Spectroscopic characterization and photophysical properties of schiff base metal complex

Article abstract of DOI:10.1016/j.molstruc.2018.06.010

In the present work, Schiff base ligand N,N-bis(salicylidene)-(3,3′-diaminobenzidine) and its zinc complex were synthesized. The coordination of Schiff base ligand with zinc metal ion seems to occur via N– azomethine followed by the deprotonation of OH gr

Full text of DOI:10.1016/j.molstruc.2018.06.010

Accepted Manuscript
Spectroscopic characterization and photophysical properties of Schiff base metal
complex
N.K. Gondia, S.K. Sharma
PII:
S0022-2860(18)30711-7
10.1016/j.molstruc.2018.06.010
MOLSTR 25297
DOI:
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Accepted Date:
16 February 2018
04 June 2018
Please cite this article as: N.K. Gondia, S.K. Sharma, Spectroscopic characterization and
photophysical properties of Schiff base metal complex, Journal of Molecular Structure (2018), doi:
10.1016/j.molstruc.2018.06.010
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ACCEPTED MANUSCRIPT
Spectroscopic characterization and photophysical properties of Schiff base metal complex
N. K. Gondia and S. K. Sharma*
Department of Applied Physics, Indian Institute of Technology (Indian School of Mines),
Dhanbad 826004, India
*Corresponding author E mail: sksharma.ism@gmail.com
Tel: +91-326-2235412; Fax: +91-326-2296563
Abstract:
In the present work, Schiff base ligand N,N-bis(salicylidene)-(3,3'-diaminobenzidine ) and its
zinc complex were synthesized. The coordination of Schiff base ligand with zinc metal ion
seems to occur via N- azomethine followed by the deprotonation of OH group. The nuclear
magnetic resonance peaks as well as important bands in the Fourier transform infra-red spectra
are discussed in relation to the structure of the ligand and complex. The optical properties like
absorption and fluorescence were studied to get the idea about the transition among molecular
orbitals as well as quantum yield. Colour coordinates of complex fall in the blue region. HOMO-
LUMO energy gap was used to determine various parameters. Thermal studies were also
performed to determine the thermal stability of the complex in the range from room temperature
to 500ºC.
Keywords: Schiff base, Absorption, Fluorescence, HOMO-LUMO
1. INTRODUCTION
During the past decades metal complexes of Schiff base are widely studied due to having good
luminescence properties, high conductivity and thermal stability. Schiff-base ligands coordinated
with metals usually linked to the imine nitrogen and aldehyde group. Luminescence properties of
the aromatic bridged azomethine metal complexes have already been reported. Among many of
the coordination compounds, zinc metal based Schiff base complexes have been found to show
the good luminescence properties due to d10 electronic configuration [1,2]. Salicylaldehyde schiff
base Zn complex generally exhibit photoluminescence as well as electroluminescence properties
and formed the basis of materials used in various display and lighting devices either as electron
transport layer or emissive layer.
The structures of salicylaldehyde Schiff bases are similar to those of 8-hydroxyquinoline ligands,
which have at least one hydroxyl group, a coordination nitrogen atom and a delocalized system.
The salicylaldimine ligand forms the basis of an extensive class of chelating ligands that has very
popular use in the coordination chemistry of transition and main group elements [3]. Metal
complexes particularly having transition metal ions at inner coordination sphere play an
important role in achieving color tunable properties with high value of luminous efficacy of
radiation [4,5]. Schiff base complexes of transition and non-transition metals have been used
extensively for various optoelectronic applications due to their ease of synthesis, wide range of
complexation ability and many useful properties like electron transportation, higher thermal
stability and ease of sublimation. These complexes have ability to show the tunable properties by
varying the substituent derivatives of the coordinated metal ion and enhance their activities [6].
The fluorescence intensity and quantum yield of metal complexes change with respect to ligand
after complexation with various metals. The coordination complex derived from condensation
reaction of diamines and salicylaldehyde generally exhibit good luminescence properties [7] and
can be used as light emitting material in organic light emitting diodes and fluorescent sensors.

