Optical and thermal properties of azo derivatives of salicylic acid thin films

Article abstract of DOI:10.1016/j.saa.2014.08.122

N-acryloyl-4-aminosalicylic acid (4-AMSA), monomer (HL) and 5-(4′-alkyl phenylazo)-N-acryloyl-4-aminosalicylic acid (HL_n) are synthesized and characterized with various physico-chemical techniques. Thin films of 5-(4′-alkyl phenylazo)-N-acryloy

Full text of DOI:10.1016/j.saa.2014.08.122

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1039–1049
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Optical and thermal properties of azo derivatives of salicylic acid thin
films
a
b
c,
⇑
c
c
c,1
M.M. Ghoneim , N.A. El-Ghamaz , A.Z. El-Sonbati , M.A. Diab , A.A. El-Bindary , L.S. Serag
a
Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt
Physics Department, Faculty of Science, Damietta University, Damietta, Egypt
Chemistry Department, Faculty of Science, Damietta University, Damietta, Egypt
b
c
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
Spectral behavior of absorption index, k, for 4-ASA, HL and HL
n
thin films.
ꢀ
ꢀ
ꢀ
We examine changes in optical
properties for 4-ASA and its
derivatives.
All the thermal parameters are
affected by amidation and
diazotization processes.
The XRD pattern of the powder and
thin film forms are polycrystalline
and amorphous, respectively.
The optical transitions are found to be
indirect allowed.
1.0
0
0
0
0
0
0
.09
.08
.07
.06
.05
.04
HL
HL
HL
HL
1
2
3
4
0
0
0
.8
.6
.4
ꢀ
ꢀ
4
-ASA
HL
HL
1
2
3
4
380
400
420
440
460
λ, nm
480
500
520
Diazotization remarkably enhances
the photoluminescence spectra.
0.2
HL
HL
HL
0
.0
2
00
250
300
350
400
450
500
λ
, nm
a r t i c l e i n f o
a b s t r a c t
0
Article history:
Received 27 May 2014
Received in revised form 28 June 2014
Accepted 24 August 2014
Available online 18 September 2014
N-acryloyl-4-aminosalicylic acid (4-AMSA), monomer (HL) and 5-(4 -alkyl phenylazo)-N-acryloyl-4-
aminosalicylic acid (HL
n
) are synthesized and characterized with various physico-chemical techniques.
) are prepared by spin coating
0
Thin films of 5-(4 -alkyl phenylazo)-N-acryloyl-4-aminosalicylic acid (HL
n
technique. The X-ray diffraction (XRD) patterns of 4-aminosalicylic acid (4-ASA) and its derivatives are
investigated in powder and thin film forms. Thermal properties of the compounds are investigated by
thermogravemetric analysis (TGA). The optical energy gap and the type of optical transition are investi-
n
. The values of fundamental energy
gap (E ) are in the range 3.60–3.69 eV for all compounds and the type of optical transition is found to be
Keywords:
Azo salicylic acid
Spin coating
Fluorescence
X-ray diffraction
gated in the wavelength range (200–2500 nm) for 4-ASA, HL and HL
g
ꢁ
indirect allowed. The onset energy gap E appeared only for azodye compounds is found to be in the
range 0.95–1.55 eV depending on the substituent function groups. The refractive index, n, shows a nor-
g
Thermal and optical properties of thin films
mal dispersion in the wavelength range 650–2500 nm, while shows anomalous dispersion in the wave-
ꢁ
length rang 200–650 nm. The dispersion parameters
e
1
,
e
L
, E
d
, E
o
and N=m are calculated. The
photoluminescence phenomena (PL) appear for thin films of 4-ASA and its derivatives show three main
emission transitions.
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
⇑
Corresponding author. Tel.: +20 1060081581; fax: +20 572403868.
E-mail address: elsonbatisch@yahoo.com (A.Z. El-Sonbati).
Abstracted from her M.Sc.
Aromatic azo compounds constitute a very important class of
organic compounds because of their widespread applications in
1
http://dx.doi.org/10.1016/j.saa.2014.08.122
386-1425/Ó 2014 Elsevier B.V. All rights reserved.
1

1
040
M.M. Ghoneim et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1039–1049
many areas of organic dyes [1], indicators [2], radical reaction ini-
tiators [3,4], and therapeutic and drug delivery agents [5–7]. They
are also important units in the area of nonlinear optics [8], optical
storage media [9], chemosensors [10], photochemical switches
when (kex = 346 nm). The increase of SA coordinating to Eu3+ ion,
leading to the enhancement of fluorescence emission intensity of
the complex SAPS–Eu(III).
Konjac glucomannan (KGM) has been interacted with
4-aminosalicylic acid (4-ASA) to produce 4-ASA-KGM blends [26].
The fluorescence spectra of both 4-ASA and 4-ASA-KGM blends
exhibited an identical emission spectra at 390 nm. It was found
that the presence of KGM cause the fluorescence quench and the
emission intensity increased proportionally with increasing of 4-
ASA concentration in aqueous solutions. Joshi et al. [27] reported
that the amino substitution at 4-position does not lead to dual
emission, unlike 5-aminosalicylic acid (5-ASA). Contrary, Suyal
et al. [28] reported that 4-ASA fluorescence emission shows dual
emission in poly (vinyl alcohol) (PVA), poly (methyl methacrylate)
(PMMA) and cellulose acetate (CA). They also reported that, 4-ASA
is present as two rotomers (P and R) and rotomer P is more stable
than R-due to strong intarmolecular hydrogen bond. Rotomer R
cannot undergo ESTPT and has normal fluorescence band. Dual
emission was observed in the polymers. They attributed the emis-
sion peaks at longer wavelength to ESIPT, while the band at
330 nm is due to those rotomer which cannot undergo ESIPT.
