Optical properties of some synthesized azo thin films

Article abstract of DOI:10.1016/j.molstruc.2012.06.004

5-(4′-Alkyl phenylazo)-2-thioxothiazolidin-4-one (PATT-L_n) have been synthesized and characterized with various physico-chemical techniques. Thin films of PATT-L_n have been prepared by the thermal deposition technique in a vacuum of 3 × 10^-5 mbar onto optical flat glass substrates. The absorption properties of thermally deposited PATT-L_n thin films were investigated in the wavelength range 190-2500 nm. The type of optical transition near the edge of the band gap is found to be direct allowed transition. The values of the energy gap for derivatives under investigation are calculated and found to be in the range 1.77-2.29 eV dependent on the nature of the substituent. They tend to the increase according to the following order p-(OCH₃ < CH₃ < H < Cl < NO₂) as expected from Hammett's constant σ^R.

Full text of DOI:10.1016/j.molstruc.2012.06.004

Journal of Molecular Structure 1027 (2012) 92–98
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Optical properties of some synthesized azo thin films
b
N.A. El-Ghamaz a, A.Z. El-Sonbati b, , Sh.M. Morgan
⇑
a Department of Physics, Faculty of Science, Mansoura University, Damietta, Egypt
b Department of Chemistry, Faculty of Science, Mansoura University, Demiatta, Egypt
h i g h l i g h t s
"
"
"
"
"
Thermal evaporation process does not affect the chemical structure of ligands.
Thermally deposited PATT-Ln thin film can be considered as completely amorphous.
The optical absorption showed many absorption bands.
The type of optical transition is found to be direct allowed transition.
The Eg values are in the range 1.78–2.29 eV according to p-(NO2 > Cl > H > OCH3 > CH3).
a r t i c l e i n f o
a b s t r a c t
5-(40-Alkyl phenylazo)-2-thioxothiazolidin-4-one (PATT-Ln) have been synthesized and characterized
with various physico-chemical techniques. Thin films of PATT-Ln have been prepared by the thermal
deposition technique in a vacuum of 3 ꢀ 10ꢁ5 mbar onto optical flat glass substrates. The absorption
properties of thermally deposited PATT-Ln thin films were investigated in the wavelength range 190–
2500 nm. The type of optical transition near the edge of the band gap is found to be direct allowed tran-
sition. The values of the energy gap for derivatives under investigation are calculated and found to be in
the range 1.77–2.29 eV dependent on the nature of the substituent. They tend to the increase according to
Article history:
Received 12 April 2012
Received in revised form 3 June 2012
Accepted 4 June 2012
Available online 12 June 2012
Keywords:
Rhodanine azodyes
Optical properties
Thin films
the following order p-(OCH3 < CH3 < H < Cl < NO2) as expected from Hammett’s constant rR
.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
Despite the importance of studying the optical absorption prop-
erties of rhodanine derivatives in solar cell applications, to the best
of our knowledge, little efforts were done to study the optical
absorption phenomena for 5-(40-alkyl phenylazo)-2-thioxothiaz-
olidin-4-one (PATT-Ln) in solution form whereas no studies on
the materials under investigation in the thin films form has been
done.
The objectives of the present work are synthesis and character-
izing the 5-(40-alkyl phenylazo)-2-thioxothiazolidin-4-one (Fig. 1),
preparing the thin films by thermal evaporation technique, and to
study the optical absorption properties of PATT-Ln thin films.
Azo compound, based on thioxothiazolidin (rhodanine), plays a
central role as chelating agents for a large number of metal ions [1]
as they form a stable six-member ring after complexation with a
metal ion and can also be used as analytical reagents [2]. Rhoda-
nine plays an important role in biological reactions [3]. Chemical
properties of rhodanine and its derivatives are of interest due to
coordination capacity and their use as metal extracting agents
[4]. Azo compounds based on rhodanine were synthesized as
potential medicinal preparations [3].
