Chiral Dibenzopentalene-Based Conjugated Nanohoops through Stereoselective Synthesis

Article abstract of DOI:10.1002/anie.202016968

Conjugated nanohoops allow to investigate the effect of radial conjugation and bending on the involved π-systems. They can possess unexpected optoelectronic properties and their radially oriented π-system makes them attractive for host–guest chemistry. Be

Full text of DOI:10.1002/anie.202016968

Angewandte
Research Articles
Chemie
How to cite: Angew. Chem. Int. Ed. 2021, 60, 10680–10689
International Edition:
German Edition:
doi.org/10.1002/anie.202016968
doi.org/10.1002/ange.202016968
Stereoselective Synthesis
Chiral Dibenzopentalene-Based Conjugated Nanohoops through Ste-
reoselective Synthesis
Mathias Hermann, Daniel Wassy, Julia Kohn, Philipp Seitz, Martin U. Betschart, Stefan Grimme,
and Birgit Esser*
Dedicated to Professor Peter Bäuerle on the occasion of his 65th birthday
Abstract: Conjugated nanohoops allow to investigate the effect
of radial conjugation and bending on the involved p-systems.
They can possess unexpected optoelectronic properties and
ular chemistry[18,19] is a rising field. The incorporation of p-
systems other than benzene allows to alter the optoelectronic
and structural properties of nanohoops.[12] This has been
their radially oriented p-system makes them attractive for host– demonstrated using donor-/acceptor-aromatics or polycyclic
guest chemistry. Bending the p-subsystems can lead to chiral
hoops. Herein, we report the stereoselective synthesis of two
enantiomers of chiral conjugated nanohoops by incorporating
dibenzo[a,e]pentalenes (DBPs), which are generated in the last
synthetic step from enantiomerically pure diketone precursors.
Owing to its bent shape, this diketone unit was used as the only
bent precursor and novel “corner unit” in the synthesis of the
hoops. The [6]DBP[4]Ph-hoops contain six antiaromatic
DBP units and four bridging phenylene groups. The small
HOMO–LUMO gap and ambipolar electrochemical character
of the DBP units is reflected in the optoelectronic properties of
the hoop. Electronic circular dichroism spectra and MD
simulations showed that the chiral hoop did not racemize even
when heated to 110 8C. Due to its large diameter, it was able to
accommodate two C60 molecules, as binding studies indicate.
aromatic hydrocarbons mostly composed of six-membered
rings.[12] We herein present the dibenzo[a,e]pentalene (DBP)-
based chiral nanohoops (+)-1 and (À)-1 consisting of six DBP
units and four phenylene rings (Figure 1A), accessed in
a stereoselective synthesis. The DBP units endow ambipolar
electrochemical behavior to the hoop,[20] they possess anti-
aromatic character,[21,22] and their lack of a dividing mirror
plane when bent makes nanohoop 1 chiral. After rubicene-
Introduction
Conjugated nanohoops have become a major research
area in recent years.[1] While hoop-shaped molecules have
fascinated chemists since the 1950s,[1–3] the interest in the field
has rapidly increased since 2008, when the first [n]cyclo-
paraphenylenes ([n]CPPs) were synthesized.[4–6] Much prog-
ress has been achieved since then.[7] Not only have [n]CPPs of
various sizes been synthesized[8–11] and their properties
extensively investigated, but also many derivatives containing
aromatic units other than benzene.[12–14] Emerging applica-
tions for nanohoops were identified,[15] and their use as
segments of carbon nanotubes[16,17] as well as in supramolec-
Figure 1. A) DBP-based chiral nanohoop 1 synthesized herein (and
derivatives 2-1 and 2-2 used for calculations); B) Synthetic strategy
using chiral, enantiopure diketone units as precursors to bent DBP
units.
[*] M. Hermann, D. Wassy, P. Seitz, Prof. Dr. B. Esser
Institute for Organic Chemistry, University of Freiburg
Albertstr. 21, 79104 Freiburg (Germany)
and
Freiburg Center for Interactive Materials and Bioinspired Technolo-
gies, University of Freiburg
E-mail: besser@oc.uni-freiburg.de
Georges-Kçhler-Allee 105, 79110 Freiburg (Germany)
Homepage: https://www.esser-lab.uni-freiburg.de
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202016968.
J. Kohn, Prof. Dr. S. Grimme
Mulliken Center for Theoretical Chemistry, University of Bonn
Beringstr. 4, 53115 Bonn (Germany)
ꢀ 2021 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
M. U. Betschart
Institut für Pharmazeutische Wissenschaften, University of Freiburg
Albertstr. 25, 79104 Freiburg (Germany)
Prof. Dr. B. Esser
Freiburg Materials Research Center, University of Freiburg
Stefan-Meier-Str. 21, 79104 Freiburg (Germany)
10680
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 10680 –10689

Angewandte
Research Articles
Chemie
containing hoops reported by the Isobe group[23] and [2]DBP-
hoops. The 2,7-bromo substituents in 3 would later allow to
[12]CPP nanohoops synthesized by us,[24] this is the third
synthetic report incorporating a non-alternant hydrocarbon
into a nanohoop and the second using an antiaromatic unit.[25]
The hoop incorporation allows to study the effect of radial
conjugation and bending on the antiaromaticity of the DBP
units. DBP possesses a small band gap due to an increased
HOMO and decreased LUMO energy in comparison to an
alternant hydrocarbon of similar size.[1] This causes its
ambipolar electrochemical character, making it attractive
for, e.g., field-effect transistors.[26–28]
perform cross-coupling reactions. Racemic 3 was synthesized
in four steps from 2-(4-bromophenyl)acetic acid, as previous-
ly reported.[38] In initial synthetic attempts to nanohoop 1 (as
mixture of stereoisomers) using racemic 3 we faced two main
difficulties: (a) Diastereomeric mixtures of intermediates
were formed, containing several of the diketone units, which
were difficult to purify and structurally characterize, and
(b) macrocyclization proceeded with low yield, likely due to
the required conformation being hard to achieve in some of
the diastereomeric precursors. To avoid these issues we
decided to enter the nanohoop synthesis with enantiomeri-
cally pure diketone 3 and therefore establish a method for its
racemic resolution. While a separation of its enantiomers by
analytical chiral HPLC was possible (see SI, Figure S88), the
low solubility of 3 in common organic solvents restricted its
resolution on a semi-preparative HPLC scale to < 50 mg. For
the syntheses envisioned herein to obtain chiral DBP-based
nanohoops, larger amounts were required. The carbonyl
functions in 3 provide a useful handle for its transformation
into diastereomers, separable on a preparative scale. We
initially tested the transformation of 3 into a bisketal using l-
(+)-diethyl tartrate, its functionalization to a bisimine using
(S)-a-methylbenzylamine or to a bishydrazone using (À)-
menthydrazide.[43] However, in the first two cases functional-
ization was unsuccessful, and in the third case the diastereo-
meric hydrazones formed from 3 were inseparable by column
chromatography or crystallization. With success, on the other
hand, proceeded the sulfoximine-mediated racemic resolu-
tion developed by Johnson,[44,45] as shown in Scheme 1. Chiral
sulfoximine (S)-4 was synthesized from thioanisol in three
steps including racemic resolution of the intermediate sulfox-
imine before N-methylation.[46–48]
Chiral conjugated nanohoops have been synthesized on
few occasions by unsymmetrically incorporating polycyclic
aromatic hydrocarbons, such as phenanthrene,[29] anthra-
cene,[30] chrysene,[31,32] anthanthrene,[33] and rubicene,[23] or
through topological design.[34–36] In most of these cases the
hoops were synthesized as racemic or diastereomeric mix-
tures, and pure stereoisomers were separated by chiral HPLC.
