Synthesis of two new azo-azomethines; Spectral characterization, crystal structures, computational and fluorescence studies

Article abstract of DOI:10.1016/j.molstruc.2015.03.043

This study describes the preparation, characterization and the photoluminescence properties of novel azo-azomethines (2-{(E)-[(4-ethylphenyl)imino]methyl}-4-[(E)-phenyldiazenyl]phenol, HL¹ and 2-{(E)-[(3-ethylphenyl)imino]methyl}-4-[(E)-phenyld

Full text of DOI:10.1016/j.molstruc.2015.03.043

Journal of Molecular Structure 1094 (2015) 183–194
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis of two new azo-azomethines; spectral characterization, crystal
structures, computational and fluorescence studies
Sevgi Eskikanbur a, Koray Sayin b, Muhammet Kose a, Huseyin Zengin c, Vickie McKee d,
Mukerrem Kurtoglu a,
⇑
a
_
Department of Chemistry, Faculty of Science and Literature, Kahramanmaraßs Sütçü Imam University, Kahramanmaraßs 46050, Turkey
b Department of Chemistry, Institute of Science, Cumhuriyet University, Sivas 58140, Turkey
c Department of Chemistry, Faculty of Science and Literature, Gaziantep University, Gaziantep 27310, Turkey
d School of Chemical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Two novel azo-azometine dyes were
synthesized.
The dyes were characterized by
analytical and spectroscopic methods.
The structures of the dyes were
determined by X-ray diffraction.
Computational calculations have
been performed using DFT methods.
The photoluminescence properties of
the azo-azomethines were also
evaluated.
a r t i c l e i n f o
a b s t r a c t
Article history:
This study describes the preparation, characterization and the photoluminescence properties of novel
azo-azomethines (2-{(E)-[(4-ethylphenyl)imino]methyl}-4-[(E)-phenyldiazenyl]phenol, HL1 and 2-{(E)-[(3-
ethylphenyl)imino]methyl}-4-[(E)-phenyldiazenyl]phenol, HL2 dyes). The dyes were characterized by
elemental analysis, spectroscopic studies such as IR, 1H and 13C NMR, mass and fluorescence spectra.
Molecular structures of the dyes were examined by X-ray diffraction analysis. The molecular structures
are mostly similar, differing mainly in the position of the ethyl group and dihedral angles between aro-
matic rings. X-ray data revealed that both HL1 and HL2 favor phenol-imine tautomer in the solid state. An
intramolecular phenol-imine hydrogen bond (O1ꢁ ꢁ ꢁN1) were observed in both compounds resulting in a
Received 26 January 2015
Received in revised form 23 March 2015
Accepted 24 March 2015
Available online 9 April 2015
Keywords:
Azo-azomethine
Enol–keto tautomer
Crystal structure
Spectroscopy
Computational study
Optical behavior
S(6) hydrogen bonding motif. Molecular packing of both compounds are determined by
pꢁ ꢁ ꢁp interac-
tions. Quantum chemical investigation of mentioned molecules were performed by using DFT hybrid
function (B3LYP) with 6-31+G(d) basis set. The compounds HL1 and HL1 gave intense light emissions upon
irradiation by Ultra-Violet light. The photoluminescence quantum yields and long excited-state lifetimes
of the compounds HL1 and HL2 were measured. The azo-azomethine dyes HL1 and HL2 have photolumi-
nescence quantum yields of 34% and 32% and excited-state lifetimes of 3.21 and 2.98 ns, respectively. The
photoluminescence intensities and quantum yields of these dyes were dependent on the position of alkyl
group on the phenyl ring.
Ó 2015 Elsevier B.V. All rights reserved.
⇑
Corresponding author.
E-mail addresses: kurtoglu01@gmail.com, mkurtoglu@ksu.edu.tr (M. Kurtoglu).
http://dx.doi.org/10.1016/j.molstruc.2015.03.043
0022-2860/Ó 2015 Elsevier B.V. All rights reserved.

184
S. Eskikanbur et al. / Journal of Molecular Structure 1094 (2015) 183–194
X-ray crystallographic results were collected using a Bruker APEX2
Introduction
CCD diffractometer and data reduction was carried out using
Bruker SAINT [35]. Bruker (1998) APEX2 and SAINT Bruker AXS
Inc. SHELXTL was used for solving and refining the structures [36].
Azo-azomethines are known to be interesting because of the
existence of both hard nitrogen and/or oxygen donor atoms in
the backbones of these compounds, some of which have interest-
ing physical and chemical properties [1] and potentially useful bio-
logical activities [2]. Further, azo dyes are a versatile class of
colored organic dyes which continue to receive a great deal of
attention, as noted in the literature, due to their biological proper-
ties and applications in various fields, such as textiles, papers,
leathers, additives, and organic synthesis [3–7]. Recent studies
show some azo compounds used in a wide variety of applications,
such as medicines, cosmetics, food, paints, plastics, shipbuilding
and automobile and cable manufacturing [8–15]. Also the applica-
tions of azo dyes has been applied in the electronics field as a
storage components in digital versatile disc-recordable devices
due to them being stable metal azo dyes [16,17].
Interest in new organic materials continues to grow owing to
their photophysical properties and usefulness in light emitting
devices [18–20]. In fact, some organic polymers have been noted
as showing strong luminescence, allowing them to be considered
for flat panel display technologies [21,22]. A number of studies
on organic ligands and their metal complexes have found use in
electroluminescent devices [23,24] and laser systems [25].
Further, various aromatic amines and polymeric arylamines have
also been recognized as active layers in organic electroluminescent
displays [26,27].
