Interaction of schiff base ligand with tin dioxide nanoparticles: Optical studies

Article abstract of DOI:10.1016/j.saa.2013.05.037

Interaction between 1,4 Bis ((2-Methyl) thio) Phenylamino methyl benzene (BMTPMB) Schiff base with tin dioxide nanoparticles (SnO₂ NPs) of various concentrations in methanol have been studied using UV-Visible and Fluorescence spectroscopic tech

Full text of DOI:10.1016/j.saa.2013.05.037

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 114 (2013) 170–174
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Interaction of Schiff base ligand with tin dioxide nanoparticles: Optical
studies
⇑
J. Suvetha Rani , V. Ramakrishnan
Department of Laser Studies, School of Physics, Madurai Kamaraj University, Madurai 625 021, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Scott plot.
ꢀ
Optical studies of Schiff base (SB)
ligand with semiconductor oxide NPs
were studied.
ꢀ
ꢀ
Weak interaction between BMTPMB
and SnO2 NPs was observed.
Scott plot was employed to determine
the molar absorptivity of the SB-NPs
system.
a r t i c l e i n f o
a b s t r a c t
Article history:
Interaction between 1,4 Bis ((2-Methyl) thio) Phenylamino methyl benzene (BMTPMB) Schiff base with
tin dioxide nanoparticles (SnO2 NPs) of various concentrations in methanol have been studied using
UV–Visible and Fluorescence spectroscopic techniques. The low value of Stern–Volmer quenching con-
stant and non-linear plot of Benesi–Hildebrand equation suggests the less affinity of SnO2 NPs towards
the adsorption of BMTPMB Schiff base. The Scott equation has been employed to determine molar
absorptivity of the Schiff base-NPs system.
Received 22 November 2012
Received in revised form 13 May 2013
Accepted 14 May 2013
Available online 25 May 2013
Keywords:
Ó 2013 Elsevier B.V. All rights reserved.
1,4 Bis ((2-Methyl) thio) Phenylamino
methyl benzene (BMTPMB)
Tin dioxide nanoparticles (SnO2 NPs)
Optical absorption
Fluorescence quenching
Scott plot
Molar absorptivity
Introduction
bases derived from aromatic amines and aromatic aldehydes have
a wide variety of applications in many fields [2]. There has been an
A Schiff base (or azomethine or imine) named after Hugo Schiff,
is a functional group with the general formula of R1R2C = N – R3,
where R is an organic side chain. These bases can be synthesized
by a nucleophilic addition of aromatic amine and a carbonyl com-
pound followed a dehydration process to yield imine [1]. Schiff
interest in the chemistry of metal complexes of Schiff bases con-
taining N and S donor atoms because of their structural features
and biological activities (antibacterial agents, antifungal agents,
antitumour drugs) [3]. Neutral Schiff bases are used to prepare
complexes with wide range of properties [4]. The preparation of
mixed donor ligands has received great attention recently. Ligands
that pair a hard donor group and a soft donor group are chiefly use-
ful in catalysis [5]. Schiff bases may become promising dye
⇑
Corresponding author. Tel.: +91 04522520343.
E-mail address: nsuvetha@yahoo.co.in (J. Suvetha Rani).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.05.037

