High Tg blue emitting materials for electroluminescent devices

Article abstract of DOI:10.1039/b501819f

A series of 2′,7′-di-tert-butyl-9,9′-spirobifluorene derivatives (flu) incorporating arylamines at the 2- and/or 7-positions were synthesized. These compounds possess a high glass transition temperature ranging from 135 to 215°C. Incorporation of spirobifluorene was found to be beneficial for raising the glass transition temperature (T_g) of the molecules. Most of the compounds are blue emitting with good solution quantum yields. Blue-emitting double-layer devices with narrow full width at half-maximum (fwhm < 68 nm) were constructed using these materials as the hole-transporting and emitting layer, and TPBI (1,3,5-tris(N-phenylbenzimidazol- 2-yl)benzene) as the electron-transporting layer (device I). In double-layer devices of the configuration ITO/flu (40 nm)/Alq₃ (40 nm)/Mg : Ag (Alq₃ = tris(8-hydroxyquinoline)aluminium) (device II), flu functioned mainly as a hole-transporting layer, and the green light characteristic of Alq₃ was detected. Two of the devices of type I have very promising performance, and one of them emits pure blue light (CIE (x,y) = (0.15, 0.09)). One of the compounds was found to be an efficient hole-injection material. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b501819f

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
High Tg blue emitting materials for electroluminescent devices{
Jiun Yi Shen,a Chung Ying Lee,ac Tai-Hsiang Huang,a Jiann T. Lin,*ab Yu-Tai Tao,*a Chin-Hsiung Chiena
and Chiitang Tsaic
Received 5th February 2005, Accepted 27th April 2005
First published as an Advance Article on the web 12th May 2005
DOI: 10.1039/b501819f
A series of 29,79-di-tert-butyl-9,99-spirobifluorene derivatives (flu) incorporating arylamines at the
2- and/or 7-positions were synthesized. These compounds possess a high glass transition
temperature ranging from 135 to 215 uC. Incorporation of spirobifluorene was found to be
beneficial for raising the glass transition temperature (Tg) of the molecules. Most of the
compounds are blue emitting with good solution quantum yields. Blue-emitting double-layer
devices with narrow full width at half-maximum (fwhm , 68 nm) were constructed using these
materials as the hole-transporting and emitting layer, and TPBI (1,3,5-tris(N-phenylbenzimidazol-
2-yl)benzene) as the electron-transporting layer (device I). In double-layer devices of the
configuration ITO/flu (40 nm)/Alq3 (40 nm)/Mg : Ag (Alq3 5 tris(8-hydroxyquinoline)aluminium)
(device II), flu functioned mainly as a hole-transporting layer, and the green light characteristic of
Alq3 was detected. Two of the devices of type I have very promising performance, and one of
them emits pure blue light (CIE (x,y) 5 (0.15, 0.09)). One of the compounds was found to be an
efficient hole-injection material.
devices. Herein, we report the synthesis and characterization of
Introduction
new blue-emitting materials. Devices fabricated from these
The pioneering work on small molecule-based and polymer-
based organic light-emitting diodes (OLEDs) by Kodak1
mance of multilayered OLEDs can be achieved through proper
materials will also be discussed. Optimization of the perfor-
and Holmes’s group2 respectively, stimulated numerous
choice of hole- and electron-transport materials so that the two
researchers to jump into this fast-growing area. Following
carriers have balanced transport speed and the excitons are
considerable progress in the past decade, more and more
confined efficiently within the emitting layer.
products have been commercialized. Low molecular weight
small molecules are generally vacuum-deposited as thin films
Experimental
in device fabrication. Therefore, film-forming properties and
their temporal stability are prerequisite to the performance and
longevity of devices. Amorphous materials possessing high
glass transition temperatures (Tg) should be beneficial for
forming glasses and avoiding crystal formation which leads to
grain boundary problems. A well known class of amorphous
materials is the starburst compounds which have nonplanar
molecular structure and exhibit a number of conformations.3
Another novel class of amorphous materials is the spiro-linked
compounds developed by Salbeck’s group.4 Intramolecular
bond rotation and vibration in the spiro-linked unit are not
possible and an increase in Tg is anticipated. The 9,99-spirobi-
fluorene unit appears to be a very promising building block for
the construction of high Tg blue-emitting materials for high-
performance blue electroluminescent devices.5 New blue-
emitting materials continue to receive considerable attention,
not only for use as a blue light source in their own right but
also as a host for downhill energy transfer to green- or red-
emitting materials. Therefore, we set out to develop high Tg,
blue-emitting materials using the 9,99-spirobifluorene unit as a
building block for efficient blue-emitting electroluminescent
General information
Unless otherwise specified, all the reactions were carried out
under nitrogen atmosphere using standard Schlenk techniques.
Solvents were dried by standard procedures. All column
chromatography was performed with the use of silica gel (230–
400 mesh, Macherey-Nagel GmbH & Co.) as the stationary
phase. The 1H NMR spectra were recorded on a Bruker
AC400 spectrometer. Electronic absorption spectra were
measured in dichloromethane using a Cary 50 Probe UV-
visible spectrophotometer. Emission spectra were recorded by
a Hitachi F-4500 fluorescence spectrometer. Emission quan-
tum yields were measured by standard methods6 with reference
to Coumarin 1 in EA (ethyl acetate). Cyclic voltammetry
experiments were performed with a BAS-100 electrochemical
analyzer. All measurements were carried out at room
temperature with a conventional three-electrode configuration
consisting of platinum working and auxiliary electrodes and a
nonaqueous Ag/AgNO3 reference electrode. The E1/2 values
a
c
a
c
were determined as 1/2(Ep + Ep ), where Ep and Ep are the
anodic and cathodic peak potentials, respectively. The solvent
in all experiments was CH2Cl2 and the supporting electrolyte
was 0.1 M tetrabutylammonium perchlorate. DSC measure-
ments were carried out using a Perkin-Elmer 7 series thermal
analyzer at a heating rate of 10 uC min21. TGA measurements
{ Electronic supplementary information (ESI) available: 1H NMR,
FAB MS and elemental analysis data for other flu compounds. See
http://www.rsc.org/suppdata/jm/b5/b501819f/
*jtlin@chem.sinica.edu.tw (Jiann T. Lin)
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were performed on a Perkin-Elmer TGA7 thermal analyzer.
