A Switchable Open/closed Polyaromatic Macrocycle that Shows Reversible Binding of Long Hydrophilic Molecules

Article abstract of DOI:10.1002/anie.201703357

In spite of wide-ranging previous studies on synthetic macrocycles, the installation of open–close functions into the frameworks remains a challenge. We present a new polyaromatic macrocycle capable of switching between open and closed forms in response to external stimuli, namely, base and acid. The macrocycle, which is prepared in three steps, has a well-defined hydrophobic cavity with a length of around 1 nm, surrounded by four pH-responsive acridinium panels. The open and closed structures were confirmed by single-crystal X-ray analysis. The cylindrical cavity can bind long hydrophilic molecules up to 2.7 nm in length in neutral water and then release the bound guests through a reversible open-to-closed structural change upon simple addition of base.

Full text of DOI:10.1002/anie.201703357

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Accepted Article
Title: A Switchable Open/closed Polyaromatic Macrocycle and its
Reversible Binding of Long Hydrophilic Molecules
Authors: Kohei Kurihara, Kohei Yazaki, Munetaka Akita, and Michito
Yoshizawa
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A Switchable Open/closed Polyaromatic Macrocycle and its
Reversible Binding of Long Hydrophilic Molecules
Kohei Kurihara,[a] Kohei Yazaki,[b] Munetaka Akita,[a] and Michito Yoshizawa*[a]
Abstract: In spite of wide-ranging previous studies on synthetic
macrocycles, installation of open-close functions into the frameworks
remains a knotty challenge. Here we present a new polyaromatic
macrocycle capable of switching between open and closed forms in
response to external stimuli, base and acid. The macrocycle,
prepared in three steps, has a well-defined hydrophobic cavity with
dimensions of ~1 nm, surrounded by four pH-responsive acridinium
panels. The open and closed structures are definitely confirmed by
X-ray single-crystal analysis. The cylindrical cavity can bind long
hydrophilic molecules up to 2.7 nm in neutral water as well as
release the bound guests through the open-to-closed structural
change by simple addition of base, in a reversible fashion.
Figure 1. a) Cartoon of an open/closed structural change of a polyaromatic
macrocycle and b) acridinium-based macrocyclic molecule 1 reported herein.
c) General reactivity of a biarylacridinium salt with base (+ROH) and acid.
Reversible open-close motions triggered by external stimuli or
forces are one of the most basic functions in our lives.
Incorporation of such mechanical motions into synthetic
Biarylacridinium frameworks contain a rigid polyaromatic
molecular rings, tubes, and cages is highly promising for the
panel with a monocationic nitrogen atom (Figure 1c, left). Unlike
development of functional nanoscale containers and machines.[1-
common polyaromatic hydrocarbons (e.g., anthracene, pyrene,
3]
There has been intensive study of covalent macrocyclic
and perylene), nucleophilic addition of alcohols or water to the
compounds, such as cyclodextrins, cyclophanes, cucurbiturils,
acridinium panel under basic conditions gives rise to flexible
and pillararenes, possessing definite cavities capable of binding
acridane panels (Figure 1c, right). The resultant neutral
wide-ranging organic molecules.[4] In addition, the electrostatic
frameworks can revert to the originals under acidic conditions.
properties of the host frameworks can be occasionally changed
We expected that the incorporation of multiple acridinium panels
by the addition of acids/bases or by oxidation/reduction
into a shape-persistent polyaromatic tubular structure[7] could
reactions. Nevertheless, open-close switching of the cylindrical
generate new pH-responsive macrocyclic molecule 1 (Figure
cavities by external stimuli (e.g., heat, light, and pH) has been
1b).[8] The tetracationic framework provides hydrophilic exterior
rarely achieved owing to their rigid constitutions.[5] Open-ended
surfaces so that the polyaromatic macrocycle is usable in water
macrocyclic and tubular structures are expected to bind various
without the attachment of typical hydrophilic substituents.
guest molecules, even when the lengths are longer than that of
Importantly, the open macrocycle, prepared in only three-step
the host cavities, in contrast to capsular and cage-like hosts with
reactions, can reversibly transform to the closed macrocycle (2)
isolated cavities.[6] Therefore, installation of open-close functions
into covalent macrocyclic hosts will open up their applications in
stimuli-responsive molecular sensors and separators for, for
example, long and complex biomolecules.
by simple addition of base (Figure 1a).
