Folding and unfolding movements in a [2]pseudorotaxane

Article abstract of DOI:10.1021/jo101762k

A new dibenzo[24]crown-8 derivative (1) was synthesized and functionalized with aromatic moieties such as naphthalene and coumarin units. These two fluorophores are known to form an effective FRET (Forster resonance energy transfer) pair, and this formed

Full text of DOI:10.1021/jo101762k

pubs.acs.org/joc
Folding and Unfolding Movements in a [2]Pseudorotaxane
†
Moorthy Suresh, Amal Kumar Mandal, Manoj K. Kesharwani, N. N. Adarsh,
†
†
§
,
†
‡
, ‡
Bishwajit Ganguly,* Ravi Kumar Kanaparthi, Anunay Samanta,* and
,†
Amitava Das*
‡
Central Salt and Marine Chemicals Research Institute (CSIR), Bhavnagar, Gujarat, India, University of
†
§
Hyderabad, Hyderabad, India, and Indian Association for the Cultivation of Science, Kolkata, India
ganguly@csmcri.org; assc@uohyd.ernet.in; amitava@csmcri.org
Received September 10, 2010
A new dibenzo[24]crown-8 derivative (1) was synthesized and functionalized with aromatic moieties
such as naphthalene and coumarin units. These two fluorophores are known to form an effective
FRET (Forster resonance energy transfer) pair, and this formed the basis for the design of this host
crown ether derivative. Results of the steady-state and time-resolved fluorescence studies confirmed
the resonance energy transfer between the donor naphthalene moiety and acceptor coumarin
fragment, while NMR spectra and computational studies support a folded conformation for the
uncomplexed crown ether 1. This was found to form an inclusion complex, a [2]pseudorotaxane type
þ
with imidazolium ion derivatives as the guest molecules with varying alkyl chain lengths ([C mim] or
4
þ
[
interwoven inclusion complex (1 [C mim] or 1 [C mim] ). This change in conformation, from the
C mim] ). The host crown ether (1) tends to adopt an open conformation on formation of the
þ
1
0
þ
3
4
3
10
1
folded to a open one, was predicted by computational as well as H NMR studies and was confirmed
by single crystal X-ray structure for one (1 [C mim] ) of the two inclusion complexes. The increase in
4
the effective distance between the naphthalene and coumarin moieties in the open conformation of
these inclusion complexes was also supported by the decrease in the effective FRET process that was
operational between naphthalene and coumarin moieties in the free molecule (1). Importantly, this
inclusion complex formation was found to be reversible, and in the presence of a stronger base/polar
solvent, such as triethyl amine/DMSO, the deprotonation/effective solvation of the cationic
þ
3
þ
þ
imidizolium ions ([C mim] or [C mim] ) resulted in decomplexation or dethreading with restora-
4
10
tion of the original emission spectra for 1, which signifies the subsequent increase in the FRET
process. Thus we could demonstrate that a molecular folding-unfolding type of movement in the
crown ether derivative could be induced by chemical input as an imidazolium ion.
5
a-j
5k-m
5n,o
Introduction
π-π interaction,
and van der Waals
hydrogen bonding,
p,q
hydrophobic,
interactions. Change(s) in any or a
5
The development of inclusion complexes such as pseudoro-
1
2
3
combination of these interactions may lead to different
conformations of the host or orientation of the host and
the guest in the assembly with respect to each other. This
opens up the possibility of achieving desired mechanical
movements such as molecular motion with an appropriate
taxanes, rotaxanes, and catenanes, where the host-guest
assembly together can perform some desired function, is of
4
current research interest in supramolecular chemistry. Forces
that are generally operational for the generation of these host-
guest assemblies are various nonbonding interactions such as
1
38 J. Org. Chem. 2011, 76, 138–144
Published on Web 12/07/2010
DOI: 10.1021/jo101762k
r 2010 American Chemical Society

Suresh et al.
JOCArticle
response of the fluorescence output signal as a consequence
of the formation of a host-guest inclusion complex.
external stimulation. Achieving such molecular motion with
an ionic input could lead to the design of a supramolecular
assembly that is capable of mimicking the function of biological
8a
However, neither change in conformation nor any molecular
movement was reported as a result of such inclusion complex
formation. To demonstrate how RET can further be used to
probe the folding-unfolding movement in a definite self-
assembled system, we have synthesized a new crown ether
derivative (1) substituted with naphthalene (nap) and cou-
marin (cou) functionality as the donor and acceptor compo-
nent, respectively (Scheme 1), as these are known to form a
6
motors at the molecular level. Recently Rebek et al. have
shown that intermolecular resonance energy transfer (RET)
between pyrene and perylene moieties occurs when mono-
functionalized pyragallol or resorcinol arene moieties form
self-assembled hexameric capsule-like architecture. It is
shown that formation of such self-assembled structures is a
random process and one out of many conformers, and
7
8
b
assemblies could show the RET process. In our attempt to
RET pair and allow us to study any conformational change
of the crown moiety on forming the inclusion complex.
