Emissive iridium(III) diimine complexes formed by double cyclometalation of coordinated triphenylphosphite

Article abstract of DOI:10.1021/ic2003723

We report on the synthesis of a new series of iridium(III) complexes functionalized with various diimine chromophores, together with a facially coordinated dicyclometalated phosphite chelate and a monodentate anionic ancillary. This conceptual design pres

Full text of DOI:10.1021/ic2003723

ARTICLE
pubs.acs.org/IC
Emissive Iridium(III) Diimine Complexes Formed by Double
Cyclometalation of Coordinated Triphenylphosphite
Yao-Yuan Chang,† Jui-Yi Hung,† Yun Chi,*,† Jong-Pyng Chyn,‡ Min-Wen Chung,§ Chia-Li Lin,§
Pi-Tai Chou,*,§ Gene-Hsiang Lee,§ Chih-Hao Chang,*,^ and Wei-Chieh Lin^
†Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
‡Department of Chemistry, R. O. C. Military Academy, Fengshan 830, Taiwan
§Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
^Department of Photonics Engineering, Yuan Ze University, Chung-Li 32003, Taiwan
S Supporting Information
b
ABSTRACT: We report on the synthesis of a new series of iridium(III)
complexes functionalized with various diimine chromophores, together with a
facially coordinated dicyclometalated phosphite chelate and a monodentate
anionic ancillary. This conceptual design presents a novel strategy in obtaining
a new class of iridium(III) diimine complexes without employment of
traditional nitrogen-containing polyaromatic cyclometalates. Additionally,
we discuss the basic charactersistics of the ground and lower-lying excited
states involved, as documented by crystal structural, photophysical studies,
and density functional theory calculations. Fabrication of the green-emitting organic light-emitting diodes with one such dopant,
[Ir(dbbpy)(tpit)NCS] (2b), where dbbpy and tpit represent di-tert-butyl-2,20-bipyridine and dicyclometalated triphenylphosphite,
respectively, was successfully made, attaining a peak external quantum efficiency (ηext), a luminance efficiency (ηl), and a power
efficiency (ηp) of 14.1%, 46.6 cd Aꢀ1, and 39.9 lm Wꢀ1, respectively.
’ INTRODUCTION
fundamental properties and use relevant rhenium(I) complexes
as emissive labels, probes, and sensors.5 Switching the metal
atom from rhenium(I) to osmium(II) has intuitively led to the
formation of osmium(II) diimine complexes with two terminal
halides, for which complexes such as class C were readily
prepared from solid-state pyrolysis of [Os(CO)3I2]2 and diimine
chelate.6
In sharp contrast to previously discussed group 6ꢀ8 transi-
tion-metal complexes, the iridium(III) complexes with both
diimine chelate and carbonyl ancillaries are not as common.7
Apparently, the higher oxidation state at the iridium(III) metal
center would reduce the π-back-bonding to the carbonyl ligand
and, consequently, destabilize this bonding interaction. This
explains why only a limited number of carbonyl-containing
iridium(III) metal complexes have been isolated and character-
ized. On the other hand, the cyclometalating chelate can be
utilized to substitute carbonyl ligand in order to stabilize diimine
coordination to the iridium(III) metal center, which then pro-
duced a large number of cationic complexes showing bright
luminescence in a degassed solution at room temperature.8 A
representative example is given by the tris-chelated iridium(III)
complexes of type D, which is typically synthesized via the
treatment of dimeric [Ir(ppy)2(μ-Cl)]2 with 2,20-bipyridine.9
In this contribution, we report the in situ preparation of a
Late-transition-metal and third-row transition-metal complexes
with diimine ligands such as 2,20-bipyridine or functionalized
entities have attracted great interest because of their appli-
cations in materials chemistry, metal catalysts, and optoelec-
tronics.1 Their characteristic robustness and higher luminescent
efficiencies also make them excellent candidates as a paradigm for
probing the basic coordination chemistry and the associated
catalytic and photophysical properties. Particularly, the diimine
metal complexes with d6 electronic configuration are ubiquitous
because of their greater thermal stability, higher luminescent
efficiency, and versatility to which the electronic properties of
the ligands can be tuned.2 Recently, studies are also extended
to the whole series of d6 metal complexes, i.e., those with W0, ReI,
OsII, and IrIII elements. Particularly, the boom of research on
osmium(II) and iridium(III) complexes is driven by the urgent
need for highly efficient, phosphorescent materials for organic
light-emitting diodes (OLEDs).3
Charge-neutral tungsten(0) complexes like class A shown in
Scheme 1 are the prototype, for which the lowest-lying electronic
absorption and emission spectra are attributed to the metal-to-
ligandcharge-transfer (MLCT) transitionsandwhichis commonly
highly solvent-dependent.4 Next, the isoelectronic rhenium(I)
complexes such as class B with a chelating 2,20-bipyridine ligand
have been assembled with one anionic halide ligand to balance
the 1þ charge of the central metal ion. With the broad ability to
functionalize bipyridine, it then clears the way to study their
Received: February 22, 2011
Published: May 04, 2011
r
2011 American Chemical Society
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|

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ARTICLE
Scheme 1. Structural Drawings of Third-Row d6 Metal
Complexes with 2,20-Bipyridine Chelates
was removed, and the residue was purified by silica gel column chro-
matography, eluting with a 3:1 mixture of CH2Cl2 and ethyl acetate. The
green crystals of [Ir(bpy)(tpit)Cl] were obtained by the slow diffu-
sion of hexane into a CH2Cl2 solution at room temperature (88 mg,
0.13 mmol, 64%).
