Substituent and solvent effects on intramolecular charge transfer of 5-arylidene-2,4-thiazolidinediones

Article abstract of DOI:10.1016/j.saa.2011.10.074

The absorption spectra of twelve 5-arylidene-2,4-thiazolidinediones were recorded in twenty one solvents in the range from 300 to 600 nm. The effect of specific and non-specific solvent-solute interactions on the absorption maxima shifts were evaluated by

Full text of DOI:10.1016/j.saa.2011.10.074

Spectrochimica Acta Part A 86 (2012) 500–507
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Substituent and solvent effects on intramolecular charge transfer of
5-arylidene-2,4-thiazolidinediones
Milica Rancˇic´ a, Nemanja Trisˇovic´ b, Milosˇ Milcˇic´ c, Gordana Usˇc´umlic´ b,∗, Aleksandar Marinkovic´ b
a Faculty of Forestry Science, University of Belgrade, Kneza Visˇeslava 1, 11030 Belgrade, Serbia
b Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade, Serbia
c Faculty of Chemistry, University of Belgrade, Studentski trg 12-16, 11001 Belgrade, Serbia
a r t i c l e i n f o
a b s t r a c t
Article history:
The absorption spectra of twelve 5-arylidene-2,4-thiazolidinediones were recorded in twenty one sol-
vents in the range from 300 to 600 nm. The effect of specific and non-specific solvent–solute interactions
on the absorption maxima shifts were evaluated by using the Catalán solvent parameter set. Furthermore,
the experimental findings were interpreted with the aid of ab initio MP2 and time-dependent density
functional (TD-DFT) methods. It was found that different substituents significantly change the extent of
conjugation in the molecules and further affect their intramolecular charge transfer character.
© 2011 Elsevier B.V. All rights reserved.
Received 30 August 2011
Received in revised form 24 October 2011
Accepted 29 October 2011
Keywords:
5-arylidene-2,4-thiazolidinediones
Absorption spectra
Solvent effects
Substituent effects
Ab initio MO calculation
1. Introduction
centrosymmetric molecular arrangement in crystalline state, the
introduction of a methoxy group into para-position of phenyl moi-
2,4-Thiazolidinedione (TZD) is an interesting structural unit
in medicinal chemistry and responsible for numerous pharma-
cological properties and biological activities, e.g., antidiabetic [1],
antiarthritic [2], antibacterial [3] and oncostatic [4]. Due to this,
structures and relative stabilities of the potential tautomeric forms
of TZD derivatives have been extensively studied from both the-
oretical and experimental point of view [5–7]. Electronic spectra
of these molecules have been investigated mostly for reasons of
the determination of their acidities and basicities, as well as the
energies of protonation and deprotonation [8,9]. An ab initio HF
and MP2 study of static hyperpolarizability of TZD has also been
reported [10].
The second-order nonlinear optical properties arise from a
non-centrosymmetric ꢀ-conjugated charge-transfer (CT) molecule
containing both electron donor and electron acceptor groups con-
nected by an electron-transmitting bridge. The importance of
heterocycles in the design of such donor–acceptor systems has been
extensively discussed [11–13]. 5-Arylidene-2,4-thiazolidinediones
are expected to be useful as second-order nonlinear optical mate-
rials from the electron deficient TZD moiety and the arylidene
part of molecule designed to be responsive to the TZD demand,
ety causes enhancement of molecular hyperpolarizability (ˇ) and
brings about a non-centrosymmetric molecular arrangement [14].
It has been established that the photophysical properties of these
derivatives are mainly governed by the polarity of the medium,
hydrogen bonding and electronic substituent effects at arylidene
moiety [15]. The absorbance and the fluorescence spectra reflect
their solvent polarity dependence, whereas the diminished flu-
orescence intensities in polar solvents indicate the formation of
hydrogen bonds resulting in radiationless decay.
In this work, twelve 5-arylidene-2,4-thiazolidinediones (Fig. 1a)
were synthesized in order to characterize their absorption bands,
as well as to study contributions of appropriate substituent- and
solvent-dependent electronic transitions observed in the corre-
sponding spectra. The absorption spectra were recorded in the
range from 300 to 600 nm in 21 solvents of different properties. The
influence of the interactions of these molecules with the surround-
ing media was investigated by solvatochromic studies, whereby
the appropriate contributions of the specific solvent–solute inter-
actions can be defined (Fig. 1a). The effects of solvent dipolarity,
polarizability and solvent–solute hydrogen bonding interactions
were evaluated by means of the linear solvation energy relationship
(LSER) model of Catalán [16], given by Eq. (1):
respectively. While 5-arylidene-2,4-thiazolidinedione takes
a
ꢁ = ꢁ0 + aSA + bSB + cSP + dSdP
(1)
∗
where SA, SB, SP and SdP characterize solvent acidity, basicity,
polarizability and dipolarity of a solvent, respectively; and a–d
Corresponding author. Tel.: +381 11 3303 869; fax: +381 11 3370 387.
