Heterometallic Zn(II)-K(I) complex with salen-type Schiff-base ligand: Synthesis, crystal structure, solid-state photoluminescent property and theoretical study

Article abstract of DOI:10.1016/j.molstruc.2019.02.092

A novel tetranuclear heterometallic complex, [KZn(3-MeOsalpn) (μ_1,1-NCS)]₂(1) [3-MeOsalpn^2- = N, N′-propylene bis(3-methoxysalicylideneiminate)] has been synthesized by the reaction of Zn(OAc)₂?2H₂O w

Full text of DOI:10.1016/j.molstruc.2019.02.092

Accepted Manuscript
Heterometallic Zn(II)-K(I) complex with salen-type Schiff-base ligand: Synthesis,
crystal structure, solid-state photoluminescent property and theoretical study
Dhrubajyoti Majumdar, Swapan Dey, Dhiraj Das, Dheeraj Kumar Singh, Sourav Das,
Kalipada Bankura, Dipankar Mishra
PII:
S0022-2860(19)30228-5
DOI:
https://doi.org/10.1016/j.molstruc.2019.02.092
MOLSTR 26249
Reference:
To appear in: Journal of Molecular Structure
Received Date: 24 December 2018
Revised Date: 17 February 2019
Accepted Date: 23 February 2019
Please cite this article as: D. Majumdar, S. Dey, D. Das, D.K. Singh, S. Das, K. Bankura, D. Mishra,
Heterometallic Zn(II)-K(I) complex with salen-type Schiff-base ligand: Synthesis, crystal structure, solid-
state photoluminescent property and theoretical study, Journal of Molecular Structure (2019), doi: https://
doi.org/10.1016/j.molstruc.2019.02.092.
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Heterometallic Zn(II)-K(I) complex with salen-type Schiff-base ligand: synthesis, crystal
structure, solid-state photoluminescent property and theoretical study
a,b
b
c
d
d,*
Dhrubajyoti Majumdar , Swapan Dey , Dhiraj Das , Dheeraj Kumar Singh , Sourav Das ,
a
a,*
Kalipada Bankura , Dipankar Mishra
a
Department of Chemistry, Tamralipta Mahavidyalaya, Tamluk-721636, West Bengal, India
b
Department of Applied Chemistry, Indian Institute of Technology (Indian School of Mines),
Dhanbad, Jharkhand 826004, India
c
Department of Chemical Sciences, IISER, Mohali, Manauli, PO 140306, India
d
Department of Chemistry; Institute of Infrastructure Technology Research and Management;
Near Khokhara Circle, Maninagar East, Ahmedabad-380026, Gujarat, India
AUTHOR EMAIL ADDRESS: dmishra.ic@gmail.com, d.sourav245@gmail.com

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ABSTRACT
A novel tetranuclear heterometallic complex, [KZn(3-MeOsalpn) (µ -NCS)] (1) [3-MeOsalpn
2
-
1
,1
2
=
N, N’-propylene bis(3-methoxysalicylideneiminate)] has been synthesized by the reaction of
’
Zn(OAc) ・2H O with the multidentate Schiff-base ligand, N, N -bis(2-hydroxy-3-
2
2
OMe
methoxybenzylidene)-propane-1,2-diamine (H L ) in the presence of KSCN. 1 is
2
characterized by elemental analysis, powder X-ray diffraction and different spectroscopic
techniques along with single crystal X-ray crystallography. Crystal structure of 1 completely
divulges a very intricate heterometallic environment of Zn(II) which is a centrosymmetric penta-
coordinated distorted square pyramidal molecule possessing a basic tetranuclear structural core
[
KZn(OAr
)
2
(µ1,1-NCS)]
2
that consisting mainly two Zn atoms and two K atoms bridged by four
-
-
ꢀ
2
-OAr (O1, O1*, O2 and O2*), and two [µ1,1-NCS] groups. In the asymmetric unit of the
-
OMe 2-
binding action ꢀ -O from one fully deprotonated [L
]
enable to construct the four
2
Ar
membered heterometallic ZnK(O) core. Complex 1 structure was optimized with the aid of
2
density functional theory at B3LYP level hybrid functional and GAUSSIAN 16 programming
package in presence of 6-31+g(d, p) basis set. ESP study of Schiff base ligand further supported
that polymerization is more favorable with potential border line Zn(II) metal ion. Hirshfeld
surface and 2D fingerprint plots have been explored in 1 to find out different types of non-
covalent supramolecular interactions. The solution and solid-state luminescent property of the
title complex 1 was reported. Fluorescence life time study further indicates that excited state
stabilities of 1 are smaller than Schiff base ligand. Finally, experimental UV-Vis electronic
transitions of 1 were compared with those obtained theoretically from TD-DFT level of
calculations.
Keywords: Schiff base, Zn(II), K(I), Fluorescence, ESP, Hirshfeld surface, DFT, TD-DFT

