1. INTRODUCTION:
Cyanine dyes were added as a new class of
dyes to the supply of commercial dyestuffs with the synthesis of the first
polymethine dye (“cyanine”) by C. Williams in 1856[1]. Cyanine dyes are
characterized by their intense and sharp absorption bands in the UV-visible
region, usually between 225 and 735 nm, with narrow half-band widths of
typically ~ 25 nm. Cyanine dyes did not become commercial dyes for coloration
purposes immediately after their discovery, since they are prone to
decolorization by light and acid. One of the most important applications of
cyanine dyes is their use as spectral sensitizers for silver halide
micro-crystals in photographic films, since their absorption spectra are easily
shifted by chemical substitution and by aggregation [2-3]. Cyanine dyes have
been used extensively to probe biological systems, such as the helical
structure of DNA, through the measurement of induced circular dichroism (ICD)
spectra [4]. The chiral environment of anionic polysaccharides and
cyclodextrins has been investigated through measurement of induced CD spectra
with different types of cyanine dyes [5- 6]. Pinacyanol (1,1'-diethyl-2,2'-carbocyanine)
chloride (PIN) is a cationic dye that belongs to the class of conjugated
cyanine dyes.
The amphipathic nature of these dyes
confers solubility in a wide range of solvents, including water and chloroform.
It can form aggregates. Pinacyanol has been used to determine
spectrophotometrically the critical micellar concentration (cmc) of
surfactants. [7]. Because of its solvatochromic behavior, it can be used as an
indicator of solvent polarity [8].
It is well-known that the local
microenvironment surrounding a dye molecule influences its electronic structure
and thus its photophysics. Changes in this local microenvironment, as the dye
interacts with other species in solution, can produce measurable spectral
shifts which can, in turn, be monitored spectroscopically [9]. This property,
known as solvatochromism, allows elucidation of the influence of the immediate
environment of the molecule within the probed system and, moreover, gives
evidence of specific interactions. Solvatochromism is caused by the
differential solvation of the ground and first excited state of the chromophore.
For instance, a reduction of the polarity of the medium produces bathochromic
shifts of peaks of maximal absorption of PIN. This means that the ground-state
molecule of PIN is better stabilized by solvation than the molecule in the
excited state.
Solvatochromism is a powerful tool to
investigate the physical-chemical properties of molecules [10]. It is well
known that the effects of solvents on physical-chemical phenomena and
spectroscopic data are better analyzed in terms of a linear combination of
solvent properties, including solvent dipolarity/polarizability - π*,
hydrogen-bond donation ability (solvent “acidity”) - α, and hydrogen-bond
acceptance ability (solvent “basicity”) β.
UV-Vis absorption spectra can be influenced
by non-specific interactions such as ion–dipole, dipole–dipole (Keesom
interaction), induced dipole-permanent dipole interactions (Debye interaction)
or by specific interaction such as hydrogen bonding with solvents. Thus,
solvents play an important role in physical and chemical processes and can
determine change in the position, intensity, and shape of absorption bands
[11-16]. Solvent polarity is a commonly used term related to the capacity of a
solvent for solvating dissolved charged or neutral, apolar or dipolar species.
Several workers have investigated the
solvent effect on dye using various equations [17]. Bhattacharya et al [18]
have studied the absorption and emission spectroscopic studies of dye in
various solvents. Bhowmik and his co-workers [19] and P. Ganguly [20] have
investigated the photophysical properties of dye in different solvents. Effect
of solvent on the spectroscopic behaviour of the dye safranin T was studied by
Khatua et al. [21]. Solvatochromic effect on the absorption and fluorescence
spectra of Rose Bengal dye in various solvents has been studied by Rauf et al.
[22]. Solvent effect on the spectral properties of Neutral Red was studied by
Rauf et al. [23]. Substituent and solvent effects on the photophysical
properties of some coumarin dyes were studied by Zakerhamidi et al [24]. J.
Estelrich [25] has also determined the solubilization site of PIN in different
solvents.
Although several reports are there on the
studies of solvent effect on dyes but still such studies are considered to be
fragmentary in nature on the basis of spectral behavior of Pinacyanol Chloride
with different solvents. Despite of numerous applications of Pinacyanol
Chloride dye in various areas, information on the spectral properties of the
dye in various solvents required for understanding its spectral behavior is
incomplete. The aim of this work is to investigate the solvent influence on the
UV-Vis absorption spectra of cyanine dye Pinacyanol Chloride and to evaluate
the intermolecular interactions occurring in solutions. The spectral
characteristic of the studied dye molecule in different solvents at room
temperature was analyzed in this paper. A linear correlation between
experimental spectral values (νmax) and the solvent parameters
or the solvatochromic empiric variables π*, α, β, have been used
to discuss the solvatochromic behavior of the analyzed dyes and to evaluate
their contributions to the solute-solvent interactions. The chemical structure
of the investigated dye is presented in scheme 1.
