Polyfluorenylacetylene for near-infrared laser protection: Polymer synthesis, optical limiting mechanism and relationship between molecular structure and properties

Article abstract of DOI:10.1039/c7ra08499d

A series of functional polyacetylenes bearing the fluorene moiety with different conjugated lengths and terminal substituents, poly[2-ethynyl-7-(4-nitrostyryl)-9,9-dioctyl-9H-fluorene] (P1), poly[2-ethynyl-7-(4-(4-nitrostyryl)styryl)-9,9-dioctyl-9H-fluorene] (P2), poly[2-ethynyl-7-(4-(4-methoxystyryl)styryl)-9,9-dioctyl-9H-fluorene] (P3), poly[2-ethynyl-7-(4-(4-methylstyryl)styryl)-9,9-dioctyl-9H-fluorene] (P4), and poly[2-ethynyl-7-(4-(4-methylstyryl)styryl)-9,9-didodecyl-9H-fluorene] (P5), were designed and prepared using [Rh(nbd)Cl]₂ as the catalyst. Their structures and properties were characterized and evaluated by FTIR, NMR, UV, FL, GPC and TGA analyses. The optical limiting properties were investigated using 450 fs laser pulses at 780 nm. These results show that the incorporation of styryl/stilbene functionalized polyfluorenylacetylenes has endowed resultant polyacetylenes with novel near-infrared laser protection properties and enhanced thermal stability. The optical limiting mechanism for laser protection was studied. It was found that the optical limiting properties mainly originated from two-photon absorption (TPA) of molecules in the resulting polyacetylenes. Additionally, it was also found that the functionalized polyacetylene with a longer fluorene-based conjugated chromophore and a stronger terminal electron acceptor group exhibits better optical limiting properties because of the larger π-electron delocalization and dipolar effect.

Full text of DOI:10.1039/c7ra08499d

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PAPER
Polyfluorenylacetylene for near-infrared laser
protection: polymer synthesis, optical limiting
mechanism and relationship between molecular
structure and properties†
Cite this: RSC Adv., 2017, 7, 53785
a
b
c
a
ad
*
*
Naibo Lin
Gang Zhao, Yan Feng, Shanyi Guang, Hongyao Xu,
and Xiangyang Liud
A series of functional polyacetylenes bearing the fluorene moiety with different conjugated lengths and
terminal substituents, poly[2-ethynyl-7-(4-nitrostyryl)-9,9-dioctyl-9H-fluorene] (P1), poly[2-ethynyl-7-
(4-(4-nitrostyryl)styryl)-9,9-dioctyl-9H-fluorene] (P2), poly[2-ethynyl-7-(4-(4-methoxystyryl)styryl)-9,9-
dioctyl-9H-fluorene] (P3), poly[2-ethynyl-7-(4-(4-methylstyryl)styryl)-9,9-dioctyl-9H-fluorene] (P4),
and poly[2-ethynyl-7-(4-(4-methylstyryl)styryl)-9,9-didodecyl-9H-fluorene] (P5), were designed and
prepared using [Rh(nbd)Cl]2 as the catalyst. Their structures and properties were characterized and
evaluated by FTIR, NMR, UV, FL, GPC and TGA analyses. The optical limiting properties were investigated
using 450 fs laser pulses at 780 nm. These results show that the incorporation of styryl/stilbene
functionalized polyfluorenylacetylenes has endowed resultant polyacetylenes with novel near-infrared
laser protection properties and enhanced thermal stability. The optical limiting mechanism for laser
protection was studied. It was found that the optical limiting properties mainly originated from two-
photon absorption (TPA) of molecules in the resulting polyacetylenes. Additionally, it was also found that
the functionalized polyacetylene with a longer fluorene-based conjugated chromophore and a stronger
terminal electron acceptor group exhibits better optical limiting properties because of the larger
p-electron delocalization and dipolar effect.
Received 1st August 2017
Accepted 2nd November 2017
DOI: 10.1039/c7ra08499d
rsc.li/rsc-advances
nanotubes,5–9 organic dyes,10 inorganic materials,11,12 p-electron
conjugated systems such as fullerene13,14 and porphyrins,
phthalocyanines,15–18 and organometallic complexes19–21 as
1. Introduction
With the rapid development of new laser technology, laser
protection in recent years is believed to be promising for both
civilians and the military.1 These works have mainly focused
on the visible range, where the nonlinear transmission
phenomena are mainly dependent on multiphoton absorption
(MPA) or reverse saturable absorption (RSA), nonlinear scat-
tering. The materials designed for laser protection involve gra-
phene families,2–4 single-walled and multi-walled carbon
optical limiting materials for protecting human eyes and
sensitive optical sensors from laser damage. Accordingly, to
obtain an ideal optical limiting implement, some major factors
should be taken into account such as fast response speed, low
transmittance at high intensities and high transmittance at low
intensities, temporal response and broad spectral characteris-
tics.22 For the classical benchmark, porphyrins, phthalocya-
nines and fullerenes can also provide excellent optical limiting
properties. However, their narrow excited-state absorption (ESA)
spectral regions and poor optical transparency characteristics
restricted practical applications in broadband optical limiting
properties. Up to now, a considerable crowd of papers have re-
ported that some materials have broad spectral optical limiting
properties under nanosecond photoexcitation.2,23–32 Specically,
metal-based materials are premeditated as novel generation
optical limiting materials displaying a high transparent char-
acteristic in the visible spectral region.33,34 However, to the best
of our knowledge, only a few papers reported the laser protec-
tion on the near-infrared spectral region has been investigated.
The relationship between molecular structure and near-infrared
aCollege of Material Science and Engineering, State Key Laboratory for Modication of
Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620,
China. E-mail: hongyaoxu@163.com
bSchool of Chemistry and Chemical Engineering, The Key Laboratory of Environment-
friendly Polymer Materials of Anhui Province, Anhui University, Hefei 230039, China
cCollege of Chemistry and Bioengineering, Donghua University, Shanghai 201620,
China. E-mail: syg@dhu.edu.cn
dDepartment of Physics, Department of Chemistry, National University of Singapore, 2
Science Drive 3, Singapore, 117542, Singapore
† Electronic supplementary information (ESI) available: FTIR, 1H NMR spectra of
M1–M3, M5 and P1–P3, P5; 13C NMR spectra of M2–M5 and P2–P5; normalized
UV-vis absorption and FL emission spectra of M5 (blue solid line) and P5 (red
dash dot) in THF solutions; open-aperture Z-scans of P1, P3–P5 at the different
pulse energy in THF solution. See DOI: 10.1039/c7ra08499d
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laser protection properties has not been understood.11,35–38 It is 2,7-Dibromouorene was obtained from TEIKO International
well known that substituted polyacetylenes (PAs) are consider- LTD. 1-Bromooctane, 1-bromododecane, tetrabutyl ammonium
able very promising laser protection materials owing to their bromide, 4-methoxybenzaldehyde, 4-methylbenzaldehyde, 4-
large third order nonlinear optical susceptibilities, fast time nitrobenzaldehyde, NaH, NaOH, KOH, MgSO4, triphenylphos-
response, good conductivity,39 well photophysical stability and phine, tetrahydrofuran (THF), dimethylsulfoxide (DMSO),
processability for optical devices.40–42 In our previous work, dimethylformamide (DMF), triethylamine (Et3N), dioxane,
many functional polyacetylene materials for laser protection in dichloromethane, petroleum ether and ethyl acetate were all
the visible optical wavelength region were designed.43–52 It was purchased from Shanghai Chemical Reagent Company.
found that the incorporation of nonlinear optical chromophore Dioxane and THF were distilled from sodium benzophenone
as a pendant group into polyacetylene backbone has endowed ketyl immediately before use. DMF was dried over anhydrous
polyacetylenes with novel optical limiting properties. The MgSO4 and freshly distilled prior to use. Et3N was distilled from
optical limiting properties were mainly inuenced by the KOH before use. Technical grade methanol was used to
molecular structure of substituted chromophore group and precipitate the polymers.
