Effects of length and number of aromatic rings in carboxylic acid ligands on structure and optical properties of lead(II) coordination polymers

Article abstract of DOI:10.1007/s11164-017-2960-1

To systematically examine the effects of the length and number of aromatic rings in carboxylic acid ligands on the structure and properties of lead(II) coordination polymers, terephthalic acid (L¹) and 4,4-biphenyl dicarboxylic acid (L²) as primary ligands and 1-(dimethylamino)propan-2-ol (L³) as ancillary ligand were used for preparation of two new lead(II) coordination polymers: [Pb₂L₂ ¹L³(DMF)]_n (1) and [PbL²L³]_n (2). Single-crystal X-ray diffraction analysis was used to determine the crystal structure of the prepared compounds. The results showed that the length of the π-conjugation system of the aromatic carboxylic acid ligands was a primary factor controlling the structure of the coordination compounds. Connecting Pb(II) atoms using ligands L¹ and L³ led to formation of compound 1 with two-dimensional network structure, while use of ligands L³ and L² resulted in formation of compound 2 with zigzag one-dimensional (1D) polymeric network structure. Study of the thermal behavior of the prepared compounds revealed that the structure and type of the ligands in the coordination compounds affected their thermal stability. Investigation of the optical properties of the compounds demonstrated that changing the length of the π-conjugation system of the aromatic carboxylic acid ligands in the structure of the coordination compounds can be used to tune their photophysical properties.

Full text of DOI:10.1007/s11164-017-2960-1

Res Chem Intermed
DOI 10.1007/s11164-017-2960-1
Effects of length and number of aromatic rings
in carboxylic acid ligands on structure and optical
properties of lead(II) coordination polymers
Alireza Hashemzadeh1 Mostafa M. Amini1
•
•
Ezzatollah Najafi2 Hamid Reza Khavasi1
•
Received: 14 January 2017 / Accepted: 18 April 2017
Ó Springer Science+Business Media Dordrecht 2017
Abstract To systematically examine the effects of the length and number of aromatic
rings in carboxylic acid ligands on the structure and properties of lead(II) coordination
polymers, terephthalic acid (L1) and 4,4-biphenyl dicarboxylic acid (L2) as primary
ligands and 1-(dimethylamino)propan-2-ol (L3) as ancillary ligand were used for
preparation of two new lead(II) coordination polymers: [Pb2L12L3(DMF)]n (1) and
[PbL2L3]n (2). Single-crystal X-ray diffraction analysis was used to determine the
crystal structure of the prepared compounds. The results showed that the length of the
p-conjugation system of the aromatic carboxylic acid ligands was a primary factor
controlling the structure of the coordination compounds. Connecting Pb(II) atoms
using ligands L1 and L3 led to formation of compound 1 with two-dimensional net-
work structure, while use of ligands L3 and L2 resulted in formation of compound 2
with zigzag one-dimensional (1D) polymeric network structure. Study of the thermal
behavior of the prepared compounds revealed that the structure and type of the ligands
in the coordination compounds affected their thermal stability. Investigation of the
optical properties of the compounds demonstrated that changing the length of the p-
conjugation system of the aromatic carboxylic acid ligands in the structure of the
coordination compounds can be used to tune their photophysical properties.
Electronic supplementary material The online version of this article (doi:10.1007/s11164-017-2960-1)
contains supplementary material, which is available to authorized users.
&
&
Mostafa M. Amini
m-pouramini@sbu.ac.ir
Ezzatollah Najafi
ezzat.najafi@gmail.com
1
2
Department of Chemistry, Shahid Beheshti University, Tehran 1983963113, Iran
Department of Chemistry, Payame Noor University (PNU), Tehran 19395-3697, Iran
123