We synthesized water-soluble polyaromatic macrocycle 1
through
nickel-catalyzed
homocoupling
of
a
diiododihydroacridine derivative followed by treatment with a HCl
solution (step iii in Figure 2a).[9] The bent precursor was
prepared in two steps starting from acridone and 1,3-
diiodobenzene (steps i and ii in Figure 2a).[10] The macrocyclic
structure of 1 was unambiguously confirmed by NMR, MS, and
Here we report a new polyaromatic macrocycle switchable
between open and closed states, reversibly (Figure 1a), in
response to external stimuli, base and acid. Macrocycle 1
designed herein is composed of four pH-responsive acridinium
panels connected alternately by meta-phenylene and meta-
biphenyl spacers (Figure 1b). Both the open and closed
structures are confirmed by X-ray crystallographic analysis. In
neutral water, the cylindrical cavity with dimensions of ~1 nm
can bind long hydrophilic molecules with a pyranose or steroid
moiety (up to 2.7 nm in length). Remarkably, the bound guests
are released from the cavity through the open-to-closed
structural change of the host framework upon addition of base.
X-ray single-crystal analyses.[9,11]
A
1H NMR spectrum of 1 in
D2O showed eleven signals in the aromatic region (Figure 3b)
due to the high symmetry (virtually D2h). The ESI-TOF MS
spectrum exhibited
a
prominent peak at m/z
=
292.1
corresponding to the [1 – 4•Cl–]4+ species (Figure S12). Single
[a]
[b]
K. Kurihara, Prof. Dr. M. Akita, Dr. M. Yoshizawa
Laboratory for Chemistry and Life Science
Institute of Innovative Research, Tokyo Institute of Technology
4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
E-mail: yoshizawa.m.ac@m.titech.ac.jp
Dr. K. Yazaki
Faculty of Engineering, Yamanashi University
4-3-11 Takeda, Kofu 400-8511, Japan
Supporting information for this article is given via a link at the end of
the document.
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crystals of the macrocycle suitable for X-ray crystallographic
analysis were obtained from the PF6 analogue 1” in the
panels to the non-planar acridone panels (Figure 3e). The
closed structure bearing the distorted four acridane panels
remains intact even at 80 ºC, as indicated by variable
temperature 1H NMR analysis (Figure S21). Notably, open
macrocycle 1 could be regenerated from closed macrocycle 2’
under acidic conditions. Addition of an aqueous HCl solution
(~60 equiv.) to a CH3OH suspension of 2’ followed by stirring for
~5 h at room temperature afforded a clear yellow solution
including the original macrocycle (93% isolated yield) (Figure 3d).
–
presence of coumarin guest 3a (Figure 2b).[12a] The molecular
structure showed that the four acridinium panels are connected
alternately by the non-substituted mono- or bisphenylene
spacers, creating a cylindrical cavity with dimensions of 1.1 × 0.8
nm (Figure 2c). The ~1 nm-length cavity is fully occupied by the
stacked two molecules of 3a (1.5 nm in length) in an antiparallel
fashion (Figure 2d). The interplanar guest-host (acridinium
panels) and guest-guest distances of 3.4-3.8 Å indicate the
existence of multiple π-π interactions.[13,14]
Figure 2. a) Synthesis of polyaromatic macrocycle 1 starting from acridone in
three steps.[11] Conditions and reagents: (i) CuI, dipivaloylmethane, K2CO3,
DMF, 160 ºC, (ii) 1,3-diiodobenzene, n-BuLi, THF, –80 ºC and then NaHCO3,
CH3OH, r.t., (iii) Ni(COD)2, 2,2’-bipyridyl, THF, 70 ºC and then CH3OH, HCl aq.,
r.t. (24% total yield).[9] b) Coumarin 3a as a guest molecule. Crystal structure
of 1”•(3a)2 (counterions and solvent molecules are omitted for clarity): c) the
top view without the guests (space-filling model) and d) the side views with the
guests (cylinder and space-filling models).
