A rare hetero-bimetallic Zn(II)/Ca(II) Schiff base complex: Synthesis, crystal structure, DFT, molecular docking and unveiling antimicrobial activity

Article abstract of DOI:10.1016/j.molstruc.2020.128951

A new hetero-bimetallic Zn(II)/Ca(II) complex, [Zn₂(L^OMe-pn)₂(?¹-NCS)₂Ca] (1) has been synthesized using a potentially hexadentate N₂O₄ proligand, H₂L^OMe-pn

Full text of DOI:10.1016/j.molstruc.2020.128951

Journal Pre-proof
A rare hetero-bimetallic Zn(II)/Ca(II) Schiff base complex: Synthesis,
crystal structure, DFT, molecular docking and unveiling antimicrobial
activity
Dhrubajyoti Majumdar , Dhiraj Das , Sudip Nag ,
Maitree Bhattacharyya , Dheeraj K. Singh , Debaprasad Parai ,
Kalipada Bankura , Dipankar Mishra
PII:
S0022-2860(20)31276-X
DOI:
Reference:
https://doi.org/10.1016/j.molstruc.2020.128951
MOLSTR 128951
To appear in:
Journal of Molecular Structure
Received date:
Revised date:
Accepted date:
14 May 2020
11 July 2020
20 July 2020
Please cite this article as: Dhrubajyoti Majumdar , Dhiraj Das , Sudip Nag , Maitree Bhattacharyya ,
Dheeraj K. Singh , Debaprasad Parai , Kalipada Bankura , Dipankar Mishra , A rare hetero-
bimetallic Zn(II)/Ca(II) Schiff base complex: Synthesis, crystal structure, DFT, molecular
docking and unveiling antimicrobial activity, Journal of Molecular Structure (2020), doi:
https://doi.org/10.1016/j.molstruc.2020.128951
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1
RESEARCH HIGHLIGHTS
A new hetero-bimetallic Zn(II)/Ca(II) complex was synthesized using hexadentate

N
2
4
O proligand




X-ray single-crystal structure of complex was determined
Fluorescence and DFT/M06 investigations were performed
Antibacterial efficacy, solvent effect and bacterial killing efficiency were examined
A docking study with DNA Gyrase and DNA Polymerase III proteins corroborate the
experimental findings

2
A rare hetero-bimetallic Zn(II)/Ca(II) Schiff base complex: Synthesis, crystal structure,
DFT, molecular docking and unveiling antimicrobial activity
a, b
c
d
d
Dhrubajyoti Majumdar , Dhiraj Das , Sudip Nag , Maitree Bhattacharyya , Dheeraj K.
e
f
a**
a*
Singh , Debaprasad Parai , Kalipada Bankura , Dipankar Mishra
a
Department of Chemistry, Tamralipta Mahavidyalaya, Tamluk-721636, West Bengal, India
Department of Applied Chemistry, Indian Institute of Technology (Indian School of Mines),
b
Dhanbad, Jharkhand 826004, India
c
Department of Chemistry, IISER, Tirupati, Rami Reddy Nagar, Karakambadi Road,
Mangalam (PO), Tirupati- 517507, Andhra Pradesh, India
d
Department of Biochemistry, University of Calcutta, 35 Ballygunge Circular Road, Kolkata
7
00019, India
e
Department of Physics; Institute of Infrastructure Technology Research and Management;
Near Khokhara Circle, Maninagar East, Ahmedabad-380026, Gujarat, India
f
L V Prasad Eye Institute, Patia, Bhubaneswar, India-751024
AUTHOR EMAIL ADDRESS: dmishra.ic@gmail.com, kalipada2010@gmail.com

3
ABSTRACT
A new hetero-bimetallic Zn(II)/Ca(II) complex, [Zn
synthesized using a potentially hexadentate N
OMe-pn
1
2
(L
)
2
(ƞ -NCS)
2
Ca] (1) has been
OMe-pn
2
O
4
proligand, H
2
L
= N,N-bis(3-
methoxysalicylidene)-2,2-dimethylpropane-1,3 diamine and structurally well characterized
by different physicochemical techniques. Single crystal X-ray diffraction study of the
complex revealed that, de-protonic form of the ligand utilizes two pockets, one is occupied by
Zn(II) ion and other is housed by a Ca(II) ion. The Zn centres are fitted into the N
compartment with distorted square pyramidal geometry while a large open O
2
O
2
2
2
O '
compartment is occupied by a Ca(II) ion in a octacoordinated environment. DFT studies were
successfully carried out using DFT/M06 functional level and 6-31G * basis set (over all the
atoms) to delineate the complex optimized structure, composition of the frontier molecular
orbitals, global reactivity, total energy frameworks, MEP, total dipole moment, average
-31
polarizability and first hyperpolarizability. First hyperpolarizability value (59.62 x 10 esu)
suggests that complex is a good NLO material. Hirshfeld surface revealed the presence of
many short intermolecular contacts like C⋯H/S⋯H/N⋯H/O⋯H/H⋯H/C⋯C and S⋯S in the
crystal structure. The experimental absorption spectra are comparable with the electronic
*
transitions assigned from TD-DFT studies. CHEF based (π→π ) luminescence property has
been measured in DMSO solvent at room temperature. Concentration dependent antibacterial
efficacy was investigated against gram-negative bacterial strains viz. Escherichia coli (ATCC
2
5922) & Pseudomonas aeruginosa (ATCC 27853) and gram-positive bacterial strains viz.
Staphylococcus aureus (ATCC 25923) & Bacillus subtilis (ATCC 6633). The bacterial
killing efficiency and solvent effect of the complex was compared with that of ligand.
Finally, a docking study with the active site of DNA Gyrase and DNA Polymerase III
proteins has been carried out to corroborate the experimental findings.

