Dielectrical, conduction mechanism and thermal properties of rhodanine azodyes

Materials Science in Semiconductor Processing 19 (2014) 150–162

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Dielectrical, conduction mechanism and thermal properties of rhodanine azodyes N.A. El-Ghamaz a, A.Z. El-Sonbati b,n, M.A. Diab b, A.A. El-Bindary b, M.K. Awad c, Sh.M. Morgan b,1 a b c

Physics Department, Faculty of Science, Damietta University, Damietta, Egypt Chemistry Department, Faculty of Science, Damietta University, Damietta, Egypt Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt

a r t i c l e in f o

abstract

Available online 4 January 2014

In this paper, we report on the differential scanning calorimetry analysis (DSC) and thermogravimetric analysis (TGA) performed for 5-(40 -derivatives phenylazo)-2-thioxothiazolidin-4-one (HLn) (n ¼ 1, R ¼ OCH3; n ¼ 2, R ¼ CH3; n ¼ 3, R¼ H; and n ¼4, R ¼ NO2) in the temperature range 46–800 1C. The values of the thermal activation energies of decomposition of HL1, HL3 and HL4 are found in the range 59.10–299.72 kJ/mol. The molecular and electronic structures of the investigated compounds (HLn) were also studied using quantum chemical calculations. The alternating current conductivity (sac) and dielectrical properties of HLn were investigated in the frequency range 0.1–100 kHz and temperature range 303–500 K. The temperature and frequency dependence of the real and the imaginary dielectrical constants are studied. The values of the thermal activation energy for derivatives under investigation were calculated at different frequencies. The values of thermal activation energies of electrical conductivity ΔE1 and ΔE2 for all ligands decrease with increasing the test frequency. The activation energies, ΔE1 and ΔE2, increase according to the following order p-(NO2 4H 4CH3 4 OCH3). This is in accordance with that expected from Hammett's substituent coefficients (sR). The conductivities are found to be dependent on the structure of the compounds. The values of sac are related to the frequency as sac α ωS where the behavior of the exponent S determines the operating conduction mechanism. The correlated barrier hopping (CBH) is the dominant conduction mechanism for HLn. The values of maximum barrier height (Wm) were calculated. & 2013 Elsevier Ltd. All rights reserved.

Keywords: Rhodanine azodyes TGA and DSC Ac conductivity Dielectrical properties Hammett's substituent coefficients (sR)

1. Introduction Rhodanine (2-thioxothiazolidin-4-one) derivatives are a good series of ligands capable of binding metal ions leading to metal complexes with increasing properties. Efforts have been made to carry out detailed studies to synthesize and elucidating the chemical structural and electronic properties of novel families of complexes with

n

Corresponding author. Tel.: þ201060081581; fax: þ 20572403867. E-mail address: [email protected] (A.Z. El-Sonbati). 1 Abstracted from her Ph.D.

1369-8001/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2013.12.005

rhodanine derivatives as a novel chelating bidentate azodyes models [1–7]. Chemical properties of rhodanine and its derivatives are of interest due to coordination capacity and their use as metal extracting agents and as analytical reagents [8,9]. Azo compounds based on rhodanine were synthesized as potential medical preparations [10–13]. The optical absorption properties of azodye rhodanine derivatives thin films have been studied by El-Ghamaz et al. [14]. They found that the values of the energy band gap, Eg for derivatives were in the range 1.77–2.29 eV depending on the nature of the substituent. They also reported that, the tendesity of Eg to increase its according to the following order p-(OCH3 oCH3 oHoCloNO2) as

