Preparation, characterization, and electroluminescence characteristics of α-diimine-type platinum(II) complexes with perfluorinated phenyl groups as ligands

Article abstract of DOI:10.1246/cl.2005.592

The α-diimine-type platinum(II) complexes having bulky perfluorinated phenyl groups were prepared and the molecular and crystal structures were revealed by X-ray analysis. They showed phosphorescence and electron-accepting properties. The organic light-em

Full text of DOI:10.1246/cl.2005.592

592
Chemistry Letters Vol.34, No.4 (2005)
Preparation, Characterization, and Electroluminescence Characteristics
of ꢀ-Diimine-type Platinum(II) Complexes with Perfluorinated Phenyl Groups as Ligands
Jun-ichi Nishida, Akihiro Maruyama, Takeshi Iwata,y and Yoshiro Yamashitaꢀ
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering,
Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8502
yCentral Reserch Labolatory, Takasago International Corporation, Nishiyawata, Hiratsuka 254-0073
(Received January 27, 2005; CL-050123)
The ꢀ-diimine-type platinum(II) complexes having bulky
These compounds were purified by recrystallization and subli-
mation.
perfluorinated phenyl groups were prepared and the molecular
and crystal structures were revealed by X-ray analysis. They
showed phosphorescence and electron-accepting properties.
The organic light-emitting diode (OLED) devices showing green
luminescence were fabricated.
Molecular structures of 1a–1d were elucidated by X-ray
analyses.9 Complexes 1a, 1b have a similar square planar geom-
etry around the metal center and the two bulky pfp or tfp rings
are twisted with the angle of ca. 70ꢁ from these square planes
(Figure 1). These molecular geometries are similar to a series
of platinum(II) complexes having electron-donating bulky mesi-
tyl (2,4,6-trimethylphenyl) groups.5 The Pt–C distances of 1a,
Electrophosphorescent materials incorporating heavy transi-
tion metals have attracted considerable attention because of their
application to highly efficient electroluminescent (EL) emitters.1
Platinum complexes such as porphyrin-type and phenylpyridine-
type platinum(II) complexes have been used for OLED de-
vices.2,3 However, the numbers of platinum complexes are less
than those of iridium ones.4 This is due to their square planar
d8 structures, which lead to close intermolecular Pt–Pt contacts
or ꢁ stacking resulting in quenching the phosphorescence. ꢀ-
Diimine-type platinum(II) complexes based on bidentate ligands
such as bipyridine and 1,10-phenanthroline have been prepared
and the properties and structures have been studied.5 However,
OLED devices using these complexes as EL emitter are rare
probably because of the less phosphorescence efficiency.6 If bul-
ky phenyl groups are introduced in these complexes, they would
weaken intermolecular interactions and increase the phosphores-
cence efficiency. Moreover, the substituents on the phenyl
groups can perturb the metal d orbitals and tune the emitting col-
or. In this context, we have prepared a series of ꢀ-diimine-type
platinum(II) complexes 1 having bulky and electron-withdraw-
ing perfluorinated phenyl groups7 and investigated their struc-
tures and physical properties. We have also fabricated OLED de-
vices based on them for the first time.
ꢀ
1b are in a range of 1.994(6)–2.016(6) A, which are similar to
those of other ꢀ-diimine-type platinum(II) complexes (1.99–
5
ꢀ
2.04 A) and slightly shorter than those of other platinum com-
7
ꢀ
plexes containing pentafluorophenyl groups (2.03–2.12 A).