4
Keywords: Antibacterial, Schiff base, Zn(II)/Ca(II), DFT, Fluorescence, Molecular docking
. Introduction
1
In recent years, the design and syntheses of hetero-bimetallic complexes using salen-type
ligands are continuously pledging immense interest due to their diverse field of applications
in coordination chemistry [1-16]. Complexation with the introduction of another new metal
centre may possibly change the topologies and the coordination environments which largely
affect various physicochemical properties like magnetic, catalytic, Photoluminescent etc.
Such classified heteronuclear complexes have been synthesized using bicompartmental N/O-
donor Schiff bases which are commonly derived from salicylaldehyde derivatives ortho
vanillin and usual diamines [17]. Curiously, Calcium is the fifth most abundant element in the
Earth's crust, and the third most abundant metal, still coordination chemistry of Ca(II)-Schiff
base is rare. The beginning of Calcium-Schiff base complexes started from 1977 by Bullock
and Tajmir-Riahi in relationship with trivalent lanthanides. Very few instances of Ca(II)-
MOFs or N/O-donor complexes are explored within the literature which are excellent
luminescent under UV/Vis excitation [18].The ring-opening polymerization, hydrogenation,
hydroboration, hydroamination, hydrosilylation and amine-borane dehydro-coupling are
other significant uses of novel Ca(II) buildings [19]. The stereochemical condition of Ca(II)
buildings is amazing, it ought to be either hexa or octa coordinated. Furthermore, polydentate
phenol-based ligands possessing unique bicompartmental nature, because of its ability to
provide two adjacent coordination sites with dissimilar environment functioning as bi-
nucleating agent [20]. Single crystal X-ray diffraction study diagnoses the mode of
2
+
encapsulation of the metal (M ) ions in the ligand proximity without exception [21-25].
Apart from conventional homonuclear complexes, our recent research focus has been on
heterometallic complexes [26-28] due to their multifaceted synthetic advantage. In this
context crown ethers/cryptands are another important class of complexing agent for s-block

5
or 3d series metal ions [21] but hexadentate N
2
O
4
proligand is effectively bound to s-
block/3d/4f metal ions [21,29-37]. Such labile ligands played not only an important role in
the development of coordination chemistry [38] but also their significant contribution in the
development of inorganic biochemistry [39], catalysis [40], electrochemical conduction
[
41,42], ion recognition [43-46], supramolecular architectures [47,48], magnetism [49,50],
optical devices [51] and luminescent materials [52-55]. Moreover, homo/heteronuclear Schiff
base complexes have been attracting much attention in the curated area due to awesome
ligating behaviour and have a wide spectrum of biological applications, e.g., DNA binding
[
[
[
56], antimicrobial [57], anticancer [58,59], antitubercular, antiangionesis [60,61], antiviral
62], antipyretic [63], antiparasitic, anti-inflammatory [64], anti-HIV and anti-convulsant
65].These biological applications are greatly influenced by the structural diversity of the
complex, the presence of azomethine linkage as well as the substituents bound on the Schiff
base ligands [66]. Remarkably, computer aided drug design (molecular docking) of Schiff
base complexes are another recent development in biological coordination chemistry [67].
Today DFT in conjunction with molecular docking analysis is inevitable to explore several
molecular interactions between the synthesized metal complex and proteins [67]. Thiocyanate
-
[
SCN ] ion is a versatile pseudo-halide that can coordinate to metal ions in an assortment of
bridging modes (Scheme S2).Along these lines, hexadentate N
2
4
O proligand in association
with pseudo-halide ions have been utilized over many years for the synthesis of
homo/heterometallic complexes [68,69] which show interesting structure and molecular
topologies, where the molecular architecture is governed not only by the nature of pseudo-
halides but also depends on the steric requirements of the ligands [70].The chemistry of
1
0
group 12 metal ions has always exhibited flexible geometry due to their d configuration and
zero crystal field stabilization energy (CFSE) [71].The selection of biologically important
2
+
Zn metal ion is not only based on the novel applications of emitting materials for organic

6
light emitting diodes [72], but also due to the fluorescent sensors for organic/inorganic
analyses [73]. In the last few years, we have explored Group 12 metal ions coordination
behaviour with compartmental salen-type ligands and their photoluminescence, DFT/TD-
DFT, molecular docking, cytotoxic effect, autophagy, apoptosis/necrosis type classical cell
death, in vitro antibacterial and anti-biofilm properties vividly [74-78].The result is the
consistent of photoluminescence, cytotoxicity, antibacterial and anti-biofilm properties yet it
is noteworthy that no exiguous number of research works have been devoted in favour of
-
hetero-nuclear complex formation with hexadentate N
2
4
O proligand and pseudo-halide [SCN
]
spacers. In spite of the fact that the broad literature survey investigates chemistry of
discrete/di/trinuclear 3d/alkali metal/salicylaldimine complexes [79] but exceptionally few
looks into their structures [80,81] and novel properties.
In this paper, we have reported the synthetic details, spectral characterizations, X-ray crystal
structure, DFT investigations, along with concentration dependent antibacterial efficacy and
fluorescence studies of complex 1 in the presence of a potentially hexadentate N
2
O
4
OMe-pn
proligand, (H
2
L
). Subsequently, molecular docking studies were performed in order to
understand the binding capability of the complex with active site of DNA Gyrase and DNA
Polymerase III proteins.
2
. Experimental section
2
.1. Materials and physical measurements
All chemicals were of analytical reagent grade and used as received without any purification.
Zn(OAc) .2H O, Ca(SCN) .4H O, ortho vanillin and 2, 2-dimethyl-1,3-propanediamine were
2
2
2
2
purchased from Sigma Aldrich Company, USA. Elemental analyses were performed on a
Perkin-Elmer 2400 CHN elemental analyzer. FT-IR and Raman spectra were recorded as
-1
KBr pellets (4000–400 cm ) using Perkin–Elmer spectrum RX 1 and BRUKER RFS 27
-1
1
13
(
4000-50 cm ). H and C NMR spectra were recorded on a Bruker 300 MHz and 75.45

7
MHz FT-NMR spectrometer using trimethyl silane as internal standard in DMSO-d
6
solvent.
EDAX and SEM experiments were performed on EDAX OXFORD XMX N Model using
Tungsten filament and JEOL Model JSM - 6390LV. UV-Vis spectra (200-1100 nm) were
determined using Hitachi model U-3501 spectrophotometer. Fluorescence spectra in DMSO
solvent were measured at room temperature using Perkin-Elmer LS50B Spectrofluorometer
model. Powder X-ray diffraction measurements were carried out by BRUKER AXS,
GERMANY X-ray diffractometer D8 FOCUS model using Cu Kα-1 radiation. Quantum
yield (Φ ) of compounds was determined using equation (1) with reference to quinine sulfate
F
as a secondary standard (Φ = 0.57 in water) [82].
F
2
S AS (Abs)R nS
R AR (Abs)S