Spectral Data of 1a. MS (FAB, 35Cl, 193Ir): m/z 657 (M ꢀ Cl)þ. 1H
NMR (500 MHz, CDCl3, 294K): δ 8.18 (d, J = 7.0 Hz, 2H), 8.00 (d, J =
5.0 Hz, 2H), 7.86ꢀ7.80 (m, 4H), 7.24 (t, J = 6.0 Hz, 2H), 6.98ꢀ6.96 (m,
4H), 6.90ꢀ6.87 (m, 2H), 6.78 (t, J = 7.5 Hz, 1H), 6.61 (t, J = 8.0 Hz,
2H), 6.51 (d, J = 8.0 Hz, 2H). 31P{1H} NMR (202 MHz, CDCl3, 294K):
δ 118.9 (s, 1P). Anal. Calcd for C28H21ClIrN2O3P: C, 48.59; N, 4.05; H,
3.06. Found: C, 48.27; N, 4.47; H, 3.32.
Selected crystal data of 1a:C28H21ClIrN2O3P; M= 692.09; monoclinic;
space group = P21/c; a = 14.0496(7) Å, b = 9.9966(5) Å, c = 18.0855(9) Å,
β = 107.663(1)ꢀ, V = 2420.3(2) Å3; Z = 4; Fcalcd = 1.899 Mg mꢀ3; F(000) =
1344; crystal size = 0.22 ꢁ 0.13 ꢁ 0.08 mm3; λ(Mo KR) = 0.710 73 Å; T =
150(2) K; μ = 5.728 mmꢀ1; 18 216 reflections collected, 5541 independent
reflections (Rint = 0.0393), GOF = 1.077, final R1 [I > 2σ(I)] = 0.0251 and
wR2 (all data) = 0.0624.
Synthesis of 2a. This complex was synthesized from dbbpy using
the procedures reported for 1a and then purified by silica gel column
chromatography using a 1:2 mixture of hexane and ethyl acetate. The
green crystals of [Ir(dbbpy)(tpit)Cl] (2a) were obtained by the slow
diffusion of hexane into a CH2Cl2 solution at room temperature (73 mg,
0.09 mmol, 46%).
distinctively different iridium(III) diimine complex bearing facially
coordinated, triphenylphosphite dicyclometalate, plus one term-
inal halide or pseudohalide ligand for maintaining the charge
neutrality of the iridium(III) complex. Such a tripodal ancillary
has only been isolated from either a circuitous route or the
treatment of iridium(I) starting materials with hindered phos-
phites to promote dicyclometalation.10 Recently, such a synthetic
stalemate was finally overcome upon employment of a distinctive
metal reagent [IrCl3(tht)3], where tht = tetrahydrothiophene,
which allowed the synthetic reaction to be conducted in hydro-
carbon, which facilitates dicyclometalation of triphenylphosphite.11
These new series of emissive iridium(III) complexes are then
studied by absorption and emission spectroscopy together with a
density functional theory (DFT) computational approach to gain
insight into the relationship regarding structural versus photo-
physical properties. The successful fabrication of OLEDs using
one of the titled complexes, [Ir(dbbpy)(tpit)NCS] (2b), as the
dopant is also reported. Details are elaborated in the following
sections.
Spectral Data of 2a. MS (FAB, 35Cl, 193Ir): m/z 769 (M ꢀ Cl)þ. 1H
NMR (500 MHz, CDCl3, 294 K): δ 8.18 (d, J = 7.2 Hz, 2H), 7.86 (d, J =
5.6 Hz, 2H), 7.73 (d, J = 1.6 Hz, 2H), 7.22 (dd, J = 8.0 and 2.0 Hz, 2H),
6.96ꢀ6.95 (m, 4H), 6.93ꢀ6.85 (m, 2H), 6.75 (t, J = 6.8 Hz, 1H), 6.68 (t,
J = 7.2 Hz, 2H), 6.48 (d, J = 8.0 Hz, 2H), 1.35 (s, 18H). 31P{1H} NMR
(202 MHz, CDCl3, 294 K): δ 119.1 (s, 1P). Anal. Calcd for C36H37ClIr-
N2O3P: C, 53.76; N, 3.48; H, 4.64. Found: C, 53.73; N, 3.81; H, 4.48.
Synthesis of 2b. 2a (50 mg, 0.06 mmol) and KSCN (60 mg, 0.6
mmol) were combined in N,N-dimethylformamide (DMF; 15 mL), and
the mixture was refluxed for 32 h. After cooling to room temperature, the
solvent was removed, and the residue was purified by silica gel column
chromatography, eluting with a 1:1 mixture of hexane and ethyl acetate.
The green crystals of [Ir(dbbpy)(tpit)NCS] (2b) were obtained by the
slow diffusion of hexane into a CH2Cl2 solution at room temperature
(42 mg, 0.06 mmol, 82%).
Spectral Data of 2b. MS (FAB, 193Ir): m/z 828 (M þ 1)þ. 1H NMR
(500 MHz, CDCl3, 294 K): δ 7.80 (d, J = 6.0 Hz, 2H), 7.78 (d, J =
0.8 Hz, 2H), 7.73 (d, J = 1.2 Hz, 2H), 7.26 (dd, J = 6.0 and 1.6 Hz, 2H),
6.99ꢀ6.94 (m, 4H), 6.91ꢀ6.87 (m, 2H), 6.75 (t, J = 7.6 Hz, 1H), 6.67 (t,
J = 8.0 Hz, 2H), 6.43 (d, J = 8.4 Hz, 2H), 1.38 (s, 18H). 31P{1H} NMR
(202 MHz, CDCl3, 294 K): δ 121.8 (s, 1P). Anal. Calcd for C37H37IrN3-
O3PS: C, 53.74; N, 5.08; H, 4.51. Found: C, 53.43; N, 5.06; H, 4.78.
