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Perylene Diimide-Embedded Double [8]Helicenes
Bo Liu, Marcus Böckmann, Wei Jiang,* Nikos L. Doltsinis, and Zhaohui Wang*
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sı Supporting Information
*
ABSTRACT: The rational design and modification of the helix is
of significance for fully promoting properties of configurationally
stable materials for various applications in chiral science. Herein, a
straightforward, sterically less demanding synthetic approach
involving hybridization between two [6]helicene subunits and a
perylene diimide (PDI) scaffold are presented, affording perylene
diimide-embedded double [8]helicenes (PD8Hs) which represent
the highest double carbohelicenes reported thus far. Due to the
structural features of PDI and [6]helicene, the PD8Hs have six
stereoisomers including two pairs of enantiomers and two
mesomers. Such structural diversity is unprecedented in the
realm of double helicenes. The absolute configuration of these
PD8Hs was unambiguously confirmed by single-crystal X-ray
diffraction analyses, revealing that the subtle configurational differences lead to great variation in the superhelical structure and
molecular packing arrangement. Due to the embedding of the PDI chromophore, the PD8Hs possess outstanding fluorescence
quantum yields of approximately 30%. Two pairs of enantiomers were resolved by chiral HPLC, and the chiroptical properties were
evaluated using circular dichroism and circularly polarized luminescence spectroscopy, of which PD8H-6R exhibited excellent
chiroptical performances in both the absorption and emission ranges with dissymmetry factors |gabs| of 0.012 and |glum| of 0.002.
■
INTRODUCTION
Helicenes, which are angularly ortho-fused aromatic subunits
arranged consecutively in a helix, have kindled great
enthusiasm among organic and material chemists due to
their inherent chirality and aesthetic structures.1−5 The
helicene skeleton can be adapted to inherently curved and
multilayered chiral π systems with exceptional chiroptical
properties for potential use in organic optoelectronics.6
Despite the broad interest in these π-conjugated helical
molecules, synthetic approaches toward more complicated
and higher helicenes remain challenging.7−9 In fact, a record
[16]carbohelicene10 was only achieved in 2015 after a 40-year
hiatus since the synthesis of [14]helicene in 1975.11 In these
triple-layered helically extended π systems, the framework
around the second layer is tightly compressed, and the steric
distortion is difficult to relieve. At the same time, another
significant challenge is to fully promote properties of
configurationally stable materials, such as supramolecular
packing, photophysical, chiroptical, conductive, and redox
properties, based on rational design and modification of the
helix.12,13
Perylene diimides (PDIs) have become the subject of
extensive research efforts, demonstrating the promise of
organic optoelectronics based on their tunable electronic
structures and properties.14−17 The PDI end-capped helicenes
with immense strain and impressive chiroptical properties have
been recently reported by Nuckolls and co-workers.18−20
© 2020 American Chemical Society
Herein, we present a straightforward, sterically less demanding
synthetic approach that involves embedding PDI into a
superhelical architecture. The lateral fusion of a representative
helicenic segment (i.e., [6]helicene) to easily prepare and
derivatize PDIs is expected to combine the desired properties
of both types of molecular structures into a unique
superhelicene. Guided by this strategy, six stereoisomers of
double [8]helicenes (PD8Hs) including two pairs of
enantiomers and two mesomeric types were precisely
constructed by combining regioisomerically pure dibrominated
PDIs with racemic [6]helicene using a palladium-catalyzed
Suzuki cross-coupling reaction followed by photocyclization
(Figure 1). To the best of our knowledge, the PD8Hs are the
highest double carbohelicenes reported, and only a restricted
number of double [7]helicenes have recently been reported.21−23 More importantly, the structural diversities have
not been found in other double helicenes.24−28 Such variegated
structural information allowed us to gain deep insight into the
relationships between molecular configuration and chiral
properties of these PD8Hs. The single-crystal X-ray diffraction
Received: January 24, 2020
Published: March 20, 2020
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RESULTS AND DISCUSSION
Synthesis and Characterization. The two essential
factors that exist for combining [6]helicene with the PDI
framework are as follows: (1) selective annulation of
[6]helicene subunit onto PDI via photocyclization and (2)
accurate functionalization of PDI derivatives. Scheme 1
illustrates the rational synthesis of atomically precise double
[8]helicenes via a two-step process from two regioisomers of
dibrominated PDI (i.e., 1,6-dibromo-PDI (minor) and 1,7dibromo-PDI (major)).29 Their mixture often results in issues
with synthesizing structurally specific difunctionalized PDI
derivatives. However, this selectivity can be utilized to
construct configurationally isomeric double [8]helicenes
using regioisomerically pure 1,6- and 1,7-dibromo-PDI,
respectively.