------------------------------------------------------------------------------(1)
2
n
R
According to equation (1): A terms denote the fluorescence area under the curve; Abs denotes
absorbance; n is the refractive index of the medium; Φ is the fluorescence quantum yield; and
subscripts S and R denote for the studied sample and reference respectively.
2
.2. X-ray crystallography
Crystals of complex 1 were grown by slow evaporation of CH
room temperature. Good quality crystals data were collected on a Bruker SMART CCD [83]
diffractometer using Mo K radiation (λ = 0.71073 Å) at 140 K. Judicious crystal data
3
OH-DMF solvent medium at
α
collection purpose, we have utilized different common programs such as SMART for
collecting frames of data, indexing reflections and determining lattice parameters, SAINT [84]
for integration of the intensity of reflections and scaling, SADAB [85] for absorption
correction and SHELXTL for space group, structure determination and least-squares
2
refinements on F . Crystal structure was fully solved after refining by full-matrix least-
2
squares methods against F using SHELXL-2014 [86] and Olex-2 software [87]. Further, all
the non-hydrogen atoms were refined with anisotropic displacement parameters and all

8
hydrogen positions were fixed at calculated positions which are refined iso-tropically. All
crystallographic figures were designed using Diamond Version 4.6.3. The crystallographic
data and full structure refinement parameters for complex 1 are clearly submitted in Table 1.
Table 1 Crystal data and full structure refinement parameters for complex
Formula
M/g
Crystal system
C
44
H
48 CaN
6
O
8
S
2
Zn
2
1023.82
Monoclinic
Space group
P2 /c
1
a/Å
24.363(2)
b/Å
c/Å
β (°)3
V/Å
Z
19.4085(9)
19.8148(17)
93.881(3)
9348.0(15)
8
-3
ρ
c
/g cm
1.455
-
1
μ/mm
1.283
4240
0.26× 0.22 × 0.20
2.26 to 25.00
-28 ≤ h ≤ 28
F(000)
Cryst size (mm )
θ range (deg)
3
Limiting indices
-
23 ≤ k ≤ 23
-
23≤ l ≤ 23
Reflns collected
Ind reflns
181406
16429[Rint = 0.1769, Rsigma = 0.0931]
Completeness to θ (%)
Refinement method
0.999
2
Full-matrix-block least-squares on F
Data/restraints/ parameters
16429/0/1145
1.070
2
Goodness-of-fit on F
Final R indices
1
R = 0.0721
[
I > 2θ(I)]
wR
2
= 0.1198
R indices (all data)
R
1
= 0.1366
wR
2
= 0.1415
-3
Largest diff. peak and hole (e ·Å )
1.08 and -0.65
2
.3. Computational methods

9
Complex geometry optimization was carried out using Gaussian 16 programming package
[
88]. All geometries in DMSO (Dimethyl Sulfoxide) have been optimized and the solvent
effect was estimated with the polarized continuum model (PCM) [88]. For complete DFT
calculations, the new highly parameterized approximate exchange-correlation energy
functional M06 (Meta-generalized gradient approximation (GGA) functional exchange) was
selected [89,90]. Calculations were performed using a valence double-zeta polarized basis set
6
-31G* over all the atoms. Considering the presence of a minima of the potential surface, the
vibrational analysis was carried out which revealed a lack of imaginary frequencies. The
time-dependent density functional theory (TD-DFT) was employed for complex 1 at M06/6-
3
1G* functional level of theory in DMSO using PCM model to obtain an insight into the
participation of various frontier molecular orbitals (FMOs) and their related contribution to
the UV-Vis absorption spectrum. These calculations were performed after geometry
optimization at ground electronic state to obtain vertical electronic transition energies. The
Gauss View 6 program was used to visualize the optimized structures, molecular electrostatic
potential (MEP) plot, graphics of the frontier molecular orbitals and theoretically calculated
UV-Vis spectra [91]. Gauss sum [92] was used in the final phase of the theoretical
computation to calculate the fractional contribution to each molecular orbital of the various
individual elements existing in the complex. The theoretical calculation of the first
hyp
hyperpolarizability ( ) and its component using finite field (FF) approach was carried out
hyp
at M06/6-31G* level of theory. For calculation of first hyperpolarizability ( ) of a variety
of chemical systems, the FF approach has been used which provide very constant results in
accordance with the experimental results [93]. The total static dipole moment was calculated
using equation (1) as
ꢃ
ꢂ
ꢃ
ꢅ
ꢃ
ꢀ
ꢁ
ꢄꢁ
ꢄ
ꢆ
(1)
The isotropic polarizability was calculated using equation (2) as:

10
ꢈꢉꢉꢊꢈꢋꢋꢊꢈꢌꢌ
ꢍ
ꢇꢀ
(
2)
hyp
The first-order hyperpolarizability ( ) was calculated using equation (3) as:
ꢑ
ꢃ
ꢃ
ꢃ ꢒ
ꢎꢏꢐ
ꢎꢏꢐ
ꢎꢏꢐ
ꢂꢅꢅ
ꢎꢏꢐ
ꢎꢏꢐ
ꢅꢅꢅ
ꢎꢏꢐ
ꢎꢏꢐ
ꢎꢏꢐ
ꢎꢏꢐ
ꢎꢏꢐ

ꢀ[(ꢂꢂꢂ ꢄꢁ
ꢄꢂꢆꢆ) ꢄ(
ꢄꢁꢅꢆꢆ ꢄꢅꢂꢂ) ꢄ(ꢆꢆꢆ ꢄꢁꢆꢂꢂ ꢄꢆꢅꢅ) ]
(
3)
2
.4. Antibacterial assay
The antimicrobial efficiency of complex (C1) was investigated comparatively with ligand
L1) against gram-negative bacterial strains viz. Escherichia coli (ATCC 25922) &
(
Pseudomonas aeruginosa (ATCC 27853) and gram-positive bacterial strains viz.
Staphylococcus aureus (ATCC 25923) & Bacillus subtilis (ATCC 6633). The antibacterial
activity was investigated through concentration dependent bacterial growth inhibition assay
using micro-dilution method [94] in 96 well plate. Briefly, all bacterial suspension was
cultured in Luria Bertini (LB) broth in 96 well plate. Complex and ligand at different
concentrations (5, 10, 25, 50, 100, 200, 500, 700, 1000 µM) were added in respective well
containing a bacterial culture and incubated for 24 h in mild shaking condition at 37°C.The
bacterial growth inhibition was evaluated by recording the optical density (O.D) at 600 nm
using microplate reader (Biorad, USA) compared to control. Herein, standard antibiotic
solution (aqueous) gentamicin sulphate (10 µg/mL) was utilized as a positive control. As the
complex and ligand were soluble in dimethyl sulfoxide (DMSO) and methanol (CH OH)
3
respectively, the solvent effect on bacterial growth [95] has also been investigated. The
bacterial cells viability was presented as percentage of relative survival. Cell viability was
determined using the following equation,
ꢟꢠꢡꢢꢣꢤꢥꢦ ꢧꢟꢠꢨꢥꢢꢩꢪ
ꢓꢔꢕꢕꢁꢖꢗꢘꢙꢗꢕꢗꢚꢛꢜꢝꢞꢀ
ꢯꢰꢱꢱ
ꢟꢠꢫꢬꢩꢭꢮꢬꢥ ꢧꢟꢠꢨꢥꢢꢩꢪ

