A general synthetic route to indenofluorene derivatives as new organic semiconductors

Article abstract of DOI:10.1021/ol0476570

(Chemical Equation Presented) A series of 2,6-disubstituted indenofluorene derivatives were obtained in high purity via a general route involving the Suzuki coupling reaction. The potential of these conjugated indenofluorenes as new organic semiconductors was demonstrated by the light-emitting diode reaching a high luminance of 1400 Cd/m² below 10 V.

Full text of DOI:10.1021/ol0476570

ORGANIC
LETTERS
2005
Vol. 7, No. 5
795-797
A General Synthetic Route to
Indenofluorene Derivatives as New
Organic Semiconductors
Tayebeh Hadizad,† Jidong Zhang,† Zhi Yuan Wang,*,† Timothy C. Gorjanc,‡,§ and
Christophe Py‡
Department of Chemistry, Carleton UniVersity, 1125 Colonel By DriVe,
Ottawa, Ontario, Canada K1S 5B6, Institute for Microstructural Sciences,
National Research Council of Canada, M50-263, 1200 Montreal Road, Ottawa,
Ontario, Canada K1A 0R6, and Department of Physics, UniVersity of Ottawa,
150 Louis Pasteur, Ottawa, Ontario, Canada K1N 6N5
wangw@ccs.carleton.ca
Received November 13, 2004
ABSTRACT
A series of 2,6-disubstituted indenofluorene derivatives were obtained in high purity via a general route involving the Suzuki coupling reaction.
The potential of these conjugated indenofluorenes as new organic semiconductors was demonstrated by the light-emitting diode reaching a
high luminance of 1400 Cd/m2 below 10 V.
Conjugated aromatic compounds are potential candidates as
organic semiconductors for use in thin-film transistors (TFTs)
and light-emitting diodes (LEDs). By incorporation of
strongly electronegative substituents such as fluorine1 and
cyano groups2 into certain aromatic molecules, some im-
portant n-type compounds have been obtained for TFT
applications. To develop organic LED devices (OLEDs) with
uniform luminescence, saturated color, and low driving
voltages, many material factors must be considered, such as
emission efficiency, color purity, carrier mobility, thermal
and chemical stability, and processability (for vapor-phase
deposition).3
Among those organic semiconductors known for transistor
and OLED applications, the fluorene-based compounds,4
oligomers,5 and polymers6 have received most attention,
owing to their unique properties, availability, and process-
ability. Since the substituted fluorenes are known to have
relatively high band gaps and low HOMO levels, they are
† Carleton University.
‡ Institute for Microstructural Sciences.
§ University of Ottawa.
(1) (a) Hoshino, S.; Nagamatsu, S.; Chikamatsu, M.; Misaki, M.; Yoshida,
Y.; Tanigaki, N.; Yase, K. Synth. Met. 2003, 137, 953. (b) Sakamoto, Y.;
Suzuki, T.; Kobayashi, M.; Gao, Y.; Fukai, Y.; Inoue, Y.; Sato, F.; Tokito,
S. J. Am. Chem. Soc. 2004, 126, 8138. (c) Bao, Z.; Lovinger, A. J.; Brown,
J. J. Am. Chem. Soc. 1998, 120, 207. (d) Barbarella, G.; Zambianchi, M.;
Antolini, L.; Ostoja, P.; Maccagnani, P.; Bongini, A.; Marseglia, E. A.;
Tedesco, E.; Gigli, G.; Cingolani, R. J. Am. Chem. Soc. 1999, 121, 8920.
(e) Katz, H. E.; Laquindanum, J. G.; Lovinger, A. J. Chem. Mater. 1998,
10, 633. (f) Dimitrakopoulos, C. D.; Purushothaman, S.; Kymissis, J.;
Callegari, A.; Shaw, J. M. Science 1999, 283, 822. (g) Sirringhaus, H.;
Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-
Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig,
P.; de Leeuw, D. M. Nature 1999, 401, 685.
(3) Leung, L. M.; Lo, W. Y.; So, S. K.; Lee, K. M.; Choi, W. K. J. Am.
Chem. Soc. 2002, 122, 5640.
(4) (a) Wong, K. T.; Chien, Y. Y.; Chen, R. T.; Wang, C. F.; Lin, Y. T.;
Chiang, H. H.; Hsieh, P. Y.; Wu, C. C.; Chou, C. H.; Su, Y. O.; Lee, G.
H.; Peng, S. M. J. Am. Chem. Soc. 2002, 124, 11576. (b) Katsis, D.; Geng,
Y. H.; Ou, J. J.; Culligan, S. W.; Trajkovska, A.; Chen, S. H.; Rothberg, L.
J. Chem. Mater. 2002, 14, 1332. (c) Beaupre, S.; Ranger, M.; Leclerc, M.
Macromol. Rapid Commun. 2002, 21, 1013. (d) Bao, Z. U. S. Pat. 6, 452,
207 B1 2002.
(5) (a) Klaerner, G.; Miller, R. D. Macromolecules 1998, 31, 2007. (b)
Meng, H.; Bao, Z.; Lovinger, A. J.; Wang, B. C.; Mujsce, A. M. J. Am.
Chem. Soc. 2001, 123, 9214.
(2) (a) Facchetti, A.; Yoon, M. H.; Stern, C. L.; Katz, H. E.; Marks, T.
J. Angew. Chem., Int. Ed. 2003, 42, 3900. (b) Pappenfus, T. M.; Burand,
M. W.; Janzen, D. E.; Mann, K. R. Org. Lett. 2003, 5, 1535. (c) Pappenfus,
T. M.; Chesterfield, R. J.; Frisbie, C. D.; Mann, K. R.; Casado, J.; Raff, J.
D.; Miller, L. L. J. Am. Chem. Soc. 2002, 124, 4184.
(6) (a) Ding, J.; Day, M.; Robertson, G.; Roovers, J. Macromolecules
2002, 35, 3474. (b) Lim, E.; Jung, B. J.; Shim, H. K. Macromolecules 2003,
36, 4288.
10.1021/ol0476570 CCC: $30.25
© 2005 American Chemical Society
Published on Web 02/09/2005

