Synthesis and optical studies of conjugated polyfluorenyl cations

Article abstract of DOI:10.1016/j.polymer.2010.09.062

Polyfluorene based precursor polymers were synthesized either by Yamamoto type or Suzuki polycondensation. These polymers were then converted to the corresponding conjugated polycations by treatment with trifluoroacetic acid. The formation of the conjugated polycations were followed by UV-vis and characterized by ¹H NMR. The absorption maxima of the polycations fall in the range of 580-690 nm. The formation of the polycations is evidenced by a red shift in absorption maxima compared to the parent polymers due to the generation of stable cations with more extended conjugation and planarity. The cationic polymers show low solubility in common organic solvents, however the solubility of the cationic salt can be greatly enhanced by controlling the cation density along the polymer chain by copolymerization.

Full text of DOI:10.1016/j.polymer.2010.09.062

Polymer 51 (2010) 5705e5711
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and optical studies of conjugated polyfluorenyl cations
,
Renchu Scaria a, Klaus Muellen b, Josemon Jacob a
*
a Centre for Polymer Science and Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
b Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 July 2010
Received in revised form
21 September 2010
Accepted 25 September 2010
Available online 7 October 2010
Polyfluorene based precursor polymers were synthesized either by Yamamoto type or Suzuki poly-
condensation. These polymers were then converted to the corresponding conjugated polycations by
treatment with trifluoroacetic acid. The formation of the conjugated polycations were followed by
UVevis and characterized by 1H NMR. The absorption maxima of the polycations fall in the range of
580e690 nm. The formation of the polycations is evidenced by a red shift in absorption maxima
compared to the parent polymers due to the generation of stable cations with more extended conju-
gation and planarity. The cationic polymers show low solubility in common organic solvents, however
the solubility of the cationic salt can be greatly enhanced by controlling the cation density along the
polymer chain by copolymerization.
Keywords:
Polyfluorene
Polycations
Ó 2010 Elsevier Ltd. All rights reserved.
Conjugated materials
1. Introduction
where the maximum photon flux is around 1.7 eV (700 nm) [16].
Copolymerization of fluorene with acceptor type materials to form
During the past two decades, the design and synthesis of new
classes of conjugated polymers have attracted attention due to their
interesting optical, electrochemical and electrical properties [1e4].
Fluorene-containing molecules represent an important class of
aromatic systems that have received considerable attention due to
their unique photophysical properties and potential for chemical
modification [5e7]. Easy dialkylation at the 9-position and selec-
tive bromination at the 2,7-positions of fluorene allow versatile
molecular manipulation to enhance solubility and extend conju-
gation easily by means of transition-metal catalyzed cross-coupling
reactions [8e10]. Due to their high fluorescence quantum yield,
excellent hole-transporting properties, good film-forming proper-
ties, and exceptional chemical stability, polyfluorene-based (PF)
conjugated polymers have been extensively studied and utilized
specifically in the field of light-emitting diodes [11e13]. Well-
defined, fluorene based oligomers and copolymers also find
application in organic thin film transistors [14,15]. However, the
large optical band gap of poly(9,9-dialkylfluorene) limits its appli-
cation in photovoltaic devices. For use in efficient solar cells, the
absorption spectrum of the conjugated polymer needs to cover
both the red and near infrared regions to harvest the maximum
photon flux. The optical band gap of poly(9,9-dialkylfluorenes) is
above 3 eV, which is too high for efficient sunlight harvesting
a donoreacceptor alternating arrangement in the polymer back-
bone is a strategy reported in literature for narrowing the band gap
for optimal sunlight collection [17e19]. Carbinols, which contain
oligo(1,4-phenylenevinyline) (OPV) chains and triaryl carbinol
based polymers on treatment with strong acid lead to the forma-
tion of corresponding carbocations which exhibit strong bath-
ochromic shift of the absorption maxima [20,21]. It is also reported
that electron impact ionization and vacuum ultraviolet photoioni-
zation of fluorenes and photoheterolysis of 9-fluorenols with alkali
metal zeolites generate corresponding fluorenyl cations having
absorption maxima at higher wavelength region [22,23]. Fluorenyl
cations can also be easily generated from their alcoholic or neutral
methoxy precursors under acidic conditions [24,25]. Polycations
derived from polyfluorenes with cationic functionalities incorpo-
rated into the polymer side chains has been widely investigated
because of their unique characteristics that are different from those
of neutral polymers [26e30]. Apart from a few limited reports in
literature, there has been no systematic study into the synthesis
and characterization of conjugated polycations in which cations are
in conjugation with the polymer backbone [31,32]. The objective of
our present study is to synthesize, and characterize conjugated
polycations bearing cations in conjugation with the polymer
backbone and investigate their effect on optical properties of the
resulting materials. In our efforts to understand the effects of
introducing conjugated cations along a polymeric network on
conjugation and optical properties, we have explored the possi-
bility of generating fluorenyl cations in the polymer chain. A
* Corresponding author. Tel.: þ91 9911061890; fax: þ91 11 2659 1421.
