Authentic-blue phosphorescent iridium(III) complexes bearing both hydride and benzyl diphenylphosphine; Control of the emission efficiency by ligand coordination geometry

Article abstract of DOI:10.1021/ic900607s

Sequential treatment of IrCl₃· nH₂O with 2 equiv of benzyl diphenylphosphine (bdpH) and then 1 equiv of 3-trifluoromethyl-5-(2-pyridyl) pyrazole (fppzH) in 2-methoxyethanol gave formation to three isomeric complexes with formula [lr(

Full text of DOI:10.1021/ic900607s

8164 Inorg. Chem. 2009, 48, 8164–8172
DOI: 10.1021/ic900607s
Authentic-Blue Phosphorescent Iridium(III) Complexes Bearing Both Hydride and
Benzyl Diphenylphosphine; Control of the Emission Efficiency by Ligand
Coordination Geometry
Yuan-Chieh Chiu,† Chen-Huey Lin,† Jui-Yi Hung,† Yun Chi,*,† Yi-Ming Cheng,‡ Kang-Wei Wang,‡
Min-Wen Chung,‡ Gene-Hsiang Lee,‡ and Pi-Tai Chou*,‡
†Department of Chemistry, National Tsing Hua University, Hsinchu 300, Taiwan, and ‡Department of
Chemistry, National Taiwan University, Taipei 106, Taiwan
Received March 30, 2009
Sequential treatment of IrCl3 nH2O with 2 equiv of benzyl diphenylphosphine (bdpH) and then 1 equiv of
3
3-trifluoromethyl-5-(2-pyridyl) pyrazole (fppzH) in 2-methoxyethanol gave formation to three isomeric complexes
with formula [Ir(bdp)(fppz)(bdpH)H] (1-3). Their molecular structures were established by single crystal X-ray
diffraction studies, showing existence of one monodentate phosphine bdpH, one terminal hydride, a cyclometalated
bdp chelate, and a fppz chelate. Variation of the metal-ligand bond distances showed good agreement with those
predicted by the trans effect. Raman spectroscopic analyses and the corresponding photophysical data are also
recorded and compared. Among all isomers complex 1 showed the worst emission efficiency, while complexes 2 and 3
exhibited the greatest luminescent efficiency in solid state and in degassed CH2Cl2 solution at room temperature,
respectively. This structural relationship could be due to the simultaneously weakened hydride and the monodentate
bdpH bonding that are destabilized by the trans-pyrazolate anion and cyclometalated benzyl group, respectively.
1. Introduction
how to conduct the tuning of emission color across the whole
visible spectrum. Such perspective has then motivated scien-
tists to investigate the red and green-emitting Ir(III) com-
plexes, for which vast technological progresses have been
made. However, among emitting materials that could afford
the required Red-Green-Blue (RGB) colors, the true-blue
The use of heavy transition metal complexes as emitting
materials in organic light-emitting devices (OLEDs), parti-
cularly the metal complexes with a Ru(II),1 Os(II),2 Ir(III),3
and Pt(II)4 central cation, is an area of increasing importance
and significance. Among these emitting materials, Ir(III)-
containing materials are the most pursued subjects.5 This
interest is motivated by their great emission efficiency, less
aggregation in solid state, and improved thermal stabilities.
However, a limiting factor of their application in OLEDs is
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Yeh, S.-J.; Chen, C.-T.; Murayama, H.; Tsuboi, T.; Li, W.-S.; Chao, I.; Liu, S.-W.;
Wang, J.-K. Adv. Funct. Mater. 2007, 17, 1887. (g) Su, S.-J.; Sasabe, H.; Takeda,
T.; Kido, J. Chem. Mater. 2008, 20, 1691. (h) Orselli, E.; Albuquerque, R. Q.;
Fransen, P. M.; Froehlich, R.; Janssen, H. M.; De Cola, L. J. Mater. Chem. 2008,
18, 4579. (i) Wang, Y.-M.; Teng, F.; Gan, L.-H.; Liu, H.-M.; Zhang, X.-H.; Fu,
W.-F.; Wang, Y.-S.; Xu, X.-R. J. Phys. Chem. C 2008, 112, 4743. (j) Yang, L.;
Okuda, F.; Kobayashi, K.; Nozaki, K.; Tanabe, Y.; Ishii, Y.; Haga, M. Inorg.
Chem. 2008, 47, 7154. (k) Stagni, S.; Colella, S.; Palazzi, A.; Valenti, G.;
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*To whom correspondence should be addressed. E-mail: ychi@mx.nthu.
edu.tw (Y.C.), chop@ntu.edu.tw (P.-T.C.).
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r
pubs.acs.org/IC
Published on Web 08/11/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 17, 2009 8165
phosphor represents the most difficult one to achieve. This is
mainly because it requires the highest excitation energy with
the shortest-wavelength phosphorescence output,6 and thus
is commonly subject to the inferiority of weak luminescence
yield, incorrect monochromaticity, and short lifespan.
So far, there is an increasing amount of research report-
ing on how to prepare blue-phosphors and investigate their
photophysical and device properties.7 Thus, having the
capability to obtain the true-blue phosphors with good
color chromaticity and excellent quantum efficiency would
be considered as one important breakthrough in OLEDs.
In this paper, we disclosed a new design concept via
incorporation of pyridyl azolate chelate, terminal hydride,
monodentate and cyclometalated phosphine chelate,8 giv-
ing successful preparation of three isomeric, blue-emitting
phosphorescent complexes 1-3. Since neither phosphine
ligand nor terminal hydride possesses lower lying π-mo-
lecular orbitals and lone pair electrons, both S1 and T1
states involved in these Ir(III) complexes are mainly
localized at the chelating pyridyl pyrazolate site, and the
corresponding S0-T1 transition resides within the higher
energy region of the visible spectra. Amid this study, an
intriguing relationship for structure versus phosphores-
cent signal intensity was explored, which may have sig-
nificant implications in the future design and preparation
of efficient true blue phosphors. Details of results and
discussion are elaborated in the following sections.
broadening and consequently renders a temporal resolution of
∼200 ps.
Confocal Raman spectra of each sample were recorded from
their single crystallines and obtained with a Thermo Nicolet
Almega XR Dispersive Raman Spectrometer (λex=780 nm) and
a Olympus BX51 Microscope. The measurements were per-
formed with 2 s exposure time, and all spectra were accumulated
over an average of 30 scans.
Synthesis of IrCl3(THT)3. The synthetic protocol was accord-
ing to a literature report with a slight modification;9 a mixture of
IrCl3 3H2O (200 mg, 0.57 mmol) and tetrahydrothiophene
3
(THT, 0.25 mL, 2.84 mmol) in 2-methoxyethanol (20 mL) was
refluxed for 12 h. After removal of solvent in vacuo, the yellow
powder was triturated with a 1:1 mixture of ethyl acetate and
hexane, and collected by filtration (271 mg, 0.49 mmol, 85%).