J. Suvetha Rani, V. Ramakrishnan / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 114 (2013) 170–174
171
sensitizer in molecular photovoltaic cells if combined with chelat-
ing activities and other properties [6]. 1,4 Bis ((2-Methyl) thio)
Phenylamino methyl benzene (BMTPMB) is a neutral, sulfur-con-
taining mixed-donor Schiff base ligand.
80 °C for 5 h. The dried precipitate was kept at 105 °C for 4 h,
and it was loaded into the alumina crucible. Then it was annealed
in a muffle furnace at 600 °C for 5 h in air to enhance the crystal-
linity of SnO2. After the heat treatment, the product appeared to
be white in color [12].
Nano-scale materials have fascinated much attention because
of their excellent and attractive properties [7]. Semiconductor
nanoparticles are the most promising ones due to their high pho-
tochemical stability and size tunable photoluminescence. Recently,
n-type inorganic semiconductor materials have been studied be-
cause of their high electron mobility. Nanostructured metal oxides
are interesting inorganic semiconductors because of the ease of
fabrication, good control of film morphology, and interfacial prop-
erties [8]. Tin dioxide (SnO2) is a n-type stable large band gap semi-
conductor (3.6 eV) which has been used in gas sensors and
optoelectronic devices [7]. Although it does not respond to visible
light excitation, it can be sensitized with organic dyes which in
their excited state can inject electrons into the conduction band
of a large band gap semiconductor [9,10]. Photosensitization is a
convenient and useful method to extend the photo response of
large band gap semiconductor [11]. In the recent years, surface
modification of nanoparticles (NPs) with organic dye molecules
has been intensively studied to obtain nanomaterials with en-
hanced optical, catalytic and sensing properties for molecular elec-
tronic application or light energy conversion system.
The electromagnetic interaction between the NPs and Schiff
base dye molecules in the close vicinity modifies the electronic
and optical properties of the surface bound molecule. As a conse-
quence, the fluorescence intensity of dye molecule can be en-
hanced or decreased. That is, fluorescence quenching or
enhancement depending on the location of the dye around the par-
ticle, its separation from the surface, and the molecular dipole ori-
entation with respect to the particle surface. Taken into account of
these features, here we report a study of interaction of BMTPMB
Schiff base with SnO2 nanoparticle.
Experimental details
Procedure
The concentration of BMTPMB in methanol was maintained at
0.04 mM throughout the experiment. The concentration of SnO2
nanoparticles in Schiff base solution was varied as 0.04 mM,
0.08 mM, 0.12 mM, 0.16 mM, 0.2 mM and 0.24 mM. For neat solu-
tion and also for each sample optical absorption and emission mea-
surements were taken.
Measurements
The crystalline phase and size of SnO2 nanoparticles annealed at
600 °C were determined by using XPERT-PRO X-ray powder dif-
fractometer with Cu Ka radiation (k = 0.1540 nm). High resolution
Scanning electron microscopy (HRSEM) was performed using ZEISS
field emission scanning electron microscope to observe the mor-
phology of the SnO2 nanoparticles. Absorbance and emission mea-
surements were carried out with a Shimadzu UV-2450 UV–Visible
spectrophotometer and Shimadzu RF-5301PC spectrofluoropho-
tometer respectively. All measurements were performed at room
temperature. The optical absorption and fluorescent measure-
ments have been repeated for five times for each set of samples.
It was noticed that the data are reproducible with an accuracy of
0.1 nm. And hence, there is a good reliability of the data.
Results and discussion
Materials and methods
Structural properties of SnO2 nanoparticles
Chemicals
X-RD pattern of SnO2 nanoparticles annealed at 600 °C is shown
in Fig. 1. The diffraction peaks around 27°, 34°, 38° and 52° are as-
signed to (110), (101), (200), (211) (PDF No. 88-0287) planes of
SnO2 respectively. The planes in the X-RD pattern confirm the
tetragonal structure of SnO2.The broad diffraction peaks indicate
that the crystalline size of the nanoparticles is small [13].
All chemicals used in this work were purchased from Merck
with 99.9% purity and were used without further purification.
Preparation of Schiff base
1,4 Bis ((2-Methyl) thio) Phenylamino methyl benzene
(BMTPMB) Schiff base was synthesized as follows: An alcoholic
solution containing terphthalaldehde (1.34 g, 10 mM) and 2-
(methylthio) aniline (2.78 g, 20 mM) taken in the 1:2 molar ratio
was magnetically stirred for about 6 h and the contents were kept
over-night. The pure yellow coloured fine crystals were filtered,
washed with alcohol and dried. The dried powder was used for
the analysis [5].
Preparation of SnO2 nanoparticles
SnO2 nanoparticles used in this study were synthesized using
chemical precipitation method as follows: A 0.1 M solution of
SnCl2ꢁ5H2O (4.5126 g) was prepared in the de-ionized water
(200 ml) to get a mixed aqueous solution. Then ammonia was
added into the mixed aqueous solution drop wise under vigorous
stirring to get the pH value of the solution in the range of 8–9.
Now, the precipitate had been formed at the bottom of the glass
beaker. The precipitate was kept at room temperature for 2 h for
ageing and then washed with deionized water. The washing was
repeated for 5–6 times. The resulting precipitate was heated at
Fig. 1. X-RD pattern of SnO2 nanoparticles.

