Cyclometalated platinum(II) complexes of lepidine-based ligands as highly efficient electrophosphors

Article abstract of DOI:10.1021/om100573r

A new series of lepidine-containing-ligand-based cyclometalated platinum(II) complexes 1-7 with general formula [Pt(L)acac] (L = lepidine-based ligand, acac = acetylacetonate) have been synthesized. The complexes have been characterized using spectral and electrochemical techniques. The single-crystal X-ray structures of complexes 2 and 5 have been successfully determined. The effects of substituents on the electronic features and redox potentials of the cyclometalated phenyl ring of the complexes are discussed. Complexes 1-7 are highly phosphorescent in fluid as well as in solid state, covering a rather broad spectral range from yellow to saturated red. These complexes were successfully used as a dopant material to fabricate the electroluminescent device, ITO/NPB (40 nm)/CBP-doped Pt complex (5 wt %) (20 nm)/BCP (10 nm)/Alq₃ (20 nm)/LiF (1 nm)/Al (150 nm), which exhibits very promising performance. The device doped with 5 wt % 3 produced a maximum external quantum efficiency of 15.2% with a maximum brightness of 26847 cd m^-2 and corresponds to a luminance efficiency of 29.83 cd A ^-1.

Full text of DOI:10.1021/om100573r

3912 Organometallics 2010, 29, 3912–3921
DOI: 10.1021/om100573r
Cyclometalated Platinum(II) Complexes of Lepidine-Based Ligands
as Highly Efficient Electrophosphors
Marappan Velusamy,† Chih-Hsin Chen,† Yuh S. Wen,† Jiann T. Lin,*,† Chao-Chen Lin,‡
Chin-Hung Lai,‡ and Pi-Tai Chou*,‡
†Institute of Chemistry, Academia Sinica, 128, Section 2, Academia Road, Taipei 115, Taiwan, Republic of
China, and Department of Chemistry, National Taiwan University, Section 4, Roosevelt Road, Taipei 106,
‡
Taiwan, Republic of China
Received June 10, 2010
A new series of lepidine-containing-ligand-based cyclometalated platinum(II) complexes 1-7
with general formula [Pt(L)acac] (L = lepidine-based ligand, acac = acetylacetonate) have been
synthesized. The complexes have been characterized using spectral and electrochemical techniques.
The single-crystal X-ray structures of complexes 2 and 5 have been successfully determined. The
effects of substituents on the electronic features and redox potentials of the cyclometalated phenyl
ring of the complexes are discussed. Complexes 1-7 are highly phosphorescent in fluid as well as in
solid state, covering a rather broad spectral range from yellow to saturated red. These complexes were
successfully used as a dopant material to fabricate the electroluminescent device, ITO/NPB (40 nm)/
CBP-doped Pt complex (5 wt %) (20 nm)/BCP (10 nm)/Alq3 (20 nm)/LiF (1 nm)/Al (150 nm), which
exhibits very promising performance. The device doped with 5 wt % 3 produced a maximum external
quantum efficiency of 15.2% with a maximum brightness of 26847 cd m-2 and corresponds to a
luminance efficiency of 29.83 cd A-1
.
Introduction
immense attention due to their versatile applications in
organic light-emitting diodes (OLEDs). Luminescent mate-
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(I),4 gold(III),5 and platinum(II)6 centers, have received
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pubs.acs.org/Organometallics
Published on Web 08/10/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 17, 2010 3913
beneficial for electroluminescent devices. Strong spin-orbit
coupling in the above-mentioned heavy metal complexes
permits efficient intersystem crossing between singlet and
triplet states and utilization of both states available for
emission.7 Consequently, the device efficiencies based on
phosphorescent metal complexes are far superior to those
based on pure organic fluorescent compounds emitting only
from the singlet excited state. Among these phosphorescent
materials, iridium(III) complexes have been extensively stu-
died because of their relatively short lifetime, high quantum
efficiency, and easy tuning of the emission color8 via suit-
able ligand alternations. Cyclometalated complexes are also
attractive due to their neutral nature, which allows high
thermal stability and vapor deposition techniques for the
creation of amorphous thin films suitable for the fabrication
of light-emitting diodes. Even though many reports have
concentrated on cyclometalated iridium(III) complexes,
there were relatively fewer studies of analogous phosphor-
escent platinum(II) complexes and their exploitation in
electroluminescent devices.6,9 A main obstacle in the deve-
lopment of highly emissive platinum(II) complexes in OLEDs
is the great tendency of these square-planar species to aggre-
gate at higher concentration, resulting in quenching of
emission in the solid state. We previously reported a new
series of lepidine-based cyclometalated iridium(III) emitters
with rich photophysical properties.10 These lepidine-based
complexes emit in the orange to red region and can be
fabricated into devices of good performance. Therefore, we
extended our studies to cyclometalated platinum complexes
with an aim to unravel the structure versus luminescence
property relationships. In this article, we present the synthe-
sis and spectral and electroluminescent characteristics of
platinum(II) cyclometalated complexes featuring aryl-sub-
stituted lepidine ligands. We observe that inhibition of stack-
ing by incorporation of suitable substituents at the aryl moie-
ties is beneficial to the emitting behavior of these complexes
in doped matrixes. As a result, those ligands that are func-
tionalized effectively to prevent intermolecular stacking
interactions display better electroluminescent parameters.
Results and Discussion
Synthesis and Characterization. The structures of the new
platinum complexes 1-7 (abbreviated as [PtC∧N(acac)])
(C∧N = cyclometalated ligands 1a-7a; acac = acetylace-
tonate) are shown in Figure 1. Some substituted lepidines
(1a-5a) used in the present work for the assembly of
luminescent platinum complexes were available from our ear-
lier study.10 However, the fluorene- and bifluorene-containing
ligands (6a and 7a) were strategically designed and synthesized
for this work as illustrated in Scheme 1. Ligands 6a and 7a were
obtained from 2-chlorolepidine and the corresponding boronic
acid (9-diethyl-9H-fluoren-3-ylboronic acid or bifluorenylbo-
ronic acid) via Suzuki’s C-C coupling reaction.11 Cyclometa-
lated complexes 6 and 7 were then obtained from the ligand 6a
and 7a by a two-step protocol essentially the same as that used
for the construction of iridium(III) cyclometalated complexes:
(a) treatment of the ligands with K2PtCl4 produced the cholro-
bridged dimers 6b and 7b; (b) subsequent reaction of 6b and 7b
with acetylacetone in the presence of sodium carbonate yielded
the desired monomeric complexes 6 and 7. Complexes 1-5
were synthesized from 1a-5a by the same synthetic protocol.
