Tert-Butylated spirofluorene derivatives with arylamine groups for highly efficient blue organic light emitting diodes

Article abstract of DOI:10.1039/c2jm14869b

A series of tert-butylated spirofluorene derivatives incorporating a diphenylaminoaryl-vinyl group was synthesized via the Horner-Wadsworth-Emmons olefination and a Suzuki cross-coupling reaction. To examine the electroluminescent properties of these materials, multilayered OLEDs were fabricated into the following device structure: ITO/DNTPD/NPB/MADN:blue dopant materials 1-14/Alq₃/Liq/Al. All devices showed efficient blue emission. In particular, one device exhibited highly efficient sky blue emission with a maximum luminance of 25100 cd m^-2 at 8.5 V, as well as luminous, power and external quantum efficiencies of 9.5 cd A^-1, 5.1 lm W^-1 and 6.7% at 20 mA cm^-2, respectively. The peak wavelength of electroluminescence was 458 and 484 nm with CIE_x,y coordinates of (0.14, 0.21) at 8.0 V. In addition, a deep blue device with CIE_x,y coordinates of (0.15, 0.15) at 8.0 V showed a luminous efficiency and external quantum efficiency of 3.8 cd A^-1 and 3.3% at 20 mA cm^-2, respectively.

Full text of DOI:10.1039/c2jm14869b

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PAPER
tert-Butylated spirofluorene derivatives with arylamine groups for highly
efficient blue organic light emitting diodes†
Kum Hee Lee,a Seul Ong Kim,a Jae Nam You,a Sunwoo Kang,a Jin Yong Lee,a Kyoung Soo Yook,b
b
Soon Ok Jeon, Jun Yeob Lee and Seung Soo Yoon
b
a
*
*
Received 29th September 2011, Accepted 20th December 2011
DOI: 10.1039/c2jm14869b
A series of tert-butylated spirofluorene derivatives incorporating a diphenylaminoaryl-vinyl group was
synthesized via the Horner–Wadsworth–Emmons olefination and a Suzuki cross-coupling reaction. To
examine the electroluminescent properties of these materials, multilayered OLEDs were fabricated into
the following device structure: ITO/DNTPD/NPB/MADN:blue dopant materials 1–14/Alq3/Liq/Al.
All devices showed efficient blue emission. In particular, one device exhibited highly efficient sky blue
emission with a maximum luminance of 25 100 cd mꢀ2 at 8.5 V, as well as luminous, power and external
quantum efficiencies of 9.5 cd Aꢀ1, 5.1 lm Wꢀ1 and 6.7% at 20 mA cmꢀ2, respectively. The peak
wavelength of electroluminescence was 458 and 484 nm with CIEx,y coordinates of (0.14, 0.21) at 8.0 V.
In addition, a deep blue device with CIEx,y coordinates of (0.15, 0.15) at 8.0 V showed a luminous
efficiency and external quantum efficiency of 3.8 cd Aꢀ1 and 3.3% at 20 mA cmꢀ2, respectively.
emission quantum yield, deep blue color and good durability are
Introduction
still rare.4,5,22–24 Therefore, development of blue-emitting mate-
rials with the satisfactory properties for the application in full
color displays became a major issue.
Conjugated small organic molecules have attracted considerable
attention in the past few decades due to their remarkable optical
and electronic properties for organic light-emitting diodes
(OLEDs).1–3 To compete with liquid crystal displays (LCDs) in
the display market, OLEDs must have high device performances,
good thermal stabilities, excellent color purity, and low
production costs. Despite the advent of first generation OLEDs,
there is still great interest in developing more efficient and stable
red, green, and blue luminescent materials with promising
properties to further improve the device performance and
stability for full color displays. A lot of red and green light-
emitting materials with both high efficiency and good color
purity for OLEDs have been developed during past two decades.
For example, many p-conjugated blue fluorescent materials
based on anthracene, stilbene, quinoline, carbazole, and pyrene
have recently been reported as efficient blue emitters.4–14 Also,
a great deal of attention has been focused on fluorene derivatives
due to their high photoluminescence (PL) quantum yield, blue
emission spectra, easy tunability of properties by substitution,
and high thermal stability.15–21 However, blue emitters with high
In this paper, we designed a series of novel tert-butylated
spirofluorene derivatives with terminal arylamino groups
(Scheme 1). In these materials, spiro-substitution is introduced to
maintain the color stability and improve their electrolumines-
cence (EL) efficiencies, in which the perpendicular arrangement
of the spiro-conformation not only hinders close packing and
intermolecular aggregation but also increases molecular rigidity
aDepartment of Chemistry, Sungkyunkwan University, Suwon, 440-746,
Korea. E-mail: ssyoon@skku.edu; Fax: +82 31 290 7075; Tel: +82 31
290 7071
bDepartment of Polymer Science and Engineering, Dankook University,
Yongin, 448-701, Korea. E-mail: leej17@dankook.ac.kr; Fax: +82 31
8021 7218; Tel: +82 31 8005 3585
† Electronic supplementary information (ESI) available: Synthesis of
blue materials 1–14 and the operation lifetime of devices. See DOI:
10.1039/c2jm14869b
Scheme 1 Structures of targeted tert-butylated spirofluorene cored
derivatives (1–14).
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without the change of the p-conjugation length of fluorophore.
