Influence from thermal elimination temperature of precursor polymer and film-forming methods on the photophysics of the poly(2,5-didodecyloxy-p- phenylenevinylene)

Article abstract of DOI:10.1002/cjoc.201180377

A series of poly(p-phenylenevinylene)s (PPVs) with good solubility were synthesized from thermal elimination of precursor poly(2,5-didodecyloxy-p- phenylenevinylene) at different temperature via Wessling method. The polymer photophysics were influenced by

Full text of DOI:10.1002/cjoc.201180377

FULL PAPER
Influence from Thermal Elimination Temperature of Precursor
Polymer and Film-forming Methods on the Photophysics of
the Poly(2,5-didodecyloxy-p-phenylenevinylene)
Wang, Shangli(王尚丽)
Zhao, Wei(赵伟)
Xu, Xiaoqian(许晓前)
Cheng, Si(程丝)
Fan, Lijuan*(范丽娟)
Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science
and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou,
Jiangsu 215123, China
A series of poly(p-phenylenevinylene)s (PPVs) with good solubility were synthesized from thermal elimination
of precursor poly(2,5-didodecyloxy-p-phenylenevinylene) at different temperature via Wessling method. The poly-
mer photophysics were influenced by the thermal elimination condition, which was consistent with NMR and IR
characterizations. The additional absorption peak at longer wavelength and the red-shifted emission maximum both
in solution and in film, for PPVs obtained at high elimination temperature, indicated the existence of longer conju-
gated blocks in these systems. The emission maximum for drop-cast film (436 nm) for PPV obtained under 200 ℃
(PPV200) was 16 nm blue shifted to the spin-coated films (452 nm) or 29 nm to the solution (465 nm). The SEM
study showed drop-cast film had the morphology of isolated conjugated particles in the matrix while blurry linear
structure was found for spin-coated film, which was consistent with the photophysics. The discussion about this
difference was carried out based on the consideration of the flexibility of the polymer chains and different conju-
gated length of PPV in different states.
Keywords poly(p-phenylenevinylene), synthetic methods, photophysics, polymer precursor, morphology
solvent-soluble PPV and the thermal elimination tem-
Introduction
perature. The thermal elimination condition had been
tracked in several other PPV systems17-20 by some
in-situ techniques, such as FTIR, UV-vis, TGA, etc.,
directly in film or in solution where the reaction took
place. These in-situ techniques gave clear pictures of the
gradual elimination of sulfonium groups and the forma-
tion of PPV. In this contribution the property studies
were carried out after the thermal elimination in solid in
glass containers and then the products were taken out
for future investigation in solution or in solid, which
was very different from those reported dynamic in-situ
studies and would provide more information for real
application for these PPVs.
Poly(p-phenylenevinylene) (PPV) is one of the most
extensively studied conjugated polymers and has been
widely used in the light emitting device.1-7 There are
several routines, such as Wessling,8 Glich,9,10 Wittig,11
and Heck Coupling12 to prepare PPV. Wessling method
was demonstrated irreplaceable advantages in some re-
spects due to the easiness in handling and characteriza-
tion of the polymer precursor. The photophysical prop-
erties of PPV had been tuned by varying the polymer
structure, such as, assembling electron-withdrawing or
electron-donating side groups onto the backbone,13,14
controlling the different conjugated length by selective
elimination.15,16 However, all these methods required
additional synthetic effort, while the properties of PPV
could be simply controlled by changing thermal elimi-
nation condition of the precursor polymer.17,18 Thus,
here we present our preparation of a poly-(2,5-dido-
decyloxy-p-phenylenevinylene) sulfonium bromide pre-
cursor polymer with a modified method compared to the
literature, and the systematical investigation into the
relationship between the properties of the organic-
Experimental
Materials
Bromododecane (C12H25Br) and tetrahydrothiophene
(THT) were purchased from Alfa Aesar Chemical Co.
N,N-Dimethylformamide (DMF), tetrahydrofuran (THF)
were from Sinopharm Chemical Reagent Co., Ltd. All
materials were of analytical grade and used as received
*
E-mail: ljfan@suda.edu.cn; Tel. and Fax: 0086-512-65880045
Received April 25, 2011; revised and accepted June 1, 2011.
Project supported by the National Natural Science Foundation of China (Nos. 50773049, 20974075) and the Program of Innovative Research Team of So-
ochow University and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
Chin. J. Chem. 2011, 29, 2175— 2181
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Wang et al.
