Synthesis of a Helical Analogue of Kekulene: A Flexible ?€-Expanded Helicene with Large Helical Diameter Acting as a Soft Molecular Spring

Article abstract of DOI:10.1021/jacs.8b09825

A π-expanded helicene that is the helically twisted analogue of kekulene was synthesized using a 6-fold ring-closing olefin metathesis (RCM) reaction as a key step. The π-expanded geometry with large helical diameter (d_h = 10.2 ?), consisting o

Full text of DOI:10.1021/jacs.8b09825

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Article
Synthesis of Helical Analogue of Kekulene: a Flexible #-Expanded
Helicene with Large Helical Diameter Acting as a Soft Molecular Spring
Yusuke Nakakuki, Takashi Hirose, and Kenji Matsuda
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09825 • Publication Date (Web): 19 Oct 2018
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Synthesis of Helical Analogue of Kekulene: a
Flexible -Expanded Helicene with Large
Helical Diameter Acting as a Soft Molecular
Spring
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Yusuke Nakakuki, Takashi Hirose , and Kenji Matsuda*
*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering,
Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan.
*
e-mail: thirose@sbchem.kyoto-u.ac.jp (T.H.), kmatsuda@sbchem.kyoto-u.ac.jp (K.M.)
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Abstract: A -expanded helicene, that is the helically twisted analogue of kekulene, was
synthesized using a 6-fold ring-closing olefin metathesis (RCM) reaction as a key step. The -
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expanded geometry with large helical diameter (d = 10.2 Å), consisting only of carbon and
h
hydrogen atoms (C54H30), was unambiguously determined by single-crystal X-ray analysis. We
found that the -expanded helicene with large helical diameter will act as a soft molecular spring
with a small spring constant. Interestingly, the mechanical properties of molecular springs roughly
satisfied a physical formula for macroscopic spring materials, i.e., the force constant of the
elongation of molecular spring (k) is inversely proportional to the third power of the helical
−3).
diameter (k  d
h
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Introduction
Synthesis of chiral compounds is one of the main concerns in chemistry to elucidate their unique
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properties.1–4 Helicenes—ortho-fused polycyclic aromatic hydrocarbons (PAHs)—are
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representative examples of helically twisted molecules, which have been widely investigated from
the viewpoint of challenging targets of (asymmetric) organic synthesis6–9 as well as the viewpoint
of chiral organic materials showing remarkable response to circularly polarized light10–13 and large
nonlinear optical (NLO) properties.14–16 Following the recent advances in organic chemistry to
construct highly strained molecules,17–20 impressive -extended helicenes have been reported in
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the last five years, including pyrene-based -extended helicenes by Collins and by Sterá and
Starý,22,23 perylene bisimide-based -conjugated helices by Nuckolls,24,25 and double and multiple-
pole helicenes consisting of two or more helicene-like subunits by Itami,26,27 Müllen,28,29
30
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Kamikawa, Coquerel and Gingras, and Wang.
By making planar PAHs nonplanar, a number of basic structures of helically twisted
molecules can be systematically designed. From this perspective, [7]helicene (C30
regarded as the helical analogue of coronene, which is the smallest helical PAHs with helical
H18) can be
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diameter (d ) of 5.1 Å, where d is defined by the longest distance between the centroids of
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polycyclic rings (Figure 1). Recently, we have reported the synthesis of hexa-peri-
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hexabenzo[7]helicene (C48
H
24),33 which is the helical analogue of hexa-peri-hexabenzocoronene.
The homogeneous -extension throughout the helicene structure eventually leads to the helically
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twisted graphenes that can be considered as a promising candidate as a helix-shaped molecular
element in molecular electronic devices.34,35
Besides the -extension of helicenes, another molecular design strategy is the expansion
of helicene framework, i.e., the increasing in helical diameter. With this approach, a number of
basic molecular structures of helically twisted PAHs can be designed (Figure 1). One distinctive
36–38
example of -expanded helicenes is angularly fused helical phenylenes, so-called heliphenes.
The synthesis of [n]heliphenes (n = 5–9) has been achieved by Vollhardt et al. in 2002 using cobalt-
catalyzed [2+2+2] cycloaddition of alkynes. Since the pioneering works by Vollhardt, the
chemistry of -expanded helicene has been largely undeveloped except for a few examples
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including indeno-fused helicenes by Wang in 2004, oligonaphthofrans by Tsubaki in 2014, and
oxahelicenes by Vacek, Stará, and Starý in 2017.41
Kekulene is a -expanded planar PAH consisting of hexagonally fused 12 benzene rings
with D6h symmetry (Figure 1). The synthesis of kekulene was reported for the first time by
Diederich and Staab in 1978.43,44 [7]Heliphene (C42
H18) can be regarded as the helical analogue of
anti-kekulene—a kekulene-like molecule showing an anti-aromatic character in which alternating
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benzene rings of kekulene are replaced by 4-membered ring.45,46 A chemical structure of the
helical analogue of kekulene has been reported as "a conceptual molecule" by Bell in 1991.47
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Recently, Tilley et al. reported the synthesis of helically twisted analogue of kekulene using
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iridium-catalyzed [2+2+2] cycloaddition of alkynes in 2017. The synthetic strategy proposed by
Tilley is beneficial for affording a series of substituted -expanded helicenes, however, the
synthesis of the basic structure of helical analogue of kekulene, especially with no substituent
group, has not been achieved for 40 years since the first report of kekulene.
