Synthesis of the end-functionalized polymethyl methacrylate incorporated with an asymmetrical porphyrin group via atom transfer radical polymerization and investigation on the third-order nonlinear optical properties

Article abstract of DOI:10.1016/j.jphotochem.2013.08.016

A novel asymmetrical zinc(II) porphyrin (ZnPor-Br) was synthesized as the initiator for the atom transfer radical polymerization (ATRP). Using CuBr/2,2′-dipyridyl as the catalyst system, the ATRP of polymethyl methacrylate (PMMA) was carried out to afford

Full text of DOI:10.1016/j.jphotochem.2013.08.016

Journal of Photochemistry and Photobiology A: Chemistry 272 (2013) 65–72
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis of the end-functionalized polymethyl methacrylate
incorporated with an asymmetrical porphyrin group via atom transfer
radical polymerization and investigation on the third-order nonlinear
optical properties
Nannan Qiua, Dajun Liub,∗, Shuangli Hanc, Xingquan Heb, Guihua Cuia, Qian Duana,∗
a School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China
b School of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun 130022, China
c School of Chemistry and Environmental Engineering, Changchun University of Changchun Institute of Optics, Fine Mechanics and Physics,Chinese
Academy of Sciences, Changchun 130033, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 May 2013
Received in revised form 26 July 2013
Accepted 8 August 2013
Available online xxx
A novel asymmetrical zinc(II) porphyrin (ZnPor-Br) was synthesized as the initiator for the atom transfer
radical polymerization (ATRP). Using CuBr/2,2ꢀ-dipyridyl as the catalyst system, the ATRP of polymethyl
methacrylate (PMMA) was carried out to afford a new linear PMMA with the end group of the asym-
metrical porphyrin (ZnPor-PMMA). The structures of the porphyrin, ZnPor-Br and as-prepared polymer
were determined by FT-IR, 1H NMR spectra and further confirmed by element analysis. Polydisper-
sity index (PDI) obtaining from the gel permeation chromatograph (GPC) indicated that the molecular
weight distribution was narrow, and the whole process of polymerization was well controlled. The
third-order nonlinear optical properties of polymers were investigated by using Z-scan technique in
the pico-second time scale. The nonlinear refracting index (n2) and the third-order nonlinear polariz-
ability [ꢀ(3)] were 4.1 × 10−10 esu and 1.64 × 10−10 esu respectively, while the molecular polarizability
(ꢁꢀ) was 1.01 × 10−28 esu (Mn = 2000). The obtained results implied that the prepared compound featured
the large nonlinear optical limiting properties and could serve as a potential candidate for the reverse
saturable absorption (RSA) and the refractive-based optical limiting applications in the laser protection
field.
Keywords:
Atom transfer radical polymerization
Zincporphyrin
Z-scan
Optical limiting
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
ꢂT, larger than the ground-state absorption cross-section, ꢂ0. As
the optical excitation intensity increases, more molecules are pro-
Since the invention of the intense light sources based on laser
mechanism in 1960s, the protection of optical sensors and human
eyes from the accidental or hostile lasers has been consider-
ably researched [1,2]. More recently, several materials and device
configurations as optical limiting (OL) materials have been devel-
oped and proposed to meet this challenge [3]. These materials
include porphyrins, porphyrin–graphene nanohybrid, phthalocya-
nines, fullerenes and octanuclear silver cluster [4–12]. Porphyrins
with large ␲-conjugated structures have been a type of extensively
investigated nonlinear optical (NLO) material and exhibit strongly
reverse saturable absorption (RSA).
moted to the excited state, thus giving rise to higher absorption at
intense light excitation. The mechanism of RSA is often described in
terms of a five-level model (Fig. 1). The RSA effect of a molecule can
be described as follows: the ground state (S0) of a molecule passes
to the first singlet state (S1) by absorption of a photon, When the
population of particle reaches a certain value on S1, it may have fur-
ther transition to higher single excited state of S2 or an intersystem
crossing (ISC) process takes place and increases the population in
the triplet state (T1) with a time constant ꢃISC. Successively, a sec-
ond photon is absorbed by the system in the state (T1) to the excited
state (T2) [13].