We have used imidazolium (1-butyl-3-methylimidazolium
develop such a system where the RET process can be
achieved with more control and as a result of some ionic
input, we have designed a simple bisbenzocrown derivative,
as this class of compounds is known to adopt different
conformations in the presence and absence of the guest
molecule. To the best of our knowledge there is only one
reference available in the literature that describes the RET
þ þ
[C mim] ) and 1-decyl-3-methylimidazolium ([C mim] ))
4 10
9
(
ions, as these are known to form a [2]pseudorotaxane type of
2f
inclusion complex with bisbenzo crown ether derivatives.
In this paper, we have described how the formation of a
þ
simple [2]pseudorotaxane adduct, between 1 and [C mim]
4
þ
or [C mim] , could induce conformational change of the
0
1
(
1) (a) Ashton, P. R.; Philp, D.; Spencer, N.; Stoddart, J. F. J. Chem. Soc.,
crown ether based host fragment and RET process opera-
tional between nap and cum fragments. Ratiometric changes
in the fluorescence readout signal as a consequence of the
RET process enabled us to read the conformational changes
or the molecular movements. Such an example is rare in the
literature.
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Stoddart, J. F.; White, A. J. P.; Williams, D. J. Angew. Chem., Int. Ed. Engl.
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Stoddart, J. F.; White, A. J. P.; Williams, D. J. Chem. Commun. 1996,
1
1
387–1388. (d) Mirzoian, A.; Kaifer, A. E. Chem.;Eur. J. 1997, 3, 1052–
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(
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Results and Discussion
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Chem.;Eur. J. 1996, 2, 640–643. (d) Leigh, D. A.; Murphy, A.; Smart, J. P.;
Slawin, A. M. Z. Angew. Chem., Int. Ed. Engl. 1997, 36, 728–732. (e) Jiang,
W.; Winkler, H. D. F.; Schalley, C. A. J. Am. Chem. Soc. 2008, 130, 13852–
A database search on the structural aspects of various
derivatives of dibenzo[24]crown-8 reveals both open and
folded conformations. In order to establish the actual con-
1
3853. (f) Li, L.; Clarkson, G. J. Org. Lett. 2007, 9, 497–500.
3) (a) Schill, G.Catenanes, Rotaxanes, Knots; Academic Press: New York,
971. (b) Nierengarten, J.-F.; Dietrich-Buchecker, C. O.; Sauvage, J.-P.
(
1
formation for the 1, its H NMR spectra was compared with
1
J. Am. Chem. Soc. 1994, 116, 375–376. (c) Armaroli, N.; Balzani, V.;
Barigelletti, F.; De Cola, L.; Flamigni, L.; Sauvage, J.-P.; Hemmert, C.
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Quintela, J. M. J. Am. Chem. Soc. 2009, 131, 920–921.
that of 2,3-dihydroxy naphthalene (N), 6,7-dihydroxy cou-
marin (C), and a physical equimolar mixture of N and C of
comparable concentrations clearly revealed an upfield shift
for H atom (6.20-6.18), H (7.65-7.55 ppm), and H , H
3
4
5
8
(
6.9-6.8 ppm) of coumarin moiety (Supporting Information).
(4) (a) Ballardini, R.; Balzani, V.; Credi, A.; Gandolfi, M. T.; Venturi, M.
Acc. Chem. Res. 2001, 34, 445–455. and references therein. (b) Hernandez,
J. V.; kay, E. R.; Leigh, D. A. Science 2004, 306, 1532. (c) Chatterjee, M. N.;
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V.; Lee, C.-F.; Kay, E. R.; Leigh, D. A. Nature 2007, 445, 523–527.
These upfield shifts owing to the shielding effect for the part
of the naphthalene moiety support a proximate distance
between coumarin and naphthalene moiety in 1 and thus
confirm the folded conformation. 2D-NOESY spectrum was
also recorded, which revealed the NOE effect between two
chromophores (Supporting Information). The observed
cross peaks for H /H at δ = 7.3 ppm and for H /H
at δ = 7.1 ppm confirm the NOE effect between nearby
H-atoms of two different aromatic moieties and thus the
folded conformation.
(
e) Sauvage, J. P. Acc. Chem. Res. 1998, 31, 611–619. (f) Raymo, F. M.;
Stoddart, J. F. Chem. Rev. 1999, 99, 1643–1963. (g) Onagi, H.; Rebek, J., Jr.
Chem. Commun. 2005, 4604–4606.
