Preservation of photoluminescence efficiency in the ordered phases of poly(2,3-diphenyl-1,4-phenylenevinylene) via disturbing the intermolecular χ-χ Interactions with dendritic aliphatic side chains

Article abstract of DOI:10.1021/ma300640n

To understand the relation between the solid-state phase structures and the photophysical properties of poly(2,3-diphenyl-1,4-phenylenevinylene) (DP-PPV) derivatives, three DP-PPV derivatives, P1-P3, were designed, synthesized via Gilch polymerization and

Full text of DOI:10.1021/ma300640n

Article
pubs.acs.org/Macromolecules
Preservation of Photoluminescence Efficiency in the Ordered phases
of Poly(2,3-diphenyl-1,4-phenylenevinylene) via Disturbing the
Intermolecular π−π Interactions with Dendritic Aliphatic Side Chains
Yu-Chun Wu,† Xiang-Kui Ren,‡ Er-Qiang Chen,‡ Hsun-Mei Lee,† Jean-Luc Duvail,§
Chien-Lung Wang,*,† and Chain-Shu Hsu*,†
†Department of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh Road, Hsin-Chu 30010, Taiwan
‡Beijing National Laboratory for Molecular Sciences, Department of Polymer Science and Engineering and Key Laboratory of
Polymer Chemistry and Physics of Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China
§Institut des Mater
́
iaux Jean Rouxel, UMR6502 CNRS, University of Nantes, F-44322 Nantes, France
S
* Supporting Information
ABSTRACT: To understand the relation between the solid-state phase structures and the photophysical properties of poly(2,3-
diphenyl-1,4-phenylenevinylene) (DP-PPV) derivatives, three DP-PPV derivatives, P1−P3, were designed, synthesized via Gilch
polymerization and characterized. Among the polymers, P1 is a reported highly emissive poly(2,3-diphenyl-5-hexyl-p-
phenylenevinylene), and P2 and P3 are novel DP-PPV derivatives, which are purposely designed to bear hydrophobic and
hydrophilic Percec-type dendrons as side chains. The bulkiness and hydrophobic−hydrophilic natures of the side chains show
strong effects on photophysical properties of the polymers. The solutions and as-casted films of P1−P3 all show remarkably high
photoluminescence (PL) efficiency (ΦPL) (>80% in chloroform solution, and >63% for the as-casted films). However, ΦPL of P1
and P3 decrease significantly to 30% after cooled their polymer melts to room temperature. Through the phase behavior analysis
by differential scanning calorimetry (DSC), and phase structure analysis by wide-angle X-ray diffraction (WAXD), the decrease of
ΦPL can be elucidated and attributed to the ordering of the solid-state structures of P1 and P3. To our surprise, ΦPL of P2 is
preserved even in an ordered solid-state phase, and it is insensitive to the structural ordering. Structural analysis of P2 revealed
that the aliphatic dendritic side chains of P2 effectively disturbing the intermolecular π−π interactions among the conjugated
backbones, which allows the preservation of ΦPL in the environment with ordered packing of DP-PPV molecules. The results of
time-resolved PL decay experiments also confirmed that P2 possesses long-lived decay time because of excitons confined more
effectively for emissive relaxation.
formation of molecular aggregates. These molecular aggregates
are lower-energy nonemissive sites in the polymer matrix,
which can trap the excitation energy and quench the relaxation
pathways of PL and electroluminescence (EL).7,8 To minimize
the formation of the aggregates and increase the ΦPL in the
solid state, various approaches, such as incorporation of meta-
substitution in the polymer backbone,9−12 polymer blends,13,14
and introduction of pendant groups,15−19 have been reported
to control the π−π interaction induced molecular aggregation.
INTRODUCTION
■
Conjugated polymers have received considerable interest due
to their potential applications as active materials in optoelec-
tronic devices, such as light-emitting diodes,1−3 thin film
transistors,4 and organic solar cells.5 Among them, poly-
(phenylenevinylene) (PPV) and its derivatives have been one
of the most extensively studied conjugated polymers because of
their excellent optoelectronic properties and processability.6
Although PPVs possess many advantages, poor solid-state
photoluminescence (PL) efficiency (ΦPL) has been a long-
lasting challenge for the PPV derivatives. For most of the
conjugated polymers, their ΦPLs are substantially lower in the
solid state than those in the solution state because of the
Received: March 29, 2012
Revised: May 6, 2012
Published: May 16, 2012
© 2012 American Chemical Society
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Scheme 1. Synthetic Route of Homopolymers P1−P3
Among these researches, introducing of bulky pendant groups
as polymer side chains has been demonstrated as a promising
way to preserve the high ΦPL in the solid state without
changing the energy gaps of the parent polymer.20,21 On the
basis of this concept, PPV derivatives, poly(2,3-diphenyl-1,4-
phenylenevinylene) (DP-PPV) and poly(2-dimethylphenylsilyl-
1,4-phenylenevinylene) were synthesized and exhibits solid-
state photoluminescence (PL) efficiency between 60 and
80%.22,15 In addition, different substituents were introduced
at C-5 position of the phenylene moieties of DP-PPV to not
only further enhance the ΦPL, but also to improve the solubility
of the polymers.22−24 Recently, we have also reported various
types of novel DP-PPV-based copolymers with excellent
polymer light emitting diode performance.25−27
among the conjugated backbones. Furthermore, the hydrophilic
dendrons of P3 are purposely designed to study the effect of
side-chain segregation on the phase structure and photophysical
properties. The phase behaviors and phase structures were
characterized with differential scanning calorimetry (DSC) and
one-dimensional (1D) and two-dimensional (2D) wide-angle
X-ray diffractometer (WAXD), and the photophysical proper-
ties were characterized with UV−vis absorption spectropho-
tometer, luminescence spectrometer, and time-resolved PL
decay measurements. The structure−property relationships of
P1−P3 were investigated through the analysis and comparison
of the phase structures solved from WAXD analysis and
photophysical date measured from the spectroscopies.
