Photocontrolled On-Surface Pseudorotaxane Formation with Well-Ordered Macrocycle Multilayers

Article abstract of DOI:10.1002/chem.201603156

The photoinduced pseudorotaxane formation between a photoresponsive axle and a tetralactam macrocycle was investigated in solution and on glass surfaces with immobilized multilayers of macrocycles. In the course of this reaction, a novel photoswitchable b

Full text of DOI:10.1002/chem.201603156

DOI: 10.1002/chem.201603156
Full Paper
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Surface Chemistry |Hot Paper|
Photocontrolled On-Surface Pseudorotaxane Formation with Well-
Ordered Macrocycle Multilayers
Felix B. Schwarz,[a] Thomas Heinrich,[a, b] J. Ole Kaufmann,[a] Andreas Lippitz,[b]
Rakesh Puttreddy,[c] Kari Rissanen,[c] Wolfgang E. S. Unger,*[b] and Christoph A. Schalley*[a]
Abstract: The photoinduced pseudorotaxane formation be-
tween a photoresponsive axle and a tetralactam macrocycle
was investigated in solution and on glass surfaces with im-
mobilized multilayers of macrocycles. In the course of this
reaction, a novel photoswitchable binding station with azo-
benzene as the photoswitchable unit and diketopiperazine
as the binding station was synthesized and studied by NMR
and UV/Vis spectroscopy. Glass surfaces have been function-
alized with pyridine-terminated SAMs and subsequently with
multilayers of macrocycles through layer-by-layer self assem-
bly. A preferred orientation of the macrocycles could be con-
firmed by NEXAFS spectroscopy. The photocontrolled depo-
sition of the axle into the surface-bound macrocycle-multi-
layers was monitored by UV/Vis spectroscopy and led to an
increase of the molecular order, as indicated by more sub-
stantial linear dichroism effects in angle-resolved NEXAFS
spectra.
TLM-based rotaxanes[11] on different substrates through layer-
by-layer self-assembly[12] using metal-ion/pyridine/terpyridine
coordination chemistry.[13] This included the implementation of
chemically switchable rotaxanes that change the macrocycle
position in a cooperative manner when adding or removing
chloride ions.[14] Although these systems work nicely, address-
ing responsive surfaces by inducing a chemical stimulus has
some limitations. It unavoidably brings impurities into the
system and, therefore, hampers an unambiguous analysis of
structural changes on the surface. Another problem could be
to address the lower layers in densely packed systems, which
bulky molecules cannot reach easily by diffusion. In order to
overcome these limitations, we aim to implement photo-
switchable molecules in these previously realized macrocycle
multilayer surfaces.
Introduction
Stimuli-responsive chemical systems capable of performing
molecular motion are of great interest for the development of
functional materials and molecular machines.[1] Immobilizing
such molecules on surfaces allows the concerted action of mo-
lecular switches and shuttles.[2] In particular, surfaces function-
alized with photoresponsive units have been of great interest
in recent research.[3] Mostly, gold and silicon substrates have
been used as they can be easily functionalized with well-stud-
ied monolayers and due to the technological importance of
these substrates.[4] Photochromic molecules like azobenzenes,
spiropyranes, or dithienylethenes bound to surfaces have, for
example, been used to control the surface wettability,[5] to
change their electrical properties,[6] or to control the adsorp-
tion of molecules on surfaces.[7] Furthermore, photoresponsive
units were utilized to actuate and control the molecular
motion in molecular rotary motors[8] and linear molecular mus-
cles.[9]
In this study, we start with a photoswitchable pseudorotax-
ane as the precursor for further studies with rotaxanes. Azo-
benzene, one of the most studied photochromic molecules,
was chosen as the photoswitchable unit due to its fast and
clean photoisomerization associated with different geometric
and physicochemical properties of the two switching states.[15]
The outstanding optomechanical properties make azobenzene
a key compound in switchable molecular and supramolecular
functional systems, which is reflected in various reported ex-
amples in solution,[16] on nanoparticles,[17] and on surfaces.[18]
However, there are only a few reports on surface-[19] or nano-
particle-bound[20] azobenzene-based (pseudo-)rotaxanes.
