Towards the Highly Efficient Synthesis and Selective Methylation of C(sp3)-Bridged [6]Cycloparaphenylenes from Fluoren[3]arenes

Article abstract of DOI:10.1002/anie.202102701

An approach to the highly efficient synthesis of C(sp3)-bridged [6]cycloparaphenylenes (C[6]CPPs) from fluoren[3]arenes (F[3]As) was developed. Consequently, F[3]As as a new kind of macrocyclic arenes were synthesized. Followed by the demethylation, triflation and intramolecular aryl–aryl coupling reactions, C[6]CPPs were then conveniently obtained. Interestingly, C[6]CPPs could be selectively methylated to produce their fully outer-methyl-substituted derivatives. The crystal structures showed the hydroxyl-substituted F[3]As had bowl-shaped conformations, and the C[6]CPPs exhibited rigid belt-shaped structures with deep cavities. Moreover, C[6]CPPs exhibited high HOMO energies and narrow energy gaps. An unclosed belt was further obtained, and it not only showed a similar narrow energy gap to those of the aromatic belts, but also displayed strong fluorescence property, which can play a vital role in the design and synthesis of new aromatic belts.

Full text of DOI:10.1002/anie.202102701

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Macrocycles Hot Paper
Towards the Highly Efficient Synthesis and Selective Methylation of
3
C(sp )-Bridged [6]Cycloparaphenylenes from Fluoren[3]arenes
Xu-Sheng Du, Da-Wei Zhang, Yan Guo, Jing Li, Ying Han, and Chuan-Feng Chen*
Abstract: An approach to the highly efficient synthesis of
C(sp3)-bridged [6]cycloparaphenylenes (C[6]CPPs) from
fluoren[3]arenes (F[3]As) was developed. Consequently,
F[3]As as a new kind of macrocyclic arenes were synthesized.
Followed by the demethylation, triflation and intramolecular
aryl–aryl coupling reactions, C[6]CPPs were then convenient-
ly obtained. Interestingly, C[6]CPPs could be selectively
methylated to produce their fully outer-methyl-substituted
derivatives. The crystal structures showed the hydroxyl-sub-
stituted F[3]As had bowl-shaped conformations, and the
C[6]CPPs exhibited rigid belt-shaped structures with deep
cavities. Moreover, C[6]CPPs exhibited high HOMO energies
and narrow energy gaps. An unclosed belt was further
obtained, and it not only showed a similar narrow energy
gap to those of the aromatic belts, but also displayed strong
fluorescence property, which can play a vital role in the design
and synthesis of new aromatic belts.
a belt-forming step and strain-inducing step. It was commonly
known that the macrocyclization was in fact the most
predominant step in the synthesis of aromatic belts.
[
15,16]
Along with the development of macrocyclic chemistry, the
strategy to synthesize the aromatic belts from the most
common macrocycles opens the way to rational design and
synthesis of such kinds of interesting structures. Recently,
resorcin[4]arenes and their derivatives were explored to
construct zig-zag hydrocarbon belts by Wangꢀs group, repre-
[
17–21]
senting a breakthrough in this area.
Very recently, Itami
[
22]
et al. reported a fully alternant methylene-bridged [6]cyclo-
paraphenylene (M[6]CPP), which was cleverly designed and
synthesized in about 10% total yield starting from ethyl-
substituted pillar[6]arene derivative. The M[6]CPP represents
a new kind of aromatic belt as the belt segment of a haeckelite
[
23]
nanotube, and also endows itself promising applications in
[24]
nanoelectronics and photonics.
However, synthesis of
M[6]CPP from the pillar[6]arene was still in a low efficiency
because of up to 12 multiple of reactive sites during the
intramolecular aryl–aryl coupling to form six new CꢀC bonds,
Introduction
[
1]
Aromatic belts as sections of nanotubes, belonging to
inner-side vertical flip of the OTf groups to adapt their
a unique class of macrocycles with radially oriented p
reactivities, and the originally poor separated yield of pillar-
[
2]
[25–32]
orbitals, have attracted an increasing interest in recent
years owing to their aesthetic structures, specific physical
properties, potential applications in supramolecular chemis-
try and materials science. More importantly, they were of the
possibilities to act as seeds for rationally chemical synthesis of
[6]arene itself. Although various macrocyclic arenes
have
hitherto been reported, the macrocycles that favor to the
formation of aromatic belts are very limited. What is more,
the synthesis of aromatic belts with conjugated structures
from the macrocyclic arenes still remains a considerable
challenge.
