This project explored synthesis of high-oxidation-potential organic compounds and understanding their stability in different battery environments. Systematic functionalization of phenothiazine (PT) core led us to develop derivatives with different oxidation potentials by incorporation of electro-donating and withdrawing substituents (Figure 1).         We have synthesized phenothiazine derivatives containing -F and -CF₃ substituents to enhance their oxidation potentials. 

We synthesized perfluorinated PT derivative (OFMEEPT, figure 2) with a methoxyethoxy ethyl group at N atom. OFMEEPT is a liquid at RT which is miscible in acetonitrile and has an oxidation potential of 1.08 V vs. Fc/Fc⁺(4.4 V vs. Li), the PT derivative with highest oxidation potential. Its charged form was isolated as SbCl₆^- salt (Figure 2). The stability of the neutral and charged forms was studied in acetonitrile using UV-vis spectroscopy. The neutral form is stable; however, the charged form slowly decays and loses most of the absorption intensity within 24 h. When analyzed using bulk electrolysis, rapid capacity fade was observed. The reason for the instability is still unclear.

Organic radical cations are used in synthetic chemistry as oxidizing catalysts or initiators and they mediate a significant number of transformations to construct C-C and C-X bonds. They are also important intermediates in organic materials science as of their usefulness in electronic and energetic applications. In order for radical cation salts to act as reagents for synthetic transformations, they should have sufficient shelf and solution stability. They can often react with the solvent, electrolyte, or with the intermediates that are formed during the reaction. Radical cation salts of various phenothiazines are synthesized by chemical oxidation using different chemical oxidizing agents such as SbCl₅, NOBF₄, NOPF₆, AgClO₄.xH2O and AgTFSI.^{1, 2} As an example different salts of MEEPT were synthesized (Figure 3) and isolated as solids which were further crystallized to produce crystalline salts. The stability of these radical cation salts was studied in solution and solid state by electrochemical and spectroscopic methods.¹ Evaluating shelf stability is another practical consideration for their use as reagents. Storing these samples at bench-top or inside a glovebox will give an idea of their stability to oxygen and moisture, which may likely affect their long-term stability in the solid state. As an example, MEEPT-BF₄ was freshly synthesized and crystallized, then stored (1) in a glass vial kept on the bench top, (2) in a glass vial wrapped in Al foil and stored in a drawer, (3) in a glass vial kept in an argon-filled glovebox and (4) in a glass vial kept in a black box in an argon-filled glovebox. The UV-vis spectrum of a freshly prepared 10 mM sample was recorded in ACN at 0, 1, 7, 14 and 21 day to to determine its stability in variable environments. The powder or crystalline sample do not show any appreciable decay in absorbance whether they are stored inside or outside the glovebox or in the presence or absence of light. Thus, can be used as shelf stable electron transfer reagents for performing organic transformations.

For objective 3, to explore how changes in oxidation state and potential impact the diffusion and aggregation of these redox-active species, we employed classical molecular dynamics (MD) simulations with a computationally less expensive forcefield. To study the interactions of the redox-active molecule (OFMEEPT) within the electrolyte and at an electrode/electrolyte interface, the MD simulations were carried out with three different systems to capture the difference between electrolyte systems (1) with neutral, (2) with charged, and (3) with no redox active organic molecules. Three layers of graphene placed along the z-axis were used to denote each electrode with 3936 C atoms in each sheet with the dimensions of 100x100 Ų with an interspacing layer distance of 3.4 Å (Figure 4). It was observed that the simulations were able to successfully capture the physics of the system as we can observe that the positively charged molecules drifting away from the high potential graphite layer and negatively charged molecules drifting towards it.  The interfacial region with an approximate thickness of 1.4 nm continues through approx. 3.1 nm to 4.5 nm at the low potential graphite layer (LPGL) through approx. 9.2 nm to 10.6 nm at the high potential graphite layer (HPGL). This region could be a crude representation of the solid-liquid and solid-solid electrochemical interfaces which could make up the electric double layer. The double-layer behavior is further supported by the distribution of the charge density at the proximity of the electrodes. We also computed the bulk and transport properties using MD simulations.

1. A. P. Kaur, O. C. Harris, N. H. Attanayake, Z. Liang, S. R. Parkin, M. H. Tang, S. A. Odom,  Chem. Mater. 2020, 32, (7), 3007-3017.

2. N. H. Attanayake, A. P. Kaur, T. M. Suduwella, C. F. Elliott, S. R. Parkin, S. A. Odom,  New J. Chem. 2020, 44, (42), 18138-18148.

Last Modified: 02/16/2023
Modified by: Aman Preet Kaur