1
Spectra data of IrCl3(THT)3. MS (FAB): m/z 562 (Mþ). H
NMR (400 MHz, CDCl3, 294 K): δ 3.64-3.58 (m, 4H), 3.23-
3.17 (m, 2H), 2.91-2.80 (m, 6H), 2.30-2.01 (m, 12H).
Synthesis of A. A 50 mL reaction flask was charged with
Ir(THT)3Cl3 (168 mg, 0.3 mmol), benzyldiphenylphosphine
(bdpH, 171 mg, 0.62 mmol) and 5-pyridyl-3-trifluoromethyl-
1H-pyrazole (fppzH, 64 mg, 0.3 mmol). To this mixture de-
gassed decalin (20 mL) was added as solvent. The solution was
first refluxed for 1 h. After that, Na2CO3 (318 mg, 3.00 mmol)
was added, and the mixture was brought to reflux for another
24 h to ensure the complete reaction. Purification was conducted
by silica-gel column chromatography eluting with a mixture of
ethyl acetate and hexane (1:3) with Rf =0.62. Crystallization
from a mixture of ethyl acetate and hexane gave colorless
needles of A (86 mg, 0.09 mmol, 35%).
1
Spectral Data of A. MS (FAB, 193Ir): m/z 956 (Mþ1)þ; H
2. Experimental Section
NMR (500 MHz, CDCl3, 294 K): δ 7.89 (d, J=8.0 Hz, 1H), 7.87
(t, J=8.0 Hz, 1H), 7.50 (d, J=6.5 Hz, 1H), 7.43 (t, J=9.0 Hz, 2H),
7.33-7.25 (m, 3H), 7.21 (d, J = 7.5 Hz, 1H), 7.15 (t, J =
8.0 Hz, 3H), 7.11-7.08 (m, 5H), 6.96 (t, J=7.5 Hz, 1H), 6.85 (t,
J=7.5 Hz, 1H), 6.82-6.79 (m, 3H), 6.77 (s, 1H), 6.72 (t, J=7.5Hz,
1H), 6.69-6.66 (m, 3H), 6.61 (t, J=9.0Hz, 2H), 6.43 (t, J=7.0 Hz,
1H), 6.32 (t, J=8.5 Hz, 2H), 6.18 (t, J=6.0 Hz, 1H), 4.05 (dd, J=
15.0, 11.0 Hz, 1H), 3.77 (dd, J=15.0, 11.0 Hz, 1H), 3.46 (dd, J=
16.5, 10.0 Hz, 1H), 2.17 (dd, J=16.5, 10.0 Hz, 1H); 19F-{1H}
NMR (470 MHz, CDCl3, 294 K): δ -60.27 (s, 3F); 31P-{1H}
NMR (202 MHz, CDCl3, 294 K): δ 6.29 (d, JPP=11 Hz, 1P), 6.18
(d, JPP=11 Hz, 1P). Anal. Calcd for C47H37F3IrN3P2: N, 4.40; C,
59.11; H, 3.91. Found: N, 4.39; C, 58.90; H, 4.09.
General Information and Materials. All reactions were per-
formed under a nitrogen atmosphere using anhydrous solvents
or solvents treated with an appropriate drying reagent. Mass
spectra were obtained on a JEOL SX-102A instrument operat-
ing in electron impact (EI) mode or fast atom bombardment
(FAB) mode. 1H and 19F NMR spectra were recorded on Varian
Mercury-400 or INOVA-500 instruments. Elemental analyses
were conducted at the NSC Regional Instrumentation Center at
National Chiao Tung University.
Spectral and Dynamic Measurement. Steady-state absorption
and emission spectra were recorded by a Hitachi (U-3310)
spectrophotometer and an Edinburgh (FS920) fluorimeter,
respectively. Emission quantum yields were measured at excita-
tion wavelength λex=350 nm in CH2Cl2 at room temperature
(RT). In this approach, Quinine sulfate with an emission yield of
Φ ∼ 0.54 ( 0.2 in 1.0 N sulfuric acid solution served as the
standard to calculate the emission quantum yield. A configura-
tion of front-face excitation was used to measure the emission of
the solid sample 1-3. The solid sample was fixed by assembling
two edge-polished quartz plates with various Teflon spacers. A
combination of appropriate filters was used to avoid the inter-
ference from the scattering light. An integrating sphere was
applied to measure the quantum yield in the solid state, in which
the solid film of 1-3 was prepared via the spin coating method
and was excited by a 325 nm He-Cd laser line. The resulting
luminescence was acquired by an intensified charge-coupled
detector for subsequent quantum yield analyses. Lifetime stu-
dies were performed by an Edinburgh FL 900 photon counting
system with a hydrogen-filled or a nitrogen lamp as the excita-
tion source. Data were analyzed using a nonlinear least-squares
procedure in combination with an iterative convolution meth-
od. Emission decays were analyzed by the sum of exponential
functions, which allows partial removal of the instrument time
Synthesis of Hydride Complexes 1-3. A 50 mL reaction flask
was first charged with IrCl3 H2O (212 mg, 0.60 mmol) and
3
bdpH (350 mg, 1.26 mmol). To this mixture degassed 2-methoxy-
ethanol (30 mL) was added as solvent. The solution was then
refluxed for 12 h and then cooled to ambient. After then, fppzH
(128 mg, 0.6 mmol) and Na2CO3 (630 mg, 6.00 mmol) were
added. The mixture was brought to reflux for another 1 h. After
cooling to RT, addition of excess of deionized water resulted in a
white precipitate, which was collected by filtration, washed with
cold methanol and diethyl ether in sequence. Further purifica-
tion was conducted by silica-gel column chromatography elut-
ing with a mixture of ethyl acetate and hexane (1:5) with Rf
values of complexes 1, 2, and 3 being 0.43, 0.16, and 0.57,
respectively. All crystalline materials were obtained from a
layered solution of CH2Cl2 and hexane at RT and were color-
less; cf. 1 (84 mg, 0.09 mmol, 15%), 2 (153 mg, 0.16 mmol, 26%),
and 3 (57 mg, 0.06 mmol, 10%).
1
Spectral Data of 1. MS (FAB, 193Ir): m/z 956 (M-1)þ; H
NMR (500 MHz, CDCl3, 294 K): δ 8.40 (t, J=7.5 Hz, 1H), 8.16
(t, J=9.0 Hz, 2H), 7.93-7.89 (m, 2H), 7.48 (t, J=7.5 Hz, 1H),
7.35-7.28 (m, 6H), 7.20 (t, J=7.5 Hz, 1H), 7.07 (t, J=8.5 Hz,
2H), 7.01-6.98 (m, 3H), 6.92 (d, J = 7.5 Hz, 1H), 6.87
(8) Chiu, Y.-C.; Hung, J.-Y.; Chi, Y.; Chen, C.-C.; Chang, C.-H.; Wu,
C.-C.; Cheng, Y.-M.; Yu, Y.-C.; Lee, G.-H.; Chou, P.-T. Adv. Mater. 2009,
21, 2221.
