What is the best approach for estimating standard electrochemical potentials, E (0) , from voltammograms that exhibit chemical irreversibility? The lifetimes of the oxidized or reduced forms of the majority of known redox species are considerably shorter than the voltammetry acquisition times, resulting in irreversibility and making the answer to this question of outmost importance. Halfwave potentials, E (1/2) , provide the best experimentally obtainable representation of E (0) . Due to irreversible oxidation or reduction, however, the lack of cathodic or anodic peaks in cyclic voltammograms renders E (1/2) unattainable. Therefore, we evaluate how closely alternative potentials, readily obtainable from irreversible voltammograms, estimate E (0) . Our analysis reveals that, when E (1/2) is not available, inflection-point potentials provide the best characterization of redox couples. While peak potentials are the most extensively used descriptor for irreversible systems, they deviate significantly from E (0) , especially at high scan rates. Even for partially irreversible systems, when the cathodic peak is not as pronounced as the anodic one, the half-wave potentials still provide the best estimates for E (0) . The importance of these findings extends beyond the realm of electrochemistry and impacts fields, such as materials engineering, photonics, cell biology, solar energy engineering and neuroscience, where cyclic voltammetry is a key tool.

Electron-deficient π-conjugated functional dyes lie at the heart of organic optoelectronics. Adding nitro groups to aromatic compounds usually quenches their fluorescence via inter-system crossing (ISC) or internal conversion (IC). While strong electronic coupling of the nitro groups with the dyes ensures the benefits from these electron-withdrawing substituents, it also leads to fluorescence quenching. Here, we demonstrate how such electronic coupling affects the photophysics of acceptor–donor–acceptor fluorescent dyes, with nitrophenyl acceptors and a pyrrolo[3,2-b]pyrrole donor. The position of the nitro groups and the donor-acceptor distance strongly affect the fluorescence properties of the bis-nitrotetraphenylpyrrolopyrroles. Concurrently, increasing solvent polarity quenches the emission that recovers upon solidifying the media. Intramolecular charge transfer (CT) and molecular dynamics, therefore, govern the fluorescence of these nitro-aromatics. While balanced donor-acceptor coupling ensures fast radiative deactivation and slow ISC essential for large fluorescence quantum yields, vibronic borrowing accounts for medium dependent IC via back CT. These mechanistic paradigms set important design principles for molecular photonics and electronics.

Molecular dipoles present important, but underutilized, methods for guiding electron transfer (ET) processes. While dipoles generate fields of Gigavolts per meter in their vicinity, reported differences between rates of ET along versus against dipoles are often small or undetectable. Herein we show unprecedentedly large dipole effects on ET. Depending on their orientation, dipoles either ensure picosecond ET, or turn ET completely off. Furthermore, favorable dipole orientation makes ET possible even in lipophilic medium, which appears counterintuitive for non-charged donor-acceptor systems. Our analysis reveals that dipoles can substantially alter the ET driving force for low solvent polarity, which accounts for these unique trends. This discovery opens doors for guiding forward ET processes while suppressing undesired backward electron transduction, which is one of the holy grails of photophysics and energy science.

Charge transfer and charge transport are by far among the most important processes for sustaining life on Earth and for making our modern ways of living possible. Involving multiple electron-transfer...