Although the rational molecular designs of the PPVs have
successfully enhanced the solid-state ΦPL of PPVs, to the best
of our knowledge, the relation between the solid-state phase
structures and the ΦPL of these highly emissive PPVs remains
unexplored. The previous molecular designs used bulky
substituents to minimize the intermolecular interactions
among the conjugated chains, and avoid the formation of
ordered structures in the solid state. However, it is still unclear
if these amorphous phases of the PPVs are a thermodynami-
cally stable phase and whether or not further ordering of the
solid-state structure would decrease the ΦPL of the materials.
More importantly, it is of great interests to know if it is possible
to preserve the ΦPL even when the ordering process of the
solid-state structure occurs. To reach in-depth understanding
about this issue, a set of three DP-PPV derivatives (P1−P3)
were synthesized via the Gilch polymerization as shown in
Scheme 1. P1 is the well-known poly(2,3-diphenyl-5-hexyl-p-
phenylenevinylene), which demonstrates the solid-state ΦPL
over 60% in the as-casted film.22 Its phase structure−
photophysical property relation is carefully studied in this
work. P2 and P3 are designed to bear hydrophobic and
hydrophilic dendrons, respectively. Dendronized polymers also
have been proved to efficiently restrain the aggregations of the
polymer backbones,28,29 and enhance the optical and electrical
properties of conjugated polymers through reducing self-
quenching process.30−34 It is expected that the bulky dendritic
pendants of P2 and P3 would block intermolecular interactions
EXPERIMENTAL SECTION
■
General Measurement and Characterization. 1H and 13C
NMR spectra were measured with a Varian-300 MHz spectrometer.
Gel permeation chromatography (GPC) data assembled from a
Viscotek T50A Differential Viscometer and a LR125 Laser
Refractometer. Three columns in series were used to measure the
molecular weights of polymers relative to polystyrene standards.
Thermal gravimetric analysis (TGA) was recorded on Perkin-Elmer
Pyris under a nitrogen atmosphere at a heating rate of 10 °C/min. The
differential scanning calorimetry (DSC) measurements were carried
out utilizing a Perkin-Elmer Pyris I DSC with a mechanical
refrigerator. The temperature and heat flow scales were calibrated at
different heating and cooling rates (1−40 °C/min) using standard
materials such as benzoic acid and indium. The one-dimensional (1D)
wide-angle X-ray diffraction (WAXD) experiments were performed on
a Philips X’Pert Pro diffractometer with a 3 kW ceramic tube as the X-
ray source (Cu Kα) and an X’celerator detector. The sample stage was
set horizontally. The reflection peak positions were calibrated with
silicon powder (2θ > 15°) and silver behenate (2θ< 10°). A
temperature control unit (Paar Physica TCU 100) in conjunction with
the diffractometer was utilized to study the structure evolutions as a
function of temperature. The heating and cooling rates in the WAXD
experiments were 1.5 °C/min. Two-dimensional (2D) WAXD
patterns were obtained using a Bruker D8Discover diffractometer
with GADDS as a 2D detector. Again, calibration was conducted using
silicon powder and silver behenate. Samples were mounted on the
sample stage, and the point-focused X-ray beam was aligned both
perpendicular and parallel to the mechanical shearing direction. The
2D diffraction patterns were recorded in a transmission mode at room
temperature. UV−vis absorption spectra were obtained with an HP
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Scheme 2. Synthetic Route of Monomers M2 and M3
8453 diode array spectrophotometer. PL emission spectra were
obtained using ARC SpectraPro-150 luminescence spectrometer. The
photoluminescence (quantum) efficiency were measured according to
the previous literatures.10,20,34 For example, the polymer films for
solid-state PL measurement were prepared by spin-cast on quartz
substrates. The thickness of the films was controlled to ensure the
intensity of their absorbance was between 0.08 and 0.10. The relative
PL efficiency of films were estimated by using Φs/Φr = (Ar/As)(Fs/
Fr)(ns2/nr2) ≈ (Ar/As)(Fs/Fr), assuming the same refractive index for
polymer films. Here Φr, Ar, and Fr are PL efficiency, absorbance at
excitation wavelength, and emission integration area for the reference,
while As and Fs are absorbance and emission integration for the sample
film.10 The PL efficiency of polymer film was calculated by assuming
the efficiency of the reference polyfluorene thin film to be 0.55.34 The
transient photoluminescence experiments have been achieved with a
homemade, time-resolved confocal microscope setup. Excitation at
400 nm was provided by a Spectra-Physics Hurricane X laser system
(82 fs, 1 kHz). The mean pump power impinging on samples was kept
well below 0.1 mW with density filters to minimize sample
photobleaching and annihilation processes. The collected emission
was temporally detected with a streak camera (Hamamatsu C7700)
coupled to an imaging spectrograph.
under vacuum for several days. For 2D WAXD experiments, the
oriented samples were prepared by mechanically shearing the P1, P2,
and P3 polymers at 320, 180, and 200 °C.
Synthesis of Monomers. All reagents and chemicals were
purchased from commercial sources (Aldrich, Lancaster, or TCI)
and used without further purification. Tetrahydrofuran (THF) was
dried by distillation from sodium/benzophenone. The synthetic route
of P1−P3 are shown in Scheme 1, and the synthetic routes for
monomers M2 and M3 are shown in Scheme 2. Compounds 1a, 1b,
2a, 2b, 3a, and 3b in Scheme 2 and M1 in Scheme 1 were synthesized
according to previous literature.22,35,36
3,4,5-Tris(dodecyloxy)-1-benzyloxypent-4-yne (4a). Into a
two-neck round-bottom flask was added sodium hydride (0.72 g,
30.0 mmol) in the presence of 0.1 equiv of tetrabutylammonium
bromide. Dendritic chloride 3a (6.8 g, 10.0 mmol) in THF (50 mL)
and pent-4-yn-1-ol (2.5 g, 30.0 mmol) were added dropwise
subsequently. The mixture was refluxed for 12 h under nitrogen
atmosphere. The excess of sodium hydride was quenched with water
(2 mL) in an ice bath. After evaporating the solvent, the crude product
was dissolved in 100 mL of ethyl acetate and extracted with brine (200
mL) twice. The combined organic layer was dried over MgSO4. After
removal the solvent under reduced pressure, the residue was purified
by column chromatography on silica gel [ethyl acetate/hexane = 1/40
(v/v)] to give a colorless liquid 4a.(5.2 g, 72%).
