Tetracene Derivatives as Potential Red Emitters for Organic LEDs

Article abstract of DOI:10.1021/ol035415e

(Equation presented) As part of our program investigating the use of ethynylated acenes in organic electronics, we have prepared a series of functionalized tetracene derivatives in search of a material with red emission suitable for use in display technologies. A number of such compounds with functionalization on the alkyne and/or the acene ring were easily prepared in one or two steps from commercially available materials. Solution fluorescence quantum efficiencies were generally good for these derivatives, which possessed emission maxima spanning the range 540-637 nm. Simple light-emitting diodes fabricated from these compounds showed that one of the derivatives did exhibit red electroluminescence.

Full text of DOI:10.1021/ol035415e

ORGANIC
LETTERS
2
003
Vol. 5, No. 23
245-4248
Tetracene Derivatives as Potential Red
Emitters for Organic LEDs
4
Susan A. Odom, Sean R. Parkin, and John E. Anthony*
Department of Chemistry, UniVersity of Kentucky, Lexington, Kentucky 40506-0055
anthony@uky.edu
Received July 28, 2003
ABSTRACT
As part of our program investigating the use of ethynylated acenes in organic electronics, we have prepared a series of functionalized tetracene
derivatives in search of a material with red emission suitable for use in display technologies. A number of such compounds with functionalization
on the alkyne and/or the acene ring were easily prepared in one or two steps from commercially available materials. Solution fluorescence
quantum efficiencies were generally good for these derivatives, which possessed emission maxima spanning the range 540−637 nm. Simple
light-emitting diodes fabricated from these compounds showed that one of the derivatives did exhibit red electroluminescence.
5
Acenes, the series of cata-condensed aromatic hydrocarbons,
are at the forefront of current research into organic electronic
devices. Pentacene and tetracene have been used as the active
stability, combined with emission (476 nm) falling outside
the range considered most exploitable in display technologies.
6
Our research into functionalized pentacene led us to
1
layer of high-performance organic field-effect transistors and
consider whether functionalized tetracene could be adapted,
by the addition of appropriate substituents, to become an
acceptable red emitter for use in electroluminescent displays.
Functionalized tetracene has several potential advantages
over the larger pentacene: The smaller aromatic unit in
tetracene, combined with the substituted alkyne functional
groups hindering Diels-Alder reactivity on the central rings,
leads to higher stability of this material over pentacene. The
increased solubility of functionalized tetracenes may make
have both been used as hole collectors in organic photovoltaic
2
cells. Acenes have also found use in organic light-emitting
diodes (OLEDs), where the typically high fluorescence
efficiency of these aromatic compounds makes them ideal
emitters. Functionalized anthracene has been reported to yield
3
efficient blue OLEDs, while substituted pentacene deriva-
3
tives act as a guest dye in Alq -based devices to produce
4
red electroluminescence. There are no published reports of
the use of tetracene as the emissive material in OLEDs,
ostensibly due to the parent compound’s poor oxidative
(3) (a) Zhang, Z. L.; Jiang, X. Y.; Zhu, W. Q.; Zheng, X. Y.; Wu, Y. Z.;
Xu, S. H. Synth. Met. 2003, 137, 1141. (b) Kondakov, D. Y.; Sandifer, J.
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S.-H. Thin Solid Films 2001, 401, 251.
(
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Phys. Lett. 2002, 80, 2925. (b) W u¨ rthner, F. Angew. Chem., Int. Ed. 2001,
0, 1037. (c) Nelson, S. F.; Lin, Y.-Y.; Gundlach, D. J.; Jackson, T. N.
4
Appl. Phys. Lett. 1998, 72, 1854. (d) Dimitrakopoulos, C. D.; Furman, B.
K.; Graham, T.; Hedge, S.; Purushothaman, S. Synth. Met. 1998, 92, 47.
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2378.
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Tennakone, K. Solar Energy 2002, 73, 103. (b) Videlot, C.; Fichou, D.;
Garnier, F. J. Chim. Phys. Phys.-Chim. Biol. 1998, 95, 1335. (c) Dahlberg,
S. C.; Musser, M. E. J. Chem. Phys. 1979, 71, 2806. (d) Matsumura, M.;
Uohashi, H.; Furusawa, M.; Yamamoto, N.; Tsubomura, H. Bull. Chem.
Soc. Jpn. 1975, 48, 1965.
(5) (a) Venedictov, E. A.; Tulikova, E. J. Tetrahedron Lett. 2003, 44,
3215. (b) Cicerone, M. T.; Ediger, M. D. J. Phys. Chem. 1993, 97, 10489.
(6) Anthony, J. E.; Eaton, D. L.; Parkin, S. R. Org. Lett. 2002, 4, 15. (b)
Anthony, J. E.; Brooks, J. S.; Eaton, D. L.; Parkin, S. R. J. Am. Chem. Soc.
2001, 123, 9482.
1
0.1021/ol035415e CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/14/2003

