Colour coding the co-conformations of a [2]rotaxane flip-switch

Article abstract of DOI:10.1039/c1cc10948k

Fine-tuning the charge transfer chromophores in a series of [2]rotaxane flip-switches yields a unique optical signal (purple colour) for one of the interactions allowing for facile determination of the position of the flip-switch equilibrium.

Full text of DOI:10.1039/c1cc10948k

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Colour coding the co-conformations of a [2]rotaxane flip-switchwz
Natalie D. Suhan, Laura Allen, Mireille T. Gharib, Elizabeth Viljoen, Sarah J. Vella and
Stephen J. Loeb*
Received 17th February 2011, Accepted 30th March 2011
DOI: 10.1039/c1cc10948k
Fine-tuning the charge transfer chromophores in a series of
[2]rotaxane flip-switches yields a unique optical signal (purple
colour) for one of the interactions allowing for facile determination
of the position of the flip-switch equilibrium.
A mechanically interlocked molecule (MIM) such as a
[2]rotaxane can operate as a molecular switch if two different
recognition sites are available on the axle for recognition and
binding of the macrocyclic wheel.1 In this type of [2]rotaxane
molecular shuttle, the two states are translational co-conformers
related by the relative positioning of the macrocycle along the
length of the axle.2 Similarly, we reported the concept of a
molecular flip-switch that operates at a single recognition site on
a simple [2]rotaxane.3 These flip-switches utilize the templating
motif between 24-crown-8 wheels and 1,2-bis(pyridinium)-
Fig. 1 A [2]rotaxane flip-switch with two possible co-conformations
unique rotaxanes4 and catenanes5 and has been incorporated
that operate at a single recognition site.
ethane axles that has been successful in creating a variety of
into metal–organic frameworks (MORFs).6
absorption spectroscopy of new rotaxane flip-switches with
An example of a [2]rotaxane flip-switch is shown in Fig. 1.
Since both the axle and the wheel are unsymmetrical, the axle
contains two different pyridinium groups and the crown
contains two different aromatic rings, there are two possible
co-conformers. At low temperature (slow exchange), 1H NMR
spectra could be attained, two sets of pyridyl protons
(Dd E 0.5 ppm) were clearly visible due to differences in
shielding and integration could be used to determine the
co-conformer distribution. Alternatively, at room temperature
(fast exchange) the weighted average positions of these peaks
can sometimes be used to estimate the distribution.3a
chromophores appropriate for easily identifying co-conformer
distributions at room temperature. 1H NMR spectroscopy was
not capable of distinguishing co-conformers in this system as
chemical shift differences for axle protons were negligible.
Fig. 2 shows a series of [2]rotaxanes in which we have
attempted to manipulate the nature of the p–p interactions
between electron-rich aromatic rings of the crown ether wheel
It was of interest to expand the scope of this switchable
MIM system but the requirement of using NMR spectroscopy
and studying systems with large differences in shielding was
limiting and impractical. It was reasoned that optical spectro-
scopy would be a much more convenient method for studying
these rapidly equilibrating systems. This communication
describes the design, synthesis and characterization by visible
Department of Chemistry and Biochemistry, University of Windsor,
Windsor, Ontario, Canada N9B 3P4. E-mail: loeb@uwindsor.ca;
Fax: +1 519 973 7098; Tel: +1 519 253 3000
w This article is part of a ChemComm ‘Supramolecular Chemistry’ web-
based themed issue marking the International Year of Chemistry 2011.
z Electronic supplementary information (ESI) available: Details of
syntheses, spectroscopy and X-ray solutions. CCDC 813693 and
813694. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1cc10948k
Fig. 2 Different coloured [2]rotaxanes with different charge transfer
interactions due to p-stacking between the axle and wheel.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 5991–5993 5991