All the complexes were characterized by NMR, high-resolution
mass spectra, and elemental analyses. Single-crystal X-ray
structural characterization was also carried out on two selected
complexes, 2 and 5. All of these complexes are soluble in
common organic solvents including dichloromethane, chloro-
form, and tetrahydrofuran, while they are insoluble in hexane
and protic solvents. They are intensely orange or red in the solid
state; however, dissolution in solvent leads to dark yellow
solutions and gives rise to intense orange or red color emission
when exposed to visible light.
Crystal Structures. Single crystals of the complexes 2 and 5
suitable for X-ray diffraction analysis were obtained by slow
diffusion of hexane to the dichloromethane solutions of the
corresponding complex. ORTEP plots of 2 and 5 are dis-
played in Figures 2 and 3, respectively. In both structures
the platinum(II) center is located in a slightly distorted
square-planar environment, as indicated by the bond angles
C(21)-Pt-O(2) [2, 173.9(2)o; 5, 174.1(1)o] and O(1)-Pt-N(1)
[2, 170.1(2)o; 5, 169.94(9)o]. The bond length of Pt-N
(7) (a) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson,
M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4. (b) Adachi, C.; Baldo,
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˚
˚
[2, 2.070(4) A; 5, 2.038(3) A] is slightly longer compared
to that of common Pt-N bonds (∼1.95-1.99 A)12 because
˚
this N atom is opposite the acetylacetonate oxygen atom
having weak trans influence. Similarly, the larger Pt-O(2) [2,
€
€
(c) Yersin, H. Top. Curr. Chem. 2004, 241, 1. (d) Kohler, A.; Bassler, H.
Mater. Sci. Eng. R 2009, 66, 71.
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Hong, J.-I. Organometallics 2005, 24, 1578. (b) Takizawa, S.; Nishida, J.;
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15, 1455.
˚
˚
2.082(3) A; 5, 2.107(2) A] bond length than Pt-O(1) [2,
˚
˚
2.010(3) A; 5, 2.003(2) A] can be attributed to the greater
trans influence of the phenyl carbon C(21), which resides in
the trans position to O(2). The bond distances and angles
within the acac ligand are comparable with those found
in other acetylacetonate Pt(II) complexes.13 In the crystal
(9) (a) Laskar, I. R.; Hsu, S.-F.; Chen, T.-M. Polyhedron 2005, 24,
881. (b) Che, C.; Chan, S.; Xiang, H.; Chan, M. C. W.; Liu, Y.; Wang, Y.
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J.; Jabbour, G. E. Adv. Mater. 2008, 20, 2405. (f) Yan, B.-P.; Cheung,
C. C. C.; Kui, S. C. F.; Xiang, H.-F.; Roy, V. A. L.; Xu, S.-J.; Che, C.-M. Adv.
Mater. 2007, 19, 3599. (g) Zhou, G.; Wang, Q.; Ho, C.-L.; Wong, W.-Y.; Ma,
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Aly, S. M.; Fortin, D.; Harrey, P. D.; Yao, B.; Xie, Z. Y.; Wang, L. X.
Macromol. Chem. Phys. 2009, 210, 1786. (i) Zhou, G.-J.; Wong, W.-Y.;
Yao, B.; Xie, Z.; Wang, L. J. Mater. Chem. 2008, 18, 1799.
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(b) Suzuki, A. J. Organomet. Chem. 1999, 576, 147.
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Organometallics 2005, 24, 4079. (b) Yin, B.; Niemeyer, F.; Williams,
J. A. G.; Jiang, J.; Boucekkine, A.; Toupet, L.; Bozec, H. L.; Guerchais, V.
Inorg. Chem. 2006, 45, 8584. (c) Li, H.; Ding, J.; Xie, Z.; Cheng, Y.; Wang,
L. J. Organomet. Chem. 2009, 694, 2777. (d) Ho, C.-L.; Wong, W.-Y.; Yao,
B.; Xie, Z.; Wang, L.; Lin, Z. J. Organomet. Chem. 2009, 694, 2735. (e) Liu,
J.; Yang, C.-J.; Cao, Q.-Y.; Xu., M.; Wang, J.; Peng, H.-N.; Tan, W.-F.; L€u,
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41, 3055. (c) Gong, J.-F.; Fan, X.-H.; Xu, C.; Li, J.-L.; Wu, Y.-J.; Song,
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5677.

3914 Organometallics, Vol. 29, No. 17, 2010
Velusamy et al.
Figure 1. Structures of the cyclometalated platinum(II) complexes featuring lepidine donors.
Scheme 1. Synthetic Pathway Leading to the Formation of Complexes 6 and 7
packing, compound 2 forms head-to-tail dimers (Figure 4) in
which the two molecules are related to each other by a center
of inversion. The two molecules have a plane-to-plane
˚
˚
separation of 3.444 A, and the Pt-Pt distance (3.249 A) is
considerably shorter than those calculated for other mono-
cyclometalated π-π stacked platinum(II) complexes in the
literature.6c,9c,14 Also, there exists a weak intermolecular
π-π interaction between the lepdine units (Figure 4).
(14) (a) Ma, B.; Li, J.; Djurovich, P. I.; Yousufuddin, M.; Bau, R.;
Thompson, M. E. J. Am. Chem. Soc. 2005, 127, 28. (b) Chassot, L.; Muller,
E.; Von; Zelewsky, A. Inorg. Chem. 1984, 23, 4249.