Rhodanine derivative attracted some attention in solar cell
application [5]. Many authors studied the light-to-electricity con-
version efficiency [6] for rhodanine derivative. They found that
2. Experimental technique
the value of conversion efficiency (g) is in the range 2.86% and
All the chemicals used were of British Drug House quality.
6.27%. A few triple rhodanine indoline derivatives showed compa-
rable conversion efficiency [5]. A double rhodanine indoline dye
has been reported to show a highest solar-to-electricity conversion
2.1. Synthesis of 5-(40-alkyl phenylazo)-2-thioxothiazolidin-4-one
(PATT-Ln)
efficiency (g = 9.0%) on titanium oxide [7].
In a typical preparation, 25 ml of distilled water containing
0.01 mol hydrochloric acid were added to aniline (0.01 mol) or
p-derivatives. To the resulting mixture stirred and cooled to 0 °C,
⇑
Corresponding author. Tel.: +20 1060081581; fax: +20 572403868.
E-mail address: elsonbatisch@yahoo.com (A.Z. El-Sonbati).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.06.004

N.A. El-Ghamaz et al. / Journal of Molecular Structure 1027 (2012) 92–98
93
a solution of 0.01 mol sodium nitrite in 20 ml of water was added
dropwise. The formed diazonium chloride was consecutively cou-
pled with an alkaline solution of 0.01 mol 2-thioxo-4-thiazolidi-
none, in 10 ml of pyridine. The colored precipitate, which formed
immediately, was filtered through sintered glass crucible, washed
several times with water and ether. The crude products was puri-
fied by recrystallization from hot ethanol, yield ꢂ65% then dried in
vacuum desiccator over P2O5.
monitored and controlled during deposition by a film thickness
monitor (TM-350 MAXTEK). The film thickness of all samples
was in the range 100–110 nm and the rate of deposition was ran-
ged 0.5–1 nm/s.
2.3. Measurements
Elemental microanalyses of the separated solid chelates for C, H,
The resulting formed ligands are: 5-(40-methoxyphenylazo)-2-
thioxothiazolidin-4-one (PATT-L1), 5-(40-methylphenylazo)-2-thi
oxothiazolidin-4-one (PATT-L2), 5-(40-phenylazo)-2-thioxothiazoli
din-4-one (PATT-L3), 5-(40-chlorophenylazo)-2-thioxothiazolidin-
4-one (PATT-L4) and 5-(40-nitrophenylazo)-2-thioxothiazolidin-4-
one (PATT-L5).
and
N were performed in the Microanalytical Center, Cairo
University, Egypt. Infrared spectra were recorded using Perkin–El-
mer 1340 spectrophotometer. 1H NMR spectra were obtained on a
JEOL FX 9000 Fourier transform spectrometer with deutrated
dimethylsulfoxide (DMSO-d6) as a solvent and TMS as an internal
reference.
HCl.H2O
NaNO2
+
-
R
NH2
HCl
R
N
N
Cl
O
O
HN
S
HN
alkaline solution
0-5 oC
+
-
R
N
N
Cl
S
S
S
N
R
N
O
N
OH
HO
HN
HN
=
S
S
S
S
N
N
N
R
R
N
H
n=1, R = OCH3 (PATT-L1); n=2, R = CH3 (PATT-L2); n=3, R = H (PATT-L3); n=4,
R = Cl (PATT-L4); and n=5, R = NO2 (PATT-L5)
The formation mechanism of azodye ligands (PATT-Ln)
The ligands were also characterized by elemental analysis (Table 1), NMR and I.R.
spectroscopy.
2.2. Preparation of 5-(40-alkyl phenylazo)-2-thioxothiazolidin-4-one
(PATT-Ln) thin films
Structural analysis of the powder and thin film was performed at
room temperature by a Philips X-ray diffractometer equipped with
utilized monochromatic Cu K
a radiation (k = 1.5418 Å). The X-ray
Thin films of PATT-Ln were prepared by thermal evaporation
technique onto optical flat and well-cleaned glass substrates. The
vacuum chamber was pumped down to 3 ꢀ 10ꢁ5 mbar at room
temperature before starting the evaporation process. A high vac-
uum coating unit (Edward’s, E306A, UK) was used to prepare thin
films of PATT-Ln. The film thickness and the rate of deposition were
tube voltage and current were 40 kV and 25 mA, respectively.
