Optical and Electrical Properties of π-Conjugated Polymers Based on Electron-Rich 3,6-Dimethoxy-9,9-dihexylfluorene Unit

Article abstract of DOI:10.1021/ma035064j

A new family of π-conjugated polymers has been developed using 3,6-dimethoxy-9,9-dihexylfluorene as an electron-rich unit. These electroactive and photoactive polymers have been prepared from nickel(0)-mediated coupling or by palladium-catalyzed Suzuki co

Full text of DOI:10.1021/ma035064j

8986
Macromolecules 2003, 36, 8986-8991
Optical and Electrical Properties of π-Conjugated Polymers Based on
Electron-Rich 3,6-Dimethoxy-9,9-dihexylfluorene Unit
Ser ge Bea u p r e´ a n d Ma r io Lecler c*
Canada Research Chair in Polymer Chemistry, De´partement de Chimie, Universite´ Laval, Quebec City,
Quebec, Canada G1K 7P4
Received J uly 23, 2003; Revised Manuscript Received September 15, 2003
ABSTRACT: A new family of π-conjugated polymers has been developed using 3,6-dimethoxy-9,9-
dihexylfluorene as an electron-rich unit. These electroactive and photoactive polymers have been prepared
from nickel(0)-mediated coupling or by palladium-catalyzed Suzuki coupling. These new 3,6-dimethoxy-
9,9-dihexylfluorene-based copolymers have demonstrated emission spanning the entire visible spectrum.
Moreover, all of these polymers show reversible electroactivity upon reduction and oxidation, and as
expected, the presence of methoxy groups onto the fluorene moiety increases the ionization potential of
the resulting copolymers by about 0.2 eV when compared with some 9,9-dihexylfluorene-based copolymers.
The reduction of the energy barrier for the injection of holes in related polymeric light-emitting devices
should contribute to the enhancement of their performances.
UV-vis absorption spectra were recorded on a Hewlett-
Packard diode-array spectrophotometer (model 8452A), and
fluorescence measurements were carried out with a Carry
Eclipse spectrometer. For solution fluorescence analyses, the
polymer concentration was about 10-6 M, giving an absorbance
always less than 0.06 to avoid any inner filter effect. The
quantum yield of fluorescence was determined by the relative
method by comparing the areas of the fluorescence spectra of
the standard and the sample. As reference substances, we used
9,10-diphenylanthracene (φF ) 0.90 ( 0.02; cyclohexane),19
PQC10 (φF ) 0.11; chloroform),20 2,2-bipyridylruthenium
chloride (φF ) 0.28; water),21 and rhodamine 101 (0.89 ( 0.02;
acidified ethanol).22 For the fluorescence spectrum in the solid
state, a thin film of polymer was applied on a triangular cell.
The cell was put at 45° of the incident beam.
In tr od u ction
Since the discovery of polymeric light-emitting diodes
based on poly(p-phenylenevinylene) by the Cambridge
group,1 the search for new photoactive and electroactive
conjugated polymers has expanded rapidly, including
new poly(phenylenevinylene)s,1-3 poly(p-phenylene)s,4,5
poly(thiophene)s,6 poly(2,7-(N-alkylcarbazole))s,7,8 and
poly(fluorene)s.9-15 Among these polymers, polyfluorene
derivatives have revealed particularly interesting opti-
cal and electrical properties and emit colors spanning
the entire visible range. It is known that an optimized
light-emitting diode requires efficient and balanced
charge injection, good and comparable mobilities for
both holes and electrons, and high luminescence quan-
tum yield. One of the drawbacks for light-emitting
diodes based on polyfluorene derivatives is the injection
of holes from the anode due to the low ionization
potential (IP) of most derivatives. The control of the
ionization potential becomes important for the optimi-
zation of PLED’s made from this class of polymers.
