Synthesis and two-photon absorption properties of conjugated polymers with N-arylpyrrole as conjugated bridge and isolation moieties

Article abstract of DOI:10.1002/pola.24685

Three novel conjugated polymers with N-arylpyrrole as the conjugated bridge were designed and synthesized, which emitted strong one- or two-photon excitation fluorescence in dilute tetrahydrofuran (THF) solution with high quantum yields. The maximal two-p

Full text of DOI:10.1002/pola.24685

Synthesis and Two-Photon Absorption Properties of Conjugated
Polymers with N-Arylpyrrole as Conjugated Bridge and Isolation Moieties
QIANQIAN LI,1 JING HUANG,1 ZHIGUO PEI,2 AOSHU ZHONG,1 MING PENG,1 JUN LIU,2
ZHENLI HUANG,2 JINGUI QIN,1 ZHEN LI1
1Department of Chemistry, Wuhan University, Wuhan 430072, China
2Key Laboratory of Biomedical Photonics of Ministry of Education, Huazhong University of Science and Technology,
Wuhan 430074, People’s Republic of China
Received 10 January 2011; accepted 19 March 2011
DOI: 10.1002/pola.24685
Published online 14 April 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Three novel conjugated polymers with N-arylpyrrole
as the conjugated bridge were designed and synthesized,
which emitted strong one- or two-photon excitation fluores-
cence in dilute tetrahydrofuran (THF) solution with high quan-
tum yields. The maximal two-photon absorption (TPA) cross-
sections of the polymers, measured by the two-photon-induced
fluorescence method using femtosecond laser pulses in THF,
were 752, 1114, and 1869 GM, respectively, indicating that the
insertion of electron-donating or electron-withdrawing moieties
into the polymer backbone could benefit to the increase of the
TPA cross-section. Their large TPA cross-sections, coupled
with the relatively high emission quantum yields, made these
conjugated polymers attractive for practical applications, espe-
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cially two-photon excited fluorescence.
2011 Wiley Periodi-
cals, Inc. J Polym Sci Part A: Polym Chem 49: 2538–2545, 2011
KEYWORDS: conjugated polymers; copolymerization; isolation;
NLO; pyrrole; two-photon absorption
INTRODUCTION Conjugated materials with two-photon
absorption (TPA) properties have gained great interest for
their potential applications in fluorescence imaging of bio-
logical samples, optical limiting, photodynamic therapy,
three-dimensional optical data storage, and microfabrica-
tion.1–11 According to various design strategies, many conju-
gated materials, including small molecules and polymers,
have been successfully prepared.12–23 Especially, for small
molecules, it is now apparent that the appropriate design
strategy involves symmetrically substituted system with a
general structure of the type of D–p–D or A–p–A, where A is
an acceptor group, D a donating one, and p a conjugating
moiety. Generally, the increase of the donor/acceptor
strength, the conjugation length, and the planarity of the p-
center can effectively improve the TPA cross-section (r) of
the corresponding molecule. Thanks to the enthusiastic
efforts of scientists, the effect of varying the electronic prop-
erties of the terminal groups and p bridge in small molecules
have been investigated in detail,24–28 giving some useful
rules to obtain molecules with high TPA activities. To be
applied in some practical applications, small TPA molecules
should be incorporated into the host polymer to avoid the
possible aggregation at high concentrations. However, their
disperse concentration in the host polymer matrix is limited
by the phase separation in a large degree. As an alternative
approach, the conjugated polymers have attracted more and
more interest, which offered enhanced TPA effects.
