Synthesis and solid-state structures of a tetrathiafulvalene-conjugated bistetracene

Article abstract of DOI:10.1002/chem.201304997

A tetrathiafulvalene (TTF)-conjugated bistetracene was synthesized and characterized in the molecular electronic structures based on the spectroscopic measurements and the single-crystal X-ray diffraction analysis. UV/Vis absorption and electrochemical measurements of 5 revealed the considerable electronic communication between two tetracenedithiole units by through-bond and/or through-space interactions. The difference in the crystal-packing structures of 5, showing polymorphism, results in a variety of intermolecular electronic-coupling pattern. Of these, the π-stacking structure of 5 A gave a large transfer integral of HOMOs (97meV), which value is beyond hexacene and rubrene, thus, quite beneficial to achieve the high hole mobility. High hole mobility: A tetrathiafulvalene (TTF)-conjugated bistetracene was synthesized and the molecular electronic structures, as well as the packing structures were characterized. A variety of crystal packing arrangements could be originated from the soft π linkage of TTF unit. The π-stacking structure affords large intermolecular orbital coupling of HOMOs, which is quite beneficial to enhance the charge-carrier mobility (see figure).

Full text of DOI:10.1002/chem.201304997

DOI: 10.1002/chem.201304997
Full Paper
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Pi Interactions
Synthesis and Solid-State Structures of a Tetrathiafulvalene-
Conjugated Bistetracene
Masataka Yamashita,[a] Daiki Kuzuhara,[a] Naoki Aratani,*[a] and Hiroko Yamada*[a, b]
Abstract: A tetrathiafulvalene (TTF)-conjugated bistetracene
was synthesized and characterized in the molecular electron-
ic structures based on the spectroscopic measurements and
the single-crystal X-ray diffraction analysis. UV/Vis absorption
and electrochemical measurements of 5 revealed the consid-
erable electronic communication between two tetracenedi-
thiole units by through-bond and/or through-space interac-
tions. The difference in the crystal-packing structures of 5,
showing polymorphism, results in a variety of intermolecular
electronic-coupling pattern. Of these, the p-stacking struc-
ture of 5A gave a large transfer integral of HOMOs (97 meV),
which value is beyond hexacene and rubrene, thus, quite
beneficial to achieve the high hole mobility.
cule that has been used in various applications.[7] Although di-
benzoTTF and dinaphthoTTF (DN-TTF) derivatives have been
widely investigated, the p-extended TTF derivatives more than
anthracene fusion, however, remain virtually unexplored,
mainly because of their chemical instability and low solubility
(Figure 1).[8,9] Hitherto practically only one example of dianthra-
cenoTTF (DA-TTF) was reported.[8]
Introduction
Acenes are typical aromatic hydrocarbons composed of linearly
fused benzene rings and of contemporary interest from a prac-
tical standpoint as functional organic materials.[1] It is known
that the electronic properties of the bulk acenes depend heavi-
ly on their molecular-packing patterns. In this context, it is in-
trinsically important to predict or control the molecular ar-
rangement of acenes in the solid state to provide the best per-
formance of the organic-molecular devices. In general, an inter-
molecular interaction between the large acenes is governed
exclusively by p–p stacking and CÀH···p interactions.[2,3] The
herringbone packing motif is believed to be responsible for
the high carrier mobility of oligoacenes in the crystalline state,
which is induced by the strong CÀH···p interactions,[4] although
a rubrene (5,6,11,12-tetraphenyltetracene) exhibits remarkable
carrier mobility mainly due to the appropriate p–p stacking.[5]
Recently, we have synthesized 13,13’-(3,5-bis(trifluoromethyl)-
phenyl)-6,6’-bipentacene from a soluble bispentacenequinone
precursor.[6] Bispentacene takes orthogonal conformation in
the solid state and a pair of slipped p–p stacking of both pen-
tacene planes results in the formation of two-dimensional grid-
like network structure in the crystal.
Figure 1. p-Extended TTF and rubrene.