[
3–7]
single walled carbon nanotubes.
The chemical synthesis of
[
8]
aromatic belts has been a long sought-after target. Up to
date, some successful examples for the synthetic segments of
Herein, we first report a new kind of macrocyclic arenes,
F[3]As, which could be conveniently synthesized in 48–67%
yield by BF ·OEt -catalyzed condensation between 2,7-
[9]
[10,11]
[12–14]
arm-chaired, chiral
and zig-zag
carbon nanotubes
3
2
have been reported. Nonetheless, the design and construction
of aromatic belts are still a hot topic and represent a challeng-
ing synthetic target.
Till now, much more attentions have been paid to
developing the strategies of highly efficient fabrication of
aromatic belts. Traditionally, the construction of an aromatic
belt mainly experienced three steps: a macrocyclization step,
alkoxyl-substituted fluorenes and paraformaldehyde. By the
demethylation and triflation of F[3]As, and then followed by
the intramolecular aryl–aryl coupling reactions (Scheme 1),
a new and powerful approach to the highly efficient synthesis
of C[6]CPPs was developed. Interestingly, the treatment of
C[6]CPPs with iodomethane in the presence of t-BuOK
selectively produced the fully outer-methyl-substituted de-
rivatives in almost quantitative yields. The crystal structures
showed the fully hydroxyl-substituted F[3]As possessed bowl-
[
*] X.-S. Du, D.-W. Zhang, Y. Guo, J. Li, Dr. Y. Han, Prof. Dr. C.-F. Chen
Beijing National Laboratory for Molecular Sciences, CAS Key
Laboratory of Molecular Recognition and Function, Institute of
Chemistry, Chinese Academy of Sciences
Beijing 100190 (China)
E-mail: cchen@iccas.ac.cn
X.-S. Du, D.-W. Zhang, Y. Guo, J. Li, Prof. Dr. C.-F. Chen
University of Chinese Academy of Sciences
Beijing 100049 (China)
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.202102701.
Scheme 1. Design for the synthesis of C[6]CPPs from F[3]As.
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shaped conformations, and the C[6]CPPs showed rigid belt-
shaped structures and deep cavities. The experimental results
and theoretical calculations indicated that all of the C[6]CPPs
exhibited high HOMO energies and narrow HOMO–LUMO
gaps. Moreover, an unclosed belt UB was obtained, and it not
only showed the similar narrow HOMO–LUMO energy gap
to those of the aromatic belts, but also displayed a strong
fluorescence property.
[39]
Figure 1. Crystal structures of a) F[3]A3 and b) F[3]A5.
Results and Discussion
dipropyl-2,7-dimethoxylfluorene subunits in F[3]A3 posi-
tioned at one side, while the other fluorene subunit took the
reversed conformation owing to the steric effect of the
adjacent methoxyl groups and the propyl groups. For F[3]A5,
it adopted the similar conformation to that F[3]A3 in the solid
state (Figure 1b). Moreover, there existed strong CꢀH···O
Synthesis and Structures of F[3]As
Synthesis of F[3]As was depicted in Scheme 2. Under the
optimized conditions (Table S1), F[3]A1 could be conven-
iently synthesized in 64% yield by one-pot condensation
between 2,7-dimethoxy-9H-fluorene and paraformaldehyde
in dichloromethane (DCM) at room temperature for 1.0 hour
hydrogen bonds of intermolecular F[3]A5 with the distance at
2.655 ꢁ, but no efficient p-p interaction was observed (Fig-
ure S77).
1
in the presence of BF ·OEt . The H NMR spectrum of
3
2
F[3]A1 showed two single peaks for the aromatic protons and
three single peaks for the methyl, the methylene groups in the
fluorene subunit and the bridged methylene groups, indicat-
ing its high symmetric structure and low inversion barrier in
solution. The MALDI-TOF HRMS at 714.2973 further
confirmed the chemical structure of F[3]A1 (Figure S12).
