Experimental and computational study of electronic

Journal of Molecular Structure 1162 (2018) 71e80

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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc

Experimental and computational study of electronic, electrochemical and thermal properties of quinoline phosphate Takoua Ben Issa a, b, Chedia Ben Ali Hassine c, Houcine Ghalla d, * , Houcine Barhoumi c, Latifa Benhamada a a

Energy and Materials Laboratory, Higher School of Science and Technology Hammam Sousse, University of Sousse, Sousse, 4054, Tunisia Faculty of Sciences, University of Monastir, Monastir, 5000, Tunisia Laboratory of Interfaces and Advanced Materials (LIMA), University of Monastir, 5000, Tunisia d Quantum Physics Laboratory, Faculty of Sciences, University of Monastir, Monastir, 5000, Tunisia b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 October 2017 Received in revised form 27 January 2018 Accepted 21 February 2018 Available online 22 February 2018

In this work, the electronic behavior, charge transfer, non linear optical (NLO) properties, and thermal stability of the quinoline phosphate (QP) have been investigated. The experimental UVeVis spectrum has been recorded in the range of 200e800 nm. Additionally, the absorption spectrum was reproduced by time-dependent density functional theory (TD-DFT) method with B3LYP functional and with empirical dispersion corrections D3BJ in combination with the 6e311þG(d,p) basis set. The electronic properties such as HOMO-LUMO energy gap and chemical reactivity have been calculated. The electrochemical characterization of the title compound is investigated using cyclic voltammetry and impedance spectroscopy methods. Finally, the thermal stability of the QP is discussed in term of differential scanning calorimetry (DSC) measurement, which showed that QP compound is thermally stable up to 150  C. © 2018 Published by Elsevier B.V.

Keywords: Quinoline phosphate TD-DFT HOMO-LUMO gap NLO Cyclic voltammetry DSC

1. Introduction Recently extended scientific research is focused on the investigation of charge transfer properties in hydrogen-bonded compounds. This field of research has received considerable attention not only for understanding numerous biophysical processes in the biological systems, but also because their applications as the electrolyte for fuel and electrolysis cells [1,2]. In particular, organic phosphate complexes attract more and more the attention of many scientists all over the world. These materials have been the subject of various structural and theoretical studies regarding to their applications in different field of the sciences [3e7]. They were potentially good candidates for NLO applications [8], photocatalysts [9,10], as well as biology [11e13] and medicinal uses [14e16]. The originality of these results depends not only to the organic or inorganic parts itself, but also to the morphology and the structure of the final compound. Moreover, the choice of the organic molecule involved in the structure of the hybrid compound is very important because it influences intensively the final result.

* Corresponding author. E-mail address: [email protected] (H. Ghalla). https://doi.org/10.1016/j.molstruc.2018.02.085 0022-2860/© 2018 Published by Elsevier B.V.

The quinoline is a good candidate regarding to its ability as a good established feature in a variety of naturally occurring and medicinally active compounds [17]. Concerning to its easy process and its potential applications, quinoline is one of the most studied heterocyclic compounds. The quinoline rings feature in a variety of naturally occurring and medicinally active compounds. These compounds were used as a treatment for parasitic infections. They are useful as antimicrobial [18e20], antitubercular [21,22], antimalarial [23e25], antiallergic [26] and antiasthmatic activity [27]. In a recent study, we have reported the X-ray diffraction and theoretical molecular structure, Hirshfeld surfaces, topological analyses and spectroscopic vibrational assignment of QP [28]. Fig. 1 shows that in the crystal packing of QP inorganic and organic components are connected through hydrogen bonding and p$$$p interactions. Additionally, the main important role in the process of self-assembly of QP salt is played by strong self-association via OeH/O (N) hydrogen bonding and also p$$$p interactions. These interactions have been proved by the topological AIM analysis and Hirshfeld surface analysis. From the literature, the electronic, electrochemical, and thermal properties of the QP are not yet reported. In the first section, the experimental UVeVis spectrum absorption has been recorded

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3. Theoretical details

Fig. 1. Crystal packing of QP complex.

within the range of wavenumbers of 200e800 nm. TD-DFT calculations have been performed in order to discuss the electronic transitions within the compound. The NBO analysis has been carried out to investigate the stability of the molecule and charge delocalization. In addition, the NLO properties of the QP compound have been discussed and compared to those found experimentally for salts of quinoline. In the second section, the dielectric behavior of QP was investigated through cyclic voltammetry and impedance spectroscopy methods. The following section undertakes the thermal behavior of the considered compound in term of DSC.

