Synthesis, structure and electroluminescent properties of cyclometalated iridium complexes possessing sterically hindered ligands

Article abstract of DOI:10.1039/b618311e

New CN donor ligands incorporating pyridine or benzoimidazole N donors and a sterically hindered cyclometalating aromatic core featuring a polyphenylenephenyl, fluoranthene, or triphenylene segment are prepared and successfully converted into heteroleptic

Full text of DOI:10.1039/b618311e

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Synthesis, structure and electroluminescent properties of cyclometalated
iridium complexes possessing sterically hindered ligands
Marappan Velusamy,a K. R. Justin Thomas,a Chih-Hsin Chen,a Jiann T. Lin,*a Yuh S. Wen,a Wan-Ting Hsieh,b
Chin-Hung Laib and Pi-Tai Chou*b
Received 15th December 2006, Accepted 13th March 2007
First published as an Advance Article on the web 30th March 2007
DOI: 10.1039/b618311e
New CN donor ligands incorporating pyridine or benzoimidazole N donors and a sterically hindered
cyclometalating aromatic core featuring a polyphenylenephenyl, fluoranthene, or triphenylene segment
are prepared and successfully converted into heteroleptic iridium(III) cyclometalated complexes with
acetylacetone auxiliary ligands. The X-ray structure of the complex, derived from a ligand containing a
fluoranthene fragment, has been solved to unveil the corresponding structure. The results clearly
demonstrate that the nature of the r-coordinating ligand segment plays a key role in dictating the
emission profile and peak position, such that the emission hue has been successfully tuned ranging from
green to red. Supplementary support of this viewpoint is also rendered by computational (DFT)
approaches. Electroluminescent devices fabricated using a complex as dopant in the PVK matrix were
found to exhibit bright greenish yellow emission with promising device characteristics (maximum
brightness 26450 cd m−2 at 30 V and a maximum current efficiency of 40 cd A−1).
devices. Mu¨llen-type dendrons are rigid and voluminous, thus
Introduction
raising the Tg and thermal stability in addition to minimising the
There is considerable interest in the synthesis and characterization
non-radiative pathways. Attempts have also been made to replace
of cyclometalated iridium(III) complexes due to their use in light-
the central phenyl core in the polyphenylenephenyl dendrons
emitting diodes (LED)1 and as biological labelling agents.2 Light-
with other extended conjugated aromatic segments. Accordingly,
emitting organometallic complexes such as Ru(II) polypyridine
fluoranthene cored bright blue emitters have been reported
complexes3 and Ir(III) complexes1,4 are explored for LED appli-
cations because they offer the possibility of reaping both singlets
and triplets formed during the LED operation.5 In contrast, while
some polymers have been reported to facilitate triplet harvesting,6
small organic molecules usually lack this feature.
recently.20 Herein, we report a series of new CN donor ligands
and their Ir(III) cyclometalated complexes (Charts 1 and 2).
In view of iridium(III) complexes, iridium(III) cyclometa-
lated complexes containing N-donor motifs such as pyridine,7
benzothiazole,8 benzoimidazole,9 quinoline,10 and pyrazole11 were
prepared. Additional anionic auxiliary ligands such as acetylace-
tone, pyridine-2-carboxalic acid,12 triazoles13 and pyrazoles14 were
also used in the synthesis of heteroleptic complexes. Recently,
dendritic15 and polymer16 based ligands have also been devel-
oped and used for the construction of Ir(III) complexes. These
macromolecules inherit additional benefits such as chromophore
isolation (preventing unwanted interchromophoric interactions
that often lead to emission quenching and emission profile
alteration), energy harvesting emission enhancement, and simple
device fabrication procedures.
Polyphenylenephenyl dendrons, often termed as Mu¨llen-type
dendrons, have been widely used in recent designs to probe
inter-chromophoric interactions such as energy and electron
transfer.17 Organic molecules incorporating these dendrons were
found to emit intense blue18 or red19 colors in electroluminescent
Chart 1
In view of the structure, these ligands can be classified into
two categories: in the first, the central phenyl/aryl of the segment
participates in cyclometalation (A in Scheme 1), and in the second,
a peripheral phenyl farthest from the centroid is involved in
coordination (B in Scheme 1). As for the case of B type ligands,
Burn et al.15 have initiated elegant work regarding the effect of
steric crowding in phosphorescent dendrimers. In this study, we
aInstitute of Chemistry, Academia Sinica, 115 Nankang, Taipei, Taiwan.
E-mail: jtlin@chem.sinica.edu.tw; Fax: +886 2 278313771; Tel: +886-2-
27898522
bDepartment of Chemistry, National Taiwan University, Taipei, Taiwan.
E-mail: chop@ntu.edu.tw; Fax: +886-2-23695208; Tel: +886-2-33663894
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Chart 2
in deoxygenated toluene solution at 298 K with a Jobin Yvon
SPEX Fluorolog-3 spectrofluorimeter. The emission spectra were
collected on samples with O.D. (optical density) <0.1 at the
excitation wavelength. The spectra were corrected for instrumental
response. UV-Visible spectra were checked before and after irradi-
ation to monitor possible sample degradation. Emission maxima
were reproducible to within 2 nm. Luminescence quantum yields
(Uem) were calculated using quinine sulfate as the primary standard
(Uem = 0.577 in 0.1 M sulfuric acid) and Ir(ppy)3 (Uem = 0.40 in
toluene) as the secondary reference.21 The lifetime was measured
with the laser photolysis technique, in which the third harmonic
of an Nd:YAG laser (8 ns, Continuum Surlite II) was used as the
excitation source, coupled with a fast response photomultiplier
(Hamamatsu model R5509-72) operated at −80 ◦C. Typically, an
average of 512 shots was acquired for each measurement.
