Strategic design and synthesis of osmium(II) complexes bearing a single pyridyl azolate π-chromophore: Achieving high-efficiency blue phosphorescence by localized excitation

Article abstract of DOI:10.1021/ic7015269

We present the strategic design and synthesis of Os(II) complexes bearing a single pyridyl azolate π-chromophore with an aim to attain high efficiency blue phosphorescence by way of localized transition. It turns out that our proposal of localized excitation seems to work well upon anchoring a single π-chromophore on the Os(II) complexes such that the control of MLCT versus ππ* (or even LLCT) transitions is more straightforward. Among the titled complexes, [Os(CO)₃(tfa)(fppz)] (1) and [Os(CO) ₃(tfa)(fbtz)] (5) (tfa = trifluoroacetate, (fppz)H = 3-(trifluoromethyl)-5-(2-pyridyl)pyrazole, and (fbtz)H = 3-(trifluoromethyl)-5- (4-tert-butyl-2-pyridyl)-1,2,4-triazole) give the anticipated blue phosphorescence with efficiencies of 0.26 (λ_max = 460 nm) and 0.27 (λ_max = 450 nm), respectively. For their halide analogues [Os(CO)₃(X)(fppz)] (2, X = Cl; 3, X = Br; 4, X = I) and phosphine-substituted isomeric derivatives [Os(tfa)(fppz)(PPh ₂Me)₂(CO)] (6-8), the localization of the excitation energy seems to populate at certain vibrational modes with weak bonding strength and hence an associated shallow potential energy surface to induce a facile radiationless transition. Furthermore, their ancillary ligands play an important role in fine-tuning not only the energy gap but also the emission intensity, i.e., in manifesting the radiationless transition pathways. Our results clearly show that there is always a tradeoff upon varying the parameters in an aim to optimize the hue and efficiency of phosphorescence toward blue.

Full text of DOI:10.1021/ic7015269

Inorg. Chem. 2007, 46, 10276−10286
Strategic Design and Synthesis of Osmium(II) Complexes Bearing a
Single Pyridyl Azolate -Chromophore: Achieving High-Efficiency Blue
π
Phosphorescence by Localized Excitation
Yi-Ming Cheng, Elise Y. Li, Gene-Hsiang Lee, and Pi-Tai Chou*
Department of Chemistry and Instrumentation Center, National Taiwan UniVersity,
Taipei 106, Taiwan
Sue-Yi Lin and Ching-Fong Shu
Department of Applied Chemistry, National Chiao Tung UniVersity, Hsinchu 300, Taiwan
Kwun-Chi Hwang, Yao-Lun Chen, Yi-Hwa Song, and Yun Chi*
Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 300, Taiwan
Received August 2, 2007
We present the strategic design and synthesis of Os(II) complexes bearing a single pyridyl azolate
π-chromophore
with an aim to attain high efficiency blue phosphorescence by way of localized transition. It turns out that our
proposal of localized excitation seems to work well upon anchoring a single -chromophore on the Os(II) complexes
such that the control of MLCT versus ππ* (or even LLCT) transitions is more straightforward. Among the titled
complexes, [Os(CO)3(tfa)(fppz)] (1) and [Os(CO)3(tfa)(fbtz)] (5) (tfa trifluoroacetate, (fppz)H 3-(trifluoromethyl)-
5-(2-pyridyl)pyrazole, and (fbtz)H 3-(trifluoromethyl)-5-(4-tert-butyl-2-pyridyl)-1,2,4-triazole) give the anticipated
blue phosphorescence with efficiencies of 0.26 (λmax 460 nm) and 0.27 (λmax 450 nm), respectively. For their
halide analogues [Os(CO)3(X)(fppz)] (2, X Cl; 3, X Br; 4, X I) and phosphine-substituted isomeric derivatives
[Os(tfa)(fppz)(PPh2Me)2(CO)] (6 8), the localization of the excitation energy seems to populate at certain vibrational
π
)
)
)
)
)
)
)
)
−
modes with weak bonding strength and hence an associated shallow potential energy surface to induce a facile
radiationless transition. Furthermore, their ancillary ligands play an important role in fine-tuning not only the energy
gap but also the emission intensity, i.e., in manifesting the radiationless transition pathways. Our results clearly
show that there is always a tradeoff upon varying the parameters in an aim to optimize the hue and efficiency of
phosphorescence toward blue.
Introduction
complexes often undergo unitary S1-T1 intersystem crossing
and exhibit bright luminescence from the triplet manifold
due to the enhanced spin-orbit coupling. Tuning of emission
over the entire near-infrared (NIR) to visible spectra could
be achieved by judicious modification of both a ligated
chromophore and the nature of the central metal atom.2
Luminescent complexes possessing third-row transition
metal elements have received considerable research atten-
tion.1 In addition to their good thermal stability, these metal
* To whom correspondence should be addressed. E-mail: ychi@
mx.nthu.edu.tw (P.-T.C.), chop@ntu.edu.tw (Y.C.). Fax: +886 3-572 0864
(P.-T.C.), +886 2-2369 5208 (Y.C.).
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10276 Inorganic Chemistry, Vol. 46, No. 24, 2007
10.1021/ic7015269 CCC: $37.00
© 2007 American Chemical Society
Published on Web 10/20/2007

Os(II) Complexes Bearing a π-Chromophore
However, there are only a few reports on the production of
much higher energy, saturated blue or even near-UV
phosphorescent materials.3 This is partly attributed to the fact
that upon increasing the emission energy, there is a gradual
decrease in admixture of the metal-to-ligand charge transfer
(MLCT) transition,4 and the reduction of metal participation
leads to the adverseness on spin-orbit coupling, such that
the state mixing (first-order approximation) between singlet
and triplet manifolds is less efficient. The net result reduces
the T1-S0 transition moment, and the radiative decay rate
constant kr is decreased accordingly. Note the emission
quantum efficiency, QY, is expressed as QY ) kr /(kr + knr).
Under identical nonradiative deactivation rate constant knr,
lowering the kr value thus causes a decrease of the corre-
sponding QY. Moreover, the increase of radiative lifetimes
is undesirable for phosphorescent OLEDs, as it would cause
saturation and triplet-triplet annihilation at higher current
and driving voltage.5
With an aim to attain the saturated blue phosphors, we
recently launched a venture project on preparation of a series
of Os(II) carbonyl complexes with two identical pyridyl
azolate chromophores. It was found that the geometrical
isomers with cis-oriented azolate fragments exhibited much
stronger room-temperature phosphorescence compared with
the corresponding trans-counterparts. The results have been
tentatively rationalized by the increased electrostatic repul-
sion in the trans-azolate arrangement, which caused shallow
potential energy surface and hence increased radiationless
deactivation rate constant.6,7 As for another observation, upon
increase of the interligand ππ* energy gap of chelating
ligands in the trisubstituted pyridyl azolate-Ir(III) com-
plexes, the lowest energy ππ* state showed significant
mixing with a thermally accessible ligand-to-ligand charge
transfer (LLCT) state.8 This unusual state mixing then
increases the radiative lifetime owing to its largely charge-
separated character and partially forbidden transition prob-
ability versus the ground state.
On the basis of the above progress, it seems that confine-
ment of the electronic excitation to a single ligated chro-
mophore may result in a possible increase of the emission
efficiency.9 However, it may conversely cause an adverse
effect in that the associated metal-ligand bond strength of
the locally excited chromophore is weaker than those metal
complexes possessing multiple isoenergetic chromophores
such that the excitation energy can be randomly dissipated.
Consequently, the presence of weak bonds may induce rapid
radiationless deactivation and then offset the presuming gain
of emission efficiency upon the energy localization (vide
supra).
Unfortunately, at the current stage, the theoretical assess-
ment of radiationless deactivation for the sophisticated
molecules like transition metal complexes is still formidable.
It is thus desirable to construct a series of prototypical
systems, for which the direct photoexcitation is preliminarily
associated with a single π-chromophore, in an aim to
examine the fundamental properties elaborated above. For a
comparison, the ligand-dependent excited-state behaviors of
the related Re(I) and Ru(II) systems were documented,10
offering certain prospective molecular designs for various
functional photonic materials, such as sensitizers, lumino-
phores, and even luminescent sensors.
Experimental Section
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 operating 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.