2. Experimental details The studied compound has been prepared by the slow evaporation method, as reported previously [28]. At room temperature, the UVeVis spectrum of QP was performed by the use of Perkin Elmer Lambda 950 spectrometer in the range between 200 and 800 nm. The reference material used for this measurement was ethanol ([QP] ¼ 5.105 mol/L). Electrochemical measurements were performed using a traditional three-electrode cell. This latter was containing a platinum wire as a counter electrode and glassy carbon electrode (GCE) as working electrodes with a geometric area of 0.07 cm2. The Ag/AgCl in saturated KCl (Ag/AgCl/(sat.KCl)) was used as a reference electrode. For this task, 2 mg of QP was dispersed in 1 mL of N, Ndimethylformamid. Then, 5 mL of the QP solution was dropped directly on the surface of a clean glassy carbon electrode GCE surface and dried at room temperature to form a QP modified GCE. The electrochemical experiments were carried out using an Autolab PGSTAT 320 N potentiostat for impedance spectroscopy spectra measurements controlled by computer with software (NOVA 1.5) for data analysis. DSC measurement was performed on heating samples from 30 to 350  C on a SETARAM apparatus (model DSC 92) at a heating rate of 5  C/min.

Starting from the crystal structure [28], the QP was optimized using ORCA 3.0 program [29] and applying the DFT method using global hybrid Generalized Gradient Approximation (GGA) functional B3LYP coupled to the empirical Becke and Johnson damping dispersion corrections D3BJ [30e32] in combination with the 6311þþG(d,p) basis set. The B3LYP-D3BJ was selected as a widely applicable method that proved to describe weak intermolecular interactions, more accurately and reliably than traditional DFT methods [33e40]. The optimization was performed with resolution of the identity approximation along with chain of spheres exchange method (RIJCOSX) [41]. The vibrational frequencies have been calculated at the same level of theory, and the absence of imaginary frequencies proves that the optimized geometry is a minimum on its potential energy surface. The optimized structure along with the atom numbering is given in Fig. 2. TD-DFT approach [42,43], at the same level of theory in the implicit salvation model COSMO [44], was applied to simulate the UVeVis spectrum and electronic properties, such as absorption wavelength, oscillator strength and HOMO-LUMO energy gap. Here ethanol (3 ¼ 24.3) was taken as implicit solvent, as it is used during the experiment. It is worthy to note that, recent research proves that TD-DFT theory has a potential role in the examination of the dynamic and static properties of the complexes within their excited states [42]. It allows for the best correlation between accuracy and computational cost. Moreover, the contribution percentages of the investigated electronic transitions have been simulated by GaussSum-3.0.1 program [45]. Finally, the optimized geometry was used to perform the NBO analysis and to determine the NLO properties with the help of Gaussian 09 package [46]. 4. Results and discussion 4.1. Electronic behavior Experimental and predicted UVeVis spectra of QP are shown in Fig. 3. A good correlation is observed between the calculated and the theoretical spectra. Experimentally, the absorption takes place in a weak zone between 250 nm and 350 nm. Furthermore, the experimental absorption maximum values have been localized nearly at 189 and 208 nm. Consequently, we can attribute these bands to the aromatic part of our compound (the quinoline sheets). These observations are similar for other studied on chromophores

Fig. 2. Optimized molecular structure of QP optimized at B3LYP-D3BJ/6-311þþG(d,p) level of theory.

T. Ben Issa et al. / Journal of Molecular Structure 1162 (2018) 71e80

73

Fig. 3. Experimental and theoretical UVeVis spectra of QP compound in ethanol solvent.