Article
Organometallics, Vol. 29, No. 17, 2010 3915
Figure 2. ORTEP diagram of complex 2 with thermal ellipsoids
˚
shown at the 30% probability level. Bond distances (A): Pt-C(21)=
1.918(5), Pt-N(1) = 2.070(4), Pt-O(1) = 2.010(3), Pt-O(2) =
2.082(3). Bond angles (deg): C(21)-Pt-N(1) = 81.2(2), C(21)-
Pt-O(1) = 89.1(2), O(1)-Pt-O(2) = 88.4(1), N(1)-Pt-O(2) =
100.9(2), O(1)-Pt-N(1)=170.0(2), C(21)-Pt-O(2)=173.9(2).
Figure 3. ORTEP diagram of complex 5 with thermal ellipsoids
˚
at the 30% probability level. Bond distances (A): Pt-C(21)=
1.955(3), Pt-N(1)=2.038(3), Pt-O(1)=2.003(2), Pt-O(2)=
2.107(2). Bond angles (deg): C(21)-Pt-N(1)=80.6(1), C(21)-
Pt-O(1)=89.4(1), O(1)-Pt-O(2)=87.93(9), N(1)-Pt-O(2)=
102.07(9), O(1)-Pt-N(1)=169.94(9), C(21)-Pt-O(2)=170.1(1).
Though close contact between the lepidine and naphthyl
units from two different molecules is noticed for 5, i.e., the
˚
distance between C30 and C6 is 3.397 A (Figure 5), the rather
˚
long Pt-Pt separation (8.603 A) precludes the formation of
transitions are also affected by the nature of the substituents
present in the aryl moieties. For example, the electron-with-
drawing CF3 results in a blue shift17 of the low-energy band
in compound 2 (λmax = 417 nm), while the electron-donating
amino group causes the opposite red shift in compounds 3
and 4 (λmax = 469 and 472 nm) when compared to the tert-
butyl-substituted ligand complex 1 (λmax = 438 nm). Lumine-
scence spectra of the complexes in toluene are displayed in
Figure 7, while pertinent spectral data are listed in Table 1.
Complexes 1-7 emit in the yellowish-orange to red region
with moderate to high quantum yields (0.15-0.99 in the
degassed solution). The emission spectral feature of 1-7 is
nearly independent of concentration prepared in the range
10-6-10-4 M, and the excitation spectrum is effectively the
same as the corresponding absorption spectrum. The results
lead us to conclude that the emission originates from the
monomeric species rather than ground-state dimer or ex-
cimer species. The solution quantum yields appear to be high
among the known cyclometalated platinum complexes docu-
mented in the literature.18 Especially, to the best of our
knowledge, cyclometalated platinum complexes that exhibit
a quantum yield of greater than 0.5 at room temperature are
extremely rare.13 The observed lifetimes, τobs, of the com-
plexes were measured to be 2.1-23 μs (see Table 1), which
fall in the range for cyclometalated platinum complexes
reported in literatures.6,12 Together with the observed emis-
sion quantum yield (Φp), the radiative lifetimes τr (τr =τobs/Φp)
are deduced to be in the range 10-30 μs for 1-7. This, in
a Pt-Pt bond. Another prominent feature is the significant
deviation of the naphthylene and the lepidine units from
coplanarity due to the steric congestion between the C(3)-H
and C(23)-H (Figure 3); the dihedral angle between the two
units is calculated to be 25.49o. The torsional motion be-
tween lepdine and naphthyl moieties upon photoexcitation is
believed to be the cause of low emission solution quantum
yield and consequently inferior electroluminescent perfor-
mance observed for complex 5 (vide infra), when compared
with other complexes.
Photophysical Properties. The absorption (Figure 6) and
luminescence spectra of compounds 1-7 were measured in
toluene, and pertinent data are collected in Table 1. The
intense absorption peaks observed in the ultraviolet region
(335-375 nm) originate from the localized lepidine ligand
π f π* transitions. The lower energy absorption bands peak-
ing in the 405-500 nm range can be assigned to the spin-
allowed singlet dπ(Pt) f π*(L) metal-to-ligand charge trans-
fer (1MLCT) (1, 2, 5-7) or (1LMCT) (3, 4) in combination
with the intraligand π(L) f π*(L) transition (vide infra).16
At the long-wavelength tail of each complex, the absorption,
in part, should be attributed to triplet metal-to-ligand charge
transfer (3MLCT) transition,15 for which the spin-forbidden
nature in transition has become partially allowed due to
the enhancement of spin-orbit coupling via the heavy metal
center (Pt(II)). The position and intensity of the MLCT/ππ*
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Evans, H. S. Inorg. Chem. 1996, 35, 4883. (b) Chassot, L.; Muller, E.;
Zelewsky, A. Inorg. Chem. 1984, 23, 4249. (c) Craig, C. A.; Garces, F. O.;
Watts, R. J.; Palmans, R.; Frank, A. J. Coord. Chem. Rev. 1990, 97, 193. (d)
Balashev, K. P.; Puzyk, M. V.; Kotlyar, V. S.; Kulikova, M. V. Coord. Chem.
Rev. 1997, 159, 109.
(16) Li, J.; Djurovich, P. I.; Alleyne, B. D.; Yousufuddin, M.; Ho,
N. N.; Thomas, J. C.; Peters, J. C.; Bau, R.; Thompson, M. E. Inorg.
Chem. 2005, 44, 1713.
(17) Babudri, F.; Farinola, G. M.; Naso, F.; Ragni, R. Chem.
Commun. 2007, 1003.
(18) (a) Farley, S. J.; Rochester, D. L.; Thompson, A. L.; Howard,
J. A. K.; Williams, J. A. G. Inorg. Chem. 2005, 44, 9690. (b) Kavitha, J.;
Chang, S.-Y.; Chi, Y.; Yu, J.-K.; Hu, Y.-H.; Chou, P.-T.; Peng, S.-M.; Lee,
G.-H.; Tao, Y.-T.; Chien, C.-H.; Carty, A. J. Adv. Funct. Mater. 2005, 15, 223.

3916 Organometallics, Vol. 29, No. 17, 2010
Figure 4. Crystal packing diagrams of complex 2.
Figure 5. Crystal packing diagrams of complex 5.
