A reversible nanoswitch as an ON-OFF photocatalyst

Article abstract of DOI:10.1039/c2cc36408e

The two states of a new nanomechanical switch were quantitatively and reversibly populated in several subsequent switching cycles using either Cu ⁺ or cyclam as chemical inputs. State II was demonstrated to cis-trans isomerise diazastilbene upo

Full text of DOI:10.1039/c2cc36408e

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Chemical Communications
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Volume 48 | Number 96 | 14 December 2012 | Pages 11701–11800
ISSN 1359-7345
COMMUNICATION
Michael Schmittel et al.
A reversible nanoswitch as an ON–OFF photocatalyst

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A reversible nanoswitch as an ON–OFF photocatalystw
Michael Schmittel,* Susnata Pramanik and Soumen De
Received 3rd September 2012, Accepted 4th October 2012
DOI: 10.1039/c2cc36408e
The two states of a new nanomechanical switch were quantita-
tively and reversibly populated in several subsequent switching
cycles using either Cu+ or cyclam as chemical inputs. State II
was demonstrated to cis–trans isomerise diazastilbene upon
irradiation selectively in the presence of stilbene, and even to
operate as a photocatalyst.
In the present communication, we describe the synthesis,
characterisation and modus operandi of an improved nanoswitch
architecture: a fixed two-state conformational nanomechanical
switch. The switch works reversibly and quantitatively in the
presence of Cu+ as a chemical stimulus liberating a zinc
porphyrin unit in the ON state that acts as a photosensitiser
and even a photocatalyst in the isomerisation of an alkene
(Scheme 1). To the best of our knowledge, there is no report
on any nanoswitch that is operated reversibly and quantitatively
by a chemical stimulus and allows to control a photoreaction in
an ON–OFF manner.
The ease at which intricate and intertwined biological
machines perform multifarious tasks with supreme efficiency
has inspired researchers worldwide to design and investigate
artificial molecular machines, switches and devices1–4 that are
controlled by external input signals. As a notable subcategory
in this wide field, molecular switches5 that trigger chemical
processes6 have recently gained importance, among them in
particular nanomechanical switches with their fascinating
modes of action. A nanomechanical switch, here called a
nanoswitch, operates via significant amplitude motions, thus
entailing large geometric and often electronic changes. In one
recent example, Rebek and coworkers7 have described a light-
responsive cavitand–piperidinium complex that is used to
catalyse a Knoevenagel condensation. Although the rate of
reaction is well modulated using the light-triggered isomerisation
of an azobenzene arm, switching does not operate in ON–OFF
mode. The switchable organocatalyst based on a rotaxane
design as presented lately by Leigh fulfills much better the
criteria of an ON–OFF switch, but the ON mode requires both
an alternate switching state and an activation of the nucleophile.8
Already these two examples demonstrate that a nanomechanical
switch with distinct ON–OFF9 functions for catalysis requires a
clear-cut OFF state, while the ON state should simply lead to a
significantly accelerated reaction with regard to that of the OFF
state. In this context, we presented lately the chemical-stimuli
responsive nanoswitch 4 for reversible ON–OFF Knoevenagel
organocatalysis.10 In the ON state of catalysis, an ‘‘autoinhibitory
segment’’ precludes the catalyst to bind to a zinc porphyrin unit of
the switch while in the presence of [Cu(2,9-di(9-anthryl)phenan-
throline)]+ as input the zinc porphyrin becomes active for binding
of the catalyst, resulting in an OFF state.
In extending our previous design and to more readily effect
formation of both switching states, we attach a sterically
encumbered phenanthroline as a second intramolecular station
to nanomachine 410 (Scheme 2). The so formed fixed two-state
nanoswitch 1 (Scheme 1) requires only one single chemical input
for switching between the two states as relocation of the arm is
intramolecular. In state I, the pyridyl-pyrimidine (py-pym)
subunit coordinates to the zinc porphyrin thus diminishing its
propensity to coordinate to added substrates. Upon switching
to state II triggered by Cu+ addition, the zinc porphyrin, a well-
known triplet photosensitiser,11 should be liberated and be
available for photosensitized isomerisation reactions of properly
chosen substrates.
Nanoswitch 1 was synthesised using a Sonogashira reaction
between 410 and 5 (Scheme 2), the latter being prepared
following a synthetic protocol reported earlier.12 ESI-MS,
1H and 13C NMR, UV-Vis, and elemental analysis techniques
were used to characterise switch 1. ESI-MS data show a
molecular ion peak at m/z 1736.5 that is attributed to 1ÁH+
with the experimental isotopic distribution matching the
1
theoretical values (Fig. S21, ESIw). In the H NMR, protons
a-H, b-H and c-H of the pyrimidine unit appear as sharp
signals at 3.32, 2.87 and 6.63 ppm, respectively (Fig. S7, ESIw),
indicating that they are immersed into the shielding region of
Center of Micro and Nanochemistry and Engineering, Organische
Chemie I, Universitat Siegen, Adolf-Reichwein-Str. 2, D-57068
¨
Siegen, Germany. E-mail: schmittel@chemie.uni-siegen.de;
Web: http://www.chemie-biologie.uni-siegen.de/oc/oc1/chef/index.
html?lang=de
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc36408e
Scheme 1 Nanoswitching controls a photosensitised isomerisation.