Recently, azo-azomethines and their metal complexes were
reported by our group [28–33]. Here we report on the synthesis
and characterization of the azo-azomethine dyes HL1 and HL2. The
compounds HL1 and HL2 were found to exhibit photoluminescence,
emitting an intense light upon UV irradiation. Compounds HL1 and
HL2 were characterized using ultraviolet–visible spectrophotome-
ter (UV–Vis), photoluminescence spectrophotometer (PL) and
Fourier transform infrared spectroscopy (FT-IR). The structures of
compounds HL1 and HL2 were analyzed by IR, 1H and 13C NMR, ele-
mental analysis and single crystal X-ray studies. Computational
studies of HL1 and HL2 are performed by using restricted B3LYP/6-
31+G(d) level of theory in vacuo. Optimized structural parameters,
vibration frequencies, frontier molecular orbitals (FMOs), molecular
electrostatic potential (MEP) maps, MEP contours and non-linear
optical (NLO) properties of relevant molecules are investigated by
Gaussian09 package program. The optical behavior of the com-
pounds in solution was also investigated.
Preparation of azo-containing azomethines (HL1 and HL2)
Azo-azomethines (HL1 and HL2) were synthesized by addition of
4-ethylaniline or 3-ethylaniline compounds (1 mmol) in methanol
(10 mL) to a methanolic solution of 2-hydroxy-5-[(E)-phenyl-
diazenyl]benzaldehyde (1 mmol). The mixtures were stirred for
about 30 min and allowed to react at room temperature for about
24 h. The colored powders were recrystallized from chloroform/
methanol (1:1) v/v solution.
HL1; Yield: 0.27 g (86%), color: orange yellow. M.p.: 212–213 °C.
Analysis Calc. for C21H19N3O: C, 76.57; H, 5.81; N, 12.76. Found: C,
76.45; H, 5.74; N, 12.76%. ESI-MS (m/z (rel. intensity)): 316(7%)
[C20H18N3O]+, 289(9%) [C18H14N3O]+, 213(100%) [C12H12N3O]+,
200(48%) [C11H10N3O]+, 185(2%) [C11H11N3]+. NMR (CDCl3 as sol-
vent, dppm); 1H: 14.06 (s, 1H, phenolic OH), 8.77 (s, 1H, CH@N),
7.15–8.07 (m, 11H, aromatic-H), 2.75–2.70 (q, 2H, CH2 of ethyl),
1.32–1.28 (t, 3H, CH3A of ethyl group). 13C: 164.18 (C@N),
161.15 (CAOH), 118.17–152.64 (C, aromatic), 29.52 (C, CH2 of
ethyl), 15.62 (C, CH3 of ethyl group). IR (KBr, cmꢂ1): 3431, 2957,
1618, 1569, 1508, 1432, 1280, 1102.
HL2; Yield: 0.20 g (82%), color: orange. M.p.: 124–125 °C.
Analysis Calc. for C21H19N3O: C, 76.57; H, 5.81; N, 12.76. Found:
C, 76.53; H, 5.88; N, 12.74%. NMR (DMSO-d6 as solvent, dppm):
1H: 13.94 (s, 1H, phenolic OH), 9.17 (s, 1H, CH@N), 7.14–8.30 (m,
11H, aromatic-H), 2.69–2.66 (q, 2H, ACH2 of ethyl group), 1.25–
1.22 (t, 3H, ACH3 of ethyl group). 13C: 164.41 (C@N), 163.12
(CAOH), 118.51–152.44 (C, aromatic), 28.57 (C, CH2 of ethyl),
15.97 (C, CH3 of ethyl group). IR (KBr, cmꢂ1): 3415, 2961, 1620,
1569, 1436, 1305, 1277, 1105.
X-ray crystallography
Data for HL1 and HL2 were collected at 150(2) K with a Bruker
Apex II CCD diffractometer using Mo Ka radiation (k = 0.71073 Å).
Structures were determined by direct methods and the refinements
were based on F2, using all reflections [36]. Non-hydrogen atoms
were refined anisotropically by atomic displacement parameters.
Hydrogen atoms bonded to carbons were placed at calculated posi-
tions by applying a riding model, where those hydrogens bonded to
oxygen and nitrogen atoms were positioned on difference maps and
refinement with temperature factors riding on the carrier atom was
carried out. Crystal data and refinement are given in Table 1.
Hydrogen bond parameters are given in Table 2 and bond lengths
and angles are given in Table 3.
Experimental
Reagents
9,10-Diphenylantracene, 3-ethylaniline, 4-ethylaniline and
2-hydroxybenzaldehyde were purchased from Aldrich Chem. Co.
and Merck and used as received. All solvents for synthesis and
analysis from commercial sources and used as received unless
otherwise noted. The azo-aldehyde compound, 2-hydroxy-5-[(E)-
phenyldiazenyl]benzaldehyde was prepared according to the
published paper [29,34].
Computational method
All computational progresses were done by using GaussView
5.0.8 [37] and Gaussian 09 IA32W-G09RevA.02 package program
[38]. Additionally, preparation of figures was done by using
ChemBioDraw Ultra Version (13.0.0.3015) [39]. Becke, 3-parame-
ter, Lee–Yang–Parr (B3LYP) hybrid function [40,41] which is one
of the hybrid density functional theory functions was selected as
computational method for investigated molecules. In calculations,
6-31+G(d) was selected as basis set. All calculations were made in
vacuo. In the IR spectra, all frequencies were scaled by 0.975 [42].
