Interactions between shape-persistent macromolecules as probed by AFM

Article abstract of DOI:10.3762/bjoc.13.95

Water-soluble shape-persistent cyclodextrin (CD) polymers with amino-functionalized end groups were prepared starting from diacetylene-modified cyclodextrin monomers by a combined Glaser coupling/click chemistry approach. Structural perfection of the neut

Full text of DOI:10.3762/bjoc.13.95

Interactions between shape-persistent macromolecules
as probed by AFM
Johanna Blass‡1,2, Jessica Brunke‡3, Franziska Emmerich1,2, Cédric Przybylski4,
Vasil M. Garamus5, Artem Feoktystov6, Roland Bennewitz1,2, Gerhard Wenz3
and Marcel Albrecht*3
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2017, 13, 938–951.
1INM-Leibniz-Institute for New Materials, Saarland University,
Campus D 2.2, D-66123 Saarbrücken, Germany, 2Physics
Department, Saarland University, Campus D 2.2, D-66123
Saarbrücken, Germany, 3Organic Macromolecular Chemistry,
Saarland University, Campus C 4.2, D-66123 Saarbrücken, Germany,
4UPMC, IPCM-CNRS UMR 8232, Sorbonne Universités, 75252 Paris
Cedex 05, France, 5Helmholtz-Zentrum Geesthacht (HZG), Centre for
Materials and Costal Research, Max-Planck-Str. 1, 21502
Geesthacht, Germany and 6Jülich Centre for Neutron Science (JCNS)
at Heinz Maier-Leibnitz Zentrum (MLZ), Forschungszentrum Jülich
GmbH, Lichtenbergstr. 1, 85748 Garching, Germany
doi:10.3762/bjoc.13.95
Received: 23 October 2016
Accepted: 24 April 2017
Published: 18 May 2017
This article is part of the Thematic Series "Spatial effects in polymer
chemistry". Dedicated to Gerhard Wegner and his work on
shape-persistent polymers.
Guest Editor: H. Ritter
Email:
© 2017 Blass et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Marcel Albrecht* - m.albrecht@mx.uni-saarland.de
* Corresponding author ‡ Equal contributors
Keywords:
AFM; cyclodextrin; inclusion complexes; molecular recognition;
polyconjugated polymers; shape persistent polymers
Abstract
Water-soluble shape-persistent cyclodextrin (CD) polymers with amino-functionalized end groups were prepared starting from
diacetylene-modified cyclodextrin monomers by a combined Glaser coupling/click chemistry approach. Structural perfection of the
neutral CD polymers and inclusion complex formation with ditopic and monotopic guest molecules were proven by MALDI–TOF
and UV–vis measurements. Small-angle neutron and X-ray (SANS/SAXS) scattering experiments confirm the stiffness of the
polymer chains with an apparent contour length of about 130 Å. Surface modification of planar silicon wafers as well as AFM tips
was realized by covalent bound formation between the terminal amino groups of the CD polymer and a reactive
isothiocyanate–silane monolayer. Atomic force measurements of CD polymer decorated surfaces show enhanced supramolecular
interaction energies which can be attributed to multiple inclusion complexes based on the rigidity of the polymer backbone and the
regular configuration of the CD moieties. Depending on the geometrical configuration of attachment anisotropic adhesion charac-
teristics of the polymer system can be distinguished between a peeling and a shearing mechanism.