In this work, we synthesized a -expanded helicene 1 (C54H30), that is the basic structure
of helically twisted analogue of kekulene, using Suzuki–Miyaura coupling and ring-closing
metathesis reactions as the key steps (Figure 1). The synthesis of non-functionalized compounds
is of importance to elucidate the origin of unique molecular properties derived from a primary
molecular structure. For this purpose, the synthesis of the simplest chemical structures as a model
compound is highly required. In the current study, we focused on the elucidation of the local
aromaticity, photophysical properties, and molecular mechanical properties of the -expanded
helicenes. The -expanded geometry with large helical diameter (d = 10.2 Å) was unambiguously
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determined by X-ray crystallography. Because of the larger helical diameter, the magnitude of
transition magnetic dipole moment of 1 was roughly three times larger than that of [7]helicene.
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We revealed that the -expanded structure of 1 consisting solely of benzene rings is remarkably
flexible even more than [7]heliphene. The flexibility of 1 is likely attributed to the -expanded
molecular framework with large helical diameter, consisting of many C–C bonds in a helical pitch,
by which the steric distortion required for stretching and racemization of helix-shaped molecule
can be effectively dispersed.
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Figure 1. Chemical structure of planar (left) and helically twisted PAHs (right). Each helically
twisted analogue can be systematically generated from a planar PAH by making an open-ring
structure so that one benzene ring is duplicated. Helical diameters defined by the distance between
the centroids of outer rings are shown in red.
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Results and Discussion
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Synthesis of Helical Analogue of Kekulene. The ring-closing olefin metathesis (RCM)
reactions are a powerful approach for the formation of polycyclic structures.48,49 Collins reported
that the RCM reaction can be applicable to the synthesis of helicenes despite the structure of
product is significantly strained.50 The applicability of RCM reaction in the synthesis of
complicated polycyclic systems has further underlined by King in the synthesis of septulene—a
heptagonal analogue of kekulene consisting of 14 benzene rings.51 In the present work, the
synthesis of the helical analogue of kekulene 1 was performed in a stepwise fashion using Suzuki–
Miyaura coupling to construct m-septiphenyl framework 9 and then using RCM reaction as the
final step (Scheme 1). 1,5-Dibromo-2,4-bis(prop-1-enyl)benzene (2) was prepared from m-xylene
by a two-step sequential bromination, hydrolysis of bromomethyl groups affording dialdehyde,
followed by a Wittig reaction (Supporting Information, Scheme S1). The dibromo derivative 2
could be selectively monolithiated and then borylated, affording 3 as a mixture of (E/Z)-isomers
1
in 71% yield. To simplify the structural analysis using H NMR, the (E/Z)-mixture of 3 was
isomerized into the (E,E)-isomer in the presence of a catalytic amount of bis(acetonitrile)dichloro-
).52 Suzuki coupling reaction of (E,E)-3 with 1-iodo-2-((E)-prop-1-
2
palladium(II) (PdCl (MeCN)
2
1
enyl)benzene (4) provided 5-bromobiphenyl derivative 5 in 91% yield as the (all-E)-isomer. In H
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NMR spectrum of 5, three doublet peaks assignable for the three olefin side groups were detected
at different chemical shifts and whose J-coupling values were always ca. 17 Hz, suggesting that
the (all-E)-isomer is dominant for 5. The (all-E)-5 was again lithiated and borylated, affording 3-
borylbiphenyl derivative (all-E)-6. The 5-borylbiphenyl (6) was used for subsequent synthesis as
the terminal fragment of the m-septiphenyl (9).
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Scheme 1. Synthesis of Helically Twisted Analogue of Kekulene 1.a
aB
pin
: bis(pinacolato)diboron; Grubbs II: Grubbs’ second generation catalyst.
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A 2-fold borylation reaction of 2 was satisfactory proceeded by a palladium-catalyzed
borylation reaction, affording 1,5-diboryl-2,4-bis(prop-1-enyl)benzene (7) in 83% yield as an
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(MeCN) to give (E,E)-7. Suzuki
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coupling reaction of (E,E)-7 with an excess amount of (E,E)-2 (6 mol equiv.) provided (all-E)-8
in 60% yield. The key precursor m-septiphenyl (9) was then synthesized by Suzuki coupling
reaction of 5,5"-dibromo-m-terphenyl (8) as the middle fragment with 2 molar equivalent of 3-
borylbiphenyl (6) as the terminal fragment in 64% yield.