Osmium reagent [Os(CO)3(tfa)2] was prepared from the direct
treatment of Os3(CO)12 with trifluoroacetic acid.18 The chelating
ligands 3-(trifluoromethyl)-5-(2-pyridyl)pyrazole (fppz)H and 3-(tri-
fluoromethyl)-5-(4-tert-butyl-2-pyridyl)-1,2,4-triazole (fbtz)H were
prepared according to the methods reported in the literature.11
Steady-state absorption and emission spectra were recorded with
a Hitachi (U-3310) spectrophotometer and an Edinburgh (FS920)
fluorometer, respectively. Quinine sulfate with an emission yield
of Φ ∼ 0.57 in 1.0 N H2SO4 served as the standard to calculate
the emission quantum yield. The emission decays of <10 µs were
measured by an Edinburgh (FS920) fluorometer using a time-
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2006, 39, 349. (h) Shih, P.-I.; Chien, C.-H.; Chuang, C.-Y.; Shu, C.-
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Inorg. Chem. 2007, 46, 4308. (j) Chen, L.; You, H.; Yang, C.; Ma,
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R.; Thompson, M. E.; Holmes, R. J.; Forrest, S. R. Inorg. Chem. 2005,
44, 7992. (b) 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.
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Connick, W. B.; Miskowski, V. M.; Houlding, V. H.; Gray, H. B.
Inorg. Chem. 2000, 39, 2585. (c) Kavitha, J.; Chang, S.-Y.; Chi, Y.;
Yu, J.-K.; Hu, Y.-H.; Chou, P.-T.; Peng, S.-M.; Lee, G.-H.; Tao, Y.-
T.; Chien, C.-H.; Carty, A. J. AdV. Funct. Mater. 2005, 15, 223. (d)
Reineke, S.; Walzer, K.; Leo, K. Phys. ReV. B 2007, 75, 125328.
(6) (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.sEur. J.
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Inorganic Chemistry, Vol. 46, No. 24, 2007 10277

Cheng et al.
correlated single photon counting technique. The data were analyzed
by the sum of exponential functions, which allows partial removal
of the instrument time broadening and consequently renders a
temporal resolution of ∼200 ps. The long-lived (g10 µs) phos-
phorescence spectra were measured by an ultrasensitive detection
system coupled with a laser excitation source. Briefly, an Nd:YAG
(355 nm, 8 ns, Continuum Surelite II) pumped optical parametric
oscillator coupled with a second harmonic device served as a tunable
excitation source. The resulting emission was detected with an
intensified charge coupled detector (ICCD, Princeton Instruments,
model 576G/1). Data were analyzed using the nonlinear least-
squares procedure in combination with an iterative convolution
method.
δ -61.17 (s, 3F). Anal. Calcd for C12H5ClF3N3O3Os: C, 27.62;
H, 0.97; N, 8.05. Found: C, 27.58; H, 1.16; N, 7.84.
Spectral data for 3: MS (EI, 192Os) m/z 567 (M+); IR (CH2-
1
Cl2) ν(CO), 2127 (vs), 2058 (vs), 2033 (vs) cm-1; H NMR (500
MHz, acetone-d6, 298 K) δ 9.10 (d, 1H, JHH ) 5.5 Hz, CH), 8.28
(dt, 1H, JHH ) 7.8, 2.0 Hz, CH), 8.19 (d, 1H, JHH ) 8.0 Hz, CH),
7.56 (ddd, 1H, JHH ) 7.0, 6.0, 1.0 Hz, CH), 7.25 (s, 1H, CH); 13
C
NMR (126 MHz, acetone-d6, 298 K) δ 172.6 (CO), 169.9 (CO),
2
164.5 (CO), 155.0, 154.7 (CH), 151.8, 145.8 (q, JCF ) 37 Hz,
1
CCF3), 142.6 (CH), 125.4 (CH), 123.0 (q, JCF ) 268 Hz, CF3),
122.3 (CH), 104.0 (CH); 19F NMR (470 MHz, acetone-d6, 298 K)
δ -61.16 (s, 3F). Anal. Calcd for C12H5BrF3N3O3Os: C, 25.45;
H, 0.89; N, 7.42. Found: C, 25.46; H, 1.03; N, 7.21.
Spectral data for 4: MS (EI, 192Os) m/z 615 (M+); IR (CH2-
Synthesis of [Os(CO)3(tfa)(fppz)] (1). [Os(CO)3(tfa)2] (70 mg,
0.14 mmol) and 3-(trifluoromethyl)-5-(2-pyridyl)pyrazole (34 mg,
0.16 mmol) were refluxed in anhydrous toluene (10 mL) for a period
of 2 h. The solvent was removed under vacuum and residue
sublimed at 140 °C and 100 mTorr. Colorless crystals were obtained
from a mixture of CH2Cl2 and diethyl ether (58 mg, 0.10 mmol,
68%).
1
Cl2) ν(CO), 2123 (vs), 2054 (vs), 2032 (vs) cm-1; H NMR (500
MHz, acetone-d6, 298 K) δ 9.13 (d, JHH ) 5.6 Hz, 1H), 8.26 (dt,
JHH ) 8.0, 1.0 Hz, 1H), 8.19 (d, JHH ) 8.0 Hz, 1H), 7.54 (ddd, JHH
) 8.0, 6.0, 2.0 Hz, 1H), 7.27 (s, 1H); 13C NMR (126 MHz, acetone-
d6, 298 K) δ 172.1 (CO), 169.8 (CO), 162.9 (CO), 155.1 (CH),
151.9 (2C), 146.0 (q, 2JCF ) 37 Hz, CCF3), 142.4 (CH), 125.3 (CH),
122.9 (q, 1JCF ) 267 Hz, CF3), 122.3 (CH), 104.1 (CH); 19F NMR
(470 MHz, acetone-d6, 298 K) δ -61.1 (s, 3F). Anal. Calcd for
C12H5F3IN3O3Os: C, 23.50; H, 0.82; N, 6.85. Found: C, 23.68;
H, 1.16; N, 6.82.
Alternative Synthesis for 1. A finely pulverized mixture of [Os-
(CO)3(tfa)2] (100 mg, 0.20 mmol) and 3-(trifluoromethyl)-5-(2-
pyridyl)pyrazole (47 mg, 0.22 mmol) was loaded into a 10 mL
Carius tube. The tube was sealed under vacuum and placed into an
oven maintained at 185 °C for 1 h. After being allowed to cool to
RT (room temperature), the tube was opened. The residue was
triturated with distilled water and then sublimed at 140 °C and 100
mTorr. Further purification was carried out by recrystallization from
a mixture of CH2Cl2 and diethyl ether, giving 85 mg of colorless
crystals (0.14 mmol, 71%).
Synthesis of [Os(CO)3(tfa)(fbtz)] (5). Finely pulverized [Os2-
(CO)6(tfa)2] (100 mg, 0.13 mmol) and 3-(trifluoromethyl)-5-(4-tert-
butyl-2-pyridyl)-1,2,4-triazole (77 mg, 0.28 mmol) were mixed
uniformly and loaded into a 10 mL Carius tube. The tube was sealed
under vacuum and placed into an oven maintained at 185 °C for 1
h. After the tube was cooled to room temperature, the material was
subjected to sublimation at 150 °C and 100 mTorr. Further
purification was carried out by recrystallization from dichlo-
romethane and diethyl ether, giving 55 mg of white crystals (0.084
mmol, 32%).
Spectral data for 1: MS (EI, 192Os) m/z 601 (M+); IR (CH2-
1
Cl2) ν(CO), 2132 (vs), 2063 (vs), 2040 (vs) cm-1; H NMR (500
MHz, acetone-d6, 298 K) δ 9.13 (d, 1H, JHH ) 5.5 Hz, CH), 8.34
(dt, 1H, JHH ) 7.9, 1.5 Hz, CH), 8.22 (d, 1H, JHH ) 7.5 Hz, CH),
Spectral data for 5: MS (EI, 192Os) m/z 658 (M+); IR (CH2-
7.34 (ddd, 1H, JHH ) 6.0, 5.5, 1.5 Hz, CH), 7.28 (s, 1H, CH); 13
C
1
Cl2) ν(CO), 2134 (vs), 2066 (vs), 2046 (vs) cm-1; H NMR (500
NMR (126 MHz, acetone-d6, 298 K) δ 173.1 (CO), 169.6 (CO),
MHz, acetone-d6, 298 K) δ 9.12 (d, 1H, JHH ) 6.0 Hz), 8.26 (d,
1H, JHH ) 2.5 Hz), 7.84 (dd, 1H, JHH ) 6.0, 2.5 Hz), 1.48 (s, 9H);
13C NMR (125.7 MHz, acetone-d6, 298 K) δ 172.1 (CO), 169.0
2
167.5 (CO), 162.7 (q, JCF ) 37 Hz, CCF3), 155.4 (CH), 155.0,
2
152.4, 146.0 (q, JCF ) 37 Hz, CCF3), 143.3 (CH), 125.4 (CH),
1
1
122.9 (q, JCF ) 266 Hz, CF3), 122.2 (CH), 115.1 (q, JCF ) 287
Hz, CF3), 104.0 (CH); 19F NMR (470 MHz, acetone-d6, 298 K) δ
-61.35 (s, 3F), -74.77 (s, 3F). Anal. Calcd for C14H5F6N3O5Os:
C, 28.05; H, 0.84; N, 7.01. Found: C, 28.04; H, 1.16; N, 7.01.
