Vibrational analysis and physical property studies of 6-Methoxy-2-[(E)-phenyliminomethyl]-phenol in the THz, IR and UV–visible spectral regions

Article abstract of DOI:10.1016/j.saa.2019.117227

Bulk single crystals of 6-Methoxy-2-[(E)-phenyliminomethyl]phenol were grown after preparing the material by Schiff base condensation of ortho-vanillin alternatively called 2-hydroxy-3-methoxybenzaldehyde and aniline. The three dimensional molecular and c

Full text of DOI:10.1016/j.saa.2019.117227

Accepted Manuscript
Vibrational analysis and physical property studies of
6-Methoxy-2-[(E)-phenyliminomethyl]-phenol in the THz, IR and
UV–visible spectral regions
K.M. Hijas, S. Madan Kumar, B.C. Manjunath, R. Nagalakshmi
PII:
S1386-1425(19)30617-1
DOI:
https://doi.org/10.1016/j.saa.2019.117227
Article
Number:
Reference:
117227
SAA 117227
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date:
Revised date:
Accepted date:
9 April 2019
30 May 2019
31 May 2019
Please cite this article as: K.M. Hijas, S.M. Kumar, B.C. Manjunath, et al., Vibrational
analysis and physical property studies of 6-Methoxy-2-[(E)-phenyliminomethyl]-phenol
in the THz, IR and UV–visible spectral regions, Spectrochimica Acta Part A: Molecular
and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117227
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ACCEPTED MANUSCRIPT
Vibrational analysis and physical property studies of 6-Methoxy-2-[(E)-
phenyliminomethyl]-phenol in the THz, IR and UV-Visible spectral regions.
K.M. Hijasa, S. Madan Kumarb, B. C Manjunathc, R. Nagalakshmia*
aIntermetallic and nonlinear optics laboratory, Department of Physics, National Institute of
Technology, Tiruchirappalli 620015, India.
bDST-PURSE lab, Mangalore university, Mangalagangotri, 574199, India
cDepartment of physics, Yuvaraja's college, University of Mysore, 570005, India.
Abstract
Bulk single crystals of 6-Methoxy-2-[(E)-phenyliminomethyl]phenol were grown after
preparing the material by Schiff base condensation of ortho-vanillin alternatively called 2-
hydroxy-3-methoxybenzaldehyde and aniline. The three dimensional molecular and crystal
structure of the title compound is confirmed by X-ray diffraction. Molecules crystallized in the
orthorhombic crystal system and noncentrosymmetric space group P212121. Geometry
optimization, vibrational analysis, Calculation of HOMO-LUMO band gap and molecular
hyperpolarizability of the proposed material have been carried out. Terahertz time domain
spectroscopic studies have been performed and the refractive index and absorption coefficient of
material is calculated in the THz regime. Molecular vibrations responsible for different THz
phonon modes are identified with the help of density functional theory based calculations.
Keywords: Crystal growth, Nonlinear optics, Second harmonic generation, Organic materials,
THz spectroscopy, Density functional theory.
*Corresponding
author:
Tel.:
+91431-2503615;
e-mail:
nagaphys@yahoo.com,
nagalakshmi@nitt.edu (R.Nagalakshmi,)

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1.Introduction
Low cost organic nonlinear optical (NLO) materials are being considered as efficient
substitutes to their inorganic counterparts because of their large values of hyperpolarizability and
less complex crystal growth process [1, 2]. Since organic materials exhibit superior electro-optic
response, they are also potential alternatives to inorganic materials for terahertz (THz) generation
and subsequent applications[3]. Major drawback of the organic NLO materials reported so far is
their low stability under laser action and moisture. The highly efficient, well established organic
NLO material DAST cannot be used for second harmonic generation (SHG) using the
technologically important widespread Nd:YAG laser of wavelength 1064 nm because of its
absorption edge in 560 nm and high absorbance at 532 nm [4, 5]. Researchers are working
towards developing high efficient NLO organic noncentrosymmetric materials with higher laser
damage threshold. Materials are also supposed to be possessed with the property of low
absorption in the optically important spectral regions so that they can be potentially applied to
second harmonic and THz generation purposes. This can be achieved by constructing new
organic molecules with donor-π-acceptor (D-π-A) system comprising different functional groups
which have lesser absorption bands in the THz and visible regions[6, 7]. But most of the time the
chiral molecular structures built in this way will crystallize in a centrosymmetric fashion which
cannot be used for second order nonlinear optical processes [8]. In that sense it is easy and
important to study the physical properties related to nonlinear optical performance of organic
materials for which noncentrosymmetric crystal structure is already reported.
Schiff base condensation is a well used chemical process for design and synthesis of
organic NLO materials. It’s an easily feasible chemical reaction to add or substitute electron
accepting or donating organic functional groups to the parent molecule and also to establish π-
bridges in it[9-11]. The large optical nonlinearity and high second harmonic generation (SHG)
efficiency of the organic material vanillin and its derivatives and that of the derivative of aniline
are well known [12]. m-nitroaniline, 2-methyl-4-nitroaniline, etc are some very familiar
examples. m-nitroanilin is reported for its terahertz generation as well [13, 14].
Vanillylideneaniline (VAN) also called 2-methoxy-4(phenyliminomethyl) phenol formed
through Schiff base condensation of vanillin and aniline has been already reported for its SHG
and high nonlinear optical activity [15, 16]. The authors have reported its detailed infrared and

