Synthesis, spectroscopic characterizations and quantum chemical computational studies of (Z)-4-[(E)-(4-fluorophenyl)diazenyl]-6-[(3- hydroxypropylamino)methylene]-2-methoxycyclohexa-2,4-dienone

Article abstract of DOI:10.1016/j.saa.2011.09.020

In this study, the molecular structure and spectroscopic properties of the title compound were characterized by X-ray diffraction, FT-IR and UV-vis spectroscopies. These properties were also investigated using DFT method. The most convenient conformation

Full text of DOI:10.1016/j.saa.2011.09.020

Spectrochimica Acta Part A 85 (2012) 85–91
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectroscopic characterizations and quantum chemical computational
studies of (Z)-4-[(E)-(4-fluorophenyl)diazenyl]-6-[(3-
hydroxypropylamino)methylene]-2-methoxycyclohexa-2,4-dienone
C¸ ig˘dem Albayraka,∗, Mustafa Odabas¸ og˘lub, Arzu Özekc, Orhan Büyükgüngörc
a Faculty of Education, Sinop University, 57100 Sinop, Turkey
b Chemical Technology Program, Pamukkale University, 20070 Kınıklı-Denizli, Turkey
c Department of Physics, Faculty of Arts and Sciences, Ondokuz Mayıs University, 55139 Kurupelit-Samsun, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, the molecular structure and spectroscopic properties of the title compound were charac-
terized by X-ray diffraction, FT-IR and UV–vis spectroscopies. These properties were also investigated
using DFT method. The most convenient conformation of title compound was firstly determined. The
geometry optimizations in gas phase and solvent media were performed by DFT methods with B3LYP
adding 6-31G(d) basis set. The differences between crystal and computational structures are due to crys-
tal packing in which hydrogen bonds play an important role. UV–vis spectra were recorded in different
organic solvents. The results show that title compound exists in both keto and enol forms in DMSO, EtOH
but it tends to shift towards enol form in benzene. The polar solvents facilitate the proton transfer by
decreasing the activation energy needed for Transition State. The formation of both keto and enol forms in
DMSO and EtOH is due to decrease in the activation energy. TD-DFT calculations starting from optimized
geometry were carried out in both gas and solution phases to calculate excitation energies of the title
compound. The non-linear optical properties were computed at the theory level and the title compound
showed a good second order non-linear optical property. In addition, thermodynamic properties were
obtained in the range of 100–500 K.
Received 3 February 2011
Received in revised form 7 September 2011
Accepted 8 September 2011
Keywords:
Schiff Base
Azo dye
Non-linear optical properties
DFT
TD-DFT
Spectral characterization
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Intramolecular proton transfer plays an important role in many
fields of chemistry [8,9]. Intramolecular proton transfer mechanism
Azo compounds have received much structural interest in chem-
istry due to their versatile applications in many different areas such
as polyester fiber [1], disperse dyes [2], as well as their use in many
biological reactions and in analytical chemistry [3]. Furthermore,
their application as industrial dyes and in biological systems where
some may be used as inhibitor for tumor growth [3] is of great
importance. Azobenzene is one of the most representative classes
of photochromic molecules with two geometric isomers, trans and
cis [4–6]. The trans-to-cis isomerization occurs by photo irradiation
with UV light and cis-to-trans isomerization proceeds with blue-
light irradiation or heating. It is generally accepted that their trans
forms are thermodynamically more stable than their cis forms [7].
which occurs in both excited and ground states is a subject of inten-
sive research [10–12]. Molecules exhibiting intramolecular proton
transfer are used as laser dyes, in higher energy radiation detectors,
memory storage devices, fluorescent probes and polymer protec-
tors [13–15]. Hence, many molecules such as o-hydroxy Schiff Bases
exhibiting intramolecular proton transfer have attracted consider-
able attention from both experimental and theoretical points of
view [16–21].
The DFT and TD-DFT (time-dependent density functional the-
ory) are of particular interest owing to give satisfactory results with
experiment by costing low computational demands compared to
the computational methods developed for the calculation of the
electronic structure and excitation energies of molecular systems
[22,23].
In this work, the crystal structure of (Z)-4-[(E)-(4-
fluorophenyl)diazenyl]-6-[(3-hydroxypropylamino)methylene]-
2-methoxycyclohexa-2,4-dienone (I) was determined by single
∗
Corresponding author. Tel.: +90 368 2715526; fax: +90 368 2715530.
E-mail address: calbayrak@sinop.edu.tr (C¸ . Albayrak).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.09.020

86
C¸ . Albayrak et al. / Spectrochimica Acta Part A 85 (2012) 85–91
Table 1
2.4. Computational procedures
Crystal data, data collection and refinement details.
All computations were performed by using Gaussian 03W
program package [26]. Full geometry optimization of the title
molecule was performed by using DFT method with Becke’s three-
parameters hybrid exchange-correlation functional (B3LYP) [27]
employing 6-31G(d) basis set [28] as implemented in Gaussian
03W.
