Conjugated polymers with pyrrole as the conjugated bridge: Synthesis, characterization, and two-photon absorption properties

Article abstract of DOI:10.1021/jp2015484

The synthesis, one- and two-photon absorption (2PA) and emission properties of two novel pyrrole-based conjugated polymers (P1 and P2) are reported. They emitted strong yellow-green and orange fluorescence with fluorescent quantum yields (Φ) of 46 and 33%

Full text of DOI:10.1021/jp2015484

ARTICLE
pubs.acs.org/JPCB
Conjugated Polymers with Pyrrole as the Conjugated Bridge:
Synthesis, Characterization, and Two-Photon Absorption Properties
Qianqian Li,†,§ Cheng Zhong,†,§ Jing Huang,† Zhenli Huang,‡ Zhiguo Pei,‡ Jun Liu,‡ Jingui Qin,† and
Zhen Li*,†
†Department of Chemistry, Hubei Key Laboratory on Organic and Polymeric Opto-Electronic Materials, Wuhan University,
430072, P. R. China
‡Key Laboratory of Biomedical Photonics of Ministry of Education, Huazhong University of Science and Technology, 430074,
P. R. China
S Supporting Information
b
ABSTRACT: The synthesis, one- and two-photon absorption (2PA) and
emission properties of two novel pyrrole-based conjugated polymers (P1 and
P2) are reported. They emitted strong yellow-green and orange fluorescence
with fluorescent quantum yields (Φ) of 46 and 33%, respectively. Their maximal
2PA cross sections (δ) measured by the two-photon-induced fluorescence
method using femtosecond laser pulses in THF were 2392 and 1938 GM per
repeating unit, respectively, indicating that the 2PA chromophores consisting of
the triphenylamine with nonplanar structure as the donor and electron-rich
pyrrole as the conjugated bridge could be the effective repeating units to enhance
the δ values.
’ INTRODUCTION
which does not extend the conjugated system as well as the planar
ones. To make up this weakness, some electron-excessive hetero-
aromatic rings, such as furan, thoiphene, and pyrrole, could be used
as the conjugation bridge instead of the benzene one to enhance
the intramolecular charge transfer.
Conjugated polymers, an important class of multiphoton absorb-
ing materials, are known to provide the advantage of a collective
optical response and have attracted increasing attention due to
their versatile applications in two-photon cellular imaging, optical
power limiting, multiphoton pumped lasing, three-dimensional
optical storage, and photodynamic therapy.1ꢀ3 Generally, in the
field of two-photon absorption materials, by extending the con-
jugation length (effective π-delocalization), conjugated polymers
with large two-photon absorption (2PA) cross sections can be
obtained as a result of their extended intramolecular charge
transfer. However, their planar structures have usually caused
aggregations and possible interchain quenching; thus, they often
have exhibited decreased fluorescence quantum yields, which has
limited their applications in some fields that require strong two-
photon excited fluorescence (TPEF), such as up-converted
lasing and bioimaging. Therefore, there is a need to develop
novel 2PA materials, which may circumvent this trade-off.
As an electron-rich, propeller-shaped molecule with C3 sym-
metry and good hole transporting ability, coupled with its bulky
volume for the prevention of possible aggregation, triphenyla-
mine is often utilized to construct 2PA materials.4 Interestingly,
in comparison with many studies of efficient fluorescent triphe-
nylamine-based branched polymers with high 2PA cross sections,
the linear ones bearing triphenylamine as the core have seldom
been reported, although they could be synthesized much easier
than the branched ones, possibly due to their nonplanar structure,
We are interested in the design and preparation of new
pyrrole-based functional materials, since in comparison with
the most-used thiophene ring, pyrrole possesses a more abun-
dant electron density, directly leading to more effective charge
transfers as a linker between the donors and the acceptors. Our
recent work partially proved this point: the pyrrole-containing
second-order nonlinear optical (NLO) chromophores showed
enhanced NLO effects (up to 3.3 times) and much more blue-
shifted maximum absorption (up to 36 nm) than their analogues
with furan or thiophene as a conjugated bridge.5 When pyrrole
moieties were introduced into the dye sensitizers as a π-bridge, a
high conversion efficiency of 7.21% was achieved for solar-cell
devices based on them, which was 91% of the standard cell from
N719 under the same test conditions.6a,b In addition, some
exciting results were reported by the Marder group on pyrrole-
based 2PA materials.7 However, conjugated polymers based on a
pyrrole moiety as the 2PA materials are still very scarce.