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Trans. 2000, 3715–3734. (b) Rebek, J.; Nemeth, D. J. Am. Chem. Soc. 1986,
4
a,f
4
c,d
108, 5637–5638. (c) Burley, S. K.; Petsko, G. A. J. Am. Chem. Soc. 1986, 108,
7995–8001. (d) Hamilton, A. D.; Engen, D. V. J. Am. Chem. Soc. 1987, 109,
5035–5036. (e) Zimmerman, S. C.; Vanzyl, C. M. J. Am. Chem. Soc. 1987,
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Science 1997, 277, 1793–1796. (g) Hunter, C. A. Angew. Chem., Int. Ed. Engl.
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Ferguson, S. B.; Diederich, F. Angew. Chem., Int. Ed. Engl. 1986, 25, 1127–
129. (j) Nishio, M.; Hirota, M. Tetrahedron 1989, 45, 7201. (k) Kotera, M.;
Lehn, J.-M.; Vigneron, J.-P. J. Chem. Soc., Chem. Commun. 1994, 197–199.
l) Sijbesma, R. P.; Meijer, E. W. Curr. Opin. Colloid Interface Sci. 1999, 4,
4–32. (m) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford
Any possibility of intermolecular interaction as a probable
reason for these upfield shifts was ignored; as no further shift
1
1
1
was observed when H NMR and 2D-NOESY spectra for 1
(
2
were recorded using an even higher concentration of 1.
þ
þ
Complexation Studies of Host 1 with [C mim] /[C mim] .
10
4
University Press: New York, 1997. (n) Ben-Naim, A. Hydrophobic Interac-
tions; Plenum Press: New York & London, 1980. (o) Bhasikuttan, A. C.;
Mohanty, J.; Nau, W. M.; Pal, H. Angew. Chem., Int. Ed. Engl. 2007, 46, 4120–
In order to examine the possibility of formation of the
interwoven complex between 1 and imidazolium ion, we
4
9
122. (p) Street, A. G.; Mayo, S. L. Proc. Natl. Acad. Sci. U.S.A. 1999, 96,
074–9076. (q) Ohta, K.; Ikejima, M.; Moriya, M.; Hasebe, H.; Yamamoto, I.
1
recorded H NMR spectra for 1 in the absence and presence
þ
of varying concentration of [C mim] in CD Cl (Figure 1).
2
J. Mater. Chem. 1998, 8, 1971–1977.
6) (a) Fletcher, S. P.; Dumur, F.; Pollard, M. M.; Feringa, B. L. Science
005, 310, 80–82. (b) Badjic, J. D.; Balzani, V.; Credi, A.; Silvi, S.; Stoddart,
4
2
(
A downfield shift of H proton from δ=8.47 ppm to δ=
I
2
8
.54 ppm indicates [C-H
3 3 3 O] interaction of the acidic H
I
J. F. Science 2004, 303, 1845. (c) Ballardini, R.; Balzani, V.; Clemente Leon,
M.; Credi, A.; Gandolfi, M. T.; Ishlow, E.; Perkins, J.; Stoddart, J. F.; Tseng,
H.-R.; Wenger, S. J. Am. Chem. Soc. 2002, 124, 12786–12795.
(8) (a) Ishow, E.; Credi, A.; Balzani, V.; Spadola, F.; Mandolini, L.
(7) (a) Barrett, E.; Dale, T. J.; Rebek, J., Jr. J. Am. Chem. Soc. 2008, 130,
344–2350. (b) Barrett, E. S.; Dale, T. J.; Rebek, J., Jr. Chem. Commun. 2007,
224–4226.
Chem.;Eur. J. 1999, 5, 984–989. (b) Lee, M. H.; Quang, D. T.; Jung, H. S.;
Yoon, J.; Lee, C.-H.; Kim, J. S. J. Org. Chem. 2007, 72, 4242–4245.
(9) Park, S.; Kazlauskas, R. J. J. Org. Chem. 2001, 66, 8395–8401.
2
4
J. Org. Chem. Vol. 76, No. 1, 2011 139

Suresh et al.
JOCArticle
1
þ
þ
FIGURE 1. H NMR spectra (500 MHz, CD
2
Cl
2
, 298 K) of (A) 1, (B) [C
4
mim] , and (C) 1 [C mim] in CD
4
2
Cl
2
.
3
SCHEME 1. Methodologies Adopted for Synthesis of 1, 1D, and 1A
þ
atom of [C mim] with the lone pair of electrons residing on
4
electron density upon complexation through formation of
[(C-H)Imidazolium OCrown] hydrogen bonding. One would
the oxygen atom of the macrocyclic crown ether backbone.
For H (δ = 7.29 ppm to 7.20 ppm) and HIV (δHIV = 4.15
ppm to 4.10 ppm) atoms, upfield shifts were observed. This
could be explained on the basis of the polarization or shift of
3 3 3
expect that stabilization of partial positive charge of the N
þ
II HII
1
atom of [C mim] by ion;dipole interaction involving the
4
nearby crown oxygen atoms shifts the electron density of the
1
40 J. Org. Chem. Vol. 76, No. 1, 2011

Suresh et al.
