To Investigate the Electrochemical Behaviors of Bis-Chalcone Derivatives

The electrochemical behavior of (2E,5E)-2,5-dibenzylidenecyclopentanone (P1) and (2E,5E)-2,5-bis(4-nitrobenzylidene)cyclopentanone (P2) known as bis-chalcone derivatives was studied by cyclic voltammetry (CV) using an indium tin oxide (ITO) as the working electrode. Repeated cyclic voltammograms measurements exhibited excellent longterm redox stability for bis-chalcone derivatives. The oxidation peak for in the anodic region for the 1st cycle of the P1 appeared at -0.20 V, while the oxidation peak of P2 was observed at -0.47 V. On the other hand, the existence of the electron attracting NO2 group at p-position on the benzene ring in the structure of P2 caused an increase in the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy gap. The surface morphology and structural features of P1 and P2 films coated onto ITO glass substrates were investigated by using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). SEM micrographs demonstrated that the P1 and P2 were homogeneously distributed over the ITO surface. After CV measurement, it was observed that the grain size of P2 increased with increasing ITO surface roughness. Moreover, the EDS results confirmed the presence of P1 and P2 on the ITO surface.


INTRODUCTION
Chalcones are generally defined as α,β-unsaturated ketones and are precursors not only for synthetic manipulations but also for important components of natural products. Chalcones and their synthetic analogues are known to show a large number of interesting biological activities (Ducki et al., 1998;Konieczny et al., 2007;Nowakowska, 2007;Katsori and Hadjipavlou-Litina, 2009;Kumar et al., 2010;Biradar et al., 2010;Rane and Telekar, 2010;Zhang et al., 2010). In recent years, the inclusion of an α,β-unsaturated ketone unit into chalcone has been seen as an effective strategy for the development of chemotherapy drugs. Therefore, a series of chalcone analogues carrying an α,βunsaturated ketone was synthesized from chalcone analogues with modest anticancer activities and were used in studies (Zhu et al., 2018;Going et al., 2018). Moreover, novel types of chalcone analogue containing metal complexes such as Pt(II), Ni(II) and Pd(II) were synthesized and DNA binding, molecular docking and antimicrobial activities of these compounds were evaluated. The results showed that it can be used as therapeutic drug candidates (Atlam et al., 2017).
In addition to biological activities, the photophysical properties of chalcone derivatives have received much attention in studies such as nonlinear optics (NLO), photorefractive polymers, fluorescent probes for the detection of metal ions and organic luminescent as well as semiconducting device applications (Fayed, 2006;Wei et al., 2011;Si et al., 2011;Teo et al., 2017;Kwong et al., 2017;Karuppusamy et al., 2017;Nohut Maşlakcı et al., 2018). Also, bis-chalcones are an interesting class of compounds because of their use as precursors to potentially bioactive and functional compounds (Xu et al., 2001;Bukhari et al., 2013;Ritter et al., 2014;Albuquerque et al., 2014;Tala-Tapeh et al., 2015). Thanks to the versatile properties of the bis-chalcone derivatives, electrochemical studies have been carried out not only to investigate pharmacological and biological activities but also to study their electrochemical properties (Yellepa and Mallapa, 2015). Recent investigations on the effect of electron-donating groups on optoelectronic properties, binding and electron transfer feasibility with benzofuran substituted chalcone derivatives have provided important information for photovoltaic applications (Coskun et al., 2019).
It is known that chalcones have a conjugated double bond in both benzene rings and a completely delocalized π-electron system. Molecules with such a system have a relatively low redox potential, while the likelihood of undergoing electron transfer reactions is quite high (Yellepa and Mallapa, 2015). It has been suggested that the reduction of the chalcones may be achieved by the isomerization of free radicals formed on the carbonyl group of the first electron atom into another free radical which can be further reduced or converted to a dimer. Furthermore, it has been observed that in cyclic voltammetric studies carried out in the DMSO solvent, α,β-unsaturated ketone is reduced to radical anion and is then consistent with a mechanism involving irreversible dimerization (Yellepa and Mallapa, 2015). Recently, it has been found that the electron affinities of a series of chalcone derivatives computed at the density functional level are linearly related to the measured voltammetric potentials in the aprotic environment (DMSO, DCM, etc.) (Quintana-Espinoza et al., 2006;Jin et al., 2014;Yellepa and Mallapa, 2015). However, much work has not been studied in literature with polar aprotic solvents such as acetonitrile (Al-Ayed, 2011).
The main goal of the present study is to report on the electrochemical properties of the bis-chalcone derivatives which contain NO2 functional group and without. The nitro group on the benzene ring is able to delocalized π-electrons in the ring to meet their charge deficiency. In this respect, while the charge is provided to the molecule, it gives the nitro group unique properties that make it an important functional group during chemical synthesis (Ju and Parales, 2010). In the literature, electrochemical studies of various chalcone derivatives have been carried out (Naik and Nandibewoor, 2012), but studies investigating the effect of the chalcone derivatives containing the nitro group have not been observed. P1 and P2 structures were selected to investigate the substitute group effects on the electrochemical properties of the chalcone derivatives.
In this study, the electrochemical properties of P1 and P2 were studied by the cyclic voltammetry (CV) technique. The HOMO and LUMO values of P1 and P2 were compared using the onset oxidation and reduction potentials obtained from the cyclic voltammograms. The morphology and elemental analysis of P1 and P2 films coated onto ITO glass substrates were investigated by using scanning electron microscopy (SEM) and energydispersive X-ray spectroscopy (EDS). Although EDS has disadvantages such as poor energy resolution of peaks and the sensitivity of the surfaces complicating bulk analysis, in most cases, it is the most widely used method in electron microscopes due to its advantages such as high data collection speed, ease of interfacing, high speed of detection and high detector efficiency (Willis et al., 2002;Girao et al., 2017). Therefore, the morphological characteristics of bis-chalcone derivatives (P1 and P2) were investigated by the SEM-EDS technique.

