Syntheses, characterizations, crystal structures, DFT/TD-DFT, luminescence behaviors and cytotoxic effect of bicompartmental Zn (II)-dicyanamide Schiff base coordination polymers: An approach to apoptosis, autophagy and necrosis type classical cell death

Received: 2 July 2019

DOI: 10.1002/aoc.5269

Revised: 4 September 2019

Accepted: 10 September 2019

F U L L P A P E R

Syntheses, characterizations, crystal structures,

DFT/TD‐DFT, luminescence behaviors and cytotoxic effect

of bicompartmental Zn (II)‐dicyanamide Schiff base

coordination polymers: An approach to apoptosis,

autophagy and necrosis type classical cell death

Dhrubajyoti Majumdar^1,2

Manas K. Santra³| Sourav Das⁷

Dipankar Mishra¹

| Yashika Agrawal³| Renjith Thomas⁴| Zakir Ullah^5,6

|

| Tapan K. Pal⁸| Kalipada Bankura¹|

¹Department of Chemistry, Tamralipta

Two new Zn (II)‐dicyanamide (dca) 1‐D chain coordination polymers (CPs),

[Zn (L^OMe)(μ₁‐dca)(μ_1,5‐dca)]_n(1) and [Zn (L^OEt)(μ₁‐dca)(μ_1,5‐dca)]_n(2) have

been successfully synthesized from bicompartmental Schiff base ligands N,N^′‐

Bis(3‐methoxysalicylidenimino)‐1,3‐diaminopropane (H₂L^OMe), N,N^′‐Bis(3‐

ethoxysalicylidenimino)‐1,3‐diaminoproane (H₂L^OEt) respectively and structur-

ally characterized using various spectroscopic protocols like ¹H NMR, IR,

Raman, UV–Vis, fluorescence as well as elemental analysis, TGA, PXRD and

SCXRD studies. X‐ray single crystal study revealed that both the complexes

have two different geometrical arrangement of Zn metal centres with distorted

square pyramidal Zn(2) and trigonal prismatic geometry Zn(1). Ab‐initio DFT

(Density functional theory) has been executed at B3LYP (Becke, 3‐parameter,

Lee‐Yang‐Parr) using DGDVP (Diffuse gradient double valence polarised) basis

set to explain FMO (Frontier molecular orbital), TD‐DFT (Time‐dependent

density functional theory) and photovoltaic efficiency in Dye Sensitized Solar

Cell (DSSC). Hirshfeld surface (HS) and 2D fingerprint plot analyses are shed

more light on the non‐covalent supramolecular interactions. The steady state

and time‐resolved fluorescence measurements have been conducted in DMSO

and solid‐state. CPs exhibited bi‐exponential decay in DMSO as well as solid‐

state where fluorescence behaviors are mainly intra‐ligand (π → π^*) in nature

with lifetimes in the range (1.11–1.06 ns). In particular, in vitro cytotoxic activ-

ities were evaluated towards MCF7 (breast cancer) cell line, MDA‐MB‐231

(breast carcinoma) cell line and MCF10A (breast epithelial) cell line using

MTT assay. CP1 had lower cytotoxic effect against MCF7 (20 μM), MDA‐MB‐

231 (15 μM) cell lines in comparison with cisplatin (42.2 8, 128.2 7 μM).

CP1 induced classical cell death apoptosis, autophagy and necrosis. Lower

Mahavidyalaya, Tamluk 721636, West

Bengal, India

²Department of Applied Chemistry,

Indian Institute of Technology (Indian

School of Mines), Dhanbad, Jharkhand

826004, India

³National Center for Cell Science, Pune

411007, Maharashtra, India

⁴Department of Chemistry, St Berchmans

College (Autonomous), Changanassery,

Kerala 686101, India

⁵Department of Chemistry, Korea

Advanced Institute of Science and

Technology (KAIST), Daejeon 34141,

Republic of Korea

⁶Center for Catalytic Hydrocarbon

Functionalization, Institute for Basic

Science (IBS), Daejeon 34141, Republic of

Korea

⁷Department of Chemistry, Institute of

Infrastructure Technology Research and

Management, Ahmedabad 380026,

Gujarat, India

⁸School of Liberal Studies, Pandit

Deendayal Petroleum University,

Gandhinagar 382421, India

Correspondence

Dipankar Mishra, Department of

Chemistry, Tamralipta Mahavidyalaya,

Tamluk‐721636, West Bengal, India.

Email: dmishra.ic@gmail.com

Appl Organometal Chem. 2019;e5269.

wileyonlinelibrary.com/journal/aoc

1 of 22

https://doi.org/10.1002/aoc.5269

2 of 22

MAJUMDAR ET AL.

Sourav Das, Department of Chemistry,

Institute of Infrastucture Technology

Research and Management, Near

Khokhara Circle, Maninagar East,

Ahmedabad‐380026, Gujarat, India.

Email: d.sourav245@gmail.com

IC₅₀value of CP1 against MDA‐MB‐231 cell line provide new insights in the

development of cancer therapeutics.

Highlights

• Two new Zn(II)‐dicyanamide CPs were synthesized and characterized

• X‐ray Crystal structures of Zn(II)‐CPs were determined

• DFT, TD‐DFT and luminescence behaviors have been explored

• Zn(II)‐CPs show noteworthy cytotoxic activity against breast cancer cell lines

• Apoptosis, autophagy and necrosis type classical cell death phenomenon was

explained

Funding information

UGC, New Delhi, Grant/Award Number:

F.PSW‐232/15‐16(ERO

KEYWORDS

cytotoxic, DFT, dicyanamide, photoluminescence, Zn (II)

1 | INTRODUCTION

disease in both developing and developed countries glob-

ally.^[31–35]High levels of Zn²⁺ions inhibit growth of mam-

Over the last few years, design and synthesis of coordina-

tion polymers (CPs) of group 12 metal ions have been

attracted tremendous attention not only of their novel

molecular architectures and topological networks^[1–3]but

also their blooming field of applications in lumines-

cence,^[4–7]chemical sensors,^[8–10]antibacterial and anti‐

biofilm,^[11–14]anti‐proliferative,^[15]organic light emitting

diode (OLED).^[16–19]Moreover, recently zinc complexes

can be used as the photosensitizer in DSSC.^[16–19]Ortho

vanillin derived Schiff base platforms have always excel-

lent coordinating ability with group 12 metal ions to func-

tion as sequential multidentate bridging blocks and thus

enable to synthesize novel CPs embedded with supramo-

lecular chemistry which flourishing their new potential

applications.^[20–22]Under such consideration, we have

ventured to explore Ortho vanillin derived two popular

Schiff base ligands (H₂L^OMe/H₂L^OEt), Zn (II)‐dicyanamide

CPs to searched for its novel applications. In human body,

Zn (II) is the 2nd most important abundant metal ions after

iron due to its steady involvement of several biochemical

applications.^[23,24]Zinc metal ions can induce Alzheimer's

disease, Parkinson's disease^[25,26]on account of its concen-

tration abnormality in human body. From biological

standpoint it constructs many active sites of hydrolytic

enzymes and function as Lewis catalyst.^[27,28]Pseudo‐

halides linked Zn (II) or M (II)‐Schiff base complexes

[where M (II) other transition metal ions] have always

excellent bioactivity owing to the presence of azomethine

linkage and active metal centres.^[29,30]Anticancer proper-

ties of Schiff bases enhanced many times after chelated

with Zn (II)/M (II) metal ions which can be well explained

on the basis of popular “Overtone's concept” and “chela-

tion theory”.^[31–35]In 21st century, Cancer is the most fatal

malian cells severely^[31–35]and increasing exposure of such

metal ions induce a typical, caspase‐independent form of

apoptosis.^[31–35]A recent study report has clearly shown

that this property can be used to kill cancer cells selec-

tively.^[31–35]Therefore, to overcome these fatal diseases

researcher in coordination chemistry domain always try

to synthetically design novel anticancer drugs with d¹⁰

metal ions in presence of Schiff base ligands and flexible

pseudo‐halide anions.^[31–37]

Due to our long‐standing relationship with ortho vanil-

lin derived bicompartmental Schiff base ligands, our

research group has already reported Zn (II)/Cd (II) com-

plexes photoluminescence, DFT/TD‐DFT, in vitro antibac-

terial and anti‐biofilm properties vividly. The outcome of

previous research works is that pseudo‐halides linked

metal complexes exhibit good photophysical, antimicro-

bial and anti‐biofilm properties^{[11–14,38–42]}but no exiguous

number of studies of photovoltaic efficiency in Dye Sensi-

tized Solar Cell (DSSC) using TD‐DFT, cytotoxic effect,

apoptosis, autophagy and necrosis type classical cell death.

