A photoswitchable rotaxane operating in monolayers on solid support

Article abstract of DOI:10.1039/c6cc08586e

A novel photoswitchable rotaxane was synthesised and its switching behaviour in solution was analysed with NMR and UV-Vis. A monolayer of rotaxanes was deposited on glass surfaces and the on-surface photoswitching was investigated. Angle-resolved NEXAFS spectra revealed a preferential orientation that reversibly changes upon switching.

Full text of DOI:10.1039/c6cc08586e

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A photoswitchable rotaxane operating in
monolayers on solid support†
Cite this:DOI: 10.1039/c6cc08586e
Felix B. Schwarz,a Thomas Heinrich,ab Andreas Lippitz,b Wolfgang E. S. Unger*b
and Christoph A. Schalley*a
Received 25th October 2016,
Accepted 24th November 2016
DOI: 10.1039/c6cc08586e
www.rsc.org/chemcomm
A novel photoswitchable rotaxane was synthesised and its switching
behaviour in solution was analysed with NMR and UV-Vis. A monolayer
of rotaxanes was deposited on glass surfaces and the on-surface photo-
switching was investigated. Angle-resolved NEXAFS spectra revealed a
preferential orientation that reversibly changes upon switching.
Photochromic molecules are widely used as key components in
stimuli-responsive molecular switches and machines. Common
photoswitches like azobenzenes, spiropyranes and dithienylethenes
offering a fast and clean photoisomerisation have been utilised in
the development of complex functional systems and materials.1–7
Especially, mechanically interlocked molecules (MIMs) like
rotaxanes have been investigated intensely in this context.1,5–7
The transfer of MIMs from solution into ordered arrays at
interfaces is of great interest, as such order is the prerequisite
for macroscopic effects through the concerted action of micro-
scopic units.8–11 Several examples for the deposition of MIMs
on surfaces are reported in the literature.12–14 However, there
are only a few examples for surface-bound photoresponsive
MIM-based systems.15–17
Fig. 1 Synthesis of photoswitchable rotaxanes Rot1, Rot2 and Rot3.
Recently, we reported the photoinduced pseudorotaxane
formation of a photoresponsive axle and a tetralactam macro-
cycle carried out in solution and on surfaces with immobilized building block for rotaxanes, serving as the photoswitch, the first
multilayers of macrocycles.18 Here, we report a photoswitchable binding site and the stopper simultaneously. The second part of
rotaxane, consisting of a tetralactam macrocycle (TLM) and a the axle contains a diamide binding site,19 which bears a bulky
photoswitchable axle. The axle is comprised of an azobenzene trityl stopper on one end and a linker unit at the other end.
photoswitch and a diketopiperazine binding site, which are Rotaxanes Rot1 and Rot2 were obtained in a one-step ether-
both attached to a rigid xanthene backbone in a way that the rotaxane synthesis from two axle building blocks and TLM 1 or 2
azobenzene photoisomerisation influences the binding strength (Fig. 1). Rot3 functionalised with a terpyridine unit at the TLM
of the adjacent site by steric hindrance. By substitution with a was synthesised in one step starting from Rot2 in a Sonogashira
suitable linker, this photoswitchable axle was used as the central coupling reaction with acetylene-functionalised terpyridine 3
(ESI†). Rot1 containing the di-tert-butyl substituted TLM 1 was
used for all solution studies due to its good solubility, while Rot3
was used for surface experiments.
