Dinaphthopentalenes: Pentalene derivatives for organic thin-film transistors

Article abstract of DOI:10.1002/anie.201003609

Extended π system: A nickel(0)-mediated reaction afforded dinaphthopentalene derivatives, whose electronic and electrochemical properties depend on the fusion patterns. Very high hole mobility was observed for an amorphous material, and the compounds are

Full text of DOI:10.1002/anie.201003609

Communications
DOI: 10.1002/anie.201003609
Molecular Electronics
Dinaphthopentalenes: Pentalene Derivatives for Organic Thin-Film
Transistors**
Takeshi Kawase,* Takeru Fujiwara, Chitoshi Kitamura, Akihito Konishi, Yasukazu Hirao,
Kouzou Matsumoto, Hiroyuki Kurata, Takashi Kubo, Shoji Shinamura, Hiroki Mori,
Eigo Miyazaki, and Kazuo Takimiya*
Since the first synthesis of dibenzopentalene 1 by Brand in
1912,[1a] pentalene derivatives have had a long history of
studies on their synthesis, structures, and electronic proper-
ties.[1,2] Recently, polycyclic conjugated systems bearing
carbocyclic five-membered rings such as fluorenes[3] and
indenes[4] have attracted much attention because of their
utility in organic electronic devices. However, the application
of pentalene derivatives to organic semiconductor devices has
remained underdeveloped to date. Despite possessing a 4np-
electron periphery, dibenzopentalenes are fairly stable com-
pounds with a planar structure. Thus, appropriate modifica-
tion would provide them with desirable electronic properties.
Last year we found a novel reaction yielding dibenzopenta-
lene derivatives from readily available o-bromoethynylben-
zenes using commercially available nickel complexes.[5a] Soon
afterwards, Levi and Tilley independently found another
efficient dibenzopentalene synthesis using a Pd0 complex.[6]
These methods would be accessible to various pentalene
derivatives.[5b] In the course of the study, we synthesized di-
(1,2)-naphthopentalenes 2 and (2,3)-isomers 3 as entirely new
p-extended pentalene derivatives from corresponding bro-
moethynylnaphthalenes. Their electronic and structural prop-
erties drastically change with their fusion patterns, which is
consistent with theoretical calculations. The structural sim-
ilarity to dichalcogenophene derivatives 4 as high-perfor-
mance semiconductors[7] promoted us to investigate solid-
state properties of 1b, 2b, and 3b. Among them, 3b showed
hole mobility on the order of 10ꢀ3 cm2 Vꢀ1 sꢀ1, which is a very
high value for amorphous materials. It is the first pentalene
derivative for organic thin-film semiconductors. Furthermore,
3b was employed as a p-type material for organic hetero-
junction photovoltaic cells.[8] Although the power-conversion
efficiency (PCE) value (0.94%) is not so high, the open-
circuit voltage (Voc = 0.96 V) is considerably high.
Treatment of bromoethynylnaphthalenes 5 and 6 with a
Ni0 complex,[9] generated from [NiCl2(PPh3)2] and zinc dust in
toluene/1,2-dimethoxyethane (DME) (4:1), furnished corre-
sponding dinaphthopentalenes 2b,c and 3b,c, respectively, in
11–20% yields (Scheme 1).[10] Toluene, DME, or THF can be
also employed as the solvent, but the yields decreased slightly.
[*] Prof. T. Kawase, T. Fujiwara, Prof. C. Kitamura
Graduate School of Engineering, University of Hyogo
2167 Shosha, Himeji, Hyogo 671-2280 (Japan)
Fax: (+81)79-267-4889
E-mail: kawase@eng.u-hyogo.ac.jp
Homepage: http://www.eng.u-hyogo.ac.jp/msc/msc4/index.html
ꢀ
Taking into account that three C C bonds form in one
reaction, the yields are not so poor.
