The heteroleptic complexes containing 2,3-diphenylquinoline derivatives as phosphorescent materials

Article abstract of DOI:10.1016/j.jpcs.2007.10.126

New types of heteroleptic iridium complexes were designed and synthesized with two different species of chelating ligands (C^{^}N) in this study. Ir(ppy)₂(4-Me-2,3-dpq), Ir(ppy)(4-Me-2,3-dpq)₂, Ir(pq)₂(4-Me-2,3-dpq

Full text of DOI:10.1016/j.jpcs.2007.10.126

ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 69 (2008) 1320–1324
www.elsevier.com/locate/jpcs
The heteroleptic complexes containing 2,3-diphenylquinoline
derivatives as phosphorescent materials
Ã
So Youn Ahna, Myung Joo Koa, Yunkyoung Hab,
aDepartment of Chemical Engineering, Hongik University, Seoul 121-791, Republic of Korea
bDepartment of Information Display Engineering, Hongik University., Seoul 121-791, Republic of Korea
Received 27 June 2007; received in revised form 23 October 2007; accepted 30 October 2007
Abstract
New types of heteroleptic iridium complexes were designed and synthesized with two different species of chelating ligands (C^N) in this
study. Ir(ppy)2(4-Me-2,3-dpq), Ir(ppy)(4-Me-2,3-dpq)2, Ir(pq)2(4-Me-2,3-dpq) and Ir(pq)(4-Me-2,3-dpq)2 were prepared, where ppy,
4-Me-2,3-dpq and pq represent 2-phenylpyridine, 4-methyl-2,3-diphenylquinoline and 2-phenylquinoline, respectively. According to
our previous study, Ir(4-Me-2,3-dpq)2(acac) showed orange-red emission at 603 nm with the luminescence efficiency of 8.10 cd/A in
electroluminescence (EL) spectra. On the other hand, Ir(ppy)2(acac), a green phosphor, has been known to have an outstanding efficiency
of more than 45 cd/A. In order to improve the luminescence efficiency, it is necessary to design a phosphorescent material which is
capable of transferring the excited energy efficiently and properly. The heteroleptic Ir(III) complexes containing two different kinds of
ligands can have a high luminescence efficiency by intramolecular energy transfer from the energy-absorbing ligand to the luminescent
ligand, leading to a decrease in quenching or energy deactivation. Thus, the luminescence characteristics of the iridium complexes
prepared herein were investigated. Ir(ppy)2(4-Me-2,3-dpq), Ir(ppy)(4-Me-2,3-dpq)2, Ir(pq)2(4-Me-2,3-dpq) and Ir(pq)(4-Me-2,3-dpq)2
exhibited the emission maxima at 512, 512, 583, and 584 nm, respectively. Two different light-emitting mechanisms were suggested to
explain these phenomena.
r 2007 Elsevier Ltd. All rights reserved.
1. Introduction
leptic iridium complexes were reported so far [11–16],
compared with the well-developed classes of homo-
leptic iridium complexes probably due to synthetic
difficulty.
Organic light-emitting diodes (OLEDs) that involve
phosphorescent metal complexes as an emitting material
have been attracting a great deal of attention because they
exhibit higher efficiency than OLEDs that use conventional
fluorescent materials [1–9]. The purpose of the present
study was the molecular design and synthesis of phosphor-
escent emitters based on heteroleptic iridium(III) phenyl-
quinoline complexes [10]. The heteroleptic Ir(III)
complexes containing two different kinds of ligands can
have a high luminescence efficiency by intramolecular
energy transfer from the energy-absorbing ligand to
the luminescent ligand. However, relatively few hetero-
Our previous study showed that a 2,3-diphenylquinoline
iridium complex, Ir(4-Me-2,3-dpq)2(acac), exhibited its
photoluminescence (PL) emission at 604 nm with a
luminescence efficiency of 8.10 cd/A. The quinoline-based
iridium complexes possess relatively short phosphorescence
lifetimes that suppress triplet–triplet (TT) annihilation
which results in improved device quantum efficiency
at a high current density [17]. The Thompson group
reported that Ir(ppy)2(acac) and Ir(pq)2(acac) have emis-
sion peaks at 516 and 599 nm, with the life time of 1.6
and 2.0 ms, respectively [10,18]. In this study, we synthe-
sized the heteroleptic complexes involving 4-Me-2,3 dpq
and ppy/pq ligands, and investigated their luminescence
properties.
Ã
Corresponding author. Tel.: +82 2 320 1490; fax: +82 2 3142 0335.
E-mail address: ykha@hongik.ac.kr (Y. Ha).
0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2007.10.126