MATERIAL AND METHOD
All chemicals ad organic solvents were obtained from commercial sources with the highest purity available. Lithium perchlorate (LiClO4) and acetonitrile (ACN) were purchased from Sigma-Aldrich. Gamry 300 Model potentiostat instrument was used to make all electrochemical measurements in a classical one-compartment, three-electrode electrochemical cell. The surface morphology of P1 and P2 were investigated using SEM (scanning electron microscopy) (FEI Quanta FEG 250).
The elemental compositions of the P1 and P2 were analyzed with EDS (Bruker EDAX/EDS).

Electrochemical Studies
For electrochemical measurements of P1 and P2, the solution of bis-chalcone derivatives was prepared in 0.1 M LiClO4/ACN electrolyte and solvent. Electrochemical measurements were made in the triple electrode system. A typical triple electrode configuration was performed with indium tin oxide (ITO) as a working electrode, Ag/AgCl as the reference electrode and a Pt wire as the counter electrode. Electrochemical measurements were performed by applying a potential scanning rate of 100 mV/s in the potential range from +2.5 to -2.5 V. The electrochemical behavior at different scan rates of the ITO electrode immersed in 0.1 M LiClO4/ACN containing 0.1 wt.% of P1 and P2 was also investigated by cyclic voltammetry (CV).

