Bidirectional chemical communication between nanomechanical switches

Article abstract of DOI:10.1002/anie.201400804

The interplay of biological machines depends critically on the bidirectionality of chemical information exchange. The implementation of such a communication procedure for abiological systems is achieved using two nanoswitches that both operate as transmitters and receivers by transfering copper ions in oxidation states +I and +II. Even at micromolar concentrations, communication in both directions is remarkably fast, occurring at t _1/2=2-3 min. Metal ion translocation triggers a 20 A relocation of the toggle at both nanoswitches, entailing major geometric and electronic changes. Life depends critically on information transfer, because ON-OFF regulation of biological machines requires error-free procedures for intra- and intercellular data exchange. Bidirectional molecular communication for the first time leads to a two-way information exchange between two nanomechanical switches within few minutes.

Full text of DOI:10.1002/anie.201400804

Angewandte
Chemie
DOI: 10.1002/anie.201400804
Nanoswitches
Bidirectional Chemical Communication between Nanomechanical
Switches**
Susnata Pramanik, Soumen De, and Michael Schmittel*
Abstract: The interplay of biological machines depends
critically on the bidirectionality of chemical information
exchange. The implementation of such a communication
procedure for abiological systems is achieved using two
nanoswitches that both operate as transmitters
and receivers by transfering copper ions in
oxidation states + I and + II. Even at micromolar
concentrations, communication in both directions
is remarkably fast, occurring at t1/2 = 2–3 min.
Metal ion translocation triggers a 20 ꢀ relocation
of the toggle at both nanoswitches, entailing major
geometric and electronic changes.
Herein, we present a procedure for reversible communi-
cation between the two nanoswitches 1 and 2. Both experi-
ence major nanomechanical reorganization at the sending and
the receiving nanoswitch and both are able to act as trans-
E
ndogeneous and exogeneous communication
is essential for life. From bacteria to animals,
intricate processes,[1] such as chemotaxis[2]
(motion towards food) and cell division,[3] are
guided by chemical signal transduction, often
even through entangled signaling cascades. Signal
transduction in biology has to be specific and
sensitive. Thus, it is a tremendous challenge for
the new field of systems chemistry to establish
fast bidirectional communication[4] using appro-
priate driving energy[4c,d] between artificial
molecular devices and switches.[5–7] An elegant
example of molecular communication that
Scheme 1. Reversible bidirectional communication between two nanoswitches (the
switching states are denoted in brackets).
involves a tandem switching being reminiscent of proton
relay processes in biology has recently been reported by
Aprahamian.[4d] In that particular case, the addition of
a zinc(II) ion changed the configuration and switching state
of a hydrazone-based switch, which additionally initiated
toggling of a second switch through translocation of a proton.
To reset the system to its initial state, the metal ion needed to
be removed by cyanide. As a consequence, reversible
communication was unilaterally actuated through chemical
inputs at the same switch while the second switch assumed
a passive and externally operated role.[4c,d]
mitters in the switching. Bidirectional communication is
realized by translocation of copper in two redox states and
may be triggered externally by chemical or electrochemical
oxidation/reduction (Scheme 1). In detail, upon oxidation of
complex [Cu(1)]+ (state 1II +), the resultant copper(II) ion
Cu
migrates to nanoswitch 2, where it detaches the azaterpyr-
idine arm from the zinc porphyrin station to form complex
[Cu(2)]2+ (state 2II 2+). At the same time, that is, after
Cu
releasing the Cu2+, the azabipyridine arm of 1 moves to the
zinc porphyrin station, generating state 1I. Thus, in the
communication process, both switches undergo a circa 20 ꢀ
nanomechanical reorganization at their arms. Reduction of
state 2II 2+ in presence of 1I resets the system by reconstitut-
Cu
ing the original switching states (1II + and 2I).
