Formation of one-dimensional helical columns and excimerlike excited states by racemic quinoxaline-fused [7]carbohelicenes in the crystal

Article abstract of DOI:10.1002/chem.201402426

A series of quinoxaline-fused [7]carbohelicenes (HeQu derivatives) was designed and synthesized to evaluate their structural and photophysical properties in the crystal state. The quinoxaline units were expected to enhance the light-emitting properties an

Full text of DOI:10.1002/chem.201402426

DOI: 10.1002/chem.201402426
Full Paper
&
Helicenes
Formation of One-Dimensional Helical Columns and Excimerlike
Excited States by Racemic Quinoxaline-Fused [7]Carbohelicenes in
the Crystal
Hayato Sakai,*[a] Sho Shinto,[a] Yasuyuki Araki,*[b] Takehiko Wada,[b] Tomo Sakanoue,[c]
Taishi Takenobu,*[c] and Taku Hasobe*[a]
Abstract: A series of quinoxaline-fused [7]carbohelicenes
(HeQu derivatives) was designed and synthesized to evalu-
ate their structural and photophysical properties in the crys-
tal state. The quinoxaline units were expected to enhance
the light-emitting properties and to control the packing
structures in the crystal. The electrochemical and spectro-
scopic properties and excited-state dynamics of these com-
pounds were investigated in detail. The first oxidation po-
tentials of HeQu derivatives are approximately the same as
that of unsubstituted reference [7]carbohelicene (Heli),
whereas their first reduction potentials are shifted to the
positive by about 0.7 V. The steady-state absorption, fluores-
cence, and circular dichroism spectra also became redshifted
compared to those of Heli. The molecular orbitals and
energy levels of the HOMO and LUMO states, calculated by
DFT methods, support these trends. Moreover, the absolute
fluorescence quantum yields of HeQu derivatives are about
four times larger than that of Heli. The structural properties
of the aggregated states were analyzed by single-crystal
analysis. Introduction of appropriate substituents (i.e., 4-me-
thoxyphenyl) in the HeQu unit enabled the construction of
one-dimensional helical columns of racemic HeQu deriva-
tives in the crystal state. Helix formation is based on intracol-
umn p-stacking between two neighboring [7]carbohelicenes
and intercolumn CH···N interaction between a nitrogen
atom of a quinoxaline unit and a hydrogen atom of a heli-
cene unit. The time-resolved fluorescence spectra of single
crystals clearly showed an excimerlike delocalized excited
state owing to the short distance between neighboring
[7]carbohelicene units.
related photochemistry, for example, chiral recognition,[2] sepa-
ration,[3] switching,[4] and optoelectronics.[5]
Introduction
Helical chirality is ubiquitous in biological molecular systems
such as DNA, proteins, and nucleic acids.[1] In general, helix for-
mation seems to be a quasi-universal option for high-order
spatial organization of matter at both microscopic and macro-
scopic levels. Such a prominent role of one-dimensional helical
arrangements in natural systems has recently stimulated the
curiosity of researchers in the fields of materials chemistry and
To construct designed helical molecular superstructures, syn-
thetic strategies such as covalent and noncovalent self-assem-
bled organization of various chiral and achiral units have been
commonly utilized. Macromolecular organization exclusively
through covalent bonding is uncommon because of the syn-
thetic difficulty and poor solubility.[6] Representative examples
are helical polyacetylenes with continuous p conjugation
through covalent bonds.[7] In contrast, supramolecular methods
utilizing intermolecular interactions such as p–p interaction-
s,[5a,8] van der Waals interactions,[5e,9] hydrogen bonds,[3b,4e,10]
and coordination bonds[4c,8e,11] are powerful tools. Supramolec-
ular techniques make it possible to achieve novel phenomena
including spectroscopic and optical properties in comparison
with those of the corresponding monomeric forms.[12] On the
one hand, p–p interactions in organized structures are espe-
cially known to yield new emissive states, such as excimers.[13]
On the other hand, excimer formation also reveals details of
the superstructure. Accordingly, appropriate organization by
both covalent and noncovalent bonding/interaction is essential
for construction of one-dimensional macromolecular helical
systems.
[a] Dr. H. Sakai, S. Shinto, Prof. T. Hasobe
Department of Chemistry, Faculty of Science and Technology
Keio University, Yokohama, Kanagawa 223-8522 (Japan)
Fax: (+81)45-566-1697
E-mail: hasobe@chem.keio.ac.jp
[b] Prof. Y. Araki, Prof. T. Wada
Institute of Multidisciplinary Research for Advanced Materials
Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577 (Japan)
Fax: (+81)22-217-5608
E-mail: araki@tagen.tohoku.ac.jp
[c] Prof. T. Sakanoue, Prof. T. Takenobu
Department of Applied Physics, Waseda University
3-4-1, Okubo, Shinjuku, Tokyo 169-8555 (Japan)
Fax: (+81)3-5286-2981
E-mail: takenobu@waseda.jp
To obtain the above photofunctionalized and helical macro-
molecular systems, carbohelicene is a good candidate as the
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402426.
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basic molecular unit. Carbohelicenes are polycyclic aromatic
hydrocarbons composed of ortho-condensed benzene rings.
