:10.1021/ol401984j

The effect of rotaxane shuttling on the fluorescence properties of a fluorophore was investigated by exploiting fluorophore-tethered [2]rotaxanes. A fluorescent boron enaminoketonate (BEK) moiety was introduced in a rotaxane via transformation of an isoxazole unit generated as a result of an end-capping reaction using a nitrile N-oxide. The rotaxane exhibited a red shift of the fluorescence maximum along with a remarkable enhancement of the fluorescence quantum yield through wheel translation to the fluorophore.

Full text of DOI:10.1021/ol401984j

ORGANIC
LETTERS
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Vol. XX, No. XX
000–000
Fluorescence Control of Boron
Enaminoketonate Using a Rotaxane Shuttle
Yasuhito Koyama,*,† Tohru Matsumura,‡ Tatsuto Yui,§ Osamu Ishitani, and
Toshikazu Takata*,‡
Catalysis Research Center, Hokkaido University, N21 W10, Kita-ku,
Sapporo 001-0021, Japan, Department of Organic and Polymeric Materials, Tokyo Institute
of Technology, 2-12-1 (H-126), Ookayama, Meguro, Tokyo 152-8552, Japan, Department
of Material Science and Technology, Faculty of Engineering, Niigata University,
Igarashi 2-8050, Niigata 950-2181, Japan, and Department of Chemistry,
Tokyo Institute of Technology, 2-12-1 (E1-9), Ookayama, Meguro, Tokyo 152-8551, Japan
yasuhito.koyama@cat.hokudai.ac.jp; ttakata@polymer.titech.ac.jp
Received July 15, 2013
ABSTRACT
The effect of rotaxane shuttling on the fluorescence properties of a fluorophore was investigated by exploiting fluorophore-tethered [2]rotaxanes.
A fluorescent boron enaminoketonate (BEK) moiety was introduced in a rotaxane via transformation of an isoxazole unit generated as a result of
an end-capping reaction using a nitrile N-oxide. The rotaxane exhibited a red shift of the fluorescence maximum along with a remarkable
enhancement of the fluorescence quantum yield through wheel translation to the fluorophore.
Boron enaminoketonates (BEKs) are a new type of
boron-chelating dye.1 Their optical properties feature a
large Stokes shift and high-molarabsorption coefficient (ε)
that are quite similar to those of boron diketonates. Boron
diketonates are useful in molecular probes,2 lasers,3 and in
optical sensing devices,4 which are capable of incorporat-
ing various organic backbones.
attracted much interest as a new concept for improving
the electro-optical properties of fluorophores via static
encapsulation, because the method enables direct modifi-
cation of such properties without any structural change
in the fluorophore itself.5,6 Considering such studies, we
envisaged that the statistical and dynamic coverage of a
fluorophore moiety on the rotaxane axle via wheel shut-
tling may diminish quenching to a certain extent by an
On the other hand, mechanically interlocked molecules,
such as rotaxanes, catenanes, and polyrotaxanes, have
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† Hokkaido University.
‡ Department of Organic and Polymeric Materials, Tokyo Institute of
Technology.
§ Niigata University.
Department of Chemistry, Tokyo Institute of Technology.
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Nagai, A.; Tanaka, K.; Chujo, Y. Chem.;Eur. J. 2013, 19, 4506–4512.
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r
10.1021/ol401984j
XXXX American Chemical Society