FULL PAPER
unless otherwise noted. Methanol and acetone were
dried over molecular sieve. 1,4-bis(dodecyloxy)benzene
was synthesized by an adapted literature procedure.15
flask. The system was deoxygenated with three cycles
of vacuum-argon cycling. Tetrahydrothiophene (2.000 g,
23.00 mmol) was added into the system. The mixture
was stirred at 55 ℃ for 22 h under argon atmosphere.
The solvent was completely removed with rotary
evaporation and the residue was purified with dry ace-
tone. The mixture was then filtered and the resulting
solid was dried in vacuum to give a white solid (1.302 g,
yield 52%). 1H NMR (CDCl3, 400 MHz) δ: 6.85 (s, 2H),
4.53 (s, 4H), 3.94—4.01 (m, 4H), 2.78—2.86 (m, 8H),
1.88—1.97 (m, 8H), 1.75—1.85 (m, 4H), 1.06—1.52
(m, 36H), 0.82—0.90 (m, 6H); IR (KBr) ν: 2917, 1627,
1510, 1475, 1415, 1398, 1322, 1233, 1110, 1029, 865,
General methods
1H NMR spectra were recorded using an Inova 400
MHz NMR spectrometer with CDCl3 as the solvent and
TMS as the internal reference. FTIR spectra of the
monomers, the precursor polymer and the PPV poly-
mers were obtained on a Nicolet 6700 FT-IR spectro-
photometer (thermo). The samples were prepared by
adding monomers, precursor polymer or PPVs into KBr,
and the mixture was ground to a fine power and pressed
to form disks. The elemental analysis was carried out on
a Carlo-Erba EA-1110. The molecular weight was de-
termined on a Waters 1515 gel permeation chromato-
graph with THF as the mobile phase and PS as the
standard. The UV-vis spectra were obtained using a Hi-
tachi U-3900/3900H spectrophotometer. The solution
UV-vis and fluorescence measurements were carried out
－
1
794, 713, 653 cm . Anal. calcd for C40H72S2O2Br2: C
59.40, H 8.91; found C 59.48, H 8.83.
The precursor polymer of poly(2,5- didodecyloxy-
p-phenylenevinylene) (pre-PPV) 1,l'-[((2,5-Didode-
cyloxy)-p-phenylene)bis(methylene)]bis[tetrahydrothio-
phenium]dibromide (1.009 g, 1.23 mmol), and 26.0 mL
of THF were placed into pre-dried 50.0 mL two-necked
round bottom flask. The system was deoxygenated by
three cycles of vacuum-argon cycling and maintained
the temperature at 0 ℃. 2.0 mL of ice-cold, Ar-purged
methanolic NaOH solution (1.25 mol/L) was added in a
slow stream to the system. The mixture was stirred at 0
℃ for 1 h followed adding of 2.0 mL of HCl (1.00
mol/L) into the system. The mixture was then dialyzed
with cellulose membrane dialysis tubing with a molecu-
lar mass cutoff of 3500 Da in water for 12 h, during
which the water was changed for every 2 h. The mixture
inside the membrane was filtered. The precursor poly-
mer was collected as a pale yellow powder and dried in
in THF with a concentration of 1×10－ mol/L with re-
5
spect to the repeating unit of precursor polymer. The
solid UV-vis and fluorescence measurements were car-
ried out with the PPV film on quartz slides, prepared
through drop-casting or spin-coating technique, with a
concentration of 1×10－ 2 mol/L with respect to the re-
peating unit of prepcusor polymer in CHCl3, and 20
mg/mL of PMMA codissolved in CHCl3. The
spin-coating was carried out on a Spin-coater KW-4A
from Chemmat Technology, started with 6 s at the rate
of 400 r/min and followed by 10 s at the rate of 1000
r/min. The drop-casting was carried out at room tem-
perature (around 20 ℃) for 8 min. Morphological stud-
ies of the films were carried out using a Hitachi S-4700
scanning electron microscope (SEM).