The steric hindrance arising in this ortho condensation pre-
cludes planarity for molecules with sequences of five or more
rings and causes the p-conjugated structures to become dis-
torted into helices with intrinsically chiral structures.[14] Conse-
quently, carbohelicenes exhibit unique optical properties, for
example, high anisotropy factor (g=De/e) with increasing the
number of ortho-condensed benzene units.[15] Sapir and co-
workers reported the detail excited-state dynamics of carbohe-
licenes. In the range of 5–9 benzene units (i.e., [5]- to [9]carbo-
helicene), the fluorescence quantum yields FFL are approxi-
mately zero, whereas the quantum yields of intersystem cross-
ing FISC exceed 0.9.[16] Thus far, the number of publications
concerning the excited-state dynamics of carbohelicenes is lim-
ited, so new synthetic strategies to control the excited-state
dynamics are essential for high fluorescence quantum yields
and subsequent further development of optical applications
such as circularly polarized luminescence.[17]
Figure 1. Chemical structures of quinoxaline-fused [7]carbohelicenes in this
study.
quinoxaline-fused [7]carbohelicene in the single crystal is note-
worthy. The helical columnar structure is mainly formed by in-
tracolumn p–p stacking interaction between two neighboring
carbohelicene units and intercolumn CH···N interaction be-
tween a nitrogen atom in a quinoxaline unit and a hydrogen
atom in a helicene unit, which is in sharp contrast to the pack-
ing motif of the corresponding enantiomer. Moreover, the exci-
merlike delocalized excited state originating from neighboring
carbohelicene units in the single crystal was clearly observed
by time-resolved fluorescence measurements.
According to the previous reports and a database of single-
crystal structures of helicenes, one-dimensional columnar heli-
cal packing driven by p–p stacking interactions is unusual, and
the major structural motif of helicenes is noncolumnar packing
driven by CH···p interactions.[1c,18] Recently, Nozaki and co-
workers reported one-dimensional column formation of chiral
l5-phospha[7]helicenes by utilizing dipole–dipole interaction in
addition to p–p stacking interactions.[19] Similar one-dimension-
al helix formation by heterochiral crystals has been already re-
ported.[20] However, one-dimensional columnar helical struc- Results and Discussion
tures of racemic helicene derivatives in the single-crystal state
Synthesis
have yet to be reported, although there are a few reports con-
cerning the corresponding one-dimensional nonhelical struc-
tures.[20c,21]
As mentioned above, quinoxalines can be simply synthesized
from diketones. We used this synthetic strategy to synthesize
a series of quinoxaline-fused [7]carbohelicene derivatives.
Quinoxalines (i.e., benzopyrazines) have a synthetic advant-
age over other aromatic compounds because of their conven-
ient synthesis from diketones and diamines.[22] Quinoxalines
show strongly electron-accepting behavior due to their high
electron affinity, which is derived from the two unsaturated ni-
trogen atoms. Accordingly, benzopyrazines are widely utilized
as light-emitting and electron-transporting materials because
of the highly polarized nature of the imine units in the pyra-
zine ring.[22] Additionally, the polarized nature of a quinoxaline
unit may have an effect on the crystal packing. Various small
organic molecules, including promising p-type organic materi-
als such as oligoacences, show a herringbone packing motif.
However, if the benzene rings are replaced by pyrazine rings,
the crystal stacking motif originating from CH···p interactions
changes to intracolumn p stacking between two neighboring
molecules and intercolumn CH···N interactions between two
neighboring columns.[23]
[7]Carbohelicene 1,2-diketone (He-Ket) was obtained from 1,2-
dimethoxy [7]carbohelicene (He-OMe; Scheme 1). First, He-
OMe was synthesized by the reported synthetic method of
Harrowven and co-workers.[24] Next, He-Ket was obtained by di-
hydroxylation of He-OMe, which was followed by dehydration.
The yield of the above two steps was 64%. Finally, quinoxa-
line-fused [7]carbohelicenes were synthesized by dehydration
of He-Ket and corresponding 1,2-diaminobenzene derivatives
under appropriate synthetic conditions (Table 1).
In view of the above points, we synthesized a series of qui-
noxaline-fused [7]carbohelicene derivatives (Figure 1). The inte-
gration of quinoxaline and [7]carbohelicene units contributes
to the control of spectroscopic behavior (e.g., fluorescence) in
the monomeric form and the structural properties in the ag-
gregate state. In particular, formation of one-dimensional heli-
cal columns by a racemic bis(4-methoxyphenyl)-substituted
Scheme 1. Synthesis of quinoxaline-fused [7]carbohelicenes.
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Table 1. Synthetic conditions and yields of new quinoxaline-fused [7]car-
bohelicenes.
T
[h]
Yield
[%]
Solvent
AcOH
1
3
65
68
HeQu
AcOH/CHCl3
(1/5 v/v)
HeQu-Ph
AcOH/EtOH
(1/2 v/v)
3
1
62
63
HeQu-Ani
Figure 2. Cyclic and differential pulse voltammograms of HeQu-Ani in CH2Cl2
with 0.1m nBu4NPF6 as supporting electrolyte. Scan rate: 0.05 VsÀ1 for CV
and 0.01 VsÀ1 for DPV.
AcOH/EtOH/CH2Cl2
(1/1/1 v/v/v)
HeQu-C16
Table 2. Electrochemical properties and energy levels of HeQu deriva-
tives and Heli.
