Star-shaped polymer PFStODO by atom transfer radical polymerization: Its synthesis, characterization, and fluorescence property

Article abstract of DOI:10.1002/pola.25055

A novel monomer bearing with pyrazoline chromophore, 3-(4-fluorophenyl)-1- phenyl-5-(4-(4-vinylbenzyloxy)phenyl-4,5-dihydro-1H-pyrazole (FStODO), was synthesized, and its atom transfer radical polymerization initiated by a tetrafunctional initiator, penta

Full text of DOI:10.1002/pola.25055

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Star-Shaped Polymer PFStODO by Atom Transfer Radical Polymerization:
Its Synthesis, Characterization, and Fluorescence Property
Pei-Yang Gu,1 Cai-Jian Lu,1 Qing-Feng Xu,1 Gao-Jie Ye,1 Wei-Qiang Chen,2 Xuan-Ming Duan,2
Li-Hua Wang,1 Jian-Mei Lu1
1Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science,
Soochow University, Suzhou, Jiangsu 215123, China
2Laboratory of Organic NanoPhotonics and Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of
Physics and Chemistry, Chinese Academy of Sciences, No.2, Beiyitiao, Zhongguancun Road, Haidian District, Beijing 100190,
China
Correspondence to: J.-M. Lu (E-mail: lujm@suda.edu.cn) or Q.-F. Xu (E-mail: xuqingfeng@suda.edu.cn)
Received 14 June 2011; accepted 8 October 2011; published online 7 November 2011
DOI: 10.1002/pola.25055
ABSTRACT: A novel monomer bearing with pyrazoline chro-
mophore, 3-(4-fluorophenyl)-1-phenyl-5-(4-(4-vinylbenzyloxy)-
phenyl-4,5-dihydro-1H-pyrazole (FStODO), was synthesized, and
its atom transfer radical polymerization initiated by a tetrafunc-
tional initiator, pentaerythritol tetrakis(2-bromoisobutyrate)
(4Bri-Bu), was studied in detail and characterized by 1H NMR,
GPC, and TG-DSC. The solution, film luminescence of mono-
mer, and its polymer were studied in detail. Compared with that
of monomer, both luminescence emission intensity and quan-
tum yield of star-shaped polymer PFStODO in DMF solution are
decreased. However, the two-photon absorption (TPA) spectra
in DMF solution measured by a femtosecond laser show an
entirely different result that the maxima TPA cross-section value
of polymer reaches to 203 GM, better than that of the monomer
itself (13 GM). More interestingly, the film of polymer shows
surprisingly white emission ranging from 400 to 700 nm
assigned to the excimer formation. We attribute this formation
of excimers to the ordered chromophore aggregates in film,
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KEYWORDS: atom transfer radical polymerization (ATRP); films;
fluorescence; functionalization of polymers; pyrazoline; star
polymers; white color emission
INTRODUCTION Pyrazoline and its derivatives, one of the
most popular heterocyclic organic luminescent materials, have
attracted growing attention due to their potential applications
are focusing on later, especially the flexible star-shaped poly-
mers, the simplest type of branched polymer.
Star-shaped polymers are studied especially from the view-
points of their potential applications for film and fabricate de-
vice with much lower synthetic cost and better long-term sta-
bility.19 With recent progress in living radical polymerization,
the synthesis of star polymers has received enough attention,
and various functional monomers are available for the star
polymer formation, which are difficult to reach in living ionic
polymerization routes, such as atom transfer radical polymer-
ization (ATRP), reversible addition–fragmentation chain
transfer polymerization, and nitroxide-mediated polymeriza-
tion.19 Among them, ATRP is a more attainable method
because of facile multifunctional initiators, relatively moder-
ate polymerization condition, and low polydispersity indices.