Actually, in most of the reported conjugated TPA polymers,
such as ladder-type poly(p-phenylene), poly[1,6-bis(3,6-
dihexadecyl-N-carbazolyl)-2,4-hexadiyne], polyfluorene, and
poly(p-phenyleneethynylene), the intramolecular charge
transfer was relatively weak, resulting in the relatively lower
r.29–32 Thus, TPA chromophores with large r were used as
building blocks or repeating units in the conjugated poly-
mers, to enhance their r values. However, the aggregation in
their concentrated solutions or solid state due to the long
conjugation chains and the planar structure of the conjuga-
tion polymers, usually quenched their emission, leading to
the TPA efficiency/fluorescence tradeoff, which, as a result,
limited their applications in some fields that required strong
two-photon excited fluorescence (TPEF), such as upcon-
verted lasing and bioimaging. To suppress the aggregation,
some hyperbranched polymers and dendrimers containing
TPA-active units have been synthesized.33–36 Their three-
dimensional architectures gave them advantages for TPA.
Recently, some exciting results have been obtained from pyr-
role-containing materials in several different research fields,
including photochemistry, second-order nonlinear optical
and TPA materials.37–47 Also, our previous research demon-
strated that N-arylpyrrole as the building block could effi-
ciently improve the stability of the resultant chromophores
compared to that of the N-alkylpyrrole-based analogs.48–50
More importantly, the introduced central aryls could hinder
Additional Supporting Information may be found in the online version of this article. Correspondence to: Z. Li (E-mail: lizhen@whu.edu.cn)
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Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 2538–2545 (2011) 2011 Wiley Periodicals, Inc.
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ARTICLE
other scientists also confirmed this effect.52–54 Thus, we
wondered, how about the results if TPA chromophore moi-
eties were bonded together through an isolation group?
Therefore, all the above points prompted us to prepare some
novel conjugated polymers, in which the N-arylpyrrole-based
TPA chromophore moieties were linked together through an
isolation group, with triphenylamine group acting as three-
dimensional bulky moiety similar to dendritic ones, generally
present in dendrimers and hyperbranched polymers, to sup-
press the aggregations as possible. Just as shown in Chart 3,
we have successfully obtained these new kinds of polymers,
P1–P3. Furthermore, the optical properties of the polymers
could be subtly tuned by introducing various substituted
conjugated aromatic rings bearing the donating and/or with-
drawing abilities (the Ar group in Chart 3). All the polymers
(P1–P3) exhibited good solubility (in common organic sol-
vents), high thermal stability, satisfied quantum yields, and
good TPA properties, with the highest r value achieved as
large as 1869 GM for P3 at 760 nm.
CHART 1 The structure of dye LI-1 with N-arylpyrrole as the
conjugated bridge. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
the p–p stacking and aggregation process (i.e., the red part
in Chart 1). For example, when N-arylpyrrole moieties were
introduced into the dye sensitizers (LI-1, Chart 1) as p
bridge, a high conversion efficiency of 7.21%, 91% of the
standard cell from N719 tested under similar measuring
conditions was obtained from the corresponding solar-cell
device. Additionally, pyrrole, as an electron-rich heterocyclic
ring, could act as the auxiliary donor to enhance the intra-
molecular charge transfer from the terminal donor groups to
the p bridge. Thus, N-arylpyrrole might be a novel conjuga-
tion moiety for optimizing TPA efficiency/fluorescence trade-
off, as different aryl groups could be conveniently introduced
through the carbon–nitrogen bond.
EXPERIMENTAL
Materials
Tetrahydrofuran (THF) was dried over and distilled from a
K–Na alloy under an atmosphere of dry nitrogen. Com-
pounds 3–5 were prepared according to the literature.55–57
All other reagents were used as received.
