Synthesis and characterization of heteroatom-bridged bis-spirobifluorenes for the application of organic light-emitting diodes

Article abstract of DOI:10.1021/ol5005214

Pure 2-iodo-9,9′-spirobifluorene was synthesized by an efficient method without troublesome iodination of 9,9-spirobifluorene (SP) or the Sandmeyer reaction of 2-amino-9,9′-spirobifluorene. A series of main group element-bridged bis-9,9′-spirobifluorene d

Full text of DOI:10.1021/ol5005214

Letter
pubs.acs.org/OrgLett
Synthesis and Characterization of Heteroatom-Bridged Bis-
spirobifluorenes for the Application of Organic Light-Emitting
Diodes
Cheng-Lung Wu,†,‡ Chao-Tsen Chen,*,† and Chin-Ti Chen*,‡
†Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
‡Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan
S
* Supporting Information
ABSTRACT: Pure 2-iodo-9,9′-spirobifluorene was synthesized by an
efficient method without troublesome iodination of 9,9-spirobifluorene
(SP) or the Sandmeyer reaction of 2-amino-9,9′-spirobifluorene. A series
of main group element-bridged bis-9,9′-spirobifluorene derivatives were
synthesized via coupling reactions of 2-iodo-9,9′-spirobifluorene and main
group element-containing precursors. These heteroatom-bridged bis-
spirobifluorenes show large triplet state energy gaps, high glass transition
temperatures, and varied charge-transporting properties advantageous to the host materials for blue phosphorescence organic
light-emitting diodes.
a
Scheme 1
rganic materials with a spiromolecular structure have
Obeen widely used in organic optoelectronics due to their
structural rigidity and high thermal or morphological
stabilities.1 In application of blue fluorescent emitter, the
specific rigid structure provided a promising design to achieve
intramolecular excimer emission.2 Specifically, 9,9′-spirobifluor-
ene compounds comprise a famous family known for high
fluorescence quantum efficiency, high glass transition temper-
ature, and ambipolar charge-transporting properties.3 In this
work, we focused our attention on the design, synthesis, and
characterization of a new series of host materials based on bis-
9,9′-spirobifluorene derivatives (SP2X) for blue phosphores-
cence organic light-emitting diodes (OLED). Main-group
elements are designed as the linking elements between two
9,9′-spirobifluorene moieties, such as SP2N (6a), SP2Si (6b),
SP2PO (6c), SP2O (6d), and SP2SO2 (6e) (Scheme 1)
because of their potential charge (electron or hole)-trans-
porting properties.
Phosphorescent OLEDs have attracted much attention
because near 100% internal quantum efficiency can be achieved
via utilization of both singlet and triplet excitons. Because of
their relatively long emission lifetime, a phosphorescence
emitter is doped into a suitable host material in order to
alleviate triplet−triplet annihilation. Therefore, an ideal host
material needs to have a high triplet energy gap (ET), greater
than ∼2.60 eV for blue phophorescent dopant FIrpic, for
instance.4 Moreover, an ideal host material should be charge-
balanced in order to enhance efficiency of the charge
recombination on phosphorescence dopant. Finally, a high
glass transition temperature (Tg) of the host material is most
needed if the morphological stability of the amorphous film is
necessary. 9,9′-Spirobifluorene compounds are one of the few
materials having both high ET (over 2.8 eV) and morphology
a
Reagents and conditions: (i) NH3(l), 50% Cu2O, 0.5% Pd(OAC)2,
NMP, 800 psi, 130 °C, 24 h; (ii) 1, Cs2CO3, Pd(OAc)2, P(t-Bu)3,
toluene, reflux, 36 h; (iii) iodobenzene, Cu, K2CO3, 18-crown-6, o-
dichlorobenzene, reflux, 18 h; (iv) (a) n-BuLi, THF, −78 °C, 1 h, (b)
Cl2Ph2Si, 12 h; (v) (a) n-BuLi, THF, −78 °C, 1 h, (b) Cl2PhP, 18 h,
(c) H2O2, CH2Cl2, rt, 24 h; (vi) 5, 10% Cu2O, Cs2CO3, Salox,
sulfolane, 140 °C, 72 h; (vii) (a) thioacetamide, 5% CuI, Cs2CO3, t-
BuOK, Salox, sulfolane, 140 °C, 72 h, (b) H2O2, CH2Cl2, HOAc, rt, 24
h.
stability (Tg > 150 °C),3 which are advantageous to
electroluminescence (EL) efficiency and stability of OLEDs.