E-mail address: goca@tmf.bg.ac.rs (G. Usˇc´umlic´).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.10.074

M. Rancˇi´c et al. / Spectrochimica Acta Part A 86 (2012) 500–507
501
Fig. 1. Structures of the investigated 5-arylidene-2,4-thiazolidinediones, where X is: H (1); Me (2); iPr (3); OMe (4); OEt (5); OH (6); NH2 (7); NMe2 (8); Cl (9); Br (10); CN
(11) and NO2 (12) with possible solvent–solute interaction modes (a), and the optimized conformation of 5-benzylidene-2,4-thiazolidinedione (b).
are the regression coefficients describing the sensitivity of the
absorption maxima to the different types of the solvent–solute
interactions. The advantage of this concept is that it gives possibility
to separate non-specific solvent effects into two terms: dipolarity
and polarizability.
Additionally, the linear free energy relationship (LFER) method-
ology was applied to the ꢁmax of the studied TZD derivatives with
the aim to get an insight into factors determining the absorption
maxima shifts. The transmission of electronic substituent effects
(Fig. 1a) was studied using the simple Hammett equation:
surface polarized continuum model (IPCM). All calculations were
done with Gaussian03 software [21].
The correlation analysis was carried out using Microsoft Excel
software that considers the 95% confidence level. The goodness of
fit is discussed using the correlation coefficient (R), the standard
error of the estimate (sd), and Fisher’s significance test (F).
3. Results and discussion
A generally accepted concept, for the relationship between
molecular structure and absorption spectrum, requires the max-
imum planarity to produce red shift due to the increase of
␲-conjugation. Based on this, a series of TZDs coupled with
para-substituted benzaldehydes was synthesized, supposing that
the obtained ␲-conjugated systems show different sensitivities
towards solvent and substituent effects.
The absorption spectra of the investigated compounds were
recorded in 21 solvents in the range from 300 to 600 nm and the
characteristic cases in representative solvents are shown in Fig. 2.
It can be seen that some absorption bands (e.g., 3, 9, 10, 11) are
broadened and split in the protic solvent ethanol. This splitting
is proposed to originate from the keto-enol tautomerization. The
main absorption is assigned to ICT transition in the keto form and
the corresponding maxima are collected in Table 1. The low-energy
shoulder appearing mostly in protic solvents arises from the enol
form. However, the further discussion is directed only to absorp-
tion maxima of the keto form of the investigated compounds due
to the higher stability and presence in all used solvents.
s = ꢂꢃ + h
(2)
where s is a substituent-dependent value: absorption frequencies
(ꢁmax), ꢂ is the proportionality constant reflecting the sensitiv-
ity of the ꢁmax to the substituent effects, ꢃ is the corresponding
substituent constant, and h is the intercept (i.e., describes the
unsubstituted member of the series) [17]. Eq. (2) attributes the
observed substituent effect to an additive blend of polar and ꢀ-
delocalization effects given as corresponding ꢃ values.
Ab initio MP2 calculations indicate non-planar conformations of
the investigated TZD derivatives. The contributions of electronic
substituent effects were further discussed in relation to the geom-
etry found by MP2 calculations. The most stable conformation
of 5-benzylidene-2,4-thiazolidinedione is presented by the struc-
ture in Fig. 1b. Time-dependent density functional theory (TD-DFT)
appears to be quite successful for the states of solvated organic
molecules corresponding to an electronic transfer from a donor
group to an acceptor moiety [18] or to twisted intramolecular
charge transfer (ICT) situations [19]. With the help of a TD-DFT
model systematically accounting for solvent effects, the absorp-
tion spectra were computed in order to obtain the best similarity
to experimental ones.
The absorption maxima of all compounds showed red shift rela-
tive to 1 and were also red-shifted with increasing solvent polarity
and hydrogen-bonding capacity. Thus, the introduction of electron
donor substituents into the arylidene part produces larger red shifts
compared to electron acceptors suggesting a more pronounced CT
interaction in a molecule.