Chemical formula
Crystal system, space group, Z
a
b
c
ˇ
V
Dx
C17H18FN3O3
Monoclinic, P 21/c, 4
˚
19.1832(8) A
˚
7.4944(4) A
˚
11.3173(5) A
103.002(4)◦
3
˚
1585.32(14) A
1.388 Mg m−3
The most convenient conformation of I was firstly investigated
in the gas phase. To determine the conformational energy pro-
files for keto form, the values of DFT energies were calculated as
the functions of the torsion angles Â1(C2C1N1N2), Â2(C8C7N2N1),
Â3(C15C16C17O3) and Â4(C9C14N3C15) from −180◦ to 180◦ var-
ied every 10◦, Â5(C10C11O2C13) from 0◦ to 360◦ varied every 10◦
and the molecular energy profiles were obtained. The geometry
optimization of the most convenient conformation was performed
by DFT methods with B3LYP adding 6-31G(d) basis set for calcula-
tions [29,30]. The optimized geometry of molecule, total molecular
energy and dipole moment were obtained from the optimization
output. Solution phase geometry optimizations were performed
with the same level of theory by the polarizable continuum model
(PCM) [31,32]. TD-DFT calculations starting from optimized geome-
tries for gas phase and solution phase were carried out at the same
theory level in order to calculate the excitation energies of enol and
keto tautomers. In addition, thermodynamic properties of I were
obtained by applying same level of theory.
˚
Radiation, ꢀ
ꢁ
T
Tmin, Tmax
F(0 0 0)
Diffractometer
Scanning mode
Scan range
Mo K␣, 0.71073 A
0.11 mm−1
293 K
0.481, 0.872
696
STOE IPDS II
ω
−23 < h < 23, −9 < k < 9, −13 < l < 13
Âmin, Âmax
2.2◦, 26.0◦
Number of measured/independent
reflections, Rint
Number of reflections with 2ꢂ(I)
Number of refined parameters
S
22,015/3107, 0.046
2399
221
1.04
R[F2 > 2ꢂ(F2)]
0.037
0.101
0.18, −0.19 e A
wR(F2)
−3
˚
ꢃꢄmax, ꢃꢄmin
crystal X-ray diffraction study. The structure of I was experimen-
tally characterized by FT-IR, UV–vis spectroscopies, investigated by
using DFT and excitation energies were carried out using TD-DFT
calculations starting from optimized geometry.
2.5. Synthesis
A mixture of 4-fluoroaniline (0.7 g, 6.5 mmol), water (20 mL)
and concentrated hydrochloric acid (1.6 mL, 19.7 mmol) was
stirred until a clear solution was obtained. This solution was
cooled to 273–278 K and a solution of sodium nitrite (0.6 g,
8.7 mmol) in water was added dropwise while the temperature
was maintained below 278 K. The resulting mixture was stirred for
30 min in an ice bath. o-Vanillin (1 g, 6.5 mmol) solution (pH 9) was
gradually added to a cooled solution of 4-fluorobenzenediazonium
chloride, prepared as described above, and the resulting mixture
was stirred at 273–278 K for 60 min in ice bath. The prod-
uct was recrystallized from ethanol to obtain solid (E)-2-hyd-
2. Experimental and computational methods
2.1. Instrumentation
The melting point was determined by StuartMP30 melting point
apparatus. FT-IR spectrum of I was recorded on a Bruker 2000 spec-
trometer in KBr disk. UV–vis absorption spectra were recorded on
a Thermo scientific BioGenesis UV-Vis spectrometer.
2.2. X-ray crystallography
roxy-3-methoxy-5-(4-fluorophenyldiazenyl)benzaldehyde.
The
compound (Z)-4-[(E)-(4-fluorophenyl)diazenyl]-6-[(3-hydroxy-
propylamino)methylene]-2-methoxycyclohexa-2,4-dienone was
prepared by refluxing a mixture of a solution containing (E)-2-
hydroxy-3-methoxy-5-(4-fluorophenyldiazenyl)benzaldehyde
(0.5 g, 1.82 mmol) prepared as described above in 20 mL
All diffraction measurements were performed at room temper-
ature (293 K) using graphite monochromated Mo K␣ radiation and
a STOE IPDS 2 diffractometer. Reflections were collected in the rota-
tion mode and cell parameters were determined by using X-AREA
software [24]. Absorption correction was achieved by the integra-
tion method via X-RED software [24]. The structure was solved
by direct methods using SHELXS-97 [25]. The refinement was car-
ried out by full-matrix least-squares method on the positional and
anisotropic temperature parameters of the non-hydrogen atoms, or
equivalents corresponding to 221 crystallographic parameters. All
non-hydrogen atom parameters were refined anisotropically and
all H atoms except for H1 were located in their idealized positions
and refined using a riding model with C–H distances in the range
ethanol and
a
solution containing 3-hydroxypropylamin
(0.137 g, 1.82 mmol) in 20 mL ethanol. The reaction mixture
was stirred for 2 h under reflux. The crystals of (Z)-4-[(E)-(4-
fluorophenyl)diazenyl]-6-[(3-hydroxypropylamino)methylene]-
2-methoxycyclohexa-2,4-dienone suitable for X-ray analysis
were obtained by slow evaporation from ethanol (yield 85%, m.p.
428–430 K).
˚
of 0.93–0.96 A. The data collection conditions and parameters of
refinement process are listed in Table 1.
3. Results and discussion
3.1. Structure determination
2.3. Supplementary data
The crystal data and the refinement details of (Z)-4-[(E)-(4-
fluorophenyl)diazenyl]-6-[(3-hydroxypropylamino)methylene]-
2-methoxycyclohexa-2,4-dienone (I) compound are given in
Table 1. The selected bond lengths and angles are given in Table 2.
The molecular structure of I is shown in Fig. 1 with the atom
numbering scheme. o-Hydroxy Schiff Bases exhibit tautomerism
by intramolecular proton transfer from the oxygen atom to the
Crystallographic data (excluding structure factors) for the struc-
ture in this paper have been deposited with the Cambridge
Crystallographic Data Centre as the supplementary publication no.