Measurements of transmittance T (k) of the thin film samples
were carried out at room temperature and at normal incidence of
light in the wavelength range 190–2500 nm in steps of 2 nm using
a double-beam spectrophotometer (JASCO model V-570-UV/Vis/
NIR).

94
N.A. El-Ghamaz et al. / Journal of Molecular Structure 1027 (2012) 92–98
A
B
C
D
F
E
G
Fig. 1. The structure of 5-(40-alkyl phenylazo)-2-thioxothiazolidin-4-one (PATT-Ln).
3. Results and discussion
Table 1
Analytical data of azorhodanine derivatives.
3.1. Characterization of the ligands
Compounda Empirical
formula
Yield
%
M.p.
(°C)
Calc. (Exp.)%
C
H
N
All five ligands PATT-L1–PATT-L5, gave satisfactory elemental
analysis (Table 1). The molecular structures of these ligands are
such that they can exist in three tautomeric forms as shown in
Fig. 1. Detailed solution and solid states studies of these ligands
were carried out to establish their geometry.
In general, the azo group can act as a proton acceptor in hydro-
gen bonds [8–10]. The role of hydrogen bonding in azo aggregation
has been accepted for some time. Intramolecular hydrogen bonds
involving OH group with the AN@NA group increased their stabil-
ities [8–10]. The strength of the hydrogen bond of compounds de-
pends on the nature of substituents present in the coupling
component from the aryl azo group. Chelating rings formed by
NHꢃ ꢃ ꢃN bonds are less stable than corresponding rings formed by
OHꢃ ꢃ ꢃN bonds [11,12].
PATT-L1
PATT-L2
PATT-L3
PATT-L4
PATT-L5
C10H9N3O2S2
Red
37.45
47.81
42.19
51.37
221
231
237
248
44.93
3.39
15.72
(44.82) (3.25) (15.85)
C10H9N3OS2
Dark orange
47.79 3.61 16.72
(47.88) (3.76) (16.61)
45.55 2.97 17.71
(45.68) (2.80) (17.85)
39.78 2.23 15.46
(39.65) (2.35) (15.58)
38.29 2.14 19.85
(38.42) (2.25) (19.98)
C9H7N3OS2
Pale yellow
C9H6N3OS2Cl
Light orange
C9H6N4O3S2
Dark yellow
66.087 245
a
The analytical data agree satisfactory with the expected formulae represented
as given in structures PATT-L1–PATT-L5 and LH. Air-stable, colored, insoluble in
water, but soluble in hot ethanol, and soluble in coordinating solvent.
3.1.1. Infrared spectra
The broad/strong absorption of a band located at ꢂ3450–3434
system. The frequencies for the N@N stretching lie in the region
1440–1435 cmꢁ1. The region between 1500 and 900 cmꢁ1 is due
CAN stretching, NAH in plane or out of plane bending and out-of-
plane CAH bending vibrations [13]. The symmetric and antisym-
metric (C@C) stretching vibration modes are expected to exist in this
and at ꢂ1710–1695 cmꢁ1 region assigned to the
m(NH) stretching
vibrations mode and carbonyl stretching vibrations are expected
to occur in this region. The three bands in the 1600–1500 cmꢁ1
region are characteristic for most six-membered aromatic ring

N.A. El-Ghamaz et al. / Journal of Molecular Structure 1027 (2012) 92–98
95
region. Thus, this ligand (PATT-Ln) contains four potential donor
3.1.2. 1H NMR spectra
sites (Fig. 1): (i) the ring nitrogen NH, (ii) the ring CS, (iii) the car-
bonyl group, and (iv) the nitrogen of azo (N@N) group. However,
considering the planarity of the ligand, it is unlikely that this ligand
could be tetradentate on a single metal center. Hence, this ligand is
expected to be bidentate and the three favorable possibilities of
donor sites are: (i) the carbonyl oxygen and (ii) the nitrogen atom
of azo (N@N) group or (i) the carbonyl oxygen and (ii) the CS. The
coordination of the ring nitrogen (NH) is unlikely due to the zwitter-
ions [4] formation, thereby lowering the electron density on N.