2
(CO), 168.9 (CO), 166.4 (CO), 165.8, 162.7 (q, JCF ) 38 Hz,
2
CCF3), 157.2 (q, JCF ) 37 Hz, CCF3), 155.7 (CH), 149.9, 125.1
(CH), 121.0 (q, 1JCF ) 270 Hz, CF3), 120.4 (CH), 115.0 (q, 1JCF
)
289 Hz, CF3), 36.7 (CMe3), 30.2 (3C); 19F NMR (470 MHz,
acetone-d6, 298 K) δ -64.60 (s, 3F), -74.88 (s, 3F). Anal. Calcd
for C17H12F6N4O5Os: C, 31.10; H, 1.84; N, 8.53. Found: C, 31.14;
H, 2.12; N, 8.33.
Synthesis of [Os(CO)3(Cl)(fppz)] (2). A mixture of [Os(CO)3-
(tfa)(fppz)] (100 mg, 0.17 mmol) and NaCl (39 mg, 0.67 mmol) in
methanol (25 mL) was heated to reflux for 4 h. After the reaction
was completed, the solvent was evaporated in vacuum; the residue
was washed with water and subjected to sublimation at 150 °C
and 100 mTorr. Further purification was carried out by recrystal-
lization from dichloromethane and diethyl ether, giving 78 mg of
white crystals (0.15 mmol, 90%). The respective bromide and iodide
substituted derivatives [Os(CO)3(Br)(fppz)] (3) and [Os(CO)3(I)-
(fppz)] (4) were prepared by employing 4 equiv of halide reagents
NaBr and NaI, respectively.
Synthesis of [Os(tfa)(fppz)(PPh2Me)2(CO)] (6-8). An aceto-
nitrile solution (5 mL) of freshly sublimed Me3NO (27 mg, 0.365
mmol) was added dropwise into a stirred solution of 1 (200 mg,
0.332 mmol) in toluene (25 mL) over a period of 5 min. This
mixture was refluxed for 1 h, PPh2Me (90 µL, 0.498 mmol) was
added, and the mixture was heated at reflux for another 12 h.
Finally, the solvent was removed under vacuum and the residue
loaded onto a silica gel column and eluted with a 1:1 mixture of
ethyl acetate and hexane, giving complexes 6 (105 mg, 0.111 mmol,
33%), 7 (93 mg, 0.098 mmol, 30%), and 8 (13 mg, 0.013 mmol,
4%) as yellow, pale yellow, and colorless powdery materials. Single
crystals of all complexes were obtained by cooling a warm, saturated
methanol solution to room temperature.
Spectral data for 2: MS (EI, 192Os) m/z 523 (M+); IR (CH2-
1
Cl2) ν(CO), 2128 (vs), 2058 (vs), 2032 (vs) cm-1; H NMR (500
MHz, acetone-d6, 298 K) δ 9.09 (d, 1H, JHH ) 6.0 Hz, CH), 8.28
(dt, 1H, JHH ) 7.9, 1.0 Hz, CH), 8.18 (d, 1H, JHH ) 7.5 Hz, CH),
7.57 (ddd, 1H, JHH ) 8.0, 6.0, 2.0 Hz, CH), 7.24 (s, 1H, CH); 13
C
Spectral data for 6: MS (FAB, 192Os) m/z 946 (M + 1+), 832
(M+ - tfa), 745 (M+ - PPh2Me); IR (CH2Cl2) ν(CO), 1955 (vs)
cm-1; 1H NMR (500 MHz, acetone-d6, 298 K) δ 8.39 (d, 1H, JHH
) 5.5 Hz), 7.62 (t, 1H, JHH ) 7.5 Hz), 7.45-7.41 (m, 4H), 7.35 (t,
NMR (126 MHz, acetone-d6, 298 K) δ 173.0 (CO), 170.1 (CO),
2
165.5 (CO), 154.9, 154.5 (CH), 151.8, 145.7 (q, JCF ) 37 Hz,
1
CCF3), 142.6 (CH), 125.4 (CH), 123.0 (q, JCF ) 266 Hz, CF3),
122.2 (CH), 104.0 (CH); 19F NMR (470 MHz, acetone-d6, 298 K)
10278 Inorganic Chemistry, Vol. 46, No. 24, 2007

Os(II) Complexes Bearing a π-Chromophore
2H, JHH ) 8.0 Hz), 7.25 (t, 6H, JHH ) 8.0 Hz), 7.17 (d, 1H, JHH
)
19 008 reflections measured with 6225 unique reflections (Rint )
7.5 Hz), 7.09-7.05 (m, 5H), 6.95-6.91 (m, 4H), 6.56 (s, 1H), 1.73
(t, 6H, JHP ) 4.0 Hz); 19F NMR (470.3 MHz, acetone-d6, 298 K)
δ -60.66 (s, 3F), -74.89 (s, 3F); 31P NMR (202.3 MHz, acetone-
d6, 298 K) δ -10.81 (s). Anal. Calcd for C38H31F6N3O3OsP2: C,
48.36; H, 3.31; N, 4.45. Found: C, 48.53; H, 3.55; N, 4.70.
Spectral data for 7: MS (FAB, 192Os) m/z 946 (M + 1+), 832
(M+ - tfa), 745 (M+ - PPh2Me); IR (CH2Cl2) ν(CO), 1937 (vs)
cm-1. 1H NMR (500 MHz, acetone-d6, 298 K) δ 8.25 (d, 1H, JHH
) 5.5 Hz), 7.34-7.30 (m, 4H), 7.28-7.24 (m, 5H), 7.17-7.05
(m, 12H), 6.95 (d, 1H, JHH ) 8.5 Hz), 6.69 (dt, 2H, JHH ) 7.5, 1.5
Hz), 2.00 (t, 6H, JHP ) 4.0 Hz); 19F NMR (470.3 MHz, acetone-
d6, 298 K) δ -60.49 (s, 3F), -74.47 (s, 3F); 31P NMR (202.3 MHz,
acetone-d6, 298 K) δ -11.72 (s). Anal. Calcd for C38H31F6N3O3-
OsP2: C, 48.36; H, 3.31; N, 4.45. Found: C,48.32; H, 3.45; N,
4.69.
0.0867); final wR2(all data) ) 0.1250, R1[I > 2σ(I)] ) 0.0716.
Computational Methodology. Calculations on the electronic
ground states of complexes 1-8 were carried out using B3LYP
density functional theory.12-13 A “double-ú” quality basis set
consisting of Hay and Wadt’s effective core potentials (LANL2DZ)12
was employed for Os atoms, and a 6-31G* basis,14 for H, C, N,
and F atoms. A relativistic effective core potential (ECP) replaced
the inner core electrons of the Os(II) element, leaving the outer
core (5s25p6) electrons and the 5d6 valence electrons. Time-
dependent DFT (TDDFT) calculations using the B3LYP functional
were then performed on the basis of the structural optimized
geometries.15 Typically, the lowest 10 triplet and 10 singlet roots
of the nonhermitian eigenvalue equations were obtained to deter-
mine the vertical excitation energies. Oscillator strengths were
deduced from the dipole transition matrix elements (for singlet states
only). The ground-state B3LYP and excited-state TDDFT calcula-
tions were carried out using Gaussian03.16 Compositions of
molecular orbitals in terms of the constituent chemical fragments
were calculated using the AOMix program.17 For the characteriza-
tion of the HOMO-x f LUMO+y transitions as partial charge
transfer (CT) transitions, the following definition of the CT character
has been used:
Spectral data for 8: MS (FAB, 192Os) m/z 945 (M+), 832 (M+
1
- tfa); IR (CH2Cl2) ν(CO), 1948 (vs) cm-1; H NMR (500 MHz,
acetone-d6, 298 K) δ 8.05 (d, 1H, JHH ) 6.5 Hz), 7.95-7.91 (m,
3H), 7.82 (dt, 1H, JHH ) 7.5, 1.5 Hz), 7.62-7.55 (m, 3H), 7.48
(dt, 2H, JHH ) 7.3, 2.5 Hz), 7.39-7.34 (m, 3H), 7.28 (s, 1H), 7.24
(dt, 1H, JHH ) 7.5, 1.5 Hz), 7.18-7.14 (m, 5H), 7.06 (dt, 2H, JHH
) 7.5, 2.5 Hz), 6.82-6.77 (m, 3H), 2.39 (d, 3H, JHP ) 10.5 Hz),
1.49 (d, 3H, JHP ) 9.5 Hz); 19F NMR (470.3 MHz, acetone-d6,
298 K) δ -60.90 (s, 3F), -75.00 (s, 3F); 31P NMR (202.3 MHz,
CT(M) ) %(M)HOMO-x - %(M)LUMO+y
(1)
