Investigation on environmental sensitivity characteristics of pyridine compounds with different position of N-atoms and various active functional groups

Article abstract of DOI:10.1016/j.molliq.2018.11.090

Environment induced effects including polarity play important role in controlling physical, biological, and chemical behavior of local molecules. In this study, due to the importance of this issue, solvatochromism and charge transfer characteristics of so

Full text of DOI:10.1016/j.molliq.2018.11.090

Accepted Manuscript
Investigation on environmental sensitivity characteristics of
pyridine compounds with different position of N-atoms and
various active functional groups
M. Khadem Sadigh, M.S. Zakerhamidi, A.N. Shamkhali, B.
Shaabani, N. Rad-Yousefnia
PII:
S0167-7322(18)34492-1
DOI:
https://doi.org/10.1016/j.molliq.2018.11.090
MOLLIQ 10000
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
30 August 2018
16 November 2018
19 November 2018
Please cite this article as: M. Khadem Sadigh, M.S. Zakerhamidi, A.N. Shamkhali, B.
Shaabani, N. Rad-Yousefnia , Investigation on environmental sensitivity characteristics
of pyridine compounds with different position of N-atoms and various active functional
groups. Molliq (2018), https://doi.org/10.1016/j.molliq.2018.11.090
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ACCEPTED MANUSCRIPT
Investigation on environmental sensitivity characteristics of
pyridine compounds with different position of N-atoms and various
active functional groups
a
a
b,c
d
d
M. Khadem Sadigh , M. S. Zakerhamidi , A. N. Shamkhali , B. Shaabani , N. Rad-Yousefnia
aResearch Institute for Applied Physics and Astronomy, University of Tabriz, Tabriz, Iran
bDepartment of Chemistry, Faculty of Science, University of MohagheghArdabili, 56199-11367
Ardabil, Iran
cNeuroscience Research Center, Iran University of Medical Sciences, Tehran, Iran
dDepartment of inorganic Chemistry, Faculty of chemistry, University of Tabriz, Tabriz, Iran
*
Corresponding Author
Mahsa Khadem Sadigh
Tel: number: +98-41-33393027
Fax: number: +98-41-33347050
E-mail: mahsa.sadigh@tabrizu.ac.ir , mahsa.sadigh@yahoo.com
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Abstract
Environment induced effects including polarity play important role in controlling physical,
biological, and chemical behavior of local molecules. In this study, due to the importance of this
issue, solvatochromism and charge transfer characteristics of some groups of pyridine compounds
were investigated in various environments with different polarity. According to the experimental
results, polarity of molecular surrounding environment, position of N-atom in pyridine rings, and
various active functional groups have profound effects on the molecular photo-physical features.
The contribution of various solvent induced effects on the solvatochromism characteristics of
samples were also investigated by using of linear salvation energy relationship concept. Moreover,
according to the experimental data and density functional theory calculations, the differences
between molecular dipole moment values in the ground and excited states are high enough to
increase the probability of charge transfer mechanism. Therefore, from the theoretical calculations
,
and quantitative experimental results of solute-solvent various interactions contributions, it is
possible to estimate the operation of pyridine compounds in various physical, chemical and
biological phenomena in the solvent media.
Keywords: Photo-physical, Pyridine, Solvent polarity, Solute-solvent interaction, Dipole
moment, Density functional theory
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1
. Introduction
The compounds containing pyridine ring, nitrogen comprising six membered aromatic rings, have
gained intense attention during the last years. These materials with interesting characteristics play
major roles in metabolism [1]. Furthermore, these compounds can be used as precursor for
synthesis of pesticides [1-2], ligand in coordination chemistry [2] and anti-cancer agents [3-5]. In
addition to the mentioned characteristics, they are found widely in pharmaceuticals [6], dyestuffs
[
2] and electronic devices [7]. First derivate of pyridine was acquired from coal tar by Anderson
in 1846 [8].Then different methods were provided by Ramsay [9], Hantzsch [10] and Chichibabin
11] for synthesis of pyridine and its derivatives. Because of their importance, they are still used
[
in industrial processes.
Various factors can modify the operation of pyridine compounds in various biological, physical
and chemical phenomena. Molecular surrounding media features as one of the most fundamental
factors can have considerable effects on the various phenomena. In real, the majority of
experimental works and natural phenomena are usually performed in the solution state. In this
case, different molecular interactions can play important roles on the response of solute molecule
in various phenomena. These interactions are divided into two different types called specific and
nonspecific interactions.
In general, solvent induced effects are usually studied by means of solvatochromic and solvent
polarity parameters [12]. There are various physical, chemical and biological processes that single
solvent polarity scales cannot explain completely the nature and degree of different solute-solvent
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interactions. Therefore, multi-parameter polarity parameters based on general multi-linear
expression (equation (1)) are used [12].
y  y  aAbBcC
(1)
0
In equation (1), y
0
describes the physicochemical property of understudy quantity in the reference
solvent or gas phase. a, b and c coefficients also indicate the solvent dependent characteristic of
understudy quantity (y) based on solvent polarity parameters (A, B, C).Two well-known sets of
multi-parameter solvent polarity scales are proposed and formulated as Kamlet-Abboud-Taft and
