Concave-Convex π-π Template Approach Enables the Synthesis of [10]Cycloparaphenylene-Fullerene [2]Rotaxanes

Article abstract of DOI:10.1021/jacs.8b08244

The cycloparaphenylenes (CPPs) are a class of strained macrocycles that until 2008 were considered beyond the reach of organic synthesis. With its cyclic array of ten para-substituted phenylene rings, [10]CPP possesses a concave π-system that is perfectly preorganized for the strong supramolecular association of convex fullerenes such as C₆₀. Although mechanically interlocked CPP architectures have been observed in the gas phase, the rational synthesis of bulk quantities has not been achieved yet, which is likely due to the fact that conventional template strategies are not amenable to CPP rings that lack heteroatoms. Here, we report the synthesis of two [2]rotaxanes in which a [10]CPP ring binds to a central fullerene bis-adduct and is prevented from dethreading by the presence of two bulky fullerene hexakis-adduct stoppers. The final step in the rotaxane synthesis is surprisingly efficient (up to ca. 40% yield) and regioselective because the fullerene acts as an efficient convex template, while [10]CPP acts as a supramolecular directing group, steering the reaction at the central fullerene exclusively toward two trans regioisomers. Comprehensive physicochemical studies confirmed the interlocked structure, shed light on the dynamic nature of the CPP-fullerene interaction, and revealed intriguing consequences of the mechanical bond on charge transfer processes. In light of recent advances in the synthesis of nanohoops and nanobelts, our concave-convex π-π templating strategy may be broadly useful and enable applications in molecular electronics or complex molecular machinery.

Full text of DOI:10.1021/jacs.8b08244

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Article
A Concave-Convex #-# Template Approach Enables the
Synthesis of [10]Cycloparaphenylene-Fullerene [2]Rotaxanes
YOUZHI XU, Ramandeep Kaur, Bingzhe Wang, Martin B. Minameyer, Sebastian
Gsänger, Bernd Meyer, Thomas Drewello, Dirk M. Guldi, and Max von Delius
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08244 • Publication Date (Web): 20 Sep 2018
Downloaded from http://pubs.acs.org on September 20, 2018
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a
b
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b
c
Youzhi Xu, Ramandeep Kaur, Bingzhe Wang, Martin B. Minameyer, Sebastian Gsänger, Bernd
c
b
b
a
Meyer, Thomas Drewello, Dirk M. Guldi * and Max von Delius *
a) Institute of Organic Chemistry and Advanced Materials, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm,
Germany.
b) Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials, Friedrich-Alexander
University Erlangen-Nꢀrnberg, Egerlandstrasse 3, 91058 Erlangen, Germany.
c) Interdisciplinary Center for Molecular Materials (ICMM) and Computer-Chemistry-Center (CCC), Friedrich-
Alexander University Erlangen-Nürnberg, Nägelsbachstraße 25, 91052 Erlangen, Germany.
ABSTRACT: The cycloparaphenylenes (CPPs) are a class of strained macrocycles that until recently were considered be-
yond the reach of organic synthesis. With its cyclic array of ten para-substituted phenylene rings, [10]CPP possesses a
concave π-system that is perfectly preorganized for the strong supramolecular association of convex fullerenes such as C .
60
Although mechanically interlocked CPP architectures have been observed in the gas phase, the rational synthesis of bulk
quantities has not been achieved yet, which is likely due to the fact that conventional template strategies are not amena-
ble to CPP rings that lack heteroatoms. Here we report the synthesis of two [2]rotaxanes, in which a [10]CPP ring binds to
a central fullerene bis-adduct and is prevented from dethreading by the presence of two bulky fullerene hexakis-adduct
stoppers. The final step in the rotaxane synthesis is surprisingly efficient (up to ca. 40% yield) and regioselective, because
the fullerene acts as an efficient convex template, while [10]CPP acts as a supramolecular directing group, steering the
reaction at the central fullerene exclusively towards two trans regioisomers. Comprehensive physicochemical studies
confirmed the interlocked structure, shed light on the dynamic nature of the CPP-fullerene interaction, and revealed
intriguing consequences of the mechanical bond on charge transfer processes. In light of recent advances in the synthesis
of nanohoops and nanobelts, our concave-convex π-π templating strategy may be broadly useful and enable applications
in molecular electronics or complex molecular machinery.