[47]. This behavior is considered as a good indication for the NLO applications. Accordingly, recent studies on salts of quinoline show that these kinds of compounds are good materials for NLO applications [47]. The computed results are given in Table 1. It seems important to mention that, the frontier molecular orbitals formed by the HOMO and the LUMO are the most important parameters. These parameters are the most popular quantum mechanical descriptors which define the reactivity of the studied compound. The energy of the HOMO is, essentially, related to the ionization potential. Also, it is useful to discuss the electrophilic behavior of the studied complex. Accordingly, the energy of LUMO is basically related to the nucleophilic activities of the compound. The gap between these two energetic states describes the molecular chemical reactivity [48,49]. As a result, the intense transitions are located at 208 (f ¼ 0.543), and 189 nm (f ¼ 0.916). The most intense band,

calculated at 189 nm, is mainly assigned to the HOMO-1 / LUMOþ3 (36%), HOMO / LUMOþ1 (24%), HOMO-6 / LUMO (12%), HOMO-1 / LUMO (12%). The HOMO / LUMO (86%) electronic transition appears at 291 nm (f ¼ 0.054). Fig. 4 illustrates the 2D color coded representation of frontier molecular orbitals of QP complex (from HOMO-1 to LUMOþ1). The brown color refers to the positive phase, which indicating the nucleophilic site. The negative phase is indicated by green color, and is related to the electrophilic site. As shown in Fig. 4, HOMO, lying at 6.88 eV, is attributed to a delocalized p orbital and located essentially on all the atoms of the quinoline ring, expect the phosphate groupments. The LUMO is assigned to the p* orbital and lying at 2.11 eV. Consequently, the HOMO-LUMO energy gap is closed to 4.77 eV. Referring to this low value, we suggest that the transition between HOMO and LUMO orbitals is probably p-p* transition type. Similar behavior is

Table 1 Calculated absorption wavelength l(nm), excitation energy E(eV) and oscillator strength f(a.u.) values of some electronic transitions of QP, calculated using TD-B3LYP(D3BJ)/ 311þþG(d,p) method in the ethanol solvent. No.

E

l

f

Major contributions (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

4.13 4.38 5.66 5.40 5.09 5.77 5.85 6.29 5.85 6.34 6.23 6.45 6.49 6.37 6.52 6.56 6.51 6.64

291 274 212 222 236 208 205 191 205 189 193 186 185 188 184 183 184 181

0.054 0.025 0.025 0.002 0.016 0.543 0.007 0.228 0.117 0.916 0.014 0.163 0.004 0.012 0.003 0.018 0.007 0.004

HOMO/LUMO (86%) HOMO-1/LUMO (59%), HOMO/LUMOþ1 (38%) HOMO/LUMOþ3 (51%), HOMO-4/LUMO (19%), HOMO-1/LUMOþ1 (19%) HOMO/LUMOþ2 (84%), HOMO/LUMOþ4 (14%) HOMO-3/LUMO (86%) HOMO-1/LUMOþ3 (38%), HOMO/LUMOþ1 (24%), HOMO-1/LUMO (18%) HOMO-1/LUMOþ2 (85%), HOMO-1/LUMOþ4 (11%) HOMO-1/LUMOþ1 (43%), HOMO/LUMOþ3 (28%) HOMO-6/LUMO (69%) HOMO-1/LUMOþ3 (36%), HOMO/LUMOþ1 (24%), HOMO-6/LUMO (12%), HOMO-1/LUMO (12%) HOMO-1/LUMOþ4 (82%), HOMO-1/LUMOþ2 (11%) HOMO-4/LUMO (28%), HOMO-6/LUMO (14%), HOMO-5/LUMO (12%), HOMO-1/LUMOþ1 (15%) HOMO-8/LUMO (76%), H-2/Lþ3 (18%) HOMO/LUMOþ6 (77%) HOMO-2/LUMOþ3 (69%), HOMO-10/LUMO (10%), HOMO-8/LUMO (16%) HOMO-2/LUMOþ2 (88%) HOMO/LUMOþ7 (58%), HOMO/LUMOþ8 (15%) HOMO/LUMOþ9 (31%), HOMO/LUMOþ8 (30%), HOMO/LUMOþ7 (11%)