Received: February 16, 2011
Revised:
June 6, 2011
Published: June 08, 2011
r
2011 American Chemical Society
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Recently, we designed a series of new pyrrole-based chromo-
phores with the D-π-D structure, which exhibited two-photon
absorption activity in the range of 730ꢀ900 nm with a large two-
photon absorption cross section (∼1000ꢀ1700 GM).6c For
example, as shown in Chart 1, PL-3 demonstrated the two-
photon absorption cross section of 1716 GM, with its quantum
yield as good as 0.41. Prompted by these results, also considering
the above-mentioned points, we further designed and synthe-
sized some pyrrole-containing conjugated polymers with differ-
ent conjugated linkers for 2PA interest (P1 and P2, Scheme 1).
Because there were many successful reports concerning the
cooperative enhancement effect observed in multibranched
2PA molecules, we constructed the polymers on the basis of
disubstitued triphenylamine moieties. The prepared polymers,
P1 and P2, exhibited good solubility in common organic solvents
and emitted strong one- and two-photon excitation fluorescence.
Their maximal 2PA cross sections, measured by the two-photon-
induced fluorescence method using femtosecond laser pulses in
THF, were 2392 and 1938 GM, respectively.
(TMS; δ = 0 ppm) as internal standard. The Fourier transform
infrared spectra were recorded on a PerkinElmer-2 spectrometer
in the region of 3000ꢀ400 cmꢀ1. UVꢀvisible spectra were
obtained using a Shimadzu UV-2550 spectrometer. Photolumi-
nescence spectra were performed on a Hitachi F-4500 fluores-
cence spectrophotometer. EI-MS spectra were recorded with a
Finnigan PRACE mass spectrometer. Elemental analyses were
performed by a Carloerba-1106 microelemental analyzer.
Thermal analysis was performed on Netzsch STA449C thermal
analyzer at a heating rate of 10 °C/min in argon at a flow rate of
50 cm3/min for thermogravimetric analysis (TGA). The ther-
mometer for measurement of the melting point was uncorrected.
A mode-locked Ti:sapphire laser (Mai Tai, Spectra-Physics Inc.,
U.S.A.) 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 (350ꢀ80
LA BK, Conoptics Inc., U.S.A.) 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 = 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 Inc., U.S.A.), which was used to record
the fluorescent spectra. Fluorescein in water was chosen as the
reference standard.
Synthesis of Compound 3. Compound 1 (1.32 g, 2.42
mmol) was suspended in 20 mL of anhydrous tetrahydrofuran
under an atmosphere of dry argon. t-BuOK (1.14 g, 11.16 mmol)
was added directly as a solid, and the resultant mixture was stirred
at room temperature for 10 min. After the addition of the
solution of compound 2 (0.91 g, 5.08 mmol) in anhydrous
tetrahydrofuran (10 mL) dropwise, the reaction mixture was
stirred at room temperature overnight, then poured into 100 mL
of water. The organic product was extracted with chloroform and
dried over anhydrous Na2SO4. After removing the solvent, the
crude product was purified through a silica gel chromatography
column by using petroleum/ethyl acetate (10/1) as eluent to
afford yellow oil 3 (1.03 g, 71.5%). 1H NMR (CDCl3) δ (ppm):
7.35ꢀ7.26 (m, 5H, ArH and ꢀCHdCHꢀ), 7.14ꢀ7.00 (m, 10
H, ArH and ꢀCHdCHꢀ), 6.84 (s, br, 6H, ArH), 6.16 (s, br, 2H,
ArH), 3.95 (t, 4H, J = 6.6 Hz, ꢀNꢀCH2ꢀ), 1.75 (s, br,
4H, ꢀCH2ꢀ), 1.31 (s, br, 12H, ꢀCH2ꢀ), 0.88 (s, br, 6H, ꢀCH3).