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Introduction
Shape-persistence is an important key feature in self-organisa- Among many supramolecular interactions, such as hydrogen
tion strategies of supramolecular building blocks resulting in bonding, π–π-interactions or hydrophobic host–guest interac-
high structural perfection of the obtained molecular assemblies tions [12-16], the interactions of cyclodextrins (CDs) with
[1], such as shape persistent macrocycles, cage compounds or hydrophobic guest molecules are of special interest, since CDs
rotaxanes [2-4]. Especially shape-persistent polymers are of sig- are readily available bio-based materials and interactions take
nificant scientific interest as their defined structural characteris- place under physiological conditions [17]. CDs are ideal candi-
tics offer various applications as sensor materials, biomimetic dates for the investigation of multivalent interactions as they
filaments or organic electronics [5-7]. Furthermore, compared combine high affinities with a versatile integrability in macro-
to polymers with flexible chains, shape persistent macromole- molecular systems [18]. CDs have already been employed for
cules with high structural rigidity are able to form stable aggre- the construction of supramolecular polymers [19-21], supramo-
gates based on multiple supramolecular interactions, which can lecular hydrogels [22,23], molecular printboards [24,25] or
be detected and quantified without the presence of side effects, multivalent interfaces [26-28] with tunable chemical and physi-
such as self-passivation or coiling processes. Dendrimers, nano- cal properties. Herein, for the first time, we present studies con-
particles and shape-persistent polymers had been previously cerning the synthesis of shape-persistent CD polymers to inves-
discussed as scaffolds for the design of multiple ligands of high tigate multivalent binding with ditopic guest molecules on the
affinity [8]. Nevertheless, well-defined model systems in which molecular level (Figure 1). The ditopic guest (shown in red
the influence of rigidity and regularity on cooperativity of colour) should act as a connector between opposing CD
binding was systematically investigated have not been reported moieties.
so far.
Only a few examples of shape-persistent CD polymers have
Rigid linear polymers have been considered as suitable scaf- been reported so far, including CD-modified conjugated
folds for the design of supramolecular systems showing oligomers and polymers composed of rigid phenylene ethynyl-
multiple interactions. A high rigidity of the macromolecule is ene (PPE) structure units which are able to form self-inclusion
maintained by rigid, linear repeat units, such as trans-etheny- complexes with tunable electrochemical properties [29-35]. The
lene, ethynylene, or p-phenylene moieties. The observed persis- synthesis of PPE, in which two β-CD rings were attached to
tence lengths of polyconjugated polymers ranged from 6 to every second phenylene group, was described by Ogoshi et al.
16 nm, depending on the side groups and the method of deter- [36] using a Sonogashira–Hagiwara coupling. We preferred a
mination [9-11].
poly-phenylene-butadiynylene backbone, synthesized by a
Figure 1: Interaction of a shape-persistent CD polymer with ditopic guests.
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Glaser–Eglington coupling, since the repeating unit is long force loading rate [46]. We have previously explored the adhe-
enough (l = 0.944 nm) to allow the connection of one CD sion characteristics of dense CD layers on an AFM tip and a
moiety at each phenylene unit. Based on the stiffness of the planar silicon surface connected by various ditopic linker mole-
polymer chain self-passivation of CD polymer modified sur- cules. In this system we were able to switch adhesion and fric-
faces is reduced to a minimum. Furthermore, the ethynyl end tion by applying external stimuli onto the responsive ditopic
groups are easily functionalized by click chemistry.
linkers [47-49]. In contrast to previous work our molecular
toolkit, based on ditopic connector molecules, allows the inde-
Isothermal titration calorimetry (ITC), fluorescence spectrosco- pendent determination of unspecific interactions between CD
py, quartz crystal microbalance (QCM), surface plasmon reso- polymers at tip and planar surface as well as the specific inter-
nance (SPR) and atomic force microscopy (AFM) have been actions to ditopic connector molecules. In the following, we
employed to quantify the strength of the multivalent interac- describe the first example of multivalent interaction of ditopic
tions [8]. Because binding affinities can be very high for multi- guest molecules with shape-persistent CD polymers covalently
valent supramolecular systems, the constituents are commonly attached to an AFM tip and a planar surface. Nano force mea-
used in low equilibrium concentrations. Since AFM even allows surements between CD and CD polymer, CD polymer and CD,
the investigation of single molecules, such as DNA [37,38] or and CD and CD at the tip and the planar surface, respectively,
molecular self-assembling based on “Dip-Pen” nanolithogra- exerted by the adamantane ditopic connector molecules were
phy [39], it was chosen as the most reliable technique to probe systematically investigated. All four configurations are
highly cooperative recognition processes.
schematically depicted in Figure 2.