2. Experimental
5-Arylidene-2,4-thiazolidinediones were synthesized by Kno-
evenagel condensation of 2,4-thiazolidinedione (TZD) with appro-
priate aryl aldehydes [20].The UV absorption spectra were
measured with a Shimadzu 1700 UV/vis spectrophotometer. The
UV/vis spectra were taken in spectro quality solvents (Fluka) at a
concentration of 1 × 10−5 mol dm−3 and showed no dependence on
the concentration.
Geometries of all twelve molecules were fully optimized with
MP2/6-31G(**) method. Global minima were found for every
molecule. Theoretical absorption spectra were calculated in gas
phase and ethanol solution with TD-DFT method for molecules
1, 8 and 12 on optimized geometry with B3LYP functional and
6-31G(**) basis set. Solvent was simulated with static isodensity
3.1. Solvent effects
The effect of various types of solvent–solute interactions on the
absorption maxima shifts was interpreted by means of the linear
solvation energy relationship (LSER) model of Catalán. The Catalán
parameters for the solvents used in correlations are listed in Table 2.
The regression values ꢁo, a, b, c and d fit at the 95% confidence level
are presented in Table 3.
The success degree of the quantification and interpretation of
solvent effects on the absorption frequencies of the investigated
molecules is shown in Fig. 3 by means of a plot of ꢁ observed (ꢁexp
)
versus ꢁ calculated (ꢁcalc) in different solvents (R = 0.997, sd = 0.16,
F = 33,627).

502
M. Rancˇi´c et al. / Spectrochimica Acta Part A 86 (2012) 500–507
Table 1
The absorption frequencies of the investigated compounds in selected solvents.
Solvent/No.
ꢁmax × 10−3 (cm−1
)
1
2
3
4
5
6
7
8
9
10
29.72b
11
12
Water
Methanol
Ethanol
2-Propanol
1-Butanol
2-Methyl-2-propanol
1-Hexanol
Ethylene glycol
Tetrahydrofuran
Disopropyl ether
Dimethyl sulfoxide
Methyl acetate
Ethyl acetate
N,N-Dimethylformamide
N,N-Dimethylacetamide
Chloroform
Anisole
Acetic acid
Acetonitrile
Pyridine
30.26b
31.06
30.96
30.91
30.92
30.86
30.90
30.56
31.00
31.11a
30.60
31.09
31.04
30.41
30.60
30.75
30.54
30.86
31.06
30.56
31.10
29.72b
30.49
30.40
30.49
30.28
30.35
30.40
29.87
30.47
30.62a
30.05
30.51
30.54
30.32
30.28
30.01
29.92
30.17
30.44
30.01
30.53
29.59b
30.40
30.35
30.26
30.30
30.21
30.32
29.83
30.41
30.58a
29.92
30.41
30.38
30.23
30.14
29.98
29.83
30.08
30.30
29.96
30.40
28.78
29.03
29.15
29.24
29.15
29.15
28.97
28.67b
29.19
29.48
28.67b
29.22
29.31
29.64a
28.95
28.84
28.70
28.69
29.24
28.87
29.37
28.61
28.94
29.11
28.94
28.92
28.90
28.99
28.56
29.10
29.38
28.54
29.14
29.24
29.64a
28.87
28.65
28.59
28.49b
28.94
28.64
29.24
28.61
28.78
28.74
28.45
28.56
28.25
28.51
28.28
28.95
29.14
28.17
29.22a
28.99
28.82
28.60
28.30
28.90
28.53
29.15
28.14b
29.03
26.54
25.67
25.51
25.54
25.59
25.32
25.38
25.35
25.93
26.50
24.83b
26.33
26.22
26.88a
25.13
25.69
26.25
26.14
26.46
25.47
26.32
23.87
24.48
24.63
24.75
24.63
24.60
24.61
23.78b
24.83
25.23
23.99
24.76
24.93
26.55a
24.37
24.06
24.21
24.10
24.51
24.37
24.91
29.85b
30.72
30.63
30.53
30.43
30.58
30.54
30.16
30.60
30.73a
30.19
30.67
30.67
30.19
30.08
30.40
30.03
30.40
30.63
30.16
30.63
30.53a
30.30
30.35
30.35
30.12
30.35
30.23
29.24
30.34
30.49
28.80
30.49
30.41
28.70
28.60b
30.05
29.94
30.35
30.40
28.60b
30.53a
29.28
29.07
28.82
29.07
28.84
29.15
28.89
28.28
28.94
29.36a
27.46
29.24
29.21
26.25
25.76b
28.80
28.74
29.11
29.15
28.36
29.28
30.44
30.40
30.35
30.30
30.40
30.40
29.90
30.47
30.66a
29.82
30.60
30.66a
29.94
29.98
30.21
29.98
30.40
30.53
30.07
30.53
Heptane
ꢄꢁ (cm−1
)
850
900
990
970
1150
1080
2050
2770
880
940
1930
3600
a
Solvent with the highest hypsochromic shift.
