Oscillating Emission of [2]Rotaxane Driven by Chemical Fuel

Article abstract of DOI:10.1021/acs.orglett.7b03996

A molecular shuttle consisting of a dibenzo-24-crown-8 macrocycle and an axle with two degenerate peripheral triazolium stations, a central dibenzyl ammonium station, and two anthracenes stoppers was exposed to 2-cyano-2-phenylpropanoic acid as a chemical fuel. Protonation/deprotonation of the amine reversibly switches the rotaxane from a static and little emissive to a dynamic fluorescent shuttling device, the latter exhibiting rapid motion (15 kHz at 25 °C). Four fuel cycles were run.

Full text of DOI:10.1021/acs.orglett.7b03996

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Oscillating Emission of [2]Rotaxane Driven by Chemical Fuel
Amit Ghosh,† Indrajit Paul,† Matthias Adlung,‡ Claudia Wickleder,‡ and Michael Schmittel*,†
†Center of Micro- and Nanochemistry and Engineering, Organische Chemie I, Adolf-Reichwein-Strasse 2, D-57068 Siegen, Germany
‡Center of Micro- and Nanochemistry and Engineering, Anorganische Chemie II, Adolf-Reichwein-Strasse 2, D-57068 Siegen,
Germany
S
* Supporting Information
ABSTRACT: A molecular shuttle consisting of a dibenzo-24-crown-8 macrocycle and
an axle with two degenerate peripheral triazolium stations, a central dibenzyl
ammonium station, and two anthracenes stoppers was exposed to 2-cyano-2-
phenylpropanoic acid as a chemical fuel. Protonation/deprotonation of the amine
reversibly switches the rotaxane from a static and little emissive to a dynamic fluorescent
shuttling device, the latter exhibiting rapid motion (15 kHz at 25 °C). Four fuel cycles
were run.
he fascinating arena of mechanically interlocked molecular
T
machines,1 such as rotaxanes2 and catenanes,3 has
received distinct recognition with the 2016 Nobel Prize in
Chemistry.4 Among these systems, [2]rotaxane-based molec-
ular shuttles, with either nondegenerate or degenerate stations,
have been intensely studied.5 Whereas nondegenerate molec-
ular shuttles are widely explored as bistate switches,1c,2b,6
systems with two identical binding sites, i.e., degenerate
molecular shuttles,7 are less known.8 For some rigid [2]-
rotaxanes, the thermal shuttling of the macrocycle has been
shown to be independent of the distance between the
degenerate stations.8a Further insight into the dynamic effects
of shuttling is thus needed for improving nanoscaled devices,
e.g., those used in molecular electronics.9
Herein we present a switchable [2]rotaxane10 in which the
degenerate shuttling is switched “ON” and “OFF” by addition
of the chemical fuel, 2-cyano-2-phenylpropanoic acid (1),
developed by Di Stefano et al.11 Switching by chemical fuel has
lately gained momentum.12 Usually, switching between two
states requires the sequential addition of a forward and
backward stimulus, like acid−base,13 reductant−oxidant,14
irradiation−heating,15 or light at two different wavelengths.16
Due to its unique features, the chemical fuel 1 acts first as acid
and after decarboxylation as strong base. Starting with R-2, fuel
1 should first generate R-1 (Figure 1) that does not show any
shuttling motion. Deprotonation of R-1 yields R-2 exhibiting
rapid shuttling (k298 = 15 kHz), which changes the electronic
properties at both triazolium stations. In contrast to static
fluorescence switches,17 cyclic emission is now possible.
In order to prepare the dynamic molecular shuttle R-2, we
resorted to the switchable rotaxane motif developed by Coutrot
et al.18 Traditionally, the stations in rotaxanes are linked by
long alkyl chains, but for the present work we synthesized the
[2]rotaxane R-1 with two degenerate triazolium stations
connected via a short dibenzylammonium bridge (see the
Supporting Information). Terminal anthracenes were used both
as stoppers and fluorescence probes. The triazolium methylene
Figure 1. Chemical structure and cartoon representation of molecular
shuttles. Intense blue color indicates strong fluorescence of anthracene
units and light color indicates weak fluorescence.
anthracene unit was utilized recently by Li et al. for reversible
switching in a [2]rotaxane resulting in two different fluorescent
states.19
In rotaxane R-1 (Figure 1), the macrocycle has a stronger
affinity toward the ammonium moiety than toward the
triazolium units. Thus, it preferentially sits on the central
station without visible shuttling. Using DBU and TFA as the
external stimuli, switching between R-1 and R-2 is highly
1
reversible, as demonstrated from H NMR and fluorescence
spectroscopic data. When R-1 is deprotonated with DBU,
Received: December 22, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03996
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
1
significant changes are observed in the H NMR indicative of
formation of R-2. The occurrence of a single set of triazolium
proton signals at 25 °C suggests rapid shuttling of the
macrocycle between both triazolium units (vide infra). Protons
1-H and 16-H (methyl protons) of the triazolium rings exhibit
an upfield shift from 8.75 to 8.56 ppm and from 4.09 to 3.05
ppm, respectively (Figure 2A). On the other hand, protons 8-H
Figure 3. Dynamic interconversion of states B−D of rotaxane R-2 (=
starting state B) fueled by 2-cyano 2-phenylpropanoic acid (1).