Furthermore, ATRP is an established technique in our group
for polymerization of functional monomers.20,21 For instance,
we utilize an orange emission monomer 4-(2-benzothiazole-
2-yl-vinyl)-phenyl methacrylate and a blue emission initiator
to obtain a white emission polymer by ATRP method.
including fluorescence probes,1 organic light-emitting diodes
2
,
organic one-dimensional nanomaterials,3 nonlinear optical
materials,4 biological activities,5–8 and so on.9–12 For example,
Yao and coworkers3 have proved 1,3,5-triphenyl-2-pyrazoline
is not only a good blue light emitter but also an electron donor
compound. Its single crystalline nanowires can emit pure blue
light, and the emission colors were tunable by doping rubene,
but in the amorphous film, such phenomenon cannot be
observed. Moreover, the easy crystallization and phase separa-
tion of pyrazoline-based small molecules would affect the
long-term stability and would limit the applications in large
area fabrications. To solve the problem, preparation of poly-
mers bearing with pyrazoline segment in its side chain or
main chain could be an effective method.13–17 Two methods
are applied in the preparation of pyrazoline-containing poly-
mers: one is pyrazoline conjugated polymer13–17 and the other
is flexible polymer bearing with pyrazoline chromphore.18 We
Additional Supporting Information may be found in the online version of this article.
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In this article, by utilizing ATRP, we reported a star-shaped
polymer with pyrazoline chromophore at side chain. A novel
blue emissive monomer with pyrazoline segment, 3-(4-fluo-
rophenyl)-1-phenyl-5-(4-(4-vinylbenzyloxy)phenyl-4,5-dihy-
dro-1H-pyrazole (FStODO), was designed and synthesized. Its
ATRP initiated by a tetrafunctional initiator, pentaerythritol
tetrakis(2-bromoisobutyrate) (4Bri-Bu), was studied in detail
5.81 (d, 1H), 5.40 (q, 1H), 5.24 (d, 1H), 5.02 (s, 2H), 3.85
(q, 1H), 3.06 (q, 1H).
ATRP Polymerization of FStODO
FStODO was initiated by tetrafunctional initiator 4Bri-Bu as
the following procedure: FStODO, initiator, CuCl, and
PMDETA were dissolved in cyclohexanone, and the mixture
was placed into a two-neck round-bottomed flask. The flask
was sealed and cycled between vacuum and Ar (g) for three
times. Samples were taken out by a syringe at different time
intervals and diluted with tetrahydrofuran (THF). The
diluted solution was passed through an alumina column to
remove the copper catalyst, and the filtrate was precipitated
by addition of methanol. The precipitation was filtrated and
dried under vacuum.
1
and characterized by H NMR, Gel Permeation Chromatography
(GPC), and Thermogravimetry - Differential Scanning Calorimetry
(TG-DSC) measurement. The luminescent property of monomer
and its star-shaped polymer were studied in detail. They show
similar emission wavelength with kmax ¼ 465 nm and the fluo-
rescence quantum yields (U) were 73% (monomer) and 40%
(polymer). Although, there is obvious emission intensity decreas-
ing in one-photon fluorescence (OPF), the cross sections of two-
photon absorption (TPA) of star-shaped polymer are much
higher than that of monomer. More interestingly, the films of
polymers prepared whether by spin coating or dispensing on
quartz are surprisingly white emissive within whole visible-light
region. It could be assigned to the formation of excimer, which is
caused by ordered aggregate of chromophores in the side chain.
Fabrication of Films
The 1,2-dichloroethane or THF solution of PFStODO (0.5
mmol/L) was spin-coated (2000 r sꢀ1, 40 s) (or dipped)
onto quartz surface and evaporated under atmosphere at
room temperature. All obtained films are transparent.
Measurements
EXPERIMENTAL
Conversions for monomer were determined by gravimetry.
