Chiroptical Switching in a Bistable Molecular Shuttle

Article abstract of DOI:10.1021/ja036665t

Although various methods for switching the positions of macrocycles in bistable rotaxane-based molecular shuttles have been developed, exploiting such movements to trigger property changes has thus far received little attention. Here we describe one of th

Full text of DOI:10.1021/ja036665t

Published on Web 10/14/2003
Chiroptical Switching in a Bistable Molecular Shuttle
Giovanni Bottari, David A. Leigh,* and Emilio M. Pe´rez
School of Chemistry, UniVersity of Edinburgh, The King’s Buildings, West Mains Road, Edinburgh EH9 3JJ, U.K.
Received June 13, 2003; E-mail: david.leigh@ed.ac.uk
Although various methods for switching the positions of mac-
rocycles in bistable rotaxane-based molecular shuttles have been
developed,1 exploiting such movements to trigger property changes
has thus far received little attention.2,3 Here we demonstrate one
of the first examples of a property change achieved through a large
amplitude translational motion in a rotaxane, a chiroptical switch
in which light-induced shuttling of the macrocycle along the thread
produces a strong induced circular dichroism (ICD) response when
the macrocycle is hydrogen-bonded to a chiral peptide station.
Chiral dipeptides have previously been shown to induce an
asymmetric response in the aromatic ring absorption bands of
intrinsically achiral components of [2]rotaxanes through tight
intercomponent binding in nonpolar solvents.4,5 The effect can be
“switched off” by adding a polar solvent (e.g., MeOH) to break
the hydrogen-bonding interactions between macrocycle and thread.
Although triggered changes in optical properties are currently
utilized in optical data storage and processing, waveguides, and
other photonics applications,6 a solvent change is clearly unlikely
to prove useful as a means of switching such real-world devices.
However, the breaking (and making) of intercomponent interactions
Scheme 1. Synthesis of Bistable Molecular Shuttles E/Z-1a
Figure 1. 1H NMR spectra (400 MHz in CDCl3 at 298 K) of (a)
fumaramide thread E-2, (b) fumaramide rotaxane E-1, (c) maleamide thread,
Z-2, (d) maleamide rotaxane Z-1. The assignments correspond to the lettering
shown in Scheme 1.
is also a feature of positionally bistable molecular shuttles.
Accordingly, it seemed possible that optical properties could be
influenced solely by switching the position of a macrocycle in a
molecular shuttle that incorporates a chiral peptide “station”.
Such a bistable shuttle, E/Z-1, is shown in Scheme 1. The idea
is that in E-1 the macrocycle resides over the strongly macrocycle-
binding fumaramide portion of the thread and the asymmetric center
is not close enough to any aromatic rings to influence their
absorption spectrum. Upon photoisomerization of the olefin station
(E-1fZ-1), the ring moves to the glycyl-L-leucine (Gly-Leu) unit,
locking the molecule in a co-conformation where aromatic rings
(principally those of the C-terminal stopper4) are held in a well-
expressed chiral environment. The change in the position of the
macrocycle should thus manifest itself in terms of a measurable
change in the CD response.
a Reaction conditions: i) N-Boc-L-leucine, 4-(dimethylamino)pyridine
(4-DMAP), 1-(3-dimethylaminopropyl)-3-ethyl-carbodiimide hydrochloride
(EDCI), CHCl3, 94%. ii) trifluoroacetic acid (TFA), CHCl3, quantitative.
iii) N-Boc-glycine, 4-DMAP, EDCI, CHCl3, 91%. iv) TFA, CHCl3,
quantitative. v) N-Boc-11-aminoundecanoic acid, 4-DMAP, EDCI, CHCl3,
82%. vi) TFA, CHCl3, quantitative. vii) fumaric acid monoethylester,
4-DMAP, EDCI, CHCl3, 85%. viii) EtOH, NaOH, 91%. ix) 4-DMAP, EDCI,
DMF, 74%. x) p-xylylenediamine, isophthaloyl dichloride, Et3N, CHCl3,
58%. xi) EITHER 254 nm, CH2Cl2, 20 min, 56% OR 254 nm, CH3CN, 20
min, 49% OR 350 nm, benzophenone, CH2Cl2, 20 min, 70% xii) EITHER
312 nm, CH2Cl2 or CH3CN, 20 min, 62% OR 400-670 nm, cat. Br2,
CH2Cl2, 2 min, >95% OR 130 °C, C2H2Cl4, 6 days, 95%.
Molecular shuttle E-1 was synthesized from thread E-2 in 58%
yield (Scheme 1). Comparison of the 1H NMR spectra of the
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C O M M U N I C A T I O N S
L-enantiomer, negative (Figure 2a). The absence of any detectable
signal for the E-rotaxane shows that the CD signal is genuinely
only generated by controlling the position of the macrocycle in the
shuttle.
The most efficient ways of interconverting the rotaxane diaster-
eomers are photochemical: 350 nm, catalytic benzophenone
sensitizer, CH2Cl2, 70% for E-1fZ-1 and 400-670 nm, catalytic
Br2, CH2Cl2, >95% for Z-1fE-1 (Scheme 1). However, a modest
