Physico-chemical studies of new luminescent chemosensor for Cu2+

Article abstract of DOI:10.1016/j.saa.2012.06.049

The spectroscopic properties of (E)-2-(2-hydroxybenzylideneamino)-3- phenylpropanoic acid (HBDPPA) has been studied in a series of different solvents. As revealed by absorbance, and emission results, the molecule undergoes fast excited-state intramolecula

Full text of DOI:10.1016/j.saa.2012.06.049

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 677–683
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Physico-chemical studies of new luminescent chemosensor for Cu2+
⇑
Jayaraman Jayabharathi , Venugopal Thanikachalam, Munusamy Vennila, Karunamoorthy Jayamoorthy
Department of Chemistry, Annamalai University, Annamalainagar, Tamilnadu 608002, India
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
The quenching of fluorescence was
observed in the presence of copper
metal ion.
"
"
ESIPT process plays main role.
Competition of intra and
intermolecular hydrogen bonding in
binary solvents.
"
"
NBO analysis revealed that the
n(N1) ? p⁄ O(30)–H(31) interaction
gives the strongest stabilization to
the system.
PES diagram, MEP and electronic
properties studied using Gaussian-
03.
a r t i c l e i n f o
a b s t r a c t
Article history:
The spectroscopic properties of (E)-2-(2-hydroxybenzylideneamino)-3-phenylpropanoic acid (HBDPPA)
has been studied in a series of different solvents. As revealed by absorbance, and emission results, the
molecule undergoes fast excited-state intramolecular proton transfer (ESIPT) to yield emission of the cor-
responding tautomeric species. A large energy barrier for interconversion of the keto-enol rotamers in the
ground and excited state have been predicted on the basis of DFT calculations and also supported by
HOMO–LUMO energies, PES calculation, NLO and NBO analysis. Hyperpolarizability and octupolar and
dipolar components ratio confirmed that HBDPPA can be used as optoelectronic materials. The ground
state equilibrium and ESIPT process of HBDPPA are essentially unaffected by the nature of the solvent.
Measurements of absorption solvatochromism have been interpreted with Marcus and Reichardt–
Dimroth solvent functions to estimate the transition dipoles. Good correlation exists between absorption
as well as fluorescence wavenumbers obtained by the multi-component linear regression employing the
Taft and Catalan solvent parameters with the experimental values. The prepared Schiff base HBDPPA can
be used as a new fluorescent sensor to detect the quantity of Cu2+ ion in any sample solution depending
on the relative intensity change.
Received 8 March 2012
Received in revised form 30 May 2012
Accepted 6 June 2012
Available online 20 July 2012
Keywords:
ESIPT
HOMO–LUMO
DFT
PES
NLO
Chemosensor
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
opment of coordination chemistry related to catalysis and enzy-
matic reactions [3], magnetism and molecular architectures [4].
Schiff bases are used as starting materials in the synthesis of
important drugs, such as antibiotics, antiallergic, antiphlogistic,
and antitumor substances [1]. On the industrial scale, they have
a wide range of applications, such as dyes and pigments [2]. Schiff
base complexes have been playing an important role in the devel-
Schiff base compounds display interesting photochromic and ther-
mochromic features in the solid state and can be classified in terms
of these properties [5]. Photo- and thermochromism arise via
H-atom transfer from the hydroxy O atom to the imine N atom
[ESIPT] [6]. Such proton-exchanging materials are used for the de-
sign of various molecular electronic devices and their investigation
during the last few years are in large extent because of their poten-
tial applicability in optical communications and many of them
show non-linear optic (NLO) behavior [7]. NLO materials have been
⇑
Corresponding author. Tel.: +91 9443940735.
