Novel oligomers of 9,9-dialkylated fluorenes for thermally stable hole-transport materials in OLEDs

Article abstract of DOI:10.1246/bcsj.80.418

Using 9,9-dialkyl-functionalized fluorenes as monomers, novel fluorene-based oligomers were prepared in order to improve the thermal stability of a typical hole-transport material (HTM), N,N′-bis(3-methylphenyl)-N, N′-diphenylbenzidine (TPD), in organic l

Full text of DOI:10.1246/bcsj.80.418

418 Bull. Chem. Soc. Jpn. Vol. 80, No. 2, 418–422 (2007)
Ó 2007 The Chemical Society of Japan
Novel Oligomers of 9,9-Dialkylated Fluorenes for
Thermally Stable Hole-Transport Materials in OLEDs
ꢀ1
ꢀ1
Makoto Kimura, Toshihiko Kurata,1 Yasuhiko Sawaki,
Yasumasa Toba,2 Toshio Enokida,2 and Katsuhiko Takagi
ꢀ1
1Departments of Applied Chemistry and Crystalline Materials Science, Graduate School of Engineering,
Nagoya University, Chikusa-ku, Nagoya 464-8603
2Toyo Ink Mfg. Co., Ltd., 27 Wadai, Tsukuba 300-4247
Received June 2, 2006; E-mail: mkimura@apchem.nagoya-u.ac.jp
Using 9,9-dialkyl-functionalized fluorenes as monomers, novel fluorene-based oligomers were prepared in order to
improve the thermal stability of a typical hole-transport material (HTM), N,N0-bis(3-methylphenyl)-N,N0-diphenylben-
zidine (TPD), in organic light emitting diodes (OLEDs). 2,7-Bis(diphenylamino)fluorene was utilized as a TPD unit
throughout the present synthetic work. 9,9-Bis(2-methylallyl)fluorene and its TPD congener underwent a new cyclopoly-
merization reaction affording six-membered oligomers under acid catalysis with p-toluenesulfonic acid. Intermolecular
Suzuki coupling of a 9,9-bis(p-bromobenzyl)fluorene analogue with the corresponding bis-boronate afforded a new
oligomer containing the TPD substructure. These oligomers (3 and 6) for use as HTMs have high glass-transition tem-
peratures (Tg’s) (147 and 167 ꢁC, respectively); the values are much higher than the Tg for TPD (63 ꢁC). These materials
are soluble in organic solvents to allow easy processing for preparing layered devices, though their electroluminescent
performances were lower than that of the single molecule TPD.
Organic light emitting diodes (OLEDs) are being actively
developed for full-color flat-panel displays and flexible dis-
plays.1 The materials for these devices need to be thermally
stable for long-life driving. For multi-layered devices, numer-
ous triphenylamine derivatives have been developed as the
hole-transport material (HTM).2–5 N,N0-Bis(3-methylphenyl)-
N,N0-diphenylbenzidine (TPD) is a typical HTM, which shows
excellent EL characteristics, but its thermal stability is insuffi-
cient as indicated by the low glass-transition temperature (Tg)
of 63 ꢁC.6 To overcome the drawback of TPD, two approaches
for thermally stable HTMs have been reported. One is based
on single-molecule materials, and another is on polymer mate-
rials.2–4 The former is favorable in material purification be-
cause of sublimation, but the latter is advantageous in prepara-
tion of thin films because of easy coating procedure. The ad-
vantage of the polymer-type approach has been illustrated with
polystyrene having triphenylamine groups as pendant groups,7
cross-linkable alkoxylated polymethacrylates,8 a polymer of
1,1-cyclohexane-bridged triphenylamines,9 and some conden-
sation polymers.10–12
Our polymeric approach utilizes 9H-fluorene as the key sub-
stance. Introduction of functional groups into fluorene is avail-
able not only at the 2 and 7 positions via aromatic electrophilic
substitutions, but also at the 9 position via facile alkylation due
to the high acidity of the methylene carbon (pKa 23) giving a
stable carbanion.13 Thus, 2,7-bis(diphenylamino)fluorene (1,
see Scheme 1) contains the N,N,N0,N0-tetraphenylbenzidine
substructure of TPD. Our interest was focused on 9,9-diallyl-
substituted derivative of 1 because the cyclopolymerization14
of several diallyl monomers which afforded the famous elec-
troconducting polymer, poly(diallyldimethylammonium chlo-
ride), has been reported.15 However, no attempt to polymer-
ize 9,9-diallylfluorene has been reported to the best of our
knowledge, although the analogous 9,9-di-2-propynylfluorene
is known to undergo oxidative coupling polymerization.16
A
relevant compound alkylated with p-bromobenzyl group is
also valuable because aromatic bromides for polymeric HTM
are available through intermolecular Suzuki coupling.17 Here,
we report the preparation of polymer-type HTMs of several
9,9-dialkylated 2,7-bis(diphenylamino)fluorenes. Some of the
resulting oligomers unexpectedly have high thermal stability
indicated by their Tg’s. Their electroluminescent (EL) charac-
teristics remain good enough for practical use.