OL based on RSA are very transparent for weak light and get
opaque for the intense light. Moreover, in the RSA process, the
absorbing material has an excited-state absorption cross-section,
Porphyrins are ubiquitously compounds with important biolog-
ical representatives such as hemes, chlorophyll and Vitamin B12
.
And the porphyryl derivatives are widely used in the organic photo-
voltaic cells, optical communication, signal processing/switching,
light-emitting materials, optical limiting materials and modulat-
ing devices [14–16]. Porphyrin molecules exhibit well flexible and
their optical properties are easily fine-tuned through proper sub-
stituents, but it is difficult for the fabrication of devices with the
∗
Corresponding authors. Tel.: +86 431 85583105; fax: +86 431 85583105.
E-mail addresses: cuiyuhan1981 0@sohu.com (G. Cui),
duanqian88@hotmail.com (Q. Duan).
1010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2013.08.016

66
N. Qiu et al. / Journal of Photochemistry and Photobiology A: Chemistry 272 (2013) 65–72
(c)
1000
2920
3432
990
728
(b)
(a)
960
809
2920
2940
3314
3435
3320
3500
4000
3000
2500
2000
1500
1000
500
Wavenumber(cm-1
)
Fig. 1. The five-level model of reverse saturable absorption.
Fig. 2. FT-IR spectra of (a) p-hydroxyethoxybenzaldehyde, (b) Por-OH, (c) Zn-Por.
single porphyrin. Previous studies on the optical limiting materi-
als usually focused on the mixtures of porphyrins and polymers
with good plasticity which is only a simple physical mixing pro-
cesses, followed by the preparation of films via dip-coating or
self-assembled methods [17–19]. In this process, it has some draw-
backs such as the unevenly dispersedness, the agglomeration of
porphyrins, and the restrictedly discretional incorporation of por-
phyrins in polymer arising from their poor solubility. To some
extent, these drawbacks have negative impact for the optical
limiting performance. So in this study, the purpose of synthesiz-
ing the end-functionalized polymethyl methacrylate incorporated
with an asymmetrical porphyrin via ATRP are as follows: firstly, the
polymethyl methacrylate could endow the porphyrin with plastic-
ity for the fabrication of devices. Secondly, we make the porphyrin
together with polymers by the chemical bonds which could avoid
the drawbacks coming from the physical mixing processes. Lastly,
we control exactly the concentration of porphyrin in polymer via
changing the molecular weight by ATRP and we study the effect
causing by molecular weight of polymer on the nonlinear optical
nonlinearity of porphyrin.
CuBr/2,2ꢀ-dipyridyl were employed as the catalyst system at 60 ◦C
during the further polymerization. The obtained polymer exhibited
narrow molecular weight distribution and the process followed a
first-order reaction kinetics. Nonlinear optical properties of por-
phyrin and polymers were also tested under pulsed laser by the
Z-scan technique.
2. Experimental
2.1. Materials
Methyl methacrylate (99%, Aldrich) was purified by the distil-
lation in vacuum. CuBr (99%, Aldrich) catalyst was successively
washed with acetic acid and ether, dried, and then stored
under the nitrogen atmosphere. Triethylamine (99%, Aldrich), p-
hydroxybenzaldehyde (99%, Aldrich), 2-bromoethyl alcohol (99%,
Aldrich), Zinc acetate (99%, Aldrich) and 2-bromopropionyl bro-
mide were available commercially and used without further
purification.
ATRP as one of the mostly investigated controlled/living rad-
ical polymerzation (CRP) methods could provide polymers with
designed structures and narrow molecular weight distributions by
using suitable starting materials [20–24]. Although much atten-
tion has been paid to the star polymers with a porphyrin or metal
porphyrin core by ATRP due to their special chemical and phys-
ical features relevant in various fields, we squinted towards the
strategy of synthesizing linear polymers with asymmetrical por-
phyrin group in order to keep large third-order nonlinearity optical
properties of the porphyrins [25,26]. The methyl methacrylate,
which was one of the green and environment-friendly materi-
als with the advantages of good chemical stability, fine solubility,
high-temperature resistance, high degree of transparency, ease
of processing, etc. and widely used in the preparation of opti-
cal limiting materials, is selected as the polymerized monomer.