Synthesis of Compound 4. DMF (0.38 g, 5.20 mmol) was
added to freshly distilled POCl3 (0.66 g, 4.32 mmol) under an
atmosphere of dry argon at 0 °C, and the resultant solution was
stirred until its complete conversion into a glassy solid. After the
addition of 1,2-dichloroethane (10 mL), the resultant colorless
solution was added dropwise at 0 °C to a yellow solution of
’ EXPERIMENTAL SECTION
Materials. Tetrahydrofuran (THF) was dried over and dis-
tilled from a KꢀNa alloy under an atmosphere of dry argon. N,N-
Dimethylformamide (DMF) was dried over and distilled from
CaH2. 1,2-Dichloroethane was dried over and distilled from
phosphorus pentoxide. Phosphorus oxychloride was freshly
distilled before use. Compounds 1,8 2,5 5, and 6,9 were prepared
according to the literature. All other reagents were used as
received.
1
Instrumentation. H and 13C NMR spectra were measured
on a Varian Mercury300 spectrometer using tetramethylsilane
Chart 1
Scheme 1
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compound 3 (1.03 g, 1.73 mmol) in 1,2-dichloroethane (10 mL).
The mixture was stirred at room temperature overnight, then
poured into an aqueous solution of sodium acetate (1 M,
100 mL) and stirred for another 2 h. The mixture was extracted
with chloroform several times, and the organic fractions were
collected and dried over Na2SO4. After removing the solvent
under vacuum, the crude product was purified through a silica gel
chromatography column by using chloroform as eluent to afford
the product. The polymer was further purified by reprecipitation
with acetone/MeOH.
P1: compound 4 (130 mg, 0.20 mmol), compound 5 (104 mg,
0.20 mmol). Orange solid (150 mg, 64.1%). Mw = 2.10 ꢁ 104,
Mw/Mn = 1.14 (GPC, polystyrene calibration). IR (thin film), υ
(cmꢀ1): 1655 (terminal CHO). 1H NMR (CDCl3) δ (ppm): 9.5
(trace terminal ꢀCHO), 7.7 (ArH), 7.5 (ArH), 7.4 (ArH),
7.3ꢀ7.0 (ArH and ꢀCHdCHꢀ), 6.6 (ArH), 4.5 (ꢀNꢀCH2ꢀ),
2.1 (ꢀCH2ꢀ), 1.8 (ꢀCH2ꢀ), 1.6 (ꢀCH2ꢀ), 1.4 (ꢀCH2ꢀ), 0.9
(ꢀCH3). 13C NMR (CDCl3) δ (ppm): 150.9, 148.0, 147.5,
146.7, 141.6, 140.8, 137.0, 133.4, 133.3, 132.7, 132.1, 129.7,
127.8, 127.0, 125.9, 125.7, 125.1, 125.0, 124.8, 124.3, 123.5,
120.7, 120.0, 116.3, 116.0, 115.7, 113.4, 107.8, 62.5, 56.2, 45.3,
43.5, 33.1, 31.7, 26.7, 26.5, 22.8, 16.7, 14.3.
