Neutral RuII-based emitting materials: A prototypical study on factors governing radiationless transition in phosphorescent metal complexes

Article abstract of DOI:10.1021/ic060066g

In addition to the metal-centered dd transition that is widely accepted as a dominant radiationless decay channel, other factors may also play important roles in governing the loss of phosphorescence efficiency for heavy-transition-metal complexes. To conduct our investigation, we synthesized two dicarbonylruthenium complexes with formulas [Ru(CO)₂(BQ) ₂] (1) and [Ru(CO)₂(DBQ)₂] (2), for which the cyclometalated ligands BQ and DBQ denote benzo-[h]quinoline and dibenzo[f,h]quinoxaline, respectively. Replacing one CO ligand with a P donor ligand such as PPh₂Me and PPhMe₂ caused one cyclometalated ligand to undergo a 180° rotation around the central metal atom, giving highly luminous metal complexes [Ru(CO)L(BQ)₂] and [Ru(CO)L(DBQ) ₂], where L = PPh₂Me and PPhMe₂ (3-6), with emission peaks λ_max in the range of 571-656 nm measured in the fluid state at room temperature. It is notable that the S₀-T ₁ energy gap for both 1 and 2 is much higher than that of 3-6, but the corresponding phosphorescent spectral intensity is much weaker. Using these cyclometalated Ru metal complexes as a prototype, our experimental results and theoretical analysis draw attention to the fact that, for complexes 1 and 2, the weaker spin-orbit coupling present within these molecules reduces the T ₁-S₀ interaction, from which the thermally activated radiationless deactivation may take place. This, in combination with the much smaller ³MLCT contribution than that observed in 3-6, rationalizes the lack of room-temperature emission for complexes 1 and 2.

Full text of DOI:10.1021/ic060066g

Inorg. Chem. 2006, 45, 8041−8051
Neutral RuII-Based Emitting Materials: A Prototypical Study on Factors
Governing Radiationless Transition in Phosphorescent Metal
Complexes†
Elise Y. Li, Yi-Ming Cheng, Cheng-Chih Hsu, Pi-Tai Chou,* and Gene-Hsiang Lee
Department of Chemistry and Instrumentation Center, National Taiwan UniVersity,
Taipei 106, Taiwan
I-Hui Lin and Yun Chi*
Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 300, Taiwan
Chao-Shiuan Liu
Department of Chemistry, SooChow UniVersity, Taipei 111, Taiwan
Received January 13, 2006
In addition to the metal-centered dd transition that is widely accepted as a dominant radiationless decay channel,
other factors may also play important roles in governing the loss of phosphorescence efficiency for heavy-transition-
metal complexes. To conduct our investigation, we synthesized two dicarbonylruthenium complexes with formulas
[Ru(CO)2(BQ)2] (1) and [Ru(CO)2(DBQ)2] (2), for which the cyclometalated ligands BQ and DBQ denote benzo-
[h]quinoline and dibenzo[f,h]quinoxaline, respectively. Replacing one CO ligand with a P donor ligand such as
PPh2Me and PPhMe2 caused one cyclometalated ligand to undergo a 180
giving highly luminous metal complexes [Ru(CO)L(BQ)2] and [Ru(CO)L(DBQ)2], where L
(3 6), with emission peaks λmax in the range of 571 656 nm measured in the fluid state at room temperature. It
is notable that the S0 T1 energy gap for both 1 and 2 is much higher than that of 3 6, but the corresponding
°
rotation around the central metal atom,
)
PPh2Me and PPhMe2
−
−
−
−
phosphorescent spectral intensity is much weaker. Using these cyclometalated Ru metal complexes as a prototype,
our experimental results and theoretical analysis draw attention to the fact that, for complexes 1 and 2, the weaker
spin
−
orbit coupling present within these molecules reduces the T1
−
S0 interaction, from which the thermally activated
3
radiationless deactivation may take place. This, in combination with the much smaller MLCT contribution than that
observed in 3−6, rationalizes the lack of room-temperature emission for complexes 1 and 2.
1. Introduction
good phosphorescence efficiency for these metal complexes.
Among the phosphorescent complexes, green-emitting com-
plexes2 have been known for years and were fabricated as
OLED components with ∼100% internal quantum efficiency,
while the red-emitting complexes are also accessible through
judicious choices of chelate chromophores to lower the
energy gap and extend the triplet-state lifetime3 as well as
One of the important research subjects toward organic light
emitting diodes (OLEDs) is the development of phospho-
rescent materials that emit all three primary colors for full-
color displays. This approach leads to the essentiality of
preparing phosphors incorporating second- and third-row
transition-metal complexes.1 The associated strong spin-
orbit coupling in heavy metals would promote singlet-to-
triplet intersystem crossing as well as enhance the subsequent
radiative transition from the triplet to the ground state, giving
(1) (a) Holder, E.; Langeveld, B. M. W.; Schubert, U. S. AdV. Mater.
2005, 17, 1109. (b) Hwang, F.-M.; Chen, H.-Y.; Chen, P.-S.; Liu,
C.-S.; Chi, Y.; Shu, C.-F.; Wu, F.-I.; Chou, P.-T.; Peng, S.-M.; Lee,
G.-H. Inorg. Chem. 2005, 44, 1344. (c) Tung, Y.-L.; Lee, S.-W.; Chi,
Y.; Chen, L.-S.; Shu, C.-F.; Wu, F.-I.; Carty, A. J.; Chou, P.-T.; Peng,
S.-M.; Lee, G.-H. AdV. Mater. 2005, 17, 1059. (d) Chou, P.-T.; Chi,
Y. Eur. J. Inorg. Chem. 2006, 3319 (DOI 10.1002/ejic.200600364).
† Dedicated to Prof. John R. Shapley on the occasion of his 60th birthday.
* To whom correspondence should be addressed. E-mail: chop@ntu.edu.tw
(P.-T.C.), ychi@mx.nthu.edu.tw (Y.C.).
10.1021/ic060066g CCC: $33.50
Published on Web 08/30/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 20, 2006 8041

Li et al.
to circumvent their intrinsic obstacle, namely, the rapid
nonradiative deactivation predicted by the energy gap law.4
Presently, researchers have turned their attention to the
preparation of the remaining blue-emitting phosphorescent
complexes.5 This task, however, is even more difficult to
achieve than those of the other two cases. One major
challenge lies in the selection of suitable chelate ligands that
are able to form complexes with sufficiently large ligand-
centered ππ* transition energies and/or metal-to-ligand
charge-transfer (MLCT) energies. Such an approach might
inevitably raise the ligand-centered transition (or MLCT) to
a region very close to or even higher than the metal-centered
dd states (or ligand-field, LF, states), such that a very efficient
radiationless decay pathway may take place through a
shallow potential energy surface or a possible T1-S0 inter-
section due to the weakness of the metal-ligand bonds.
To circumvent this obstacle, a few attempts have been
made through the use of strong field ancillary ligands such
as CO or cyanide with an aim to increase the dd transition.6
This, in combination with the incorporation of third-row
metal elements, further strengthens the metal-ligand bond-
ing.7 More recently, a series of blue-emitting pyridyl azolate
osmium carbonyl complexes8 as well as iridium complexes
with 2,4-difluorophenylpyridyl, pyrazolyl, and even N-
heterocyclic carbene ligands have been reported.9 These
exquisite works demonstrate the feasibility of achieving a
saturated blue color or even near-UV phosphorescence.10
Despite this perspective, however, almost all of these blue
phosphorescent complexes are inevitably subject to a pro-
nounced decrease in the luminance efficiency at room
temperature.
