Synthesis, two-photon absorption, and optical power limiting of new linear and hyperbranched conjugated polyynes based on bithiazole and triphenylamine

Article abstract of DOI:10.1002/pola.24608

Two new linear and hyperbranched conjugated polymers P1 and P2 have been synthesized by Sonogashira coupling reaction, in which the main chain consists of bithiazole moieties as electron acceptors and triphenylamino groups as donors. The conjugated polymers were characterized by TGA, UV-vis absorption, fluorescence emission, electrochemical cyclic voltammetry, and two-photon absorption measurements. They exhibited excellent solubility in organic solvents and high thermal stability (5% of weight loss at 299 °C). The two-photon absorption cross sections (σ) measured by the open aperture Z-scan technique using 140 femtosecond (fs) pulse were determined to be 1014 and 552 GM per repeating unit for P1 and P2, respectively. This result shows that the σ of linear conjugated P1 is higher than that of hyperbranched P2, indicating that the linear polymer offers better intramolecular charge transfer ability. In THF, P1 and P2 exhibit intense frequency up-converted fluorescence under the excitation of 140 fs pulses at 800 nm with the peaks located at 580 and 548 nm, respectively. Meanwhile, the optical limiting behaviors for the polymers were studied by using a focused 800 nm laser beam of 140 fs duration. It was found that these polymers also exhibit good optical-limiting properties and make them potential candidates for optical limiters in the photonic fields.

Full text of DOI:10.1002/pola.24608

Synthesis, Two-Photon Absorption, and Optical Power Limiting
of New Linear and Hyperbranched Conjugated Polyynes Based
on Bithiazole and Triphenylamine
YIHUA JIANG,1 YAOCHUAN WANG,2 JIABAO YANG,1 JIANLI HUA,1 BING WANG,1 SHIQIONG QIAN,2 HE TIAN1
1Key Laboratory for Advanced Materials, Institute of Fine Chemicals and Department of Chemistry,
East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China
2Department of Physics, Fudan University, Shanghai 200433, China
Received 12 December 2010; accepted 27 January 2011
DOI: 10.1002/pola.24608
Published online 4 March 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Two new linear and hyperbranched conjugated
polymers P1 and P2 have been synthesized by Sonogashira
coupling reaction, in which the main chain consists of bithia-
zole moieties as electron acceptors and triphenylamino groups
as donors. The conjugated polymers were characterized by
TGA, UV–vis absorption, fluorescence emission, electrochemi-
cal cyclic voltammetry, and two-photon absorption measure-
ments. They exhibited excellent solubility in organic solvents
and high thermal stability (5% of weight loss at 299 ꢀC). The
two-photon absorption cross sections (r) measured by the
open aperture Z-scan technique using 140 femtosecond (fs)
pulse were determined to be 1014 and 552 GM per repeating
unit for P1 and P2, respectively. This result shows that the r of
linear conjugated P1 is higher than that of hyperbranched P2,
indicating that the linear polymer offers better intramolecular
charge transfer ability. In THF, P1 and P2 exhibit intense fre-
quency up-converted fluorescence under the excitation of 140
fs pulses at 800 nm with the peaks located at 580 and 548 nm,
respectively. Meanwhile, the optical limiting behaviors for the
polymers were studied by using a focused 800 nm laser beam
of 140 fs duration. It was found that these polymers also
exhibit good optical-limiting properties and make them poten-
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tial candidates for optical limiters in the photonic fields. 2011
Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49:
1830–1839, 2011
KEYWORDS: bithiazole; conjugated polymers; dyes/pigments;
synthesis; triphenylamine; two-photon
INTRODUCTION p-Conjugated organic molecules are an im-
portant class of two photon absorption (2PA) materials,
which have attracted increasing attention owing to their
potential application in the two-photon dynamic therapy,1,2
three-dimensional optical data storage,3–5 up-converted
lasing,6,7 optical power limiting materials,8–10 bioimaging,
etc.11,12 The strategy for the designing of molecules with
large 2PA cross sections (r) has been investigated both
experimentally and theoretically,13–16 including symmetrical
donor-conjugated bridge-donor (D-p-D), acceptor-conjugated
bridge-acceptor (A-p-A) and donor-conjugated bridge-
acceptor-conjugated bridge-acceptor-conjugated bridge-donor
(D-p-A-p-A-p-D), or asymmetrical donor-conjugated bridge-
acceptor (D-p-A) molecules, macrocycles, dendrimers, multi-
branched molecules, and polymers. These studies reveal that
this enhancement in r has been correlated with intramolecu-
lar charge transfer from the terminal donor groups to the p-
bridge.17 And the magnitude of r can thus be controlled
through the modification of the molecular structure in such
a way as to affect the amount of intramolecular charge trans-
fer. Specifically, r is increased when the conjugation length
of the p-system is increased and when electron acceptors
(A) are attached to the p bridge, to form molecules of
general structure D-A-D.18–21 Meanwhile, this is a beneficial
feature especially for the broadband optical limiting applica-
tions based on 2PA. Optical limiting is a significant area to
protect the human eyes and delicate optical instruments
from intense laser beams. An ideal optical limiting material
should strongly attenuate intense and potentially dangerous
laser beams, while exhibiting high transmittance for low
intensity ambient light. Up to now, a number of organic
materials have been found to exhibit large 2PA cross section
and excellent optical limiting properties.22,23 Thus, to realize
full potential applications, new compounds with strong two-
photon activities are crucially desired.
