Computational study on non-linear optical property of Wittig based Schiff-Base ligands (both Z & E isomers) & Copper(II) complex

Article abstract of DOI:10.1016/j.molstruc.2018.04.090

96 new Wittig based Schiff-Base (WSB) ligands (both E & Z conformation) containing fused aromatic rings were designed and screened for NLO property. Wittig based precursor aldehydes were synthesized and spectroscopically confirmed. WSB ligands and their copper(II) complexes were also designed, optimized and their NLO property was studied using a GAUSSIAN09 computer program. For Optimization & Hyperpolarizability (β) calculations, DFT based B3LYP method was applied with LANL2DZ basis set for metal ion and 6-31G* basis set for C, H, N & O atoms. The study presents the Structure-Activity relationship (SAR) between WSB ligands and β. The study revealed that WSB Ligands of the category N₂, which are those derived from precursor aldehyde, (2-hydroxy-5-(2-(naphthalen-1-yl)vinyl)benzaldehyde) encoded as PA-2, showed higher β values over N₁, which are derived from, 2-hydroxy-5-(2-(naphthalen-1-yl)vinyl)benzaldehyde, encoded as PA-1. Among all ligands, Ligand-14, (E?4-(2-Pyren-1-yl-vinyl)-2-([1,2,4]triazol-4-yliminomethyl)-phenol), of N₂ category showed highest β value (6.968 × 10^?30 e.s.u). After complexing with Cu(II), encoded as Complex-1 ([(4-(2-Pyren-1-yl-vinyl)-2-([1,2,4]triazol-4-yliminomethyl)-phenoxy)₂Cu], which has C₂ symmetry, the β value (0.310 × 10^?30 e.s.u) unexpectedly decreased. However, it is still high as compared to that of urea (β = 0.091 × 10^?30 e.s.u). The output of TD-DFT also supports the obtained results. Different factors affecting β value especially their geometrical isomeric effect were successfully analyzed.

Full text of DOI:10.1016/j.molstruc.2018.04.090

Accepted Manuscript
Computational study on non-linear optical property of Wittig based Schiff-Base
ligands (both Z & E isomers) & Copper(II) complex
Bathula Rajasekhar, P.K. Muhammad Hijaz, Toka Swu
PII:
S0022-2860(18)30540-4
10.1016/j.molstruc.2018.04.090
MOLSTR 25163
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 5 February 2018
Revised Date: 18 April 2018
Accepted Date: 27 April 2018
Please cite this article as: B. Rajasekhar, P.K. Muhammad Hijaz, T. Swu, Computational study on non-
linear optical property of Wittig based Schiff-Base ligands (both Z & E isomers) & Copper(II) complex,
Journal of Molecular Structure (2018), doi: 10.1016/j.molstruc.2018.04.090.
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ACCEPTED MANUSCRIPT

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Computational Study on Non-linear Optical property of Wittig based Schiff-
Base ligands (both Z & E isomers) & Copper(II) complex
Bathula Rajasekhar a, Muhammad Hijaz P. K a and Toka Swu * a
a Department of Chemistry, Pondicherry University, Puducherry, India-605014.
E-mail: tokaswu.che@pondiuni.edu.in
Abstract
96 new Wittig based Schiff-Base (WSB) ligands (both E & Z conformation) containing fused aromatic
rings were designed and screened for NLO property. Wittig based precursor aldehydes were synthesized and
spectroscopically confirmed. WSB ligands and their copper(II) complexes were also designed, optimized and their
NLO property was studied using a GAUSSIAN09 computer program. For Optimization & Hyperpolarizability (
calculations, DFT based B3LYP method was applied with LANL2DZ basis set for metal ion and 6-31G* basis set
for C, H, N & O atoms. The study presents the Structure-Activity relationship (SAR) between WSB ligands and
The study revealed that WSB Ligands of the category N2, which are those derived from precursor aldehyde, (2-
hydroxy-5-(2-(naphthalen-1-yl)vinyl)benzaldehyde) encoded as PA-2, showed higher values over N1, which are
derived from, 2-hydroxy-5-(2-(naphthalen-1-yl)vinyl)benzaldehyde, encoded as PA-1. Among all ligands, Ligand-
β)
β
.
β
14, (E-4-(2-Pyren-1-yl-vinyl)-2-([1,2,4]triazol-4-yliminomethyl)-phenol), of N2 category showed highest
β
value
(6.968x10-30 e.s.u).
After complexing with Cu(II), encoded as Complex-1 ([(4-(2-Pyren-1-yl-vinyl)-2-
([1,2,4]triazol-4-yliminomethyl)-phenoxy)2Cu], which has C2 symmetry, the
β
value (0.310x10-30 e.s.u)
β
= 0.091x10-30 e.s.u). The output of
unexpectedly decreased. However, it is still high as compared to that of urea (
TD-DFT also supports the obtained results. Different factors affecting β value especially their geometrical isomeric
effect were successfully analysed.
Keywords: Nonlinear Optical Property; Hyperpolarizability; DFT & TD-DFT; Wittig based Schiff-Base ligands;
Copper(II) complex.
1. Introduction
In last few decades, molecular-based NLO materials such as organic compounds,
polymers and coordination complexes [1-3] which possess metal ion and organic ligand have
drawn considerable focus as compared with the atomic-based materials such as LiNbO3, KH2PO4
(KDP), gallium arsenide (GaAs), α-quartz (SiO2) and β-barium borate (BBO) [4]. Nonlinear
optical property can be defined as the interaction between electromagnetic radiation and
materials which results in a change in the properties of laser beam like frequency, phase,
amplitude, and polarization [5]. However, organic compounds and polymers have disadvantages
like low thermal stability, mechanical weakness, volatility, etc. which is not so in the case of
coordination complexes. Also, some other advantages of complexes are that they can exhibit
optical bands in the Visible region on absorption of radiowave, and a metal ion which is having
diversity in oxidation states as well as in redox properties can be used as a central metal ion for
complexation and optimize its NLO properties by modifying the ligands [6-19].
Coordination complexes exhibit NLO property but among them also, Schiff-Base
complexes seemed to be prominent chromophores showing significantly large nonlinear response