Results and Discussion
Synthesis of the shape-persistent CD
polymer
Our synthetic approach for the preparation of modified
poly(phenylene butadiynylene)s bearing one CD molecule per
repeat unit started from 2,5-dibromo-4-methylbenzoic acid (2)
The investigation of cooperativity of multiple host–guest inter-
actions using AFM has been reported by several groups [40-45].
Huskens and co-workers measured the supramolecular interac-
tions between a β-CD-modified planar surface and mono-, di-
and trivalent adamantane guest molecules attached to an AFM
tip and found enhancement factors up to 2, depending on the
Figure 2: Schematic representation of tip and surface modifications realized in this study (bottom). Blue lines symbolize the CD polymers, red circles
the complexed ditopic linkers.
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[50,51], which was esterified to 3 with tert-butanol catalyzed by formation, which might prevent the accessibility of the
H2SO4 (Scheme 1). The TMS-protected diacetylene derivative CD-moieties located on the polymer backbone, was avoided by
4 was prepared by Sonogashira reaction of 3 with trimethyl- the presence of pyridine as a non-polar solvent. Both NOESY
silylacetylene. Subsequent deprotection of the TMS groups NMR experiments and circular dichroism (results not shown)
using tetra-n-butylammonium fluoride and saponification of the do not indicate any significant interaction of the CDs and the ar-
tert-butyl ester with trifluoroacetic acid resulted in the corre- omatic backbone. Compared to monomer 7, peak broadening
sponding benzoic acid 6. The latter was coupled to and the disappearance of the 1H NMR signals of the acetylene
6-monoamino-6-deoxy-β-CD [52] using N,N’-dicyclohexyl- protons at 4.54 and 4.36 ppm indicate the formation of polymer
carbodiimide (DCC) and 1-hydroxybenzotriazole (HOBt) 8. The presence of the conjugated backbone was confirmed by
applying a procedure known for terephthalic acid [53]. The re- UV–vis and fluorescence measurements in water. Compared to
sulting product, monomer 7, was easily isolated due to its low 7, a characteristic bathochromic shift could be observed both in
solubility in water which was attributed to self-inclusion be- the absorption and emission spectra of polymer 8 (Figure 3)
tween hydrophobic phenyl moieties and β-CD rings leading to showing the presence of the extended polyconjugated π-system.
daisy chains [54].
Quantitative information about the molecular weight distribu-
The polymerization of 7 was performed through Glaser cou- tion of 8 was obtained by MALDI–TOF measurements using an
pling in pyridine catalyzed by Cu(I)/Cu(II). After removal of ionic liquid matrix (HABA/TMG2) [55]. A representative
low molecular weight material by ultrafiltration polymer 8 was MALDI spectrum, shown in Figure 4, exhibits a wide range of
isolated as a light orange solid in 91% yield. Polyrotaxane broad signals starting from the signal of the dimer at
Scheme 1: Synthesis of the CD polymer. a) conc. HNO3, reflux, 6 d; b) tert-butanol, cat. H2SO4, MgSO4, CH2Cl2, rt, sealed vessel, 4 d; c) TMSA,
PdCl2 (10 mol %), CuI (5 mol %), PPh3 (0.5 equiv), Et3N, 80 °C, 48 h; d) TBAF, THF, −20 °C, 30 min; e) TFA, CH2Cl2, rt, 18 h; f) 6-monoamino-6-
deoxy-ß-CD, DCC, HOBt, DMF, rt, 8 d; g) cat. CuCl, cat. Cu(OAc)2, pyridine, 60 °C, 24 h.
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Beilstein J. Org. Chem. 2017, 13, 938–951.
of the repeat unit originating from unmodified benzoic acid de-
rivative 6, e.g., at 2,621.33 and 2,786.52 Da (Figure 4). The MS
analysis reveals the high structural perfection of the polymer 8
where at most one CD entity per polymer molecule is missing.