acetone-d6, 298 K) δ -15.15 (d, JPP ) 11.1 Hz), -15.97 (d, JPP
11.1 Hz).
)
Here %(M)HOMO-x and %(M)LUMO+y are electronic densities
on the metal in HOMO-x and LUMO+y. If the excited state, e.g.,
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-ray Crystallography. Single-crystal X-ray diffraction data
was measured on a Bruker SMART CCD diffractometer (2θmax
e55.0°, ω scan mode) equipped with a graphite monochromator.
The data collection was executed using the SMART program. Cell
refinement and data reduction were accomplished using the SAINT
program. The structures were solved using the SHELXTL/PC
package and refined using full-matrix least squares. An empirical
absorption correction was applied with the SADABS routine (part
of the SHELXTL program). The structure was solved by direct
methods using the SHELXTL suite of programs. All non-hydrogen
atoms were refined anisotropically by full-matrix least-squares on
F2. Hydrogen atoms were placed in calculated positions and allowed
to ride on the parent carbon atoms.
Selected crystal data for 1: C14H5F6N3O5Os, M ) 599.41;
triclinic, space group P1h; a ) 9.9557(1), b ) 12.0647(2), c )
15.7194(3) Å; R ) 85.4865(10), â ) 74.0517(11), γ ) 71.1197-
(11)°; V ) 1717.64(5) Å3; T ) 150(2) K; Z ) 4; µ(Mo KR) )
7.521 mm-1; 24 159 reflections measured with 7880 unique
reflections (Rint ) 0.0606); final wR2(all data) ) 0.0844, R1[I >
2σ(I)] ) 0.0326.
CTI(M) )
[CI(i f j)]2(%(M)i - %(M)j)
(2)
∑
i,a
Here CI(i f j) are the appropriate coefficients of the Ith eigenvector
of the CT matrix. Accordingly, one can very effectively use the
(12) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (b)
Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(13) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt,
W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt,
W. R. J. Chem. Phys. 1985, 82, 299.
(14) Hariharan, P. C.; Pople, J. A. Mol. Phys. 1974, 27, 209.
(15) (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.
(16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J.
W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.
Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
(17) (a) Gorelsky, S. I. AOMix: Program, for Molecular, Orbital, Analysis;
http://www.sg-chem.net/; University of Ottawa: Ottawa, Canada, 2007.
(b) Gorelsky, S. I.; Lever, A. B. P. J. Organomet. Chem. 2001, 635,
187-196.
Selected crystal data for 6: C38H31F6N3O3OsP2, M ) 943.80;
monoclinic, space group P21/c; a ) 12.5949(8), b ) 18.2927(11),
c ) 16.1595(10) Å; â ) 101.664(2)°; V ) 3646.2(4) Å3; T ) 150-
(2) K; Z ) 4; µ(Mo KR) ) 3.658 mm-1; 21 772 reflections
measured with 6406 unique reflections (Rint ) 0.0841); final wR2-
(all data) ) 0.1730, R1[I > 2σ(I)] ) 0.0930.
Selected crystal data for 7: C38H31F6N3O3OsP2, M ) 943.80;
triclinic, space group P1h; a ) 9.4227(7), b ) 11.2810(8), c )
17.8178(13) Å; R ) 77.385(2), â ) 84.591(2), γ ) 75.979(2)°; V
) 1791.5(2) Å3; T ) 150(2) K; Z ) 2; µ(Mo KR) ) 3.722 mm-1
;
)
23 330 reflections measured with 8215 unique reflections (Rint
0.0326); final wR2(all data) ) 0.0637, R1[I > 2σ(I)] ) 0.0248.
Selected crystal data for 8: C38H31F6N3O3OsP2, M ) 943.80;
triclinic, space group P1h; a ) 10.2656(7), b ) 12.4189(8), c )
14.7477(9) Å; R ) 92.357(1), â ) 92.615(1), γ ) 109.768(1)°; V
) 1764.3(2) Å3; T ) 150(2) K; Z ) 2; µ(Mo KR) ) 3.780 mm-1
;
Inorganic Chemistry, Vol. 46, No. 24, 2007 10279

Cheng et al.
MO compositions in terms of fragment orbital contributions to probe
the nature of electronic transitions. More details about the basis of
this method could be found in the manual of the AOMix program.
pyrazolate chelate in 1-4 is replaced by the tert-butyl-
substituted pyridyl triazolate, forming derivative 5. In
comparison to 1-4, we expect a profound increase on the
associated emission energy gap due to its increase (decrease)
of the electronic density in the pyridyl and the triazolate
moieties, respectively.
Results and Discussion
Preparation and Characterization. The blue-emitting
complex [Os(CO)3(tfa)(fppz)] (1) was prepared by treatment
of Os(II) reagent [Os(CO)3(tfa)2] (tfa ) trifluoroacetate)18
and 3-(trifluoromethyl)-5-(2-pyridyl)pyrazole (fppz)H in re-
fluxing toluene or using the so-called solid-state pyrolysis
technique.19 Both methods were initially utilized for the
preparation of emissive Os(II) complexes.20 After isolation
and purification of 1, its IR ν(CO) data showed three CO
stretching bands at 2132, 2063, and 2040 cm-1 in CH2Cl2,
which are characteristic for a facial-[Os(CO)3] unit. This
complex was then characterized with single-crystal X-ray
diffraction analyses. As depicted in Figure 1, the structure
consists of a central Os(II) atom chelated by an fppz ligand,
together with the attachment of one terminal tfa anion and
three CO ligands. Its coordination arrangement is akin to
that of the diketonate complexes [Os(CO)3(tfa)(hfac)], hfac
) hexafluoroacetylacetonate,21 and the 8-quinolinolate de-
rivative [Os(CO)3(tfa)(O∧N)], O∧N ) quinolinolate; the latter
even shows the room-temperature dual fluorescence and
phosphorescence arising from inefficient singlet-triplet
intersystem crossing.22
Last, we also conducted the phosphine substitution reaction
with an aim to probe the influence of ancillary ligands on
their structural and photophysical properties. The results of
this attempt led to the successful isolation of three PPh2Me-
substituted complexes 6-8 in 30, 33, and 4% yields,
respectively. Spectroscopic investigation revealed that com-
plexes 6-8 were isomeric, as all possess one pyridyl
pyrazolate chelate, one carbonyl, and one terminal tfa ligand
attached to the central Os(II) atom, together with two PPh2-
Me ligands (vide infra). It is also noted that although the
crystalline samples are indefinitely stable in air at room
temperature, they turned unstable upon being dissolved in
both chlorinated solvents and even nonchlorinated solvent
such as acetone. A color change from light yellow to dark
green was observed within 24 h for the solutions prepared
for NMR analyses, showing high chemical reactivity that is
in sharp contrast to that of the hydride analogues.23
Other halide-substituted derivatives 2-4 were synthesized
via an anion exchange reaction, which involved direct
treatment of parent tfa complex 1 with the alkali metal salts
NaCl, NaBr, and NaI in refluxing methanol solution. These
complexes were then identified using spectroscopic methods,
for which the distinct IR ν(CO) patterns are essentially
identical with that of 1 and show a slightly decreasing in
stretching frequency with a sequence of Cl g Br g I, which
is consistent with the trend of π-donor strength and/or the
electronegativity of the halides. As for the photophysical
characteristics, the lowest lying electronic transition for 1-4
may be largely attributed to the unique fppz ligand (vide
infra). Accordingly, as for further strategic design, the pyridyl
Complexes 6 and 7 were also characterized through crystal
structural analyses. Figures 2 and 3 reveal their molecular
structures. The PPh2Me ligands in both complexes adopt a
trans-configuration, with Os-P distances in the range of
2.367-2.378 Å, which are comparable to that of the red-
emitting Os(II) complex [Os(fppz)2(PPh2Me)2] (2.362 Å)24
and the corresponding porphinato (2.369 Å) and benzotria-
zolato analogues (2.353 Å).25,26 Moreover, it is notable that
the pyridyl group in 6 is located trans to the strong
π-accepting CO ligand. It exhibits a lengthened Os-N(py)
distance in 6 (Os-N(1) ) 2.149 Å), compared with that of
(18) (a) Deeming, A. J.; Meah, M. N.; Randle, N. P.; Hardcastle, K. I. J.
