Synthesis and characterization of fluorene based electron transporting materials for polymer light-emitting diodes

Article abstract of DOI:10.1080/15421406.2011.566029

We synthesized fluorene based alternating copolymers comprised of several electron withdrawing groups via Suzuki coupling polymerization. Physical properties were characterized using the UV-vis spectroscopy, photoluminescence (PL) spectroscopy. The absorption peaks of copolymers were red-shifted compared to that of corresponding fluorene homopolymer. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15421406.2011.566029

This article was downloaded by: [University of Tennessee, Knoxville]
On: 20 December 2014, At: 21:59
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Molecular Crystals and Liquid Crystals
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/gmcl20
Synthesis and Characterization of
Fluorene Based Electron Transporting
Materials for Polymer Light-Emitting
Diodes
a b
a b
a b
a b
Ho-Ik Ahn
, Chil-Sung Choi
, Jin-Woo Oh
& Nakjoong Kim
a
Department of Chemistry , Hanyang University , Seoul, South Korea
b
Department of Chemistry , Research Institute for Natural Sciences,
Hanyang University , Seoul, Korea
Published online: 16 May 2011.
To cite this article: Ho-Ik Ahn , Chil-Sung Choi , Jin-Woo Oh & Nakjoong Kim (2011) Synthesis and
Characterization of Fluorene Based Electron Transporting Materials for Polymer Light-Emitting Diodes,
Molecular Crystals and Liquid Crystals, 539:1, 28/[368]-32/[372], DOI: 10.1080/15421406.2011.566029
To link to this article: http://dx.doi.org/10.1080/15421406.2011.566029
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“
Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &
Conditions of access and use can be found at http://www.tandfonline.com/page/terms-
and-conditions

Mol. Cryst. Liq. Cryst., Vol. 539: pp. 28=[368]–32=[372], 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.566029
Synthesis and Characterization of Fluorene Based
Electron Transporting Materials for Polymer
Light-Emitting Diodes
1
HO-IK AHN, CHIL-SUNG CHOI,
,2
1,2
1
JIN-WOO OH, AND NAKJOONG KIM
,2
1,2
1
Department of Chemistry, Hanyang University, Seoul, South Korea
Department of Chemistry, Research Institute for Natural Sciences,
Hanyang University, Seoul, Korea
2
We synthesized fluorene based alternating copolymers comprised of several electron
withdrawing groups via Suzuki coupling polymerization. Physical properties were
characterized using the UV-vis spectroscopy, photoluminescence (PL) spectro-
scopy. The absorption peaks of copolymers were red-shifted compared to that of
corresponding fluorene homopolymer.
Keywords Electron withdrawing group; PLEDs; polyfluorene
1
. Introduction
During the past decade, polymer light-emitting diodes (PLEDs) have attracted
continuous attention because of their low costs, processability, and the possibility
of flexible fabrication [1]. To achieve high efficiency, low-work-function metals (such
as Ca or Ba) are widely used as a cathode to facilitate electron injection. However,
these metals are very sensitive to moisture and oxygen forming detrimental quench-
ing sites at interfacial area between the electronluminescence (EL) layer and the
cathode [2,3]. Recently, ammonium-functionalized cationic polyflourenes surfac-
tants and their precursors have been found to exhibit excellent electron-injection
(or transport) ability in PLEDs [4,5]. Their good solubility in polar solvents such
as alcohol renders them as ideal materials for electron transporting layer in solution
processed, multilayered PLEDs. In this work, we designed and synthesized several
fluorene based alcohol-soluble copolymers having various electron withdrawing
groups as an electron transporting layer. The polymers were synthesized by
palladium-catalyzed Suzuki coupling reaction and their electronic properties are
compared.
Address correspondence to Nakjoong Kim, Department of Chemistry, Hanyang Univer-
sity, 17 Haengdang-Dong, Seongdong-Gu, Seoul 133-791, South Korea. E-mail: kimnj@
hanyang.ac.kr
28=[368]