Results and Discussion
Monomers and Oligomers. At the beginning of this study,
9,9-diallylfluorene18 was chosen as a promising monomer for
the cyclopolymerization because it should polymerize similar-
ly to diallyldimethylammonium chloride.15 Attempts to poly-
merize 9,9-diallylfluorene in refluxing benzene using AIBN
as radical initiator were unsuccessful, i.e., only the starting ma-
terial was recovered. Use of 9,9-bis(2-methylallyl)fluorene,19
(9,9-bis(2-methyl-2-propenyl)fluorene), as a substrate in the
presence of an acid catalyst p-toluenesulfonic acid through
cationic polymerization afforded oligomers with 6-membered
rings albeit in low yield. When 2,7-bis(diphenylamino)-9,9-
bis(2-methylallyl)fluorene (2), which is a TPD congener, was
used as a substrate, oligomeric material 3 was obtained in
97% yield probably due to its low solubility in solvents
(Scheme 1).
As for the degree of polymerization, up to 5-mers were de-
1
tected with APCI-MS analysis. In the H NMR spectrum of 3,
Published on the web February 13, 2007; doi:10.1246/bcsj.80.418

M. Kimura et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 2 (2007)
419
Scheme 1. a) 2-Methylallyl chloride (2.5 equiv), NaH in DMF, rt, 6 h, 72%. b) TsOH (10 mol %), o-dichlorobenzene, 130 ꢁC, 1 day,
then reprecipitation with MeOH, 97%.
olefinic protons that had appeared at 4.2 and 4.4 ppm in 2
became saturated protons at 2.1 and 2.3 ppm. The broadened
methyl signal from 1.1–1.3 ppm contained 4–6 peaks. The
structure of 6-membered rings is formed via a cationic poly-
merization mechanism: (1) initial protonation of a methylallyl
group generating a stable tertiary carbocation, (2) which un-
dergoes sequentially both intramolecular cyclization leading
to a 6-membered ring and intermolecular C–C bond formation
as the propagation step, followed by (3) a termination step
such as alkylation of aminophenyl rings (presumably at the
para positions) with the resulting carbocations and their depro-
tonation to an inner olefin. An analogous di-2-propynyl TPD
congener was subjected to oxidative polymerization, similar
to 9,9-di-2-propynylfluorene,16 in the presence of oxygen and
the copper complex; however, the yield of the polymeric
material was low (at best 35% yield). The degree of polymer-
ization was about 5, whereas 50 is reported in the case of sim-
ple 9,9-di-2-propynylfluorene.16 In the 1H NMR spectra of this
TPD congener, the peaks for the terminal acetylenic protons
completely disappeared; however, new olefinic proton peak
did not appear. In other words, no cyclopolymerization lead-
ing to any cyclic polyene derivative took place. Accordingly,
this oligomer was considered to contain only intermolecular
linkages.
Another polymer of TPD congeners was obtained by Suzuki
cross-coupling polymerization.17 Starting dibromide 4 was
prepared via dialkylation of 1 with p-bromobenzyl bromide,
and 4 was then converted into diboronate 5 using a conven-
tional procedure (Scheme 2). Bifunctional Suzuki coupling re-
action between 4 and 5 in the presence of a palladium catalyst
with tri-t-butyl phosphine ligands20 in refluxing toluene gave
polymeric material 6 that was mainly comprised of a 4-mer
based on the gel permeation chromatography (GPC) analysis
using polystyrene standards (Mn 2800, Mw=Mn 1.67).
