Superstructure-dependent optical and electrical properties of an unusual face-to-face, π-stacked, one-dimensional assembly of dehydrobenzo[12]annulene in the crystalline state

Article abstract of DOI:10.1002/chem.200800228

To develop a novel π-conjugated molecule-based supramolecular assembly, we designed and synthesized trisdehydrotribenzo[12]annulene ([12]DBA) derivative 2 with three carboxyl groups at the periphery. Recrystallization of 2 from DMSO gave a crystal of the solvate 2·3 DMSO. Crystallography analysis revealed, to our surprise, that a face-to-face π-stacked one-dimensional (1D) assembly of 2 was achieved and that the DMSO molecule played a significant role as a structure-dominant element in the crystal. This is the first example of [12]DBA to stack completely orthogonal to the columnar axis. To reveal its superstructure-dependent optical and electrical properties, 2 and its parent molecule 1, which crystallizes in a herringbone fashion, were subjected to fluorescence spectroscopic analysis and charge-carrier mobility measurements in crystalline states. The 1D stacked structure of 2 provides a red-shifted, broadened, weakened fluorescence profile (λ_max = 545 nm, φ_F = 0.01), compared to 1 (λ_max = 491 nm, φ_F = 0.12), due to strong interactions between the p orbitais of the stacked molecules. The charge-carrier mobility of the single crystal of 2·3 DMSO, as well as 1, was determined by flash photolysis time-resolved microwave conductivity (FP-TRMC) measurements. The single crystal of 2-3 DMSO revealed significantly-anisotropic charge mobility (Σμ = 1.5×10^-1 cm²V^-1s^-1) along the columnar axis (crystallographic c axis). This value is 12 times larger than that along the orthogonal axis (the a axis).

Full text of DOI:10.1002/chem.200800228

DOI: 10.1002/chem.200800228
Superstructure-Dependent Optical and Electrical Properties of an Unusual
Face-to-Face, p-Stacked, One-Dimensional Assembly of
Dehydrobenzo[12]annulene in the Crystalline State
Ichiro Hisaki,*[a] Yuu Sakamoto,[a] Hajime Shigemitsu,[a] Norimitsu Tohnai,[a]
Mikiji Miyata,*[a] Shu Seki,[b] Akinori Saeki,[c] and Seiichi Tagawa[c]
Abstract: To develop a novel p-conju-
gated molecule-based supramolecular
assembly, we designed and synthesized
trisdehydrotribenzo[12]annulene
nal to the columnar axis. To reveal its
superstructure-dependent optical and
electrical properties, 2 and its parent
molecule 1, which crystallizes in a her-
ringbone fashion, were subjected to
fluorescence spectroscopic analysis and
charge-carrier mobility measurements
in crystalline states. The 1D stacked
structure of 2 provides a red-shifted,
broadened, weakened fluorescence
profile (lmax =545 nm, fF =0.01), com-
pared to 1 (lmax =491 nm, fF =0.12),
due to strong interactions between the
p orbitals of the stacked molecules.
The charge-carrier mobility of the
single crystal of 2·3DMSO, as well as
1, was determined by flash photolysis
time-resolved microwave conductivity
(FP-TRMC) measurements. The single
crystal of 2·3DMSO revealed signifi-
([12]DBA) derivative 2 with three car-
boxyl groups at the periphery. Recrys-
tallization of 2 from DMSO gave a
crystal of the solvate 2·3DMSO. Crys-
tallographic analysis revealed, to our
surprise, that a face-to-face p-stacked
one-dimensional (1D) assembly of 2
was achieved and that the DMSO mol-
ecule played a significant role as a
“structure-dominant element” in the
crystal. This is the first example of
[12]DBA to stack completely orthogo-
cantly-anisotropic
charge
mobility
(Æm=1.5”10ꢀ1 cm2 Vꢀ1 sꢀ1) along the
columnar axis (crystallographic c axis).
This value is 12 times larger than that
along the orthogonal axis (the a axis).
Keywords: charge-carrier mobility ·
dehydroannulenes · fluorescence ·
pi
interactions
·
solid-state
structures
Introduction
Dehydrobenzo[n]annulenes ([n]DBAs), in which n denotes
the number of p electrons in the cyclic pathway, are a family
of carbon-rich p-conjugated cyclic systems involving ben-
zene rings and acetylene units.[1] The chemistry of [n]DBAs
was intensively investigated during the 1960s and 1970s
mainly from the viewpoint of ring currents induced in cyclic
p systems by magnetic fields. Subsequently, they have been
studied from the various aspects, as unique organometallic
ligands by Youngs et al.,[2] as precursors of carbon nanopar-
ticles by Vollhardt and Bunz et al.,[3] and as partial units of
non-natural carbon allotropes, such as the so-called graph-
yne and graphdiyne by Tobe and Haley et al.[4] Furthermore,
various functionalized [n]DBAs have been synthesized, and
their optical and optoelectronic properties have been inves-
tigated in solution for applications as functional materials.[5]
To develop a significant property of [n]DBA in the solid
state, on the other hand, the molecular arrangement is a cru-
cial factor in addition to the molecular structure. So far,
[a] Dr. I. Hisaki, Y. Sakamoto, H. Shigemitsu, Dr. N. Tohnai,
Prof. Dr. M. Miyata
Department of Material and Life Science
Graduate School of Engineering, Osaka University
2-1 Yamadaoka, Suita, Osaka, 565-0871 (Japan)
Fax : (+81)6-6879-7404
E-mail: hisaki@mls.eng.osaka-u.ac.jp
miyata@mls.eng.osaka-u.ac.jp
[b] Prof. Dr. S. Seki
Division of Applied Chemistry
Graduate School of Engineering, Osaka University
2-1 Yamadaoka, Suita, Osaka, 565-0871 (Japan)
[c] Dr. A. Saeki, Prof. Dr. S. Tagawa
The Institute of Scientific and Industrial Research
Osaka University, 8-1 Mihogaoka, Ibaraki
Osaka, 567-0047 (Japan)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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some well-designed supramolecular architectures based on
[n]DBA have been reported in crystals,[6] in liquid crystals,[7]
in gels,[5f,g] in vesicles,[8] and on surfaces.[9] However, reveal-
ing the relationship between the superstructures and physi-
cal properties, such as optical, electrical, and electronic
properties, and the development of novel functional materi-
als based on [n]DBAs still remain as challenges.