It is widely recognized that mechanically interlocked
architectures such as catenanes and rotaxanes are ideal
components for the construction of molecular
machines. Yet, the mechanical bond can also be har-
nessed to stabilize otherwise highly reactive chemical
or to generate unprecedented functional prop-
and gather fundamental insights on pho-
to/redox-active building blocks that the mechanical bond
1
–5
6–8
9
1
0–14
motifs
erties
1
5–19
2
0–26
holds in dynamic spatial proximity.
A class of macro-
cycles that seem predestined to give rise to such function-
al behaviour, are the cycloparaphenylenes (CPPs). These
strained “nanohoops”, which were first synthesized in
exhibit highly unusual optoelectronic proper-
For instance, a red-shifted emission is observed
upon shortening the conjugation length, which is in stark
contrast to the behavior observed in linear oligo-
2
7–29
2
ties.
008,
30–34
Figure 1. Representative example of Mꢀllen’s gas-phase
observation of CPP catenanes and structure of the [10]CPP-
fullerene [2]rotaxane reported herein. Y: macrocyclic spacer.
phenylenes.
Moreover,
[10]CPP
strongly
binds
35,36
fullerenes and has been shown to mediate charge
transport both in solution and in thin film.
A first step towards CPP-based mechanically inter-
locked molecules was recently achieved by Müllen and
coworkers who analyzed the insoluble remainder from a
CPP synthesis by ion-mobility mass spectrometry and ob-
37,38
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a
Scheme 1. Retrosynthetic Strategy and Synthesis of Key Precursors.
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a
Reaction conditions: (i) S3, CBr , DBU, DCM, RT, 12 h, 90%; (ii) C , CBr , BTTP, toluene, RT, 12 h, 56%; (iii) S4, DBU, DCM, RT,
4
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4
1
0 h, 75%; (iv) n-BuLi, CH I, DMF, THF, -120 ⁰C, 12 h, 62%; (v) n-BuLi, isopropyl pinacol borate, THF, -78 ⁰C, 2 h, 90%; (vi)
3
Pd(OAc) , S-Phos, K PO , DMF/H O, 90 ⁰C, 12 h; (vii) sodium naphthalenide, THF, -78 ⁰C, 2 h, 15% (two steps).
2
3
4
2
3
6
erved large macrocycles, as well as catenanes (Fig. 1, top)
systems is roughly 3.4 Å, which is the optimal distance
for - interactions. Hence, a fullerene that possesses
3
9
36
and most likely also a trefoil knot. The rational synthesis
of a mechanically interlocked compound based on one or
more CPP rings and the study of the properties of such a
compound is however still elusive. Herein, we report the
four rigid substituents along its equator should be effec-
tive at preventing [10]CPP from dethreading. As a specific
fullerene addition reaction, we chose the well-established
26,40–42
48,49
use of a concave-convex π-π
which enabled us to achieve the synthesis of two
2]rotaxanes comprising a [10]CPP macrocycle, a fullerene
template strategy,
Bingel reaction,
both for functionalizing the fullerene
stoppers and the central fullerene binding site, because
this reaction reliably generates octahedral hexakis-
addition patterns and most importantly, it can be carried
out at low temperature, which we expected to be im-
portant for the crucial final reaction step.
[
bis-adduct binding site and two fullerene hexakis-adduct
stoppers (Fig. 1, bottom).
The synthesis of key building blocks is shown in
Scheme 1. Fullerene diethyl malonate pentakisadduct 1
was prepared according to a modified, three-step proce-
Synthesis strategy and preparation of precursors. A
key consideration for the synthesis of rotaxanes is the
choice of reaction that forges the mechanical bond, i.e.
the reaction that leaves the macrocycle kinetically
trapped on the linear “thread”. Due to the high strain
5
0
dure starting from C . Two “half-threads” (2, 3) were
60
synthesized from 1 by means of high-yielding Bingel reac-
tions with a symmetric dimalonate macrocycle (abbrevi-
ated as “Y”) that, based on molecular modelling, would
allow dynamic motion of [10]CPP along the thread of the
final rotaxane. Half-thread 2 was transformed into key
compound 4, which bears the central fullerene binding
site, in a Bingel reaction whose selectivity for the mono-
addition product was increased by utilization of excess
C . As a final building block, [10]CPP was prepared as
3
0
energy present in [10]CPP (58 kcal/mol) the ring-closing
step is not the final step in its synthesis and the initially
obtained macrocycle exhibits a rectangular rather than
circular shape. As such, we chose to avoid a “clipping”
approach and instead to opt for a “stoppering” strategy.