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The addition of electron donating group into the salicylidene moiety of Zn(II) complex results in
the enhancement of quantum yield and emission of light in blue region [8-10].
Efforts have been put to develop a new cost effective material with ease of synthesis and good
luminescent properties. The present work describes the synthesis and spectroscopic
characterization of Zn(II) complex derived from 3,3'-diaminobenzidine and salicylaldehyde.
Absorption and fluorescence spectra were recorded to get the quantum yield and CIE parameters.
Zn complex was found to exhibit higher luminescence intensity compared to ligand. CIE
parameters, results of thermal studies and HOMO-LUMO energy gap suggest the suitability of
synthesized complex in organic light emitting devices.
2. EXPERIMENT
The reagent used in the present studies 3,3'-diaminobenzidine and salicylaldehyde were
purchased from Sigma Aldrich. Zinc nitrate was purchased from Merck chemical company.
Ethanol was used as a solvent. All the solvents and metal salt were used without further
purification. The Schiff-base ligand and its zinc complex were synthesized by the condensation
reaction method.
2.1 Synthesis of ligand and complex
3,3'-diaminobenzidine (100 mg, 0.466 mmol) was dissolved in 20 ml of ethanol in a round
bottom flask. Salicylaldehyde (0.198 ml, 1.866 mmol) was added with continuous stirring. The
mixture was refluxed for three hours and then allowed to cool. The final product was collected
after filtration and orange colour Schiff base ligand N,N–bis(salicylidene)-(3,3'-
diaminobenzidine) was obtained.
The zinc complex was prepared by mixing the synthesized ligand (50 mg, 0.079 mmol) in
ethanol (20 ml) and zinc nitrate (30.02 mg, 0.158 mmol) with continuous stirring [11]. The
yellow precipitates confirm the formation of complex. These precipitates were filtered and
washed several times with ethanol and then dried over silica gel to obtain the desired complex.
The following are the proposed structures (Fig.1 and Fig.2) of the prepared ligand and complex:
Fig.1 (a) Chemical structure (b) Optimized molecular structure of ligand
Fig.2 (a) Chemical structure (b) Optimized molecular structure of complex

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2.2 Measurements:
Fourier Transform Infra-red spectra were obtained in KBr background in the wave number range
4000-400 cm-1 using Perkin Elmer make spectrum RX1 FTIR 2000 spectrometer. Nuclear
Magnetic Resonance spectra were recorded on JEOL JNM-EXCP 400 MHz NMR spectrometer.
Absorption spectra were obtained on Agilent Cary 5000 UV-VIS NIR spectrophotometer.
Fluorescence emission spectra were measured on Hitachi F-2500 Fluorescence
spectrophotometer in the wavelength range 200-800 nm. HOMO-LUMO and other quantum
chemical parameters were computed by Gaussian computational chemistry program using semi
empirical and DFT approach . Quantum yield of ligand and complex was calculated by using the
standard quinine sulphate which has a fixed and known fluorescence quantum yield. Photometric
parameters were calculated using colour calculator programme developed by Osram Sylvania.
3. RESULTS AND DISCUSSION
3.1 FTIR Studies
The Fourier Transform Infra-red (FTIR) spectra of ligand and complex are shown in Fig.3. The
positions of functional groups for the prepared ligand and complex are listed in Table 1. The
absorption band at 1613 and 1610 cm–1 in ligand and complex is due to presence of azomethine
group. Bands at 3460 and 3425 cm-1 are due to presence of water molecule. A slight shift in peak
position was observed after complex formation. The shifting of bands corresponding to nitrogen
atom of amine (azomethine) group, hydroxyl group of salicylaldehyde and C-H indicates the
coordination of metal to the ligand resulting formation of complex [12-13].
Compound O-H
C=N
C-OH C-H
Ligand
3460
3425
1613
1610
1268
1182
804
828
Complex
Fig.3 FTIR spectra of ligand and complex
Table 1: Position of functional groups (cm-1)
in FTIR spectra
3.2 NMR Studies:
The Nuclear Magnetic Resonance (NMR) spectra of ligand and complex (Fig.4 & Fig.5) were
obtained with DMSO as a solvent. In HNMR spectra it was found that the OH signal, which
appears at 13.02 ppm in the ligand gets disappear in the complex indicating removal of OH
proton during chelation process with metal ion. Two doublets appears at d 7.69 (H), 7.63 (H)
belong to the aromatic proton of benzidine after complexation both shifted to 7.86 (H), 7.45(H)
slightly and downfield and upfield respectively. Two triplets at 7.93 (H), 7.43 (H) ppm are
originating from salicyl moiety. Two doublets and one singlet appear at multiplet 7.01 (H) ppm
of salicyl and benzidine moiety respectively [14,15]. The proton signals obtained for ligand and