them more amenable to inexpensive solution deposition for
device fabrication. Inducing bathochromic shifts using a wide
variety of substituents will also allow the “fine-tuning” of
the emission color. We report here the synthesis of a series
of substituted tetracene derivatives, their solution absorption
and emission spectra, and the electroluminescence charac-
teristics of the derivatives with properties amenable to device
fabrication.
Table 1. Synthetic Yield and Spectroscopic and
Electrochemical Data for Compounds 3-8
%
λmaxabs λmaxem Φrel λmaxEL Eox (mV)a
compd
yield
(nm)
(nm)
(%)
(nm)
vs SCE
tetracene
475
534
556
571
597
580
617
476
540
567
586
643
604
637
720
910
830
568
806
750
610
3
4
5
6
7
8
98
98
55
49
88
17
79
81
69
10
19
36
543
595
Following the precedent of our recent work with ethyny-
lated pentacene derivatives, we sought to prepare a series of
ethynylated tetracenes with various substituents on the
alkynes. The most prevalent method for the ethynylation of
aromatic compounds involves Sonogashira coupling between
604
656
a
Performed in 0.1 M solution of Bu4NPF6 in dichloromethane, Pt
electrode, scan rate of 150 mV/s, ferrocene as internal standard.
7
a terminal alkyne and an aryl bromide or iodide. Unfortu-
nately, this method results in the formation of ring-fused side
products when applied to halides on the peri position of
linearly fused systems. We avoid these confounding side
8
11
oxygen, many of the ethynylated materials were stable for
products by adopting an alternative procedure to palladium-
catalyzed coupling reactions. The addition of a lithium
acetylide to an acenequinone, followed by deoxygenation
with stannous chloride, provides the diethynyl acene in good
months under identical conditions. This behavior mimics that
observed for the silylethynylated pentacenes and is likely
due to an increase in oxidation potential (Table 1).12
All of the tetracene derivatives possess the characteristic
strong â-band absorption at approximately 290 nm, and as
expected, this band shifts only slightly with changes in
9
to excellent yield. Following this procedure, the desired
ethynylated tetracenes 3-8 are easily prepared from 5,12-
tetracenequinone 1 and 6,11-dimethoxy-5,12-tetracenequino-
ne 2 (Scheme 1, for yields see Table 1).10
1
3
substitution. The long-wavelength absorptions are more
sensitive to substitution and undergo significant variations
14
upon change in functionality. Simple ethynylation produces
a bathochromic shift of 55-115 nm, depending on the alkyne
substituent (compounds 3-6). The addition of two π-electron-
donating methoxy groups to the acene backbone further shifts
Scheme 1
1
5
these values by ∼50 nm. The sensitivity of this long-
wavelength band to substitution thus allows the tuning of
absorption over the range of 475 nm (unsubstituted tetracene)
to 617 nm (compound 8). Absorption spectra showing the
long-wavelength region for the ethynylated tetracene deriva-
tives 3-8, along with that for unsubstituted tetracene, are
represented by the black traces in Figure 1.
Along with the superior solubility of these materials in
common organic solvents, the most notable difference
between these derivatives and the parent hydrocarbon tet-
racene is the generally improved oxidative stability of the
alkyne-functionalized materials. While solutions of tetracene
decolorize within 48 h when left exposed to light and
(
7) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
4
467.
(
(
(
8) Dang, H.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2001, 123, 355.
9) See e.g. Hanhela, P. J.; Paul, D. B. Aust. J. Chem. 1981, 34, 1701.
10) Comound 8 did not crystallize readily from any solvent. To attain
Figure 1. Absorbance (black traces), photoluminescence (PL, blue
traces), and electroluminescence (EL, red traces) spectra for
compounds 3-8, taken in hexane.
material pure enough for device fabrication, repeated chromatography on
silica led to significant decomposition, and thus a low yield, of product 8.
4246
Org. Lett., Vol. 5, No. 23, 2003

