Thermally stable oxadiazole-containing polyacetylenes: Relationship between molecular structure and nonlinear optical properties

Article abstract of DOI:10.1039/b807004k

Two novel high molecular weight functional polyacetylenes bearing oxadiazole groups as pendants, poly(2-(4-decyloxyphenyl)-5-(4-ethynylphenyl)-1, 3, 4-oxadiazole) (P1) and poly(2-(4-butyloxyphenyl)-5-(4-ethynylphenyl)-1, 3, 4-oxadiazole) (P2) were designe

Full text of DOI:10.1039/b807004k

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Thermally stable oxadiazole-containing polyacetylenes: Relationship
between molecular structure and nonlinear optical properties†
a
a
Xin Wang,ab Shanyi Guang, Hongyao Xu, Xinyan Su,b Junyi Yang,c Yinglin Song,c Naibo Lina
*
and Xiangyang Liud
Received 24th April 2008, Accepted 16th June 2008
First published as an Advance Article on the web 29th July 2008
DOI: 10.1039/b807004k
Two novel high molecular weight functional polyacetylenes bearing oxadiazole groups as pendants,
poly(2-(4-decyloxyphenyl)-5-(4-ethynylphenyl)-1, 3, 4-oxadiazole) (P1) and poly(2-(4-
butyloxyphenyl)-5-(4-ethynylphenyl)-1, 3, 4-oxadiazole) (P2) were designed and synthesized by
a multistep reaction route. The structures and properties were characterized and evaluated with FTIR,
NMR, UV, TGA, GPC, optical-limiting (OL) and nonlinear optical (NLO) analyses. The
incorporation of oxadiazole into polyacetylene significantly endows polyacetylenes with higher thermal
stability and optical limiting property. The solubility, stereoregularity and optical properties of
resultant polyacetylenes are obviously affected by flexible terminal alkoxy chain length. The cis olefinic
structure content in polyacetylene backbone increases with increasing terminal alkoxy group length.
The functional polyacetylene with the higher cis olefinic structure content shows higher thermal
stability, better optical limiting property and larger third-order nonlinear optical performance. The
optical limiting mechanism of resulting polymers was investigated, which is mainly originated from
larger excitation state absorption cross-section of molecules to result in reverse saturable absorption.
electron-withdrawing chromophore, which shows weak ground
Introduction
state electronic absorption in the visible region wavelengths more
Recently, third-order nonlinear optical (NLO) materials based
than 400 nm,11,16 good thermal stability17 as well as large second
on p-conjugated structure have received much attention because
of their potential applications in optical switching, optical bist-
ability, optical modulator and other all-optical devices.1,2 Apart
order nonlinear susceptibility.18,19 Based on Schuling’s nonlinear
optical theory, the third-order nonlinear optical susceptibility (g)
0
of molecules depends on ge , a term related to the movement of
from this, it is also a type of important materials for optical
electron, and b, the second-order nonlinear optical suscepti-
limiting application due to their large nonlinear optical proper-
bility.20 In this paper, we design and prepare two novel soluble
ties, high damage threshold, and untrafast NLO response.3–5
oxadiazole-containing PAs (Scheme 1) by directly incorporation
Polyacetylene (PA) is a typical p-conjugated polymer with
high c(3). Nevertheless, it is difficult to process because of its
nonfusibility and insolubility.6–9 Therefore, much attention has
of oxadiazole into polyacetylene mainchain, which is expected
that incorporating oxadiazole into PA can endow resulting
polymer with high thermal stability, novel optical properties and
been being paid to the synthesis of PA derivatives with good
enhanced third-order nonlinear optical properties, and carefully
processibility, high thermal stability and large NLO suscepti-
bility.10,11 Our previous work has substantiated that incorpora-
optical properties and their molecular structure.
tion of azobenzene into PAs have endowed resultant polymer
investigate the relationship between the thermal stability, the
with novel optical properties, flexible terminal groups signifi-
Results and discussion
cantly improving solubility of resulting PAs.12–15 However, the
strong ground state electronic absorption in visible region of
azobenzene-containing PAs limits its application in laser limiting
field. Therefore, it is very important to design new functional PAs
for optical limiting application. Oxadiazole is a well-known
Scheme 1 illustrates the synthetic procedures used for the prep-
aration of our new polymers. Synthesis details are available in
the electronic supplementary information.