We anticipated the efficient combination of the advantages of
polymethylmethacrylate with the third-order nonlinear optical
properties of porphyrins in the present strategy.
2.2. Characterization instruments
1H NMR spectra were carried out on a Bruker Avance 400 MHz
spectrometer at room temperature, using CDCl3 as solvent.
Element analysis was obtained on a Carlo Erba-MOD1106 instru-
ment. Molecular weights (Mn) and polydispersity (Mw/Mn) were
measured on a gel permeation chromatograph (GPC) using a
WAT044207 differential refractometer at 35 ◦C. Infrared spectra
were determined by measuring samples in KBr disks on a Shimadzu
IR-8400S spectrometer. Ultraviolet–visible (UV–vis) spectra were
collected on a UV mini 1240 (Shimadzu) spectrophotometer.
2.3. Synthesis of p-hydroxyethoxybenzaldehyde
2.3 g (0.1 mol) sodium metal was cut into small pieces and then
added into 90 mL ethanol quickly at reflux temperature. Under
the protection of nitrogen, 12.2 g (0.1 mol) p-hydroxybenzaldehyde
was added into the above solution with stirring for 15 min. 7.5 mL
2-bromoethyl alcohol was slowly added into the reaction vessel
under stirring. The mixture was further stirred for 5–6 h at reflux
temperature, then evaporated to give the pale yellow oily matter
[29]. The obtained mixture was purified by column chromatogra-
phy (SiO2, petroleum ether/ethyl acetate: 3:1 v/v as the eluent) to
give a purple p-hydroxyethoxybenzaldehyde (Scheme 1).
Based on the above strategy, the asymmetrical ZnPor-Br was
synthesized as the starting material for ATRP in order to avoid the
metalation of the porphyrin by copper(II) during the polymeri-
zations [27]. While the copper(II) is the open shell paramagnetic
ion with unfilled orbitals involving spin–orbit coupling through
coulombic exchange terms, which led to the decrease of the triplet
lifetime and consequently scarce efficiency in the increase of pop-
ulation in the absorbing excited state. On the other hand, Zn2+
,
IR: 3400 cm−1
( (
OH), 2850–2925 cm−1 CH2 ) (Fig. 2(a)). 1H
Cd2+ and Pb2+ ions have closed shells and the OL performances are
favourable in these complexes. The third-order nonlinear optical
properties of zinc(II)porphyrin are highest to our knowledge [28].
NMR: 9.86 (S, 1H), 7.83 (d, 2H), 7.00 (d, 2H), 4.17 (t, 2H), 4.00 (t,
2H), 2.87 (d, 1H) (Fig. 3(A)).

N. Qiu et al. / Journal of Photochemistry and Photobiology A: Chemistry 272 (2013) 65–72
67
OH
+
Na
OH
O
ONa
OH
H
N
reflux
OH
Br
ONa
+
CHO
CHO
re
CHO
OH
fl
u
OH
x
O
O
Zn
O
NH
N
N
N
N
N
O
O
(CH3COO)2Zn
DMF
O
O
O
O
O
O
O
O
HN
O
O
N
O
O
O
O
O
Scheme 1. Synthetic route of Zn-Por.
IR: 3430 cm−1
(
OH), 1750 cm−1 (C O), 729 cm−1, 810 cm−1
,
2.4. Synthesis of 5,10,15,20-tetra(p-hydroxyethylphenyl)
porphyrin tripropionate (Por-OH)