In addition, incorporation of a diphenylaminoaryl-vinyl group
into the spiro-core unit is beneficial for improving the hole
mobilities due to electron-rich triarylamines25 and thus enhances
EL performances of devices using them. In seven blue materials
(2–8) with the same diphenylaminoaryl-vinyl group at the C2
position of the spirofluorene backbone, various aromatic end-
capping groups or heteroatom containing aryl groups at the C7
position of the spirofluorene core unit were introduced to study
systematically the effect on the EL properties of end-capping
groups. Three materials (9, 10 and 11) had diphenylamino-fluo-
renyl-vinyl, diphenylamino-biphenyl-vinyl and di-tert-butyl-
phenyl-carbazole-vinyl at the C2 position of the tert-butylated
spirofluorene core unit. Materials 1, 9, 10 and 11 would show the
effect on the EL properties of triarylamine groups. Compound 12
had diphenylaminostyryl at the C4 position of the tert-butylated
spirofluorene core unit. In compounds 13 and 14, the spiro-
fluorene core units were replaced by dispiroanthracene and dis-
piroindenofluorene. Materials 12–14 were designed to study
systematically the effect of the structural change in core units of
spiro-compounds on the EL properties. As will be seen below,
these materials show highly efficient blue emission, which
depends on the various aromatic end-capping groups, triaryl-
amine groups and the structure of the core unit through steric
and electronic effects.
Scheme 2 Synthesis of 1–11. Conditions: (a) n-BuLi, DMF, THF,
ꢀ78 ꢁC, 2 h; (b) t-BuOK (1.2 eq.), THF, 0 ꢁC to room temperature, 1 h;
(c) n-BuLi, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, THF,
ꢀ78 ꢁC to room temperature, 3 h; (d) Pd(PPh3)4, Aliquat 336, K2CO3,
toluene, 120 ꢁC, 16 h; (e) NaBH4 (4 eq.), ethanol, 80 ꢁC, 3 h; and (f) I2,
P(OEt)3, 12 h. See the Experimental section for full details.
Results and discussion
Scheme 1 shows the targeted tert-butylated spirofluorene deriv-
atives (1–14) and Schemes 2 and 3 present their synthetic
protocol. The preparation of the mono-formylated compound 15
was accomplished in 40% yield by a lithium–halogen exchange
reaction of bromospirobifluorene with a stoichiometric amount
of n-BuLi, and the subsequent addition of dimethylformamide at
ꢀ78 ꢁC. A subsequent Horner–Wadsworth–Emmons reaction of
15 with the corresponding phosphonate 16 provided compound 1
in an isolated yield of 76%. Similar Horner–Wadsworth–
Emmons reactions of compound 15 with the corresponding
phosphonates, diethyl (40-(diphenylamino)-9,90-diethylfluorenyl-
7-yl)methylphosphonate, compounds 21 and 23, afforded
compounds 9, 10, and 11 in 93, 40, and 68% yield, respectively.
For the preparation of compounds 2–8, the mono-bromo-
compound 18 was prepared by the mono-formylation of 2,7-
dibromo-20,70-di-tert-butyl-9,90-spirobifluorene with n-BuLi/
DMF (1 eq.) and subsequent Horner–Wadsworth–Emmons
reaction with compound 16. The mono-bromo-compound 19
was conveniently converted to its 2,3-dimethyl-2,3-butanediol
(pinacol) boronic ester 19, and subsequent Suzuki cross-coupling
reaction of compound 19 with the corresponding bromoarylenes
provided compounds 2–8 in 35–91% yield. The intermediates 25,
27 and 30 for the synthesis of compounds 12–14 were prepared
using the following standard protocols, as shown in Scheme 3.
Horner–Wadsworth–Emmons reactions of compounds 25, 27
and 30 with the corresponding phosphonate 16 afforded
compounds 12, 13 and 14 in 91, 80, and 54% yield, respectively.
After purification by column chromatography and recrystalli-
zation, these newly synthesized blue-emitting materials (1–14)
were purified further by train sublimation under reduced pres-
sure (<10ꢀ3 Torr) and fully characterized by 1H- and 13C-NMR,
infrared, and low- and high-resolution mass spectroscopy. High-
pressure liquid chromatography was carried out to check the
purity of the materials. These analyses showed that the purity of
blue-emitting materials (1–14) was at least above 99.0%.
Fig. 1 and 2 show the UV-Vis absorption and photo-
luminescence (PL) spectra of compounds 1–14 in dichloromethane
with a sample concentration of approximately 1 ꢂ 10ꢀ5 M.
Scheme 3 Synthesis of 12–14. Conditions: (a) n-BuLi, THF, ꢀ78 ꢁC to
room temperature; (b) acetic acid, H2SO4; (c) n-BuLi, DMF, THF,
ꢀ78 ꢁC, 2 h; (d) t-BuOK (1.2 eq.), THF, 0 ꢁC to room temperature, 1 h;
(e) H2SO4, dichloromethane; and (f) Pd(PPh3)4, Aliquat 336, K2CO3,
toluene, 120 ꢁC. See the Experimental section for full details.
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Fig. 1 UV spectra of the blue fluorescent materials (1–14).
Fig.
2
PL spectra of the blue fluorescent materials (1–14) in
dichloromethane.