Catalan equations. Based on Kamlet-Abboud-Taft equation, A, B and C solvent polarity parameters
indicate hydrogen bond donor (HBD) acidity (α) [13], hydrogen bond acceptor (HBA) basicity (β)
[
14] and dipolarity/polarizability (π*) [15], respectively. On the other hand, in Catalan
formulation, A, B and C describe acidity (SA), basicity (SB) and polarity/polarizability (SPP) [16-
7] of selected solvents.
1
In addition to molecular surrounding environment features, molecular electronic structure,
molecular charge distribution characteristics and intra- and intermolecular interactions can be
depend on the position of atoms in the aromatic rings and active functional groups in the molecular
structures of selected samples.
Unfortunately, during the last years, it has not been reported a comprehensive experimental and
TDDFT study on the selected materials in various solvent media with different polarity. Our aim
of this work is investigation of media polarity effects on the linear optical properties and dipole
moment characteristics of pyridine compounds. In this case, at first, solvatochromism behavior of
pyridine compounds is studied at room temperature by UV-Visible spectroscopic technique. Then
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observable spectral shifts accompanied with theoretical methods are used for calculation of
molecular dipole moments in the ground and excited states. By using these experimental and
theoretical data for interactional behavior of pyridine compounds, it is possible to estimate the
molecular dominant resonance structures and their operation in various physical, chemical and
biological phenomena in solvent media.
2
. Experimental
2
.1. Materials
In this work, the pyridine compounds were synthesized according to the common methods in the
laboratory as follows:
Sample I: The compound 1 was prepared according to a literature method [18]. First, 2 ml of
methanol solution of 4-pyridinecarboxaldehyde (1.88 ml, 20.0 mmol) was drop-wise added to a
1
0 ml methanol solution of 3-aminobenzoicacid (2.74 g, 20.0 mmol). Then, the mixture was
refluxed for about 3 h. The acquired solid product was filtrated and washed with methanol before
-1
drying in air. Yield: 61% (1.54 g). FT-IR (KBr, cm ): 3443(m), 2885 (w), 1696 (s), 1608 (s),
1
411(m), 1298 (vs), 1267 (s), 1229 (m), 1201(m), 1076 (m), 1048(m),824(m).
Sample II: The compound 2 was prepared according to a literature method [19-21]. First, 20.0
mmol (1.88 ml) of 4-pyridinecarboxaldehyde in 5 ml acetonitrile was added drop-wise to an
acetonitrile solution of 1, 4-phenylendiamine (1.08 g, 10.0 mmol). Then, the mixture was stirred
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for about 3 h at room temperature. The formed solid product was separated and filtered off, and
washed with acetonitrile (3-5 ml) and hexane (3-5 ml). Finally, it was dried in air to give sample
-1
II. Yield: 78% (2.23 g). FT-IR (KBr, cm ): 3028 (w), 2921 (w), 2888 (w), 2855 (w), 1627 (s),
1
8
595 (s), 1551 (s), 1486 (s), 1412 (s), 1323 (s), 1221 (m), 1191(s), 1100 (m), 984 (w), 891 (m),
15 (s), 603 (m).
Sample III: The compound 3was prepared according to a literature method [22-23]. This sample
was also prepared with the similar method to the synthesis of sample II, except that 3- pyridine
carboxaldehyde (20.0 mmol, 1.88 ml) was added instead of 4-pyridinecarboxaldehyde. Yield: 68%
-1
(
(
(
1.95 g). FT-IR (KBr, cm ) = 2924 (w), 2887 (w), 1619 (s), 1570 (s), 1489 (m), 1422 (m), 1363
w), 1330 (w), 1287 (w), 1236 (w), 1194 (m), 1129 (w), 1104(w), 1022 (m), 973 (w),881 (m), 836
s), 800 (m), 701 (s).
Moreover, the selected solvents with high purity were prepared from Merck. The solvent polarity
parameters and polarity functions of them were also summarized in Table 1 [12, 16].
2
.2. Absorption and emission spectroscopy
First, diluted pyridinesolutions were prepared in selected solvents with diverse polarity. In real,
for investigation of solute-solvent interaction effects and regardless of solute-solute interactions,
the diluted sample solutions were prepared. Then, two spectroscopic devices were used for
investigation of absorption and emmesion spectra of pyridine solutions.Double beam Shimadzu
UV-2450 Scan Spectrophotomete is used for recording absorption spectra and JASCO FP-6200
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Spectrofluorometer was selected for studying emession characteristics of samples.In this case, the
optical length of quartz rectangular cuvettes for measurements were about 1 cm.
2
.3. Calculation of ground and excited state dipole moments
,
The values of samples dipole moments give important information about their geometrical
structures and charge distribution features. In addition, this parameter can be useful for designing
of nonlinear materials. Various procedures such as fluorescence polarization [24], Stark splitting
[
25], electric dichroism [26], microwave conductivity [27] and solvatochromism method [28-29]
can be used for calculation of dipole moment values. Among the mentioned methods, the method
based on solvent induced spectral shifts is usually used due to its simplicity. In this case, Kawaski
et al. [30-31] by using of quantum mechanical perturbation theory expressed simple expressions
for relation between solvent polarity parameters and difference and sum of maximum absorption
-1
and fluorescence wavelengths (in cm ) in diverse solvent environments. Such solvent dependent
relations can be written according to the Eq. 2 and 3.
a  m f (,n)const. (2)
f
1
a  m [ f (,n)2g(n)]const.
(3)
f
2
By definition of solvent polarity functions in the form of Eq. (4) and (5) and linear relation between
the difference and sum of maximum wave numbers and solvent polarity functions, it is possible to
obtain m
1
and m slopes (Eq. (6) and (7)) .
2
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2