1H NMR spectra were measured using INOVA 400 MHz
NMR spectrometer, CDCl3 or DMSO-d6 as solvent, and tetra-
methylsilane as the internal standard at ambient tempera-
ture. The purity was determined with a Waters515 HPLC
apparatus: a mixture of methanol and water (methanol:-
water ¼ 80:20, v/v) was used as the eluent at a flow rate
of 0.8 mL/min at 30 ꢁC with a C18 column and with a
Waters 996 detector. Molecular weights and the polydisper-
sity relative to PSt were measured using Waters1515 GPC
with THF as a mobile phase at aꢁflow rate of 1 mL/min and
with column temperature of 30 C. UV–vis absorption spec-
tra of the polymers and initiator in DMF solutions were
determined on a Shimadzu RF540 spectrophotometer. Room
temperature emission and excitation spectra were carried
out using Edinburgh-920 fluorescence spectra photometer,
and fluorescence lifetimes were measured with a Fluo-
CuCl (Sinopharm Chemical Reagent, 98.5%) was purified in
hydrochloric acid, washed with methanol, and dried under
vacuum to afford
a white powder. Cyclohexanone and
N,N,N0,N00,N00-pentamethyldiethylenetriamine (PMDETA) were
distilled under vacuum. 4-Vinylbenzyl chloride (TCI, 90%)
was obtained by column chromatography (aluminum oxide,
neutral). 2-Bromo-2-methylpropionyl bromide and 40-fluoroa-
cetophenone (Alfa Aesar, 99%), tetramethylolmethane, 4-
hydroxybenzaldehyde, phenylhydrazine (Sinopharm Chemical
Reagent). All other reagents and solvents were analytically
pure and were used as received without further purification.
Synthesis of Initiator
4Bri-Bu was synthesized as described in the corresponding
references.22 1H NMR (DMSO-d6, 400 MHz), d (ppm): 4.28 (s,
8H), 1.91 (s, 24H).
R
rologV-3 spectrometer. Wide-angle X-ray diffraction (XRD)
Preparation of Monomer and Polymers
patterns were taken on an X0 Pert-Pro MPD X-ray
diffractometer.
Monomer is prepared in three steps (Supporting Informa-
1
tion) and characterized by H NMR.
Two-photon excited fluorescence (TPEF) spectra were
recorded on a SD2000 spectrometer (Ocean Optical) with ex-
4-4(Vinylbenzyloxy)benzaldehyde (BzDD)
1H NMR (DMSO-d6, 400 MHz), d (ppm): 9.8 (s, 1H), 7.87 (d,
2H), 7.51 (d, 2H), 7.44 (d, 2H), 7.20 (d, 2H), 6.76 (m, 1H),
5.85 (d, 1H), 5.28 (d, 1H), 5.23 (s, 2H).
citation using
a femtosecond laser (Tsunami, Spectra-
Physics) with 100-fs pulse width and 82 MHz repetition
rate. TPA cross sections were determined by the TPEF
method.23 It is assumed that the quantum efficiencies after
two-photon excitation are the same as those after one-pho-
ton excitation. The TPA cross sections are obtained by cali-
bration against fluorescein with known Ud value in aqueous
NaOH solution (pH ¼ 11) at concentrations of 1.0 ꢂ 10ꢀ4 M.
The samples were dissolved in solvents at concentrations of
about 1.0 ꢂ 10ꢀ4 M. The error of TPEF measurement is
about 15%. To ensure that the measured signals were solely
due to TPA, the dependence of TPEF on the incident inten-
sity was verified in each case to be quadratic. Then, the TPA
1-(4-Fluorophenyl)-3-(4-(4-vinylbenzyloxy)phenyl)
prop-2-en-1-one (FPhPP)
1H NMR (CDCl3, 400 MHz), d (ppm): 8.06 (t, 2H), 7.80 (d,
1H), 7.61 (d, 2H), 7.42 (m, 5H), 7.18 (t, 2H), 7.01 (d, 2H),
6.73 (q, 1H), 5.78 (d, 1H), 5.28 (d, 1H), 5.11 (s, 2H).
3-(4-Fluorophenyl)-1-phenyl-5-(4-(4-vinylbenzyloxy)
phenyl-4,5-dihydro-1H-pyrazole (FStODO)
1H NMR (DMSO-d6, 400 MHz), d (ppm): 7.79 (t, 2H), 7.46 (d,
2H), 7.38 (d, 2H), 7.19 (m, 6H), 7.97 (t, 4H), 6.69 (t, 2H),
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SCHEME 1 Synthetic scheme of FStODO and PFStODO.
cross sections d were calculated on the basis of the following
expression:
first-order with respect to monomer. The number average
molecular weights (Mn) increases linearly with the conver-
sion and the polydispersity indices (PDIs) keep relatively
narrow in all cases (Mw/Mn ¼ 1.20–1.33), indicating the
Cr nr Fs Ur
ds ¼ dr
(1)
Cs ns Fr Us
where d is TPA cross section, C and n are the concentration
and refractive index of the sample solution, respectively, and
F is the integrated area under the TPEF spectrum.