Many ways have been used to modulate the ionization
potential (IP), electron affinity (EA), and band gap of
polyfluorenes derivatives.13,15-18 We report here a new
synthetic strategy to modulate the IP of polyfluorenes
derivatives by using two new monomers: 2,7-dibromo-
3,6-dimethoxy-9,9-dihexylfluorene and 2,7-bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)-3,6-dimethoxy-9,9-
dihexylfluorene. Here, we report the synthesis, the
optical properties, and the electrochemical properties of
a series of well-defined copolymers having 3,6-dimethoxy-
9,9-dihexylfluorene moiety.
Thermogravimetric analysis (TGA) was performed on a
Mettler Toledo TG50 under heating rate of 20 °C/min and a
nitrogen flow rate of 200 mL/min. Differential scanning
calorimetry (DSC) was performed on Perkin-Elmer DSC7
module in conjunction with the Perkin-Elmer thermal analysis
controller TAC7/DX at a heating rate of 20 °C/min.
Cyclic voltammetry was carried out in a two-compartment
cell with a Solartron potentiostat (model 1287) with platinum
electrodes at a scan rate of 50 mV/s against an Ag/Ag+ (0.1 M
AgNO3 in acetonitrile) reference electrode in an anhydrous and
argon-saturated solution of 0.1 M Bu4NBF4 in acetonitrile.
Syn th esis. 3,6-Dimethoxy-9-fluorenone (1).23,24 2,5′′′′-Diiodo-
3,3′,4′′′,3′′′′-tetramethyl-3′′,4′′-dihexyl-2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′-
quinquethiophene-1′′,1′′dioxide,15,21 2,5-bis(octyloxy)-1,4-ben-
zenedicarboxaldehyde,25 2,7-bis(4,4,5,5-tetramethyl-1,3,2-di-
oxaborolan-2-yl)-9,9-dihexylfluorene26 (monomer A), and tet-
rakis(triphenylphosphine)palladium(0)27 were synthesized fol-
lowing procedures reported in the literature. 1,4-Dibromoben-
zene, 2,5-dibromothiophene, and 2-thiopheneacetonitrile were
purchased from Aldrich and used without further purification.
Acetonitrile (Aldrich) was dried over CaH2 under an inert
atmosphere (nitrogen) for 24 h. Tetrabutylammonium tet-
rafluoroborate (Bu4NBF4) was recrystallized three times in a
50/50 mixture of methanol/water and dried at 100 °C under
reduced pressure, as reported in the literature.28
Exp er im en ta l P a r t
Gen er a l. 1H and 13C NMR spectra were recorded on a
Bruker AMX300 in deuterated chloroform solutions at 298 K.
Number-average (Mn) and weight-average (Mw) molecular
weight were determined by size exclusion chromatography
(SEC) with HPLC Pump using a Waters 515 differential
refractometer. The calibration curve was made with a series
of monodispersed polystyrene standards in THF (HPLC grade,
Aldrich).
3,6-Dimethoxyfluorene (2). In a 250 mL round-bottom flask,
3,6-dimethoxy-9-fluorenone (10.0 g, 41.6 mmol) was dissolved
in dichloromethane (165 mL) and cooled to 0 °C. The solution
was treated under stirring with TiCl4, which was added by
syringe through a septum. A solution of dimethylamine-
borane complex (4.90 g, 83.2 mmol) in dichloromethane (21
mL) was added to the cold solution, and the mixture was
* To whom correspondence should be addressed.