Figure 3. a) Schematic representation of the open/closed structural change of
polyaromatic macrocycle 1. 1H NMR spectra (400 or 500 Hz, D2O, r.t.) of b) 1,
c) closed macrocycle 2’ (in toluene-d8), and d) 1 regenerated from 2’. e)
Crystal structure of 2’ (space-filling model; solvent molecules are omitted for
clarity). f) UV-visible spectra (H2O, r.t.) of 1 after alternate addition of (i) HCl aq.
and (ii) NaOH aq. (~20 equiv. based on 1) and g) the open-closed switching
cycles of 1 under basic/neutral conditions, monitored by the UV-visible spectra
(plot of the absorption intensity at 434 nm).
An open-to-closed structural change of rigid polyaromatic
macrocycle 1 was carried out under basic conditions in the
presence of nucleophiles. For example, when macrocycle 1 and
excess NaHCO3 were stirred in a mixed solvent of CH3CN and
2-ethyl-1-butanol for 15 h at 60 ºC, the yellow solution turned
colorless to give a white precipitate. 1H NMR analysis of the
collected precipitate indicated the formation of closed
macrocycle 2’ in 94% isolated yield.[9] The NMR spectrum of 2’
in toluene-d8 showed three broad signals derived from the
attached ethylbutyloxy groups in the range of 3.23 to 0.82 ppm
(Figure 3c). The aromatic signals of 2’ were also broadened,
most probably due to the restricted motion of the compressed
structure. The infrared (IR) spectroscopy and elemental analysis
(E.A.) of the product also proved the full conversion of the
acridinium rings into the acridane rings (Figure S22). The closed
structure of 2’ was finally evidenced by X-ray crystallographic
analysis.[12b] In the crystal structure, the open cavity found in 1
completely disappeared due to flipping of the biphenyl rings
inward through the structural change of the planar acridinium
Next we demonstrated that the open-closed switching
process of 1 can be repeated several times in aqueous solutions.
Addition of an aqueous NaOH solution (~20 equiv.) to open
macrocycle 1 in H2O gave rise to closed macrocycle 2 (R = -OH)
as a white precipitate within 1 h at room temperature. The
resultant macrocycle reverted into the original one by the
treatment with an aqueous HCl solution (~20 equiv. based on 2)
under similar conditions. The pH-responsive structural change
was easily monitored by UV-visible spectroscopic analysis
(Figure 3f). The absorption bands (λmax = 434 nm) derived from
the acridinium panels of 1 disappeared under basic conditions
due to the formation of 2 precipitated out of the solution.[9,15] The
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Figure 4. a) Schematic representation of the binding of hydrophilic molecules
3b-d by open macrocycle 1 in water. 1H NMR spectra (400 MHz, D2O, r.t.) of
b) 3b, c) 1•(3b)2, d) 3c, e) 1•3c, and f) 1•3d. g) Plots of the integrated heat for
the ITC titrations of 3c (left) and 3d (right) into a H2O solution of 1. Black
squares and black lines are experimental and calculated values, respectively.
h) An optimized structure of 1•3d (space-filling model).
bands were recovered through neutralization upon addition of
acid. Repeatability of the spectroscopic changes (five times)
revealed that the opening-closing cycles of 1 occur efficiently in
water at ambient temperature (Figure 3g).