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1
. Introduction
Non-covalent interactions popularly known as supramolecular interactions refer to a different
variety of interactions that are not actually in true sense covalent nature. Such non-covalent
interactions associated various effects like hydrogen bonding, electrostatic interactions, van der
Waals forces and cation-π interactions played essential roles in coordination chemistry especially
in supramolecular chemistry [1-6], molecular biology [7], chemical processes [8] and material
science [9]. Among them, cation-π interactions effect is one of the most important processes for
synthetic aspect of homo or heterometallic complexes that is basically involved into an electron
rich aromatic π-cloud donor centers with a proper cationic charge density and size of particular
alkali metal cations [10]. Such interaction is a better example of noncovalent bonding between a
+
monopole cation (M ) and a quadruple aromatic ring π cloud system [11a-b]. Such complexed
system particularly gets importance of molecular recognition, enzyme catalysis and stability of
protein structures. The design and synthesis of novel heterometallic complexes involving with
-
[
SCN] ions in terms of supramolecular interactions have attracted much attention for inorganic
chemists in that respect that introduction of second metal opens up new concept of topology
-
[
11c-d]. Again due to versatile bridging propensity of [SCN] anions [12a-e], different
dimensional homo or heterometallic novel complexes have been resulted with 3d/4f block metals
12f-j] which further opens up a new avenue for attraction for crystal growth and design,
[
inorganic and material chemistry [13].Such blooming fields of research work in conjunction with
salen-type o-phenoxido groups containing tetradentate ligands (N O ) are further fascinating in
2
2
that respect it can strongly coordinate not only p-and d-block metal ions but also s-block alkali-
metal ions to generate vast number of heterometallic novel complexes [14]. MOFs based Zn(II)

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+
+
and alkali metal ions (Na , K ) containing heterometallic complex belongs one of the important
class of compounds which are useful in biomimetic analysis [15], small-molecules activation
[
16], electron storage 17] and they are always well known to carry polar class of organometallics
18]. (N O ) salen-type ligand basically has a common compartmental system [19] which has
[
2
2
been targeted to prepare different kinds of heterometallic 3d/alkali metal [20] or 3d-4f metal
complexes [21]. Till date copper (II)-sodium(I) and nickel (II)-sodium(I) heterometallic as well
as discrete 3d-4f complexes have been well explored in literature with salen or salpn-type
compartmental ligand [22]. A closer look of ligand structure divulges that transition metal ions
+
are placed in the N
2
O
2
compartment along with mobile trapping of alkali metal ions (M ) in the
O compartment. Thus self-assembly cation-π interactions class of complex formation with
4
OMe
Zn(II)-K(I) in presence of Schiff base ligand (H2L ) and KSCN is truly unveiled research.
Therefore, in this juncture our main purpose to explore the function of multisite coordinating
OMe
salen-type ligand (H2L ) as prime building block to synthesize different kinds of novel cation-
π interaction class of complexes. Thus, a common target-oriented reaction is carried out in
OMe
presence of ligand (H
2
L
), zinc acetate dihydrate and KSCN in methanol solvent and
successfully isolated an air stable cation-π interaction class of complex (1).
Herein, we report the synthesis, crystal structure, solid-state photoluminescent property,
fluorescence lifetime, ESP, Hirshfeld surface and computational TD-DFT calculations of
heterometallic complex [KZn(3-MeOsalpn) (µ -NCS)] (1).
1
,1
2

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2
. Experimental Section
.1. Materials and physical measurements
2
All chemicals for used the present research work were of analytical grade, purchased from
commercial sources and used as received without further purification. Elemental analysis was
determined with the help of Perkin–Elmer CHN analyzer 2400. IR spectra were recorded as KBr
-1
pellets within the range 4000–400 cm on a Perkin–Elmer spectrum RX 1. FT-Raman spectra
-1
were determined with the help of BRUKER RFS 27 between 4000-50 cm . The electronic UV-
visible spectra (200-1100 nm) were recorded in DMSO solvent on a Hitachi model U-3501
spectrophotometer. Fluorescence spectra were measured using Perkin-Elmer LS50B
spectrofluorometer model at room temperature (298K).X-ray powder diffraction measurements
was carried out using BRUKER AXS, GERMANY X-ray diffractometer using radiation Cu K-
alpha-1. Quantum yield (Φ) have been calculated using popular equation (1) where quinine
sulfate used as the secondary standard (Φ = 0.57 in water) [23]
2
ΦS AS (Abs)R nS
=
×
×
-------------------------------------------------(1)
2
R
ΦR AR (Abs)S
n
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 parameters for the studied sample and reference respectively.
Fluorescence lifetime measurements were recorded by using common JOBIN-VYON M/S
OMe
fluorimeter. The average fluorescence life time decay profile for ligand (H2L ) and complex 1
has been determined using common literature method (Equation S1).

