Tri-, tetra- and pentamers of 9,9'-spirobifluorenes through full ortho -linkage: High triplet-energy pure hydrocarbon host for blue phosphorescent emitter

Article abstract of DOI:10.1021/ol1024184

4,4'-Dibromo-9,9'-spirobifluorene was first synthesized, and from this intermediate, three fully ortho-linked tri-, tetra-, and pentamers of 9,9'-spirobifluorenes were constructed. The full ortho-linkage impedes the π conjugation of fluorene units, and guarantees their high triplet energies (E_T = 2.80 eV). A device with the trimer as the first pure hydrocarbon host material for blue phosphor FIrpic shows a maximum current efficiency of 25 cd/A.

Full text of DOI:10.1021/ol1024184

ORGANIC
LETTERS
2010
Vol. 12, No. 24
5648-5651
Tri-, Tetra- and Pentamers of
9,9′-Spirobifluorenes through Full
ortho-Linkage: High Triplet-Energy Pure
Hydrocarbon Host for Blue
Phosphorescent Emitter
Cong Fan,‡ Yonghua Chen,† Pei Gan,‡ Chuluo Yang,*,‡ Cheng Zhong,‡
Jingui Qin,‡ and Dongge Ma*,†
Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic
Materials, Wuhan UniVersity, Wuhan 430072, People’s Republic of China, and State Key
Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, People’s Republic of China
clyang@whu.edu.cn; mdg1014@ciac.jl.cn
Received October 7, 2010
ABSTRACT
4,4′-Dibromo-9,9′-spirobifluorene was first synthesized, and from this intermediate, three fully ortho-linked tri-, tetra-, and pentamers of
9,9′-spirobifluorenes were constructed. The full ortho-linkage impedes the π conjugation of fluorene units, and guarantees their high triplet energies (ET )
2.80 eV). A device with the trimer as the first pure hydrocarbon host material for blue phosphor FIrpic shows a maximum current efficiency of 25 cd/A.
Spiro compounds have been widely applied in organic opto-
electronics due to their rigid structures, good solubility and
amorphous nature.1 Among them, 9,9′-spirobifluorene and its
derivatives have evoked considerable interest for application
in organic light-emitting diodes (OLEDs), photovoltaic cells and
field-effect transistors.2 Apart from the general properties of
spiro compounds, 9,9′-spirobifluorene and its derivatives exhibit
high quantum efficiency and nondispersive ambipolar carrier
transporting properties,3 making them desirable host materials
in OLEDs. Up to the present, most of the 9,9′-spirobifluorene-
based small molecules and polymers are constructed by para-
linkage,4 which are usually derived from 2-bromo-9,9′-
spirobifluorene or 2,7-dibromo-9,9′-spirobifluorene. However,
we note that the modifications on the para-linked oligo(9,9′-
spirobifluorene)s usually bring about the π conjugation between
chromophores, leading to the depreciation of the triplet energy.5
Therefore, their usage as host materials for phosphorescent
emitters is limited, especially for blue phosphorescent emitters.
‡ Wuhan University.
† Chinese Academy of Sciences.
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10.1021/ol1024184  2010 American Chemical Society
Published on Web 11/11/2010

We have previously reported that the oligo(9,9′-spirobifluorene)
24TSF by partial ortho-linkage exhibits a higher triplet energy
(ET ) 2.55 eV) than its para-linked isomer (ET ) 2.28 eV),
and 24TSF can be utilized as full-hydrocarbon host material
for green and red electrophosphorescence.6 In this letter, we
report the elegant synthesis of new tri-, tetra- and pentamers of
9,9′-spirobifluorenes by full ortho-linkage. The molecular design
impedes the π conjugation of fluorene units, and guarantees
their high triplet energies (ET ) 2.80 eV). We also report their
unique photophysical properties as well as their application as
host materials for blue phosphorescent OLEDs.
with 1 equiv of n-BuLi by a lithium-halogen exchange
reaction, then reacted with carbon dioxide followed an
dehydration in concentrated sulfuric acid to afford 4-bromo-
9-H-fluoren-9-one (3).7,8 Afterward, 3 was treated with 1
equiv of (2′-bromobiphenyl-2-yl) lithium (2), followed by
an intramolecular ring-closure reaction to afford the desired
4 in a good isolated yield of 87%. The fully ortho-linked
trimer (6) was synthesized by Suzuki coupling reaction of 4
with 2 equiv of 9,9′-spirobifluorene-4-yl pinacol boronic ester
(5)6a with an isolated yield of 56% (Scheme 1).
To build up the fully ortho-linked tetramer (10), a single
ortho-brominated dimer, 4-(4′-bromo-9,9′-spirobifluorene-4-yl)-
9,9′-spirobifluorene (8) is needed. As shown in Scheme 2, there
To obtain the fully ortho-linked oligo(9,9′-spirobifluo-
rene)s, 4,4′-dibromo-9,9′-spirobifluorene (4) is a key inter-
mediate, which cannot be synthesized from the direct
Scheme 2. Synthesis of the Single ortho-Brominated Dimer 8
Scheme 1. Synthesis of the Key Intermediate
4,4′-Dibromo-9,9′-spirobifluorene 4 and the Fully ortho-Linked
Trimer 6
bromination of 9,9′-spirobifluorene due to the regioselectiv-
ity. An alternative synthetic route is adopted as shown in
Scheme 1. 2,2′-Dibromobiphenyl (1) was initially treated
are two synthetic routes for 8. Though only one step reaction
in route A, the yield was low (ca. 20%) due to the formation
of the trimer 6. In route B, 4-(9,9′-spirobifluorene-4-yl)-9-H-
fluoren-9-one (7) was first obtained by combination of 3 and 5
through Suzuki coupling reaction, and then the dimer 8 can be
achieved from 7 according to the same strategy mentioned
above, with an excellent yield of 91%. Subsequently, 8 was
conveniently converted to its pinacol boronic ester 9. Accord-
ingly, the tetramer 10 can be synthesized by the combination
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Org. Lett., Vol. 12, No. 24, 2010
5649