1
yellow solid 4 (0.78 g, 69.2%). Mp = 144ꢀ145 °C H NMR
(CDCl3) δ (ppm): 9.45 (s, 2H, ꢀCHO), 7.39 (d, 4H, J = 8.1 Hz,
ArH), 7.34ꢀ7.26 (m, 3H, ArH and ꢀCHdCHꢀ), 7.16ꢀ7.07
(m, 8H, ArH), 6.92 (d, 2H, J = 4.5 Hz, ArH), 6.86 (d, 2H, J = 16.2
Hz, ꢀCHdCHꢀ), 6.55 (d, 2H, J = 3.6 Hz, ArH), 4.45 (t, 4H, J =
8.1 Hz, ꢀNꢀCH2ꢀ), 1.75 (s, br, 8H, ꢀCH2ꢀ), 1.31 (s, br, 12H,
ꢀCH2ꢀ), 0.86 (s, br, 6H, ꢀCH3). 13C NMR (CDCl3) δ (ppm):
178.7, 147.7, 147.1, 141.5, 132.5, 132.2, 131.3, 129.8, 127.9,
125.5, 124.3, 124.0, 113.8, 107.9, 45.3, 31.7, 31.6, 26.5, 22.8, 14.3.
IR (thin film), υ (cmꢀ1): 1652 (ꢀCHO). MS (EI), m/z [M+]
651.17, calcd 651.38. Anal. Calcd for C44H49N3O2: C, 81.07; H,
7.58; N, 6.45;. Found: C, 81.07; H, 7.26; N, 6.18.
P2: compound 4 (130 mg, 0.20 mmol), compound 6 (87.4
mg, 0.20 mmol). Red solid (120 mg, 55.2%). Mw = 2.28 ꢁ 104,
Mw/Mn = 1.12 (GPC, polystyrene calibration). IR (thin film), υ
(cmꢀ1): 1652 (terminal ꢀCHO). 1H NMR (CDCl3) δ (ppm):
9.5 (trace terminal ꢀCHO), 7.3 (ArH), 7.2ꢀ7.0 (ArH and
ꢀCHdCHꢀ), 6.9 (ArH), 6.6 (ArH), 4.5 (ꢀNꢀCH2ꢀ), 4.1
(ꢀOꢀCH2ꢀ), 1.8 (ꢀCH2ꢀ), 1.3 (ꢀCH2ꢀ), 0.9 (ꢀCH3). 13
C
General Procedure for Preparation of Polymers P1 and P2.
To ice-cold solutions of compound 4 (1 equiv) and compounds
5ꢀ6 (1 equiv) in dry THF was added t-BuOK (2 equiv) (1.0 M
in THF solution) dropwise under an atmosphere of dry argon.
The reaction mixture was stirred overnight at room temperature.
The THF was evaporated, and methanol was added to precipitate
NMR (CDCl3) δ (ppm): 151.9, 148.3, 147.8, 147.5, 146.9,
146.5, 141.9, 134.1, 133.6, 133.0, 132.4, 131.0, 130.0, 128.5,
128.0, 127.3, 126.9, 126.1, 125.9, 125.4, 125.1, 124.6, 124.1,
123.7, 121.5, 118.4, 118.0, 116.0, 115.1, 113.7, 109.8, 109.4,
108.3, 108.1, 62.5, 56.8, 56.7, 45.5, 43.8, 31.9, 30.2, 27.0, 26.8,
23.1, 16.9, 14.5.
’ RESULTS AND DISCUSSION
Synthesis and Characterization. The synthetic route to the
polymers is given in Scheme 1. Compound 1 was synthesized
according to the previous literature in three-step sequences: a
Vismeier reaction of triphenyl amine yielded the aldehyde, then
the deoxidization of the aldehyde group with NaBH4, followed
by the reaction with triethyl phosphate.8 The intermediate 3 was
obtained by means of a double WittigꢀHornerꢀEmmons
olefination of compounds 1 and 2 in the presence of t-BuOK
in THF with satisfactory yields (71.5%). The conversion of
compound 3 to the corresponding bisaldehydes 4 was achieved
by the Vilsmeier reaction, then the preparation of the polymers
P1 and P2 followed by WittigꢀHornerꢀEmmons olefination
using the required bisphosphonates with the yields of 64.1% and
55.2% for fluorenylene and 1,4-dimethoxyphenylene moieties,
respectively. By utilizing this approach, different aryl moieties
could be conveniently introduced into this system through
CdC bonds.