E-mail address: jacob@polymers.iitd.ac.in (J. Jacob).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.09.062

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R. Scaria et al. / Polymer 51 (2010) 5705e5711
NaH, THF
CH3I
2-bromo 9,9-dioctylfluorene (2)
Br
Br
Br
Br
R
Br
Br
R
n-BuLi, THF, -78 0
C
HO
O
MeO
R
R
3
R = -C8H17
1
4
Scheme 1. Synthetic route for monomer 4.
carbocation centered at the 9-position of fluorene in a conjugated
material can be stabilized by extensive delocalization of the charge
along the polymer backbone. Here, we report on the synthesis and
characterization of conjugated polycations based on fluorene
bearing alkylfluorene sidechains. Also, we report on copolymeri-
zation studies whereby the cation density along the polymer chain
has been systematically varied.
2.3. Synthesis
2.3.1. Synthesis of 2,7-dibromo-9-(9,9-dioctylfluorene-2-yl)-9-
hydroxyfluorene (3)
To a solution of 2 (1.084 g, 2.308 mmol) in dry THF (5 mL) kept
at ꢁ78 ꢀC, n-BuLi in hexane (1.4 mL, 1.6 M, 2.260 mmol) was added
dropwise and the mixture was stirred for 2 h at the same
temperature. 2,7-dibromofluorenone (0.600 g, 1.775 mmol) was
added to the reaction mixture and the temperature was gradually
increased to room temperature. After stirring for 20 h the reaction
was quenched by the addition of water. The product was extracted
with diethyl ether, washed with saturated brine and dried over
Na2SO4. The crude so obtained was purified by column chroma-
tography with 0e1% ethylacetate in hexane as eluent to obtain 3 as
2. Experimental
2.1. Materials
Dry solvents stored in a glove box were used for specified reac-
tions. 2,7-dibromofluorenone (1), 2-bromo 9,9-dioctylfluorene (2),
2,7-dibromo-9,9-dioctylfluorene (5), 2,7-bis(4,4,5,5-tetramethyl-
1,3,2 dioxaboralan-2-yl)-9,9-dioctylfluorene (6) were synthesized
in accordance with literature procedure [33e35].
a
colorless oil (0.600 g, 46%). 1H NMR (300 MHz, CDCl3):
d
¼ 7.60e7.17 (m, 12H), 6.88 (d, J ¼ 7.5 Hz, 1H), 2.43 (s, 1H), 1.84 (t,
J ¼ 8.1 Hz, 4H), 1.34e0.54 (m, 30H). 13C NMR (75 MHz, CDCl3):
d
¼ 152.13, 151.16, 150.89, 140.82, 140.50, 140.38, 137.39, 132.29,
128.33, 127.05, 126.69, 123.84, 123.29, 122.76, 122.47, 121.49,
119.73, 119.62, 119.53, 83.57, 55.11, 40.20, 31.74, 30.02, 29.21, 23.87,
22.57, 14.09. Elem. Anal. Calc. For C42H48Br2O: C, 69.23, H, 6.64;
Found: C, 68.87, H, 7.04.