To utilize the cis–trans photoisomerization of azobenzene for
controlling the threading and dethreading of a pseudorotaxane
with a Hunter/Vçgtle-type TLM, the photoswitch was attached
to a rigid xanthene backbone resulting in a fixed position rela-
tive to the diketopiperazine binding station to influence its
binding strength by steric hindrance. While the azobenzene
Recently, we reported the deposition of ordered multilayers
of Hunter/Vçgtle-type tetralactam macrocycles[10] (TLM) and
[a] F. B. Schwarz, T. Heinrich, J. O. Kaufmann, Prof. C. A. Schalley
Institut für Chemie und Biochemie, Freie Universität Berlin
Takustrasse 3, 14195 Berlin (Germany)
E-mail: c.schalley@fu-berlin.de
[b] T. Heinrich, A. Lippitz, Dr. W. E. S. Unger
BAM-Federal Institute for Materials Research and Testing
Unter den Eichen 44-46, 12203 Berlin (Germany)
E-mail: wolfgang.unger@bam.de
[c] Dr. R. Puttreddy, Acad. Prof. K. Rissanen
University of Jyvaskyla, Department of Chemistry
Nanoscience Center, P.O. Box. 35, 40014 Jyvaskyla (Finland)
Supporting information for this article can be found under http://
dx.doi.org/10.1002/chem.201603156.
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blocks the binding station in its trans configuration, the bind-
ing site becomes available upon switching to the cis isomer. In
solution studies by Jeong et al.,[21] a similar photoswitchable di-
amide station exhibited binding to nitro-substituted macrocy-
cles. For our purposes, such a substitution on the macrocycles
isophthaloyl diamide units is not suitable as it would block the
functionalization necessary for surface binding. To design a suit-
able molecule for binding in our TLM multilayer surfaces, we
have introduced diketopiperazine as the binding station,
which exhibits a higher binding affinity towards TLM.[22]
its trans configuration with the phenyl ring, and is nearly co-
planar to the xanthene-backbone with a root mean square de-
viation (RMSD) of 0.140 Š. The angle between the mean planes
Results and Discussion
The photoswitchable binding site 8 was synthesized starting
from commercially available xanthene 1 by following literature
known procedures[21b,23] to give the azobenzene-substituted
xantheneamine 4. The synthesis was optimized to achieve
higher product yields (for details see the Supporting Informa-
tion). The diketopiperazine binding station was attached to
amine 4 in four steps[24] starting with the reaction of 4 with
bromoacetic acid ethyl ester to give secondary amine 5. Com-
pound 5 was then acetylated with chloroacetyl chloride and
subsequently converted to iodide 6 in a Finkelstein reaction.
Treatment of 6 with methanolic ammonia induced ring closure
and afforded the diketopiperazine 7. Methylation of 7 yielded
the final product 8 with an overall yield of 14% over 10 steps
(Scheme 1). Tetralactam macrocycles were synthesized accord-
ing to literature known procedures.[25]
Single crystals of trans-7 suitable for X-ray analysis (Support-
ing Information) were obtained by the slow evaporation of
a solution of trans-7 in methanol. Figure 1a shows the section
of the asymmetric unit of trans-7, crystallized in the triclinic
space group PÀ1 with selected atom labeling. The asymmetric
unit contains two trans-7 and two water molecules. The re-
stricted rotation of the diketopiperazine, due to steric hin-
drance with the xanthene-substituted azobenzene, favors the
keto-groups as binding stations in 8@9. The azobenzene is in
Figure 1. a) Crystal structure of 7 (thermal ellipsoids are shown at the 50%
probability level) and b) MM2 force-field model of the pseudorotaxane 8@9
with axle 8 displayed in space-filling mode and macrocycle 9 in a ball-and-
stick model (top left: side view, bottom right: top view). See the Supporting
Information for a crystal structure of 9.
Scheme 1. Synthesis of photoswitchable binding station 8: a) 1. Br2, CHCl3, rt, 24 h, quant., 2. nBuLi, CO2, THF, À788C, 2 h, 62%; b) 1. DPPA, TEA, BnOH, tolu-
ene, 808C, 22 h, 79%, 2. KOH, EtOH, 908C, 16 h, 98%; c) Nitrosobenzene, HOAc, CHCl3, rt, 17 h, 54%; d) Ethyl bromoacetate, NaHCO3, DMF, 908C, 20 h, 76%;
e) 1. Chloroacetyl chloride, TEA, CH2Cl2, 08C!rt, 12 h, 83%, 2. NaI, acetone, rt, 16 h, 99%. f) NH3 in MeOH, MeOH, rt, 12 h, 89%. g) KOtBu, MeI, THF, 08C!rt,
98%. DPPA=diphenylphosphoryl azide; TEA=triethylamine.
