Vapor-phase processable novel nonplanar donor - Acceptor quateraryls for blue OLEDs

Article abstract of DOI:10.1021/ol8008182

(Figure Presented) A novel series of thermally stable blue light emitting quateraryls with a piperidine donor and a nitrile acceptor was prepared from a ketene-S,S-acetal under mild conditions without using an organometal catalyst. The performance of a bl

Full text of DOI:10.1021/ol8008182

ORGANIC
LETTERS
2008
Vol. 10, No. 12
2553-2556
Vapor-Phase Processable Novel
Nonplanar Donor-Acceptor Quateraryls
for Blue OLEDs#
Atul Goel,*,† Manish Dixit,† Sumit Chaurasia,† Amit Kumar,†
Resmi Raghunandan,‡ P. R. Maulik,‡ and R. S. Anand§
DiVisions of Medicinal and Process Chemistry and Molecular and Structural Biology,
Central Drug Research Institute, Lucknow 226001, India, and Department of Electrical
Engineering, Indian Institute of Technology, Kanpur 208016, India
agoel13@yahoo.com
Received April 10, 2008
ABSTRACT
A novel series of thermally stable blue light emitting quateraryls with a piperidine donor and a nitrile acceptor was prepared from a ketene-
S,S-acetal under mild conditions without using an organometal catalyst. The performance of a blue quateraryl 6e was investigated by fabricating
a multilayer OLED with a configuration of ITO/PEDOT:PSS (40 nm)/quateraryl (60 nm)/BCP (6 nm)/Alq3 (20 nm)/LiF (0.5 nm)/Al (200 nm), which
exhibited blue emission with a low turn on voltage of 4 V at a brightness of 0.22 cd/m2.
Small organic light emitting diode (OLED) displays such as
displays of mobile phones, mp3 players, and car radios are
on the verge of establishing themselves in the market with
great potential to expand in the near future. For full-color
flat-panel displays, three primary colors, red, green, and blue,
with equal performance are required.1 Several green and red
light emitting organic molecules are commercially available;
however, substances suitable for long-term use as blue light
emitters are still far from the standards of color purity and
thermal stability.2
renes),3 ladder- (II, biphenalenes, phenylenes),4 and poly-
fluorene5 dendritic systems (III) have been synthesized
(Figure 1). Among them, fluorene-based oligomers and
polymers are the most promising family of blue light
emitters, but they are prone to oxidation (keto defect or
(2) For an extensive review, see: (a) Tour, J. M. Chem. ReV. 1996, 96,
537. (b) Pu, L. Chem. ReV. 1998, 98, 2405. (c) Martin, R. E.; Diederich, F.
Angew. Chem., Int. Ed. 1999, 38, 1350. (d) Berresheim, A. J.; Muller, M.;
Mullen, K. Chem. ReV. 1999, 99, 1747. (e) Roncali, J. Acc. Chem. Res.
2000, 33, 147. (f) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning,
A. P. H. J. Chem. ReV. 2005, 105, 1491. (g) Schwab, P. F. H.; Smith, J. R.;
Michl, J. Chem. ReV. 2005, 105, 1197. (h) Forrest, S. R.; Thompson, M. E.
Chem. ReV. 2007, 107, 924.
For blue OLEDs, several tailor-made multiple π-conju-
gated molecular architectures such as spiro- (I, spirofluo-
(3) (a) Wu, C.-C.; Liu, T.-L.; Hung, W.-L.; Hung, W.-Y.; Lin, Y.-T.;
Wong, K.- T.; Chen, R.-T.; Chen, Y.-M.; Chien, Y.-Y. J. Am. Chem. Soc.
2003, 125, 3710. (b) Salbeck, J.; Yu, N.; Bauer, J.; Weissortel, F.; Bestgen,
H. Synth. Met. 1997, 91, 209. (c) Fungo, F.; Wong, K.-T.; Ku, S.-Y.; Hung,
Y.-Y.; Bard, A. J. J. Phys. Chem. B 2005, 109, 3984. (d) Chiang, C.-L.;
Wu, M.-F.; Dai, D.-C.; Wen, Y.-S.; Wang, J.-K.; Chen, C.-T. AdV. Funct.