(9) John, D.; Salazar, K. V.; Scott, B. L.; Baker, R. T.; Sattelberger, A. P.
Organometallics 2001, 20, 296.

8166 Inorganic Chemistry, Vol. 48, No. 17, 2009
Chiu et al.
Table 1. Crystal Data and Structure Refinement Parameters for Complexes 1, 2, and 3
complex
1
2 0.75 ꢀ CH2Cl2 0.5 ꢀ H2O
3
3
3
empirical formula
formula weight
temperature
crystal system
space group
a
b
c
C
47H39F3IrN3P2
C47.75H41.50Cl1.50F3IrN3O0.50P2
C47H39F3IrN3P2
956.95
956.95
150(2) K
triclinic
P1
12.3595(8) A
13.3107(8) A
˚
13.7068(8) A
1029.65
150(2) K
triclinic
P1
13.3687(6) A
18.0579(8) A
150(2) K
monoclinic
P21/c
14.6300(6) A
14.9847(6) A
˚
˚
˚
˚
˚
˚
˚
19.5186(9) A
˚
18.2547(8) A
R
75.969(1)°
69.893(1)°
74.047(1)°
100.571(1)°
100.911(1)°
104.355(1)°
β
92.581(1)°
γ
volume, Z
density (calculated)
absorption coefficient
F(000)
3
3
3
˚
˚
˚
2008.4(2) A , 2
4348.9(3) A , 4
3997.8(3) A , 4
1.582 Mg/m3
1.573 Mg/m3
1.590 Mg/m3
3.472 mm-1
1904
3.455 mm-1
952
3.287 mm-1
2050
crystal size (mm3)
reflections collected
independent reflections
max. and min transmission
data /restraints /parameters
Goodness-of-fit on F2
final R indices [I g 2σ(I)]
R indices (all data)
largest diff. peak and hole
0.30 ꢀ 0.12 ꢀ 0.12
0.22 ꢀ 0.12 ꢀ 0.05
0.24 ꢀ 0.15 ꢀ 0.15
26147
47094
30448
9195 [R(int) = 0.0284]
0.6819 and 0.4238
9195/0/521
1.036
R1 = 0.0236, wR2 = 0.0553
R1 = 0.0262, wR2 = 0.0563
15332 [R(int) = 0.0654]
0.8529 and 0.5316
15332/2/1050
1.079
R1 = 0.0531, wR2 = 0.1092
R1 = 0.0671, wR2 = 0.1167
9183 [R(int) = 0.0388]
0.6240 and 0.4896
9183/0/535
1.110
R1 = 0.0318, wR2 = 0.0645
R1 = 0.0375, wR2 = 0.0665
3
3
3
˚
˚
˚
1.164 and -0.668 e/A
1.725 and -1.723 e/A
1.042 and -1.041 e/A
(t, J=7.5 Hz, 1H), 6.76-6.72 (m, 4H), 6.63 (s, 1H), 6.62-6.44
(m, 7H), 6.07 (d, J=8.0 Hz, 2H), 5.16 (dd, J=15.0, 11.0 Hz, 1H),
3.69 (t, J=15.0 Hz, 1H), 2.96 (dd, J=15.0, 6.0 Hz, 1H), 2.88 (dd,
J = 15.0, 6.0 Hz, 1H), -17.84 (dd, J = 17.5, 14.5 Hz, 1H).
19F-{1H} NMR (470 MHz, CDCl3, 294 K): δ -60.04 (s, 3F).
31P-{1H} NMR (202 MHz, CDCl3, 294 K): δ 20.05 (d, JPP=11.1
Hz, 1P), 4.09 (d, JPP = 11.1 Hz, 1P). Anal. Calcd for
C47H39F3IrN3P2: N, 4.39; C, 58.99; H, 4.11. Found: N, 4.36;
C, 58.66; H, 4.40
Spectral Data of 2. MS (FAB, 193Ir): m/z 957 (Mþ); 1H NMR
(500 MHz, CDCl3, 294 K): δ 7.60-7.50 (m, 6H), 7.36 (t, J=
7.0 Hz, 1H), 7.29 (t, J=7.0 Hz, 1H), 7.23-7.20 (m, 2H), 7.15-
7.05 (m, 6H), 6.99-6.95 (m, 3H), 6.93 (s, 1H), 6.80 (t, J=8.0 Hz,
1H), 6.76 (t, J=7.0 Hz, 1H), 6.68-6.62 (m, 3H), 6.69 (d, J=
5.5 Hz, 1H), 6.53 (t, J=8.5 Hz, 2H), 6.44-6.39 (m, 3H), 6.23 (dd,
J=8.0, 5.0 Hz, 1H), 6.03 (d, J=8.5 Hz, 2H), 4.01 (dd, J=16.0,
11.0 Hz, 1H), 3.35 (dd, J=16.0, 11.0 Hz, 1H), 2.67 (dd, J=14.5,
5.0 Hz, 1H), 2.13 (dd, J=14.5, 5.0 Hz, 1H), -19.31 (dd, J=25.5,
11.0 Hz, 1H). 19F-{1H} NMR (470 MHz, CDCl3, 294 K):
δ -60.08 (s, 3F). 31P-{1H} NMR (202 MHz, CDCl3, 294 K):
δ 18.65 (d, JPP=11.1 Hz, 1P), 4.15 (d, JPP=11.1 Hz, 1P). Anal.
Calcd for C47H39F3IrN3P2: N, 4.39; C, 58.99; H, 4.11. Found:
N, 4.35; C, 59.12; H, 4.52.
structure was determined using the SHELXTL/PC program
and refined using full-matrix least-squares (Table 1). The crys-
talline sample of 3 crystallized with 3 equiv of CH2Cl2 and 1
equiv of H2O in the unit cell.
Computational Methodology. Geometry optimization of
the singlet ground state (S0) and lowest-lying triplet excited
state (T1) were performed using the density functional theory
(DFT) at the B3LYP level.10 Restricted and unrestricted form-
alisms were adopted in the singlet and triplet geometry optimi-
zation calculations, respectively. The 6-31G(d) basis set was
employed for H, C, N, F atoms and a double-ξ quality basis
set, LANL2DZ,11 consisting of Hay and Wadt0s effective core
potentials (ECP), was employed for the Ir atom. Further-
more, vibrational frequencies were then calculated based on
the optimized ground and lowest triplet state geometries of
all complexes to verify that each of the structures is at its
global minimum. Calculated Ir-H stretching frequency of
the titled complexes compared to the experimental Raman
spectral results were also shown in the Results and Discussion
section.