S. Shinamura, H. Mori, Dr. E. Miyazaki, Prof. K. Takimiya
Graduate School of Engineering, Hiroshima University
Higashi-Hiroshima, Hiroshima, 729-8527 (Japan)
E-mail: ktakimi@hiroshima-u.ac.jp
The dinaphthopentalenes were obtained as fairly stable
crystalline substances. They show different colors: com-
pounds 2 are reddish brown, whereas compounds 3 are
orange. Figure 1 shows their absorption spectra in CH2Cl2. In
contrast to the fusion pattern, the substituent effects at the 3-
and 6-positions are small. The first and second intense
absorption bands (250–400 and 400–550 nm) are almost
identical to each other. The difference in their colors is due
to the presence of the weak, long-wavelength absorption
band.
A. Konishi, Dr. Y. Hirao, Dr. K. Matsumoto, Prof. H. Kurata,
Prof. T. Kubo
Graduate School of Science, Osaka University
1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043 (Japan)
[**] We thank Prof. Don Tilley (Department of Chemistry, University of
California, Berkeley) for helpful discussions. This work was
supported by Hyogo prefecture and a Grant-in-Aid for Scientific
Research on Innovative Areas (No. 21108521A01, “pi-Space”) from
the Ministry of Education, Culture, Sports, Science and Technology
(Japan).
The electronic properties of unsubstituted 2a and 3a
together with dibenzopentalene 1a were calculated with the
TD-DFT(RB3LYP/6-31G**) calculation embedded in the
Gaussian03 software package.[11] The resultant molecular
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003609.
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Scheme 1. Synthesis of 2b,c and 3b,c.
Figure 2. Energy diagrams of 1a–3a (eV); TD-DFT(RB3LYP/6-31G**)
and molecular orbitals of a) LUMO, b) HOMO, and c) HOMOꢀ1 of
2a, and d) LUMO, e) HOMO, and f) HOMOꢀ1 of 3a.
Figure 1. Absorption spectra of 2b,c and 3b,c in CH2Cl2.
orbitals and energy diagrams (eV) are shown in Figure 2. The
longest absorption bands (S0!S1 bands) of 2a are attribut-
able to the HOMO!LUMO transitions, which are symme-
try-forbidden as is typical for 4np-electron systems. In
contrast, the calculation of 3a shows that the HOMOꢀ1
possesses same symmetry as that of the HOMO of 2a. Thus,
the order of the energy levels is reversed by changing the
fusion pattern. The TD-DFT calculations also indicate that
the longest absorption band of 3a (S0!S1 band) is attribut-
able to a HOMOꢀ1!LUMO transition, which is symmetry-
forbidden. An allowed HOMO!LUMO transition occurs at
higher energy, and proximity in energy of HOMOꢀ1 and
HOMO would lead to near overlap of the both transitions,
which is consistent with the observed absorption spectra of 3.
Conclusively, the HOMO–LUMO gaps of 2a and 3a drasti-
cally change with the fusion patterns (Figure 2a). The value of
3a (3.06 eV) is almost comparable to that of 1a (3.13 eV),
whereas the value of 2a (2.42 eV) is considerably smaller than
those of 1a and 3a.
Good single crystals of 2b and 3b suitable for single-
crystal X-ray diffraction studies were obtained from hexane
and CH2Cl2/hexane solutions, respectively.[12] Structural anal-
ysis revealed that they have a planar structure (Figure 3).
Bond lengths of 2b and 3b are summarized in Figure S4a in
the Supporting Information. The reported molecular struc-
tures of dibenzopentalenes are characterized by large bond
alternation in the pentalene moiety and relatively small bond
Figure 3. ORTEP drawings (50% probability) of a) top view and b) side
view of two molecules of 2b; c) a top view and d) side view of two
molecules of 3b.
alternation in the six-membered rings. The large bond
alternation in the pentalene skeletons of 2b and 3b is similar
to that of 1b, and the degrees are largely enhanced.