Velusamy et al.
combination with their drastically oxygen quenching dyna-
mics, ensures the origin of emission to be the phosphorescence.
As for the emission intensity, among 1-7 the naphthyl-
containing complex 5 exhibits a relatively low quantum yield
(Φp ∼0.15). Because the radiative lifetimes of all 1-7 are
of the same magnitude (10-30 μs), 5 (knr ∼3.9 ꢀ 105 s-1, see
Table 1) possesses the largest nonradiative decay rate con-
stant among the title complexes. The radiationless deactiva-
tion pathways for 5 may be attributed to several factors. First
of all, the π-stacking interactions present even in solutions
via the planar naphthyl channels may serve as a deactivation
channel. However, since both quantum yield and observed
lifetime are concentration independent in solution (vide
supra), this proposal may thus be discarded at least in the
solution. In addition, the solvent quenching from the void
axial position should apply for all title Pt(II) complexes and,
hence, cannot be the sole deactivation factor for 5. Alter-
natively, the result may tentatively be rationalized by the
steric hindrance created between C(3)-H and C(23)-H
(Figure 3), the result of which distorts the planarity of the
C∧N ligand, as evidenced by the dihedral angle between the
naphthylene and the lepidine units being 25.49o (vide supra).
Upon excitation, the torsional motion between lepidine and
naphthyl moieties may induce a radiationless deactivation
channel. This, in combination with the rather small emission
gap (see Figure 6) and thus effective operation of the energy
gap law,19 leads to an increase of the nonradiative quenching
rate substantially.
MO Calculations. In order to gain more insight into the
fundamental basis of the transition, time-dependent DFT
has been performed. All pertinent data including energy gaps
calculated and corresponding assignments of each transition
are listed in Table S1 of the Supporting Information. Taking 1
as an example, the calculated energy gaps of the S0 f S1 (435 nm)
(19) Siebrand, W. J. Chem. Phys. 1967, 47, 2411.

Article
Organometallics, Vol. 29, No. 17, 2010 3917
Figure 6. Absorption spectra of complexes 1-7 recorded in
toluene.
Figure 7. Emission spectra of complexes 1-7 recorded at room
temperature in toluene.
Table 1. Absorption, Emission, and Redox Properties of the Cyclometalated Platinum(II) Complexesa
λ
abs, nm,
10-3 M-1 cm-1
compd
λ
em, nm
Φp
τ
obs, μs
kr ꢀ 10-4
8.638
knr ꢀ 10-4
16.1
Eox, mV
Ered, mV
-2195
HOMO/LUMO, eV
5.13/2.50
ΔE, eV
1
358 (11.502),
407 (8.296),
438 (5.313)
336 (6.907),
352 (6.633),
417 (5.732)
343 (9.977),
382 (5.213),
452 (6.828),
469 (6.932)
352 (8.423),
396 (7.247),
453 (9.221),
472 (9.496)
373 (11.797),
423 (7.537),
465 (6.045)
374 (12.481),
415 (6.832),
453 (7.352)
318 (40.176),
372 (18.655),
417 (8.246),
451 (9.162)
619
0.35
4.0
330
2.63
2
3
617
0.70
0.87
7.3
23
9.572
3.796
4.12
0.57
374
-2006
-2197
5.17/2.48
5.02/2.55
2.69
2.48
597, 639
221, 663
4
593, 635
0.99
16
6.079
0.608
323, 631
-2201
5.12/2.65
2.48
5
6
7
641, 680
600, 646
588, 633
0.15
0.74
0.60
2.1
12
6.910
6.045
8.590
39.4
2.12
5.76
349
335
690
-2050
-2119
-2027
5.15/2.74
5.14/2.63
5.49/3.02
2.41
2.51
2.47
7.0
a Absorption and emission spectra were recorded in toluene. Electrochemical parameters were deduced for dichloromethane solutions.
and S0 f T1 (528 nm) transitions (see Figure 8) are close to
the observed onsets of the absorption and phosphorescence
spectra (see Figures 6 and 7). Similar results are obtained for the
rest of the complexes (see Table S1). These results indicate that
the TDDFT calculation, in a qualitative manner, can well
predict the lowest Franck-Condon absorption in the singlet
manifold as well as the phosphorescence energy gap based on
the ground-state geometries of the studied complexes. As for
the transition characteristics, frontier orbital analyses indi-
cate that for 1, in view of emission, the S0 f T1 transition is
primarily attributed to HOMO f LUMO and HOMO-1 f
LUMO (see Figure 8), in which the HOMO of 1 is mainly
localized at the central Pt(II) atom and phenyl moiety of the
C∧N ligand, while the HOMO-1 is mainly localized at the
Pt(II) atom and acac ligand. In comparison, the electron
density of the LUMO is distributed at the lepidine moiety.
Therefore, the lowest lying electronic transition of 1 possesses
significant MLCT character. For complexes 3and 4, the phenyl
moiety is attached by a more electron donating substituent,
such that its π-electron energy should be lifted. Accordingly,
the HOMO in 3 and 4 is mainly located at the phenyl moiety,
as evidenced by the corresponding HOMO frontier orbitals
shown in Table S1. The prominent π-π* character for 3 and 4
is consistent with the longer lifetime observed for both com-
plexes (vide supra). It is also interesting to note that the lowest
triplet state of 3 has certain LMCT character, whereas that
of 4 has MLCT character. Careful analyses of frontier orbi-
tals indicate that the S0-T1 transition for 3 is solely from the
HOMO f LUMO, for which the contribution of metal
d-character in the LUMO is non-negligible. Conversely, the
S0-T1 transition for 4 has 7% of HOMO-2 f LUMO, where
HOMO-2 possesses prominent metal d-character. Due to the
electron-withdrawing CF3 group, which lowers the π-electron
energy in the phenyl ring, the HOMO in 2 is then partly located
at the acac site (see Table S1 of SI). Upon fusing the phenyl
ring, forming the naphthalene-containing complex 5, due to the

3918 Organometallics, Vol. 29, No. 17, 2010
Velusamy et al.
