Piperidine based effective chemosensor for Zn(II) with the formation of binuclear Zn complex having specific Al(III) detection ability in aqueous medium and live cell images

Article abstract of DOI:10.1016/j.jphotochem.2021.113302

A schiff base chemosensor HL [4-chloro-(((2-(piperidine-1-yl)imino)methyl)phenol], constructed by one step condensation of 2-aminoethyl piperidine and 5- Chloro Salicylaldehyde is illustrated to have its usefulness for selective and sensitive detection of Zn(II) among other metal ions in aqueous solution (HEPES buffer) displaying the colour changes from light blue to turquoise under UV light. The specific detection phenomena of Zn (II) is monitored by the remarkable fluorescence enhancement and notable change in NMR spectral data of fluorescence active HL after addition of Zn(II) to it. The impressive high binding constant in 10⁶ orders, determined by electronic spectral titration after introduction of Zn(II) to HL assure the formation of a Zinc complex during sensing phenomena. In order to check whether the nuclearity of the constructed complex can be increased or not fluorescence enhancement phenomenon is further monitored in presence of different bridging ligands and the results display that only in presence of azide a significant enhancement of Zn(II)-HL domain is exposed where as for other bridging ligand the spectra remain unaltered. This observation insists to conduct the reaction between HL and ZnCl₂ in presence of sodium azide which lead the formation of a Zn complex [Zn₂(HL)₂(N₃)₄] (complex 1) employing azide as the secondary anionic residue instead of bridging. The single crystal structure of the complex unveils the unique protonation of the piperidine N during the crystallization of the dinuclear motif of the complex 1. Furthermore the Zn complex due to the presence of remarkable luminescence property have been effectively used for selective sensing of Al(III) ion as metalloreceptor in HEPES buffer solution via turn of fluorescence with remarkable fluorescence quenching constant. In aqueous solution, complex 1 induces a 1:2 complex formation with Al(III) ion indicated by Job's plot analysis. The DFT study suggests the formation of bimetal complex during sensing of Al(III) with complex 1 as metalloreceptor. The in vitro cell imaging study gives a authentic hints for in vivo biomedical application of HL as a selective and easy Zn(II) sensor. The two step sensing phenomena of HL and complex 1 is further established by advanced molecular logic gate formation.

Full text of DOI:10.1016/j.jphotochem.2021.113302

Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113302
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology, A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Piperidine based effective chemosensor for Zn(II) with the formation of
binuclear Zn complex having specific Al(III) detection ability in aqueous
medium and live cell images
Manik Dasa, Biplab Koleyb, Uttam Kumar Dasc, Arijit Bagd, Soumik Lahae,
,
Bidhan Chandra Samantaf, Indranil Choudhurig, Nandan Bhattacharyyag, Tithi Maitya *
a Department of Chemistry, Prabhat Kumar College, Contai, Purba Medinipur, West Bengal, 721404, India
b Department of Chemistry, IIT KGP, Kharagpur, India
c Depatment of Chemistry, School of Physical Science, Mahatma Gandhi Cental University, Bihar, India
d School of Natural and Applied Sciences, Maulana Abul Kalam Azad University of Technology, West Bengal, India
e IICB, Kolkata, West Bengal, India
f Department of Chemistry, Mugberia Gangadhar Mahavidyalaya, Purbamadinipur, India
g Department of Biotechnology, Panskura Banamali College, West Bengal, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
A schiff base chemosensor HL [4-chloro-(((2-(piperidine-1-yl)imino)methyl)phenol], constructed by one step
condensation of 2-aminoethyl piperidine and 5- Chloro Salicylaldehyde is illustrated to have its usefulness for
selective and sensitive detection of Zn(II) among other metal ions in aqueous solution (HEPES buffer) displaying
the colour changes from light blue to turquoise under UV light. The specific detection phenomena of Zn (II) is
monitored by the remarkable fluorescence enhancement and notable change in NMR spectral data of fluores-
cence active HL after addition of Zn(II) to it. The impressive high binding constant in 106 orders, determined by
electronic spectral titration after introduction of Zn(II) to HL assure the formation of a Zinc complex during
sensing phenomena. In order to check whether the nuclearity of the constructed complex can be increased or not
fluorescence enhancement phenomenon is further monitored in presence of different bridging ligands and the
results display that only in presence of azide a significant enhancement of Zn(II)-HL domain is exposed where as
for other bridging ligand the spectra remain unaltered. This observation insists to conduct the reaction between
HL and ZnCl2 in presence of sodium azide which lead the formation of a Zn complex [Zn2(HL)2(N3)4] (complex
1) employing azide as the secondary anionic residue instead of bridging. The single crystal structure of the
complex unveils the unique protonation of the piperidine N during the crystallization of the dinuclear motif of
the complex 1. Furthermore the Zn complex due to the presence of remarkable luminescence property have been
effectively used for selective sensing of Al(III) ion as metalloreceptor in HEPES buffer solution via turn of
fluorescence with remarkable fluorescence quenching constant. In aqueous solution, complex 1 induces a 1:2
complex formation with Al(III) ion indicated by Job’s plot analysis. The DFT study suggests the formation of
bimetal complex during sensing of Al(III) with complex 1 as metalloreceptor. The in vitro cell imaging study
gives a authentic hints for in vivo biomedical application of HL as a selective and easy Zn(II) sensor. The two step
sensing phenomena of HL and complex 1 is further established by advanced molecular logic gate formation.
Sensor
Fluorescence
DFT
Living cell image
Molecular logic gate
1. Introduction
community due to some special advantages like high sensitivity, easy
visualization, short response time for detection [1–7] in comparison to
Since the last few decade the easy and selective detection of physi-
ologically and environmentally concern metal ions with the organic
chemosensors have achieved a special attention to the scientific
the other detection methods like atomic absorption spectrometry,
inductively coupled plasma-atomic emission spectrometry (ICPAES) and
voltammetry which led to an expensive instrumentation and high
* Corresponding author.