Electrochemical properties of quinoxaline-fused [7]carbo-
helicenes
Helicene Eox[a,b] [V]
Ered1
Ered2[a] [V] EHOMO[c] [eV] ELUMO[c] [eV]
[a]
HeQu
1.47 (À5.74 eV)[d] À1.34 À1.78
À5.47
À5.44
À5.37
À5.36
À2.26
À2.29
À2.19
À1.49
The electrochemical properties of quinoxaline-fused [7]carbo-
helicenes were investigated by using cyclic voltammetry (CV)
and differential pulse voltammetry (DPV) to examine the sub-
stituent effects on the reduction and oxidation potentials. A
typical voltammogram of HeQu-Ani in CH2Cl2 containing 0.1m
nBu4NPF6 is shown in Figure 2. Supporting Information Fig-
ure S1 shows cyclic and differential pulse voltammograms of
HeQu and HeQu-Ph, respectively. The electrochemical poten-
tials of quinoxaline-fused [7]carbohelicenes and reference Heli
are summarized in Table 2. Two reversible redox couples, corre-
sponding to the first and second reduction processes of HeQu
derivatives, were observed, whereas one quasireversible oxida-
tion peak was found in all of the cyclic voltammograms for
HeQu derivatives. The first reduction (Ered1) and oxidation (Eox)
potentials of HeQu-Ani in CH2Cl2 were determined to be
À1.31 V and 1.43 V versus SCE, respectively. These redox po-
tentials are quite similar to those of HeQu (Ered1 =À1.34, Eox =
1.47 V) and HeQu-Ph (Ered1 =À1.28, Eox =1.47 V). Compared to
the reported first reduction (Ered1 =À2.02 V) and oxidation
(Eox =1.42 V) potentials of reference compound Heli, the first
oxidation potentials of HeQu derivatives are similar to that of
Heli, whereas the corresponding first reduction potential is sig-
nificantly shifted to positive direction by about 0.7 V.
HeQu-Ph 1.47 (À5.74 eV)[d] À1.28 À1.72
HeQu-Ani 1.43 (À5.70 eV)[d] À1.31 À1.73
Heli
1.42[e] (À5.69 eV)[d] À2.02[e]
[a] Volts vs. SCE. [b] Determined by DPV. [c] Calculated by DFT at the
B3LYP/6-31G* level of theory. [d] Calculated HOMO levels (EHOMO) from the
following equation:
MeCN.[26]
E
HOMO =E1/2(Fe/Fe+)+4.80.[25] [e] Reported value in
The HOMO and LUMO levels calculated by the DFT method
at the B3LYP/6-31G* level of theory also support the above
trends observed in electrochemical data (Figure 3, Supporting
Figure 3. Molecular orbitals of HeQu-Ani and Heli (B3LYP/6-31G* level of
theory).
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Information Figure S2, and Table 2). The experimental results
are comparable to those determined by DFT calculations
(Table 2). The HOMO and LUMO are localized on helicene and
quinoxaline units, respectively (Figure 3). This trend is totally
different from the molecular orbitals of Heli. In contrast to the
similar HOMO levels of HeQu derivatives and Heli, the LUMO
levels of HeQu derivatives are strongly stabilized as compared
to that of Heli. The decrease in HOMO–LUMO gap may have
an effect on the spectroscopic properties (see below).
CD spectra of quinoxaline-fused [7]carbohelicene deriva-
tives
The separation of enantiomers from racemic mixtures was ac-
complished by HPLC on a chiral column. Then, we measured
CD spectra of HeQu derivatives to assign the helicity (Figure 5
Steady-state absorption spectra of quinoxaline-fused [7]car-
bohelicene derivatives
The steady-state spectroscopic properties of the quinoxaline-
fused [7]carbohelicenes was studied by means of absorption,
circular dichroism (CD), and fluorescence measurements.
Figure 4 shows absorption spectra of HeQu derivatives. The
Figure 5. CD spectra of a) P-HeQu in THF, b) M-HeQu in THF, c) P-HeQu-Ani
in THF, d) M-HeQu-Ani in THF, and e) P-Heli in CHCl3. Spectrum e was repro-
duced from ref. [15b].
and Supporting Information Figure S4). In all cases, they dem-
onstrate mirror-image values of the (+) and (À) enantiomers
within the experimental error. A decreasing trend in De values
of HeQu derivatives was observed over the wavelength range
of 250–300 nm compared to Heli, whereas the De value in-
creased in the range of 400–500 nm. Thus, attachment of qui-
noxaline to helicene units induced chirality in both UV and visi-
ble regions because of the expanded p-conjugation. We also
explored in greater detail the relations among the molecular
absorption coefficients e and anisotropy factors gCD of these
molecules at absorption maximum wavelength (Table 3). For
example, e and De of Heli at 351 nm are 11800mÀ1 cmÀ1 and
263mÀ1 cmÀ1, respectively. Thus, gCD was estimated to be 22.3ꢁ
Figure 4. UV/Vis spectra of a) 2.0 mm HeQu in THF, b) 2.0 mm HeQu-Ani in
THF, and c) Heli in CH2Cl2. Spectrum c was reproduced from ref. [27].
spectra of HeQu (Figure 4a) and HeQu-Ani (Figure 4b) are sig-
nificantly redshifted and broadened compared to the corre-
sponding Heli reference system (Figure 4c). The spectrum of
HeQu-Ph is nearly identical to that of HeQu-Ani (Supporting In-
formation Figure S3). Considering the above electrochemical
data and DFT calculations (Table 2), a plausible reason for the
redshifts is lowering of the LUMO relative to the HOMO levels
on introducing an electron-withdrawing quinoxaline unit,
which leads to a decrease in the HOMO–LUMO gap. Moreover,
we also calculated the molar extinction coefficients e0–0 from
the 0–0 absorption bands (Table 4). The absorption peak of
Heli e0–0 =474mÀ1 cmÀ1) at 410 nm was originally assigned to
a symmetry forbidden transition.[27] However, these transitions
become allowed when the symmetry is reduced. Therefore,
the e0–0 values of HeQu derivatives increase strongly with in-
creasing bulkiness of the substituents.
