Syntheses and structures of novel heteroarene-fused coplanar π-conjugated chromophores

Article abstract of DOI:10.1021/ol061791y

(Chemical Equation Presented) We have synthesized a series of novel coplanar chromophores in which heteroarenes, namely, thiophene, benzothiophene, and carbazole, were fused to neighboring phenylene ring(s) through intramolecular annulation via sp³

Full text of DOI:10.1021/ol061791y

ORGANIC
LETTERS
2006
Vol. 8, No. 22
5033-5036
Syntheses and Structures of Novel
Heteroarene-Fused Coplanar
π
-Conjugated Chromophores
Ken-Tsung Wong,* Teng-Chih Chao, Liang-Chen Chi, Ying-Ying Chu,
Akula Balaiah, Shih-Feng Chiu, Yi-Hung Liu, and Yu Wang
Department of Chemistry, National Taiwan UniVersity, Taipei 106, Taiwan
kenwong@ntu.edu.tw
Received July 20, 2006
ABSTRACT
We have synthesized a series of novel coplanar chromophores in which heteroarenes, namely, thiophene, benzothiophene, and carbazole,
were fused to neighboring phenylene ring(s) through intramolecular annulation via sp3-hybridized carbon atoms bearing two p-tolyl groups as
peripheral substituents. The molecular configurations of the
π-conjugated backbones were determined by X-ray crystallographic analysis; the
heteroarene-fused molecular frameworks of these novel molecules exhibit nearly coplanar conformations.
Polymeric and oligomeric π-conjugated materials possessing
coplanar π-conjugated backbones, such as the ladder-type
poly(p-phenylene)s (LPPPs),1 have become the subject of a
considerable degree of interest for their use in optoelectronics
applications because of their enhanced degree of π-conjuga-
tion,2 highly efficient photoluminescence, and remarkable
hole mobility.3 The superior physical properties of LPPPs
can be ascribed to the conformational rigidity of the coplanar
molecular backbone that is obtained through intramolecular
annulation. More importantly, the physical properties of
ladder-type materials can be tailored by changing the degree
of coplanarity between consecutive backbone units or, more
effectively, by flattening the π-conjugated molecular frame-
work by incorporating heteroatoms, such as silicon,4 sulfur,5
selenium,6 boron,7 and nitrogen,8 as bridging atoms. Alter-
natively, the physical behavior of π-conjugated materials can
be modulated, often dramatically, by incorporating hetero-
arenes directly as constituents of the conjugated backbones,
facilitating ready manipulation of the electronic structures.9
In this Letter, we report the syntheses and solid-state
structures of novel coplanar chromophores featuring embed-
ded heteroarenes as constituents of coplanar conjugated
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10.1021/ol061791y CCC: $33.50
© 2006 American Chemical Society
Published on Web 09/28/2006