surements and device performances. Interestingly, oligomers
3 and 6 have Tg’s much higher than 100 ꢁC, namely 147 and
167 ꢁC, respectively. These unexpectedly high Tg values are
comparable to those of the TPD-pendant polymers (132–151
ꢁC)7 and of the TPD-PPV copolymer with Mw ¼ 35810 (130
ꢁC).11 Regarding the Tg, 3 and 6 having TPD moiety are supe-
rior to the polymers of 1,1-cyclohexane-bridged triphenyl-
amines (Tg ¼ 107{134 ꢁC).9 Formaldehyde or benzaldehyde-
condensed polymers TPD-FA and TPD-BzA with high molec-
ular weights (Mw > 104) have higher Tg’s 183 and 239 ꢁC, re-
spectively.10 In the single-molecule materials, triphenylamine
derivatives, like TPD, have been reported to show Tg’s up to
145 ꢁC with an increase in number of the triphenylamine unit
from 2 to 5.22 From our recent research, a trispirocycle-incor-
porated trimeric TPD analogue has a Tg of 170 ꢁC.23 The sim-
ple TPD congener 1 has a Tg of 88 ꢁC. Therefore, the present
oligomer-based method can be used to prepare HTM’s with a
high Tg. The high molecular weight is the dominant factor for
raising Tg in accord with requirements reported elsewhere.24 In
order to design an HTM with high Tg, structural aspects may
also be important for increasing Tg based on the high Tg for
3, which has an incorporated spirocycle, and 6, which has a
large number of the aromatic moieties.
Cyclic voltammetric (CV) measurements of 3 and 6 in
dichloromethane were carried out for the design of devices.
Figure 1 shows a cyclic voltammogram of bis(2-methylallyl)
oligomer 3. Both oligomer 3 and 6 have two reversible waves,
which correspond to the successive formation of the corre-
sponding cation radical and dication. The ratios of cathodic
current to anodic current25 for 3 and 6 with the first peak were
0.90 and 1.00, respectively (Table 1); this means that cation
radicals formed are highly stable and unreactive. In other
words, one may expect efficient hole-transport ability in the
solid state. The difference in the oxidation potentials is small
among TPD, 3 and 6 (see Table 1). TPD congener 1 is oxi-
dized at 0.60 V vs Ag/AgCl reference electrode (being only
by 0.03 V lower than TPD), whereas 3 and 6 at 0.64 and 0.66
V, respectively. In relation to the energy diagram for a device,
the HOMO levels of 3 and 6 are shifted to the level lower by
0.01–0.03 eV, or the nearly the same as that of TPD. UV and
Properties of Oligomers and Devices. For practical use in
EL devices, it is crucial that the Tg of HTM be high. For ex-
ample, ꢀ-NPD having a high Tg of 95 ꢁC was first commercial-
ly used in place of TPD with Tg 63 ꢁC.21 The Tg’s of oligomers
3 and 6 were obtained by DSC analyses, and they are listed
in Table 1 together with the results from electrochemical mea-

420 Bull. Chem. Soc. Jpn. Vol. 80, No. 2 (2007)
Novel Oligomers of 9,9-Dialkylated Fluorenes
Scheme 2. a) p-Bromobenzyl bromide, NaH in DMF, rt 6 h, 45%. b) BuLi, THF ꢂ70 ꢁC 1 h; B(OMe)3, then H3Oþ. c) 1,3-Propane-
diol, PhMe reflux overnight, 86% (2 steps). d) [Pd(PPh3)4] (3 mol %), K2CO3/H2O, PhMe reflux 2 days, then reprecipitation with
MeOH, 76%.
Table 1. Properties and Device Characteristics of Oligomeric HTMs 3 and 6
Oligomer
ThermalaÞ
ElectrochemicalbÞ
DevicecÞ
Tg/ꢁC
Eox/V
ipc=ipa
L300/V
Lmax/cd mꢂ2
ꢂe/lm wꢂ1
ꢂc/cd Aꢂ1
3
6
147
167
0.64
0.66
0.90
1.00
13
9.5
2600
6300
0.18
0.78
0.75
2.3
a) DSC analysis, 10 ꢁC minꢂ1. b) CV conditions, 1 mM (M = mol dmꢂ3), in dichloromethane, Bu4NClO4
0.1 M, scan rate 0.2 V sꢂ1, vs Ag/AgCl electrode. Eox means the half oxidation potential, ipc=ipa being
the ratio of cathodic to anodic peak current. c) The device structure, ITO/HTM 50 nm/Alq3 50 nm/Mg–
Ag. L300, Lmax, ꢂe, and ꢂc are already defined in the text.
