Fluorescent conjugated polymer nanoparticles and aggregates based on rapid precipitation and self-assembled π-conjugated systems

Article abstract of DOI:10.1016/j.polymer.2018.12.039

Conjugated copolymers containing fluorene and diphenylamine moieties with octyl and triethylene glycol side chains have been synthesized via Suzuki-Miyaura cross-coupling reaction and their structures have been characterized. Conjugated polymer nanoparticles and self-organization of the polymers have been prepared by rapid precipitation and solvent diffusion methods, respectively. The conjugated polymers exhibited well-defined spherical amorphous structures and crystalline microspheres with a diameter ranging from several nanometer to micrometer. Self-assembled conjugated polymer aggregates were obtained in a selective good solvent/poor solvent mixture. Alternating conjugated polymers are generally difficult to assemble into well-defined spheres due to their rigid and planar backbones, however, alternating polymers containing dioctyl and triethylene glycol side chains tends to form microspheres or sheet-like structures depending on the different solvent mixtures. The emission maximum of the dispersed polymers in water was significantly red-shifted with a dramatic reduction in the photoluminescence quantum yield. In addition, the emission maximum of the self-assembled copolymers was barely red shifted with a small reduction in the photoluminescence quantum yield.

Full text of DOI:10.1016/j.polymer.2018.12.039

Polymer174(2019)45–51
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Fluorescent conjugated polymer nanoparticles and aggregates based on
rapid precipitation and self-assembled π-conjugated systems
Alis Shano Godana, Chin-Yang Yu∗
Department of Materials Science and Engineering, National Taiwan University of Science and Technology, 43, Section 4, Keelung Road, Taipei, 10607, Taiwan
H I G H L I G H T S
Copolymers containing fluorene and diphenylamine with different chains have been prepared.
Conjugated polymer nanoparticles have been prepared in the aqueous media.
Polymers with micrometer sized particles were obtained from solvent diffusion method.
Self-assembled polymers exhibited a relatively high photoluminescence quantum yield.
•
•
•
•
A R T I C L E I N F O
A B S T R A C T
Keywords:
Conjugated copolymers containing fluorene and diphenylamine moieties with octyl and triethylene glycol side
chains have been synthesized via Suzuki-Miyaura cross-coupling reaction and their structures have been char-
acterized. Conjugated polymer nanoparticles and self-organization of the polymers have been prepared by rapid
precipitation and solvent diffusion methods, respectively. The conjugated polymers exhibited well-defined
spherical amorphous structures and crystalline microspheres with a diameter ranging from several nanometer to
micrometer. Self-assembled conjugated polymer aggregates were obtained in a selective good solvent/poor
solvent mixture. Alternating conjugated polymers are generally difficult to assemble into well-defined spheres
due to their rigid and planar backbones, however, alternating polymers containing dioctyl and triethylene glycol
side chains tends to form microspheres or sheet-like structures depending on the different solvent mixtures. The
emission maximum of the dispersed polymers in water was significantly red-shifted with a dramatic reduction in
the photoluminescence quantum yield. In addition, the emission maximum of the self-assembled copolymers was
barely red shifted with a small reduction in the photoluminescence quantum yield.
Fluorene and diphenylamine moieties
Triethylene glycol side chains
Conjugated polymer nanoparticles
Self-organization
Sheet-like structures
1. Introduction
precipitation method and miniemulsion method [10–16]. The different
solubility of the hydrophilic and hydrophobic segments of the amphi-
Nanometer-sized particles have attracted much attention due to
their tremendous applications such as drug and gene delivery [1],
biosensors [2], gas sensors [3], fluorescence imaging and cellular
tracking [4,5]. Molecular self-assemblies have an intriguing role in
nature such as the formation of molecular crystals and the formation of
an ordered array of supramolecular architectures by the interaction of
noncovalent forces [6,7]. The interactions of self-assembled supra-mo-
lecules include ionic, hydrogen and coordination bonds as well as dis-
persion and van der Waals forces which lead to ordered nanostructures
upon equilibration between aggregated and non-aggregated states
[8,9]. Self-assembled polymer colloids can be prepared by either direct
polymerization from the corresponding monomers or post poly-
merization techniques such as vapor diffusion method, interface
philic copolymers in organic solvents have a tendency to induce self-
assembly into micelles in a selected solvent [17,18]. Hydrophilic
components such as polymers containing oligo(ethylene glycol) chains
have been widely applied in cosmetic and pharmaceutic applications
due to their biocompatibility and non-toxicity.