Mater. 2005, 15, 231. (e) Itkis, M. E.; Chi, X.; Cordes, A. W.; Haddon,
R. C. Science 2002, 296, 1443.
† Medicinal and Process Chemistry, Central Drug Research Institute.
‡ Molecular and Structural Biology, Central Drug Research Institute.
§ Indian Institute of Technology.
# CDRI Communication No. 7003.
(1) (a) Marsitzky, D.; Mullen, K. In AdVances in Synthetic Metals,
Twenty Years of Progress in Science and Technology; Bernier, P., Lefrant,
S., Bidan, G., Eds.; Elsevier: New York, 1999; p 1. (b) Mullen, K.; Wegner,
G. Electronic Materials: The Oligomer Approach; Wiley-VCH: Weinheim,
NY, 1998. (c) Miyata, S.; Nalwa, H. S. Organic Electroluminescent
Materials and DeVices; Gordon and Breach Publishers: Amsterdam, 1997.
(4) (a) Scherf, U.; Mullen, K. Macromol. Chem., Rapid Commun. 1991,
12, 489. (b) Muller, M.; Morgenroth, F.; Scherf, U.; Soczka-Guth, T.;
Klarner, G.; Mullen, K. Philos. Trans. R. Soc. London, Ser. A 1997, 355,
715.
10.1021/ol8008182 CCC: $40.75
Published on Web 05/23/2008
2008 American Chemical Society

with amine donor and nitrile acceptor moieties and examined
their optical properties. In this letter, we report a new series
of thermally stable donor-acceptor nonplanar quateraryls
and their potential use in preparing multilayer blue OLED
devices.
Benzene rings functionalized with polyaryl groups have
been synthesized either by the coupling of biaryltriflate
compounds with Grignard reagents in the presence of a
palladium catalyst in moderate to good yields10 or by the
iterative coupling of aryl boronic acid with aromatic halides11
separately. However, it has been observed that iterative
coupling on functionally crowded aryl trihalides to prepare
triarylated-benzene compounds places constraints on the
choice of reagents/catalysts.12 Our aim to prepare blue light
emitting compounds with donor-acceptor groups was
achieved by preparing a key intermediate R-oxo-ketene-S,S-
acetal13 1 from easily accessible precursors methyl cyanoac-
etate, carbon disulfide, and methyl iodide, following the
Tominagaprotocol.14Substrate1,onMichaeladdition-cyclization
reaction with various substituted acetophenones 2a-e under
alkaline conditions, furnished 6-aryl-2H-pyran-2-ones 3a-e
in excellent yields (Scheme 1). In order to prepare com-
pounds with N,N-dialkylamino functionality, a good leaving
methylsulfanyl group of lactones 3a-e was replaced with
an amine (piperidine) to furnish 6-aryl-2-oxo-4-piperidin-1-
yl-2H-pyran-3-carbonitriles (4a-e) in good yields. These
compounds 4a-e were reacted with functionalized deoxy-
benzoins 5 to yield 5′-(piperidin-1-yl)-[1,1′;2′,1′′;3′,1′′′]quater-
phenyl-4′-carbonitriles (6a-e) in excellent yields (Scheme
1).
Figure 1. Various π-conjugated frameworks for blue light emitting
materials (I-III), small molecule polyphenylphenyl dendron (IV),
and new donor-acceptor quateraryl (V). F, fluorenyl; P, phenalenyl;
Ar, aryl; D, donor; A, acceptor.
green-emission defect)6 resulting in an additional undesirable
low-energy broadband in the EL spectrum, which not only
reduces emission efficiency but also destroys the blue color
purity.6 The blue light emitting materials generally have a
low affinity for the electrons from the cathode in OLEDs
due to the large band gap energy. Recently Komatsu and
co-workers7 elucidated electronic properties of fluorene
having a radical cation and dication through computational
and experimental studies, which provided insights for low
electrical conductivity observed for poly(2,7-fluorene)s.
