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Oct 28, 2015 - G. Sumathi and T. Sreenivasulu Reddy*. *Department of Chemistry ... The interference from Mo (VI), Zr (IV), Th (IV) and Ti. (IV) was eliminated ...

JCBPS; Section A; February 2016 – April 2016, Vol. 6, No. 2; 376-386.

E- ISSN: 2249 –1929

Journal of Chemical, Biological and Physical Sciences An International Peer Review E-3 Journal of Sciences Available online at www.jcbsc.org Section A: Chemical Sciences

CODEN (USA): JCBPAT

Research Article

Electrical conductivity, optical properties and antioxidant activity of the hybrid [2, 3-(CH3)2C6H3NH3]6P6O18.2H2O Ramzi Fezai1 *, Ali Mezni2 and Mohamed Rzaigui1 1

Laboratoire de Chimie des Matériaux LR13ES08, Faculté des Sciences de Bizerte7021 Zarzouna, Université de Carthage, Tunisia. 2

Département de science de la vie, Faculté des Sciences de Bizerte 7021 Zarzouna, Université de Carthage Tunisia. Received: 20 January 2016; Revised: 25 January 2016; Accepted: 03 February 2016

Abstract: The present study reports investigation of Layers structure of the hybrid compound [2,3-(CH3)2C6H3NH3]6P6O18.2H2O. The impedance spectroscopy has been used to characterize this material in the temperature range 373-413 K. Electrical properties and dielectric measurements dependence on both temperature and frequency of this compound have been reported. The direct current conductivity process is thermally activated and the spectra follow the Arrhenius law with two activation energy 1.34 eV for T < 348 K and 0.72 eV for T > 348 K. The frequency dependence of AC conductivity follows Jonscher's universal law. The luminescent properties of this material have been carried out at room temperature based on UV absorption spectroscopy spectrum. This compound was also screened for its antioxidant activity using 1,1-diphenyl-2picrylhydrazyl (DPPH), hydroxyl radical and reducing power methods and with ascorbic acid as control. Keywords: Hybrid material; Dielectric properties; Optical properties; Antioxidant activity.

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INTRODUCTION Chemistry of the solid state is a vast area of research that continues to develop looking for new materials with valorization on the applied or fundamental level. Study of the electrical properties of these compounds is an interesting field of research because they combine layered structures suitable for mixed ionic-electronic conductivity 1-3. In this work, we report the study of electrical properties and antioxidant activity of [2,3(CH3)2C6H3NH3]6P6O18.2H2O which is characterized by a layered structure 4. Antioxidants are used to prevent the formation of Reactive oxygen species (ROS) as HO•, H2O2, NO, which are generated in living organisms during metabolism 5-8. Excessive amounts of ROS are harmful because they can initiate biomolecular oxidations which lead to cell injury and death, and create oxidative stress which results in numerous diseases and disorders such as cancer, diabetes, mycocardial infarction, stroke, etc 9-11. MATERIAL AND METHODS Synthesis: Crystals of the title compound were obtained according to the following chemical reaction: 6[2,3-(CH3)2C6H3NH2]+H6P6O18+2H2O