Because both strong-field metal elements and ligands are
selected in assembling this class of metal complexes, it is
reasonable to expect that the metal-centered dd state will be
inaccessible from the lowest emissive triplet state. Thus, the
dominant radiationless deactivation generalized by a quench-
ing mechanism incorporating dd transition may be ground-
less.6,11 As such, the call for alternative, convincing expla-
nations to account for this ubiquitous observation is urgent.
Bearing this challenge in mind, we then made an assiduous
effort to design and synthesize a new series of RuII complexes
possessing cyclometalated chromophores and other strong-
field ligands, aimed at probing the radiationless pathways
in correlation with their chemical structures.12 It is notable
that RuII complexes are well suited for this approach mainly
because of their relatively small ligand field and weaker
metal-ligand bonding. This, in combination with its less
heavy atom effect and hence the weaker spin-orbit coupling,
leads us to believe that, under the same ligand configuration,
the induction of radiationless transition in RuII complexes is
expected to be more drastic than that of the third-row metal
congeners. Accordingly, it becomes more plausible to explore
the undermining factors causing such intriguing phenomena,
i.e., loss of the emission intensity, from fundamental aspects.
As an equally important issue, once the radiationless channels
are inhibited, the prevailing of RuII complexes to the third-
row transition-metal complexes toward OLED application
are apparently due to the lower cost and greater abundance
of Ru.
(2) (a) Hua, F.; Kinayyigit, S.; Cable, J. R.; Castellano, F. N. Inorg. Chem.
2005, 44, 471. (b) Huang, W.-S.; Lin, J. T.; Chien, C.-H.; Tao, Y.-T.;
Sun, S.-S.; Wen, Y.-S. Chem. Mater. 2004, 16, 2480. (c) Tokito, S.;
Iijima, T.; Tsuzuki, T.; Sato, F. Appl. Phys. Lett. 2003, 83, 2459.
(d) Lo, S.-C.; Namdas, E. B.; Burn, P. L.; Samuel, I. D. W. Macro-
molecules 2003, 36, 9721. (e) Adachi, C.; Baldo, M. A.; Forrest, S.
R.; Thompson, M. E. Appl. Phys. Lett. 2000, 77, 904.
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Y.-M.; Chou, P.-T.; Peng, S.-M.; Liu, C.-S. J. Mater. Chem. 2005,
15, 460. (b) Anthopoulos, T. D.; Frampton, M. J.; Namdas, E. B.;
Burn, P. L.; Samuel, I. D. W. AdV. Mater. 2004, 16, 557. (c)
Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.; Kamatani,
J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada, S.;
Hoshino, M.; Ueno, K. J. Am. Chem. Soc. 2003, 125, 12971. (d)
Adachi, C.; Baldo, M. A.; Forrest, S. R.; Lamansky, S.; Thompson,
M. E.; Kwong, R. C. Appl. Phys. Lett. 2001, 78, 1622.
2. Experimental Section
General Information and Materials. Elemental analyses and
mass spectroscopy (operating in fast atom bombardment, FAB,
mode) were carried out at the NSC Regional Instrument Centre at
(4) (a) Kober, E. M.; Caspar, J. V.; Lumpkin, R. S.; Meyer, T. J. J. Phys.
Chem. 1986, 90, 3722. (b) Perkins, T. A.; Pourreau, D. B.; Netzel, T.
L.; Schanze, K. S. J. Phys. Chem. 1989, 93, 4511.
1
National Chiao Tung University, Hsinchu, Taiwan. H and 13C
NMR spectra were recorded on a Varian Mercury 400 or an Inova
500-MHz instrument; chemical shifts are quoted with respect to
internal standard Me4Si. Details of the fabrication and characteriza-
tion of electroluminescent devices are as reported previously.1b All
synthetic manipulations were performed under a N2 atmosphere,
while solvents were used as received. Benzo[h]quinoline (BQ) was
purchased from TCI Japan, while dibenzo[f,h]quinoxaline (DBQ)
was prepared from condensation of phenanthrene-9,10-dione with
ethylenediamine.13 The RuII metal complex [Ru(CO)2(BQ)2] (1) was
(5) (a) Yeh, S.-J.; Wu, M.-F.; Chen, C.-T.; Song, Y.-H.; Chi, Y.; Ho,
M.-H.; Hsu, S.-F.; Chen, C.-H. AdV. Mater. 2005, 17, 285. (b) Ren,
X.; Li, J.; Holmes, R. J.; Djurovich, P. I.; Forrest, S. R.; Thompson,
M. E. Chem. Mater. 2004, 16, 4743. (c) Karatsu, T.; Nakamura, T.;
Yagai, S.; Kitamura, A.; Yamaguchi, K.; Matsushima, Y.; Iwata, T.;
Hori, Y.; Hagiwara, T. Chem. Lett. 2003, 32, 886. (d) Holmes, R. J.;
D’Andrade, B. W.; Forrest, S. R.; Ren, X.; Li, J.; Thompson, M. E.
Appl. Phys. Lett. 2003, 83, 3818. (e) Tanaka, I.; Tabata, Y.; Tokito,
S. Chem. Phys. Lett. 2004, 400, 86.
(6) (a) Anderson, P. A.; Keene, F. R.; Meyer, T. J.; Moss, J. A.; Strouse,
G. F.; Treadway, J. A. J. Chem. Soc., Dalton Trans. 2002, 3820. (b)
Koike, K.; Okoshi, N.; Hori, H.; Takeuchi, K.; Ishitani, O.; Tsubaki,
H.; Clark, I. P.; George, M. W.; Johnson, F. P. A.; Turner, J. J. J.
Am. Chem. Soc. 2002, 124, 11448.
(7) (a) Lee, C.-L.; Das, R. R.; Kim, J.-J. Chem. Mater. 2004, 16, 4642.
(b) Nazeeruddin, M. K.; Humphry-Baker, R.; Berner, D.; Rivier, S.;
Zuppiroli, L.; Graetzel, M. J. Am. Chem. Soc. 2003, 125, 8790.
(8) (a) 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. 2004, 10, 6255. (b) Wu, P.-C.; Yu, J.-K.; Song,
Y.-H.; Chi, Y.; Chou, P.-T.; Peng, S.-M.; Lee, G.-H. Organometallics
2003, 22, 4938.
(10) (a) Li, J.; Djurovich, P. I.; Alleyne, B. D.; Tsyba, I.; Ho, N. N.; Bau,
R.; Thompson, M. E. Polyhedron 2004, 23, 419. (b) Yang, C.-H.; Li,
S.-W.; Chi, Y.; Cheng, Y.-M.; Yeh, Y.-S.; Chou, P.-T.; Lee, G.-H.;
Wang, C.-H.; Shu, C.-F. Inorg. Chem. 2005, 44, 7770. (c) 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.
(11) Zalis, S.; Farrell, I. R.; Vlcek, A. J. Am. Chem. Soc. 2003, 125, 4580.
(12) 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.
(9) Sajoto, T.; Djurovich, P. I.; Tamayo, A.; Yousufuddin, M.; Bau, R.;
Thompson, M. E.; Holmes, R. J.; Forrest, S. R. Inorg. Chem. 2005,
44, 7992.
(13) Steel, P. J.; Caygill, G. B. J. Organomet. Chem. 1990, 395, 359.