To shed light on the feasibility and regioselectivity of the
photocyclization reaction between the [6]helicene subunit and
the PDI scaffold, a model compound (i.e., P8H) was designed
(Schemes 1 and S1−S2). A Suzuki cross-coupling reaction
between 1-bromo-PDI 230 and racemic 131 using Pd(dppf)Cl2
as the catalyst and K3PO4 as the base afforded rac-5 in a high
yield of 93%. Then, a toluene solution of rac-5 containing a
catalytic amount of I2 was further irradiated by an LED lamp at
100 °C, affording P8H as a racemate in 92% yield. To our
delight, the cyclization occurred at the 1-position of [6]helicene, and the excellent yields corroborated the high
controllability and efficiency of the regioselective photocyclization reaction between [6]helicene and PDI.
The two helicenic fragments fused onto the PDI skeleton
may adopt the same or opposite configuration based on the
good stability of [6]helicene and its derivatives.31,32 The
former case would lead to a chiral PD8H, and the latter would
result in a meso-type achiral PD8H. Accordingly, when
regioisomerically pure 1,6-dibromo-PDI 333 and 1,7-dibromo-PDI 433 were used as the starting materials in the Suzuki
cross-coupling and cyclization processes even at a lower
Figure 1. Concept for the design of perylene diimide-embedded
double [8]helicenes.
analysis indicated that the subtle configurational differences
lead to great variation in the superhelical structure and
molecular packing arrangement. The PD8Hs possess outstanding fluorescence quantum yields of approximately 30% by
the embedding of the PDI chromophore. The two pairs of
enantiomers were successfully resolved by chiral high-performance liquid chromatography (HPLC), and the chiroptical
properties were investigated using circular dichroism (CD) and
circularly polarized luminescence (CPL) spectroscopy. The
results indicated relatively high dissymmetry factors of |gabs| =
0.012 and |glum| = 0.002 for PD8H-6R.
Scheme 1. Synthetic Routes to Perylene Diimide-Embedded Double [8]Helicenes
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Molecular Configuration and Crystal Packing Arrangement. To confirm the helicene architecture, single
crystals of P8H that were suitable for X-ray crystallography
were obtained by slow evaporation of methanol into a
dichloroethane solution of racemic P8H. The single-stranded
helicene crystals contain two pairs of P- and M-enantiomers in
the unit cell with helicenic subunits facing each other and PDI
moieties facing out. Intense intermolecular π−π interactions of
3.31−3.39 Å between the homoenantiomers (red dotted lines)
are observed in the PDI blades, with interactions of 3.31−3.40
Å between the heteroenantiomers (black dotted lines) in the
helicenic wings. The dihedral angle for C17−C30−C29−C34
is 24.9°. The average perpendicular separation between the
two pairs of terminal overlapping rings, which is defined as the
average of the distances from the centroid of ring A or B to the
plane of ring C or D and that from the centroid of ring C or D
to the plane of ring A or B, are 3.28 and 3.19 Å, respectively
(Figure 3).
In addition, crystals of the PD8Hs were successfully grown
by vapor diffusion of methanol into chloroform solutions for
PD8H-6R, PD8H-7R, and PD8H-7M as well as a toluene
solution for PD8H-6M, allowing us to gain deeper insight into
their configurational differences using X-ray diffraction
analyses. The structural information can be classified into
two categories according to the location of the two helicenic
wings relative to the PDI plane, as discussed below.