2
1 n 1

2
 1 2n 1
(4)
f

,n 

2
2

2  1  2 n 1 
a 2 1
1

1


3

3
2

a 2n 1
2
2
n 1   n 1 
1

2

2
2

2
n 1
n 2n 1



g(n) 
(5)
2
n 1
1

3
2
a 2n 1
2
2
(   )
e
g
m 
(6)
(7)
1
3
hca
2
e
2
g
2
(   )
m 
2
3
hca
In these relations, ε, n, a, and α symbols indicate dielectric permittivity, refractive index, spherical
cavity radius of solute and average polarizability, respectively.
3
For an isotropic polarizability of solute molecule, the condition of 2α/a =1can be satisfied.
Therefore, the equations (4) and (5) are converted to Bakhshiev solvent polarity functions (Eq.8
and 9).
2
2
2
n 1  1 n 1
fBK

,n



(8)
(9)
2

2

n  2   2 n  2


4
3
 n 1 
gBK


n 

2
2

2
(n  2)


With approximation of parallel dipole moments of ground and excited states, we can write:
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1/2
3
m  m hca 
2
1
g
e


(10)
(11)




2
2m1
1/2
3
m  m hca 
2
1




2
2m1
m  m
1
2
e

g

m m

(12)
1
2
m  m
1
2
Where h is Plank’s constant and c is velocity of light in vacuum. In addition, μ
g
and μ
e
indicate
ground and excited state’s dipole moments, respectively. Onsager cavity radius (a) of pyridine
compounds was also determined theoretically according to their optimized geometry.
2
.4. Theoretical methods
In this work, the molecular structures of selected samples were optimized by density functional
theory (DFT) using M06-2X hybrid meta-functional and 6-31G (d, p) basis set [32-34]. By using
these optimized structures (Fig. 1), time-dependent density functional theory (TDFT)was
employed for investigation of molecular vertical excitations [35]. In addition, the molecular
orbitals included in λmax transition of these molecules are shown in Fig.1. These molecular orbitals
are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital
(
LUMO).
It is useful to know that all of DFT calculations were performed by NWChem quantum chemistry
program [36]. Furthermore, Onsager radii of the samples were determined by the same method
[
37]. All of obtained theoretical results for dipole moment values, the most probable excitation
wavelengths, and related oscillator strengths can be found in Table 3. Along with these results, for
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precise investigation of solvent induced effects, molecular electrostatic potential (MEP) and
solvent accessible surface (SAS) were observable in Fig. 2 and Fig. 3, respectively.
3
. Result and discussion
3
.1. The solvent effect on the absorption and fluorescence spectra
In this section, spectroscopic technique as an applicable method is used for investigation of media
polarity effects. In this case, the absorption and emission spectra of the pyridine parent molecules
in various solvent media were recorded at room temperature according to Fig. 4-6. Moreover, for
precise investigation of mentioned figures, the absorption spectra of samples are plotted in
different solvent media, separately that they are available in supplementary data.
Pyridine compounds with different position of N-atoms and various active functional groups in
their chemical structures indicate various active bands in the range of 270-370 nm (Table 4-
6
).Hence pyridine and C=N groups can have significant roles in molecular behavior.
The selected samples indicate two active absorption bands in UV region of electromagnetic
spectrum. By changing the position of N-atoms in pyridine rings, the observable absorption bands
are modified. In contrast to sample II and III, high energy band of sample I occur in the limited of
solvents. As seen in Fig.4-6, similar behaviors are also observed in the emission characteristics of
selected samples. In this case, the fluorescence spectra were recorded by excitation the samples at
fixed wavelength (maximum absorption wavelength). According to DFT calculations, these
behaviors can be related to transition between various molecular orbitals that for all of the samples