Zn (II) metal ions always have wide spectrum of flexible

geometry due to its d¹⁰configuration and zero crystal field

stabilization energy value (CFSE).^[43–47]Therefore, the key

factors for novel syntheses of Zn (II)‐CPs are always careful

selection of organic Schiff base ligands in conjunction with

pseudo‐halide anions.^[48]In this juncture, flexible

dicyanamide, a larger pseudo‐halide ion‐rod is under

active consideration since it exhibits versatile coordination

motifs to reinforce the CPs formation.^[49–55]Most of the

dicyanamide complexes, μ_1,5‐dca bridging mode is popular

and common whereas terminal coordination motif is rare

due to its long chain length and negative charge delocaliza-

tion over the symmetrical molecule.^[56,57]Herein a dual

MAJUMDAR ET AL.

3 of 22

combination of terminal along with μ_1,5‐dca enabled 1‐D

chain CPs have been presented.

Biotechnology. AKT (9272), pAKT‐473 (4058), pAKT‐308

(9275), PUMA (4976), p‐mTOR (2974), pS6K (9205) and

LC3B (27758) were obtained from Cell Signaling. Antibody

against Alpha‐Tubulin (T5168) was obtained from Sigma.

Elemental analyses were performed on a Perkin‐Elmer

2400 CHN elemental analyzer. FT‐IR and Raman spectra

were recorded as KBr pellets (4000–400 cm⁻¹) by using

Perkin–Elmer spectrum RX 1 and BRUKER RFS 27

(4000–50 cm⁻¹). ¹H NMR spectra of ligands and CPs were

recorded on a Bruker 300 MHz FT‐NMR spectrometer

using trimethylsilane as internal standard in DMSO sol-

vent. Energy dispersive X ray (EDX) experiments was per-

formed on EDAX OXFORD XMX N (model) using

Tungsten filament. UV–Visible spectra (200–1100 nm)

were determined using Hitachi model U‐3501 spectropho-

tometer. Fluorescence spectra for Schiff base ligands and

CPs in DMSO solvent (spectroscopic grade) were measured

by using Perkin‐Elmer LS50B Spectrofluorometer model at

room temperature (298 K). Fluorescence lifetime measure-

ments were recorded by using JOBIN‐VYON M/S Fluorim-

eter. CPs fluorescence lifetime were determined using

Equation S1. Thermogravimetric analyses (TGA) were car-

ried out on a TGA‐5OH analyzer from ambient tempera-

ture to 1000 °C at a temperature rate of 30 °C/min in a

flowing 10 ml/min under environment of nitrogen atmo-

sphere using a platinum cell. Powder X‐ray diffraction

measurements were carried out by BRUKER AXS, GER-

MANY X‐ray diffractometer model using Cu Kα‐1

In this article, we report syntheses, characterizations,

single crystal structures, DFT/TD‐DFT, photophysical

and antiproliferative activities of 1–2. The thermal stabil-

ities of metal–organic frameworks have been investi-

gated by TGA. In addition, apoptosis, autophagy and

necrosis type classical cell death phenomena were inves-

tigated in detail.

2 | EXPERIMENTAL

2.1 | Materials and instrumentation

All research chemicals and solvents were of reagent grade

and used as received without further purification. Zn

(OAc)₂.2H₂O, Sodium dicyanamide, o‐vanillin and 1,3‐

diaminopropane were purchased from Sigma Aldrich

Company, St. Louis, Missouri, USA. The chemicals MTT

and Propidium Iodide were also purchased from Sigma

Aldrich Company, St. Louis, Missouri, USA. The antibod-

ies against pMEK (sc7995), MEK (sc43612), pERK

(sc16982), ERK (K23‐sc94), BAX (sc23959), BCL2

(sc7382), Caspase 8 (sc70501), Caspase 9 (sc56073), PARP

(sc56198), mTOR (sc1549), Beclin (sc48341), p62

(sc28359), Goat anti‐rabbit IgG (sc‐2004) and Goat anti‐

mouse IgG (sc‐2005) were obtained from Santa Cruz

SCHEME 1 Synthetic schemes for Schiff base platforms and CP1‐CP2

4 of 22

MAJUMDAR ET AL.

radiation. Quantum yield (Φ) have been determined using

Equation (1) where quinine sulphate is used as secondary

standard (Φ = 0.57 in water).^[58]

isolated by filtration and air dried. Yield: 0.458 g, Anal.

Calc. for C₂₃H₂₀N₈O₄Zn₂: C, 45.79; H, 3.34; N, 18.58.

Found: C, 45.75; H, 3.29; N, 18.49%. IR (KBr cm⁻¹

)

selected bands: ν(C═N), 1620 vs, ν(C ≡ N), 2290 s,

2180 s, 2090 m, FT‐Raman (cm⁻¹) selected bands:

ν(C═N), 1628 vs, ν(C ≡ N), 2274 vs, 2235 vs, 2169 s,

UV–Vis λ_max(DMSO): 276 nm, 358 nm.

Φ_SA_SðAbsÞ_Rn²_S

¼

×

(1)

Φ_RA_RðAbsÞ_Sn²_R

According to Equation (1), A terms denote the fluores-

cence 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.

2.3.2 | [Zn (L^OEt)(μ₁‐dca)(μ_1,5‐dca)]_n(2)

CP2 was prepared by adopting similar procedure as 1

except Schiff base solution used is (H₂L^OEt). Yield:

0.460 g, Anal. Calc. for C₂₅H₂₄N₈O₄Zn₂: C, 45.56; H,

3.83; N, 17.75 Found: C, 45.51; H, 3.78; N, 17.70%. IR

(KBr cm⁻¹) selected bands: ν(C═N), 1620 vs, ν(C ≡ N),

2300 s, 2210 s, 2170 s, FT‐Raman (cm⁻¹) selected bands:

ν(C═N), 1631 vs, ν(C ≡ N), 2264 vs, 2236 vs, 2182 m,

UV–Vis λ_max(DMSO): 275 nm, 356 nm.

2.2 | Syntheses of Schiff base ligands

Two Schiff base ligands were synthesized in similar ways

as previously literature reported method (Scheme 1).^[59]

Briefly, ligands (H₂L^OMe/H₂L^OEt) are 1:2 condensation

product of 1,3‐diaminopropane (0.0371 g, 0.5 mmol) with

3‐methoxy‐2‐hydroxybenzaldehyde (0.152 g, 1 mmol) and

3‐ethoxy‐2‐hydroxybenzaldehyde (0.166 g, 1 mmol) in

(50 ml) methanol at 70 °C for 1 hr. The yellow or orange

colored ligand separated out upon cooling the solution

which was collected and dried. N, N^′‐Bis(3‐

methoxysalicylidenimino)‐1,3‐diaminopropane (H₂L^OMe):

Yield: (90%), Anal. Calc. for C₁₉H₂₂N₂O₄: C, 66.65; H,

6.48; N, 8.18 Found: C, 66.71; H, 6.42; N, 8.08%. IR (KBr

cm⁻¹) selected bands: ν(C═N), 1640, ν(C‐O_phenolic) 1254,

ν(O‐H) 3449, UV–Vis (λ_max/nm): 330 nm.

2.4 | X‐ray crystallography

The good quality single crystals for CPs were selected and

mounted on a Bruker SMART CCD^[60]diffractometer

using Mo K_αradiation at λ = 0.71073 Å. Crystal data col-

lection purpose different common programs were oper-

ated e.g. SMART program used for collecting frames of

data, indexing reflections, and determining lattice param-

eters, SAINT^[61]for integration of the intensity of reflec-

tions and scaling, SADAB^[62]for absorption correction

and popular SHELXTL for space group and structure

determination and least‐squares refinements on F ². The

crystal structure was fully solved and refined by full‐

matrix least‐squares methods against F²using the com-

mon program SHELXL^[63]and Olex‐2 software.^[64]All

the non‐hydrogen atoms were refined with anisotropic

displacement parameters. All H atoms were generated

geometrically, and positions were fixed at calculated posi-

tions and refined isotopically. Crystallographic diagrams

were constructed using latest Diamond software.^[65]The

selected crystallographic data, experimental conditions

and relevant structural refinements for CPs were clearly

summarized in Table 1. Crystallographic data (excluding

structure factors) for CP1‐CP2 have been deposited with

the Cambridge Crystallographic Data Centre as supple-

mentary publication numbers CCDC 1901797–1 901 798.

N, N^′‐Bis(3‐ethoxysalicylidenimino)‐1,3‐diaminopro-

ane (H₂L^OEt): Yield: (88%), Anal. Calc. for C₂₁H₂₆N₂O₄:

C, 68.09; H, 7.07; N, 7.56 Found: C, 67.18; H, 7.12; N,

7.90%. IR (KBr cm⁻¹) selected bands: ν(C═N), 1641, ν(C‐

O

_phenolic) 1256, ν(O‐H) 3448, UV–Vis (λ_max/nm): 340 nm.

