Construction of Helical Structures with Multiple Fused Anthracenes: Structures and Properties of Long Expanded Helicenes

Article abstract of DOI:10.1002/chem.202004720

Polycyclic aromatic compounds consisting of four or five fused anthracene units were synthesized by PtCl₂-catalyzed cycloisomerization as novel long expanded helicenes. These compounds have helical structures with significant stacking of the te

Full text of DOI:10.1002/chem.202004720

Accepted Article
Accepted Article
Title: Construction of helical structures with multiple fused anthracenes:
structures and properties of long expanded helicenes
Authors: Kei Fujise, Eiji Tsurumaki, Kan Wakamatsu, and Shinji Toyota
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Construction of helical structures with multiple fused
anthracenes: structures and properties of long expanded
helicenes
Kei Fujise,[a] Eiji Tsurumaki,[a] Kan Wakamatsu,[b] Shinji Toyota*[a]
[a]
[b]
K. Fujise, Dr. E. Tsurumaki, Prof. Dr. S. Toyota
Department of Chemistry, School of Science
Tokyo Institute of Technology
2–12–1 Ookayama, Meguro-ku, Tokyo 152-8551 (Japan)
E-mail: stoyota@chem.titech.ac.jp
Prof. Dr. K. Wakamatsu
Department of Chemistry, Faculty of Science
Okayama University of Science
1–1 Ridaicho, Kita-ku, Okayama 700-0005, (Japan)
Supporting information and the ORCID identification numbers for the authors of this article can be found via a link at the end of the document.
Abstract: Polycyclic aromatic compounds consisting of four or five
fused anthracene units were synthesized by PtCl2-catalyzed
cycloisomerization as novel long expanded helicenes. These
compounds have helical structures with significant stacking of the
terminal anthracene moieties at 0.33 nm interlayer distance. In the
UV-vis and fluorescence spectra, the absorption and emission bands
were red-shifted as the number of fused anthracene units was
increased. The characteristic broad and long-lived emission bands of
the long analogs are explained by the excimer-like stabilization of the
excited state. These photophysical data as well as their cyclic
voltammetric data are discussed on the basis of the π-conjugation and
interlayer π···π interactions in the molecular structures and the
molecular orbitals. The barrier and mechanism of helical inversion are
also reported.
properties of the π-conjugated system. We were interested in how
the structures and the photophysical properties are influenced by
the introduction of additional anthracene units into the helical
system, and designed [4]HA and [5]HA having four and five
anthracene units, respectively. The synthesis of these
compounds is challenging because their structures can be
regarded as a substructure of helically twisted nanographenes[9]
or molecular springs.[6,10] We herein report the synthesis of these
long helical anthracenes and compare their structures, properties,
and dynamic behavior involving helical inversion with those of
anthracene (A), [2]HA (anthra[2,1-a]anthracene), and [3]HA.
In the chemistry of helicenes, researchers have continued
their efforts to synthesize long analogs by adding benzene units
to realize novel structures and properties.[1,2] The helical structure
of the longest helicene, [16]helicene, has more than 2.5 turns, and
some benzene units are triply layered.[3] Recently, we reported
new aromatic compound [3]HA (hereinafter, [n]HA refers to a
helical anthracene with n fused anthracene units) as an expanded
helicene,[4] which is defined as a helicene with extra linearly fused
benzene rings into angularly fused units (Figure 1).[5,6] Even
though the racemization via helical inversion takes place rapidly
for [3]HA, the barrier is sufficiently enhanced by the introduction
of diphenyl substituents enough to enable resolution of its
enantiomers at room temperature. We found that these
enantiomers exhibited considerably intense circular dichroism
and circularly polarized luminescence activities for simple organic
molecules.[7,8] In [3]HA, the number of turns is approximately one
and the terminal benzene rings interact with each other but do not
completely overlap. The introduction of additional anthracene
units into [3]HA should increase the number of helical turns
exceed one, leading to significant interactions between the
anthracene units in a face-to-face fashion. Such interactions
across the helical turns should influence the structures and
Figure 1. Structures of anthracene (A) and helical anthracenes ([n]HA), and a
schematic presentation of a helical structure. Fused sides are highlighted in red.
