Synthesis of a pentalene centered polycyclic fused system

Article abstract of DOI:10.1021/ol200213h

A novel pentalene-centered polycyclic 24π-electron system, IB1, was synthesized via a Pd-catalyzed homocoupling reaction. The geometry structure was studied by X-ray diffraction and theoretical method. The HOMO level of IB1 was studied by electrochemical

Full text of DOI:10.1021/ol200213h

ORGANIC
LETTERS
2011
Vol. 13, No. 6
1520–1523
Synthesis of a Pentalene Centered
Polycyclic Fused System
Xiaodong Yin,† Yongjun Li,† Yulan Zhu,‡ Yuhe Kan,‡ Yuliang Li,*,† and Daoben Zhu*,†
Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory
of Organic Solids, Center for Molecular Sciences, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, P. R. China, Graduate University of Chinese
Academy of Sciences, Beijing 100190, P. R. China, and Jiangsu Key Laboratory for
Chemistry of Low-dimensional Materials, Huaiyin Normal University, Huai’an, Jiangsu
223300, P. R. China
ylli@iccas.ac.cn; zhudb@iccas.ac.cn
Received January 24, 2011
ABSTRACT
A novel pentalene-centered polycyclic 24π-electron system, IB1, was synthesized via a Pd-catalyzed homocoupling reaction. The geometry
structure was studied by X-ray diffraction and theoretical method. The HOMO level of IB1 was studied by electrochemical experiment and DFT
methods. The IB1 molecule shows a strong electro-donating property and can form a charge transfer complex with electro-acceptor TCNQ,
indicating fascinating potential in the field of organic electronics.
Linear polyacenes with extended π-conjugation, such as
pentacene and rubrene, have become a primary focus in the
field of organic electronics. This species has been widely
used in the construction of organic electronic devices (e.g.,
OFET, OLED)1 and is considered to be among the most
promising organic semiconductors with incredible carrier
mobility. However, the poor solubility and oxidative de-
gradation restrict the further application of these materials.
Pentalene (as shown in Figure 1), as a 4nπ-electron system,
provides corresponding stable dianion and dication species
leading to highly amphoteric redox properties.2 In particu-
lar, the more stable extended pentalene derivatives,3 and
related polyindenoindenes,4 though scarcely studied, repre-
sent an interesting alternative to classic polyacenes in
several fields. Among them, the dichalcogenophene deriva-
tives of pentalene have demonstrated extraordinary poten-
tial as high-performance semiconductors.5 Recently,
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H.; Kubo, T. Chem.;Eur. J. 2009, 15, 2653. (b) Kawase, T.; Fujiwara,
T.; Kitamura, C.; Konishi, A.; Hirao, Y.; Matsumoto, K.; Kurata, H.;
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† Institute of Chemistry, Chinese Academy of Sciences.
‡ Huaiyin Normal University.
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2004, 84, 3061. (b) Podzorov, V.; Pudalov, V. M.; Gershenson, M. E.
Appl. Phys. Lett. 2003, 82, 1739. (c) Podzorov, V.; Sysoev, S. E.;
Loginova, E.; Pudalov, V. M.; Gershenson, M. E. Appl. Phys. Lett.
2003, 83, 3504. (d) Podzorov, V.; Menard, E.; Borissov, A.; Kiryukhin,
V.; Rogers, J. A.; Gershenson, M. E. Phys. Rev. Lett. 2004, 93, 086602.
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r
10.1021/ol200213h
Published on Web 02/24/2011
2011 American Chemical Society