In the crystal structure of PD8H-6M and PD8H-7R, the two
helicenic wings are located on the same side of the PDI core.
In PD8H-6M, the two helicene parts exhibit opposite
configuration, which results in a meso-type achiral double
[8]helicene. However, in PD8H-7R, the two helicene moieties
possess the same configuration, leading to a chiral double
[8]helicene. Both configurations may suffer from severe steric
repulsion, resulting in the two helicene subunits being
separated from each other, and one wing largely deviating
from the PDI core to maintain a compact helical structure in
another strand with comparable distances of single-stranded
helicene P8H. This result can be rationalized by the increased
distances 3.69 and 3.92 Å for PD8H-6M as well as 3.72 and
3.83 Å for PD8H-7R between the two pairs of overlapping
rings. As defined by the selected dihedral angles, the angles of
torsion for C82−C83−C84−C85 increased to 30.6° in PD8H6M and 33.9° for C93−C92−C132-C133 in PD8H-7R
(Figure 4). This result coincides with the aforementioned 1H
NMR results, which indicated the presence of stronger
deshielding effects in both conformers.
In the crystal structure of PD8H-6R and PD8H-7M, the two
helicenic wings are located on the upper and lower sides of the
PDI plane. As expected, PD8H-6R, which possesses two
helicenic wings with identical configuration, is a chiral
superhelicene with a screw axis across the center of the PDI
core. However, a meso-type achiral structure is formed in
PD8H-7M due to the opposite configuration wings. This
configurational feature can contribute to releasing the steric
distortion in contrast to PD8H-6M and PD8H-7R and even
the highest [16]carbohelicene.10 Accordingly, the mean
vertical distances between the two pairs of overlapping rings
are nearly the same for PD8H-6R and PD8H-7M (Figure 4).
These distances are significantly shorter than that in double
[7]helicene and comparable to that in π-extended double
[7]helicene,22,23 indicating strong intramolecular π−π stacking
interactions. The selected dihedral angles for C4−C5−C32−
reaction temperature, six stereoisomeric PD8Hs including two
racemates (PD8H-6R and PD8H-7R) and two mesomers
(PD8H-6M and PD8H-7M) were formed (Scheme 1; for
details, see Schemes S3−S7 in the Supporting Information).
For the photocyclization step, the first ring-closing reaction
was complete in only 2 h for all PD8Hs. However, the reaction
time for the second ring closure differed significantly. In
general, much more time is required to finish the conversion of
the precursors to PD8H-6M and PD8H-7R, which both
possess two helicenic wings on the same side of the PDI
skeleton, than to PD8H-6R and PD8H-7M, which both
possess two helicenic wings on the upper and lower sides of
the PDI plane, due to the severe steric congestion in the
former. It is worth noting that the ratios of racemic and
mesomeric types are close to 1:1 for both compounds 6 and 7,
originating from the racemic [6]helicene and its configurational stability at the reaction temperature.32 Nevertheless, the
polarity of racemate and mesomer for 6 is much closer than
that of 7, thus leaving a mixture of single-bonded compounds 6
for direct photocyclization.
All PD8Hs as well as the P8H were unambiguously
characterized using NMR spectral, high-resolution mass
spectral, and single-crystal X-ray diffraction analyses. PD8H6R, PD8H-6M, and PD8H-7R are soluble in common organic
solvents, and PD8H-7M was reasonably soluble in dichloromethane and toluene. Thermal gravimetric analysis indicated
that these compounds exhibit good thermal stability with
decomposition temperatures with 5% weight loss higher than
420 °C (Figure S5 and Table 1).
