Synthesis and characterization of enhanced azo-azomethine doped PANI/HCl conducting polymers for electrochemical applications

Materials and methodsAnalytical-grade precursors from Oxford lab fine chem LLP, such as aniline(99%), were utilized. Glacial acetic acid(aojinchem 99.85%), Ammonium peroxydisulfate (APS)(sigma-aldrich, 98.0%), KOH(99%), KCl(99.0%), NaNO2(≥ 97.0%), 4-aminophenol(≥ 98.0%), 2-hydroxy-3-methoxybenzaldehyde(≥ 98%) of Sigma Aldrich precursor compounds are used.Synthesis methodsSynthesis of polyaniline emeraldine salt in the presence of hydrochloric acid medium (PANI/HCl)In a 250 mL conical flask, aniline (0.01 mol, 1 mL) and HCl (125 mL) were added and cooled to 0–5 °C (solution A). To the solution A added solution B was prepared from (NH4)2S2O8 (2.5 g, 0.011 mol, 25 ml D.W) in 30 mins with stirred vigorously. Continue string mixture solution at 0–5 °C for 4 h then overnight stirring at room temperature. Filtrate the dark green precipitate product (polyaniline emeraldine salt) and washed with D.W frequently then by small amount of acetone one time. Keep the product in a vacuum oven for 24 h at 55 °C25.General procedure for Synthesis of 2-hydroxy-5-((4-hydroxyphenyl)diazenyl)-3-methoxybenzaldehyde (3)In 100 mL conical flask, prepared a solution of 4-aminophenol (1.0913 g, 10 mmol) in a mixture of HCl(%37, 10 mL) and D.W(10 mL) and cooled it to 0–5 °C, then dropwise added 12 ml solution of NaNO2 (1.207 g, 17.5 mmol) and continue stirring at 0–5 °C for 1 h to formation of 4-aminopheno diazonium chloride salt solution (solution A). To formation the solution of potassium 2-formyl-6-methoxyphenolate, in separate beaker prepared 15 mL of 2-hydroxy-3-methoxybenzaldehyde (1.521 g, 10 mmol) in 10% KOH solution and keep temperature range at 0–5 °C (solution B). Added solution B gradually to solution A with vigorously stirring at 0–5 °C until gelled solution formed then added 25 mL of cold D.W. Leaved overnight stirring at room temperature, filtrate the collected orange product and recrystallized in a suitable solvent mixture.2-hydroxy-5-((4-hydroxyphenyl)diazenyl)-3-methoxybenzaldehyde (3)Light-brown solid, Yield 85.5%, m.p. 46 °C. IR cm−1: 3080.62 (Csp2–H aromatic),2974.06–2940.98 ((Csp3–H),2884.94–2839.88(CH. aldehyde)1636.88 (C=O), 1588.61 (C=C aromatic);1453.08 (–N = N–), and 838.16 (aromatic substituted).; 1H NMR (DMSO–d6, δ ppm): 3.860(3H,s,OCH3), 6.823–6.849 (2H, dd, arH, J = 13 Hz), 6.871–6.897(H, dd, arH, J = 13 Hz), 7.341–7.372(H, dd, arH, J = 15.5 Hz), 7.542–7.573 (2H, dd, arH, J = 13 Hz), 7.873–7.899(H, dd, arH, J = 13 Hz), 10.018 (1H,s, HC = N). 10.358 (1H,s, ar–OH), 12.106 (1H,s, HC=O). 13C NMR (DMSO–d6, δ ppm): 56.561(C, s,OCH3), 115.126 (2CH, arCH), 124.055(CH, arCH), 129.416(2CH, arCH), 130.506(C, arC), 131.318(C, arC–N=N), 132.012(C, arC–N=N), 151.535(arC, arC–OH), 153.411(arC, arC–OCH3), 157.426(arC, arC–OH), 179.26(C, HC=O). Mass Spectrum [ESI]: m/z 272.11(Found) [L + H]+General procedure for the synthesis of the desired azo-azomethine (4)The general method for synthesis of azo-azomethine(4) was used with a slight modification32. In a 250 mL flask, 4-aminobenzoic acid (1 mmol, 1 eq.) was dissolved in boiling ethanol; then, 2-hydroxy-5-((4-hydroxyphenyl)diazenyl)-3-methoxybenzaldehyde (3) (1 mmol 1 eq.) and a catalytic amount of glacial acetic acid (1 ml) were added. The reaction mixture was sonicated until the completed of the reaction (50 min at 60 °C); progress of the reaction was monitored by TLC (ethyl acetate: diethyl ether (1:1), ratio). From the resulting mixture, the solvent was evaporated by rotary evaporation to obtain a solid product and purified by recrystallization in solvent mixture (Scheme 1).Scheme 1Schematic synthesis of desired products.2-hydroxy-5-((Z)-(4-hydroxyphenyl)diazenyl)-3 methoxybenzylidene)amino)benzoic acid (4)Dark-yellow, yield 82.21%, m.p. 98.5 °C, IR cm−1: 3466.42(OH, Ar–OH), 2841.71–2937.99(–OCH3, asymmetric stretching), 1651.86(–COOH, Ar–C=O) 1617.63 (C=N), 1548.52 (C=C aromatic), 1465.61 (N=N); 785.79 (aromatic substituted). ).;1H NMR (DMSO–d6, δ ppm): 3.866(3H,s,OCH3), 6.929–6.955 (2H,d, arH, J = 13 Hz),7.117 (H,s, arH,), 7.249–7.256 (2H,d, arH, J = 3.5 Hz), 7.276–7.282 (2H, d, arH, J = 3 Hz), 7.353–7.382 (2H,d, arH, J = 2.9 Hz), 7.503 (H,s, arH,), 8.028 (1H,s, HC=N), 8.942(1H,s,phenolic –OH), 9.042(1H,s,phenolic –OH), 10.293(1H,s,carboxylic –OH). 