Hirshfeld surface analysis, enrichment ratio, energy frameworks and third-order nonlinear optical studies of a hydrazone derivative for optical limiting applications

Article abstract of DOI:10.1016/j.molstruc.2021.131019

Organic materials with good third-order nonlinear optical (NLO) properties are used in optoelectronics for protecting human eyes and sensors from high intensity laser light. In this regard, herein we report the synthesis, structural analysis, and third-or

Full text of DOI:10.1016/j.molstruc.2021.131019

Journal of Molecular Structure 1245 (2021) 131019
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Hirshfeld surface analysis, enrichment ratio, energy frameworks and
third-order nonlinear optical studies of a hydrazone derivative for
optical limiting applications
Laxminarayana Kamatha, Anthoni Praveen Menezesb,∗, Raghavendra Bairyc,
Shashi Kumar Kumara swamya, A. Jayaramaa
a Department of Physics, Alva’s Institute of Engineering & Technology, Shobhavana Campus, Mijar, Moodabidri, 574225, India
b Department of Physics, Mangalore Institute of Technology & Engineering (MITE), Moodabidri, 574 225 India
c Department of Physics, N.M.A.M. Institute of Technology, Nitte, Udupi, 574110, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Organic materials with good third-order nonlinear optical (NLO) properties are used in optoelectronics
for protecting human eyes and sensors from high intensity laser light. In this regard, herein we re-
port the synthesis, structural analysis, and third-order NLO properties of an organic molecule N’-[(E)-
(4-fluorophenyl)methylidene]biphenyl-4-carbohydrazide (FMBC) containing hydrazone moiety. The forma-
tion of the synthesized molecule is confirmed by identifying the functional groups through FTIR spectral
analysis. The material possesses good thermal stability before the melting point of 242.5 ◦C and trans-
parent in the entire visible region of the EM spectrum. The photoluminescence study revealed the blue
light-emitting property of FMBC. The crystal structure is stabilized by N-H···O, C-H···O, and C-H···F hy-
drogen bond interactions. The interconnects in the crystal packing were envisioned quantitatively using
Hirshfeld surfaces (HS) by 2D fingerprint plots. The main contribution to the HS comes mainly from C-H,
H-H, and O-H interconnects which cover about 80 % of the total HS surface. The enrichment ratios show
that the favorable contacts accountable for the crystal packing are C···H, N···H, O···H, and F···H. The break-
down of the interaction energies obtained for various molecular pairs shows the nature and strength of
the interactions. The calculation of 3D energy frameworks suggests that contacts formed in the structure
are largely due to the dispersion force followed by the electrostatic potential. Third-order NLO coeffi-
cients were extracted under the continuous wave (CW) regime using z -scan technique. The material is
an excellent optical limiter with a threshold value of 3.42 kJ/cm2.
Received 21 October 2020
Revised 19 May 2021
Accepted 30 June 2021
Available online 4 July 2021
Keywords:
Hydrazone
Hirshfeld analysis
Enrichment ratio
Interaction energy
nonlinear refraction
Optical Limiting
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
to their versatile properties which include high optical nonlinear
and electro-optic coefficients, fast response, flexibility in modifying
The ultimate goal of crystal engineering is to synthesize crys-
talline materials for application on nonlinear optics (NLO) and
optoelectronic systems with specific physicochemical properties.
Materials possessing excellent third-order nonlinear optical prop-
erties could be used in optical switching, optical signal processing
optical limiting, and such other applications [1,2,3]. Materials with
substantial nonlinear refractive indices are appropriate for light
transmission based devices whereas those with large nonlinear
absorption are best suited for optical switching and sensor pro-
tection applications [4]. Among the various types of materials
investigated for NLO properties, organic materials are in focus due
the structure, ease of fabrication, and incorporation in devices for
various photonic applications [5,6,7]. Recently, the organic material
N-acyl hydrazone attracted extensive interest due to its ease of
synthesis, presence of various isomers, and various potent prop-
erties. The hydrazone moiety, -C(=O)-NH-N=CH- in an organic
molecule, leads to the nucleophilic and electrophilic effect in the
molecule [8,9]. Attachment of the electron-donating/ withdrawing
groups across this bridge results in a push-pull structure with
improved molecular polarizability and optical nonlinearity.
Schiff bases are an important class of organic materials with ex-
citing third-order NLO properties. The aromatic rings substituted
on either side of the π− electron bridge (–C=N-) enhances the
delocalization of the π− conjugated electron in these type of ma-
terials. The magnitude of the NLO coefficients largely depends on
the length of conjugation, molecular confirmations, and electron
∗
Corresponding author.
E-mail address: mapraveen.80@gmail.com (A.P. Menezes).
https://doi.org/10.1016/j.molstruc.2021.131019
0022-2860/© 2021 Elsevier B.V. All rights reserved.