11
The absorbance values of treated cells and untreated cells are denoted as ODSample and
ODControl respectively. The whole assay was performed with independent triplicate
experimental setup to obtain the standard mean values. Further, the killing efficiency of
complex and ligand were determined in a comparative way by the spread plate method.100
µL of culture suspension was collected from well and spread over the LB agar plates with
appropriate dilutions and then incubated at 37ºC for another 24 h under static condition. The
killing efficiency was determined by counting bacterial colonies compared to control.
2
.5. Molecular docking
DNA gyrase and DNA polymerase III (Pol III) are two essential proteins required for
bacterial DNA replication with high efficiency and accuracy which leads to bacterial cell
viability [96,97]. DNA gyrase catalyzes the ATP-dependent negative supercoils into DNA
and Pol III carried out highly processive DNA replication during cell division in tight
coordination between synthetic strands. These two proteins of bacteria serve as the potential
targets in the development of new antibacterial compounds. Molecular docking was
performed using Escherichia coli DNA gyrase (GyrB) and DNA Pol III protein. The
crystallographic structure of both proteins: GyrB (PDB ID: 5MMN, resolution=1.9 Å) and
Pol III (PDB ID: 2HNH, resolution=2.3Å) were downloaded from RCSB Protein Data Bank
(
PDB) [https://www.rcsb.org/pdb].
.6. Synthesis of N, N-bis(3-methoxysalicylidene)-2,2-dimethylpropane-1,3 diamine
Schiff base has been synthesized in our laboratory following the literature method (Scheme
) [98]. It was obtained by the reflux condensation of 2, 2-dimethyl-1, 3-propanediamine
2
1
(
0.0511 g, 0.5 mmol) with 3-methoxy-2-hydroxybenzaldehyde (0.152 g, 1 mmol) in (50 mL)
°
methanol at 70 C for 3 h. Subsequently solvent was removed under vacuum to give a yellow
powder product. N, N-bis(3-methoxysalicylidene)-2,2-dimethylpropane-1,3 diamine: Yield:
0
.158 g (85.4 %), Anal. Calc. for C21
H
26
N
2
4
O : C, 68.09; H, 7.07; N, 7.56 Found: C, 68.06; H,

12
-
1
7
3
1
.05; N, 7.53 %. IR (KBr cm ) selected bands: ν(C꞊N), 1652 vs, ν(C-Ophenolic) 1252 s, ν(O-H)
1
1
2
3
378 s, H NMR (DMSO-d6, 300 MHz): δ (ppm): 3.76 (s, 3H ), 6.79-7.01 (m, 1H , 1H ,
4
5
6
7
8
H ), 8.50 (m, 1H ), 13.86 (m,1H ), 3.45-3.46 (s, 2H ), 0.96 (s, 6H ), (Scheme1, Fig. S3A),
1
3
11
1
C NMR (DMSO-d6, 75.45 MHz): δ (ppm):15.2 ( CH
3
), 64.2-67.0 (O- CH
3
),116.3-147.6
3
5
7
8
10
9
(
(
Arom- C- C),152.2 ( C-OH), 167.3 ( CH=N), 24.0 ( C-(CH
3
)
2
, 36.1 ((CH
3
)
2
-C-( CH
2
2
) ),
Fig. S3B), UV-Vis (λmax /nm): 382 nm.
OMe-pn
Scheme 1 Synthesis of compartmental Schiff base ligand (H
.7. Synthesis of complex as single crystal
2
L
)
2
To the methanolic solution (25 mL) of zinc acetate dihydrate (0.2195 g, 1 mmol), (10 mL)
ligand solution was added drop by drop and the resulting solution was stirred for 1 h. Then
(
10 mL) methanolic solution of Ca (SCN)
2
2
.4H O (0.228 g, 1 mmol) was added to it. The
0
overall reaction mixture was refluxed for 30 min at 85 C followed by adding few drops of
DMF. Stirring was continued for about 1.5 h. Colourless filtrate was kept for crystallization
by slow evaporation at room temperature. Single crystal, suitable for X-ray diffraction was
obtained after 25 days on slow evaporation of the solution in open atmospheric condition.
Colourless block shaped crystals were isolated by filtration and air dried. Yield: 0.53 g, (51.8
%
), Anal. Calc. for C44
H48 CaN
6
O
8
S
2
Zn : C, 51.61; H, 4.73; N, 8.21. Found: C, 51.59; H,
2

13
-
1
4
2
.70; N, 8.25 %. IR (KBr cm ) selected bands: ν(C꞊N), 1624 vs, ν(C≡N), 2215 s, ν(SCN)
-1
090 vs, FT-Raman (cm ) selected bands: ν(C꞊N), 1632 vs, ν(C≡N), 2191 s, ν(SCN) 2106
1
1
2
3
4
vs, H NMR (DMSO-d6, 300 MHz): δ (ppm): 3.76 (s, 3H ), 6.78-7.10 (m, 1H , 1H , 1H ),
5
6
7
13
8
.42 (m, 1H ), 3.35 (s, 2H ), 0.96 (s, 6H ), (Fig. S3C), C NMR (DMSO-d6, 75.4 MHz): δ
1
1
1
3
5
8
(
ppm):14.9 ( CH
3
), 64.5-65.7 (O- CH
3
), 115.5-151.7 (Arom- C- C), 164.4-166.6 ( CH=N),
1
0
2
3
3
0.5 ( C-(CH
3 2
)
. (Fig. S3D), UV-Vis λmax (DMSO): 370 nm.
. Results and discussion
.1. Synthetic aspect
Schiff base was synthesized by the condensation of 2, 2-dimethyl-1, 3-propanediamine with
ortho vanillin (3-methoxy-2-hydroxybenzaldehyde) in MeOH at (1:2 M) ratio (Scheme1)
[
98]. The complex was derived in moderate yield by taking the following in situ procedure
where (1:1:1 M) ratio of Zn(II) acetate dihydrate, hexadentate N proligand and
Ca(SCN) .4H O in minimum volume methanolic solution under stirred and reflux condition
2
O
4
2
2
followed by a few drops of DMF (Scheme 2). After few days moderately air stable block
shaped crystals appeared at the junction of the solutions which are insoluble in water.
Interestingly, Zn(II) is placed in inner N
2
O
2
compartment and Ca(II) is placed in an outer
proligand. Since Ca(II) centres were well
’
O
2
O
2
compartment of the respective N
2
O
4
surrounded by the alkoxy group, therefore pseudo-halides (thiocyanate) were unable to
coordinate Ca(II) centre. Further, such stereochemical difference is probably due to the larger
size Ca(II) metal ion (ionic radius 126 pm) compared to that of small size Zn(II) ion (ionic
radius 74 pm).Hence it is difficult to accommodate large size Ca(II) in the inner N
2
O
2
’
compartment compared to that of external cavities O
2
O
2
[87].This stereochemical
environment is completely comparative with already detailed salen-type bi-compartmental
1
2
M (II)/M (II)-Schiff base complexes [52,98,99]. Moreover, Salicylaldehyde derivatives
ortho vanillin) are useful precursor for novel syntheses of Schiff base since additional
(

