Novel axially substituted lanthanum phthalocyanines: Synthesis, photophysical and nonlinear optical properties

Article abstract of DOI:10.1016/j.dyepig.2020.108407

Materials with excellent nonlinear optical (NLO) properties are usually applied in optoelectronics and optical limiting devices. Photoinduced intramolecular electron transfer (PET) and energy transfer (ET) play an important role in enhanced NLO properties

Full text of DOI:10.1016/j.dyepig.2020.108407

Dyes and Pigments 179 (2020) 108407
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Novel axially substituted lanthanum phthalocyanines: Synthesis,
photophysical and nonlinear optical properties
,
,*
Bolong Lia, Zengduo Cuia, Yuping Hanb **, Jiale Dinga, Zhenhua Jianga, Yunhe Zhanga
a Engineering Research Center of Super Engineering Plastics, Ministry of Education, College of Chemistry, Jilin University, Changchun, 130012, PR China
b Depantment of Urology, China-Japan Union Hospital of Jilin University, Chang Chun, 130012, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Materials with excellent nonlinear optical (NLO) properties are usually applied in optoelectronics and optical
limiting devices. Photoinduced intramolecular electron transfer (PET) and energy transfer (ET) play an important
role in enhanced NLO properties. Through axially connected anthraquinone to phthalocyanine, two types of
novel lanthanum phthalocyanine (LaPc-Aqn and HLaPc-Aqn) were designed to effectively improve PET/ET
process, and thus leading to excellent NLO properties. Hyperbranched phthalocyanines possess enhanced NLO
properties owing to their special three-dimensional structures that have more donor–acceptor (D–A) systems in
their cell structure than lanthanum phthalocyanines. The uniform polyphenylsulfone composite film(HLaPc-Aqn-
PPSU) was prepared by a simple solution casting method according to the “like dissolves like” theory. The values
Phthalocyanine
Anthraquinone
Polyphenylsulfone
Nonlinear optics
Optical limiting
Electron transfer
of large third-order nonlinear susceptibility (Im[
χ
(3)]) and low limit threshold (Ilim) were 1.51 � 10À 9 esu and
0.015 J/cm2, respectively, demonstrating that the film had great optical limiting performance. Such research can
contribute to the development of novel phthalocyanine materials for nonlinear optics and optical limiting
devices.
1. Introduction
the intersystem crossing (ISC) between singlet states and triplet states
through enhancing spin-orbit coupling [9]. Chen et al. synthesized a
Nonlinear optical (NLO) materials have attracted great attention due
to their applications in photonics, telecommunication systems, data
processing, and optical power limiting (OPL) [1–3]. Organic materials
with fast response time and large optical nonlinearities have attracted
growing attention, compared with a variety of other promising materials
with outstanding NLO performance. Such organic materials effectively
attenuate laser pulses to protect human eyes and optical components
from exposure to intense light beams [4,5]. In recent years, metal
phthalocyanines have been widely studied owing to their 18-electron
conjugated macrocycle structure, photoelectric properties, and chemi-
cal and thermal stability. Meanwhile, several efforts have been made to
successfully regulate the photophysical and NLO properties of phthalo-
series of hyperbranched phthalocyanines with different substituents,
and the Z-scan result indicated that NLO properties could be tailored
further
by
placing
substituents
[7].
In
addition,
phthalocyanine-modified carbon-based nanomaterials have been
extensively studied owing to their multiple optical limiting mechanisms
[10,11]. Wang et al. synthesized a series of graphene hybrid materials
that have been demonstrated to show a superior NLO response origi-
nating from the contribution of multiple optical limiting mechanisms
[12]. Nevertheless, poor dispersion stability and machinability of the
hybrid materials has restricted their prospective application in OPL
devices. Moreover, it has been suggested that lanthanide double-decker
phthalocyanines show modified NLO properties owing to their expanded
cyanine through metal coordination, substitutional effects and
π-con-
π electron system and the presence of the heavy lanthanide central rare
jugate size [6–8].
earth atom [13]. However, the extensive study and further application
of double-decker phthalocyanines have been restricted because of the
difficulty in preparation and low yield.
The tuning of the NLO properties of phthalocyanines (Pcs) can be
achieved by varying the centric atoms and placing substituents on the Pc
ring. Previous studies have shown that Pcs could be coordinated with
rare earth atoms such as lanthanum, holmium and ytterbium, helping
Another important representative of organic NLO materials,
anthraquinone dye that possesses a highly delocalized π-electron system
* Corresponding author.
** Corresponding author
E-mail addresses: blli17@mails.jlu.edu.cn, 1545855940@qq.com (Y. Han), zhangyunhe@jlu.edu.cn (Y. Zhang).
https://doi.org/10.1016/j.dyepig.2020.108407
Received 2 January 2020; Received in revised form 26 March 2020; Accepted 26 March 2020
Available online 4 April 2020
0143-7208/© 2020 Elsevier Ltd. All rights reserved.

