Synthesis and photophysics of new donor-acceptor copolymers based on fluorene and phenylquinolines

Article abstract of DOI:10.1080/07370650601013104

The synthesis and photophysics of two new donor-acceptor conjugated A-B type copolymers based on conjugated 2,6-disubstitued-4-phenyl-quinoline and 2-substitued-4-phenyl-quinoline, and 9,9-dialkyl-fluorene linked through the 1,4 and 1,3,5 phenyl rings via the Friedlnder condensation and Suzuki coupling reaction are presented. The band gap energy of the polymers 1 and 2 measured from the thin films were 430nm and 380nm corresponding to 2.88eV and 3.26eV, respectively. The photoluminescence (PL) spectra of the thin films of the polymers 1 and 2 showed maximum peaks at 427 and 397nm corresponding to the deep blue and UV ranges respectively. The red shift of the polymer 1 relative to the polymer 2 can be accounted for by the increased conjugation length because the quinoline unit is oriented to include the phenyl ring in the backbone of polymer 1. Both polymers have similar trends of irreversible oxidation. The ionization energy of polymers 1 and 2 were measured by cyclic voltammetry (CV) and were found to be 5.60 and 5.65eV, respectively.

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Synthesis and Photophysics
of New Donor-Acceptor
Copolymers Based on Fluorene
and Phenylquinolines
Brent Siemssen a , Ki-Won Kim b , Min-Sook Kim b
,
Byung-Soo Kim b , Seong-Jin Cho b , Dong-Kyu Park b
, Hyung-Suk Woo b & Tae-Woo Kwon b
a Department of Chemistry, University of
Washington, Seattle, WA, USA
b Department of Chemistry and Center for Organic
Devices & Advanced Materials, Kyungsung University,
Busan, Korea
Available online: 17 Oct 2011
To cite this article: Brent Siemssen, Ki-Won Kim, Min-Sook Kim, Byung-Soo Kim,
Seong-Jin Cho, Dong-Kyu Park, Hyung-Suk Woo & Tae-Woo Kwon (2006): Synthesis
and Photophysics of New Donor-Acceptor Copolymers Based on Fluorene and
Phenylquinolines, Molecular Crystals and Liquid Crystals, 462:1, 159-167
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Mol. Cryst. Liq. Cryst., Vol. 462, pp. 159–167, 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/07370650601013104
Synthesis and Photophysics of New Donor-Acceptor
Copolymers Based on Fluorene and Phenylquinolines
Brent Siemssen
Department of Chemistry, University of Washington,
Seattle, WA, USA
Ki-Won Kim
Min-Sook Kim
Byung-Soo Kim
Seong-Jin Cho
Dong-Kyu Park
Hyung-Suk Woo
Tae-Woo Kwon
Department of Chemistry and Center for Organic Devices & Advanced
Materials, Kyungsung University, Busan, Korea
The synthesis and photophysics of two new donor-acceptor conjugated A-B type
copolymers based on conjugated 2,6-disubstitued-4-phenyl-quinoline and 2-substi-
tued-4-phenyl-quinoline, and 9,9-dialkyl-fluorene linked through the 1,4 and 1,3,5
phenyl rings via the Friedla¨nder condensation and Suzuki coupling reaction are
presented. The band gap energy of the polymers 1 and 2 measured from the thin
films were 430 nm and 380 nm corresponding to 2.88 eV and 3.26 eV, respectively.
The photoluminescence (PL) spectra of the thin films of the polymers 1 and 2
showed maximum peaks at 427 and 397 nm corresponding to the deep blue and
UV ranges respectively. The red shift of the polymer 1 relative to the polymer 2
can be accounted for by the increased conjugation length because the quinoline
unit is oriented to include the phenyl ring in the backbone of polymer 1. Both
polymers have similar trends of irreversible oxidation. The ionization energy of
polymers 1 and 2 were measured by cyclic voltammetry (CV) and were found to
be 5.60 and 5.65 eV, respectively.
We gratefully acknowledge support from the Ministry of Information and Communi-
cations, Korea, through the support program of the Information Technology Research
Center (ITRC). We thank Prof. Samson A. Jenekhe and Jessica for helpful discussions
and determining the photophysics.
Address correspondence to Tae-Woo Kwon, Department of Chemistry and Center for
Organic Devices & Advanced Materials, Kyungsung University, Busan 608-736, Korea.
E-mail: twkwon@star.ks.ac.kr
159