Instrumentation
NMR spectra were collected using a Bruker Advance 400 MHz.
spectrometer. Mass spectrum of HL1 was recorded on a Thermo
Fisher Exactive + Triversa Nanomate spectrometer. IR spectra were
obtained (4000–400 cmꢂ1) using a Perkin Elmer Spectrum 100 FTIR
spectrophotometer. Carbon, hydrogen and nitrogen elemental
analyses were performed with a Model CE-440 elemental analyzer.
Photoluminescence studies
The fluorescence spectra of the synthesized azo-azomethine
dyes HL1 and HL2 were obtained using a Perkin Elmer LS55

S. Eskikanbur et al. / Journal of Molecular Structure 1094 (2015) 183–194
185
Table 1
IR spectra
Crystallographic data for the dyes HL1 and HL2.
The positions of some prominent bands in the IR spectra of HL1
and HL2 and their assignments based on extensive data available
for related compounds are given in the experimental section. The
IR spectra of dyes HL1 and HL2 are given in Fig. 1. In the IR spectra
of dyes HL1 and HL2, broad peaks at 3431 and 3415 cmꢂ1 were
Identification code
HL1
HL2
Empirical formula
Formula weight
Crystal size (mm3)
Crystal color
Crystal system
Space group
Unit cell a (Å)
b (Å)
C21H19N3O
329.39
C21H19N3O
329.39
0.57 ꢃ 0.25 ꢃ 0.09
0.67 ꢃ 0.28 ꢃ 0.09
Yellow
Orange
Monoclinic
P2/c
12.4546(17)
6.5006(9)
21.408(3)
90
100.482(2)
90
1704.4(4)
4
0.081
14,486
99.6%
3506 [0.0347]
0.0396, 0.0961
0.0587, 0.1064
1034612
Monoclinic
P2(1)/n
7.1885(7)
6.4536(7)
36.892(4)
90
90.306(2)
90
1711.5(3)
4
0.081
14,430
99.5%
3493 [0.0206]
0.0362, 0.0938
0.0439, 0.0986
1034613
attributed to the
observed at 1618 cmꢂ1 for HL1 and 1620 cmꢂ1 for HL2 can be
assigned to the (C@N) group. Absence of (C@O) absorption in
the IR spectra of the prepared dyes, together with the appearance
of new
(C@N) absorption in the range of 1618–1620 cmꢂ1 clearly
m(OAH) vibrations, respectively [32]. A band
m
m
c (Å)
a
(°)
b (°)
(°)
m
indicated that the new azo-azomethine compounds had formed in
each case. Aliphatic (CAH) vibration bands were observed in the
range of 2850–2960 cmꢂ1. The symmetric N@N stretching mode,
leads to a medium band at 1569 cmꢂ1 for both the synthesised
dyes and while the (CAN) stretching mode gives an intense band
at 1280 and 1277 cmꢂ1, for HL1 and HL2, respectively.
c
Volume (Å3)
Z
Abs. coeff. (mmꢂ1
Refl. collected
)
Completeness to h = 28.02°
Ind. Refl. [Rint
R1, wR2 [I > 2
]
r
(I)]
R1, wR2 (all data)
CCDC number
NMR spectra
1H and 13C NMR spectra of the synthesized azo-azomethine
dyes were recorded. The resonance of protons has been assigned
on the basis of their integration and multiplicity pattern. The 1H
and 13C NMR spectra of HL1 are shown in Figs. 2 and 3. Singlets
in HL1 and HL2 dyes at 8.77 (HL1) and 9.17 ppm (HL2) are assignable
to the azomethine protons. The multisignals within the 8.07–7.15
(HL1) ppm and 8.30–7.14 (HL2) ppm range are assigned to the aro-
matic protons of the three rings in the synthesized dyes. The peak
observed at 14.06 and 13.94 ppm in the compounds is attributable
to the AOH signal of salicylaldehyde derivatives molecule in HL1
and HL2, respectively. NMR spectra for HL1 and HL2 were recorded
in two different solvents (HL1 in CDCl3 and HL2 in DMSO-d6). By
examining the chemical shifts of the phenolic (PhOH) and azome-
thine (AC@NA) proton signals, there was no significant change in
the spectra. In addition to these, the proton NMR spectra of the
azo-azomethines display two signals at 2.75–2.70 ppm (for HL1)
and 2.69–2.66 ppm (for HL2) as triplet and 1.32–1.28 (HL1) and
1.25–1.22 ppm (HL2) as quartet with an integration equivalent to
five hydrogens corresponding to the ArACH2A and ArACACH3
groups, respectively.
Table 2
Hydrogen-bond parameters (Å, °) for the dyes HL1 and HL2.
DAHꢁ ꢁ ꢁA DAH Hꢁ ꢁ ꢁA Dꢁ ꢁ ꢁA
DAHꢁ ꢁ ꢁA
HL1 O(1)AH(1A)ꢁ ꢁ ꢁN(1) 0.933(17) 1.749(18) 2.5957(15) 149.4(15)
HL2 O(1)AH(1A)ꢁ ꢁ ꢁN(1) 0.930(18) 1.755(18) 2.6035(14) 150.2(16)
Table 3
Selected bond lengths and angles (Å, °) for the dyes.