In terms of synthetic strategy, we were able to prepare SP2X
series from 2-bromo-, 2-hydroxyl-, or 2-iodo-9,9′-spirobifluor-
ene (i.e., 1, 4, or 5 in Scheme 1). Whereas 1 and 4 can be
readily prepared according to the reported procedures,5 we
have encountered difficulty in the preparation of 5. In fact, 5 is
Received: February 18, 2014
Published: April 10, 2014
© 2014 American Chemical Society
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Letter
a previously unknown compound, and no synthesis report can
be found in the literature.
Table 1. Physical Properties of the SP2X Series
SP
λmax (nm) 297,
SP2N
308,
SP2Si
295,
SP2PO
SP2O
296,
SP2SO2
In general, there are three synthetic strategies used to prepare
monosubstituted 9,9′-spirobifluorene (SP). The first one we
tried is similar to the preparation of 1, i.e., synthesis from 2-
iodo-9-fluorenone by reacting with a Grinard reagent of 2-
bromobiphenyl.5 However, such reaction conditions were too
harsh to obtain intact 5. The other plausible procedure is a
direct iodination on 9,9′-spirobifluorene. The reaction we tried
yielded complicate products including mono- and multiple di-
or triiodo species, which are virtually inseparable from each
other, or pure 2 was only obtained in very low yields.
Alternatively, 5 can be prepared by the replacement of the
amino group of 2-amino-9,9′-spirobifluorene (2) by potassium
iodide following a conventional Sandmeyer procedure.6
However, in addition to biphenyl-, azobenzene-, and phenol-
type side products, Sandmeyer reactions are often plagued with
position isomers of halide substituents. In this case, 5 was
obtained together with 3-iodo-9,9′-spirobifluorene and 9,9′-
spirobifluorene as well. The separation of 5 from these side
products has been proven difficult and hampers further
materials application.
a
ab
307,
319
298,
309
309
369
308
309
a
fl
λmax (nm) 328
420
339
348
344
390
b
Eg (eV)
3.91
3.10
3.82
3.72
−5.88
3.37
3.57
c
HOMO
(eV)
−5.64
−5.49
−5.87
−5.55
−5.71
d
LUMO
−1.73
−2.39
−2.05
−2.16
−2.18
−2.14
(eV)
e
Tg (°C)
82
153
242
346
442
2.55
132
223
283
459
2.86
156
280
324
429
2.81
149
g
145
219
289
426
2.83
e
Tc (°C)
145
206
257
2.97
e
Tm (°C)
255
414
2.82
f
Td (°C)
h
ET (eV)
a
b
Measured in dichloromethane. Optical gap energy estimated from
c
the onset of the absorption spectrum in thin film state. Measured in
the solid state by a low-energy photoelectron spectrometer (Riken-
d
e
Keiki AC-2). LUMO = Eg + HOMO. Obtained by differential
f
scanning calroimetry (DSC). Obtained by thermogravimetric analysis
(TGA). Not observed. Measured in 2-methyltetrahydrofuran at 77K.
g
h
Accordingly, a renovated and reliable synthesis has been
successfully developed for the preparation of 5 in the present
study (Scheme 2).
electron on amine nitrogen donor, SP2N display significant red-
shifting in absorption wavelength. Similarly, the lone-pair
electron on aryloxy oxygen donor of SP2O causes absorption
wavelength red-shifting but to a smaller extent compared with
that of SP2N, although SP2O exhibits some weak absorption
bands extended beyond 350 nm. On the other hand, solution
(in dichloromethane) PL spectra of SP2X series are all red-
shifted when compared with that of SP. Although the most red-
shifting compound is SP2N similar to that observed in
absorption spectra, the second most red-shifting compound is
SP2SO2 instead of SP2O. Different red-shifting trends found for
absorption and PL spectra is not uncommon. The absorption
energy is basically determined by Frank−Condon excited
states, which are transformed to other energy-stabilized states
before light emission based on Kasha’s rule. Such energy-
stabilized states (or light-emitting states) are dependent on the
nature of surrounding matrix of the compound. Therefore, the
second most red-shifting PL spectra of SP2SO2 may be
attributed to the relatively large molecular dipole moment
among the series. The light-emitting state of a compound with
larger dipole moment is more susceptible to the surrounding
matrix which is dichloromethane solvent in this case. A different
red-shifting trend has been observed for the thin film samples,
with PL (fluorescence) wavelengths of 335, 432, 390, 380, 387,
and 393 nm for SP, SP2N, SP2Si, SP2PO, SP2O, and SP2SO2,
respectively. For gauging the difference of ET in solution and
thin film, the time-delayed neat film PL (phosphorescence)
spectrum was also recorded for SP2N. When compared with
the wavelength (∼486 nm) of the solution PL spectrum, the
highest energy emission band appears as a shoulder sideband
(∼498 nm) in the PL spectrum of SP2N neat film (Figure 1).
