Synthesis and characterization of cyclometalated iridium(III) complexes containing benzoxazole derivatives and different ancillary ligands

Article abstract of DOI:10.1016/j.jorganchem.2008.06.034

The synthesis, structures, electrochemistry, and photophysics of a series of cyclometalated iridium(III) complexes based on benzoxazole derivatives and different β-diketonate ligands are reported. These complexes have a general formula C^∧N₂Ir(LL′) [where C^∧N is a monoanionic cyclometalating ligand; 2-phenylbenzoxazolato (pbo), 2-(4-chlorophenyl)benzoxazolato (cpbo), 2-phenyl-5-chlorobenzoxazolato (pcbo), 2-(3,5-difluorophenyl)benzoxazole (fpbo), or 2-(2-naphthyl)benzoxazolato (nbo), and LL′ is an ancillary ligand; acetylacetonate (acac), dibenzoylmethanate (dbm), or 1,1,1,5,5,5-hexafluoroacetylacetonate (hfacac)]. The complexes (pcbo)₂Ir(acac) (3), (dfpbo)₂Ir(acac) (4), (cpbo)₂Ir(dbm) (7), (dfpbo)₂Ir(dbm) (8), and (dfpbo)₂Ir(hfacac) (9) have been structurally characterized by X-ray crystallography. All of the complexes show reversible oxidation between 0.45 and 1.07 V, versus Fc/Fc⁺, and have short luminescence lifetime (τ = 0.1-1.3 μs) at room temperature. Except complex 9, the radiative decay rate (k_r) and nonradiative decay rate (k_nr) of the (C^∧N)₂Ir(LL′) complexes have been determined by using the lifetime and quantum efficiency. The k_r ranges between 2.0 × 10³ and 3.0 × 10⁵ s^-1 and k_nr spans a narrower range of values (5.0 × 10⁵ to 7.0 × 10⁶ s^-1).

Full text of DOI:10.1016/j.jorganchem.2008.06.034

Journal of Organometallic Chemistry 693 (2008) 3117–3130
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and characterization of cyclometalated iridium(III) complexes
containing benzoxazole derivatives and different ancillary ligands
*
Tsun-Ren Chen
Department of Applied Chemistry and Life Science, National Pingtung University of Education, Pingtung 90003, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 June 2008
Received in revised form 28 June 2008
Accepted 28 June 2008
Available online 4 July 2008
The synthesis, structures, electrochemistry, and photophysics of a series of cyclometalated iridium(III)
complexes based on benzoxazole derivatives and di^fferent b-diketonate ligands are reported. These com-
plexes have a general formula C^N2Ir(LL0) [where C N is a monoanionic cyclometalating ligand; 2-phen-
ylbenzoxazolato (pbo), 2-(4-chlorophenyl)benzoxazolato (cpbo), 2-phenyl-5-chlorobenzoxazolato
(pcbo), 2-(3,5-difluorophenyl)benzoxazole (fpbo), or 2-(2-naphthyl)benzoxazolato (nbo), and LL0 is an
ancillary ligand; acetylacetonate (acac), dibenzoylmethanate (dbm), or 1,1,1,5,5,5-hexafluoroacetylaceto-
nate (hfacac)]. The complexes (pcbo)2Ir(acac) (3), (dfpbo)2Ir(acac) (4), (cpbo)2Ir(dbm) (7),
(dfpbo)2Ir(dbm) (8), and (dfpbo)2Ir(hfacac) (9) have been structurally characterized by X-ray crystallog-
raphy. All of the complexes show reversible oxidation between 0.45 and 1.07 V, versus Fc/Fc+, and have
Keywords:
Benzoxazole derivatives
Iridium complex
Self-organized
short luminescence lifetime (s = 0.1–1.3 ls) at room temperature. Except complex 9, the radiative decay
Ancillary ligand
rate (kr) and nonradiative decay rate (knr) of the (C^N)2Ir(LL0) complexes have been determined by using
the lifetime and quantum efficiency. The kr ranges between 2.0 ꢀ 103 and 3.0 ꢀ 105 sꢁ1 and knr spans a
narrower range of values (5.0 ꢀ 105 to 7.0 ꢀ 106 sꢁ1).
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
investigate the effect of benzoxazole derivatives on the electro-
chemical and photophysical properties of iridium complexes, sev-
Recently, iridium complexes have been studied extensively be-
cause of their potential applications. For example, these complexes
can be used as emissive dopants in organic light emitting devices
(OLEDs) [1,2], catalysts for chemical reactions [3,4], photocatalysts
forCO2 reduction,photooxidants, andbiologicalreagents[5]. Several
research groups have reported that the heavy metal, iridium, in the
complex can induce the intersystem crossing (ISC) by strong spin–
orbitcoupling, whichleadstothemixing ofthesingletandtripletex-
cited states, and results in high phosphorescence efficiencies [6,7].
Benzoxazole and their derivatives have been comprehensively
studied in recent years because of their high quantum yields [8],
high non-linear optical effectiveness, biological activities [9,10],
photo-stable properties [11], and capabilities to ligate transition
metals [12]; hence, the benzoxazole compounds and their metallic
complexes have been widely applied for use in chemistry and
medicine [13]. In addition, the substituent in position 2 of the
benzoxazole ring may be quite easily changed to modify the photo-
physical properties of the compound. For example, an electron-
donor substituent on the phenyl ring of 2-phenylbenzoxazole can
shift the absorption and emission spectra to the longer wave-
lengths, and an electron-withdrawing group can shift the photo-
physical spectra to the shorter wavelengths [9]. Therefore, to
eral kinds of benzoxazole derivatives were synthesized in this
paper and used as cyclometalating ligands (abbreviated as C^N)
to prepare a series of cyclometalated iridium(III) complexes, 1–9
(see Chart 1). Because benzoxazole derivatives possess high extinc-
tion coefficients and exhibit high fluorescence quantum yields
[14,15], metal complexes containing benzoxazole derivatives as
chelating ligands are expected to exhibit special properties and
high quantum effici^encies. The complexes reported here have a
general formula (C N)2Ir(LL0), in which the LL0 represents an
ancillary ligand, and three kinds of b-diketonate ligands (acetylace-
tonate (acac), dibenzoylmethanate (dbm), and 1,1,1,5,5,5-hexa-
fluoroacetylacetonate (hfacac), respectively) are used as the
ancillary ligands to investigate the effect on the character of the cor-
responding iridium complexes. The photophysical properties of the
complexes reported here in acetonitrile and dichloromethane were
also studied in order to find out the relationships between structural
properties and the spectroscopic properties, which can enable us to
design more rational probes possessing required properties [9].
2. Results and discussion
2.1. Synthesis of materials
Scheme 1 shows the synthesis of the ligands and the iridium
complexes. By Philips0 condensation, a series of benzoxazole
* Tel.: +886 8 7226141 800.
E-mail address: trchen@mail.npue.edu.tw
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.06.034

3118
T.-R. Chen / Journal of Organometallic Chemistry 693 (2008) 3117–3130
Cl
O
N
O
O
O
N
O
N
O
O
O
O
Ir
O
N
O
O
Ir
Ir
O
N
O
O
Ir
Ir
F
F
Cl
2
2
2
2
2
4
5
3
1
2
F
F
F
O
N
O
O
O
N
F
F
Ir
Ir
O
N
O
N
O
O
O
O
O
Ir
Ir
O
F
F
F
F
F
Cl
2
2
2
2
6
7
8
9
Chart 1. Molecular structures of complexes 1–9.
R3
R3
R1
IrCl3. nH2O
Y
HO
H2N
Y
O
N
O
PPA
+
R2
R2
Δ
OH
H2O / 2-ethoxyethanol
R1
S2
S1
L1~L5
Y
Y
Y
O
O
R'
Cl
N
O
O
N
O
N
O
R'
R'
Ir
Ir
Ir
O
K2CO3 / 2-ethoxyethanol
Cl
R3
R1
R1
R3
R3
R1
R'
R2
R2
R2
2
2
2
D1~D5
1~9
L 1 and D1: R1 = R2 = R3 = Y = H
L 2 and D2: R1 = R3 = Y = H, R2 = Cl
L 4 and D4: R2 = Y = H, R1 = R3 = F
L 3 and D3: R1 = R2 = R3 = H, Y = Cl
L 5 and D5: R1 = Y = H, R2-R3 =
1: R1 = R2 = R3 = Y = H, R' = CH3
2: R1 = R3 = Y = H, R2 = Cl, R' = CH3
4: R2 = Y = H, R1= R3 = F, R' = CH3
6: R1 = R3 = Y = H, R2 = Cl, R' = C6H5
3: R1 = R3 = R2 = H, Y = Cl, R' = CH3
5: R1 = Y = H, R2-R3 =
, R' = CH3
7: R2 = Y = H, R1= R3 = F, R' = C6H5
9: R2 = Y = H, R1= R3 = F, R' = CF3
8: R1 = Y = H, R2-R3 =
,R' = C6H5
Scheme 1. Synthesis of iridium complexes.
derivative ligands including 2-Phenylbenzoxazole (pbo) (L1), 2-(4-
Chlorophenyl)benzoxazole (cpbo) (L2), 2-Phenyl-5-chlorobenzoxa-
zole (pcbo) (L3), 2-(3,5-difluorophenyl)benzoxazole (fpbo) (L4),
and 2-(2-Naphthyl)benzoxazole (nbo) (L5) were obtained and
characterized. The cyclometalated Ir(III) chloro-bridged dimers
(pcbo)2Ir(
and (nbo)2Ir(
l
-Cl)2Ir(pcbo)2 (D3), (dfpbo)2Ir(
l
-Cl)2Ir(dfpbo)2 (D4),
l
-Cl)2Ir(nbo)2 (D5) were isolated and characterized
by 1H NMR, 13C NMR, FAB-MS spectrometry, and elemental analy-
ses. The desired iridium complexes (1–9) were synthesized by the
reaction of cyclometalated Ir(III) chloro-bridged dimers with an
appropriate ancillary ligand, acetylacetonate (acac), dibenzoyl-
methanate (dbm), or 1,1,1,5,5,5-hexafluoroacetylacetonate (hfa-
cac); all of the iridium complexes were purified and identified by
1H NMR, 13C NMR, elemental analyses, and FAB-MS spectrometry;
having a general formula (C^N)2Ir( -Cl2)Ir(C^N)2 (D1–D5) were
l
synthesized by the method reported by Nonoyama [1]. Except for
the D1 ((pbo)2Ir( -Cl2)Ir(pbo)2), too unstable to be purified, the
other chloro-bridged dimers (cpbo)2Ir( -Cl2)Ir(cpbo)2 (D2),
l
l