8042 Inorganic Chemistry, Vol. 45, No. 20, 2006

Neutral RuII-Based Emitting Materials
prepared using Ru3(CO)12 and BQ in a mixture of 1,2-dimethoxy-
ethane and octane following literature procedures.14
anhydrous diethylene glycol monoethyl ether (DGME) and then
immersed into an oil bath maintained at ∼130 °C. Freshly sublimed
Me3NO (109 mg, 1.46 mmol) dissolved in 12 mL of DGME was
dropwise added over a period of 5 min, followed by the addition
of PPh2Me (500 µL, 2.63 mmol). The resulting mixture was then
stirred at 160 °C for 24 h. Finally, the solvent was evaporated under
vacuum and the residue placed into 30 mL of CH2Cl2. The solution
was washed with distilled water (150 mL × 2), dried over MgSO4,
and evaporated to dryness. The residue was purified by silica gel
column chromatography using ethyl acetate/hexane (1:5) as the
eluent. Recrystallization was conducted from a mixture of CH2Cl2
and methanol at room temperature, giving yellow crystalline solids
(238 mg, 0.348 mmol) in 60% yield. Complex [Ru(CO)(BQ)2-
(PPhMe2)] (4) was prepared in 80% yield using similar procedures.
Spectral Data for 3. MS (FAB, 102Ru): observed m/z [assign-
Cyclic voltammetry (CV) measurements were performed using
a BAS 100 B/W electrochemical analyzer. The oxidation and
reduction measurements were recorded, respectively, in anhydrous
CH2Cl2 and anhydrous THF solutions containing 0.1 M TBAPF6
as the supporting electrolyte, at a scan rate of 100 mV s-1. The
potentials were measured against an Ag/Ag+ (0.01 M AgNO3)
reference electrode with the ferrocene/ferrocenium couple as the
internal standard.
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 excitation wavelength λexc ) 480 nm in CH2Cl2 at room
temperature. 4-(Dicyanomethylene)-2-methyl-6-[p-(dimethylamino)-
styryl]-4H-pyran (DCM, Φr ) 0.44) in a methanol solution was
used as the reference, and the equation
1
ment] 686 [M+]. IR (CH2Cl2): ν(CO) 1909 (s) cm-1. H NMR
(500 MHz, acetone-d6): δ 9.01 (dd, 1H, J ) 5.0 and 1.0 Hz), 8.52
(dd, 1H, J ) 1.5 and 1.0 Hz), 8.30-8.29 (m, 1H), 7.85 (d, 1H, J
) 7.8 Hz), 7.78-7.67 (m, 5H), 7.56-7.40 (m, 7H), 7.27 (d, 1H, J
) 7.5 Hz) 7.02-6.81 (m, 6H), 6.70 (dd, 1H, J ) 8.0 and 5.5 Hz),
6.64-6.60 (m, 2H), 1.32 (d, 3H, J ) 6.5 Hz). 31P NMR (202 MHz,
acetone-d6): δ 18.35 (s). Anal. Calcd for C40H29N2OPRu: C, 70.06;
H, 4.26; N, 4.09. Found: C, 69.75; H, 4.57; N, 4.01.
ηs2ArIs
Φs ) Φr
2
(
)
ηr AsIr
was used to calculate the emission quantum yields, where Φs and
Φr are the quantum yields of the unknown and reference samples,
η is the refractive index of the solvent, Ar and As are the absorbance
of the reference and the unknown samples at the excitation
wavelength, and Is and Ir are the integrated areas under the emission
spectra of interest, respectively. For the phosphorescence lifetime
measurements in the microsecond region, a third harmonic of an
Nd:YAG laser of 355 nm was used as the excitation source. For
this approach, emission decay was detected with a photomultiplier
tube and averaged over 500 shots using an oscilloscope, and laser
energy was reduced to e 1 mJ pulse-1 to prevent possible photo-
chemical decomposition. For the nanosecond lifetime measure-
ments, the fundamental train of pulses from a Ti-sapphire oscillator
(82 MHz, Spectra Physics) was used to produce second harmonics
(375-425 nm) as an excitation light source. The signal was detected
by a time-correlated single-photon-counting system (Edinburgh OB
900-L).
Spectral Data for 4. MS (FAB, 102Ru): observed m/z [assign-
1
ment] 624 [M+]. IR (CH2Cl2): ν(CO) 1909 (s) cm-1. H NMR
(400 MHz, CD2Cl2): δ 8.42 (dt, 1H, J ) 5.6 and 1.6 Hz), 8.35 (d,
1H, J ) 5.2 Hz), 8.26 (dd, 1H, J ) 6.6 and 1.6 Hz), 7.91 (d, 1H,
J ) 8.0 Hz), 7.82 (d, 1H, J ) 8.8 Hz), 7.00 (d, 1H, J ) 8.8 Hz),
7.64-7.54 (m, 3H), 7.47 (d, 1H, J ) 8.8 Hz), 7.30-7.23 (m, 7H),
7.13 (dt, 1H, J ) 4.8 and 1.6 Hz), 6.98 (td, 1H, J ) 7.4 and 1.2
Hz), 6.95-6.83 (m, 2H), 1.56 (d, 3H, J ) 7.2 Hz), 0.61 (d, 3H, J
) 6.4 Hz). 31P NMR (202 MHz, CDCl3): δ 18.32 (s). Anal. Calcd
for C35H27N2OPRu: C, 67.41; H, 4.36; N, 4.49. Found: C, 67.52;
H, 4.46; N, 4.74.
Preparation of [Ru(CO)(DBQ)2(PPh2Me)] (5) and [Ru(CO)-
(DBQ)2(PPhMe2)] (6). The synthesis procedures were essentially
identical with those described for 3, using similar ratios of 1, freshly
sublimed Me3NO, and the phosphine ligand PPh2Me or PPhMe2.
Dark-red 5 and 6 were obtained from a mixture of CH2Cl2 and
methanol at room temperature. Yield: 68-70%.
Synthesis of [Ru(CO)2(DBQ)2] (2). A mixture of Ru3(CO)12
(111 mg, 017 mmol), dibenzo[f,h]quinoline (250 mg, 1.09 mmol),
1,2-dimethoxyethane (2 mL), and octane (15 mL) was heated at
reflux for 12 h, after which time a dark-brown precipitate had
deposited from the brown solution. The solvent was then removed
using a rotary evaporator, and the residue was treated with a mix-
ture of 1 mL of diethylamine and 3 mL of methanol to remove the
probable side product [Ru4H4(CO)12]. The remaining solid was
collected by filtration and washed with methanol (3 × 2 mL)
and hexane (3 × 3 mL) three times. The expected DBQ complex
2 was obtained as a yellow-brown solid (83 mg, 0.34 mmol,
80%).
Spectral Data of 2. MS (FAB, 102Ru): observed m/z [assign-
ment] 616 [M+]. IR (CH2Cl2): ν(CO) 2015 (s), 1948 (s) cm-1. 1H
NMR (400 MHz, CD2Cl2): δ 9.04 (d, 2H, J ) 7.6 Hz), 8.73 (d,
2H, J ) 8.4 Hz), 8.55 (d, 2H, J ) 7.2 Hz), 8.45 (d, 2H, J ) 8.0
Hz), 8.37 (d, 2H, J ) 2.8 Hz), 7.90-7.81 (m, 4H), 7.77 (d, 2H, J
) 2.8 Hz), 7.70 (t, 2H, J ) 7.6 Hz). Anal. Calcd for C34H18N4O2-
Ru: C, 66.34; H, 2.95; N, 9.10. Found: C, 66.37; H, 3.01; N, 8.95.