HL1
Calc. (HL1)
HL2
Calc. (HL2)
N(1)AC(9)
C(11)AO(1)
N(2)AN(3)
C(6)AN(1)
C(14)AN(2)
N(3)AC(16)
1.2860(16)
1.3456(16)
1.2558(15)
1.4185(16)
1.4292(18)
1.4294(18)
1.292
1.340
1.260
1.410
1.419
1.001
1.2864(16)
1.3451(15)
1.2574(14)
1.4191(15)
1.4196(16)
1.4260(17)
1.292
1.340
1.260
1.411
1.419
1.001
C(6)AN(1)AC(9)
N(3)AN(2)AC(14)
N(2)AN(3)AC(16)
121.43(12)
114.41(12)
112.71(12)
121.45
115.41
115.24
120.18(11)
114.05(11)
113.90(11)
121.33
115.39
115.24
13C NMR spectra further supported the structural characteriza-
tion of the azo-azomethines. The number of signals found corre-
sponds with the presence of magnetically nonequivalent carbon
atoms, which were assigned by comparison with literature values.
In the 13C NMR spectra of dyes (HL1 and HL2), a signal for carbon of
CH@N groups of each compounds appears at 164.18 and
164.41 ppm for HL1 and HL2, respectively. The carbon atom of
CAOH groups was observed at 161.15 and 163.12 ppm for HL1
and HL2, respectively. All the other aromatic carbon atoms were
observed in the range of 152–118 ppm. Two aliphatic signals
(29.52 and 15.62 ppm for HL1 and 28.57 and 15.97 ppm for HL2)
were assigned to the ethyl group carbon atoms (ArACH2CH3).
Both 1H and 13C NMR spectra indicated that the compounds did
not contain significant impurity.
spectrometer, where samples were prepared in spectrophotomet-
ric grade acetonitrile and analyzed using 1 cm optical cells.
Solution concentrations of HL1 and HL2 in acetonitrile were
1.0 ꢃ 10ꢂ5 mol/L with excitations at 328 nm wavelength. The
quantum yields of HL1 and HL2 were calculated using the standard
9,10-diphenylantracene [43–45].
Results and discussion
Synthesis
Treatment of the azo-aldehyde with two different aromatic
amines, in refluxing MeOH yielded the novel azo-azomethine dyes
(Scheme 1). The air-stable azo-azomethine dyes (HL1 and HL2) are
soluble in DMSO, DMF, MeOH, EtOH, MeCN, and CHCl3, and
insoluble in H2O. Elemental analysis data are provided in the
experimental section and were found to be good agreement with
the calculated values. The infrared, 1H and 13C NMR and mass spec-
tra of azo-azomethine dye HL1 in CDCl3 is shown in Figs. 1–3,
respectively. The data are in good agreement with single crystal
X-ray structures.
Mass spectra
ESI mass spectrum of the azo-azomethine dye HL1 was recorded
in MeOH. The molecular ion signal was not observed. The ESI mass
spectrum of the dye showed fragmentation signals at m/z 316(7%)
[C20H18N3O]+, 289(9%) [C18H14N3O]+, 213(100%) [C12H12N3O]+,
200(48%) [C11H10N3O]+ and 185(2%) [C11H11N3]+. Mass spectrum
and fragmentation pattern of the azo-azomethine dye HL1 is shown
in Figs. S1 and S2, respectively.

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S. Eskikanbur et al. / Journal of Molecular Structure 1094 (2015) 183–194
OH
NH2
N
N
i
O
ii
OH
N
R2
R1
N
N
azo form
O
N
R2
R1
O
N
NH
R2
R1
NH
N
N
hydrozone form
enamine form
Scheme 1. Preparation of azo-azomethine dyes (HL1, R1 = ACH2CH3 R2 = AH and HL2, R1 = AH R2 = ACH2CH3) and possible tautomeric equilibriums [(i): (1) NaNO2, HCl, 0 °C,
(2) salicylaldehyde; (ii): 4-ethylanilineor 3-ethylaniline, MeOH and reflux].
Fig. 1. IR spectra of the dyes HL1 and HL2.
X-ray structures
compounds so as to evaluate the aromaticity of the phenol ring in
the solid state [46,47]. HOMA indexes were in the range of 0.900–
0.990 or 0.500–0.800 this showing that the rings were either aro-
matic or non-aromatic, respectively. HOMA index values for the ring
C10AC15 were found to be 0.939 and 0.944 for HL1 and HL2, respec-
tively. These results revealed that the ring C10AC15 shows aromatic
character for both compounds in the solid state. The azomethine
(AC9@N1A) linkages are 1.2860(16) and 1.2864(16) Å for HL1 and
HL2, respectively; similar to those observed for related azo-azome-
thine compounds [48,49]. The diazenyl (AN2@N3A) bond distances
are 1.2558(15) and 1.2574(14) Å for HL1 and HL2, respectively.
Aromatic rings (C10AC15) and (C16AC21) adopt the trans
Molecular structures of HL1 and HL2 azo-azomethine dyes with
atom numbering are shown in Fig. 4. The molecular structures are
similar by large, differing mainly in the position ethyl group and
dihedral angles between aromatic rings. For both structures, all
bond lengths and angles are within the normal ranges. All bond
lengths and angles in the phenyl rings have normal Csp2–Csp2 val-
ues with small distortions. X-ray analysis indicated that the azo-
azomethine dyes HL1 and HL2 favor the phenol-imine tautomer in
the solid state. The harmonic oscillator model of aromaticity
(HOMA) index for the ring C10AC15 are calculated for both

S. Eskikanbur et al. / Journal of Molecular Structure 1094 (2015) 183–194
187
Fig. 2. 1H NMR spectrum of HL1 in CDCl3.
Fig. 3. 13C NMR spectrum of HL1 in CDCl3.