E-mail address: jtchalam2005@yahoo.co.in (J. Jayabharathi).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.06.049

678
J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 677–683
attractive in recent years with respect to their future potential
applications in the field of optoelectronic such as optical commu-
nication, optical computing, optical switching, and dynamic image
processing [8,9]. Literature survey revealed that the DFT has a great
accuracy in reproducing the experimental values in geometry, di-
pole moment, vibrational frequency, etc. [10]. In this paper we re-
port the photophysical studies of o-hydroxy Schiff base containing
amino acid as component and also its ESIPT process. Density func-
tional theory (DFT) calculation [11] on energy, dipole moment of
various species, HOMO, HOMO-1, LUMO and LUMO+1 energies,
PES, NLO, NBO and molecular electrostatic potential surface
(MEP) studies have been performed to supplement the experimen-
tal results.
neutral ethanol shows band at 348.5 nm. From the solid state
UV–Vis spectrum of Schiff base (HBDPPA) the optical band gap va-
lue is found as 2.39 eV [13]. The smaller band gap indicates easy
electronic transitions from bulk to vacuum energy level of the
compound and has high electrical conductivity as well as fluores-
cence property. Electronic absorption spectrum has been calcu-
lated using TD–DFT based on the B3LYP/6-31G (d,p). The
calculated absorption wavelength is displayed in Table 1 which
are in comparable with the experimental absorption maximum.
Fig. 2 displays the emission spectrum Schiff base in various sol-
vents. With increasing the solvent polarity red shift was observed.
Schiff base (HBDPPA) as a chemosensor
The changes in the fluorescence properties of HBDPPA caused
by different metal ions such as Co2+, Hg2+, Ni2+ and Cu2+ were mea-
sured in ethanol. The fluorescence of HBDPPA quenched markedly
with the gradual addition of Cu2+ but the fluorescence properties of
HBDPPA was slightly influenced by other metal ions (Fig. 3). The
fluorescence intensity of HBDPPA was linearly reduced with in-
crease in concentration of Cu2+. The quenching in fluorescence
intensity of Schiff base HBDPPA by the addition of Cu2+ cation indi-
cates the complexation of Schiff base (HBDPPA) with Cu2+, the
Schiff base (HBDPPA) has bidentate sites and forming the expected
Cu2+ complex (Fig. S4) [14]. The possible reason for the fluores-
cence quenching is the formation of a ground state non-fluorescent
complex and the enhancement of spin–orbit coupling [15] for
HBDPPA–Cu2+ is presumed resulting in the fluorescence quench-
ing. According to the obtained results, Schiff base HBDPPA can be
used as a new fluorescence sensor to detect the quantity of Cu2+
ion in any sample solution depending on the relative intensity
change.
Experimental
Materials and methods
L-Phenylalanine (Sigma–Aldrich Ltd.), salicylaldehyde (S.D. fine)
and all other reagents used without further purification.
Optical measurements and composition analysis
NMR spectrum was recorded for Schiff base on a Bruker
400 MHz. The ultraviolet–visible (UV–Vis) spectra were measured
on UV–Vis spectrophotometer (Perkin Elmer, Lambda 35) and cor-
rected for background due to solvent absorption. Photolumines-
cence (PL) spectra were recorded on a (Perkin Elmer LS55)
fluorescence spectrometer. MS spectrum was recorded on a Varian
Saturn 2200 GCMS spectrometer.
Computational details
Excited state intramolecular proton transfer (ESIPT) process
Quantum mechanical calculations were used to carry out the
optimized geometry, NLO, NBO, PES, HOMO–LUMO and TD–DFT
with Guassian-03 program using the Becke3–Lee–Yang–Parr
(B3LYP) functional supplemented with the standard 6-31G(d,p) ba-
sis set [11,12].
The fluorescence spectra of HBDPPA in dioxane contain two
emission bands and the emission peak at shorter wavelength at
342.4 nm is assigned to isomer I (Fig. S5) and the small at higher
wavelength 367.3 nm reveal that only the isomer II of HBDPPA.
However in the case of hydroxyl containing solvent (EtOH), a short
wavelength emission band appears for HBDPPA. This result corre-
sponds to the data obtained earlier [16–19] for compounds demon-
strating ESIPT and can be explained by the presence of
intermolecular hydrogen bonding with solvent molecule leading
to the stabilization of solvated isomer IV in which ESIPT is
impossible.
