A water-soluble pH-triggered molecular switch

Article abstract of DOI:10.1021/ja409006y

A bistable donor-acceptor [2]catenane, which is composed of a crown ether containing a hydroquinone unit and a 1,5-diaminonaphthalene unit, interlocked mechanically by cyclobis(paraquat-p-phenylene) as its tetrachloride, exists as a mixture of translational isomers, both in the solid state and in aqueous solution. UV/vis and ¹H NMR spectroscopies demonstrate that this isomeric mixture can be switched in water in the presence of hydrochloric acid to afford a single diprotonated derivative in which only the hydroquinone unit resides inside the cavity of the tetracationic cyclophane. Treatment with 1,4-diazabicyclo[2.2.2]octane resets the molecular switch.

Full text of DOI:10.1021/ja409006y

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Communication
A Water Soluble pH-Triggered Molecular Switch
Sergio Grunder, Psaras Lamar McGrier, Adam C Whalley,
Megan M Boyle, Charlotte L. Stern, and J. Fraser Stoddart
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 07 Nov 2013
Downloaded from http://pubs.acs.org on November 12, 2013
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A Water Soluble pH-Triggered Molecular Switch
Sergio Grunder,† Psaras L. McGrier,† Adam C. Whalley, Megan M. Boyle, Charlotte Stern, and J. Fraser
Stoddart*
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Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States
ABSTRACT: A bistable donor-acceptor [2]catenane,
which is composed of a crown ether containing a hydro-
quinone unit and a 1,5-diaminonaphthalene unit, inter-
locked mechanically by cyclobis(paraquat-p-phenylene) as
its tetrachloride, exists as a mixture of translational iso-
mers, both in the solid state and in aqueous solution.
1
UV/Vis and H NMR spectroscopies demonstrate that this
isomeric mixture can be switched in water in the presence
of hydrochloric acid to afford a single diprotonated deriva-
Figure 1. The translational isomerism between 1A4+ and 1B4+ and
tive in which only the hydroquinone unit resides inside the
the pH-triggered switching of the bistable [2]catenane 14+ which
cavity of the tetracationic cyclophane. Treatment with 1,4-
exists both in solution (78:22) and in the solid state (32:68) as a
diazabicyclo[2.2.2]octane resets the molecular switch.
mixture of 1A4+ and 1B4+, respectively. Acid converts 14+ into 1-
6+
6+
H2 (protonation of the DAN nitrogen atoms) and base 1-H2
into 14+ . These three compounds are illustrated as X-ray crystal
structures in their tubular representations. In addition to disor-
dered Cl– counterions, solvent molecules and hydrogen atoms
are omitted for the most part for the sake of clarity.
Mechanically interlocked molecules1 (MIMs), which em-
brace a large family of bistable catenanes2 and rotaxanes3,
have been identified as prototypes for the construction of
artificial molecular switches4 (AMSs) and machines5 on
account of our ability to exhibit control over the relative
intramolecular movements of their mechanically inter-
locked components. This attribute has rendered bistable
MIMs indispensible compounds for incorporation into in-
tegrated systems such as molecular actuators6, molecular
electronic devices7, and drug delivery vehicles.8
In the search for more application-driven devices derived
from MIMs, we are looking currently towards their operation
in aqueous solutions where most biological processes based on
motor-proteins express their functions, e.g., myosin9, kinesin10,
ATP-ase11 and bacterial flagella.12 Only a relatively few
examples13 of MIMs that can be switched in water have been
reported. For example, MIMs based on π-electron-rich
tetrathiafulvalene (TTF) units can be switched between two
states chemically in water by utilizing an amine as the trigger13b
to induce pirouetting of a tetracationic cyclophane containing
two different π-electron-deficient recognition units, e.g., a
bipyridinium (BIPY2+) and a diazapyrenium (DAP2+) unit. The
system, however, relies on a reversible coordination process
which attenuates the electron deficiency of the DAP2+ unit to
induce the pirouetting motion. In order to develop AMSs
which operate in water, it is desirable to create MIMs that can
be switched between two states by varying the pH of the
aqueous solution14 as opposed to relying upon a coordination
event. Herein, we present an acid-base switchable [2]catenane
O
O
HN
O
O
O
OTs
OH
HN
O
O
O
a
HN
O
O
O
OTs
HN
O
O
O
OH
2
3
4
N
N
N
N
Br
N
O
N
N
2 PF6
O
O
HN
O
O
Br
N
6
5
b
HN
O
O
O
4 Cl
ꢀꢁ‡ꢁ4Cl
Scheme 1. The synthesis of 1⋅4Cl. Conditions and reagents: a)
CsCO3, DMF, 80° C, 35%; b) (i) DMF, RT, 4d, (ii) MeCN, TBACl,
RT, 48%.