HOMO–LUMO gap can be safely estimated to be smaller than
0.1 eV. Therefore, the band diagram involving HOMO and
LUMO levels for 1, 3, and 6 is almost the same as that for typi-
cal TPD (ꢂ5:5 and ꢂ2:1 eV vs vacuum level, respectively).
Multilayered EL devices for 3 and 6 were fabricated on
transparent indium–tin oxide (ITO) glass (anode) via spin-
coating, followed by successive vapor deposition of aluminum
tris(8-quinolinolate) (Alq3, electron-transport material) and
metal cathode. The device configuration was ITO/HTM (3
or 6) 50 nm/Alq3 50 nm/Mg–Ag. Upon application of a direct
current, emission of green light due to Alq3 was observed
at ꢁmax ꢃ 530 nm. Figure 2 illustrates the luminance—applied
voltage characteristics of oligomers 3 and 6. The devices using
3 and 6 had a practical level of luminance 200–300 cd mꢂ2 at
9–14 V of applied voltage (Table 1). However, these oligomer-
ic materials are significantly inferior in EL characteristics to
the single molecules studied by the authors;22 for example,
TPD congeners like 1 can acquire practical luminance L300
which is the voltage required for the luminance of 300 cd mꢂ2
,
,
Fig. 1. CV of oligomer 3 measured in dichloromethane
containing tetrabutylammonium perchlorate. Values are
plotted for calibration vs ferrocene reference electrode.
at only about 5 V and has a maximum luminance (Lmax) of
20000 cd mꢂ2. The Lmax for oligomers 3 and 6 is generally sev-
eral thousands cd mꢂ2, similar to those of 9,9-fluorene-bridged
triphenylamines.26 On the basis of easy processability to afford
thin films as well as high thermal stability as described before,
HTMs 3 and 6 are useful enough. Benzylic-type oligomer 6
visible absorption spectra of 3 and 6 were recorded in chloro-
form in order to estimate the band gap of the HTM. Simple
TPD congener 1, oligomers 3 and 6 have absorption maxima
at 362, 367, and 369 nm, respectively. The differences in the

M. Kimura et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 2 (2007)
421
Attempts to Polymerize 9,9-Diallylfluorene. A solution of
9,9-diallylfluorene18 (1.23 g, 5.0 mmol), AIBN (86 mg, 0.5 mmol)
in benzene (10 mL) was heated at reflux for 3 h. The insoluble sol-
id, which was also insoluble in chloroform, was filtered and dried
(5 mg). Analysis of the filtrate showed that the most (>90%) of
the starting material remained unchanged. Similar attempts with
9,9-diallyl-2,7-bis(diphenylamino)fluorene, prepared similarly as
described below, in benzene or o-dichlorobenzene resulted in
recovery of the starting material.
9,9-Bis(2-methylallyl)fluorene was prepared by the alkylation
described below in 91%. However, this substance is a colorless
solid, mp 64–66 ꢁC, and not a liquid as reported.19 A solution of
bis(2-methylallyl)fluorene (0.683 g, 2.5 mmol) and p-toluenesul-
fonic acid monohydrate (TsOH, 95 mg, 0.5 mmol) in benzene
(2 mL) was heated at reflux for 2 h. Precipitation with methanol
gave a yellowish solid (0.12 g).
10000
1000
100
10
1
Oligomer3
Oligomer6
0.1
0
5
10
15
Preparation of 2,7-Bis(diphenylamino)fluorene (1). A solu-
tion of 2,7-dibromofluorene (16.2 g, 50 mmol), diphenylamine
(18.6 g, 110 mmol), and sodium t-butoxide (11.5 g, 120 mmol) in
o-xylene (100 mL), to which a prepared solution of palladium(II)
acetate (0.33 mmol) and tri-t-butylphosphine20 in o-xylene was
added as the catalyst, was refluxed at about 120 ꢁC under nitrogen
atmosphere for 6 h. After cooling, extraction with chloroform and
the usual work-up gave a crude material. Recrystallization from
benzene–ethanol (4:1) gave brownish powder of 1 (17.9 g, 71%
yield): mp 233–235 ꢁC; 1H NMR: ꢄ 3.75 (s, 2H), 7.68–7.49 (m,
26H).
2,7-Bis(diphenylamino)-9,9-bis(2-methylallyl)fluorene (2).