10.1021/ma035064j CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/04/2003

Macromolecules, Vol. 36, No. 24, 2003
π-Conjugated Polymers 8987
allowed to warm to room temperature. Stirring was continued
for 30 min. Thereafter, the solution was poured in 200 mL of
aqueous 1 N HCl for the destruction of excess reductant and
hydrolysis. Phases were separated, and the aqueous layer was
extracted three times with dichloromethane. The combined
extracts were washed with brine and dried with sodium
sulfate. The solvent was removed under reduced pressure, and
the crude product was purified by recrystallization from
ethanol to afford the title product as white needles (7.05 g,
75%); mp 89-91 °C. 13C NMR (75 MHz, CDCl3, ppm): 159.22;
2-Thiophene-(1-cyano)vinylene-2,5-dioctyloxy)-1,4-phenylene-
(2-cyano)vinylene-5-thiophene (5). In a 100 mL round-bottom
flask, 50 mL of methanol is added to a mixture of 1.75 g of
2,5-bis(octyloxy)-1,4-benzenedicarboxaldehyde and 1.15 mL of
2-thiopheneacetonitrile (2.4 equiv). Once the monomers were
in solution, potassium tert-butoxide was added in catalytic
amount. The mixture was stirred for 3 h at room temperature.
The solid was filtered and washed with a large amount of
methanol to afford the title product as an orange powder (85%);
mp 120-122 °C. 13C NMR (75 MHz, CDCl3, ppm): 151.51;
139.92; 133.28; 128.16; 127.21; 126.46; 125.44; 117.23; 110.88;
106.01; 69.40; 31.80; 29.37; 29.29; 29.22; 26.22; 22.70; 14.12.
1H NMR (300 MHz, CDCl3, ppm): 7.87 (s, 2H); 7.83 (s, 2H);
7.39 (d, 2H, J ) 3.5 Hz); 7.32 (d, 2H, J ) 5.1 Hz); 7.08 (m,
2H); 4.10 (t, 4H, J ) 6.2 Hz); 1.84 (qt, 4H, J ) 6.7 Hz); 1.53
(m, 4H); 1.37-1.26 (m, 16H); 0.88 (t, 6H, J ) 6.2 Hz). HRMS:
Calculated for C36H44N2O2S2: 600.2844. Found: 600.2840.
1
142.99; 136.52; 125.59; 113.37; 104.85; 55.58; 35.43. H NMR
(300 MHz, CDCl3, ppm): 7.42 (d, 2H, J ) 8.2 Hz); 7.29 (d, 2H,
J ) 2.2 Hz); 6.89 (dd, 2H, J ) 2.2 and 8.2 Hz); 3.91 (s, 6H);
3.76 (s, 2H). HRMS: Calculated for C15H14O2: 226.0994.
Found: 226.0998.
3,6-Dimethoxy-9,9-dihexylfluorene (3). To a solution of 3,6-
dimethoxyfluorene (10.0 g 44.2 mmol) in anhydrous THF (220
mL) at -78 °C, n-butyllithium (38.0 mL; 95 mmol; 2.5 M in
hexane) was added dropwise. The mixture was stirred at -78
°C for 45 min, and 1-bromohexane (14.9 mL, 106.1 mmol) was
added dropwise to the mixture. The solution was allowed to
warm to room temperature and stirred for 5 h. The mixture
was poured into water and extracted several times with diethyl
ether. The organic extracts were washed with brine and dried
over magnesium sulfate. The solvent was removed under
reduced pressure, and the excess of 1-bromohexane was
removed by distillation under reduced pressure to afford the
3,6-dimethoxy-9,9-dihexylfluorene as a viscous oil (16.3 g,
93%). 13C NMR (75 MHz, CDCl3, ppm): 159.20; 143.89; 142.28;
123.53; 113.49; 104.78; 55.57; 53.86; 40.62; 31.71; 29.91; 23.93;
2-Bromothiophene-(1-cyano)vinylene-2,5-dioctyloxy)-1,4-phen-
ylene-(2-cyano)vinylene-5-bromothiophene (6). In a 100 mL
round-bottom flask, 40 mL of DMF was added to monomer 5.