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expected from Hammett's substituent coefficients (sR). The optical values of the energy band gap (Eg) for all derivatives near the absorption edge were found to be direct allowed transition. Azodye rhodanine derivatives have been studied the antimicrobial activity and compared with the standard antibacterial and antifungal drugs [15]. The Proton-ligand dissociation constants of the rhodanine azodye compounds and metal–ligand stability constants have been determined [16]. Many authors studied the light-to-electricity conversion efficiency for rhodanine derivative [17]. It is found that the value of conversion efficiency (η) is in the range 2.86–6.27%. A few triple rhodanine indoline derivatives showed comparable conversion efficiency [17a]. A double rhodanine indoline dye has been reported to show a highest solar-to-electricity conversion efficiency (η¼ 9.0%) on titanium oxide [17b]. The DC electrical conductivity of 5-(4-R-phenylazo)-3phenyl-2-thioxothiazolidin-4-one and their complexes 2þ with Co2 þ , Cu2 þ , and UO2 have been studied [18]. It is found that the electrical conductivity is thermally activated depending on the nature of the complexes. To the best of our knowledge, there are no data concerning the thermal decomposition, ac conductivity and dielectrical properties in the available literature for 5-(40 -derivatives phenylazo)-2-thioxothiazolidin-4-one (HLn)

151

(Fig. 1). Because of this reason, the aim of the present work is to study the thermal decomposition, ac conductivity, dielectrical properties and conduction mechanism of HLn, as well as the effect of substituents on these properties. 2. Experimental details 2.1. Synthesis of 5-(40 -derivatives phenylazo)-2thioxothiazolidin-4-one (HLn) The standard chemical aniline (99.5%), 4-derivatives anilines (alkyl: OCH3 (99.0%), CH3 (96.0%), NO2 (97.0%) and 2-thioxo-4-thiazolidinone (98.0%); Aldrich chemical Co.) were used without any further purification. The experimental technique has been described previously [19]. 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. The resulting mixture stirred and cooled to 0 1C and a solution of 0.01 mol sodium nitrite in 20 ml of water was added dropwise. The formed diazonium chloride was consecutively coupled with an alkaline solution of 0.01 mol 2-thioxo-4-thiazolidinone, in 10 ml of pyridine. The colored precipitate, which was formed immediately, was filtered through sintered glass crucible, washed several times with water and ether then dried in a vacuum desiccator over P2O5. The ligands were

Fig. 1. The structures of 5-(40 -derivatives phenylazo)-2-thioxothiazolidin-4-one (HLn).

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Table 1 Analytical data of azodye rhodanine derivatives (HLn). Compound Empirical formula

HL1 HL2 HL3 HL4

C10H9N3O2S2 Red C10H9N3OS2 Dark Orange C9H7N3OS2 Pale Yellow C9H6N4O3S2 Dark Yellow

Yield %

M. P. 1C Calc. (Exp.)%

37.45

221

47.81

231

42.19

237

66.09

245

C

H

N

44.93 (44.82) 47.79 (47.88) 45.55 (45.68) 38.29 (38.42)

3.39 (3.25) 3.61 (3.76) 2.97 (2.80) 2.14 (2.25)

15.72 (15.85) 16.72 (16.61) 17.71 (17.85) 19.85 (19.98)