L. Kamath, A.P. Menezes, R. Bairy et al.
Journal of Molecular Structure 1245 (2021) 131019
Fig. 1. Scheme for the synthesis of FMBC.
donor/acceptor groups. The azomethine group/ hydrazone core be-
ing an electron donor, the substitution of a more functional group
on the hydrazone moiety further enhances the number of donor
sites and thus the molecular nonlinearity [10]. The integration of
various substituents with appropriate donor/acceptor strength into
the aromatic rings also boosts the molecular binding affinities via
intermolecular interactions and π−electron delocalization and as
a consequence the optical nonlinear behavior increases. Keeping
these aspects in mind, we synthesized a Schiff base, N’-((E)- (4-
fluoro Phenyl)methylidene)biphenyl-4-carbohydrazide (FMBC) hav-
ing large conjugation length [11]. Since the material crystallizes in
centrosymmetric C2/c space group second-order nonlinearity hard
to observe. Therefore, the third-order nonlinear properties such as
nonlinear absorption (β), nonlinear refractive index (n2), and the
nonlinear optical susceptibility (χ(3)) were extracted by perform-
ing a single beam z-scan experiment. Herein, we report the syn-
thesis, thermal, photoluminescence, linear, and nonlinear optical
studies of FMBC. The structural details obtained from single-crystal
X-ray diffraction is also provided for ready reference. The contri-
bution of intermolecular interactions to crystal packing and non-
linear behavior is analyzed through the Hirshfeld surface analysis
(HS), interaction energy calculations, enrichment ratios, 3D energy
framework, and 2D fingerprint plots.
tial thermal (DT) analysis of FMBC. The sample was scanned in the
temperature range 30°C - 600 °C at the heating rate of 10 °C/min in
the nitrogen atmosphere using SDT Q600 V20.9 Build 20 thermal
analyzer. The linear optical absorption/transmission spectrum was
recorded in the wavelength region 200 nm to 900 nm using a UV-
1601PC UV-Visible spectrophotometer. The specimen is dissolved
in DMF (0.01M) and taken in a 1 mm thick quartz cuvette and
exposed to the radiation. The photoluminescence (PL) study was
carried out on FMBC using the FluoroMax-4CP spectrometer. The
sample was dissolved in DMF and the experiment was conducted
in the range 400 nm to 800 nm at room temperature by exciting
the sample with a radiation of wavelength 380 nm. The signal was
detected using the R928P photomultiplier tube with a resolution
of 0.2 nm. The interatomic contacts present in the solid structure
which contribute to the stabilization of bulk structure were an-
alyzed by Crystal Explorer 17.5. The Hirshfeld surfaces (HSs), 2D
fingerprint plots (FPs) interaction energies and energy 3D-frame
works of FMBC were obtained using the crystallographic informa-
tion file (.cif) as the input [12]. The third-order nonlinear optical
parameters were evaluated by performing the classical Z scan ex-
periment [13]. A continuous-wave laser beam of wavelength 532
nm with 200 mW output power from a diode-pumped solid-state
laser source was focused onto a 1 mm thick cuvette containing the
0.01M solution of FMBC prepared in DMF. The laser beam of peak
input intensity of I0 = 8.48 × 107W/m2 was focused using a lens
of focal length 28.6 cm. The thin sample approximation is appli-
cable in this case as the sample thickness is much smaller than
the equivalent Rayleigh length z0 (8.86 mm). The beam waist at
the focus was estimated to be equal to 38.75 microns. The sample
is moved using a computer-controlled translational stage and far-
field light intensity variation is noted with and without aperture in
front of the detector using Laser Probe Rj-7620 Energy Meter with
pyroelectric detectors.
2. Materials and methods
2.1. Synthesis
The title compound, N’-[(E)-(4-fluorophenyl) methylidene]
biphenyl-4-carbohydrazide (FMBC) was synthesized using 4-
fluorobenzaldehyde and biphenyl-4-carbohydrazide as the starting
materials. The chemicals with 99.99% purity were procured from
Sigma Aldrich Inc. and were used without further purification.
Equimolar quantities of the precursors were dissolved separately
in 30 ml of ethanol. Two drops of Conc. HCl acid was added, and
the mixture was refluxed for 3 h. The mixture is then allowed
to cool, the solid precipitate formed was washed with water
and dried. Shown in Fig. 1 is the synthesis process schematic.
The composition of the product - Found (calculated): C: 75.40 %
(75.46%); H: 4.78 % (4.75 %); N: 8.73% (8.80%). The single crystals
were grown by slow evaporation of the solvent method at ambient
temperature choosing DMF as the solvent.
3. Results and discussions
The recorded IR spectrum of the title compound is displayed in
Fig. 2. The spectrum shows a broad absorption band at 3403 cm−1
corresponding to the N-H vibration and the weak band at 2937
cm−1 represents the presence of aromatic C-H stretching vibration.