This was possible since racemization barriers were high
enough to slow down conformational isomerization at room
temperature. One example has been reported using a chiral
catalyst, which allowed obtaining an enantiomeric excess of
a cyclophenylene,[37] and another where an enantioenrich-
ment was obtained for a cyclochrysenylene.[31] We herein
present a unique and novel synthetic strategy, which enabled
the stereoselective synthesis of (+)-1 and (À)-1 as one of the
first reports of a stereoselective nanohoop synthesis. This was
possible by using a novel bent precursor for nanohoop
synthesis, as shown in Figure 1B, a bent and chiral diketone
unit, which can be transformed into DBPs once incorporated
into the hoop. In the synthesis of dibenzopentalenophanes, we
had demonstrated before that this method is suitable to
introduce strain and to strongly bend DBP units.[38] Herein we
used both enantiomers of this diketone, synthesized by
racemic resolution, as the only bent precursor in the synthesis
of conjugated nanohoops (+)-1 and (À)-1. Bending a pref-
erably planar p-system into a hoop shape is one of the highest
challenges in nanohoop synthesis.[11] Several strategies lead-
ing to a bent array of six-membered rings have been reported
in the context of CPP syntheses.[4–6,39–41] Our strategy reported
herein adds to this pool and enables the stereoselective
synthesis of chiral nanohoops containing antiaromatic and
electrochemically ambipolar DBP units.
After deprotonation with ethyl magnesium bromide,
diastereoselective, cerium trichloride-mediated addition[49]
of (S)-4 to 3 afforded diastereomers 5 and 6 in high yields,
which were easily separable (DRf = 0.4) on large scale using
Results and Discussion
Racemic resolution of diketone 3
We recently reported on the synthesis of the DBP-
containing nanohoops [2]DBP[12]CPP,[24] accessed using
Itamiꢀs L-shaped diphenylcyclohexane units[42] as bent pre-
cursors to terphenyl units together with tetrafunctionalized
DBPs. In order to synthesize a nanohoop that is chiral and
contains a higher ratio of DBP units, we wanted to refrain
from using bent terphenylene precursors and instead employ
diketone 3, which possesses a naturally bent structure (107.28
between two six-membered rings)[38] and is a chiral precursor
to DBPs, making it ideally suited to access strained nano-
Scheme 1. Racemic resolution of 2,7-dibromo-diketone 3 and molec-
ular structure of bis-adduct 6 in the solid state (displacement
ellipsoids are shown at 50% probability; hydrogen atoms and co-
crystallized chloroform molecule are omitted for clarity).
Angew. Chem. Int. Ed. 2021, 60, 10680 –10689 ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH www.angewandte.org 10681

Angewandte
Research Articles
Chemie
an automated column chromatography system. The addition
of the sulfoximine selectively occurred from the convex side
of diketone 3. This can be seen in the molecular structure of 6
in the solid state, shown in Scheme 1, resolved by X-ray
diffraction on single crystals, grown from chloroform/n-
pentane by solvent layering. Thermolysis in toluene allowed
removing the chiral auxiliary and furnished (R,R)-3 and (S,S)-
3 in excellent yields and ees. Their absolute configuration was
determined through the molecular structures of 6 as well as
(S,S)-3 (see SI) in the solid state. This method of racemic
resolution allowed accessing both enantiomers of 3 in 1.2 g
scale.