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[20-36]. They have advantage of simple synthetic procedure, thermal stability, easy generation of
polarizability and low energy charge transitions [37-39]. Schiff-Base coordination complexes
have shown high nonlinearity than the Schiff-Base ligands alone because metal centre can act as
an electron donor and/or acceptor and thereby enhancing the intramolecular charge transfer [40].
Among all Schiff-Base coordination complexes, Ni(II) (closed shell system) complexes are
studied extensively [25]. De Bella et. al reported that unpaired electron system like radicals and
open shell metal complexes like Cu(II), Co(II) Schiff-Base complexes have higher β
(hyperpolarizability) value indicating the enhancement of nonlinearity [41-44]. Also, Copper(II)
Schiff-Base complexes with extended conjugation are found to be a class of promising NLO
materials [46]. Keeping all these in mind, we have designed Wittig based Schiff bases (WSB)
and also have synthesized some precursors.
2. Materials and Methods
2. 1. Materials
Salicylaldehyde, paraformaldehyde and diethyl ether were purchased from Loba Chemie,
Hydrochloric acid from Avra, Triphenylphosphine, potassium-tert-butoxide from Spectrochem,
1-pyrenecarboxaldehyde from Sigma Aldrich. They were used without further purification.
Ethanol, Dichloromethane and all other solvents were purified by using standard methods.
2. 2. Physical measurements
ESI-MASS spectrum measurements were carried out using 6530B AGILENT Q-
TOFLC/MS analyser. Ultra-Violet (UV-Vis) spectra were measured on a Shimadzu UV-2450 PC
spectrophotometer at room temperature. Infrared (IR) spectra were recorded on a Nicolet iS10
instrument using KBr pellets. The 1H, 13C-NMR (Nuclear Magnetic Resonance) spectra obtained
from Bruker 400 MHz spectrometer and chemical shift values were reported in δ units (parts per
million) relative to Me4Si as an internal standard.
2. 3. Computational methods
Hyperpolarizability calculations of all the molecules N1, 2 (both E & Z) were performed
using GAUSSIAN09 [47]. For structural optimization & Hyperpolarizability calculations, DFT
based B3LYP method was used and LANL2DZ basis set was used for metal ion & 6-31G* for C,
H, N & O atoms. Hyperpolarizability calculations were performed for Complex-1 by giving this
keyword ‘# b3lyp/gen geom=connectivity gfinput pseudo=read polar=(DCSHG,Cubic)
CPHF=RdFreq’ in route section and 0.1 ω frequency at bottom of XYZ coordinates. TD-DFT
analysis was carried out for all ligands (see Table-S9, “ S ” stands for Supplementary material)
using keyword [# td=(nstates=20) b3lyp/6-31g(d) geom=connectivity] and electronic transitions
with oscillatory strength ≥ 0.05 were taken into consideration because these transitions were
contributing more to the β value of the studied molecules.