electron interaction between polyacetylene backbone and
substituted chromophore. The interaction generally increases
with the decrease of space length between substituted chro-
2.2 Instrumentation
mophore group and polyacetylene backbone.45 The functional FTIR spectra of KBr disks were recorded with a Thermo Nicolet
polyacetylene containing long conjugated chromophore with 5700 spectrometer at room temperature, 32 scans were collected
C]C double bond as p electron conjugation bridge exhibited at a resolution of 1 cmꢀ1. 1H NMR (400 MHz) and 13C NMR (100
better optical limiting property than that with C]N or N]N MHz) were recorded on a Bruker DMX-400 spectrometer
double bond as p-electron conjugation bridge under the same utilizing tetramethylsilane (TMS, 0.00 ppm) as the internal
linear transmittance.47,51 Their optical limiting mechanism was standard in chloroform-d (CDCl3) at room temperature.
attributed to the reverse saturated absorption (RSA) of Elementary analyses were conducted on Vario EL-III elementary
molecules.
analysis apparatus. UV-vis spectra were recorded on a Shimadzu
Thus, it was reported that polyacetylenes may be a potential UV-265 spectrometer using a 1 cm square quartz cell. Fluores-
near-infrared laser protection materials.53 However, to our cence spectra were obtained on a Shimadzu RF-5301PC spec-
knowledge, the polyacetylenes with optical limiting properties trouorimeter equipped with a 450 W Xe lamp and a time-
for near-infrared optical region have seldom been reported. In correlated single-photon counting card. Weight-average (Mw)
particular, the relationship between the structure and near- and number-average (Mn) molecular weights, and polydispersity
infrared optical limiting properties of the molecules, so far, index (PDI, Mw/Mn) were determined by gel permeation chro-
has not been understood. It is known that uorene is a well matograph (GPC) using a Waters 510 HPLC equipped with
near-infrared TPA chromophore.54,55 It is expected that the a Rheodyne 7725i injector with a stand kit, a set of styragel
incorporation of uorene-based chromophores as the pendant columns (HT3, HT4, and HT6; molecular weight range 102–107),
groups into polyacetylene could endow the polyacetylenes with a column temperature controller, a Waters 486 wavelength tunable
novel optical properties. Therefore, in the work, we designed UV-vis detector, a Waters 410 differential refractometer, and
and synthesized a series of novel polyacetylenes that contain a system DMM/scanner possessing an 8-channel scanner option.
uorene pendant with different conjugation length and All polymer solutions were prepared in THF (ca. 2 mg mLꢀ1
)
terminal substituent such as electron withdrawing or electron and ltered through 0.45 mm PTFE syringe-type lters before
donating groups. At the same time, the alkyl chains at nine injected into the GPC system. THF was used as the eluent at
positions extending away from the conjugated p-system of u- a ow rate of 1.0 mL minꢀ1. The column temperature was
orene was introduced for improving the solubility of resultant maintained at 30 ꢁC and the working wavelength of the UV
functional polyacetylenes in common organic solvents and detector was set at 254 nm. A set of mono-disperse polystyrene
provide good compatibility with other matrices.56 The relation- standards (waters) was used for calibration purposes. Differ-
ship between near-infrared optical limiting properties and ential scanning calorimetry (DSC) was performed on a TA
molecular structure and the optical limiting mechanism were Instruments Q2000 equipped with a liquid nitrogen cooling
investigated in detail.
accessory (LNCA) unit under a continuous nitrogen purge
(50 mL minꢀ1). The scan rate was 10 ꢁC minꢀ1 within the
temperature range 30–250 ꢁC. Samples (4–6 mg) were weighed
and sealed in aluminum pans. Thermogravimetric analysis
(TGA) was carried out using a NETZSCH Instruments 449F3
2. Experimental section
2.1 Materials
Unless otherwise noted, all commercial reagents were used as thermogravimetric analyzer with a heating rate of 20 ꢁC minꢀ1
received. 4-Vinylbenzyl chloride, triethyl phosphite, 2-methyl-3- from 20 to 800 ꢁC under a continuous nitrogen purge
butyn-2-ol,
p-nitrostyrene,
palladium(II)
acetate,
bis- (100 mL minꢀ1). Samples (15–25 mg) were loaded in alumina
(triphenylphosphine) palladium(II) chloride [PdCl2(PPh3)2], pans. The thermal degradation temperature (Td) was dened
norbornadiene rhodium(I) chloride dimer ([Rh(nbd)Cl]2), WCl6, as the temperature of 5% mass loss.
TaCl5, MoCl5 and tetraphenyltin (Ph4Sn) were all purchased
The molecular two-photon absorption (TPA) cross sections
from Aldrich, and used as received without further purication. were measured by a Z-scan technique. The laser pulses were
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produced by a mode-locked Ti:sapphire laser (Quantronix, 5.77 (d, J ¼ 17.6 Hz, 1H, –CH]CH2), 5.26 (d, J ¼ 12.0 Hz, 1H,
IMRA), which seeded a Ti:sapphire regenerative amplier, and –CH]CH2), 3.86 (s, 3H, Ar-OCH3).
focused onto a 1 mm-thick quartz cuvette containing the solu-
4-Ethenyl-40-methyl-1,2-stilbene (1c). The synthesis process
tions of these molecules with the minimum beam waist of 25 ꢂ is similar to compound 1a by using 4-methylbenzaldehyde. The
5 mm. The incident and transmitted laser pulse energy were product was white crystal. Yield: 44%. IR (KBr), n (cmꢀ1): 3021,
monitored by moving the cuvette along the propagation direc- 970, 992, 898 (]C–H), 2915, 2855 (CH3), 1624 (C]C), 1513, 835
tion of the laser pulses. The maximum on-axis irradiance of the (Ar). 1H NMR (400 MHz, CDCl3), d (TMS, ppm): 7.48 (d, J ¼
laser pulses was ꢃ120 GW cmꢀ2 at the focus point. The Z-scan 8.4 Hz, 2H, Ar-H meta to CH3), 7.43 (t, J ¼ 13.6 Hz, 4H, Ar-H–
experimental system was calibrated using a piece of cadmium CH]CH2) 7.19 (d, J ¼ 8.0 Hz, 2H, Ar-H ortho to CH3), 7.08 (d, J ¼
sulde (CdS) bulk crystal as a reference because it possesses 6.0 Hz, 2H, –CH]CH–), 6.74 (q, J ¼ 18.2 Hz, 1H, –CH]CH2),
large TPA at the wavelength of 780 nm and has been well 5.77 (d, J ¼ 17.6 Hz, 1H, –CH]CH2), 5.26 (d, J ¼ 11.2 Hz, 1H,
investigated in our laboratory. The TPA coefficient of CdS was –CH]CH2), 2.39 (s, 3H, Ar-CH3).