The preparation and design of novel π-conjugated backbones offer
challenges to improve the device performance as the solubility is de-
creased through the strong π−π interactions in the solid state. In order
to overcome this issue, flexible side chains are usually introduced to
afford better solubility for enhanced device fabrication [19,20]. The
solubility as well as optoelectronic application of the conjugated
polymers were maintained by equipping a rigid conjugated polymer
backbone with an alkyl side chain such as oligo(ethylene glycol) chains
∗ Corresponding author.
E-mail address: cyyu@mail.ntust.edu.tw (C.-Y. Yu).
https://doi.org/10.1016/j.polymer.2018.12.039
Received 18 October 2018; Received in revised form 19 December 2018; Accepted 25 December 2018
Availableonline29April2019
0032-3861/©2019ElsevierLtd.Allrightsreserved.

A.S. Godana and C.-Y. Yu
Polymer174(2019)45–51
and fluoroalkyl chains [19,21] for efficient organic light emitting
diodes, organic field-effect transistors and organic photovoltaics de-
vices which can be deposited in polar nonhalogenated solvents [22,23].
The introduction of alkyl chains to the organic materials increases van
der Waals interactions between these chains and the solvent. This in-
creases the total interaction energy between the organic molecules and
solvent as well as the vibrational motions of alkyl chains that may
destroy the molecular arrangement in the solid state by decreasing the
interactions between π-conjugated systems [20]. Oligo(ethylene glycol)
chains are well known for their hydrophilic properties due to many
ether groups [24] and is often used to increase molecule/polymer so-
lubility in water or polar organic solvents [25,26]. The side-chain-end
functionalities of conjugated polymers in water/polar organic solvent
mixtures are one of the important classes of polymers which can be
processed from water or other polar solvents for interface modification
of organic electronics [27]. As mentioned by B. Meng and co-worker
[28], oligo(ethylene glycol) chains are more flexible than alkyl chains
due to less steric hindrance between the two lone electron pairs in two
oxygen atoms [29]. The nature of the flexible side chains on conjugated
polymer backbones alter the optical, electronic and charge transport
behavior by the self-organization of polymer backbones in the solid
state [30,31]. A high degree of solubility of the polymers in common
organic solvents may increase the molecular weight during the poly-
merization and enhance the processability of thin films [32,33].
The synthetic design of the dispersible nanoparticles of the π-con-
jugated polymers in aqueous systems is still a big challenge for fluor-
escence imaging because of the aggregation of the hydrophobic com-
ponents in aqueous media decreasing the light harvesting ability of the
polymers. Previously, we have successfully prepared alternating copo-
lymers containing fluorenes, PEGylated carbazoles and diphenylamines
by Suzuki-Miyaura cross-coupling reaction and their dispersion nano-
particles in aqueous media [34]. Although the self-organized structures
of the polyfluorenes [35–40] and the photophysical properties of the
amphiphilic polyfluorenes [41–45] have been intensively investigated,
there is no report on the alkyl and tri(ethylene glycol) substituted
fluorene and diphenylamine copolymers. Herein, we report the synth-
esis of fluorescent conjugated polymers containing tri(ethylene glycol)
and octyl side chains with various segments through Suzuki-Miyaura
cross-coupling reaction and their structural characterizations. The op-
tical properties of the as prepared polymers in water dispersion and
self-organization from good solvent/poor solvent mixtures were stu-
died. The particle sizes, morphologies and self-organization of the
conjugated polymers were also investigated.
Malvern Zetasizer Nano ZS at a temperature of 25 °C. The SEM images
of the conjugated polymer nanoparticles were obtained by field-emis-
sion scanning electron microscopy (FESEM, JSM 6500F, JEOL).
Synthesis of 2,7-dibromo-9,9-bis(2-(2-methoxyethoxy)ethyl)-9H-
fluorene (M1). A solution of 1-(2-(2-methoxyethoxy)ethoxy)-2-bro-
moethane (6.08 g, 26.8 mmol), 2,7-dibromofluorene (4.00 g,
12.4 mmol) and potassium iodide (67.60 mg, 0.27 mmol) in dimethyl
sulfoxide (40 mL) was cooled to 0 °C under a nitrogen atmosphere.