RESULTS AND DISCUSSION
Characterization of Polymerization
Preparation of monomer and polymer is presented in
Scheme 1. The monomer FstODO was prepared by the reac-
tion of a,b-unsaturated ketones and hydrazines using ethanol
as solvent. It is easy to dissolve in common organic solvents
such as ketone, THF, and DMF. The ATRP of FStODO was per-
formed using 4Bri-Bu as a tetrafunctional initiator in cyclo-
hexanone at 100 ꢁC with [FStODO]0:[4Bri-Bu]0:[CuCl]0:[PM-
DETA]0 ¼ 70:1:1:2, [FStODO]0 ¼ 0.4454 M, and the results
are shown in Figures 1 and 2. According to Figure 1, the lin-
earity of the semilogarithmic plot of ln([M]0/[M]) versus the
polymerization time indicates that the polymerization is
FIGURE 1 Kinetic plot for the ATRP of FStODO initiated
by 4Bri-Bu in cyclohexanone solution ([FStODO]0:[4Bri-Bu]0:
[CuCl]0:[PMDETA]0 ¼ 70:1:1:2, [FStODO]0 ¼ 0.4454 M, 100 ꢁC).
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FIGURE 2 Dependence of Mn(GPC) (g/mol) and PDI with conver-
FIGURE 3 The GPC traces of the PFStODO. (a) Mn(GPC) ¼ 10,000
g/mol, PDI ¼ 1.20; (b) Mn(GPC) ¼ 12,300 g/mol, PDI ¼ 1.28; (c)
Mn(GPC) ¼ 13,900 g/mol, PDI ¼ 1.26; (d) Mn(GPC) ¼ 16,200 g/mol,
sion for the polymerization of FStODO initiated by 4Bri-Bu in
cyclohexanone
solution
([FStODO]0:[4Bri-Bu]0:[CuCl]0:[PM-
DETA]0 ¼ 70:1:1:2, [FStODO]0 ¼ 0.4454 M, 100 ꢁC).
PDI ¼ 1.30; (e) Mn(GPC) ¼ 17,100 g/mol, PDI ¼ 1.32; (f) Mn(GPC)
18,000 g/mol, PDI ¼ 1.33.
¼
polymerization is well controlled (Fig. 2). The Mn(GPC)s were
trails indicative of transfer processes. Therefore, the Mn(GPC)s
deviated from the Mn(th)s that may be caused by the lower
hydrodynamic volumes of star-branched polymers compared
with their linear analogs.24–26
lower than the correspondent Mn(th)s, in which the Mn(th)
was calculated according to Mn(th) Mw,4Bri-Bu
s
þ
¼
([FStODO]0/[4Bri-Bu]0) ꢂ Mw,FStODO ꢂ Conversion (Mw,4Bri-Bu
and Mw,FStODO were the total molecular weight of 4Bri-Bu
and FStODO and [FStODO]0 and [4Bri-Bu]0 means the initial
concentrations of FStODO and 4Bri-Bu, respectively). The
GPC chromatograms (Fig. 3) display narrow, single peaks
and demonstrate that there are no low molecular weight
Characterization of Star-Shaped Polymer
The star-shaped polymer was also characterized by H NMR
1
spectra and TG-DSC measurement. Figure 4 exhibits the 1H
NMR spectra of PFStODO, in which the signals of hydrogen
FIGURE 4 1H NMR spectrum in DMSO-d6 of PFStODO (Mn(GPC) ¼ 17,100 g/mol, PDI ¼ 1.32).