borylation furnished (R,R)- and (S,S)-11 in excellent yields of
96% and 93%, respectively. (R,R)- and (S,S)-11 were then
cross-coupled with an excess of (R,R)- and (S,S)-3, respec-
tively, to give (R,R)3- and (S,S)3–12 in good yields of 57%
3
respective 48% (the superscript indicates the presence of
three diketone units with the respective (R,R)- or (S,S)-
configuration). The surplus of (R,R)- or and (S,S)-3 employed
in this step could be recovered during purification of the
product. Quantitative Miyaura-borylation of (R,R)3- and
(S,S)3-12 led to (R,R)3- and (S,S)3-13, which were the
compounds required to perform the macrocyclization. Due
to its success in the synthesis of strained [n]CPPs, we chose
Jastiꢀs oxidative homocoupling of boronic esters as the hoop-
forming reaction.[50] Subjecting (R,R)3- and (S,S)3-13 to these
conditions furnished the dimeric hoops (R,R)6- and (S,S)6-14
in respectable yields of 32% and 26%, respectively, which
could be purified by column chromatography. Interestingly,
the smaller hoops, resulting from an intramolecular cycliza-
tion of (R,R)3- and (S,S)3-13, were not detected. This must
have been due to the conformation of the open precursors
(R,R)3- and (S,S)3-13, which can take on a U-shape, ideally
suited for a cyclodimerization (for calculated structure of
(R,R)3-13 see SI, Figure S124), but not an intramolecular
reaction. Larger oligomers were also formed (as indicated by
analytical GPC, see SI, Figure S83), but could not be isolated
in pure form. In the last two steps we converted all six
diketone units in 14 into DBPs. 4-(n-Hexyl)phenyl substitu-
ents to the DBPs were chosen to impart sufficient solubility to
both 16 and 1. Cerium trichloride-mediated addition[38,49] of
the Grignard-reagent of 15 (for synthesis see SI) afforded
Synthesis of DBP-based nanohoops (+)-1 and (À)-1 and reference
compound 7
With sufficiently large amounts of enantiomerically pure
diketone 3 in hand, we embarked on the stereoselective
nanohoop synthesis using both enantiomers (R,R)-3 and
(S,S)-3 (Scheme 2). From the molecular design we incorpo-
rated four diethyl-substituted phenylene groups in total into
the hoop as spacers between DBP units, since preliminary
investigations had shown that a linear trimer of 3 (in analogy
to 12 without the two bridging arylene units) was too
insoluble for further functionalization. Suzuki–Miyaura-cou-
pling of boronic ester 8 (for synthesis see SI) with (R,R)-3 or
(S,S)-3 afforded (R,R)- and (S,S)-9 in excellent yields of 95%
and 93%, respectively, which were then transformed into
dibromides (R,R)- and (S,S)-10. The following Miyaura-
Scheme 2. Stereoselective synthesis of the chiral, DBP-based nanohoops (+)-1 and (À)-1 (the stereocenters assigned by (R) or (S) are indicated
by an asterisk, for 14, 16 and 1, the structures of the (R,R)6/(+)-isomers are shown).
10682 www.angewandte.org ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angew. Chem. Int. Ed. 2021, 60, 10680 –10689

Angewandte
Research Articles
Chemie
dodecaols (R,R)6-(+)- and (S,S)6-(À)-16 in high yields of 82%
and 87%, respectively. This corresponds to a yield of 98%
respective 99% for the individual addition to each of the
twelve carbonyl groups. The last critical step was the 12-fold
water elimination to furnish conjugated hoops 1. We had
previously optimized this step in the synthesis of strained
DBP-phanes and found Burgessꢀs reagent (17) to be the mild
method of choice.[38] The same procedure worked cleanly for
(R,R)6-(+)- and (S,S)6-(À)-16 and gave DBP-based chiral
hoops (+)-1 and (À)-1 in high yields of 86% and 90%,
respectively. As for the previous step, this corresponds to
impressive yields of 99% for each individual water elimina-
tion step. The identity and purity of (+)-1 and (À)-1 as well as
cyclic precursors (R,R)6- and (S,S)6-14 and (R,R)6-(+)- and
(S,S)6-(À)-16 was confirmed by 1D- and 2D-NMR spectros-
copy (1H, 13C, COSY, HSQC, for (+)-1 also ROESY) and
HRMS.
Figure 2. Selected regions of the 1H NMR spectra of hoop (+)-1 and
reference compound 7 (CDCl3, 500 MHz, 300 K).
For comparison of the optoelectronic properties we
synthesized 7, representing a planar reference compound
for nanohoop 1 (Scheme 3). Its synthesis started from 18,
which was obtained in analogy to 12 (Scheme 2), but using
racemic 3 as starting material (see SI for details). Mesityl
groups were attached to the two outer diketone rings in 18
using a Suzuki–Miyaura-coupling reaction to furnish 19.
Cerium trichloride-mediated addition of the Grignard-re-
agent of 15 to the diketone units in 19 followed by acid-
catalyzed hexafold water elimination afforded reference
compound 7 in good yield of 63%.
The DBP protons highlighted in green were more differ-
entiated in hoop (+)-1 compared to reference compound 7
due to the higher rigidity of the hoop, the same held for the
DBP protons highlighted in yellow. A stronger splitting was
also observed for the ethyl protons (highlighted in blue) in
(+)-1 compared to 7. A VT-NMR experiment did not reveal
significant changes in signal width or splitting with increasing
temperature (see SI).
To evaluate the structural properties of 1 by DFT
calculations all alkyl groups were replaced by methyl groups,
furnishing 2-1 (its stereochemistry corresponding to the D2-
symmetric-enantiomer drawn in Figure 1 or 1a in Figure 4).
The geometry of 2-1 was initially assessed using molecular
dynamics (MD) simulations (with the OPLS3 Force Field as
implemented in Schrodinger 2017), followed by geometry
optimization at the PBEh-3c[51]-level of theory (see SI for
details).
The calculated structure of 2-1 shows its near D2
symmetry and circular geometry (Figure 3). Its diameter
averages to 2.5 nm, which makes [6]DBP[4]CPP-hoop 2-
1 slightly larger than the [2]DBP[12]CPP-hoop recently
reported by our group with 2.1 nm diameter.[24] The DBP
Scheme 3. Synthesis of reference compound 7.
Molecular structure of DBP-based nanohoops (+)-1 and (À)-1
Due to their mirror-symmetric structure as enantiomers,
the NMR spectra of hoops (+)-1 and (À)-1 were identical (see
1
SI). Comparison of the H NMR spectra of nanohoop (+)-
1 with planar reference compound 7 confirmed the high
symmetry and more rigid structure of the hoop (Figure 2,
some protons were assigned for comparison, for full assign-
ment see SI). The doublets of doublets (highlighted in purple)
for the DBP units, located next to the mesityl groups in 7 and
next to a neighboring DBP unit in hoop (+)-1, were shifted
downfield by about 0.5 ppm in the hoop compared to planar 7.
Figure 3. Calculated structure of 2-1 (PBEh-3c, H-atoms are omitted
for clarity) and geometrical parameters.
Angew. Chem. Int. Ed. 2021, 60, 10680 –10689 ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH www.angewandte.org 10683

Angewandte
Research Articles
Chemie
units are bent by VDBP = 32.98 on average, which is slightly
less than in the [2]DBP[12]CPP-hoop mentioned above and
significantly less than the bends of up to VDBP = 91.98
reported by our group for DBP-phanes.[38] In comparison,
the four phenylene units in 2-1 experience a bend of only 7.18,
which is lower than that in [18]CPP of 8.08 with similar
diameter.[52] While the phenylene units are rotated more
strongly out of the hoop shape relative to the DBP units
(dihedral angle VDBP-Ph-av. = 53.78), the dihedral angles be-
would be 1a*. 1a is also the conformation we depicted in
Figure 1 and Scheme 2 for simplicity.
