A series of iridium complexes equipped with inert shields: Highly efficient bluish green emitters with reduced self-quenching effect in solid state

Article abstract of DOI:10.1016/j.ica.2010.08.038

In this paper, we synthesize a series of cyclometalated ligands and their corresponding Ir(III) complexes using pentane-2,4-dione as the auxiliary ligand. We discuss the photophysical properties of these Ir(III) complexes in detail, including their UV-Vis

Full text of DOI:10.1016/j.ica.2010.08.038

Inorganica Chimica Acta 365 (2011) 78–84
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A series of iridium complexes equipped with inert shields: Highly efficient
bluish green emitters with reduced self-quenching effect in solid state
b
a
b
Wei He a, , Da-jian Zu , De-min Liu , Ran Cheng
⇑
a College of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, PR China
b School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 May 2010
Received in revised form 11 August 2010
Accepted 19 August 2010
Available online 27 August 2010
In this paper, we synthesize a series of cyclometalated ligands and their corresponding Ir(III) complexes
using pentane-2,4-dione as the auxiliary ligand. We discuss the photophysical properties of these Ir(III)
complexes in detail, including their UV–Vis absorption spectra, photoluminescence spectra in solid and
liquid states, luminescence decay lifetimes, and luminescence quantum yields. The correlation between
self-quenching effect and molecular structure is also investigated. It is found that these Ir(III) complexes
are solid-emitting ones due to their reduced self-quenching in solid state. Theoretical calculation and
experimental data reveal that the following two reasons should be responsible for the reduced self-
quenching in solid state: (1) pentane-2,4-dione, phenyl, and triphenylamine moieties serve as inert
shields for the excited state Ir(III) complexes; (2) the radiative decay process in these Ir(III) complexes
is accelerated by the introduction of electron-donors, and thus partly immune from self-quenching
caused by intermolecular action.
Keywords:
Iridium complex
Bluish green
Self-quenching
Inert shield
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
charge-carrier and energy transportation, the intense self-quench-
ing can be avoided. However, this doped device structure at the
Since the first report of phosphorescent porphyrin platinum as a
highly efficient emitter in organic light emitting diodes (OLEDs), the
scope and diversity of study on transition-metal complexes have
continued to expand at an exponential rate due to the potential
advantage of achieving a maximum internal quantum efficiency of
100% [1]. Particularly, the second- and third-row transition-metal
complexes incorporating chelating chromophores, such as
2,20-bipyridine and 2-phenyl pyridine, have attracted a great deal
of study [2]. In this series, Ir(III) cores are particularly promising be-
cause of their favorable short phosphorescence lifetimes, well-sui-
ted energy levels, thermal stability, and environmental inertness.
In exploring high efficiency phosphorescent emitters and moving
towards materials with the required color gamut for full color dis-
plays, many efforts have been devoted to develop tricolor-emitting
phosphorescent materials, and a good harvest of Ir(III) complexes
covering the whole visible region has been achieved [3–7]. Unfortu-
nately, most of the reported phosphorescent emitters based on
Ir(III) complexes suffer badly from self-quenching at high concen-
tration, leading to luminescence decrease or even absence [8,9].
The host–guest emitting system has been raised and used to over-
come this disadvantage: by doping and thus isolating emitter
molecules into host materials which are responsible for both
same time complicates device fabrication procedure, leading to
somewhat poor repeatability.
Recently, several research groups have reported phosphores-
cence OLEDs with non-doped device structures, and exciting re-
sults have been realized. For example, Burn and co-workers
report a class of phosphorescent dendrimers composed of an Ir(III)
core, metal-bonded phenylene dendrons, and 2-ethylhexyloxy sur-
face groups which, however, are not sufficient enough to eliminate
the interactions between emitting cores [10]. What’s more, Huang
and Cheng also demonstrate novel Ir(III)-based emitters for non-
doped phosphorescent OLEDs, and
a luminous efficiency of
34.7 cd/A with an external quantum efficiency of 10.3% is finally
reported [11,12]. Those reports enlighten a practicable way of sim-
plifying OLED fabrication procedure.
Despite their exciting device performances, such phosphorescent
Ir(III)-based emitters for non-doped device structures are still rare,
and the correlation between self-quenching effect and molecular
structure is far from developed. Guided by above considerations,
we devote our initial effort to the synthesis of Ir(III)-based emitters
with reduced self-quenching effect, as well as to the correlation be-
tween self-quenching effect and molecular structure. In this paper,
we synthesize a series of cyclometalated ligands (C^N ligands)
equipped with various moieties, and their corresponding Ir(III) com-
plexes. The photophysical properties, including UV–Vis absorption
spectra, PL spectra in solid and liquid states, luminescence decay
⇑
Corresponding author. Tel.: +86 27 87692130; fax: +86 27 87694034.
