Nature of the Interaction of <i>N</i>,<i>N</i>′‑Diphenyl-1,4-benzoquinonediimine with Iron Oxide Surfaces and Its Mobility on the Same Surfaces

Short chain aniline oligomers are of interest for applications in organic electronics and as corrosion inhibitors for steel, requiring an improved understanding of their interactions with metal oxide films. Here we investigate the interactions of <i>N</i>,<i>N</i>′-diphenyl-1,4-benzoquinonediimine (B2Q1, oxidized form of an aniline dimer) with iron­(III) oxides. B2Q1 transforms into its semiquinone form when interacting with α-Fe₂O₃. The resulting charge transfer between B2Q1 and α-Fe₂O₃ is demonstrated with mid-IR, visible, and Raman spectroscopy. Atomic force microscopy shows the first layer of B2Q1 to be oriented face-on. Thermal analysis also confirms this orientation for submonolayer coverage, whereas molecules start standing up on their edges upon multilayer formation. Thermal analysis shows that the first monolayer of B2Q1 is chemisorbed on the α-Fe₂O₃ surface, and the following multilayers are strongly interacting with each other. The behavior of the oxidized aniline dimer B2Q1 is in stark contrast to its reduced counterpart (DPPD), which also undergoes charge transfer to iron oxide (in opposite direction). B2Q1 interacts more weakly with the surface, causing it to be more mobile. The mobility of B2Q1 provides a clue toward understanding the self-healing behavior of polyaniline corrosion-inhibiting films on steel

Nature of the Interaction of <i>N</i>,<i>N</i>′‑Diphenyl-1,4-phenylenediamine with Iron Oxide Surfaces

Redox-active polymers and small molecules are of great interest in coatings, such as corrosion inhibitors for steel and other metals. In this work the interaction of the redox-active phenyl-capped aniline dimer (<i>N</i>,<i>N</i>′-diphenyl-1,4-phenylenediamine, DPPD) with iron oxide surfaces was investigated with the aim to understand the corrosion inhibition and self-healing properties of polyaniline and aniline oligomers on iron oxide surfaces. Raman, mid-IR, and visible spectroscopies all show that reduced DPPD transforms into the semiquinone form by interacting with α-Fe₂O₃. Thermal gravimetric analysis (TGA) was used to quantify the strength of these interactions, clearly within the chemisorption range. TGA analysis, mid-IR spectroscopy, and atomic force microscopy showed the DPPD molecules to be standing on their edge on the surface and changing their orientation to standing on end upon initiation of multilayer formation. DPPDand hence other reduced oligoanilines or polyanilineare therefore shown to strongly interact with iron oxide surfaces through hydrogen bonding and charge transfer to the surface. A full understanding of coatings will ultimately require the study of all oxidation states and their surface interactions. Here we provide the most detailed understanding to date of the reduced state as a first step

Surface Mobility and Nucleation of a Molecular Switch: Tetraaniline on Hematite

Understanding the dynamics of organic thin film formation is crucial to quality control in organic electronics and smart coatings. We have studied the nucleation and growth of the reduced and the oxidized states of phenyl-capped aniline tetramer (PCAT) deposited on hematite(1000) surfaces by physical vapor deposition. The fully reduced PCAT molecules form 2D islands on the surface, whereas the fully oxidized molecules form 3D islands. Through scaled island size distribution, it was found that the critical island sizes, <i>i</i>, for the reduced and oxidized molecules are <i>i</i> = 4 and 5, respectively. From the dependence of the island density on substrate temperature, the activation energies for the diffusion of the molecules away from the critical cluster were calculated to be 1.30 and 0.55 eV, respectively. At low temperatures, the reduced and the oxidized PCAT molecules form compact islands on the surface. At higher temperatures, the reduced islands become dendritic, whereas the oxidized islands become slightly dendritic. The attempt frequencies for surface diffusion of the reduced and the oxidized islands were estimated to be about 5 × 10²⁵ and 8 × 10¹¹ s^–1, respectively. The former value is in line with the high degree of surface wetting by the reduced PCAT, whereas the latter value shows the higher degree of intermolecular interaction in the fully oxidized PCAT and the low degree of its interaction with the iron oxide surface. Interconversion between oxidized and reduced islands through exposure to a reducing environment, and its impact on island morphology was examined. We also found that the presence of Fe²⁺ defects on the hematite surface did not impact the nucleation and growth of the molecular islands, likely due to a discrepancy in time scale. This study elucidates the interactions between an oligoaniline-based molecular switch (PCAT) and hematite surfaces as a function of molecular oxidation state, with applications in molecular electronics, chemical sensors, and smart coatings

Reagent-Free Quantification of Aqueous Free Chlorine via Electrical Readout of Colorimetrically Functionalized Pencil Lines

Colorimetric methods are commonly used to quantify free chlorine in drinking water. However, these methods are not suitable for reagent-free, continuous, and autonomous applications. Here, we demonstrate how functionalization of a pencil-drawn film with phenyl-capped aniline tetramer (PCAT) can be used for quantitative electric readout of free chlorine concentrations. The functionalized film can be implemented in a simple fluidic device for continuous sensing of aqueous free chlorine concentrations. The sensor is selective to free chlorine and can undergo a reagent-free reset for further measurements. Our sensor is superior to electrochemical methods in that it does not require a reference electrode. It is capable of quantification of free chlorine in the range of 0.1–12 ppm with higher precision than colorimetric (absorptivity) methods. The interactions of PCAT with the pencil-drawn film upon exposure to hypochlorite were characterized spectroscopically. A previously reported detection mechanism relied on the measurement of a baseline shift to quantify free chlorine concentrations. The new method demonstrated here measures initial spike size upon exposure to free chlorine. It relies on a fast charge built up on the sensor film due to intermittent PCAT salt formation. It has the advantage of being significantly faster than the measurement of baseline shift, but it cannot be used to detect gradual changes in free chlorine concentration without the use of frequent reset pulses. The stability of PCAT was examined in the presence of free chlorine as a function of pH. While most ions commonly present in drinking water do not interfere with the free chlorine detection, other oxidants may contribute to the signal. Our sensor is easy to fabricate and robust, operates reagent-free, and has very low power requirements and is thus suitable for remote deployment