(HQ) unit and a 1,5-diaminonaphthalene (DAN) unit, both
competing16 to reside inside the cavity of the CBPQT4+ ring in
aqueous solution. The [2]catenane 14+ exists as two
translational isomers 1A4+ (DAN inside CBPQT4+) and 1B4+
(HQ inside CBPQT4+) in the ground state. Circumrotation of
the crown ether with respect to the CBPQT4+ ring is induced
(Figure 1) by protonation of the nitrogen atoms on the DAN
unit. In order to probe the extent of the Coulombic effect upon
the circumrotation process (Figure 1), UV/Vis spectroscopy
was performed (Figure 2) on pseudorotaxanes consisting of
HQ-gly and DAN-gly as the guests and CBPQT4+ as the host.
Because the electron-rich guests are not soluble in water before
complexation, the comparative binding study was conducted
1⋅4Cl (Scheme 1) which can undergo (Figure 1) mechanically
induced motion in water – simply by altering the pH of its
aqueous solution – between the 14+ state and the protonated 1-
H26+ state.
The bistable [2]catenane consists of two mechanically
interlocked rings, one, an electron-deficient tetracationic
cyclophane, cyclobis(paraquat-p-phenylene)15 (CBPQT4+)
and the other a crown ether containing a hydroquinone
with CBPQT4+⋅4PF6 in MeCN. The formation of the charge-
transfer
(CT)
complexes
CBPQT4+⊂HQ-gly
and
CBPQT4+⊂DAN-gly in MeCN results in characteristic CT
absorption bands (solid lines in Figure 2) at ca. 476 and ca. 720
nm, respectively. Upon addition of an excess of trifluoroacetic
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–
render the bistable [2]catenane soluble in water, the PF6
anions in 1⋅4PF6 were exchanged with Cl– anions using
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tetrabutylammonium chloride (TBACl) to afford 1⋅4Cl after
purification by HPLC (See Supplementary Information (SI)).
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Analysis (Figure 3a) of 1⋅4Cl by H NMR spectroscopy in D2O
at room temperature revealed the presence of two translational
isomers, a major (78%) one 1A4+, where the DAN unit is
located inside the cavity of the CBPQT4+ ring, and a minor
(22%) one 1B4+, where the HQ unit resides inside the cavity. By
utilizing 1H NMR correlation spectroscopy (COSY),
heteronuclear multiple bond correlation (HMBC), and
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heteronuclear
single-quantum
correlation
(HSQC)
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experiments, all the 1H signals for the two translational
isomers, 1A4+ and 1B4+, could be assigned: see Figure 3b and
Figures S1–S4 in the SI. The resonances for 1A4+ at 8.96/8.80,
Figure 2. Partial absorption spectra of the pseudorotaxanes formed
between the guests HQ–gly (red) and DAN–gly (magenta) before
(solid) and after (dashed) addition of excess of TFA. In both cases,
the major absorption band corresponds to the CT interaction be-
tween the guest and the host. On addition of TFA, the CT band
complex for the DAN–gly complex disappears on account of the
protonation of the secondary amino groups, a reaction that leads to
the dissociation of the pseudorotaxanes. No change is observed in
the case of the HQ-gly complex since it does not become protonat-
ed under the conditions of the experiment.
7.23/7.20 and 8.03/7.99 ppm are assigned to H 1/H 2, H 1/H 2
α
β
β
and Ha/Hb on the CBPQT4+ ring, respectively, whαile the signals
for the DAN unit, located inside the CBPQT4+ ring, appear at
6.00 (H3/7), 5.94 (H2/6) and 1.40 (H4/8) ppm, and for the HQ unit
alongside the CBPQT4+ ring at 6.13 (HHQ) ppm. The two
heterotopic sets (H 1/H 2) of eight α protons and the two
α
α
heterotopic sets (H 1/H 2) of eight β protons on the two
acid (TFA), the UV/Vis spectrum of the CBPQT4+⊂HQ-gly
complex at ca. 720 nm disappeared entirely, indicating
dissociation of the donor-acceptor (DA) complex. The
disruption of the DA complex in MeCN suggests that an
analogous Coulombic interaction between the DAN unit and
the CBPQT4+ ring in the bistable [2]catenane 1⋅4Cl will occur
in water, thus inducing the circumrotation process (Figure 1)
to occur on the addition of acid.
