Mixed ligand cobalt and palladium complexes containing triphenylphosphine and a hydrazone: Synthesis and application in non-linear optics

Article abstract of DOI:10.1177/1747519820943534

Mixed ligand complexes of cobalt and palladium containing triphenylphosphine and a hydrazone derived from furfural and hydrazine hydrate have been designed, synthesized, evaluated, and characterized from their spectral properties, elemental analysis, and magnetic susceptibility measurements. The spectral techniques suggest that the complexes exhibit square planar geometry. The monomeric properties of the complexes are evaluated from their magnetic susceptibility values. The complexes were subjected to z-scan analysis for third-order non-linear optical measurements. Non-linear transmission measurements performed using laser pulses at 532 nm in nanosecond indicate that the complexes may show good potential as optical limiters.

Full text of DOI:10.1177/1747519820943534

943534CHL0010.1177/1747519820943534Journal of Chemical ResearchRamakrishna
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Journal of Chemical Research
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Mixed ligand cobalt and palladium complexes
containing triphenylphosphine and a hydrazone:
Synthesis and application in non-linear optics
https://doi.org/10.1177/1747519820943534
DOI: 10.1177/1747519820943534
journals.sagepub.com/home/chl
Dileep Ramakrishna
Abstract
Mixed ligand complexes of cobalt and palladium containing triphenylphosphine and a hydrazone derived from furfural
and hydrazine hydrate have been designed, synthesized, evaluated, and characterized from their spectral properties,
elemental analysis, and magnetic susceptibility measurements. The spectral techniques suggest that the complexes exhibit
square planar geometry. The monomeric properties of the complexes are evaluated from their magnetic susceptibility
values. The complexes were subjected to z-scan analysis for third-order non-linear optical measurements. Non-linear
transmission measurements performed using laser pulses at 532nm in nanosecond indicate that the complexes may
show good potential as optical limiters.
Keywords
furfural, metal complexes, mixed ligands, non-linear optical, optical limiting, z-scan
Date received: 20 May 2020; accepted: 30 June 2020
Introduction
Department of Chemistry, School of Engineering, Presidency University,
Bangalore, India
The expansion of innovative and simple materials that dis-
play non-linear optical (NLO) activities is of prime impor-
tance in laser technology. Materials derived from specific
molecules that have considerable potential for fabrication
Corresponding author:
Dileep Ramakrishna, Department of Chemistry, School of Engineering,
Presidency University, Yelahanka, Bangalore 560064, Karnataka, India.
of a device and its commercialization are of significant Email: dileep.r@presidencyuniversity.in; dileep.618@gmail.com