Selected crystal data of 2b: C37H37IrN3O3PS; M = 826.93; orthor-
hombic; space group = Pbca; a = 17.9687(8) Å, b = 17.0664(8) Å, c =
21.9546(10) Å, V = 6732.6(5) Å3; Z = 8; Fcalcd = 1.632 Mg mꢀ3; F(000)
= 3296; crystal size = 0.25 ꢁ 0.20 ꢁ 0.15 mm3; λ(Mo KR) = 0.710 73 Å;
T = 150(2) K; μ = 4.117 mmꢀ1; 49 982 reflections collected, 7728
independent reflections (Rint = 0.0756), GOF = 1.060, final R1
[I > 2σ(I)] = 0.0344 and wR2 (all data) = 0.0666.
Synthesis of 2c. Method 1. Complex 2a (50 mg, 0.06 mmol) and
AgOTf (24 mg, 0.09 mmol) were combined in toluene (25 mL), and the
mixture was stirred at room temperature for 12 h. After the addition of
3,5-bis(triflouromethyl)-1H-pyrazole (bfpzH; 63 mg, 0.31 mmol) and
sodium acetate (25 mg, 0.31 mmol), the mixture was stirred at room
temperature for another 12 h. Finally, the solvent was removed, and the
residue was purified by silica gel column chromatography, eluting with a
2:1 mixture of hexane and ethyl acetate. The green crystals were
obtained as [Ir(dbbpy)(tpit)(bfpz)] (2c) by the slow diffusion of hexane
into a CH2Cl2 solution at room temperature (25 mg, 0.03 mmol, 41%).
’ EXPERIMENTAL SECTION
General Procedures. All reactions were performed under an argon
atmosphere, and solvents were distilled from appropriate drying agents
prior to use. Commercially available reagents were used without further
purification unless otherwise stated. All reactions were monitored using
precoated thin-layer chromatography (TLC) plates (0.20 mm with
fluorescent indicator UV254). Mass spectra were obtained on a JEOL
SX-102A instrument operating in electron impact (EI) or fast atom
bombardment (FAB) mode. 1H and 13C NMR spectra were recorded on
a Varian INOVA-500 instrument. Elemental analysis was carried out
with a Heraeus CHN-O rapid elementary analyzer. Cyclic voltammetry
(CV) measurements were performed using a BAS 100B/W electro-
chemical analyzer. For convenience, hereafter, the abbreviations for
various chelates utilized in this study, i.e., 2,20-bipyridine, di-tert-butyl-
2,20-bipyridine, 2,20-biquinoline, and dicyclometalated triphenylpho-
sphite, are defined as bpy, dbbpy, bq, and tpit, respectively.
Synthesis of 1a. [IrCl3(tht)3] (110 mg, 0.20 mmol), tpitH2
(62 mg, 0.20 mmol), bpy (31 mg, 0.20 mmol), and sodium acetate
(82 mg, 1.00 mmol) were added in decalin (15 mL), and the mixture was
heated to 180 ꢀC for 8 h. After cooling to room temperature, the solvent
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ARTICLE
Method 2. A mixture of 2a (50 mg, 0.06 mmol), bfpzH (63 mg,
0.31 mmol), and potassium acetate (30 mg, 0.31 mmol) were combined
in decalin (15 mL), and the mixture was heated at 180 ꢀC for 24 h. After
cooling to room temperature, the solvent was removed, and the residue
was purified by silica gel column chromatography, eluting with a 1:2
mixture of hexane and ethyl acetate. Recrystallization from hexane gave
the green crystals as 2c (27 mg, 0.03 mmol, 45%).
DFT with the B3LYP hybrid functional.13 Restricted and unrestricted
formalisms were adopted in the singlet and triplet geometry optimiza-
tions, respectively. A “double-ζ” quality basis set consisting of Hay and
Wadt’s effective core potentials (ECPs; LANL2DZ)14 was employed for
the IrIII metal atom, and a 6-31G* basis set,15 for the rest of the atoms.
The relativistic ECP replaced the inner-core electrons of the IrIII metal
atom, leaving only the outer-core valence electrons (5s25p65d6) to be
concerned with. Time-dependent DFT (TDDFT) calculations using the
B3LYP functional were then performed based on the optimized
structures at ground states.16 Moreover, considering the solvation effect,
the calculations were then combined with an integral equation formal-
ism polarizable continuum model (IEF-PCM; in dichloromethane),
implemented in Gaussian 03.17 Typically, 10 lower triplet and singlet
roots of the nonhermitian eigenvalue equations were obtained to
determine the vertical excitation energies. Oscillator strengths were
then deduced from the dipole transition matrix elements (for singlet
states only). All calculations were carried out using Gaussian 03.18
Spectral Data of 2c. MS (FAB, 193Ir): m/z 973 (M þ 1)þ. 1H NMR
(500 MHz, acetone-d6, 294 K): δ 8.19 (d, J = 6.0 Hz, 2H), 8.09 (d,
J = 1.5 Hz, 2H), 7.54 (dd, J = 6.0 and 1.5 Hz, 2H), 7.51 (d, J = 7.5 Hz,
2H), 6.94ꢀ6.83 (m, 7H), 6.73ꢀ6.70 (m, 2H), 6.52 (s, 1H), 6.47 (d,
J = 8.5 Hz, 2H), 1.37 (s, 18H). 31P{1H} NMR (202 MHz, acetone-d6,
294 K): δ 119.8 (s, 1P). 19F{1H} NMR (470 MHz, acetone-d6, 294 K):
δ ꢀ56.14 (s, 3F), ꢀ61.84 (s, 3F). Anal. Calcd for C41H38F6IrN4O3P:
C, 50.67; N, 5.76; H, 3.94. Found: C, 50.33; N, 5.26; H, 3.65.
Synthesis of 3a. This complex was synthesized from bq using a
procedure similar to that reported for 1a and then purified by silica gel
column chromatography using a 3:1 mixture of CH2Cl2 and ethyl
acetate. The red crystalline solids of [Ir(bq)(tpit)Cl] (3a) were obtained
by the slow diffusion of hexane into a CH2Cl2 solution at room
temperature (56 mg, 0.08 mmol, 42%).