1
vacuum overnight. H NMR (CDCl3, 400 MHz) δ:
6.82—6.93 (m, 2H), 4.36—4.47 (m, 1H), 3.91—4.00
(m, 4H), 3.47 (s, 2H), 2.80—2.87 (m, 4H), 1.91—1.97
(m, 4H), 1.71—1.83 (m, 4H), 1.18—1.51 (m, 36H),
0.84—0.92 (m, 6H); IR (KBr) ν: 2921, 1618, 1513,
1467, 1417, 1384, 1307, 1218, 1108, 1041, 952, 896,
Synthesis
1,4-Bis(bromomethyl)-2,5-bis-(dodecyloxy)ben-
zene Formaldehyde (0.145 g, 48.30 mmol), 1,4-
bis(dodecyloxy)benzene (5.100 g, 11.20 mmol), HBr
(17.5 mL, 50 wt% in acetic acid) and 30.0 mL of acetic
acid were placed into a round bottom flask. The mixture
was stirred under room temperature for 30 min and 80
℃ for 3.0 h. Then the solvents were completely re-
moved under reduced pressure. The crude product was
purified by redissolving it in chloroform and reprecipi-
tated out of ethanol to give a white power (5.450 g,
yield 77%). 1H NMR (CDCl3, 400 MHz) δ: 6.85 (s, 2H),
4.53 (s, 4H), 3.93—4.02 (m, 4H), 1.76—1.87 (m, 4H),
1.20—1.52 (m, 36H), 0.84—0.92 (m, 6H); IR (KBr) ν:
2918, 1627, 1510, 1473, 1413, 1313, 1226, 1033, 863,
－
1
865, 825, 721 cm . Anal. calcd for (C36H63SO2Br2)n: C
67.60, H 9.86; found C 68.45, H 10.52.
Thermal elimination
The precursor polymers were heated at 60, 100, 120,
140, 160 and 200 ℃ for 2 h in vacuum to a series of
poly(2,5-didodecyloxy-p-phenylenevinylene)s (PPVs)
with various elimination extent. The PPVs were named
as PPV60, PPV100, PPV120, PPV140, PPV160, and
PPV200 respectively according to the thermal elimina-
tion temperature. All the PPVs were powder in different
shades of yellow.
－
1
719, 690 cm . Anal. calcd for C32H56O2Br2: C 60.76, H
8.86; found C 60.62, H 9.08.
Results and discussion
1,l'-[((2,5-Didodecyloxy)-p-phenylene)bis(methyl-
The PPVs were synthesized based on the Wessling
method as shown in Scheme 1. The precursor polymer
was prepared by a modification of previous literature
ene)]bis[tetrahydrothiophenium]dibromide
1,4-
Bis-(bromomethyl)-2,5-bis(dodecyloxy)benzene (1.950
g, 3.10 mmol), and anhydrous methanol (20.3 mL) were
placed into pre-dried 50.0 mL two-necked round bottom
1
reports.15 The H NMR spectra of the monomers,
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Chin. J. Chem. 2011, 29, 2175— 2181

Influence on the Photophysics of the Poly(2,5-didodecyloxy-p-phenylenevinylene)
Scheme 1
CH2Br
OC12H25
C
12H25Br
HCHO/HBr
HOAC
THT
C12H25
O
OC12H25
HO
OH
C12H25
O
DMF/KOH
MeOH
BrH2C
C12H25
O
C12H25
O
C
12H25O
SBr
S
Br
C12H25
O
SBr
(1) NaOH/MeOH
vacuum
heat
(
(2) HCl
)
OC12H2x5
SBr
n
(3) Dialysis Water
OC12H25
y
OC12H25
OC12H25
n = x + y
1,4-bis(bromomethyl)-2,5-bis(dodecyloxy)benzene, and
1,l'-{[(2,5-didodecyloxy)-p-phenylene]bis(methyl-
ene)}bis[tetrahydrothiophenium] dibromide, were
shown in Figure 1. The peaks around δ 2.00—2.80
demonstrated the successful replacement of the Br on
the benzyl bromide by the tetrahydrothiophenium group.
The reaction condition for the synthesis of the precursor
polymer was optimized. Initially methanol was used as
the solvent for the polymerization. However, it was
proven not the best option since some of the tetrah-
drothiophenium could be substituted by the methoxy
group.16 Thus THF was used as the alternative to re-
duce the side reactions. After the polymerization, the
whole reaction mixture was placed into a cellulose
membrane dialysis tubing with a molecular mass cutoff
of 3500 Da in water for 12 h, during which the water
was changed for every 2 h. The mixture inside the
membrane was filtered and the solid was dried in vac-
uum. Then the thermal elimination was carried out by
placing the precursor polymers in vacuum under various
temperatures in different batches. The resultant poly-
mers, namely PPV60, PPV100, PPV120, PPV140,
PPV160 and PPV200, according to the thermal elimin-
tion temperature, were very soluble in common organic
1
solvents such as THF and chloroform. The H NMR
spectra of the pre-PPV and the PPVs prepared under
different elimination temperatures were shown in Figure
2. Basically, there was negligible change for the peaks
belong to the protons on the alkoxyl side group (around
δ 4.00 and between δ 0.80—1.80) and the protons on the
bezenze ring (between δ 6.75—7.00). The disappearing
of the peaks for the protons on the tetrahydrothiophe-
nium (δ 2.80 and 2.00) in the PPVs was reasonable, as
well as the decrease in the peak for the proton next to
Figure 1 1H NMR spectra of monomers of PPV.
