Divalent Osmium Complexes
A R T I C L E S
Figure 2. Structures of strong π acid ligands.
Figure 3. Synthetic scheme for cis-1,2-vinylenebis(diphenylarsine).
(such as arsines, phosphines, DMSO, CN, and CO; see Figure
2) in combination with polypyridyl ligands may yield Os(II)
complexes with quantum yields approaching 24%. These strong
π acid ligands (arsines and phosphines) strongly back-bond with
the Os, but their σ* or d accepting orbitals are very high in
energy21 compared to the polypyridyl π*; thus, the charge
transfer to ligand bands are dominantly Os (t2g) f bpy (π*).
By reducing the number of polypyridyl ligands, there is less
ability to accept an electron; thus, there is a shift to higher energy
in the absorption and emission bands from the tris(polypyridyl)
counterparts. This makes the mixed-ligand system suitable for
OLED applications by blue-shifting the emission from the
infrared (750-850 nm) for the Os(II)tris(polypyridyl) complexes
to orange-red (600-650 nm) for the mixed ligand system. These
changes in structure cause an increase in radiative decay and
leads to emission yields up to 45%.
Using triplet-based emitting centers in organic and polymer
OLEDs eliminates the 25% limit for maximum internal quantum
efficiency, which is the expected singlet exciton fraction
generated by electrical injection, and potentially allows for
displays with 100% internal quantum efficiency.22 Strong back-
bonding with a metal center, which exhibits a large spin-orbit
coupling constant, facilitates intersystem crossing by breaking
down the spin selection rules, thus leading to stronger triplet-
state emission. Improved phosphorescence provides a possibility
to design a high-efficiency OLED device. Triplet-harvesting red
and green OLEDs based on Pt and Ir complexes have demon-
strated very high external quantum efficiency.23-27 Europium
complexes also show triplet emission and have been used in
red OLEDs.28-30 The characteristic of the lowest excited states
(triplet states) of these heavy-metal complexes can be systemati-
cally varied from largely ligand-centered (LC) to metal-to-
Figure 1. Polypyridyl ligands and numbering position on the ring: 2,2′-
bipyridine is on the left and 1,10-phenanthroline is on the right.
bands of tris(polypyridyl) Os(II) complexes are greatly red-
shifted from their Ru(II) counterparts and occur in the near-
infrared, which makes them unsuitable for use as OLEDs. This
is due to the fact that Os(II) is more easily oxidized than
Ru(II). According to the gap law, decreasing the energy
difference between the excited and ground states enhances
nonradiative decay of the excited state. Thus, red-shifting the
emission band could decrease emission quantum yields; how-
ever, this does not have to be the case.
In designing a synthetic strategy, the above-discussed prob-
lems must be taken into account. It has been repeatedly shown
that the use of phenyl groups on the polypyridyl ligand (Figure
1) greatly increases quantum yields. For example, the quantum
yield for emission of Ru(II)tris(phenanthroline) is 1.9%, while
the yield for Ru(II)tris(22) is 36.6%,17 despite a red shift in
emission from 595 nm for the phenanthroline complex to 610
nm for the 22 complex. Similar results can be observed for
Ru(II) tris complexes of both 21 and 26. The reason for this is
that the phenyl groups undergo a conformational change18 in
the excited state, contributing to and extending to the π system.
In this conformation there is a barrier to C-C bond rotation
due to the extended π system. This reduces C-C bond rotation
and vibration of the substituent, both of which may lead to an
increase in nonradiative decay. This barrier would not exist for
other substituents such as a methyl or other aliphatic groups.
The substitution also replaces a C-H bond vibration para to
the nitrogen, which may quench luminescence. Therefore,
phenyl or other aromatic derivatives of polypyridyls were used
in this study to increase quantum yields.
(19) Kober, E.; Caspar, J.; Sullivan, P.; Meyer, T. Inorg. Chem. 1988, 27, 4587.
(20) Kober, E. M.; Sullivan, B. P.; Dressick, W. J.; Caspar, J. V.; Meyer, T. J.
J. Am. Chem. Soc. 1980, 102 (24), 7383.
(21) Periodic Correlations; Rich, R. L., Ed.; Benjamin: New York, 1965; p 6.
(22) Adachi, C.; Baldo, M.; Thompson, M. E.; Forrest, S. R. J. Appl. Phys.
2001, 90 (10), 5048.
(23) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.;
Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151.
(24) Kwong, R. C.; Sibley, S.; Dubovoy, T.; Baldo, M. A.; Forrest, S. R.;
Thompson, M. E. Chem. Mater. 1999, 11, 3709.
(25) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest,
S. R. Appl. Phys. Lett. 1999, 75, 4.
(26) Tsutsui, T.; Yang, M.; Yahiro, M.; Nakamura, K.; Watanabe, T.; Tsuji,
T.; Fukuda, Y.; Wakimoto, T.; Miyaguchi, S. Jpn. J. Appl. Phys. 1999,
38, L1502.
(27) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Lamansky, S.; Thompson, M. E.;
Kwong, R. C. Appl. Phys. Lett. 2001, 78, 1622.
(28) Kido, J.; Hayase, H.; Hongawa, K.; Nagai, K.; Okuyama, K. Appl. Phys.
Lett. 1994, 65, 2124.
While Os(II)tris(22) and similar derivatives exhibit some
increase in quantum yield, the emission is still in the infrared,
making them unsuitable for use as OLEDs. Meyer and co-
workers19,20 showed that that the use of strong π acid ligands
(16) Fetterolf, M. L.; Offen, H. W. J. Phys. Chem. 1985, 89 (15), 3320.
(17) Alford, P.; Cook, M.; Lewis, A.; McAuliffe, G.; Skarda, V.; Thomson, A.;
Glasper, J.; Robbins, D. J. Chem. Soc., Perkin Trans. 2 1985, 705.
(18) Damrauer, N.; McCusker, J. K. Inorg. Chem. 1999, 38, 4268.
(29) McGehee, M. D.; Bergstedt, T.; Zhang, C.; Saab, A. P.; O’Regan, M. B.;
Bazan, G. C.; Srdanov, V. I.; Heeger, A. J. AdV. Mater. 1999, 11, 1349.
(30) Jiang, X.; Jen, A. K.-Y.; Huang, D.; Phelan, G.; Londergan, T.; Dalton, L.
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