For better understanding the ESIPT mechanism in HBDPPA, we
have performed DFT calculation of electron density for the keto
and enol isomers of the HBDPPA molecule in the ground and the
excited states (Table 2) which reveal that excitation of enol isomer
(I) leads to an increase in the electron density at N atom and de-
crease at O atom resulting in ESIPT and formation of the excited
keto isomer in excited state. Then, the excited keto isomer emits
luminescence and returns to the ground state keto form, which is
characterized by a large positive charge at the N atom and negative
charge at the O(23) atom. As a result, a reverse process occurs in
the ground state of the molecule producing an initial molecule in
the form enol form.
General procedure for the synthesis of (E)-2-(2-
hydroxybenzylideneamino)-3-phenyl propanoic acid (HBDPPA)
The Schiff base (HBDPPA) was synthesized according to the pro-
cedure reported in literature [13]. A solution of L-phenylalanine
(1 mmol) and salicylaldehyde (1.5 mmol) in 20 ml absolute etha-
nol was refluxed for 2 h. The resulting precipitate was filtered off
and purified by column chromatography. Yield: 60 %, m.p:
189 °C. Anal. calcd. for C16H15NO3: C, 71.36; H, 5.61; N, 5.20; O,
17.72. Found: C, 71.34; H, 5.62; N, 5.19; O, 17.74. MS: m/z
269.11, calcd. 269.9. 1H NMR: (400 MHz, CDCl3): d3.27 (d, 2H),
d4.39 (t, 1H), d5.1 (s, 1H), d6.85 (d, 1H), d6.76 (t, 2H), d7.12 (t,
2H), d7.21 (m, 2H), d7.45 (s, 1H), d11.4 (s, 1H). 13C NMR
(400 MHz, CDCl3): 38.1, 67.2, 116.1, 121.5, 124.6, 126.1, 127.8,
128.7, 130.6, 132.6, 139.5, 160.9, 161.1, 177.5.
Results and discussion
Absorption and emission spectra of Schiff base (HBDPPA)
Competition between intramolecular and intermolecular hydrogen
bonding
The absorption spectra of Schiff base (Fig. 1) largely depend on
the solvent polarity and red shift was observed in hydroxyl sol-
vents. Larger red shift is due to the stabilizing interaction between
the hydroxyl group of the alcohol and the Schiff base so that there
Fluorescence spectra of HBDPPA exhibits two emission peaks at
342.4 and 367.3 nm (cis enol) respectively in pure dioxane. With
the addition of water, the emission intensity decreases in enol
when water fraction is increased to 20% (v/v), the emission almost
is greater delocalization of the
p electron cloud of the aromatic sys-
tem, CH=N and N=N bonds. The absorption spectra of HBDPPA in

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 677–683
679
Fig. 1. Normalized absorption spectra of HBDPPA.
Table 1
UV–Vis absorption (kabs), excitation energy (Eexc) and oscillator strength (f) for HBDPPA calculated by TD–DFT method.
Parameter
HOMO–LUMO
HOMO-1-LUMO
HOMO-1-LUMO + 1
HOMO–LUMO + 1
275.91
4.4937
0.1090
kabs(nm)
Eexc(eV)
f
340.95
3.6365
0.0023
340.95
3.6365
0.0023
282.64
4.3866
0.0259
Fig. 2. Normalized emission spectra of HBDPPA.
vanished. With further increasing water fraction to 40% (v/v), a
new emission band at 350.6 nm appeared in addition to the normal
emission. This band intensity increased with increasing water frac-
tion and turned to be the main emission when water fraction is
high up to 80% (v/v) (Fig. 4). The emission decreases with increas-
ing water fraction in the mixed solvent is due to formation of the
intermolecular hydrogen bonding between HBDPPA and water. In
the initial stage, the presence of small amount of water in the diox-
ane solution must give rise to the solvation of HBDPPA. The inter-
molecular hydrogen bonding between HBDPPA and water
definitely disrupts the ground state intramolecular hydrogen
bonding isomers but increases the quantity of species IV, in which
ESIPT is inhibited and therefore, the trans enol emission decreases
with addition of water and finally vanished [20].

680
J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 677–683
Table 3
Relative energies and dipole moment for HBDPPA.