Figure 1. Normalized UVꢀvis spectra of the polymers in THF.
Concentration: 1.0 ꢁ 10ꢀ6 M in repeating unit. Inset: photographs of
the polymers in THF, P1 (left) and P2 (right).
The polymers P1 and P2 were soluble in common organic
solvents, such as THF, chloroform, DMF, and DMSO. The
Table 1. Some Characterization Data of Polymers
polymers yield (%) Mw ( ꢁ 104)a Mw/Mn
λ
(nm)b
λ
(nm)c
λ
(nm)d
λ
(nm)e
λ
max
tp (nm)f Φ (%)g δ (GM) h Td (°C) i
a
abs
max
abs
max
em
max
em
max
tp
P1
P2
64.1
55.2
2.10
2.28
1.14
1.12
469
489
469
489
533
569
555
597
760
760
46
33
2392
1938
369
362
a Determined by GPC in DMF on the basis of a polystyrene calibration. b 1.0 ꢁ 10ꢀ6 M per repeating unit in THF, wavelength of maximum absorbance.
c UVꢀvis spectra of polymers in thin film, wavelength of maximum absorbance. d 1.0 ꢁ 10ꢀ6 M per repeating unit in THF, wavelength of maximum
intensity. e PL spectra of polymers in thin film, wavelength of maximum intensity. f 2.0 ꢁ 10ꢀ5 M per repeating unit in THF, wavelength of maximum
g
2PA cross sections. Quantum yields in THF solution using fluorescein in water (ΦF = 90%, pH = 11) as a standard. h 2PA cross sections, 1 GM
(G€oppertꢀMayer) = 10ꢀ50 cm4 s photonꢀ1, the experimental uncertainty on δmax is on the order of 10ꢀ15%. i The 5% weight loss temperature of
polymers detected by the TGA analyses under argon at a heating rate of 10 °C/min.
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monomer and polymers were characterized by FT-IR and 1H and
13C NMR. The strong peak of the aldehyde group of the
momomer was observed at about 1652 cmꢀ1 in the FT-IR
spectra, and in those of the polymers, the peak around 1652 cmꢀ1
could still be observed due to the presence of the terminal
planar structure than the phenylene moieties, the two methyloxy
groups substituted on the phenyl ring would increase the
electron density of the phenylene moieties, thus benefitting the
intramolecular charge transfer. As shown in Figure 1, the color
of the polymers in THF solution changed from yellow to orange
for P1 and P2, suggesting that the different aryl moieties incorpo-
ratedintotheconjugatedsystemthroughtheCdC bond could tune
the electron properties of the corresponding polymers expediently.
With the same trend as observed in their UVꢀvisible absorp-
tion, P2 exhibited red-shifted emission spectra compared with
that of P1. The same red-shifted absorption and emission spectra
from P1 to P2 indicated that the optical properties of these
conjugated materials were highly dependent on the nature of the
active building blocks. P1 and P2 emitted strong yellow-green
and orange fluorescence with the PL quantum yields (Φ) of 46
and 33%, respectively, by using fluorescein in water (ΦF = 90%,
pH = 11) as the reference. The relatively higher Φ values might
be due to the twist structure of the triphenylamine group and the
electron-rich property of the pyrrole ring.
To explore the optical properties of these polymers in the solid
state, their absorption and PL properties were investigated
(Figure 3), and the corresponding data are summarized in
Table 1. The absorption spectra of dilute solutions and solid
film were similar, and the maximum absorption wavelengths
were the same, which partially disclosed that there was little
ground-state aggregation in the solid state. Subsequently, to
confirm the result, several experiments were conducted. First, the
absorption spectra of polymers in different concentrations were
tested (Figures S5 and S6 of the Supporting Information). The
shape of the absorption spectra changed only a little accompa-
nied by the growth of the concentrations, and the maximum
absorption wavelengths were the same as that in the solid film,
indicating that the ground-state interactions between polymer
chains were similar from the dilute solutions, to concentrated
solutions, then to solid films. Second, the excitation spectra of the
long wavelength bands in thin films mostly remained unchanged
upon the excitation of different emission wavelengths (Figure S7
of the Supporting Information), and the same phenomena were
observed in their solutions (Figure S8 of the Supporting In-
formation), further indicating that the aggregate formation was
inhibited in the ground state.
