1,3,6,8-tetrasubstituted pyrenes: Solution-processable materials for application in organic electronics

Article abstract of DOI:10.1021/ol1007179

(Equation Presented). A series of star-shaped organic semiconductors have been synthesized from 1,3,6,8-tetrabromopyrene. The materials are soluble in common organic solvents allowing for solution processing of devices such as light-emitting diodes (OLEDs). One of the materials, 1,3,6,8-tetrakis(4- butoxyphenyl)pyrene, has been used as the active emitting layer in simple solution-processed OLEDs with deep blue emission (CIE = 0.15, 0.18) and maximum efficiencies and brightness levels of 2.56 cd/A and >5000 cd/m², respectively.

Full text of DOI:10.1021/ol1007179

ORGANIC
LETTERS
2010
Vol. 12, No. 15
3292-3295
1,3,6,8-Tetrasubstituted Pyrenes:
Solution-Processable Materials for
Application in Organic Electronics
Prashant Sonar,*,† Mui Siang Soh,† Yuen Hsia Cheng,† John T. Henssler,‡ and
Alan Sellinger*,†,§
Institute of Materials Research and Engineering (IMRE), Agency for Science,
Technology and Research (ASTAR), 3 Research Link, Republic of Singapore 117602,
and Department of Chemistry and Macromolecular Science and Engineering Center,
UniVersity of Michigan, 930 North UniVersity, Ann Arbor, Michigan 48109-1055
sonarp@imre.a-star.edu.sg; aselli@stanford.edu
Received April 1, 2010
ABSTRACT
A series of star-shaped organic semiconductors have been synthesized from 1,3,6,8-tetrabromopyrene. The materials are soluble in common
organic solvents allowing for solution processing of devices such as light-emitting diodes (OLEDs). One of the materials, 1,3,6,8-tetrakis(4-
butoxyphenyl)pyrene, has been used as the active emitting layer in simple solution-processed OLEDs with deep blue emission (CIE ) 0.15,
0.18) and maximum efficiencies and brightness levels of 2.56 cd/A and >5000 cd/m2, respectively.
The quest for new and improved organic semiconductors
continues with emphasis on optimizing key properties such
as luminescence, absorption, energy band gaps, charge
transport, and stability.1,2 With regard to device fabrication,
organic semiconductors can be classified into two general
families: those that can be processed into devices by vacuum
deposition (small molecules) or by solution processing
(polymers). Small molecules are advantageous because they
can be (1) purified by common techniques such as recrys-
tallization, chromatography, and sublimation and (2) vacuum
deposited in multilayer stacks, both important for device
lifetime and efficiency.3 However, vacuum-deposition tech-
niques require costly processes that are limited to practical
substrate size and relatively low yields in the manufacture
of high volume products using masking technologies.4 On
the other hand, polymers are generally of lower purity than
small molecules but can be used to achieve larger display
† Institute of Materials Research and Engineering (IMRE).
‡ University of Michigan.
§ Current address: Department of Materials Science and Engineering and
the Center for Advanced Molecular Photovoltaics (CAMP), Geballe
Laboratory for Advanced Materials, 476 Lomita Mall, Stanford University,
Stanford, CA 94305-4045.
(1) (a) Shirota, Y.; Kageyama, H. Chem. ReV. 2007, 107, 953. (b) Lloyd,
M. T.; Anthony, J. E.; Malliaras, G. G. Mater. Today 2007, 10, 34. (c)
Garnier, F. Acc. Chem. Res. 1999, 32, 209. (d) Dimitrakopoulos, C. D.;
Malenfant, P. R. L. AdV. Mater. 2002, 14, 99
(2) (a) Shirota, Y. J. Mater. Chem. 2000, 10, 1. (b) Mitschke, U.; Ba¨
uerle, P. J. Mater. Chem 2000, 10, 1471
.
(3) Anthony, J. E.; Heeney, M.; Ong, B. S. MRS Bull. 2008, 33, 698.