The strong band at 1645 cm−1 indicates the –C=O- group while
the C-F stretching is confirmed by the weak band observed at 1089
cm−1. The medium intensity peak at 1395 cm−1 confirms the C-
N stretching vibrations. Thus, the analysis of the FT-IR spectrum
confirms the formation of the title compound, FMBC.
2.2. Characterization and computational techniques
The formation of the compound was confirmed by identifying
the functional groups present in FMBC through FT-IR spectroscopy.
The spectrum is recorded using the pellet technique wherein the
sample is mixed with KBr powder and a disk is formed. The spec-
trum is recorded in the wavenumber range 4000-400 cm–1 using
a SHIMADZU-8400S FT-IR spectrometer with a spectral resolution
of 0.1 cm–1. To explore the temperature range over which the ma-
terial can be used, the melting point and the decomposition tem-
perature was determined by thermogravimetric (TG) and differen-
Fig. 3 depicts 1D 1H (Hydrogen) NMR spectra of the title
compound – FMBC. The 1H – Chemical shifts are referenced to
DimethylSulfoxide (DMSO-d6 peak 2.49 ppm, solvent reference)
and were analyzed in accordance with standard 1H Chemical shift
database. The occurrence of crowded aromatic hydrogen signals in
the 1H chemical shift range of 7 – 8 ppm indicates signal arising
from 13-methine protons (C-1H) from the aromatic ring of dibenzyl
moiety. The upfield shifted aliphatic methine (C-1H) is found at 3.3
ppm which arises from the carbon linking dibenzyl and flouroben-
2

L. Kamath, A.P. Menezes, R. Bairy et al.
Journal of Molecular Structure 1245 (2021) 131019
Fig. 4. TG/DTA plot of FMBC.
Fig. 2. FT-IR spectrum of FMBC.
the Tauc’s plot [14]. The curve of (αhν)2 against photon energy is
shown in Fig. 6. In this case, the absorption coefficient α is deter-
zyl moiety. Further extreme downfield shifted amide (N-1H) signal
at 11.94 ppm arising from the hydrogen attached to linking nitro-
gen, double validates the formation of the FMBC compound.
The thermal stability of the material for high-temperature ap-
plications is investigated by Thermogravimetric and differential
thermal analysis. The recorded thermograms are shown in Fig. 4.
The sharp endothermic peak observed a 242.5 ◦C is the temper-
ature at which the material undergoes melting. The absence of
weight loss at the melting is evident from the TGA curve. More-
over, the material starts losing its weight at about 275 ◦C. The
material undergoes decomposition at 359.5 ◦C The major weight
A
mined using the relation, α = 2.303( / ), A being the absorbance
t
and ‘t’ the sample thickness. The linear nature of the curve can be
considered as evidence for the indirect transition taking place in
the material [15]. The optical band gap is obtained by extrapolat-
ing the linear portion of the curve to the energy axis as shown in
Fig. 6 and the value is found to be 3.44 eV. The large transmittance
in the visible region is the consequence of this wider bandgap.
The emission and excitation properties of the title crystal were
examined by Photoluminescence (PL) spectral studies. This is a
non-destructive technique of identifying defects by the mecha-
nism of color center creation [16]. The recorded spectrum of FMBC
shown in Fig. 7 shows a broad high-intensity emission peak at cen-
tered on 431 nm. The broad emission peak in the PL spectrum is
ascribed to the strong intra-molecular interaction present within
FMBC crystal. The broad emission peak confirms the light emission
in the blue region and can be used as blue light-emitting mate-
rial in optoelectronics such as for blue LEDs. Furthermore, the sin-
gle color emission peak observed in the explored region suggests
the good crystallinity and structural perfection of the grown FMBC
crystals [17].
loss of about 95 % occurs in the temperature range of 310◦C
−
− 380 ◦C. Up to the melting point no phase transitions were found
and thus FMBC forms a useful candidate in high-temperature ap-
plications.
Linear optical absorption studies were carried out to explore
the transmission window of the crystal for optical applications. The
UV-VIS absorption and transmission spectra of FMBC are reported
in Fig. 5 which reveals that the crystal has a broad transmission
window covering the entire visible region of the EM spectrum.
The minimum absorption in the visible region of the EM spec-
trum enables the material to be used for NLO applications because
the larger the absorption, the greater will be the loss of conver-
sion efficiency in the region of interest. Using the linear absorption
data, the optical bandgap of the material is obtained by plotting
The single-crystal X-ray diffraction study of FMBC, reported by
G. Dutkiewiez et al [11] showed that FMBC crystallizes in the mon-
oclinic crystal system with the centrosymmetric C2/c space group.
˚
˚
The unit cell parameters are, a = 37.163(5) A. b = 10.692(2) A,
Fig. 3. 1H (Hydrogen) NMR spectrum of FMBC.
3

L. Kamath, A.P. Menezes, R. Bairy et al.
Journal of Molecular Structure 1245 (2021) 131019
Fig. 5. UV-VIS spectrum of FMBC.