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S. Eskikanbur et al. / Journal of Molecular Structure 1094 (2015) 183–194
Fig. 4. Molecular structures of azo-azomethine dyes HL1 and HL2 with atom numbering. Intra-molecular hydrogen bonding is shown as dashed lines.
configuration with regard to the azo double bond (AN@NA) with
the torsion angle C(14)AN(2)AN(3)AC(16) of 178.99(10) and
ꢂ179.89(9)° for HL1 and HL2, respectively.
cell packing of both compounds are determined by
interactions (Figs. 7 and 8).
p–p stacking
An intramolecular phenol-imine hydrogen bond (O1ꢁ ꢁ ꢁN1)
exists in both compounds resulting in a S(6) hydrogen bonding
motif. In the structures of HL1 and HL2, both outer rings twisted
with respect to the central benzene ring. The mean planes of
C1AC6 and C16AC21 to the central (C10AC15) ring are at
26.60(5) and 2.67(7)° for HL1 and at 34.91(3) and 16.98(4)° for
HL2, respectively. These differences reflect the different inter-
molecular interactions in the lattice. In HL1, there are two sets of
Thermodynamic parameters and optimized structures
Molecules HL1 and HL2 are optimized at RB3LYP/6-31+G(d) level
and thermodynamic parameters of the three possible isomers are
listed in Table 4. Thermodynamic parameters are total energy
(ETotal), enthalpy energy (H), Gibbs free energy (G) and entropy (S).
According to Table 4, the lower energy and the higher entropy values
mean that molecule is more stable. Azo form more stable than other
forms in each molecule. Additionally, the total energy, enthalpy and
Gibbs free energy of HL2 are lower than in HL1. Therefore, HL2 is more
stable than HL1. These molecules are structural isomers of each
other. Optimized structures of mentioned molecules are repre-
sented in Fig. S3. Some optimized structural parameters of mole-
cules HL1 and HL2 are given in Table 3. According to Table 3,
structural parameters are very close to each other and these results
mean that optimized structures of mentioned molecules are similar
to each other. These calculated results and their experimental values
are used to plot the distribution graph and correlation coefficient of
this graph is calculated as 0.9997. This value implies that there is a
well agreement between experimental and calculated results.
p–p stacking interactions in the structure (Fig. 5). First, C1AC11
section is stacked with the same section of an adjacent molecule
under symmetry operation of 2 ꢂ x, ꢂy, 1/2 ꢂ z; C1 and C11 are
separated by 3.441 Å; C5 and C9 are separated by 3.406 Å.
Second, C9AC17 section is stacked with the same section of the
neighboring molecule under symmetry operation of 1 ꢂ x, y, 1/
2 ꢂ z; C9 and C17 are separated by 3.388 Å. In HL2, there are face
to face and edge to edge phenyl–phenyl stacking interactions were
observed. The C9AC14 section is stacked with the same section of a
neighboring molecule; C9 and C13 are separated by 3.393 Å (sym-
metry code: ꢂx, 2 ꢂ y, ꢂz). The C14AC15 edge is stacked with the
same section of a neighboring molecule; C14 and C15 are sepa-
rated by 3.477 Å, (symmetry code: 1 ꢂ x, 2 ꢂ y, ꢂz) (Fig. 6). Unit
Fig. 5.
p–p
stacking interactions in HL1. Hydrogen atoms are omitted for clarity.

S. Eskikanbur et al. / Journal of Molecular Structure 1094 (2015) 183–194
189
Fig. 6.
p–p
stacking interactions in HL2. Hydrogen atoms are omitted for clarity.
Fig. 7. Packing diagram of HL1 viewing down the b axis,
p–p interactions are shown as dashed.
Fig. 8. Packing diagram of HL2 viewing down the b axis,
p–p interactions are shown as dashed lines and hydrogen atoms are removed for clarity.
Calculated IR spectra
is bigger than 25 D. Intensities of asymmetric CAH frequencies is
less than 25 D. Therefore, there are mainly symmetric CAH fre-
quencies in Table 5. There is a good agreement between experi-
mental and calculated frequencies except OAH frequency.
Functional groups in molecules can be easily determined by
using vibration frequencies. Investigated vibration frequencies
are scaled by 0.975 to obtain anharmonic frequencies. Some anhar-
monic frequencies are examined in detail and given in Table 5.
There are 44 atoms in each molecule and there must be 126 vibra-
tion modes which is calculated with (3Nꢂ6). In calculated results,
126 vibration modes are observed and some vibration modes are
examined in detail. In Table 5, intensity of vibration frequencies
Some molecular orbitals and contour diagrams
Highest occupied molecular orbital (HOMO) and lowest unoc-
cupied molecular orbitals (LUMO) play an important roles in
chemical processes and reactions. LUMO, HOMO, HOMOꢂ1 and

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S. Eskikanbur et al. / Journal of Molecular Structure 1094 (2015) 183–194
Table 4
Thermodynamic parameters of investigated molecules at RB3LYP/6-31+G(d) in vacuo.
Structures
Isomer
ETotal (a.u.)
H (a.u.)
G (a.u.)
S (J molꢂ1 Kꢂ1
)
HL1
Azo form
Enamine form
Hydrazone form
ꢂ1051.168419
ꢂ1050.790199
ꢂ1050.760853
ꢂ1051.168529
ꢂ1050.790463
ꢂ1050.761080
ꢂ1050.791824
ꢂ1050.789255
ꢂ1050.759909
ꢂ1050.791958
ꢂ1050.789519
ꢂ1050.760136
ꢂ1050.867941
ꢂ1050.865865
ꢂ1050.838323
ꢂ1050.868141
ꢂ1050.865990
ꢂ1050.838311
670.285170
673.987380
689.850480
HL2
Azo form
Enamine form
Hydrazone form
670.858380
672.762640
687.747940
Table 5
Vibration modes, calculated frequencies and their assignments for each molecule at B3LYP/6-31+(d) level in vacuo.