determined to be 6.4 ꢂ 0.6 cm GWꢀ1, which was in good
2,7-Dibromo-9,9-dioctyluorene (2a). 1-Bromooctane (5.60 g,
agreement with theoretical values within the experiment 29.0 mmol) was added by using a syringe to a mixture of 2,7-
uncertainty. The investigation of the optical limiting properties dibromouorene (3.88 g, 12.0 mmol), tetrabutyl ammonium
of the samples was carried out by using the same laser system as bromide (0.0225 g, 0.0975 mmol) and 3.75 mL of 50% aqueous
in the Z-scan experiment. The experimental arrangement was NaOH in DMSO (50 mL). The reaction mixture was stirred at
similar with that in the literature.57 The samples were contained room temperature for 3 h. The mixture was poured into H2O
in quartz cells with a thickness of 1 mm and positioned at the (500 mL), and then was extracted three times with dichloro-
focal point. Two-photon uorescence (TPF) measurements of methane. The combined organic layers were dried over anhy-
the polymers in solid form were carried out using the same laser drous MgSO4 and decolor by active carbon. The solvent was
system described above. The details of the two-photon tech- removed under reduced pressure. The crude product was puri-
nique experimental setup are described elsewhere.58
ed by recrystallization by hexane as the solvent to yield
a colorless crystal. Yield: 89%. IR (KBr), n (cmꢀ1): 2955, 2920,
2851 (CH2, CH3); 1467, 1448, 1416, 812 (Ar). 1H NMR (400 MHz,
CDCl3), d (TMS, ppm): 7.51 (d, 2H, J ¼ 7.7 Hz), 7.45 (m, 4H), 1.90
2.3 Monomer synthesis (cf. Scheme 1)
4-Ethenyl-40-nitro-1,2-stilbene (1a). 3.9 mL 4-vinylbenzyl (m, 4H, –CH2(CH2)6CH3), 1.25–0.86 (m, 20H, –CH2(CH2)5C2H5),
chloride (25 mmol)ꢁand 5.5 mL triethyl-phosphite (30 mmol) 0.83 (t, 6H,
J
¼
7.2 Hz, –(CH2)7CH3), 0.58 (s, 4H,
were heated at 110 C for 6 h in a solvent-free system. Excess –(CH2)6CH2CH3).
triethylphosphite was removed by vacuum distillation, and the
2,7-Dibromo-9,9-1-didodecyluorene (2b). The synthesis
resultant phosphonate was obtained as the thick oil, which was process is similar to compound 2a by using 1-bromododecane.
used for the Wittig–Horner reaction without further purica- Yield: 78%. IR (KBr), n (cmꢀ1): 2935, 2923, 2850 (CH2, CH3);
1
tion. Aer the crude phosphonate in 50 mL of dry THF was 1464, 1449, 1416, 808 (Ar). H NMR (400 MHz, CDCl3), d (TMS,
cooled to 0 ꢁC, 1.2 g NaH (50 mmol) were added slowly and ppm): 7.51 (d, 2H, J ¼ 7.7 Hz), 7.45 (m, 4H), 1.90 (m, 4H,
reacted for 30 min. 3.78 g 4-nitrobenzaldehyde (25 mmol) in –CH2(CH2)10CH3), 1.30–1.00 (m, 36H, –CH2(CH2)9C2H5), 0.87 (t,
10 mL THF was then added dropwise, and the contents were J ¼ 7.2 Hz, 6H, –(CH2)11CH3), 0.59 (s, 4H, –(CH2)10CH2CH3).
stirred under nitrogen for 12 h. The mixture was added slowly to
2-Bromo-7-(4-nitrostyryl)-9,9-dioctyl-9H-uorene (3). 2,7-
150 mL of ice-chilled water, and the red brown solid was ltered Dibromo-9,9-dioctyluorene (1.096 g, 2.0 mmol), Pd(OAc)2
and rinsed twice with water and ethanol. The crude product was (4.48 mg, 2.0 ꢄ 10ꢀ2 mmol), triphenylphosphine (10.50 mg, 4.0
recrystallized from THF/ethanol (2 : 1 v/v) to provide red brown ꢄ 10ꢀ2 mmol) and p-nitrostyrene (0.3 mL, 2.0 mmol) were dis-
crystal powder. Yield: 53%. IR (KBr), n (cmꢀ1): 3021, 997, 927, solved in 20 mL DMF and 4 mL Et3N. The resultant mixture was
1
972 (]C–H), 1629 (C]C), 1589, 850 (Ar), 1514, 1338 (NO2). H combined in a screw cap vial and heated for 48 h at 110 ꢁC
NMR (400 MHz, CDCl3), d (TMS, ppm): 8.24 (d, J ¼ 8.8 Hz, 2H, under a nitrogen atmosphere. Aer cooling to room tempera-
Ar-H ortho to NO2), 7.65 (d, J ¼ 8.8 Hz, 2H, Ar-H meta to NO2), ture, the precipitate was ltered and the ltrate was concen-
7.54 (d, J ¼ 8.4 Hz, 2H, Ar-H meta to vinyl), 7.46 (d, J ¼ 8.4 Hz, trated. The mixture was extracted with CH2Cl2 and dried over
2H, Ar-H ortho to vinyl), 7.30–7.14 (q, J ¼ 18.7 Hz, 2H, –CH] MgSO4. The crude product was puried by silica gel column
CH–), 6.79–6.72 (q, J ¼ 9.5 Hz, 1H, –CH]CH2), 5.82 (d, J ¼ chromatography using petroleum ether/dichloromethane (6 : 1
17.6 Hz, 1H, –CH]CH2), 5.33 (d, J ¼ 11.2 Hz, 1H, –CH]CH2). v/v) as the eluent. Product was obtained as a yellow solid. Yield:
4-Ethenyl-40-methoxy-1,2-stilbene (1b). The synthesis 62%. IR (KBr), n (cmꢀ1): 2975, 2922 (CH2, CH3), 1604, 1512, 856
process is similar to compound 1a by using 4-methox- (Ar), 1517, 1346 (NO2), 1062 (Ar-Br). 1H NMR (400 MHz, CDCl3),
ybenzaldehyde. The product was gray crystal. Yield: 39%. IR d (TMS, ppm): 8.23 (d, J ¼ 8.8 Hz, 2H), 7.70–7.42 (m, 8H), 7.35 (d,
(KBr), n (cmꢀ1): 3001, 970, 992, 905 (]C–H), 2954, 2834 (CH3), J ¼ 16.0 Hz, 1H), 7.21 (d, J ¼ 16.0 Hz, 1H), 1.91 (m, 4H,
1624 (C]C), 1601, 1514, 1462, 840 (Ar), 1254, 1036 (C–O–C). 1H –CH2(CH2)6CH3), 1.25–0.86 (m, 20H, –CH2(CH2)5C2H5), 0.84 (t,
NMR (400 MHz, CDCl3), d (TMS, ppm): 7.48 (d, J ¼ 8.8 Hz, 4H, J ¼ 7.2 Hz, 6H, –(CH2)7CH3), 0.58 (m, 4H, –(CH2)6CH2CH3).
Ar-H–CH]CH2) 7.42 (d, J ¼ 8.4 Hz, 2H, Ar-H meta to OCH3),
2-Bromo-7-(4-(4-nitrostyryl)styryl)-9,9-dioctyl-9H-uorene (4a).
7.11–6.96 (q, J ¼ 13.6 Hz, 2H, Ar-H ortho to OCH3), 6.92 (d, J ¼ The synthesis process is similar to compound 3 by using
8.8 Hz, 2H, –CH]CH–), 6.74 (q, J ¼ 17.6 Hz, 1H, –CH]CH2), 4-ethenyl-40-nitro-1,2-stilbene (1a). The crude product was
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puried by silica gel column chromatography using petroleum hydroxide (144 mg, 3.6 mmol) was added with stirring to
ether/dichloromethane (5 : 1 v/v) as the eluent. Product was ob- a solution of 2-(2-methyl-3-yn-2-ol)-7-(4-nitrostyryl)-9,9-dioctyl-
tained as a yellowish-red solid. Yield: 79%. IR (KBr), n (cmꢀ1): 9H-uorene (0.39 g, 0.5 mmol) in dry dioxane (20.0 mL) under
3023, 962 (]C–H), 2955, 2924, 2852 (CH2, CH3), 1588, 1460, 816 nitrogen. The reaction mixture was then heated at 80 ꢁC under
1
(Ar), 1514, 1338 (NO2), 1063 (Ar-Br). H NMR (400 MHz, CDCl3), reux for 4 h. Aer cooling to room temperature and the
d (TMS, ppm): 8.25 (d, J ¼ 8.4 Hz, 2H) 7.68–7.46 (m, 12H), 7.33– removal of the solvent with a rotary evaporator, the raw product
7.17 (q, 4H), 1.98 (m, 4H, –CH2(CH2)6CH3), 1.25–1.09 (m, 20H, was extracted with dichloromethane three times, dried over
–CH2(CH2)5C2H5), 0.84 (t, J ¼ 6.8 Hz, 6H, –(CH2)7CH3), 0.65 (m, MgSO4 and recrystallized from dichloromethane/ethanol
4H, –(CH2)6CH2CH3).
mixture (2 : 1 v/v). Product was obtained as a yellow powder.