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Raman spectroscopic analysis along with computational details in [9]. In this report , we are
introducing another NLO crystal for second order processes which is obtained by transferring the
hydroxyl group of aldehyde moiety of VAN from para position to ortho position (Compare the
molecular structure given under geometry optimization section of this manuscript with figure 4
of reference [9]). Here the title material 6-Methoxy-2-[(E)-phenyliminomethyl]-phenol is
synthesized by Schiff base condensation of 2-Hydroxy-3-methoxybenzaldehyde alternatively
called ortho-Vanillin with aniline. The synthesis procedure and noncentrosymmetric crystal
structure is already reported by Yu-Ye Yu [17]. We have grown a bulk crystal of the material
with a dimension of size 7 X 4 X 3 mm3 by slow evaporation solution growth technique and its
structure is confirmed by single crystal X-ray diffraction method. A study on physical properties
which affects the nonlinear optical performance of the material and detailed vibrational analysis
has been carried out along with theoretical calculations. Refractive index and absorption
coefficient of the material in the technically important 0-2THz region is also studied. Molecular
vibrations responsible for THz phonon mode absorption in this range are investigated as well.
The first order molecular hyperpolarizability calculation and experimentally observed SHG show
that the grown crystal is a potential candidate for second order nonlinear optical processes.
2.Synthesis and crystallization
The title material, 6-Methoxy-2-[(E)-phenyliminomethyl]phenol (OVAN) is prepared by
Schiff base condensation of ortho-vanillin alternatively called 2-hydroxy-3-
methoxybenzaldehyde and aniline. Measured amount of ortho-vanillin is diluted in 20 ml of
ethanol and stirred for 10 minutes. Equimolar amount of aniline is added to this drop by drop
using a micropipette. The solution is stirred and refluxed for two hours and then set to cool. It is
then transferred to a beaker and covered with perforated aluminum foil. The prepared solution is
thus allowed for slow evaporation to form the crystals of red colour. This is recrystallised several
times to get the purest crystal. Finally crystal of the size 7 X 4 X 3 mm3 has been harvested.
Photograph of the as grown crystal is shown in Fig.1a. Reaction scheme of the title material is
given in Fig. 1b.

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Fig. 1: a) Photograph of as grown OVAN crystal. b) Schiff's base condensation reaction of ortho-vanillin and Aniline in ethanol
solvent to form 6-Methoxy-2-[(E)-phenyliminomethyl]phenol.
3.Experimental details
Single crystal X-ray diffraction experiment was performed on a Rigaku Saturn724
diffractometer using graphite monochromated Mo-Ka radiation at a temperature of 296 K. A
complete data set was processed using Crystal Clear software. Perkin Elmer Frontier FTIR/FIR
spectrometer was used to record FTIR spectrum in the wavenumber range of 4000 cm-1 - 400 cm-
1. JASCO V670 UV-vis-NIR spectrophotometer was used to record the UV-Visible absorption
spectrum in the range 200 -1000 nm. Kurtz-Perry powder technique was employed to make a
qualitative measurement of SHG efficiency. THz time domain spectroscopic studies (THz-TDS)
were carried out using a TERA K8 terahertz spectrometer manufactured by Menlosystems. To
decrease the absorption of THz radiation by water vapor, nitrogen gas was filled in to the
spectrometer till the humidity reads less than 7%. The refractive index and absorption coefficient
of VAN in the 0-2 THz regime were calculated by inbuilt software TeraMat attached with the
spectrometer setup.
4.Computational details
Optimized molecular geometry of OVAN is computed using B3LYP (Becke, three-
parameter, Lee-Yang-Parr) functional with 6-311++g(d,p) basis set. Gaussian 16, revision B.01
software package was used for computation and GaussView 06 program for visualization. [18,
19]. Most stable conformation obtained out of geometry optimization was further confirmed by a
potential energy scanning over the possible dihedral angles. A vibrational analysis was carried
out using the same level of theory and the FTIR spectrum of the material is simulated. VEDA 04

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program was used to calculate the potential energy distribution of vibrational wavenumbers in
the IR and THz region.
5. Results and discussions
5.1. X-ray diffraction analysis and geometry optimization
CCDC number
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
1914825
C14H13NO2
227.25
293(2)
orthorhombic
P212121
a/Å
6.0826(5)
b/Å
9.1620(8)
c/Å
21.0100(16)
α/°
90
β/°
90
γ/°
90
Volume/Å3
1170.87(16)
Z
4
ρcalcg/cm3
1.289
0.087
μ/mm-1
F(000)
480.0
Crystal size/mm3
Radiation
0.33 × 0.29 × 0.23
MoKα (λ = 0.71073)
3.878 to 50.054
-7 ≤ h ≤ 6, -10 ≤ k ≤ 10, -25 ≤ l ≤ 25
9765
2057 [Rint = 0.0663, Rsigma = 0.0467]
2057/0/156
1.079
2Θ range for data collection/°
Index ranges
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indexes [I>=2σ (I)]
Final R indexes [all data]
Largest diff. peak/hole / e Å-3
Flack parameter
R1 = 0.0805, wR2 = 0.2182
R1 = 0.1068, wR2 = 0.2575
0.35/-0.28
0.3(10)
Table 1. Crystal data and structure refinement for OVAN.

