Non-amine-based furan-containing oligoaryls as efficient hole transporting materials

Article abstract of DOI:10.1039/b207489c

A new class of highly stable furan-based hole transporting oligomeric materials, synthesized from the corresponding propargylic dithioacetals, serve as efficient hole transporting materials in electroluminescent devices. The performance of the devices usi

Full text of DOI:10.1039/b207489c

Non-amine-based furan-containing oligoaryls as efficient hole
transporting materials†
Ling-Zhi Zhang,a Chieh-Wei Chen,b Chin-Fa Lee,ac Chung-Chih Wu*b and Tien-Yau Luh*ac
a Department of Chemistry, National Taiwan University, Taipei, Taiwan 106
b Department of Electrical Engineering, Graduate Institute of Electro-optical Engineering and Electronics
Engineering, National Taiwan University, Taipei, Taiwan 106
c
Institute of Chemistry, Academia Sinica, Taipei, Taiwan 115. E-mail: tyluh@chem.sinica.edu.tw;
Fax: +886-2-2651-1488; Tel: +886-2-2789-8500
Received (in Cambridge, UK) 31st July 2002, Accepted 22nd August 2002
First published as an Advance Article on the web 16th September 2002
A new class of highly stable furan-based hole transporting
oligomeric materials, synthesized from the corresponding
propargylic dithioacetals, serve as efficient hole transporting
materials in electroluminescent devices. The performance of
the devices using these furan materials is comparable with or
somewhat better than those employing the conventional
triarylamines (e.g. a-NPD).
hexaaryls containing symmetrically two furan moieties 2a–c
were synthesized.‡ The physical properties of 2a–c are
compared with those of 1 and tris(8-oxyquinoline)aluminium
(Alq3) (Table 1). A thin film of 2a–c on glass or a NaCl pellet
was thermally stable, no decomposition being observed at 300
°C. Neither weight loss nor spectroscopic variation (NMR, IR,
UV-vis, and fluorescence) was observed when the samples,
under nitrogen atmosphere, were heated at 200 °C for 24 h or
irradiated with a 200 W sunlamp at 150 °C for 24 h. The solid
samples 2a–c remained intact under ambient conditions for over
one year. A solution of 2a–c (e.g. in CHCl3) was stable at room
temperature in the dark, but decomposed slowly in the presence
of ambient light.
Triarylamines, such as a-NPD (4,4A-bis[N-(1-naphthyl)-N-
phenylamino]biphenyl, 1), have been extensively employed as
the hole transporting (HT) material in OLEDs (organic light
emitting devices) covering a wide range of wavelengths and
generating very bright emission.1–7 The use of oligoaryls such
as thiophene derivatives in optoelectronic applications
abounds,8 whereas the corresponding furan-containing oligo-
mers or polymers have been only sporadically explored.9–12 It is
interesting to note that furan has a similar HOMO energy level
and a relatively high lying LUMO energy level in comparison
with those of thiophene.13 We felt that these properties could be
used in electroluminescent applications.
These furan-containing oligoaryls showed bright fluores-
cence in the blue light region with quantum yields in the range
of 0.91–0.95. The emission frequencies for 2a–c exhibited
slight red shifts in the solid in comparison with those in
solution.
Compounds 2a–c showed a reversible two-electron redox
wave as examined by cyclic voltammetry (Table 1). As
expected, the electron donating butyl substituent may cause a
reduction in the oxidation potential. The first oxidation potential
for pentaaryl 2a was somewhat lower than those for hexaaryls
2b and c and the reverse was true for the second oxidation
potentials. Similar redox behavior of diphenyl-a-oligopyrroles
has been reported.15
We recently reported a convenient synthesis of 2,3,5-trisub-
stituted furans from the corresponding propargylic dithioacetals
(eqn. 1).14 According to this protocol, a series of penta- and
(1)
Based on the electrochemical and photophysical data, the
HOMO and LUMO energy levels of 2a–c were estimated, and
they fit very well with the frontier orbital energies for Alq3.
Typical electroluminescent results are outlined in Table 2. A
† Electronic supplementary information (ESI) available: experimental
details. See http://www.rsc.org/suppdata/cc/b2/b207489c/
Table 1 Physical properties of 2 and related compounds
a
Compd. Tg
Tc
Td
lmax/nmb
lem/nmb
Ffc
E1/mV E2/mVd
Eox1/2/mVd HOMO/eV LUMO/eV
2a
2b
2c
1
Alq3
88
98 308
434
73 339
245, 383 (253, 395)
244, 374 (245, 375)
240, 368 (238, 371)
273, 342 (345)
439, 457 (461, 480)
436, 459 (450, 478)
424, 448 (440, 464)
463 (441)
0.91
0.92
0.95
0.16
596
675
612
349
866
793
788
623
548
645
550
297
5.35
5.45
5.35
5.10
5.86
2.45
2.51
2.34
2.02
3.05
96 nae
23
102
179
382
262, 388 (262, 395)
503 (520)
a Decomposition temperature at 5% weight loss measured by TGA under N2 atmosphere. b Measured in CHCl3 solution and in solid form (values for solid
in parentheses). c Quantum yield obtained relative to Coumarin I. d Measured in CH2Cl2, all oxidation potentials are relative to ferrocene/ferrocenium half
cell. e na: not available.