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2
.2. X-ray crystallography
The good quality crystal data for heterometallic complex (1) have been collected on a Bruker
SMART CCD [24] diffractometer (Mo K radiation, λ = 0.71073 Å). The program SMART was
α
used for collecting frames of data, indexing reflections, and determining lattice parameters,
SAINT [25] for integration of the intensity of reflections and scaling, SADAB [26] for absorption
correction, and SHELXTL for space group and structure determination and least-squares
2
refinements on F . The crystal structure of hetero-metallic complex 1 was fully solved and
2
refined by full-matrix least-squares methods against F by using the program SHELXL-2014 [27]
and Olex-2 software [28]. All the non-hydrogen atoms were refined with anisotropic
displacement parameters. Hydrogen positions were fixed at calculated positions and refined
isotropically. Different types of crystallographic diagrams constructed for 1 with the aid of latest
Diamond software [29]. A details summary of the crystallographic data and full structure
refinement parameters for 1 are submitted in Table 1. Crystallographic data (excluding structure
factors) of 1 have been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication numbers. CCDC number 1849917. Copies of the data can be
obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K.:
http://www.ccdc.cam.ac.uk/cgi-bin/catreq.cgi, e-mail:data_request@ccdc.cam.ac.uk, or fax: +44
1
223 336033.

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Table1 Crystal data and full Structure refinement parameters for 1
Formula
M/g
Crystal system
C H K N O S Zn (1)
40 40 2 6 8 2 2
1005.84
Triclinic
ꢀ
P1
Space group
a/Å
7.7741(13)
b/Å
c/Å
11.4612(17)
12.5333(17)
80.808(9)
β (°)
3
V/Å
1
070.80(3)
Z
1
-3
ρc/g cm
1.560
-
1
ꢀ
/mm
1.471
516
0.2× 0.1 × 0.1
0.971 to 24.80
-8 ≤ h ≤ 9
F(000)
Cryst size (mm )
3
θ range (deg)
Limiting indices
-13≤ k ≤ 13
-
14≤ l ≤ 14
Reflns collected
Ind reflns
9266
3582 [Rint = 0.0640, Rsigma = 0.0734]
Completeness to θ (%)
Refinement method
0.971
2
Full-matrix-block least-squares on F
Data/restraints/ parameters
3582/447/303
2
Goodness-of-fit on F
1.068
Final R indices
R1 = 0.0534
[
I > 2θ(I)]
wR
R1 = 0.0937
wR = 0.1311
2
= 0.1130
R indices (all data)
2
-3
Largest diff. peak and hole(e ·Å )
0.387 and -0.394
CCDC
1849917

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2
.3. Computational methodologies
The prediction of some important theoretical molecular properties of heterometallic complex (1)
are computed in the gas phase using density functional theory (DFT) calculations. The electronic
excitation energies and molecular intensities of the lowest-energy spin allowed transitions were
analyzed by time dependent density functional theory (TD-DFT). In the present research works
all the computational calculations were performed using the GAUSSIAN 16 programming
package[30].For generating the input molecular structure and analysis of all computed results
GaussView 6 software were used. The geometrical optimization of 1 was computed in the gas
phase using density functional theory (DFT) and B3LYP hybrid functional [31-33]. Generally,
B3LYP is popular method and superior to HF and MP2 method, which have Becke’s exchange
functional, the Lee–Yang–Parr correlation functional as well as a HF exchange term. Today,
DFT calculations with the B3LYP method provided the researchers a meaningful and nice
correlation with experimental results [34-37a]. The 6-31+g(d,p) basis set was used in all
calculations purpose. The electronic excitation energies and corresponding intensities of the
prominent lowest-energy spin allowed transitions were also computed using the time dependent
density functional theory (TD-DFT) concept. Moreover, the electrostatic potential (ESP)
mapping surfaces for 1 was also shown for better understanding the charge distributions through
shape, size and color code.
OMe
2
.4. Synthesis of Schiff base ligand (H2L
)
OMe
The Schiff base ligand (H2L ) was synthesized by the condensation of 1 mmol (0.152g) 3-
methoxysalicylaldehyde with 0.5 mmol (0.0371g) 1,2-propanediamine (Pn) in (50 mL) of
methanol for ca. 1 h and allowed to cool, crystallized from hot methanol (Scheme 1). Yield:

