Syntheses and properties of graphyne fragments: Trigonally expanded dehydrobenzo[12]annulenes

Article abstract of DOI:10.1002/chem.201300838

We present herein the synthesis and properties of the largest hitherto unknown graphyne fragment, namely trigonally expanded tetrakis(dehydrobenzo[12] annulene)s (tetrakis-DBAs). Intramolecular three-fold alkyne metathesis reactions of hexakis(arylethynyl)DBAs 9 a and 9 b using Fuerstner's Mo catalyst furnished tetrakis-DBAs 8 a and 8 b substituted with tert-butyl or branched alkyl ester groups in moderate and fair yields, respectively, demonstrating that the metathesis reaction of this protocol is a powerful tool for the construction of graphyne fragment backbones. For comparison, hexakis(arylethynyl)DBAs 9 c-g have also been prepared. The one-photon absorption spectrum of tetrakis-DBA 8 a bearing tert-butyl groups revealed a remarkable bathochromic shift of the absorption cut-off (λ _cutoff) compared with those of previously reported graphyne fragments due to extended π-conjugation. Moreover, in the two-photon absorption spectrum, 8 a showed a large cross-section for a pure hydrocarbon because of the planar para-phenylene-ethynylene conjugation pathways. Hexakis(arylethynyl)- DBAs 9 c-e and 9 g and tetrakis-DBA 8 b bearing electron-withdrawing groups aggregated in chloroform solutions. Comparison between the free energies of 9 e and 8 b bearing the same substituents revealed the more favorable association of the latter due to stronger π-π interactions between the extended π-cores. Polarized optical microscopy observations, DSC, and XRD measurements showed that 8 b and 9 e with branched alkyl ester groups displayed columnar rectangular mesophases. By the time-resolved microwave conductivity method, the columnar rectangular phase of 8 b was shown to exhibit a moderate charge-carrier mobility of 0.12 cm² V^-1 s^-1. These results indicate that large graphyne fragments can serve as good organic semiconductors. Copyright

Full text of DOI:10.1002/chem.201300838

FULL PAPER
Expanded graphyne fragment: Trigo-
nally expanded dehydrobenzo[12]an-
nulenes (DBA; see figure) can be syn-
thesized by three-fold alkyne metathe-
sis reactions catalyzed by a Mo nitride
complex. Tetrakis-DBAs show a large
two-photon cross-section, enhanced
aggregation properties in solution, the
formation of columnar rectangular
mesophases in the condensed state,
and moderate charge-carrier mobility.
These results indicate that large graph-
yne fragments may serve as promising
organic semiconductors.
Graphyne Fragments
K. Tahara, Y. Yamamoto, D. E. Gross,
H. Kozuma, Y. Arikuma, K. Ohta,
Y. Koizumi, Y. Gao, Y. Shimizu,*
S. Seki,* K. Kamada,* J. S. Moore,*
Y. Tobe* ....................... &&&& — &&&&
Syntheses and Properties of Graphyne
Fragments: Trigonally Expanded
Dehydrobenzo[12]annulenes
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DOI: 10.1002/chem.201300838
Syntheses and Properties of Graphyne Fragments: Trigonally Expanded
Dehydrobenzo[12]annulenes
Kazukuni Tahara,[a] Yuki Yamamoto,[a] Dustin E. Gross,[b] Hiroyoshi Kozuma,[a]
Yoko Arikuma,[a] Koji Ohta,[c] Yoshiko Koizumi,[d] Yuan Gao,[b] Yo Shimizu,*[c]
Shu Seki,*[d] Kenji Kamada,*[c] Jeffrey S. Moore,*[b] and Yoshito Tobe*[a]
Abstract: We present herein the syn-
thesis and properties of the largest
hitherto unknown graphyne fragment,
namely trigonally expanded tetrakis-
rakis-DBA 8a bearing tert-butyl groups
revealed a remarkable bathochromic
Comparison between the free energies
of 9e and 8b bearing the same sub-
stituents revealed the more favorable
association of the latter due to stronger
p–p interactions between the extended
p-cores. Polarized optical microscopy
observations, DSC, and XRD measure-
ments showed that 8b and 9e with
branched alkyl ester groups displayed
columnar rectangular mesophases. By
the time-resolved microwave conduc-
tivity method, the columnar rectangu-
lar phase of 8b was shown to exhibit a
moderate charge-carrier mobility of
0.12 cm2 Vꢀ1 sꢀ1. These results indicate
that large graphyne fragments can
serve as good organic semiconductors.
shift of the absorption cut-off (lcutoff
)
compared with those of previously re-
ported graphyne fragments due to ex-
tended p-conjugation. Moreover, in the
two-photon absorption spectrum, 8a
showed a large cross-section for a pure
hydrocarbon because of the planar
para-phenylene-ethynylene conjugation
pathways. Hexakis(arylethynyl)-DBAs
9c–e and 9g and tetrakis-DBA 8b
bearing electron-withdrawing groups
aggregated in chloroform solutions.