To gain better insight into the nature of spectroscopic proper-
ties, time dependent DFT (TD-DFT) calculations12 were executed
employing the optimized geometries at the lowest singlet and
triplet states, and then applied to probe the absorption and
emission properties. Typically, the lowest triplet and 10 singlet
roots of the non-hermitian eigenvalue equations were obtained
to determine the vertical excitation energies. Oscillator strengths
were deduced from the dipole transition matrix elements (for
singlet states only). All calculations were carried out using
Gaussian 03.13
Spectral Data of 3. MS (FAB, 193Ir): m/z 957 (Mþ); 1H NMR
(500 MHz, CDCl3, 294 K): δ 8.00-7.89 (m, 2H), 7.41-7.39 (m,
3H), 7.26 (d, J=6.5 Hz, 1H), 7.21 (t, J=7.5 Hz, 2H), 7.11 (t, J=
7.5 Hz, 1H), 7.05-7.01 (m, 3H), 7.00-6.97 (m, 5H), 6.92-6.86
(m, 6H), 6.81 (t, J=7.5 Hz, 1H), 6.79 (t, J=7.0 Hz, 1H), 6.73-
6.72 (m, 3H), 6.66 (t, J=7.0 Hz, 2H), 6.45-6.40 (m, 3H), 6.06 (t,
J=6.0 Hz, 1H), 4.06 (t, J=14.0 Hz, 1H), 3.90 (dd, J=14.5,
8.5 Hz, 1H), 3.57 (dd, J=14.5, 8.5 Hz, 1H), 3.28 (t, J=14.0 Hz,
1H), -18.86 (t, J=17.6 Hz, 1H). 19F-{1H} NMR (470 MHz,
CDCl3, 294 K): δ -60.04 (s, 3F). 31P-{1H} NMR (202 MHz,
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W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt, W. R.
J. Chem. Phys. 1985, 82, 299.
CDCl3, 294 K): δ 23.58 (d, JPP=369 Hz, 1P), 13.38 (d, JPP
=
369 Hz, 1P). Anal. Calcd for C47H39F3IrN3P2: N, 4.39; C, 58.99;
H, 4.11. Found: N, 4.45; C, 58.87; H, 4.40.
X-ray Diffraction Studies. Single crystal X-ray diffraction
data of 1, 2, and 3 were measured on a Bruker SMART Apex
(12) (a) Jamorski, C.; Casida, M. E.; Salahub, D. R. J. Chem. Phys. 1996,
104, 5134. (b) Petersilka, M.; Grossmann, U. J.; Gross, E. K. U. Phys. Rev. Lett.
1996, 76, 1212. (c) Bauernschmitt, R.; Ahlrichs, R.; Hennrich, F. H.; Kappes,
M. M. J. Am. Chem. Soc. 1998, 120, 5052. (d) Casida, M. E. J. Chem. Phys.
1998, 108, 4439. (e) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem.
Phys. 1998, 109, 8218.
˚
CCD diffractometer using (Mo KR) radiation (λ=0.71073 A).
The data collection was executed using the SMART program.
The hydride of each complexes was located in the difference
Fourier and refined independently. Cell refinement and data
reduction were performed with the SAINT program. The
(13) Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.

Article
Inorganic Chemistry, Vol. 48, No. 17, 2009 8167
Compositions of molecular orbitals in terms of the constituent
been successfully utilized for preparation of numerous blue
phosphorescent complexes by taking advantage of its higher
lying π* orbital on the pyridyl segment.19
chemical fragments were calculated using the AOMix program.14
For the characterization of the HOMO-x f LUMOþy transi-
tions as partial charge transfer (CT) transitions, the following
definition of the CT character has been used:
To move on one more step, because of their multisubsti-
tuted nitrogen atoms at the azolate site and the CF3 sub-
stituent, 2-pyridyl azoles, such as 3-trifluoromethyl-5-
(2-pyridyl) pyrazole (fppzH) and its analogues, are expected
to offer an even wider ππ* energy gap than dfppyH.
Examples for high efficiency, true-blue phosphorescent
materials have been demonstrated in the Os(II) comp-
lexes known as [Os(fppz)2(CO)2],20 [Os(CO)3(tfa)(fppz)],
tfa=trifluoroacetate, and analogues21 in our recent studies.
Subsequently, we also synthesized a class of hetero-
leptic Ir(III) complexes assembled using both fppz chromo-
phore and non-chromophoric, ancillary chelates with an
excessively large ligand-centered ππ* energy gap.6a Depend-
ing on the intrinsic characteristics of the non-chromopho-
ric ligands, RT true-blue phosphorescence was noted in
both fluid and solid states;22 for example, the stronger ligand
field strength, non-conjugated ancillary chelates are found
to be capable of inducing the light output from the afore-
mentioned fppz chelate with little loss of its emission effi-
ciency.8,23
CTðMÞ ¼ %ðMÞHOMO-x -%ðMÞLUMOþy
where %(M)HOMO-x and %(M)LUMOþy are electronic den-
sities on the metal in HOMO-x and LUMOþy. If the excited
state, for example, S1 or T1, is formed by more than one one-
electron excitation, then the metal CT character of this excited
state is expressed as a sum of CT characters of each participating
excitation, i f j:
X
2
CTIðMÞ ¼
½CIði f jÞꢁ ð%ðMÞi -%ðMÞjÞ
i,a
where CI(ifj) are the appropriate coefficients of the I-th eigen-
vector of the CI matrix. Accordingly, one can very effectively use
the MO compositions in terms of fragment orbital contributions
to analyze the nature of electronic transitions.
3. Results and Discussion
Traditionally, the blue phosphorescent transition metal
complexes were synthesized employing a wide-energy-gap
cyclometalated ligand, such as 4,6-difluorophenyl pyridine
(dfppyH),15 and the respective functionalized cyclometa-
lates.16 It is believed that, upon forming metal complexes,
as the electron withdrawing fluoro substituents are located at
the meta-positions with respect to the central metal atom, a
larger stabilization of the metal dπ orbitals is expected. On the
other hand, since these fluoro substituents are also located at
the ortho and para position relative to the pyridyl group, it is
doubtless that the resonance (mesomeric) effects would exert
an opposite influence to the anticipated inductive effect,17
giving a much reduced influence on the energy level of the
adjacent pyridyl π* orbital, and hence manifesting excessive
ligand-centered ππ* energy required for blue phosphores-
cence. Although such properties have not been explicitly
elaborated in literature,18 the respective dfppy chelates have
(14) (a) Gorelsky, S. I. AOMix: Program for Molecular Orbital Analysis;
University of Ottawa: Ottawa, Ontario, Canada, 2007; http://www.sg-chem.net/.
(b) Gorelsky, S. I.; Lever, A. B. P. J. Organomet. Chem. 2001, 635, 187.
(15) Li, J.; Djurovich, P. I.; Alleyne, B. D.; Tsyba, I.; Ho, N. N.; Bau, R.;
Thompson, M. E. Polyhedron 2004, 23, 419.
(16) (a) Lo, S.-C.; Shipley, C. P.; Bera, R. N.; Harding, R. E.; Cowley, A.
R.; Burn, P. L.; Samuel, I. D. W. Chem. Mater. 2006, 18, 5119. (b) Lee, S. J.;
Park, K.-M.; Yang, K.; Kang, Y. Inorg. Chem. 2009, 48, 1030.