Table 1 summarizes their optical and photoluminescent proper-
ties. In the dilute dichloromethane solutions, compounds 1–14
exhibit strong absorption with their maximum absorption peaks
in the range 360–399 nm, whereas the emission maxima of
compounds 1–14 were in the range 432–483 nm in the blue region
of the visible spectrum with high quantum yields of 56–99%.
Suitable spectral overlap was observed between the emission
spectrum of the common blue host (MADN) and the absorption
spectra of these materials. Particularly good spectral overlap was
observed in compounds 1–9 and 14. This suggests that these blue
materials can effectively accept energy from the MADN host
existence of a heavy selenium atom in compound 8 decreased the
fluorescence quantum yield compared to that of the other
heteroatom containing compounds. Secondly, significant changes
in optical properties were observed upon the introduction of
various aryl substituents at the C2 position of the spirobifluorene
core unit. Compound 9 exhibited longer absorption (ca. 9 nm) and
emission (ca. 20 nm) wavelengths than those of compound 1
due to the extended p-conjugation by the fluorene group between
the spirobifluorene and diphenylamine. Interestingly, compound
10 with a biphenyl group between the spirobifluorene and
diphenylamine exhibited a shorter absorption wavelength but
much longer emission wavelength compared to compound 1.
Compared to the PL spectra of compounds 1, 9 and 10, the
spectrum of compound 11 showed hypsochromic shifts (31–
51 nm) due to a difference in the electronic properties of the
diphenylaminoarene and phenylcarbazole moieties. Thirdly,
compared to compound 1 with a diphenylaminostyryl group at
the C2 position of the spirobifluorene core unit, compound 12
showed that the UV-Vis absorption spectra was blue-shifted by
approximately 15 nm due to the interruption of p-conjugation
between the core unit and diphenylaminostyryl group as a result
of the incorporation of the same diphenylaminostyryl group at
the C4 position. Fourthly, among compounds 1–14, 13 exhibited
the shortest absorption and emission wavelength due to an
interruption of the p-conjugation of compound 13 by the 9,10-
dihydroanthracene core. On the other hand, when a longer
conjugation emitting core group was introduced (14), reasonable
€
material by Forster energy transfer. Several conclusions could be
made by examining these spectra comparatively. Firstly, the
introduction of an aryl group into the conjugated C7 position of
spirobifluorene leads to a small red-shift. By comparing unsub-
stituted
1 with 3,5-di-tert-butylphenyl substituted 2, small
red-shifts resulting from the extended conjugation length by
introduction of phenyl ring at the C7 position were observed for
compound 2. The UV and PL spectra indicated that compound 2
was red-shifted by approximately 6 and 4 nm, respectively.
Compared to the unsubstituted compound 1, the absorption
spectra of compounds 3–8 with heteroatom containing aryl groups
at the C7 position were shifted to longer wavelengths by
approximately 6–11 nm. This suggests that heteroatoms did not
have any substantial effect on the effective conjugation length. For
example, compounds 3 and 8, containing a tetraphenylmethane
and diphenyl selenide group at the C7 position of spirobifluorene,
showed the same maximum emission peaks at 472 nm. The
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Table 1 Physical properties for the blue fluorescent materials (1–14)
Ff sola b
(%)
fwhma/
nm
labs filmc/
nm
lem filmc/
nm
lem ꢀ labs film/
Ff filmd
(%)
Eg
eV
EHOMO
eV
a
a
e
/
f
/
g
ELUMO /
eV
,
labs
nm
/
lem
nm
/
Compounds
nm
1
2
3
4
5
6
7
8
385
391
393
393
396
392
394
392
394
376
360
370
370
399
463
467
472
474
468
471
476
472
483
475
432
468
443
474
99
88
97
99
97
90
95
75
85
72
56
65
99
78
65
67
68
70
67
68
71
70
67
75
67
74
62
69
388
391
392
397
398
394
398
394
395
377
358
315
372
400
474
460
475
476
495
494
487
478
485
474
454
455
439
484
86
69
83
79
97
100
89
84
90
97
96
140
67
84
47
47
67
90
54
42
56
46
53
98
44
96
99
57
2.91
2.87
2.86
2.84
2.86
2.84
2.84
2.84
2.83
2.92
3.09
2.93
3.03
2.84
ꢀ5.55
ꢀ5.38
ꢀ5.53
ꢀ5.49
ꢀ5.56
ꢀ5.54
ꢀ5.55
ꢀ5.50
ꢀ5.55
ꢀ5.63
ꢀ5.45
ꢀ5.59
ꢀ5.57
ꢀ5.80
ꢀ2.64
ꢀ2.51
ꢀ2.67
ꢀ2.65
ꢀ2.70
ꢀ2.70
ꢀ2.71
ꢀ2.66
ꢀ2.72
ꢀ2.71
ꢀ2.36
ꢀ2.66
ꢀ2.54
ꢀ2.96
9
10
11
12
13
14
a
Measured in a CH2Cl2 solution. b The photoluminescence quantum yields (Ff) of these compounds (1–14) in CH2Cl2 were determined using a BDAVBi
solution as the standard (Ff ¼ 86% in dichloromethane). c Measured in film. d Fluorescent quantum yields of thin films were determined using BDAVBi
film as a standard (Ff ¼ 30% measured by the calibrated integration sphere system). e Eg is the band-gap energy estimated from the intersection of the
f
absorption and photoluminescence spectra. The HOMO energy level was determined using a low-energy photoelectron spectrometer (Riken-Keiki,
AC-2). g LUMO ¼ HOMO + DE.
red-shifts in both the absorption (ca. 8 nm) and emission (ca.