In aqueous media, polyaromatic macrocycle 1 exhibited a
binding ability toward water-soluble molecules, esculin (3b),
sodium cholate (3c), and CHAPS (3d), whose lengths are longer
than that of the cavity (Figure 4a). Simple mixing aqueous
macrocycle 1 (1.2 µmol) with 3b (2.3 µmol), with a length of 1.3
nm, in D2O (1.0 mL) at room temperature led to the quantitative
formation of 1:2 host-guest complex 1•(3b)2 within 5 min. In the
1H NMR spectrum of the product (Figure 4c), proton signals
derived from the coumarin moiety (HA-D) of 3b were observed in
0.12 ppm). These characteristic shifts, caused by aromatic
shielding effects, indicate selective binding of the hydrophobic
coumarin part of 3b by the cylindrical cavity of 1 through the
hydrophobic effect and π-stacking interactions. The 1H NMR
Job’s plot identified the host-guest stoichiometry of the binding
to be 1:2 (Figure S29). ESI-TOF MS analysis also confirmed the
host-guest ratio (Figure S30).[16] In the same way, hydrophilic
steroid derivatives 3c and 3d (1.5 and 2.7 nm in length,
respectively) were quantitatively accommodated within 1 to give
1:1 host-guest complexes 1•3c and 1•3d, respectively. Upfield
shifts of the methyl signals (HA-C) on the steroid moieties were
definitely observed in the 1H NMR spectra (Figure 4e,f). The
relatively large binding constant (Ka = 3.9 × 104 M–1) and
thermodynamic parameters (ΔH = 3.89 kcal mol–1 and ΔS = 34.0
cal mol–1 K–1) toward the 1•3c complex were determined by
isothermal titration calorimetry (ITC; Figure 4g, left).[9] On the
other hand, relatively long compound 3d was bound by 1 with a
moderate binding constant (7.3 × 103 M–1; Figure 4g, right).[17]
The optimized structure of 1•3d by semiempirical calculations
(PM6 level) reveals that most of the steroid moiety of 3d is
encircled by the acridinium panels of 1 (Figure 4h).[18,19]
the range of 7.45 to 5.68 ppm with large upfield shifts (Δδmax
=
0.64 ppm). On the other hand, those derived from the pyranose
moiety appeared around 5.0 and 4.7 ppm with slight shifts (Δδ <
We further examined the release of the bound guests from
the cavity of
1 through the pH-responsive open-to-closed
structural change at room temperature (Figure 5a). Addition of a
base (NaHCO3, ~20 equiv. based on 1) to the aqueous solution
of 1•(3b)2 evoked the rapid color change from yellow to colorless,
accompanying the precipitation of a white solid. The 1H NMR
spectrum showed the disappearance of the host signals and the
large downfield shifts of the aromatic signals of guest 3b (Δδmax
= +0.48 ppm) (Figure 5b,c). Similarly, the steroid signals (e.g.,
HA-C) of 3d bound in 1 were shifted downfield (Δδmax = +1.09
ppm) upon addition of the base (Figure 5e,f). These spectral
changes stem from the release of the bound guests from open
macrocycle 1 in water.
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Acknowledgements
This work was supported by JSPS KAKENHI (Grant No.
JP25104011/JP26288033/JP17H05359) and “Support for
Tokyotech Advanced Researchers (STAR)”. We thank Dr.
Yoshihisa Sei (Tokyo Institute of Technology), Kenji Yoza
(Bruker AXS), and Dr. Takane Imaoka (Tokyo Institute of
Technology) for supporting X-ray crystallographic analysis and
ITC experiments. K.Y. thanks the JSPS for
Fellowship for Young Scientists.
a Research
Keywords: macrocycle, acridinium, catch and release, water,
pH
[1]
[2]
J.-M. Lehn, Supramolecular Chemistry: Concepts and Perspectives,
Wiley-VCH, Weinheim, Germany, 1995.
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[3]
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2693.
a) F. Diederich, Cyclophanes, RSC, Cambridge, U.K., 1991; b) J. Szejtli,
T. Osa (Eds.), Comprehensive Supramolecular Chemistry:
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W. M. Nau, Chem. Soc. Rev. 2015, 44, 394–418; g) T. Ogoshi,
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Vermeulen, M. Juríček, J. C. Barnes, R. M. Young, M. R. Wasielewski,
J. F. Stoddart, Acc. Chem. Res. 2016, 49, 262–273.
Figure 5. a) Schematic representation of pH-responsive catch and release of
guests 3b and 3d by macrocycle 1 in water. 1H NMR spectra (400 MHz, D2O,
r.t.) of 1•(3b)2 b) before and c) after addition of NaHCO3, and d) after further
addition of HCl aq. 1H NMR spectra (400 MHz, D2O, r.t.) of 1•3d e) before and
f) after addition of NaHCO3 and g) after further addition of HCl aq.