2-Bromo-7-(4-(4-methoxystyryl)styryl)-9,9-dioctyl-9H-uorene Yield: 85%. Mp: 120.0 ꢁC. IR (KBr), n (cmꢀ1): 3310 (^C–H), 2974,
(4b). The synthesis process is similar to compound 3 by using 4- 2922 (CH2, CH3), 2103 (C^C), 1602, 1512, 850 (Ar), 1516, 1343
1
ethenyl-40-methoxy-1,2-stilbene (1b). The crude product was (NO2). H NMR (400 MHz, CDCl3), d (TMS, ppm): 8.26 (d, J ¼
puried by silica gel column chromatography using petroleum 8.0 Hz, 2H), 7.61–7.53 (m, 4H), 7.51–7.42 (m, 4H), 7.28 (d, J ¼
ether/dichloromethane (10 : 1 v/v) as the eluent. Product was 16.0 Hz, 1H), 7.12 (d, J ¼ 16.0 Hz, 1H), 3.18 (s, 1H, –C^CH), 1.96
obtained as a yellow solid. Yield: 68%. IR (KBr), n (cmꢀ1): 3022, (m, 4H, –CH2(CH2)6CH3), 1.04 (m, 20H, –CH2(CH2)5C2H5), 0.61
963 (]C–H), 2953, 2926, 2852 (CH2, CH3), 1603, 1512, 1458, 825 (t, J ¼ 7.2 Hz, 6H, –(CH2)7CH3), 0.56 (m, 4H, –(CH2)6CH2CH3).
(Ar), 1250, 1033 (C–O–C), 1061 (Ar-Br). 1H NMR (400 MHz, Anal. calcd for C39H37NO2: C 84.90, H 6.75, N 2.53. Found: C
CDCl3), d (TMS, ppm): 7.66 (d, J ¼ 7.6 Hz, 1H) 7.57–7.46 (m, 84.89, H: 6.77, N 2.51.
11H), 7.21–6.92 (m, 6H), 3.87 (s, 3H, Ar-OCH3), 1.98 (m, 4H,
–CH2(CH2)6CH3), 1.23–1.08 (m, 20H, –CH2(CH2)5C2H5), 0.83 (t, This compound was prepared as the similar approach of M1, using
J ¼ 7.2 Hz, 6H, –(CH2)7CH3), 0.65 (m, 4H, –(CH2)6CH2CH3). 4a instead of 3, as the starting material. Product was recrystallized
2-Ethynyl-7-(4-(4-nitrostyryl)styryl)-9,9-dioctyl-9H-uorene (M2).
2-Bromo-7-(4-(4-methylstyryl)styryl)-9,9-dioctyl-9H-uorene from dichloromethane/ethanol mixture (2 : 1 v/v), and obtained as
(4c). The synthesis process is similar to compound 3 by using 4- an orange-red powder. Yield: 65%. Mp: 136.1 ꢁC. IR (KBr),
ethenyl-40-methyl-1,2-stilbene (1c). The crude product was n (cmꢀ1): 3318 (^C–H), 3022, 961 (]C–H), 2923, 2851 (CH2, CH3),
puried by silica gel column chromatography using petroleum 2104 (C^C), 1584, 1466, 821 (Ar), 1511, 1339 (NO2). 1H NMR (400
ether/dichloromethane (20 : 1 v/v) as the eluent. Product was MHz, CDCl3), d (TMS, ppm): 8.25 (d, J ¼ 8.4 Hz, 2H), 7.70–7.50 (m,
obtained as a yellow solid. Yield: 65%. IR (KBr), n (cmꢀ1): 3023, 12H), 7.32–7.17 (q, 4H), 3.18 (s, 1H, –C^CH), 2.00 (m, 4H,
962 (]C–H), 2953, 2926, 2852 (CH2, CH3), 1514, 1458, 820 (Ar), –CH2(CH2)6CH3), 1.24–1.08 (m, 20H, –CH2(CH2)5C2H5), 0.84 (t, J ¼
1062 (Ar-Br). 1H NMR (400 MHz, CDCl3), d (TMS, ppm): 7.66 (d, J 7.2 Hz, 6H, –(CH2)7CH3), 0.65 (m, 4H, –(CH2)6CH2CH3). 13C NMR
¼ 7.6 Hz, 1H) 7.57–7.44 (m, 11H), 7.20 (m, 4H), 7.11 (d, 2H), 2.39 (75 MHz, CDCl3), d (ppm): 151.7, 151.0, 146.8, 143.9, 141.5, 140.4,
(s, 3H, Ar-CH3), 1.98 (m, 4H, –CH2(CH2)6CH3), 1.25–1.09 (m, 138.1, 136.7, 135.4, 132.9, 131.2, 129.9, 127.6, 127.5, 127.0, 126.8,
20H, –CH2(CH2)5C2H5), 0.84 (t, J ¼ 6.8 Hz, 6H, –(CH2)7CH3), 0.65 126.5, 126.0, 125.8, 124.2, 120.9, 120.4, 120.3, 119.6 (Ar and –CH]
(m, 4H, –(CH2)6CH2CH3).
CH–), 84.7 (–C^CH), 77.2 (–C^CH), 55.1, 40.4, 31.8, 30.0, 29.2,
2-Bromo-7-(4-(4-methylstyryl)styryl)-9,9-didodecyl-9H-uorene 23.7, 22.6, 14.1 (–(CH2)7CH3). Anal. calcd for C47H53NO2: C 85.02,
(4d). The synthesis process is similar to compound 3 by using H 8.05, N 2.11. Found: C 85.09, H: 8.01, N 2.07.
4-ethenyl-40-methyl-1,2-stilbene (1c) and 2,7-dibromo-9,9-
2-Ethynyl-7-(4-(4-methoxystyryl)styryl)-9,9-dioctyl-9H-uorene
didodecyluorene (2b). The crude product was puried by silica (M3). This compound was prepared as the similar approach of
gel column chromatography using petroleum ether/ M1, using 4b instead of 3, as the starting material. Product was
dichloromethane (25 : 1 v/v) as the eluent. Product was ob- recrystallized from dichloromethane/ethanol mixture (1 : ꢁ1 v/v),
tained as a yellow solid. Yield: 54%. IR (KBr), n (cmꢀ1): 3023, 965 and obtained as a yellow powder. Yield: 73%. Mp: 135.8 C. IR
(]C–H); 2926, 2852 (CH2, CH3); 1514, 1457, 821 (Ar), 1061 (Ar- (KBr), n (cmꢀ1): 3310 (^C–H), 3021, 968 (]C–H), 2926, 2852
Br). 1H NMR (400 MHz, CDCl3), d (TMS, ppm): 7.66 (d, J ¼ (CH2, CH3), 2104 (C^C), 1605, 1512, 1462, 830 (Ar), 1255, 1038
8.0 Hz, 1H) 7.57–7.44 (m, 11H), 7.26–7.07 (m, 6H), 2.39 (s, 3H, (C–O–C). 1H NMR (400 MHz, CDCl3), d (TMS, ppm): 7.70–7.64 (q,
Ar-CH3), 1.98 (m, 4H, –CH2(CH2)10CH3), 1.23–1.08 (m, 36H, J ¼ 7.8 Hz, 2H) 7.57–7.49 (m, 10H), 7.26–6.92 (m, 6H), 3.87 (s, 3H,
–CH2(CH2)9C2H5), 0.88 (t, J ¼ 7.2 Hz, 6H, –(CH2)11CH3), 0.66 (m, Ar-OCH3), 3.17 (s, 1H, –C^CH), 2.00 (m, 4H, –CH2(CH2)6CH3),
4H, –(CH2)10CH2CH3).