JOCArticle
1
0
imidazolium ring toward positive N atom. Thus, the protons
3
(HIII and H ) that are nearer to the N atom are influenced
by the increased electron density on the N atom of [C mim]
Information. However, the structure is good enough to
confirm the proposed interwoven complex formation in the
case of 1 [C mim] and validate the optimized structure for
4
IV 3
þ
þ
3
4
3
moiety. Thus, as a consequence of this polarization of
þ
the interwoven complex (vide infra, Figure 3b).
aromatic electrons, signals for H at 3.91 ppm in [C mim]
V
4
Computational Studies
is found to be downfield shifted to δ = 3.94 ppm in the
hydrogen-bonded adduct; the positive inductive effect of -CH3
partially stabilizes the positive charge on the N atom.
3
þ
The interwoven complex formation (1 [C mim] ) observed
4
3
through NMR and fluorescence spectral data was also
examined by molecular modeling studies. The conforma-
More interestingly, downfield shift (ΔδH = 0.24 ppm
3
þ
and ΔδH = 0.11 ppm) for H and H , with respect to that of
tional study of 1 and its interwoven complexes 1 [C mim]
4
4
4
3
3
þ
1, was observed for the [2]pseudorotaxane, which suggested
that the two arene fragments move away from each other
when [C mim] was included in the host crown moiety. Little
4
downfield shifts were also observed for H , H , and H .
8
and 1 [C mim] was evaluated with molecular mechanics
10
3
and DFT calculations. Initially, the conformational search
þ
þ þ
was performed for 1, 1 [C mim] , and 1 [C mim] with the
4 10
3
3
1
1
c
d
MM2 force field in CH Cl medium. The selected confor-
2 2
Further, the NOEs observed between H /H and H /H in 1
4
mers were taken for further optimization using the DFT
12
(GGA/BLYP/DNP) method in CH Cl employing a
c
4
þ
f
were absent in the interwoven complex, 1 [C mim] . All of
4
3
2
2
1
3
these observations suggest that the complex adopts a different
conformation compared to that in 1 with lesser (if any) π-π
stack interaction. This indicates an unfolding of the crown
COSMO solvation model. The lowest energy conformer
at the GGA/BLYP/DNP level of 1 was found to be the
folded structure having naphthalene and coumarin moieties
þ
˚
moiety of 1 in the [2]pseudorotaxane complex, [1 C mim] .
4
close to each other (6.77 A), which corroborates the feasi-
bility of FRET (Figure 3a). However, in the interwoven
3
Thus, the inclusion complex formation makes the crown
moiety change its conformation from a folded one to an
unfolded one. Associated shifts were also observed for
þ
þ
complex between 1 and [C mim] /[C mim] , the crown
10
4
ether conformation changes from a folded to an unfolded
situation, where the naphthalene and coumarin moieties
þ
aliphatic hydrogen atoms of 1 and [C mim] ; however, due
4
˚
˚
to the complicated nature of the overlapping spectra it was
difficult to assign the extent of the shift for individual hydrogen
move apart (15.09 A/15.88 A), and reduces the possibility
of energy transfer between these two moieties (Figure 3b/3c).
The calculated interwoven complex was found to be similar
þ
atoms. Similar studies were repeated using [C mim] instead
þ
10
þ
of [C mim] , and analogous unfolding of the crown con-
4
to that of the X-ray crystal structure obtained for 1 [C mim]
4
3
formation was also observed (Supporting Information).
The formation of the [2]pseudorotaxane was also con-
firmed by the ESI mass spectra: a strong m/z signal at 705.72
(Figure 2). These results further corroborate the reduction in
the FRET process, when the interwoven complex is formed.
Thus, it may appear from the above discussion that there is
an apparent disagreement between the optimized structure
for the lowest energy uncomplexed crown in which both
chromophores are positioned with a larger distance of
þ þ
100%) for 1 [C mim] and 789.75 (100%) for 1 [C mim]
4 4
(
3
3
confirms the 1:1 stoichiometry (Supporting Information).
Further, evidence for the proposed [2]pseudorotaxane for-
mation was obtained from the single crystal X-ray structure
for 1 [C mim]PF , though the R-factor (gt) was high (18%)
˚
∼6.7 A and the observed NMR spectral shifts due to π-π
4
6
interactions. Other low energy conformers for the uncom-
plexed crown showed the smaller distances between the
chromophores, which might be attributed to the observed
NMR signals in the experimental studies (Supporting Infor-
mation). Further, note that the π-π interaction between
3
(
Figure 2). Despite our best efforts, we could not develop a
better quality single crystal with a lower R-factor (gt), and we
have provided details about the structure in the Supporting
˚
substituted aromatic moieties can be observed within ∼7 A
1
4
as reported in the literature.