The infrared spectra of PATT-Ln give interesting results and con-
clusions. The ligands gives two bands at ꢂ3200–3040 cmꢁ1 due to
asymmetric and symmetric stretching vibrations of NAH group
and intramolecular hydrogen bonding NHꢃ ꢃ ꢃO systems (Fig. 1D),
respectively. When the OH group (Fig. 1C) is involved in intramo-
lecular hydrogen bond, the Oꢃ ꢃ ꢃN and Nꢃ ꢃ ꢃO bond distances are the
same. But, if such mechanism is happened in case of intermolecu-
lar hydrogen bond, the Oꢃ ꢃ ꢃO and Oꢃ ꢃ ꢃN bond distances are differ.
The broad absorption of a band located at ꢂ3400 cmꢁ1 is as-
The NMR spectroscopy was used to differentiate stereoisomers.
El-Sonbati, Diab and coworkers [16,1,17] investigated the NMR
spectra of azo rhodanine and its derivatives with various transition
metal salts. The 1H NMR spectra are in agreement with the pro-
posed structures. Signal for CH (ꢂ4.42 ppm), favors formation of
an intramolecular hydrogen bond with the N@N (azodye) group.
Electron-withdrawing substituents reduce the intramolecular
hydrogen bond as indicated by the marked shift of the hydroxyl sig-
nal to a higher field in the p-NO2 and p-Cl compounds. Electron-
donating substituents give the opposite effect, arising from the
increasing basicity of the azo-nitrogen. The broad signals assigned
to the OH protons at ꢂ11.36–11.88 ppm are not affected by dilu-
tion. The previous two protons disappear in the presence of D2O.
According to El-Sonbati et al. [16], hydrogen bonding leads to a
large deshielding of the protons. The shifts are in the sequence: p-
(NO2 > Cl > H > OCH3 > CH3). In the meantime, the 1H NMR of the
PATT-L1/PATT-L2 exhibits signals at d(ppm) [3.3 (s, 3H, CH3)]/[3.9
(s, 3H, OCH3)]. The aromatic protons have resonance at 7.10–
7.45 ppm for the ligands.
signed to mOH. The low frequency bands indicate that the hydroxy
hydrogen atom is involved in keto , enol (A , B) tautomerism
through hydrogen bonding (Fig. 1C). Bellamy [14] made detailed
studies on some carbonyl compounds containing ANH-group. The
On the basis of all the above spectral data, an internally hydro-
gen boned azo–enol structure has been proposed for the ligand
(Fig. 1).
D
m
NH values were used to study the phenomena of association.
On the other hand, the OH group (Fig. 1B) exhibits more than one
3.1.3. Structural analysis and optical characterization of 5-(40-alkyl
phenylazo)-2-thioxothiazolidin-4-one (PATT-Ln) thin films
absorption band. The two bands located at 1330 and 1370 cmꢁ1 are
assigned to in-plane deformation and that at 1130 cmꢁ1 is due
m
The IR spectra of 5-(40-alkyl phenylazo)-2-thioxothiazolidin-4-
one (PATT-Ln) before and after thermal evaporation were mea-
sured. Study of these figures shows that there are no changes in
the chemical composition for all derivatives. In addition, there is
a small shift in some absorption peaks. These shifts can be attrib-
uted to change in the bond length.
CAOH.
However, the 860 cmꢁ1 band is probably due to the out-of-
plane deformation of the AOH group. On the other hand, the two
bands located at 650 and 670 cmꢁ1 are identified as dC@O and NH.