Table 1. Summary of Optical and Thermal Properties of
PD8Hs with PDI as Reference
compd
λmax
[nm]a
εmax
[M−1 cm−1]a
Egopt
[eV]b
Φfl
[%]c
Tdeg
[°C]d
PDI
PD8H-6R
PD8H-6M
PD8H-7R
PD8H-7M
527
566
556
557
563
87 600
26 300
23 400
25 000
25 200
2.30
2.11
2.15
2.15
2.13
89
33
29
31
34
403
427
420
426
439
Article
Measured in a dilute CHCl3 solution (1.0 × 10−5 M). bCalculated by
the onset of absorption in a CHCl3 solution according to Egopt (eV) =
(1240/λonset). cMeasured in a dilute CHCl3 solution (1.0 × 10−5 M)
and determined using the absolute quantum yield method.
d
Decomposition temperature determined by TGA corresponding to
5% weight loss at 10 °C/min under a nitrogen flow.
a
The 1H NMR spectra of the PD8Hs contain well-resolved
proton signals, which were assigned by COSY and NOESY
experiments (Figure 2, Figures S44−S51). The two singlet
peaks in the lower field (δ = 8.6−10.6 ppm) could be ascribed
to the two signals of the protons on the PDI core. Three
groups of signals appear at a relatively higher field of δ = 4.6−
5.8 ppm for PD8H-6R and PD8H-7M, indicating an enhanced
shielding effect induced by the spatial overlap between the
terminal benzene rings and the PDI skeleton. In contrast, a
lower field proton displacement was observed for PD8H-6M
and PD8H-7R due to the deshielding effect of the strong steric
repulsion of the terminal rings caused by the two helicenic
wings. Additionally, the signals indicate the symmetric feature
of these double [8]helicenes, further demonstrating the high
controllability and efficiency of the regioselective photocyclization.
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Figure 2. 1H NMR spectra of PD8H-6R (a), PD8H-7M (b), PD8H-6M (c), and PD8H-7R (d) recorded in C2D2Cl4 at 373.2 K.
interactions of 3.22−3.40 Å between the heterochirally
helicenic wings along with C−H···O contacts (2.40 Å).
Single crystals of enantiopure (P,P)- and (M,M)-PD8H-6R
were also cultivated by vapor diffusion of methanol into a
chloroform solution. As shown in Figures 5 and S14, both
enantiomerically pure (P,P)- and (M,M)-PD8H-6R exhibit
comparable packing arrangements. In stark contrast to racemic
PD8H-6R, only C−H···π contacts were observed in the
stacking structures of the two enantiomers, which hamper
intermolecular π−π overlap.34 Therefore, the two enantiomers
stacked with an incompact packing motif without any π−π
interactions.
Optical and Electrochemical Properties. To investigate
the variation of electronic properties of these PD8Hs with
variegated configurations, UV−vis absorption measurements
were performed in a CHCl3 solution with the parent PDI and
racemic [6]helicene for comparison. As shown in Figure 6,
four PD8Hs exhibit comparable absorption spectra in the
300−600 nm range with the characteristics of both the PDI
and [6]helicene subunits where the maximum absorption
peaks are markedly red-shifted. In addition, the absorption
intensity in the long-wavelength region assigned to the
perylene core diminishes substantially with a molar extinction
coefficient of 23 000−26 000 M−1 cm−1 in contrast to the
parent PDI. However, the absorption intensity associated with
the [6]helicene subunits increases in all PD8Hs. Among the
PD8Hs, PD8H-6R exhibits a maximum red-shifted absorption
with the longest wavelength peak at 566 nm, which is 39 nm
red-shifted relative to that of PDI due to the generation of a
superhelicene architecture that provides the most efficient πconjugation. In addition, the absorption spectra reveal slight
red shifts of a few nanometers for PD8H-6R (λmax = 566 nm)
and PD8H-7M (λmax = 563 nm) compared to those of their
Figure 3. Single-crystal structure (a) and packing arrangement (b) of
P8H.
C33 in PD8H-6R is 28.4°, and the C11−C13−C18−C19
dihedral angle in PD8H-7M is 29.5°.