Synthesis and Characterization of Fluorene Based Electron
29=[369]
2
. Experimental
2
.1. Materials
Tetrakis(triphenylphosphine)palladium(0) was purchased from STREM Chemicals
Co. All other chemical reagents were purchased from Aldrich Co. and used without
0
further
purification.
The
2,2 -(9,9-bis(6-bromohexyl)-9H-fluorene-2,7-diyl)-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) was synthesized according to proce-
dures in published paper [6].
2
.2. General Procedure for the Conjugated Polymer Synthsis
0
2
,2 -(9,9-bis(6-bromohexyl)-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolane) (1 g, 1.35 mol), the dibromo monomers (R1, R2, R3) (1.35 mmol), tetra-
kis(triphenylphospine)palladium (0.0202 mmol) were placed in a 100 mL round
bottom flask. A mixture of dried THF (10 mL) and aqueous 2 M potassium carbon-
ate solution (10 mL) were added to the flask. The mixture was violently stirred at
ꢀ
85 C for 36 h and then precipitated into methanol. The polymers were dried under
vacuum for 24 h, dissolved in 7 mL THF=DMF (1:1) mixture and then treated with
ꢀ
excess diethanolamine while stirring at 85 C for 36 h. The mixture was extracted with
chloroform, concentrated, and then precipitated from methanol. Finally, we
obtained three types of polymers, referred to as P1, P2 and P3. (yields: 60%, 50%
1
and 62%, respectively). P1: H NMR (400 MHz, CDCl , d): d 7.90(m, 2H),
3
7
0
2
.47(br, 6H), 3.30(br, 4H), 3.08(br, 4H), 2.03(br, 4H), 1.62(m, 3H), 1.11(br, 10H),
1
.77(br, 4H) P2: H NMR (400 MHz, CDCl , d): d 7.80(br, 6H), 3.30(br, 4H),
3
1
.19(br, 6H), 1.68(br, 4H), 1.25(br, 6H), 0.84(br, 6H) P3: H NMR (400 MHz,
CDCl , d): d 7.96(br, 6H), 3.28(br, 4H), 2.18(br, 6H), 1.69(br, 4H), 1.24(br, 4H),
3
1
.15(br, 4H), 0.92(br, 3H), 0.84(br, 2H).
2
.3. Measurements
1
H NMR spectra were recorded on a Varian Mercury 400 NMR spectrometer using
CDCl as a solvent and tetramethylsilane as a standard. UV-vis spectra were mea-
3
sured by an Optizen 2120UV spectrometer and photoluminescence (PL) spectra were
measured by using ISS PC1 photon-counting spectrofluorometer. The electrochemi-
cal experiments were carried out using the BAS-100D photoelectron spectrometer.
3
. Results and Discussion
The synthetic route is shown in Scheme 1. The physical data of polymers are sum-
marized in Table 1. UV-vis absorption spectra of synthesized polymers are shown
in Figure 1. P1 and P2 exhibit absorption maxima at 321 nm and 383 nm, respect-
ively. Two absorption peaks at 309 nm and 425 nm are observed in the absorption
spectra of P3. Optical band gaps estimated from the absorption edges of each poly-
mer are in order of P1 (3.42 eV), P2 (2.78 eV), P3 (2.51 eV) [7]. Figure 2 shows the
photoluminescence spectra of P1, P2 and P3. Three emission peaks of P1, P2 and
P3 are observed at 393 nm, 444 nm and 538 nm, respectively. We were determined
HOMO energy levels of P1 (ꢁ5.06 eV), P2 (ꢁ5.36) and P3 (ꢁ5.37) using a BAS-100D
photoelectron spectrometer; we estimated the LUMO energy levels P1 (ꢁ2.63 eV),
P2 (ꢁ2.72 eV) and P3 (ꢁ1.46 eV) by adding the optical energy gap to the obtained