Trisdehydrotribenzo[12]annulene ([12]DBA) 1,[18] which is
the target molecule in this study, is also known to crystallize
in a herringbone fashion; this arrangement is functionally
Among the various molecular arrangements observed in
supramolecular assemblies, a p-stacked, one-dimensional
(1D) assembly is one of the most fundamental and fascinat-
ing ones, because the interactions between the p orbitals of
the stacked molecules provide the pathway for charge or ex-
citon migration to exhibit functional properties. Thus far,
the optical, electrical, and electronic properties of p-stacked
aggregates have been examined experimentally and theoret-
ically for a large number of p-conjugated compounds includ-
ing heterocyclic compounds, such as porphyrin and phthalo-
cyanine, and polycyclic aromatic hydrocarbons (PAHs), such
as triphenylene and hexabenzocoronene.[10] In most cases, p-
stacked 1D aggregates are achieved in liquid crystals or in
gels by using molecules with discotic cores the periphery of
which is functionalized by long aliphatic chains. Moreover,
to effectively achieve these, well-designed hydrogen bonds
are also applied in some cases. For example, Müllen et al.
reported the control of the molecular arrangement of hexa-
benzocoronene derivatives through the hydrogen bond of
the urea moiety.[11]
In the crystalline state, on the other hand, a p-stacked 1D
assembly is often difficult to achieve, because p–p interac-
tions and CH–p interactions, which are unfavorable for p-
stacked 1D assembly, play a significant role when discotic
compounds such as PAHs crystallize. In connection with
this, Desiraju and Gavezzotti have classified the molecular
arrangements of PAHs into the following four categories,
herringbone, sandwich herringbone, b, and g structures, and
they have established the relationship between molecular
structure and molecular arrangement on the basis of the
crystal structures of 32 PAHs.[12] They have proposed signifi-
cant guidelines for the molecular arrangement of PAHs in
crystals to rationalize and predict crystal structures of PAH
from the molecular structures. However, the construction of
face-to-face stacked 1D assembly in crystals has remained
difficult, and even though large discotic compounds tend to
stack in 1D columns, the cores are still tilted (in many cases,
by 40–608) with respect to the columnar axis.[13] In addition,
organic crystals, especially single crystals, have other diffi-
culties associated with material functionalization; for exam-
ple, contingent and time-consuming growth processes and
brittleness of the resulting crystals. However, crystallograph-
ically determined and thoroughly ordered molecular ar-
rangements in organic crystals are fascinating from the view-
point of not only fundamental chemistry, but also for the de-
velopment of functional materials. Indeed, field-effect tran-
sistor (FET) properties of promising compounds, such as an-
thracene,[14] rubrene,[15] pentacene,[16] and others,[17] have
been examined with the single crystals to develop organic
semiconductor materials.
less useful arrangement due to small p–p interactions.[19]
Thus, in order to improve optical and electrical properties of
crystalline materials based on 1, it is necessary to control its
molecular arrangement in the solid state. To modulate the
molecular arrangement of 1, we introduced carboxyl groups
into its periphery for the following two reasons. First, hydro-
gen bonds of the carboxyl groups, based on the supramolec-
ular synthon theory,[20] can provide well-defined networked
lattices to keep them in a co-planar arrangement. Second,
any compound capable of hydrogen bonding with carboxylic
groups can be examined by high-throughput screening to
modulate the arrangement of the [12]DBA core. Indeed, we
have successfully controlled the arrangement of anthracene
chromophores in the crystalline state and have been able to
change their emission properties on the basis of this crystal-
engineering strategy.[21] In the present system, we serendipi-
tously succeeded in constructing and crystallographically an-
alyzing the face-to-face p-stacked 1D assembly of [12]DBA
2 in the crystalline state, in which the plane of 2 was com-
pletely orthogonal to the columnar axis. Although, Vollhardt
et al. reported the face-to-face stacking of [12]DBA deriva-
tives to form dimers,[22] the present system is the first exam-
ple of [12]DBA derivatives to form a face-to-face stacked
1D columnar assembly. Furthermore, we revealed that the
1D assembly of 2 shows superstructure-dependent emission
behavior, which yields a red-shifted, broadened, weakened
fluorescence spectral profile, and significantly anisotropic
charge mobility along the columnar axis (ꢁ12 times larger
than that along the orthogonal axis).
Herein, we describe the synthesis of 2, its crystal structure
with the 1D columnar assembly, the role of the DMSO mol-
ecule as a “structure-dominant element”, and its superstruc-
ture-dependent fluorescence and electrical properties. The
fluorescence properties of 2 were evaluated both in solution
and as a crystalline powder, and compared with those of its
parent compound 1. The charge-carrier mobility of the
single crystal of 2, as well as 1, was estimated by flash pho-
tolysis time-resolved microwave conductivity (FP-TRMC)
measurements.