990 cm−1 (prophine), 3317 cm−1, 969 cm−1(N H) (Fig. 2(b)). 1H
NMR: 8.86 (s, 8H), 8.11 (d, 8H), 7.27 (d, 8H), 4.61 (t, 6H), 4.44 (m,6H),
4.35 (t, 2H), 4.14 (t, 6H), 2.50 (q, 6H), 2.28 (s, 1H), 1.27 (t, 9H), −2.75
(s,2H) (Fig. 3(B)). Anal. Calcd for C62H59N4O11: C, 70.61; H, 5.49; N,
5.54. Found: C, 71.61; H, 5.71; N, 5.48.
60 mL (0.1 mol) propionic acid was added into the reac-
tion vessel, heated to the reflux temperature. 6 g (0.036 mol)
p-hydroxyethoxybenzaldehyde was added into the reflux-
ing propionic acid under stirring, 2.8 mL (0.036 mol) new
steamed pyrrole was added dropwise into the above mix-
ture under stirring. The mixture was reacted for 50 min at
reflux temperature, then added by 70 mL methanol and pro-
cessed the crystallization in the refrigerator. The obtained
crystals were purified by column chromatography (SiO2,
dichloromethane/methanol 50:1 v/v as the eluent) to obtain
2.5. Synthesis of 5,10,15,20-tetra(p-hydroxyethylphenyl)
zinc(II)porphyrin tripropio-nionate (Zn-Por)
100 mg (0.15 mol) 5,10,15,20-tetra(p-hydroxyethylphenyl)
porphyrin tripropionate was added into DMF, then heated to the
reflux temperature. 80 mg (0.36 mol) zinc acetate was added to
above solution under stirring at reflux temperature. The reaction
was remained for 2 h. Subsequently, the mixed solution was
a
purple 5,10,15,20-tetra(p-hydroxyethylphenyl) porphyrin
tripropionate (Scheme 1).
Fig. 3. 1H NMR spectra of (A) p-hydroxyethoxybenzaldehyde, (B) Por-OH, (C) ZnPor with CDCl3 as solvent.

68
N. Qiu et al. / Journal of Photochemistry and Photobiology A: Chemistry 272 (2013) 65–72
over Na2SO4, filtered, and then evaporated to obtain zinzolin solid
(Scheme 2).
IR: 2850–2950 cm−1 CH2, CH3), 1730 cm−1 (C O), 740 cm−1
,
(b)
740
810
(
810 cm−1, 990 cm−1 (prophine), 1000 cm−1 (Zn N) (Fig. 4(a)). The
1H NMR spectra of ATRP initiator. ı, 8.85 (s, 8H), 8.13 (d, 8H), 7.30
(d, 8H), 4.73 (s, 2H), 4.63 (s, 6H), 4.58 (d, 1H), 4.53 (d, 6H), 4.49 (d,
6H), 2.53–2.48 (q, 6H), 1.83 (d, 3H), 1.24 (t, 9H) (Fig. 5(A)). Anal.
Calcd for C64H59BrN4O12 Zn: C, 62.83; H, 4.77; N, 4.40. Found: C,
62.93; H, 4.87; N, 4.59.
990
2950
1730
720
(a)
940
795
2960
3000
990
1760
2.7. Synthesis of ZnPor-PMMA
The ZnPor-PMMA were synthesized as below (Scheme 3): a
mixture of CuBr (14.4 mg, 0.1 mmol) and 2,2ꢀ-dipyridyl (15.6 mg,
0.1 mmol) in DMF (1 mL) was placed on one side of an H-
shaped glass ampoule and stirred at room temperature, while
methyl methacrylate (0.14 mL, 2 mmol) and the indicator (22 mg,
0.02 mmol) in DMF (1.5 mL) were placed on the other side
of the ampoule. Nitrogen was bubbled through both mixtures
for 15 min to remove the oxygen. Three freeze-pump-thaw
cycles were then performed to degas the solution. Both mix-
tures were mixed and placed in an oil bath at 60 ◦C for 2 h.
The polymerization reaction was terminated by the exposure
to air. The reaction mixture was purified through a neutral
Al2O3 column using THF as eluent to remove the copper com-
plex. After precipitated three times by adding the polymer
solution of DMF into methanol and water (v:v = 1:1), fuchsia poly-
mer was collected by filtration and dried in a vacuum oven
overnight.
4000
3500
2500
2000
1500
1000
500
-1
Wavenumber(cm
)
Fig. 4. FT-IR spectra of (a) ZnPor-Br and (b) ZnPor-PMMA.
evaporated under vacuum, then added into 150 mL deionized
water. The zinzolin solid precipitated, followed by the filtering and
drying at 65 ◦C under vacuum (Scheme 1).