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
Fꢁrstnerꢂs Mo catalyst furnished tetra-
kis-DBAs 8a and 8b substituted with
tert-butyl or branched alkyl ester
groups in moderate and fair yields, re-
spectively, demonstrating that the
metathesis reaction of this protocol is a
powerful tool for the construction of
graphyne fragment backbones. For
comparison, hexakis(arylethynyl)DBAs
9c–g have also been prepared. The
one-photon absorption spectrum of tet-
Keywords: alkynes · dehydroben-
zoannulenes · graphyne · metathe-
sis · self-assembly
Introduction
Highly conjugated carbon-rich molecules have attracted in-
creased attention because of their potential utility in the
field of organic materials science.[1] They show intriguing op-
tical and electronic properties and are capable of building
up highly organized supramolecular nanostructures in one,
two, and three dimensions.[2] Among them, hexadehydroben-
zo[12]annulene (DBA, 1, Figure 1),[3] which can be regarded
as the smallest unit of a hitherto unknown carbon allotrope,
g-graphyne,[4,5] and its derivatives are currently the subject
of intense interest. This is especially so in supramolecular
chemistry, including the formation of one-dimensional col-
umnar stacking[6] or rosette-shaped packing in three-dimen-
sional crystals,[7] liquid crystals,[8] vesicles,[9] and two-dimen-
sional (2D) molecular networks at the liquid–solid inter-
face.[10] On the other hand, large g-graphyne fragments, such
as multiply fused DBAs 2–7, have been synthesized to inves-
tigate their optical and electronic properties as well as local
aromaticity.[11,12] One of the largest known fragments is the
trefoil-type triply-fused DBA 7, which exhibits a relatively
large two-photon absorption cross-section among hydrocar-
bons of 1300 GM at 572 nm, due to the three para-phenyle-
neethynylene conjugation pathways locked in the planar
[a] Dr. K. Tahara, Y. Yamamoto, H. Kozuma, Y. Arikuma, Prof. Y. Tobe
Department of Frontier Materials Science
Graduate School of Engineering Science, Osaka University
1-3 Machikaneyama, Toyonaka, Osaka 560-8531 (Japan)
Fax : (+81)6-6850-6229
E-mail: tobe@chem.es.osaka-u.ac.jp
[b] Dr. D. E. Gross, Y. Gao, Prof. J. S. Moore
Department of Chemistry
University of Illinois at Urbana-Champaign
Urbana, Illinois 61801 (USA)
E-mail: moore@illinois.edu
[c] Dr. K. Ohta, Dr. Y. Shimizu, Dr. K. Kamada
Research Institute for Ubiquitous Energy Devices National Institute
of Advanced Industrial Science and Technology (AIST)
Ikeda, Osaka 563-8577 (Japan)
E-mail: yo-shimizu@aist.go.jp
k.kamada@aist.go.jp
[d] Dr. Y. Koizumi, Prof. S. Seki
Department of Applied Chemistry, Graduate School of Engineering
Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871 (Japan)
E-mail: seki@chem.eng.osaka-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300838.
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as for the graphdiyne series a large fragment consisting of
four [18]DBA units is known (Figure 2).[19d] Consequently, to
design organic materials based on graphyne fragments, it is
necessary to extend their structural size limit and to system-
atically study structure–property relationships using discrete
molecular species.[20]
Recently, alkyne metathesis has become
a powerful
method for the construction of arylene-ethynylene macrocy-
cles by virtue of a remarkable improvement in catalyst effi-
ciency and stability.[21] The first synthesis of an arylene-ethy-
nylene macrocycle by the use of alkyne metathesis was re-
ported by Adams and Bunz et al.,[22] whereby meta-cyclo-
phenylene-ethynylene (m-CPE) hexamers were synthesized
in up to 6% yield from m-dipropynylbenzene derivatives by
´
an in situ generated molybdenum catalyst. Miljanic and
Whitener et al. demonstrated the syntheses of DBAs and a
bow-tie-shaped bis-DBA from the corresponding propynyl-
benzene precursors catalyzed by a tungsten carbyne com-
plex.[12f] The maximum yields of the DBAs and bis-DBA
were 55% and 6%, respectively. On the other hand, Moore
et al. reported that a trialkoxymolybdenum alkylidyne com-
plex showed high reactivity towards propynylbenzene deriv-
Figure 1. Chemical structures of parent DBA 1 and multiply fused DBAs
2–7.
framework.[13] It is of further in-
terest to develop viable synthet-
ic approaches to even larger g-
graphyne fragments and to gain
insight into the issue of how
many DBA units are needed to
confer the graphyne-like char-
acter. Graphyne has been re-
ported to exhibit nonlinear op-
tical properties[14] and conduc-
tivity when doped with potassi-
um metal.[15] Recently, graph-
yne and graphyne nanoribbons
Figure 2. Structures of known multiply fused dehydrobenzoannulenes with trefoil shapes.
have become intriguing subjects
of theoretical studies,[16] and
one of the interesting predictions is a remarkable hydrogen-
storage capacity when they are decorated with Li or Ca.[17]
As such, large graphyne fragments are intriguing synthetic
targets.
atives.[23] In this case, the reaction side-product 2-butyne had
to be removed under reduced pressure in order to drive the
reaction equilibrium toward the product and to prevent de-
activation of the catalyst. An alternative method for remov-
ing the reaction side-product was developed by the same au-
thors that utilized benzoylbiphenyl groups as the terminal
groups of the alkyne in the precursors, which were convert-
ed into insoluble alkyne by-products. Multigram quantities
of m-CPEs were obtained by the so-called precipitation-
driven alkyne metathesis reactions.[24] Further modification
of the Mo catalyst was achieved by changing the co-catalyst
(Silanol-POSS)[25] or using a silica support.[26] Recently,
Fꢁrstner et al. reported that molybdenum nitride and alkyli-
dyne complexes bearing triphenylsilyloxy ligands represent-
ed superior alkyne metathesis catalysts in terms of stability
to both air and side-product (2-butyne), reaction efficiency,
and functional group tolerance.[27] Such a catalyst can be
used for the synthesis of m-CPEs.[28]
For the synthesis of multiply fused DBAs 2–7, the Sono-
gashira–Hagihara-type coupling reaction,[12a,d] the Stephens–
Castro-type reaction,[12c] double-elimination following the
formation of a single-bond linkage by intramolecular pinacol
coupling reaction,[12e,g] and an alkyne metathesis reaction[12b,f]
have been employed as pivotal steps for the construction of
the DBA subunits. However, in view of the low efficiency in
the construction of the twelve-membered ring, the syntheses
of the graphyne fragments are generally difficult compared
to those of multiply fused dehydrobenzo[18]annulene
([18]DBA) derivatives, graphdiyne fragments[18] that are
constructed by oxidative coupling of terminal alkynes.[19]
Indeed, large graphyne fragments composed of more than
three DBA units have not been synthesized thus far, where-
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Y. Shimizu, S. Seki, K. Kamada, J. S. Moore, Y. Tobe et al.