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Amines
Homoaromatic amines (1-7)
NH2 NH2
NH2
NH2
NH2
CH3
NH2
OH
OH
OH
NH2
OH
O
N
NH2
Heteroaromatic amines (8-14)
H
N
O
H2N
NH2
H2N
NH2
N
NH2
N
N
N
NH2
NH2
N
N
N
N
N
N
N
H
N
H
Aliphatic cyclic amines (15-20)
NH2
NH2
N
NH2
N
NH2
NH2
NH2
N
N
H
N
N
H
N
H
Aliphatic acyclic amines (21-23)
OH
N
N
H2N
H2N
H2N
Note: Black color groups are interacting in the condensation reaction (Step-4) of the Scheme-1.
Figure-1. Library of Amines used in the reaction step-4 of scheme-1.
2. 4. Synthetic procedure of precursor aldehyde (PA-2), 2-Hydroxy-5-(2-pyren-1-yl-vinyl)-
benzaldehyde of Z-isomer (e)
Step-1: Synthesis of 5-Chloromethyl-2-hydroxy-benzaldehyde (a).
30g of salicylaldehyde, 13.6g of paraformaldehyde and 150 mL of concentrate
hydrochloric acid were mixed and stirred for 7 hr and followed by workup with 0.5% NaHSO4
solution. A pale pink color precipitate was obtained and recrystallized with diethyl ether. Yield
50 %.
Step-2: Synthesis of phosphonium chloride salt (b).
10g of (a) dissolved in 100 mL of toluene was added to 10g of triphenylphosphine
dissolved in 100 mL toluene. The mixture was refluxed for 2 hours and after removal of solvent,
a white color phosphonium chloride salt (PPT) was obtained. Yield 95%.

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Step-3: Synthesis of Wittig based Precursor Aldehyde (e)
Dissolved 4.4 mmole of ligand (b) in 80 mL of distilled ethanol to it added 4.5mmole of
sodium methoxide directly. To this reaction mixture, 1-pyrenecarboxaldehyde dissolved in 80ml
ethanol was added. It was refluxed for 11hr under an inert atmosphere, cooled to room
temperature, and on addition of 370ml of 5% HCl solution, 700 ml of distilled water, yellow
color precipitate was observed. This precipitate was filtered and dried in room atmosphere. Yield
55 %.
Wittig based precursor aldehyde (e) was synthesized according to the literature [45] and
used for 2-Hydroxy-5-[2-(4-nitro-phenyl)-vinyl]-benzaldehyde preparation.
The Wittig reaction between phosphonium salt and pyrenecarboxaldehyde gave the
expected product (e) which was confirmed by NMR, IR, UV-vis absorption, and MASS spectral
analysis. FT-IR (as KBr pellet, cm-1, Figure-S1) spectra: broad peak at 3433 (broad, ν(OH)),
1677 (strong, ν(C=O & alkene C=C)), 1588 & 1495 (strong, ν(aromatic C=C)), 704 (strong,
1
13
ν(=C-H bending was assigned for Z conformation to the alkene C=C). H, C, and DEPT-135
analysis give the information regarding the proton, carbon skeleton structure and number of C-H,
1
CH2 and CH3 groups present in the obtained product. H NMR (400 MHz, CDCl3, Figure-S2) δ
10.79 (s, 1H), 9.44 (d, J = 9.3 Hz, 1H), 8.46 (d, J = 7.9 Hz, 1H), 8.35 – 8.32 (m, 1H), 8.34 – 8.30
(m, 2H), 8.28 (d, J = 8.1 Hz, 1H), 8.24 (d, J = 8.9 Hz, 1H), 8.12 (d, J = 3.8 Hz, 1H), 8.10 (d, J =
2.5 Hz, 1H), 7.67 (d, J = 8.1 Hz, 1H), 7.64 (d, J = 7.9 Hz, 1H), 7.53 (d, J = 6.8 Hz, 1H), 7.45 (d,
J = 6.5 Hz, 1H), 7.30 (d, J = 8.6 Hz, 1H), 5.29 (s, 1H). Chemical shifts at 7.67 δ and 7.64 δ are
indicative of the compound being Z conformation, otherwise it would be close to 15 δ. 13C-NMR
spectra (Figure-S3) of (e) shows twenty peaks in the range of δ 0-220 ppm. 193 (CO), peaks
from 135 to 128 were assigned for pyrenecarboxaldehyde carbons and peaks from 127 to 124
were identified as salicylaldehyde ring carbons. Alkene double bonded carbons chemical shifts
appeared at 124 & 123. DEPT-135 spectra (Figure-S4) of Wittig based precursor aldehyde (e)
showed all positive peaks. The peaks at 193 & from 132 to 127 are indications of
pyrenecarboxaldehyde C-H groups, peaks from 127 to 126 were assigned for salicylaldehyde
ring C-H groups and peaks at 124, 123 may be assigned to C-H groups of alkene double bond
(Figure-9 & 10). UV-Visible (Dichloromethane as a solvent, λmax; nm, Figure-S5): lower energy
electron transitions as moderately sharp peaks at 498 nm, 394 nm, and 362 nm were identified as
n→π* and charge transfer from HOMO →LUMO. Another two peaks at 286 nm, 232 nm were
assigned for π→π* electron transitions. ESI MASS: 349 (M+H) (Figure-S6)
3. Results and Discussions
For convenience, the Wittig-based Schiff Base (WSB) Ligands in question are grouped in to
two categories:
The first category is encoded as N1, which are WSB Ligands derived from the precursor
aldehydes, 2-hydroxy-5-(2-(naphthalen-1-yl)vinyl)benzaldehyde (encoded as PA-1) (both E &
Z). To be more specific, PA-1, Z-isomer is denoted as (c) and E-isomer as (d). The second
category is N2, which are derived from the precursor aldehyde, 2-hydroxy-5-(2-(pyren-1-
yl)vinyl)benzaldehyde, encoded as PA-2 (both E & Z). PA-2, Z-isomer is denoted as (e) and E-