Accordingly, our synthesis is based on the covalent cap-
ture of a pseudorotaxane by means of an addition reac-
tion to the central fullerene in the final step of the syn-
thesis (Scheme 1). A second strategic consideration relates
to the choice of a “stopper”, which needs to be sufficiently
bulky to prevent the macrocycle from dethreading. With
a chain of 40 carbon atoms constituting its macrocycle
and a diameter of approximately 13.7 Å , [10]CPP repre-
sents an unusually large, albeit relatively shape-persistent
60
illustrated in Scheme 1, according to a modified published
36
procedure.
Rotaxane synthesis and characterization. With
compound 4 in hand, we proceeded to prepare the corre-
sponding pseudorotaxane and to investigate the thermo-
dynamics of the [10]CPPfullerene inclusion complex. As
shown in Fig. 2c (top), addition of one equivalent of
[10]CPP to 4 resulted in a pronounced shift and slight
nanoring, in comparison with the vast majority of report-
ed [2]rotaxanes.
fullerene hexakis-adducts as stoppers, based on the ra-
tionale that in the supramolecular inclusion complex
between [10]CPP and C , the distance between the two 
43–47
As a result, we chose to utilize two
1
broadening of the H NMR signal of [10]CPP, indicating
that the two compounds are subject to a relatively strong
60
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Figure 2. Synthesis of the [10]CPP-Fullerene-Rotaxane. a, Preparation of pseudorotaxane 5 by addition of [10]CPP to 4 and
subsequent covalent capture by means of a Bingel reaction with malonate 3, leading to two [2]rotaxane isomers: trans-2-6 and
-
7
trans-3-6. Reaction conditions: (i) BTTP, DCM, -60 ⁰C → RT, 12 h. b, Fluorescence titration between [10]CPP (conc. 7.2  10 M)
-
6
and 4 (conc. range 0 - 2.5 ×10 M) in toluene. Summary of association constants that were obtained by fitting the data according
to a 1:1 model or alternatively a combined 1:1, 1:2 and 2:1 model, as well as schematic illustration of the three corresponding equi-
1
libria. c, Partial H NMR spectra (400 MHz, CDCl , 298 K) of [10]CPP (red), pseudorotaxane 5 (light green), crude mixture of final
3
Bingel reaction (dark green) and isolated [2]rotaxane isomers trans-2-6 (blue) and trans-3-6 (purple). d, UV-Vis spectra of pseu-
dorotaxane 5 (light green), trans-2-6 (blue) and trans-3-6 (purple), including a more concentrated spectrum of the “fullerene bis-
adduct fingerprint region” (inset) and schematic structures of trans-2 and trans-3 fullerene bis-adduct regioisomers. e, Calculat-
ed DFT geometries of the trans-2-6 (left) and trans-3-6 (right) [2]rotaxanes. Oxygen atoms: light red; carbon atoms: dark grey or
dark red ([10]CPP ring). For computational details, see Supporting Information (Fig. S11). f, MALDI mass spectra of the pseu-
dorotaxane 5 (top) and trans-3-6 (bottom) in the positive-ion mode using DCTB as matrix. g, MALDI MS/MS spectra of the
•+
•+
radical cations of pseudorotaxane 5 (top) and trans-3-6 (bottom). For further details Fig. S12, S13.
3
7
non-covalent association (in CDCl ). The fluorescence of [10]CPP complexes with other fullerene monoadducts.
3
[
10]CPP with a maximum at 466 nm allowed us to carry
Upon close inspection, we noticed however that the fit
did not optimally reproduce the experimental data. As a
result, we tested whether a 2:1 or a 1:2 binding model
would lead to better agreement. Interestingly, we could
only obtain a near-perfect fit for the experimental data,
when 1:2 and 2:1 equilibria were considered in addition to
out a quantitative study of this association by fluores-
cence quenching titration (in toluene; Fig. 2b). When the
titration data was fitted according to a 1:1 binding model,
an association constant of 1.8  10
which is in very good agreement with published data on
6
-1
M was obtained,
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the 1:1 mode of binding (Fig. 2b for schematic structures calculated structures), respectively, which evidently al-
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of the corresponding complexes and equilibria, as well as
a summary of all binding constants). Although it is intri-
guing that both 2:1 and 1:2 complexes are ostensibly
formed in highly dilute solution, it should be stressed that
at the more concentrated conditions that were used for
the [2]rotaxane synthesis, the 1:2 and 2:1 complexes are
only present in very small amounts (<10%; Fig. S3).
6
lows for the association of [10]CPP throughout the course
of the Bingel reaction, whereas the 90° angle in the e
isomer presumably leads to steric repulsion and com-
pletely shuts down this regioisomeric pathway (vide infra
for more results pertaining to the question of regioselec-
tivity).