[2,3-(CH3)2C6H3NH3]6P6O18.2H2O

Cyclohexaphposphoric acid (H6P6O18) was produced by passing through a cation-exchange resin (Amberlite IR 120) the lithium salt Li6P6O18.6H2O, prepared from LiH2PO4 12. The mixture, hexaphosphoric acid and an ethalonic solution of 2,3-dimethylaniline, was stirred and subjected at room temperature. After three days, colorless monocrystals are obtained with suitable sizes for DRX investigation. Materials and measurements: The electrical conductivity measurements were performed as a function of temperature (373 K to 413 K) and frequency (5Hz to 13MHz) with 5 °C steps, using a Hewlett-Packard 4192A analyzer. A two-electrode configuration was used. The finely grain sample was pressed into pellet of 13 mm diameter and 1.13 mm thickness using a hydraulic press. Both pellet surfaces were coated with silver pastes to act as electrodes and platinum wires attached to the electrodes were used as current collectors. The UV absorption spectrum was recorded at room temperature with a Perkin Elmer Lambda 11 UV/Vis spectrophotometer in the range of 200-400 nm. Solid-state emission spectrum was recorded for the solid sample at room temperature. The sample was loaded into a cell (1 cm diameter) which was then fixed on a bracket at room temperature with Perkin-Elmer LS55 spectrofluorometer (λex = 280 nm). The slit widths used for the excitation and emission measurements were 2.5 and 11.5 nm respectively. The scan speed was 1200 nm/min. RESULTS AND DISCUSSION Electrical properties: Impedance analysis: Complex impedance spectra 𝑍𝑍 ′′ versus 𝑍𝑍 ′ [𝑍𝑍 ′ ′ = f (𝑍𝑍 ′ )] recorded at various temperatures are presented in Fig. 1. As temperature increases, the radius of the arc corresponding to the bulk resistance of the sample decreases indicating an activated thermal conduction mechanism. So, the complex impedance spectrum shows that the title compound follows the Cole–Cole law 13. The sample of [2,3-(CH3)2C6H3NH3]6P6O18.2H2O could be fitted with series network (RP1//CPE1)–(RP2//CPE2) as shown in 377

J. Chem. Bio. Phy. Sci. Sec. A, February 2016 – April 2016; Vol.6 No.2; 376-386.

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Fig. 1. Where the RP1 (Rg), RP2 (Rgb) represent the bulk resistance and the fractal capacitance of the interface CPE1, CPE2 represent the effect of electrode (capacity of the fractal interface CPE).

Fig. 1: Complex impedance diagrams (- Z'' vs Z') for [2,3-(CH3)2C6H3NH3]6 P6O18. 2H2O at various temperatures. Fig. 2 (a) and (b) shows the variation of real (𝑍𝑍 ′ ) and imaginary (𝑍𝑍 ′′ ) parts of impedance with frequency at various temperatures. Indeed, the magnitude of 𝑍𝑍 ′ decreases with the increase in both frequency and temperature indicating an increase in AC conductivity of the material. All the curves merge in the highfrequency region (>106 Hz), and then 𝑍𝑍 ′ becomes independent of frequency. Moreover, the variation of 𝑍𝑍 ′′ with frequency at various temperatures reveals that the 𝑍𝑍 ′′ values reach two maximums (𝑍𝑍 ′′ max), which shifts to higher frequencies with increasing temperature. Such behavior indicates the presence of two relaxation processes in the system. One related to the grain and the second is due to the presence of grain boundaries.

(a)

(b)

Fig. 2: Plots of the real (Z′) and imaginary parts (Z′′) of impedance vs. log(f) of [p(F)C6H4NH3]12[P6O18]2.5H2O at various temperatures.

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Electrical conductivity: The frequency–temperature dependence of AC conductivity of the sample is shown in Fig. 3. Besides, the electrical response of the low conductivity materials is usually characterized by the well known universal dynamic response 14: σ(ω) = σ(0) + Aωn (*)

Fig. 3: The frequency dependence of the AC conductivity σAC at various temperatures in the structure of [2,3-( CH 3 ) 2 C 6 H 3 NH 3 ] 6 P 6 O 1 8 .2H 2 O. Where σ(0) correspond to the frequency independent DC (or low-frequency) conductivity, ω the angular frequency of measurement, A is fitting parameter, which is, principally, temperature dependent. The exponent n is related to the interaction of the transferring charge entities with the matrix. We note that the frequency-dependent conductivity in the present work shows two limited regions containing each one two distinct regimes: plateau and dispersion 15. So the (*) equation can be expressed as the following relation: σ(ω) = σ(0) + Aωn1 + σ' (0) + Bωn2 Fig. 4 shows the variation of ln(σDCT) with the inverse of absolute temperature (103/T). The bulk e conductivity of the material was calculated from Rg by the relation: σ = , (e/S represents the S Rg

geometrical ratio sample) and was evaluated from the complex impedance plots of the sample at selected temperatures. This plot is explained by Arrhenius relation: σ=

σ0 T

ex p �-

Ea � kT

Where σ0 is the pre-exponential factor, K the Boltzmann’s constant and Ea is the thermal activation energy for the ion migration. Two regions are observed and separated by a discontinuity in the temperature range 348 ± (5) K. This discontinuity is an agreement with the dehydration observed in the DSC curves at 348 K 4. Two mechanisms of conductivity are obvious. The first one, corresponding to the starting hydrated compound, occurs before 348 ± (5) K. The second mechanism, corresponding to the 379

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anhydrous phase obtained after dehydration and before the decomposition of our product (470 K). The values of the activation energies determined in regions I and II are 1.34 and 0.72 eV, respectively. The conduction is explained by the thermally activated mechanism.