Mass spectra (FAB) were recorded on a Hewlett-Packard
GC/MS 5995 spectrometer. Elementary analyses were
2 H, meta-C6H5), 7.02 (t, J 5 8,3 Hz, 1 H, H-7 of
spirobifluorene ), 7.93 (d, J 5 7.7 Hz, 2 H, ortho-C6H5),
6.76–6.73 (m, 3 H, para-C6H5, H-19 and H-89 of spirobifluor-
ene ), 6.60 (d, J 5 7.5 Hz, 1 H, H-8 of spirobifluorene), 6.34 (d,
J 5 1.9 Hz, 1 H, H-1 of spirobifluorene), 1.16 (s, 18 H, CH3).
Anal. Calcd for C39H37N: C, 90.13; H, 7.18; N, 2.70. Found:
C, 89.66; H, 7.31; N, 2.76%.
performed on
a
Perkin-Elmer 2400 CHN analyzer.
2-Bromo-(29,79-di-tert-butyl)-9,99-spirobifluorene,5d 6-bromo-
9-ethyl-9H-carbazole-3-carbonitrile7a and 5-[N-(3-methylphenyl)-
N-phenylamino]-2-(tri-n-butylstannyl)-thiophene7b were prepared
according to the published procedures.
Compounds bis[(29,79-di-tert-butyl)-9,99-spirobifluoren-2-yl]-
phenyl-amine (2), [(29,79-di-tert-butyl)-9,99-spirobifluoren-2-yl]-
{4-[5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2-yl]-phenyl}-phenyl-
amine (3), 6-(N-[(29,79-di-tert-butyl)-9,99-spirobifluoren-2-yl]-
N-phenylamino)-9-ethyl-9H-carbazole-3-carbonitrile (4), and
6-(N-[(29,79-di-tert-butyl)-9,99-spirobifluoren-2-yl]-N-(naphthalen-
1-yl)amino)-9-ethyl-9H-carbazole-3-carbonitrile (5) were synthe-
sized by similar procedures, and only the preparation
of compound 2 will be described in detail. The data for
4 and 5 are deposited as electronic supplementary informa-
tion (ESI).{
[(29,79-Di-tert-butyl)-9,99-spirobifluoren-2-yl]-phenyl-amine
(1). 2-Bromo-(29,79-di-tert-butyl)-9,99-spirobifluorene (2.53 g,
5.0 mmol), aniline (0.61 g, 6.5 mmol), Pd(OAc)2 (23 mg,
0.1 mmol), P(tBu)3 (20 mg, 0.1 mmol), sodium tert-butoxide
(0.72 g, 7.5 mmol) and toluene (50 mL) were charged in a two-
necked flask kept under nitrogen. The mixture was heated to
reflux for 12 h. After cooling, it was quenched with 5 mL of
water. The solvent was removed under vacuum and the residue
was extracted with dichloromethane–water. The organic layer
was dried over MgSO4 and filtered. Evaporation of the solvent
left a brown residue that was chromatographed through silica
Compound 2. Compound 1 (0.52 g, 1.0 mmol), 2-bromo-
(29,79-di-tert-butyl)-9,99-spirobifluorene (0.51 g, 1.0 mmol),
Pd(OAc)2 (5 mg, 0.02 mmol), P(tBu)3 (4 mg, 0.02 mmol),
sodium tert-butoxide (0.14 g, 1.5 mmol) and toluene (20 mL)
were charged in a two-necked flask kept under nitrogen.
The mixture was heated to reflux for 12 h. After cooling,
it was quenched with 2 mL of water. The solvent was
removed under vacuum and the residue was extracted with
gel using
a dichloromethane–hexane mixture as eluent.
Compound 1 was obtained as a white solid in 85% yield.
FAB MS: m/z 519 (M+). 1H NMR (acetone-d6): d 7.87–7.79
(m, 4 H, H-4, H-5, H-49 and H-59 of spirobifluorene), 7.44 (dd,
J 5 8.1, 1.8 Hz, 2 H, H-39 and H-69 of spirobifluorene), 7.35–
7.31 (m, 2 H, N-H and H-6 of spirobifluorene), 7.18 (dd,
J 5 8.2, 2.0 Hz, 1 H, H-3 of spirobifluorene), 7.08 (t, J 5 7.4 Hz,
Fig. 1 Structures of the compounds.
2456 | J. Mater. Chem., 2005, 15, 2455–2463
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dichloromethane–water. The organic layer was dried over
MgSO4 and filtered. Evaporation of the solvent left a deep-
brown residue that was chromatographed through silica
C72H67N: C, 91.38; H, 7.14; N, 1.48. Found: C, 91.07; H,
7.45; N, 1.56%.
gel using
a
dichloromethane–hexane mixture as eluent.
Compound 3. White solid. Yield: 75%. FAB MS: m/z 796
1
Compound 2 was obtained as a white solid in 80% yield.
FAB MS: m/z 945 (M+). 1H NMR (acetone-d6): d 7.85 (d,
J 5 7.5 Hz, 2 H, H-5 of spirobifluorene), 7.78 (d, J 5 8.0 Hz,
2 H, H-4 of spirobifluorene), 7.69 (d, J 5 7.8 Hz, 4 H, H-49
and H-59 of spirobifluorene), 7.38–7.33 (m, 6 H, H-6, H-39 and
H-69 of spirobifluorene), 7.06 (t, J 5 6.8 Hz, 2 H, H-7 of
spirobifluorene), 6.93 (d, J 5 8.2 Hz, 2 H, H-3 of spirobi-
fluorene), 6.86 (t, J 5 7.2 Hz, 2 H, meta-C6H5), 6.71–6.65 (m,
7 H, H-19, H-89, para-C6H5 and ortho-C6H5), 6.69 (d, J 5
7.6 Hz, 2 H, H-8 of spirobifluorene), 6.33 (s, 2 H, H-1 of
spirobifluorene ), 1.16 (s, 36 H, CH3). Anal. Calcd for
(M + H)+. H NMR (acetone-d6): d 8.04–7.96 (m, 4 H, H-4,
H-5, H-49 and H-59 of spirobifluorene), 7.85 (d, J 5 6.9 Hz,
2 H, C6H4), 7.75 (d, J 5 8.4 Hz, 2 H, C6H4), 7.65 (d, J 5 6.9 Hz,
2 H, C6H4), 7.43–7.40 (m, 3 H, H-6, H-39 and H-69 of
spirobifluorene), 7.25 (t, J 5 8.0 Hz, 2 H, meta-C6H5), 7.19
(dd, J 5 8.1, 2.0 Hz, 2 H, ortho-C6H5), 7.12–7.04 (m, 3 H, H-3,
H-7 of spirobifluorene and para-C6H5), 6.99 (d, J 5 8.4 Hz,
2 H, C6H4), 6.80 (d, J 5 1.6 Hz, 2 H, H-19 and H-89 of
spirobifluorene), 6.65 (d, J 5 7.5 Hz, 1 H, H-8 of spirobi-
fluorene), 6.51 (d, J 5 1.8 Hz, 1 H, H-1 of spirobifluorene),
1.37 (s, 9 H, CH3), 1.20 (s, 18 H, CH3). Anal. Calcd for
Scheme 1 Reagents and conditions: (i) 2 mol% Pd(OAc)2, 2 mol% P(tBu)3, NatBuO; (ii) 2 mol% PdCl2(PPh3)2, DMF.