Potassium hydroxide (2.28 g, 40.8 mmol) was added to the mixture and
the mixture was allowed to warm to room temperature and was stirred
for 16 h. The reaction mixture was then poured into water and ex-
tracted with ethyl acetate and the organic layers dried under reduced
pressure. The crude product was purified by silica column chromato-
graphy, eluting with ethyl acetate/hexane (2:3) to give a colorless solid
(4.94 g, 65%). 1H NMR (600 MHz, CDCl3): δ 7.52 (d, J = 1.8 Hz, 2H),
7.50 (d, J = 8.4 Hz, 2H), 7.46 (dd, J = 8.4, 1.8 Hz, 2H), 3.52 (m, 4H),
3.48 (m, 4H), 3.38 (m, 4H), 3.33 (s, 6H), 3.20 (m, 4H), 2.78 (t,
J = 7.2 Hz, 4H), 2.34 (t, J = 7.2 Hz, 4H). 13C NMR (150 MHz, CDCl3,
δ): 150.95, 138.46, 130.66, 126.72, 121.64, 121.21, 71.88, 70.48,
70.45, 70.06, 66.79, 58.99, 51.91, 39.49. HR-MS (EI, [M]+): calculated
for [C27H36Br2O6]+: m/z 614.0879, found m/z 614.0879.
Synthesis of N-octyl-4,4′-dibromodiphenylamine (M2). A mixture of
bis(4-bromophenyl)amine) (4.90 g, 15.0 mmol), potassium tert-but-
oxide (2.02 g, 18.0 mmol) and 120 mL of anhydrous THF was stirred for
30 min at room temperature under
a nitrogen atmosphere. 1-
Bromooctane (63.47g, 18.0 mmol) was added and the mixture was
stirred for 24 h at 60 °C. After cooling to room temperature, the reaction
mixture was poured into water and then extracted with di-
chloromethane. The combined organic layers were washed with water
several times and dried over anhydrous MgSO4. The crude product was
purified by column chromatography (hexane/DCM) 7:3, v/v) to give a
colorless oil (5.25 g, 80%). 1H NMR (600 MHz, CDCl3): δ 7.37 (d,
J = 8.4 Hz, 4H), 6.87 (d, J = 8.4 Hz, 4H), 3.64 (m, 2H), 1.65 (m, 2H),
1.32–1.27 (m, 10H), 0.87 (t, J = 7.2 Hz, 3H). 13C NMR (150 MHz,
CDCl3, δ): 146.77, 132.27, 122.57, 113.86, 52.45, 31.78, 29.35, 29.25,
27.26, 27.01, 22.62, 14.08. HR-MS (EI, [M]+): calculated for
[C20H25Br2N]+: m/z 437.0354, found m/z 437.0352.
Synthesis of 4,4′-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-
N-octyl diphenylamine (M3). To
a solution of N-octyl-4,4′-di-
bromodiphenylamine (5.0 g, 11.3 mmol) in anhydrous THF (150 mL)
was added dropwise 11.8 mL (29.4 mmol) of n-butyllithium (2.5 M in
hexane) at −78 °C under a nitrogen atmosphere. After the mixture was
stirred for 2 h at −78 °C, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-diox-
aborolane (8.4 g, 45.2 mmol) was added and the mixture was warmed
to room temperature and stirred overnight. The resulting mixture was
poured into water, extracted with ether, and then dried over anhydrous
MgSO4. After the evaporation of the solvent, the crude product was
recrystallized from hexane and dried under vacuum to give white
crystals (yield 3.70 g, 60%). 1H NMR (600 MHz, CDCl3): δ 7.70 (d,
J = 8.4 Hz, 4H), 7.00 (d, J = 8.4 Hz, 4H), 3.72 (m, 2H), 1.65 (t,
J = 7.8 Hz, 2H), 1.28 (s, 24H), 1.27–1.22 (m, 10H), 0.88 (t, J = 7.2 Hz,
3H). 13C NMR (150 MHz, CDCl3, δ): 150.11, 136.04, 119.96, 83.51,
52.06, 31.79, 29.34, 29.24, 27.40, 27.01, 24.86, 22.62, 14.08. HR-MS
(EI, [M]+): calculated for [C32H49Br2NO4]+: m/z 533.3848, found m/z
533.3850.
