Synthesis and surfactochromicity of 1,4-diketopyrrolo[3,4-c]pyrrole(DPP)- based anionic conjugated polyelectrolytes

Article abstract of DOI:10.1002/pola.27062

Two novel anionic conjugated copolyelectrolytes PSDPPPV and PSDPPPE were synthesized via Heck/Sonogashira coupling reactions and characterized by FT-IR, ¹H NMR, UV-vis, and PL spectroscopy. The two polymers are respectively constituted of 2,5-diethoxy-1,4-phenyleneethynylene (DPV) and 2,5-diethoxy-1,4-phenyleneethynylene (DPE) with 1,4-diketo-2,5-bis(4- sulfonylbutyl)-3,6-diphenylpyrrolo[3,4-c]pyrrole (SDPP) which is a novel water soluble diketopyrrolopyrrole derivative. PSDPPPV and PSDPPPE show broad absorption band in visible region and they exhibit strong fluorescence quenching in aqueous solution. The fluorescence of their aqueous solutions can be enhanced in the presence of cationic surfactant or polymer nonionic surfactant. Fluorescence enhancement by introduction of polyvinylpyrrolidone (PVP) shows linear response. This result provides a controllable method to increase fluorescence intensity of dipyrrolopyrrole-based conjugate polyelectrolytes in aqueous phase. The optical properties suggested that PSDPPPV and PSDPPPE which are negatively charged conjugated polymers can assemble with positively charged photovoltaic materials to form ionic photoactive layer.

Full text of DOI:10.1002/pola.27062

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Synthesis and Surfactochromicity of 1,4-Diketopyrrolo[3,4-c]pyrrole(DPP)-
Based Anionic Conjugated Polyelectrolytes
Haoyue Shen, Chun Kou, Min He, Han Yang, Kuan Liu
College of Life and Basic Science, Sichuan Agricultural University, Yaan 625014, People’s Republic of China
Correspondence to: K. Liu (E-mail: cosmicer@live.cn)
Received 22 August 2013; accepted 5 December 2013; published online 22 December 2013
DOI: 10.1002/pola.27062
ABSTRACT: Two novel anionic conjugated copolyelectrolytes
PSDPPPV and PSDPPPE were synthesized via Heck/Sonogashira
coupling reactions and characterized by FT-IR, 1H NMR, UV-vis,
and PL spectroscopy. The two polymers are respectively consti-
tuted of 2,5-diethoxy-1,4-phenyleneethynylene (DPV) and 2,5-di
ethoxy-1,4-phenyleneethynylene (DPE) with 1,4-diketo-2,5-bis
(4-sulfonylbutyl)-3,6-diphenylpyrrolo[3,4-c]pyrrole (SDPP) which
ionic surfactant. Fluorescence enhancement by introduction of
polyvinylpyrrolidone (PVP) shows linear response. This result
provides a controllable method to increase fluorescence inten-
sity of dipyrrolopyrrole-based conjugate polyelectrolytes in
aqueous phase. The optical properties suggested that PSDPPPV
and PSDPPPE which are negatively charged conjugated poly-
mers can assemble with positively charged photovoltaic materi-
C
V
is
a
novel water soluble diketopyrrolopyrrole derivative.
als to form ionic photoactive layer. 2013 Wiley Periodicals, Inc.
PSDPPPV and PSDPPPE show broad absorption band in visible
region and they exhibit strong fluorescence quenching in aque-
ous solution. The fluorescence of their aqueous solutions can be
enhanced in the presence of cationic surfactant or polymer non-
J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 739–751
KEYWORDS: conjugated polymers; conjugated polyelectrolytes;
copolymerization; diketopyrrolopyrrole; optics; surfactochromicity
A general strategy to synthesize conjugated polymers with
excellent coverage of visible region is to combine dye mole-
cules with another monomer via copolymerization.26,27
DPP, 1,4-diketopyrrolo[3,4-c]pyrrole, an appropriate
copolymerization monomer precursor, is a kind of tradi-
tional hydrogen bond pigment along with indigo com-
pounds and quinacridone, and its derivatives are already
commercialized as high performance dyes and pigments.28
DPPs are composed of large conjugated system as chromo-
phore and a lactam structure as auxochromes with excep-
tional light, weather and heat stability. Thus, DPPs have
been considered as electron acceptor unit to optimize the
energy-level structure and reduce the band gap of conju-
gated polymers.29,30 These DPP-based polymers exhibit
outstanding optical properties such as red light emis-
sion,31,32 large-scale visible light covered absorption33,34
and large Stokes shift.35 DPPs are always modified with
functional side chains to adapt common solvent via the
alkylation reaction on its nitrogen atom. These soluble
polymers provide a possibility to become film by tradi-
tional process and the film can be used as the active layer
of the devices. For aforementioned advantages, a variety of
DPP-based polymers have been investigated thoroughly in
application of bulk heterojunction semiconductor devices
such as solar cells.36–38
INTRODUCTION Conjugated polyelectrolytes (CPEs) which are
functionalized conjugated polymers (CPs) with ionic side-chain,
have received considerable interest in past decades because of
their main chain controlled electronic/optical properties,1–3
surfactochromicity,4–6 and environment-friendly solvent proc-
essability.7–9 With designed backbones and appropriate regions
of absorption or emission, variety of CPEs have attracted exten-
sive interest in photoelectronic applications such as polymer
light-emitting diodes (PLEDs),10,11 chemosensors12–14 and pho-
tovoltaic devices.15,16 With the excellent solubility in water,
CPEs are advanced used in biological fluorescent sensors17–19
and interface controlled bulk heterojunction solar cells.20,21
During the recent years, CPEs based self-assembly solar cell has
been focus in application of photovoltaic materials.22–24 In
these devices, the polyelectrolytes are assembled with other
organic electrolytes23,24 or inorganic quantum dots25 by inti-
mate electrostatic attraction to form electron acceptor-donor
pair in the photoactive layer. The power conversion efficiency of
these photovoltaic devices is enhanced because of the extensive
interfacial charge transfer area between the combined electron
acceptor and electron donor.25 However, owing to the hardness
of purification and limited synthesis strategies, conjugated poly-
electrolytes only appeared with several kinds of main chain
structure. Design and synthesis of novel CPEs with excellent
optical properties is an important mission in recent research.