’ RESULTS AND DISCUSSION
Synthetic Strategy. Synthesis of the required iridium(III)
metal complexes was accomplished using the [IrCl3(tht)3] metal
reagent, which is highly soluble in nonpolar, higher-boiling
hydrocarbon solvents such as decalin. Moreover, the experiments
were performed using procedures reported for the synthesis of
relevant iridium(III) complexes employing benzyldiphenylpho-
sphine cyclometalates.19 Thus, upon the addition of an equimo-
lar amount of tpitH2 and diimine such as bpy, dbbpy, and bq, we
were able to obtain a series of novel diimine complexes, 1a, 2a,
and 3a, in high yields. Scheme 2 depicts the structural drawings of
these iridium(III) complexes. It is believed that the added
diimine and phosphite would easily replace all three tht ligands
and then form certain unknown intermediates at the beginning of
the reaction. Moreover, the phosphite coordination would bring
two of its phenyl substituents near the central IrIII atom and
then facilitate subsequent double cyclometalation with assis-
tance from sodium acetate added to the mixture. Further
cyclometalation of the last phenyl group is considered to be
infeasible because of the geometrical constraint imposed by the
phosphite-chelating conformation.
In addition to the tripodal phosphite, it is interesting to note
that all of the complexes 1aꢀ3a possess a terminal chloride
ligand at the sixth coordination site, a result that is in sharp
contrast to the previous reaction pattern involving phosphine, for
which the removal of all chloride in the presence of acetate has
been documented.20 However, anion metathesis was successfully
achieved using a higher-boiling, polar solvent under extended
heating or upon the addition of a silver reagent as the chloride
scavenger. Accordingly, complex 2a reacts with KNCS and
bis(trifluoromethyl)-1H-pyrazole to form thiocyanate and
Spectral Data of 3a. MS (FAB, 35Cl, 193Ir): m/z 757 (M ꢀ Cl)þ. 1H
NMR (500 MHz, CDCl3, 294 K): δ 8.49 (d, J = 9.5 Hz, 2H), 8.25 (d, J =
5.0 Hz, 1H), 8.23 (d, J = 9.5 Hz, 3H), 7.97 (d, J = 8.5 Hz, 2H), 7.70 (d, J =
8.0 Hz, 2H), 7.46 (t, J = 7.5 Hz, 2H), 7.27 (t, J = 8.0 Hz, 2H), 6.84ꢀ6.83
(m, 4H), 6.76ꢀ6.74 (m, 2H), 6.71 (t, J = 7.5 Hz, 1H), 6.32 (t, J = 8.0 Hz,
2H), 6.51 (d, J = 8.0 Hz, 2H). 31P{1H} NMR (202 MHz, CDCl3, 294 K):
δ 113.4 (s, 1P). Anal. Calcd for C36H25ClIrN2O3P: C, 54.58; N, 3.54;
H, 3.18. Found: C, 54.36; N, 3.98; H, 3.49.
Single-Crystal X-ray Diffraction Studies. Single-crystal X-ray
diffraction data were measured with a Bruker SMART Apex CCD diffract-
ometer using Mo KR radiation (λ = 0.710 73 Å). The data collection was
executed using the SMART program. Cell refinement and data reduction
were performed with the SAINT program. The structure was determined
using the SHELXTL/PC program and refined using full-matrix least squares.
Photophysical Investigation. Steady-state absorption, emission,
and phosphorescence lifetime measurements in solution and the solid
state were described in our previous reports.12 To determine the
photoluminescence quantum yield in solution, the samples were de-
gassed by three freezeꢀpumpꢀthaw cycles. 4-(Dicyanomethylene)-
2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran (DCM; λmax = 615 nm,
exciton) in methanol, with a quantum yield of ∼0.44, served as the
standard for measurement of the quantum yield. Lifetime studies were
performed with an Edinburgh FL 900 photon-counting system using
a hydrogen lamp as the excitation source. Data were analyzed using
a nonlinear least-squares procedure in combination with an iterative
convolution method. The emission decays were fitted by the sum of
exponential functions with a temporal resolution of ∼300 ps after
deconvolution of the instrument response function.
Computational Methodology. Calculations on the electronic
singlet and triplet states of all titled complexes were carried out using
Scheme 2. Structural Drawings of the Iridium(III) Diiminephosphite Complexes
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pyrazolate derivatives 2b and 2c in moderate yields, respectively.
Moreover, the thiocyanate-substituted 2b appears to be reason-
ably stable in solution, but the pyrazolate complex 2c can be
partially converted back to its parent complex 2a in a chlorinated
solvent such as CHCl3 at room temperature within approxi-
mately 24 h.
Structural Determination. Single crystals suitable for X-ray
structural analysis were grown by the slow diffusion of hexane
into the CH2Cl2 solution at room temperature. The X-ray struc-
tural determinations were conducted on complexes 1a and 2b to
confirm their coordination features. Figures 1 and 2 show their
ORTEP diagrams, together with the selected metric parameters
that are listed in the caption. As one can clearly perceive, the IrIII
metal center in both complexes formed a distorted octahedral
coordination arrangement. The phosphite tripod occupies the
facial dispositions, among which the P atom is tilted toward both
phenyl cyclometalates because of an internal constraint setting
up by the oxygen linkers. The bpy chelate and the dual phenyl
cyclometalates adopt the square-planar geometry. This planar
arrangement then allows the remaining phenyl pendent of
phosphite to stack favorably with bpy. The associated intramo-
lecular ππ-stacking interaction is confirmed by the short phenyl-
to-pyridine hexagonal center-to-center contact of 3.573 Å, as
observed in 1a. On the other hand, for complex 2b, because of
the presence of tert-butyl substituents on the bipyridine, the
relevant center-to-center contact now increased to 3.651 Å,
showing a reduction of the ππ-stacking interaction attributed
to the increased steric demand. Finally, the ambidentate thio-
cyanate ligand of complex 2b is linked to the IrIII metal atom via
its N atom and retained a linear arrangement; such a bonding
mode is identical with that observed in many structurally char-
acterized ruthenium(II) and iridium(III) complexes.21
Electrochemistry. The electrochemical behavior of these
iridium(III) metal complexes was investigated by CV using
ferrocene as the internal standard. The respective redox data
are listed in Table 1. During the anodic scan in CH2Cl2, all
iridium(III) metal complexes exhibited an irreversible oxidation
peak occurring in the region 0.89ꢀ1.01 V, which is assigned as
the metal-centered oxidation process. As a result, these potentials
are less affected by variation of diimine chelate. This viewpoint is
confirmed by the fact that only smaller variations from 0.89 to
0.95 V are observed for the set of chloride complexes 1a, 2a, and
3a. On the other hand, variation of the anionic ligand from
chloride (2a) and bfpz (2c) to thiocyanate (2b) has produced a
notable change from 0.89 and 0.90 to 1.01 V. This was attributed to
the much reduced electron-donating capability of thiocyanate in 2b.
