No abstract

For redox active organic molecules (ROMs) used in grid-scale energy storage applications, such as redox flow batteries, solubility is an essential physicochemical property. Specifically, solubility is directly proportional to the volumetric energy density of an energy storage device and thus affects its corresponding spatial footprint. Recently pyridiniums have been introduced as a class of ROMs with high persistence in multiple redox states at low potentials. Unfortunately, solubility of pyridinium salts in non-aqueous media remains low (generally less than 1 M), and relatively few practical molecular design strategies exist for generalized improvement of ROM solubility. Herein, we convey the extent to which discrete, attractive interactions between C-H groups and the p-electrons of an aromatic ring (C-H···pi interactions) can describe the solubility of N-substituted pyridinium salts in a non-aqueous solvent (acetonitrile). We find a direct correlation between the extent of crystalline C-H···pi interactions for each pyridinium salt and its solubility in acetonitrile (R2 = 0.93, solubility range = 0.3 – 2.1 M). The presence of C-H···pi interactions reveals how large disparities in solubility between (e.g.) N-(p-tolyl)-4-phenyl-2,6-dimethylpyridinium (0.32 ± 0.03 M) and N-(p-tolyl)-4-(p-tolyl)-2,6-dimethylpyridinium (1.06 ± 0.03 M) tetrafluoroborate may arise despite differing in structure by only three atoms. The correlation presented in this work highlights a surprising consequence of disrupting strong electrostatic interactions with weak dispersion interactions, showing how minimal structural change can have dramatic effects on ROM solubility.

Successful application of emerging bioelectrocatalysis technologies depends upon an efficient electrochemical interaction between redox enzymes as biocatalysts and conductive electrode surfaces. One approach to establishing such enzyme-electrode interfaces utilizes small redox-active molecules to act as electron mediators between an enzyme active site and electrode surface. While redox mediators have been successfully used in bioelectrocatalysis applications ranging from enzymatic electrosynthesis to enzymatic biofuel cells, they are often selected using a guess-and-check approach. Herein, we identify structure-function relationships in redox mediators that describe the bimolecular rate constant for its reaction with a model enzyme, glucose oxidase (GOx). Based on a library of quinone-based redox mediators, a quantitative structure-activity relationship (QSAR) model is developed to describe the importance of mediator redox potential and projected molecular area as two key parameters for predicting the activity of quinone/GOx-based electroenzymatic systems. Additionally, rapid scan stopped-flow spectrophotometry was used to provide fundamental insights into the kinetics and the stoichiometry of reactions between different quinones and the flavin adenine dinucleotide (FAD+/FADH2) cofactor of GOx. This work provides a critical foundation for both designing new enzyme-electrode interfaces and understanding the role that quinone structure plays in altering electron flux in electroenzymatic reactions.

Redox flow batteries (RFBs) are a strong candidate for grid-scale energy storage applications. Recent pursuits for chemical systems involve focus on organic species, due to their chemical abundance, and non-aqueous solvent systems, due to an expanded electrochemical stability window. Currently, RFBs suffer from limitations that prevent them from being economically competitive when scaled. Among the critical properties hindering RFB expansion, one limitation is low system energy densities. The energy density of a RFB system is dependent on voltage, electrons transferred, and concentration of the anolyte and catholyte. Electrolyte advancements have focused on optimizing energy density by targeting species that maximize divergence of anolyte/catholyte redox potentials, increase species solubility, and feature reversible multi-electron transfer. Strategic structural engineering of redox-active materials is necessary to tune these distinct qualities. Understanding the relationship between molecular design and these variables, and then developing strategies to predict structures with optimal characteristics could help identify promising electrolyte candidates. Our work is focused on understanding and predicting the solubility trends of pyridinium anolyte materials. The pyridiniums in this series all feature low reduction potentials and a broad range of solubility in acetonitrile. By carefully aligning experimental data to DFT and other modeled parameters we are investigating the predictive parameters involved in controlling electrolyte solubility.

Enzymatic bioelectrocatalysis involves the application of enzymes to facilitate the conversion of chemical to electrical energy. This approach has been widely applied in the fields of biosensing, biofuel cells, and to a lesser degree the synthesis of fine chemicals. While direct electrochemical communication between the enzymes and electrodes is generally preferable, it is often the case that efficient electron transfer rates can only be achieved when redox mediators are employed to shuttle electrons reversibly from the redox site of the enzyme to the electrode interface. Unfortunately, there still exist many oxidoreductases for which no effective exogenous mediator is known. This may be due to insufficient understanding of the role that molecular structure of the mediator plays in determining its ability to facilitate electron transfer to a given enzyme. Consequently, selection and/or design of novel redox mediators remains a challenge that is often accomplished using a “guess and check” approach to maximize electron transfer rates. Developing a rapid, and convenient method of predicting the role played by other structural and chemical features beyond redox potential of a mediator is an important goal in advancing mediated bioelectrocatalysis. This talk will describe our recent efforts in identifying structure - function relationships of redox mediators that control efficient electron transfer rates. I will highlight results from stopped-flow spectrophotometry experiments which review the nature of electron transfer events between quinone redox couples in altering bioelectrocatalytic activity.