2.3 | Synthesis of CPs

2.3.1 | [Zn (L^OMe)(μ₁‐dca)(μ_1,5‐dca)]_n(1)

To the methanolic solution (25 ml) of zinc acetate

dihydrate (0.2195 g, 1 mmol), Schiff base ligand (H₂L^OMe

)

solution was added drop by drop and the resulting solu-

tion was stirred for 15 mins. An aqueous methanolic solu-

tion (5 ml) of sodium dicyanamide (0.0891 g, 1 mmol)

was then added. The overall reaction mixture was

refluxed for 25 min at 60 °C. 10 ml of Dichloromethane

(DCM) were also added into the reaction mixture. The

stirring was continued for about 3 hr. Finally, the light‐

yellow filtrate was kept for crystallization by slow evapo-

ration at room temperature. Single crystal, suitable for X‐

ray diffraction were obtained after 10 days on slow evap-

oration of the solution in open atmosphere. Crystals were

2.5 | DFT calculations

CPs are subjected to high level computational studies

after optimizing the geometry with B3LYP functional

and the Split Valance Polarisation (DGDVP) basis set,

MAJUMDAR ET AL.

5 of 22

TABLE 1 Crystallographic parameters for CP1 and CP2

Formula

C₂₃H₂₀N₈O₄Zn₂(1)

C₂₅H₂₄N₈O₄Zn₂(2)

M/g

603.21

631.26

Crystal system

Space group

a/Å

Monoclinic

Orthorhombic

P2₁2₁2₁

9.2767(8)

12.2635(10)

23.804(2)

90

P2₁/n

8.805(2)

b/Å

15.613(4)

c/Å

18.541(5)

α (°)

90

β (°)

90.068(9)

90

γ (°)

90

V/Å³

2534.5(12)

2708.1(4)

4

Z

4

ρ_c/g cm⁻³

μ/mm⁻¹

F (000)

1.581

1.548

1.939

1.818

1224

1288

Cryst size (mm³)

θ range (deg)

Limiting indices

0.052× 0.031 × 0.022

0.993

0.2× 0.2 × 0.1

0.996

‐10 ≤ h ≤ 10

−18 ≤ k ≤ 18

−21 ≤ l ≤ 21

29 021

−11 ≤ h ≤ 11

−14 ≤ k ≤ 14

−16 ≤ l ≤ 28

20 195

Reflns collected

Ind reflns

4466[R_int= 0.049, R_sigma= 0.0340]

0.993

4744[R_int= 0.0320, R_sigma= 0.0318]

Completeness to θ (%)

Refinement method

Data/restraints/parameters

Goodness‐of‐fit on F ²

Final R indices

0.996

Full‐matrix‐block least‐squares on F²

4466/18/324

4744/0/354

1.037

1.023

R₁= 0.0431

R₁= 0.0242

[I > 2θ(I)]

wR₂= 0.1018

R₁= 0.0662

wR₂= 0.0625

R₁= 0.0281

R indices (all data)

wR₂= 0.1132

1.060 and − 0.691

wR₂= 0.0646

0.361and −0.321

Largest diff. Peak and hole (e ·Å⁻³

)

which is among Karlsruhe basis sets. We commonly used

popular Gaussian 09 W^[66–68]for simulations and Gauss

View 5.0^[66–68]for visualizations. Frequency calculations

are performed to make sure that there are no imaginary

frequencies and the simulated structure refers to a mini-

mum energy conformation. Electronic transition, being

a time resolved process, the electronic spectra can be

modelled computationally by TD‐DFT using new hybrid

exchange–correlation functional using the Coulomb‐

attenuating method called CAM‐B3LYP functional using

the same basis set used for geometry optimizations.

PCM model is used to create a solvent atmosphere of

the DMSO solvent. HS and 2D fingerprint plots were

generated using Crystal Explorer 17^[66–68]using cif files

for CPs obtained as a result of single crystal X‐ray diffrac-

tion analysis. HS were calculated with B3LYP level func-

tional and 6‐31G(d) basis set using the TONTO Abinitio

and DFT code fixed within the Crystal Explorer 17.

2.6 | Experimental of cytotoxicity

2.6.1 | Cell culture

CPs including synthesized Schiff base ligands were dis-

solved in DMSO to prepare 25 mM stock solutions. Further

6 of 22

MAJUMDAR ET AL.

dilutions were also made in DMSO. During treatment, the

final concentration of DMSO was maintained <0.1%.

Breast epithelial cell line MCF10A was grown in

DMEM/F12 (Dulbecco's Modified Eagle Medium) supple-

mented with 5% horse serum, 20 ng/ml EGF, 0.5 mg/ml

hydrocortisone, 100 ng/ml cholera toxin and 10 μg/ml

insulin. Breast cancer cell line MCF7 was grown in DMEM

(high glucose) (Dulbecco's Modified Eagle Medium) and

MDA‐MB‐231 was grown in RPMI as monolayer with

10% FBS (Foetal Bovine Serum) 100 U/ml penicillin and

100 μg/ml streptomycin. All the cell lines were grown at

37°C in a humid, 5% CO₂regulated incubator.

through cell strainers and proceeded for FACS acquisition

on BD FACS Cali bur. The raw data was then analyzed

using Cell Quest pro software.

2.6.4 | Colony formation assay

Colony formation assay was performed to check the long‐

term growth of cells upon treatment of the drug as

described previously.^[70]Briefly, MCF7 and MDA‐MB‐

231 cells were seeded in 35 mm plates (5 × 10³cells) and

were grown in DMEM (Dulbecco's Modified Eagle

Medium) and RPMI media supplemented with 10% FBS

in 5% CO₂at 37 °C, respectively. After 48–72 hrs of growth,

MCF7 cells were treated with 20 μM of CP1 and MDA‐MB‐

231 cells were treated with 15 μM of CP1. Media was

changed after 48 hrs of treatment and cells were allowed

to form colonies for 2 weeks. Then, the cells were fixed

with 3.7% formaldehyde solution followed by staining

with 0.5% crystal violet for 15 mins. Stained cells were then

washed with PBS three times with gentle rocking at room

temperature to remove the residual crystal violet solution.

The images were obtained using the Amersham Imager

600 (GE Healthcare Lifesciences) and the experiment

was repeated three times.

2.6.2 | Growth inhibition by MTT assay

The cytotoxic effect of CPs and ligands was determined

using MTT (3‐[4, 5 dimethylthiazol‐2‐yl]‐2,5‐diphenyltet-

razolium bromide) assay as described previously.^[69]

Briefly, the cells were seeded (3 × 10³cells per well) in 96

well plates. After 24 hrs, cells were treated with varying

concentrations (0–100 μM) of respective compounds for

48 hrs in triplicates. After 48 hrs of drug treatment, 20 μL

MTT solution (5 mg/ml MTT stock) was added to each well

of the 96 well plate and incubated further for 3.5 hrs in

humid 5% CO₂incubator. Media containing MTT solution

was then replaced by MTT solvent (4 mM HCl and 0.1%

Triton X‐100 in iso‐propanol), incubated for 15 min at

room temperature with gentle shaking to ensure the com-

plete dissolution of Formazan. Absorbance was measured

at 570 nm using a Thermo Scientific Multiskan GO Elisa

plate reader. All experiments were carried out three times

in triplicate and the percentage of viable cells was calcu-

lated as the mean with respect to the vehicle treated cells.

2.6.5 | Cell lysate preparation and

immunoblotting

Cells were harvested and washed twice with ice‐cold PBS.

Cells were then lysed with whole cell lysis buffer (50 mM

Tris pH 7.4, 200 mM NaCl, 50 mM NaF, 1 mM Na₃VO₄,

0.5% Triton X‐100 and protease inhibitor cocktail) in ice

for 30 mins as described previously.^[70]Cell lysates were

centrifuged at high speed (16 000xg) and supernatants

were transferred to new tubes. Protein concentration was

measured by Bradford method using bovine serum albu-

min as a standard.^[71]The protein samples were resolved

using SDS‐PAGE with Tris‐Glycine (25 mM Tris, 192 mM

Glycine) running buffer containing 0.1% SDS. Separated

proteins were transferred onto the PVDF membrane with

transfer buffer (Tris‐Glycine and 20% Methanol). Mem-

brane was incubated with indicated primary antibodies

overnight at 4 °C. The primary antibody was then

removed, and blots were incubated for 1 hr with secondary

antibody. Blots were then developed by chemilumines-

cence method.

2.6.3 | S‐cell cycle analysis by fluorescence

activated cell sorting (FACS)

The cells were treated with different concentrations of CPs

for 48 hours and then collected for FACS analysis.

