Molecular-wire behavior of OLED materials: Exciton dynamics in multichromophoric Alq3-oligofluorene-Pt(II)porphyrin triads

Article abstract of DOI:10.1021/ja064471i

Donor-bridge-acceptor triads consisting of the Alq₃ complex, oligofluorene bridge, and Pt^II tetraphenylporphyrin (PtTPP) were synthesized. The triads were designed to study the energy level/distance-dependence in energy transfer both

Full text of DOI:10.1021/ja064471i

Published on Web 08/31/2006
Molecular-Wire Behavior of OLED Materials: Exciton Dynamics in
Multichromophoric Alq3-Oligofluorene-Pt(II)porphyrin Triads
Victor A. Montes,† Ce´sar Pe´rez-Bol´ıvar,† Neeraj Agarwal,† Joseph Shinar,‡ and
Pavel Anzenbacher, Jr.*,†
Center for Photochemical Sciences, Bowling Green State UniVersity, Bowling Green, Ohio 43403, and
Ames Laboratory-USDOE and Department of Physics and Astronomy, Iowa State UniVersity, Ames, Iowa 50011
Received July 7, 2006; E-mail: pavel@bgnet.bgsu.edu
A successful development of flat displays utilizing organic light-
emitting diodes technology (OLED)1 depends on the availability
of organic semiconductors capable of emitting light with high
electron-to-photon quantum efficiency. Because of the spin statistics
of hole-electron recombination2 that produces three triplets per one
singlet, triplet emitters are preferred over singlet emitters to increase
the device efficiency. While the energy of singlet states is easily
harnessed by a plethora of fluorescent chromophores such as
conjugated polymers (e.g., polyfluorene)3 and complexes of metals
with low spin-orbit coupling (e.g., AlIII trisquinolinolate, Alq3),4
triplet excitons are more difficult to utilize.5,6 Generally, two types
of materials may be used: electroluminescent complexes such as
cyclometalated Ir(III) complexes or blends of singlet electrolumi-
nophores (e.g., Alq3) doped with a triplet emitter, for example PtII
octaethylporphyrin.7,8 The success in the preparation of materials
of high charge-to-photon conversion efficacy by blending/doping
depends on our ability to harvest the energy of triplet states. Alas,
solution-processing of blends is often plagued by defects originating
from phase separation of the components.9 Single-molecular
Figure 1. Structure and energy alignments in triads 1a-d.
materials with a donor and acceptor (dopant) in a predefined
solid-state photonic properties are of practical importance for
distance to facilitate effective triplet energy transfer would be of
fabrication of OLEDs as they could yield super-effective doping.
practical advantage.10
In this work, we present donor-bridge-acceptor triads 1a-d
The energy transfer (ET) between a donor (D) and acceptor (A)
or between a host and dopant involves usually components of both
(Figure 1) consisting of Alq3, oligofluorene bridge, and PtII
tetraphenylporphyrin (PtTPP), in which ET is facilitated by energy
Fo¨rster and Dexter processes.11 However, in conjugated materials
alignment of the components. Here, Alq3- and the fluorene
involving donor-bridge-acceptor (DBA) systems, the Dexter
exchange mechanism, usually limited to a short distance (<10 Å),
may show only a weak dependence on the D-A distance and is
fragments appear to form a single fluorophore owing to strong
electronic coupling facilitating the singlet exciton migration to the
porphyrin (singlet energy: Alq3-fluorene ) 2.25 eV, PtTPP ) 2.0
then regarded as superexchange. Both Dexter exchange and
superexchange mechanisms allow for singlet-singlet and triplet-
triplet processes.11c In the superexchange regime, the efficacy of
the singlet and triplet ET from a donor to an acceptor decreases
exponentially with the D-A distance, with the rate constant for
these processes being limited by the rate of the Marcus theory of
electron transfer.12 Since the rate of triplet-triplet ET depends on
both electron and hole transfers generated during the ET event
within the D-A pair,12b the superexchange mechanism is of
practical importance to OLEDs utilizing a host-dopant emissive
layer.