DFT calculations provided insight into the relative
energies and interconversion barriers of the 14 diastereomers
1a–1n shown in Figure 4. The structures of 1a–1n were
optimized using the semiempirical extended tight-binding
model GFN2-xTB[53] with tight convergence criteria and
applying the implicit solvation model GBSA[54] with toluene
solvent. This method is particularly well-suited to explore the
conformational space of molecular systems. B97-3c[55] DFT
single-point energies were computed on all optimized struc-
tures, applying the implicit solvent model D-COSMO-RS[56]
with toluene solvent. With 9.0 kcalmolÀ1 (maximum energy
for 1n) the energy range is substantial, and 1a as the D2-
symmetrical isomer as well as 1c with one DBP unit rotated
relative to the others are the most stable diastereomers with
relative energies of 1.2 and 0.0 kcalmolÀ1, respectively.
Energetic barriers for the rotation of the two symmetrically
inequivalent DBP units were estimated at the B97-3c-
(toluene)//GFN2-xTB(gas phase) level of theory using a sim-
plified molecular structure of the hoop, namely 2-2 (Figure 1).
In 2-2 the 4-(n-hexyl)-phenyl substituents R on the DBP units
were replaced by H atoms to allow for a transition state
calculation, since the actual hoop was too large for such
a calculation. The calculated values for one DBP rotation
amounted to 8.9 and 10.8 kcalmolÀ1, and they thus mostly
reflect the dihedral strain occurring through rotation of the
DBP units with respect to the neighboring aryl or DBP rings.
Using these estimated values in two possible pathways for
complete racemization of hoop 2-2a to its enantiomer 2-2a*
(see SI, Figure S132), in which all six DBP units have to
rotate, provided estimated values of 16 or 18 kcalmolÀ1 for
the simplified molecular structure of the hoop 2-2. In the
actual hoop 1, including all hexylphenyl substituents R on the
DBP units, both energetic as well as entropic contributions
would significantly increase the rotational (free energy)
barriers for the DBP units. Interestingly, a molecular dynam-
ics simulation of the full system 1a at 400 K at the GFN2-xTB/
GBSA(toluene) level of theory showed that while the hoop is
conformationally highly flexible and its shape changes from
circular to oval, no rotation of a DBP unit through the hoop
occurred. This can be seen in the animation, provided as
additional material online. In comparison, the same calcu-
lation with highest energy conformer 1n (relative energy
9 kcalmolÀ1) led to almost a complete rotation of one DBP
unit to furnish a lower energy conformer (see animation as
additional material).
tween two DBP units are comparably small (VDBP-DBP-av.
=
36.18). The calculated strain energy of 2-1 amounts to
44.8 kcalmolÀ1 (see SI for homodesmotic equation used).
This is larger than in the slightly smaller [2]DBP[12]CPP-
hoop (36.5 kcalmolÀ1) reported by us[24] or in [18]CPP of
similar size (31.7 kcalmolÀ1).[52] The reason may lie in the
larger dihedral angles between the DBP and phenylene units
in 2-1 of 548 due to the ortho-substituents on the latter
compared to large [n]CPPs, where the dihedral angles amount
to 368 on average. This implies a lower conjugation and hence
lower conjugative stabilization in 2-1, responsible for its
relatively high strain energy.
Stereoisomerism in DBP-based nanohoops (+)-1 and (À)-1
We next assessed the stereoisomerism in 1. In theory, 14
diastereomers 1a–1n are possible by rotation of one or
several DBP units through the hoop (A or B conformation,
Figure 4), of which ten are pairs of enantiomers and four are
meso compounds. In the most symmetric (D2-symmetric)
enantiomer 1a, all DBP units are oriented the same way (A
conformation). Its enantiomer (all DBPs in B conformation)
We next turned to optical rotation and electronic circular
dichroism (ECD) measurements to assess the chirality of the
synthesized hoops (+)-1 or (À)-1. The ECD spectra for (+)-
1 or (À)-1 possess a mirror image relationship, confirming
their enantiomeric relationship (Figure 5A). The ECD spec-
trum of (+)-1 shows a positive Cotton effect between 500–
350 nm followed by a negative Cotton effect from 350–300
(Figure 5A). In addition, a negative Cotton effect appears
between 700–500 nm, corresponding to the HOMO–LUMO
transition of the DBP units, which is symmetry-forbidden and
therefore weak (see also below, discussion of the UV/Vis
spectra). The signals correspond well to the UV/Vis absorp-
Figure 4. 14 possible diastereomers 1a–1n of hoop 1 with relative
energies in kcalmolÀ1 (B97-3c//GFN2-xTB; for the pairs of enantio-
mers, one enantiomer each is shown).
10684 www.angewandte.org ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angew. Chem. Int. Ed. 2021, 60, 10680 –10689

Angewandte
Research Articles
Chemie
Figure 6. A) UV/Vis absorption spectra of (+)-1 and reference com-
pound 7 (CH2Cl2) and photos of (++)-1 as powder and in solution;
B) cyclic voltammogram of (+)-1 (0.16 mm in CH2Cl2 with 0.1 m n-
Bu4NPF6, scan rate 0.1 VsÀ1, glassy carbon electrode); C) Selected
orbital energies and D) plots for 2 (B3LYP-D3/def2-TZVP//PBEh-3c).