Solvent with the highest bathochromic shift.
b
Fig. 2. The absorption spectra of 5-arylidene-2,4-thiazolidinediones in ethanol (a) and anisole (b).
The correlation results obtained according to Eq. (1) (Table 3)
imply that the solvent polarizability is the principal factor influ-
encing the shift of ꢁmax, whereas the solvent dipolarity, acidity
and basicity have moderate to low contribution to solvatochromic
properties of the investigated compounds. Negative values of
the coefficients c and d indicate higher solvent dipolarity and
polarizability effects in the electronic excited state compared
with the ground state. Therefore, the neutral resonance structure
(Figs. 4 and 5; structure I) clearly dominates over the zwitterionic
ones (Fig. 4; structures II–IV and Fig. 5; structures V and VI) in the
ground state, whereas in the excited state the reverse situation is
found. This behavior is in fact typical for ICT processes.
A low variation in the c values is observed for the electron
donor substituted derivatives (2–8), indicating a lower contribu-
tion of extended resonance transmission. Obviously, the UV/vis
absorption of these compounds is more substituent-dependent
than solvent-dependent (Table 1). Although the TZD moiety acts as
an electron acceptor group, the introduction of different electron
Fig. 3. The plot of ꢁexp against ꢁcalc
.

M. Rancˇi´c et al. / Spectrochimica Acta Part A 86 (2012) 500–507
503
Table 2
Solvent parameters [16].
(Fig. 4; structure IV) causing larger localized ꢀ-electronic density
shifts. The largest red shifts of compounds 7 and 8 are consistent
with the strongest electron-donating properties of the NMe2 and
NH2 groups.
Solvent
SP
SdP
SA
SB
Water
Methanol
Ethanol
2-Propanol
1-Butanol
2-Methyl-2-propanol
1-Hexanol
Ethylene glycol
Tetrahydrofuran
Disopropyl ether
Dimethyl sulfoxide
Methyl acetate
Ethyl acetate
N,N-Dimethylformamide
N,N-Dimethylacetamide
Chloroform
Anisole
Acetic acid
Acetonitrile
Pyridine
0.681
0.608
0.633
0.633
0.674
0.632
0.698
0.777
0.714
0.645
0.830
0.645
0.656
0.759
0.763
0.783
0.820
0.651
0.645
0.842
0.635
0.997
0.904
0.783
0.808
0.655
0.732
0.552
0.910
0.634
0.286
1.000
0.637
0.603
0.977
0.987
0.614
0.543
0.676
0.974
0.761
0.000
1.062
0.605
0.400
0.283
0.341
0.145
0.315
0.717
0.000
0.000
0.072
0.000
0.000
0.031
0.028
0.047
0.084
0.689
0.044
0.033
0.000
0.025
0.545
0.658
0.830
0.809
0.928
0.879
0.534
0.591
0.666
0.647
0.527
0.542
0.613
0.65
The opposite is observed for the electron acceptor substi-
tuted derivatives (9–12). The molecular polarizability is also
conformation-dependent property and could be to an appropriate
extent a consequence of the transmission of electronic substituent
effects. The large polarizability of these compounds indicates that
the ꢀ-electronic polarization is not necessarily transmitted through
the whole ꢀ-network, but the polarization of each localized ꢀ-unit
is of higher significance. This is ascribed to the destabilization of
the TZD moiety by electron acceptor groups causing the change in
the electronic distribution to be more sensitive to solvent effects.
Dipolar solvent–solute interactions, assigned with the d term,
are of importance only if a strong electron donor (except compound
6) or acceptor substituent is present in the arylidene part of the
molecule. However, the electron donors influence dipolar charac-
teristics of the investigated molecules differently from the electron
acceptors. Electronic substituent effects, polar (field/inductive) and
resonance, transmitted through the molecule, cause the separa-
tion of charges susceptible to solvent dipolar interactions. It seems
that the higher contribution of the solvent dipolar effect towards
the electron acceptor substituted derivatives 11 and 12 is due to
two distinct ꢀ-electronic entities: the arylidene and TZD moieties
that possess dipolar characteristic differently oriented in space. It
is typical for styrene derivatives that the zwitterionic state of the
derivatives with an electron acceptor substituent has a much larger
0.071
0.299
0.390
0.286
0.581
0.083
Heptane
donor substituents does not significantly influence the variation in
the mobility of the ꢀ-electrons, i.e., enhance the molecule polariz-
ability. It is expected that the highly ꢀ-conjugated structures have
the highest polarizability. A greater extent of resonance interac-
tion is operative within more planar ꢀ1- and ꢀ2-resonance units
Fig. 4. Resonance structures of electron donor substituted 5-arylidene-2,4-thiazolidinediones.
