You et al.
complexes embedded in hyperbranched structures12(d) have been
enthusiastically studied with an aim of enhanced processibility.
A widely used approach to enhancing the solubility of Ir(III)
complexes has been to attach solubilizing alkyl moieties or to
introduce dendritic architectures.13 However, the introduction
of alkyl groups perturbs charge carrier transport and lowers the
glass transition temperature (Tg). Hence, sterically hindered all-
aromatic substituents, which give rise to less side effects, are
considered better solubilizing groups.10b As a successful example
of this approach, we have found that the green electrophospho-
rescence from the well-known complex Ir(ppy)3 (Ir(III) tris(2-
phenylpyridinato-N, C2′)) can be improved remarkably by
introducing tetraphenylsilyl group.14 We found that compared
to Ir(ppy)3, the modified Ir(ppy)3 complex containing three
identical 4-tetraphenylsilane-substituted ligands showed higher
solubility and higher electrophosphorescence efficiency (32.8
cd/A) when doped in a poly(N-vinylcarbazole) (PVK) layer.
In addition to the improvement in solubility and phospho-
rescence efficiency that can be achieved by tetraphenylsilyl
substitution, we wished to elucidate the phosphorescence color
tuning effect of this type of substitution. Hence, in the present
study, we sought to synthesize a saturated red-emitting Ir(III)
complex with tetraphenylsilyl substitution and, additionally, to
investigate the optical role of the tetraphenylsilyl substituents.
To this end, we introduced a tetraphenylsilyl group into the
2-(2′-benzo[b]thienyl)pyridine ligand of the well-known red-
emitting Ir(III) complex, btp2Iracac (Ir(III) bis(2-(2′-benzo[b]-
thienyl)pyridinato-N,C3′) (acetylacetonato),5(a) 2, in Scheme 1).
To obtain deeper red phosphorescence than that of 2, we
attached a tetraphenylsilyl group to the 5-position of the pyridine
ring (see Chart 1). This modification should extend the
π-conjugation in the ligand structure, causing a red-shift in the
phosphorescence emission.
This approach is supported by quantum chemical calculations
based on density functional theory (DFT), which predict that
the lowest unoccupied molecular orbital (LUMO) of 1 is
stabilized by 0.095 eV compared to that of 2. This stabilization
is attributed to elongation of the region containing the LUMO
electronic population, which is localized on the pyridine ring
in 2, but on both the pyridine ring as well as the additional
phenyl ring of tetraphenylsilane in 1 (see Chart 1). We also
find that energy levels of the molecular orbitals neighboring
the frontier orbitals (i.e., a set of HOMO-1 and HOMO-2, and
a set of LUMO+1 and LUMO+2) approach those of the highest
occupied molecular orbital (HOMO) or LUMO (see bottom of
Chart 1). Considering the fact that excitations between molecular
orbitals other than HOMO and LUMO also significantly
contribute effective transitions,15 this prediction further supports
stabilization of the phosphorescent state of 1. Thus, tetraphenyl-
silyl introduction on the Ir(III) complex appears to lower the
optical transition energy, making it possible to achieve deep-
red phosphorescence. These findings indicate that the introduc-
tion of structural motifs utilizing arylsilane substituents is a
convenient way to achieve both enhanced processability and
deep-red phosphorescence of Ir(III) complexes.
(5) (a) Lamansky, S.; Djurovich, P. I.; Abdel-Razzaq, F.; Garon, S.;
Murphy, D. L.; Thompson, M. E. J. Appl. Phys. 2002, 92, 1570. (b)
Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.; Kamatani, J.;
Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada, S.; Hoshino,
M.; Ueno, K. J. Am. Chem. Soc. 2003, 125, 12971-12979. (c) Ragni, R.;
Plummer, E. A.; Brunner, K.; Hofstraat, J. W.; Babudri, F.; Farinola, G.
M.; Naso, F.; De Cola, L. J. Mater. Chem. 2006, 16, 1161-1170. (d)
Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Kwong, R.;
Tsyba, I.; Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E. Inorg. Chem.
2001, 40, 1704-1711. (e) Anthopoulos, T. D.; Frampton, M. J.; Namdas,
E. B.; Burn, P. L.; Samuel, I. D. W. AdV. Mater. 2004, 16, 557-560. (f)
Yeh, S.-J.; Wu, M.-F.; Chen, C.-T.; Song, Y.-H.; Chi, Y.; Ho, M.-H.; Hsu,
S.-F.; Chen, C. H. AdV. Mater. 2005, 17, 285-289. (g) Li, J.; Djurovich,
P. I.; Alleyne, B. D.; Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E.
Polyhedron 2004, 23, 419-428. (h) Tamayo, A. B.; Alleyne, B. D.;
Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N. N.; Bau, R.; Thompson,
M. E. J. Am. Chem. Soc. 2003, 125, 7377-7387.
(6) (a) Gong, X.; Lim, S.-H.; Ostrowski, J. C.; Moses, D.; Bardeen, C.
J.; Bazan, G. C. J. Appl. Phys. 2004, 95, 948. (b) Adachi, C.; Baldo, M.
A.; Thompson, M. E.; Forrest, S. R. J. Appl. Phys. 2001, 90, 5048. (c)
Baldo, M. A.; Adachi, C.; Forrest, S. R. Phys. ReV. B 2000, 62, 10967.
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Appl. Phys. Lett. 2003, 83, 569. (b) Ren, X.; Li, J.; Holmes, R. J.; Djurovich,
P. I.; Forrest, S. R.; Thompson, M. E. Chem. Mater. 2004, 16, 4743. (c)
Kawamura, Y.; Shozo, Y.; Forrest, S. R. J. Appl. Phys. 2002, 92, 87. (d)
Lamansky, S.; Kwong, R. C.; Nugent, M.; Djurovich, P. I.; Thompson, M.