(17) (a) Cheng, Y.-M.; Yeh, Y.-S.; Ho, M.-L.; Chou, P.-T.; Chen, P.-S.;
Chi, Y. Inorg. Chem. 2005, 44, 4594. (b) Hwang, K.-C.; Chen, J.-L.; Chi, Y.; Lin,
C.-W.; Cheng, Y.-M.; Lee, G.-H.; Chou, P.-T.; Lin, S.-Y.; Shu, C.-F. Inorg. Chem.
2008, 47, 3307.
With these precedent experiences in mind, we then carried
out the reaction of IrCl3 nH2O with 2 equiv of benzyldiphe-
3
nylphosphine (bdpH) in 2-methoxyethanol, followed by treat-
ment with the aforementioned fppzH ligand in the presence of
Na2CO3. Our strategy was to allow a prior coordination of
bdpH ligands to the Ir(III) metal center, affording an Ir(III)
(18) Nazeeruddin, M. K.; Gratzel, M. Struct. Bonding (Berlin) 2007, 123,
113.
(19) (a) Coppo, P.; Plummer, E. A.; De Cola, L. Chem. Commun. 2004,
1774. (b) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.;
Ho, N. N.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2003, 125, 7377.
(b) Wu, L.-L.; Yang, C.-H.; Sun, I.-W.; Chu, S.-Y.; Kao, P.-C.; Huang, H.-H.
Organometallics 2007, 26, 2017. (c) Yang, C.-H.; Cheng, Y.-M.; Chi, Y.; Hsu,
C.-J.; Fang, F.-C.; Wong, K.-T.; Chou, P.-T.; Chang, C.-H.; Tsai, M.-H.; Wu, C.-C.
Angew. Chem., Int. Ed. 2007, 46, 2418. (d) De Angelis, F.; Fantacci, S.; Evans,
N.; Klein, C.; Zakeeruddin, S. M.; Moser, J.-E.; Kalyanasundaram, K.; Bolink,
H. J.; Graetzel, M.; Nazeeruddin, M. K. Inorg. Chem. 2007, 46, 5989. (e) He, L.;
Duan, L.; Qiao, J.; Wang, R.; Wei, P.; Wang, L.; Qiu, Y. Adv. Funct. Mater. 2008,
18, 2123. (f) Di Censo, D.; Fantacci, S.; De Angelis, F.; Klein, C.; Evans, N.;
Kalyanasundaram, K.; Bolink, H. J.; Graetzel, M.; Nazeeruddin, M. K. Inorg.
Chem. 2008, 47, 980.
(20) (a) Wu, P.-C.; Yu, J.-K.; Song, Y.-H.; Chi, Y.; Chou, P.-T.; Peng,
S.-M.; Lee, G.-H. Organometallics 2003, 22, 4938. (b) Yu, J.-K.; Hu, Y.-H.;
Cheng, Y.-M.; Chou, P.-T.; Peng, S.-M.; Lee, G.-H.; Carty, A. J.; Tung, Y.-L.; Lee,
S.-W.; Chi, Y.; Liu, C.-S. Chem.;Eur. J. 2004, 10, 6255.
(21) Cheng, Y.-M.; Li, E. Y.; Lee, G.-H.; Chou, P.-T.; Lin, S.-Y.; Shu,
C.-F.; Hwang, K.-C.; Chen, Y.-L.; Song, Y.-H.; Chi, Y. Inorg. Chem. 2007,
46, 10276.
(22) Song, Y.-H.; Chiu, Y.-C.; Chi, Y.; Cheng, Y.-M.; Lai, C.-H.; Chou,
P.-T.; Wong, K.-T.; Tsai, M.-H.; Wu, C.-C. Chem.;Eur. J. 2008, 14, 5423.
(23) Chang, C.-F.; Cheng, Y.-M.; Chi, Y.; Chiu, Y.-C.; Lin, C.-C.; Lee,
G.-H.; Chou, P.-T.; Chen, C.-C.; Chang, C.-H.; Wu, C.-C. Angew. Chem.,
Int. Ed. 2008, 47, 4542.

8168 Inorganic Chemistry, Vol. 48, No. 17, 2009
Chiu et al.
Figure 2. ORTEP diagram of 2 with thermal ellipsoids shown at 30%
˚
Figure 1. ORTEP diagram of 1 with thermal ellipsoids shown at 30%
˚
probabilitylevel; selected bond lengths(A) follow: Ir(1)-C(10)=2.104(8),
probability level; selected bond lengths (A) follow: Ir-C(10)=2.102(3),
Ir(1)-P(1)=2.247(2), Ir(1)-P(2)=2.360(2), Ir(1)-N(1)=2.229(6), and
Ir-P(1) = 2.2437(7), Ir-P(2) = 2.3303(6), Ir-N(1) = 2.124(2), and
˚
Ir(1)-N(2)=2.061(7) A; the hydride is shown in the calculated position.
˚
Ir-N(2)=2.107(2) A; the hydride is shown in the calculated position.
complex with hypothetical formula of IrCl3(bdpH)2, for which
its analogue IrCl3(PPh3)2, probably as a 5-coordinate complex
in solution, has been independently prepared from reaction of
IrCl(PPh3)3 with chlorine.24 Afterwards, the phosphine co-
ordination could then facilitate the subsequent cyclometala-
tion at both of the benzyl and relevant sites.25 The remaining
empty coordination site, or sites occupied by either solvent
molecule or chlorides, would be substituted by the incoming
fppz chelate, giving a proposed Ir(III) complex A (see above
Scheme) possessing per se three chelating ligands.
However, the reaction did not proceed in accordance with
the proposed pathway. Instead, after single cyclometalation,
serendipitously, it affords three air stable, isomeric hydride
complexes 1, 2, and 3 with yields of 15%, 26%, and 10%,
respectively. The existence of hydride is unambiguously
1
identified by the H NMR signals observed in the range
Figure 3. ORTEP diagram of 3 with thermal ellipsoids shown at 30%
˚
δ -17.84 to -19.31, while their spectral pattern is consistent
with the structures in that the hydride is coupled to two
strongly coupled, but distinctive, phosphorus nuclei and thus
appears as either a doublet of doublet or a triplet in the
1H NMR spectra. The source of hydride has not been
confirmed in this reaction, but the 2-methoxyethanol solvent
could plausibly be the major supplier, as evidenced by earlier
reports on the related Ir(III) system26 as well as the isoelec-
tronic Os(II) complexes.27 Finally, to our disappointment,
none of these hydride complexes underwent H2 elimination
to afford the proposed target complex, namely, the fully
cyclometalated product A. Instead, complex A was synthe-
sized by treatment of a mixture of dbpH and fppzH with
Ir(THT)3Cl3 in high boiling decalin solvent and then served
as a controlled protocol.
probability level; selected bond lengths (A) follow: Ir-C(10)=2.067(3),
Ir-P(1) = 2.2744(9), Ir-P(2) = 2.3168(9), Ir-N(1) = 2.175(3), and
˚
Ir-N(2)=2.110(3) A; the hydride is shown in the calculated position.