(4) Shtein, M.; Peumans, P.; Benziger, J. B.; Forrest, S. R. AdV. Mater.
2004, 16, 1615.
.
10.1021/ol1007179  2010 American Chemical Society
Published on Web 06/29/2010

Scheme 1. Synthesis of Compounds 1-4
sizes at much lower costs using technologies such as ink
conjugated organic chromophores. In this approach, the
active components are electronically connected through
the conjugated planar pyrene core. Tetrafunctional pyrene-
based materials, with their extended delocalized π-electrons,
jet and screen printing.5 Our interest lies in materials that
combine the advantages of both small molecules and
polymers, specifically monodisperse star-shaped materials
with high purity and solution processability as will be
explained in this paper. Previous reports from our group
describe the use of multifunctionalized insulating inor-
ganic cores, such as octavinylsilsesquioxane and cyclic
triphosphazenes, as platforms from which organic chro-
mophores were attached for application in solution pro-
cessable OLEDs.6 In this method, the conjugated com-
ponent is electronically isolated, with the core serving as
an inert rigid platform to impart amorphous behavior and
enhanced thermal properties.
(7) (a) Sagara, Y.; Mutai, T.; Yoshikawa, I.; Araki, K. J. Am. Chem.
Soc. 2007, 129, 1520. (b) Lackowicz, J. R. In Principles of Fluorescence
Spectroscopy, 2nd ed.; Kluwer Academic/Plenum Publishers: New York,
1999; pp 595-614. (c) Birks, J. B. In Photophysics of Aromatic Molecules;
Wiley-Interscience: London, 1970.
(8) (a) Moorthy, J. N.; Natarajan, P.; Venkatkrishnan, P.; Huang, D. F.;
Chow, T. J. Org. Lett. 2007, 9, 5215. (b) Mikroyannidis, J. A. Synth. Met.
2005, 155, 125. (c) Venkataramana, G.; Sankararaman, S. Org. Lett. 2006,
8, 2739. (d) Zhang, H. J.; Wang, Y.; Shao, K. Z.; Liu, Y. Q.; Chen, S. Y.;
Qiu, W. F.; Sun, X. B.; Qi, T.; Ma, Y. Q.; Yu, G.; Su, Z. M.; Zhu, D. B.
Chem. Commun. 2006, 7, 755. (e) Sienkowska, M. J.; Farrar, J. M.; Zhang,
F.; Kusuma, S.; Heiney, P. A.; Kaszynski, P. J. Mater. Chem. 2007, 17,
1399. (f) Liu, F.; Lai, W. Y.; Tang, C.; Wu, H. B.; Chen, Q. Q.; Peng, B.;
Wei, W.; Huang, W.; Cao, Y. Macromol. Rapid Commun. 2008, 29, 659.
(g) Gingras, M.; Placide, V.; Raimundo, J. M.; Bergamini, G.; Ceroni, P.;
Balzani, V. Chem.sEur. J. 2008, 14, 10357. (h) Xia, R. D.; Lai, W. Y.;
Levermore, P. A.; Huang, W.; Bradley, D. D. C. AdV. Funct. Mater. 2009,
19, 2844. (i) Zhao, Z. J.; Li, J. H.; Chen, X. P.; Wang, X. M.; Lu, P.;
Yang, Y. J. Org. Chem. 2009, 74, 383. (j) Halleux, V. de.; Calbert, J. P.;
Brocorens, P.; Cornil, J.; Declercq, J. P.; Bredas, J. L.; Geerts, Y. AdV.
Funct. Mater. 2004, 14, 649. (k) Faulkner, L. R.; Bard, A. J.; Tachikaw,
H. J. Am. Chem. Soc. 1972, 94, 691. (l) Tang, C.; Liu, F.; Xia, Y. J.; Xie,
L. H.; Wei, A.; Li, S. B.; Fan, Q. L.; Huang, W. Org. Electron. 2006, 7,
155. (m) Hayer, A.; Halleux, V. de.; Kohler, A.; El-Garoughy, A.; Meijer,
E. W.; Barbera, J.; Tant, J.; Levin, J.; Lehmann, M.; Gierschner, J.; Cornil,
J.; Geerts, Y. H. J. Phys. Chem. B 2006, 110, 7653. (n) Oh, H. Y.; Lee, C.;
Lee, S. Org. Electron. 2009, 10, 163. (o) Kim, H. M.; Lee, Y. O.; Lim, S.;
Kim, J. S.; Cho, B. R. J. Org. Chem. 2008, 73, 5127.