C
V
2013 Wiley Periodicals, Inc.
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However, few publications concerning DPP-based conjugated
polyelectrolytes were reported. The only discussed cationic
one was obtained by indirect method.39 It was synthesized
though copolymerization of a non-electrolyte DPP unit and
the aminated fluorene unit first and then made the ami-
nated side chain into ammonium via quaternization reaction.
The polyelectrolyte derived from this indirect strategy is
considered as an incomplete charged product because not
all the side chains of the polymer are translated into cati-
onic group. DPP unit of this polymer is still a traditional oil-
soluble N-alkyl substitution form but not directly charged.
On the other hand, ionic DPP derivatives always appeared
as small molecules. They are mostly modified with car-
boxyl40 or sulfuric acid ester41 on their N-alkyl side chain
by multistep reactions with low yield and unsatisfied purity.
Recently, alkylation with alkane sultone is considered as a
novel strategy to make hydrogen bond pigments into elec-
trolytes with high yield and purity. This process was only
realized on the functionalization of quinacridone.42 This
novel method is expected to applied in alkylation of DPP to
acquire a sulfonyl modified DPP which can adapt wide pH
conditions and have perfect water solubility. Furthermore
this kind of novel DPP electrolyte could act as an important
DPP-based monomer which can couple with other p-
conjugated organic molecules to make different anionic
DPP-based CPEs.
In our contribution, a novel negatively charged DPP mono-
mer
1,4-diketo-2,5-bis(4-sulfonylbutyl)-3,6-bis(4-bromophe
nyl)pyrrolo[3,4-c]pyrrole (monomer 3) has been designed,
synthesized and purified. This water soluble DPP derivative
has been copolymerized with two traditional monomers
based on phenylenevinylene (PV)/phenyleneethynylene (PE)
structure via palladium catalyzed Heck/Sonogashira coupling
reactions to afford two novel anionic alternating conjugated
polyelectrolytes PSDPPPV and PSDPPPE. As same as other
DPP-based conjugated polymers, PSDPPPV and PSDPPPE
exhibit red emission and large Stokes shift. On the other
hand, similar as other conjugated polyelectrolytes, the optical
properties of these two polymers show solvent-dependency,
and the fluorescence of these two polymers can be enhanced
by the addition of oppositely charged surfactant and non-
ionic polymer surfactant polyvinylpyrrolidone (PVP).
PSDPPPV and PSDPPPE with excellent optical properties and
electrolyte modified side chains are suggested to be a kind
of photovoltaic materials which have potential in interface
control and self-assembly to enhance the photoelectric con-
version efficiency in application of bulk heterojunction solar
cells.
SCHEME 1 Synthetic routes of monomer 1–3, PSDPPPV and
PSDPPPE.
4-Bromobenzonitrile, diisopropyl succinate, potassium tert-
butoxide, and 1,4-butane sultone were obtained from
domestic chemical companies. All the chemicals were used
as received. tert-Amyl alcohol and triethylamine were dis-
tilled over calcium hydride. Toluene was distilled over
sodium with benzophenone as indicator. N,N-Dimethylforma-
mide was purified by distillation under vacuum.
Pd(PPh3)2Cl2 was synthesized according to method
described in literature.43
EXPERIMENTAL
The synthetic routes of the monomer 1–3, PSDPPPV and
PSDPPPE are illustrated in Scheme 1. The following com-
pounds were synthesized according to the procedure in the
literature: 1,4-diethyloxy-benzene,44 1,4-bisbromomethyl-2,5-
FT-IR spectra were recorded on a Shimadzu FTIR-8400S IR
spectrometer. 1H NMR measurements were carried out on a
Bruker Avance II-300MHz spectrometer. UV-vis spectra were
performed on a UNICO-2102 UV-vis spectrophotometer. Pho-
toluminescence (PL) spectra were recorded on a Hitachi
F-4600 luminescence spectrophotometer.
diethyloxy-benzene,45
2,5-diethyloxy-1,4-xylylene-bis(tri-
phenyl phosphonium bromide)46 and 1,4-diiodo-2,5-diethy-
loxy-benzene.47
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Monomers’ Synthesis
acetic acid (20 mL) was added. The resulting mixture was
refluxed for 1 h, and the precipitate was filtered and washed
with hot methanol and hot water repeatedly until the filtrate
was colorless. The crude product was dried under vacuum
with P2O5 to afford a fresh red powder. Yield: 6.39 g,
75.37%.
1,4-Divinyl-2,5-diethyloxy-benzene (Monomer 1)
Under argon atmosphere, a mixture of 2,5-diethoxy-1,4-xylyl-
ene-bis(triphenyl phosphonium bromide) (6.57 g, 7.50 mmol),
dichloromethane (100 mL) and formaldehyde aqueous
ꢀ
(30 mL) was stirred at 0 C, then aqueous solution of NaOH
(20 wt %, 65 mL) was added dropwise in 1 h. The reaction
mixture was stirred at room temperature for 24 h. The organic
phase was separated and the aqueous layer was extracted
three times with dichloromethane. The combined organic
layer was washed with water and dried over anhydrous
sodium sulfate. Subsequently, the solvent was removed by
rotary evaporation, and the residue obtained was purified by
recrystallization from ethanol to afford a pale yellow crystal.
Yield: 1.05 g, 64.13%.
IR (KBr): m 5 3147 (vs; m (N-H)), 1643 (s; m(C5O)), 1602 (s),
1554 (s; m(ArC5C)), 1394 cm21 (vs; m(C-N)).