10À3 [15b]
Table 3.
.
The date of the HeQu derivatives are summarized in
Table 3. Optical properties of HeQu derivatives and Heli.
Helicene
lobs [nm]
E [mÀ1 cmÀ1
]
De [mÀ1 cmÀ1
]
gCD
HeQu
351
357
360
351
20200
21200
26900
11800
121
103
77.5
263
6.21ꢁ10À3
4.85ꢁ10À3
2.88ꢁ10À3
22.3ꢁ10À3
HeQu-Ph
HeQu-Ani
Heli[a]
[a] Reported value in CHCl3.[15b]
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Steady-state fluorescence spectra of quinoxaline-fused
[7]carbohelicene derivatives
bearing aryl groups decrease slightly compared to that of pris-
tine HeQu, introduction of quinoxaline units enhanced the FFL
values slightly compared to Heli.
Fluorescence spectra of HeQu derivatives were measured in
THF at an excitation wavelength of 330 nm (Figure 6 and Sup-
porting Information Figure S5). The fluorescence spectra of
HeQu (Figure 6a) and HeQu-Ani (Figure 6b) are similar and
they are significantly redshifted by about 70 nm relative to
that of reference Heli (Figure 6c). This trend is also similar to
that in absorption measurements, and is attributable to the de-
crease in HOMO–LUMO gap because of the lowered of LUMO
levels.
To further investigate the light-emitting properties of HeQu
derivatives, fluorescence lifetime measurements were per-
formed. The fluorescence decays of HeQu derivatives were ex-
amined in THF solution with pulsed 390 nm laser light
(Figure 7). The fluorescence lifetimes tFL, which were evaluated
from monoexponential fittings, are summarized in Table 4. tFL
values of HeQu (2.70 ns), HeQu-Ph (2.46 ns), and HeQu-Ani
(2.05 ns) are much shorter than that of Heli (13.8 ns).[13e,18a,b] To
discuss the excited-state dynamics in detail, the net rate con-
Figure 6. Fluorescence spectra of a) 2.0 mm HeQu in THF, lex =330 nm,
b) 2.0 mm HeQu-Ani in THF, lex =330 nm, and c) Heli in 1,4-dioxane. Spec-
trum c was reproduced from ref. [28].
Figure 7. Typical fluorescence decays of a) 2.0 mm HeQu in THF, b) 2.0 mm
HeQu-Ph in THF, and c) 2.0 mm HeQu-Ani in THF. The excitation wavelength
was 390 nm.
stants of fluorescence emission kFL and other processes kother
were determined (Table 4). The kFL values of HeQu derivatives
are one order of magnitude greater than that of Heli. This is in
sharp contrast to small increases of kother values in HeQu deriv-
atives relative to Heli. It is well-known that kFL is significantly
related to the molar extinction coefficient for absorption.[30]
Therefore, we estimated the molar extinction coefficients of 0–
0 absorption bands e0–0 (Table 4). HeQu derivatives have large
e0–0 values relative to that of Heli. The increasing trend in e0–0
values agrees well with that in kFL. On the basis of these re-
sults, we can conclude that introduction of quinoxaline units
successfully contributes to improved light-emitting properties
of HeQu derivatives.
Quantum yields and fluorescence lifetimes of quinoxaline-
fused [7]carbohelicene derivatives in solution
To evaluate the light-emitting properties of HeQu derivatives,
first we determined their absolute fluorescence quantum
yields FFL (Table 4). The FFL values of HeQu (0.07), HeQu-Ph
(0.05), and HeQu-Ani (0.05) are larger than that of Heli (0.02).
In particular, the FFL value of HeQu is about four times larger
than that of Heli. Although the FFL values of HeQu derivatives
Table 4. Spectroscopic properties of HeQu derivatives and Heli.[a]
[c]
Helicene FFL tFL[d] [ns] kFL ꢁ106 [sÀ1
]
kother ꢁ107 [sÀ1 e0–0 [mÀ1 cmÀ1
] ]
HeQu
0.07 2.70
25.6
19.5
23.4
1.52
34.5
38.7
46.4
7.09
14500 (434 nm)
25000 (447 nm)
41800 (452 nm)
474 (410 nm)
HeQu-Ph 0.05 2.46
HeQu-Ani 0.05 2.05
Heli[b]
Analyses of single-crystal structures
Single-crystal structures were determined by X-ray analysis to
evaluate the aggregate properties of HeQu derivatives. We pre-
pared single crystals of HeQu derivatives for X-ray diffraction
analysis by vapor diffusion at room temperature (see Experi-
mental Section), and the crystal data are summarized in Sup-
porting Information Table S1. Reference compound rac-Heli
0.02 13.8
[a] FFL: fluorescence emission quantum yield measured in 2.0 mm THF so-
lution, tFL: fluorescence lifetime, kFL: fluorescence emission rate constant,
kother: total rate constant of other pathways, e0À0: molar extinction coeffi-
cient of 0–0 absorption band. FFL =kFL tFL =kFL/(kFL+kother). [b] Reported
value in 1,4-dioxane.[13e,28,29] [c] Excited at 330 nm. [d] Excited at 390 nm.
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As discussed above,[23] quinoxalines form intracolumn
p-stacked structures in the aggregate state. The p-
stacking distance dp was estimated to be 4.2 ꢂ
(yellow line in Figure 8c). The distance of the CH···p
interaction between 4-methoxyphenyl group and hel-
icene was calculated to be 2.7 ꢂ (blue line in Fig-
ure 8c).