2.2. Characterization
1H and 13C NMR spectra were taken on a Bruker 300 MHz
spectrometer operating respectively at 300 MHz for 1H and 75 MHz
for 13C using deuterated solvents with tetramethylsilane (TMS) as
a reference for chemical shifts. Elemental analyses were measured
on Elementar Vario EL III elemental analyzer. UVevis absorption
spectra were recorded in chloroform solution on a PerkineElmer
Lambda 25 Spectrophotometer. Photoluminescence measurements
were carried out with a Fluorolog HORIBAJOBIN YVON spectro-
photometer using a xenon-arc lamp as a source, in chloroform
solution. Thermal degradation was studied by TGA on a Per-
kineElmer Pyris 7 thermal analysis system in a dynamic atmo-
sphere of nitrogen at a heating rate of 20 ꢀC/min. The molecular
weights of the polymers were determined by Gel Permeation
Chromatography against polystyrene standard in THF at 30 ꢀC.
2.3.2. Synthesis of 2,7-dibromo-9-(9,9-dioctylfluorene-2-yl)-9-
methoxyfluorene (4)
Compound 3 (0.550 g, 0.755 mmol) was dissolved in dry THF
(4 mL) and NaH (0.072 g, 3.020 mmol) was added to this solution.
The reaction mixture was stirred for 30 min at room temperature.
Methyl iodide (0.470 mL, 7.550 mmol) was added to this solution
and after stirring at room temperature for 18 h the product was
extracted with diethyl ether and dried over Na2SO4. The crude so
obtained was purified by column chromatography with 0e1%
ethylacetate in hexane as eluent to obtain 4 as a white solid
(0.480 g, 85%). 1H NMR (300 MHz, CDCl3):
6.89 (d, J ¼ 7.8 Hz, 1H), 3.02 (s, 3H), 1.92 (t, J ¼ 9 Hz, 4H), 1.20e0.59
d
¼ 7.60e7.39 (m, 12H),
R
R
Ni(COD)2,COD , 2.2'-bipyridine
toluene, DMF, 75 0C, 48 h
n
Br
*
n
Br
Br
R
+
Br
m
*
m
R
R
R
MeO
MeO
R
R
R = -C8H17
4
5
P1-OMe ; m = 1, n = 0
P2-OMe (a) ; m = 1, n = 1
P3-OMe ; m = 1, n = 2
P4-OMe ; m = 1, n = 6
P5-OMe ; m = 1, n = 10
Scheme 2. Synthesis of polymers via Yamamoto polycondensation.

R. Scaria et al. / Polymer 51 (2010) 5705e5711
5707
R
R
O
O
Pd(PPh3)4, 2M K2CO3
B
B
Br
R
Br
+
*
*
n
O
toluene, 85 0C, 48 h,
O
R
R
R
MeO
MeO
R
R
R = -C8H17
6
4
P2-OMe (b)
Scheme 3. Synthesis of P2-OMe (b) via Suzuki polycondensation.
(m, 30H). 13C NMR (75 MHz, CDCl3):
d
¼ 151.03, 150.93, 149.03,
a Schlenk tube. The solution was purged under nitrogen for 30 min,
and then stirred for 48 h at 85 ꢀC. Bromobenzene (45
L,
140.70,140.62,138.74,132.33,128.68,126.98,126.68,124.04,122.77,
122.45, 121.39, 120.07, 119.60, 119.38, 55.11, 51.73, 40.25, 31.78,
30.05, 29.23, 23.86, 22.60, 14.10. Elem. Anal. Calc. For C43H50Br2O: C,
69.54, H, 6.79; Found: C, 69.400, H, 6.91.
m
0.42 mmol) was added as end capping agent to the mixture and
stirred for 12 h under the same conditions. After cooling to room
temperature, the viscous solution was poured into 300 mL of
methanol. The precipitated polymer was isolated by filtration. It
was further purified by Soxhlet extraction with acetone followed by
reprecipitation from methanol. The polymer was dried under
reduced pressure to yield P2-OMe as a pale yellow solid (0.382 g,
2.3.3. Synthesis of poly[2,7-9-(9,9-dioctylfluorene-2-yl)-9-
methoxyfluorene], P1-OMe
Ni(COD)2 (0.338 g, 1.228 mmol), COD (0.150 mL, 1.228 mmol)
and 2,20-bipyridine (0.192 g, 1.228 mmol) were added to a Schlenk
tube containing a mixture of DMF (1.4 mL) and toluene (1 mL) in
a glove box. The resulting solution was heated under nitrogen at
60 ꢀC for 30 min and then a solution of 4 (0.380 g, 0.520 mmol) in
toluene (4.6 mL) was added. The reaction mixture was heated for
48 h at 75 ꢀC. After cooling to room temperature, the viscous
solution was poured into 250 mL of methanol and the precipitated
polymer was isolated by filtration. It was further purified by
extraction with acetone in a Soxhlet apparatus. The polymer was
redissolved in chloroform and washed with saturated aqueous
EDTA solution and distilled water. The organic layer was extracted,
concentrated to minimum volume and poured into 250 mL of
methanol causing the precipitation of the polymer to yield PI-OMe
as a pale yellow powder (0.166 g, 54% yield). 1H NMR (300 MHz,
97%). 1H NMR (300 MHz, CDCl3):
(s, 3H, br), 1.98 (t, 8H, br), 1.06e0.78 (m, 60H, br). UVevis (CHCl3)
lmax: 383 nm. GPC: Mw ¼ 151.1 kDa, Mn ¼ 23.9 kDa, PDI ¼ 6.38.