Results and Discussion
Synthesis and crystallization of the [12]DBAs: [12]DBA 2
was synthesized with a 71% yield by the hydrolysis of the
corresponding methyl ester 3, which is derived from the iod-
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I. Hisaki, M. Miyata et al.
ization of the diethyltriazene derivative 4[23] with a 69%
yield and desilylation followed by modified Castro–Stephens
cyclization[24] of the resultant iodoethynylbenzene derivative
6 with a 39% yield in two steps, as shown in Scheme 1.
that is, in a herringbone fashion. In the case of 2, recrystalli-
zation from DMSO yielded a yellow columnar single crystal
2·3DMSO suitable for X-ray single-crystal analysis. To our
surprise, molecules of
2 solvated with DMSO stack
co-facially to give 1D columnar assembly. [12]DBA 3 yield-
ed a needle-like crystal from dichloromethane. Its diameter
was less than 0.01 mm, which is too thin for crystallographic
analysis in our laboratory. However, powder X-ray diffrac-
tion (PXRD) patterns of the bulk crystal of 3 (see, Figure S1
in Supporting Information) exhibited a similar profile as
that for 2·3DMSO, indicating that molecules of 3 also aggre-
gate in a p-stacked 1D columnar structure, though we have
not described the aggregate of 3 in this manuscript.
Scheme 1. Synthesis of [12]DBA 2. a) CH3I, 1208C, 17 h, 69%; b) TBAF,
THF, RT; c) K2CO3, CuI, PPh3, DMF, 1208C, 24 h, 39% for 2 steps; d)
KOH, THF, RT, 19 h, 71%.
Crystal structure of 1 and 2·3DMSO: In the crystalline
state, 1 is packed in a herringbone fashion as already de-
scribed in the literature (Figure 1a).[19] For a clear interpre-
tation of the intermolecular interaction working in the crys-
tal, the molecule of 1 in the crystal was drawn by the Hirsh-
feld surface[26] mapped with de between 1.0 and 2.6 Š, for
which de denotes the distance from the surface to the near-
est nucleus in another molecule. The surfaces (Figure 1b)
show two sets of two significant orange spots (labeled i) on
the benzene ring and the center of the trigonal structure of
one side (left) and on the two benzene rings of the other
side (right), as well as green flat regions (labeled ii) on the
[12]DBA 1 was also prepared from 1-iodo-2-ethylylbenzene
as the reference compound. To investigate the molecular ar-
rangement in the solid state, [12]DBAs 1, 2, and 3 were then
recrystallized from various organic solvents with or without
a second component capable of hydrogen bonding. Slow
evaporation of a solution of 1 in hexane/diisopropyl ether
yielded pale yellow single crystals[25] in which molecule 1
was arranged in the same way as described in the literature;
Figure 1. Structural features of 1 and 2 in the crystals. a) Packing diagram of 1 with a space-filling model. The Hirshfeld surface plots of b) 1 and c) 2.
The surfaces are mapped with de between 1.0 (red) and 2.6 (blue) Š. The orange spots labelled i and flat green regions labelled ii in b) and c) represent
CH–p and p–p contacts, respectively. d) Diagrams of 2·3DMSO with 50% thermal ellipsoids for non-hydrogen atoms. Symmetry codes: A: x, y, z; B:
1ꢀy, 1+xꢀy, z; C: 1ꢀy, 1+xꢀy, 1.5ꢀz. e) A side view of the 1D columnar assembly of 2·3DMSO with a space-filling model for 2 and a ball-and-stick
model for DMSO. f) A selected scaffold of the DMSO molecules. g) A hexagonal packing diagram of 2·3DMSO. DMSO is depicted in green, 2 is depict-
ed in red or blue in e)–g).
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Dehydroannulenes
FULL PAPER
surface of both sides. The orange spots and the green re-
gions indicate CH–p contacts and p–p stacking, respectively.
Thus, these profiles imply that 1 is packed in the crystal with
a large CH–p and tiny p–p interactions.
which denotes the maximum diameter of the trans-confor-
mation sulfoxides (Figure 2), of ethylenesulfoxide of 4.9 Š is
much smaller than that for DMSO, which is 7.0 Š, while the
The crystal structure of 2·3DMSO is shown in FigCAHTREuUNG re 1d–
g. The carboxylic group of 2 is solvated with the DMSO
molecule through a strong hydrogen bond O(3A)H···O(1B),
in which the distances between the O(1B) and O(3A) atoms
and between the H and O(1B) atoms are 2.58 and 1.77 Š,
respectively, and two weak hydrogen bonds C(9B)H···O(2A)
and C(9C)H···O(2A), in which distances between H and
O(2A) atoms and O(2A) and C(9B) or C(9C) atoms are
2.64 and 3.42 Š, respectively, and an angle of C(9B) or
Figure 2. Space-filling models of diethylsulfoxides with gauche and anti
conformations. The distances dgauche and danti denote the diameters of the
molecules in the respective conformations.
ꢀ
C(9C) H···O(2A) is 137.48 (symmetry codes: A: x, y, z; B:
1ꢀy, 1+xꢀy, z; C: 1ꢀy, 1+xꢀy, 1.5ꢀz; Figure 1d). All
atoms of the molecule 2, including those of the carboxyl
groups, and sulfur and oxygen atoms of DMSO are laid out
on the same plane, leading to the C3h symmetrical structure.