CCDC 787956. Copies of the data can be obtained, free of charge, on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax:
+44 1223 336033 or e-mail: deposit@ccdc.cam.ac.uk).

C¸ . Albayrak et al. / Spectrochimica Acta Part A 85 (2012) 85–91
87
Table 2
Table 3
´
˚
´
◦
˚
◦
The selected bond lengths, angles and torsion angles (A, ).
Hydrogen bonding geometry (A, ).
X-ray
1.2573(16)
DFT/B3LYP enol form DFT/B3LYP keto form
D–H· · ·A
D–H
H· · ·A
D· · ·A
∠D–H· · ·A
C10–O1
N3–C14
N1–N2
N1–C1
1.3339
1.2835
1.2600
1.3171
N3–H1· · ·O1
N3–H1· · ·O3
O3–H3A· · ·O1i
0.91(2)
0.91(2)
0.82
1.98(2)
2.25(2)
2.01
2.6774(15)
2.7897(16)
2.8092(13)
132.1(17)
117.4(16)
164
1.2871(18)
1.2605(16)
1.4287(17)
1.4056(17)
124.07(12)
122.94(12)
121.59(11)
1.2645
1.2678
1.4152
1.4146
Symmetry code: (i) −x + 2, −y + 1, −z + 1.
N2–C7
1.4073
1.3996
C14–N3–C15
O1–C10–C9
O1–C10–C11
119.8978
122.5454
118.1173
−179.4368
128.5611
123.2298
121.0173
−169.0142
In the title compound, the aromatic rings adopt a trans con-
figuration around the azo bridges. The dihedral angle between
the planes of two aromatic rings is 27.08(8)◦. The N N length of
C9–C14–N3–C15 −179.08(14)
˚
1.2605(16) A corresponds to double bond character and agrees well
with previously reported values [39,40].
Two significant intramolecular interactions are noted between
atom O1 and nitrogen atom N3 and between atom O3 and nitro-
gen atom N3. These interactions constitute a six-membered ring
˚
S(6). The O1· · ·N3 distance of 2.6774(14) A and O3· · ·N3 distance
˚
of 2.7897(16) A are indicative of strong intramolecular hydrogen
bonding (Table 3). This length is clearly shorter than the sum of the
van der Waals’ radii for N and O [41].
The intermolecular O3–H3a· · ·O1 hydrogen bond is important
in the crystal packing of I, giving rise to the formation R66(8) motif
(Fig. 1) [42]. The intermolecular hydrogen bond geometry and the
details of O3–H3a· · ·O1 interactions are given in Table 3. Further-
more, the distance between the ring centroids from Cg1 and to Cg1ii
Fig. 1. A partial packing diagram for I with N–H· · ·O and O–H· · ·O hydrogen bonds
shown as dashed lines [symmetry code: (i) −x + 2, −y + 1, −z + 1].
˚
is 3.746 A. These weak ␲–␲ interactions formed between aromatic
rings further stabilize the structure (Fig. S1 in Supplementary data).
In order to determine the conformational energy profiles for
keto form, the values of DFT energies were calculated as the func-
tions of selected torsion angles Â1(C6C1N1N2), Â2(C12C7N2N1),
Â3(C15C16C17O3), Â4(C9C14N3C15) and Â5(C10C11O2C13). The
calculated molecular energy profiles as the functions of torsion
angles Â1, Â2, Â3, Â4 and Â5 are given in Supplementary data. The
molecular energy profiles as a function of Â1 and Â2 show two max-
ima at −90◦ and +90◦ because of decreasing of conjugation between
aromatic rings. The molecular energy profile as a function of Â3
shows three low-energy conformers at +50◦ (conformer 1), −30◦
(conformer 2), and −80◦ (conformer 3). The energies of three con-
formers are close to each other. The conformer 1 has the lowest
energy. The energies of conformer 2 and conformer 3 are higher
than that of conformer 1 by 0.264 kcal/mol and 0.075 kcal/mol,
respectively. The molecular energy profile as a function of Â4 shows
two minima at −180◦ and 180◦. The calculated molecular energy
profile as a function of Â5 has two maxima at 120◦ and 240◦ which
have 3.8 kcal/mol energy barriers. In additional, there are two local
minima at 60◦ and 300◦ and a minimum at 180◦ which corre-
sponds to conformer with the lowest energy. After determining
the most convenient conformation, the geometry optimizations
pertaining to enol and keto forms of I were performed again by
DFT methods with B3LYP adding 6-31G(d) basis set for calcu-
lations to compare each other. The optimized geometry of keto
form (Fig. S2 in Supplementary data) deviates from experimen-
nitrogen atom. As a result of this, o-hydroxy Schiff Bases can exist
in two tautomeric structures as enol and keto form in the solid
state. As shown in Fig. 2, depending on their forms, two types of
intramolecular hydrogen bonds are possible (a) N–H· · ·O in keto
form and (b) O–H· · ·N in enol form. As it can be seen in Fig. 1,
I exists in keto form. The C14–N3 bond length of 1.2871(18) Å
and C10–O1 bond length of 1.2573(17) Å are consistent with the
distances of the C–N single bond and the C–O double bond reported
in previous studies [33,34]. While C9–C14, C7–C8, C11–C12 dis-
tances are 1.4152(18), 1.3657(19), 1.3566(19) Å; C9–C10, C10–C11
distances are 1.4410(18), 1.4581(18) Å (Supplementary data). The
contraction of C10–O1, C9–C14, C7–C8, C11–C12 distances and
the elongation of C9–C10, C10–C11 distances show that I exists
in keto form. As an another way of confirming if I exists in keto
form, the harmonic oscillator model of aromaticity (HOMA) index
is calculated by using Eq. (1) for rings [35,36].