14
alkoxy group attached to salicylaldehyde not only enhanced the ligand denticity but
accelerated the discrete or polymeric complex formations [71]. Binding versality of N
proligand with metal ions is another responsible factor for its selection of the current research
work (Scheme S1). Scheme 3 totally discloses N proligand basic imitates since it
comprises two imines, two phenols and two methoxy groups. After deprotonation, N
2
O
4
2
O
4
2
O
2
2
+
imine chelating location is produced which is facilitated with M metal ions (Scheme 3)
100]. In most of the hetero-metallic complexes entangling capability of metal ions (Scheme
) by inward N and external O ’ compartments was investigated strikingly [52,83,99]
[
3
2
O
2
2
O
2
however, till date hetero-nuclear Zn(II)/Ca(II) complex with particular hexadentate N
2
O
4
proligand is cruelly lacking in the literature [18,101].
Scheme 2 Synthetic route for hetero-bimetallic complex. A=Zn(OAc)
B=Ca(SCN) .4H O, C=Few drops of DMF
2
.2H O,
2
2
2

15
Scheme 3 Existing compartmental ligand showing N
2
O
2
vs O
2
2
O pocket
4
ǀ X-ray crystallography
.1. Crystal structure of [Zn
OMe-pn
1
4
2
(L
)
2
(ƞ -NCS) Ca] (1)
2
The complex is structurally characterized by single crystal X-ray diffraction, and it reveals
that it crystallizes in monoclinic space group P2 /c (Z=8). The complete crystallographic
parameters are shown in Table 1 and selected some important bond distances (Å) and angles
°) are shown in Table S1.The ORTEP view of asymmetric unit (Fig.1) contains two
1
(
molecular units which cooperate by a few C-H∙∙∙∙π type non-covalent associations portrayed
in Fig.3 reflecting Ca-O bond distances (Å). A close investigation into the complex crystal
OMe-pn
structure reveals that the asymmetric unit contains two Schiff base ligands (L
), (where
OMe-pn
H
2
L
is the diprotic proligand), two Zn atom and one Ca atom (Fig.2). Further from
proligand has two pockets, one is
occupied by Zn(II) ions and other is occupied by Ca(II) ion. In each molecular unit the Zn(II)
ligand structural point of view, the hexadentate N
2
O
4
1
-
centre is fitted into the N
2
O compartment with the ƞ fashion of NCS co-ligand with
2
distorted square pyramidal geometry. This structural motif is identical with previously
reported heterometallic complexes [52,83,99,102].

16
Fig.1. ORTEP view of the asymmetric unit (Hydrogen atoms are omitted for clarity)
Fig.2. Two molecular units in the asymmetric unit (Hydrogen atoms are omitted for clarity)

17
Fig.3. C-H∙∙∙∙π type non-covalent interactions
In complex 1, two salen ligands are almost in perpendicularly (83.15° and 80.92°) oriented by
Ca(II) metal ion. The Ca(II) metal ion in each unit is coordinated to eight oxygen atoms, half
is coming from phenolic group and half is coming from methoxy group. Overall, this
stereochemical environment is similar with previously reported Ca(II)-Salen complexes
[
99,101]. In the asymmetric unit (Fig.2), Ca1 is coordinated to O1 to O8 oxygen atoms and
Ca2 is coordinated to O9 to O16 oxygen atoms. The equatorial plane of all the Zn(II) ions are
occupied by N compartment of salen ligand and apical position is occupied by the N atom
2
O
2
of thiocyanate group. The Zn(II) centres (Zn1-Zn4) are displaced towards axial
N3/N6/N12/N9 atom by 0.523 Å, 0.512 Å, 0.520 Å, and 0.512 Å, respectively from their
respective mean equatorial planes. The synthesized complex exhibit awesome C-H∙∙∙∙π type
non-covalent interactions (Table2) which is rehashed all through the 3-D sheet of Packing
diagram (Fig.S5A).
Table 2 C-H∙∙∙∙π type non-covalent interactions

18
Components
C12-H12A····π
C55-H55····π
Distance (Å)
3.182 (3)
3.83 (3)
Angle (°)
160.65(5)
154.61(5)
128.22(5)
124.95(5)
C13-H13A····N8
C53-H53A····N4
3.34 (5)
3.49 (5)
Further, the structural environment of two zinc metal centres were calculated from Addison
parameter Tau (ґ), (ґ =׀β-α׀/06° erwhw β, α are the two largest angles around zinc metal atom;
ґ =0 for a perfect square pyramidal and 1 for a perfect trigonal bipyramidal geometry
[
103].The calculated Addison tau (τ) parameters are 0.098, 0.145, 0.063, and 0.21 (ideal
value of tau for square pyramidal geometry is 0) respectively, for four Zn(II) centres. The
large open O ' compartment is occupied by octacoordinated Ca(II) metal ion, resulting a
2
O
2
tri-nuclear hetero-bimetallic system. The average distances between two heteroatoms (Zn and
Ca) is 3.530 Å {Zn1-Ca1 = 3.5195(12), Ca1-Zn2 = 3.5318(13), Zn3-Ca2 = 3.5237(13), Zn4-
Ca2 = 3.5453(13)} and the average angle between them is 106.33 ° {Zn1-O1-Ca1 =
1
1
1
05.79(16) °, Ca1-O3-Zn1 = 105.20(15)°, Ca1-O5-Zn2 = 106.96(18)°, Zn2-O7-Ca1 =
06.83(18)°, Zn3-O9-Ca2 = 105.66(16)°, Zn3-O11-Ca2 = 106.21(16)°, Ca2-O13-Zn4 =
07.38(17)°, Ca2-O15-Zn4 = 106.62(17)°}. The bond lengths of Zn–Nthiocyanato are in the
range of ~1.981–2.091 Å which are comparable with literature reported Zn(II) complexes
Table S3). In the meantime, to underline as-blended complex structure, we have
demonstrated an auxiliary correlation with literature reported hetero-nuclear complexes
Table S5A-S5B).To the best of our knowledge this is the first ever example of hetero-
bimetallic system where two terminal Zn(II) centres are appropriately fit into the N
compartment followed by large open O ' compartment possessed by Ca(II) ion.
.2. SEM-EDAX
(
(
2
O
2
2
O
2
4