1,4-Diketo-2,5-bis(4-sulfonylbutyl)-3,6-bis(4-bromophenyl)
pyrrolo[3,4-c]pyrrole (monomer 3)
Under argon atmosphere, 1,4-diketo-3,6-bis(4-bromophenyl)-
pyrrolo[3,4-c]pyrrole (4.46 g, 10 mmol) and potassium tert-
butoxide (1.76 g, 22 mmol) in dry DMF (150 mL) were
ꢀ
heated to 60 C; 1,4-butane sultone (3.10 mL, 30 mmol) wꢀas
1H NMR (300 MHz, CDCl3, d): 7.00–7.10 (dd, J1 5 11.13 Hz,
J2 5 17.76 Hz, 2H; vinyl-H), 7.00 (s, 2H; Ar-H), 5.73 (d,
J 5 17.7 Hz, 2H; vinyl-H(terminal)), 5.26 (d, J 5 11.1 Hz, 2H;
vinyl-H(terminal)), 4.04 (q, J 5 7.0 Hz, 4H; O-CH2), 1.43 (t,
J 5 7.0 Hz, 6H; -CH3); IR (KBr): m 5 3090 (m; m(5C-H)), 2980
(vs), 2924 (vs; mas(-CH3, -CH2-)), 2875 (vs; ms(-CH3, -CH2-)),
1620-1429 (s; m(ArC5C)), 1392 (s; d(-CH3)), 1269 (s; m(-CH3)),
1205 (vs; m(C-O)), 999 (s), 910 cm21(s; c(5C-H)).
added dropwise in 10 min. The mixture was stirred at 90 C
for 12 h and subsequently the hot solution was poured into
ethanol (1 L). The precipitate was collected by filtration and
dissolved in hot water (200 mL). Then the solution was fil-
trated to remove the oil soluble impurity. The solvent was
removed by rotary evaporation and the residue obtained
was purified by recrystallization from aqueous solution of
potassium chloride to give an orange crystal. Finally this
ꢀ
hydrated crystal was dried under vacuum at 120 C for 12 h
1,4-Bis(3-methyl-3-hydroxybut-1-ynyl)-2,5-diethyloxy-
benzene (Monomer 2)
to give a dark red solid. Yield: 1.55 g, 19.51%.
1H NMR (300 MHz, D2O, d) : 7.60 (d, J 5 8.5 Hz, 4H; DPP
aromatic H), 7.44 (d, J 5 8.4 Hz, 4H; DPP aromatic H), 3.67
(t, J 5 6.0 Hz, 4H; N-CH2), 2.69 (t, J 5 7.3 Hz, 4H; S-CH2),
1.49 (m, 8H; methylene); IR (KBr): m 5 2937 (vs; mas(-CH3,
-CH2-)), 2872 (vs; ms(-CH3, -CH2-)), 1676 (s; m(C5O)), 1603
(s), 1551 (s; m(ArC5C)), 1371 (vs; m(C-N)), 1184 (vs), 1045
(vs),1007 (vs, m(sulfonate)), 735 cm21(w, d(-(CH2)n-)).
Under argon atmosphere, a solution of 1,4-diiodo-2,5-diet-
hoxybenzene (5.86 g, 14.02 mmol), Pd(PPh3)2Cl2 (0.03 g,
0.04 mmol) and CuI (0.06 g, 0.32 mmol) in dry triethylamine
(100 mL) was stirred in a three-neck flask at room tempera-
ture. 2-methyl-3-butyn-2-ol (3.62 mL, 0.05 mol) was added
droꢀpwise in 20 min, and the reaction mixture was stirred at
90 C for 24 h. Subsequently, the precipitate was filtered and
the solvent was removed under vacuum. The residue was
purified by column chromatography on silica using ether as
the eluent and then the crude product was recrystallized
from hexane/chloroform (1:1) to afford a white crystal.
Yield: 1.20 g, 25.90%.
Polymers’ Synthesis
PSDPPPV
Under argon atmosphere, monomer 1 (0.22 g, 1.00 mmol),
monomer 3 (0.79 g, 1.00 mmol), palladium(II) acetate
(0.0135 g, 0.06 mmol), tri(o-tolyl)phosphine (0.0730 g, 0.24
mmol) and triethylamine (5 mL) were dissolved in a mixture
of H2O (3 mL) and dimethylsulfoxide (8 mL). The reaction
mixture was stirred at 100 ꢀC for 18 h and subsequently
cooled to room temperature and then filtered. The filtrate
was poured into the mixed solvent (ethyl ether/acetone/
methanol 5 5/4/1). The precipitate was isolated by filtration
and then redissolved in deionized water (10 mL). The result-
ing solution was dialyzed against deionized H2O for 3 days
using a 4kD MWCO cellulose membrane (Spectra/Por Cellu-
lose Ester Membrane). Finally the solvent was removed by
rotary evaporation and the crude product was further dried
1H NMR (300 MHz, CDCl3, d): 6.86 (s, 1H; Ar-H), 4.02 (q,
J 5 7.0 Hz, 2H; -CH2-), 2.24 (s, 1H; -OH), 1.64 (s, 6H; -CH3),
1.43 (t, J 5 7.0 Hz, 3H; -CH3 (ethoxy)); IR (KBr): m 5 3541
(s), 3450 (s), 3292 (s; m(O-H)), 2980 (vs), 2930 (vs; mas(-CH3,
-CH2-)), 2870 (vs; ms(-CH3, -CH2-)),2223 (w; m(CꢁC)), 1502
(s), 1475 (s; m(ArC5C)), 1392 (s; d(-CH3)), 1273 (s; m(-CH3)),
1219 (vs; m(C-O)), 1159 (s; m(C-O)), 1047 (w), 864 (w), 590
cm21(w; d(Ar-H)).
14-Diketo-3,6-bis(4-bromophenyl)pyrrolo[3,4-c]pyrrole
Under argon atmosphere, sodium (2.21 g, 96 mmol), FeCl3
(0.10 g) and dry tert-amylalcohol (50 mL) were stirred and
heated to 90 ꢀC until the sodium was dissolved completely.
The solution was cooling to 50 ꢀC and 4-bromobenzonitrile
(8.74 g, 48 mmol)ꢀwas added portionwise. Then the mixture
was heated to 90 C again. A solution of succinic acid diiso-
propyl ester (3.88 g, 19 mmol) in dry tert-amylalcohol (20
mL) was added dropwise in 1 h. After stirring at 90 ꢀC for
24 h, the dark red mixture was cooled to 60 ꢀC, and then
ꢀ
under vacuum with P2O5 at 50 C for 2 days to afford a dark
purple powder. Yield: 0.34 g, 39.94%.