Sodium hydride (NaH 60% in oil, 0.96 g, 24 mmol) was washed
repeatedly with hexane. A mixture of NaH, 1 (5.01 g, 10 mmol),
and 2-methylallyl chloride (2.5 mL, 25 mmol) in DMF (30 mL)
was stirred at room temperature (rt) for 6 h. Work-up, including
extraction with benzene and purification by recrystallization
from benzene–ethanol (4:1), afforded a faint purple colored solid
(4.38 g, 72%). Repeated recrystallization gave an analytical sam-
ple of 2: mp 215–217 ꢁC; APCI-MS m=z 609 (M þ 1); 1H NMR: ꢄ
1.10 (s, 6H), 2.63 (s, 4H), 4.28 (s, 2H), 4.48 (s, 2H), 7.0–7.5 (m,
26H); Anal. Found: C, 88.72; H, 6.85; N, 4.43%. Calcd for
C45H40N2: C, 88.78; H, 6.62; N, 4.60%.
Applied voltage/V
Fig. 2. Luminance–applied voltage characteristics for oli-
gomer 3 ( ) and 6 ( ). The device structure is shown
in the footnote c) in Table 1.
was better than 3 because of its lower L300 of 9.5 V and its
higher Lmax of 6300 cd mꢂ2 (see Table 1). This statement is
further supported by efficiencies; oligomer 6 had a luminous
power efficiency ꢂe of 0.78 lm Wꢂ1 and a current efficiency
ꢂc of 2.3 cd Aꢂ1. Both values for 6 are larger than those for
3. Based on the large differences, it should be possible to im-
prove these values by fine tuning the structure via substituents,
molecular weights, terminal groups, and device structures. The
difference between 3 and 6 in EL performance may be under-
stood in terms of the higher content of electroconductive aro-
matic rings in 6; it seems necessary to elucidate not only chem-
ical aspects, such as ꢃ-electron interaction,27 in the solid state
but also physical aspects of the surface phenomena, including
band bending.28
To summarize the present approach to polymers, it is con-
cluded that oligomers like 6 are advantageous due to their high
Tg (167 ꢁC) for the purpose of long-life driving in devices as
well as facile processing of thin-film devices, though the de-
vice performance must be improved. For the molecular design
of thermally stable HTMs, it is noted that oligomers with a low
number22 of the degree of polymerization can bring about
Tg’s > 100 ꢁC for practical use.
Polymerization of 2. A solution of 2 (1.83 g, 3 mmol) and
TsOH (57 mg, 0.3 mmol) in o-dichlorobenzene (10 mL) was heat-
ed at 130 ꢁC for 1 day. The reaction mixture was poured into
methanol (50 mL). The resulting solid was filtered and dissolved
in THF. Reprecipitation with methanol gave a greenish solid of
1
3 (1.79 g): H NMR: ꢄ 1.15–1.30 (br s, 6H), 1.61 (br s, 4H), 2.13
(br s, 2H), 2.33 (br s, 2H), 7.0–7.4 (m, 26H); Anal. Found: C,
89.51; H, 6.17; N, 4.32%. Calcd for (C45H40N2)n: C, 88.78; H,
6.62; N, 4.60%.
Experimental
General. Commercially available reagents including dry sol-
vents were used as received. NMR spectra were recorded on a
200 or 500 MHz spectrometer in CDCl3 using TMS as an internal
standard. GPC analysis using polystyrene standards was carried out
with a dilute solution of the oligomers in THF solvent. DSC analy-
ses were carried out on a Perkin-Elmer DSC 7000 model under ni-
trogen; samples were heated to over 300 ꢁC, cooled rapidly to room
temperature and heated again with heating rate of 10 ꢁC minꢂ1 for
record. CV measurements were carried out on a BAS-50W electro-
chemical analyzer with dichloromethane solutions (10 mL, 0.01 M)
containing tetrabutylammonium perchlorate (0.1 M) under nitro-
Preparation of 9,9-Di-p-bromobenzyl-2,7-bis(diphenyl-
amino)fluorene (4). The alkylation described in 2 was carried
out with p-bromobenzyl bromide (6.25 g, 25 mmol) on 1 (5.01 g,
10 mmol). Purification by recrystallization from benzene–ethanol
(5:1) gave a pinkish white solid of 4: mp 210–212 ꢁC; APCI-
MS m=z 837 (M ꢂ 1, 50%), 839 (M þ 1, 100), 841 (M þ 3, 50);
1H NMR ꢄ 3.15 (s, 4H), 6.50 (d-like, 4H), 7.0–7.55 (m, 30H);
Anal. Found: C, 72.88; H, 4.62; N, 3.27%. Calcd for C51H38-
Br2N2: C, 73.03; H, 4.57; N, 3.34%.