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u¼ S¼

m2 2h

(5)

1

(6)

h

At the same level, hardness, h, and softness, S, are significant parameters to evaluate the stability or the reactivity of a molecular system. These parameters are considered as important factors, which can describe the polarizability of a molecular system and give a qualitative indication of its electronic distortion. A molecular system is qualified hard if the value of its energy gap is high, whereas it considered soft if its energy gap is low [51]. The values of all the thermodynamic parameters mentioned above are listed in Table 2. The IE and the EA of QP compound are 6.88 eV and 2.11 eV, respectively, which evidently designate that this compound is stable. The QP has high value of the energy gap (4.77 eV), higher hardness (2.38 eV), and a lower softness (0.21 eV). These quantities prove the stability of the title compound. The computed dipole moment value is 5.66 D. This value indicates that the investigated compound is a polar structure. 4.2. Natural bond orbital analysis Fig. 4. Selected frontier molecular orbitals of QP computed with TD-B3LYP-D3BJ)/6311þþG(d,p) in ethanol solvent.

observed for the HOMO / LUMO þ1 and HOMO-1 / LUMO transitions. Additionally, the HOMO-1 / LUMOþ1 transition imply an electron density transfer of the QP compound regarding the lowest value of the HUMO-LUMO energy gap which is lying at 6.32 eV. At this level of our study it seems important to mention that, the value of the frontier orbital gap can serve to give more information about the kinetic stability, the polarizability and also the chemical reactivity of the studied compound. In this context, the concept of softness and hardness of the electrophile and nucleophile attack, respectively, of the compound is directly related to the values of HOMO ad LUMO energies. Hence, the lower value of frontier orbital gap makes it more reactive and less stable. Therefore, this value is owing to an electron withdrawing groups that enter into conjugation and also, thermodynamically support the occurrence of an electron transfer. Additionally, hardness and electronegativity are potentially useful to predict the chemical behavior of the studied complex. Numerous thermodynamic parameters can be calculated using the values of HOMO and LUMO energies. With regard to the “Koopmans' theory”, ionization potential (IE), electron affinity (EA), chemical hardness (h), global electronegativity (c), global electrophilicity index (u), and global softness (S) can be calculated using the following expressions [50]:

IE ¼ EHOMO EA ¼ ELUMO

(1) (2)

h¼

IE  EA 2

(3)

c¼

IE þ EA 2

(4)

NBO analysis is considered as one of the most popular methods for investigating the electronic transitions, the charge transfer and the conjugative interaction in molecular systems [52]. Hence, the most important parameter to analyze those interactions is the stabilization energy E(2) accompanying with electron delocalization between donor and acceptor [53]. According to the second-order perturbation theory; for any donor NBO (i) and acceptor NBO (j) its stabilization energy E(2) can be calculated using the following expression:

Eð2Þ ¼ qi

F 2 ði; jÞ εi  εj

(7)

where, qi is the donor occupancy, εi,j are diagonal elements, and F(i,j) is the off diagonal NBO Fock matrix element. The stabilization energy for the significant donoreacceptor interactions of QP calculated at B3LYP/6-311þþG(d,p) level are summarized in Table 3. Accordingly, the Fock matrix analysis yields various types of donor-acceptor interactions and the correspondents stabilization energies. Referring to the results illustrated in Table 3, several intermolecular hyperconjugative interactions are found with a high value of energy and contribute to the stability of the QP complex. It seems important to note that an increase of the occupancy in (CeC),

Table 2 HOMO-LUMO energy gap, chemical potential, electronegativity, global hardness, global softness, electrophilicity index and dipole moment of QP calculated using TD-B3LYP(D3BJ)/311þþG(d,p) in ethanol solvent. Function

Value

EHOMO (eV) ELUMO (eV) DEHOMOeLUMO gap (eV) EHOMOe1 (eV) ELUMOþ1 (eV) DEHOMOe1eLUMOþ1 gap (eV) Chemical potential m (eV) Electronegativity c (eV) Global hardness h (eV) Global Softness z (eV)1 Global Electrophilicity index (j) Dipole moment (D)