B. Li et al.
Dyes and Pigments 179 (2020) 108407
in its nucleus [14]. As typical electron-acceptor, anthraquinones contain
various types of chemical reaction sites that are widely used to modify
material structures [15,16]. Pcs as a good electron donor can form a
donor–acceptor (D–A) structure with anthraquinone through covalent
connection. Fluorescene quenching and effective charge transfers in
molecules caused by formation of D–A structure exert important sig-
nificance for improving NLO properties [10,17].
obtained on a FEI Nova Nano 450 field mission SEM system and samples
were aurum coated. X-ray photoelectron spectra was measured on PHI-
1600 X-ray photoelectron spectrometer using Mg Kα (1253.6 eV) radi-
ation as the radiation source. Fluorescence spectra were recorded on a
FLS 920 steady-transient spectrophotometer with a time-correlated
single-photon counting system using a 400 nm laser source. The elec-
trochemical data were obtained from Bio-Logic SP-150 electrochemical
work station.
For practical application, Pcs or other organic materials are often
embedded in transparent polymers and the polymers could be used as
substrates to turn materials into optical limiters [16]. Solid state optical
limiters expand the use of materials and facilitate transportation. In
addition, embedding the Pcs within a polymer thin film for OPL appli-
cations adds some protection to the Pcs against degradation [18]. Sakai
Y et al. prepared polymer optics material through the covalent bonding
of phthalocyanine and polymer [19]. However, the compounds exhibit
low grafting ratio, which was a major obstacle to their application.
Sekhosana K E et al. directly and physically mixed double-layer phtha-
locyanine materials into poly(methyl methacrylate) (PMMA) to prepare
films, which presented excellent nonlinear properties and had a simple
preparation process [13]. However, the phthalocyanines in the com-
posite films prepared by this method cannot be uniformly dispersed,
affecting the optical properties of the films.
2.3. Synthesis
2.3.1. Synthesis of 4-phenoxyphthalonitrile (1), Scheme 1
First, the solution of 4- nitrophthalonitrile (3.46 g, 20 mmol) and
phenol (1.88 g, 20 mmol) in 50 mL anhydrous N, N-Dimethylformamide
(DMF) was stirred for 30 min under N2. Then, dry potassium carbonate
(K2CO3) (5.52 g, 40 mmol) was added into above mixture products and
stirred for 6 h at 80 �C. After cooling to room temperature, the mixture
was poured into a large amount of deionized water. The reaction
product was collected by filtration, and the solid was washed with
deionized water until the filtrate became neutral, and crystallized by
acetonitrile. Compound 1 was prepared according to previously re-
ported methods [20]. Yield: 3.56 g (81%). FT-IR (KBr, cmÀ 1): 2233,
1564, 1421, 1251, 1212, 1156, 1075, 872, 784, 694, 523. 1H NMR
(DMSO‑d6 6) δ 8.12 (d, 1H), 7.81 (d, J ¼ 2.6 Hz, 1H), 7.56–7.47 (m, 2H),
7.41–7.30 (m, 2H), 7.22 (dd, J ¼ 9.9, 2.2 Hz, 2H) ppm. The detailed
spectra of compound 1 are listed in the Supporting Information (Fig. S1
and Fig. S2).
In this work, two kinds of phthalocyanines, phenyl lanthanum
phthalocyanine (compound 2) and hyperbranched lanthanum phthalo-
cyanine (compound 5), were prepared. Next, lanthanum phthalocyanine
derivatives (compounds 3 and 6) were obtained by axially connecting
anthraquinone to compounds 2 and 5, respectively. Open aperture (OA)
Z-scan technique was used to investigate the NLO performance of the
samples. Because of the extended π-electron system and the formation of
a D–A structure, the phthalocyanines axially bonded with anthraqui-
none, resulting in an effective Photoinduced intramolecular electron
transfer (PET)/energy transfer (ET) process and excellent NLO perfor-
mance. To facilitate practical application, compounds 5 and 6 were
embedded in polyphenylsulfone(PPSU) films. The PPSU composite films
possessed stronger NLO properties than solutions. As a kind of special
engineering plastic, PPSU features good thermal stability, superior heat
deflection temperature and flame resistance. Based on the principle of
“like dissolves like”, hyperbranched phthalocyanine and its derivatives
can be well-dispersed with PPSU to form uniform and stable films.
2.3.2. Synthesis of lanthanum(III) 2,9,16,23-tetrakis (4- phenoxy)
phthalocyanine chlorine (2), Scheme 1
First, the solution of compound 1 (400 mg, 1.82 mmol) and LaCl3
(147.2 mg, 0.6 mmol) in n-pentanol (30 mL) was stirred for 30 min
under N2. Then, 0.5 mL of 1, 8-Diazabicyclo[5.4.0]undec-7-ene (DBU,
3.35 mmol) was added into above mixture products and was stirred for
8 h at 140 �C [9]. After cooling to room temperature, the product was
precipitated out of solution by methanol. The reaction product was
collected by filtration, and the solid was washed with methanol,
deionized water in turn. Yield: 0.259 g (54%). FT-IR (KBr, cmÀ 1): 1650,
1563, 1412, 1233,1154, 1090, 1015, 944, 724. 1H NMR (DMSO‑d6) δ
7.99–7.65 (m, 5H), 7.64–7.54 (m, 4H), 7.53–7.31 (m, 10H), 7.31–7.04
(m,10H), 7.03–6.87 (m, 3H) ppm. MALDI-TOF-MS (m/z): calcd for
2. Experimental section
C
56H32ClN8O4: 1054.13; found:1054.2 (Mþ), 1019.3 (Mþ–Cl). elemental
2.1. Materials
analysis calcd (%) for C56H32ClN8O4La: C 63.74, H 3.06, N 10.62; found:
C 64.02, H 3.04, N 10.36. The detailed spectra of compound 2 are listed
in the Supporting Information (Fig. S1 and Fig. S3).