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-1
8
6%, FT-IR (KBr cm ) selected bands: 1630 (C=N), 2887(C–H sp3), 1145 and 1254 (C–O),
UV-Vis λmax (DMSO): 283, 334 nm.
OMe
Scheme1. Synthetic scheme for ligand (H
2
L
)
2
2
.5. Synthesis of [KZn(3-MeOsalpn) (µ1,1-NCS)] (1)
To the methanolic solution (25 mL) of zinc acetate dihydrate (0.2195g, 1 mmol), dark orange
OMe
yellow solution of Schiff base ligand (H2L ) was added directly followed by mixing a solution
of KSCN (0.0972 g, 1 mmol) in minimum volume of methanol solvent (10 mL) with constant
stirring for 3 h. Few drops of CH CN solution were then added, stirred for 30 minutes and the
3
resulting mixture was additionally refluxed for 2 hour at 60 ̊C . Finally yellow colored filtrate was
kept in refrigerator for crystallization by slow evaporation. After 10 days yellow colored single
crystal suitable for X-ray crystallography was obtained. Deep yellow crystals was isolated by
filtration and air dried. Yield: 0.358 g (68%), Anal. Calc. for C40
40 2 6 8 2 2
H K N O S Zn : C, 47.76;
-1
H,4.01; N,8.35. Found: C, 47.66; H, 3.98; N, 8.31 %. FT-IR (KBr cm ) selected bands: ν1632
-1
(
vs), 2900-2832 (w), 2071(vs), 1215(vs), 1241-1215(vs). FT-Raman (cm ) selected bands: ν
2
066 (vs), 260(w), 204 (w), 353, 306(w), UV-Vis λmax (DMSO): 482, 488 nm.

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3
. Results and discussion
3
.1. Synthetic methodology
The tetradentate Schiff-base ligand was synthesized using the literature method [37b]. The
heterometallic complex 1 was synthesized by stirring Zn(OAc) .2H O with Schiff base ligand in
2
2
methanol followed by slow addition of methanolic solution of KSCN in 1:1:1 molar ratio
Scheme 2). Complex 1 was successfully isolated in crystalline form from slow evaporation of
CH OH-CH CN mixture. The versatile coordination behavior of Schiff base ligand with different
metal ions shown in Scheme S1(a). Air stable heterometallic complex 1 obtained in
(
3
3
+
2
M
moderate yield (68%), successfully characterized by the elemental microanalysis, powder X-ray
diffraction, single X-ray crystallography, FT-IR, FT-Raman, UV-Vis and fluorescence
spectroscopy. To the best of our knowledge, this is the first example of heterometallic complex
containing intricate environment of Zn(II)-K(I) in presence of Schiff base ligand and ancillary
-
[
SCN] ion as co-ligand reflecting (µ -NCS) bridging propensity. Positively, the synthetic
1
,1
approach of 1 opens up a new window to all inorganic chemists in connection of novel cation-π
interactions class of complex formation.
Scheme2. Synthetic scheme for complex 1

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3
.2. Spectral Characterizations
OMe
1
13
Schiff base ligand (H2L ) formation is well characterized by FT-IR, H NMR and C NMR
-1
spectroscopic studies (Fig.S4). The C=N stretching value 1630 cm indicate the well-known
-1
Schiff base formation which is shifted to 1632 cm in 1 supported strong coordination of the
1
imino nitrogen with Zn(II) metal center [38]. In 1, strong FT-IR peak observed near at 2071 cm
-
is positively assigned N-bonded bridging propensity of ambidentate [SCN] ion (Fig.S1). To
-
corroborate the bridging propensity of [SCN] ion during complex 1 formation, FT-IR spectrum
of 1 and other well reported Zn(II)-NCS complexes were systematically compared (Table S2).
-
Further in 1, to compare µ-1,3 and µ-1,1 bridging propensity of [SCN] ion, FT-Raman spectrum
data was utilized in conjunction with FT-IR spectrum study. Generally µ-1,3 and µ-1,1 bridging
-1
-1
purpose ν (CN) is shifted to higher or lower wave numbers that is ≥2100 cm or ≤ 2000 cm .
as
-1
Since FT-Raman band for 1 is observed near at 2066 cm , hence µ-1,1 bridging mode is more
common (Fig.S3). FT-Raman spectra of 1 and other reported Zn(II)-NCS complexes were
-
further established µ-1,1 bridging mode of [SCN] ion(Table S3-S4).FT-Raman bands near at
-1
-1
(
260, 204 cm ) and (353, 306 cm ) confirmed the bonding nature of Zn-N and Zn-O in complex
1
(Fig.S1).This is comparable to other reported Zn(II) complexes (Table S4). Ar-O stretching
1
frequencies in 1, near at 1241-1215 cm is similar to the other reported salen-type ligands [39].
3
.3. UV-Vis spectra
*
The free ligand shows high intense bands at 283 and 334 nm which are assigned due to π→π
*
and n→π . Complex 1 also exhibits ligand–based transition at 482, 488 nm presumably due to
*
*
10
π→π or n→π type of transitions [40, 41]. Since Zn(II) metal have filled d configuration hence
no metal centric d-d transition was commonly observed in complex 1 spectrum (Fig.S2).