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complex were found to be in the expected region and shows good agreement with the values
obtained for HC = N and –OH signals.
In 13C spectrum of ligand, carbon signal appear at 163.70 (C-N), 143.02, 141.45, 138.45, 133.52
and 132.55 ppm which indicate the carbon signals of benzidine moiety, 164.80 (C-O), 160.43
(C=N), 125.80, 120.09, 119.36, 117.97 and 116.68 ppm indicate to the carbon signal of salicyl
moiety. After complexation with Zn(II), 13C signal shifted to upfield and downfield which
appeared at 172.35, 163.46, 162.57, 139.76, 138.49, 136.29, 134.40, 125.65, 123.12, 119.49,
116.87, 114.54 and 113.00 ppm. The chemical shift is the change in position of signals in NMR
spectra. Nucleiꢀwhich exhibit downfield shift feelꢀstronger magnetic field due
toꢀtheꢀremovalꢀofꢀelectronꢀdensity whereas upfield shift in nuclei indicate the addition of
electron density.
HC1
HL1
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
ppm
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
ppm
Fig.4 (a) HNMR spectra of Ligand
Fig.4 (b) HNMR spectra of complex
HL1
HC1
180
175
170
165
160
155
150
145
140
135
130
125
120
115
110
ppm
180
175
170
165
160
155
150
145
140
135
130
125
120
115
110
105
ppm
Fig.5 (a) CNMR spectra of Ligand
Fig.5 (b) CNMR spectra of complex

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3.3 FESEM and EDX Studies:
Field Emission Scanning Electron Microscope (FFSEM) images and Energy Dispersive X-ray
(EDX) spectra of ligand and complex are shown in Fig.6 (a, b). The ligand shows a rod like
morphology. The complex shows fiber like shape with variable diameter. Significant change in
surface morphology was observed after introduction of zinc ions [16]. It was observed that the
rod diameter decreases after formation of complex. The EDX spectra were used to measure the
elemental composition of C, N, O and Zn in the synthesized ligand/complex [17] and the values
in atomic /weight percentage are shown in Table 2.
Ligand
Element Wt% Atomic%
67.06
76.62
C
N
O
7.73
7.57
17.83
50.58
6.21
15.29
70.49
7.42
Complex C
N
O
15.80
18.83
16.53
4.82
Zn
Fig.6 FESEM images and EDX spectra of
a) Ligand b) complex
Table: 2 Elemental composition
of ligand and complex
3.4 TEM Studies
TEM studies were performed in order to get the idea of particle size and morphology. TEM
images of ligand and complex are shown in Fig. 7 (a, b). These images show the change in
morphology and particle size after complex formation. The agglomeration of particles are
observed in case of complex. The average particle size of ligand and complex was found to be
130 and 60 nm respectively.
Fig.7 (a) TEM image and size distribution of ligand