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Journal of Chemical Research 00(0)
interest in industrial and academic research laboratories.1
The investigation of metal–organic compounds as NLO
materials is making its way toward many applications
including satellite communications. Novel NLO materials
are crucial to the improvement of optoelectronic technolo-
gies and devices used in communications, ultra-fast elec-
tro-optical information processing, high-density data
storage, light variation, switching, and so on. Since the
mid-1980s, these materials have continued to be at the fore-
front of research activities in the field of laser optics. Still,
the challenge remains for scientists to design and prepare
new non-linear optically active materials which are required
for practical NLO applications in the form of optoelectronic
devices.2,3
Scheme 1. Synthesis of the ligand.
P
Cl
N
CoCl2(PPh3)2
Co
Acetone-MeOH
Reflux
NH2
O
2
N
NH2
O
Many organic and inorganic materials have been compre-
hensively used for NLO applications.4–9 For a NLO material
used for optical devices, optimal dipole moments of an active
chromophore and matrices should be present.10,11 Also, the
other criteria of the molecule being non-symmetric should be
satisfied. A derivative based on ferrocene exhibiting NLO
properties was reported for the first time that belonged to the
class of metal–organic compounds.12 In organic compounds
with a metal, increased optical activity would be observed.
This is due to two reasons. First, the high level of the p con-
jugated charge transfer and second, the growth in the extent
of conjugation that will have a pivotal role on the delocaliza-
tion of p-electrons in the presence of excitation light. Hence,
as a result, the χ3 and β values are enhanced with the differ-
ent types of bonds in the molecule.13
1
P
Cl
PdCl2(PPh3)2
Pd
NaOAc, EtOH
RT
NH2
N
O
3
Scheme 2. Synthesis of the complexes.
respectively. The synthesized complex exhibits a non-
symmetric structure which is one of the criteria for a NLO
material. The metal atom is surrounded by different chem-
ical environments which contribute to the non-symmetric
nature of the molecule.
In recent years, organometallic and coordination com-
plexes have been widely studied for their NLO properties.
The combined properties of flexibility and thermal stability
of organic materials and the mechanical strength of inor- Results and discussion
ganics make these compounds potentially show NLO activ-
ities. Metal complexes (organometallic and coordination),
Electronic spectral studies
in particular, signify themselves as a trending class of mate- The UV-Vis spectra of the complexes in DMF are depicted
rials in this area of research. Due to the larger variety of in Figure 1, exhibiting bands in the region between 250 and
structures and their stability under normal environmental 500nm. The electronic transitions due to the intraligand
conditions, metal–organic compounds serve as better NLO interactions appear between 260 and 350nm, being more
materials compared to organic molecules. Also, metal com- toward the visible region. A relatively low intensity band is
plexes have a range of electronic properties because of the seen in the range 380–495nm due to the forbidden d→d
electron-deficient central metal atom surrounded by elec- transition. Other cobalt and palladium complexes exhibit-
tron-donating organic moieties.14 In 1987, a report by ing similar electronic spectral performance have been
Green et al. described the second-harmonic generation described in the literature.16–18
(SHG) efficiency on a derived ferrocene complex that
proved it to be an excellent type.9 Subsequently, the utiliza-
FTIR spectral studies
tion of metal complexes as NLO materials has attracted the
attention of many researchers. Consequently, a systematic The Fourier transform infrared spectrum of the ligand dis-
exploration has been performed on various new metal com- plays an intense sharp peak around 1610–1620cm−1,
plexes in this field. Initial and more topical literature on the assigned to the ν(C=N) vibration. As a result of complex
studies of metal complexes and their NLO activities indi- formation as the nitrogen atom shares its pair of the elec-
cate the scope of research in this field. Noticeably, the NLO trons with the electron-deficient metal atom, this peak
properties of several organometallic compounds have been shifts to a lower wavenumber.16,17,19,20 The band in the
given substantial attention.15
region 1315–1330cm−1, assigned to ν(C−O) in the free
The following section of the paper reports the design, ligand, is shifted to a higher wavenumber in the complexes
synthesis, and NLO measurements of metal M [M=Co(II) suggesting the coordination of oxygen to the metal ion.8,16,17
and Pd(II)] complexes of a mixed ligand system contain- The 550- and 470-cm−1 bands in the complexes are assigned
ing triphenylphosphine and a hydrazone derived from fur- to ν(M−O) and ν(M−N) frequencies, respectively.16,17 The
fural and hydrazine. The synthesis of the ligand (1) and FTIR spectra are provided in the Supporting Information
the complexes (2 and 3) is shown in the Schemes 1 and 2, (Supplemental Figure S1).

