Properties of Tb(tb-pmp)3 and Gd(tb-pmp)3
J. Phys. Chem. A, Vol. 106, No. 16, 2002 4021
metal energy transfer that can occur during the lifetimes of both
the triplet and the singlet states. In particular, the very rapid S1
ligand-to-metal energy transfer competes effectively with ligand
intersystem crossing, and also with other singlet emission
radiative and nonradiative quenching processes. All such
alternative pathways that deactivate the singlet state potentially
lead to device losses if, as is the case here, nonradiative losses
from the triplet state occur more rapidly than energy transfer to
the metal.
states, slow energy transfer to the lanthanide could cause a major
loss of quantum efficiency. Whether this will happen depends
on the details of the electronic structure of the ligand.
Acknowledgment. We thank Stan Botchway and Mike
Towrie of Rutherford Appleton Laboratories for their help with
TCSPC measurements, as well as the EPSRC for access to Mass
Spectrometry facilities. J.M.R. thanks the Swiss National
Science Foundation for financial support. V.C, S.E.W and M.G
were supported by Opsys Ltd.
The quantum yield and lifetime measurements as a function
of temperature imply that there is the potential for an increase
in device efficiency by a factor of about 6. We have, however,
observed no evidence for triplet-triplet energy migration, nor
for Tb-Tb energy transfer in these materials, even at very high
exciton densities. Both these processes are potential mechanisms
for decreasing the quantum yield in electroluminescent devices.
This result is striking because it has been argued that, in
electroluminescent devices, triplet-triplet (bi-exciton) annilation
may be a general mechanism limiting the current density (J0),
and therefore brightness, at which the device efficiency begins
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2
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3
densities, so an alternative quenching process must operate.
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(
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providing that the current is space-charge limited.30 Simply put,
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by interactions with another excited molecule, but with a
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devices, the most mobile carriers are ligand-based holes, so
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of an f-electron from an excited [Tb(III)(tb-pmp)3]* molecule
to a ligand-based hole on an adjacent molecule. The resultant
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1
Andrews with excitation at 30 800 cm
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(
Tb(IV)(tb-pmp)3]+ species would then relax to a Tb(III)
[
(
configuration with a hole on a ligand.
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(
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(