Figure 8. Calculated lower lying transitions for complex 1 in both singlet and triplet manifolds and the corresponding frontier orbitals.
energy-lifting of the π-electron, the HOMO is then located at the
naphthalene moiety. For all complexes the LUMO is located at
the lepidine moiety. Although the complexes were used as the
dopants instead of neat film in the OLEDs (vide infra), a fair
comparison was still made between the calculated and the
crystal structures, taking advantage of the available crystal
structures for 2 and 5. Selected bond distances and angles for 2
and 5 are listed in Table S2 (see SI). The average deviations of
metal-ligand distance (Pt-N1, Pt-C21, Pt-O1, Pt-O2) and
ligand-metal-ligand angle (N1-Pt-C21, O1-Pt-O2) are
12.44% and 18.93% for 2 and 4.12 and 2.33% for 5, respec-
tively. The larger discrepancy between the calculated and
crystal structures for complex 2 can be attributed to its greater
intermolecular interaction in the crystal (vide supra).
complexes, as they exhibited reversible oxidation processes.
The reduction of the lepidine ligand was observed as an
irreversible wave from -2.01 to -2.20 V. The oxidation
potential of the present complexes increases in the order 3 <
4 < 1 < 5 < 6 < 2 < 7 and is consistent with the nature
of the substituent on the cyclometalated phenyl ring. For
example, the strong electron donating substituent (dimethyl-
amino) containing compound 3 exhibits negatively shifted
(Eox = 0.22 V) oxidation potentials when compared with
tert-butyl-substituted compound 1 (Eox = 0.33 V). On the
contrary, compound 2, containing the electron-withdrawing
CF3 substituent, shows a positively shifted oxidation poten-
tial (0.37 V) when compared with compound 1. An addi-
tional oxidation wave observed for compounds 3 (Eox
=
Electrochemical Studies. To ensure the title complexes are
suited for the application of OLEDs, the electrochemical
properties of the complexes were studied by cyclic voltam-
metric method, and the redox potentials of the compounds
are compiled in Table 1. The redox potentials were corrected
relative to an internal ferrocene reference (Fc/Fcþ). All the
compounds displayed an irreversible oxidation potential
ranging from 0.221 to 0.690 mV, which is attributed to the
oxidation of the platinum-phenyl center. This is in sharp
contrast to that observed for the corresponding iridium(III)
0.66 V) and 4 (Eox = 0.63 V) is attributed to the oxidation of
the amine groups.10,13 The first oxidation potentials were
used to calculate the highest occupied molecular orbital
(HOMO) energy level of the molecules and compared with
ferrocene (4.8 eV). These HOMO values with the absorption
spectra were then used to obtain the lowest unoccupied
molecular orbital (LUMO) energy levels.20
(20) Thelakkat, M.; Schmidt, H.-W. Adv. Mater. 1998, 10, 219.

Article
Organometallics, Vol. 29, No. 17, 2010 3919
Table 2. Electroluminescent Parameters for the Devices with the Configuration ITO/NPB (40 nm)/CBP-Doped Pt Complex (5 wt %)
(20 nm)/BCP (10 nm)/Alq3 (20 nm)/LiF (1 nm)/Al (150 nm)a
L
max, cd/m2
compd Von,b V (V at Lmax, V)
λ
em, nm CIE (x,y) fwhm, nm ηext,,max, % η η
p,max, lm/W ηc,max, cd/A L,c cd/m2 ext,c % ηp,c lm/W ηc,c cd/A
1
2
3
4
5
6
7
5.0
4.5
6.0
4.5
5.0
5.0
6.5
56 727 (17.0)
23 892 (19.0)
26847 (25.5)
24 011 (17.5)
6130 (15.5)
576
578
598
594
632
590
588
0.50, 0.45
0.53, 0.46
0.60, 0.40
0.56, 0.40
0.55, 0.35
0.58, 0.40
0.59, 0.40
98
92
84
86
76
74
82
6.72
7.90
15.21
7.90
2.18
6.47
6.2
10.37
10.75
11.79
11.72
1.38
18.15
20.53
29.83
20.53
2.20
12484
9308
11265
8068
1360
7240
4528
4.63
3.60
5.77
4.11
1.35
3.50
2.86
3.56
2.08
1.57
2.20
0.39
7.30
0.823
12.50
9.34
11.31
8.10
1.36
2.04
4.63
28 988 (16.5)
19 523 (19.5)
6.52
12.29
13.48
5.30
a Lmax, maximum luminance; L, luminance; Von, turn-on voltage; V, voltage; ηext,max, maximum external quantum efficiency; ηp,max, maximum power
efficiency; ηcmax, maximum current efficiency; ηext, external quantum efficiency; ηp, power efficiency; ηc, current efficiency; fwhm, full width at half-
maximum. b Von was obtained from the x-intercept of log(luminance) vs applied voltage plot. c At a current density of 100 mA/cm2.
Figure 9. EL spectra of the devices 1-7.
Electrophosphorescent Properties. OLEDs using these pla-
tinum complexes were fabricated by a high-vacuum ther-
mal evaporation method onto glass substrates. The device
structure used in these studies is similar to that developed by
Thompson and Forrest:21 ITO/NPB (40 nm)/CBP-doped Pt
complex (5 wt %) (20 nm)/BCP (10 nm)/Alq3 (2 nm)/LiF
(1 nm)/Al (150 nm). Here, NPB (4,40-bis[N-(1-naphthyl)-N-
phenylamino]biphenyl) acts as the hole-transporting layer,
BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) acts
as the hole-blocking layer, CBP (4,40-bis(N-carbazolyl)bi-
phenyl) acts as the host for the platinum emitter, and Alq3
(tris(8-hydroxyquinoline)aluminum(III)) acts as the electron-
transporting layer. Two different doping concentrations, 5
and 15 wt %, were tested for the device performance. Devices
with 15% dopant had somewhat broadened and red-shifted
EL spectra. This observation together with the lower device
efficiencies at 15% doping level suggests that there is non-
negligible intermolecular interaction at higher doping concen-
tration. Therefore, we will focus our discussion on the device
with 5 wt % dopant. The turn-on voltages of the devices range
from 4.5 to 6.0 V. The EL spectra of these devices are displayed
in Figure 9, and the device performance characteristics are
collected in Table 2. The devices for complexes 1 and 2 exhibit
Figure 10. I-V-L characteristics observed for the devices of
the complexes (5% dopant).
devices 1 and 5 are slightly contaminated with those charac-
teristic of NPB (∼450 nm) and Alq3 (∼510 nm), respectively.