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CHEM. COMMUN., 2002, 2336–2337
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Table 2 Electroluminescence data for the compounds
b
c
d
e
f
g
h
i
j
Devicea
lEL/nm
Von
V20
Vmax
B20
Bmax
h20
hmax
lm/W20
lm/Wmax
1
2
3
4
5
6
7
8
9
476
530
540
538
532
535
537
542, 578
542, 578
4.3
2.5
2.5
2.5
2.5
2.0
2.0
2.0
2.0
8.3
8.3
11.5
9.6
9.0
6.8
6.8
7.9
8.4
11.5
12.0
16.0
12.5
13.0
11.5
12.0
18.0
7
770
750
1050
740
870
710
2050
1970
260
18 600
16 100
9000
16 650
30 400
30 800
182 800
180 400
0.1
1.2
1.2
1.7
1.2
1.4
1.2
2.8
2.6
0.2
1.3
1.3
1.8
1.4
1.5
1.3
2.8
2.7
0.1
1.5
1.0
2.1
1.3
2.1
1.6
4.1
3.8
0.3
1.6
1.1
2.2
1.4
3.6
2.7
10.1
9.3
a Device configurations (thickness of each layer is described in the text): device 1: ITO/PEDT+PSS/2a/Mg+Ag/Ag; devices 2–5: ITO/PEDT+PSS/HTL/Alq3/
Mg+Ag/Ag (HTL for device 2: 2a; device 3: 2b; device 4: 2c; device 5: 1); devices 6–7: ITO/PEDT+PSS/HTL/Alq3/LiF/Al (HTL for device 6: 2a; device
7: 1); devices 8–9: ITO/PEDT+PSS/HTL/Alq3+3/Alq3/LiF/Al (HTL for device 8: 2a; device 9: 1). b Turn-on voltage (V) at which emission starts to be
detectable. c Voltage (V) taken at a current density of 20 mA cm22 d Voltage (V) at the maximum brightness. e Brightness (cd m22) taken at V20. f Maximal
.
. .
brightness. g External quantum efficiency at 20 mA cm22 h Maximal external quantum efficiency. i Luminous efficiency at 20 mA cm22 j Maximal
luminous efficiency.
comparable with or somewhat better than those employing the
conventional a-NPD 1.
We thank the Ministry of Education, Academia Sinica, and
the National Science Council of the Republic of China for
support and Professor Suit Tong Lee of the City University of
Hong Kong for stimulating discussions.
Notes and references
‡ All new compounds gave satisfactory spectroscopic and analytical data.
The details are described in the ESI.†
1 C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913.
2 For a recent review, see Y. Shirota, J. Mater. Chem., 2000, 10, 1 and
references therein.
3 (a) S. A. VanSlyke, C. H. Chen and C. W. Tang, Appl. Phys. Lett., 1996,
69, 2160; (b) D. F. O’Brien, P. E. Burrows, S. R. Forrest, B. E. Koene,
D. E. Loy and M. E. Thompson, Adv. Mater., 1998, 10, 1108.
4 (a) J. Kido and Y. Iizumi, Appl. Phys. Lett., 1998, 73, 2721; (b) C. H.
Chen and C. W. Tang, Appl. Phys. Lett.,, 2001, 79, 3711.
5 C.-C. Wu, C.-W. Chen, Y.-T. Lin, H. -L Yu, J.-H. Hsu and T.-Y. Luh,
Appl. Phys. Lett., 2001, 79, 3023.
Fig. 1 A comparison of the performance of devices 2–5 with theconfigura-
tion ITO/PEDT+PSS/HTL (40 nm)/Alq3 (60 nm)/Mg+Ag(800 nm)/Ag
(150 nm), where HTL is 2a (device 2, solid line), or 2b (device 3, dash line),
or 2c (device 4, dotted line) or 1 (device 5, dash-dotted line).
6 (a) H. Heil, J. Steiger, R. Schmechel and H. Seggern, J. Appl. Phys.,
2001, 90, 5357; (b) M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov,
S. Sibley, M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151.
7 (a) C. W. Ko and Y. T. Tao, Appl. Phys. Lett., 2001, 79, 4234; (b) Y. Liu,
J. Guo, H. Zhang and Y. Wang, Angew. Chem., Int. Ed., 2002, 41,
182.