Analogous to 1b, 3b possesses exo-butadiene conjugations
with regard to the pentalene skeleton. The averaged bond
length of the 5–6 fusion of 3b (1.441 ꢀ) is significantly longer
than the corresponding one of 1b (1.425 ꢀ). The longer bond
length reflects an unfavorable effect from the 4np cyclic
conjugation of the pentalene p system. In contrast, the bond
length of the 5–6 fusion of 2b (1.407 ꢀ) is considerably
shorter than those of 1a, which indicate that 2a possesses a
pentalene 8p-electron system. Taking into account a reso-
nance contribution, exo-butadiene conjugations with regard
to the pentalene skeleton destroy an aromatic sextet in the
exterior six-membered rings. In this context, the counter-
balance between aromatic stabilization of the exterior
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Table 1: Redox potentials of 2b and 3b (CH2Cl2).[a]
benzene and anti-aromatic destabilization of pentalene
ox
ox
red
red
p system should determine the bond alternation. These
results are also consistent with the predicted bond lengths in
optimized structures of 2a and 3a (RB3LYP/6-31G**,
Figures S1 and S3 in the Supporting Information).
Compound
E
[V]
E
[V]
E
[V]
E
[V]
2,1/2
1,1/2
1,1/2
2,1/2
2b
3b
1.14[b]
1.11[b]
0.55
0.73
ꢀ1.52
ꢀ1.79
ꢀ2.06[b]
ꢀ2.26[b]
[a] V vs. Ag/Ag+ in 0.1m nBu4NClO4/CH2Cl2, scan rate 100 mVsꢀ1, 258C,
ferrocene was used as a standard. [b] Peak potentials.
The phenyl substituents of 2b are tilted 558 from the
pentalene plane; the protons of the substituents direct toward
the pentalene p plane of neighboring molecules. The resulting
CH–p interactions build a slipped parallel stacking arrange-
ment in the crystal. In contrast, two phenyl groups of 3b direct
toward the p plane of the phenyl substituents of neighboring
molecules to form a parallel stacking arrangement of the
double CH–p interactions; the molecules construct a one-
dimensional columnar structure in the crystal (Figure S9 in
the Supporting Information).
The redox properties of 2b and 3b were examined by
cyclic voltammetry. The cyclic voltammograms of 2b and 3b
exhibit reversible first oxidation and reduction waves for
solutions in CH2Cl2 (Figure 4), whereas the corresponding
dibenzopentalene 1b shows pseudoreversible oxidation
waves under similar conditions. Thus, the extension of
conjugation stabilizes their oxidation states. The redox
potentials (Table 1) indicate their highly amphoteric redox
properties. The electrochemical properties also vary with
their fusion pattern; 2b possesses higher electron-donating
and -accepting properties than 3b and 1b.
were examined by X-ray diffraction (XRD), which showed no
peaks (Figure S5 in the Supporting Information). The results
revealed the formation of amorphous films on the surfaces.
OFETs were fabricated in a “top-contact” configuration on a
heavily doped n+-Si(100) wafer with 200 nm thick thermally
grown SiO2.[13] The characteristics of the OFET devices were
measured at room temperature in air. Whereas 1b and 2b
showed little mobility under the measured conditions, 3b had
a hole mobility of 1.8 ꢁ 10ꢀ3 cm2 Vꢀ1 sꢀ1 with current on/off
ratio (Ion/Ioff) of 105 at room temperature (Figure 5). The
mobility is a very high value for amorphous materials.
Takimiya and co-workers reported that the hole mobilities
of two structural isomers 7 and 8 were 102 to 103 times lower
than that of 4.[14] Higher HOMO energy level of 4 than those
of 7 and 8 probably accounted for the difference; however,
We next investigated the solid-state properties of 2b and
3b together with 1b. These pentalenes readily formed a good
thin film on an n-doped Si wafer with 200 nm thermally grown
SiO2. To obtain information on the film structures, the films
Figure 5. FET characteristics of 3b-based OFET on OTS-treated sub-
strate at room temperature: a) output characteristics (top) and
b) transfer characteristics at VDS =ꢀ60 V (bottom). VG =gate voltage,
VDS/IDS =drain-source voltage/current.
Figure 4. Cyclic voltammograms of a) 2b and b) 3b in CH2Cl2 (V vs.