Sample Preparation. For DSC experiments, a typical mass of 5
mg was encapsulated in a sealed aluminum pan with the pan weight
identically matched to the reference pan. For 1D WAXD powder
experiments, the samples were cast from chloroform solution and
dried at room temperature under a vacuum. The films were then kept
1H NMR (CDCl3, δ, ppm): 0.86−0.94 (m, 9H), 1.20−1.46 (m,
54H), 1.70−1.87 (m, 8H), 1.94 (s, 1H), 2.29−2.35 (t, 2H), 3.54−3.58
(t, 2H), 3.90−3.99 (m, 6H), 4.40 (s, 2H), 6.52 (s, 2H). 13C NMR
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(CDCl3, δ, ppm): 14.10, 15.30, 22.68, 26.10, 28.62, 29.36, 29.41,
29.65, 29.69, 29.75, 30.31, 31.92, 68.45, 68.59, 69.05, 73.25, 73.38,
83.93, 106.07, 133.42, 137.56, 153.12. MS (EI, C48H86O4): calcd,
727.19; found, 726.
1H NMR (CDCl3, δ, ppm): 0.83−0.88 (t, 9H), 1.17−1.43 (m,
54H), 1.69−1.89 (m, 6H), 2.03−2.08 (m, 2H), 2.94−3.00 (t, 2H),
3.55−3.59 (t, 2H), 3.89−3.97 (m, 6H), 4.38 (s, 2H), 4.40 (s, 2H),
4.42 (s, 2H), 6.47 (s, 2H), 6.89−7.12 (m, 10H), 7.59 (s, 1H). 13C
NMR (CDCl3, δ, ppm): 14.08, 22.68, 26.12, 29.22, 29.35, 29.45, 29.64,
29.69, 29.74, 30.07, 30.32, 31.04, 31.91, 59.41, 63.43, 69.06, 69.53,
73.16, 73.98, 105.98, 126.60, 126.84, 127.33, 127.56, 130.09, 130.29,
133.66, 133.91, 136.52, 137.46, 138.17, 138.73, 139.80, 140.16, 141.47,
153.16. MS (FAB, C66H102O6): calcd, 991.51; found, 991.
2,3-Diphenyl-5-(3,4,5-tris(2-(2-ethoxyethoxy)ethoxy)-1-ben-
zyloxypropyl)-1,4-bis(hydroxymethyl)benzene (6b). A solution
of 5b (5.4 g, 5.9 mmol) in dry THF (40 mL) was added dropwise
under nitrogen atmosphere to a stirred suspension of LiAlH4 (2.3 g,
59.0 mmol) in dry THF (20 mL) cooled to 0 °C in an ice bath. The
reaction mixture was stirred and refluxed for 12 h. The excess of
hydride was quenched under cooling in an ice bath with 15%
NaOH(aq) (2 mL) followed by H2O (10 mL). The THF was
removed by evaporation and the resulting mixture was dissolved in 100
mL of ethyl acetate and extracted with brine (200 mL) twice. The
organic phase was then dried over MgSO4, evaporated, and purified
with column chromatography [silica gel, ethyl acetate as eluent] to
yield 6b as a colorless oil (3.9 g, 75%).
3,4,5-Tris(2-(2-ethoxyethoxy)ethoxy)-1-benzyloxypent-4-
yne (4b). Into a two-neck round-bottom flask was added sodium
hydride (1.27 g, 52.8 mmol) in the presence of 0.1 equiv of
tetrabutylammonium bromide. Dendritic chloride 3b (9.2 g, 17.6
mmol) in THF (50 mL) and pent-4-yn-1-ol (4.5 g, 52.8 mmol) were
added dropwise subsequently. The mixture was refluxed for 12 h under
nitrogen atmosphere. The excess of sodium hydride was quenched
with water (2 mL) in an ice bath. After evaporating the solvent, the
crude product was dissolved in 100 mL of ethyl acetate and extracted
with brine (200 mL) twice. The combined organic layer was dried over
MgSO4. After removal the solvent under reduced pressure, the residue
was purified by column chromatography on silica gel [ethyl acetate/
hexane = 2/1 (v/v)] to give a yellow liquid 4b. (8.5 g, 85%)
1H NMR (CDCl3, δ, ppm): 1.14−1.23 (t, 9H), 1.80−1.86 (m, 2H),
1.95 (s, 1H), 2.31−2.34 (t, 2H), 3.47−3.87 (m, 28H), 4.12−4.18 (m,
6H), 4.39 (s, 2H), 6.57 (s, 2H). 13C NMR (CDCl3, δ, ppm): 15.14,
15.26, 28.55, 66.64, 68.56, 68.76, 69.70, 69.87, 69.90, 70.49, 70.52,
70.86, 72.29, 72.96, 83.85, 106.98, 133.91, 137.68, 152.59. MS (EI,
C30H50O10): calcd, 570.71; found, 570.
Diethyl 2,3-diphenyl-5-(3,4,5-tris(dodecyloxy)-1-benzyloxy-
propyl) terephthalate (5a). A solution of 2,5-bis(ethoxycarbonyl)-
3,4-diphenylcyclopentadienone (2.7 g, 7.1 mmol) and 4a (5.2 g, 7.1
mmol) in 20 mL of toluene was refluxed for 12 h. The mixture was
then cooled to room temperature. After removal of the toluene under
reduced pressure, the residue was purified by column chromatography
on silica gel [ethyl acetate/hexane = 1/10 (v/v)] to produce an orange
oil 5a. (6.2 g, 81%)
1H NMR (CDCl3, δ, ppm): 1.12−1.27 (t, 9H), 2.04−2.09 (m, 2H),
2.97−3.02 (t, 2H), 3.45−3.87 (m, 26H), 4.13−4.19 (m, 6H), 4.40 (s,
2H), 4.43 (s, 2H), 4.44 (s, 2H), 6.63 (s, 2H), 6.93−7.15 (m, 10H),
7.46 (s, 1H). 13C NMR (CDCl3, δ, ppm): 15.10, 15.14, 29.27, 31.49,
59.47, 63.33, 66.62, 68.73, 69.40, 69.70, 69.82, 69.88, 70.51, 70.81,
72.31, 72.74, 107.09, 126.34, 127.35, 127.50, 128.08, 129.98, 130.16,
133.98, 135.68, 137.68, 138.39, 138.43, 139.01, 139.64, 140.92, 142.57,
152.57. MS (FAB, C48H66O12): calcd, 835.03; found, 835.
2,3-Diphenyl-5-(3,4,5-tris(dodecyloxy)-1-benzyloxypropyl)-
1,4-bis(chloromethyl)benzene (M2). Into two-neck round-bottom
flask was added 6a (4.6 g, 4.6 mmol) dissolved in 20 mL of CH2Cl2.