The starting materials, 3,4-bis(4-methoxy-phenyl)-2,5-diphenyl-
cyclopenta-2,4-dienone,19 7,9-diphenyl-cyclopenta[a]acenaphthy-
len-8-one,22 1,3-diphenyl-cyclopenta[l]-phenanthren-2-one,23 and
2-ethynyl-pyridine,24 were prepared by adopting the literature
procedures.
Scheme 1
made further investigations to probe both A and B type ligands,
and found that, depending on the cyclometalating segment, the
emission properties can be fine-tuned ranging from green to red.
Consequently, the nature of the r-coordinating ligand segment
plays a key role in dictating the emission properties.
Experimental
Methods and materials
Synthesis of the ligands 1–4
All reactions and manipulations were carried out under N2 with
the use of standard inert atmosphere and Schlenk techniques.
Solvents were dried by standard procedures. The entire column
chromatographic studies were performed under N2 with the use of
silica gel (230–400 mesh, Macherey–Nagel GmbH & Co.) as the
Ligands 2[{3ꢀ,4ꢀ-(4ꢀꢀ-methoxyphenyl)-2ꢀ,5ꢀ-diphenyl}phenyl]pyri-
dine (1), 2-(7,10-diphenyl-fluoranthen-8-yl)-pyridine (2), and
2-(1,4-diphenyl-triphenylen-2-yl)-pyridine (3) were synthesized
from 2-ethynylpyridine on reaction with 3,4-bis-(4-methoxy-
phenyl)-2,5-diphenyl-cyclopenta-2,4-dienone, 7,9-diphenyl-cyclo-
penta[a]acenaphthylen-8-one, and 1,3-diphenyl-cyclopenta[l]-
phenanthren-2-one, respectively. Ligand 2-[(3ꢀ,4ꢀ-bis(4ꢀꢀ-methoy-
phenyl)-2ꢀ,5ꢀ-diphenyl)-biphenyl-4-yl]-1-phenyl-1H-benzo(d)imida-
zole 4 was synthesized from 3,4-bis-(4-methoxy-phenyl)-2,5-
diphenyl-cyclopenta-2,4-dienone in reaction with 2-ethynyl-
1-phenyl-1H-benzoimidazole. The detailed procedure for the
preparation of ligand 1 is discussed here and the same procedure
is followed for the other ligands.
1
stationary phase. The H NMR spectra were obtained by using
Bruker AC300 or AMX400 spectrometers. Mass spectra were
recorded on a JMS-700 double focusing mass spectrometer (JEOL,
Tokyo, Japan). Elemental analyses were performed on a Perkin-
Elmer 2400 CHN analyzer. Cyclic voltammetry experiments were
performed with a BAS-100 electrochemical analyzer at room
temperature with a conventional three-electrode configuration
consisting of a glassy carbon working electrode, a platinum wire
auxiliary electrode, and a non-aqueous Ag/AgNO3 reference
electrode. The solvent in all experiments was dichloromethane, and
the supporting electrolyte was 0.1 M tetrabutylammonium hex-
Synthesis of ligand 1. A stirred mixture of 2-ethynylpyridine
(0.61 g, 6 mmol) and 3,4-bis-(4-methoxy-phenyl)-2,5-diphenyl-
cyclopenta-2,4-dienone (2.2 g, 5 mmol) in degassed diphenyl ether
(15 mL) was refluxed for 15 h. At the completion of the reaction
water was added to quench the reaction. The product was extracted
with diethyl ether. The organic layer was collected, dried over
anhydrous MgSO4, and evaporated under vacuum. The solid was
a
afluorophosphate. The E1/2 values were determined as 1/2(Ep
+
c
a
c
Ep ), where Ep and Ep are the anodic and cathodic peak poten-
tials, respectively. The potentials are quoted against the ferrocene
internal standard. Electronic absorption spectra were obtained
on a Perkin Elmer Lambda 900 UV-Vis-NIR spectrophotometer
for dichloromethane solutions. Emission spectra were recorded
3026 | Dalton Trans., 2007, 3025–3034
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adsorbed on silica gel and purified by column chromatography,
using a CH2Cl2–hexane mixture as eluent to give 1 as a white solid
(2.0 g, 80%). 1H NMR (d, CDCl3): 3.61 (s, 3H, OCH3), 3.63 (s, 3H,
OCH3), 6.40–6.47 (m, 4H, OCH3-Ar), 6.63–6.71 (m, 4H, OCH3-
Ar), 6.85–6.87 (m, 3H, Ar), 6.93–6.94 (m, 3H, Ar), 7.04–7.13 (m,
6H, Ar), 7.33 (t, J = 7.3 Hz, 1H, pyridyl), 7.73 (s, 1H, central Ar),
8.55 (d, J = 4.56 Hz, 1H, pyridyl). 13C NMR (d, CDCl3): 55.0,
55.1, 112.4, 112.6, 121.3, 125.5, 125.9, 126.2, 127.2, 127.6, 130.1,
131.2, 131.5, 132.5, 132.6, 132.7, 135.3, 139.5, 140.1, 140.6, 141.5,
141.7, 142.0, 148.9, 157.3, 157.5, 158.5; FAB MS(m/z): 520.1 (M +
H)+. FAB-HRMS calc. for C37H30NO2 [M + H]+ : 520.2277 found:
520.2290.