also characterized by elemental analysis (Table 1), 1H NMR and IR spectroscopy [1,2]. 2.2. Characterization Elemental microanalyses of the separated solid chelates for C, H, and N were performed in the Microanalytical Center, Cairo University, Egypt. Melting or decomposition points were carried out on a melting point apparatus. The infrared spectra were recorded as KBr discs using a Perkin-Elmer 1340 spectrophotometer. 1H NMR spectra were obtained on a JEOL FX 9000 Fourier transform spectrometer with deuterated dimethylsulfoxide (DMSO-d6) as a solvent and TMS as an internal reference. Ultraviolet–Visible (UV–vis) spectra of the compounds were recorded in nuzol solution using a Unicom SP 8800 spectrophotometer. X-ray diffraction analysis of the powder HL2 was performed at room temperature by a Philips X-ray diffractometer equipped with utilized monochromatic Cu Kα radiation (λ ¼1.5418 Å). The X-ray tube voltage and current were 40 kV and 30 mA, respectively. Thermal properties were investigated using Shimadzu Thermal Analyzer with a scan rate 10 1C/min in air atmosphere in the temperature range from 50 to 800 1C. The molecular structures of the investigated compounds were optimized initially with PM3 semiempirical method so as to speed up the calculations. The resulting optimized structures were fully re-optimized using an initio Hartree–Fock (HF) [20] with 6–31 G basis set. The molecules were built with the GaussView 3.09 and optimized using Gaussian 03Wprogram [21]. The corresponding geometries were optimized without any geometry constraints for full geometry optimizations. Frequency calculation was executed successfully, and no imaginary frequency was found, indicating minimal energy structures. Quantum chemical parameters such as the highest occupied molecular orbital energy (EHOMO), the lowest unoccupied molecular orbital energy (ELUMO), energy gap (ΔE), dipole moment (μ) and the total energy for the investigated molecules were calculated. Ac conductivity measurements are performed on the samples in the form of pellets of thickness 0.4–0.8 mm and compressed at a pressure of 12 t/cm2. The surface of each sample was covered by a layer of silver, then was held between two copper electrodes and inserted vertically into a cylindrical electric furnace. The ac conductivity

measurements of samples are measured as a function of temperature range from 303 to 500 K and frequency range 0.1–100 kHz using Stanford research systems Model SR 720 LCR METER. The temperature is measured by NiCr– NiAl thermocouple. The range of temperature for electrical measurements is chosen according to TGA and DSC measurements. 3. Results and discussion 3.1. Structure of the ligands The molecular structures of these ligands can exist in three tautomeric forms (as shown in Fig. 1). The infrared spectra of HLn give interesting results and conclusions. The ligands gives two bands at 3200–3040 cm  1 due to asymmetric and symmetric stretching vibrations of N–H group and intramolecular hydrogen bonding NH…O systems (Fig. 1-D), respectively. When the OH group (Fig. 1-C) is involved in intramolecular hydrogen bond, the O…N and N…O bond distances are the same. But, if such mechanism is happened in case of intermolecular hydrogen bond, the O…O and O…N bond distances differ. The broad absorption band located at  3400 cm  1 is assigned to νOH. The low frequency bands indicate that the hydroxy hydrogen atom is involved in keto 3 enol (A3 B) tautomerism through hydrogen bonding (Fig. 1-C). Bellamy [22] made detailed studies on some carbonyl compounds containing –NH– group. The ΔνNH values were used to study the phenomena of association. On the other hand, the OH group (Fig. 1-B) exhibits more than one absorption band. The two bands located at 1330 and 1370 cm  1 are assigned to in-plane deformation and that at 1130 cm  1 is due νC-OH. Similar to the other investigated compounds, the different modes of vibrations of C–H and C–C band are identified by the presence of characteristic bands in the low frequency side of the spectrum in 600–200 cm  1. The infrared spectra of ligands shows medium broad band located at  3460 cm  1 due the stretching vibration of some sort of hydrogen bonding. El-Sonbati et al. [1,2]

3600 3000

Intensity (A.U)

152

2400 1800 1200 600 0 10

20

30

40

50

60

70

2θ (Degree) Fig. 2. X-ray diffraction pattern for HL2 Powder form.

80

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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 –N═N– system and hydrogen atom of the hydroxy hydrogen atom (Fig. 1-C). This is evident by

153

the presence of a broad band centered at 3460 cm  1. (2) Hydrogen bonding of the OH.N type between the hydroxy hydrogen atom and the N-ph group (Fig. 1-C). (3) Intermolecular hydrogen bonding is possible forming cyclic dimer through NH.O¼C (G), OH.N═N (F) or OH. OH (E) (Fig. 1).

Fig. 3. The calculated molecular structures of the investigated compounds (HLn).