Figure 5. A) Electronic circular dichroism (ECD) spectra of (+)-1 and
(À)-1 (CHCl3, 208C) with spectrum simulated for enantiomer 1a
(sTDA-xTB level out of 100 snapshots of a GFN2-xTB/GBSA(toluene)
MD-simulation of 100 ps length); B–D) ECD-spectra of (+)-
1 (3.34”10À5 m in PhCl) during (B) heating at a rate of 28CminÀ1, (C)
holding the temperature at 1108C and (D) cooling at a rate of
spectra for all 14 stereoisomers (see SI, Figure S115). Chiral
hoops 1a–1j showed Cotton effects of the same signs at
similar wavelengths, only with different intensities, depending
on the number of DBP units in each orientation (A or B in
Figure 4). 1a with all DBP units oriented the same way had
the highest intensities, followed by 1b and 1c with one unit
rotated, and 1d–1i as well as 1j had the lowest calculated
intensities of the CD-spectroscopic bands. Exemplarily, the
simulated spectrum for 1a is shown in Figure 5A. It
corresponds well in shape to the experimental ECD-spectrum
of (+)-1. Measurement of specific optical rotation for the
DBP hoops gave [a]2D5 values of opposite sign with + 5.48 (c =
28CminÀ1
.
tion spectrum of (+)-1 shown in Figure 6A (an overlay of
both spectra can be found in the SI, Figure S111). The ECD
spectrum of (À)-1 shows the opposite signs for the Cotton
effects at the same wavelengths as in (+)-1 (Figure 5A),
confirming its mirror-symmetric structure. To potentially
assign the ECD-spectrum of (+)-1 to one or several of the
stereoisomers 1a–1n (see Figure 4), we simulated ECD
Angew. Chem. Int. Ed. 2021, 60, 10680 –10689 ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH www.angewandte.org 10685

Angewandte
Research Articles
Chemie
0.44, CHCl3) for (+)-1 and À2.48 (c = 0.42, CHCl3) for (À)-1.
These rotation values are low and at the instrumental
detection limit (see SI), but their opposite sign confirms the
mirror image geometry for both hoops.
several orbitals, as we previously assigned for small molecule
DBP derivatives using TDDFT calculations.[49,57] Electroni-
cally, the DBP units in hoop 1 are intermediate between 2,7-
[49]
and 5,10-arylalkinyl-substituted DBPs,[57] since the bands
ECD-spectra were also measured for linear hexaones
(R,R)3-12 and (S,S)3-12, cyclic dodecaones (R,R)6-14 and
(S,S)6-14 and cyclic dodecaols (R,R)6-(+)-16 and (S,S)6-(À)-
16, displaying strong Cotton effects and mirror image
relationships for each of the respective pairs of enantiomers.
These can be found in the SI. (R,R)6-(+)-16 and (S,S)6-(À)-16
provided specific optical rotation values of [a]2D5 =+ 82.68 (c =
1.03, CHCl3) and À76.88 (c = 0.98, CHCl3), respectively.
These are significantly larger than in (+)-1 or (À)-1 likely
due to the presence of discrete stereocenters in 16. However,
a quantitative evaluation of optical rotation values in differ-
ent molecules is not possible.
between 400–500 nm are relatively large in comparison to the
absorption maximum at 322 nm due to significant conjugation
to the 5,10-aryl substituents. The slight bathochromic shift of
all bands in hoop (+)-1 compared to reference compound 7
indicates a stronger conjugation in the hoop. Most noteworthy
is the lowest energy band, corresponding to the HOMO !
LUMO single excitation and well visible in the inset in
Figure 6A. This transition is forbidden in planar and centro-
symmetric DBP derivatives, where both orbitals are of au
symmetry (Laporteꢀs rule). Since the DBP units in hoop
1 deviate more strongly from planarity than in planar 7, this
shoulder at 500–600 nm had a significantly higher intensity for
the hoop. The optical band gap of (+)-1 amounted to 1.66 eV.
This value is strongly bathochromically shifted compared to
reference compound 7 with 1.87 eV, to [2]DBP[12]CPP hoop
recently reported by us with 1.83 eV[24] as well as to small
molecule DBP derivatives[57] and indicates strong conjugation
around the hoop. Nanohoop (++)-1 showed no fluorescence,
which has been observed for other DBP derivatives.[49,57]
The cyclic voltammogram of (+)-1 demonstrates its
ambipolar electrochemical character due to the DBP units[20]
(Figure 6B). A reversible reduction occurred at a half-wave
potential of E1/2 = À1.71 V, and two quasi-reversible oxida-
tions appeared at E1/2 = 0.63 and 0.80 V (all vs. Fc/Fc+).
Compared to the [2]DBP[12]CPP hoop recently reported by
our group and a 2,5,7,10-tetramesityl-substituted DBP[24] both
reduction and oxidation were facilitated (shifted to higher
respective lower absolute potential) in (+)-1. For reference
compound 7 the first oxidation occurred at a similar potential
as in the hoop (measured in CHCl3 for solubility reasons, see
SI for CV). Based on this and the UV/Vis data, the
HOMO[58,59] and LUMO[60] energies for (+)-1 were estimated
to À5.35 eV and À3.69 eV, respectively.
We performed temperature-dependent ECD spectra to
gain insight into the conformational behavior of hoop (+)-
1 (Figure 5B–D). We chose chlorobenzene as solvent for
these experiments, since it allowed accessing higher temper-
atures. The ECD spectrum of (++)-1 in chlorobenzene at 208C
(Figure 5B, blue line) slightly differs to that in chloroform
(Figure 5A) in that a longer wavelength shoulder around
530 nm is present. To investigate the temperature-depend-
ence of the ECD-signals, we heated the sample from 20 to
1108C (at a rate of 28C per min) and recorded ECD spectra at
regular intervals (Figure 5B). All spectra in Figure 5 are UV/
Vis-corrected to rule out concentration effects. With increas-
ing temperature, the intensities of all bands slightly decreased.
The largest change happened at around 608C, where in
particular the shoulder around 530 nm disappeared. Upon
further heating, the signal intensity further decreased for all
bands. We then held the temperature at 1108C for 18 h, during
which time no further change occurred (Figure 5C). Lastly,
the solution was again cooled to rt, whereby the intensities of
the CD bands slightly increased again (Figure 5D). However,
the shoulder band at 530 nm did not reappear. These results
show that optical activity of (+)-1 is not lost, even after
prolonged heating to 1108C, and complete racemization did
not occur. The temperature-dependent changes we observed
in the ECD spectra of (+)-1 were mostly reversible, with the
exception of the shoulder at 530 nm. This result is in line with
the MD simulations mentioned above, which showed that in
1a at a simulated temperature of 400 K none of the DBP units
rotated. Simulation of the ECD spectrum of 1a at 400 K
confirmed a slightly lower intensity of the bands compared to
300 K (see SI, Figure S116). It may be that higher temper-
atures are required to observe a racemization of the hoop,
however, the CD spectrometer we used only allowed heating
to 1108C as the highest temperature.