Propidium Iodide staining was performed for the total

DNA content of the cells as described previously.^[70]

Briefly, the cells were washed with phosphate buffer saline

(PBS), trypsinized and centrifuged at 1500xg for 3 mins at 4

°C. The cells were then fixed and permeabilized using

900 μL of 95% chilled ethanol which was added drop wise

along with continuous vertexing and cells were then stored

overnight at 4 °C. The fixed cells were centrifuged at 1500 x

g for 3 mins and washed twice with PBS. The cells were dis-

solved and stained with 500 μL staining solution (900 μL

PBS, 2 mM MgCl₂, 50 μL PI stock solution (5 mg/ml PI in

PBS), 50 μL RNase A (1 mg/ml RNase A) (Sigma)) and

incubated at 37 °C for 20 mins. Cells were then passed

2.6.6 | DNA fragmentation assay

The assay was performed as per the protocol described

previously.^[72]Briefly, the cells were harvested, washed

MAJUMDAR ET AL.

7 of 22

with ice‐cold PBS and lysed in NP40 lysis buffer (20 mM

EDTA, 50 mM Tris‐Cl, pH 7.5 and 1% NP40) for 10 sec-

onds in ice. Cell lysates were centrifuged at 1600xg for

5 mins and the supernatant was collected. The pellet

was resuspended in NP40 lysis buffer and centrifuged to

obtain the second batch of supernatant. Both the super-

natants were mixed and SDS (final concentration 1%)

was added. RNase A was added to the mixture (final con-

centration 5 μg/μl) and incubated at 56 °C for 2 hr. Pro-

teinase K (final concentration 2.5 μg/μl) was added and

the mixture was further incubated for 2 hr and 37 °C.

Half volume of ammonium acetate (10 M) was added

followed by glycogen. The DNA was precipitated with

2.5 volume of ethanol and incubated at −20 °C overnight.

The mix was then centrifuged at 5000xg for 30 mins; pel-

let was dissolved in gel loading buffer and separated on

3% agarose gel.

3 | RESULTS AND DISCUSSIONS

3.1 | Synthesis

Schiff base ligand platforms were synthesized by the con-

densation of 1,3‐diaminopropane with o‐vanillin in MeOH

at 1:2 M ratio.^[59]Using dicyanamide spacer, CPs derived

in a good yields by taking the following procedure where

1:1:1 M ratio of zinc acetate dihydrate, Schiff base (H₂L-

^OMe/H₂L^OEt) and sodium dicyanamide [NaN (CN)₂] in

methanolic solution under stirred condition (Scheme 1).

The binding versatile ability of ligands and dicyanamide

spacer with metal ions (M²⁺

) are submitted in

Scheme S1‐S2. The crystalline form of CPs was only iso-

lated from slow evaporation of mixed solvent (CH₃OH‐

DCM) at room temperature. A close inspection of ligands

structure completely divulges that it comprises two

SCHEME 2 Similarities of Ω‐shape H₂vanen ligand with other compartmental ligands

TABLE 2 Compartmental ligands coordination features with Zn (II) and dicyanamide anion

Salen‐type ligands

Ligands pocket used

Zn (II)‐dca complexes/CPs

Ref

1. [N, N^′‐bis(3‐methoxysalicylidenimino)‐

N₂O₂and O₂O₂

Terminal and μ_1,5‐dca bridging mode. 1‐D CPs

This work

1,3‐diamino propane] [H₂L^OMe

]

[N, N^′‐bis(3‐ethoxysalicylidenimino)‐1,3‐

damino propane] [H₂L^OEt

]

2. N, N^′‐bis(2‐hydroxy‐

N₂O₂and O₂O₂

Terminal and μ_1,5‐dca bridging mode. Di‐nuclear.

11–14

3‐methoxybenzylidene) ‐propane‐1,

Isostructural

2‐diamine) [H₂L^OMe

]

N, N^′‐bis(2‐hydroxy‐3‐

methoxybenzylidene)‐propane‐1,2

‐diamine) [H₂L^OEt

3. N, N^′‐ethylene

]

N₂O₂and Ω‐shape O₂O₂

Terminal and μ_1,5‐dca bridging. Zn₄/Zn₃‐nuclear

56,57

bis(3methoxysalicylaldimine)

neutral metal complexes

N, N^′‐ethylene

bis(3‐ethoxysalicylaldimine)

[H₂vanen‐type ligands]

8 of 22

MAJUMDAR ET AL.

imines, two phenols and (‐OR) groups. After deproton-

in CP2) and effective strong intensity band (at

2090 cm⁻¹in CP1 and 2170 cm⁻¹in CP2) are indicative

of the presence of dicyanamide spacer in the CPs. IR

spectra with previously reported Zn (II)‐dicyanamide

complexes were compared in Table S3. Raman spectral

data's (previously reported Zn (II)‐dicyanamide com-

plexes) were additionally explore to support the presence

of bridging propensity of [N (CN)₂]⁻anions (Table S4).

Ar‐O stretching frequencies of CPs observed near at

1280–1220 cm⁻¹and 1300–1250 cm⁻¹which is similar to

the previously reported salen‐type ligands.^[78]UV–Vis

spectra (Figure S2) of Schiff bases at 330 and 340 nm

are assigned due to π → π^*and n → π^*type electronic

transitions. CPs in DMSO solvent exhibit ligand–based

transition at 275 nm, 358 nm and 275 nm, 356 nm

(Figure S2) due to π → π^*or n → π^*type of transitions.^[79]

Herein no characteristic broad d‐d absorption band was

assigned in the UV–Vis spectra.

ation of either [L^{OMe 2−}or [L^{OEt 2−}

] , the N₂O₂imine‐based

]

chelating sites are available to bind M²⁺ions whereas CPs

formation with dicyanamide spacer in presence of zinc

metal is still unveiled of research. Apart from, ligands

structural platforms are close resemblance with Ω‐shape

H₂vanen‐type or other compartmental ligands

(Scheme 2).^{[11–14,56,57]}In this context our research group

highlighted the coordination motifs of compartmental

ligands with Zn (II)‐nodes and dicyanamide spacer

(Table 2). The general stoichiometries of CPs have been

corroborated thoroughly by the microanalytical results.

CPs were yellow coloured solid, non‐volatile, stable on

prolonged exposure to light and air, insoluble in water

and successfully characterized using various spectroscopic

protocols like FT‐IR, FT‐Raman, ¹H NMR, UV–Vis along

with EDAX, powder X‐ray diffraction, TGA, X‐ray crystal-

lography and fluorescence spectroscopy.

3.2 | Spectroscopic characterizations

3.3 | ¹H NMR spectroscopy

FT‐IR spectra of CPs were carefully analyzed and com-

pared with those of the corresponding bicompartmental

Schiff base ligands (H₂L^OMe/H₂L^OEt) and different well‐

known stretching vibrations are listed in Table 3. Thus,

Schiff base formations are characterized by IR, and UV–

Vis spectroscopic studies (Figure S1, Figure S2). The char-

acteristic imines (C=N) stretching vibration of two Schiff

base ligands are found to be 1640 cm⁻¹and 1641 cm⁻¹

respectively. The absence of the N‐H stretching band near

at 3150–3450 cm⁻¹positively confirmed the condensation

of all the primary amine groups. Well defined stretching

bands at 3449 and 3448 cm⁻¹in the spectra of two Schiff

base ligands are due to O‐H stretching which disappeared

in Zn (II)‐CPs.^[73]In CP1 and CP2, IR and FT‐Raman

(C=N) stretching vibration bands are shifted to

1620 cm⁻¹(for IR) and 1628, 1631 cm⁻¹(for Raman)

respectively (Figure S3). The above mention data directly

confirmed the coordination of the imine nitrogen atom to

Two Schiff base formations and its effective coordination

mode with Zn (II) in CPs has been confirmed by ¹H NMR

spectroscopic study. The ¹H NMR proton numbering

scheme of ligands are represented in Scheme 3 and the

corresponding NMR spectra of ligands and CPs are

clearly submitted in Figure S1A. For free ligands NMR

spectra, no broad peak was identified in the region δ

the zinc metal centers.^[74–76]Dicyanamide anion [N (CN)

−

]

showed three sharp characteristic band preferably in

2

the region 2300–2170 cm^{−1 [77]}

Two medium intensity

.

SCHEME 3 ¹H NMR proton numbering scheme of Schiff base

ligands

bands (at 2290, 2180 cm⁻¹in CP1 and 2300, 2210 cm⁻¹

TABLE 3 FT‐IR spectral data (cm⁻¹) of Schiff base ligands and CPs

Compounds

(H₂L^OMe

ν(C=N)

ν(C‐O_Phenolic

)

ν(O‐H)/H₂O

ν(N‐H)

ν (Zn‐N)

ν (dca)

)

1640

1641

1620

1254

3449

3150–3450 absence

‐

(H₂L^OEt

)

1256

3448

3150–3450 absence

‐

CP1

1280–1220

1300–1250

‐

453

445

2290–2180 s 2090 vs

2300–2210 s 2170 vs

CP2

MAJUMDAR ET AL.