Conversion of 4 into the Corresponding Borate 5. To a so-
lution of dibromide 4 (2.10 g, 2.5 mmol) in THF cooled at ꢂ70 ꢁC,
butyllithium (in hexane 1.6 M, 3.8 mL, 6.1 mmol) was added drop-
wise for 10 min. After the resulting slurry was stirred for 1 h, tri-
methyl borate (0.84 mL, 7.5 mmol) was added, and the mixture
gen atmosphere, and were recorded at a scan rate of 0.2 V sꢂ1
.
The voltammogram in Fig. 1 was plotted after calibration using
ferrocene and recording the oxidation potential, an average of peak
oxidation and reduction potentials vs Ag/AgCl electrode.

422 Bull. Chem. Soc. Jpn. Vol. 80, No. 2 (2007)
Novel Oligomers of 9,9-Dialkylated Fluorenes
was allowed to warm up to rt over 4 h. The reaction mixture was
treated with benzene (100 mL) successive washing with water and
brine, drying over MgSO4, and removal of solvents to give a crude
boric acid. This was admixed with 1,3-propanediol (0.43 mL,
6 mmol) and toluene (100 mL), and refluxed overnight. Workup
and purification by column chromatography on silica gel using
benzene as an eluant gave 5 as a white solid (1.84 g, 86%): mp
140–142 ꢁC; 1H NMR ꢄ 2.40 (quintet, J ¼ 7 Hz, 4H), 3.25 (s, 4H),
4.12 (t, J ¼ 7 Hz, 8H), 6.5–6.7 (m, 4H), 7.0–7.4 (m, 30H); Anal.
Found; C, 80.74; H, 6.06; N, 3.32%. Calcd for C57H50B2N2O4: C,
80.67; H, 5.94; N, 3.30%.
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Polymerization by Suzuki Coupling of 4 and 5. A mixture
of bromide 4 (1.00 g, 1.20 mmol) and boronate 5 (1.02 g, 1.20
mmol), [Pd(PPh3)4] catalyst (41 mg, 0.036 mmol), K2CO3 (0.433
g, 3.6 mmol), and water (6 mL) with toluene (30 mL) was heated
at reflux for 3 days. The resulting reaction mixture was poured
into methanol (100 mL). The solid that formed was collected by
filteration and dried, and redissolved in THF (30 mL). Reprecipi-
1
tation gave 6 as a white powder (1.45 g): H NMR ꢄ 3.22 (br s,
4H), 6.77 (br d, 4H), 6.95–7.30 (m, 30H); Anal. Found: C, 90.09;
H, 5.96; N, 3.94%. Calcd for (C51H38N2)n: C, 90.23; H, 5.64; N
4.13%.
Devices. An ITO-coated glass substrate (sheet resistance is
10 ꢀ sqꢂ1) was cleaned with a UV-ozone chamber. A 50 nm thick
hole-transport layer consisting of the oligomer was formed onto
the ITO anode substrate by spin coating of a chloroform solution.
Then, an Alq3 layer of 50 nm was deposited under a vacuum of
5 ꢄ 10ꢂ7 Torr. Finally, a magnesium–silver alloy (10:1 ratio by
weight, 500 nm) was deposited as a cathode. The emission area
of the device was 4 mm2. The luminance–current–voltage charac-
teristics of the devices were measured with an Advantest R6243
current/voltage measurement unit and a Minolta CS-100 Chroma-
meter under nitrogen atmosphere at room temperature (Fig. 2).
The emission was brilliant green, x ¼ 0:30 and y ¼ 0:53 in the
CIE color coordinate (JIS Z8110).
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This study was supported in part by a Grant-in-Aid for Sci-
entific Research from Ministry of Education, Culture, Sports,
Science and Technology of Japan. We thank Professor Masami
Kamigaito and Dr. Kohtaro Satoh at Nagoya University for
their molecular weight measurements.
23 M. Kimura, S. Kuwano, Y. Sawaki, H. Fujikawa, K. Noda,
Y. Taga, K. Takagi, J. Mater. Chem. 2005, 15, 2393; See also: M.
Kimura, S. Inoue, K. Shimada, S. Tokito, K. Noda, Y. Taga, Y.
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Y. Taga, M. Kimura, Y. Sawaki, Appl. Phys. Chem. 2000, 77, 160;
S. Tokito, K. Noda, K. Shimada, S. Inoue, M. Kimura, Y. Sawaki,
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