Upon switching to the cathodic sweep in tetrahydrofuran
(THF), a single quasi-reversible peak potential ranging from
ꢀ2.33 to ꢀ2.78 V was also detected. As revealed by previous
studies, the reduction occurred mainly at the π* orbital of the
diimine chelate. Accordingly, the lowest reduction peak potential
of ꢀ2.33 V is detected in complex 3a, for which the extended π
conjugation of bq would afford the lowest potential for the
incoming electron. In contrast, all of the complexes 2aꢀ2c would
afford many negative reduction potentials because of the pre-
sence of two electron-donating tert-butyl groups on bipyridine
versus a simple bipyridine or biquinoline chelate.
Figure 1. ORTEP diagram of 1a with ellipsoids shown at the 30%
probability level. Selected bond distances (Å): IrꢀP(1) = 2.1441(10),
IrꢀCl(1) = 2.4455(9), IrꢀC(7) = 2.051(4), IrꢀC(1) = 2.054(4),
IrꢀN(1) = 2.134(3), and IrꢀN(2) = 2.145(3).
Photophysical Properties. The photophysical properties of
these iridium(III) complexes can be systematically tuned via
modification of the diimine chromophores and/or the ancillary
ligands. Figure 3 displays the absorption and emission spectra of
all titled complexes, 1a, 2aꢀ2c, and 3a, recorded in degassed
CH2Cl2 at room temperature, while pertinent numerical data are
Figure 2. ORTEP diagram of 2b with ellipsoids shown at the 30%
probability level. Selected bond distances (Å): IrꢀP(1) = 2.1486(10),
IrꢀN(3) = 2.086(3), IrꢀC(7) = 2.048(4), IrꢀC(1) = 2.054(4),
IrꢀN(1) = 2.123(3), and IrꢀN(2) = 2.125(3).
Table 1. Electrochemical and Photophysical Data of Iridium(III) Complexes at Room Temperature
UV/vis data, nm (ε ꢁ 10ꢀ3, Mꢀ1 cmꢀ1
)
PL λmax (nm)
QY a (%)
τobsa (μs)
kr (sꢀ1
)
knr (sꢀ1
)
Eox
Ered
b
b
1a
2a
2b
2c
3a
265sh (15.4), 302sh (14.6), 308 (15.7), 432 (0.67)
267sh (15.1), 298sh (13.3), 305 (13.9), 415 (0.64)
275sh (15.5), 297 (14.9), 308 (14.5), 410 (0.70)
283sh (11.8), 300 (14.6), 399 (0.91)
574
0.48
0.86
0.59
0.29
0.11
0.84
5.77 ꢁ 105
6.32 ꢁ 105
3.91 ꢁ 105
2.2 ꢁ 105
6.15 ꢁ 105
1.0 ꢁ 105
2.76 ꢁ 105
5.4 ꢁ 105
2.74 ꢁ 106
0.92
0.89
1.01
0.90
0.95
ꢀ2.53
ꢀ2.66
ꢀ2.57
ꢀ2.78
ꢀ2.33
557 [517]
552 [500]
562
1.36 [1.39]
1.50 [1.96]
1.32
274 (48.1), 353sh (17.9), 370 (23.0), 487 (1.47)
646
0.33
3.39 ꢁ 105
a Samples were degassed and recorded in CH2Cl2 at room temperature with ε in Mꢀ1 cmꢀ1, while photophysical data recorded in the solid state are
depicted in square brackets. b All electrochemical potentials were measured in a 0.1 M TBAPF6/CH2Cl2 and THF solution for oxidation and reduction
reactions and are reported in volts using Fc/Fcþ as the reference. Both Eox and Ered are referred to the irreversible peak potential. The glassy carbon and
goldꢀmercury alloy were selected as the working electrodes of oxidation and reduction processes, respectively.
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summarized in Table 1. In general, all of the complexes 1a and
2aꢀ2c exhibit allowed absorption bands in the UV region of
<300 nm (and <400 nm for 3a), for which ε values at the peak
maxima are measured to be >1.5 ꢁ 104 Mꢀ1 cmꢀ1 and can thus
be attributed to a ligand-centered ππ* transition. The broad band
at longer wavelength of up to 400 nm for complexes 1a and
2aꢀ2c (500 nm for 3a), as revealed by their relatively lower
extinction coefficients (<3 ꢁ 103 Mꢀ1 cmꢀ1), is assigned to the
tail of the ππ* transition overlapping with the MLCT transition
in the singlet manifold. Furthermore, complex 3a showed a
significant gain in absorptivity, and all peaks are very red-shifted
compared to those of other complexes, manifesting the influence
of extended π conjugation on the quinoline (cf. pyridine) moiety.