Cu
[*] S. Pramanik, S. De, Prof. Dr. M. Schmittel
Center of Micro and Nanochemistry and Engineering
Organische Chemie I, Universitꢀt Siegen
We designed the reversible switching on the basis of three
orthogonal coordination motifs, that include [Cu(phen1)-
(phen2)]+ (HETPHEN), [Cu(phen)(terpy)]+ (HETTAP),
and pyridine!zinc porphyrin interactions[8] (phen = phenan-
throline, terpy = terpyridine). In the recently reported nano-
switch 1,[9] we combined HETPHEN and pyridine!zinc
porphyrin interactions, while in the new nanoswitch 2 the
latter interaction is opposed to a HETTAP complexation site.
As demonstrated earlier, Cu+ favors the HETPHEN coordi-
nation, while Cu2+ ought to prefer the pentacoordinated
Adolf-Reichwein-Strasse 2, 57068 Siegen (Germany)
E-mail: schmittel@chemie.uni-siegen.de
[**] We are grateful to the Deutsche Forschungsgemeinshaft and the
Universitꢀt Siegen for financial support. We thank Debrata Samanta
for PM6 calculations, Dr. T. Paululat for NMR measurements, and
Kun Chen for the biferrocenyl derivative.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201400804.
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Scheme 2. Selectivities of model ligands towards Cu+.
HETTAP situation, in analogy to procedures elaborated by
Sauvage in redox-controlled molecular machinery.[10]
Before preparation of 2 made enough sense, we needed at
first to evaluate orthogonal functioning of its prospective
azaterpyridine arm face to face with the azabipyridine unit in
1. Thus, in a model study (Scheme 2) a 1:1:1:1 mixture of 3, 4,
5,[11] and Cu+ was reacted in [D2]dichloromethane, furnishing
[Cu(3)(4)]+ and [Cu(3)(5)]+ in a ratio of 78:22, as derived
from 1H NMR spectroscopy (Supporting Information, Fig-
+
Cu
ure S1). As a result, preferentially [Cu(1)]+ (state 1II
)
should form from a 1:1:1 mix of Cu+, 1, and 2. The relative
ratio of [Cu(1)]+ vs. [Cu(2)]+, however, would even be
enlarged if intramolecular binding in 2I were stronger than
that in 1I. To predict partitioning of Cu+ to both switches, we
undertook computations at the PM6 level. The minimized
structure of switch 1I shows the azabipyridine 4N-terminus to
be significantly off the ideal axial coordination to the zinc
porphyrin, suggesting some amount of strain in the molecule
(see Computational Studies in the Supporting Information).
Conversely, switch 2I may experience less strain[12] because
Figure 1. 1H NMR (400 MHz, 298 K, CD2Cl2) of a) switch 2; b) switch
1; c) [Zn(2)]2+; d) [Cu(2)]+; e) [Zn(1)]2+; f) [Cu(1)]+; g) mixture of 1 and
2 in presence of Zn2+; and h) mixture of 1 and 2 in presence of Cu+.
4
the azaterpyridine N-atom coordinates almost perpendicu-
larly (at 87.48) to the zinc porphyrin.
Synthesis and characterization of 1 and [Cu(1)]+ have
been reported recently.[9] Nanoswitch 2 was prepared in 30%
yield by Sonogashira coupling from its direct precursors
(Supporting Information, Scheme S1) and was fully charac-
1
terized using H NMR, IR, and UV/Vis spectroscopy, ESI-
MS, and elemental analysis. The ESI mass spectrum displays
a molecular ion peak at m/z = 1813.7 that is diagnostic for
[2CH]+ with the observed isotopic distribution neatly matching
the theoretical one (Supporting Information, Figure S3). In
the 1H NMR, the pyrimidine protons a-H and b-H show up in
the aliphatic region at 3.57 and 2.72 ppm (Figure 1a; Sup-
porting Information, Figure S2), respectively, clearly suggest-
ing coordination of the 4N nitrogen to the zinc porphyrin and
thus state 2I (Scheme 3). Furthermore, the UV/Vis absorption
band at 429 nm (Soret band, c(2) = 10ꢀ6 m) corroborates the
pyrimidine unit to be bound to the zinc porphyrin. Concen-
tration independence of the wavelength and the extinction
coefficient provides convincing evidence for intramolecular
coordination (Supporting Information, Figure S4).