c
11730 Chem. Commun., 2012, 48, 11730–11732
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addition of one equiv. of cyclam the band at 550 nm completely
shifts to 563 nm (Fig. S24, ESIw). This change indicates convin-
cingly that the py-pym unit is now again coordinated to the zinc
porphyrin. Secondly, after addition of one equivalent of cyclam
1
to 2, similarly the H NMR titration showed protons a-H and
b-H (Scheme 1) of the py-pym unit to reappear in the aliphatic
region at 3.33 and 2.88 ppm, respectively, proposing its
coordination to the zinc porphyrin. Moreover, the peaks at
5.93, 6.16, 6.26 and 6.37 ppm of protons d-H and e-H disappear
and regenerate at 6.97 and 7.03 ppm (Fig. S9, ESIw). Thus, both
UV-Vis and NMR titration data unambiguously provide
evidence for quantitative switching from state II to state I, a
result that is still rare for nanoswitches. Reversibility of this
process was checked up to four cycles and monitored by
1H NMR (Fig. 1).
Scheme 2 Synthesis of nanoswitch 1 from 4.10
the porphyrin ring current. The diagnostic shifts and their
concentration independence (Fig. S2, ESIw) indicate that the
py-pym unit is intramolecularly coordinated to the zinc porphyrin.
The UV-Vis spectrum of 1 displays an absorption maximum at
429 nm of the Soret band that is typical for a mono-coordinated
porphyrin. Like for the self-locked molecule 4,10 the combined
1H NMR and UV-Vis data unambiguously support intra-
molecular coordination between the pyrimidine and the zinc
porphyrin units in 1, representing state I.
After proving quantitative reversible switching between
states I and II, we considered to use 2 (state II) with its
liberated zinc porphyrin to command a chemical process. As
a reference reaction we envisaged the photoinduced cis–trans
isomerisation of azastilbenes, reported by Whitten.11 However,
azastilbene already binds weakly to the intramolecularly bound
zinc porphyrin of state I (log K = À1.09 Æ 0.01, Fig. S27, ESIw)
thus partly displacing the intramolecular arm. It is thus not
suited to keep state I, our chosen OFF state for photochemical
activity, intact. In contrast, 1 equiv. of the weaker binding
diazastilbene trans-3 (at 0.18 mM) (Fig. S10, ESIw) does not
displace the arm and likewise cis-3 (Scheme 1) should have
suitable binding properties.15
Switching to state II was achieved by addition of 1 equiv. of
[Cu(CH3CN)4]PF6 as an external stimulus to furnish complex
2 (Scheme 1). 2 was fully characterised by ESI-MS, NMR,
UV-Vis, and elemental analysis. The peak at m/z 1798.4 in the
ESI-MS (Fig. S22, ESIw) strongly supports formation of 2
with the experimental isotopic distribution neatly matching
1
the computed one. The H NMR spectrum is informative in
two ways with regard to establishing switching state II. Firstly,
the downfield shifts of the pyrimidine protons a-, b- and c-H
(Fig. S7, ESIw) from 3.32, 2.87, and 6.63 to 7.31, 7.31 and
7.67 ppm, respectively, clearly indicate formation of 2. Likewise,
the characteristic upfield shifts and splitting of the mesityl protons
at the phenanthroline (d- and e-H) from 6.97 and 7.03 to
5.93, 6.16, 6.26 and 6.37 ppm, respectively, are comprehensibly
diagnostic of the formation of heteroleptic13 complex 2. Thus,
spectroscopic evidence suggests a clean toggling of the pyridyl-
pyrimidine unit from the porphyrin station (state I) to the
phenanthroline station (state II).
To probe for its photochemical suitability, cis-3 was subjected
to isomerisation using ZnTPP as a photosensitiser at a 1 : 1
ratio. Irradiation at 419 nm (lmax = 422 nm) in DCM for
30 min led to 72(Æ3)%16 of trans-3 (at 0.10 mM concentration,
Fig. S12, ESIw). As a control, we made sure that cis-3 is stable
to unsensitised irradiation at 419 nm for 30 min. In contrast,
To further investigate the switching process, a UV-Vis
titration of a solution of 1 (10À4 M) against [Cu(CH3CN)4]
PF6 (2.5 Â 10À3 M) was carried out in dichloromethane. The
UV-Vis absorption maximum at 563 nm, characteristic for a
mono-coordinated porphyrin, completely shifts to 550 nm
upon addition of only 1 equiv. of Cu+ (Fig. S23, ESIw). To
further corroborate the switching to state II, i.e. 2, an 1H-NMR
titration of a solution of 1 (1.05 mM) was performed with
[Cu(CH3CN)4]PF6. The results clearly indicate full switching to
state II upon addition of one equiv. of Cu+ (Fig. S8, ESIw),
because the upfield shifted mesityl protons of 2 (due to shielding
by the py-pym ring current) appear, while at the same time the
original downfield shifted mesityl protons of 1 disappear
completely.
To assess reversibility of the switching process, we choose
cyclam10,14 to remove Cu+. Because [Cu(cyclam)]PF6 is insoluble
in the reaction mixture, it precipitates and sedimentates; thus, the
removal of the copper salt as waste in each repeating step is quite
convenient. The ensuing reversal of the switching, i.e. a change
from state II to I, was confirmed by UV-Vis and NMR spectro-
scopic data. Firstly, a UV-Vis titration with 2 (10À4 M) and
cyclam (2.5 Â 10À3 M) in DCM demonstrates that upon
Fig. 1 Partial 1H NMR spectra (400 MHz, CD2Cl2, 298 K) depicting
the reversibility of 4 cycles of switching between 1 and 2 (1.2 mM). (a),
(c), (e) and (g) correspond to 1 (state I) whereas (b), (d), (f) and (h)
represent 2 (state II), obtained after successive addition of Cu+ and
cyclam (d and e: mesityl protons of the phenanthroline; c: c-H protons
at the pyrimidine ring). The four cycles start with (a).
c
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Chem. Commun., 2012, 48, 11730–11732 11731