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The presence of broad band located at  3200 cm  1 is strong indication by νNH (Fig. 1-D). In general, the low frequency of such region from its normal position is, again due to hydrogen bond property gathered with keto 3 enol tautomerism. In general, hydrogen bonding involving both NH and OH groups are proton donors and both –N and –O atoms are proton acceptors. It is of interest since much multiplicity of proton donor and acceptor sites are prevalent in biological systems. Both intra- and intermolecular OH.N and NH.O may form leading to a number of structures in simultaneous equilibrium. Again the three bands located at 1380, 1340 and 1310 cm  1 identified as δOH gathered with the two bands at 1240 cm  1 assigned as νC-O are strong indication to keto 3 enol equilibrium. The presence of a medium band at 1605 cm  1 assigned to νC═N illustrates the tracing of keto structure (Fig. 1-D). The 1H NMR spectra showed that, signal for CH (  4.42 ppm), favoring formation of an intramolecular hydrogen bond with the –N¼N– (azodye) group. Electron-withdrawing substituents reduce the intramolecular hydrogen bond as indicated by the marked shift of the hydroxyl signal to higher field in the p-NO2 compound. 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 11.36– 11.88 ppm are not affected by dilution. The previous two

protons disappear in the presence of D2O. Absence of –CH proton signal of the ligand moiety indicated the existence of the ligand in the azo-enol form. In the meantime, the 1 H NMR of the HL2/HL1 exhibits signals at δ(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. On the basis of all the above spectral data, an internally hydrogen bond azo-enol structure has been proposed for the ligand (Fig. 1). The electronic absorption spectra of the ligands exhibit mainly five bands (A–D, F). The band A located at 26,360– 26,280 cm  1 can be assigned to the n–πn transition of the CS group. The band B within 30,560–30,260 cm  1; can be assigned to n–πn transition within the CO group. The band C within 32,980–33,180 cm  1 could be assigned to the H-bonding and association. The band D located at 40, 250–3990 cm  1 could be assigned to Ph–Phn, π–πn corresponding to the aromatic system. The band F located at Table 2 The calculated quantum chemical parameters for the investigated compounds (HLn). Compound HOMO (a.u) LUMO (a.u) ΔE (a.u) m (D) T.E (a.u) HL1 HL2 HL3 HL4

 0.327  0.337  0.348  0.367

0.048 0.047 0.044  0.004

0.375 0.384 0.432 0.363

5.358  1491.580 4.554  1416.775 3.950  1377.752 3.712  1581.099

Fig. 4. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the investigated compounds (HLn).

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29,620–29,350 cm  1. These latter bands can be assigned to phenyl rings overlapped by composite broad π–πn of azo structure. The band B transition disappears with the simultaneous appearance of new bands, being attributed to π–πn (C ¼C) as sequences of enolization. Moreover, the band A transition shifts slightly to lower energy and remains almost constant. The X-ray diffraction (XRD) pattern of the as-synthesized HL2 powder is shown in Fig. 2. Many peaks were observed which indicate that the polycrystalline nature of the as-synthesized HL2 ligand.

60

200

40

300

20

400

0

500

-20 TGA

100

Weight loss (%)

DSC

HL3

80

100

300

40

400

20

500

0

3.2. Thermal analyses Thermal analysis plays an important role in determining thermal stability of the organic material [29,30]. The

600

-20 TGA

100

DSC

HL4

80

Weight loss (%)

600 0

200

60

Heat Flow Endo Down (mW)