Then 1 M SOCl2 (18.6 mL, 18.6 mmol) was added dropwise via a
dropping funnel. The reaction was stirred at room temperature for 2 h.
The solvent was removed and the crude product was redissolved in
CH2Cl2, washed twice with brine and dried over MgSO4. Evaporation
of the solvent yielded the crude product, which was purified by column
chromatography [silica gel, ethyl acetate/hexane = 1/30 (v/v)]. The
pure fractions were collected, and after evaporation resulted in a
viscous oil (2.0 g, 42%)
1H NMR (CDCl3, δ, ppm): 0.80−1.00 (m, 15H), 1.06−1.44 (m,
54H), 1.67−1.83 (m, 6H), 1.98−2.03 (m, 2H), 2.77−2.82 (t, 2H),
3.50−3.54 (t, 2H), 3.85−3.99 (m, 10H), 4.40 (s, 2H), 6.53 (s, 2H),
6.91−7.09 (m, 10H), 7.67 (s, 1H). 13C NMR (CDCl3, δ, ppm): 13.52,
14.09, 22.67, 26.11, 29.35, 29.37, 29.43, 29.64, 29.69, 29.73, 30.07,
30.32, 31.03, 31.90, 61.03, 69.05, 69.46, 73.08, 73.37, 106.01, 126.50,
126.83, 127.19, 127.22, 129.08, 129.77, 130.11, 133.52, 133.59, 137.21,
137.53, 137.90, 138.16, 138.71, 138.89, 139.73, 153.12, 168.49, 168.67.
MS (FAB, C70H106O8): calcd, 1075.59; found, 1075.
Diethyl 2,3-diphenyl-5-(3,4,5-tris(2-(2-ethoxyethoxy)-
ethoxy)-1-benzyloxypropyl) terephthalate (5b). A solution of
2,5-bis(ethoxycarbonyl)-3,4-diphenylcyclopentadienone (3.8 g, 10.0
mmol) and 4b (5.7 g, 10.0 mmol) in 20 mL of toluene was refluxed for
12 h. The mixture was then cooled to room temperature. After
removal the toluene under reduced pressure, the residue was purified
by column chromatography on silica gel [ethyl acetate/hexane = 3/1
(v/v)] to produce an orange oil 5b. (7.5 g, 82%)
1H NMR (CDCl3, δ, ppm): 0.84−0.88 (t, 9H), 1.18−1.44 (m,
54H), 1.67−1.82 (m, 6H), 2.05−2.10 (m, 2H), 2.95−3.00 (t, 2H),
3.58−3.62 (t, 2H), 3.89−3.98 (m, 6H), 4.31 (s, 2H), 4.38 (s, 2H),
4.45 (s, 2H), 6.56 (s, 2H), 6.96−7.18 (m, 10H), 7.46 (s, 1H). 13C
NMR (CDCl3, δ, ppm): 14.10, 22.68, 26.10, 29.05, 29.35, 29.42, 29.64,
29.69, 29.74, 30.32, 31.05, 31.91, 41.66, 44.60, 69.06, 69.53, 73.22,
73.38, 106.05, 126.69, 126.74, 127.36, 127.46, 130.09, 130.29, 133.46,
133.91, 136.01, 137.56, 138.17, 138.67, 139.80, 141.43, 143.20, 153.16.
MS (FAB, C66H100Cl2O4): calcd, 1028.40; found, 1027. Anal. Calcd for
C66H100Cl2O4: C, 77.08; H, 9.80. Found: C, 77.18; H, 9.89.
2,3-Diphenyl-5-(3,4,5-tris(2-(2-ethoxyethoxy)ethoxy)-1-ben-
zyloxypropyl)-1,4-bis(chloromethyl)benzene (M3). Into a two-
neck round-bottom flask was added 6b (3.4 g, 4.0 mmol) dissolved in
20 mL of CH2Cl2. Then, 1 M SOCl2 (16.0 mL, 16.0 mmol) was added
dropwise via a dropping funnel. The reaction was stirred at room
temperature for 2 h. The solvent was removed and the crude product
was redissolved in CH2Cl2, washed twice with brine and dried over
MgSO4. Evaporation of the solvent yielded the crude product, which
was purified by column chromatography [silica gel, ethyl acetate/
hexane = 1/1 (v/v)]. The pure fractions were collected, and after
evaporation resulted in a viscous oil (2.2 g, 63%)
1H NMR (CDCl3, δ, ppm): 0.79−0.88 (t, 6H), 1.11−1.25 (t, 9H),
1.98−2.05 (m, 2H), 2.78−2.83 (t, 2H), 3.44−4.00 (m, 30H), 4.09−
4.19 (m, 6H), 4.40 (s, 2H), 6.59 (s, 2H), 6.92−7.10 (m, 10H), 7.68 (s,
1H). 13C NMR (CDCl3, δ, ppm): 14.12, 15.14, 29.27, 31.49, 60.92,
66.62, 68.73, 69.40, 69.70, 69.82, 69.88, 70.51, 70.81, 72.31, 72.74,
105.96, 126.59, 126.85, 127.18, 127.27, 129.38, 129.77, 130.13, 133.53,
133.59, 137.26, 137.53, 137.90, 138.16, 138.71, 139.39, 140.17, 152.55,
168.34, 168.87. MS (FAB, C52H70O14): calcd, 919.10; found, 919.