These shorter bond lenghs seem favorable for protecting the
platinum center. The phenyl substituents at the 4-, 7-positions
of the phenanthroline are also considered to disturb intermolec-
ular interactions. Although the complex 1a forms a dimer struc-
ture between the planar phenanthroline units, the interplanar dis-
ꢀ
tance is ca. 3.62 A, which is longer than that of a 3,4,7,8-tetra-
methylphenanthroline derivative.5b On the other hand, 1c, 1d
having methyl groups at the 2-, 9-positions of phenanthroline
adopt unique bending structures as depicted in Figure 1b, where
the dihedral angles between the N–Pt–N plane and the phenan-
throline are ca. 24ꢁ. The Pt–N distances are slightly longer than
ꢀ
those of 1a, 1b [1a; 2.082(5)–2.083(5) A, 1b; 2.082(6)–
ꢀ
ꢀ
ꢀ
2.083(5) A, 1c, 2.102(5)–2.124(5) A, 1d; 2.101(3)–2.131(3) A],
which can be attributed to the steric hindrance of the methyl
groups.
(a)
(b)
Figure 1. Molecular structures of (a) 1a and (b) 1c.
Complexes 1a, 1b show weak emission in solution at room
temperature. The emission maxima in dichloromethane ap-
peared at 515 nm in 1a and 509 nm in 1b, which are much
blue-shifted than that of a platinum complex with dimesityl
groups and 4,7-diphenylphenanthroline (660 nm in toluene and
620 nm in solid).5c This result indicates that the introduction of
electron-withdrawing fluorine and trifluoromethyl substituents
at the phenyl ring induces blue-shifts of emission. On the other
hand, the emission of 1c, 1d in solution could not be observed at
room temperature. However, all of the complexes 1 showed
Pentafluorophenyl (pfp) derivatives 1a, 1c were prepared in
96 and 47% yields, respectively, by the reaction of the corre-
sponding 4,7-diphenylphenanthrolines with bis(pfp)(cod)plati-
num(II).8 4-Trifluoromethyl-2,3,5,6-tetrafluorophenyl (tfp) de-
rivatives 1b, 1d were obtained in 72 and 40% yields, respective-
ly, by the reaction of the corresponding diphenylphenanthrolines
with bis(tfp)(cod)platinum(II) which was prepared by the reac-
tion of dichloro(cod)platinum(II) with (tfp)lithium in 54% yield.
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.4 (2005)
593
Table 1. Emission maxima and redox potentialsa of 1
ꢆem (nm)
References and Notes
1
a) M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S.
Sibley, M. E. Thompson, and S. R. Forrest, Nature, 395,
151 (1998). b) D. F. O’Brien, M. A. Baldo, M. E. Thompson,
and S. R. Forrest, Appl. Phys. Lett., 74, 442 (1999).
C. Adachi, M. A. Baldo, S. R. Forrest, S. Lamansky, M. E.
Thompson, and R. C. Kwong, Appl. Phys. Lett., 78, 1622
(2001).
a) V. Adamovich, J. Brooks, A. Tamayo, A. M. Alexander,
P. I. Djurovich, B. W. D’Andrade, C. Adachi, S. R. Forrest,
and M. E. Thompson, New J. Chem., 26, 1171 (2002). b)
B. W. D’Andrade, J. Brooks, V. Adamovich, M. E.
Thompson, and S. R. Forrest, Adv. Mater., 14, 1032 (2002).
c) J. Brooks, Y. Babayan, S. Lamansky, P. I. Djurovich,
I. Tsyba, R. Bau, and M. E. Thompson, Inorg. Chem., 41,
3055 (2002).
E1=2 (V)
Solutionb
Solidc
1a
1b
1c
1d
515
509
534
508
484sh, 507
501
ꢂ1:31
ꢂ1:29
ꢂ1:45
ꢂ1:41
2
3
d
—
—
d
aIn CH2Cl2 containing 0.1 mol dmꢂ3 n-Bu4NPF6, vs SCE, Pt
electrode. In CH2Cl2. In crystal. Not observed.
b
c
d
strong photoluminescence in the solid state. The emission max-
ima are summalized in Table 1. The differences of the emission
maxima between in solid and in solution are small, indicating
that the intermolecular interactions are weak in the solid state
as expected.
4
a) C. Adachi, M. A. Baldo, S. R. Forrest, and M. E.
