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Geometrically Precisely Defined Multinanometer ExpansionContraction Motions in a Resorcin[4]arene Cavitand Based Molecular Switch.

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Angewandte
Chemie
developing a rigid molecular system in which both states
adopt a precisely defined geometry and report here a first
example based on a bridged resorcin[4]arene scaffold. These
cavitands, first introduced by Cram and co-workers,[8] can be
reversibly switched between a contracted (vase) and an
expanded (kite) conformation upon variation of temperature[8–10] or pH,[11] or upon metal-ion addition.[12]
Recent advances in the synthesis of partially or differently
bridged resorcin[4]arene cavitands[13, 14] enabled the preparation of 1, in which two opposite cavity walls are greatly
extended by attachment of rigid oligo(phenylene ethynylene)
arms. A donor–acceptor BODIPY (dipyrrometheneboron
difluoride) dye pair[15] is attached to the oligomeric termini to
allow observation of the switching process by fluorescence
resonance energy transfer (FRET).[16] BODIPY dyes were
chosen for the targeted pH-induced switching, since their
emission behavior features low sensitivity to pH and environmental polarity, as we could confirm in model studies.
Computer model building,[17] based on X-ray crystal structures of bridged resorcin[4]arene cavitands in both vase and
kite conformations,[12, 13b] suggested a 1000 % difference in
the distance between the dye pair in the contracted ( 7 :)
and expanded ( 7 nm) states (Figure 1), which should allow
convenient detection of different FRET behavior in the two
states.[16a]
Molecular Devices
Geometrically Precisely Defined
Multinanometer Expansion/Contraction
Motions in a Resorcin[4]arene Cavitand Based
Molecular Switch**
Vladimir A. Azov, Anna Schlegel, and
Franois Diederich*
The invention of molecular devices that undergo reversible
controlled mechanical movements on the nanometer scale
and thereby mimic natural phenomena such as muscle
expansion/contraction is a hot target in contemporary
chemistry.[1] Elegant examples of molecular and supramolecular switches with bistable conformations of proFigure 1. Donor–acceptor dye-labeled cavitand 1 and its extensions in the confoundly different geometry have been described in recent
tracted (vase) and expanded (kite) states. In the models, the alkyl chains (“legs”)
of the cavitand are omitted.
years.[2–7] On the other hand, the conformational space
available in the contracted and expanded states of the
reported systems often is rather large and the geometric
differences not always well defined. We became interested in
The synthesis of 1 proceeded in a highly convergent
manner. Sonogashira cross-coupling[18] of 2[15b] with Me3Si-C
CH afforded 3, which was deprotected to give 4. A second
[*] Dr. V. A. Azov, A. Schlegel, Prof. Dr. F. Diederich
Laboratorium f+r Organische Chemie
cross-coupling with aryl iodide 5[19] provided 6, which was
ETH-H1nggerberg, HCI
deprotected to yield the donor dye-appended 7. A similar
8093 Z+rich (Switzerland)
route led from 8[15a] (via 9!10!11) to the acceptor building
Fax: (+ 41) 1-632-1109
block 12 (Scheme 1).
E-mail: diederich@org.chem.ethz.ch
The cavitand core 13 was prepared by bridging 14[13, 14]
[**] This research was supported by the Swiss National Science
with imide 15 (Scheme 2). 1H NMR investigations revealed
Foundation (NFP-47 “Supramolecular Functional Materials” and
reversible vase–kite switching of 13 induced either by changes
NCCR “Nanoscience”). We thank Rolf HCfliger for providing highin temperature or pH. Upon addition of trifluoroacetic acid
resolution mass spectra.
(TFA) to a solution of 13 in CH2Cl2 or when the temperature
Supporting information for this article is available on the WWW
was lowered from 60 8C to 60 8C, the signals of the methine
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 4635 –4638
DOI: 10.1002/anie.200500970
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4635
Communications
Supporting Information). Upon lowering the
temperature to 60 8C or upon addition of
TFA (up to 0.27 m), they both undergo
expansion to the kite forms as clearly evidenced by the characteristic large upfield
shift ( 5.6! 3.6 ppm) of the resorcinarene methine protons.[20]
Whereas the fluorescence spectrum of
bis(donor dye)-appended 17 is nearly identical in the contracted and expanded forms (see
the Supporting Information), the emission
behavior of target compound 1 differs dramatically in the two states. The UV/Vis
spectrum of 1 (Figure 3 a) in CHCl3 (c =
1.0 F 105 m) displays three strong absorption
bands assigned to the oligo(phenylene ethynylene) spacers (lmax 332 nm), the donor dye
(529 nm), and the acceptor dye (619 nm);
these bands are not affected by the addition
of TFA. The emission spectrum in CHCl3 (c =
0.5 F 107 m, lexc = 490 nm, 20 8C) features two
strong peaks at 542 nm (donor dye) and
630 nm (acceptor dye) with an integral ratio
of the donor/acceptor fluorescence intensity
Scheme 1. Synthesis of the donor dye (7) and acceptor dye (12) building blocks. a) Me3Siof 45:55. In view of the prevalence of the
CCH, [Pd(PPh3)4], CuI, Et(iPr)2N, tetrahydrofuran (THF), 20 8C, 1–3 d; b) Bu4NF, THF,
contracted vase conformation under these
78 8C, 30 min; c) 5, [Pd(PPh3)4], CuI, Et(iPr)2N, THF, 20 8C, 1–3 d.