Chem. Soc., Dalton Trans. 1989, 2211. (b) Deeming, A. J.; Randle,
N. P.; Bates, P. A.; Hursthouse, M. B. J. Chem. Soc. Dalton Trans.
1988, 2753.
(19) Gong, J.-H.; Hwang, D.-K.; Tsay, C.-W.; Chi, Y.; Peng, S.-M.; Lee,
G.-H. Organometallics 1994, 13, 1720.
(20) Yu, J.-K.; Cheng, Y.-M.; Hu, Y.-H.; Chou, P.-T.; Chen, Y.-L.; Lee,
S.-W.; Chi, Y. J. Phys. Chem. B 2004, 108, 19908.
(21) (a) Chen, Y.-L.; Li, S.-W.; Chi, Y.; Cheng, Y.-M.; Pu, S.-C.; Yeh,
Y.-S.; Chou, P.-T. Chem. Phys. Chem. 2005, 6, 2012. (b) Chen, Y.-
L.; Sinha, C.; Chen, I.-C.; Liu, K.-L.; Chi, Y.; Yu, J.-K.; Chou, P.-T.;
Lu, T.-H. Chem. Commun. 2003, 3046.
(23) 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.
(24) Tung, Y.-L.; Wu, P.-C.; Liu, C.-S.; Chi, Y.; Yu, J.-K.; Hu, Y.-H.;
Chou, P.-T.; Peng, S.-M.; Lee, G.-H.; Tao, Y.; Carty, A. J.; Shu, C.-
F.; Wu, F.-I. Organometallics 2004, 23, 3745.
(25) Xie, J.; Huang, J.-S.; Zhu, N.; Zhou, Z.-Y.; Che, C.-M. Chem.sEur.
J. 2005, 11, 2405.
(22) Cheng, Y.-M.; Yeh, Y.-S.; Ho, M.-L.; Chou, P.-T.; Chen, P.-S.; Chi,
Y. Inorg. Chem. 2005, 44, 4594.
(26) Olby, B. G.; Robinson, S. D.; Hursthouse, M. B.; Short, R. L. J. Chem.
Soc., Dalton Trans. 1990, 621.
10280 Inorganic Chemistry, Vol. 46, No. 24, 2007

Os(II) Complexes Bearing a π-Chromophore
Figure 4. ORTEP drawing of complex 8 with thermal ellipsoids shown
at the 30% probability level. Selected distances: Os-P(1) ) 2.336(3), Os-
P(2) ) 2.310(3), Os-N(1) ) 2.141(9), Os-N(2) ) 2.109(8), Os-C(1) )
1.839(12), and Os-O(2) ) 2.118(8) Å.
Figure 1. ORTEP drawing of complex 1 with thermal ellipsoids shown
at the 30% probability level. Selected distances: Os(1)-C(1) ) 1.906(5),
Os(1)-C(2) ) 1.961(5), Os(1)-C(3) ) 1.938(5), Os(1)-O(4) ) 2.098-
(3), Os(1)-N(1) ) 2.139(4), and Os(1)-N(2) ) 2.061(4) Å.
Figure 2. ORTEP drawing of complex 6 with thermal ellipsoids shown
at the 30% probability level. Selected distances: Os-P(1) ) 2.367(3), Os-
P(2) ) 2.378(4), Os-N(1) ) 2.149(10), Os-N(2) ) 2.035(11), Os-O(2)
) 2.112(10), and Os-C(1) ) 1.832(15) Å.
Figure 5. UV-vis absorption and emission spectra of Os(II) complexes
1-5 in room-temperature acetonitrile. Note that the normalized emission
spectra were acquired under the degassed condition.
Figure 3. ORTEP drawing of complex 7 with thermal ellipsoids shown
at the 30% probability level. Selected distances: Os-P(1) ) 2.3784(8),
Os-P(2) ) 2.3752(8), Os-N(1) ) 2.079(3), Os-N(2) ) 2.118(3), Os-
C(1) ) 1.845(3), and Os-O(2) ) 2.120(2) Å.
its geometric isomer 7 (Os-N(1) ) 2.079 Å), for which the
pyridyl unit is located trans to the somewhat σ-donating tfa
ligand. The reverse alternation of the Os-N bond distances
is also detected for the pyrazolate segments, i.e., 2.035 Å in
6 versus 2.118 Å in 7, indicating that the tfa ligand is a poor
trans-directing ligand compared with the CO ligand within
this class of complexes.
Figure 6. UV-vis absorption and emission spectra of Os(II) complexes
1 and 6-8 in room-temperature acetonitrile. Note that the normalized
emission spectra were acquired under the degassed condition.
determination. As shown in Figure 4, this complex depicts
a distinctive configuration with the phosphine ligands located
at the cis-disposition and showing notably stronger Os-P
Finally, the structure of the third, nonemissive counterpart
8 was also subjected to the single-crystal X-ray structural
Inorganic Chemistry, Vol. 46, No. 24, 2007 10281

Cheng et al.
Table 1. Selected Photophysical Properties of Os(II) Complexes 1-8 in Acetonitrile at RT
compd
UV/vis data/nm (10-3ꢀ)
PL λmax (nm)
QY
τ
obs (µs)
kr
knr
1
2
3
4
5
6
7
8
247 (19), 292 (9.6), 320 (9.6)
260 (14), 291 (8.8), 316 (6.9)
263 (9.6), 306 (6.9), 318 (6.5)
260 (11), 301 (9.0)
460
457
458
457
446
550
520
0.26
0.03
0.003
<10-4
0.27
143
25.6
2.07
0.3
46.2
0.024
0.035
1.8 × 103
1.2 × 103
1.5 × 103
∼3.0 × 102
5.8 × 103
8.3 × 104
8.6 × 104
5.2 × 103
4.0 × 104
4.8 × 105
3.3 × 106
1.6 × 104
4.2 × 107
2.8 × 107
307 (6.7)
254 (27), 270 (21), 312 (11), 398 (1.5)
270 (18), 292 (14), 330 (6.7), 366 (3.4)
273 (13), 309 (8.5), 328 (7.3), 360 (3.3, sh)
0.002
0.003
interactions, Os-P(1) ) 2.336(3), and Os-P(2) ) 2.310(3)
Å, due to the absence of trans phosphine-to-phosphine linear
alignment. Moreover, the variation of Os-N distances in 8
was found to be comparable with the respective Os-N
lengths observed in both complexes 6 and 7, showing similar
trans-influences exerted on the fppz ligand.
tentatively assigned to 1MLCT mixed, in part, with the 1ππ*
transition in character. This assignment is considerably
different from that of 1-5, in which a ππ* ILCT is dominant
for the lowest lying transition. Such a salient difference lies
in the relative stronger σ-donating and less efficient π
accepting properties of PPh2Me in 6-8, which pushes up
the dπ orbital of Os(II) metal center and accordingly increases
Photophysical Properties. The absorption and lumines-
cence spectra recorded for 1-5 in acetonitrile are depicted
in Figure 5, while pertinent data are listed in Table 1. Several
remarks can be pointed out from the corresponding spec-
troscopic and dynamic measurements. As shown in the
absorption spectra, complexes 1-4 bearing different axial
anions, i.e., either tfa or halide ions, share spectral similarities
in the lower lying electronic transitions. For example, they
all exhibit the S0 f S1 transition band in the UV region of
ca. 300-350 nm with high absorptivity of ꢀ ∼ 1.0 × 104
M-1 cm-1 at their peak maxima of ∼320 nm. The results
can be rationalized by the S0 f S1 transition for 1-4 being
ascribed to the dominant ligand-based ππ* transition from
pyrazolate to the pyridyl moiety of the fppz chelate (not
shown here).6a Another support of this viewpoint is given
by the comparative study between 1-4 and 5. Adding a tert-
butyl group on the pyridyl moiety and substitution of
pyrazolate site by triazolate to 1, forming complex 5, gives
rise to an S0 f S1 transition at 305 nm, which is significantly
blue-shifted from that (∼320 nm) of 1-4. The results are
best rationalized by the S0 f S1 transition for 1-5 being
largely attributed to azolate (HOMO) pyridyl (LUMO) ππ*
transition, such that a significant increase of the associated
S0 f S1 energy gap is expected for 5 due to its profound
decrease (increase) of the electron density in the HOMO
(LUMO) level. Further direct support will be elucidated in
the section on computational approaches.