Integration of the relative distribution of the most intense ions
of each population allowed to estimate both the number aver-
age molecular weight, Mn, and the mass average molecular
weight, Mw, of 8,765.77 Da, and 22,023.56 Da, respectively.
These values result in a polydispersity index PDI = Mw/Mn of
2.59 typical for normal distributions. From the value of Mw an
average contour length L = 17 nm of the macromolecule was
calculated. A more detailed analysis of the MS data is provided
in Supporting Information File 1.
SANS and SAXS measurements of the CD
polymer
Structural characteristics of the CD polymer 8 have been inves-
tigated by small-angle neutron and X-ray scattering experi-
ments (SANS/SAXS). SANS data (KWS-1, JCNS at Heinz
Figure 3: Absorption spectra of monomer 7 (solid red line) and
polymer 8 (solid blue line) in water. Emission spectra of monomer 7
(dotted red line) and polymer 8 (dotted blue line) in water excited at
290 nm and 335 nm, respectively.
m/z 2,621.33 Da detected as [M + Na]+ and ending at the 38mer Maier-Leibnitz Zentrum [56]) for a polymer concentration
at m/z 48,196.23 Da for a S/N ratio ≥3, with an average range from 0.005 to 0.03 g/cm3 are presented in Figure 5.
1297.4 mass units shift corresponding to one additional SANS intensities are normalized to polymer concentration and
repeating unit. Among each discrete envelope, one to three therefore scattering intensities depend on polymer chain mass
supplementary ions, have been detected with a constant (or mass of chain aggregates), square of scattering contrast,
165.2 mass unit shift, revealing the presence of small quantities conformation of polymer chain, and interaction between the
Figure 4: Positive linear MALDI–TOF spectrum of polymer 8 using HABA/TMG2 matrix.
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chains (aggregates). There are only minor differences in scat- where the apparent molar weight, Mapp, is connected with the
tering for concentrations up to 0.02 g/cm3 indicating no signifi- real molar weight, M, via a structure factor S(0) (interaction
cant aggregation between polymer chains with increasing con- among polymer chains) as M × S(0) = Mapp and Δρm is the
centration which would lead to highly ordered polymer species. difference in neutron scattering length density between polymer
The decrease of scattering intensity for the highest concentra- and solvent normalized to the density of polymer. The local
tion of 0.03 g/cm3 can be attributed to interaction of polymer structure of the polymer cylindrical cross-section was extracted
chains. The SAXS curve measured at 0.03 g/mL shows a simi- by applying indirect Fourier transformation (IFT) [58] to the ex-
lar shape as the neutron data (Supporting Information File 1, perimental data from the high-q range. Detailed information
Figure S1).
applying this method is presented in Supporting Information
File 1. The resulting parameters for the concentration depen-
dence of I(0), scattering at “zero angle” of a cylindrical cross-
section of polymer ICS(0), radius of gyration Rg,app, and radius
of gyration of a cylindrical cross-section Rg,CS are presented in
Table 1. The ratio between I(0) and ICS(0) provides the apparent
contour length of the polymer chain. SAXS and SANS indicate
a contour length of 130–160 Å, i.e., 15 monomer units with the
length of one unit of Lmon = 9.2 Å and the chemical composi-
tion C54H75NO35 (molecular weight 1298.17 g/mol). Rg,CS has
been used to calculate the cross-section diameter of the homo-
geneous cylinder to be 30 Å.
Scattering intensities do not change significantly with concen-
tration indicating that the value of S(0) is close to 1. We
consider the values for Lapp and Mapp as lower limits. They are
probably affected by the inexact determination of the scattering
contrast.
Figure 5: SANS data for polymer 8 and fit by cylindrical model (solid
line).
The low-q range of scattering data has been analyzed with a The apparent mass and contour length of polymer 8 with values
Debye function. The apparent radius of gyration Rg,app and the of about 18 kDa and 130–160 Å are in the same range as those
scattering at “zero angle”, I(0), were obtained by fitting the obtained by MALDI measurements (Mw = 22 kDa, L = 170 Å)
scattering data for q < 0.02 Å−1 [57]:
and confirm the structural characteristics of the stiff CD
polymer.