T.-R. Chen / Journal of Organometallic Chemistry 693 (2008) 3117–3130
3119
and further, complexes 3, 4, and 7–9 were characterized by a sin-
gle-crystal X-ray analysis.
ORTEP diagrams in Fig. 1. All the crystals were obtained from a
mixed solution of dichloromethane and n-hexane. Complexes 3,
4, 7, 8, and 9 belong to the monoclinic space group C2/c, the mono-
ꢀ
2.2. Structures of 3, 4, 7, 8, and 9
clinic space group P21/c, the triclinic space group P1, the triclinic
ꢀ
space group P1, and the monoclinic space group P21/c, respectively.
The crystallographic data for all the structures reported here
have been deposited in the Cambridge Database. Single-crystal
structures of complexes 3, 4, 7, 8, and 9 are represented with
The diversity of space groups for this series of complexes suggests
that the packing of these complexes is very sensitive to the differ-
ent substituents of the ligands [2a]. As depicted in Fig. 1, these five
Fig. 1. ORTEP diagrams of 3, 4, 7, 8, and 9 with thermal ellipsoids drawn at 30% probability level. The hydrogen atoms have been omitted for clarity.

3120
T.-R. Chen / Journal of Organometallic Chemistry 693 (2008) 3117–3130
complexes adopt a distorted-octahedral coordination geometry
around iridium, consisting of two benzoxazolate ligands and one
b-diketonate ligand. Because of the smaller bite angles for the 2-
aryl benzoxazolate chelating interaction, the crystal structures of
these complexes are distorted octahedrons and prefer the cis-C–
C, trans-N–N, and cis-O–O chelate disposition.
The selected bond distances and angles of complex 3, 4, 7, and 9
are listed in Table 2, which shows that the bond length of the Ir–C
bond in complex 3 (Ir–Cav = 2.005(4) Å) is shorter than those in
complexes 4 (Ir–Cav = 2.029(4) Å), 7 (Ir–Cav = 2.021(5) Å), and 9
(Ir–Cav = 2.025(3) Å) because complex 3 has a stronger donating
and back bonding interaction between aryl fragments of pcb and
the iridium atom. Fig. 2a shows that the molecules of complex 3
are interlinked to form a one-dimensional linear structure through
Clꢂ ꢂ ꢂH interaction (Clꢂ ꢂ ꢂH = 2.702 Å) in the packing diagram, and
then, through C–Hꢂ ꢂ ꢂO interaction (Oꢂ ꢂ ꢂH = 2.424 Å), two linear
chains are interlinked to form a self-complementary double chain,
Fig. 2b [16]. In the self-complementary double chain, a valid stack-
ing interaction between phenyl moiety and benzoxazole moiety of
pcb is observed, in which the stacking distance (h) is 3.38 Å, the
ring shift (r) is 1.46 Å, the inclination (h) is 3.7°, and the direction
angle (
of pcb carry a formal negative charge, the highest occupied molec-
ular orbital (HOMO) is principally composed of the orbital of the
W) is 66.7°, respectively [17]. Because the phenyl fragments
p
phenyl moiety of pcb, whereas the benzoxazolate fragment of pcb
is formally neutral and is the principal contributor to the lowest
unoccupied molecular orbital (LUMO) [18]. That is to say, the
self-complementary double chain in the packing diagram of com-
plex 3 is not only stabilized by the C–Hꢂ ꢂ ꢂO interaction but also
by the
p– p
pꢃ interaction between the HOMO ( ) the LUMO (pꢃ).
Interestingly, the self-complementary double chain exhibits a heli-
cal structure, Fig. 2c. These phenomena reveal that complex 3
Table 1
Crystal data for complexes 3, 4, 7, 8, and 9
Ir(P5CBO)2(PDO)
34H28Cl2IrN2O4
Ir(FPBO)2(PDO)
Ir(FPBO)2(DBM)
Ir(NBO)2(DBM)
Ir(FPBO)2(HFPDO)
Empirical formula
Fw
C
C31H19F4IrN2O4
751.68
C41.50H24ClF4IrN2O4
918.28
C52H38IrN2O4
947.04
C31H13F10IrN2O4
859.63
791.68
Temperature (K)
Radiated used (k[Å])
Crystal size (mm3)
Crystal system
Space group
a (Å)
297(2)
0.71073
297(2)
0.71073
297(2)
0.71073
297(2)
0.71073
295(2)
0.71073
0.46 ꢀ 0.17 ꢀ 0.16
monoclinic
C2/c
23.7943(15)
12.9749(8)
22.1626(14)
90
112.9200(10)
90
6302.0(7), 8
1.669
0.38 ꢀ 0.36 ꢀ 0.35
monoclinic
P21/c
16.7347(16)
18.2736(17)
8.8533(8)
90
100.711(2)
90
2660.2(4), 4
1.877
0.60 ꢀ 0.44 ꢀ 0.25
0.58 ꢀ 0.50 ꢀ 0.44
0.46 ꢀ 0.35 ꢀ 0.24
monoclinic
P21/c
17.5756(12)
18.7959(14)
8.8512(6)
90
100.6360(10)
90
2873.8(3), 4
1.987
triclinic
triclinic
ꢀ
ꢀ
P1
P1
12.1208(15)
12.8648(16)
13.1540(17)
95.468(3)
114.883(2)
98.239(2)
1813.6(4), 2
1.682
12.366(5)
13.553(6)
14.406(6)
93.145(8)
109.520(7)
113.283(7)
2041.0(15), 2
1.541
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3), Z
qcalc. (g cmꢁ3
l
)
(mmꢁ1
)
4.447
5.089
3.821
3.321
4.753
F(000)
3112
1456
898
946
1648
Scan range h (°)
Number of total reflections
2.13–26.02
17592
2.23–26.02
14750
1.73–26.15
10245
1.93–26.19
11426
2.17–26.01
15924
Number of unique reflections (Rint
)
6183 (0.0283)
6183/2/388
0.986
5223 (0.0255)
5223/0/379
1.002
7078 (0.0201)
7078/0/496
1.136
7896 (0.0605)
7896/2/532
1.265
5638 (0.0257)
5638/0/487
1.143
Number of data/restraints/parameters
Goodness-of-fit on F2
R1, wR2 [I >
R1, wR2 (all data)
r
2(I)]
0.0277 and 0.0697
0.0427 and 0.0787
0.0244 and 0.0671
0.0296 and 0.0696
0.0318 and 0.0995
0.0352 and 0.1020
0.0595 and 0.1731
0.0627 and 0.1767
0.0212 and 0.0482
0.0258 and 0.0496
Table 2
Selected bond distance (Å) and bond angles for complexes 3, 4, 7, 8, and 9
Complex
Ir–C (Å)
Ir–O (Å)
Ir–N (Å)
N–Ir–N (°)
C–Ir–C (°)
O–Ir–O (°)
C–Ir–N (°)
N–Ir–O (°)
C–Ir–O (°)
3
2.006(4)
2.005(4)
2.144(3)
2.141(3)
2.037(3)
2.033(3)
174.18(13)
174.88(11)
176.84(12)
174.6(2)
91.13(17)
87.69(12)
96.01(15)
95.77(15)
80.08(15)
79.92(15)
95.57(12)
94.38(12)
89.85(12)
88.60(12)
175.62(14)
173.82(14)
91.81(15)
89.78(14)
4
7
8
9
2.030(3)
2.028(4)
2.124(3)
2.120(3)
2.053(3)
2.033(3)
91.47(14)
93.56(18)
91.3(3)
89.07(12)
89.77(12)
88.2(2)
99.80(13)
95.44(13)
80.04(13)
79.45(13)
95.62(11)
94.03(12)
89.48(11)
86.44(12)
174.63(12)
173.03(12)
92.73(13)
87.24(13)
2.031(5)
2.011(5)
2.125(3)
2.095(3)
2.028(4)
2.026(4)
98.29(18)
97.82(17)
79.84(18)
79.67(18)
95.87(14)
95.54(15)
86.60(14)
86.46(14)
175.16(14)
174.40(14)
90.61(15)
86.39(15)
2.023(7)
1.986(6)
2.135(5)
2.122(5)
2.070(6)
2.034(6)
97.3(3)
94.8(3)
80.6(3)
79.9(3)
96.9(2)
96.9(2)
87.6(2)
86.3(2)
176.2(2)
173.7(2)
94.4(3)
86.2(2)
2.030(3)
2.019(3)
2.150(2)
2.139(2)
2.061(2)
2.035(2)
174.96(9)
90.19(11)
88.04(8)
100.41(11)
95.29(11)
79.62(11)
79.67(11)
97.02(9)
95.17(9)
88.02(9)
85.19(9)
175.30(9)
173.00(10)
94.97(10)
87.12(10)

T.-R. Chen / Journal of Organometallic Chemistry 693 (2008) 3117–3130
3121
Fig. 2. The diagrams of 3 showing the linear molecular chain linked by the C–Hꢂ ꢂ ꢂCl interaction (a), the self-complementary double chain constructed by the C–Hꢂ ꢂ ꢂO
hydrogen bonds (b), and the helical structure built up by the C–Hꢂ ꢂ ꢂCl interaction, C–Hꢂ ꢂ ꢂO hydrogen bonds, and the
pꢃ interaction (c), respectively.
p
–
could be self-organized and sensitive to polar surroundings, which
also suggests that this complex could be a promising material used
as intrinsic or extrinsic fluorescent probes in the biophysical stud-
ies of the polarity of the microenvironment of peptides, proteins
and lipid bilayers [19]. A detailed study of these properties is cur-
rently underway and will be presented in a future report.
of the other dfpbo ligand to form a one-dimensional linear struc-
ture (Fꢂ ꢂ ꢂH = 2.500 Å). Table 2 shows that the bond length of Ir–O
of complex 4 is shorter (2.122 (3) Å) than that of complex 3
(2.143(3) Å), which implies that the negative charge on the oxygen
atoms of the acac ligand in complex 4 is more delocalized into the
iridium ion than that in complex 3; hence, the charge on the oxy-
gen atoms needed for the intermolecular interaction between the
oxygen atom and the hydrogen atom is weakened. Therefore, the
Fig. 3a shows the most positive (or acidic) hydrogen atom of a
dfpbo ligand in complex 4, H9, is interacting with the fluorine atom

3122
T.-R. Chen / Journal of Organometallic Chemistry 693 (2008) 3117–3130
Fig. 3. (a) A packing diagram showing the linear chain of 4 linked by the C–Hꢂ ꢂ ꢂF hydrogen bonds. A pseudo-axis (
diagram. (b) A packing diagram showing the linear chain of 7 linked by the C–Hꢂ ꢂ ꢂO hydrogen bonds. A pseudo-axis (
showing the projection of the linear packing diagram of 7 visualized by an end-on view of the pseudo-axis ( ).
a
) of the linear chain is designated to analyze this packing
a
) of the linear chain is also designated. (c) A diagram
a
C–Hꢂ ꢂ ꢂO interaction has not been observed in the packing diagram
of complex 4, and then the self-complementary double chain has
not been observed in complex 4. The stacking parameters of the
rial fluorine atom of the other dfpbo ligand, whereas the linear
chain in the packing diagram of complex 7 is constructed through
the Oꢂ ꢂ ꢂH interaction between an equatorial hydrogen atom of a
dfpbo ligand and an equatorial oxygen atom of the other dfpbo
ligand (Oꢂ ꢂ ꢂH = 2.524 Å).
packing diagram of 4 (h = 3.39 Å, r = 1.67 Å, h = 3.4°,
W = 63.7°)
show that the
p–
pꢃ interaction between the HOMO on the difluo-
rophenyl ring and the LUMO on the benzoxazole moiety of dfpbo of
4 is weaker than that of the corresponding interaction of complex 3
[17].
In Fig. 3b, the complex 7 molecules are also interlinked to form
a one-dimensional linear structure but this packing diagram is
quite different from that of complex 4. To analyze these packing
A projection of the linear packing diagram of 7 visualized by
an end-on view of the pseudo-axis (a) (Fig. 3c) shows that there
are two different sets of phenyl groups of dbm in the packing
diagram designated as sets A and B, in which the dihedral angle
between the phenyl ring of set A and the six-membered b-dike-
tonate chelate ring is 15° and the dihedral angle between the
phenyl ring of set B and the six-membered b-diketonate chelate
ring is 24.7°, i.e. the phenyl rings of set A are sterically different
from the phenyl rings of set B in the packing diagram of 7. And
because of the different steric effect of phenyl rings, the bond
diagrams, the pseudo-axes (a) of the linear chains are designated
in Fig. 3a and b. The linear chain in the packing diagram of complex
4 is built up by the Hꢂ ꢂ ꢂF interlinking resulting from the interaction
between an axial hydrogen atom of a dfpbo ligand and an equato-

T.-R. Chen / Journal of Organometallic Chemistry 693 (2008) 3117–3130
3123
length of Ir–O1 (2.095(3) Å) is distinct from that of Ir–O2
(2.125(3) Å); and further, by the trans influence, the bond length
of Ir–C28 (2.031(5) Å) is found to be longer than that of Ir–C41
(2.011(5) Å). These data suggest that, for complex 7, the confor-
mation of the lumophore, composed of two dfpbo ligands and
the central metal, iridium, can be affected by the steric effect
of phenyl rings of the dbm ligand. Fig. 4 shows the 1H NMR
spectrum of complex 7 at various temperatures in acetone-d6,
in which the chemical shift of the protons (Ha) on the phenyl
groups of dfpbo ligands are further deshielded at lower temper-
ature, whereas the chemical shift of the protons (Hb) on the
benzoxazole groups of the dfpbo ligands are further shielded
as the temperature is lowered. This result indicates that for com-
plex 7, the variation of temperature gives rise to a variation of
the electron density on the phenyl and benzoxazole group of
the dfpbo ligand. Because this phenomenon is not obviously ob-
served for complex 4, the temperature-dependent chemical shift
must be due to the effect of the ancillary ligand. On the other
hand, the conformation of the lumophore of complex 7 could
be perturbed by the variation of temperature, and would be
more sensitive to temperature than is the corresponding com-
plex, 4, having the same cyclometalated ligands (fpbo) as com-
plex
7 but with a different ancillary ligand (acac). The
integration ratio of the intensity of photoluminescence at low
temperature (I77K) to that at room temperature (IRT) for complex
7 is much larger (I77K/IRT = 12.96) than that for complex 4 (I77K
/
IRT = 0.78), which proves that the lumophore of complex 7 is
much more sensitive to temperature than for complex 4.
Hb
O
Ha
F
N
Ph
Ph
O
O
Ir
F
F
N
F
O
Ha
Hb
Fig. 4. An 1H NMR spectrum of complex 7 at various temperatures.

3124
T.-R. Chen / Journal of Organometallic Chemistry 693 (2008) 3117–3130
Complex 8 possesses a similar packing diagram and the same
dbm as an ancillary ligand display higher reduction potentials than
do the complexes containing acac ligand, which indicates that the
frontier molecular orbitals are affected by the phenyl groups of
dbm in the (C^N)2Ir(dbm) complexes, and these experimental data
concur with a theoretical study: the HOMOs of the (C^N)2Ir(dbm)
and (C^N)2Ir(acac) complexes are both composed of d(Ir) and
space group as complex 7. As depicted in Fig. 1 and Table 2, the
dihedral angle between the phenyl ring of set A and the six-mem-
bered b-diketonate chelate ring (15.2°) is also obviously different
from that between the phenyl ring of set B and the six-membered
b-diketonate chelate ring (24.6°); and the bond length of Ir–C49
(2.032 (7) Å) is also distinct from that of Ir–C32 (1.986 (6) Å). The
integration ratio of the intensity of photoluminescence at different
temperature for complex 8 is also quite larger (I77K/IRT = 7.03) than
that for complex 5 (I77K/IRT = 0.73), which is composed of the same
C^N ligand (nbo) as complex 8 but has a different ancillary ligand.
These data reveal that the conformation of the lumophore of com-
plex 8 is also more sensitive to temperature than for complex 5.
p
(C^N), whereas the LUMO is only localized on the pꢃ(C^N) orbital
for (C^N)2Ir(acac) complexes and is composed of pꢃ(C^N) and
pꢃ(dbm) for (C^N)2Ir(dbm) complexes; therefore the LUMO orbital
of (C^N)2Ir(dbm) complexes can be affected by the ancillary ligand,
dbm [20].
2.4. Photophysical properties
2.3. Electrochemical properties
The absorption and photoluminescence spectra of complexes
1–9 in CH2Cl2 solutions are depicted in Figs. 6 and 7; the photo-
physical data of all the iridium complexes reported here are
summarized in Tables 3 and 4. The strong absorption bands in
the ultraviolet region at about 240–310 nm with high extinction
The electrochemical properties of 1–9 in CH2Cl2 are summa-
rized in Table 3. All complexes show reversible oxidation waves
(see Fig. 5) in the range of 0.45–1.07 V versus ferrocene/ferroce-
nium. The HOMO and LUMO levels of all complexes were calcu-
lated according to the equation HOMO/LUMO (eV) = ꢁ(4.8 +
Eonset) [1]. When an electron-withdrawing group such as F or Cl
was introduced into the cyclometalating ligand, the E1/2(ox) of Ir(III)
complexes shifted more positively than the E1/2(red) does, which
leads to an increase of energy gap between the HOMO and LUMO
levels. For complexes with the same C^N cyclometalated ligands,
the oxidation potentials of (C^N)2Ir(dbm) complexes are similar
to those of (C^N)2Ir(acac) complexes, but the complexes containing
coefficients (
localized p–
e
> 4.0 ꢀ 104 cmꢁ1 Mꢁ1) are assigned to the ligand
pꢃ transitions by the comparison with the absorption
spectra of the corresponding C^N ligands. The intense absorption
bands at lower energy (320–420 nm,
= 5000–25000 cmꢁ1 Mꢁ1
e
)
can be assigned to the typical spin-allowed metal to ligand
charge-transfer (1MLCT) transitions; the weaker absorption bands
at the still lower energy (413–475 nm,
e )
= 1300–5000 cmꢁ1 Mꢁ1
are assigned to the spin-forbidden metal to ligand charge-transfer
(3MLCT) transitions. The energies of these MLCT transitions are
Table 3
UV–Vis absorption and electrochemical properties of complexes 1–9
Absorptionak (nm) {
e
, 104 cmꢁ1 Mꢁ1
}
E1/2 (V)
E1/2 (V)
D
E
(V)
HOMO (eV)
LUMO (eV)
E1MLCT (V)
ox
red
a
Complex
1/2
1
2
3
4
5
6
7
8
9
262 (4.12), 295 (2.1), 338 (0.49), 370 (0.25), 416 (0.17), 443 (0.13)
255 (4.11), 300 (4.26), 336 (1.14), 379 (0.73), 410 (0.50), 445 (0.40)
258 (4.09), 298 (4.10), 338 (1.16), 379 (0.76), 409 (0.53), 440 (0.40)
250 (4.00), 299 (3.84), 338 (1.55), 420 (0.70), 445 (0.35), 505 (0.031)
245(5.58), 310 (5.88), 360 (1.81), 408 (0.56), 475 (0.32)
258 (3.58), 300 (3.73), 334 (1.5), 379 (0.99), 416 (0.6), 439 (0.38)
255 (4.68), 299 (4.7), 334 (2.25), 413 (0.94), 445 (0.44), 502 (0.05)
245 (5.10), 303 (5.64), 364 (2.44), 406 (1.20), 453 (0.48)
0.45
0.70
0.73
0.78
0.55
0.73
0.78
0.54
1.07
ꢁ2.30
ꢁ2.11
ꢁ2.32
ꢁ2.13
ꢁ2.08
ꢁ2.13
ꢁ2.04
ꢁ1.99
ꢁ2.12
2.75
2.81
3.05
2.91
2.63
2.86
2.82
2.53
3.19
ꢁ5.25
ꢁ5.50
ꢁ5.53
ꢁ5.50
ꢁ5.35
ꢁ5.53
ꢁ5.58
ꢁ5.34
ꢁ5.87
ꢁ2.50
ꢁ2.69
ꢁ2.48
ꢁ2.67
ꢁ2.72
ꢁ2.67
ꢁ2.76
ꢁ2.81
ꢁ2.68
2.98
3.02
3.03
2.95
2.61
2.98
2.78
2.74
3.00
248 (3.17), 296 (3.51), 326 (1.99), 413 (0.57), 504 (0.035)
a
Estimated from the 1MLCT absorption bands, E1MLCTðeVÞ ¼
, (h: Planck’s constant, c: velocity of light in vacuum).
hc
k1 MLCT
1
2
3
4
6
7
8
9
9
1.0
0.8
0.6
0.4
0.2
0.0
8
7
6
4
3
2
1
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1. 8
Voltage (vs Fc/Fc+)
Fig. 5. Cyclic voltammograms of 1–9, measured in CH2Cl2.

T.-R. Chen / Journal of Organometallic Chemistry 693 (2008) 3117–3130
3125
1
2
3
4
5
6
7
8
9
6
5
4
3
2
1
0
1.0
5
0.5
0.0
6
400
500
8
3
Wavelength ( nm )
4
7
9
2
1
300
400
500
600
Wavelength (nm)
Fig. 6. Room-temperature absorption spectra of 1–9, measured in CH2Cl2.
1.0
0.8
0.6
0.4
0.2
0.0
PL at 77K
5
1
2
3
4
5
6
7
8
8
4
6
7
3
2
1
450
500
550
Wavelength (nm)
600
650
Fig. 7. Emission spectra of 1–9 at 77 K in CH2Cl2.
only weakly solvatochromic, undergoing a blue-shift in a polar
(acetonitrile) solvent, which indicates that the polarity of the
ground state is higher than that of the excited state for the com-
plexes reported here. The much weaker, lowest energy absorption
the dominant ligand-localized vibrations in the excited state and
the ground state, and this explanation also suggests that the incre-
ment of the SM value indicates less 1MLCT character in the T1 state
[7].
bands (505–515 nm,
e
= 40–500 cmꢁ1 Mꢁ1) can be identified as a
Table 4 shows that, at 77 K, complexes including dbm as an
ancillary ligand have a larger SM value than do the complexes con-
taining acac as an ancillary ligand, which indicates that the T1 state
of complexes possessing dbm ligand become more ligand-localized
(3LC) in character (less 1MLCT character in the T1 state). Therefore,
the decreased 1MLCT admixture leads to the decrease in quantum
triplet transition (T1) on the basis of the small energy shift (350–
900 cmꢁ1) between absorption and emission at room temperature.
As shown in Table 4, the photoluminescence of all complexes
are sensitive to oxygen, which indicates that the emitting states
of these complexes have triplet character [1]. Two prominent vib-
ronic features are present in the emission spectra (Fig. 7) and the
Huang–Rhys factor, SM value, can be estimated from the intensity
ratio of this first major vibrational transition to the highest energy
peak [21]. The SM value is a measure of vibronic coupling between
the ground and the excited state and is proportional to the degree
of structural distortion that occurs in the excited state relative to
the ground state and can quantify the degree of electron-vibrations
coupling [7]. A large SM value indicates a strong coupling between
yield of (C^N)2Ir(dbm) complexes (
U is in a lower range between
0.0015 and 0.0045 for (C^N)2Ir(dbm) complexes and in a higher
range between 0.02 and 0.15 for (C^N)2Ir(acac) complexes). Inter-
estingly, the Huang–Rhys factor of the complexes containing
dbm as an ancillary ligand increases with increasing temperature
(Fig. 8), which indicates that the emitting states of this kind of
complex have more 3LC character with increasing temperature,
and therefore the quantum yield is decreased. As described above,

3126
T.-R. Chen / Journal of Organometallic Chemistry 693 (2008) 3117–3130
Table 4
Photophysical properties of complexes 1–8
Complex
Emission at RT
Emission at 77 Kb
b
b
b
e
kmaxa, kmax (nm)
sa
,
sb
(ls)
Ua
,
Ub
kra, kr (105 sꢁ1
)
knra, knr (106 sꢁ1
)
kmax (nm)
Eem (0–0) (104 cmꢁ1
)
⁄
xM (cmꢁ1
)
SM
1
2
3
4
5
6
7
8
531
530
530
531
537
530
530
530
568
566
528
529
531
531
579
579
1.30
0.07
0.54
0.72
529
1.89
1.86
1.86
1.89
1.77
1.90
1.90
1.75
1360
1341
1074
1421
1538
1401
1401
1388
0.45
1.10
0.14
1.41
0.65
0.84
0.30
0.95
0.12
0.04
0.13
0.06
0.15
0.05
0.02
1.09
2.86
0.92
0.92
1.79
1.67
0.21
0.80
6.86
0.62
1.45
1.01
3.17
1.03
528
538
529
565
527
527
572
0.53
050
0.42
0.22
0.60
0.58
0.24
1.03
0.42
0.81
0.0045
0.004
0.04
0.97
2.37
1.23
0.095
0.019
0.0015
a
Measured in dichloromethane solution at a concentration of 1 ꢀ 10ꢁ6 M.
b
Measured in acetonitrile solution at a concentration of 1 ꢀ 10ꢁ6 M. The emission energy, Eem (0–0), was obtained from the maximum emission wavelength and the energy
of ⁄xM was obtained from the energy difference (in cmꢁ1) of the first two emission spectra. The Huang–Rhys factor, SM value, was estimated from the intensity ratio of the
first major vibrational transition to the highest energy peak at the 77 K emission spectra.
1.8
77 K
1.6
1.4
1.2
1.0
0.8
0.6
temperature increase
RT
500
550
600
Wavelength (nm)
Fig. 8. Emission spectra of complex 7 at various temperatures.
ative decay rates of the (C^N)2Ir(LL0) complexes range between
2.0 ꢀ 103 and 3.0 ꢀ 105 sꢁ1. The nonradiative decay rates span a
narrower range of values (5.0 ꢀ 105 to 7.0 ꢀ 106 sꢁ1). As shown in
Table 4, the kr values of (C^N)2Ir(acac) complexes are more than
ten times as large as those of (C^N)2Ir(dbm) complexes, which indi-
cates that the higher quantum yield of (C^N)2Ir(acac) complexes is
due to the higher kr values.
the emission intensities of complex 7 and 8 are sensitive to tem-
perature. A resembling phenomenon is also observed in the com-
parison between complex 6 and complex 2 (I77K/IRT is 9.27 for
complex 6 and 1.17 for complex 2, respectively).
For luminescent molecules, the quantum yield of emission (
is generally given by
U)
U
¼ kr
s
Furthermore, the radiative decay rate can be evaluated by
where kr is the rate constant for radiative decay, and
s
is the mea-
4E3em
3ꢀh
!
2
sured experimental lifetime of the emitting state [22]. Experimen-
tally, when the quantum yield has been determined, the radiative
decay rate (kr) and the nonradiative decay rate (knr) are given by [7]
kr ¼
j hwe j d j wgij
!
2
where Eem is the emission energy and j< we j d j wg > j describes
the probability for an excited to ground state transition. Because
the emission energies of (C^N)2Ir(acac) complexes are not obviously
different from those of the corresponding (C^N)2Ir(dbm) complexes,
the variations in the radiative decay rate of (C^N)2Ir(LL0) complexes
are m!ostly due the change of the transition probability,
U
s
ð1 ꢁ
UÞ
kr ¼ ; and knr
¼
s
The lifetime, kr, and knr values of complexes 1–8 have been obtained
and listed in Table 4; all of the (C^N)2Ir(LL0) complexes have short
2
luminescence lifetime (s = 0.3–2 ls) at room temperature. The radi-
j< we j d j wg > j ; in other words, the transition probabilities of

T.-R. Chen / Journal of Organometallic Chemistry 693 (2008) 3117–3130
3127
1.0
0.8
0.6
0.4
0.2
0.0
Spectra of 4
PL
UV in CH2Cl2
EX in CH2Cl2
EX in CH3CN
PL in CH2Cl2
EX in CH2Cl2
UV
300
200
400
500
600
700
Wavelength (nm)
Fig. 9. Absorption (UV), excitation (EX), and emission (PL) spectra of complex 4 at room-temperature.
(C^N)2Ir(acac) complexes must be higher than that of (C^N)2Ir(dbm)
complexes.
lumophore of complexes, and the sensitivities of complexes to
temperature and solvent polarity. The excited states of complexes
possessing dbm as an ancillary ligand become more ligand-local-
ized than those containing acac do, and the less 1MLCT character
in the T1 transition for (C^N)2Ir(dbm) complexes leads to the de-
crease in radiative decay rate, the lifetime of excited states and
the quantum efficiencies. The (C^N)2Ir(acac) complexes reported
here are highly emissive at room temperature and stable to air,
humidity, and most organic solvents. Most of the complexes re-
ported here could be promising materials used as biophysical prep-
arations or electroluminescent materials.
As described in Table 4, the quantum efficiencies of all com-
plexes reported here are obviously reduced when a polar solvent
is used. Sajoto et al. have reported that the decline in quantum effi-
ciencies of some iridium complexes is due to a significant decrease
in the radiative rates, which comes about from an increased sepa-
ration between the MLCT and LC energies as a result of the de-
creased HOMO energy [18]. For the iridium complexes reported
here, their radiative rates (kr) and Eem are not obviously changed
in different solvent so that the decrease in quantum efficiencies
could not be attributed to the decrease of radiative rate or the in-
crease of emission energy. Tamayo et al. reported that because of
the efficient intersystem crossing (ISC) between the singlet and
triplet excited states brought about by the strong spin–orbit cou-
pling of the Ir(III) metal ion, d6 Ir(III) complex can exhibit long ex-
cited-state lifetime and high luminescence efficiencies [23], which
implies that the efficient ISC leads to the excited states possessing
more triplet character, and hence, a longer lifetime, in which the
4. Experimental
4.1. Materials and equipment
Acetylacetone and dibenzoylmethane were obtained from Alfa
Aesar. 1,1,1,5,5,5-hexafluoroacetylacetone and 2,4-difluorobenzoic
acid were purchased from Matrix, and IrCl3 ꢂ nH2O was from Seed-
chem Co. All other chemicals were purchased from Acros and used
as received.
NMR spectra were measured on a Bruker Avance-400 MHz or a
Mercury 300 MHz NMR spectrometer. UV–Vis spectra were ob-
tained from a Cary 50 Probe spectrophotometer. Elemental analy-
ses (CHN) were obtained from an Elementar vario EL III analyzer.
Mass spectra were taken with a Finnigan/Thermo Quest MAT
95XL instrument with electron impact ionization for organic com-
pounds or fast atom bombardment for metal complexes. Emission
spectra were obtained from a Hitachi-F7000 spectrometer. The
quantum efficiency (UPL) was carried out at room temperature in
dichloromethane solution. A solution of 10-methyl-9(10)-acridone
v
vibrational perturbation {j<T1 jHsojSn > j, where Hso is the spin–
orbit coupling} is required for the effective mixing of singlet states
with T1 [22]. The excitation spectrum of complex 4, Fig. 9, shows
that the absorption band of MLCT transition contributes to a part
of the photoluminescence in a polar solvent (acetonitrile) and this
contribution is increased with decreasing polarity of solvent
(dichloromethane), in which the vibrational structure can be ob-
served in the excitation spectrum. Since the probability of
v
j<T1 jHsojSn > j increases with decreasing solvent polarity, the low-
er SM values were observed in a low polar environment, which also
implies that the T1 states of complexes possess more 1MLCT char-
acter in dichloromethane; therefore, a longer excited-state lifetime
and a higher luminescence efficiency were observed in dichloro-
methane than that in acetonitrile.
in dichloromethane (U = 0.95) was used as a reference. Phospho-
rescence lifetime measurements were performed on an Edinbrugh
Instrument model OB920 spectrometer.
3. Conclusion
4.2. Electrochemical measurements
This^work reports detailed synthesis and characterization of
nine (C N)2Ir(LL0) complexes composed of benzoxazole derivatives
and b-diketonate ligands. The effects of C^N cyclometalated ligands
and b-diketonate ligands on the electrochemical and photophysical
properties were studied. The ancillary ligands obviously affect the
electrochemical, photophysical properties, the conformation of the
Cyclic voltammetry were performed using a CHI voltammetric
analyzer. Anhydrous and nitrogen-purged CH2Cl2 was used as the
solvent, and 0.1 M tetra(n-butyl)ammonium hexafluorophosphate
(Bu4N+ ꢂ PF6ꢁ) was used as the supporting electrolyte. A glassy-
carbon rod was used as the working electrode, a platinum-wire

3128
T.-R. Chen / Journal of Organometallic Chemistry 693 (2008) 3117–3130
was used as the counter electrode, and a Ag/Ag+ electrode as the
reference electrode. The redox potentials are based on values mea-
sured from differential pulsed voltammetry and are reported rela-
tive to a ferrocene/ferrocenium (Fc/Fc+) redox couple used as an
internal standard [23].
2H), 7.89–7.86 (m, 1H), 7.83–7.80 (m, 1H), 7.62–7.54 (m, 3H),
7.40–7.25 (m, 2H). 13C NMR (75 MHz, CDCl3, 298 K; d (ppm)):
163.4, 151.1, 142.4, 134.9, 133.2, 129.1, 129.0, 128.3, 128.1,
128.0, 127.1, 125.4, 124.8, 124.6, 124.194, 120.2, 110.8. Anal. calc.
for C17H11NO (MW = 245.28): C, 83.24; H, 4.52; N, 5.71. Found: C,
83.25; H, 4.57; N, 5.71%. MS (FAB; m/z): 245.0839.
4.3. Synthesis of benzoxazole derivative ligands (L1–L5)
4.9. Synthesis of cyclometalated Ir(III)l-chloro-bridged dimers (D2–
D5)
The benzoxazole derivative ligands, L1–L5, were prepared by
Philips’ condensation (as shown in Scheme 1) as follows: [24]
one equiv of benzoic acid (or substituted benzoic acid) and 1.05
equiv of an appropriate aminophenol were added to polyphospho-
ric acid (10 g/mmol of benzoic acid), and then the mixture was stir-
red at 200 °C for 5 hours. After cooling to room temperature, the
mixture was slowly poured into pure water with well stirred; the
precipitate was collected by filtration, washed with pure water
(3 ꢀ 100 ml), and dried to yield a crude product. The crude product
was purified by column chromatography over silica gel using n-
hexane–CH2Cl2 (1:1–1:5) as eluent to obtain the corresponding
ligand.
Cyclometalated Ir(III) chloro-bridged dimers (D2-D5) were syn-
thesized according to previous papers (as shown in Scheme 1) [1–
3]. A mixture of 2-ethoxylethanol and water (3:1, v/v) was added
to a flask containing iridium trichloride (IrCl3ꢄ3H2O, 2 mmol) and
an appropriate benzoxazole derivative (L2–L5, 5 mmol), and then
the mixture was refluxed for 24 hours under nitrogen. After cool-
ing to room temperature, the mixture was poured into 20 ml of
pure water, and the dimer precipitate was filtered off, washed
with deionized water, and followed by drying at 60 °C in a vacuum
oven.
4.4. Synthesis of 2-phenylbenzoxazole (L1)
4.10. Synthesis of (cpbo)2Ir(l-Cl)2Ir(cpbo)2 (D2)
White crystal. 70% yield. 1H NMR (300 MHz, CDCl3, 298 K; d
(ppm)): 8.28–8.25 (m, 2H), 7.80–7.76 (m, 1H), 7.60–7.56 (m, 1H),
7.53–7.51 (m, 3H), 7.37–7.33 (m, 2H). 13C NMR (75 MHz, CDCl3,
298 K; d (ppm)): d163.2, 151.0, 142.3, 131.7, 129.1, 127.8, 127.4,
Yellow solid. 85% yield. 1H NMR (300 MHz, DMSO-d6, 298 K; d
(ppm)): 8.76–8.73 (m, 2H), 8.09–8.54 (m, 4H), 7.95 (d, J = 8.4 Hz,
2H), 7.84–7.60 (m, 8H), 7.48 (dd, J = 8.2, 2.1 Hz, 2H), 7.38 (t,
J = 8.4 Hz, 2H), 7.14 (t, J = 8.4 Hz, 2H), 7.03 (dd, J = 8.1, 1.8 Hz,
2H), 6.40 (d, J = 7.8 Hz, 2H), 6.0 (d, J = 2.1 Hz, 2H). Anal. calc. for
125.3, 124.8, 120.263, 110.8. Anal. calc. for
C13H9NO
(MW = 195.22): C, 79.98; H, 4.65; N, 7.18. Found: C, 79.99; H,
4.65; N, 7.20%. MS (FAB; m/z): 195.0686.
C52H28Cl6Ir2N4O4 (MW = 1369.98): C, 45.59; H, 2.06; N, 4.09.
Found: C, 45.67; H, 2.10; N, 4.11%. MS (FAB; m/z): 1367.9515.
4.5. Synthesis of 2-(4-chlorophenyl)benzoxazole (L2)
4.11. Synthesis of (pcbo)2Ir(l-Cl)2Ir(pcbo)2 (D3)
White crystal. 55% yield. 1H NMR (300 MHz, CDCl3, 298 K; d
(ppm)): 8.17 (d, J = 8.7 Hz, 2H), 7.77–7.74 (m, 1H), 7.57–7.54 (m,
1H), 7.48 (d, J = 8.7 Hz, 2H), 7.38–7.33 (m, 2H). 13C NMR
(75 MHz, CDCl3, 298 K; d (ppm)): 162.2, 150.9, 142.2, 137.9,
129.4, 129.0, 125.8, 125.5, 124.9, 120.3, 110.8. Anal. calc. for
Yellow solid. 70% yield. 1H NMR (300 MHz, DMSO-d6, 298 K; d
(ppm)): 8.22 (d, J = 1.8 Hz, 4H), 7.65 (dd, J = 7.8, 1.2 Hz, 4H), 7.36
(d, J = 9 Hz, 4H), 7.1 (dd, J = 8.7, 2.1 Hz, 4H), 6.87 (dt, J = 7.4,
1.2 Hz, 4H), 6.68 (dt, J = 7.4, 1.2 Hz, 4H), 6.17 (d, J = 7.8 Hz, 4H).
Anal. calc. for C52H28Cl6Ir2N4O4 (MW = 1369.98): C, 45.59; H,
2.06; N, 4.09. Found: C, 44.46; H, 2.10; N, 4.09%. MS (FAB; m/z):
1367.9504.
C13H8ClNO (MW = 229.67): C, 67.98; H, 3.51; N, 6.10. Found: C,
67.98; H, 3.53; N, 6.11%. MS (FAB; m/z):229.0303.
4.6. Synthesis of 2-phenyl-5-chlorobenzoxazole (L3)
4.12. Synthesis of (fpbo)2Ir(l-Cl)2Ir(fpbo)2 (D4)
White crystal. 65% yield. 1H NMR (300 MHz, CDCl3, 298 K; d
(ppm)): 8.23–8.20 (m, 2H), 7.66 (d, J = 8.7 Hz, 1H), 7.58 (d,
J = 1.8 Hz, 1H), 7.55–7.49 (m, 3H), 7.32 (dd, J = 8.7, 1.8 Hz, 1H).
13C NMR (75 MHz, CDCl3, 298 K; d (ppm)): 163.9, 151.1, 141.1,
132.0, 130.9, 129.2, 127.8, 126.9, 125.5, 120.7, 111.4. Anal. calc.
for C13H8ClNO (MW = 229.67): C, 67.98; H, 3.51; N, 6.10. Found:
C, 67.91; H, 3.52; N, 6.10%. MS (FAB; m/z): 229.0300.
Dark yellow solid. 75% yield. 1H NMR (300 MHz, DMSO-d6,
298 K; d (ppm)): 8.18 (d, J = 8.18, 8.1 Hz, 4H), 7.31 (dd, J = 7.6,
2.7 Hz, 4H), 7.22–7.12 (m, 8H), 6.99 (dt, J = 7.1, 1.5 Hz, 4H), 6.14
(dt, J = 9.6, 2.7 Hz, 4H). 13C NMR (75 MHz, CDCl3, 298 K;
d
(ppm)): 174.9, 169.2, 169.1, 167.5, 167.4, 160.3, 160.2, 158.7,
158.6, 149.1, 140.8, 133.6, 133.5, 133.4, 133.3, 125.1, 124.4,
118.0, 115.3, 115.0, 110.9, 108.6, 108.4, 107.2, 107.0, 106.8, 31.8,
4.7. Synthesis of 2-(3,5-difluorophenyl)benzoxazole (L4)
31.1, 22.8, 14.3. Anal. calc. for
C52H24F8Cl2Ir2N4O4 (MW =
1376.12): C, 45.38; H, 1.76; N, 4.07. Found: C, 44.89; H, 1.70; N,
3.98%. MS (FAB; m/z): 1376.0319.
White crystal. 85% yield. 1H NMR (300 MHz, CDCl3, 298 K; d
(ppm)): 7.80–7.74 (m, 3H), 7.59–7.56 (m, 1H), 7.41–7.36 (m, 2H),
7.00–6.96 (m, 1H). 13C NMR (75 MHz, CDCl3, 298 K; d (ppm)):
165.1, 164.9, 161.8, 161.7, 160.9, 151.0, 141.9, 130.4, 130.3,
130.1, 126.1, 125.2, 120.6, 110.9, 110.8, 110.7, 110.6, 107.3,
107.0, 106.7. Anal. calc. for C13H7F2NO (MW = 231.20): C, 67.53;
H, 3.05; N, 6.06. Found: C, 67.50; H, 3.08; N, 5.99%. MS (FAB; m/z):
231.0496.
4.13. Synthesis of (nbo)2Ir(l-Cl)2Ir(nbo)2 (D5)
Red solid. 65% yield. 1H NMR (300 MHz, DMSO-d6, 298 K; d
(ppm)): 8.93 (d, J = 8.4 Hz, 2H), 8.63 (s, 2H), 8.48 (s, 2H), 8.38 (s,
2H), 8.16 (d, J = 7.5 Hz, 2H), 8.11 (d, J = 7.2 Hz, 2H), 7.93 (d,
J = 7.2 Hz, 2H), 7.83–7.80 (m, 4H), 7.71–7.57 (m, 8H), 7.37 (t,
J = 7.8 Hz, 2H), 7.25–7.23 (m, 4H), 7.14 (t, J = 7.5 Hz, 2H), 7.02–7.0
(m, 2H), 6.5 (d, J = 7.5 Hz, 2H), 6.37 (s, 2H). Anal. calc. for
4.8. Synthesis of 2-(2-naphthyl)benzoxazole (L5)
White crystal. 65% yield. 1H NMR (300 MHz, CDCl3, 298 K; d
(ppm)): 8.76 (s, 1H), 8.31 (dd, J = 8.7, 1.8 Hz, 1H), 7.99–7.94 (m,
C68H40Cl2Ir2N4O4 (MW = 1432.43): C, 57.02; H, 2.82; N, 3.91.
Found: C, 56.28; H, 3.82; N, 2.85%. MS (FAB; m/z): 1432.1689.

T.-R. Chen / Journal of Organometallic Chemistry 693 (2008) 3117–3130
3129
4.14. Synthesis of iridium complexes 1–9
7.51–7.32 (m, 8H), 6.29–6.22 (m, 2H), 5.31 (s, 1H), 1.87 (s, 6H).
13C NMR (75 MHz, CDCl3, 298 K; d (ppm)): 185.7, 149.9, 139.6,
126.2, 125.6, 117.1, 111.7, 108.9, 108.8, 108.6, 108.5, 107.2,
106.9, 106.8, 106.4, 101.7, 28.2. Anal. calc. for C31H19F4IrN2O4
(MW = 751.72): C, 49.53; H, 2.55; N, 3.73. Found: C, 49.51; H,
2.67; N, 3.74%. MS (FAB; m/z): 752.2380.
Complexes 1–9 were synthesized according to a method given
previously in the literature (as shown in Scheme 1) [1]. One equiv
of
a l-
cyclometalated Ir(III) chloro-bridged dimer [(C^N)2Ir(
Cl2)Ir(C^N)2], 5 equiv of silver carbonate, and 2.5 equiv of an appro-
priate ancillary ligand were added to 2-ethoxyethanol (10 ml/
mmol of Ir(III) chloro-bridged dimer), and then the mixture was
stirred and refluxed under nitrogen for 24 hours. After that, the
mixture was cooled to ambient temperature and concentrated un-
der reduced pressure. The residue was taken up in dichlorometh-
ane, and the silver salt was removed by filtration through celite.
The resultant solution was concentrated and further purified by
column chromatography over silica gel using n-hexane–CH2Cl2
(1:1–1:9) as eluent to obtain the corresponding iridium complex.
The iridium complexes were characterized by 1H NMR, 13C NMR,
elemental analysis, and FAB-MS spectrometry. Moreover,
complexes 3, 4, and 7–9 were further identified by a single-crystal
X-ray analysis.
0
4.19. Synthesis of bis[2-(2-naphthyl)benzoxazolato-N,C2 ]iridium(III)
(acetylacetonate) (5)
Red solid. 80% yield (based on the corresponding dimer). 1H
NMR (300 MHz, CDCl3, 298 K; d (ppm)): 8.21 (s, 2H), 7.79–7.77
(m, 2H), 7.68–7.63 (m, 4H), 7.55–7.45 (m, 6H), 7.23–7.12 (m,
6H), 6.82 (s, 1H), 1.85 (s, 6H). Anal. calc. for
C39H27IrN2O4
(MW = 779.87): C, 60.06; H, 3.49; N, 3.59. Found: C, 60.02; H,
3.50; N, 3.55%. MS (FAB; m/z): 780.2987.
4.20. Synthesis of bis[2-(4-chlorophenyl)benzoxazolato-
0
N,C2 ]iridium(III) (dibenzoylmethanate) (6)
0
4.15. Synthesis of bis[2-phenylbenzoxazolato-N,C2 ]iridium(III)
(acetylacetonate) (1)
Red solid. 82% yield (based on the corresponding dimer). 1H
NMR (400 MHz, CDCl3, 298 K; d (ppm)): 7.84–7.80 (m, 4H), 7.73–
7.70 (m, 2H), 7.65–7.60 (m, 4H), 7.51–7.15 (m, 12H), 6.96–6.93
(m, 2H), 6.63 (s, 1H). 13C NMR (75 MHz, CDCl3, 298 K; d (ppm)):
179.7, 177.2, 150.0, 149.7, 147.9, 140.2, 139.5, 138.2, 137.1,
134.2, 130.4, 129.3, 128.3, 126.9, 126.8, 126.5, 126.2, 125.6,
125.0, 124.7, 122.2, 121.9, 121.8, 117.1, 116.6, 115.0, 113.2,
111.5, 96.2, 95.3. Anal. calc. for C41H25Cl2IrN2O4 (MW = 872.78):
C, 56.42; H, 2.89; N, 3.21. Found: C, 56.12; H, 2.90; N, 3.16%. MS
(FAB; m/z): 872.0827.
Orange solid. 60% yield (based on the corresponding dimer). 1H
NMR (300 MHz, CDCl3, 298 K; d (ppm)): 7.69–7.65 (m, 4H), 7.56–
7.53 (m, 2H), 7.47–7.39 (m, 4H), 6.90–6.85 (m, 2H), 6.75–6.70
(m, 2H), 6.51 (d, J = 7.5 Hz, 2H), 5.26 (s, 1H), 1.85 (s, 6H). 13C
NMR (75 MHz, CDCl3, 298 K; d (ppm)): 185.5, 178.4, 150.4, 147.0,
139.0, 134.7, 131.4, 130.7, 126.2, 126.0, 125.0, 121.3, 117.1,
116.9, 111.9, 111.6, 101.5, 28.4. Anal. calc. for C31H23IrN2O4
(MW = 679.75): C, 49.73; H, 2.83; N, 3.74. Found: C, 49.67; H,
2.90; N, 3.69%. MS (FAB; m/z): 680.2825.
4.21. Synthesis of bis[2-(3,5-difluorophenyl)benzoxazolato-
0
N,C2 ]iridium(III) (dibenzoylmethanate) (7)
4.16. Synthesis of bis[2-(4-chlorophenyl)benzoxazolato-
0
N,C2 ]iridium(III) (acetylacetonate) (2)
Red solid. 85% yield (based on the corresponding dimer). 1H
NMR (300 MHz, CDCl3, 298 K; d (ppm)): 7.87–7.84 (m, 4H), 7.60
(d, J = 8.1 Hz, 2H), 7.45–7.32 (m, 12H), 7.20–7.15 (m, 2H), 6.71 (s,
1H), 6.36–6.29 (m, 2H). 13C NMR (75 MHz, CDCl3, 298 K; d
(ppm)): 179.9, 149.9, 140.1, 139.5, 130.8, 128.6, 127.2, 126.3,
125.5, 117.2, 111.7, 108.8, 108.4, 107.1, 106.8, 106.7, 106.4, 95.6.
Anal. calc. for C41H23F4IrN2O4 (MW = 875.85): C, 56.22; H, 2.65;
N, 3.20. Found: C, 56.25 H, 2.66; N, 3.21%. MS (FAB; m/z): 876.3012.
Orange solid. 70% yield (based on the corresponding dimer). 1H
NMR (300 MHz, CDCl3, 298 K; d (ppm)): 7.73–7.38 (m, 10H), 7.41–
7.22 (m, 1H), 6.93–7.90 (m, 2H), 6.41 (d, J = 1.8 Hz, 1H), 5.26 (s,
1H), 1.85 (s, 6H). 13C NMR (75 MHz, CDCl3, 298 K; d (ppm)):
185.8, 150.3, 147.9, 138.6, 138.3, 137.5, 134.3, 129.4, 127.0,
126.9, 126.7, 126.4, 125.4, 122.3, 122.1, 117.2, 116.8, 111.9,
101.9, 101.6, 28.3, 27.9, 27.7. Anal. calc. for C31H21Cl2IrN2O4
(MW = 748.64): C, 49.73; H, 2.83; N, 3.74. Found: C, 49.64; H,
2.89; N, 3.67. MS (FAB; m/z): 748.1827.
0
4.22. Synthesis of bis[2-(2-naphthyl)benzoxazolato-N,C2 ]iridium(III)
(dibenzoylmethanate) (8)
4.17. Synthesis of bis[2-phenyl-5-chlorobenzoxazolato-
Red solid. 78% yield (based on the corresponding dimer). 1H
NMR (400 MHz, CDCl3, 298 K; d (ppm)): 8.23 (s, 2H), 7.86–7.83
(m, 4H), 7.72–7.69 (m, 4H), 7.56 (d, J 8.4 Hz, 2H), 7.43–7.35 (m,
4H), 7.29–7.15 (m, 12H), 6.92 (s, 2H), 6.68 (s, 1H). Anal. calc. for
0
N,C2 ]iridium(III) (acetylacetonate) (3)
Orange solid. 50% yield (based on the corresponding dimer). 1H
NMR (300 MHz, CDCl3, 298 K; d (ppm)): 7.69–7.64 (m, 4H), 7.45–
7.39 (m, 4H), 6.92–6.85 (m, 2H), 6.78–6.72 (m, 2H), 6.47 (d,
J = 7.5 Hz, 2H), 5.24 (s, 1H), 1.82 (s, 6H). 13C NMR (75 MHz, CDCl3,
298 K; d (ppm)): 185.7, 179.1, 150.6, 147.0, 137.8, 134.6, 131.8,
130.7, 130.1, 127.0, 126.3, 126.2, 121.7, 121.5, 117.2, 112.5,
112.4, 101.5, 28.4. Anal. calc. for C31H21Cl2IrN2O4 (MW = 748.64):
C, 49.73; H, 2.83; N, 3.74%. Found: C, 49.70; H, 2.85; N, 3.70%..
MS (FAB; m/z): 748.2119.
C49H31IrN2O4 (MW = 904.01): C, 65.1; H, 3.46; N, 3.10. Found: C,
65.56; H, 3.37; N, 3.16%. MS (FAB; m/z): 904.
4.23. Synthesis of bis[2-(3,5-difluorophenyl)benzoxazolato-
0
N,C2 ]iridium(III) [1,1,1,5,5,5-hexafluoroacetylacetonate] (9)
Red solid. 90% Yield (based on the corresponding dimer). 1H
NMR (300 MHz, CDCl3, 298 K; d (ppm)): 7.71 (d, J = 8.1 Hz, 2H),
7.52–7.25 (m, 8H), 6.32–6.27 (m, 2H), 6.10 (s, 1H). 13C NMR
(75 MHz, CDCl3, 298 K; d (ppm)): 149.8, 138.8, 126.8, 126.2,
116.3, 112.1, 109.2, 109.1, 108.8, 107.7, 107.4, 107.3, 106.9, 92.9.
Anal. calc. for C31H13F10IrN2O4 (MW = 859.65): C, 43.31; H, 1.52;
N, 3.26. Found: C, 43.40; H, 1.52; N, 3.24%. MS (FAB; m/z):
860.2679.
4.18. Synthesis of bis[2-(3,5-difluorophenyl)benzoxazolato-
0
N,C2 ]iridium(III) (acetylacetonate) (4)
Orange solid. 80% yield (based on the corresponding dimer). 1H
NMR (300 MHz, CDCl3, 298 K; d (ppm)): 7.66 (d, J = 7.5 Hz, 2H),

3130
T.-R. Chen / Journal of Organometallic Chemistry 693 (2008) 3117–3130
(e) L.-L. Wu, C.-H. Yang, I.-W. Sun, S.-Y. Chu, P.-C. Kao, Y. Cao, H.-H. Huang,
4.24. X-ray crystallography
Organometallics 26 (2007) 2017;
(f) F.-M. Hwang, H.-Y. Chen, P.-S. Chen, C.-S. Liu, Y. Chi, C.-F. Shu, F.-I. Wu, P.-T.
Chou, S.-M. Peng, G.-H. Lee, Inorg. Chem. 44 (2005) 1344;
(g) C.-H. Yang, S.-W. Li, Y. Chi, Y.-M. Cheng, Y.-S. Yeh, P.-T. Chou, G.-H. Lee,
C.-H. Wang, C.-F. Shu, Inorg. Chem. 44 (2005) 7770;
(h) H.-C. Li, P.-T. Chou, Y.-H. Hu, Y.-M. Cheng, R.-S. Liu, Organometallics 24
(2005) 1329.
The diffraction data of complexes 3, 4, and 7–9 were collected
on a Bruker SMART APEX CCD diffractometer with graphite-mono-
chromatized Mo Ka X-ray radiation (k = 0.71073 Å) at room tem-
perature. All the calculations for the structure determination
were carried out using SHELXTL package (version 5.1) [25]. Initial
atomic positions were located by Patterson methods using
XS, and the structures of the complexes were refined by the
least-square method using SHELXL-97. The structural analyses for
complexes 4 and 7 proceeded smoothly, but disordered solvent
molecules were observed from different maps for complex 3, a
slight packing disorder of one of the two nbo ligands was observed
for complex 8, and a packing disorder of F atoms of the ancillary
ligand, hfpdo, was also observed for complex 9. The final residuals
of the final refinement were R1 = 0.0277, wR2 = 0.0697 for 3,
R1 = 0.0244, wR2 = 0.0671 for 4, R1 = 0.0318, wR2 = 0.0995 for 7,
R1 = 0.0595, wR2 = 0.1731 for 8, and R1 = 0.0212, wR2 = 0.0482 for
9, respectively. Basic information pertaining to crystal parameters
and structure refinement are summarized in Table 1. Selected bond
distances and angles are listed in Table 2.
[3] E. Mieczy’nska, M. Anna, A.M. Trzeciak, J. J’ozef, I. Kownacki, B.J. Marciniec,
Mol. Catal. A 237 (2005) 246.
[4] (a) K. Ogata, A. Toyota, J. Organomet. Chem. 692 (2007) 4139;
(b) W.H. Bernskoetter, E. Lobkovsky, P.J. Chirik, Organometallics 24 (2005)
6250.
[5] P. Mura, A. Casini, G. Marcon, L. Messori, Inorg. Chim. Acta 312 (2001) 74.
[6] (a) I.J. Kim, Y.H. Kim, Y.S. Kim, Colloids Surf. A 313 (2008) 426;
(b) S. Lamansky, P. Djurovich, D. Murphy, A.-R. Feras, R. Kwong, I. Tsyba, M.
Bortz, B. Mui, R. Bau, M.E. Thompson, Inorg. Chem. 40 (2001) 1704.
[7] L. Jian, P.I. Djurovich, B.D. Alleyne, M. Yousufuddin, N.N. Ho, J.C. Thomas, J.C.
Peters, R. Bau, M.E. Thompson, Inorg. Chem. 44 (2005) 1713.
[8] A. Rzeska, J. Malicka, K. Guzow, M. Szabelski, W. Wicz, J. Photochem. Photobiol.
A 146 (2001) 9.
[9] K. Guzowa, D. Szmigiel, D. Wr’oblewski, M. Milewska, J. Karolczak, W. Wiczk, J.
Photochem. Photobiol. A 187 (2007) 87.
[10] E.-D.H. Safa, A.F. Tarek, N.Z. Saleh, J. Photochem. Photobiol. A 177 (2006)
238.
[11] A.F. Tarek, J. Photochem. Photobiol. A 121 (1999) 17.
[12] (a) A. Decken, A.G. Robert, J. Inorg. Biochem. 99 (2005) 664;
(b) X.-F. He, C.M. Vogels, A. Decken, S.A. Westcott, Polyhedron 23 (2004)
155;
(c) K. Tanaka, K.N. Sano, T. Kurushima, J. Fluor. Chem. 127 (2006) 1073.
[13] (a) K. Guzowa, M. Szabelskia, J. Karolczakb, W. Wiczka, J. Photochem.
Photobiol. A 170 (2005) 215;
(b) S.-I. Um, Dyes Pigments 75 (2007) 185.
[14] K. Guzowa, D. Szmigiel, D. Wr’oblewski, M. Milewskaa, J. Karolczak, W. Wiczk,
J. Photochem. Photobiol. A 187 (2007) 87.
Acknowledgement
This work was supported by the National Science Council of the
Republic of China (Grant No. NSC 95-2113-M-153-001-MY3).
[15] M. Guan, L. Gaob, S. Wang, C. Huanga, K. Wang, J. Lumin. 127 (2007) 489.
[16] (a) K.-Y. Ho, W.-Y. Yu, K.-K. Cheng, C.-M. Che, Chem. Commun. (1998) 2101;
(b) Z.K. Chan, T.-R. Chen, J.-D. Chen, J.-C. Wang, C.W. Liu, Dalton Trans. (2007)
3450;
(c) Z.K. Chan, T.-R. Chen, Y.-F. Tsai, J.-D. Chen, J.-C. Wang, Polyhedron 26
(2007) 3450.
[17] M.L. Glówka, D. Martynowski, K.J. Kozlowska, Mol. Struct. 474 (1999) 81.
[18] T. Sajoto, P.I. Djurovich, A. Tamayo, M. Yousufuddin, R. Bau, M.E. Thompson,
Inorg. Chem. 44 (2005) 7992.
[19] K. Guzowa, M. Szabelskia, J. Karolczakb, W. Wiczka, J. Photochem. Photobiol. A
170 (2005) 215.
[20] T. Liu, B.-H. Xia, X. Zhou, H.-X. Zhang, Q.J. Pan, J.-S. Gao, Organometallics 26
(2007) 143.
[21] D.P. Rillema, C.B. Blanton, R.J. Shaver, D.C. Jackman, M. Boldaji, S. Bundy, L.A.
Worl, T. Meyer, J. Inorg. Chem. 31 (1992) 1600.
Appendix A. Supplementary material
CCDC 685736, 685737, 685738, 685739, 685740 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2008.06.034.
References
[22] N.J. Turro, Modern Molecular Photochemistry, University Science Books, Mill
Valley, CA, 1991.
[1] Z. Zhao, C.-Y. Jiang, M. Shi, F.-Y. Li, T. Yi, Y. Cao, C.-H. Huang, Organometallics
25 (2006) 3631.
[23] A.B. Tamayo, B.D. Alleyne, P.I. Djurovich, S. Lamansky, I. Tsyba, N.N. Ho, R. Bau,
M.E. Thompson, J. Am. Chem. Soc. 125 (2003) 7377.
[24] Í. Ören, Ö. Temiz, Í. Yalçin, E. Sener, N. Altanlar, Eur. J. Pharm. Sci. 7 (1998)
153.
[2] (a) Y.-T. Huang, T.-H. Chuang, Y.-L. Shu, Y.-C. Kuo, P.-L. Wu, C.-H. Yang, I.-W.
Sun, Organometallics 24 (2005) 6230;
(b) X. Gong, S. Wang, D. Moses, G.C. Bazan, A.J. Heeger, Adv. Mater. 17 (2005)
2053;
(c) L. Chen, C. Yang, J. Qin, J. Gao, D. Ma, Inorg. Chim. Acta 359 (2006) 4207;
(d) R.R. Das, C.-L. Lee, Y.-Y. Noh, J.-J. Kim, Opt. Mater. 21 (2002) 143;
[25] G.M. Sheldrick, SHELXTL
Madison, WI, 1997.
, version 5.1, Bruker Analytical X-ray System, Inc.,