Preparation of [Ru(CO)(BQ)2(PPh2Me)] (3). A 50-mL reaction
flask was first charged with 1 (300 mg, 0.58 mmol) and 20 mL of
Spectral Data for 5. MS (FAB,102Ru): observed m/z [assign-
1
ment] 788 [M+]. IR (CH2Cl2): ν(CO) 1928 (s) cm-1. H NMR
(400 MHz, CD2Cl2): δ 9.22 (dd, 1H, J ) 7.8 and 1.6 Hz), 8.91 (d,
1H, J ) 8.0 Hz), 8.77-8.75 (m, 2H), 8.69 (d, 1H, J ) 8.0 Hz),
8.52 (d, 1H, J ) 7.6 Hz), 8.41 (d, 1H, J ) 7.6 Hz), 8.23 (d, 1H,
J ) 8.0 Hz), 7.98 (d, 1H, J ) 8.0 Hz), 7.86-7.44 (m, 11H), 7.10-
7.06 (m, 2H), 6.91-6.90 (m, 1H), 6.76-6.69 (m, 3H), 6.56-6.52
(m, 2H), 1.41 (d, 3H, J ) 7.5 Hz). 31P NMR (202 MHz, CDCl3):
δ 18.26 (s). Anal. Calcd for C46H31N4OPRu: C, 70.13; H, 3.97;
N, 7.11. Found: C, 69.86; H, 3.71; N, 6.41.
Spectral Data for 6. MS (FAB, 102Ru): observed m/z [assign-
1
ment] 726 [M+]. IR (CH2Cl2): ν(CO) 1924 (s) cm-1. H NMR
(500 MHz, CD2Cl2): δ 9.21 (dd, 1H, J ) 7.8 and 1.0 Hz), 8.99
(dd, 1H, J ) 7.8 and 1.0 Hz), 8.86 (d, 1H, J ) 2.4 Hz), 8.81 (d,
1H, J ) 8.3 Hz), 8.70 (d, 1H, J ) 3.0 Hz), 8.60 (d, 1H, J ) 8.5
Hz), 8.45 (d, 1H, J ) 7.0 Hz), 8.32 (d, 1H, J ) 8.0 Hz), 8.25 (d,
1H, J ) 2.5 Hz), 8.00 (d, 1H, J ) 7.5 Hz), 7.86-7.67 (m, 5H),
7.49 (dd, 1H, J ) 2.5 and 1.0 Hz), 7.22-6.89 (m, 7H), 1.76 (d,
3H, J ) 7.5 Hz), 1.03 (d, 3H, J ) 7.5 Hz). 31P NMR (202 MHz,
CD2Cl2): δ 18.22 (s). Anal. Calcd for C41H29N4OPRu: C, 67.85;
H, 4.03; N, 7.72. Found: C, 68.17; H, 4.36; N, 7.94.
(14) (a) Patrick, J. M.; White, A. H.; Bruce, M. I.; Beatson, M. J.; Black,
D. S. C.; Deacon, G. B.; Thomas, N. C. J. Chem. Soc., Dalton Trans.
1983, 2121. (b) Bruce, M. I.; Liddell, M. J.; Pain, G. N. Inorg. Synth.
1989, 26, 171.
X-ray Structural Measurements. Single-crystal X-ray analysis
was measured on a Bruker SMART Apex CCD diffractometer using
Inorganic Chemistry, Vol. 45, No. 20, 2006 8043

Li et al.
Table 1. X-ray Structural Data of Complex 3
the nonhermitian eigenvalue equations were obtained to determine
the vertical excitation energies. Oscillator strengths were deduced
from the dipole transition matrix elements (for singlet states only).
The excited-state TDDFT calculations were carried out using
Gaussian03, as described in our previous publications.19
empirical formula
mol wt
C40H29N2OPRu
685.69
cryst syst
space group
T, K
monoclinic
P21/c
150(1)
a, Å
b, Å
c, Å
9.5823(5)
17.7987(10)
18.2986(10)
96.148(1)
3102.9(3)
4
1.468
1400
0.593
0.30 × 0.25 × 0.20
-11 < h < 12,
-23 < k < 23,
-23 < l < 23
25 478
7127 [R(int) ) 0.0375]
7127/0/407
1.110
3. Results
Synthesis and Characterization. It has been reported that
the direct reaction of BQ with Ru3(CO)12 in a mixture of
1,2-dimethoxyethane and octane yielded a cyclometalated
RuII complex 1, which contains two mutually orthogonal
C,N-chelating BQ ligands (see Scheme 1).14 The X-ray
structural analysis of 1 revealed that the central Ru atom
has approximate octahedral coordination, for which the two
mutually cis CO ligands lie trans to the N atoms of the
cyclometalated BQ ligands and the cyclometalated C atoms
are located at the opposite dispositions.
Moreover, treatment of a more conjugated heteroaromatic
DBQ with Ru3(CO)12 under similar conditions led to the
isolation of a second derivative complex 2. The structure of
2 was readily characterized by NMR analyses, the results of
which revealed several multiplets between δ 9.94 and 7.70
due to the aromatic proton resonances, while its final
structural identification was achieved by the observation of
two sharp IR ν(CO) stretching bands at 2015 and 1948 cm-1,
attributed to the cis-oriented CO ligands. To the best of our
understanding, complex 2 is one of the few known examples
to contain cyclometalated DBQ ligands. Well-known ex-
amples include the recently reported orange-emitting complex
[Ir(DBQ)2(acac)], for which the N atoms in two DBQ ligand
chelates are located at the mutual trans disposition.20 In sharp
contrast, however, RuII complexes 1 and 2 possess a cis
orientation between the two DBQ N atoms.
For preparation of the luminescent complexes (vide infra),
phosphine-substituted RuII derivatives 3-6 were synthesized
in two consecutive steps, employing an excess of decarbo-
nylation reagent Me3NO, followed by addition of the phos-
phine ligand. The resulting mixture was then stirred at
160 °C for 24 h, and the product was isolated by routine
chromatography and recrystallization. These metal complexes
were found to be highly soluble in most organic solvents
such as acetone and CHCl3 and have been characterized using
various spectroscopic methods including FAB MS, IR, and
1H and 31P NMR (see the Experimental Section). Importantly,
these complexes showed only one sharp IR ν(CO) stretching
signal in the range 1928-1909 cm-1 and are in good
agreement with the retention of a single carbonyl ligand. It
is also notable that the amount of Me3NO added in the initial
reaction mixture is sufficient to remove both CO ligands in
the parent complexes 1 and 2; this observation implies that
the second, remaining CO ligand is essentially inert to the
Me3NO reagent.
â, deg
V, Å3
Z
Dc, g cm-3
F(000)
µ(Mo KR), mm-1
cryst size, mm
h, k, l ranges
reflns collected
indep reflns
data/restraints/parameters
GOF on F 2
R1, wR2 with I > 2σ(I)
D map, max/min, e/Å-3
0.0348, 0.0840
0.510/-0.384
Table 2. Selected Bond Distances (Å) and Angles (deg) for Complex 3
Ru-P(1)
Ru-C(2)
Ru-N(1)
C(1)-O(1)
2.3789(6)
2.079(2)
2.208(2)
1.157(3)
Ru-C(1)
Ru-C(15)
Ru-N(2)
1.833(3)
2.041(2)
2.151(2)
P(1)-Ru-C(2)
N(2)-Ru-C(1)
N(2)-Ru-C(15)
176.09(6)
170.34(9)
80.10(8)
N(1)-Ru-C(15)
N(1)-Ru-C(2)
Ru-C(1)-O(1)
165.80(8)
78.58(8)
176.1(2)
µ(Mo KR) radiation (λ ) 0.710 73 Å). The data collection was
executed using the SMART program. Cell refinement and data
reduction were made by the SAINT program. The structure was
determined using the SHELXTL/PC program and refined using full-
matrix least squares. All non-H atoms were refined anisotropically,
whereas H atoms were placed at the calculated positions and
included in the final stage of refinements with fixed parameters.