1
aldehyde group. This fact was further confirmed by their H
NMR spectra. As measured by TGA, the polymers were found to
exhibit very good thermal stability, with the initial decomposition
temperature above 360 °C under the atmosphere of argon, where
over 95% of their mass was retained (Figure 1 and Table 1).
One-Photon Physical Properties. Absorption and emission
spectra of polymers were measured in THF solution at a concen-
tration of 1.0 ꢁ 10ꢀ6 M per repeating unit (Figure 1 and Figure 2),
with the corresponding photophysical data summarized in Table 1.
UVꢀvis spectra of polymers P1 and P2 showed broad absorp-
tion maxima at 469 and 489 nm, respectively, which should be
assigned to the πꢀπ* transition of the conjugated system,
indicating that 1,4-dimethoxyphenylene moieties as the conju-
gated bridge could contribute more to the effective charge transfer
from the donor groups to the π-bridge than the fluorenylene
group. Although the fluorenylene group possessed the larger
Figure 2. Normalized fluorescence spectra of the polymers in THF.
Concentration: 1.0 ꢁ 10ꢀ6 M in repeating unit. Inset: fluorescence
photographs of the polymers in THF under UV illumination, P1 (left)
and P2 (right).
To get an insight into the molecular structure and electron
distribution of the polymers, the geometries and frontier orbitals
Figure 3. Normalized solid-state absorption spectra (left) and normalized solid-state PL spectra (right) of the polymers.
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Figure 4. The repeating units of polymers P1 and P2 optimized at the B3LYP/6-31+G(D) level.
Figure 5. Two-photon absorption cross section (δ) of polymers in THF (left) and the dependence of logarithmic output fluorescence intensity (up-
conversion signal) on logarithmic input laser power under the excited wavelength of 800 nm (right). Concentration: 2.0 ꢁ 10ꢀ5 M in repeating unit.
of the repeating units of polymers P1 and P2 have been
optimized by using time-dependent DFT (TDDFT) calculations
with the Gaussian 03 program (Figure 4 and Figure S4 of the
Supporting Information). It was easily seen that the nonplanar
structure of triphenylamine as the core led to the twisted
configuration of the repeating units, which could inhibit the
aggregated formation.
The high fluorescence of polymers in solution was partially
quenched in the solid state, with their maximum emissions at 555
and 597 nm, respectively, which were about 22 and 28 nm red-
shifted from those in solutions. Considering all the above facts, the
long-wavelength emission in the solid state should be ascribed to
the formed excimers rather than the possible ground-state aggrega-
tions. However, the increase in the effective conjugation length in
the solid states could also result in the red-shifted emission spectra.
Two-Photon Absorption Properties. The two-photon cross
sections of polymers in THF at a concentration of 2 ꢁ 10ꢀ5 M in
repeating unit were measured by the two-photon induced
fluorescence technique, using femtosecond laser pulses.10 The
femtosecond laser pulses (100 fs) were employed to avoid
possible complications due to the excited-state excitation, and
the thermooptical effect could be ignored. To confirm the
occurrence of nonlinear absorption, we have investigated the
strong fluorescence emission from the polymer solutions under
different laser intensities, and the output fluorescence intensity
was nonlinearly increased with the input laser power. The plot of
logarithmic input laser power gave a power law dependence of
exponent 1.92 (Figure 5), which was indicative of a two-photon
excitation process. The 2PA spectra of the polymers in THF were
presented in Figure 5, with the excitation wavelength in the range
of 730ꢀ900 nm. The relevant spectroscopic parameters are
summarized in Table 1. The δ values were calculated according
to the following equation:11
Fs Φrnrcr
δ2s
¼
ꢁ
δ
2r
Fr Φsnscs
The subscripts s and r stand for the measured sample and
reference molecule, respectively; F is the integrated fluorescence
intensity measured at the same power as the excitation beam; Φ
is the fluorescence quantum yield; c is the number density of the
molecules in the solution. The δ2r term is the 2PA cross section
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Table 2. Some Parameters of Polymers for Normalized TPA
Cross Sections
information is available free of charge via the Internet at http://
pubs.acs.org.