HL1
HL2
Mode
Frequencya
Intensityb
Assignment
Mode
Frequencya
Intensityb
Assignment
38
42
47
48
50
69
72
677.6
762.5
828.8
830.7
857.0
1107.6
1156.4
35.5600
37.9463
26.5903
86.7409
29.4914
104.5261
77.04073
x
x
x
x
x
a
CAHaro.
CAHaro.
CAHaro.
CAHaro.
CAHaro.
CAHaro.
CAHaro.
37
42
45
47
54
70
71
676.8
761.5
791.2
828.5
910.6
38.1762
37.5733
25.0943
84.5103
36.6279
102.4422
37.3202
x
x
x
x
x
a
x
m
x
m
m
x
m
x
m
CAHaro.
CAHaro.
CAHaro.
CAHaro.
CAHaro.
CAHaro.
CAHaro.
C@N
,
,
x
x
OAH
OAH
,
,
,
m
x
m
C@C
OAH
C@N
1107.2
1143.1
c
,
,
a
a
CAHaro.
CAHaro.
75
1194.4
34.4862
m
c
m
m
C@N,
OAH
CAN,
C@N,
c
CAHaro.
72
1150.2
35.3132
CAHaro.
C@N
77
80
1214.9
1254.8
47.0413
25.1170
c
c
CAHaro.
CAHaro.
74
76
1163.1
1200.3
30.5264
28.1855
C@N,
OAH,
x
x
CAHaro.
CAHaro.
m C@N
CAC,
OAH,
C@C,
CAO,
C@C
81
88
1292.9
1361.1
1415.7
1472.3
1488.8
1509.3
1515.2
1580.5
1592.2
1606.8
1609.7
72.0548
84.1590
43.1273
30.6050
51.7085
65.4677
90.3296
83.5044
10.8366
79.1341
114.6744
m
c
C@O,
c
CAHaro.
77
81
1215.2
1293.0
1360.4
1415.4
1471.7
1487.2
1488.4
1514.8
1577.5
1605.9
1608.1
53.4042
80.1526
82.2848
59.1197
28.6087
29.8704
47.9597
73.7771
144.6035
243.7911
30.7644
x
CAHaro.
CAN
x CAHaro.
m
CAHaliph.
,
m
C@C
m
m
x
90
m
c
m
c
m
c
m
C@C,
OAH
N@N,
OAH
C@C,
OAH
C@C,
c
CAHaro.
CAHaro.
CAHaro.
CAHaro.
C@C
88
CAHaliph.
94
c
90
x
x
m
m
m
x
x
m
OAH,
m
C@C
CAHaro.
97
c
c
94
C@C,
CAO
C@C,
OAH
x
a
CAHaro.
CAHaro.
99
97
100
102
103
104
105
m
c
c
m
m
N@N,
m
98
OAH,
C@C
x
CAHaro.
CAHaro.
,
c
OAH
OAH,
C@N
C@C,
m
C@C
100
101
104
105
m
N@N, x
CAHaro.
OAH
m
C@C
x
x
m
m
x
m
,
C@C
c
CAHaro.
OAH,
C@N,
m
x
CAHaro
c
m
c
m
m
c
m
m
c
CAHaro.
C@C,
CAHaro.
,
c
OAH
C@C,
m
C@N
m
C@N
,
CAHaro
c
OAH
C@C,
N@N
a
CAHaro.
C@C,
N@N
m
C@N
m
106
107
1628.3
1637.0
60.3676
CAHaro.
C@C
,
c
OAH
106
107
1628.6
1636.3
42.5376
m
m
m
c
C@C,
C@N,
C@N,
x
m
CAHaro.
C@C
395.1317
C@N,
m
C@C
384.4145
CAHaro.
CAHaro., x OAH
108
109
110
112
113
114
121
2964.0
2965.0
2988.4
3030.0
3035.2
3059.6
3123.0
47.9279
49.4335
67.0623
69.0246
35.6848
765.4599
29.8244
ms CAHaliph.
ms CAHaliph.
ms CAHaliph.
ms CAHaliph.
mas CAHaliph.
108
109
110
112
113
114
121
2965.1
2966.0
2989.1
3030.8
3036.1
3061.0
3122.6
39.3719
41.7539
62.5699
60.3878
37.1072
757.8592
29.2336
ms CAHaliph.
ms CAHaliph.
ms CAHaliph.
mas CAHaliph.
mas CAHaro.
m
OAH
m OAH
ms CAHaro.
mas CAHaro.
ms CAHaro.
ms CAHaro.
mas CAHaro.
ms CAHaro.
124
3132.0
25.1931
124
3131.8
25.0559
Vibration modes:
m
, stretching;
a, scissoring;
c
, rocking;
x
, wagging; d, twisting. Abbreviations: s, symmetric; as, asymmetric; aro., aromatic; aliph., aliphatic.
a
In cmꢂ1
.
b
In D (10ꢂ40 esu2 cm2).
HOMOꢂ2 are important to determine the reactivity of molecules.
Contour diagrams of mentioned molecular orbitals and their
energies are represented in Fig. 9.
