Novel photoswitchable rotaxanes

Article abstract of DOI:10.1039/b702737k

A photoresponsive rotaxane based on the photoheterolysis of an acridane unit which is at the same time a bulky end group has been developed. The Royal Society of Chemistry.

Full text of DOI:10.1039/b702737k

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Novel photoswitchable rotaxanes{
Werner Abraham,*a Karin Buck,a Marziena Orda-Zgadzaj,a Sebastian Schmidt-Scha¨ffera and
Ulrich-W. Grummtb
Received (in Cambridge, UK) 21st February 2007, Accepted 4th May 2007
First published as an Advance Article on the web 21st May 2007
DOI: 10.1039/b702737k
To introduce this photoswitch into rotaxanes, we have
A photoresponsive rotaxane based on the photoheterolysis of
an acridane unit which is at the same time a bulky end group
has been developed.
synthesized the molecular axle 2 (Scheme 2) (see ESI{). The
tetracationic cyclophane cyclobis(paraquat-4,49-bisphenylene) 3
was used as a ring component of the rotaxanes. Therefore, the
differing electron donor strengths of the recognition stations within
the axle will play a role. These two stations are the acridane which
is, at the same time, the endcap group. A second photoinactive
station is represented by anisole which is a relatively weak electron
donor. An anisole unit is present both in station A (acridane) of 2
and as a substituent in compound 1. Accordingly, the question
arises as to whether there are sufficient differences of the
interaction energies between 3 and the two stations.
Biological machines are able to transform chemical energy into
linear or rotational motion. In order to simulate a linear motion
with synthetic systems, bistable rotaxanes having two recognition
stations of differing interaction strengths with the ring which
may be an appropriate starting point.1 An outer stimulus causes
a shift of the ring from one station to the next. Chemical energy,
or optimally, light energy, weakens the noncovalent interaction
between the ring component and the first station on which the ring
resides,2 and thus, the second station comes into play. The original
equilibrium, the so-called co-conformation,3 is altered and the ring
moves due to Brownian motion toward the station that now has
the stronger affinity for the ring. The reset can take place either
photochemically or thermally.4
The rotaxane 4 was formed by the esterification of the OH-
group of the pseudorotaxane 1/3 (Scheme 3). The synthesis route
leads in only three steps to a rotaxane with a 1.8 nm distance
between the two recognition stations.
According to the 1H NMR spectrum of 4, ring 3 resides
exclusively on station B. The proton resonances of station B are
strongly upfield shifted by 2.5 and 3.9 ppm.
Here we present a novel photoswitch based on bond fission for
changing the co-conformation of a rotaxane.
Photoexcited N-methyl-9-phenyl-9-methoxy-9,10-dihydroacri-
dine (N-methyl-9-phenyl-9-methoxyacridane) I forms an acridi-
nium ion II (Scheme 1),5 thus drastically changing both the
electronic properties and the shape of the molecule. The acridane
compound is reformed by the thermal attack of the leaving group
methoxide upon the acridinium compound.
Two-dimensional spectra (ROESY) show cross peaks between
ring protons and protons of station B. It is noteworthy that the
protons of the adjacent propoxy group are also upfield shifted by
The rate constant of the thermal reaction II A I depends
strongly on the solvent polarity.5
Scheme 1
aInstitute for Chemistry, Humboldt-University, Berlin, Germany.
E-mail: abraham@chemie.hu-berlin.de; Fax: +49 30 20937266;
Tel: +49 30 20937348
bInstitute for Physical Chemistry, Friedrich-Schiller-University, Jena,
Germany. E-mail: cug@un-jena.de; Fax: +49 30641 948302;
Tel: +49 3641 948350
{ Electronic supplementary information (ESI) available: Full synthetic
details pertaining to the preparations of new compounds. UV-vis and
NMR spectra of selected compounds. Assignment of protons of rotaxanes
4 and 6a. Transient decay curves and absorption spectra. See DOI:
10.1039/b702737k
Scheme 2 Components for rotaxane synthesis.