ꢀ
ꢂ
n
ꢁ
˛
n
2
HOMA = 1 −
(Ri − Ropt
)
(1)
i=1
n is the number of bonds in ring, ˛ is the constant equal to 257.7 and
Ropt is equal to 1.388 A for CC bonds. For the purely aromatic com-
˚
pounds HOMA index is equal to 1 but, for non-aromatic compounds
it is equal to 0. The HOMA indices in the range of 0.900–0.990
or 0.500–0.800 show that the rings are aromatic or the non aro-
matic, respectively [37,38]. We calculated HOMA index of C1–C6
and C7–C12 rings. The calculated HOMA indices for C1–C6 and
C7–C12 rings are 0.908 and 0.528, respectively. These results also
indicate that the title compound exists in keto form.
˚
tal geometry with an r.m.s. of 1.575 A. While the dihedral angle
between the rings is 0.64◦ in the optimized geometry, it is 27.08◦
for experimentally obtained geometry. This means that the opti-
mized geometry of I is planar while X-ray crystal structure of I is
non-planar. This result can be attributed to the presence of crys-
tal packing effects. The hydrogen-bonding properties in the crystal
packing are fundamental in determining the crystallographically
observed conformation. The selected bond lengths and angles for
the optimized structure and X-ray geometry of the molecule are
listed in Table 2 and Supplementary data. As shown in Table 2,
C10–O1 and C14–N3 distances for the keto form at the optimized
˚
˚
geometry are 1.26 A and 1.3171 A while these distances for the
˚
enol form are 1.3339 and 1.2835 A. In addition, while C9–C14,
Fig. 2. Keto and enol tautomeric forms of I.

88
C¸ . Albayrak et al. / Spectrochimica Acta Part A 85 (2012) 85–91
Table 4
The experimental and the calculated vibrational frequencies (cm−1).
Assignments
Experimental
DFT/B3LYP
N–H str.
3353
3117
C–H str. (aromatic)
C–H (CH2) str
C–H (CH3) str
3057
3085, 3070
3007, 2956, 2952, 2935
3037, 2962, 2906
1645
2918, 2836
2957, 2883
1647
C10–O1 + C14–N3 str.
C
C
C
N
C str. (aromatic)
C str. (aromatic)
C str. (aromatic)
N str.
1607, 1575
1546, 1527
1497
1611, 1598, 1579
1562, 1531
1491
1418
1460
C–H bend. (aromatic)
C–H bend. (aromatic)
CH2, CH3 bend.
CH2 bend.
1138
833
1454
1389
1132
984
1450, 1441
1405, 1341
1139, 739
CH3, CH2 bend.
1022, 674
Str., stretching; bend., bending.
3.3. UV–vis absorption spectra
Fig. 3. FT-IR spectrum of I.
o-Hydroxy Schiff Bases can exist in two forms as keto form
containing N–H· · ·O intramolecular hydrogen bond and enol form
containing O–H· · ·N hydrogen bond in the solid state. The previ-
ous studies indicated that o-hydroxy Schiff Bases containing azo
group can exist in keto and enol forms in both the solid [40,45]
solution states [46]. To investigate the behavior of I in solution,
its UV–vis electronic spectra in three organic solvents with dif-
ferent polarity (DMSO, EtOH and benzene) were measured in the
wavelength range 200–600 nm at room temperature. The charac-
teristic UV–vis absorption bands of I in DMSO, EtOH and benzene
are given in Table 5. UV–vis spectra are shown in Fig. 4. Exami-
nation of the results indicates that the UV–vis electronic spectra
of I are largely dependent on the nature of the solvent. The UV–vis
absorption spectra of I display mainly two absorption bands arising
from ␲ → ␲* transitions. These absorption bands are observed at
350–390 nm followed by a shoulder above 400 nm in benzene. The
absorption band in the range of 350–390 nm shows bathochromic
shift while the one above 400 nm rises with the increase in the
polarity of solvent.
C7–C8, C11–C12, C9–C10, C10–C11 distances for enol form are
˚
˚
1.4582, 1.3896, 1.3827 A, 1.4163, 1.4271 A, these lengths for keto
˚
˚
form are 1.4086, 1.3772, 1.3679 A, 1.4642, 1.4684 A, respectively
(Table S1 in Supplementary data). The bond lengths obtained from
the optimization for keto form are near to the experimental ones.
The differences between the crystal and optimized geometries arise
from inter and intramolecular hydrogen bonds and crystal packing
effects.
