Suzuki coupling approach for the synthesis of Phenylene - Pyrimidine alternating oligomers for blue light-emitting material

Article abstract of DOI:10.1021/ol017066z

Conjugated oligomers with an alternating phenylene-pyrimidine structure have been synthesized by the successive Suzuki coupling reaction starting from 2-bromo-5-iodopyrimidine. The photoluminescence properties and quasi-reversible redox behavior of these

Full text of DOI:10.1021/ol017066z

ORGANIC
LETTERS
2002
Vol. 4, No. 4
513-516
Suzuki Coupling Approach for the
Synthesis of Phenylene−Pyrimidine
Alternating Oligomers for Blue
Light-Emitting Material
,†
,‡
Ken-Tsung Wong,* Tsung Shi Hung,† Yuting Lin,‡ Chung-Chih Wu,*
Gene-Hsiang Lee,† Shie-Ming Peng,† Chung Hsien Chou,† and Yuhlong Oliver Su†
Department of Chemistry and Graduate Institute of Electro-Optical Engineering,
National Taiwan UniVersity, Taipei 106, Taiwan
kenwong@ccms.ntu.edu.tw
Received November 19, 2001
ABSTRACT
Conjugated oligomers with an alternating phenylene−pyrimidine structure have been synthesized by the successive Suzuki coupling reaction
starting from 2-bromo-5-iodopyrimidine. The photoluminescence properties and quasi-reversible redox behavior of these oligomers make
them applicable as an active material for a light-emitting device. Blue light-emitting electroluminescent devices with an external quantum
efficiency up to 1.8% have been fabricated.
Since the pioneering work1 of Tang and VanSlyke in the
thin film OLED device, conjugated organic materials with
high morphological stability have received considerable
attention due to their promising potential for the industrial
manufacture of light-emitting displays.2 However, most of
the organic solids used for OLED exhibit several orders
higher of hole mobility than electron mobility. Therefore,
the poor quantum efficiency of the OLED device can be
primarily ascribed to the imbalance of electron-hole re-
combination in the active layer. To create an efficient device,
new materials with high electron transporting capability are
demanded. Over the past decade, many activities have been
carried out on this issue, resulting in great contributions to
the molecular design for fulfilling the requirements of high
electron affinity, high electron mobility, and morphology
stability. The electronic properties of organic solids now can
be finely tailored by modifying the main chain structure with
highly electronegative groups3 or introducing the heteroaryl
moiety as a functional subunit. For example, 1,3,4-oxadia-
zole-containing molecules4 have been extensively investi-
gated as an electron transporting material in OLED. More-
over, benzimidazole,5 phenanthroline,6 pyridine,7 pyrazine,8
quinoline,9quinoxaline,10 pyrazolopyridine,11 silole,12 thiophene-
(3) (a) Moratti, S. C.; Cervini, R.; Holmes, A. B.; Baigent, D. R.; Friend,
R. H.; Greenham, N. C.; Gru¨ner, J.; Hamer, P. J. Synth. Met. 1995, 71,
2117. (b) Go´mez, R.; Segura, J. L.; Mart´ın, N. Chem. Commun. 1999, 619.
(c) Heidenhain, S. B.; Sakamoto, Y.; Suzuki, T.; Miura, A.; Fujikawa, H.;
Mori, T.; Tokito, S.; Taga, Y. J. Am. Chem. Soc. 2000, 122, 10240. (e)
Uckert, F.; Tak, Y.-H.; Mu¨llen, K.; Ba¨ssler, H. AdV. Mater. 2000, 12, 905.
(4) (a) Huang, W.; Meng, H.; Yu, W.-L.; Gao, J.; Heeger, A. J. AdV.
Mater. 1998, 10, 593. (b) Chung, S.-J.; Kwon, K.-Y.; Lee, S.-W.; Jin, J.-I.;
Lee, C. H.; Lee, C. E.; Park, Y. AdV. Mater. 1998, 10, 1112. (c) Peng, Z.;
Zhang, J. AdV. Mater. 1999, 11, 1138.