7 nm) spectra were observed in comparison to compound 2.
The other important factor is the optical properties in the solid
state. The UV-Vis absorption, the PL spectra, and the PL
quantum yields of the thin film were evaluated and are listed in
Table 1 (the BDAVBi film was used as a standard and Ff ¼ 30%
was measured by the calibrated integrating sphere system26). The
maximum emission peaks of compounds 1–14 in the thin films
were similar to those in a dilute solution. The small shifts might
be explained by the fact that the effective suppression of the
intermolecular interactions of their p-systems is due to the
sterically bulky and twisted characteristics by the tert-butylated
spiro-system of these materials. According to the Table 1, the
Stokes’ shift for compound 12 (140 nm) is larger than compound
1 (86 nm) due to a change of the molecular position from
compound 1 with a diphenylaminostyryl group at the C2 posi-
tion of the spirobifluorene unit to compound 12 with a diphe-
nylaminostyryl group at the C4 position of the same unit.
Presumably, this large Stokes shift is assumed to be the conse-
quence of the restriction of rotational freedom of compound 12
in the ground state, which is induced by the repulsion between
hydrogen atoms of the vinyl position and C5 position of the
spirobifluorene unit of compound 12 and released by the
extended bond-length around the spirobifluorene unit of
compound 12 in the excited state.
results suggest that the end-capping groups did not have any
substantial effect on the electronic structures of blue materials 3–
8. On the other hand, the structural changes in the emitting core
unit induced large differences in the HOMO/LUMO energy
levels. For example, compared to the HOMO energy level of
compound 1, 11 showed a higher HOMO energy level of 0.1 eV,
whereas the HOMO energy level of compound 10 was 0.08 eV
lower than that of compound 1. In addition, the LUMO energy
Table 1 and Fig. 3 present the highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) energy levels of compounds 1–14, obtained from a low-
energy photoelectron spectrometer and UV-Vis absorption and
PL measurements. The HOMO energy levels for compounds 1–
14 varied from ꢀ5.38 to ꢀ5.80 eV and the LUMO energy levels,
which were calculated by subtracting the optical band gap from
the HOMO energy levels, ranged from ꢀ2.36 to ꢀ2.96 eV.
Interestingly, compounds 3–8 with the different aryl end-capping
groups, such as tetraphenylmethane, tetraphenylsilicon, triphe-
nylnitrogen, diphenyloxygen, diphenylsulfur and diphenylsele-
nium, show similar HOMO and LUMO energy levels. These
Fig. 3 Energy levels of the blue fluorescent materials (1–14) in the
MADN host.
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level of compound 13 was 0.1 eV higher than that of compound 1
due to a shortening of the p-conjugation length. In addition,
compared to compound 2, the HOMO/LUMO energy levels of
compound 14 were decreased by 0.42 and 0.45 eV, respectively,
due to the extended p-conjugation of the indenofluorene back-
bone in compound 14 compared to the fluorene backbone in
compound 2.
To better understand the observed properties of organic light
emitting diodes at the molecular level, density functional theory
(DFT) calculations for compounds 1–14 were carried out using
the exchange functional of Becke’s three parameterized Lee–
Yang–Parr (B3LYP) employing 6-31G* basis sets using a suite of
Gaussian03 programs.27 The torsion angles in the molecular
structures and electron distributions in the HOMOs and LUMOs
were important keys for explaining the electronic transition
properties because they reflect a certain degree of effective
conjugation. As a result of optimization, the calculated dihedral
angles between the phenyl moiety and spirobifluorene moiety for
compounds 1–8, 10, and 12 were 12.54, 2.12, 7.50, 10.24, 2.58,
9.78, 9.89, 6.88, 2.49, and 41.73ꢁ, respectively. The dihedral angle
between the di-tert-butylcarbazole and spirobifluorene moiety of
compound 11 was calculated to be 4.24ꢁ, and the dihedral angles
between the phenyl and the dispiroanthracene moiety (dis-
piroindenofluorene moiety) for compounds 13 and 14 were
calculated to be 1.45 and 4.54ꢁ, respectively. The calculated
torsion angles for compounds 2–8 were smaller than that of
compound 1. This indicates that compounds 2–8 have more
delocalized chromophores than compound 1, hence the absorp-
tion wavelengths of compounds 2–8 were shifted to a longer
wavelength. In contrast to the results of compounds 2–8, the
calculated torsion angle for compound 12 was approximately 20ꢁ
larger than for compound 1. Therefore, its absorption wave-
length showed a blue-shift. The structures of compounds 9 and
10 were replaced and adducted by the di-ethyl fluorene moiety
and phenyl moiety compared to compound 1. In these cases, the
experimental and theoretical absorption wavelengths showed
a red-shift due to the increase in delocalization area.