IR: 3430 cm−1
CH2,
(
OH), 1750 cm−1 (C O), 2850, 2925 cm−1
810 cm−1 960 cm−1 990 cm−1
(
CH3), 729 cm−1
,
,
,
(prophine), 1000 cm−1 (Zn N) (Fig. 2(c)). 1H NMR: 8.86 (s,
8H), 8.11 (d, 8H), 7.27 (d, 8H), 4.61 (t, 6H), 4.44 (m,6H), 4.35 (t,
2H), 4.14 (t, 6H), 2.50 (q, 6H), 2.28 (s, 1H), 1.27 (t, 9H) (Fig. 3(C)).
Anal. Calcd for C62H59N4O11Zn: C, 67.61; H, 5.40; N, 5.09. Found:
C, 67.93; H, 5.37; N, 4.59.
2.6. Synthesis of 5,10,15,20-tetra(p-bromopropanoyloxy-
ethylphenyl) zincporphyrin tripropionate (ZnPor-Br)
IR: 2850–2960 cm−1 CH2, CH3), 1760 cm−1 (C O), 740 cm−1
( ,
810 cm−1, 990 cm−1 (prophine), the peak of carbonyl, methylene
and methyl are stronger than ATRP initiator (shown in Fig. 4). Fig. 5
shows the 1H NMR spectra of the polymer(ZnPor-PMMA): peak
at 3.53 ppm is assigned to the hydrogen (P OCH3), 0.78 0.89 ppm
are assigned to the hydrogen (P CH3), 1.20 ppm is assigned to the
hydrogen ( CH3), and broad peaks at 1.75–2.0 ppm are assigned to
the hydrogen (P CH2 ), peaks at 7.65 and 8.22 ppm are assigned
to the hydrogen (Ar H) the signals of the porphyrin are located at
8.98 ppm.
5, 10, 15, 20-tetra(p-hydroxyethylphenyl) zincporphyrin tripro-
pionate (100 mg, 0.1 mmol) was added into dichloromethane, and
cooled to 0 ◦C in an ice bath. Then ␣-bromopropionyl bromide
(0.25 mL, 0.2 mmol) and triethylamine (0.28 mL, 0.2 mmol) were
added dropwise. After reacting for 1 h at 0 ◦C, the solution was fur-
ther stirred for 20 h at room temperature. Subsequently, the mixed
solution was purified by extraction. The organic layer was dried
Fig. 5. 1H NMR spectra of (A) ZnPor-Br, and (B) ZnPor-PMMA with CDCl3 as solvent.

N. Qiu et al. / Journal of Photochemistry and Photobiology A: Chemistry 272 (2013) 65–72
69
Br
OH
O
O
O
O
O
Br
N
N
N
N
N
N
N
N
O
Br
CH2Cl2, 0
O
O
O
Zn
O
Zn
O
O
O
O
O
O
O
O
O
O
O
O
O
Scheme 2. Synthetic route of ZnPor-Br.
O
O
Br
O
O
Br
O
n
O
O
O
O
N
N
N
N
N
N
N
O
O
O
O
Bpy/
O
Zn
O
O
Zn
O
O
O
CuBr/
DMF
O
O
O
O
N
O
O
O
O
O
Scheme 3. Synthetic route of ZnPor-PMMA.
3. Results and discussion
demonstrated that the polymerizations were performed in a con-
trolled process.