2. Experimental section
Materials and instrumentation. Chemicals, reagents and solvents
from commercial sources were analytical or spectrophotometric grade
and used without further purification unless otherwise noted. 1H nu-
clear magnetic resonance (1H NMR) spectra were recorded on a Bruker
600 MHz. Chemical shifts are reported in ppm relative to the indicated
residual solvent. All coupling constants (J values) are reported in hertz
(Hz) and the following abbreviations are used to indicate multiplicity:
s = singlet, d = doublet, dd = doublet of doublets, t = triplet and
m = multiplet, br = broad signals. Molecular weights of polymers were
determined by gel permeation chromatography (GPC) using a Waters
ACQUITY Advanced Polymer Chromatography with RI detector in THF
solution calibrated with low polydispersity polystyrene standards. The
flow rate was set as a 0.8 mL/min in 45 °C. The UV–vis absorption
spectra were recorded using a Jasco (V-670) UV–Vis–NIR spectro-
photometer and fluorescence spectra were recorded a Jasco FP-8500
fluorescence spectrophotometer. The fluorescence quantum yields (Φ)
of the conjugated polymers and conjugated polymer nanoparticles were
performed in THF and water solutions, respectively, relative to the
quinine sulfate (Φ = 54.6% in 0.1 M H2SO4) as a standard. The hy-
drodynamic size of the nanoparticle dispersions were measured using a
Synthesis
of
4-bromo-N-(4-bromophenyl)-N-(2-(2-(2-methox-
yethoxy)ethoxy)ethyl)aniline (M4). Bis(4-bromophenyl) amine (4.0 g,
12.30 mmol) and potassium hydroxide powder (3.45 g, 61.50 mmol)
were dissolved in 50 mL of dimethyl sulfoxide. A solution of 1-(2-(2-
methoxyethoxy)ethoxy)-2-bromoethane (TEG-Br) (4.0 g, 18.0 mmol) in
10 mL DMSO was added dropwise by syringe and stirred for 36 h at
room temperature. After quenching the reaction mixture by the addi-
tion of distilled water, the crude compound was extracted with ethyl
acetate and washed with brine. The combined organic layers were dried
over MgSO4 and concentrated using a rotary evaporator. The crude
compound was purified by column chromatography with hexane/EA
46

A.S. Godana and C.-Y. Yu
Polymer174(2019)45–51
(7:3) as the eluent to give the pure compound in a yield of 3.47 g (60%).
1H NMR (600 MHz, CDCl3, δ): 7.33 (d, 4H, J = 9.0 Hz), 6.91 (d, 4H,
J = 9.0 Hz), 3.86 (t, 2H, J = 6.0 Hz), 3.64 (t, 2H, J = 6.0 Hz),
3.58–3.59 (m, 6H), 3.50 (m, 2H), 3.36 (s, 3H). 13C NMR (150 MHz,
CDCl3, δ): 146.55, 132.24, 122.74, 114.11, 71.91, 70.75, 70.68, 70.61,
68.01, 59.04, 51.78. HR-MS (EI, [M]+): calculated for
[C19H23Br2NO3]+: m/z 471.0045, found m/z 471.0044.
Synthesis of the alternating copolymer P1: 2,7-dibromo-9,9-bis(2-
(2-methoxyethoxy)ethyl)-9H-fluorene (M1) (0.37 g, 0.60 mmol), 4,4′-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-N-octyl diphenyla-
mine (M3) (0.32 g, 0.60 mmol) and a drop of Aliquat 336 were dis-
solved in toluene (12 mL) and then the mixture was degassed for
copolymers P2 and P3 using monomers M1, M3, M2 and M1, M3, M4,
respectively. The crude polymer was further purified by a Soxhlet ex-
traction in methanol and acetone for 24 h to remove catalyst, by-pro-
duct and oligomers before fully solubilizing in hot dichloromethane.
The molecular weights of the polymers P1, P2 and P3 were de-
termined by gel permeation chromatography (GPC) in THF solution,
calibrated against narrow polydispersity index polystyrene standards,
using refractive index (RI) detection. The results of the number average,
weight average molecular weights, polydispersity index (PDI) and re-
action yields were summarized in Table 1. The measured number
average molecular weight (Mn) of the copolymers P1, P2 and P3 was
20,800, 8000 and 11,700 g/mol and the PDI was 1.93, 1.33 and 1.50,
respectively. The higher Mn value of 20,800 g/mol for polymer P1
compared to that of P2 (8000 g/mol) and P3 (11,700 g/mol) can be
possibly attributed to the better solubility of the resulting polymers. In
addition, the reaction yield of P1, P2 and P3 is 60, 47 and 43%, re-
spectively.
30 min.
A tetrakis (triphenylphosphine) palladium (0) (35 mg,
0.03 mmol) was added to the reaction mixture followed by the addition
of aqueous sodium carbonate (2 M, 6 mL) and was degassed for an
additional 30 min. The reaction mixture was stirred and heated at reflux
for 56 h before cooling to room temperature. The resulting polymer was
precipitated by pouring into distilled water; the crude compound was
extracted with dichloromethane, washed with brine, and dried over
MgSO4. The solvent was removed in vacuo and the resulting crude
powder was purified by Soxhlet extraction with methanol and acetone
for a day before redissolving in chloroform. The solid was then dried
under vacuum to give a yield of 60%. 1H NMR (600 MHz, CDCl3, δ):
7.74 (d, 2H), 7.65–7.60 (m, 8H), 7.17 (d, 4H), 3.80 (br, 2H), 3.50–3.49
(m, 4H), 3.45–3.43 (m, 4H), 3.40 (t, 4H), 3.30 (s, 6H), 3.24 (t, 4H), 2.86
(t, 4H), 2.47 (br, 4H), 1.70 (br, 2H), 1.31–1.29 (m, 10H), 0.89 (t, 3H).