Fig. 4 B3LYP/6-31G* calculated HOMOs and LUMOs of the blue
fluorescent materials (1–14).
compound 13, the electrons of the HOMO were distributed over
the diphenylamino-styryl-partial anthracene moiety whereas the
electrons of the LUMO were distributed over the styryl-anthra-
cene moiety. The electrons of the HOMO of compound 14 were
distributed over the diphenylamino-styryl-indenofluorene
moiety, whereas the electrons of the LUMO were distributed
over the styryl-indenofluorene moiety. The distribution of elec-
trons in the LUMO orbitals, the LUMOs for compounds 2–4
and 6–8, were more delocalized than compound 1, as shown in
Fig. 1. The more delocalized areas of the LUMOs induced
a decrease in the energy levels of LUMOs, hence compounds 2–4
and 6–8 showed smaller energy gaps than compound 1. In
contrast to 2–4 and 6–8, the HOMO of compound 5 was
apparently more delocalized than compound 1. The calculated
excitation energies (Eexcitation) for 1–8 were calculated to be
2.9732, 2.8994, 2.8881, 2.8672, 2.8271, 2.8847, 2.8584, and
2.8572 eV, respectively, which corresponded to absorption
wavelengths of 417.01, 427.61, 430.34, 432.42, 438.55, 429.80,
433.75, and 433.94 nm, respectively. These Eexcitation values are
well consistent with the experimental results of compounds 1–8.
In the experiment, the observed result for the absorption prop-
erty of compound 11 showed a blue-shift compared to compound
1. The calculated Eexcitation and corresponding wavelength for
compound 11 were calculated to be 3.2431 eV and 382.30 nm,
respectively. The tendency of the theoretical result of compound
11 is consistent with the experimental data. The calculated
Eexcitation for compounds 12 and 13 were 3.1148 and 3.1364 eV,
respectively, which correspond to absorption wavelengths of
398.05 and 395.01 nm, respectively. In the HOMOs of
Fig. 4 shows the molecular structures and highest occupied
and lowest unoccupied molecular orbitals. Time dependent DFT
calculations of the optimized structures were carried out to better
understand the absorption properties of compounds 1–14. The
HOMO to LUMO transition contributed solely to the electronic
transition for compounds 1–14. The HOMO and LUMO shapes
for compounds 1–4, 6–8, and 12 appeared similar. The electrons
of the HOMOs of compounds 1–4, 6–8, and 12 were distributed
over the diphenylamino-styryl-fluorene moiety, whereas the
electrons of LUMO were distributed over the styryl-fluorene
moiety. The electrons of the HOMO of compound 5 were
distributed mostly over the molecule whereas the electrons of the
LUMO were distributed over the molecule except for both
diphenylamino moieties. The electrons of the HOMOs of
compounds 9 and 10 were distributed over the diphenylamino-
fluorenevinyl-fluorene (diphenylamino-biphenylvinyl-fluorene)
moiety, whereas the electrons of the LUMO were distributed
over the fluorenevinyl-di-tert-butyl spirofluorene (phenylvinyl-
di-tert-butyl spirofluorene) moiety. The electrons of the HOMO
of compound 11 were distributed over the di-tert-butylphenyl-
carbazolevinyl-fluorene moiety whereas the electrons of the
LUMO were distributed over vinyl-spirofluorene. For
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compounds 12 and 13, the electrons were slightly delocalized on
chromophores compared to compound 1. The calculated
Eexcitation value for compound 9 was approximately 0.127 eV
lower than compound 1, and its Eg value corresponded to an
absorption wavelength of 435.53 nm because the length of
chromophore was longer than compound 1. The calculated
Eexcitation for compound 14 was 2.8106 eV, which corresponded
to an absorption wavelength of 441.13 nm. The Eexcitation for
compound 14 was smaller than that for compound 1 because the
length of the absorption chromophore was longer than that of
compound 1. All calculated absorption wavelengths were in good
agreement with the experimental trend except for compound 10.
The experimentally observed absorption wavelength of
compound 10 showed a blue shift, but the gas phase calculation
showed a red-shift. It was reported that when a phenyl moiety
was added to the molecular structure, the gas phase calculations
sometimes produce the wrong absorption property due mainly to
an over-evaluation of the conjugation, hence causing too much
of a red-shift. This might be one reason for the inconsistency for
compound 10, and the other possible reason would be the solvent
effect. When the solvent effect by the polarized continuum model
(PCM) method was included, the TDDFT calculations gave
excitation energies of 2.8001 and 2.8003 eV for compounds 1 and
10, respectively, which correspond to absorption wavelengths of
442.78 and 442.75 nm, respectively. Therefore, to obtain more
accurate absorption properties of compound 10, further calcu-
lations at a higher level of theory considering the extended
conjugated systems will be needed, which are beyond the scope of
this study.
Fig. 5 EL spectra of devices 1A–14A.
To examine the electroluminescent properties of compounds 1–
14, the devices were fabricated using thermal deposition in the
following configuration: ITO/N,N0-diphenyl-N,N0-bis-[4-(phenyl-
m-tolyl-amino)-phenyl]-biphenyl-4,40-diamine (DNTPD) (60 nm)/
4,40-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (NPB) (30 nm)/
conjugation system due to the distorted structure at the biphenyl
moiety, in comparison with dopant 1 in device 1A. Interestingly,
compared to device 2A, the CIE coordinates of device 14A were
red-shifted compared with the fluorene backbone in dopant 2 in
device 2A due to the extended p-conjugation of the indeno-
fluorene backbone in dopant 14 in device 14A. Interestingly,
devices 1A–10A, 12A and 14A exhibited blue emission with two
emission peaks, whereas devices 11A and 13A showed the EL
spectra with only one emission peak. The two emission peaks are
due to different energy transfer from the MADN host to dopant
materials. In the case of 11A and 13A, the overlap of UV-Vis
absorption of dopant materials with PL emission of MADN is
small, while other materials showed rather wide overlap between
UV-Vis absorption of dopants and PL emission of MADN.