The relative molecular weight and the polydispersity index (PDI)
of the target polymer were obtained through the gel permeation
chromatography (GPC). After a series of purifications, GPC traces of
ZnPor-PMMA (Fig. 6) exhibited relatively symmetric and showed
void tailing at either side, which suggested the absence of any
small molecule residues, such as the starting material, monomer
or other byproducts in the final product. Moreover, the polymers
possessed the low PDI with values between 1.04 and 1.09, which
The UV–vis spectrum of 5,10,15,20-tetra(p-hydroxy-
ethylphenyl) porphyrin tripropionate exhibited a strong Soret
band at 416 nm and four Q-bands between 500 and 700 nm, which
were typical for a simple free-base porphyrin. The Soret band of the
zincporphyrin red shifted to 421 nm and the number of Q-bands
peaks reduced from four to two Q-bands between 500 and 600 nm
due to the introduction of zinc ion (Fig. 7(a)). Meanwhile, the Soret
band of ZnPor-PMMA showed a bathochromic shift as compared
to that of ZnPor-Br in the UV–vis spectrum (Fig. 7(b)), which arose
from the increased conjugacy by the introduction of the long chain
segment in PMMA.
We performed the Z-scan measurements using a Q-switched,
mode-locked Nd3+:YAG laser source (ꢄ = 532 nm) and delivering
21 ps-duration (FWHM) pulses at 1 Hz repetition frequency. The
temporal profile of the pulses was nearly the space Gaussian line-
shape and their spatial mode was close to TEM00. The Gaussian
beam of the beam waist with 40 ␮m radius was focused on the
sample cell with the thickness of 2 mm by using a convex lens of f
30 cm. The maximum on-axis light intensity was 0.3 GW cm−2 and
the linear transmittance S of the aperture was 0.1.
M
M
=2000,M /M =1.04
n,GPC
w
n
=4400,M /M =1.05
n,GPC
n,GPC
w
n
M
=6700,M /M =1.09
w
n
Figs. 8 and 9 showed the closed-aperture and open-aperture
Z-scan curves of zincporphyrin, ZnPor-Br and ZnPor-PMMA. The
solid lines were generated theoretically using the Z-scan theory for
a nonlinear phase shift less than ␲ [30].
From the Z-scan curve of samples with closed-aperture we
can see that the valley was prior to the peak in Normalized
transmittance curve becausing of the self-focusing effects, which
indicate the nonlinear refractive indexes of the samples n2 were
positive. Furthermore, The phenomenon of the peak and valley
14
15
16
17
18
19
20
Retention time (min)
Fig. 6. GPC traces of zincporphyrin-end-functionalized polymethacrylate.

70
N. Qiu et al. / Journal of Photochemistry and Photobiology A: Chemistry 272 (2013) 65–72
3.0
2.5
2.0
1.5
1.0
0.5
0.0
3.5
(a)
(b)
3.0
2.5
2.0
ZnPor-Br
zincporphyrin
1.5
1.0
0.5
0.0
porphyrin
ZnPor-PMMA
300
400
500
600
700
300
400
500
600
700
Wavelength(nm)
Wavelength(nm)
Fig. 7. UV–vis spectra of (a) porphyrin and zincporphyrin and (b) ZnPor-Br and ZnPor-PMMA in CH2Cl2 (3 × 10−4 mol L−1).
1.15
1.10
1.05
1.00
0.95
0.90
0.85
1.05
1.00
0.95
0.90
0.85
0.80
a(2)
a(1)
-60 -40 -20
0
20
40
60
-60 -40 -20
0
20
40
60
Z/mm
Z/mm
1.05
1.10
b(2)
b(1)
1.08
1.00
0.95
0.90
0.85
0.80
0.75
0.70
1.06
1.04
1.02
1.00
0.98
0.96
0.94
-60 -40 -20
0
20
40
60
-60 -40 -20
0
20
40
60
Z/mm
Z/mm
1.05
1.00
0.95
0.90
0.85
0.80
1.08
1.06
1.04
1.02
1.00
0.98
0.96
0.94
c(2)
c(1)
-60 -40 -20
0
20
40
60
-60 -40 -20
0
20
40
60
Z/mm
Z/mm
Fig. 8. Normalized transmissivity Z-scan curve of nonlinear absorption of samples with closed-aperture(1) and open-aperture(2). (a) ZnPor; (b) ZnPor-Br; (c) ZnPor-PMMA
(Mn,GPC = 2000).