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TABLE 1 Photophysical Data for FStODO and PFStODO in
Dilute DMF Solutions
Ue
dfmax
a
b
c
d
Compounds
kmax
kmax
kmax
kmax
FStODO
PFStODO
a
360
360
460
460
730
730
462,482
462,482
0.73
0.40
13
203
The maximum wavelength of the UV–vis absorption spectra (nm).
The maximum wavelength of the fluorescence emission spectra (nm).
The maximum wavelength of the TPA spectra (nm).
b
c
d
The maximum wavelength of the two-photon fluorescence emission
spectra (nm).
e
Fluorescence quantum yield.
f
Two photon cross section (GM).
FIGURE 5 Powder and film XRD pattern of PFStODO.
as to PFStODO) with the concentration increasing at first
and then decrease with continuous concentration increasing.
As shown in Figure 6, the fluorescence intensity decreases
from monomer to polymer with the number of the pyrazo-
line chromophore increasing. Accordingly, the fluorescence
quantum yields [solution fluorescence quantum yield of the
target compound was measured in DMF dilute solutions
using quinine bisulfate (Uf ¼ 0.546 in 0.1 N H2SO4) as
standards] of polymer (0.40) are obviously smaller than
that of monomer (0.73), which can be attributed to the
reabsorption of emitted photons11,12 and the self-quenching
due to the intramolecular and intermolecular interactions of
polymer.27–29 The fluorescence lifetime of FStODO and
PFStODO in DMF solution were also studied as shown in
Figure 7. There is biexponential decay, and the lifetime value
of the monomer is observed to be longer than that of poly-
mer. The result with the quantum yield may attribute to the
electron transition of the monomer that is easier than that
of the polymers.
atom from the vinyl group (5.5–6.0 ppm) were disappeared,
and a broad signal of methylene below 2.0 ppm was formed.
Furthermore, the signal of methyl groups adjacent to the
bromine atoms in initiator at 1.91 ppm (Supporting Informa-
tion) is disappeared in polymer, and new signal at 1.23 ppm
may be assigned to those methyl groups and the chemical
shift can be due to the longer distance of terminal bromine
and methyl group. Hence, the 1H NMR spectrum along with
GPC results indicates the successful synthesis of star
polymer.
The PFStODO also exhibits good thermal stability, with a
glass transition temperature of about 145 ꢁC and an onset
decomposition temperature of about 290 ꢁC, determined by
differential scanning calorimetry and thermogravimetric
analysis, respectively (Supporting Information).
TPEF spectra at the same concentration and the same
pumped power under 720 nm excitation are depicted in Fig-
ure 8(a). The emission pattern of the monomer and polymer
are similar but different with those in OPF. There is a com-
parable broad band with two maximum wavelengths of kem1
¼ 462 nm and kem1 ¼ 482 nm in TPEF, whereas only one
XRD Analysis
XRD analysis was performed to clarify the aggregate of pow-
der and film. According to Figure 5, the XRD results show
that the powder has a broad diffraction peak at 19ꢁ, whereas
that of film has a relatively sharp peak with a slightly red-
shifted to 21ꢁ. It indicates that the bulk pyrazoline groups at
the side chains were in more ordered aggregates in film.
Optical Properties in Organic Solution
All photophysical data of FstODO and its polymer are listed
in Table 1. PFStODO exhibits good solubility in organic sol-
vents such as CHCl3, THF, and DMF. As shown in Figure 6,
absorption spectra of both monomer and its polymer in DMF
solution are similar with kmax ¼ 360 nm, which can be
assigned to the p–p* transition localized on the pyrazoline
ring system.
The OPF in solution (5 ꢂ 10ꢀ5 mol/L) was first studied
with kex at 360 nm for FStODO and 340 nm for polymer (5
ꢂ 10ꢀ6 mol/L). The fluorescence of monomer and its poly-
mer in DMF solution with different concentrations are also
investigated (Figures were provided in Supporting Informa-
tion). In all cases, the emission intensity reaches to a highest
value (5 ꢂ 10ꢀ5 mol/L as to FStODO and 5 ꢂ 10ꢀ6 mol/L
FIGURE 6 Linear absorption (left) and OPF (right) spectra of
FStODO and PFStODO in DMF.
