Synthesis and Characterization of Oligo(9,9-dihexyl-2,7-fluorene ethynylene)s: For Application as Blue Light-Emitting Diode

Article abstract of DOI:10.1021/ol010069r

(Formula Presented) Highly soluble and strongly blue fluorescent oligo(9,9-dihexyl-2,7-fluorene ethynylene)s (OFEs) were synthesized by a Pd/Cu-catalyzed Sonogashira coupling reaction. An organic light-emitting diode using pentamer 15 as the emitting material was successfully fabricated and emitted a bright blue light.

Full text of DOI:10.1021/ol010069r

ORGANIC
LETTERS
2001
Vol. 3, No. 13
2005-2007
Synthesis and Characterization of
Oligo(9,9-dihexyl-2,7-fluorene
ethynylene)s: For Application as Blue
Light-Emitting Diode
,†,‡
Sang Ho Lee,† Toshikazu Nakamura,‡ and Tetsuo Tsutsui*
CREST, Japan Science and Technology Corporation (JST), Japan, and Department of
Applied Science for Electronics and Materials, Graduate School of Engineering
Sciences, Kyushu UniVersity, Kasuga, Fukuoka, 816-8580, Japan
tsuigz@mbox.nc.kyushu-u.ac.jp
Received April 13, 2001
ABSTRACT
Highly soluble and strongly blue fluorescent oligo(9,9-dihexyl-2,7-fluorene ethynylene)s (OFEs) were synthesized by a Pd/Cu-catalyzed Sonogashira
coupling reaction. An organic light-emitting diode using pentamer 15 as the emitting material was successfully fabricated and emitted a bright
blue light.
Monodisperse, well-defined π-conjugated oligomers have
recently become a subject of intense research in material
science.1 For example, they can be used as nondefected
structures for electronic devices such as organic light-emitting
diodes (OLEDs),2 solar cells,3 and field-effect transistors
(FETs)4 and as models1c to understand the fundamental
properties of their analogous polydisperse polymers. Devel-
opment of synthetic methodology makes it possible to design
a variety of soluble monodisperse oligomers, which permit
color and charge injection tuning through their conjugation
length control, as well as the introduction of electron-
donating or -withdrawing groups to the parent π-conjugated
system.5
Oligofluorenes are important model compounds for poly-
fluorenes, which are the promising blue light-emitting
materials with extremely high photoluminescence, quantum
yields, and thermal and oxidative stability.6 The structure-
property relationship of polyfluorene was obtained by
investigating the electronic properties of each type of
oligofluorene.7 However, there were, to our knowledge, no
reports on determination of the effective conjugation length
for poly(2,7-fluorene ethynylene) (PFE). Herein we report
a successful synthesis of a series of OFEs and their optical
properties and offer the possibility of an OLED application
using pentamer 15 as blue light-emitting material.
The syntheses of OFEs started from commercially avail-
able fluorene. Electrophilic monoiodination8 of fluorene
afforded 2-iodofluorene, which was subsequently alkylated
to give 2-iodo-9,9-dihexylfluorene (2) in 57% overall yield.
† CREST, Japan Science and Technology Corporation.
‡ Kyushu University.
(1) For an extensive review, see: (a) Tour, J. M. Chem. ReV. 1996, 96,
537-553. (b) Pu, L. Chem. ReV. 1998, 98, 2405-2494. (c) Martin, R. E.;
Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 1350-1377. (d) Berresheim,
A. J.; Muller, M.; Mullen, K. Chem. ReV. 1999, 99, 1747-1786. (e) Roncali,
J. Acc. Chem. Res. 2000, 33, 147-156.
(2) (a) van Hutten, P. F.; Wildeman, J.; Meetsma, A.; Hadziioannou, G.
J. Am. Chem. Soc. 1999, 121, 5910-5918. (b) Goodson, T., III; Li, W.;
Gharavi, A.; Yu, L. AdV. Mater. 1997, 9, 639-643.
(3) (a) Noma, N.; Tsuzuki, T.; Shirota, Y. AdV. Mater. 1995, 7, 647-
648. (b) Eckert, J.-F.; Nicoud, J.-F.; Nierengarten, J.-F.; Liu, S.-G.;
Echegoyen, L.; Barigelletti, F.; Armaroli, N.; Ouali, L.; Krasnikov, V.;
Hadziioannou, G. J. Am. Chem. Soc. 2000, 122, 7467-7479.
(4) Schon, J. H.; Dodabalapur, A.; Kloc, C.; Batlogg, B. Science 2000,
290, 963-965.
(5) (a) Maddux, T.; Li, W.; Yu, L. J. Am. Chem. Soc. 1997, 119, 844-
845. (b) Detert, H.; Sugiono, E. Synth. Met. 2000, 115, 89-92.
(6) (a) Pei, Q.; Yang, Y. J. Am. Chem. Soc. 1996, 118, 7416-7417. (b)
Grell, M.; Bradley, D. D. C.; Inbasekaran, M.; Woo, E. P. AdV. Mater.
1997, 9, 798-802. (c) Kreyenschmidt, M.; Klaerner, G.; Fuhrer, T.;
Ashenhurst, J.; Karg, S.; Chen, W. D.; Lee, V. Y.; Scott, J. C.; Miller, R.
D. Macromolecules 1998, 31, 1099-1103. (d) Lee, J.-I.; Klaerner, G.;
Miller, R. D. Chem. Mater. 1999, 11, 1083-1088.
(7) (a) Klaerner, G.; Miller, R. D. Macromolecules 1998, 31, 2007-
2009. (b) Lee, S. H.; Tsutsui, T. Thin Solid Films 2000, 363, 76-80.
(8) Suzuki, H.; Nakamura, K.; Goto, R. Bull. Chem. Soc. Jpn. 1966, 39,
128-131.
10.1021/ol010069r CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/25/2001