E-mail address: titlipkc2008@gmail.com (T. Maity).
https://doi.org/10.1016/j.jphotochem.2021.113302
Received 8 December 2020; Received in revised form 22 March 2021; Accepted 12 April 2021
Available online 19 April 2021
1010-6030/© 2021 Elsevier B.V. All rights reserved.

M. Das et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113302
quantity sample [8–10]. At the same time recently in this chemosensor
development research area, a new concept has been launched where
metal organic complexes with the capability of taking another metal ion
along with impressive photoluminescence property has been utilized for
easy and selective detection of metal ions as metelloreceptor [11–13].
Ru(II)/Re(I)/Ir(III)/Pt(II) based complexes are mainly exposed in this
developing area [14–17]. But the design of fluorescence active metal-
loreceptors based on 3d-metal complexes, particularly Zn(II) complexes,
are now in the embryonic stage [18,19].
electronic titration among HL and Zn(II) displays impressively high
binding constant providing an idea regarding complex formation. To
inspect whether more than one nuclearity complex is obtained or not
during sensing, several bridging ligands are introduced to Zn(II)-HL
adduct and the spectral alteration of fluorescence enhancement is
observed. Interestingly in presence of azide anion noticeable spectral
change is observed which influence to conduct a reaction among HL and
ZnCl2 in presence of NaN3, leading to the formation of a binuclear
complex [Zn2(HL)2(N3)4](complex 1) with azide as a secondary anionic
residue instead of bridging. Consecutively complex 1 have further ex-
poses to detected the physiologically and environmentally concern Al
(III) ion as metalloreceptor due to the presence of notable luminescence
property. The sensing phenomena of Al(III) is also monitored by
adopting the same technique as like Zn(II) sensing. DFT study gives an
authentic prove of Zn/Al bimetal complex formation during the sensing
of Al(III) with complex 1.
From biological point of view Zn is a very crucial trace element not
only for human body but also for plants and animals [20,21]. It is the
second abundant element in human body among different biologically
significant metal ions due to its active participation in several biological
processes [22–24]. Its deficiency as well as excess accumulation can
generate various human health hazards in the form of nausea, vomiting,
loss of appetite, stomach cramps, diarrhea, and headaches [25,26].
Beside that over accumulation of this essential metal to the environment
as a result of industrial revolution led to some environmental problems
which introduce this essential element in a new perspective as ‘pollut-
ants’. At the same time after oxygen and silicon aluminum is the third
most wide spread (8.3 % by weight) metallic element in the Earth’s
crust. As a part of life Aluminum stays with us in the form of utensil,
building equipment and different packaging items and as a result of
which high quantity of free aluminum ions releases Al(III) to the envi-
ronment, making it toxic [27,28]. In addition the abnormal accumula-
tion of aluminum ion in human body may cause several life threatening
diseases like Alzheimer’s disease, Parkinson’s disease, osteomalacia etc
[29–31].
To get a satisfactory regarding the in vivo biomedical application of
HL probe for selective detection of Zn(II), living cell image study has
been carried out. Furthermore the use of two step advanced logic gate
formation represents the sensing process of HL and [Zn2(HL)2(N3)4].
2. Results and discussion
2.1. Synthesis and general characterization of complexes
The Zn complex, [Zn2(HL)2(N3)4], complex 1 has been synthesized
by the reaction of ZnCl2 with the Schiff base ligand (HL) prepared in situ
via condensation of 2-amino ethyl piperidine and 5-chloro Salicylalde-
hyde followed by the addition of Sodium azide (Scheme 1). In the initial
stage the FTIR spectrum of ligand HL and complex 1 are recorded to get
an introductory idea regarding the existence of the bonds (Fig S1 in
supplementary information file). For ligand a band at 3521 cmꢀ 1 is
Under this circumstance the selective detection of Zn (II)/Al(III) is
very much indispensible for healthy body and environment. As previ-
ously discussed the fabrication of effective chemosensors should be the
best choice for easy, sensitive and selective detection of Zn(II) and Al
(III).
–
observed which stands for O H stretching frequency. Within the range
of 1730ꢀ 1740 cmꢀ 1 the characteristic C N stretching is observed in HL
–
–
Literature survey exposed several good evidence of individual
development of fluorescence active organic probe and metal organic
complexes for selective detection Zn(II)/Al(III). As for example, Das
et al., [32] Naskar et. al [33], Xu et al. [34], have developed several
fluorescence based Schiff base organic chemo sensor for selective
detection of Zn(II) and Song et al. [35], have also developed different Zn
(II) metalloreceptors for the same purpose. In a parallel way Dwivedi
and coworkers [36], Borase and coworkers, [37], Das and coworkers
[38], Liu and coworkers [39] have exposed separate organic chemo-
sensor for selective detection of Al(III) but no report has been found to
fabricate new metal complex for selective detection of Al(III). All these
report highlight the individual construction of either an effective
organic probe or metalloreceptor for detection of Zn(II) or Al(III).
Interestingly very few report has been found where organic probe as
well as the metalloreceptor, constructed by using mother organic probe,
have been used consecutively for the selective detection of Zn(II) and Al
(III) respectively. In effective organic probe development area for se-
lective sensing of metal ions Schiff base attracts a special attention.
Literature survey also revealed the excellent coordinating power of
piperidine based N, N, O donor Schiff base ligand to form metal com-
plexes having different potential application in magnetic field, interac-
tion with macromolecule like DNA, anticancer drug development area
etc [40,41]. But very few report where the luminance property of those
metal complexes, constructed with this tri dented ligand alone are
focused to detect the biologically and environmentally sensitive cations.