1.23–1.08 (m, 20H, –CH2(CH2)5C2H5), 0.84 (t, J ¼ 7.2 Hz, 6H,
2-Ethynyl-7-(4-nitrostyryl)-9,9-dioctyl-9H-uorene (M1). 2- –(CH2)7CH3), 0.65 (m, 4H, –(CH2)6CH2CH3). 13C NMR (75 MHz,
Methyl-3-butyn-2-ol (0.8 mL, 8.0 mmol) was added to a solution CDCl3), d (ppm): 159.4, 151.7, 151.0, 141.6, 140.0, 137.0, 136.9,
of PdCl2(PPh3)2 (28.00 mg, 4.0 ꢄ 10ꢀ2 mmol), CuI (2.00 mg, 1.0 136.4, 131.2, 130.2, 128.8, 128.2, 128.0, 127.7, 126.8, 126.6, 126.5,
ꢄ 10ꢀ2 mmol), and 3 (0.38 g, 0.5 mmol) in 20 mL of triethyl- 126.2, 125.7, 120.7, 120.3, 120.1, 119.6, 114.2 (Ar and –CH]CH–
amine under a nitrogen atmosphere. The mixture was reacted at ), 84.8 (–C^CH); 77.2 (–C^CH), 55.3 (Ar-OCH3), 55.1, 40.4, 31.8,
ꢁ
80 C for 48 h. Aer cooling to room temperature, the precipi- 30.0, 29.2, 23.7, 22.6, 14.1 (–(CH2)7CH3). Anal. calcd for C48H56O:
tate was ltered and the ltrate was concentrated. The crude C 88.84, H 8.70. Found: C 88.80, H 8.64.
product was puried by aluminum oxide column using ethyl
2-Ethynyl-7-(4-(4-methylstyryl)styryl)-9,9-dioctyl-9H-uorene
acetate/petroleum ether mixture (1 : 6 v/v) as eluent. The (M4). This compound was prepared as the similar approach of
resulting solid 2-(2-methyl-3-yn-2-ol)-7-(4-nitrostyryl)-9,9-dioc- M1, using 4c instead of 3, as the starting material. Product was
tyl-9H-uorene, was a yellow powder in 60% yield. Sodium recrystallized from dichloromethane/ethanol mixture (1 : 1 v/v),
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ꢁ
and obtained as a yellow powder. Yield: 67%. Mp: 144.2 C. IR precipitant was dried under vacuum at 30 ꢁC to a constant
(KBr), n (cmꢀ1): 3316 (^C–H), 3022, 963 (]C–H), 2926, 2852 weight.
(CH2, CH3), 2106 (C^C), 1514, 1464, 825 (Ar). 1H NMR (400
Poly[2-ethynyl-7-(4-nitrostyryl)-9,9-dioctyl-9H-uorene] (P1).
MHz, CDCl3), d (TMS, ppm): 7.70–7.64 (q, J ¼ 8.0 Hz, 2H), 7.58– Deep red solid. Yield: 60%. GPC: Mw ¼ 10 100, PDI ¼ 1.384
7.44 (m, 10H), 7.26–7.07 (m, 6H), 3.17 (s, 1H, –C^CH), 2.39 (s, (polystyrene calibration, Table 1, no. 1). IR (KBr), n (cmꢀ1): 2927,
3H, Ar-CH3), 2.01 (m, 4H, –CH2(CH2)6CH3), 1.23–1.08 (m, 20H, 2857 (CH2, CH3), 1610, 856 (Ar), 1513, 1340 (NO2). 1H NMR (400
–CH2(CH2)5C2H5), 0.84 (t, J ¼ 6.8 Hz, 6H, –(CH2)7CH3), 0.65 (m, MHz, CDCl3), d (TMS, ppm): 8.32 (d, 2H), 8.07–6.92 (br, 10H),
4H, –(CH2)6CH2CH3). 13C NMR (75 MHz, CDCl3), d (ppm): 151.7, 5.43 (s, 1H), 1.93 (br, 4H), 1.05 (br, 22H), 0.81 (br, 8H).
151.0, 141.6, 140.1, 137.6, 137.0, 136.9, 136.6, 134.6, 131.2,
Poly[2-ethynyl-7-(4-(4-nitrostyryl)styryl)-9,9-dioctyl-9H-uo-
129.4, 128.9, 128.6, 128.0, 127.3, 126.8, 126.7, 126.5, 126.4, rene] (P2). Deep red solid. Yield: 38%. GPC: Mw ¼ 4910, PDI ¼
125.8, 120.8, 120.3, 120.2, 119.6 (Ar and –CH]CH–), 84.8 1.259 (polystyrene calibration, Table 1, no. 3). IR (KBr), n (cmꢀ1):
(–C^CH), 77.2 (–C^CH), 55.1, 40.4, 31.8, 30.0, 29.2, 23.7, 22.6, 3022, 961 (]C–H); 2923, 2851 (CH2, CH3), 1584, 1466, 821 (Ar),
14.1 (–(CH2)7CH3), 21.3 (Ar-CH3). Anal. calcd for C48H56: C 1511, 1339 (NO2). 1H NMR (400 MHz, CDCl3), d (TMS, ppm):
91.08, H 8.92. Found: C 91.11, H 8.90.
8.27 (d, J ¼ 8.4 Hz, 2H) 7.68–7.54 (br, 12H), 7.32–7.17 (br, 4H),
2-Ethynyl-7-(4-(4-methylstyryl)styryl)-9,9-didodecyl-9H-uo- 7.02 (s, 1H), 2.07 (br, 4H, –CH2(CH2)6CH3), 1.28–1.08 (br, 20H,
rene (M5). This compound was prepared as the similar –CH2(CH2)5C2H5), 0.91–0.81 (br, 10H, –(CH2)6CH2CH3). 13C
approach of M1, using 4d instead of 3, as the starting material. NMR (75 MHz, CDCl3), d (ppm): 151.7, 151.0, 146.8, 143.9,
Product was recrystallized from dichloromethane/ethanol 141.5–138.1, 136.7, 135.4, 132.9, 131.2, 129.9, 127.6–124.2,
mixture (1 : 1 v/v), and obtained as a yellow powder. Yield: 120.9–119.6 (Ar and –CH]CH–), 55.1, 40.4, 31.8, 30.0–29.2,
65%. Mp: 104.8 ꢁC. IR (KBr), n (cmꢀ1): 3308 (^C–H); 3023, 960 23.7, 22.6, 14.1 (–(CH2)7CH3).
(]C–H), 2926, 2851 (CH2, CH3), 2104 (C^C), 1514, 1463, 826
Poly[2-ethynyl-7-(4-(4-methoxystyryl)styryl)-9,9-dioctyl-9H-
(Ar). 1H NMR (400 MHz, CDCl3), d (TMS, ppm): 7.70–7.64 (q, J ¼ uorene] (P3). Deep red solid. Yield: 65%. GPC: Mw ¼ 96 200,
8.0 Hz, 2H) 7.57–7.44 (m, 10H), 7.22–7.11 (m, 6H), 3.17 (s, 1H, PDI ¼ 1.585 (polystyrene calibration, Table 1, no. 5). IR (KBr), n
–C^CH), 2.39 (s, 3H, Ar-CH3), 2.00 (m, 4H, –CH2(CH2)10CH3), (cmꢀ1): 3021, 958 (]C–H), 2926, 2852 (CH2, CH3), 1605, 1512,
1.33–1.01 (m, 36H, –CH2(CH2)9C2H5), 0.88 (t, J ¼ 6.8 Hz, 6H, 1462, 824 (Ar), 1251, 1036 (C–O–C). 1H NMR (400 MHz, CDCl3),
–(CH2)11CH3), 0.65 (m, 4H, –(CH2)10CH2CH3). 13C NMR (75 d (TMS, ppm): 7.57–7.49 (br, 12H), 7.26–6.92 (br, 6H), 7.02 (s,
MHz, CDCl3), d (ppm): 151.7, 151.0, 141.6, 140.1, 137.6, 137.0, 1H), 3.87 (s, 3H, Ar-OCH3), 2.07 (br, 4H, –CH2(CH2)6CH3), 1.28–
136.9, 136.6, 134.6, 131.2, 129.4, 128.9, 128.6, 128.0, 127.3, 1.08 (m, 20H, –CH2(CH2)5C2H5), 0.91–0.81 (br, 10H, –(CH2)6-
126.8, 126.7, 126.5, 126.4, 125.8, 120.8, 120.3, 120.2, 119.6 (Ar CH2CH3). 13C NMR (75 MHz, CDCl3), d (ppm): 159.4, 151.7,
and –CH]CH–), 84.8 (–C^CH), 77.2 (–C^CH), 55.1, 40.4, 31.9, 151.0, 141.6, 140.0, 137.0–136.4, 131.2, 130.2, 128.8–125.7,
30.0, 29.6, 29.5, 29.3, 29.2, 23.7, 22.6, 14.1 (–(CH2)11CH3), 21.3 120.7, 120.3, 120.1, 119.6, 114.2 (Ar and –CH]CH–), 55.3 (Ar-
(Ar-CH3). Anal. calcd for C56H72: C 90.26, H 9.74. Found: C OCH3); 55.1, 40.4, 31.8, 30.0, 29.2, 23.7, 22.6, 14.1
90.19, H 9.80.