This outcome may be due to the different carrier mobility of 1
and 5 from others since the HOMO and LUMO levels of the
two complexes do not significantly deviate from others.
Low efficiency of the device from 5 may be due to the low
solution quantum yield of 5, which in turn probably arises
from the detrimental torsional deactivation (vide supra). The
maximum external quantum efficiencies of the devices for
a strong yellow emission at 576 and 578 nm respectively. In
contrast, the devices of the complexes 3, 4, and 5 have a bright
orange emission at 598, 594, and 590 nm, respectively. No
residual emission from the CBP host (∼400 nm) was discern-
other complexes fall in the range from 6.7 to 15.2% and
ible, indicating that energy transfer from the host (CBP) to the
dopant (platinum complex) or exciton trapping in the host is
maximum brightness from 26 700 to 56 700 cd m-2. Inter-
estingly, compound 3 showed a remarkably high external
very efficient in all the devices. However, the EL spectra of
quantum efficiency of 15.2% and brightness of 26800 cd
m
-2. To the best of our knowledge, these data are among the
best performances of platinum(II) complex-based OLED
(21) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Appl.
Phys. Lett. 2000, 77, 904.

3920 Organometallics, Vol. 29, No. 17, 2010
Velusamy et al.
focusing mass spectrometer (JEOL, Tokyo, Japan). Cyclic
voltammetry experiments were performed with a BAS-100
electrochemical analyzer at room temperature with a conven-
tional three-electrode configuration consisting of a platinum
working electrode, a platinum wire auxiliary electrode, and a
nonaqueous Ag/AgNO3 reference electrode. The solvent in all
experiments was dichloromethane, and the supporting electro-
lyte was 0.1 M tetrabutylammonium hexafluorophosphate. The
a
c
c
E1/2 values were determined as 1/2(Ep þ Ep ), where Epa and Ep
are the anodic and cathodic peak potentials, respectively. The
potentials are quoted against the ferrocene internal standard.
Electronic absorption spectra were obtained on a Perkin-Elmer
Lambda 900 UV-vis-NIR spectrophotometer for toluene
solutions. Emission spectra were recorded in deoxygenated
toluene solution at 298 K with a Jobin Yvon SPEX Fluoro-
log-3 spectrofluorometer. Luminescence quantum yields (Φem
)
were calculated using Ir(ppy)3 (Φem = 0.40 in toluene) as the
reference. The lifetime was measured with the laser photolysis
technique, in which the third harmonic of an Nd:YAG laser
(8 ns, Continuum Surlite II) was used as the excitation source,
coupled with a fast response photomultiplier (Hamamatsu
model R5509-72) operated at -80 °C. Typically, an average
of 512 shots was acquired for each measurement. The ligands,
2-(4-tert-butylphenyl)-4-methylquinoline (1a), 4-methyl-2-(4-
(trifluoromethyl)phenyl)quinoline (2a), N,N-dimethyl-4-(4-me-
thylquinolin-2-yl)aniline (3a), 4-(4-methylquinolin-2-yl)-N,N-
diphenylaniline (4a), and 4-methyl-2-(naphthalen-1-yl)quino-
line (5a), were prepared by adopting literature procedures.
2-(9,9-Diethyl-9H-fluoren-2-yl)-4-methylquinoline (6a) and
2-(20,70-di-tert-butyl-9,90-spirobi[fluorene]-2-yl)-4-methylqui-
noline (7a) were prepared by Suzuki couplingreactions according
to the reported procedures, and only the preparation of 6a and 7a
will be described in detail.
Figure 11. Observed variation of the external quantum efficiency
against current density in the electrophosphorescent devices.
devices.15 The device luminance and efficiencies of the
present complexes are comparable to those obtained for
iridium-based devices.6,12 Figure 10 presents the current
density-voltage and luminance-voltage characteristics
(I-V-L) of the devices fabricated, and Figure 11 shows
the external quantum efficiency of the devices as a function of
current density. The EL spectra of the compounds 1, 2, 5, and
6 were blue-shifted by 10-43 nm when compared with the PL
spectra of the compounds in toluene solution. The blue shift in
electroluminescence spectra may be attributed to the mini-
mization of intermolecular interactions at the doping concen-
trations and absence of solvent-induced relaxation processes.
However, the EL spectra observed for compounds 3 and 4 are
similar to the PL spectra. This probably points to the fact that
the incorporation of a trigonal amine functionality in com-
pounds 3 and 4 efficiently prevents the approach of two mole-
cules from stacking interactions. Lastly, the order of external
quantum efficiency (5 < 7 < 6 < 1 < 2 < 4 < 3) is roughly
opposite that of the magnitude of knr (5 > 1 > 7 > 2 > 6 > 4
> 3), while the values of kr remain much the same among the
whole series, suggesting that the overall device performance is
governed by the knr in this case.
2-(9,9-Diethyl-9H-fluoren-2-yl)-4-methylquinoline (6a). A stir-
red mixture of 2-chlorolepidine (0.88 g, 5.0 mmol), 9-diethyl-
9H-fluoren-3-yl-boronic acid (1.46 g, 5.5 mmol), Pd(PPh3)4
(75 mg), Na2CO3 (0.75 g), toluene (10 mL), THF (10 mL), and
H2O (2 mL) was heated at reflux for 24 h. When the reaction was
completed, water was added to quench the reaction. The pro-
duct was extracted with diethyl ether. The organic layer was
collected, dried over anhydrous MgSO4, and evaporated under
vacuum. The solid was adsorbed on silica gel and purified by
column chromatography using CH2Cl2/hexane as the eluent.
1
Yield: 75%. H NMR (δ, CDCl3): 0.37 (t, 6H, J = 7.24 Hz),
2.02-2.17 (m, 4H), 2.78 (s, 3H), 7.29-7.37 (m, 3H), 7.50-7.55
(m, 1H), 7.68-7.83 (m, 4H), 7.9-8.0 (m, 1H), 8.12-8.21 (m,
3H). FAB-MS: m/z 364.21 [M þ H]þ.