of the dimer 8 and its pinacol boronic ester 9 through Suzuki
coupling reaction with an isolated yield of 50% (Scheme 3).
from 244 °C (trimer), 280 °C (tetramer), to 326 °C
(pentamer). The very high Tg of the oligomers enable them
to be highly amorphous solids. The three oligomers show
irreversible oxidation process in the cyclic voltammetry like
the case of the single 9,9-spirobifluorene unit, possibly due
to the unblocked electrochemical active sites of the mol-
ecules.9 Their HOMO levels evaluated from the onsets of
the oxidation potentials are equal (-6.08 eV, relative to
vacuum level).
Scheme 3. Synthesis of the Fully ortho-Linked Tetramer 10
Figure 1. Absorption and emission spectra of the three oligomers
in CH2Cl2 solution at room temperature and their phosphorescence
spectra in film at 77 K.
Based on the obtained intermediates, the fully ortho-linked
pentamer (12) can be facilely constructed (Scheme 4). After
converting 4 to 4,4′-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)-9,9′-spirobifluorene (11), the pentamer 12 was afforded
by treating 2 equiv of the dimer 8 and 1 equiv of 11 through
Suzuki coupling reaction with a good yield of 80%. All the
Figure 1 shows the absorption and fluorescence spectra
of the three oligomers in CH2Cl2 solution at room temper-
ature and their phosphorescence spectra in film at 77 K. The
oligomers show almost identical electronic spectra with the
maximum absorption at 308 nm, which is attributed to
the π-π* transition of the fluorene unit.10 This contrasts to
the fully para-linked oligo(9,9′-spirobifluorene)s,11 which
show consistent red shift in the peak wavelength from 334
nm (dimer), 350 nm (trimer) to 360 nm (tetramer) with the
backbone length. The optical band gaps (Eg) determined from
the onsets of the absorption spectra are 3.93 eV. Similarly,
the three oligomers exhibit the same fluorescence spectra
with the maximum emissions at 374 nm. Their triplet
energies (ET) are determined to be the same level of 2.80
eV by the highest-energy vibronic sub-band of the phos-
phorescence spectra. Compared with the para-linked oligo(9,9′-
spirobifluorene), namely SSS (ET ) 2.28 eV),5a or the
partially ortho-linked oligo(9,9′-spirobifluorene), namely
24TSF, (ET ) 2.55 eV),6a the fully ortho-linked trimer shows
the significantly higher triplet energy. Moreover, the triplet
energies remain invariant with the extension of the spirobi-
fluorene units. This indicates that full ortho-linkage of 9,9′-
spirobifluorene completely impedes the π conjugation of
fluorene units. The optical data are collected in Table 1.
1
compounds have been fully characterized by H NMR, 13C
NMR, mass spectrometry, and elemental analysis (see Sup-
porting Information).
Scheme 4. Synthesis of the Fully ortho-Linked Pentamer 12
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97, 211. (b) Poriel, C.; Liang, J. J.; Berthelot, J. R.; Barriere, F.; Cocherel,
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The glass transition temperatures (Tg) of the three oligo-
mers increase with the enlargement of the molecular system
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5650
Org. Lett., Vol. 12, No. 24, 2010