(–(CH2)7CH3).
Poly[2-ethynyl-7-(4-(4-methylstyryl)styryl)-9,9-dioctyl-9H-u-
orene] (P4). Deep red solid. Yield: 67%. GPC: Mw ¼ 152 000,
PDI ¼ 2.269 (polystyrene calibration, Table 1, no. 6) IR (KBr),
2.4 Polymerization
n (cmꢀ1): 3022, 963 (]C–H), 2926, 2852 (CH2, CH3), 1514, 1464,
1
Scheme 2 all the polymerization reactions and manipulations 825 (Ar). H NMR (400 MHz, CDCl3), d (TMS, ppm): 7.55–7.41
were performed under puried nitrogen using Schlenk tech- (br, 12H), 7.21–7.12 (br, 6H), 7.02 (s, 1H), 2.39 (s, 3H, Ar-CH3),
niques either in vacuum-line system or an inert-atmosphere 2.07 (br, 4H, –CH2(CH2)6CH3), 1.28–1.10 (br, 20H, –CH2(CH2)5-
glove box, except for the purication of the polymers, which C2H5), 0.91–0.81 (br, 10H, –(CH2)6CH2CH3). 13C NMR (75 MHz,
were done in open air. Typical procedure is given below: CDCl3), d (ppm): 151.0, 141.0, 137.5–134.6, 129.4–126.4, 120.6
1.0 mmol of the monomer was added into a baked 20 mL
Schlenk tube with a side arm. The tube was evacuated under
Table 1 Polymerization summary of P1–P5a
vacuum and then ushed with dry nitrogen three times through
the side arm. Three milliliters of dioxane was injected into the
tube to dissolve the monomer. The catalyst solution was
prepared in another tube by dissolving 4.6 mg (0.01 mmol)
[Rh(nbd)Cl]2 and 2.02 mg (0.02 mmol) Et3N in 2 mL of dioxane,
which was transferred to the monomer solution using a hypo-
dermic syringe. The reaction mixture was stirred at 60 ꢁC under
nitrogen for 4 h. The mixture was then diluted with 5 mL of
dioxane and added dropwise to 200 mL of methanol under
stirring. The precipitate was centrifuged and redissolved in THF.
The THF solution was added dropwise into 200 mL of methanol
to precipitate the polymer. The dissolution–precipitation
process was repeated three times, and the nally isolated
b
c
Entry
Polymer
Yield (%)
Mw
Mw/Mn
1
2
3
4
5
6
7
P1
P10
P2
P20
P3
P4
P5
60
67
38
54
65
67
84
10 100
12 760
4910
1.384
2.121
1.259
2.080
1.585
2.269
1.882
9650
96 200
152 000
257 600
a
Unless otherwise specied, the polymerization was carried out in
dioxane under nitrogen with [Rh(ncb)Cl]2-Et3N as a catalyst for 4 h at
b
60 ꢁC. Estimated by GPC in term of polystyrene calibration.
c
Polymerized in THF.
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(Ar and –CH]CH–), 55.0, 40.4, 31.9–29.2, 23.9–22.6, 14.1
(–(CH2)7CH3), 21.3 (Ar-CH3).
Poly[2-ethynyl-7-(4-(4-methylstyryl)styryl)-9,9-didodecyl-9H-
uorene] (P5). Deep red solid. Yield: 84%. GPC: Mw ¼ 257 600,
PDI ¼ 1.882 (polystyrene calibration, Table 1, no. 7). IR (KBr),
n (cmꢀ1): 3023, 960 (]C–H); 2926, 2851 (CH2, CH3), 1514, 1463,
1
826 (Ar). H NMR (400 MHz, CDCl3), d (TMS, ppm): 7.56–7.38
(br, 12H), 7.22–7.11 (br, 6H), 7.02 (s, 1H), 2.39 (s, 3H, Ar-CH3),
2.07 (br, 4H, –CH2(CH2)10CH3), 1.28–1.11 (br, 36H,
–CH2(CH2)9C2H5), 0.91–0.81 (br, 10H, –(CH2)10CH2CH3). 13C
NMR (75 MHz, CDCl3), d (ppm): 151, 141, 137.5–134.6, 129.4–
126.4, 120.8–119.6 (Ar and –CH]CH–), 55.1, 40.4, 31.9–29.2,
23.7, 22.6, 14.1 (–(CH2)11CH3), 21.3 (Ar-CH3).
Scheme 2 Synthetic routes to the polymers P1–P5.
rst attempted to polymerize these monomers (M1–M5) using
the classical metathesis catalysts, such as WCl6–Ph4Sn, TaCl5–
Ph4Sn and MoCl5–Ph4Sn. However, only trace polymeric prod-
ucts were obtained. On the contrary, [Rh(nbd)Cl]2/Et3N catalyst
effectively works for these monomers and the results are listed
in Table 1. It was found that P1 and P2 were partly insoluble in
dioxane, only giving molecular weight of the soluble part of the
polymers (Table 1, entry 1 and 3). However, the yield of P10 and
P20 was increased by using THF as the polymerization solvent
(Table 1, entry 2 and 4). P1 and P2 exhibited the low polymeri-
zation yield in dioxane, which may be mainly originated from
the enwrapped effect of insoluble polymer to prevent further
polymerization of monomers. Moreover, the polymerization
yield of P2 in dioxane or P20 in THF was almost lower than that
of corresponding P1 or P10, hinting that polymerization activity
of monomer M2 may be lower than that of M1. The result
exhibited that polymerization activity of functionalized acety-
lene monomer decreased with the increase of conjugated chain
length of substituted chromophore group. P3, P4, and P5 were
3. Results and discussion
3.1 Synthesis and polymerization
We designed and synthesized one styryl-substituted uorene
derivate M1 and four different stilbene-substituted uorene
derivates M2–M5 with different terminal substituted group
(NO2, OCH3 and CH3) and side alkyl chain (Scheme 1). The
Wittig Horner reaction of these different benzaldehyde deriva-
tives with 4-vinylbenzyl chloride gave stilbene derivatives 1a–1c.
2,7-Dibromouorene were alkylated at the 9 position by 1-bro-
mooctane or 1-bromododecane in strongly basic conditions to
form 2a–2b. 3 was prepared from 2a and p-nitrostyrene by Heck
reaction, and then 4a–4d were made from 2a–2b and 1a–1c by
the same reaction. The resultant products (3 and 4a–4d) were
then reacted with 2-methyl-3-butyn-2-ol using Pd(PPh3)2Cl2 as
the catalyst, the obtained seminished products with 2-propyl-
2-ol group were cleaved in dioxane/NaOH solution to give the
desirable monomers M1–M5 in high yields, respectively.