Due to the importance of azo compounds and salicylic acid
derivatives and their applications, the present work aims to inves-
tigate the dispersion, absorption and emission spectra of 4-ASA
and its azo derivatives to produce new materials having promising
technological applications such as nonlinear optics devices, solar
cells and light emitting display LED.
[
11] and electronic devices [12].
Salicylic acid and its derivatives are widely used in many
applications such as, pharmaceutical industry [13,14], skin-care
products [15], food antiseptic, pesticide, cleanser textiles [16],
analgesic, antipyretic, anti-flammatory bowel diseases (IBD) and
anti-tuberculosis drug [17,18].
4
-Aminosalicylic acid (4-ASA) was conjugated with various car-
rier molecules to get new azo derivatives of 4-ASA [17,18]. Azo sal-
icylic acid derivatives are presented remarkable anti-inflammatory
activity and antioxidant activity which attributed to the co-antioxidant
effects of 4-ASA and substituted phenol with higher reducibility,
according to the superoxide dismutase (SOD) activity [18].
4
-Aminosalicylic acid has been successfully intercalated into
zinc layered hydroxide (ZLH) using zinc oxide suspension by Sai-
fullah et al. [19]. The X-ray diffraction patterns and FTIR analyses
indicated that the molecule has been intercalated into the ZLH
interlayer space with an average basal spacing of 24 Å. Further-
more, they discussed that TG curve of 4-ASA showed two stages;
the first one in the temperature range 101–150 °C was attributed
to dehydration. The second stage in the temperature range 150–
2
35 °C was related to combustion of 4-ASA. The thermal behavior
has been changed with intercalated 4-ASA anions (decomposition
occurs at 227 °C) compared to 4-ASA anions. 4-ASA-ZLH has been
remarkably enhanced thermal stability. Rotich et al. [20] discussed
the thermal properties of 4-ASA, which gives a two stage mass loss
with discontinuity after about 30% mass loss at close to the melting
point (130 °C) of 4-ASA. The weight loss at discontinuity close to
Experimental
Materials
calculated CO
the latter stage of the TG curve. The corresponding enthalpy
vap) for the two stages, in the temperature region 130–150 °C
2 2
content but the evolution of CO occurs during
Acryloyl chloride (AC) (Aldrich Chemical Co., Inc.) was used
(DH
without further purification. It was stored below ꢂ18 °C in a tightly
and 150–180 °C, have been calculated and found to be 250 ± 40
3 3
glass-stoppered flask. Aniline and 4-R-aniline (R@OCH , CH , Cl
ꢂ1
and 50 ± 5 kJ mol , respectively.
2
and NO ) (Aldrich Chemical Co., Inc.) also was used without further
A series of aromatic molecules which display strong internal
hydrogen bonding has been studied as regards the existence of
purification. 4-Aminosalicylic acid (Aldrich chemical Co., Inc.) was
purified by recrystallization from hot ethanol and filtering. All
other chemicals and solvents were purified by standard
procedures.
phototropic tautomerism both in the ground and excited state
⁄
[
21–23]. Transition energies of all tautomers for the allowed pAp
transition shows that keto-hydrazone tautomers absorbed at
longer wavelengths when compared with enol-azo tautomers. Lar-
ger Stokes shifts can be ascribed to excited state intramolecular
proton transfer (ESIPT) processes, whereas smaller Stokes shifts
can be regarded as belonging to normal emission bands arising
from the primary excited structure [21,22]. A proton-transferring
for 1,4-bis-p-sulfonylazo-2,3 dihydroxy naphthalene (SADN), and
its possible binuclear copper(II) complexes have been studied in
aqueous solution [23]. The results provided evidence that SADN
undergoes both single and double excited state intramolecular pro-
ton transfer (ESIPT) phenomena.
Synthesis of N-acryloyl-4-aminosalicylic acid (HL)
N-Acryloyl-4-aminosalicylic acid – (4-AMSA) – monomer (HL)
was performed by the amidation reaction of equimolar amounts
of AC and 4-aminosalicylic acid in dry benzene until the evolution
of hydrogen chloride cased forming a gray powder of HL monomer
[
29] as shown in Scheme 1.
0
Synthesis of 5-(4 -alkyl phenylazo)-N-acryloyl-4-aminosalicylic acid
Three azopyrrole compounds have been synthesized and their
structures have been determined by single crystal X-ray diffrac-
tion, H NMR and UV–Vis spectra by Chen and Yin [24]. They
(HL
n
)
1
0
5-(4 -Alkyl phenylazo)-N-acryloyl-4-aminosalicylic acid (HL )
n
reported that it is cooperative intramolecular hydrogen bonds that
influence the azopyrrole tautomerisation from the azo to the
hydrazone form, only if adding a hydrogen bond acceptor on the
benzene ring to the ortho-position of azo linkage. It is also neces-
sary to add a hydrogen bond donor to the pyrrole side and build
two cooperative intramolecular hydrogen bonds.