At first, we investigated toggling between the two states
2+
2I!2II
of the new nanoswitch using one equiv of Zn2+ as
Scheme 3. Reversible toggling between two states of switch 2. Zn-
(OTf)2 has to be added in CD3CN owing to solubility problems.
Zn
a diamagnetic input. The 1H NMR for [Zn(2)]2+ = 2II
2+
Zn
(Figure 1c; Supporting Information, Figure S5) exhibits
downfield shifts Dd of 4.72 and 5.88 ppm for the pyrimidine
protons a-H and b-H, respectively, suggesting the formation
of a zinc(II) phenanthroline terpyridine complex. This assign-
ment is further corroborated by diagnostic upfield shifts of the
mesityl protons from 6.98 and 7.05 ppm to 6.07, 6.18, 6.24, and
6.27 ppm, as the protons are now shielded by the azaterpyr-
idine unit. Splitting into four signals arises owing to constitu-
tional and geometrical unsymmetry in the complex. Likewise,
2
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the characteristic downfield shifts Dd of phenanthroline
protons 4-H and 7-H by 0.74 and 0.70 ppm, respectively,
verify formation of 2II 2+. A shift of the Q-band from 562 to
Zn
550 nm in the UV/Vis titration of 2 (10ꢀ4 m) with one equiv of
Zn2+ (2.5 ꢁ 10ꢀ3 m) indicates a detachment of the azaterpyr-
idine arm from the zinc porphyrin station (Supporting
Information, Figure S13). The ESI mass spectrum shows
a molecular ion peak at m/z = 939.1, the experimental isotopic
distribution of which precisely matches the theoretical pattern
for [Zn(2)]2+ (Supporting Information, Figure S7).
Toggling was completely reversed to state 2I upon removal
of zinc(II) ions from 2II 2+ by complexation with one equiv of
Zn
cyclam. Hereupon, the azaterpyridine arm reverts back to the
zinc porphyrin station as proven by NMR and UV/Vis
spectroscopy and in comparison with state 2I. In the
1H NMR spectrum (c = 0.55mm), for example, protons a-H
and b-H reappear at their initial positions (3.57 and 2.72 ppm)
as do the mesityl protons at 6.98 and 7.05 ppm when 1 equiv of
cyclam was added. Reversibility of the process was demon-
1
strated over two and a half cycles by H NMR spectroscopy
(Supporting Information, Figure S12). For additional support,
2+
a UV/Vis titration of [Zn(2)]2+ = 2II
(10ꢀ4 m) against
Zn
cyclam (2.5 ꢁ 10ꢀ3m) was conducted. The shift of the Q-
band from 550 to 562 nm indicated formation of 2I. However,
owing to the lower concentrations in the UV/Vis titration,
1.5 equiv of cyclam was required (Supporting Information,
Figure S14).
Figure 2. ESI mass spectra of (top) a mixture of 1, 2 and Cu+ (1:1:1)
suggesting a ratio of both complexes [Cu(1)]+/[Cu(2)]+ ꢁ90:10, as
corroborated by NMR integration; (bottom) a mixture of 1, 2, and
Zn2+ (1:1:1) showing exclusive formation of [Zn(2)]2+, as substantiated
by the NMR data (Figure 1g).