Having established that [10]CPP binds to 4 with K  10
A
-
1
M in toluene, we turned our attention towards the con-
version of 5 into [2]rotaxane 6 by means of a final Bingel
reaction. When we subjected pseudorotaxane 5 to an
equimolar amount of half-thread 2 under standard reac-
tion conditions, we did indeed observe spectroscopic
evidence that was indicative for the formation of the de-
sired [2]rotaxane species, albeit in low yield. A thorough
analysis of the corresponding MALDI mass spectra re-
vealed that the low yield was primarily due to the for-
0
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mation of side products, in which addition of CBr frag-
2
ments had occurred to the central fullerene, as a result of
the large excess of CBr . To alleviate this problem, we
4
decided to circumvent the use of CBr by using brominat-
4
ed half-thread 3 (Fig. 2) as a pre-activated reagent for the
addition reaction. To our delight, we found that an
equimolar amount of this precursor led to a clean mixture
of rotaxane(s) 6 (ca. 16%) and unreacted [10]CPP (ca.
8
4%). By using a twofold excess of half-thread 3, we were
1
Figure 3. a, Experimental (left) and simulated (right) H
able to increase the combined (NMR) yield of 6 to ap-
proximately 40% ( H NMR stacked plot in Fig. 2c).
NMR spectra of pseudorotaxane 5 at various temperatures,
including the determined rates (k) for the decomplexa-
tion/complexation process. b, Eyring plot.
1
1
Analysis of the crude mixture by H NMR, MALDI mass
spectrometry and HPLC suggested that a mixture of two
regioisomeric [2]rotaxanes had formed. The observation
of a mixture of isomers in such a reaction is not a surpris-
ing result, because eight chemically distinct double bonds
Evidence for [2]rotaxane structure. Not surprisingly
for a molecule featuring a molecular weight of 5.3 kDa, as
well as two flexible spacer groups, we were unable to grow
single crystals for X-Ray crystallography. Therefore, we
aimed at providing evidence for the rotaxane structure of
trans-2-6 and trans-3-6 by other means. The relative posi-
5
1
are available for the second addition step in 5. Purifica-
1
tion of these two compounds by HPLC ( H NMR stacked
plot in Fig. 2c) and extensive analysis of the correspond-
1
tion and shape of the [10]CPP H NMR peaks correspond-
1
13
ing UV-Vis (Fig. 2d), H and C NMR spectra led us to
conclude that the major product is the trans-3-6 (ca. 26%
isolated yield) and the minor product is the trans-2-6 (ca.
ing to pseudorotaxane 5 and rotaxanes 6 is a first indica-
tion to this end (Fig. 2c). More conclusive evidence was
obtained by variable-temperature NMR (VT-NMR). The
1
0% isolated yield). This result is unusual in light of the
[10]CPP signal in pseudorotaxane 5 splits into two peaks
49,52
literature on fullerene bis-addition reactions,
owing to
at low temperature due to “freezing” of the complexa-
the fact that generally a more diverse isomeric mixture is
observed, and the so called e isomer (e for equatorial),
whose formation we did not observe at all, is typically the
major reaction product in Bingel reactions. The unex-
pected regioselectivity can be explained by considering
the angle between the two substituents in different regi-
oisomers. For instance, in the trans-1 regioisomer, the two
addends are located diametrically opposed on the C ,
‡
tion/decomplexation equilibrium (Fig. 3a; G
= 57.3 kJ
298
-1
mol ; t = 1.2 ms), whereas no such peak splitting is ob-
1
/2
served in rotaxane trans-3-6 (Fig. S7). VT-NMR line-shape
45
analysis allowed us to obtain a comprehensive kinetic
picture of the interaction between [10]CPP and the fuller-
‡
-1
‡
ene mono-adduct 4 (Fig. 3b; H = 33.0 kJ mol ; S = -
8
-1 -1
1.5 J mol K ), and we believe it is reasonable to assume
60
that the dynamic [10]CPP translation in the rotaxanes will
be on the same millisecond timescale. In contrast to
pseudorotaxane 5, we were also unable to remove the
hence the angle is 180°, which should be ideal for [10]CPP
binding. As such, our inability to isolate the trans-1 rotax-
ane is likely a result of the statistical, steric and electronic
factors that have previously been invoked to explain the
[
10]CPP ring from rotaxane trans-3-6 by addition of excess
5
3
C60 or silica gel chromatography.