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C57H53N3O: C, 86.00; H, 6.71; N, 5.28. Found: C, 85.74; H,
6.67; N, 5.17%.
d 7.81 (d, J 5 8.2 Hz, 2 H, H-49 of spirobifluorene), 7.63 (d,
J 5 8.1 Hz, 2 H, H-4 of spirobifluorene), 7.35 (dd, J 5 8.1,
1.9 Hz, 2 H, H-39 of spirobifluorene), 7.11 (t, J 5 7.5 Hz , 4 H,
meta-C6H5), 7.00–6.90 (m, 4 H, H-3 of spirobifluorene and
C6H4CH3), 6.91–6.85 (m, 8 H, ortho-C6H5, para-C6H5 and
C6H4CH3), 6.73 (d, J 5 8.1 Hz, 4 H, H-19 of spirobifluorene
and C6H4CH3), 6.66 (d, J 5 8.1 Hz, 2 H, C6H4CH3), 6.40 (d,
J 5 2.1 Hz, 2 H, H-1 of spirobifluorene), 2.05 (s , 6 H, CH3),
1.20 (s, 18 H, CH3). Anal. Calcd for C59H54N2: C, 89.58; H,
6.88; N, 3.54. Found: C, 89.85; H, 7.18; N, 3.54%.
Compounds
4,49-bis{N-[(29,79-di-tert-butyl)-9,99-spirobi-
fluoren-2-yl]-phenylamino}biphenyl (6), 2,7-bis{N-[(29,79-di-
tert-butyl)-9,99-spirobifluoren-2-yl]-phenylamino}-(29,79-di-tert-
butyl)-9,99-spirobifluorene (7), and 2,6-di-tert-butyl-9,10-bis-
{4-[(29,79-di-tert-butyl)-9,99-spirobifluoren-2-yl]amino-phenyl}-
anthracene (8) were synthesized by a similar procedure as
described for compound 2, except that only 0.5 equivalent of
dibromo compounds was used in the reaction. The data for
6–8 are deposited as ESI.{
N,N9-Diphenyl-[(29,79-di-tert-butyl)-9,99-spirobifluorene]-2,7-
diamine (13). Compound 13 was prepared by a similar
procedure as described for compound 1 except that 2,7-
dibromo-(29,79-di-tert-butyl)-9,99-spirobifluorene and Pd(dba)2
were used instead of 2-bromo-(29,79-di-tert-butyl)-9,99-spiro-
bifluorene and Pd(OAc)2, respectively, and the reaction time
was 48 h. Compound 13 was obtained as a grey solid in 66%
Compounds 2,7-bis[(N-phenyl)-3-tolylamino]-(29,79-di-tert-
butyl)-9,99-spirobifluorene (9), 2,7-di-carbazolyl-(29,79-di-
tert-butyl)-9,99-spirobifluorene (10), 2,7-bis[(N-naphthalenyl)-
phenylamino]-(29,79-di-tert-butyl)-9,99-spirobifluorene (11), and
2,7-bis[(N-phenyl)-4-cyanophenylamino]-(29,79-di-tert-butyl)-9,99-
spirobifluorene (12) were synthesized by a similar procedure,
and only the synthesis of compound 9 is described. The data
for 10–12 are deposited as ESI.{
1
yield. FAB MS: m/z 610 (M+). H NMR (acetone-d6): d 7.78
(d, J 5 8.1 Hz, 2 H, H-4 of spirobifluorene), 7.72 (d, J 5
8.0 Hz, 2 H, H-49 of spirobifluorene), 7.42 (d, J 5 8.1 Hz,
2 H, H-39 of spirobifluorene), 7.26 (s, 2 H, NH), 7.16 (d,
J 5 8.2 Hz, 2 H, H-3 of spirobifluorene), 7.07 (t, J 5 7.5 Hz,
4 H, meta-C6H5), 6.92 (d, J 5 8.6 Hz, 4 H, ortho-C6H5),
6.81 (s, 2 H, H-19 of spirobifluorene), 6.71 (t, J 5 6.9 Hz,
2 H, para-C6H5), 6.34 (s, 2 H, H-1 of spirobifluorene), 1.17
(s , 18 H, CH3). Anal. Calcd for C45H42N2: C, 88.48; H,
6.93; N, 4.59. Found: C, 88.47; H, 6.85; N, 4.60%.