8 (a) J. M. Tour, Acc. Chem. Res., 2000, 33, 791; (b) Electronic Materials:
The Oligomer Approach, ed. K. Müllen and G. Wegner, Wiley-VCH,
Weinheim, 1998; (c) U. Mitschke, E. M. Osteritz, T. Debaerdemaeker,
M. Sokolowski and P. Bäuerle, Chem. Eur. J., 1998, 4, 2211.
9 For a review see: A. Gandini and M. N. Belgacem, Prog. Polym. Sci.,
1997, 22, 1203 and references therein.
10 H. Saadeh, T. Goodson and L. Yu, Macromolecules, 1997, 30, 4608.
11 (a) R. E. Niziurski-Mann and M. P. Cava, Adv. Mater., 1993, 5, 547; (b)
R. E. Niziurski-Mann, C. Scordillis-Kelley, T.-L. Liu, M P. Cava and R.
T. Carlin, J. Am. Chem. Soc., 1993, 115, 887; (c) A. Hucke and M. P.
Cava, J. Org. Chem., 1998, 63, 7413; (d) J. K. Politis, J. C. Nemes and
M. D. Curtis, J. Am. Chem. Soc., 2001, 123, 2537.
12 G. Dufresne, J. Bouchard, M. Belletete, G. Durocher and M. Leclerc,
Macromolecules, 2000, 33, 8252; C. Adachi, T. Tsutsui and S. Saito,
Appl. Phys. Lett., 1990, 56, 799.
single layer device constituted of ITO/PEDT+PSS/2a (120 nm)/
Mg+Ag (800 nm)/Ag (150 nm) (device 1), exhibited a blue
emission at 476 nm.16 Double layered devices with the
configuration ITO/PEDT+PSS/HTL (40 nm)/Alq3 (60 nm)/
Mg+Ag (800 nm)/Ag (150 nm) were fabricated. The HT
materials used here were 2a–c (devices 2–4) and 1 (device 5).
The brightness–voltage curves are compared in Fig. 1. All of the
devices exhibited typical emission of Alq3 at ca. 535 nm having
similar turn-on voltage and external quantum efficiency.
When LiF/Al was used as the cathode in the devices with the
configuration ITO/PEDT+PSS/HTL (40 nm)/Alq3 (60 nm)/LiF
(0.5 nm)/Al (150 nm), where HTL was either 2a (device 6) or 1
(device 7), the turn-on voltages were lowered to 2.0 V. The
external quantum efficiency of device 6 was 1.5% and the
maximum brightness for both devices 6 and 7 was increased to
3 3 104 cd m22
.
When N,N-dimethylquinacridone (DMQA 3) was employed
as the dopant,17 multilayer devices ITO/PEDT+PSS/HTL (40
nm)/Alq3+3 (20 nm)/Alq3 (40 nm)/LiF (0.5 nm)/Al (150 nm),
where HTL was either 2a (device 8) or 1 (device 9), were
fabricated. The optimal concentration of 3 in Alq3 was found to
be 0.5%, providing an external quantum efficiency of 2.8% and
a maximum luminance of 182 800 cd m22 at 18 V for device 8.
The external quantum efficiency and the maximal luminance for
device 9 were 2.7% and 180 200 cd m22 at 19.5 V,
respectively.
In summary, we have depicted, for the first time, a new class
of highly stable furan-based hole transporting oligomeric
materials for electroluminescent devices. The performance of
the devices by using these furan materials appeared to be
13 S. Yamaguchi and K. Tamao, J. Chem. Soc., Dalton Trans., 1998,
3693.
14 C.-F. Lee, L.-M. Yang, T.-Y. Hwu, A.-S. Feng, J.-C. Tseng and T.-Y.
Luh, J. Am. Chem. Soc., 2000, 122, 4992.
15 J. A. E. H. van Haare, L. Groenendaal, H. W. I. Peerlings, E. E. Havinga,
J. A. J. M. Vekemans, R. A. J. Janssen and E. W. Meijer, Chem. Mater.,
1995, 7, 1984.
16 For the use of poly(ethylenedioxy)thiophene+polystyrene sulfonate
(PEDT+PSS) for efficient hole injection in OLEDs, see: A. Elschner, F.
Bruder, H.-W. Heuer, F. Jonas, A. Karbach, S. Kirchmeyer, S. Thurm
and R. Wehrmann, Synth. Met., 2000, 111–112, 139. For a recent review
on the use of PEDT+PSS, see: L. Groenendaal, F. Jonas, D. Freitag, H.
Pielartzik and J. R. Reynolds, Adv. Mater., 2000, 12, 481.
17 J. Shi and C. W. Tang, Appl. Phys. Lett., 1997, 70, 1665.
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