19
To find out the chemical composition of complex 1, SEM-EDAX analysis was performed.
The chemical composition of 1 was confirmed from EDAX profile analysis. The weight
percentage (%) contribution of the elements is submitted in Table S4 and their EDAX profile
in Fig.S6. The calculated and EDAX values of essential elements present in complex are
nearly good agreement. EDAX profile contained only predictable elements and there is no
impurity of other elements. Therefore, the empirical formula of 1 is formulated as
C
44
H48CaN
6
O
8
S
2
Zn . Interestingly, the EDAX profile of complex reflects the highest peak of
2
C and thereafter O, N, Zn which further agreed the empirical formula of 1. Moreover, the
important structural morphological features are supported by SEM investigation. SEM
images characterized the size and morphological structure. Herein, micrograph of SEM
(
Fig.S7A) clearly explores that the morphology of 1 is plate shape shown up to 50 µm.
.3. Powder X-ray diffraction
4
Powder X-ray diffraction patterns for 1 were recorded at room temperature. The X-ray
powder diffraction patterns for complex were recorded experimentally by scanning the
0
0
synthesized complex in the range (2θ=4 -50 ) using 1-D Lynxeye detector at ambient
conditions. Fig.S7B show well defined sharp PXRD peaks due to their crystalline nature. The
experimental PXRD patterns (Fig.S7B) of the bulk materials of heteronuclear complex are
similar with the patterns simulated from single crystal X-ray diffraction data (CIF) obtained
from CCDC Mercury software consisting that single crystals and bulk material are the same.
The results of PXRD analysis can provide the confirmation of the phase purity of each bulk
samples.
5
. DFT analyses
5
.1. Hirshfeld surface

20
Apart from SCXRD analysis, short intermolecular contacts in the crystal structure of the
complex can be quantified by Hirshfeld surface and 2D fingerprint plot. Hirshfeld surface
(
HS) is one of the emerging attempts to define the space occupied by a molecule in a crystal
after proper partitioning the crystal electron density into molecular fragments [104]. HS and
-D fingerprint plots were generated to visualize, explore and quantify the short
intermolecular interactions in the supramolecular packing where Crystal Explorer program
105] has been commonly used. Herein, such analyses revealed the presence of important
2
[
short intermolecular contacts like C⋯H/S⋯H/ N⋯H/O⋯H/H⋯H/C⋯C and S⋯S in the
crystal structure. The HS mapped with the dnorm property has been shown in the Fig.4 and the
2
-D fingerprint plots with characteristics features of their short interactions are represented in
Fig.4(a-c). The dominant interactions according to HS mapped with dnorm are the red spot. In
, relative contributions to the HS area due to close intermolecular contacts are as follows:
C⋯H, 45.8%; S⋯H, 33.8%; N⋯H, 9.2%; O⋯H, 3.2%; H⋯H, 51.0%; C⋯C, 0.6%; and S⋯S
.8 % (Table 3). HS analysis divulges that the assembly dominated by H⋯H and C⋯H
1
0
supramolecular contacts along with minor contributions C⋯C/S⋯S. In terms of 2-D
fingerprint plots (Fig.4), the intermolecular interactions appear as distinct spikes. Details
breakdown of HS and 2-D fingerprint plots of different contributions are submitted in Fig.S8.
Fig.4. Hirshfeld surface dnorm property: red spot represents closet contacts and blue spot most
distant contacts. Graphical view of Hirshfeld surface and 2-D fingerprint plots with

21
chacteristics features of C⋯H 4(a) S⋯H 4(b) N⋯H 4(c) contacts, d
i
and d are the distances
e
from the surface to the nearest atoms interior and exterior to the surface
Table 3 Different type of interactions on HS in complex 1
SL. No.
Type of interactions
C···C (i.e. π··· π)
C···H (i.e. C-H··· π)
S···H (i.e. C-S··· H)
N···H
Major contribution (%)
1
2
3
4
5
6
7
.
.
.
.
.
.
.
0.6
45.8
33.8
9.2
3.2
51.0
0.8
H··· O
H···H
S···S
5
.2. Total energy Frameworks (TEF)
The Crystal Explorer program [105] has been used for energy framework calculation in
favour of synthesized complex. The molecular packing in the unit cell was explored by
means of energy frameworks analysis. Accordingly, the interaction energies between
different molecular pairs are represented via cylindrical tubes connecting the centres of mass
of the interacting molecules. The radius of the cylindrical tube is directly proportional to the
interaction energies. Further insight can be gained when the topology, which is represented
by cylindrical tubes, is disintegrated into electrostatic and dispersive components [106,107].
In complex 1, total-energy-framework diagram exhibits a relatively isotropic topology of
interactions where the dispersion component (-74.5 KJ/mol) is dominating over others. The
occurrence of dimer in the asymmetric unit of complex is certainly stabilizing, with a total
energy of -106.5 KJ/mol. Fig.5(a-c) reflects electrostatic, dispersive, and total energy
contribution whereas Fig.6(a-c) in the asymmetric unit shows electrostatic, dispersive, and
total energy contribution. The calculated energy frameworks output is submitted in Table S6.