ARTICLE IN PRESS
S.Y. Ahn et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1320–1324
1321
2. Experimental section
(160 ml). The resultant dark-yellow precipitate was
collected by filtration. The solid was chromatogra-
All reactions were carried out under a dry argon
atmosphere.
phed on a silica-gel column with dichloromethane
(yield:80%).
2.1. Synthesis of ligand
2.2. Synthesis of iridium(III) acac complexes
2.1.1. 4-Me-2,3-dpq
This dpq-ligand derivative was obtained according to
the Friedlander reaction with 1-(2-amino-phenyl)-ethanone
(1.352 g, 10.0 mmol) and 1,2-diphenyl-ethanone (1.962 g,
10.0 mmol) in 30 ml of glacial acetic acid. The solution
was heated to reflux for 6 h at 110 1C. The reaction
mixture was then cooled and dripped slowly under stirr-
ing into an ice-cold solution of 50% ammonium hydroxide
2.2.1. Ir(C^N)2(acac)
The cyclometallated Ir(III) m-chloro-bridged dimer,
(C^N)2Ir(m-Cl)2Ir(C^N), was synthesized according to
the Nonoyama method with a slight modification. The
following reaction of the dimer with 2,4-pentandione gave
Ir(C^N)2(acac) (yield Ir(ppy)2(acac): 71%; Ir(pq)2(acac):
70%; Ir(4-Me-2,3-dpq)2(acac): 70%).
O
CH3COOH,H2SO4
+
N
reflux
O
NH2
Cl
Cl
O
O
O
O
(C^N)
(C^N)2 Ir
(C^N)2 Ir
Ir (C^N)2
IrCl3 nH2O
reflx
reflux
N
N
(C^N) =
N
,
,
N
N
N
O
(C^N) =
N
(C^N)2
Ir
(C^N)2 Ir
O
,
glycerol, reflux
O
(C^N)'
glycerol, reflux
N
N
N
N
Ir
Ir
(C^N)'
(C^N)' =
O
,
2
2
Fig. 1. Synthesis of the ligand and iridium complexes.