Thompson, Appl. Phys. Lett., 77, 904 (2000). b) S. Tokito,
T. Iijima, Y. Suzuri, H. Kita, T. Tsuzuki, and F. Sato, Appl.
Phys. Lett., 83, 569 (2003). c) S. Lamansky, P. Djurovich,
D. Murphy, F. Abdel-Razzaq, H.-E. Lee, C. Adachi, P. E.
Burrows, S. R. Forrest, and M. E. Thompson, J. Am. Chem.
Soc., 123, 4304 (2001). d) A. Tsuboyama, H. Iwawaki, M.
Furugori, T. Mukaide, J. Kamatani, S. Igawa, T. Moriyama,
S. Miura, T. Takiguchi, S. Okada, M. Hoshino, and K. Ueno,
J. Am. Chem. Soc., 125, 12971 (2003).
Complexes 1 having the electron-withdrawing perfluorinat-
ed phenyl groups are expected to show electron-accepting prop-
erties. To prove this point, their redox potentials were measured
by cyclic voltammetry. All of the voltammograms exhibited a
reversible one-electron reduction wave at ꢂ1:29 to ꢂ1:45 V vs
SCE (Table 1), which are lower than those of similar ꢀ-diimine
complexes having nonsubstituted phenyl groups.5 The electron-
withdrawing ligand is considered to lower the levels of d orbitals
of platinum, which leads to blue-shifts of emission. On the other
hand, the reduction waves of 1c, 1d having dimethyl groups
were observed at lower potentials than those of 1a, 1b. This is
attributed to the electron-donating effects of methyl groups.
Preliminary OLED devices of 1a–1c were fabricated by a
thermal deposition method onto a clean glass substrate with in-
dium-tin-oxide (ITO). A 40-nm-thick layer of 4,40-bis[N-(1-
naphthyl)-N-phenylamino]biphenyl (NPD) for hole transport, a
35-nm-thick layer of 4,40-N,N0-dicarbazolylbiphenyl (CBP) con-
sisting of 6% complexes 1, a 10-nm-thick layer of 2,9-dimethy-
4,7-diphenylphenanthroline (BCP) for hole-block and a 35-nm-
thick layer of tris(8-hydroxyquinoline)aluminium (Alq3) for
electron transport, a 0.5-nm-thick LiF and a 100-nm-thick Al
layer as cathode electrode were successively deposited. The
EL spectrum of 1a is similar to the emission spectrum in solution
with a little red-shift (4 nm) of the maximum (519 nm). The
Commission Internationale de l’Eclairage (CIE) coordinate of
the device of 1a (x ¼ 0:329, y ¼ 0:551) corresponds to the green
region. The external quantum efficiency (ꢂext) and the power ef-
ficiency at 100 cd mꢂ2 were 2.1% and 2.4 lm Wꢂ1, respectively.
The maximum luminance of 4795 cd mꢂ2 was obtained at 15 V.
Devices using 1b, 1c showed the EL peak maxima at 516 nm
(x ¼ 0:297, y ¼ 0:513) and 523 nm (x ¼ 0:329, y ¼ 0:514),
respectively. The devices showed lower ꢂext values of 0.82%
5
a) A. Klein, E. J. L. McInnes, and W. Kaim, J. Chem. Soc.,
Dalton Trans., 2002, 2371. b) A. Klein, J. van Slageren, and
´ ˇ
S. Zalis, Eur. J. Inorg. Chem., 2003, 1927. c) K. E. Dungey,
B. D. Thompson, N. A. P. Kane-Maguire, and L. L. Wright,
Inorg. Chem., 39, 5192 (2000).
6
7
S.-C. Chan, M. C. W. Chan, Y. Wang, C.-M. Che, K.-K.
Cheung, and N. Zhu, Chem.—Eur. J., 7, 4180 (2001).
a) R. Uson, J. Fornies, J. Gimeno, P. Espinet, and R. Navarro,
J. Organomet. Chem., 81, 115 (1974). b) R. Uson, J. Fornies,
M. Tomas, J. M. Casas, and C. Fortun˜o, Polyhedron, 8, 2209
(1989).