In contrast to P-HeQu-Ani, rac-HeQu-Ani exhibits
one-dimensional and helical column formation along
the crystallographic c axis by virtue of intracolumn in-
teraction between nearest two helicene units (Fig-
ure 9a and b). In the intracolumn direction, face-to-
face and p–p interactions of helicene units occur
(Figure 9c). The P-HeQu-Ani and M-HeQu-Ani con-
formers are alternately stacked in the same column.
The p-stacking distance dp was estimated to be 3.6 ꢂ
(yellow line in Figure 9c). In addition, intercolumn
CH···N interactions between nitrogen atoms in qui-
noxaline units and hydrogen atoms in helicene units
was observed. The CH···N distance is 3.4 ꢂ (black line
in Figure 9b). This is in sharp contrast to the forma-
tion of p-stacked dimers of quinoxaline units in P-
HeQu-Ani. Thus, utilization of racemic HeQu-Ani suc-
cessfully induced intracolumn p stacking between
two helicene units in P- and M-HeQu-Ani and inter-
column CH···N interaction between two neighboring
columns to form one-dimensional columnar struc-
tures.
Figure 8. Single-crystal structures of P-HeQu-Ani. Red: P enantiomer, green: pyrazine
ring. a) Packing mode along the b axis. Hydrogen atoms have been omitted for clarity.
b) Packing mode along the a axis. Hydrogen atoms have been omitted for clarity. c) p-
stacking between two neighboring P-HeQu-Ani with intermolecular distance. Yellow line:
p-stacking distance dp. Blue line: CH···p interaction distance.
shows a nonhelical crystal structure based on CH···p interac-
tions (see Supporting Information Figure S6). The crystal struc-
tures of P-HeQu-Ani and rac-HeQu-Ani are depicted in Fig-
ures 8 and 9, respectively. P-HeQu-Ani forms dimers by virtue
of p-stacking between two neighboring quinoxaline units and
CH···p interactions between helicene units (Figure 8a and b).
To investigate the substituent effect (i.e., 4-methoxyphenyl
group) on one-dimensional column formation, we also com-
pared the crystal structures of rac-HeQu-Ph and rac-HeQu-Ani
(Figure 9 and Supporting Information Figure S7). In contrast to
rac-HeQu-Ani, rac-HeQu-Ph shows dimer formation by virtue of
p stacking between two neighboring quinoxaline units and
Figure 9. Single-crystal structures of rac-HeQu-Ani. Red: P enantiomer, blue: M enantiomer, green: pyrazine ring. a) Packing mode along c axis. Hydrogen
atoms have been omitted for clarity. b) A view of the neighboring columns showing intermolecular distances. Black line: CH···N interaction distance. c) Helical
columnar structure of rac-HeQu-Ani with intermolecular p-stacking distance dp.
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CH···p interaction between helicene units of P- and M-HeQu-
Ph (Supporting Information Figure S7a and b). This comparison
indicates that the relatively bulky 4-methoxyphenyl group
hampered p–p interactions between quinoxaline units and in-
duced the above-mentioned one-dimensional column forma-
tion (Figure 9).
solution (Figure 6a and b). This band monotonously decreases
with increasing time. In contrast, we found a new band with
a peak maximum around 590 nm in addition to the above mo-
notonous decrease of fluorescence intensity at 550 nm for
single-crystal rac-HeQu-Ani. The new band may be ascribed to
the excimerlike delocalized excited state (Figure 10e).
To further check the substituent effects on the aggregate
properties, the crystal structure of rac-HeQu was also deter-
mined (Supporting Information Figure S8). As a consequence,
in this case, we could not find one-way column formation of
racemic helicenes. A plausible
To further discuss the excited-state dynamics, we analyzed
decay lifetimes in two different wavelength regions: 510–
530 nm and 600–650 nm (Figure 10b, d, and f). The decay
curves of the fluorescence intensity could be fitted as double
reason is the occurrence of mo-
lecular packing formations based
on CH···p interactions. In particu-
lar, in the case of rac-HeQu, the
crystal structure shows conglom-
erates based on CH···p interac-
tions. Moreover, in the case of
rac-HeQu-C16 with long alkyl
chains, typically dozens of nano-
sized fibrous structures (see the
TEM image in Supporting Infor-
mation Figure S9) were obtained
by
a
simple drop-casting
method using methylcyclohex-
ane. Therefore, we can conclude
that introduction of relatively
bulky
4-methoxyphenyl-linked
quinoxaline onto [7]carboheli-
cene unit induced one-dimen-
sional and helical column forma-
tion in the racemic single crystal.
Time-resolved fluorescence
spectra of HeQu derivatives in
single crystal states
To evaluate the intermolecular
interactions in the excited states,
time-resolved fluorescence spec-
tra and the corresponding
decay-time profiles were mea-
sured (Figure 10). Figure 10a and
c show time-resolved spectra of
rac-HeQu and P-HeQu-Ani single
crystals. To clarify the spectral
shapes, the fluorescence intensi-
ties of these spectra were equal-
ly normalized. On laser-pulse ex-
citation at 390 nm, we first ob-
served
a
broad fluorescence
spectrum with a maximum peak
at around 550 nm. This is proba-
bly attributable to the mono-
meric HeQu derivatives by com-
parison with the steady-state
Figure 10. Time-resolved fluorescence spectra and fluorescence lifetime decays of single crystals. a) spectra of rac-
HeQu, b) decays of rac-HeQu, c) spectra of P-HeQu-Ani, d) decays of P-HeQu-Ani, e) spectra of rac-HeQu-Ani, and
f) decays of rac-HeQu-Ani. The excitation wavelength was 390 nm. A: monomer for comparison, B: 1.00 ns, C:
fluorescence spectrum in THF 1.50 ns, D: 2.00 ns, E : 2.50 ns, and F: 3.00 ns.