were prepared in our laboratories by diazotization reaction. In a
typical preparation [30–34], 25 ml of distilled water containing
0.01 mol hydrochloric acid are added to aniline (0.01 mol) or p-
derivatives. To the resulting mixture stirred and cooled to 0 °C, a
solution of 0.01 mol sodium nitrite in 20 ml of water was added
dropwise. The formed diazonium chloride is consecutively coupled
with an alkaline solution of 0.01 mol N-acryloyl-4-aminosalicylic
acid, in 10 ml of pyridine. The colored precipitate, which formed
immediately, is filtered through sintered glass crucible, washed
several times with water and ether. The crude products was
purified by recrystallization from hot mixture of ethanol and water
Fluorescence spectra of rare earth complex polystyrene with
side chain bonded salicylic acid (SA), SAPS–Eu(III), have been
investigated [25]. There were two absorption peaks at 273 and
⁄
3
15 nm, which attributed to
pAp
electron transition of benzene
ring of polystyrene backbone and the carboxyl group of SA ligand.
The excitation spectrum of SAPS–Eu(III) was obtained at 618 nm
2 5
then dried in vacuum desiccator over P O .

M.M. Ghoneim et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1039–1049
1041
H
H
C
H
C
C
H
C
NH2
H
H
O
O
H
N
C
C
+
20 o
C
OH
0
-
O
Cl
C
OH
HO
O
C
HO
Scheme 1. The formation mechanism of N-acryloyl-4-aminosalicylic acid (HL).
The resulting formed ligands are:
diffractometer0 (GNR APD-2000 PRO) with CuK
a-radiation
(
k = 1.540598 ÅA ) in the range of diffraction angle (2h° = 4–70°).
0
ꢀ
5-(4 -methoxyphenylazo)-N-acryloyl-4-aminosalicylic acid
(
The applied voltage and the tube current are 40 KV and 30 mA,
respectively. Measurements of transmittance, T(k), and reflectance,
R(k), of the thin film samples are carried out at room temperature
and at normal incidence of light in the wavelength range 200–
2500 nm in steps of 2 nm using a double-beam spectrophotometer
(JASCO model V-570-UV/Vis/NIR). Thermal properties were inves-
tigated using Simultaneous Thermal Analyzer (TGA) STA 6000 with
a scan rate 10 °C/min in air atmosphere from ambient tempera-
tures to 600 °C. Photoluminescence spectra are recorded on Fluo-
rimeter spectrophotometer (model 6285, for excitation and
emission spectra, 200–700 nm).
1
HL ),
0
ꢀ
ꢀ
5-(4 -phenylazo)-N-acryloyl-4-aminosalicylic acid (HL
2
),
acid
0
5-(4 -chlorophenylazo)-N-acryloyl-4-aminosalicylic
(
3
HL ),
0
ꢀ
5-(4 -nitrophenylazo)-N-acryloyl-4-aminosalicylic
HL ),
acid
(
4
which are shown in Scheme 2.
Preparation of thin films
n
Thin films of 4-ASA, HL and HL with good homogeneity are
Determination of optical constants of thin films
prepared by spin coating technique [35] onto optical flat and
well-cleaned glass substrates. The coating system is locally
constructed in our laboratory. The speed of spin coating machine
was adjusted to 2800 rps. 0.001 mol of all compounds are
dissolved in ethanol and dropped onto the rotating substrate to
form thin films at room temperature.
The absolute values of transmittance, T(k), and reflectance, R(k),
which are used to calculate the optical constants are calculated
according to the relation [36]:
ꢀI
ꢁ
ft
TðkÞ ¼
ð1 ꢂ R
g
Þ;
ð1Þ
I
g
Analysis techniques
and
ꢀ
ꢁ
hꢂ
ꢂ
ꢃꢃi
Elemental microanalyses of the separated compounds for C, H,
and N were determined on Automatic Analyzer CHNS Vario ELIII,
Germany. The analyses were repeated twice to check the accuracy
of the analyzed data. The FTIR spectra are recorded as KBr disks
using Perkin-Elmer 1340 spectrophotometer. X-ray diffraction pat-
terns of the powder and the thin film forms, are recorded on X-ray
I
fr
2
RðkÞ ¼
R
Al 1 þ 1 ꢂ ðR
g
Þ
g
ꢂ TR ;
ð2Þ
I
Al
g
where Ift and I are the intensities of light passing through film–
glass system and reference glass substrate, respectively. Ifr and IAl
are the intensities of the light reflected from the sample and that
NaNO / HCl.H O
2
2
N+ Cl-
R
NH2 + HCl
R
N
H
H
C
C
H
H
H
C
C
H
C
C
H
O
O
N
H
O
O
N
N+ Cl-
alkaline solution
- 5 o
R
N
+
R
N
N
0
C
OH
OH
C
C
HO
HO
0
n 3 2
Scheme 2. The formation mechanism of 5-(4 -alkyl phenyl azo)-N-acryloyl-4-aminosalicylic acid (HL ); R@OCH (n = 1), H (n = 2), Cl (n = 3) and NO (n = 4).

1
042
M.M. Ghoneim et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1039–1049
Results and discussion
(
f)
Characterization of ligands
Microanalytical data are in good agreement with the stoichiom-
etry of the proposed structures.