13C NMR (DMSO–d6, δ ppm): 56.559(C, OCH3), 116.466 (CH, arC), 119.682(CH, arC), 120.501 (2CH, arCH), 122.026 (CH, arC), 122.978 (2CH, arCH), 123.160 (2CH, arCH), 130.214 (C, arC), 131.239 (2CH, arCH), 130.214 (C, arC), 131.239 (C, arC), 148.833 (C, arC–OCH3), 151.192 (C, arC–OH), 160.764 (C, arC–N=CH), 168.644 (CH, HC=N), 177.151(C, arC–OH), 192.392(C, arCOOH). HRMS (ESI) (Cal): for C21H17N3O5 [M + H] +, m/z: 391.12 (100.0%), 392.12 (22.7%), 393.12 (2.5%), 392.11 (1.1%), 393.12 (1.0%):(found): 391.114 (100.0%), 392.118 (18.4%), 393.123(4.5%), 394.127(1.6%).Synthesis of doped solid polymers (AA(1–3)/PANI/HClIn a 250 mL conical flask, aniline (0.01 mol, 1 mL), (5 × 10–4 M, 1.25 × 10–4 M and 6.25 × 10–4 M separately) of azo-azomethine (4) and HCl (125 mL) were added and cooled to 0–5 °C (solution A). To the solution A added solution B was prepared from (NH4)2S2O8 (2.5 g, 0.011 mol, 25 ml D.W) in 30 mints with stirred vigorously. Continue string mixture solution at 0–5 °C for 4 h then overnight stirring at room temperature. Filtrate the green to orange colour precipitate product and washed with D.W frequently then by small amount of acetone one time. Keep the product in a vacuum oven for 24 h at 55 °C25 (Table 1).Table 1 The composition of azo-azomethine/PANI/HCl systems.Electrochemical characterizationCyclic voltammetry measurements were performed with a μStat-i 400 s Potentiostat/Galvanostat/Impedance Analyzer connected with a three-electrode. The working electrode was glassy carbon, counter electrode was graphite electrode, and saturated electrode, calomel electrode (SCE) with 0.1 M KCl was Saturated electrode used as a reference electrode. electrochemical impedance spectroscopy used to study of charge transfer process of azo-azomethine/PANI/HCl doped polymers. In an electrolytic cell made of 0.1 M KCl/Ethanol, impedance was carried in a three-point electrode akin to the one used in CV at a frequency of 0.01–105 Hz and an AC amplitude of 5 mV at ambient temperature. The working electrode was doped polymers by casting the DMF solution of doped polymers on Glassy carbon electrode with doped polymer film. Dimensions are radius (r) = 0.15 cm, thickness (t) = 0.03539 cm. Nyquist plots were extracted from this spectroscopy.Spectroscopy, thermal and structural characterization techniquesFTIR spectra of the polymers were obtained by using a Thermo Scientific’s Nicolet iS50 series. UV–vis data of the DMSO polymer solution collected by using UV/Visible Scanning Spectrophotometers of the Jenway 67 Series in the range of 250–900 nm. With a TESCAN accelerating potential of 30 kV (MIRA III, Czech Republic), FESEM observations of doped polymers were made. The structure of doped polymers were characterized using a TEM (JEOL 2010). Thermal investigations of doped polymers were carried out in a zero air and argon atmosphere using a TA Instruments- Q600 (USA) (20–800 °C at 10 °C/ min. For characterization of synthesized azo dye (3) and azo-azomethine (4) utilizing a DMSO-d6 BRUKER AVENE 500 MHz NMR spectrometer, 1H and 13C-NMR have been identified. Internal standard (Me4Si) is used to represent the chemical changes in ppm. Hz is applied to J values and HRMS data was gathered using an Agilent LC/MSD TOF mass spectrometer. The XRD patterns of polymers in solid form recorded by using an X-ray diffractometer (PHILIPS, PW1730 Company, Netherlands). At room temperature, measurements were performed using copper-target K radiation (λ = 1.54056 nm). The scanning rate from 5° to 90° (2θ) was 1°/min, with 0.05-degree step size. Voltage = 40 kV, current = 30 mA. The XY plot (2θ vs. intensity) was also generated using Origin 2021. Smoothing was achieved through adjacent averaging involving 50 points. Subsequently, a linear baseline was subtracted to normalize Y to zero. Following this, a Gaussian fitting was applied to identify and measure the areas and full width at half maximum (FWHM) of multiple peaks. The degree of crystallinity (C%) was determined by calculating the ratio of the area of crystalline peaks to the total peak area. Additionally, the d-spacing (D) corresponding to the most intense crystalline peak was calculated using the Debye–Scherrer (powder) method based on Bragg’s relation33,34.$$n\lambda =2D\mathit{sin}\theta$$
(1)
Here, n is an integer, \(\lambda\) represents the wavelength of the X-ray, specifically 1.54056 Å for Cu target, and \(\theta\) denotes the angle between the incident and reflected rays. The crystallite size (T) of the most intense crystalline peak was calculated using the Scherrer equation34.$$T=\frac{k\lambda }{B\mathit{cos}\theta }$$
(2)
where K stands for the shape factor representing the average crystallite (∼ 0.9), and B denotes the full width at half maximum of the crystalline peak in radians. The inter-chain separation length (R) corresponding to the most intense crystalline peak was determined using the relationship formulated by Klug and Alexander35.$$R=\frac{5\lambda }{8\mathit{sin}\theta }$$
(3)