The calculated frontier molecular orbitals of 2-1 are
distributed over several DBP units (Figure 6D and SI for
images of other orbitals). From the LUMO up to LUMO + 5
and the HOMO down to HOMO–11, these orbitals lie very
close in energy (Figure 6C, DE ꢀ 0.15 eV). Hence the redox
events visible in Figure 6B could be processes involving more
than one electron, which could explain the higher current for
the reduction and second oxidation compared to the first
oxidation.
NICS (nucleus independent chemical shift) values[61]
provided information on the (anti)aromatic character of the
DBP units in 2-1. NICS(1)iso above/below the five- and six-
membered rings amounted to average values of 4.6 and À6.3,
respectively (calculated with the GIAO method on the
B3LYP/6-31G* level of theory). A comparison with the
NICS(1) values for the unsubstituted DBP of 5.9 for the five-
and À6.2 for the six-membered ring[62] shows that the
antiaromaticity in the central pentalene unit in hoop 2-1 is
slightly reduced while the aromaticity of the six-membered
rings remains unaffected.
Electronic structure of DBP-based nanohoop 1
The optoelectronic properties of hoop 1 are dominated by
the DBP units and reflect its ambipolar character. The UV/
Vis absorption spectra of (+)-1 and reference compound 7
showed several bands, characteristic for substituted DBPs
(Figure 6A).[27,38,49,57] Most constitute transitions involving
10686 www.angewandte.org ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angew. Chem. Int. Ed. 2021, 60, 10680 –10689

Angewandte
Research Articles
Chemie
Complexation of fullerene-C60 by (+)-1
DBP and phenylene units amount to 3.24 Š on average in the
1:1 complex, which is close to double the van-der-Waals
radius of carbon (1.7 Š).
The calculations also furnished association free energies
consisting of the following contributions [Eq. (1)]:
Due to their hoop shape and large p-surface, conjugated
nanohoops are well-suited to bind guest molecules, such as
fullerenes. A strong binding of C60 (diameter 0.71 nm[63]) by
nanohoops has been reported on several instances[18,19] for
hoops with diameters of 1.3–1.5 nm, such as [10]CPP.[64] With
2.5 nm, the diameter of hoop 1 (calculated for 2-1) in
spherical shape significantly exceeds these values, however,
our initial MD simulations indicated the hoop structure to be
somewhat flexible. Indeed, NMR titration experiments[65]
performed in triplicate with C60 in [D8]toluene showed a shift
DG DGRRHO DE DdGsolv
,
1
where DGRRHO is the thermostatistical contribution in the
rigid-rotor-harmonic-oscillator approximation calculated
with GFN-FF,[70] DE is the gas phase association energy and
dGsolv the solvation free energy of each species, the sum of the
latter two terms calculated at the GFN2-xTB level of theory.
The association free energy for the complexation of the first
C60 molecule by (+)-1 of À5.3 kcalmolÀ1 matches very well
with the experimental value of À5.1 kcalmolÀ1, while binding
of the second C60 molecule is slightly stronger in the
calculation (À6.5 kcalmolÀ1) compared to the experiment
(À3.2 kcalmolÀ1).
1
in some of the H NMR-resonances with increasing concen-
tration of C60 (Figure 7A). The spectra indicated a fast
Conclusion
We herein reported the stereoselective synthesis of two
enantiomers of the chiral conjugated nanohoop 1. These
hoops contain six dibenzo[a,e]pentalene (DBP) and four
arylene units. Stereoselectivity was achieved by using bent
and chiral diketone precursors to DBP units in enantiomer-
ically pure form, thereby introducing a new bent “corner”
unit for nanohoop synthesis. Electronic circular dichroism
spectra and MD simulations showed that— inspite of its
conformational flexibility regarding the outer shape— hoop
(+)-1 did not racemize even when heated to 1108C. The
antiaromaticity of the DBP units was reflected in the
ambipolar electrochemical properties of the hoop allowing
for a reversible reduction and two quasi-reversible oxidations.
Due to its large size of 2.5 nm the [6]DBP[4]Ph-hoop could
accommodate up to two fullerene-C60 molecules, shown
through NMR-based binding studies and DFT calculations.
1
Figure 7. A) Selected regions of H NMR spectra during titration of
(+)-1 with C60 (0 to 12 equiv. from bottom to top, [D8]toluene,
300 MHz, 300 K); B) Calculated equilibrium structures of 1ꢁC60
(GFN2-xTB/GBSA (toluene)).
exchange between free and complexed species, as only one set
of signals was visible. To obtain binding constants, we used
hoop (+)-1 and a 1:2 ((+)-1:C60) binding model,[66–68] which
considered the formation of 1:1 and 1:2 complexes. This
model provided a better fit of the experimental data using
nonlinear regression and an online tool[69] than a 1:1 model.
The resulting binding constants were K11 = (5.4 Æ 0.7) ”
103 mÀ1 and K12 = (1.1 Æ 1.0) ” 102 mÀ1 for the complex forma-
tion between hoop (+)-1 and one respective two C60
molecules. These values lie three to four orders of magnitude
below association constants with C60 reported for smaller
nanohoops, such as [10]CPP, due to the large size of (+)-1. In
the statistical case a ratio of 4:1 between K11 and K12 would be
expected;[66] hence our results indicate an anti-cooperative
situation for the binding of the second fullerene molecule.
This is somewhat surprising, since both C60 molecules are well
accommodated within the hoop, and only a small distortion of
the latter is required (see Figure 7B).
Acknowledgements
M.H. thanks Dr. D. Kratzert for X-ray structure solution. This
research was funded by the Deutsche Forschungsgemein-
schaft (DFG, German Research Foundation)— project num-
bers 230408635, 434040413, INST 40/467-1 FUGG and INST
39/1081-1 FUGG) and the state of Baden-Württemberg
through bwHPC. Open access funding enabled and organized
by Projekt DEAL.
Conflict of interest
The authors declare no conflict of interest.