8
9
G. B. Deacon and K. T. Nelson-Reed, J. Organomet. Chem.,
322, 257 (1987).
Crystal data for 1a: C36H16N2F10Pt, Mr ¼ 861:61, monoclinic
ꢀ
P21=c, a ¼ 13:919ð5Þ, b ¼ 16:383ð6Þ, c ¼ 14:994ð7Þ A, ꢃ ¼
ꢁ
105:39ð3Þ , V ¼ 3296ð2Þ A , Z ¼ 4, Dcalcd ¼ 1:736 g/cm3,
ꢀ 3
.
R1 ¼ 0:042, wR2 ¼ 0:131, S ¼ 1:72. 1b: C38H16N2F14Pt
ꢁ
CHCl3, Mr ¼ 1081:00, triclinic P1, a ¼ 11:237ð4Þ, b ¼
ꢀ ¼ 106:23ð3Þꢁ,
ꢃ ¼
ꢀ
11:509ð5Þ,
c ¼ 16:302ð5Þ A,
ꢁ
ꢁ
ꢀ 3
107:87ð3Þ , ꢄ ¼ 91:41ð3Þ , V ¼ 1912ð1Þ A , Z ¼ 2, Dcalcd
¼
1:877 g/cm3, R1 ¼ 0:054, wR2 ¼ 0:159, S ¼ 1:27. 1c:
C38H20N2F10Pt, Mr ¼ 889:66, monoclinic P21=c, a ¼
ꢁ
ꢀ
12:606ð7Þ, b ¼ 15:025ð9Þ, c ¼ 17:131ð9Þ A, ꢃ ¼ 101:31ð5Þ ,
V ¼ 3181ð3Þ A , Z ¼ 4, Dcalcd ¼ 1:857 g/cm3, R1 ¼ 0:047,
ꢀ 3
wR2 ¼ 0:107, S ¼ 0:98. 1d: C40H20N2F14Pt, Mr ¼ 989:68,
monoclinic C2=c, a ¼ 30:10ð1Þ, b ¼ 15:249ð6Þ, c ¼
for 1b and 0.91% for 1c at 100 cd mꢂ2
.
In conclusion, we have prepared novel ꢀ-diimine-type plat-
inum(II) complexes having perfluorinated phenyl groups and
succeeded in fabricating their OELDs exhibiting green lumines-
cence. ꢀ-Diimine-type complexes having bulky phenyl groups
are attractive candidates for EL emitters and the structures would
be easily modified to improve the EL performance.
ꢁ
ꢀ
ꢀ 3
16:600ð7Þ A, ꢃ ¼ 114:34ð4Þ , V ¼ 6941ð5Þ A , Z ¼ 8,
Dcalcd ¼ 1:894 g/cm3, R1 ¼ 0:030, wR2 ¼ 0:077, S ¼ 0:95.
The details of the crystal data have been deposited with Cam-
bridge Crystallographic Data Centre as supplementary publi-
cation no. CCDC 264434-264437.
10 1H NMR Data (300 MHz, CDCl3); 1a: ꢅ 8.73 (d, 2H,
J ¼ 5:1 Hz), 8.06 (s, 2H), 7.75 (d, 2H, J ¼ 5:1 Hz), 7.60 (m,
10H); 1b: 8.65 (d, 2H, J ¼ 5:1 Hz), 8.09 (s, 2H), 7.79 (d,
2H, J ¼ 5:1 Hz), 7.56 (m, 10H); 1c: 7.90 (s, 2H), 7.55 (m,
12H), 2.43 (s, 6H); 1d: 7.92 (s, 2H), 7.55 (m, 12H), 2.43 (s,
6H).
This work was supported by the 21st Century COE program
and a Grant-in-Aid for Scientific Research of Priority Areas
(No. 15073212) of the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
Published on the web (Advance View) March 19, 2005; DOI 10.1246/cl.2005.592