Finally, rebinding of the released guests to the open
macrocycle was readily achieved by adding acid to the resultant
suspension of 2 and 3b in water. Addition of an aqueous HCl
solution (~20 equiv. based on 1) caused rapid color change of
the solution from colorless to yellow, accompanied with the
[5]
Reports on switchable open/closed macrocycles without cylindrical
cavities: a) S. Shinkai, A. Yoshioka, H. Nakayama, O. Manabe, J.
Chem. Soc., Perkin Trans. 2, 1990, 1905–1909; b) H. Kanazawa, M.
Higuchi, K. Yamamoto, J. Am. Chem. Soc. 2005, 127, 16404–16405; c)
G. Ma, Q. Zhou, X. Zhang, Y. Xu, H. Liu, New J. Chem. 2014, 38, 552–
560; d) S. T. J. Ryan, J. del Barrio, R. Suardíaz, D. F. Ryan, E. Rosta,
O. A. Scherman, Angew. Chem. Int. Ed. 2016, 55, 16096–16100.
Reviews on covalent molecular hosts: a) L. R. MacGillivray, J. L.
Atwood, Angew. Chem. Int. Ed. 1999, 38, 1018–1033; b) M. Mastalerz,
Angew. Chem. Int. Ed. 2010, 49, 5042–5053; c) G. Zhang, M.
Mastalerz, Chem. Soc. Rev. 2014, 43, 1934–1947.
1
dissolution of the precipitate, at room temperature. The H NMR
spectrum of the product (Figure 5d) was virtually identical to that
of 1•(3b)2 (Figure 5b). The original 1•3d complex was also
regenerated exclusively by the same way (Figure 5g).
[6]
[7]
In conclusion, we have developed a new polyaromatic
macrocycle providing (i) a open/closed switching function within
the framework, (ii) a binding capability toward long hydrophilic
molecules with the lengths longer than that of the cavity, and (iii)
a releasing ability of the bound molecules in water at ambient
temperature. The structures of the open and closed macrocycles
are successfully confirmed by NMR, MS, and X-ray single-
crystal analyses. In addition, reversible catch and release of the
long molecules by the macrocycle are revealed by detailed NMR
analysis. The key to the characteristics of the present
macrocycle arises from the installation of pH-responsive
polyaromatic panels (i.e., acridinium ring) into a macrocyclic
structure, which has never been reported so far.[20] Therefore,
the guest release can be accomplished through not typical
electrostatic repulsion or solvent effects but mechanical motions.
Further studies on the finite open/closed switchable macrocycle,
e.g., incorporation of them into infinite polymer and inorganic
matrixes, could exploit novel stimuli-responsive sensing and
separating materials.
Shape-persistent molecular tubes with anthracene panels: a) K.
Hagiwara, Y. Sei, M. Akita, M. Yoshizawa, Chem. Commun. 2012, 48,
7678–7680; b) K. Yazaki, Y. Sei, M. Akita, M. Yoshizawa, Nat.
Commun. 2014, 5, 5179; c) K. Hagiwara, M. Akita, M. Yoshizawa,
Chem. Sci. 2015, 6, 259–263; d) K. Hagiwara, M. Otsuki, M. Akita, M.
Yoshizawa, Chem. Commun. 2015, 51, 10451–10454; e) M. Yoshizawa,
J. K. Klosterman, Chem. Soc. Rev. 2014, 43, 1885–1898.
[8]
[9]
To the best of our knowledge, there have been no reports on pH-
responsive molecular hosts with multiple acridinium rings. Acridinium-
containing rotaxanes: a) W. Abraham, K. Buck, M. Orda-Zgadzaj, S.