100

Geometrical structures and electronic properties of the investigated compound and their protonated forms were calculated by optimizing their bond lengths, bond angles and dihedral angles. The calculated molecular structures with the optimized bond lengths are shown in Fig. 3. According to the frontier molecular orbital theory, FMO, the chemical reactivity is a function of interaction between HOMO and LUMO levels of the reacting species [23]. The EHOMO often associated with the electron donating ability of the molecule to donate electrons to appropriated acceptor molecules with low-energy, empty molecular orbital. Similarly, ELUMO indicates the ability of the molecule to accept electrons. The lower value of ELUMO indicates the high ability of the molecule is to accept electrons [24,25]. While, the higher is the value of EHOMO of the inhibitor, the easer is its offering electrons. The HOMO and LUMO are shown in Fig. 4. The HOMO–LUMO energy gap, ΔE, which is an important stability index, is applied to develop theoretical models for explaining the structure and conformation barriers in many molecular systems. The smaller is the value of ΔE, the more is the reactivity of the compound has [24,26,27]. The dipole moment, m, the first derivative of the energy with respect to an applied electric field, was used to discuss and rationalize the structure [28]. The calculated quantum chemical parameters are collected in Table 2.

-120 0

60

120

40

240

20

360

0

480

Heat Flow Endo Down (mW)

Weight loss (%)

80

0

DSC

HL1

Heat Flow Endo Down (mW)

TGA

100

155

600

-20 0

100

200

300

400

500

600

700

800

Temperature (°C) Fig. 5. TGA and DSC thermographs of azodye rhodanine derivatives.

Fig. 6. The relation between lnK and 1/T for HL1, HL3 and HL4.

Table 3 The thermal analyses data for HL1, HL3 and HL4. Compound

HL1 HL3 HL4

First stage

Second stage

Third stage

Temperature (1C)

Weight loss (%)

Temperature (1C)

Weight loss (%)

Temperature (1C)

Weight loss (%)

200 145 112

61.9 59.8 5.4

320 400 225

38.1 39.9 33.6

– – 284

– – 58.7

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TGA and DSC curves for HLn (where n ¼1, 3 and 4) are shown in Fig. 5. It is clear that the change of substituent affects the thermal properties of HLn. The TGA curve for HL1 and HL3 shows that there are two steps of the loss of masses, while for HL4 there are three steps. The temperature intervals and the percentage of loss of masses are listed in Table 3. The DSC data for HLn (where n¼1, 3 and 4) show many endothermic and exothermic peaks (Fig. 5). These peaks can be explained in terms of TGA curves. The DSC curve for HL1, HL3 and HL4 show only one exothermic peak at 220, 190 and 245 1C, respectively, which correspond to the first loss of masses step in TGA curve. The temperature corresponding to the exothermic peaks for HL1, HL3 and HL4 can be considered as the phase transition temperature. These results are in agreements with the melting temperature which are listed in Table 1. The DSC charts for HL3 and HL4 show only one endothermic peak at 180 and 120 1C, respectively. These endothermic peaks can be attributed to the loss of residual solvent trapped in the ligands matrices. More information about the decomposition process such as the rate constant of the thermal degradation, K, in the initial stages of decomposition and the thermal

activation energies of degradation, Ea, can be extracted from the TGA curves. The rate constant of the thermal decomposition is given by the Arrhenius Eq. (1) [31]: ln K ¼ ln A  Ea =RT;

ð1Þ

where A is a constant, R is the gas constant (¼8.314 J K/mol) and T is the absolute temperature. The values of the thermal activation energies of degradation, Ea, for HL1, HL3 and HL4 are calculated from the slope of the straight line obtained from the plot lnK versus 1/T as shown in Fig. 6. The values Ea for HL1, HL3 and HL4 are found in the range 59.10–299.72 kJ/mol. The ligand HL4 is more thermally stable than HL1 and HL3. This can be attributed to the fact that the effective charge experienced by the d-electrons increases due to the electron withdrawing p-substituent NO2 while it decreases by the electron donating character of OCH3. 3.3. Ac measurements analyses The dielectrical permittivity ε can be considered as a complex quantity and is given by eq. (2) below: ε ¼ εr iεi ;

Fig. 7. Temperature and frequencies dependence of the real dielectrical constant, εr, for (a) HL1, (b) HL2, (c) HL3 and (d) HL4.