A. Hashemzadeh et al.
Keywords Lead(II) coordination polymer Á Effect of ligand Á Photoluminescence Á
Thermal behavior
Introduction
During the last two decades, great effort has been devoted to preparation of novel
metal coordination compounds with multidimensional networks, which involves
self-assembly of organic multitopic ligands with suitable functional groups and
metallic centers. Concomitantly, this interest is also due to their unique structural
topologies and potential applications in many fields, such as lasers, telecommuni-
cation technology, transistors, and optical sensors for special probes, and in
bioanalysis [1–4]. Investigation of the molecular structure of a large number of
coordination compounds of main and transition metal ions has shown that selection
of an appropriate ligand and knowledge of the nature and number of binding modes
and geometry are important to improve the molecular recognition ability of such
coordination compounds. Changing the length, symmetry, and flexibility of the
organic ligands provides the ability to form new classes of compounds with unique
arrangements and applications [5–7].
Studies have shown that the length and number of aromatic rings of the
carboxylic acid ligands can affect the coordination ability and mode of related
carboxylic groups, also providing convenient conditions for formation of intra- and/
or internetwork p–p stacking and C–H–p interactions to form molecular solids,
which are characterized by strong interactions between single molecular units
[8–13]. These noncovalent weak interactions have been deeply investigated in the
fields of structural chemistry, structural biology, and pharmaceutical science
[14–18].
Due to their widespread use as ligands for synthesis of coordination polymers,
the chemistry of organic carboxylic acids is considered very important. To date,
many coordination compounds formed of carboxylate groups with main and
transition metal ions with interesting structure and topology have been reported.
Moreover, because of their conjugated p-system, coordination compounds with
aromatic carboxylic acid ligands are currently of interest for development of
fluorescent materials and application as model compounds for electroluminescence
(EL), chemosensors, and photoinduced electron transfer (PET) sensors [19–22]. To
the best of the authors’ knowledge, the relationship between the length and number
of aromatic rings of aromatic carboxylic acid ligands and the structure, optical
properties, and thermal behavior of resulting metal coordination polymers has not
been widely studied. In continuation of our interest in this direction, two new
lead(II) coordination polymers with 4,4-biphenyl dicarboxylic acid and terephthalic
acid ligands with different aromatic rings were synthesized in this work. The crystal
structure of the prepared compounds was determined, and their optical properties
and thermal behavior investigated.
123

Effects of length and number of aromatic rings in…
Synthesis
Two new lead(II) coordination polymers were synthesized using terephthalic acid
(L1) and 4,4-biphenyl dicarboxylic acid (L2) ligands (Scheme 1) under hydrother-
mal conditions. It is noteworthy that efforts to prepare crystals of compounds 1 and
2 suitable for structural analysis by X-ray diffraction were not successful. To solve
this problem, 1-dimethylamino-2-propanol (L3) was introduced as ancillary ligand
into the structure of the prepared lead(II) coordination polymers. Use of this
ancillary ligand led to formation of high-quality crystals suitable for structural
analysis. This result shows that inclusion of ancillary ligands can be a useful
approach to improve the crystal quality of coordination compounds. The crystal
structure of the coordination compounds was assessed to evaluate the effects of the
length and number of aromatic rings of the carboxylic acid ligands on their
structure.
Characterization of lead coordination polymers 1 and 2
The structure of the prepared compounds was verified by elemental (CHN) analysis
and single-crystal X-ray diffraction analysis. The distinct difference between the
infrared (IR) spectra of complexes 1 and 2 and the free ligands as well as the
presence of the stretching vibration bands of Pb–O at about 750 cm-1 and Pb–N at
about 420 cm-1 unambiguously indicate coordination of the aromatic carboxylic
acid ligands and 1-dimethylamino-2-propanol to Pb(II) ion. Relatively weak
Scheme 1 Line drawings of prepared lead(II) coordination polymers
123