Therefore, different absorption modes are excited in dopant
materials except for 11A and 13A, resulting in two EL emission
peaks. However, the absorption mode of 11A and 13A is simple
due to narrow overlap, inducing a single EL emission peak. This
also caused the shift of EL spectra of dopant 11 and 13 to long
wavelength. As can be seen in Fig. 1, the PL emission of 11 and
13 was observed at short wavelength compared to that of other
dopant materials, but the EL emission peak position was similar
to that of other dopants. As explained above, this behavior is due
to the excitation of the low energy emission mode due to small
spectral overlap. The current density and luminance of the
devices increased at high doping concentrations in most cases
(Fig. 6) due to the high hole mobility of the triarylamine moieties
x
% dopant:MADN host (30 nm)/tris(8-hydroxyquinoline)
aluminium (Alq3) (20 nm)/LiF (1.0 nm)/Al (200 nm). To improve
the electroluminescent efficiency and color purity, a host-dopant
emitting system was adopted using these materials 1–14 as dopant
ataconcentrationrange of5, 7.5, and10%inthegeneralfluorescent
blue host MADN.
Fig. 5 shows the EL spectra of the blue devices 1A–14A with
a 5% doping concentration. Compared to the CIE coordinates of
device 1A doped with the dopant without C7 aryl-groups, those
of devices 2A–8A containing the dopant with C7 aryl-substitu-
ents were slightly red-shifted in the y coordinates, which is a good
agreement with the UV-Vis and PL spectra of the corresponding
dopants 1–8. The CIE coordinates of devices 3–8 were in similar
ranges at 5, 7.5, and 10% doping, respectively. This suggests that
the tert-butylated spirobifluorene moieties effectively prevented
an intermolecular interaction and concentration quenching.
Consequently, these devices can maintain stable color purity
even at high doping concentrations. Compared to device 1A,
although device 9A using dopant 9 with a fluorenyl spacer group
showed a red-shift in the CIE coordinates, device 10A doped
using dopant 10 with a biphenyl spacer group showed a blue-
shift. Presumably, these observations reflect the differences in the
PL spectra of dopants 1, 9, and 10. Dopant 9 in device 9A had
a planar fluorene structure that ensured a longer conjugation
length, whereas dopant 10 in device 10A had a shorter
5150 | J. Mater. Chem., 2012, 22, 5145–5154
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of the dopants. All devices had low turn-on voltages of 4.0–5.0 V,
in which stable blue emission was observed from the whole device
area (Table 2).
As shown in Fig. 7, the EL efficiencies of devices 1A–14A were
sensitive to the structural changes in the dopants in the devices.
In particular, devices 1A–9A and 14A exhibited higher external
quantum efficiency than those of devices 10A–13A. In devices
10A–13A, energy transfer from the MADN host to the corre-
sponding dopants was not effective compared to the other
devices 1A–9A and 14A because of the poor spectral overlap
between the emission spectrum of the MADN host and the
absorption spectra of 10–13, as shown in Fig. 1. Consequently,
the contribution from the weak emission of the MADN host and
exciplexes between the MADN host and dopants 10–13 would
decrease the EL efficiencies of the devices 10A–13A. Among
devices 1A–8A, device 1A showed the lowest EL efficiency.
Compared to dopant 1 in device 1A, dopants 2–8 in devices 2A–
8A have in addition bulky end-capping groups. Presumably,
these bulky groups prevented the intermolecular interaction and
concentration quenching, and led to improved EL efficiency.
Furthermore, compared to 2–8, the poor spectral overlap
between the emission spectrum of the MADN host and the
absorption spectra of dopant 1 and the ineffective energy transfer
from the MADN host to dopant 1 in device 1A would have
partially contributed to the lower EL efficiency of device 1A than
devices 2A–8A. Among the devices, device 4A using the
tetraphenylsilyl moiety and device 14A using the
Fig. 7 External quantum efficiencies as a function of luminance of the
devices 1A–14A.
dispiro-indenofluorene moiety showed the highest efficiencies.
For example, a comparison of the EQE of device 3A (5.9% EQE
at 20 mA cmꢀ2) and device 4A (7.2% EQE at 20 mA cmꢀ2
)
showed that the change from carbon to silicon in the dopants
increases the device EQE by 22%. In addition, the external
quantum efficiency of device 4A showed only a mild decrease
with increasing luminance. For example, device 4A showed
a slight decrease in EQE when the luminance was increased to
20 000 cd mꢀ2. Compared to device 3A, the improved EQE of
device 4A might be due to the silicon effect.10 The tetraphe-
nylsilicon group is more bulky than the tetraphenylmethane due
to the longer carbon–silicon bond than carbon–carbon bond.