4-Nitrophtalonitrile, phenol, 4,40-dihydroxydiphenylsulfone, 1-
hydroxyanthraquinone, N, N-Dimethylformamide (DMF), lanthanum
chloride hydrate(LaCl3), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), n-
pentanol, acetone, methanol and silver triflate were purchased from
Aladdin Industry Company and used without further purification. PPSU
was purchased from SOLVAY (model: RADEL R-5000) and used directly.
2.3.3. Synthesis of lanthanum(III) 2,9,16,23-tetrakis (4- phenoxy)
phthalocyanine derivative (3), Scheme 1
First, the solution of compound 2 (211 mg,0.2 mmol) and silver
triflate (0.1 g) in anhydrous DMF (35 mL) was stirred for 12 h under N2.
Then 1-hydroxyanthraquinone (67.2 mg, 0.3 mmol) in anhydrous DMF
(15 mL) was added into above mixture products and stirred for 24 h at
60 �C under N2. The reaction mixture was poured into a large amount of
deionized water. The product was collected by filtration and washed
several times with acetone and deionized water to remove the excess
reactants. Finally, the blue-green sample was obtained in the vacuum
drying oven [19]. Yield: 0.192 g (74%). FT-IR (KBr, cmÀ 1):1650, 1629,
1585, 1473, 1419, 1227, 1156, 1074, 1010, 937, 879, 831, 723, 694,
m/z ¼ 1242. 1H NMR (DMSO‑d6 6) δ 8.21–8.28 (m, 2H), 7.99–7.65 (m,
9H), 7.64–7.54 (m, 4H), 7.53–7.38 (m, 11H), 7.31–7.04 (m, 10H),
7.03–6.88 (m,3H) ppm. MALDI-TOF-MS (m/z): calcd for C70H39N8O7La:
1242.20; found: 1242.2 (Mþ). elemental analysis calcd (%) for
2.2. Instruments
IR spectra (KBr pellets) were recorded on a Nicolet iS10 FTIR spec-
trophotometer. The 1H NMR spectra were recorded on a Bruker
AVANCEIII600 instrument with dimethylsulfoxide-d 6 (DMSO‑d6) as the
solvent. Gel permeation chromatograms (GPC) employing polystyrene
as a standard were obtained with an Agilent PL-GPC220 instrument with
DMF as the eluent at a flow rate of 0.1 mL minÀ 1. UV–vis absorption
spectra were performed on UV2501-PC spectrophotometer. All MALDI-
TOF-MS spectra were measured on a Brucker Autoflex speed TOF/TOF
mass spectrometer. The nonlinear optical measurements were per-
formed using an open aperture Z-scan technique employing a Q-
switched Nd: YAG laser of 6 ns pulses at 532 nm with a repetition of 10
Hz. All liquid samples were placed in a 5 mm quartz cell and moved
along the Z direction. Scanning electron microscopy (SEM) images were
C
70H39N8O7La: C 67.64, H 3.16, N 9.01; found: C 67.39, H 3.18, N 8.76.
The detailed spectra of compound 3 are listed in the Supporting Infor-
mation (Fig. S1 and Fig. S4).
2

B. Li et al.
Dyes and Pigments 179 (2020) 108407
Scheme 1. Synthetic route of compounds 2 and 3.
2.3.4. Synthesis of bis-[4-(3,4-dicyanophenoxy)phenyl]sulfone (4),
Scheme 2
2.3.6. Synthesis of hyperbranched lanthanum (III) phtalocyanine derivative
(6), Scheme 2
First, the solution of 4- nitrophthalonitrile (6.92 g, 40 mmol) and
4,40-Sulfonyldiphenol (5.01 g, 20 mmol) in 50 mL anhydrous DMF was
stirred for 30 min under N2. Then, dry potassium carbonate (K2CO3)
(11.04 g, 80 mmol) was added into above mixture products and stirred
for 6 h at 80 �C. After cooling to room temperature, the mixture was
poured into a large amount of deionized water. The reaction product
was collected by filtration, and the solid was washed with deionized
water until the filtrate become neutral, and crystallized using acetoni-
trile. Compound 4 was prepared according to previously reported
methods [21]. Yield: 8.14 g (81%). FT-IR (KBr, cmÀ 1): 2236, 1578,
1484, 1313, 1250, 1151, 953, 682. 1H NMR (DMSO‑d6 6) δ 8.18 (d, J ¼
8.7 Hz, 1H), 8.07 (d, J ¼ 8.8 Hz, 2H), 8.02 (d, J ¼ 2.5 Hz, 1H), 7.62 (dd,
J ¼ 8.7, 2.6 Hz, 1H), 7.37 (d, J ¼ 8.8 Hz, 2H) ppm. The detailed spectra
of compound 4 is listed in the Supporting Information (Fig. S5 and
Fig. S6).