To evaluate the unidirectional communication, nano-
switches [Zn(1)]2+ (Figure 1e) and [Cu(2)]+ (Figure 1d)
were prepared separately.[13] Upon mixing of [Zn(1)]2+ and
2 in [D2]dichloromethane, translocation of the zinc ion from
1II 2+ to 2I thus generating states 1I and 2II 2+ was monitored
by H NMR spectroscopy and ESI-MS. For solubility pur-
poses, the mixture had to be heated first at 408C for a few
seconds to dissolve compound 2. A subsequently recorded
1H NMR spectrum of the reaction mixture (Figure 1g) was
devoid of the three sets of mesityl protons that are indicative
for [Zn(1)]2+ showing instead signals typical for 1, for example
the two singlets at 6.97 and 7.03 ppm. Indicative of 1,
pyrimidine protons a’-H and b’-H now emerge at 3.31 and
spectra in Figure 1h and the Supporting Information, Fig-
ure S18), the selectivity was increased to 97:3 ([Cu(1)]+/
[Cu(2)]+) by adding 2% (v/v) CD3CN as a weakly coordinat-
ing solvent (Supporting Information, Figure S20).[12] In the
translocation process, the three peaks related to mesityl
protons of [Cu(2)]+ at 6.08, 6.14, and 6.32 ppm decrease and
a new set with four mesityl protons at 5.92, 6.14, 6.26, and
6.34 ppm emerges for [Cu(1)]+. Additionally, a new set of
peaks appears at 3.55 and 2.71 ppm, which is characteristic for
free 2 with its azaterpyridine protons a-H and b-H being
located in the shielding region of the zinc porphyrin ring. The
translocation of Cu+ was equally seen in the ESI mass spectra.
Upon changing from dichloromethane (Figure 2, top) to
dichloromethane/2% acetonitrile (Supporting Information,
Figure S22) the peak of complex [Cu(1)]+ at m/z = 1799.2 is
further enlarged.
Initial kinetic measurements for metal ion translocation
were conducted at 258C. At first, the kinetics of Zn2+
translocation from [Zn(1)]2+ to 2 was evaluated (at c =
2.56 mm) in CDCl3. When the NMR spectrum was recorded
6 min after mixing, it indicated that 90% translocation had
already taken place. The translocation was clearly completed
16 minutes after mixing (Supporting Information, Fig-
ure S16). A kinetic analysis at c = 2.5 ꢁ 10ꢀ6 m for each of
[Zn(1)]2+ and 2 showed a pseudo-first-order course, providing
Zn
1
Zn
2.88 ppm, respectively. The mesityl protons related to
2+
Zn
[Zn(2)]2+ = 2II
appear upfield at 6.07, 6.18, 6.24, and
6.27 ppm as four peaks. Protons a-H and b-H show up at 8.29
and 8.60 ppm, respectively (Supporting Information, Fig-
ure S15). ESI-MS data corroborate the full metal ion
exchange, displaying only [Zn(2)]2+ at m/z = 939.1 (Figure 2,
bottom). As a result, a battery of data clearly shows
quantitative translocation of Zn2+ between [Zn(1)]2+ and 2
to afford [Zn(2)]2+ and 1, that is, a directional communication
2+
between nanoswitches in states 1II
and 2I generating the
Zn
switches in the final states (1I and 2II 2+).
Zn
Reverse metal-ion translocation was studied by a similar
procedure; however, now using copper(I) ions. Complex
[Cu(2)]+ was prepared separately in [D2]dichloromethane,
then 1 was added. The communication of nanoswitches 1I and
t
1/2 = 161 s (258C; see “Translocation of metal ions” in the
2II + by copper(I) ion translocation led to the nanoswitches in
Supporting Information). Similarly, the Cu+ translocation was
followed in the 1H NMR after adding one equivalent of
nanoswitch 1 to [Cu(2)]+ (both at 2.78 mm). Six minutes after
mixing, the system had already reached its final equilibrium
Cu
states 1II + and 2I. While in CD2Cl2, the translocation stopped
Cu
at 90% conversion, that is, the mixture contained 90% of
+
Cu
+
1
[Cu(1)]+ = 1II
and 10% of [Cu(2)]+ = 2II
(see H NMR
Cu
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position (Supporting Information, Figure S19). A more
accurate analysis by UV/Vis spectroscopy showed t1/2 = 102 s
(258C) and pseudo-first-order behavior (both reactants used
at c = 5 ꢁ 10ꢀ6 m). The two metal ion translocations are thus
kinetically independent of the concentration of the metal-free
nanoswitch, implying that the rate-limiting step involves an
intramolecular dissociation of the metal complex = state II.