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J. Suvetha Rani, V. Ramakrishnan / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 114 (2013) 170–174
Fig. 2. SEM image of SnO2 nanoparticles.
Fig. 4. Absorption spectrum of BMTPMB in methanol.
Fig. 2 shows the HRSEM image of the SnO2 nanoparticles. The
spherical grain morphology of SnO2 nanoparticles is observed.
The average particle size of the SnO2 nanoparticles obtained from
HRSEM is around 30 nm.
Optical absorption and emission studies
Molecular structure of BMTPMB is shown in Fig. 3. In this struc-
ture, the outer benzene rings are twisted away from coplanarity
with the central benzene ring due to steric repulsion between
the hydrogen atoms attached to the cyno group and CH group of
the outer benzene ring [6]. The absorption spectrum of BMTPMB
in methanol is presented in Fig. 4. A broad absorption band in
the region 300–400 nm observed in the spectrum suggests the
presence of
p–
pꢂ transitions involving the molecular orbitals lo-
cated at the phenolic chromophores. And the bands in the UV re-
gion are due to n–pꢂ transitions relating the imine chromophore
orbitals and phenyl ring electrons [14]. The broad band at
378 nm is taken for the present investigation. Fig. 5 shows the
UV–Visible spectrum of SnO2 nanoparticles in methanol. Fig. 6
exhibits the absorbance spectra of BMTPMP with various concen-
trations of SnO2 nanoparticles. As the concentration of SnO2 NPs
varies, a change in optical density occurs without change in
absorption maximum. This may due to the interaction of Schiff
base compound and SnO2 nanoparticles. Fig. 7 shows the emission
spectrum of BMTPMB Schiff base in methanol. Emission spectra of
Schiff base with SnO2 NPs of various concentrations are displayed
in Fig. 8. A decrease in fluorescence intensity is observed as the
Fig. 5. UV–Visible spectrum of SnO2 NPs in methanol.
Fig. 6. Absorption spectrum of BMTPMB in various concentrations of tindioxide
Fig. 3. Molecular structure of BMTPMB.
nanoparticles.