E-mail address: heweiwilliam1005@163.com (W. He).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.08.038

W. He et al. / Inorganica Chimica Acta 365 (2011) 78–84
79
lifetimes, and luminescence quantum yields, are discussed in detail.
The correlation between self-quenching effect and molecular struc-
ture is also investigated.
imidazol-2-yl)-9H-carbazole (L5) were synthesized according to the
literature procedure [15]. A typical synthetic procedure for L1 is de-
scribed as follows.
L1. The mixture of 10 mmol of 1,2-phenylenediamine, 20 mmol
of benzaldehyde, 0.2 mmol of iodine, and 20 mL of THF:H2O
(V:V = 1:1) was stirred for 5 h at 80 °C. After cooling, the mixture
were extracted with CH2Cl2 and further purified by column chro-
matography on silica gel to give the pure product as white powder.
1H NMR (CDCl3, 300 MHz) d [ppm]: 5.47 (s, 2H), 7.10 (d, 2H), 7.31
(m, 2H), 7.35 (m, 2H), 7.46 (m, 4H), and 7.70 (d, 2H).
L2. The synthetic procedure for L2 is similar with that of L1 ex-
cept that benzaldehyde was replaced by 4-methoxy-benzaldehyde.
1H NMR (CDCl3, 300 MHz) d [ppm]: 3.76 (s, 3H), 3.84 (s, 3H), 5.39
(s, 2H), 6.85 (d, 2H), 6.96 (d, 2H), 7.04 (d, 2H), 7.23 (m, 2H), 7.27 (d,
2H), and 7.62 (d, 2H).
2. Experimental
All starting materials, including I2, pentane-2,4-dione (Hacac),
benzene-1,2-diamine, IrCl3ꢀ3H2O, benzaldehyde, 4-methoxy-
benzaldehyde, 4-dimethylamino-benzaldehyde, triphenylamine,
9H-carbazole, polyvinylpyrrolidone (PVP), and bromoethane were
purchased from Aldrich Chemical Co. and used without further
purification unless otherwise stated. Organic solvents were
carefully dried and distilled prior to use. 4-Diphenylamino-benzal-
dehyde and 9-ethyl-9H-carbazole-3-carbaldehyde were synthe-
sized according to the literature procedures [13,14].
L3. The synthetic procedure for L3 is similar with that of L1 ex-
cept that benzaldehyde was replaced by 4-dimethylamino-benzal-
dehyde. 1H NMR (CDCl3, 300 MHz) d [ppm]: 2.93 (s, 6H), 3.01 (s,
6H), 5.37 (s, 2H), 6.68 (m, 2H), 6.73 (d, 2H), 7.02 (d, 2H), 7.20 (d,
2H), 7.64 (d, 2H), and 7.83 (d, 2H).
L4. The synthetic procedure for L4 is similar with that of L1 ex-
cept that benzaldehyde was replaced by 4-diphenylamino-benzal-
dehyde. 1H NMR (CDCl3, 300 MHz) d [ppm]: 5.43 (s, 2H), 6.98 (d,
6H), 7.05 (d, 2H), 7.08 (m, 4H), 7.12 (m, 8H), 7.15 (m, 3H), 7.23
(m, 3H), and 7.31 (d, 6H).
2.1. Synthesis of ligands
Scheme 1 shows the synthetic procedure for C^N ligands and their
corresponding Ir(III) complexes. 1-Benzyl-2-phenyl-1H-benzo[d]-
imidazole (L1), 1-(4-methoxybenzyl)-2-(4-methoxyphenyl)-1H
-benzo[d]imidazole (L2), 4-(1-(4-(dimethylamino)benzyl)-1H-ben-
zo [d]imidazol-2-yl)-N,N-dimethylaniline (L3), 4-(1-(4-(diphenyl-
amino) benzyl)-1H-benzo[d]imidazol-2-yl)-N,N-diphenylaniline (L4),
and 9-ethyl-3-(1-((9-ethyl-9H-fluoren-2-yl) methyl)-1H-benzo[d]-
Scheme 1. A synthetic procedure for C^N ligands and their corresponding Ir(III) complexes.

80
W. He et al. / Inorganica Chimica Acta 365 (2011) 78–84
L5. The synthetic procedure for L5 is similar with that of L1
Their initial structures were optimized by MOPAC 2009 with
PM6 Hamilton.
except that benzaldehyde was replaced by 9-ethyl-9H-carbazole-
3-carbaldehyde. 1H NMR (CDCl3, 300 MHz) d [ppm]: 1.41 (d, 6H),
4.32 (d, 4H), 5.73 (s, 2H), 7.17 (m, 5H), 7.39 (d, 8H), and 7.93 (d,
5H).