UV-vis, Fluorescence, and TCSPC Studies. The absorp-
tion spectrum of 1 in dichloromethane shows maxima at 280
nm (π f π*; log ε = 3.14, S f S ), a typical band pattern for
0
1
naphthalene moiety, and at 340 nm (π f π*; log ε = 3.28,
S f S ) for the coumarin moiety. Further, the absorption
0
1
spectra of 1 in different solvents of varying polarity did not
show any new features indicative of ground state interaction
between the naphthalene and coumarin subunits of 1
When excited at 280 nm, where naphthalene absorbs
predominantly (Figure 4), the fluorescence spectrum of 1
þ
FIGURE 2. Molecular structure of 1 [C
4
mim] .
3
þ þ
4
FIGURE 3. GGA/BLYP/DNP optimized lowest energy conformer of (a) 1, (b) 1 [C mim] , and (c) 1 [C10mim] .
3
3
J. Org. Chem. Vol. 76, No. 1, 2011 141

Suresh et al.
JOCArticle
-
7
þ
FIGURE 4. Fluorescence spectra of 1 (1.0 ꢀ 10 M) in dichloromethane upon addition of increasing concentration of (a) [C
4
mim] and
þ
(
b) [C10mim] ; λexc was 280 nm. Inset: overlap spectra for the emission spectrum of 1D and absorption spectrum of 1A.
consisted of emission predominantly from the coumarin
moiety (λmax = 420 nm) in addition to the typical weak
naphthalene emission at 340 nm. In order to understand the
process better, emission spectra of two model compounds,
with τ and τ given as 0.85 ( 0.01 ns and 0.32 ( 0.005 ns. The
1
2
decay parameters and in particular the presence of 0.32 ns
component with both the 320 and 470 nm emission bands and
negative pre-exponential factor associated with the later
emission provide convincing evidence of the energy transfer
process in 1. Comparison of the decay constants for 1 and 1D
(donor crown) 1D and (acceptor crown) 1A, in CH Cl
solution were recorded (Supporting Information). Emission
2 2
maximum for 1D was observed at 320 nm (λexc=280 nm, φ=
clearly reveals that the short lifetime (τ = 0.32 ( 0.002 ns
1
0
.35 with respect to naphthalene in cylcohexane, τ =9.61
(95.65%) of the naphthalene moiety in 1 is the consequence of
fluorescence quenching due to the RET process. Based on the
steady-state and time-resolved emission data, the Forster
1
ns), whereas that for 1A was at 420 nm (λexc=340 nm, φ =
.1 with respect to quinine sulfate in 0.1 M H SO , τ=0.9 ns).
The quantum yields were calculated using eq 1.
0
2
4
distance (R ) was evaluated using the standard equation and
o
ꢀ
˚
found to be 26.9 A with the energy transfer efficiency (Φ) of
96%. Thus, the energy transfer from photoexcited naphthalene
to nearby coumarin confirms the proximity of two chromo-
0
f
2
2
φ ¼ φ ðIsample=IstdÞðAstd=AsampleÞðη sample=η stdÞ
ð1Þ
f
0
where φf is the absolute quantum yield of the reference
compound, Isample and Istd are the integrated emission intensities,
Asample and A are the absorbance at the excitation wave-
phores, which is favorable for high energy transfer efficiency.
þ
On addition of an increasing amount of [C mim] or
4
std
þ
[
C mim] to a solution of 1 in CH Cl , a gradual decrease
2
2
2
10
2
length, and η
and η are the respective refractive indices.
std
sample
in the fluorescence emission at 420 nm (λexc = 280 nm) with
concomitant enhancement at 340 nm was registered
Further, examination of the emission spectra of 1D and the
absorption spectra of 1A suggests a significant spectral overlap
and thus supports the possibility of resonance energy transfer
from the photoexcited naphthalene moiety to the coumarin
fragment in 1, following the Forster theory by weak dipole-di-
pole induced interaction. To ascertain that such a process is
operational in 1, we have carried out time-resolved emission
studies using the time correlated single photon counting
(Figure 4). The ratiometric emission response of 1 with the
revival of naphthalene emission at 340 nm at the expense of
coumarin emission with the increase in concentration of
þ þ
C mim] or [C mim] suggests the formation of the interwo-
4 10
[
ven[2]pseudorotaxane complex. This causes a change in con-
formation of the crown ring in 1 from a folded to an un-
folded one. Thus, this conformational change partially inter-
rupts the interaction of coumarin and naphthalene moieties
that existed in the folded conformation and reduces the
(
TCSPC) technique (Supporting Information).