Mn = 20,800, Mw = 40,200, PDI = 1.93.
Synthesis of the alternating copolymer P2: A similar procedure was
used as that described for the synthesis of P1 using 2,7-dibromo-9,9-bis
(2-(2-methoxyethoxy)ethyl)-9H-fluorene (M1) (0.18 g, 0.30 mmol), N-
octyl-4,4′-dibromodiphenylamine (M2) (0.13g, 0.30 mmol), 4,4′-bis
(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-N-octyl diphenylamine
(M3) (0.32 g, 0.60 mmol), a drop of Aliquat 336 and tetrakis (triphe-
nylphosphine) palladium (0) (35 mg, 0.03 mmol). P2 was obtained in a
yield of 47%. 1H NMR (600 MHz, CDCl3, δ): 7.74–7.72 (br, 2H),
7.64–7.60 (m, 8H), 7.52–7.50 (br, 8H), 7.17–7.09 (m, 12H), 3.78–3.76
(br, 6H), 3.50–3.49 (m, 4H), 3.45–3.44 (m, 4H), 3.41–3.39 (m, 4H),
3.30 (t, 6H), 3.24 (t, 4H), 2.86 (br, 4H), 2.47 (br, 4H), 1.74 (br, 6H),
1.32–1.28 (m, 30H), 0.89–0.88 (m, 9H). Mn = 8000, Mw = 10,600,
PDI = 1.33.
Synthesis of the alternating copolymer P3: A similar procedure was
used as that described for the synthesis of P1 using 2,7-dibromo-9,9-bis
(2-(2-methoxyethoxy)ethyl)-9H-fluorene (M1) (0.18 g, 0.30 mmol),
4,4′-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-N-octyl diphe-
nylamine (M3) (0.32 g, 0.60 mmol), 4-bromo-N-(4-bromophenyl)-N-(2-
(2-(2-methoxyethoxy)ethoxy)ethyl)aniline (M4) (0.14 g, 0.30 mmol), a
drop of Aliquat 336 and tetrakis (triphenylphosphine) palladium (0)
(35 mg, 0.03 mmol). P3 was obtained in a yield of 43%. 1H NMR
(600 MHz, CDCl3, δ): 7.74–7.73 (br, 2H), 7.64–7.60 (m, 8H), 7.53–7.50
(m, 8H), 7.17–7.12 (m, 12H), 4.02 (br, 4H), 3.76 (br, 4H), 3.65–3.62
(m, 4H), 3.54–3.53 (br, 2H), 3.50–3.48 (m, 4H), 3.45–3.43 (m, 4H),
3.41–3.39 (m, 4H), 3.37–3.36 (m, 2H), 3.30 (t, 9H), 3.25–3.23 (m, 4H),
2.86 (br, 4H), 2.47 (br, 4H), 1.75 (br, 4H), 1.32–1.29 (m, 20H), 0.88 (t,
6H). Mn = 11,700, Mw = 17,500, PDI = 1.50.
3.2. Characterization of polymers by NMR spectroscopy
The 1H NMR spectra of P1, P2 and P3 are shown in Fig. 1. The 1H
NMR spectrum of P1 recorded in CDCl3 shows a doublet at 7.74 ppm
corresponding to the hydrogens of fluorene rings meta to the carbon
bonded to two triethylene glycol monomethyl ether (TEG) chains. The
signals between 7.65 and 7.60 ppm are assigned to the hydrogens of the
fluorene rings ortho and para to the carbon bonded to two TEG chains
and to the hydrogens of phenyl rings bonded to the carbons meta to the
nitrogen. The hydrogens of phenyl rings bonded to the carbons ortho to
the nitrogen appear at 7.17 ppm. The signals between 3.50 and
2.40 ppm integrating to 30 hydrogens are associated with the hydro-
gens of the triethylene glycol chains. A broad signal at the 3.87 ppm
indicates alpha hydrogens of the octyl group attached on the nitrogen.
The signals below 2.00 ppm correspond to the remaining hydrogens of
the octyl chain. A broad peak at 7.74 ppm for P2 is associated to the
hydrogens of fluorene rings meta to the carbon bonded to two triethy-
lene glycol monomethyl ether chains.