Here, we report the straightforward synthesis, characteriza-
tion, photophysical, and thermal properties of multifunc-
tionalized solution processable materials using a conjugated
core, 1,3,6,8-tetrabromopyrene, from which to attach the
(5) (a) Krebs, F. C. Sol. Energy Mater. Sol. Cells 2009, 93, 394. (b)
Loo, Y. L.; McCulloch, I. MRS Bull. 2008, 33, 653. (c) Brabec, C. J.;
Durrant, J. R. MRS Bull. 2008, 33, 670. (d) Sirringhaus, H.; Ando, M. MRS
Bull. 2008, 33, 676.
(6) (a) Lo, M. Y.; Sellinger, A. Synlett 2006, 18, 3009. (b) Lo, M. Y.;
Ueno, K.; Tanabe, H.; Sellinger, A. Chem. Rec. 2006, 6, 157. (c) Lo, M. Y.;
Zhen, C. G.; Lauters, M.; Jabbour, G. E.; Sellinger, A. J. Am. Chem. Soc.
2007, 129, 5808. (d) Sudhakar, S.; Sellinger, A. Macromol. Rapid Commun.
2006, 27, 247.
Org. Lett., Vol. 12, No. 15, 2010
3293

Table 1. Optical, Electrochemical, and Thermal Properties of Compounds 1-4
CIE coordinatesa e
,
UV-vis
PL
compound
λmax (nm)a λmax (nm)b λmax (nm)a λmax(nm)b bandgapc (eV) HOMOd
x
y
Tmf (°C) Tdg (°C)
Py-BtC6 (1)
Py-PhOC4 (2)
Py-TtC9 (3)
375, 451
306, 394
347, 429
307, 452
389, 470
290, 396
364, 442
380, 471
530
433
490
541
605
462
585
635
2.32
2.83
2.44
2.25
-5.15
-5.25
-5.30
-5.33
0.48
0.15
0.41
0.59
0.50
0.10
0.56
0.41
460
252
100
436
299
451
Py-BtzTC8 (4)
a Measured in chloroform solutions. b Measured from thin films on glass. c Measured from the absorption onset of the UV-vis spectrum. d Measured
from the oxidation onset of the cyclic voltammetry (CV). e Obtained from chromameter measurements upon excitation at 365 nm. f Obtained from DSC
measurement. g Obtained from TGA measurement (temperature at 5% weight loss under nitrogen, 10 °C/min ramp rate).
discotic shape, high photoluminescence efficiency, and good
hole-injection/transport properties, have the potential to be
a very interesting class of materials for opto-electronic
applications.7 Previously, there have been reports on the
synthesis of tetrafunctionalized pyrene materials,8 but very
few of these offer OLED device results,8a,f,i,n and none to
our knowledge report deep blue OLEDs from solution
processing. One very attractive aspect of these materials is
that they can be accessed from relatively simple synthetic
routes as shown in Scheme 1. We selected bithiophene,
phenylene, thienothiophene, and benzothiadiazole-thiophene
as the chromophores with the aim of preparing electron-rich
materials for application in OLEDs, organic photovoltaics
(OPV), and organic thin-film transistors (OTFTs). To achieve
solution processability, alkyl groups were incorporated on
the conjugated moieties. As illustrated in Scheme 1, 1 (Py-
BtC6) and 2 (Py-PhOC4) were synthesized and purified as
red and white powders by Suzuki-Miyaura cross-coupling
reaction of 1,3,6,8-tetrabromopyrene with commercially
available 2-(5-(5-hexylthiophen-2-yl)thiophene-2-yl)-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane and 4-butoxyphenylboronic
acid, respectively. Compounds 3 (Py-TtC9) and 4 (Py-
BtzTC8) were synthesized and purified as dark yellow and red
powders by coupling tributyl(6-nonylthieno[3,2-b]thiophene-
2-yl)stannane and 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)-7-(5-octylthiophene-2-yl)benzo[c][1,2,5]thiadiazole9 with
1,3,6,8-tetrabromopyrene using Stille and Suzuki-Miyaura
cross-coupling routes, respectively. All of the compounds
are readily soluble in common organic solvents such as
CHCl3, CH2Cl2, THF, and toluene, which allows for puri-
fication by column chromatography (using hexane/CH2Cl2
mixtures) and solution processing. 1H and 13C NMR spectra,
elemental analysis, and MALDI-TOF were employed to
confirm the structure and purity of all compounds (Support-
ing Information).