Therefore, new chemical entities that may overcome the
drawbacks of existing blue light emitters are urgently
required for preparing B-OLEDs.
Recently, a great deal of attention has been focused on
synthesizing small molecule polyphenylphenyl dendrons (IV)
for preparing B-OLEDs.8 Studies have also shown that the
presence of strong electron-donating N,N-dialkylamine func-
tionalities in π-conjugated materials enhances the device
efficiency by elevating the HOMO energy levels of the
molecules for ease of hole injection/transporting in the film,8
and the presence of the cyano group influences photophysical
and electroluminescent properties by lowering the energy of
the LUMO, thus exhibiting a relatively low threshold voltage
and high quantum efficiency in LED devices.9 Considering
these aspects, we designed quateraryls (V) functionalized
A plausible mechanism, depicted in Scheme 2, suggests
that the reaction is initiated by the Michael addition of an
anion, generated from a molecule of deoxybenzoin 5, to the
2H-pyran-2-one 4 followed by intramolecular cyclization to
form a bicyclic intermediate. This bicyclic intermediate on
decarboxylation, protonation, and dehydration furnished
quateraryl 6 in excellent yield.
To examine the solid state molecular organization, the
crystallizations of these quateraryls in various solvent systems
were attempted. All the quateraryls except 6c were amor-
phous substances. The X-ray structural analysis of compound
6c (Figure 2) revealed that the aryl rings are arranged
(5) (a) Kottas, G. S.; Clarke, L. I.; Horinek, D.; Michl, J. Chem. ReV.
2005, 105, 1281. (b) Setayesh, S.; Grimsdale, A. C.; Weil, T.; Enkelmann,
V.; Mullen, K.; Meghdadi, F.; List, E. J. W.; Leising, G. J. Am. Chem.
Soc. 2001, 123, 946. (c) Sun, D.; Rosokha, S. V.; Kochi, J. K. Angew.
Chem., Int. Ed. 2005, 44, 5133. (d) Chen, C.-T.; Chiang, C.-L.; Lin, Y.-C.;
Chan, L.-H.; Huang, C.-H.; Tsai, Z.-W.; Chen, C.-T. Org. Lett. 2003, 5,
1261. (e) Wakamiya, A.; Ide, T.; Yamaguchi, S. J. Am. Chem. Soc. 2005,
127, 14859.
Hamer, P. J. Synth. Met. 1995, 71, 2117. (c) Wu, C.-J. J.; Xue, C.; Kuo,
Y.-M.; Luo, F.-T. Tetrahedron 2005, 61, 4735. (d) Hwu, J. R.; Chuang,
K.-S.; Chuang, S. H.; Tsay, S.-C. Org. Lett. 2005, 7, 1545.
(10) (a) Satoh, T.; Kawamura, Y.; Miura, M.; Nomura, M. Angew.
Chem., Int. Ed. Engl. 1997, 36, 1740. (b) Kamikawa, T.; Hayashi, T. Synlett
1997, 163.
(11) (a) Blake, A. J.; Cooke, P. A.; Doyle, K. J.; air, S.; Simpkins, N. S.
Tetrahedron Lett. 1998, 39, 9093. (b) Bahl, A.; Grahn, W.; Stadler, S.;
Feiner, F.; Bourhill, G.; Bra¨uchle, A. R.; Jones, P. G. Angew. Chem., Int.
Ed. Engl. 1995, 34, 1485.
(6) (a) Li, J. Y.; Ziegler, A.; Wegner, G. Chem. Eur. J. 2005, 11, 4450.
(b) Scherf, U.; List, E. J. W. AdV. Mater. 2002, 14, 477. (c) Romaner, L.;
Pogantsch, A.; de Freitas, P. S.; Scherf, U.; Gaal, M.; Zojer, E.; List, E. J. W.
AdV. Funct. Mater. 2003, 13, 597. (d) Kulkarni, A. P.; Kong, X.; Jenekhe,
S. A. J. Phys. Chem. B 2004, 108, 8689. (e) List, E. J. W.; Guentner, R.;
de Freitas, P. S.; Scherf, U. AdV. Mater. 2002, 14, 374.