However, these small molecules have to be incorporated into
polymer to form thin films for many practical applications.
In many cases, the disperse concentration of 2PA materials
in the host polymer matrix is limited and the phase separa-
tion would easily happen under intense laser irradiation. To
minimize phase segregation during blending, a few 2PA con-
jugated polymers have been reported. Moreover, conjugated
Correspondence to: J. Hua (E-mail: jlhua@ecust.edu.cn) or H. Tian (E-mail: tianhe@ecust.edu.cn)
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Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 1830–1839 (2011) 2011 Wiley Periodicals, Inc.
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been used as acceptor unit to copolymerize with the donor
unit. Therefore, p-conjugated polymers incorporating with
bithiazole moieties have been demonstrated to be as new n-
type transporting materials.36–43 Obviously, bithiazole unit is
simple in structure in comparison with other acceptor units
such as benzothiadiazole etc. However, only a limited num-
ber of bithiazole based polymers have been explored,44–46
and their applications on 2PA have not been reported yet.
With this aim in mind, here we would like to report the one-
and 2PA properties of new linear and hyperbranched conju-
gated polymers P1 and P2 with triphenylamine and thiazole
groups in the main chain, in which the 2PA cross section of
the polymers were performed by open-aperture Z-scan
experiment using 140 femtosecond (fs) pulse.
EXPERIMENTAL
Materials Synthesis
THF was pre-dried over 4 Å molecular sieves and distilled
under argon atmosphere from sodium benzophenone ketyl
immediately prior to use. Triethylamine and pyridine were
distillated under normal pressure and dried over potassium
hydroxide. N,N-dimethyl formamide (DMF) and dichlorome-
thane (DCM) were reflux with calcium hydride and distilled
before used. All other chemicals were purchased from
Aldrich and used as received without further purification.
CHART 1 The structures of linear and hyperbranched polymers
(P1 and P2). [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
Instruments
FT-IR spectra were recorded on a Nicolet Magna-IR550 spec-
polymers are of particular interest due to their extended deloc-
alization of p-electrons along the polymer backbone, in which
some large electron-delocalized aromatic units are founded to
show impressive 2PA properties, such as poly(9,9-didecyl-2,7-
1
trometer. H and 13C NMR spectra were recorded on a Bruker
AM-500 spectrometer using chloroform-d as solvent and tetra-
methylsilane (d ¼ 0) as internal references. The UV/Vis spec-
tra were recorded on a Varian-Cary 500 spectrophotometer
with 2 nm resolution at room temperature. Molecular weights
and polydispersity indices of the polymers were estimated in
THF by a waters associated gel permeation chromatograph
(GPC) system. A set of monodisperse polystyrene standards
covering the molecular weight range of 103–107 was used for
the molecular weight calibration. The fluorescence spectra
were taken on a Varian-Cary fluorescence spectrophotometer.
The cyclic voltammograms of polymers were obtained with a
Versastat II electrochemical workstation (Princeton applied
research) using a normal three-electrode cell with a Pt work-
ing electrode, a Pt wire counter electrode, and saturated calo-
mel electrode (SCE) reference electrode in saturated KCl solu-
tion. TPA cross section of polymer P1 and P2 was measured
by femtosecond open-aperture Z-scan technique according to
previously described method.47
diphenyl-aminofluorene),24
a methylsubstituted ladder-type
poly(p-phenylene) (m-LPPP),25 poly(9,9-bis(6N,N,N-triethylam-
monium) hexylfluorenephenylene) (PFP),26,27 hyperbranched
poly(aroylarylene),28 and linear poly(aroyltriazole)s (PATAs).29
Especially, polymers with new structures (such as polymers
with donors and acceptors) can multiply the variety of the 2PA
materials, and it will be helpful to gain more information about
their structures-properties relationships.30,31
In this article, two new linear and hyper-branched conju-
gated polymers (P1 and P2, Chart 1) with triphenylamine
and thiazole groups in the main chain were synthesized and
used for two photon absorption materials. Triphenylamine is
an electron-rich, special propeller starburst molecular
displaying an octupolar feature and which can be linked to
different electron withdrawing end groups via p-conjugated
units. Meanwhile, the triphenylamine in the main chain can
improve the hole transporting ability of the materials and
the bulky volume of triphenylamine also can prevent the
aggregation of the materials, resulting in a more highly solu-
ble amorphous polymer. Many studies have explored this
design, developing efficient fluorescent triphenylamine deriv-
atives with high TPA cross-sections.32–35 However, thiazole is
a well-known electron-deficient unit because it contains one
electron-withdrawing nitrogen of imine (C¼¼N) in place of
the carbon atom at the 3-position of thiophene. Bithiazole
(BTz) with two thiazole rings connected together have also
Two-photon excited fluorescence (TPF) was excited by the fs
pulses with different intensities at wavelength of 800 nm.
