Phosphorescence quenching by conjugated polymers

Article abstract of DOI:10.1021/ja0343297

Energy transfer between phosphors and conjugated polymers was investigated using a fluorene trimer (F₃) as a model conjugated material. The phosphors studied were bis-cyclometalated iridium complexes (FP, PPY, BT, PQ, and BTP), with triplet energies of 2.6, 2.4, 2.2, 2.1, and 2.0 eV, respectively (based on phosphorescence spectra). Stern-Volmer analysis of luminescent quenching shows that energy transfer from either FP or PPY to F3 is an exothermic process with Stern-Volmer quenching constants (k_qSV) of near 10⁹ M^-1 s^-1 while energy transfer from BT, PQ, and BTP is endothermic (k_qSV = 10⁷-10⁶ M^-1 s^-1). On the the basis of above results, the triplet energy of F₃ is estimated to be less than 2.3 eV (530 nm). This study suggests that conjugated polymers, which typically have lower T₁ energies than F₃, should also quench phosphorescent emission in thin films and organic light-emitting diodes (OLEDs) incorporating these and related phosphorescent dopants. Copyright

Full text of DOI:10.1021/ja0343297

Published on Web 06/06/2003
Phosphorescence Quenching by Conjugated Polymers
Madhusoodhanan Sudhakar, Peter I. Djurovich, Thieo E. Hogen-Esch, and Mark E. Thompson*
Department of Chemistry, UniVersity of Southern California, Los Angeles, California 90089
Received January 24, 2003; E-mail: met@usc.edu
Efficient organic light-emitting diodes (OLEDs) have been
fabricated with low-molecular weight materials.1 Hole-electron
recombination in these OLEDs leads to the formation of both singlet
and triplet excited states (excitons) within the molecular thin film.2
For most compounds, only the singlet state is emissive, leading to
a significant limitation in the OLED efficiency. Incorporation of
phosphorescent compounds into the OLED gives a substantial
improvement in device efficiency, since both the singlet and triplet
excitons are trapped at the phosphor. This approach has led to
OLEDs with external quantum efficiencies (photon/electron) of
roughly 20% which correspond to internal efficiencies of nearly
100%.3
Conjugated polymers have also been used to prepare OLEDs,
e.g. poly(phenylenevinylenes),4a polyfluorenes,4b and poly(p-
phenylenes).4c While these devices can have good power efficiencies
(lum/W or Wopt/Welect), the polymer-based OLEDs tend to give
external quantum efficiencies of less than 5%. Several groups have
attempted to increase the quantum efficiencies of conjugated
polymer-based OLEDs by incorporating phosphorescent dopants.5
However, while the efficiencies of conjugated polymer OLEDs are
improved by phosphor doping, values are still markedly lower than
those of small-molecule-based devices (<5%).5 In contrast, phosphor-
doped OLEDs fabricated with nonconjugated polymers (e.g. poly-
(vinylcarbazole), PVK) can have good external quantum efficien-
cies,6 with some as high as 8%.7
To understand why phosphor-doped OLEDs give low quantum
efficiencies with conjugated polymers, we have carried out a
luminescent quenching study of phosphorescent emission using a
model polyfluorene oligomer as a triplet energy quencher (F3 shown
in Figure 1). The phosphorescent cyclometalated Ir complexes used
in this study had a range of emission energies, spanning from blue
to red (Figure 2).8 A fluorene trimer, rather than a polymer, was
chosen as a quencher for several reasons. F3 is estimated to have
a triplet energy somewhat higher than that of the polymer, roughly
about 540-580 nm.9 In addition, unlike polyfluorenes, F3 has
negligible absorption in the region where the phosphors absorb
strongly (i.e., 435 nm), allowing the direct excitation of the
phosphor. Also, on the basis of solution electrochemistry of F3 and
the complexes, the HOMO and LUMO orbitals of each of the
phosphors are expected to fall within the HOMO/LUMO gap of
F3.10 This precludes exciplex formation or excited-state electron
transfer as the origin of luminescent quenching. Last, F3 is more
soluble than a polymer and has a discrete molecular weight, making
for straightforward Stern-Volmer analysis of the quenching
phenomena.
Figure 1. Schematic energy level diagram showing energy transfer between
F3 (left) and the iridium complexes (right).
Figure 2. Stern-Volmer plot for quenching of the five phosphors by F3.
(Legend: 9 FP, 2 PPY, [ BT, b PQ, 1 BTP). Table: triplet energy8,10 of
phosphors and their Stern-Volmer quenching constants.11 The estimated
errors in the kqSV values are (10%.
The luminescent lifetimes were recorded in 2-methyltetrahydro-
furan as a function of F3 concentration and plotted using standard
Stern-Volmer analysis (i.e., τ0/τ versus [F3], Figure 2).10,11
Quenching rate constants were derived from the slope of linear
fits to the data for each complex. The Stern-Volmer quenching
rates given in Figure 2 were measured using direct excitation of
the complexes with 435 nm light; however, similar rates are
obtained when the solutions are excited using 373 nm light and
energy is transferred from the F3 singlet to the triplet levels of the
complex, as illustrated by the gray arrow in Figure 1. It can be
seen from the data in Figure 2 that quenching of FP and PPY
emission by F3 is an exothermic process, occurring at near diffusion
controlled rates, while quenching of the yellow, orange, and red