Fig. 7. PL spectrum of FMBC.
almost on a single plane as indicated by the dihedral angle of
◦
2.83 between their mean planes. Thus, the FMBC molecule as a
whole has a nearly planar structure.
The analysis of calculated Hirshfeld surfaces is now an invalu-
able tool, as it provides further insight into the various intermolec-
ular interactions that influence the packing of molecules in crys-
tals. Crystal Explorer 17.5 computer program was used to gener-
ate the Hirshfeld Surface (HS) and 2D fingerprint plots of FMBC
[18]. The intermolecular interactions with the adjacent molecules
are evaluated by the colour shading range of -0.542 (red region) to
1.524 (blue region) in terms of distances ‘de’ (the distance between
the surface and the nearest nucleus outside the surface) and ‘di’
(distance between the surface and the nearest nucleus inside the
surface). These distances allow us to analyze the intermolecular in-
teractions through the mapping of dnorm. The STO-3G (Slater-type
orbitals simulated by 3 Gaussians) basis set at the Hartree-Fock
theory wave function calculations were used to obtain electrostatic
potentials mapped over the Hirshfeld surface over the range of -
Fig. 6. Tauc Plot of FMBC.
˚
˚
0.030 A (red) to 0020 A(blue). To calculate the electrostatic po-
tential, the experimental geometry obtained from the single-crystal
X-ray diffraction study was used with TONTO integrated into Crys-
tal Explorer program [19]. These plots help in understanding the
crystal packing by visualizing the various intermolecular interac-
tions. The short but strong interatomic contacts appear as red spots
on the dnorm surface (Fig. 11a) while other weak interactions with
positive dnorm value are represented in blue. The surfaces mapped
over shape index and curvedness properties shown in Fig. 11b and
Fig. 11c helps to identify the different packing modes and the in-
terconnections between nearby molecules. Through the shape in-
dex map of the HS, one can visualize the existence of π-π stacking
interactions. From Fig. 11(b), it is clear that there are no such inter-
actions present in FMBC which is evident from the absence of ad-
jacent red and blue triangular regions. This is further confirmed by
◦
˚
c = 8.98(2) A, β = 101.18(2) . The structure of the FMBC molecule
with 50 % probability ellipsoids is shown in Fig. 8. A detailed
description of the structure can be found in the literature [11].
The structure is stabilized by N-H···O, C-H···O, and C-H···F interac-
tions as shown in Fig. 9. The N-H···O hydrogen bond interactions
help the molecules to pack into the infinite chains along crystallo-
graphic c direction as shown in Fig. 9 and Fig. 10. Also, there exists
a highly directional C-H···F interaction which helps the molecules
to form a stable crystal structure. The atoms of the C=N–N–C=O
bridge namely C8, N8, N9, C10, and O11 are coplanar [11], and this
◦
plane makes an angle of 13 with the phenylene ring and makes
◦
an angle of 22.05
with the mean plane through the biphenyl
rings of the benzoyl arm. The fragments of the biphenyl rings are
twisted by an amount of 23.14 ◦. The terminal benzene rings lie
Fig. 8. Molecular structure of FMBC shown with 50 % probability ellipsoids.
4

L. Kamath, A.P. Menezes, R. Bairy et al.
Journal of Molecular Structure 1245 (2021) 131019
Fig. 9. Packing diagram of FMBC showing hydrogen bond interactions.
Fig. 10. Packing diagram of FMBC showing orientation of dipoles along the c–
direction.
Fig. 12. Hirshfeld surfaces mapped over (a) dnorm and (b, c) electrostatic potential
of FMBC showing N-H…O hydrogen bond interactions as dashed lines.
the donors with positive electrostatic potential are shown as blue
regions.
The 2D fingerprint .plot is a useful method to represent the
complex information incorporated in a crystal. It provides a vi-
sual summary of the frequency of the different combinations of de
and di values across the Hirshfeld surface of a molecule. A finger-
print plot, which is divided into certain specific interatomic con-
tact, provides information about that specific intermolecular inter-
action. The fraction of the various intermolecular interactions that
contribute to the overall HS of FMBC is shown in Fig. 13 and Fig. 14
along with the portion of the HS covered by each type while the
relative contributions are summarized in Fig. 15. The main contri-
bution to the surface arises from C···H and H···H interactions cover-
ing 37.9 % and 32.5 % of the HS respectively. The reciprocal contacts
on H···O, H···F intermolecular interactions appeared as two sharp
Fig. 11. Hirshfeld surfaces mapped over (a) dnorm, (b) shape index, and (c) curved-
ness.