We previously reported on theoretical studies based on
DFT calculations on the graphyne fragments at the B3LYP/
6-31G* level of theory; the parent trigonally expanded tet-
planarized para-phenyleneethynylene conjugation pathways,
the enhanced aggregation properties of 8b in solution due
to strong p–p interactions, the formation of columnar rec-
tangular mesophases of 8b in the condensed phase, and a
moderate charge-carrier mobility of 8b. These results indi-
cate that large g-graphyne fragments serve as promising or-
ganic semiconductors.
rakis(dehydrobenzo[12]annulene)
(tetrakis-DBA,
8c,
Figure 3) was determined to have the smallest HOMO–
LUMO gap (2.29 eV) among the multiply fused DBAs 2–
7.[11a] Though a structurally related trefoil dehydrobenzoan-
nulene consisting of a central [12]DBA and peripheral dehy-
Results and Discussion
Synthesis of trigonally expanded tetrakis-DBAs
Synthetic strategy: Our synthetic strategy for obtaining tetra-
kis-DBAs relies on stepwise construction of the DBA rings
to circumvent obstacles in the construction of four DBA
units, formation of the central DBA ring by Sonogashira–
Hagihara coupling reaction, forming hexaethynyl-DBA 17,
and subsequent construction of the three peripheral DBA
units. A double-elimination reaction after the intramolecular
pinacol coupling of 9g and subsequent chlorination were ini-
tially attempted. However, this reaction sequence resulted
in the formation of a complex mixture of products. There-
fore, we chose the alkyne metathesis reaction, which has
proved to be successful for the construction of three periph-
eral DBA units.[12f]
Synthesis of arylene-ethynylene macrocycles by alkyne meta-
thesis reactions: We tested the efficiency of alkyne metathe-
sis catalysts for the construction of the DBA framework by
using simple precursor 10. We first investigated a highly
active trialkoxymolybdenum alkylidyne complex with Sila-
nol-POSS as a co-catalyst.[25] One drawback of this catalyst
system, in relation to our current studies, is that it has the
potential to scramble the internal alkynes present in the
pre-constructed central DBA ring of compounds 9.[31]
Indeed, such a scrambling effect was observed when 10 was
subjected to these conditions, giving rise to a complex mix-
ture of tert-butyl-substituted DBAs. Thus, we chose an alter-
native catalyst, namely molybdenum nitride and an alkyli-
dyne complex bearing triphenylsilyloxy ligands, which was
superior in terms of stability to both air and side-product (2-
butyne), reaction efficiency, and functional group tolerance.
Despite these advantageous properties, these catalytic sys-
tems have not been shown to react in a reversible way with
“internal” diphenylacetylenes. In fact, by optimizing the re-
action conditions (see the Supporting Information,
Table S1), we found that DBA 11 could be obtained in ex-
cellent yield by the reaction of 10 in 1,2,4-trichlorobenzene
(TCB) at 1508C (Scheme 1). Surprisingly, no detectable
amounts of scrambled products were formed under these re-
action conditions. Applying the optimized reaction condi-
tions, we started the synthesis of tetrakis-DBA derivatives.
Figure 3. Chemical structures of trigonally expanded tetrakis-DBAs 8a–c
and reference compounds (AE)6-DBAs 9a–g.
dro[14]annulene units has been reported by Haley and co-
workers (Figure 2),[29] all-[12]DBA trefoil has hitherto re-
mained unknown. In addition, tetrakis-DBA is expected to
show a large two-photon absorption cross-section due to the
planar tri(para-phenyleneethynylene) conjugation path-
way.[13,30] Moreover, stacking interactions between the large
planar p-conjugated cores are expected to enhance the self-
assembly properties, not only in solution but also in the con-
densed phase, giving rise to columnar liquid-crystalline ma-
terials when appropriately decorated at the periphery. We
therefore envisaged tetrakis-DBA derivatives as interesting
target molecules for studies of their optoelectronic proper-
ties and self-assembly behavior.
Herein, we describe the syntheses and properties of tetra-
kis-DBA derivatives 8a,b. Hexakis(arylethynyl)dehydroben-
zo[12]annulene ((AE)6-DBA) derivatives 9c–g have also
been prepared as reference compounds. The alkyne linkages
in tetrakis-DBAs 8a,b were introduced in moderate to fair
yields by three-fold alkyne metathesis reactions of 9a and
9b catalyzed by an Mo nitride complex, demonstrating the
power of alkyne metathesis for the construction of the mul-
tiple DBA units of graphyne fragments. Among the optical
and physical properties of 8a and 8b, particularly notewor-
thy are the large two-photon cross-section of 8a due to the
Syntheses of trigonally expanded tetrakis-DBAs: The synthe-
sis of tetrakis-DBAs began with hexakis[(triisopropylsilyl)e-
thynyl]DBA 16, which was prepared in 20% overall yield
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three-fold symmetries of the tetrakis-DBAs are evident
from their NMR spectra, thus ruling out alternative prod-
ucts derived from metathesis scrambling.