9 of 22

5.0–8.0 ppm indicating positively the absence of free ‐NH₂

group function. This situation is further confirmed the

formation of Schiff base ligands during condensation pro-

cess.^[80]Ligands aromatic proton peak is assigned in

between δ 6.75 and 6.95 ppm. H⁵and H⁶protons are

consisting that single crystals and bulk material are the

same. Further, results of PXRD study can provide the con-

firmation of the phase purity of each bulk sample.

attached to the imino carbon of (H₂L^OMe) and (H₂L^OEt

)

4 | SINGLE‐CRYSTAL X‐RAY

DIFFRACTION STUDIES

are downfield shifted (δ 8.46–8.52 ppm) since these two

protons are influenced by the close vicinity of phenolic ‐

OH and imino nitrogen groups. The methylene protons

assigned as H⁷for (H₂L^OMe) and H⁸for (H₂L^OEt) are close

vicinity with imino nitrogen, such protons are deshielded

and appear as a 2H triplet at δ 3.8–3.9 ppm. Three meth-

ylene protons (H¹) since attached to the aromatic oxygen

(H₂L^OMe) appear as a 3H singlet at δ 3.84 ppm whereas

for (H₂L^OEt) these hydrogens are intervened by a ‐CH₂

group appear at upper field as a triplet at δ 1.28 ppm. Fur-

ther, for (H₂L^OEt) due to intervening ‐CH₂group protons

(H²) appear as upper field quartet at δ 3.97 ppm. This pro-

ton character is paramagnetic influenced of the aromatic

4.1 | Crystal structure of CP1

Single crystal X ray diffraction study of CP1 reveals that 1

is a neutral and consist of two crystallographic indepen-

dent zinc metal ions with the formula [ZnL^OMe(dca)₂]

where, dca = dicyanamide ion. The CP1 crystallized in

the monoclinic space group P2₁/n (Z = 4). In this com-

plex all the metal ions are in full occupancy and charge

is balanced by the ligand, [L^{OMe 2−}

and the co‐ligand,

]

dicyanamide anion. The ORTEP diagram of asymmetric

unit of CP1 is given in the Figure 1 whereas selected bond

angle and bond length parameters of CPs are given in the

Table S1‐S2. From structural point of view of (H₂L^OMe), it

1

oxygen. CP1 and CP2 was also characterized by H NMR

spectra. Comparison of ¹H NMR spectra of CPs with

respect to free Schiff base ligands indicates that the pro-

ton signal corresponding to OH group of ligands are

completely disappeared in both CPs which may be due

to deprotonation. Further, signals of H³‐H⁶are broadened

compared to the free ligand (H₂L^OEt) possibly due to

spin–lattice relaxation caused by paramagnetic impurities

in the CPs.^[81]

3.4 | EDAX analysis

The weight (%) contribution of the elements of

dicyanamide modulated CPs were further confirmed

from the EDAX profile analyses. The weight (%) contribu-

tion of the elements present in CPs were clearly submit-

ted in Table S5 and the related EDAX profiles are

shown in Figure S4. The calculated and EDAX values of

CPs are nearly good agreement. Therefore, the empirical

formula of CP1 and CP2 is formulated as C₂₃H₂₀N₈O₄Zn₂

and C₂₅H₂₄N₈O₄Zn₂. EDAX profile shows highest peak of

only C and thereafter O, Zn which further established the

empirical formula of dicyanamide linked zinc metal CPs.

FIGURE 1 ORTEP diagram for CP1

3.5 | Phase purity studies

Powder X‐ray diffraction patterns for CPs were recorded at

room temperature. The X‐ray powder diffraction patterns

for CPs were recorded experimentally after small (2θ) scan-

ning from 4⁰to 50⁰. The experimental PXRD patterns

(Figure S5) of the bulk materials for CPs are similar with

the patterns simulated from single X‐ray crystal diffraction

data (CIF 1 & 2) obtained from CCDC Mercury software

FIGURE 2 1‐D Polymeric structure of 1

10 of 22

MAJUMDAR ET AL.

contains two unsymmetrical tetradentate pockets [N2O2

and O4] which contains two Zn (II) metal ions. Apart

from, this coordination action provided by the ligand

102.7° respectively. The distance between Zn(1) and Zn

(2) metal centers is 3.165 Å which is comparable to other

Zn—Zn separation of double phenoxo‐bridged Zn₂com-

plexes.^{[11–14,84,85]}All the bond angles and distances are

comparable to those reported in the literature (Table S7).

[L^{OMe 2−}

] , the di‐nuclear zinc framework is further

attached by dicyanamide ligand like a trunk of the insects

(Figure 1). A close investigation into the crystal structure

of CP1 reveals that the asymmetric unit further grows by

the dicyanamide ligand offers a one dimensional (1‐D)

polymeric chain (Figure 2). The two nearest 1D polymers

further undergoes several non‐covalent interactions

(hydrogen bonding and C‐H····π) and displays a two‐

dimensional polymeric framework (Figure S6). The coor-

4.2 | Crystal structure of CP2

Single crystal X‐ray diffraction study of 2 reveals that this

CP crystallizes in the orthorhombic space group, P2₁2₁2₁

(Z = 4) and asymmetric unit (Figure S7) contains two

crystallographic independent zinc metal ions (Figure

S7). The ORTEP diagram of asymmetric unit of CP2 is

given in the Figure 4. The structure is initially 1‐D poly-

meric unit which undergoes several non‐covalent interac-

tions with the nearby another 1‐D polymeric unit and

ultimately furnish a 2‐D layer (Figure S8) identical with

dination fashion of the ligands [L^{OMe 2−}

and dicyanamide

]

anions are shown in the Figure S6A which displays a

fully deprotonated ligand coordinated with two Zn (II)

ions in μ₂– η1: η2: η1: η1: η2: η1 mode. The μ₂‐phenolate

oxygen atom acts as bridging ligand to construct Zn2(O)2

core whereas dicyanamide anion acts as a bridge between

the two such di‐nuclear core by using its μ_1,5coordination

mode to afford a 1‐D polymer (Figure 2). This type of

dicyanamide bridging fashion is identical with previously

reported bi‐nuclear Schiff base complexes^[82,83]and

another dicyanamide ligand simply attached to the one

of the zinc centers in the bimetallic core as a μ₁mono

dentate fashion. Two structurally independent Zn (II)

centers are found in the single X‐ray crystal structure.

Zn(2) shows five coordination in a distorted square pyra-

midal geometry (Figure 3) which is composed by two

imine nitrogen atoms and two phenolate oxygen atoms

from deprotonated coordinating ligand, one N atom from

dicyanamide anion where Addison parameter tau ()^[82,83]

value is 0.22 ( = |β‐α|/60⁰) where β and α are the two larg-

est angles around the central atom; = 0 for perfect square

pyramidal and 1 for a perfect trigonal geometry). The cal-

culated tau () value of CP1 has been compared with liter-

ature reported Zn (II)‐dicyanamide complexes (Table S6).

On the other hand, Zn(1) is six coordinated satisfied from

4O, 2 N with a trigonal prismatic geometry (Figure 3).

The bond lengths of zinc metal coordinated phenoxy

atom are of range 2.011–2.043 Å while the metal coordi-

nated nitrogen atoms are of range 2.045–2.063 Å. The

bond angle Zn1‐O2‐Zn2 and Zn1‐O1‐Zn2 are 102.9° and

FIGURE 4 ORTEP diagram for CP2

FIGURE 3 Zinc metal coordination

environment in CP1

MAJUMDAR ET AL.

11 of 22

CP1. The binding mode of the main ligand, (H₂L^OEt) and

co‐ligand dicyanamide anion in the CP2 is same as in 1

and the metal coordination environment (Figure S9 and

Figure S10). Selected some important bond angle and

bond lengths of CPs are given in the Table 4. Supplemen-

tary, the coordination motifs of present investigated

ligands have been compared with identical compartmen-

tal ligands in terms of ligand anionic donor centers, CPs

formation, dicyanamide spacer bridging flexibility

(Table S8).

the crystal packing. Total electrostatic interactions are

determined as a sum of electrostatic interaction, polariza-

tion, dispersion and exchange repulsion which all are cal-

ibrated against dispersion corrected DFT.^[86–88]For CP1,

the D_norm, shape Index, fragment patch and curved Index

are provided in Figure 5. The total volume of the surface

is 623.69 A³, globularity is 0.682 and asphericity 0.030.

This shows that the CP1 deviates from perfect spherical

shape whereas for CP2, globularity value is 0.695 and

asphericity value is 0.02. These values suggest that CP1

is more deviate from the sphericity than CP2 (Figure 6).

2‐D fingerprint plots (Figure 5 and Figure 6) provides

information about different types of possible interactions

between CPs and other atoms of the surround molecules

(Table S9). For CP1, the highest interaction is between

hydrogen atoms and there is no interaction between O

atoms and Zn atoms, due to their high electronegativity

and electro positivity. For CP2, the results are slightly dif-

ferent from that of CP1 where no interaction observed

between O atoms, Zn atoms and N atoms. The highest

interaction is between the hydrogen atoms and also

between C and H followed by N and H.