Complexes 1, 2, and 3 were further characterized through
crystal structural analyses. Figures 1, 2, and 3 depict their
molecular structures, together with the metric parameters that
reveal the systematic variation of bond strengths caused by
geometrical isomerism. Salient differences among their
X-ray crystal structures are summarized: (i) The monodentate
bdpH ligand in all derivatives is found to coordinate to the axial
position relative to the square plane defined by the fppz ligand.
(ii) TheIr-P(2) distance of monodentate bdpH ligand increases
in the order of 3 < 1 < 2 (see Table 2), for which the strongest
Ir-P bonding occurred with its trans-ligand being switched
from Ir-C(benzyl) site to PPh2 fragment. (iii) The hydride ligands
in 1, 2, and 3 were located at the trans disposition to the
pyrazolate N(2), pyridyl N(1), and pyridyl N(1) atom, respec-
tively. Because of the trans-effect, the longest Ir-N(1) distance
observed in 2 suggested it may possess the strongest Ir-H
bonding among all three iridium complexes. (iv) The chelating
bdp ligand seems to possess a notably stronger Ir-P bonding
interaction compared with the monodentate bdpH ligand. This
is revealed by the shortened Ir-P(1) distance in all derivative
(24) Bennett, M. A.; Milner, D. L. J. Am. Chem. Soc. 1969, 91, 6983.
(25) (a) Verstuyft, A. W.; Redfield, D. A.; Cary, L. W.; Nelson, J. H.
Inorg. Chem. 1977, 16, 2776. (b) Klein, H.-F.; Beck, R.; Florke, U.; Haupt, H.-J.
Eur. J. Inorg. Chem. 2003, 853. (c) Frech, C. M.; Leitus, G.; Milstein, D.
Organometallics 2008, 27, 894.
(26) Ionkin, A. S.; Marshall, W. J. Organometallics 2004, 23, 6031.
(27) (a) Sullivan, B. P.; Caspar, J. V.; Meyer, T. J. Organometallics 1984,
3, 1241. (b) Hsu, F.-C.; Tung, Y.-L.; Chi, Y.; Hsu, C.-C.; Cheng, Y.-M.; Ho, M.-L.;
Chou, P.-T.; Peng, S.-M.; Carty, A. J. Inorg. Chem. 2006, 45, 10188.
(c) Dickinson, P. W.; Girolami, G. S. Inorg. Chem. 2006, 45, 5215.

Article
Inorganic Chemistry, Vol. 48, No. 17, 2009 8169
a
˚
Table 2. Selected Bond Lengths (A) of Ground State Optimized Geometries for Complex 1-3
Ir-H(1)b
Ir-C(10)
Ir-P(1)
Ir-P(2)
Ir-N(1)
Ir-N(2)
1
2
3
1.61
1.58
1.59
2.108 [2.102]
2.108 [2.105]
2.098 [2.067]
2.307 [2.244]
2.326 [2.247]
2.334 [2.274]
2.439 [2.330]
2.431 [2.360]
2.396 [2.317]
2.136 [2.124]
2.268 [2.229]
2.243 [2.175]
2.171 [2.107]
2.078 [2.062]
2.146 [2.110]
a The data shown in square bracket are obtained from X-ray diffraction studies. b The Ir-H distances obtained from X-ray structural determination
were removed because of the poor accuracy.
performed in the solid state and gas phase, respectively.
Upon geometry optimization, the Ir-H bond distance is
˚
calculated to be 1.61, 1.58, and 1.59 A (see Table 2) for 1, 2,
and 3, respectively, which is not only in line with the ν(Ir-H)
bands detected in Raman spectroscopy but also shows good
agreement with the anticipated Ir-H distances documented
in literature reports.29 For the controlled experiment, as
shown in Figure 4, complex A failed to show any vibrational
peak at the region between 2000-2400 cm-1, which falls
within the typical region expected for ν(Ir-H) bands.30
Unfortunately, all these hydride complexes 1-3 failed to
undergo H/D exchange with D2O or CD3OD,31 even in
presence of added CF3CO2D or NaOD. Thus, confirmation
of these Ir-H peaks by classical isotope labeling experiments
was not conducted further.
Having proven structural differences among complexes
1-3, we decided to investigate the photophysical properties
in an attempt to decipher the factors that govern their, for
example, relative emission quantum yield. All pertinent
photophysical data are listed in Table 3. Figure 5 depicts
the absorption and emission spectra measured in CH2Cl2
solution and 77 K cryogenic matrix, while the respective solid
state emissions at RT are depicted in Figure 6. In a qualitative
manner, their identical spectral profile shown in Figure 5
implicates similar type of transition characteristics for all
complexes: The <300 nm region in the absorption spectra of
1-3 all displays an intense ligand-centered ππ* transition.
Subsequently, the absorption bands ranging from 300-400
nm are plausibly attributed to the spin-allowed and/or spin-
forbidden MLCT mixed with ππ* transitions. Additional
supports for these assignments are provided in TD-DFT
calculation and molecular orbital analyses (vide infra).
All 1-3 complexes exhibit true-blue emission peaked
centered at around 460 nm in RT CH2Cl2. This, in theory,
is expectable because of their geometrical isomerism in
nature. The luminescence spectra of 2 and 3 reveal clearly
the feature of ππ* vibronic progression. For 1, because of the
much weaker intensity (vide infra), the lack of structural
feature for emission in solution may plausibly result from the
wide openness of both excitation and emission slits of the
fluorimeter. This viewpoint is further supported by the
emission spectra acquired in the 77 K CH2Cl2 matrixes, in
which all three complexes, exhibit strong emissions posses-
sing obvious vibronic fine structure. The radiative lifetime,
deduced from the observed lifetime divided by the respective
Figure 4. Raman spectra of complexes A, 1, 2, and 3 in solid in the
.
region of 2000-2400 cm-1
complexes, as well as those reported in other phosphine sub-
28
˚
stituted Ir(III) derivatives (2.434-2.307 A). Moreover, even in
3, where both phosphine fragments are located at the trans-
disposition, the chelating Ir-P(1) bond is still found to
˚
be shortened by 0.0424(9) A. (v) The Ir-C(10) distances of
bdp chelate followed a trend of 1 ∼ 2 > 3, showing a stronger
trans effect exerted by the phosphine fragment versus the
pyrazolate anion. (vi) The Ir-N(2) distances of the pyrazolate
sites followed a sequence of 3 > 1 > 2. This observation
is in good agreement with a better σ-donating effect of the
phenyl group versus hydride, as well as a π-accepting effect of
phosphine.