Therefore, compared to the tetraphenylmethane group, the tet-
raphenylsilicon end-capping group prevented effective molecular
aggregation and self-quenching of the dopant materials in the
devices. Consequently, the EQE of device 4A is better than that
of device 3A. Interestingly, the external quantum efficiency of
device 8A showed a large decrease with increasing luminance. In
addition, device 8A showed a large decrease in EQE with
increasing doping concentrations. Presumably, compared to the
other dopants, the relatively easy oxidation of the selenium atom
in dopant 8 of device 8A would contribute to these decreases in
EQE of device 8A with increasing luminance and doping
concentrations. In addition, a comparison of the EQE of device
14A (6.7% EQE at 20 mA cmꢀ2) with 2A (5.0% EQE at 20 mA
cmꢀ2) showed that the replacement of the emitting core unit from
Fig. 6 J–V–L characteristics of devices 1A–14A.
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the fluorene to indenofluorene increased the device EQE by 34%.
Furthermore, compared to device 2A, the luminous and power
efficiency of device 14A increased by 44% and 55%, respectively.
The blue material 14 has a unique, double spiro-annulated
structure connected indenofluorene core unit with an extended
conjugation system, whereas material 2 has a well-known spi-
robifluorene core. The degree of overlap of the emission spec-
trum of MADN host material absorption with the absorption
spectrum of dopant 14 in device 14A is more effective than that of
dopant 2 in device 2A, as shown in Fig. 1. Therefore, the
improved efficiency in device 13A indicates that the effective
blue emission with the excellent device performance. In parti-
cular, using 14 as a dopant in the emitting layer, a highly efficient
blue OLED was fabricated, showing a maximum luminance,
luminous efficiency, power efficiency, external quantum effi-
ciency and CIEx,y coordinates of 25 100 cd mꢀ2 at 8.5 V, 9.5 cd
Aꢀ1 at 20 mA cmꢀ2, 5.1 lm Wꢀ1 at 20 mA cmꢀ2, 6.7% at 20 mA
cmꢀ2, and (x ¼ 0.14, y ¼ 0.21) at 8.0 V, respectively. Moreover, in
the case of device 10A, deep-blue emission with CIEx,y (0.15,
0.15) was achieved with a luminance external quantum efficiency
of 10 610 cd mꢀ2 and 3.3%, respectively. This study clearly
demonstrated the excellent properties of tert-butylated spiro-
fluorenes with triarylamine groups when applied as blue-emitting
materials in OLEDs.
€
Forster energy transfer between the host and dopant plays an
important role in the highly efficient blue OLEDs. In addition,
the blue fluorescent dopant materials synthesized in this work
showed better device stability than the common blue dopant
material, N-phenyl-N-(4-styryl-p-terphenyl)benzenamine (BD 1)
(see Fig. S1†).
Experimental section
General information
The 1H and 13C NMR spectra were recorded on a Varian (Unity
Inova 300Nb) spectrometer using CDCl3 as the solvent. The FT-
IR spectra were recorded using a Bruker VERTEX70 FT-IR
spectrometer. The low- and high-resolution mass spectra were
measured using a Jeol JMS-AX505WA spectrometer in FAB
mode or Jeol JMS-600 spectrometer in EI mode. The UV-Vis
absorption measurements of these blue emitting materials in
dichloromethane (10ꢀ5 M) were acquired using a Sinco S-3100 in
a quartz cuvette (1.0 cm path). The photoluminescence spectra
were measured on an Amincobrowman series 2 luminescence
spectrometer. The fluorescence quantum yield of the blue mate-
rials were determined in dichloromethane at 293 K using
Conclusions
This study demonstrated the efficient synthesis of highly emissive
materials containing tert-butylated-spirofluorenes and triaryl-
amine, and examined their photophysical and electroluminescent
properties. These compounds have a three-dimensional confor-
mation which is beneficial for preventing concentration
quenching and excimer formation due to the tert-butylated spiro-
systems. The introduction of triarylamine groups at the emitting
core unit, such as fluorene, anthracene and indenofluorene,
improves the hole mobilities of the devices using them. All
devices using them as dopants in the emitting layer showed bright
Table 2 Electroluminescent data of the devices 1–14
b c
hL /cd Aꢀ1
,
b c
hp /lm Wꢀ1
,
Vona/V
EL/nm
Lmax (V at Lmax)/cd mꢀ2