We note that totally 7 different singlet peaks were detected in the aromatic region (6.8–
1
8
.0 ppm) for H NMR spectrum of (all-E)-8, which has more than 4 aromatic H-atoms expected
from the chemical structure of 8. This result suggested a slow rotation dynamics of the m-
phenylene framework compared to the time scale of NMR measurement at room temperature.
Indeed, the 7 peaks observed in CDCl at 25 °C started to merge into 4 singlet peaks at 50 °C with
3
the intensity ratio of 1:2:2:1, supporting the existence of rotation isomers for (all-E)-8 (Figure S1
1
in the Supporting Information). Likewise, H NMR spectrum of (all-E)-9 was remarkably
complicated probably due to the slow rotation dynamics of the m-oligophenyl with bulky olefins;
there are 20 different rotamers for m-septiphenyl 9 expected by considering the 6 rotatable C–C
single bonds (Table S1).
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Finally, a 6-fold RCM reaction of 9 using Grubbs’ second generation catalyst (Grubbs II)
was successfully proceeded, affording helical analogue of kekulene 1 in 71% yield. The structure
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of 1 was fully characterized by H NMR, C NMR, HMQC/HMBC measurements, and high-
resolution mass spectrometry (see the Supporting Information for details). In contrast to the
extremely low solubility nature of kekulene43,44 and septulene (ca. 1 mg/20 mL in 1,2,4-
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trichlorobenzene at 100 °C), the helical analogue 1 was moderately soluble in organic solvents
such as tetrahydrofuran (THF), 1,2,4-trichlorobenzene, and carbon disulfide (CS ) (ca. 1 mg/mL
2
1
in each solvent). The H NMR of 1 (in CS
2
/CDCl = 2/1) showed characteristic three singlet peaks
3
at 10.75, 10.71, and 10.48 ppm with the intensity ratio of 1:2:2, assignable to the inner protons of
helical structure. Theoretically estimated chemical shifts for 1 were 11.47 ppm for the three inner
protons of the rings E, G, and I, and 11.22 ppm for the rings C and K, calculated at the GIAO-
B3LYP/6-311g(2d,p) level of theory.
Nomenclature for -Expanded Helicenes. According to the IUPAC nomenclature, the
name of 1 would be dinaphtho[1′′,2′′:6,7;1′′′,2′′′:6′,7′]diphenanthro[3,2-a:2′,3′-j]anthracene. As
seen in this example, the IUPAC nomenclature tends to provide very complicated name, especially
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for a polycyclic system, from which it is difficult to estimate the exact molecular structure. We
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propose an alternative nomenclature that is helpful to provide an intuitive name for the -expanded
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helicenes, based on the corannulene nomenclature proposed by Agranat. In the corannulene
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
m
nomenclature, like corannulene[n ,ꞏꞏꞏ], the numbers in square brackets (n) denote the size of n-
membered rings and the superscript numbers (m) designate the number of outer carbon atoms
unshared by the neighboring rings. Accordingly, [7]helicene, [7]heliphene, and 1 are named
2
2
2
2
2
0
2
0
2
0
2
0
2
0
2
0
helicene[6,6 ,6 ,6 ,6 ,6 ,6],
helicene[6,4 ,6 ,4 ,6 ,4 ,6 ,4 ,6 ,4 ,6 ,4 ,6],
and
2
1
2
1
2
1
2
1
2
1
2
helicene[6,6 ,6 ,6 ,6 ,6 ,6 ,6 ,6 ,6 ,6 ,6 ,6], respectively. The long names can be shortened to
helicene[6,(62)
,6]), helicene[6,(4 ,6 )
0
2
,4 ,6], and helicene[6,(6 ,6 ) ,6 ,6]) as a simplified form.
0
2
1
2
5
5
5
Thus, the structure of 1, i.e., the size and number of polycyclic rings and the connectivity consisting
of the alternating angular (m = 2) and linear (m = 1) annulations, could be clearly indicated by the
proposed nomenclature.