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units was observed because of the short distance between
two helicene units. Such synthetic and supramolecular strat-
egies for helicene derivatives will provide a new perspective
for future development of optoelectronic applications.
Table 5. Fluorescence lifetimes of assemblies of HeQu derivative.
Helicene
tFL [ns], 510–530 nm
tFL [ns], 600–650 nm
0.30 (89.8%)
1.14 (10.2%)
0.73 (21.9%)
1.83 (78.1%)
0.88 (90.0%)
1.96 (10.0%)
0.84 (79.4%)
2.97 (20.6%)
0.86 (55.0%)
2.90 (45.0%)
1.05 (82.8%)
4.21 (17.2%)
rac-HeQu-Ani
P-HeQu-Ani
rac-HeQu
Experimental Section
General and materials
All solvents and reagents of the best grade available were pur-
chased from commercial suppliers such as Tokyo Chemical Industry,
Kanto Chemical Co., Inc., Nacalai Tesque, Wako Pure Chemical In-
dustries, and Sigma-Aldrich. All commercial reagents were used
without further purification. Column flash chromatography was
performed on silica gel (Kanto Chemical Silica Gel 60N, 40–50 mm
or 100–210 mm). Preparative recycling gel-permeation chromatog-
raphy was performed with an HPLC apparatus (Japan Analytical In-
dustry LC-9204) by using chloroform as eluent at room tempera-
ture. This system is equipped with a pump (JAI PI-60, flow rate
3.5 mLminÀ1), a UV detector (JAI UV-3740), and two columns
(JAIGEL-2H and JAIGEL-1H, both 40ꢁ600 mm). Racemic [7]carbohe-
licene derivatives were separated by using hexane/chloroform (2/1,
v/v) as eluent at room temperature on a DAICEL CHIRALPAK IA
column. 1H NMR and 13C NMR spectra were acquired on a JEOL
ECX-400, AL-400, or ALPHA-400 spectrometer by using the solvent
peak as the reference standard, with chemical shifts given in parts
per million (ppm). CDCl3 was used as NMR solvent. The 1H NMR
and 13C NMR spectra are shown in Supporting Information Fig-
ure S10–S31. MALDI-TOF mass spectra were recorded on a Bruker
Ultraflex instrument. The DFT calculations of molecular orbitals
were performed with Gaussian 03 at the B3LYP/6-31G* level of
theory. The molecular geometries were optimized at the same
level.[31]
exponentials, and the fluorescence lifetimes are listed in
Table 5. In the 510–530 nm region, the tFL value of rac-HeQu-
Ani crystal (0.30 ns) is much shorter than those of P-HeQu-Ani
(0.73 ns) and rac-HeQu (0.88 ns) crystals. A plausible mecha-
nism of the strong quenching process may be singlet–singlet
annihilation of helicene units. As discussed above, in rac-
HeQu-Ani crystals, adjacent face-to-face and p–p interaction of
helicene units was observed, for which the dp value of 3.6 ꢂ is
smaller than that of 4.2 ꢂ in P-HeQu-Ani (Figures 8c and 9c).
Furthermore, the molecular packing formation of rac-HeQu is
based on CH···p interactions. Hence, in the case of rac-HeQu-
Ani, singlet–singlet annihilation and the subsequent excimer-
like delocalized excited state continuously occur in the rac-
HeQu-Ani crystal. Thus, we have successfully observed a pecu-
liar photophysical process in the one-dimensional helical col-
umnar structure.
Conclusion
We designed and synthesized a series of quinoxaline-fused
[7]carbohelicenes. The electrochemical and spectroscopic prop-
erties of these compounds were evaluated by electrochemical
methods, steady-state and time-resolved spectroscopy, and
DFT calculations. In contrast to the similar first oxidation poten-
tials of HeQu derivatives and Heli, the first reduction potentials
of HeQu derivatives were shifted to the positive (ꢀ0.7 V) by
introducing quinoxaline units. The steady-state absorption,
fluorescence, and CD spectra became redshifted compared to
the reference Heli. These trends were explained by the HOMO
and LUMO levels and molecular orbitals calculated by DFT
methods. We further investigated the absolute fluorescence
quantum yields FFL of HeQu derivatives. The maximum FFL of
HeQu derivatives is about four times larger than that of refer-
ence Heli. The enhancement of FFL is plausibly attributable to
relative acceleration of the net rate constant of fluorescence
emission kFL.