Rotamer
Ground state
(D)
Excited state
(D)
l
E(eV)
l
E(eV)
I
5.70
0.10
9.97
4.20
(0.00)
0.14
(2.20)
4.84
II
4.80
9.27
(0.05)
(5.11)
Values in the parentheses corresponds to ethanol.
and 4.6 kcal/mol in ethanol. In the excited state the barrier for
HBDPPA in the isolated molecule and in the ethanol medium are
11.0 and 9.5 kcal/mol, respectively. The barriers for interconver-
sion in the excited state are much higher than that in the ground
state [21].
Molecular electrostatic potential
Fig. 3. Fluorescence spectra of HBDPPA in the presence of various metal ions.
The MEP is related to the electronic density and is a very useful
descriptor for determining the sites for electrophilic and nucleo-
philic reactions as well as hydrogen bonding interactions [22]. To
predict reactive sites for electrophilic and nucleophilic attack for
the investigated molecule, the MEP at the B3LYP/6-311++G(d,p)
optimized geometry was calculated. The negative (red and yellow)
regions of the MEP are related to electrophilic reactivity and the
positive (blue) regions to nucleophilic reactivity, as shown in
Fig. 6. As can be seen from the figure, this molecule has several pos-
sible sites for electrophilic attack. Negative regions in the studied
molecule are found around the hydroxyl oxygen O(23) atom. Also,
a negative electrostatic potential region is observed around the
nitrogen atom N(3). The negative V(r) values are ꢀ0.121 a.u. for hy-
droxyl oxygen atom (23), which is the most negative region, and
0.0546 a.u. for nitrogen atom N(3), which is the least negative re-
gion. However, a maximum positive region is localized on the alde-
hydic C–H bond with a value of +0.0106 a.u., indicating a possible
site for nucleophilic attack. According to these calculated results,
the MEP map shows that the negative potential sites are on elec-
tronegative atoms as well as the positive potential sites are around
the hydrogen atoms. These sites give information concerning the
region from where the compound can undergo non-covalent
interactions.
Table 2
Electron density of atoms N(3) and O(31) for HBDPPA.
Compound
HBMBA
Atom
I
I*
III
N(3)
O(30)
ꢀ0.492
ꢀ0.588
ꢀ0.545
ꢀ0.501
0.441
ꢀ0.608
*
corresponds to excited state
The MEP is best suited for identifying sites for intra- and inter-
molecular interactions [23]. When an intramolecular interaction
takes place the electrostatic potential of the negative atom be-
comes less negative and the positive region on the other atom be-
comes less positive. For the MEP surface in the studied molecule,
the weak negative regions associated with the nitrogen atom
N(3) and also the weak positive region by the nearby hydrogen
atom are indicative of intramolecular (N(3)ꢁ ꢁ ꢁH(31)–O(30)) hydro-
gen bondings.
Fig. 4. Competition of intra and intermolecular hydrogen bonding of HBMBA in
binary solvents.
Natural bond orbital (NBO) analysis
Potential energy surface (PES) studies of HBDPPA
NBO analysis revealed that the n(N(3) ? ⁄(O(30)–H(31)) inter-
actions give the strongest stabilization to the system of the title
compound by 36.82 kcal/mol (gas); 40.66 kcal/mol (chloroform)
and 46.02 kcal/mol (ethanol) and strengthen the intramolecular
O(30)–H(31)ꢁ ꢁ ꢁN(3) hydrogen bond in gas phase. The E(2) value
due to the n(N(3) ? ⁄(O(30)–H(31) orbital interaction increases
with increasing dielectric constant of solvent. This result can be
used to explain the enhancement of the O–Hꢁ ꢁ ꢁN hydrogen bond
strength with increase of solvent polarity. Thus, it is apparent that
O–Hꢁ ꢁ ꢁN interactions significantly influence crystal packing with
this molecule.