160
B. Siemssen et al.
¨
Keywords: 9,9-dialkyl-fluorene; donor-acceptor (D-A) molecules; Friedlander conden-
sation; OLED; 4-phenyl-quinoline; Suzuki coupling reaction
INTRODUCTION
Conjugated polymers with donor-acceptor architectures are currently
of interest for their potential applications in advanced semiconducting
electronic devices such as polymeric light-emitting diodes (PLEDs)
and organic solar cells [1]. Donor-acceptor type conjugated polymers
have their own characteristic properties because the built-in intramol-
ecular charge transfer can enhance the advantage of manipulation of
the electronic structure (HOMO=LUMO levels), leading to small band
gap semiconductors [2]. Since the potential utilization of polyquino-
lines was first introduced by Stille and coworkers [3], a variety of con-
jugated polyquinoline have been extensively designed, synthesized
and characterized with the aim of achieving stable PLEDs with high
brightness and external quantum efficiency (EQE) [4]. Most conju-
gated polyquinolines are known to be thermally stable semiconducting
materials and well investigated as an excellent electron-acceptor
which can be utilized in photonic and electronic application like
PLEDs [5]. Previous studies have shown that 4-phenyl or alkyl quino-
lines are excellent building blocks for the design of electron-acceptor
(n-type) conjugated linear-chain polymers and dendrimers with high
electroluminescence [6], electron transport [7], electrochemilumines-
cence (ECL) [8], nonlinear optical [9], and other advanced electronic
properties. However, hole injection and electron donating properties
of the 4-phenyl and alkyl quinolines still need improvement [10].
One of the challenges of improving hole injection and transport
(electron-donating) properties are introducing donor architectures
such as polyfluorene [11]. Polyfluorenes have been shown to be p-type
(hole transport) conducting polymers, and have been recently used in
PLEDs by copolymerizing with an electron-deficient (n-type) building
block while retaining the blue emission to improve the electron-
transport properties [12].
In this paper, we report the syntheses and photophysics of two new
donor-acceptor polymers based on 2,6-disubstitued-4-phenyl-quinoline
or 2-substitued-4-phenyl-quinoline and 9,9-dialkyl-fluorene linked via
1,4 or 1,3,5 connecting phenyl rings as shown in Figure 1. The quino-
line moiety was chosen as the acceptor building block because it
showed good electron transport (n-type) properties in oligoquinoline
[13], and polyquinoline based PLEDs [14]. Different configurations

Donor-Acceptor Copolymers Based on Fluorene and Phenylquinolines
161
FIGURE 1 (a) Normalized optical absorption and PL emission spectra of
10ꢀ5 M solutions in CHCl3 and thin films of copolymer 1 and 2. The excitation
wavelength was 350 nm. Normalized PL emission spectra of the polymer thin
films annealed at 100ꢁC in air.
of the quinoline units were introduced to control chain rigidity and
conjugation length. 9,9-Dialkyl-fluorene was chosen as the donor
building block.
EXPERIMENTAL
1H and 13C NMR spectra were recorded on a JEOL (Japan), JNM ECP-
400 at 400 MHz or a Bruker-AF301 at 300 MHz. The spectra were
recorded using CDCl3 as the solvent at room temperature. Cyclic vol-
tammetry (CV) experiments were performed using a Perkin ElmerTM
Potentiostat=Galvanostat Model 263A. A three-electrode cell was used
in all experiments. Platinum wire was used for counter electrodes.
Working electrodes were made of indium-tin-oxide (ITO) and Ag=AgCl
(a saturated 3.5 M KCl solution) was used as a reference electrode. The
polymer was coated onto the ITO working electrode by spin coating of
the viscous polymer solution in CHCl3. Electrolyte solutions of 0.1 M
Bu4NClO4 in DMF were used in our experiments. Cyclic voltammo-
grams were obtained at a scan rate of 50 mV=sec. UV-vis spectra were
recorded on a UV-1601 PC, UV-visible spectrophotometer (SHI-
MADZU). Photoluminescence spectra were measured on a RF-5301
PC spectrofluorophotometer (SHIMADZU).
Polymerization of 6 with 7 by Suzuki Coupling
6-Bromo-2-(4-bromo-phenyl)-4-phenylquinoline, 6, was prepared
according to the literature methods [15]. To a two necked flask, 0.5 gram