Instrumentation
1H and 13C NMR spectra were measured on a Varian Mer-
cury300 spectrometer using tetramethylsilane (d ¼ 0 ppm)
as internal standard. The Fourier transform infrared spectra
were recorded on a Perkin-Elmer-2 spectrometer in the
region of 3000–400 cmꢀ1. UV–visible spectra were obtained
using a Shimadzu UV-2550 spectrometer. Photoluminescence
spectra were performed on a Hitachi F-4500 fluorescence
spectrophotometer. EI-MS spectra were recorded with a Fin-
nigan PRACE mass spectrometer. Molecular weights (Mw and
Mn) and polydispersity indices (Mw/Mn) of the polymers
were estimated by a Waters 1515 gel permeation chromatog-
raphy (GPC) system equipped with interferometric refrac-
tometer detector, using a set of monodisperse polystyrenes
as calibration standards and THF as the eluent at a flow rate
of 1.0 mL/min. Elemental analyses were performed by a
On the other hand, in 2006, for the first time, we attempted
to link two pieces of donor–p–acceptor blocks together
through an isolation group to yield a new type of chromo-
phore, which we termed as ‘‘H‘‘-type chromophores (Chart
2).51 After embedded into the polymeric system, the result-
ant polyurethanes containing this ‘‘H’’-type chromophore
(PS2, Chart 2) exhibited enhanced second-order nonlinear
optical effects in comparison with their linear analogs (PS1,
Chart 2), realizing the effect of ‘‘one plus one greater than
two,’’ partially due to the presence of the isolation group,
which weakened the aggregation of the donor–p–acceptor
blocks. Our other examples and some cases reported by
CHART 2 The structures of polymers PS1 and PS2. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
SYNTHESIS OF CONJUGATED POLYMERS, LI ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
CHART 3 Schematic representation of monomers and polymers. [Color figure can be viewed in the online issue, which is available
at wileyonlinelibrary.com.]
CARLOERBA-1106 microelemental analyzer. Thermal analysis
was performed on NETZSCH STA449C thermal analyzer at a
General Procedure for Preparation of Polymers P1–3
t-BuOK (2 equiv.; 1.0 M in THF solution) was added drop-
wise to ice-cold solutions of compound 2 (1 equiv.) and one
of compounds 3–5 (1 equiv.) in dry THF. The reaction mix-
ture was stirred overnight at room temperature. THF was
evaporated, and methanol was added to precipitate the prod-
uct. The polymers were further purified by reprecipitation
with acetone/MeOH.
3
ꢁ
heating rate of 10 C/min in argon at a flow rate of 50 cm /
min for thermogravimetric analysis. The thermometer for the
measurement of the melting point was uncorrected. A mode-
locked Ti:sapphire laser (Mai Tai, Spectra-Physics, Santa
Clara, CA) was used as the excitation source. The average
output power, pulse width, and repetition rate were 0.5 W,
100 fs, and 80 MHz, respectively. After passing through a
Pockel cell (350-80 LA BK, Conoptics, Danbury, CT) that was
used to control the power of the laser, the laser was focused
on the cell (polished on all sides) by a focusing lens (f ¼ 6
cm). The emission light was collected by an objective lens
(Olympus, Japan) and then was focused by another objective
lens on a fiber Spectrometer (HR2000, Ocean Optics), which
was used to record the fluorescent spectra. Fluorescein in
water was chosen as the reference standard.
P1
Compound 2 (121.2 mg, 0.10 mmol), compound 3 (52.3 mg,
0.10 mmol). Orange solid (120 mg, 67.0%). Mw ¼ 0.30 ꢂ
104, Mw/Mn ¼ 1.52 (GPC, polystyrene calibration). IR (thin
film), u (cmꢀ1): 1663 (terminal-CHO).
1H NMR (CDCl3) d (ppm): 10.0 and 9.45 (trace terminal-
CHO), 7.8 (ArH), 7.6–7.5 (ArH), 7.2 (ArH), 7.0 (ArH and
ACH¼¼CHA), 6.8–6.5(ArH and ACH¼¼CHA), 2.1(ACH2A), 1.6
(ACH2A), 1.1(ACH2A), 0.7(ACH3). 13C NMR (CDCl3)
d
Synthesis of Compound 2
(ppm): 147.4, 130.3, 130.0, 129.6, 128.9, 128.4, 128.0, 127.8,
127.4, 126.5, 125.0, 124.7, 124.5, 124.1, 123.9, 123.7, 123.0,
121.7, 120.6, 55.7, 40.7, 31.7, 29.9, 24.1, 22.8, 14.3.