1H NMR (300 MHz, DMSO-d6, d) : 7.80-7.95 (DPP aromatic
H), 7.48-7.65 (vinyl-H), 7.42 (DVB aromatic H), 4.20 (O-CH2),
3.72-3.76 (N-CH2), 2.33 (S-CH2), 1.24,1.47 (-CH3 and -CH2-);
IR (KBr): m 5 3045 (m; m(5C-H)), 2931 (vs; mas(-CH3, -CH2-)),
2872 (vs; ms(-CH3, -CH2-)), 1664 (s; m(C5O)), 1593 (s), 1541
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(s; m(ArC5C)), 1388 (s; d(-CH3)), 1367 (ms; m(C-N)), 1180 (vs),
1041 (vs; m(sulfonate)), 964 (m; c(5C-H, trans)), 725 cm21
(w; d(-(CH2)n-)).
PSDPPPE
Under argon atmosphere, monomer 2 (0.0991 g, 0.30 mmol),
monomer 3 (0.2384 g, 0.30 mmol), Pd(PPh3)2Cl2 (0.0184 g,
0.0263 mmol), CuI (0.0190 g, 0.1 mmol), PPh3 (0.0378 g,
0.14 mmol) and potassium carbonate (1.14g, 8.25mmol)
were dissolved in dimethylsulfoxide (10 mL). The reaction
mixture was stirred at 100 ꢀC for 14 h and subsequently
cooled and then filtered. The filtrate was poured into the
mixed solvent (ethyl ether/ methanol 5 1/1). The precipitate
was isolated by filtration and then redissolved in deionized
water (10mL). The resulting solution was dialyzed against
deionized H2O for 3 days using a 4kD MWCO cellulose mem-
brane (Spectra/Por Cellulose Ester Membrane). Finally the
solvent was removed by rotary evaporation and the crude
FIGURE 1 IR spectra of (a) 1,4-diketo-3,6-bis(4-bromophenyl)-
pyrrolo[3,4-c]pyrrole and (b) monomer 3.
ꢀ
product was further dried under vacuum with P2O5 at 50 C
for 2 days to afford a dark purple powder. Yield: 0.0461 g,
18.15%.
tion from potassium chloride aqueous solution which pro-
vide high concentration of potassium ion.
1H NMR (300 MHz, DMSO-d6, d): 7.98-6.97 (Ar-H), 4.16-4.04
(O-CH2), 3.74 (N-CH2), 2.37-2.34 (CH2-SO3 ), 1.52-1.47 (-CH2-),
1.41-1.33 (-CH3); IR (KBr): m 5 2976 (vs), 2926 (vs; mas(-CH3,
-CH2-)), 2874 (vs; ms(-CH3, -CH2-)), 2201 (w; m(CꢁC)), 1662 (s;
m(C5O)) 1510 (s), 1472 (s; m(ArC5C)), 1390 (s; d(-CH3)), 1275
(s; m(-CH3)), 1209 (vs; m(C-O)), 1171 (vs), 1041 (vs; m(sulfo-
nate)), 845 cm21 (w; d(Ar-H)).
In order to increase the solubility of polymers in water,
monomer 1 and monomer 2 have been modified with short
alkoxyl substituent like ethyoxyl. In their synthetic route,
most of the solvents which are used for reactions and recrys-
tallizations should be high polar or large quantity. To synthe-
size PSDPPPV, the traditional aqueous phase Heck coupling
reaction which reported in the reference46 has been success-
fully achieved in this polymerization. On the other hand, the
synthesis of PSDPPPE was completed by modifying the pro-
cedure of Heck-Sonogashira coupling reaction in oil soluble
system.49 The traditional phase transfer system of the sol-
vent and the base has been replaced by strong polar solvent
DMSO and potassium carbonate which is a solid weak base,
respectively, and this reaction condition makes this polymer-
ization faultless. Water solubility of PSDPPPV and PSDPPPPE
are respectively around 4.7 mg mL21 ([repeat unit] 5 5.52 3
1023 M) and 4.6 mg mL21 ([repeat unit] 5 5.42 3 1023 M)
at room temperature.
-
RESULTS AND DISCUSSION
Synthesis of Monomer 3 and Polymers
Monomer 3 was obtained via N-alkylation reaction with
alkane sultone as same as the synthesis of oil soluble deriva-
tives of DPP with alkyl halide.48 Here, reaction yield and
product purity not only depend on temperature but also the
variety of solvent and the strength of the base. DMF was
used as the solvent because it is the most used one which
has the similar chemical structure with DPP. Potassium tert-
butoxide was adopted as the base because it is a strong base
which can make lactams of DPP into nitrogen anions imme-
diately in the solution and this ion can accelerate the nucleo-
philic substitution process. Although tert-butoxide anion can
make this alkylation fast and afford a higher yield rate, the
pyrrole rings of DPP can be easily destroyed in this situation
and give a variety of impurities. The usual impurities such as
single substitution DPP and ring-opened product which are
hardly soluble in water can be removed by filtration. And
other water soluble impurities which are considered as sul-
fonate species have been removed completely by recrystalli-
zation. The result shows that monomer 3 has an excellent
solubility in polar solvent and can be crystallized in oversa-
turated aqueous solution. However, reducing the solubility of
the target product by admixture of organic solvent such as
ethanol or acetone can not give the crystal from recrystalli-
zation. Finally, pure N-sulfonylalkyl DPP hydrate crystal with
low concentration was successfully obtained by recrystalliza-
The Structure Characterizations of Monomers and
Polymers
Figure 1 shows the FT-IR spectra of 1,4-diketo-3,6-bis(4-bro-
mophenyl)pyrrolo[3,4-c]pyrrole and monomer 3. Compared
with Figure 1(a), the absent absorption peak at 3147 cm21
in Figure 1(b) is attributed to the N-H vibration of the lac-
tam structure. In addition, two new absorption peaks at
1184 cm21 and 1007 cm21 in Figure 1(b) are attributed to
the sulfonate. Those transformations prove that monomer 3
has been successfully translated into a sulfonate functional-
ized molecular with hydrophilic side chain via alkylation on
nitrogen atoms. The absorption peaks at 1643 cm21 and
1394 cm21, which have been characterized in both Figure
1(a,b) are due to the typical C5O and C-N stretching modes
of DPP unit. This result proves that the DPP lactam ring has
been preserved in monomer 3 during the synthesis in the
presence of the strong base potassium tert-butoxide.