Herein, we prepared, for the first time, a TTF-conjugated bis-
tetracene 5 (Figure 1). The advantage of using such additional
benzene rings is increase of intermolecular p–p interactions,
which leads to a large transfer integral between molecules in
the solid state.[10] To enhance the solubility of the compounds
in this study, four phenyl groups were introduced at 5,5’,12,12’
positions. During its crystallization process, we have serendipi-
tously discovered its polymorphism (5-A and 5-B). These two
crystal structures were predicted to have different electronic
properties as a crystalline material. Based on the crystal data,
the transfer integrals of 5-A and 5-B were estimated.
Tetrathiafulvalene (TTF) is a representative of the electron
donor and an important and potential functional organic mole-
[a] M. Yamashita, Dr. D. Kuzuhara, Prof. Dr. N. Aratani, Prof. Dr. H. Yamada
Graduate School of Materials Science
Nara Institute of Science and Technology (NAIST)
8916-5 Takayama-cho, Ikoma, Nara 630-0192 (Japan)
Fax: (+81)743-72-6042
E-mail: aratani@ms.naist.jp
hyamada@ms.naist.jp
[b] Prof. Dr. H. Yamada
CREST (Japan) Science and Technology (JST)
Chiyoda-ku, 102-0075 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304997.
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Results and Discussion
Synthesis and solid-state structures
The route for the synthesis of the tetraphenyl TTF-conjugated
bistetracene 5 starts from a Diels–Alder reaction of the qui-
none 1[11] with 1,3-diphenylisobenzofurane to form an adduct,
then subsequent dehydration provided the tetracenequinone
(2; Scheme 1). The quinone 2 was reduced with NaBH4 and
SnCl2 to give tetracene thioketone (3), which was interconvert-
ed to ketone with Hg(OAc)2 in 78% yield.
Scheme 1. Synthesis of a TTF-conjugated bistetracene 5.
1
The structures of 3 and 4 were confirmed by their H NMR
and high-resolution FAB mass spectroscopy data. Slow vapor
diffusion of methanol into a chloroform solution of 3 and 4
gave good crystals suitable for X-ray diffraction analysis
(Figure 2).[12] Both structures revealed a slightly curved but per-
fectly planar skeleton unambiguously, in which the phenyl
groups take perpendicular orientation. In the solid state, 3
forms a face-to-face antiparallel dimeric structure with a stack-
ing distance of 3.54 ꢁ (Figure S10 in the Supporting Informa-
tion). A short intermolecular S···S contact of 3.44 ꢁ was also ob-
served. Compound 4 also forms the face-to-face antiparallel di-
meric structure (Figure S11 in the Supporting Information). The
interplanar distances of 3.50 and 3.59 ꢁ were observed.
Figure 2. Crystal structures of 3 and 4: a) top view of 3; b) side view of 3;
c) top view of 4; d) side view of 4. Solvent molecules are omitted for clarity.
Thermal ellipsoids represent 50% probability.
The structure of 5 was unambiguously revealed by single-
crystal X-ray diffraction analysis (Figure 3).[12] Interestingly, re-
crystallization of 5 from toluene and ethanol exhibited poly-
morphism, giving rod-like crystals (5-A) and sheet-like ones (5-
B), both of which included two toluene molecules in their unit
cells (Figure S15 in the Supporting Information). The structure
analyzed from the rod-like crystal is shown in Figure 3a–c.
Compound 5-A is perfectly planar and forms face-to-face stack-
ing columnar structure with the interplanar distance of 3.74 ꢁ
along the a axis. The phenyl units are slightly tilting from an
orthogonal arrangement (64 and 708). The neighboring TTF
parts are deviated by about 4.8 ꢁ.
Then, we tried to prepare a target compound 5 from 3 and
4 based on the phosphonate-induced cross-coupling reaction.
After several experiments, we obtained 5 in 70% yield as a red
solid. The solubility of 5 in CH2Cl2 is 0.051 mgmLÀ1. High-reso-
lution atmospheric-pressure chemical ionization (APCI) mass
spectroscopy detected the parent ion peak at m/z=908.2018
1
(calcd for C60H36S4 =908.1700 [M]+). The H NMR spectrum of 5
in CD2Cl2 exhibited only aromatic protons at d=8.04, 7.60–
To our surprise, the diffraction analysis of 5-B exhibited
smoothly curved S-shaped planar structure as shown in Fig-
7.54, 7.45, and 7.18 ppm.