According to the similar synthetic approach to that of
F[3]A1, we could also easily obtain F[3]A2 and F[3]A3 in
Synthesis and Structures of C[6]CPPs
The demethylation of F[3]As was then performed. As
shown in Scheme 3, 2,7-OH-F[3]As could be easily obtained
in high yields by treating F[3]As with boron tribromide in
dichloromethane at 08C. For the demethylation of F[3]A4,
two isomeric products as a mixture were obtained in high total
yield as well, but they could not be separated by column
chromatography. It was found that 2,7-OH-F[3]As have good
6
7% and 54% yield starting from 2,7-dimethoxy-9,9-dime-
thylfluorene and 9,9-dipropyl-2,7-dimethoxylfluorene, re-
spectively. If 2,7-dimethoxy-9-propyl-9H-fluorene was used
as the precursor, F[3]A4 as a mixture of two isomers was
obtained in 61% total yield. Compared to F[3]A1 with the
poor solubility in common solvents including chloroform and
dichloromethane, F[3]A2 showed a poor solubility as well,
but F[3]A3 and F[3]A4 had very good solubilities in common
solvents due to introducing the propyl groups. Under the
same reaction conditions, the F[3]A5 was also obtained in
1
solubilities in acetone and DMSO. The H NMR spectral
analyses of 2,7-OH-F[3]As indicated the fully hydroxyl-
substituted macrocycles adopted an averaged C3h symmetry
in solution due to a fast conformational interconversion. By
slow diffusion of CH Cl into the acetone solution of 2,7-OH-
2
2
F[3]A2, we also obtained the single crystal of 2,7-OH-
4
8% yield. As expected, F[3]A5 showed markedly increased
solubility in chloroform compared with F[3]A1. The variable
1
temperature H NMR spectra of F[3]A5 showed that a free
rotation of the fluorene subunit in solution occurred due to
the low inversion barrier (Figure S25).
We obtained the crystal structures of macrocycles F[3]A3
and F[3]A5. As shown in Figure 1a, it was found that two 9,9-
Scheme 2. Synthesis of F[3]As.
Angew. Chem. Int. Ed. 2021, 60, 2 – 10
Scheme 3. Synthetic routes of C[6]CPPs.
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adopted an unsymmetric conformation (Figure S82), in which
the two fluorene subunits were positioned in one side while
another one was at the other side. This case was very similar to
that of F[3]A5.
With 2,7-OTf-F[3]As in hand, we then performed the
synthesis of the aromatic belts. Consequently, by the treat-
ment of 2,7-OTf-F[3]A1 with 6.0 equiv of Ni(cod) /2,2’-
2
bipyridyl (bpy) at 808C for 2 h, it was found that the aryl–
aryl coupling reaction could efficiently occur to produce the
aromatic belt C[6]CPP1 in 90% yield (Scheme 3), which was
[
22]
markedly higher than that from pillar[6]arene probably due
to the preorganized structure of the macrocyclic precursor,
the free outer rotation of the OTf groups, and the formation
of only three new CꢀC bonds in our system.
Figure 2. Crystal structures. a) Top view and b) side view of 2,7-OH-
F[3]A2; c) top view and d) side view of i,i,i-2,7-OH-F[3]A4; and e) top
view and (f) side view of o,o,i-2,7-OH-F[3]A4. Blue lines represent the
intramolecular hydrogen bonding interactions. Hydrogen atoms not
Under the same conditions, C[6]CPP2 could also be
separated as a red solid starting from 2,7-OTf-F[3]A2
[
39]
involved in the non-covalent interactions were omitted for clarity.
(Table 1, entry 1), but the yield was only 24%. Meantime,
F[3]A2. As shown in Figure 2a, the crystal structure showed
that three of the fluorene subunits in 2,7-OH-F[3]A2
positioned at the same side to form a bowl shaped structure,
in which intramolecular hydrogen bonding interactions be-
tween the adjacent hydroxyl groups with an average distance
of 1.97 ꢁ played a vital role (Figure 2a). The upper and lower
rim of the bowl had an approximately diameter of 10.1 ꢁ and
Table 1: The Optimization of aryl–aryl coupling reaction from 2,7-OTf-
F[3]A2.
Entry
T [8C]
t [h]
Yield [%] of C[6]CPP2/UB
1
2
3
4
5
80
100
80
80
80
2
2
4
8
12
24/36
29/0
47/19
62/0
5
.4 ꢁ, and the cavity depth was measured about 4.5 ꢁ.