As for the emission spectra, the significant variation of the
intensity between aerated and degassed solutions, i.e., the great
influence of O2 quenching, together with the long radiative
lifetime (approximately microseconds; see Table 1), indicates
that the emission originates from the triplet manifold, i.e., the
phosphorescence. All of the titled complexes exhibit broad,
structureless emission within the range of 450ꢀ750 nm for 1a
and 2aꢀ2c and 550ꢀ850 nm for 3a. It is noteworthy that the
spectral profiles of both absorption and emission for 1a, 2aꢀ2c,
and 3a are independent of the concentrations prepared, elim-
inating the possible ππ interaction of the titled complexes in
solution in both ground and excited states.
Figure 3. Absorption and luminescence spectra of 1a, 2aꢀ2c, and 3a in
a degassed CH2Cl2 solution at room temperature. The inset shows the
absorption spectra of complexes 1a and 2aꢀ2c in the region of
350ꢀ450 nm.
Apparently, the extension of π conjugation on biquinoline in
3a substantially decreases the lowest unoccupied molecular
orbital (LUMO; π*) energy (cf. the bipyridine moiety) and,
hence, reduces the emission gap. In addition, 3a gives the lowest
quantum yield among all of the studied complexes; cf. Φ1a
=
0.48, Φ2a = 0.86, Φ2b = 0.59, Φ2c = 0.29, and Φ3a = 0.11. For 3a,
the reduction of the ππ* gap should enlarge the energetic
3
difference between the emitting MLCT/ππ* state and the
higher-lying 3MC dd state; the latter commonly induces a
nonradiative pathway due to weakening of the metalꢀligand
3
bond. Accordingly, suppression of the MC dd population, in
theory, should lead to a higher quantum yield in 3a. While the
dicyclometalated phosphite and chloride ancillaries are identical
among complexes 1a, 2a, and 3a, the apparent opposite result,
i.e., the lowest Φp for 3a among all titled complexes, is thus
intriguing. The main quenching process for a red emitter such as
3a with emission maximized at 650 nm may be attributed to high-
frequency vibrational deactivation, i.e., the operation of the
energy gap law.22 In addition, other deactivation pathways asso-
ciated with substantial geometric distortion of the diimine ligand
occurring between the ground and emitting states cannot be
eliminated. Support of this viewpoint is provided in the section
on the theoretical approach (vide infra).
Figure 4. Frontier orbitals involved in the lowest-lying electronic
transitions for 1a, 2aꢀ2c, and 3a. Calculations were done with
incorporation of the PCM model in CH2Cl2.
Table 2. Calculated Wavelength (λ), Oscillator Strength (f), and Orbital Transition Analyses of 1a, 2aꢀ2c, and 3a Incorporated
with the PCM Model of Dichloromethanea
states
λcal
f
assignments
HOMO f LUMO (98%)
MLCT (%)
1a
2a
2b
2c
3a
T1
S1
T1
S1
T1
S1
T1
S1
T1
S1
477
0
33.76
469.8
0.0053
HOMO f LUMO (98%)
33.76
467.8
0
HOMO f LUMO (98%)
33.82
465.3
0.0061
0
HOMO f LUMO (98%)
33.82
464 [645.6]
460.6 [643.1]
453.3
HOMO f LUMO (98%) [HOMO f LUMO (100%)]
HOMO f LUMO (99%) [HOMO f LUMO (100%)]
HOMO f LUMO (95%)
32.35 [4.62]
32.68 [4.62]
33.62
0.0074 [0.0001]
0
446.9
0.0067
0
HOMO f LUMO (98%)
34.68
545.3
HOMO f LUMO (94%)
32.08
521.9
0.035
HOMO f LUMO (97%)
33.11
a Data shown in brackets are calculated in the gas phase.
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Figure 5. Absorption and luminescence spectra of 2a (a and b) and 2b (c and d) resolved in different degassed solvents at room temperature.
and emission based on the S0-optimized geometries of all
complexes.
As for the transition characteristics, frontier orbital analyses
indicate that the lowest singlet excited states (S1) for all com-
plexes are primarily attributed to the HOMO f LUMO transition
(cf. Table 2). As depicted in Figure 4, the HOMO of all titled
complexes is mainly localized at the central IrIII metal atom and
phenyl cyclometalates of phosphite, while the LUMO is located
at the diimine moiety, implying that the lowest-lying absorption
transition mainly involves the MLCT transition and interligand
(phenyl f diimine) charge transfer (LLCT). Because of the
Figure 6. Optimized structures at the ground state (S0) and the lowest
triplet state (T1) for 3a, respectively.
large separation in terms of distance and orientation between
chromophores, the latter may cause a significant change of the
dipole moment between ground and excited states. This is clearly
In order to gain in-depth information about the absorptive
and emissive properties, TDDFT was then performed via use
of the functional B3LYP and the 6-31G(d)/LANL2DZ basis
set. Also, the PCM model in CH2Cl2 is incorporated to take the
solvation effect into account (see the Experimental Section for
details). Figure 4 depicts the selected frontier orbitals involved
in lower-lying transitions of the titled complexes. The calcu-
lated energy gaps and corresponding assignments of each
transition are listed in Table 2. The calculated S0 f S1
transition (in terms of wavelength) of all studied complexes
(1a, 469.8 nm; 2a, 465.3 nm; 2b, 460.6 nm; 2c, 446.9 nm; 3a,
521.9 nm) are close to the observed onsets of the absorption
spectra depicted in Figure 3. Moreover, the calculated energy
gaps of the T1 states (1a, 477 nm; 2a, 467.8 nm; 2b, 464 nm; 2c,
453.3 nm; 3a, 545.3 nm) are qualitatively in agreement with
the onsets (i.e., blue edge) of their emission spectra. This result
indicates that the TDDFT calculation works well in predicting
the lowest FranckꢀCondon excited state for both absorption
demonstrated by the strong solvent-polarity-dependent absorp-
tion spectra for, e.g., 2a and 2b, shown in Figure 5 (vide infra).