were filtered off, washed with water (10 mL), methanol (10 mL),
and diethyl ether (20 mL), and dried. These insoluble products
were dissolved again in dichloromethane (20 mL), adsorbed on
silica gel, and rapidly purified by column chromatography using a
hexane–dichloromethane mixture (3 : 2). The yellow solid obtained
was recrystallized from a dichloromethane–hexane mixture. Yield:
1
0.10 g (80%). H NMR (d, CDCl3): 1.56 (s, 6H, CH3-acac), 3.55
(s, 6H, OCH3), 3.59 (s, 6H, OCH3), 5.02 (s, 1H, CH-acac), 5.84
(d, 2H, J = 8.4 Hz, Ar), 6.21–6.22 (m, 2H, Ar), 6.31–6.39 (m,
16H, Ar), 6.50–6.57 (m, 4H, Ar), 6.62–6.73 (m, 8H, Ar), 7.04–
7.12 (m, 10H, Ar), 7.72–7.74 (m, 2H, Ar). 13C NMR (d, CDCl3):
14.2, 28.6, 31.0, 53.5, 54.9, 55.0, 99.5, 111.5, 111.8, 111.9, 119.2,
123.1, 123.9, 125.4, 125.5, 126.0, 128.0, 128.1, 130.1, 130.7, 131.0,
132.1, 132.5, 132.6, 132.7, 133.0, 134.0, 134.3, 134.6, 135.3, 139.8,
142.5, 143.8, 145.6, 149.1, 156.4, 156.5, 167.7, 184.0, 206.9; FAB
MS(m/z): 1328.2 (M+). Anal. Calcd for C79H63IrN2O6: C, 71.42;
H, 4.78; N, 2.11. Found: C, 71.21; H, 4.88; N, 2.05%.
Ligand 2. 1H NMR (d, CDCl3): 6.71 (d, 1H, J = 7.2 Hz,
pyridyl), 7.0–7.05 (m, 2H, pyridyl), 7.26–7.30 (m, 2H, Ar), 7.34–
7.42 (m, 7H, Ar), 7.46–7.54 (m, 3H, fluoranthene and Ar), 7.59
(s,1H, central Ar), 7.68–7.74 (m, 4H, fluoranthene), 8.56 (d 1H,
J = 4.5 Hz, pyridyl). 13C NMR (d, CDCl3): 121.4, 123.4, 123.5,
125.5, 126.8, 127.0, 127.6, 127.7, 127.8, 127.9, 128.7, 128.8, 129.3,
129.4, 129.8, 130.4, 131.3, 133.3, 135.5, 135.9, 136.1, 136.6, 137.0,
138.2, 138.3, 139.2, 140.8, 148.9, 158.6. FAB MS(m/z): 431 (M+).
FAB-HRMS calc. for C33H22N [M + H]+ : 432.1752 found:
432.1760.
Complex 6. Yield: 55%. 1H NMR (d, CDCl3): 1.59 (s, 6H, CH3-
acac), 4.87 (s, 1H, CH-acac), 5.53 (d, 2H, J = 7.2 Hz, Ar), 6.08 (d,
2H, J = 7.2 Hz, Ar), 6.27 (d, 2H, J = 8.5 Hz, Ar), 6.59–6.63 (m,
4H, Ar), 6.76–6.89 (m, 2H, Ar), 6.9–7.02 (m, 10H, Ar), 7.14–7.16
(m, 2H, Ar), 7.47–7.55 (m, 6H, Ar), 7.61–7.62 (m, 4H, Ar), 7.72–
7.74 (m, 4H, Ar), 8.13–8.15 (m, 2H, Ar). 13C NMR (d, CDCl3):
14.2, 22.7, 28.5, 31.7, 99.6, 118.9, 121.5, 122.6, 123.4, 124.9, 125.5,
125.9, 126.9, 127.2, 127.4, 127.5, 129.0, 129.2, 129.5, 129.6, 129.7,
130.2, 130.6, 133.5, 134.9, 136.0, 137.2, 137.4, 138.0, 142.1, 143.0,
144.5, 146.5, 149.0, 149.5, 167.3, 184.5. FAB MS(m/z): 1152.3
(M+). Anal. Calcd for C71H47IrN2O2: C, 74.00; H, 4.11; N, 2.43.
Found: C, 73.72; H, 4.24; N, 2.36%.
Ligand 3. 1H NMR (d, CDCl3): 6.63 (d, 1H, J = 7.2 Hz,
pyridyl), 6.98 (m, 1H, pyridyl), 7.08–7.20 (m, 7H, Ar), 7.34–7.51
(m, 9 H, Ar), 7.75 (d, 1H, J = 8.43 Hz, pyridyl), 7.85 (s, 1H,
central Ar), 8.42 (d, 2H, J = 8.2 Hz, Ar), 8.62 (d, 1H, J = 4.9 Hz,
pyridyl). 13C NMR (d, CDCl3): 121.3, 123.3, 123.4, 125.4, 125.7,
126.6, 126.9, 127.0, 127.3, 128.8, 129.1, 129.8, 130.0, 130.3, 130.8,
131.2, 131.6, 131.7, 131.8, 132.0, 132.1, 135.1, 136.7, 138.7, 142.1,
144.5, 149.2, 159.6. FAB MS(m/z): 457.1 (M+). FAB-HRMS calc.
for C35H24N [M + H]+: 458.1909 found: 458.1909.