ð2Þ

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where εr is the real dielectrical constant and εi is the imaginary dielectrical constant. The real dielectrical constant, εr, can be calculated from the measured capacitance in parallel mode by Eq. (3) [32,33] below: εr ¼

Cpd ; εo Α

ð3Þ

where Cp is the capacitance of the samples measured in parallel mode, εo is the permittivity of free space, A is the cross sectional area and d is the thickness of the samples. The imaginary part of the dielectrical constant, εi, can be calculated from the calculated values of εr and the measured values of loss tangent (tan δ) using Eq. (4) [32,33] below: εi ¼ εr tan δ:

ð4Þ

the value of εr and εi are calculated in the frequency range 0.1–100 kHz and temperature range 303–500 K. The temperature dependence of εr and εi for 5-(40 -derivatives phenylazo)-2-thioxothiazolidin-4-one (HLn) at different frequencies is shown in Figs. 7 and 8. It is clear from these figures that both of εr and εi, in general, decrease

157

with increasing frequency and increases with increasing temperature for all azodye rhodanine derivatives (HLn). This behavior is in agreement with the expected behavior of the organic and inorganic materials [32,34]. The rate of change of εr depends on the nature of the substituents. At low temperature region (303–350 K) the values of εr for HLn are almost independent of temperature (Fig. 7), while at higher temperature region (350–500 K) a stronger dependence on temperature is observed for all HLn derivatives. Fig. 8 shows that the behavior of εi for the ligand HL1 is weakly depend on temperature in the temperature range 303–350 K, while εi for the ligands HL2–HL4, is sensitive to temperature in the range 400–450 K. According to Giuntini et al. [35], the values of εi at a certain temperature are given by Eq. (5) below: εi ¼ ðεo  ε1 Þ2π 2 Nðne2 =εo Þ3 kB Tωm ðτo Þm ðW m Þ  4 ;

ð5Þ

where εo, ε1 are the static and optical dielectrical constants of the material, respectively, N is the concentration of the density of states, n is the number of electrons that hop and Wm is the maximum barrier height. Eq. (5) can be

Fig. 8. Temperature and frequencies dependence of the imaginary dielectrical constant, εi, for (a) HL1, (b) HL2, (c) HL3 and (d) HL4.

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reduced to the form as: εi ¼ Β ωm ;

ð6Þ

The power m can be calculated from the slope of the obtained straight lines of Figs. 9–12 for different temperatures then m is plotted as a function of temperature, as shown in the inset of Figs. 9–12. It is clear from the figures that m decreases with the increase of the temperature. The obtained behavior of m is consistent with the results obtained for many organic compounds [36,37]. The ac conductivity, sac, for HLn derivatives can be calculated from Eq. (7) [32] below: ð7Þ

where so is the pre-exponential constant, ΔE is the thermal activation energy and kB is Boltzmann's constant. The relation between lnsac as a function of (1/T) for 5-(40 derivatives phenylazo)-2-thioxothiazolidin-4-one (HLn) are show in Fig. 14(a–d) in the temperature and frequency 308 333 363 373 393 403 413 423 433

0.8

logεi

0.6

0.4

Fig. 10. Plot of logεi versus logω at different temperatures for HL2 (Inset: The relation between m and T).

2.5

313 333 343 353 363 373 383 393 403 413 423

2.0

1.5 logεi

where ω is the angular frequency (ω¼2πf (where f is the frequency)). The calculated values of sac of the HLn under consideration as a function of temperature are investigated in the temperature range 303–500 K and in frequency range 0.1–100 kHz and are show in Fig. 13. The calculated values of sac are found to increase with increasing temperatures and frequencies. The sac in the temperature range 310–400 K for HL4 has the higher value than that of other azodye rhodanine derivatives for all test frequencies (about twice the value at 100 kHz and about 10 times at 10 kHz). This behavior can be attributed to the strong withdrawing nature of the NO2 as a function group [38,39]. It is important to note that the existence of a methyl and/or methoxy group enhances the electron density on the coordination sites and simultaneously decreases the values of ac conductivity. It is found that the change of substituents affects the values of ac conductivity for HLn. This behavior can be attributed to the resonance effects of the various substituents of azodye rhodanine derivatives studied. The ac electrical conductivity, sac, as a function of temperature is given by Eq. (8) [32,40,41] below:    ΔE sac ¼ so exp ; ð8Þ kB Τ

1.0 0.5

0.0 -0.5 3

4

5

6

logω

Fig. 11. Plot of logεi versus logω at different temperatures for HL3 (Inset: The relation between m and T).