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lectrolyte PSDPPPE which has ethynylene structure in main
chain has been successfully obtained.
1
Figure 3 shows the H NMR spectra of monomer 1, monomer
1
3 and PSDPPPV. And Figure 4 gives the H NMR of monomer
2, monomer 3 and PSDPPPE. In the 1H NMR spectrum of
monomer 3, the doublets at 7.60 ppm and 7.44 ppm are attrib-
uted to the AB spin-spin coupling system of two kinds of aro-
matic hydrogen in bromophenyl unit. Doublets at 3.67 ppm
and 2.69 ppm are respectively attributed to the hydrogen of
N-CH2 and S-CH2. The signals of other alkyl hydrogen in the
middle of the carbon chain of sulfonylbutyl unit appears in the
1
region of 1.60 ppm to 1.36 ppm. Here, the H NMR spectrum
of monomer 3 without impure signal obviously shows that
monomer 3 has got a considerable purity. It is identified as a
valuable monomer to synthesize DPP-based conjugated copo-
lyelectrolytes via the synthetic route in Scheme 1.
FIGURE 2 IR spectra of (a) monomer 1, (b) PSDPPPV,
(c) monomer 2 and (d) PSDPPPE.
1
Figure 2 shows the FT-IR spectra of monomer 1, PSDPPPV,
monomer 3 and PSDPPPE. The absorption peak at 3090
cm21 in Figure 2(a) and 3045 cm21 in Figure 2(b) are
attributed to the C-H vibration of the C5C double bond.
These peaks intuitively prove that the vinyl in monomer 1
has been coupled with monomer 3 successfully to give the
vinylene structure in the main chain of PSDPPPV via Heck
coupling reaction. In Figure 2(b), the absorption peak at
1674, 1367 cm21 are attributed to lactam unit, and 1180,
1041 cm21 are attributed to sulfonate. These signals prove
that sulfonate functionalized DPP unit has been preserved in
PSDPPPV. Similarly, these characteristic absorption peaks
which are attributed to sulfonate modified DPP structure can
also be observed in the IR spectrum of PSDPPPE. Further-
more, the O-H stretching vibration at 3292 cm21 of mono-
mer 2 is disappeared in Figure 2(d), and the CꢁC vibration
at 2201 cm21 has been obviously conserved. These results
prove that another DPP-containing anionic conjugated polye-
In the H NMR spectrum of monomer 1 (Fig. 3), the quartet
at 4.04 ppm with triplet at 1.43 ppm is attributed to the
hydrogen of ethyloxy unit. As same, the H NMR spectrum of
1
monomer 2 in Figure 4 also shows these peaks. The other
structure signals of monomer 1 and monomer 2 are as same
as their homologs which have longer hydrocarbon chains
linked to the oxygen atoms.35,46 In the 1H NMR spectra of
PSDPPPV (Fig. 3), the doublets of terminal vinyl of monomer
1 at 5.73 ppm and 5.26 ppm are absent. It is verified that all
the vinyl has been transformed into vinylene structure in its
backbone. In Figure 4, the singlet at 2.24 ppm and 1.64 ppm
which attributed to the hydrogen of the hydroxyl unit and
the methyl hydrogen in monomer 2 are invisible in the 1H
NMR of PSDPPPE. This transformation indicated that the iso-
propanol protection unit in monomer 2 has been removed
and the ethynylene structure was introduced into the back-
bones of PSDPPPE. The signals of aromatic unit of PSDPPPV
and PSDPPPE in the range of 7.98 to 6.97 ppm both show
FIGURE 3 1H NMR spectra of monomer 1, monomer 3 and PSDPPPV.
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FIGURE 4 1H NMR spectra of monomer 2, monomer 3 and PSDPPPE.
larger chemical shift compared with their monomers because
of the stronger deshielding effect of their p-conjugated back-
bones. Above mentioned translation indicated that the poly-
merizations in Scheme 1 were accomplished.
band which can cover most visible light region ideally. In
the UV-vis spectrum of PSDPPPV, The absorption peak at
421 nm is associated with 1,4-phenylenevinylene segments
and the absorption maximum at 524 nm is attributed to the
electron transitions of the largest p-conjugated backbones.
Compared with its monomers, the absorption maximum of
PSDPPPV exhibits a great red shift due to the longer conjuga-
tion length which has been acquired via the coupling
reaction. On the other hand, PSDPPPV exhibits pale pink
color fluorescence and shows red-shifted emission maximum
at 609 nm compared to monomer 3. As same as other
DPP-based conjugated polymer,31,32 the strength of fluores-
cence is decreased observably after the polymerization of
monomer 3. This phenomenon suggests that there is a
As some references reported, the molecular weight data is
not available from common techniques such as gel permea-
tion chromatography (GPC) or light scattering method due to
the aggregation phenomenon of CPEs.3 It is difficult to find
an appropriate solvent which lead to weak aggregation of
CPEs with high concentration to finish the GPC measure-
ment. Furthermore, aggregated CPEs may be easily adsorbed
on the chromatographic column to give the imprecise results.
In this investigation, solutions of PSDPPPV and PSDPPPE
were dialyzed by 4kD MWCO cellulose membrane, which is
the normal method to remove the oligomers and impurities.
This approach can ensure that the molecular weight of
PSDPPPV and PSDPPPE are more than 4000 (at least four
constitutional repeating units). However, the actual molecu-
lar weights of these polyelectrolytes are much more than we
imagined. Moreover, the absorption/emission peaks in
ultraviolet-visible (UV-vis) and photoluminescence (PL) spec-
tra of the polymers exhibit remarkable red shift compared
with the monomers. These results also prove that the poly-
merization of PSDPPPV and PSDPPPE has been successfully
completed with the eligible molecular weight.