The ethynylated tetracene derivatives are all fluorescent
in solution, with relative fluorescence quantum yields ranging
derivatives (4, 6) evaporated before decomposing, both
leaving an insoluble black residue on the evaporation boat.
All of the derivatives containing the bulky tri(isopropyl)-
silyl group did yield homogeneous films by evaporation.
Electroluminescence (EL) produced by the devices made
from compound 5 (with λmax 595 nm) was so weak that it is
not shown in Figure 1.
16
from 10% to 81% (versus a rubrene standard). Solution
photoluminescence spectra, represented as the blue traces
in Figure 1, showed the expected fine-structure typically
mirroring the absorption features and emission λmax ranging
from 540 to 637 nm.17 For use in a red-emitting OLED, the
material should have as narrow an emission as possible, close
to the low-wavelength end of the red portion of the spectrum
The EL spectra for compounds 3, 7, and 8 are shown as
the red traces in Figure 1. Derivative 3 exhibits a solid-state
electroluminescence maximum at 543 nm nearly identical
to the solution-state fluorescence and yields a bright orange
OLED. The methoxy homologue 7 has a beautiful red-orange
emission, with a maximum at 604 nm identical to the solution
emission, but with an additional strong emission at 670 nm
and significant tailing to longer wavelengths. Such behavior
is frequently attributed to aggregation, particularly involving
interaction between π-faces of adjacent molecules.20 To
further investigate the solid-state order of 7 we acquired the
X-ray crystal structure of this compound. Because films
formed by the thermal evaporation of acenes are typically
polycrystalline, we expect a strong correlation between the
order of the molecules in the thin film and the order in the
(
typically ∼620 nm). Emission at wavelengths much longer
than this value is less important due to the poor spectral
response of the human eye at longer wavelengths. Both
compounds 6 and 8 satisfy these criteria.
Determination of the electroluminescence spectra of these
materials must involve the construction of a light-emitting
device. Because we are interested in the color of the
emission, rather than optimum device performance, we used
a highly simplified device structure. While most small-
3
molecule OLEDs incorporate Alq as an electron-transport
layer, electroluminescence from this compound can alter the
color of the emissive layer.18 To maintain color purity, the
functionalized tetracene was used as both the emissive layer
and the electron transport layer in our devices. We did find
that incorporation of a lithium fluoride “buffer layer” between
the cathode and the acene significantly improved device
performance without altering the emission wavelength of the
2
1
single crystal. As shown in Figure 3, there is indeed
19
acene. The simple device configuration is shown in Figure
, and the specifics of device fabrication and performance
2
are reported in the Supporting Information.
Figure 2. Configuration of simple tetracene-based devices.
To form working devices, the emissive material must be
sufficiently volatile under high vacuum to be deposited onto
the substrate. Unfortunately, neither of the aryl-ethynylated
(
11) Fritzsche, J. Comptes Rendus Acad. Sci. 1867, 69, 1035.
(12) The tetracene derivatives also exhibited a generally higher thermal
stability versus tetracene, undergoing decomposition under TGA analysis
air atmosphere) at between 20 and 50 ° C higher temperature than the
parent hydrocarbon.
13) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Inter-
science: New York, 1970.
14) Not surprisingly, the shifts in absorption follow the predictions made
by applying the Woodward-Fieser rules to tetracene.
15) The magnitude of this shift agrees with the empirical rules for
absorption spectroscopy, each methoxy group inducing a shift of ∼25 nm.
16) Fery-Forgues, S.; Lavabre, D. J. Chem. Educ. 1999, 76, 1260.
(
Figure 3. Crystal structure of compound 7, showing π-interaction
(
between adjacent molecules in the crystal.
(
(
π-interaction between adjacent molecules in the solid state.
Excimer formation arising from these interactions may be
the cause of the undesired long-wavelength emission. Be-
(
Rubrene quantum yield is essentially 1 in nonpolar solvents (all measure-
ments done in hexane): Mattoussi, H.; Murata, H.; Merritt, C. D.; Lizumi,
Y.; Kido, J.; Kafafi, Z. H. J. Appl. Phys. 1999, 86, 2642.
(
17) The thin-film PL spectra of these compounds were essentially
(18) (a) Troadec, D.; Veriot, G.; Moliton, A. Synth. Met. 2002, 127, 165.
(b) Aziz, H.; Popovic, Z. D. Appl. Phys. Lett. 2002, 80, 2180.
identical to EL spectra in all cases and have been omitted for clarity.
Org. Lett., Vol. 5, No. 23, 2003
4247