The ground-state geometries of the three species, I, II, and III for
HBDPPA was optimized using the DFT/6-31G (d,p) method. Com-
plete optimization of all the geometrical parameters gives the
ground state energy of each species. The energy of the excited state
was calculated using the standard CIS method. Table 3 gives the
energies and dipole moment of the species I, II, and III in the
ground and the excited state. The potential energy (PE) curves
for the interconversion of isomers I and II of HBDPPA in the ground
(Fig. 5a) and the excited states (Fig. 5b) reveal that for isolated
HBDPPA in the ground state the barrier for interconversion is 5.3

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 677–683
681
Fig. 5. PES for the interconversion of the rotamers of HBMBA in the ground and excited state.
hydroxyl oxygen than imine nitrogen and hence transfer of proton
from hydroxyl oxygen to imine nitrogen in the ground state (ESIPT)
is quite impossible.
Hyperpolarizability Vs absorption maximum of HBDPPA by DFT
method
To determine the transference region and hence to know the
suitability of HBDPPA for microscopic nonlinear optical applica-
tions, the absorption maximum was compared with b-components
of HBDPPA [24,25]. All the absorption bands are due to
p ?
p⁄
transitions. The increased transparency in the visible region might
enable the microscopic NLO behavior with non-zero values [26].
The observed red shift in absorption with increasing solvent
polarity accompanied with the upward shifts non-zero values in
the b-components and hence has rather well microscopic NLO
behavior [27]. Urea is one of the prototypical molecules used in
the study of the NLO properties of molecular systems. Therefore
it was used frequently as a threshold value for comparative pur-
Fig. 6. MEP surface diagram of HBDPPA.
poses. The calculated values of ltot, atot and btot for the HBDPPA
HOMO–LUMO energies of HBDPPA by DFT method
are 6.34 D, 25.4 ꢂ 10ꢀ31 and 54.76 ꢂ 10ꢀ31 e.s.u which are greater
than those of urea. These results indicate that the title compound
is a good candidate of NLO material.
Analysis of the electron density of HOMO and LUMO of HBDPPA
can through some light on the ground and excited states proton
transfer processes. HOMO of enol form predicts that intramolecu-
lar hydrogen bonded (IMHB) ring system (Fig. S9) posses bonding
character over hydroxyl oxygen and nitrogen, with a larger elec-
tron density over the hydroxyl oxygen. Electron density of HOMO
of keto tautomer around IMHB ring shows antibonding character
over the hydroxyl oxygen and nitrogen atoms. Again comparison
of HOMO of enol form with HOMO of keto form shows much larger
electron density on hydroxyl oxygen. In the later case HOMO elec-
tron density for both the enol and keto form shows less electron
density over the phenyl ring.
By considering LUMO of enol and keto forms, LUMO of the enol
tautomer possess high electron density on hydroxyl oxygen. After
tautomerization, LUMO of keto-tautomer still shows larger elec-
tron density on nitrogen and comparatively smaller density on hy-
droxyl oxygen .Thus it favors the transfer of a proton from
hydroxyl oxygen to imine nitrogen in the excited state (ESIPT).
Again our calculation of electron density over the proton transfer
co-ordinate in the excited states (i.e., LUMO electron densities)
shows that there is a shift of p-electron distribution from hydroxyl
oxygen to imine nitrogen. Analysis of electron density of HOMO for
enol tautomer predicts imine nitrogen has slightly higher bonding
character than hydroxyl oxygen. On the other hand, HOMO elec-
tron density of keto tautomer shows greater bonding character of
Octupolar and dipolar components and lbo
The electrostatic first hyperpolarizability (b) of the o-hydroxy
Schiff base has been calculated by using Gaussian-03 package
[11], it is found that the chromophore show larger
lbo value
(347.6 ꢂ 10ꢀ31 e.s.u), which is attributed to the positive contribu-
tion of their conjugation. o-hydroxy Schiff base exhibits larger
non-linearity and so their absorption is red-shifted. Therefore,
the hyperpolarizability is a strong function of the absorption max-
imum. This o-hydroxy Schiff base derivative possess more appro-
priate ratio of off-diagonal versus diagonal b-tensorial
component [r = 1.92] which reflects the non-linearity and the larg-
est
l
bo values (347.6 ꢂ 10ꢀ31 e.s.u).