It is also noticeable that nobody utilizes chloro substituted small easily
developing N, N, O donor Schiff base ligand for selective detection of
metal ions.
as well as in complex 1. The sharp bands ranging from 1310 cmꢀ 1 to
1440 cmꢀ 1 indicate the vibration of skeleton benzene in both ligand and
complex. The UV–vis spectroscopic analysis of the ligand (HL) and
complex 1 (Figure S2 in Supporting information file) clarify that two
absorption bands are observed at around 325 nm (
€
= 9636 M-1 cm-1)
and 420 nm (
€
= 3433 M-1 cm-1) in case of ligand (HL). The band can-
tered at 420 nm may be attributed to the n–π* transition of the azo-
methine group where as the band at 325 nm arises due to the π-π*
transition. The absorption bands in complex 1 shifted toward lower
frequencies, 385 (
€
= 11,006 M-1 cm-1) nm due to CT transition advo-
cating the coordination of azomethine nitrogen and phenolato oxygen
with the metal centre.
The electrospray ionization mass spectra (positive mode, m/z up to
1200 amu) of the HL is recorded in the form of its methanolic solutions
(Figure S3 in supporting information file). Ligand HL shows one peak at
m/z 268.4154 amu for species [HL + H]+ (Figure S3 in the supple-
mentary information file). (Calculated m/z = 268.4112 amu) and
complex 1 shows one major peak at m/z = 407.0526 amu for dinuclear
species [C28H38N13Cl2O2Zn2Na]2+ (calculated m/z = 406.8680 amu
(figure S4 in supporting information).
2.2. Crystal structure description
To obtain the solid state structure the single crystal of complex 1 is
subjected to SXRD analyses. The crystallographic data and the refine-
ment parameters for complex 1 is given in Table 1.
Keeping all these points in mine in the present work one fluorescence
active chemosensor HL, constructed by one step condensation of 2-ami-
noethyl piperidine and 5- chloro Salicylaldehyde, have been demon-
strated by utilizing its easy and selective sensing behavior to Zn(II). The
sensing phenomena are optically and spectroscopically monitored. The
2.3. [Zn2(LH)2(N3)4]
The X-ray quality single crystals of Zinc complex is crystallized in a
centro symmetric monoclinic space group ‘P21/c’ from DMSO/acetoni-
trile solvent system by slow evaporation at room temperature. The
2

M. Das et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113302
Scheme 1. Synthesis of HL and complex 1.
ring of one ligand and the hydrogen atom (H1B) of the pipyridnium
Table 1
–
moiety of the other ligand. The C H⋯Π interactions and overall par-
The crystallographic data and refinement parameters of Complex 1.
allel packing of the metal complex is shown in Fig. 1.
Complex 1
CCDC No.
2,032,970
C28 H38 Cl2 N16 O2 Zn2
832.38
2.4. Optical and UV–vis spectroscopic signature of ligand HL
Empirical formula
Formula weight
Crystal size/mm
Crystal system
Space group
a /Å
As the synthesized ligand HL has potential luminescence property so
we initiate our work to examine the sensing ability HL in presence of
various cations like Co(II), Fe(III), Na(I), Ni(II), Cu(II), Cd(II), Mn(II), Zn
(II) and Hg(II) in HEPES buffer medium (pH 7.4). Under UV lamp
(λ = 365 nm) a significant luminescence colour change from light blue to
turquoise is visualized with naked eye after selective addition of Zn(II)
to HL(Fig. 2). No such remarkable colour changes are obtained for other
analytes. Before work it is prerequisite to check the stability of HL in
HEPES buffer solution at a fixed pH value 7.4 and the stability is
ascertained by means of time–scan UV–Vis experiment (Figure S5 sup-
plementary section). At the same time to resolve the arising question
that why the pH has been fixed to 7.4 during the entire works, fluores-
cence intensity of HL and HL-Zn(II) adduct at different pH are recorded
and the results are summarized in Figure S6. The pictograph revels that
HL-Zn(II) adduct exhibits highest fluorescence intensity at pH 7.4.
The optical colour changes provide an indication of Zn(II) sensing by
HL. For further monitoring the selective sensing phenomena of HL to Zn
(II), in next step the electronic titration is performed among HL and Zn
(II) in HEPES buffer medium with the measurement of binding constant.
Here the alteration of absorbance of HL are recorded after incremental
addition of Zn(II) to it and the experimental results are summarized in
Fig. 3.
0.56 × 0.30 × 0.21
Monoclinic
P21/c
8.6118(5)
25.1200(13)
8.6612(5)
90.00
b/Å
c /Å
0
α
/
β/0
109.144(2)
90.00
γ/0
Volume/Å3
Z
1770.05(17)
2
D
calc /gcmꢀ 3
1.562
F(000)
856
μ
MoK
α
/mmꢀ 1
1.559
Temperature/K
296(2)
Rint
0.0306
Range of h, k, l
ꢀ 7/10, -29/29, -10/10
2.632/24.998
15,502 / 3109/ 2818
3109/0/231
1.020
θ min/max/◦
Reflections collected/unique/ observed [I>2
Data/restraints/ parameters
Goodness of fit on F2
σ(I)]
R1 = 0.0257
wR2 = 0.0699
R1 = 0.0295
wR2 = 0.0735
Final R indices [I>2σ(I)]
R indices (all data)
The figure clearly display that After addition of incremental con-
centration of Zn(II) to HL according to the expectation the absorbance of
HL at 328 nm gradually decrease with generation of a new band at
380 nm and two isosbestic points at 275 nm and 345 nm indicating the
formation of only one species during titration. The calculated binding
constant for HL-Zn(II) adduct is found to be 5.44 × 106 Mꢀ 1 indicating a
Zn(II) complex formation as the ligand has the donar centre to coordi-
nate with the metal centre.