X-ray Crystallography. The -expanded helical structure of 1 was unambiguously
determined by X-ray crystallography (Figure 2). To the best of our knowledge, this is the first
report of the single crystal structure of non-functionalized helical analogue of kekulene, which
provides fundamental insights into molecular structure, local aromaticity, and the nature of
intermolecular interactions of -expanded helicenes. A single crystal suitable for X-ray analysis
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was obtained by a slow vapor diffusion of hexane into a concentrated solution of 1 in
tetrahydrofran (THF). The crystal was a racemate (monoclinic, space group I2/a) composed of 1:1
mixture of enantiomers. The two enantiomers, i.e., (P)-1 and (M)-1, were alternately stacked along
the b-axis so that the quasiplanar part of helical molecule, i.e., the seven middle rings from D to J,
were overlapped each other, where the interplanar distances were alternately 3.32 and 3.35 Å
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
Figure 2b and 2c). For the racemic dimer pair with the shortest interplanar distance (dFGH–FGH
=
3
.32 Å), the adjacent (P)-1 and (M)-1 were interacted each other so that five benzenoid rings, i.e.,
rings from F to J, were overlapped (Figure 2c). The -stacked racemic column structure is
36
commonly seen in the other -expanded helicenes, i.e., [7]heliphene by Vollhardt and a
42
selenophene-annulated expanded helicene by Tilley, but not seen in the crystal structures of
54
[
7]helicene. The -stacked racemic column of 1 along the b-axis were arranged in a herringbone
manner, where the molecular plane defined by the central three rings F, G, and H formed an angle
of 29° with the vertical plane of the b-axis (Figure 2b). The herringbone angle (HB)—the
inclination of the molecular planes in adjacent columns—was therefore 58° for 1. Similar
44
herringbone-type crystal packing was reported for kekulene (HB = 86°) and septulene (HB
=
51
36–38
and Tilley.42
5
5°), however, it is not seen in the -expanded helicenes reported by Vollhardt
In this context, the crystal structure of 1 can be regarded as having both features of -expanded
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8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
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5
5
5
5
5
5
5
5
5
5
6
helicenes, e.g., heliphenes, and disk-shaped planar PAHs, e.g., kekulene43,44 as well as coronene
and ovalene.55
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 2. Crystal structure of helical analogue of kekulene 1 (C54
H30). (a) ORTEP drawing of 1
recorded at 93 K, showing 50% probability thermal ellipsoids. Hydrogen atoms are omitted for
ꢁ
clarity. (b) Crystal packing structure of 1 viewed normal to the ꢀ100ꢂ face. (P)-1 and (M)-1 are
colored in red and blue, respectively. Co-crystalized solvent molecules, i.e., hexane and THF, are
omitted for clarity (see the Supporting Information for details). (c) Crystal packing structure of the
-stacked racemic column of 1. Left, the side view of the racemic column, in which green dashed
lines represents the least-square planes defined by the central rings F, G, and H. Right, the top
view of the racemic dimer with the shortest interplanar distance of dFGH–FGH = 3.319 Å, viewed
nomal to the plane defined by the rings F, G, and H. Five aromatic rings (i.e., rings F–J) were well
overlapped each other in the closest dimer pair. (d) Bond length in angstrom of the unit determined
by X-ray crystallography.
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9
The bond length determined by X-ray crystallography provides fundamental information
on the electronic structure of 1. The representative values of bond length larger than 1.43 Å or
shorter than 1.38 Å are shown in Figure 2d. Obviously, there are two types of six-membered rings
in 1. The bond lengths for the inner rim and outer edges in the even-numbered rings—i.e., the
angular rings B, D, F, H, J, and L—were 1.460 ± 0.002 and 1.342 ± 0.002 Å, respectively, which
are almost identical to the bond lengths of butadiene (1.47 and 1.35 Å for single- and double-bonds,
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
56
respectively). On the other hand, the odd-numbered rings—i.e., the terminal rings A and M, and
the linearly fused rings C, E, G, I, and K—had intermediate bond lengths within the range of 1.36–
1
.43 Å, which was close to the typical length for aromatic rings (ca. 1.39 Å). On the basis of the
57
bond lengths, the values of the harmonic oscillator model of aromaticity (HOMA) for each type
of rings were HOMAeven = 0.39 ± 0.04 and HOMAodd = 0.89 ± 0.02, suggesting the olefinic
character for the angular (even) rings and the benzenoid character for the terminal and linear (odd)
rings. On the basis of the structural analysis, the electronic structure of 1 is best represented by the
Clar’s sextet notation (Figure 2d), which is fully consistent with the X-ray structures of the planar
44
51
macrocyclic analogues, i.e., kekulene and septulene.
Local Aromaticity. The local aromaticity of helical analogue of kekulene 1 was
investigated by the nucleus-independent chemical shifts (NICS)58 calculated at the GIAO-
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2
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6
7
8
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
B3LYP/6-311g(2d,p) level. Negative NICS(0) values with strong intensity were found at the
center of the odd numbered rings, i.e., A and M rings, −9.11 ppm; C and K rings, −9.24 ppm; E
and I rings, −8.91 ppm; G ring, −8.94 ppm, whereas negative values with moderate intensity at the
even numbered rings, i.e., B and L, −4.20 ppm; D and J, −2.37 ppm; F and H, −2.53 ppm
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
Supporting Information, Figure S5). The result of the calculated local aromaticity was in a good
agreement with the bond length analysis based on the HOMA index; large negative NICS values
were found in the rings with a large HOMA value. The alternately localized aromatic character of
44
51
1
is consistent with the planar analogues, i.e., kekulene and septulene, which is likely attributed
to the ring connectivity consisting of the alternating angular and linear annulations.