Basic Optical Properties of the Monomers and the
Polymers
In this part, all the UV-vis and PL spectra have been respec-
tively normalized to the same maximum values for better com-
parison. The photo of monomers and polymers in different
solvent is shown in Figure 5. And all the absorption and emis-
sion maxima of polymers and monomers are listed in Table 1.
FIGURE 5 Photo of the solutions, (a) monomer 1 in chloroform,
(b) monomer 2 in DMSO, (c) monomer 3 in water, (d) PSDPPPV
in DMSO, (e) PSDPPPE in DMSO, (f) PSDPPPV in water and
(g) PSDPPPE in water.
Figure 6 shows the UV-vis and PL spectra of monomer 1,
monomer 3 and PSDPPPV. PSDPPPV shows broad absorption
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TABLE 1 Optical Properties of Monomer 1–3, PSDPPPV, PSDPPPE
Polymers
Solvent
UV-Vis (nm)
PL (nm)
Stokes Shift (nm)
Monomer 1
Monomer 2
Monomer 3
Chloroform
344
404
60
DMSO
335
382
534/554a
47
69/ 89a
Water
465
DMSO
476
548
72
PSDPPPV
Water
547
–
–
DMSO
524
609
85
b
b
LAS/water: 16 lg mL21
CTAB/water: 16 lg mL21
PVP/water: 16 lg mL21
Water
547b
544b
558b
522
–
–
721b
668b
636
177b
110b
114
PSDPPPE
DMSO
507
593
86
LAS/water: 16 lg mL21
CTAB/water: 16 lg mL21
PVP/water: 16 lg mL21
522b
528b
530b
616b
641b
636b
94b
113b
106b
a
b
The concentration of monomer 3 is higher than 0.75 mmol L21
.
The concentration of PSDPPPV or PSDPPPE is fixed on 0.01 mg mL21
.
remarkable fluorescence self-quenching on polymer main-
chain. In the backbone of PSDPPPV, DPP is considered as an
acceptor unit due to the electron deficient lactam structure
and phenylenevinylene (PV) segment is considered as a
weak electron donor. The energy of excitons is partly dissi-
pated by the electron transfer between the copolymerization
units rather than translated into light emission. So, PSDPPPV
has a similar framework with donor-acceptor structure
CPs35,50 which has narrow band gap, large Stokes shift and
weak fluorescence.
maximum at 507 nm and emission maximum at 593 nm,
and those peaks both have a shorter wavelength than that of
PSDPPPV. The difference suggests that the C5C double bond
own a better conjugation degree compared to the CꢁC triple
bond segments. In the mainchain of PSDPPPE, energy of exci-
tons may not be dissipated effectively as PSDPPPV due to
the CꢁC triple bond. As reference reported, p-phenylene
ethynylene (PPE) typed conjugated polymers always show
shorter wavelength of absortion/emission maxima and
higher fluorescence quantum efficiency compared to the
p-phenylene vinylene (PPV) typed CPs35 which have the sim-
ilar structures.
Figure 7 shows the UV-vis and PL spectra of monomer 2,
monomer 3 and PSDPPPE. As same as PSDPPPV, the UV-vis
and PL maxima of PSDPPPE both show large red shift com-
pared to its monomers. PSDPPPE exhibits the absorption
In addition, the optical properties of PSDPPPV and PSDPPPE
vary dramatically with the solvent composition. Figure 8
FIGURE 6 UV-vis and PL spectra of monomer 1 (in chloro-
form), monomer 3 (in water) and PSDPPPV (in DMSO). [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
FIGURE 7 UV-vis and PL spectra of monomer 2 (in DMSO),
monomer 3 (in water) and PSDPPPE (in DMSO). [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
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FIGURE 9 UV-vis and PL spectra of PSDPPPE in different sol-
vents. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
FIGURE 8 UV-vis and PL spectra of PSDPPPV in different sol-
vents. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
The p-p staking of PSDPPPE may less than PSDPPPV due to its
lower conjugation level.35 When the solvent is changed
from DMSO to water, the absorption maximum of PSDPPPE is
red-shifted by 15 nm as short as 23 nm of PSDPPPV. This
difference may be due to the rigidity of the CPEs. DMSO is
considered as a good solvent that makes the backbones free
and independently, and water is considered as a pool solvent
that makes the backbones form J-aggregates. PSDPPPE has
more rigid backbones, so there is an unobvious change of its
conformation when the solvent changes. The conjugated level
only increased a little after its aggregation compared to
PSDPPPV.
shows the UV-vis and PL spectra of PSDPPPV in water and
DMSO. In contrast, when the solvent is changed from DMSO
to water the absorption maximum has been red-shifted from
524 nm to 547 nm, and its fluorescence has been quenched
completely. This transformation suggests that PSDPPPV has
been translated to a higher conjugated state and the excitons
may have different dissipation forms in aqueous solution. In
water, PSDPPPV is aggregated strongly by p-p stacking as
same as other reported conjugated polyelectrolytes51–53 and
this stacking has been entitled the J-aggregate.54 It is a
model that the backbones of the polyelectrolytes have been
stacked mutually with conjugated layers because of the
hydrophobicity. And the hydrophilic electrolyte side chains of
them have been stretched freely between the backbones and
water molecules. J-aggregate causes the conjugated polyelec-
trolytes to have a higher conjugation degree and the excitons
can transfer easily between the mainchains. This new form
of energy dissipation due to the exciton transfer may be one
reason why the fluorescence of PSDPPPV is quenched
completely.
Fluorescence Enhancement by Surfactant
Another important research about conjugated polyelectrolyte
(CPE) was focused on its surfactochromicity.4 With the
involvement of oppositely charged surfactant, the homoag-
gregates between conjugated mainchains are broken and the
heteroaggregates obviously form between polymer and sur-
factant. When the J-aggregate has been destroyed, excitons
can not transfer easily between the mainchains, resulting in
the increasing fluorescence strength of CPE due to the lower
energy dissipation. On the other hand, the conjugation
degree of the polymer has been changed because of the new
shape of backbones upon the addition of surfactant. The UV-
vis and PL spectra of CPEs always show complex changes
with the involvement of surfactant in a stepwise manner.