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3
.4 X-ray powder diffraction
The crystalline phase purity of complex 1 was confirmed by X-ray powder diffraction pattern
analysis. The X-ray powder diffraction pattern for complex 1 was recorded experimentally after
0
0
considering effectively 2θ scan from 4 to 50 . As clearly shown in the experimental PXRD
patterns (Fig.S4A), the major peak positions of the bulk solids of 1 agree very well with the
patterns simulated from single-crystal diffraction data (cif file) obtained from Mercury software.
Hence the study of X-ray powder diffraction is further established the phase purity of the bulk
crystal samples of the heterometallic complex 1.
4
. X-ray crystal structure
4
.1. Crystal growth
X-ray diffraction quality yellow colored single crystals have grown for complex 1 in mixed
solvent medium (CH OH-CH CN) by slow evaporation at room temperature. Full
3
3
Crystallographic data and detailed structural refinement parameter of 1 was presented in Table 1.
.2. Crystal structure of [KZn(3-MeOsalpn) (µ -NCS)] (1)
4
1
,1
2
Single-crystal X-ray diffraction data divulges that molecular structure of 1 represent a neutral
heterometallic complex 1 and crystallized in the triclinic space group P-1 (Z = 1). For complex 1,
the asymmetric unit has one-half of the whole molecule, [KZn(3-MeOsalpn) (µ1,1-NCS)]
2
(1)
(
Fig.1) and the entire molecule is produced by an inversion element which situated at the center
of the molecule. A perspective aspect of 1 is depicted in Fig.2. Selected some important bond
lengths and bond angle values of 1 are summarized in Table S1A.

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.
Fig.1. Asymmetric unit of heterometallic complex 1
Fig.2. Molecular structure of heterometallic complex 1
The centrosymmetric molecule possesses a [KZn(O ) (NCS)] core (Fig.2) comprising two Zn
Ar 2
2
-
-
2
atoms and two K atoms bridged by four ꢀ -OAr (O1, O1*, O2 and O2*), and two µ1,1-NCS
groups. The formation of heterometallic assembly of asymmetric unit of complex 1 involves
OMe 2-
mainly the action of only one fully deprotonated ligand [L ] each of which are hexadentate
1
and feats all the available coordinating centers. In asymmetric unit of 1, ligand displays ꢀ –η :
2
2
1
1
2
1
η : η : η : η : η coordination mode [Scheme S1(b)] to hold one Zn atom and one K atom in its
-
inner and outer coordination sphere respectively. In this binding action ꢀ -O (O1 and O2)
2
Ar

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holds together Zn1 and K1 atom forming a ZnK(O)
2
four membered rings. Finally, two such
-
dimeric subunits where Zn atom is stitched to K atom by bridging µ -NCS groups to construct
1
,1
mainly a heterometallic tetranuclear assembly. In 1, Zn-NCS bond distances fits within the range
of 2.063-2.074 (Å) which is comparable [1.955-2.226 (Å)] to other reported Zn(II)-NCS
complexes (Table S5(A)).The tetranuclear assembly thus formed is supported by the
OMe 2-
-
coordination action of [L ] and SCN ligands, featuring a bent butterfly shaped topology
[
42,43] (Fig.3). Furthermore, the tetranuclear assembly has an eight-member macrocycle
Zn ) resemblance with S crown-shaped structure [44, 45] (Fig. 4).
(
2
K
2
O
2
N
2
8
Fig.3. Butterlfly shape of heterometallic complex 1