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Optical Properties at Film
Emission at a higher concentration (0.5 mmol/L) of polymer
in DMF is measured. Except the band at 450 nm, a new
band at 560 nm appeared in the emission pattern, indicating
the excimer formation at high concentration (Supporting In-
formation).32 Therefore, optical property of films of PFStODO
is studied to confirm the excimer formation. The films of
PFStODO are prepared by two methods: one is dipping the
1,2-dichoroethane (DCE) solution or THF solution (0.5
mmol/L) onto quartz surface, and the other is by spin coat-
ing using DCE solution. Figure 8 shows the absorption (left)
and emission (right) pattern of PFStODO films. Compared to
that of PFStODO solution, the main absorption band at 360
nm remains, but the new small absorption band at 460 nm
appears ascribing to the aggregates of chromophore mole-
cules.33 More importantly, emission of films is quite different
and appears as two main broad emission bands at 450 and
560 nm. It covers almost the whole visible-light spectrum
(400–700 nm). The inner figure of Figure 8 is white
FIGURE 7 Typical fluorescence decay curves associated with
lamp profile for the monomer (a) and its polymer (b) in ambi-
ent environment. Curve is shown in biexponential fits. Excita-
tion wavelength is kept at 360 nm.
broad band with kem ¼ 460 nm in OPF. Generally, most
reported emission patterns of OPF and TPEF are similar.
However, the chromophore aggregation or different excita-
tion process will affect emission pattern. Herein, we tempo-
rarily attribute it to the molecule aggregation.
The inset figure of Figure 8(a) depicts the logarithmic plot of
the TPEF integral versus the pumped power. The slope value
obtained (k ¼ 1.82 and 1.96), close to 2, indicates that TPA
is the main excitation mechanism in the femtosecond
regime.30,31
The two-photon cross sections of the monomer and poly-
mers in DMF at different excitation wavelengths are shown
in Figure 8(b). The monomer and its polymer display a grad-
ual increasing trend in TPA cross section with the maxima
TPA cross-section value of 13 GM for FStODO and 203 GM
for PFStODO. It is clear that the maxima TPA cross-section
value increases with the number of pyrazoline chromophore.
Obviously, the polymerization improves the TPA cross-sec-
tion value.
FIGURE 8 (a) TPF spectra of FStODO and PFStODO in DMF so-
lution at 720 nm, inset is the dependence of output fluores-
cence intensity (Iout) of FStODO and PFStODO on the input
laser power (Iin) at 720 nm in DMF solution. (b) TPA cross sec-
tions of FStODO and PFStODO in 710–780 nm regions.
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FIGURE 9 Linear absorption (left) and OPF (right) spectra of
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emission of transparent film under 365 nm UV light. The
shorter band is assigned to emission of pyrazoline chromo-
phore itself, whhereas the longer band might be caused by
formation of excimer in film.34–37 The quantum yield (QY)
and fluorescent lifetime of film are also measured according
to ref. 38; the QY of film is 1%, and the lifetime is 1.56 ns.
The shorter lifetime confirm the formation of excimer. Com-
pared with the XRD results, we considered that excimer for-
mation is probably due to the ordered aggregates of pyrazo-
line chromophore at the side chains.
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obtained only by adjusting molecular aggregates (Fig. 9).
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In summary, a novel blue fluorescent monomer with pyrazo-
line segment was synthesized, and its star-shaped polymer
was obtained by ATRPs. The polymer emits strong blue fluo-
rescence at 465 nm with the fluorescence quantum yields
0.40. Moreover, polymer shows obvious two-photon fluores-
cence with the cross-section value of 203 GM, much higher
than that of monomer (13 GM). More interestingly, the film
of PFStODO can emit white-light due to the excimer forma-
tion caused by the chromophore aggregates. Although the
emission mechanism is still glancing, it might be a novel
approach for designing new white emission materials tuned
by molecule aggregates.
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authors also thank Chinese Natural Science Foundation (Nos.
20876101, 200909044, and 21071105), major project of col-
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project funded by the Priority Academic Program Development
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