regioisomeric outcomes of Bingel reactions. In the two
isolated products, trans-2-6 and trans-3-6, the angle be-
tween substituents is 144° and 120° (Fig. 2d and Fig. 2e for
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Figure 4. Electron transfer dynamics in trans-2-6. a, Differential absorption spectra obtained by fs-TAS pump-probe experi-
ments (exc = 320 nm, 140 nJ) of trans-2-6 in dichloromethane with several time delays between 1 to 5500 ps. b, Deconvoluted
1*
1*
species associated spectra (SAS) in dichloromethane with bound ([10]CPPC ) (SAS1 – black), unbound ([10]CPPC ) (SAS2
60
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•+
•-
•+
•-
3*
3*
–
red), bound [10]CPP C60 (SAS3 – blue), unbound [10]CPP C
(SAS4 – green) and ([10]CPP) & (C ) (SAS5 – pink)
60
60
obtained via target analysis in GloTarAn. c, Population dynamics of SAS1, SAS2, SAS3, and SAS4. d, Differential absorption spec-
tra obtained by ns-TAS pump-probe experiments (exc = 320 nm, 140 nJ) of trans-2-6 in dichloromethane with several time delays
•+
•-
between 0.001 and 10 µs. e, Deconvoluted species associated spectra (SAS) in dichloromethane with bound [10]CPP C (SAS3
60
•
+
•-
3*
3*
– blue), unbound [10]CPP C (SAS4 – green), and ([10]CPP) & (C ) (SAS5 – pink) obtained via target analysis in GloTarAn.
60
60
f, Population dynamics of SAS3, SAS4, and SAS5. g, Lifetimes of two charge-separation processes, namely CS (bound) and CS
1
2
(unbound), and two charge recombination processes, namely CR (bound) and CR (unbound) reported for the trans-2-6, and
1
2
trans-3-6. h, Schematic illustration of CR (bound) and CR (unbound) involved in trans-2-6, and trans-3-6.
1
2
The structural difference between the pseudorotaxane 5
on one hand and the rotaxane isomer trans-3-6 on the
other hand, is clearly reflected in the respective MALDI
mass spectra (Fig. 2f). In all cases, the molecular radical
cation is observed. However, the molecular ion of the
pseudorotaxane 5 is much less abundant than those ions,
which correspond to decomposition products of 5, i.e. the
radical cations of 4 and [10]CPP. This is indicative of the
non-covalent bonding between 4 and [10]CPP. In the case
of rotaxane 6, using trans-3-6 as example, the molecular
ion produces the most intense signal, with only minor
decomposition, as expected for the mechanically inter-
locked architecture in 6. We did not observe mass spec-
trometric evidence for the formation of [3]rotaxanes, in
which two CPP rings would be trapped on the thread.
This finding may be due to the fact that the presence of
two CPP rings in the corresponding pseudo[3]rotaxane
loss of the rather weakly bound CPP ring, the molecular
ion of rotaxane 6 features the loss of malonate ligands,
while the CPP ring remains attached to the fullerene
thread. This behavior is fully consistent with a [2]rotaxane
structure, where the CPP nanoring cannot dissociate
without breaking a number of covalent bonds in the
stopper (Fig. S12, S13).
To investigate to which extent the [10]CPP ring is mo-
bile along the thread of the rotaxane, once it has dissoci-
ated from the central fullerene, we performed ROESY
NMR experiments (Fig. S8). In these spectra, we observed
NOE contacts between the [10]CPP protons and various
CH groups of the macrocyclic spacer, indicating some
2
mobility along the thread, at least in the vicinity of the
central fullerene. However, we did not observe NOE con-
tacts between the [10]CPP protons and the CH protons
2
within the ethyl ester substituents on the fullerene stop-
pers, indicating that there is most likely no local thermo-
dynamic minimum for the macrocycle in the vicinity of
the two stoppers.
(
see Fig. 2b) does not allow sufficient space for the Bingel
reaction to proceed.