crystal structure of the complex 1 is shown in Fig. 1. The asymmetric
unit contains one Zn, one ligand and two azide anions. The oxidation
state of zinc is (+2). Although two azide anions and one phenoxide
moiety of the ligand coordinate with zinc, but the protonation of the
pipyridyl N-atom of the ligand balances the overall charge of the metal
(zinc) ion of the complex. The protonation of the pipyridyl N-atom of the
ligand was also established from X-ray crystal structure. The protonation
further stabilizes the dinuclear unit introducing hydrogen bonding
interaction within the structure involving the azide nitrogen atoms. The
intra-molecular hydrogen bonding interaction is present between atom
2.5. 1H NMR spectral titration
–
N1 (of pipyridinium moiety) and atom N3 (of azide) involving N H⋯N
◦
Further, 1H NMR studies (Fig. 4) in CDCl3 solvent also give a
conformational support of the formation of coordination modes of the
ligand to Zn(II). The 1H NMR spectrum of the HL exhibits signal at δ
13.25 ppm corresponding to phenolic ꢀ OH as expected. Upon addition
of 0.5 equiv of ZnCl2 the peak intensity of the phenolic ꢀ OH signals
decreases considerable and with 1 equiv addition of ZnCl2 the peak in-
tensity of the ꢀ OH signal is vanished indicating the formation of Zn
–
[N⋯N = 2.989; ∠N H⋯N = 125.78 ] interaction of the ligand. The
central Zn adopts distorted trigonal bipyramidal geometry (τ ~ 0.78)
where two axial positions of the bipyramid are occupied with one oxy-
gen (phenolato) and one nitrogen (imine) atom coming from the ligand
backbone and the three basal sites are coordinated with one oxygen
(phenolato) and two nitrogen atoms of the added azide residue (Fig. 1).
–
There is strong C H⋯ Π interactions (3.044 Ǻ) between chlorophenyl
3

M. Das et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113302
–
Fig. 1. a) Asymmetric Molecular plot of complex 1 b) Polyhedron view of central metal ion c) the C H⋯Π interaction present in the crystal (all the H-atom except
H1B, azide moieties are hidden for better clarity); d) The overall parallel packing of the complex is shown (all the H-atom are hidden for better clarity).
Fig. 2. Under UV the visible colour changes of HL after addition of four equivalent of different metal ions separately showing a significant colour changes for Zn(II)
in HEPES buffer medium.
Fig. 3. UV–vis spectral changes of sensor HL(0.5 × 10ꢀ 4M) in HEPES buffer (pH 7.4) solutions upon addition of Zn(II) ions (0-55 × 10-5M) (b) Benesi–Hildebrand
plot of absorbance titration curve of HL with [Zn(II)].
complex with the ligand HL. At the same time, the azomethine proton
signal shifts to 8.31 ppm from 8.33 ppm. Other aromatic and aliphatic
protons display very little shifting. The results at a glance suggest the
coordination of one azomethine-N and two phenolate-O of HL with Zn
(II) to form the Zn complex which is a good support to the obtained
structure of the complex 1.
2.6. Fluorescence study
After fruitful optical, UV and NMR spectral operation in the next
level we have moved to investigate the fluorescence intensity change of
HL after introduction of different metal ions separately to it (Fig. 5 and
S7 in supporting information file). The graphical representation displays
that the ligand HL revels the emission maxima at 492 nm and the
remarkable enhancement of fluorescence intensity is visualized only in
4

M. Das et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113302
Fig. 4. 1H NMR titration spectra of HL in CDCl3 with the absence and presence of ZnCl2.
information file). To get a quantitative assessment of the relationship
between the changes in fluorescence intensity of HL with the amount of
Zn(II) a detailed fluorometric titration is performed in the same working
medium(Fig. 6). The titration result clearly unveils a considerable
enhancement of fluorescence intensity at 492 nm with blue shift of
12 nm after incremental addition of Zn(II) to HL. The limit of detection
(LOD) is found to be 5.34 × 10ꢀ 8 M, calculate by using the equation
DL = K × σ/S, where K = 3, σ is the standard deviation of the blank
solution and S is the slope of the calibration curves (Fig. 6).
The presence of reversibility is considered to be an important tool in
sensing field for practical application. Here the recognition process
regarding the presence of reversibility of HL is executed by incremental
addition of ethylenediaminetetraaceticacid (EDTA) to HL-Zn(II) probe
and simultaneously the fluorescence intensity is recorded. The titration
graph (Fig. 7) depicts that the fluorescence intensity of HL-Zn(II) probe
at 492 nm gradually decreases after gradual addition of EDTA indicating
the demetallation from the receptor along with the recovery of the free
receptor for practical application of HL as sensor.
Fig. 5. Fluorescence spectral change HL ligand (4 × 10–5 M) at 492 nm in the
presence of different cations showing considerable enhancement for Zn(II) ion
in HEPES buffer (pH = 7.4) solution.
In this Zn(II) detection phenomena by chemosensor HL the impres-
sive binding constant obtain from electronic titration, abolish of ꢀ OH
peck, visualized in NMR titration and the amazing enhancement of
fluorescence intensity along with shifting of spectral positions give a
guaranty regarding the Zn(II) complex formation. During complex for-
mation, to inspect whether the nuclearity of the complex can be
increased or not with the help of bridging ligand, the fluorescence in-
tensity changes of HL-Zn(II) adduct is further monitored with separate
introduction of several bridging ligands to this adduct and the fluores-
cence intensities are recorded (in Figs. 8 and S9).
presence of Zn(II) which is well
Accordance with the introduction of d10 system to HL showing
chelation enhanced fluorescence, CHEF. After coordination, fluores-
cence emission arises as a result of π-π* ligand-centered charge transfer
(LCT) and the role of the zinc ion is to freeze the favorable re-emissive
conformation. This is the most probable path way, followed by the
ligand HL to show strongly emissive power upon zinc coordination.
Interestingly the exposure of other analyts towards the spectral change is
found to be negligible. Collaterally upon addition of the mixture of all
aforementioned metal ions to HL (competitive testing) the emission
spectra of the ligand nearly do not alter, except on exposure to Zn(II),
showing a specific enhancement in the fluorescence intensity and this
demonstrate the efficient and selective sensing behaviour of HL for Zn
(II) over other competitive essential metal ions (fig S8 in supporting
2.7. Response of complex 1 towards Al(III) by fluorescence spectroscopy
The newly synthesized Zn complex possesses potential luminescent
character as perceive from the fluorescence responses. The luminescent
5

M. Das et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113302
Fig. 6. (a) Impressive fluorescence intensity changes of HL (5 × 10ꢀ 5 M) upon incremental addition of Zn(II) (1.0-12.0 × 10ꢀ 5 M) in HEPES buffer (pH = 7.4)
solution (λex=360 nm, λem=492 nm). Inset: visual colour change observed upon addition of Zn(II) to HL solution under UV light (λ = 365 nm) (b) Change of
Fluorescence emission intensity of HL at 492 nm as a function of Zn(II) concentration for detection limit calculation.