The b-tensor decomposed in a sum of dipolar (J¼12Db) and octu-
polar (J¼32Db) tensorial components and the ratio of these two com-
ponents strongly depends on their ‘r’ ratios. The octupolar and
dipolar components of the b- tensor has been calculated using
the following equations:
2
2
jjj¼12Dbjj ¼ ð3=4Þ½ðbxxx þ bxyyÞ þ ½ðbyyy þ byxxÞ ꢃ
ð8Þ
ð9Þ
2
2
jjJ¼32Dbjj ¼ ð1=4Þ½ðbxxx ꢀ 3bxyyÞ þ ½ðbyyy þ byxxÞ ꢃ

682
J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 677–683
Fig. 7. Correlation of the absorption and fluorescence wave numbers obtained by the multi-component linear regression.
Table 4
Adjusted coefficients ((tx)0, ca, cb and cc) and correlation coefficients (r) for the multilinear regression analysis of the absorption tab and fluorescence tfl wavenumbers and Stokes
shift (
D
t
ss) of HBDPPA with the solvent polarity/polarizability, and the acid and base capacity using the taft (p⁄
,
a
and b) and the Catalan (SPPN, SA and SB) scales.
(tx
)
(
tx)
0
cmꢀ1
(
p⁄
)
c
a
cb
r
kab
kfl
(3.08 0.024) ꢂ 104
(2.62 0.019) ꢂ 104
(0.32 0.005) ꢂ 104
ꢀ(11.81 4.99) ꢂ 103
ꢀ(10.42 4.09) ꢂ 103
ꢀ(1.40 0.95) ꢂ 103
cNSPP
(23.457 14.92) ꢂ 103
(20.72 12.22) ꢂ 103
(2.73 2.84) ꢂ 103
cSA
ꢀ(13.11 11.16) ꢂ 103
ꢀ(11.49 9.15) ꢂ 103
ꢀ(16.28 21.29) ꢂ 103
cSB
0.78
0.87
0.78
r
D
tss
=
=
tab
–
tfl
(tx
)
(tx)
0 cmꢀ1
kab
kfl
(3.03 0.017) ꢂ 104
(2.70 0.014) ꢂ 104
(0.33 0.034) ꢂ 104
ꢀ(17.62 6.15) ꢂ 103
ꢀ(13.79 5.06) ꢂ 103
ꢀ(3.83 1.20) ꢂ 103
(56.24 25.77) ꢂ 103
(43.29 20.98) ꢂ 103
(12.95 5.03) ꢂ 103
ꢀ(52.88 26.78) ꢂ 103
ꢀ(40.69 21.80) ꢂ 103
ꢀ(12.18 5.28) ꢂ 103
0.85
0.86
0.82
D
tss
tab–
tfl
Marcus and Reichardt–Dimroth solvent functions - solvatochromism
Solvent shifts in the absorption and emission of o-hydroxy
Schiff base have been measured in different solvents. The emission
peaks are observed around 341.5, 365 nm in n-hexane (non-polar),
389.8 nm in ethanol (polar protic solvent), 375.6 nm in CH2Cl2
(medium polarity) and 395.4 nm in acetonitrile (a strongly polar
aprotic solvent). The peaks shift may be due to the stronger inter-
action between the solvent and the excited state molecules. The
excited state of o-hydroxy Schiff base has been more stabilized in
polar solvents than in non-polar solvents. This leads to red shift
of emission with increasing solvent polarity. Measurements of
absorption solvatochromism have been interpreted with Marcus
and Reichardt–Dimroth solvent functions to estimate the transi-
tion dipoles in the excited state. The linear correlation (Fig. S10)
of absorption energy with Reichardt–Dimroth solvent ET parame-
ters indicates that the dielectric solute solvent interactions are
responsible for the observed solvatochromic shift. Similarly the ob-
served linear correlation (Fig. S11) has been observed between
absorption energy and Marcus [28] optical dielectric solvent func-
tion [(1ꢀDop)/(2Dop + 1)] reveal that the transition dipoles are asso-
ciated with absorption and the direction of excited dipole is
opposite to that of the ground state dipole.
Fig. 8. Stabilization of the resonance structures of the chromophore.