and the electron-poor pyridinium rings of the axle. The intent
was to produce a unique charge transfer chromophore and
therefore a unique visible colour that could be used to identify
and quantify one particular co-conformer.7
[2]Rotaxanes 1 and 2, containing dibenzo-24-crown-8,
DB24C8, were previously shown to exhibit charge transfer
absorptions giving rise to pale yellow and yellow-orange
solutions, respectively; 2 showed a clear maximum at 433 nm
in MeNO2 solution. It was rationalized that since these
absorption bands arise from charge transfer between the
HOMO on the crown ether wheel and the LUMO on the
pyridinium axle, the energy of the band could be manipulated
by increasing or decreasing this HOMO–LUMO gap.8
For this purpose, the new crown ether 4,5,40,50-tetramethoxy-
dibenzo-24-crown-8, (MeO)4DB24C8, was prepared in 43%
yield from the known tetraiodo species I4DB24C8 via a Cu(I)
catalyzed reaction in MeOH solution under mW assisted
conditions (ESIz). [2]Rotaxane, 3, containing (MeO)4DB24C8
and ester stoppers was prepared from the [2]pseudorotaxane
formed between (MeO)4DB24C8 and 1,2-bis((4-phenyl-
methanol)4-pyridinium))ethane by reaction with 3,5-dimethyl-
benzoic anhydride using published conditions.9 [2]Rotaxane 4
was prepared by stoppering the [2]pseudorotaxane formed between
(MeO)4DB24C8 and 1,2-bis(4,40-bipyridinium) ethane with
4-tert-butylbenzyl bromide employing mW assisted conditions.9
Gratifyingly, solutions of the four [2]rotaxanes containing
the four different pairs of charge transfer components yielded
four different colours with the colour of 4 being distinctly
different (purple, lmax = 551 nm in MeNO2) from the other
three; Fig. 3. The visible absorption spectra of these solutions
are shown in Fig. 4.
Fig. 4 Visible absorption spectra of [2]rotaxanes 1–4 in MeNO2
solution (10ꢀ3 M).
Crystals of 3 (yellow) and 4 (purple) containing the crown
ether (MeO)4DB24C8 were grown by diffusion of iPr2O into
MeCN solutions of the compounds and their structures are
shown in Fig. 5. (The structures of both 1 and 2 containing
DB24C8 were determined previously.)9 It is clear in both
X-ray structuresy that the methoxy-substituted aromatic rings
are involved in significant p-stacking interactions with the
aromatic groups of the pyridinium axle as designed.
Fig. 5 Ball-and-stick representations of the cationic portion of the
X-ray crystal structures of [2]rotaxanes 3 (left) and 4 (right) (atoms:
red = O, blue = N, black = C. Bonds: axle = gold, wheel = silver).
With the visible absorption properties of these four [2]rotaxanes
established, it was important to incorporate these features into
a [2]rotaxane flip-switch and test the validity of using optical
spectroscopy to quantify the co-conformer distribution in such
a system. To this end, two test [2]rotaxanes 5 and 6 and the
[2]rotaxane flip-switch, 7 were prepared utilising the crown ether
(MeO)2DB24C8 which contains both benzo and dimethoxy-benzo
aromatic groups (ESIz). By design, the [2]rotaxanes 5 and 6 must
contain equal contributions of two chromophores since the ends
of the axles are identical while rotaxane 7 could potentially display
all four chromophores since the ends of the axle are different. The
distribution of co-conformers for 7 is however unknown, since
there may be a preference for a co-conformer with a particular set
of p–p interactions (Fig. 6).
As designed, solutions of [2]rotaxanes 5 and 6 displayed
colours indicative of equal mixing of the individual chromo-
phores of 1 with 3 and 2 with 4, respectively; Fig. 7 (left). The
visible absorption spectra of 5 and 6 were then successfully
simulated10 based on equal contributions from their two
constituent chromophores; 1 plus 3 for 5 and 2 plus 4 for 6
(ESIz). These results served to verify the spectral modelling
criteria and using the same methodology it was possible to
simulate the spectrum of 7 (Fig. 8) and solve for the relative
distribution of conformers A and B. It was determined that
at room temperature, in the MeNO2 solvent, the [2]rotaxane
Fig. 3 From left to right: MeNO2 solution (10ꢀ3 M) of [2]rotaxanes
1, 2 containing DB24C8 and 3, 4 containing (MeO)4DB24C8.
c
5992 Chem. Commun., 2011, 47, 5991–5993
This journal is The Royal Society of Chemistry 2011

isomers determined by visible absorption spectroscopy. Fine-
tuning of the molecular interactions in the system to produce a
distinct colour that was easily observed by the naked eye and
quantifiable by optical spectroscopy bodes well for the detec-
tion of such isomer ratios in solid state materials such as liquid
crystals or metal–organic frameworks.
Notes and references
y Crystal data11 for [3][OTf]2.(MeCN)2: C78H88F6N4O22S2, M =
1611.64, T = 173(2) K, monoclinic, space group P21/c, a =
16.268(2), b = 12.443(1), c = 20.841(2) A, b = 110.648(1)1, V =
3947.7(7) A3, rcalc = 1.356 g cmꢀ3, m = 0.158 mmꢀ1, Z = 2,
reflections collected = 36 743 (Rint = 0.0942), final R indices [I 4
2sI]: R1 = 0.0827 , wR2 = 0.2020, R indices (all data): R1 = 0.1275,
wR2 = 0.2400, GoF = 1.015 with data/variables/restraints = 6942/
506/0.
Crystal
data
for
[4][OTf]4.(iPr2O)2(MeCN)2(H2O):
C92H126F12N6O27S4, M = 2104.23, T = 173(2) K, monoclinic, space
group P21/c, a = 13.845(2), b = 36.778(4), c = 20.786(2) A, b =
96.962(2)1, V = 10.506(2) A3, rcalc = 1.330 g cmꢀ3, m = 0.186 mmꢀ1
,
Z = 4, reflections collected = 100 488 (Rint = 0.1698). R indices [I 4
2sI]: R1 = 0.0990, wR2 = 0.2029, R indices (all data): R1 = 0.2350,
wR2 = 0.2715, GoF = 0.999 with data/variables/restraints = 18 466/
1325/348.
Fig. 6 The co-conformers (A and B) of [2]rotaxane flip-switch 7.
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Fig. 7 Left: MeNO2 solutions (10ꢀ3 M) of [2]rotaxanes 5 and 6.
Right: MeNO2 solution (10ꢀ3 M) of [2]rotaxane flip-switch 7.
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Fig. 8 Visible absorption spectrum of [2]rotaxane flip-switch 7 and
simulated contributions from 1–4 (MeNO2 solution at 10ꢀ3 M).
flip-switch 7 prefers co-conformer A over co-conformer B with
a distribution ratio A : B of 60 : 40. In co-conformer A, the
interaction of the most electron-rich crown aromatic ring
containing the MeO groups with the most electron-poor
pyridinium ring presumably stabilizes this arrangement of
the axle and wheel components.
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In summary, we have demonstrated that the rapid
equilibration between co-conformers inherent in a [2]rotaxane
flip-switch can be probed and ultimately the distribution of
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 5991–5993 5993