Importantly, the subtle configurational differences lead to
great variation in the molecular packing arrangements. In the
racemic crystals of PD8H-6R, the (P,P)- and (M,M)enantiomers are separately packed in an alternating fashion
along the a-axis, forming close π-stacking superstructures with
multiple π−π interactions of 3.29−3.39 Å within columns and
3.18 Å between columns. The stacking columns are also
supported by C−H···O contacts of 2.48 Å. However, the
molecules of PD8H-7R are arranged in an offset head-to-tail
stacking array with a π-sequence of (P,P)- and (M,M)-isomers
alternatively along the a-axis. Intense π−π interactions of
3.22−3.40 Å connect the heteroenantiomers between the PDI
cores and the less-distorted wing within columns. In contrast
to PD8H-7R, a unique pair is formed in the mesomeric crystals
of PD8H-6M with PDI core staggered stacking, where close
π···π contacts of 3.34 Å within the PDI pair along with π···π
contacts of 3.27−3.38 Å between the heterochirally helicenic
wings are observed. For the other mesomer, PD8H-7M
exhibits a stacking structure that is primarily sustained by π−π
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Figure 4. Single crystal structures of PD8H-6M (upper left), PD8H-7R (upper right), PD8H-6R (lower left), and PD8H-7M (lower right).
Purple, green, and gray atoms represent carbon, and blue and orange atoms represent N and O, respectively. (M,M)-Helical is shown in green, and
(P,P)-helical is shown in violet. The dotted lines represent the intermolecular π−π interactions (red: between the homoenantiomers; black:
between the heteroenantiomers). Hydrogen atoms and alkyl chains are omitted for clarity.
crowded diastereomers PD8H-6M (λmax = 556 nm) and
PD8H-7R (λmax = 557 nm). Compared with those in the
solutions, the absorption spectra in films in general do not
change much probably due to their twisted helical configurations, which impede the aggregation in solid state (Figure
S6 and Table S1). Analysis of the contributing molecular
orbitals (MOs) and time-dependent density functional theory
(TDDFT) eigenvectors indicates that the LUMOs of PD8Hs
are primarily located on the PDI core, and the HOMOs as well
as the subsequent occupied molecular orbitals (MOs) exhibit
dominant contributions from the two helicenic wings (Tables
S8−S13). Further inspection reveals that the lowest energy
bands for the PD8Hs encompass the excitations to S1, S2, S3,
and S4, which are characterized by transitions into the LUMO
from HOMO, HOMO−1, HOMO−2, and HOMO−3,
respectively. In addition, these transitions carry small oscillator
strengths due to the inferior orbital overlap in contrast to that
of PDI. The optical energy gaps (Egopt) estimated from their
absorption edges are located at 2.11−2.15 eV. Remarkably, the
PD8Hs are fluorescent with absolute quantum yields (Φfl) of
approximately 30% (Tables 1 and S3), emphasizing that the
embedded PDI chromophore, not helicene units, mainly
contributes to the luminescence. On the other hand, the
decreased fluorescence relative to parent PDI is probably due
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and PD8H-7R into their corresponding enantiopure isomers
using chiral HPLC with a CHIRALPAK IE and IC column,
demonstrating their good configurational stability. Their
chiroptical properties were investigated using CD and CPL.
As shown in Figures 7 and S10, the two isolated enantiomers
Figure 5. Molecular packing diagram of enantiomerically pure (P,P)PD8H-6R (left) and (M,M)-PD8H-6R (right). The blue dotted lines
represent the intermolecular C−H···π contacts.
Figure 7. CD and CPL (Ex at 360 nm) spectra of PD8H-6R in
CHCl3 (1 × 10−5 M).
of both racemates revealed perfect mirror images of each other.