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Fig. 2: Crystal packing view of OVAN along a-axis
Synthesis and crystal structure of the material OVAN is already reported[17]. To confirm
the crystal structure we carried out single crystal X-ray diffraction experiment on the grown
samples and the structure details obtained were found to be in agreement with the earlier
structure report [17]. Also the crystallographic data obtained is in accordance with the ‘Tables of
bond lengths determined by X-ray and neutron diffraction. part 1 .bond lengths in organic
compounds’ by Frank H. Allen et. al [20]. The details of the crystal data and structure refinement
is presented in Table 1. Fig. 2 shows the crystal packing view of the grown crystals along a-axis.
As shown in Table 1, the material is found to be crystallized in orthorhombic crystal system with
noncentrosymmetric space group P212121.
Fig. 3a shows the optimized molecular arrangement of OVAN molecule in gas phase.
Fig. 3b shows the orientation of a single molecule in the crystalline unit cell which is deduced
out of the single crystal X-ray diffraction structure solution. Fig. 3a is generated using density
functional theory (DFT) calculations in Gaussian software. This is the most stable conformation
of the molecule’s geometry which is confirmed after a potential energy surface scanning over the
possible dihedral angles. Fig. 4 shows the variation of potential energy of the molecule when the
dihedral angles C21-C20-N19-C17, C2-C3-O9-C10, H8-O7-C2-C1 are rotated through 360° with
rotation steps of 10°. In all the three cases the lowest energy value is found as -746.705 Hartree.
In all this three cases molecular orientation is similar to that obtained from the self consistent
field equation solution which is shown in Fig. 3a. A comparison of the DFT optimized
geometrical parameters with the experimental values has been carried out. Details are tabulated
as Table A1 in appendix. The calculated geometrical parameters are found to be in good

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agreement with the single crystal XRD measurements output. Most of the calculated bond
lengths are same as the experimental observations up to two decimal places. C-H bond lengths
are calculated to be 1.08 Å but it is found as 0.93 Å and 0.96 Å experimentally. This is expected
as the DFT calculations are done in gas phase without considering intermolecular crystalline
effects. Bond length O7-H8 has been calculated as 0.99 Å is found experimentally only of length
0.82 Å. This is the shortest bond because it makes an intermolecular hydrogen bonding with the
nitrogen atom N19 [9]. It must be also in intermolecular hydrogen bonding relationships with the
neighboring molecules in the unit cell. This is further confirmed by the difference in the
simulated and experimental values of dihedral angle C1-C2-O7-H8 = 0.36 (simulated) and -
138.0 (experimental) as visible in Fig. 3a and 3b. Also there exists a strong intramolecular
hydrogen bonding between H8--O9 atoms in the molecule. The crystal packing diagram in Fig. 2
shows the unit cell of the crystal comprise of 4 molecules also linked through an intermolecular
hydrogen boding between O9--H29 between neighboring molecules. This O9--H29 hydrogen
bonding can be further confirmed by the abnormal difference in the simulated and experimental
bond angle value of O9-C10-H11 = 105.74 (simulated) and 109.5 (experimental). The existence
of C17-N19 bond of length 1.28 Å is a confirmation of the occurrence of intended Schiff base
condensation reaction between the ortho-vanillin and aniline moieties of the title material.
Different dihedral angle values will help us to see how planar the molecule is. The two rings in
the molecule exist in different planes. The difference in the dihedral angle values C17-N19-C20-
C25 = 144.14, C2-C1-C17-N19 = -0.62 confirms that the two rings are not coplanar. But the
bridge atoms connecting the two rings C1, C17, N19 and C20 are sharing almost the same spatial
plane as the dihedral angle C1-C17-N19-C20 value reads 177.23. The dihedral angle C2-C1-
C17-N19 = -0.62 also show that first three of the above mentioned bridge atoms lie in the same
plane of ortho-vanillin ring. The dihedral angle values C2-C3-O9-C10 = -179.80, C4-C3-O9-
C10 = 0.2 shows that the carbon atom C10 of the methoxy group lies in the same plane of the
ring. This in turn enlights into the vibrational analysis of the molecule. It is proved that the
methoxy group lying in the same plane with the attached ring structure may cause a higher
wavenumber value for its asymmetric and symmetric stretching. This is discussed in the section
methoxy group vibration in vibrational analysis part in this report. The dihedral angle value C3-
O9-C10-H11 = 179.87 shows even the hydrogen atom H11 is also in the same plane of the ring
and methoxy carbon. But the other two hydrogen atoms of the methoxy group lies out of plane