Fluorescence decay traces (λexc = 280 nm) monitored at
20 nm could be best fitted to a biexponential function with
3
þ
RET. Fluorescence decay traces recorded for 1 [C mim]
4
3
time constants of τ = 0.32 ( 0.002 ns (95.65%) and τ =
þ
1
2
and 1 [C mim] following excitation with a 280 nm source
10
3
8.68 (0.02 ns (4.35%), and the decay trace monitored at 470 nm
are shown in Supporting Information Figures 25 and 26.
The decay traces, monitored at (acceptor emission), could
be best fitted to a biexponential model with decay time
constants with τ =0.33 ( 0.03 ns (42.5%) and τ =1.1 (
0.01 ns (57.5%) for the complex 1 [C mim] and τ =0.25 (
0.01 ns (70%) and τ = 1.05 ( 0.01 ns (30%) for complex
was best fitted to the biexponential function with a negative
pre-exponential factor, I(t) = 1.61exp (-t/τ ) - 0.61exp (-t/τ )
1
2
1
2
3
þ
(
10) Crystal dimension: 0.14 ꢀ 0.04 ꢀ 0.02 mm ; T = 100(2) K; mono-
3
4
1
clinic, space group C2/m ; a = 32.186(4), b = 11.5459(13), c = 12.7286(15)
ꢀ
˚
ꢀ
˚
3
-3
2
A; β = 104.996(3)°; V = 4569.1(9) A ; Z = 4, Fcalcd = 1.229 g cm ; μ =
þ
-1
0
.136 mm ; F(000) = 1764; θmin/max = 1.31°/25.00°; Rint = 0.0852; range of
1 [C mim] . Comparison of these decay parameters with
those for 1 suggests that the component for the coumarin
moiety has gone down from 100% to 30% with an associated
10
3
h, k, l = -30/38, -13/13, -14/15; 11,533 reflections collected of which 4223
were unique, 2412 observed [I > 2σ(I)] reflections, 266 parameters were
2
= 0.1827, wR2 = 0.4524; goodness of fit on F = 1.492; CCDC
refined; R
1
þ
þ
7
75359 contains the supplementary crystallographic data for this paper.
11) (a) Allinger, N. L. J. Am. Chem. Soc. 1977, 99, 8127–8134. (b)
Burkert, U.; Allinger, N. L. Molecular Mechanics; ACS Monograph 177;
increase in the naphthalene component for 1 [C mim] .
10
3
(
The extent of these changes is a little less for 1 [C mim] .
4
3
Thus, TCSPC studies support the unfolding of the crown
conformation of 1 on formation of a [2]pseudorotaxane com-
plex. Residual emission for the coumarin moiety (λexc=280
and λemi = 420 nm) suggests the both folded and unfolded
conformation exist in equilibrium in the solution. This is also
indicative of a modest affinity constant value for the inter-
woven complex formation.
American Chemical Society: Washington, DC, 1982.
(
12) (a) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.;
Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1992, 46, 6671–6687.
b) Becke, A. D. J. Chem. Phys. 1997, 107, 8554–8560. (c) Lee, C. L.; Yang,
W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789.
13) (a) Neese, F. J. Chem. Phys. 2003, 119, 9428. (b) Neese, F. Curr. Opin.
Cell Biol. 2003, 7, 125–135.
14) (a) Hunter,C.A.;Lawson,K.R.;Perkins,J.;Ursh,C.J. Chem. Soc., Perkin
Trans. 2 2001, 651–669. (b) Burley, S. K.; Petsko, G. A. Science 1985, 229, 23–28.
(
(
(
1
42 J. Org. Chem. Vol. 76, No. 1, 2011

Suresh et al.
JOCArticle
The energy transfer efficiencies (Φ) between naphthalene
in 75 mL of dry DMF was added dropwise for 2 h at 60 °C. Then
the temperature was raised to 80 °C, and the mixture was
allowed to stir for 5 days. The solvent was removed under
þ
þ
and coumarin moiety in 1 [C mim] and 1 [C mim]
were evaluated. For 1 [C mim] , Φ is 85.2%, whereas for
3
4
þ
3
10
3
4
þ
reduced pressure and extracted three times with CHCl and
3
1
1
[C mim] , Φ is 76%. The association constants (K ) of
3
10
a
þ þ
:1 complex for 1 [C mim] and 1 [C mim] are found to
4
water. Organic layers were combined and dried over anhydrous
sodium sulfate. Solvent was removed under reduced pressure to
give crude product that was purified on a silica gel column, using
chloroform/methanol (98:2 v/v) as an eluent to yield 1D (0.85 g,
3
3
10
-1
-
1
-1
be 162.92 M (ΔG°=-3.22 kcal mol ) and 232.27 M
-
1
(
affinity for 1 [C mim] explains the relatively lower resi-
ΔG° = -3.01 kcal mol ), respectively. Higher binding
þ
1
3
10
62%), as a white powder. H NMR (500 MHz, CD Cl , δ ppm)
2
2
dual emission for the folded conformation (λ =280 nm,
exc
7.680-7.634 (2H, dd, J=3.2), 7.344-7.297 (2H, dd, J=3.2); 7.1
(2H, s), 6.878 (4H, s), 4.267 (4H, t, J=4.4), 4.162 (4H, t, J=4.4),
λemi=420 nm). Addition of polar solvents such as DMSO to
the inclusion complex resulted in retrieval of emission of 1
due to the effective solvation of imidazolium cations by polar
solvent. Thus, addition of DMSO to the pseudorotaxane
complex dethreads the imidazolium ion from the crown and
makes the system reversible (Supporting Information).