Moreover, because the translocation of both Cu+ and Zn2+
ions proceeds within a few minutes, the communication is of
rather similar speed in both directions.
After these promising results, we checked for bidirec-
tional reversible redox-controlled copper ion translocation. A
solution (1:1:1) of 1, 2, and [Cu(CH3CN)4]B(C6F5)4 in
dichloromethane (c = 0.80 mm), thus of [Cu(1)]+ and 2,
served as the starting point.[15] A scan to anodic potential in
a cyclic voltammetry (CV) experiment revealed a small
oxidation wave at 0.25 VSCE for [Cu(2)]+/2+ (10%) and a large
oxidation current at circa 0.75 VSCE for [Cu(1)]+/2+ (Support-
ing Information, Figure S33). Unfortunately, no Cu2+ trans-
location from [Cu(1)]2+ to 2 generating [Cu(2)]2+ was
recognized on the CV time scale in the reverse sweep, even
when we used low scan rates, such as 50 mVsꢀ1. Equally, the
translocation did not show up when the CV was stopped at the
anodic switching potential for 60 s before starting the
reductive scan. Such finding is in agreement with the slow
rate of Zn2+ translocation taking place only within minutes
(see above), as most likely Cu2+ will behave similarly owing to
analogous coordination preferences. On the other side,
exclusive formation of [Cu(2)]2+ was detected in ESI-MS
investigations 10 minutes after adding switch 2 to [Cu(1)]2+
(Supporting Information, Figure S27). Because of the low
concentrations (0.80 mm) and inadequate time scale of CV
experiments, metal ion transfer is thus not detectable after
oxidation in a single CV scan.
Figure 3. ESI-MS signature (top) of a solution containing 1, 2, and Cu+
(1:1:1) as obtained after oxidation with one equiv of TBPA+C in CH2Cl2;
(bottom) after reduction of the above solution with one equiv of
dmfc.[14]
two-electron oxidation produces a diamagnetic species. In
CD2Cl2, a [Cu(1)]+/[Cu(2)]+ ratio of 88:12 was observed,
while in presence of 2% CD3CN the ratio was improved to
95:5 (Supporting Information, Figure S30).[20]
Unfortunately, all attempts to monitor the kinetics of Cu2+
translocation by UV/Vis spectroscopy were met with failure
owing to the fact that the absorbance changes are too small,
but the rate of the process should to be similar to that of the
Zn2+ translocation (see above).
In conclusion, we report herein on several examples of
highly selective communication between two nanomechanical
switches and their kinetics, as effected by intermolecular
metal ion translocation. Even at micromolar concentrations,
communication in both directions is fast occurring at t1/2 = 2–
3 min with monomolecular metal-ion–ligand dissociation
being the rate determining step. The metal ion translocation
results in a circa 20 ꢀ relocation of the toggle at both
nanoswitches and thus to major geometric and electronic
changes, similar as those in biological systems, that may allow
to even link communication with switching ON/OFF (orga-
no)catalysis.[21]
A reversible bidirectional communication is possible
using chemical or electrochemical oxidation/reduction.
Accordingly, oxidation of [Cu(1)]+ and 2 with TBPA+C leads
to Cu2+ translocation furnishing 1 and [Cu(2)]2+, while after
reduction with dmfc or BFD the translocation of Cu+ yields
the starting state = ([Cu(1)]+ and 2).
In summary, our work opens new vistas for systems
chemistry, such as how to interconnect molecular computer
and storage units or artificial synapses by chemical exchange
processes. Such intermolecular switching is distantly reminis-
cent of that found in neural networks, relying on electro-
chemical signals between regions of the brain and spinal cord.
Furthermore, communication between nanoswitches may be
coupled to chemical follow-up processes.