Fig. 5. Resonance structures of electron acceptor substituted 5-arylidene-2,4-thiazolidinediones.

504
M. Rancˇi´c et al. / Spectrochimica Acta Part A 86 (2012) 500–507
Table 3
Regression fits to the solvatochromic parameters.
No.
ꢁo × 10−3
a × 10−3
b × 10−3
c × 10−3
d × 10−3
Ra
sdb
Fc
Solvents excluded from
the correlatione
(cm−1
)
(cm−1
)
(cm−1
)
(cm−1
)
(cm−1
–
)
d
1
32.64
0.18)
−0.24
–
–2.47
0.25)
0.928
0.948
0.914
0.949
0.952
0.930
0.977
0.939
0.930
0.938
0.927
0.918
0.08
0.09
0.11
0.08
0.09
0.14
0.13
0.15
0.11
0.11
0.29
0.48
49.4
50.3
28.6
45.5
31.1
19.2
68.6
28.0
29.7
38.9
32.3
14.7
W, DMF
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
0.08)
(
(
(
(
(
(
(
(
(
(
(
2
32.15
0.21)
−0.51
0.21
−2.65
–
–
–
0.07)
(
(
0.08)
0.27)
3
31.89
0.26)
−0.49
0.22
0.10)
−2.44
–
0.08)
0.34)
4
31.06
0.22)
−0.38
–
−2.60
−0.14
DMF, AcOH
0.08)
0.28)
(
(
(
(
(
0.09)
5
30.85
0.22)
−0.30
0.18
−2.50
−0.28
tBuOH, DMF, AcOH
tBuOH, DMF, Chl, Hp
DMF, Chl, Hp
DMF
0.09)
(
(
(
(
(
0.09)
0.32)
0.11)
6
31.83
0.39)
−0.66
−0.93
−3.02
−0.43
0.13)
0.18)
0.50)
0.20)
7
30.28
0.34)
−0.33
−2.13
−3.16
−1.31
0.12)
0.15)
0.44)
0.18)
8
26.78
0.36)
−0.62
0.38
0.14)
−2.88
−0.48
0.14)
0.50)
0.17)
9
32.25
0.26)
−0.44
0.20
0.10)
−2.63
–
MeOH, DMAc, Chl
Hp
0.09)
0.34)
10
11
32.80
0.25)
−0.39
–
−2.93
−0.50
0.09)
0.36)
(
(
(
0.14)
36.82
0.71)
–
−1.02
−7.44
−1.44
Hp
(
0.28)
0.91)
0.35)
12
40.36
1.79)
1.16
−1.69
−10.93
2.10)
−4.60
iPr2O, DMSO, AcOH, Py, Hp
(
0.46)
(
0.53)
(
0.82)
a
Correlation coefficient.
Standard deviation.
Fisher test of significance.
Negligible values with high standard errors.
Abbreviations taken from www.chemnetbase.com.
b
c
d
e
dipole moment than those of the parent molecule and derivatives
with an electron donor substituent [22]. The better stabilization
of the excited state relative to the ground state in dipolar solvents
results in red shift and depends mainly on the solute dipole moment
change during the transition.
the stabilization of the solute in the excited state. High values of
the coefficient b for electron acceptor substituted compounds 11
and 12 are a consequence of the strong electron-accepting prop-
erties of NO2 and CN groups, and the contribution of the both
carbonyl groups polarization (Fig. 4; structures II and III) contribut-
ing an increased NH proton-donating ability. Although substituents
with +M effect (compound 6 and 7) cause an increase of the elec-
tron density at the TZD moiety, their proton-donating capability
is significant, regardless if it comes from substituent or the TZD
moiety. Positive sign and lower values of the coefficient b for other
compounds (Table 3), considering the electron donor substituted
compounds, clearly indicate the significance of proton-donating
capabilities of OH and NH2 substituents.