6.88 2.11 4.77 7.37 1.05 6.32 4.49 4.49 2.38 0.21 4.24 5.66

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Table 3 Second order perturbation theory analysis of Fock Matrix in NBO basis of QP. Donor NBO (i)

Occupancy

Acceptor NBO (i)

Occupancy

E(2)a

E(j)-E(i)b

F(i,j)c

LP (1) O2 LP (2) O2 LP (2) O2 LP (2) O3 LP (2) O4 LP (2) O4 LP (3) O4 LP (2) O5 LP (1) N9 p (N9 e C13) p (N9 e C13) p (C10 eC11) p (C10 eC11) p (C12 eC17) p (C12 eC17) p (C16 eC18) p (C16 eC18) LP (1) N9 LP (1) N9 LP*(1) C14 LP*(1) C14 LP (1) C15 LP (1) C15 p* (N9 eC13)

1.95198 1.91103 1.91103 1.93499 1.81928 1.81928 1.80754 1.93451 1.82848 1.80872 1.80872 1.72602 1.72602 1.71504 1.71504 1.73684 1.73684 1.82848 1.82848 0.96095 0.96095 1.01074 1.01074 0.31420

s* (P1 e O4) s* (P1 e O3) s* (P1 e O5) s* (P1 e O5) s* (P1 e O3) s* (P1 e O5) s* (P1 e O2) s* (P1 e O3) s* (O2 e H8)

0.07976 0.18669 0.17648 0.17648 0.18669 0.17648 0.15132 0.18669 0.12099 0.96095 0.22809 0.96095 0.24500 1.01074 0.31420 1.01074 0.23907 0.02317 0.03978 0.31420 0.23907 0.22809 0.24500 0.22809

7.77 10.66 7.62 10.76 22.32 13.12 22.29 10.35 52.38 39.54 9.66 45.02 17.20 37.99 26.60 40.99 17.31 7.91 8.37 57.14 50.02 59.88 54.35 95.25

0.96 0.55 0.56 0.57 0.49 0.50 0.53 0.57 0.73 0.21 0.35 0.14 0.29 0.16 0.27 0.15 0.30 0.89 0.89 0.12 0.15 0.14 0.15 0.02

0.078 0.070 0.060 0.072 0.093 0.072 0.098 0.071 0.177 0.103 0.052 0.089 0.064 0.086 0.076 0.088 0.064 0.078 0.080 0.091 0.100 0.104 0.101 0.083

a b c

LP*(1) C14 p* (C12 e C17) LP*(1) C14 p* (C16 e C18) LP (1) C15 p* (N9 e C13) LP (1) C15 p* (C10 e C11) s* (C12 e C13) s* (C14 e C15) p* (N9 e C13) p* (C10 e C11) p* (C12 e C17) p* (C16 e C18) p* (C12 e C17)

E(2) means energy of hyperconjucative interactions (>5 kcal/mol). Energy difference between donor and acceptor i and j NBO orbitals (in a.u.). F(i,j) is the Fock matrix element between i and j NBO orbitals (in a.u.).

(CeN) and (CeH) leads to decrease the electron density of the correspondent antibond orbital. Also the electron density of the bonds of the quinoline rings is in the range between 1.7 and 1.8 e. This fact can be explained by the strong electronic delocalization and the tautomeric effect within the quinoline rings. Here, the highest values of the stabilization energies are assigned to the intermolecular transitions LP (1) C15/p* (C12eC17) and LP (1) C15/p* (C16eC18), which are found to be 59.88 and 54.35 kJ/mol, respectively. Furthermore, the intermolecular OeH/N hydrogen bond is formed by the orbital overlap between the LP(N) and s*(OeH), which results in an intermolecular charge transfer and support the stabilization of the H-bonded systems. This fact influences the ED of the OeH anti-bonding orbital, the nature and the strength of the OeH bonds involving in the hydrogen bonded system. The results of the NBO analysis exhibit a strong OeH/N intermolecular hydrogen bonding in the QP complex associates the organic sheets to the inorganic moieties. This result is previously proven by the AIM topological analysis [28]. In addition, the interaction between the lone pair donor orbital LP(N9) and s*(O2eH8) anti-bonding orbital is seen to give a strong stabilization, 52.38 kJ/mol. The LP(O)/s*(PeO) interaction between the O4

lone pair and the P1eO2 anti-bonding orbital shows an important stabilization energy of 22.29 kJ/mol than this between lone pair donor orbital LP(O4) and the P1eO2 anti-bonding orbital.