A. Hashemzadeh et al.
stretching vibration bands of aromatic and aliphatic C–H bonds were observed in
the IR spectra of the complexes in the range of 3000–3050 and 2930–3000 cm-1
,
respectively. The Pb–O stretching vibration band appeared at wavenumber similar
to values found for other lead(II) complexes [23–26].
The 1H nuclear magnetic resonance (NMR) spectrum of complex 2 was recorded
to provide information on metal–ligand binding and additional structural details in
solution, while that of complex 1 was not recorded because of its insolubility in
1
common organic solvents. The H NMR spectrum of complex 2 exhibited the
anticipated aliphatic and aromatic peaks with exact integration values, albeit shifted
somewhat downfield in comparison with those of the free ligands. This shift can be
related to electron density transfer from the ligands to the lead atom as acceptor.
According to single-crystal X-ray structural analysis, both compounds crystallized
in monoclinic system in space group P21/c (Table 1). The asymmetric unit of
lead(II) coordination polymer 1 has two crystallographically independent lead
atoms, Pb1 and Pb2, two terephthalic acid (L1) ligands, one 1-dimethylamino-2-
propanol (L3) ligand, and one DMF molecule (Fig. 1). The coordination number of
seven for Pb1 is achieved through five oxygen atoms from terephthalate and the
nitrogen and oxygen atoms of chelating 1-dimethylamino-2-propanol ligand. In
contrast, Pb2 is six-coordinated by five carbonyl oxygen atoms from four ligands
and one oxygen atom from DMF, resulting in a different coordination environment
from that of Pb1 (Fig. 1). The coordination environment of the Pb1 ion can be
described as a distorted mono-capped trigonal prism, while each Pb2 cation lies at
the center of a distorted octahedron.
The coordination sphere of both Pb(II) atoms resulting from the arrangement of
the donor oxygen and nitrogen atoms obviously indicates an apparent vacancy,
revealing that the lone pair of electrons is stereochemically active for Pb1 and Pb2
in the structure of the prepared lead(II) complex (Fig. 2). Comparison of the Pb–O
bond lengths opposite versus adjacent to the putative lone pair, for both lead ions,
confirms this property. As shown in Scheme 2, the terephthalate anions adopt two
different coordination modes. In mode a, anion L1 is coordinated to three
symmetry-related Pb2 atoms in a chelating and bis-monodentate mode through
carboxylate groups. In mode b, carboxylate groups act in hexadentate bridging
mode, coordinated to one Pb2 and three symmetry-related Pb1 ions. The Pb1ÁÁÁPb1
1
˚
and Pb2ÁÁÁPb2 distances separated by the L ligand were 4.655 and 3.787 A. The
˚
Pb–O bond length lay in the range of 2.315(18)–2.84(3) A, while that of Pb–N was
˚
2.73(2) A (Table S1, Electronic Supporting Information). As analyzed using
Cambridge Structure Data 2017 with the aid of Conquest (version 1.19) and
Mercury (version 3.9) software (Figs. S1, S2), these values are in accordance with
those reported for other Pb(II)–O and Pb(II)–N donor complexes. Comparison of
these Pb–O bond distances with similar carboxylate-based ligands also reveals that
these values are similar to previously reported values (Fig. S3). In the structure of
the prepared complexes, weak inter- and intramolecular interactions are the
principal agent controlling the packing. As shown in Fig. 1, linking of Pb atoms by
L1 ions leads to formation of two-dimensional (2D) layers. Connection between the
2D coordination layers via hydrogen-bonding interactions (Table S2) and C–HÁÁÁp
123

Effects of length and number of aromatic rings in…
Table 1 Crystal data and refinement details for lead(II) complexes
Complex
1
2
Empirical formula
Formula weight
Crystal color
C24H28N2O10Pb2
918.88
C19H21NO5Pb
550.57
Colorless
Yellow
Crystal size (mm)
Temperature (K)
0.14 9 0.13 9 0.10
298(2)
0.21 9 0.20 9 0.19
298(2)
˚
Wavelength (A)
0.71073
0.71073
Crystal system
Space group
Monoclinic
P21/c
Monoclinic
P21/c
˚
a (A)
11.9186 (15)
16.642 (2)
14.666 (2)
107.333 (10)
2776.8 (7)
4
11.5003 (11)
12.1076 (11)
14.9719 (15)
110.023 (7)
1958.7 (3)
4
˚
b (A)
˚
c (A)
b (8)
3
V (A )
˚
Z
Absorption coefficient (mm-1
Calculated density (mg/m3)
F(000)
)
12.166
8.641
2.198
1.867
1720
1056
h range for data
1.79–27.00
-15 B h B 15,
-21 B k B 21,
-16 B l B1
6072
1.88–29.26
15 B h B 15,
-16 B k B 15,
-20 B l B 16
5268
Index ranges
Reflections collected
Unique reflections
5861
2766
Restraints/parameters
Goodness of fit on F2
Final R indices [I [ 2r(I)]
R indices (all data)
0/332
1/241
0.916
0.884
R1 = 0.0963, wR1 = 0.1939
R2 = 0.2036, wR2 = 0.2310
R1 = 0.0567, wR1 = 0.1020
R2 = 0.1297, wR2 = 0.1192
˚
interactions with distance of 2.867 A (Fig. 3) led to growth of the structure of the
complex into a three-dimensional (3D) coordination polymer.
The asymmetric unit of 2 contains one unique Pb(II) atom, one biphenyl-4,40-
dicarboxylate (L2), and one 1-dimethylamino-2-propanol (L3) in which the Pb(II)
ion is coordinated through four carboxylate oxygen atoms from L2 in a bidentate
chelating coordination mode and two nitrogen and oxygen atoms from L3 (Fig. 4).
Compound 2 forms a zigzag 1D polymeric network made up of a hemidirected
coordination sphere of Pb(II) caused by stereochemically active lone pairs of Pb(II)
˚
with PbÁÁÁp interactions with the nearby aromatic ring (distance 3.659 A) (Figs. 5,
6). In the chain, the PbÁÁÁPb distance bridged by the biphenyl-4,40-dicarboxylic acid
˚
is about 15.79 A. Checking the Pb–O and Pb–N distances using the Cambridge
Structural Database 2017 with use of Conquest (version 1.19) and Mercury (version
123