(1)
(2)
The flexibility of chains of polymer 8 was determined by means
of a Holtzer plot [59]. Detailed information and the correspond-
ing data are presented in Supporting Information File 1. The
absence of a characteristic inflection point, where the scattering
intensity changes from q−1 as for rigid cylinder to q−2 (or to
q−5/3 when self-avoidance is important) as for flexible chains,
where x= q2 Rg,app2. The scattering intensity is given by
Table 1: Structural parameters of polymer 8 (apparent radius of gyration, scattering at zero angle, radius of gyration of polymer cross-section, scat-
tering at zero angle of polymer cross-section, apparent contour length obtained from the ratio between I(0) and ICS(0), and calculated apparent mass
of polymer 8, obtained from the length of monomer unit Mapp = Mmon × Lapp/Lmon).
Conc, g/mL
Rg,app, Å
I(0), cm2·g−1
Rg,CS, Å
ICS(0), Å−1·cm2·g−1
Lapp, Å
Mapp, kDa
0.03 (SAXS)
0.005
38.0 ± 1.5
37.4 ± 3.5
31.6 ± 2.5
32.7 ± 1.6
34.6 ± 1.5
680 ± 10 a.u.
16.2 ± 0.3
15.0 ± 0.2
14.9 ± 0.2
13.0 ± 0.1
10.2 ± 0.5
9.8 ± 0.5
9.7 ± 0.5
9.5 ± 0.5
9.4 ± 0.5
5.23 ± 0.05 a.u.
0.099 ± 0.002
0.119 ± 0.002
0.117 ± 0.002
0.100 ± 0.002
130
164
126
127
130
18
23
18
18
18
0.01
0.02
0.03
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indicates that polymer chains are short and rigid, i.e., that the ITC measurements considering a two-step sequential complex-
persistence length is of the same order as the contour length of ation with guest 10. Further information is provided in Support-
the polymer.
ing Information File 1. Incomplete complexation with cationic
guest molecules is indicated by a significant lower binding con-
The SAXS data has been analyzed by models representing the stant of 670 M−1 for the second binding complexation step,
expected shape of polymers. It was assumed that there is no which is strongly inhibited as a result of the electronic repul-
interaction between aggregates, which means that the scattering sion of charged guest molecules in close proximity to each
intensities depend only on the size and shape of the aggregates other. In contrast, a pronounced reduction of the solubility of
[60]. Details are shown in Supporting Information File 1.
CD polymer 8 was observed in the presence of ditopic guest 9,
which was attributed to the interconnection of polymer chains
The scattering data could be described (Figure 5 above and through the complexation of the ditopic guest. The very low
Figure S1 in Supporting Information File 1) by a population of concentration of connector 9 necessary for the almost complete
rigid cylinders of length 110 ± 5 Å and radius of cross-section precipitation of the host polymer 8 can be explained by the high
of 12 ± 2 Å. Neglecting the interaction between polymer chains integrability of the host–guest system based on the shape-persis-
in the model leads to the slightly lower length values.
tence of the polyconjugated polymer backbone of 8.
Complexation of monotopic and ditopic
guests
In contrast to monomer 7, polymer 8 was soluble in water up to
a concentration of 0.15 mM (based on the repeating unit). This
allows the investigation of the complexation of ditopic and
monotopic guests, 9 and 10, respectively. The solubility of the
host polymer 8 as a function of the concentration of both guests
9 and 10 (Scheme 2) was determined by UV−vis spectroscopy
using the extinction coefficient ε of 8 (14,800 M−1 cm−1) at
425 nm. A more detailed description of the solubility measure-
ments is presented in Supporting Information File 1.