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isomer as (f). WSB coordinated with Cu(II) is encoded as Complex-1. Library of Wittig based
precursor aldehydes & Amines used are given in Scheme-1 & Figure-1 respectively.
3. 1. Structure-activity relationship (SAR) analysis
Table-1 & Figure-2 to 5 present change in β value with change in the structure of Wittig
based Schiff-base (WSB) ligand and also the effect of HOMO-LUMO energy gap (∆E,
Kcal/mol) and electronic effects on β value. Library of Wittig based precursor aldehydes &
Amines used are given in Scheme-1 & Figure-1 respectively. From this study, the key
observations are noted here.
1. First hyperpolarizabilities (βtotal) of both Z & E isomer Wittig based Schiff-Base ligands were
sensitive to the HOMO-LUMO energy gap (∆E). The general trend is that, with decrease in ∆E,
the β value increases. In each case, βtotal = β value is enhanced greatly as ∆E decreases and
polarization increases. E isomer WSB ligands have shown higher β value compared to Z isomer
(majority of molecules showing similar results were taken into account for convenience).
2. WSB ligands (N1,
both Z & E isomers) derived from aromatic (homoaromatic &
2
heteroaromatic) amines showed high β value as compared to aliphatic amines. In each case, a
majority number of molecules having β >1 were taken into account for convenience. Among all,
WSB ligands derived from 9H-Purin-6-ylamine and [1,2,4]triazol-4-ylamine exhibited highest β
values.
3. Enhancement in β value was observed for WSB ligands (Ligands-2&4) derived from
homoaromatic amines having –OH, -CO-NH2 (in the context of resonance –OH is strong
electron releasing & -CO-NH2 is an electron withdrawing) groups at para position against –CH3
group (Ligand-3) which is attributed to high polarization capability of the former groups. This
trend is observed for N1, 2 both Z & E isomers. The general trend of ∆E relationship with β (β
value enhanced greatly as ∆E decreases) was observed for N2 (both E & Z isomer) WSB ligands.
However, in the case of N1 (both Z & E isomer) WSB ligands, β value is highly influenced by
the presence of ER such that the effect of ∆E was not observed.
4. It was also observed that the presence of ER and EW groups in WSB ligands (Ligand-1 to
Ligand-4) were contributing to the polarization and that the contribution of EW is found to be
greater than that of ER. The presence of the ER & EW groups in the molecules (Ligand-2,3 &4
form N1, 2 both Z & E isomers) leads to higher β value over Ligand-1.
5. Strong ER group (–OH) at para positioned in WSB ligands, Ligand-2 (N1, 2 both Z & E
isomer; except N2, E isomer) showed high β values as compared to Ligand-5 (–OH group
positioned at ortho to the imine group). Further, these results were cross-examined with ∆E