3. Results and discussion
3.1. Absorption of Ir(III) complexes
2.2. Synthesis of Ir(III) complexes
As mentioned above, we introduce various moieties into C^N li-
gands to investigate the correlation between self-quenching effect
and molecular structure. Here, we firstly inspect their effect on
UV–Vis absorption of the corresponding Ir(III) complexes. Fig. 1
shows the UV–Vis absorption spectra of the five Ir(III) complexes,
Ir1–Ir5, in CH2Cl2 solutions with a concentration of 1 ꢂ 10ꢁ5 mol/
L. It is observed that each absorption spectrum is composed of an
intense multiple absorption band in ultraviolet region from 220
to 340 nm and a weak absorption band ranging from 340 to
A typical synthetic procedure for the Ir(III) complexes of Ir1–Ir5
is described as follows [16]. Of IrCl3ꢀ3H2O (0.68 mmol) and
1.8 mmol of C^N ligand (L1, L2, L3, L4, or L5) was added into the
mixed solvent of 2-ethoxyethanol (15 mL) and water (5 mL). The
mixture was refluxed for 48 h under N2 atmosphere. After cooling,
a small quantity of cold water was added to give solid product. The
dried product of chloro-bridged dimmer was mixed with 2.1 mmol
of anhydrous sodium carbonate, 25 mL of 2-ethoxyethanol, and
2.1 mmol of Hacac. The mixture was refluxed for 16 h. After cool-
ing, water as added to give colored product. The crude product
was further purified on silica gel to give pure samples.
520 nm. The high energy absorption bands are assigned to spin-al-
*
lowed p–p transitions of the C^N ligands according to the litera-
ture reports [16,17]. As for the low energy ones, they are
experimentally assigned to the absorption of singlet and triplet
metal-to-ligand-charge-transfer (MLCT) transitions. For example,
the broad absorption of Ir1 ranging from 340 to 514 nm is com-
posed of 1MLCT and 3MLCT transitions, and their corresponding
absorption intensity values are found to be similar. Above phe-
nomenon suggests that the 3MLCT transition is strongly allowed
by the effective mixing of singlet-triplet with higher lying spin-al-
lowed transitions on the C^N ligand, and this mixing is facilitated
by the strong spin–orbit coupling of iridium center [16,17]. Simi-
larly, the other four complexes also demonstrate 1MLCT and 3MLCT
absorption bands. In addition, the optical absorption edges (kedg) of
the five Ir(III) complexes occupy a narrow region of 500–530 nm as
shown by the inset of Fig. 1, which may be caused by their similar
C^N ligands. Thus, it is expected that various moieties incorporated
by inert chain of methylene exhibit no obvious effects on the onset
electronic transition of these Ir(III) complexes.
Ir1. 1H NMR (CDCl3, 300 MHz) d [ppm]: 2.10 (d, 6H), 3.49 (m,
1H), 5.45 (s, 4H), 6.84 (d, 4H), 6.96 (m, 4H), 7.06 (m, 2H), 7.10
(m, 8H), and 7.36 (d, 4H). Anal. Calc. for C45H37N4O2Ir: C, 62.99;
H, 4.34; N, 6.53. Found: C, 63.13; H, 4.72; N, 6.31%.
Ir2. 1H NMR (CDCl3, 300 MHz) d [ppm]: 2.06 (d, 6H), 3.50 (m,
1H), 3.70 (s, 6H), 3.70 (s, 6H), 5.47 (s, 4H), 6.62 (d, 4H), 6.76 (d,
4H), 6.97 (d, 2H), 7.05 (m, 4H), 7.11 (d, 4H), and 7.36 (d, 4H). Anal.
Calc. for C49H45N4O6Ir: C, 60.17; H, 4.64; N, 5.72. Found: C, 59.84;
H, 4.91; N, 6.01%.
Ir3. 1H NMR (CDCl3, 300 MHz) d [ppm]: 2.12 (d, 6H), 3.04 (s,
12H), 3.12 (s, 12H), 3.55 (m, 1H), 5.46 (s, 4H), 6.46 (m, 4H), 6.89
(d, 4H), 6.94 (d, 2H), 7.24 (d, 4H), 7.80 (d, 4H), and 7.99 (d, 4H).
Anal. Calc. for C53H57N8O2Ir: C, 61.78; H, 5.58; N, 10.88. Found: C,
61.94; H, 5.97; N, 10.59%.