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from the ring. This is evident from the dihedral angle values C3-O9-C10-H12 = 61.16, C3-O9-
C10-H13 = -61.42. C1-C17 is the largest C-C bond length observed which is equal to 1.45 Å.
Most of the other C-C bond lengths are around 1.41 Å. This shows that the molecule is highly
conjugated and there is a large delocalization of π electrons. C3-C4 (1.39 Å), C5-C6 (1.38 Å),
C21-C22 (1.39 Å), C22-C23 (1.39 Å), C23-C24 (1.39 Å), C24-C25 (1.39 Å), are the shortest C-
C bonds observed. It means π electron population is higher at these positions. The largest π
electron concentration is located at C17-N19 where we have the shortest bond length of 1.28 Å.
The bond lengths C2-O7 (1.34 Å) and C3-O9 are also comparatively shorter as the two oxygen
atoms make major contribution to the π electron delocalization in the ring.
Fig. 3: a).Optimized molecular structure of 6-Methoxy-2-[(E)-phenyliminomethyl]phenol using B3LYP/6-
311++G(d,p) level of DFT. b) Orientation of a single molecule in the crystal unit cell obtained from single crystal
Fig. 4: Two dimensional potential energy scan curves of OVAN at B3LYP/6-311++G(d,p) level

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5.2. Vibrational analysis
Fig. 5: a) Simulated B3LYP/6-311++G(d, p) level IR spectrum. b) FT-IR spectrum of OVAN recorded in the range 4000-400 cm-1
5.2.1. Hydroxyl group vibrations
Fig. 5a and 5b show the simulated and experimental IR absorption bands for OVAN.
Major absorption peaks and vibrational assignments are detailed in Table 2. The sharp band at
3065 cm-1 in the simulated spectrum stands for the absorption due to O-H stretching vibrations.
Experimentally a very weak absorption band is found at 3080 cm-1. This is usually expected in
and around 3600 cm-1 without the influence of intra/inter- molecular hydrogen bonds[21, 22].
The experiment was performed using solid powder sample which has all its crystalline nature
prevalent. Therefore in the experimentally observed spectrum O-H stretching band is found to be
weakened and shifted towards a lower wavenumber. This is due to the effect of strong
intramolecular N19-H8 and intermolecular C-H--O hydrogen bonds[17]. The sharp band at 1195
cm-1 supports this argument as it confirms the existence of strong intermolecular hydrogen
bonding[23]. Absorption due to O-H in-plane and out-of-plane bending vibrations are observed
at 1443 cm-1 and 690 cm-1 respectively. The out-of-plane bending vibration can vary from 800
cm-1 to 300 cm-1 according to the strength of prevailing hydrogen bonds with the O-H atoms[24].

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Therefore the O-H out-of-plane bending vibration located at 694 cm-1 further reaffirms the
existence of strong hydrogen bonds in the crystal.
5.2.2. Methoxy group vibrations
The dihedral angle value C2-C3-O9-C10 = -179.80 shows that carbon atom of methoxy
group lies in the plane of phenyl ring. This indicates that the asymmetric and symmetric
stretching wavenumbers of methoxy group will possess a high value. This is found to be satisfied
as we observed two clear absorption peaks at 2956 cm-1 and 2837 cm-1 corresponding to –CH3
asymmetric and symmetric stretching respectively. Presence of a methoxy group can be
confirmed with the existence of the –CH3 symmetric stretching band in between 2840 cm-1 to
2815 cm-1 [23]. Sharp peak at 1464 cm-1 and 1409 cm-1 is attributed to the asymmetric and
symmetric deformation of methoxy group respectively[9, 24]. C-O-C stretching vibration
associated with methoxy group (C10-O9-C3) is expected in the range 1310 cm-1 – 1020 cm-1
[24]. In the experimental spectrum asymmetric and symmetric C-O-C stretching is found at 1251
cm-1 and 1075 cm-1 respectively. Absorption bands in the region 580 cm-1 to 505 cm-1 is a
specification of aromatic compounds with methoxy group. This is spotted as the C-O-C in-plane
deformation bands at 549 cm-1 and 555 cm-1 in the experimental spectrum.
5.2.3. C=N Vibrations
In case of the title material, a characteristic peak for C=N vibration can show that the
proposed chemical reaction has occurred and the synthesis procedure is successful. The sharp
peak is found experimentally at 1613 cm-1.