The repetition rate of the laser pulses is 1 kHz and the pulse
duration is 140 fs. For both open-aperture Z-scan and TPF
measurement, the concentration of P1 and P2 in THF were
6.5 mg/mL.
Monomer Synthesis
1-Bromo-2-octanone (4)
In a 250 mL two-necked round-bottom flask was added 2-
Octanone (40 mL, 0.251 mol), urea (25.0 g, 0.417 mol), and
SYNTHESIS OF CONJUGATED POLYYNES P1 AND P2 WITH BITHIAZOLE, JIANG ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
glacial acetic acid (125 mL) with an ice bath for cooling. A
solution of bromine (14.0 mL, 0.275 mmol) in glacial acetic
acid (40 mL) was added dropwise to the flask, and the solu-
tion was stirred overnight at room temperature. Water
(250 mL) was then added to the solution, and the solution
was extracted with dichloromethane. The combined extracts
were washed with 10% sodium carbonate and brine and
then dried over anhydrous magnesium sulfate. The pure
product was obtained after vacuum distillation (28.591 g,
yield 55%). 1H NMR (CDCl3, 400 MHz), d (TMS, ppm): 3.88
(s, 2H), 2.62 (t, 2H, J ¼ 7.2 Hz), 1.58 (m, 2H), 1.35–1.15 (m,
6H), 0.86 (t, 3H, J ¼ 6.8 Hz).
mg (0.02 mmol) of CuI, and 4 g (48 mmol) of 2-methylbut-
3-yn-2-ol, the mixture was heated to reflux for 12 h under
argon atmosphere. The solvent was removed by rotary evap-
oration. The residue was extracted with chloroform and
water. The combined organic phases were dried over anhy-
drous MgSO4 and concentrated using a rotary evaporator.
The crude product was purified by column chromatography
on silica (dichloromethane:ethyl acetate ¼ 10:1, v/v) to
afford 2.7 g yellow powder (yield: 55%). 1H NMR (CDCl3,
400 MHz), d (TMS, ppm): 7.28 (d, 6H, J ¼ 8.4 Hz), 7.07 (d,
3H, J ¼ 8.4 Hz), 6.97 (d, 4H, J ¼ 8.4 Hz), 1.61 (s, 12H).
4-Ethynyl-N-(4-ethynylphenyl)-N-phenylaniline (2)
4,40-Dihexyl-2,20-bithiazole (5)
A modified version of a previously reported method was
used.30 In a 100 mL two-necked round-bottom flask, 2.6 g
(6.4 mmol) of 7 and 2 g (36 mmol) of potassium hydroxide
were added under argon atmosphere. The mixture was
heated to reflux for 6 h. Solvent was removed by rotary
evaporation. The residue was extracted with chloroform and
water. The combined organic phases were dried over anhy-
drous MgSO4 and concentrated using a rotary evaporator.
The residue was purified by column chromatography on
silica (petroleum ether: dichloromethane ¼ 1:1, v/v) to
afford 0.804 g of product as a white powder (yield: 29%).
1H NMR (CDCl3, 400 MHz), d (TMS, ppm): 7.26 (d, 4H, J ¼
8.4 Hz), 7.19 (d, 2H, J ¼ 8.4 Hz), 7.09 (m, 1H), 7.00 (d, 2H,
J ¼ 8.4 Hz), 6.90 (d, 4H, J ¼ 8.4 Hz), 3.06 (s, 2H). 13C NMR
(100 MHz, CDCl3), d (TMS, ppm): 146.6, 145.5, 132.2, 128.6,
124.5,122.1, 114.9, 82.6, 75.6. HRMS (m/z) [M] Calcd for
In a 250-mL two-necked round-bottom flask was added 4
(10.906 g, 52.66 mmol), dithiooxamide (3.165 g, 26.33
mmol), and absolute ethanol (140 mL). The solution was
heated to reflux for 4 h, and after cooling, it was poured
onto crushed ice. The mixture was extracted with dichloro-
methane and then dried over anhydrous magnesium sulfate.
After the evaporation of the solvent, the product was
obtained as a brown crystal (7.178 g, yield: 81%). 1H NMR
(CDCl3, 400 MHz), d (TMS, ppm): 6.92 (s, 2H), 2.76 (t, 4H,
J ¼ 8.0 Hz), 1.72 (m, 4H), 1.40–1.15 (m, 12H), 0.87 (t, 6H,
J ¼ 8.0 Hz).