this study (1-4) demonstrated absorption λmax values that
are significantly red-shifted. In thin film absorption, com-
pounds 1, 3, and 4 show red shifts of 13-19 nm, whereas
compound 2 shows similar absorbance compared to their
respective dilute soultions. Solution PL spectra for 2 and 3
show deep blue and sky blue emission, respectively, at 433
and 490 nm, whereas 1 and 4 exhibit green and orange
emission at 530 and 541 nm, respectively. The bathochromic
shifts of the solution PL λmax range from 40 to 148 nm for
compounds 1-4 compared to the unsubstituted pyrene core
(PLmax ) 393). In thin film PL, all the compounds are red-
shifted 29-95 nm compared their respective dilute solutions
(Table 1). This is likely due to aggregation in the solid state
versus dilute solutions. CIE coordinates for dilute solutions
of 1-4 are also shown in Table 1, indicating the excellent
deep blue coordinates for 2 (0.15, 0.10).
The electrochemical properties of compounds 1-4 were
investigated by cyclic voltammetry (CV) in anhydrous
dichloromethane (Supporting Information). The HOMO
levels or ionization potentials (IP) were calculated by using
onset
the known equation IP ) Eox
+ 4.4.10 The results are
presented in Table 1. The calculated HOMO values are in
the range of -5.15 to -5.33 eV, indicating that the materials
are suitable for application in OLEDs. For example, these
energy levels match quite well with commonly used hole
injection/transport layers and anodes such as PEDOT:PSS
(-5.1 eV) and ITO (-4.9 eV). This fine-tuning of electro-
chemical properties demonstrates the significant effect of
attaching the different conjugated moieties to the central
pyrene core.
The thermal properties of the tetrafunctional pyrene deriva-
tives were analyzed by differential scanning calorimetry (DSC)
and thermogravimetric analysis (TGA) and are shown in Table
1. Only 2 and 3 show melting points (Tm ) 252 and 100 °C,
respectively) in DSC heating cycles up to 300 °C, whereas 1
and 4 do not show any thermal transitions in this range. The
thermal decomposition (Td) temperatures (5% weight loss in
nitrogen atmosphere) were observed above 400 °C for all
compounds except 3, which shows Td onset at 299 °C.
The strong PL emission, tunable energy levels, excellent
solubility, enhanced thermal properties, and good film-
forming properties make these materials promising candidates
for application in solution-processed devices. As a starting
point, 2 was selected as a potential deep blue emitting
material for application in OLEDs, as this is still a highly
The photophysical properties of the compounds were
measured by UV-vis absorption and photoluminescence
(PL) spectroscopy in chloroform and in thin films. All four
compounds exhibit prominent short and long wavelength
absorption peaks (see Table 1 and Supporting Information).