yield (see the Supporting Information). Compound 2 can be
prepared in tens of grams and becomes a building block for
a series of indenofluorene derivatives.
Scheme 1
The subsequent Suzuki reaction was carried out to couple
compound 2 with the corresponding boronic acids in the
presence of Pd(PPh3)4 in NMP at 90 °C (see the Supporting
Information). The resulting products were filtered and washed
with water. Compounds 3a-h were obtained in modest
yields (7-44%), without optimization, after purification by
vacuum sublimation.
Thermogravimetric analysis confirmed good thermal sta-
bility of these indenofluorene derivatives, as 5% weight loss
of 3a was observed at 411 °C in nitrogen. The differential
scanning calorimetry (DSC) revealed the melting transition
of 392 °C for 3c, while the crystallization temperature upon
cooling was found to be 338 °C. High melting behavior was
also observed for other compounds (Table 1). Despite their
stable to photo- and electro-oxidation and the related devices
have shown high on/off ratios with good aging characteristics.4d
Fluorene is a rigid and planar molecule, due to the
methylene bridging unit at the 2,2′-positions of the biphenyl
moiety, with high thermal stability.7 Since its electronic and
optical properties arise from its planar biphenyl moiety, it is
conceivable that the structurally related compound, indeno-
fluorene, should have some unique electronic and optical
characteristics attributable to a planar, longer conjugated
p-terphenyl moiety.
Table 1. Properties of Compounds 3a-h
mpa (Tc, °C)
λabs,b nm
λPL,b nm
Φc
3a
3b
3c
3d
3e
3f
362
>400
365, 381
350, 365
346, 356
350, 360
353
358
311, 345
355
396, 416, 443
378, 398, 421
376, 394, 417
383-401, 430
385, 403, 432
428
44
34
48
43
59
78
18
55
392 (338)
397 (385)
>400
389 (380)
337 (281)
>400
3g
3h
393
394, 403, 432
A number of different synthetic routes are available for
the syntheses of indenofluorene derivatives that are mainly
functionalized at the 9,9′-positions with the alkyl groups.8
Each one is unique, but not general enough to allow for the
synthesis of a series of structurally related indenofluorenes
for probing the structure-property relationships. We report
herein a facile synthetic route to a new series of indeno-
fluorene derivatives with aryl groups at the terminal 2,6-
positions and the preliminary results of organic LED using
one of eight indenofluorene derivatives synthesized.
In this two-step approach (Scheme 1), a key step involves
the Suzuki coupling reaction, and 2,6-dibromoindenofluorene
(2) is a building block for a variety of indenofluorene
derivatives. Indenofluorene was readily available in tens or
even hundreds of grams in four steps, starting with the Suzuki
reaction of 2,5-dibromo-p-xylene with phenylboronic acid
in quantitative yield. Oxidation of 2,5-diphenyl-p-xylene by
potassium permanganate yielded 2,5-diphenylterephthalic
acid (92% yield), which was treated in concentrated H2SO4
to afford the corresponding diketone in 96% yield, which
was then reduced to indenofluorene by the modified Wolf-
Kishner reduction.8a Indenofluorene was easily brominated
with Br2 in 1,1,2,2-tetrachloroethane (TCE) at room tem-
perature to afford 2,6-dibromoindenofluorene (2) in 77%
a Melting points are measured by DSC. Tc ) crystallization temperature.
b In DMF. c Quantum yield (%) in reference to quinine sulfate (1 µM in 1
N H2SO4).
high melting characteristics, these compounds can be readily
sublimed under vacuum at elevated temperatures.
The UV-vis absorption and photoluminescence (PL) of
all eight compounds in solution (DMF) were studied (Table
1). Two absorption peaks (λmax) were observed for 3a-d
and 3g, whereas 3e, 3f, and 3h had only one broad peak.
The peaks at the longer wavelength (345-381 nm) cor-
respond to the π-π* transition of the conjugated aromatic
rings, and those at a shorter wavelength (311-365 nm) are
attributed to the electronic transition of each aromatic unit.
The presence of the thienyl rings caused a red shift (Figure
1). Correspondingly, PL peaks appeared around 400 nm for
the samples in solution.
Energy level of 3a and 3b could be determined using the
data from cyclic voltammetry and absorption spectra.9
However, like many conjugated organic compounds, the
electrochemical processes of 3a and 3b were not reversible.
By calculating with the HOMO-LUMO band gap measured
from the absorption spectra, the LUMO level is 2.0 eV for
3a and 2.6 eV for 3b (Table 2). The HOMO levels of 3a-h
are ∼5.1-5.8 eV.
(7) (a) Loy, E. D.; Koene, E. B.; Thompson, M. E. AdV. Funct. Mater.
2002, 12, 245. (b) Setayesh, S.; Marsitzky, D.; Mullen, K. Macromolecules
2000, 33, 2016. (c) Okumoto, K.; Ohara, T.; Noda, T.; Shirota, Y. Synth.
Met. 2001, 121, 1655.
(8) (a) Merlet, S.; Birau, M.; Wang, Z. Y. Org. Lett. 2002, 4, 2157. (b)
Harvey, R. G.; Pataki, J.; Cortez, C.; Raddo, P. D.; Yang, C. J. Org. Chem.
1991, 56, 1210.
The electroluminescent (EL) properties of compound 3a
were tested in the following OLED structure: ITO/3a (500
(9) Wang, L.; Ng, M.; Yu, L. Phys. ReV. 2000, 62, 4973.
Org. Lett., Vol. 7, No. 5, 2005
796