DFT calculations provided insight into the molecular
structures of the 1:1- and 1:2-complexes (Figure 7B). Struc-
tures were optimized at the GFN2-xTB level of theory, as
described above. The nearest distances between C60 and the
Keywords: antiaromaticity ·chiral macrocycles ·
chiral resolution ·cycloparaphenylenes ·fullerenes
Angew. Chem. Int. Ed. 2021, 60, 10680 –10689 ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH www.angewandte.org 10687

Angewandte
Research Articles
Chemie
[34] Y. Segawa, M. Kuwayama, Y. Hijikata, M. Fushimi, T. Nishihara,
J. Pirillo, J. Shirasaki, N. Kubota, K. Itami, Science 2019, 365,
272 –276.
[1] R. Gleiter, G. Haberhauer, Aromaticity and Other Conjugation
Effects, Wiley-VCH, Weinheim, 2012.
[2] A. Schrçder, H.-B. Mekelburger, F. Vçgtle, in Cyclophanes (Ed.:
E. Weber), Springer-Verlag, Berlin, Heidelberg, 1994, pp. 179 –
201.
[35] K. Senthilkumar, M. Kondratowicz, T. Lis, P. J. Chmielewski, J.
´
Cybinska, J. L. Zafra, J. Casado, T. Vives, J. Crassous, L.
´
Favereau, M. Ste˛pien, J. Am. Chem. Soc. 2019, 141, 7421 –7427.
[3] K. Tahara, Y. Tobe, Chem. Rev. 2006, 106, 5274 –5290.
[4] R. Jasti, J. Bhattacharjee, J. B. Neaton, C. R. Bertozzi, J. Am.
Chem. Soc. 2008, 130, 17646 –17647.
[5] H. Takaba, H. Omachi, Y. Yamamoto, J. Bouffard, K. Itami,
Angew. Chem. Int. Ed. 2009, 48, 6112 –6116; Angew. Chem.
2009, 121, 6228 –6232.
[6] S. Yamago, Y. Watanabe, T. Iwamoto, Angew. Chem. Int. Ed.
2010, 49, 757 –759; Angew. Chem. 2010, 122, 769 –771.
[7] S. E. Lewis, Chem. Soc. Rev. 2015, 44, 2221 –2304.
[8] Y. Segawa, A. Yagi, K. Itami, Phys. Sci. Rev. 2017, 2, 1 –24.
[9] E. S. Hirst, R. Jasti, J. Org. Chem. 2012, 77, 10473 –10478.
[10] M. R. Golder, R. Jasti, Acc. Chem. Res. 2015, 48, 557 –566.
[36] S. Nishigaki, Y. Shibata, A. Nakajima, H. Okajima, Y. Masu-
moto, T. Osawa, A. Muranaka, H. Sugiyama, A. Horikawa, H.
Uekusa, H. Koshino, M. Uchiyama, A. Sakamoto, K. Tanaka, J.
Am. Chem. Soc. 2019, 141, 14955 –14960.
[37] J. Nogami, Y. Tanaka, H. Sugiyama, H. Uekusa, A. Muranaka,
M. Uchiyama, K. Tanaka, J. Am. Chem. Soc. 2020, 142, 9834 –
9842.
[38] M. Hermann, D. Wassy, D. Kratzert, B. Esser, Chem. Eur. J. 2018,
24, 7374 –7387.
[39] C. Huang, Y. Huang, N. G. Akhmedov, B. V. Popp, J. L. Petersen,
K. K. Wang, Org. Lett. 2014, 16, 2672 –2675.
[40] Z. Sun, N. Miyamoto, S. Sato, H. Tokuyama, H. Isobe, Chem.
Asian J. 2017, 12, 271 –275.
´
[11] M. A. Majewski, M. Ste˛pien, Angew. Chem. Int. Ed. 2019, 58,
86 –116; Angew. Chem. 2019, 131, 90 –122.
[41] Q. Huang, G. Zhuang, H. Jia, M. Qian, S. Cui, S. Yang, P. Du,
Angew. Chem. Int. Ed. 2019, 58, 6244 –6249; Angew. Chem.
2019, 131, 6310 –6315.
[42] H. Omachi, S. Matsuura, Y. Segawa, K. Itami, Angew. Chem. Int.
Ed. 2010, 49, 10202 –10205; Angew. Chem. 2010, 122, 10400 –
10403.
[12] M. Hermann, D. Wassy, B. Esser, Angew. Chem. Int. Ed. 2021,
https://doi.org/10.1002/anie.202007024; Angew. Chem. 2021,
https://doi.org/10.1002/ange.202007024.
[13] D. Wu, W. Cheng, X. Ban, J. Xia, Asian J. Org. Chem. 2018, 7,
2161 –2181.
[43] R. B. Woodward, T. P. Kohman, G. C. Harris, J. Am. Chem. Soc.
1941, 63, 120 –124.
[14] A.-F. Tran-Van, H. A. Wegner, Beilstein J. Nanotechnol. 2014, 5,
1320 –1333.
[44] C. R. Johnson, J. R. Zeller, J. Am. Chem. Soc. 1982, 104, 4021 –
4023.
[45] C. R. Johnson, J. R. Zeller, Tetrahedron 1984, 40, 1225 –1233.
[46] A. Tota, M. Zenzola, S. J. Chawner, S. S. John-Campbell, C.
Carlucci, G. Romanazzi, L. Degennaro, J. A. Bull, R. Luisi,
Chem. Commun. 2017, 53, 348 –351.
[15] E. J. Leonhardt, R. Jasti, Nat. Rev. Chem. 2019, 3, 672 –686.
[16] R. Jasti, C. R. Bertozzi, Chem. Phys. Lett. 2010, 494, 1 –7.
[17] Y. Segawa, A. Yagi, K. Matsui, K. Itami, Angew. Chem. Int. Ed.
2016, 55, 5136 –5158; Angew. Chem. 2016, 128, 5222 –5245.
[18] Y. Xu, M. von Delius, Angew. Chem. Int. Ed. 2020, 59, 559 –573;
Angew. Chem. 2020, 132, 567 –582.
[47] J. Brandt, H.-J. Gais, Tetrahedron: Asymmetry 1997, 8, 909 –912.
[48] C. S. Shiner, A. H. Berks, J. Org. Chem. 1988, 53, 5542 –5545.
[49] J. Wilbuer, D. C. Grenz, G. Schnakenburg, B. Esser, Org. Chem.
Front. 2017, 4, 658 –663.
[50] E. R. Darzi, B. M. White, L. K. Loventhal, L. N. Zakharov, R.