5,50-Dibromo-4,40-dihexyl-2,20-bithiazole (1)
In a 100 mL round-bottom flask, 5 (1 g, 2.97 mmol) and
NBS (1.335 g, 7.43 mmol) were dissolved in a mixture of gla-
cial acetic acid (10 mL) and N,N-dimethylformamide
(10 mL). After 2 h of stirring in the dark, a gray solid pre-
cipitated in the reaction mixture. The precipitate was fil-
tered, washed with methanol, and then dried to produce
the dibromo product (1.292 g, yield: 88%). 1H NMR (CDCl3,
400 MHz), d (TMS, ppm): 2.72 (t, 4H, J ¼ 8.0 Hz), 1.68 (m,
4H), 1.40–1.15 (m, 12H), 0.87 (t, 6H, J ¼ 8.0 Hz). 13C NMR
(CDCl3, 100 MHz), d (TMS, ppm): 159.9, 157.4, 31.5, 29.5,
28.8, 28.6, 22.6, 14.1. HRMS (m/z) [M] Calcd for
C22H15N: 293.1204, Found: 293.1192.
Tris(4-iodophenyl)amine (8)
A modified version of a previously reported method was
used.30 To a stirred solution of 10 g (40 mmol) of triphenyl-
amine and 13.5 g (80 mmol) of potassium iodide in 340 mL
acetic acid with 30 mL distilled water at 80 ꢀC was added
ꢀ
12 g (56 mmol) potassium iodate. After stirring at 80 C for
6 h, the mixture was poured into distilled water to induce
precipitation of the crude product. The precipitation was fil-
tered off and recrystallized with chloroform to give 22.8 g of
C18H26Br2N2S2: 419.9904, Found: 491.9958.
4-Iodo-N-(4-iodophenyl)-N-phenylaniline (6)
1
white solid (yield: 69%). H NMR (CDCl3, 400 MHz), d (TMS,
A modified version of a previously reported method was
used.30 To a stirred solution of 15 g (60 mmol) of triphenyl-
amine and 13.5 g (80 mmol) of potassium iodide in 340 mL
acetic acid with 30 mL distilled water at 80 ꢀC was added
ppm): 7.54 (d, 6H, J ¼ 8.8 Hz), 6.82 (d, 6H, J ¼ 8.8 Hz).
4,40,400-(4,40,400-Nitrilotris(benzene-4,1-diyl))
tris(2-methylbut-3-yn-2-ol) (9)
ꢀ
12 g (56 mmol) potassium iodate. After stirring at 80 C for
A modified version of a previously reported method was
used.30 In a 250 mL two-necked round-bottom flask, 10 g
(12 mmol) of 8 was dissolved in 100 mL of triethylamine at
room temperature. After addition of 2.9 mg (0.01 mmol) of
Pd(PPh3)2Cl2, 2.9 mg (0.01 mmol) of triphenylphosphine, 5.2
mg (0.02 mmol) of CuI, and 5 g (60 mmol) of 2-methylbut-
3-yn-2-ol, the mixture was heated to reflux for 12 h under
argon atmosphere. The Solvent was removed by rotary evap-
oration. The residue was extracted with chloroform and
water. The combined organic phases were dried over anhy-
drous MgSO4 and concentrated using a rotary evaporator.
The crude product was purified by column chromatography
on silica (dichloromethane:ethyl acetate ¼ 10:1, v/v) to
afford 3.3 g yellow powder (yield: 55%). 1H NMR (CDCl3,
6 h, the mixture was poured into distilled water to induce
precipitation of the crude product. The precipitation was fil-
tered off and recrystallized with chloroform to give 25 g of
white solid (yield: 50 %). 1H NMR (CDCl3, 400 MHz), d
(TMS, ppm): 7.41 (d, 4H, J ¼ 8.8 Hz), 7.20 (d, 2H, J ¼ 8.8
Hz), 6.67 (m, 3H), 6.41 (d, 4H, J ¼ 8.8 Hz).
4,40-(4,40-(Phenylazanediyl)bis(4,1-phenylene))
bis(2-methylbut-3-yn-2-ol) (7)
A modified version of a previously reported method was
used.30 In a 250 mL two-necked round-bottom flask, 6 g (12
mmol) of 6 was dissolved in 100 mL of triethylamine at
room temperature. After addition of 2.9 mg (0.01 mmol) of
Pd(PPh3)2Cl2, 2.9 mg (0.01 mmol) of triphenylphosphine, 5.2
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ARTICLE
400 MHz), d (TMS, ppm): 7.30 (d, 6H, J ¼ 8.4 Hz), 6.98 (d,
6H, J ¼ 8.4 Hz), 1.62 (s, 18H).
Tris(4-ethynylphenyl)amine (3)
A modified version of a previously reported method was
used.30 In a 100 mL two-necked round-bottom flask, 3.12 g
(6.4 mmol) of 9 and 3 g (54 mmol) of potassium hydroxide
were added under argon atmosphere. The mixture was
heated to reflux for 6 h. Solvent was removed by rotary
evaporation. The residue was extracted with chloroform and
water. The combined organic phases were dried over anhy-
drous MgSO4 and concentrated using a rotary evaporator.
The residue was purified by column chromatography on
silica (petroleum ether: dichloromethane ¼ 1:1, v/v) to
afford 0.604 g of product as a white powder (yield: 30%).