Figure 1. UV absorption (25 µM for all) and PL of 3a (0.5 µM)
3d (0.25 µM), and 3h (0.5 µM) in DMF, excited at 350, 355, and
365 nm, respectively.
Figure 2. JLV (current density in black circles, luminance in red
squares) characteristics of an ITO/3a/PBD/LiF/Al OLED. Inset: EL
(taken at 9V, solid red line) of OLED and PL of 3a (350 nm
excitation, dashed black line).
Å)/PBD (500 Å)/LiF (15 Å)/Al. PBD or 2-(4-biphenylyl)-
5-phenyl-1,3,4-oxadiazole is an electron transport material
with a lower HOMO than that of 3a, which should facilitate
the charge recombination in 3a.10 The combination of a thin
LiF layer and Al has been shown to form an efficient
cathode.11 The ITO surface was treated in a UV-ozone oven
for 10 min, and then the structure was thermally evaporated
without exposure to air in an Edwards Auto-306 multisource
chamber with a base vacuum better than 10-6 Torr. A quartz
substrate was placed beside the OLED substrate to collect
the deposited 3a, for comparison of thin film PL with its
EL spectrum.
meter. Figure 2 shows typical current density-voltage and
luminance-voltage characteristics of the OLED. The OLED
reaches a high luminance of 1400 Cd/m2 below 10 V for a
current density around 1600 A/m2. The quantum luminous
efficiency is about 1 Cd/A at 9 V but decreases at higher
voltages. The inset of Figure 2 shows the EL spectrum taken
at 9 V, which is independent of the voltage and is in
agreement with the PL taken on the quartz witness sample
excited at 350 nm. Due to the polycrystalline nature of the
deposited film of 3a, the observed absorption was broader,
have multiple bands and shifted to longer wavelengths in
comparison with the spectrum of 3a in solution (Figure 1).
As a result, the PL spectrum of thin film was also broader
and the device emitted the whitish yellow color.
Table 2. HOMO and LUMO Level of 3a and 3b
Eox a (V) HOMO (eV) λonset (nm) Eg (eV) LUMO (eV)
3a
3b
0.55
1.18
5.1
5.8
399
383
3.1
3.2
2.0
2.6
Acknowledgment. This work was supported by the
Natural Sciences and Engineering Research Council of
Canada and the National Capital Institute of Telecom-
munications.
a Redox potential versus NHE; performed in propylene carbonate
containing 0.1 M LiClO4.
Supporting Information Available: Syntheses and char-
acterizations of compounds 2 and 3 and CV of thin films of
3a and 3b. This material is available free of charge via the
Internet at http://pubs.acs.org.
The OLED was tested using a programmable Keithley
2400 SourceMeter unit and a Minolta LS-110 Luminance
(10) Adachi, C.; Tsutsui, T.; Saito, S. Appl. Phy. Lett. 1989, 55, 1489.
(11) Hung, L.; Tang, C.; Mason, M. Appl. Phys. Lett. 1997, 70, 152.
OL0476570
Org. Lett., Vol. 7, No. 5, 2005
797