a
¨
¨
Institut fur Chemie und Biochemie, Freie Universitat Berlin, Takustr. 3, 14195
Berlin, Germany. E-mail: c.schalley@fu-berlin.de
b BAM – Federal Institute for Materials Research and Testing, Unter den Eichen 44-46,
12203 Berlin, Germany. E-mail: wolfgang.unger@bam.de
The formation of Rot1 was followed by 1H NMR spectro-
scopy, confirming the threaded structure with the TLM being
located at the diamide and not at the diketopiperazine binding
site (Fig. 2d(i and ii) and ESI†). The mechanically interlocked
† Electronic supplementary information (ESI) available: Experimental proce-
dures, surface preparation, additional NMR, MS, XPS and NEXAFS data. See
DOI: 10.1039/c6cc08586e
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Fig. 2 (a) Photoswitching of Rot1. (b) UV-Vis spectra of trans-Rot1 in CH2Cl2 (black), Rot1 after 10 min irradiation at l1 = 365 nm (green), Rot1
after irradiation at l1 = 365 nm and l2 = 470 nm for 10 min each (red), and Rot1 after 10 min irradiation with 365 nm and subsequent equilibration for
2 d at 30 1C in the dark (blue, overlapping with black spectrum). (c) Reversibility of photoswitching of Rot1 in CH2Cl2 tested by alternating irradiation at
l1 = 365 nm and l2 = 470 nm for 10 min in each step. (d) 1H NMR spectra of the free axle in CDCl3 at RT (i), trans-Rot1 in CDCl3 at RT (ii), cis : trans 2 : 1
at RT (iii) and cis : trans 2 : 1 at 228 K (iv).
structure of Rot1 was confirmed by IRMPD ESI-FTICR-MS after irradiation at l1 = 365 nm for 10 min (Fig. 2d(ii and iii)). In
experiments (ESI†).
the spectrum after irradiation, a second set of signals for cis-Rot1
The photoswitching of Rot1 was studied in solution by is observed with a ratio of cis- : trans-Rot1 of ca. 2 : 1. The protons
UV-Vis and NMR spectroscopy. Photoisomerisation of the azo- a and b of the macrocycle undergo a small shift upfield in cis-
benzene group in Rot1 from trans to cis was carried out by Rot1 compared to trans-Rot1, indicating a different binding
irradiation with an LED lamp at a wavelength of l1 = 365 nm. situation. The protons c and d of the diketopiperazine site are
Back-switching was carried out by irradiation at l2 = 470 nm or shifted upfield by 0.73 and 0.15 ppm, while the protons e, f and g
thermal equilibration. The UV-Vis spectrum of trans-Rot1 in of the diamide site are shifted downfield by 0.05, 0.06 and
CH2Cl2 displays a broad absorption band at ca. 520 nm and a 0.42 ppm. This leads to the conclusion that the macrocycle is
characteristic absorption band for the p - p* transition of moving from the diamide towards the diketopiperazine binding
1
azobenzene at 322 nm (Fig. 2b).20 Irradiation at l1 = 365 nm site. In comparison with the H NMR spectrum of the free axle
leads to a decrease in intensity of the p - p* absorption band, and the shifts observed in a previous binding study with the
indicating the formation of cis-Rot1. Irradiating the sample corresponding pseudorotaxane,18 the shifts of the binding site
at l2 = 470 nm induces back-switching to trans-Rot1 up to protons c–g upon switching of Rot1 are smaller than expected.
ca. 90%. In both cases, the photostationary state was reached Likely, the macrocycle undergoes a shuttling motion between the
after 10 minutes. Complete back-isomerisation to trans-Rot1 two binding sites which is fast on the NMR time scale (see below).
was accomplished by equilibrating the sample in the dark at
The binding constants for both binding sites in Rot1 were
35 1C over two days. Reversibility was investigated by alternating determined by NMR titrations using structurally analogous
irradiation at l1 = 365 nm and l2 = 470 nm over five cycles model compounds containing one binding site each. For the
(Fig. 2c). 1H NMR spectra of Rot1 were measured before and diketopiperazine site, binding constants of 1650 Æ 170 MÀ1
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when the azobenzene is in its cis configuration and o15 MÀ1
when the azobenzene is in the trans configuration were obtained.18
For the diamide site, a binding constant of 1400 Æ 140 MÀ1 was
determined (ESI†). Although the diketopiperazine represents
the favoured binding site, the small difference in binding
energy will rather lead to a molecular shuttle than to a rotaxane
switch with two distinct positions of the macrocycle.