A. Hashemzadeh et al.
Fig. 1 ORTEP view of lead(II) complex 1
Fig. 2 Presence of apparent vacancy in coordination sphere of Pb1 and Pb2 in 1
3.9) software revealed that they have similar values to other Pb(II)–O and Pb(II)–N
donor complexes (Table S3; Figs. S1–S3). The ligands from adjacent chains are
also paired to furnish strong O–HÁÁÁO and CHÁÁÁO hydrogen bonds (Table S4). These
interactions show a significant effect on the formation and stabilization of the three-
dimensional supramolecular structure.
123

Effects of length and number of aromatic rings in…
Scheme 2 Coordination modes
of carboxylate groups of L1
anion in 1
Fig. 3 C–HÁÁÁp interaction in complex 1
Fig. 4 ORTEP view of coordination polymer 2
123

A. Hashemzadeh et al.
Fig. 5 Zigzag state of 1D coordination polymer 2
Fig. 6 PbÁÁÁp interactions in structure of coordination polymer 2 with hemidirected Pb(II) coordination
sphere
It is clear that the only variable factor in the preparation of these compounds was
the length and number of aromatic rings of the carboxylic acid ligands. The results
of crystal structure analysis showed that this factor had a significant effect on the
coordination number and geometry of the corresponding Pb(II) atoms in the
structure of the resulting coordination polymers. These factors also influence the
coordination mode of the carboxylate groups of the respective ligands. Ligands L1
and L2 have completely different coordination modes in the structure of compounds
1 and 2, respectively. The most important point when comparing the structure of
compounds 1 and 2 is the connection of Pb(II) atoms by L1 ligand in the structure of
compound 1. This connection led to formation of one 2D supramolecular network,
while the connection of lead atoms by L2 ligand in the structure of compound 2
resulted in a zigzag 1D polymeric network. As a result, choice of a suitable ligand
with intermediate flexibility, length, and several coordination sites can lead to
formation of new coordination polymers with unforeseen and fascinating structure.
It seems essential to choose a proper ligand in order to control the structure of the
compounds for design and preparation of coordination polymers with particular
structural topology. Development of such ligands opens a new horizon for research
into supramolecular coordination compounds with fascinating structural topology,
outstanding physical properties, and applications.
123