First, the solution of compound 5 (330 mg) and silver triflate (0.1 g)
in anhydrous DMF (35 mL) was stirred for 12 h under N2. Then 1-
hydroxyanthraquinone (79 mg, 0.35 mmol) in anhydrous DMF (15
mL) was added into above mixture products and stirred for 24 h at 60 �C.
The reaction mixture was poured into a large amount of deionized
water. The product was collected by filtration and washed several times
with acetone and deionized water to remove the excess reactants [19].
Finally, the dark green solid was filtered and dried in vacuum, affording
hyperbranched lanthanum phthalocyanine powder was recorded as
compound 6. Yield: 0.231 g (68%). FT-IR (KBr, cmÀ 1): 2236, 1647,
1633, 1582, 1483, 1230, 1151, 1105, 835, 777, 724, 682. 1H NMR
(DMSO‑d6 6) δ 8.48–7.87 (m, 16H), 7.86–7.70 (m, 6H), 7.70–7.04 (m,
11H), 7.04–6.60 (m, 4H) ppm. GPC: Mn, 10600; Polydispersity, 1.20.
The detailed spectra of compound 6 is listed in the Supporting Infor-
mation (Fig. S5, Fig. S7 and Fig. S9).
2.3.5. Synthesis of hyperbranched lanthanum (III) phtalocyanine (5),
Scheme 2
2.4. Fabrication of composite films
First, the solution of compound 4 (2.259 g, 4.5 mmol) and anhydrous
LaCl3 (557 mg, 1.5 mmol) in 40 mL anhydrous DMF and 7 mL n-pen-
tanol were stirred for 30 min under N2. Then, 0.5 ml of DBU (3.35 mmol)
was added into above mixture and stirred for 8 h at 140 �C. After cooling
to room temperature, the product was precipitated out of solution by
methanol. The reaction product was collected by filtration, and the solid
was washed with methanol, deionized water in turn [9]. The dark green
solid was filtered and dried in vacuum, affording hyperbranched
lanthanum phthalocyanine powder was recorded as compound 5. Yield:
1.64 g (71%). FT-IR (KBr, cmÀ 1): 2234, 1580, 1481, 1299, 1230, 1105,
1099, 835, 724, 683. 1H NMR (DMSO‑d6 6) δ 8.47–7.85 (m, 12H),
7.84–7.71 (m, 4H), 7.71–7.06 (m, 10H), 7.06–7.61 (m, 4H) ppm. GPC:
Mn, 10100; Polydispersity, 1.13. The detailed spectra of compound 5 is
listed in the Supporting Information (Fig. S5, Fig. S7 and Fig. S8).
Briefly, PPSU (0.3 g) and 6 mg of compounds 2, 3, 5, or 6 were,
respectively, dissolved in NMP (12 mL) and stirred for 6 h until homo-
geneous mixtures of Pc-polymer solution were obtained. Drop the so-
lution on the 8 � 8 cm glass plate, and then bake the glass plate
containing the sample polymer solution in a vacuum oven at 80 �C for 8
h to completely evaporate NMP to obtain the film sample. A series of
films with the thickness of 25 μm were obtained. The prepared thin films
are represented as 2-PPSU, 3-PPSU, 5-PPSU and 6-PPSU, respectively.
3. Results and discussions
3.1. Synthesis and characterization
The procedure in Scheme 1 is a synthesis of 4-phenoxyphthalonitrile.
Compound 2 was prepared by the condensation of 4-phenoxyphthaloni-
trile and anhydrous LaCl3. 1-Hydroxyanthraquinone was attached to
compound 2 through axial covalent bonding under the conditions
3

B. Li et al.
Dyes and Pigments 179 (2020) 108407
Scheme 2. Synthetic route of compounds 5 and 6.
catalyzed by silver triflate to synthesize compound 3. The process for the
preparation of the compounds 5 and 6 are illustrated in the same method
as shown in Scheme 2. Fig. 1 shows the 1H NMR spectra of 1-hydroxyan-
thraquinone(Aqn), 2 and 3. Compared with compound 2, we found that
compound 3 exhibited the new bands at approximately 8.21–8.28 ppm
and 7.99–7.65 ppm, which were attributed to the H of the anthraqui-
none groups. Meanwhile, the peak on the hydroxyl hydrogen atom of
anthraquinone disappeared at 12.45 ppm, which proved the successful
synthesis of compound 3.
does not exist in the phthalocyanine derivatives, was observed in com-
pounds 3 and 6; this indicates that Cl was substituted. Fig. 2b–e depicts
the high-resolution XPS spectra of C 1s. The spectrum of C 1s can be
fitted to four peaks, C-C at 284.5 eV, C-N at 285.8 eV, C¼O at 288.2 eV
and C-O at 288.4 eV, corresponding to different carbon species. For the
spectra of compounds 2 and 5, a species corresponding to C¼O was not
found, while anthraquinone in compounds 3 and 6 corresponding to
C¼O species was found at 288.2 eV. The relevant FT-IR, 1H NMR, GPC
and MALDI-TOF-MS spectra are shown in Fig. S1-S9. The image of the
solution sample is shown in Fig. S10. All results mentioned above
confirmed that compounds 2, 3, 5 and 6 were successfully synthesized.