3
0=[370]
H.-I. Ahn et al.
ꢀ
Scheme 1. Synthetic route of P1, P2 and P3. (a) NaBr, H SO , EDC, 80 C, 5 h, (b) 1,6-
2
4
ꢀ
dibromohexane, TBAB, 50% KOH solution, 80 C, 1 h, (c) 1.7 M t-BuLi, 2-isopropoxy-4,4,5,5-
ꢀ
ꢀ
tetramethyl-[1.3.2]-dioxaborolane ꢁ78 C, 3 h, (d) Pd(PPh
3 4 2 3
) , 2 M K CO , THF 85 C, 36 h,
ꢀ
(
e) HN(CH
2
CH
2
OH)
2
, THF=DMF 1:1, 85 C, 36 h.
Table 1. Summary of the optical properties of the polymers
Cyclic Volatammetry Absorption spectroscopic data
c
b
kPL
(
kmax
nm) (nm) potential (V)
Oxidation
a
HOMO
(eV)
LUMO
(eV)
Optical
energy gap (eV)
P1
P2
P3
393
444
538
321
383
309
0.66
0.96
0.97
ꢁ5.06
ꢁ5.36
ꢁ5.37
ꢁ2.63
ꢁ2.72
ꢁ1.46
3.42
2.78
2.51
a
b
c
Determined by photoelectron spectrometer (BAS-100D).
Determined by UV-vis absorption spectroscopy using 2120UV spectrometer.
Determined by photoemission spectroscopy using ISS PC1 photon-counting spectrofluo-
rometer.
Figure 1. The absorption spectra of P1, P2 and P3 in THF.

Synthesis and Characterization of Fluorene Based Electron
31=[371]
Figure 2. The emission spectra of P1, P2 and P3 in THF.
Figure 3. Cyclic voltammograms of the anodic oxidation of P1, P2 and P3.
ꢁ
3
HOMO energy levels. The cyclic voltammograms of ꢂ10 M CH CN solutions of
3
ꢁ
1
polymers are shown in Figure 3. Cyclic voltammetry (CV) at a scan rate of 100 mVs
was applied to study the oxidation behavior of polymers. The absorption peaks and
emission peaks of copolymers were red-shifted compared to that of corresponding
fluorene homopolymer. The band gaps of polymers were controlled by using
electron withdrawing group.
4
. Conclusion
We successfully synthesized alcohol-soluble alternating copolymers comprising flu-
orene group and electron withdrawing groups. All these polymers can be processed

3
2=[372]
H.-I. Ahn et al.
from polar solvents such as methanol, which is suitable for multilayer of solution
processed PLEDs. Band gap of the polymer can be controlled by introducing proper
electron withdrawing monomers, as shown at absorption spectra and emission spec-
tra. These polymers are expected to improve the PLED performance when using as
an electron injection=transporting material.
Acknowledgment
This work was supported by the Korea Science and Engineering Foundation
(KOSEF) grant funded by the Korea government (MEST) (Grant No. R11-
2
007-050-01003-0) and by the Research fund of HYU (HY-2010-HYURS).
References
[
[
[
1] Burroughes, J. H., Mackay, K., Burn, P. L., & Holmes, A. B. (1990). Nature, 347, 539.
2] Parker, I. D., Cao, Y., & Yang, C. Y. (1999). J. Appl. Phys., 85, 2441.
3] Huang, F., Niu, Y. H., Ka, J. W., Lir, M. S., & Jen, A. K.-Y. (2007). Adv. Mater., 19,
2010.
[
[
[
[
4] Kulkarni, A. P., Tonzola, C. J., & Jenekhe, S. A. (2004). J. Mater. Chem., 16, 4556.
5] Hung, L. S., Tang, C. W., & Mason, M. G. (1997). Appl. Phys. Lett., 70, 152.
6] Liu, B., & Bazan, G. C. (2006). J. Am. Chem. Soc., 128, 1188.
7] Hwang, D. H., Park, M. J., & Kim, S. K. (2004). J. Mater. Res., 19, 2081.