d
¼ 7.80e7.13 (m, 19H, br), 3.18
2.3.6. Synthesis of P3-OMe
P3-OMe was prepared according to the procedure reported for
P1-OMe using 4 (0.160 g, 0.215 mmol), 5 (0.235 g, 0.423 mmol), Ni
(COD)2 (0.392 g, 1.423 mmol), COD (0.175 mL, 1.423 mmol), 2,20-
bipyridine (0.223 g, 1.423 mmol), toluene (6.8 mL) and DMF
(1.7 mL). The polymer was obtained as a pale yellow solid (0.25 g,
84%). 1H NMR (300 MHz, CDCl3):
(s, 3H, br), 1.99 (t, 12H, br), 1.24e0.80 (m, 90H, br). UVevis (CHCl3)
lmax: 381 nm. GPC: Mw ¼ 34.1 kDa, Mn ¼ 14.1 kDa, PDI ¼ 2.62.
d
¼ 7.67e7.26 (m, 25H, br), 3.16
CDCl3):
d
¼ 7.57e6.98 (m, 13H, br), 2.95 (s, 3H, br), 1.83 (t, 4H, br),
2.3.7. Synthesis of P4-OMe
0.89e0.56 (m, 30H, br). UVevis (CHCl3) lmax: 372 nm. GPC:
Mw ¼ 4.2 kDa, Mn ¼ 2.9 kDa, PDI ¼ 1.45.
P4-OMe was prepared according to the procedure reported for
P1-OMe using 4 (0.100 g, 0.135 mmol), 5 (0.440 g, 0.808 mmol), Ni
(COD)2 (0.597 g, 2.167 mmol), COD (0.265 mL, 2.167 mmol), 2,20-
bipyridine (0.339 g, 2.167 mmol), toluene (9.6 mL) and DMF
(2.4 ml). The polymer was obtained as a yellow solid (0.290 g, 74%).
2.3.4. Synthesis of poly[2,7-(9,9-dioctylfluorene)-co-20,70-9-(9,9-
dioctylfluorene-2-yl)-9-methoxyfluorene], P2-OMe(a)
P2-OMe(a) was prepared according to the procedure reported
for P1-OMe using 4 (0.150 g, 0.202 mmol), 5 (0.110 g, 0.202 mmol),
Ni(COD)2 (0.256 g, 0.929 mmol), COD (0.114 mL, 0.929 mmol), 2,20-
bipyridine (0.64 g, 0.929 mmol), toluene (4.3 mL) and DMF (1.1 mL).
The polymer was obtained as a yellow solid (0.150 g, 76%). 1H NMR
1H NMR (300 MHz, CDCl3):
d
¼ 7.82e6.97 (m, 49H, br), 3.20 (s, 3H,
(300 MHz, CDCl3):
(t, 8H, br), 1.01e0.56 (m, 60H, br). UVevis (CHCl3) lmax: 375 nm.
GPC: Mw ¼ 8.3 kDa, Mn ¼ 3.8 kDa, PDI ¼ 2.1.
d
¼ 7.58e6.95 (m, 19H, br), 3.02 (s, 3H, br), 1.88
2.3.5. Synthesis of poly[2,7-(9,9-dioctylfluorene)-co-20,70-9-(9,9-
dioctylfluorene-2-yl)-9-methoxyfluorene], P2-OMe(b)
The monomers 4 (0.338 g, 0.404 mmol) and 6 (0.274 g,
0.424 mmol), Pd (PPh3)4 (14 mg, 2 mol %), toluene (4 mL), Aliquat
336 (3 drops) and 1.2 mL of 2 M Na2CO3 solution were added to
Table 1
The initial feed ratio and ratio as determined by 1H NMR for the two comonomers.