60/0
Moreover, it was also found that acetone and H O molecules
2
existed in the crystal structure, and the intermolecular
hydrogen bonds between 2,7-OH-F[3]A2 and the solvent
molecules played an important role in formation of the stable
bowl shaped structure as well (Figure S79).
we obtained one intermediate containing two triflate groups,
named an unclosed belt (UB), in 36% yield as well (Fig-
ure 5a). Interestingly, we found that UB could produce the
aromatic belt C[6]CPP2 in 98% yield in the presence of
By slow evaporation of 2,7-OH-F[3]A4 in CH Cl and
2
2
acetone mixed solution, we obtained two different crystal
structures for the isomers. In one crystal structure, three
propyl groups were all positioned inside of the cavity, named
i,i,i-2,7-OH-F[3]A4 (Figure 2c and d). The other crystal
structure showed that the two propyl groups positioned
outside of the cavity while the other one was inside of the
cavity, which was named as o,o,i-2,7-OH-F[3]A4 (Figure 2e
and f). Similar to that of 2,7-OH-F[3]A2, both i,i,i-2,7-OH-
F[3]A4 and o,o,i-2,7-OH-F[3]A4 adopted the bowl shaped
structures in the solid state (Figure 2), in which the intra-
molecular hydrogen bonding interactions between the adja-
cent hydroxyl groups played a predominant role. These
macrocycles with specific structures will be used as a new kind
of promising synthetic hosts for wide applications in supra-
molecular chemistry.
Ni(cod) /bpy, which indicated that the yield of C[6]CPP2
2
could be improved by increasing the reaction temperature or
prolonging the reaction time. Consequently, we optimized the
aryl–aryl coupling reaction and the results were summarized
in Table 1. It was found that if the temperature rose from
808C to 1008C, C[6]CPP2 was only separated in 29% yield
(Table 1, entry 2). So, we still carried out the reaction at 808C,
but prolonged the reaction time up to 4 hours, the yield of
C[6]CPP2 was markedly improved while the yield of UB was
decreased (Table 1, entry 3). When the reaction time was
prolonged 8 hours, we found that C[6]CPP2 could be
obtained in 62% yield, while no UB was separated (Table 1,
entry 4). If we continued to prolong the reaction time, no
increase of the yield was observed (Table 1, entry 5). In the
case of 2,7-OTf-F[3]A3, no aromatic belt C[6]CPP3 was
obtained even we prolonged the reaction time for more than
24 hours, which was probably resulted from the prominent
steric hindrance of the propyl groups. Both C[6]CPP1 and
C[6]CPP2 showed good solubility in common solvents, such
as chloroform and dichloromethane. The chemical structures
As shown in Scheme 3, 2,7-fluoren[3]arene triflates (2,7-
OTf-F[3]As) were easily prepared in almost quantitative
yields by the treatment of 2,7-OH-F[3]As with trifluoroacetic
anhydride in CH Cl in the presence of 4-dimethylaminopyr-
2
2
1
13
idine (DMAP). The H NMR and C NMR spectra of 2,7-
OTf-F[3]A1 (Figure S39,40) and 2,7-OTf-F[3]A2 (Fig-
ure S43,44) showed only one set of signals, which were similar
to those of their precursors. However, for 2,7-OTf-F[3]A3
1
of the aromatic belts were confirmed by their H NMR,
1
3
C NMR and HRMS spectra (see Supporting Information for
details).
In the H NMR spectrum of C[6]CPP1 in CD Cl , the
1
(
Figure S47,48), its NMR spectra displayed more than one set
2
2
of signals, which could be attributed to the increased steric
hinderance of the triflate groups in 2,7-OTf-F[3]A3. The X-
ray crystal structure of 2,7-OTf-F[3]A2 showed that it
methylene groups showed broad signals for the aliphatic
protons which was hard to be distinguished. However, it was
surprisingly found that if a mixture solvents of C D and
6
6
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1
CD Cl (4:1, v/v) was used, a distinct split H NMR spectrum
an average CꢀC single bond that connected the phenylene
2
2
of C[6]CPP1 was observed (Figure S51), in which one singlet
at 7.73 ppm belonged to the aromatic proton, while the inner
and outer protons of the methylene groups showed two
doublets at 4.15 and 3.83 ppm, respectively. This result also
indicated its high D3d symmetric structure. Moreover, the red
crystal of C[6]CPP1 could be obtained by slow diffusion of
units of 1.478 ꢁ. Comparatively, both of the diameter and the
average CꢀC single bond length were smaller than those of
[33,34]
3
[6]CPP,
demonstrating that the C(sp )-bridged [6]CPP
possessed a condensed belt structure. Moreover, C[6]CPP2
with the inner substituted methyl groups showed both a deep-
er cavity (5.200 ꢁ) than that of C[6]CPP1 and a shrunken
upper rim. These structural properties made the encapsulated
n-hexane difficult to thread the cavity (Figure S85). This clear
observation was in agreement with the result from 2D
NOESY analysis, in which CH Cl positioned at the non-
methanol into a CH Cl2 solution of C[6]CPP1 at room
2
temperature. The crystal structure showed C[6]CPP1 had
a cylindrical shape-like structure with an average diameter
and cavity depth 7.766 ꢁ and 3.971 ꢁ, respectively, and an
average CꢀC single bond that connected the phenylene units
2
2
methylated side.
was 1.475 ꢁ, which were all consistent with those of the
The optimized structure of UB was first calculated by
using B3LYP/6-31G(d) level of theory, which showed its fish-
like and symmetrical structure (Figure 4b). As shown in
[22]
Itamiꢀs report.