In cases of donor > bridge > acceptor energy alignment, the
ET is weakly dependent on the bridge length, which then behaves
as an incoherent wire.11c,13 Recent findings suggest that molecular-
wire behavior is achieved in cases with properly aligned singlet14
or triplet energy levels.15 Because of the 3:1 triplet-singlet exciton
ratio in OLEDs, the alignment of the triplet levels in particular
could result in lossless energy transfer and improved emission of
the acceptor. While such triads are of fundamental interest, their
3
eV). The corresponding triplet levels are Alq3, E ) 2.17 ( 0.10
eV;16 fluorene bridge F1-F4, 2.86 to 2.18 ( 0.10 eV;17 and PtTPP,
1.91 eV.18 Synthesis of 1a-d is outlined in the Supporting
Information (SI).
The UV-vis absorption spectra of triads 1a-d revealed typical
features of these chromophores with Soret and Q-bands being
centered at 405 and 510 nm, respectively. The π-π* absorption
band at 340-370 nm originating from the oligofluorene frag-
ment increased in intensity with an increasing number of fluorene
units. Alq3 absorption (λmax ) 380-420 nm)19 is overlapped
with the Soret band of the PtTPP (Figure 2). The PtTPP Q-bands
(490-540 nm) overlap with Alq3 emission (490-580 nm), which
accounts for effective ET according to both the Fo¨rster and Dexter/
superexchange model.
Excitation in the Q-band resulted only in phosphorescence from
the PtTPP (λmax ) 670 and 740 nm, τ ) 43.6 ( 0.8 µs, Φph ) 7.5
( 0.5%). When excited at 420 nm (Soret and π-π* quinolinolate
bands) the emission of 1a-d is dominated by PtTPP phosphores-
cence while 1c-d show <1% contribution from Alq3 fluorescence
(λmax ≈ 540 nm). This behavior is consistent with nearly complete
† Bowling Green State University.
‡ Iowa State University.
9
12436
J. AM. CHEM. SOC. 2006, 128, 12436-12438
10.1021/ja064471i CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Figure 4. Uncorrected PL spectra of 1a-d in thin films (1% w/w in
polystyrene) and EL spectra of 1c-d OLEDs.
Figure 2. UV-vis absorption spectra of compounds 1a-d and their
corrected emission spectra when excited at 420 nm.
Figure 5. (left) Luminance curves of 1a-d OLEDs; (right) chromacity
diagram and CIE coordinates for the EL demonstrating their high color
purity; (right bottom) 1d-based OLED showing saturated red color.
Figure 3. Transient absorption spectroscopy data (pumping at 475 nm):
(left) decay traces of 1a-d monitored at 520 nm showing ET from Alq3-
fluorene to PtTPP; (right) distance dependence of the energy transfer rate
for 1a-d used to calculate the distance-attenuation factor â.
In the transient spectroscopy experiments, the assignment of the
lowest electronic state was supported by nanosecond UV-vis
transient absorption spectroscopy, which showed no evidence of
endothermic energy transfer5 from the platinum porhyrin centers
upon selective excitation of the Q-bands at 532 nm.20 Although
we were unable to either induce effective intersystem crossing (ISC)
in the D-B section of the triad or selectively sensitize its triplet
state to observe the triplet transfer processes by transient absorption
spectroscopy, we believe that the energy transfer process described
above indicates that the triplet transfer would exhibit similar wirelike
behavior resulting in equally fast, or perhaps even faster, rates. This
assertion is based on the following: first, in these rigidly linked
D-B-A systems the superexchange mechanism is available for
both singlet-singlet and triplet-triplet processes.21 Thus, the
incohererent wire/superexchange mechanism is also available for
triplet excitons generated during the hole-electron recombination.
Second, the delocalization of triplet excitons in conjugated materials
is considered to be shorter than that of singlets,10a which puts more
importance on the triplet energy alignment of the individual
components. Triads 1c and 1d bearing terfluorene and quaterfluo-
rene bridges show good alignment of the DBA triplet levels, which
allows for the bridge-mediated hopping mechanism of energy
transfer.15 This hypothesis was further supported by the data
obtained from OLEDs fabricated using triads 1a-d.
energy transfer from Alq3 to PtTPP even though the D-A distance
considerably exceeds the Fo¨rster radius (24 Å).