Similar to the other investigated compounds, the different
modes of vibrations of CAH and CAC band are identified by the
presence of characteristic bands in the low frequency side of the
Because of the high similarity of the X-ray diffraction, XRD, pat-
terns for all thin films of PATT-Ln ligands, we will suffice by men-
tioning the XRD patterns of PATT-L2 as representative results for
the rest of the ligands. The XRD patterns of the as-synthesized
PATT-L2 powder and the thermally deposited thin films, measured
at room temperature, are shown in Fig. 2. The XRD pattern of the
powder (Fig. 2a) show many peaks which indicate the polycrystal-
line nature of the as-synthesized PATT-L2 ligand. The XRD pattern
of the thermally deposited thin film (Fig. 2b) show a broad peak at
around 2h = 23° indicating a completely amorphous structure. In
general, the thermally deposited PATT-Ln thin film can be consid-
ered as completely amorphous.
spectrum in 600–200 cmꢁ1
.
The infrared spectra of ligands shows medium broad band
located at ꢂ3460 cmꢁ1 due the stretching vibration of some sort
of hydrogen of hydrogen bonding. El-Sonbati et al. [10,13,15,16]
made detailed studies for the different types of hydrogen bonding
which are favorable to exist in the molecule under investigation:
(1) Intramolecular hydrogen bond between the nitrogen atom
of the AN@NA system and hydrogen atom of the hydroxy
hydrogen atom (Fig. 1C). This is evident by the presence of
a broad band centered at 3460 cmꢁ1
.
The measured transmittance T (k) of PATT-Ln thin films in the
wavelength range 190–2500 nm are shown in Fig. 3. This figure
(2) Hydrogen bonding of the OHꢃ ꢃ ꢃN type between the hydroxy
hydrogen atom and the N-ph group (Fig. 1C).
(3) Intermolecular hydrogen bonding is possible forming cyclic
dimer through NHꢃ ꢃ ꢃO@C (G), OHꢃ ꢃ ꢃN@N (F) or OHꢃ ꢃ ꢃOH
(E). (Fig. 1).
The presence of broad band located at ꢂ3200 cmꢁ1 is strong
indication by mNH (Fig. 1D). In general, the low frequency of such
region from its normal position is, again due to hydrogen bond
property gathered with keto , enol tautomerism.
In general, hydrogen bonding involving both NH and OH groups
are proton donors and both AN and AO atoms are proton accep-
tors. It is of interest since much multiplicity of proton donor and
acceptor sites are prevalent in biological systems. Both intra- and
intermolecular OHꢃ ꢃ ꢃN and NHꢃ ꢃ ꢃO may form leading to a number
of structures in simultaneous equilibrium.
(a)
(b)
Again the three bands located at 1380, 1340 and 1310 cmꢁ1
identified as dOH gathered with the two bands at 1240 cmꢁ1 as-
10
20
30
40
2θο
50
60
70
signed as
m
CAO are strong indication to keto , enol equilibria.
The presence of a medium band at ꢂ1605 cmꢁ1 assigned to
C@N illustrates the tracing of keto structure (Fig. 1D).
m
Fig. 2. X-ray diffraction pattern for PATT-L2 derivative: (a) Powder form and (b)
thermally evaporated thin film.

96
N.A. El-Ghamaz et al. / Journal of Molecular Structure 1027 (2012) 92–98
1.0
0.8
0.6
0.4
0.2
0.0
11
10
(PATT-L1)
(PATT-L2)
(PATT-L3)
(PATT-L4)
(PATT-L5)
(PATT-L1)
(PATT-L2)
(PATT-L3)
(PATT-L4)
(PATT-L5)
9
1.5
1.8
hν, eV
2.1
200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
wavelength (λ), nm
Fig. 5. Plot of ln
a
versus photon energy h
m
, for PATT-Ln thin films.