1
equimolar amount of acid 1, the H NMR recorded after 45 s
revealed a mixture of rotaxanes R-1 and R-2 in a ratio of 2:3
(see Figure S26). After 9 min (Figure 4), all proton signals
Figure 2. (A) Partial 1H NMR spectra (400 MHz, CD2Cl2, 25 °C) (a)
of R-1, (b) after addition of 1 equiv of DBU to R-1 (generating R-2),
and (c) after addition of 1 equiv of TFA (regenerating R-1). (B)
Switching fluorescence reversibly by toggling between R-1 and R-2
(λexc = 252 nm, 10−5 M) in CH2Cl2 at 25 °C.
1
Figure 4. Partial H NMR spectra (CD2Cl2, 25 °C) recorded after
mixing equimolar amounts of rotaxane R-2 (c = 1.5 mM) and acid 1 at
various times (after start). Marked signals are assigned to protons
○
▲
using the following symbols: , rotaxane R-1; , rotaxane R-2; *,
and 9-H are shifted downfield, while all anthracene stopper
protons display upfield shifts owing to shielding effects by the
crown-ether ring. These 1H NMR shifts clearly indicate
complexation between the crown ether and the triazolium
rings. Upon protonation with TFA the macrocycle returns back
benzyl proton of compound 4.
corresponding to R-1 had disappeared. A new parallel signal at
3.94 ppm increasingly emerged that is characteristic for the
benzylic proton of 4. Finally, full back transformation of R-1
into R-2 is clearly proven by the appearance of all R-2 peaks
with original intensity.
1
to the ammonium site; see the H NMR spectra in Figure 2A.
As shown in Figure 2B, both rotaxanes R-1 and R-2 exhibit
typical anthracene emission patterns but at quite different
intensity. In R-1, fluorescence is almost quenched due to a
strong PET from the excited anthracene to the triazolium
units.19 When 1 equiv of base was added to afford R-2, the
emission intensity immediately increased ca. 10-fold (Figure
2B). Apparently, association of the macrocycle with the
triazolium rings fully or partly prevents quenching by PET.
Addition of 1 equiv of acid to R-2 decreases the fluorescence
intensity again to the original value. Therefore, the switching
Following the fuel-driven transformation (c = 10−5 M,
CH2Cl2) by fluorescence spectrometry revealed additional
insight. As indicated in Figure 2B, rotaxane R-1 exhibits low
and rotaxane R-2 ca. 10-fold increased fluorescence intensity.
After the chemical fuel 1 had been added to the solution of R-2,
spectra were measured in 1 min intervals (Figure 5B and
Figures S40 and S41). Right after addition, the fluorescence
intensity decreased drastically within the first minute. As the
low fluorescence intensity of state A is not reached, the
protonation of R-2 by acid 1 apparently furnishes a mixture of
R-1 and R-2 (calculated as 3:1: state C). Over the course of ca.
25 min, the fluorescence intensity increases constantly (state C
→ D). The concentration dependence of the recovery toward
state D indicates that not only decarboxylation but also final
deprotonation of R-1 contribute to the overall rate. In the
course of four fueled cycles, each ignited by addition of 1 equiv
of acid 1 (Figure 5C), the fluorescence drops rapidly after each
addition and recuperates over ca. the same time, but never to
the original level as defined by state B (Figure 5B). With
increasing cycles, a slight decrease of both the high-level and
low-level emission intensity is visible that most likely finds an
1
can readily be monitored by means of either H NMR or
luminescence spectroscopy.
Inspiration to use a chemical fuel has come from the report
by Di Stefano in which acid 1 undergoes protonation, smooth
decarboxylation, and proton back transfer in the presence of an
acid−base switchable catenane at room temperature.11a
Similarly, we use acid 1 as a chemical fuel to drive the
switchable molecular shuttle. Addition of 100 mol % of acid 1
1
to rotaxane R-2 in CD2Cl2 causes a drastic change in the H
NMR within a few seconds with concomitant formation of R-1.
Reformation of R-2 is observed in about 9 min following the
steps depicted in Figure 3.
To investigate the fuel-driven reversible cycle between R-2
and R-1, we started from rotaxane R-2. After addition of an
B
DOI: 10.1021/acs.orglett.7b03996
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Letter
(around its own central axis) increasing partial shielding of the
triazolium unit via both benzene rings of the macrocycle. The
signal of proton 1-H at 8.56 ppm was simulated with the aid of
WINDNMR22 allowing the determination of the exchange
frequency k at various temperatures. At 25 °C the exchange
frequency k298 was calculated to be 15 kHz. The activation
parameters obtained for the dynamic process in R-2 are ΔH⧧ =
31.3 kJ mol−1, ΔS⧧ = −60.0 J mol−1 K−1, while the free energy
barrier at 25 °C was calculated to be 49.2 kJ mol−1.