a
f
Neff
E10 (eV) b E20 (eV) c
n d
δmax (GM) e δ/δmax
’ AUTHOR INFORMATION
P1
P2
40.5
36.5
2.64
2.54
2.66
3.26
3.26
3.26
1.405
1.405
1.405
2392
1938
1716
0.0032
Corresponding Author
*Phone: 86-27-62254108. Fax: 86-27-68755363. E-mail: lizhen@
whu.edu.cn, lichemlab@163.com.
0.0023
0.0022
PL-3 41.6
a The effective number of electrons in the molecules or the repeating
unit of polymers. b The transition energy between the one-photon
Author Contributions
§These authors contributed equally to this manuscript.
em
max
excited state and the ground state, which is given by E10 = 1240/λ
.
c The transition energy between the two-photon excited state and the
ground state, which is given by E20 = 2 ꢁ 1240/λmaxtp. d The refractive
index of THF. e Two-photon absorption cross sections in repeating unit,
’ ACKNOWLEDGMENT
1 GM (G€oppertꢀMayer) = 10ꢀ50 cm4 s photonꢀ1. The normalized
f
We are grateful to the National Science Foundation of China
(no. 21002075 and 21034006), the National Fundamental Key
Research Program (no. 2011CB932702), for financial support.
two-photon absorption cross sections.
of the reference molecule. Here, fluorescein was chosen as the
reference molecule.
It could be easily seen that P1 and P2 exhibited broad 2PA
bands with the maximum 2PA cross sections (δ) around 760 nm.
The maximum 2PA cross sections of polymers P1 and P2 were
2392 and 1938 GM per repeating unit, respectively. The large δ
value implied that the 2PA chromophores consisting of the
triphenylamine and pyrrole could be the effective building blocks
(repeating units). P1 exhibited a little larger δ value than P2,
indicating that, in this system, the extension of conjugation
length was more effective in enhancing 2PA cross-section than
the incorporation of electron-donating moieties, which might be
due to the strong electron-rich property of the pyrrole ring.
According to Kuzyk’s electron rules,12 the normalized 2PA
cross sections were calculated to determine the 2PA cross
sections per electron, with the data summarized in Table 2.
The trend for the normalized 2PA cross sections was similar to
the absolute values. Since PL-3 has a similar structure with the
repeating unit of polymer P2, it could be seen that the 2PA cross
sections of P2 in the repeating unit were larger than the single
chromophore, suggesting that it was a good approach for the
enhancing of the 2PA cross sections to connect the chromophore
moieties together to construct the corresponding conjugated
polymers.
’ REFERENCES
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’ CONCLUSIONS
In this work, two novel conjugated polymers, P1 and P2, with
triphenylamine as the donor and pyrrole and fluorenylene or 1,4-
dimethoxyphenylene moieties as the connecting unit, were
synthesized by WittigꢀHornerꢀEmmons olefination. They
were soluble in common organic solvents and emitted strong
one- and two-photon excitation fluorescence in dilute THF
solution. The 2PA properties were studied by the two-photon
induced fluorescence measurement technique, and the 2PA cross
sections of 2392 and 1938 GM were obtained for the polymers
P1 and P2 per repeating unit, respectively. The results showed
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and pyrrole would be promising candidates for the further
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’ ASSOCIATED CONTENT
S
Supporting Information. FT-IR spectra, TGA curves,
b
UVꢀvis spectra, PL spectra and excitation spectra of polymers,
and Frontier orbitals of the repeating units of polymers. This
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The Journal of Physical Chemistry B
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