A mixture of compound 1 (1.50 g, 2.89 mmol), 2,20-(9,9-
dihexyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-diox-
aborolane) (0.69 g, 1.38 mmol), sodium carbonate (3.06 g,
28.9 mmol), and a catalytic amount of tetrakis(triphenylphos-
phine) palladium (Pd(PPh3)4) was carefully degassed and
charged with nitrogen. Subsequently, THF (40 mL) and deoxi-
dized water (20 mL) were added by syringe. The reaction
mixture was stirred at 80 ꢁC for 48 h. After cooled to room
temperature, the organic layer was separated, dried over
sodium sulfate, and evaporated to dryness. The crude prod-
uct was purified through a silica gel chromatography column
by using petroleum/ethyl acetate (6/1) as an elꢁuent to afford
a yellow solid 2 (1.65 g, 47.1%). mp: 144–145 C.
P2
Compound 2 (181.7 mg, 0.15 mmol), compound 4 (65.8 mg,
0.15 mmol). Red solid (150 mg, 61.0%). Mw ¼ 0.77 ꢂ 104,
Mw/Mn ¼ 1.57 (GPC, polystyrene calibration). IR (thin film),
u (cmꢀ1): 1665 (terminal-CHO).
1H NMR (CDCl3) d (ppm): 10.0 (trace terminal-CHO), 7.8
(ArH), 7.6 (ArH), 7.4 (ArH), 7.2 (ArH), 7.0 (ArH and
ACH¼¼CHA), 6.8–6.5(ArH and ACH¼¼CHA), 3.9–3.7
(AOACH3), 2.1(ACH2A), 1.6 (ACH2A), 1.1(ACH2A),
0.7(ACH3). 13C NMR (CDCl3) d (ppm): 152.2, 147.8, 147.5,
146.9, 140.6, 139.3, 136.8, 132.3, 129.5, 128.0, 127.2, 127.0,
126.5, 125.1, 124.7, 124.6, 124.0, 123.3, 123.1, 121.6, 120.6,
116.6, 107.9, 56.6, 55.7, 40.7, 31.7, 29.9, 24.1, 22.8, 14.3.
1H NMR (CDCl3) d (ppm): 10.03 (s, 1H, ACHO), 9.44 (s, 1H,
ACHO), 7.81 (s, br, 8H, ArH), 7.68–7.56 (m, 4H, ArH), 7.48
(d, 2H, J ¼ 7.2 Hz, ArH), 7.43 (d, 2H, J ¼ 7.5 Hz, ArH), 7.26–
7.20 (m, 8H, ArH), 7.11–6.74 (m, 24H, ArH and ACH¼¼CHA),
6.53 (d, 2H, J ¼ 16.8 Hz, ACH¼¼CHA), 2.07 (s, br, 4H,
ACH2A), 1.07 (s, br, 16H, ACH2A), 0.74 (s, br, 6H, ACH3).
13C NMR (CDCl3) d (ppm): 186.4, 178.4, 152.2, 148.5, 148.3,
147.5, 142.4, 142.1, 141.8, 140.7, 140.0, 139.2, 138.1, 136.1,
134.0, 132.2, 130.4, 129.9, 129.6, 128.9, 128.4, 128.1, 127.8,
126.6, 125.8, 125.1, 124.8, 123.7, 123.1, 122.5, 121.8, 121.6,
120.6, 120.3, 115.2, 114.2, 113.8, 110.1, 109.0, 108.1, 55.7,
40.8, 31.8, 30.0, 24.1, 22.9, 14.3. MS (EI), m/z [Mþ1]þ:
1211.3, calcd, 1210.6. Anal. Calcd for: C87H78N4O2: C, 86.25;
H, 6.49; N, 4.62; Found: C, 86.68; H, 6.42; N, 4.56.
P3
Compound 2 (181.7 mg, 0.15 mmol), compound 5 (64.3 mg,
0.15 mmol). Dark red solid (180 mg, 73.2 %). Mw ¼ 2.47 ꢂ
104, Mw/Mn ¼ 1.88 (GPC, polystyrene calibration). IR (thin
film), u (cmꢀ1): 1666 (terminal-CHO).