All polymerization reactions were carried out under nitrogen
atmosphere using a standard Schlenk vacuum-line system. We
Scheme 1 Synthetic routes of monomer molecules M1–M5.
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soluble in dioxane and gave dark red polymers with high
molecular weight (Mw ¼ 9 ꢄ 104–2.5 ꢄ 105) in high polymeri-
zation yield (Table 1, entry 5, 6, 7), and P5 obtained the highest
molecular weight and yield, hinting that the longer exible alkyl
side chain on uorene was benecial to polymerization of
monomer.
3.2 Solubility
It's reported that PAs have weak solubilities in common
solvents.25 However, when the styryl/stilbene–uorene pendant
with exible alkyl group as side tailor was incorporated into the
PA backbone chain, all the polymers were readily soluble in
various common organic solvents such as chloroform, toluene,
THF, and 1,2-dichloroethane.
Fig. 2 1H NMR spectra of M4 and P4.
3.3 Structural characterization of the polymers
All the polymers and corresponding monomers were charac-
terized by standard spectroscopic methods, from which satis-
factory analysis data were obtained (see the Experimental
section for details). An example of the FTIR spectrum of P4 is
given in Fig. 1. For comparison, the FTIR spectrum of its cor-
responding monomer M4 is also given in the same gure. As
can be seen from Fig. 1, M4 clearly exhibits two characteristic
absorptions at 3319 and 2104 cmꢀ1, respectively, corresponding
to the ^C–H and C^C stretching vibrations in the mono-
substituted acetylene. The acetylenic absorption bands disap-
pear in the spectrum of P4 and the characteristic vibration
intensity of aromatic ring is signicantly enhanced, suggesting
that the triple bond in the monomer has changed to double
bond in the polymer. Similar results (Fig. S1–S4†) were found in
other monomers (M1–M3 and M5) and their polymers (P1–P3
and P5), suggesting that the triple bonds of these monomers
have been transferred to double bond polyacetylene backbone.
Fig. 2 shows the 1H NMR spectra of M4 and P4 in
chloroform-d, respectively. The absorption peak of the acetylene
proton in M4 at 3.17 ppm as a singlet peak, completely disap-
pears in the spectrum of its polymer P4. Meanwhile, all the
corresponding characteristic absorption peaks of monomer M4
signicantly were widened in the spectra of P4 with a widened
and enhanced peak at 7.02 ppm corresponding to the olen
protons and aromatic protons absorption, further proving that
the M4 has been polymerized to yield the functional poly-
acetylene P4. The results are consistent with that of FTIR
spectra. Alternatively, no absorptions are found in the cis-olen
absorption region of 5.20–6.20 ppm, suggesting that these
polymers possess a predominantly trans conformation.59,60 The
high trans content may result from the strong steric hindrance
of the pendants.61
Compared to that of M4, the signals at 77.2 and 84.8 ppm
corresponding to the acetylenic carbons disappear in the 13C
NMR spectrum of P4 (Fig. S9†), and the relevant absorption
peak intensity of the carbon atoms of the aromatic pendants in
polymers is signicantly enhanced because of the absorption
overlapping effect of the olen carbon atoms of the poly-
1
acetylene backbone. Thus, the H NMR and 13C NMR spectra
data further conrm that the acetylene polymerization has
taken place. The similar results were found in polymerization of
other monomers. The satisfactory spectral data are given in the
ESI (Fig. S10–S12†).
3.4 Linear optical properties
The linear optical properties of the polymers and corresponding
monomers were investigated by UV-vis absorption and uores-
cence in THF solutions at 20 ꢁC in extremely diluted solution (2
ꢄ 10ꢀ5 M) and these spectra are depicted in Fig. 3. It can be seen
from Fig. 3 that the maximum absorption wavelengths of P1–P5
are located at 400, 406, 388, 385 and 385 nm, respectively, which
are assigned to the p–p* electronic transitions of the corre-
sponding conjugated styryl/stilbene–uorene chromophores.
Obviously, the maximum absorption wavelengths of the poly-
mers changes at the different terminal substituents or the
length of conjugated systems, but aren't inuenced by the
length of alkyl exible chain. The absorption band of the nitro-
substituted polymers P1 and P2 shows signicantly red-shis,
which results from the larger p-electron delocalization and
Fig. 1 FTIR spectra of M4 and P4.
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Fig. 3 Normalized UV-vis absorption and FL emission spectra of the monomers (solid line) and corresponding polymers (dash dot) in THF
solutions (spectra of M5 and P5 were showed in Fig. S13†).
stronger dipolar effect of NO2 group. P2 with longer conjugated
chromophore substituent shows larger red-shi at maximum
absorption wavelength than P1. The absorption band of the
methyl-substituted polymers P4 and P5 with the different alkyl
exible chain exhibit the almost same maximum absorption
wavelength. Although the polymers and their corresponding
monomers show nearly the same lmax, the polymers show
signicant broad backbone absorption at longer wavelength
than 420 nm, which result from the double contribution of
conjugated double-bond backbones of polyacetylene and
enhancement conjugation effect between the double-bond
backbones of the polyacetylenes and styryl/stilbene–uorene
pendant groups. These results further support that the mono-
mers have been converted to polymers.
An interesting feature for the polymers P1–P5 is that they all
possess luminescent properties upon photo irradiation while
PA itself is nonluminescent.62 When excited at relative
maximum absorption wavelength in THF solution, P1–P5 emit
uorescence with maximum emitting peak at around 515 nm,
541 nm, 453 nm, 450 nm and 447 nm respectively, which are
higher than that of the corresponding monomers (Fig. 3). The
red-shi of uorescence emitting peak probably results from
a strong pendant–pendant intermolecular interaction and/or
intramolecular interaction between styryl/stilbene–uorene
pendants and the polyacetylene conjugation main chain.
Similar phenomena are found by Balcar.63
3.5 Optical limiting properties and mechanism
Many optical limiting materials for visible optical region have
been investigated. However, the optical limiting materials for
the near-infrared optical region were rarely reported. Thus, we
investigated the near infrared optical limiting properties of
resultant polymers by a mode-locked Ti:sapphire laser (Quan-
tronix, IMRA) at 780 nm using 450 femtosecond laser pulses at 1
kHz repetition rate. For avoiding the inuence from thermally
induced scattering effects, the optical limiting properties of the
polymers in THF solution were measured at low unit concen-
tration (1 mM). Fig. 4 shows the optical limiting properties of
P1–P5 in THF and the results are summarized in Table 1S.† For
comparison, the optical limiting property of poly-
phenylacetylene (PPA) at the same condition is also made. The
results were shown in Fig. 4. At low incident irradiance, the
transmittances of all the polymer solutions remain unchanged
with the incident irradiance obeying the Beer–Lambert law.
However, the transmittances of the solutions start to deviate
from the linear transmission with a further increase in the
incident irradiance, and a nonlinear relationship is observed
between the output and input irradiance, while that of PPA still
rises linearly because of the laser-induced photolysis of the
polyacetylene chains,68 suggesting that the incorporation of
conjugated styryl/stilbene–uorene group into polyacetylene
backbone has endowed the resultant polyacetylenes with novel
laser protection characteristics at 780 nm for 450 femtosecond
pulses laser. It is also found that the limiting thresholds of the
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s2 ¼ hnb/N
(2)
where N is the number of molecules per cm3 and hn is the
excitation photon energy. The TPA cross-section s2 is expressed
in Goppert Mayer (GM) units, in which 1 GM ¼ 1 ꢄ 10ꢀ50 cm4 s
¨
per photon. The Z-scan data (or the transmittance as a function
of z position) were normalized to the linear (small-signal)
transmittance. The nonlinear-optical signal from the solvent
was negligible, compared to the signals from the fullerene
derivatives. The Z-scan results conrmed that the samples had
surprisingly good photostability, which was veried by indif-
ference between the linear absorption spectra measured before
and aer intense laser irradiation in the Z-scans. In addition,
the TPA coefficients b, could be extracted from the best tting
between the Z-scan theory and the data, and the TPA cross
sections were then calculated from the denition (2).