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Fig.7 (b) TEM image and size distribution of complex
3.5 UV- Vis studies:
The electronic spectra of ligand and complex in the UV-visible region were recorded in DMSO
solvent with 10-2 M concentration and the bands assigned are shown in Fig. 8. The absorbance
bands in the spectrum of ligand observed at 274 nm and 350 nm are due to n-π* transition of
lone pair of electron of azomethine group and anti-bonding π orbital. After coordination,
significant bathochromic shift was observed in the spectrum of Zn complex. The Zn complex,
shows shifting of band towards red region and were observed at 311 nm and 420 nm. Slight shift
in the position of these bands was observed as a result of complexation. This red shifting of band
is an indication of intermolecular charge transfer in complex due to π electron conjugation
between ligand and Zn2+ ion [18,19]. Shifting of absorption bands from 274 nm (ligand) to 311
nm (complex) and 350 nm (ligand) to 420 nm (complex) are due to the participation of lone pair
electrons of azomethine nitrogen and conjugation effect with π bond of benzene ring. This is due
decrease in energy which cause bathochromic shift. This shifting of bands also confirm the
formation of complex from its ligand.
Fig.8 UV-Vis spectra of ligand and complex Fig.9 Fluorescence spectra of ligand and complex
3.6 Fluorescence studies
The fluorescence spectra of ligand and complex (Fig.9) were recorded at room temperature in the
range 400-800 nm. Both the developed compounds show intense blue emission after excitation
with 343 nm wavelength source. The fluorescence emission intensity of complex is higher than
the ligand. This increase in intensity is due to the chelation of ligand to metal ion center.

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Chelation of ligand to metal ion via complexation results an increment in rigidity of complex and
hence loss of energy reduces [20]. This is due to the occurrence of less number of non-radiative
intraligand transitions from excited state to ground state.
The fluorescence intensity of ligand may probably be quenched by the occurrence of a photo
induced electron transfer (PET) process due to the presence of lone pair of electrons in the
ligand. PET is a process in which an excited electron is transferred from donor to acceptor. After
the electron transfer, fluorescence intensity of acceptor is quenched due to formation of radical
anion. PET process is prevented after the complex formation which results intensity
enhancement by the coordination of Zn(II) ions.
3.7 Quantum yield:
The fluorescence quantum yield (ϕ) of ligand and complex was calculated by comparative
William’s method using the following equation [21,22]. The detail can be seen in the previous
work by authors [23].
2
η푆2
푚푆
ϕS= ϕST
2
η푆1
푚푆푇
Here, the terms
and
represent the slopes of samples and standard reference.
and
휂푆1 휂푆2
푚푆
푚푆푇
are the refractive index of solvents (H2SO4 and DMF). The integrated fluorescence intensities
were plotted as a function of concentration of ligand, complex and standard reference quinine
sulphate. The slopes of curves were calculated to obtain the quantum yield (Fig. 10). Slope and
quantum yield values of ligand and complex are tabulated in Table 3.
Fig.10 Plot of concentration vs intensity
Fig. 11 Chromaticity diagram of ligand and
of ligand, complex and standard
complex
3.8 CIE parameters:
The International Commission on Illumination (CIE) suggested a set of parameters in order to
quantify the colour emission from a material using chromaticity diagram. In the present work,
this diagram (Fig. 11) was plotted using colour calculator software. Photometric characteristics
of prepared ligand and complex were studied from this diagram. Colour coordinates (x, y) for
both compounds fall in the blue region with luminous efficacy of radiation (LER) values as
shown in Table 3. High value of LER is always favorable for application of a complex in various
display devices.