The flask was put in the dark, and then 724 mg (2.1 equiv) of
NBS was added in one step. The solution was warmed to 80
°C and stirred for 24 h. The mixture was allowed to cool to
room temperature and poured in 30 mL of water. The red solid
was filtered and washed with methanol. The crude product
was purified by recrystallization from chloroform to afford the
1
title product as red needles (73%); mp 162-164 °C. H NMR
(300 MHz, CDCl3, ppm): 7.77 (s, 2H); 7.71 (s, 2H); 7.12 (d,
2H, J ) 3.9 Hz); 7.02 (d, 2H, J ) 4.0 Hz); 4.07 (t, 4H, J ) 6.2
Hz); 1.84 (qt, 4H, J ) 6.7 Hz); 1.53 (m, 4H); 1.37-1.26 (m,
16H); 0.88 (t, 6H, J ) 6.2 Hz). HRMS: Calculated for
1
22.74; 14.16. H NMR (300 MHz, CDCl3, ppm): 7.19 (m, 4H);
6.84 (dd, 2H, J ) 2.2 Hz and J ) 6.1 Hz); 3.89 (s, 6H); 1.88
(m, 4H); 1.12-1.03 (m, 12H); 0.75 (t, 6H, J ) 6.7 Hz); 0.63
(m, 4H). HRMS: calculated for C27H38O2: 394.2872. Found:
394.2878.
C
36H42N2O2S2Br2: 756.1054. Found: 756.1064.
Gen er a l P r oced u r e of P olym er iza tion . The synthesis of
all copolymers are shown in Schemes 2-4. Poly(2,7-(3,6-
dimethoxy-9,9-dihexylfluorene) (P 1) was synthesized by nickel-
(0)-mediated polymerization.29 All others copolymers (P 2-P 6)
were synthesized by palladium-catalyzed Suzuki couplings.10,15
Here is a typical procedure for the synthesis of the copolymers.
To a mixture of carefully purified 2,7-bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-3,6-dimethoxy-9,9-dihexylfluorene (mon-
omer B) or 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
9,9-dihexylfluorene (monomer A), dihalide compound (1.00
equiv), and freshly prepared Pd(0)(PPh3)4 was added a de-
gassed mixture of THF ([monomer ) 0.25 M]) and aqueous 2
M K2CO3 (3/2 in volume). The mixture was vigorously stirred
at 85-90 °C for 48-72 h. The whole mixture was then poured
slowly into cold methanol. The precipitate material was
collected by filtration and washed with methanol. The solid
was washed for 24 h in a Soxhlet apparatus using acetone to
remove oligomers and catalyst residues. The resulting poly-
mers were soluble in THF and CHCl3.
2,7-Dibromo-3,6-dimethoxy-9,9-dihexylfluorene (4). To a so-
lution of 3,6-dimethoxy-9,9-dihexylfluorene (16.3 g, 41.3 mmol)
in chloroform (65 mL) at 0 °C, ferric chloride (5% mol) and
bromine (4.4 mL, 86 mmol) were added. The solution was
allowed to warm to room temperature and stirred for 24 h.
The resulting slurry was poured into water and washed several
times with a sodium thiosulfate aqueous solution. The aqueous
layers were extracted with chloroform, and the organic layers
were combined and dried over magnesium sulfate. The solvent
was removed under reduced pressure, and the crude product
was purified by recrystallization from a mixture of ethanol/
tetrahydrofuran to afford the title product as white needles
(15.0 g, 66%); mp 171-173 °C. 13C NMR (75 MHz, CDCl3,
ppm): 155.35; 144.58; 140.81; 127.70; 111.28; 103.28; 56.61;
1
54.63; 40.29; 31.67; 29.73; 23.83; 22.72; 14.14. H NMR (300
MHz, CDCl3, ppm): 7.44 (s, 2H); 7.18 (s, 2H); 4.00 (s, 6H);
1.86 (m, 4H); 1.15-1.04 (m, 12H); 0.77 (t, 6H, J ) 6.6 Hz);
0.56 (m, 4H). HRMS: calculated for C27H36O2Br2: 550.1082.
Found: 550.1076.
