Angewandte
Chemie
of the recorded force–extension curves, however, two rupture
events were observed. These double rupture events were due
either to the mechanochemical ring opening of macrocycle 14
or to the attachment of two polymer chains. In the case of
mechanochemical ring opening, the same polymer chain was
stretched before and after the first rupture event, thus
resulting in identical slopes of the curve (Figure 1). If multiple
polymer chains were attached, the slope of the force–
extension curve changed after the first rupture, and these
curves were discarded. After close examination of all of the
force–extension curves with double ruptures, only three
curves remained in which the slope was the same before
and after the first rupture event. In these curves, the length
increase is measured as described in Figure 1. The results are
summarized in Table 1, with a conservative uncertainty of
Æ 0.2 nm for the length change, based on the uncertainty of
positioning the parallel fit lines in the force–extension curves.
Figure 2. COGEF calculation of the length increase resulting from
mechanochemical ring opening of macrocycle 14.
Table 1: Rupture force and elongation measured from force–extension
curves featuring a double rupture event.
Force–exten-
sion curve
Rupture
Force [nN]
Elongation Dx
(exp) [nm]
Elongation Dx
(COGEF) [nm]
The initial state was represented by two phenyl rings
linked by a 1,2,3-triazole; the final state by two phenyl rings
linked by the safety line (Figure 2). The PEG chains were not
included in the calculations. For both configurations, a series
of relaxed scans was performed. The pulling forces were
obtained from the first derivatives of the energy–distance
curves. From the resulting force–distance curves, the length
difference between the initial and final configurations, taken
at the experimental force value, yielded our theoretical
estimate for the length change upon mechanochemical ring-
opening. This theoretical estimate is 1.05 Æ 0.20 nm for a force
of 2.05 nN, which is slightly shorter than the experimental
value. However, there are several effects that contribute to
significant uncertainties. The conditions for the calculations
were set as under vacuum at 0 K, while the experiments were
performed in solution at room temperature. Rupture of the
CÀN bond instead of triazole cycloreversion will slightly
1
2
3
1.11Æ0.01
1.21Æ0.02
2.05Æ0.03
1.2Æ0.2
1.2Æ0.2
1.4Æ0.2
1.01Æ0.20
1.01Æ0.20
1.05Æ0.20
[6,7]
According to Bielawski and co-workers,
this length
increase should be assigned to a mechanochemical retro-click
reaction of macrocycle 14. However, bond ruptures between
the triazole unit and its phenyl anchors, with the triazole ring
remaining intact, would lead to the same AFM response.
These events thus cannot be distinguished here, and the
single-molecule nature of the experiment precludes the use of
standard spectroscopic techniques to further differentiate
between the possible products.
The observation of only three ring-opening events in
several thousand force–extension curves clearly shows that
the aryl–triazole–aryl region is mechanically stronger than the
change the geometry of the aryl groups. The high force of
2.05 nN applied to the bond deforms the binding potentials
and increases their anharmonicity, which in turn leads to
thermal expansion of the PEG chain. The effect of the lever
[
11]
silane surface anchor.
rupture forces are scattered over a range of more than
In force-ramp experiments, the
[12]
[22]
1
nN,
so it is entirely reasonable that in rare cases the
arms was also shown to be significant. These effects justify
mechanically stronger bond breaks first. With the safety line
concept described here, we can unambiguously identify these
events and measure the rupture force. Unfortunately, the
events were so rare in this case that a quantitative statistical
analysis was not possible.
a conservative estimate of Æ 0.20 nm for the uncertainty.
The described combination of tailor-made mechanophore
synthesis, AFM experiments, and quantum chemical calcu-
lations shows that arbitrary bonds can be embedded in
a macrocycle and selectively addressed through an external
mechanical force applied through PEG linkers. Even very few
rupture events of the mechanophore can be unambiguously
identified through the characteristic length increase together
with the unchanged slope of the force–extension curve before
and after the rupture event. With the present molecular
design, we cannot determine whether it was really mechano-
chemically induced retro-click reactions of the 1,2,3-triazole
ring that took place or merely bond ruptures next to it. We can
state, however, that the force required to induce either of the
two reactions is in the nN region. The present technique
opens a wide range of possibilities for the design of
Among the quantum-chemical methods available to
[
2,20,21]
describe covalent mechanochemistry,
the constrained
[
20]
geometries simulate external force (COGEF) method is
ideally suited to describe AFM experiments performed in
force-ramp mode. In this study, COGEF calculations were
used to determine the expected elongation associated with
mechanochemical ring-opening of 14 and subsequent stretch-
ing of the (CH2)10 safety line. Since only the length difference
before and after mechanochemical ring-opening was of
interest here, only the force-induced structural deformability
of the initial and final states was modelled.
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3
These are not the final page numbers!
Schütze, Doreen