difference in the electronic absorption spectra of E-1 and Z-1 (Figure
2b) results in different photostationary states for the isomers at 254
(56% Z-1 in CH2Cl2; 49% Z-1 in CH3CN) and 312 nm (38% Z-1
in either CH2Cl2 or CH3CN), meaning that even for this first-
generation system a large net change (>1500 deg cm2 dmol-1) in
elliptical polarization can be achieved solely through irradiation
with light of different wavelengths. The photoisomerizations are
highly reproducible with five complete switching cycles (E-1fZ-
1fE-1) carried out with no decomposition detectable by NMR or
CD spectroscopy (Figure 2c).
In conclusion, we have demonstrated a system where a large
amplitude displacement of a macrocycle along a thread elicits a
chiral optical response. In addition to this being a novel mode of
switching to be explored for possible photonic and data storage
applications, control of the expression of chirality in switchable
interlocked systems through hiding or revealing chiral subunits
could find important applications in areas where chiral transmission
from one chemical entity to another underpins a physical or
chemical response, such as the seeding of supertwisted nematic
liquid crystalline phases or asymmetric synthesis.
Acknowledgment. This work was supported by the European
Union Future and Emerging Technology Program MechMol and
the EPSRC. D.A.L. is an EPSRC Advanced Research Fellow
(AF/982324).
Figure 2. (a) CD spectra (0.1 mM in CHCl3) at 298 K of Z-1 (blue), E-1
(purple), E-2 (green), and Z-2 (red). (b) UV-vis spectra of E-1 (purple)
and Z-1 (blue) in CHCl3 at 298 K, [ꢀ] ) mM-1‚cm-1. Dotted line represents
the Z-1/E-1 absorption ratio. (c) Percentage of E-1 in the photostationary
state (from 1H NMR data, 400 MHz, CD3CN, 298 K) after alternating
irradiation at 254 nm (half integers) and 312 nm (integers) for five complete
cycles. The right-hand Y axis shows the CD absorption at 246 nm.
Supporting Information Available: Experimental procedures and
spectral data for all new compounds (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
References
E-rotaxane and thread (Figure 1, a and b) shows the excellent
discrimination of the macrocycle toward the different stations. While
the Gly-Leu protons are only slightly affected by the aromatic
shielding effect of the macrocycle, the E-olefin protons are
significantly shifted upfield (∼1.2 ppm), confirming that the co-
conformer having the macrocycle over the olefin station is the major
translational isomer in E-1.
Light-induced EfZ isomerization7 of the fumaramide unit
reverses8 the macrocycle-binding affinity of the two stations in the
molecular shuttle because the cis-olefin (maleamide) is largely self-
satisfying in terms of H-bonding and the amide carbonyl groups
are no longer suitably orientated for binding to the macrocycle.7
The 1H NMR spectra of Z-1 and Z-2 (Figure 1, c and d) reveal the
concomitant change in the position of the macrocycle. The Z-olefin
protons Hd′ and He′ occur at almost identical chemical shifts in
rotaxane and thread, whereas the Gly methylene protons are now
shielded by 0.9 ppm.
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(2) For a recent example of a light-driven mechanical motion which elicits a
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J. Am. Chem. Soc. 2003, 125, 5612-5613.
(3) For systems where a large amplitude translational motion in a rotaxane
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(5) For an alternative chiral amino acid-containing rotaxane system see: (a)
Smukste, I.; Smithrud, D. B. J. Org. Chem. 2003, 68, 2547-2558. (b)
Smukste, I.; House, B. E.; Smithrud, D. B. J. Org. Chem. 2003, 68, 2559-
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(6) (a) Irie, M., Ed. Photochromism: Memories and Switches (Special Issue).
Chem. ReV. 2000, 100, 1638-1890. (b) Feringa, B. L., Ed. Molecular
Switches; Wiley-VCH: Weinheim, 2001.
(7) Gatti, F. G.; Leo´n, S.; Wong, J. K. Y.; Bottari, G.; Altieri, A.; Farran
Morales, A. M.; Teat, S. J.; Frochot, C.; Leigh, D. A.; Brouwer, A. M.;
Zerbetto, F. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 10-14.
(8) Altieri, A.; Bottari, G.; Dehez, F.; Leigh, D. A.; Wong, J. K. Y.; Zerbetto,
F. Angew. Chem., Int. Ed. 2003, 42, 2296-2300.
The shuttle design works remarkably well. When the macrocycle
is tightly bound close to the Leu residue in Z-1, the aromatic rings
of the rotaxane do, indeed, experience a well-expressed chiral
environment as evidenced by CD spectroscopy. Of the two
rotaxanes and two threads, only rotaxane Z-1 gives a CD response,
which is both strong (-13k deg cm2 dmol-1) and, for the
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