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Figure 3. Crystal structures of a) top view; b) side view; and c) packing diagram of 5-A; d) top view; e) side view; and f) packing diagram of 5-B; g) top view;
h) side view; and i) packing diagram of 5-C. Solvent molecules are omitted for clarity. Thermal ellipsoids represent 50% probability.
ure 3d–f. This clearly indicates the flexibility of the TTF-conju-
gated tetracene framework. The phenyl groups displayed a per-
pendicular conformation (78 and 888). Importantly, in the crys-
tal, a slipped p–p stacking of tetracene planes resulted in the
formation of one-dimensional chain-like structure (Figure 3 f).
The interplanar distance is averagely 3.77 ꢁ. This is a new po-
tential structural motif for extended TTF or bridged acene sem-
iconducting materials.
shift (408 cmÀ1), indicating essentially the same but slightly en-
hanced electronic structure of 4 compared with tetracene. The
absorption spectrum of 5 (lmax =510 nm) becomes very broad
with less vibronic structure and exhibits redshifted absorption-
edge compared to 4, suggesting the considerable electronic
communication between two tetracenedithiole units by
through-bond and/or through-space interactions. Moreover,
the fluorescence spectrum of 5 is remarkably redshifted, re-
flecting the red-shifted absorption spectrum. The fluorescence
quantum yields of 4 and 5 are 66 and 23%, respectively, thus
the larger molecular size and the ethene-linkage of 5 would
lead to enhance nonradiative process.
Interestingly, the subtle change of the crystallizing solvent
from toluene/ethanol to o-xylene/ethanol (just one methyl
group addition for crystalline solvent) results in the drastic al-
ternation of molecular arrangement in the crystal. From o-
xylene and ethanol, the plate-shaped orange single crystals of
5 (5-C) were formed. The structure of 5-C is shown in Fig-
ure 3g–i, which is totally different from those of 5-A and 5-B,
including ten o-xylene molecules in the unit cell. The main
skeleton of the bistetracene is bent at the central TTF unit, and
the half plane of bistetracene stacked each other to form di-
meric structure with a distance of 3.55 ꢁ surrounded by many
o-xylene molecules (Figure 3i and S14 in the Supporting Infor-
mation).
Spectroscopic analysis
UV/Vis absorption and fluorescence spectra of 4 and 5 in
CH2Cl2 are shown in Figure 4. Compared to tetracene (lmax
=
475 nm in CHCl3), 4 exhibits distinctly redshifted absorption at
l=514 nm with the clear vibrational bands and a small Stokes
Figure 4. UV/Vis absorption and fluorescence spectra of 4 and 5 in CH2Cl2.
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tures (Figure 7). Curiously, DFT
calculations revealed that the
lowest-energy structure of 5 is
a V-shaped conformer, and the
energy of the planar molecule
(5-A) is 9.9 kcalmolÀ1 higher,
which difference corresponds to
an inversion barrier. Notably, this
is similar to the bowl-to-bowl in-
version barrier of corannulene
(11.5 kcalmolÀ1).[15] Even the
energy of the S-shaped confor-
mer is 6.5 kcalmolÀ1 higher than
that of the optimized one, indi-
Figure 5. Polarized absorption spectra of the single crystals of a) 5-A; b) 5-B; and c) 5-C with photographs of each
crystal at 08C.
The intramolecular interaction in 5 was also confirmed by
cyclic voltammetry (CV). The CV of 4 in PhCN displayed reversi-
ble oxidation potential at 0.52 V (vs. ferrocene/ferrocenium+
ion couple) and irreversible one at 0.96 V (Figure S9 in the Sup-
porting Information). On the other hand, the CV of 5 exhibits
three oxidation potentials at 0.38, 0.61, and 0.65 V as fully re-
versible waves. From the first oxidation potential, we could es-
timate HOMO level to be À5.18 eV.[13] Reduction potentials
were not detected at these conditions.
cating that the lattice energy based on van der Waals interac-
tions is large enough to transform the TTF part. Thus, the dif-
ference in the packing structures of 5-A, 5-B, and 5-C could be
originated from the soft p linkage of TTF unit. The herringbone
structure is not favorable for 5 with the peripheral phenyl
rings.
