Preparation and electroluminescent characteristics of a series of cyclometalated Ir(III) complexes based on phenylpyridines with a diphenylamino group

Article abstract of DOI:10.1246/cl.2005.1378

A series of cyclometalated iridium complexes with a strong electron-donating and bulky diphenylamino group was prepared and the phosphorescence wavelengths were found to be greatly dependent on the substituent positions: some of them afforded high perform

Full text of DOI:10.1246/cl.2005.1378

1378
Chemistry Letters Vol.34, No.10 (2005)
Preparation and Electroluminescent Characteristics of a Series of Cyclometalated Ir(III)
Complexes Based on Phenylpyridines with a Diphenylamino Group
Jun-ichi Nishida, Hidenori Echizen, Takeshi Iwata,y and Yoshiro Yamashitaꢀ
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering,
Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8502
yCentral Reserch Labolatory, Takasago International Corporation, Nishiyawata, Hiratsuka 254-0073
(Received August 5, 2005; CL-051011)
A series of cyclometalated iridium complexes with a strong
tion of 2-chloro-4-iodopyridine with diphenylamine. Ligands
L2a and L2c with a diphenylamino group as R4 were prepared
using the Suzuki coupling reaction of meta-diphenylamino-
phenylboronic ester 5, which was obtained from 3-bromo-
phenyldiphenylamine by a conventional method. Ligand L2b
was obtained by the Suzuki coupling reaction of a ppy derivative
7 with a bromo substituent. Ligands L3a and L3b with a di-
phenylamino group as R5 were prepared by the coupling reaction
of para-diphenylaminophenylboronic ester 6 with the corre-
sponding bromopyridines. Ir complexes 1–3 with these and acac
ligands were prepared via the dichloro-bridged dimers according
to the method reported by Nonoyama.8 Purification was per-
formed via chromatography, as described in the Supporting
Information.
Remarkable differences in the absorption spectra of the Ir
complexes 1–3 were observed.9 Complex 3a with a diphenyl-
amino group as R5 exhibits a strong absorption at 350–400 nm
ascribed to the ꢀ–ꢀꢀ transition of the ligand. The absorption
maximum is red-shifted to 418 nm in the complex 3b with an
electron-withdrawing trifluoromethyl group as R1. On the other
hand, the complex 2a with a diphenylamino group as R4 shows
the absorption around 300 nm with a weak absorption at 482 nm.
Other derivatives of 2 show the similar weak absorptions (2b;
501 nm, 2c; 580 nm). In contrast, complex 1 having a diphenyl-
amino group on the pyridine ring exhibits a broad absorption at
electron-donating and bulky diphenylamino group was prepared
and the phosphorescence wavelengths were found to be greatly
dependent on the substituent positions: some of them afforded
high performance electroluminescence (EL) devices.
Phosphorescent materials incorporating heavy transition
metals have attracted considerable attention for applications as
highly efficient EL emitters.1 Cyclometalated iridium complexes
such as Ir(ppy)3 (ppy; 2-phenylpyridine) and (ppy)2Ir(acac)
(acac; acetylacetonate) are excellent phosphorescent materials
whose EL devices have nearly quantitative internal quantum ef-
ficiencies.2,3 Color tuning is important for fabricating EL devices
and has been accomplished by introducing substituents in the
ppy ligand. For example, the substituent effects of electron-with-
drawing fluoro, trifluoromethyl, and pentafluorophenyl groups
have been systematically investigated4,5 and these groups were
found to be effective for shifting the emission maxima to shorter
wavelengths. On the other hand, electron-donating diphenyl-
amino groups have often been used as units of hole transporting
materials.6 When this group is introduced in the ppy ligand of
Ir complexes, it would strongly affect the energy levels of
the Ir complexes leading to significant phosphorescence color
changes. Introduction of the diphenylamino group is also expect-
ed to enhance the EL efficiency owing to the good hole-trans-
porting property as well as the bulky size suppressing a self-
quenching process.7 We report here the synthesis of a series of
(ppy)2Ir(acac) complexes 1–3 bearing a diphenylamino group
at the different positions and their physical properties involving
the EL behavior (Scheme 1).
Ph2N
(a)
L1
N
R2
R3
R1
4
Cl
The syntheses of new ppy ligands L1–L3 are shown in
Scheme 2. Thus, the ppy ligand L1 having a diphenylamino
group as R2 was prepared by the Suzuki coupling reaction of
phenylboronic acid with (2-chloro-4-pyridyl)diphenylamine
(4), which was prepared by the Cu(I)-catalyzed substitution reac-
Ph2N
Ph2N
N
O
O
(b)
B
L2a; R1 = R2 = R3 = H
7; R 1 = Br
5; m-isomer
6; p-isomer
(d)