1H NMR (CDCl3, 400 MHz), d (TMS, ppm): 7.39 (d, 6H, J ¼
8.4 Hz), 7.01 (d, 6H, J ¼ 8.4 Hz), 3.06 (s, 3H). 13C NMR (100
MHz, CDCl3), d (TMS, ppm): 147.1, 133.5, 124.0, 117.0, 83.5,
77.0. HRMS (m/z) [M] Calcd for C24H15N: 317.1204, Found:
317.1192.
Polymerization
P1. In a 50 mL two-necked round-bottom flask, 370.5 mg
(0.75 mmol) of 1, 202 mg (0.69 mmol) of 2, 2.9 mg (0.01
mmol) of Pd(PPh3)2Cl2, 2.9 mg (0.01 mmol) of triphenyl-
phosphine, 5.2 mg (0.02 mmol) of CuI, 12 mL of THF, and 4
mL of triethylamine were added under the argon atmo-
sphere. The mixture was heated to reflux for 24 h under
argon atmosphere. The solvent was removed by rotary
evaporation. The residue was extracted with chloroform and
water. The combined organic phases were dried over anhy-
drous MgSO4 and concentrated using a rotary evaporator.
The residue was dissolved by 5 mL of DCM and added drop-
wise to a 450 mL methanol through a cotton filter under
stirring. The precipitate was allowed to stand overnight, and
then filtered with Gooch crucible. The polymer was washed
with methanol and dried in a vacuum oven to a constant
weight to afford 250 mg (56%) yellow powder was col-
lected. Mw: 7811, Mw/Mn: 2.7 (GPC). 1H NMR (400 MHz,
CDCl3), d (TMS, ppm):7.53, 7.39, 7.30, 7.14, 7.07, 2.90, 2.76,
1.78, 1.33, 0.89. 13C NMR (100 MHz, CDCl3), d (TMS, ppm):
162.7, 158.5, 157.5, 147.6, 146.4, 132.6, 128.3, 125.8, 123.4,
122.9, 116.4, 115.9, 96.2, 79.6, 31.6, 30.5, 29.6, 29.1, 28.9,
28.8, 28.7, 22.6, 14.1.
SCHEME 1 The synthetic procedure of polymers (P1 and P2).
with methanol and dried in a vacuum oven to a constant
weight to afford 150 mg (45%) yellow powder was col-
lected. Mw: 11847, Mw/Mn: 1.9 (GPC). 1H NMR (400 MHz,
CDCl3), d (TMS, ppm):7.37, 7.03, 2.83, 2.70, 1.71, 1.27, 0.82.
13C NMR (100 MHz, CDCl3), d (TMS, ppm):161.7, 159.1,
157.6, 156.5, 132.8, 131.7, 123.3, 123.1, 106.0, 97.7, 78.1,
30.6, 29.4, 28.5, 28.0, 27.8, 27.8, 27.7, 21.6, 13.1.
P2. In a 50 mL two-necked round-bottom flask, 370.5 mg
(0.75 mmol) of 1, 128 mg (0.4 mmol) of 3, 2.9 mg (0.01
mmol) of Pd(PPh3)2Cl2, 2.9 mg (0.01 mmol) of triphenyl-
phosphine, 5.2 mg (0.02 mmol) of CuI, 12 mL of THF, and 4
mL of triethylamine were added under the argon atmo-
sphere. The mixture was heated to reflux for 24 h under
argon atmosphere. The Solvent was removed by rotary evap-
oration. The residue was extracted with chloroform and
water. The combined organic phases were dried over anhy-
drous MgSO4 and concentrated using a rotary evaporator.
The residue was dissolved by 5 mL of DCM and added drop-
wise to a 450 mL methanol through a cotton filter under
stirring. The precipitate was allowed to stand overnight, and
then filtered with Gooch crucible. The polymer was washed
RESULTS AND DISCUSSION
Synthesis
The synthesis of the monomers 1, 2, and 3 is shown in
Scheme 1. The monomer 1 was prepared according to previ-
ously reported methods,48 in which 2-octanone was bromi-
nated, then reacted with dithiooxamide to produce 4-hexyl-
2-(4-hexylthiazol-2-yl)thiazole 5. The treatment of compound
5 with NBS resulted in selective dibromination at position of
the bithiazole moiety, which yielded monomer 1. Monomers
2 and 3 were prepared by Sonogashira coupling reaction
and hydrolyzation of 2-methylbut-3-yn-2-ol with 4-iodo-N-
SYNTHESIS OF CONJUGATED POLYYNES P1 AND P2 WITH BITHIAZOLE, JIANG ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 1 1H NMR spectrum of monomers 1, 3 and their hyperbranched polymer P2 in chloroform-d (400 MHz). The solvent peak
is marked with an asterisk.