2-(20,70-Di-tert-butyl-9,90-spirobi[fluoren]-7-yl)-4-methylquino-
line (7a). To a mixture of (20,70-di-tert-butyl-9,90-spirobi[fluoren]-
7-yl)boronic acid (2.83 g, 6.0 mmol), chlorolepidine (0.88 g, 5.0
mmol), Pd(PPh3)4 (75 mg), and Na2CO3 (1.6 g, 15 mmol) were
added toluene (15 mL) and THF (15 mL), and the resulting
mixture was stirred and heated to reflux under nitrogen atmo-
sphere for 24 h. The reaction mixture was poured into water and
extracted with dichloromethane, and the combined extracts were
dried overMgSO4 andfiltered. The crude product was purified by
column chromatography on silica gel using dichloromethane/
hexane as the eluent to afford 7a as a white solid (1.7 g, 60%
yield). 1H NMR (δ, CDCl3): 1.11 (s, 18H), 2.70 (s, 3H), 6.64 (d,
2H, J = 7.6 Hz), 6.69 (d, 2H, J = 1.5 Hz), 6.77 (s, 1H), 7.00-7.05
(m, 1H), 7.13-7.27 (m, 4H), 7.24-7.27 (m, 1H), 7.40-7.44 (m,
1H), 7.50 (d, 2H, J = 8.0 Hz), 7.58 (d, 1H, J = 7.4 Hz), 7.91 (d,
1H, J = 7.5 Hz), 8.18 (s, 1H), 8.30 (d, 1H, J = 8.7 Hz). FAB-MS:
m/z 570.75 [M þ H]þ.
Conclusions
In conclusion, we have synthesized and characterized a
series of new cyclometalated platinum(II) complexes. These
complexes exhibit very high solution phosphorescence quan-
tum yields up to ∼99%. Single-crystal X-ray structural
determination on two complexes reveals the presence of
π-π interaction and/or Pt-Pt interaction. OLEDs using
these complexes as the dopants exhibit intense yellow to red
electroluminescence, and high external quantum efficiency
of up to 15.2% can be achieved.
Experimental Section
Methods and Materials. All reactions and manipulations were
carried out under N2 with the use of standard inert atmosphere
and Schlenk techniques. Solvents were dried by standard pro-
cedures. The entire column chromatographic studies were per-
formed under N2 with the use of silica gel (230-400 mesh,
Macherey-Nagel GmbH & Co.) as the stationary phase. The 1H
NMR spectra were obtained by using Bruker AMX400 spectro-
meters. Mass spectra were recorded on a JMS-700 double-
Preparation of the Complexes. General procedure for the
construction of orthometalated platinum(II) complexes: A mix-
ture of the ligand (3 mmol, 1.09 g) and K2PtCl4 (1 mmol 0.415 g)
was refluxed in 2-ethoxyethanol (6 mL) and water (3 mL) for
24 h. After cooling to room temperature, the mixture was added
to water, and the precipitate collected was washed with water

Article
Organometallics, Vol. 29, No. 17, 2010 3921
and an ether/hexane mixture and vacuum-dried. The precipitate
then reacted with acetylacetone (0.10 g, mmol) and Na2CO3
(0.16 g, mmol) in 2-methoxyethanol (10 mL) at 90 °C for 18 h.
The solvent was removed under reduced pressure, and the
residue was purified by column chromatography on silica gel
using CH2Cl2/hexane as the eluent to afford a yellow product
with an isolated yield of 70%.
were placed at calculated positions. Crystal data and structure
refinement parameters are given in Table SX of the Supporting
Information.
LED Fabrication and Measurements. Prepatterned ITO sub-
strates with an effective individual device area of 3.14 mm2 were
cleaned as described in a previous report. Compound BCP (2,9-
dimethyl-4,7-diphenyl-1,10-phenanthroline) was purchased from
Aldrich and used as received. Alq3 (tris(8-hydroxyquinolinato)-
aluminum), NPB (4,40-bis[N-(1-naphthyl)-N-phenylamino]bi-
phenyl), and CBP (4,40-biscarbazolylbiphenyl) were synthesized
accordingtoliteratureproceduresandweresublimedpriortouse.
In a vacuum chamber with <10-4 Pa, the following materials
were sequentially deposited onto the substrate to construct the
device: 40 nm of NPB as the hole-transporting layer, 30 nm of
the complex-doped (5-15%) CBP as the emitting layer, 20 nm of
BCP as hole- and exciton-blocking layer, 30 nm of Alq3 as the
electron-transporting layer, and a cathode composed of 1 nm of
lithium fluoride and 150nm of aluminum. The current-voltage-
brightness (I-V-B) characteristics of the EL devices were mea-
sured at ambient conditions with a Keithey 2400 Source meter.
Light intensity was measured with a Newport 1835 optical meter.
Computational Methodology. All calculations are performed
with a Gaussian 03 program.22 The geometries of all title
complexes are calculated by the hybrid B3LYP functional.23
For simplicity, the ethyl groups in 6 were replaced by methyl
groups, and the complex was labeled as 60 (see Table S1 in
Supporting Information). For the metal Pt, the effective core
potential LANL2 is used and combined a valence double-ξ basis
set, i.e., LANL2DZ. 6-31G* is used to describe the other
elements. After gaining the converged geometries, vibrational
frequency analyses were performed to confirm that the number
of imaginary frequencies is zero. On the basis of the optimized
geometries, TDB3LYP is performed to obtain the vertical
excitation energies.24
1. 1H NMR (δ, CDCl3): 1.38 (s, 9H), 2.0 (s, 3H), 2.01 (s, 3H),
2.73 (s, 3H), 5.53 (s, 1H), 7.15-7.17 (m, 1H), 7.44-7.52 (m, 2H),
7.57 (s, 1H), 7.66-7.75 (m, 2H), 7.86-7.88 (m, 1H), 9.49 (d, 1H,
J = 8.6 Hz). FAB-HRMS: calcd for C25H27NO2Pt 568.1690,
found 568.1694. Anal. Calcd for C25H27NO2Pt: C, 52.81; H,
4.79; N, 2.46. Found: C, 52.78; H, 4.69; N, 2.35.