1
Complex 7. Yield: 70%. H NMR (d, CDCl3): 1.54 (s, 6H,
CH3-acac), 4.98 (s, 1H, CH-acac), 5.75–5.81 (m, 2H, Ar), 6.15–
6.28 (m, 2H, Ar), 6.40–6.41 (m, 1H, Ar), 6.43–6.49 (m, 3H, Ar),
6.50–6.83 (m, 10H, Ar), 6.98–7.01 (m, 5H, Ar), 7.22–7.41 (m, 12H,
Ar), 7.53–7.54 (m, 2H, Ar), 7.71–7.73 (m, 1H, Ar), 8.05–8.07 (m,
1H, Ar), 8.28–8.34 (m, 4H, Ar), 8.54–8.56 (m, 1H, Ar). 13C NMR
(d, CDCl3): 28.6, 28.8, 99.1, 99.6, 119.3, 120.8, 122.9, 123.4, 124.7,
125.0, 125.1, 125.5, 127.3, 127.6, 128.8, 129.2, 129.7, 130.3, 130.4,
130.5, 131.1, 131.2, 131.9, 132.2, 132.6, 134.3, 141.7, 143.5, 144.2,
145.0, 146.5, 147.8, 148.8, 150.1, 151.4, 167.2, 167.4, 183.8, 184.5.
FAB MS(m/z): 1204.3 (M+). Anal. Calcd for C75H51IrN2O2: C,
74.79; H, 4.27; N, 2.33. Found: C, 74.82; H.4.13; N, 2.24%.
Ligand 4. 1H NMR (d, CDCl3): 3.62 (s, 3H, OCH3), 3.65
(s, 3H, OCH3), 6.44–6.52 (m, 4H, OCH3-Ar), 6.68–6.77 (m, 4H,
OCH3-Ar), 6.81–6.82 (m, 2H, Ar), 6.93–6.94 (m, 3H, Ar), 7.08–
7.18 (m, 7H, Ar), 7.25–7.28 (m, 4H, Ar), 7.31–7.34 (m, 1H, Ar),
7.36–7.39 (m, 2H, Ar), 7.46–7.48 (m, 3H, Ar), 7.53 (s, 1H, central-
Ar), 7.89 (d, 1H, J = 8.2 Hz, benzimidazole-Ar). 13C NMR
(d, CDCl3): 55.1, 110.5, 112.4, 112.7, 119.0, 119.8, 123.2, 123.5,
125.8, 126.3, 127.2, 127.5, 127.6, 127.8, 128.6, 128.9, 129.9, 130.0,
131.2, 131.6, 132.4, 132.5, 132.6, 132.8, 137.0, 137.2, 139.7, 139.8,
140.0, 141.2, 141.8, 142.0, 143.0, 143.3, 152.4, 157.3, 157.5. FAB
MS(m/z): 710.1 (M+). FAB-HRMS calculated for C51H39N2O2
[M + H]+ : 711.3012 found: 711.3019.
1
Complex 8. Yield: 60%. H NMR (d, CDCl3): 1.94 (s, 6H,
CH3-acac), 3.69 (s, 6H, OCH3), 3.73 (s, 6H, OCH3), 5.33 (s, 1H,
CH-acac), 6.22–6.25 (m, 2H, Ar), 6.40–6.42 (m, 2H, Ar), 6.37–
6.64 (m, 12H, Ar), 6.65–6.86 (m, 16H, Ar), 7.04–7.06 (m, 6H, Ar),
7.17–7.23 (m, 8H, Ar), 7.28–7.35 (m, 6H, Ar), 7.47–7.50 (m, 2H,
Ar), 7.60–7.68 (m, 8H, Ar). 13C NMR (d, CDCl3): 28.5, 31.7, 55.0,
101.4, 110.2, 112.2, 112.3, 112.5, 116.7, 122.7, 123.8, 125.3, 125.9,
126.8, 127.5, 127.9, 128.5, 129.6, 130.1, 130.2, 131.4, 132.5, 132.7,
133.1, 133.6, 135.9, 136.6, 138.7, 139.4, 140.1, 140.5, 140.7, 141.2,
142.1, 148.5, 157.0, 157.3, 164.2, 184.9. FAB MS(m/z): 1711.5
(M+). Anal. Calcd for C107H81IrN4O6: C, 75.11; H, 4.77; N, 3.27.
Found: C, 74.83; H, 4.83; N, 3.16%.
Synthesis of the complexes 5–8
Complexes 5–8 were synthesized by similar procedures, as followed
for complex 5.
Synthesis of complex 5. A mixture of 1 (0.62 g, 1.2 mmol),
IrCl3·xH2O (0.15 g, 0.5 mmol), 2-methoxyethanol (6 mL), and
distilled water (3 mL) was heated at reflux for 12 h. After
cooling, the yellow precipitate which formed was filtered off and
washed with methanol (50 mL), diethyl ether (50 mL), and hexane
(20 mL). The dried chlorobridged dimer was suspended in 2-
methoxyethanol (5 mL) and treated with acetylacetone (0.10 g,
1 mmol) and anhydrous Na2CO3 (0.16 g, 1.5 mmol). The reaction
mixture was stirred at 80 ◦C for 8 h. The insoluble products
LED Fabrication and measurements
Compound BCP (2.9-dimethyl-4,7-diphenyl-1,10-phenanthro-
line) was purchased from Aldrich and used as received. The
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◦
˚
electron-transporting materials, Alq3 (tris(8-hydroxyquinoline)
aluminium)25 and PPO (2-(4-tert-butylphenyl)-5-(pyren-1-yl)-
1,3,4-oxadiazole),26 were synthesized according to literature proce-
dures and were sublimed prior to use. Prepatterned ITO substrates
with an effective individual device area of 3.14 mm2 were cleaned
as described in a previous report.27 A PVK:Ir complex (device I)
or PVK:Ir Complex:PPO film (device II) was spin cast first. In
device I, a 10-nm-thick BCP acting as a hole and exciton blocking
layer (HBL) and 20-nm-thick Alq3 layer as an electron transport
layer were then deposited sequentially. Finally, a thin layer of
Table 2 Selected bond lengths (A) and angles ( ) for 6
Ir–C(11)
Ir–C(61)
Ir–N(1)
1.995(3)
1.996(3)
2.016(3)
Ir–N(2)
Ir–O(1)
Ir–O(2)
2.017(3)
2.128(2)
2.140(2)
C(11)–Ir–C(61)
C(11)–Ir–N(1)
C(61)–Ir–N(1)
C(11)–Ir–N(2)
C(61)–Ir–N(2)
N(1)–Ir–N(2)
C(11)–Ir–O(1)
C(61)–Ir–O(1)
96.77(13)
80.69(13)
103.68(13)
104.46(13)
80.26(13)
173.25(11)
173.17(12)
86.83(12)
N(1)–Ir–O(1)
N(2)–Ir–O(1)
C(11)–Ir–O(2)
C(61)–Ir–O(2)
N(1)–Ir–O(2)
N(2)–Ir–O(2)
O(1)–Ir–O(2)
92.83(10)
81.84(10)
87.12(12)
173.31(12)
82.28(10)
93.52(11)
89.87(9)
˚
˚
LiF (10 A) followed by aluminium (1000 A) was deposited as the
cathode. The I–V curve was measured on a Keithley 2400 Source
meter in the ambient environment. Light intensity was measured
with a Newport 1835 optical meter.