The bistable [2]catenane was obtained (Scheme 1), after
work-up, as 1⋅4PF6 from the template-directed reaction17
between the template 4 and 5⋅2PF6 and 6 in MeCN. In order to
β
β
bipyridinium (BIPY2+) units, as well as the two heterotopic sets
(Ha/Hb) for the eight paraxyxylene protons are a consequence
of the DAN unit imposing its local C2h symmetry on the BIPY2+
units which are rendered equivalent as a result of rapid
pirouetting of the HQ unit around the CBPQT4+ ring on the 1H
NMR time scale at room temperature. In the case of 1B4+, all
eight of the α protons (Hʹα) and all eight of the alpha protons
(Hʹβ) on the two equivalent BIPY2+ units are homotopic and so
only two resonances are observed at 8.86 and 7.68 ppm,
respectively.
Figure 3. (a) The equilibrium in D2O between 1A4+ (structural formula showing the proton assignments) and 1B4+ (solid-state structure) of
1⋅4Cl. (b) Partial 1H NMR (600 MHz, D2O, 298 K) spectrum of the equilibrium mixture of the major (1A4+) and minor (1B4+) isomers. The
peaks labeled with primes correspond to 1B4+. From integration, the ratio of 1A4+ to 1B4+ was found to be 78:22. (c) 1H NMR titration with
DCl in D2O. Increasing the [DCl] to 214 mM results in complete protonation of the DAN unit and the conversion of the 1A4+ / 1B4+ isomer-
6+
ic mixture to 1–H2
.
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Figure 5. Acid/base switching of the [2]catenane 1⋅4Cl moni-
tored by 1H NMR spectroscopy. Addition of 1.15 M DCl in D2O
leads to protonation of the DAN unit and build-up (pale green
stripe) of the HQ unit inside the CBPQT4+ ring. This process can
be reversed on addition of 1.15 M DABCO in D2O. Repeating
this procedure of acid/base addition leads to a reversible inter-
conversion between 14+ and 1-H26+, which has been driven
through five complete cycles without any visible signs of de-
composition.
Figure 4. UV/Vis Titration of 1⋅4Cl in H2O (5 x 10-4 M). Addi-
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tion of 0.6 M aqueous HCl (in 0.5 µL aliquots) leads to protona-
tion of the DAN unit which is obliged to leave the cavity of the
CBPQT4+ ring, resulting in a decrease in the CT band at ca. 720
nm. During the addition of the HCl a CT band appears at ca. 476
nm, indicating that the HQ unit is now occupying the cavity of
the CBPQT4+ ring.
nation) of the nitrogen atoms, the resonances (HP2/6, HP
,
3/7
HP4/8) shift downfield as Coulombic repulsion in 1A4+ in-
duces the crown ether to circumrotate through the
CBPQT4+ ring until 100% of the HQ units reside inside it.
All the resonances for this translational isomer (1-D26+, Fig-
The eight paraxyxylene protons Hʹa/b are all homotopic and
resonate at 7.71 ppm, as are the eight CH2 protons which
resonate at 5.71 ppm. The signal for the HQ unit appears at
3.35 ppm, while those for the alongside DAN unit resonate at
7.00 (Hʹ3/7), 6.85 (Hʹ2/6) and 6.21 (Hʹ4/8) ppm.
Single crystals of the [2]catenane were obtained by slow
diffusion of Me2CO into an aqueous solution of 1⋅4Cl. X-
Ray analysis18 revealed19,20 the presence (Figure 1) of 1A4+
and 1B4+ in the same crystal in a ratio of 32:68, respectively,
with the two translational isomers packed randomly in a
[π⋅⋅⋅π] stacking mode. Red single crystals of 1-D2⋅6Cl were
obtained by cooling a D2O solution of the [2]catenane con-
taining DCl down to 277 K. As expected, X-ray analysis18
revealed21 (Figure 1) that the HQ unit resides inside the
CBPQT4+ ring. Somewhat surprisingly, however, the crystal
1
ure 1) were confirmed in the COSY H NMR spectra (Fig-
ure S8 in the SI). Back titrating 1-D2•6Cl with a 1.15 M so-
lution of DABCO restored the distribution of 1A4+ to 1B4+
to that of a 78:22 ratio. The reproducibility and stability of
this acid-base switch was established by performing
1
(Figure 5) five complete cycles, monitored by H NMR
spectroscopy (Figure S11).