Photophysical properties
One-photon absorption and emission spectra: The one-
photon absorption (UV/Vis) spectrum of tetrakis-DBA pro-
vides useful insight into the electronic structure of the new
p-system. Figure 4(a) displays the absorption spectrum of
tetrakis-DBA 8a, together with those of (AE)6-DBA 9c,
tris-DBA 7, bis-DBA 2, and the parent DBA 1 for compari-
son. The spectrum of tetrakis-DBA 8a shows a strong band
with its maximum at 360 nm (lmax) accompanied by a weak
broadened band extending to 550 nm (lcutoff). The former is
attributable to the p-bis(phenylethynyl)benzene chromo-
phore,[32] while the latter weak band is due to the forbidden
S0-S1 transition of the D3h symmetric structure of 8a.[33] That
is to say, TD-DFT calculations on the parent tetrakis-DBA
at the B3LYP/6-31G* level of theory under D3h symmetric
constraints indicated that the S0-S1 transition is forbidden
(Table S2 and Figure S1). Additionally, a significant batho-
chromic shift of the lcutoff value of tetrakis-DBA 8a was ob-
served compared with those of substituted tris-DBAs 6 and
7 (lcutoff =525 nm for 6; lcutoff =500 nm for 7), indicative of
Scheme 1. Synthesis of DBA 11 by alkyne metathesis catalyzed by a Mo
nitride complex.
from 1,2-dibromo-3,4-diiodobenzene (Scheme 2; see also the
Supporting Information). Desilylation of 16 with tetrabuty-
lammonium fluoride (TBAF) followed by a six-fold cross-
coupling catalyzed by palladium with the corresponding aryl
iodide (see the Supporting Information) gave the penulti-
mate (AE)6-DBAs 9a and 9b in yields of 31% and 48%
(from compound 16), respectively. As reference compounds,
(AE)6-DBAs 9c–g were synthesized according to the same
protocol. Next, a three-fold alkyne metathesis reaction was
performed to construct the three peripheral DBA units. Spe-
cifically, treatment of 9a and 9b with 63 and 70 mol% of
Fꢁrstnerꢂs Mo nitride complex at 1508C in TCB afforded
tetrakis-DBAs 8a and 8b as orange solids in yields of 19%
and 56%, respectively (Scheme 3). The lower isolated yield
of 8a was most probably due to physical loss during isola-
tion owing to its low solubility in common organic solvents
(i.e., less than 0.1 mgmLꢀ1 in
CH2Cl2), in contrast to 8b,
which is reasonably soluble. Al-
ternatively, it may also have
been due to the different side-
products of the metathesis be-
cause of the different alkyne
terminal groups in 9a (Me) and
9b (Et); it has been reported
that the deactivation rate of an
Mo catalyst by 2-butyne (gener-
ated from 9a) is faster than
that by 3-hexyne (generated
from 9b).[23] Note that the Scheme 3. Synthesis of trigonally expanded tetrakis-DBAs 8a,b.
Scheme 2. Synthesis of (AE)6-DBAs 9a–g. TIPSA=triisopropylsilylacetylene, TIPS=triisopropylsilyl, TMSA=trimethylsilylacetylene, TMS=trimethyl-
silyl.
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Y. Shimizu, S. Seki, K. Kamada, J. S. Moore, Y. Tobe et al.
brational transition band.[12g] These features are associated
with the structural difference between the ground and excit-
ed states, which becomes more significant in larger p-conju-
gated cores. On the other hand, in spite of the large p-conju-
gated framework, the fluorescence quantum yields of 8a
and 9c were determined as 0.20 and 0.22, respectively,
which are comparable to those of the bow-tie-shaped bis-
DBA (0.19–0.21) and tris-DBA 6 (0.27).
Two-photon absorption cross-section: Two-photon absorp-
tion measurements of tetrakis-DBA 8a were performed in
chloroform at concentrations of 0.6–0.8 mm by using the
open-aperture Z-scan method (see the Supporting Informa-
tion). Tetrakis-DBA 8a was found to have a broad two-
photon absorption spectrum with two peaks, centered at
around 640 and 740 nm (Figure 4(c)). The peak at 640 nm
showed a two-photon absorption cross-section of around
900 GM while that at 740 nm had a value of 500 GM, where
1 GM=10ꢀ50 cm4 s·photonꢀ1·moleculeꢀ1. This spectral shape
is similar to that of 7[12e] in terms of peak positions and rela-
tive peak intensities (the peaks are located at 610 and
750 nm for 7). In contrast, the magnitude of the spectrum of
8a is almost twice as large as that of 7 at all wavelengths.
This larger two-photon absorption cross-section is consid-
ered to arise from the elongated p-conjugation path of 8a,
with three “phenyleneethynylene” units, compared to that
of 7 with only two such units. It is worth noting that the
peak wavelengths of 8a do not show a significant bathochro-
mic shift upon extension of the conjugation. Similar behav-
ior of an increase in the two-photon absorption intensity
without a spectral shift was observed for compounds having
ethylene and butadiylene p-conjugation linkers.[34]
Figure 4. (a) Absorption and (b) emission spectra of DBA 1 (black),
rhombic-shaped bis-DBA 2 (green), trefoil-shaped tris-DBA 7 (red),
trigonally expanded tetrakis-DBA 8a (blue), and (AE)6-DBA 9c
(orange) measured in dichloromethane or chloroform at room tempera-
ture. (c) Two-photon cross-section of 8a in chloroform at room tempera-
ture.
