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Unformatted text preview: behaviour of the various complexes is the same, but they exhibit profoundly different emission properties, we conclude that the specific nature of the bridge is the key to the colour switching phenomenon. It is well known that electron conduction, stabiliz- ation and delocalization on such bridging ligands depend on the geometry, energetics and structural features of the molecules 19–21 . Furthermore, use of different polymers with a higher-energy LUMO level (the HOMO level is chosen to be the same)—so that electron transfer mediated by the Ru complex cannot populate their excited state—did not result in green emission at reverse bias. Symmetric devices with Au as anode and cathode have symmetric emission properties (that is, they show red emission at both forward and reverse bias), clearly indicating that an asymmetry in the device’s charge injection behaviour is needed for the differentiation of the two possible mechanisms of light emission in the [Ru(ph) 4- Ru] 4 þ PPV system. As the work functions of the ITO and Au contacts are energetically comparable, and the red/green switching behaviour is also seen when Au is replaced by Al, influences other than simple energetics must also have a role in allowing the stepwise electron transfer at reverse bias; we intend to study these influences in greater detail in the near future. A Received 19 July; accepted 12 November 2002; doi:10.1038/nature01309. 1. Baldo, M. A. et al. Highly efficient phosphorescent emission from organic electroluminescent devices. Nature 395, 151–154 (1998). 2. Buda, M., Kalyuzhny, G. & Bard, A. J. Thin-film solid-state electroluminescent devices based on tris(2,2-bipyridine)ruthenium(II) complexes. J. Am. Chem. Soc. 124, 6090–6098 (2002). 3. Wu, A., Yoo, D., Lee, J.-K. & Rubner, M. F. Solid-state light emitting devices based on the tris-chelated ruthenium(II) complex. 3. High efficiency devices via a layer-by-layer molecular-level blending approach. J. Am. Chem. Soc. 121, 4883–4891 (1999). 4. Faulkner, L. R. & Bard, A. J. Electroanalytical Chemistry (ed. Bard, A. J.) 1–95 (Marcel Dekker, New York, 1977). 5. Rudmann, H., Shimada, S. & Rubner, M. F. Solid-state light emitting devices based on the tris- chelated ruthenium(II) complex. 4. High-efficiency light-emitting devices based on derivatives of the tris(2,2-bipyridyl) ruthenium(II) complex. J. Am. Chem. Soc. 124, 4918–4921 (2002). 6. Handy, E. S., Pal, A. J. & Rubner, M. F. Solid-state light emitting devices based on the tris-chelated ruthenium(II) complex. 2. Tris(bipyridyl)ruthenium(II) as a high-brightness emitter. J. Am. Chem. Soc. 121, 3525–3528 (1999). 7. Elliott, C. M., Pichot, F., Bloom, C. J. & Rider, L. S. Highly efficient solid-state electrochemically generated chemiluminescence from ester-substituted trisbipyridineruthenium(II)-based polymers....
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This note was uploaded on 07/17/2008 for the course EEOB 700 taught by Professor Wolfe during the Winter '05 term at Ohio State.
- Winter '05