N. Qiu et al. / Journal of Photochemistry and Photobiology A: Chemistry 272 (2013) 65–72
71
1.05
d(2)
1.08
1.06
1.04
1.02
1.00
0.98
0.96
0.94
d(1)
1.00
0.95
0.90
0.85
0.80
-60 -40 -20
0
20
40
60
-60 -40 -20
0
20
40
60
Z/mm
Z/mm
1.05
1.00
0.95
0.90
0.85
0.80
0.75
1.04
1.02
1.00
0.98
0.96
0.94
0.92
0.90
e(1)
e(2)
-60 -40 -20
0
20
40
60
-60 -40 -20
0
20
40
60
Z/mm
Z/mm
Fig. 9. Normalized transmissivity Z-scan curve of nonlinear absorption of samples with closed-aperture(2) and open-aperture(1). (d) ZnPor-PMMA (Mn = 4400); (e) ZnPor-
PMMA (Mn = 6700).
exhibited asymmetrical, the valley was restrained while the peak
was enhanced showed that the samples had favourable nonlinear
absorption properties [31]. The nonlinear refractive coefficient n2
could be obtained by the followed equations:
where
ˇI0(t)Leff
(1 + z2/z02)
q0(z, t) =
z = 0
(3.5)
(3.6)
ꢀ
ꢀ
ꢅTp−v = 0.406(1 − S)0.25 ꢅꢆ0
,
(3.1)
(3.2)
ꢀ
ꢀ
4(1 − T(0))
ˇ(MKS) =
ꢇꢆ0(t) = kꢅn0(t)Leff
,
I0Leff
cn0
I(0) was the instantaneous light strong at the focus of the convex
lens (about z (axis) = 0 point), while ˇ was negligible. On the style
to take an approximate
n2(esu) =
ꢁ(m2/W)
(3.3)
40ꢈ
While the ꢇTp−v was the peak and valley difference in the Z-scan
Normalized transmittance, S is the linear transmittance of the aper-
ture in the absence of a sample, ꢇꢆ0 was the laser induced phase
shift and Leff = (1 − e−␣L)/˛ was the effective propagation length in
the solution. ˛ and L represented the linear absorption coefficient
and length of sample respectively, and n2 (esu) and ꢁ (m2/W) were
nonlinear refractive coefficients of Gaussian system of units.
On the other hand, from the Z-scan curves of nonlinear absorp-
tion for the samples with open-aperture we could found that the
samples exhibited reverse saturable absorption (RSA) at 532 nm.
In addition, a reduction in the transmission exhibited at the focus
lens. The phenomenon was typical for an induced nonlinear absorp-
tion of the incident laser beam, which could be attributed to RSA,
while the excited state absorption cross section of the T1 → T2 tran-
sition was larger than the ground state absorption cross section
S1 → S2. Normally, the Nonlinear absorption coefficient ˇ is utilized
to measure the magnitude of the nonlinear absorptive effect.
We can calculate the Nonlinear absorption coefficient ˇ from
the data of open-aperture Z-scan curves for the samples. The Nor-
malized transmittance curve could be expressed as:
z
3/2[1 − T(0)]
I(0)Leff
ˇ =
(3.7)
in which T(0) was the open aperture transmittance at z = 0 point. In
the case of the initial linear transmittance of the sample was 75%,
we can fit and calculate the experimental data.
The nonlinear absorptive and refractive coefficients were
related to the real part of the third order susceptibility through
the below equation:
ꢀ(R3)(MKS) = 2n2ε0cꢁ
(3.8)
0
In Eq. (3.8), c was the light velocity, and ε0 = 10−9/36ꢈ was the
free space permittivity.
The nonlinear absorptive coefficient ˇ was related to the imag-
inary part of the third order susceptibility through the followed
function.
n20ε0c2
ω
ꢀ(I3)(MKS) =
ˇ
(3.9)
∞
m
ꢁ
[−q0(z, 0)]
T(z, S = 1) =
m = 0, 1
(3.4)
where n0 was the refraction index of solvent, c was the light veloc-
ity, and ε0 = 10−9/36ꢈ was the free space permittivity.