backbones. Because of their intrinsic coplanarity, ladder-type
materials usually exhibit strong intermolecular π-π interac-
tions, and hence, they tend to be crystalline. Unfortunately,
non-homogeneous crystalline systems that feature grain
boundaries of various sizes often cause problems in device
applications. In our present approach, the introduction of aryl
groups as peripheral substituents of the coplanar backbone
impedes the intermolecular interactions between the mol-
ecules quite efficiently, leading to their exhibiting amorphous
morphologies, which is crucial for practical applications in
organic light-emitting diodes (OLEDs).
bridged by the two sp3-hybridized carbon atoms was almost
coplanar. The p-tolyl groups occupying the top and bottom
faces of the molecular plane were arranged with a dihedral
angle of ca. 15.3°.10
The acid-mediated intramolecular ring closure between a
tertiary hydroxyl group and adjacent arene(s) is a useful
reaction for establishing the coplanarity of a conjugated
backbone. This versatile synthetic strategy provides the
flexibility to introduce various heteroarenes as constituent
parts of the conjugated backbone to effectively manipulate
the energy levels, as well as the electronic behavior, of the
resulting coplanar molecules. Along this line, Pd-catalyzed
Negishi coupling reaction of diethyl 2,5-dibromoterephthalate
with 2-thienyl zinc chloride, which was generated in situ
through transmetalation of a corresponding 2-thienyl Grig-
nard reagent with ZnCl2, afforded the doubly coupled product
4 in 73% yield (Scheme 2).11
Scheme 1 depicts the syntheses of three thiophene-im-
planted coplanar chromophores that feature aryl substituents.
Scheme 1
Scheme 2
The pinacol boronates of the diaryl-substituted inde-
nothiophenes9 1 were reacted with ethyl 2-iodobenzoate in
the presence of catalytic amounts of Pd(PPh3)4 and co-
catalyst t-Bu3P to yield the coupling products 2 in good
yields. The additions of excess amounts of aryl Grignard
reagents, which we prepared in advance from the corre-
sponding aryl bromides, onto the ester group of compounds
2 gave the corresponding alcohols as intermediates. Without
further purification, these crude alcohols were subjected to
acid-catalyzed intramolecular annulation to afford the co-
planar phenylene-thiophene-phenylene systems 3 in good
isolated yields. The flexibility of this synthetic sequence
allowed us to introduce different aryl substituents, which we
expected to exert subtle influences on the molecular mor-
phologies and physical properties.
After the double addition and acid-promoted intramolecu-
lar cyclization reaction sequence, we isolated a new chro-
mophore, p-thiophene-phenylene-thiophene (5), in 65%
yield (Scheme 2); we confirmed its coplanar structure
through X-ray crystallographic analysis (Figure S-1, Sup-
porting Information).10
The degree of π-conjugation can be extended effectively
by increasing the degree of coplanarity of a backbone unit.
Thus, the Suzuki coupling of diethyl 2,5-dibromoterephtha-
late with benzothiophene boronic acid afforded the bis-
(benzothienyl)phenylene 6 in excellent yield. A subsequent
ring-closure strategy provided the new coplanar chromophore
7, which can be regarded as a coplanar distyryl-p-phenylene
flattened through the incorporation of two different bridging
atoms (i.e., carbon and sulfur) (Scheme 3). This compound
serves as a new member of the family of coplanar oligo-
(phenylenevinylene) (OPV) derivatives that contain bridging
silicon and silicon,4 silicon and carbon,12 and sulfur and
carbon5b atoms.
We characterized the molecular structure of 3c using X-ray
crystallography (Figure 1). The π-conjugated molecular plane
(10) Crystal packing diagrams and the analyses of possible intermolecular
interactions of molecules 3c, 5, 7, 9, and 15 are provided in the Supporting
Information.
(11) A polymer related to this structure has been reported previously.
Forster, M.; Annan, K. O.; Scherf, U. Macromolecules 1999, 32, 3159.
(12) (a) Xu, C.; Wakamiya, A.; Yamaguchi, S. J. Am. Chem. Soc. 2005,
127, 1638. (b) Yamaguchi, S.; Xu, C.; Yamada, H.; Wakamiya, A. J.
Organomet. Chem. 2005, 690, 5365.
Figure 1. (a) Top and (b) side views of the solid-state structure
of 3c.
5034
Org. Lett., Vol. 8, No. 22, 2006

Scheme 3
Scheme 4
We confirmed the nearly flat molecular structure of 9 using
X-ray crystallography (Figure 3); the two fused thiophene
X-ray crystallographic structural analysis of 7 con-
firmed the relative coplanarity of this molecule’s backbone
(Figure 2).
Figure 3. (a) Top and (b) side views of the solid-state structure
of 9.
Figure 2. (a) Top and (b) side views of the solid-state structure
of 7.
rings were twisted by 5.6 and 2.2°, respectively, in relation
to the central phenylene ring.10
Carbazole-based conjugated oligomers and polymers are
often used as hole-transporting materials, as well as host
materials,16 in organic light-emitting devices (OLEDs). Much
research effort has been exerted toward the realization of
carbazole derivatives for optoelectronic applications. We
applied our present synthetic protocol to the synthesis of
coplanar chromophores containing carbazole units as core
structural features. Recently, interesting carbazole-based
coplanar molecules were reported that featured phenylene
substituents at the C2 and C7 positions of their carbazole
units; their molecular planes were subsequently flattened at
the electron-rich sites (e.g., C3 and C6) of the carbazole
moiety.17 We became intrigued to extend the π-conjugation
of carbazole in a different mannersthrough intramolecular
ring closure of aryl substitutions at the C3 and C6 positions.
This approach led us to prepare the interesting carbazole-
containing coplanar chromophore 15 (Scheme 5).
Interestingly, the bridging carbon atoms appear somewhat
deviated from the planar structure, leading to a slightly
twisted molecular plane having a dihedral angle of 6.7°
between the central and terminal phenylene units (rings 1
and 4).10
Both theoretical calculations13 and practical examples14
have indicated that the energy levels, as well as the electronic
properties, of π-conjugated molecules can be modulated
when they adopt different molecular geometries. Using a
similar synthetic pathway (Scheme 4), we synthesized the
m-thiophene-phenylene-thiophene 9 as a novel coplanar
molecule to serve as a counterpart when determining the
physical properties of the linear isomer 5. Negishi coupling
of thienyl zinc chloride and diethyl 4,6-dibromoisophthalate
afforded the diester 8 in excellent yield. Subsequent addition
of a p-tolyl Grignard reagent onto the ester groups of 8,
followed by intramolecular annulation, gave the target
product 9 in an isolated yield of 58%.15
The electron-rich C3 and C6 positions of carbazole are
functionalized quite readily; thus, we treated 3,6-dibromo-
(13) Avilov, I.; Marsal, P.; Bre´das, J.-L.; Beljonne, D. AdV. Mater. 2004,
16, 1624.
(14) (a) Brunner, K.; van Dijken, A.; Borner, H.; Bastiaansen, J. J. A.
M.; Kiggen, N. M. M.; Langeveld, B. M. W. J. Am. Chem. Soc. 2004, 126,
6035. (b) van Dijken, A.; Bastiaansen, J. J. A. M.; Kiggen, N. M. M.;
Langeveld, B. M. W.; Rothe, C.; Monkman, A.; Bach, I.; Sto¨ssel, P.;
Brunner, K. J. Am. Chem. Soc. 2004, 126, 7718.
(16) (a) Tsai, M.-H.; Lin, H.-W.; Su, H.-C.; Wu, C.-c.; Fang, F.-C.; Liao,
Y.-L.; Wong, K.-T.; Wu, C.-I. AdV. Mater. 2006, 18, 1216. (b) Yeh, S.-J.;
Wu, M.-F.; Chen, C.-T.; Song, Y.-H.; Chi, Y.; Ho, M.-H.; Hsu, S.-F.; Chen,
C.-H. AdV. Mater. 2005, 17, 285. (c) Tokito, S.; Iijima, T.; Suzuri, Y.;
Kita, H.; Tsuzuki, T.; Sato, F. Appl. Phys. Lett. 2003, 83, 569. (d) Holmes,
R. J.; Forrest, S. R.; Tung, Y.-J.; Kwong, R. C.; Brown, J. J.; Garon, S.;
Thompson, M. E. Appl. Phys. Lett. 2003, 82, 2422 .
(15) The coplanar molecules 5 and 9 may serve as useful core structures
for developing potential π-conjugated oligomers or polymers because
halogenations can be performed readily at the C2 positions of terminal
thiophene moieties. We are currently investigating such reactions.
(17) (a) Qin, S.; Liu, L.; Wang, B.; Shen, F.; Zhang, W.; Li, M.; Ma, Y.
Macromolecules 2005, 38, 6782. (b) Patil, S. A.; Scherf, U.; Kadashchuk,
A. AdV. Funct. Mater. 2003, 13, 609. (c) Mishra, A. K.; Graf, M.; Grasse,
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Org. Lett., Vol. 8, No. 22, 2006
5035

