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• Oxidation of gallic acid (GA) on gold electrode is examined in a broad pH range. • Oxidation mechanism of GA based on bi-cubic scheme is proposed. • pKa value of the final ortho-quinone product is experimentally determined. • Spectroelectrochemical studies identify the different intermediates and the products. • The kinetics of gallic acid redox process involves a disproportionation reaction. A clear understanding of redox mechanism of antioxidants, like gallic acid (GA) is important to decipher their radical scavenging functioning in the biochemical environment of the body. Herein, electrooxidation process of GA is reexamined in an aqueous medium through a combination of cyclic voltammetric and spectroelectrochemical studies on gold electrode in wide pH and concentration ranges. These studies reveal redox process of GA as an irreversible two electrons process, resulting in the formation of ortho-quinone, where the second electron transfer is the rate limiting step. The redox process of GA at different pH is presented in the form of a bicubic scheme and the associated thermodynamic parameters are analyzed using Laviron's theory. It allows for the first time the determination of the acidity constant of the corresponding ortho-quinone (pK a = 5.33). The number of exchanged electrons and protons depends on the pH conditions, such that (2e−, 2H+) are exchanged at pH < 4.24 and pH > 5.33, while (2e−, 1H+) are exchanged at 4.24 ≤ pH ≤ 5.33. A careful analysis using the theoretical treatment proposed by Amatore and Saveant for an ECE-DISP mechanism demonstrates the existence of a DISP2 mechanism leading to the formation of ortho-quinone through disproportionation of the semiquinone radicals, made possible thanks to their stabilization. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Zinc porphine is electropolymerized into two structural forms (pZnP-1 & pZnP-2) • Monomers are orthogonally linked in pZnP-1 while ZnP rings are fused in 2D in pZnP-2. • pZnP-1 shows low conductivity while pZnP-2 displays high conductivity. • pZnP-1/LuPc 2 device shows interfacial charge transport contrary to bulk transport in pZnP-2/LuPc 2. • Heterojunction sensor based on pZnP-1 detects NH 3 gas below environmental guidelines. Owing to high interfacial conductivity, organic heterostructures hold great promises to augment the electrical performances of electronic devices. In this endeavor, the present work reports fabrication of novel polyporphine/phthalocyanine heterostructures and investigates the modulation of charge transport induced by structural change of polyporphine and its implication on ammonia sensing properties. Polyporphines materials are electrosynthesized by oxidation of zinc(II) porphine monomer that corresponds to the fully unsubstituted porphyrin. At less-positive anodic potential, low conducting meso , meso -singly-linked type-1 polymer (pZnP-1) is formed in which a monomer unit stays orthogonal to its neighbors. At higher anodic potential, monomer units are fused in the 2D plane to produce β , β - meso - meso - β , β -triply-fused type-2 polymer (pZnP-2), having a π-conjugated structure and high conductivity. Association of these polymers in organic heterojunction devices with lutetium bis-phthalocyanine (LuPc 2) reveals non-linear current–voltage (I-V) characteristics typical for interfacial accumulation of charges in the heterostructure for pZnP-1 and a linear I-V behavior for pZnP-2. Characterization of these heterojunctions by impedance spectroscopy further confirms the predominance of interfacial charge transport in pZnP-1/LuPc 2 which is improved with increasing bias, while largely bulk charge transport independent of bias prevails in pZnP-2/LuPc 2 device. Different regimes of charge transport influence ammonia-sensing properties of the devices, such that pZnP-1/LuPc 2 demonstrates highly sensitive, reversible and stable response, while pZnP-2/LuPc 2 shows low and unstable response. [ABSTRACT FROM AUTHOR]