313 323 333 363 403 413 423 433 443 453

2.0

1.5 logεi

sac ¼ ω εo εi

1.0

0.5

0.0 3

4

5

6

logω

Fig. 12. Plot of logεi versus logω at different temperatures for HL4 (Inset: The relation between m and T)

0.2

0.0 2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

logω

Fig. 9. Plot of logεi versus logω at different temperatures for HL1 (Inset: The relation between m and T).

ranges under consideration. It is clear from this figure that HLn derivatives show a semiconductor behavior each of them is characterized by two thermal activation energies (ΔE1 and ΔE2) depending on the temperature range. In Fig. 14(a), lnsac is weakly dependent on temperature for HL1, while the ligands HL2–HL4 show a strong dependence on temperature at higher temperatures.

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The values of the thermal activation energy, ΔE1 and ΔE2, for HLn at different frequencies are determined in the low and high temperature regions, respectively, and listed in Table 4. It is clear that there is a variation in the values of ΔE1 and ΔE2 according to the effect of the substituents [42]. This behavior has been observed from the DC electrical conductivity of some rhodanine azodye compounds and their complexes [18]. The effect of the substituents on the thermal activation energies ΔE1 and ΔE2 can be confirmed in terms of Hammett's substituent coefficients (sR) are shown in Figs. 15 and 16, respectively. It is clear that the values of ΔE1 and ΔE2 increase with increasing sR. The increase of ΔE1 and ΔE2, with respect of the substituent is in the following order p-(NO2 4H4CH3 4OCH3) [14,18,38]. This can be attributed to the fact that the effective charge experienced by the d-electrons increases due to the electron withdrawing p-substituent NO2 while it decreases by the electron donating character of OCH3 and CH3. This is in accordance with that expected from Hammett's substituent coefficients (sR) as shown in Figs. 15 and 16. The value of sac at certain temperature is related to the test frequency by Eq. (9) [32] below: sac ¼ Αn ωs ;

ð9Þ

159

where An is a constant depending on temperature, ω is the angular frequency and S is an exponent in which its value and it is behavior with temperature determines the dominant conduction mechanism. In order to determine the conduction mechanism for a material, theoretical model has been reported to correlate the conduction mechanism with the behavior of the frequency exponent S [43]. The value of S is the slope of the straight line obtained from the relation between logsac and logω. The values of S as a function of temperature for HLn are shown in Fig. 17. The values of S are found to decrease with increasing temperature [44–47]. The general behavior of S for all azodye rhodanine derivatives appear to be consistent with the hopping process of charge carriers between localized sites and suggests that the correlated barrier hopping (CBH) model is the best suitable model for conduction mechanism [44,45]. In CBH model, the electrons in charged defect states, hop over the Coulomb barrier whose height, W, is given by Eq. (10) [32,47] below: W ¼ Wm þ

8e2 ; εεo R

ð10Þ

where Wm is the maximum barrier height (the energy required to move the electron from a site to infinity), R is the distance between hoping states.

Fig. 13. The relation of conductivity (sac) as a function of temperature for (a) HL1, (b) HL2, (c) HL3 and (d) HL4 at different frequencies.

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Fig. 14. Dependence of lnsac versus 1/T for (a) HL1, (b) HL2, (c) HL3 and (d) HL4 at different frequencies.