A
signal appears at
7.65–7.59 ppm corresponds to the hydrogens of the fluorene rings ortho
and para to the carbon bonded to two TEG chains and to the hydrogens
of phenyl rings attached to the carbons meta to the nitrogen. In addi-
tion, the hydrogens of the aromatic rings of diphenylamine bonded to
the carbons ortho to the nitrogen were found at 7.10 ppm. The signals
below 4.00 ppm are attributed to the TEG and octyl chain hydrogens.
The 1H NMR spectrum of P3 observed a broad signal at 7.80 ppm is
associated to the hydrogens of the fluorene rings meta to the carbon
bonded to two TEG chains. Signals appear at 7.65–7.59 ppm correspond
to the hydrogens of the fluorene rings ortho and para to the carbon
bonded to two TEG chains and to the hydrogens of phenyl rings at-
tached to the carbons meta to the nitrogens. In addition, signals be-
tween 7.20 and 7.05 ppm correspond to the hydrogens of the aromatic
rings of diphenylamine bonded to the carbons ortho to the nitrogens.
Again, the signals below 4.00 ppm are attributed to the hydrogens of
the TEG and octyl chains. The signals associated with the hydrogens of
the corresponding polymers are fully characterized. The results of 1H
NMR spectra of all polymers confirmed the microstructure of the
polymer backbone. In addition, the position of peaks corresponded to
the hydrogens were confirmed by 2D 1H-1H COSY spectra in the Figure
S1-S3 in supporting information. The mole percent of each monomer
unit can be calculated and the microstructures of the copolymers can be
evaluated from the integration areas of the corresponding peaks from
the 1H NMR spectra of the copolymers.
3. Results and discussion
3.1. Polymerization
The synthetic routes for monomers M1, M2, M3 and M4 can be
found in Scheme S1 in supporting information. The synthetic routes of
the copolymers are shown in Scheme 1. Alternating copolymer P1 was
prepared by Suzuki-Miyaura cross-coupling reaction using equimolar
amounts of the comonomers such as M1 and M3 in the presence of Pd
(PPh3)4 in a mixture of toluene and aqueous K2CO3. A similar procedure
was used as that described for P1 by the synthesis of random
3.3. Conjugated polymer nanoparticles and aggregates
Conjugated polymer nanoparticles were prepared by rapid pre-
cipitation method. A polymer solution in THF (1 mg/mL) was rapidly
injected into water with the relative amount of polymer solution and
47

A.S. Godana and C.-Y. Yu
Polymer174(2019)45–51
Scheme 1. Synthetic routes to conjugated copolymers P1, P2 and P3 (R1 = –CH2CH2OCH2CH2OCH2CH2OCH3, R2 = -octyl).
Table 1
The number average, weight average molecular weights, polydispersity index
and reaction yields of polymers P1-P3.
Polymer Reaction Condition
Mn (g/mol)a Mw (g/mol)a PDIa Yield (%)b
P1
P2
P3
Toluene, 110 °C, 56 h 20,800
Toluene, 110 °C, 56 h 8000
Toluene, 110 °C, 56 h 11,700
40,200
10,600
17,500
1.93 60
1.33 47
1.50 43
a
Determined by GPC in THF solution calibrated with monodisperse poly-
styrene standards using RI detection.
b
After Soxhlet extraction.
non-solvent (water) in a volume ratio of 1: 4 for each experiment with
vigorous stirring at room temperature. Residual THF was removed from
the resultant nanoparticle suspensions by blowing argon over the sus-
pension. Self-assembled polymeric aggregates were prepared by dis-
solving 2 mg of polymers into 2 mL of THF. The vial containing a
polymer solution was then placed in a 50 mL vial containing 5 mL of a
nonsolvent such as acetone or methanol and the outer vial was capped.
The nonsolvent vapors gradually diffused into the solution of the
polymers to give a suspension after 4 days at room temperature.
Scanning electron microscopy (SEM) images were taken from air-dried
suspension of polymers drop-cast onto the glass slide (Fig. 2). Con-
jugated polymer P1 from THF/MeOH mixture afforded regular sheet-
like features (Fig. 2a) possibly due to slow nucleation of polymer
backbones in the THF/MeOH system. The SEM image of the conjugated
polymer P1 from the mixture of THF/acetone showed nanoaggregates
instead of well-dispersion structures (Fig. 2b). In addition, the SEM
image of the conjugated polymer nanoparticles P1 from the dispersion
method in water showed mainly a spherical geometry (Fig. 2c). The
SEM images of the conjugated polymer P2 from THF/MeOH mixture
exhibited regular sheet-like features (Fig. 2d). A clear spherical geo-
metry was observed for P2 from a solvent combination of THF/acetone
(Fig. 2e). Conjugated polymer nanoparticle P2 formed by the dispersion
method in water was not as smooth as the spheres of P1 due to the
formation of partial aggregates (Fig. 2f). An irregular aggregate struc-
ture (Fig. 2g) was observed for the P3 from the THF/MeOH mixture.