(
e)
d)
FT-IR spectra
(
The Fourier transform infrared spectrum (FT-IR) of 4-ASA in
comparison with its derivatives is presented in Fig. 1. The main
absorption peaks are tabulated in Table 1. The IR spectrum for 4-
(
c)
ASA (Fig. 1(a)) shows NH
2
group absorption peaks at 3474 and
ꢂ1
3
390 cm . The OH stretching broad bands of the phenolic and car-
ꢂ1
boxylic acid groups at 3006 and 2889 cm , respectively. This
broadening is an evidence for the presence of weak hydrogen bond
between OH and carboxylic groups [40]. The presence of hydrogen
(
b)
bond facilitates excited state hydrogen transfer. The C@O group
ꢂ1
appears at 1680 cm . For HL (Fig. 1(b)), the bands for NH
2
group
ꢂ1
at 3494 and 3390 cm disappear due to amidation reaction and
(
a)
ꢂ1
a new broad band appears at 3458 cm
corresponding to NH
group. The more broadening due to overlap between NH, OH and
carboxyl groups may causes the probability of hydrogen transfer
to increase in the presence of C@O group which directly attached
ꢂ1
to NH. Absorption band at 1504 ± 5 cm (N@N) appears as a con-
4000
3200
2400
sequence of diazotization process (Fig. 1(c–f)). The previously dis-
cussed broad band (3458–2950 cm ) is red shifted with more
Wave number, cm-1
ꢂ1
broadening. This may be attributed to extra hydrogen transfer of
azo-hydrazone tautomer between N@N and o-NH (Scheme 3(III)).
A distinguished absorption band appears related to substituent
function groups attached with azo linkage. These absorption bands
1 2 3 4
Fig. 1. The IR spectrum of; (a) 4-ASA, (b) HL, (c) HL , (d) HL , (e) HL and (f) HL .
from the reference mirror, respectively. RAl is the reflectance of Al-
mirror and R is the reflectance of the glass.
appears for
2 3 3
t(NO ), t(OCH ) and d(OCH ) at 1330, 2838 and
g
ꢂ1
1
465 cm , respectively.
The refractive index, n, is an important parameter for optical
materials design and it includes valuable information for optical
TGA analysis of 4-ASA, HL and HL
n
materials with higher efficiency. The complex refractive index
⁄
(
n ) can be expressed as:
The thermal behavior of 4-ASA, HL and its azo derivatives (HL
n
)
nꢁ ¼ n ꢂ ik;
ð3Þ
were investigated using thermogravemeteric analysis (TGA). TGA
curves (Fig. 2) show that all compounds decompose in two steps
in the temperature range (100–600 °C). Weight loss percentage in
each stage of HL and its azo derivatives is presented in Table 2. It
can be seen clearly that the mass losses obtained from the TG curves
and that calculated for the corresponding molecule or molecules as
well as the final decomposition product are in good agreement for
all of the decomposition steps. 4-ASA TG thermogram has been pre-
viously reported by Saifullah and Rotich [19,20] showing two
decomposition steps. The first one occurred in the temperature
range 101–150 °C near to melting point with mass loss 30%, attrib-
uted to decarboxylation of 4-ASA [20]. The second stage (151–
where n is the real part of the refractive index and k is the absorp-
tion index. The values of absorption coefficient, , (=4 k/k) of thin
films can be calculated from the T(k) and R(k) according to the rela-
a
p
tion [37–39]:
2
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffi 3
u
u
t
!
ꢀ
ꢁ
2
4
1
d
ð1 ꢂ RÞ
2T
ð1 ꢂ RÞ
2
4
5
a
¼
ln
þ
þ R
;
ð4Þ
2
4T
where k is the wavelength of the incident light on the sample, d is
the thickness of the film ꢃ (100) nm.
The refractive index, n of thin films is calculated according to
the relation:
2
35 °C) is due to thermal combustion of 4-ASA [19]. Accordingly,
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
the first stage occurred in the temperature range 170–185 °C is
attributed to decarboxylation mechanism. The second stage in the
temperature range 185–255 °C is mainly due to combustion of a
u
u
t
!
!
ð1 þ RÞ
ð1 ꢂ RÞ
4R
2
n ¼
þ
ꢂ k
;
ð5Þ
2
ð1 ꢂ RÞ
6 4
part of the monomer (C H N) and at higher Temperature
Table 1
The important IR spectra bands of 4-ASA, HL and its azo derivatives.
ꢂ1
Compound
Assignment (cm
)
t
2
(NH )
t
(OH Phenolic,
t
(C@O)
t
(N@N)
Carboxylic and NH)
4
-ASA
3474, 3390
3006, 2889
1652
–
–
1504
1504
1500
1509
HL
HL
HL
HL
HL
–
–
–
–
–
3400, 3050, 2950
3458, 3351, 3207
3440, 3388, 3205
3460, 3342, 3158
3397, 3338, 3174
1679, 1612
1614, 1604
1610, 1600
1627, 1585
1658,1635
1
2
3
4

M.M. Ghoneim et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1039–1049
1043
HC
C
CH2
HC
C
CH2
HC
C
CH2
O
O
O
H
H
NH
R
N
N
N
R
N
N
N
R
N
N
O
H
O
H
O
H
HO
O
HO
O
HO
O
I
III
II
n
Scheme 3. Possible ESIPT of azo derivatives HL .
1
1
10
00
E
a
)) ꢃ 1. A plot of the left-hand side of Eq. (6) against 1/T gives a
4
HL
HL
-ASA
slope from which E , is the energy of activation, was calculated, and
a
ꢄ
ꢀ
ꢁꢅ
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
2
W
W
a
c
hE
a
log log
¼
ꢂ log 2:303;
ð7Þ
HL3
2
2:303RTs
where h = T ꢂ T
g
, w
c
= w
a
ꢂ w, w
a
mass loss at the completion reac-
/w )] vs. h
(d)
c)
tion; w = mass loss up to time t. The plot of log [log(w
a
c
2
was drawn and found to be linear from the slope (E
which E was calculated.