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relation with β and was found that except for N2, E isomer, the β value of all other ligands were
influenced by ER groups over ∆E.
6. Introduction of extended (fused) aromatic moiety in WSB ligands, Ligand-6 & Ligand-7 (N1, 2
both Z & E isomer; except N2, E isomer) enhances the β value as compared to Ligand-5. Though,
∆E relationship with β differs in the case of N2 (both Z & E isomers), it was observed in general
that the introduction of fused aromatic rings causes a reduction in the ∆E.
7. Among Ligand-6 & Ligand-7, high β value was observed for Ligand-6 (N1,2 Z isomer
category only) which have –NH2 group positioned at the 1st carbon of the naphthalene moiety
whereas in the case of Ligand-7, N1,2 where –NH2 group is positioned at the 3rd carbon, only E
isomer exhibited high β value as compared to Ligand-6. Further, comparison of ∆E between
Ligand-6 & 7 showed that the Ligand-6 (N1,2 both Z & E isomers) exhibited higher value.
8. In all cases, a comparative study from Ligand-1 to Ligand-7 concludes that β value was highly
influenced by the amine structure. WSB ligands [Ligand-1 to 5] derived from ER group
containing homoaromatic amines (Figure-1) greatly enhanced the β value as compared to fused
aromatic ring containing amines [Amines-6&7] indicating that the electronic effects are
dominating the β value over ∆E effect.
9. Comparative study between the WSB ligands, Ligand-1 & Ligand-8 (N1,2 both Z & E isomers)
revealed that the presence of nitrogen atom in the aromatic ring (in Ligand-8) causes
enhancement of β value which may be due to the difference in the electronegativity between
carbon and nitrogen and thereby causing polarization in the system at given frequency (ω=0.1
Hartrees). Polarization may also be affected by the presence of lone pair in nitrogen atom. These
results are well supported by ∆E relationship with β value.
10. WSB ligands (Ligand-9 & Ligand-11) showed a positive impact for the –R–CH2N (‘N’ is
part of the –C=N group and ‘R’ is pyridine ring) moiety position on β value. –CH2N moiety
placed at para position in Ligand-9 showed higher β value as compared to Ligand-11 in which –
R–CH2N moiety is placed at ortho position (N1,2 both Z & E isomers). Except N1, E isomer, all
WSB ligands of these group did not follow the general trend of higher β value for lower ∆E.
11. Among all, WSB ligands derived from non-aromatic amines, Ligand-19 (N1,2 only Z
category isomers) & Ligand-20 (N1,2 only E category isomers) showed high β value. This also
must be the effect of nitrogen atom in the ring. For lower ∆E, higher β trend was observed in
N1,2 Z isomer ligands only.
12. Among WSB ligands derived from aliphatic five-membered cyclic ring containing amines,
Ligand-15 (repeated trend was taken for the consideration, N1,2 both Z & E isomers, except N2, Z
isomers only) showed high β value as compared to six-membered cyclic ring containing amines
(Ligand-16). Here, ∆E relation with β, general trend was observed in the case of N1,2 (Z isomers

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only) whereas in the case of N1,2 E isomer ligands, it was not so. This anomaly is explained in
section 3.2 together with other molecules showing similar result.
13. A gradual increment in the β value of Ligands-16 to 19 (N1,2 both Z & E isomer) is due to
increase in the number of tertiary amines in the cyclic ring. N1,2 Z isomer WSB ligands explicitly
follow the ∆E relation with β whereas in other cases this trend was not observed.
14. WSB ligands (from Ligand-21 to Ligand-23) derived from aliphatic acyclic amines, Ligand-
21 (N1,2 both Z & E isomers, except Ligand-20, N2 E isomer) showed high β value. HOMO-
LUMO trend was observed for N1 (both Z & E isomers) category ligands only.
Table-1. ∆E and β values of WSB ligands (N1,2 both Z & E isomers).
N1 [Z isomers]
(β)x10-30
N1 [E isomers]
(β)x10-30
N2 [Z isomers]
(β x10-
N2 [E isomers]
(β)x10-
Ligands
∆E
(Kcal/mol)
85.089
83.947
84.292
83.947
79.404
83.846
81.362
80.006
80.025
91.094
83.834
89.538
91.414
72.087
80.740
91.803
91.935
92.224
89.657
89.074
92.337
90.868
91.345
91.056
∆E
(Kcal/mol)
80.627
79.623
79.592
79.322
75.130
77.659
78.029
75.689
75.137
85.792
74.528
86.018
85.240
68.115
75.808
86.990
86.727
87.022
85.616
87.266
85.723
87.335
87.461
87.448
∆E
(Kcal/mol)
78.023
78.644
78.312
78.462
73.386
76.411
76.887
74.252
74.045
78.964
74.465
77.992
78.996
66.471
73.700
78.764
78.324
78.488
77.421
77.088
78.770
79.140
78.613
78.419
∆E
(Kcal/mol)
73.832
73.863
74.032
74.051
69.012
71.811
72.413
71.623
69.313
74.748
69.144
74.139
74.566
62.354
68.686
74.396
74.183
74.302
73.430
74.365
74.553
75.036
74.616
74.440
e.s.u
0.586
0.807
2.376
1.006
2.245
0.556
0.913
0.554
1.272
1.661
1.732
1.479
1.355
3.713
2.762
0.596
0.568
0.361
0.681
1.577
0.693
1.085
0.972
0.941
e.s.u
1.493
0.752
1.313
1.254
2.772
1.124
0.680
1.197
3.212
3.251
2.481
1.247
1.235
4.664
5.025
0.508
0.463
0.870
0.376
0.905
2.005
1.353
0.695
0.552
30 e.s.u
1.065
1.005
2.474
1.248
2.776
1.070
1.253
1.043
1.438
2.176
1.811
1.723
1.639
4.424
2.910
0.851
0.866
0.322
0.887
1.837
0.626
1.759
1.159
1.199
30 e.s.u
2.577
1.130
1.961
1.383
2.852
2.035
1.216
1.257
3.543
4.191
3.864
1.599
1.518
5.611
6.968
0.879
0.851
1.121
0.884
1.292
2.173
0.745
1.208
0.847
Pre. Ald
Ligand-1
Ligand-2
Ligand-3
Ligand-4
Ligand-5
Ligand-6
Ligand-7
Ligand-8
Ligand-9
Ligand-10
Ligand-11
Ligand-12
Ligand-13
Ligand-14
Ligand-15
Ligand-16
Ligand-17
Ligand-18
Ligand-19
Ligand-20
Ligand-21
Ligand-22
Ligand-23