the absence of flat regions and the dark blue edges on the curved-
ness map. The concave-shaped regions in bright red (red hollow
with shape index <1) indicate the acceptor atoms while convex
regions in blue color (blue bumps-shape with shape index >1) ap-
pear around the donor atoms on the HS mapped with shape in-
dex [20]. Fig 12(a) is the dnorm surface showing N-H…O interaction
with a dark red spot on N-H atom and the oxygen atom due to
∼
=
∼
=
needles with de + di
2.28 Å for H···O and with d + d
2.82 Å
e
i
for H···F interaction. The H···C interaction appeared as the sym-
∼
=
metrical wing, de + di
3.28 Å with significant contribution to
the packing of the crystal. The other major contributor for crystal
…
the presence of strong H9 O11 hydrogen bond in the title com-
packing is H···H interatomic contact, appeared as a diffused needle
∼
=
pound. The electrostatic potential surface displayed in Fig. 12 (b)
and Fig. 12(c) further establishes the N-H…O interaction where the
atoms involved in strong hydrogen bonds are seen as dark-blue
and dark-red regions. The red regions in these plots represent the
negative electrostatic potential corresponding to the acceptors and
with de + di
2.44 Å.
Surface contact data derived from the HS analysis are used to
derive enrichment ratios (ER) that enable to analyze the tendency
of chemical species to interact in pairs in forming crystal packag-
ing and are tabulated in Table 1 [21]. The ER values for C···H, N···H,
5

L. Kamath, A.P. Menezes, R. Bairy et al.
Journal of Molecular Structure 1245 (2021) 131019
Fig. 13. 2D fingerprint plots for FMBC delineated into F···F, C···O, C···C, and H···N contacts.
Fig. 14. 2D fingerprint plots for FMBC delineated into H···F, H···O, H···H, and C···H intercontacts.
O···H, and F···H are greater than unity indicating that these pairs
have a larger propensity to form interconnects in the crystal struc-
ture whereas the pairs with random contacts fewer than 0.9 % are
ignored. The C···C and O···C inter-contacts have ER values 0.62 and
0.61 respectively and are less likely to form contacts in the crystal.
The H···H self-contact is also not favoured with an ER value of 0.74.
Furthermore, the intermolecular interaction energies between
the molecular pairs in FMBC were calculated using the Crystal Ex-
plorer 17.5 software. These energy values were further used to vi-
sualize and analyze the predominant interactions in the molecu-
lar crystals packing as 3D energy frameworks [22,23]. A cluster of
molecules was built around a selected reference molecule within a
radius of 3.8 Å and the interaction energy was calculated by gener-
ating the wave function using the most accurate B3LYP/6-31G(d,p)
level of theory. By applying crystallographic symmetry operations,
the encircling molecules in the shell around the central molecule
were obtained. Within the defined radius (shell) there are a total
of ten molecules that are involved in the dimer formation. These
molecular energy interaction pairs enable the total energy to be
divided into different components such as electrostatic (Eele), dis-
persion (Edis), repulsion (Erep), and polarization (Epol). Mackenzie’s
method was adopted to partition the total energy benchmark with
1.057, 0.871, 0.618, and 0.740 as the scale factors, for electrostatic,
dispersion, repulsion, and polarization respectively [22]. The calcu-
lated interaction energies between the dimers within the default
shell are given in Table 2. Fig. 16 is the visualization of the interac-
6

L. Kamath, A.P. Menezes, R. Bairy et al.
Journal of Molecular Structure 1245 (2021) 131019
Table 1
Hirshfeld surface contacts, the derived random contacts, and ER values for the FMBC by atom type. The data
values obtained from Crystal Explorer 17.5 program are written in italics at the top of the table. The table
does not include the ER for random contacts less than 0.9 % as they do not mean.
Atoms
H
C
N
O
F
H
32.5
37.9
6.8
Contacts (%)
C
2.5
N
-
-
-
-
O
9.2
1.3
-
-
-
F
8.7
1.0
Surface %
61.27
43.77
26.51
4.23
7.00
7.05
0.74
1.43
1.61
1.31
1.23
24.57
3.58
5.03
5.55
H
C
N
O
F
Random Contacts (%)
4.01
1.28
2.12
2.13
0.10
0.34
0.34
0.28
0.56
0.28
H
C
N
O
F
Enrichment of contacts
0.62
-
/
/
/
0.61
-
/
/
/
Table 2
Interaction Energies (kJ/mol) with symmetry operations (symop) for FMBC. R is the distance between molecular cen-
˚
troids (mean atomic position) in A. Total energies are the sum of the four energy components, scaled appropriately. The
energy model used CE-B3LYP ... B3LYP/6-31G(d,p) electron densities with K_ele (1.057), K_pol (0.740), K_disp (0.871) and
K_rep (0.618). Refer to Fig. 14 for color codes.