22
Fig.5. (a) Electrostatic, (b) dispersive and (c) total energy contribution
Fig.6. (a) Electrostatic, (b) dispersive and (c) total energy contribution in the asymmetric unit
.3. MEP
5
Molecular electrostatic potential (MEP) were calculated to comprehend the relationship
between the molecular structure and reactivity towards nucleophilic and electrophilic regions.
Because of the electron density distribution, MEP is described as the potential experienced by
a unit positive charge near the molecule. MEP indicates the relative polarity of the molecule
as positive potential can predict electrophilic attack sites while the adverse potential indicates
sites for nucleophilic attack. The expression for the MEP at any point r in the space near the
molecule is given by equation (4)

23
ꢆꢳ
ꢷꢜꢸꢹꢞꢺꢸꢹ
ꢖꢁꢜꢲꢞꢀ∑ꢴ
ꢧ∫
ꢹ
ꢵꢳꢶꢮꢴ
ꢴꢮ ꢶꢮꢴ
(
4)
Where, Z
A
is the charge on the nucleus A studied at R and ρ(r’) the electron density. V(r)
A
defines the net electrostatic effect of the total charge distribution (electrons + nuclei) of the
molecule at point r. MEP mapping has recently played a notable part in the geometric
structure, charge density analysis and polarity of various molecular systems [102,108,109].
MEP was generated from the DFT optimized structure of the synthesized complex to
visualize electrophilic and nucleophilic regions (Fig.S8A). Moreover, MEP map distribution
purpose Multiwfn program [110] and the FChk type file was used as the input program using
at B3LYP/6-31G (d, p) level of theory. The red colour on the molecular surface in the Fig.8A
shows the negative MEP (areas prone to nucleophilic attack) while the blue colour on surface
shows positive MEP (electrophilic attack). The potential on MEP surface of the complex is in
the following order of yellow < orange < red < green < blue. The green colour on the surface
indicates the region with zero potential. Further it is evident that the regions around the
terminal Sulphur atoms of the two thiocyanate co-ligands have the highest negative
electrostatic potential with a value of -48.9 kcal/mol and the highest positive charge is found
near the azomethine hydrogen atoms with a value of +46.1 kcal/mol (Fig.S8B). This is
common nature found widely among the salen-type complexes [111,112]. The region around
the Zn metal centre has a positive charge of +21.6 kcal/mol while the benzenoid rings of the
complex have a charge of -26.4 kcal/mol. The area around calcium metal ion where it is
bonded by eight oxygen atoms appears to have a negative charge of about -36.5 kcal/mol.
5
.4. Global reactivity
Global reactivity descriptors can be taken as a tool for reactivity and regioselectivity of the
synthesized compound towards various reactions [113] which are also carefully calculated in

24
Table S6A. On performing Mulliken charge analysis (Fig.S8B), it reveals that the highest
positive charges are concentrated on the metal centres. Out of the three metal centres, only Ca
centre has a charge of +1.17 and the two Zn centres have a charge of +0.96 each. The highest
negative charges are concentrated on the donor hetero atoms (S, N, O) and among them
phenolic oxygens of the salen-type ligand exhibited the highest charge (-0.77) followed by
methoxy oxygen atoms in these ligands (-0.58). The next highest charge is observed in
nitrogen atoms of the thiocyanate co-ligand (-0.57) and by the azomethine nitrogen atoms (-
0
.46). The least negative charge is observed in sulphur atoms (-0.29) of the coordinated
thiocyanate moiety.
.5. TD-DFT
5
TD-DFT computations were executed in DMSO solvent to quantify the contributions of
various frontier molecular orbitals (FMO) towards absorption spectra (Table S6B). It is
evident that major molecular orbital contributor in charge transfer (CT) region in complex 1
is HOMO-LUMO (Table S6B). The computed TD-DFT study of complex 1 exhibits the
electronic transitions which are mainly due to the intra-molecular charge transfer. The
HOMO of the complex is mainly composed of π-orbitals whereas LUMO consists of π*-
orbitals. The analysis of calculated spectrum disclosed one strong electronic transition at 365
nm (experimental 370 nm) with oscillator strength 0.33 (Fig.S9 and Table S6B). The
electronic transition is primarily due to the transition of an electron from HOMO to LUMO
(
48%) and HOMO to LUMO-1 (22%).
.6. Energy and HOMO-LUMO analysis
5
Synthesized complex was optimized in DMSO solvent using a PCM model including solvent
effect at M06/6-31G* level of theory. Fig.S10 shows the optimized geometry of complex 1.
The mapping of the frontier molecular orbitals (FMOs) has provided an interesting outcome

25
for complex. HOMO (highest occupied molecular orbital) value determines a molecule's
capacity to donate electron while LUMO (lowest unoccupied molecular orbital) value
determines the electron-accepting nature. HOMO‐LUMO density plot (Fig.7) was used for
probing of FMOs of the complex as it is well known that the non-linear optical (NLO)
characteristics of π‐conjugated complexes are derived from the delocalization of the π‐
electron cloud [114]. A close inspection of the HOMO-LUMO energy plot divulges that
HOMO of 1 is primarily contributed from the methoxy group (donor) and oxygen atoms
which is coordinated to the Ca-atom at the centre of the molecule. In LUMO, the electron
density is mainly focused on the moieties which are attached to the methoxy group and other
-
moieties which are bridged with nitrogen atoms connected to the zinc-atoms. The [SCN ]
spacer makes the bridging is strong and stable since it is not contributing much in HOMO or
LUMO and it affects the stability of the complex. The electronic energy of HOMO-LUMO is
tabulated in Table S6C. The energy of HOMO is -5.73 eV whereas of LUMO is -1.46 eV.
The energy gap ∆E of HOMO-LUMO (ꢻꢼꢀꢼꢽꢾꢿꣀ ꢧꢼꣁꣀꢿꣀ) is 4.27 eV (marked by blue
line, Fig.7) which is slightly high making the complex stable and can be correlated with the
band gap of the complex. The distinction between the conductive band and the valence band
is usually referred as the band gap. In order to check the semiconducting properties of
complex, theoretical UV-Vis absorption spectra were also calculated and shown in Fig. S9
(
wavelength range of 200-600 nm). Here the theoretical band gap is in the range of 3-6 eV
which is defined for the wide-band gap semiconductors [115,116]. Therefore, complex
possesses semiconducting property. Because its bandgap is about 4.27 eV (wide-bandgap
semiconductor), it functions at much greater voltages, frequencies and temperatures than
usual semiconductor-based products. Based on these theoretical bandgap, compound 1 can be
suitable for next-generation semiconductor devices.