Chin. J. Chem. 2011, 29, 2175— 2181
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Wang et al.
FULL PAPER
the saturated carbon on the polymer backbone (δ 4.50
and 3.50) and the emerging of the peak for the vinyl
protons (between δ 6.60 and 6.75) with increasing the
elimination temperature. Interestingly, as least seen
from the NMR spectra, the formation of the double
bond did not come together with the lose of the tetrahy-
drothiophenium group, in the case of using low elimina-
tion temperature. Significant NMR signal for double
bonding formation could only be seen in PPVs obtained
under 160 and 200 ℃. Even in the case of PPV200, the
peaks for the saturated proton still existed, indicating
the backbone was not fully conjugated. However, the
peaks for the PPV200 appeared much broaden than
other PPV, very likely due to the rigidity caused by the
higher conjugation extent. Figure 3 shows the IR spectra
of this series of polymers. A sharp and weak absorption
－
1
around 1680 cm , which could be attributed to the C＝
C stretching vibration, appeared in the case of PPV160
and PPV200, which was consistent with NMR spectra,
indicating the formation of double bonds in main chain.
The molecular weights of the pre-PPV and PPVs were
determined by the GPC. All the polymers had a similar
Mn, between 4.30×105 and 4.70×105, and a PDI
around 1.70. That means there was no significant deg-
radation of the main chain.
Figure 2 1H NMR spectra of (a) pre-PPV; (b) PPV60; (c) PPV100; (d) PPV120; (e) PPV140; (f) PPV160; (g) PPV200.
Figure 3 FTIR spectra (% transmittance) of (a) pre-PPV; (b) PPV60; (c) PPV100; (d) PPV120; (e) PPV140; (f) PPV160; (g) PPV200.
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Influence on the Photophysics of the Poly(2,5-didodecyloxy-p-phenylenevinylene)
The photophysics of PPVs, both in THF solution and
film, was studied. Both the spin-coated and drop-cast
films were prepared from solutions with a relatively low
concentration of PPVs, which was based on the consid-
eration of making the photophysics of film relatively
comparable to the photophysics in dilute solution.
drop-casting, using PMMA as the inert matrix due to its
transparency within the UV-vis range. Basically, the
profiles of absorbance and fluorescent emission spectra
for these polymers in films (Figure 5 and Figure 6) were
similar to those in solution. In the UV-vis spectra of
spin-coated films (Figure 5 top), PPVs obtained below
140 ℃ had exactly same profile as the pre-PPV. The
absorbance at higher wavelength gradually appeared
and increased when the elimination temperature was
140 ℃ and higher. In the emission spectra of
spin-coated film (Figure 5 bottom), pre-PPV, PPV60,
PPV100, PPV120 and PPV140 had very similar profile
while PPV160 showed small red-shifting and broade-
ing and PPV200 displayed even greater red-shifting and
broadening. This phenomenon is consistent with sol-
tion spectra, except the small difference in the case of
PPV160 due to the restricted movement effect in the
solids. In the drop-cast film, the absorbance spectra
were similar to those in solutions or in spin-coated films.
However, the emission of PPV160 and PPV200 showed
less red-shifting and less broadening, which was not
common since the polymer chains in drop-cast film
usually has well-organized structure and more
π-stacking resulted in red-shifting, since the slow
evaporation process allows the reorganization of the
molecules.21 Thus, detailed discussion related to this
abnormality will be carried out in the following para-
graphs.
Figure 4 Normalized UV-vis absorbance (top) and fluorescence
emission (λ ＝357 nm) (bottom) spectra of pre-PPV and PPVs in
ex
THF.