† Department of Chemistry.
‡ Graduate Institute of Electro-Optical Engineering.
(1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
(2) (a) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int.
Ed. 1998, 37, 402. (b) Shirota, Y. J. Mater. Chem. 2001, 10, 1.
(5) Tao, Y. T.; Balasubramaniam, E.; Danel, A.; Tomasik, P. Appl. Phys.
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10.1021/ol017066z CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/31/2002

1-oxide,13 thiophene-1,1-dioxide,13,14 and triazole15 have been
incorporated into the π-conjugated systems for improving
the electron mobility. Among these heteroaromatics, a
pyrimidine-containing molecule is one of the least exploited
materials for OLED. Polypyrimidine has been shown to
exhibit high electron affinity,16 which makes it an attractive
candidate for incorporation into a π-conjugated system. An
elegant step-by-step synthetic strategy for the synthesis of
pyrimidine-containing oligophenylenes has been reported by
Gommper et al.17 We report in this Letter a new approach
for the synthesis of phenylene-pyrimidine alternating oli-
gomers by the Suzuki coupling reaction. The resultant
oligomers could serve as an active electron transporting
material in the OLED device.
bromo-substituted carbon of 2 with diboronic acid was
promoted by adding a bulky trialkyphosphine, PtBu3, as a
cocatalyst. The corresponding oligomers 3a-c were isolated
in good yields and characterized with satisfactory spectral
analyses.19
To investigate the electronic properties of the phenylene-
pyrimidine alternating oligomers, 3a-c, we first examined
the absorption spectra for each oligomer in dilute solution
(CHCl3). The vacuum-deposited thin films of 3a-c were also
inspected for comparison. Two absorption bands were
observed, which were red-shifted when compared to those
of the analogous pentaphenylene.20 The resultant bathochro-
mic shift is presumably due to the intramolecular charge
transfer21 in our oligomers with a donor-acceptor alternating
arrangement. In the solid film, the relative intensity of the
long wavelength absorption is increased and distinctly red-
shifted while the short wavelength absorption stays almost
unchanged. The red-shifted absorption could result from the
molecules with a more coplanar conformation in the solid
film. The terminal substituents of oligomers 3a-c have no
substantial influences on the absorption wavelength; the
oligomers terminated with electron-donating butoxy groups
(3b) only exhibit a slightly bathochromic shift (ca. 17 nm)
in short wavelength absorption when compared to that of
3a. The λmax for oligomers 3a-c are summarized in Table
1.
A two-step synthesis of phenylene-pyrimidine alternating
oligomers by a Pd-catalyzed cross-coupling reaction starting
from 2-bromo-5-iodopyrimidine 1 is outlined in Scheme 1.
Scheme 1
Oligomers 3a-c exhibit strong blue fluorescence in dilute
solution with emission maximum centered at 419 nm
irrespective of the nature of the terminal substituents. The
quantum yields are 0.37, 0.54, and 0.37 for 3a, 3b, and 3c,
respectively. It is worth noting that, in the solid film, 3a
and 3b exhibit PL spectra without a distinct red shift when
compared to those in solution. The identity of the emission
behavior suggests that the excited state of 3a and 3b could
have a similar conjugation length in dilute solution and in
the solid film. A comparison of the photoluminescence
spectrum of 3c in solution and in the solid film is shown in
Figure 1. For 3c, the most intensive emission peaks in the
solid film are almost overlapped with the emission peak in
solution. In addition, three weak emission peaks at 461, 495,
and 545 nm were also detected. The long wavelength
emission peaks could be possibly attributed to the intermo-
The selective coupling reaction of 118 on the iodo-substituted
carbon with aryl boronic acids was carried out under standard
aqueous Suzuki coupling conditions to afford 2a-c in
excellent yields. However, a second Suzuki coupling on the
(6) Naka, S.; Okada, H.; Onnagawa, H.; Tsutsui, T. Appl. Phys. Lett.