Poly(2,7-(3,6-dimethoxy-9,9-dihexylfluorene)) (P 1). 1H NMR
(300 MHz, CDCl3, ppm): 7.37 (m, 4H); 3.94 (s, 6H); 1.98 (m,
4H); 1.17 (m,12H); 0.97 (m, 4H); 0.82 (m, 6H).
2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-3,6-di-
methoxy-9,9-dihexylfluorene (Monomer B). To a solution of 2,7-
dibromo-3,6-dimethoxy-9,9-dihexylfluorene (5.00 g, 9.1 mmol)
in anhydrous THF (90 mL) at -78 °C was added TMEDA (2.8
mL, 19.1 mmol) and n-butyllithium (7.6 mL, 19.1 mmol, 2.5
m in hexane). The mixture was stirred at -78 °C for 45 min.
2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (5.5 mL,
27.2 mmol) was added rapidly to the solution. The mixture
was allowed to warm to room temperature and stirred over-
night. The mixture was poured into water and extracted with
diethyl ether. The organic extracts were combined and washed
with brine. The solvent was removed under reduced pressure,
and the crude product was purified by recrystallization from
an EtOH/THF mixture to afford the title as white powder (2.26
g, 39%); mp 165-166 °C. 13C NMR (75 MHz, CDCl3, ppm):
164.04; 144.98; 143.46; 130.43; 102.05; 83.32; 56.14; 53.90;
Poly[2,7-(3,6-dimethoxy-9,9-dihexylfluorene)-co-alt-2,7-(9,9-
dihexylfluorene)] (P 2). 1H NMR (300 MHz, CDCl3, ppm): 7.83
(m, 2H); 7.69 (m, 2H); 7.60 (m, 2H); 7.39 (m, 4H); 4.01 (s, 6H);
2.06 (m, 8H); 1.18 (m, 24H); 0.85 (m, 20H).
Poly[2,7-(3,6-dimethoxy-9,9-dihexylfluorene)-co-alt-2,5-thio-
1
phene] (P 3). H NMR (300 MHz, CDCl3, ppm): 7.62 (m, 4H);
7.31 (m, 2H); 4.11 (s, 6H); 2.01 (m, 4H); 1.10 (m, 12H); 0.79
(m, 10H).
Poly[2,7-(3,6-dimethoxy-9,9-dihexylfluorene)-co-alt-p-phen-
1
ylene] (P 4). H NMR (300 MHz, CDCl3, ppm): 7.75 (m, 4H);
7.38 (m, 4H); 4.03 (s, 6H); 1.98 (m, 4H); 1.14 (m, 12H); 0.83
(m, 10H).
Poly[2,7-(3,6-dimethoxy-9,9-dihexylfluorene)-co-alt-2,5-(3,3′,-
4′′′,3′′′′-tetramethyl-3′′,4′′-dihexyl-2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′-quin-
1
1
39.96; 31.46; 29.59; 24.85; 23.56; 22.52; 14.03. H NMR (300
quethiophene-1′′,1′′dioxide) (P 5). H NMR (300 MHz, CDCl3,
MHz, CDCl3, ppm): 7.54 (s, 2H); 7.15 (s, 2H); 3.93 (s, 6H);
1,91 (m, 4H); 1.38 (s, 24H); 1.10-1.01 (m, 12H); 0.75 (t, 6H,
J ) 6.6 Hz); 0.58 (m, 4H). HRMS: Calculated for C39H60B2O6:
646.4576. Found: 646.4565.
ppm): 7.66 (m, 2H); 7.61 (m, 2H); 7.47 (m, 2H); 7.29 (m, 2H);
4.09 (m, 6H); 2.70 (m, 4H); 2.33 (m, 12H); 2.00 (m, 4H); 1.62-
1.56 (m, 8H); 1.36 (m, 8H); 1.08 (m, 12H); 0.92 (m, 6H); 0.78-
0.69 (m, 10H).