Pd/Cu-catalyzed cross-coupling of 2 and 3-methyl-1-butyn-
3-ol provided 3 (88%), which was then converted to
2-ethynyl-9,9-dihexylfluorene (4) by a base-promoted depro-
tection in 87% yield. The long dihexyl moieties provide good
solubility for the longer oligomers. 2,7-Diethynyl-9,9-di-
hexylfluorene (8), which is a monomer to construct the longer
OFEs, was prepared by diiodination of fluorene, a displace-
ment reaction with hexyl bromide, cross-coupling using Pd/
Cu catalyst, and finally deprotection (Scheme 1).
methane, and THF. This is necessary not only to prepare
oligomers with the sufficiently long rod length but also to
fabricate organic light-emitting diodes (OLEDs) by the spin-
coating method.
To investigate the electronic properties of each type of
oligomer structure and to determine the effective conjugation
length for the corresponding polymer, absorption spectra for
each oligomer were examined. The OFEs exhibit a distinct
red shift as their rod lengths increase (Figure 1a). The
Scheme 1a
a (a) I2, H5IO6, AcOH, H2SO4, H2O, reflux, 4 h; (b) C6H13Br,
triethylbenzylammonium chloride, 50% aqueous NaOH, DMSO,
rt, 6 h; (c) 3-methyl-1-butyn-3-ol, PdCl2(PPh3)2, PPh3, CuI, tri-
ethylamine, reflux, 6 h; (d) KOH, 2-propanol, reflux, 3 h.
Our strategy for the potential blue light-emitting OFEs was
based on the synthetic route above (Scheme 2). Pd/Cu-
catalyzed cross-coupling of 2 and 4 generated dimer 9 in
62% yield. Treatment of 4 with 6 in the presence of Pd/Cu
catalyst afforded trimer 10 (28%) and byproduct iodo dimer
11 (24%) after purification of silica gel, respectively. The
latter further reacted with 3-methyl-1-butyn-3-ol to give 12,
followed by the cleavage of the polar group to afford dimer
13 with the terminal acetylene unit. Treatment of 11 with
13 or 8 using a Sonogashira coupling reaction gave the
desired tetramer 14 (67%) or pentamer 15 (19%), respec-
tively. Pentamer 15, which is the longest monodisperse OFEs
in the present π-conjugated system, is still highly soluble in
common organic solvents such as chloroform, dichloro-
Figure 1. (a) Normalized absorption spectra of 9, 10, 14, and 15
in chloroform. (b) Plot of optical band gap versus the number of
repeating units (n) for 9, 10, 14, 15, and PFE.
maximum peaks observed for these oligomers approach that
(2.88 eV)9 for PFE. By inspecting the edge of the absorption
spectra, the energy band gaps were plotted as a function of
oligomer length (Figure 1b). This saturated curve indicates
that the oligomers prepared are well defined and largely
nondefected, and the calculated effective conjugation length
of PFE is about 10 fluorene units, which corresponds to a
(9) Lee, S. H.; Nakamura, T.; Tsutsui, T., unpublished results.
2006
Org. Lett., Vol. 3, No. 13, 2001

Scheme 2a
a (a) PdCl2(PPh3)2, PPh3, CuI, triethylamine, reflux, 24 h; (b) 3-methyl-1-butyn-3-ol, PdCl2(PPh3)2, PPh3, CuI, triethylamine, reflux, 6 h;
(c) KOH, 2-propanol, reflux, 6 h.
total of 20 aromatic rings. As shown in Figure 2, pentamer
15 exhibits strong blue fluorescence with an emission
maximum at 402 nm and a weaker peak centered on 445
nm, which is bathochromically shifted by 50 nm compared
to dimer 9. The PL efficiencies of 9, 10, 14, and 15 are 49,
52, 63, and 64, respectively. Apparently, increasing conjuga-
tion length in the π-conjugated system increases the quantum
yield of fluorescence. Pentamer 15, with the highest quantum
efficiency in this study, was tried to fabricate OLED.10 The
electroluminescence (EL) spectrum of pentamer 15 shows
blue light emission with a maximum peak at 424 nm and a
weaker peak at 449 nm, in which the maximum peak is red-
shifted by 22 nm compared to the PL spectrum of pentamer
15 in CHCl3. This result indicates strong intermolecular
interaction in the solid state.
In summary, we have synthesized a new series of soluble
oligo(9,9-dihexyl-2,7-fluorene ethynylene)s with strong blue
fluorescence and successfully fabricated an organic light-
emitting diode (OLED) using pentamer 15, yielding blue light
emission.
Acknowledgment. We thank the Core Research for
Evolutional Science and Technology, Japan Science and
Technology Corporation (CREST/JST) for financial support
of this work.
Supporting Information Available: Experimental pro-
cedure and characterization for all compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL010069R
(10) OLED was fabricated on an indium tin oxide (ITO)-coated glass
substrate by spin-coating of pentamer 15 in chloroform as the emitting layer
(60 nm) and then by vacuum thermal deposition (below 1 × ∼10-6 Torr)
of Ca as the cathode.
Figure 2. Normalized photoluminescence spectra of 9, 10, 14, and
15 in chloroform and electroluminescence (EL) spectrum of 15.
Org. Lett., Vol. 3, No. 13, 2001
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