Since C[6]CPP2 showed good solubility in benzene, its
H NMR spectrum in C D was then performed. As shown in
1
1
Figure 4c, UB exhibited a comparatively complex H NMR
6
6
Figure 3, with the aid of 2D NOESY NMR analysis, two
spectrum in CD Cl . According to the 2D NOESY spectrum
2
2
1
2
singlets H and H at 7.71 ppm and 7.75 ppm were assigned to
the aromatic protons close to -C(CH ) and -CH , respective-
(Figure 4d), the NOE signals between aromatic protons and
the inner protons of the methylene and methyl groups
enabled us to give a full assignment of the six singlets for
3
2
2
1
13
ly. The H- C HSQC spectrum further distinguished the inner
and outer protons (H and H respectively) of the methylene
4
i
4o
1
13
the aromatic protons. With the aid of H- C HSQC spectrum
(Figure 4e), H7/H8 and H9/H10 could be easily distinguished
to the outer and inner protons of the two methylene groups,
respectively. Moreover, two singlets H13 and H14 at 1.82 ppm
and 1.18 ppm were assigned to the methyl protons of one
C(CH ) , while two singlets H11 and H12 at 1.38 ppm and
group (Figure 3e). Moreover, it was also found that CH Cl2
2
could be encapsulated in the cavity of C[6]CPP2 with a high-
field shift of the proton signal from 4.27 to 3.48 ppm, and the
2
crossing signal between the proton of CH Cl and H implied
2
2
it might position at the non-methylated side.
3
2
By slow diffusion of n-hexane into a CH Cl solution of
1.14 ppm was assigned to the methyl protons of another
2
2
C[6]CPP2 at room temperature, we also obtained the red
single crystals of C[6]CPP2. As shown in Figure 3b. the
crystal structure showed C[6]CPP2 had a cylindrical shape-
like structure as well with an average diameter of 7.772 ꢁ and
C(CH ) . Interestingly, it was further found that UB showed
3
2
Figure 3. a) Chemical structure of C[6]CPP2. b) Crystal structure of
1
C[6]CPP2 from top view and side view. c) H NMR spectrum
Figure 4. a) Chemical structure of UB. b) DFT optimized structure of
1
(
400 MHz, 298 K) of C[6]CPP2 in C D . d) Partial 2D NOESY spectrum,
and e) partial H- C HSQC spectrum of C[6]CPP2 in C D .
UB. c) H NMR spectrum (500 MHz, CD Cl , 298 K) of UB. d) Partial
6
6
2
2
1
13
[39]
1
13
[39]
2D NOESY spectrum of UB. e) Partial H- C HSQC spectrum of UB.
6
6
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strong fluorescence under 365 nm UV lamp probably due to
the two strong electron-deficient triflate groups, which was
obviously different from those of the aromatic belts C-
[
6]CPPs with fully carbon conjugated structures. Molecule
UB could not only be very helpful for us to understand the
formation process of the aromatic belt, but also be of great
meaningfulness to further design new systems with specific
structure and property.
Selective Methylation of C[6]CPPs
Post modification of aromatic belts is vital for their further
functionalization and application researches. Because of the
activation of conjugated rigid structure, the methylene groups
of C[6]CPP1 possessed higher reactivity than the monomer
2
,7-dimethoxy-9H-fluorene. Consequently, we tried the meth-
ylation of C[6]CPP1 with CH I at 08C in the presence of
3
excess t-BuOK. As shown in Scheme 4, it was interestingly
1
Figure 5. a) Chemical structure and the H NMR spectrum (500 MHz,
C D :CD Cl =4:1, 298 K) of C[6]CPP4. b) Chemical structure and the
6
6
2
2
1
H NMR spectrum (500 MHz, C D , 298 K) of C[6]CPP5. c) Top view
6
6
and side view of crystal structure of C[6]CPP4. d) Top view and side
view of crystal structure of C[6]CPP5. Hydrogen atoms are omitted for
clarity.
[
39]
Scheme 4. Selective post-methylation of C[6]CPP1-2.