As for (iii), because of the weak scattering factors for
hydrogen versus nearby iridium, the hydride position could
not be exactly located either by difference Fourier or by mean
of least-squares refining.26 Thus, comparison of such struc-
tural data is only qualitative and must be treated with
extreme caution. To provide further support of the trend of
Ir-H bond strengths, both Raman experiments and calcula-
tions of corresponding vibrational frequencies of 1-3 were
performed (see Experimental Section for details). Figure 4
showsthe results of Ramanspectra for the crystalline samples
of 1-3, in which Ir-H stretching frequencies of 2110, 2200,
and 2150 cm-1 are resolved for complexes 1, 2, and 3,
respectively. The assignment of this band is qualitative in
accordance with the calculated Ir-H stretching frequencies
of 2181 cm-1 (1), 2254 cm-1 (2), and 2232 cm-1 (3). Note that
the observed discrepancy, in part, may be attributed to the
fact that the experimental and theoretical approach are
(29) (a) Salomon, M. A.; Braun, T.; Krossing, I. Dalton Trans. 2008, 5197.
(b) Alvarez, E.; Paneque, M.; Petronilho, A. G.; Poveda, M. L.; Santos, L. L.;
Carmona, E.; Mereiter, K. Organometallics 2007, 26, 1231.
(30) (a) Carlo Preti, C.; Tosi, G. Transition Met. Chem. 1977, 2, 1.
(b) Socol, S. M.; Yang, C.; Meek, D. W.; Glaser, R. Can. J. Chem. 1992, 70,
2424. (c) Albinat, A.; Venanzi, L. M. Coord. Chem. Rev. 2000, 200-202, 687.
(d) Acha, F.; Garralda, M. A.; Ibarlucea, L.; Pinilla, E.; Torres, M. R. Inorg.
Chem. 2005, 44, 9084.
(28) (a) Li, J.; Djurovich, P. I.; Alleyne, B. D.; Yousufuddin, M.; Ho,
N. N.; Thomas, J. C.; Peters, J. C.; Bau, R.; Thompson, M. E. Inorg. Chem.
2005, 44, 1713. (b) Lyu, Y.-Y.; Byun, Y.; Kwon, O.; Han, E.; Jeon, W. S.; Das,
R. R.; Char, K. J. Phys. Chem. B 2006, 110, 10303. (c) Chin, C. S.; Eum, M.-S.;
Kim, S. Y.; Kim, C.; Kang, S. K. Eur. J. Inorg. Chem. 2007, 372.
(31) Frost, B. J.; Mebi, C. A. Organometallics 2004, 23, 5317.

8170 Inorganic Chemistry, Vol. 48, No. 17, 2009
Chiu et al.
Table 3. Photophysical Data of Complexes of 1-3 Recorded in CH2Cl2 Solution and in Solid State
abs/nm (ε ꢀ 103 M-1 cm-1
)
λem/nm, 298 K, [77 K]
Φem
τobs (ns)
τrad (μs)
kra
knr
a
λ
1
2
3
275 (18.9), 320 (9.0), 367 (1.6)
273 (20.2), 325 (8.9)
271 (26.4), 316 (7.7), 354 (1.5)
459 [449, 470]
457 [447, 463]
448 [440, 463]
4.0 ꢀ 10-4 (5.1 ꢀ 10-4 ,b
1.1 ꢀ 10-3 (0.62)b
)
2.9 (5.2)b
7.3 (10.2)
9.2 (11.5)
9.3 (6.9)
1.3 ꢀ 105
1.0 ꢀ 105
1.1 ꢀ 105
3.4 ꢀ 108
9.9 ꢀ 107
1.1 ꢀ 107
10.1 (7100)b
90.2 (103)b
9.7 ꢀ 10-3 (0.015)b
a kr (radiative decay rate constant) and knr (non-radiative decay rate constant) were calculated according to the equations, kr=Φ/τobs and knr
=
(1/τobs) - kr. b Data measured in solid state.
Figure 6. RT solid state emission spectra of complexes 1, 2, and 3.
Figure 5. UV/visabsorptionandemissionspectraofcomplexes1, 2, and
3 in CH2Cl2 solution at 298 K. The solid circle shows the corresponding
emission spectra in a 77 K CH2Cl2 matrix.
In sum, among all isomers, complex 1 shows the worst
emission efficiency in both solution and solid, while com-
plexes 2 and 3 exhibit the greatest luminescent efficiency in
solid state and in degassed CH2Cl2 solution at RT, respec-
tively. In solution, since all of the radiative lifetimes are
nearly identical (∼10 μs), the difference in emission yield
should lie in the great changes in the non-radiative decay
Q.E. in degassed condition, is around ∼10 μs (1: 7.3 μs,
2: 9.2 μs, 3: 9.3 μs) for all complexes (see Table 3). These
results render confirmation that the emission originates from
3
a triplet state manifold with a great percentage of ππ*
rate constant, knr, which is in the order of 1 (3.4 ꢀ 108 s-1
)
transition character. Further support of this assignment is
given by the fact that the observed spectral profile also
showed good agreement with those of the relevant Os(II)
complex with formula [Os(fppz)2(CO2)].20 In sharp contrast,
the potassium salt of fppz chelate, namely, [(fppz)K], or even
the boron metal complex [B(C6F5)2(fppz)] exhibits normal
short-lived, fluorescence maximized at 372 and 380 nm,
respectively. The results manifest a fast rate of S1 f T1
intersystem crossing enhanced by Ir (or Os) spin-orbit
coupling.
> 2 (9.9 ꢀ 107 s-1) > 3 (1.1 ꢀ 107 s-1) (see Table 3).
According to frontier orbital analyses, the repulsive d-d
transition does not appear up to as high as S5 (or T5) state
(not shown here); thus its role as the major quenching
pathway at RT can be eliminated.32 Alternatively, we
suspect that the seemingly weak bonding between Ir metal
and hydride (cf. C-H bond), together with low frequency
vibrations associated with the motions of dangling dbpH
phosphine, may be responsible for the dominant non-
radiative relaxation. Thus, as indicated by their bond
distances (see Table 2), the much weakened Ir-H and
Ir-P(bdpH) bond strength because of the trans-effect in 1
may qualitatively explain its worst emission efficiency in
both solution and solid at RT. Conversely, the relatively
stronger Ir-P bonding of the monodentate dbpH ligand in
3 may account for its least quenching effect and hence the
highest emission intensity in solution among all titled
complexes. Alternatively, the large amplitude motion in
solid state associated with dangling dbpH motion has been
highly restricted. This may lead to the unfrozen Ir-H
stretching serving as a principal process for the non-
radiative deactivation. Accordingly, the strongest Ir-H
bonding and hence highest stretching frequency in 2, as
revealed by the Raman spectroscopic analysis (vide supra),
should significantly suppress the non-radiative decay and
Despite the seemingly identical spectral profiles among 1-
3, however, drastically different emission intensity was re-
solved in solution. In terms of emission quantum yield, it
ranges from very weak in 1 (4.0 ꢀ 10-4), weak in 2 (1.1ꢀ10-3
)
to moderate intensity (∼0.01) in 3. The difference in emission
intensity goes up to as large as 25 fold between 1 and 3, which
is also clearly shown in the emission appearance viewed with
the naked eyes (see Table of Contents (TOC)). We also
performed luminescence studies in solid state. As shown in
Figure 6 and the TOC illustration, while the solid state
emission of 1 is still weak, relatively stronger emission
intensity was observed for the solid sample of 2 and 3.