Such results illustrate that the rigid and bulge chemical
structure of SP greatly hinders the molecular aggregation in
condense phase and inhibits ET from largely decreasing. Except
for SP2N, ET’s of SP2X series are all greater than 2.80 eV, which
is sufficiently high for the host material of blue phosphorescent
dopant FIrpic. Somewhat different from those red-shifting
trends observed for absorption and fluorescence, the second
most red-shifting phosphorescence is not from SP2SO2 or
SP2O but from SP2PO. Similar to the dipolar sulfone group of
SP2SO2, phosphine oxide of SP2PO has dipolar characteristics.
Scheme 2. Preparation of 2-Iodo-9,9′-spirobifluorene (5)
After modification from our previous synthesis of 2,2′-
dibromo-9,9′-spirobifluorene,7 compound 5 was readily pre-
pared from 4-(trimethylsilyl)-2′-bromobiphenyl (7) with a two-
step reaction. Compound 5 prepared by our new method was
pure with high isolation yields of 83%. The better reactivity of
the Csp2−I bond facilitates the iodine replacement with oxygen
or sulfur in the following synthesis of SP2O and SP2SO2, where
the bromo congener 1 fails to convert.
For the preparation of SP2N, one key precursor 2-amino-
9,9′-spirobifluorene (2) was synthesized cleanly with high
isolation yields. Our synthesis of 2 is different from that of
conventional reduction of 2-nitro-9,9′-spirobifluorene or 2,2′-
dinitro-9,9′-spirobifluorene.8 The synthesis of 2 was achieved
smoothly via copper-mediated amination of 1 under a high
pressure of ammonia. For the copper-mediated amination
reacion herein, an addition of 0.5 mol % of palladium(II)
acetate was found very effective in promoting reaction yields.
SP2Si and SP2PO were facilely synthesized from treating
lithium−halogen exchanged 1 with dichlorodiphenylsilane and
dichlorophenylphosphine, respectively.
The UV−vis absorption and photoluminescence (PL)
spectra of SP2X series were studied in dichloromethane (Figure
S24, Supporting Information), and data are shown in Table 1.
Except for SP2N and SP2O, the absorption spectra of SP2X
series exhibit little red-shifting compared with that of SP. The
results can lead us to conclude that silicon, phosphine oxide,
and sulfur dioxide bridging moieties can break up the extension
of π-conjugation between two 9,9′-spirobifluorene moieties.
However, due to the π-conjugation nature of the lone-pair
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Organic Letters
Letter
°C), which are prone to submicrometer-size aggregation
scattering visible light.
Four SP2X-hosted FIrpic dopant OLED devices were
fabricated by sequential thermal vacuum deposition of thin
film layer of organic material and LiF−Al as the final cathode
electrode on ITO (indium−tin-oxide)-coated glass substrate,
ITO/NPB (40 nm)/TCTA (10 nm)/SP2X:10% FIrpic (20
nm)/TPBI (30 nm)/LiF (1 nm)/Al (100 nm). As clearly
shown in the data (Figure 3 and Table 2), OLEDs having
Figure 1. Absorption and fluorescence (in CH2Cl2) and phosphor-
escence (in 2-Me-THF, 77K) spectra of SP2N. Time-delayed neat film
PL spectrum at 15 K is also included.
The electrochemical properties of the SP2X series should be
highly informative about the energy level of frontier molecular
orbitals. However, cyclic voltammetry (CV) measurement only
acquired the oxidation potentials of SP2N and SP2O and
featureless reduction voltammograms were obtained for all
SP2X series (see Figure S25, Supporting Information). Hence,
we estimated the highest occupied molecular orbital (HOMO)
energy level of SP2X series in solid state by a low energy
photoelectron spectrometer (Riken-Keiki AC-2). The lowest
unoccupied molecular orbital (LUMO) energy level was
calculated from the HOMO energy level and the optical gap
(absorption onset energy) of the materials measured in thin
film state. Based on our measurement, HOMO energy levels
varied from −5.49 to −5.88 eV, of which SP2N and SP2O have
the higher levels than the other three consistent with the
electron-donating nature of the bridging heteroatom. Excluding
SP2N and SP2O, SP2PO and SP2SO2 are two materials with
deeper LUMO energy levels, which are in accordant with the
non-π-conjugated electron withdrawing nature of phenyl-
phosphine oxide and sulfone, respectively. Differently, SP2N
and SP2O have arylamine and aryloxy as a π-conjugated
bridging donor, respectively. Such bridging donors potentially
induce intramolecular charge transfer (ICT) which decreases
LUMO energy level as well.