4.3 | Hirshfeld surface

HS analysis is generally used to study the intermolecular

forces between different molecules and intra molecular

interaction within a molecule in a crystal lattice. Molecu-

lar electrostatic potential mapping on the HS acts as a

tool to study electrostatic complementarily between

nearby molecules, which provides basic knowledge about

TABLE 4 Selected important bond distances (Å) and angles (°)

for CP1 and CP2

Bond lengths (Å) Value (Å) Bond angles (°) Value (°)

1

4.4 | FMO

Frontier molecular orbital (FMO) provides useful infor-

mation about the reactivity and stability of dicyanamide

modulated zinc metal CPs. Visualizations of the HOMO

and LUMO can provide some interesting insights about

the nature of the CPs and its reactivity and other phys-

icochemical characteristics. Figure S11‐12 provides the

pictorial representations of the frontier molecular

orbitals of the CP1 and CP2. It shows that for both

CPs, HOMO is predominantly placed over the Zn atoms

and LUMO is delocalized over the aromatic ortho vanil-

lin condensed Schiff base ligands coordinated to the

central metal atoms. The energy values obtained from

FMO's can be used to generate other different reactivity

parameters. These data can be used to find other fron-

tier orbital data as follows, ionization potential: I = ‐

E_HOMO, electron affinity A = ‐E_LUMO, hardness η = (I‐

A)/2, and electrophilicity index ω = μ²/2η.^[89]For CP1,

energy of HOMO and LUMO are −8.7827

and − 2.9777 eV respectively, the vertical excitation

energy gap, which is the difference between HOMO

and LUMO is 2.8050 eV. The ionization energy is

5.7827 and electron affinity A, 2.977, global electronega-

tivity 4.3802, hardness 1.4025, chemical potential

−4.3802, electrophilicity index 6.8402 and global soft-

ness 0.7130 (eV). Data for CP2 is only slightly different

where HOMO is at −5.8110 eV, LUMO at −2.9315,

energy gap 2.8795, ionization energy 5.110, electron

Zn1‐N1

Zn1‐N2

Zn1‐N6

Zn1‐O1

Zn1‐O2

Zn1‐N8

Zn2‐O2

Zn2‐O1

Zn2‐O4

Zn2‐N6

Zn2‐N3

2

2.063(5)

2.045(4)

1.943(6)

2.043(2)

2.040(3)

2.025(4)

2.006(3)

2.011(3)

2.540(4)

1.973(4)

1.949(5)

N1‐Zn1‐N2

O1‐Zn1‐O2

N8‐Zn1‐N2

N8‐Zn1‐N1

O1‐Zn1‐N1

O2‐Zn1‐N2

O1‐Zn2‐O2

N3‐Zn2‐N6

N3‐Zn2‐O4

N6‐Zn2‐O1

N6‐Zn2‐O2

96.5(2)

75.4(1)

106.0(2)

100.5(2)

88.0(1)

89.3(1)

75.4(1)

119.3(2)

80.9(1)

109.1(1)

113.5(1)

Zn1‐O1

Zn1‐O2

Zn1‐O4

Zn1‐N8

Zn1‐N3

Zn2‐N2

Zn2‐N1

Zn2‐N6

Zn2‐O1

Zn2‐O4

2.012(2)

2.574(3)

2.001(3)

1.980(4)

1.938(4)

2.048(3)

2.058(3)

2.018(4)

2.026(3)

2.079(3)

O1‐Zn1‐O4

N3‐Zn1‐N8

O2‐Zn1‐O4

O1‐Zn1‐O2

O1‐Zn2‐O4

N1‐Zn2‐N2

O1‐Zn2‐N6

N1‐Zn2‐N6

N2‐Zn2‐N6

‐

146.5(1)

124.8(2)

146.5(1)

67.8(1)

76.6(1)

94.7(1)

100.6(1)

105.3(1)

104.8(1)

‐

12 of 22

MAJUMDAR ET AL.

FIGURE 5 Hirshfeld surface analysis for CP1: d_norm, Shape index, curvedness and different 2D fingerprint plots

affinity 2.9315, electronegativity 4.3712, global hardness

1.4398, chemical potential −4.3712, electrophilicity

4.5 | TD‐DFT

index 6.6358 and global softness 0.6946 (all values in

eV). High energy gap, ionization energy, lower electron

affinity, electronegativity, high hardness, low electrophi-

licity index and softness are for CP2, which means that

CP2 is more stable than CP1. Partial density of states

(PDOS) of CPs are provided in Figure S13‐14 which

indicates that there is not much orbital overlap in the

core molecular orbitals.

The simulated UV–Vis spectral data can be used to deter-

mine whether the CPs utilized in a Dye Sensitized Solar

Cell (DSSC) as a potential photosensitizer, which absorbs

energy from light and undergoes electronic transition and

later transferred the gained energy to the TiO2 semicon-

ductor, which completes the electronic circuit in the solar

cell. The ability of the CPs, called the light harvesting effi-

ciency (LHE) to absorb the light depends on its oscillator

MAJUMDAR ET AL.

13 of 22

FIGURE 6 Hirshfeld surface analysis for CP2: d_norm, Shape index, curvedness and different 2D fingerprint plots

strength (f) by the relation following (LHE) = 1‐10^‐f.Φ_inject

or the electron injection efficacy is a very important

parameter that decides the efficiency of the DSSC. It is

almost synonymous to the driving force ΔG_inject. The elec-

trons injecting from the excited states of CPs to the semi-

conductor is obtained from the difference between the

excited state oxidation potential (E*_Oxi) and E_CBis the

ground state reduction potential of TiO₂conduction band

(E_CB= −4.0 eV). ΔG_injectis calculated by using the popu-

lar equation, ΔG_inject= E*_Oxi− E_CBwhere E*_Oxiis calcu-

lated from the ground state redox potential of sensitizers

(E_Oxi) and vertical transition energy corresponding to

the maximum absorption (E₀₀) can be written as:

E*_Oxi= E_Oxi− E₀₀. For CP1, two major electronic transi-

tions are observed at wavelength 325.79 nm and

267.26 nm with oscillating strengths 0.38 and 0.6396

respectively. The experimental values are 358 nm and

276 nm. The first transition is due to HOMO‐1 to LUMO

(43%) and HOMO to LUMO+1 (51%), while the second

transition is contributed by HOMO‐5 to LUMO +1

(29%), HOMO‐4 to LUMO (39%) and HOMO‐3 to LUMO

(13%) (Table S10A†). Both transitions are from singlet

ground state HOMO orbitals to the excited state orbital

of A symmetry. In case of CP2, there are again two

absorption wavelengths at 327.88 nm and 268.68 nm with

an oscillator strength of 0.3732 and 0.6237 respectively.

The experimental UV–Vis values are at 356 nm and

275 nm. Symmetrically, they are from the singlet state

to the states of A symmetry. The transition at 327.88 nm

is contributed by HOMO‐1 to LUMO (44%) and HOMO

14 of 22

MAJUMDAR ET AL.

to LUMO+1 (51%) and for the transition at 268.68 is con-

tributed by HOMO‐5 to LUMO +1 (29%), HOMO‐4 to

LUMO (25%) and HOMO‐3 to LUMO (20%) (Table

S10B). The light harvesting efficiency (LHE) for transition

1 is 0.5861 and for the second is 0.7707, which means that

during the second transition 77.07% of the incident solar

energy is absorbed by the CP1. ΔG_injectvalues are calcu-

lated and provided in the Table S11. For CP1, the values

are −2.0234 ev and − 2.8569 eV. The data clearly indi-

cates that the second transition is more feasible than the

other with more negative Gibb's free energy values. For

CP2, the free energy of injection values are −1.9709 eV

and − 2.4262 eV which is less than CP1. Generally, free

energy values can predict the spontaneity of a process.

More negative the free energy, more spontaneous or fea-

sible the process is. Therefore, finally it can be concluded

that CP1 is having more use in DSSC's than CP2.