Polymer
Initial feed ratio (m:n)
Ratio in the polymer (m:n)
P1-OMe
P2-OMe (a)
P3-OMe
P4-OMe
P5-OMe
1:0
1:1
1:2
1:6
1:10
1:0
1:1
1:1.96
1:6.06
1:10.03
Fig. 1. UVevis spectra of polymers in chloroform solution.

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R. Scaria et al. / Polymer 51 (2010) 5705e5711
2.4.2. P2 (a)
1H NMR (300 MHz, CDCl3/CF3COOD):
2.03 (t, 8H, br), 1.27e0.79 (m, 60H, br). UVevis (CDCl3/CF3COOD).
lmax: 682 nm, 481 nm, 323 nm.
d
¼ 7.69e7.26 (m,19 H, br),
2.4.3. P2 (b)
1H NMR (300 MHz, CDCl3/CF3COOD):
2.02 (t, 8H, br), 1.10e0.79 (m, 60H, br). UVevis (CDCl3/CF3COOD).
lmax: 682 nm, 481 nm, 349 nm.
d
¼ 7.74e7.26 (m, 19H, br),
2.4.4. P3
1H NMR (300 MHz, CDCl3/CF3COOD):
2.12 (t, 12H, br), 1.12e0.84 (m, 90H, br). UVevis (CDCl3/CF3COOD).
lmax: 687 nm, 480 nm, 384 nm.
d
¼ 7.64e7.26 (m, 25H, br),
2.4.5. P4
1H NMR (300 MHz, CDCl3/CF3COOD):
2.28 (t, 28H, br), 1.43e0.81 (m, 210H, br). UVevis (CDCl3/CF3COOD).
lmax: 587 nm, 324 nm.
d
¼ 7.67e7.00 (m, 49H, br),
2.4.6. P5
Fig. 2. PL spectra of precursor polymers in chloroform solution.
1H NMR (300 MHz, CDCl3/CF3COOD):
2.15 (t, 28H, br), 1.17e0.83 (m, 330H, br). UVevis (CDCl3/CF3COOD).
lmax: 606 nm, 325 nm.
d
¼ 7.67e7.25 (m, 73H, br),
br), 2.10 (t, 28H, br), 1.13e0.81 (m, 210H, br). UVevis (CHCl3) lmax
385 nm. GPC: Mw ¼ 179.1 kDa, Mn ¼ 32.2 kDa, PDI ¼ 5.59.
:
2.4.7. M4
1H NMR (300 MHz, CDCl3/CF3COOD):
d
¼ 7.59e7.25 (m, 13H br),
2.3.8. Synthesis of P5-OMe
P5-OMe was prepared according to the procedure reported for
P1-OMe using 4 (0.050 g, 0.067 mmol), 5 (0.367 g, 0.673 mmol), Ni
(COD)2 (0.469 g, 1.703 mmol), COD (0.209 mL, 1.703 mmol), 2,20-
bipyridine (0.226 g, 1.703 mmol), toluene (8 mL) and DMF (2 mL).
The polymer was obtained as a yellow solid (0.245 g, 82%). 1H NMR
1.96 (t, 4H, br), 1.27e0.62 (m, 30H, br). UVevis (CDCl3/CF3COOD).
lmax: 693 nm, 468 nm, 315 nm.
3. Results and discussion
(300 MHz, CDCl3):
d
¼ 7.76e6.99 (m, 73H, br), 3.13 (s, 3H, br), 2.04
(t, 44H, br), 1.06e0.74 (m, 330H, br). UVevis (CHCl3) lmax: 386 nm.
GPC: Mw ¼ 119.8 kDa, Mn ¼ 30.2 kDa, PDI ¼ 3.93.