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Figure-2. First Hyperpolarizability (β) of N1, Z & E isomer Wittig based Schiff-Base ligands
Figure-3. N1, Z & E isomer Wittig based Schiff-Base ligands HOMO-LUMO energy gap (∆E)

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Figure-4. First Hyperpolarizability (β) of N2, Z & E isomer Wittig based Schiff-Base ligands
Figure-5. N2, Z & E isomer Wittig based Schiff-Base ligands HOMO-LUMO energy gap (∆E)
3. 2. TD-DFT Analysis of selected WSB ligands.
UV-Visible analysis supports the reason for the ligands of high ∆E to exhibit higher β
value than the expectation. The analysis revealed that higher the number of electronic transitions
and higher the oscillator strength (either one or both can be taken into the consideration), higher
is the β value and it agrees with the reports of other researchers as well [48]. Except for few
ligands highlighted in bold, this anomaly was successfully interpreted using TD-DFT study
(Table-2).

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Table-2. Summary view of the TD-DFT calculations for WSB ligands.
Number of
Electronic
Molecule
Ligand-2
State
∆E (eV)
3.6928
3.5464
4.7856
3.3915
5.1683
5.1949
3.5779
4.4947
3.3776
3.5228
3.4952
3.4891
3.3759
3.4964
3.504
λ (nm)
335.75
349.6
ƒ
transitions a
10
Excited State 3
Excited State 2
Excited State 18
Excited State 3
Excited State 18
Excited State 18
Excited State 2
Excited State 8
Excited State 2
Excited State 2
Excited State 2
Excited State 2
Excited State 1
Excited State 2
Excited State 2
Excited State 2
Excited State 3
Excited State 4
Excited State 1
Excited State 1
Excited State 1
Excited State 1
Excited State 1
Excited State 1
Excited State 8
Excited State 7
Excited State 1
Excited State 16
Excited State 1
Excited State 1
Excited State 1
Excited State 1
0.4112
0.3016
0.3151
0.428
Ligand-4
Ligand-6
9
7
259.08
365.57
239.9
Ligand-7
8
(N1, Z)
7
0.3548
0.504
Ligand-9
Ligand-11
Ligand-22
Ligand-23
Ligand-13
Ligand-14
Ligand-15
Ligand-16
Ligand-18
Ligand-19
Ligand-22
Ligand-23
Ligand-6
7
238.67
346.53
275.85
367.08
351.95
354.73
355.35
367.26
354.61
353.83
353.77
390.99
369.8
6
0.1747
0.1285
0.6557
0.7621
0.571
8
8
6
12
12
10
8
0.5612
0.6602
0.6842
0.6254
0.6019
0.441
(N1, E)
9
9
3.5046
3.1711
3.3527
3.1523
3.1548
3.1624
3.1269
3.0368
3.036
6
Ligand-7
5
0.4518
0.5272
0.547
5
393.31
393
Ligand-9
Ligand-11
Ligand-21
Ligand-22
Ligand-9
Ligand-11
Ligand-13
Ligand-14
Ligand-15
Ligand-16
Ligand-18
Ligand-19
Ligand-22
Ligand-23
6
(N2, Z)
9
392.06
396.5
0.5303
0.5148
0.8331
0.9446
0.7225
0.3579
0.9922
0.4368
0.9827
0.9824
0.9711
0.9741
8
6
408.28
408.37
318.68
305.95
409.43
246.15
416.5
8
7
3.8905
4.0524
3.0282
5.0369
2.9768
3.0302
3.0379
3.0305
6
8
(N2, E)
8
11
6
409.17
408.13
409.12
9
7
NX, Z or E, where x = 1& 2, and Cis or Z isomer and Trans or E isomer.
a Number of electronic transitions with oscillator strength ≥0.05
Detailed information (TD-DFT) provided in the Table-S9.