The molecules stack in a face-to-face fashion, in which the
plane of the [12]DBA cores is orthogonal to the columnar
axis, to yield the A,B-type 1D columnar structure. The
[12]DBA cores, A and B colored in red and blue in the
figure, are staggered by 20.198, and the distance between
them is 3.52 Š (Figure 1e). The columns are parallel-packed
into a hexagonal pattern (Figure 1g) through weak but fa-
vorable CH–S interactions. The Hirshfeld surface of 2,
mapped with de between 1.0 and 2.6 Š (Figure 1c), exhibits
a large flat green region, indicating that the p–p interaction
predominantly occurs over the whole p surface of the mole-
cule. The orange spots on the edge of the molecule are due
to CH–S contacts.
others have larger values (8.2 to 9.4 Š). Thus, if 2 solvated
by ethylenesulfoxide formed the 1D columnar assembly, the
resulting structure should leave an unfavorable void space
for crystallization. On the other hand, the three bulky sulf-
oxides probably do not allow for the construction of 1D
stacked crystal structures like 2·3DMSO, although the diam-
eter of the gauche conformation (dgauche) is comparable to
that of DMSO. These indicate that DMSO molecules specif-
ically work as the glue for construction of the face-to-face
1D structure of 2. In addition, other molecules capable of
participating in a hydrogen bond with 2 were subjected to
co-crystallization with 2. However, to our dismay, they have
not yielded single crystals suitable for X-ray diffraction anal-
ysis yet.
Stability of the crystal of 2·3DMSO: At ambient tempera-
ture, the DMSO molecules remained in the crystal when the
crystals were placed under vacuum (ꢁ1 Pa) for several
hours. To evaluate the thermal stability of the crystal, ther-
mal gravimetric (TG) analysis was carried out, given the
fact that DMSO molecules are released from the crystal at
about 1118C (Figure S2 in the Supporting Information). Fur-
thermore, the crystalline powder was subjected to DMSO
desorption/absorption experiments, revealing that the crystal
structure collapsed with loss of the DMSO molecules, while
exposure of the resultant powder to DMSO vapors allowed
for the recovery of the original structure. Figure 3 shows the
change in the PXRD pattern of the crystalline powder upon
desorption/absorption. The PXRD pattern of the crystalline
powder of 2·3DMSO (Figure 3b), which is in good agree-
ment with that simulated from the single-crystal data (Fig-
ure 3a), changed into a broadened, different pattern upon
removal of the DMSO molecules by heating at 1308C for
1 h in vacuo (Figure 3c). The PXRD pattern in Figure 3c has
a significantly broadened peak around 2q=26.18 (3.41 Š)
that can probably be attributed to the partly-surviving p
stacked aggregate of 2. On the other hand, the correspond-
ing peak in the pattern in Figure 3b is observed at 2q=25.08
(002 diffraction peak), the d-spacing of which is 3.56 Š, indi-
cating that the 1D assembly of 1 shrinks by about 4% along
the columnar axis (c axis in the crystal structure) by replace-
ment of the appended DMSO molecules, which is also im-
plied by its solid-state fluorescent spectrum (vide infra).
It is worth noting that the DMSO molecules bound to the
periphery of 2 form well-fitted scaffolds with the intermo-
lecular CH···O=S interactions (Figure 1f), preventing
[12]DBA cores from forming CH–p interactions or slipped
stacking. To confirm the steric effect of the DMSO mole-
cules, other sulfoxides, such as ethylene-, methylethyl-, di-
ethyl-, and dimethoxysulfoxides, were also subjected to co-
crystallization with 2 under the same conditions as in the
case of DMSO. No crystals were obtained from these sys-
tems: Ethylenesufoxide yielded a powder-like precipitate
that did not exhibit any significant PXRD pattern, and the
others also yielded amorphous films. This is probably due to
steric misfit of the sulfoxides scaffolds. The approximate
lengths of the sulfoxides are listed in Table 1. The danti value,
Table 1. Approximate lengths[a] of sulfoxides subjected to co-crystalliza-
tion with [12]DBA 2.
Substituents
danti [Š]
dgauche [Š]
Precipitates
R1, R2 =ꢀCH2CH2-
4.9
7.0
8.2
9.4
8.6
–
–
7.0
7.9
–
powder
single crystal
film
film
film
R1 =R2 =Me
R1 =Me, R2 =Et
R1 =R2 =Et
R1 =R2 =OMe
[a] Molecular structures with anti conformation were optimized by HF/6–
31G* level and those with gauche conformation were modeled based on
MM calculations.
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I. Hisaki, M. Miyata et al.
Figure 3. Change in the PXRD patterns of 2·3DMSO upon desorption/
absorption. a) Simulated pattern from single-crystal X-ray diffraction
data. b) Observed pattern of crystalline powder, c) after heating at
1308C for 1 h in vacuo, and d) after exposure to vaporous DMSO at
408C for 43 h
Figure 4. Normalized UV/Vis (dashed line) and fluorescence (solid line)
spectra of 1 (gray) and 2 (black) in solution. The concentrations of 1 and
2 in the solutions are 5.0 and 2.9 mm, respectively. Excitation wavelengths
for 1 and 2 are 303 and 292 nm, respectively.
spectively, based on quinine sulfate reference. The cause of
the observed red-shifted absorption spectrum of 2 was deter-
mined by the time-dependent density functional theory
(TDDFT) calculation at the B3LYP/6-31+G* level as
shown in Figure 5.[27] [12]DBA 2 shows the signals ascribed
Comparison of the low angle diffraction peaks between the
patterns in Figure 3b and c indicates that rearrangement of
the columnar structure also occurred upon removal of the
DMSO molecules. Especially, the d-spacing (26 Š) of a new
peak at 2q=3.48 in the pattern in Figure 3c is consistent
with the length of the dimeric species of 2 derived by the
self-complementary hydrogen bond of carboxylic moieties.
Interestingly, when the destructured powder was exposed to
DMSO vapor at 408C for 43 h, the original PXRD pattern
completely recovered as shown in the pattern in Figure 3d.
Thus, these results clearly indicate that DMSO is an excel-
lent “structure-dominant element” for construction of the
crystal with 1D columnar assembly of 2.