Compound 9. 2,7-Dibromo-(29,79-di-tert-butyl)-9,99-spirobi-
fluorene (0.88 g, 1.5 mmol), phenyl-m-tolyl-amine (0.55 g,
3.0 mmol), Pd(OAc)2 (15 mg, 0.06 mmol), P(tBu)3 (12 mg,
0.06 mmol), sodium tert-butoxide (0.43 g, 4.5 mmol) and
toluene 30 mL were charged in a two-necked flask kept under
nitrogen. The mixture was heated to reflux for 48 h. After
cooling, the solvent was removed under vacuum and the
residue was extracted with dichloromethane–water. The
organic layer was dried over MgSO4 and filtered. The residue
obtained after evaporation of the solvent was chromato-
graphed through silica gel using a dichloromethane–hexane
mixture as eluent. Compound 9 was obtained as a white solid in
80% yield. FAB MS: m/z 792 (M + H)+. 1H NMR (acetone-d6):
Compounds 2,7-bis[(N-phenyl)-pyrenylamino]-(29,79-di-tert-
butyl)-9,99-spirobifluorene (14), 2,7-bis[N-(6-cyano-9-ethyl-9H-
carbazol-3-yl)-phenylamino]-(29,79-di-tert-butyl)-9,99-spirobifluor-
ene (15), and 2,7-bis{(4-[5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2-
yl]-phenyl)phenylamino}-(29,79-di-tert-butyl)-9,99-spirobifluorene
Table 1 Physical data for the compounds
Compd.
Tga/uC
Tdb/uC
lmaxc/nm
lem (wf)d/nm
Eox (DEp)e/mV
HOMO, LUMOf/eV
2
3
4
5
6
7
8
9
10
11
12
14
15
16
17
NPB
ITO
TPBI
Alq3
Mg : Ag
a
145
146
154
178
205
210
215
na
405
378
414
440
480
430
470
430
450
421
407
400
442
430
410
na
366, 315, 345, 303
373, 315, 345, 303
353, 315, 303
350, 328, 315, 302
371, 346, 315, 303
394, 358, 315, 302
396, 354, 315, 303
378, 304
343, 335, 314
379, 350, 315
371; 335, 316
415; 392, 362; 336
375, 312
414 (0.73)
469 (0.49)
452 (0.11)
463 (0.10)
423 (0.82)
422 (0.65)
488 (0.48)
402 (0.64)
368 (0.68)
480 (0.10)
464 (0.27)
530 (0.13)
469 (0.09)
502 (0.51)
476 (0.25)
450
535 (100)
712 (95)
5.15, 2.06
5.33, 2.44
5.10, 2.00
5.12, 2.05
5.05, 2.08
4.96, 2.04
5.21, 2.42
4.95, 1.84
5.28, 1.93
5.31, 2.32
5.26, 2.23
5.04, 2.39
4.93, 1.98
5.12, 2.22
4.99, 2.38
5.23, 2.20
4.70 (EF), na
6.20, 2.70
6.09, 2.95
na, 3.70 (EF)
b
523 (100), 1147 (i)
535 (110), 1170 (i)
434 (98), 675 (90)
340 (100), 652 (95)
595 (130), 987 (154)
412 (165), 742 (120)
760 (93), 1119 (210)
477 (91), 804 (90)
722 (131), 901 (123)
472 (120), 780 (116)
391 (86), 668 (90)
601 (102), 832 (105)
453 (109)
na
135
139, 194 (Tc), 287 (Tm)
185
na
180
na
103
387, 315
406, 314
342, 271
+340 (66)
129
177
na
428
304
385
376
510 (0.15)
+1300 (i)
na
Obtained from DSC measurements, Tg: glass transition temperature; Tc: crystallization temperature; Tm: melting temperature. Obtained
c
d
e
from TGA measurements. Measured in CH2Cl2 solution. Measured in CH2Cl2 solution; wf fluorescence quantum yield. Measured in
CH2Cl2. All Eox data are reported relative to Ag/Ag+. The concentration of the compounds used in this experiment was 2.5 6 1024 M and the
scan rate was 100 mV s21. i: irreversible. HOMO energy was calculated with reference to ferrocene (4.8 eV). Solvent to vacuum correction
f
was not applied. Band gap was derived from the observed optical edge and LUMO energy was derived from the relation, Band gap 5
HOMO 2 LUMO. na: not observed or not available.
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Fig. 3 Absorption spectra of compounds 3,and 8 in dichloromethane
solution.
H-4 of spirobifluorene), 7.69 (d, J 5 8.0 Hz, 2 H, H-49 of
spirobifluorene), 7.49 (d , J 5 8.0 Hz, 2 H, H-39 of spirobi-
fluorene), 7.36 (dd , J 5 8.4, 1.8 Hz, 2 H, H-3 of spirobi-
fluorene), 7.19 (t, J 5 7.3 Hz, 4 H, meta-C6H5), 7.11–7.05
(m, 6 H, spirobifluorene and C6H5 ), 6.97 (t, J 5 3.1 Hz,
2 H, C6H4), 6.91–6.88 (m, 6 H, spirobifluorene, C6H5, and
thiophene), 6.81–6.80 (m, 4 H, H-19 of spirobifluorene,
thiophene), 6.66 (d, J 5 1.3 Hz, 2 H, C6H4), 6.52 (d, J 5
2.8 Hz, 2 H, H-1 of spirobifluorene), 1.5 (s, 6 H, CH3), 1.41
(s, 18 H, CH3). Anal. Calcd for C67H58N2S2: C, 84.24; H,
6.12; N, 2.93. Found: C, 84.02; H, 5.93; N, 2.88%.
LED fabrication and measurement
Electron-transporting materials TPBI (1,3,5-tris(N-phenyl-
bezimidazol-2-yl)benzene) and Alq3 (tris(8-hydroxyquinoline)-
aluminium) were synthesized according to literature
procedures,8 and were sublimed twice prior to use.
Prepatterned ITO substrates with an effective individual device
area of 3.14 mm2 were cleaned as described in a previous
report.9 Double-layer EL devices using compounds 2–8 as the
hole-transport and emitting layer and TPBI or Alq3 as the
electron-transport layer were fabricated. The devices were
prepared by vacuum deposition of 40 nm of the hole-
transporting layer, followed by 40 nm of TPBI or Alq3. An
alloy of magnesium and silver (ca. 10 : 1, 50 nm) was deposited
as the cathode, which was capped with 100 nm of silver. Three-
layer devices using compound 9 as the hole-injection layer,
compound 12 or 16 as the hole-transporting and emitting
layer, and TPBI as the electron-transport layer was also
fabricated. I–V curves were measured on a Keithley 2400
Source Meter in ambient environment. Light intensity was
measured with a Newport 1835 Optical Meter.
Fig. 2 Cyclic voltammograms of compounds
6 (a) and 8 (b).
Ferrocene was added to the solution as an internal standard. All
potentials are in volts vs. Ag/AgNO3 (0.01 M in CH3CN; the scan rate
is 80 mV s21).