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3. 3. Comparison of Hyperpolarizabilities (β) between Z & E isomer Wittig based Schiff-Base
ligands
Figure-S7 & S8, describes the geometrical isomeric effect on NLO property (β=βtotal x 10-30
e.s.u). From these figures, the key observations are noted here,
1. E isomer WSB ligands showed higher β value compared to Z isomers (the majority of the
molecules showed this trend and thus, they alone were taken into account for convenience).
2. WSB ligands derived from aromatic (homoaromatic & heteroaromatic) amines showed higher
β value as compared to aliphatic amines. Among these, WSB derived from heteroaromatic
amines showed higher β values compared to homoaromatic amines.
3. N2 (both Z & E isomers) showed higher β value than N1. It showed that introduction of fused
aromatic rings or planar aromatic moiety in the Wittig precursor aldehydes enhances the β value.
With this knowledge, the present work produced synthetic procedure & structural
characterization for Precursor Aldehyde (e) also (section 2. 4).
4. It was observed that the presence of strong electron releasing groups & electron rich atoms in
the system leads to change in β value and that in some cases these changes do not correlate with
∆E relationship with β.
3. 4. E isomer Wittig based Schiff-Base copper(II) complex (Complex-1).
Hyperpolarizability calculations were carried out for all the molecules using
GAUSSIAN09. Complex-1 (Figure-6) was optimized using DFT based B3LYP method in which
LANL2DZ basis set was used for metal ion & 6-31G* for C, H, N & O atoms. Frontier
molecular orbitals of alpha and beta electrons of Complex-1 are presented in Figure-7 with
HOMO-LUMO energy comparison, and selected bond lengths & angles are given in Table-3 &
4.
Figure-6. Optimized structure of Copper(II) Schiff-Base complex (Complex-1)

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Table-3. Selected bond lengths (Å) for the Complex-1.
N1 – Cu= 2.0175
C2 – C3 = 1.4239
C8 – C9 = 1.4648
C15 – C16 = 1.3940
C20 – C21 = 1.4319
N1 – N2 = 1.3979
C27 – N2 = 1.3711
O1 – Cu=1.9126
C3 – C4 = 1.3753
C9 – C10 =1.4663
C16 – C17 = 1.3934
C21 – C22 = 1.4286
N2 – C26 = 1.3709
C1 – C7= 1.4184
C4 – C5 = 1.4269
C10 – C11=1.4080
C18 – C19 = 1.3628
C21 – C23 = 1.4294
C26 – N3 = 1.3085
C7 – N1 = 1.3147
C5 – C6 = 1.3863
C11 – C12 = 1.3861
C19 – C20 = 1.4384
C23 – C24 = 1.4275
N3 – N4 = 1.3874
C1 – C2 = 1.4397
C6 – C1 = 1.4206
C13 – C14 = 1.3620
C20 – C10 = 1.4247
C23 – C25 = 1.4299
N4 – C27 = 1.3085
Table-4. Selected bond lengths (Degree) for the Complex-1.
C7 – N1 – Cu = 126.0361 C2 – O1 – Cu = 131.2398 O1 – N1 – Cu = 90.2692
C7 – N1 – N2 = 114.5957
C1 – C2 – O1 = 124.0486 C6 – C5 – C8 = 123.9313 C5 – C8 – C9 = 126.6482 C8 – C9 – C10 = 125.2541
C1 – C7 – N1 – N2 = -179.6775
C6 – C5 – C8 – C9 = 19.9311
C8 – C9 – C10 – C11 = 29.8163
C7 – N1 – N2 – C26 = 95.6203
C5 – C8 – C9 – C10 = -176.4652
C19 – C20 – C10 – C9 = 2.9690
Figure-7. HOMO-LUMO energy comparison of alpha and beta electrons of Complex-1
Cartesian coordinates were used for calculating the dipole moment ( ), polarizability (
and first static hyperpolarizability ( ) by using standard equations [49-51] given below:
µ
α)
β
2
2
2 1/2
µ
α
β
= (µx
= 1/3 (αxx + αyy + αzx)
total = [(βxxx βxyy
xzz) 2 + (βyyy
+ µy + µz )
+
+
β
+
βyzz
+
β
yxx) 2 + (βzzz
αyy
zxx) were obtained from GAUSSIAN09 frequency job output file (Table-5) and
+
βzyy
+
β
zxx) 2] 1/2
The polarizability and hyperpolarizability tensors (αxx
βyxx, βzzz βzyy
,
,
αzx
,
βxxx, βxyy, βxzz, βyyy
, βyzz,
,
,
β