FIGURE 7 Ligand‐centered Fluorescence emission spectra for

CP1 and CP2

enhancement of Quenching effect” (CHEQ) is recom-

mended.^[99]The nature of emission spectra of ligands

and CPs are comparable with previously reported salen‐

type ligands^[73,84,85]as well as Zn (II)‐dicyanamide com-

plexes (Table S12). The low quantum yield of Schiff base

ligands is ascribed due to fast Photoinduced electron

transfer (PET) from nitrogen lone pair to the conjugated

phenolic moiety. L‐M complexation prevents PET process

thereby enhances quantum yield (ɸ) in the CPs.^[100–102]

Since ligands exhibit chrematistic fluorescence emission,

hence solid‐state fluorescence spectroscopic studies have

been undertaken mainly for CPs.^[103]Under solid‐state,

photo excitation at similar wavelength shows photolu-

minescence with the main emission intense peak near

at 464 nm and 462 nm (Figure 8). This type of electronic

transitions is assigned due to intra‐ligand (π → π^*)

5 | PHOTOLUMINESCENCE

Photoluminescence properties of Schiff base ligands and

CPs are monitored in spectroscopic grade DMSO solvent

(Table 5). In DMSO, the fluorescence emission spectra

(Figure 7) revealing that free ligands are practically weak

fluorescence with respect to CPs. Upon photo excitation

at 332 nm, 338 nm, Schiff base ligands exhibit fluorescent

emission maxima centered at 390 nm and 423 nm. Simi-

larly, CPs upon photo excitation at wavelength 359 nm,

356 nm shows fluorescence maxima with the major emis-

sion peak at 470 nm and 475 nm. The increased fluores-

cence emission intensity of CPs over free ligands may

probably due to N, O‐donor centers effective coordination

with the Zn (II) metal ion, thereby increasing the confor-

mational rigidity via chelation effect [CHEF] and subse-

quently loss of energy by radiation less thermal

vibration. Besides, the active fluorescence nature of CPs

compared to free ligands may be due to d¹⁰configuration

zinc metal ion is difficult or hard to oxidize or reduce.

This type of fluorescence nature may be assigned due to

the intra‐ligand (π → π^*) transition or L → M charge

transfer popularly referred as CHEF (Chelation enhanced

fluorescence)^[90–98]whereas for quenching “Chelation

FIGURE 8 Solid‐state Fluorescence emission spectra for CP1 and

CP2

TABLE 5 Summary of steady state and time‐resolved photoluminescence decay

λ_ex

(nm)

λ_em

(nm)

λ_em(nm)

[Solid‐state]

Quantum

Yield (ɸ)

Compounds

τ₁[ns]

τ₂[ns]

<τ > [ns]

χ²

CP1

CP2

(H₂L^OMe

(H₂L^OEt

359

356

332

338

470

475

390

423

464

462

‐

0.01211

0.01453

0.00352

0.00359

0.76 (99%)

0.63 (99%)

7.87% (1%)

7.92 (1%)

1.11

1.06

1.27

1.26

)

‐

MAJUMDAR ET AL.

15 of 22

transitions. Therefore, CPs fluorescence nature are more

active in solution than solid‐state. Interestingly, the fluo-

rescence emission maxima of CPs are more

bathochromically red shifted in DMSO solvent than

solid‐state environment. Therefore, in DMSO, the fluores-

cence behaviours of CPs are mainly ligand centred

whereas in solid‐state it is totally crystal packing depen-

dent.^[103]To get a better understanding on emitting prop-

erties of CPs, time‐resolved fluorescence spectroscopic

studies have been measured in DMSO (Figure S15). The

fluorescence lifetime for CPs have been measured upon

excitation at 359 nm and 356 nm. The nature of decay

profile will be best fitted to bi‐exponential nature (with

acceptable comparable χ²values) which are comparable

with reported Zn (II)‐salen‐type complexes.^{[73,103–108]}In

DMSO, a regular trend observed for fluorescence life-

times where CPs shows two different decay times having

maximum (99%) and minimum (1%) contribution in

amplitude. The fluorescence lifetime value further

divulges that importance of excited states stabilities of

CP1 are greater than CP2. The bi‐exponential decay pro-

files for CPs are best explained as due to competition of

different excited states that is π → π^*excited states

involved in the emission features and preferably CT char-

acter that quenches the fluorescence.^[104,105]

349.77 °C which corresponds to the loss of the μ_1,5‐

dicyanamide linker units. A second mass loss of about

20.1% (calculated 21.5%) is observed in the range 440–

652°C which corresponds to the loss of the methoxy units

in the CP. On further heating up to 1000 °C the CP con-

tinues to disintegrate. The thermal behavior of CP2 is dif-

ferent when compared to that of 1. In 2, a continues of

decomposition of the CP can be seen in the broad temper-

ature range from 211 to 685 °C which includes the disin-

tegration of the μ_1,5‐dicyanamide linker units, diimine

spacer unit and ethoxy units from the CP and the which

has a total mass loss of about 60.07% (calculated 59.21%,

Figure S16[b]). On further heating the CP continues to

decompose. It can be seen that the thermal stability of

CP1 is relatively high when compared to that of 2 where

the later exhibits dismal thermal stability where there is

continues decomposition starting from ca. 200 °C.

7 | CYTOTOXICITY ANALYSIS

We screened synthesized ligands S1B (H₂L^OMe)/S2C

(H₂L^OEt) and their corresponding dicyanamide modu-

lated zinc metal CPs, S2A for CP1 and S2D for CP2

respectively against three cancerous cell lines using

MTT assay viz., MCF7 (breast cancer), MDA‐MB‐231

(breast carcinoma) and MCF10A (breast epithelial) to

determine their toxicity and inhibitory concentrations

(IC₅₀). Based on the analysis of cytotoxic activity of four

synthesized compounds, CP1 to be most potent against

MCF7 and MDA‐MB‐231 cancerous cell lines. The CP1

exhibited the IC₅₀value at 20 μM for (MCF7) and 15 for

(MDA‐MB‐231) whereas the other showed a high IC₅₀

6 | THERMAL ANALYSIS

Thermal characteristics of the CPs were studied using

TG‐DTG techniques under nitrogen atmosphere. In the

CP1, a mass loss of about of about 11.34% (calculated

11.98%, Figure S16[a]) in the temperature range 207–

FIGURE 9 A–C Cytotoxic effect against breast cancer cell lines of ligands and CP1‐CP2

16 of 22

MAJUMDAR ET AL.

value with (H₂L^OMe) at 50 μM (MCF7), (H₂L^OEt) at 47 μM

(MCF7) and CP2 at 60 μM concentrations (MCF7)

(Figure 9(A)). Further, four screened compounds IC₅₀

values have been compared with previously reported Zn

(II)‐Schiff base complexes as well as anticancer drug cis-

platin (Table 6).^[109–116]Among the four compounds, only

CP1 was found to be good cytotoxicity with MCF7 cell

line. We then further proceed with CP1 and performed

a similar screen in MDA‐MB‐231 cell line. The results

revealed an even lower IC₅₀value (15 μM) of CP1 in this

cell line (Figure 9(B)‐9(C)). Therefore, in between MCF7

and MDA‐MB‐231 cell lines, CP1 is active only with

MDA‐MB‐231 over CP2 or free Schiff base ligands. Fur-

ther, since two Schiff bases are identical structure except

intervened ‐CH₂‐moiety with respect to OH group at

ortho position, their cytotoxic variation in MCF7 and

MDA‐MB‐231 cell lines are significantly small.^[110]The

cytotoxic behavior of zinc metal CPs is preferentially

dependent on their ability to bind with DNA. The

potency of cytotoxic effect may also be attributed with

positive charge of the metal atom (here Zn²⁺ion) which

increased the acidity of proton of coordinated ligands

and causes stronger hydrogen bonding, thereby enhanc-

ing biological activity. Generally, damage of DNA struc-

ture results in the impairment of its function, which

eventually causes cell death. A clear variation of cytotox-

icity was observed particularly in presence of variable

coordinated substituents and different nature of metal

ions (our synthesized CPs possess common Zn²⁺ion).

This is basically due to the alternation in binding ability

of DNA.^[117,118]Synthesized both CPs are dicyanamide

bridged 1‐D chain polymer, therefore CP1 cytotoxic vari-

ation against MCF7 and MDA‐MB‐231 cell lines are

nearly identical. Further, it was construed that in CP2

greater electron releasing substituents effect of Schiff base

ligand (H₂L^OEt

)

significantly enhances cytotoxicity

against MCF7 breast cancer cell line.^[110]This difference

may be attributed due to the introduction of ‐OEt with

TABLE 6 IC₅₀values for CP1‐CP2 and previously reported Zn (II)/cisplatin complexes against breast cancer cell lines (MCF7, MDA‐MB‐

231, MCF10A) using MTT assay

IC₅₀values (μM)

Compounds

Ref

MCF7

MDA‐MB‐231

MCF10A

CP1

CP2

(H₂L^OMe

(H₂L^OEt

20

60

50

47

15

‐

40

‐

This work

57

)

‐

)

‐

[Zn₄(L^OMe)₂(μ₁‐dca)₂(μ_1,5‐dca)] [Zn₃(L^OEt

(H₂O)(μ₁‐dca)(μ_1,5‐dca)]

)

46.6 10.69

47.8 10.78

‐

2

[Zn(L₂)(OAC)₂.2H₂O]

[Zn(L₁).2H₂O]

[Zn(L₂).2H₂O]

[Zn(L₃).2H₂O]

[ZnL¹(OAc)₂]PF₆

[ZnL¹(OAc)₂](BPh₄

[Zn(L²)₂]

16.8 0.21

6.7 0.18

4.3 0.31

58.9 0.62

16.21

‐

109

110

‐

111

112

14.72

‐

2.0

[Zn(L³)₂]

[Zn(L)]

14.9–22.2

26 1.8

1.05

‐

113

114

115

[Zn(L)₂]

[Zn (Br)₂L¹]

[Zn (Br)₂L²]

[Zn (Br)₂L³]

[Zn(I)₂L¹]

1.064

2.319

0.899

[Zn(I)₂L²]

1.265

[Zn(I)₂L³]

3.049

Cisplatin

42.2

8

128.2

7

‐

116

MAJUMDAR ET AL.