8988 Beaupre´ and Leclerc
Macromolecules, Vol. 36, No. 24, 2003
Sch em e 1. Syn th esis of 3,6-Dim eth oxy-9,9-d ih exylflu or en e Un itsa
a
Reagents and conditions: (i) TiCl4-dimethylamine-borane complex/CH2Cl2; (ii) n-BuLi/1-bromohexane/THF; (iii) Br2/FeCl3/
CHCl3; (iv) n-BuLi/2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane/THF.
Ta ble 1. Molecu la r Weigh ts a n d Th er m a l An a lysis Da ta
of P 1-P 6
Sch em e 2. Syn th esis of P olym er s P 1 a n d P 2
polymer
Mn
Mw Mw/Mn
Tg (°C)
Td (°C)
P 1
P 2
P 3
P 4
P 5
P 6
5600
41 200
2400
2700
2500
9200
176 000
3100
3700
3600
41 000
1.6
4.3
1.3
1.4
1.4
3.0
130
128
65
60
111
407
412
399
224
260
388
13 300
rene, was obtained by treating 4 with n-BuLi/2-isopro-
poxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. 2-Bromo-
thiophene-(1-cyano)vinylene-2,5-dioctyloxy)-1,4-phen-
ylene-(2-cyano)vinylene-5-bromothiophene (6) was pre-
pared in good yield from 2-thiopheneacetonitrile and 2,5-
bis(octyloxy)-1,4-benzenedicarboxaldehyde through a
Knoevanegel reaction followed by direct bromination
with NBS. Monomer A was synthesized using an
already published procedure.26
Syn th esis a n d Ch a r a cter iza tion of P olym er s. All
copolymers were prepared in good yields (45-75%, see
Schemes 2-4) and are highly soluble in most common
organic solvents such as chloroform, tetrahydrofuran,
and toluene. The number-average molecular weight (Mn)
and weight-average molecular weigth (Mw) of the co-
polymers ranged from 2400 to 41 200 and 3100 to
176 000, respectively, with a polydispersity indexes
ranging from 1.3 to 4.3. These data are summarized in
Table 1. We have observed that the copolymers (P 3-
P 6) obtained from monomer B present lower molecular
weight than the one obtained by monomer A. We can
attribute this effect to a lower reactivity of the monomer
B due to the fluorene ring bearing two electron-donating
groups. It seems that polymerization via Suzuki cou-
plings of some electron-rich monomers tend to give low
molecular weight polymers.33
The thermal stability of the polymers was evaluated
by thermogravimetric analyses (TGA) under a nitrogen
atmosphere. The data are summarized in Table 1. All
copolymers, except P 4 and P 5, exhibit very good ther-
mal stabilities up to 350-400 °C. Differential scanning
calorimetry (DSC) measurements were also performed
on all copolymers. The DSC data are also reported in
Table 1. It is interesting to note that P 1, P 2, and P 5
present a glass transition temperature higher than poly-
(2,7-(9,9-dihexylfluorene), a well-studied polymer.34 More-
over, it is worth noting that all these polymeric mate-
rials are amorphous, which should lead to a good
mechanical stability up to 100 °C. This parameter may
become particularly important for their future utiliza-
Poly[2,7-(3,6-dimethoxy-9,9-dihexylfluorene)-co-alt-2,5-bis(2-
thienyl-2-cyanovinyl)-1-(2′-octyloxy)-5′-octyloxybenzene) (P 6).
1H NMR (300 MHz, CDCl3, ppm): 7.95 (m, 4H); 7.60 (m, 2H);
7.43 (m, 4H); 7.31 (m, 2H); 4.14 (m, 10H); 1.92 (m, 8H); 1.59
(m, 4H); 1.37-1.10 (m, 24H); 0.85-0.78 (m, 20H).