Enantiomerically pure PD8H-6R exhibits an impressive
Cotton effect in the range 380−450 nm with a corresponding
|Δε| of 388 M−1 cm−1 at 405 nm. In addition, enantiomerically
pure PD8H-7R exhibits a prominent Cotton effect in the
visible region with a corresponding |Δε| of 460 M−1 cm−1 at
382 nm. Notably, the absorption dissymmetry factor |gabs| can
reach as high as 0.012 at 411 nm for PD8H-6R, which is
relatively high compared to those of recently reported helicene
derivatives.12,37,41,42 Nevertheless, PD8H-7R only exhibits a
maximum |gabs| of 7.6 × 10−3 at 403 nm (Figure S10), implying
that the superhelicene structure of PD8H-6R exhibited a better
synergistic role in chirality transference than that of PD8H-7R.
The simulated CD spectra based on TDDFT calculations at
the B3LYP/6-31G* level are in agreement with the
experimental data, confirming the first and second fractions
assigned as the (P,P)- and (M,M)-isomer, respectively, for both
PD8H-6R and PD8H-7R (Figure S17). The absolute
configurations of the two fractions of PD8H-6R were also
confirmed by X-ray diffraction analysis.
Due to their unique properties (i.e., chirality and high
fluorescence quantum yields), PD8H-6R and PD8H-7R were
predicted to be CPL-active. As expected, both PD8H-6R and
PD8H-7R exhibit CPL activities with luminescence dissymmetry factors |glum| of up to 2 × 10−3 and 5 × 10−4,
respectively, which are both in the range of those for previously
reported chiral organic molecules.12,43−46 In addition, the |glum|
factor of PD8H-6R exhibits a remarkable 4-fold increase
compared to that of PD8H-7R (Figures 7 and S11).
To assess their resistance to thermally induced racemization,
the single enantiomers of PD8H-6R and PD8H-7R in
diphenyl ether at 200 °C were heated for 6 h. Monitored by
recording the CD and NMR spectra, no isomerization was
observed during this process for these two molecules (Figure
S12), indicating good configurational stability due to the
compact helical structure by the substantial overlaps between
the terminal benzene rings.18 Considering the stereoisomeric
character of all the PD8Hs, the dynamic behavior was studied
by evaluating the isomerization process between (M,M)-
Figure 6. Room-temperature UV−vis absorption spectra of PD8H6R (red), PD8H-6M (blue), PD8H-7R (green), PD8H-7M (violet),
PDI (dark cyan), and [6]helicene (dark gray) in CHCl3 (1 × 10−5
M).
to the intramolecular charge transfer (ICT) process from the
helicenic wings to the PDI core as evidenced by DFT
calculations. The fluorescence of PD8Hs is relatively high in
the carbohelicenes family,35−40 demonstrating the potential for
use as a good emitter in optoelectronics.
The electrochemical properties of the PD8Hs were
investigated using cyclic voltammetry in CH2Cl2 solutions
(Figure S3), and the half-wave reduction and oxidation
potentials versus Fc/Fc+ are summarized in Table S2. All the
PD8Hs feature two well-defined, reversible reductive waves
and one oxidative peak within the scanning range, and the
configurational differences have a slight effect on their redox
properties. The first half-wave reduction and oxidation
potentials for four compounds are located at approximately
−1.50 and 1.00 V, respectively. The reductive potentials are
more negative in contrast to that of PDI, implying that the
lateral fusion of the electron-rich helicenic wings substantially
changes their redox properties. Accordingly, both the LUMO
and HOMO levels for the PD8Hs increased with the LUMOs
estimated to be −3.44, −3.48, −3.48, −3.42 eV and the
HOMOs estimated to be −5.75, −5.73, −5.76, −5.75 eV in
sequence. The electrochemical HOMO−LUMO gaps
(Egelectro) in the range 2.25−2.33 eV are in good agreement
with their Egopt.