The fact that a bistable [2]catenane can be induced to
undergo large amplitude, relative motions, while being
switched between two different states by changing the pH of an
aqueous solution raises the possibility of developing integrated
nanobiomechanical systems at a level commensurate with
molecular prosthetics.22
packing (Figure S15) in the SI is governed by [π⋅⋅⋅π] stack-
ing of the tetradeuterated DAN2+ units located intra- and
intermolecularly between two BIPY2+ with a plane-to-
ASSOCIATED CONTENT
Supporting Information. Synthesis and characterization of
all compounds and additional experimental results. This
material is available free of charge via the Internet at
http://pubs.acs.org.
centroid distance of only 3.22 Å in the latter case.
In order to probe the switching of the [2]catenane in H2O
using acid/base triggers, titration experiments employing HCl
and DABCO (1,4-diazabicyclo[2.2.2]octane) have been
monitored (Figure 4 and Figures S12 and S13 in the SI) by
AUTHOR INFORMATION
UV/Vis spectroscopy. A neutral 0.5 mM solution of 1⋅4Cl
Corresponding Author
* stoddart@northwestern.edu
displays a green color, arising from an absorption band at ~720
nm, which can be attributed to a DAN⋅CBPQT4+ charge
transfer (CT) interaction. Upon the stepwise addition of 0.6 M
HCl, this CT band starts to disappear gradually as a new band
emerges at ~476 nm producing a red color that can be
Author Contributions
†These authors contributed equally to this work.
associated with an HQ⋅CBPQT4+ CT interaction. The evolution
of both CT bands supports the argument that the CBPQT4+
ring is encircling the DAN unit at neutral pH and the HQ unit
at acidic pHs. The switching which occurs by circumrotation
of the crown ether through the CBPQT4+ ring can be reversed
by the stepwise addition of 0.6 M DABCO to a red solution of
ACKNOWLEDGMENTS
The research was enabled by the National Center for Nano-
technology research at the King Abdulaziz City of Science
and Technology (KACST) in Saudi Arabia. The authors
thank Dr. Turki S. Al-Saud and Dr. Nezar H. Khdary at
KACST for their interest in the research. S. G. thanks the
Swiss National Science Foundation for financial support.
P.L.M acknowledges the National Science Foundation
(NSF) and Georgia Tech Facilitating Academic Careers in
Engineering and Science (FACES) committee for a post-
doctoral fellowship.
1-H2⋅6Cl.
1H NMR titration experiments were also performed on the
[2]catenane in D2O using DCl and DABCO as the acid/base
triggers. A 1.15 M DCl/D2O solution was added in 1.5 µL
1
aliquots to a 3.5 mM solution of 1⋅4Cl in D2O and H NMR
spectra (Figure 3c) were recorded at room temperature after
equilibration. Initially, only 22% (1B4+) of the HQ units
reside inside the CBPQT4+ ring. Upon deuteration (proto-
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Center under supplementary publication nos. CCDC-
956146, 956144 and 956145, respectively.
(19) Both translational isomers 1B4+and 1A’4+ were obtained
from one crystal in a 68:32 ratio of HQ-inside:DAN-inside
the CBPQT4+.ring Their crystal structures are shown in Fig-
ure 1. The observation that the ratio of the two translational
isomers is inversed in solution and the solid state is current-
ly unknown. The crystal packing of 1B4+/1A4+ is random.
(5) (a) Balzani, V.; Credi, A.; Raymo, F.M.; Stoddart, J.F. Angew.
Chem. Int. Ed. 2000, 39, 3349–3391. (b) Kay, E.R.; Leigh,
D.A.; Zerbetto, F. Angew. Chem. Int. Ed. 2007, 46, 72–191;
(c) Balzani, V.; Credi, A.; Venturi, M. Molecular Devices
and Machines; Wiley: Weinheim, 2008. (d) Hirose, K. J. Incl.