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of these rings is 86.78. The diketopiperazine rings in two crys-
tallographically independent molecules constitutes a RMSD of
0.132 and 0.58, and are perpendicular to the xanthene back-
bone. All the bond parameters in the xanthene backbone of 7
and C55ÀN5 (1.440(4) Š) are close to the reported values,[26]
while C40ÀN7 (1.423(4) Š), N7ÀN8 (1.261(4) Š), and N8ÀC37
(1.429(4) Š) are similar to trans-azobenzene.[27]
the azobenzene at 323 nm,[28] which decreases upon irradiation
with l1 =365 nm, indicating the formation of cis-8. The photo-
stationary state with a cis- to trans-8 ratio of 2:1 was reached
after approximately 10 min. After irradiation with l2 =470 nm
for 10 min or equilibration in the dark over 24 h, the UV/Vis
spectrum was hardly different from the initial one. Reversibility
was tested by alternating irradiation over five cycles (Fig-
ure 2c). UV/Vis analysis of 8 was also carried out with a 1:1
mixture of 8 and 9, in which, as expected, no shifts of absorp-
tion maxima could be observed since the binding mode does
not significantly influence the chromophor’s electronic proper-
ties.
MM2 force-field modeling shows cis-8 to nicely fit into the
cavity of the TLM (Figure 1b). The modeled structure is in
good agreement with the crystal structure of 7 in which the
xanthene unit is almost planar, while the diketopiperazine unit
is turned out of plane.
The photoisomerization of 8 and subsequent pseudorotax-
ane formation (Figure 2a) was studied first in solution by UV/
Vis and NMR spectroscopy. The di-tert-butyl-substituted TLM 9
(for an X-ray structure, see the Supporting Information) was
used for solution studies due to its good solubility. Photoiso-
merization of 8 from trans to cis was carried out by irradiating
a solution of trans-8 in CH2Cl2 with an LED lamp at a wave-
length of l1 =365 nm. The back isomerization to the trans con-
figuration was performed by either irradiation with an LED
lamp at l2 =470 nm or by equilibration in the dark at room
temperature for 24 h.
Besides investigating the switching process and its reversibil-
ity by UV/Vis spectroscopy, NMR spectroscopy was applied to
gain structural information and evidence for switchable pseu-
dorotaxane formation. The NMR spectrum of TLM 9 (Figure 2d,
I) shows the signals for the inner isophthalic acid protons a at
7.98 ppm and the amide protons b at 7.29 ppm. Upon addition
of one equivalent trans-8 (Figure 2d, II), the amide protons un-
dergo only a very small shift of Dd=0.04 ppm. After 10 min ir-
radiation at l1 =365 nm (Figure 2d, III), a second set of signals
for cis-8 appears with a ratio of cis- to trans-8 of approximately
2:1. Due to a fast exchange of axle 8 in the cavity of macrocy-
cle 9, averaged signals are observed. The macrocycle protons a
and b are now significantly shifted downfield by Dd=
The UV/Vis spectrum of trans-8 in CH2Cl2 (Figure 2b) shows
the characteristic absorption band for the p!p* transition of
Figure 2. a) Photoswitchable pseudorotaxane formation. b) UV/Vis spectra of cis- and trans-8 in CH2Cl2. c) Reversibility of photoswitching in CH2Cl2 tested by
alternating irradiation at l1 =365 nm and l2 =470 nm for 10 min in each step. d) NMR analysis (400 MHz, 298 K, CDCl3) of pseudorotaxane switching. I: TLM 9.
II: 9+trans-8, 1:1. III: 9+8 after 10 min irradiation at l1 =365 nm. IV: 9+8 after 24 h in the dark at rt.