Fig.4: Variation of the ln(σT) versus 1000/T for [2,3-(CH3)2C6H3NH3]6P6O18.2H2O. Dielectric studies: The dielectric constant (or relative permittivity) ε of a dielectric solid, placed in an alternating electric field of angular frequency ω, is a complex quantity because the orientational polarization lags behind the polarizing electric field with the increase in frequency of the applied field. The complex relative permittivity or dielectric constant (ε*); ε*= ε' - j ε'' was calculated from: 𝜺𝜺′ = −

⍵𝑪𝑪𝟎𝟎

𝒁𝒁"

𝟐𝟐 (𝒁𝒁"𝟐𝟐 +𝒁𝒁′ )

; 𝜺𝜺" =

𝒁𝒁′

𝟐𝟐

⍵𝑪𝑪𝟎𝟎 (𝒁𝒁"𝟐𝟐 +𝒁𝒁′ )

Where C0 = ε0S/e is the capacitance, ε0 is the permittivity of the vacuum, ε' and ε'' are the real and imaginary parts of ε*, respectively. The study of the dielectric properties is an important source for valuable information about conduction processes 16. Fig. 5 (a) and (b) shows that ε' and ε'' decreases with increasing frequency for all temperatures. At low frequencies, as the temperature increases, ε' and ε'' shows a dispersive behavior. The dielectric profile plot is higher at low frequencies possibly because of the different types of polarization effects. These effects may be caused by one or more of the contribution polarization factors that is, i.e., electronic, atomic, ionic, interfacial factors.

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Fig.5: Variation of dielectric complex permittivity (ε' and ε'') for [2,3-( CH 3 ) 2 C 6 H 3 NH 3 ] 6 P 6 O 1 8 .2H 2 O at several temperatures Fig. 6 (a) and (b) shows that at low temperature, the variations of ε' and ε'' with temperature are almost constants. This is may be explained by the restricted reorientational motions of the cation which cannot orient themselves with respect to the direction of applied electric field. At high temperature an asymmetrical dielectric loss peak is observed. The dielectric anomaly appears when the jumping frequency of localized charge carries approximately becomes equal to that of the externally applied electric field. Further, as the frequency increases, the peak located around 400 K becomes less pronounced, this can be explained by the fact that beyond a certain frequency of external field the charge carriers cannot follow the alternating electric field 17.

(a)

(b)

Fig.6: Dielectric loss, ε' (a) & ε '' (b), as a function of temperature at different frequencies. The dielectric loss was plotted as log (ε'') vs. log (f) as it was reported in Fig. 7. From this figure, the obtained curves are straight lines at various temperatures with different slopes. The complex permittivity follows a fractional power law of frequency 18-20: ε* = A*ωm Where A* is a complex constant and -1 < m < 0. 381

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Fig.7: log(ε'') vs. log(f) for [2,3-(CH3)2C6H3NH3]6P6O18.2H2O at different temperatures. Optical properties: UV absorption spectroscopy: The UV–Visible spectrum of [2,3-(CH3)2C6H4NH3]6P6O18.2H2O, in a mixture aceton & methanol solution (Fig. 8a), exhibits strong broad band centered around 280 nm which can be attributed to the π–π* transition related to aromatic ring of the 2,3-dimethylanilinium cation. The gap energy determined from the optical diffuse reflectance spectrum (Fig. 8b) was 5.56 eV. This result indicates that the title compound is a semiconductor with wide band gap 21.

(a)

(b)

Fig. 8: UV-Visible absorption spectrum (a) and UV diffuse reflectance spectrum (b) of the studied material.

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Luminescent properties: The fluorescence emission in the solid state has a big importance in the field of optoelectronics, particularly in light emitting diodes and solid state lasers 22. The emission spectrum of the studied material at an excitation wavelength of 280 nm (Fig.9) shows a luminescent peak at 454 nm which corresponds to π – π* transition of the aromatic ring of the 2,3-dimethylammonium cation.