cause neat films of 7 also exhibited the same long-wavelength
absorption, this feature is not likely caused by interaction
between 7 and the hole-transport layer (TPD).
By careful functionalization of the tetracene backbone,
it is indeed possible to tune the photoluminescence and
electroluminescence emission to a significantly longer wave-
length, while simultaneously improving the stability and
solubility of the emissive molecule over the parent hydro-
carbon. Compound 8 can be used to prepare a red-emitting
diode, using a very simple device structure. Further inves-
tigation of the ethynylated acenes, to improve device per-
formance, is currently underway.
Compound 8 shows a relatively sharp electroluminescence
emission maximum at 656 nm and produces a deep red-
colored emission. The solid-state electroluminescence is
shifted almost 20 nm relative to the solution emission, a
behavior also seen in the ethynylated pentacenes. However,
there is no broadening of the emission peak characteristic
of aggregate formation. The origin of this bathochromic shift
in the ethynylated tetracenes and pentacenes is currently
under investigation.
Acknowledgment. This work was supported by the
Office of Naval Research and the National Science Founda-
tion. We also thank Dr. Zakya Kafafi, Naval Research
Laboratory, for helpful discussions.
(
19) (a) Hung, L. S.; Tang, C. W.; Mason, M. G. Appl. Phys. Lett. 1997,
0, 152. (b) Jabbour, G. E.; Kawabe, Y.; Shaheen, S. E.; Wang, J. F.;
Morrell, M. M.; Kippelen, B.; Peyghambarian, N. Appl. Phys. Lett. 1997,
1, 1762. (c) Shaheen, S. E.; Jabbour, G. E.; Morrell, M. M.; Kawabe, Y.;
7
Supporting Information Available: Experimental pro-
cedures, characterization and crystallographic data for com-
pound 7, and device fabrication procedures. This material is
available free of charge via the Internet at http://pubs.acs.org.
7
Kippelen, B.; Peyghambarian, N.; Nabor, M. F.; Schlaf, R.; Mash, E. A.;
Armstrong, N. R. J. Appl. Phys. 1998, 84, 2324.
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D.; Setayesh, S.; M u¨ llen, K. Adv. Mater. 2001, 13, 1096. (b) Merlet, S.;
Birau, M.; Wang, Z. Y. Org. Lett. 2002, 4, 2157.
(
21) See, for example: Shtein, M.; Mapel, J.; Benziger, J. B.; Forrest,
S. R. Appl. Phys Lett. 2002, 81, 268 and references therein.
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