The XPS spectra of compounds 2, 3, 5 and 6 are shown in Fig. 2. As
shown in Fig. 2a, there were three peaks near 284 eV, 399 eV and 532
eV, corresponding to C 1s, N 1s and O 1s species, respectively [11]. The
peak of La 4 d was observed at 97.6 eV corresponding to the orbit d of La,
and two peaks were observed at 836 eV and 853 eV, corresponding to
3d5/2 and 3d3/2 species, respectively, of La. However, compared with
compounds 2 and 5, no peak related to Cl 2p, which is an element that
3.2. Photophysical parameters
Fig. 3 shows the UV-Vis absorption spectra of the samples in the DMF
solution. In UV–Vis spectra, all samples exhibited typical
4

B. Li et al.
Dyes and Pigments 179 (2020) 108407
phthalocyanine spectral behavior with two characteristic absorption
peaks, one in the UV region of 300–400 nm (B band), another in the
visible region of about 600–700 nm (Q band) due to the π-π* transition
[22,23]. Compound 2 had two narrow peaks in the Q band of the
spectrum at 667 and 699 nm (Fig. 3a), owing to the oxygen in the
phenoxy group in one phthalocyanine structure liable to be coordinated
with La3þ of another phthalocyanine molecule to form Pc aggregates
[24,25]. The structure of compound 3 increased the distance between
the two molecules, which destroyed the structural features of the for-
mation of aggregates. Reduced aggregation was known to positively
affect photophysical parameters [26,27]. The absorption of compounds
5 and 6 are shown in Fig. 3b. Compound 5 had a hyperbranched
structure with a highly branched three-dimensional molecular structure,
which had difficulty in forming aggregates. Compound 6 showed B
bands appearing in the region of 319–406 nm, and the Q band appeared
in the region of 570–730 nm. These observations suggested the axial
connection of the oxime group increaseed the conjugated system of the
phthalocyanine, resulting in a change in its electronic state.
Given that the as-prepared phthalocyanine derivatives contained
electron donor (Pc) and acceptor (Aqn) units, we studied the photo-
induced intramolecular events by steady-state fluorescence spectra,
and the results are shown in Fig. 4a. The efficient PET/ET played a
crucial role in nonlinear optics [11,28]. At the excitation wavelength of
610 nm, the maximum emission peaks of compounds 2, 3, 5 and 6
correspond to approximately 706 nm, 709 nm, 689 nm and 696 nm,
respectively. It was clear that fluorescence quenching had taken place in
compounds 3 and 6 relative to that of compounds 2 and 5. This was due
to the PET between the anthraquinone and the phthalocyanine ring,
which meant the electron mobility of the entire phthalocyanine ring
increased, and thus the molecule underwent fluorescence quenching
[16,29]. The symmetry of the whole electron cloud was decreased after
being axially connected, which facilitated the flow of the electron cloud.
According to Fig. 4b and Table 1, the fluorescence lifetime(τF) and
fluorescence quantum yield(ΦF) of compound 3 was lower than those of
compound 2. Compared with compound 3, the introduction of anthra-
quinone group in compound 6 has less effect on the fluorescence
quantum yield, making the fluorescence quantum yield decreased
slightly.
Fig. 1. 1H NMR spectra of 1-hydroxyanthraquinone (Aqn), compound 2 and
compound 3.
The energy level model was used to describe the interaction of light
with the phthalocyanines in terms of PET/ET process (Fig.S11). There
were several competing processes that could occur at different energy
levels. However, another process usually happened from S1 state to
charge separated state (CSS) by the PET when the D–A structure existed
in the materials [11]. In addition, the fluorescence lifetime (τF) of
compounds 3 and 6 were lower than those of compounds 2 and 5
(Table 1), respectively, suggesting that axially substituted phthalocya-
nines possessed a higher energy level of the S1 state than the unmodified
lanthanum phthalocyanines. Therefore, the energy gap in axially
substituted phthalocyanines between the S1 state and theT1 state was
much lower than that of the unmodified lanthanum phthalocyanines,
resulting in a higher value of the nonlinear absorption effect [30].
3.3. Nonlinear optical properties
To further study the OL performance of compounds 2, 3, 5 and 6 OA
Z-scan measurements were performed. All the samples were dispersed in
DMF at a concentration of 0.3 mg/mL. The Q-switched Nd:YAG laser
provided a 4 ns laser pulse at 532 nm of second harmonic, with a
repetition rate of 10 Hz and a pulse energy of 200 μJ. The transmitted
beam of all samples along the Z position decreased, indicating that the
sample exhibited positive nonlinear absorption, the shape of the Z-scan
signal indicated that the samples had effective nonlinear optical
parameters.
The normalized transmittances of compounds 2, 3, 5 and 6 at the
focus (maximum input flux) were reduced to 0.93, 0.90, 0.87, and 0.79,
respectively. When the sample moved to focus, compound 5 exhibited a
Fig. 2. XPS spectra of compounds 2, 3, 5 and 6.
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B. Li et al.
Dyes and Pigments 179 (2020) 108407
Fig. 3. UV–vis absorption spectra of compounds 2, 3, 5 and 6 in DMF at concentration 2.5 � 10À 5 M.