Addition of hydrophilic guest 10 caused an increase in solu-
bility of host polymer 8 in water (Figure 6). The surprisingly
steep initial slope of the phase solubility diagram, m = 1.4
(repeating unit/guest) could be well represented by a model
where every second CD moiety has to be complexed by the
hydrophilic guest to significantly improve the solubility in
Figure 6: Solubility of polymer 8 in the presence of ditopic connector 9
(black graph) and 1-aminoadamantane hydrochloride 10 (red graph),
respectively in water at 25 °C.
water. Binding constants of about 40,000 M−1, which were in Attachment of polymer 8 to silicon surfaces
the same range as literature values for the incorporation of Planar silicon wafers, as well as the silicon AFM tip, were first
adamantane derivatives into β-CD, [61] were obtained using functionalized by a polysiloxane monolayer bearing isothio-
Scheme 2: Ditopic and monotopic guest molecules.
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cyanate groups, which smoothly react with amines forming While the pull-off force is similar, the overall appearance of the
stable thiourea links [48]. Monolayers of β-CD or β-CD- force curves differs for the three polymer configurations. The
polymer were obtained by attachment of monoamino β-CD or interaction distance varies significantly for the different config-
amino-modified CD polymer 12, synthesized from polymer 8 urations. The CD–CD configuration has the shortest and the
(Scheme 3) through Cu(I)-catalyzed azide–alkyne cycloaddi- polymer–polymer configuration the longest range of interac-
tion (CuAAC) with the triethylene glycol linker 11 (N3-TEG- tions. The interaction range can be quantified by the tip–sur-
NH2) which had been prepared in a five-step procedure [62,63]. face distance at which the last rupture occurs, referred to as
maximum rupture length. The histograms of the maximum rup-
Probing multivalent interactions by AFM
ture length for all four configurations are presented in Figure 7.
The adhesive forces of 12, due to supramolecular interactions For the CD–CD configuration, the most probable maximum
with ditopic guest 9, between a planar silicon surface and an rupture length of 5 nm corresponds to the combined height of
AFM tip both modified with the CD polymer 12 or the monolayers on tip and surface, each of about 2.5 nm. The
6-monoamino-6-deoxy-β-CD were systematically investigated typical rupture length for the CD–12 configuration is 10 nm,
by AFM. While adhesion was very weak in pure water, signifi- while it is 29 nm for the 12–CD configuration. The difference in
cant adhesion took place over a wide range of distances in a maximum rupture length indicates a difference in the detach-
10 μM solution of ditopic guest 9 (Figure 7a–d). For compari- ment mechanism. In the CD–12 configuration, the polymers
son, we also investigated the adhesion forces between CD and bind to the sloped facets of the asperity of the AFM tip. Upon
12, 12 and CD, and CD and CD at the tip and the planar sur- pulling, the polymers are sheared from the tip apex by rupturing
face, respectively, caused by the adamantane connector 9. all bonds simultaneously leading to one large rupture peak at a
Adhesive forces were recorded as function of the tip–surface small tip–surface distance. For the 12–CD configuration, a force
distance upon retracting of the tip from the surface for all four plateau observed in the force-distance curve in Figure 7c
configurations. The pull-off force required to detach the tip reveals the peeling of a polymer chain from the CD-coated sur-
from the surface in the presence of connector molecules was of face resulting in a rupture length similar to the length of the
the order of 500 pN for the CD–CD configuration and about polymer chains.
1 nN for all configurations involving CD polymers (12). These
values are significantly higher than the pull-off forces of about For the 12–12 configuration, many additional small detachment
250 pN measured in control experiments for all configurations. events lead to a broadening of the pull-off curve and reveal the
The graphical summary in Figure 7a suggests that the pull-off rupture of bonds for tip–surface distances as large as 110 nm in
forces for the 12–12 configuration are slightly higher than for Figure 7d. The broad distribution of rupture length, which
the 12–CD and for the CD–12 configuration.
extends to roughly the double of that of the 12–CD configura-
Scheme 3: Synthesis of amino functionalized polymer 12.