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0.21 ppm and Dd=0.86 ppm due to hydrogen bonding to cis-
8. The axle protons c, d, and e are shifted upfield by Dd=0.44,
0.88, and 0.49 ppm due to the shielding effect of TLM 9, clearly
evidencing pseudorotaxane formation. After leaving the
sample for 24 h in the dark, the spectrum was virtually indistin-
guishable from the initial one (Figure 2d, IV). When the back
isomerization was carried out photochemically by irradiation at
l2 =470 nm, the photostationary state with a cis- to trans-8
ratio of 1:9 was reached after approximately 10 min, as deter-
mined by NMR (for the spectrum see the Supporting Informa-
tion).
Binding constants of trans- and cis-8 to TLM 9 have been de-
termined by NMR titrations. For trans-8, a very small binding
constant of <15mÀ1 was obtained (Supporting Information),
while cis-8 has a binding constant of 1,650Æ165mÀ1 (Figure 3).
Figure 3. NMR titration of TLM 9 with trans-8 (a) and cis-8 (c) in CDCl3
at room temperature.
After studying pseudorotaxane formation and its switchabili-
ty in solution, the concept was transferred to surfaces. Multi-
layers of tetralactam macrocycles have been prepared by
metal-coordination-based layer-by-layer self-assembly on glass
substrates according to previously established procedures.[29]
As the template layer, a self-assembled monolayer of PDS
(Figure 4a) was deposited on cleaned glass surfaces. The
Figure 4. a) Diterpyridine-TLM 10, PDS-SAM, metal ions used for the self-as-
sembly of macrocycle multilayers. b) Surface UV/Vis spectra showing multi-
layer deposition. c) UV/Vis spectra of surfaces before and after immersion in
trans-8 solution. d) UV/Vis spectra of surfaces before and after immersion in
cis-8 solution. The increase of absorption in 4d is in line with the solution
UV/Vis spectra of cis-8 in Figure 2b.
surfaces were then treated with
a 1 mm solution of
Pd(CH3CN)4(BF4)2 in CH2Cl2 to coordinate the pyridyl endgroups
of the SAM with PdII ions, enabling the attachment of the next
organic layer. By alternating deposition of di-terpyridine-TLM
10 and Fe(BF4)2·6H2O, a controlled growth of macrocycle layers
was achieved, monitored by UV/Vis spectra recorded after
each deposition step (Figure 4b).
The experiment was then repeated with the same surfaces
immersed into the same solution, but now under continuous
irradiation with UV light of a wavelength of l1 =365 nm to
form cis-8.
To avoid light-induced damage on the surfaces, the irradia-
tion was carried out in a way that no light reached the surfaces
directly, but only the solution was irradiated. After washing the
surfaces for 10 min with CH2Cl2, UV/Vis spectra were recorded.
The spectrum of the surface with only the PDS-SAM showed
no change before and after deposition of 8. The spectra of the
surfaces with one, three, and five layers of TLM 10 showed
a substantial increase of the absorption around 350 nm, indi-
cating the deposition of cis-8 (Figure 4d).
To show that cis-8 binds to the cavity of the macrocycles on
the surface while trans-8 does not, surfaces with one, three,
and five layers of TLM 10 have been prepared. A surface with
only the SAM was used as the control. The surfaces were im-
mersed in a 1 mm solution of trans-8 (large excess) in CH2Cl2
for 24 h, followed by immersing the surface in CH2Cl2 for
10 min to remove any unspecifically bound molecules. For all
four surfaces, no increase of the p–p* band in the UV/Vis spec-
tra before and after immersion to the solution of trans-8 was
observed, clearly showing that no additional chromophores
are deposited on the surface and therefore that trans-8 does
not bind to the macrocycle multilayers (Figure 4c).
Interestingly, the increase of absorption after the deposition
of 8 on the surfaces does not linearly increase with a higher
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number of macrocycle layers. While the absorption increases
for three layers relative to one layer, it does not increase any
further for five layers (Supporting Information). This indicates
that the multilayers are rather densely packed and that not all
macrocycles are accessible for the binding of cis-8. In a previous
study, we used a squaraine as the guest molecule that was
bound to macrocycle multilayers and a linear increase of
bound molecules with an increasing number of layers was ob-
served.[30] This can be explained by the differences between
the molecular structure of the guests. While the squaraine mol-
ecule used in the former study is relatively small, has a linear
form without stopper-units and can therefore easily slip
through the voids in the surface or the macrocycle cavities,
guest molecule 8 used here is stoppered on one end through
the xanthene unit, so that the accessibility of deeper macrocy-
cle layers is significantly hampered.