Spherical assemblies were formed upon the diffusion of acetone into the
polymer solution of P2 (Fig. 2h) with a relatively high content of di-
phenylamine moieties. In addition, irregular aggregates (Fig. 2i) were
observed for P3 dispersed in water. The relatively high contents of TEG
or octyl substituted diphenylamine moieties of P2 and P3 disturbed the
planarity of polymer backbones to form twisted configurations, leading
to disordered inter-chain packing which contributed to a spherical
morphology [46–48]. The polymers with a relatively high content of
diphenylamine moieties possibly aggregate due to low affinity for the
nonsolvents which minimizes the contact area with the hydrophobic
polymer, and contributes to the gradual growth of a spherical geometry
[49]. Both the average size and the morphology of the polymers were
strongly depended on the solubility of the polymers in a good solvent as
well as the rate of diffusion of the poor solvent.
Fig. 1. H1 NMR spectra of polymers (a) P1, (b) P2 and (c) P3 in CDCl3.
As shown in Fig. 2, the well-defined spherical structure and uniform
particle distribution of the copolymers P1 prepared from rapid pre-
cipitation in water can be observed. The average hydrodynamic dia-
meter of copolymers P1-P3 dispersed in water determined by dynamic
light scattering (DLS) measurement is shown in Fig. 3. A range of
75–90 nm (Fig. 3a) for the rapidly precipitated copolymer P1 was
measured, which is in agreement with the particle size calculated by FE-
SEM images of the copolymer P1. The average hydrodynamic diameter
of rapidly precipitated copolymers P2 (Fig. 3b) and P3 (Fig. 3c) is 421
and 350 nm, respectively which is relatively large compared to that of
48

A.S. Godana and C.-Y. Yu
Polymer174(2019)45–51
Fig. 2. SEM images of P1 from (a) THF/MeOH, (b) THF/acetone and (c) water dispersion, P2 from (d) THF/MeOH (e) THF/acetone and (f) water dispersion and P3
from (g) THF/MeOH (h) THF/acetone and (i) water dispersion.
Fig. 3. Histograms of size distribution of the polymers (a) P1 (b) P2 and (c) P3 dispersed in water obtained by DLS analysis.
Fig. 4. XRD patterns of cast films of (a) P1, (b) P2 and (c) P3 from THF/MeOH, THF/acetone, water dispersion and powder.
P1 possibly due to the nature of the aggregate features of copolymers
P2 and P3. The highly hydrophobic nature of random copolymers P2
and P3 containing a relatively large content of hydrophobic moieties
unable to precipitate rather than forming aggregates in aqueous media.
X-ray diffraction (XRD) is one of the most utilized techniques to
confirm the differences in the crystallinity of copolymers. As shown in
the Fig. 4, the XRD of the blank sample did not show any diffraction
peaks as we expected. The self-assembled copolymers with a combi-
nation of solvents, such as THF/MeOH and THF/acetone were crystal-
lized and the XRD 2theta peaks were around 6ο and 10ο. It is worth
noting that a relatively strong peak intensity was observed for P2
prepared from the THF/acetone mixture compared to that of P1 and P3
possibly due to the a reorientation of the octyl chain taking place at the
interface between THF and acetone. The neat polymer powders and
49

A.S. Godana and C.-Y. Yu
Polymer174(2019)45–51
Table 2
Photophysical properties of P1-P3.
a
b
c
a
b
c
d
e
f
g
Polymer
λabs (nm)
λabs (nm)
λabs (nm)
λPL (nm)
λPL (nm)
λPL (nm)
ΦPL (%)
ΦPL (%)
ΦPL (%)
ΦPL (%)
P1
P2
P3
385
375
376
397
381
386
383
420
417
436
440
440
444
443
440
440
438
438
55
30
28
0.59
0.33
0.25
42
22
20
30
19
15
a
In dilute THF solution (5 × 10−6 M).
Polymer thin films spin-coated from a polymer THF solution.
Polymer nanoparticle dispersion from THF/water mixture.
Quantum yields from polymer solution.
Quantum yields from polymer dispersed in water.
Quantum yields of self-assembled polymers from THF/MeOH and.