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Experimental IR Relative
Scaled
Wavenumbera (cm-1)
Wavenumber
(cm-1)
Intensityb
Assignments
(Calculated)
3065
2957
2914
1619
1470
1448
1413
1358
1251
1186
3080
2956
2837
1613
1464
1443
1409
1361
1251
1195
88.52
12.77
20.56
45.22
5.26
O7-H8 stretching
CH3 asymmetric stretching.
CH3 symmetric stretching.
C=N vibrations
O-CH3 asymmetric deformation
O-H in plane bending
2.33
6.70
O-CH3 Symmetric deformation
CH3 in-plane bending
7.79
100
Asymmetric C-O-C stretching
Strong intermolecular hydrogen bonds,
O-H bending.
32.33
1094
1072
680
1091
1075
690
5.61
7.25
8.13
2.49
0.68
C-O asymmetric stretching
Symmetric C-O-C stretching
O-H out of plane deformation
C-O-C in plane deformation
C-O-C in plane deformation
569
549
542
535
Table 2. Calculated vibrational wavenumbers and measured IR band positions and assignments for VAN
aObtained from the wave numbers calculated at B3LYP/6-311++G(d,p) using scaling factors 0.9673
bRelative absorption intensities normalized with highest peak absorption equal to 100.

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5.3. THz spectroscopy
Fig. 6: a) THz transmission signal recorded in the time domain with and without the sample. b) Fast Fourier transformed
frequency domain spectrum obtained by dividing amplitude of the transmitted wave through the sample by reference
amplitude. Y-axis is plotted on a logarithmic scale to the base 10
Fig. 6a shows the amplitude of transmitted THz signal in the time domain. Black line
represents blank measurements and red line shows the measurement on the sample. Fig. 6b
shows the measured THz absorption spectrum in the frequency domain which is calculated by
dividing the amplitude of THz signal transmitted through the sample by the amplitude obtained
in the blank measurements. Three major absorption dips are found in this spectrum at 0.89 THz,
1.19 THZ and at 1.55 THz. In many cases it has been reported that absorptions in the range 0-2
THz is due to intermolecular vibrations and their assignments are generally difficult [25-27].
Jongtaek Kim et al reported assignments for molecular vibrations of OH1 crystal for its THz
absorption [7]. In their report it was shown that vibrational absorption spectrum calculated in the
gas phase for a single molecule of OH1 is showing considerable similarity to the absorption
spectrum calculated for OH1 crystal [7]. Based on this we have performed calculations of THz
phonon mode vibrations of OVAN in the gas phase and tried to assign them to its THz
absorption modes. THz phonon mode vibrations of OVAN was simulated by Gaussian software
package[18]. The simulated vibrations were visualized using GaussView program[19]. The
phonon mode absorptions were assigned to different molecular vibrations using VEDA 04
program[28]. In the simulation THz phonon mode absorptions are found at 1.00 THz, 1.34 THz
and at 1.84 THz. This can be considered analogous to the experimental THz absorption values
mentioned above as 0.89 THz, 1.19 THz and at 1.55 THz. The small shift in the calculated value

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is due to the non consideration of crystalline effects. According to the Gaussian outputs the
observed THz absorptions are attributed to different molecular vibrations as in Table 3.
Frequency in THz
Relative
Intensityb
Vibrational assignments (PED %)
Only contributions ≥ 10% are listed
Calculateda
Observed
0.89
(Calculated)
0.99
1.34
1.84
0.11
0.68
0.13
τC17-N19-C20-C25(37), τC2-C1-
C17-N19 (15), τC1-C17-N19-C20
(20),
1.19
1.55
τC17-N19-C20-C25(32), τC5-C6-
C1-C17 (16), τC1-C17-N19-C20
(18),
δ C17-N19-C20(33), δ C1-C17-N19
(21),
Table 3. Calculated THz vibrational wavenumbers, measured THz absorption band positions (cm-1) and assignments for OVAN
Abbreviations: δ: bending, τ: torsion
aObtained from the wave numbers calculated at B3LYP/6-311++G(d,p) using scaling factors 0.9673
bRelative absorption intensities normalized with highest peak absorption equal to 100.
Fig. 7: a) Absorption coefficient of OVAN in the 0-2THz range. b) Refractive index of OVAN in the 0-2 THz region c)
Extinction coefficient of OVAN in the 0-2 THz region.

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Absorption coefficient and THz refractive index of the material is given in Fig. 7a and 7b
as obtained from the TeraMat software. There is a moderate absorption of THz radiation after 0.5
to 2 THz by the title material. Fig. 7a shows that maximum THz absorption is found as 49 cm-1
at 1.54 THz. There are three peak values of absorption coefficient 30 cm-1, 36 cm-1, and 49 cm-
1at 0.89 THz, 1.19 THz, 1.55THz which correspond to the phonon mode absorptions we already
discussed. Refractive index of the material in the 0.2 to 2 THz regime is found varying in the
range 1.85 to 2.0. The imaginary part of refractive index (Extinction coefficient) is calculated
and depicted in Fig. 7c. From Fig. 7a and 7c, we can see that behavior of variation of extinction
coefficient is similar to that of the absorption coefficient. Value of extinction coefficient varies
in the range 0.1 to 0.8.
5.4. UV-Visible spectroscopy
Absorbance in the UV-Visible spectral range has been measured using a UV
spectrometer from 200nm to 800nm wavelength region. The calculated % transmittance and the
Tauc plot has been given in Fig. 8a and 8b [29, 30]. In Fig. 8a the lower transmission cut off is
marked at 500nm. It means the absorption of light is maximum by the material below this value.
The material is nearly 80% transparent to light above 500 nm. The material’s good transparency
of green light (> 500 nm) makes it suitable for second harmonic generation using the widespread
1064 nm Nd-YAG lasers. We extrapolated the (αhυ)2 Vs hυ graph shown in the Fig. 8b to the X
–axis to meet it at the value of 2.01 eV which represents the optical band gap of the material. The
optical band gap of VAN which has a very close structure with the title material was reported as
3.49 eV in [15].
Fig. 8: a) UV transmission spectrum of OVAN measured in the range 200-800 nm. b) Tauc plot of OVAN