3.2. FT-IR absorption spectrum
FT-IR spectrum of I is given in Fig. 3. The (N–H) and (O3–H)
stretching vibrations which broaden owing to the formation of
strong intramolecular and intermolecular hydrogen bonding in
the structure are at 3353 cm−1. The result obtained from X-ray
diffraction study indicates that C10–O1 bond of I is the double
bond in character. As a result of this, the sharp absorption band
at 1647 cm−1 corresponds to (C O) stretching. Depending on the
X-ray and IR results, I in the solid state exists in keto form. The
other characteristic IR absorption band of I is (N N) stretching
frequency at 1418 cm−1. The aromatic C–H stretching, C–H in-
plane bending and C–H out-of-plane bending vibrations appear in
3000–3100 cm−1, 1100–1500 cm−1 and 800–1000 cm−1 frequency
ranges, respectively [43]. The absorption band at 3057 cm−1 corre-
sponds to the aromatic C–H stretching vibrations of I. In addition,
in plane bending and out-of-plane C–H vibrations were observed
at 1138 cm−1 and at 833 cm−1 for I, respectively. The asymmet-
ric and symmetric stretching vibrations of the aliphatic CH2 and
CH3 group of I were observed at 2957, 2918, 2883, 2836 cm−1. The
To investigate the effect of pH on tautomeric forms, the UV–vis
spectra in EtOH are also recorded by dropping H2SO4 and adding
NaOH. When the solution was exposed to acid, the absorption band
at 368 nm followed by shoulder at 406 nm disappeared (Fig. 5) and
a new absorption band appeared at 328 nm. Keto form cannot exist
deformation modes of these groups were observed at 1454 cm−1
bending modes were observed at 1389 cm−1, 1022 and 674 cm−1
,
.
The absorption bands observed at 1600–1400 cm−1 are assigned
to C–C stretching vibrations of the aromatic compounds. The C–C
stretching modes of aromatic rings of I are observed at 1607, 1575,
and 1546, 1527 and 1497 cm−1. These results are in agreement with
the literature [43].
The vibrational frequencies of I were calculated by using the
same level of theory. The scale factor of 0.9613 was applied to
vibrational frequencies [44]. Vibrational bands have been made by
using Gaussview. The experimental and the calculated frequencies
are given in Table 4. The calculated results by frequency analysis
for N N and N–H stretching show deviations from experimental
values due to the differences between the calculated structure and
crystal structure.
Fig. 4. The solvent effect on UV–vis spectra of I in (- - - -) DMSO, (—) EtOH, (– · · –)
benzene.

C¸ . Albayrak et al. / Spectrochimica Acta Part A 85 (2012) 85–91
89
Table 5
For enol and keto forms wavelength, oscillator strength, major contributions of calculated transitions.
Experimental
Calculated
Keto form
Major contribution
Enol form
Major contribution
ꢀ (nm)
ꢀ (nm) (f)
ꢀ (nm) (f)
–
387.48 (0.5087)
428.57 (0.2716)
H → L + 1 (70%) H → L (8%)
H → L (70%) H → L + 1 (9%)
Gas phase
Benzene
EtOH
377.43 (0.7014)
393.70 (0.9041)
391.84 (0.8840)
394.95 (0.9127)
H → L (77%)
H → L (81%)
H → L (80%)
H → L (81%)
358
397.10 (0.3911)
434.83 (0.6688)
H → L + 1 (79%)
H → L (78%)
368
406
394.05 (0.2947)
425.08 (0.7777)
H → L + 1 (75%) H → L (6%)
H → L (74%) H → L + 1 (7%)
386
410
395.37 (0.2710)
427.50 (0.8411)
H → L + 1 (77%)
DMSO
H → L (76%) H → L + 1 (6%)
in acidic media because both phenolic oxygen and nitrogen atoms
are protonated. Therefore, the absorption band disappeared above
400 nm in acidic media corresponds to absorption band formed by
keto form. The addition of grainy NaOH to the solution caused small
changes in the absorption spectrum in EtOH (Fig. 5). The absorp-
tion band at 368 nm showed bathochromic shift and intensity of
shoulder at 406 nm increased.
The keto form is more polar than enol form, it can be stabilized
easily by polar solvents as EtOH and DMSO. As a result of this, I exists
in both keto and enol forms in EtOH and DMSO. Since benzene is
a solvent of low polarity a shift towards enol form was observed
in the absorption band of I. The previous computational studies
also show that the polar solvents facilitate the proton transfer by
decreasing the activation energy needed for Transition State [30].
TS structure of the title compound is stabilized by DMSO and EtOH,
and as a result of this, the activation energy needed to overcome
the barrier of intramolecular proton transfer between keto form
and TS structures decreases. This decrease in the activation energy
enables the formation of both keto and enol forms in DMSO and
EtOH.
increases. While the dipole moment also increases, the energy gap
(ꢃE) between the highest occupied molecular orbital (HOMO) and
the lowest-lying unoccupied molecular orbital (LUMO) increases
for keto form with the increase in the polarity of the solvent. HOMO
and LUMO called Frontier molecular orbitals (FMO) are the most
important orbitals because they play important roles in reactions
between molecules and in electronic spectra of a molecule. The
frontier molecular orbitals for both enol and keto form of I are
shown in Fig. 6 for gas phase.
In addition, the first 10 spin-allowed singlet–singlet excita-
tions for both enol and keto forms of I were calculated by TD-DFT
approach. TD-DFT calculations were started from optimized geom-
etry using the same level of theory and carried out in gas phase
to calculate excitation energies. The percentage contributions of
molecular orbitals to formation of the bands were extracted from
output by using SWizard Program [47]. For both enol and keto form
of I, wavelength (ꢀ), oscillator strength (f) larger than 0.25, major
contributions of calculated transitions are given in Table 5. From
Table 5, in view of TD-DFT calculations corresponding excitation
energy at 377.43 nm arises from HOMO → LUMO (77%) transition
in gas phase for enol form. The excitation energy from HOMO to
LUMO (70%) and HOMO to LUMO + 1 (9%) is at 428.57 for keto form.