The entropy of activation (
and the free energy change of activation (
using the following equations:
a
/2.303RT ) of
s
(
a
⁄
⁄
D
S ), enthalpy of activation (
D
H )
⁄
D
G ) were calculated
(
b)
a)
ꢄ
ꢀ
ꢁꢅ
Ah
k T
B s
ꢁ
(
D
S ¼ 2:303 log
R;
ð8Þ
is
100
200
300
400
500
600
where k
B
is the Boltzmann constant, h is the Plank’s constant, T
s
Tempreature,°C
the peak temperature and A is the Arrhenius factor. The enthalpy
⁄ ⁄
of activation, DH , and Gibbs free energy, DG , calculated from:
Fig. 2. The TG curves of thin films; (a) 4-ASA, (b) HL, (c) HL
2
, and (d) HL
3
.
ꢁ
(
> 300 °C) loss acrylaldehyde molecule (C
3
H
3
O) [19]. All azo deriv-
D
D
H ¼ E
a
ꢂ R
m
T
m
ð9Þ
ð10Þ
G for the decom-
atives show first TG step in the temperature range ꢄ100–250 °C
losing azo part, while in the temperature region ꢄ250–300 °C, for
the second TG stage attributed to decarboxylation mechanism. At
higher temperatures HL and its azo derivatives loss the acrylalde-
ꢁ
ꢁ
ꢁ
G ¼
D
H ꢂ T
m
DS
⁄
⁄
⁄
D
The calculated values of E
a
, A,
D
S ,
D
H and
position steps are summarized in Table 3. According to obtained
kinetic data, entropy was found to be positive. So, the activated
compound can be thought to own less ordered structures than
3 3
hyde molecule (C H O) accompanied by 4-ASA combustion.
The kinetic and thermodynamic parameters such as activation
⁄
⁄
⁄
energy (E
of the decomposition (
Redfern (CR) and Horowitz–Metzger (HM) methods [41a,b],
respectively.
a
), enthalpy (
D
H ), entropy (
D
S ) and free energy change
the reactant, while the negative values of activation entropies
D
G ) were evaluated by applying the Coast–
⁄
D
S indicate a more ordered activated compounds than the reac-
tants and/or the reactions are slow [41c].
The non-spontaneous nature of the degradation reactions of the
compounds were supported by the positive value of
2
3
⁄
D
W
f
ꢄ
ꢀ
ꢁꢅ
G in the
log
ðW
2
f
ꢂWÞ
AR
hE
m
1 ꢂ 2Rm
T
m
E
a
⁄
4
5
degradation steps. The positive value of
D
H at a particular temper-
log
¼ log
ꢂ
m m
2:303R T
;
ð6Þ
T
a
E
a
m
ature indicated that the process was endothermic nature. The high
values of the activation energies reflect the thermal stability of the
compounds. So, it is clear from TG analysis that HL more stable
than its azo derivatives. Also indicate that thermal stability of
f
where W is the mass loss at the completion of the reaction, W is the
mass loss up to temperature T
activation energy in kJ mol , h is the heating rate and (1 ꢂ (2R
; R
m m
is the gas constant, E
a
is the
ꢂ1
m m
T /
HL
3
> HL
2
[42].
Table 2
n
Weight loss percentage of HL and its azo derivatives (HL ).
Compound
First stage
Second stage
Remaining Wt.% after ꢃ 300 °C
T
S
Wt. loss%
Found
Assignments loss of:
T
S
Wt. loss%
Found
Assignments loss of:
Calc.
Calc.
HL
170
85
110
29.9
39.9
38.2
28.0
39.5
40.3
CO
2
+ H
2
O
185
195
190
43.7
12.5
12
43.0
14
12.7
C
CO
CO
6
H
4
N
26.4
47.6
49.8
HL
HL
2
C
C
6
H
H
5
N
N
2
+ H
Cl
2
O
2
3
6
4
2
2

1
044
M.M. Ghoneim et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1039–1049
Table 3
Kinetic and thermodynamic data of the thermal decomposition for HL and its azo derivatives (HL
n
).
Comp. Decomp.