Self-assembling behavior of tetrakis-DBA and hexakis-
AHCTUNGTREG(NNUN arylethynyl)-DBAs
Self-association behavior in solution: The self-association be-
havior of shape-persistent phenylene-ethynylene macrocy-
clic molecules in solution due to p–p stacking as well as sol-
vophobic interactions has been extensively studied in order
to elucidate the structural and electronic factors in both the
core p-systems and peripheral substituents that affect the
self-assembling propensity. Systematic investigations on the
self-association of m-CPEs have revealed the importance of
structural planarity of the p-cores, as well as the presence of
electron-withdrawing peripheral substituents such as ester
groups attached to the p-cores and unbranched alkyl groups
on the substituents, for achieving a strong tendency for self-
association in apolar solvents.[35] When such compounds are
solubilized in polar solvents, solvophobic interactions play a
major role in the assembly of large aggregates. Presumably
because of the low association propensity of small DBA de-
rivatives, there seems to be no report on the self-association
of ortho-phenyleneethynylene macrocycles such as DBA
and its homologues, except for that on dehydrobenzo[14]an-
nulene derivatives bearing strongly electron-withdrawing/
donating groups by Haley et al.[30f] Although DBA deriva-
extended p-conjugation. The HOMO–LUMO gap calculat-
ed for 8a from the lcutoff value (2.25 eV) was also consistent
with the value estimated by DFT simulation (2.29 eV).[11a]
Attempts to determine the electrochemical HOMO/LUMO
levels of 8b and 9e were not successful, because in cyclic
voltammetry only irreversible and reversible reduction
waves were observed (Epa =ꢀ2.21 V for 8a; E1/2 =ꢀ1.68 V
for 9e in THF, nBu4NClO4 as electrolyte), and no oxidation
waves were detected (Figures S2 and S3).
Figure 4(b) shows the emission spectra of tetrakis-DBA
8a, (AE)6-DBA 9c, tris-DBA 7, bis-DBA 2, and the parent
DBA 1. The emission spectrum of 8a is bathochromically
shifted compared with those of the other multiply fused
DBAs. In the emission profile of 8a, the relative intensity of
the 0–0 vibrational transition band is weak compared with
the most intense 0–1 band. The same tendency is also ob-
served in 9c, but the other multiple DBA molecules, except
for trapezoid-type tris-DBA 5, exhibit a most intense 0–0 vi-
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tives bearing tetrathiafulvalene (TTF) units rather than ben-
zene rings have been shown to self-aggregate in solution,[36]
we focus here on hydrocarbon backbones. In this context,
we became interested in investigating the self-association
behavior of tetrakis-DBAs and (AE)6-DBAs in solution.
Prior to studying tetrakis-DBA 8b, the self-association be-
havior of (AE)6-DBA derivatives was investigated by con-
was observed for the formyl derivative 9g, presumably be-
cause of intermolecular hydrogen-bonding or dipole–dipole
interactions between the formyl groups. A similar associa-
tion-enhancing effect of formyl groups has been reported
for a pyrene derivative (Figure S7).[40] Dodecyl-substituted
(AE)6-DBA 9 f did not show a concentration-dependent
1
change in its H NMR spectrum, consistent with previous re-
1
centration-dependent H NMR measurements. For example,
sults.[35] Most importantly, tetrakis-DBA 8b exhibited a
larger association constant than that of (AE)6-DBA 9c (Fig-
ure S8), clearly showing the role of the large planar p-core
of the former, which enhances self-association by p–p stack-
ing interactions.
the signals of the inner aromatic protons Ha and peripheral
aromatic protons Hb (Figure 3) of butyl ester-substituted
(AE)6-DBA 9c in CDCl3 at 308C shifted from d=7.57 to
7.04 ppm and from d=8.02 to 7.55 ppm, respectively, on in-
creasing the solute concentration from 0.15 to 19.1 mm (Fig-
ure S4), indicating face-to-face-type self-association. The
chemical shift change was analyzed assuming a monomer–
dimer equilibrium, and from the least-squares curve-fitting
(see the Supporting Information) the equilibrium constant
(K) and the free energy (DG) of dimerization were deter-
mined.[37] Table 1 lists the results, together with those simi-
larly determined for (AE)6-DBAs 9d–g and tetrakis-DBA
8b. Because the association constants determined by follow-
ing the concentration-dependent shifts of Ha and Hb are
not equal, average values are also included. The larger the
K values, the greater the discrepancy becomes, most likely
due to the formation of higher aggregates. The data in
Table 1 nevertheless serve as a reasonable measure of the
aggregation propensities of the DBA derivatives in solution.
Similar observations have been reported previously.[30f,35,38]
The self-association propensity of dodecyl ester-substitut-
ed (AE)6-DBA 9d is similar to that of 9c, indicating that
the alkyl chain length does not influence the association be-
havior in a polar solvent (Figure S5). On the other hand, the
branched alkyl chains in 9e suppress the self-association of
(AE)6-DBA (Figure S6).[39] The largest association constant
Self-assembly in the condensed phase: Disk-like planar con-
jugated molecules often show columnar liquid-crystalline be-
havior.[41] In general, the transition temperature, the thermal
stability of the condensed phase, and the domain size are
dependent on molecular features such as the size and shape
of the p-conjugated core,[42] and the number, length, and
branching of the peripheral alkyl chains.[43] For DBA deriva-
tives, the formation of the liquid-crystalline phase is also in-
fluenced by the number and nature of the alkyl chains at
the periphery.[8,44] For example, DBAs with three alkoxy
groups displayed liquid-crystalline behavior while those with
six alkoxy groups did not, in contrast to the triphenylene de-
rivatives with smaller p-cores.[45] In the cases of tetrakis-
DBA and (AE)6-DBA, we expected the formation of stable
liquid-crystalline phases because of their extended p-conju-
gated systems.[46]
First, we investigated the mesomorphism of (AE)6-DBA
9d having C12 chains by TGA, DSC, and XRD measure-
ments, as well as by polarized optical microscopy (POM) for
optical texture observations of the mesophases. The TGA
measurement indicated that 9d showed gradual thermal de-
composition above 1508C (Figure S9), and thus the isotropi-
zation temperature could not be determined. DSC measure-
ment of 9d revealed that a phase transition was observed at
538C (Figure S10). Birefringence due to soft states of matter
was clearly observed at 1908C, indicating a mesomorphism
of 9d (Figure S11 (a)). However, definitive assignment of
the mesophase by XRD was hampered due to the presence
of some unidentified diffractions.