Scheme 5
Figure 4. (a) Top and (b) side views of the solid-state structure
of 15.
position of the carbazole and, sequentially, forces the second
annulation to occur at the C7 position. In Figure 4, we
observe that the terminal phenylene rings are twisted slightly
from the molecular plane of 15, leading to a nearly flat
structure having dihedral angles of 6.7 (between rings 1 and
2), 6.8 (rings 2 and 3), and 2.0° (rings 3 and 4).10
In summary, we have achieved an efficient syntheses of a
series of novel coplanar chromophores that feature phenylene
ring(s) fused to heteroarenes, namely, thiophene, ben-
zothiophene, and carbazole. The carbon atoms bearing the
two p-tolyl groups flatten their respective phenylene(s) and
adjacent heteroarene(s) units to afford nearly coplanar
molecular configurations of the conjugated backbones, which
we confirmed through X-ray crystallography. The structure-
property relationships and potential applications in OLEDs
of these new compounds are currently under investigation
and will be reported in due course.
9-phenylcarbazole with n-BuLi at -78 °C, followed by
quenching of the dilithiated intermediate with 2-isopropoxy-
4,4,5,5-tetramethyl-[1,3,2]-dioxaborolane, to obtain the bis-
(pinacol boronate) 11 in 82% yield. Suzuki coupling of the
diboronate 11 with ethyl 2-iodobenzoate afforded the diester
12 in excellent isolated yield (92%). Subsequent double
addition of aryl lithium onto the ester groups of 12, followed
by acid-mediated intramolecular annulation, gave the novel
coplanar carbazole-cored chromophore 15 having two bridg-
ing carbon atoms attached to the C4 and C7 positions,
respectively, of the central carbazole moiety. We confirmed
the structure of 15 through X-ray crystallographic analysis
(Figure 4). We believe that the acid-promoted intramolecular
Friedel-Crafts alkylation of the diol 13 occurred initially at
the C4 position of the carbazole to form the alcohol 14. The
presence of the two p-tolyl groups attached to the bridging
carbon atom presents a sterically congested environment that
inhibits the second ring closure from occurring at the C5
Acknowledgment. This study was supported financially
by the National Science Council and the Ministry of
Economic Affairs, Taiwan.
Supporting Information Available: Detailed experi-
mental procedures; spectroscopic characterization and NMR
spectra of new compounds; crystal data, crystal packing, and
crystal CIF files of compounds 3c, 5, 7, 9, and 15. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL061791Y
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Org. Lett., Vol. 8, No. 22, 2006