The most conclusive evidence for the rotaxane struc-
ture was provided by MS/MS spectra (Fig. 2g). In MS/MS
experiments, the chosen precursor ion was mass-selected
and allowed to dissociate, producing a daughter ion mass
spectrum of the individually selected precursor ion. While
the molecular ion of pseudorotaxane 5 shows the efficient
Electron transfer studies with the rotaxane iso-
mers. All rotaxanes (Fig. S14) exhibit in dichloromethane
the characteristic absorption features of the fullerene
mono-adduct / fullerene bis-adduct, on one hand, and of
the fullerene hexakis-adduct, on the other hand, with
ACS Paragon Plus Environment

Journal of the American Chemical Society
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•
+
•-
prominent absorptions around 270 and 280 nm (ε ≈ 2.2 ×
[10]CPP C charge-separated states in 0.84 ± 0.02 and
60
5
-1
-1
5
-
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
0 M cm ), between 310 nm and 400 nm (ε ≈ 1.65 × 10 M
10.5 ± 0.3 ps, respectively. Notable is the fact that only the
charge separated state originating from the “unbound”
state features distinct fingerprints, while they are some-
what broadened in the case of the “bound” state as pre-
cursor. The difference is the appreciably stronger elec-
tronic coupling between [10]CPP and C60 in the “bound”
relative to the “unbound” case. For trans-3-6, the corre-
sponding CS and CS occur with 0.9 ± 0.02 and 12.9 ± 0.5
1
-1
4
-1
-1 50,54
cm ) and around 700 nm (ε ≈ 1.0 × 10 M cm ).
In
addition, the [10]CPP absorption is discernable at 340 nm
5
-1
-1
(ε ≈ 1.3 × 10 M cm ). In line with previous studies, the
singlet excited state energies of the fullerene mono-
adduct, the fullerene bis-adduct, the fullerene hexakis-
adduct, and [10]CPP are 1.76, 1.77, 1.89, and 3.46 eV, re-
5
5–58
spectively.
1
2
•
+
•-
ps. Both [10]CPP C charge-separated states, that is,
To determine the energetics of charge transfer reac-
tions between [10]CPP and the different fullerenes in the
pseudorotaxane 5, trans-2-6, and trans-3-6, we recorded
differential pulse voltammograms (Fig. S15) in argon-
60
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
“bound” and “unbound” are subject to a parallel decay
(Fig. 4g) and exhibit drastically different lifetimes. For
trans-2-6 and trans-3-6, the two products are the triplet
excited state of the fullerene bis-adduct, on one hand, and
the ground state, on the other hand. As far as the first,
short-lived component is concerned, it is constituted by
the one-electron oxidized, “bound” [10]CPP, which is
locked in close proximity to the fullerene bis-adduct, and
decays with a time constant (CR ) of 1.62 ± 0.2 ns (Fig. 4h
and S20). The second, long-lived component stems from
unbound” [10]CPP occupying a more distant location
relative to the central fullerene bis-adduct, which is a
unique consequence of the mechanically interlocked
architecture (Fig. 4h). From fs-TAS, only an upper limit
of >6 ns was extrapolated for the underlying time con-
+
saturated dichloromethane vs. Fc / Fc . For pseudorotax-
ane 5, the first and second reduction at ‒1.05 and ‒1.39 V,
respectively, are centered on the mono-adduct, while the
third reduction due to the hexakis-adduct is noted at ‒1.55
V. The first [10]CPP centered oxidation is at +0.75 V. For
trans-2-6 and trans-3-6, the first reduction at ‒1.23 V is
centred on the bis-adduct and, therefore shifted by 0.18 V
relative to the mono-adduct in the pseudo rotaxane, fol-
lowed by the second reduction and third reduction, which
involve the hexakis- and bis-adduct, respectively, and,
which coalesce at ‒1.50 V. Correspondingly, we derive
energies of the charge-separated state of 1.82 eV for pseu-
dorotaxane 5 and 1.98 eV for trans-2-6 / trans-3-6.
1
“
stant (CR ). An accurate determination of CR was
2
2
63
achieved by ns-TAS (Fig. 4 and S20), where, next to the
fast CR , CR evolved with a lifetime of 20.3 ± 6 ns for
To obtain insights into the photoproducts formed in
the rotaxanes (trans-2-6 / trans-3-6) upon photoexcita-
tion, we turned to transient absorption spectroscopy on
the femto-, pico-, nano-, and microsecond timescales (fs-
TAS and ns-TAS). We selected an excitation wavelength
of 320 nm (3.87 eV), where all fullerene adducts as well as
1
2
trans-2-6. 1.56 ± 0.2 and 26.9 ± 8.6 ns were the two life-
times for trans-3-6. On even longer time scales, only the
triplet excited state absorption features of [10]CPP and
the fullerene bis-adduct dominate the transient absorp-
tions.
[10]CPP absorb. Considering the extinction coefficients,
the light partitioning at 320 nm is 1 to 1. Within a few
picoseconds after excitation, an ultrafast evolution of the
transient absorption features characteristic of the one-
electron oxidized [10]CPP and the one-electron reduced
fullerene bis-adduct at around 540 and 1045 nm evolved
59-61
(
Fig. 4, S20-S22).