The experimental absorption bands are observed at 350–390 nm
and above 400 nm in all solvents. From TD-DFT calculations, we
can say that the absorption band in the range greater than 400 nm
belongs to HOMO → LUMO and HOMO → LUMO + 1 transitions of
the keto form and the absorption band in the range of 350–390 nm
arises from that of enol form of I.
For the comparison of the experimental and theoretical results,
the total energy, dipole moment and frontier molecular orbital
energies are calculated by using DFT/B3LYP method with 6-31(d)
basis set by adding the polarizable continuum model (PCM) in sol-
vent media for both enol and keto form. Results obtained from
used solvents having different polarities are shown in Table 6. As
shown in Table 6, the total energies decrease with the increase in
the polarity of the solvent, in other words, the stability of molecule
Fig. 5. UV–vis spectra of I (- - - -) with acid and (– · · –) with NaOH in EtOH.
Fig. 6. The molecular orbital surfaces of I for enol and keto forms in gas phase.

90
C¸ . Albayrak et al. / Spectrochimica Acta Part A 85 (2012) 85–91
Table 6
The energies and dipole moments of keto and enol forms in gas phase and solution.
Gas phase
Benzene
EtOH
DMSO
Keto form
Etot (a.u.)
ꢁ (D)
−1148.34313885
3.7313
−1148.35408718
5.4692
−1148.36990069
6.5582
−1148.37049854
6.6853
ELUMO (eV)
EHOMO (eV)
ꢃE (eV)
−1.917
−1.900
2.010
5.186
3.176
2.009
5.186
3.177
−5.082
−5.056
3.165
3.156
Enol form
Etot (a.u.)
ꢁ (D)
−1148.34190617
1.7548
−1148.35034933
1.7811
−1148.36243628
1.8649
−1148.36298190
1.8667
ELUMO (eV)
EHOMO (eV)
ꢃE (eV)
−2.034
−2.106
−2.216
−2.225
−5.478
−5.536
−5.633
−5.639
3.444
3.430
3.417
3.414
Table 7
3.4. Non-linear optical (NLO) properties
Calculated dipole moments (D), polarizability and first hyperpolarizability compo-
nents (a.u.) for the title compound.
The non-linear optical properties play an important role for the
design of materials in modern communication technology, signal
processing, optical switches and optical memory devices [48]. The
non-linear optical properties of the organic molecules arise from
delocalized ␲ electrons that move along molecule. The increase
of the conjugation on molecule leads to an increase in its non-
linear optical properties. One another way to increase non-linear
optical properties is to add donor and acceptor groups. Acceptor
group is opposite of donor group in the organic molecules contain-
ing donor and acceptor groups and ␲ electron cloud moves from
donor group to acceptor group. If the donor and acceptor groups are
powerful, delocalization of ␲ electron cloud on organic molecules
increases and as a result of this the polarizability and first hyper-
polarizability of organic molecules increase [49]. The energy gap
between HOMO and LUMO has an important role in getting polar-
izability of a molecule [50]. The increment of the strength of the
donor and acceptor groups increases the non-linear optical prop-
erties of organic molecules due to the decrease the energy gap
between HOMO and LUMO. The molecules having a small energy
gap are more polarizable than molecules having a large energy gap.
In addition, UV–vis spectra can be used to correlate with polariz-
ability. A small HOMO–LUMO gap means small excitation energy.
Absorption bands of molecules having a small energy gap shift
towards the visible region. Quantum chemical calculations can be
used to describe the relationship between the electronic struc-
ture of molecules and their non-linear optical properties. The title
compound, a Schiff Base containing azo group, has the delocalized
␲ electron system, electron-donating substituent –OH group and
electron-withdrawing substituent –F atom. In order to investigate
the effect of ␲ electron system and donor–acceptor group on its
non-linear optical property I was computationally studied by DFT
(B3LYP) theory level.
ꢁx
ꢁy
ꢁz
−1.0389341
−0.9760553
0.3507316
ˇxxx
ˇxxy
ˇxyy
ˇyyy
ˇxxz
ˇxyz
ˇyyz
ˇxzz
ˇyzz
ˇzzz
4397.8559237
−728.5872868
−365.79141
−273.5402732
146.4112794
−58.2468345
−25.2137522
−20.5961183
−4.2741836
42.6944248
˛
509.0315178
21.0290349
234.7209847
14.575595
7.4710347
99.3389395
xx
˛
˛
˛
˛
˛
xy
yy
xz
yz
zz
respectively [51]. In addition, it is found that the first hyperpolariz-
ability and polarizability of I are greater than those of related Schiff
Bases [51–53]. That the energy gap between HOMO and LUMO of
I is 3.165 eV for gas phase shows that the title compound has a
smaller energy gap, thus, the absorption bands in the electronic
spectra are shifted towards the visible region. The movement of ␲
electron cloud from electron donating −OH group to electron with-
drawing −F substituent (at opposite side of donor group) through
N
N bridge and the increase of ␲ electron system by adding C N
bond to molecule bond lead to an increase of the conjugation and
consequently an increase in its non-linear optical properties. These
results show that the title compound can be used as a good non-
linear optical material.
3.5. Thermodynamic properties
The heat capacity (Cp0,m), entropy (Sm0 ) and enthalpy (Hm0 ) that
are the standard thermodynamic functions were performed using
DFT/B3LYP method with 6-31G(d). The results obtained from
the basis of vibrational analysis are shown in Table 8. The heat
capacities, entropies and enthalpies were obtained by increasing
temperature from 100 K to 500 K. As the results show increase of
temperature increases heat capacities, entropies and enthalpies
due to increasing intensities of molecular vibration.