According to Fig. 9, electrons which are belong to HOMO are
mainly localized on N@N bond; HOMOꢂ1 and HOMOꢂ2 electrons
are mainly localized on molecule for each molecule. As for the
LUMO, if molecules accept electrons, these electrons will be mainly
localized on left side of molecules which is according to Fig. 9. In
chemical reactions, electron can be transferred from HOMO or
HOMOꢂ1 or HOMOꢂ2 to appropriate molecular orbitals. The

S. Eskikanbur et al. / Journal of Molecular Structure 1094 (2015) 183–194
191
Fig. 9. Contour diagrams and energies of mentioned molecular orbitals of HL1 and HL2 at B3LYP/6-31+G(d) level in vacuo.
energy differences are important to determine the molecular
orbitals which contain the transferred electrons. The energy
differences between ‘‘LUMO and HOMO’’, ‘‘HOMO and HOMOꢂ1’’
and ‘‘HOMOꢂ1 and HOMOꢂ2’’ are calculated as 343.26, 42.14,
According to MEP maps in Fig. 10, electrophilic active regions are
around the oxygen atom and nitrogen atoms which are connected
to each other with double bonds in each molecule. Additionally,
p
-electrons in benzene ring on left side of molecule are slightly
6.80 kJ molꢂ1
,
respectively for HL1; 345.33, 40.09 and
active in HL2. These results are supported with MEP contours and
14.34 kJ molꢂ1, respectively for HL2. These results imply that elec-
trons can easily be transferred from HOMO or HOMOꢂ1 or
HOMOꢂ2 to appropriate molecular orbitals.
the last result are seen better with MEP contours.
Non-linear optical (NLO) properties
NLO is important property in providing the key functions of fre-
quency shifting, optical modulation, optical switching, optical logic
and optical memory for the technologies in areas such as telecom-
munications, signal processing and optical interactions [33].
Therefore, NLO is located on the foreground in current research.
Some quantum chemical descriptors which are total static dipole
Molecular electrostatic potential (MEP) maps and contours
Molecular electrostatic potential (MEP) maps are related to
electron density on molecule surface. These maps can be used
to predict the active sites on molecules. Additionally, MEP con-
tours are related to electron density on molecular plane. Steric
effects and active sites can be easily determined by using of these
contours. MEP maps and contours of investigated molecules are
calculated at the B3LYP/6-31+G(d) level and represented in
Fig. 10.
Different values of electrostatic potential at MEP maps are
shown by different colors1 which are mainly red, yellow, green, light
blue and blue. The colors between red and green in MEP maps are
related to electrophilic reactivity while the colors between green
and blue in MEP maps is related to nucleophilic reactivity.
moment (l), the average linear polarizability (
a), the anisotropy
of the polarizability (
D
a) and first hyperpolarizability (b) have
been used for explaining the NLO properties in many computa-
tional studies. The total static dipole moment, the average linear
polarizability, the anisotropy of the polarizability and first hyper-
polarizability are calculated by using the Eqs. (1)–(4), respectively
and given in Table 6.
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
l
a
¼
¼
lx2
þ
ly2
þ
lz2
ð1Þ
ð2Þ
1
3
1
For interpretation of color in Fig. 10, the reader is referred to the web version of
ð
axx
þ
ayy
þ
azz
Þ
this article.

192
S. Eskikanbur et al. / Journal of Molecular Structure 1094 (2015) 183–194
Fig. 10. MEP maps and contours of studied molecules at B3LYP/6-31+G(d) level in vacuo.
In Table 6, mentioned parameters of studied molecules and urea
Table 6
are calculated at B3LYP/6-31+G(d) level. NLO properties can be
affected from polarizability, anisotropy of the polarizability and
hyperpolarizability. NLO properties increase with increasing the
linear polarizability, anisotropy of the polarizability and first
hyperpolarizability. According to Table 6, all values of each men-
tioned molecules are greater than their urea values. Therefore,
NLO properties of HL1 and HL2 are better than urea. According to
Table 6, the general ranking of NLO properties should be as
follows:
The calculated dipole moment (l), average linear polarizability (a), the anisotropy of
the polarizability
molecules.
(Da) and hyperpolarizability (b) for urea and investigated
Parameters
Urea
HL1
HL2
a
lx
0.0000
0.0000
ꢂ0.9291
ꢂ0.77351
ꢂ0.44968
0.296679
a
ly
ꢂ0.5715
lza
ꢂ1.8058
23.6560
0.0000
36.1116
0.0000
0.0000
37.7298
0.0000
ꢂ0.1945
b
axx
535.1930
ꢂ146.8840
374.9131
2.9888
508.4758
b
ayy
ꢂ154.703
b
azz
380.9196
b
HL1 > HL2 > Urea
According to this ranking, molecule HL1 is the best candidate for
NLO properties.
axy
ꢂ0.90323
ꢂ19.6799
165.7135
b
axz
ayz
18.8451
b
157.5689
1247.6970
ꢂ1562.9124
1677.0764
ꢂ1,646.3768
ꢂ276.5585
ꢂ231.9426
ꢂ197.3607
58.2370
ꢂ88.6901
1.107993
37.69924
100.1942
2.00 ꢃ 10ꢂ26
b
bxxx
byyy
bzzz
bxyy
bxxy
bxxz
bxzz
byzz
567.623091
ꢂ1406.7708
1599.13189
ꢂ1604.1212
357.70877
ꢂ292.43902
243.764081
62.5269691
ꢂ108.3215
0.942624
b
0.0000
0.0000
0.0000
b
Fluorescence measurements
b
b
ꢂ34.3289
The photophysical properties of the azo-azomethine dyes HL1
and HL2 were explored. Excitation and emission spectra were stud-
ied for solutions of the dyes which were excited at 328 nm. Dyes
HL1 and HL2 gave intense light emission upon irradiation by UV
light and their photoluminescence spectra in MeCN is shown in
Fig. 11. Maximum luminescent intensity was observed at 402 nm
and the full width at half maximum was 74 nm for compound
HL1. The dye HL1exhibited a photoluminescence quantum yield of
34% and a long excited-state lifetime of 3.21 ns. Maximum lumi-
nescent intensity was observed at 397 nm and the full width at half
maximum was 57 nm for compound HL2. Dye HL2 exhibited a pho-
toluminescence quantum yield of 32% and a long excited-state
lifetime of 2.98 ns. The photoluminescence intensity and quantum
yield of dye HL1 were higher than that of the dye HL2 due to the
position of alkyl group on the phenyl ring. An explanation for the
high quantum yields is that the phenyl ring alkyl group para-
b
0.0000
b
36.2531
0.0000
0.0000
1.8058
2.9522
b
b
byyz
la
ac
36.2900
D
bd
ac
10.7676
99.9594
4.31 ꢃ 10ꢂ28
1.51 ꢃ 10ꢂ26
a
b
c
In Debye.