Polymerization
aCollege of Material Science and Engineering & State Key Laboratory for
Modification of Chemical Fibers and Polymer Materials, Donghua
University, Shanghai, 200051, China. E-mail: hongyaoxu@163.com;
Fax: +86-21-67792874; Tel: +86-21-67792874
bSchool of Chemistry and Chemical Engineering, Anhui University, Hefei,
230039, China
cDepartment of Physics, Suzhou University, Suzhou, 215008, China
dDepartment of Physics, National University of Singapore, Singapore
117542
We first tried to polymerize the monomers by WCl6-, TaCl5- and
MoCl5-Ph4Sn, and MoCl5. The polymerization of 6a and 6b
catalyzed by TaCl6-Ph4Sn, MoCl5-Ph4Sn, MoCl5 at 80 ꢀC or 60
ꢀC in dioxane or toluene were made to get only trace amount
polymeric products, while MoCl5 for 6a did not work (Table 1,
runs 1–4). The polymerization of 6a and 6b in the presence of
WCl6-Ph4Sn under nitrogen for 24 h produced a pale yellow
powdery solid in a low yield (Table 1 and Table 2, runs 5–6)
with low molecular weight. The reaction product of 6a from the
† Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b807004k
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Scheme 1 Synthetic routes to P1 and P2
WCl6-Ph4Sn catalyst does not exhibit absorption peaks of acet-
ylene and olefin protons, but shows a few new peaks in the
aromatic proton absorption region, hinting that they had
undergone some kind of W-catalyzed trimerization reaction (no
spectra. The polymerized product of 6b by using WCl6-Ph4Sn as
catalyst was not further investigated due to its poor solubility.
It is wondrously found that the [Rh(nbd)Cl]2-Et3N catalyst
worked for the polymerization of 6a and 6b in high yield, and the
results are listed in Table 1 (run 7–9) and Table 2 (run 7–9),
triple bond left),21–23 which have been confirmed by HNMR
1
Table 1 Polymerization summary of 6a
Table 2 Polymerization summary of 6b
Temp
ꢀC)
Yield
(%)
Temp
ꢀC)
Yield
(%)
e
e
e
e
Run Catalyst
(
Solvent
Mw
Mw/Mn
Run Catalyst
(
Solvent
Toluene
Dioxane Trace
Mw
Mw/Mn
1
MoCl5-
Ph4Sna
60
Dioxane trace
1
2
TaCl5-Ph4Sna
80
60
Trace
MoCl5-
Ph4Snb
MoCl5-
Ph4Sna
2
3
TaCl5-Ph4Snb
MoCl5-
80
60
Toluene
Toluene
trace
trace
3
60
Toluene
Toluene
Trace
Trace
Ph4Snb
MoCl5
c
c
4
5
6
7
60
60
60
Toluene
Toluene
0
25
4
5
6
7
MoCl5
60
60
60
30
WCl6-Ph4Snb
WCl6-Ph4Snb
3200 1.07
3210 1.04-
14300 1.30
WCl6-Ph4Sna
WCl6-Ph4Sna
[Rh(nbd)Cl]2-
TEAd
Dioxane 24
2210 1.24
3160 1.04
16900 1.52
Dioxane 21
Toluene 92
Toluene
Toluene
26
91
(Rh(nbd)Cl)2- 60
TEAd
8
9
(Rh(nbd)Cl)2- 60
TEAd
(Rh(nbd)Cl)2- 30
TEAd
Dioxane 93
Dioxane 89
25400 1.71
25300 1.54
8
9
[Rh(nbd)Cl]2-
TEAd
[Rh(nbd)Cl]2-
TEAd
60
30
Dioxane 92
Dioxane 91
29300 1.36
32700 1.32
a
a
Carried out under nitrogen for 6 h; [M]0 ¼ 0.20M, [cat.] ¼ [cocat.] ¼
Carried out under nitrogen for 24 h; [M]0 ¼ 0.20M, [cat.] ¼ [cocat.] ¼
b
b
0.020 M. Carried out under nitrogen for 24 h; [M]0 ¼ 0.20M,
0.020 M. Carried out under nitrogen for 6 h; [M]0 ¼ 0.20M,