Fig. 4. Fluorescence spectra (a) and Fluorescence decay curves (b) of compounds 2, 3, 5 and 6 in DMF at concentration 2.5 � 10À 5 M.
Table 1
Fluorescence properties and nonlinear optical parameters of compounds 2, 3, 5
and 6 in DMF.
2
3
5
6
λmax(nm)
ΦF
706
709
689
696
0.17
0.11
0.06
0.05
τF (ns)
5.32
5.26
5.11
4.87
I0
(
μ
J)
200
200
200
200
LT (%)
80
80
84
76
α0 (cmÀ 1
)
0.43
0.44
0.34
0.55
βeff (cm GWÀ 1
Im[χ(3)](esu)
)
0.43
0.51
0.60
1.52
1.41 � 10À 13
1.67 � 10À 13
1.97 � 10À 13
4.99 � 10À 13
more prominent two-photon absorption (TPA) processes than com-
pound 2. Furthermore, the nonlinear absorption coefficients of two
axially-connected phthalocyanines were larger than phthalocyanines in
Fig. 5. It was clear that the compounds 3 and 6 exhibit much better NLO
performances than compounds 2 and 5, which meant that the intro-
duction of the anthraquinone enhanced the NLO response of phthalo-
cyanines. The corresponding parameters were obtained by fitting the
numerical data, and the results are shown in Table 1. The nonlinear
absorption coefficient βeff (cm GWÀ 1) could be calculated as equation (1)
through the experimental data:
Fig. 5. Open-aperture Z-scan curves of compounds 2, 3, 5 and 6 in DMF at
532 nm.
TOA refers to the measured hole transmittance, and Leff is the effec-
h
.�
�i
tive thickness of the sample. Leff ¼ [1-exp(-αL)]/α, α is the linear ab-
m
1 þ ðz=z0Þ2
∞
À βeffI0Leff
X
sorption rate of the sample. I0 is the axial light intensity of the samples
and z is the position of the sample. z0 (z0 ¼ πω20/λ) is the Rayleigh range,
where ω0 is the beam waist at focal point (z ¼ 0) and λ is the wavelength
TOA
¼
(1)
ðm þ 1Þ3=2
m¼0
6

B. Li et al.
Dyes and Pigments 179 (2020) 108407
of incident light [31,32].
expanded flow range of electrons, resulting in the improvement of NLO
properties of phthalocyanines [38]. Compared with compound 3
(0.550), compound 6(0.587) had a larger discrepancy ΔE between the
LUMO level of phthalocyanine and anthraquinone. Thus, large LUMO
levels of phthalocyanines were thought to be the important factors for
the enhanced NLO effect. The LUMO energy level further confirmed that
ET can occur between phthalocyanine moieties acting as electron donor
and anthraquinone moiety acting as the electron acceptor [39].
The third-order nonlinear susceptibility Im[
χ
(3)] is a parameter of the
firmness of a molecular material against perturbation caused by a laser
pulse, and is expressed in terms of βeff using equation (2) [33]:
�
�
�
Im χð3Þ ¼ n20ε0cλβeff
2π
(2)
where n0 is the linear refractive index, c is the speed of light in vacuum,
and ε0 is the permittivity of a vacuum. According to the definition of Im
[
χ
(3)], the change rule of Im[
χ
(3)] under the same measurement condi-
3.5. Optical limiting properties of composite films
tions is consistent with that of βeff [34]. Therefore, the larger the value of
Im[
χ
(3)], the better the nonlinear optical absorption performance of the
From a practical application point of view, an effective way to get
optical limiting devices that can be easily transported and stored is to
realize the transformation of NLO properties of materials from organic
compound solutions to solid films. Previous studies have noted that the
phthalocyanines embedded in polymer as films have better optical
limiting performance than phthalocyanine solution [40]. The uniform
dispersion of phthalocyanine in polymer films is important for stable
performance of optical limiting properties. The hyperbranched phtha-
locyanines can be uniformly dispersed in the composite film, and the
reduction of the aggregate can ensure that the material effectively plays
the optical limiting effect. When laser irradiated, the uniform composite
films ensure that each part of the film has the same nonlinear optical
response. In addition, reducing the aggregation also helps to improve
the PET/ET process of phthalocyanine and further enhance nonlinear
optical performance. The optical microscopy images of composite films
are shown in Fig. 6. For the sake of observation, the composite films
were placed on the paper with text. Clearly, compounds 2 and 3 had
aggregation tendencies in thin films. Compounds 5 and 6 were
well-dispersed in PPSU films, and no obvious inhomogeneity was
observed. Hyperbranched phthalocyanines and PPSU both had sulfone
groups in their structures. The highly similar structures made for
excellent uniform films. Based on the “like dissolves like” theory, com-
pounds 5, and 6 significantly improved their homogeneity and disper-
sion stability in the polymer-based materials.
material.