Temperature-dependent NMR spectra in a range between
300 K and 228 K have been recorded to investigate the shuttling
of cis-Rot1 (ESI†). Upon cooling the sample, the peak for the
1
macrocycle protons a at 8.52 ppm in the H NMR spectra gets
broadened, undergoes coalescence at approximately 243 K and
splits into two signals at 8.63 and 8.50 ppm upon further
cooling, indicating a shuttling motion which is slow on the
1H NMR timescale. At 228 K, the signals for protons c and d of
the diketopiperazine site split into two sets at 5.20 and 4.12 ppm
for c respectively 4.24 and 3.87 ppm for d. The signals for the
protons e, f, and g for the diamide site split as well into signals at
2.93 and 2.86 ppm for e, 2.25 and 2.86 ppm for f and 2.11 and
Fig. 3 (a) Surface deposition of Rot3, (b) transmission UV-Vis spectra of
surfaces, PDS (black), PDS-Pd-Rot3 (blue), PDS-Pd-Rot3 1 h 365 nm (red)
and PDS-Pd-Rot3 1 h 365 nm, 1.5 h 470 nm (green; superimposing the
blue curve), (c) reversibility of the on-surface photoswitching tested over
five cycles.
3.59 ppm for g (Fig. 2d(iv)). This implies that three different of 1 h, while it took 1.5 h in case of l2 = 470 nm. After
species are present: trans-Rot1 with the macrocycle being located irradiation at l1 = 365 nm, the UV-Vis spectra of the surface
at the diamide site, cis-Rot1 with the macrocycle being located at showed a decrease in absorbance in the region between 300 and
the diketopiperazine site and cis-Rot1 with the macrocycle at the 500 nm, indicating the formation of cis-Rot3. After irradiation at
diamide site. Integration of the signals in the NMR spectrum at l2 = 470 nm, the absorbance of the surface increased up to about
228 K yields an overall ratio of Rot1 with the macrocycle at the 90% of its initial value, indicating almost complete back-switching
diketopiperazine site to Rot1 with the macrocycle at the diamide to trans-Rot3. Reversibility was tested over five switching cycles
site of approximately 2 : 3. Due to the very complex spectra and (Fig. 3c). In a second approach, surfaces were functionalised
broad signals, a more detailed analysis of the shuttling behavior with a covalently bound monolayer of rotaxanes by azide–alkyne
was not straightforward.
click chemistry, which resulted in a similar photoswitchability,
After the rotaxane switching was investigated in solution, but a less dense packing (ESI†).
the concept was transferred to surfaces by following previously
Contact angle measurements were conducted for a monolayer
established procedures for surface deposition.21 A monolayer of of Rot3 that reveal a strong and reversible change in polarity upon
rotaxanes was deposited on surfaces using metal–ion/pyridine/ photoswitching of the surfaces (ESI†). To prove successful rotax-
terpyridine coordination chemistry.22–27 Cleaned glass slides ane deposition and the structural effects of the rotaxane switching
have been used as the substrates for UV surface experiments as well, angle-resolved C K-edge NEXAFS spectroscopy was con-
and contact angle measurements, while silicon substrates were ducted for the pristine rotaxane monolayer of Rot3 on silicon, the
used for XPS and NEXAFS experiments. A self-assembled monolayer after 1 h irradiation with l1 = 365 nm, and after 1 h
monolayer of PDS was deposited on the surface by immersing light irradiation with l1 = 365 nm with subsequent 1.5 h irradia-
the substrate in a 5 mM solution of PDS. The surfaces were then tion at l2 = 470 nm as seen in Fig. 4. In the 551 C–K edge spectrum
treated with a 1 mM solution of tetrakis(acetonitrile)palladium(II) of the pristine rotaxane layer, the characteristic p* resonance
tetrafluoroborate to coordinate the pyridyl endgroups of PDS with splitting for pyridines and benzene is visible.27 Furthermore, the
palladium(II) ions, followed by immersion in a 1 mM solution of peak at 285.4 eV is significantly more intense than the peak at
Rot3 to create a monolayer of rotaxanes on the surface (Fig. 3a). 284.9 eV. Together with the intense p* resonance in the N–K edge
The deposition of Rot3 was followed by UV-Vis spectroscopy spectrum (Fig. S28, ESI†) at 399.4 eV, which is characteristic for
(Fig. 3b). As the glass slides absorb light below 300 nm, only the Pd-coordinated pyridine,28 with a shoulder at 398.6 eV (p*OQC–N
)
region above this value could be used. A substantial increase characteristic for the axle molecule,18 and with XPS data (Fig. S30
in absorbance was detected after deposition of Rot3, which and S31, ESI†) this clearly indicates the successful deposition
resembles the UV-Vis spectra of Rot1 in solution and therefore of the rotaxane. The pristine monolayer of trans-Rot3 shows a
indicates a successful monolayer formation. XPS measurements relatively small, but clearly visible linear dichroism for the C K-edge
show a molecular ratio of Rot3 to PDS of 1 : 15.