162
B. Siemssen et al.
(0.995 mmol) of 9,9-dihexyl-fluorene-2,3-bis(trimethylene boronate),
(0.44 g, 0.995 mmol), 1 mol% of tetrakis(triphenylphosphine) pal-
ladium(0) (0.012 g) were dissolved in dry toluene (6 mL) at 60ꢁC
under an argon atmosphere. Aliquat 336 (0.01 mL) in 0.5 mL of dry
toluene and 4 mL of 2M K2CO3 were added via syringe. The reaction
mixture was heated at 80ꢁC for 48 h. The polymerization mixture was
extracted with a solution of EDTA in CHCl3. Volatiles were removed
under reduced pressure. The reaction mixture was slowly poured into
500 mL of methanol. The precipitated polymer was filtered off and
further purified by soxhlet extraction through methanol for 24 h to
remove low molecular weight oligomers and inorganic impurities.
The resulting polymer was redissolved in chloroform and recrystal-
lized from methanol. The precipitated polymer was collected,
vacuum filtered, and dried in a vacuum oven to yield 0.62 g
(66%). 1H NMR (400 MHz, CDCl3): d 8.38 (1 H, bs), 8.20 (1 H, bd),
8.12–7.61 (17 H, m), 2.04 (4 H, bs), 1.08 (16 H, bs), 0.76 (6 H, bs)
ppm. 13C NMR (CDCl3): d156.0, 148.5, 147.8, 144,1, 138.6, 137.5,
136.5, 124.4, 130.2, 129.7, 128.5, 128.2, 127.8, 127.2, 126.9, 126.0,
125.4, 124.3, 123.5, 35.7, 31.7, 29.1, 28.9, 22.7, 14.1 ppm. Tg: 168ꢁC.
IR (KBr) ¼ 3058.ꢀ613 (aromatic CꢀH), 2924.27 (aliphatic CꢀH),
=
1588.72 (C N) cm .
Synthesis of 10 via Friedla¨nder Synthesis
3,5-Dibromoacetophenone, 9 was prepared as described in the litera-
ture [16] with 62% yield. To a 500 mL round bottomed flask was added
5.12 g (18.5 mmol) of 9 (MW¼292.8 g molꢀ1), 4.04 g (20.5 mmol) of
2-aminobenzophenone (MW¼197.24 g molꢀ1), and 18.6 g (74.3 mmol)
of diphenyl phosphate (MW¼250.19 g molꢀ1). Reaction mixture was
stirred and heated slowly to 150ꢁC and allowed to run overnight.
400 mL of CH2Cl2 was added. CH2Cl2 layer was washed twice with
200 mL of 10% aqueous NaOH solution and then washed five times
with 300 mL of water. CH2Cl2 layer was dried over MgSO4 and roto-
evaporated to give a crude oil which was purified by column chromato-
graphy using silica gel (50:50 hexanes:chloroform, v=v) to give 6.19 g of
light orange solid in 77% yield. m.p. ¼ 183–184ꢁC. 1H NMR (300 MHz,
CDCl3): d 8.32 (1H, s), 8.26 (1H, d), 7.94 (1H, d), 7.79 (2H, m), 7.58
(8H, m) ppm. 13C NMR (75 MHz, CDCl3): d153.56, 149.83, 148.70,
142.97, 138.00, 134.58, 129.96, 129.53, 129.31, 128.71, 130.24, 128.67,
127.06, 126.15, 125.75, and 123.48 ppm. IRꢀ(1KBr) ¼ 3083.90 (aromatic
=
CꢀH), 1580.43 (C N), 1540.71, 853.07 cm . Mass spectrum: CIMS,
m=s 439 (Mþ).