1H NMR (CDCl3) d (ppm): 10.0 (trace terminal-CHO), 7.9–7.6
(ArH), 7.2 (ArH), 7.0–6.5 (ArH and ACH¼¼CHA),
2.1(ACH2A), 1.6 (ACH2A), 1.1(ACH2A), 0.7(ACH3). 13C
NMR (CDCl3) d (ppm): 152.2, 147.7, 140.8, 139.0, 133.3,
132.7, 129.6, 128.3, 128.0, 127.3, 126.8, 126.3, 124.9, 124.7,
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ARTICLE
SCHEME 1 The synthetic route to polymers P1–3.
123.5, 121.7, 120.6, 115.7, 55.7, 40.7, 31.7, 30.0, 24.1, 22.8,
14.3.
such as THF, chloroform, DMF, and DMSO. The strong peak
of the aldehyde group in monomer 2 appeared at about
1660 cmꢀ1 in its FTIR spectroscopy (Supporting Information
Fig. S1). Also, in the FTIR spectra of the polymers, the peak
around 1660 cmꢀ1 could be observed due to the presence of
some terminal aldehyde groups (Supporting Information
Fig. S2). The same phenomenon could be seen in their 1H
NMR spectra (Supporting Information Figs. S3–S5). All the
polymers exhibited very good thermal stability, and their ini-
tial decomposition temperatures were above 320 ꢁC under
argon, where over 95% of their mass was retained (Sup-
porting Information Fig. S6 and the detail data shown in
Table 1). The molecular weights of P1–P3 were determined
by GPC with THF as an eluent and polystyrene standards as
calibration standards. As shown in Table 1 and Experimental
section, these three polymers possessed different molecular
weights, and from P1 to P3, the molecular weights
increased. This should be ascribed to the different monomer
structure of one of the compounds 3–5.
RESULTS AND DISCUSSION
Synthesis and Characterization
The synthetic route to the polymers was given in Scheme 1.
The preparation of compound 1 was presented in the Sup-
porting Information. 4-Bromophenylpyrrole (compound S1),
the important starting material, was synthesized from 2,5-
dimethoxy THF and 4-bromoaniline in high yield, then 4-bro-
mophenylpyrrole-2-carbaldehyde (compound S2) was
obtained through the normal Vilsmeier reaction, which
reacted with compound S3 under Wittig–Hornor reaction to
yield compound S4. Again, by another Vilsmeier reaction, an
aldehyde group was introduced to the pyrrole ring. Then,
under the normal palladium-catalyzed Suzuki coupling of
compound 1 and 2,20-(9,9-dihexyl-9H-fluorene-2,7-diyl)bis
(4,4,5,5-tetramethyl-1,3,2-dioxaborolane), the bisaldehyde 2
was obtained with fluorene moiety as the isolation moieties.
Thus, in this reaction, various aromatic rings could be incor-
porated as the isolation moieties, and in this article, we
chose fluorene as the example. The Wittig–Horner–Emmons
olefination reaction between compound 2 and the required
bisphosphonates with t-BuOK as catalyst, were chosen to
synthesize polymers P1–P3. By using this approach, various
bisphosphonates containing different electron-donating or
electron-withdrawing groups could be conveniently intro-
duced into the polymers to tune their optical properties.
One-photon Physical Properties
The UV–Vis absorption spectra of the polymers in THF at
dilute concentration were shown in Figure 1, and the data
were summarized in Table 1. The absorption maximum
(kmax) of polymer P2 was located at 461 nm, which was sim-
ilar to the corresponding small molecules shown in Chart 3,
indicating that the fluorene unit together with the two phe-
nyls as the isolation moieties would inhibit the p stacking of
the conjugation systems efficiently. When the conjugation
bridge changed from 1,4-dimethoxyphenylene to fluoreny-
lene one, the kmax value of P1 blue-shifted to 410 nm. This
was reasonable. While the larger planar structure of the fluo-
renylene unit gave the effective charge transfer from the
The purified polymerization products gave satisfactory spec-
troscopic data corresponding to the expected molecular
structures. They were soluble in common organic solvents
TABLE 1 Some Characterization Data of Polymers
tp
abs
abs
em
em
Yield
(%)
Mw
(ꢂ104)a
k
k
k
k
max
k
U
(%)g
r
Td
(ꢁC)i
max
max
max
max
a
Polymers
Mw/Mn
(nm)b
(nm)c
(nm)d
(nm)e
(nm)f
(GM)h
P1
P2
P3
a
67.0
61.0
73.2
0.30
0.77
2.47
1.52
1.57
1.88
410
461
508
414
460
518
523
559
640
538
589
716
760
760
760
60.7
39.3
13.3
752
1114
1869
330
368
323
f
Determined by GPC in THF on the basis of a polystyrene calibration.