Local Structural Deformation. A careful comparison of the X-ray structures suggested
that the structural deformation of 1 is significantly smaller than that of [7]helicene (Figure 3). The
vertical distances (d) from the centroid of the teriminal rings A and M to the least-squares plane
defined by the central three ring F, G, and H were dA–FGH = 1.65 and dM–FGH = 1.66 Å for 1. The
sum of the two values was dA/M–FGH = 3.31 Å, which is close to the typical - distance (3.4 Å).
The sum of the vertical distances was about 10% larger for [7]helicene (dA/G–CDE = 3.67 Å),
suggesting that the helical geometry of [7]helicene is largely distorted. Difference in the helical
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distortion was also suggested by the angle between the two planes () defined by the terminal
rings; the terminal rings of 1 arranged more parallelly (AM = 18.2°) than those of [7]helicene (AG
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
=
32.8°). The local structural deformation is directly reflected to the torsion angle along the helical
inner rim (). The torsion angles of 1 were  = 0.9, 5.4, 3.0, 3.6, 7.5, and 7.9° along the six fjord
regions from the rings ABC to rings KLM. The mean value was <AM> = 5 ± 2° for the 6 torsion
angles of 1, which is significantly smaller than that of [7]helicene (<AG> = 22 ± 3°). Thus, the
local structural distortion of 1 was remarkably small compared to [7]helicene, likely due to the -
expanded helicene structure with large helical diameter.
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Figure 3. Comparison of crystal structures between (a) 1 and (b) [7]helicene. The angle between
the two planes (AM) of the terminal rings is highlighted in green. The torsion angle along the
helical inner rim (<AM>) is highlighted in red.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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8
9
0
1
2
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4
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7
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
UV-vis Absorption and Emission in Solution. UV-vis absorption and emission spectra
of 1 was measured in chloroform (Figure 4). Compound 1 showed a strong absorption band at 320
5
−1
−1
nm ( = 1.4 × 10 M ꞏcm ) and structured bands at 375 and 395 nm with moderate intensities (
4
4
−1
−1
=
3.5 × 10 and 3.8 × 10 M ꞏcm , respectively). Upon excitation with UV light (ex = 320 nm),
a structured emission spectrum was recorded with maximum peak wavelengths at 448, 461, 481,
and 515 nm, which is commensurate with a blue emission. The absorption and fluorescence spectra
of 1 were quite similar in shape to those of kekulene43,44 and septulene (Figure S7). The similarity
of absorption and emission spectra in the open-ring (1) and the closed-ring analogues (kekulene
and septulene) suggests that the contribution of the macrocyclic conjugation in the closed-ring
structure, i.e., superaromaticity, to the electronic structures is almost negligible in both the ground
and excited states. Thus, the experimental results of the absorption and emission spectra of non-
functionalized derivative 1 is a convincing evidence supporting the theoretical result of very small
superaromatic stabilization energy (SSE) for kekulene and septulene.45,59 The fluorescence
51
quantum yield (
f
) and mean fluorescence lifetime (<
f
>) of 1 were 
f
= 0.04 and < > = 33 ns
f
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(
Table S7). On the basis of the two parameters, the fluorescence emission (k
f
) and nonradiative
−1
−1
decay rate constants (knr) of 1 were determined at k = 0.001 ns and knr = 0.03 ns , respectively,
f
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
, 0.002 ns−1;
which are comparable to those of [7]helicene in chloroform (
f
, 0.04; <
f
>, 26 ns; k
f
nr, 0.04 ns−1).60
k
Experimentally observed absorption spectrum of 1 was well reproduced by time-
dependent density functional theory (TD-DFT) calculations at the B3LYP/6-311g(2d,p) level of
theory (Figure 4). According to the calculations, the absorption band appeared at 395 nm was
attributed to a combination of the HOMO−1 → LUMO (48%) and the HOMO → LUMO+1 (47%)
transitions (f = 0.380, max,calc = 402 nm). The strong absorption band appeared at 320 nm was
attributed to a symmetry allowed transition (f = 1.250, max,calc = 327 nm) corresponded to the S
0
–
S
18 transition consisting of complicated contributions of many transitions including the HOMO →
LUMO+3 (28%), the HOMO−3 → LUMO (19%), and the HOMO−1 → LUMO+2 (15%)
transitions.