Ramakrishna
3
Figure 1. UV spectra of complexes 2 and 3.
point. The experiment proceeds by the sample being placed
in the laser beam at different positions relative to the focus
(various values of z). This allows the measurement the light
transmission. The intensity-dependent transmission of the
sample can thus be measured since the sample experiences
dissimilar laser intensities at each position. These data give
the way to calculate the NLA coefficient.
NMR spectral studies
1
The H NMR spectra of the complexes exhibit multiplets
around 6.9–7.9ppm which are assigned to the protons of the
phenyl groups present in triphenylphosphine (Supplemental
Figures S2 and S5). A sharp peak at around 8.5ppm in the
complex has been assigned to azomethine proton of the
hydrazone where the nitrogen of −C=N− is participating in
Using an Nd:YAG laser, at a wavelength of 532nm,
laser pulses at 5-nanosecond intervals were performed to
irradiate the samples in 1-mm cuvettes. This provides the
SHG of the crystal. The third-order NLO properties were
assessed at concentrations and the solvent same as that in
UV-Vis studies (i.e. 0.01M and DMF). DMF was selected
as the solvent to increase the solubility and also to mini-
mize the overlap between the absorption spectra of the
metal complexes and the wavelength of the z-scan meas-
urement. Most importantly, DMF does not show any NLO
response and hence ensures that the absorption obtained is
that of the solute.28 Both complexes, CoL and PdL, showed
good non-linear transmission. At higher intensities, the
transmission in both samples was reduced (Figures 3 and
4), indicating that these samples may conduct as optical
limiters. It is a known fact that optical limiters find uses in
protecting light-sensitive devices like cameras from acci-
dental exposure to intense lasers. Optical limiters also play
a similar and very important role in protecting human eyes
in these situations.
complex formation.21–26 The H NMR spectra of the com-
1
plexes are provided in the Supporting Information. In the
13C NMR spectra of the complexes (Supplemental Figures
S3 and S6), resonances were observed in the 151- to 155-
ppm series was due to the azomethine carbon. The chemical
shifts observed in the regions 145–150, 162–165, and 142–
145ppm are due to C–N, C–O, and C–P, respectively.16
Also, additional information was derived from the 13C NMR
spectra of complexes, which showed the existence of car-
bons with six different chemical environments (130–
135ppm). The three quaternary carbons arising from the
triphenylphosphine aromatic units were in the same mag-
netic environments.16 The 31P NMR spectra exhibited sin-
glets at 22.3 and 23.1ppm (Supplemental Figures S4 and
S7), which suggests that one triphenylphosphine molecule
coordinates with the metal ion in the complexes.16,17
Magnetic susceptibility measurements
The complexes were subjected to magnetic susceptibility
measurements to obtain further information on their struc-
tures. The magnetic susceptibility measurements showed
that the complexes are diamagnetic in nature and confirmed
a tetra-coordinated configurations.26
It was observed that the absorption process of a two-
photon type gives the best fit to the obtained open-aperture
z-scan data. The data obtained for a two-photon absorption
process are numerically fitted to the non-linear transmis-
sion equation,29 given by
T = (1− R)2 exp(−αL) / π q
(
)
0
NLO measurements
+∞
(1)
2


The open-aperture z-scan technique, developed by Sheik
Bahae et al.,27 is widely used to measure the non-linear
transmission of light through samples. This technique
ln 1+ q0 exp(−t ) dt
∫




−∞
allows the determination of the non-linear absorption (NLA) where T is the transmission; L is the length; R is the surface
coefficient. A Gaussian laser beam is focused spatially reflectivity of the sample; α is the linear absorption coeffi-
through the sample using a lens. The beam is propagated in cient; q0 =β(1−R)I0Leff, where β is the two-photon absorp-
the direction of the z-axis, and the focal point is chosen from tion coefficient; I0 is the on-axis peak intensity; and
the origin, z=0. At the origin, the laser beam, which has Leff =1−exp(−αl/α).
maximum energy density, will be reduced proportionally to
A knife-edge measurement reveals that the laser beam is
each side of the positive and negative values of the focal Gaussian spatially (Figure 2). The diameter of the beam