To investigate the dynamics of the energy transfer we studied
the triads 1a-d using femtosecond transient absorption spectros-
copy. Unfortunately, selective excitation of the Alq3 moiety is not
possible because of high absorption of PtTPP. However, excitation
at 475 nm (mainly residual π-π* absorption of Alq3) allowed us
to monitor the disappearance of an absorption band from the Alq3-
fluorene singlet centered at 520 nm along with additional bleaching
from the PtTPP ground-state Q-band.20 The lifetime of the Alq3-
fluorene singlet ranged from 14 to 860 ps. The estimated rates of
1
1
ET (Alq3-fluorene) to PtTPP were (1a) 7.1 × 1010 s-1; (1b) 1.9
× 1010 s-1; (1c) 2.7 × 109 s-1; (1d) 1.0 × 109 s-1. The observed
energy transfer rates are 2 orders of magnitude higher than the rates
calculated from the Fo¨rster model for the D-A distances (19-40
Å), which indicates strong participation of the superexchange
mechanism.
The exponential dependence of the energy transfer rate constant
on the D-A distance yielded an attenuation factor â of 0.21 (
0.02 Å-1 (Figure 3), which is higher than â values (â ) 0.07-
0.09 Å-1) observed for incoherent-wire behavior,13b,15 yet signifi-
cantly lower than values (â ) 0.32-0.66 Å-1) derived for
superexchange in triads with p-phenylene or fluorene bridges. This
observation is further confirmed by an experiment where excitation
was performed at 340 nm (π-π* absorption of fluorene). We did
not observe any transient absorption features attributable to fluorene
models, which is consistent with the single Alq3-Fn fluorophore.
Also, the dynamics were very similar to the ones observed upon
excitation at 475 nm. We conclude that the triads 1a-d display
very fast energy transfer in the mixed incohererent wire/superex-
change mechanism.
The materials 1a-d were incorporated as emitter and electron-
transport layer (ETL) into simple unoptimized double-layer OLEDs
(ITO/PEDOT:PSS/1a-d/CsF:Al)20 fabricated by solution process-
ing. All four OLEDs afforded saturated red color almost identical
to the emission from thin films (Figure 4).
The OLEDs showed pure red (CIE X, Y: 0.706, 0.277) emission
demonstrating effective energy transfer taking place in the emissive
layer comprised of 1a-d (Figure 5, right).
Remarkably, the turn-on voltages as well as the maximum
external quantum efficiencies were found to be highly dependent
9
J. AM. CHEM. SOC. VOL. 128, NO. 38, 2006 12437

C O M M U N I C A T I O N S
Table 1. Triplet Energy of the Bridge (ET),17 Turn-on Voltages
(Vturn-on), and Quantum Efficiency (ηMax) Values for OLEDs
Fabricated Using 1a-d2 a
emission with high color purity. To the best of our knowledge this
is the first example of molecular wire behavior demonstrated to
take place in solid state and in functional OLEDs.
ET
±
0.10
Vturn
ηmax
(%)c
-
(V)b
on
Acknowledgment. Support from the NSF (Grant DMR-0306117
to P.A.), A. P. Sloan Foundation (P.A.), BGSU (TIE grant to P.A.),
Ohio Laboratory for Kinetic Spectrometry, and a McMaster
fellowship (V.M.) is acknowledged. We are grateful to Prof. M.
A. J. Rodgers and Dr. E. Danilov for helpful discussions and
assistance.