Specific solvent–solute interactions through hydrogen bonding,
i.e., solvent acidity and basicity, can be attributed mainly to the car-
bonyl and NH groups of the TZD moiety. Low negative values of the
coefficient a for compounds 1–10 indicate moderate to low con-
tribution of the solvent acidity to the stabilization of the excited
state of the investigated compounds. It might be a consequence of
the long-range transmission of substituent effects, which supports
the larger polarization of both carbonyl groups and, in that way,
enhances the hydrogen bonding capabilities of carbonyl oxygens.
The highest contribution of the solvent acidity to solvatochromic
properties was observed for compounds 6 and 8. Such observa-
tion indicates that the overall hydrogen-bond accepting ability of
these compounds originate from the appropriate contributions of
the substituent lone pair and the TZD moiety proton-accepting
sites. These observations indicate that solvent achieves the higher
extent of hydrogen bonding with nucleophilic sites at the TZD moi-
ety, and its low dependence on substituent effects was observed
(Table 3). Moreover, solvent-dependent hypsochromic shifts of
ꢁmax observed for compound 12 indicates that this molecule is more
susceptible to the solvent HBD stabilization in the ground state,
due to a higher contribution of the structures II and III (Fig. 4). Basi-
cally, deviation from the general trend of the value of the coefficient
a for compound 12 (Table 3) is a consequence of the appropriate
geometry and transmission mode of substituent effects.
3.2. Substituent effects
In an attempt to assess substituent effects on ICT of the inves-
tigated molecules, the principles of liner free energy relationships
(LFER) were applied to the UV/vis spectral data using Eq. (2). The
correlation results are given in Table 4.
The correlation results (Table 4), obtained separately for the
electron donor and electron acceptor substituted derivatives,
reflect different transmission modes of electronic substituent
effects through differently oriented ꢀ-electronic units (Fig. 1a). The
introduction of an electron donor substituent enhances the positive
solvatochromism of the molecules and also results in a higher sus-
ceptibility of the absorption maxima shifts to electronic substituent
effects. Concerning the electron acceptor substituted derivatives,
a significantly lower sensitivity is observed. Higher sensitivities of
their absorption frequencies to substituent effects in aprotic dipolar
solvents (DMSO, DMF and DMAc) can be explained by the fact that,
The highest contribution of the solvent HBA effect was observed
for compound 12, while it was lower for 6, 7 and 11, in line with

M. Rancˇi´c et al. / Spectrochimica Acta Part A 86 (2012) 500–507
505
Table 4
Regression fits according to Eq. (2) for the electron donor and acceptor substituted derivatives.
Solvent
Electron-donor substituted derivatives (1–8)
Electron-acceptor substituted derivatives (1, 9–12)
ꢁo × 10−3
ꢂ × 10−3
R
sd
F
ꢁo × 10−3
ꢂ × 10−3
R
sd
F
(cm−1
)
(cm−1
)
(cm−1
)
(cm−1
)
Water
30.71
0.32)
7.32
0.74)
0.970
0.988
0.989
0.987
0.990
0.988
0.988
0.991
0.988
0.990
0.988
0.985
0.990
0.980
0.983
0.993
0.977
0.983
0.982
0.991
0.990
0.54
0.39
0.37
0.39
0.34
0.38
0.39
0.33
0.37
0.32
0.41
0.40
0.33
0.33
0.47
0.30
0.49
0.45
0.45
0.32
0.32
97
30.14
0.10)
−1.16
0.957
0.989
0.989
0.994
0.998
0.990
0.993
0.991
0.994
0.997
0.987
0.997
0.994
0.967
0.977
0.992
1.000
0.999
0.997
0.993
0.999
0.14
0.16
0.17
0.10
0.08
0.13
0.13
0.16
0.12
0.08
0.29
0.07
0.11
0.61
0.58
0.13
0.03
0.04
0.08
0.15
0.05
22
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
0.25)
Methanol
31.42
0.23)
8.38
0.54)
245
263
232
304
255
243
346
244
288
241
200
294
149
172
401
127
174
162
343
308
31.13
0.12)
−2.60
90
91
0.27)
Ethanol
30.39
0.22)
8.29
0.51)
31.08
0.13)
−2.83
0.30)
2-Propanol
31.30
0.23)
8.00
0.53)
30.96
0.08)
−2.39
180
413
102
136
113
178
292