4
Journal of Chemical Research 00(0)
large absorptive optical non-linearity at strong optical inten-
sities. NLA in the molecules can originate due to multiple
components such as on-photon molecular absorption, exci-
tation wavelength, fluence and intensity of laser beam, and
pulse duration. The different possible NLA mechanisms are
saturable absorption (SA), excited-state absorption (ESA),
reverse SA (RSA), 2-photon absorption (2PA), and so on.
The molecule exhibiting small one-photon absorption at
532nm, so ESA is the major process in the existing
NLA.28,30–33 Furthermore, 2PA play a dominating role when
molecules subjected to a strong nanosecond laser beam.34–36
The normalized dip describes the existence of RSA mecha-
nism in the molecule.37
The non-linear transmission curves and z-scans are
shown in Figures 3 and 4. The data indicate that both com-
plexes behave as good optical limiters, with results reveal-
ing that the NLO transmission was reduced at higher
intensities. The numerically calculated values of the two-
photon absorption coefficient are 5×10−11 mW−1 for CoL
(1) and 9×10−12 mW−1 for PdL (2), respectively.
Figure 2. Spatial profile of the laser beam (Nd:YAG).
1.0
0.9
Conclusion
Novel cobalt and palladium complexes of a mixed ligand
containing triphenylphosphine and a hydrazone derived
from furfural and hydrazine have been synthesized, and
structural characterization was performed by elemental
analysis, IR, NMR, UV spectroscopy, and mass spectrom-
etry. Elemental analysis confirmed the molecular formulae
of the ligand and complexes. Keeping in view the favored
geometries and on the basis of all the above studies, square
planar structures have been put forward for both the cobalt
and palladium complexes. Preliminary investigations on
the NLO measurements of both complexes performed at
532nm using nanosecond laser pulses show promising
responses. Both the samples display an excellent optical-
limiting capability. By virtue of being optical limiters, these
complexes can possibly serve as materials with potential
applications in the field of optoelectronics and photonics.
Future work on the optical power limiting and optical
switching studies of the complexes in the form of thin films
will be reported shortly.
1.0
0.8
0.9
0.8
0.7
0.7
0.6
-10
10
-5
0
5
10
0.6
Z (mm)
4
105
4x105
Input laser fluence (J/m2)
Figure 3. Non-linear optical transmission in CoL.
1.00
0.96
1.00
0.96
0.92
0.92
Experimental
0.88
-10
-5
0
5
10
Materials
0.88
Z (mm)
4
5
5
Analytical grade chemicals were used for the synthesis of
all the compounds. A standard procedure available in the
literature was used to purify the solvents.38 Furfural, hydra-
zine hydrate, and triphenylphosphine were purchased from
Merck and were used without further purification.
CoCl2·6H2O and PdCl2 were obtained from Spectrochem
10
10
Input laser fluence (J/m2)
4x10
Figure 4. Non-linear optical transmission in PdL.
(full width at half maximum) was found to be 7.82mm (by chemicals, and were used without further purification. The
calculation). The Rayleigh range in our experiment was starting complex, [CoCl2(PPh3)2], was prepared by heating
1.8mm.
a solution of CoCl2·6H2O and triphenylphosphine in glacial
The pattern of the open-aperture characteristics describes acetic acid.39 The other starting material, [PdCl2(PPh3)2],
the intensity-dependent absorptive optical non-linearity pre- was synthesized by refluxing anhydrous PdCl2 and triphe-
sent in the samples with a symmetric dip at z=0 affirming nylphosphine in tetrahydrofuran for 5h.40

Ramakrishna
5
λmax (nm) intraligand interactions: 225, 275, 339, d→d for-
bidden transition: 447.
Physical measurements
The electronic spectra of solutions of the complexes in
DMF were recorded on a GBC UV-Vis double beam spec-
trophotometer in the 200- to 800-nm range. FTIR spectra
were obtained using a Thermo Nicolet Avatar FTIR spec-
trometer, with samples prepared with KBr powder in the
frequency range 400–4000cm−1. The C, H, N, S, and O
composition analysis was performed on a Thermo Flash
elemental analyzer. NMR spectra were collected on a
Bruker AMX 400 instrument with TMS as the internal
standard. The magnetic moment measurements were
obtained using a Sherwood Scientific instrument.
Acknowledgements
The author would like to thank the IISc, Bangalore for the NMR
analyses and RRI, Bangalore for the NLO measurements.
Declaration of conflicting interests
The author declared no potential conflicts of interest with respect
to the research, authorship, and/or publication of this paper.
Funding
The author received no financial support for the research, author-
ship, and/or publication of this paper.
Synthesis of the hydrazone: (E)-(furan-2-
ylmethylene) hydrazine (1)
ORCID iD
Dileep Ramakrishna
https://orcid.org/0000-0003-0801-2623
The hydrazone was prepared using a procedure reported in
the literature.41,42 Hydrazine was slowly added to a metha-
nolic solution of furfural and stirred for about 30min at
room temperature.
Supplemental material
Supplemental material for this paper is available online.
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