2000, 76, 197.
(7) (a) Yamamoto, T.; Maruyama, T.; Zhou, Z.-H.; Ito, T.; Fukuda, A.;
Kubota, K. J. Am. Chem. Soc. 1994, 116, 4832. (b) Wang, Y. Z.; Epstein,
A. J. Acc. Chem. Res. 1999, 32, 217.
(8) Peng, Z.; Galvin, M. E. Chem. Mater. 1998, 10, 1785.
(9) Saito, N.; Kanbara, T.; Nakamura, Y.; Yamamoto, T.; Kubota, K.
Macromolecules 1994, 27, 657.
(10) (a) Yamamoto, T.; Sugiyama, K.; Kushida, T.; Inoue, T.; Kanbara,
T. J. Am. Chem. Soc. 1996, 118, 3930. (b) Redecker, M.; Bradley, D. D.
C.; Jandke, M.; Strohriegl, P. Appl. Phys. Lett. 1999, 75, 109.
(11) Tao, Y. T.; Balasubramaniam, E.; Danel, A.; Tomasik, P. Appl. Phys.
Lett. 2000, 77, 933.
(12) (a) Tamao, K.; Uchida, M.; Izumizawa, T.; Furuukawa, K.;
Yamaguchi, S. J. Am. Chem. Soc. 1996, 118, 11974. (b) Yamaguchi, S.;
Endo, T.; Uchida, M.; Izumizawa, T.; Furukawa, K.; Tamao, K. Chem.
Eur. J. 2000, 6, 1683.
(13) Suh, M. C.; Jiang, B.; Tilley, T. D. Angew. Chem., Int. Ed. 2000,
39, 2870.
(18) (a) Goodby, J. W.; Hird, M.; Lewis, R. A.; Toyne, K. J. Chem.
Commun. 1996, 2179. (b) During the preparation of this manuscript, the
Suzuki coupling reaction of halogenated pyrimidne was reported: Scho-
maker, J. M.; Delia, T. J. J. Org. Chem. 2001, 66, 7125.
(19) Representative experimental procedure: Bisboronic acids (pre-
pared by the literature method: Hu, Q.-S.; Huang, W.-S.; Vitharana, D.;
Zheng, X.-F.; Pu, L. J. Am. Chem. Soc. 1997, 119, 12454) (422 mg, 1
mmol), 2b (615 mg, 2.0 mmol), Na2CO3 (2.0 mL, 2 M in H2O, 4.0 mmol),
Pd(PPh3)4 (23 mg, 1 mol %), and PtBu3 (0.2 mL, 0.05 M in toluene, 5 mol
%) in argon-saturated toluene (10 mL). The mixture was refluxed overnight
under argon. The mixture was allowed to cool to room temperature and
diluted with chloroform (10 mL). The organic solution was dried over
MgSO4. The concentrated crude product was recrystallized from CHCl3
with MeOH to afford 3b as a light yellow solid (550 mg, 70%) (for
characterization data, see the Supporting Information).
(14) Barbarella, G.; Favaretto, L.; Sotgiu, G.; Zambianchi, M.; Bongini,
A.; Arbizzani, C.; Mastragositino, M.; Anni, M.; Gigli, G.; Cingolani, R.
J. Am. Chem. Soc. 2000, 122, 11971.
(20) Zhao, M.; Cui, Y.; Samoc, M.; Prasad, P. N.; Unroe, M. R.;
Reinhardt, B. A. J. Chem. Phys. 1991, 95, 3991.
(15) Burn, P. L.; Grice, A. W.; Tajbakhsh, A.; Bradley, D. D. C.; Thomas,
A. C. AdV. Mater. 1997, 9, 1171.
(21) (a) Yamamoto, T.; Zhou, Z.-H, Kanbara, T.; Shimura, M.; Kizu,
K. Maruyama, T.; Nakamura, Y.; Fukuda, T.; Lee, B.-L.; Ooba, N.; Tomaru,
S.; Kurihara, T.; Kaino, T.; Kubota, K.; Sasaki, S. J. Am. Chem. Soc. 1996,
118, 10389. (b) Zhang, Q. T.; Tours, J. M. J. Am. Chem. Soc. 1998, 120,
5355.