Computational methodology
To obtain the excitation energies of complex 6, we applied time-
dependent density function theory (TDDFT), in which the B3LYP
functional was adopted. The geometry of complex 6 was based
upon its X-ray structure. The basis set used is LANL2DZ for
iridium, and 6-31G for the other elements. The calculation was
performed with the Gaussian 03 program.28 Additionally, though
being time consuming, the solvent effect on the absorption energies
in CH2Cl2 are also considered by using an implicit solvation model
IEFPCM with the same DFT functional and basis set.29 To save
the computational resource, we only consider the solvent effect on
the absorption energy of the lower lying states up to S2 and T2.
Structural determination of complex 6
Red crystals of 6 were grown from a dichloromethane solution
layered with hexane at room temperature. Relevant crystal data
collection and refinement are summarized in Table 1. Selected
bond lengths and bond angles are given in Table 2. The space
¯
group (P1) was determined from the systematic absence of specific
reflections; successful refinement of the structure confirmed the
space group assignment. Direct methods were used to locate the
Ir atom, whereas subsequent cycles of least-squares refinements
and difference Fourier map were used to locate the remaining
non-hydrogen atoms. Hydrogen atoms were placed at calculated
positions except for the O–H bonds of the water molecule which
Results and discussion
˚
were fixed at 0.85 A. All calculations were performed using the
Synthesis and structure
SHELX software package.
The structures of ligands and complexes are depicted in Charts 1
and 2. The ligands were synthesized by employing a Diels–
Alder [4 + 2] cycloaddition reaction22 involving the appropriately
substituted cyclopentadienone and the corresponding terminal
acetylene in diphenyl ether at 200 ◦C (Scheme 2).
CCDC reference number 294665.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b618311e
Table 1 Crystal data and structure refinement details for 6
Empirical formula
C71H47IrN2O2·3CH2Cl2·H2O
M
1425.10
100 (1)
T/K
˚
k/A
0.71073
Triclinic
¯
P1
12.2327(4)
13.1476(4)
20.8703(6)
83.244(2)
73.8030(10)
77.3430(10)
3139.30(17)
2
1.508
2.433
Crystal system
space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
V/A
Z
3
˚
Calcd density/Mg m−3
l/mm−1
Scheme 2
F(000)
1432
Crystal size/mm
0.38 × 0.24 × 0.20
1.02 to 25.03, −13 ≤ h ≤ 14,
−15 ≤ k ≤ 15, 0 ≤ l ≤ 24
11103
h range, data collection/◦
The ligands were obtained in moderate yields (60–80%) and
were either colorless or yellow. The required complexes were
obtained in two steps: (i) chloro-bridged dinuclear complexes by
the treatment of the ligands with IrCl3·xH2O in 2-methoxyethanol.
(ii) These dinuclear complexes were cleaved by reaction with
acetylacetone in the presence of sodium carbonate to obtain
the mononuclear heteroleptic complexes, featuring the sterically
demanding CN donor ligands and acetylacetonate auxiliary
No. of reflns collected
No. of indep. reflns
Refinement method
11103
Full-matrix least-squares on F2
11103/2/687
1.071
No.of data/restraints/parameters
Goodness-of-fit on F2
Final R indices (I > 2r(I)): R1, wR2
R indices (all data): R1, wR2
0.0278, 0.0755
0.0293, 0.0768
3028 | Dalton Trans., 2007, 3025–3034
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ligand. The as prepared complexes were purified by column
chromatography, and the yield ranged from 55 to 80%.
absorption spectra of the complexes. The absorption profiles of the
ligands and the complexes recorded in dichloromethane solution
are displayed in Fig. 2 and 3. All the ligands except 2 possess
p–p* transition bands below 350 nm (Fig. 2).31 The absorption
wavelengths of the p–p* transition bands are in the order of 1 < 4 ≈
3 < 2, consistent with the degree of electronic delocalization in the
ligands. The peak wavelength at 374 nm for ligand 2 may originate
from the fluoranthene segment.22 Compound 3 also possesses a
fused aromatic segment, triphenylene. However, its associated p–
p* transition bands appear to be at shorter wavelengths because
the fluoranthene segment has a smaller p–p* transition energy
than the triphenylene analogue.32 In comparison to compound
4, the shorter absorption wavelength in compound 1 may be
attributed to its less effective conjugation length since no more
than three aromatic rings are linked in para form for favourable
conjugation. Comparing Fig. 2 and 3, it is obvious that the
ligand centered peaks for all the complexes, to certain degrees, are
perturbed due to the presence of the metal center. In complexes
5 and 8, an additional three less intense transitions were noticed,
All the new compounds were thoroughly characterized by
NMR (1H and 13C), mass spectra and elemental analyses. The
spectral characteristics were in accordance with the structural
compositions expected for the compounds. Since no broadening
of the peaks was noticed in the 1H NMR spectra it is believed that
there exists only one isomer for these complexes. The structure of
complex 6 was also confirmed by single crystal X-ray diffraction
studies. In addition to the molecule 6, each asymmetric unit also
contains three dichloromethane and one water molecules. As for
the molecular structure displayed in Fig. 1, the iridium adopts an
octahedral coordination environment with cis-C–C, cis-O–O and
trans-N–N donor dispositions. The average Ir–C, Ir–N and Ir–O
˚
distances are 1.996 (3), 2.017 (3), and 2.134 (2) A, respectively.