68
0.18)
1-Butanol
31.28
0.20)
8.13
0.47)
30.96
0.06)
−2.69
0.13)
2-Methyl-2-propanol
1-Hexanol
31.24
0.22)
8.29
0.52)
30.95
0.09)
−2.26
0.22)
31.30
0.23)
8.30
0.53)
31.01
0.10)
−2.66
0.23)
Ethylene glycol
Tetrahydrofuran
Disopropyl ether
Dimethyl sulfoxide
Methyl acetate
Ethyl acetate
N,N-Dimethyl formamide
N,N-Dimethyl acetamide
Chloroform
30.95
0.19)
8.43
0.45)
30.65
0.12)
−2.99
0.28)
31.39
0.17)
7.90
0.51)
31.09
0.19)
−2.71
0.20)
31.18
0.19)
7.44
0.44)
29.96
0.06)
−2.29
0.13)
31.01
0.24)
8.67
0.56)
30.82
0.21)
−4.19
0.51)
31.45
0.23)
7.79
0.55)
31.15
0.05)
−2.42
385
155
29
0.12)
31.42
0.19)
7.71
0.45)
31.15
0.08)
−2.43
0.19)
30.25
0.20)
5.44
0.45)
29.96
0.19)
−5.76
1.07)
31.18
0.27)
8.35
0.64)
31.16
0.43)
−6.62
42
1.02)
31.05
0.17)
8.22
0.41)
30.83
0.10)
−2.56
129
2769
1175
359
133
688
0.23)
Anisole
30.88
0.28)
7.47
0.66)
30.54
0.02)
−2.31
0.04)
Acetic acid
31.07
0.26)
8.00
0.61)
30.90
0.03)
−2.27
0.07)
Acetonitrile
31.40
0.26)
7.83
0.61)
31.11
0.06)
−2.49
0.13)
Pyridine
30.93
0.18)
7.95
0.43)
30.69
0.11)
−2.92
0.25)
Heptane
31.46
0.19)
7.71
0.44)
31.11
0.04)
−2.34
0.09)
at high relative permittivity of surrounding medium, the energy
necessary to bring about charge separation in the excited state is
relatively small, which gives rise to a higher susceptibility to elec-
tronic substituent effects.
In addition, the geometries of the investigated molecules were
fully optimized by the use of ab initio MP2 method. The signifi-
cant stability of the diketo tautomer was obtained and results of
geometry calculation are presented in Table 5.
It can be seen that the geometric features of the substituted
derivatives are very similar to those of the unsubstituted one. How-
ever, the relationship between structure and the main absorption
band is not fully consistent with the above-mentioned concept. The
main absorption bands of the electron-donor substituted deriva-
tives (2–8) appear at longer wavelengths (∼20–100 nm) compared
to those of 1. An electron donor substituent supports an electron
density shift from the ꢀ1-unit to the TZD moiety causing whole
molecule planarization. A decrease in the C4–C5 bond length, which
is a part of the enone system (ꢀ2-unit), indicates a greater extent
of the ꢀ,ꢀ-delocalization. Thus, the carbonyl groups are slightly
longer in the electron donor substituted derivatives. This result
is an additional support for the extended conjugation operative
in the ꢀ1- and ꢀ2-unit, i.e., the ꢀ-electron density shifts towards
C4 O carbonyl group increasing both carbonyl bond lengths and
supporting n, ꢀ-conjugation in the ꢀ3-unit.
The results are quite the opposite for the electron acceptor
substituted compounds (9–12). The deviation from the planarity
increases with increasing electron acceptor ability of the arylidene
substituent. An increase in the C4–C5 bond length indicates that

506
M. Rancˇi´c et al. / Spectrochimica Acta Part A 86 (2012) 500–507
Table 5
Elements of the optimized geometries of investigated compounds obtained by the use of ab initio MP2 method.
No. Interatomic distance (Å)
Torsion angle
C2–N3
N3–C4
C4–C5
C5–C6
C6–C7
C4
O
C2
O
ꢅ (◦)
1
2
3
4
5
6
7
8
9
1.3867
1.3867
1.3866
1.3862
1.3862
1.3863
1.3857
1.3855
1.3866
1.3866
1.3867
1.3867
1.3909
1.3912
1.3913
1.3918
1.3919
1.3916
1.3926
1.3931
1.3906
1.3905
1.3898
1.3897
1.4925
1.4919
1.4918
1.4904
1.4903
1.4908
1.4891
1.4882
1.4931
1.4933
1.4949
1.4949
1.3505
1.3507
1.3508
1.3513
1.3513
1.3511
1.3517
1.3524
1.3510
1.3510
1.3515
1.3519
1.4573
1.4560
1.4558
1.4536
1.4534
1.4542
1.4521
1.4501
1.4557
1.4558
1.4560
1.4558
1.2224
1.2225
1.2225
1.2229
1.2229
1.2228
1.2230
1.2233
1.2222
1.2222
1.2220
1.2221
1.2138
1.2140
1.2140
1.2142
1.2143
1.2141
1.2145
1.2148
1.2133
1.2133
1.2127
1.2126
27.36
26.60
26.42
23.89
23.79
24.52
22.52
20.99
25.96
26.04
26.57
26.53
10
11
12
Table 6
Calculated energies of the HOMO and LUMO orbitals and energy gap for compounds 1, 8 and 12 in gas phase and ethanol.