Next, tetrakis-DBA 8b and (AE)6-DBA 9e with branched
alkyl chains were investigated, because it is known that the
branched alkyl chains lower the isotropic temperature and
increase the domain size.[43a,e] TGA measurements on 8b
and 9e showed gradual thermal decomposition above 1508C
(Figures S12 and S13), similar to what was seen for 9d; thus,
the isotropization temperatures could not be determined.
However, DSC measurements revealed that phase transi-
tions occurred at lower temperatures, ꢀ28C for 8b and
108C for 9e (Figures S14 and S15), which corresponded to
the melting points evidenced by POM observations (Fig-
ure S11 (b, c)). Consequently, three and two mesophases
were detected for 8b and 9e, respectively. The phase-transi-
tion parameters are summarized in Table 2. In order to de-
termine the molecular spatial orderings in the mesophases,
Table 1. Association constants and the corresponding free energies for
hexakis(arylethynyl)DBAs 9c–g and tetrakis-DBA 8b determined assum-
ing a monomer–dimer model.[a]
Compound
Hydrogen atom[b]
Kassoc [mꢀ1
]
DG [kJmolꢀ1
]
Ha
Hb
134ꢁ2
174ꢁ4
154
ꢀ12.3ꢁ0.1
ꢀ13.0ꢁ0.1
ꢀ12.7
9c
average value
Ha
Hb
93.3ꢁ1.5
126ꢁ2
110
ꢀ11.4ꢁ0.1
ꢀ12.2ꢁ0.1
ꢀ11.8
9d
9e
average value
Ha
Hb
13.9ꢁ0.7
12.5ꢁ0.8
13.2
ꢀ6.6ꢁ0.1
ꢀ6.4ꢁ0.1
ꢀ6.5
average value
[c]
[c]
[c]
9 f
9g
–
–
–
Ha
1800ꢁ130
ꢀ18.9ꢁ0.2
Ha
Hb
247ꢁ15
594ꢁ34
421
ꢀ13.9ꢁ0.2
ꢀ16.1ꢁ0.2
ꢀ15.0
8b
average value
[a] In CDCl3 at 308C. [b] Ha and Hb correspond to the inner and periph-
eral aromatic protons, respectively (see Figure 3). [c] No association was
observed.
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Y. Shimizu, S. Seki, K. Kamada, J. S. Moore, Y. Tobe et al.
Table 2. Phase-transition parameters of 8b, 9d, and 9e.
to photo-carrier generation through triplet excited-
state migration. A relatively high charge-carrier
generation yield has often been observed for iso-
lated columnar assemblies upon photo-excita-
tion.[49] Clear kinetic traces of the transient absorp-
tion were observed for both compounds in the
solid state. The transient absorption (S1-Sn) of the
Compound
Phase-transition temperature [8C] (DG [kJmolꢀ1])
9d
9e
8b
cryst. 53 (71) M1[a] (decomp: >1258C)
cryst. 10 (16) M2[b] 55 (4.9) M3[b] (decomp: >1508C)
cryst. ꢀ2 (11) M4[a] 4 (17) M5[b] 178 (8.1) M6[a] (decopm: >1258C)
[a] Unidentified phase. [b] Colr phase.
XRD analyses were performed. The XRD patterns observed
from all mesophases for the non-aligned samples showed a
broad halo at around 0.45 nm, originating from the molten
alkyl chains (Figures S16 and S17). Moreover, it turned out
that 8b and 9e adopted columnar mesophases with rectan-
gular lattices (P21/a symmetry) (Table S8 and Figure S18).
Note that, among them, only tetrakis-DBA 8b exhibited a
strong correlation of 0.34 nm, which corresponds to the
stacking periodicity of molecules within a column. This is
due to the planarity and rigidity of the p-conjugated core,
which enhances the stacking order by the acetylene linkages
connecting the peripheral phenyl groups in 8b.
The observed mesophases exhibited relatively high ther-
mal stability above 3008C, though thermal decomposition
gradually took place. Moreover, the introduction of
branched alkyl chains at the periphery led to a lowering of
the melting temperature.[43a,e] However, in all cases, no clear-
ing points were observed before the thermal decomposition.
The branched alkyl chains in 8b and 9e are presumably not
large enough to reduce the clearing temperature to a value
below the decomposition temperature, probably because of
the large facial area generated by the trigonally expanded
p-conjugated core.
Figure 5. Kinetic traces of conductivity transients (blue lines) observed
upon 355 nm excitation at 296 K with an excitation photon density of
9.1ꢃ1015 photonscmꢀ2 of 8b (a) and 9e (b) films cast on a quartz sub-
strate. Transient absorption kinetic traces were also measured (a) for 8b
at 650 nm and (b) for 9e at 600 nm, respectively, displayed in red.
Charge-carrier mobility in the liquid crystals: Disc-shaped p-
conjugated molecules, when assembled to form columnar or-
dering in 3D crystals and liquid-crystalline materials, often
show semiconducting properties,[47] which are dependent on
the intrinsic molecular features and molecular packing.