 *transitions are dominant (Fig.1). Moreover, the theoretical results from DFT calculation
are accessible in Table 3.
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In general, environment effects on the solvatochromism characteristics of materials are usually
studied in the term of polarity parameter. According to Lippert equation, general solute-solvent
interactions can be described in terms of the orientation polarizability parameter (∆f). Hence, for
studying the solvent effects on the spectral characteristics of pyridine compounds, the maximum
absorption and emission wavelengths are plotted as a function of orientation polarizability (∆f) of
solvents. In this case, we focus on general solvent interactions effects on the spectral behavior of
low and high-energy bands of pyridine compounds. The linear shown in Fig.7 and 8, help to the
reader to recognize the blue and red shifts of used samples in different solvents with various
polarity.
As can be seen in Fig. 7, in sample II and sample III, maximum absorption wavelengths tend to be
shifted toward the long wavelength region by increasing orientation polarizability (∆f).In this case,
similar behavior occurs in emission process. In sample I, by increasing the solvent polarity
parameter (∆f), two active absorption bands shift toward the low wavelength region and negative
solvatochromism process occur.
Moreover, the revealed differences between the absorption and emission of selected samples in
various solvents media lead to phenomenon that is well known as Stokes shift. In real, fluorescence
process occurs slower than the absorption process and at these circumstances; solvent molecules
have enough time to reorient around to the solute molecules in the excited state. Therefore,
emission process occurs at the relaxed state. In this case, the increasing of Stokes shifts (ν
a
b
-ν )
accompanied with increasing the polarity of organic solvents is due to reorientation of solvent
1
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molecules around to the solute molecules (Fig. 8). It should be noted that opposite behavior is
observed in the first band of sample III.
A comparison among the experimental results indicates that the observable different spectral shifts
is related to different position of N-atoms in pyridine ring of compounds and different active
functional groups in their chemical structures. These differences can induce various solute-solvent
interactions that lead to observation of various spectral behaviors.
Although above explanations can describe the general solvent effects qualitatively, they cannot
describe completely the contribution of specific and nonspecific solute-solvent interaction’s
effects. For this reason in the next section, we will use the multi-parameter solvent polarity scales.
3
.2. Correlation with multi-parameter solvent polarity scale
In order to investigate the various solute-solvent interaction’s contribution on the spectral
characteristics of pyridine compounds, multi-parameter solvent polarity scales that are known as
Kamlet- Abboud- Taft and Catalan formulation, were used. In the first step, all solvents were
2
selected for multi-parameter analysis. Then according to the statistical factors (R and significance
of F-test) and visual inspection, the appropriate solvents were selected for obtaining the best
results. Multiple linear analysis of absorption, emission and stokes shift data with multi-parameter
polarity scales show the satisfactory correlation for selected collection of solvents. The obtained
results are summarized in Table 7 and 8. For the better comparison of data in Table 7 and 8, we
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transformed them into contribution percentages of various solvent polarity parameters (Table 7 (b)
and 8 (b)).
The results from the multiple linear analyses (Table 7) indicate that by transition to the excited
state, solvent dipolarity/polarizability and solvent hydrogen bond acceptor abilities play significant
role on the spectral shifts of high- and low energy bands of sample I, respectively. In similar
analysis to the sample I, solvent specific interactions with high contribution have important effect
on the observable absorption properties of low- and high energy bands of sample II. In comparison
to the ground state, by excitation to the excited state, solvent hydrogen bond donor and solvent
dipolarity/polarizability abilities play significant role on the photo-physical behavior of high- and
low energy bands, respectively. In addition to these solvent polarity parameters, other polarity