ARTICLE IN PRESS
S.Y. Ahn et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1320–1324
1322
2.3. Synthesis of heteroleptic Ir(III) complexes
2.3.1. Ir(ppy)2(4-Me-2,3-dpq)
Ir(ppy)2(4-Me-2,3-dpq)
Ir(ppy)(4-Me-2,3-dpq)2
Ir(ppy)2(acac) (1 mmol) and 4-Me-2,3-dpq (2.3 mmol)
were dissolved in 20 ml of glycerol in a 50 ml flask. The
mixture was refluxed for 10 h at 210 1C. The reaction
mixture was poured into 30 ml of 2 N HCl to give the crude
solid. Column chromatography on silica followed by
recrystallization in dichloromethane/hexanes yielded a
yellow powder of Ir(ppy)2(4-Me-2,3-dpq) (yield 21%.
FAB–MS: calculated 795; found 795).
1.0
0.5
2.3.2. Ir(ppy)(4-Me-2,3-dpq)2,
Ir(pq)2(4-Me-2,3-dpq),
Ir(pq)2(4-Me-2,3-dpq), Ir(pq)(4-Me-2,3-dpq)2
These heteroleptic complexes were prepared from the
reaction of the corresponding dimer with the hetero-ligand,
according to the procedure above.
0.0
1.5
400
wavelength (nm)
600
Ir(ppy)(4-Me-2,3-dpq)2: yield 18%. FAB–MS: calcu-
lated 935; found 935.
Ir(pq)2(4-Me-2,3-dpq): yield 20%. FAB–MS: calculated
895; found 895.
Ir(pq)2(4-Me-2,3-dpq)
Ir(pq)(4-Me-2,3-dpq)2
Ir(pq)(4-Me-2,3-dpq)2: yield 19%. FAB–MS: calculated
985; found 985.
1.0
0.5
0.0
2.4. Optical measurements
UV–Vis absorption spectra were measured on a Hewlett
Packard 8425A spectrometer. PL spectra were measured
on a Perkin-Elmer LS 50B spectrometer. UV–Vis and PL
spectra of iridium complexes were measured in 10ꢀ5 M
dilute CH2Cl2 solution.
400
wavelength (nm)
600
3. Results and discussion
Fig. 2. UV–Vis absorption spectra of the heteroleptic iridium complexes.
The synthesis of ligands and heteroleptic iridium
complexes was straightforward, according to the proce-
dures reported by Friedlander and by Nonoyama with a
slight modification, as summarized in Fig. 1.
The photoluminescence PL spectra of the heteroleptic
complexes are shown in Fig. 3. The emission maxima for
both Ir(ppy)2(4-Me-2,3-dpq) and Ir(ppy)(4-Me-2,3-dpq)2
appeared at 512 nm. Against our expectation, these
emission peaks are similar to those of Ir(ppy)3 (514 nm).
On the other hand, both Ir(pq)2(4-Me-2,3-dpq) and
Ir(pq)(4-Me-2,3-dpq)2 showed the PL peaks at 583 nm.
To explain these emission characteristics, the HOMO–
LUMO levels of the related complexes, Ir(C^N)2(acac),
where C^N ¼ ppy, pq, and 4-Me-2,3-dpq, are investigated
and the simple energy diagrams of these complexes are
shown in Fig. 4. Based on the HOMO–LUMO levels of
these related iridium complexes, two different luminescent
mechanisms are suggested. In both Ir(ppy)2(4-Me-2,3-dpq)
and Ir(ppy)(4-Me-2,3-dpq)2, MLCT absorptions occur
at both ppy and 4-Me-2,3-dpq ligands because HOMO
levels of Ir(ppy)2(acac) and Ir(4-Me-2,3-dpq)2(acac) are
very similar [18,19]. However, for the photoluminescence
the inter-ligand energy transfer (ILET) time from the
The UV–Vis absorption spectra of heteroleptic com-
plexes, Ir(ppy)2(4-Me-2,3-dpq) and Ir(ppy)(4-Me-2,3-dpq)2,
and Ir(pq)2(4-Me-2,3-dpq) and Ir(pq)(4-Me-2,3-dpq)2 were
investigated and compared as shown in Fig. 2. The intense
bands appearing between 200 and 320 nm are assigned to
1
the spin-allowed (p-p*) transitions of the ligands in the
complex. Due to the perturbation iridium metal, these
transitions are shifted with respect to those of the free
ligands. The bands extending into the visible region from
320 to 430 nm and the weak bands at the longer wavelength
can be assigned to the spin-allowed metal–ligand charge-
transfer band (1MLCT), and both spin–orbit couplings
3
3
enhanced p2pꢁ and MLCT transitions, respectively. In
detail, triplet absorption peaks for Ir(ppy)2(4-Me-2,3-dpq)
and Ir(ppy)(4-Me-2,3-dpq)2 appeared at 456, 483, 520, and
581 nm, while the absorption peaks for Ir(pq)2(4-Me-2,3-
dpq) and Ir(pq)(4-Me-2,3-dpq)2 appeared at 440, 487,
and 503 nm.
3
3
ppy MLCT state to the 4-Me-2,3 dpq MLCT state and

