Synthesis, characterization, photophysical and electrochemical properties of new phosphorescent dopants for OLEDs

Article abstract of DOI:10.1016/j.tetlet.2008.02.158

New heteroleptic iridium(III) complexes of 2-(p-substituted-phenyl)-pyridine were synthesized and characterized. These complexes have two cyclometalated ligands (C^∧N) and a bidentate ancillary ligand (LX), that is, (C^∧N)₂I

Full text of DOI:10.1016/j.tetlet.2008.02.158

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 2710–2713
Synthesis, characterization, photophysical and electrochemical
properties of new phosphorescent dopants for OLEDs
*
Neeraj Agarwal , Pabitra K. Nayak
Department of Chemical Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India
Received 23 November 2007; revised 27 February 2008; accepted 29 February 2008
Available online 4 March 2008
Abstract
New heteroleptic iridium(III) complexes of 2-(p-substituted-phenyl)-pyridine were synthesized and characterized. These complexes
^
^
have two cyclometalated ligands (C N) and a bidentate ancillary ligand (LX), that is, (C N)
2
Ir(LX). LX was either acetylacetonate
^
or 5-nitro-8-hydroxy quinolate. Substitution on the p-phenyl of C N ligands was used to alter the electronic properties of these
complexes. The (C N) Ir(acac) complexes show phosphorescence with good quantum yields and microsecond lifetimes and also show
^
2
photoluminescence over a wide visible range (kmax = 503–620 nm). The HOMO level and triplet energy level of these complexes were
also determined. These data indicate their potential use as emitting materials for organic light emitting diodes, OLEDs.
Ó 2008 Elsevier Ltd. All rights reserved.
Organic light emitting devices (OLEDs) have become
very attractive mainly due to their potential use in flat
the cyclometalating ligand should alter the phosphorescent
properties of the iridium(III) complexes. Alternatively,
heteroleptic Ir(III) complexes having ancillary ligands such
1
,2
3
panel displays, and for oxygen sensing purposes. Heavy
metal complexes, particularly those containing Pt and Ir
have attracted considerable attention as they can induce
inter system crossing by strong spin–orbit coupling, leading
1
0
as acetylacetonate, and picolinate were synthesized. The
emission properties in most of the heteroleptic complexes
are still dominated by cyclometalating ligands. Also, the
electron-donating or electron-withdrawing character of
the ancillary ligand in heteroleptic complexes can be used
as a tool to increase or decrease the electron density on
the metal center which subsequently will affect the emission
and the lifetime of the excited state.
4
to mixing of the singlet and triplet excited states. The spin
forbidden nature of radiative relaxation becomes allowed
in heavy metal complexes, and hence results in high phos-
phorescence efficiency. Homoleptic and heteroleptic irid-
ium(III) complexes have been studied extensively
5
,6
resulting in improved materials for device purposes.
In the literature, several structurally different cyclometa-
lating ligands, namely, 2-phenylpyridine, 2-thienylpyridine,
2-quinolinylpyridine, 2-benzothienylpyridine were used to
OLEDs prepared with Ir(III) complexes as dopants are
the most efficient OLEDs reported having nearly 100%
7
9
internal phosphorescence efficiency.
achieve wide range emission of Ir(III) complexes. Irid-
For practical applications such as display technology,
tuning of emission in the visible spectrum is required. A
powerful color tuning has been obtained by employing
ium(III) complex of 2-phenylpyridine, that is, Ir(ppy) -
(acac) has been studied extensively. In this project, we
2
5
aimed to tune a wide range of emission by very simple
chemical modification of the 2-phenylpyridine ligand in
Ir(III) complexes. Herein, we report the synthesis, charac-
terization, photophysical, and electrochemical properties
of iridium(III) complexes 1–4 of 2-(p-methoxy/acetyl-phen-
yl)-pyridine having acetylacetonate or 5-nitro-8-hydroxy-
quinolate as the ancillary ligand. These easily achieved
^
different cyclometalating ligands (C N) in Ir(III) com-
8
,9
plexes. Structural change or chemical modification of
*
Corresponding author. Tel.: +91 22 22782864; fax: +91 22 22804610.
E-mail address: nagarwal@tifr.res.in (N. Agarwal).
0
040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.02.158