external species and by energy transfer, even though the
wheel cannot stay on the fluorophore.
mixed solvent of CH3CNÀH2O to give the β-enaminoke-
tone, followed by the treatment with BF3 OEt2 in the pres-
3
We have previously reported a powerful and highly
reliable catalyst-free click end-capping reaction of an
alkyne-terminated pseudorotaxane with a stable nitrile
N-oxide7 to give a [2]rotaxane8À10 accompanied by the
formation of a promising isoxazole skeleton as a BEK
precursor. Considering our previous work, we describe
herein the fluorescence control of a BEK placed at the end
of the rotaxane axle, which was demonstrated through a
comparison between two types of BEKs with a through-
space linkage: (i) a BEK-rotaxane with the wheel located at
the far position and (ii) a BEK-rotaxane with the freely
movable wheel. It was discovered that the high mobility of
the wheel resulted in a red shift of the fluorescence max-
imum along with a remarkable enhancement of the fluor-
escence quantum yield (ΦF), indicating that the mobile
wheel stabilizes the excitation state of the fluorophore.
Scheme 1 shows the synthesis of [2]rotaxane 5. Treat-
ment of sec-ammonium salt 1 in the presence of dibenzo-
24-crown-8-ether (DB24C8, 2) gave pseudorotaxane 3
possessing an alkyne moiety at the terminus. A subsequent
catalyst-free 1,3-dipolar cycloaddition reaction of the stable
nitrile N-oxide 4 with 3 proceeded smoothly via isoxazole
formation to give the corresponding [2]rotaxane 5 in an
excellent yield (95% yield). The structure of 5was determined
by 1H and 13C NMR, IR, and ESIÀTOF MS analysis, and 5
was confirmed to exist as a single regioisomer.11
ence of Et3N to afford the BEK-containing [2]rotaxane
6. Moreover, the removal of the hydrogen bonds in 5
was performed by acetylation of 5 to give the nonionic
[2]rotaxane 7. The sequential transformation of the iso-
xazole moiety of 7 to the BEK function was performed
in the same manner togive8. The structures of 6 and 8 were
1
determined by H NMR, 19F NMR, IR, and MALDIÀ
TOF MS analysis.11
Scheme 1. Synthesis of [2]Rotaxane 5
The transformation of the isoxazole skeleton of 5
to a BEK functioning as a fluorophore was performed
(Scheme 2). The selective NÀO bond cleavage of the
isoxazole moiety of 5 was performed using Mo(CO)6 in a
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Oxides, Nitrones, and Nitronates in Organic Synthesis, 2nd ed.; Feuer, H.,
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Figure 1 shows 1H NMR spectra of 6 and 8. In spectrum
(A), the characteristic signals of the benzyl protons (c and
d) of the axle component of 6 appear as broad peaks due to
geminal coupling, supporting the assignment of the inter-
locked structure of 6, in which the ammonium moiety is
localized at the center of the crown ether moiety in
accordance with the literature.12 As can be seen in spec-
trum (B), the signals of the benzyl protons (c and d) are
upfield-shifted, and the signals of the acetyl group can be
observed, confirming the N-acetylated structure of 8. In
addition, certain signals of the axle portion of the molecule
are shifted due to the ring current effect from 2, and these
shifts provide critical information about the increased
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(11) See Supporting Information.
B
Org. Lett., Vol. XX, No. XX, XXXX

Scheme 2. Synthesis of Fluorescent [2]Rotaxanes via Selective Transformation of Different Isoxazoles to BEK
Figure 1. 1H NMR spectra of (A) 6 and (B) 8 (400 MHz, CDCl3,
298 K).
mobility of the wheel component that is separated from the
ammonium moiety. Signals h and i are downfield-shifted,
indicating deshielding of these protons from the aromatics
of 2. In contrast, the methoxy signal (OMe) is upfield-
shifted, implying CHÀπ interactions ofthe methoxy group
with the π-plane of the wheel aromatic substituents. It
should be noted that 8 is a nonstationary rotaxane con-
Figure 2. UVÀvis (A) and fluorescence spectra (B) of 6 (blue
sisting of the wheel that is capable of free movement along
the axle, because 8 does not have any strong interaction
between the components.8,13
line) and 8 (red line) (CH2Cl2, 293 K, 20 μM). The excitation
wavelengths of 6 and 8 at (B) are 364 and 384 nm, respectively.
To evaluate the effect of the wheel mobility on the
optical properties of compound 8, the UVÀvis absorption
spectra of 6 and 8 in CH2Cl2 were measured (Figure 2A).
The absorption spectrum of 6 exhibits a maximum at
364 nm, while the absorption maximum of 8 appears at
384 nm. The 20 nm red shift of the absorption maximum of
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Korean Chem. Soc. 2004, 25, 1641–1644.
Org. Lett., Vol. XX, No. XX, XXXX
C