To investigate the solid-state properties of each crystal, UV/
Vis absorption spectra of the single crystals of 5-A, 5-B, and 5-
C have been measured (Figure 5). The peak absorption wave-
lengths of the crystalline state were all at approximately
530 nm, which are distinctly longer than that observed in solu-
tion, thus indicating intermolecular interactions and deforma-
tion effects of 5 within the crystalline packing structure. The
changes in optical density seen on rotating the crystal samples
under the polarized microscope indicate that the bistetracenes
are anisotropically oriented in the crystal. Especially, the molec-
ular orientation in 5-B stands perpendicular to the substrate
based on the crystal analysis, resulting in the large difference
of its absorption spectra depending on the angle of the polar-
ized light. Those in 5-A and 5-C are obliquely oriented to the
substrate.
Figure 6. Molecular-orbital diagrams of DN-TTF, DA-TTF, and DT-TTF.
Theoretical calculations
To understand their electronic features, MO calculations of the
model compounds DN-TTF, DA-TTF, and DT-TTF were per-
formed at B3LYP/6-31G(d) level by using Gaussian 09 package
(Figure 6).[14] At a glance, the HOMOs of DN-TTF and DA-TTF
are localized on the TTF unit, whereas that of DT-TTF is local-
ized on the tetracene units. Although the energy level of the
orbital, which has a large MO coefficient on the TTF part,
slightly lowers when the acene becomes larger, the degenerat-
ed occupied MOs localized on the acene parts (HOMOÀ1 and
HOMOÀ2 for DN-TTF and DA-TTF) rise steeply with the in-
crease of the fused benzene rings, becoming HOMO and
HOMOÀ1 at the tetracene stage in reverse.
Figure 7. Relative potential-energy diagrams of the various conformers of 5
based on the B3LYP/6-31G* level of theory.
The p stacking structure can maximize the intermolecular or-
bital overlap and thus, the structures of 5-A and 5-B are ex-
pected to have large intermolecular orbital couplings of the
HOMOs in the molecular p stacks. Intermolecular transfer inte-
gral V (meV) values between the HOMOs of neighboring mole-
cules were calculated with the Amsterdam density functional
(ADF) program package,[16] and the calculated V values are
shown in Figure 8. As was expected, V values in the stacking
Next question to be answered was why does 5 take various
conformations in the solid state. To understand the nature of
the TTF-conjugated bistetracene 5, we have calculated the po-
tential energy of each conformer based on the crystal struc-
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NAIST, for the X-ray diffraction analysis and Ms. Y. Nishikawa,
NAIST, for the mass spectroscopy.
Keywords: density functional calculations · pi interactions ·
polymorphism · tetracenes · X-ray diffraction
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Figure 8. The calculated transfer integrals of a) 5-A; b) hexacene; and c) ru-
brene.
direction are very large for 5-A (ca. 97 meV) and moderate for
5-B (ca. 16 meV), whereas V values in the transverse directions
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structures of 5 are most likely one dimensional. It is notewor-
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[12] Crystallographic data for 3: C31H18S3·CHCl3, Mw =606.00, monoclinic,
space group P21/c (No. 14), a=8.49920(10), b=16.9929(3), c=
19.5302(4) ꢁ, b=104.6960(10)8, V=2728.39(8) ꢁ3, 1calcd =1.475 gcmÀ3
,
Conclusion
Z=4, R1 =0.0333 [I>2.0s(I)], Rw =0.0980 (all data), GOF=1.032. Crystal-
lographic data for 4: C31H18OS2, Mw =470.57, monoclinic, space group
P21/c (No. 14), a=12.3693(2), b=27.5630(5), c=13.7529(3) ꢁ, b=
103.4210(10)8, V=4560.80(15) ꢁ3, 1calcd =1.371 gcmÀ3, Z=8, R1 =0.0398
[I>2.0s I]], Rw =0.1065 (all data), GOF=1.023. Crystallographic data for
The TTF-conjugated bistetracene 5 could be synthesized and
characterized in the molecular electronic structures based on
the crystal data. Due to the flexible TTF unit and the peripheral
phenyl groups, 5 exhibits a variety of molecular structures with
face-to-face interactive manner in crystals. The p–p interaction
affords large intermolecular orbital coupling of HOMOs in 5-A,
resulting in the anisotropic 1D electronic structure as a whole.