4.3. NLO properties Polarizabilities and hyperpolarizabilities provide profound information about the main criteria for the technological innovations in several areas as signal processing, optical interconnections and communication [54e56]. These parameters may be used to predict the NLO properties of a molecular system. Various optical parameters for QP have been calculated at DFT-B3LYP/6-311þþG(d,p) level based on the finite field approach. These parameters and their corresponding values are reported in Table 4. The calculated dipole moment is found to be 1.847a.u. The direction of dipole moment is mainly carried out for the x axis (mx ¼ 1.817 a.u). The average polarizability is calculated to be 14.26  1024 esu. The first order hyperpolarizability b, which is an important factor in NLO system, is about 0.597  1030 esu. For the title compound, the values of the dipole moment and the first order hyperpolarizability are 3.36 murea and 1.6 burea, respectively (murea ¼ 0.540 a.u,

Table 4 The dipole moment components (a.u.), polarizabilities a and average polarizability a0 (1024 esu), anisotropy of polarizability Da (1024 esu), and hyperpolarizabilities and the first hyperpolarizability b0 (1030 esu). Dipole moment

mx my mz m

Polarizability 1.817 0.304 0.138 1.847

axx axy ayy axz ayz azz a0 Da

Hyperpolarizability 27.503 2.318 25.894 0.210 0.141 13.091 22.163 14.267

bxxx bxxy bxyy byyy bxxz bxyz byyz bxzz byzz bzzz b0

0.080 0.159 0.323 0.816 0.150 0.095 0.107 0.458 0.064 0.001 0.597

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(a) (b)

I(μA)

50

0

-50

-0,5

0,0

0,5

1,0

E(V) 4 Fig. 5. Cyclic voltammograms of (a) GCE and (b) GCE/QPperformed with 1 mM ofFe(CN)3 6 /Fe(CN)6 at potential sweep rate of 100 mV/s vs. Ag/AgCl/(sat. KCl).

burea ¼ 0.372  1030 esu). These results point to good NLO properties of the studied complex. These properties may be explained by the charge transfer existing within the studied compound; especially from the conjugation of the p-electrons on the quinoline rings. 4.4. Electrochemical analysis To give more information on the electrochemical behavior of the studied complex, we report in this part of our investigation the

results of the electrochemical impedance spectroscopy (EIS) and the cyclic voltammetry. The modified electrode surface was characterized through the cyclic voltammetry and impedance spectroscopy measurements in 1 mM of [Fe(CN)6]3as redox probe. The obtained results are shown in Figs. 5 and 6. Moreover, the electrode surface coverage [57,58] has been calculated applying the following expression:

q¼1

RCT RCT

(8)

(a) (b)

2500

2000

-Z''( )

1500

1000

500

0 0

500

1000

1500

2000

2500

3000

3500

Z'( ) 4 Fig. 6. Nyquist plots of the (a) GCE and (b) GCE/QP modified performed with 1 mM Fe(CN)3 6 /Fe(CN)6 .