To probe the redox-initiated toggling on a slower time
scale, we decided to use chemical redox agents. For oxidation,
we chose tris(4-bromophenyl)aminium hexachloroantimo-
+
ꢀ
nate (TBPAC SbCl6 )[16] and for reduction, decamethylferro-
cene (dmfc)[17] or a 3-alkyl-1,1’-biferrocenylene (BFD).[18,19]
To a mixture of [Cu(1)]+ and 2 in dichloromethane (prepared
+
as above), one equivalent of TBPAC was added. After 3 min,
a reductive scan CV analysis that was started at + 1.0 VSCE
(Supporting Information, Figure S34) showed
a sharp
increase in current at 0.25 VSCE, suggesting increased forma-
tion of [Cu(2)]2+. In ESI-MS, the molecular ion peak at m/z =
939.1, assigned to the dicharged [Cu(2)]2+, shows up almost
exclusively (Figure 3, top). The resultant solution of [Cu(2)]2+
was now reduced by adding 1 equiv of dmfc (Supporting
Information, Figure S35). An oxidative scan started at
ꢀ0.9 VSCE 2 minutes after mixing exhibited a lower current
signal for [Cu(2)]+ and a larger signal for [Cu(1)]+ (includes
Cu+/2+ and por0/+C redox transitions) than in the initial state.
Thus, reduction of [Cu(2)]2+ with dmfc led to translocation of
Cu+ ions to 1. An ESI mass spectrum recorded 10 minutes
after mixing displayed peaks at m/z = 1799.2 (major) and
1876.2 (minor) (Figure 3, bottom), indicative of [Cu(1)]+ and
[Cu(2)]+, respectively, thus confirming return to the starting
state. For quantification, backward translocation was moni-
1
tored by H NMR using BFD as reducing agent because its
4
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Published online: && &&, &&&&
Keywords: chemical communication · nanoswitches ·
.
orthogonal coordination · redox reactions · reversible switching
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2011, 5, 622; f) G. Haberhauer, Angew. Chem. 2011, 123, 6539;
Angew. Chem. Int. Ed. 2011, 50, 6415; g) H. Li, A. C. Fahren-
bach, A. Coskun, Z. Zhu, G. Barin, Y.-L. Zhao, Y. Y. Botros, J.-P.
[14] Masses are depicted above m/z = 350 to exclude the intense peak
of dmfc+ in the mass spectrum.
[15] CV spectra of the compounds [Cu(1)]+ and [Cu(2)]+ are shown
in the Supporting Information, Figures S31 and S32, respectively.
[16] F. A. Bell, A. Ledwith, D. C. Sherrington, J. Chem. Soc. C 1969,
2719.
[17] N. G. Connelly, W. E. Geiger, Chem. Rev. 1996, 96, 877.
[18] a) R. Breuer, M. Schmittel, Organometallics 2012, 31, 6642;
b) R. Breuer, M. Schmittel, Organometallics 2013, 32, 5980.
[19] Oxidation potentials: E1/2 (dmfc)[17] = ꢀ0.160 VSCE; E1/2 (BFD) =
0.09 VSCE and E1/2 (BFD+C)[18] = 0.76 VSCE; reduction potential:
E1/2 (TBPA+C) = 1.06 VSCE (W. Schmidt, E. Steckhan, Chem. Ber.
1980, 113, 577).
[20] To monitor the reverse metal ion translocation by NMR
spectroscopy, we started from a solution of [Cu(2)]2+ and
1 (one equiv each) and added 3-(11-bromoundecyl)-1,1’-biferro-
cenylene as reductant (Supporting Information, Figure S30).
[21] M. Schmittel, S. De, S. Pramanik, Angew. Chem. 2012, 124, 3898;
Angew. Chem. Int. Ed. 2012, 51, 3832.
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Nanoswitches
S. Pramanik, S. De,
M. Schmittel*
&&&& — &&&&
Bidirectional Chemical Communication
between Nanomechanical Switches
Life depends critically on information
transfer, because ON-OFF regulation of
biological machines requires error-free
procedures for intra- and intercellular
data exchange. Bidirectional molecular
communication for the first time leads to
a two-way information exchange between
two nanomechanical switches within few
minutes.
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!