Also, as shown in Table 2, the calculated % MLCT for both S1
and T1 states is >30% for all titled complexes, resulting in a strong
spinꢀorbit coupling, i.e., a great mixing between the S1 and T1
states, due to the direct involvement of the heavy IrIII metal atom.
Support of this viewpoint is also given by the small energy gap
between S1 and T1 (see Table 2) as well as the close onset
between S0ꢀS1 absorption and phosphorescence (see Figure 3).
As for the chemical substitution effect, compared with complexes
1a and 2aꢀ2c exhibiting more blue-shifted emission profiles, this
can be rationalized by an increase of the LUMO energy via the
tert-butyl substituents imposed on the diimine moiety.
Careful examination of the optimized structures of 3a reveals
an intriguing geometry in the lower-lying excited states. While
the optimized structure of 3a in the ground state (S0) shows a
twisted, nonplanar configuration along the biquinolinyl moiety,
the relaxed geometry in the T1 state reveals a planar structure.
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the electron density of the HOMO is then redistributed and
located at the central IrIII metal atom and phenyl cyclometalates
of the phosphite tripod (see Figures 4 and 7). Compared with the
experimental data, the computation incorporating the solvation
model seems to give more reliable results. First of all, in the gas
phase, the calculated S1 state for 2b is located at 643 nm, which is
very red-shifted with respect to the onset of the experimental
lowest-lying absorption (∼450ꢀ470 nm). Alternatively, calcula-
tion involving CH2Cl2 solvation predicts a close S1 energy of
460 nm. Note that the calculated blue shift of the S0 f S1
absorption upon an increase in the polarity (gas f CH2Cl2) is
consistent with the experimental observation (see Figure 7).
Second, in the gas phase, a small proportion of MLCT in both S1
(4.62%) and T1 states (4.62%) (see Table 2) is estimated. The
dominant ππ* character for the T1 state predicts a less allowed
T1ꢀS0 phosphorescence and, hence, a much smaller radiative
decay rate constant. This is apparently contradictory to the
Figure 7. HOMO of 2b calculated (a) in the gas phase and (b) in
CH2Cl2 incorporated with the PCM model.
experimentally resolved kr value of as large as 3.91 ꢁ 105 sꢀ1
.
Alternatively, the >33% MLCT contribution for the T1 state
calculated in CH2Cl2 (see Table 2) well-rationalizes the experi-
mental data.
Finally, as shown in Figure 5, the negative absorption solva-
tochromism for 2a and 2b can be rationalized in terms of a
reduced excited-state dipole moment upon MLCT/LLCT
excitation.4 We then calculate the dipole moment of, e.g., 2a in
both S0 and S1 states. The dipole moment of the excited state
(S1) is calculated via the HellmannꢀFeynman theory, which can
be estimated by the analytical derivative of the energy of the
excited state with respect to an applied electric field. All details
were described in our previous reports.23 As a result, the
calculated dipole moment of 15.45 D in S0 is larger than that
(3.18 D) in S1. In addition, the dipole orientation of the S0 state is
52.7ꢀ with respect to that of the S1 state. Thus, a substantial
change of the dipole moment of 13.75 D between S0 and S1 is
deduced, supporting the absorption solvatochromism. Despite
the solvent-polarity-dependent absorption, however, the emis-
sion profiles of 2a and 2b reveal nearly solvent-polarity indepen-
dence (see Figure 5). We attribute the results to compensation of
an offset between absorption and emission. A more blue shift in
the absorption results in a greater difference in the dipole moment
between the ground and excited states. Thus, the emission is
subject to larger solvent relaxation/stabilization, giving virtually
nearly solvent-polarity-independent emission spectra.
Figure 8. Structural drawing of the materials and schematic structures
of green-emitting OLEDs.
OLED Device Fabrication. Complex 2b is next selected as the
dopant for the fabrication of phosphorescent OLEDs because of
its higher stability and photoluminescence quantum efficiency.
The devices consist of a simplified three-layer architecture of
indiumꢀtin oxide (ITO)/TAPC (30 nm)/CBP doped with
8 wt % 2b (25 nm)/electron-transport layer (ETL; 50 nm)/LiF
(0.8 nm)/Al (150 nm), for which the configurations and
structural drawings of essential materials are shown in Figure 8.
Several different types of electron-transport materials were
utilized for investigating the effect of the carrier balance,
including 1,3,5-tris[(3-pyridyl)phen-3-yl]benzene (TmPyPB),
2,20,200-(1,3,5-benzinetriyl)tris(1-phenyl-1H-benzimidazole),
2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, bis(2-methyl-8-
quinolinolate)-4-(phenylphenolato)aluminum, and 3-(4-biphenylyl)-
4-phenyl-5-(tert-butylphenyl)-1,2,4-triazole (TAZ). The electron
mobility of these materials ranges from ∼10ꢀ6 (TAZ) to
∼10ꢀ3 cm2 Vꢀ1 sꢀ1 (TmPyPB).24 However, all devices gave
very similar maximum external quantum efficiencies of 11ꢀ13%,
showing that the electron mobility is not the dominant factor for
This difference is evidenced by the dihedral angle —N1ꢀ
C1ꢀC2ꢀN2 of 10.55ꢀ and 0.06ꢀ at the S0 and T1 states, respec-
tively (see Scheme 2 and Figure 6). Thus, despite the seemingly
more π conjugation in 3a, the substantial structural differences
between the optimized S0 and T1 states may thus execute the
T1 f S0 radiationless transition; that is, the high-density, low-
frequency vibrations associated with the changes of geometry in
S0 may couple with the T1 lower-lying vibronic states to channel
into the deactivation pathway.22c
It should be noted that the above computational approach
incorporates the PCM model to take the solvation effect (e.g., in
CH2Cl2) into consideration. Amid the calculation, we noticed
that the solvation effect plays a crucial role in influencing the
electronic distribution, especially for 2b possessing a thiocyanate
ancillary. In the gas phase, the electron densities of the highest
occupied molecalar orbital (HOMO) calculated for 2b are
mainly distributed on the thiocyanate group (see Figure 7). This
result is in sharp contrast to that obtained in CH2Cl2, in which
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Figure 9. (a) EL spectra, (b) CIE coordinates, (c) current densityꢀvoltage (IꢀV) characteristics, (d) luminescence vs voltage, (e) external quantum
efficiency vs luminance, and (f) power efficiencies vs luminance for devices A and B.