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Banaszak, M.; Astumian, R.D.; Stoddart, J.F.; Grzybowski,
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Crystal parameters for 1B4+/1A’4+: C68H76Cl4N6O8, Mr
=
1247.15, yellow plates, crystal size 0. 33 x 0.31 x 0.01 mm,
orthorhombic, space group Pccn, a = 29.0878(6), b =
10.0431(2), c = 26.8906(5) Å, α= 90.00 °, V = 7855.6(3) Å3, Z
= 4, ρcalc = 1.055, T = 100(2) K, R1(F2 > 2σF2) = 0.1293, wR2 =
0.3903.
(6) Liu, Y.; Flood, A.H.; Bonvallet, P.A.; Vignon, S.A.;
Northrop, B.H.; Tseng, H.-R.; Jeppesen, J.O.; Huang, T.J.;
Brough, B.; Baller, M.; Magonov, S.; Sobres, S.D.; Goddard,
III, W.A.; Ho, C.-M.; Stoddart, J.F.; J. Am. Chem. Soc. 2005,
127, 9745–9759.
(20) Slow diffusion of i-Pr2O into a MeCN solution of 1⋅4PF6
produced brown single crystals suitable for X-ray analysis.
Crystal parameters for 1 • 4PF6: C80H94F24N12O8P4, Mr =
1931.55, brown plates, crystal size 0.21 x 0.19 x 0.02 mm,
Triclinic, space group P-1, a = 13.34(4), b = 13.59(3), c =
25.053(4) Å, α= 86.557(8) β = 81.545(10)° γ = 76.912(9), V =
4374.1(19) Å3, Z = 2, ρcalc = 1.467, T = 100(2) K, R1(F2 >
2σF2) = 0.0431, wR2 = 0.1201.
(7) (a) Collier, C.P.; Mattersteig, G.; Wong, E.W.; Luo, Y.; Bev-
erly, K.; Sampaio, J.; Raymo, F.M.; Stoddart, J.F.; Heath, J.R.
Science 2000, 289, 1172–1175. (b) Green, J.E.; Choi, J.W.;
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E.; Luo, Y.; Sherry, B.A.; Xu, K.; Shin, Y.S.; Tseng, H.-R.;
Stoddart, J.F.; Heath, J.R. Nature 2007, 445, 414–417. (c)
Heath, J.R. Annu. Rev. Mater. Res. 2009, 39, 1–23.
(21) Crystal parameters for 1-D26+•6Cl: C68H78Cl4N6O16, Mr =
1377.16, Red plates, crystal size 0.45 x 0.39 x 0.02 mm, mon-
oclinic, space group P2(1)/c, a = 29.60(4), b = 9.899(14), c =
28.00(4) Å, α= 90.00 β = 102.00(3)°, V = 8025(20) Å3, Z = 4,
ρcalc = 1.140, T = 100(2) K, R1(F2 > 2σF2) = 0.1285, wR2 =
0.3748.
(8) Li, Z.; Barnes, J.C.; Bosoy, A.; Stoddart, J.F.; Zink, J.I. Chem.
Soc. Rev. 2012, 41, 2590–2605.
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M.; Holmes, K.; Milligan, R. Science 1993, 261, 58–65.
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Nature 1998, 380, 550–555.
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conjugate Chem. 1994, 5, 504–507. (b) Shimoboji, T.;
Larenas, E.; Fowler, T.; Kulkarni, S; Hoffman, A.S.; Stayton,
P.S. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 16592–16596. (c)
Boyle, M.M.; Smaldone, R.A.; Whalley, A.C.; Ambrogio,
M.W.; Botros, Y.Y.; Stoddart, J.F. Chem. Sci. 2011, 2, 204–
210. (d) Coskun, A.; Banaszak, M.; Astumian, R.D.;
Stoddart, J.F.; Grzybowski, B.A. Chem. Soc. Rev. 2012, 41,
19–30.
(11) Abrahams, J.-P.; Leslie, A.G.W.; Lutter, R.; Walker, J.E. Na-
ture 1994, 370, 621–628.
(12) (a) Berg, H.C.; Anderson, R.A. Nature 1973, 245, 380–382.
(b) Silverman, M.; Simon, M. Nature 1974, 249, 73–74.
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