As manifested in Table 1, compound 6 shows superior NLO proper-
ties to other samples, and its βeff reached 1.52 cm GWÀ 1, which was 2.5
times higher than that of compound 5. In particular, the βeff of com-
pound 3 was 1.2 times larger than that of compound 2. Compounds 3
and 6 exhibited a modified NLO performance in comparison with that of
compounds 2 and 5, which further proved the existence of a cumulative
effect due to the axial bonding between anthraquinone and Pcs. The Im
[
χ
(3)] of compounds 2, 3, 5 and 6 reached 1.41 � 10À 13 esu, 1.67 �
10À 13 esu, 1.97 � 10À 13 esu and 4.99 � 10À 13 esu, respectively
(Table 1). The anthraquinone in compounds 3 and 6 could reduce the
symmetry of the electron cloud system and facilitate its flow [35]. It is
clear that βeff of compound 6 was higher than that of compound 3 for the
following reasons. The introduction of the anthraquinone group as the
electron acceptor can form D–A structure with every phthalocyanine
ring in compound 6, thus compound 6 has several D–A systems in a cell
structure, which promotes the effective PET/ET process. Furthermore,
the anthraquinone axial connection of phthalocyanines could increase
the electron cloud density and show obvious fluorescence decay, which
would be beneficial for the electron transfer process. It has been verified
by fluorescence spectra and fluorescence quantum yield data.
3.4. Electrochemical properties
To further observe the micro-morphology of the films, Fig. 7 and
Fig. S13 display the SEM images of 2-PPSU, 3-PPSU, 5-PPSU and 6-
PPSU. Apparently, there was a significant dispersion difference wherein
a certain amount of compound 2 deposition occurred in 2-PPSU, and a
significant amount of agglomerated particles were deposited on one side
of the 2-PPSU (Fig. 7a). The deposition of 3-PPSU was also obvious
(Fig. 7b). In contrast, 5-PPSU and 6-PPSU were dispersed evenly in the
films (Fig. 7c and d), and no obvious precipitation area was observed
due to the distinct of hyperbranched structure. The hyperbranched
phthalocyanine and its derivatives had a sulfone group structure, the
PPSU polymer also had this structure, which allowd compounds 5 and 6
to be well-dispersed in the PPSU films and to show good optical
properties.
In order to gain insight into the effect on the NLO properties due to
axially connecting anthraquinone to phthalocyanine, the molecular
orbital energy levels of phthalocyanine and anthraquinone were ob-
tained and compared. According to the cyclic voltammetry spectra
(Fig. S12) and UV–vis spectrum (Fig. 3), the lowest unoccupied molec-
ular orbital level of compounds 2 and 3 were obtained by using equation
(3) [36].
ELUMOðeVÞ ¼ À eð4:8 À EFC þ EOXÞ þ Eg
(3)
where Eg is the HOMO–LUMO gap of Pc; Eg ¼ hc/λ ¼ 1240/λ, λ being the
absorption edge(Table S1); Eox is the onset of oxidation potential of Pc;
EFC ¼ (Eox þ Ered)/2 ¼ 0.5 is the energy level of ferrocene used as a
standard, and the LUMO levels of the anthraquinone as obtained by
The nonlinear optical properties of 5-PPSU and 6-PPSU were studied
under the same conditions as the solution using Z-scan(Fig. 8a). We
place the films sample vertically and clamp them on the mold vertically,
and the mold moves uniformly along the Z axis direction. A significant
decrease in the transmittance of composite films at the focal can be
observed. The linear transmission values of 64% and 59% were
respectively measured for 5-PPSU and 6-PPSU. The NLO properties of
composite films were immensely improved and the βeff of 5-PPSU and 6-
PPSU could reach 2678.29 cm GWÀ 1 and 3542.58 cm GWÀ 1, respec-
tively. The considerable enhancement of NLO performance was derived
from the stabilization of the polymer and the conjugated electrons of a
large number of benzene rings in the PPSU. In addition, 6-PPSU has the
superior NLO performance of compound 6 and uniform dispersion in
PPSU. The OA Z-scan of polymer-based films showed that the intro-
duction of the anthraquinone structure gave the material a greater
normalized transmittance valley depth, indicating that the enhanced
NLO properties stemmed from the intracomplex PET/ET process [38].
Incident intensity threshold (Ilim) is defined as the input strength
E
LUMO (eV) ¼ À e(4.8À EFC þ Ered) [37], Ered being the onset of reduction
potential of anthraquinone.
It is found that the LUMO energy level of anthraquinone (ELUMO
¼
À 3.68 eV) was lower than that of compounds 2 and 5 as shown in
Table 2. The electrons in the LUMO of phthalocyanine could easily
transit to the low-energy LUMO of anthraquinone and led to the
Table 2
Eg, LUMO Energy Levels, and ΔE data of the samples.
Eg(eV)
ELUMO(eV)
ΔE (eV)a
2
1.701
1.736
1.715
1.714
2.672
À 3.13
–
–
3
0.550
5
À 3.10
–
6
–
0.587
Aqn
À 3.68
–
a
ΔE ¼ ELUMO (Pc) À ELUMO (Aqn).
7

B. Li et al.
Dyes and Pigments 179 (2020) 108407
Fig. 6. Optical microscope images of 2-PPSU (a), 3-PPSU (b), 5-PPSU (c), and 6-PPSU (d).
Fig. 7. SEM images of 2-PPSU (a), 3-PPSU (b), 5-PPSU (c) and 6-PPSU (d).
Fig. 8. Open-aperture Z-scan curves(a) and Optical limiting plots of normalized transmittance versus incident fluence (b) for PPSU films at 532 nm.