Fig. 7. (a) Fluorescence spectral change of HL (30
μM) upon incremental addition of Zn(II) followed by EDTA in HEPES buffer solution at pH 7.4 and (b) The
variation of fluorescence intensity at 492 nm after alternative addition of Zn(II) and EDTA to HL solution.
selectively detect Al(III) among several cations like Fe(III), Na(I), Ni(II),
Cu(II), Cd(II), Mn(II), Mg(II), Al(III) and Hg(II) in HEPES buffer medium
(pH 7.4). This detection phenomenon is optically ascertained along with
florescence changes in HEPES buffer medium (Fig S11 in supplementary
information file). The fluorometric titration profile of complex 1
(4 × 10ꢀ 5 M) by Al(III) (0.3ꢀ 6 × 10ꢀ 5 M) (Fig. 9) in HEPES buffer so-
lution at pH 7.4 illustrates a rapid diminish of fluorescence intensity
after addition of Al(III) to complex 1 with blue shift of 11 nm. The
quenching constant value (KSV) is calculated to be 6.7 × 104 M-1 using
Stern-Volmer equation, F◦/F = KSV[Q] +1; where F◦ and F are the
emission intensity in absence and presence of analyte, [Q] is the con-
centration of the added analyte and KSV is the quenching constant value.
The detection limit of complex 1 for Al(III) ion was found to be
1.64 × 10ꢀ 8 M (shown in figure S12 in supporting information file).
From the quenching constants and detection limit values it can further
be conclude that the complex 1 has Potential and selective detection
efficacy towards Al(III) ion. The quenching constant values are very
much comparable with other Al(III) sensing organic probes [36–38]
indicating the strong bonding association between the studied analyte
and complex 1.
Fig. 8. Change of fluorescence intensity of HL-Zn(II) adduct in presence of
different bridging ligand (10
tensity in presence of azide.
μM) showing enhancement of fluorescence in-
property of complex 1 is further explore in different solvents namely
DMSO, THF, EtOH, MeOH, DMF, DCM and MeCN and the fluorescence
intensity change of the complex 1 in different solvents are shown in fig
S10 in supporting file.
From the figure one can easily conclude the luminescent property of
the complex depends on the nature of the solvents and in DMSO solvent
it exhibits its highest fluorescence intensity. At the same time it exhibits
a strong emission at 459 nm upon excitation of 370 nm in HEPES buffer
medium at pH 7.4. Due to the exposure of photoluminescence property
complex 1 can be re-utilized for the second step detection procedure.
Keeping this point in mind we further move for the demonstration of
complex 1 by utilizing its sensing behavior towards metal ion where it
2.8. Time resolved fluorescence decay
For understanding the fluorescence quenching mechanism of com-
plex 1 after the addition of Al(III) the time resolved fluorescence decay
study have been carried out with the concentration variation of Al(III) in
HEPES buffer solution (Table S1 in revised supporting file). The Life time
decay profiles of complex 1 in presence and absence of Al(III) are
condensed in Fig. 10. The life time fluorescence decay curve of complex
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M. Das et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113302
Fig. 9. (a) Alteration of fluorescence spectra of complex 1 (4 × 10ꢀ 5 M) at 459 nm upon incremental addition of Al(III) solution (0.3-8.0 × 10ꢀ 5 M) in HEPES buffer
(pH = 7.4) solution (λex=360 nm, λem=459 nm). Inset: visual colour change observed with addition of Al(III) to complex 1 solution under UV light (λ = 365 nm) (b)
Stern Volmer plot for the detection of quenching constant of complex 1 after addition of Al(III).
2.9. UV vis spectroscopic study
The selective detective phenomena of complex 1 towards Al(III) is
further assessed by performing UV–vis spectral titration with the
determination of the respective host-guest binding constants value by
adopting the same procedure like Zn(II) sensing by HL.
Fig. 11 display the titration result. As expected the peak at 382 nm of
free complex gradually decreases with the appearance of a new band at
349 nm after addition Al(III). An isosbestic point at 367 appeared on the
titration curve indicating the presence of equilibrium which further
imply the conversion of the free chemosensor to its bimetal complexes.
The binding constant for complex 1 – Al(III) adduct is obtained to be
1.37 × 105
M
ꢀ 1. During sensing of Al(III) by complex 1 the 1:2 ratio
determination from the Jobs plot rationalized the fact of Zn/Al bimetal
complex formation during sensing (S13 in supporting information file).
To optimize the Zn-Al bimetal structure DFT study has been performed.
2.10. DFT calculation for optimization of Zn-Al complex structure
Fig. 10. Time-resolved fluorescence decay of Complex 1 in the absence and
In the foregoing discussion we have successfully explained the se-
lective sensing phenomena of complex 1 towards Al(III) by utilizing
several indicating the formation of a single complex during detection. In
the next step the structural details of the newly formed complex should
be highlighted. The ground state geometry of the complex between
complex 1 and Al(III) is optimized with the help of DFT study. From the
experiment (JOBS plot) It is seen that during sensing Al(III) can fabricate
a complex by maintaining 2:1 stoichiometric ratio and this is an indi-
cation regarding the formation of a Zn-Al bimetal complex. The theo-
retically optimized structure of the Zn-Al complex is exposed in scheme
2 .
presence Al(III) via concentration dependent manner in HEPES buffer.
1 and Complex 1-Al(III) adduct with variation of Al(III) concentration is
fitted by mono exponential function with acceptable value of
χ
2. The
average life time value (
τ
1) for complex 1 is found to be 8.07 ns and upon
addition of Al (III) with different concentration to the system no sig-
nificant changes are visualized. It indicates static quenching mechanism
of the complex 1 after addition of Al(III).