c
c
[cat.] ¼ [cocat.] ¼ 0.020 M. Carried out under nitrogen for 24 h; [M]0
[cat.] ¼ [cocat.] ¼ 0.020 M. Carried out under nitrogen for 24 h;
d
d
¼ 0.20M, [cat.] ¼0.020 M. Carried out under nitrogen for 6 h;
[M]0 ¼ 0.20M, [cat.] ¼0.020 M. Carried out under nitrogen for 6 h;
e
e
[M]0 ¼ 0.20M, [cat.] ¼ 0.0020M, [cocat.] ¼ 0.0040M. Determined by
[M]0 ¼ 0.20M, [cat.] ¼ 0.0020 M, [cocat.] ¼ 0.0040M. Determined by
GPC in THF (calibration: polystyrene).
GPC in THF (calibration: polystyrene).
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respectively. All polymerization using [Rh(nbd)Cl]2-Et3N as
a catalyst was carried out under nitrogen for 6 h, to yield black
yellow or yellow solids. To optimize the polymerization process,
we studied the polymerizations of 6a and 6b under different
reaction temperature and solvent, finding that the resulting
polymers showed the higher molecular weight and yeild (Table 1,
runs 8–9; Table 2, runs 8–9) when polymerization was carried out
ꢀ
in dioxane at 60 C.
Solubility
We first synthesized polyacetylene containing diphenyl oxadia-
zole chorophore and found that polyacetylene containing
diphenyl oxadiazole chorophore is insoluble by directedly incor-
poration of diphenyl oxadiazole chorophore into polyacetylene
mainchain. However, we have found that substituent of flexible
terminal group effectively improves solubilty of resultant poly-
mers in our previous azobenzene-containing PA.14 Thus, we
designed and synthesied oxadiazole functionalized polyacetylenes
with flexible terminal group. It is found that the solubility of
resultant PAs is obviously improved and affected by the length of
the flexible alkoxy chain length. and solubility of resultant PAs
increases with increasing the chain length of flexible terminal
alkoxy group. For example, polyacetylene containing diphenyl
oxadiazole chorophore without flexible terminal alkoxy chain is
insoluble, P2 with flexible terminal alkoxy group (n ¼ 4) is
partially soluble in common organic solvent such as THF, CHCl3,
toluene and dioxane while P1 with longer flexible terminal alkoxy
group (n ¼ 10) completely soluble in above common organic
solvents. This observation will provide important foundation for
molecular design of soluble functioanl polyacetylenes.
Fig. 2 1HNMR(CDCl3) spectra of 6a and P1 (Table 1, run 8). The
solvent peaks are marked with asterisks.
the hC–H stretching vibration in the monosubstituted acetylene
molecules. However, the characteristic ns (hC–H) absorption
band disappeared in the spectrum of P1 (Table 1, run 8) and the
relative intensity of the stretching band at 1611 cmꢁ1 in P1
significantly increases, which is assigned to C]C group, indi-
cating that ChC in the monomer has changed into C]C group
in the polymer and the Rh catalyst has initiated the polymeri-
zation of acetylene. The similar results were also observed in
spectrum of P2.