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5.5. Frontier Molecular orbital calculations
A molecular orbital is a mathematical function that describes the behavior of an electron
or pair of electrons with in a molecule. These functions are typically plotted as surfaces around
the molecular structure. Although these orbitals are actually mathematical conveniences and not
physical quantities, they are very useful for qualitative description of bonding and reactivity[31].
While designing materials for nonlinear optical applications, material structures with relatively
low HOMO-LUMO band gap is usually preferred. High charge transfer is associated with such
molecules of low HOMO-LUMO band gap and hence high nonlinear optical response is
observed with them. The exact correlation functional to calculate HOMO eigenvalue, LUMO
eigenvalue and the value of energy gap between them is not known. It has been reported that
more accurate LUMO eigenvalues can be calculated by first calculating HOMO-LUMO energy
gap and adding HOMO eigenvalue to it. Zhang and Musgrave reported a linear correlation
equation to get corrected values of these parameters. Here we performed the frontier molecular
orbital calculation using the B3LYP functional and 6-311++g(d,p) basis set of DFT. The output
obtained is then corrected using the correlation equation (1) introduced by Zhang and
Musgrave[32]. The results are tabulated in Table 4 comparing with the geometrically similar
material VAN for which the calculations are already reported by the authors in [9]. Fig. 9(a) and
9(b) shows the distribution of HOMO and LUMO orbitals over the molecular structure. They are
shown as transparent surfaces in order to see their orientation more easily. The positive and
negative orbital lobes are displayed in different colors. It can be seen that a major part of HOMO
orbital surface is distributed over the ortho-vanillin moiety of the title material whereas LUMO is
distributed uniformly over both the rings. But there is no contribution from the methoxy ring
towards the LUMO orbital. As per Table 4 corrected HOMO, LUMO eigenvalues and their band
gap are found as -8.49 eV, -5.88 eV and 2.6 eV respectively. The HOMO-LUMO band gap of
2.6 eV is quite small and it favors good intermolecular charge transfer and hence high nonlinear
optical performance as well. This electronic band gap was reported as 2.85 eV for the
geometrically similar VAN. The colour difference of the two crystals VAN and OVAN can be
accounted by this difference of 0.25 eV in the electronic band gap. The only difference in the
molecular structure of VAN and OVAN is the position of –OH group in the vanillin moiety.

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-HOMOcorr= A + B x (-HOMOcal)
(1)
Fig. 9: a) HOMO plot of OVAN. b) LUMO plot of OVAN. Both generated using B3LYP functional and 6-311++g(d,p) basis set.
Calculated value (eV)
Corrected value (eV)
OVAN
VAN
OVAN
VAN
HOMO eigen value
LUMO eigen value
HOMO-LUMO energy
gap
-5.89
-1.99
3.90
-5.92
-1.76
4.16
-8.49
-5.88
2.60
-8.52
-5.68
2.85
Table. 4: HOMO energy, LUMO energy and HOMO-LUMO gap values calculated using B3LYP functional and corrected by
equation (1).
5.6. Hyperpolarizability calculations and second harmonic generation
The search for materials suitable for nonlinear optical applications focuses on ones
having a very large hyperpolarizability. The dynamic first order hyperpolarizability of the
molecule at a specified input laser wavelength will represent the second harmonic generation
efficiency of the material with the same wavelength of incident light. But computing the
nonlinear optical related properties of organic molecules is computationally intensive.
Conventional hybrid DFT functionals perform adequately with for compounds with moderate
electron delocalization. However they are known to overestimate these properties for systems