Optical properties of the DBAs: The 1D p-stacked structure
is especially attractive for examining the optical and electri-
cal properties, as reported for PAHs.[10] To reveal the super-
structure-dependent optical properties of [12]DBA systems,
the electronic spectra of 2 were measured in the crystalline
state and in DMSO, and compared with those of the parent
compound 1. First, to clarify self-association behaviors of 1
Figure 5. Calculated UV/Vis spectra of 1 (gray bar) and 2 (black bar) a)
in vacuo and b) in DMSO. The excitation energy has been calculated
with the TDDFT method using the B3LYP/6-31+G* basis set. The struc-
tures are optimized at the B3LYP/6–31G* level. The values in the
DMSO solutions were obtained using the IEFPCM scheme.
1
and 2 in DMSO, H NMR spectra of 1 and 2 were measured
in various concentrations, yielding no changes in chemical
shifts up to concentrations of 2 mm (see Figures S3 and S4 in
the Supporting Information). Similarly, the UV/Vis spectra
of 1 and 2 do not show any hypochromic spectral change of
the absorption bands in the range of 5.9–1.5 mm and 10–
2.5 mm, respectively (see Figures S5 and S6 in the Supporting
Information). These results indicate that 1 and 2 are well
dispersed in the DMSO. Next, we compare the optical prop-
erties of 1 and 2. In solution, 1 and 2 show absorption
maxima at 292 and 303 nm, respectively, and lower energy
bands at around 345 and 356 nm, respectively (Figure 4).
The spectral profile of 2 is similar to that of 1, but is slightly
red-shifted by about 10 nm. The fluorescence emission spec-
trum of 2 with bands at 483, 500, and approximately 530 nm
also shows a spectral profile similar to that of 1 with 469,
484, and 515 nm, but is red-shifted by about 15 nm. The
fluorescence quantum yields of 1 and 2 are 0.15 and 0.07, re-
to p–p* transitions (HOMO!LUMO+1 and HOMOꢀ1!
LUMO) at 333 nm which is red-shifted by 29 nm relative to
the corresponding signal of 1 (304 nm). Moreover, calcula-
tion with the integral equation formalism version of the po-
larizable continuum model (IEFPCM)[28] accounting for the
electrostatic interactions between the [12]DBA molecules
and the solvent; that is, DMSO, further gives a red-shifted
value of 34 nm in the absorption spectrum of 2. Thus, al-
though quantitative evaluation is difficult due to the large
difference between the observed and calculated absorption
spectra, the calculation results qualitatively indicate that the
observed red-shifted absorption spectrum of 2 is due to both
the effect of perturbation of the carboxyl groups and the
solvent effect of DMSO.
In the crystalline state, although the relative difference in
the wavelength between the maximum absorption bands of
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Dehydroannulenes
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1 and 2 remains at about 10 nm, the spectral profiles of 1
and 2 are strongly broadened and red-shifted by approxi-
mately 120 nm relative to those found in solution, as shown
in Figure 6, resulting in much smaller values of the Stokes
porting Information) which is slightly red-shifted compared
to 2·3DMSO due to stronger interactions between the
neighboring molecules. These results strongly indicate that
the arrangement of [12]DBA cores crucially affects the
solid-state optical properties.
Electrical properties of the DBAs: The molecular arrange-
ment of p-conjugated discotic compounds is significant for
the conductive properties of the crystal. Especially, one-di-
mensional conductivity of materials depends on the interac-
tion between the delocalized
p orbitals of adjacent
[12]DBAs. To evaluate charge mobility of the crystals 1 and
2·3DMSO, their single crystals were subjected to flash pho-
tolysis time-resolved microwave conductivity (FP-TRMC)
measurements.[29] The TRMC technique, which can predict
the nanometer-scale mobility of charge carriers generated
by laser pulse irradiation under a low oscillating microwave
electric field, has been recently applied to assess the instinct
mobility of p-conjugated polymers and p–p-stacked discotic
materials, because the technique is not affected by chemical
or physical defects in the material or the organic/metal-elec-
trode interface.[30] Moreover, we investigated the anisotropy
of the charge transport in the single crystals, because al-
though the present system has a sophisticated p-stacked 1D
column, the column is insulated from the neighbors only by
DMSO molecules. In general, in the case of liquid crystal[31]
and graphitic nanotube[30d] systems of discotic cores having
long alkyl chains, peripheral long hydrocarbon chains insu-
late the conductive parts, resulting in highly anisotropic 1D
charge transport, while in the case of crystals reported so
far, the anisotropy tends to remain relatively low (the ratio
is within five) due to close packed p-conjugated molecules.
Figure 7 displays photographs of the single crystals, their
crystalline indices, and the corresponding molecular ar-
rangements. The crystal of 1 exhibits a block-like shape with
dimensions of 0.4”0.7”1.0 mm and the crystal of 2·3DMSO
exhibits a hexagonal column-like shape with dimensions of
Figure 6. Normalized UV/Vis (dashed line), fluorescence (solid line), and
excitation (inset) spectra of 1 (gray) and 2·3DMSO (black) in the crys-
tals. Excitation wavelengths for the fluorescence spectra of 1 and 2 are
449 and 414 nm, respectively. Excitation spectra of 1 and 2 are recorded
at 491 and 500 nm, respectively.
shift than those in the solution due to rigidly packed crystal
structures. The fluorescence excitation spectra of 1 and 2 in
Figure 6 (inset) are in agreement with their absorption spec-
tra. It is worth noting that the fluorescence spectra of 1 and
2 show completely different profiles. The spectrum of the
crystalline solid of 1 shows a more unambiguous vibrational
structure (bands at 474, 491, 500, 519 and 528 nm) than that
in solution, though they lie in a similar wavelength region.
This is caused by molecular packing with tiny p–p interac-
tions and/or emission from two or more excitation states.