Donor-Acceptor Copolymers Based on Fluorene and Phenylquinolines
163
Polymerization of 2-(3,5-Dibromophenyl)-4-Phenylquinoline,
10, with 9,9-Dioctyl-Fluorene-2,3-bis(Trimethylene Boronate)
by Suzuki Coupling
The following reagents were added to a 500 mL of round bottomed flask
under a blanket of argon: 1.311 g (2.35 mmol) of 9,9-dioctyl-fluorene-2,
3-bis(trimethylene boronate) (MW ¼ 558.4 g molꢀ1), 1.036 g (2.36 mmol)
of 2-(3,5-dibromophenyl)-4-phenylquinoline (MW ¼ 439.1 g molꢀ1),
5.15 g (48.6 mmol) of Na2CO3 (MW ¼ 105.88 g molꢀ1), 0.176 g of aliquot
336, 35 mL of toluene, and 24 mL of water. After stirring this reaction
mixture under argon, 58.31 mg (0.05 mmol) of tetrakis(triphenylpho-
sphine)palladium(0) was added. Reaction mixture was refluxed for
3 days then washed with three times of 200 mL water. Toluene layer
was separated and dried over MgSO4, then filtered to remove MgSO4.
The volatiles were rotoevaporated to give a crude product. The product
was dissolved in THF, precipitated dropwise into MeOH, and repeated
three times. 1.201 g of polymer was isolated in 51.2% yield. Further
purification of 0.61 g of crude polymer by soxhlet extracted with acetone
for three days, leaving 0.58 g polymer, having removed 4.6% of the mass
1
as oligomers. H NMR (300 MHz, CDCl3): d 8.51 (2H, m), 7.70 (15, m),
7.19 (2H, m), 2.09 (4H, bs), 1.12 (24H, m), 0.69 (6H, m) ppm. 13C
NMR (75 MHz, CDCl3): d 157.12, 151.93, 148.90, 149.38, 143.05,
140.39, 40.49, 55.57, 140.40, 140.15, 138.40, 130.20, 129.62, 128.66,
128.64, 126.49, 125.94, 125.72, 125.51, 121.89, 120.15, 119.70, 31.74,
30.10, 29.26, 23.95, 22.59, and 14.03 ppm. IR (KBr) ¼ 3060.53 (aromaꢀti1c
=
CꢀH), 2925.49 (aliphatic CꢀH), 1587.85 (C N), 1542.55, 1466.76, cm .
RESULTS AND DISCUSSION
The synthetic approaches to polymers 1 and 2 are illustrated in
Scheme 1. The synthetic strategy for the preparation of 1 involved
the introduction of bromine functionality on the amino benzophenone
ring of commercially available 2-aminobenzophenone, 3, in tetrabutyl-
¨
ammonium tribromide=CHCl3 solution [17]. Friedlander condensation
between 4 and 4-bromoacetophenone, 5, in the presence of diphenyl-
phosphate (DPP) gave 6-bromo-2-(4-bromophenyl)-4-phenylquinoline,
6 which was immediately reacted with 9,90-dihexylfluorene-2,7-
bis-(trimethylene boronate), 7 to obtain polymer, 1, in 66% yield
(Scheme 1). The synthesis of polymer 2 started from the acetylation
reaction of commercially available 1,3,5-tribromobenzene, 8, using
one equivalent n-BuLi, and N,N-dimethylacetamide in diethylether
at ꢀ78ꢁC. 3,5-Dibromoacetylbenzene, 9, was obtained in 62% yield.
Condensation of 9 with 2-aminobenzophenone in the presence of

164
B. Siemssen et al.
SCHEME 1 Molecular structures and synthetic route of polymers 1 and 2.
DPP at 120ꢁC gave 2-(3,5-dibromo-phenyl)-4-phenylquinoline, 10 in
80% yield. The acetyl carbonyl peak (1710 cmꢀ1) of 5 and 9 completely
disappeared in the FT-IR spectra and new strong imine functional
peaks from the quinoline ring (1400–1600 cmꢀ1) were observed in 6
1
and 10. H and 13C NMR are another excellent confirmation method
of the completion of the cyclization reaction to produce the quinoline
¨
moiety via Friedlander condensation. The three protons and carbon
of the acetyl group also completely disappeared in the 1H and 13C
NMR of 6 and 10. DOFPQ polymer 2 was successfully obtained in
51.2% yield by treating 10 with 9,90-dioctyl-fluorene-2,3-bis(trimethy-
lene boronate), Na2CO3, aliquot 336 and tetrakis(triphenyl phosphine)
palladium(II) (Scheme 1). Both polymers 1 and 2 were soluble in
a number of organic solvents such as CH2Cl2, toluene, THF and
1
ethyl acetate and were characterized using FT-IR, H and 13C NMR
which are in good agreement with the proposed structures of
1 and 2.

