Recent Developments on Electroactive Organic Electrolytes for Non-Aqueous Redox Flow Batteries: Current Status, Challenges, and Prospects

The ever-increasing threat of climate change and the depletion of fossil fuel resources necessitate the use of solar- and wind-based renewable energy sources. Large-scale energy storage technologies, such as redox flow batteries (RFBs), offer a continuous supply of energy. Depending on the nature of the electrolytes used, RFBs are broadly categorized into aqueous redox flow batteries (ARFBs) and non-aqueous redox flow batteries (NARFBs). ARFBs suffer from various problems, including low conductivity of electrolytes, inferior charge/discharge current densities, high-capacity fading, and lower energy densities. NARFBs offer a wider potential window and range of operating temperatures, faster electron transfer kinetics, and higher energy densities. In this review article, a critical analysis is provided on the design of organic electroactive molecules, their physiochemical/electrochemical properties, and various organic solvents used in NARFBs. Furthermore, various redox-active organic materials, such as metal-based coordination complexes, quinones, radicals, polymers, and miscellaneous electroactive species, explored for NARFBs during 2012-2023 are discussed. Finally, the current challenges and prospects of NARFBs are summarized.

Tetrathiafulvalenes as anchors for building highly conductive and mechanically tunable molecular junctions

The interface between molecules and electrodes has great impact on charge transport of molecular devices. Precisely manipulating the structure and electronic coupling of electrode-molecule interface at a molecular level is very challenging. Here, we develop new molecular junctions based on tetrathiafulvalene (TTF)-fused naphthalene diimide (NDI) molecules which are anchored to gold electrodes through direct TTF-Au contacts formed via Au-S bonding. These contacts enable highly efficient orbital hybridization of gold electrodes and the conducting π-channels, yielding strong electrode-molecule coupling and remarkably high conductivity in the junctions. By further introducing additional thiohexyl (SHe) anchors to the TTF units, we develop molecular wires with multiple binding sites and demonstrate reversibly switchable electrode-molecule contacts and junction conductance through mechanical control. These findings show a superb electrode-molecule interface and provide a new strategy for precisely tunning the conductance of molecular devices towards new functions.

Design of novel tetra-hetero[8]circulenes: a theoretical study of electronic structure and charge transport characteristics

Tetra-hetero[8]circulenes were theoretically designed and emerged as good candidates for ambipolar organic semiconductors.

Charge transport parameters and structural and electronic properties of octathio[8]circulene and its plate-like derivatives

The first fully heterocyclic circulene very recently isolated, C(16)S(8), was studied by means of high accurate methods, allowing reliable predictions and interpretations of the structural and electronic properties of organic molecules bearing sulfur and selenium atoms. The changes induced by the oxidation process and the S/Se substitution on some of its properties and the infrared (IR) spectra were analyzed, allowing a comprehensive assignment of the bands observed in the case of C(16)S(8). The results confirmed the planarity and a large surface area of C(16)S(8), which remain in C(16)S(4)Se(4) and C(16)Se(4) derivatives, favoring their use for H(2) adsorption. The molecules were shown to have a strong aromatic character, while the IR spectrum of C(16)S(8) was elucidated, toward its possible application for a better understanding of the new class of materials; the IR signal associated to the asymmetric stretching of the CC bonds can be used as a structural signature to identify the neutral from the radical forms whose structural planarity was found to resist against the oxidation process. Some of the electronic and physical properties characterizing good electron-donating (ED) and charge-transporting (CT) capacity such as the frontier molecular orbital energies (E(HOMO), E(LUMO)), the ionization potential (IP), and the reorganization energy (lambda(h)/lambda(e) for hole/electron) were calculated and the influence of the cyclic structure of C(16)S(8) on them discussed. C(16)S(8), C(16)S(4)Se(4), and C(16)Se(4) were found to display a comparable/much lower lambda(h) and higher IP and E(LUMO) than those for some of the already well-known field-effect transistors (FET) materials such as pentacene, anthracene, and DT-TTF; further investigation for this issue is strongly recommended.

Tuning First Molecular Hyperpolarizabilities through the Use of Proaromatic Spacers

In this paper, we describe the second-order nonlinear optical properties of a series of 1,3-dithiole-based electron donor-acceptor systems incorporating proaromatic donor and spacer groups. Modification of the proaromaticity of the quinoid spacer gives rise to NLO-phores with mubeta values ranging from -2000 x 10(-)(48) esu to +3000 x 10(-)(48) esu. Quite surprisingly, compounds with a p-benzoquinoid spacer and a strong acceptor group show negative mubeta values, usually associated to zwitterionic ground states, and yet they are largely quinoid, as evidenced by crystallographic data and theoretical calculations. Progressive benzoannulation of the spacer and introduction of alkylsulfanyl substituents on the dithiole donor unit result in a shift to more positive mubeta values. DFT and ab initio calculations verify these empirical trends.

A Study by Spectroelectrochemical FTIR and Density Functional Theory Calculations of the Reversible Complexing Ability of an Electroactive Tetrathiafulvalene Crown

We report on the study of the electrochemically targeted complexation/expulsion of a metal cation (Ba2+) by a crown ether tetra(thiomethyl)tetrathiafulvalene derivative (crown-TTM-TTF). Real time, in situ FTIR spectroelectrochemistry was used to obtain spectroscopic evidence of this electrochemically triggered phenomenon. Density functional theory calculations allowed the spectral information collected to be assigned. Both experimental and theoretical results clearly show that neutral crown-TTM-TTF complexes well Ba2+. Complexation is evidenced by a significant downshift of the frequency corresponding to the asymmetric stretching of the C-O-C ether groups. Concerning the cation crown-TTM-TTF, the spectroscopic signal of the complex form was difficult to identify, first because of the rather low value of the complexation constant and second because the vibration modes involving the oxygen atoms (which are the most affected by the complexation) were found by calculation to occur in the lower spectral region (<1000 cm(-1)), which is not accessible in our experimental conditions. In the case of the dication crown-TTM-TTF, it is now clear that the complex form does not exist, which means that the electrochemical formation of the dication necessarily involves the expulsion of the barium ion.