(12) (a) Hassan, J.; Se´vignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M.
Chem. ReV. 2002, 102, 1359. (b) Goel, A.; Singh, F. V.; Kumar, V.; Reichert,
M.; Gulder, T. A. M.; Bringmann, G. J. Org. Chem. 2007, 72, 7765. (c)
Goel, A.; Singh, F. V.; Dixit, M.; Verma, D.; Raghunandan, R.; Maulik,
P. R. Chem. Asian J. 2007, 2, 239.
(7) (a) Nishinaga, T.; Inoue, R.; Matsuura, A.; Komatsu, K. Org. Lett.
2002, 4, 4117. (b) Yamazaki, D.; Nishinaga, T.; Komatsu, K. Org. Lett.
2004, 6, 4179.
(13) (a) Dieter, R. K. Tetrahedron 1986, 42, 3029. (b) Junjappa, H.;
Ila, H.; Asokan, C. V. Tetrahedron 1990, 46, 5423. (c) Tominaga, Y. Trends
Heterocycl. Chem. 1991, 2, 43.
(8) Huang, C.; Zhen, C.-G.; Su, S. P.; Loh, K. P.; Chen, Z.-K. Org.
Lett. 2005, 7, 391.
(9) (a) Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend, R. H.;
Holmes, A. B. Nature 1993, 365, 628. (b) Moratti, S. C.; Cervini, R.;
Holmes, A. B.; Baigent, D. R.; Friend, R. H.; Greenham, N. C.; Gru¨ner, J.;
(14) (a) Tominaga, Y.; Ushirogouchi, A.; Matsuda, Y. J. Heterocycl.
Chem. 1987, 24, 1557. (b) Tominaga, Y.; Ushirogouchi, A.; Matsuda, Y.;
Kobayashi, G. Chem. Pharm. Bull. 1984, 32, 3384.
2554
Org. Lett., Vol. 10, No. 12, 2008

Scheme 1. Synthesis of Quateraryls
Figure 2. ORTEP diagram of 6c with a thermal ellipsoid plot (50%
probablity).
coefficient, Stokes shift and the fluorescent quantum yield
of quateraryls 6a-e in tetrahydrofuran solution. The different
fluorophoric groups at the para-position of the peripheral
B-phenyl ring of the quateraryls (6a-d) did not make any
remarkable difference in the longest absorption maximum
(345-351 nm). A strong red shift (>50 nm) was observed
when a hydroxy group was attached at position 2 of the
peripheral D-phenyl ring (6e, 406 nm). The increase in the
wavelength and intensity of aborption of 6e may be due to
the presence of an auxochrome (OH) directly in conjugation
with the π-system of the chromophore.
The thermal properties of quateraryls were investigated
using thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC). Compound 6e exhibited good
thermal stability, showing less than 5% decomposition up
to 300 °C under nitrogen and losing about 50% weight at
about 360 °C. In the DSC, 6e showed a melting temperature
(Tm) of 215.91 °C in the first heating scan at the rate of 5
°C/min. Rapid cooling at the rate of 25 °C/min to room
temperature and subsequent slow heating scans detected a
cofacially in a half-circular array around the central benzene
ring. The mean plane angles for the twist of the peripheral
phenyl rings B, C, and D from the plane of the central
benzene ring A are 47.9°, 64.1°, and 54.5°, respectively, with
an average twist of 55.49°, giving each molecule a propeller
conformation. The crystal packing showed weak C-H···π
interactions, but no π-π interaction was observed, which
supports developing nonplanar molecular blocks.
Figure 3 shows the normalized UV-vis and PL spectra
of quateraryls (6a-e). Table 1 shows the λmax of their UV
and fluorescence spectral data along with the extinction
Scheme 2. Plausible Reaction Mechanism
Figure 3. Normalized absorbance and fluorescence spectra of
quateraryls 6a-e in THF (∼10-5 M).