N
Symop
R
E_ele
E_pol
E_dis
E_rep
E_tot
1
-x, -y, -z
17.99
6.21
-3.9
-14.7
-0.7
-3.4
-1.2
-53.7
-3.8
-3.8
0.5
-0.4
-3.1
-0.3
-2.7
-0.4
-13.0
-1.1
-2.2
-0.2
-0.2
-23.6
-7.5
5.2
-7.8
0
-x+1/2, y+1/2, -z+1/2
x+1/2, y+1/2, z
-x+1/2, -y+1/2, -z
-x, y, -z+1/2
-33.5
-5.7
16.8
2.9
-36.6
-4.1
1
19.52
5.88
0
-35.7
-8.2
18.1
2.6
-25.5
-7.1
1
17.64
4.07
1
x, -y, z+1/2
-72.3
-16.7
-54.1
-4.8
82.5
8.9
-78.4
-13.9
-37.7
-2.8
0
-x+1/2, y+1/2, -z+1/2
-x+1/2, -y+1/2, -z
-x, -y, -z
7.34
1
5.05
24.4
1.5
0
0
19.60
18.87
-x, y, -z+1/2
-0.6
-85.3
-6.9
2.2
-5.4
Total Energy
-245.4
165.1
-219.3
Fig. 16. Color coding of the adjacent molecules concerning the central molecule
(shown with HS) viewed along a,b,c axes.
Fig. 15. The relative contribution of intermolecular contacts in FMBC to the Hirsh-
feld surface. Inset is the full 2D fingerprint plot.
An extended characteristic established by the calculation of in-
teraction energies is the simulated 3D energy framework. These
energy frameworks enable one to have a better understanding
of the topology of the overall interaction energies among vari-
ous parts of the crystal. The framework is obtained by generat-
ing a cluster of molecules within a 1 × 1 × 1 unit cell. The gen-
erated frameworks are presented in Fig. 17. The relative strengths
of molecular packing interaction energy in all directions are indi-
cated by the cylindrical shapes where the radii of the rods are pro-
tion energies via the color codes. The total interaction energies de-
lineated into electrostatic, polarization, dispersion, and exchange-
repulsion respectively are -85.3 kJ/mol, -23.6 kJ/mol, -245.4 kJ/mol,
and 165.1 kJ/mol. The total interaction energy is found to be -219.3
kJ/mol. These energy values show that the overall stability of the
structure is mainly due to the dispersion force followed by the
electrostatic potential force.
7

L. Kamath, A.P. Menezes, R. Bairy et al.
Journal of Molecular Structure 1245 (2021) 131019
Fig. 17. Energy framework diagrams for Eele (red tubes), Edis(green tubes), Etot(blue tubes) for a cluster of molecules within 1 × 1 × 1 unit cell of FMBC viewed down the a,b,c-
axes.
portional to the magnitude of the interaction energy in a different
direction. The energy frameworks for electrostatic, dispersion and
total energy components are shown in red, green, and blue colored
tubes. All the frameworks were adjusted to the same cylinder scale
factor of 150. Weak and insignificant contacts less than the 5 kJ /
mol threshold were not included in the calculation.
Third-order NLO parameters (n2 and β) were estimated by the
well known single beam Z scan experimental procedure. The non-
linear refractive index is determined by measuring the intensity
of the light transmitted by the sample as it moves along the fo-
cal length of the beam. In this case, the light intensity is detected
after passing it through an aperture (close aperture mode). The
beam experiences a nonlinear phase shift ꢀ∅ as it passes through
the sample which leads to the variation in the amount of light
falling on the detector. The shape of the curve and thus the rel-
ative position of the peak and valley on the Z-axis depend on the
nonlinear phase shift [24]. Fig. 18(a) confirms that the normalized
transmittance exhibits a transmission maximum (peak) before the
focus and a transmission minimum (valley) after the focus. This
peak-valley configuration is a signature of self – defocusing effect
in FMBC (negative nonlinear refractive index). Further, the curve is
asymmetric and thus it can be inferred that when the sample is
scanned with a CW laser beam, the laser beam gets distorted due
to the spatial thermal variation. Hence the nonlinear refraction ob-
served in FMBC is of thermal origin. The magnitude and sign of n2
can be obtained by fitting the experimental closed aperture nor-
malized transmittance with the theoretical equation,
ꢀ
ꢃ
4xꢀ∅
(
)
ꢁ
ꢂꢁ
ꢂ
T z
( )
=
1 −
(1)
x2 + 1 x2 + 9
Fig. 18. (a) Closed aperture curve, (b) open aperture curve of FMBC.
Where x = z/z0 and ꢀ∅ is the nonlinear phase shift. The magni-
tude of n2 can be found from the relation, ꢀ∅ = kn2I0Le f f . In this
equation, k is the wave vector (k = 2π/λ), Le f f = [1 − e−αL]/α, is
the effective thickness of the sample, and α is the linear absorp-
tion coefficient.
The experimental open aperture data were fitted with the
equation (2) to extract the β value[13] [25].