T. Ben Issa et al. / Journal of Molecular Structure 1162 (2018) 71e80

In this equation, R CT is the charge-transfer resistance measured at the bare working electrode and RCT is the charge-transfer resistance. All these values are illustrated in Table 5. The cyclic voltammograms of [Fe(CN)6]3-/4- redox system present a quasi-reversible redox reaction at the bare and the modified electrodes with the peak-to-peak separation (DE) of 78 and 154 mV for the bare GC and the GC/QP electrodes, respectively. Besides, the EIS gave detailed

Table 5 Electrochemical parameters for bare and modified GC electrode obtained from the analysis of impedance data with the equivalent circuit. Surfaces

RS (U)

CPE (mF)

W ( U)

Rtc (U)

Q (%)

Equivalent circuit

Bare GCE QP/GCE

151 98.722

6.02 26.526

429 386.84

159 1305.3

e 87

[R(Q[RW])] [R(Q[RW])]

313.15 K 323.15 K 333.15 K 343.15 K 353.15 K 363.15 K 373.15 K 383.15 K

80

40

Current (μA)

77

0

-40

-80 -1,0

-0,5

0,0

0,5

1,0

Potential (V)

(a) 2500 313.15 K 323.15 K 333.15 K 343.15 K 353.15 K 363.15 K 373.15 K 383.15 K

2000

-Z''( )

1500

1000

900

600

500 300

500

0 0

500

1000

1500

2000

1000

2500

1500

3000

3500

Z'( )

(b) Fig. 7. Cyclic voltammograms (a) and electrochemical impedance plots (b) of modified electrode at differenttemperatures.

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T. Ben Issa et al. / Journal of Molecular Structure 1162 (2018) 71e80

Fig. 8. Randles-type equivalent circuit.

Table 6 Electrochemical parameters for modified GC electrode obtained at different temperatures from the analysis of impedance data with the equivalent circuit. T ( C)

Rs (U)

CPE (mF)

Rtc (U)

W (mF)

313.15 323.15 333.15 343.15 353.15 363.15 373.15 383.15

98.722 71.276 60.826 56.502 50.969 45.487 36.264 35.125

26.526 14.732 11.485 10.89 11.073 11.085 11.285 11.1

1305.3 846.77 741.32 654.88 605.53 565.96 512.3 489.91

386.84 660.49 697.21 699.39 639.77 568.11 568.13 601.27

information about the change of the interface properties during the modification process, as shown in Fig. 6. As a consequence, all these results were in good agreement with the variation of the charge transfer resistance after the surface modification, as shown in Table 5. Also, the EIS signal is equivalent to 87% coverage for the modified electrode related to the formation of compact layer. In order to discuss the influence of increasing the temperature on the electrochemical properties of the studied compound, all the measurements mentioned above were performed in the range between 313.15 and 383.15 K. Fig. 7 illustrates the cyclic voltammograms and the impedance plots of the QP-modified GCE. Then, the impedance spectra were fitted to equivalent circuits using NOVA 1.5 as presented in Fig. 8. Besides, numerous parameters can be calculated from the numeric simulation of impedance spectra, such as the solution resistance (Rs), the charge transfer resistance (RTC), the constant phase element (CPE) and the diffusion impedance (W). All the simulation

Fig. 10. DSC curve of QP.

data are given in Table 6. As a results, the EIS measurements show a decrease of the charge transfer resistance when the temperature increases. This behavior can be attributed to the electron transfer kinetics generating charge accumulation at the surface of the modified electrode. Accordingly, the same electrochemical performances were observed by cyclic voltammetry. Moreover, to understand the conduction phenomenon, the Arrhenius modeling equation was used to determine the activation energy [59]: Ea

I ¼ AeRT

(9)

where I is the anodic peak current, A is a constant, Ea is the activation energy for adsorption or desorption, R is the universal gas constant and T is the temperature. In Fig. 9, we report the schema of Ln(I) vs. 1/T which is determined using the Arrhenius equation. Accordingly, the estimated values of the activation energies (Ea) were calculated to be 1.31 kJ/ mol (0.013 eV) and 4.71 kJ/mol (0.048 eV). We can deduce that the conductivity of the studied compound increases with the temperature demonstrating that the material has semiconductor behavior [60,61]. Finally, as a practical application of the developed probe, the modified electrode by the quinoline phosphate can be used for ion sensor for environmental applications. For instance, this modified electrode can be used for metal ion complexation such as Zn2þ and Cu2þ. 4.5. Thermal analysis

Fig. 9. Arrhenius type plot of the quinoline phosphate layer deposited on GCE at different temperatures.