The red crystals of C[6]CPP4 were obtained by slow
diffusion of methanol into a mixed solution of C[6]CPP4 at
room temperature (Figure S87). The crystal structure of
C[6]CPP4 showed an obvious elliptic deformation and
cylindrical-like cavity with the average diameter and cavity
depth of 7.769 ꢁ and 5.166 ꢁ, respectively (Figure 5c). The
six methyl groups were all positioned outside of the cavity.
The average CꢀC single bond that connected the phenylene
found that C[6]CPP4 in which the six methyl groups were all
positioned outside was obtained in 98% yield as the only
product. No reactions occurred in the presence of t-BuONa or
NaH. Similarly, the treatment of C[6]CPP2 with CH I in the
3
presence of excess t-BuOK also selectively produced C-
[
6]CPP5 in 96% yield, in which the three methyl substituents
were all positioned outside. The selective methylation of
C[6]CPPs might be attributed to the big steric hindrance
effect that impeded the inner protons to take place the
methylation. The fully outer-methyl-substituted C[6]CPPs
could also make them as a new kind of useful synthetic hosts
for wide potential applications in supramolecular chemistry
and materials science.
units was 1.491 ꢁ. Moreover, aromatic belt C[6]CPP4 could
pack into nanotube structures with CH Cl and H O mole-
2
2
2
cules inside its cavity. Interestingly, six adjacent molecules of
C[6]CPP4 could also self-assemble into a six membered ring-
2
like structure by virtue of the intermolecular C(sp )-H···p
interaction with an average distance of 2.878 ꢁ, which further
packed into tube-like architectures with benzene and CHCl3
molecules encapsulated inside the cavity (Figure S88). Be-
1
The H NMR spectrum of C[6]CPP4 in C D :CD Cl (4:1,
6
6
2
2
v/v) showed one singlet at 7.64 ppm for the aromatic protons,
3
3
one quartet at 4.04 ppm for the inner protons of C(sp )-bridge
sides, C(sp )-H···Cl hydrogen bonding between C[6]CPP4
and one doublet at 1.55 ppm for the outside methyl groups
and CHCl with the distance of 2.876 ꢁ, and the strong Cꢀ
3
(
Figure 5a), which indicated its high D symmetry. In the
H···p interaction between benzene and CHCl3 with the
distance of 2.865 ꢁ also played significant roles in formation
of the architectures in the solid state.
3
d
1
case of C[6]CPP5, its H NMR spectrum in C D (Figure 5b)
6
6
exhibited two singlets at 7.66 ppm and 7.72 ppm for the
aromatic protons H1 and H2, respectively. Moreover, one
quartet at 4.17 ppm for the inner hydrogen H3, one doublet at
Similarly, it was found that C[6]CPP5 (Figure 5d) showed
a cylindrical shape structure in the solid state as well, in which
an average diameter and cavity depth of 7.764 ꢁ and 5.828 ꢁ,
respectively, were observed. Moreover, the average CꢀC
1
.68 ppm for the newly formed methyl groups, and two
singlets at 1.80 ppm and around 1.67 ppm for the outer and
inner -CH , respectively, were observed, which was consistent
single bond that connected the phenylene units was 1.480 ꢁ.
3
with its D3h symmetric structure.
The main interactions in the packing mode of C[6]CPP5 were
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3
found to be the intermolecular C(sp )-H···p interactions
between the adjacent molecules of C[6]CPP5 with the
distances of 2.829 ꢁ and 2.880 ꢁ, and the intermolecular
The electrochemical properties of the aromatic belts were
carried out by cyclic voltammetry in CH Cl . Consequently, it
2
2
was found aromatic belts C[6]CPP1, C[6]CPP2 (Figure 6b),
C[6]CPP4 and C[6]CPP5 showed quite similar oxidation
potential, in which two reversible oxidation processes were
observed. However, the curve of the ferrocene/ferrocenium
2
C(sp )-H···p interaction with an average distance of 2.834 ꢁ
(
Figure S90).