Quantum yields of self-assembled polymers from THF/acetone.
b
c
d
e
f
g
Fig. 5. Normalized absorption (solid lines) and fluorescence (dashed line) spectra of P1-P3; (a) conjugated polymer solution (in THF), (b) solid state and (c) water
dispersion.
polymer nanoparticles prepared by rapid precipitation for P1, P2 and
P3 do not show any diffraction peaks indicating the amorphous nature
of the polymers.
P1 is mainly caused by the aggregation of conjugated polymer upon
nanoparticle formation which increases the interactions between seg-
ments of the polymer chains [54]. The fluorescence quantum yields of
polymer nanoparticles were significantly quenched compared to that of
the solution (0.59%, 0.33% and 0.25% in aqueous media versus 55%,
30% and 28% for P1, P2 and P3 in THF, respectively). The nature of
aggregation of polymer chains (interchain interactions) in the aqueous
media and self-aggregation driven by the poor solubility as well as π-π
interactions quenched the fluorescence [55]. The nature of conjugated
polymer backbone influenced spectroscopic properties, such as ab-
sorption and emission efficiency, as consequence of the internal orga-
nization of aggregation. A higher percentage of a hydrophobic segment
in the polymer chain will have stronger hydrophobic-hydrophobic in-
teractions between polymer chains, which quench the emission in-
tensity in aqueous media. However, the fluorescence quantum yields of
polymers P1-P3 from THF/MeOH or THF/acetone were much higher
compared to that of the polymers prepared by rapid precipitation. The
absorption and photoluminescence maximum of self-assembled ag-
gregates (Figure S4 and Table S1) is slightly red shifted in a range of
1–2 nm in comparison to solution. The smaller Δλmax values between
polymer solution and self-assembled aggregates compared with
polymer nanoparticles derived from rapid precipitation imply that the
interchain interactions of self-assembled aggregates are relatively weak.
3.4. Photophysical properties of the copolymers
The details of photophysical properties of the conjugated polymer in
solutions, in solid state, as well as rapid precipitation and self-assembly
from THF/MeOH and THF/acetone are shown in Table 2 and Table S1.
The absorption and fluorescence spectra of P1-P3 in solution, in solid
state and in water dispersion are shown in Fig. 5. The conjugated
polymer P1 containing an alternating fluorene-diphenylamine structure
with a relatively high molecular weight exhibits a longer absorption
wavelength (385 nm) in THF solution compared to that of P2 (375 nm)
and P3 (376 nm). The solid state absorption maximum of the polymers
for P1, P2 and P3 is at 397, 381 and 386 nm, respectively which is more
red-shifted compared to that of solution possibly due to the aggregation
formation. The dense aggregation characteristics of rapid precipitation
for polymers P2 and P3 show a red shifted absorption maxima com-
pared to the solution and the solid state [50,51]. However, the ab-
sorption maximum of conjugated polymer nanoparticles for P1 was
blue shifted with a longer wavelength tail compared to the solution and
the solid state. This phenomenon can be attributed to the densely
packed structure of the particles causing a decrease in the conjugation
length of the polymer backbones due to a high degree of bending [52].
The solution photoluminescence emission maximum of polymers
P1-P3 was 436, 440 and 440 nm, respectively. The large stokes shift for
P2 (65 nm) and P3 (64 nm) compared to P1 (51 nm) suggests a more
planar structure in the excited state due to redistribution of the electron
density in the excited state [53]. The solid state emission maximum of
polymers P1-P3 was 444, 443 and 440 nm, respectively which is red-
shifted compared to that of solution. This could be attributed to ag-
gregation in solid state. In comparison to the solution, the emission
maximum of the polymer nanoparticles for P2 and P3 was blue shifted
by 2 nm and the polymer nanoparticles for P1 was red shifted by 4 nm.
The red-shift in the emission maximum for the polymer nanoparticles of
4. Conclusion
In summary, π-conjugated copolymers containing fluorene and di-
phenylamine with alkyl and TEG side chains have been prepared by
Suzuki-Miyaura cross-coupling reaction and their structures were fully
characterized. The shapes and structures of the polymers were highly
depended on the planarity of π-conjugated system, the molecular
weights and the solvent combination. These factors influenced the in-
ternal organization of aggregation and the interactions between copo-
lymers and surrounding media. Such parameters are powerful compo-
nents for determining morphologies and optical properties and are
useful tools for developing strategies for converting π-conjugated
50

A.S. Godana and C.-Y. Yu
Polymer174(2019)45–51
polymers into colloidal architectures. Self-assembled polymers with
micrometer sized particles from the solvent diffusion method exhibited
a relatively high photoluminescence quantum yield in comparison with
that of the conjugated polymer nanoparticles formed from the rapid
precipitation method.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
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