3094 | Chem. Commun., 2007, 3094–3096
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Accordingly, the proton resonances both of the acridane protons
and the methoxy substituent are downfield shifted, leading to the
conclusion that the ring occupies station A in the acridane
rotaxane. To shift the equilibrium toward the photoactive rotaxane
6 in concentrated solutions, a base such as NaHCO3 or Pri2EtN
should be added along with the nucleophile (Scheme 5). On a
preparative scale, rotaxanes 6 are obtained in excess, with less than
15% 4.
In contrast to the rotaxane 4, the proton signals in the 1H NMR
spectra of the ring 3 and of the aryl protons of stations A and B of
rotaxanes 6 are broad or are even merged in the baseline at room
temperature. However, the proton signals of the propoxy chain
of station B are sharp, and therefore assignable. These proton
resonances are low-field shifted compared to those of rotaxane 4
and are identical with the proton resonances of compound 5. On
the other hand, the protons of the RX-group are low-field shifted
due to their location at the edge of the ring. Both findings, taken
together, clearly demonstrate the occupancy of station A by the
1
ring. H NMR spectra recorded at lower temperature reveal that
Scheme 3 Rotaxane synthesis.
the tetracationic cyclophane 3 encircles station A. At 333 K,
signals are much sharper; the aryl protons of the two stations
appear and can be assigned by ROESY spectra. The protons of
station A are upfield shifted by 2.7 and 5.5 ppm (ESI{). Due to the
asymmetry of the axle, the bipyridinium protons appear as four
broad singlets and the protons of the methylene groups of 3 are
split into two doublets of the AB-system (ESI{).6
Rotaxanes 6 in dilute solutions are able to repair themselves.
The proportion of the acridinium rotaxane 4 remaining after the
synthesis is decreased when 4 is dissolved in methanol because the
equilibrium favors the acridane. In contrast, preparation of
methanol solutions of the acridane compounds 2a and 2b forms
40 and 55% acridinium ions 1 by dissociation. These differences in
behavior of rotaxanes and free axles are caused by the additional
free energy attained by the interaction of the tetracationic ring and
the phenyl substituent of the acridane moiety.
0.5 and 0.2 ppm. Thus, these protons may serve as probes of the
interaction between the ring and station B.
To generate the photoactive rotaxane, compound 4 must react
with nucleophiles such as methanol. This can be easily accom-
plished by using methanol as the solvent because acridane
compounds are ionogen compounds in methanol. Thus far,
however, this equilibrium has never been studied. The equilibrium
between the acridinium rotaxane 4 and the acridane rotaxane 6
favors 6 (80%) in diluted solution (1024…1025 M) (Scheme 4). In
contrast, the corresponding equilibrium of the molecular thread 1
is poised toward the acridinium compound (95%).
Evidently the rotaxane 4 gains free energy by conversion into
the rotaxane 6 due to the interaction of the ring with station A
instead of station B. This surprising finding can be explained by an
additional edge-to-face interaction of 3 with the acridane ring.
The photoexcitation of the molecular axle 2a results in the
corresponding acridinium ethoxide in solutions of both acetonitrile
and alcohols. Three isosbestic points of UV-vis spectra recorded
after consecutive irradiations of the solution indicate a simple
uniform photoreaction. The quantum yield is 0.5 in acetonitrile
solution. In alcoholic solutions, the thermal back reaction is
pseudo-first order. The lifetime of the acridinium alkoxide depends
strongly on the nature of the solvent. In a mixture of ethanol–
acetonitrile (5 : 1), the lifetime is 1200 s, 170 s in ethanol, and 97 s
in methanol solution. Also, the rotaxanes 6 undergo photoreaction
by excitation with 313 nm light. In each case, a transient exhibiting
the UV-vis spectrum of the acridinium ion was detected (ESI{).
Scheme 4 Equilibria of ionogen compounds.
Scheme 5 Synthesis of acridane rotaxanes.
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wavelength of 313 nm. This inner filter absorbs 50% of the
incoming light. A comparison of the absorbance at 360 nm
observed after the excitation of 2a and 6a results in a decrease of
the quantum yield to 20% of that of the 2a axle alone. Thus, based
on a quantum yield 0.5 (see above) for 2a, the efficiency of the
rotaxane photoheterolysis is in the range of 0.1. Ten switch cycles
do not result in any irreversible degradation of the acridane
rotaxane.