Fig. 3. Plot of transmittance (T) versus wavelength (k) for PATT-Ln thin film.
absorption spectrum gives a useful informations about the energy
band structure of the compounds under investigation. The absorp-
tion coefficient spectrum reveals many peaks and shoulder in the
following Vis.Q-band, Soret B-band, Variable-N-band and C-band
[20,21] with values of the order of 104 cmꢁ1, indicating transitions
of charge carriers from bonding to antibonding molecular orbitals,
except PATT-L1 which shows no peak in the photon energy range
2.14–3.26 eV. The Q-band exists in the visible region of spectra
while others (B, N and C) exist in the UV region of spectra. This
behavior can be attributed to the orbital molecular transition from
bonding to antibonding molecular orbital [1]. The peak and
shoulder positions for the PATT-Ln thin films are listed in Table 2.
The bands are appearing at 6.29–6.32 eV and the shoulders at
4x105
(PATT-L1)
(PATT-L2)
(PATT-L3)
(PATT-L4)
(PATT-L5)
3x105
2x105
1x105
0
5.00–4.78 eV and are assigned to p–
pꢄ transitions between orbitals
largely localized in phenyl ring [17]. The shoulders located around
3.5–3.6 eV and the band at 4.2 eV were assigned to n–pꢄ transitions
of conjugated heterocyclic rhodanine ring and N@N band respec-
tively [22,23]. The shoulders located around 2.9–3.2 eV are ascribed
to n–pꢄ transition [17]. The shoulders in the range 2.28–2.45 eV and
the bands in the range 2.72–3.77 eV are assigned to intramolecular
charge transfer (ICT) and transition within the whole molecule. This
various types of transition are attributed to either withdrawing or
donating functional groups [10,17].
0
2
4
6
hν, eV
, as a function of photon energy, h
Fig. 4. Absorption coefficient,
a
m
, for PATT-Ln thin
films.
shows a strong absorption in the wavelength range 190–600 nm
while at longer wavelength up to 2500 nm, the value of T (k)
increases gradually until reaching a value, approximately unit. This
behavior indicates that these derivatives are transparent at wave-
length range 600–2500 nm. The absorption coefficient,
lated from T (k) according to the following equation [18,19]:
The spectral distribution of the absorption coefficient
divided into two regions. In the first region, where the absorption
coefficient
is less than 104 cmꢁ1, there is usually an Urbach tail
[24,25] where
a is
a
a, is calcu-
a
depends exponentially on the photon energy h
m
according to the following relation:
4pkf
;
a
¼
ð1Þ
1
Ea
ꢁ EꢄÞ;
ð3Þ
k
ln
where
a
¼ ln ao
þ
ðh
t
where the absorption index, kf, can be calculated from the relation:
m
is the frequency of the incident light,
a
o is the pre-exponen-
2:303k ꢃ logð1=TÞ
tial constant, h is Planck’s constant and Ea is the width of the tails of
the localized states in the gap region. The existence of the tail can be
attributed to the amorphous nature of the as-deposited thin films.
kf ¼
;
ð2Þ
4pd
where d is the thickness of the film.
The absorption coefficient, , of PATT-Ln thin films as a function
) is shown in Fig. 4. In general, the
a
m
Therefore, by plotting the relation between ln
a and hm, as shown
of incident photon energy (h
in Fig. 5, the values of Ea for the films under investigation can be
Table 2
The absorption bands of (PATT-Ln) thin films.
N
Vis.Q-band (eV)
Soret B-band (eV)
Variable-N band (eV)
C-band (eV)
1
2
3
4
5
2.28 (sh)
2.37 (sh)
2.33 (sh)
2.37 (sh)
2.46 (sh)
–
–
3. 52 (sh)
3.62 (sh)
3.63 (sh)
3.57 (sh)
3.7–3.5 (sh)
4.21 (P)
4.20 (P)
4.22 (P)
4.25 (P)
4.22 (P)
5 (sh)
6.29 (P)
5.6 (P)
6.29 (P)
6.29 (P)
6.33 (P)
2.74 (P)
2.72 (P)
2.74 (P)
2.77 (sh)
2.99 (sh)
2.96 (sh)
2.99 (sh)
3.21 (sh)
4.77 (sh)
4.76 (sh)
4.79 (sh)
4.78 (sh)

N.A. El-Ghamaz et al. / Journal of Molecular Structure 1027 (2012) 92–98
97
Table 3
tion process, where x = 1/2 and 3/2 for direct allowed and forbidden
transitions, respectively, and x = 2 and 3 for indirect allowed and
forbidden transitions, respectively. The best fit for our data was
found to be of the value of x = 1/2, indicating that the transition is
The values of the energy gap and the width of the tails of the
localized states in the gap region for (PATT-Ln) thin films.