8

B. Li et al.
Dyes and Pigments 179 (2020) 108407
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when the linear transmittance of the material is 0.5 (50%). Therefore,
the low Ilim signified that the material played a role in optical limitation
field and has broad prospective applications. The optical limiting curve
of the composite material in PPSU thin films is given by plotting the
energy density versus normalized transmittance (Fig. 8b). At low energy
density, the transmittance remained almost constant and did not change
with incident energy density, following Lambert-Beer’s law. At high
energy density (above ~ 0.002 J/cm2), the transmittance decreaseed as
the energy density of the incident light increased and deviated from
linearity. The Ilim value of 5-PPSU and 6-PPSU were 0.019 J/cm2 and
0.015 J/cm2, respectively, which showd excellent optical limiting per-
formance. 6-PPSU demonstrated superior optical limiting properties,
originating from a synergy between the polymer and phthalocyanine
and the excellent nonlinear optical properties of compound 6 itself.
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In this study, we have designed two novel lanthanum phthalocyanine
derivatives modified with anthraquinone though axial substitution. The
fluorescence quenching, short fluorescence lifetime, low fluorescence
quantum yield, and discrepancy of LUMO levels have proved that novel
axially substituted lanthanum phthalocyanines have an effective PET/
ET process. Owing to the accumulation effect of the D–A system, highly
branched structure, and the PET/ET process, hyperbranched lanthanum
phtalocyanine derivative not only exhibited NLO properties superior to
those of unmodified hyperbranch phthalocyanine, but also out-
performed lanthanum phthalocyanine derivative. In addition, the com-
posite films prepared by solution casting exhibited remarkable
uniformity owing to the “like dissolves like” theory. The PPSU-based
composite films possessed excellent NLO properties and optical
limiting effects. Our results suggest that these phthalocyanines and their
composite films show strong NLO properties and optical limiting per-
formance that can make them suitable candidates for optical limiting
devices.
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anthraquinone dyads bridged with a triazine group. Dyes Pigments 2010;85(1–2):
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nonlinearity through axial bonding of pyridine anchored methine-bridged
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Declaration of competing interest
The authors declare no conflict of interest.
[21] Peng X, Sheng H, Guo H, Kimiyoshi N, Yu X, Ding H, Qu X, Zhang Q. Synthesis and
properties of a novel high-temperature diphenyl sulfone-based phthalonitrile
polymer. High Perform Polym 2014;26(7):837–45.
CRediT authorship contribution statement
[22] Arslanoglu Y, Mertsevim A, Hamuryudan E, Gul A. Near-IR absorbing
phthalocyanines. Dyes Pigments 2006;68(2–3):129–32.
Bolong Li: Writing - original draft, Conceptualization, Formal
analysis. Zengduo Cui: Data curation, Writing - original draft. Yuping
Han: Data curation. Jiale Ding: Validation. Zhenhua Jiang: Resources.
[23] Nas A, Kahriman N, Kantekin H, et al. The synthesis of novel unmetallated and
metallated phthalocyanines including (E)-4-(3-cinnamoylphenoxy) groups at the
peripheral positions and photophysicochemical properties of their zinc
phthalocyanine derivatives. Dyes Pigments 2013;99(1):90–8.
[24] Zhang XF, Xi Q, Zhao J. Fluorescent and triplet state photoactive J-type
phthalocyanine nano assemblies: controlled formation and photosensitizing
properties. J Mater Chem 2010;20(32):6726.
Yunhe Zhang: Writing
-
review
&
editing, Conceptualization,
Supervision.
[25] Kameyama K, Morisue M, Satake A, et al. Highly fluorescent self-coordinated
phthalocyanine dimers. Angew.Chemie 2005;117(30):4841–4.
[26] Bian YH, Chen JS, Xu ST, Zhou YL, Zhu LJ, Xiang YZ, Xia DH. The effect of a
hydrogen bond on the supramolecular self-aggregation mode and the extent of
metal-free benzoxazole-substituted phthalocyanines. New J Chem 2015;39:
5750–8.
Acknowledgements
This work was financially supported by National Natural Science
Foundation of China (No. 51973080 and 51573070) and Jilin Province
Science and Technology Development Plan Project (20190302051GX).
[27] Shirk JS, Pong RGS, Flom SR, Heckmann H, Hanack M. Effect of axial substitution
on the optical limiting properties of indium phthalocyanines. J Phys Chem A 2000;
538:1438–49. 104.
Appendix A. Supplementary data
�
�
� �
[28] Torre GDL, Vazquez Purificacion, Agullo-Lopez F, et al. Role of structural factors in
the nonlinear optical properties of phthalocyanines and related compounds. Chem
Rev 2004;104(9):3723–50.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2020.108407.
[29] Qiumei G, Limin C, Sujian P, et al. Morpholinyl dendrimer phthalocyanine:
synthesis, photophysical properties and photoinduced intramolecular electron
transfer. Dalton Trans 2018;10:1039.
[30] Chen J Xu Y, Cao M, et al. Strong reverse saturable absorption effect of a
nonaggregated phthalocyanine-grafted MA-VA polymer. J Mater Chem C 2018;6:
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10