These values are close to that observed for Ir(ppy)2(acac) (2.003
(9), 2.010 (9), 2.146 (6)).30 A slight deviation of the perfect
octahedron is evident from the C–Ir–C (96.77 (13)◦), O–Ir–O
(89.87 (9)◦), and N–Ir–N (173.25 (11)◦) angles. The C–Ir–C angle
is slightly wider when compared to that observed for Ir(ppy)2(acac)
(95.2 (5)◦). Consequently, the N–Ir–N angle is narrowed (3◦) as
compared to that observed in the Ir(ppy)2(acac) complex. We
attribute the above observations to the steric effect of the rigid,
bulky ligand. The steric dictation on the coordination sphere is
more evident from the N–Ir–C bite angles (av. 80.48 (13)◦) and
the N–Ir–C inter-ligand angles (av. 104.07 (13)◦). The former is
slightly shorter (cf. 81.7 (4)◦), and the latter is wider (cf. 95.8 (4)◦)
than in Ir(ppy)2(acac). All other distances and angles of the ligand
are within the normal range. It is important to note that the pyridyl
ring of each cyclometalated ligand is somewhat parallel to one of
the dangling phenyl rings from the other cyclometalated ligand,
˚
and the distance between the centroids of the two rings is ∼3.7 A.
Such a p–p interaction may result in low emission quantum yields
for the complexes 5–7 in solution (vide infra). In comparison, the
intermolecular p–p interactions seem negligible.
Fig. 2 Absorption spectra of the ligands 1–4 recorded in CH2Cl2
solutions.
Fig. 1 Molecular structure of the complex 6 (the solvent molecules and
hydrogen atoms are removed for clarity). The thermal ellipsoids are drawn
at the 30% probability level.
Absorption spectra
Complexes 5–8 are either deep yellow or red in color, depending
on the cyclometalating ligands. This is clearly manifested in the
Fig. 3 Absorption spectra of the complexes 5–8 recorded in CH2Cl2 (inset
shows the zoomed MLCT region for the complexes 5 and 8).
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while in complexes 6 and 7, a prominent red-shifted absorption was
identified above 450 nm. These bands were reasonably ascribed
to the MLCT transitions.1,4 It is interesting to compare the
MLCT absorption peak wavelengths in the control complexes
Ir(ppy)2(acac)33 and Ir(pbi)2(acac)9 with those of the complexes
prepared in the current work. The salient spectral features derived
from absorption studies enable us to gain insights into the role
of the sterically hindered ligand on the electronic structure of the
metal fragment. There is only slight red-shift (ca. 10 nm) in the
MLCT transitions of 5 and 8 when compared to the corresponding
transitions with the control complexes. Apparently, the peripheral
phenyl rings do not lead to much conjugation because they are
skew with respect to the central phenyl rings, as evidenced from
the structure of 6 (vide supra). In contrast, better conjugation is
expected in ligands 2 and 3 due to the presence of polyaromatic
segments. Accordingly, the resulting HOMO–LUMO energy gap
leads to more pronounced red-shifted MLCT absorption peaks for
their corresponding complexes 6 and 7 compared to the control
complex Ir(ppy)2(acac).34,15a
Fig. 5 Emission spectra of the complexes 5–8 recorded in toluene
solution, 3% 5 doped in PVK film and EL spectra of device II.
yields of complexes 5–7 stem from the close contact of the pyridyl
ring and the dangling phenyl ring nearby (vide supra). Support
of this viewpoint is given by the significantly large non-radiative
decay rate constant deduced for 5–7 (see Table 3). Complexes 5
and 8 display phosphorescence lifetimes similar to those of the
control complexes (Ir(ppy)2(acac) and Ir(pbi)2(acac). However, a
significant two-fold (7) and four-fold (6) enhancement in lifetime is
observed for the complexes which possess extended polyaromatic
conjugation in the r-coordinating ligand segment. This result
clearly supports the involvement of the ligand in the excited state
properties of the complexes.36 These lifetime values appear to fall in
the range of bis-cyclometalated31 and tris-cyclometalated21 iridium
complexes. The distinct nonradiative decay constant for 5 among
the four complexes accounts for its relatively small quantum yield.
Emission spectra
The luminescence spectra of the ligands and complexes were mea-
sured in dichloromethane and toluene, respectively. The spectra
are displayed in Fig. 4 and 5, and the pertinent photophysical
data are collected in Table 3. The complexes emit green or
red in both solution and the solid state at room temperature.