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Compound Slope
Quantum yield Color coordinates LER
(lm/w)
x
y
Ligand
51.72
3.77
0.15
0.15
139
Complex
Standard
209.13
777.95
15.28
0.54
0.14
-
0.24
-
206
-
Table: 3 CIE parameters of ligand and complex
3.9 Quantum chemical parameters:
The highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals
(LUMO) are the energy levels (Fig.12, 13) known as frontier molecular orbitals (FMO). FMO
have been found to play an important role in determining quantum cehmical parameters,
electrical and optical properties of organic molecules. The HOMO-LUMO energy gap
determines the stability and reactivity of a molecule. It is high for hard molecule and small for
soft molecule. Lower value of energy gap indicates the ease of charge transfer within the
molecule [24,25]. HOMO-LUMO energies of ligand and complex as well as various quantum
chemical parameters calculated using the following relations are given in Table 4:
Ionization potential (IP) = -EHOMO
Electron affinity (EA) = - ELUMO
Electronegativity (χ) = (IP + EA)/2
Compound
HOMO
LUMO
Energy gap
IP
(Semi
empirical)
EA
χ
(Semi
(Semi
empirical)
empirical)
Semi-
DFT
Semi-
DFT
Semi
DFT
empirical
empirical
empirical
-0.290
-0.248
-0.208 -0.057
-0.198 -0.185
-0.080 0.233
-0.090 0.063
0.128
0.108
0.290
0.248
0.057
0.185
0.1735
0.2165
Ligand
Complex
Table:4 Quantum chemical parameters (in eV) of ligand and complex
The low value of HOMO-LUMO gap of complex indicates its more reactiviy and ease of charge
transfer compared to ligand. The low value of ionization potential for complex indicates its weak
tendency to remove even the most loosely bound electrons and hence the stability of complex is
high. The high value of electron affinity and electronegativity for complex indicate its ability to
accept or attract electron easily and help in charge transfer process as well as enhances the
electrical conductivity. Small value of HOMO-LUMO gap indicates that the complex is a soft
molecule.
Density functional theory (DFT) was also used to calculate the value of HOMO-LUMO gap. For
this purpose, basic set 6-31G and calculation method RB3LYP was used. HOMO-LUMO
energies of ligand and complex as calculated by DFT approach are shown in Table 4. Figures 12
and 13 show a similar type of plots with a slight variation in HOMO-LUMO structures compared
to plots obtained by semi empirical approach. The ground state properties of many-electron
system depend on the electron density n(x,y,z). The allowed energy states of many-electron
system can be determined. In DFT, the electron density is a function of space and time which

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deals directly with wave function. The comparison of HOMO and LUMO values calculated by
two approaches is not always correct. This is because the HOMO’s values are found by the
actual states of electronic interactions, while the LUMO’s are virtual states and the specification
of these states is not always accurate [26].
Fig.12 HOMO-LUMO structure of ligand and complex (Semi empirical)
Fig.13 HOMO-LUMO structure of ligand and complex (DFT)
3.10 Thermal studies:
Thermal studies were performed to know about the thermal stability and presence of water
molecule with respect to inner coordination sphere of complex. The thermo gravimetric (TG)
curve (Fig.13) indicates decomposition of complex in three successive stages within the
temperature range 100 to 800ºC. The first decomposition in the temperature range 100-250ºC
denotes the dehydration process due to removal of water [27-28]. Decomposition of the
coordination sphere takes place in the second stage in the temperature range 250-450ºC. The
third decomposition in the temperature range 450-800ºC corresponds to the complete loss in
mass of the complex. This thermal decomposition is also confirmed by presence of differential
thermal analysis (DTA) peaks in the region 100-250ºC, 250-450ºC and 450-800ºC.

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Fig.13 TG/DTA curves of complex
Conclusions:
 NMR studies confirm the coordination of Schiff base ligand with Zn(II) metal ions via N-
azomethine group followed by the deprotonation of OH group. The difference in peaks of
FTIR spectra of ligand and complex corresponding to C-OH and C=N vibrations indicate
the formation of complex. Significant change in surface morphology was observed after
inclusion of Zn ions.
 Enhancement in fluorescence intensity, quantum yield and LER after formation of
complex suggests its applications in display devices. The color coordinates of complex
was observed in the blue region. Low value of HOMO-LUMO gap and thermal stability
upto the 500ºC also supports the above application.
Acknowledgement
One of the author Mr. N. K. Gondia gratefully acknowledge the Council of Scientific and
Industrial Research (CSIR), New Delhi for awarding him Junior Research Fellowship (Award
No 09/085(0110)/2013-EMR-1).
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36.

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Highlights
 Synthesis of Schiff base ligands N,N-bis(salicylidene)-(3,3'-diaminobenzidine) and
zinc metal complexes.
 Structural, optical, theoretical and thermal studies of ligands and complexes.
 Colour coordinates of complex observed in the blue region.
 Enhancement in fluorescence intensity, quantum yield and LER of complex suggests
its applications in display devices.