(16) were synthesized by a similar procedure as described for
compound 2. Instead of compound 1, compound 13 was
used in the coupling reaction with appropriate aryl halides,
and Pd(dba)2 was used as the catalyst instead of Pd(OAc)2.
The data for 14–16 are deposited as ESI.{
2,7-Bis(5-[N-phenyl-N-(m-tolyl)amino]-2-thienyl-(29,79-di-
tert-butyl)-9,99-spirobifluorene (17). To a mixture of 5-[N-(3-
methylphenyl)-N-phenylamino]-2-(tri-n-butylstannyl)thiophene
(0.72 g, 1.3 mmol, 90% purity), 2,7-dibromo-(29,79-di-tert-
butyl)-9,99-spirobifluorene (0.30 g, 0.5 mmol) and PdCl2(PPh3)2
(14 mg, 0.02 mmol) was added 4 mL of dry dimethylform-
amide (DMF). The solution was slowly warmed to 80 uC
and stirred for 16 h. After cooling, methanol (5 mL) was
added to precipitate the product. The solid was filtered,
washed with methanol (2 6 5mL), and then chromato-
graphed using dichloromethane–hexane (1 : 4) as eluent.
Yellow powdery 17 was isolated in 23% yield. FAB MS: m/z
954 (M+). 1H NMR (CDCl3): d 7.74 (d , J 5 8.0 Hz, 2 H,
Results and discussion
Syntheses
The compounds synthesized in this study are illustrated in
Fig. 1. Palladium catalyzed aromatic C–N coupling10 of
[(29,79-di-tert-butyl)-9,99-spirobifluoren-2-yl]-phenyl-amine (1)
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with mono- and di-bromo aromatic compounds provided 2–4
and 6–8, respectively. Coupling of 2-bromo-(29,79-di-tert-
butyl)-9,99-spirobifluorene with appropriate secondary amine
led to formation of 5. Similarly, reaction of 2,7-dibromo-
(29,79-di-tert-butyl)-9,99-spirobifluorene with amines resulted
in formation of 9–12, and the reaction of N,N9-diphenyl-
(29,79-di-tert-butyl)-9,99-spirobifluorene-2,7-diamine (13) with
bromo aromatic compounds gave 14–16. Double thienylation
of 2,7-dibromo-(29,79-di-tert-butyl)-9,99-spirobifluorene with
5-[N-(3-methylphenyl)-N-phenylamino]-2-(tri-n-butylstannyl)-
thiophene, using Stille’s cross-coupling reaction, yielded com-
pound 17. Representative examples are outlined in Scheme 1.
state, which persisted in the subsequent heating cycles.
Compound 12 exhibited sequential glass transition, crystal-
lization, and melting behavior upon repetitive heating and
cooling cycles. In contrast, neither Tm nor Tg (glass transition
temperature) was detected for compound 15. Compounds 9,
10, and 17 melt at 332, 355 and 234 uC, respectively. However,
no glass transition, crystallization, and melting were noticed
for these compounds after fast cooling of the melt. As
expected, incorporation of spirobifluorene is beneficial
to raising Tg of the molecules. For instance, replacement of
both 1-naphthyl units in 2,6-di-tert-butyl-9,10-bis{4-(1-
naphthylphenylamino)phenyl}anthracene (Tg 5 138 uC)11
and 1,4-bis(1-naphthylphenylamino)-biphenyl (a-NPD, Tg
5
100 uC)12 by spirobifluorene increases the Tg by 77 uC (8,
Tg 5 215 uC) and 105 uC (6, Tg 5 205 uC), respectively.
Comparison of Tgs among the compounds 4 (154 uC), 9-ethyl-
6-(naphthalen-1-yl-phenyl-amino)-9H-carbazole-3-carbonitrile
(95 uC),7a 9-ethyl-6-[(9-ethyl-9H-carbazol-3-yl)-phenyl-amino]-
9H-carbazole-3-carbonitrile (115 uC),7a and 9-ethyl-6-(phenyl-
pyren-1-yl-amino)-9H- carbazole-3-carbonitrile (132 uC)7a
indicates that the order of effectiveness in raising Tg is
2-spirobifluorenyl . 1-pyrenyl . 3-carbazolyl . 1-naphthyl.
These observations can be attributed to the more restricted
Thermal properties
The thermal properties of the new compounds were deter-
mined by DSC and TGA measurements (Table 1). Compounds
2, 5, 6, and 7 exhibited a glass transition in the first heating
cycle and no crystallization exotherm and melting endotherm
were noticed. Compounds 3, 4, 8, 11, 14 and 16 exhibited
melting isotherms (Tm: 3, 273 uC; 4, 310 uC; 8, 352 uC; 11,
322 uC; 14, 328 uC; 16, 345 uC) during the first heating cycle,
but rapid cooling of the melt led to the formation of a glassy
Table 2 Electroluminescent data of the devicesa
2
3
4
5
6
7
8
12
16
Von,/V
3.5
2.5
3.3
3.0
4.7
4.2
3.3
3.9
2.8
2.5
2.7
3.3
2.8
3.4
3.8
5.0
3.4
3.0
3.3
2.8
3853 (11)
16207 (15) 3064 (12.5) 6762 (13)
Lmax/cd m22
(V at Lmax/V)
1805 (11);
21637 (12) 15262 (14)
8390 (13.5) 5522 (13.5) 7395 (15)
25749 (15)
6327 (12)
32175 (15) 21032 (12) 18157 (12)
4429 (13.5) 24247 (15) 1336 (11)
8119(13.5)
440
516
7208 (11.5)
460
510
lem/nm
410
520
450
518
458
524
458
524
422
518
424
518
472
518
446
462
CIE (x,y)
fwhm/nm
gext,max (%)
0.16, 0.06
0.29, 0.54
0.15, 0.09
0.27, 0.44
0.15, 0.13
0.31, 0.55
0.15, 0.13
0.31, 0.55
0.17, 0.06
0.28, 0.54
0.16, 0.05
0.29, 0.53
0.14, 0.22
0.29, 0.52
0.15, 0.08
0.25, 0.41
0.15, 0.10
62
116
66
1.2
0.47
2.3
0.48
0.46
1.2
0.88
1.3
0.15, 0.16
0.24, 0.40
0.15, 0.17
68
122
68
1.2
0.54
2.4
1.2
0.86
2.2
1.7
1.4
48
98
62
110
62
96
64
98
52
94
48
98
60
92
1.1
0.86
4.2
1.3
1.4
1.4
1.1
1.4
2.0
1.1
1.6
0.69
3.4
1.0
g
p,max/lm W21 0.35
1.9
2.1
2.0
0.61
1.6
0.74
4.5
0.71
2.2
0.61
1.4
2.4
1.7
g
c,max/cd A21
0.44
2.7
3.6
3.5
1.5
4.4
1.2
2.0
1.0
3.5
0.74
2.2
5.3
3.1
2.0
725
1250
1684
1.0
3.4
L/cd m22
gext (%) *
*
416
2620
2893
3452
1418
4368
1066
4430
849
2863
607
2065