The
target
compounds
were
synthesized
by
cycloaromatization of terminal alkyne precursors prepared by the
Suzuki-Miyaura coupling of suitable arene building units, as was
adopted for the synthesis of [3]HA.[4] The synthetic routes are
shown in Scheme 1. Compound [2]HA, which had been
synthesized by photocyclization previously,[11] was synthesized
by the cycloaromatization approach. The precursor of [2]HA was
prepared by the Suzuki-Miyaura coupling of
1 and 1-
1
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chloroanthracene (2)[12] followed by desilylation. Compound 3 was
heated at 80 °C for 24 h in the presence of PtCl2 and P(C6F5)3 to
give [2]HA in 71% isolated yield.[13] Compound 6, a substituted
interlayer distance between the terminal anthracene units, which
was calculated from the coordinates of the carbon atoms around
benzene rings B and K, is 0.36–0.37 nm and regarded as the pitch
of the central helix. This value is comparable to the calculated
pitches of [n]helicenes (ca. 0.37 nm for n = 11–14).[3,9a,10a,15] The
shortest nonbonding C···C distance between the layered
anthracene units, 0.332 nm for C6···C22, is comparable to the
sum of the van der Waals radii of C(sp2) atoms. This molecule has
three [4]helicene substructures around benzenes B–E, E–H, and
H–K, in which the torsion angles along the inner carbon rims are
in the range of +8.6 to +21.4°. Therefore, the stereochemistry of
this structure is P for the overall molecule (global helicity), and
P,P,P for the three [4]helicene moieties (local helicity). We
calculated the HOMA (Harmonic Oscillator Model of Aromaticity)
index for each benzene ring from the X-ray structures. The values
are 0.72–0.80 for the terminal and linearly fused benzene rings (A,
B, E, H, K, and L) and 0.29–0.47 for the angularly fused benzene
rings (C, D, F, G, I, and J), indicating that the π-conjugation is less
effective in the latter.
analog of [2]HA, was prepared from
1
and 1,8-
dichloroanthracene (4) in a similar manner followed by Pd-
catalyzed borylation. The Suzuki-Miyaura coupling of 6 and
dibromobenzene 7 in 2:1 ratio followed by desilylation afforded
terminal alkyne 8. Cyclization of this precursor afforded [5]HA in
44% isolated yield as an orange solid. For the precursor of [4]HA,
two kinds of aryl units 9 and 6 were introduced into 7 by coupling
reactions. Cyclization of 10 afforded [4]HA in 46% isolated yield
as a yellow solid. Although [4]HA and [5]HA were less stable than
[3]HA and slowly decomposed in solution under ambient
conditions, we were able to isolate these long analogs and
measure their spectroscopic data by taking the necessary
precautions. Compounds [4]HA and [5]HA were characterized by
1H NMR and mass spectral measurements. These compounds
1
showed signals in the aromatic region of the H NMR spectra as
expected for the symmetric structures.[14] The signals due to the
inner hydrogen atoms in the nonterminal anthracene units were
observed at 12.6 ppm for [4]HA and 12.5 and 13.3 (central) ppm
for [5]HA (cf. 12.11 ppm for [3]HA).[4] These hydrogen atoms
were extraordinarily deshielded as ordinary neutral aromatic
compounds because of the enhanced ring current effect of the
aromatic moieties.
Cl
1.
Bpin
2
condition B
71%
condition A
2. Bu4NF, THF
Cl
SiiPr3
[2]HA
1
3
Cl
1.
1. condition B
85%
4
Cl
Bpin
condition A
2. condition C
96%
2. Bu4NF, THF
Figure 2. X-ray structures of [4]HA (ORTEP: 50% probability) and calculated
structures of [4]HA and [5]HA at M06-2X/6-31G(d,p) level. The structures of P
global helicity are shown.
5
6
Br
Br
1. 6, condition A
condition B
44%
87%
iPr3Si
SiiPr3
[5]HA
2. Bu4NF, THF
96%
7
The X-ray structure of [4]HA was reproduced by DFT
calculations at the M06-2X/6-31G(d,p) level (Figure 2).[16,17] The
global minimum structure of C2 symmetry has the local helicity of
P,P,P, which is slightly more stable than the other energy-
minimum structure of the P,P,M form. The calculations of [5]HA
gave two energy-minimum structures having comparable
energies; one is C2 symmetric (Figure 2) and the other is less
symmetric.[18] The two terminal anthracene units on both sides are
nearly double layered with each other. The typical short interlayer
C···C distances (ca. 0.33 nm) are shown in the optimized
structures. The number of helical turns of [5]HA is ca. 1.5, which
is unambiguously smaller than the ideal value (1.67) expected for
the planar triangular structure due to torsional deformation. The
torsion angles around the four [4]helicene substructures are in the
range of +10.7° to +17.2°, meaning that the local helicity is
P,P,P,P. In the other stable structure, one of the nonterminal
[4]helicene units is nearly coplanar.
8
Bpin
1.