N. Agarwal, P. K. Nayak / Tetrahedron Letters 49 (2008) 2710–2713
2711
modifications of the ligands alter the photophysical and
electrochemical properties of Ir(III) complexes. Electro-
chemical study of 1–4 shows the effect of different ligands
on the EHOMO and triplet energy levels.
and 69% yields, respectively. Complexes 3–4 were synthe-
1
2
sized using a recently reported synthetic method. l-
Chloro bridged 7 or 8 and 5-nitro-8-hydroxyquinoline were
refluxed in a mixture of dichloromethane, ethanol, and tri-
ethylamine at 80 °C for 16–20 h under a nitrogen atmo-
sphere. The evaporation of the solvent followed by
washing with ethanol afforded pure 3–4 as orange solids
in ꢀ80% yield. Compounds 1–8 were characterized by
Ir(III) complexes 1–4 were synthesized as outlined in
Scheme 1. New cyclometalated ligands were synthesized
1
1
by Suzuki–Miyaura coupling of 2-bromopyridine and
aryl boronic acids catalyzed by Pd(PPh ) in a biphasic
3
4
1
13
13–15
aqueous solution of K CO and DME. Reflux of the reac-
H NMR, C NMR, and ES-MS techniques.
All
2
3
tion mixture at 80 °C for 16 h and purification of the crude
compounds 1–8 were stable in air for several days.
Compounds 1 and 2 showed intense absorptions from
product by column chromatography gave pure ligands,
p–p*, MLCT, and MLCT transitions. Compounds 3
and 4 showed an intense absorbance at 450–462 nm, which
is attributed to the quinolinolate ligand (Table 1). Deaer-
ated dilute solutions of 1–4 in toluene were used to record
the emission spectra. Excitation of 1–4 at 450 nm gave
emission in the green to red region (Fig. 1). A considerable
bathochromic shift was observed in the emissions from
1–4. Iridium complex 1 emits at 503 nm while 4 shows
emission at 620 nm. Phosphorescence quantum yields
1
3
5
–6, in good yields. Cyclometalating ligands 5–6 were used
to prepare the l-chloro bridged precursors 7–8. A solution
of 5 or 6 (2.3 equiv) and iridium chloride (1.0 equiv) in 2-
ethoxyethanol was refluxed for 20 h at 120 °C. The solvent
was evaporated, and the solid residue was washed several
times with methanol to afford pure 7–8. Iridium(III) com-
plexes 1 and 2 were prepared by allowing 7 or 8 to react
with acetylacetone in 2-ethoxyethanol at 120 °C for 15–
1
8 h. The solvent was evaporated by vacuum distillation
1
6
and the crude product was washed four times with metha-
nol to afford 1 (yellow solid) and 2 (orange solid) in 75%
(/PL) were obtained with respect to rhodamine 6G. /PL
of 1–2 were higher as compared to 3–4. Ir(III) complexes
R
R
Pd(PPh3)4
K2CO3
N
IrCl .3H O
N
3
2
Cl
Cl
+
Ir
2
Ir
Br
N
DME/H2O
2-ethoxyethanol
120 C, 20 h
º
º
N
B(OH)2 80 C, 16 h
R
R
2
R = OCH , 5, (62%)
3
R = COCH3, 6, ( 55%)
R = OCH , 7, (54%)
3
R = COCH3, 8, (61%)
R
R
Na CO3
2
N
acetylacetone
Cl
N
Ir
Ir
2
-ethoxyethanol
Cl
Ir
N
N
º
1
20 C, 15 h
O
O
R
R
2
2
R = OCH , 1, (75%)
3
R = COCH3, 2, (69%)
7
or 8
R
R
NO2
N
N
Cl
Cl
CH Cl , ethanol, TEA
2 2
Ir
Ir
2
Ir
N
+
O
º
120 C, 15-18 h
N
N
N
OH
R
R
2
NO2
R = OCH , 3, (82%)
R = COCH3, 4, (81%)
3
7
or 8
Scheme 1. Synthetic scheme for compounds 1–8.