In addition to UV/Vis spectroscopy, the surfaces have been
analyzed with XPS (see the Supporting Information) and
NEXAFS spectroscopy. As glass slides are not suitable for these
methods, silicon surfaces have been used that have been pre-
pared in the same way as the glass slides used before. Surfaces
with three layers of macrocycle with and without incorporated
cis-8 have been prepared and analyzed. In addition, a thin film
of 8 was deposited on a cleaned silicon surface via a drop-
coating from CH2Cl2.
The N K-edge NEXAFS spectrum of the macrocycle multilay-
er in Figure 5a shows three clearly visible peaks that are as-
signed to the terpyridine moiety (398.7 and 399.3 eV) as well
as to the amide nitrogens of the macrocycle (400.6 eV). In the
spectrum of dropcoated 8 (Figure 5d), two resonances for the
excitation into p* orbitals at 398.6 and 401.9 eV are found that
can be assigned to the azobenzene- and diketopiperazine-ni-
trogen atoms. While these resonances are absent in the N K-
edge NEXAFS spectra of the pristine macrocycle multilayers
and those treated with trans-8 (Figure 5b), they are present in
the spectra of the multilayers treated with cis-8 (Figure 5c),
clearly showing the presence of cis-8 on the surface and indi-
cating the incorporation of the guest into the macrocycle mul-
tilayers (Figure 5). To get a more detailed insight to the pack-
ing of macrocycles with and without cis-8, angle-dependent C
K-edge NEXAFS spectra have been recorded (Figure 6). The
pristine macrocycle multilayer (Figure 6a) shows effects analo-
gous to those observed in earlier studies of macrocycles on
a terpyridine-terminated template with gold as the sub-
strate.[30] The p* resonance exhibits splitting into two charac-
teristic peaks at 285.1 and 285.4 eV, which show a clear linear
dichroism in the 90–308 difference spectrum in line with the
earlier results. This also includes a significant change of the rel-
ative heights of the two peaks.[31] When the multilayer is treat-
ed with trans-8 solution (Figure 6b), the spectra closely resem-
ble those of the pristine multilayer with comparable effects. In
contrast, the surface treated with cis-8 exhibits different spec-
tra (Figure 6c). The signal at 285.4 eV seems to be superim-
posed by a second one and, furthermore, a signal at 288.3 eV
shows up. When we look at the CÀK edge of dropcoated 8
(Figure 6d), we can observe three characteristic signals at
photon energies of 285.4, 287.2, and 288.3 eV, two of which
Figure 5. N K-edge NEXAFS spectra of a) a three-layered macrocycle multi-
layer, b) the multilayer treated with a solution of trans-8 in the dark, c) the
multilayer treated with a solution of cis-8, and d) the dropcoated 8. Spectra
were taken at an incidence angle of the linearly polarized synchrotron light
of 558.
can be assigned to the ‘new’ signals in the spectrum of the
multilayer treated with a solution of cis-8. That we do not ob-
serve the third peak at 287.2 eV in this spectrum can be ex-
plained by its relatively low intensity and that it is therefore su-
perimposed by the spectral contributions of the macrocycle
multilayer. Together with the results from the NÀK edge spec-
tra, this is a clear evidence for the presence of the guest 8 on
the surface. If we look at the 90–308 difference spectrum of
this sample, an increase— when compared to the one of the
original multilayer— of the linear dichroism in the area in
which the p* resonances appear becomes apparent; this indi-
cates the incorporation of the guest into the cavity of the mac-
rocyles.