Ag/Ag+ in 0.1m nBu4NClO4/CH2Cl2, scan rate 100 mVsꢀ1, 258C; Cp/
Cp+ =0.19 V.
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of alumina using hexane/CH2Cl2 (1:1) as an eluent to remove
insoluble materials. The crude product was then purified by column
chromatography on silica gel.
the HOMO energy level of 3b (ꢀ5.44 eV) is lower than that
of 2b (ꢀ5.26 eV) and comparable to that of 1b (ꢀ5.52 eV).[15]
These results indicate that the linear fusion pattern in
polycyclic conjugated systems plays an important role in the
solid-state properties.
Received: June 14, 2010
Published online: September 10, 2010
Moreover, 3b was also applied to an electron-donor layer
for a heterojunction organic thin-film solar cell.[16] Fullerene
and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)
were used as a electron-acceptor and an exciton-blocking
layers, respectively.[8] These layers were deposited on the
indium–tin oxide (ITO) substrates (see the Supporting
Information). The device was fabricated with a structure of
ITO/3b (40 nm)/C60 (30 nm)/BCP (10 nm)/Al (100 nm) and
showed a PCE value of 0.94% and a VOC value of 0.96 V
(Figure 6). The PCE value is lower than those of pentacene
(2.7%) and tetracene (2.3%). However, the VOC value is
considerably higher than those of pentacene (0.58 V) and
tetracene (0.36 V), although the conditions were somewhat
different.[17]
Keywords: materials science · molecular electronics ·
photophysics · polycycles · semiconductors
.
[1] a) K. Brand, Ber. Deutsch. Chem. Ges. 1912, 45, 3071; b) S.
Wawzonek, J. Am. Chem. Soc. 1940, 62, 745; c) M. P. Cava, R.
Pohlke, M. J. Mitchell, J. Org. Chem. 1963, 28, 1861; d) W. Frank,
R. Gompper, Tetrahedron 1987, 28, 3083; e) J. Blum, W.
Baidossi, Y. Badriech, R. E. Hoffman, J. Org. Chem. 1995, 60,
4738; f) J. Yang, M. V. Lakshmikantham, M. P. Cava, J. Org.
Chem. 2000, 65, 6739; g) M. Saito, M. Nakamura, T. Tazima, M.
Yoshioka, Angew. Chem. 2007, 119, 1526; Angew. Chem. Int. Ed.
2007, 46, 1504; h) G. Babu, A. Orita, J. Otera, Chem. Lett. 2008,
37, 1296; i) M. Saito, Symmetry 2010, 2, 950.
[2] C. C. Chuen, S. W. Fenton, J. Org. Chem. 1958, 23, 1538; I.
Willner, J. Y. Becker, M. Rabinovitz, J. Am. Chem. Soc. 1979,
101, 395; M. Rabinovitz, I. Willner, A. Minsky, Acc. Chem. Res.
1983, 16, 298; D. V. Preda, L. T. Scott, Org. Lett. 2001, 3, 1489;
J. K. Kendall, H. Shechter, J. Org. Chem. 2001, 66, 6643; M.
Saito, M. Nakamura, T. Tajima, Chem. Eur. J. 2008, 14, 6062.
[3] J. Jacob, S. Sax, T. Piok, E. J. W. List, A. C. Grimsdale, K.
Mꢂllen, J. Am. Chem. Soc. 2004, 126, 6987; K.-T. Wong, L.-C.
Chi, S.-C. Huang, Y.-L. Liao, Y.-H. Liu, Y. Wang, Org. Lett. 2006,
8, 5029; S. Y. Cho, A. C. Grimsdale, D. J. Jones, S. E. Watkins,
A. B. Holmes, J. Am. Chem. Soc. 2007, 129, 11910; D. Thirion, C.
Poriel, F. Barriere, R. Metivier, O. Jeannin, J. Rault-Berthelot,
Org. Lett. 2009, 11, 4794; N. Cocherel, C. Poriel, L. Vignau, J.-F.