E. Org. Electron. 2001, 2, 53.
(8) (a) Kappaun, S.; Eder, S.; Sax, S.; Saf, R.; Mereiter, K.; List, E. J.
W.; Slugovc, C. J. Mater. Chem. 2006, 16, 4389. (b) Kwon, T.-H.; Cho,
H. S.; Kim, M. K.; Kim, J.-W.; Kim, J.-J.; Lee, K.-H.; Park, S. J.; Shin,
I.-S.; Kim, H.; Shin, D. M.; Chung, Y. K.; Hong, J.-I. Organometallics
2005, 24, 1578. (c) Kim, Y. H.; Ahn, J. H.; Shin, D. C.; Kwon, S. K.
Polymer 2004, 45, 2525. (d) Yang, C.-H.; Cheng, Y.-M.; Chi, Y.; Hsu,
C.-J.; Fang, F.-C.; Wong, K.-T.; Chou, P.-T.; Chang, C.-H.; Tsai, M.-H.;
Wu, C.-C. Angew. Chem., Int. Ed. 2007, 46, 2418. (e) Yang, C.; Zhang,
X.; You, H.; Zhu, L.; Chen, L.; Zhu, L.; Tao, Y.; Ma, D.; Shuai, Z.; Qin,
J. AdV. Funct. Mater. 2007, 17, 651.
(9) (a) Yang, C.-H.; Cheng, Y.-M.; Chi, Y.; Hsu, C.-J.; Fang, F.-C.;
Wong, K.-T.; Chou, P.-T.; Chang, C.-H.; Tsai, M.-H.; Wu, C.-C. Angew.
Chem., Int. Ed. 2007, 46, 2418. (b) Yang, C.-H.; Tai, C.-C.; Sun, I-W. J.
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Results and Discussion
(10) (a) Wong, W.-Y.; Zhou, G.-J.; Yu, X.-M.; Kwok, H.-S.; Tang, B.-
Z. AdV. Funct. Mater. 2006, 16, 838. (b) Ding, J.; Gao, J.; Cheng, Y.; Xie,
Z.; Wang, L.; Ma, D.; Jing, X.; Wang, F. AdV. Funct. Mater. 2006, 16,
575.
The tetraphenylsilyl-substituted ligand 4 was synthesized by
successive palladium-catalyzed Suzuki-Miyaura reactions.
Specifically, lithiation of 1,4-dibromobenzene with n-BuLi (1.6
M in hexane) followed by substitution with a triphenylsilyl-
chloride gave the bromine-terminated tetraphenylsilane in
quantitative yield. Then, a further lithiation with n-BuLi and
subsequent reaction with trimethylborate afforded the corre-
sponding methyl boronate, which was further transformed to
the corresponding boronic acid by treatment with aqueous 2 N
HCl. Another part of the ligand, 2-(2′-benzo[b]thienyl)-5-
bromopyridine, was easily obtained via Suzuki-Miyaura reac-
tion of 5-bromo-2-iodopyridine and 2-benzo[b]thiopheneboronic
acid. Exclusive coupling at the iodine of pyridine was observed,
which was confirmed by mass spectrometry. Finally, the
remaining bromine of the pyridine was coupled with triphenyl-
(11) Noh, Y.-Y.; Lee, C.-L.; Kim, J.-J. J. Chem. Phys. 2003, 118, 2853.
(12) (a) Sandee, A. J.; Williams, C. K.; Evans, N. R.; Davies, J. E.;
Boothby, C. E.; Ko¨hler, A.; Friend, R. H.; Holmes, A. B. J. Am. Chem.
Soc. 2004, 126, 7041. (b) Chen, X.; Liao, J.; Liang, Y.; Ahmed. M. O.;
Tseng, H.-E.; Chen, S.-A. J. Am. Chem. Soc. 2003, 125, 636. (c) You, Y.;
Kim, S. H.; Jung, H. K.; Park, S. Y. Macromolecules 2006, 39, 349. (d)
Hecht, S.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2001, 40, 74.
(13) (a) Anthopoulos, T. D.; Frampton, M. J.; Namdas, E. B.; Burn, P.
L.; Samuel, I. D. W. AdV. Mater. 2004, 16, 557 and references therein. (b)
Bolink, H. J.; Cappelli, L.; Coronado, E.; Gra¨tzel, M.; Ort´ı, E.; Costa, R.
D.; Viruela, P. M.; Nazeerudin, M. K. J. Am. Chem. Soc. 2006, 128, 14786.
(c) Wong, W.-Y.; Ho, C.-L.; Gao, Z.-Q.; Mi, B.-X.; Chen, C.-H.; Cheah,
K.-W.; Lin, Z. Angew. Chem., Int. Ed. 2006, 45, 7800. (d) Lepeltier, M.;
Le Bozec, H.; Guerchais, V. Organometallics 2005, 24, 6069. (e) Wong,
W.-Y.; Zhou, G.-J.; Yu, X.-M.; Kwok, H.-S.; Tang, B.-Z. AdV. Funct. Mater.
2006, 16, 838. (f) King, S. M.; Al-Attar, H. A.; Evans, R. J.; Congreve, A.;
Beeby, A.; Monkman, A. P. AdV. Funct. Mater. 2006, 16, 1043.
(14) You, Y.; An, C.-G.; Lee, D.-S.; Kim, J.-J.; Park, S. Y. J. Mater.
Chem. 2006, 16, 4706.
(15) Hay, P. J. J. Phys. Chem. A 2002, 106, 1634.
6242 J. Org. Chem., Vol. 72, No. 16, 2007