Donor-Acceptor Copolymers Based on Fluorene and Phenylquinolines
165
ABSORPTION AND PHOTOLUMINESCENCE SPECTROSCOPY
The UV-vis and photoluminescence (PL) spectra of the two copolymers
in dilute CHCl3 solution (1 ꢂ 10ꢀ5 M) and in thin films are shown in
Figure 1. In thin films, the band gap energies of the polymers were
estimated from the absorption edges and were found to be 430 nm
and 380 nm, respectively. It is, of course, clear that the absorption
edge of the solution phase was blue shifted due to the higher degree
of the freedom in polymer chain geometry, allowing for twisting and
blending. The PL spectra showed the maximum emission peaks of
polymers 1 and 2 at 427 nm and 397 nm, respectively. The red shift
of polymer 1 relative to that of polymer 2 is likely due to increased con-
jugation length in the monomer when the quinoline unit is included as
part of the polymer backbone. We note that DOFPQ polymer 2 emits
almost ultra-violet light. In order to estimate the ionization potential
(IP) of the polymers, we performed cyclic voltammetry (CV), shown
in Figure 2 and determined the IP of polymers 1 and 2 as 5.60 eV
and 5.65 eV, respectively. It is likely that both polymers have similar
trends of irreversible oxidation. From the measured values of the band
gap energies and IP, the electron affinity (EA) of polymers 1 and 2
were calculated and were found to be 2.80 eV and 2.30 eV, respectively.
The optical data as well as the electrochemical data are summarized
and shown in Table 1.
GPC analysis of the polymer was determined on a Younglin 9200
GPC system equipped with RI750F detector using THF as solvent.
FIGURE 2 Cyclic voltammetry of oxidation of a 100-nano-thick polymer 1
and 2 film spin-coated over an ITO electrode with 0.1 M Bu4NClO4 in DMF
as the supporting electrolyte. Scan rate 50 mV=s.

166
B. Siemssen et al.
TABLE 1 Photophysical and Electrochemical Properties of Polymers 1 and 2
Abs
abs
PL
max
PL
k
max
k
log emax
k
k
max
max
(soln) (105 Mꢀ1
(nm)
(film) (soln) (film)
(nm)
E
IP
Eg
EA
onset
ox
Polymer
cmꢀ1
)
(nm)
(nm) (V)a
(eV)b (eV)c (eV)d
DHFPQ 1
DOFPQ 2
389
335
0.93
1.41
397
335
432
395
435
397
0.80
0.85
5.60
5.65
2.80
3.35
2.80
2.30
aAll potentials are vs SCE reference.
bIP were calculated from IP ¼ 4.8 þ E
onset
ox
(CV).
cEg was calculated from UV kmax ¼ 1240=Eonset, Eonset of 1 ¼ 442 nm, Eonset of
2 ¼ 374 nm.
dEA ¼ IP ꢀ Eg.
The weight-average molecular weights, based on polystyrene stan-
dards, were 35,000 for DHFPQ, 1 and 26,300 for DOFPQ, 2 with poly-
dispersity indices (Mw=Mn) of 1.74 and 1.92, respectively. The thermal
behavior of polymers 1 and 2 was measured by differential scanning
calorimetry (DSC) analysis which was performed on a SETARAM
DSC 131 at a heating rate of 10ꢁC=min. Introduction of quinoline
structure in the polyfluorene backbone significantly enhanced the
thermal stability. DSC revealed that the glass transition temperatures
(Tg) of DHFPQ, 1 and DOFPQ, 2 were 168 and 148ꢁC which are much
higher Tg than that of typical polyfluorene (55ꢁC) [18]. Synthetic
yields, molecular weights distribution, and thermal properties of
DHFPQ, 1, and DOFPQ, 2, are summarized in Table 2.
SUMMARY
In summary, we described the syntheses and photophysics of two new
donor-acceptor copolymers, 1 and 2, containing phenyl quinolines and
9,9-di-n-alkyl fluorene units in the main chain. The band gap energies
for polymers 1 and 2 were 2.88 and 3.26 eV, respectively. Emission
peaks from PL measurements of polymers 1 and 2 were found at
TABLE 2 Molecular Weights Distribution and Thermal Properties of
Polymers 1 and 2
ꢁ
Polymer
Yield (%)
Mw (ꢂ104)a
Mw=Man
DSC (Tg, C)
DHFPQ 1
DOFPQ 2
66
51
3.50
2.63
1.74
1.92
168
148
aMw and PDI (Mw=Mn) of the polymers were determined by GPC using polystyrene as
a standard.

Donor-Acceptor Copolymers Based on Fluorene and Phenylquinolines
167
427 nm (deep blue) and 397 nm (UV range), respectively. CV results
estimate the IP of polymers 1 and 2 as 5.60 and 5.65 eV, respectively
while EA was calculated as 2.8 and 2.3 eV, respectively. Optical data
showed the emission wavelength could be controlled by incorporating
or excluding the quinoline moieties into the backbone of the polymer.
The PL spectra in Figure 1 shows that our material emits at near UV.
This indicates that the fluorene-quinoline copolymers could be useful
for developing UV emitting LED. We are currently investigating
electroluminescent properties of these polymers.
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