+
couple (Fc/Fc ) was overlapped with that of C[6]CPPs, as
Photophysical and Electrochemical Properties of C[6]CPPs and
UB
shown in Figure S95-98. The half-wave potentials of C-
[6]CPP2 could not be given accurately. In the case of UB,
cyclic voltammetry in CH Cl2 exhibited two reversible
2
The photophysical properties of C[6]CPPs were mea-
sured in CH Cl . Compared to C[6]CPP1 (Figure 6a, solid
oxidation processes with half-wave potentials of 0.15 V and
0.29 V vs. the Fc/Fc , as shown in Figure 6d. The HOMO
+
2
2
red line) with two major absorption bands at 347 nm and
energy of UB was calculated to be ꢀ4.27 eV. As a result, the
3
6
70 nm and a very weak broad absorption band at 400–
00 nm, C[6]CPP2, C[6]CPP4 and C[6]CPP5 showed bath-
LUMO energy of UB was approximately ꢀ1.79 eV.
ochromic shifts with increasing numbers of methyl group at
3
C(sp ) atoms. As a result, the major absorption bands of
Theoretical Calculations
C[6]CPP2 and C[6]CPP4 containing six methyl groups at
3
53 nm and 354 nm were observed, respectively, and C-
To further understand the differences observed in the
absorption spectra of C[6]CPPs, the time-dependent density
functional theory (TD-DFT) calculations were carried out by
the B3LYP/6-31G(d) level of theory. The highest occupied
molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) of C[6]CPPs showed that the
frontier molecular orbitals were all delocalized over the
phenylene backbone (Figure 7, Figure S100–103). The HO-
MO-1 and HOMO-2, LUMO + 2 and LUMO + 3 were
degenerate owing to the high symmetry of C[6]CPPs. Major
electronic transitions for C[6]CPPs were summarized in
Table S12–15, in which the observed absorption bands at
around 347 nm and 370 nm for C[6]CPPs could be assigned to
electronic transitions from the HOMO-1 and HOMO-2 to
LUMO, and from the HOMO to LUMO + 2 and LUMO + 3.
And the weak broad absorption band at 400–600 nm was
attributed to the symmetry forbidden HOMO–LUMO tran-
sitions. Moreover, the calculated maximum absorptions of
C[6]CPP1, C[6]CPP2, C[6]CPP4 and C[6]CPP5 were at 432,
[
6]CPP5 with nine methyl groups showed the maximum
absorption band at 357 nm.
In the case of UB, major absorption bands at 335, 358, 423,
53 and 482 nm, respectively, were found. Interestingly, UB
4
also showed strong fluorescence property with the maximum
emission at 522 nm (Figure 6c), and the fluorescence quan-
tum yield was determined to be 0.21. This result was obviously
different from those of C[6]CPPs, in which there was no
detectable fluorescence. Energy gap of UB was approximate-
[35]
ly calculated to be 2.48 eV,
possessed a similar energy gap (E ) to that of C[6]CPP1
which suggested the UB
g
(
2.67 eV).
3
47, 346 and 350 nm, respectively. These trends were in line
with the experimental results. The HOMO–LUMO energy
gaps of C[6]CPPs were calculated to be 2.61–2.67 eV, which
were in agreement with the experimental values as well.
Figure 6. a) UV–Vis absorption spectra of C[6]CPP1 (solid red line),
C[6]CPP2 (solid blue line), C[6]CPP4 (dash red line) and C[6]CPP5
ꢀ
5
(
dash blue line); [C[6]CPPs]=4.0ꢀ10 M. b) Normalized cyclic vol-
tammetry of C[6]CPP2, the measurement was carried out in 0.1 M
ꢀ
1
TBAP/CH Cl at room temperature at a scan rate of 100 mVs
,
2
2
+
potentials were recorded versus Fc/Fc . c) UV–Vis absorption and
fluorescence spectra of UB, [UB]=1.0ꢀ10 M. d) Normalized cyclic
voltammetry of UB, the measurement was carried out in 0.1 M TBAP/
ꢀ5
ꢀ
1
CH Cl at room temperature at a scan rate of 100 mVs , potentials
Figure 7. The comparison of the calculated frontier molecular orbitals
of C[6]CPPs.
2
2
+
were recorded versus Fc/Fc .
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Angewandte
Research Articles
Chemie
These narrow HOMO–LUMO energy gaps were close to that
which the oscillator strength (f) was 0.1282. And the observed
strongest absorption band at 358 nm could be assigned to
electronic transitions from the S to S , where the f value was
[
36]
of C , implying a promising application of these C[6]CPPs
6
0
in optoelectronic materials.
0
5
For a clear description of aforementioned properties of
UB, TD-DFT calculations were also taken into account.
Firstly, the calculated energies of the HOMO and LUMO and
the corresponding major electronic transitions for UB were
displayed in Figure 8. Similar to those of C[6]CPPs, the
HOMO and LUMO of UB were mainly delocalized over the
1.0408. Secondly, the strong fluorescence of UB was theoret-
ically analyzed (Figure S107), and the f value at 603 nm was
0.1441 (single electron calculation in vacuum), confirming an
efficient S ! S transition.