The crystallographic refinement parameters of 3 are summarized
in Table 1, and the selective bond distances and angles are listed
in Table 2.
Theoretical Approach. Time-dependent density functional
theory (TDDFT)15 calculations using the B3LYP16 functional were
performed based on the structures obtained from single-crystal X-ray
diffraction data. A “double-ú” quality basis set consisting of Hay
and Wadt’s effective core potentials (ECPs; LANL2DZ)17 was
employed for Ru atoms and the 6-31G* basis set18 for H, C, N,
and P atoms. A relativistic ECP replaced the inner-core electrons
of RuII, leaving the outer-core (4s24p6) electrons and the 4d6 valence
electrons. Typically, the lowest 10 triplet and 10 singlet roots of
(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.
Figure 1 depicts the ORTEP diagram of 3, showing octa-
hedral arrangement around the RuII metal center. However,
to our surprise, the BQ ligand orientation is distinctive from
(16) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (b)
Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(17) (a) Hay, P. J.; Wadt, R. W. 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.
(19) 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.
(20) Duan, J.-P.; Sun, P.-P.; Cheng, C.-H. AdV. Mater. 2003, 15, 224.
(18) Hariharan, P. C.; Pople, J. A. Mol. Phys. 1974, 27, 209.
8044 Inorganic Chemistry, Vol. 45, No. 20, 2006

Neutral RuII-Based Emitting Materials
Scheme 1
that of its parent complex 1, namely, one cyclometalated C
atom is shifted to a new position trans to the PPh2Me
phosphine ligand, while the second C atom resides trans to
the N atom of the second BQ ligand. This unique structural
behavior demonstrates that one BQ ligand undergoes a 180°
rotation around the RuII center during the course of phosphine
substitution. A similar coordination arrangement was ob-
served from the X-ray structural determination of a PPh3
derivative prepared using UV irradiation.21
ligand. Apparently, the trans competition through metal-C
σ bonding is more pronounced in this class of complexes,
although the CO ligand was among the best π-acceptor
ligands but has failed to impose a sufficient amount of trans
effect compared to the metal-C σ bond that exerted to the
first BQ ligand. Moreover, the Ru-C distance of the second
BQ ligand [Ru-C(15) ) 2.041(2) Å] is shorter than that of
the other Ru-C bond [Ru-C(2) ) 2.079(2) Å], which is,
in turn, much shorter than those of the mutually trans-oriented
Ru-C bonds of its parent complex 1 [2.12-2.13(1) Å].12
Again, this increase in the Ru-C bond distances could be
attributed to the bonding competition exerted by their trans
ligands, for which the metal-ligand bond strengths follow
the order of, i.e., cyclometalated C atom (C) > phosphine
(P) > pyridine (N).
Electrochemistry. The redox potentials of the RuII com-
plexes were determined from cyclic voltammograms, and
the data are summarized in Table 3. It is believed that the
oxidation occurred mainly at the metal site, with minor
contributions from the cyclometalated chelate and other
ancillary ligands. For 3 and 4 possessing BQ ligands, an
oxidation potential at 0.13 V was acquired in CH2Cl2, while
DBQ complexes 5 and 6 exhibited a higher oxidation
potential at 0.35 V due to the presence of the quinoxaline
fragment, which reduced the electron donation to the metal
atom with the presence of an additional N atom. Moreover,
variation of phosphine has caused almost no change to the
oxidation potential. Finally, the corresponding parents 1 and
2 give an irreversible oxidation half-wave at 0.74 and 0.95
V, which are in good agreement with their electron-deficient
nature induced by the carbonyl ligands.
As for the reduction behavior, the dicarbonyl complexes
1 and 2 give three closely spaced, reversible reduction peaks
in the THF solution, a result of two one-electron reductions
at each of the cyclometalated ligands as well as the possible
reduction at the metal site to give RuI species. It is possible
that this metal reduction is coupled with the [Ru(CO)2]
fragment, allowing an easy delocalization of electron density
Figure 1. ORTEP diagram of 3 with thermal ellipsoids shown at the 50%
probability level.
For the metric parameters, in 3, the Ru-N(1) distance
[2.208(2) Å] of the first BQ ligand is obviously longer than
the respective Ru-N(2) distance [2.151(2) Å] and the Ru-N
distances observed in 1 (2.148 and 2.161 Å), showing a large
labilization effect attributed to its trans Ru-C(15) σ bond
versus that of the trans CO ligand exerted to the second BQ
(21) Zhang, Q.-F.; Cheung, K.-M.; Williams, I. D.; Leung, W.-H. Eur. J.
Inorg. Chem. 2005, 4780.
Inorganic Chemistry, Vol. 45, No. 20, 2006 8045

Li et al.
Table 3. Photophysical and Electrochemical Properties for Complexes 1-6 in Degassed CH2Cl2 at Room Temperature
ox
1/2
red
λmabasx/nm (ꢀ × 10-3
320 (7.2), 381 (3.8),
395 (4.2)
)
λemmax/nm
Φ
k
obs (s-1
)
kr (s-1
)
E
E
1/2
1
2
3
4
5
6
-[485, 521, 563,
-[4.1 × 102]a
0.74 [irr]
0.95 [irr]
0.13
-2.41, -2.64, -2.77
-1.91, -2.11, -2.34
-2.57, -2.92
617 (sh)]a
332 (14), 376 (7),
421 (3.8)
562 [526]a
∼5.0 × 10-4
0.24
3.7 × 106 [7.9 × 103]a
∼1.9 × 103
3.3 × 104
2.1 × 104
3.3 × 104
2.9 × 104
310 (14), 370 (7.4),
456 (2.4)
571 [553]a (539)b
575 [556]a (542)b
655 [632]a(642)b
656 [633]a (645)b
1.4 × 105 [3.8 × 103]a
(2.5 × 107)b
312 (18), 378 (9.2),
458 (3)
0.18
1.2 × 105 [5.5 × 103]a
(2.0 × 107)b
0.13
-2.59, -2.86
344 (13), 359 (12),
387 (8), 506 (1.4)
0.008
4.2 × 106 [1.7 × 104]a
0.35
-2.07, -2.32
(1.4 × 107)b
343 (11), 362 (16),
387 (11), 513 (1.8)
0.004
7.1 × 106 [1.8 × 104]a
0.35
-2.09, -2.32
a Data in square brackets are measured in a CH2Cl2 matrix at 77 K, and “sh” denotes “shoulder”. b Data in parentheses are measured in a thin solid film
at room temperature.
to the CO ligands. However, no further attempt was made
to support this hypothesis. On the other hand, the phosphine-
substituted complexes 3-6 exhibit only two reversible
reduction signals between the narrow ranges of -2.57 to
-2.92 and -2.07 to -2.32 V for the pair of BQ and DBQ
complexes, respectively. This finding led us to propose that
the observed reduction was strongly associated with both
BQ and DBQ groups, while the ancillary phosphine ligand
has very little influence on the reduction potential.
Photophysical Measurements. The photophysical proper-
ties of these RuII complexes can be systematically varied by
modifying the cyclometalated ligand chromophores and the
ancillary ligands; the respective data are listed in Table 3. It
is notable that the lowest absorption of their parent BQ and
DBQ complexes 1 and 2 appear at λmax ∼395 and 420 nm,
respectively, which are tentatively assigned to the ligand-
centered ππ*, mixed with small amounts of MLCT transi-
tions. The slightly higher extinction coefficient and occur-
rence of multiple peak maxima in the higher energy region
for the DBQ complex 2 are obviously caused by the extended
π conjugation of the DBQ ligands.