1
Fig. 2 shows the H NMR spectra of 6a and P1. The absorp-
tion peak of the acetylene proton in 6a is located at d 3.25 ppm as
a singlet peak, but completely disappears in the spectrum of its
polymer P1. Alternatively, P1 shows a broad peak at d 5.9–6.68
ppm corresponding to the olefin and aromatic protons absorp-
tion, which further proves the changing ChC to C]C. The
1HNMR analysis also provided valuable information of the
stereoregularity of the resulting PAs. As shown in Fig. 2, a new
cis olefin absorption peak (d 5.92 ppm) appears in the spectrum
of P1.21,23 Based on the Eq (1) used by Tang and Xu for evalu-
ating the stereostructure of the PA derivative,14,24 we calculate
the cis content of the polymers based on the following equation:
Structural characterization
The structures of P1 and P2 were characterized by spectroscopic
techniques, and both gave satisfactory data corresponding to
their expected molecular structures. The FTIR spectra of 6a and
P1 are shown in Fig. 1 As can be seen in Fig. 1, 6a clearly exhibits
a strong characteristic absorption band near 3272 cmꢁ1, due to
cis content(%) ¼ [A5.92/((A + A5.92)/9)] ꢂ 100
(1)
where A ¼ A7.69 + A6.93 + A6.68, corresponding to the peak areas
of the eight aryl protons of the polymers and the peak areas of
trans olefinic proton, and A5.92 corresponds to the peak areas of
cis olefinic proton. Then, the cis olefin contents can be estimated
based on integration areas of the peak, i.e., the cis contents of P1
and P2 are 93.1% and 81.8 %, respectively. The increase of cis
content of P1 compared with P2 may be caused by the cis-trans
isomerization during the polymerization due to the higher stereo-
hindrance of longer alkoxyl chain.21,23,25–28
Optical properties
The photophysical characteristics of the P1 and P2 were studied
Fig. 1 Fig. 1 FT-IR (KBr pellet) spectra of 6a and P1 (Table 1, run 8).
4206 | J. Mater. Chem., 2008, 18, 4204–4209
by UV-vis absorption in THF solutions. The absorption spectra
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alkyne)s has endowed the polymers with high thermal stability. It
may be due to the ‘jacket effect’ of the aromatic oxadiazole
pendants, namely, the rigid aromatic oxadiazole groups may
have formed a protective ‘‘jacket’’ via the strong electronic
interactions among the polarized aromatic oxadiazole side
chains, shielding the polymer main chains from thermal attack.
Similar phenomenon is also found by Tang and our previous
work.14,19,26,30,31 Simultaneously, it is also found that the thermal
stability of resultant polymers is affected by stereoregularity of
PA main chain. The thermal stability increases with the increase
of cis olefin content of polymer, implying that thermal stability
could be adjusted by alkoxy chain length. It is consistent with
that found in Tang and our previous work.19,26
Optical limiting properties
Fig. 5 shows the optical responses of P1(c ¼ 2.2 mg/mL, T ¼
67%), P2 (c ¼ 1.8 mg/mL, T ¼67%) along with that of PPA
(c ¼ 4.6 mg/mL, T ¼ 67 %) solutions to 532 nm laser pulses. It
can be seen from Fig 5 that, at very low input fluence, the output
fluence of all the solutions linearly increases with the incident
fluence, obeying the Beer- Lambert law, but at high input fluence,
the transmittance of P1 and P2 solution decreased, and
a nonlinear relationship is observed between the output and
input fluence, showing significant OL property. In contrast, the
transmittance of the PPA solution continually increases instead
of decrease due to the laser induced photolysis of the PA chains,32
indicating that the incorporation of oxadiazole endows PA with
novel OL property. Simultaneously, we measured the UV-vis
absorption spectrum of the P1 and P2 solutions before and after
the laser irradiation and found that the pattern and its intensity
of UV-vis absorption spectra have almost no change, hinting
that the P1 and P2 possesses well photostability.