All five compounds of the SP2X series are thermally stable
with Td 414−459 °C, which ensure the materials safely
manageable in vacuum-thermal-deposition process. Moreover,
all six compounds of the SP2X series essentially inherited the
high Tg nature of 9,9′-spirobifluorene because high Tg in a
range of 130−150 °C was observed (see Figure S26,
Supporting Information). Figure 2 exhibits the photo image
of thin film samples of SP2X series on a glass substrate, which
were prepared by vacuum-thermal deposition. Whereas thin
films of SP2N, SP2PO, SP2O, and SP2SO2 are transparent, both
thin films of SP and SP2Si are totally opaque and semiopaque,
respectively. Nontransparent feature of the samples can be
attributed to the relatively low Tg of SP (82 °C) and SP2Si (132
Figure 3. Electroluminescence characteristics of OLEDs.
Table 2. Characteristics of OLEDs
max L
L, EQE, voltage
max efficiency
(%, cd/A, lm/W) CIE (x, y)
a
b
(cd/m2) (cd/m2, %, V)
SP2Si
5845
7579
6405
6200
208, 5.0, 6.6
2043, 4.0, 9.4
191, 4.5, 6.2
1598, 3.8, 8.6
258, 5.9, 4.9
2204, 5.0, 7.5
272, 6.1, 5.1
2164, 5.0, 7.6
5.6, 12.6, 8.1
4.5, 9.6, 6.1
6.0, 13.1, 8.9
6.1, 13.4, 8.7
(0.17, 0.38)
SP2O
(0.17, 0.39)
(0.17, 0.41)
(0.19, 0.42)
SP2PO
SP2SO2
a
b
At 2 and 20 mA/cm2, respectively. At ∼7 V.
SP2PO or SP2SO2 as the host material exhibit much better
performance than SP2Si and SP2O, in terms of external
quantum efficiency (%), luminous efficiency (cd/A), or power
efficiency (lm/W). We may attribute such results to the bipolar
properties of host materials, SP2PO and SP2SO2. Both
phenylphosphine oxide and sulfur dioxide bridging moieties
are electron deficient functional groups that enhance the
electron transporting ability of SP2X host materials, of which
hole transporting is the main characteristic. Considering the
charge balance of OLEDs, SP2PO and SP2SO2 can be expected
to be better host materials than SP2Si or SP2O.
In order to understand the hole and electron transporting
nature of SP2X series, we also fabricated two kinds of single-
charge carrier-dominated devices for exploring the nature of
material charge carrier in terms of the current density of such
devices. The hole-dominated device has a configuration of
ITO/NPB(40 nm)/TCTA(10 nm)/SP2X(15 nm)/NPB(20
nm)/Al(100 nm). The electron-dominated device has a
configuration of ITO/BCP(10 nm)/SP2X(15 nm)/TPBI(20
Figure 2. Photo images of thin film samples on glass substrate: SP,
SP2N, and SP2Si (top row from left to right); SP2PO, SP2O, and
SP2SO2 (bottom row from left to right).
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nm)/LiF(1 nm)/Al(100 nm). As shown in the inset of Figure
4, we used high-lying LUMO NPB to limit the electron carrier
in a hole-dominated device and low-lying HOMO BCP and
TPBI to limit the hole carrier in electron-dominated device.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: chintchen@gate.sinica.edu.tw.
*E-mail: chenct@ntu.edu.tw.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Science Council (NSC 101-2113-M-
001-004 -MY2), Academia Sinica, and National Taiwan
University for financial support.
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Considering the current density of hole-dominated devices
studied herein (Figure 4 top), the results indicate that the hole
mobility of SP2X series is descending in order of SP2N ≫ SP2O
> SP2Si > SP2PO and SP2SO2. The situation of the current
density of electron-dominated device is quite similar but in a
reverse order (Figure 4 bottom). The magnitude of current
density clearly is decreasing in the order of SP2PO > SP2SO2 >
SP2Si > SP2O > SP2N, which is accordant with the anticipated
order of material electron mobility based on the structure
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In summary, we have successfully synthesized and charac-
terized a series of heteroatom-bridged bis-spirobifluorene
derivatives SP2X. Regarding material synthesis, we were able
to synthesize previously unknown 2-iodo-9,9′-spirobifluorene.
We have reported an improved method of high-yield synthesis
of 2-amino-9,9′-spirobifluorene. Except for SP2N, these bis-
spirobifluorene new compounds have sufficiently high ET as
host material for FIrpic blue phosphorescent dopant in OLED.
Most of them are morphologically stable with Tg > 140 °C. We
have demonstrated that the performance of such OLEDs is
highly dependent on charge carrier nature. Based on our device
data, bipolar SP2PO and SP2SO2 outperform SP2Si and SP2O
because of their electron-transporting characteristics.
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthetic details, structural characterizations, physical charac-
terizations, and OLED fabrication and measurement. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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