Taft and Catalan solvatochromism
jjJ¼32Dbjj
The parameter q2D
½
q2D ¼ j
ꢃ is convenient to compare the rel-
jjJ¼12Dbjj
Solvent properties mainly affect the photophysical properties of
compounds are polarity, H-bond donor capacity and electron do-
nor ability. Different scales for such parameters can be found in
ative magnitudes of the octupolar and dipolar components of b. The
observed positive q2D values, reveal that the b- tensor components
cannot be zero and o-hydroxy Schiff base are dipolar components.
Since most of the practical applications for second order NLO chro-
mophores based on their dipolar components, this strategy is more
appropriate for designing highly efficient NLO chromophores.
the literature, Kamlet and Taft [29] propose the p⁄
,
a
and b scales,
shown as
ꢄ
ꢄ
y ¼ y0 þ aa þ bbb þ cp p ðKamlet ꢀ TaftÞ
a
ð1Þ

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 677–683
683
Catalan et al. [30] suggest the SPPN, SA and SB scales to described
Acknowledgments
the polarity/polarizability, the acidity and basicity of the solvents
as shown as
One of the authors Dr. J. Jayabharathi is thankful to DST [No. SR/
S1/IC-73/2010], UGC (F. No. 36-21/2008 (SR)) and DRDO (NRB-213/
MAT/10-11) for providing funds to this research study. Mr. K. Jaya-
moorthy is thankful to DST [No. SR/SI/IC-73/2010] for fellowship.
y ¼ y0 þ aSASA þ bSBSB þ cSPPSPP ðCatalanÞ
ð2Þ
The correlation of the absorption (Fig. 7a) and fluorescence
(Fig. 7b) wave numbers obtained by the multi-component linear
regression employing the Taft and Catalan solvent parameters with
the experimental values given in Table 4. The polarity/polarizabil-
ity of the solvent coefficient [C or CNSPP] value correlating the sol-
Appendix A. Supplementary data
p
⁄
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.06.049.
vatochromic shifts with the solvent polarity. The coefficient
controlling the basicity of the solvent (Cb or CSB), has a negative va-
lue for o-hydroxy Schiff base. This is interpreted in terms of the sta-
bilization of the resonance structures of the chromophore (Fig. 8).
Resonance structure ‘‘II’’ has the positive charge located at the car-
bon atom and it will be stabilized in acidic solvents. Because these
resonance structures is predominant in the S1 state and the stabil-
ization of the S1 state with the solvent basicity would be more
important than that of the S0 state. Consequently, the energy gap
between the S1 and S0 states decreases and the absorption and
fluorescence wavelengths shift to longer wavelength with increas-
ing solvent basicity.
References
[1] R.W. Layer, Chem. Rev. 63 (1963) 489–510.
[2] A.E. Taggi, A.M. Hafez, H. Wack, B. Young, D. Ferraris, T. Lectka, J. Am. Chem.
Soc. 124 (2002) 6626–6635.
[3] P.S. Dixit, K. Srinivasan, Inorg. Chem. 27 (1988) 4507–4509.
[4] H. Miyasaka, N. Matsumoto, H. Okawa, N. Re, E. Gallo, C. Floriani, J. Am. Chem.
Soc. 118 (1996) 981–994.
[5] M.D. Cohen, G.M.J. Schmidt, S. Flavian, J. Chem. Soc. (1964) 2041–2051.
[6] E. Hadjoudis, M. Vitterakisp, I.M. Mavridis, Tetrahedron 43 (1987) 1345–1360.
[7] L. Feringa, W.F. Jager, B. De Lange, Tetrahedron 49 (1993) 8267–8310.
[8] D.R. Kanis, M.A. Ratner, T.J. Marks, Chem. Rev. 94 (1994) 195–242.
[9] B.K. Lee, J.M. Stout, C.E. Dykstra, J. Mol. Struct.: THEOCHEM 400 (1997) 57–68.
[10] F.D. Proft, P. Geerlings, Chem. Rev. 101 (2001) 1451–1464.
[11] Gaussian 03 program, (Gaussian Inc., Wallingford CT) 2004.