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Fig.4. S
Geometry distortion knowledge of penta-coordinated Zn(II)-K(I) complex 1 can be well
obtained from structural index or Addison parameter tau ( ) [46] which represents the relative
8
crown-shaped structure of complex 1
ґ
amount of trigonally in the crystal structures (for square pyramid, ґ = 0; trigonal bipyramid, ґ =1;
ґ = (α-β)/60º, where α and β are the two largest angles around the central metal). Overall, Zn
atom is five coordinated with N O environment in a distroted squre pyramidal geometry (
τ
3
=
0.02) that is comparable to other literature reported complexes [46]. InTable S5(B), a details
comparison of possible 5-coordinated geometry distrotion (ґ) values of some important reported
Zn-NCS or M(II)-NCS complexes [where M(II) other than Zn(II) metal ions] have been
submittd.On the other hand, K atom is also penta-coordinated and possesses O N environment in
4
a distorted geometry Scheme S1(b). Another bulging feature of this paricular complex is the
+
presence of K ···Cπ(arene) intramolecular interactions with a capping aromatic ring of another
+
tetramaric complex from the hitched o-vaniline part of ligand.The K ···Cπ(arene) nature of
complex 1 further reveals its organometallic nature since the aforesaid bond distances [2.995-
+
3
.489(Å)] are nearly comparable to other reported (K ) nature of organmetllics (Table S6).
+
Further, the solid state structure of 1 revealed the presence of mentioned K ···Cπ(arene)
intramolecular interactions leading to the formation of extended 1D chains (Fig.5).The distance
+
of K ···Cπ(arene) interaction is 3.123 (13) Å, are not rare for potassium and the above bond
+
distances are nearly identical with other K ···Cπ(arene) nature of organometallics (Table S6).
To enhance the flourishing chemistry of thiocyanate linked Zn(II) Schiff base complexes, we
have also explore the coordination chemistry involving identical salen-type Schiff base ligand
OMe1
+
(H
2
L
) in conjunction with trapping of alkali metal ions (M ) (Table S7-S8).

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+
Fig.5. K ···Cπ(arene) intra molecular interactions leading to the formation of extended 1D chain
4
.3. Hirshfeld surface
The Hirshfeld surface for 1 mapped with the dnorm property has been shown in the Fig.6 and the
2
D fingerprint plot analysis [47-51] with all characteristics features of their short interactions are
represented in Fig.7.
Fig.6.Hirshfeld surface mapped with the dnorm property; red spots represent the closest contacts
and blue color the most distant contacts

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(a)
(b)
(c)
Fig.7.Graphical view of Hirshfeld surface with 2D fingerprint plot analysis with characteristic
features of C H {i.e. C-H π} (a), K C {i.e. K π} (b), S H {i.e. C-S H} (c) contacts; di
and d are the distances from the surface to the nearest atoms interior and exterior to the surface
⋯
⋯
⋯
⋯
⋯
⋯
e
respectively
Different important relative contributions [1-7] to the HS area due to close intermolecular
contacts of 1 are clearly shown in Table 2. The relative breakdown contributions to the Hirshfeld
surface area of 1 was presented in Fig.S5B.
Table 2 Hirshfeld surface different interactions contribution in (%) of 1
SL. Number
Types of interactions
C (i.e. π π)
H (i.e. C-H π)
H (i.e. C-S H)
C (i.e. K π)
C-H
Contribution in %
0.1%
1.
2.
3.
4.
5.
C
⋯
⋯
C
⋯
⋯
22.9%
S
⋯
⋯
12.1%
K⋯
⋯
3.9%
⋯
O
4.2%

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6
.
.
C-H
⋯
N
4.6%
7
H⋯
H
42.5 %
A close inspection of Table 2 further divulges that 2D fingerprint plots for heterometallic
complex 1 exhibits several types of intermolecular interactions e.g. H H, C H (i.e. C-H π)
and S H (i.e. C-S H) but H H interactions (42.5 %) significantly are well dominated and
⋯
⋯
⋯
⋯
⋯
⋯
highest one. Apart from, solid state X-ray structure of 1 also revealed the presence of
+
K ···Cπ(arene) types intramolecular interactions that leads to the formation of extended 1D
+
chains (Fig.5). Similar types of K ···Cπ(arene) intramolecular interaction also discloses by
Hirshfeld surface analysis where percentage of contribution is about only 3.9%. Thus in
+
connection of K ···Cπ(arene) intramolecular interaction, a nice correlation was envisaged in the
solid state X-ray crystal structure of 1.
4
.4. ESP analysis
The electron density iso-surface on which the ESP surface was computed and mapped is depicted
OMe
in Fig.8 for ligand (H
2
L
) (Fig.8a) and complex 1 (Fig. 8b). These shown surfaces clearly
indicate the size, shape, charge density and site or direction of chemical reactivity of the
optimized molecules. The different values of the ESP surface are shown by different colors
coding. The red, blue and green regions provide the most negative electrostatic potential, the
most positive electrostatic potential and zero potential, respectively. The potential increases in
the color code as shown in the top panel of mapped optimized structure. Recently, ESP mapping
performed a tremendous vital role for understanding the reduction property of Schiff base ligand
reactivity and its polarity [52, 53].