Compounds 1 and 4 show red-shifted wavelength absorption
maxima (λmax) at 451 and 452 nm in comparison to 2 and 3
(λmax ) 394 and 429 nm, respectively) due to their slightly
more extended conjugation lengths. When compared to
molecular pyrene (abs λmax ) 338 nm), all compounds in
3294
Org. Lett., Vol. 12, No. 15, 2010

sought after goal in the OLED community.11 The structure
of the OLED is as follows: indium tin oxide (ITO)/PEDOT:
PSS (50 nm)/ 2 (50 nm)/1,3,5-tris(phenyl-2-benzimidazolyl)-
benzene (TPBI) (20 nm)/Ca (20 nm)/Ag (100 nm) where
PEDOT/PSS and TPBI act as hole-injecting/transport and
electron-injecting/transporting layers, respectively. The OLED
device data is shown in Figure 1. The maximum brightness
efficiency numbers are quite promising for unoptimized small
molecule solution processed blue OLEDs.12 The turn-on
voltage for the device of around 3 V is quite low, suggesting
that the barrier for hole injection from PEDOT/PSS is low,
which is expected from the measured HOMO levels. The
nondoped OLED devices show efficient bright blue light
emission with CIE coordinates (0.15, 0.18) (EL spectrum
shown in Figure 1b).
In summary, we report here the possibilities of tetrafunc-
tionalized pyrene-based materials for application in solution-
processed organic electronic devices. The introduction of
different moieties (bithiophene, phenylene, thienothiophene,
and benzothiadiazole-thiophene) has a substantial influence
on the resultant opto-electronic and thermal properties.
OLEDs based on 1,3,6,8-tetrakis(4-butoxyphenyl)pyrene (2)
as the active emitter show efficiencies of 2.56 cd/A, deep
blue emission (CIE ) 0.15, 0.18), low turn-on voltages (3.0
V), and a maximum brightness of 5015 cd m-2 at 11 V.
These findings are quite encouraging for unoptimized OLEDs
and suggest that further improvements are likely. For
example, optimizing the layer thicknesses and doping the
emitters into suitable hosts may further improve device
efficiency and color coordinates.8a We are currently testing
these pyrene-based materials for further application in
OLEDs and organic field effect transistors (OFETs).
Acknowledgment. We acknowledge the Institute of
Materials Research and Engineering (IMRE) and the Agency
for Science, Technology and Research (A*STAR) under the
Visiting Investigator Program (VIP) for financial support.
We thank Dr. Samarendra P. Singh from IMRE for assistance
in helpful discussions.
Supporting Information Available: Experimental pro-
cedures, spectroscopic data, thermal analysis, and cyclic
voltammograms for 1-4. This material is available free of
charge via the Internet at http://pubs.acs.org.
OL1007179
Figure 1. (a) Brightness and efficiency characteristics and (b) EL
(9) (a) Zhang, X. N.; Kohler, M.; Matzger, A. J. Macromolecules 2004,
37, 6306. (b) Sonar, P.; Singh, S. P.; Lecle`re, P.; Surin, M.; Lazzaroni, R.;
Lin, T. T.; Dodabalapur, A.; Sellinger, A. J. Mater. Chem. 2009, 19, 3228.
(10) Agrawal, A. K.; Jenekhe, S. A. Chem. Mater. 1996, 8, 579.
(11) Turker, L.; Tapan, A.; Gumus, S. Polycyclic Aromat. Compd. 2009,
29, 139.
spectrum of OLED devices based on (2). OLED device configu-
ration: ITO/PEDOT (50 nm)/(2) (50 nm)/TPBI (20 nm)/Ca (20 nm)/
Ag (100 nm) (CIE ) 0.15, 0.18).
(12) (a) Zhang, M.; Xue, S.; Dong, W.; Wang, Q.; Fei, T.; Gu, C.; Ma,
Y. Chem. Commun. 2010, 46, 3923. (b) Wang, L.; Jiang, Y.; Luo, J.; Zhou,
Y.; Zhou, J.; Wang, J.; Pei, J.; Cao, Y. AdV. Mater. 2009, 21, 4854.
and luminescence efficiency are 5015 cd/m2 (at 11 V) and
2.56 cd/A (at 10 cd/m2), respectively (Figure 1a). These
Org. Lett., Vol. 12, No. 15, 2010
3295