Fig. 4 Optical limiting properties of P1–P5, and polyphenylacetylene
(PPA) for femtosecond laser pulse.
As thermally induced scattering effects and exited-states
absorption caused by high irradiance may inuence the accu-
racy of overall TPA cross sections measured, we undertook the
Z-scans of P1–P5 in THF with different irradiances. As an
example, the results of P2 are shown in Fig. 5. Aer calculation
from denition (2), it is found that the TPA cross sections (s2) of
the polymers are independent of laser irradiance and remain
nearly constant across the irradiance range of interest, indi-
cating the observed nonlinear absorption is induced from
a pure third-order nonlinear process (Table 1S†). Other
nonlinear mechanisms such as thermally induced scattering
and excited-state absorption, in particular singlet excited-state
absorption, are negligible in our experiments. The similar
phenomena (Fig. S14–S17†) were also observed in other poly-
mers. Furthermore, it is found that TPA cross sections of the
polymers P1–P5 are obviously larger than those of PPA.42 Thus,
the attachment of styryl/stilbene–uorene moieties into the PA
main chain has endowed the resultant polyacetylenes with
enhanced the nonlinear optical properties signicantly, which
may be attributed to the enhanced conjugation effect between
substituent uorene pendants and conjugated backbone of
polyacetylene. The similar results were also found in oxadiazole-
resultant polymers, which are dened as the incident irradiance
uence where the transmittance starts to deviate from linearity,
are signicantly inuenced by molecular structures. As seen in
Table 1S,† polymer P5 (34.9 GW cmꢀ2) with longer tail alkyl
chain length in side position of uorene pendant exhibits better
optical limiting properties than P4 (47.6 GW cmꢀ2). Moreover,
polymer P3 shows the better optical property when the terminal
methyl group of stilbene–uorene pendant of P4 is replaced by
methoxy group. The limiting threshold of P2 (16.7 GW cmꢀ2) is
better than P1 (21.4 GW cmꢀ2), which should be attributed to
the larger electron conjugation of chromophoric moiety in P2. It
can be found that P2 shows the best optical limiting properties
(lowest optical limiting thresholds) among all the polymers,
owing to the longer conjugated chromophore substituent and
larger dipolar effect of strong electron acceptor NO2.
The optical limiting mechanisms of organic compounds are
oen based on TPA or RSA. Generally, TPA based optical
limiting effect can be yielded in principle under the laser irra-
diation of picosecond or shorter pulses, while RSA is achieved
on a nanosecond or longer time scale, owing to the different
excited state lifetimes involved in a multilevel energy process.64
In this work, the polymers are excited by 450 femtosecond laser
pulses at 780 nm. Therefore, we consider that the optical
limiting properties of the polymers may mainly originate from
TPA.
For investigation of optical limiting mechanism, we per-
formed open-aperture Z-scans with the same laser. The Z-scan
experiments of the polymer solution were carried out at
780 nm using femtosecond laser pulses (450 fs, 1 kHZ). In
theory, the normalized transmittance for the open aperture can
be written as65
m
N
X
½ꢀq0ðzÞꢅ
Tðz; s ¼ 1Þ ¼
;
for jq0j\1
(1)
3=2
m¼0 ðm þ 1Þ
where q0(z) ¼ a2I00(t)Leff/(1 + z2/z0 ) with b is the TPA coefficient,
I0 is the intensity of laser beam at focus (z ¼ 0), Leff ¼ [1 ꢀ
exp(ꢀa0L)]/a0 is the effective thickness (a is the linear absorp-
tion coefficient, L is the sample thickness, z0 is the Rayleigh
range of the laser beam, and z is the sample position). The TPA
cross-section s2 is calculated by55
2
Fig. 5 Open-aperture Z-scans of P2 at the different pulse energy in
THF solution. Solid lines are the theoretical fit for two-photon
absorption.
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based or azobenzene-based chromophore functionalized
polyacetylenes.45,49
Fig. 6 shows the TPF emission spectra of all polymers (P1–P5)
in solid form. The maximum TPF peaks of P1–P5 located at
562 nm, 584 nm, 512 nm, 506 nm and 506 nm, respectively.
Comparing the TPF curves of P1–P5 with those of one-photon
uorescence of P1–P5 in THF (Fig. 3), red shis can be identi-
ed, supporting that the re-absorption effects are involved in
the nonlinear process in the solid polymers. It is well known
that the uorescence emission intensity presented a square
dependence of the uorescence on the input intensity to prove
the TPA nature. When the sample is represented by TPA without
stimulated emission, self-quenching, or any dark states inu-
encing the emission intensity, the signal intensity (Iuor) of the
detected uorescence from the sample cell is given by55
Fig. 7 TGA thermograms of P1–P5.
2
Ifluor f I0 bl0
(3)
and 345 ꢁC for P3, P4 and P5, respectively, which are much
higher than that for PPA (225 C).55 Thus, the incorporation of
ꢁ
here I0 is the input light intensity, l0 and b are the optical path
length and TPA coefficient of the medium, respectively. There-
fore, Fig. 6(a) and (b) showed the two-photon uorescence (TPF)
spectra at different input light intensities and the log–log plots
of the signal intensity (Iuor) of the detected uorescence vs. the
input light intensity (I0) of P1–P5, respectively. The results dis-
played that the log–log plots of the detected TPF intensity are
linear with the input light intensity, and the slopes are
approximately 2, further conrming that the optical limiting
properties are mainly attributed to the molecular TPA absorp-
tion of resultant polyacetylenes.66
the rigid styryl/stilbene–uorene groups into polyacetylenes
signicantly improves the thermal stability of resultant poly-
acetylenes. The enhancement of the thermal stability may be
due to the “jacket effect” of the styryl/stilbene–uorene
pendants. Namely, the alternating-double-bond backbone of
the polymers may be surrounded by a rigid “jacket” formed
through the strong intra- and interchain molecular electronic
interaction of the styryl/stilbene–uorene groups, shielding the
polymer main chains from the thermal attack. Similar
phenomena were also found by Tang,59,62 Masuda67 and our
previous work.45–48,68,69
3.6 Thermal property
The thermal stability of the resulting polymers was evaluated by
thermogravimetric analysis (TGA) under nitrogen atmosphere.
4. Conclusions
It is well known that poly(1-alkyne)s such as poly(1-butyne) and In summary, a series of uorene-containing functionalized
poly(1-hexyne), are so unstable that even the isolation process of polyacetylenes with different molecular structure was success-
the polymer products from the polymerization reactions leads fully prepared using [Rh(nbd)Cl]2-Et3N as the catalyst. The
to degradation.59,62 However, as shown in Fig. 7, all the uorene- incorporation of conjugated styryl/stilbene–uorene group into
containing polyacetylenes exhibit good thermal stability. Td polyacetylene backbone has endowed the resultant poly-
(dened as the temperature of 5% weight loss) for P1 is as high acetylenes with novel near-infrared laser protection. The optical
as 357 ꢁC, while Td for P2 with longer conjugated pendant is at limiting properties for near infrared laser protection are inu-
327 ꢁC. When para-position of stilbene–uorene substituted by enced by their molecular structures. The functional poly-
different groups, the resulting polymers showed Td at 407, 397 acetylene with longer conjugation structure and stronger
Fig. 6 (a) Two-photon fluorescence (TPF) spectra of P1–P5; (b) the log–log plots of the signal intensity (Ifluor) of the detected fluorescence vs.
the input light intensity (I0) of P1–P5.
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
This research was nancially supported by the National Natural 26 Z. Yang, A. D. Xia, J. S. Ma, Q. S. Li, Z. K. Wu, C. L. Liu and
Science Fund of China (Grant No. 21671037, 21471030, and
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