Eco-friendly preparation and testing of electroactive quinoxalines

2.1. Materials and instruments

All the starting materials and solvents (CH₃COOH) were obtained from Aldrich. ¹H NMR (500 MHz) and ¹³C NMR (125 MHz) spectra were recorded on a Bruker AVANCZ 500 spectrometer, using CDCl₃ as solvent at 298 K and tetramethylsilane (TMS) as the internal standard. Molecular weights and elemental composition of quinoxalines were characterized by the Flash EA 1112, CHNS–O elemental analysis instrument. High-purity water was obtained by distillation prior to use. The UV-Visible Spectroscopy was performed on the TU-1810 (PERSEE, CHINA). Electrochemical measurements were performed with an electrochemical work station. For all the voltammetry experiments, the solvents and electrolytes were added to the single-cell (degassed with N₂ for 5 min) containing working, reference, and counter electrodes protected under N₂. Ferrocene was used as the internal reference for the cyclic voltammogram experiments.

2.2. General synthesis method and characterization data

2.2.1. Synthesis

The reaction Scheme 1 shows the one-step preparation of Quinoxalines. For the first product as an example: 1 mmol of Benzil (210.2 mg) and 1 mmol of aromatic ortho-diamine (e.g., benzene-1,2-diamine, 108.1 mg) were dissolved at 35°C in two different round-bottom flasks (50 mL) containing CH₃COOH (15 mL) with the help of a stirrer. The ultrasound (US) instrument was arranged (vertical ultrasound-horn attached to the VCX-750) and turned on as indicated (Scheme 1). The two solutions were allowed to mix under the Ultrasound irradiation (300 watts, 20 kHz) for 45 min in a high-strength glass flask (Boro 3.3). The product separates as a solid that allows easier filtration and filtration as explained below.

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Scheme 1.

(a) Eco-friendly reaction scheme of quinoxalines: Chemical structures of 2,3-diphenylquinoxaline (Qx-1), 2,3-diphenylpyrido[3,2-b]pyrazine (Qx-2), 6-methyl-2,3-diphenylquinoxaline (Qx-3), 6,7-dimethyl-2,3-diphenylquinoxaline (Qx-4), 2,3-diphenylbenzo[g]quinoxaline (Qx-5); and (b) Proton Nuclear Magnetic Resonance (¹H-NMR) Characterization.

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Figure 1.

The UV-Visible absorption spectra (solvent: THF) of (a): Qx-1, (b) Qx-2, (c) Qx-3, (d) Qx-4, (e) Qx-5, and (f) summarized data of the 5 quinoxalines (Qx-1, 2, 3, 4, 5).

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2.3. Materials characterization

3.1. Synthesis and proposed mechanism

Scheme 1 clearly shows the eco-friendly reaction scheme of quinoxalines, while Scheme 2 shows the proposed mechanism for the synthesis of quinoxalines inspired by previous research. The important steps include benzil protonation that allows the nucleophilic attack of benzene-1,2-diamine, followed by the multiple proton-transfer steps. This allows the dehydration step that contributes to aromatization. The completion of this step gives the solid product (e.g., 2,3-diphenylquinoxaline). This allows filtration that separates the product. The product is washed with dilute acetic acid, followed by washing with dilute NaOH. This not only removes the residual acetic acid of the product but also neutralizes the acetic acid used in the reaction. Hence, there are no waste products that can harm the environment.

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Scheme 2.

Proposed mechanism for the synthesis of quinoxalines [20, 21].

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3.2. UV-Vis spectroscopy

The great scholar Eugene Sawicki et al. (1957), in their research (UV-Vis absorption spectra of quinoxaline derivatives), reported the UV-Visible absorption spectra for the simple quinoxalines [19]. The λ_max with logε facilitates the comparison of UV-Vis absorption bands because the logarithm of the molar absorptivity (ε) indicates how strongly a substance absorbs light at the λ_max. The underlying principle is the Beer-Lambert Law: A = ε·c·l; where A is the absorbance, c is the concentration (mol/L or mol/dm³), l is the path length (cm), and ε is the molar absorptivity. Therefore, we followed his footsteps (Figure 1). The UV-Vis spectrum for Qx-1 displays (Figure 1a) two absorption bands (π → π* transitions) characteristic: (a) a high-energy, high-intensity band at λ_max​ = 244 nm (logε = 4.47); and (b) A lower-energy, moderately intense band at λ_max​ = 341.5 nm (logε = 3.97). The band at 341.5 nm corresponds to the lowest energy transition, thereby originates from the electronic transition between the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The UV-Vis of Qx-2 (Figure 1b) is like Qx-1, but due to an additional nitrogen atom in the central core, it shows enhanced absorption. The red-shifted λ_max at 347.5 nm compared to the Qx-1 (341.5 nm) suggests a smaller HOMO-LUMO gap, indicating increased conjugation from the extra nitrogen. The high log ε (4.50 at 215 nm) indicates a stronger π→π* transition. The Qx-3 and Qx-4 (Figures 1c,d) have the addition of one and two methyl groups, respectively. Methyl groups are weak electron-donating groups (auxochromes), causing the decrease in the HOMO-LUMO energy reflected by the absorption at a longer wavelength. The UV-Vis of Qx-5 (Figure 1e) has a lower energy absorption band at λ_max​ = 385 nm. This absorption is closer to the visible region of the spectrum, which explains the molecule’s yellow color. The multiple bands observed are typical for large, polycyclic aromatic systems [20,21]. In summary, the UV-Vis spectra of the five quinoxalines (Qx-1 to Qx-5) revealed systematic changes in their absorption that correlate with their molecular structures and observed colors. Qx-1, Qx-2, Qx-3, and Qx-4 all appear white (λ_max ranging from 225 nm to 352.5 nm), indicating no significant absorption in the visible spectrum. However, Qx-5 exhibits a distinct yellow color due to an absorption band at 385 nm (logε = 4.03), which extends into the visible region (440 nm) and absorbs blue light caused by the electronic conjugation modifications in Qx-5 [22,23].