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Figure 7: Characteristic force curves recorded during retraction of the AFM tip from the surface. Four functionalizations are compared: (a) cyclo-
dextrin (CD) layers on tip and surface, (b) CD layer on the tip and polymers (12) on the surface, (c) polymers (12) on the tip and CD layer on the sur-
face, and (d) polymers (12) on tip and surface. Black curves represent control experiments in pure water, red curves experiments in solution contain-
ing ditopic connector 9. The maximum negative force is referred to as the pull-off force. The histograms summarize the distribution of maximum rup-
ture length for every configuration.
tion, indicates that individual long polymer chains interlock, the maximum rupture length reflects the final detachment of
explaining also the characteristic stretching events in the force- supramolecular bonds at the end of stretched polymer chains at-
distance curve. The most probable maximum rupture length for tached to AFM tip and surface.
the 12–12 system is 38 nm, which is double of the average
polymer length of 17 nm predicted from the MALDI–TOF and The higher sensitivity of our AFM set up compared to the
SANS/SAXS results. The agreement confirms the picture that MALDI–TOF instrument allowed us to even detect single rup-
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Beilstein J. Org. Chem. 2017, 13, 938–951.
ture events at a distance up to 250 nm, which proved that some schematic drawings in Figure 2. The shearing configuration
individual chains had a length of at least 125 nm. Compared to (CD–12) leads to simultaneous rupture of all bonds, while the
MALDI–TOF measurements in which the small number of high peeling configuration (12–CD) involves bending of the polymer
molecular weight polymer chains are hardly detectable, AFM and consecutive rupture. The strongest adhesion is offered by
experiments overemphasize the few longest polymer chains the supramolecular interlocking of polymers attached to tip and
probing the interactions of the regularly spaced CDs in CD surface. Supramolecular interconnection between two CD
polymer 12 and ditopic connector molecules. Due to this obser- polymer 12 molecules through the ditopic guest 9 is expected to
vation AFM is an excellent detection tool for analysing cooper- be superior to the one between CD polymer 12 and CD because
ative effects in ordered supramolecular systems.
of the higher regularity of the CD spacing at the polymer com-
pared to the spacing within the CD monolayer. We conclude
The differences between the four configurations of functionali- that the regularity of the CD polymer 12 allows to establish a
zation can be further quantified by integration of the force much higher number of supramolecular bonds with the
curves, resulting in the work of separation which has been em- connector 9 giving rise to about a fivefold enhancement of the
ployed before as a suitable parameter for the quantification of work of separation.
polymer detachment [45]. In line with the characteristic shape
of the example force curves, the work of separation increased Many force curves exhibit a well-defined last rupture event. A
significantly in the order CD–CD, CD–12, 12–CD and 12–12 representative example is shown in Figure 9a, where the force
configuration (Figure 8). The relative increase in the work of drops from around 63 pN to zero at a distance of 110 nm. The
adhesion from control experiments to measurements of the spe- distribution of rupture forces for the last rupture events, shown
cific interactions caused by the connector molecule 9 was even in Figure 9b, has a clear maximum at 63 pN, determined by a
higher than the respective increase in pull-off force due to the Gaussian fit to the distribution, and a weak second maximum at
very short range of the non-specific adhesive interactions.
about double this value.
The significant difference in the interaction range and thus in We conclude that 63 ± 10 pN is the rupture force for a single
the work of separation between CD–12 and 12–CD configura- bond between our supramolecular polymers 12 established by
tion can be explained by the asymmetry between curved tip and the ditopic guest 9. The value agrees with rupture force
flat surface and the resulting difference in the detachment mech- measured for adamantane–CD complexes with CD molecules in
anism. Polymers attached to the surface bind to the side faces of the surface layers when the stiffness of the AFM cantilever is
the tip with its nanometer-scale apex radius. Upon retraction, taken into account [64].
the force acts along the polymer and shears the polymer off the
tip, with all bonds rupturing more or less simultaneously. In Force curves like those shown in Figure 7 can be repeated on
contrast, polymers attached to the tip bind to the flat surface the same spot of one sample many times with very similar
such that upon retraction the polymer is peeled from the sur- results. The repeatability confirms the reversibility of the under-
face by the orthogonal force, one bond breaking after another. lying interactions. It is difficult to estimate the number of supra-
The different detachment scenarios are depicted in the molecular bonds contributing to pull-off forces of 1 nN in
Figure 8: Graphical summary of experimental results for the four configurations of CD attachment introduced in Figure 2: (a) pull-off forces,
(b) work of separation. The error bars indicate standard deviations for averages over different lateral positions on the functionalized surface.