On the other hand, the crystalline solid of 2·3DMSO shows
a strongly broadened, structureless spectrum (lmax =545 nm)
that is red-shifted by nearly
50 nm relative to the maximal
emission band in the solution.
This indicates that the molecule
has p–p interaction with the ad-
jacent molecule and that the ex-
citon may delocalize along the
p–p stacked 1D columns to
form excited oligomeric species.
Indeed, the emission quantum
efficiency of the crystalline
2·3DMSO (fF =0.01) is much
smaller than that of 1 (fF =
0.12). Furthermore, the powder
sample after removal of the
DMSO molecules (Figure 3c),
which implies shorter p-stack
distance than 2·3DMSO, exhib-
its an emission maximum at
Figure 7. Single crystals of 1 and 2·3DMSO used in the PF-TRMC measurement. Photograph of the crystals a)
1 and d) 2·3DMSO. Crystal shapes with indices for b) 1 and e) 2·3DMSO. Molecular arrangements of c) 1 and
550 nm (Figure S7 in the Sup- f) 2·3DMSO corresponding to the photographs.
Chem. Eur. J. 2008, 14, 4178 – 4187
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www.chemeurj.org
4183

I. Hisaki, M. Miyata et al.
0.3”0.3”1.3 mm. The microwave electric field was applied
along the crystallographic a and c axes directions in the
single crystals of both 1 and 2·3DMSO to investigate aniso-
tropic conductivity. With regards to the p-orbital overlap-
ping of the annulene molecules, compound 1, which aligns
co-planarly along the c axis, is slightly p-stacked along the a
axis, while 2, which is packed co-planarly in the ab plane, p
stacks to form a 1D structure along the c axis, as described
above.
(ꢁ3 mm thick) of 1 and 2 that were cast from solutions of
the samples in DMSO on Al substrates and overcoated by
an Au semitransparent electrode, under excitation at 355 nm
with a power density of 5 mJcmꢀ2. It should be noted that
the transient was obtained under an applied +5 V bias volt-
age between the electrode (Figure S8 in the Supporting In-
formation), while no transient was observed under a nega-
tive bias, suggesting that the major charge carriers are holes.
The value for 1 (6.5%) was slightly larger than that for 2
(4.5%), corresponding to the fact that the HOMO level of 1
(ꢀ5.30 eV) is higher than that of 2 (ꢀ5.82 V) or 2 solvated
by DMSO (ꢀ5.47 eV) (Figure S9 in the Supporting Informa-
tion). Assuming that the values of f in the single crystals
are the same as those in the films, the single crystal of 1 ex-
hibits the minimum mobility of 3.3”10ꢀ2 cm2 Vꢀ1 sꢀ1 along
the a axis and the single crystal of 2·3DMSO exhibits mini-
mum mobilities of 1.5”10ꢀ1 and 1.3”10ꢀ2 cm2 Vꢀ1 sꢀ1 along
the c and a axes, respectively.[32]
Figure 8 shows the kinetic traces of the conductivity
(fÆm) of the single crystals of 1 and 2·3DMSO obtained
from TRMC measurements, in which f and Æm denote pho-
It is not easy to compare the values obtained in this study
with the reported values in other promising systems such as
rubrene by the time of flight or field-effect transistor meth-
ods.[15,17] However, the single crystal of 2·3DMSO yielded a
larger value compared with that of the single crystal of ru-
brene (Æm=5.2”10ꢀ2 cm2 Vꢀ1 sꢀ1) obtained by the FP-
TRMC method.[30e,33] Furthermore, it is worth noting that
significant anisotropy of charge transport is observed in the
single crystal of 2·3DMSO. The mobility along the c axis is
ꢁ12 times larger that that along the a axis, although the
[12]DBA molecules lie coplanarly in the crystallographic ac
plane and are insulated from the adjacent molecules only by
DMSO molecules. The observed high anisotropic conductiv-
ity is presumably due to not only spatial but also the ener-
getic barrier of the DMSO for the charge-carrier transport.
The anisotropic effect observed in the present system is sig-
nificantly higher than that in reported crystalline systems[17a]
and comparable to the graphitic systems.[30e,31] Thus,
[12]DBA 2 is expected to be another candidate for organic
semiconductors.
Figure 8. Conductivity transients for the single crystals of a) 2·3DMSO
and b) 1. The transients colored by gray and black in a) are along the a
and c axes, respectively. Those by gray and black in b) are along the c
and a axes, respectively. The inset in a) shows the partially expanded
curve of the conductivity of 2·3DMSO along the c axis. The spike ob-
served at 0.8 ms is just noise.
Conclusion
tocarrier generation yield (quantum efficiency) and sum of
mobilities for negative and positive carriers, respectively.
The transient curves of 2 are shown in Figure 8a; the gray
and black curves correspond to the conductivity along the
crystallographic a and c axes, respectively. Upon irradiation
of a laser pulse with an excitation wavelength of 355 nm, the
single crystal of 2·3DMSO revealed a strong transient con-
ductivity fÆm with a peak of 6.9”10ꢀ3 cm2 Vꢀ1 sꢀ1 at 81 ns
after irradiation along the c axis and weak conductivity with
a peak of 6.0”10ꢀ4 cm2 Vꢀ1 sꢀ1 at 106 nm along the a axis.
On the other hand, the single crystal of 1 showed only a
weak transient conductivity peak of 1.9”10ꢀ3 cm2 Vꢀ1 sꢀ1
along the a axis and no conductivity along the c axis.