The emission peak wavelength of the bulky complexes is red-
shifted when compared to that of the control complexes, and the
magnitude of the shift is dependent on the conjugation of the
cyclometalating aromatic segment.1b,13b,35 Accordingly, complexes
5 and 8 are green-emitting species, while complexes 6 and 7 emit in
the red due to the presence of the fluoranthene and triphenylene
segments. As supported by the theoretical approach (vide infra),
the emission is principally derived from the MLCT and p–p* states
of the cyclometalated ligand. Moreover, the emission quantum
yield of the complexes seems to be dependent on the interaction
mode between metal and ligand sites. Type “A” complexes (5–
7) possess low quantum yield, while the type “B” complex (8)
displays moderate emission efficiency. The low solution quantum
Electrochemical properties
Electrochemical characteristics of the complexes were studied by
using cyclic voltammetric methods in dichloromethane and the
redox potentials are reported in Table 3. All complexes exhibit
a quasi-reversible one-electron oxidation wave ranging from 0.25
to 0.29 V vs. ferrocene, which is attributed to the oxidation of
iridium(III). The larger peak-to-peak separation (DEp) observed in
the present study suggests a significant structural reorganization
during the redox process.37 The pyridine-containing complexes,
5–7 have a higher oxidation potential (0.25–0.27 V) than the
control complex, Ir(ppy)2(acac) (0.46 V),33 while complex 8 has
an oxidation potential (0.29 V) similar to that of Ir(pbi)2(acac)
(0.30 V).9 We attributed this observation to the influence of the
stericallyhinderedmoietyontheHOMO. Apparentlythesterically
hindered moiety is closer to the cyclometalated phenyl ring in 5–
7. All complexes exhibit an irreversible reduction wave attributed
to the reduction of the ligand. Complexes 6 and 7 have more
positive potential, possibly due to the more delocalized nature of
the fluoranthene and triphenylene segments. The differences in
the oxidation and reduction potentials for the complexes are in
the order of 6 < 7 < 8 < 5. Though the irreversible nature of the
reduction peak prevents us from more accurate estimation, this
trend, however, is consistent with the absorption and emission
spectra (vide supra).
Fig. 4 Emission spectra of the ligands 1–4 recorded in CH2Cl2 solution
at room temperature.
3030 | Dalton Trans., 2007, 3025–3034
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MO Calculations
To gain more insight into the photophysical properties of the
complexes, TDDFT calculations (see Experimental section) on
the molecular orbitals involved in the transitions was carried
out for 6. The features of the two lowest unoccupied (LUMO
and LUMO + 1) and the two highest occupied (HOMO and
HOMO − 1) frontier orbitals mainly involved in the transition
are depicted in Fig. 6, while the descriptions and energy gaps of
each transition are listed in Table 4. Qualitatively, the stronger
singlet absorptions for 6 are calculated at ∼478, 412, and 390 nm,
the results of which are in agreement with the observed absorption
spectral profiles. Moreover, the lowest triplet state is calculated to
be ∼572 nm, which is also consistent with the 0–0 onset (∼580 nm)
of the corresponding phosphorescence spectrum (see Fig. 5),
supporting the validity of the TDDFT approach. Evidently, the
electron densities of both the HOMO and HOMO − 1 are located
largely on the metal and the fluoranthene segment of 6, whereas
those of the LUMO and LUMO + 1 are distributed largely on
the pyridyl moiety and partly on the fluoranthene segment and
nonbonding d orbital of the central metal, indicating that the
lowest transition is ascribed to metal-to-ligand charge transfer
(MLCT) mixed with p (fluoranthene) → p* (pyridine) character.
The dominance of the HOMO → LUMO transition (see Table 4),
in which MLCT has a great contribution, qualitatively explains
the broad, featureless emission band with fast radiative decay
time. Accordingly, it is reasonable to expect that by introducing
more extended polyaromatic conjugation into the r-coordinating
ligand segment, such as in 6 and 7, there should be more pp*
contribution to the T1 state in complex 7. Since the T1 → S0
transition with a pp* configuration does not directly involve the
heavy metal (Ir(III)) dp orbital as well as lacking the orbital flipping
that can be borrowed by the flip of spin, the transition dipole is
small hence the resulting radiative lifetime is expected to be longer,
consistent with that observed experimentally (see Table 3). The
Fig. 6 Selected frontier orbitals for complex 6.
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Table 4 Calculated energy levels of the lower lying transitions of complex 6
State
S1
E/eV
k/nm
(f )b
Assignments
2.49
2.45a
2.59
2.50
3.01
2.17
2.16
2.22
2.20
498.9
507.0a
478.3
495.0
412.0
571.9
574.1
557.5
562.9
0.0008
0.0028a
0.0582
0.0971
0.1534
0.0000
0.0000
0.0000
0.0000
HOMO → LUMO (+90%)
HOMO → LUMO (+88%a
HOMO → LUMO + 1 (+90%)
HOMO → LUMO + 1 (+90%)
HOMO − 1 → LUMO (+90%)
S2
S3
T1
HOMO → LUMO (+76%); HOMO → LUMO + 1 (+8%); HOMO − 1 → LUMO + 1 (+6%)
HOMO → LUMO (+69%); HOMO → LUMO + 1 (+16%); HOMO − 1 → LUMO + 1(6%)
HOMO → LUMO + 1 (+71%); HOMO → LUMO (+9%); HOMO − 1 → LUMO (+8%)
HOMO → LUMO + 1 (+65%); HOMO → LUMO (16%); HOMO − 1 → LUMO (7%)
T2
a The values in Italic type are the results considering solvent effects. b Oscillator strength.
aforementioned theoretical approaches were performed under gas
conditions. In order to evaluate the solvation effect, the absorption
energy gap with the addition of CH2Cl2 was also calculated and
listed in Table 4 (see Experimental section for details). As shown in
Table 4, upon considering the solvent effect, the absorption peaks
in both the singlet and triplet states are only slightly red-shifted
and are closer to those observed experimentally. Nevertheless,
the involved frontier orbitals remain unchanged. Considering the
time-consuming computation upon incorporating the solvation
effect, theoretical approaches in the gas phase seem to be sufficient
for the rationalization.