Bergamini, J. Rault-Berthelot, Org. Lett. 2010, 12, 452.
[4] X. Zhu, C. Mitsui, H. Tsuji, E. Nakamura, J. Am. Chem. Soc.
2009, 131, 13596; H. Zhang, T. Karasawa, H. Yamada, A.
Wakamiya, S. Yamaguchi, Org. Lett. 2009, 11, 3076.
[5] a) T. Kawase, A. Konishi, K. Hirao, K. Matsumoto, H. Kurata, T.
Kubo, Chem. Eur. J. 2009, 15, 2653; b) A. Konishi, T. Fujiwara, N.
Ogawa, K. Hirao, K. Matsumoto, H. Kurata, T. Kubo, C.
Kitamura, T. Kawase, Chem. Lett. 2010, 39, 300.
[6] Z. U. Levi, T. D. Tilley, J. Am. Chem. Soc. 2009, 131, 2796.
[7] a) K. Takimiya, H. Ebata, K. Sakamoto, T. Izawa, T. Otsubo, Y.
Kunugi, J. Am. Chem. Soc. 2006, 128, 12604; b) K. Takimiya, Y.
Kunugi, Y. Konda, H. Ebata, Y. Toyoshima, T. Otsubo, J. Am.
Chem. Soc. 2006, 128, 3044; c) T. Yamamoto, K. Takimiya, J. Am.
Chem. Soc. 2007, 129, 2224.
[8] N. S. Sariciftci, D. Braun, C. Zhang, V. I. Srdanov, A. J. Heeger,
G. Stucky, F. Wudl, Appl. Phys. Lett. 1993, 62, 585; P. Peumans, V.
Bulvoic, S. R. Forrest, Appl. Phys. Lett. 2000, 76, 2650; P.
Peumans, S. R. Forrest, Appl. Phys. Lett. 2001, 79, 126.
Figure 6. A J–V characteristic for a ITO/3b/C60/BCP/Al solar cell
device under 100 mWcmꢀ2 AM 1.5 G illumination. J=current density,
V=bias.
In conclusion, a nickel(0)-mediated reaction of bromoe-
thynylnaphthalenes afforded corresponding dinaphthopenta-
lene derivatives as entirely new p-extended pentalene deriv-
atives. The wide applicability to produce a variety of novel p-
conjugated systems with pentalene skeletons has been
demonstrated. The dependence of the electronic and electro-
chemical properties upon the fusion patterns is consistent
with theoretical calculations. Compound 3b showed very high
hole mobility (1.8 ꢁ 10ꢀ3 cm2 Vꢀ1 sꢀ1) for an amorphous mate-
rial, and is thus suitable for organic heterojunction photo-
voltaic cells; the device showed PCE of 0.94% and a
VOC value of 0.96 V. The first pentalene derivative for organic
thin-film transistors is now demonstrated. p-Extended penta-
lenes would serve as a good platform for materials applicable
to organic electronics.
[9] a) M. Zembayashi, K. Tamao, J.-I. Yoshida, M. Kumada,
Tetrahedron Lett. 1977, 18, 4089; b) H. Matsumoto, S.-i. Inaba,
R. D. Rieke, J. Org. Chem. 1983, 48, 840; c) M. Iyoda, H. Otsuka,
K. Sato, N. Nisato, M. Oda, Bull. Chem. Soc. Jpn. 1990, 63, 80;
d) J. Hassan, M. Sevignon, C. Gozzi, E. Schulz, M. Lemaire,
Chem. Rev. 2002, 102, 1359.
[10] See the Supporting Information.
[11] The molecular geometries of the p-extended pentalenes were
fully optimized at the RB3LYP/6-31G** level of theory embed-
ded in the Gaussian03 software package. The time-dependent
calculations were performed on the optimized geometries.
Theoretical estimation of S0!S1 absorption bands in 2a and
3a were performed with TD-RB3LP/6-31G**//RB3LYP/6-
31G** method.