J. Suvetha Rani, V. Ramakrishnan / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 114 (2013) 170–174
173
ing form collisional encounters between the fluorophore and
quencher. In both cases, molecular contact is required between
the fluorophore and the quencher. Collisional quenching of fluores-
cence has been thoroughly studied during the last decades because
it is of considerable interest for physical chemistry as well as for
biochemistry and biophysics where quenching is frequently used
to elucidate structure and functional features of the systems [15].
Under the experimental condition, it is important to note that exci-
tation wavelength of the Schiff base does not coincide with the
absorption peak of SnO2 NPs. Therefore, it can be concluded that
the absence of Forster energy transfer between Schiff base and
SnO2 NPs. The quenching of Schiff base indicates that the nanosize
particles are responsible for this effect. And no discernible change
in shapes of the fluorescence spectra was observed upon quench-
ing. To illustrate the quenching effect of SnO2 NPs on the fluores-
cence intensity of Schiff base, Stern–Volmer equation, F0/
F = 1 + Ksv[Q] where F0 and F are the fluorescence intensities of
Schiff base in absence and presence of quencher [Q] respectively,
is applied. The plot of F0/F vs [SnO2] has been shown in Fig. 9a.
The relative intensity of the Schiff base is found to increase with
increasing SnO2 NPs concentration. The plot is linear with Ksv = 52 -
Mꢃ1. The linearity of the S–V plot indicates that only one type of
quenching occurs in the system. In the presence of quencher, the
absorption spectra of Schiff base remain unaltered. This indicates
that static quenching does not occur [16]. The Stern–Volmer
quenching constant is low indicating the adsorption of only a
few probe molecules on the quencher surface. The quenching of
the fluorescence of probe by SnO2 NPs in methanol also suggests
that the electrostatic effect between the sulfur containing mixed-
donor dye and the n-type quencher may be responsible for the
low quenching effect. As BMTPMB molecule exhibits TICT (Twisted
intramolecular charge transfer) in the excited state [4], the outer
benzene rings in the probe molecule are twisted such that they
come into coplanarity with the central benzene ring. Now all the
benzene rings come to the same plane and this geometrical
arrangement may restrict the collisions between the fluorophore
and the SnO2 NPs. This could also be a possible reason for the ob-
served low quenching effect.
Fig. 7. Fluorescence spectra of BMTPMB in methanol.
To observe one-to-one binding between a Schiff bases (Guest)
and the NPs (Host), the Benesi–Hildebrand method can be em-
ployed [10]. The dependence of the fluorescence intensity upon
quencher concentration is also presented as a plot between [D]/A
and 1/[SnO2] (Fig. 9b). Benesi–Hildebrand plot deviates from its
linear nature and exhibits scatter plot characteristics. This suggests
a weak association between the probe and SnO2 NPs. Hence, the
association constant could not be deduced. The Scott’s equation
[17] which is given below as
Fig. 8. Fluorescence spectra of BMTPMB in various concentrations of tindioxide
nanoparticles.
concentration of SnO2 NPs increases. This decrease suggests the
existence of the interaction of probe molecule and SnO2 NPs.
Fluorescence quenching is the decrease of the quantum yield of
fluorescence from a fluorophore induced by a variety of molecular
interactions with a quencher molecule. Fluorescence quenching
can be static, resulting from the formation of a ground state com-
plex between the fluorophore and a quencher or dynamic, result-
½Aꢄ0½Dꢄ0=A ¼ ð1= Þ½Dꢄ0 þ 1=K;
e
where [A]0 is the concentration of Schiff base and [D]0 concentration
of SnO2 NPs and A is the absorbance of the Schiff base – NPs system,
e
is the molar absorptivity and K is the association constant. This is
made use of presenting a plot (Fig. 9c) of [A]0[D]0/A vs [D]0, which is
Fig. 9a. Relative fluorescence intensity of BMTPMB in various concentrations of tindioxide nanoparticles.

174
J. Suvetha Rani, V. Ramakrishnan / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 114 (2013) 170–174
Fig. 9b. [D]/[A] vs 1/[SnO2].
Fig. 9c. Scott plot for the BMTPMB–SnO2 NPs system.
linear with the intercept at origin as expected. Hence only molar
Madurai Kamaraj University for the help on the preparation of
SnO2 nanoparticles.
absorptivity
e can be evaluated. From the slope, e is deduced as
9.03 ꢅ 103 Mꢃ1 mꢃ1. The lower molar absorptivity compared to
the neat Schiff base solution (1.5 ꢅ 106 Mꢃ1 mꢃ1) could be due to
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The authors thank CSIR, Government of India for the financial
assistance in the form of a research scheme to one of the authors
(VRK). Further the financial support received from UGC, Govern-
ment of India under DRS Phase III to have Shimadzu UV-2450
UV–visible spectrophotometer facility is gratefully acknowledged.
One of the authors (JS) would like to express her sincere thanks
to the Principal and Management of the Lady Doak College for their
encouragement and Ms. P. Sangeetha, Department of Laser Studies,