4661
2947
1404
1410
2833
1.1
0.54
2.1
0.67
0.59
1.3
1.4
1.4
1.1
0.83
3.4
1.3
1.3
1.4
1.0
1.4
1.7
0.88
1.3
0.65
3.0
0.97
0.47
2.0
gp/lm W21
*
0.23
1.5
1.1
1.3
0.49
1.3
0.46
1.7
0.49
1.5
0.35
1.2
1.5
1.0
0.30
0.44
0.69
0.73
1.3
gc/cd A21
*
0.42
2.6
2.9
3.5
1.4
4.4
1.1
4.4
0.85
2.8
0.62
2.1
4.7
2.9
1.7
2.9
a
The first, second and third values are for the devices I (2–8, 12 and 16), II (2–8, 12 and 16) and III (12 and 16), respectively. Lmax, maximum
luminance; L, luminance; Von, turn-on voltage; V, voltage; gext,max, maximum external quantum efficiency; gp,max, maximum power efficiency;
g
c,max, maximum current efficiency; gext, external quantum efficiency; gp, power efficiency; gc, current efficiency; fwhm, full width at half
maximum. *, at a current density of 100 mA cm22. Von was obtained from the x-intercept of log(luminance) vs. applied voltage plot.
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motion4 and the higher molecular mass13 of the spirobi-
fluorene unit.
(vide supra) and by comparison with ferrocene (4.8 eV).16
These data together with absorption spectra were then used
to obtain the LUMO (lowest unoccupied molecular orbital)
energy levels (Table 1).17 According to the HOMO and
LUMO energy levels obtained, the compounds in this
study (flu) appear to be appropriate as hole-transporting and
emitting materials.
Electrochemical properties
Electrochemical properties of the new compounds were
investigated by cyclic voltammetry, and the redox potentials
of the compounds are collected in Table 1. The cyclic
voltammograms for selected compounds are shown in Fig. 2.
The arylamines connected to the 9,99-spirobifluorene unit
exhibit quasi-reversible oxidation waves. The presence of an
Compounds with higher solution quantum yields were
subjected to fabrication of two types of double-layer devices
using TPBI (1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene) or
Alq3 (tris(8-hydroxyquinoline)aluminium) as the electron-
transport materials: (I) ITO/flu (40 nm)/TPBI (40 nm)/Mg :
Ag; (II) ITO/flu (40 nm)/Alq3 (40 nm)/Mg : Ag. The
performance parameters of all devices are collected in
Table 2. The current–voltage–luminance (I–V–L) charac-
teristics are shown in Fig. 4 and Fig. 5. Fig. 6 shows the EL
spectra of the devices I and II. Blue light is emitted from flu
in device I. Apparently TPBI (HOMO 5 6.2 eV) effectively
serves as a hole blocker due the large HOMO energy gap (0.9–
1.2 eV) between TPBI and flu. The relatively smaller LUMO
energy gap (0.3–0.7 eV) between the electron-transporting
layer (ETL) and the hole-transporting layer (HTL) also
electron-withdrawing oxadiazole unit in
3 results in a
significant anodic shift of the potential. The two amines in 6,
7, and 9–16 are electronically coupled and two quasi-reversible
one-electron redox processes (supported by Osteryoung
square-wave voltammetry) were observed. Such an electronic
coupling significantly lowers the first oxidation potential of the
diamine compounds when compared to their monoamine
analogues: 16 (601 mV) vs. 3 (712 mV); 15 (391 mV) vs. 4
(523 mV). The biphenyl entity between the two amines is more
planar in 7 than in 6 due to the constraint of the fluorene in the
former, and the electronic coupling between the two amines in
7 is expected to be better than that in 6. Consequently, the 7/7+
couple occurs at a lower potential than the 6/6+ couple.
Moreover, the difference of the oxidation potential between
the 7+/72+ couple and the 7/7+ couple is larger than that
between the 6+/62+ couple and the 6/6+ couple. In contrast, the
two amines in compounds 8 and 17 are oxidized simulta-
neously due to lack of communication between the two
amines. Such an outcome can be attributed to the non-
planarity and/or long length of the spacer between amines. The
second one-electron oxidation wave in 8 may be due to the
formation of diphenylanthracene cation.11,14 The irreversible
oxidation wave observed at .1.1 eV for 4 and 5 is attributable
to the oxidation of the carbazolyl moiety.12 It is interesting
that interposition of the electron-rich thienyl units15 in 17 not
only lowers the oxidation potential of the amines, but also
blocks the electronic communication of the two amines.
Optical properties
The absorption and luminescence data of the compounds are
presented in Table 1. Representative absorption spectra are
shown in Fig. 3. The absorptions that fall within lmax # 440–
300 nm are attributed to nAp* and pAp* transitions. The
compounds are moderately to strongly emissive in dichloro-
methane or toluene. Except for 10, the emission colors vary
from purple blue to blue green. In general the absorption and
emission wavelengths for each compound are not very sensitive
to the solvent polarity, and the variation between toluene and
CH2Cl2 is within 5 nm, indicating low polar character of the
compounds. Only compounds possessing a strong donor
(amine) and acceptor (oxadiazole), 3 (422 nm in toluene;
469 nm in CH2Cl2) and 16 (441 nm in toluene; 502 nm in
CH2Cl2), exhibit a larger solvatochromic effect in emission.