8 from that of 6 can be assumed to be the result of
the increasing conjugation length of the BEK moiety
and/or πÀπ stacking between the BEK and crown
ether moieties. The ε values of 6 and 8 were 2.5 Â 104
(DFT) at the B3LYP/6-31G level.1,11 The calculated struc-
ture of 8 indicates increased rigidity around the BEK
moiety of 8 in the excitated state with the support of
the wheel translation, which restricts the conformational
freedom of the fluorophore. In addition, the naphthalene
skeleton of 8 barely affected the π-extension of the
emissive framework because of the distorted structure.
The improved ΦF of 8 with respect to 6 appears to be
the result of the steric protection of the fluorophore by
the mobile wheel.15 Although this work has focused on
the synthesis of the fluorescence control system, future
studies will concentrate on the evaluation of the steric
protection effect of the mobile wheel on the optical
properties of fluorophore.
In conclusion, we have described the usefulness of a
statistical stabilization method using a rotaxane system
through a comparison of the fluorescence properties be-
tween two types of BEKs with a through-space linkage.
The rotaxane-based BEKs were readily prepared via se-
quential transformation of an isoxazole-containing rotax-
ane synthesizedvia a stable nitrileN-oxide-based click end-
capping reaction. The statistical stabilization can be clearly
distinguished from conventional stabilization methods,
such as thermodynamic and kinetic stabilization. The
present achievement will provide crucial insights for not
only supramolecular chemistry but also polymer chemis-
try, because the sequential transformation from isoxazole
to BEK would be versatile and the concept of dynamic
steric protection should be applicable to the construction
of unprecedented polymers with chemically unstable func-
tionalities. The development of stimuli-responsive fluor-
escent switching materials exploiting a combination of this
system with polyrotaxane network chemistry will be an
important subject for future work.
and 2.9 Â 104 MÀ1 cmÀ1, respectively.
3
The fluorescence emission spectra of 6 and 8 when
excited at their absorption maxima are shown in Figure 2
B. The fluorescence maxima of 6 and 8 appear at 440
and 454 nm, indicating that the Stokes shifts are suffi-
ciently large (1.3 Â 105 and 1.4 Â 105 cmÀ1, respecitvely)
and similar to that of conventional boron diketonates.
The ΦF’s of 6 and 8 were determined on the basis of the
fluorescent intensity of 9,10-diphenylanthracene as a re-
ferencestandard(ΦF =97%).14 Asaresult, 8was foundto
exhibitanenhanced ΦF (ΦF = 12%) that isapproximately
4-fold higher than that of 6 (ΦF = 3%). The model
compound 9 containing a partical structure of the rotaxanes
was also synthesized as a control sample (Figure 3). It turned
out that the UVÀvis and fluorescence spectra of 9 were in
good accordance with those of 611 and the ΦF of 9 was
determined to be a similar value to that of 6 (9: ΦF = 4%).
This result suggests that the reason for the dramatic change of
the optical properties of 8 originates not from the absence of
the ammonium group but from the statistical existence of the
crown ether moiety.
Figure 3. Structure and ΦF of model compound 9.
Acknowledgment. This work was financially supported
by a Grant-in-Aid for Scientific Research from MEXT,
Japan (Nos. 24685023 and 25102510).
To clarify that the enhancement of ΦF accompanies
increased wheel mobility, the energy-minimized structure
of 8 was calculated using the density-functional theory
Supporting Information Available. Full experimental
details for all new compounds are provided, including 1H
NMR, 13C NMR, IR, UVÀvis, and fluorescence spectra
of the rotaxanes. This material is available free of charge
via the Internet at http://pubs.acs.org.
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Coord. Chem. Rev. 2010, 254, 2449–2458.
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€
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F.; Meinardi, F.; Anderson, H. L.; Beljonne, D.; Caciali, F.; Samori, P.
Adv. Funct. Mater. 2011, 21, 834–844. (b) Taylor, P. N.; Hagan, A. J.;
Anderson, H. L. Org. Biomol. Chem. 2003, 1, 3851–3856.
The authors declare no competing financial interest.
D
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