Note Added after ASAP Publication: In the version published
on the Internet August 31, 2006, there were formatting errors in
the Supporting Information on pages 26 and 29. The final version
published September 1, 2006, and the print version are correct.
compound
(eV)
1a
1b
1c
1d
2.86
2.43
2.25
2.18
10.0
4.3
3.9
3.9
0.012
0.039
0.131
0.204
a The performance of the OLEDs using 1c-d improves with better triplet
energy alignment of the components. b Defined as the bias needed to obtain
a luminance of >0.2 cd/m2. c External maximum efficiency of the diodes.22
Supporting Information Available: Procedures and characteriza-
tion of compounds 1a-d and synthetic precursors. Additional transient
spectra, lifetime fits, and construction of the OLEDs. This information
is available free of charge at http://pubs.acs.org
References
(1) (a) Organic Electroluminescence; Kafafi, Z. H.; Ed. CRC Press: Boca
Raton, FL, 2005. (b) Shinar, J. Organic Light-Emitting DeVices - A
SurVey; Springer: Berlin, 2003.
(2) (a) Koehler, A.; Wilson, J. Org. Electron. 2003, 4, 179. (b) Wilson, J. S.;
Dhoot, A. S.; Seeley, A. J. A. B.; Khan, M. S.; Koehler, A.; Friend, R.
H. Nature 2001, 413, 828.
(3) Scherf, U.; List, E. J. W. AdV. Mater. 2002, 14, 477.
(4) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
(5) (a) Sudhakar, M.; Djurovich, P. I.; Hogen-Esch, T. E.; Thompson, M. E.
J. Am. Chem. Soc. 2003, 125, 7796. (b) Evans, N. R.; Devi, L. S.; Mak,
C. S. K.; Watkins, S. E.; Pascu, S. I.; Koehler, A.; Friend, R. H.; Williams,
C. K.; Holmes, A. B. J. Am. Chem. Soc. 2006, 128, 6647.
(6) Avilov, I.; Marsal, P.; Bre´das, J.-L.; Beljonne, D. AdV. Mater. 2004, 16,
1624.
(7) (a) Baldo, M. A.; Forrest, S. R.; Thompson, M. E. In Organic Electrolu-
minescence; Kafafi, Z. H. Ed.; Taylor & Francis: Boca Raton, FL, 2005;
p 274. (b) Holder, E.; Langeveld, B. M. W.; Schubert, U. S. AdV. Mater.
2005, 17, 1109.
(8) (a) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.;
Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151. (b) Cleave, V.;
Yahioglu, G.; Le Barny, P.; Friend, R. H.; Tessler, N. AdV. Mater. 1999,
11, 285. (c) Cleave, V.; Yahioglu, G.; Le Barny, P.; Hwang, D.-H.;
Holmes, A. B.; Friend, R. H.; Tessler, N. AdV. Mater. 2001, 13, 44.
(9) (a) Chen, F.-C.; He, G.; Yang, Y. Appl. Phys. Lett. 2003, 82, 1006. (b)
Chen, F.-C.; Chang, S.-C.; He, G.; Pyo, S.; Yang, Y.; Kurotaki, M.; Kido,
J. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 2681.
(10) (a) Sandee, A. J.; Williams, C. K.; Evans, N. R.; Davies, J. E.; Boothby,
C. E.; Kohler, A.; Friend, R. H.; Holmes, A. B. J. Am. Chem. Soc. 2004,
126, 7041. (b) Chen, X.; Liao, J.-L.; Liang, Y.; Ahmed, M. O.; Tseng,
H.-E.; Chen, S.-A. J. Am. Chem. Soc. 2003, 125, 636.
(11) (a) Dexter, D. L. J. Chem. Phys. 1953, 21, 836. (b) Fo¨rster, T. Discuss.
Faraday. Soc. 1959, 27, 7. (c) Speiser, S. Chem. ReV. 1996, 96, 1953.
(12) (a) Marcus, R. A. Discuss. Faraday. Soc. 1960, 29, 21. (b) Closs, G. L.;
Johnson, M. D.; Miller, J. R.; Piotrowiak, P. J. Am. Chem. Soc. 1989,
111, 3751
(13) (a) Weiss, E. A.; Tauber, M. J.; Kelley, R. F.; Ahrens, M. J.; Ratner, M.