step
Parameter
CR method
HM method
ꢂ1
⁄
⁄
⁄
ꢂ1
⁄
⁄
⁄
DG
(kJ mol )
E
a
A (s
)
D
S
D
H
D
G
E
a
A s
D
S
D
H
ꢂ1
ꢂ1 ꢂ1
ꢂ1
ꢂ1
ꢂ1
ꢂ1 ꢂ1
ꢂ1
ꢂ1
(kJ mol
)
(J mol
K
)
(kJ mol
)
(kJ mol
)
(kJ mol
)
(J mol
K
)
(kJ mol
)
3
6
6
2
37
7
2
HL
HL
HL
1st
341
84.6
2.58 ꢅ 10
2.27 ꢅ 10
4.49 ꢅ 10
337
80.3
131
146
344
94.5
5.17 ꢅ 10
2.50 ꢅ 10
4.74 ꢅ 10
340
90.2
123
146
2
2
2
2
2nd
ꢂ1.28 ꢅ 10
ꢂ1.26 ꢅ 10
ꢂ2.49 ꢅ 10
ꢂ1.08 ꢅ 10
ꢂ2.11 ꢅ 10
ꢂ1.08 ꢅ 10
6
2
2
7
2
3
1st
69.8
48.7
2.29 ꢅ 10
66.4
42.6
118
226
77.4
67.4
7.60 ꢅ 10
1.53 ꢅ 10
ꢂ96.7
74.1
61.3
114
216
2
2nd
1.49
ꢂ2.11 ꢅ 10
7
2
2
7
2
1st
80.6
43.1
1.81 ꢅ 10
1.02 ꢅ 10
77.3
38.8
120
148
75.1
47.1
9.39 ꢅ 10
1.96 ꢅ 10
ꢂ94.6
71.9
42.8
109
149
2
2nd
ꢂ2.06 ꢅ 10
0
.8
.7
.6
.5
.4
.3
.2
.1
.0
0
0
0
0
0
0
0
0
(
e)
(
a)
(d)
(c)
4
-ASA
HL
HL
(
b)
1
HL2
HL3
HL4
2
00 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
(
a)
λ, nm
1
0
20
30
40
50
60
70
0.13
2
θ°
4-ASA
(
b)
HL
HL
Fig. 3. The XRD patterns of powder forms; (a) 4-ASA, (b) HL, (c) HL
HL
3
, (d) HL
4
, and (e)
0.12
1
HL2
HL3
2
.
0
0
0
0
0
.11
.10
.09
.08
.07
HL4
(
d)
(
c)
200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
λ, nm
(
b)
Fig. 5. Spectral behavior of (a) Transmittance, T, and (b) Reflectance, R, for 4-ASA,
HL and HL thin films.
n
(
a)
Saifullah et al. and Xu et al. [19,26] and differ from 5-ASA [43].
HL is also polycrystalline structure (Fig. 3b). The XRD patterns for
azo derivatives indicate a mixture of polycrystals and amorphous
structure (Fig. 3c–e). The XRD patterns of thin films (Fig. 4) indicate
that all thin films of 4-ASA and its derivatives are amorphous.
1
0
20
30
40
50
60
70
2
θ°
3 4
Fig. 4. The XRD patterns of thin film forms; (a) 4-ASA, (b) HL, (c) HL , and (d) HL .
X-ray diffraction
Optical properties
The X-ray diffraction patterns (XRD) of 4-ASA and its azo
derivatives in powder and thin film forms are shown in Figs. 3
and 4, respectively. The XRD patterns of the powder forms indicate
that all the samples are polycrystalline with different structure and
various degree of crystallinity. Moreover the XRD pattern of 4-ASA
Optical constants of 4-ASA and its derivatives thin films
The absolute values of transmittance, T(k), and reflectance, R(k),
n
for thin films of 4-ASA, HL and HL in the wavelength range 200–
2500 nm are shown in Fig. 5(a and b) respectively. There is a strong
absorption in the wavelength range 200–400 nm (T + R < 1) and
very weak absorption in the wavelength range 600–2500 nm
(Fig. 3a) found to be, perfectly, identical to the one reported by

M.M. Ghoneim et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1039–1049
1045
4
-ASA
0.448
0.440
0.432
4
-ASA
HL
HL
HL
HL
1
2
2
1
1
.1
.0
.9
.8
1
HL2
HL3
HL4
HL2
HL3
HL4
0.424
0.416
0
0
0
.408
.400
.392
200
400
600
800
1000
1200
1400
λ, nm
0
.0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
(hν)
2, eV2
Fig. 6. Spectral behavior of refractive index, n, for 4-ASA, HL and HL
n
thin films.
2
ꢂ1
n
m thin films.
)2 for 4-ASA, HL and HL
Fig. 9. Plot of (n ꢂ 1) vs. (h
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
0
0
0
0
0
0
.09
.08
.07
.06
.05
.04
HL1
HL2
HL3
The spectral behavior of the calculated values of refractive
index, n, for thin films of 4-ASA, HL and HL is shown in Fig. 6.
n
HL4
The spectral behavior of n for all samples shows normal dispersion
behavior at the wavelength ranged 650–2500 nm, while it shows
anomalous dispersion at k < 650 nm. This behavior has been
reported for many organic compounds [37,45]. The spectral distri-
bution of absorption index, k, for all samples in the wavelength
range 200–600 nm is shown in Fig. 7. All the materials under inves-
tigation have two peaks at 230 nm to 300 nm. Additional peak
appears at the wavelength range 380–520 nm for azo derivatives
only.
4
-ASA
HL
HL
1
380
400
420
440
460
480
500
520
HL2
HL3
HL4
λ, nm
Dispersion analysis
The obtained data of refractive index, n, in the non absorbing
region can be analyzed in the terms of single oscillator model to
2
00
250
300
350
λ, nm
400
450
500
obtain the lattice dielectric constant,
to effective mass ratio (N/m ), the dispersion energy, E
e
L
, the carrier concentration
, the single
⁄
d
Fig. 7. Spectral behavior of absorption index, k, for 4-ASA, HL and HL
n
thin films.
o 1
oscillator energy, E , and the dielectric constant at infinity, e ,
which describes the contribution of the free carriers and the lattice
vibration modes of the dispersion via the following procedure:
3
3
3
3
3
3
3
3
.55
.50
.45
.40
.35
.30
.25
.20
0
0
2
2
The real part of the dielectric constant
the wavelength of the incident light in the non absorbing region by
e
(
e
= n ꢂ k ) related to
4
HL
HL
HL
HL
HL
-ASA
the following relation [37,45]:
1
2
3
4
e2N
0
2
e
¼
e
L
ꢂ 4
k ;
2
ð11Þ
ꢁ
p
e
o
m c
where e is the charge of the electron,
e
o
, is the permittivity of free
space and c is the velocity of light. Fig. 8 shows the relation between
0
2
e
and k in the non-absorbing region for 4-ASA and its derivatives
thin films. The value of
e
L
can be obtained from the intersection
0
of the extrapolation of the straight line with the
e
axis, the value
⁄
of N/m can be calculated using the slope of the straight line.