hextb c (%)
CIEd (x,y)
,
Device
Doping (%)
1A
2A
3A
3B
3C
4A
4B
4C
5A
5B
5C
6A
6B
6C
7A
7B
5.0%
5.0%
5.0%
7.5%
10%
5.0%
7.5%
10%
5.0%
7.5%
10%
5.0%
7.5%
10%
5.0%
7.5%
10%
5.0%
7.5%
10%
4.5
4.5
4.5
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
4.5
4.5
4.5
4.5
4.5
4.5
5.0
5.0
5.0
4.5
4.5
4.5
4.5
5.0
4.0
4.0
449, 470
452, 477
455, 481
455, 481
455, 481
457, 482
458, 483
457, 483
456, 483
456, 484
457, 484
455, 480
455, 481
457, 481
457, 483
458, 483
458, 484
457, 483
457, 483
457, 483
454, 480
448, 467
456
9002 (8.5)
9614 (8.5)
4.5/4.5
6.7/6.6
8.6/7.8
8.8/8.2
8.4/8.2
10.2/10.0
10.9/10.1
9.5/9.1
9.8/9.7
9.6/9.0
8.8/8.7
9.3/8.6
9.5/9.1
9.0/8.2
9.7/9.1
10.1/9.7
9.8/9.2
8.7/8.7
8.7/8.6
7.8/7.8
8.5/8.4
3.9/3.8
3.1/2.8
3.3/3.3
3.3/3.2
10.2/9.5
1.4/1.1
2.7/2.4
3.9/3.3
5.0/3.0
4.8/3.2
4.0/3.3
5.4/3.9
5.3/3.9
4.5/3.5
5.2/3.8
5.0/3.5
4.3/3.6
5.5/3.5
5.3/3.5
4.9/3.4
5.8/3.7
5.7/3.8
5.1/3.8
4.6/4.4
4.4/4.2
4.1/3.8
4.8/4.4
2.5/2.1
2.1/1.5
2.1/1.8
2.1/1.7
5.4/5.1
1.0/0.5
3.7/3.7
5.1/5.0
6.4/5.9
6.2/5.9
6.1/6.1
7.3/7.2
7.4/7.1
6.6/6.4
7.0/7.0
6.4/6.2
6.0/6.0
6.9/6.5
6.5/6.4
6.5/6.0
6.8/6.5
6.7/6.5
6.6/6.3
6.2/6.2
5.7/5.8
5.1/5.1
6.3/6.1
3.3/3.3
2.3/2.2
2.6/2.6
2.5/2.5
7.3/6.7
1.6/1.5
0.15, 0.16
0.15, 0.19
0.15, 0.20
0.15, 0.21
0.15, 0.20
0.15, 0.21
0.15, 0.22
0.15, 0.22
0.15, 0.21
0.15, 0.23
0.15, 0.22
0.14, 0.20
0.15, 0.22
0.15, 0.21
0.15, 0.21
0.15, 0.24
0.15, 0.23
0.15, 0.21
0.15, 0.23
0.15, 0.23
0.15, 0.20
0.15, 0.15
0.16, 0.16
0.16, 0.16
0.16, 0.17
0.14, 0.21
0.15, 0.08
18 580 (9.0)
31 860 (9.5)
33 210 (9.5)
19 460 (9.0)
19 880 (9.0)
17 260 (9.0)
22 740 (9.0)
22 130 (9.0)
29 100 (9.0)
24 980 (9.5)
24 980 (9.5)
29 820 (9.0)
29 650 (9.0)
31 120 (9.5)
29 820 (9.0)
23 980 (9.5)
19 690 (10.0)
15 080 (10.0)
14 320 (8.5)
10 610 (8.5)
6578 (8.5)
7C
8A
8B
8C
9A
5.0%
5.0%
5.0%
5.0%
5.0%
5.0%
10A
11A
12A
13A
14A
MADN-only
456
459
458, 484
442
7230 (8.5)
9503 (9.0)
25 100 (8.5)
954
a
b
c
d
The turn-on voltage at a brightness of 1 cd mꢀ2
.
Maximum values. Measured at a current density of 20 mA cmꢀ2
.
Commission Internationale
ꢀ
d’Enclairage (CIE) coordinates at 8.0 V.
5152 | J. Mater. Chem., 2012, 22, 5145–5154
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BDAVBi as a reference (F ¼ 86%)10 and in film on quartz plate
50 nm using BDAVBi as a standard (Ff ¼ 30% measured by the
calibrated integration sphere system).26 The HOMO energy levels
were determined with a low energy photoelectron spectrometer
(Riken-Keiki, AC-2). The energy band gaps were determined
from the intersection of the absorption and photoluminescence
spectra. The LUMO energy levels were calculated by subtracting
the corresponding optical band gap energies from the HOMO
energy values.
High Efficiency White OLED Materials and Their Devices) and
Development of Core Technologies for Organic Materials
Applicable to OLED Lighting with High Color Rendering Index
by MKE.
Notes and references
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Device fabrication and characterization
The device configuration of the blue devices was indium tin oxide
(ITO,
150
nm)/N,N0-diphenyl-N,N0-bis-[4-(phenyl-m-tolyl-
amino)-phenyl]-biphenyl-4,40-diamine (DNTPD, 60 nm)/N,N0-
di(1-naphthyl)-N,N0-diphenylbenzidine (NPB, 30 nm)/MADN:
dopants (30 nm)/tris(8-hydroxyquinoline) aluminium (Alq3,
20 nm)/LiF (1.0 nm)/Al (200 nm). DNTPD and NPB were hole
injection and hole transport materials, respectively. Alq3 was
used as an electron transport layer and LiF/Al as a cathode. All
organic materials except for dopants were deposited at a depo-
sition rate of 1 A sꢀ1. Current (I)–voltage (V)–luminance (L)
ꢁ
characteristics and electroluminescence (EL) spectra of the
devices were measured with a Keithley 2400 source measurement
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This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and Technology
(20110004655). JYL acknowledges the NRF Grant (no. 2010-
0001630) funded by MEST, Republic of Korea. J.Y. Lee
acknowledges the financial support from the Fundamental R&D
Program for Core Technology of Materials (grant no.
M2009010025) funded by the MKE, GRRC program of
Gyeonggi Province (GRRC Dankook2011-B01: Development of
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This journal is ª The Royal Society of Chemistry 2012