Figure 1. Structure of pentalene and IB1.
Scheme 1. Synthesis Route of IB1
Figure 2. (a) Molecular structure of IB1, ellipsoids drawn at
50% probability level; (b) IB1 packing along the a-axis; (c) IB1
packing from side view.
2 equiv of 7 occurred utilizing a Pd(0) catalyst in an inert
atmosphere to afford IB1 as purple needle crystals in 20%
yield. Different from traditional polyacenes, IB1 showed
incredible solubility and thermostability. A TGA experi-
ment reveals that the decomposition temperature is ca.
350 °C (Figure S1).
After careful purification by alumina column, IB1 was
recrystallized by slow evaporation of a solution in acetone
to afford a small amount of crystalloids suitable for X-ray
diffraction. Consistent with the reported molecular struc-
ture of dibenzopentalenes,8 the molecular structure
(Figure 2) reveals that the fused ring system is essentially
planar with a slight dihedral angle of ca. 10° between the
planes of indole and pentalene. The two benzene rings
substituted on the pentalene have a torsion angle of ca. 51°
to the plane of pentalene, and they experience steric
hindrance due to the neighboring alkyl chains, of which
tremendous progress has been made on synthetic methodo-
logies for accessing indenoindenes and dibenzopenta-
lenes,6,7 thus promoting research on their application for
organic electronics.6b
In this work, we synthesized a pentalene-centered, six-
ring fused system with two outstretched units of indole,
named IB1 (Figure 1). This molecule shows a strong
electro-donating property with a high HOMO level. Inter-
estingly, IB1 can be oxidized to a radical cation by oxidative
reagents and form a charge transfer complex with TCNQ,
which renders enormous potential as an organic semicon-
ductor/conductor.
IB1 was synthesized via the Pd-catalyzed homocoupling
of 3-bromo-1-hexyl-2-(phenylethynyl)-1H-indole (7) which
was obtained from bare indole in six steps (Scheme 1). First,
phenylsulfyl-protected indole was brominated and iodi-
nated successively to obtain 4. Then deprotection occurred
in methanol with aqueous sodium hydroxide, which was
followed by condensation with 1-bromohexane to obtain 6.
6 was coupled with phenylacetylene via a Sonagashira cross
coupling reaction to afford 7. IB1 was obtained in a similar
method as reported by Tilley’s group.7b In the presence of
hydroquinone, Cs2CO3, and CsF, a coupling reaction with
˚
the nearest distance is ca. 3 A (Figure S2). Due to the
shielding effect of the phenyl ring, chemical shifts of
hydrogen on the alkyl chain were split from each other
and display a high-field shift in varying degrees (Figure
S2). Especially, thehydrogenonthe carbon connectedwith
the nitrogen atom has a marked high-field shift of about
1 ppm. The slightly high-field shift of the hydrogen on the
benzene of indole might be due to the paratropicity of
pentalene.
In accordance with the reported structures of pentalene
˚
derivatives, the alternating long (1.442(3) and 1.443(3)A
for C(2)-C(3) and C4-C(1A), respectively) and short
˚
(1.398(3) and 1.405(3) A for C(1)-C(2) and C(3)-C(4),
respectively) bonds in the central pentalene core were seen,
whereas the peripheral benzene bonds are relatively homo-
˚
geneous (1.382-1.399 A). At the same time, the fully
structural optimization of IB1 was carried out by Gaussian
03 at the RB3LYP/6-31þG* level. The calculated result
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Chem., Int. Ed. 2007, 46, 1504.
Org. Lett., Vol. 13, No. 6, 2011
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Figure 3. NICS grid plot of IB1 (GIAO-RB3LYP/6-31þG*)
(red, positive value; green, negative value); the size of the sphere
relates to its absolute value.
matches well with the crystallographic data with the
˚
exception of some small deviations (less than 0.01 A) on
C(3)-C(4) and C(5)-C(6) (Figure S3, Table S1). Such
good agreement suggests the good feasibility of the mole-
cular model we optimized for further study.
IB1 exhibited a column packing mode along the a-axis
(Figure 2), and the major contacts in the unit cell are
C-H...π between hydrogens on the phenyl ring, alkyl
chains, and the central pentalene ring. The average dis-
Figure 4. (a) TD-DFT calculated electro spectra of IB1 com-
pared with experimental electronic spectra (in acetonitrile, 1 ꢀ
10-5 M). (b) The molecular orbitals and energy diagrams (eV) of
IB1 calculated with the B3LYP/6-31þG* method.
˚
tance between two neighboring molecules is 3.83 A.
Nucleus-independent chemical shift (NICS) calculation
was used to investigate the antiaromatic character of IB1.
NICS was introduced by Schleyer and co-workers as a
measure of electron delocalization and induced ring
current.9 A negative NICS value is indicative of the pre-
sence of diatropic ring current, while a positive value is
related to paratropic ring current. A series of NICS values
with different positions relative to IB1 were calculated at
the GIAO-RB3LYP/6-31þG* level (Figure 3, Table S2).
The results showed that the NICS values of pentalene ring
and benzene of indole were þ28 and -7.8, respectively.
The highly positive value of pentalene ring, which is even
higher than pentalene itself (þ18.42), indicated a high
antiaromaticity, whereas the benzene ring showed slightly
less aromaticity than that of benzene itself (-11.31).
The photophysical property of IB1 was studied by both
experimental and theoretical methods. The electronic
spectra of IB1 were measured in different solvents
(Figure S4). The results show that IB1 has two main
absorption bands at 380 and 540 nm without a solvent-
chromic effect. The absorptions at 280 and 530 nm in
hexane are due to the bad solubility of IB1 which leads to
formation of aggregation. Theoretical calculation of the
excited state was carried out by the TD-DFT method at the
B3LYP/6-31þG*level (Figure4). The resultindicated that
the absorption band located at about 530 nm is attribu-
table to HOMO-1fLUMO transition, while the transi-
tion of HOMOfLUMO is symmetry forbidden which is
typical for 4nπ-electron systems (Table S2). We also
investigated the molecular orbitals of IB1 at B3LYP/6-
31þG* (Figure 4), which indicated that IB1 has an un-
usually high HOMO of about -4.4 eV, and the gap between
HOMO and LUMO is 1.85 eV. The distribution of the
HOMO reveals the delocalization of electrons on the main
part of IB1, which might be due to its good planarity.
Upon comparison with the reported HOMO of acenes like
rubrene and tetracene,10 the unusually high HOMO of IB1
should be dueto thehighercomposition oftheantibonding
orbital in the HOMO of IB1.
Subsequently, electrochemical experiments were carried
out to study the redox property of IB1 (Figure S5, Table S4).
Oxidative (two) and reductive (one) processes were ob-
served with good reversibility. Interestingly, the first oxi-
dation revealed an extremely low oxidative potential which
is -270 mV versus that of ferrocene, indicating a strong
electro-donating property of IB1 with a HOMO level of ca.
-4.5 eV. The formation of IB12þ occurs at a potential ca.
340 mV more positive than that for IB1þ, and the forma-
tion of IB1- occurs at the potential of -1.76 V versus Fcþ/
Fc. The DPV result shows that all of these redox processes
are equally one-electron processes.
(9) (a) Chen, Z. F.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.;
Schleyer, P. V. Chem. Rev. 2005, 105, 3842. (b) Fallah-Bagher-Shaidaei,
H.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. V. Org.
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1072.
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Org. Lett., Vol. 13, No. 6, 2011