62
Multiwavelength and global target analyses of the fs-
TAS and ns-TAS data necessitated a five species and three
species kinetic model, respectively, to fit the data for
trans-2-6 / trans-3-6. From the VT-NMR data, which are
shown in Fig. 3, we postulate that the dissociation of
[10]CPP from the central fullerene bis-adduct and the
microscopic reversibility of this process, are very slow,
that is, on the millisecond timescale, when compared to
the electron transfer processes studied by fs-TAS and ns-
TAS. Hence, we partitioned the data fitting into “bound”
and “unbound” states, where [10]CPP is located on the
central fullerene bis-adduct or somewhere along the
thread, respectively. Based on the differences in associa-
tion constants for [10]CPP in the two regioisomers, which
infer that in trans-2-6 the “bound” state is less populated
than in trans-3-6, we used a 9-to-1 ratio for trans-2-6 and
a 7-to-3 ratio for trans-3-6.
Figure 5. Model study on the effect of [10]CPP on the
regioselectivity of fullerene bis-addition. a, Reaction
sequence for [10]CPP-directed bis-addition and subsequent
removal of [10]CPP by addition of excess C60. Reaction condi-
tions: (i) diethyl bromomalonate, DBU, CH Cl , RT, 10h, 46%.
2
2
b, reaction outcome in the presence of [10]CPP (red bars) in
4
9
comparison with published data in the absence of [10]CPP
(black bars). c, structures of trans-2-8[10]CPP and trans-3-
8[10]CPP, including corresponding substituent angle and
For trans-2-6, the “bound” and “unbound” states trans-
form
in
dichloromethane
into
two
different
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Journal of the American Chemical Society
binding constant K (fit according to 1:1 model). For further
details see Fig. S23-S26.
products are formed. The calculated geometries (DFT) of
trans-2-6 and trans-3-6 shown in Fig. 2e lend support to
this reasoning, because the [10]CPP ring appears to have a
certain degree of freedom by rotation around the vertical
axis in the trans-2 isomer, whereas there is not much
room for such a rotation in the trans-3 isomer.
A
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Further support for the notion of charge separation in a
“
bound” and “unbound” state came from changing the
solvent dielectric constant from 8.93 for dichloromethane
to 6.97 in 2-methyltetrahydrofuran. Considering that the
charge recombination is placed in the normal region of
the Marcus parabola as in fullerene-ferrocene conju-
64
gates, CR is in 2-methyltetrahydrofuran with 6.2 ± 3.4
2
We have described the first synthesis of [2]rotaxanes
featuring a shape-persistent cycloparaphenylene macro-
cycle devoid of heteroatoms. Key to the successful synthe-
sis was the use of the strong concave-convex π-π interac-
tion between [10]CPP and a fullerene mono-adduct and
the ability of the [10]CPP ring to significantly improve the
outcome of the stoppering reaction, which is reminiscent
and 8.3 ± 4.9 ns for trans-2-6 and trans-3-6, respectively,
much faster than in dichloromethane (vide supra). In
stark contrast, no differences between the two solvents
are, however, seen for CR due to the close [10]CPP-to-C
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
60
proximity in the “bound state”: 1.75 ± 0.9 ns for trans-2-6
65
and 1.49 ± 0.3 ns for trans-3-6. (Fig. S21, S22)
Preliminary insights into the role of [10]CPP as su-
pramolecular directing group. Intrigued by the CPP-
directed trans-3 and trans-2 regioselectivity during the
final step of the rotaxane synthesis and inspired by recent
progress toward regioselectivity using tethered reagents
of Stoddart’s use of a cyclodextrin ring to enhance the
69,70
rate of CuAAc “click” reactions
or Leigh’s use of mac-
rocyclic ligands in his “active metal template”
71,72
approach . However, in the present example, the modu-
lation of reactivity leads to drastically improved regiose-
lectivity, rather than reactivity enhancement or compo-
nent preorganization. These findings may have techno-
logical importance, because the most effective fullerenes
66
and endohedral fullerenes, we decided to investigate the
generality of this effect with the help of model monoma-
lonate 7. Addition of one equivalent of [10]CPP to 7 led to
the corresponding inclusion complex 7[10]CPP, which
served as our starting material for a second addition reac-
tion (Fig. 5, top). When analyzing the reaction outcome
by HPLC, we observed the trans-3 bis-adduct (trans-3-8,
ca. 24%) as major product and trans-2-8 as side product
73–75
in organic photovoltaics (PC BM, ICBA and Bis-PCBM
)
71
are employed as regioisomeric mixtures, despite recent
evidence for dramatic differences in the performance
76
between regioisomers. To this end, it is worth emphasiz-
ing that we developed a simple strategy for the efficient
recycling of the costly [10]CPP template.