2,3-Diphenyl-5-(3,4,5-tris(dodecyloxy)-1-benzyloxypropyl)-
1,4-bis(hydroxymethyl)benzene (6a). A solution of 5a (6.0 g, 5.6
mmol) in dry THF (40 mL) was added dropwise under nitrogen
atmosphere to a stirred suspension of LiAlH4 (2.2 g, 56.0 mmol) in
dry THF (20 mL) cooled to 0 °C in an ice bath. The reaction mixture
was stirred and refluxed for 12 h. The excess of hydride was quenched
under cooling in an ice bath with 15% NaOH(aq) (2 mL) followed by
H2O (10 mL). The THF was removed by evaporation and the
resulting mixture was dissolved in 100 mL of ethyl acetate and
extracted with brine (200 mL) twice. The organic phase was then
dried over MgSO4, evaporated, and purified with column chromatog-
raphy [silica gel, ethyl acetate/hexane = 1/3 (v/v)) as eluent] to yield
6a as a white solid (4.9 g, 89%)
1H NMR (CDCl3, δ, ppm): 1.14−1.28 (t, 9H), 2.06−2.17 (m, 2H),
2.96−3.01 (t, 2H), 3.49−3.88 (m, 26H), 4.13−4.19 (m, 6H), 4.33 (s,
2H), 4.40 (s, 2H), 4.46 (s, 2H), 6.62 (s, 2H), 6.98−7.18 (m, 10H),
7.47 (s, 1H). 13C NMR (CDCl3, δ, ppm): 15.15, 29.05, 31.05, 41.69,
44.60, 66.65, 68.82, 69.59, 69.74, 69.88, 69.92, 70.54, 70.87, 72.33,
72.94, 106.99, 126.69, 126.75, 127.28, 127.37, 127.46, 130.11, 130.21,
130.29, 133.90, 133.98, 136.02, 138.19, 138.67, 141.40, 143.21, 152.65.
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MS (FAB, C48H64Cl2O10): calcd, 871.92; found, 871. Anal. Calcd for
C48H64Cl2O10: C, 66.12; H, 7.40. Found: C, 65.98; H, 7.23.
Synthesis of Polymers. Scheme 1 outlines the synthetic route of
polymers P1−P3.
DP-PPV derivatives, P1−P3 (Scheme 1) in good yields. Yields,
molecular weight and polydispersity index (PDI) of resulting
polymers are reported in Table 1. Because of the substituents at
P1. To a solution of M1 (1.95 g, 4.74 mmol) in THF (300 mL) and
potassium tert-butoxide (8.51 g, 75.8 mmol) in THF (174 mL) was
added. The resulting mixture was stirred at room temperature for 12 h
under a nitrogen atmosphere. A solution of 2,6-di-tert-butylphenol (6
equiv) as end-capping agent in THF (20 mL) was then added and
stirred for 6 h. The resulting mixture was poured into methanol, and
the precipitate was filtered off and washed with water. The resulting
polymer was dissolved in THF, precipitated again in methanol, and
washed by Soxhlet extraction with methanol for 2 days. After drying
under vacuum for 24 h, the polymer was obtained as a yellow solid
(0.92 g, 55%).
Table 1. Molecular Weights and Thermal Properties of
Polymers P1−P3
yield
(%)
Mn
(kDa)
Mw
(kDa)
PDI (Mw/
Mn)
Ti
(°C)
Td
polymer
(°C)
P1
P2
P3
55
50
41
182
135
58.8
418
389
147
2.30
2.88
2.50
308
169
189
419
347
318
C-5 position of the phenylene moieties, P1−P3 are highly
soluble in common organic solvents, such as chloroform,
toluene, and chlorobenzene and can be polymerized to
relatively high molecular weight. Number-average molecular
weights (Mn) of the polymers are in the range from 59 to 180
kDa (Table 1). Transparent and self-standing films can be cast
from their solutions. P2 and P3 show lower yield and molecular
weight than that of P1, which implies that the bulkier pendant
groups may result in larger steric hindrance and a less efficient
polymerization process.
Thermal properties. In the TGA measurements, the 5%
weight loss temperatures of P1−P3 were observed above 310
°C at a heating rate of 10 °C/min (Figure S7, Supporting
Information), suggesting the good thermal stability of the
polymers. Figure 1 shows the DSC thermal diagrams of P1−
1H NMR (CDCl3, δ, ppm): 0.76−0.89 (m, 3H), 1.20−1.27 (m,
6H), 2.17−2.53 (m, 2H), 3.40−3.49 (m, 2H), 5.77−6.06 (m, 2H),
6.25−7.00 (m, 10H), 7.60 (s, 1H). GPC (THF, polystyrene standard):
Mn = 182 kDa, Mw = 418 kDa, PDI = 2.30.
P2. To a solution of M2 (0.96 g, 0.94 mmol) in THF (70 mL) and
potassium tert-butoxide (1.68 g, 15.0 mmol) in THF (24 mL) was
added. The resulting mixture was stirred at room temperature for 12 h
under a nitrogen atmosphere. A solution of 2,6-di-tert-butylphenol (6
equiv) as end-capping agent in THF (20 mL) was then added and
stirred for 6 h. The resulting mixture was poured into methanol, and
the precipitate was filtered off and washed with water. The resulting
polymer was dissolved in THF, precipitated again in methanol, and
washed by Soxhlet extraction with methanol for 2 days. After drying
under vacuum for 24 h, the polymer was obtained as a yellow solid
(0.45 g, 50%).
1H NMR (CDCl3, δ, ppm): 0.86−0.97 (m, 9H), 1.06−1.44 (m,
54H), 1.72−1.88 (m, 6H), 2.47−2.58 (m, 2H), 3.21−3.43 (m, 2H),
3.82−4.02 (m, 8H), 4.23−4.32 (m, 2H), 5.99−6.05 (m, 2H), 6.30−
6.50 (m, 2H), 6.58−7.18 (m, 10H), 7.60 (s, 1H).
GPC (THF, polystyrene standard): Mn = 135 kDa, Mw = 389 kDa,
PDI = 2.88.