Designing a large transition magnetic dipole moment (TMDM) of electronic transitions
61
is crucial to create chiral molecules with an excellent chiroptical properties. The dissymmetry
factor (g), that is a measure of the selective response of chiral molecules to the left and right
2
circularly polarized light (CPL), can be expressed as follows; g = 4R/(D + G) = 4(||ꞏ|m|ꞏcos)/(||
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8
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
+
|m| ), where R, D, and G represent the rotatory, electric dipole, and magnetic dipole strengths,
respectively,  and m transition electric dipole moment (TEDM) and TMDM, respectively, and 
the angle of TEDM and TMDM.62,63 The g value is always zero for achiral molecules, and ideal
molecules that selectively respond to only either the left- or right-handed CPL have the maximum
g value (i.e., g = +2 and −2 for the left- and right-handed CPL, respectively). According to this
equation, the g value takes the maximum value when the two vectors of TEDM and TMDM have
the same magnitude and the directions of the two vectors are parallel (i.e., g = ±2 when || = |m|
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
64
and cos = 1). Considering the electronic transitions of organic molecules, the magnitude of
TEDM is usually several orders of magnitude larger than TMDM, therefore, the large g value can
be logically designed by making the magnitude of TMDM large or making TEDM small (i.e., g ~
(
4|m|/||) when || >> |m| and cos = 1). The magnitude of TEDM can be well controlled by
molecular design in intuitive manner because TEDM is approximately proportional to the root of
oscillator strength (f), however, no clear relationship between the molecular structure and the
magnitude of TMDM has not been established yet.
Interestingly, TD-DFT calculations suggested that the transition magnetic dipole moment
(
TMDM) of -expanded helicene 1 was significantly larger than the other helical molecules; the
−1
magnitude of TMDM was |m| = 3.149, 4.800, and 9.039 ergꞏGauss for [7]helicene, [7]heliphene,
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7
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and 1, respectively (Table 1). For the corresponding planar PAHs, the TMDM became large in a
similar manner as increasing the diameter of macrocyclic structures; |m| = 3.669, 5.542, and 9.918
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
−1
ergꞏGauss for coronene, anti-kekulene, and, kekulene, respectively. From these results, the large
TMDM observed for -expanded helicene 1 is likely attributed to the large helical diameter. In
this context, the pure enantiomers of the -expanded helicene, i.e., (P)-1 and (M)-1, are expected
to show an excellent chiroptical properties because of the large TMDM. For the investigation of
circular dichroism (CD) and circularly polarized luminescence (CPL), we tried to isolate the
enantiomers of 1 using chiral HPLC, however, no separation was detected under any condition
tested.
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Figure 4. Absorption (black solid line) and fluorescence (red dashed line) spectra of 1 in
−7
chloroform (c = 7 × 10 M, ex = 320 nm). Blue bar represents the oscillator strengths of
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
electronic transitions of 1 calculated at the TD-B3LYP/6-311g(2d,p) level of theory.
Table 1. Transition dipole moments and chiroptical properties of planar and helical PAHs.a
| b
|m| c
|
cos(m) d
|g| e
Compd
20
−1
/
10− esuꞏcm / 10−20 ergꞏGauss
coronene
anti-kekulene
kekulene
0.00
0.00
3.669
5.542
9.918
3.149
4.800
9.039
–
–
–
–
0.00
–
–
[7]helicene
290.02
298.33
569.80
0.681
0.469
0.419
0.0296
0.0302
0.0266
[
7]heliphene
1
a
Calculated at the TD-B3LYP/6-311g(2d,p) level of theory. The electronic transition having
large transition magnetic dipole moment (|m|) was chosen in this table (See the Supporting
Information) i.e., The values of the S –S transition for anti-kekulene and [7]heliphene, the S –S
transition for [7]helicene and 1, and the S –S transition for
0
1
0
2
transition for kekulene, the S
0
–S
3
0
7
coronene.
Helical Inversion Process. According to the literature,36,37 the barrier for the helical
‡
−1
inversion of a [7]heliphene derivative is remarkably low (G = 12.6 kcalꞏmol at −27 °C

‡
−1
determined by low-temperature H NMR study) compared to [7]helicene (G = 41.7 kcalꞏmol ).