Effects of length and number of aromatic rings in…
Powder X-ray diffraction analysis and thermal study of prepared
coordination polymers
The high purity of the prepared compounds was confirmed by comparison of their
powder X-ray diffraction patterns with those simulated from their single-crystal data
(Fig. S4). Investigation of the thermal behavior of compounds used to fabricate
optical devices is essential, as they must show relatively high thermal stability. To
evaluate the effects of the structure and ligand of the coordination polymers on the
thermal behavior, thermal analysis of the prepared compounds was performed in air
atmosphere from room temperature up to 800 °C (Figs. S5, S6). Decomposition of
the framework and loss of organic components from the prepared complexes
resulted in an intense exothermic peak in the differential thermal analysis (DTA)
curves together with significant weight loss in their thermogravimetric analysis
(TGA) curves in the temperature range of 400–550 °C. The residue after thermal
analysis can be attributed to formation of lead(II) oxide particles. These thermal
analysis results show that the structure of coordination compounds can have a
significant impact on their thermal stability. As illustrated by the thermograms in
Figs. S1 and S2, coordination polymer 1 exhibited relatively better thermal stability
than coordination polymer 2. Furthermore, these results show that the prepared
coordination polymers had appropriate thermal stability for fabrication of optical
devices such as organic light-emitting diodes (OLEDs).
Optical properties
Brief literature review reveals that control of the chemical and physical properties of
materials is critical for device performance, so much effort has been made by
scientists to control them [27–31]. Over the past two decades, according to
increasing demand for optical devices, synthesis and design of compounds with
good optical properties have attracted much attention.
Control of the optical properties of luminescent compounds is essential for
preparation of optical devices to address applications effectively. Considerable
efforts have already been made in this field, including the introduction of different
substituents on ligands used to prepare coordination compounds, altering the central
metal, and use of ancillary ligands [32–36]. In this work, to prepare coordination
compounds with rare structural topology and control their optical properties, two
ligands with similar structure but different p-conjugation systems, viz. 4,4-biphenyl
dicarboxylic acid and terephthalic acid, were used to prepare two new lead(II)
coordination polymers. The solid-state absorption spectra of the prepared coordi-
nation polymers and related ligands are shown in Fig. 7. Bands observed in the
absorption spectra of the ligands can be related to spin-allowed p–p* transitions.
The similar absorption spectra of the ligands and corresponding complexes reveal
that the origin of the absorption is the same for both. Comparison of the absorption
spectra of the ligands reveals that the absorption band of 4,4-biphenyl dicarboxylic
acid is red-shifted relative to that of terephthalic acid, which could be due to the
different p-conjugation length of these ligands, the only variable factor in their
structures. The similar shift in the absorption spectra of the complexes indicates that
123

A. Hashemzadeh et al.
Fig. 7 Solid-state absorption
spectra of prepared coordination
polymers and related ligands
the optical properties of the ligands affect the properties of the compounds. Overall,
the results show that the ligand chosen can strongly influence the optical properties
of the resulting coordination compound and thereby the fabrication of optical
devices.
The bandgap of the prepared compounds was calculated from the absorption edge
[37] of 329 and 342 nm for compound 1 and 2, respectively (Fig. 8), yielding values
of 3.77 and 3.62 eV. The wider bandgap of complex 1 relative to complex 2 shows
that the absorption properties of these optical compounds can be controlled over a
broad wavelength range via the length of the p-conjugation system of the ligand in
their structure.
The room-temperature solid-state photoluminescence spectra showed peaks at
430 and 470 nm for complex 1 and 2, respectively (Fig. 8), corresponding to spin-
allowed p–p* transitions of the ligands. The maximum in the emission spectrum of
complex 1, similar to the absorption spectra, showed a blue-shift relative to that of
complex 2.
The absolute quantum yield of the prepared complexes 1 and 2 in solid state was
0.54 and 0.62, respectively, as calculated according to Moreno [38]. The higher
quantum yield of complex 2 relative to complex 1 shows that the p-conjugation
Fig. 8 Room-temperature
solid-state photoluminescence
spectra of prepared complexes
123