P3. To a solution of M3 (1.15 g, 1.32 mmol) in THF (100 mL) and
potassium tert-butoxide (2.37 g, 21.1 mmol) in THF (32 mL) was
added. The resulting mixture was stirred at room temperature for 12 h
under a nitrogen atmosphere. A solution of 2,6-di-tert-butylphenol (6
equiv) as end-capping agent in THF (20 mL) was then added and
stirred for 6 h. The resulting mixture was poured into methanol, and
the precipitate was filtered off and washed with water. The resulting
polymer was dissolved in THF, precipitated again in methanol, and
washed by Soxhlet extraction with methanol for 2 days. After drying
under vacuum for 24 h, the polymer was obtained as a yellow solid
(0.43 g, 41%).
1H NMR (CDCl3, δ, ppm): 0.98−1.19 (m, 9H), 2.40−2.46 (m,
2H), 3.20−3.28 (m, 2H), 3.45−3.72 (m, 26H), 3.81−4.11 (m, 8H),
4.31−4.41 (m, 2H), 6.01−6.05 (m, 2H), 6.44−6.47 (m, 2H), 6.54−
7.15 (m, 10H), 7.60 (s, 1H).
GPC (THF, polystyrene standard): Mn = 58.8 kDa, Mw = 147 kDa,
PDI = 2.50.
Figure 1. DSC thermal diagrams of (a) P1, (b) P2 and (c) P3 at a
scan rate of 10 °C/min. The two thermal scans of each sample include
a cooling scan from the I phase and the subsequent heating scan.
RESULTS AND DISCUSSION
■
Synthesis. Scheme 2 outlines the synthetic route for the
monomers. Dendritic benzyl chlorides 3a,b were prepared
through the sequential LiAlH4 reduction and SOCl2-mediated
chlorination of dendritic benzoate 1a,b. Substitution of the
chloro functional group of the corresponding dendritic
chlorides with pent-4-yn-1-ol in the presence of sodium
hydride and tetrabutylammonium bromide (TBAB) afforded
compound 4a,b. The Diels−Alder reaction of 2,5-bis-
(ethoxycarbonyl)-3,4-diphenylcyclopentadienone with 4a,b
gives 5a,b in high yields. 5a,b are then reduced with LiAlH4
to give 6a,b and then reacted with SOCl2 in methylene chloride
to produce the monomers, M2 and M3. The obtained
monomers were then polymerized via Gilch route to obtain
P3, which recorded the cooling scan from their isotropic (I)
melts and the subsequent heating scans at 10 °C/min. The I
phases of P1−P3 were confirmed by the amorphous halos
observed in their wide-angle X-ray diffraction (WAXD)
patterns (Figure S8, Supporting Information). During the
cooling, all the polymers show a I phase to an ordered phase
transition. The transition temperatures are found at 234 °C
(latent heat (ΔH) = −6.22 J/g) for P1, 139 °C (ΔH = −2.93
J/g) for P2, and 126 °C (ΔH = −2.31 J/g) for P3. Upon the
subsequent heating, the endothermic thermal transitions, which
represents the isotropization of P1−P3 were observed at
temperature (Ti) of 308 °C (ΔH = 6.23 J/g), 169 °C (ΔH =
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2.96 J/g) and 189 °C (ΔH = 2.35 J/g), respectively. The nearly
identical latent heats during the cooling and heating scans of
each polymer indicate that the three DP-PPV derivatives all
exhibit an enantiotropic phase behavior. The much lower Tis of
P2 and P3 compared to that of P1 indicates that the flexible
dendritic side chains of P2 and P3 significant decreases the
isotropization temperature. A smaller supercooling was also
observed in the phase transition of P2, suggesting a smaller
kinetic energy barrier for P2 to transform from its I phase to an
ordered phase.
stronger intermolecular interactions among the polymer chains
in the thin-film state. As shown in Table 2, in the chloroform
solution, P2 possesses a remarkably high ΦPL, 86%, and both
the ΦPLs of P2 and P3 are further enhanced from their
predecessor, P1. These results demonstrate that the incorpo-
ration of appropriate dendritic moieties as polymer side chains
can lead to less chain aggregation, and higher ΦPL in the DP-
PPV derivatives. Accompanying with the decrease of
intermolecular distance in the thin-film state, the ΦPLs of P1
and P3 dropped to 65% and 63%, while P2 keeps its ΦPL at
77%. To further investigate how the structural ordering in the
thin-film affects ΦPL, the thin films of P1−P3 were thermally
treated according to their DSC thermograms. These polymer
films were first heated to temperature above the Ti of the
corresponding polymer, and then cooled slowly to room
temperature under a nitrogen atmosphere to allow the
structural development of each polymer. After the thermal
treatment, the ΦPLs of P1 and P3 decreased significantly to
33% and 38%. However, the ΦPL of P2 was kept at remarkable
high value of 71%. Therefore, it is obvious the ΦPL of P2 is
much less sensitive to the structural ordering in the solid state,
and the aliphatic dendritic side chains provides the polymer a
better ability to preserve its ΦPL in the solid state.
Optical Properties. Figure 2 shows the UV−vis absorption
and PL emission spectra of polymers in chloroform and thin
Time-resolved PL decay experiments are shown in Figure 3.
The PL lifetimes of the three polymers in chloroform solution
has no obvious difference. However, in the thin-film state, the
PL lifetime of P2 is longer than those of P1 and P3 even after
thermal treatment. Therefore, the excitons generated in P2 are
more effectively confined for emissive relaxation, which results
in a higher ΦPL and longer lifetime.37−39
Analysis of 1D-WAXD and 2D-WAXD Patterns. To gain
insights about the phase structures of P1−P3 and identify the
structure−property relationships of these polymers, a series of
1D WAXD patterns have been acquired as shown in Figure 4.