The total static dipole moment ꢁ, the average linear polarizabil-
ity ˛, and the first hyperpolarizability ˇ can be calculated by using
Eqs. (2)–(4), respectively [48].
ꢁ = (ꢁ2x + ꢁ2y + ꢁz2)1/2
(2)
(3)
(4)
Table 8
1
3
Thermodynamic properties of title compound at different temperature.
˛ = (˛xx + ˛yy + ˛zz
)
T (K)
Hm0 (kcal/mol)
Sm0 (cal/mol K)
Cp0,m (cal/mol K)
1/2
2
2
ˇ = [(ˇxxx + ˇxyy + ˇxzz
)
+ (ˇyyy + ˇxxy + ˇyzz
)
+ (ˇzzz + ˇxxz + ˇyyz)2]
100
200
298.15
300
400
2.328
7.280
14.521
14.680
24.494
36.504
99.744
133.090
162.225
162.757
190.829
217.543
35.054
59.785
83.800
84.251
107.667
127.936
The dipole moment, polarizability and the first hyperpolar-
izability were calculated using polar = ENONLY at the level of
B3LYP/6-31G(d) and the results obtained from calculation were
given in Table 7.
500
Cp0,m = 7.92782 + 0.27564T − 6.99837 × 10−5
T
2, R2 = 0.99974.
2, R2 = 0.99988.
T
2, R2 = 0.99998.
The calculated polarizability ˛ and first hyperpolarizability ˇ of I
3
are 41.603 A and 35.76 × 10−30 cm5/esu that are greater than those
Sm0 = 65.44074 + 0.35617T − 1.0472 × 10−4
T
˚
Hm0 = −0.33858 + 0.01463T + 1.1823 × 10−4
of urea (˛ and ˇ of urea of 3.8312 A3 and 0.37289 × 10−30 cm5/esu),
˚

C¸ . Albayrak et al. / Spectrochimica Acta Part A 85 (2012) 85–91
91
The correlation equations between heat capacities, entropies,
enthalpies and temperature are shown in Table and
Figs. S8–S10 in Supplementary data can be used for analyzing heat
capacities, entropies and enthalpies in different temperature.
[17] A. Grabowska, K. Kownacki, L. Kaczmarek, J. Lumin. 60/61 (1994)
886–890.
8
[18] K. Ogawa, J. Harada, T. Fujiwara, S. Yoshida, Phys. Chem.
3425–3427.
A 105 (2001)
[19] D. Guha, A. Mandanl, A. Koll, A. Filarowski, S. Mukherjee, Spectrochim. Acta A
56 (2000) 2669–2677.
[20] M. Zgierski, A. Grabowska, J. Chem. Phys. 113 (2000) 7845–7852.
[21] M. Kletski, A. Milov, A. Metelisa, M. Knyazhansky, J. Photochem. Photobiol. A
110 (1997) 267–270.
4. Conclusion
[22] V.N. Nemykin, J.G. Olsen, E. Perera, P. Basu, Inorg. Chem. 45 (2006) 3557–3568.
[23] G. Menconi, N. Kaltsoyannis, Chem. Phys. Lett. 415 (2005) 64–68.
[24] Stoe & Cie, X-AREA (Version 1.18) and X-RED32 (Version 1.04), Darmstadt,
Germany, 2002.
In this work, the compound (Z)-4-[(E)-(4-fluorophenyl)
diazenyl]-6-[(3-hydroxypropylamino)methylene]-2-methoxycy-
clohexa-2,4-dienone was experimentally characterized by means
of X-ray diffraction, FT-IR and UV–vis spectroscopic techniques. It
has been determined that the title compound is in keto form in
the solid state both on the basis of X-ray and FT-IR spectroscopic
data. UV–vis spectra were recorded in different organic solvents.
The results show that compound exists in both keto and enol
forms in DMSO, EtOH but it tends to shift towards enol form in
benzene. Since the polar solvents facilitate the proton transfer by
decreasing the activation energy needed for Transition State, title
compound exists in both keto and enol forms in DMSO and EtOH.
The geometry optimization was performed using DFT method with
B3LYP applying 6-31G(d) basis set. There are differences between
crystal and computational structures because of crystal packing in
which the hydrogen-bonding properties are fundamental in deter-
mining the crystallographically observed conformation. TD-DFT
calculations starting from optimized geometry were carried out
in both gas and solution phase to calculate excitation energies. In
addition, the non-linear optical properties were computed and
the changes of thermodynamic properties were obtained with the
same level of theory.
[25] G.M. Sheldrick, Acta Crystallogr. A 64 (2008) 112–122.
[26] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
J.A. Montgomery, T.J. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J.
Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson,
H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian,
J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J.
Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A.
Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels,
M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman,
J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A.
Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham,
C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen,
M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Revision E. 01, Gaussian, Inc.,
Wallingford, CT, 2004.
[27] P.J. Stephens, F.J. Devlin, C.F. Chabalowski, M.J. Frisch, J. Phys. Chem. 98 (1994)
11623–11627.
[28] R. Ditchfield, W.J. Hehre, J.A. Pople, J. Chem. Phys. 54 (1971) 724–728.
[29] A.V. Zakharov, N. Vogt, Struct. Chem. 22 (2011) 305–311.
[30] N. Tezer, N. Karakus, J. Mol. Model. 15 (2009) 223–232.