A face-to-face stacked 1D columnar assembly composed of
[12]DBA macrocyclic cores was successfully constructed by
co-crystallization of the carboxylic derivative 2 with DMSO,
and was characterized by single-crystal X-ray analysis. The
1D stacked structure of 2 in the solid state was revealed to
show the red-shifted, broadened, weakened fluorescence
spectrum, relative to the parent compound 1, which crystal-
lizes in a herringbone fashion with only a very small amount
of p orbital overlap. We also revealed that the single crystal
of 2·3DMSO has significantly anisotropic charge mobility
(Æm=1.5”10ꢀ1 cm2 Vꢀ1 sꢀ1) along the columnar axis. The
value is 12 times larger than that along the orthogonal axis.
Thus, [12]DBA 2 can be another candidate for organic semi-
conductor materials. These characteristics are strongly de-
pendent on the supramolecular structure in the assemblies.
To determine the value of the charge-carrier mobility Æm,
the value of f was determined by the conventional direct-
current current integration (DC-CI) method using thin films
4184
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Chem. Eur. J. 2008, 14, 4178 – 4187

Dehydroannulenes
FULL PAPER
The values of f for the films of 1 and 2 were determined by conventional
DC current integration using 3 mm thick films sandwiched by Al and sem-
itransparent-Au electrodes under excitation at 355 nm with a power den-
sity of 5.0 mJcmꢀ2. Other details of the apparatus are described else-
where.[30]
Experimental Section
General methods: 1H and 13C spectra were measured by a JEOL spec-
trometer (270 MHz for 1H and 67.5 MHz for 13C). MS data were obtained
from a JEOL JMS-700 instrument. UV/Vis spectra in the solid state and
in solution were measured on a JASCO V-550 spectrometer. Thermogra-
vimetric analysis was performed on a Rigaku TAS100 system and Rigaku
Thermoplus TG8120 with about 10 mg of sample from 30 to 2508C at a
heating rate of 58Cminꢀ1. Emission spectra in the solid state and in solu-
tion were measured using a JASCO FP-6500 spectrofluorometer with an
accessory and a cell for solid samples from JASCO. FT-IR spectra of the
synthesized compounds in a KBr pellet were recorded using a Horiba
FT-720 spectrometer.
Methyl 4-iodo-3-trimethylsilylethynylbenzenoate (5): Diethyltriazene 4
(100 mg, 0.302 mmol), which was synthesized according to the report by
Haley et al.,[23] was dissolved in methyl iodide (2 mL) in an autoclave
under a nitrogen atmosphere. The reaction vessel was placed in an oil
bath at 1208C for 17 h. After cooling to room temperature, the solvent
was removed under reduced pressure. The residue was subjected to
column chromatography on silica gel (dichloromethane) and washed with
Na2S2O3 aqueous solution to yield 5 (74.6 mg, 69%) as a yellow crystal-
line powder. M.p. 458C; 1H NMR (270 MHz, CDCl3): d=8.08 (d, J=
2.2 Hz, 1H; ArH,), 7.92 (d, J=8.3 Hz, 1H; ArH), 7.60 (dd, J=2.0,
8.2 Hz, 1H; ArH), 3.91 (s, 3H; OCH3), 0.29 ppm (s, 9H, SiCH3);
13C NMR (67.5 MHz, CDCl3): d=165.8, 138.9, 133.3, 130.1, 129.9, 129.8,
Crystal structure determination: X-ray diffraction data were collected on
a Rigaku R-AXIS RAPID diffractometer with a 2D area detector using
graphite-monochromatized CuKa radiation (l=1.54187 Š). Direct meth-
ods (SIR-97) were used for the structure solution.[34] All calculations
were performed with the observed reflections [I>2s(I)] by the program
CrystalStructure crystallographic software packages[35] except for refine-
ment, which was performed using SHELXL-97.[36] All non-hydrogen
atoms were refined with anisotropic displacement parameters and hydro-
gen atoms were placed in idealized positions and refined as rigid atoms
with the relative isotropic displacement parameters.
107.2, 105.5, 100.0, 52.4, ꢀ0.2 ppm; IR (KBr): n˜ =2954, 2160, 1720 cmꢀ1
;
HR-MS (FAB): m/z calcd for [M]+ C13H15O2SiI: 357.9886; found:
357.9880.
Dehydrobenzo[12]annulene 3: A 1m solution of tetrabutylammonium
fluoride in THF (0.7 mL, 0.7 mmol) was added to
a solution of 5
(500 mg, 1.40 mmol) dissolved in THF (20 mL). After stirring for 1 h at
room temperature, the solvent was removed under reduced pressure. The
residue was extracted with dichloromethane and washed with water.
After drying the organic layer with anhydrous MgSO4, the solvent was
evaporated under reduced pressure to yield iodophenylacetylene 6 as an
oil. The resulting material was used for the following step without further
purification. A solution of iodophenylacetylene dissolved in DMF (5 mL)
was added to a mixture of K2CO3 (580 mg, 4.20 mmol), CuI (80 mg,
0.42 mmol), PPh3 (110 mg, 0.419 mmol). The mixture was stirred for 24 h
in an oil bath at 1208C under a nitrogen atmosphere. The mixture was
poured into H2O and extracted with chloroform. After drying the organic
layer over anhydrous MgSO4, the solvent was evaporated under reduced
pressure. The residue was purified by column chromatography on silica
gel (chloroform) to yield 3 (86.1 mg, 39%) in the form of an ocher
powder. M.p. (decomp) 2408C; 1H NMR (270 MHz, CDCl3): d=8.03 (s,
3H; ArH), 7.88 (d, J=8.1 Hz, 3H; ArH), 7.43 (d, J=8.1 Hz, 3H; ArH),
3.94 ppm (s, 9H; OCH3); 13C NMR (67.5 MHz, CDCl3): d=165.4, 133.3,
132.1, 130.7, 130.2, 129.9, 126.3, 94.8, 92.7, 52.5 ppm; IR (KBr) n˜ =3448,
2923, 1720 cmꢀ1; HR-MS (EI): m/z calcd for [M]+ C30H18O6: 474.1103;
found: 474.1099.