p* and s* resonances.29 Irradiation with light at l1 = 365 nm results
To investigate on-surface switching, a surface functionalised in a decrease of the linear dichroism in the C K-edge which is
with a monolayer of Rot3 was irradiated with an LED lamp at a regained upon subsequent irradiation with l2 = 470 nm. In context
wavelength of l1 = 365 nm or l2 = 470 nm from a distance of of the solution study and the fact that the position of the macro-
about 20 cm. Different irradiation times were investigated and cycle is fixed in trans-Rot3, but variable in cis-Rot3, a higher
it was found that in case of l1 = 365 nm, no further changes order in the monolayer when the rotaxane is switched to its
in the absorbance spectra occurred after an irradiation time trans-state can be assumed. The relatively small magnitude of
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Notes and references
1 N. Katsonis, M. Lubomska, M. M. Pollard, B. L. Feringa and
P. Rudolf, Prog. Surf. Sci., 2007, 82, 407–434.
2 W. R. Browne and B. L. Feringa, Annu. Rev. Phys. Chem., 2009, 60,
407–428.
3 M.-M. Russew and S. Hecht, Adv. Mater., 2010, 22, 3348–3360.
4 R. Klajn, Pure Appl. Chem., 2010, 82, 2247.
5 M. Baroncini, G. Ragazzon, S. Silvi, M. Venturi and A. Credi, Pure
Appl. Chem., 2015, 87, 537.
6 R. H. El Halabieh, O. Mermut and C. J. Barrett, Pure Appl. Chem.,
2004, 76, 1445.
7 S. Erbas-Cakmak, D. A. Leigh, C. T. McTernan and A. L. Nussbaumer,
Chem. Rev., 2015, 115, 10081–10206.
8 A. Coskun, M. Banaszak, R. D. Astumian, J. F. Stoddart and
B. A. Grzybowski, Chem. Soc. Rev., 2012, 41, 19–30.
9 M. M. Boyle, R. A. Smaldone, A. C. Whalley, M. W. Ambrogio,
Y. Y. Botros and J. F. Stoddart, Chem. Sci., 2011, 2, 204–210.
10 J. J. Davis, G. A. Orlowski, H. Rahman and P. D. Beer, Chem. Commun.,
2010, 46, 54–63.
11 A. C. Fahrenbach, S. C. Warren, J. T. Incorvati, A.-J. Avestro, J. C. Barnes,
J. F. Stoddart and B. A. Grzybowski, Adv. Mater., 2013, 25, 331–348.
12 V. Balzani, A. Credi and M. Venturi, ChemPhysChem, 2008, 9, 202–220.
13 E. Coronado, P. Gavina and S. Tatay, Chem. Soc. Rev., 2009, 38,
1674–1689.
14 Y. Liu, A. H. Flood, P. A. Bonvallet, S. A. Vignon, B. H. Northrop,
H.-R. Tseng, J. O. Jeppesen, T. J. Huang, B. Brough, M. Baller, S. Magonov,
S. D. Solares, W. A. Goddard, C.-M. Ho and J. F. Stoddart, J. Am.
Chem. Soc., 2005, 127, 9745–9759.
Fig. 4 551 C K-edge NEXAFS spectrum of the pristine rotaxane mono-
layer and its 901–301 difference spectrum. Difference spectra after 1 h
irradiation with l = 365 nm, and after 1 h irradiation with l = 365 nm
followed by 1.5 h irradiation with l = 470 nm (peak assignments are
given in eV).
15 I. Willner, V. Pardo-Yissar, E. Katz and K. T. Ranjit, J. Electroanal.
Chem., 2001, 497, 172–177.