17 of 22

respect to hydroxyl group in o‐vanillin. Finally, the over-

all discussion implies that the electronic effect may be

one of the influential factors in determining the antican-

cer activities of dicyanamide modulated Zn (II)‐CPs.

observed a decrease in the pro‐apoptotic protein levels

PUMA and BAX and lack of cleaved caspase‐8 and

caspase‐9 in presence of CP1 (Figure 11(A)) indicating that

CP1 does not inhibit the growth of cancer cells through

induction of apoptosis. Furthermore, the 89 kDa cleaved

product of PARP was not seen in the nuclear cytoplasmic

fractionation (Figure 11(B)) ascertaining the earlier obser-

vation. To corroborate the finding, we performed the DNA

fragmentation assay and did not observe the ladder of

fragmented DNA in the cells treated with CP1 (Figure 11

(C)). All these data depict that CP1 does not induce apopto-

sis. To decipher the cause of reduction in cell number in

presence of CP1, we checked for the other mode of cell

death. To this end, we checked for necrosis by observing

the pattern of PARP cleavage but did not find a 55 kDa

cleavage product which is a marker of necrosis. We further

checked for the possibility of autophagic cell death induc-

tion by the CP1. Though we have detected a reduction in

p‐mTOR and pS6K levels (Figure 12(A)), which is a charac-

teristic of autophagy initiation. The autophagy driven cell

death is associated with increased levels of Beclin as well

as LC3BII and decreased levels of p62. However, we

obtained contrasting results wherein the expression levels

of Beclin and LC3BII were decreased and the levels of

p62 were increased (Figure 12(B)) denoting the absence

of autophagy‐induced cell death. Cumulatively, these

observations infer that CP1 may utilize some other non‐

classical death mechanism to limit the number of breast

cancer cells. Maintenance of normal cell cycle is essential

for the cellular growth and proliferation. Since CP1

7.1 | Colony formation assay

To further identify its effect on the non‐cancerous normal

breast epithelial cells, we screened CP1 in MCF10A cell

lines and observed a high IC₅₀value (40 μM) denoting

the lesser toxicity of CP1 on the normal cells. The low

IC₅₀values obtained for CP1 in both MCF7 and MDA‐

MB‐231 cell lines prompted us to check the effect of this

CP on the long‐term growth of these cells. We, therefore,

performed the colony formation assay in both the cell

lines. We found that treatment of cells with their effective

IC₅₀concentrations led to a reduction in the number as

well as the size of colonies formed (Figure 10).

7.2 | Apoptosis/autophagy/necrosis

Apoptosis, autophagy and necrosis are recognized as the

classical cell death pathways which involve different

mechanisms and morphological features.^[119,120]The

smaller and lesser number of colonies observed in the pres-

ence of CP1 led us to check if the complex induced any

form of cell death. We treated MDA‐MB‐231 cells with

two different concentrations of CP1 and checked the

expression levels of proteins involved in apoptosis. We

FIGURE 10 Colony formation assay of

CP1

FIGURE 11 A–C Effect of CP1 against MDA‐MB‐231 cells

18 of 22

MAJUMDAR ET AL.

FIGURE 12 A–B The autophagic cell

death induction by CP1

inhibits the cell proliferation, we went ahead to check if 1

interferes with the normal functioning of the cell cycle

thereby inhibiting the cellular growth. The cell cycle pro-

file was examined through FACS analysis. Results demon-

strated a normal cell cycle profile with no apparent

differences observed in any phases of the cell cycle show-

ing that 1 does not control the cell cycle (Figure 13). In

addition to the cell cycle, cell proliferation is largely con-

trolled by the multitudes of signaling pathways function-

ing in the cells. Since, we found that CP1 does not alter

the cell cycle, we went ahead to decipher its effect on major

proliferative pathways of the cell in the MDA‐MB‐231

cells. We observed a decrease in the levels of phospho‐

and total levels of MEK and ERK with concomitant

increase in the dose of CP1 (Figure 14). Furthermore, a

similar observation was made for PI3K/AKT pathway as

well. The phospho‐AKT473 and the total AKT levels were

found to decrease in MDA‐MB‐231 cells (Figure 14).

FIGURE 14 Major proliferative pathways of MDA‐MB‐231 cells

of CP1

These observations indicate towards the negative regu-

lation of two major cell proliferation pathways by CP1.

The inhibition of MAPK and PI3K/AKT pathways may

play a partial role for the observed reduction in growth

of cells. To further discern the effect of CP1 on normal

breast epithelial cells, we treated both the non‐malignant

breast cells MCF10A and the malignant cells MDA‐MB‐

231 with the IC₅₀dose of CP1 for MDA‐MB‐231 cells.

FIGURE 15 Major proliferative pathways of MDA‐MB‐231 cells

of CP1

FIGURE 13 FACS analysis of CP1

MAJUMDAR ET AL.

19 of 22

We observed the expected reduction in expression levels

of MAPK/ERK pathway and the PI3K/AKT pathway in

MDA‐MB‐231 cells but did not detect any difference in

the MCF10A cells (Figure 15) indicating the utility of

CP1 in cancer therapeutics.

ACKNOWLEDGMENTS

This research work did not receive any specific grant

from funding agencies in the public, commercial or not‐

profit sectors. Dr. Dipankar Mishra gratefully acknowl-

edges the financial grant sanctioned by UGC, New Delhi,

in his favor vide Minor Research Project (F.PSW‐232/15‐

16(ERO). D. J. Majumdar thanks Mr. Surajit Mondal,

Research Scholar, IIT‐ISM, Dhanbad‐826004, India for

¹H NMR spectroscopic help and Dr. S. S. Sreejith, Cochin

University of Science and Technology, Kochi‐682022,

Kerala, India for Thermogravimetric analysis help.

8 | CONCLUSIONS

This research article describes the syntheses and struc-

tural aspects of the dicyanamide‐bridged 1‐D chain zinc

metal coordination polymers (1–2). The successful syn-

theses of CPs were only achieved in presence of ortho

vanillin derived Schiff base ligands after utilizing prop-

CONFLICT OF INTEREST

−

Author declare no potential conflict of interest.

erly their compartmental N₂O₂and O₄donor sets.

Solid‐state X‐ray single crystal study revealed two differ-

ent geometrical environment of Zn metal centers with

distorted square pyramidal Zn(2) and trigonal prismatic

Zn(1) geometry. DFT using B3LYP and DGDVP basis

sets were successfully applied to explain FMO, HS, TD‐

DFT and photovoltaic efficiency in Dye Sensitized Solar

Cell (DSSC). The Steady state and time‐resolved fluores-

cence measurements were conducted in DMSO and

solid‐state condition at room temperature. The thermal

stabilities of CPs have been examined by TGA. CPs as

well as Schiff base ligands evinced potential cytotoxic

effects on breast cancer cells using MTT assay, MCF7,

MDA‐MB‐231 and MCF10A. In vitro cytotoxicity assess-

ment confirmed CP1 to be most potent against MDA‐

MB‐231 cell line and even anticancer drug cisplatin.

Supplementary, programmed cell death apoptosis,

autophagy and necrosis were successfully highlighted

in this research article. To the best of our knowledge

dicyanamide‐bridged Zn (II)‐CPs photovoltaic efficiency

in Dye Sensitized Solar Cell and cancerous activities

are seems to be rare in literature. Thus, the prospective

study in the present manuscript deals a new insight of

CP1 in the development of cancer therapeutics.

ORCID

Dhrubajyoti Majumdar

7750

Sourav Das https://orcid.org/0000-0002-3346-5598

Dipankar Mishra https://orcid.org/0000-0002-0216-8400

https://orcid.org/0000-0002-9785-

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SUPPORTING INFORMATION

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How to cite this article: Majumdar D, Agrawal

Y, Thomas R, et al. Syntheses, characterizations,

crystal structures, DFT/TD‐DFT, luminescence

behaviors and cytotoxic effect of bicompartmental

Zn (II)‐dicyanamide Schiff base coordination

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necrosis type classical cell death. Appl Organometal

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Syntheses, characterizations, crystal structures, DFT/TD-DFT, luminescence behaviors and cytotoxic effect of bicompartmental Zn (II)-dicyanamide Schiff base coordination polymers: An approach to apoptosis, autophagy and necrosis type classical cell death

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