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with significant electron delocalization. In order to ameliorate this effects we used the Coulomb-
attenuating model employing B3LYP hybrid functional (CAM-B3LYP) in our calculations along
with 6-311++g(d,p) basis set. When comparing the observed experimental values with this
predicted hyperpolarizability values from Gaussian, we multiply the predicted values by a factor
of ½ [31]. Therefore both these values are separately shown in Table 5. Table 5 also shows a
comparison of dipole moment, polarizability and wavelength dependent hyperpolarizability at
1064 nm of the title material with those of the geometrically similar VAN and the benchmark
urea. Dipole moment of OVAN molecule is 1.8 times higher than that of VAN, but it is 1.8 times
lesser than that of urea. Polarizability of OVAN is exactly similar to that of VAN and it is 6
times higher than that of urea. Interestingly molecular hyperpolarizability of OVAN is 10 times
higher than that of urea. But it is found 2.5 times lesser than that of geometrically similar VAN.
A semi-qualitative measurement of the second harmonic generation efficiency of the material
was calculated employing the widely used Kurtz-Perry powder technique [33]. The powdered
sample was filled in a capillary tube and placed in the beam path of a 1064 nm laser radiation
with a pulse width of 10 ns and repetition rate of 10 Hz comprising an input energy of 1.2
mj/pulse. The observed SHG intensity is tabulated in Table 5 comparing with VAN and Urea
along with their hyperpolarizability values. As expected from the hyperpolarizability values the
observed second harmonic generation output for OVAN is lesser than that for VAN.
Material
Dipole
moment
(esu)
Polarizability Hyperpolarizability Hyperpolarizability/2 Observed
(esu)
at 1064 nm. (esu)
(esu)
SHG
Intensity
(mV)
OVAN 2.428 x 10- 3.0005 x 10-
9.7316 x 10-30
2.434 x 10-29
9.49 x 10-31
4.8658 x 10-30
1.217 x 10-29
4.745 x 10-31
5
18
23
VAN
Urea
1.3625 x 10- 3.056 x 10-23
10
12
18
4.4755 x 10- 4.906 x 10-24
18
Table. 5: Comparison of dipole moment, polarizability, hyperpolarizability and experimental second harmonic generation output
values of OVAN, VAN and urea.

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6. Conclusion
Bulk single crystals of 6-Methoxy-2-[(E)-phenyliminomethyl]phenol is grown by slow
evaporation solution growth technique using ethanol solvent. Theoretically simulated vibrational
bands in the IR region are found in good agreement with the experimental output. The material is
transparent to wavelength above 500 nm enabling it for second harmonic generation using
widespread 1064 nm Nd:YAG lasers. The optical band gap of the material is obtained as 2.01
eV. THz refractive index of the material is found varying in the range 1.85 to 2.0. Material
shows moderate absorption of THz in the measured range of 0-2 THz. The HOMO-LUMO
energy gap of the material is only 2.6 eV. This is close to the value of geometrically similar
VAN. First order molecular hyperpolarizability is calculated as 9.7316 x 10-30 which is 10 times
higher than that of urea. The low HOMO-LUMO band gap and high hyperpolarizability value
shows that the material can be used for second order nonlinear optical processes including
second harmonic generation. Kurtz-Perry experimental results shown in Table 5 confirm the
good second harmonic generation capability of the material.
Appendix
Bond length
Bond Angle
Calc
Dihedral Angle
Calc
Parameters Calc
XRD Parameters
1.401 C2-C1-C6
1.383 C2-C1-C17
1.436 C6-C1-C17
1.394 C1-C2-C3
1.347 C1-C2-O7
1.361 C3-C2-O7
1.368 C2-C3-C4
1.393 C2-C3-O9
0.930 C4-C3-O9
XRD
118.7
120.5
120.5
118.8
121.4
119.8
120.4
114.5
125.0
120.7
119.6
119.6
118.8
120.5
120.7
Parameters
XRD
0.7
-179.2
174.7
-5.2
C1-C2
C1-C6
C1-C17
C2-C3
C2-O7
C3-C4
C3-O9
C4-C5
C4-H14
C5-C6
C5-H15
C6-H16
O7-H8
O9-C10
C10-H11
C6-C1-C2-C3
C6-C1-C2-O7
C17-C1-C2-C3
C17-C1-C2-O7
C2-C1-C6-C5
C2-C1-C6-H16
C17-C1-C6-C5
C17-C1-C6-H16
C2-C1-C17-H18
C2-C1-C17-N19
C6-C1-C17-H18
C6-C1-C17-N19
C1-C2-C3-C4
C1-C2-C3-O9
O7-C2-C3-C4
1.413
1.412
1.45
1.417
1.337
1.39
1.359
1.405
1.082
1.378
1.083
1.085
0.994
1.42
119.78
0.17
-179.78
-179.89
0.16
120.8
119.41
119.24
122.52
118.23
119.64
115.17
125.19
120.94
119.95
119.11
119.93
119.45
120.61
-1.0
-0.17
178.4
-175.5
4.0
179.8
-0.1
-6.0
173.9
-1.1
179.88
179.89
-0.06
-179.74
-0.62
0.21
179.33
-0.06
1.34
C3-C4-C5
0.931 C3-C4-H14
0.931 C5-C4-H14
0.820 C4-C5-C6
C4-C5-H15
0.960 C6-C5-H15
1.43
-177.6
178.8
179.94
179.89
1.089