device performances. Upon careful inspection of the carrier
balance in this system, one can see that TAPC possesses a higher
transport in the emitting layer, after considering the electron
mobility of all electron-transport materials.
hole mobility (∼10ꢀ2 cm2 Vꢀ1
s
ꢀ1) than that of all tested
One optimized device was next fabricated using TAZ as the
ETL; cf. device A. It exhibited peak electroluminescent (EL)
efficienciesof13.0%photon/electron, 40.3cdAꢀ1, and40lmWꢀ1
for the forward directions. Under a more practical brightness of
100 cd mꢀ2, the forward viewing efficiencies remained high,
at around 10.9%, 33.9 cd Aꢀ1, and 21.3 lm Wꢀ1. However, the
predominantly green emission of 2b was accompanied with
a notable near-UV emission at higher current densities, which is
electron-transport materials.25 In addition, the host material,
CBP, possesses a bipolar property, implying that the dopant
played a key role in manipulating the carrier balance in the
emitting layer.26 Hence, one could expect that dopant 2b func-
tions as effective hole traps in the emitting layer, thereby hinder-
ing excessive hole transport.27 Thus, the lower hole mobility of
CBP doped with 2b would be in favor of balancing carrier
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Table 3. EL Characteristics of Green-Emitting OLEDs Using Complex 2b with Different ETL
ηext (%)
ηl (cd Aꢀ1
)
ηp (lm Wꢀ1
)
Von (V)
max. B.
CIE coordinates
max.
102 nit
max.
102 nit
max.
102 nit
1 nit
nit (V)
102 nit
103 nit
Device A (TAZ)
13.0
14.1
10.9
12.7
40.3
46.6
33.9
42.0
40.0
39.9
21.3
22.0
3.1
3.6
4124 (13.2 V)
6125 (14.8 V)
(0.302, 0.561)
(0.298, 0.564)
(0.329, 0.545)
(0.302, 0.557)
Device B (UGH2/TAZ)
Abbreviations: ηext = external quantum efficiency; ηl = luminescence efficiency; ηp = power efficiency; Von = turn-on voltage; max. B = maximum
brightness; nit = cd mꢀ2
.
attributed to the carrier recombination in the ETL (TAZ is known
to exhibit near-UV emission at ∼400 nm).24 To diminish the
unwanted emission and to furtherimprove the efficiency, the wide-
gap material UGH2, which possesses a rather large triplet energy
gap of 3.18 eV, was employed to give optimal carrier/exciton
confinement.28 The modifiedarchitecture(i.e., deviceB) consisted
of ITO/TAPC (30 nm)/CBP doped with 8 wt % 2b (25 nm)/
UGH2 (3 nm)/TAZ (47 nm)/LiF (0.8 nm)/Al (150 nm).
Figure 9 shows the EL spectra of these two green-emitting devices.
Obviously, the near-UV emission generated from TAZ in device A
was effectively reduced after employment of a thin layer of UGH2.
Such an outcome indicates the successful confinement of carrier
recombination or blockage of exciton diffusion into the charge-
transport layer.
serious consideration. As for the practical application, fabrication
of thegreen-emitting OLEDswithdopant 2b has been successfully
made, attaining a peak external quantum efficiency (ηext), a
luminance efficiency (ηl), and a power efficiency (ηp) of 14.1%,
46.6 cd Aꢀ1, and 39.9 lm Wꢀ1, respectively. We thus believe that
the new ligand design and corresponding synthetic strategy should
add an additional avenue to promote the research field relevant to
phosphorescent OLEDs.
’ ASSOCIATED CONTENT
S
Supporting Information. X-ray crystallographic data file
b
(CIF) of the studied complexes 1a and 2b. This material is
available free of charge via the Internet at http://pubs.acs.org.
The currentꢀvoltageꢀluminance (IꢀVꢀL) characteristics
and other EL performance data are also depicted in Figure 9 and
Table 3. The peak external quantum efficiency (ηext), luminance
efficiency (ηl), and power efficiency (ηp) of device B were
increased to 14.1%, 46.6 cd Aꢀ1, and 39.9 lm Wꢀ1, respectively.
Such high external quantum efficiencies implied a high internal
quantum efficiency of nearly 70% in device B (assuming ∼20%
optical out-coupling efficiency from the devices). Again, these
results confirmed the effectiveness of the buffer layer for
achieving effective carrier/exciton confinement. Upon an in-
crease in the practical brightness to 100 cd mꢀ2, ηext, ηl, and
ηp remained above 12.7%, 42.0 cd Aꢀ1, and 22.0 lm Wꢀ1, which
are consistent with less significant efficiency roll-offs, with
Commission Internationale de L’Eclairage (x, y) coordinates
(CIEx,y) of (0.298, 0.564). In the mean time, the luminance
reached its maximum of 6125 cd mꢀ2 at a driving voltage of
14.8 V.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: ychi@mx.nthu.edu.tw (Y.C.), chop@ntu.edu.tw (P.-T.C.),
chc@saturn.yzu.edu.tw (C.-H.C.).
’ ACKNOWLEDGMENT
This work was supported by the National Science Council of
Taiwan. We are also grateful to the National Center for High-
Performance Computing for computer time and facilities.
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