The normalized absorbance and emission spectra of
pre-PPV and PPVs were shown in Figure 4. Basically,
the spectra for PPVs obtained below 140 ℃ were very
similar. Those polymers had one absorption maximum
around 300 nm and double emission peaks around 405
and 427 nm. The absorbance shoulder around 354 nm
gradually appeared in the PPV140, PPV160 and
PPV200. In the emission spectra, PPV200 displayed a
broad emission with a peak around 465 nm and a small
shoulder around 405 nm. However, the PPV140 and
PPV160 still have very similar emission to other PPVs
with very slight emission increase in the longer-wave-
length area. Thus, it is very likely that vinyl formation
on the backbone took place only in the case of using
relatively high elimination temperature. In addition,
high conjugation degree was obtained only in PPV200
which displayed significant red-shift in the emission
maximum. The broad emission of PPV200 also indi-
cated the existence of many conjugated blocks with
various conjugated length.
Figure 5 Normalized UV-vis absorbance (top) and fluorescence
emission (λ ＝357 nm) (bottom) spectra of pre-PPV and PPVs in
ex
The PPV films were prepared by spin-coating and
spin-coated film.
Chin. J. Chem. 2011, 29, 2175— 2181
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Wang et al.
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Figure 6 Normalized UV-vis absorbance (top) and fluorescence
emission (λ ＝357 nm) (bottom) spectra of pre-PPV and PPVs in
ex
Figure 7 Normalized UV-vis absorbance (top) and fluorescence
drop-cast film.
emission (λ ＝357 nm) (bottom) spectra of PPV200 in different
ex
states.
Figure 7 shows the comparison of the absorbance
and emission spectra of PPV 200 in various states. All
the absorbance spectra had the similar profile, with
peaks around 300 and 354 nm. However, there was sig-
nificant difference in the emission, with peak around
465 nm in solution, 452 nm in spin-coated film and 436
nm in drop-cast film. In most conjugated polymer sys-
tems the emission peaks had the reversed sequence
compared to our system. That is, the drop-cast film
would be the most red-shifted one and the solution
would be the most blue-shifted one, which was ex-
plained by the enhanced intermolecular π-stacking, ri-
gidification and enhanced conjugation or well-organized
structure in solid, especially in drop-cast film.22-25 Thus,
the PPVs here were different from other conjugated
polymer systems.
and had long conjugation length, while the PPV200 in
drop-cast film was in a curved state and therefore had
short conjugation length and blue-shifted emission. Ad-
ditional information from these studies was that the
PPV200 was more flexible than other common conju-
gated polymer, which resulted in the easiness of chain
folding instead of π stacking induced aggregation. The
NMR spectrum of PPV200 in Figure 2 also showed that
there existed quite a few relatively flexible saturated
bonding on the polymer backbone together with the
rigid conjugated blocks.
Conclusions
A series of PPVs were synthesized via Wessling
route from the same polymer precursor but under dif-
ferent thermal elimination temperature in vials, using
common vacuum oven. These polymers were demon-
strated to be very soluble in common solvent and ready
for all characterization and future study. NMR and IR
characterization indicated that only PPVs obtained un-
der high thermal elimination temperature, such as 160
and 200 ℃, had significant formation of vinyl bonding
on the polymer backbone. Photophysical study of these
polymers, both in solution or in films, basically was
consistent with the NMR and IR study. The emergence
of new absorption peak at longer wavelength in UV-vis
To further clarify this issue, the morphology study
was carried out on the PPV200 drop-cast film and
spin-coated film (Figure 8). Interestingly, the drop-cast
film showed some isolated domain embedded in the
matrix while the spin-coated film had relatively smooth
feature with some blurry linear structure on the surface.
It was well-known the state of the polymer in spin-
coated film is much closer to its state in solution due to
the quick evaporation of solvent and limited reorganiza-
tion of polymer chains. Correlating the photophysics
with the morphology, one conclusion could be made
that the PPV200 in solution was in an extended state
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Chin. J. Chem. 2011, 29, 2175— 2181

Influence on the Photophysics of the Poly(2,5-didodecyloxy-p-phenylenevinylene)
Figure 8 SEM of PPV200 films prepared by drop-casting (left) and spin-coating (right)
11 Tachelet, W.; Jacobs, S.; Ndayikengurukiye, H.; Geise, H. J.;
spectra and the red-shifting of the emission maximum
were reasonable since longer conjugated blocks existed
for the PPVs obtained under higher elimination tem-
perature. The SEM images of PPV200 films suggested
the existence of saturating part on the backbone resulted
in the relative flexibility of the polymer. The difference
in morphology of drop-cast film and spin-coated film
matched their difference in photophysics, which might
be useful in adjusting the photophysics of these poly-
mers simply by controlling the film-forming process
instead of synthesizing polymers with different chemi-
cal structures.
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