2. EXPERIMENTAL:
Pinacyanol Chloride was purchased from
Sigma-Aldrich (USA) and was used as such. The solvents used were HPLC grade
products from E. Merck, Germany. Absorption spectra of PIN in water, and
different solvents were recorded using a Lambda 25 spectrophotometer
(Perkin-Elmer, USA).
A stock solution of PIN of concentration 10-5
mol dm-3 was prepared using double distilled water and wrapped
with black paper kept at 40C. Refractive index (n), dielectric
constant (ε) and the solvatochromic parameters (π*, α, β)
were taken from literature [10-11, 15, 26-27]. They are listed in Table 1
in which the solvents are arranged in the increasing order of polarity.
Fig.1: Visible absorption spectra of dye
(PIN) in various solvent solution.
Table 1:
Dielectric constants (ε, refractive index (n) and polarity scales (π*,
α, β of solvents) at 298 K
|
Solvents
|
ε
|
n
|
π*
|
α
|
β
|
|
Water
|
80.1
|
1.3330
|
1.09
|
1.17
|
0.18
|
|
Methanol
|
33.10
|
1.3288
|
0.60
|
0.93
|
0.62
|
|
Ethanol
|
25.30
|
1.3611
|
0.54
|
0.83
|
0.77
|
|
Propanol
|
20.80
|
1.3855
|
0.52
|
0.78
|
0.85
|
|
Acetone
|
21.01
|
1.3588
|
0.71
|
0.08
|
0.48
|
|
Ethylene glycol
|
41.40
|
1.4318
|
0.92
|
0.90
|
0.52
|
|
Chloroform
|
4.81
|
1.4459
|
0.58
|
0.44
|
0.0
|
|
DMF
|
38.25
|
1.4305
|
0.87
|
0.0
|
0.69
|
|
DMSO
|
47.24
|
1.4770
|
1.0
|
0.0
|
0.76
|
3. RESULTS AND DISCUSSION:
3.1. Solvent effects on the UV-Vis
absorption spectra
Absorption spectrum of the dye solution was
recorded in different solvents with the aim to probe the effects of various
solvents and correlate various absorption parameters to dye spectra in various
solvents. For this purpose solvents of different types were selected, firstly
the non-hydrogen-bond donating solvents (also called as non-HBD type of
solvents) such as acetone, DMF and DMSO; and secondly the hydrogen-bond
donating solvents (also called as HBD type solvents) such as water, ethanol,
methanol, chloroform, ethylene glycol and propan-1-ol. Table 2 contains
the wavelengths in the visible absorption maxima of the studied PIN dye in
different solvents. The solvent effect on the wavenumbers νmax
in the electronic absorption band maximum of PIN in representative solvents is
illustrated in (Fig. 1).
One can see from this table that the
absorption maximum of the dye is affected by solvent type and has a maximum
shift of Δλ = 20 nm for the solvents used in this work. Thus
this change in spectral position can be used as a probe for various types of
interactions between the solute and the solvent.
Fig.2: Visible absorption spectra of PIN as a
function of solvent polarizability (π*).
The analysis of solvent effect on spectral
properties of dye solutions were carried out by using the spectral position in
above mentioned solvents and correlating these with the Kamlet-Taft solvent
properties namely, π*, α, β, n and ε. Since the shift in
λmax values with solvent type reflects dye-molecule
interactions, an attempt was made to study this phenomenon in detail. The
spectral position of dye in various solvents has revealed interesting results.
Since all the solvents used in this work were polar in nature, one would expect
that the dye would bind more strongly to a more polar solvent and thus cause
the spectra to shift to longer wavelengths.
An increase in λmax values
with π* (dipolarity/polarizability) as shown in (fig. 2) also
indicates that dye interaction becomes different with increasing capability of
a given solvent to form H bonds in solution. The absorption data of dyes in
various solvent was also analyzed in terms of various polarity scales. The
first method involves the transformation of λmax (nm) of dye in
various solvents into molar transition energies {ET(dye), kcal/mole}
by using the following relationship [28]
ET(dye) =
28,591/λmax (1)
The ET(dye) values signify
transition energy which also reflects the stabilization of the dye in its
ground state in a given solvent. This may be due to either hydrogen bond formation
or dye-solvent interaction. Therefore, ET(dye) provides a direct
empirical measure of dye solvation behavior. The values are shown in Table 2.
Fig.3: Plot of absorption value (in wavenumber) of
PIN in various solvents vs the ET (30) values.
The absorption values were also related to
the solvent polarity parameter, namely ET(30), which also considers
other interactions besides those of specific nature. The values of ET(30)
were obtained from the literature for various solvents used in this work and
are listed in Table 2 [31]. (Fig. 3) shows the correlation between the
absorption value (in wavenumber) and ET(30) for the dye studied in
this work. A linear correlation of absorption energy covering a range of ET(30)
indicates the presence of specific nature of interactions between the solute
and solvents.
The spectral band shifts were also related
to solvent parameter φ(ε, n) which is given as follows [29]
(2)
The function takes into account two
important properties of the solvents namely the dielectric constant and the
refractive index and is a sum of two independent terms
namely f(ε, n) and g(n) which are
given as follows:
(3)
(4)
where, ε is the dielectric constant
and n is the refractive index and both these quantities reflect the freedom of
motion of electrons in the solvent and the dipole moment of the molecules.