Ir4. 1H NMR (CDCl3, 300 MHz) d [ppm]: 2.07 (d, 6H), 3.55 (m,
1H), 5.55 (s, 4H), 6.85 (d, 6H), 6.85 (d, 6H), 6.90 (m, 4H), 7.04 (m,
16H), 7.18 (m, 8H), 7.20 (m, 10H), and 7.20 (d, 12H). Anal. Calc.
for C93H73N8O2Ir: C, 73.16; H, 4.82; N, 7.34. Found: C, 72.98; H,
5.11; N, 7.54%.
3.2. Theoretical calculations on Ir(III) complexes
Ir5. 1H NMR (CDCl3, 300 MHz) d [ppm]: 1.46 (d, 12H), 2.09 (d,
6H), 3.52 (m, 1H), 4.03 (d, 8H), 5.68 (s, 4H), 6.86 (m, 10H), 7.362
(d, 14H), and 7.64 (d, 10H). Anal. Calc. for C77H65N8O2Ir: C, 69.71;
H, 4.94; N, 8.45. Found: C, 69.53; H, 5.07; N, 8.67%.
In order to get a further understanding on the electronic nature
of these Ir(III) complexes, we perform a DFT/TD-DFT calculation,
which has been proved to be a powerful tool to investigate the
electronic properties of transition-metal complexes, on two typical
Ir(III) complexes of Ir1 and Ir4 at B3PW91/SBKJC level [18]. The se-
lected geometric parameters are listed in Table 1, along with the
graphic presentations of frontier molecular orbitals (MOs) for Ir1
and Ir4 shown in Fig. 2A and B. The calculated structural parame-
2.3. Methods and measurements
1H NMR spectra were recorded on a Bruker AVANCE 300 MHz
spectrometer. Element analyses were performed using a Vario Ele-
ment Analyzer. UV–Vis absorption spectra were obtained with a
Shimadzu UV-3101PC spectrophotometer. Solid state photolumi-
nescence (PL) spectra of the five Ir(III) complexes were measured
in powders with a Hitachi F-4500 fluorescence spectrophotometer.
Solid state PL quantum yields were measured in films using the
Hitachi F-4500 fluorescence spectrophotometer equipped with an
integrating sphere. All films for PL record were obtained by evapo-
rating their corresponding CH2Cl2 solutions on quartz substrates.
PL quantum yields in solutions were measured with the Hitachi
F-4500 fluorescence spectrophotometer according to the literature
procedure [13]. PL decay data were measured by a quanta ray DCR-
3 pulsed Nd:YAG laser system in solution excited by laser pulse at
wavelength 355 nm. The Nd:YAG laser possesses a line width of
1.0 cmꢁ1, pulse duration of 10 ns and repetition frequency of
10 Hz. A Rhodamine 6G dye pumped by the same Nd:YAG laser
was used as the frequency-selective excitation source. Time-
dependent density functional theory (TD-DFT) calculations were
performed on Ir1 and Ir4 with GAMESS at RB3PW91/SBKJC level.
Fig. 1. UV–Vis absorption spectra of the five Ir(III) complexes in CH2Cl2 solutions
with a concentration of 1 ꢂ 10ꢁ5 mol/L.

W. He et al. / Inorganica Chimica Acta 365 (2011) 78–84
81
Table 1
Selected geometric parameters of Ir1 and Ir4 calculated at RB3PW91/SBKJC level.
Ir1
Ir4
Bond length
(Å)
Bond angle
(°)
82.3
81.8
91.2
176.0
88.6
85.7
93.6
Bond length
(Å)
Bond angle
(°)
82.5
82.1
91.5
87.3
175.3
93.5
86.3
Ir–N1
Ir–C1
Ir–N2
Ir–C2
Ir–O1
Ir–O2
2.035
1.929
2.063
1.968
2.129
2.182
N1–Ir–C1
N2–Ir–C2
O1–Ir–O2
N1–Ir–O1
N1–Ir–O2
N2–Ir–O1
N2–Ir–O2
Ir–N1
Ir–C1
Ir–N2
Ir–C2
Ir–O1
Ir–O2
2.034
1.933
2.057
1.976
2.179
2.126
N1–Ir–C1
N2–Ir–C2
O1–Ir–O2
N1–Ir–O1
N1–Ir–O2
N2–Ir–O1
N2–Ir–O2
Fig. 2B. HOMO (up) and LUMO (down) of Ir4 calculated at RB3PW91/SBKJC level.
HOMOꢁ4 (MO 129), however, the contributions from Ir center de-
crease largely. Correspondingly, the contributions from acac to MO
130 and MO 129 increase. On the other hand, the lowest unoccu-
pied molecular orbital (LUMO, MO 134) and LUMO+1 (MO 135)
*
are essentially
p orbitals from L1, along with the minor contribu-
tions from acac and Ir center.