Resu lts a n d Discu ssion
Syn th esis of Mon om er s. The synthesis of the fluo-
rene derivatives is shown in Scheme 1. 3,6-Dimethoxy-
9-fluorenone (1) was prepared in a good yield following
a multiple-step synthesis.23,24 In our first attempts to
obtain 3,6-dimethoxyfluorene (2), an 1,2-ethanedithiol/
FeCl3-SiO2/Raney nickel30,31 treatment was utilized,
but low yields were obtained. We found an efficient one-
step reaction to obtain, in a high yield, compound 2: 3,6-
dimethoxy-9-fluorenone is treated with titanium tetra-
chloride in the presence of a dimethylamine-borane
complex.32 This mild and fast deoxygenation reaction
is useful to reduce diaryl ketone to methylene in the
presence of methoxy groups and is more efficient than
other reduction reactions such as Wolf-Kishner or
Clemmensen reaction. This direct reduction of (1) af-
forded 3,6-dimethoxyfluorene (2) in 75% yield. Alkyla-
tion of 2 with n-BuLi/Br-CH2(CH2)4CH3 afforded com-
pound 3 in 93% yield. Direct bromination of 3,6-
dimethoxy-9,9-dihexylfluorene (3) with bromine and
catalytic amount of ferric chloride afforded monomer 4
in 66% yield. Monomer B, 2,7-bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-3,6-dimethoxy-9,9-dihexylfluo-

Macromolecules, Vol. 36, No. 24, 2003
π-Conjugated Polymers 8989
Sch em e 3. Syn th esis of Cop olym er s fr om Su zu k i Cr oss-Cou p lin g Rea ction
Sch em e 4. P olym er s fr om 3,6-Dim eth oxy-9,9-d ih exylflu or en e Un it
Ta ble 2. Op tica l Da ta a n d th e F lu or escen ce Qu a n tu m Yield
absorption λmax (nm)a fluorescence λmax (nm)a
solution film solution film
PL efficiency solution
polymer
φF
P 1
P 2
P 3
P 4
P 5
P 6
374
388
444
374
482 (388)
538
366
380
446
366
478 (392)
563 (601)
412 (436)
420 (445)
498 (530)
415 (438)
645 (702)
616
430
434
510
424
662
656 (702)
0.48
0.93
0.30
0.85
0.21
0.32
a
The data in parentheses are the wavelengths of shoulders and subpeaks.
tion in light-emitting diodes. For P 3 and P 4, the low
glass transition temperature observed is mainly due to
the low molecular weight of the copolymers. In the case
of P 6, no glass transition temperature has been ob-
served.
Op tica l P r op er ties. Obviously, UV-vis absorption
and photoluminescence properties of these polymers
have been investigated and are summarized in Table
2. All spectroscopic properties were measured both in
solution (chloroform) and as thin film. Absorption and
fluorescence spectra of all copolymers in solution are
shown in Figure 1 and Figure 2. It is important to note
that all polymers were fluorescent both in solution and
as thin film. Furthermore, we have been able to
modulate the maximum emission wavelength in order
to obtain the three primary colors (blue, green, and red).
Moreover, it is interesting to compare the absorption
and emission spectra of P 1, P 3, and P 4 with their

8990 Beaupre´ and Leclerc
Macromolecules, Vol. 36, No. 24, 2003
F igu r e 1. UV-vis absorption spectra of P 1-P 6 in solution.
F igu r e 3. Cyclic voltammograms of P 1-P 6.
This red shift can be attributed to a stronger push-
pull effect between the fluorene ring and the thiophene
ring, or it may also be explained by a more planar
conformation of the ground state of P 3 due to an
anomeric effect between the methoxy group and the
hydrogen atom of the thiophene ring. However, there
is no difference between P 3 and PTF in their emission
spectra. We can suppose that these two polymers adopt
the same molecular geometry in the excited state, and
there is no significant effect from the methoxy groups
on the fluorene moiety. However, the quantum yield of
fluorescence for P 3 is lower than for PFT, 0.30 for the
former and 0.49 for the latter. Finally, Liu et al.34 have
reported for poly(2,7-(9,9-dihexylfluorene)-alt-(1,4-phe-
nylene) (PFP) an absorption maximum at 370 nm and
an emission maximum at 408 (431) nm. We observed
almost the same results for P 4 that is 374 nm for the
absorption maximum and 415 (438) nm for the emission
maximum. Unlike P 1 and P 3, the fluorescence quantum
yield is not affected by the methoxy group and φF
remains quite high at 0.85.