In summary, we have described a new class of tunable rotaxanes
incorporating an acridane moiety. The complete change of the ring
position following the photochemical and thermal reactions are an
advantage of the new system. Both ends of the rotaxanes can easily
be modified to introduce functional groups which are able to
immobilize such rotaxanes on surfaces and nanoparticles. Work is
currently in progress to realize these functionalities.
Scheme 6 Molecular shuttle 6b.
We gratefully acknowledge the Deutsche Forschungs-
gemeinschaft for financial support.
The lifetime of the transient of rotaxane 6a in ethanol solution is
1200 s. The acridinium methoxide intermediate formed from
rotaxane 6b possesses a lifetime of 130 s, while irradiation of a
methanol solution of the acridane rotaxane 6c with the thiolate
leaving group generates the acridinium thiolate which has a 180 s
lifetime. Analysis of the 1H NMR spectra of the rotaxanes 4 and 6,
respectively, has revealed that in the conversion from the acridane
to the acridinium rotaxane, a complete change of the location of
the ring occurs. Accordingly, the photoreaction of 6 leads to a
change of the site of the ring within the rotaxane. In other words, a
shuttle process is induced by irradiation (see Scheme 6). The reset
of the system occurs thermally and the lifetime of the intermediate
state of the rotaxanes can be controlled by the solvent properties.
The lifetime of the rotaxanes in their acridinium state is long
enough to allow the ring movement along the molecular axle.7
Therefore it is reasonable to conclude that both the forward and
the back reaction is accompanied by ring motion.
Notes and references
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K. Beizai and F. Vo¨gtle, Acc. Chem. Res., 2001, 34, 465; (c) E. R. Kay,
D. A. Leigh and F. Zerbetto, Angew. Chem., Int. Ed., 2007, 119, 72.
2 (a) V. Balzani, M. Venturi and A. Credi, Molecular Devices and
Machines, Wiley-VCH, Weinheim, 2003; (b) W. R. Brown and
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3 M. C. T. Fyfe, P. T. Glink, S. Menzer, J. F. Stoddart, A. J. P. White and
D. J. Williams, Angew. Chem., Int. Ed. Engl., 1997, 36, 2068.
4 For new photoswitchable rotaxanes, see: (a) E. M. Perez, D. T. F.
Dryden, D. A. Leigh, G. Teobaldi and F. Zerbetto, J. Am. Chem. Soc.,
2004, 126, 12210; (b) Y. Li, H. Li, Y. Li, H. Liu, S. Wang, X. He,
N. Wang and D. Zhu, Org. Lett., 2005, 7, 4835; (c) H. Murakami,
A. Kawabuchi, R. Matsumoto, T. Ido and N. Nakashima, J. Am. Chem.
Soc., 2005, 127, 15891; (d) V. Balzani, M. Clemente-Leon, A. Credi,
B. Ferrer, M. Venturi, A. H. Flood and J. F. Stoddart, Proc. Natl. Acad.
Sci. USA, 2006, 103, 1178.
5 T. M. Grigor9eva, V. L. Ivanov and M. G. Kuzmin, Zh. Org. Khim.,
1981, 17, 423.
6 J. O. Jeppesen, K. A. Nielsen, J. Perkins, S. A. Vignon, A. Di Fabio,
R. Ballardini, M. T. Gandolfi, M. Venturi, V. Balzani, J. Becher and
J. F. Stoddart, Chem. Eur. J., 2003, 9, 2982.
However, the efficiency of the photoreaction suffers due to two
problems. The charge transfer interaction between ring 3 and
station A is connected with a charge transfer state which opens
additional channels for decay of the excited state of the rotaxane 6.
Secondly, the ring component 3 also absorbs light at the excitation
7 V. Balzani, M. Go`mez-Lo`pez and J. F. Stoddart, Acc. Chem. Res., 1998,
31, 405.
3096 | Chem. Commun., 2007, 3094–3096
This journal is ß The Royal Society of Chemistry 2007