Fig. 2A. HOMO (up) and LUMO (down) of Ir1 calculated at RB3PW91/SBKJC level.
The singlet and triplet electronic transitions involving frontier
molecular orbitals (MO) of 129–136 are thus assigned to be MLCT
ones, admixed with large contributions from innerligand charge-
transfer (ILCT) and ligand-to-ligand-charge-transfer (LLCT). The
singlet onset electronic transition of S0 ? S1 is essentially the elec-
tronic transition from MO 133 to 134 with transition energy of
431 nm. While, the triplet onset electronic transition of S0 ? T1
consists of electronic transitions of MO 133 to 134 and MO133 to
135, with transition energy of 508 nm. The calculated S0 ? T1 en-
ergy is so close to the kedg obtained from the absorption spectrum
of Ir1. Similar case is also observed for Ir4 as shown in Table 2B.
ters fit well with those obtained from single crystals, confirming
the correctness of these optimized structures [16,19]. Not surpris-
ingly, the Ir(III) ion occupies the center of a distorted octahedral
environment, which is consistent with literature reports [16,19].
The calculated percentage composition of frontier molecular
orbitals for Ir1 and Ir4, as well as their singlet (S1) and triplet
(T1) excitation energy values, are shown in Tables 2A and 2B. As
for Ir1, the highest occupied molecular orbital (HOMO) of Ir-POP
(MO 133) is found to have a dominant character of Ir, admixed
with large contributions from L1 ligand, but the contribution from
acac is slim. The cases for HOMOꢁ1 (MO 132), HOMOꢁ2 (MO 131)
are similar with that for HOMO. As for HOMOꢁ3 (MO 130) and
The occupied frontier MOs have a dominant Ir character, while
*
the unoccupied MOs are essentially
p orbitals from L4. The calcu-
lated S0 ? T1 energy of Ir4 is also close to the kedg obtained from

82
W. He et al. / Inorganica Chimica Acta 365 (2011) 78–84
Table 2A
Calculated percentage composition of frontier MOs for Ir1, as well as the first five singlet and triplet excitation energy values.
MO&transition
Energy
Character
Contribution (%)
Ir
C^N ligand
acac
136(V)
135(V)
134(V)
133(O)
132(O)
131(O)
130(O)
129(O)
ꢁ0.678 eV
ꢁ1.233 eV
ꢁ1.276 eV
ꢁ4.865 eV
ꢁ5.050 eV
ꢁ5.412 eV
ꢁ5.731 eV
ꢁ6.076 eV
acac&L1
L1
L1
L1&Ir
L1&Ir
Ir&L1
L1&acac
L1&acac
5.8
4.6
2.0
39.6
41.0
46.2
2.5
46.7
90.6
95.7
47.4
44.7
42.0
58.5
66.9
47.6
4.8
2.3
13.0
14.3
11.9
39.0
23.9
9.2
S0 ? S1
S0 ? S2
S0 ? S3
S0 ? S4
S0 ? S5
431 nm
425 nm
402 nm
396 nm
368 nm
(IL&ML)CT
(IL&ML)CT
(IL&ML)CT
(IL&ML)CT
(IL&ML&LL)CT
133 ? 134(86.1)/133 ? 135(10.3)
133 ? 135(85.9)/133 ? 134(9.8)
132 ? 134(74.3)/132 ? 135(21.7)
132 ? 135(71.2)/132 ? 134(19.5)
133 ? 136(94.7)
S0 ? T1
S0 ? T2
S0 ? T3
S0 ? T4
S0 ? T5
508 nm
496 nm
443 nm
439 nm
425 nm
(IL&ML&LL)CT
(IL&ML&LL)CT
(IL&ML)CT
(IL&ML)CT
(IL&ML&LL)CT
133 ? 134(41.7)/133 ? 135(38.3)/130 ? 135(5.4)
133 ? 135(25.3)/132 ? 134(19.5)/133 ? 134(18.2)/132 ? 135(9.5)/130 ? 134(8.3)
132 ? 134(47.9)/132 ? 135(18.4)/133 ? 134(9.2)/131 ? 135(8.4)/133 ? 135(6.9)
132 ? 135(41.3)/133 ? 134(18.1)/131 ? 134(13.1)/133 ? 135(12.7)
132 ? 136(41.7)/130 ? 136(23.1)/131 ? 136(9.7)/129 ? 136(7.3)/133 ? 136(4.6)
Table 2B
Calculated percentage composition of frontier MOs for Ir4, as well as the first five singlet and triplet excitation energy values.