Electr och em ica l P r op er ties. As shown in Figure
3 and summarized in Table 3, all copolymers synthe-
sized from 3,6-dimethoxy-9,9-dihexylfluorene exhibit
quasi-reversible oxidation and reduction electrochemical
processes. P 1-P 6 show good reversibility during the
p-doping process as well as in the n-doping process; this
is an advantage for polymeric light-emitting devices.
The goal of this work was to investigate the influence
of the substitution of the fluorene ring with two electron-
donating groups on the ionization potential (IP). We
have been able to estimate, by using a well-established
electrochemical method,15 the HOMO-LUMO energy
levels, and the results are summarized in Table 3. To
appreciate the modulation of the ionization potential,
we will compare P 1, P 3, and P 4 with similar copolymers
F igu r e 2. Fluorescence spectra of P 1-P 6 in solution.
similar copolymers but that are not bearing methoxy
groups at the 3,6-positions. For instance, Liu et al.34
have reported for poly(2,7-(9,9-dihexylfluorene) (PDHF)
an absorption maximum at 379 nm and an emission
maximum at 415 (438, 469) nm. For P 1 we have
obtained almost the same valuess374 nm for the
absorption maximum and 412 (436) nm for the emission
maximum. In this case, the substitution by methoxy
groups at the 3,6-positions of the fluorene ring does not
affect the absorption or emission spectra. However, a
decrease of the quantum yield of fluorescence (φF) from
0.82 for PDHF to 0.48 for P 1 observed. On the other
hand, Ranger and Leclerc35 have reported for poly(2,7-
(9,9-dioctylfluorene)-alt-(2,5-thiophene) (PFT) an ab-
sorption maximum at 431 nm and an emission maxi-
mum at 496 nm, in solution. We report a red shift of 15
nm between P 3 and PFT for the absorption maximum.
Ta ble 3. Electr och em ica l P r op er ties of P 1-P 6
n-doping (V vs SCE)a
p-doping (V vs SCE)a
energy levels (eV)
LUMO
b
polymer
Eonset
Epc
Epa
Eonset
Epa
Epc
HOMO
Eg
P 1
P 2
P 3
P 4
P 5
P 6
-2.02
-2.10
-1.87
-2.17
-1.31
-1.08
-2.47
-2.29
-2.11
-2.44
-1.51
-1.42
-2.21
-2.11
-2.11
-2.23
-1.27
-1.27
1.25
1.28
0.81
1.16
0.88
0.92
1.49
1.48
1.03
1.42
1.04
1.13
1.24
1.34
0.93
1.21
0.78
1.00
5.65
5.67
5.21
5.56
5.28
5.32
2.38
2.30
2.53
2.23
3.09
3.32
3.27
3.37
2.68
3.33
2.19
2.00
a
b
Epa and Epc stand for anodic peak potential and cathodic peak potential. Eg stands for the band gap energy.

Macromolecules, Vol. 36, No. 24, 2003
π-Conjugated Polymers 8991
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Con clu sion
Six copolymers containing the 3,6-dimethoxy-9,9-
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mers exhibit good thermal stability and relatively high
glass transition temperature. P 1, P 2, and P 4 emit blue
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Ack n ow led gm en t. This work was supported by
operating and strategic grants from the Natural Sci-
ences and Engineering Research Council of Canada
(NSERC). We thank J ean-Franc¸ois Morin for the syn-
thesis of 2-thiophene-(1-cyano)vinylene-2,5-dioctyloxy)-
1,4-phenylene-(2-cyano)vinylene-5-thiophene and 2-
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Refer en ces a n d Notes
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MA035064J