parameters with lower contribution play minor role on the photo-physical behavior of two active
bands of sample II in the excited state. Therefore, absorption and fluorescence characteristics of
low- and high energy bands are due to both specific and nonspecific interactions with different
contributions.
In contrast to sample I and II, solvent hydrogen bond acceptor and solvent dipolarity/polarizability
abilities have the same contribution in the absorption properties of high-energy band of sample III.
By excitation to the excited state, the contribution of solvent dipolarity/polarizability ability is
decreased and the contributions of solvent hydrogen bond donor and acceptor abilities are
increased. The similar analysis for low-energy band indicates that in the ground state, solvent
1
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hydrogen bond acceptor ability and in the excited state, solvent dipolarity/polarizability ability
have dominant contribution.
As discussed in previous section, the difference between the absorption and emission wavelengths
leads to Stokes shift. Hence, investigation on the linear correlation between Stokes shift data and
multi-parameter solvent polarity scales will give important information about the solvent polarity
effects on the molecular reorientation. The experimental results (Table 7) indicate that in the most
cases, the increment of dominant solvent polarity parameters increase stokes shifts of used
samples. In this case, reorientation of solute molecules is increased in various solvent media.
Now, we exploit of Catalan multi-parameter solvent polarity scales for description of solvent
effects. The advantage of this formulation in comparison to the Kamlet–Taft solvatochromic
model is separation of nonspecific solvent effects into two dipolarity and polarizability terms.
Therefore, in Eq. 1, the coefficient c is divided into c and d that they are describe the contribution
of solvent dipolarity (SP) and polarizability (SdP) abilities, respectively.
According to the obtained experimental results (Table 8), in the most cases, there are good
correlation between absorption, emission and stokes shift data with Catalan multi-parameter
solvent scales. The data in parenthesis in Table 7 and 8 indicate the number of selected solvents in
multi-parameter analysis.
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In high-energy band of sample I, solvent dipolarity parameter has dominant role in its absorption
characteristic. Moreover, the multi-linear analysis indicates that in the excited state, the correlation
between Catalan solvent polarity parameters and first emission band of this sample is weak. In
low-energy band of sample I, solvent acidity in the ground state and solvent basicity in the excited
state as effective solvent polarity parameters play a significant role in the molecular photo-physical
behavior. In sample II, solvent dipolarity have high contribution in the ground and excited states
of two observable active bands. The similar solvent effect is also observed on the absorption
characteristics of sample III. Furthermore, the analysis of solvent effects on the emission
characteristics of two active bands of sample III reveals this fact that solvent acidity and solvent
dipolarity with high contribution have significant effect on the excited state properties of low and
high-energy bands, respectively. Moreover, similar analysis for stokes shifts data (Table 8)
indicate that by increasing dominant solvent polarity parameters (except the high-energy band of
sample III), the reorientation of solute molecules are increased in various solvent media.
In addition to the solvent effect on the spectral properties of used samples, it can effect on the
stability of ground and excited states of solute molecules. Through the investigation on the stability
of ground and excited states by changing each of solute-solvent contribution coefficient sign, it is
clear that absorption is an instantaneous event and absorption spectroscopy give only information
about the ground state of the molecules. Increasing the positive coefficients in Eq. (1) leads to
stability of ground state. In contrast, fluorescence lifetime is usually longer than solvent relaxation
time. Hence, fluorescence spectra illustrate the excited state of molecule. Therefore, increasing the
negative coefficients in Eq. (1) increases the stability of excited state [38-39]. According to data
in Table 7 and 8, in both selected models, dominant solvent polarity parameters lead to stability of
ground and excited state of sample I. Dominant solvent polarity parameters also stabilize the
1
5