The detected thermal phenomena with DSC studies are various. These manifestations can be classified in two categories: (i) endothermic phenomena are numerous such as sublimation, fusion, dehydration, and vaporization, and (ii) exothermic ones are almost the crystallization, the decomposition and the adsorption. For the QP complex, the curve corresponding to DSC is given in Fig. 10. This curve exhibits a significant weight loss in the region of 140e220  C. In this range of temperature, two thermal phenomena are occurred. The first one, noticed in the area of 142e160  C with a maximum at 159  C, is an endothermic manifestation indicating the beginning of the fusion. An additional treatment has been performed in order to predict the melting point of our anhydrous compound, which is represented by the endothermic peak appearing in Fig. 10. Further thermal treatment on a Kofler bench was performed from room temperature up to 165  C to confirm the range of the melting of our

T. Ben Issa et al. / Journal of Molecular Structure 1162 (2018) 71e80

investigated compound. Then, the compound changes to be a slightly brown liquid indicating the start of the decomposition. The second phenomenon is located between 160 and 215  C with a maximum at 164  C. This manifestation is exothermic. Accordingly, the peak found at 164  C may be accredited to the crystallization of an infinite chain of polyphosphate (H3PO4)n. Furthermore, referring to our results reported previously [28], the structure is made by two types of H-bonding; OeH,,,O (moderately strong, linking the inorganic fragment in the mineral chains) and OeH,,,N (considered as weak, connecting the organic part to the mineral chains). These interactions may be reorganized during the process of fusion by the breaking of the weak H-bonds. At this moment, the phosphate chains are liberated. Then, the increase of temperature leads to the crystallization of polyphosphate. This process is accompanied by an important weight loss which may be ascribed to the combustion of the organic groups. This result is similar to those found in previous investigations on organic phosphate [62]. Finally, the chemical stability of the studied anhydrous QP compound, below the melting point (159  C), can be explained by the H-bonding network proven by the X-ray diffraction. The above thermal analysis results are consistent with structural geometry [28]. 4.6. Correlation between electric, thermal and structural properties Increasing the temperature affects the structural properties of the title compound. So, this procedure can favors the vibration of the alternate chains formed by the phosphate groupments and quinoline sheets and can influence the orientation of the H3PO4 groups and leads to the reorientations of these latter. Therefore, it can cause a higher migration of the Hþ proton in the hydrogen bonding network. Besides, the strong conductivity of the QP may be explained by the 3D motions of protons Hþ [63]. 5. Conclusions Organic-inorganic hybrids have a great interest in different field of the sciences regarding to their extraordinary new properties, multifunctional character, and their numerous applications as well as optics, semiconductors, catalysis, and medical applications. The present work is devoted to investigate the electronic, electrochemical and thermal properties of the organic-inorganic hybrid quinoline phosphate. The optical absorption spectrum shows two bands observed at the weak zone 250e350 nm. The transparency of QP crystal in the visible region reveals its suitability for optical applications. The HOMO-LUMO energy gap calculated by TDB3LYP-D3BJ approach is found to be 4.77 eV. The stability of QP has been evaluated in term of second order perturbation energy. Additionally, the value of the first order hyperpolarizability indicates that the title compound may be used as a material for nonlinear optical applications which is in good agreement with the result given by the UVeVis measurement. The electrochemical analysis reveals that the conductivity of the studied compound increases with the temperature indicating that the material shows semiconductor behavior. Finally, the DSC curve shows that QP is thermally stable up to 159  C. Accordingly, it can be exploited for any application well below 159  C. References [1] R.L. Costa, P.G. Grimes, Electrolysis as a source of hydrogen and oxygen, Chem. Eng. Prog. 63 (1967) 56e58. [2] T. Takahashi, S. tanase, O. Yamamoto, S. Yamauchi, Proton conduction in triethylenediamine-and hexamethylenetetramine-sulfate, J. Solid State Chem. 17 (1976) 353e361. [3] I. Chaabane, F. Hlel, K. Guidara, Electrical study by impedance spectroscopy of

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