0.08

Table 4 The values of the activation energies for ac electrical conductivity of azodye rhodanine derivatives at different frequencies. F HL1 (kHz) ΔΕ1 (eV)

ΔΕ2 (eV)

ΔΕ1 (eV)

ΔΕ2 (eV)

ΔΕ1 (eV)

ΔΕ2 (eV)

ΔΕ1 (eV)

ΔΕ2 (eV)

0.1 1 10 100

0.320 0.239 0.082 0.028

0.037 0.029 0.024 0.011

0.959 0.811 0.516 0.201

0.046 0.040 0.034 0.013

0.988 0.817 0.525 0.206

0.062 0.055 0.045 0.018

2.251 1.725 1.247 0.661

HL3

HL4

0.06

The frequency exponent, S, in CBH model is given by Eq. (11) [32,47] below:

ΔE1 (eV)

0.038 0.022 0.018 0.005

HL2

0.1 kHz 1 kHz 10 kHz 100 kHz

HL3 HL1

0.04

0.02

0.00 -0.4

6kB T
 S ¼ 1  wm kB T ln 1=ωτo

HL4

-0.2

0.0

0.2

0.4

0.6

0.8

ð11Þ Fig. 15. The relation between ΔE1 (eV) and Hammett's substituent coefficients (sR).

First approximation to Eq. (11) gives the expression S ¼ 1

6kB T Wm

ð12Þ

The maximum barrier height, Wm, as a function of temperature for HLn is plotted in Fig. 18. The value of

maximum barrier height, Wm, decreases with increasing the temperature for HL1–HL3, while it increases with increasing the temperature for HL4 up to 370 K then starts

N.A. El-Ghamaz et al. / Materials Science in Semiconductor Processing 19 (2014) 150–162

2.4

4. Conclusion

HL4

0.1 kHz 1 kHz 10 kHz 100 kHz

ΔE2 (eV)

1.6

HL3 0.8

HL1

0.0

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Fig. 16. The relation between ΔE2 (eV) and Hammett's substituent coefficients (sR).

Frequency exponent (s)

1.0

0.8

0.6

HL1 HL2

In summary, 5-(40 -derivatives phenylazo)-2-thioxothiazolidin-4-one (HLn) ligands were synthesized by azo coupling reaction. The thermal properties of the ligands (HLn) are investigated using TGA and DSC. For HL1 and HL3 there are two steps of the loss of masses, while for HL4 there are three steps. The thermal activation energies of decomposition of HL1, HL3 and HL4 are found in the range 59.10–299.72 kJ/mol. The real and imaginary parts of the dielectrical constant εi and εr are found to increase with increasing the temperature and decreases with increasing frequency for all HLn. The calculated values of sac are found to increase with increasing temperatures and frequencies for all HLn. The effect of methoxy and methyl group is to decrease the ΔE1 and ΔE2, while the effect of nitro group is to increase ΔE1 and ΔE2. This can be attributed to the fact that the effective charge experienced by the d-electrons increases due to the electron withdrawing p-substituent NO2 while it decreases by the electron donating character of OCH3 and CH3. It is found that the change of substituents affects the thermal properties, dielectrical properties, ac conductivity and the values of the thermal activation energy of HLn. The response of the exponent, S, with the temperature shows different behavior depending on the substituent. The values of maximum barrier height (Wm) are calculated. The correlated barrier hopping (CBH) is found to be the operating conduction mechanism for HLn.

HL3 HL4

0.4 280

References

320

360

400

440

480

T (K)

Fig. 17. Plot of the frequency exponent, S, versus T for azodye rhodanine derivatives (HLn).

HL1 HL2

2.0

HL3 HL4

1.5

Wm (eV)

161

1.0

0.5

0.0 300

330

360

390

420

450

480

T (K)

Fig. 18. The maximum barrier height, Wm, as a function of temperature for azodye rhodanine derivatives (HLn) according to CBH conduction model.

to decrease with increasing the temperature. Moreover, the optioned values of Wm for HL1 are greater than that of HL2–HL4.

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