MO&transition
Energy
Character
Contribution (%)
Ir
C^N ligand
acac
259(V)
258(V)
257(O)
256(O)
255(O)
254(O)
ꢁ1.165 eV
ꢁ1.227 eV
ꢁ4.653 eV
ꢁ4.759 eV
ꢁ5.010 eV
ꢁ5.099 eV
433 nm
426 nm
411 nm
406 nm
391 nm
L4
L4
L4&Ir
L4
L4&Ir
L4&Ir
(IL&ML)CT
(IL&ML)CT
(IL&ML)CT
(IL&ML)CT
(IL&ML)CT
3.3
2.1
14.1
9.5
29.9
27.5
92.7
95.4
80.8
89.1
58.0
59.4
3.3
2.5
5.1
1.4
12.1
13.1
S0 ? S1
S0 ? S2
S0 ? S3
S0 ? S4
S0 ? S5
257 ? 258(84.2)&254 ? 258(8.0)
257 ? 259(81.4) & 255 ? 259(7.0)
256 ? 258(75.1) & 255 ? 258(10.2)
256 ? 259(75.4) & 255 ? 259(18.2)
255 ? 258(69.7) & 256 ? 258(10.5) & 254 ? 258(9.8)
S0 ? T1
S0 ? T2
S0 ? T3
S0 ? T4
S0 ? T5
532 nm
527 nm
446 nm
434 nm
429 nm
(IL&ML)CT
(IL&ML)CT
(IL&ML)CT
(IL&ML)CT
(IL&ML)CT
257 ? 259(59.0) & 257 ? 258(22.6) & 256 ? 259(4.9)
256 ? 258(61.9) & 257 ? 259(9.6) & 256 ? 259(7.7) & 257 ? 258(5.8)
257 ? 258(45.7) & 255 ? 258(13.2) & 257 ? 259(12.0) & 254 ? 258(9.0) & 256 ? 258(7.2)
256 ? 259(34.1) & 254 ? 259(24.7) & 255 ? 259(16.3) & 254 ? 258(7.8) &256 ? 258(5.0)
255 ? 259(35.6) & 255 ? 258(26.9) & 254 ? 259(9.8) & 256 ? 259(3.8)
the absorption spectrum of Ir4, confirming that the low energy re-
gion of Ir1 and Ir4 absorption spectrum is composed of both singlet
and triplet charge-transfer transitions as we expected.
It is also observed that the contributions from acac to frontier
MOs, including both occupied and unoccupied ones, are slim.
What’s more, phenyl and triphenylamine moieties incorporated
by the inert chain of methylene give no contribution to frontier
MOs as shown in Fig. 2A and B. It is thus expected that the moieties
of acac, phenyl, and triphenylmine should be isolated from photon
absorption and emitting processes. Consequently, above men-
tioned moieties may serve as inert shields for excited state Ir(III)
complexes, and then suppress the phosphorescence self-quenching
caused by intermolecular interaction.
3.3. PL properties and photophysical parameters of Ir(III) complexes
Fig. 3. PL spectra of the five Ir(III) complexes in CH2Cl2 solutions with
concentration of 1 ꢂ 10ꢁ5 mol/L.
a
Fig. 3 shows the PL spectra of the five Ir(III) complexes, Ir1–Ir5,
in CH2Cl2 solutions with a concentration of 1 ꢂ 10ꢁ5 mol/L. Their
photophysical parameters, including kedg, emission peaks (kem),
luminescence decay lifetimes (s), PL quantum yields (U), radiative
and non-radiative constants (Kr and Knr), are summarized in Table
3. Given their similar optical absorption edges above mentioned, it
is reasonable to see that their emission peaks are similar with each
other. Their main emission peaks occupy a narrow region of 490–
520 nm. As for Ir1, for example, the emission consists of a main
peak of 511 nm and a shoulder peak of 544 nm. The Stokes shift

W. He et al. / Inorganica Chimica Acta 365 (2011) 78–84
83
Table 3
Summarized photophysical parameters of the five Ir(III) complexes.
Complex
kedg (nm)
kem (nm)
s
(ns)
U (liquid)
U (solid)
Kr (ꢂ106 sꢁ1
)
Knr (ꢂ106 sꢁ1
)
Ir1
Ir2
Ir3
Ir4
Ir5
514
502
520
530
500
511, 544
491, 519
510, 542
520, 557
502
97.5
84.2
50.1
96.5
48.8
0.36
0.59
0.50
0.71
0.46
0.28
0.50
0.46
0.67
0.42
3.7
7.0
10.2
7.4
6.5
7.9
9.7
3.0
13.7
12.1
values between kedg and kem are slim, and there are even overlap
between the absorption edges and emission spectra, suggesting
that the emissive states of the five Ir(III) complexes are highly re-
stricted ones and suffer from no serious geometric distortion.