No.
Gas phase
Ethanol
EHOMO (eV)
ELUMO (eV)
Egap (eV)
EHOMO (eV)
ELUMO (eV)
Egap (eV)
1
8
12
−6.37
−5.46
−6.94
−2.26
−1.89
−3.56
4.11
3.57
3.72
−6.25
−5.37
−6.20
−2.18
−1.96
−3.05
4.07
3.41
3.16
two opposite electron accepting effects operate in the ꢀ2-unit: elec-
tron accepting arylidene group and TZD carbonyl groups causing a
non-planar geometrical adjustment together with low sulfur posi-
tive resonance participation as a response to electronic demand of
the electron deficient environment (Fig. 5; structure VI). The nor-
mal carbonyl groups polarization is suppressed and causes a slight
bond length decrease. Nevertheless, the longer ꢆmax are obtained
than for 1.
Mechanism of electronic excitations and changes in the over-
all charge distribution in both ground and excited states of the
investigated molecules were studied by calculation of the energy
gaps between the HOMO and LUMO orbitals for the unsubstituted
derivative (1) and derivatives having a strong electron donor (8)
and a strong electron acceptor substituent (12) in the arylidene
part, and results are presented in Fig. 6 and Table 6.
solvent effects through TD-DFT calculations leads to a decrease in
energy gaps by 0.08–0.56 (Table 6). Furthermore, the ICT character
can be clearly observed during the orbital transition process from
the HOMO to LUMO. It can be noticed that the electron densities of
the HOMO and LUMO orbitals for 8 are separately populated on the
phenyl ring and the TZD moiety, respectively, confirming its strong
ICT character. The electron density of the LUMO for 1 is localized
on both the phenyl and TZD rings. Due to this, its ICT character is
weaker than that of 8. It should be noted that the electron density of
the LUMO orbital for 12 is dominantly populated on the phenyl ring
due to strong electron accepting character of the nitro substituent
causing ꢀ-electron density shift to arylidene moiety regardless of
oppositely oriented, but weaker effects of the TZD moiety.
Obviously, the ICT process is more feasible in compounds 8 and
12. For compound 8, the reason is higher planarity, while, in com-
pound 12, higher rotation around the bond connecting the electron
donor and acceptor group can decouple the orbitals of these two
groups providing means by which nearly complete charge transfer
from the donor to the acceptor can occur. The highly polarizable
Compound 1 has the largest energy gap with 4.11 eV, while
somewhat smaller energy gaps for compounds 8 and 12 with 3.56
and 3.72 eV, respectively, were observed. This is consistent with red
shift of their absorption maxima relative to those of 1. Inclusion of
Fig. 6. The molecular orbitals and energy gaps between HOMOs and LUMOs of compounds 1, 8 and 12.

M. Rancˇi´c et al. / Spectrochimica Acta Part A 86 (2012) 500–507
507
twisted excited state of compound 12 can be, under appropriate sol-
vent conditions, preferentially stabilized with respect to the ground
state (Table 3). Due to this, the excited and ground states are closer
and more rapid internal conversion is permitted. Variation of sub-
stituent patterns clearly indicates that contributions of both twist
and donor–acceptor character are involved in the ICT mechanism
of the investigated molecules.
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4. Conclusions
Solvent effects on the absorption maxima shifts of the investi-
gated compounds were successfully evaluated by using the Catalán
solvent parameter set. Solvent polarizability is the principal fac-
tor which influences the shift of absorption maxima, whereas the
solvent dipolarity, acidity and basicity are less important. The
absorption maxima undergo a bathochromic shift with increasing
solvent polarizability indicating that the excitation state is more
polarizable than the ground state. Specific interactions through
hydrogen bonding, expressed by solvent acidity and basicity, can
be attributed mainly to the carbonyl moiety and NH hydrogen
and they are only slightly affected by the arylidene substituent.
Moreover, different substituents significantly change the extent of
conjugation in the molecules and further affect their ICT character.
The correlation results imply that the solvatochromic properties
are the consequence of the overall effect of the molecule geom-
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through ꢀ-conjugated systems. The results obtained in this study
help in assessing the potential application of the investigated com-
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