To relate the ΦPL and the lifetime of the as-casted samples and
thermally treated samples to their phase structures, two kinds of
solid samples of P1−P3 were prepared. The “dried samples”
were made by completely evaporating the solvent (chloroform)
from the respective polymer solutions, and are used to identify
the possible lyotropic ordered structure in the as-casted films
formed during the solvent evaporation. The thermally treated
samples were prepared via cooling the individual polymer melts
to room temperature, and are used to study the corresponding
structural changes, which result in the significant drops in the
ΦPL of the thermally treated P1 and P3. The 1D WAXD
patterns of the dried samples are shown in the bottom of
Figures 4a−c. Diffuse halos are observed for the dried samples
of P1 and P3. The halos indicates that P1 and P3 were trapped
in a relatively amorphous state with random chain orientations
during the solvent evaporation process. On the other hand, the
dried sample of P2 shows a Bragg diffractions located at 2θ of
2.38° (d-spacing: 3.71 nm), and an amorphous halo centered at
19.61° (d-spacing: 0.45 nm). The Bragg diffraction in the low
2θ angle region represents an ordered packing of P2 molecules
at a length scale of a P2 molecular dimension (vide infra), while
the halo in the high 2θ angle region can be attributed to a
short-range order at the segmental level within molecules.40
The ordered phase of P2 is therefore a mesophase. The 1D
WAXD patterns of the dried samples indicate that the solvent
evaporation process allows the formation of the lyotropic
mesophase of P2, but results in amorphous phases and random
chain packing of P1 and P3. Since the structure evolutions of
P1−P3 were clearly observed in the DSC measurements,
Figure 2. UV−vis and PL spectra of (a) P1, (b) P2 and (c) P3.
film state. Table 2 summarizes the UV−vis absorption maxima
and PL emission maxima of all polymers in different states. In
the chloroform solution, the absorption maxima of P1−P3 are
located in the range from 352 to 362 nm, and the PL emission
maxima are located in the range from 477 to 481 nm, which is
attributed to the π−π* transition of the polymers. Thus, the
modification on the side chain structures of P1−P3 shows
limited effects on their energy gaps. Red shift of the absorption
and PL maxima were observed in the thin-films of all the
polymers, which indicates a closer intermolecular distance and
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Table 2. Optical Properties of Polymers P1−P3
UV−vis (nm)
PL (nm)
a
b
a
b
polymer
CHCl3
Film-1
Film-2
CHCl3
Film-1
Film-2
c
d
d
P1
P2
P3
352
359
362
367
377
375
381
376
374
481 (0.80)
488 (0.65)
498 (0.33)
477 (0.86)
477 (0.82)
496 (0.77)
497 (0.63)
501 (0.71)
497 (0.38)
a
b
c
Film-1 was treated before thermal annealing. Film-2 was treated after thermal annealing. Solution PL quantum efficiency were measured in
d
CHCl3, relative to 9,10-diphenylanthracene (ΦPL = 0.90). PL quantum efficiency were estimated relative to polyfluorene thin film as a standard
(ΦPL = 0.55).
Figure 3. Time-resolved PL decay of polymers (a) in CHCl3 solution, (b) in film state before thermal treatment, and (c) in film state after thermal
treatment following 400 nm excitation.
WAXD pattern of thermal treated samples of P1−P3 were
further investigated. The 1D WAXD patterns of these samples
are shown as the top patterns in Figures 4a-c. Bragg diffractions
were observed for the thermally treated P1 and P3,
representing the formation of ordered phases of P1 and P3
during cooling the samples from their melts. The diffraction
peaks of P1 are found at 2θ = 6.89°, 14.01°, 15.32°, and 18.35°
Figure 4. Set of 1D WAXD patterns of (a) P1, (b) P2 and (c) P3. The
which corresponds to d-spacings of 1.28, 0.63, 0.58, and 0.48
bottom patterns in each figure are the 1D WAXD patterns of the dried
nm. For P3, the diffraction peaks are found at 2θ = 2.76°, 8.32°,
samples recorded at 30 °C. The top patterns are the 1D WAXD
and 14.05°, which corresponds to d-spacings of 3.20, 1.06, and
patterns of the thermally treated samples, which were cooled from the
0.63 nm. Interesting, in Figure 4b, cooling P2 from its melt
only leads to a stronger and sharper diffraction peak located at
identical 2θ angle (2.38°). Therefore, the ordered structures
observed in the dried sample and the thermally treated sample
I phases to room temperature, and recorded at 30 °C.
Figure 5. 2D WAXD patterns of the ordered phases of (a) P1, (b) P2, and (c) P3 after shearing. Solid arrow indicates the direction of mechanical
shearing force applied on the sample. The incident X-ray beam was along the normal direction of the 2D patterns. Schematic representations of the
molecular dimensions along the chain and the lateral directions are illustrated in (d) for P1, (e) for P2, and (f) for P3, respectively.
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Figure 6. Schematic illustration of the phase structures of the ordered phases of (a) P1, (b) P2, and (c) P3. The conjugated polymer backbone is
represented as a blue board, the domain occupied with the aliphatic side chains of P2 is represented as a green board, and the hydrophilic domain
occupied with the oligo(ethylene oxide) chains of P3 is represented as a red board.
of P2 are identical, and the thermal treatment led to an increase
in the overall fraction and average domain sizes of the ordered
domain in the sample of P2.
interaction in the ordered phases of P1 and P3 could be higher
than it is in the ordered phase of P2.
The lateral packings of P1−P3 were further investigated by
the comparison between the lateral dimensions and the
equatorial diffractions of P1−P3. For each polymer, the
diffraction arc at the lowest 2θ angle on the equator represents
the largest d-spacing of the periodic structure on the lateral
direction. In Figure 5a−c, such diffraction peaks are located at
2θ = 5.48° (d-spacing: 1.61 nm) for P1, 2θ = 2.38° (d-spacing:
3.71 nm) for P2, and 2θ = 2.76° (d-spacing: 3.20 nm) for P3.
As shown in Figure 5d, the lateral dimension of P1 is 1.68 nm,
which matches well with observed d-spacing of 1.61 nm. The
significantly larger lateral d-spacings observed in P2 (3.71 nm)
and P3 (3.20 nm) clearly indicate that the much bulkier
dendritic side chains of P2 and P3 increase the lateral
interchain distance tremendously. Most importantly, for P2,
the lateral interchain distance of 3.71 nm is much smaller than
the lateral dimension of P2 (5.60 nm) with its dendritic side
chains fully extended. Therefore, instead of extending, the
aliphatic side chains of P2 have to fold and the domain
occupied with the aliphatic side chains is amorphous as
confirmed by the amorphous halo centered at 19.61° shown in
Figure 4b. The randomness in the domain occupied with the
dendritic alkyl chains disturbs the stacking of the conjugated
backbones, and results in a quasi-long-range ordered lamellar
structure. Thus, although P2 molecules have preferable chain
orientation, their conjugated backbones are not well stacked,
which decreases the degree of intermolecular π−π interactions.