The PL quantum yields are determined in CH2Cl2 solutions with
quinine sulfate in 1.0 M sulfuric acid (Ur = 0.546) as the reference
standard according a literature procedure [13]. Sample and stan-
dard solutions are degassed with no fewer than four freeze-
pump-thaw cycles. Quantum yield (U) is determined according to
the following expression:
2
Us
¼
UrðBr=BsÞðns=nrÞ ðDs=DrÞ
ð1Þ
where the subscripts s and r refer to sample and reference standard
solution, respectively, n is refractive index of the solvent, D is the
integrated intensity, and
U is luminescence quantum yield. The
quantity B is calculated by B = 1 ꢁ 10AL, where A is the absorption
coefficient at the excitation wavelength and L is the optical length.
The PL quantum yields of the five Ir(III) complexes shown in Table 3
confirm that they are all efficient emitters, and we attribute the
causation of these high PL quantum yields to the strong and effi-
cient spin–orbit coupling in Ir(III) complexes.
Even in dilute solutions, where the self-quenching effect is lar-
gely and effectively reduced, the five Ir(III) complexes still exhibit
short-lived luminescence decay lifetimes on the scale of ꢃ100 ns as
shown in Table 3. We thus come to a hypothesis that the instinct
fast-decay processes are responsible for these short-lived emissive
states. In order to confirm this hypothesis, we calculate the corre-
sponding Kr and Knr values as follows:
Fig. 4. Solid state PL spectra of the five Ir(III) complexes in powders.
sive states remains to be unaffected in solid state, and the self-
quenching effect in solid state is largely and effectively reduced.
In order to further confirm the reduced self-quenching effect in
solid state, we consult the PL quantum yields of doped thin films
with various doping concentrations. We select the PL quantum
yields of Ir4 doped in PVP as the representative data which are
measured to be 0.705 for the 20 wt% doped film, 0.694 for the
40 wt% doped film, 0.687 for the 60 wt% doped film, 0.684 for the
80 wt% doped film, respectively. It is observed that the PL quantum
yield changes slightly from 0.705 for a light dopant concentration
of 20 wt% to 0.684 for a heavy dopant concentration of 80%, further
confirming the reduced self-quenching effect in solid state.
Based on above analysis and data, we attribute the reduced self-
quenching to the following two reasons. (1) As mentioned above,
theoretical calculation reveals that the contributions from acac,
phenyl, and triphenylamine moieties to frontier MOs are slim,
and they are thus isolated from excited state processes. The above
mentioned moieties may serve as inert shields for excited state Ir(-
III) complexes during photon absorption and emitting processes,
and then suppress the phosphorescence self-quenching caused
by intermolecular interaction. (2) Owing to the C^N ligands
equipped with electron-donor moieties, the radiative decay pro-
cess is instinctively fast, making the excited state some what invul-
nerable towards the phosphorescence self-quenching caused by
intermolecular interaction. Above finding may be useful when
designing efficient solid state emitters for non-doped phosphores-
cent OLEDs.
Kr ¼
U
=
s
ð2Þ
ð3Þ
Knr ¼ ð1 ꢁ
UÞ=s
The Kr values are found to be typically two orders of magnitude
bigger than literature ones, confirming the correctness of our
hypothesis [16,20]. In addition, it is observed that the
U values
of Ir2–Ir5 are obviously larger that that of Ir1, correspondingly, Kr
values of Ir2–Ir5 are obviously larger that that of Ir1. Above data
suggest that C^N ligands with electron-donor moieties are positive
on increasing the radiative decay process, and thus improve the
luminescence quantum yield.
3.4. Analysis on the causation of reduced self-quenching
Considering these short-lived emissive states, it is thus ex-
pected that the self-quenching caused by intermolecular interac-
tion may be ineffective, leading to reduced self-quenching and
consequently solid state emission from the five Ir(III) complexes.
The solid emission spectra of the five Ir(III) complexes shown in
Fig. 4 confirm the correctness of above expectation. The solid state
emission spectra of the five Ir(III) complexes measured in powders
are similar with the corresponding ones measured from dilute
solutions, with very slight spectral shift. In addition, the PL quan-
tum yields measured in solid state films suggest that all efficient
PL emissions remain in solid state, and the ratio of U(solid)/U(li-
quid) increases with the size of inert shields, as shown in Table
3. Above data suggest that the electronic transition nature of emis-
4. Conclusions
In this paper, we synthesize a series of C^N ligands and their
corresponding Ir(III) complexes using acac as the auxiliary ligand.