In yet another approach, despite their structural similarities,
three isomers, 6-8, which are formed by replacing two
terminal CO ligands in 1 with PPh2Me, show intrinsically
different absorption spectra. As depicted in Figure 6, the
S0-S1 absorption spectral features are quite different among
6-8, in which the peak wavelengths of 398 nm (6, shoulder),
366 nm (7), and 360 nm (8) appear to be far red-shifted
with respect to that of 1-4 (320 nm) and 5 (305 nm) (see
Table 1 for comparison). The results showed good consis-
tence with the general trend of photophysical properties that
occurred after phosphine substitution.27 Moreover, supported
by their extinction coefficients of <3000 M-1 cm-1,
the corresponding S0 f S1 transition for 6-8 can be
1
the associated HOMO energy level as well as the MLCT
contribution.24
Furthermore, the S0 f S1 transition energy tends to be
increased in the order of 6 < 7 < 8, the results of which
can be rationalized by the fact that the remaining CO
possesses a stronger π-accepting strength than that of PPh2-
Me. As opposed to 7 and 8, in which the unique CO ligand
is located opposite to the pyrazolate moiety, the CO ligand
of 6 is in the trans position with respect to the pyridyl
fragment. Since pyrazolate and pyridyl moieties, to a certain
extent, determine the HOMO and LUMO energy levels,
respectively (vide supra), the decrease of the electron density
trans to CO such as pyridyl (LUMO in 6) and pyrazolate
fragment (HOMO in 7 and 8) should lead to a decrease
(increase) of the S0 f S1 energy gap for 6 (7 and 8),
rationalizing the lowest S0-S1 energy gap for 6. Moreover,
as for 7 versus 8, realizing that both π-accepting PPh2Me
are located at the mutually trans disposition in 7, a lesser
degree of π-back-donation from the Os(II) metal center to
trans-disposed phosphines is thus anticipated. Accordingly,
both the better σ-donation from phosphine ligands and the
poor competition of the dπ-electron in 7 would increase the
relative electron density at the metal center, giving rise to a
lower S0-S1 gap.
As for the luminescence properties, owing to its significant
O2 quenching and/or the rather long radiative lifetime of tens
to hundreds of microseconds (see Table 1), the origin of
emission for the titled compounds being ascribed to phos-
phorescence is unambiguous. In a good correlation with the
trend of absorption spectra, independent of the axial anions,
complexes 1-4 all exhibit the expected unique blue emission
with peak wavelengths at ∼445-460 nm in acetonitrile,
while 5 revealed a significant blue shift with respect to that
of 1-4 due to the replacement of pyrazolate by triazolate
moiety. The similar trend for both S0 f S1 (absorption) and
T1 f S0 (phosphorescence) transitions leads us to conclude
that the frontier orbitals involved in the S0-T1 transition must
be the same as that of S0-S1, i.e. largely incorporating the
azolate (HOMO) pyridyl (LUMO) ππ* transition. Qualita-
tively, prompt evidence of this viewpoint is given by the
distinct vibronic progression of the phosphorescence in 1-5
(27) Li, E. Y.; Cheng, Y.-M.; Hsu, C.-C.; Chou, P.-T.; Lee, G.-H.; Lin,
I.-H.; Chi, Y.; Liu, C.-S. Inorg. Chem. 2006, 45, 8041.
10282 Inorganic Chemistry, Vol. 46, No. 24, 2007

Os(II) Complexes Bearing a π-Chromophore
(Figure 5), a phenomenon symbolizing the transition domi-
nated by the intraligand ππ* character.
s-1 and 8.6 × 104 s-1 for 6 and 7, respectively, which is
greater than that of 1-5 by more than 1 order of magnitude
(see Table 1).
Another interesting point is that complexes 1 and 5,
bearing tfa anions, exhibit much higher emission yield (1,
QY ) 0.26; 5, QY ) 0.27) than those of 2-4, bearing
halides. With emission quantum yield and observed lifetime
provided, both radiative and nonradiative decay rate are also
deduced and listed in Table 1. The radiative decay rates, kr,
calculated as 1 (1.8 × 103 s-1), 2 (1.2 × 103 s-1), 3 (1.5 ×
103 s-1), 4 (3.0 × 102 s-1), and 5 (5.8 × 103 s-1) are rather
small, manifesting their dominant ππ* origin. Theoretically,
the smaller radiative decay rate corresponds to less allowed
S0-T1 transition. With application of the first-order ap-
proximation, it also implies the reduction of the spin-orbit
coupling integral, most likely from the lesser dπ contribution
of the heavy metal atom.28 In other words, compared with
the examples documented in our previous studies,23,29 these
results lead to a proposal involving much reduced MLCT
contribution for complexes 1-5 in the lowest energy triplet
manifold T1.
In brief, for the titled complexes 1-8, the trend of energy
gap, measured by either absorption (S0-S1) or phosphores-
cence (S0-T1), and the contribution of types of frontier
orbitals can be well explained by the electronic properties
and relative positions of the axial anion. Nevertheless, a
puzzle pending resolution lies in the extremely weak (QY
3
, 0.01) for 6-8 despite their great MLCT contribution,
i.e., a consequence of the short radiative decay time.
Accordingly, the nonradiative decay rate, knr, for 6 and 7
was also deduced and listed in Table 1. As for 1-5 with
ππ* dominance versus 6 and 7 with great MLCT contribu-
tion, the difference in knr of more than 2 orders of magnitude
is much greater than the corresponding difference in kr. In
other words, though kr is significantly increased in 6 and 7
by promoting the MLCT contribution, the accompanied knr
increases as well with an even more pronounced increment.
Note Angelis et al.31 have recently studied some phospho-
rescent Ir(III) complexes for OLED applications and the
results revealed a perfectly linear correlation between the
nonradiative deactivation rate constant and energy-gap law.32
Generally, the energy-gap law works well under the circum-
stance of a large difference in energy gap and perhaps the
same excited-state characters, such as pure ππ*. In contrast,
in this study, we noticed that we could not obtain a linear
proportional relation for ln(knr) versus the emission energies
among the titled Os(II) complexes. This “unusual observa-
tion” may actually be normal if light can be shed on the
fundamental basis. The radiative decay rate constant is
proportional to the electronic coupling matrix and the
Franck-Condon weighed density of the state expressed as
While the radiative decay rates among 1-5 are on the
same order of magnitude, the main difference in emission
quantum yield lies in the more than 1 order of magnitude
difference in the nonradiative decay rate knr, namely 5.2 ×
103 s-1 (1) and 1.6 × 104 s-1 (5), compared to the halide-
substituted complexes; cf. 4.0 × 104 s-1 (2), 4.8 × 105 s-1
(3), and 3.3 × 106 s-1 (4). Due to the high-energy emission,
we tentatively rationalize the results by the weakening of
the Os(II)-halide bonding upon excitation, such that the
excited-state potential energy surface (PES) tends to be
shallower than those of tfa-anchored complexes 1 and 5. The
net result not only destabilizes the molecular framework but
also increases the radiationless deactivation process plausibly
due to the resulting shallow PES in the T1 state that may
intersect with PES of the ground state. A parallel support is
also given by the correlation between halide complexes and
knr, which shows a tendency of decreasing the bond strength
in the order of Os(II)-Cl > Os(II)-Br > Os(II)-I,
corresponding to the increase of knr in a trend of 2 < 3 < 4.