16 P. Wan, Y. Jiang, Y. Wang, Z. Wang and X. Zhang, Chem. Commun.,
2008, 5710–5712.
the linear dichroism effect might be due to a rather sparse
packing in the monolayer resulting in lower preferential orientation
of the immobilized molecules. Due to the exclusively photo-induced 17 J. Berna, D. A. Leigh, M. Lubomska, S. M. Mendoza, E. M. Perez,
P. Rudolf, G. Teobaldi and F. Zerbetto, Nat. Mater., 2005, 4, 704–710.
18 F. B. Schwarz, T. Heinrich, J. O. Kaufmann, A. Lippitz, R. Puttreddy,
modification of the rotaxane states, the observed differences in
linear dichroism can be associated with the switching states of
K. Rissanen, W. E. S. Unger and C. A. Schalley, Chem. – Eur. J., 2016,
Rot3. However, the analysis does not provide a quantification of
the on-surface switching process.
22, 14383–14389.
19 L. Kaufmann, E. V. Dzyuba, F. Malberg, N. L. Low, M. Groschke,
B. Brusilowskij, J. Huuskonen, K. Rissanen, B. Kirchner and
C. A. Schalley, Org. Biomol. Chem., 2012, 10, 5954–5964.
In the present study, we developed a photoswitchable rotaxane
and analysed its switching behavior in solution as well as in a 20 J. Griffiths, Chem. Soc. Rev., 1972, 1, 481–493.
21 C. H. H. Traulsen, V. Kunz, T. Heinrich, S. Richter, M. Holzweber,
monolayer of rotaxanes deposited on glass and silicon surfaces.
NEXAFS spectroscopy revealed a preferential orientation in the
A. Schulz, L. K. S. von Krbek, U. T. J. Scheuschner, J. Poppenberg,
W. E. S. Unger and C. A. Schalley, Langmuir, 2013, 29, 14284–14292.
¨
monolayer, which reversibly changes upon photo switching of the 22 J. J. Richardson, M. Bjornmalm and F. Caruso, Science, 2015, 348, 6233.
rotaxane. In combination with chemically switchable rotaxanes,
we are aiming for multi-stimuli responsive surface systems
˜´
23 V. Pinon and M. Weck, Langmuir, 2012, 28, 3279–3284.
24 Y. Chaikin, H. Leader, R. Popovitz-Biro, A. Vaskevich and I. Rubinstein,
Langmuir, 2011, 27, 1298–1307.
capable of performing concerted switching of distinct layers 25 P. C. Mondal, J. Yekkoni Lakshmanan, H. Hamoudi, M. Zharnikov
and T. Gupta, J. Phys. Chem. C, 2011, 115, 16398–16404.
26 G. de Ruiter, M. Lahav and M. E. van der Boom, Acc. Chem. Res.,
resulting in potential macroscopic effects.
We are grateful to the Deutsche Forschungsgemeinschaft
2014, 47, 3407–3416.
(SFB 765) for funding. F. B. S. thanks the Beilstein Institut for a 27 N. Tuccitto, I. Delfanti, V. Torrisi, F. Scandola, C. Chiorboli,
V. Stepanenko, F. Wurthner and A. Licciardello, Phys. Chem.
Chem. Phys., 2009, 11, 4033–4038.
28 S. Richter, J. Poppenberg, C. H. H. Traulsen, E. Darlatt, A. Sokolowski,
PhD fellowship. The authors thank Dr A. Nefedov (Karlsruhe
Institute of Technology, KIT) from the HE-SGM Collaborate
Research Group at the synchrotron facility BESSY II and
Dr A. Vollmer and M. Mast (all HZB/BESSY II) for support
during our activities at HZB. We thank Prof. Biprajit Sarkar for
providing the catalyst for surface reactions.
D. Sattler, W. E. S. Unger and C. A. Schalley, J. Am. Chem. Soc., 2012,
134, 16289–16297.
29 T. Heinrich, C. H. H. Traulsen, M. Holzweber, S. Richter, V. Kunz,
S. K. Kastner, S. O. Krabbenborg, J. Huskens, W. E. S. Unger and
C. A. Schalley, J. Am. Chem. Soc., 2015, 137, 4382–4390.
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