We discuss the photophysical properties of these Ir(III) complexes
in detail, including their UV–Vis absorption spectra, PL spectra in
solid and liquid states, luminescence decay lifetimes, and lumines-
cence quantum yields. It is found that these Ir(III) complexes are
solid-emitting ones due to their reduced self-quenching in solid

84
W. He et al. / Inorganica Chimica Acta 365 (2011) 78–84
[5] C.F. Chang, Y.M. Cheng, Y. Chi, Y.C. Chiu, C.C. Liu, G.H. Lee, P.T. Chou, C.C. Chen,
C.H. Chang, C.C. Wu, Angew. Chem., Int. Ed. 47 (2008) 4542.
[6] H. Wiedenhofer, S. Schützenmeier, A. von Zelewsky, H. Yersin, J. Phys. Chem.
99 (1995) 13385.
[7] J. Schmidt, H. Wiedenhofer, A. von Zelewsky, H. Yersin, J. Phys. Chem. 99
(1995) 226.
[8] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H.-E. Lee, C. Adachi, P.E.
Burrows, S.R. Forrest, M.E. Thompson, J. Am. Chem. Soc. 123 (2001) 4304.
[9] H.J. Bolink, E. Coronado, S.G. Santamarira, M. Sessolo, N. Evans, C. Klein, E.
Baranoff, K. Kalyanasundaram, M. Graetzel, M.K. Nazeeruddin, Chem.
Commun. 31 (2007) 3276.
[10] J.P.J. Markham, I.D.W. Samuel, S.C. Lo, P.L. Burn, M. Weiter, H. Bässler, J. Appl.
Phys. 95 (2004) 438.
[11] Z.W. Liu, M. Guan, Z.Q. Bian, D.B. Niu, Z.L. Gong, Z.B. Li, C.H. Huang, Adv. Funct.
Mater. 16 (2006) 1441.
[12] J.Q. Ding, J. Gao, Y.X. Cheng, Z.Y. Xie, L.X. Wang, D.G. Ma, X.B. Jing, F.S. Wang,
Adv. Funct. Mater. 16 (2006) 575.
state. Theoretical calculation and experimental data reveal that (1)
phenyl, and triphenylamine moieties incorporated by the inert
chain of methylene, as well as acac, serve as inert shields for the
excited state Ir(III) complexes; (2) the radiative decay process in
these Ir(III) complexes is accelerated by the introduction of elec-
tron-donors, and thus partly immune from self-quenching caused
by intermolecular action. Above two reasons are attributed to be
responsible for the reduced self-quenching effect as mentioned,
and this conclusion may be useful when designing efficient solid
state emitters for non-doped phosphorescent OLEDs.
Acknowledgments
[13] L. Zhang, B. Li, B. Lei, Z. Hong, W. Li, J. Lumin. 128 (2008) 67.
[14] Y. Wang, B. Li, Y. Liu, L. Zhang, Q. Zuo, L. Shi, Z. Su, Chem. Commun. 39 (2009)
5868.
[15] C.X. Li, J.W. Hu, P.P. Sun, Y. Pan, C.H. Cheng, Chinese J. Inorg. Chem. 23 (2007)
392.
We gratefully acknowledge the financial support by the Educa-
tion Department of Hubei Province and the Program for Innovative
Research Team of Huazhong University of Science and Technology.
[16] L. Zhang, B. Li, L. Shi, W. Li, Opt. Mater. 31 (2009) 905.
[17] K. Zhang, Z. Chen, C. Yang, Y. Zou, S. Gong, J. Qin, Y. Cao, J. Phys. Chem. C 112
(2008) 3907.
[18] L. Yang, J.K. Feng, A.M. Ren, M. Zhang, Y.G. Ma, X.D. Liu, Eur. J. Inorg. Chem. 10
(2005) 1867.
[19] M.S. Eum, C.S. Chin, S.Y. Kim, C. Kim, S.K. Kang, N.H. Hur, J.H. Seo, G.Y. Kim, Y.K.
Kim, Inorg. Chem. 47 (2008) 6289.
[20] S. Reineke, K. Walzer, K. Leo, Phys. Rev. B 75 (2007) 125328-1.
References
[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] A. Lavie-Cambot, M. Cantuel, Y. Leydet, G. Jonusauskas, D.M. Bassani, N.D.
McClenaghan, Coord. Chem. Rev. 252 (2008) 2572.
[3] J.P. Duan, P.P. Sun, C.H. Cheng, Adv. Mater. 15 (2003) 224.
[4] X.W. Chen, J.L. Liao, Y.M. Liang, M.O. Ahmed, H.E. Seng, S.A. Chen, J. Am. Chem.
Soc. 125 (2003) 636.