ꢄ
ꢅ
I0Le f f β
1
T z = 1 −
( )
√
(2)
1 + x2
2
2
8

L. Kamath, A.P. Menezes, R. Bairy et al.
Journal of Molecular Structure 1245 (2021) 131019
Table 3
Third-order nonlinear optical parameters of FMBC.
λ
n2
β Re χ3 Imgχ3 χ3
γh
(m)
×10−9
(cm2/W) (cm/W) (esu) (esu) (esu) (esu)
×10−8 × 10−6 × 10−7 × 10−7 × 10−7 × 10−26
532 −1.568 2.22 6.724 3.896 6.72 4.877
Table 4
The absorption cross-sections and optical limiting threshold value of FMBC.
[34,35]. This kind of charge transfer increases the π−electron den-
sity through improved conjugation, which in turn contributes to
the improvement in the NLO property of a material. Besides, the
hydrogen bonds, optimized alignment of the molecular dipoles in
the bulk, molecular planarity play an important role in the opti-
cal nonlinearity of the material. From the X-ray diffraction studies
and the HS analysis, we found that the crystal structure of FMBC
is stabilized by the strong N-H…O and C-H…F hydrogen bond inter-
action. The benzoyl and the phenylene moieties are interconnected
via a planar C=N–N–C=O chain and this planar backbone enables
an effective flow of charge from the ends of the molecule towards
the center [11]. None the less, these hydrogen bond interactions
help the molecules to pack in an optimized manner so that there
is addition of the individual dipole moments which in turn con-
tributes to the NLO properties of the material. In FMBC, the molec-
ular dipoles are stacked into layers along c-direction (Fig. 9) but
with an angle inclination of 66.380 between the adjacent dipoles
(Fig. 10). Thus, though the molecules are nearly planar, the net
dipole moment within a unit cell is quite small and hence ob-
served β value of FMBC is less than some of the earlier reported
materials (Table 5). Furthermore, optical excitation of the sample
results in π − π∗ electronic transition within the aromatic part of
the molecule, which has a longer lifetime in the excited states.
These long lifetimes lead to the enhanced frequency of two photon
resonance transitions which enhances the NLA via improved delo-
calization of π-electron and degree of π-conjugation [36]. Thus,
the observed optical nonlinearity of FMBC can be attributed to its
crystal structure consisting of planar molecules held in position by
N-H…O and C-H…F hydrogen bond interaction.
σ2
σ21 σex σg
OL Threshold
(cm4/W) (cm4S) (cm2) (cm2 ) (J/cm2)
×10−25 × 10−43 × 10−20 × 10−23 ×103
3.69
1.38 1.92 6.1 3.42
The experimental and theoretically fitted open aperture curves
are shown in Fig. 18(b). It is evident from the shape of the plot
that the title compound exhibits two-photon absorption with re-
verse saturation (Table 4). The positive NLA coefficient of FMBC is
confirmed from the fact that the light intensity is least at the focus
(Z = 0) and increases symmetrically on either side of it [26]. The
estimated values of n2 and β were used to determine the molec-
ular second-order hyperpolarizability γhwhich is the indication of
induced nonlinear molecular polarization. The quantity γhis related
to the macroscopic third-order nonlinear optical susceptibility χ(3)
through the equation,
3
χ( )
γh =
(3)
ꢁꢁ
ꢂ ꢂ
4
NAC n20 + 2 /3 × 10−3
Here, C is the concentration (mole/cm3), NA is the Avogadro
number, and n0 is the linear refractive index. χ(3) is obtained us-
ing the relations [27],
1
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
/
ꢆ
ꢆ
ꢆ
ꢆ
ꢁ
ꢂ
ꢁ
ꢂ
2
2
2
3
( )
3
3
χ
=
Reχ( ) + Imχ( )
(4)
(5)
(6)
ꢁ
ꢂ
The ratio σex/σg is the figure of merit (FOM) for a material,
the magnitude of which determines the optical limiting (OL) prop-
erty. Optical limiting (OL) is the ability of a material to allow low-
intensity radiation to pass through while blocking high-intensity
radiation after a certain threshold. These types of devices are very
useful in protecting human eyes, optical sensors, and such sensi-
tive systems from intense laser beams. From Table 4, it is clear
that FOM for FMBC is much greater than unity and thus we be-
lieve that it is a good material candidate for optical limiting de-
vices for controlling CW laser intensities of μW to KW power. The
OL behavior was extracted using the open aperture z scan experi-
mental technique by plotting normalized transmittance against the
input fluence and is shown in Fig. 19. In the case of an open aper-
ture z scan experiment employing a Gaussian beam, the position-
dependent fluence at point z along the beam was calculated using
the equation,
ε0c2n20n2
3
Reχ( ) e.s.u
=
(
)
π
ꢁ
ꢂ
ε0C2βn0λ
3
Imχ( ) e.s.u
=
(
)
π
The magnitudes of the various third-order nonlinear optical pa-
rameters thus obtained are presented in Table 3 and Table 4. The
values of molecular two-photon absorption coefficient(σ2), molec-
ular two-photon absorption coefficient(σ21), excited-state absorp-
tion coefficient(), and ground-state absorption coefficient () were
estimated by using the equations presented in the literature [26].