conditions, as demonstrated by 1H NMR
spectroscopy, this relatively low FRET efficiency is rather surprising. We tentatively explain it by either
protons in the resorcin[4]arene core shifted upfield from
a) the dynamic behavior of the cavitand,[20] with constant
5.6 ppm to 3.6 ppm, which is characteristic for the
expansion process (see the Supporting Information). Sonowagging vibrations separating the two dyes one from another
gashira cross-coupling of 13 with 7 afforded a mixture of the
for a statistically important amount of time, or b) unfavorable
three cavitands 13, 16, and 17, which was separated by gel
orientations of the transition dipole moments of the dyes.
permeation chromatography (GPC, Biorad-BioBeads SX-1,
CH2Cl2 ; see the Supporting Information). A second crosscoupling of 16 with 12 provided the target structure 1 as a
stable, high-melting dark-violet solid after GPC purification.
The reversed sequence of dye addition to 13 (first 12 followed
by 7) was much less fruitful due to the poor solubility of the
singly labeled intermediate. The spectroscopic data fully
support the structural assignment for 1. The base peak in the
high-resolution matrix-assisted laser-desorption-ionization
mass spectrum (HR-MALDI-MS, matrix: 3-hydroxypicolinic
acid (3-HPA)) corresponds to the molecular ion after loss of
one F atom at m/z 2880.2161 ([MF]+, C188H156B2F3N14Oþ12 ;
calcd 2880.2160).
To test distance-dependent FRET between the chosen
donor and acceptor BODIPY dye pair, we also prepared the
three oligo(phenylene ethynylene)s 18 a–c of different lengths
(Figure 2). Fluorescence spectroscopy in very dilute CHCl3
solution (c = 0.5 F 107 m) showed that the FRET efficiency
strongly decreases with increasing length of the rigid spacer
from about 98 % for 18 a (distance r = 19 : between the B
atoms of the dyes), to 85 % for 18 b (r = 32 :), and to
35 % for 18 c (r = 53 :). Importantly, addition of TFA (up
to c = 0.3 m) does not affect the emission behavior.
The two bis(BODIPY dye)-appended cavitands 1 and 17
Figure 2. FRET intensities of 18 a–c in CHCl3 (c = 0.5 H 107 m). The
were shown by 1H NMR spectroscopy to adopt the contracted
higher energy band is the donor emission and the lower energy band
(vase) conformation in CD2Cl2 or CHCl3 above 25 8C (see the
the acceptor emission.
4636
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2005, 44, 4635 –4638
Angewandte
Chemie
Experimental Section
Compound 1: Cavitand 16 (13.2 mg,
5.73 mmol), 12 (9.3 mg, 13 mmol,
2.25 equiv), [Pd(PPh3)4] (1.3 mg,
0.11 mmol), and CuI (0.22 mg,
0.11 mmol) were mixed together in
a Schlenk apparatus filled with Ar.
CHCl3 (0.3 mL) and THF (0.3 mL)
were added, and the mixture was
degassed by freeze-pump-thaw
cycles.
Et(iPr)2N
(0.02 mL,
120 mmol) was added and the mixture degassed again, then stirred at
35 8C for 2 d. The mixture was
concentrated to dryness, and flash
chromatography (SiO2 ; CH2Cl2/
EtOAc gradient 0!0.5 %) yielded
crude 1. Further purification by
repeated GPC (Biorad BioBeads
SX-1, CH2Cl2), afforded 1 (8.2 mg,
49 %) as a dark-violet solid. M.p.
