2716 Organometallics, Vol. 29, No. 12, 2010
Jung et al.
Chart 1. Molecular Structures of Representative Silole Deriva-
tives Being Used for OLEDs: (a) Type 1, (b) Type 2, (c) Type 3,
(d) Type 4, (e) Type 5a)
Therefore, there is still a strong need for a preparative method
of DTS-based π-conjugated compounds. We have recently
developed palladium(II)-catalyzed cross-coupling using in-
dium reagents.13 This method complements the existing syn-
thetic methods due to some advantageous properties of indium
reagents such as availability, ease of preparation and handling,
high reactivity, operational simplicity, and low toxicity.
For OLED application, molecules should have high thermal
stabilities, high quantum yields, and good film-forming proper-
ties (glassy nature) in order to obtain high efficiency in OLED
performance.14 Highly π-conjugated compound derivatives
bearing a naphthyl segment were not only regarded as promis-
ing host materials but also showed high thermal stability and
efficiency.15 In these molecules, the naphthyl group could play a
key role in enhancing thermal stability and uniform film-
forming properties.15a Therefore, we expected that the combi-
nation of a highly fluorescent DTS unit and a naphthyl segment
would be the best pathway to develop efficient materials.
Herein, we describe the results of our investigation on the prepa-
ration, structural characterization, electrochemical behavior,
optical properties, and the fabrication of multilayer light-emit-
ting devices of a series of DTS-based π-conjugated compounds
bearing a naphthyl moiety.
a Ar: either phenyl or functionalized aromatic group.
dithienosiloles (type 3),7 2,3,4,5-tetrafunctionalized siloles
(type 4),8 and 2,5-functionalized spiro-bisiloles (type 5),9 as
shown in Chart 1.
The external quantum efficiency (EQE) of OLEDs fabri-
cated by using siloles as emissive or electron-transporting
materials ranges from 0.0045% to 8%.4a,5a Such results
indicate that the efficiency of silole-based OLEDs is quite
sensitive to both the device structure and the molecular
structure of the siloles. Therefore, continuous efforts for
the development of suitable silole derivatives for application
in OLEDs and the finding of optimized device conditions are
still required.
Recently, a two thiophene fused silole, dithienosilole
(DTS), has attracted much attention due to its usefulness
as a building block that can be incorporated into conjugated
molecules and polymers.10 Although DTS has a number of
advantages, such as the ease of introducing substituents at
different positions of the DTS ring, high photoluminescence
quantum efficiency in both solution and the solid state, and
easy tuning of emission energy, etc., there have been only a
few reports on DTS-based OLED performance due to
synthetic limitations of DTS derivatives.8
Result and Discussion
Syntheses and Characterization of Dithienosilole-Bearing
Naphthyl Groups. The synthetic route and details for three
new DTS derivatives are shown in Scheme 1. In our initial study,
we have attempted syntheses of 1-3using typical cross-coupling
methods, Kumada,11 Stille,12 Negishi,16 and Suzuki coupling,17
under mild or reflux conditions according to previous reports.
All of these coupling reactions, however, gave 1-3 in poor to
moderate yields. Even undesired compounds, silanol and oxo-
bridged siloxane-bithiophene, were obtained as major compo-
nents when used Suzuki coupling. This result is presumably
attributed to ring cleavage of the Si-C(thiophene) bond in the
presence of base. As an alternative method, we examined a new
cross-coupling reaction using indium reagent as a nucleophile in
the presence of various palladium catalysts. At first, we exam-
ined the stoichiometry of tri(2-naphthyl)indium and the catalytic
activity of several palladium catalysts, such as Pd(0) and Pd(II),
under varied reaction conditions. Of the conditions screened, the
best results were obtained with tri(2-naphthyl)indium (1.0 equiv)
in the presence of Pd(dppf)Cl2 (dppf=1,10-bis(diphenylphos-
phino)ferrocene) in THF (70-80 °C), producing 1-3 in mode-
rate to good yield (50-60%) (details are presented in the Experi-
mental Section; see Scheme 1).
To prepare DTS-based π-conjugated compounds, typical
coupling reactions, such as Kumada11 and Stille couplings,12
are mainly adopted. These reactions, however, need harsh
reaction conditions and gave either poor or moderate yields.
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All compounds are stable in air and soluble in common
organic solvents but only slightly soluble in hexane and
1
pentane. The structures of 1-3 have been confirmed by H
NMR, 13C NMR, and mass and elemental analyses, including
X-ray diffraction analysis of 3. In order for molecular organic
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