Table 1. Optical Properties of the Three Oligomers
λabs, max λPL, max
HOMO LUMO
ET
(nm)a
(nm)a Φsol (%)b
(eV)c
(eV)d (eV)e
trimer
tetramer
pentamer
308
308
308
374
374
374
63
58
60
6.08
6.08
6.08
2.15
2.15
2.15
2.80
2.80
2.80
a Measured in CH2Cl2 solution. b Quantum yields in CH2Cl2 solution
(9,10-diphenylanthracene in cyclohexane as reference, ΦF ) 0.90).
c Determined from the onsets of oxidation potentials in CV. d Deduced from
HOMO and Eg. e Determined from the phosphorescence spectra in film.
The triplet energies of the three oligomers are high enough
for blue phosphorescent emtters, hence we fabricated the
devices by using the trimer as host for blue and green
phosphorescent emitters. The device configurations are as
follows: blue device A: ITO/MoO3 (10 nm)/NPB (80 nm)/
TCTA (5 nm)/ trimer:FIrpic (20 nm)/TPBi (30 nm)/LiF (1
nm)/Al (120 nm); green device B: ITO/MoO3 (10 nm)/NPB
(80 nm)/TCTA (5 nm)/ trimer:Ir(ppy)3 (20 nm)/TPBi (40
nm)/LiF (1 nm)/Al (120 nm). In these devices, 1,4-bis(1-
naphthylphenylamino)-biphenyl (NPB) is used as the hole-
transporting material; 4,4′,4′′-tri(N-carbazolyl)triphenylamine
(TCTA) is used as exciton blocking material; 1,3,5-tris(N-
phenylbenzimidazol-2-yl)benzene (TPBi) is utilized as elec-
tron transporting as well as hole blocking material; irid-
ium(III) [bis(4,6-difluorophenyl)pyridinato-N,C2′]picolinate
[FIrpic] or iridium(III) fac-tris(2-phenylpyridine) [Ir(ppy)3]
is used as the triplet emitter, with optimized doping levels
of FIrpic and Ir(ppy)3 at 8% and 11%, respectively; MoO3
and LiF serve as hole- and electron-injecting materials,
respectively. Current density-voltage-luminance (J-V-L)
characteristics and current efficiency, power efficiency versus
current density of the devices are shown in Figure 2. Device
A displays a typical emission from the blue phosphor FIrpic
with CIE coordinates (0.16, 0.33), and achieves a maximum
current efficiency of 25 cd/A, a maximum power efficiency
of 17 lm/W, and a maximum luminance of 13 073 cd/m2 at
15 V. Device B reveals a maximum current efficiency of 66
cd/A, a maximum power efficiency of 46 lm/W, and a
maximum luminance of 47 885 cd/m2 at 15 V, which are
comparable with the results we recently reported.12 Notice-
ably, the efficiencies (ηc) of the devices decay gently with
the increasing current density. At the brightness of 100 cd/
m2 and 1000 cd/m2, the current efficiencies are still as high
as 23 cd/A and 19 cd/A for blue device A, 65 cd/A and 54
cd/A for green device B, respectively. This could be
attributed to the effective separation of the guest molecules
by the unique three-dimensional conformation of the host.
Figure 2. (a) J-V-L characteristics. (b) Current efficiency and
power efficiency versus current density of the devices.
In summary, we have successfully synthesized three novel
fully ortho-linked oligo(9,9′-spirobifluorene)s from the in-
termediate 4,4′-dibromo-9,9′-spirobifluorene. We note that
the glass transition temperatures of the three oligomers
significantly increase with the enlargement of the molecular
system; however their absorption and emission peaks show
no red shift and their high triplet energies remain invariant.
This indicates that full ortho-linkage of 9,9′-spirobifluorene
completely impedes the π conjugation of fluorene units. The
devices with trimer as host material and FIrpic or Ir(ppy)3
as guest emitter show maximum external quantum efficien-
cies of 11.6 and 17.3% for blue and green electrophospho-
rescence, respectively. This represents the first example of
using a pure hydrocarbon small molecule as an efficient host
for blue phosphorescent emitter.
Acknowledgment. We thank the National Natural Science
Foundation of China (Project Nos. 90922020, 50773057),
and the National Basic Research Program of China (973
Program 2009CB623602, 2009CB930603) for financial sup-
port.
Supporting Information Available: General experimental
information; synthesis and characterization of all intermedi-
ates and the three oligomers; DSC and CV curves of the
three oligomers; EL spectra. This material is available free
of charge via the Internet at http://pubs.acs.org.
(12) (a) Tao, Y.; Wang, Q.; Yang, C.; Wang, Q.; Zhang, Z.; Zou, T.;
Qin, J.; Ma, D. Angew. Chem. Int. Ed. 2008, 47, 8104. (b) Jiang, Z.; Chen,
Y.; Fan, C.; Yang, C.; Wang, Q.; Tao, Y.; Zhang, Z.; Qin, J.; Ma, D. Chem.
Commun. 2009, 3398. (c) Fan, C.; Chen, Y.; Jiang, Z.; Yang, C.; Zhong,
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OL1024184
Org. Lett., Vol. 12, No. 24, 2010
5651