Specific solvent effects occur by interactions of the solvent and the
chromophores. (Fig. 4A and 4B) shows the trend when the spectral
position (λmax) of the dyes in various solvents (non-protic and
protic) were plotted against the solvent polarity parameter φ(ε, n).
3.2. Absorption correlation studies
The position of absorption bands changes
with solvent nature. The spectral shifts induced by solvents were analyzed by
solvatochromic parameters such as (n, ε), (π*, α and β),
respectively, using multiple linear regression (MLR) method. The effect of
solvent on the absorption energy may described by the model proposed by Kamlet-
Abboud and Taft [30].
Table 3:
Multiple correlation coefficients for PIN from Kamlet-Taft equation
|
Effect
|
Coefficient
|
Std Error
|
t-value
|
R2
|
R2
adjusted
|
|
|
16674.41 (νo)
|
156.530
|
106.525
|
|
|
|
π*
|
-425.45 (s)
|
207.920
|
-2.046
|
0.928
|
0.713
|
|
Α
|
-248.45 (a)
|
113.338
|
-2.192
|
|
|
|
Β
|
242.0 (b)
|
118.291
|
2.045
|
|
|
Kamlet considered the total solvent effect
to be composed of three independent contributions; solvent polarity (π*),
acidity (α) and basicity (β) for hydrogen bond acceptor (HBA)
solvents. These contributions are gathered in one equation as follows:
(5)
Where ν is the wave number at maximum
absorption, s, a and b are regression factors, whose values depend on the
extent of contribution of each solvent parameter (π*, α, β) to
the predicted values ν'.
The solvent independent
correlation coefficients, (νo, s, a and b) have
been determined by multiple linear regression analysis, using Microcal Origin
6.0 statistics program. Multivariate
regression was used to fit the absorption energies to the Kamlet- Abboud and
Taft relationship. The results are shown in Tables 3 and reveal that the
solvent polarizability factor (π*) and hydrogen bond accepting abilities
of the solvent have the most significant effect on the absorption energy. The
hydrogen bond donating ability of the solvents have a lesser effect as
indicated by the smaller α coefficient. The coefficients s, a and b from
Eq. (5) (Table 3), can indicate the strength of the different solute-solvent
interactions.The solvents H2O,
DMSO and DMF did not fit the model well and were excluded from the regression
analysis.
Fig.5: A plot of absorption wavelength vs
predicted absorption wavelength for the solvents methanol, ethanol, propanol,
acetone, ethylene glycol.
Fig. 5 shows a plot of absorption wavelength vs predicted
absorption wavelength for the solvents methanol, ethanol, propanol, acetone,
ethylene glycol, chloroform. The data fits the Kamlet- Abboud and Taft model well with an R2
value of 0.928.
McRae’s equation [31] has been used here to
find out the dipole moment in
the excited and ground states
(6)
where, is the frequency (cm-1) of absorption maxima,
μe and μg are the permanent dipole moments in
the excited and ground states of the solute molecule respectively, h is
Planck’s constant, c is the speed of light in a vacuum and 
is
the Onsager cavity radius assuming the solvent to be a medium of continuous
dielectric constant (ε) and refractive index (n); F is solvent
polarity function which is given by:
(7)
Fig.6: Plot
of vs solvent polarity (F) for various solvents.
By plotting vs F gives us a linear graph with a positive slope as
shown in (Fig.6). The results of solvent polarity function, F was
presented in Table 4.
Table 4: Solvent polarity function, F for PIN in
various solvents using McRae’s equation
|
Solvents
|
F
|
|
Water
|
1.9269
|
|
Methanol
|
1.8265
|
|
Ethanol
|
1.7718
|
|
Propanol
|
1.7368
|
|
Acetone
|
1.7392
|
|
Ethylene glycol
|
1.8617
|
|
Chloroform
|
1.1176
|
|
DMF
|
1.8509
|
|
DMSO
|
1.8781
|
4. CONCLUSION:
The electronic absorption spectrum of PIN
molecule was recorded in solvents having different physical-chemical
properties. A bathochromic shift (positive solvatochromism) of these compounds
was observed upon increasing the solvent polarity. The spectral shifts in the
electronic absorption spectra of the studied dye were quantitatively expressed
by means of LSER model. Thus, correlations (MLR analysis) between wavenumber in
the maximum of absorption band (νmax) of the compound and the
solvent parameters (ε, n) or (π*, α, β) show that a major
contribution to the solvatochromism is due mainly to the non-specific
solute-solvent interactions and generally, they are stronger than the specific
interactions by hydrogen bonds. The results have shown that the absorption
maximum of dye is dependent on the solvent polarity.
5. ACKNOWLEDGEMENT:
The authors are thankful to the Department
of Chemistry, Tripura University, for providing laboratory facilities and also
to U.G.C, Govt. of India for supporting financial assistance through Research
Eligibility Test.
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