In atomic unit (a.u.).
In Å3.
d
In cm5/esu.
h
i
1=2
1
2
2
2
Da ¼
ðaxx ꢂ ayyÞ þ ðayy ꢂ azzÞ þ ðazz ꢂ axxÞ þ 6a2xz þ 6ax2y þ 6ay2z
pffiffiffi
2
ð3Þ
substitution led to greater p-electron delocalization forming large
h
i
1=2
2
2
2
conjugated system in the dye HL1 structure.
b ¼ ðbxxx þ bxyy þ bxzzÞ þ ðbyyy þ bxxy þ byzzÞ þ ðbzzz þ bxzz þ byzz
Þ
Fig. 11 also shows the comparison of excitation and emission
ð4Þ
spectra of the dyes HL1 and HL2 in the solvent MeCN. The

S. Eskikanbur et al. / Journal of Molecular Structure 1094 (2015) 183–194
193
in vacuo. HL2 is more stable than HL1. There is a well agreement
between experimental structural parameters and theoretical. IR
spectra of each molecule were calculated and good agreement is
obtained between experimental and calculated frequencies except
OAH vibration. Some vibration modes which have higher intensity
than 25D are examined in detail. NLO properties of HL1 are better
than HL2. The azo-azomethine dyes HL1 and HL2 were found to dis-
play an intense photoluminescence at 402 and 397 nm, respec-
tively. The photoluminescence intensities and quantum yields of
these compounds are affected by the position of alkyl group on
the phenyl ring. The para-substituted alkyl group on the phenyl
ring revealed better electron donating effect and thus higher fluo-
rescence emission. These compounds are attractive materials
which can be used in fabrication of electroluminescent devices.
800
700
600
500
400
300
200
100
0
Ex. of HL1
Ex. of HL2
Em. of HL1
Em. of HL2
200
260
320
380
440
500
560
Wavelength (nm)
Acknowledgements
Fig. 11. Photoluminescence spectra of dyes HL1 and HL2 in acetonitrile; samples
were excited at 328 nm.
We would like to thank the Departments of Chemistry at
Kahramanmaras Sutcu Imam University, Cumhuriyet University,
Gaziantep University, and Loughborough University. The numeri-
cal calculations reported in this paper are performed at TUBITAK
ULAKBIM, High Performance and Grid Computing Center (TRUBA
Resources).
Table 7
Photoluminescence data for the dyes HL1 and HL2.
Entity kmax Ex (nm)
In
Ex
kmax Em (nm)
In
Em
/
(%)
sf
(ns)
f
HL1
HL2
333
(286;324;342)
331
704
639
402
695
633
34
3.21
2.98
(386;445;476;513)
397
Appendix A. Supplementary material
32
(284;321;339)
(383;436;475;512)
CCDC 1034612 and 1034613 contain the supplementary crystal-
lographic data for HL1 and HL2, respectively. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif,
by e-mailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre 12 Union Road
Cambridge CB2 1EZ, UK. Fax: +44 (0)1223 336033. Supplementary
data associated with this article can be found, in the online version,
at http://dx.doi.org/10.1016/j.molstruc.2015.03.043.
kmax Ex: maximum excitation wavelength; In Ex: maximum excitation intensity;
kmax Em: maximum emission wavelength; In Em: maximum emission intensity; /f:
quantum yield; sf: excited-state lifetime.
photoluminescence intensity and quantum yield of compound HL2
decreased with respect to that of the compound HL1 due to the
ortho-position of alkyl group on the phenyl ring. Compound HL2
exhibited an excitation maximum wavelength at 331 nm, slightly
shifting to 333 nm for compound HL1. Decreases in excitation
intensity of HL2 was observed being dependent on the ortho-
substitution of alkyl group on the phenyl ring. With excitation at
328 nm, the compound HL1 showed an emission maximum at
402 nm, which also shifted to 397 nm upon changing from para-
position of alkyl group on the phenyl ring to ortho-position.
Thus, the fluorescence emission intensity of the compound HL1
increased as compared to that of compound HL2. This increase of
emission intensities is due to the better electron donating group
effect of para-substitution of alkyl group on the phenyl ring.
Compound HL1 allows for energy transfer from the excited state
of the dye HL1 to ground state, hence decreasing the non-radiated
transition of the compound HL1 excited state and increasing the
fluorescence emission. Also, upon changing from ortho-position
of alkyl group on the phenyl ring to para-position, the maximum
photoluminescence peak shifted from 397 to 402 nm, and shoulder
peaks become more noticeable. The photoluminescence data for
the compounds HL1 and HL2 are given in Table 7. The photolumi-
nescent properties of these dyes may provide promising optical
and electronic applications.
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