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excited state of low-energy band of sample II. Moreover, the similar behavior is observed in low-
energy band of sample III.
In addition to the solvent effects on the spectral characteristics of pyridine compounds, they can
alter the electronic structure and charge distribution properties of the solute molecules. For this
reason, in the next section, the dipole moments of solute molecules are calculated and investigated
in the ground and excited states.
3
.3. Estimation of ground and excited state dipole moment
For calculation of dipole moments of pyridine compounds, first solvent polarity functions, f(ε,n)
and f(ε,n)+2g(n), were calculated (Table 2). Then the spectral shifts ν -ν and ν +ν of the selected
sample were plotted as a function of solvent polarity functions f(ε,n) and f(ε,n)+2g(n), respectively,
a
f
a
f
(Fig. 8). Hence, the points in Fig. 9 indicate the spectral shifts of samples in specific solvents with
2
certain solvent polarity parameters. Moreover, according to the statistical factors (R ), we deleted
some solvents and selected the proper solvent collections. Finally, using the slopes of these lines
and equations of (10) and (11) the ground and excited state dipole moments of samples were
calculated. The obtained experimental results in various solvent media can be seen in Table 9.
The revealed experimental results indicate that the first excited state dipole moment values are
higher than the ground state dipole moments in solvent media. On the other hand, dipole moments
of the high-energy bands are smaller than the low-energy bands for three mentioned samples. The
1
6