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C10-H12
C10-H13
C17-H18
C17-N19
N19-C20
C20-C21
C20-C25
C21-C22
C21-H30
C22-C23
C22-H29
C23-C24
C23-H28
C24-C25
C24-H27
C25-H26
0.960 C1-C6-C5
0.960 C1-C6-H16
0.931 C5-C6-H16
1.285 C2-O7-H8
1.420 C3-O9-C10
1.399 O9-C10-H11
1.383 O9-C10-H12
1.367 O9-C10-H13
0.930 H11-C10-H12
122.5
118.8
118.7
109.5
117.0
109.5
109.4
109.5
109.5
109.5
109.5
118.3
123.4
118.3
121.0
124.0
117.1
119.0
121.2
119.3
119.3
119.7
120.2
120.2
119.3
120.3
120.4
121.8
119.1
119.1
118.9
120.4
120.5
-
O7-C2-C3-O9
C1-C2-O7-H8
C3-C2-O7-H8
C2-C3-C4-C5
C2-C3-C4-H14
O9-C3-C4-C5
O9-C3-C4-H14
C2-C3-O9-C10
C4-C3-O9-C10
C3-C4-C5-C6
C3-C4-C5-H15
H14-C4-C5-C6
H14-C4-C5-H15
C4-C5-C6-C1
C4-C5-C6-H16
2.3
-138.0
42.1
1.096
1.096
1.096
1.288
1.409
1.403
1.402
1.393
1.084
1.394
1.084
1.395
1.084
1.391
1.084
1.084
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
120.46
118.92
120.62
107.14
118.38
105.74
111.46
111.46
109.33
109.33
109.43
116.24
122.74
121.02
121.18
122.84
118.0
119.12
120.18
119.84
119.94
120.43
119.53
120.03
119.54
120.2
120.25
120.35
120.05
119.6
120.35
118.75
120.9
-
-0.11
0.36
-179.6
-0.05
-179.98
179.95
0.02
-179.8
0.2
2.0
-177.7
-178.3
-2.0
164.6
-11.7
-3.0
176.9
176.9
-3.0
1.37
H11-C10-H13
0.06
0.931 H12-C10-H13
-179.96
179.99
-0.03
0.05
-180
-179.93
0.02
179.87
61.16
-61.42
177.23
-3.69
-38.26
144.14
-179.17
-1.32
1.38
C1-C17-H18
0.930 C1-C17-N19
1.38
H18-C17-N19
3.0
0.930 C17-N19-C20
0.931 N19-C20-C21
-177.3
-177.3
3.0
-170.2
69.8
-50.2
-175.8
4.3
H15-C5-C6-C1
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
N19-C20-C25
C21-C20-C25
C20-C21-C22
C20-C21-H30
C22-C21-H30
C21-C22-C23
C21-C22-H29
C23-C22-H29
C22-C23-C24
C22-C23-H28
C24-C23-H28
C23-C24-C25
C23-C24-H27
C25-C24-H27
C20-C25-C24
C20-C25-H26
H15-C5-C6-H16
C3-O9-C10-H11
C3-O9-C10-H12
C3-O9-C10-H13
C1-C17-N19-C20
H18-C17-N19-C20
C17-N19-C20-C21
C17-N19-C20-C25
N19-C20-C21-C22
N19-C20-C21-H30
C25-C20-C21-C22
C25-C20-C21-H30
N19-C20-C25-C24
N19-C20-C25-H26
C21-C20-C25-C24
C21-C20-C25-H26
C20-C21-C22-C23
C20-C21-C22-H29
H30-C21-C22-C23
H30-C21-C22-H29
C21-C22-C23-C24
C21-C22-C23-H28
H29-C22-C23-C24
H29-C22-C23-H28
C22-C23-C24-C25
C22-C23-C24-H27
30.7
-150.8
-179.1
1.0
2.5
-1.59
-177.5
-178.7
-1.0
-2.8
177.4
-1.0
179.0
179.0
-1.0
176.26
179.82
-0.97
2.12
-178.67
0.18
179.37
-177.67
1.52
C24-C25-H26
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
0.0
0.72
179.8
179.8
0.0
0.0
179.9
179.8
-178.47
0.61
-0.18
179.35

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-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
H28-C23-C24-C25
H28-C23-C24-H27
C23-C24-C25-C20
C23-C24-C25-H26
H27-C24-C25-C20
H27-C24-C25-H26
179.9
0.0
2.0
-178.5
-178.4
1.0
-179.26
0.27
-1.25
179.55
179.21
0.02
Table A1. DFT optimized and experimental geometrical parameters
Acknowledgement
Authors aknowledge the THz-TDS facility extended by Institute of physics, University of
Pecs, Hungary. KM Hijas acknowledge Stipendium Hungaricum fellowship program by Tempus
public foundation, Hungary which enabled him to take a six month partial study program in
Institute of physics, University of Pecs, Hungary. KM Hijas also acknowledge GATE fellowship
by MHRD, Govt of India for the financial support of his PhD course.
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Graphical abstract

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Highlights





Good quality NLO crystals of 6-Methoxy-2-[(E)-phenyliminomethyl]-phenol
THz spectroscopic studies have been carried out in the 0-2 THz region
Molecular vibrations responsible for THz phonon modes have been identified
THz Absorption coefficient and refractive index of the material is calculated.
An organic material with HOMO-LUMO energy gap of only 2.6 eV.