Electroluminescent properties
The HOMO energy values derived from the oxidation potential
(Table 3) fall in the range 5.05–5.09 eV. This is sufficiently
high to facilitate hole injection in the molecular layer. Due to
the favourable redox and emission characteristics, the complexes
are perceived as suitable for electroluminescent applications.
Subsequently, the complexes were applied in organic light-emitting
diodes as dopants. Two types of devices were fabricated using 5 and
8 as dopant in a polyvinyl carbazole (PVK) layer (Fig. 7). They
are: (I) ITO/3% 5 (or 8) in PVK (70 nm)/BCP (10 nm)/Alq3
(20 nm)/LiF (1 nm)/Al (100 nm), and (II) ITO/3% 5 (or 8)
and 40% PPO in PVK (100 nm)/LiF (1 nm)/Al (100 nm).
The first device adopts a multi-layer architecture consisting of
Fig. 7 Device configurations and the structure of the constituent
an Ir doped hole-transport layer, a hole-blocking layer, and
an electron-transport layer. In the second device, a single-layer
configuration containing a blend of hole-transporting PVK and
electron-transporting PPO was used to dope the Ir complex. It
was found that the latter device did not require a hole-blocking
layer to restrict the recombination zone inside the PVK layer. The
EL spectra of the two devices closely resemble the solid state PL
realized for the PVK : Ir complex blend (Fig. 5). A slight red-tailing
noticed in the EL spectra of the devices may be attributed to the
medium perturbation. The pertinent parameters are collected in
Table 5, and the I–V–L characteristics observed for devices 5 are
plotted in Fig. 8. Device II for 5 exhibited maximum brightness,
26450 cd m−2 at 30 V, and a maximum current efficiency of 40 cd
A−1. The better performance of device II as compared to that of
I is ascribed to the improved balance in the charge transport in
the HTL : ETL blend layer.38 The maximum external quantum
efficiency (I, 4.4%; II, 6.69%) for both devices of 5 are comparable
to that (∼10%) of the vacuum-deposited multi-layer device of
Ir(ppy)2(acac).33 However, the maximum power efficiency for both
materials.
devices (I, 7.64 lm W−1; II, 6.69 lm W−1) are inferior to that (∼18 lm
W−1) of Ir(ppy)2(acac). This may be due to the low phospho-
rescence quantum yield (7.7%) of 5 compared to Ir(ppy)2(acac)
(34%). In comparison, the device performance appears to be
better than the single-layer device of the complex composed of
a fac-tris(2-phenylpyridyl)iridium(III) core and similar sterically
crowded dendron (2.9%) reported by Burn’s group.15a We speculate
that the emission of the iridium core is well encapsulated by
the three somewhat insulating ligands to allow efficient carrier
injection. For both devices of 8, the maximum external quantum
efficiency (I, 8.64%; II, 3.85%) and maximum power efficiency
(I, 9.51 lm W−1; II, 5.79 lm W−1) seem to be inferior to those
(∼16.7%; 20 lm W−1) of the vacuum-deposited multi-layer device
of Ir(pbi)2(acac).9 The cause of the low efficiency for the devices
of 8 may be due to both encapsulation of the emitting core with
somewhat insulating ligands as well as the imbalance in the carrier
mobility. However, more studies are needed to understand the
3032 | Dalton Trans., 2007, 3025–3034
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Table 5 Electroluminescent data for 5 and 8a
Device
I (5)
II (5)
I (8)
II (8)
V
on/V
8.0
8.0
26498 (30)
550
6.5
7125 (22.5)
550
5.5
5407 (14.5)
548
L
max/cd m−2 (V at Lmax, (V))
15264 (28.5)
566
kem/nm
CIE (x,y)
fwhm/nm
0.44, 0.54
92
0.43, 0.56
84
0.40, 0.54
72
0.41, 0.56
70
gext
,
(%)
11.53
7.64
40.12
15210
4.40
1.71
15.32
11.17
6.69
40.23
23058
6.41
2.61
23.11
8.64
9.51
30.33
6961
1.99
0.99
6.99
3.85
5.79
13.99
5119
1.42
1.18
5.15
max
gp,max/lm W−1
gc,max/cd A−1
Lb/cd m−2
b
gext (%)
gpb/lm W−1
b
gc /cd A−1
a Legends: Lmax, maximum luminance; L, luminance; Von, turn-on voltage; V, voltage; gext,max, maximum external quantum efficiency; gp,max, maximum
power efficiency; gc,max, maximum current efficiency; gext, external quantum efficiency; gp, power efficiency; gc, current efficiency; fwhm, full width at half
maximum. b At a current density of 100 mA cm−2. Von was obtained from the x-intercept of log(luminance) vs. applied voltage plot.
structural characterization and photophysical behaviour of the
corresponding complexes have been investigated. The results, in
combination with theoretical approaches, address the correlation
between the nature of the r-coordinating ligand segment and the
absorption/emission characteristics such as spectral profile, peak
wavelength and relaxation dynamics. For complexes 5–8, a wide
spectral tunability ranging from green to red has been achieved.
Electroluminescent devices using a selected complex (5) as dopant
in a polymer or polymer : ETL blend matrix are demonstrated
with promising device parameters. Work focusing on promoting
device engineering and its associated physical and photophysical
properties is underway.
Acknowledgements
Financial support from Academia Sinica and National Taiwan
University is gratefully acknowledged.
Fig. 8 I–V–L characteristics of the devices I and II containing the
complex 5 as a dopant.
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3034 | Dalton Trans., 2007, 3025–3034
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