Temporary and permanent trapping of the metastable twisted conformer of an overcrowded chromic alkene via encapsulation

Article abstract of DOI:10.1021/ja308101a

An overcrowded alkene with an anti-folded conformation was converted to its twisted conformer, accompanied by a dramatic color change from yellow to deep purple, by inclusion in a self-assembled T_d-symmetric coordination cage. The shape of the caged cavity was suitable and desirable for trapping of the twisted conformer. The twisted conformation was temporarily memorized in the alkene even after guest ejection. Permanent trapping of the twisted conformation was achieved by bromination of the twisted conformer formed in situ in the cage.

Full text of DOI:10.1021/ja308101a

Communication
pubs.acs.org/JACS
Temporary and Permanent Trapping of the Metastable Twisted
Conformer of an Overcrowded Chromic Alkene via Encapsulation
Hiroki Takezawa, Takashi Murase, and Makoto Fujita*
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656,
Japan
S
* Supporting Information
Scheme 1. Schematic Representation of Inclusion of
Overcrowded Alkene 1a into Td-Symmetric Cage 2
ABSTRACT: An overcrowded alkene with an anti-folded
conformation was converted to its twisted conformer,
accompanied by a dramatic color change from yellow to
deep purple, by inclusion in a self-assembled Td-symmetric
coordination cage. The shape of the caged cavity was
suitable and desirable for trapping of the twisted
conformer. The twisted conformation was temporarily
memorized in the alkene even after guest ejection.
Permanent trapping of the twisted conformation was
achieved by bromination of the twisted conformer formed
in situ in the cage.
terically overcrowded alkenes with distorted CC bonds
S
have received considerable attention because of their
chromic behavior caused by unimolecular conformational
changes.1−3 Bis-tricyclic aromatic ene 1 (Figure 1) is one of
Figure 1. Cartoon representations of anti-folded and twisted
conformers of overcrowded alkenes 1.
Supporting Information), exhibits a yellow color characteristic
of the anti-folded conformation.6 Solid af-1a (in excess) was
suspended in an aqueous solution of cage 2, and the mixture
was heated under microwave conditions at 100 °C for 1 h.
Water-insoluble af-1a gradually dissolved, and the solution
turned deep purple. This color change suggested the inclusion
of af-1a into 2 and its conformational change from the anti-
folded to the twisted form. After removal of excess af-1a by
the widely studied overcrowded alkenes with chromic proper-
ties.3−6 Its two major conformers, anti-folded and twisted, show
distinctly different colors: colorless or yellow for anti-folded
conformers, but deep-colored for twisted ones.5,6 Although a
variety of derivatives of 1 have been synthesized to date,5−7
their dynamic conformational behavior is still difficult to predict
and control. Here we demonstrate that Td-symmetric self-
assembled cage 2 is a suitable host for fixing and stabilizing the
twisted conformer of an overcrowded alkene (Scheme 1). The
overcrowded alkene examined here, 1a, exists as the anti-folded
conformer in the solid state. We found that this anti-folded
guest (hereafter, af-1a) was induced to adopt the metastable
twisted conformation (hereafter, t-1a) upon inclusion in the
cavity of 2, along with a sharp color change from yellow to deep
purple.8 The stabilization of t-1a is ascribed to the perfect
complement of its C2 symmetry to the cage’s Td symmetry.9
The guest alkene af-1a, synthesized in six steps from 9-
fluorenone-2,7-dicarboxylic acid and 9-xanthone (see the
1
filtration, H NMR spectroscopy revealed the formation of the
inclusion complex 2·(t-1a) in ∼10% yield (Figure 2). The guest
signals were significantly shifted upfield as a result of shielding
by the aromatic panels of cage 2.
The twisted conformation of the guest in the cage was
1
elucidated by symmetry analysis of the cage signals in the H
NMR spectrum. After guest inclusion, the Td symmetry of cage
2 was reduced, and three sets of pyridyl protons were observed
(Figure 2b). This splitting pattern is predicted when the guest
Received: August 15, 2012
Published: October 8, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
∼570 nm (Figure 3b). Thus, the twisted conformation of 1a