Effects of length and number of aromatic rings in…
length of the ligand is an important parameter to control the optical properties of the
resulting coordination compound. The thermal stability and optical property results
reveal that these complexes have good potential as fluorescent materials for
preparation of luminescence devices such as light-emitting diodes.
Experimental
Materials and methods
Lead(II) nitrate, terephthalic acid (L1), and 1-dimethylamino-2-propanol (L2) were
purchased from Sigma-Aldrich and used without further purification. All solvents
were dried before use according to standard procedures [39]. Elemental analysis of
compounds was carried out using a Thermo Finnigan Flash-1112EA microanalyzer.
A Bomem MB-series FT-IR instrument was used to record infrared spectra. NMR
spectra were obtained on a Bruker AVANCE 300 operating at 300.3 MHz. Solid-
state luminescence and absorption spectra of the prepared compounds were obtained
on USB2000 and Shimadzu 2100 spectrometers, respectively. X-ray diffraction
patterns were obtained on a STOE diffractometer using Cu Ka radiation at 60 keV
and 15 mA with scan rate of 3° min-1. Thermal analysis (TGA) was carried out on
a Bahr STA-503 instrument in air at heating rate of 10 °C min-1
.
Crystallographic measurements were carried out on a STOE IPDS-II diffrac-
tometer using graphite-monochromated Mo Ka radiation at 100 K. Unit cell
parameters and orientation matrices were obtained by least-squares refinement. The
structures were solved by the direct method and refined using the full-matrix least-
squares procedure on F2. SHELXL97 crystallographic software was used for all
refinements [40].
Preparation of biphenyl-4,40-dicarboxylic acid
Acetyl chloride (8.6 mL, 0.12 mol) was added to solution of anhydrous aluminum
chloride (13.32 g, 0.1 mol) in CH2Cl2 (20 mL) and stirred for 20 min at 0 °C.
Solution of biphenyl (6.16 g, 40 mmol) in CH2Cl2 (80 mL) was added to the
resulting acetylating complex during 1 h at 0 °C with vigorous stirring. Stirring the
resulting acetylated AlCl3 complex for 0.5 h at 0 °C then overnight at room
temperature led to formation of 1,10-([1,10-biphenyl]-4,40-diyl)bis(ethan-1-one).
Biphenyl-4,40-dicarboxylic acid was obtained by reflux of the prepared diketone
(5.0 g, mmol), fuming HNO3 (10 mL), and acetic acid (100 mL) for 6 days.
Biphenyl-4,40-dicarboxylic acid was filtered and washed with water and ethanol
several times, then dried at room temperature (yield 75 %, m.p. [ 270°C). FT-IR
1
(KBr, cm-1): 766, 843, 1102, 1111, 1178, 1279, 1396, 1530, 1607, and 1696. H
NMR (300 MHz, DMSO-d6): d = 8.03, 8.06 (d, 2H), 7.85, 8.87 (d, 2H), 13.06 (s,
1H) ppm.
123

A. Hashemzadeh et al.
Synthesis of complexes 1 and 2
To prepare complex 1, [Pb2L21L3(DMF)]n, Pb(NO3)2 (0.04 g, 0.12 mmol), and
1-(dimethylamino)propan-2-ol (1.0 mL) were added to 1:2 methanol/DMF
(15.0 mL) solution of terephthalic acid (0.02 g, 0.12 mmol) in a 20-mL glass tube.
The solution was transferred to a glass vessel and heated hydrothermally from room
temperature to 80 °C at 2 °C/min, kept at 80 °C for 72 h, then allowed to cool to
room temperature during 18 h. After filtration, yellow needle-like crystals were
obtained (yield 73 %, m.p. [ 270°C). Anal. Calc. (%) for C24H28N2O10Pb2: C,
31.37; H, 3.07; N, 3.05. Found (%): C, 31.36; H, 3.06; N, 3.08. IR (KBr, cm-1):
3030 (w), 2991 (w), 1664 (m), 1535 (s), 1369 (s), 818 (w), 751 (m), 429 (w).
[PbL2L3]n (2) was synthesized in the same manner but with biphenyl-4,40-
dicarboxylic acid replacing terephthalic acid (yield 80 %, m.p. [ 270°C). Anal.
Calc. (%) for C19H21N1O5Pb1: C, 41.45; H, 3.84; N, 2.54. Found (%): C, 41.48; H,
3.82; N, 2.52. IR (KBr, cm-1): 3035 (w), 2966 (w), 1662 (m), 1577 (s), 1384 (s),
1
851 (m), 767 (m), 414 (w). H NMR (CDCl3, ppm): 8.04, 4.35, 3.75, 2.24, 1.10,
0.07.
Conclusions
4,4-Biphenyl dicarboxylic acid was prepared to investigate the effect of the length
of the p-conjugation system of aromatic carboxylic acid ligands on the structure,
optical properties, and thermal behavior of two new lead(II) coordination polymers
with 4,4-biphenyl dicarboxylic acid or terephthalic acid as primary ligand and
1-(dimethylamino)propan-2-ol (L3) as ancillary ligand. Study of the crystal
structure of the prepared compounds revealed that the length of the p-conjugation
system of the aromatic carboxylic acid ligand affected the topology of the resulting
coordination compound. Differential thermal and thermogravimetric analyses
revealed that the thermal behavior of the coordination compounds depended on
their crystal structure. Based on the optical properties and quantum yield of the
coordination compounds, it can be concluded that altering the length of the p-
conjugation system may represent a new approach to control their optical properties
and enable their application for fabrication of optical devices.
Acknowledgement We thank the Graduate Study Councils of Shahid Beheshti University and Payame
Noor University for supporting this study.
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