Particularly, as shown in the TOC, 2 exhibits very strong
emission in solid. Quantum yield of the titled complexes in
solid was then measured to be 5.1 ꢀ 10-4, 0.62 and ∼0.015 for
1, 2, and 3, respectively (see Table 3). Note that the observed
lifetime for 2 in solid is measured to be as long as 7.1 μs, firmly
supporting its phosphorescence origin.
(32) Zalis, S.; Farrell, I. R.; Vlcek, A. J. Am. Chem. Soc. 2003, 125, 4580.

Article
Inorganic Chemistry, Vol. 48, No. 17, 2009 8171
Table 4. Calculated Vertical Excitation Energy and Orbital Transition Analyses
for Complexes 1-3a
MLCT
(%)
cpd
states
T1
λcal
f
assignments
1
2
514.2
393
0
HOMO-1fLUMO (89%)
HOMOfLUMO (16%)
0.0112 HOMOfLUMO (97%)
9.11
26.86
S1
S2
S3
T1
353.9 0.0015 HOMOfLUMOþ1 (83%) 23.75
344.8 0.0161 HOMO-1fLUMO (81%) 4.87
522.9
0
HOMO-1fLUMO (77%)
HOMOfLUMO (24%)
HOMO-3fLUMO (14%)
14.52
24.00
S1
S2
S3
T1
374.1 0.0154 HOMOfLUMO (97%)
354.6 0.0243 HOMO-1fLUMO (85%) 19.33
352.2 0.0014 HOMOfLUMOþ1 (87%) 18.23
18.08
3
517.1
0
HOMO-1fLUMO (97%)
HOMO-2fLUMO (14%)
S1
S2
S3
381.3 0.0005 HOMOfLUMO (98%)
36.44
352.1 0.0201 HOMO-1fLUMO (87%) 29.86
350.7 0.0018 HOMOfLUMOþ1 (78%)
30.21
HOMOfLUMOþ2 (10%)
a The absorption in singlet manifold was calculated based on the
optimized S0 geometries, while triplet manifold was calculated based on
the optimized T1 geometries, respectively.
a result, the transition energy and character for T1 are
listed in Table 4, while the associated frontier orbitals are
depicted in Supporting Information, Figure S1. The results
of molecular orbital analyses (see Table 4, Figure 7, and
Supporting Information, Figure S1) indicate that both S1
and T1 states share a somewhat different transition pattern
(Table 4). For complexes 1-3, the S1 transition is mainly
involves the HOMO f LUMO transition. On the other
hand, also shown in Table 4, the T1 state is dominated by
the HOMO-1 f LUMO transition for all complexes.
Nevertheless, for T1 and lower lying singlet excited state
(with largest oscillator strength), the electron densities transfer
is mainly from the central metal atom and the pyrazolate
moiety of the fppz ligand )plus the benzyl moiety of the bdp
cyclometalate in 1 predominantly) to the pyridyl fragment of
the fppz chelate. The result provides further evidence for the
aforementioned assignments of the lowest-energy absorption
bands of 1-3. Moreover, the calculated small percentages of
MLCT contribution for the T1 f S0 transitions (<20%, see
Table 4) well supports their phosphorescence originating from
a dominant ππ* transition, resulting in good vibronic pro-
gression and relatively long radiative lifetimes (vide supra).
Figure 7. Selected frontier molecular orbitals mainly involved in the
singlet manifold transition.
hence make this compound the best phosphor among all
isomers in the solid state.
The structural and spectroscopic predictions using the
DFT method were then performed to gain more insight into
the titled complexes. First of all, comparisons between the
calculated structures and the single crystal X-ray diffraction
results were made to validate the theoretical approaches
adopted in this study (see Table 2). The calculated trend of
Ir-H bond strength among all complexes appears to be in
qualitative agreement with the Raman spectral data (vide
supra). Similarly, other structural data comparisons between
the calculations and the experiments also revealed consistent
results, suggesting the adequacy of the theoretical approach
adopted in this study.
Next, employing the optimized ground state (S0) struc-
ture of 1-3, the TD-DFT method was then applied to
investigate the gain insight into their electronic transition
properties in the singlet manifold. The pertinent assign-
ment of the frontier orbitals involved in the transition is
listed in Table 4 and Figure 7. It seems that all lower lying
transitions match well with respect to the experimental
absorption data. For example, as for the lower lying
transitions (e.g., S1-S3), the largest oscillator strength
(f), that is, the S3 state for 1 and S2 states of 2 and 3, are
all calculated to be around ∼350 nm, which is close to the
shoulder observed experimentally (see Figure 5). More-
over, the calculated S0 f S1 transition is located at 393 nm
(1), 374 nm (2), and 381 nm (3), and is qualitatively in
agreement with the observed onsets (∼400 nm) of the
absorption spectra recorded in CH2Cl2 solution. These
results indicate that TD-DFT calculation works well
in estimating the absorptive transition of the Franck-
Condon excited state. Furthermore, to have a better in-
sight into the nature of phosphorescence, the structure of
the lowest triplet-state (T1) for 1-3 was also optimized,
and the corresponding TD-DFT calculation was then
performed on the basis of these optimized structures. As
4. Conclusion
In conclusion, syntheses, spectroscopic measurements, and
theoretical computations were performed on the three true-
blue isomeric Ir(III) complexes 1-3. In accordance to the
geometry isomerism, they reveal a nearly identical emission
profile but a great difference in view of luminescence intensity.
This is demonstrated by the fact that complex 1 shows the
worst emission efficiency, while complexes 2 and 3 exhibit the
greatest luminescent efficiency in solid state and in degassed
CH2Cl2 solution at RT, respectively. For the phosphores-
cence, the transition characters of these complexes are mainly
3
attributed to the ππ* mixed with small extent (<20%) of
3MLCT, the composition of which is further confirmed by the
TD-DFT approaches. The largest knr, that is, the highest
quenching efficiency, of 1 can be tentatively rationalized by
the weakened dative bonding between iridium metal and

8172 Inorganic Chemistry, Vol. 48, No. 17, 2009
Chiu et al.
hydride as well as the monodentate benzyl diphenylpho-
sphine. Since the true-blue phosphors are both emergent
and challenging issues from the viewpoint of syntheses, this
structural relationship would encourage scientists to initiate
comparative research work on how to design better blue
phosphors and to improve the efficiency of all phosphorescent
OLEDs by controlling the coordination geometry.
Affairs of Taiwan. We are also grateful to the National
Center for High-Performance Computing for computer
time and facilities.
Supporting Information Available: Figures of frontier orbitals
based on the T1 optimized geometries, and the Cartesian
coordinates of fully optimized structures, X-ray crystallo-
graphic data files (CIF) of complexes 1-3. This material
is available free of charge via the Internet at http://
pubs.acs.org.
Acknowledgment. This work was supported by the
National Science Council and Ministry of Economic