3.1. Synthesis of monomers and precursors for conjugated
polycations
The synthetic approach toward the synthesis of monomer 4 is
shown in Scheme 1. Fluorenol derivative 3 was obtained in 46%
yield by reacting the lithium reagent formed by dropwise addition
of n-BuLi into a THF solution of 2-bromo 9, 9-dioctylfluorene (2)
at ꢁ78 ꢀC with 2,7-dibromofluorenone (1). Alkylation of 3 with
methyl iodide under basic condition generated monomer 4 in 85%
yield. The monomer 2,7-dibromo-9-(9,9-dioctylfluorene-2-yl)-9-
methoxyfluorene 4 bearing substituted fluorenyl group at 9-posi-
tion was polymerized by Yamamoto type polymerization to yield
polymer P1-OMe of high solubility.
2.4. General procedure for the synthesis of conjugated polycations
The polymer/model compound (20 mg) was dissolved in chlo-
roform (3 mL) and trifluoroacetic acid (0.1e0.2 mL) was added to
the chloroform solution. Within 2e3 min, the solution turned dark
green. 1H NMR spectra of the conjugated polycations were taken in
deuterated chloroform after generating the cations with deuterated
TFA. The UVevis spectra were taken from this solution after
diluting it to very low concentration (10ꢁ6 M).
The synthetic routes toward polymers by Yamamoto and Suzuki
polycondensation methods are outlined in Schemes 2 and 3,
respectively. The homopolymer P1-OMe (m ¼ 1, n ¼ 0) and random
copolymers P2-OMe (a) (m ¼ 1, n ¼ 1), P3-OMe (m ¼ 1, n ¼ 2),
P4-OMe (m ¼ 1, n ¼ 6) and P5-OMe (m ¼ 1, n ¼ 10) were prepared by
nickel(0)-mediated Yamamoto type polycondensation. The alter-
nating copolymer P2-OMe (b) was prepared by standard Suzuki
polycondensation of 2,7-dibromo-9-(9,9-dioctylfluorene-2-yl)-9-
methoxyfluorene (4) with 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxa-
boralan-2-yl)-9,9-dioctylfluorene (6) in presence of Pd(PPh3)4. The
copolymers having different molar feed ratios of monomer 5 were
synthesized as a means to control the density of cations along the
polymer chain to understand its effect on solubility and optical
properties. After precipitation in methanol, the polymers were
further purified by Soxhlet extraction and then by reprecipitation.
All the polymers show good solubility in THF, CHCl3 and CH2Cl2.
The polymers were characterized by 1H NMR, UVevis and
photoluminescence (PL) spectroscopy, molecular weight determi-
nation and TGA. The actual ratio of monomers 4 and 5 in the
2.4.1. P1
1H NMR (300 MHz, CDCl3/CF3COOD):
d
¼ 7.67e7.24 (m, 13H, br),
1.95 (t, 4H, br), 1.02e0.74 (m, 30H, br). UVevis (CDCl3/CF3COOD).
lmax: 690 nm, 478 nm, 357 nm.
Table 2
Molecular weights and optical properties of precursor polymers.
Polymers
Yield (%) Mn (kDa) PDI
Td (ꢀC)a labs (nm)b lem (nm)c
P1-OMe
P2-OMe (a) 76
P2-OMe (b) 97
P3-OMe
P4-OMe
P5-OMe
54
2.9
3.8
1.45 346
2.1 357
372
375
383
381
385
386
417
418
419
418
419
419
23.7
14.1
32.2
30.2
6.37 339
2.41 382
5.56 380
3.96 393
84
74
82
a
The temperature at 5% weight loss measured by TGA with a heating rate of
20 ꢀC/min under nitrogen atmosphere.
b
UVevis absorption maxima recorded in chloroform solution.
Photoluminescence emission maxima recorded in chloroform solution at
c
lexc ¼ 370 nm.

R. Scaria et al. / Polymer 51 (2010) 5705e5711
5709
R
R
R
R
CHCl3, TFA
*
*
*
+
n
*
n
- CH3OH
-
R
TFA
MeO
R
R
R
R = - C8H17
P2-OMe (b)
P2 (b)
Scheme 4. Conversion of precursor polymer P2-OMe (b) to the conjugated polycation P2 (b).
copolymers (m:n) as determined from 1H NMR analysis are in good
agreement with initial feed ratios, and are summarized in Table 1.