Syntheses
He-Ket: 6.80 mL of BBr3 solution (ca. 1.0m in CH2Cl2) was added
dropwise over 10 min to
a solution of He-OMe (113 mg,
0.26 mmol) in dry CH2Cl2 (150 mL) on an ice bath. The resulting so-
lution was stirred at room temperature for 19 h under nitrogen at-
mosphere. The reaction mixture was quenched with water. Then,
the product was extracted with CH2Cl2, washed with brine, and
dried over anhydrous Na2SO4. After solvent evaporation, the crude
product as a black solid (190 mg) was dissolved in CH2Cl2 (55 mL)
and NEt3 (2.0 mL, 14.4 mmol). Next, the solution was stirred at
508C for 43 h. After cooling, the solution was poured onto water,
and then the organic layer was extracted with CH2Cl2, washed with
brine, dried over anhydrous Na2SO4, and evaporated. Finally, flash
column chromatography on silica gel with hexane/ethyl acetate
(10/1 v/v) as eluent afforded He-Ket as a red solid (67.6 mg,
1
0.17 mmol, 64%). H NMR (400 MHz, CDCl3): d=8.31 (d, J=8.1 Hz,
2H), 7.89 (d, J=8.1 Hz, 2H), 7.54 (d, J=8.8 Hz, 2H), 7.47 (d, J=
8.8 Hz, 2H), 7.26–7.24 (m, 2H), 6.99 (dd, J=7.4, 3.7 Hz, 2H), 6.64 (d,
J=8.3 Hz, 2H), 6.41 ppm (dd, J=7.6, 3.8 Hz, 2H); 13C NMR
(100 MHz, CDCl3): d=182.5, 139.2, 137.9, 132.0, 131.0, 130.6, 129.4,
129.2, 127.7, 127.1, 126.5, 125.8, 125.6, 125.2, 124.0; MALDI-TOF
MS: m/z=409 [M+H]+ (Supporting Information Figures S18 and
S19).
In the single-crystal structures, by introducing appropriate
substituents (i.e., 4-methoxyphenyl groups) onto the HeQu
unit, one-dimensional helical column formation, which is based
on intracolumn p stacking between two [7]carbohelicene units
and intercolumn CH···N interactions between two neighboring
columns, was successfully observed in the racemic single-crys-
tal state. This is in sharp contrast to formation of p-stacked
dimers of quinoxaline units by P-HeQu-Ani. Moreover, in the
time-resolved fluorescence measurements, an excimerlike delo-
calized excited state between neighboring [7]carbohelicene
HeQu: He-Ket (30.1 mg, 0.074 mmol) and o-phenylenediamine
(9.68 mg, 0.09 mmol) were dissolved in acetic acid (6.5 mL) and the
mixture stirred at 1008C for 1 h. After neutralization with dilute
aqueous NaOH solution, the organic phase was extracted with
CH2Cl2, washed with brine, and dried over Na2SO4, and the solvent
Chem. Eur. J. 2014, 20, 10099 – 10109
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was evaporated. The product was purified by silica gel column
chromatography with hexane/ethyl acetate (1/1 v/v) as eluent to
afford HeQu as a yellow solid (22.9 mg, 0.073 mmol, 65%). H NMR
Electrochemical measurements
Cyclic voltammograms were recorded on an Iviumstat 20 V/2.5 A
potentiostat by using a three-electrode system. A platinum elec-
trode was used as the working electrode, a platinum wire as the
counter electrode, and a saturated calomel electrode as reference
electrode. Ferrocene/ferrocenium redox couple was used as inter-
nal standard. All solutions were purged wit hnitrogen gas prior to
electrochemical measurements.
1
(400 MHz, CDCl3): d=9.56 (d, J=7.8 Hz, 2H), 8.45 (dd, J=6.3,
3.4 Hz, 2H), 8.16 (d, J=8.3 Hz, 2H), 7.92 (dd, J=6.8, 3.4 Hz, 2H),
7.74 (d, J=8.8 Hz, 2H), 7.50 (d, J=8.3 Hz, 2H), 7.30 (d, J=7.3 Hz,
2H), 6.99–6.96 (m, 4H), 6.45 ppm (dd, J=7.1, 3.5 Hz, 2H); 13C NMR
(100 MHz, CDCl3): d=142.9, 142.3, 133.5, 131.7, 129.7, 129.5, 129.4,
129.2, 128.5, 128.4, 128.2, 126.6, 125.6, 125.4, 124.9, 123.7,
122.8 ppm; MALDI-TOF MS: m/z=480 [M]+ (Supporting Informa-
tion Figures S20 and S21).
Spectroscopic measurements
UV/Vis absorption spectra were recorded on a PerkinElmer Lamda
750 UV/VIS/NIR spectrophotometer. CD spectra were recorded on
Jasco J-820 instrument. Fluorescence spectra were recorded on
PerkinElmer LS-55 spectrofluorophotometer. The absolute fluores-
cence quantum yields were measured with a Hamamatsu Photon-
ics C9920-02 system equipped with an integrating sphere and
a red-sensitive multichannel photodetector (PMA-12) at an excita-
tion wavelength of 330 nm. Fluorescence lifetimes were measured
on a Hamamatsu photonics C5680 with laser light (Hamamatsu
photonics M10360, laser diode head, 390 nm) as excitation source.
HeQu-Ph: He-Ket (12.7 mg, 0.031 mmol) and compound 14
(10.5 mg, 0.040 mmol) were dissolved in acetic acid (2 mL) and
CHCl3 (10 mL) and the solution was stirred at 1008C for 3 h. After
3 h, the solvent was evaporated. Next, flash column chromatogra-
phy on silica gel with hexane/ethyl acetate (10/1 v/v) as eluent was
carried out. Final purification was performed by preparative recy-
cling gel permeation chromatography to afford HeQu-Ph as
a yellow solid (13.3 mg, 0.021 mmol, 68%). 1H NMR (400 MHz,
CDCl3): d=9.55 (d, J=8.3 Hz, 2H), 8.50 (s, 2H), 8.16 (d, J=8.3 Hz,
2H), 7.74 (d, J=8.8 Hz, 2H), 7.50 (d, J=8.8 Hz, 2H), 7.37–7.34 (m,
12H), 7.02–6.95 (m, 4H), 6.46 ppm (dd, J=16.6, 8.3 Hz, 2H);
13C NMR (100 MHz, CDCl3): d=143.3, 143.1, 140.6, 133.5, 131.7,
130.5, 130.1, 129.7, 129.5, 129.2, 128.5, 128.5, 128.2, 128.1, 127.2,
126.6, 125.7, 125.4, 124.9, 123.7, 122.9 ppm; MALDI-TOF MS: m/z=
631 [MÀH]+ (Supporting Information Figures S26 and S27).