Edgirect (eV)
N
Ea (eV)
allowed direct, and the value of Eph is taken as zero. The dependence
1
2
3
4
5
1.78
2.223
2.23
2.24
2.29
0.537
0.685
0.709
1.316
0.419
2
of the optical absorption coefficient (
a
hm)
versus hm near the
absorption edge is presented in Fig. 6. It is clear from this figure that
the relation of ( is linear in the fundamental absorp-
ahm
)2 versus h
m
tion region [20]. The intercept of extrapolation of the linear part to
zero absorption with photon energy axis is taken as the value of the
fundamental optical energy gap [20,29]. The values of Eg for PATT-Ln
are determined and listed in Table 3. It is clear from Table 3, that
there is a variation of the value of Eg according to the effect of the
function group. The effect of methoxy group is reducing the Eg to
a value of 1.77 eV, that is because of methoxy group is a strong
donating function group, while the effect of nitro group as strong
withdrawing function group is to raise the Eg to about 2.29 eV
[30]. A similar effect of the substituent OCH3 reported by Leontie
and Danac [31].
1.5x1011
1.0x1011
5.0x1010
(PATT-L5)
1.5
2.0
2.5
3.0
hν , eV
The dependence of the values of Eg and Ea for PATT-Ln on the
nature of function group can be explained as follow:
(PATT-L1)
(PATT-L2)
(PATT-L3)
(PATT-L4)
The energy gap, Eg, increases according with the following order
p-(OCH3 < CH3 < H < Cl < NO2) [32]. This can be attributed to the
fact that the effective charge experienced by the d-electrons in-
creases due to the electron withdrawing p-substituent 4,5 while
it decreases by the electron donating character of 1,2. This is in
accordance with that expected from Hammett’s constant rR as
0.0
1.6
2.0
2.4
hν , eV
shown in Fig. 7. Correlating the Eg values with
r
R, it is clear that
2
R. It is important to note
Fig. 6. Plot of (
a
h
m)
versus photon energy h
m
, for PATT-Ln thin films.
these values increase with increasing
r
that the existence of a methyl and/or methoxy group enhances
the electron density on the coordination sites and simultaneously
decreases the values of energy gap.
2.4
2.3
4. Conclusions
PATT-L2
PATT-L5
PATT-L4
2.2
2.1
2.0
1.9
1.8
1.7
PATT-L3
5-(40-Alkyl phenylazo)-2-thioxothiazolidin-4-one (PATT-Ln)
ligands were synthesized by azo coupling reaction. The thin films
of PATT-Ln were prepared by thermal evaporation technique. The
values of Eg were found in the range 1.78–2.29 eV, where they
increase with increasing the effect of high electron withdrawing
characters p-(OCH3 < CH3 < H < Cl < NO2), and the values of the
width of the tails of the localized states in the gap region were
calculated and found to be 0.5369, 0.6849, 0.7087, 1.3159 and
0.4199 eV for PATT-L1, PATT-L2, PATT-L3, PATT-L4 and PATT-L5,
respectively. The optical values of energy gap (Eg) for all derivatives
near the absorption edge were found to be direct allowed
transition.
PATT-L1
-0.4
-0.2
0.0
0.2
0.4
0.6
R
0.8
Hammett's substituent coefficient (σ
)
Fig. 7. The relation of Eg (eV) and Hammett’s substitution coefficient (r
R).
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