Org. Lett., Vol. 10, No. 12, 2008
2555

Table 1. Photophysical Properties of Quateraryls 6a-e
λmax;abs
(nm)a
ꢀ (× 104
λmax;em
(nm)c
∆ νj
(cm-1
Tm
(°C)e
b
d
compd
M-1 cm-1
)
)
Φf
6a
6b
6c
6d
6e
351
345
347
346
406
2.80
2.85
2.97
2.61
1.39
424
433
429
425
447
4900
5900
5500
5400
2300
171
194
157
173
216
0.48
0.38
0.43
0.45
0.51
a Longest wavelength absorption maximum. b Molar extinction coefficients.
c Fluorescence emission maximum. d Stokes shift. e Melting temperatures
by DSC. f Fluorescence quantum yield relative to harmine in 0.1 M H2SO4
as a standard (Φ ) 0.45).
Figure 4
spectrum)
. ILV characteristics of device of 6e (insert shows the EL
signal of >200 °C corresponding to the glass transition
temperature (Tg). The high Tg is likely to be good for long
lifespan device operations. Compounds 6a-e produced
efficient blue fluorescence and exhibited good fluorescent
quantum yields15 (Φ, 0.38-0.51).
spectrum of 6e shows a sharp peak emission at 489 nm, with
the CIE coordinates of (0.2358, 0.3670), corresponding to
bluish emission. To the best of our knowledge, this is the
first report on donor-acceptor quateraryls utilized as emis-
sive layers in an OLED device.
Multilayer devices were fabricated to investigate the
performance of a blue light emitting material (6e) with the
device configuration of ITO/PEDOT:PSS (40 nm)/quateraryl
6e (60nm)/BCP (6 nm)/Alq3 (20 nm)/LiF (0.5 nm)/Al (200
nm). The patterned ITO glass plate was cleaned in 6:1:1
RCA-I solution, rinsed in DI water a number of times, and
then dried. The ITO surface was treated in ozone for 15 min.
Immediately, the first layer of poly(3,4-ethylenedioxythio-
phene) doped with poly(styrenesulfonic acid) (PEDOT:PSS)
was spin-coated on the patterned ITO to form a hole injection
layer. The PEDOT-PSS was vaccuum-dried at 120 °C for
1 h. All other organic layers and metal layers were sublimed
in high vacuum (∼1-5 × 10-6 mbar). Then the devices were
sealed with a covering glass plate using UV epoxy. The ILV
characteristics of the sealed devices were obtained using the
Keithley source-measure unit. EL spectra were recorded with
an Ocean Optics USB2000 miniature fiber optic spectrom-
eter.
In summary, we have prepared new nonplanar small
molecule quateraryls with donor-acceptor groups from
ketene-S,S-acetals in excellent yields through carbanion-
induced ring transformation of 2H-pyran-2-ones as key steps.
These molecules exhibited blue fluorescence, high quantum
yields and good thermal stability with a glass transition
temperature >200 °C, which indicates that these new
quateraryl scaffolds with donor-acceptor functionalities are
good candidates for developing blue organic light emitting
diodes (B-OLEDs).
Acknowledgment. A.G. acknowledges financial support
by the Department of Science and Technology (DST), New
Delhi, India under Ramanna Fellowship Scheme (SR/S1/
RFPC-10/2006). R.S.A. thanks the MHRD for financial
support for device experimental work. S.C., M.D., A.K., and
R.R. are thankful to CSIR, New Delhi for senior research
fellowships.
The current density-voltage and luminance-voltage
characteristics for a device of 6e are plotted in Figure 4,
and the EL spectra of 6e are shown as an insert. The turn on
voltage (at a brightness of 0.22 cd/m2) of 6e is 4 V. The EL
Supporting Information Available: Complete experi-
mental details, characterization data, UV-vis and fluores-
cence spectra, DSC, TGA, X-ray data (CIF, 6e) for the
compounds 6a-e. This material is available free of charge
via the Internet at http://pubs.acs.org.
(15) (a) Williams, A. T. R.; Winfield, S. A.; Miller, J. N. Analyst 1983,
108, 1067. (b) Pardo, A.; Reyman, D.; Poyato, J. M. L.; Medina, F. J. Lumin.
1992, 51, 269.
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