ARTICLE IN PRESS
S.Y. Ahn et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1320–1324
1323
-1
512nm
1.0
0.8
0.6
0.4
0.2
0.0
-1.270
Ir(ppy)2(4-Me-2,3-dpq)
-2
-1.783
Ir(ppy)(4-Me-2,3-dpq)2
-2.50
-3
-4
-4.754
-4.798
-5.11
-5
-6
Ir(ppy)2(acac) Ir(4-Me-2,3-dpq)2(acac)
Ir(pq)2(acac)
Fig. 4. The HOMO and LUMO levels of the related iridium complexes.
300
400
500
600
700
800
wavelength (nm)
4. Conclusion
583nm
1.0
0.8
0.6
0.4
0.2
0.0
The heteroleptic iridium complexes, Ir(ppy)2(4-Me-2,3-
dpq), Ir(ppy)(4-Me-2,3-dpq)2, Ir(pq)2(4-Me-2,3-dpq) and
Ir(pq)(4-Me-2,3-dpq)2, were synthesized and their lumines-
cence properties were investigated. The PL maxima of
Ir(ppy)x(4-Me-2,3-dpq)y and those of Ir(pq)x(4-Me-2,3-
dpq)y were observed at 512 and 583 nm, respectively. Two
different light-emitting mechanisms were suggested for
these emissions.
Ir(pq)2(4-Me-2,3-dpq)
Ir(pq)(4-Me-2,3-dpq)2
Acknowledgement
This work was supported by the Seoul Research and
Business Development Program (10555).
300
400
500
600
700
800
wavelength (nm)
References
Fig. 3. PL spectra of the heteroleptic iridium complexes in a 10ꢀ5 M
CH2Cl2 solution.
[1] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley,
M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151.
[2] D.F. O’Brien, M.A. Baldo, M.E. Thompson, S.R. Forrest, Appl.
Phys. Lett. 74 (1999) 442.
intra-radiative lifetime of the 4-Me-2,3-dpq ligand-based
state can be longer than that of the ppy ligand-based state
(16 ms). As a result, the green emissions occur mainly from
[3] M.A. Baldo, S. Lamansky, P.E. Burrows, M.E. Thompson,
S.R. Forrest, Appl. Phys. Lett. 75 (1999) 4.
[4] S. Lamansky, P. Djurovich, D. Murphy, F. Adbel-Razzaq, H.E. Lee,
C. Adachi, P.E. Burrows, S.R. Forrest, M.E. Thompson, J. Am.
Chem. Soc. 123 (2001) 4304.
3
the MLCT state of ppy in the complexes. On the other
hand, the PL peaks of both Ir(pq)2(4-Me-2,3-dpq) and
Ir(pq)(4-Me-2,3-dpq)2 (583 nm) show the hypsochromic
shift, compared with those of Ir(pq)2(acac) (599 nm) and
Ir(4-Me-2,3dpq)2(acac) (603 nm). A different light-emitting
mechanism can explain these hypsochromic shifts. The
ILET time from the 4-Me-2,3dpq 3MLCT to the pq
3MLCT states and intra-radiative lifetime of the 4-Me-2,3-
[5] M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, Y. Taga, Appl. Phys.
Lett. 79 (2001) 156.
[6] C. Adachi, M.A. Baldo, M.E. Thompson, S.R. Forrest, J. Appl.
Phys. 90 (2001) 5048.
[7] T. Tsuzuki, N. Shirasawa, T. Suzuki, S. Tokito, Adv. Mater. 15
(2003) 1455.
[8] A. Tsuboyama, H. Iwasaki, M. Furugori, T. Mukaide, J. Kamatani,
S. Igawa, T. Moriyama, S. Miura, T. Takiguchi, S. Okada,
M. Hoshino, K. Ueno, J. Am. Chem. Soc. 125 (2004) 12971.
[9] T. Tsuzuki, S. Tokito, Adv. Mater. 19 (2007) 276.
[10] F.-I. Wu, H.-J. Su, C.-F. Shu, L. Luo, W.-G. Diau, C.-H. Cheng,
J.-P. Duan, G.-H. Lee, J. Mater. Chem. 15 (2005) 1035.
[11] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, R. Kwong,
I. Tsyba, M. Bortz, B. Mui, R. Bau, M.E. Thompson, Inorg. Chem. 40
(2001) 1704.
3
dpq MLCT state of the heteroleptic complexes might be
comparable and the emission from both states can be
combined. Then, the energy gap can become larger due to
combination of the transition from the higher LUMO of
dpq-based energy state and the transition to the lower
HOMO of pq-based energy state. As a result, the energy
gap is extended, leading to a blue shift in the emission
wavelength.
[12] T.D. Anthopoulos, M.J. Frampton, E.B. Namdas, P.L. Burn,
I.D.W. Samuel, Adv. Mater. 16 (2004) 557–560.

ARTICLE IN PRESS
S.Y. Ahn et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1320–1324
1324
[13] S.-J. Yeh, M.-F. Wu, C.-T. Chen, Y.-H. Song, Y. Chi, M.-H. Ho,
S.-F. Hsu, C.H. Chen, Adv. Mater. 17 (2005) 285.
[17] M.A. Baldo, C. Adachi, S.R. Forrest, Phys. Rev. B 62 (2000) 10967;
F.-C. Chen, Y. Yang, M.E. Thompson, J. Kido, Appl. Phys. Lett. 80
(2002) 2308.
[14] J. Li, P.I. Djurovich, B.D. Alleyne, I. Tsyba, N.N. Ho, R. Bau,
M.E. Thompson, Polyhedron 23 (2004) 419–428.
[18] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, R. Kwong,
I. Tsyba, M. Bortz, B. Mui, R. Bau, M.E. Thompson, Inorg. Chem. 40
(2001) 1704.
[15] M.K. Nazeeruddin, R. Humphry-Baker, D. Berner, S. Rivier,
L. Zuppiroli, M. Graetzel, J. Am. Chem. Soc. 125 (2003) 8790.
[16] Y. You, S.Y. Park, J. Am. Chem. Soc. 127 (2005) 12438.
[19] G.Y. Park, Master Dissertation, Hongik University, 2006.