Wemple and Didomenico [46] analyzed the refractive index
data below the interband absorption edge using a single oscillator
d o
equation to define the dispersion energy parameters E and E
which measure the average strength of the interband optical tran-
sitions according to the relation:
6
6
6
6
6
6
0
1x10
2x10
3x10
4x10
5x10
6x10
λ2, nm2
9 vs. k2 for 4-ASA, HL and HL
Fig. 8. Plot of
e
n
thin films.
2
o
E E
d
n ꢂ 1 ¼
;
2
ð12Þ
2
E ꢂ ðh
mÞ
o
(
6
T + R ꢃ 1). The interference appeared in the wavelength range
00–2500 nm is due to the multiple reflection which occur at the
where hm is the energy of the incident light. Experimental verifica-
interface between film-substrate and the back reflection of the thin
film [44]. The absorption phenomena in the high wavelength range
can be attributed to scattering of the light [35].
tion of the above equation can be obtained by rearranging relation
2
ꢂ1
2
(12) and plotting (n ꢂ 1) vs. (h
m
o
) and fitting it to a straight line,
as shown in Fig. 9. The values of E
and E are determined from the
d

1
046
M.M. Ghoneim et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1039–1049
Table 4
The value of the optical parameters of 4-ASA and its derivatives thin films.
⁄
54
ꢂ1
ꢂ3
ꢁ
Compound
-ASA
e
1
e
L
E
d
(eV)
E
0
(eV)
N/m ꢅ 10 (kg
m
)
E
g
(eV)
E
(eV)
g
4
3.26
3.30
3.24
3.34
3.31
3.34
3.30
3.34
3.30
3.38
3.38
3.40
15.54
19.32
14.42
14.43
12.18
17.57
6.88
8.40
6.42
6.17
5.27
7.57
2.10
2.52
2.95
1.55
3.32
6.06
3.60
3.65
3.69
3.63
3.66
3.60
–
–
1.07
1.06
1.55
0.94
HL
HL
HL
HL
HL
1
2
3
4
5
5
5
5
5
5
5
5
4
4
4
4
4
.5x10
.0x10
.5x10
.0x10
.5x10
.0x10
.5x10
.0x10
.0x10
2.4x10
Table 5
HL1
The peak position of photoluminance (PL) spectra of solutions and thin films.
HL
HL
2
4
3
3
2
2
1
1
5
2.0x10
3
Compound Photoluminance peak position
HL4
ꢂ5
Solution (10 mol/L)
Thin film
1
1.6x10
1
2
3
2
3
4-ASA
HL
HL1
409 (P) 432 (P) 488 (Sh) 514 (P)
407 (P) 430 (P) 490 (Sh) 512 (P)
538 (P)
536 (P)
581 (Sh)
579 (Sh)
579 (Sh)
4
3
1.2x10
–
–
–
–
–
–
–
–
–
–
–
–
508 (Sh) 521 (P)
HL
2
HL
3
HL
4
512 (P)
512 (P)
512 (P)
522 (Sh) 580 (Sh)
522 (Sh) 580 (Sh)
522 (Sh) 580 (Sh)
8.0x10
4-ASA
HL
1.0
1.5
2.0
hν, eV
2.5
3.0
HL
H2
1
HL3
HL4
coefficient,
of incident photon energy (h
a
, of thin films of 4-ASA and its derivatives as a function
0
.0
m
) is shown in Fig. 10. The absorption
0
.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
hν, eV
coefficient spectra reveal two peaks in the UV region of incident
light for all compounds, and a peak in the visible region of incident
light for the azo derivatives only. This behavior can be attributed to
the orbital molecular transition from bonding to antibonding
Fig. 10. The absorption coefficient,
for 4-ASA, HL and HL thin films.
a
, as a function of the incident photon energy hm
n
molecular orbitals. The band appearing at 5.55 eV is assigned to
⁄
pAp
transitions of aromatic benzene ring and the band, appearing
close to the first one, located around 4.50 eV attributed to the stabil-
⁄
1
1
1
1
600
400
200
000
HL
HL
HL
HL
1
2
3
4
ization of the higher lying nA
p
state by substitution. Also this state
2
2
1
50
00
50
may have intramolecular charge transfer character (ICT) [27,47].The
decrease in energy gap between two bands for groups with more
electron donating character (strongly donating amino group at
-position) indicates a stronger interaction and hence a lowering
of this state [27]. The bands located around 2.65–2.88 eV is assigned
transitions for azodye derivatives function group N@N. The
variation of the peak position can be attributed to either withdraw-
ing or donating functional groups [48].