Crystal data 1: C24H12, Mr =300.36, 0.8”0.8”0.6 mm, a=11.7275(3), b=
10.9297(3), c=12.4438(3) Š, a=908, b=98.4239(18)8, g=908, V=
1577.81(7) Š3, T=213 K, monoclinic, space group P21/c (No. 14), Z=4,
mACHTREUNG , , 8293 reflections collected,
(CuKa)=0.5492 mmꢀ1 1calcd =1.264 gcmꢀ3
2764 unique (Rint =0.087) reflections, The final R1 and wR2 values were
0.052 [I>2.0s(I)] and 0.124 (all data), respectively.
Crystal data 2·3DMSO: C33H30O9S3, Mr =666.77, 0.2”0.2”0.6 mm, a=
16.5993(4), b=16.5993(4), c=7.0364(3) Š, a=908, b=908, g=1208, V=
3
¯
1679.05(8) Š , T=213 K, hexagonal, space group P62c (No. 190), Z=2,
mACHTREUNG
(CuKa)=2.458 mmꢀ1, 1calcd =1.319 gcmꢀ3, 16108 collected, 1124 unique
(Rint =0.074) reflections, The final R1 and wR2 values were 0.069 [I>
2.0s(I]) and 0.173 (all data), respectively.
CCDC-679449 (1) and 664534 (2) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/data_request/cif.
Powder X-ray diffraction (PXRD): PXRD data were collected on a
Rigaku RINT-1100 or RINT-2000 using graphite-monochromatized Cu
Ka?radiation (l=1.54187 Š) at room temperature.
Dehydrobenzo[12]annulene 2: 10% KOH aqueous solution (10 mL) was
added to a solution of DBA 3 (86.1 mg, 0.181 mmol) in THF (20 mL).
The mixture was stirred for 19 h at room temperature. The aqueous layer
was separated and a 2m HCl aqueous solution was added to the aqueous
layer. The resulting precipitation was gathered by a centrifuge, rinsed
with water several times, and dried under vacuum to yield 2, (55.5 mg,
71%) as an orange powder. M.p. (decomp) 2078C; 1H NMR (270 MHz,
[D6]DMSO): d=13.33 (brs, 3H; OH), 7.92 (d, J=1.4 Hz, 3H; ArH),
7.85 (dd, J1 =1.8, 8.2 Hz, 3H; ArH), 7.61 ppm (d, J=8.4 Hz, 3H; ArH);
13C NMR (67.5 MHz, [D6]DMSO): d=165.5, 132.7, 132.5, 131.4, 130.2,
Flash-photolysis time-resolved microwave conductivity (FP-TRMC)
measurements: Nanosecond laser pulses from a Nd:YAG laser (third har-
monic generation, THG (355 nm) from Spectra Physics, GCR-130,
FWHM 5–8 ns) were used as excitation sources. The power density of the
laser was set at 1.0–20 mJ/cm2. For time-resolved microwave conductivity
(TRMC) measurements, the microwave frequency and power were set at
ꢁ9.1 GHz and 3 mW, respectively, so that the motion of the charge carri-
ers was not disturbed by the low electric field of the microwaves. The
TRMC signal picked up by a diode (rise time <1 ns) is monitored by a
digital oscilloscope. All the above experiments were carried out at room
temperature. The transient photoconductivity (Ds) of the samples is re-
lated to the reflected microwave power (DPr/Pr) and sum of the mobili-
ties of charge carriers as given in Equations (1) and (2).
129.2, 125.2, 94.2, 92.3 ppm; IR (KBr) n˜ =3401, 2924, 2214, 1689 cmꢀ1
HR-MS (EI): m/z calcd for [M]+ C27H12O6: 432.0634; found: 432.0639.
;
1 DPr
Acknowledgement
ð1Þ
ð2Þ
hDsi ¼
Ds ¼ e
A
Pr
This work was supported by a Grant-in-Aid for Scientific Research from
the Ministry of Education, Culture, Sports, Science, and Technology,
Japan.
X
mꢀN
In these equations A, e, f, N, and Æm represent a sensitivity factor, ele-
mentary charge of an electron, photocarrier generation yield (quantum
efficiency), number of absorbed photons per unit volume, and sum of
mobilities for negative and positive carriers, respectively. The number of
photons absorbed by the sample was estimated based on steady state ab-
sorption spectra of the thin solid films of the corresponding compounds.
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[32] The value of f determined by DC-CI experiment under high electric
field strength (1.6”104 Vcmꢀ1) is probably overestimated. There-
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Dehydroannulenes
FULL PAPER
fore, the actual value of the mobilities may be larger than that deter-
mined in this study.
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[35] CrystalStructure 3.8, Crystal Structure Analysis Package, Rigaku
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[33] Although the FP-TRMC measurement of the single crystal of ru-
brene gave the mobility of 5.2”10ꢀ2 cm2 Vꢀ1 sꢀ1 as shown in the liter-
ature of reference [31e], the value is approximately three orders of
magnitude smaller than reported highest FET mobility for the re-
brene single crystal. In connection with this discrepancy, several rea-
sons including underestimation of the extinction coefficients of
charged species in the crystal phase have been speculated. Thus, the
value is expected to be minimum value of the mobility of rubrene
single crystal.
[36] G. M. Sheldrick, SHELXL-97, Program for crystal structure refine-
ment, University of Gçttingen (Germany), 1997.
Received: February 5, 2008
Published online: April 9, 2008
Chem. Eur. J. 2008, 14, 4178 – 4187
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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