2712
N. Agarwal, P. K. Nayak / Tetrahedron Letters 49 (2008) 2710–2713
Table 1
Photophysical and electrochemical data of 1–4
a
sol
b
c
d
Complexes
Absorbance (nm) (loge)
Plsol (nm)
/
s
(ls)
S
0
–T
1
(eV)
E
HOMO (eV)
1
T (eV)
Ir(ppy)
2
(acac)
345 (3.8), 412 (3.4), 460 (3.3), 497 (3.0)
371 (3.8), 407 (3.6), 434 (3.0), 482 (3.0)
366 (3.3), 407 (3.2), 439 (3.1), 505 (3.1)
349 (3.6), 462 (3.8)
516
503
569
620
619
0.34
0.23
0.11
<0.1
<0.1
1.6
1.1
1.7
12.1
13.7
—
2.51
2.25
2.04
2.04
—
—
1
2
3
4
a
ꢁ5.21
ꢁ5.37
ꢁ5.37
ꢁ5.50
ꢁ2.70
ꢁ3.12
ꢁ3.33
ꢁ3.46
322 (4.1), 450 (4.2)
Quantum yield in toluene.
Pl (Photoluminescence) life time.
Determined from the Pl spectrum.
Triplet energy level.
b
c
d
2
2
1
1
5000
0000
5000
0000
30
+
Fc/Fc
1
2
3
4
2
1
0
0
0
+
4/4
-
-
10
20
5
000
0
3
50
400
450
500
550
600
-1.0
-0.5
0.0
0.5
1.0
1.5
Wavelength (nm)
Voltage vs Ag/AgNO3
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
Fig. 2. Cyclic voltammogram of
rate = 0.2 V/s).
4 recorded in acetonitrile (scan
1
2
3
4
sponding oxidation peak potentials. The EHOMO values
of 1–4 were calculated using as earlier reported equa-
1
7,18
tion
and are listed in Table 1. The HOMO levels of
1–4 lie in the range of ꢁ5.2 to ꢁ5.5 eV. The triplet energy
levels of 1–4 promise the potential use of these compounds
as triplet harvesters to produce green to red emissions in
OLEDs.
In conclusion, new heteroleptic Ir(III) complexes of 2-p-
phenylpyridine were synthesized and characterized. Photo-
physical studies of these complexes show emission maxima
in the green to red region. Electrochemical properties were
evaluated to determine the HOMO levels and triplet levels
of these compounds which suggest that they may serve as
green-red emitters in OLEDs.
5
00
550
600
650
700
750
Wavelength (nm)
Fig. 1. Absorption (top) and emission (bottom) spectra of 1–4 recorded in
toluene (kex = 450 nm).
1
–4 show single photoluminescence life times (ꢀ1–14 ls,
Table 1).
Solution electrochemistry methods provide an insight on
Acknowledgements
the HOMO and LUMO levels and stability of the radical
anion/cations. Complexes 1–4 were characterized by
reversible oxidation in cyclic voltammetry in acetonitrile
We are thankful to Professor N. Periasamy for helpful
discussions. The National Facility for high field NMR at
TIFR is acknowledged for NMR spectra.
(
concn [1–4] ꢀ 1 mM, tetrabutylammonium hexafluoro-
phosphate 0.1 M as supporting electrolyte). The peak
potential was determined versus ferrocene as an internal
standard. Complexes 1–4 were not reducible in the electro-
chemical range accessible in acetonitrile (Fig. 2). The
HOMO levels of 1–4 were determined from the corre-
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2713
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7
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3
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1
4
afforded pure compound 1 as a green solid (0.041 g, 75%).
NMR (500 MHz, CD Cl ) d 1.84 (s, 6H, CH
OCH ), 5.31 (s, 1H), 5.72 (s, 2H), 6.48 (s, 2H), 7.15 (m, 2H), 7.55 (d,
2H, J = 8.5 Hz), 7.76–7.80 (m, 4H), 8.47 (d, 2H, 2-pyridyl,
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7
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168.23, 184.55; ES-MS (m/z) calcd for C29 Ir, 659.7528;
3
) d 28.08, 53.50, 106.29,
6
7
8
27 2 4
H N O
+
+
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(0.032 g, 0.025 mmol) and 5-nitro-8-hydroxy-quinoline (0.012 g,
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9
was filtered and washed with ethanol to give pure compound 4 as an
1
orange solid (0.031 g, 77%). H NMR (500 MHz, CDCl
3
) d 2.33 (s,
3 3
3H, CH ), 2.38 (s, 3H, CH ), 6.81 (s, 1H), 6.95 (t, 1H, J = 10 Hz), 7.05
1
1
1
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J = 10 Hz), 7.80–7.86 (m, 4H), 8.00 (d, 1H, J = 10 Hz), 8.05 (d, 1H,
J = 10 Hz), 8.65 (d, 1H, J = 10 Hz), 8.70 (d, 1H, J = 10 Hz), 9.55 (d,
1H, J = 10 Hz); C NMR (125 MHz, CDCl ) d 26.86, 115.78, 120.21,
3
122.37, 123.87, 124.38, 125.91, 127.55, 129.68, 131.36, 132.07, 132.99,
135.39, 137.42, 137.77, 142.82, 147.05, 148.61, 149.17, 166.61, 167.71,
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found 774.9623 (M ), 584.9683.
H
25
N
4
O
5
Ir, 774.1454;
1
+
1
3
) d 3.85
(
s, 3H, OCH
3
), 6.98 (d, 2H, J = 9.0 Hz, aryl), 7.15–7.17 (m, 1H,
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13
aryl), 8.63 (d, 1H, J = 5.0 Hz, 2-pyridyl); C NMR (125 MHz,
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57.07, 161.43. Elemental Anal. Calcd for C12 11NO: C, 77.81; H,
.99; N, 7.56. Found: C, 76.55; H, 6.25; N, 7.81.
3
1
5
H