1; R2 = Ph2N, R1 = R3 = R4 = R5 = H
R2 = R3 = H
L2b; R 1 = Ph
R2 = R3 = H
L2c; R1 = H
R2 ; R3 = Benzo
(c)
R1
2a; R4 = Ph2N, R1 = R2 = R3 = R5 = H
2b; R4 = Ph2N, R1 = Ph,
R2 = R3 = R5 = H
2c; R4 = Ph2N, R1 = R5 = H,
L3a,b
R2
N
R3
O
Ir
R2; R3 = Benzo
O
Scheme 2. Preparation of ligands L1–L3 by the Suzuki cou-
pling reactions (NaOHaq, Pd(PPh3)4, toluene). (a) L1: phenyl-
boronic acid, 69%; (b) L2a: 5, 2-bromopyridine, 55%; 7: 5,
2,5-dibromopyridine, 74%; L2c: 5, 1-chloroisoquinoline, 61%;
(c) L2c: phenylboronic acid, 50% (d) L3a: 6, 2-bromopyridine,
95%; L3b: 6, 2-bromo-5-trifluoromethylpyridine, 84%.
3a; R5 = Ph2N, R1 = R2 = R3 = R4 = H
3b; R5 = Ph2N, R1 = CF3,
R4
R5
2
R
2 = R3 = R4 = H
Scheme 1.
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.10 (2005)
1379
Table 1. Emission maximaa and redox potentialsb of com-
plexes 1–3 and (ppy)2Ir(acac)
and yellow color, respectively. The external quantum efficiency
(ꢁext) and power efficiency of the device using 3a were 11.8%
and 23.7 lm Wꢁ1 at 100 cd mꢁ2, respectively, which are higher
than those of (ppy)2Ir(acac) (11.4% and 22.4 lm Wꢁ1) fabricated
under the same conditions. The device using complex 2a
also showed a high performance (ꢁext: 7.6%, 7.4 lm Wꢁ1) at
pa
pc
Complex
ꢂmaxem/nm
E1
E1
(ppy)2Ir(acac)
1
516
518
585
611
671
530
555
þ0:85
þ0:70
þ0:58
þ0:58
þ0:55
þ0:78
þ0:96
ꢁ2:10
ꢁ2:24
ꢁ2:00
ꢁ1:84
ꢁ1:58
ꢁ2:12
ꢁ1:70
2a
2b
2c
3a
3b
100 cd mꢁ2
.
In conclusion, we have prepared a series of cyclometalated
iridium complexes with an electron-donating diphenylamino
group and fabricated high performance EL devices. They
showed remarkable color changes of phosphorescence depend-
ing on the substitution positions. Since the diphenylamino group
has been used as a unit of dendrimers,10 the present systems
would be possibly developed to dendrimers.
aIn CH2Cl2. bnBu4NPF6 in DMF, V vs SCE, scan rate
100 mV/s, Pt electrode. The oxidation waves of 1–3 were
reversible in their CVs.
350–380 nm with no clear red-shift of end-absorption.
This work was supported by the 21st Century COE program
and a Grant-in-Aid for Scientific Research on Priority Areas
(No. 15073212) of the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
Photoluminescence spectra9 of these derivatives were meas-
ured and the emission maxima are summarized in Table 1. A
comparison of 1, 2a, and 3a containing only a diphenylamino
group at the different positions demonstrates a clear positional
effect. Thus, complex 1 shows almost the same emission maxi-
mum as (ppy)2Ir(acac). In contrast, red-shifts are observed in 2a
(69 nm) and 3a (14 nm). It is noteworthy that the difference de-
pending on the substitution position reaches to 67 nm. Further-
more, extention of ꢀ-conjugation in 2 induces remarkable
red-shifts to afford red-color emission in 2b (611 nm) and 2c
(671 nm). Introduction of an electron-withdrawing trifluoro-
methyl group on the pyridyl ring also brings about a red-shift
as observed in 3b.
References and Notes
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2
These substituent effects are considered to be closely related
to the HOMO and LUMO energy levels of the complexes. In
order to investigate the energy levels, the redox potentials were
examined by cyclic voltammetry (CV) and differential pulse vol-
tammetry (DPV). The redox peaks measured by DPV are listed
in Table 1. The complexes 1–3 except 3b show lower oxdation
potentials than (ppy)2Ir(acac) owing to the electron-donating di-
phenylamino group. Complexes 2a–2c show lower oxidation po-
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Preliminary organic light emitting devices of complexes 2a,
3a, and (ppy)2Ir(acac) were prepared by a thermal deposition
method onto a clean glass substrate with an indium-tin-oxide
(ITO). A 40-nm-thick film of N,N0-di(naphthalen-1-yl)-N,N0-di-
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thick layer of 4,40-di(carbazol-9-yl)biphenyl (CBP) consisting
of 6% the Ir complex, a 10-nm-thick layer of 2,9-dimethy-4,7-di-
phenylphenanthroline (BCP) as hole block layer and a 35-nm-
thick of tris(8-hydroxyquinoline)aluminium (Alq3) as electron
transporting layer, and 0.5-nm-thick LiF and 100-nm-thick Al
layers as cathode electrode were successively deposited. The
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tionale de l’Eclairage (CIE) coordinates of 2a (x ¼ 0:559,
y ¼ 0:436) and 3a (x ¼ 0:379, y ¼ 0:609) mean dark orange
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Absorption and phosphorescence spectra are depicted in
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Published on the web (Advance View) September 10, 2005; DOI 10.1246/cl.2005.1378