2.0 ꢂ 10ꢀ5 M per repeating unit in THF, wavelength of maximum 2PA
b
1.0 ꢂ 10ꢀ6 M per repeating unit in THF, wavelength of maximum
cross-sections.
g
absorbance.
c
Quantum yields in THF solution using fluorescein in water (UF ¼ 90%,
Absorption spectra of polymers in thin film, wavelength of maximum
pH ¼ 11) as a standard.
h
absorbance.
TPA cross-sections, 1 GM (Go¨ ppert-Mayer) ¼ 10ꢀ50 cm4 s photonꢀ1
,
d
1.0 ꢂ 10ꢀ6 M per repeating unit in THF, wavelength of maximum
the experimental uncertainty on rmax is of the order of 10–15%.
i
intensity.
e
The 5% weight loss temperature of polymers detected by the TGA
analyses under argon at a heating rate of 10 ꢁC/min.
PL spectra of polymers in thin film, wavelength of maximum intensity.
SYNTHESIS OF CONJUGATED POLYMERS, LI ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 1 UV–Vis spectra (left) and PL spectra (right) of the polymers in THF. Concentration: 1.0 lM in repeating unit. Inset: Photo-
graphs of the polymers in THF (left) and fluorescence photographs of the polymers in THF (right) under UV illumination. From the
left to right: P1, P2, and P3.
terminal donor groups to the p bridge, the 1,4-dimethoxy-
phenylene unit with two methoxy groups substituted on the
phenyl ring could increase the electron density of the bridge
and benefit the intramolecular charge transfer. Therefore, P1
demonstrated blue-shifted absorption in comparison to P2.
Replacement of the 1,4-dimethoxyphenylene unit of P2 with
1,4-dicyanophenylene in P3, the kmax value of P3 further
red-shifted to 508 nm, indicating that the cyano substitution
on the phenyl act as the acceptor group and facilitated the
charge transfer between the terminal donor group and the p
system. This suggested that their UV–Vis absorption mainly
reflected the strong intramolecular charge transfer effect.
indicating that their emission range could be well tuned by
adjusting the nature of the conjugation bridge moieties. The
fluorescence quantum yields (U) of P1–P3 were tested to be
in the range of 0.13–0.60. The high U value of the polymer
P1 may be due to the excellent electron properties of triphe-
nylamine and pyrrole and the effect of the isolation moieties.
The relatively lower quantum yield of P3 was probably
caused by its lower energy of the emitting states, which
might facilitate the nonradiative pathways. As the polymers
emitted different fluorescent color with relatively higher
quantum yields, they might be used in organic light-emitting
devices.
The one-photon emission spectra of the polymers followed
the same trend with the absorption spectra. While the conju-
gation bridge changed from fluorenylene to 1,4-dimethoxy-
phenylene, then to 1,4-dicyanophenylene, the emission maxi-
mum were located at 523, 559, and 640 nm, respectively,
Figure 2 shows the optical properties of these polymers in
the solid state, with the corresponding data summarized in
Table 1. The trend of the solid-state spectra was similar to
their solution ones; however, the spectra were red-shifted at
different degrees. Generally, the conjugated systems became
FIGURE 2 Normalized solid-state
absorption spectra (left) and nor-
malized solid-state photolumi-
nescence spectra (right) of the
polymers. [Color figure can be
viewed in the online issue, which
is available at wileyonlinelibrary.
com.]