To study on-surface switching, surfaces with one, three and
five layers of macrocycles with bound cis-8 have been irradiat-
ed first with light of l2 =470 nm and then l1 =365 nm over
different times (1–30 min). None of these irradiations induced
any changes in the absorption spectra of the surfaces, showing
that the bound azobenzenes do not undergo cis-to-trans pho-
toisomerization on the surface. Additionally, we tried to
remove cis-8 from the multilayer by washing the surfaces with
different solvents. Immersing the surfaces in CH2Cl2 over 24 h
had no effect on the surface, showing that 8 is quite strongly
bound in the macrocycles. Next, the surfaces were irradiated in
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Figure 7. UV/Vis absorbance at 346 nm of surfaces: a) Removing 8 from five-
layer surface. b) Reversibility of cis-8 deposition on a five-layer surface versus
no deposition on a SAM surface.
cis-to-trans backswitching. This is in line with solid-state stud-
ies, showing the (photo-)isomerization of azobenzenes not to
take place unless the molecules are specially designed to do
so,[32] as the isomerization reaction requires a relatively large
free volume[33] and is, therefore, hindered in densely packed
structures.
Conclusion
Figure 6. C K-edge NEXAFS spectra acquired at an incidence angle of the lin-
early polarized synchrotron light of (grey) 558 and (black) the difference be-
tween the spectra acquired at 90 and 308 of a) a three-layered macrocycle
multilayer, b) the multilayer treated with a solution of trans-8, c) the multi-
layer treated with a solution of cis-8, and d) a cleaned surface drop-coated
with a solution of 8. e) Enhanced p*-resonances in the 90–308difference
spectra displayed on identical scales.
A photoswitchable binding station for tetralactam macrocycles
based on azobenzene as the photoswitchable element and di-
ketopiperazine as the binding station has been developed. The
cis–trans isomerization of the molecule and subsequent pseu-
dorotaxane formation with a TLM was analyzed by UV/Vis and
NMR spectroscopy. By using surfaces with multilayers of tetra-
lactam macrocycles, a photocontrolled on-surface pseudoro-
taxane formation could be achieved, confirmed by UV/Vis, XPS,
and NEXAFS spectroscopy. Additionally, NEXAFS spectra re-
vealed an increase in the molecular order on the surface after
pseudorotaxane formation. The ordered and densely packed
nature of the multilayers on the surface prevents isomerization
of the incorporated azobenzene guest molecules, illustrating
the significant differences between solution and surface.
As pseudorotaxanes are precursors of mechanical inter-
locked molecules like rotaxanes, this work opens a path to-
wards photoswitchable rotaxanes embedded in multilayers on
surfaces leading in combination with chemically switchable ro-
taxanes to multi-stimuli responsive surface systems. The quite
densely packed and oriented nature of the multilayers on the
surface may result in concerted movement and potential mac-
roscopic effects.
CH2Cl2 with visible light of l2 =470 nm to find out if the back
isomerization to trans-8 is facilitated in a solution environment,
but even after 48 h no change in the UV/Vis spectra was ach-
ieved. Guest cis-8 could finally be removed from the multilay-
ers by treating the surfaces with DMF, which acts as a competi-
tive binder to the macrocycle. After 1 h in DMF, cis-8 was com-
pletely removed from the multilayers, as followed by UV/Vis
spectroscopy (Figure 7a). Reversibility was tested by alternat-
ing deposition and removal of 8 by 24 h immersion in a solu-
tion of 8 under UV irradiation and 1 h washing with DMF (Fig-
ure 7b). Although cis-to-trans isomerization of 8 occurs easily
in solution by both equilibration in the dark or irradiation with
visible light at l2 =470 nm, it does not happen when cis-8 is
incorporated in macrocycle multilayers on a solid support. It
appears that in the densely packed[30] multilayers, the freedom
of cis-8 to change its structure is too limited, thus blocking the
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Acknowledgements
We are grateful to the Deutsche Forschungsgemeinschaft (SFB
765), the German Academic Exchange Service (DAAD) and the
Academy of Finland (K.R.: Project Nos. 263256 and 292746).
F.B.S. thanks the Beilstein Institut for a Ph.D. fellowship. The au-
thors thank Dr. A. Nefedov (Karlsruhe Institute of Technology,
KIT) from the HE-SGM Collaborate Research Group at the syn-
chrotron facility BESSY II and Dr. O. Schwartzkopf, Dr. A. Vollm-
er, and M. Mast (all BESSY II) for support during our activities at
HZB.
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Keywords: azobenzene · photochemistry · pseudorotaxanes ·
supramolecular chemistry · surface chemistry
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Received: July 1, 2016
Published online on August 19, 2016
Chem. Eur. J. 2016, 22, 14383 –14389
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