Schmidt-Schäffer, U.-W. Grummt, Chem. Commun. 2007, 3094–3096;
b) A. Vetter, W. Abraham, Org. Biomol. Chem. 2010, 8, 4666–4681.
See the Supporting Information.
[10] K. Yazaki, Y. Sei, M. Akita, M. Yoshizawa, Chem. Eur. J. 2016, 22,
17557–17561.
[11] The NMR spectra of 1 were fully characterized by using highly soluble
analogous macrocycle 1’ having BF4– counterions.
[12] a) Single crystals of 1”•(3a)2 were obtained by slow diffusion of
benzene into an acetonitrile solution of 1” in the presence of 3a (2
equiv.) for 2 d at room temperature; b) Single crystals of 2’ (R = -
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CH2CH(CH2CH3)2) were obtained by slow concentration of a toluene
solution of 2’ for 2 d at room temperature.
[16] However, these data cannot exclude the formation of a 1:1 host-guest
complex (1•3b) as a minor product. The poor solubility of neutral guest
3b in water precludes the ITC measurement.
[17] 1H NMR Job’s plots identified the formation of 1:1 host-guest
complexes 1•3c and 1•3d (Figure S33 and S37). Thermodynamic
parameters ΔH and ΔS for the 1•3d complex were calculated to be 3.11
kcal mol–1 and 28.1 cal mol–1 K–1, respectively.[9]
[13] a) C. Janiak, J. Chem. Soc., Dalton Trans. 2000, 3885–3896; b) C. A.
Hunter, K. R. Lawson, J. Perkins, C. J. Urch, J. Chem. Soc., Perkin
Trans. 2, 2001, 651–669; c) J. K. Klosterman, Y. Yamauchi, M. Fujita,
Chem. Soc. Rev. 2009, 38, 1714–1725.
[14] The antiparallel orientation of the guest molecules is presumably
attributed to the dipole-dipole interaction in the cavity. 1H NMR spectra
of 1•(3a)2 in CD3CN showed broadened host and guest peaks (Figure
S42a,b), indicating strong interactions between open macrocycle 1 and
3a. In contract, closed macrocycle 2” with ammonium groups showed
no host-guest interaction with 3a under the same conditions (Figure
S42c,d).
[18] Geometry optimization of 1•3d was carried out in water without
counterions by PM6 calculations with Gaussian 09 software. The
solvent effect was taken into account using the self-consistent reaction
field (SCRF) method.
[19] M. J. Frisch et al., Gaussian 09, Gaussian, Inc., Wallingford, CT, 2009.
[20] Related metallocage systems: a) N. Kishi, M. Akita, M. Yoshizawa,
Angew. Chem. Int. Ed. 2014, 53, 3604–3607; b) D. Preston, A. Fox-
Charles, W. K. C. Lo, J. D. Crowley, Chem. Commun. 2015, 51, 9042–
9045; c) V. Croué, S. Goeb, G. Szalóki, M. Allain, M. Sallé, Angew.
Chem. Int. Ed. 2016, 55, 1746–1750.
[15] Closed macrocycle 2 (R = -H) insoluble in general organic solvents
(e.g., acetone, CHCl3, toluene, and DMSO) was characterized by FT-IR
(Figure S22), E.A., and MALDI-TOF MS (Figure S25) analyses.
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Entry for the Table of Contents
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K. Kurihara, K. Yazaki, M. Akita, and M.
Yoshizawa*
Page No. – Page No.
A Switchable Open/closed
Polyaromatic Macrocycle and its
Reversible Binding of Long
Hydrophilic Molecules
Here we present a new polyaromatic macrocycle capable of switching between
open and closed forms in response to external stimuli. The macrocycle has a well-
defined hydrophobic cavity with dimensions of ~1 nm, surrounded by four pH-
responsive acridinium panels. The cylindrical cavity can bind long hydrophilic
molecules in neutral water as well as release the bound guests through the open-
to-closed structural change by addition of base, in a reversible fashion.
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