Jasti, J. Am. Chem. Soc. 2017, 139, 3106 –3114.
[19] D. Lu, Q. Huang, S. Wang, J. Wang, P. Huang, P. Du, Front.
Chem. 2019, 7, 668.
[20] M. Hermann, T. Bçttcher, M. Schorpp, S. Richert, D. Wassy, I.
Krossing, B. Esser, Chem. Eur. J. 2021, 27, 4964 –4970.
[21] H. Hopf, Angew. Chem. Int. Ed. 2013, 52, 12224 –12226; Angew.
Chem. 2013, 125, 12446 –12449.
[51] S. Grimme, J. G. Brandenburg, C. Bannwarth, A. Hansen, J.
Chem. Phys. 2015, 143, 054107.
[52] Y. Segawa, H. Omachi, K. Itami, Org. Lett. 2010, 12, 2262 –2265.
[53] E. Caldeweyher, S. Ehlert, A. Hansen, H. Neugebauer, S.
Spicher, C. Bannwarth, S. Grimme, J. Chem. Phys. 2019, 150,
154122.
[22] M. Schmidt, D. Wassy, M. Hermann, M. T. Gonzµlez, N. Agräit,
L. A. Zotti, B. Esser, E. Leary, Chem. Commun. 2021, 57, 745 –
748.
[23] S. Hitosugi, S. Sato, T. Matsuno, T. Koretsune, R. Arita, H. Isobe,
Angew. Chem. Int. Ed. 2017, 56, 9106 –9110; Angew. Chem.
2017, 129, 9234 –9238.
[54] J. Zhu, Y. Shi, H. Liu, J. Phys. Chem. B 2002, 106, 4844 –4853.
[55] J. G. Brandenburg, C. Bannwarth, A. Hansen, S. Grimme, J.
Chem. Phys. 2018, 148, 064104.
[24] D. Wassy, M. Pfeifer, B. Esser, J. Org. Chem. 2020, 85, 34 –43.
[25] B. Esser, Phys. Chem. Chem. Phys. 2015, 17, 7366 –7372.
[26] T. Kawase, T. Fujiwara, C. Kitamura, A. Konishi, Y. Hirao, K.
Matsumoto, H. Kurata, T. Kubo, S. Shinamura, H. Mori, E.
[56] A. Klamt, J. Phys. Chem. 1995, 99, 2224 –2235.
Miyazaki, K. Takimiya, Angew. Chem. Int. Ed. 2010, 49, 7728 – [57] D. C. Grenz, M. Schmidt, D. Kratzert, B. Esser, J. Org. Chem.
7732; Angew. Chem. 2010, 122, 7894 –7898.
[27] M. Hermann, R. Wu, D. C. Grenz, D. Kratzert, H. Li, B. Esser, J.
Mater. Chem. C 2018, 6, 5420 –5426.
2018, 83, 656 –663.
[58] From the onset of the first oxidation peak, assuming an
ionization energy of 4.8 eV for ferrocene.
[28] K. Sekine, J. Schulmeister, F. Paulus, K. P. Goetz, F. Rominger,
M. Rudolph, J. Zaumseil, A. S. K. Hashmi, Chem. Eur. J. 2019,
25, 216 –220.
[59] B. W. D’Andrade, S. Datta, S. R. Forrest, P. Djurovich, E.
Polikarpov, M. E. Thompson, Org. Electron. 2005, 6, 11 –20.
[60] ELUMO = EHOMO + Eg,Opt
.
[29] P. Sarkar, Z. Sun, T. Tokuhira, M. Kotani, S. Sato, H. Isobe, ACS
Cent. Sci. 2016, 2, 740 –747.
[61] P. v. R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao, N. J. R.
van E. Hommes, J. Am. Chem. Soc. 1996, 118, 6317 –6318.
[62] T. Kawase, A. Konishi, Y. Hirao, K. Matsumoto, H. Kurata, T.
Kubo, Chem. Eur. J. 2009, 15, 2653 –2661.
[63] R. D. Johnson, D. S. Bethune, C. S. Yannoni, Acc. Chem. Res.
1992, 25, 169 –175.
[64] T. Iwamoto, Y. Watanabe, T. Sadahiro, T. Haino, S. Yamago,
Angew. Chem. Int. Ed. 2011, 50, 8342 –8344; Angew. Chem.
2011, 123, 8492 –8494.
[30] J. Wang, G. Zhuang, M. Chen, D. Lu, Z. Li, Q. Huang, H. Jia, S.
Cui, X. Shao, S. Yang, P. Du, Angew. Chem. Int. Ed. 2020, 59,
1619 –1626; Angew. Chem. 2020, 132, 1636 –1643.
[31] S. Hitosugi, W. Nakanishi, T. Yamasaki, H. Isobe, Nat. Commun.
2011, 2, 492.
[32] S. Hitosugi, T. Yamasaki, H. Isobe, J. Am. Chem. Soc. 2012, 134,
12442 –12445.
[33] T. Matsuno, S. Kamata, S. Hitosugi, H. Isobe, Chem. Sci. 2013, 4,
3179 –3183.
[65] D. Brynn Hibbert, P. Thordarson, Chem. Commun. 2016, 52,
12792 –12805.
10688 www.angewandte.org ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angew. Chem. Int. Ed. 2021, 60, 10680 –10689

Angewandte
Research Articles
Chemie
[66] P. Thordarson, Chem. Soc. Rev. 2011, 40, 1305 –1323.
[67] R. Hahn, F. Bohle, W. Fang, A. Walther, S. Grimme, B. Esser, J.
Am. Chem. Soc. 2018, 140, 17932 –17944.
[70] S. Spicher, S. Grimme, Angew. Chem. Int. Ed. 2020, 59, 15665 –
15673; Angew. Chem. 2020, 132, 15795 –15803.
[68] R. Hahn, F. Bohle, S. Kotte, T. J. Keller, S.-S. Jester, A. Hansen,
S. Grimme, B. Esser, Chem. Sci. 2018, 9, 3477 –3483.
[69] www.supramolecular.org.
Manuscript received: December 21, 2020
Revised manuscript received: February 16, 2021
Accepted manuscript online: February 17, 2021
Version of record online: March 23, 2021
Angew. Chem. Int. Ed. 2021, 60, 10680 –10689 ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH www.angewandte.org 10689