(4-iodophenyl)-N-phenylbenzenamine and tris(4-iodophenyl)-
amine, respectively, which can be readily synthesized by sub-
stitution of triphenylamine with potassium iodide and pota-
ssium iodate in the presence of acetic acid. Polycoupling of
the monomers 1 and 2/3 was attempted under the Sono-
gashira coupling reaction conditions to afford polymers P1
and P2 with weight-average molecular weights (0.78 ꢁ 104
and 1.18 ꢁ 104 Da, respectively). The polymers have good
solubility in common organic solvents such as chloroform,
THF, and chlorobenzene at room temperature. Introduction
of long flexible alkylated groups and nonplanar triphenyl-
amine units has become a popular approach to boosting.
the methylene group adjacent to the bithiazole which
coupled with the monomer 3 by Sonogashira reaction. No
other unexpected signals are found, and all the resonance
peaks can be readily assigned. This confirms the proposed
structure of the P2. The molecular structures of the poly-
mers are further characterized by 13C NMR spectroscopy. An
example of 13C NMR spectrum of P2, along with that of
its monomer 1 and 3, is shown in Figure 2. The acetylenic
carbon atoms adjacent to the protons of monomer 3 reso-
nate at d 83.5 and 77.0. After polycoupling with 5,50-
dibromo-4,40-dihexyl-2,20-bithiazole (1), the resonance peak
of acetylenic carbon atoms of monomer 3 are absent in the
spectrum of its polymer P2. In replacement, new two peaks
due to the resonances of electron-deficient bithiazole (BTz)
carbon atoms and the phenyl carbon atoms appear at d 97.7
and 78.1, respectively (Fig. 2).
Structural Characterization
The purified polymerization product gives satisfactory spec-
troscopic data corresponding to the expected molecular
structures (see Experimental Section for details). The 1H
NMR spectra of hyperbranched polymer P2 and its mono-
mers 1 and 3 are shown in Figure 1. Monomer 1 peaks are
readily assignable: the signals from d 1.9 to 0.7 due to its
hexyl chain on bithiazole. The signal of the methylene group
adjacent to the bithiazole unit appears around 2.7 ppm.
Monomer 3 spectrum shows the resonance signals at d 7.4,
7.0, and 3.1 are due to the absorptions of the aromatic and
acetylenic protons, respectively. The 1H NMR spectra of P2
displays apparent changes that the peaks of polymer are
broader than those of monomers 1 and 3, suggesting that
the polymer possesses an irregular and somewhat rigid
steric structure. Moreover, the acetylene protons of monomer
3 resonate at 3.1 ppm, which completely disappears upon
Sonogashira coupling reaction with monomer 1. It suggests
that all triple bonds have been consumed during the poly-
merization reaction. The new signals appeared on 2.9 due to
Thermal and Optical Properties
Thermal stability of the polymers is evaluated by thermogra-
vimetric analysis (TGA) under nitrogen, as shown in Figure
3. The TGA analysis reveals that the onset temperatures with
5% weight-loss (Td) of P1 and P2 are 299 and 268 ꢀC,
respectively. This indicates that the thermal stability of the
conjugated polymers is enough for the applications in two
photon absorption materials. Moreover, the rigidity of the
main chain of the polymer P1 would be preserved due to its
linear structure, leading to an increase in Td compared with
that of polymer P2.
Normalized one-photon absorption spectra of P1 and P2 in
THF at dilute concentrations are shown in Figure 4 and the
corresponding optical data of the copolymers are summar-
ized in Table 1. In general, the extension of the p-systems
exerts an important influence on the absorption spectra. The
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ARTICLE
FIGURE
2
13C NMR spectrum
and their
of monomers 1,
3
hyperbranched polymer P2 in
chloroform-d (100 MHz). The
solvent peak is marked with an
asterisk.
absorption maximum of P1 at 429 nm is red-shifted by 12
nm relative to that of with P2 at 417 nm due to the better
planarity in linear copolymer structure than hyper branched
one. It suggests linear copolymer has stronger ICT effect
than hyper branched one. The photoluminescence (PL) spec-
tra of P1 and P2 in THF were obtained by the excitation at
420 nm (Fig. 4), in which their fluorescence maxima appear
at 526 and 529 nm, respectively. The two polymers have
similar the emission maximums due to their same donor and
acceptor in spite of different molecular structure. The fluo-
rescence quantum efficiencies of P1 and P2 were deter-
mined in ethanol using Rhodamine B as reference (Table 1),
and the fluorescence quantum yield (Uf) of P1 and P2 is
0.54 and 0.39, respectively. The Uf value of P1 was higher
than that of P2. This might be due to a more efficient energy
transfer from the triphenylamine diyne to the bithiazole unit
in linear conjugated polymer. The UV absorption and fluores-
cence spectra of solid films of P1 and P2 were also meas-
ured, their absorption maxima occur at 427 and 425 nm,
respectively. Compared with the solution spectra, the absorp-
tion maxima of films are blue-shifted by 2 nm for P1 and
red-shifted 8 nm for P2, respectively. And there is similar
emission maxima data in the PL spectra comparing the data
between the solution and the film.