Br
Br
9
condition B
46%
condition A 61%
iPr3Si
SiiPr3
[4]HA
2. 6, condition A
91%
3. Bu4NF, THF
98%
7
10
Scheme 1. Synthesis of target compounds [2]HA, [4]HA, and [5]HA. Condition
A: Pd2(dba)3·CHCl3, SPhos, and K2CO3 in toluene/H2O/1,4-dioxane. Condition
B: PtCl2 and P(C6F5)3 in toluene. Condition C: (Bpin)2, Pd2(dba)3, SPhos, and
KOAc in 1,4-dioxane. For detailed conditions, see Supporting Information.
A single crystals of [4]HA suitable for X-ray analysis was
obtained as racemic crystals and the structures of P-helicity are
shown in Figure 2. The molecule takes a helical structure of
approximately C2 symmetry. The terminal anthracene units are
stacked on top of each other but do not completely overlap. The
The stability of the helical molecules was analyzed by using
the homodesmotic reaction shown in Scheme 2, where the
products
consisted
of
planar
and
non-strained
2
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benzoanthracenes.[10a,19] The enthalpies of reaction (–H)
calculated at the M06-2X/6-31G(d,p) level were 35.5 ([2]HA),
63.3 ([3]HA), 69.7 ([4]HA), 67.5 ([5]HA), and 65.5 ([6]HA) kJ mol–
1. The positive values mean that the reactants involving an [n]HA
molecule are less stable than the products. The increase of the
enthalpy of reaction from [2]HA to [3]HA is mainly attributed to
the increased steric strain resulting from the out-of-plane
deformation. In contrast, the values are nearly the same for [n]HA
(n = 3–6) even though a deformed anthracene unit is added
successively. This indicates that the destabilization by the steric
strain is almost cancelled by the stabilization by the π···π
interactions in [4]HA and the long analogs. This factor is
supported by the calculated binding energy of a face-to-face
stacked anthracene dimer (35–42 kJ mol–1).[20] Therefore,
attractive interactions should play an important role in the layered
anthracene moieties in the long analogs. These structural and
thermochemical features are well visualized by noncovalent
interaction (NCI) analysis (Figure 3).[21] The green isosurface
showing weak interactions spreads over the stacked anthracene
moieties in [4]HA and [5]HA, in contrast to the very limited
isosurface in [3]HA.
that decreased as the number of anthracene units increased
(Table 1). In the fluorescence spectra, [4]HA and [5]HA showed
weak emission bands (f < 0.10) at long wavelengths (500–650
nm). These emission bands are significantly broad and long-lived
compared with the emission bands of the other compounds.[22] In
addition, [4]HA and [5]HA have large Stokes shifts of 2,400 cm–
1. These features mean that the excited state of the long analogs
should be stabilized by the interlayer interactions in an excimer-
like manner.[23,24] In fact, the interlayer separations in [4]HA and
[5]HA are narrowed by ca. 0.004–0.009 nm in the optimized
structures at S1 excited state compared with those at S0 ground
state (Figure S10).
Figure 4. UV-vis (left) and fluorescence (right) spectra of anthracene A (purple),
[2]HA (blue), [3]HA (green), [4]HA (orange), and [5]HA (red) in CHCl3 at room
temperature.
2
+
–DH
n–2
+
n–2
n–2
In order to investigate the electrochemical properties, we
conducted cyclic voltammetry measurements of the series of
compounds. Compounds [4]HA and [5]HA exhibited two
reversible oxidation waves, whereas [2]HA and [3]HA exhibited
[n]HA
Scheme 2. Homodesmotic reaction for estimation of strain and attractive
energies of [n]HA.
1
only one wave (Figure S7). The first oxidation potential Eox
continuously decreased as the number of anthracene units
increased. These data mean that the long analogs are readily
oxidized to form relatively stable oxidized species. This trend is
consistent with the calculated HOMO levels that increased as the
number of anthracene units increased (Table 1 and Figure S9).