emission (from BT, PQ, and BTP, respectively) is an endothermic
process, occurring at rates well below those of FP or PPY. This
indicates that the triplet energy of the trimer is less than 2.3 eV, as
predicted from Ba¨ssler’s data.9
A schematic energy level diagram for F3 and the Ir phosphors is
shown in Figure 1. The singlet state of F3 is significantly higher in
energy than any of the phosphor triplet states, but the F3 triplet
energy is expected to fall somewhere in the middle of the phosphor
energy range. Hence, phosphorescence quenching should occur by
energy transfer between the excited phosphor and the triplet state
of F3.
Upon excitation of Ir phosphor/F3 mixtures, the Ir complex can
either relax directly to the ground state (via radiative or nonradiative
pathways) or transfer energy to the F3 triplet state. Similar behavior
in the type of thin films used in phosphorescent OLEDs, where
high dopant concentrations (5-10 wt %) are employed, should lead
to pseudo-first-order reaction conditions for energy transfer from
phosphor to quencher. It is therefore worth noting that emission
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C O MMU N I C A T I O N S
from BTP is efficiently quenched by F3 (kqSV ) 1.9 × 106 M-1
s-1) despite having a lower triplet energy. Using this kqSV value,
the half-life for BTP emission quenching in a doped thin film of
F3 (ca. 1 M) is estimated to be 0.4 µs. Since the phosphorescent
half-life of an excited BTP molecule is 4 µs (based on a luminescent
lifetime of 5.8 µs),8 near complete phosphorescence quenching is
predicted for a BTP-doped F3 film. Quite the contrary, BTP
emission is observed for 5 wt % doped polyfluorene thin films.5e
This discrepancy comes about because of significant differences
between concentrated solid films and the dilute solution mixtures
used for our studies. First, the doping levels of phosphors in the
solid films are considerably higher than the µM to mM concentra-
tions used in the quenching experiments. The high doping concen-
trations promote phosphor aggregation;12 consequently, the reduced
intermolecular contact between F3 molecules and the dopant
complexes decreases the phosphorescent quenching rate. Second,
energy transfer between the triplets of the phosphor and F3 is most
likely an electron exchange or “Dexter” transfer process. Dexter
transfer requires a good intermolecular overlap between the pertinent
molecular orbitals of the donor and acceptor.11 Since molecular
motion is inhibited in amorphous solid films, a dopant-F3 config-
uration that gives poor Dexter energy transfer cannot readily reorient
to a configuration appropriate for efficient energy transfer. Last,
in fluid solution, the F3 and phosphor can physically separate after
energy transfer. This physical diffusion enhances quenching by
suppressing reverse energy transfer (F3 to phosphor), especially for
those phosphors (e.g., BTP and PQ) where energy transfer to the
F3 triplet state is an endothermic process. In a rigid matrix, such as
a doped thin film, physical diffusion cannot occur. In this case,
separating excited F3 and the phosphor involves energy migration
from the excited F3 to an adjacent F3, a slower process than the
competing back energy transfer. These three factors together lead
to kqSV values in doped thin films that are lower than those measured
in dilute fluid solutions.
phosphorescence quenching is less favorable due to the absence of
any low-energy triplet states.6,7 Although doped OLEDs with
moderate efficiencies have been prepared by blending conjugated
polymers with red phosphors,5c-e it becomes increasingly difficult
to use this strategy for devices emitting at higher energy since it
requires conjugated host polymers with high triplet-state energies.
A solution to this dilemma is either to use nonconjugated polymers,
such as PVK, or to design new conjugated polymers with higher
triplet energies. Such new polymer systems may then allow
fabrication of highly efficient (>10% external) polymer-based
phosphorescent OLEDs.
Acknowledgment. We thank The Universal Display Corpora-
tion (M.E.T.) and the NSF-DMR (T.H.E.) for financial support of
this work.
Supporting Information Available: Phosphorescence spectra from
the phosphors, synthesis and characterization of F3, electrochemical
properties of the compounds, and data for the Stern-Volmer analysis
(PDF). This material is available via the Internet at http://pubs.acs.org.
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The results obtained from our quenching studies highlight an
important criterion that needs to be considered when designing
polymer-based phosphorescent OLEDs. Energy transfer from either
singlet or triplet levels of a conjugated polymer to a phosphorescent
dopant certainly looks appealing at the outset. However, one needs
also to consider the emission quenching brought about by the low-
energy triplet states of the polymer since even endothermic transfer
can effectively quench phosphorescence. High-molecular weight
polyfluorenes have conjugation lengths that are greater than that
of F3; thus, triplet-state energies are lower for the polymers (2.1
eV) than for the oligomer (2.3 eV).9 Hence, one can expect that
phosphorescence quenching will be more efficient in polyfluorenes
than in F3. With nonconjugated polymers such as PVK, however,
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