Fig. 9: Emission spectrum for the title compound In vitro antioxidant activity: In this study we showed the antioxidant capacity of the title compound (2,3-DMA) at various concentrations. This capacity was determined, in vitro, by 1,1-diphenyl2-picrylhydrazyl (DPPH), hydroxyl scavenging ability, and reducing power using ascorbic acid as control. DPPH free radical scavenging activity: The free radical scavenging activity of the title compound (2,3DMA) was assessed on the basis of the free radical scavenging effect of the stable 1,1-diphenyl-2picrylhydrazyl (DPPH) by modified method (Braca et al., 2002) 23. This hybrid phosphate was tested at 0.25, 0.5 and 1 mg/mL for preliminary study of the DPPH free radical scavenging activity. The results are depicted in Fig. 10. This latter showed potent activity when compared with ascorbic acid used as a standard; the percentage of scavenging DPPH radicals is around 80% in all concentrations and the IC50(2,3-DMA) = 0.523 mg/mL at 1 mg/mL, compared with the observed percentages for ascorbic acid with % inhibition around 90% and IC50(AA) = 0.447 mg/mL.

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Fig. 10: DPPH Radical Scavenging Activity. 2,3-DMA (Tested compound). AA (Ascorbic acid). Degradation of deoxyribose (Fenton’s reaction): This assay was determined with the method of deoxyribose degradation described by Halliwell and Gutteridge (1981) 24. The ability of 2,3-DMA to prevent the formation of hydroxyl radicals, results of decomposition of deoxyribose as the Fenton’s reaction, was studied at various concentration (0.25, 0.5and 1 mg/mL). The results was summarized in Fig. 11, all concentrations of this material scavenge HO• produced in Fe2+/H2O2 induced decomposition of deoxyribose in Fenton’s reaction. The HO• scavenging ability of 2,3-DMA compound is higher on the highest concentration (1 mg/mL), the percentage of inhibition was 88.67% ± 0.59 and the IC50 = 0.446 mg/mL compared to that of ascorbic acid at the same concentration (1 mg/mL) the percentage of scavenging was 96,16% ± 0.46 and IC50 = 0.435 mg/mL.

Fig. 11: HO• radical scavenging ability of 2, 3-DMA and AA (Ascorbic acid).

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Reducing property: The reducing power of the new compound 2,3-DMA was assayed according to the method of Pulido et al (2000) 25. We report in Fig. 12 the reducing property of 2,3-DMA at various concentrations. It was seen from this study that at the highest concentration (1 mg/mL), the synthesized compound has the high reducing power 51.22% ±0.35 with IC50 = 0.793 mg/mL, compared with that of the ascorbic acid at the same concentration (68.22% ±1.32, IC50 = 0.372 mg/mL).

Fig.12: Reducing power. 2, 3-DMA (Tested compound). AA (Ascorbic acid). CONCLUSION A detailed analysis of the arcs of complex impedance diagrams reveals the presence of a series of two parallel bulk resistances and constant phase elements (CPE) as an equivalent electrical circuit in different regions. Besides, the frequency dependent (AC) conductivity has been interpreted in terms of Jonscher’s law. Moreover, the temperature dependence of conductivity (DC) was analyzed using the Arrhenius approach. It increases with increasing temperature indicating a semiconductor behavior. The dielectric loss ε'' indicates that its level of increase with temperature is due to an increase in the polarization of the system with temperature. Electronic properties show an important Gap energy confirming the semiconductor behavior and the observed blue photoluminescence. In addition, this product shows significant antioxidant activity demonstrated by scavenging DPPH radicals, hydroxyl radical scavenging and reducing power compared to that of ascorbic acid. REFERENCES 1. F. Huguenin, M. Ferreira, V. Zucolotto, F. C. Nart, R. M. Torresi, O. N. Oliveira, Chem. Mater. 2004, 16, 2293–2299. 385

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Corresponding author: Ramzi Fezai; Laboratoire de Chimie des Matériaux, Faculté des Sciences de Bizerte 7021 Zarzouna, Tunisia.

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