Electroluminescent properties
The HOMO (highest occupied molecular orbital) energy levels
of the compounds were calculated from cyclic voltammetry
Fig. 4 Current density vs. applied electric voltage characteristics of
the device (a) I; (b) II.
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Fig. 5 Luminance vs. current density characteristics of the device (a)
I; (b) II.
Fig. 6 EL spectra of devices I and II for selected compounds.
facilitates injection of the electrons from TPBI into the flu
layer. These devices exhibit very narrow fwhm (full width at
half-maximum), ranging from 48–69 nm. The devices of 3 and
8 appear to have very impressive performance compared to
competitive non-doping blue-emitting OLEDs of different
device structures reported recently.5a,b,18 It is also worth noting
that the device of 3 has pure blue emission. In devices II, the
HOMO energy gap (0.7–1.0 eV) between flu and Alq3
(HOMO 5 6.00 eV) is not sufficiently large to block the holes
from flu. On the other hand, the large LUMO energy gap (0.9–
1.3 eV) between Alq3 (LUMO 5 3.30 eV) and flu disfavors
injection of the electron from Alq3 to the flu layer. Conse-
quently, most of devices II emit the characteristic green light of
Alq3. Contamination of Alq3 green light with flu emission was
observed in the devices of compounds with lower LUMO
energy levels (3, 12, 16).
at half-maximum (fwhm) of the EL spectra (Fig. 7 and Table 2)
of device III are almost the same as those of device I. Device
III appears to have smaller turn-on voltage than its two-layer
congener, which may be attributed to the smaller hole injection
Compound 9 appears to have relatively higher HOMO and
LUMO energy levels among all the compounds studied.
Further, it is highly transparent to visible light. Therefore,
we also tested the possibility of using 9 as a hole-injection
layer7,19 for optimization of the device performance. A three-
layer device III was fabricated: ITO/9 (15 nm)/12 or 16
(40 nm)/TPBI (40 nm)/Mg : Ag. The profile and the full width
Fig. 7 Current density and luminance vs. applied potential charac-
teristics of EL spectra (inset) of the devices ITO/9/16/TPBI/Mg : Ag.
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6 J. N. Demas and G. A. Crosby, J. Phys. Chem., 1971, 75, 991.
7 (a) K. R. Justin Thomas, M. Velusamy, J. T. Lin, Y.-T. Tao and
C.-H. Chuen, Adv. Funct. Mater., 2004, 14, 387; (b) I.-Y. Wu,
J. T. Lin, Y.-T. Tao, E. Balasubramaniam, Y.-Z. Su and C.-W. Ko,
Chem. Mater., 2001, 13, 2626.
8 (a) C. H. Chen and J. Shi, Coord. Chem. Rev., 1998, 171, 161; (b)
J. Shi, C. W. Tang and C. H. Chen, US Patent 5,645,948, 1997; (c)
A. Y. Sonsale, S. Gopinathan and C. Gopinathan, Indian J. Chem.,
1976, 14, 408.
9 E. Balasubramaniam, Y. T. Tao, A. Danel and P. Tomasik, Chem.
Mater., 2000, 12, 2788.
10 (a) M. Nishiyama, T. Yamamoto and Y. Koie, Tetrahedron Lett.,
1998, 39, 617; (b) J. F. Hartwig, Angew. Chem., Int. Ed., 1998, 37,
2047; (c) J. F. Hartwig, Synlett, 1997, 329.
barriers at ITO/9 and 9/12 (or 16) interfaces. There is negligible
difference in current density between devices III and I up to a
drive voltage of ca. 9 V. However, the overall performance
parameters of device III are nearly doubled compared to those
of device I. It is suspected that dissipation of the electrons in
the anode is suppressed due to the high-lying LUMO of 9.
Indeed, a closely related derivative of 9, 2,7-bis(diphenyl-
amino)-(29,79-di-tert-butyl)-9,99-spirobifluorene (SpiroI), was
successfully used as hole-transporting material in a device of
structure ITO/SpiroI/Alq3/LiF/Al.20
11 K. Danel, T.-H. Huang, J. T. Lin, Y.-T. Tao and C.-H. Chuen,
Chem. Mater., 2002, 14, 3860.
Conclusions
12 B. E. Koene, D. E. Loy and M. E. Thompson, Chem. Mater., 1998,
10, 2235.
13 (a) K. Naito, Chem. Mater., 1994, 6, 2343; (b) K. Naito and
A. Miura, J. Phys. Chem., 1993, 97, 6240.
14 M. S. Workentin, L. J. Johnston, D. D. M. Wayner and
V. D. Parker, J. Am. Chem. Soc., 1994, 116, 8279.
15 I. D. L. Albert, T. J. Marks and M. A. Ratner, J. Am. Chem. Soc.,
1997, 119, 6575.
16 J. Pommerehne, H. Vestweber, W. Guss, R. F. Mahrt, H. Ba¨ssler,
M. Porsch and J. Daub, Adv. Mater., 1995, 7, 551.
17 S. Janietz, D. D. C. Bradley, M. Grell, C. Giebeler, M. Inbasekaran
and E. P. Woo, Appl. Phys. Lett., 1998, 73, 2453.
18 (a) H.-T. Shih, C.-H. Lin, H.-H. Shih and C.-H. Cheng, Adv.
Mater., 2002, 14, 1409; (b) L.-H. Chan, R.-H. Lee, C.-F. Hsieh,
H.-C. Yeh and C.-T. Chen, J. Am. Chem. Soc., 2002, 124, 6469; (c)
Y.-T. Tao, E. Balasubramaniam, A. Danel, A. Wisla and
P. Tomasik, J. Mater. Chem., 2001, 11, 768; (d) W.-B. Im,
H.-K. Hwang, J.-G. Lee, K. Han and Y. Kim, Appl. Phys. Lett.,
2001, 79, 1387; (e) Y. Liu, J. Guo, J. Feng, H. Zhang, Y. Li and
Y. Wang, Appl. Phys. Lett., 2001, 78, 2300; (f) Y.-H. Kim,
D.-C. Shin, S.-H. Kim, C.-H. Ko, H.-S. Yu, Y.-S. Chae and
S.-K. Kwon, Adv. Mater., 2001, 13, 1690; (g) L.-H. Chan,
H.-C. Yeh and C.-T. Chen, Adv. Mater., 2001, 13, 1637; (h)
S. Tokito, K. Noda, H. Tanaka, Y. Taga and T. Tsutsui, Synth.
Met., 2000, 111–112, 393; (i) L. M. Leung, W. Y. Lo, S. K. So and
W. K. Choi, J. Am. Chem. Soc., 2000, 122, 5640; (j) Y.-T. Tao,
E. Balasubramaniam, A. Danel and P. Tomasik, Appl. Phys. Lett,
2000, 77, 933; (k) Y. Kijima, N. Asai and S.-I. Tamura, Jpn. J.