1
3
4.003 (4H, t, J=4.4), 3.934 (4H, t, J=4.6), 3.882 (8H, s).
NMR: 150.9, 131.1, 128.0, 125.9, 123.2, 116.1, 109.7, 73.1, 71.9,
1.1. Elemental analysis calcd for C28 : C, 67.45; H, 6.87.
C
7
34 8
H O
Found: C, 67.2; H, 6.8. ESI-MS: calcd for C H O 498.56,
28 34
8
þ
found 521.58 [M þ Na] . Mp: 90 °C.
Synthesis of 1A. Esculetin (6,7-dihydroxy coumarin) (0.52 g,
.9 mmol) was dissolved in 100 mL of freshly dried DMF in a
2
Conclusion
two-neck round-bottom flask. To this solution was added
Cs CO powder (1.9 g, 5.8 mmol).Then the reaction mixture
2
3
In brief, we have shown that inclusion of imidazolium
ions, having a C10 or C -alkyl chain, in nonpolar media
turned from yellow to pale green color. This mixture was
allowed to stir for 15 min, and compound D (ditosylate of
oligo(ethylene glycol) derivative of benzene) (2.0 g, 2.9 mmol)
dissolved in 75 mL of dry DMF was added dropwise for 2 h at
40 °C. Then temperature was raised to 80 °C, and the mixture
was allowed to stir for 5 day. The solvent was removed under
4
forms respective [2]pseudorotaxane complexes with a poly-
ether macrocycle, which in the absence of any inclusion
adopts a folded conformation. The inclusion process induces
the macrocyclic moiety to unfold to a different extent based
on the alkyl chain length. Appropriate choice of the two
chromophores belonging to a FRET pair allowed us to probe
different degrees of mechanical movement on formation of
3
reduced pressure and extracted three times with CHCl and
water. Organic layers were combined and dried over anhydrous
sodium sulfate. Solvent was removed under reduced pressure to
give crude product that was purified on a silica gel column, using
[
2]pseudorotaxane complexes. Thus, the controlled move-
chloroform/methanol (98:4 v/v) as an eluent to yield 1A (0.8 g,
5
ment of molecular components by selective chemical stimuli
can be exploited in designing nanomolecular devices.
1
4%), as a pale yellow powder. H NMR (500 MHz, CD Cl ,
2
2
δ ppm) 7.611-7.592 (1H, d, J=9.5), 6.909 (1H, s,), 6.883 (4H, s),
.822 (1H, s), 6.223-6.204 (1H, d, J=9.5), 4.182 (2H, t, J=4.5),
4.143 (2H, t, J=4.5), 4.108 (4H, s), 3.909 (2H, t, J=4.5), 3.879
6
Experimental Section
1
3
(
1
1
2H, t, J=4.5), 3.855-3.843 (4H, q), 3.764 (8H, s). C NMR:
Synthesis of 1. Esculetin (6,7-dihydroxy coumarin) (0.49 g,
62.9, 154.6, 152.1, 150.8, 147.7, 145.2, 123.3, 116.1, 115.3, 112.9,
32 10
03.0, 73.0, 71.7, 71.2, 31.6. Elemental analysis calcd for C27H O :
2
.7 mmol) was dissolved in 100 mL of freshly dried DMF in a
two-neck round-bottom flask. To this solution was added
Cs CO powder (1.8 g, 5.4 mmol). This mixture was allowed
to stir for 15 min, and the compound B (ditosylate of oligo-
ethylene glycol) derivative of naphthalene) (2.0 g, 2.7 mmol)
dissolved in 75 mL of dry DMF was added dropwise for 2 h at
C, 62.78; H, 6.24. Found: C, 62.43; H, 6.18. ESI-MS: calcd for
2
3
þ
C
27
H
32
O
10 516.54, found 517.24 [M þ 1] . Mp: 180 °C.