Electrochemical Results
The electrochemical properties of P1 and P2 were examined by cyclic voltammetry in 0.1 M LiClO4/ACN using ITO glass as the working electrode, Ag/AgCl as the ref-erence electrode and platinum (Pt) as the counter electrode ( Figure 2). Electrochemical measurements were carried out at a scan rate of 100 mV/s in the potential range from +2.5 to -2.5 V. The repeated oxidation and reduction peaks during the coatings of P1 and P2 onto ITO surfaces were recorded due to their changes in Lidoped and undoped states without significant decomposition at molecular structures during the reversible redox process from +2.5 to -2.5 V. The electroactivity of bischalcone derivatives (P1 and P2) enhanced with increasing scan numbers in Figure 2a-b when the potential of between ± 2.5 V was scanned. This indicates that the presence of the oxidation process which can be consecutive during the oxidation peak of both P1 and P2. The oxidation peak for in the anodic region for the 1st cycle of the P1 was observed at -0.20 V (the corresponding oxidation peak current of 1700 µA), while the reduction peak appeared at -2.02 V (the corresponding reduction peak current of -2400 µA). In addition, the oxidation peak at the anodic region for the 1st cycle of P2 appeared at -0.47 V (the corresponding oxidation peak current of 1400 µA), while the reduction peak was observed at -2.03 V (the corresponding reduction peak current of -1500 µA) (Figure 2).
When the nitro group with electronegative property is attached to a benzene ring, it can delocalize the π-electrons of the ring to compensate for the lack of charge or can prevent the transfer of electrons (Saby et al., 1997;Ju et al., 2010). As expected, the oxidation and reduction potentials of P1 and P2 are strongly influenced by the substituents on the phenylene ring in chalcone derivatives (Al-Ayed, 2011;Erasmus, 2011). The fact that the oxidation peaks of P2 have more negative values compared to P1 could be explained by the electron attracting properties of the NO2 group. On the other hand, it is evident that the structure of the NO2 group on the benzene ring does not significantly affect the reduction potential of P2.
Depending on this situation, due to the electron attracting NO2 group, P2 was observed at -0.47 V with a slightly more negative potential than the oxidation peak observed in the anodic region of P1. The oxidation peak of the 20th cycle in the cyclic voltammogram of P1 has a shift towards smaller negative potentials as compared with the oxidation peak in the 1st cycle. For P2, the oxidation peak of the 20th cycle showed a similar negative potential shift as compared with the oxidation peak of the 1st cycle. The difference between oxidation and reduction potential in 1st cycle for P1 is about -1.82 V.
At the 20th cycle, this difference has changed as  -2.17 V. Moreover, while the difference between oxidation and reduction potential in 1st cycle for P2 was about -1.56 V, it is determined to  -1.91 V in the 20th cycle. Under these electrochemical conditions, it was observed that both samples well covered the ITO surface at the end of the 20th cycle. The highest occupied molecular orbital (HOMO)--the lowest unoccupied molecular orbital (LUMO) energy gap, also known as the electrochemical band gap (Eg elec ), can be calculated from the difference between initial oxidation and reduction potentials using the cyclic voltammograms (Muto et al., 2001;Jin et al., 2014;Cogal et al., 2014;Asiri et al., 2014). In order to obtain an understanding of the structure-property relationship of the compounds, the calculations of the HOMO energy levels of these compounds were performed front the onset oxidation potential of the cyclic voltammogram, using the following equations (1), (2), (3) (Jin et al., 2014;Cogal et al., 2014;Selinova et al., 2017): HOMO = -[Eox onset versus Ag/AgCl -E1/2(Fc/Fc + ) versus Ag/AgCl + 4.8]eV (1) LUMO = -[Ered onset versus Ag/AgCl -E1/2(Fc/Fc + ) versus Ag/AgCl + 4.8]eV (2) Eg elec = (LUMO -HOMO)eV where E1/2(Fc/Fc + ) is half-wave potential (0.57 mV) of the ferrocene/ferrocenium (Fc/Fc + ) (Jin et al., 2014;Cogal et al., 2014). Eox onset and Ered onset is the oxidation and reduction onset potentials, respectively. The energy value of 4.8 eV is known to be the value determined below the vacuum level of the onset oxidation potentials (Eox onset ) of ferrocene/ferrocenium (Fc/Fc + ) and is used as a calibration reference (Jin et al., 2014;Cogal et al., 2014;Selinova et al., 2017). HOMO energy levels are calculated to be -5.83 and -5.93 eV, while LUMO energy levels are determined to be -3.08 and -3.0 eV for P1 and P2, respectively. Table 1 shows the onset data of the oxidation and reduction potentials of P1 and P2 and also the HOMO and LUMO energy levels of these compounds. The obtained HOMO-LUMO energy level values are in accordance with the literatüre (Jin et al., 2014). The HOMO and LUMO values of P2 are increased due to the electron-withdrawing NO2 group at pposition on the benzene ring.   Figure  4a-b show graphs of the square root of the scan rates versus the oxidation (A) and reduction (B) peak currents of CVs obtained at different scan rates of P1 and P2, respectively. The ITO glass electrodes coated with P1 and P2 are continuously cycled throughout the reversible oxidation and reduction process, as shown in Figure  3a-b. It was observed that the oxidation and reduction peak currents increased linearly as the scan rates increased by 200 mV/s, as can be seen in Figure 4. This redox process showed that the electrochemical behaviors of P1 and P2 are reversible up to 200 mV/s. In addition, for both examples, the linearity of the plots shows that the electrochemical process can be controlled by the diffusion step at scan rates lower than 200 mV/s (Erasmus, 2011;Yellepa and Mallapa, 2015;Nohut Maşlakcı et al., 2018).

SEM-EDS Results
The morphologies of the bis-chalcone derivatives were examined with SEM, as shown in Figure 5. The SEM images of the samples show that the ITO surface is covered. For P2, the formation of granular structures on the surface is observed. The average diameter and cross-sectional depending grain size of P2 particles were determined to be 61.7 ± 5.60 nm and 712.3 nm, respectively (Figure 5c-d). The increase in surface roughness of ITO after the CV measurement is due to enhanced deposition along with the increase in grain size of the P2 film. Grain size can increase, with increasing substrate temperature or post-deposition on the surface (Kumar et al., 2009;Pammi et al., 2011). However, after measuring the CV of P1, it is observed that there are not too many particles on the ITO surface. The cross-sectional for P1 was measured as 323.8 nm (Figure 5b). The obtained SEM results showed that P2 covered the ITO surface more smoothly.
The EDS analysis was performed to confirm the film coatings of bis-chalcone derivatives (P1 and P2) on the ITO glass substrates, as shown in Table 2. The weight percentage of carbon (C) of P1 film onto the ITO glass substrate was 5.30 %, while the contents of carbon and nitrogen (N) of the P2 film were 4.07 %, and 0.53 %, respectively. The presence of nitrogen comes from the NO2 group in the P2 structure. Moreover, the reason for the presence of the tin (Sn), indium (In) and part of the oxygen (O) are the ITO glass substrate used as the working electrode (Choi et al., 2014).

CONCLUSION
In summary, the P1 and P2 bis-chalcone derivatives were further investigated by the electrochemical method for the first time. SEM micrographs demonstrated that the P1 and P2 were homogeneously distributed over the ITO surface. The P1 and P2 showed that long-term stability between +2.5 and -2.5 V in electrochemical studies. The HOMO and LUMO energy levels are promising values as -5.83 and -3.08 for P1, and -5.93 eV and -3.0 eV for P2, respectively. These bis-chalcone derivatives can provide potential applications for future designs of sensitizers with various electron-withdrawing groups for the photovoltaic and semiconducting device applications.