(16) Kanbara, T.; Kushida, T.; Saito, N.; Kuwajima, I.; Kubota, K.;
Yamamoto, T. Chem. Lett. 1992, 583.
(17) Gommper, R.; Mair, H.-J.; Polborn, K. Synthesis 1997, 696.
514
Org. Lett., Vol. 4, No. 4, 2002

Table 1. Physical Properties of Phenylene-Pyrimidine Alternating Oligomers 3a-c
1
1
c
E2pc, E2pa
∆E (mV)d
Td (°C)f
a
b
a
b
compound
λmax
λmax
emax
emax
Epc, Epa
3a
3b
3c
290, 352
307, 352
296, 352
292, 362
307, 392
299, 370
419
419, 430
419
423
417, 434
410, 428
-2.00, -1.73
-2.04, -1.77
-2.08, -1.83
-2.18, -1.91
-2.21, -1.95
-2.28, -2.04
180
170
200
444
457
441
a In nm, 5.0 × 10-5 M in CHCl3. b In nm, vacuum deposited thin film. c V vs Ag/AgCl in THF with n-Bu4NClO4 as a supporting electrolyte. d ∆E ) E1pc
- E2pc. e Detected by the DSC analysis of liquid N2 quenched melt sample, heating rate ) 10 °C/min. f Detected by TGA analysis.
lecular interactions occurring in the solid state or the
formation of excimer.
phenyl ring and the central phenyl ring may also contribute
to the crystallization. Due to the large terminal groups in
3c, the weak intermolecular interactions are characterized
by inspecting the shortest intermolecular contact distance,
3.66 Å, between the nitrogen in one molecule and the C4
carbon in the pyrimidine of neighboring molecule.
Figure 2a provides views of the crystal packing diagram
of 3a. The terminal phenyl ring and pyrimidine ring are in
a coplanar plane (with angles between the least-squares
planes of <3.3°) due to the lack of ortho-ortho interactions,
while the central phenyl ring is rotated by 42.5° with respect
to the pyrimidine ring. The 3a crystals stack each other in
such a way that the coplanar phenyl-pyrimidine segments
are closely lined up, but in opposite direction, with the same
segment of the neighboring molecule. Although these
coplanar segments could overlap to a large extent, the closest
interatomic (least-squares interplane) distance between the
terminal phenyl ring in one molecule and the pyrimidine ring
in another molecule is calculated to be 3.49 Å (3.55 Å),
which is expected to have intermolecular interactions. In
contrast to 3a, 3c crystallized in a different manner. The
crystal packing diagram for 3c is shown in Figure 2b. The
five aryl rings of 3c are not coplanar to one another. The
tBu-substituted phenyl ring is twisted by -19.2° to the
adjacent pyrimidne ring, while the central phenyl ring is
rotated by 25.2° with respect to the pyrimidine ring. 3c stacks
in such a way that the pyrimidine rings from different
molecules are closely packed to one another as the dipolar
orientation is in opposite direction. The crystal packing is
driven by this dipole-dipole interaction between the pyri-
midine rings. However, the packing between the terminal
The electrochemical properties of oligomers 3a-c were
investigated by cyclic voltammetry in THF. The data are
summarized in Table 1. Two quasi-reversible cathodic
reduction couples were observed with the potential difference
(∆E ) E1 - E2pc) 180 mV, 170 mV, and 200 mV for 3a,
pc
3b, and 3c, respectively. These two reductions were believed
to occur at the pyrimidine rings successively. The large
potential difference between the two reductions indicates that
the first radical anion, as it was formed, could efficiently
delocalize along the π-conjugation in the molecule, resulting
in the difficult formation of the dianion species. Figure 3
shows a comparison of the cyclic voltammogram of oligo-
mers 3a-c. The electron-donating butoxy group in 3b
slightly increased the first reduction potential by 0.08 V when
compared to that of 3a. 3c, with the less electron-donating
tBu groups, exhibits a reduction potential between that of
3a and 3b. These results indicate that the pyrimidine moiety
Figure 1. A comparison of the absorption and emission spectra
of 3c in solution (5.5 × 10-5 M in CHCl3) and thin film.