Time-resolved microwave conductivity (TRMC) measure-
ments can provide direct information on the local motion of
charge carriers in these columnar stacks without using elec-
trodes, which tend to localize the charge carriers at the elec-
trode–semiconductor interfaces. The direct interaction be-
tween charge carriers and the probing microwave reveals
the effective mass of the charge carriers along the conjugat-
ed columnar core stacks,[48] and hence the value of charge-
carrier mobility reflecting the local oscillating motion of
charge carriers in the potential induced by the alternating
electric field of the probing microwave.
Upon excitation of 8b and 9e in the solid state, clear con-
ductivity transients were observed at 296 K, at which both
compounds adopted a columnar structure (Colr phase), as
shown in Figure 5. The conductivity transient shows a rapid
rise within the time constant of the present TRMC measure-
ment apparatus (~50 ns) for 8b, suggesting a predominant
contribution from the singlet excited state to produce free
charge carriers. Considerable evolution of the conductivity
transient was, however, observed for 9e, which may be due
singlet excited state of 8b decayed rapidly within 1 ms (Fig-
ure 5(a)), and largely overlapped kinetic traces were ob-
served for both transient conductivity and optical absorption
at 650 nm. After the decay processes of the singlet excited
states, the transient absorption spectra for 8b indicated si-
multaneous bleaching of the steady-state S0-S1 transition at
400 nm with an isosbestic point (Figure S19), suggesting that
effective photo-ionization occurs in the present system. The
photo-ionization yield was calculated based on the assump-
tion of 100% conversion of the steady-state absorption
bleaching into the radical cations monitored at 650 nm, with
a
molar extinction coefficient of 8b at 400 nm of
38500mꢀ1 cmꢀ1, leading to an initial photo-ionization yield
(f) of 2.1ꢃ10ꢀ4. An almost identical value of f was estimat-
ed for 9e in the solid state upon excitation at 355 nm, f
ꢂ1.9ꢃ10ꢀ4 (Figure 5(b)). These values are an order of mag-
nitude higher than the yields observed for conjugated mo-
lecular crystal systems,[50] reflecting suppression of inter-col-
umnar charge recombination and/or isolation of positive
ꢀ
charges in the columnar cores from the exposure of O2 su-
peroxide anion produced by electron capture in the photo-
ionization processes. The yield of photo-carrier generation
was also determined by a photo-current integration method
(Figure S20),[51] and the values of f derived were 1.2ꢃ10ꢀ4
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Towards Larger Graphyne Fragments
FULL PAPER
and 9.5ꢃ10ꢀ5 for 8b and 9e, respectively. The lower values
of f compared to those derived from transient absorption
spectroscopy were due to charge trapping in the long-range
translational motion of charge carriers in the photo-current
integration regime, leading to their underestimation. The
minimum values of intracolumnar mobility were determined
by the transient conductivity, with f values from transient
absorption spectroscopy of Sm=0.12 cm2 Vꢀ1 sꢀ1 for 8b and
Sm=0.058 cm2 Vꢀ1 sꢀ1 for 9e, respectively. The value for 8b
is smaller than those of hexabenzocoronene derivatives,[52]
but larger than those of triphenylene derivatives.[53] The 1D
stacks of carboxylated DBA 1 in the 3D crystal were shown
to exhibit comparable mobility.[6a] Taking into account the
low ordering of 8b (non-aligned sample), the mobility of
multiply fused DBAs is sufficiently large for potential appli-
cations as organic semiconductors.
AHCTUNGTRENNUNG
1H (400 MHz, 300 MHz, and 270 MHz) and 13C (100 MHz, 75 MHz, and
67.5 MHz) NMR spectra were measured on JEOL JNM AL-400, Bruker
AV400M, Varian Mercury 300, or JEOL JNM-GSX-270 spectrometers.
Spectra measured in CDCl3 were referenced to residual solvent protons
for the 1H NMR spectra (d=7.26 ppm) and to solvent carbons for the
13C NMR spectra (d=77.0 ppm). Preparative HPLC was undertaken with
a JAI LC-908 chromatograph using 600 mmꢃ20 mm JAIGEL-2H and
2.5H GPC columns with CHCl3 as eluent. IR spectra were recorded on a
JASCO FT/IR-410 spectrometer. Mass spectra were recorded on a JEOL
JMS-700 spectrometer in EI or FAB ionization modes or an AXIMA-
CFR in LD ionization mode. Elemental analyses were carried out on a
Perkin–Elmer 2400II analyzer.
UV/Vis–NIR absorption and fluorescence measurements: UV/Vis spectra
were recorded on a Hitachi U-3310 spectrometer. Fluorescence spectra
were measured on a JASCO FP750-DS/O spectrometer. All solutions for
measurements were prepared in spectrophotometric grade dichlorome-
thane or chloroform. The concentrations of each solution were as fol-
lows: 1.00ꢃ10ꢀ5 m for 8a and 4.02ꢃ10ꢀ6 m for 9c in UV/Vis absorption
measurements and 1.01ꢃ10ꢀ6 m for 8a and 2.01ꢃ10ꢀ7 m for 9c in emission
measurements. All measurements were performed at room temperature.
Considering the relatively low association constants determined from the
1H NMR spectra, the solute molecules are expected to exist as monomers
at these concentrations.
Conclusion
In conclusion, the largest g-graphyne fragments yet assem-
bled, trigonally expanded tetrakis-DBAs, consisting of four
DBA subunits, have been synthesized by intramolecular
three-fold metathesis cyclization. The present results dem-
onstrate the potential of alkyne metathesis for the construc-
tion of multiply fused DBA backbones. In the one-photon
absorption spectrum of tetrakis-DBA 8a, a remarkable
bathochromic shift of the absorption cut-off (lcutoff) com-
pared with those of previously reported graphyne fragments
was observed because of the extended p-conjugation. More-
over, 8a displayed a large two-photon absorption cross-sec-
tion among pure hydrocarbons due to the three planar para-
phenyleneethynylene unit conjugation pathways. Hexa-
Computational details: All theoretical calculations were performed with
the Gaussian 09 package. The B3LYP functional with the 6–31G* basis
set was used for geometry optimization of 8c. Excited-state calculations
(TD-DFT) were performed on the optimized structure (B3LYP/6–31G*
level of theory). The major electronic transitions and representative fron-
tier molecular orbitals for 8c are shown in Table S2 and Figure S1.