Appl. Phys., 1999, 38, 5274; (l) Z. Gao, C. S. Lee, I. Bello, S. T. Lee,
R.-M. Chen, T.-Y. Luh, J. Shi and C. W. Tang, Appl. Phys. Lett.,
1999, 74, 865; (m) X. T. Tao, H. Suzuki, T. Wada, S. Miyata and
H. Sasabe, J. Am. Chem. Soc., 1999, 121, 9447; (n) N. Tamoto,
C. Adachi and K. Nagai, Chem. Mater., 1997, 9, 1077; (o)
C. Hosokawa, H. Higashi, H. Nakamura and T. Kusumoto, Appl.
Phys. Lett., 1995, 67, 3853.
In summary, we have developed a series of 29,79-di-tert-butyl-
9,99-spirobifluorene derivatives containing arylamines. The
new compounds possess a high glass temperature which may
even exceed 200 uC. Most of the compounds are blue emitting
with good solution quantum yields. Blue-emitting EL devices
possessing narrow full width at half-maximum (fwhm , 68 nm)
were successfully fabricated using these molecules as emitting
and hole-transporting materials. Two of the double-layer
devices using TPBI as the electron-transporting layer have very
impressive performance compared to competitive non-doping
blue-emitting OLEDs reported in the literature. One of them
emits pure blue light (CIE (x,y) 5 (0.15, 0.09)).
Acknowledgements
This work was supported by Academia Sinica and the
National Science Council.
Jiun Yi Shen,a Chung Ying Lee,ac Tai-Hsiang Huang,a Jiann T. Lin,*ab
Yu-Tai Tao,*a Chin-Hsiung Chiena and Chiitang Tsaic
aInstitute of Chemistry, Academia Sinica, Taipei, Taiwan 115,
Republic of China. E-mail: jtlin@chem.sinica.edu.tw;
Fax: +(2)27831237
bDepartment of Chemistry, National Central University, Chungli,
Taiwan 320, Republic of China
cDepartment of Chemistry, Chinese Culture University, Taipei, Taiwan,
Republic of China
References
19 (a) D. Liu, C.-G. Zhen, X.-S. Wang, D.-C. Zou, B.-W. Zhang and
Y. Cao, Synth. Met., 2004, 146, 85; (b) Q. Huang, J. Cui,
J. G. C. Veinot, H. Yan and T. J. Marks, Appl. Phys. Lett., 2003,
82, 331; (c) Y. Gao, L. Wang, D. Zhang, L. Duan, G. Dong and
Y. Qiu, Appl. Phys. Lett., 2003, 82, 155; (d) Z. B. Dang, X. M. Ding,
S. T. Lee and W. A. Gambling, Appl. Phys. Lett., 1997, 74, 2227;
(e) E. W. Forsythe, M. A. Abkowitz and Y. Gao, J. Phys. Chem. B,
2000, 104, 3948; (f) S. A. Van Slyke, C. H. Chen and C. W. Tang,
Appl. Phys. Lett., 1996, 69, 2160; (g) Y. Shirota, Y. Kuwabara,
D. Okuda, R. Okuda, H. Ogawa, H. Inada, T. Wakimoto,
H. Nakada, Y. Yonemoto, S. Kawami and K. Imai, J. Lumin.,
1997, 72–74, 985; (h) Y. Shirota, Y. Kuwabara, H. Inada,
T. Wakimoto, H. Nakada, Y. Yonemoto, S. Kawami and
K. Imai, Appl. Phys. Lett., 1994, 65, 807.
1 C. W. Tang and S. A. Van Slyke, Appl. Phys. Lett., 1987, 51, 913.
2 J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks,
K. MacKay, R. H. Friend, P. L. Burn and A. B. Holmes, Nature,
1990, 347, 539.
3 (a) Y. Shirota, J. Mater. Chem., 2000, 10, 1; (b) K. Naito and
A. Miura, J. Phys. Chem., 1993, 97, 6240.
4 (a) K. H. Weinfurtner, F. Weisso¨rtel, G. Harmgarth and J. Salbeck,
Proc. SPIE, 1998, 3476, 40; (b) J. Salbeck, N. Yu, J. Bauer,
F. Weisso¨rtel and H. Bestgen, Synth. Met., 1997, 91, 209.
5 (a) C.-C. Wu, Y.-T. Lin, K.-T. Wong, K.-T. Chen and Y.-Y. Chien,
Adv. Mater., 2004, 16, 61; (b) K.-T. Wong, Y.-Y. Chien,
R.-T. Chen, C.-F. Wang, Y. T. Lin, H.-H. Chiang, P.-Y. Hsieh,
C.-C. Wu, C.-H. Chou, Y. O. Su, G.-H. Lee and S.-M. Peng, J. Am.
Chem. Soc., 2002, 124, 11576; (c) C.-C. Wu, Y. T. Lin,
H.-H. Chiang, T. Y. Cho, C.-W. Chen, K.-T. Wong, Y. L. Liao,
G. -H. Lee and S.-M. Peng, Appl. Phys. Lett., 2002, 81, 577; (d)
Y.-H. Kim, D.-C. Shin, S.-H. Kim, C.-H. Ko, H.-S. Yu, Y.-S. Chae
and S. K. Kwon, Adv. Mater., 2001, 13, 1690.
20 (a) M. Kimura, S.-I. Inoue, K. Shimada, S. Tokito, K. Noda,
Y. Taga and Y. Sawaki, Chem. Lett., 2000, 29, 192; (b) S. Tokito,
K. Noda, K. Shimada, S.-I. Inoue, M. Kimura, Y. Sawaki and
Y. Taga, Thin Solid Films, 2000, 363, 290.
This journal is ß The Royal Society of Chemistry 2005
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