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atomic units (a.u.) were converted into electrostatic units (e.s.u.) (α; 1 a.u. = 0.1482 x 10-24 e.s.u,
β
; 1 a.u = 8.6393 x 10-30 e.s.u) and results are presented in tabular format (Table-6).
Figure-8. C2 axis passes through the central metal
Table-5. Tensors of the Ligand-14 & its copper(II) complex
Urea *
Ligand-14
Dipole moment ( ) tensors
Complex-1
Dipole moment ( ) tensors
µ
µ
Dipole moment (µ) tensors
X = -5.2358
Y = -3.2589
Z = -0.4887
X = 0.0000
Y = 0.0000
Z = 0.4386
X = 0.0000
Y = 0.0000
Z = -3.4646
Polarizability (α) tensors
Polarizability (α) tensors
Polarizability (α) tensors
XX = -214.5895
YY = -185.7891
ZZ = -184.8137
XX = -427.8444
YY = -297.0102
ZZ = -356.5230
XX = -24.2588
YY = -17.6558
ZZ = -24.6862
Hyperpolarizbility (
β
) tensors
Hyperpolarizbility (
β
) tensors
Hyperpolarizbility (β) tensors
XXX = -563.3878
YYY = -103.8030
ZZZ = 0.0411
XYY = -183.3157
XXY = -244.8851
XXZ = -65.7905
XZZ = 27.2527
YZZ = -2.1183
XXX = 0.0000
YYY = 0.0000
ZZZ = 21.6291
XYY = 0.0000
XXY = 0.0000
XXZ = -18.4375
XZZ = 0.0000
YZZ = 0.0000
YYZ = 32.7387
XYZ = -211.8146
XXX = 0.0000
YYY = 0.0000
ZZZ = -12.2493
XYY = 0.0000
XXY = 0.0000
XXZ = 0.8154
XZZ = 0.0000
YZZ = 0.0000
YYZ = 0.7980
XYZ = -3.5256
YYZ = -34.2102
XYZ = -10.1595
*
urea molecule was structurally optimized and hyperpolarizability calculations were carried out (see 2. 3.
Computational methods) using 0.1 ω frequency.

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Hyperpolarizability of copper(II) complex (Complex-1) was obtained from ‘Polar’
calculations using the GAUSSIAN09 program and presented in Table-6. Complex-1 exhibited
high β value as compared to its Schiff-Base ligand (Ligand-13, N2 & E isomer) revealing the
importance of metal ion in achieving higher molecular NLO response. β value is also supported
by low ∆E. In comparison with standard organic compound, urea (0.091816x10-30 e.s.u) [52,53],
Ligand-14 & Complex-1 with C2 symmetry showed much higher β value. However, very
interestingly Ligand-14 showed higher β value than Complex-1. Generally, metal complexes
show higher β value compared to the ligands [24] but in this case, it is vice versa most probably
because of the symmetry of Complex-1 which is likely to be governing the β value. This infers
that higher the symmetry, lower the β value. Marder & Nicoud et al also reported that molecules
which are having centrosymmetry element do not exhibit high β value [54]. However, in this
report, symmetry effect on β value was not studied systematically.
Table-6. µ, α, βtotal and ∆E of Ligand-14 & Copper(II) complex (N2, E isomer) and reference
molecules
Molecules
Dipole moment
(µ, Debye)
6.1865
Polarizability
(α x 10-24 e.s.u)
-28.908
Hyper Polarizability
(βtotal x 10-30 e.s.u)
6.968
LUMO-HOMO energy gap
(∆E; Kcal/mol)
68.6864
Ligand-14
Complex-1
0.4386
-53.420
0.310
62.606(Alpha)
38.416(beta)
Urea
3.4646
-3.290
0.091
189.242
4. Conclusions
We have successfully designed and screened 96 novel molecules and analyzed different
factors affecting β value especially their geometrical isomeric effect.
Schiff-Base ligands derived from lone pair rich heteroaromatic amines with nitrogen as
the heteroatom exhibited higher β value than other amines. Introduction of fused planar aromatic
rings in precursor aldehydes enhances the β value of their Schiff-Base ligands (N2 both Z & E
isomers) as compared to others (N1, Z & E isomers). Among the WSB of N1,2, N2 showed higher
β value. E isomer WSB ligands showed higher β value compared to Z isomer. Schiff-Base
ligands derived from homoaromatic amines containing electron withdrawing group (-NH2) at
para position showed higher β value over electron releasing group (-OH). Comparison between
aliphatic five and six-membered rings, Ligand-15 & Ligand-16 respectively, revealed that five-
membered ring has a higher β value over the six-membered ring. With increase in the number of
tertiary amines in the cyclic ring (Ligand-16 -19), β value also increases. High β value producing
molecule (e) was chosen and successfully synthesized and characterized. This study also
revealed the importance of symmetry of the molecule and accordingly, experimental studies are
underway.

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Conflict of interest
All authors declare no conflict of interest.
Acknowledgment
The authors thank DST-FIST, India for the ESI-MS and SXRD facilities. The authors also
acknowledge the NMR facility provided by the CIF, Pondicherry University. Bathula Rajasekhar
(BR) also acknowledges Pondicherry University for providing University Research Fellowship.
BR is especially indebted to Mr. Pothabathina Achhamma Rayappa and Mr. Bathula
Mariyamma Chinna Venkaiah families who have been supportive in achieving his career goals.
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Designed and screened 96 Wittig based Schiff bases for hyperpolarizability (β)
Precursor aldehydes were synthesized and spectroscopically characterized.
Geometrical isomeric effect plays an important role in β value.
Electron withdrawing group showed higher β value than electron releasing group
Aliphatic five-membered ring has a higher β value than six-membered ring.