> 300 8C
(decomp.);
Rf = 0.32
(SiO2 ; CH2Cl2/EtOAc 99.4:0.6);
1
H NMR (300 MHz, CDCl3): d =
0.91–1.02 (m, 18 H), 1.26–1.54 (m,
50 H), 2.23 (s, 6 H), 2.22–2.36 (m,
12 H), 2.49–2.54 (m, 10 H), 2.86 (br t,
J = 7, 4 H), 5.62 (t, J = 8.1, 2 H), 5.71
(t, J = 8.1, 2 H), 7.24–7.62 (m, 38 H),
7.66–7.74 (m, 4 H), 7.86–7.91 (m,
4 H), 8.25 (s, 4 H), 8.80 ppm (br d,
J = 7.8, 2 H); 13C NMR (125 MHz,
CDCl3): d = 12.10, 12.55, 12.75,
Scheme 2. Synthesis of target compound 1. a) K2CO3, Me2SO, 35 8C, 2 d; b) 7
(0.75 equiv), [Pd(PPh3)4], CuI, Et(iPr)2N, THF, 35 8C, 2 d; c) 12 (2.25 equiv), [Pd(PPh3)4], CuI, Et(iPr)2N, THF, 35 8C, 2 d.
Upon addition of TFA, (Figure 3 b) the acceptor fluorescence vanishes nearly completely, whereas the donor fluorescence doubles in intensity. In the expanded state, with a
separation of donor and acceptor of 7 nm, the FRET
efficiency is dramatically reduced. This result could only be
obtained since the expanded conformation of 1 is very rigid,
strongly limiting the available conformational space. Intermolecular FRET is clearly not effective at the low concentration range of the experiment (c = 0.5 F 107 m). The spectral
changes are reversed upon reneutralization with base (Et(iPr)2N).
Thus, the large-scale expansion/contraction movement of
1 (Figure 1) is clearly proven by both 1H NMR spectroscopy
and FRET measurements. We are now further investigating
the dynamics of the switching motion in single-molecule
studies using confocal fluorescence microscopy.[21]
Angew. Chem. Int. Ed. 2005, 44, 4635 –4638
www.angewandte.org
Figure 3. a) UV/Vis spectrum of 1 in CHCl3 at 20 8C (c = 1.0 H 105 m).
b) Fluorescence spectra of 1 in CHCl3 at 20 8C (c = 0.5 H 107 m)
recorded at different concentrations of TFA.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4637
Communications
14.29, 14.81, 17.26, 18.33, 18.36, 20.66, 22.89, 28.16, 28.20, 29.58, 29.59,
30.70, 32.09, 32.10, 32.39, 32.89, 34.42, 34.51, 90.57, 90.61, 90.76, 91.00,
91.35, 91.40, 119.02, 123.13, 123.19, 123.50, 123.83, 123.97, 124.00,
125.37, 127.57, 128.32, 128.55, 128.60, 128.86, 129.21, 129.33, 129.66,
129.72, 130.73, 131.59, 131.87, 131.89, 131.93, 131.98, 132.41, 132.47,
132.55, 133.24, 133.92, 134.92, 135.93, 135.98, 136.29, 136.37, 136.72,
137.08, 137.68, 138.31, 139.28, 140.03, 140.92, 141.72, 151.18, 152.28,
152.46, 153.31, 154.36, 159.08, 161.68 ppm; 19F NMR (CDCl3,
282.5 MHz): d = 134.5 (q, J = 33.5), 145.1 ppm (q, J = 32.5); IR
(neat): ñ = 2957 (w), 2924 (m), 2855 (w), 1739 (s), 1524 (m), 1479 (m),
1444 (m), 1411 (s), 1362 (s), 1326 (s), 1275 (m), 1231 (m), 1190 (s),
1158 (s), 1116 (m), 1103 (m), 1083 (m), 1018 (m), 979 (m), 898 (m), 837
(m), 760 (m), 708 (m), 669 (w), 604 cm1 (w); UV/Vis (CHCl3): lmax
(e) = 332 (184 000), 529 (72 000), 571 (21 000), 619 nm (98 000);
Fluorescence (CHCl3, lexc = 490 nm): lmax = 542 nm, 630 nm; HRMALDI-MS
(matrix:
3-HPA):
2880.2161
([MF]+,
þ
C188H156B2F3N14O12, calcd 2880.21599).
[7]
[8]
[9]
[10]
[11]
[12]
[13]
Received: March 16, 2005
[14]
.
Keywords: cavitands · conformation analysis · FRET
(fluorescence resonance energy transfer) · molecular devices
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Angew. Chem. Int. Ed. 2005, 44, 4635 –4638
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