Fig. 3 UV-vis absorption spectra of P1 (Table 1, run 8) and P2 (Table 2,
run8) in THF solutions with a concentration of 1 ꢂ 10ꢁ5M.
of P1 and P2 in THF are given in Fig. 3. P1 shows a strong
absorption at 312 nm, which is attributable to the oxadiazole
chromophore units, and the absorption at 400 nm with weak
intensity may result from the absorption of the conjugated
backbone of PA.11,29 The UV-vis absorption spectrum of P2 is
similar to that of P1 and the maximum absorption is at 311 nm.
In addition, the Egs of P1 and P2 from the absorption edge of
solution samples are 3.30 and 3.33 eV, respectively,19 indicating
the ground-state electronic transitions are little affected by
alkoxy chain length.
Thermal properties
It is well known that poly(1-alkyne)s such as poly(1-butyne) and
poly(1-hexyne), are so unstable that even isolation process of the
polymer products from the polymerization reactions leads to
degradation.25,30 However, the two polymers exhibit good
thermal stability. As shown iꢀn Fig. 4, the Tds (weight loss 5%) of
P1 and P2 are 353 and 346 C, respectively, indicating that the
incorporation of rigid diaryl-oxadiazole group into poly(1-
Furthermore, it is also seen that the transmitted fluence of the
P1 solution starts to deviate from linearity when the incident
fluence reaches 0.625 J/cm2 (defined as limiting threshold, i.e., the
incident fluence at which the output fluence starts to deviate from
linearity). With a further increase in the incident fluence, the
Fig. 5 Optical limiting properties of P1 and P2 solution (P1: Table 1,
run 8 and P2: Table 2, run 8) with a linear transmission of 67%. Data for
PPA with the same transmission are given for comparison.
Fig. 4 TGA thermograms of P1 (Table 1, run 8) and P2 (Table 2, run 8)
measured under nitrogen at a heating rate of 10 ꢀC /min.
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Table 3 Summary of optical limiting and nonlinear optical properties of
P1 (Table 1, run 8) and P2 (Table 2, run 8)
transmitted fluence reaches a plateau and is saturated at 0.454 J/
cm2 (defined as the limiting amplitude, i.e., the maximum output
intensity). However, the limiting amplitude value of P2 is 0.713 J/
cm2. The result suggests P1 with longer terminal alkoxy chain and
higher cis olefin content exhibits better OL performance than P2
with shorter terminal alkoxy chain and lower cis olefin content.
The OL mechanisms of organic compounds are often based on
two-photon absorption (TPA) or reverse saturable absorption
(RSA). Generally, TPA-based OL effect can be yielded in prin-
ciple under the laser irradiation of picosecond or shorter pulses.
RSA is achieved on a nanosecond or longer time scale, rather
than a picosecond time scale, owing to the different excited state
lifetimes involved in a multilevel energy process.33 In the present
work, the molecules are excited by the laser with 4 ns pulse width
at 532 nm wavelength and the transmittance of all these poly-
mers’ solution decreases with the increase of the incident fluence.
Therefore, we can consider that the OL properties of P1 and P2
may be originated from RSA.
NLO Propertiesb
OL
Amplitude
(J/cm2)a
c(3)
b (ꢂ 10ꢁ11m/W) n2 (ꢂ 10ꢁ19m2/W) (ꢂ 10ꢁ12esu)
P1 0.454
P2 0.713
8.54
4.75
16.10
6.94
4.67
2.15
a
b
Maximum output intensity. Measured by the Z-scan technique with
an 13-ns Nd:YAG laser system at a 1-Hz repetition rate and a 532-nm
wavelength.