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ARTICLE
collected in Table 1. The rmax value gradually increased in
the order of P1 < P2 < P3, reaching a maximum value of
1869 GM for P3 per repeating unit. The main difference in
these polymers was the core of the p bridge, the results sug-
gested that the 1,4-dicyanophenylene group played a crucial
role for the enhancement of r values, and the different types
of the conjugation bridge might involve different types of
states on two-photon excitation. The smaller energy gap of
P3 would give the higher probability of two-photon excita-
tion, disclosing that the strategy through the incorporation
of the cyano substitution on the conjugation bridge to form a
large p system with D–p–A–p–D backbone in the repeating
unit could be an effective approach to enhance the r values.
In addition, the electron-rich ring of pyrrole could act as the
auxiliary donor, which made the structure of the repeating
unit further change into the D–D0–A–D0–D type, and benefit
the intramolecular charge transfer between the conjugation
bridge and the two end groups. The N-arylpyrrole group in
the polymers was also helpful to TPEF, which might make
the conjugated polymers in good arrangement and avoid the
loss of the energy by p–p stacking, and the branched aro-
matic rings linked to pyrrole ring through carbon–nitrogen
bond might have the effective electronic coupling to the
backbone, and enhance the r value.
FIGURE 3 Two-photon absorption cross-section (r) of polymers
in THF.
more planar in the solid state. This led to an increase in the
effective conjugation length and the red shifted absorption
and photoluminescence (PL) spectra, in comparison with
those in solutions. Unlike the little changes in their maxi-
mum absorption wavelengths, their maximum emission
wavelengths red-shifted at a larger degree: P3 exhibited the
emission wavelength as large as 76 nm, while those of P1
and P2 were 15 and 30 nm, respectively, although the aro-
matic rings in the three polymers changed from the big one
of fluorene moieties in P1 to much smaller benzene rings in
P2 and P3. Analyzing the results carefully, it was seen that
from P1 to P3, the molecular weights increased, indicating
the possible increase in p-p stacking among the long poly-
mer chains, which might account for the increased red-
shifted emission. The little change of P1 from the solution to
the solid film partially indicated the hindrance effect of the
possible aggregation derived from these types of polymers.
Also, no matter the large red-shifted maximum emission
wavelengths of the three polymers, the curve of their emis-
sion spectra in the solid state did not change much in com-
parison with those in solutions, and the shoulder emissions
of the p-p aggregation were not so obvious.
CONCLUSIONS
We have synthesized three novel conjugated polymers bear-
ing N-arylpyrrole as conjugated bridge and isolation moieties
by Wittig–Horner–Emmons olefination reaction. All the poly-
mers exhibited good solubility, and their optical properties
could be subtly tuned by different types of the conjugation
bridges in repeating units. Thanks to the good electron prop-
erties of triphenylamine and pyrrole, and in the presence of
the isolation moieties, all the polymers emitted strong fluo-
rescence and possessed relatively higher quantum yields.
They demonstrated strong TPA properties, and the maximal
r was 1869 GM for P3 per repeating unit, indicating that the
incorporation of the electron-rich pyrrole ring and the with-
drawing 1,4-dicyanophenylene moiety into the conjugated
system to form D–D0–A–D0–D structure was an effective
approach for the enhancement of r values.
The authors are grateful to the National Science Foundation of
China (no. 21002075 and 21034006) for financial support.
REFERENCES AND NOTES
Two-Photon Absorption Properties
The TPA cross-sections (r) were determined with the two-
photon-induced fluorescence measured technique by using
femtosecond laser pulses, which could avoid possible compli-
cations due to the excite-state excitation. In all cases, the
output intensity of TPEF was linearly dependent on the
square of the input laser intensity, thereby confirming the
nonlinear absorption. Moreover, the one-photon emission
and TPEF spectra overlapped with each other, indicating that
the emission occurred from the same excited states, regard-
less of the mode of excitation. The TPA spectra of the poly-
mers in THF were presented in Figure 3, and the data were
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