A.; Wasielewski, M. R. J. Am. Chem. Soc. 2005, 127, 11842. (b)
Goldsmith, R. H.; Sinks, L. E.; Kelley, R. F.; Betzen, L. J.; Liu, W.;
Weiss, E. A.; Ratner, M. A.; Wasielewski, M. R. Proc. Natl. Acad. Sci.
2005, 102, 3540. (c) Wasielewski, M. R. J. Org. Chem. 2006, 71, 5051.
(14) Tinnefeld, P.; Heilemann, M.; Sauer, M. Chem. Phys. Chem. 2005, 6,
217
(15) Welter, S.; Lafolet, F.; Cecchetto, E.; Vergeer, F.; De Cola, L. Chem.
Phys. Chem. 2005, 6, 2417.
(16) Burrows, H. D.; Fernandes, M.; de Melo, J. S.; Monkman, A. P.;
Navaratnam, S. J. Am. Chem. Soc. 2003, 125, 15310.
(17) Wasseberg, D.; Dudek, S. P.; Meskers, S. C. J.; Janssen, R. A. J. Chem.
Phys. Lett. 2005, 411, 273.
(18) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin
Complexes; Academic Press: London, 1992.
(19) Montes, V. A.; Pohl, R.; Shinar, J.; Anzenbacher, P., Jr. Chem.sEur. J.
2006, 12, 4523.
(20) For more details see Supporting Information.
(21) Michl, J.; Bonacˇic´-Koutecky´, V. Electronic Aspects of Organic Photo-
chemistry; Wiley-Interscience: New York, 1990; p 85.
(22) O’Brien, D.; Bleyer, A.; Lidzey, D. G.; Bradley, D. D. C.; Tsutsui, T. J.
Appl. Phys. 1997, 82, 2662.
Figure 6. Schematic triplet energy alignment in materials 1a-d.
on the structure and alignment of the triplet energies of the
components of the emissive materials (Table 1). Thus 1a, with the
poorest alignment of the 3donor-3bridge-3acceptor levels, requires
the highest driving voltage and shows the lowest efficacy (ηmax) of
the electron-to-photon conversion. In the materials 1c-d with longer
oligofluorenes, turn-on voltages as low as 3.9 V were observed.
Also, the quantum efficiency (ηmax) was found to be an order of a
magnitude better in materials 1c-d with good triplet level align-
ment than that from compounds bearing shorter fluorene bridges
(1a-b) (Table 1). Significantly higher luminance is obtained from
optimized devices with advanced architectures, particularly when
a hole-blocking layer is employed. Here, we prefer simple double-
layer devices because the observed effects are easier to attribute to
the effective energy transfer.
The trend in the electroluminescence performance is attributed
to the stepwise decrease in the triplet energy of the oligofluorene
moiety as the bridge length increases.17 As depicted in Figure 6,
the triplet energies for fluorene and bifluorene are higher than that
of Alq3, thereby acting as a barrier for the triplet-triplet energy
transfer. In contrast, the triplet levels from terfluorene and quater-
fluorene are situated closer to the triplet level of Alq3 facilitating
the exchange to the incoherent hopping mechanism for triplet-
triplet energy transfer,15 as the data from femtosecond transient
spectroscopy suggest. Hence, the better device performance for
1c-d agrees with effective intramolecular triplet energy transfer
at long distances mediated by the wirelike mechanism.23
In conclusion, a series of multichromophoric emitters designed
to study energy level/distance-dependence in energy transfer were
prepared and their photophysical properties studied both in solution
and in the solid state. The materials show effective singlet and triplet
energy transfer. An improved OLED output was obtained for
systems having longer oligofluorene bridges, which showed better
alignment of triplet energy levels despite the longer donor-acceptor
distance. In the OLEDs, the order of a magnitude increase in
efficacy for materials with better triplet level alignment appears to
be due to facile triplet energy transfer. The materials exhibit red
(23) We believe that the external quantum efficacy and the higher luminance
in 1c-d were not due to intermolecular energy transfer. In fact, the dilute-
solution, thin film PL and EL were very similar, suggesting that no strong
intermolecular interaction takes place.
JA064471I
9
12438 J. AM. CHEM. SOC. VOL. 128, NO. 38, 2006