Characterization of the liquid-crystalline phase: Differential scanning cal-
orimetry (DSC) measurements were performed on a TA Instruments
DSC2920. The characteristics of the liquid-crystalline materials were ob-
served by polarization microscopy on an Olympus BH2 microscope
equipped with a Mettler FP90 hot stage. X-ray diffraction analysis of
liquid-crystalline materials was performed using a Rigaku RINT2000
(CuKa radiation) diffractometer.
Two-photon absorption cross-section: The two-photon absorption spec-
trum of 8a in chloroform was measured using the open-aperture Z-scan
method. A femtosecond optical parametric amplifier (typically 120 fs)
operating at a repetition rate of 1 kHz was used as the light source to
scan the excitation wavelength range from 590 to 820 nm. Details of this
set-up have been reported previously.[55] The Rayleigh range of this opti-
cal set-up was 6–7 mm for the wavelengths employed and much larger
than the optical pathlength of the sample solution held in a quartz cuv-
ette (2 mm), which satisfied the “optically thin” condition needed for
quantitative analysis. The recorded data were analyzed by the curve-fit-
ting procedure with the theoretical model of the transmittance of a spa-
tially and temporally resolved Gaussian pulse through two-photon ab-
sorptive media. From the curve-fitting, the two-photon absorbance of the
sample q0 =a(2)I0Leff was obtained, where a(2) is the two-photon absorp-
tion coefficient, I0 is the peak optical intensity at the focal point, and Leff
is the effective pathlength of the sample. The proportionality relationship
between q0 and I0 was confirmed by varying the incident power over the
range 0.1–0.8 mW for each measurement, which verified that the ob-
served signal was indeed due to the two-photon absorption process. Con-
ditions of I0 ꢃ 250 GWcmꢀ2 at the focal point of the set-up were main-
tained to avoid higher-order nonlinear optical processes. Finally, the two-
photon absorption cross-section was calculated using the convention
ACHTUNGTRENNUNGkis(phenylene-ethynyl)-DBAs 9c–e and tetrakis-DBA 8b
exhibited self-association behavior in chloroform: 8b
showed a stronger tendency for association than the corre-
sponding 9e due to stronger p–p interactions between the
extended p-cores. In the condensed state, 9e and 8b dis-
played liquid-crystalline phases. XRD analyses revealed the
formation of columnar rectangular phases. The columnar
rectangular phase of 8b showed moderate charge-carrier
mobility, as determined by the TRMC method, suggesting
that the graphyne fragments may be promising candidates
as organic semiconductors. With this powerful method for
the construction of graphyne fragments and the present
knowledge on the structure–property relationship of tetra-
kis-DBAs, the rational design and synthesis of larger graph-
yne fragments of optoelectronic interest should be possible.
s
(2) =Epha(2)/N, where N is the number density of the molecules in the
Experimental Section
sample and Eph is the photon energy. The obtained s(2) was calibrated
with our in-house standard compound, MPPBT,[33] measured at the same
time.
General procedure for the synthesis of tetrakis-DBAs and hexakis(aryle-
thynyl)-DBAs: All manipulations, except for the syntheses of 15, 18a,
18b, 20, and 23, were performed in an inert gas (nitrogen or argon) at-
mosphere. All solvents were distilled or passed through activated alumina
and copper catalyst in a Glass Contour solvent purification system prior
to use. All commercially available reagents were used as received. [Pd-
Time-resolved microwave conductivity (TRMC) measurements: Besides
the symmetry-forbidden S0-S1 transitions of tetrakis-DBA 8b and (AE)6-
DBA 9e at around 400–500 nm depending on the intramolecular conju-
gation, excitation light pulses at 355 nm from a Spectra Physics INDI-
Chem. Eur. J. 2013, 00, 0 – 0
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Y. Shimizu, S. Seki, K. Kamada, J. S. Moore, Y. Tobe et al.
HG nanosecond Nd:YAG laser were used to induce photo-carrier gener-
ation, whereby the extinction coefficients of the molecules per number of
DBA moieties were almost identical. The power density of the excitation
light source was set at 5.1–10.2 mJcmꢀ2 (9.1–18ꢃ1015 photonscmꢀ2). The
probing microwave frequency and power were set at ~9.1 GHz and
3 mW, respectively. Tetrakis-DBA 8b and (AE)6-DBA 9e were cast on a
quartz substrate of thickness 2.5 mm from solutions in chloroform, and
the TRMC signals were monitored upon excitation and averaged over
256 shots. Transmittance of excitation light pulses at 355 nm through the
film was calculated based on the electronic absorption spectra of the
compounds. All of the above experiments were carried out at 296 K.
Transient photoconductivity (Ds) of the samples was derived directly
from the transient traces of reflected microwave power change (DPr) rel-
ative to steady-reflected microwave power (Pr), and converted into the
sum of the mobilities of charge carriers according to:
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Acknowledgements
This work is supported by a Grant-in-Aid for Scientific Research from
the Ministry of Education, Culture, Sports, Science, and Technology,
Japan and the research at UIUC was supported by the National Science
Foundation, grant no. CHE-1010680.
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Received: March 4, 2013
Revised: May 14, 2013
Published online: && &&, 0000
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