Chiral Resolution and Chiroptical Properties. We
succeeded in the optical resolution of racemate PD8H-6R
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Institute of Chemistry, Chinese Academy of Sciences, Beijing
100190, China; Key Laboratory of Organic Optoelectronics and
Molecular Engineering, Department of Chemistry, Tsinghua
University, Beijing 100084, China; University of Chinese
Academy of Sciences, Beijing 100049, China; orcid.org/
0000-0001-5786-5660; Email: wangzhaohui@
mail.tsinghua.edu.cn
PD8H-6R and (P,M)-PD8H-6M through DFT calculation at
the B3LYP/6-31G* level of theory. From the results, (M,M)PD8H-6R is thermodynamically more stable than (P,M)PD8H-6M by 2.1 kcal/mol. A plausible isomerization pathway
between the chiral isomer and meso isomer was proposed to
proceed through a transition state with the terminal overlapping rings oriented in a face-to-face pattern in one helicenic
wing. Accordingly, the isomerization barrier was calculated to
be 41.5 kcal/mol (Figure S25), which is slightly lower than
that of [8]helicene (42.0 kcal/mol, Figure S24).
Authors
Bo Liu − Beijing National Laboratory for Molecular Sciences,
CAS Key Laboratory of Organic Solids, CAS Research/
Education Center for Excellence in Molecular Sciences, Institute
of Chemistry, Chinese Academy of Sciences, Beijing 100190,
China; University of Chinese Academy of Sciences, Beijing
100049, China
Marcus Böckmann − Institute for Solid State Theory and
Center for Multiscale Theory & Computation, University of
Muenster, 48149 Muenster, Germany
Nikos L. Doltsinis − Institute for Solid State Theory and Center
for Multiscale Theory & Computation, University of Muenster,
48149 Muenster, Germany
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c00954
■
CONCLUSION
In summary, for the first time, we have achieved a
straightforward, sterically less demanding synthetic approach
of the highest double carbohelicene PD8H by hybridization of
two [6]helicene fragments to a PDI plane. Furthermore, such
structural diversity has not been previously observed in other
double helicenes. The single-crystal X-ray diffraction analysis
unambiguously indicated that the subtle configurational
differences lead to great variation in the superhelical structure
and molecular packing arrangement. The PD8Hs possess
outstanding fluorescence quantum yields of approximately 30%
by the embedding of the PDI chromophore. Two pairs of
enantiomers of PD8H-6R and PD8H-7R were separated, and
the chiroptical properties were studied based on CD and CPL
spectroscopy. PD8H-6R exhibited excellent chiroptical responses compared to PD8H-7R in both absorption and
emission with dissymmetry factors |gabs| of 0.012 and |glum| of
0.002 as a result of the superhelicene structure of PD8H-6R.
These excellent chiroptical properties as well as the outstanding fluorescence quantum yield make PD8H-6R a
promising candidate for applications in chiral optoelectronics.
From the standpoint of synthetic chemistry, these diverse
double [8]helicenes enrich the helicene family. The work on
synthesizing higher double helicenes is currently underway.
■
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was financially supported by the National Natural
Science Foundation of China (NSFC) (21790361, 21734009,
and 51673202), the National Key R&D Program of China
(2017YFA0204701), and the Youth Innovation Promotion
Association of Chinese Academy of Sciences (No. 2017048).
■
ASSOCIATED CONTENT
sı Supporting Information
*
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c00954.
Details of synthetic methods and characterizations,
spectroscopic details, and characterization (PDF)
Data for P8H (CIF)
Data for PD8H-6R (CIF)
Data for PD8H-6M (CIF)
Data for PD8H-7R (CIF)
Data for PD8H-7M (CIF)
Data for (P,P)-PD8H-6R (CIF)
Data for (M,M)-PD8H-6R (CIF)
■
Article
AUTHOR INFORMATION
Corresponding Authors
Wei Jiang − Beijing National Laboratory for Molecular Sciences,
CAS Key Laboratory of Organic Solids, CAS Research/
Education Center for Excellence in Molecular Sciences, Institute
of Chemistry, Chinese Academy of Sciences, Beijing 100190,
China; University of Chinese Academy of Sciences, Beijing
100049, China; orcid.org/0000-0002-0153-7796;
Email: jiangwei@iccas.ac.cn
Zhaohui Wang − Beijing National Laboratory for Molecular
Sciences, CAS Key Laboratory of Organic Solids, CAS
Research/Education Center for Excellence in Molecular Sciences,
7098
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