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Vollhardt reported that the low configurational stability of [7]heliphene is likely associated with
the flexible molecular framework of phenylenes that consists of alternating 6- and 4-membered
rings.38 Based on this background, the racemization dynamics of 1, consisting solely of 6-
membered benzene rings, is of primary interest in the -expanded helicene chemistry. DFT
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
‡
calculations suggested that the activation barrier for the helical inversion of 1 were G = 54.3
−1
−1
kJꞏmol (13.0 kcalꞏmol ) at 25 °C calculated at the B3LYP/6-311g(2d,p) level of theory (Figure
‡
−1
5
). The corresponding thermodynamic parameters were determined at H = 41.1 kJꞏmol and
‡
−1
−1

S = −44.4 Jꞏmol ꞏK , from which the half-life of the helical inversion process of 1 was
‡
estimated to be only 1/2 = 0.4 ms at 25 °C (Tables 2 and S9). To compare the G value with the
other reference molecules, the activation barriers were also calculated for [7]heliphene and
‡
−1
[
7]helicene in the same conditions, which were G = 74.3 and 170.0 kJꞏmol (17.7 and 40.6
−1
kcalꞏmol ), respectively. Thus, the racemization barrier of 1 was suggested to be even smaller
than that of [7]heliphene (Figure S14). We note that there was no configurational disorder between
(
P)-1 and (M)-1 in the single crystalline structure (Figure 2), suggesting that the helical inversion
process of 1 can be well suppressed in the solid state or in the rigid matrixes.
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2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 5. Racemization process between (P)-1 and (M)-1 and the relative Gibbs free energy
(
kJꞏmol−1) calculated at the B3LYP/6-311g(2d,p) level of theory.
Table 2. Thermodynamic parameters for the helical inversion process.a
‡
−1
‡
−1ꞏ −1
‡
−1
−1

(
H / kJꞏmol
S / Jꞏmol K
G / kJꞏmol (kcalꞏmol )
50 °C 25 °C 100 °C
compd
−1
−1ꞏ −1
kcalꞏmol )
(calꞏmol K )
−
1
41.1 (9.8)
−44.4 (−10.6)
−21.9 (−5.2)
−16.7 (−4.0)
51.0 (12.2) 54.3 (13.0) 57.7 (13.8)
72.6 (17.4) 74.3 (17.7) 75.9 (18.1)
168.7 (40.3) 170.0 (40.6) 171.2 (40.9)
[7]heliphene
67.7 (16.2)
165.0 (39.4)
[7]helicene
a
Calculated at the B3LYP/6-311g(2d,p) level of theory.
Molecular Spring Constants. Flexibility of molecular framework was investigated
based on the change of the potential energy (Eel) upon elongation of molecular springs (Figure 6).
The distance between the two carbon atoms at the end of molecule (r) in equilibrium structure was
5.15, 5.33, and 5.59 Å for 1, [7]heliphene, and [7]helicene, respectively. The potential energy
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surface was scanned from the equilibrium structure with 10 steps in 0.2 Å increments. By fitting
2
the change of potential energy using the function of spring energy, i.e., Eel = (1/2)ꞏkꞏ(r) , the
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
−1
force constants (k) of the molecular springs was determined as k = 0.33, 0.86, and 3.07 Nꞏm for
, [7]heliphene, and [7]helicene, respectively. It is noted that the force constant of 1 (0.33 Nꞏm−1)
1
−1
−1
was almost one-tenth of [7]helicene (3.07 Nꞏm ) and less than half of [7]heliphene (0.86 Nꞏm ),
suggesting that the -expanded helicene 1 can be regarded as a soft molecular spring with a small
spring constant. Interestingly, the intensity ratio of the force constants of molecular spring,
(
1/2.6/9.3 for 1/[7]heliphene/[7]helicene), roughly satisfied the force constant formula of
macroscopic spring materials—i.e., the spring constant is inversely proportional to the third power
of the diameter (k  d
−3); the force constant ratio of 1/2/8 was expected from the helical diameter
h
= 10.2/8.2/5.1 Å) according to the force constant formula.65
of molecules (d
h
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1
1
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2
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2
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2
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3
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3
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3
4
4
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5
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 6. Increase of energy upon elongation of molecular spring of helical analogue of kekulene 1
(
green circle), [7]heliphene (blue square), and [7]helicene (red triangle) calculated at the B3LYP/6-
3
11g(2d,p) level of theory.
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Conclusion
In conclusion, a -expanded helicene (1)—that is the helically twisted analogue of
kekulene—was successfully synthesized using a ring-closing olefin metathesis reaction. The
electronic state of 1 was very similar to the cyclic planar PAH analogues, i.e., kekulene and
septulene, as evidenced by the alternately localized aromatic character and the absorption and
emission properties of 1. We found that the transition magnetic dipole moment of 1 was
approximately three times larger than that of [7]helicene, which is likely attributed to the -
expanded molecular geometry with the large helical diameter. Associated with the small local
deformation, the activation barrier for the helical inversion significantly decreased with increasing
the helical diameter of helix-shaped molecules. The flexibility of the -expanded helicene
framework is likely attributed to the number of C–C bonds in a helical pitch, which appears to be
a similar origin of the mechanical properties of the macroscopic spring materials. Since the -
expanded helicene having a larger helical diameter is consisting of more C–C bonds per helical
pitch, the steric distortion required for the elongation of molecular spring can be effectively
distributed throughout the molecular structure. From this perspective, -expanded helicenes are
promising candidates for molecular springs with a small spring constant. Synthetic strategy to
extend the -expanded helicene framework along the helical axis and the elucidation of unique
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
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5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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9
0
1
2
3
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8
9
0
1
2
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0
1
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0
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1
1
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2
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6
chiroptical properties of the helical analogue of kekulenes are currently under investigation in our
group.