Abovementioned properties are considered as the character-
istic of CPEs which can verify that the polymers have elec-
trolyte side chains and the potential to be self-assembly
materials.25
Figure 9 shows the UV-vis and PL spectra of PSDPPPE in these
two different solvents. The absorption maximum of PSDPPPE
also shows an obvious red shift form 507 nm to 522 nm in
aqueous solution. However, the fluorescence of PSDPPPE does
not exhibit drastically quenching, and it can be seen that the
emission maximum has been red shifted from 593 nm to 636
nm in aqueous solution. The red shifts of absorption and emis-
sion maxima of PSDPPPE are also attributed to the J-aggregate
as same as other CPEs.51,52,54 And the incomplete fluorescence
quenching of PSDPPPE may be mainly due to the higher fluo-
rescence quantum efficiency of phenyleneethynylene (PE) seg-
ment than phenylenevinylenes (PV) segment. Following the
previous studies, polyphenyleneethynylenes (PPEs) always
have higher fluorescence quantum efficiency (about two
times) than polyphenylenevinylenes (PPVs).55,56 As another
reason, PSDPPPE may have less intermolecular exciton migra-
tion than PSDPPPV in aqueous solution. It is because the
aggregation degree of PSDPPPE may be lower than PSDPPPV.
Figures 10 and 11 show the UV-vis and PL spectra of
PSDPPPV and PSDPPPE in aqueous solution upon the addi-
tion of sodium dodecyl benzene sulfonate (SDBS), respec-
tively. The optical spectra show inconspicuous response with
the complex of anionic surfactant. This result suggests that
SDBS can not break the J-aggregate of PSDPPPV and
PSDPPPE in their aqueous solution effectively because the
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FIGURE 12 UV-vis and PL spectra of PSDPPPV in aqueous
solution (0.01 mg mL21
under different concentration of
FIGURE 10 UV-vis and PL spectra of PSDPPPV in aqueous
solution (0.01 mg mL21
) under different concentration of
)
CTAB. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
SDBS. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
anion side chains when the CTAB concentration is under 12
lg mL21. In this process, CTAB bilayers avoid the exciton
transfer between the backbones and there is a considerable
enhancement of the fluorescence emission of this polymer
because the energy of exciton can not dissipate between
mainchains. However, it is believed that the homoaggregates
of PSDPPPV has been completely destroyed when there is
overmuch CTAB. When the concentration of CTAB is higher
than 12 lg mL21, the involvement of CTAB can not lead to
responses anymore. In this situation, surfactant bilayers have
already surrounded the entire polymer to form heteroaggre-
gates and make the conjugated mainchain independently
with the surrounded surfactant in aqueous solution.
negative charged surfactant exhibits repulsive force with
these anionic CPEs.
Compared with SDBS, addition of hexadecyl trimethyl ammo-
nium bromide (CTAB) generates strong response in the emis-
sion spectra of PSDPPPV and PSDPPPE. Figure 12 shows the
UV-vis and PL spectra of PSDPPPV in aqueous solution under
different concentration of CTAB. With this cationic surfactant,
an emission peak appears progressively at 721 nm. This phe-
nomenon suggests that CTAB has the ability to interrupt the
J-aggregate of PSDPPPV. However, this effect levels off at a
surfactant concentration of 12 lg mL21. On the other hand,
the absorption spectra of PSDPPPV exhibits unobvious blue-
shift and the absorbance of the spectra are decreased gradu-
ally upon the addition of CTAB. These properties are similar
to other anionic CPEs which were described by Wu et al.57
As same as the model discussed, the CTAB bilayers insert
into the homoaggregates of PSDPPPV gradually between the
The optical spectra of PSDPPPE show similar response com-
pared to PSDPPPV in the presence of CTAB. Figure 13 gives
the UV-vis and PL spectra of PSDPPPE in aqueous solution
under different concentration of CTAB. There is also a large
FIGURE 11 UV-vis and PL spectra of PSDPPPE in aqueous
FIGURE 13 UV-vis and PL spectra of PSDPPPE in aqueous
solution (0.01 mg mL21
) under different concentration of
solution (0.01 mg mL21
) under different concentration of
SDBS. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
CTAB. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
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FIGURE 14 UV-vis and PL spectra of PSDPPPV in aqueous
solution (0.01 mg mL21) under different concentration of PVP.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
FIGURE 15 Linear analysis between intensity of emission maxi-
mum of PSDPPPV (0.01 mg mL21) and concentration of PVP in
aqueous solution.
fluorescence enhancement and the emission peak can be at
3.4 times as strong as the original intensity by the addition of
CTAB up to 8 lg mL21. On the other hand, the absorption
spectra do not show obvious blue-shift as same as PSDPPPV.
In contrast, these two polymers show different limited point
of fluorescence enhancement (PSDPPPV at 12 lg mL21 and
PSDPPPE at 8 lg mL21) and different feature of absorption
response. This result may be attributed to the different rigidity
of these two polymers. The conformation of the conjugated
mainchain of PSDPPPE may own a lower distorted degree
than PSDPPPPV upon the complex of CTAB. Although these
two polymers exhibit independent mainchains in both DMSO
and CTAB aqueous solution, PSDPPPV and PSDPPPE shows
different wavelength of their absorption and emission maxi-
mum. This disagreement may suggest that the optical proper-
ties solvent-dependency of the PSDPPPV and PSDPPPPE is not
only caused by the aggregation phenomena.
As same as CTAB, the presence of non-ionic polymer surfactant
also causes an interesting response in the optical spectra of
PSDPPPV and PSDPPPE. Figure 14 shows the UV-vis and PL
spectra of PSDPPPV in aqueous solution under different con-
centration of polyvinylpyrrolidone (PVP). As shown, not only
the absorbance but also the PL intensity of PSDPPPV can be
enhanced by the addition of PVP. The fluorescence enhance-
ment of PSDPPPV exhibits linear response with the increasing
concentration of PVP compared to that of CTAB. Figure 15
gives the linear fit plot of the relationship between the concen-
tration of PVP and the PL intensity of PSDPPPV. In this analysis,
the excitation wavelength has been fixed on 540 nm and the
detection wavelength has been fixed on 668 nm. The result
shows excellent correlation coefficient (R50.9965) when the
concentration of PVP is between 0 lg mL21 and 16 lg mL21
This regular response may be attributed to the quantificational
.