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standardised conditions a yield of E72% isomerisation. More-
over, the reversibly operating nanoswitch can discriminate
between stilbene and diazastilbene in the photoisomerisation.
These results demonstrate that even complex structures such as
1/2 may be operated as ON–OFF photosensitisers.
We are grateful to the Deutsche Forschungsgemeinschaft
and the Universitat Siegen for financial support.
¨
Notes and references
1
Fig. 2 Partial H NMR (400 MHz, CD2Cl2, 298 K) of a mixture of
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cis-3 and cis-stilbene after irradiation in presence of 2 with 419 nm
light for 30 min in DCM producing 72% of trans-3.
cis-stilbene does not isomerise at all under both conditions
(Fig. S13, ESIw).
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ON–OFF photosensitiser, the 1 : 1 mixture of pure cis-3 and 1
(0.10 mM) was irradiated at 419 nm for 30 min in DCM
resulting in no isomerisation. Thus, state I is suitable as an
OFF state (Fig. S14, ESIw). Upon addition of stoichiometric
amounts of [Cu(CH3CN)4]B(C6F5)4, the nanoswitch is moved
to state II,17 thus freeing the zinc porphyrin of 2 for coordination
with cis-3 (log K = 3.13 Æ 0.04; Fig. S28, ESIw). Rewardingly,
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and trans-3 (28 : 72 (Æ4)) upon irradiation at 419 nm for 30 min
(at 0.10 mM; Fig. S15, ESIw).
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isomerisation nanoswitch. Besides, both states are found to be
inactive in photoisomerising cis-stilbene (Fig. S16 and S17,
ESIw). In a competition experiment, both states of the nano-
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toggle for photoisomerisation, but additionally selective in
isomerising cis-3 in the presence of cis-stilbene (Fig. 2).
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as determined by H NMR (Fig. S20, ESIw). Addition of one
equivalent of cyclam generates state I, i.e. 1, with the effect that
irradiation for 20 min does not yield any further isomerisation.
As a consequence, nanoswitch 1/2 acts not only as a photo-
sensitiser but alike ZnTPP, as described by Whitten earlier,11 as
a photocatalyst.
11 D. G Whitten, P. D. Wildes and C. A. DeRosier, J. Am. Chem.
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16 Because the yield of the photoisomerisation depends very much on
the efficiency of degassing of the solvent, we report herein the
maximum yield in our setting.
In conclusion, we illustrate the design, synthesis and char-
acterisation of a fixed two-state nanomechanical switch acting
as a photocatalyst. Switching is accomplished quantitatively
and reversibly using Cu+ and cyclam as input signals. Catalytic
photosensitised isomerisation of cis-3 is achieved in state II (free
zinc porphyrin station), while state I is photochemically inactive.
It is truly remarkable to see the photoefficiency of state II closely
match that of the parent ZnTPP. Both show under identical and
17 After switching 2 with [Cu(CH3CN)]PF6 the received state II salt
was not fully soluble.
c
11732 Chem. Commun., 2012, 48, 11730–11732
This journal is The Royal Society of Chemistry 2012