Fig. 11. UV–vis spectral changes of sensor complex 1 (8 × 10ꢀ 5 M) in HEPES buffer (pH 7.4) solutions upon addition of Al(III) ions (0-16 × 10ꢀ 5M) (b) Bene-
si–Hildebrand plot of absorbance titration curve of Complex 1 with Al(III) for determination of binding constant.
7

M. Das et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113302
Scheme 2. preparation and structure of Zn/Al bimetal complex during sensing of Al(II) by complex 1.
The scheme clearly shows that Al(III) holds the centre of an octa-
observed with red shift and at the same time a new band is appeared at
349 nm which is a good agreement with the theoretical results where
two band is seen at 384 nm (f = 0.04, HOMO-2→LUMO+2) and 347 nm
(f = 0.0858, HOMO-1→LUMO+3). In Zn complex, electronic transfer
takes place from benzene moiety to the hydrocarbon tail whereas in the
Zn-Al bi metal complex, electron transfer takes place from the metal
moiety to the benzene moiety.
hedral geometry where two pendent azides of Zn(II) center (complex 1)
act as a bridge among Zn(II) and Al(III)(Zn/Al bimetal complex).
A TDDFT study has been performed in aqueous solvent for complex 1
and newly formed bi metal complex to judge the absorption phenomena
and FMO features. By comparing the theoretical and experimental
UV–vis spectra of complex 1 one can easily conclude that the complex 1
exist in dimer state. In addition after incorporation Al(III) into complex
1 a significant decreases of energy gap between the HOMO to LUMO is
visualized (shown in Fig. 12). As a result of which the absorption
maxima shifted to the longer wavelength. It is also observed that the
nature of absorption is also different for two different complexes. For
complex 1 experimentally two absorbance is noticed centered at 382 nm
and 336 nm. Theoretically these two peaks appear at 373 nm (f = 0.01,
HOMO→LUMO) and 346 nm (f = 0.1175, HOMO→LUMO+3) which is
well fitted with the experimental result. On the other hand after incor-
porating Al(III) a hypochromism of absorption maxima at 382 nm is
2.11. Molecular logic gate
Incremental addition of Zn(II) into the probe HL emission intensity
enhanced significantly while further introducing micro molar NaN3 the
emission intensity increases impressively. Now Zn(II)-NaN3 adduct can
act as an chemical input (A) into the HL. Again ejecting micro molar Al
(III) as chemical input (B) into the Zn(II)-NaN3 adduct fluorescence in-
tensity further quenched. On the basis of this spectral study we forward
our direction of this experiment to investigate its use in numerous logic
Fig. 12. Frontier orbital involved in complex 1(Zn complex) and Zn/Al bimetal cmplex.
8

M. Das et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113302
gates by successive addition of inputs such as Zn(II)-NaN3 adduct and Al
(III). This molecular switching behavior of the receptor HL towards
guest inputs Zn(II)-NaN3 adduct, Al(III) instigate us to construct a
Boolean logic gates and arithmetic calculation at molecular level by
monitoring their emission intensity at 459 nm as output. We had
investigated this experiment in the manner of emission intensity
enhancement for complex only and quenching of emission intensity
through sensing Al(III) ignoring emission intensity of the probe because
HL had a potential emission intensity below the threshold value of the
Zn(II) complex, its output assumed as “0′′. So to construct a simpler
molecular logic device we initiated this experiment from ejection of Zn
(II)-NaN3 adduct into HL. We considered Inputs as “1′′ for their presence
and “0′′ for their absence. Outputs are considered as “1′′ when the
emission intensity is above the certain threshold value (25 % maximum
of the complex) and “0′′ when it is below the threshold value (Fig. 13a).
When none of the input was introduced to the probe, no character-
istic emission intensity was recorded (ignoring the emission intensity of
HL) indicating output signal “0′′ (off state). Addition of the input A Zn
(II)-NaN3 to the probe HL fluorescence intensity enhanced indicating
output signal “1′′ (on state) at 459 nm. Further addition of input B Al
(III), no significant enhancement in emission intensity was visualized, so
results no output signal. Again in the presence of both inputs simulta-
neously, no output signal observed denoted as “0′′ (off state). It is
noticed that HL shows the high emission intensity output signal in the
manner with addition of input A. It appears to consider the importance
of “AND” operation and “NOT” operation with input B. Again when the
both inputs are introduced to HL then the output emission intensity
value decreases, suggesting the off state as per the truth table (Fig.13b).
This combined experiment is an “INHIBIT” logic gate, which consists a
particular arrangement of logical functions “AND” and “NOT” (Fig.13c).
considered the binary state “0′′ (Fig. 14b). So, it can be concluded that
we have successfully built a successive logic circuit on the basis of
“Write ꢀ Read ꢀ Erase ꢀ Read” property. Hence we can say that by
using the same solution of the complex the write ꢀ erase cycles could be
repeated many times without significant decrease in emission intensity.
In the summarization, this system could be properly utilized in tech-
nological field. So, by applying molecular successive logic function
circuits can be designed which will show a comparable conduct to the
logic devices of conventional semiconductors and it can be expected that
it be a better technique for construction of molecular microprocessors of
integrated circuits.
2.13. Application of HL for the sensing of Zn (II) in live cell
In the previous discussion it is already highlighted the favorable
binding affinity of HL with Zn (II) in HEPES buffer medium via turn on
fluorescence. But this achievement applies a force to explore HL with
respect to the fluorescence imaging of live cell especially for selective
detection of intracellular zinc and the fluorescence imaging study has
been performed. The images are shown in Fig. 15.
However to materialize these compounds it is prerequisite to check
the cytotoxic effect of HL, and ZnCl2 on normal cell line(HEK cell)(S14
in supplementary file) as well as in cancer cell line (HeLa)by using MTT
assay study in does dependent manner. Up to 100
μM, these two
aforementioned materials exhibit no cytotoxicity suggesting the main-
tenance of the working concentration below 100 M. The fluorescence
μ
microscopic study reveals more or less no fluorescence when the cells
are treated with only HL. Upon incubation with HL followed by ZnCl2
striking switch ꢀ ON fluorescence (Fig.15) is appeared inside the cells.