Figure 2. UV-vis absorption and emission spectra of complexes 1-6 in
a CH2Cl2 solution, for which the emissions of 1 and 2 were taken in a 77
K matrix, while those of 3-6 were recorded at room temperature.
the slightly electron-deficient RuII metal core in 5. Further
support for these assignments is given in the Discussion
section.
Moreover, the respective PPh2Me- and PPhMe2-substituted
BQ complexes showed the occurrence of red-shifted, less
intense transitions at λmax 456 nm for 3 and 458 nm for 4,
which are tentatively assigned to the transition incorporat-
ing a state mixing among singlet and triplet metal-ligand
charge transfer (1MLCT and 3MLCT) and, to a certain extent,
Although they have the largest S0 f S1 absorption gap
among complexes 1-6, to our surprise, both 1 (Φ ∼ 0) and
2 (Φ e 5.0 × 10-4) are nearly nonemissive in a room-
temperature CH2Cl2 solution, as well as showing significant
temperature-dependent emissive behavior. For example, upon
cooling of the solution from 298 to 203 K, the emission yield
gradually increased from null to 8 × 10-4 for 1. The emission
possesses a well-resolved vibronic progression with a 0-0
transition peak at ∼ 485 nm. Both 1 and 2 revealed decent
emission signals in the 77 K solid CH2Cl2 matrix, and data
are depicted in Figure 2 and Table 3.
In sharp contrast to their parent complexes, i.e., 1 and 2,
complexes 3-6 showed strong to weak luminescence in
a room-temperature, degassed CH2Cl2 solution with peak
wavelengths located at 571, 575, 655, and 656 nm, respec-
tively (see Figure 2). Owing to the rapid quenching of
luminescence in the aerated solution for, e.g., complex 3 (not
shown here), the assignment of the emission mainly origi-
nating from the triplet manifold is unambiguous. Moreover,
for complexes 3-6, the partial overlap between the emission
3
the intraligand ππ transitions. The close energetics and
1
3
absorptivity between MLCT and MLCT bands suggest
3
that the MLCT transition, induced by the spin-orbit cou-
pling and the proximal energy levels with respect to 1MLCT,
is greatly enhanced and becomes partially allowed.22
Following the same principle, the MLCT transitions of both
complexes 5 (506 nm) and 6 (513 nm) can be assigned, for
which the more electron-rich PPhMe2-substituted 6 has
induced a slightly further bathochromic shift compared with
(22) (a) 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. (b) Chang, S.-Y.; Kavitha, J.; Li,
S.-W.; Hsu, C.-S.; Chi, Y.; Yeh, Y.-S.; Chou, P.-T.; Lee, G.-H.; Carty,
A. J.; Tao, Y.-T.; Chien, C.-H. Inorg. Chem. 2006, 45, 137.
8046 Inorganic Chemistry, Vol. 45, No. 20, 2006

Neutral RuII-Based Emitting Materials
Figure 3. Selected frontier orbitals of 1 involved in the lower-lying transitions.
onset and the lowest-energy absorption bands, in combination
with a broad, structureless spectral profile, leads us to
conclude that the luminescence originates primarily from the
3MLCT state.23 In comparison to the BQ complex 3, its DBQ
to 3, possessing PPh2Me, a very slightly bathochromic shift
of only ∼4 nm was observed for the PPhMe2-substituted
derivative 4. A similar small red shift was resolved between
5 (PPh2Me) and 6 (PPhMe2; see Table 3). This is apparently
caused by the decrease of the π-accepting strength of
PPhMe2 versus the PPh2Me ligand. However, the variation
seems significantly lower than those expected based on the
electronic properties of phosphine ligands.25 One possible
explanation is that the apparent variation viewed from peak
maxima is much reduced because of the broad, structureless
spectral profiles. It is also noteworthy that the phosphores-
cence quantum yields for 5 (8.0 × 10-3) and 6 (4.0 × 10-3)
counterpart 5 exhibits a ∼84-nm bathochromic shift in λmax
,
the result of which can qualitatively be rationalized by an
increase of π conjugation and the incorporation of a C4N2
hexagon for the DBQ heterocyclic ligands.24 In comparison
(23) Vlcek, A., Jr. Coord. Chem. ReV. 1998, 177, 219.
(24) Song, Y.-H.; Yeh, S.-J.; Chen, C.-T.; Chi, Y.; Liu, C.-S.; Yu, J.-K.;
Hu, Y.-H.; Chou, P.-T.; Peng, S.-M.; Lee, G.-H. AdV. Funct. Mater.
2004, 14, 1221.
Inorganic Chemistry, Vol. 45, No. 20, 2006 8047

Li et al.
Table 4. Calculated Energy Levels of the Lower-Lying Transitions of 1
in CH2Cl2 are significantly lower than those of yellow-
emitting complexes 3 (0.24) and 4 (0.18) (see Table 2). For
3-6, the difference could be qualitatively rationalized using
an empirical energy gap law.4 Upon a decrease in the energy
gap, rapid quenching may take place via the T1-S0 inter-
system crossing through coupling of certain high-frequency
vibration motions and subsequent solvent collisional deac-
tivation in the fluid solution. In sharp contrast, although the
T1-S0 energy gap for both 1 and 2 is larger than that of
3-6, the corresponding phosphorescence is much weaker.
Certainly, the results for 1 and 2 are opposite to the empirical
energy gap law and must be associated with other mech-
anisms of deactivation. Details regarding types of specific
modes involved in the radiationless deactivation are elabo-
rated in the Discussion section.
assignment
[nm]
E [eV]
f
T1
HOMO f LUMO (+33%)
HOMO-1 f LUMO (+20%)
HOMO f LUMO+2 (+9%)
HOMO-3 f LUMO (9%)
HOMO-1 f LUMO+2 (+7%)
HOMO f LUMO+1 (7%)
HOMO-4 f LUMO (6%)
HOMO-1f LUMO+1 (6%)
471.4
2.63
∼ 0
T2
HOMO f LUMO+1 (+15%)
HOMO-1 f LUMO+1 (13%)
HOMO-3 f LUMO+1 (12%)
HOMO-1 f LUMO (10%)
HOMO-1 f LUMO+3 (8%)
HOMO f LUMO (+7%)
452.8
2.74
∼0
HOMO-4 f LUMO+1 (+7%)
HOMO f LUMO+3 (+6%)
HOMO-2 f LUMO+1 (6%)
4. Discussion
Singlet States
S1
S2
HOMO f LUMO (+75%)
HOMO-1 f LUMO (13%)
HOMO-2 f LUMO (+8%)
391.4
3.17
3.25
0.0083
0.0149
In light of the search for blue phosphorescent materials
for fabrication of OLEDs, tuning of the emission color
becomes an urgent task, the feasibility of which may be made
by systematic variation of the ligand chromophores as well
as the ancillary ligands. However, a fundamental question
is promptly raised regarding the inferior factors that might
induce the rapid radiationless decay processes, giving rise
to low quantum efficiency for most of the authentic blue
phosphors documented in the literature.8-10 Apparently, the
room-temperature nonemissive complex 1 (with a 485-nm
emission peak at 77 K) falls in this category. Following the
HOMO-1 f LUMO (+57%
HOMO f LUMO+1 (+17%)
HOMO f LUMO (+11%)
HOMO-2 f LUMO (6%)
382.0
Giving another example of the PPh2Me-substituted com-
plex 3 (see Figure 4 and Table 5), the lowest singlet state
(S1) with an energy gap of 457 nm consists of a highest
occupied molecular orbital (HOMO) f lowest unoccupied
molecular orbital (LUMO) transition, while the lowest triplet
state (T1), with a lower gap of 507 nm, mainly involves
HOMO f LUMO+1 transition, for which the electron
densities of the HOMO are located on the C6 hexagon of
one BQ ligand trans to the CO ligand and the central Ru
atom, whereas those of the LUMO+1 are primarily distrib-
uted on the same BQ ligand. The calculated lowest excited
singlet and triplet states at 457 and 507 nm, respectively,
are also qualitatively in agreement with those (the absorption
peak maximum at 456 nm and the phosphorescence onset
of ∼510 nm) observed experimentally. We believe that the
small deviation of theoretical prediction from experimental
results is mainly due to its limitation in predicting the min-
ute nuclear motion during the electronic transitions as well
as the negligence of the solvation energy in the current
theoretical approach. Qualitative consistency between theo-
retical approaches and experimental results is also seen in
the rest of the metal complexes, for which the theoretical
results of the DBQ complex 5 are revealed in Figure 5 and
Table 6. As for a general trend, the S0-S1 and S0-T1 energy
gaps for complexes filled with dual ancillary CO ligands are
much larger than those anchored by one CO and one
phosphine, showing the expected hypsochromic shift imposed
by the stronger π-accepting CO ligands.