DT-TTF without the peripheral phenyl groups is a next attrac-
tive target, because previous studies of acene-TTF hybrid have
demonstrated that the high carrier mobility was achieved by
the elongation of acene units.[17]
¯
5 A: C62H36S4·2(C7H8), Mw =1093.42, triclinic, space group P1 (No. 2), a=
5.9266(8), b=13.6535(18), c=17.760(2) ꢁ, a=110.147(3), b=94.314(3),
g=90.948(3)8, V=1344.0(3) ꢁ3, 1calcd =1.351 gcmÀ3, Z=1, R1 =0.0808
[I>2.0s(I)], Rw =0.2055 (all data), GOF=1.018. Crystallographic data for
¯
5-B: C62H36S4·2(C7H8), Mw =1093.42, triclinic, space group P1 (No. 2), a=
10.9635(3), b=10.9709(3), c=12.5854(3) ꢁ, a=112.5040(10), b=
99.3790(10), g=94.2860(10)8, V=1364.10(6) ꢁ3, 1calcd =1.331 gcmÀ3, Z=
1, R1 =0.0358 [I>2.0s(I)], Rw =0.0954 (all data), GOF=1.057. Crystallo-
graphic data for 5-C: C62H36S4·5(C8H10), Mw =1439.95, triclinic, space
¯
group P1 (No. 2), a=13.0068(3), b=13.7967(3), c=22.8243(6) ꢁ, a=
101.1170(10), b=90.1660(10), g=106.6900(10)8, V=3842.27(16) ꢁ3,
1calcd =1.245 gcmÀ3, Z=2, R1 =0.0488 [I>2.0s(I)], Rw =0.1212 (all data),
GOF=1.038. CCDC 936781 (3), CCDC 936782 (4), CCDC 936783 (5-A),
CCDC 936784 (5-B), and CCDC 948526 (5-C) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
This work was partly supported by Grants-in-Aid for Scientific
Research (Nos. 25288092, 25107519 “AnApple”, and 25620061),
for Young Scientists (A) (No. 23685030), PRESTO program by
JST, the Green Photonics Project in NAIST supported by MEXT,
and the program for promoting the enhancement of research
universities. We thank Prof. T. Kawai and Prof. T. Nakashima,
NAIST, for the spectroscopic measurements of single crystals,
Prof. T. Takenobu and Mr. W. Takahashi, Waseda University, for
vapor deposition and helpful discussion, Prof. S. Seki and Dr. D.
Sakamaki, Osaka University, for fruitful discussion, Mr. S. Katao,
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the measurement probably due to the presence of solvent molecules
in the crystal.
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Received: December 21, 2013
Published online on && &&, 0000
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FULL PAPER
High hole mobility: A tetrathiafulvalene
(TTF)-conjugated bistetracene was syn-
thesized and characterized in the molec-
ular electronic structures, as well as the
packing structures. A variety of crystal
packing of compound could be origi-
nated from the soft p linkage of TTF
unit. The p-stacking structure affords
large intermolecular orbital coupling of
HOMOs, which is quite beneficial to en-
hance the charge-carrier mobility (see
figure).
&
Pi Interactions
M. Yamashita, D. Kuzuhara, N. Aratani,*
H. Yamada*
&& – &&
Synthesis and Solid-State Structures of
a Tetrathiafulvalene-Conjugated
Bistetracene
Chem. Eur. J. 2014, 20, 1 – 7
www.chemeurj.org
7
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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