The computed values of n2, β, χ3, and γh are compared with
those of some of the well characterized organic materials under
the CW regime and are tabulated in Table 5.
The observed good third-order NLO coefficients of FMBC under
investigation can be understood from the molecular structure and
stacking of molecules in the crystal structure. The molecule con-
sists of a hydrazone group (-C(=O)-NH-N=CH-) as can be seen in
Fig. 8. The benzene ring with fluoro group attached to its para-
position (phenylene group) at one end of this chain acts as an
electron donor moiety. The carbonyl group (-C=O-) is an electron-
withdrawing group that attracts the charges from the biphenyl
group at the other end of the hydrazone chain. Thus, FMBC is
a donor-π-acceptor-π-donor (D-π-A-π-D) type structure where
charges flow ends of the molecule towards the center [29]. Since a
strong electron donor is placed at the para position of the benzene
ring, there is an effective transfer of charge from that ends to the
center of the molecule resulting in the delocalization of charges
1
)
3
/
/
1/2
2
2
F z = 4 ln2
Ein/π w0 1 + z/z
(7)
( )
(
(
)
0
where Ein is the input laser energy, w0 is the beam waist at the
focus and z0 = πw20/λ Rayleigh range. For lower intensity/fluence
of the incident radiation, the transmittance exhibits almost linear
behavior to the incident intensity (Fig. 19). As the input intensity
increases beyond the threshold (3.42 kJ/cm2), the sample absorbs
more light and hence limits the intensity of the transmitted beam.
It is astonishing to note that FMBC has a better OL capacity com-
pared to some of the previously reported materials (Table 5). Ther-
mal defocusing, self diffraction and reverse saturable absorption
(RSA) might cause optical limitation in a material. Sinceσex ꢀ σg,
9

L. Kamath, A.P. Menezes, R. Bairy et al.
Journal of Molecular Structure 1245 (2021) 131019
Table 5
Comparison of NLO parameters of FMBC with reported organic materials under similar experimental conditions.
n2
β χ3
γh
OL Threshold
Melting Point
Compound (cm2/W ) (cm/W ) (esu) (esu)
(J/cm2)
(°C)
×10-8×10-6×10-7×10-26×103
[∗]FMBC
−1.568 2.22 6.72 4.877 3.42
242.5
9.0
5.1
4.1
1.5
-
[28]Br-ANC
−0.408
−0.23
−4.21
−0.252
7.8
2.33
2.0
-
-
191.16
170.12
160
[29]3CAMC
0.81
-
6.13
5.32
1.85
145
11.2
7.41
1.92
4.12
[25]DMAP4P
[27]MFNP
2.41
1.44
-
0.593
6.75
0.1044
1.57
4.64
-
136
[30]NPTMP −0.437
[31]FMC
−.0.1466
−0.45
−1.31
−0.246
45
0.126
3.8
-
[32]F3BC
55
108.5
177
147.7
[32]F3NC
12
11.3
1.78
[33]2ATN
33.5
and the nonlinear optical susceptibility (χ(3)) were estimated by
single beam z scan experiment under CW regime and are found to
be 2.22 × 10−6 cm/W, −1.568 × 10−8 cm2/W, and 6.72 × 10−7esu
respectively. Furthermore, FMBC shows reverse saturable absorp-
tion as the condition of σg < σex was satisfied and thus capable
of limiting the high power radiations with a limiting threshold
of 3.24 kJ/cm2. The exceptional results make FMBC, a potential
material for optical limiting and switching applications.
CRediT author statement: Author contributions
Laxminarayana Kamath: Conceptualization, Acquisition of data
Anthoni Praveen Menezes: Conceptualization, Methodology
Analysis and/or interpretation of data, drafting the original
manuscript
Raghavendra Bairy: Software, Validation, and Revision.
K. Shashi Kumar: Revising the manuscript
Fig. 19. Optical Limiting behavior of FMBC.
A. Jayarama: Supervision, revising the manuscript, and editing.
the condition for RSA, the observed OL property of FMBC is as-
cribed to RSA [37]. The one-photon FOM, W = n2I/αλ and two-
photon FOM, T = βλ/n2 for FMBC were calculated from n2 and
β values. For all-optical switching applications, W must be greater
than unity and T must be less than unity [29]. In the present inves-
tigation, the determined values are satisfying this target condition
and hence FMBC could be a potential material for optical switching
applications.
This statement is signed by (on behalf of all the authors)
Anthoni Praveen Menezes (signed on 21-10-2020)
Declaration of competing interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
4. Conclusion
References
A Schiff based hydrazone derivative, FMBC was synthesized
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are reported in this article. The synthesized compound was
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