26
Fig.7. Frontier molecular orbitals of 1 (∆E: Energy gap between HOMO and LUMO)
5
.7. NLO activity
The energy gap ∆E (4.27 eV) is linked to the transfer of electron density from the HOMO to
the LUMO and its value is important for the determination of the NLO property of the
molecular system. If a molecule possesses a smaller energy gap generally shows high NLO
activity [117]. Herein, theoretical calculations support the mentioned findings. In complex 1,
various parameters such as electric dipole moment (μ), polarizability (α) and first
hyp
hyperpolarizability (β ) were computed at the M06/6‐31G* level of theory (Table4). Since
hyp
-31
the first hyperpolarizability (β ) value is equal to 59.62 × 10 esu, synthesized compound
exhibit good NLO activity which may be due to the increased distribution of π-electronic
charge at the ground state by the donor and acceptor moieties under the impact of an
hyp
electrical field. In addition, β is often associated with the charge transfer bands resulting
from the electron cloud movement through the π-electronic framework from donor to the
acceptor level, and such electronic clouds interact with external fields, resulting in an

27
hyp
hyp
increase in β . The α and β values are directly indicated that synthesized complex exhibits
good NLO activity.
Table 4 DFT computed dipole moment (μ), average polarizability (α) and first
hyp
hyperpolarizability (β )
-22
hyp
-31
Complex
μ (Debye)
α (10 esu)
β
(10 esu)
OMe-pn
1
[
Zn
2
(L
)
2
(ƞ -NCS)
2
Ca] (1)
14.44
0.58
59.62
6
. Spectroscopic analyses
.1. FT-IR and Raman characterizations
6
Ligand and complex were characterized by the IR spectroscopic methods. The characteristic
-1
imines (C=N) stretching vibration of synthesized Schiff base was found to be 1652 cm
-
1
[
(
118] (Fig.S1). FT-IR and Raman (C=N) stretching vibration bands are shifted to 1624 cm
-
1
for IR) (Fig.S2) and 1632 cm (for Raman) respectively (Fig.S3). These spectral data
confirms the coordination mode of the imine nitrogen atom to the Zn(II) metal centre [119].
-1
-1
Complex displayed strong bands at 2090 cm (for FT-IR) and 2106 cm (for FT-Raman)
[
120,121]. These spectral values are directly attributable to ν(SCN) binding mode. Aliphatic
-1
-1
C-H stretching resonances are observed at 2932-2964 cm (for FT-IR) and 2916 cm (for
-
1
FT-Raman) respectively. Ar-O stretching frequency near 1219 cm is identical with
-
previously reported salen-type ligands [122]. Herein, the bridging mode of [SCN ] spacers in
the light of IR/Raman spectra were thoroughly compared with previously reported Zn(II)-
Schiff base complexes (Table S2A-Table S2B).
6
.2. UV-Vis spectra
The UV-Vis absorption spectra of the Schiff base and its complex were recorded in DMSO
*
solvent. Free ligand exhibit band at 382 nm (Fig. S4) which are assigned due to π→π and

28
*
n→π type transitions whereas complex displayed sharp single ligand–based UV domain at
*
*
3
70 nm attributed to L→M charge transfer transition (π→π /n→π ) (Fig.S4) [119,123].The
spectral domain supports the coordination mode of the ligand with Zn(II) metal ions which
are blue-shifted 12 nm. The blue-shift may be due to the structural environment of ligand and
*
the perturbation of the intraligand π→π transition by the Zn(II) ions which are nearly
comparable with previously reported Zn(II) salen-type complexes [123b]. Here no metal
1
0
centric d-d broad absorption band was assigned due to d electronic configuration and
diamagnetic nature of Zn(II) metal ion.
1
13
6
.3. H and C NMR spectra
1
13
H and C NMR spectra has been used to extract the formation of Schiff base ligand and its
1
13
coordination mode with Zn(II) ion in as-synthesized complex (Scheme 1). H and C NMR
spectral data of ligand and the complex was provided in the experimental section. Free Schiff
base no broad peak was identified in the region δ 5.0-8.0 ppm indicating the absence of the
free -NH group. Moreover, the identified broad peak at δ 13.86 ppm corresponds to the
2
6
5
phenolic (OH ) protons. The protons (H ) attached to the imino carbon are strongly downfield
shifted (δ 8.5 ppm) [124] due to the influence of the combined effect of phenolic -OH and
imino N groups in its close vicinity. The peaks near the range δ 6.79-7.01 ppm corresponds to
2
4
1
the aromatic protons (H -H ). The three methyl protons OCH
3
(H ) attached to the aromatic
8
oxygen appear at δ 3.76 ppm. Apart from, (C-(CH
3
)
2
group protons (H ) appear at δ 0.96
7
ppm. Since methylene protons (H ) being very close to the imino nitrogen, are deshielded and
appear at δ 3.46 ppm. In case of complex, the coordination mode of azomethine (HC=N)
nitrogen was assigned by the downfield shifting of the azomethine proton signal from 8.50
6
ppm in free Schiff base to 8.42 ppm. Moreover, the OH proton (OH ) signal at the Schiff base
1
disappeared in the H NMR spectra of the synthesized hetero-nuclear complex, indicating
1
3
deprotonation and coordination of the oxygen with Zn(II) ion [125].The C NMR spectra of

29
8
Schiff base ligand showed characteristic azomethine ( CH=N) carbons at 167.3 ppm. The
1
3
11
1
3
other C NMR peaks observed at 15.2 ( CH
3
), 64.2-67.0 (O- CH
3
), 116.3-147.6 (Arom- C-
5
7
10
9
C), 152.2 ( C-OH), 24.0 ( C-(CH
3
)
2
and 36.1((CH
3
)
2
-C-( CH
2
)
2
) ppm respectively
(
Fig.S3B). Downfield shifting of the azomethine carbons in the free ligand 167.3 ppm to
64.4-166.6 ppm in its Zn(II) complex was due to shifting of electron density towards the
1
Zn(II) ion. This downfield shifting also confirmed the coordination of the azomethine to the
zinc metal atom (Fig.S3D) [125,126].
7
. Steady-state photoluminescence
Fluorescence spectral properties of Schiff base ligand and complex are determined in DMSO
solvent at room temperature. According to Fig.8 the free ligand demonstrated a fluorescence
peak at ca 489 nm upon excitation at ca 370 nm which could be attributed to intra-ligand
*
(
π→π ) type transition whereas for complex 1 photo excitation at similar wavelength shows
red shifted fluorescence maxima with the major emission peak at ca 505 nm. The
enhancement of fluorescence emission intensity of 1 over free ligand probably may be due to
N/O-donor centres coordination with the Zn(II) metal ion, thereby increasing the
conformational rigidity via chelation effect and subsequently loss of energy by radiation less
thermal vibration. Meanwhile, the active fluorescence spectral change compared to free
1
0
ligand may be due to d electronic configuration of Zn(II) metal ion which is difficult to
oxidize or to reduce. This fluorescence behaviour may be attributed to the intra-ligand
*
(
π→π ) type transition or L→M charge transfer referred as CHEF (Chelation enhanced
fluorescence) [127-131].The enhancement of emission intensity of complex (about 16 nm)
over ligand is supported by quantum yield (Φ ) values with reference to quinine sulphate
Table5). Generally, fluorescence emission of Schiff base ligands containing an aromatic ring
F
(
enhance or quench in contact with metal ions. Herein a close inspection of quantum yield
value confirmed that variations are negligible. This is due to the excitation wavelength of