Electrochemical Properties
Figure 5 shows the cyclic voltammetry diagrams of the com-
pounds using 0.1 M tetrabutylammonium hexafluorophos-
phate as supporting electrolyte in acetonitrile solution with
platinum button working electrodes, a platinum wire counter
electrode, and an SCE reference electrode. The SCE reference
electrode was calibrated using a ferrocene/ferrocenium (Fc/
FIGURE 4 Normalized one-photon absorption spectra and
emission spectra of P1 and P2 in THF and film. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
FIGURE 3 TGA thermogram of P1 and P2 recorded under nitro-
gen at a heating rate 20 ꢀC/min.
SYNTHESIS OF CONJUGATED POLYYNES P1 AND P2 WITH BITHIAZOLE, JIANG ET AL.
1835

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 One-Photon and Two-Photon Properties of P1 and P2
TABLE 2 Electrochemical Properties of P1 and P2
abs
max
em
max
a
b
c
k
k
EHOMO
E0-0
ELUMO
(nm)
(nm)
Polymer
(eV)
(eV)
(eV)
rTPA
kTPF
Polymer
THF
Film
THF
Film
Ufa
(GM)b
(nm)c
P1
P2
a
ꢂ5.21
ꢂ5.35
2.51
2.57
ꢂ2.70
ꢂ2.78
P1
P2
a
429
417
427
425
526
529
529
529
0.54
0.39
1014
552
550
548
EHOMO were measured in acetonitrile with 0.1 M tetrabutylammonium
hexafluorophosphate (TBAPF6) as electrolyte [working electrode: Pt;
reference electrode: SCE; calibrated with ferrocene/ferrocenium (Fc/Fcþ)
as an external reference. Counter electrode: Pt wire].
The PL quantum yield (Uf) was estimated with rhodamine B in ethanol
as a standard.
b
b
E0-0 was estimated from the intercept of the normalized absorption
Two-photon absorption (TPA) cross section excited at 800 nm (140 fs)
in 10ꢂ50 cm4 s per photon (GM).
The maximum two-photon fluorescence (TPF) excited at 800 nm.
and emission spectra.
c
c
ELUMO is estimated by subtracting E0-0 to HOMO.
Fcþ) redox couple as an external standard. The electrochemi-
cal properties as well as the energy level parameters of P1
and P2 are list in Table 2. It can be speculated from Figure 5
that the first half-wave potentials (Eox ) of P1 and P2 were
0.41 and 0.55 V, respectively. Therefore, the ground state
oxidation potential corresponding to the HOMO energy levels
are ꢂ5.21 and ꢂ5.35 (vs. vacuum) according to the
equations HOMO ¼ ꢂe (Eox þ 4.8) (eV),49 respectively. It is
clear that the HOMO levels of P1 are higher than that of P2,
which may be affected by their molecular weights and
polydispersities.
resulting in higher nonlinear optical response. The linear co-
polymer P1 has stronger charge-transfer ability compared
with the hyperbranched P2, as the result of higher r value.
The copolymer P1 and P2, which have the same donor,
acceptor and p conjugation, imply the molecular structure in
polymers is another important key to improve the 2PA cross
section. Moreover, the linear structure is more effective than
hyper branched structure on improving the 2PA properties
(about two folds in this case).
Under the excitation of 140 fs, 800 nm pulses, P1 and P2 in
THF emit intense frequency up-converted fluorescence with
the peaks located at 548 and 547 nm, respectively (Table 1).
The fluorescence could even be seen by naked eye under ex-
citation of unfocused laser pulses with energy of several lJ.
The two photon fluorescence spectra of the polymers under
different laser intensity are shown in Figure 7. The linear de-
pendence of fluorescence intensity on the square of the exci-
tation intensity as shown in the inset confirms that 2PA is
the main excitation mechanism of the intense fluorescence
emission. Intense two-photon fluorescence was even
observed at the excitation intensity of about 8.9 and 7.8
Two-photon Absorption Properties
2PA cross sections of the P1 and P2 were determined by
femtosecond open aperture Z-scan technique according to
previously described method.50 Figure 6 shows the open-
aperture Z-scan data of P1 and P2, and 2PA coefficient got
by data fitting. The 2PA cross section r can be calculated by
using the equation of r ¼ hmb/N0, where N0 ¼ NAC is the
number density of the absorption centers, NA is the Avoga-
dro constant and C represents the solute molar concentra-
tion. The values of 2PA cross section (r) for P1 and P2 are
1014 and 552 GM at wavelength of 800 nm, respectively
(Table 1), in which the r of linear conjugated polymer P1 is
higher than that of hyperbranched P2. It is well-known that
greater degree of charge transfer in donor-acceptor system,
FIGURE 6 Open aperture Z-scan results for P1 and P2 solutions
in THF, respectively, using 800 nm fs pulses (scattered circle
experimental data, straight line theoretic fitted data). [Color fig-
ure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
FIGURE 5 Cyclic voltammograms of P1 and P2 in THF contain-
ing 0.1 mol/L of tetrabutylammonium hexafluorophosphate
(TBAPF6).