0
1
2
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6
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8
9
0
1
2
3
4
5
6
7
8
9
0
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9
0
Experimental Section
Synthesis of 2,2′′′′′′,4′,4′′,4′′′,4′′′′,4′′′′′,6′,6′′,6′′′,6′′′′,6′′′′′-hexadeca((E)-prop-1-
enyl)-m-septiphenyl (9). To a solution of (all-E)-8 (64 mg, 0.10 mmol), (all-E)-6 (88 mg, 0.22
mmol), and K
2
CO
3
(90 mg, 0.65 mmol) in toluene (4 mL) and water (1.5 mL) was added Pd(PPh
)
3 4
(
40 mg. 0.035 mmol) under N
2
atmosphere. The reaction mixture was stirred at 100 °C for 17 h.
After cooling to room temperature, the reaction mixture was washed with water and extracted with
CH
2
Cl
2
. The combined organic layer was dried over MgSO , filtered, and evaporated to dryness.
4
The crude product was purified by silica gel column chromatography (hexane/CH
2
Cl = 85/15) to
2
1
give (all-E)-9 (66 mg, 0.065 mmol, 64%) as a white solid. H NMR (500 MHz, CDCl
3
, ): 1.53–
1
.91 (m, 36H), 5.98–6.38 (m, 24H), 6.80–7.38 (m, 11H), 7.48–7.59 (m, 2H), 7.64–7.78 (m, 5H);
HRMS–APCI–Orbitrap (m/z): [M + H] calcd for C78H
79+, 1015.6176; found, 1015.6155.
+
Synthesis
of
Dinaphtho[1′′,2′′:6,7;1′′′,2′′′:6′,7′]diphenanthro[3,2-a:2′,3′-
j]anthracene (1). A solution of (all-E)-9 (36 mg, 0.035 mmol) and 2,6-dichloro-1,4-benzoquinone
(
10 mg, 0.057 mmol) in 1,2,4-trichlorobenzene (2.5 mL) and toluene (2.5 mL) was degassed by
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Freeze-Pump-Thaw cycles four times. Grubbs 2nd generation catalyst (14 mg, 0.016 mmol) was
then added under N atmosphere and the reaction mixture was stirred at 70 °C for 3 days. After
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
cooling to room temperature, CHCl (6 mL) and MeOH (6 mL) were added to the reaction mixture.
3
The resulting precipitates were filtered by suction, washed with CHCl
3
(20 mL) and hexane (60
1
mL), and dried under reduced pressure to give 1 (17 mg, 0.025 mmol, 71%) as a yellow solid. H
NMR (600 MHz, CS
2
/CDCl
3
= 67:33, ): 6.34 (t, J = 7.4 Hz, 2H), 6.71 (t, J = 7.3 Hz, 2H), 7.44
(
d, J = 7.4 Hz, 2H), 7.65 (d, J = 8.8 Hz, 2H), 7.88 (d, J = 8.8 Hz, 2H), 7.90-7.96 (m, 8H), 8.406 (s,
2H), 8.410 (s, 2H), 8.42 (s, 1H), 8.79 (d, J = 8.0 Hz, 2H), 10.48 (s, 2H), 10.71 (s, 2H), 10.75 (s,
13
1
H); C NMR (151 MHz, CS
2
/CDCl = 67:33, ): 116.1 (CH), 117.1 (CH), 117.2 (CH), 122.6
3
(CH), 126.2 (CH), 126.3 (CH), 127.2 (CH), 127.4 (CH), 127.5 (CH), 127.6 (CH), 127.7 (CH),
127.9 (CH), 128.0 (CH), 128.2 (4°), 129.3 (4°), 129.4 (4°), 129.8 (4°), 130.9 (4°), 131.0 (4°), 131.2
+
31+, 679.2420; found,
(
4°), 131.5 (4°); HRMS–APCI–Orbitrap (m/z): [M + H] calcd for C54
H
6
79.2409.
X-ray Crystallography. X-ray crystallographic analysis was performed on a Rigaku
XtaLab P200 diffractometer equipped with a Dectoris PILATUS 200 K detector, using a VariMax
Mo Optic with Mo−Kα radiation (λ = 0.71075 Å) and a confocal monochromator. The data
collection and cell refinement were performed using CrysAlisPro software. The structures were
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