Here an intense green fluorescence is observed in the cytoplasmic region
but not in the nucleus. Same experiment is done in presence of azide
also. But no significant change is visualized in image quality (S15 in
supporting file). Before this study in presence of azide, the MTT assay of
complex 1 on the two aforementioned cell line is again perform
(Fig. S14). Results Show that complex 1 do not exert any adverse
2.12. Molecular memory device
All the information gathers by adopting the method is well equipped
and it can be further congregated through successive logic Circuits.
These circuits reserve the following response loop. In this memory de-
vice one input is regarded as the “memory element” by considering the
output signal as input. The memory device works on a binary logic gate
function: either it will be “0′′ or “1′′ alternatively the two crisp states.
The model we have introduce for the advancement of a significant
mimicking of the memory element, we take Zn(II)-NaN3 adduct and Al
(III) remarked as the set (S) and reset (R) inputs correspondingly and the
photoluminescence was recorded at 459 nm as the output signal
(Fig. 14a). When Zn(II)-NaN3 Adduct acts as input In this memory
function, the device recognized the binary state “1′′ while under the
reset situation Al(III) acts as input, As a result we noticed that the device
cytotoxic effect up to 100 μM concentration The fluorescence micro-
scopic analysis strongly implies that the HL can readily cross the
membrane barrier, permeate into HEK cells, and rapidly detect intra-
cellular Zn(II). Additionally it is disclose here that the bright field im-
ages of treated cells do not expose any gross morphological changes,
indicating the cell viability. All these fact open up the avenue for future
in vivo biomedical applications of the sensor HL.
3. Conclusion
From the all above discussion it can be concluded that the
Fig. 13. (a) The emission intensity variation graph of Receptor and Complexes (b) truth table for advance-level molecular logic gate, and (c) circuit diagram of
the Complex.
9

M. Das et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113302
Fig. 14. (a) Consecutive logic circuit of a memory unit and (b) schematic presentation of the reversible logic operation for the memory element with a
write ꢀ read ꢀ erase ꢀ read kind of manners.
Fig. 15. Fluorescence confocal microscopic images of HL probe in presence and absence of Zn(II).
condensation reaction of 5-Chloro salisylaldehyde and 2 amino ethyl
piperidine produces a Schiff base chemosensor which can specifically
detect Zn(II) among several cations via turn on fluorescence in aqueous
solution. The sensing event is further standardized by adopting the
electronic and NMR titration methods. The significantly high binding
constant value between HL and Zn(II), obtain from UV vis spectroscopic
titration technique assure the formation of a Zn(II) complex. To get a
strong idea whether any higher nuclearity complex can be obtained or
not with the help of bridging ligands during sensing, the fluorescence
study of HL-Zn(II) adduct is further monitor in presence of several
bridging ligand. Interestingly in presence of azide a significant alteration
of the fluorescence intensity is observed. Getting inspiration from
considerable high binding constant among HL and Z(II) ion and notable
fluorescence changes of HL-Zn(II) in presence of azide a reaction is
conducted between HL and Zn(II) in presence of azide and as a result of
which a binuclear zinc complex having azide as secondary anionic res-
idue is produced. According to the hints, getting from fluorescence
experiment during sensing of HL to Zn(II), complex 1 exhibits an
impressively high photo luminance property which helps for further
utilization of complex 1 for selective detection of Al(III) among different
cations with the formation of Zn/Al bimetal complex whose structure is
authenticated by DFT study. Furthermore the two step sensing phe-
nomena is utilized to formulate an INHIBIT logic circuit which is addi-
tionally extended to construct a molecular memory device. At last to
confirm the biomedical application of HL as a sensor the fluorescence
microscopic image of HEK cell is taken after incubation with HL fol-
lowed by ZnCl2 and striking switch ꢀ ON fluorescence is appeared inside
the cells confirming its in vivo use as sensor.
4. Experimental section
4.1. Materials and physical measurements
All the reagents and solvents used in this synthesis were commer-
cially available. Reagent grade chemicals were used in this experiment.
So no further purification was needed. 2-aminoethyl piperidine, 5-
10

M. Das et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113302
chloro Salicylaldehyde, sodiam azide, were procured from Sigma
Aldrich Chemicals. ZnCl2 was purchased from Merck. Elemental ana-
lyses were performed using a Perkin-Elmer 240C elemental analyzer. For
FTIR spectrum data collection ATR mode Bruker Tensor -27 was used.
Electronic Absorption spectra were collected using Perkin Elmer UV–vis
Lambda 365 spectrophotometer. The fluorescence experiments were
performed by using Parkin Elmer Fluorescence spectrometer FL6500.
complex 1 were poured separately to HEPES buffer (pH 7.4) to attain the
experimental concentration. After that a required amount of metal ion
stock solutions (5 × 10ꢀ 3 M) were added to it using a micropipette and
Spectral data were recorded.
4.6. Time-resolved fluorescence study
Lifetime measurement study were carried out with a Horiba Jobi-
nYvon Fluorocube-01 NL timecorrelated single photon counting
(TCSPC) set up where a picosecond delta diode (DD-
375 L) worked at λex =370 nm and a repetition rate of 1 MHz as
excitation source.
4.2. Synthesis of ligand (HL)
0.1 mL (0.7 mmol) of 2-aminoethyl piperidine was added separately
to the methanolic solution of 0.11 g (0.7 mmol) 5- Chloro Salicylalde-
hyde under sturring condition. After complete addition of the amine to
aldehyde (1:1) the resultant solution was refluxed for 3 h. The colour of
the solution turned to yellow from colourless. The clear solution was
4.7. Computational details
directly used for complex synthesis.