was retained in the solid state.
The encapsulated t-1a was easily ejected upon the addition of
4 equiv of 1-adamantanol (3), an efficient guest for 2, to the
solution of 2·(t-1a) at room temperature (Figure 4). The
Figure 2. 1H NMR spectra (500 MHz, 300 K) of (a) 1a in CDCl3 and
(b) 2·(t-1a) in D2O (* labels denote signals of free cage 2).
possesses D2d symmetry.10 The C2 symmetry of t-1a can appear
to be D2d symmetry if racemization (via rotation around the
central CC bond) and vertical flipping of the guest in the
cavity are fast on the NMR time scale. The fast racemization
process in the cage is supported by the observation of the
methylene protons (h) as a singlet. In contrast, af-1a cannot
show apparent D2d symmetry on any occasion.
UV−vis measurements in both solution and the solid state
also revealed the selective capture of t-1a by cage 2 (Figure 3).
Figure 4. Ejection of t-1a from the inclusion complex 2·(t-1a) by guest
exchange. (a) Schematic representation of the ejection of t-1a. (b)
Photographs of the guest exchange. Upon addition of 3 to the solution
of 2·(t-1a), the included t-1a was ejected and precipitated. The as-
precipitated 1a had a purple color (left) but turned yellow upon
heating (right), indicating that the metastable twisted conformation
was temporarily “memorized” in the as-precipitated 1a and then
“erased” upon heating.
purple solution color turned colorless because of formation of
the complex 2·(3)4 by guest exchange. To our surprise, the
ejected 1a precipitated as a purple solid (Figure 4b, left),
indicating that the as-precipitated 1a retained the metastable
twisted conformation formed when af-1a was converted to t-1a
via inclusion complex 2·(t-1a). Interestingly, the purple color
changed to yellow upon heating at 70 °C for 15 min (Figure 4b,
right), indicating regeneration of the more stable af-1a. Thus,
the temporarily “memorized” twisted conformation of as-
precipitated t-1a could be easily “erased” upon heating.
We could also achieve permanent trapping of the twisted
conformation of t-1a by bromination of the aromatic core
(Figure 5). When an aqueous solution of the inclusion complex
2·(t-1a) ([2] = 10 mM, 10% inclusion of t-1a, 50 mL) was
treated with 2 equiv of N-bromosuccinimide (NBS), the
solution color immediately changed from dark purple to green.
After the mixture was stirred at room temperature for 5 min,
cage 2 was decomplexed by aqueous HCl, and the product was
extracted with chloroform. NMR measurements of the
extracted mixture revealed the formation of tetrabrominated
product t-4 (29% NMR yield).11 After isolation by column
chromatography, compound t-4 was obtained as a deep-blue
Figure 3. UV−vis spectra and photographs of (a) 2 (in H2O, [2] = 1.0
mM, black dotted line) and 2·(t-1a) (in H2O, [2] = 1.0 mM, 10%
inclusion of 1a, blue solid line) and (b) 2 (black dotted line), af-1a
(green dot-dashed line), and 2·(t-1a) (blue solid line) in the solid
state.
Yellow-colored af-1a showed a strong absorption at ∼400 nm
in the solid state, diagnostic of the anti-folded conformation.
However, after inclusion in the cage in water (Figure 3a), the
absorption at ∼400 nm disappeared and a new strong
absorption diagnostic of the twisted conformation appeared
at ∼550 nm.6b Even after evaporation of the water, the resulting
powdered inclusion complex showed a similar absorption at
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Journal of the American Chemical Society
Communication
properties of guests via cage-driven conformational twisting.
Even better, the resulting conformation of the guest is easily
predicted from the cavity shape.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, physical properties, and crystallo-
graphic data (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
mfujita@appchem.t.u-tokyo.ac.jp
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the CREST Project of the
Japan Science and Technology Agency (JST) and also in part
by the Global COE Program “Chemistry Innovation through
Cooperation of Science and Engineering” from MEXT, Japan.
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■
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Figure 5. (a) Schematic representation of the bromination of t-1a
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