FIGURE 16 The assumed model of the heteroaggregates between PVP and PSDPPPV. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
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FIGURE 17 UV-vis and PL spectra of PSDPPPE in aqueous
solution (0.01 mg mL21)under different concentration of PVP.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
FIGURE 18 Linear analysis between intensity of emission maxi-
mum of PSDPPPE (0.01 mg mL21) and concentration of PVP in
aqueous solution.
interaction between PVP and PSDPPPPV. Otherwise, the
absorption maximum of PSDPPPV exhibits red shift and the
peak which has the largest wavelength is narrowed gradually
after the addition of PVP. Abovementioned properties have a
notable difference with the responses caused by CTAB. It is
suggested that the heteroaggregates between PSDPPPV and
PVP have a different form with the heteroaggregates between
PSDPPPV and CTAB. Here CTAB is a cationic surfactant, thus
heteroaggregates between PSDPPPV and CTAB was caused by
the electrostatic force between the alkylammonium cation on
CTAB and the sulfonate anions on the side chain of PSDPPPV.57
However, PVP is a nonionic polymer surfactant. Heteroaggre-
gates between PSDPPPV and PVP was caused by the dispersion
force between polar groups like lactam structure on both main
chains of PSDPPPV and PVP.58,59
PSDPPPV in aqueous solution under different concentration
of polyvinylpyrrolidone (PVP) and Figure 18 gives the linear
fit plot of the relationship between the concentration of PVP
and the PL intensity of PSDPPPE. In the linear analysis of
PSDPPPE, the excitation wavelength has been fixed on
450 nm and the detection wavelength has been fixed on
633 nm. The fluorescence enhancement of PSDPPPE also
shows excellent correlation coefficient (R 5 0.9906). When
the concentration of PVP is up to 16 lg mL21, the absorp-
tion maximum of PSDPPPE is red-shifted by 8 nm as short
as 14 nm in the response of PSDPPPV. This difference sug-
gests that there is only a little change in the distorted level
of PSDPPPE due to the phenyleneethynylene (PE) unit which
is more structurally rigid than phenylenevinylene (PV) unit.
Although there is an obvious response in PL spectra
of PSDPPPV and PSDPPPPE by the addition of PVP, the wave-
lengths of their emission maxima of these two polymers
never shift in this process. Thus, the fluorescence peaks
at 668 nm of PSDPPPV and 633 nm of PSDPPPE are respec-
tively attributed to their intrinsic emission due to the
constant wavelength and the linear relationship of their
PL intensity upon the concentration of PVP. In conclusion,
fluorescence enhancement by PVP provides a controllable
method to enhance the fluorescence of PSDPPPV and
PSDPPPE in conceivable confine. And this process is
predicted to be a method that can avoid the lower fluores-
cence quantum efficiency of DPP-based conjugate
polyelectrolytes.
The assumed model of the heteroaggregates between PVP and
PSDPPPV is shown in Figure 16. In this figure, SDPP and DPV
segments are respectively stand for 1,4-diketo-2,5-bis(4-sulfo-
nylbutyl)-3,6-bis(4-bromophenyl)pyrrolo[3,4-c]pyrrole
unit
and 2,5-diethoxy-1,4-phenyleneethynylene unit in the main-
chain of PSDPPPV. Polyvinylpyrrolidone, which has the similar
lactam structure with DPP unit can attract PSDPPPV back-
bones and interrupt the J-aggregate directly by dispersion
force. This attraction is different from the indirect ionic inter-
action between anion side chains and cationic surfactant. The
exciton transfer between the mainchains has been avoided
efficiently by the inserted PVP chains. And the fluorescence of
PSDPPPV has been recovered because the energy can not dis-
sipate by exciton transfer between backbones anymore. On
the other hand, PVP may provide a flexible support to the con-
jugated backbones and this effect could prevent the main-
chains twisting overmuch. So, the presence of PVP also
decreases the distorted degree of the conjugated backbones
and makes PSDPPPV into a higher conjugation level.
CONCLUSIONS
Two novel water soluble anionic DPP-based conjugated polye-
lectrolytes PSDPPPV and PSDPPPE have been synthesized suc-
cessfully. The polymers are dark purple solid and exhibit
excellent solubility in water and DMSO. In DMSO, PSDPPPV
exhibits absorption maximum at 524 nm and emission maxi-
mum at 609 nm, and PSDPPPE shows absorption maximum at
The optical spectra of PSDPPPE also show linear response
with PVP. Figure 17 gives the UV-vis and PL spectra of
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14 S. W. Thomas, G. D. Joly, T. M. Swager, Chem. Rev. 2007,
107, 1339–1386.
507 nm and emission maximum at 593 nm. The optical proper-
ties show obvious solvent-dependency. Aqueous solutions of
PSDPPPV and PSDPPPE respectively exhibit their absorption
maximum at 547nm and 522nm which show a notable red shift
compared to their DMSO solutions. The emission of PSDPPPV
and PSDPPPE both show remarkable fluorescence quenching in
aqueous solution. PSDPPPV exhibits no fluorescence, but the
intrinsic emission at 668 nm in aqueous solution appears by
addition of PVP. PSDPPPE shows the emission maximum at
636 nm in aqueous solution which also has a large red shift
compared to its DMSO solution. Furthermore, the addition of
cationic surfactant CTAB and polymer nonionic surfactant PVP
can effectively increase the emission intensity of their aqueous
solution in a certain range. Fluorescence enhancement caused
by PVP shows linear response with the increasing concentra-
tion of PVP and provides a controllable method to enhance the
fluorescence of DPP-based conjugate polyelectrolytes in aque-
ous phase. This strategy could make the polymers be water
soluble conjugated polymer materials with red light emitting.
Finally, these two polymers have favourable coverage of the
visible light region and this property suggests they are promis-
ing material in applications of self-assembly solar cells.
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