Figure 6 also reveals the emission spectra of 6 and 7 in
degassed acetonitrile solution at RT. In good agreement with
the difference in their absorption spectra, due to the lowering
of LUMO by the π-accepting effect exerted by the CO
ligand, the phosphorescence in 6 is red-shifted by ∼30 nm
with respect to that of 7. Attempts to acquire emission of 8
unfortunately failed. Taking account of the instrument
sensitivity, the upper limit of QY for 8 is estimated to be
10-4. Moreover, compared with those of 1-5, a salient
difference in the spectral feature can be promptly pointed
out for both 6 and 7, in which the phosphorescence spectrum
is broadened and lacks vibronic progression, a feature that
2
2
kr | T1ΦT 0|Her|S0ΦS m | ) | T1|Her|S0 | |
1
0
2
2
ΦT 0|ΦS m | ) | T1|Her|S0 | FCT 0 S m
,
(3)
1
0
1
0
where Her denotes the electric dipole operator created by the
electric magnetic field. FCT 0,S m specifies the Franck-
1
0
Condon overlap factor between the vibrational wave function
(Φ) of T1 at the vibrational quanta of n ) 0 and that of S0
at n ) m. Note that S0 and T1 in eq 3 only denote the
electronic wave function. In addition, to simplify the discus-
sion, instead of the summation of entire vibrational states in
S0, only one vibronic state, i.e., m, in S0 is considered for
the transition.
In another approach, using a first-order approximation, the
radiationless decay constant knr can be expressed as
(30) (a) Tung, Y.-L.; Chen, L.-S.; Chi, Y.; Chou, P.-T.; Cheng, Y.-M.; Li,
E. Y.; Lee, G.-H.; Shu, C.-F.; Wu, F.-I.; Carty, A. J. AdV. Funct. Mater.
2006, 16, 1615. (b) Field, J. S.; Haines, R. J.; Ledwaba, L. P.; McGuire,
R.; Munro, O. Q., Jr.; Low, M. R.; McMillin, D. R. Dalton Trans.
2007, 192.
(31) Angelis, F. D.; Fantacci, S.; Evans, N.; Klein, C.; Zakeeruddin, S.
M.; Moser, J.-E.; Kalyanasundaram, K.; Bolink, H. J.; Gra1tzel, M.;
Nazeeruddin, M. K. Inorg. Chem. 2007, 46, 5989.
(32) (a) Kober, E. M.; Caspar, J. V.; Lumpkin, R. S.; Meyer, T. J. J. Phys.
Chem. 1986, 90, 3722. (b) Henry, B. R.; Siebrand, W. In Organic
Molecular Photophysics; Birks, J. B., Ed.; Wiley: New York, 1973;
Vol. 1, Chapter 4. (c) Freed, K. F.; Jortner, J. J. Chem. Phys. 1970,
52, 6272. (d) Bixon, M.; Jortner, J. J. Chem. Phys. 1968, 48, 715.
3
manifests the dominance of the MLCT character.30 The
3MLCT-enhancing T1-S0 transition can be firmly supported
by the deduced radiative decay rate constant of 8.3 × 104
(28) Yutaka, T.; Obara, S.; Ogawa, S.; Nozaki, K.; Ikeda, N.; Ohno, T.;
Ishii, Y.; Sakai, K.; Haga, M.-A. Inorg. Chem. 2005, 44, 4737.
(29) Chen, K.; Cheng, Y. M.; Chi, Y.; Ho, M. L.; Lai, C. H.; Chou, P. T.;
Peng, S. M.; Lee, G. H. Chem. Asian J. 2007, 2, 155.
Inorganic Chemistry, Vol. 46, No. 24, 2007 10283

Cheng et al.
4π2FE
h2
4π2FE
h2
2
2
knr
| T1ΦT 0|Hnr|S0ΦS (E) | )
| T1|Hnr|S0 | |
1
0
4π2FE
h2
2
2
ΦT 0|ΦS (E) | )
| T1|Hnr|S0 | FCT 0,S (E) (4)
1
0
1
0
where Hnr is any nuclear kinetic energy operator that induces
the jump between two PES’s.32
S0 and T1 in eq 4 only denote the electronic wave function.
E in ΦS (E) specifies the vibrational energy between the zero
0
point energies of T1 and S0. For nonlinear, polyatomic
molecules such as the titled Os(II) complexes, due to the 3n
- 6 degrees of vibrational freedom, there should be a large
number of ΦS (E) states being isoenergetic with respect to
0
Φ
T 0, which may be further broadened by solvent perturba-
1
tion, merging into a continuum of state density FE. FCT 0,S (E)
1
0
is the Franck-Condon overlap factor, which describes the
overlap of the continuum of vibrational states ΦS (E) with
0
respect to ΦT 0
.
1
One can thus view the Franck-Condon factor as a vertical
transition in kr, while it is a horizontal transition in knr. The
2
2
electronic coupling factors | T1|Her|S0 | and | T1|Hnr|S0 |
for kr and knr, respectively, involve the wave functions of
initial (T1) and final (S0) electronic states, such that the
radiationless transition is subject to the same multiplicity
selection rules as the radiative transition.33 In other words,
the stronger S0-T1 mixing due to the greater 3MLCT
contribution,34 in parallel, leads to an increase of both kr and
knr. In the cases of 6-8, a further increase of the radiationless
transition perhaps can be qualitatively rationalized by the
introduction of PPh2Me, in which both methyl and phenyl
torsional motions should drastically increase the density of
the isoenergic states σ in knr and hence lead to an increase
of the radiationless deactivation rate constants. Moreover,
the complexes 6 and 7 are akin to the hydride complexes of
formula [Os(H)(fptz)(PPh2Me)2(CO)] reported in the litera-
ture.22 Yet the quantum yield for the hydride complexes is
orders of magnitude higher than that of 6 and 7 with the tfa
ligand. These data indicate that the current tfa complexes 6
and 7 possess much larger nonradiative rates than those found
for the aforementioned hydride derivatives ((0.1-2.5) × 106
s-1). This discrepancy can be tentatively rationalized by the
difference in frontier orbitals involved in the low-lying
transitions. Unlike the hydride complexes, as shown in Figure
8, the HOMO for 6 and 7 has a substantial contribution from
the tfa ligand. As the T1 f S0 mainly involves LUMO f
HOMO transition, the deactivation pathways should be
subject to the associated tfa motions, and the quenching of
emission is expected to be significant due to the relatively
weak bonding to the tfa ligand.
Figure 7. Highest occupied (HOMO) and lowest unoccupied (LUMO)
molecular orbitals of complexes 1, 2, 4, and 5. Note that the frontier orbitals
of bromide complex 3 were omitted because they appeared to be quite
similar to those of chloride derivative 2.
approaches. Theoretical confirmation of the underlying basis
for the photophysical properties of the studied complexes
was provided by the density functional theory (DFT) MO
calculations. With the use of the TD-B3LYP method, incor-
porating the B3LYP/LANL2DZ and 6-31G* optimized
geometry, the vertical (i.e., Franck-Condon) excitation
energy from the ground-state to low lying excited states was
calculated. For validation of the theoretical method adopted
here, comparisons between the optimized geometrical pa-
rameters and X-ray data for complexes 1 and 6-8 are listed
in the Supporting Information. The calculated structures are
found to be in good agreement with experimental data. For
example, in complex 1, the values of Os-O(tfa), Os-C(car-
bonyl) and Os-N(azolate and pyridine) distances of 2.107,
1.916-1.949, 2.077, and 2.174 Å are calculated, to be
comparable with experimental values of 2.098, 1.907-1.962,
2.061, and 2.140 Å, respectively. Figure 7 depicts the features
of the lowest unoccupied (LUMO) and the highest occupied
(HOMO) frontier orbitals for complexes 1, 2, 4, and 5, which
are mainly involved in the lower lying transitions. Note that
the HOMO-2 and HOMO-3 orbitals were added to complex
4 because the orbitals mainly involved in the S0 f T1
transition are both HOMO-2 and HOMO-3 instead of
Theoretical Approaches
Supplementary supports of the above-mentioned results
and rationalization are rendered via the computational
(33) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience:
London and Colchester, U.K., 1970; p 151.
(34) (a) Azumi, T.; Miki, H. Top. Curr. Chem. 1997, 191, 1. (b) Solomon,
E. I.; Lever, A. B. P. Inorganic Electronic Structure and Spectroscopy;
John Wiley & Sons: New York, 1999; Vol. I, Chapter 1.