(m + 1)3/2
m=0

72
N. Qiu et al. / Journal of Photochemistry and Photobiology A: Chemistry 272 (2013) 65–72
Table 1
4. Conclusion
Nonlinear optical parameters of porphyrin, zincporphyrin, ZnPor-Br and P1 as eval-
uated by Z-scan.a
In summary, we synthesized a well-defined end-functionalized
polymethyl methacrylate incorporated with an asymmetrical por-
phyrin group via atom transfer radical polymerization (ATRP)
with controlled molecular weights and narrow polydispersity. The
method showed a potential to resolve the phenomenon of the
uneven dispersedness and agglomeration of porphyrins in poly-
mer arising from the mixed method. In addition, the solubility
and concentration of porphyrins in polymer could be improved
significantly, which benefited the properties of optical limiting.
The Z-scan test results indicated that the end-functionalized poly-
methyl methacrylate with the asymmetrical porphyrin group
(ZnPor-PMMA) showed large nonlinear optical limiting properties.
As a result, the target polymer could act as a significant and promis-
ing candidate for the nonlinear optical materials.
Zincporphyrin
ZnPor-Br
P1b
n2 (esu)
18.9 × 10−10
580 × 10−11
55 × 10−17
5.2 × 10−10
680 × 10−11
15 × 10−17
4.1 × 10−10
400 × 10−11
12 × 10−17
ˇ (m2/W)
ꢁ (m2/W)
ꢀ(3) (esu)
ꢁꢀ (esu)
4.76 × 10−10
2.1 × 10−28
2.57 × 10−10
1.64 × 10−10
1.39 × 10−28
1.01 × 10−28
a
Z-scan measurements using a Q-switched, mode-locked Nd3+:YAG laser source
(ꢄ = 532 nm) delivering 21 ps-duration (FWHM) pulses at a 1 Hz repetition fre-
quency.
b
P1: the number-average molecular weight of ZnPor-PMMA is 2000.
The absolute value of ꢀ(3) could be obtained by:
ꢂ
ꢀ
ꢀ
ꢀ
2
(3)
ꢀ
ꢀ
=
(ꢀ(R3)
)
2 + (ꢀ(I3)
)
(3.10)
The molecular polarizability (ꢁꢀ) was also used to quantify the
Acknowledgements
nonlinear absorption and it was related to ꢀ(3) through the equa-
We are grateful to Jilin Science & Technology Department
(20070556, 20090326, 20070508), Science and Technology Bureau
of Changchun City project (2008280) Foundation for Strategical
Research for financial support. The authors would like to thank all
reviewers of this article for their comments and suggestions.
tion:
ꢀ
ꢀ
ꢀ
ꢀ
(3)
ꢀ ꢀ
ꢀ
(esu)
ꢀ
ꢀ ꢀ
ꢁ
(esu) =
(3.11)
NL4
where N was the number of molecules in 1 mL solution, L = (n02
+
2)/3, and n0 was the refraction index of the solvent. All the calcula-
tive results of the nonlinear absorption and refraction were showed
in Tables 1 and 2.From Fig. 8 and Table 1, it could be seen that the
third order susceptibility (ꢀ(3)) and molecular polarizability (␥ꢀ) of
P1 diminish marginally which arise from the deformability of the
porphyrin declined owing to the obstructing and bundle of the long
chain segments (PMMA). However, the reduced magnitude seemed
to be minor relative to that of the mixed method. In general, it
could be concluded that P1 has the favourable third-order nonlin-
ear optical limiting properties. In order to study the relationship
between the molecular weight of polymers and optical limiting
effect, we performed the Z-scan measurements on the number-
average molecular weight are 4400 and 6700 respectively, the
results showed in Fig. 9 and Table 2.From Fig. 9 and Table 2, we could
see that the third order nonlinear optical limiting properties of the
polymer showed a close relationship with the molecular weight.
While the molecular weight of the polymer increased, the non-
linear refraction diminished gradually. Finally the phenomenon of
nonlinear refraction disappeared when the molecular weight of the
polymer achieved 6700, which could be attributed to that the non-
polar segments of PMMA led to the polarizability of the molecule
and the deformability of the porphyrin declined. As a result, the
nonlinear refractive index decreased until the phenomenon of non-
linear refractive disappeared.
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c