clear isosbestic points also support a clean conversion to
the radical cation.
Furthermore, IB1 can form a charge transfer complex
with TCNQ in polar solvents. The titration of IB1 with
TCNQ was carried out in acetonitrile (1 ꢀ 10-5 M,
Figure 5b). As the amount of TCNQ increased, absorption
of the IB1 radical cation at 370, 530, and 690 nm appeared
aswellasthetypical absorptionof theTCNQradical anion
at 700-800 nm and 800-900 nm.11 The solution, which
was light-purple initially, turned to dark brown. ESR
spectroscopy was carried out to confirm the formation of
a radical complex in acetonitrile (Figure 5b). The result
showed that the resonance occurred at 3485 G under a
frequency of ca. 9.8 GHz, indicating the g factor is 2.0049.
Theoretical calculation was carried out at the B3LYP/6-
31þG* level with the BSSE method to study the interac-
tion between the neutral molecule of TCNQ and IB1
(Figure S6). The result revealed a strong interaction be-
tween IB1 andTCNQwithadipole-dipolebindingenergy
of about 54 kJ/mol, which is much stronger than that of a
typical hydrogen bond. Such a strong interaction is the
driving power of the intermolecular electron transfer.
In conclusion, a six-ring fused system including penta-
lene, IB1, was synthesized and the structure was confirmed
by X-ray diffraction. Theoretical calculation was used to
study the geometric structure and MO energy level of IB1
which showed good agreement with the experimental
results. The alternating bond lengths and positive NICS
value support the strong antiaromaticity of IB1. Electro-
chemical experiments and theoretical calculations showed
that IB1 has an extremely high HOMO level indicating a
strong electro-donating property. As a consequence, a
charge-transfer complex can be formed with the TCNQ
molecule as an electron acceptor. Further research on the
application of IB1 for organic electronics is still in progress.
Figure 5. (a) UV-vis titration of IB1 (1 ꢀ 10-5 M) with Cu2þ
.
Inset: TD-DFT calculation for the IB1 radical cation at the
UB3LYP/6-31þG* level. (b) Titration of IB1 in acetonitrile (1 ꢀ
10-5 M) with TCNQ; ESP spectra of [IB1][TCNQ].
As a strong electro-donor, IB1 can be oxidized easily by
oxidative chemicals and form a radical cation. Titration of
IB1 with Cu2þ was carried out in a solution of IB1 in
acetonitrile (1 ꢀ 10-5 M, Figure 5). As the amount of Cu2þ
increased, the absorption at 380 and 540 nm blue-shifted
for ca. 20 nm, and the intensity increased by about 50%
and 100% respectively. Another relatively weak absorp-
tion peak appears at 690 nm. TD-DFT calculation
(UB3LYP/6-31þG* level) result (Table S6) for the IB1
radical cation supported the blue shift mentioned above, as
well as the new absorption at 690 nm which could be
assigned to the transition from 159β to 161β (SOMO). The
Acknowledgment. This work was supported by the Na-
tional Nature Science Foundation of China (20831160507,
20721061) and the National Basic Research 973 Program
of China.
Supporting Information Available. Detailed experimen-
tal procedures, characterization data, crystal structure
tabulated data, UV-vis absorption spectra, electrochem-
istry data, and results of theoretical calculation. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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Org. Lett., Vol. 13, No. 6, 2011
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