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increase of excited state dipole moments of high and low-energy bands of sample I in solvent
media with different polarity are about 3.55 and 3.77 Debye. The similar calculations indicate that
these values are higher in sample II and III in comparison to sample I. The differences between
ground and excited state dipole moments are about 6.83 and 6.34 Debye in high-energy band of
sample II and sample III, respectively. These changes are also about 7.33 and 8.99 Debye for low-
energy band of sample II and sample III, respectively. These results are in good agreement with
DFT calculations. It is noticeable that DFT calculations have been performed in gas phase. Hence,
dipole moments calculated by DFT method are usually different from experimental values.
However, the trend of DFT data is in agreement with experimental ones. As it can be observed in
Table 3, the differences between ground and excited state dipole moments (Δμ) indicate that
excited states have higher dipole moments in comparison with their ground states. In addition,
DFT calculations predict higher Δμ values for lower energy bonds of calculated samples. The
observable changes in dipole moment values of samples are highly sensitive to the possible
resonance structures in various solvent media. In real, the various solute-solvent interactions can
affect the electronic structure of solute molecules that the statistical weight for contribution of
resonance structures depends highly on the molecular interactions. These resonance structures are
obvious from HOMO and LUMO molecular orbital of these samples; see Fig. 1. As it can be seen
from this figure, location of π bonds is different in HOMO and LUMO orbitals.
Moreover, the large differences between ground and excited state dipole moments increase the
probability of charge transfer in solute molecules that are depend on the position and presence of
active groups in the molecular chemical structures.
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7