Such phase structure explains the lack of the diffraction arc in
the high angle region of P2 (Figure 5b). On the other hand, the
hydrophilic dendrons of P3 are not as effective in hindering
intermolecular π−π stacking as the hydrophobic dendrons of
P2 do. It may be due to the hydrophilic dendrons tend to
further segregate from the hydrophobic conjugated polymer
backbones. The segregation enhanced the lateral order and
resulted in the long-range ordered lateral lamellar structure of
P3, but made the hydrophilic dendrons less effective in
blocking the π−π stacking of conjugated chains. A periodicity
of d-spacing of 5.1 nm is therefore observed in the ordered
phase of P3. The detailed structural determination of the
ordered phases of the polymers is further investigated and will
be published elsewhere.
A combination of the DSC and 1D WAXD results provides
the information regarding the structure evolution of P1−P3.
To study the detailed supramolecular structures and packing
symmetry, 2D WAXD of P1−P3 were measured. Figure 5
shows the 2D WAXD patterns and corresponding molecular
dimensions of P1−P3. The samples were sheared at temper-
atures slight higher than the Tis of the respective polymers and
then cooled to room temperature to be used in the following
2D WAXD measurements. Along the meridian direction, a
diffraction arc with d-spacing of 0.65 nm (2θ = 13.61°) was
observed for all three polymers. Since P1−P3 share the same
DP-PPV structure on their conjugated backbones, and the
length of a DP-PPV repeat unit is 0.65 nm along the chain
direction as shown in Figure 5d−f, the diffraction arc
corresponds to the periodic arrangement of the DP-PPV
units along the polymer chain direction. It is also confirmed
that, in the sheared samples of P1−P3, the polymer chains are
aligned with the shear direction. The diffraction arcs on the
equator can thus be used to identify the ordered structures
along the lateral direction of the polymer chain. In Figure 5a,
multiple diffraction arcs were observed along the equator and in
the quadrant for ordered phase of P1, while in Figure 5, parts b
and c, diffraction arcs were only observed on the equator for
the ordered phase of P2 and P3. The results confirm the
ordered phase of P1 as a crystalline phase, and those of P2 and
P3 as liquid crystalline mesophases. For P2, the single
diffraction arc along the equator may indicate a lamellar
structure with quasi-long-range order along the lateral direction
of the conjugate backbone. As for P3, in Figure 5c, a set of
three diffractions arcs were observed on the equator located at
2θ = 2.76° (d-spacing: 3.20 nm), 2θ = 8.32° (d-spacing: 1.06
nm), and 2θ = 14.05° (d-spacing: 0.63 nm). These three
diffractions have scattering vector (q) ratio of 1:3:5, which is an
indication of the presence of a long-range ordered lamellar
structure with its lamellar normal perpendicular to the chain
axis. Notably, at the high 2θ angle region on the equator, a
strong diffraction arc located at 2θ = 18.4° corresponding to a
d-spacing of 0.48 nm was observed in P1 (Figure 5a), and a
weak diffraction arc located at 2θ = 17.4° corresponding to d-
spacing of 0.51 nm was observed in P3 (Figure 5c), while only
amorphous hole was observed in this region in P2 (Figure 5b).
The d-spacings fall into the distance of periodic π−π stack of
conjugate molecules. Thus, the extend of intermolecular π−π
On the basis of the 2D WAXD analysis, proposed molecular
packings in the ordered phases of P1−P3 are illustrated in
Figure 6. Along the chain direction, the periodic repetition of
the DP-PPV units separated with spacing of 0.65 nm can be
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seen in the ordered phases of P1−P3. The lateral interchain
distance is 1.61 nm for P1, 3.71 nm for P2, and 3.20 nm for P3,
which increases as the bulkiness of the pendant groups increase.
Periodic arrangement along the π−π stacking direction can be
observed in the ordered phases of P1 (Figure 6a) and P3
(Figure 6c), but it is absent in the ordered phase of P2 (Figure
6b) due to the bulkiness and random arrangement of the
aliphatic dendritic pendants of P2. Thus, the intermolecular
π−π interactions of P2 are strongly disturbed. Most
importantly, such effect remains even when the ordering of
the solid-state structure takes places, and may explain why the
photophysical properties of P2, such as ΦPL and PL relaxation
time are much less sensitive to its solid-state phase structure.
The correlation between the ΦPL and phase structure of P1−
P3 can be found when we compare the phase structures of P1−
P3 and their ΦPL shown in Table 2. The ΦPLs of the as-casted
P1−P3 films are 0.65, 0.77, and 0.63. After the thermal
treatment, the ΦPLs of the P1−P3 films became 0.33, 0.71, and
0.38. On the basis of the previous studies, the less ordered
structure of conjugated polymers used in PLED normally
results in a less extend of intermolecular π−π interactions and
thus, higher ΦPL.34 On the contrary to that, P2, which possesses
the most ordered structure among the three dried samples,
shows the highest ΦPL in the as-casted thin films of P1−P3. In
addition, the structural ordering induced by the thermal
treatment led to a significant decrease in the ΦPL of P1 and
P3, but showed only negligible effect on ΦPL of P2. This
phenomenon indicates the ordering of DP-PPV molecules does
not necessarily leads to a decrease in ΦPL, as long as it does not
involve a significant increase in the extend of intermolecular
π−π interactions. The effective hindering of the intermolecular
π−π interactions by the dendritic side chains in P2 allows the
preservation of ΦPL in the environment with ordered packing of
DP-PPV molecules.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H spectra, TGA thermograms and WAXD patterns of the I
phases. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Telephone: +886-3513-1523. Fax: +886-3513-1523. E-mail:
(C.-L.W.) kclwang@nctu.edu.tw; (C.-S.H.)cshsu@mail.nctu.
edu.tw.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the National Science Council and the “ATP
Program” of the Ministry of Education, Taiwan, for financial
support.
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In summary, novel DP-PPV polymers bearing hydrophobic
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