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OMe
In case of Schiff base ligand (H2L ), nitrogen and oxygen are two electronegative atoms,
therefore, these regions indicate the most negative electrostatic potential (near to red colour,
Fig.8a). In case of 1, the shape and size of ESP surfaces are significantly changed which is
indication of charge re-distribution phenomenon (Fig.8b). In complex 1, the electron density is
mainly concentrated on the N-C-S functional group. This result clearly indicates that the
polymerization through repeated units at this electronegative position N-C-S may take place
which is closed to our synthetic approach and experimental observation. Complex 1 may play a
very important role to functionalize any drugs or other new materials through the N-C-S group
and their potential applications may be check in future study.
Fig.8.Electron density calculation from the total SCF density having isovalue MO= 0.020000,
density=0.000400. This is mapped with electrostatic potential from total SCF density

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OMe
(a) Schiff base ligand (H
2
L
) (b) Complex 1. The red color is corresponding to the
most electronegativity and the blue color shows the most positive election density
.5. TD-DFT analysis
4
DFT optimized structure (1) predominantly stabilized through many types of important
interactions, such as O─K, O─Zn (left), O─Zn (right) and S-C≡N─Zn metallic type of bonds
whereas the weak secondary interactions of K─Zn (3.41Å) play the crucial role to stabilize the
complex structure(Fig.S5A).DFT calculated bond distance values are close resemblance with
experimental data’s one that found in the crystal structure of 1 (Table S1B). Time dependent-
density functional theory of 1 was performed using DFT calculations in the gas phase. The
computed most probable electronic transitions and corresponding vertical excitation along with
the oscillator strength are presented in Table 3. The absorption spectra in a TD-DFT computed
result is due to many possible single excitations. The TD-DFT calculations of 1 are computed
where the most prominent absorption bands at 1915 nm, 578 nm and 482 nm which is given rise
due to Homo →Lumo, Homo-1→Lumo and Homo→Lumo+1, transitions respectively. The
calculated absorption peak at 1915 nm falls in IR spectral range which would not be our focus of
study here since the experimental UV-Visible absorption band occurs at 482 and 488 nm. The
shifting of charge density from HOMOs to LUMOs in the forms of surface maps is shown in
Fig.S7. In 1, both the HOMOs (HOMO and HOMO-1) are responsible for the transitions and the
electron density is mainly concentrated on right hand side molecular moieties, while in LUMO
orbitals (LUMO and LUMO + 1) the electron density is shifted towards the left-hand side
moieties of molecules (Fig.S7). Overall, there is a nice correlation observed between
experimental UV-Visible absorption peak and TD-DFT level of calculations absorption peak.

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Table 3 TD-DFT computed important electronic transitions for 1
Excitation
energy (eV)
λ
Exp
Wavelength
(nm)
Oscillator
strength(f)
Key transitions
for complex 1
(nm)
0.64
2.40
2.57
1914.8
577.55
482.34
0.70675
0.69142
0.68416
Homo →Lumo
Homo-1→Lumo
488, 482
Homo→Lumo+1
4
.6. Photoluminescent property
OMe
Photoluminescent property of Schiff base ligand (H L ) and complex 1 were recorded in
2
DMSO solvent and solid-state individually at room temperature (Table 4). The fluorescence
spectra in DMSO are compared in Fig.9A, revealing strongly that Schiff base ligand is
practically non-emissive but the heterometallic complex 1 exhibit strong fluorescent behavior.
OMe
Upon photo excitation at 333 nm, Schiff base ligand (H
2
L
) exhibits a fluorescent emission
centered at 441 nm in DMSO and less intense emission peak at 366 nm in the solid-state
respectively. Similarly, complex 1 upon photo excitation at wavelength 340 nm shows
photoluminescence with the main emission peak at 480 nm in DMSO and less intense peak at
4
10 nm in the solid-state respectively [Fig.9B]. The small emission intensity of ligand may be
due to the occurrence of PET processes in the presence donor atoms lone pair within it [54].
Thus, complex 1 shows good luminescent behavior in solution as compared to solid-state. The
*
luminescence behavior in all cases may be attributed to an intraligand π→π emission [55, 56]
and overall emission spectra are bathochromically shift. Similar type of emission spectra of 1
and Schiff base ligand are observed with other ligands [57] and reported some important Zn(II)-
NCS complexes (Table S9). Quantum yield (ɸ) of Schiff base ligand and 1 have been determined