(
ca. 20%), which is similar to the outcome of the rotaxane
synthesis and confirms the generality of this effect. In
addition to the trans-3 and trans-2 regioisomers, we also
observed the trans-1 product, albeit in only ca. 2% yield.
Presumably, this isomer was also formed in the rotaxane
synthesis, but we failed to isolate trans-1-6 due to the
smaller reaction scale. When these results are compared
to published reports on the synthesis of 8 in the absence
Our electron transfer studies revealed that the presence
of the mechanically interlocked [10]CPP ring does not
affect the reduction potential of the central fullerene bis-
+
adduct, as the observed -1.21 V (vs. Fc/Fc ) is precisely
77
what is expected for a fullerene bisadduct. However,
transient absorption spectroscopy suggests that the me-
chanical bond leads to an interesting situation, where a
relatively well-defined “unbound” supramolecular state
can be probed experimentally. As one would expect based
on the increased distance in the corresponding charge
separated state, we found that charge recombination is
much slower in the “unbound” state compared to the
“bound” state and that the dielectric constant of the sol-
vent strongly affects the kinetics of this recombination
process. We anticipate that this apparent ability of the
49
of [10]CPP, it is striking that the presence of [10]CPP
completely suppresses the formation of the trans-4, e, cis-
3
and cis-2 regioisomers, which in the absence of [10]CPP
account for a total of 40% of the product mixture.
Next, we proceeded to test our hypothesis that the al-
tered regioselectivity in the presence of [10]CPP correlates
with the ability of [10]CPP to maintain a strong non-
6
7
covalent interaction once the bis-addition has occurred.
Fluorescence titrations, in which [10]CPP was treated with
trans-2-8 and trans-3-8, revealed that the association
constant for the trans-2 isomer is approximately 20 times
higher than for the trans-3 isomer. This finding explains
why the trans-2/trans-3 ratio is higher in the presence of
[10]CPP ring to act as a relay of (positive) charge, together
with our new template strategy for the synthesis of car-
bon-rich rotaxanes, may lead to further advances in mo-
lecular electronics or machinery.
68
[
10]CPP, and it also explains why the trans-1/trans-2
appears to be unchanged, because CPP binding in these
two products is already almost as strong as in mono-
adducts. In terms of the aforementioned substituent an-
gles, these results imply that 180° (trans-1) and 144°
(trans-2) allow for optimal reaction kinetics, whereas 120°
(trans-3) does already lead to a decreased rate in the addi-
tion reaction, while 110° (trans-4), 90° (e) and <90° (cis
isomers) seem to inhibit the addition reaction so dramati-
cally that no observable quantities of the corresponding
Supporting Information. Synthesis and characterization
data. Details on mass spectrometric, variable-temperature
NMR and transient absorption studies. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
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*
dirk.guldi@fau.de
(25)
de Juan-FernándezFern, L.; M, P. W.; Puthiyedath, A.; en
Nieto-Ortega, B.; Casado, S.; Ruiz-GonzálezGonz, L.; P, E.
M.; Guldi, D. M. Chem. Sci. 2018, 9, 6779–6784.
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*max.vondelius@uni-ulm.de
(
(
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The authors declare no competing financial interest
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Synthetic Carbon Allotropes”, projects A7, B10, C1 and Z1),
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Deepsing Syangtan is acknowledged for precursor syntheses
during a DAAD RISE project. Giorgio Baggi (University of
Windsor) is acknowledged for helpful advice regarding NMR
line-shape analysis. Dr. Harald Maid is acknowledged for
recording ROESY NMR spectra.
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trans-1.
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Yamago, S. J. Am. Chem. Soc. 2011, 133, 8354–8361.
The HOMO-LUMO transition in CPP features little or no
oscillator strength and the strong absorption is assigned to
the sum of HOMO‒2 to LUMO, HOMO‒1 to LUMO,
HOMO to LUMO+1, and HOMO to LUMO+2 transitions.
1
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Spectroelectrochemial oxidation of [10]CPP (Fig. S16) and
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pulse-radiolytic reduction of the fullerene bis-adduct reveal
prominent features at 520 and 1045 nm, respectively. The
earlier is accompanied by a broad NIR absorption spanning
from 900 to 1800 nm.
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Please note the 1030 to 1090 nm artifact in the differential
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We believe it is reasonable to presume that the same
correlation also applies for the rate-determining transition
state of the Bingel reaction.
Please note that the model system deviates slightly from
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