Regardless of the chiral structures of [4]HA and [5]HA, we
failed to resolve their enantiomers by chiral HPLC at room
temperature. Thus, the helical inversion process was analyzed by
DFT calculations. The global minimum structure of [4]HA was
converted into its enantiomeric structure by the stepwise
mechanism shown in Figure 5. The first inversion takes place at
a terminal [4]helicene moiety with a relatively low barrier, and the
second inversion takes place at the central [4]helicene moiety via
the Cs symmetric transition state. The following inversion at the
remaining terminal [4]helicene moiety completes the
enantiomerization. The barrier to helical inversion was calculated
to be G 95.3 kJ mol–1 for [4]HA. This barrier is not sufficiently
high to resolve the enantiomers at room temperature. The helical
inversion of [5]HA takes place in a stepwise fashion via four
intermediates. The barrier to the overall process was estimated to
be ca. 110 kJ mol–1, even though the least stable transition state
structure could not be fully optimized. Because of this high barrier,
the unsuccessful resolution was attributable to poor separation as
well as low stability. The barriers to helical inversion increased in
the order of [2]HA (21.8 kJ mol–1) < [3]HA (25.2 kJ mol–1)[4] <<
[4]HA < [5]HA. The introduction of the fourth unit considerably
enhanced the barrier because of the destabilization of the
transition state by steric interactions between the terminal
moieties. These barriers of [4]HA and [5]HA are lower than those
Figure 3. NCI plots (isosurface value 0.5) of stable C2 symmetric structures of
[3]HA, [4]HA, and [5]HA. Green isosurface shows regions having noncovalent
(van der Waals) interactions. Blue and red isosurfaces show regions having
attractive and repulsive interactions, respectively (See also Figure S13).
The UV-vis and fluorescence spectra of [4]HA and [5]HA were
measured in CHCl3 at room temperature. These data were
compared with those of the other compounds, as shown in Figure
4 and Table 1. Compounds [4]HA and [5]HA generated broad
and continuous shoulder bands from 330 nm to 500 nm in the UV-
vis spectra. Because the absorption maxima at the longest
wavelength were not clear, we compared the onset wavelengths.
The wavelengths were red-shifted from 391 nm (A) to 514 nm
([5]HA) as the number of anthracene units increased. This
tendency is consistent with the calculated HOMO-LUMO gaps
3
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Table 1. Photophysical, electrochemical, and DFT calculation data of anthracene (A) and [n]HA analogs.
UV-vis[a]
max [nm]
379
FL[a]
Stokes shift
DFT [e]
CV
HOMO–
Eox [V]
[cm–1
]
HOMO [eV]
LUMO [eV]
[c]


onset [nm (eV)] [b]

em [nm]
f
f [ns] [d]
LUMO [eV]
A
391 (3.17)
455 (2.72)
478 (2.59)
497 (2.50)
514 (2.41)
383, 404
443, 471
467, 501
505, 537
526, 556
0.12
2.0
276
–5.26
–5.03
–4.92
–4.81
–4.72
–4.65
–1.61
–1.90
–1.94
–2.05
–2.06
–2.03
3.66
3.13
2.97
2.76
2.67
2.62
+1.00 [f]
[2]HA 437
[3]HA 459
[4]HA 450
[5]HA 467
[6]HA
0.46
16.4
9.6
310
+0.67
0.18
373
+0.61
0.084
0.052
14.2
16.9
2420
2400
+0.48, +0.79
+0.39, +0.68
[a] Measured in CHCl3 at 1.0  10–5 mol L–1 (UV-vis) and 1.0–2.9  10–5 mol L–1 (FL). [b] Determined from band onset at  = 100 L mol–1 cm–1. [c] Absolute
fluorescence quantum yield. [d] Fluorescence lifetime. [e] At B3LYP/6-31G(d,p)//M06-2X/6-31G(d,p) level. [f] An Irreversible wave.
of [n]helicenes (n = 8–10, 180–210 kJ mol–1) as helical structures
with ca. 1.5 turns.[25] Therefore, the fused anthracene system
having a large helical diameter is flexible during the helical
inversion process compared with the parent helicene system.
resolve their enantiomers, we need to modify the structures to
enhance the racemization barrier as well as the stability and
solubility. Further extension of the anthracene units remains a
challenge in the generation of enantiopure helical anthracenes
and in testing their performance as molecular springs, for example
their contraction-expansion behavior, by spectroscopic
measurements at high pressure. The synthesis and structural
modification of such molecules having a large number of turns are
in progress.
Acknowledgements
This work was partly supported by JSPS KAKENHI Grant Number
JP20H02721 and Nagase Science and Technology Foundation.
The authors thank Professor Osamu Ishitani and Dr. Yusuke
Tamaki of Tokyo Institute of Technology for the measurements of
fluorescence spectra.
Keywords: Arenes • Molecular structure • Electronic spectra •
Helical inversion • π···π interactions
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10.1002/chem.202004720
Chemistry - A European Journal
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Entry for the Table of Contents
Helical Molecules
K. Fujise, E. Tsurumaki, K. Wakamatsu, S. Toyota*
Construction of helical structures with multiple fused anthracenes: structures and properties of long expanded
helicenes
Helical structures are lengthened by fusing of four and five anthracene units. Significant interlayer π···π interactions are evidenced
by molecular structures and spectroscopic data.
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