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In this case, sample I with an electron-withdrawing group (COOH) in its terminal part tends to
pull non-bonding electrons on the nitrogen atoms toward itself and leads to local intramolecular
charge transfer. A comparison between dipole moment values of this sample with two other used
samples reveals that increasing of conjugated length can lead to increment of dipole moments of
sample II and III. On the other hand, the differences dipole moment values of sample II and III can
be related to different position of nitrogen atoms in their pyridine rings.
Other results can be obtained by considering molecular electrostatic potential (MEP) and solvent
accessible surface (SAS) of these molecules. MEP describes the regions of used molecules with
positive and negative charges. According to Fig 2, the density of positive charges in sample I and
II are more than sample III. Therefore, depend on the contribution of positive and negative charges
on the solute molecules, the solute-solvent interactions can be modify. Under this condition, in
contrast to positively charge regions, the negatively charged regions operate as a good electron
donor agent in solvent media. Moreover, solvent accessible surface (SAS) values give useful
information. As can be seen in Fig 3, the largest SAS value is related to sample II. This large
interaction surface can increase the stability the structural form of sample II in comparison two
others samples.
Therefore, solute-solvent interactions and induced resonance structures can be modified by
presence of various active groups, position of N atoms, various contribution of negatively and
positively charges on the solute molecules and solvent accessible surfaces while they have
significant effects on dipole moment values.
1
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4
. Conclusions
The experimental results of spectroscopic and quantum chemistry methods reveal the interesting
characteristic of pyridine compounds. The experimental results indicate that various solute-solvent
interactions, position of atoms and active functional groups have profound effects on the
interactional behavior of selected samples.
For precise investigation of specific and nonspecific solute-solvent interaction’s contribution on
the photo-physical behaviors of samples, Kamlet-Abboud-Taft and Catalan models were
employed, separately. According to the results, in most cases, there were good correlations
between spectral data with Catalan multi-parameter solvent scales that it gives a possibility to
separate non-specific solvent effects into dipolarity and polarizability terms. Furthermore, the
dynamic behavior of pyridine samples as widely used compounds in pharmaceuticals was studied
by stokes shifts results in various environments.
According to the results, dipole moment values are dependent on the solute-solvent interactions
and induced resonance structures properties. In this case, presence of various active groups,
position of N atoms, solvent accessible surfaces, and various contribution of negatively and
positively charges on the solute molecules have significant effects on the dipole moment values.
Moreover, the large differences between the ground and excited state dipole moments increase the
probability of intramolecular charge transfer process by excitation.
Moreover, the majority of works are usually focus on the compounds with N atoms outside of
aromatic rings while in this work, the effect of N atoms inside the aromatic rings and the effect of
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their position was investigated. On the other hand, these materials with pyridine rings can play
important role in pharmaceuticals as an anti-cancer agent. Hence, investigation of the operation of
these materials in various solvent environments can give useful information about their behaviors
in various phenomena.
Therefore,, dipole moment values, interactional behavior, and recognizing environment polarity
effects on the electronic structure of selected pyridine compounds, can give valuable information
about how these samples can operate in various physical, chemical and biological phenomena. In
other words, obtained results of operation of pyridine compounds in various solvent environments
can be expanded to the biologically important environments with similar polarity characteristics.
Acknowledgment:
This work was supported by university of Tabriz.
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Figure captions:
Fig. 1. The calculated optimized structures of a) Sample I, b) sample II, and c) sample III and their
involved HOMO and LOMO molecular orbital in the most probable vertical excitation by TDFDT
method.
Fig. 2. MEP for optimized structures of a) sample I, b) Sample II, and c) sample III.
Fig. 3. SAS for optimized structures of a) sample I, b) sample II, and c) sample III.
Fig.4.a) Absorption and b) fluorescence spectral of sample I.
Fig.5. a) Absorption and b, c) fluorescence spectral of sample II.
Fig.6.a) Absorption and b, c) fluorescence spectral of sample III.
Fig.7. Variation of a) maximum absorption, b) emission wavelengths and c) stokes shift of used
samples as a function of orientation polarizability parameter (∆f).
Fig.8. Variation of stokes shifts of a) sample I, b) sample II, and c) sample III as a function of
orientation polarizability parameter (∆f).
Fig. 9. The variation of ν
a
-ν
f
with f(ε,n) and ν
a
+ν with f(ε,n)+2g(n) for a) sample I, b)sample II
f
and c)sample III.
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List of Tables:
Table 1. Molecular name, molecular structure and molecular weight of used samples.
Table 2. Solvent polarity parameters, physical properties and polarity functions of employed
solvents.
Table 3. TDDFT results for pyridine samples with dipole moments (∆μ) of the most probable
excitations, their wavelengths λmax, oscillator strengths (f), and involved molecular orbitals.
Table 4. Absorbance and fluorescence data of sample I in various solvents.
Table 5. Absorbance and fluorescence data of II in various solvents.
Table 6. Absorbance and fluorescence data of III in various solvents.
Table 7. a) Regression fit to Kamlet-Abboud-Taft multi-parameter solvent polarity scales for
absorbance, fluorescence and stokes shift of used samples b) Percentage contribution of various
solute-solvent interactions.
Table 8. a) Regression fit to Catalan multi-parameter solvent polarity scales for absorbance,
fluorescence and stokes shift of used samples b) Percentage contribution of various solute-solvent
interactions.
Table 9. Dipole moments and essential parameters for measuring them.
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Molecular structure and Molecular name
Molecular weight (g/mol)
2
26.23
(
E)-3-(pyridin-4-ylmethyleneamino)benzoic acid
(
Sample I)
286.33
1
4
1
4
(
N E,N E)-N ,N -bis(pyridin-4-ylmethylene)benzene-1,4-diamine
Sample II)
(
286.33
1
4
1
4
(
N E,N E)-N ,N -bis(pyridin-3-ylmethylene)benzene-1,4-diamine
Sample III)
Table 1. Molecular name, molecular structure and molecular weight of used samples.
(
2
9