In theory, the normalized transmittance for the open aperture
can be written as:35
m
N
X
½ ꢁ q0ðz; 0Þꢃ
Tðz; s ¼ 1Þ ¼
forjq0j\1
(2)
3=2
ðm þ 1Þ
m¼0
The OL property of RSA molecules can be evaluated by the
ratio of the excited-state absorption cross-section (sex) to the
ground state absorption cross-section (s0) of molecules, which
was defined as sex/s0 ¼ ln Tsat/ln T0. Tsat is the saturated trans-
mittance for high degrees of excitation.34 The larger value of sex/
s0, the better OL performance. In our experimental set up,
although we are unable to reach the saturable transmittance for
these compounds, we can use the transmittance at 1.4 J/cm2 to
calculate the lowest bound for sex/s0. Based on the experiment
data illustrated in Fig. 5, the calculated values of sex/s0 for P1
and P2 are 2.81 and 1.68, respectively, further confirming that
their OL mechanism are mainly originated from large excitation
state absorption cross-section to result in reverse saturable
absorption.
where q0(z) ¼ a2I(t)Leff/(1 + Z2/Z0 ), a2 is the nonlinear
absorption coefficient, I0(t) is the intensity of laser beam at focus
(z ¼ 0), Leff ¼ |1 ꢁ exp(ꢁa0L)|/a0 is the effective thickness with a0
the linear absorption coefficient and L the sample thickness, z0 is
the diffraction length of the beam, and z is the sample position.
Thus, the nonlinear absorption coefficients of the polymers can
be determined by fitting the experimental data using Eq. (2).
The normalized transmission for the closed aperture Z-scan is
given by the following:35
2
4Df0x
ðx2 þ 9Þðx2 þ 1Þ
T ¼ 1 þ
(3)
where x ¼ z/z0 and Df is on-axis phase change caused by the
nonlinear refractive index of the sample and Df ¼ 2pI0(1 ꢁ
eꢁLa )n2/la0. Thus, the nonlinear refractive coefficients of the
0
Nonlinear optical properties
polymers can be determined by fitting the experimental data
The nonlinear absorption coefficients of PAs were measured by
using Z-scan technique. The results of Z-scan with and without
an aperture showed that these PAs have both nonlinear
absorption (Fig. 6-A) and nonlinear refraction (Fig. 6-B).
using Eq.(3).
The c(3) can be calculated by Eq.(4):35
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2
2
2
8
2 2
ꢀ
ꢀ
ꢀ
ꢀ
cn0
9 ꢂ 10 30n0c
4pu
ð3Þ
c
¼
$n2
þ
$b
(4)
80p
where e0 is the permittivity of vacuum, c is the speed of light, n0 is
the refractive index of the medium and u ¼ 2pc/l. Therefore, the
results can be calculated and are listed in Table 3. From Table 3,
the nonlinear absorption coeffinces of the P1 and P2 are 8.54 ꢂ
10ꢁ11 and 4.75 ꢂ 10ꢁ11m/W, respectively. The b values of P1 and
P2 (9.5 ꢂ 10ꢁ11m/W)19 increased with the cis olefin content of
those increasing. In addition, the nonlinear susceptibilities c(3) of
the P1 and P2 are 4.67 ꢂ 10ꢁ12 and 2.15 ꢂ 10ꢁ12 esu, respectively,
which enhanced with an increase in the alkoxy chain length.14
Conclusion
Two functional PAs containing oxadiazole groups with different
terminal alkoxy group are designed and prepared by a multistep
reaction route. The incorporation of rigid aromatic oxadiazole
group into polyacetylenes had endowed the polyacetylenes with
high thermal stability, which may be owing to ‘jacket effect’ of
the aromatic oxadiazole pendants. The solubility of resultant
Fig. 6 Z-scan data of (A) open and (B) closed apertures of P1. (Table 1,
run 8).
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length. The oxadiazole-containing PA with longer terminal
alkoxy group chain shows better solubility than PA with shorter
terminal alkoxy chain. Simultaneously, the stereoregularity and
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larger stereo-hindrance of alkoxy chain. Polymer with higher cis
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Acknowledgements
This research was financially supported by the National Natural
Science Fund of China (Grant Nos. 90606011 and 50472038),
Ph.D. Program Foundation of Ministry of Education of China
(No.20070255012) and Shanghai Leading Academic Discipline
Project (No. B603).
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