[12] H.B. Schlegel, J. Comput. Chem. 3 (1982) 214–218.
Conclusion
[13] O. Oter, K. Ertekin, R. Kilincarslan, M. Ulusoy, B. Cetinkaya, Dyes Pigm. 74
(2007) 730–735.
[14] D.S. McClure, J. Chem. Phys. 20 (1952) 682–686.
[15] K. Das, N. Sarkar, A.K. Ghosh, D. Majumdar, D.N. Nath, K. Bhattacharyya, J. Phys.
Chem. 98 (1994) 9126–9132.
[16] J. Jayabharathi, V. Thanikachalam, M. Venkatesh Perumal, N. Srinivasan,
Spectrochim. Acta A 83 (2011) 200–206.
[17] J. Jayabharathi, V. Thanikachalam, K. Saravanan, M. Venkatesh Perumal,
Spectrochim. Acta A 79 (2011) 1240–1246.
[18] J. Jayabharathi, V. Thanikachalam, N. Srinivasan, K. Saravanan, J. Fluoresc. 21
(2011) 595–606.
[19] J. Jayabharathi, V. Thanikachalam, K. Jayamoorthy, M. Venkatesh Perumal,
Spectrochim. Acta A 79 (2011) 6–16.
[20] J. Jayabharathi, V. Thanikachalam, K. Jayamoorthy, Spectrochim. Acta Part A
Mol. Biomol. Spectrosc. 89 (2012) 168–176.
Intermolecular hydrogen bonding of HBDPPA with water giving
rise to IV impedes the ESIPT process. ESIPT in o-hydroxy Schiff base
confirmed by DFT calculation. PES calculation reveals that the bar-
riers for interconversion in the excited state are much higher than
that in the ground state. However, the intermolecular hydrogen
bonding with alcohol is not strong enough to compete with the
intramolecular hydrogen bonding in HBDPPA. The fluorescence of
HBDPPA quenched markedly with the gradual addition of Cu2+
but the fluorescence properties of HBDPPA was influenced by other
metal ions. Therefore Schiff base HBDPPA can be used as a new
fluorescence sensor to detect the quantity of Cu2+ ion in any sam-
ple solution depending on the relative intensity change. Molecular
orbital coefficient analyses suggest that the electronic spectrum
[21] J. Jayabharathi, V. Thanikachalam, N. Srinivasan, K. Jayamoorthy, M. Venkatesh
Perumal, J. Fluoresc. 21 (2011) 1813–1823.
[22] N. Okulik, A.H. Jubert, Internet Electron. J. Mol. Des. 4 (2005) 17–30.
[23] P. Politzer, M.C. Concha, J.S. Murray, Int. J. Quantum Chem. 80 (2000) 184–192.
[24] A. Karakas, A. Elmali, H. Unver, I. Svoboda, J. Mol. Struct. 702 (2004) 103–110.
[25] H. Unver, A. Karakas, A. Elmali, J. Mol. Struct. 702 (2004) 49–54.
[26] S.G. Prabhu, P.M. Rao, S.I. Bhat, V. Upadyaya, S.R. Inamdar, J. Cryst. Growth 233
(2001) 375–379.
[27] F. Volmer, W. Rettig, J. Photochem. Photobiol., A: Chem. 95 (1996) 143–155.
[28] A.P. Wilde, R.J. Watts, J. Phys. Chem. 95 (1991) 622–629.
[29] M.J. Kamlet, R.W. Taft, J. Am. Chem. Soc. 98 (1976) 377–383.
[30] J. Catalan, V. Lopez, P. Perez, J. Fluoresc. 6 (1996) 15–22.
correspond to the
p ?
p⁄ electronic transition. The MEP map
shows that the negative potential sites are on electronegative
atoms while the positive potential sites are around the hydrogen
atoms. These sites give information about the region from where
the compound can undergo non-covalent interactions. NBO analy-
sis revealed that the n(N1) ? p⁄ O(30)–H(31) interaction gives the
strongest stabilization to the system. The predicted nonlinear opti-
cal (NLO) properties of the title compound are much greater than
those of urea.