The electronic absorption and photoluminescence emission
3.2. Synthesis and characterization of conjugated polycations
The absorption maxima of all precursor polymers reported in
this study is in the range of 370e385 nm. Generation of cations can
be used as a means to red shift the absorption and extend the
spectral window. In order to study the effect of cation formation on
absorption spectra of conjugated polymers, polycations were
generated from the precursor polymers by treatment with tri-
fluoroacetic acid (TFA). The general route for the conversion of
precursor polymers to their cationic form is shown in Scheme 4.
To convert the methoxy precursors to the cationic form, the
polymer was dissolved in chloroform and treated with TFA. Within
2e3 min, the formerly colourless solution turned dark green. The
polycation generated from precursor polymer P1-OMe is named as
P1, P2-OMe as P2 and so on and the one from the monomer 4 is
named as M4 (the model compound). The cations were character-
ized by 1H NMR and UVevis spectroscopy. For example, the 1H
NMR spectra of P1-OMe and P1 in chloroform solution are shown
in Figs. 3 and 4 respectively.
(
lexc ¼ 370 nm) spectra of the precursor polymers in chloroform
solution are depicted in Figs. 1 and 2, respectively.
The observed absorption and emission maxima are compa-
rable to that of poly (9,9-dialkyl fluorene-2,7-diyl) reported in
literature [36,37]. This implies that replacing the two flexible
alkyl chains on fluorene unit with methoxy and alkylated fluorene
groups does not alter the electronic structure of the polyfluorene
backbone.
The molecular weights of the polymers were determined by Gel
Permeation Chromatography. The weight-average molecular
weights (Mw) were found to be 4.2, 8.3, 151.1, 34.1, 179.1 and
119.8 kDa with PDI of 1.45, 2.1, 6.37, 2.41, 5.56 and 3.96 respectively
for the precursor polymers P1-OMe to P5-OMe. The homopolymer
P1-OMe has low yield (54%) and low molecular weight
(Mn ¼ 2.9 kDa) compared to the copolymers which is due to the
formation of insoluble gel during polymerization. In the case of
copolymers no such gel formation is observed. P3-OMe also has
significantly low Mw compared to P4-OMe and P5-OMe. Since the
latter two have higher amount of alkyl solubilizing groups, it
appears that the 9,9-dioctyl flourene content has a direct bearing
on the molecular weight and solubility of the resulting copolymers.
The thermal stability of the polymers was evaluated by thermog-
ravimetric analysis (TGA). The decomposition temperature (Td) of
the polymers based on 5% weight loss is given in Table 2. The onset
decomposition temperature of the polymers are in the range of
346e393 ꢀC which is less than that of poly(9,9-dioctyl fluorene 2,7-
yl) [37]. This could be attributed to the presence of varying ratios of
methoxy group and flexible alkyl chain in the polymer. The physical
and optical properties of the various polymers are summarized in
Table 2.
Compared to the 1H NMR spectrum of P1-OMe, there is signif-
icant displacement of signals in P1, upon conversion to the poly-
cationic form. The signal a corresponding to eOMe at 2.95 ppm
completely disappears and an additional signal at 3.59 ppm cor-
responding to the eCH3 resonance of methanol, released on cation
formation, appears in 1H NMR of P1. This indicates that the
conversion of methoxy form to the cationic form is complete.
Similar observations were made in the NMR spectra for all the other
cationic polymers as well. The structures of all polycations were
investigated by generating cations in NMR experiments, however, it
is possible to remove the generated methanol by evaporation under
vacuum and obtain spectra of the polycation alone (See Supporting
information Fig.S1 and Fig.S2). This confirmed quantitative
conversion of the methoxy precursors to the polycationic form. The
molecular structure of the polycations can be confirmed by 2D NMR
analysis. But in this case the solubility of none of the polycations is
Fig. 3. 1H NMR spectrum of P1-OMe (300 MHz; CDCl3).
Fig. 4. 1H NMR spectrum of P1 (300 MHz; CDCl3/CF3COOD).

5710
R. Scaria et al. / Polymer 51 (2010) 5705e5711
Fig. 6. UVevis spectrum of P2-OMe (b) in CHCl3 with addition of TFA.