Preparation of single crystals and structure analysis
Single crystals of rac-HeQu and rac-HeQu-Ani for were prepared by
vapor diffusion of CHCl3 into MeOH solutions at room temperature.
Those of P-HeQu-Ani and rac-HeQu-Ph were prepared by vapor dif-
fusion of 1,2-dichloroethane into MeOH solutions at room temper-
ature. Crystal structure analysis was carried out on a Rigaku R-AXIS
RAPID diffractometer with graphite-monochromated MoKa radia-
tion. The structures were solved by direct methods (SHELXS-97).
CCDC 982754, 982755, 982756 and 982757 contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
HeQu-Ani: He-Ket (26.0 mg, 0.064 mmol) and compound 15
(15.5 mg, 0.048 mmol) were dissolved in acetic acid (2 mL) and
EtOH (4 mL) and the solution stirred at 1008C for 3 h. After cooling,
the reaction mixture was filtered, and the solid was washed with
EtOH and Et2O. Then, the solid was dissolved in CH2Cl2 and the sol-
vent was evaporated. Flash column chromatography on silica gel
with hexane/ethyl acetate (1/1 v/v) as eluent afforded the crude
product. Washing with hexane and solvent evaporation afforded
the product HeQu-Ani as a yellow solid (27.3 mg, 0.039 mmol,
Transmission electron microscopy
1
62%). H NMR (400 MHz, CDCl3): d=9.53 (d, J=8.3 Hz, 2H), 8.43 (s,
TEM measurements were performed on Tecnai Spirit (FEI Company)
by applying a drop of 20 mm HeQu-C16 in methylcyclohexane to
a copper TEM grid. TEM images were recorded at an accelerating
voltage of 120 kV.
2H), 8.14 (d, J=8.3 Hz, 2H), 7.73 (d, J=8.8 Hz, 2H), 7.49 (d, J=
8.5 Hz, 2H), 7.33–7.29 (m, 6H), 7.01–6.94 (m, 4H), 6.89 (dd, J=6.7,
2.0 Hz, 4H), 6.45 (dd, J=16.6, 8.3 Hz, 2H), 3.86 ppm (s, 6H);
13C NMR (100 MHz, CDCl3): d=143.0, 142.9, 133.5, 133.1, 131.7,
131.2, 130.1, 129.8, 129.5, 129.2, 128.4, 128.2, 126.6, 125.7, 125.4,
125.0, 123.7, 122.9, 113.6, 55.3 ppm; MALDI-TOF MS: m/z=693
[M+H]+ (Supporting Information Figure S28 and S29).
Acknowledgements
This work was partially supported by Grant-in-Aid for Scientific
Research (Nos. 23108721 & 23681025 to T.H.), MEXT-Supported
Program for the Strategic Research Foundation at Private Uni-
versities, 2009–2013, and Mitsubishi Foundation. We are grate-
ful to Mr. Shohei Katao (NAIST) for single-crystal X-ray diffrac-
tion analysis.
HeQu-C16
: He-Ket (13.2 mg, 0.032 mmol) and compound 16
(23.8 mg, 0.040 mmol) were dissolved in acetic acid (5 mL), EtOH
(5 mL), and CH2Cl2 (5 mL) and the solution stirred at 1008C for 1 h.
After cooling, the reaction mixture was filtered, and the solid was
washed with water. After dissolving in CH2Cl2 and solvent evapora-
tion, the product HeQu-C16 was obtained as a yellow solid
1
(22.8 mg, 0.020 mmol, 63%). H NMR (400 MHz, CDCl3): d=9.42 (d,
J=8.3 Hz, 2H), 8.32(s, 2H), 8.03 (d, J=8.3 Hz, 2H), 7.64 (d, J=
8.8 Hz, 2H), 7.41 (d, J=8.3 Hz, 2H), 7.22 (d, J=8.3 Hz, 6H), 6.93–
6.86 (m, 4H), 6.80 (d, J=8.8 Hz, 4H), 6.37 (dd, J=7.6, 3.8 Hz, 2H),
3.92 (t, J=6.6 Hz, 4H), 1.75–1.72 (m, 4H), 1.42–1.39 (m, 4H), 1.20–
1.20 (m, 48H), 0.81 ppm (t, J=6.8 Hz, 6H); 13C NMR (100 MHz,
CDCl3): d=158.4, 143.0, 142.7, 141.5, 133.4, 132.9, 131.7, 131.1,
130.0, 129.7, 129.5, 129.2, 128.3, 128.1, 126.6, 125.6, 125.3, 124.9,
123.6, 122.8, 114.1, 68.0, 31.9, 29.7, 29.6, 29.5, 29.4, 26.1, 22.7,
14.1 ppm; MALDI-TOF MS: m/z=1114 [M+H]+ (Supporting Infor-
mation Figures S30 and S31).
Keywords: electrochemistry · fluorescence · helical structures ·
stacking interactions · supramolecular chemistry
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Full Paper
Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V.
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Received: February 28, 2014
Published online on July 8, 2014
Chem. Eur. J. 2014, 20, 10099 – 10109
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10109
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim