Highly Luminescent Tetradentate Bis-Cyclometalated Platinum Complexes: Design, Synthesis, Structure, Photophysics, and Electroluminescence Application

Article abstract of DOI:10.1021/ic1002226

N,N-Di(6-phenylpyridin-2-yl)aniline (L1), N,N-di(6-(2,4-difluorophenyl) pyridin-2-yl)aniline (L2), N,N-di(3-(pyridin-2-yl)phenyl)aniline (L3), N,N-di(3-(1H-pyrazol-1-yl)phenyl)aniline (L4), N,N-di(3-(3-methyl-1H-pyrazol-1- yl)phenyl)aniline (L5), and N,N-di(3-(4-methyl-1H-pyrazol-1-yl)phenyl)aniline (L6) undergo cyclometalation to produce two types of tetradentate bis-cyclometalated platinum(II) complexes: CN*NC platinum complexes 1 and 2 and N^{^}C*C^{^}N platinum complexes 3-6, respectively, where an XY (X, Y = C or N) denotes a bidentate coordination to the platinum to form a five-membered metallacycle and X*Y denotes a coordination to form a six-membered metallacycle. The crystal structures of 1, 3, and 5 were determined by the single-crystal X-ray diffraction analysis, showing distorted square-planar geometry, that is, two CN coordination moieties are twisted. Complex 5 showed much greater distortion with largest deviation of 0.193 from the mean NCCNPt coordination plane, which is attributed to the steric interaction between the two 3-methyl groups on the pyrazolyl rings. Density functional theory (DFT) calculations were carried out on the ground states of 1 and 3-6. The optimized geometries are consistent with the crystal structures. The highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of the molecules displayed a localized characteristic with the contribution (18-45%) of the platinum metal to the HOMOs. All complexes are emissive at ambient temperature in fluid with quantum yields of 0.14 to 0.76 in 2-methyltetrahydrofuran. The emission of the complexes covers from blue to red region with λ_max ranging from 474 to 613 nm. Excimer emission was observed for 1 and 2 at high concentration of the complexes. The emission lifetime at infinite dilution for 1 and 2 was determined to be 7.8 and 11.4 μs, respectively. Concentration quenching was observed for 3 and 4, but the excimer emission was not observed. The life times for 3-6 were determined to be in the range of micro seconds, but those of 4-6 (3.4-5.7 μs) were somewhat shorter than that of 3 (7.6 μs). The highly structured emission spectra, long life times, and DFT calculations suggested that the emissive state is primarily a ³LC state with metal-to-ligand charge-transfer (MLCT) admixture. The ZFS of 23 cm^-1 for the emissive triplet state was observed directly by high resolution spectroscopy for 1 in a Shpol'skii matrix, which also suggested an emission from a triplet ligand centered (³LC) state with admixture of MLCT character. Complex 1 was incorporated into an organic light-emitting diode (OLED) device as an emitter at 4 wt % in the mixed host of 4,4',4 -tris(N-carbazolyl)triphenylamine (TCTA) and 2,2',2 -(1,3,5-benzenetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBI) and demonstrated excellent performance with maximum external quantum efficiency of 14.7% at the current density of 0.01 mA/cm^-1.

Full text of DOI:10.1021/ic1002226

Inorg. Chem. 2010, 49, 5107–5119 5107
DOI: 10.1021/ic1002226
Highly Luminescent Tetradentate Bis-Cyclometalated Platinum Complexes:
Design, Synthesis, Structure, Photophysics, and Electroluminescence Application^
Dileep A. K. Vezzu,† Joseph C. Deaton,‡ James S. Jones,§ Libero Bartolotti,† Caleb F. Harris,† Alfred P. Marchetti,
Marina Kondakova,‡ Robert D. Pike,§ and Shouquan Huo*,†
†Department of Chemistry, East Carolina University, Greenville, North Carolina 27858,
‡Eastman Kodak Company, Rochester, New York 14650, §Department of Chemistry, College of William and
Mary, Williamsburg, Virginia 23185, and Department of Chemistry, University of Rochester, Rochester,
New York 14627
Received February 4, 2010
N,N-Di(6-phenylpyridin-2-yl)aniline (L1), N,N-di(6-(2,4-difluorophenyl)pyridin-2-yl)aniline (L2), N,N-di(3-(pyridin-2-yl)-
phenyl)aniline (L3), N,N-di(3-(1H-pyrazol-1-yl)phenyl)aniline (L4), N,N-di(3-(3-methyl-1H-pyrazol-1-yl)phenyl)aniline
(L5), and N,N-di(3-(4-methyl-1H-pyrazol-1-yl)phenyl)aniline (L6) undergo cyclometalation to produce two types of
tetradentate bis-cyclometalated platinum(II) complexes: C∧N*N∧C platinum complexes 1 and 2 and N∧C*C∧N platinum
complexes 3-6, respectively, where an “X∧Y” (X, Y = C or N) denotes a bidentate coordination to the platinum to form a
five-membered metallacycle and “X*Y” denotes a coordination to form a six-membered metallacycle. The crystal
structures of 1, 3, and 5 were determined by the single-crystal X-ray diffraction analysis, showing distorted square-planar
geometry, that is, two C∧N coordination moieties are twisted. Complex 5 showed much greater distortion with largest
˚
deviation of 0.193 A from the mean NCCNPt coordination plane, which is attributed to the steric interaction between the
two 3-methyl groups on the pyrazolyl rings. Density functional theory (DFT) calculations were carried out on the ground
states of 1 and 3-6. The optimized geometries are consistent with the crystal structures. The highest occupied molecular
orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of the molecules displayed a localized
characteristic with the contribution (18-45%) of the platinum metal to the HOMOs. All complexes are emissive at
ambient temperature in fluid with quantum yields of 0.14 to 0.76 in 2-methyltetrahydrofuran. The emission of the
complexes covers from blue to red region with λmax ranging from 474 to 613 nm. Excimer emission was observed for 1
and 2 athigh concentration ofthe complexes. The emissionlifetime atinfinite dilution for 1 and 2 was determinedtobe 7.8
and 11.4 μs, respectively. Concentration quenching was observed for 3 and 4, but the excimer emission was not
observed. The life times for 3-6 were determined to be in the range of micro seconds, but those of 4-6 (3.4-5.7 μs)
were somewhat shorter than that of 3 (7.6 μs). The highly structured emission spectra, long life times, and DFT
3
calculations suggested that the emissive state is primarily a LC state with metal-to-ligand charge-transfer (MLCT)
admixture. The ZFS of23 cm-1 for the emissive tripletstate was observed directly by high resolution spectroscopy for1 in
a Shpol’skii matrix, which also suggested an emission from a triplet ligand centered (3LC) state with admixture of MLCT
character. Complex 1 was incorporated into an organic light-emitting diode (OLED) device as an emitter at 4 wt % in the
mixed host of 4,40,400-tris(N-carbazolyl)triphenylamine (TCTA) and 2,20,200-(1,3,5-benzenetriyl)tris(1-phenyl-1-H-
benzimidazole) (TPBI) and demonstrated excellent performance with maximum external quantum efficiency of 14.7%
at the current density of 0.01 mA/cm-1
.
Introduction
complexeshavebeenused as oxygensensors,1 inthe detection
of cyanogen halides,2 in cation recognition,3 and in pH
sensing.4 They have also been used as biological labeling
Cyclometalated complexes of platinum have recently been
the focus of the research for a variety of applications from
catalysts to advanced materials. Cyclometalated platinum
(2) Thomas, S. W., III; Venkatesan, K.; Muller, P.; Swager, T. M. J. Am.
Chem. Soc. 2006, 128, 16641–16648.
^ Work carried out at Eastman Kodak Company, Rochester, NY 14650, was
done prior to December, 2009.
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€
J. Inorg. Chem. 2003, 2749–2752. (b) Lanoe, P.-H.; Fillaut, J.-L.; Toupet, L.; Williams,
*To whom correspondence should be addressed. E-mail: huos@ecu.edu.
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1998, 333, 224–227. (b) Evans, R. C.; Douglas, P.; Williams, J. A. G.; Rochester,
D. L. J. Fluoresc. 2006, 16, 201–6.
J. A. G.; Bozec, H. L.; Guerchais, V. Chem. Commun. 2008, 4333–4335.
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2845–2849. (b) Koo, C.-K.; Ho, Y.-M.; Chow, C.-F.; Lam, M. H.-W.; Lau, T.-C.;
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r
2010 American Chemical Society
Published on Web 04/28/2010
pubs.acs.org/IC

5108 Inorganic Chemistry, Vol. 49, No. 11, 2010
Vezzu et al.
reagents,5 photooxidation catalysts,6 and anticancer agents.7
More recently, the cyclometalated platinum complexes have
been investigated for photogeneration of hydrogen from
water.8 In particular, the platinum complexes that emit
phosphorescent light at ambient temperature have shown
attractive applications in highly efficient organic light-
emitting diode (OLED) devices.9 In an OLED device, both
singlet and triplet excitons are generated in about 1 to 3 ratio
by charge recombination.10 Transitions from triplet excited
states to the singlet ground state are spin forbidden, therefore
only the singlet excitons, that is, about 25% of total excitons
generated in the OLED, can be harvested with the use of a
fluorescent emitter. However, this issue can be addressed by
the use of a phosphorescent emitter, which can harness
both singlet and triplet excitons and maximize the internal
quantum efficiency.11 Room-temperature phosphorescence
emitted from platinum complexes is attributed to the large
spin-orbital coupling constant of platinum, which enables
an efficient singlet-triplet intersystem crossing and makes
the triplet transitions allowed. Cyclometalating ligands in the
platinum complexes play an important role as they serve as
a strong σ donating ligand that can induce large d orbital
splitting and raise the dx2-y2 to higher energy than the anti-
bonding ligand π* orbital(s), which minimizes the undesired
non-radiative d-d transition and improves phosphorescence
quantum efficiency. The involvement of the platinum in the
electronic transitions is essential to the phosphorescent
emission; therefore they are usually characterized as either
MLCT (metal to ligand charge transfer) or LC (ligand
centered) transitions with MLCT admixture.
(2,6-diphenylpyridine),14 and N∧C∧N (1,3-dipyridyl-
benzene)15 ligands. Some of the complexes, particularly
those based on N∧C∧N ligands, have shown potential as
highly efficient triplet emitters in OLED devices.16
The photophysics of bidentate bis-cyclometalated plati-
num complexes has been extensively investigated.17 How-
ever, their application to OLED devices is rather limited by
their photophysical characteristics and thermal stability.
For example, cis-Pt(ppy)2 was reported to be nearly none-
missive at room temperature.17a On the other hand, Pt(thpy)2
(Hthpy =2-(2-thienyl)pyridine) was emissive at room tem-
perature but unstable toward sublimation,18 thus unsuitable
for the vapor deposition process required for producing highly
efficient small molecule OLED devices. The lack of room
temperature phosphorescence and thermal stability is at least
partially attributed to the flexibility of the molecules and the
D2d distortion resulting from the steric hindrance between the
bidentate ligands in those bis-cyclometalated platinum com-
plexes; therefore, the design of a more rigid structure would
address those issues and improve the emission efficiency. Here
we report design, synthesis, structure, and photophysical studies
of highly emissive tetradentate bis-cyclometalated platinum com-
plexes as well as their potential application as phosphorescent
emitters in vapor-deposited OLED devices.19 Platinum(II)
complexes with tetradentate N2O2 ligands have been reported
as robust and good phosphorescent emitters.19b-d
Results and Discussion
Design and Synthesis of the Ligands and the Complexes.
One way of making the bidentate bis-cyclometalated
platinum complexes such as cis-Pt(ppy)2 more rigid is to
form a tetradentate cyclometalating ligand by linking
There are typically two classes of cyclometalated platinum
complexes that have been extensively investigated, that is,
bidentate cyclometalated complexes such as cis-Pt(ppy)2
(Hppy =2-phenylpyridine) and (ppy)Pt(acac)12 and tridentate
cyclometalated complexes such as those derived from
tridentate C∧N∧N (6-phenyl-2,20-bipyridine),13 C∧N∧C
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Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 5109
Scheme 1. Tetradentate Cyclometalating Ligands with Different Coordination Geometries
Scheme 2. Synthesis of Tetradentate Bis-cyclometalated Platinum Complexesa
a Reagents and conditions: (a) Pd(dba)2 (2%), DPPF (2%), NaOtBu (1.2 equiv), toluene, reflux. (b) Pd(OAc)2 (2%), PPh3 (8%), DME, 2 M Na2CO3
(aq), reflux. (c) K2PtCl4 (1 equiv), AcOH, reflux. (d) Pd(PPh3)4 (5%), THF, 50 ꢀC. (e) CuI (5%), trans-N,N0-dimethylcyclohexanediamine (20%),
toluene, reflux.
the two bidentate cyclometalating ligands together. The
tetradentate ligands can coordinate to platinum through
two carbon-platinum covalent bonds and two nitrogen-
platinum coordinative bonds to form a bis-cyclometalated
complex. It can be readily envisaged that such tetradentate
cyclometalating ligands can have different coordination
geometries, that is, C∧N*N∧C, N∧C*C∧N, C∧C*N∧N,
and C∧N*C∧N, as shown in Scheme 1, where an “X∧Y”
(X, Y = C or N) denotes a bidentate coordination to the
platinum to form a five-membered metallacycle and
“X*Y” denotes a coordination to the platinum to form a
six-membered metallacycle. In this paper we will report on
the first two types of complexes (C∧N*N∧C)Pt and
(N∧C*C∧N)Pt.
have chosen an amine linker in this study, mainly because
of the high modularity in ligand design and the simplicity
in ligand syntheses. As demonstrated in Scheme 2, the
synthesis of the tetradentate C∧N*N∧C and N∧C*C∧N
ligands L1-L6 can be efficiently achieved by using the
combination of the palladium-catalyzed C-C and palladium
or copper-catalyzed C-N bond cross coupling reactions.
The C∧N*N∧C ligands L1 and L2 was prepared by two
steps of Pd-catalyzed cross-coupling reactions. The C-N
coupling20 of aniline with excess amounts of 2,6-dibromo-
pyridine produced intermediate dibromide as the major
product, which was readily separated from oligomeric
byproducts by column chromatography. The tetradentate
cyclometalating ligands L1 and L2 were then prepared in
Although there are many ways of linking two bidentate
cyclometalating ligands to form a tetradentate ligand, we
(20) Yang, J.-S.; Lin, Y.-H.; Yang, C.-S. Org. Lett. 2002, 4, 777.

5110 Inorganic Chemistry, Vol. 49, No. 11, 2010
Vezzu et al.
Scheme 3. Undesired Cyclometalation of NCCN Ligand
good yields by the Pd-catalyzed C-C cross coupling21 of
the dibromide with phenylboronic acid and 2,4-difluoro-
phenylboronic acid, respectively. The N∧C*C∧N ligand
L3-L6 were prepared similarly using 1,3-dibromobenzene
as one of the starting materials except for the second step.
The preparation of L3 involved the Negishi coupling22 of
the dibromide intermediate with the 2-pyridylzinc halide
reagent generated in situ from the treatment of 2-bromo-
pyridine with n-butyllithium followed by transmetalation
with ZnCl2, while L4-L6 were prepared using a Cu(I)-
catalyzed carbon-nitrogen bond formation protocol de-
veloped by Buchwald et al.23 It is known that bidentate bis-
cyclometalated platinum complex cis-Pt(ppy)2 could not
be prepared by direct cyclometalation reactions because
the direct metalation withK2PtCl4 produced only chloride-
bridged dinuclear monocyclometalated complexes. Rather,
it was prepared from the transmetalation of Pt(SEt)2Cl2
with the corresponding organolithium reagents.12a In con-
trast, the reaction of the ligands L1 and L2 with K2PtCl4 in
acetic acid gave the desired bis-cyclometalated platinum
complexes 1 and 2 in high yields, which may be attributed to
the fact of the second C-Pt bond formation being an
intramolecular process that is generally considered to be
more favorable than an intermolecular reaction. Complexes
3-6 were prepared similarly. The yields of 3-6 were
significantly lower than those of 1 and 2. This can be
attributed to the side reactions associated with the cyclopla-
tination occurring at unwanted sites as shown in Scheme 3,
which may also produce an oligomeric complex. The com-
plexes were easily purified by column chromatography and
1 was further purified by sublimation for OLED device
fabrications. All new compounds were characterized by 1H
and 13C NMR, MS, and satisfactory elemental analysis.
X-ray Crystal Structure. The molecular structure of 1,
3, and 5 were determined by single-crystal X-ray diffrac-
tion analysis, which confirmed the tetradentate coordina-
tion structure (Figure 1). The selected bond lengths and
angles are listed in Table 2. The nonplanarity of the
platinum coordination of the complexes is shown in
Supporting Information, Figure S1. For complexes 1
and 3, the two phenylpyridine rings are twisted probably
because of the steric interaction of the two rings in the
open end (tail) of the tetradentate complexes. The com-
plexes have a slightly twisted planar geometry around the
platinum coordination center with the largest deviation of
0.083 and 0.101 A from the NCCNPt coordination plane
for 1 and 3, respectively. It is interesting to note that the
(21) Lohse, O.; Thevenin, P.; Waldvogel, E. Synlett 1999, 45.
(22) (a) Smith, A. P.; Savage, S. A.; Love, J. C.; Fraser, C. L. Org. Synth.
2002, 78, 51–62. (b) Alami, M.; Peyrat, J.-F.; Belachmi, L.; Brion, J.-D. Eur.
J. Org. Chem. 2001, 4207–4212.
(23) Antilla, J. C.; Baskin, J. M.; Barder, T.; Buchwald, S. L. J. Org.
Chem. 2004, 69, 5578–5587.
Figure 1. ORTEP drawing of molecular structure of 1, 3, and 5.

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 5111
Table 1. Crystal Data and Structure Refinement Details for 1, 3 1/3CH2Cl2, and 5
3
1
(3 ꢀ 3) CH2Cl2
5
3
empirical formula
formula weight
temperature, K
wavelength, A
crystal system
space group
a, A
b, A
c, A
R, deg
β, deg
C28H19N3Pt
592.55
296(2)
1.54178
monoclinic
P21/c
13.09670(10)
15.123492)
11.24900(10)
90
108.9899(4)
90
2106.79(4)
4
1.868
12.616
1144
C85H59Cl2N9Pt3
1862.58
100(2)
1.54178
triclinic
P1
11.3262(11)
15.7878(14)
20.0657(18)
81.958(5)
83.676(5)
83.412(6)
3513.1(6)
C26H21N5Pt
598.57
296(2)
1.54178
monoclinic
P21/c
7.75100(10)
28.5715(4)
10.22690(10)
90
106.1765(4)
90
2175.16(5)
4
12.250
12.250
1160
γ, deg
volume, A3
Z
2
1.761
12.064
1800
density (calculated), Mg/m3
absorption coefficient, mm-1
F(000)
crystal size, mm
θ range for data collection, deg
limiting indices
0.40 ꢀ 0.19 ꢀ 0.18
3.57 to 66.98
-15 e h e 15
-17 e k e 18
-13 e l e 13
22540
0.68 ꢀ 0.61 ꢀ 0.05
2.23 to 67.00
-12 e h e 13
-18 e k e 18
-23 e l e 23
56175
0.42 ꢀ 0.22 ꢀ 0.19
3.09 to 66.99
-9 e h e 9
-29 e k e 33
-12 e l e 12
23476
reflections collected
independent reflections (Rint
completeness to θ = 67.00ꢀ
absorption correction
)
3584 (0.0295)
95.5%
numerical
12144 (0.0535)
96.9%
numerical
3797 (0.0351)
98.1%
numerical
max. and min transmission
refinement method
data/restraints/parameters
goodness-of-fit on F2
0.2136 and 0.0811
full-matrix least-squares on F2
3584/0/290
0.984
0.5619 and 0.0450
full-matrix least-squares on F2
12144/0/887
1.037
0.2008 and 0.0787
full-matrix least-squares on F2
3797/0/292
1.050
final R indices [I > 2σ(I)]
R indices (all data)
R1 = 0.0157, wR2 = 0.0423
R1 = 0.0172, wR2 = 0.0436
0.397 and -0.334
R1 = 0.0530, wR2 = 0.1512
R1 = 0.0572, wR2 = 0.1588
2.610 and -2.792
R1 = 0.0230, wR2 = 0.0635
R1 = 0.0237, wR2 = 0.0641
1.123 and -0.642
largest diff. peak and hole, e A-3
Table 2. Selected Bond Lengths (A) and Angles (deg) for Compound 1, 3, and 5
complex 1 complex 3
complex 5
1.955(4)
1.956(4)
2.093(3)
2.097(3)
Pt(1)-C(11)
Pt(1)-C(12)
Pt(1)-N(1)
Pt(1)-N(2)
2.000 (3)
2.007(3)
2.051(2)
2.050(2)
Pt(1)-C(11)
Pt(1)-C(22)
Pt(1)-N(1)
Pt(1)-N(2)
1.960(7)
1.959(8)
2.125(7)
2.130(6)
Pt(1)-C(22)
Pt(1)-C(10)
Pt(1)-N(5)
Pt(1)-N(1)
C(11)-Pt(1)-C(12)
C(11)-Pt(1)-N(2)
C(12)-Pt(1)-N(2)
C(11)-Pt(1)-N(1)
C(12)-Pt(1)-N(2)
N(1)-Pt(1)-N(2)
C(1)-N(3)-C(22)
C(1)-N(3)-C(28)
C(22)-N(3)-C(28)
103.86(11)
173.77(9)
82.02(10)
81.85(10)
171.75(10)
92.52(8)
131.4(2)
114.9(2)
113.7(2)
C(22)-Pt(1)-C(11)
C(22)-Pt(1)-N(1)
C(11)-Pt(1)-N(1)
C(22)-Pt(1)-N(2)
C(11)-Pt(1)-N(2)
N(1)-Pt(1)-N(2)
C(21)-N(3)-C(10)
C(21)-N(3)-C(23)
C(10)-N(3)-C(23)
93.6(3)
C(22)-Pt(1)-C(10)
C(22)-Pt(1)-N(5)
C(10)-Pt(1)-N(5)
C(22)-Pt(1)-N(1)
C(10)-Pt(1)-N(1)
N(5)-Pt(1)-N(1)
C(9)-N(3)-C(17)
C(9)-N(3)-C(11)
C(17)-N(3)-C(11)
92.34(16)
80.63(15)
166.99(14)
167.12(14)
80.29(15)
108.54(13)
126.6(3)
170.7(3)
80.7(3)
81.0(3)
173.8(3)
105.1(2)
126.3(6)
117.0(6)
116.7(6)
117.1(3)
116.2(3)
unit cell of the complex 3 contains three independent
molecules with two being essentially identical, but the
third one being nearly flat with the largest deviation of
only 0.023 A from the coordination plane (Supporting
Information, Figure S1, 3a). The head-to-tail stacking of
molecules in the crystal packing of complex 1 suggests a
weak π-π stacking, if any, since the distance between two
adjacent molecules is about 4.0 A. There is no Pt-Pt
interaction shown in the crystal packing of all three
complexes. The N-phenyl ring is nearly perpendicular to
the coordination square plane. Complex 5 has a much
more severely distorted square planar geometry com-
pared to 1 and 3 (Supporting Information, Figure S1).
To minimize the steric interaction of the two methyl
groups and to accommodate the square-planar geometry
of the Pt(II) complex, the pyrazolyl rings are not only
twisted but also bent relative to the phenyl ring that is
connected to it through the C-N bond. The largest
deviation from the NCCNPt coordination plane is
0.193 A.
There are small but distinct differences in the lengths of
the C-Pt and N-Pt bonds in the structures of these
complexes. In complex 1 the average of the Pt-C bond
lengths (2.004(5) A) is very close to that reported for
cis-Pt(ppy)2 (1.993(13) A), while the average of the Pt-N
bonds (2.051(1) A) is somewhat shorter than that in
cis-Pt(ppy)2 (2.127(2) A). On the other hand, the average
Pt-C bond length (1.960(1) A) in complex 3 is slightly
shorter than that reported for cis-Pt(ppy)2, while the
average Pt-N bond length (2.128(4) A) is nearly identical
to that in cis-Pt(ppy)2. The corresponding bond lengths
for 5 are similar to those of 3. The relative shorter Pt-N
bonds in 1 and Pt-C bond in 3 may be attributed to the
formation of an energetically favorable six-membered

5112 Inorganic Chemistry, Vol. 49, No. 11, 2010
Vezzu et al.
Figure 2. Density functional theory calculation (DFT) of orbit density for the HOMOs (bottom) and LUMOs (top) of 1 and 3-6.
Table 3. Calculated Energies (eV) and Energy Gaps of HOMO and LUMO
Orbitals of Complexes
metallacycle in the tetradentate platinum complex, that
is, less strained geometry when compared to other small
or medium sized rings except for the five-membered ring.
In addition, the electron delocalization within the metal-
lacycle may be also responsible for the stronger Pt-N
bonds in 1 and Pt-C bonds in 3. The fact that the endo
amino nitrogen-carbon bonds are shorter than the exo
nitrogen-carbon in 1 and 3 is indicative of an extended
conjugation through the amino nitrogen. In addition, the
nitrogen adopts a planar geometry (a sum of 360 deg of
the three CNC angles around the nitrogen) rather than
a normal trigonal pyramidal geometry, indicating the
feature of a sp2 hybridized N for extended conjugation.
The longer Pt-N bond (2.128(4) A) in 3 relative to the
Pt-C bond (2.004(5) A) in 1 and the shorter Pt-C bond
(1.960(1) A) in 3 relative to the Pt-N bond (2.051(1) A) in
1 may be responsible for the presence of the flat molecule
in the crystal packing of 3 because the open end is more
open; thus, the steric effect in 3 is less significant.
All three twisted structures have approximately C2
symmetry with deviation from the tertiary N that leans
slightly toward one side of the coordination plane, in-
dicating that they are chiral. However, the isomerization
between the two enantiomers may be too fast for them to
be isolated under normal conditions. The presence of the
flat structure in the crystals of 3 further indicates a free
interconversion of two enantiomers. The steric hindrance
between the two methyl groups in 5 may restrict such
isomerization, and the complex might be resolvable.
DFT Calculations. Density functional theory (DFT)
calculations were performed on complexes 1, 3, 4, 5, and
6. The quantum mechanical computer software program
G03 and the B3LYP24,25 exchange-correlation functional
were used to perform all calculations. The DEF2_TZVP
basis set26 was used for platinum, while the cc-pvdz basis
set27 was used for all other atoms. The optimized ground
state geometries of 1, 3, and 5 compare satisfactorily to
those determined by X-ray crystallography. The crystal
structures of 4 and 6 were not determined; however, the
optimized geometries of 4, 5, and 6 provide a clear com-
parison, showing severe distortion of the square planar
coordination of the platinum complex 5 as compared to 4
complex
1
3
4
5
6
LUMO
HOMO
ΔE
-1.70
-5.27
3.57
-1.78
-4.56
2.78
-1.31
-4.64
3.33
-1.14
-4.57
3.43
-1.22
-4.55
3.33
and 6 because of the steric interaction of the two 3-methyl
groups (Supporting Information, Figure S2).
The orbital densities of the highest occupied molecular
orbitals (HOMOs) and lowest unoccupied molecular
orbitals (LUMOs) are shown in Figure 2. The composi-
tion of the frontier molecular orbitals has been very useful
to interpreting the nature of the excited states and the
substituent effect on the photophysical properties of
platinum complexes.28 From the composition of the
HOMOs and LUMOs of the complexes, it was estimated
that the platinum metal makes about a 45% contribution
to the HOMO of 1 and 17-18% contribution to the
HOMOs of 3-6. Notably, the triarylamino nitrogen is
the major contributor to the HOMOs of 3-6, accounting
for about 24-25%. The rest of the HOMO orbital density
is localized in the phenyl rings that are connected to the
pyridyl or pyrazolyl ring. The LUMOs for 1 and 3-6 are
largely localized in the pyridyl or pyrazolyl rings with
small contributions from the phenyl rings. It is interesting
to note that 3- and 5-carbons in the pyrazolyl rings
contribute significantly to the LUMOs of the complexes
4, 5, and 6 while there is essentially no contribution from
the 4-carbon atom. Both HOMO and LUMO orbital
densities have π symmetry. The calculated energies of the
HOMOs and the LUMOs and the energy gaps are listed
in Table 3. The HOMOs of 3-6 are significantly higher
than that of 1, reflecting the electron-donating effect of
the amino nitrogen atom that contributes significantly to
the HOMOs of 3-6 as mentioned above. The LUMO
energies of 4-6 are much higher than that of 1, while 3 has
similar LUMO energy to that of 1, which is consistent
with the poorer electron acceptability of the pyrazolyl
ring compared to the pyridine ring.29
Photophysical Properties. The photophysical data for
all complexes are summarized in Table 4.
(24) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(28) Yersin, H.; Donges, D. Top. Curr. Chem. 2001, 214, 81.
(29) Develay, S.; Blackburn, O.; Thompson, A. L.; Williams, J. A. W.
Inorg. Chem. 2008, 47, 11129–11142.
(25) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B. 1988, 37, 785.
(26) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297.
(27) Dunning, T. H. J. Chem. Phys. 1989, 90, 1007.

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 5113
Table 4. Photophysical Data of Complexes 1-6.a
λ
abs/nm
(ε/M-1 cm-1
λem/nm
(298 K)
λ
em/nm
λem/nm
(77 K)
complex
)
(solid state)
τ/μs
Φd
1
2
3
4
5
6
324 (25229), 342 (26272), 384 (4903), 407 (5969)
268 (29196), 329 (18970), 387 (3891)
274 (28533), 324 (17411), 338 (17435), 409 (9656), 507(1350)
302 (34401), 335 (16243), 351 (28521), 370 (8493), 421 (3367), 442 (2880)
304 (21396), 334 (10307), 348 (13294), 365 (5021), 408 (1656), 428 (1401)
307 (28076), 342 (10328), 357 (18193), 376 (4722), 428 (1835), 451 (1560)
512, 548
488, 523
613
484, 512
474
486, 516
514, 551,595
541, 583
741, 782
509
467, 488, 540
491, 517, 554
501, 539
481, 517
594, 637
477, 510
462, 491
481, 513
7.6b
11.4b
7.6b
4.9b
3.4c
5.7c
0.74(0.03)
0.75(0.01)
0.14(<0.01)
0.56(<0.01)
0.37(<0.01)
0.63(<0.01)
a All measurement were carried out in a solution of 2-methyltetrahydrofuran, The excitation wavelength of 350 nm was used for all compounds except
for 3 where an excitation of 421 nm was used. Solid state emissions were measured with pure powder samples. b Emission lifetime at infinite dilution
extrapolated from the Stern-Volmer expression. c Average of three measurements. d Quinine sulfate solution in 0.1 N H2SO4 (Φ 0.55) was used as a
reference. The data in parentheses are the values measured in aerated solutions.
dilute solution, the 514 nm band may be attributed to the
monomer complex emission. The long wavelength band
may be attributed to the excimer30,31 rather than aggre-
gation of ground state molecules since the absorption of
the complex was found to follow Beer’s law in the
concentration range used in the experiment. Excitation
spectra registered at emission wavelengths of 514 and
710 nm, respectively, are identical, indicating that the
emissions arise from the same ground state species.
These assignments were further supported by following
the time evolution of the emission bands (inset Figure 3)
in which the monomer was observed to follow a single
exponential decay, while the excimer emission initially
increased after the excitation pulse and subsequently
decayed. The observed decay rate followed a linear
dependence upon concentration (Supporting Informa-
Figure 3. Absorption (L1: pink; 1: black), lowest energy absorption (1,
green), and normalized emission (1: blue 5.13 ꢀ 10-5 M; red 1.54 ꢀ 10-4
tion, Figure S3) since the monomer emission was quenched
asa result of excimer formation. The observed decay rate of
the monomer was fit to a Stern-Volmer equation (eq 1),31
where kq is the self-quenching (excimer formation) rate
constant, [Pt] is the concentration of the platinum complex,
and k0 is the decay rate for the monomer complex.
M) spectra (365 nm excitation) and phosphorescence decay of 1 (inset,
4.62 ꢀ 10-4 M) in 2-MeTHF at room temperature.
C∧N*N∧C Platinum Complexes 1 and 2. The absorp-
tion spectra of L1 and 1 are shown in Figure 3. The
emission spectra of 1 at two concentrations are shown in
Figure 3 and normalized to the main peak at 514 nm.
Emission decay curves of 1 at two wavelengths are shown
in the inset of Figure 3. Overlapping absorption bands
from 250 to 360 nm may be assigned as dominantly ligand
1π-π* transitions in view of their high extinction coeffi-
cient (17,000 to 35,000 M-1 cm-1) and similar energies to
the free ligand (L1) absorption bands. A band at 408 nm is
significantly lower in energy than the free ligand absorp-
tions and has extinction (5964 M-1 cm-1) more typical of
1MLCT transitions. When the absorbance was measured
in a long path length cell on an expanded scale, a weak
band at 504 nm was found. The low extinction of this
band (83 M-1 cm-1) suggests a transition to the triplet
excited state enabled by spin-orbit coupling. The weak
band at 504 nm appears to coincide with the high energy
origin of the intense green emission observed at 514 nm in
solution. Thepresence ofaweakbandat504nm intheexci-
tation spectrum of 1 (Supporting Information, Figure S7)
further supports the assignment of the triplet excited state.
The quantum yield of dilute, deoxygenated solution of 1 in
2-methyltetrahydrofuran was found to be 0.74, which is
among the highest reported for cyclometalated platinum
complexes. When the emission of a concentration series of
1 was recorded, a broad emission at longer wavelength
grew at the expense of the first emission band at 514 nm
and its associated vibronic sideband (548 nm) as the
concentration increased. Being the dominant emission in
kobs ¼ kq½Ptꢁ þ k0
ð1Þ
The self-quenching rate constant kq was 3.8 ꢀ 109
M-1sec-1, well within the range reported for other emis-
sive platinum complexes.31 The monomer emission life-
time (1/k0) was found to be 7.8 μs, which is consistent with
the emission from a 3π-π* state with MLCT admixture.
The mixed state is also supported by the DFT calculation,
which showed a contribution to the HOMO of 1 from
both platinum and the ligand. The localized HOMO
and LUMO also suggest an intraligand charge transfer
(ILCT) characteristic of the excited states.
The spectral properties of 2 are broadly similar to those of
1, except shifted to higher energy resulting from the electron-
withdrawing effect of the fluorine substituents (Figure 4).
The weak, lowest energy absorption was found at 480 nm
(48 M-1 cm-1) and the first more intense absorption at
387 nm (3891 M-1 cm-1). The quantum yield of 2 in a
deoxygenated solution of 2-methyltetrahydrofuran was
determined to be 0.75. An observed decay time of 11.4 μs
was determined for the monomer emission at infinite dilu-
(30) (a) Ma, B.; Djurovich, P. I.; Thompson, M. E. Coord. Chem. Rev.
2005, 249, 1501–1510. (b) Farley, S. J.; Rochester, D. L.; Thompson, A. L.;
Howard, J. A. K.; Williams, J. A. G. Inorg. Chem. 2005, 44, 9690–9703.
(31) (a) Connick, W. B.; Gray, H. B. J. Am. Chem. Soc. 1997, 119, 11620.
(b) Connick, W. B.; Geiger, D.; Eisenberg, R. Inorg. Chem. 1999, 38, 3264.

5114 Inorganic Chemistry, Vol. 49, No. 11, 2010
Vezzu et al.
Figure 4. Absorption (L2: pink; 2: black), lowest energy absorption
(2, green), and normalized emission (2: blue 3.84 ꢀ 10-6 M; red 3.08 ꢀ
10-5 M) spectra (365 nm excitation) and phosphorescence decay of 2
(inset, 3.08 ꢀ 10-5 M) in 2-MeTHF at room temperature.
Figure 6. Emission of 1 in Shpol’skii Matrix (n-octane). Blue: 8 K; red:
vibrational satellites at 2 K. Excitation was at 365 nm using cw Ar ion
laser.
Figure 7. High-Resolution Spectra of 1 in Shpol’skii Matrix in the
region of site A and site B. Blue, 40 K and zero field; green, 2 K and zero
field; red, 2 K and 28 kG. Excitation was at 365 nm using cw Ar ion laser.
Figure 5. 77 K emission spectra of 1 and 2 in 2-methyltetrahydrofuran.
intensity, which was not observable in zero field. This
line is more highly forbidden than the others in zero field,
but under the influence of the magnetic field it gains
intensity by mixing another, more allowed state. This
behavior has been well documented for the lowest triplet
sublevel of a number of other Pt(II) complexes.28,32
Therefore, the 512.42 nm line is assigned by analogy as
the lowest energy triplet sublevel, the 512.24 nm line as the
middle, and the 511.82 nm line that was thermally depo-
pulated as the highest energy triplet sublevel for site B.
The total zero field splitting between the highest and
lowest triplet sublevels is thus found to be 23 cm-1, while
the spacing between the middle and lowest sublevels is
7 cm-1. The weak peak at 510.94 nm and the one at
511.1 nm that grew in at high field are attributed to a
different site (A). The highest sublevel for this site was too
weak to detect with confidence, but might be the little
bump at 510.5 nm, giving a ZFS for site A of 23 cm-1
between highest and lowest sublevels, and 6 cm-1 between
middle and lowest. The ZFS values found for 1 are within
the range of values found for bis(C∧N)-cyclometalated
Pt complexes such as Pt(ppy)2 (32 cm-1) and Pt(thpy)2
(16 cm-1).28,32a A precise interpretation of the difference
in ZFS between 1 and Pt(ppy)2 cannot easily be made
because it is also known that guest-host interactions can
tion extrapolated from the Stern-Volmer plot (Supporting
Information, Figure S4). The kq (2.8 ꢀ 109 M-1sec-1) was
similar to that for 1. The emission of both complexes 1 and 2
sharpened at 77 K (Figure 5) and displayed vibronic
progression of 1407 and 1447 cm-1, respectively. The
emission spectra of 1 and 2 in pure solid were recorded
(Supporting Information, Figure S9), and they resemble
those in solution except for the red shift. The red-shifted
emission may be due to ground state intermolecular inter-
actions in pure solid state rather than the excimer formation
as observed in the solution at higher concentration, since
both the emission energy and band shape are quite different
from those of the excimer emission.
High resolution emission spectra were obtained for 1 in
a Shpol’skii matrix of n-octane. As indicated in Figure 6,
the spectrum at 8 K comprises (0,0) origin emission lines
from several distinct sites in the matrix overtop a broad
background of inhomogeneously broadened emission. At
longer wavelengths, there are vibrational satellites that
are relatively low in intensity relative to the more intense
site origin lines. At 2 K, a new set of vibrational satellites
grow in (site origins at 2 K not shown for clarity). The
origin region around site B is shown on an expanded
scale in Figure 7. At 40 K, there is a very weak feature at
510.94 nm, and two more intense lines at 511.82 and
512.24 nm. At 2 K, the line at 511.82 nm has lost intensity
relative to the 512.24 nm line. Application of a magnetic
field at 2 K causes a new line at 512.42 nm to gain
(32) (a) Strasser, J.; Homeier, H. H. H.; Yersin, H. Chem.. Phys. 2000,
255, 301. (b) Rausch, A. F.; Murphy, L.; Williams, J. A. G.; Yersin, H. Inorg.
Chem. 2009, 48, 11407–11414.

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 5115
Figure 10. 77 K emission spectra for 3-6 in 2-methyltetrahydrofuran.
Figure 8. Absorption spectra of 3-6 measured in 2-methyltetrahedro-
furan at room temperature.
N∧C*C∧N Platinum Complexes 3-6. The absorption
spectra and emission spectra of 3-6 are shown in Figure 8
and 9, respectively. These complexes displayed very simi-
lar absorption spectra except for red-shifted absorptions
of 3. As in the absorption spectra of 1 and 2, the higher
energy strong absorption bands can be assigned to ligand
1π-π* transitions. However, there are a series of rela-
tively weak low-energy absorptions (380-470 nm for 4-6
and 490-570 nm for 3) that are markedly different from
those of complexes 1 and 2. These bands have an extinc-
tion coefficient between 2000 and 4000 M-1 cm-1, within
the range for a charge transfer band. They may be
assigned as either MLCT, ILCT, or mixed charge transfer
(MMLLCT) transitions. The lowest energy absorptions
were tentatively assigned as MMLLCT transitions, that
is, a mixed d-π* (Pt), p-π* (the amino nitrogen), and
π-π* charge transfer transition. The assignment of p-π*
transitions is based on the calculated HOMO character of
these complexes which shows significant contribution
from the 2pz orbital of the amino nitrogen. It should be
mentioned that the moderate solvent effect (10-15 nm
hypsochromic shifts from toluene to acetonitrile) was
observed for the low energy charge transfer absorption
bands for compounds 1, and 4-6, while the effect for
compound 3 appears somewhat larger (19-26 nm)
(Supporting Information, Tables S2-S6). The solvent
effect for 2 was not examined because of its poor solubi-
lity. The moderate effect might be due to the twisted
geometry of the molecules and the dipole moment change
induced by the charge transfer being moderate. The weak,
lowest energy absorptions observed for 1 and 2 were not
detected for the complexes 3-6.
Complexes 3-6 emitted intensely at room temperature
in a solution of 2-methyltetrahydrofuran with quantum
yields of 0.14, 0.56, 0.37, and 0.63, respectively. The order
of the emission energy is consistent with the HOMO-
LUMO gaps calculated for 3-6. Complex 3 emitted red
light with λmax of 613 nm. When replacing the pyridine
ring in complex 3 with a pyrazolyl ring the emission was
shifted to 484 nm (complex 4), which is consistent with the
poorer electron accepting ability of the pyrazolyl ring.
Introduction of a methyl group to the 4-position of the
pyrazolyl ring did not induce any appreciable shift of the
emission wavelength (484 nm, 6); however, the methyl
group at the 3-position blue-shifted the emission by about
10 nm (474 nm, 5). This is consistent with the results of
Figure 9. Room temperature emission spectra of 3-6 in 2-methyltetra-
hydrofuran.
influence the value of ZFS and may differ from site to site,
even in the same host.33
The behavior of the vibrational satellites may be under-
stood by analogy to that of numerous other Pt complexes
that have been thoroughly studied by Yersin and co-
workers.28,32b Because the electronic origins of the middle
and upper sublevels of the triplet state were found to be
relatively allowed in zero magnetic field, the vibrational
satellites observed at 8 K (Figure 6), also in zero field,
should be the totally symmetric, Franck-Condon modes
of the complex. At 2 K, a new set of vibrational satellites
grew in that must be associated with the lowest sublevel as
it dominates the Boltzman distribution at 2 K. Because
the electronic origin of the lowest sublevel was found to be
more highly forbidden at 2 K in zero field, it is concluded
that the new vibrational satellites observed at 2 K result
from a spin-vibronic intensity-gaining mechanism for the
lowest sublevel, and that these satellites are therefore
unsymmetrical, Herzberg-Teller modes. A preliminary
analysis of the totally symmetric satellite intensities re-
lative to the respective electronic origin reveals that they
have a Huang-Rhys parameters less than 0.1, indicating
little geometry change between the ground and excited
state. Analogous observations were recently reported for
a tridentate N∧C∧N-Pt complex and were ascribed to the
rigidity imposed by the multidentate ligand.32b
(33) Rausch, A. F.; Thompson, M. E.; Yersin, H. J. Phys. Chem. A 2009,
113, 5927.

5116 Inorganic Chemistry, Vol. 49, No. 11, 2010
Vezzu et al.
Table 5. Performance Data of OLED Devices with 4% of 1 in TPBI-TCTA Host
current density
(mA/cm2)
V
L
ηc
EQE
(%)
λmax
(nm)
CIE
(x, y)
[V] [cd/m2] [cd/A]
0.01
0.1
1
3.31
4.33 49
5
50.0
48.9
45.5
37
14.7
14.2
13.1
10.6
512
512
512
512
0.318, 0.622
0.315, 0.623
0.316, 0.623
0.316, 0.623
5.92 455
8.64 3698
10
Figure 11. EL spectra of devices with various dopant concentrations in
CBP host at the current density of 20 mA/cm2.
DFT calculations, which showed significant contribution
of the 3-carbon to the LUMO of the complexes. An
electron donating group directly attached to this carbon
would be expected to raise the LUMO energy. While
complexes 3 and 5 showed poorly resolved emission at
room temperature, all complexes 3-6 displayed better
resolved spectra at 77 K (Figure 10). The main vibronic
progression was in the range of 1136 to 1383 cm-1. The
emission spectra of 3-6 in pure solid state are shown in
Supporting Information, Figure S10. Complexes 3 and 4
showed broad emissions, while 5 and 6 displayed resolved
spectra. All spectra are red-shifted when compared with
those in fluid state except for that of 5 where the first band
is slightly blue-shifted. The red shift may be due to the
intermolecular interaction in the solid state, although the
excimer formation particularly in the case of 3 cannot be
completely ruled out. The solvent effect on emission of 1
and 3 is moderate (7-8 nm hypsochromic shifts from
toluene to acetonitrile) while the effect on emission of 4-6
is minimal (Supporting Information, Tables S2-S6). The
concentration quenching was observed for complex 3
and 4 but not for 5 and 6. The emission decay time of 3
(7.6 μs) at infinite dilution in 2-methyltetrahydrofuran is
very similar to that of 1. The emission lifetime of 4 at
infinite dilution was estimated to be 4.9 μs. The quenching
constant for both 3 and 4 was found to be the same, 8.0 ꢀ
108 M-1sec-1, which is smaller than those of 1 and 2. The
emission decay time for 5 and 6 were estimated to be 3.4 and
5.7 μs, respectively. The quantum yield and emission life-
time of 4-6 suggest a similar radiative decay rate to those of
complexes 1 and 2. This seems to be inconsistent with the
DFT calculations that indicate a much smaller platinum
contribution to the HOMOs of 3-6 (Supporting Informa-
tion, Table S1). There could be several possible considera-
tions. For example, the triplet emission might not be origi-
nated solely from the HOMO. It is interesting to note that
the emissions of 4-6 occurred at higher energy than that of 1
even though 4-6 have smaller calculated HOMO-LUMO
gaps. On the other hand, the energy gap between the lowest
triplet state and the first singlet state might be smaller for
the complexes 4-6, which would indicate greater mixing of
singlet character into the triplet state. However, as factors
that affect radiative decay can be very complicated, a
detailed time dependent DFT study in combination with
a study of high resolution emission in a Shpol’skii matrix
may be necessary to further elucidate the nature of the
emissive transition states for these complexes.
Figure 12. Comparison ofELspectraofDevicewithdifferent structures
(normalized at 512 nm).
OLED Application. Complex 1 was selected to demon-
strate the utility of the highly emissive tetradentate cyclo-
metalated platinum complexes as a phosphorescent emitter
in OLED devices. Complex 1 has good thermal stability
and could be sublimed without decomposition; therefore,
it is suitable for fabricating OLED devices using vapor
deposition technology. Complex 1 was first tested as a do-
pant in the CBP34 host, a commonly used host for phos-
phorescent emitters. Different dopant concentrations were
examined. The device structure is ITO/CFx (1 nm)/ NBP
(120 nm)/ CBPþ x wt % 1 (35 nm)/BAlq (10 nm)/Alq
(40 nm)/Mg:Ag (220 nm). The performance of the devices
was very poor. The maximum external quantum efficiency
of about 2.2% was demonstrated by the device with 4%
dopant concentration at 6 mA/cm2. The low efficiency may
be attributed to the unbalanced charge injection and the
diffusion of excitons into the NPB layer. As can be seen
from the electroluminescence spectra shown in Figure 11,
an emission from the blue region (possibly emission from
NPB) was presented and became more significant at lower
dopant concentration. At higher dopant concentration, a
broad emission at longer wavelength is growing, which
may be due to the formation of excimer. However, at the
concentration of 4% or less, the excimer emission is negligible.
To improve the performance of the device, a recently
developed device configuration35 with the use of mixed
host and exciton blocking layer was employed to fabricate
the devices. The devices have the following structure of
layers: ITO/CFx (1 nm)/ NBP (65 nm)/ TCTA (10 nm)/
(TPBIþ 30 wt % TCTA)þ 4wt%1(35nm)/TPBI(10nm)/
Alq (40 nm)/ Mg:Ag (220 nm). NPB and Alq were used as
hole and electron transporting layers, respectively. The use of
the mixed host (TPBI and 30% wt % TCTA) is to facilitate
the injection of both charges. A layer of TCTA (10 nm)
(34) Adachi, D.; Kwong, R. C.; Djurovich, P.; Adamovich, V.; Baldo,
M. A.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 2001, 79, 2082.
(35) Kondakova, M. E.; Pawlik, T. D.; Young, R. H.; Giesen, D. J.;
Kondakov, D. Y.; Brown, C. T.; Deaton, J. C.; Lenhard, J. R.; Klubek, K. P.
J. Appl. Phys. 2008, 104, 094501.

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 5117
(4,40,400-tris(N-carbazolyl)triphenylamine) was used between
hole transporting layer of NPB and the emissive layer to
prevent triplet excitons from diffusing to the NPB layer. A
thin layer of TPBI (2,20,200-(1,3,5-benzenetriyl)tris(1-phenyl-
1-H-benzimidazole)) was used as the hole-blocking layer. All
of these modifications are to confine the charge recombina-
tion. The performance of the devices was improved substan-
tially, and the data are summarized in Table 5. The
maximum efficiency of 14.7% was achieved at the current
density of 0.01 mA/cm2, which decreased gradually to 10.6%
with the increase of the current density to 10 mA/cm2. This is
typical for a phosphorescent dopant and may be explained
by the triplet-triplet annihilation that becomes more severe
at higher current density.36 The devices emitted bright green
color with a maximum emission of 512 nm, which is almost
identical to the emission in solution of 2-methyltetrahedro-
furan. The color remains unchanged with the increase of current
density. From the Figure 12, it can be seen that the emission in
the blue region observed for the devices with CBP as host was
completely eliminated in the modified device structure.
were carried out under nitrogen atmosphere and anhydrous
conditions. Tetrahydrofuran (THF) and 2-methyltetrahydro-
furan were distilled from sodium and benzophenone under
nitrogen before use. All other anhydrous solvents were pur-
chased from Aldrich Chemical Co. and were used as received.
All other reagents were commercially available and used as
received. Mass spectra were measured on a Waters ZMD mass
spectrometer. Accurate mass measurements were made on a
JEOL HX-110 mass spectrometer operated in Electron Impact
(EI) ionization. NMR spectra were measured on a Mercury VX
300 or a Varian 500 spectrometer. Spectra were taken in CDCl3
or CD2Cl2 using TMS as standard for 1H NMR chemical shifts
and the solvent peak (CDCl3, 77.0 ppm; CD2Cl2, 53.8 ppm) as
standard for 13C NMR chemical shifts.
Preparation of N,N-di(2-bromopyrid-6-yl)aniline. To
a
250 mL dry, nitrogen flashed flask were charged 2,6-dibromo-
pyridine (11.85 g, 50 mmol), sodium tert-butoxide (4.8 g, 50
mmol), Pd2(dba)3 (0.366 g, 0.4 mmol), DPPF (0.443 g, 0.8 mmol),
aniline (1.8 mL, 20 mmol), and anhydrous toluene (150 mL). The
mixture was stirred at 80-90 ꢀC overnight. After cooling to room
temperature, the reaction mixture was poured into water, and
the aqueous phase was extracted with ethyl acetate (EtOAc) (3 ꢀ
100 mL). The combined organic extracts were washed with water
(200 mL) and brine (200 mL) and dried over MgSO4. Filtration
and evaporation produced dark brown oil, which was purified by
chromatography on silica gel with a mixture of dichloromethane
and heptane (v/v = 1:1) to providea yellow solid, 4.83 g, 60%. 1H
NMR (300 MHz, CDCl3) δ 7.3-7.45 (m, 4H), 7.28(t, J =7.2 Hz,
1H), 7.19 (d, J = 7.3, 2H), 7.09 (d, J = 7.6 Hz, 2H), 6.92 (d, J =
8.2 Hz, 2H). 13C NMR (75 MHz, CDCl3) δ 157.1, 143.4, 139.6,
139.4, 129.8, 127.9, 126.7, 122.0, 114.8. MS: m/z calcd 402.932;
found 402.931.
Conclusion
Six tetradentate bis-cyclometalated platinum complexes
with general coordination patterns of (C∧N*N∧C)-Pt and
(N∧C*C∧N)-Pt were synthesized, and their structures were
confirmed by the X-ray crystal structure analysis. All com-
plexes are emissive at room temperature in solution. Most of
the complexes emitted with very high quantum yields, and
some are among the highest reported for the platinum
complexes. The complexes emitted in a wide spectral range
from blue to red. The emissive states were characterized as
primarily ligand-centered triplet states with MLCT admix-
ture based on their photophysical properties including the
absorption and emission spectra and lifetime of the emissive
state. The ZFS of an emissive triplet state is characteristic of
the degree of MLCT character in the transition28 and was
observed directly by high resolution spectroscopy for 1 in a
Shpol’skii matrix. The ZFS found for 1 (23 cm-1) was within
the range of values found for bis(C∧N)-cyclometalated Pt
Preparation of N,N-di(2-phenylpyrid-6-yl)aniline (L1). Repre-
sentative Procedure. To a 250 mL flask were charged N,N-di-
(2-bromopyrid-6-yl)aniline (2.44 g, 6 mmol), phenylboronic acid
(2.18 g, 18 mmol), triphenylphosphine (0.314 g, 1.2 mmol), and
ethylene glycol dimethyl ether (35 mL). A homogeneous solution
was formed. To this solution was added 2 M K2CO3 (30 mL, 60
mmol). After the mixture was purged with nitrogen, Pd(OAc)2
(0.067 g, 0.3 mmol) was added. The mixture was refluxed for 4 h
and then cooled to room temperature. The reaction mixture was
transferred into a separating funnel, and the organic layer was
separated and retained. The aqueous phase was extracted with
ethyl acetate (EtOAc) (3 ꢀ 50 mL). The combined organic layers
were washed with water (100 mL) and brine (100 mL) and dried
over MgSO4. Filtration and evaporation produced a dark brown
solid, which was purified by chromatography on silica gel with
dichloromethane and heptane (v/v = 4:1) and recrystallization
from heptane and dichloromethane to provide L1 as white crystals,
1.66 g, 69%. 1H NMR (500 MHz, CD2Cl2) δ 7.89 (d, J = 8 Hz,
4H,), 7.67 (t, J = 8 Hz, 2H,), 7.46 (d, J = 8 Hz, 2H), 7.45 (d, J =
8 Hz, 2H), 7.42-7.32 (m, 7H), 7.31 (t, J = 8 Hz, 2H), 7.07 (d, J =
8 Hz, 2H). 13C NMR (125 MHz, CD2Cl2) δ 158.1, 155.9, 145.6,
139.7, 138.6, 129.9, 129.8, 129.1, 18.4, 127.1, 126.1, 115.7, 114.7.
Anal. Calcd for C28H21N3: C, 84.18; H, 5.30; N, 10.52; found: C,
84.21; H, 5.25; N, 10.47. MS: m/z calcd 399.1735; found 399.1731.
N,N-di(2-(2,4-difluorophenyl)pyrid-6-yl)aniline(L2). Yield,
75%. 1H NMR (500 MHz, C2D2Cl4) δ 7.83(dt, 4JH-F = 8.8 Hz;
J = 7.7 Hz, 2H), 7.73 (t, J = 7.8 Hz, 2H), 7.55 (d, J = 7.5 Hz, 2H),
7.5 (t, J = 7.5 Hz, 2H), 7.38 (d, J = 7.5 Hz, 2H), 7.35 (t, J =
7.5 Hz, H), 7.12 (d, J = 7.8 Hz, 2H), 6.98 (m, 2H), 6.94 (m, 2H).
13C NMR (126 MHz, C2D2Cl4,) δ163.3, 161.3, 157.8, 150.7, 145.0,
138.6, 132.5, 129.9, 127.9, 126.1, 123.8, 118.8, 116.1, 112.2, 104.8.
19F NMR (470 MHz, C2D2Cl4, CFCl3@ 0.00 pm) δ -109 (m, 1F),
-110.9 (m, 1F). Anal. Calcd for C28H17F4N3: C, 71.33; H, 3.63; N,
8.91; found: C, 71.41; H, 3.58; N, 8.87. MS: m/z calcd 471.1359;
found 471.1333.
complexes such as Pt(ppy)2 (32 cm-1) and Pt(thpy)2 (16 cm-1
)
3
that have also been described as emitting from LC states
having significant admixture of MLCT character.28,32a In
view of the similar nature of excited triplet states between 1
and Pt(ppy)2, the high efficient room temperature emission
from 1, in contrast to the nonemissive nature of Pt(ppy)2,
may be mainly attributed to the rigidity of the tetradentate
platinum complex. DFT calculations also suggested an ILCT
characteristic of the excited states because of the localized
characteristic of the HOMOs and LUMOs of the complexes.
The application of the complexes in OLEDs was demon-
strated by incorporating 1 into an emissive layer of an OLED
using a mixed host and an exciton blocking layer. The devices
demonstrated high performance with a maximum external
efficiency of 14.7% and showed little evidence of excimer
emission at moderate concentrations.
Experimental Section
Synthesis. General Procedures. All reactions involving
moisture- and/or oxygen-sensitive organometallic complexes
Preparation of Platinum Complex (1). Representative Pro-
cedure. Amixtureofthe tetradentate C∧N*N∧C ligand L1 (0.40 g,
(36) Adachi, D.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Appl.
Phys. Lett. 2000, 77, 904.

5118 Inorganic Chemistry, Vol. 49, No. 11, 2010
Vezzu et al.
1 mmol), K2PtCl4 (0.42 g, 1 mmol), Bu4NCl (few crystals), and
glacial acetic acid (60 mL) was degassed and refluxed under
nitrogen for 45 h. After cooling to room temperature, the yellow
to orange precipitates were collected by filtration and washed with
water and methanol, and dried in air. The crude materials were
purified by flash chromatography on silica gel with dichloro-
methane as eluting solvent; the first yellow band was collected
and evaporated to give 0.47 g of yellow crystals, 79%. 1H NMR
(500 MHz, CD2Cl2) δ8.32 (m, J = 7.7Hz, 3JPt-H 25 Hz, 2H), 7.82
(d, J = 7.8 Hz, 2H), 7.75-7.68 (m, 4H), 7.67(t, J = 7.7 Hz, 1H),
7.64 (d, J = 7.8 Hz, 2H), 7.46 (d, J = 7.8 Hz, 2H), 7.41 (d, J =
Complex (3). This complex was prepared following the general
procedure described for complex 1. Yield 30.5%. 1H NMR (500
MHz, CD2Cl2) δ 8.97 (d, J = 5.5 Hz, 2H), 8.01 (s, 1H), 7.99 (s,
1H), 7.94 (td, J = 7.5, 1.5 Hz, 2H), 7.66 (t, J = 7.5 Hz, 2H), 7.53
(tt, J = 7.5, 1.5 Hz, 1H), 7.45-7.42 (m, 2H), 7.34 (d, J = 7.5 Hz,
2H), 7.34-7.32 (m, 2H), 7.00 (t, J = 7.5 Hz, 2H), 6.28 (d, J = 8.5,
2H). 13C NMR (125 MHz, CD2Cl2) δ 166.6, 147.9, 146.7, 145.8,
142.7, 138.5, 131.8, 131.2, 128.6, 128.0, 123.5, 122.6, 120.0, 118.9,
116.2. MS m/z 592; Anal. Calcd for C28H19N3Pt 1/3(CH2Cl2): C,
3
54.81; H, 3.19; N, 6.77. Found: C, 54.95; H, 3.10; N, 6.76.
Preparation of N,N-di(3-(1H-pyrazol-1-yl)phenyl)aniline L4.
General Procedure. To a 25 mL dry, nitrogen flushed flask were
charged N,N-di(3-bromophenyl)aniline (200 mg, 0.5 mmol,),
pyrazole (101 mg, 1.5 mmol), potassium carbonate (150 mg, 1.1
mmol), copper iodide (17 mg, 0.1 mmol), trans-N,N0-dimethyl-
cyclohexanediamine (66 μL, 0.4 mmol), and toluene (5 mL). The
mixture was refluxed for 5 days. After cooling to room tem-
perature, 20 mL of ethyl acetate was added, and the precipitate
formed was filtered out. The filtrate was concentrated, and the
product was separated by column chromatography with hexane
and ethyl acetate (v/v = 5:1) to give a solid of 301 mg, yield 80%.
1H NMR (500 MHz, CD2Cl2) δ 7.86 (d, J = 2.5 Hz, 2H), 7.62 (d,
J = 2.0 Hz, 2H), 7.47 (t, J = 2.5 Hz, 2H), 7.38-7.30 (m, 6H),
7.20-7.15 (m, 2H), 7.10 (tt, J = 8.0, 1.0 Hz, 1H), 7.04-6.98 (m,
2H), 6.40 (t, J = 2.5 Hz, 2H). 13C NMR (75 MHz, CDCl3) δ
148.7, 147.0, 141.3, 141.0, 130.2, 129.6, 126.9, 124.9, 123.8,
122.0, 114.8, 113.8, 107.5. Anal. Calcd for C24H19N5: C, 76.37;
H, 5.07; N, 18.55. Found: C, 76.09; H. 4.94; N, 18.44.
N,N-di(3-(3-methyl-1H-pyrazol-1-yl)phenyl)aniline (L5). Yield
68%.1H NMR (500 MHz, CD2Cl2) δ 7.74 (d, J = 2.5 Hz, 2H),
7.43 (t, J = 2 Hz, 2H), 7.36-7.28 (m, 6H), 7.15 (d, J = 6.5 Hz,
2H), 7.09 (tt, J = 7.5, 1.5 Hz, 1H), 6.96 (t, J = 2.5 Hz, 1 H), 6.94
(t, J = 2.0 Hz, 1H), 6.20 (d, J = 2.5 Hz, 2H), 2.27 (s, 6H). 13C
NMR (75MHz, CDCl3) δ 150.5, 148.6, 147.1, 141.3, 130.2, 129.5,
127.5, 124.7, 123.5, 121.7, 114.5, 113.6, 107.5, 13.7. Anal. Calcd
for C26H23N5: C, 77.01; H, 5.72; N, 17.27. Found: C, 76.61; H,
5.58; N, 16.96.
7.3 Hz, 2H), 7.21 (t, J = 7.5 Hz, 2H), 6.51 (d, J = 8.7 Hz, 2H). 13
C
NMR (125 MHz, CD2Cl2) δ 164.3, 149.6, 149.3, 147.7, 143.8,
137.4, 136.2, 132.2, 131.0, 130.3, 129.8, 124.4, 123.7, 114.7, 112.4.
Anal. Calcd for C28H19N3Pt: C, 56.76; H, 3.23; N, 7.09. Found: C,
56.63; H, 3.20; N, 7.06. MS: m/z calcd 592.1227; found 592.1240.
Complex (2). This complex was prepared using the same pro-
cedure, yielding the crude greenish yellow crystalline materials,
66%, which were purified by sublimation to give yellow needles,
57%. 1H NMR (500 MHz, C2D2Cl4) δ 8.07 (d, J = 8.0 Hz, 2H),
7.86 (t, J = 8.6 Hz, 2H), 7.8 (t, J = 7.3 Hz, 2H), 7.77 (m, 2H),
7.76 (m. 1H), 7.46 (d, J = 7.3 Hz, 2H), 6.79 (m, 2H), 6.59 (d, J =
8.6 Hz, 2H); 13C NMR (126 MHz, C2D2Cl4) δ 138.3, 132.1,
130.3, 130.2, 117.6, 116.3, 114.3, 99.9 (because of very poor
solubility, only partial carbon signals were observed); 19F NMR
(470 MHz, C2D2Cl4, CFCl3@ 0.00 pm) δ -107.4 (m, 2F),
-109.1 (m, 2F). Anal. Calcd for C28H15F4N3Pt: C, 50.61; H,
2.28; N, 6.32. Found: C, 50.53; H, 2.35; N, 6.32. MS: m/z calcd
664.0852; found 664.0873.
N,N-di(3-bromophenyl)aniline. To a 100 mL dry, nitrogen
flushed flask was charged with aniline (186 mg, 2 mmol), di-
bromobenzene (1.40 g, 6 mmol), sodium tert-butoxide (576 mg,
6 mmol), Pd(dba)2 (92 mg, 0.16 mmol), DPPF (89 mg, 0.16
mmol) in toluene (10 mL). The reaction mixture was refluxed
overnight. After cooling to room temperature, 20 mL of ethyl
acetate was added and stirred for a while. The precipitate
formed was filtered, and the filtrate was evaporated. The crude
mixture was purified by chromatography on silica gel with a
mixture of hexane and ethyl acetate (v/v=15:1) to give a white
solid, 182 mg, yield 45%. 1H NMR (300 MHz, CDCl3), δ
7.4-6.8 (m, 13 H). 13C NMR (75 MHz, CDCl3), δ 148.7,
146.6, 130.5, 129.6, 126.5, 125.9, 125.1, 124.2, 122.9, 122.3.
Anal. Calcd for C18H13Br2N: C, 53.63; H, 3.25; N, 3.47. Found:
C, 53.79; H, 3.16; N, 3.44.
N,N-di(3-(4-methyl-1H-pyrazol-1-yl)phenyl)aniline (L6). Yield
85%. 1H NMR (500 MHz, CD2Cl2) δ 7.65 (s, 2H), 7.48-7.42 (m,
4H), 7.36-7.28 (m, 6H), 7.18 (d, J = 7.5 Hz, 2H), 7.13 (t, J =
7 Hz, 1H), 6.88 (d, J = 6.5 Hz, 2H), 2.12 (s, 6H). 13C NMR
(75 MHz, CDCl3) δ 148.6, 147.1, 141.7, 141.3, 130.2, 129.5, 125.5,
124.8, 123.6, 121.6, 118.2, 114.3, 113.3, 8.9. Anal. Calcd for
C26H23N5: C, 77.01; H, 5.72; N, 17.27. Found: C, 77.22; H,
5.80; 17.29.
N,N-di(3-(pyridin-2-yl)phenyl)aniline (L3). To a 250 mL,
3 necked dry, nitrogen flushed flask was charged with n-BuLi
(1.6 M in hexanes, 6 mL, 9.6 mmol) and cooled to -78 ꢀC. To
this, a solution of 2-bromopyridine (0.8 mL, 8 mmol) in 7 mL of
ether was added dropwise. After stirring for 30 min, the reaction
mixture was warmed to 0 ꢀC, and zinc chloride solution (1.0 M in
diethyl ether, 8 mL, 8 mmol) was added dropwise. The mixture
was then warmed to room temperature. N,N-di(3-bromophenyl)-
aniline (800 mg, 2 mmol), Pd(PPh3)4 (230 mg, 0.2 mmol), and
8 mL of diethyl ether were added, and the reaction mixture was
refluxed for 36 h. After cooling, the reaction mixture was quen-
ched by water and extracted with ethyl acetate (3 ꢀ 75 mL). The
combined organic phases were washed with water (100 mL),
brine (100 mL), dried over MgSO4, filtered, and evaporated.
The crude product was purified by chromatography on silica
gel with a mixture of petroleum ether and ethyl acetate (v/v =
Complex 4. Following general procedure, this complex was
prepared in 25% yield. 1H NMR (500 MHz, CD2Cl2) δ 8.10 (d,
J = 3.0 Hz, 2H), 7.89 (d, J = 1.5 Hz, 2H), 7.66 (t, J = 8.0 Hz,
2H), 7.53 (tt, J = 7.0, 1.0 Hz, 1H), 7.31 (d, J = 7.5 Hz, 2H),
6.96-6.88 (m, 4H), 6.64 (t, J = 2.5 Hz, 2H), 6.04 (dd, J = 8.0,
2.0 Hz, 3JPt-H 17 Hz, 2H). 13C NMR (75 MHz, CD2Cl2) δ 146.2,
145.3, 143.4, 139.7, 131.7, 131.3, 128.2, 127.1, 123.6, 115.6,
114.9, 107.5, 103.5. MS m/z 570.11 (Mþ); Anal. Calcd for
C24H17N5Pt: C, 50.53; H, 3.00; N, 12.28. Found: C, 50.33; H,
2.88; N, 12.24.
Complex 5. Following general procedure, this complex was
1
prepared in 33% yield. H NMR (500 MHz, CD2Cl2) δ 8.01
(d, J = 3.0 Hz, 2H), 7.63 (tt, J = 8.0, 1.5 Hz, 2H), 7.5 (tt, J =
7.5 Hz, 1H), 7.26 (dt, J = 7.0, 1.0 Hz, 2H), 6.92-6.84 (m, 4H),
6.42 (d, J = 2.5 Hz, 2H), 5.97 (dd, J = 7.5, 2.0 Hz, 2H), 2.54 (s,
6H). 13C NMR (75 MHz, CD2Cl2) δ 151.3, 146.7, 145.4, 142.9,
131.7, 131.2, 129.6, 128.0, 123.5, 114.2, 113.8, 107.9, 102.4, 16.1.
MS m/z 598.14(Mþ); Anal. Calcd for C26H21N5Pt: C, 52.17; H,
3.54; N, 11.70. Found: C, 52.15; H, 3.48; N, 11.63.
1
2:1) to give a solid, 597 mg, yield 75%. H NMR (500 MHz,
CD2Cl2), δ 8.61 (d, J = 4.0 Hz, 2H), 7.77 (s, 2H), 7.66-7.63 (m,
4H), 7.60-7.54 (m, 2H), 7.35 (t, J = 7.5 Hz, 2H), 7.25 (t, J =
7.5 Hz, 2H), 7.20-7.14 (m, 6H), 7.00 (t, J = 7.0 Hz, 1H). 13
C
Complex 6. Following general procedure, this complex was
prepared in 25% yield. 1H NMR (500 MHz, CD2Cl2) δ 7.88 (s,
2H), 7.69 (s, 2H), 7.64 (t, J = 8.0 Hz, 2H), 7.51 (tt, J = 7.0,
1.5 Hz, 1H), 7.29 (d, J = 7.5 Hz, 2H), 6.90-6.82 (m, 4H), 5.99
(dd, J = 7.0, 1.0 Hz, 3JPt-H 17 Hz, 2H), 2.24 (s, 6H). 13C NMR
NMR (75 MHz, CDCl3), δ 157.2, 149.6, 148.3, 147.8, 140.8,
136.7, 129.7, 129.3, 125.0, 124.1, 122.8, 122.73, 122.1, 121.6,
120.7. Anal. Calcd for C28H21N3: C, 84.18; H, 5.30; N, 10.52;
found: C, 84.11; H, 5.18; N, 10.45.

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 5119
(75 MHz, CD2Cl2) δ 146.5, 145.3, 143.3, 139.8, 131.7, 131.2,
128.0, 125.8, 123.4, 118.2, 115.6, 114.5, 103.2, 9.4. MS m/z
598.14(Mþ); Anal. Calcd for C26H21N5Pt: C, 52.17; H, 3.54;
N, 11.70. Found: C, 51.71; H, 3.45; N, 11.49.
molecule was found in the difference map. Platon Squeeze was
not found to significantly improve the structure, so the non-
Squeeze results were used. Hydrogen atoms were placed in
calculated positions and allowed to be refined isotropically as
riding models. Structure solutions, refinement, and the calcula-
tion of derived results were performed using the SHELXTL
package of computer programs.40 Details of the X-ray experi-
ments and crystal data are summarized in Table 1.
Photophysical Experiments. Absorptionspectra wererecorded
using a HP-8453 or Varian dispersive spectrometer (Cary 5E)
diode array UV/vis spectrophotometer, using 1 cm path-length
quartz cuvettes. A 10 cm path length quartz cuvette was used for
recording the weak lowest energy absorption spectra of 1 and 2.
The steady state emission spectra were measured using a PTI
QM-4CW system.
OLED Fabrication. Bottom-emitting OLEDs, were fabri-
cated on ∼1.1-mm thick glass substrates precoated with a
transparent indium tin oxide (ITO) conductive layer having a
thickness of ∼22 nm and a sheet resistance of ∼68 X/square. The
substrates were cleaned and dried using a commercial glass
scrubber tool. The ITO surface was subsequently treated with
oxygen plasma, and then conditioned as a modified anode by
decomposing CHF3 gas in a plasma treatment chamber to
deposit an ∼1 nm-thick layer of CFx. Six substrates in each
experiment were then transferred into a vacuum chamber for
sequential deposition of all organic and metal layers on top of
the substrates by a thermal evaporation method without break-
ing vacuum (∼10-6 Torr). The deposition rates and doping
concentrations of materials were controlled and measured
in situ using calibrated thickness monitors. The deposition rate
of the organic host materials was ∼0.4 nm/s, and the deposition
rates of the dopant materials were adjusted according to the
volume ratio in the host materials. The Al cathode formed on
top of the organic layers has a thickness of about 210 nm. Each
OLED has an emission area of 1 mm2. The devices were trans-
ferred from the vacuum chamber into a nitrogen-filled glovebox for
encapsulation before testing. The EL characteristics of all the fabricated
devices were evaluated using a constant current source (Keithley 2400
SourceMeter) and a photometer (Photo Research SpectraScan PR
650) at room temperature. The external quantum efficiencies (EQE) of
the devices were calculated with the assumption of a Lambertian
angular distribution of light emitted from the devices.
Quantum yields were measured using comparative method at
room temperature in a 2-methyltetrahydrofuran solution. The
solution was deoxygenated by purging with nitrogen gas for
20 min. A long necked 1 cm quartz cuvette was used for measure-
ments. The cuvette was cooled with a dry ice-acetone bath to
prevent solvent loss. A solution of quinine sulfate dihydrate in
0.1 N H2SO4 was used as a reference. The optical density of both
sample and reference solutions was maintained below 0.1 AU at
the excitation wavelength. Phosphorescence lifetime measurements
were performed on the same fluorimeter equipped with variable
high rep rate pulsed xenon source for excitation and were limited
to lifetimes >0.4 μs. Samples of compound 1 in a Shpol’skii matrix
of n-octane for high resolution spectroscopy were prepared as
previously described.37 Time-resolved measurements for com-
plexes 1 and 2 were carried out at a variety of wavelengths
(sample dependent) on the Low Temperature Photo-Luminescence
(LTPL) instrument. The LTPL setup consisted of an ISA mono-
chromator (model 750M) for the emission monochromator, a
variety of optical lenses and filters, and a PMT with associated
electronics. The excitation source was a Continuum Minilite Nd:
YAG laser excitation (3-5 ns pulse width at 355 nm, ≈3 mJ per
pulse). The RCA PMT detects emission decays after each flash at a
given wavelength as selected by the 750 M monochromator. The
decays are averaged using a Tektronix digital oscilloscope (model
TDS 3032) and acquired by computer. Normal decay curves are the
average of 256 events (flashes).
Acknowledgment. We thank Robert Schultz and Thomas
Marchincin for part of photophysical measurements and
Dr. Denis Kondakov for helpful discussion of photophyiscal
experiments. We also thank Dr. Thomas Jackson for HRMS
measurements and Dr. William C. Lenhart for part of NMR
measurements. Financial support from the Research Corpora-
tion for Science Advancement (SH), Burroughs Wellcome
Fellowship (D.A.K.V.), and East Carolina University is grate-
fully acknowledged. R.D.P. acknowledges NSF (CHE-
0443345) and the College of William and Mary for the purchase
of the X-ray equipment.
X-ray Crystallography. Crystals were grown by diffusing
hexanes to a dichloromethane solution of the complexes. A crystal
was selected and mounted on a glass fiber. All measurements
were made using graphite-monochromated Cu KR radiation
(1.54178 A) on a Bruker-AXS three-circle diffractometer,
equipped with a SMART Apex II CCD detector. In each case,
initial space group determination was based on a matrix con-
sisting of 120 frames. The data were reduced using SAINTþ,38a
and empirical absorption correction was applied using SA-
DABS.38b Structures were solved using direct methods. Least-
squares refinement for all structures was carried out on F2. All
non-hydrogen atoms were refined anisotropically, except for the
solvent CH2Cl2 carbon in 3. In the case of 3, three significant
(>2 e/A3) peaks were present; however, each was very close to a
platinum atom. Platon39 detected a void having a volume of
232.0 A3 in the unit cell of 3. However, no additional solvent
Supporting Information Available: Crystallographic data in
CIF format for 1, 3, and 5. Experimental molecular geo-
metries for 1, 3, and 5 and DFT calculated geometries for
4-6. Calculated energies and Pt character of molecular
orbitals. Stern-Volmer plots, excitation spectra of 1 and 2,
solid state emission spectra of 1-6, and solvent effects. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(37) Marchetti, A. P.; Deaton, J. C.; Young, R. Y. J. Phys. Chem. A 2006,
110, 9828–9838.
(38) (a) SAINT PLUS; Bruker Analytical X-ray Systems: Madison, WI,
2001. (b) SADABS; Bruker Analytical X-ray Systems: Madison, WI, 2001.
(39) Spek, A. L. J. Appl. Crystallogr. 2002, 36, 7.
(40) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112.