Fig. 5. UVevis spectra of polycations in chloroform/TFA solution.
polycations. The UVevis spectra of polymers were taken at different
intervals of time after addition of trifluoroacetic acid. Once the
cation is generated as indicated by the dark green color change, the
observed absorption maxima of the conjugated polycations did not
show any variation with time or amount of added trifluoroacetic
acid. As an example variation in absorption spectra of P2-OMe(b) in
chloroform solution (5 mg/mL) upon addition of varying amount of
trifluoroacetic acid is shown in Fig. 6. It can be seen that the
intensity of long wavelength absorption band of polycations
high enough to get satisfactory data by 13C and 1He13C HSQC NMR
spectra to obtain 1He13C connectivity in the polycations. The
300 MHz 1He1H 2D COSY spectra of P2-OMe (b) and P2(b) is given
in Supporting information (Fig.S3 and Fig.S4)
The acid catalyzed removal of methoxy group at the C-9 position
fluorene units is facilitated by the increase in stability of cations
through extensive conjugation along the backbone of the polymer
and also along the alkylfluorene sidechain. This is also accompanied
by a red shift in absorption maximum for the polycations compared
to their methoxy form. For comparison of the photophysical
properties of the polymers UVevis spectrum of monomer 4 as
a model compound was taken on treatment with trifluoroacetic
acid (TFA) which is depicted as M4 in Fig. 5 along with UVevis
spectra of polycations (P1eP5). The photophysical properties of
conjugated cations are summarized in Table 3.
The absorption spectra of polycations (P1eP3) reveal two new
long wavelength absorption bands around 480 nm and 680 nm
along with strong peaks around 320e380 nm, while the spectra of
P4 and P5 exhibit long wavelength absorption bands between 580
and 610 nm along with strong peaks around 325 nm. These values
are in good agreement with the electronic absorption spectrum of
fluorenyl cation generated from the monomer 4 which is having
long wavelength absorption bands at 468 and 693 nm and another
strong absorption band at 315 nm. Formation of cation at C-9
increases with addition of TFA, but the absorption maxima (lmax
)
remains almost same. Similar results have been reported in litera-
ture [38,39]. The observed time independent absorption maxima of
polycations suggest complete conversion of precursor polymers to
the cationic form instantaneously with extended conjugation and
planarity. Also, the polycations P2 (a) and P2 (b) generated from
precursor copolymers made respectively by Yamamoto and Suzuki
polycondensation did not show any significant difference in optical
properties.
3.3. Stability studies
Applications of polycations are possible only if they are stable
enough in the working condition of device. We have conducted
preliminary studies on the stability of the polyconjugated cations in
solution by monitoring the intensity of absorption in UVevis
spectra. The polycations are stable in air at room temperature for
a time period of 24 h as indicated by their color stability and
intensity of absorption. After two weeks, the color of the polymer
solutions kept in air faded, along with a slight decrease in intensity
of the long wavelength absorption band. But polycation solutions
kept under nitrogen atmosphere did not show any such variation in
color and intensity of absorption. Thus, the polycations show
enhanced stability in solution under nitrogen as evidenced by the
reproducibility of absorption spectra over a two week period. It is
also observed that the phenomenon of cation formation is revers-
ible and upon addition of methanol, the polycations recover their
initial color.
position enhances the
p conjugation compared to the methoxy
form, resulting in a red shift in absorption maxima for the cationic
polymers. As a result of cation formation the electronic structure of
the conjugated polymer is modified which resulted in new elec-
tronic transitions within the band structure of the conjugated
polycation to give additional bands in the absorption spectra of the
Table 3
Absorption data for conjugated polycations.
Polymer/Model
l1 (nm)
l2 (nm)
lmax (nm)
M4
P1
P2(a)
P2(b)
P3
315
357
323
349
384
324
325
468
478
481
481
480
693
690
679
682
687
587
606
3.4. Effect of cation density on solubility and optical properties of
conjugated polycations
The trifluoroacetate salt P1 shows low solubility in common
organic solvents. Copolymers with varying cation density along the
polymer chain were synthesized to study its effect on the solubility
P4
P5

R. Scaria et al. / Polymer 51 (2010) 5705e5711
5711
of the conjugated cationic salt. The polymers with relatively low
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Acknowledgements
The authors acknowledge the Department of Science and
Technology, New Delhi, India and the Max Planck Society, Germany
for generous financial support of this work.
Appendix. Supporting information
Supporting information associated with this article can be
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