4
100
⁄
to nAp
5
0
8
00
00
00
00
0
4-ASA
HL
0
6
4
2
0.0
0.4
0.8
1.2
1.6
hν, (ev)
2.0
2.4
2.8
3.2
HL
1
g
To determine the energy gap, E , and the type of optical transi-
tion responsible for optical absorption, the experimental data can
be treated according to the following relation [36]:
HL2
HL4
HL3
m
ahm
¼ Bðh
m
ꢂ E
g
Þ ;
ð13Þ
where B is a constant which depends on the electronic transition
probability, E is the optical energy gap of the material, m is the
power which characterizes the transition process, where m = 1/2
and 3/2 for direct allowed and forbidden transitions, respectively,
and m = 2 and 3 for indirect allowed and forbidden transitions,
respectively. The best fit for our data which are shown in Fig. 11,
ensuring the occurrence of indirect allowed transition. The funda-
and the onset energy gap E for 4-ASA and
its derivatives are tabulated in Table 4. The onset E appears only
for the azodye derivatives. The variation of the values of Eg can be
attributed to either withdrawing or donating functional groups. It
g
is those fundamental optical energy gaps E nearly have the same
values (3.60–3.69) eV. The onset energy gaps have values in the
range 0.94–1.55 eV depending on the substituent group.
0
1
2
3
4
5
6
hν, eV
g
Fig. 11. Plot of (
a
h
m
)1/2 vs. h
m
for 4-ASA, HL and HL
n
thin films.
ꢂ1
slope (= (E
o
E
d
)
) and the intersection (= (E
e
o
/E
can be obtained from the intersection of the
extrapolated straight line with the Y-axis when it equals
d
)) with the vertical
axis. The values of
1
ꢁ
mental energy gap E
g
g
ꢁ
ꢂ1
⁄
g
2
1
2
ðn ꢂ 1Þ and
e
1
¼ n . The values of
e
L
,
e
1
, N/m , E
o
and E
d
are
ꢁ
1
listed in Table 4. It is clear that,
e
L
>
e
1
for all compounds under
study. This trend was observed for other materials [37,45] and
was attributed to the contribution of a small concentration of free
carriers.
Absorption properties of thin films
Emission properties
In general, the absorption spectrum gives useful information
about the energy band structure of materials. The absorption
The photoluminescence (PL) spectra of 4-ASA and its derivatives
as solution and thin film forms are investigated. A comparison

M.M. Ghoneim et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1039–1049
1047
Fig. 12. Excited state intramolecular proton transfer (ESIPT) pathway of 4-ASA at kmax = 365 nm.
H
C
H
C
H
C
H
N
H
N
H
N
H
H
H
C
C
C
C
C
C
H
O
H
H
O
O
hυ
O
H
O
H
O
H
C
C
C
O
O
H
HO
O
HO
O
Rotomer R
Rotomer P
H
H
C
O
H
C
C
O
H
O
H
N
O
H
C
H
C
H
N
H
C
H
O
O
H
O
C
C
O
H
Scheme 4. Intra- and inter-molecular hydrogen bond of HL.
ꢂ5
between PL spectra of solution (10 mol/L, using methanol as a
solvent) and thin films are presented in Table 5. No significant
emission peaks for the solution of azodye derivatives, while all
the thin films samples of 4-ASA and its derivatives have PL spectra.
The PL spectra for thin films of 4-ASA and HL have the same behav-
ior as that of the solutions, but with red shift. This shift can be
attributed to the effect of solvent [49]. The PL spectra of 4-ASA,
n
HL and HL thin films are shown in Fig. 12. The photoluminescence

1
048
M.M. Ghoneim et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1039–1049
be lower than
1
e for all compounds which indicate the existence of
6
.4
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
HL
1
free carriers. The optical transition are found to be indirect allowed
with values energy gap, E
transition and onset energy gap E , of 0.94–1.55 eV for azo deriva-
tives only. All of the 4-ASA and its derivatives revealed emission
spectra in the wavelength rang 500–600 nm. Diazotization
remarkably enhances the PL spectra, due to the presence of an
extra ESIPT.
HL2
HL3
5.6
4.8
4.0
g
, of 3.60–3.69 eV for fundamental optical
ꢁ
HL4
g
3.2
2.4
1
0
.6
.8
HL
4
-ASA
4
80 500 520 540 560 580 600 620 640
λ, nm
References
[
[
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Fig. 13. Emission spectra for 4-ASA, HL and HL
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[
[
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5
80 nm.
The possible excited intramolecular proton transfer pathway of
-ASA and its derivatives at kmax = 365 nm are shown in Fig. 12. 4-
(
(
4
[
12] (a) W.M.F. Fabian, L. Antonov, D. Nedeltcheva, F.S. Kamounah, P.J.J. Taylor,
Phys. Chem. A 108 (2004) 7603–7612;
ASA (P-form) undergoes ESIPT. 4-ASA has short lifetime, and low
quantum yield which attributed to stabilization of the S
2
state of
(b) S. Pieraccini, G. Gottarelli, R. Labruto, S. Masiero, O. Pandoli, G. Spada,
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⁄
nAp
indicating an increased deactivation [27]. Emission fluores-
⁄
cence bands could be observed as the absorption is from N to N
(
d) Y.G. Hong, J.D. Gu, Appl. Microbiol. Biotechnol. 88 (2010) 637–643.
⁄
⁄
⁄
(
(
N is excited state normal form) and the emission is from T to T
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T is excited state tautomeric form).
[
The amidation process slightly enhances the fluorescence spec-
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and inter-molecular hydrogen bond transfer of HL is shown in
Scheme 4. It seems that HL easily undergoes ESIPT with rotomer
P. Also it can formed dimer structure through intermolecular
hydrogen bond between two molecules.
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[
[
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