3
3
initial population to the low-lying MLCT or ππ* excited
states, one widely accepted radiationless deactivation channel
may be ascribed to their rapid crossing to the adjacent or
3
even lower metal-centered dd state. Because of its anti-
bonding character, the potential energy surface of the dd state,
theoretically, is expected to be shallow, and it may intercept
with the potential energy surface associated with the ground
state and, in an extreme case, may even undergo bond
dissociation. The net results should cause rapid energy
dissipation through metal-ligand bond stretching. This, in
3
combination with the forbidden nature of the S0 f dd
3
transition, further signifies the importance of the dd state
in manipulating the radiationless pathways.
To gain detailed insights into this fundamental issue,
theoretical approaches (TDDFT, see the Experimental Sec-
tion) on the photophysical properties were performed for the
RuII complexes studied. Figure 3 depicts the features of the
selected occupied and unoccupied frontier orbitals mainly
involved in the lower-lying transitions for complex 1, while
the descriptions and energy gaps of each transition are listed
in Table 4. As shown in Table 4, the lowest singlet S0 f S1
transition of 391 nm is in good agreement with the ππ*
transition of 395 nm obtained from its absorption spectra.
Likewise, the estimated S0 f T1 transition occurring at 471
nm is consistent with the resolved 485 nm emission in the
77 K CH2Cl2 matrix.
Theoretical results provide valuable clues to the explora-
tion of the correlation between each electronic transition and
its corresponding frontier orbital contribution. A similar trend
was also resolved for these complexes, in which the S0 f
T1 transition is dominated by either RuII f BQ (for 1, 3,
and 4) or RuII f DBQ (for 2, 5, and 6) MLCT in combin-
ation with ππ* transitions within BQ or DBQ ligands. This
(25) (a) Vogler, A.; Kunkely, H. Coord. Chem. ReV. 2002, 230, 243. (b)
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. (c) Tsubaki, H.; Tohyama, S.;
Koike, K.; Saitoh, H.; Ishitani, O. Dalton Trans. 2005, 385.
8048 Inorganic Chemistry, Vol. 45, No. 20, 2006

Neutral RuII-Based Emitting Materials
Figure 4. Selected frontier orbitals of 3 involved in the lower-lying transitions.
Table 5. Calculated Energy Levels of the Lower-Lying Transitions of 3
3
3
incorporating the dπdσ* (or πdσ*) state being responsible
for the lack of room-temperature emission for dicarbonyl
complexes 1 and 2.
assignment
[nm]
506.9
483.6
E [eV]
2.45
f
T1
T2
HOMO f LUMO+1 (+88%)
∼0
∼0
On the other hand, the typical lowest-energy transition in
the triplet manifold involves a great extent of ππ* and MLCT
mixing. Because MLCT incorporates a dπ f π* transition,
its key role in enhancing the spin-orbit coupling is obvious;
namely, the more 3MLCT contribution, the greater the spin-
orbit coupling, and hence the faster radiative rate of the
3MLCT f S0 transition.10,27 We thus carefully examined the
percentage of 3MLCT contribution to the T1 state. As shown
in Table 7, it appears to us that the percentages of MLCT
contribution in the T1 state of 1 (10.2%) and 2 (12.4%) are
significantly lower than those of 3-6 (> 40%). The results
can be rationalized by the greater π-accepting properties of
CO than those of PPh2Me and PPhMe2 ligands, such that
complexes 1 and 2 with dual CO ligands render a much
reduced electron density in the dπ orbital, giving a lesser
amount of the MLCT contribution.
Further support of the above viewpoint is given by two
additional observations. First, from the steady-state approach,
the observation of vibronic progression of phosphorescence
for 1 in 77 K CH2Cl2 provides indirect evidence that
phosphorescence should contain an excess of ππ* character.
Conversely, 3-6 showed a broad, diffusive phosphorescence
even at 77 K, strongly indicating their dominance of the
3MLCT character. Moreover, as for the time-resolved meas-
HOMO-1 f LUMO (+66%)
HOMO-2 f LUMO (+15%)
HOMO-4 f LUMO (+8%)
HOMO-1 f LUMO+3 (+7%)
2.56
Singlet States
S1
S2
HOMO f LUMO (+96%)
457.3
441.9
2.71
2.81
0.0002
0.0217
HOMO f LUMO+1 (+89%)
characteristic is somewhat similar to those of Ir(ppy)3 and
its derivatives, which were also attributed to a 3ππ* manifold,
which, to a great extent, was mixed with the 3MLCT
character.26 Upon careful examination of the associated
frontier orbitals involved in each transition of our complexes,
to our surprise, the calculated lowest-lying T1 state and even
the S0 f T2 or higher energy transitions have no contribution
from the metal-centered dπdσ* and/or ligand-to-metal πdσ*
states. Instead, the unoccupied orbitals with dσ* character are
ascribed to LUMO+4 for 1 (Figure 3) and even higher for
other complexes. For the case of 1, the dπdσ* state is higher
in energy than the T1 state by 1.5 eV. This gap is so large
that, even under the excited-state geometry relaxation, a
chance of the intersection of the potential energy surface
between dπdσ* and T1 states seems to be rather slim and the
associated dπdσ* triplet state is strictly thermally inaccessible.
These results simply discard a generally accepted mechanism
(27) Yutaka, T.; Obara, S.; Ogawa, S.; Nozaki, K.; Ikeda, N.; Ohno, T.;
(26) Hay, P. J. J. Phys. Chem. A 2002, 106, 1634.
Ishii, Y.; Sakai, K.; Haga, M. Inorg. Chem. 2005, 44, 4737.
Inorganic Chemistry, Vol. 45, No. 20, 2006 8049

Li et al.
quantum yield and hence the emission intensity. Experimen-
tally, this is apparently true and can qualitatively explain
the nonemissive (weak-emissive) property for 1 and 2.