(
Computational Methods
6
0 °C. Then temperature was raised to 80 °C, and the mixture
was allowed to stir for 5 days. The solvent was removed under
An exhaustive conformational search was performed in
CH
modeling program Macromodel using MM2 force field for
000 Monte Carlo steps. Conformations whose molecular me-
2 2
Cl as solvent (dielectric constant = 8.9) with the molecular
reduced pressure and extracted three times with CHCl and
3
15
11
water. Organic layers were combined and dried over anhydrous
sodium sulfate. Solvent was removed under reduced pressure to
give crude product that was purified on a silica gel column, using
5
chanics energy is within 50 kJ/mol compared to calculated
global energy minimum (GEM) of a compound were stored.
Energy minimizations were performed with the Polak-Ribiere
conjugate gradient (PRCG) method, which involves the use of
chloroform/methanol (98:2 v/v) as an eluent to yield 1 (0.87 g,
1
8%), as a white powder. H NMR (500 MHz, CD
5
7
7
2
Cl
2
, δ ppm)
.654-7.635 (2H, dd, J = 3.5), 7.561-7.542 (1H, d, J = 9.5),
1
6
˚
first derivatives with convergence criterion set to 0.05 kJ/A-mol.
Conformational search of crown 1 produced total 2564 unique
.311-7.292 (2H, dd, 3.5), 7.103-7.096 (2H, d, J=3.5), 6.833
(
(
(
1H, s), 6.795 (1H, s), 6.181-6.162 (2H, d, 9.5), 4.228-4.201
4H, q, J=4.5), 4.172 (2H, t, J=4.5), 4.128 (2H, t, J=4.5), 3.938
4H, t, 4.5), 3.893 (4H, t, J = 4.5), 3.810-3.804 (8H, m).
conformers, which includes 397 conformers with good con-
vergence. However, conformational search of 1 [C mim] and
þ
1
3
4
3
C
þ
1
[C10mim] produced totals of 1993 and 2428 unique confor-
NMR: 161.8, 152.8, 150.2, 149.0, 145.9, 143.3, 129.4, 126.4,
3
mers, and only 30 and 91 conformers were of good convergence,
respectively. Unique conformers with good convergence, includ-
ing lowest energy conformer, were stored for further higher level
DFT calculations.
1
6
24.4, 113.5, 111.74, 110.7, 107. 9, 101.1, 71.6, 70.2, 69.9, 69.5,
9.2. Elemental analysis calcd for C31 10: C, 65.71; H, 6.05.
34
H O
Found: C, 65.23; H, 5.96. (ESI-MS) calcd for C H O 566.60,
31
34 10
þ
found 589.52 [M þ Na] . Mp: 160 °C. HRMS: calcd 589.2050
þ
The selected conformers were optimized with generalized
for [1 þ Na] ; found 589.2023 (Supporting Information).
1
2
gradient approximation (GGA) using the BLYP functional
integrated in density functional program DMol3 (version 4.1.2)
Synthesis of 1D. Catechol (2,3-dihydroxy benzene) (0.3 g,
2
.7 mmol) was dissolved in 100 mL of freshly dried DMF in a
two-neck round-bottom flask. To this solution was added
Cs CO powder (1.8 g, 5.4 mmol). This mixture was allowed
(15) Mohamdi, F.; Richards, N. G. J.; Guida, W. C.; Liskamp, R.;
2
3
Lipton, M.; Caufield, C.; Chang, G.; Hendrickson, T.; Still, W. C.
J. Comput. Chem. 1990, 11, 440–467.
(16) Polak, E.; Ribiere, G. Rev. Fr. Inf. Rech. Oper. 1969, 16-R1, 35–43.
to stir for 15 min, and compound B (ditosylate of oligo(ethylene
glycol) derivative of naphthalene) (2.0 g, 27.32 mmol) dissolved
J. Org. Chem. Vol. 76, No. 1, 2011 143

Suresh et al.
JOCArticle
17
by Accelrys Inc. The physical wave functions are expanded in
terms of numerical basis sets. We used a DNP (double numerical
polarized) basis set, which is comparable to the 6-31G** basis
set. The conductor-like screening model (COSMO) was employed
A.K.M. thank CSIR, and M.K.K. and R.K.K. thank
UGC, New Delhi for a Senior Research Fellowship.
1
3
for solvent calculations, and CH
2
Cl
2
were used as solvent.
Supporting Information Available: Synthetic details for
1
13
reaction intermediates, characterization data ( H NMR, C,
ESI-MS, 2D-NOESY spectra), and spectroscopic results
(UV-vis, fluorescence, and time correlated single photon
counting data). This material is available free of charge via the
Internet at http://pubs.acs.org.
Acknowledgment. DST (Grant No. SR/S1/IC-07/2009)
and CSIR India have supported this work. M.S. and
(
17) Delley, B. J. Chem. Phys. 2000, 113, 7756–7763.
1
44 J. Org. Chem. Vol. 76, No. 1, 2011