1836
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
which polymers show linear response of transmitted energy
obeying Beer’s law at low incident energy and the transmis-
sion decreases significantly as the incident fluence increases.
Moreover, the optical limiting threshold, defined as the input
fluence value, at which the transmission decreases to be half
of the linear transmission, is found to be 0.038 J/cm2 for P1.
As we chose a proper power to carry out the measurement,
at which, pure THF solvent shows no detectable optical limit-
ing behavior, indicating that the solvent contribution is negli-
gible, so the observed optical limiting properties should be
contributed by the polymers.
From the characteristic curves in Figure 8, the characteristic
curves illustrate a two-photon absorbing medium can be
used to stabilize the laser pulse fluctuation. As the incident
fluences increase from ꢃ0.0010 J/cm2 to ꢃ0.044 J/cm2 (an
increase of about 44 times) for both two polymers, the
transmitted output fluences only show 20-fold and 29-fold
for P1 and P2, respectively, which fits the theoretically pre-
dicted optical limiting behavior based on 2PA. In comparison
FIGURE 7 TPF intensities of hyper branched polymer P1 and
P2 under different excitation power density, respectively. Inset
is TPF intensity versus the square of the excitation power
density.
GW/cm2 (P1 and P2, respectively), indicating the large 2PA
cross section and high two-photon fluorescence quantum
yields. This is an important prerequisite for 2PA-based appli-
cations such as fluorescence microscopy and upconverted
lasing.
2PA-Based Optical Limiting Properties
Following some laser-based applications such as optical com-
munication, optical fabrication, and manufacturing, it was
recognized that delicate optical instruments, especially the
human eye, could be easily damaged from sudden exposure
to high incident intensities of laser beam. However, reducing
such fluctuation and stabilizing the pulsed laser signals could
be achieved by means of strongly multiphoton absorbing
medium. Since two polymers have comparatively large 2PA
at 800 nm, the optical limiting properties of two copolymers,
at molecular concentrations of 6.5 mg/mL dissolved in THF
solutions, were measured by using open-aperture z-scan
technique, with 800 nm laser pulses of 140 fs pulse dura-
tion. The measured relationship between the incident and
output fluence for two polymers are shown in Figure 8, in
FIGURE 8 Measured output intensity versus input intensity of
the 800 nm laser pulses based on polymers P1 and P2; the
points are the experimental data, while the solid lines are the
theoretical fits. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
SYNTHESIS OF CONJUGATED POLYYNES P1 AND P2 WITH BITHIAZOLE, JIANG ET AL.
1837

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
11 Wang, X.; Pudavar, H. E.; Kapoor, R.; Krebs, L. J.; Bergey, E.
J.; Liebow, C.; Prasad, P. N.; Nagy, A.; Schally, A. V. J Biomed
Opt 2001, 6, 319–324.
to P1, P2 shows inferior optical power limiting performance,
indicating that the optical-limiting performance of polymers
based bithiazole moiety is sensitive to the change in its
molecular structure, offering the opportunity to tune its optical
limiting properties through molecular engineering endeavor.
12 Larson, D. R.; Zipfel, W. R.; Williams, R. M.; Clark, S. W.;
Bruchez, M. P.; Wise, F. W.; Webb, W. W. Science 2003, 300,
1434–1435.
CONCLUSIONS
13 Nguyen, K. A.; Day, P. N.; Pachter, R. Theor Chem Acc 2008,
120, 167–175.
In this work, two new linear and hyperbranched conjugated
polymers P1 and P2 are synthesized by Sonogashira cou-
pling reaction, in which the synthesis is very convenient and
the yields of polymers are relatively high. The polymers have
shown not only good solubility in common organic solvents
but also high thermal stability. Using open aperture Z-scan
technique, the polymers show large two photon absorption
cross session (1014 for P1, 552 for P2) and excellent optical
limiting properties. The result shows the linear copolymer
structure is more effective than hyper branched one on
improving 2PA properties due to its better intramolecular
charge transfer ability. Thanks to their conjugated electronic
structure, the polymers based on bithiazole emit yellow-
green light efficiently. Together with their excellent optical
power-limiting properties, these novel linear and hyper-
branched polymers would be a promising direction for
further development of novel 2PA polymer materials.
14 Zein, S.; Delbecq, F.; Simon, D. Phys Chem Chem Phys
2009, 11, 694–702.
15 Chakrabarti, S.; Ruud, K. Phys Chem Chem Phys 2009, 11,
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16 Rudberg, E.; Salek, P.; Helgaker, T.; Agren, H. J Chem Phys
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17 Albota, M.; Beljonne, D.; Bre´das, J. L.; Ehrlich, J. E.; Fu, J.
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This work was supported by National Basic Research 973 Pro-
gram, the Fundamental Research Funds for the Central Univer-
sities (WJ0913001), Ph.D. Programs Foundation of Ministry of
Education of China (20090074110004) and Scientific Commit-
tee of Shanghai (10520709700).
17, 1890–1893.
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