10284 Inorganic Chemistry, Vol. 46, No. 24, 2007

Os(II) Complexes Bearing a π-Chromophore
dominated by ILCT (ππ*, azolate f pyridyl moiety). In
other words, one can also envisage that the HOMO’s of 1-5
are apparently much less located at the central Os(II) atom,
leading to very small MLCT contribution. For example,
results of the calculation show e1% MLCT contribution for
1-5, consistent with the conclusion drawn from the experi-
mental results, i.e., the emission bands with vibronic progres-
sions and rather long radiative decay time. The calculated
energy gap is also consistent with experimental results,
revealing a trend of similar S0-S1 and S0-T1 energy gaps
for 1-4, while a significant blue shift in both was resolved
for 5. Moreover, the HOMO of 4 showed significant
contribution from the axial iodide substituent, which impli-
cated the coexistence of the so-called halide-to-ligand charge
transfer (XLCT) transition documented in the literature.35
However, due to the poor overlap between the pπ orbital of
iodide and the π* orbital of the fppz chromophore, the
transition dipole between the iodide pπ and the ligand π*
orbitals should be small.
Finally, as depicted in Figure 8, the lowest lying transitions
for complexes 6-8 are apparently dominated largely by
MLCT mixed with ππ* and LLCT transitions in character.10
As compared to complexes 1-5 (MLCT ∼ 1%), the much
larger extents of the calculated MLCT contributions of 6
(55%) and 7 (58%) are unambiguously responsible for the
relatively much shorter radiative lifetime due to the direct
involvement of MLCT in spin-orbit coupling. An equally
important result is that the calculation also correctly predicts
that the S0-S1 (or S0-T1) energy gap is in the order of 8 >
7 > 6, giving firm support for our early explanation (vide
supra).
Figure 8. Highest occupied (HOMO) and lowest unoccupied (LUMO)
molecular orbitals of complexes 6-8.
HOMO. The descriptions and the energy gaps of all com-
plexes are listed in Table 2. Obviously, the calculated S1
and T1 energy levels for the titled complexes are qualitatively
consistent with those 0-0 onsets of the absorption (S1) and
phosphorescence (T1) spectra. Thus, the theoretical level
adopted here should be suitable for studying the photophysi-
cal properties of these complexes in a qualitative manner.
Deviation of current theoretical approach from the experi-
mental results may plausibly be explained by the negligence
of solvation effects in the gas phase ab initio approach.
On the basis of the frontier orbital analyses, the lowest
singlet and triplet excited states for these complexes are
Conclusion
In summary, Os complexes with single π-chromophore
were designed and synthesized in an aim to attain optimum
blue phosphorescence. Our proposal of confining the excita-
tion energy seems to work well upon anchoring of a single,
Table 2. Calculated Excitation Properties of the Lowest Lying Excited States of Complexes 1-8a
compd
state
λ (nm)
f
assignment
contents
1
T1
S1
T1
S1
T1
S1
438.1
343.8
436.5
346.5
437.5
362.0
∼0
0.0553
∼0
0.042
HOMO f LUMO (+94%); HOMO-1 f LUMO (+7%)
HOMO f LUMO (+87%); HOMO-1 f LUMO (+12%)
HOMO f LUMO (+92%); HOMO f LUMO+3 (9%)
HOMO f LUMO (+71%); HOMO-1 f LUMO (+18%)
HOMO f LUMO (+85%); HOMO f LUMO+3 (+10%)
HOMO-1 f LUMO (+64%); HOMO f LUMO (+23%);
HOMO-2 f LUMO (6%)
MLCT e 1%
MLCT e 1%
MLCT ∼ 1.5%
MLCT ∼ 3%
MLCT e 1%
MLCT ∼ 12%
2
3
∼0
0.0045
0.0477
S3
T1
345.0
436.4
HOMO f LUMO (+54%); HOMO-2 f LUMO (+17%);
HOMO-1 f LUMO (10%); HOMO-3 f LUMO (8%)
HOMO-2 f LUMO (+85%); HOMO-2 f LUMO+3
(+11%); HOMO-3 f LUMO (+10%)
MLCT ∼ 3.6%
MLCT e 1%
4
5
∼0
S1
S3
T1
392.4
347.9
403.0
0.0004
0.0435
∼0
HOMO f LUMO (+92%)
MLCT ∼ 13%
MLCT e 1%
MLCT e 1%
HOMO-2 f LUMO (+76%); HOMO-3 f LUMO (9%)
HOMO f LUMO (+89%); HOMO f LUMO+2 (+7%);
HOMO f LUMO+3 (+7%)
S1
T1
S1
T1
S1
T1
S1
329.3
542.1
505.9
524.6
470.2
457.0
399.5
0.0751
HOMO f LUMO (+89%)
HOMO f LUMO (+97%); HOMO-1 f LUMO (+3%)
HOMO f LUMO (+95%)
HOMO f LUMO (+93%); HOMO f LUMO+3 (8%)
HOMO f LUMO (+77%); HOMO-1 f LUMO (+17%)
HOMO f LUMO (+96%); HOMO-4 f LUMO+1 (+5%)
HOMO f LUMO (+89%); HOMO-1 f LUMO (+3%)
MLCT e 1%
MLCT ∼ 55%
MLCT ∼ 54%
MLCT ∼ 58%
MLCT ∼ 57%
MLCT ∼ 45%
MLCT ∼ 43%
6
7
8
∼0
0.0128
∼0
0.0203
∼0
0.0639
a Notice that the oscillator strengths (f) of the S1 states of both complexes 3 and 4 were found to be trivial; therefore, another singlet excited state (S3)
with a much more prominent oscillator strength was discovered and listed here.
Inorganic Chemistry, Vol. 46, No. 24, 2007 10285

Cheng et al.
higher energy gap π-chromophore in the Os(II) complexes;
consequently, the control of MLCT versus ππ* (or LLCT)
excited-state characters becomes much more straightforward.
However, it is now necessary to mull over other adverse
effects in that the localization of the excitation energy seems
to be more facile to populate at certain vibrational modes
with weak metal-ligand bond strength, i.e., with the shallow
PES, inducing the unwanted radiationless transition. In
addition, in comparison to the complexes with dual or
multiple chromophores, axial anionic ligands play an im-
portant role for 1-8 in fine-tuning not only the energy gap
but also the emission intensity, i.e., in manipulating the
radiative versus the radiationless transition pathways. Our
results clearly show that there is a tradeoff upon varying the
parameters in an aim to tune both emission wavelength and
intensity. Switching from dual or multiple chromophores to
the single chromophore would localize the energy and
consequently improve the emission bandwidth; it conversely
may induce the radiationless transition due to confinement
of the excitation energy in a local chromophore with certain
weak bonding modes.
character in the excited-state and hence the shorter radiative
lifetime in the OLED applications (vide supra), a strategic
design may be needed to avoid the possible involvement of
metal-centered dd*, σπ*,10 XLCT transitions as well as the
incorporation of strong trans effect as the lowest lying
transition that causes the shallow PES. This, in combination
with a more rigid, robust structure to reduce the density of
the accepting states, should provide a preliminary rule of
thumb for attaining decent emission intensity, at least for
the Os(II) relevant complexes bearing single chromophore.
The optimization of such a strategic design can be previewed
via the computational approach, such that the frontier orbitals
involved in the lowest lying transition can be analyzed in a
qualitative manner to increase the feasibility. We hope that
the results and discussion presented here can make some
contributions in the field for the development of all phos-
phorescent OLEDs. Especially, we believe that current
progress in assembling and gaining photophysical back-
grounds of the Os(II)-containing emissive molecules warrant
a similar applicability to the related isoelectronic Ir(III)
materials and/or the square planar Pt(II) system with d8-
configuration.
3
In yet another approach, increasing MLCT is a general
way to enhance the phosphorescence QY to find better
OLED dopants, especially when the blue hue is to be paid
much attention. However, it will also be subject to an
increase of the radiationless transition due to a good S0-T1
mixing. Owing to the advantage of increased 3MLCT
Acknowledgment. We thank the support from the fol-
lowing research grants: NSC 91-2119M-002-016; NSC 93-
2752-M-002-002-PAE; 94-EC-17A-08-S1-042.
Supporting Information Available: X-ray crystallographic data
files (CIF) for complexes 1, 6, 7, and 8 and a table showing the
optimized geometrical parameters obtained from DFT calculation.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(35) (a) Nieuwenhuis, H. A.; Stufkens, D. J.; Vlcˇek, A., Jr. Inorg. Chem.
1995, 34, 3879. (b) Slageren, J. V.; Hartl, F.; Stufkens, D. J.; Martino,
D. M.; Van Willigen, H. Coord. Chem. ReV. 2000, 208, 309. (c) Chen,
Y.-L.; Lee, S.-W.; Chi, Y.; Hwang, K.-C.; Kumar, S. B.; Hu, Y.-H.;
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IC7015269
10286 Inorganic Chemistry, Vol. 46, No. 24, 2007