2. 1H NMR (δ, CDCl3): 2.01 (s, 3H), 2.02 (s, 3H), 2.75 (s, 3H),
5.55 (s, 1H), 7.31-7.33 (m, 1H), 7.54-7.58 (m, 2H), 7.61 (s, 1H),
7.70-7.72 (m, 1H), 7.89-7.96 (m, 2H), 9.54 (d, 1H, J = 8.6 Hz).
FAB-HRMS: calcd for C22H18F3NO2Pt 580.0937, found
580.0939. Anal. Calcd for C22H18F3NO2Pt: C, 45.52; H, 3.13;
N, 2.41. Found: C, 45.45; H, 3.10; N, 2.50.
3. 1H NMR (δ, CDCl3): 1.99 (s, 3H), 2.0 (s, 3H), 2.70 (s, 3H),
3.09 (s, 6H), 5.52 (s, 1H), 6.50-6.53 (m, 1H), 7.01 (d, 1H, J =
2.6 Hz), 7.39-7.45 (m, 3H), 7.62-7.66 (m, 1H), 7.80-7.83
(m, 1H), 9.41 (d, 1H, J = 8.6 Hz). FAB-HRMS: calcd for
C23H24N2O2Pt 555.1486, found 555.1489. Anal. Calcd for
C23H24N2O2Pt: C, 49.73; H, 4.35; N, 5.64. Found: C, 49.80;
H, 4.40; N, 5.70.
4. 1H NMR (δ, CDCl3): 1.63 (s, 3H), 1.97 (s, 3H), 2.71 (s, 3H),
5.43 (s, 1H), 6.78 (dd, 1H, J = 2.4 Hz), 7.02-7.06 (m, 2H),
7.19-7.28 (m, 8H), 7.35 (d, 1H, J = 8.5 Hz), 7.45-7.48 (m, 2H),
7.63-7.68 (m, 1H), 7.83-7.85 (m, 1H), 9.45 (d, 1H, J = 8.6 Hz).
FAB-HRMS: calcd for C33H28N2O2Pt 679.1799, found
679.1809. Anal. Calcd for C33H28N2O2Pt: C, 58.32; H, 4.15;
N, 4.12. Found: C, 58.29; H, 4.20; N, 4.10.
5. 1H NMR (δ, CDCl3): 2.01 (s, 3H), 2.05 (s, 3H), 2.80 (s, 3H),
5.55 (s, 1H), 7.38-7.40 (m, 1H), 7.48-7.56 (m, 2H), 7.63 (d, 1H,
J = 8.4 Hz), 7.69-7.72 (m, 1H), 7.83-7.84 (m, 1H), 7.90-7.97
(m, 2H), 8.18 (s, 1H), 8.45 (d, 1H, J = 8.6 Hz), 9.42 (d, 1H, J =
8.6 Hz). FAB-HRMS: calcd for C25H21NO2Pt 562.1220, found
562.1214. Anal. Calcd for C25H21NO2Pt: C, 53.38; H, 3.76; N,
2.49. Found: C, 53.45; H, 3.80; N, 2.55.
Acknowledgment. We thank the National Science
Council, Academia Sinica, and National Taiwan Uni-
versity for supporting this work.
Supporting Information Available: HOMO and LUMO plots
and calculated energy levels of the lower lying transitions of the
complexes, X-ray structural data, and CIF files giving crystal-
lographic data for 2 and 5. This material is available free of
charge via the Internet at http://pubs.acs.org.
6. 1H NMR (δ, CDCl3): 0.36 (t, 6H, J = 7.24 Hz), 2.02-2.11
(m, 10H), 2.78 (s, 3H), 5.56 (s, 1H), 7.30-7.34 (m, 3H),
7.49-7.54 (m, 2H), 7.68-7.72 (m, 2H), 7.79 (d, 1H, J = 6.8
Hz), 7.90 (d, 1H, J = 8.2 Hz), 8.05 (s, 1H), 9.50 (d, 1H, J = 8.6
Hz). FAB-HRMS: calcd for C32H31NO2Pt 656.2003, found
656.2005. Anal. Calcd for C32H31NO2Pt: C, 58.53; H, 4.76; N,
2.13. Found: C, 58.45; H, 4.70; N, 2.0.
(22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc.:
Wallingford, CT, 2004.
1
7. H NMR (δ, CDCl3): 1.10 (s, 18H), 2.02 (s, 3H), 2.15 (s,
3H), 2.60 (s, 3H), 5.59 (s, 1H), 6.65 (d, 1H, J = 7.5 Hz), 6.72 (d,
2H, J = 1.5 Hz), 6.94 (s, 1H), 7.04-7.08 (m, 1H), 7.32-7.39 (m,
4H), 7.44-7.48 (m, 1H), 7.63-7.67 (m, 1H), 7.73 (d, 2H, J = 8.1
Hz), 7.80 (d, 1H, J = 7.4 Hz), 7.93 (d, 1H, J = 7.5 Hz), 8.21 (s,
1H), 9.43 (d, 1H, J = 8.9 Hz). FAB-HRMS: calcd for
C48H45NO2Pt 862.3098, found 862.3098. Anal. Calcd for
C48H45NO2Pt: C, 66.81; H, 5.26; N, 1.62. Found: C, 66.75; H,
5.19; N, 1.59.
Crystal Structure Analysis. The single-crystal X-ray diffrac-
tion experiments were carried out using graphite-monochro-
˚
mated Mo KR radiation (λ = 0.71073 A) on a Bruker SMART
CCD diffractometer. The data collection was performed using
the SMART program. Cell refinement and data reduction were
performed with the SAINT program. The structure was deter-
mined using the SHELX software package. All non-hydrogen
atoms were refined anisotropically, whereas hydrogen atoms
(23) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Lee, C.; Yang, W.;
Parr, R. G. Phys. Rev. B 1988, 37, 785.
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