[31] S. Miertus, E. Scrocco, J. Tomasi, Chem. Phys. 55 (1981) 117–129.
[32] M. Cossi, N. Rega, G. Scalmani, V. Barone, J. Comput. Chem. 24 (2003) 669–681.
[33] C¸ . Albayrak, M. Odabas¸ og˘lu, O. Büyükgüngör, P. Lönnecke, Acta Crystallogr. C
60 (2004) o318–o320.
[34] A. Özek, C¸ . Albayrak, M. Odabas¸ og˘lu, O. Büyükgüngör, Acta Crystallogr. C 62
(2006) o173–o177.
[35] J. Kruszewski, T.M. Krygowski, Tetrahedron Lett. 13 (1972) 3839–3842.
[36] T.M. Krygowski, J. Chem. Inf. Comput. Sci. 33 (1993) 70–78.
[37] A. Filarowski, A. Koll, T. Glowiak, J. Chem. Soc. Perkin Trans. 2 (2002) 835–842.
[38] A. Filarowski, A. Kochel, M. Kluba, F.S. Kamounah, J. Phys. Org. Chem. 21 (2008)
939–944.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2011.09.020.
[39] N. Karadayı, C¸ . Albayrak, M. Odabas¸ og˘lu, O. Büyükgüngör, Acta Crystallogr. E
62 (2006) o3695–o3696.
[40] S. Salehzadeh, M. Mahdavian, M. Khalaj, Acta Crystallogr. E 67 (2011) o606.
[41] A. Bondi, J. Phys. Chem. 68 (1964) 441–451.
References
[42] M.C. Etter, Acc. Chem. Res. 23 (1990) 120–126.
[1] G. Hallas, J.H. Choi, Dyes Pigments 40 (1999) 119–129.
[2] J. Koh, A.J. Greaves, Dyes Pigments 50 (2001) 117–126.
[3] S.S. Kandil, Transit. Metal Chem. 23 (1998) 461–465.
[4] M-J. Lee, B.W. Yoo, S.T. Shin, S.R. Keum, Dyes Pigments 51 (2001) 15–24.
[5] S. Aramaki, G.H. Atkinson, J. Am. Chem. Soc. 114 (1992) 438–444.
[6] N. Tamai, H. Miasaka, Chem. Rev. 100 (2000) 1875–1890.
[7] M. Azuki, K. Morihashi, T. Watanabe, O. Taskahashi, O. Kikuchi, J. Mol. Struct.
(Theochem) 542 (2001) 255–262.
[8] T. Sugavara, I. Takasu, Adv. Phys. Org. Chem. 32 (1999) 219–265.
[9] H. Zolinger, Color Chemistry: Synthesis, Properties and Application of Organic
Dyes and Pigments, VCH, Weinheim, 1991.
[10] F.L. Carter, Molecular Electronic Devices II, Marcel Decker, New York, 1987.
[11] H. Durr, H. Bouas-Laurenet (Eds.), Photochromism: Molecules and Systems,
Elsevier, Amsterdam, 1990.
[12] I. Willner, S. Rubin, Angew. Chem. Int. Ed. Engl. 35 (1996) 367–385.
[13] S. Nagaoka, N. Hirota, M. Sumitani, K. Yoshira, J. Am. Chem. Soc. 105 (1985)
4220–4426.
[14] A. Sytnik, J.C. Del Vale, J. Phys. Chem. 99 (1995) 13028–13032.
[15] G.M.J. Schmidt, in: D. Ginsburg (Ed.), Solid State Photochemistry, VCH, Wein-
heim, 1976.
[43] R.M. Silverstein, F.X. Webster, D.J. Kiemle, Spectrometric Identification of
Organic Compounds, 7th ed., John Wiley & Sons, New York, 2005.
[44] J.B. Foresman, A. Frisch, Exploring Chemistry with Electronic Structure Meth-
ods, 2nd ed., Gaussian, USA, 1996.
[45] C.C. Ersanlı, C¸ . Albayrak, M. Odabas¸ og˘lu, C. Thöne, A. Erdönmez, Acta Crystallogr.
C 60 (2004) o133–o135.
[46] M. Odabas¸ og˘lu, C¸ . Albayrak, R. Özkanca, F.Z. Aykan, P. Lönnecke, J. Mol. Struct.
840 (2007) 71–89.
[47] S.I. Gorelsky, SWizard Program, Revision 4.5. http://www.sg.chem.net/, Uni-
versity of Ottawa, Ottawa, Canada, 2010.
[48] D. Sajan, J. Hubert, V.S. Jayakumar, J. Zaleski, J. Mol. Struct. 785 (2006) 43–53.
[49] K.S. Thanthiriwatte, K.M. Nalin de Silva, J. Mol. Struct. (Theochem) 617 (2002)
169–175.
[50] R.G. Pearson, Proc. Natl. Acad. Sci. 83 (1986) 8440–8441.
[51] Y.X. Sun, Q.L. Hao, W.X. Wei, Z.X. Yu, L.D. Lu, X. Wang, Y.S. Wang, J. Mol. Struct.
(Theochem) 904 (2009) 74–82.
[52] M. Jalali-Hevari, A.A. Khandar, I. Sheikshoaie, Spectrochim. Acta A 55 (1999)
2537–2544.
[53] H. Tanak, J. Mol. Struct. (Theochem) 950 (2010) 5–12.
[16] K. Kownacki, R. Mordzinski, R. Wilbrandt, A. Grabowsks, Chem. Phys. Lett. 227
(1994) 270–276.