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Figure 9: (a) Detail of the end of a force curve for a polymer-functionalized tip retracted from a polymer-functionalized surface. This is the last
unbinding event which occurs at a tip–surface distance of about 110 nm, indicating the possible range of interaction for polymer functionalization of tip
and surface. (b) Distribution of force drops for the last unbinding event with a maximum at 63 pN and a possible second maximum at about double
this value.
Figure 7. Based on the single bond rupture force one could to the one for the CD–CD system previously described [48].
assume contributions by 16 supramolecular bonds or even more This spike force may be enhanced by the multivalency effects
since it is unlikely that all bonds are loaded to their rupture discussed above, but its strength indicates that more than one
force. As long as we have no experimental means to exactly de- polymer molecule might be involved.
termine the number of polymers molecules involved we cannot
evaluate the number of interconnections per polymer. Since a
significant number of single rupture events at large tip–surface
distances require forces of around 200–250 pN (Figure 7d) up
to four bonds per polymer pair appear reasonable.
Probing multivalent interactions by friction
AFM
Finally, friction force experiments have been performed for the
12–CD configuration. The tip of the AFM slides in contact
across the surface, where polymers attached to the tip may
interact with the CD layer on the surface. A characteristic result
is presented in Figure 10. The average friction force increases
by a factor of 2.5 due to the supramolecular interactions in com-
parison with control experiments in water. The friction force
curve exhibit peculiar spikes when adamantane connector mole-
cules are present. These spikes represent an irregular stick-slip
Figure 10: Characteristic result of a friction experiment for a polymer-
motion of the tip. When one or several polymers are bound to
the surface, the tip is stuck and the increasing force leads to
torsion of the cantilever until the force is large enough to detach
the polymers and drag them further across the surface. The
functionalized tip sliding on a surface carrying a CD layer. Lateral
forces are plotted as a function of lateral tip position when sliding
500 nm back and forth with a velocity of 45 nm/s.
highest friction force spikes of the 12–CD configuration exhibit Conclusion
a force drop of 2 nN, similar to the highest pull-off forces for In conclusion, regular water-soluble shape-persistent CD poly-
the same system. Shearing of a series of bonds, as described for mers based on poly(phenylene butadiynylene) were prepared by
adhesion in the CD–12 configuration, is also the mechanism a straightforward Glaser coupling/click chemistry approach,
underlying friction in the 12–CD configuration. Stick spike which can be attached to planar silicon surfaces as well as AFM
forces of 2 nN are enhanced by at least a factor of 3 compared tips. Structural perfection of the resulted polymers was con-
948

Beilstein J. Org. Chem. 2017, 13, 938–951.
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firmed by MALDI–TOF measurements revealing the presence
of high molecular weight materials with up to 38 repeat units.
High integrability of the scaffold was proven by UV–vis sup-
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adamantane connectors. Small-angle neutron scattering and
X-ray experiments reveal the presence of stiff cylindrical
polymer chains with contour lengths of about 13–16 nm, which
corresponds to the values obtained by MALDI and AFM mea-
surements. Hard substrates with the shape-persistent polymers
and interconnected by ditopic guest molecules require about
five times higher separation energies than those functionalized
with conventional CD monolayers. This significant enhance-
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favored by the rigidity of the polymer backbone and the regular
spacing of the CD moieties. The range of adhesive interactions
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MALDI–TOF and the average contour length determined by
SANS/SAXS. In addition, force microscopy experiments em-
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