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in DMSO solvent (Table 4). Further in 1, no emission emanating from metal-centered
10
MLCT/LMCT excited states are possible since it is hard to oxidize or reduce especially d
electronic configuration of Zn(II) metal ion. The photoluminescent emission nature of
heterometallic Zn(II)-K(I) metal complex 1 may therefore be assigned mainly due to the L→M
charge transfer. Therefore, the enhancement of the emission intensities of 1 may be due to the
metal-ligand chelation effect [CHEF] [58-61] or most probably the increase in conformational
OMe
rigidity of the salen-type ligand (H
2
L
) upon effective chelation via N,O-donor center with
zinc metal ion [54,55]. Since 1 mainly exhibit ligand-centered fluorescence emission spectra,
-
therefore pseudo-halide [SCN] anion shows no significant effect on fluorescence emission [62a].
As a comparative manner to concentrate the importance of excited state stabilities of ligand
OMe
(
H2L ) and 1, we are further interested to calculate the fluorescence lifetime in DMSO solvent.
The decay profiles of free ligand and 1 have been generated in Fig.S6. The nature of decay
profile of 1 as well as free Schiff base ligand can be best fitted to bi-exponential manner (with
2
acceptable comparable χ values) [62b] (Fig.S6). The calculated average fluorescence lifetime of
1
(0.68-5.86 ns) is smaller than free Schiff base ligand (2.78-9.44 ns). As a result, the importance
of excited state stabilities of ligand is maximum in comparison with complex 1.
.7. Fluorescence mechanism by DFT/TDDFT
This behavior can be understood by the performed DFT calculation on neat Schiff base ligand
4
OMe
OMe
(
H2L ) and L-M complex. In the ESP mapping surfaces of neat ligand (H2L ), N and O
atoms shows the red color (higher electronegativity as shown in Fig. 8a), however, in the L-M
complex, the significant charge density was transferred from ligand to metal (Fig. 8b). Moreover,
the atomic charge density was also analyzed for the case of neat ligand and L-M complex
respectively (Fig. S8A). These atomic charge density gives a clear evidence of charge transfer

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from L→M and therefore, the photoluminescent emission of 1 may be assigned mainly due to
the L→M charge transfer.
OMe
Table 4 Photoluminescence and time resolved photoluminescence decay of (H2L ) and 1
2
Compounds λex (nm)
λem
λem
(ɸ)
τ1[ns]
τ2[ns]
τ3[ns] <τ>[
χ
(nm)
(nm)
Solid-
n]
(
state)
410
1
340
333
480
441
0.0345
0.0353
0.68
0.01
5.86
0.39
2.35
(
0.4%)
(99.5% (0.1%)
)
1.24
Ligand
366
2.78
0.73
9.44
(
17.4%) (81.5% (1.1%)
OMe
(H
2
L
)
)
1
.21
11 00 00 00 00 00
88 00 00 00 00
Complex
Ligand
66 00 00 00 00
44 00 00 00 00
22 00 00 00 00
0
0
44 00 00
44 55 00
55 00 00
55 55 00
66 00 00
66 55 00
Wavelength (nm)
Fig.9A. Ligand-centered emission spectra of ligand (H2L ) and complex 1
OMe

ACCEPTED MANUSCRIPT
OMe
Fig.9B. Solid-state emission spectra of ligand (H2L ) and complex 1
5
. Conclusion
The present communication deals primarily with the synthesis of Schiff base ligand,
heterometallic complex, spectral characterizations, PXRD, X-ray crystal structure along with
some supporting well known theoretical approach devoted to complex 1. X-ray crystal study
discloses that in 1, the environment of Zn(II) metal is centrosymmetric phenoxido-type penta-
coordinated distorted square pyramidal that further assisted by the action of one fully
OMe 2-
deprotonated salen-type ligand [L ] . Photoluminescent property of ligand and 1 have been
discussed both in DMSO and solid-state as a concise comparative approach. Fluorescence
lifetime study further explored the importance of excited state stabilities of ligand is maximum
than that of 1.DFT study have been carried out in the title complex 1 with respect to complex
structure optimization, ESP and photoluminescent behavior via L→M charge transfer. Non-
covalent supramolecular interactions in 1 was analyzed by means of Hirshfeld surface and 2D

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-
fingerprint plot. Polymerization of salen-type ligand with zinc metal ion and ancillary [SCN]
anion is nicely corroborated with the help of ESP study. Finally, simulated TD-DFT level of
electronic transitions compared with experimental UV-Vis electronic transitions.
Acknowledgements
The present research work did not receive any specific grant from funding agencies in the public,
commercial or not-profit sectors. Dr. Dipankar Mishra gratefully acknowledges the financial
grant sanctioned by UGC, New Delhi, in his favour vide Minor Research Project (F.PSW-
2
32/15-16(ERO). Dr. Dhiraj Das also thanks to Dr. Angshuman Roy Choudhury and IISER
Mohali for taking up the Gaussian 09 software facility.
Appendix A. Supplementary material
All supplementary data’s related to this article can be associated in ESM file. CCDC number
1
849917 contains the supplementary crystallographic data (excluding structure factors) in CIF
format for the structure reported of heterometallic Zn(II)-K(I) complex. Copies of the data can be
obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K.:
http://www.ccdc.cam.ac.uk/cgi-bin/catreq.cgi, e-mail: data_request@ccdc.cam.ac.uk, or fax: +44
1
223 336033
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