Nevertheless, from the quantitative viewpoint, the deduced
radiationless decay rates of 1 [g 4.2 × 106 s-1, assuming a
detection limit of Φ ∼ 10-4 in our current system and 2400
µs (at 77 K) for the radiative decay time] and 2 (∼ 3.7 ×
106 s-1) are larger than those deduced from 3 (1.04 × 105
s-1) and 4 (9.6 × 104 s-1), which possess the same BQ and
DBQ ligands. For rationalization, our first attempt is the
weakening of the Ru-CO bonds upon excitation if the net
result of MLCT is to induce an electron transfer to the BQ
or DBQ chelates trans to the CO ligands. However, thorough
frontier orbital analysis clearly indicates that the Ru-CO
back-π-bonding character is present only in the very stable
occupied molecular orbital of HOMO-5 (Figure 3) and even
lower ones (not shown here). Thus, it is very unlikely that
the strength of such a strong Ru-CO bond, upon excitation,
could be drastically weakened; consequently, the associated
Ru-CO stretching modes should not act as a main radia-
tionless deactivation pathway. In fact, strong to moderate
emission has been reported in numerous carbonyl-containing
third-row transition-metal complexes, in which MLCT
transition occurs at the ligand chromophore trans to the
ancillary CO ligands.28
Figure 5. Selected frontier orbitals of 5 involved in the lower-lying
Alternatively, in our opinion, it is more plausible to
correlate the rapid radiationless deactivation in 1 and 2 with
respect to the increase of the vibrational modes channeling
into the radiationless deactivation pathways. As depicted in
Tables 4-6, it is obvious that, despite a very simple
contribution, i.e., HOMO f LUMO+1, to the T1 state in 3
and 5, a much more complicated frontier orbital contribution
was observed in 1, incorporating HOMO to HOMO-4 and
LUMO to LUMO+2 (see Table 4). Careful analyses
indicated that the associated frontier orbitals spread out to
both BQ ligands simultaneously rather than to only one BQ
site trans to the unique CO ligand for 3 and 4. Multiple
contributions were observed for T1 in complex 2 (see the
Supporting Information). The results can be tentatively
rationalized by the two BQ and DBQ chelate ligands being
subjected to identical coordinating environments in both 1
and 2. In contrast, a simplified T1 state was also noted in 5
and 6. The radiationless decay rates were deduced to be
4.12 × 106 and 7.12 × 106 s-1, both more than 1 order of
magnitude larger than those of 3 and 4, and can be
rationalized by the greater degree of vibrational freedom in
combination with radiationless transition governed by the
empirical energy gap law.4 Accordingly, despite the same
order of magnitude in radiative decay time with respect to 3
and 4, greatly inferior phosphorescence quantum yields were
observed for 5 and 6.
transitions.
Table 6. Calculated Energy Levels of the Lower-Lying Transitions of 5
assignment
[nm]
539.9
507.6
E [eV]
2.30
f
T1
T2
HOMO f LUMO+1 (+98%)
∼0
∼0
HOMO f LUMO (+88%)
HOMO-1 f LUMO (7%)
2.44
Singlet States
S1
S2
HOMO f LUMO (+96%)
505.0
485.4
2.46
2.55
0.0002
0.0166
HOMO f LUMO+1 (+90%)
Table 7. Percentage of the MLCT State Contributing to the Lowest
Electronic Transition in Singlet and Triplet Manifolds
complex
state
S1
1
2
3
4
5
6
11.0
10.1
13.1
12.4
44.1
40.5
45.4
46.4
45.3
46.2
43.9
48.3
T1
urement, although the radiative decay time of 1 could not
be deduced because of the nearly nonemissive properties at
ambient temperature, the decay time of 2400 µs measured
at 77 K is far longer than the 30-50 µs deduced for
complexes 3-6. A rather similar long radiative decay time
of 540 µs was also deduced for complex 2. For 1 and 2,
according to the exceedingly longer radiative decay time,
its dominant 3ππ character in the T1 state is obvious,
consistent with the theoretical approach. The emission
quantum yield Φ is defined as Φ ) kr/kobs ) kr/(kr + knr), in
which subscripts r, nr, and obs denote radiative, nonradiative,
and observed decay components, respectively. Thus, for
complexes possessing a small kr value, such as 1 and 2, the
knr value should quite effectively influence the emission
As for the thermal activation on the nonradiative deactiva-
tion, we have performed a temperature-dependent study on
(28) (a) Van Slageren, J.; Stufkens, D. J. Inorg. Chem. 2001, 40, 277. (b)
Yam, V. W.-W. Chem. Commun. 2001, 789. (c) Cheng, Y.-M.; Yeh,
Y.-S.; Ho, M.-L.; Chou, P.-T.; Chen, P.-S.; Chi, Y. Inorg. Chem. 2005,
44, 4594. (d) Chen, Y.-L.; Lee, S.-W.; Chi, Y.; Hwang, K.-C.; Kumar,
S. B.; Hu, Y.-H.; Cheng, Y.-M.; Chou, P.-T.; Peng, S.-M.; Lee, G.-
H.; Yeh, S.-J.; Chen, C.-T. Inorg. Chem. 2005, 44, 4287.
8050 Inorganic Chemistry, Vol. 45, No. 20, 2006

Neutral RuII-Based Emitting Materials
5. Conclusion
In conclusion, a series of new RuII complexes bearing
cyclometalated BQ and DBQ ligands have been designed
and synthesized with an aim to investigate the associated
photoluminescence properties. Our results solidify the cor-
relation between MLCT and radiative lifetime and hence the
efficiency of radiationless deactivation. Complexes 1 and 2,
3
possessing much less MLCT contribution because of the
electron deficiency in the dπ orbital, render a great extent of
the 3ππ* character for T1 with an exceedingly long radiative
lifetime. This, in combination with perhaps the increase of
the deactivating modes, gives rise to an essentially nonemis-
sive (very weak emission) property. Under the premise of
this proposed mechanism, one may be able to analyze the
lower lying electronic transitions and the corresponding
frontier orbital analyses, providing a guideline to predict not
only the phosphorescence properties such as peak wavelength
and transition properties but also the quantum efficiency in
a qualitative manner. We thus believe that the results
presented here may turn out to be of great importance in the
design of luminescent materials incorporating both second-
and third-row transition-metal elements.
Figure 6. Plot of the logarithm of knr(T) vs 1/T in the range of 220-150
K (see the text for the definition of knr).
complex 1. Because of its lack of emission at ambient
temperature, the experiment was performed in a temperature
range of 220-150 K. As a result, the plot of the logarithm
of knr(T) vs 1/T, in which knr(T) ∼ kobs - kr, rendered a
sufficiently straight line (see Figure 6). As a result, a ∆Ea
value of 2.78 kcal mol-1 and a preexponential factor of
1.7 × 108 s-1 were deduced. The results indicate that the
nonradiative decay rate is dominated by rather low frequency
motions, possibly involving the geometry distortion motions
coupled with the T1-S0 intersystem crossing. Although
specific modes inducing radiationless transition, at this stage,
are pending resolution, the overlap of multiple frontier
orbitals in the case of 1 (vide supra) should statistically
increase the number of active vibrational modes involved in
the radiationless deactivation.
Acknowledgment. The authors are grateful for financial
support from the National Center for High-Performance
Computing, The Ministry of Economy, and the National Sci-
ence Council of Taiwan. We also thank Prof. Ching-Fong
Shu for helpful advice on the CV measurements.
Supporting Information Available: X-ray crystallographic data
file (CIF) of complex 3 and the calculated energy levels and
associated frontier orbitals of the DFT calculation on complexes
1-6. This material is available free of charge via the Internet at
http://pubs.acs.org.
IC060066G
Inorganic Chemistry, Vol. 45, No. 20, 2006 8051