Experimental Section
A suspension of bromoethynylnaphthalenes (5 and 6: 1 mmol), Zn
powder (1.5 mmol, 0.098 g), [NiCl2(PPh3)2] (1.0 mmol, 0.654 g) in
toluene (4 mL), and DME (1 mL) was heated at 808C for 24 h under
N2. The dark reddish reaction mixture was passed through a column
Angew. Chem. Int. Ed. 2010, 49, 7728 –7732
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[12] Single crystals of 2b and 3b for X-ray crystallographic analysis
[13] H. Hoppe, S. Saricifei, J. Mater. Chem. 2006, 16, 45; G. Li, V.
Shrotriya, Y. Yao, J. Huang, J. Mater. Chem. 2007, 17, 3126.
[14] T. Yamamoto, S. Shinamura, E. Miyazaki, K. Takimiya, Bull.
Chem. Soc. Jpn. 2010, 83, 120.
were obtained from n-hexane/CH2Cl2 and n-hexane solutions,
respectively. X-ray crystal structure analysis was performed on a
Rigaku RAXIS-RAPID imaging plate diffractometer (MoKa
radiation, l = 0.71075 ꢀ). The structures were solved by direct
methods with SHELXS. Non-hydrogen atoms were refined
anisotropically. All hydrogen atoms were located by calculation.
CCDC 728891 (2b) and 728892 (3b) 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. Crystallo-
graphic data for 2b: C36H22; Mr = 454.57, a = 14.880(1) ꢀ, b =
4.9221(4) ꢀ, c = 15.904(1) ꢀ, b = 95.316(2), V= 1159.8(2) ꢀ3,
monoclinic, space group P21/c, Z = 2, m(MoKa) = 0.74 cmꢀ1, T=
200 K, Dcalcd = 1.302 gcmꢀ3, F(000) = 476.00, R1 = 0.041, wR2
(all data) = 0.109, GOF = 1.07, refl./param. = 10658/649. Crys-
tallographic data for 3b: C36H22; Mr = 454.57, a = 21.1838(8) ꢀ,
[15] The HOMO energy levels were evaluated from the observed
oxidation potentials by using the equation EHOMO
=
ꢀ(Eox+4.71) eV; G. D. Sharma, P. Suresh, J. A. Mikroyannidis,
M. M. Stylianakis, J. Mater. Chem. 2010, 20, 561 – 567.
[16] Recent developments of heterojunction photovoltaic devices
using small molecules: C.-Q. Ma, E. Mena-Osteritz, T. Debaer-
demaeker, M. M. Wienk, R. A. J. Janssen, P. Bꢃuerle, Angew.
Chem. 2007, 119, 1709; Angew. Chem. Int. Ed. 2007, 46, 1679;
R. D. Kennedy, A. L. Ayzner, D. D. Wanger, C. T. Day, M.
Halim, S. I. Khan, S. H. Tolbert, B. J. Schwartz, Y. Rubin, J. Am.
Chem. Soc. 2008, 130, 17290; Y. Matsuo, Y. Sato, T. Niinomi, I.
Soga, H. Tanaka, E. Nakamura, J. Am. Chem. Soc. 2009, 131,
16048.
b = 9.8827(4) ꢀ,
4663.1(3) ꢀ3, monoclinic, space group P21/c, Z = 8, m(MoKa) =
0.74 cmꢀ1 calcd = 1.295 gcmꢀ3
T= 200 K, F(000) = 1904.00,
c = 22.7484(9) ꢀ,
b = 101.7421(1),
V=
[17] a) J. E. Anthony, Angew. Chem. 2008, 120, 460 – 492; Angew.
Chem. Int. Ed. 2008, 47, 452 – 483; b) S. Yoo, B. Domereq, B.
Kippelen, Appl. Phys. Lett. 2004, 85, 5427; c) C.-W. Chu, Y. Shao,
V. Shrotriya, Y. Yang, Appl. Phys. Lett. 2005, 86, 243506.
,
D
,
R1 = 0.049, wR2 (all data) = 0.227, GOF = 0.68, refl./param. =
10655/649.
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