To get some insight into the effect of shuttling on the
photophysical properties, we performed fluorescence studies for
both R-1 and R-2 in dichloromethane from +25 to −75 °C. In
both cases, the emission wavelength did not alter, indicating
that the fluorescing state remains the same. In R-1, where
shuttling is restricted, the fluorescence intensity continuously
increased as expected from the well-known temperature
effect.23 However, in the dynamic rotaxane R-2 the
fluorescence remained roughly constant from +25 to −20 °C.
Only, below −20 °C it increased as expected by the
temperature effect (Figure 7). This unforeseen behavior
Figure 5. Four cycles of the shuttling of R-2 upon addition of fuel 1.
(A) Photographs of one oscillation cycle. (B) First cycle. (C)
Fluorescence intensity changes at 421 nm vs time (λexc = 252 nm, c =
10−5 M) in CH2Cl2 at 25 °C over four cycles.
explanation in the fuel byproducts that absorb at the excitation
wavelength (λexc = 252 nm).
In order to investigate the translational oscillation in shuttle
20
1
R-2, the H VT-NMR was analyzed. At room temperature,
1
rotaxane R-2 exhibits the H NMR spectrum of a compound
with 2-fold symmetry, as it shows only one set of proton signals
for the triazolium stations and anthracene stoppers. The proton
signals of 1-H and 16-H (of the triazolium groups) appear as
broad singlets at 8.56 and 3.05 ppm, respectively. Upon
lowering the temperature, the singlet corresponding to proton
1-H broadens almost to baseline at 0 °C (see Figure S28).
Lowering the temperature further, the signal first disappears
and then reappears near −30 °C. The emergence of two very
broad peaks indicates that now the symmetry is broken. As the
temperature is further decreased, the signals of proton 1-H
develop into two sharp singlets with equal intensity (Figure 6)
Figure 7. Variable-temperature normalized fluorescence intensity of R-
1 and R-2 monitored at 421 nm (λexc = 252 nm, 1 × 10−5 M) in
CH2Cl2.
deserves a comment. In the low-temperature regime (−20 to
−75 °C), the macrocycle is located on one of the triazole units
and thus prevents PET quenching at this site. In return, the
other triazolium unit behaves as in R-1 triggering strong PET
quenching of the nearby anthracene luminescence. This
kinetically slow system acts like a normal fluorophore, reflecting
the temperature effect.
In contrast, the constant fluorescence intensity (from +25 to
−20 °C) may be associated with the fast oscillating dynamics of
the ring between the triazolium stations. As there is no increase
of fluorescence, the slowing of the ring oscillation between +25
and −20 °C that should be associated with a decrease of
fluorescence apparently is more or less counterbalanced by the
inherent temperature effect on fluorescence, which should lead
to increased emission intensity upon lowering the temperature.
In conclusion, we have synthesized a switchable molecular
[2]rotaxane that is static and hardly emissive in the protonated
state and a luminescent oscillating shuttle after deprotonation.
Figure 6. Experimental (left) and simulated (right) partial VT 1H
NMR spectra (CD2Cl2, 600 MHz) of R-2 shows the splitting of the
signal of proton 1-H (magenta asterisk).
at 9.10 and 8.70 ppm (at −75 °C). The signal at 8.70 ppm
corresponds to proton 1-HUC (UC = uncomplexed) as it
matches with that of proton 1-H in rotaxane R-1 (8.75 ppm)
(Figure 2A). The initial distinct downfield shift of the
triazolium proton 1-HC (C = complexed) may be attributed
to hydrogen bonding between 1-HC and the oxygen atoms of
DB24C8 in a restricted arrangement.21 The slight upfield shift
of proton 1-HC in the temperature range −30 to −75 °C may
be explained by slowing the rotational dynamics of the crown
1
The kinetic parameters obtained from variable-temperature H
NMR indicate an exchange frequency of 15 kHz at 25 °C.
Moreover, the shuttling motion is reversibly and dynamically
switched “ON” and “OFF” by the iterative addition of the
chemical fuel 1, as observed by both 1H NMR and fluorescence
C
DOI: 10.1021/acs.orglett.7b03996
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Letter
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b03996.
Synthetic procedures and full spectral characterization of
all new compounds, fueled reaction cycles, and
fluorescence studies (PDF)
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AUTHOR INFORMATION
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Corresponding Author
*E-mail: schmittel@chemie.uni-siegen.de.
ORCID
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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This work was funded by the DFG (Schm 647/20-1). Support
by Dr. T. Paululat (Univ. of Siegen) for VT-NMR is gratefully
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