3.3. Electrochemical properties

Figure 2 shows positive and negative scans (cyclic voltammetry curves) performed separately for the 5 molecules (Qx-1, Qx-2, Qx-3, Qx-4, and Qx-5). The cyclic voltammetry data have been summarized in Figures 3(a, b). The E_HOMO and E_LUMO levels were estimated from the onset potentials by comparison to ferrocene (4.8 eV versus vacuum). Electron affinities (LUMO levels) were estimated from the onset of the reduction wave (EA = E_LUMO = – (E_onset ^red – V_FOC + 4.8) eV). Ionization Potential (HOMO levels) were estimated from the onset of the oxidation wave (IP = E_HOMO = – (E_onset ^ox – E_FOC + 4.8) eV) [24,25].

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Figure 2.

(a,b) The cyclic voltammograms of the 5 quinoxalines: Qx-1, Qx-2, Qx-3, Qx-4, Qx-5 were recorded using a glassy carbon electrode as the working electrode, Pt wire as the counter electrode, and TBAPF₆ in CH₃CN as the supporting electrolyte.

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Figure 3.

(a) The cyclic voltammograms data of the five quinoxalines (Qx-1, 2, 3, 4, 5); (b) The HOMO-LUMO energy levels of the five quinoxalines (Qx-1, 2, 3, 4, 5) and Electrochemical Band gaps of the Qx-1, 2, 3, 4, 5 (details are given in the text).

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The cyclic voltammetry data for the Qx-1 to Qx-5 revealed their electrochemical properties, crucial for applications in Organic Electronics. The Qx-1 has the highest oxidation potential (positive-scan: E_ox^onset =1.61 V) and a more negative reduction potential (negative-scan: E_red^onset = -1.82 V). This information provided an ionization potential: IP (E_HOMO) of -6.31 eV, electron affinity: EA (E_LUMO) of -2.83 eV, and the largest (E_g = |IP - EA| or E_HOMO-E_LUMO) energy gap (E_g = 3.48 eV). The Qx-2, Qx-3, and Qx-4 exhibit intermediate properties, with Qx-2 showing a relatively low oxidation potential (positive-scan: E_ox^onset = 1.28 V) and Qx-4 the most negative reduction potential (E_red^onset = -1.96 V). For the sake of comprehension, we explain the data (Figure 3a) of Qx-1 in detail. The data has two sections: (Reduction: n-doping) and (Oxidation: p-doping). From Figures 2(b) and 3(a) (Qx-1): The E_ox^onset = 1.61 V is the potential where oxidation begins and proceeds (Qx-1 becomes positive). The E_pa = 1.66 V is the anodic peak potential, which shows the maximum oxidation current and returns (reverse-scan). There was no return peak (reverse-scan), hence a cathodic peak potential E_pc was not observed, suggesting an irreversible oxidation process. All five Quinoxalines showed the same property (Figures 2b and 3a), which leads to the conclusion that all five quinoxalines cannot be used as the positive electrode in the charge-storage materials. From Figure 2(a) and 3(a) (Qx-1): The potential where reduction begins and proceeds is the E_red^onset = -1.82 V (Qx-1 becomes negative). The cathodic peak potential (current is the highest) for the reduction is E_pc = -1.96 V. Fortunately we were able to observe the anodic peak potential (Qx-1 becomes neutral) on the reverse scan at anodic peak potential E_pa = -1.89 V. This enabled us to calculate the formal reduction potential E_red^o = -1.92 V (average of E_pc and E_pa = (-1.96 + -1.89)/2 ≈ -1.92 V). The formal reduction potential (standard reduction potential) represents the voltage at which a chemical species gains electrons (reduced) under standard conditions, measured in volts (V). This indicates the thermodynamic favorability of a redox reaction, critical in energy storage devices (batteries and supercapacitors). All five quinoxalines indicate this reversible reduction property; therefore, all five quinoxalines have the potential to be used as the negative electrode in charge-storage materials, as evident by the previously published research [26-28].

Since energy density scales with the square of the voltage (E = ½CV²), a wider potential window enhances energy storage capacity and to operating at higher voltages without degradation. Based on this information, Qx-3 and Qx-4 are good candidates to widen the negative potential window, while Qx-5 is suitable to shorten the negative potential window in a charge storage cell [29-33]. Figure 3(b) clearly indicates that all five quinoxalines have different potential windows useful for energy storage materials. The Qx-5 stands out with the least negative reduction potential (E_red^onset = -1.58 V), indicating it is the easiest to reversible electro-reduction, with an IP: E_HOMO = -5.80 eV, EA (E_LUMO) of -3.12 eV, and the smallest energy gap (E_g = 2.68 eV). This is due to the structural differences that change the electronic conjugation, thereby affecting the E_HOMO and E_LUMO levels (Figure 3b) [34,35].