1
0
phenylene backbone, and the UB possessed degenerate Conclusion
HOMO-1 and HOMO-2, and degenerate LUMO + 2 and
LUMO + 3 as well. It was found that the HOMO energy of
UB was ꢀ4.36 eV, which was consistent with the result from
cyclic voltammetry. The HOMO–LUMO energy gap of UB is
In summary, we have designed and synthesized a new kind
of macrocyclic arenes, F[3]As in up to 67% yield by the
BF ·OEt catalyzed one-pot condensation between 2,7-alkox-
3
2
2
.62 eV, which is the same level as those of C[6]CPPs and
yl-substituted fluorenes and paraformaldehyde. Starting from
F[3]As, an approach to highly efficient synthesis of C[6]CPPs
was developed. Thus, the C[6]CPP1 was easily obtained in
90% yield, which was markedly higher than that from
pillar[6]arene probably due to the preorganized structure of
the macrocycle, the free outer rotations of the OTf groups,
and the formation of only three new CꢀC bonds in our system.
close to the experimental result from absorption spectrum.
The absorption spectrum of UB was simulated according to
the analysis of major electronic transitions (Figure S105),
which matched well with the experimentally observed Fig-
ure 6c. The major electronic transitions of the main absorp-
tion bands for UB were analyzed (Figure S106). Thus, the
observed maximum absorption band at 482 nm for UB could
be assigned to electronic transitions from the S to S , among
[37–38]
Similarly, C[6]CPP2 could also be synthesized in good yields
from F[3]A2. For F[3]A3, no corresponding aromatic belt
was obtained due to the steric effect of the inner n-propyl
groups to impede the ring-closing reactions. Interestingly, we
also found the selective methylation of C[6]CPP1 and
C[6]CPP2 gave their fully outer-methyl-substituted deriva-
tives in more than 96% yields. The X-ray crystal structures of
the fully hydroxyl-substituted 2,7-OH-F[3]As showed bowl-
shaped structures, and the methyl-substituted C[6]CPPs
exhibited cylindrical shaped structures with deep cavities.
The experimental results and theoretical calculations indi-
cated C[6]CPPs all exhibited high HOMO energies and
narrow HOMO–LUMO gaps. Moreover, a precursor of
C[6]CPP2, the unclosed belt UB, was also obtained, and it
not only showed the similar narrow HOMO–LUMO energy
gap to those of the aromatic belts, but also displayed strong
fluorescence property. We believe that these easily available
aromatic belts and their precursors with specific cavity
structures and properties could be promising candidates as
new kinds of macrocyclic hosts and find wide applications in
supramolecular chemistry and materials science.
0
1
Acknowledgements
We thank the National Natural Science Foundation of China
(22031010, 21772205 and 21521002) for financial support. We
also thank Dr. Tong-Ling Liang for her helps in X-ray crystal
structure analysis.
Conflict of interest
The authors declare no conflict of interest.
Keywords: [6]cycloparaphenylene · aromatic belt ·
Figure 8. Calculated frontier molecular orbital profiles, energy diagram,
and major electronic transitions of UB.
fluoren[3]arene · intramolecular aryl–aryl coupling · macrocycles
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Angewandte
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[
[
[
[
[
1
2
7
2
Manuscript received: February 22, 2021
Accepted manuscript online: March 29, 2021
1
Version of record online: && &&
,
&&&&
Angew. Chem. Int. Ed. 2021, 60, 2 – 10
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&&&&
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Angewandte
Research Articles
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Research Articles
Macrocycles
Starting from fluoren[3]arenes, a new
kind of easily available macrocyclic
arenes, a powerful approach to the highly
X.-S. Du, D.-W. Zhang, Y. Guo, J. Li,
3
Y. Han, C.-F. Chen*
&&&& — &&&&
efficient synthesis of C(sp )-bridged
[
6]cycloparaphenylenes (C[6]CPPs) with
Towards the Highly Efficient Synthesis
and Selective Methylation of C(sp )-
Bridged [6]Cycloparaphenylenes from
Fluoren[3]arenes
aesthetic conjugated belt structures was
developed. Moreover, the bridged meth-
ylene of C[6]CPPs could be selectively
methylated to produce their fully outer-
methyl-substituted derivatives in ꢁ96%
yields.
3
&
&&&
www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 10
These are not the final page numbers!