To understand the Zn/Al bimetal complex formation and reason
behind the change of absorption properties the Density Functional
Theory (DFT) study using Gaussian 09 [46] package of program have
been performed. The structure of complex 1 and Zn-Al bimetal complex
have been optimized different complexes to their respective electronic
ground states without any symmetry constraints. Optimized structures
have been checked by frequency computation. To compute the optical
absorbance, Time Dependent DFT (TD-DFT) [47] computation is per-
formed. As the working solvent is HEPES buffer so water solvent is also
used in computational study. To include the effect of salvation, contin-
uum polarized model (CPM) [48] is used in all calculations. For DFT
calculation, B3LYP [49] functional is used with LANL2DZ basis [50].
LANL2DZ basis which includes the relativistic correction to the basis set
is used due to the presence of Zn and Al metal. It is noticed that the
B3LYP functional with LANL2DZ basis gives very good results for ge-
ometry optimization as well as electronic absorption spectra [51]. In the
optimized structure the colour used for the identification of the different
atoms are as follows. Dark gray for C, Light gray for H, blue for N, green
for Cl, blue gray for Zn(II) and light pink for Al(III)
ꢀ 1
–
–
FT-IR (KBr pellet):
ν
(C N) 1734 cm
,
ν
(O H) 3456,
ν
(skeletal
–
vibration) 1440 cmꢀ 1, 1540 cmꢀ 1
ν
(Aromatic C N stretch) 1377 cm
.
ꢀ 1
–
UV–vis : 325 nm(ᴨ- ᴨ*), 420(n- ᴨ*).
4.3. Synthesis of [Zn2(HL)2(N3)4]
0.095 g (0.7 mmol) CuCl2 was dissolved in methanol (15 mL) and
added to 25 mL methanolic solution of HL1 ligand (0.248 g, 0.7 mmol)
under continuous stirring condition. The resultant solution was then
refluxed for additional 3 h. The light yellow coloured solution formed
was filtered and from the filtrate single crystals were appeared within
three day by slow evaporation. Yield 0.436 g (75 %). Anal. Calc.: For
C
28H38Cl2N16O2Zn2 (832.42): C 40.36; H 4.56; N 26.90 % Found: C
ꢀ 1
–
40.39; H 4.52; N, 27.10 %., FT-IR data (KBr pellet):
ν
(C N) 1705 cm
,
–
ν
(skeletal vibration) 1427 cmꢀ 1,1310 cmꢀ 1.1H NMR [D6-DMSO, 25 ◦C]:
δ = 8.56(s, 2 H), 7.14ꢀ 7.58(m, 6 H), 3.10–3.64 (t, 4 H) 2.69–2.91 (m,
12 H), 1.46ꢀ 1.63(m, 12 H).
Caution! Azides are potentially explosive and should be handled in
small amounts and with proper precautions.
4.8. Fluorescence imaging study
Fluorescent imaging studies were done as per Goswamiet al., [52]
with modifications. Human embryonic kidney 293 [HEK-293] cells were
seeded in a 24 well cell culture plate at a density of 5 × 104 cells per well
using Dulbecco’s Modified Eagle’s medium (DMEM) supplemented with
10 % Foetal bovine serum (FBS).Cells were incubated at 37 ◦C in an
atmosphere of air with 5% CO2.Next day, the cultured media were
removed, and cells were washed with phosphate-buffered saline (PBS,
pH 7.4) twice. The cells were incubated with ligand HL (50 mM) with
PBS (pH 7.4) for 10 min, subsequently added ZnCl2 (25/50 mM), and
washed with PBS. The cells were observed under a fluorescence micro-
scope (DeWinter) at 400X resolution and captured images using the
microscope camera. The images were analyzed using Image J 1.49v
software.
4.4. X-Ray crystallography
geting the structures of Zn complex single crystal X-ray diffraction
(SCXRD) analyses study was carried out using Bruker D8 QUEST
diffractometer. The crystallographic data and refinement parameters of
the complex were given in Table 1. Reflection data were measured at
296 K, using graphite monochromatic MoKα radiation with a PHOTON-II
detector. The collected data were reduced using the program APEX-III
and an empirical absorption correction was carried out using SADABS.
[42] The structure was solved using direct methods and refined using
the full-matrix least-squares method on F2 using the WINGX software
package. [43,44] The molecular graphics were created using SHELXT
[45]. All non-hydrogen atoms were refined with anisotropic parameters.
CIF file of complex for the structure reported have been deposited with
the Cambridge Crystallographic Data Centre (CCDC). CCDC- 2,032,970,
contains the supplementary crystallographic data for the paper. Copies
of the data can be obtained, free of charge on application to the CCDC,
12 Union Road, Cambridge, CB2 1EZ UK [Fax: 44 (1233) 336 033
e-mail: deposit@ccdc.cam.ac.uk].
The experiment is repeated in presence of presence of azide also.
4.9. In vitro cytotoxicity assay
Anti proliferative effect of ligand HL and complex 1 on HeLa cell line
(human cervical carcinoma cell) were measured by MTT assay. Cells
were seeded at a density 2 × 105 cells/well in a 24 well plate. For
normal, we used HEK cell line at same concentration. After 24 h of cell
seeding, cells were exposed to complex and ligand at different concen-
tration for 24 h. After incubation, cells were washed with 1 X PBS.
Thereafter, they were treated with 0.5 mg/mL MTT solution (SRL) and
incubated for 3ꢀ 4 hrs at 37 ◦C until a purple colour formazan product
developed. The resulting product was dissolved in DMSO and OD was
measured at 570 nm using a micro plate reader (Biorad). Rate of survival
was calculated using following formula. [52,53]
4.5. Sample preparation for fluorescence and UV spectral study
The stock solution of ligand HL and complex 1 (3 × 10–2 M) was
prepared in methanol and DMSO medium respectively and the stock
solution of various metal ions (5 × 10–3M) was prepared in triple
distilled water using their perchlorate or chloride salts. For entire fluo-
rescence and absorption studies, in quartz optical cell of 1 cm optical
path length appropriate amount of stock solution of HL (10–2 M) and
11

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