Figure 2. The molecular packing diagram of 3a (a) and 3c (b).
The long octyl chains are omitted for clarity.
Org. Lett., Vol. 4, No. 4, 2002
515

We have studied the electroluminescent (EL) properties
of these pyrimidine-containing oligomers. The typical device
structure was ITO/PEDT-PSS (300 Å)/NCB (450 Å)/3b or
3c (500 Å)/Mg:Ag (10:1)/Ag. It was found that 3b and 3c
function well as the electron-transport layer in the device. It
is consistent with the electrochemical results since it is found
that these pyrimidine derivatives in general have a strong
electron-accepting reduction tendency. Their electron affini-
ties estimated from electrochemical results are about 3.0 eV,
comparable or even slightly larger than that of Alq3, a widely
used electron-transport material. Among the compounds
studied (3b and 3c), the 3c device exhibits substantially lower
operation voltage than the 3b device (10 vs 15 V at 100
cd/m2); this may be due to the higher morphological stability
and lower reduction potential of 3c. Oligomers 3b and 3c
also function as efficient blue emitters in the devices,
exhibiting blue EL similar to their PL with external EL
quantum efficiencies of 1.3-1.8% (photon/electron) and
brightness over 2000 cd/m2. These results are rather encour-
aging since these devices are nondoped; optimization of
device structures and material properties may further enhance
the blue-emitting device performance.
Figure 3. Cyclic voltammogram of 3a-c in THF (1.0 × 10-3
M).
could serve as a electron-accepting center once it was
incorporated into a π-conjugated system. In addition, the
reduction potential could be finely tuned by introducing
substituents with different electronic nature at the terminal
position.
In conclusion, a series of phenylene-pyrimidine alternat-
ing oligomers with interesting photophysical and electronic
properties have been successfully synthesized using the
Suzuki coupling reaction. Further modification of the central
linkage by introducing a rigid skeleton for creating pyrimi-
dine-containing glassy materials is currently under investiga-
tion and will be reported in due course.
The thermal property and the morphological stability of
3a-c were investigated by the TGA and DSC analysis,
respectively. 3a-c exhibited high thermal stability; the TG
thermograms revealed that no weight loss was detected below
380 °C for 3a-c. The temperature corresponding to the
completed weight loss of 3a-c is summarized in Table 1.
The DSC analysis of the liquid nitrogen quenched melt-
samples of 3a-c showed that only 3a and 3c exhibited a
relatively low glass transition temperature (Tg) at 27.7 and
31.5 °C, respectively. DSC analysis also indicated that there
were several phase transitions of 3a-c before the crystalline
sample completely melted. Oligomers 3a and 3b melt at
150.5 and 160.2 °C, respectively, while 3c with bulky
terminal groups showed a higher melting temperature at
194.7 °C. The low Tg and low melting temperature are
presumably due to the introduction of two octyloxyl side
chains at the central phenylene ring as a solubilizing group.
Acknowledgment. We are grateful to the National
Science Council and the Ministry of Education of Taiwan
for financial support.
Supporting Information Available: 1H and 13C NMR
spectra of 3a, 3b, and 3c and detailed experimental proce-
dures, full characterization, X-ray crystallographic data (CIF)
for 3a and 3c. UV-vis and PL spectra for 3a and 3b. OLED
device characteristics of 3b and 3c. This material is available
free of charge via Internet at http://pubs.acs.org.
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