Carbonyl-Based Redox-Active Compounds as Organic Electrodes for Batteries: Escape from Middle-High Redox Potentials and Further Improvement?

Extracting─from the vast space of organic compounds─the best electrode candidates for achieving energy material breakthrough requires the identification of the microscopic causes and origins of various macroscopic features, including notably electrochemical and conduction properties. As a first guess of their capabilities, molecular DFT calculations and quantum theory of atoms in molecules (QTAIM)-derived indicators were applied to explore the family of pyrano[3,2-b]pyran-2,6-dione (PPD, i.e., A0) compounds, expanded to A0 fused with various kinds of rings (benzene, fluorinated benzene, thiophene, and merged thiophene/benzene). A glimpse of up-to-now elusive key incidences of introducing oxygen in vicinity to the carbonyl redox center within 6MRs─as embedded in the A0 core central unit common to all A-type compounds─has been gained. Furthermore, the main driving force toward achieving modulated low redox potential/band gaps thanks to fusing the aromatic rings for the A compound series was discovered.

Seeking for Optimal Excited States in Photoinduced Electron-Transfer Processes─The Case Study of Brooker's Merocyanine.

Material design enters an era in which control of electrons in atoms, molecules, and materials is an essential property to be predicted and thoroughly understood in view of discovering new compounds with properties optimized toward specific optical/optoelectronic applications. π-electronic delocalization and charge separation/recombination enter notably into the set of features that are highly desirable to tailor. Diverse domains are particularly relying on photoinduced electron-transfer (PET), including fields of paramount importance such as energy production through light-harvesting, efficient chemoreceptive sensors, or organic field-effect transistors. In view of completing the arsenal of strategies in this area, we selected Brooker's merocyanine─a typical [D-π-A] compound─as the case study and examined from time-dependent density functional theory the opportunity offered by selected excited states to reach a suited manipulation of the charge transfer (CT) extent. In addition to the consideration of diagnostic tools able to spot the charge amount (i.e., magnitude of electron fraction) transferred upon excitation (qCT), the spatial extent associated with such an electronic transition or CT length (DCT), as well as the corresponding variation in dipole moment between the ground and the excited states (µCT), further analysis of the excitation process was undertaken. The advantage of going beyond the above-mentioned molecular indicators─which can be considered as PET global indices─was explored on the basis of a partitioning of the electron density. Relevant insight was gained on the relation these global indices have with the evolution of (local) features characterizing either chemical bond or electron delocalization upon vertical excitations.

Atomic Group Decomposition of Charge Transfer Excitation Global Indexes.

A model for decomposing the Le Bahers, Adamo, and Ciofini Charge Transfer (CT) Excitations global indexes ( J. Chem. Theory Comput. 2011, 7, 2498-2506) into molecular subdomains contributions is presented and a software, DOCTRINE (atomic group Decomposition Of the Charge TRansfer INdExes) for the implementation of this novel model has been coded. Although our method applies to any fuzzy or to any disjoint exhaustive partitioning of the real space, it is here applied using a definition of chemically relevant molecular subdomains based on the Atoms in Molecules Bader basins. This choice has the relevant advantage of associating intra or inter subdomain contributions to rigorously defined quantum objects, yet bearing a clear chemical meaning. Our method allows for a quantitative evaluation of the subdomain contributions to the charge transfer, the charge transfer excitation length and the dipole moment change upon excitation. All these global indexes may be obtained either from the electron density increment or the electron density depletion upon excitation. However, the subdomain contributions obtained from the two distributions generally differ, therefore allowing to distinguish whether the contribution to a given property of a given subdomain is dominated by one of the two distributions or if both are playing a significant role. As a toy system for the first application of our model, a typical [D-π-A, π = conjugated bridge] compound belonging to the merocyanine dyes family is selected, and the first four excited states of this compound in a strongly polar protic solvent and in a weakly polar solvent are thoroughly investigated.

Breaking the Structure of Liquid Hydrogenated Alcohols Using Perfluorinated tert-Butanol: A Multitechnique Approach (Infrared, Raman, and X-ray Scattering) Analyzed by DFT and Molecular Dynamics Calculations.

The state of aggregation at room temperature of tert-butanol (TBH) and perfluoro tert-butanol (TBF) liquid mixtures is assessed by vibrational spectroscopy (Raman and infrared) and X-ray diffraction and analyzed using density functional theory (DFT) and molecular dynamics (MD) simulations. It is shown that larger clusters (mostly tetramers) of TBH are destroyed upon dilution with TBF. Small oligomers, monomers, and mainly heterodimers are present at the equimolar concentration. At variance with slightly interacting solvents, the signature of hetero-oligomers is shown by the appearance of a new broad band detected in the infrared region. The same spectral observation is detected for mixtures of other hydrogenated alcohols (methanol and 1-butanol). The new infrared feature is unaffected by dilution in a polar solvent (CDCl3) in a high-concentration domain, allowing us to assign it to the signature of small hetero-oligomers. MD simulations are used to assess the nature of the species present in the mixture (monomers and small hetero-oligomers) and to follow the evolution of their population upon the dilution. Combining MD simulations with DFT calculations, the infrared spectral profile is successfully analyzed in equimolecular mixtures. This study shows that TBF is a structure breaker of hydrogen-bonded alcohol networks and that the TBF (donor)-TBH (acceptor) heterodimer is the dominant species in an extended range of concentration, centered in the vicinity of the equimolar fraction.

A tool for deciphering the redox potential ranking of organic compounds: a case study of biomass-extracted quinones for sustainable energy.

Carbonyl compounds have emerged as promising organic electrodes for sustainable energy storage. Accelerating the process of performant materials discovery relies on the possibility of developing methodologies to enable scanning of various sets of candidates. The genesis of this educated guess strategy must be privileged to reduce the search space of experiments, accelerate this research area and contribute to sustainable effort. To address this challenge, we built a quantitative structure-activity relationship to unveil the origin of the redox potential magnitude as a function of both structural features and complexation effects. The potential of this prediction model was demonstrated on various ortho-quinones directly derived from naturally occurring catechols. In addition to the modulation provided by substituent changes, the possibility of applying various types of alkaline(-earth)-ion electrochemistry was examined thoroughly. The power of partitioning the total molecular energy into additive atomic group contributions is highlighted, and the construction of this robust strategy provides guidance towards rational selection of the most suitable compound/metal-ion couples. An upshift/downshift of the redox potential by switching from Li to Mg/Na is revealed, while the identification of the relative role played by the various components of the systems as well as electrostatic interactions is clearly identified. These results, particularly the evidence of the different substituent effects on the single/double reduction potentials and as a function of the type of electrochemistry (Li/Na/Mg), have important implications for designing new electroactive compounds with tailored redox properties.

Understanding chemical reactions of CO2 and its isoelectronic molecules with 1-butyl-3-methylimidazolium acetate by changing the nature of the cation: the case of CS2 in 1-butyl-1-methylpyrrolidinium acetate studied by NMR spectroscopy and density functional theory calculations.

NMR spectroscopy ((1)H, (13)C, (15)N) shows that carbon disulfide reacts spontaneously with 1-butyl-1-methylpyrrolidinium acetate ([BmPyrro][Ac]) in the liquid phase. It is found that the acetate anions play an important role in conditioning chemical reactions with CS2 leading, via coupled complex reactions, to the degradation of this molecule to form thioacetate anion (CH3COS(-)), CO2, OCS, and trithiocarbonate (CS3 (2-)). In marked contrast, the cation does not lead to the formation of any adducts allowing to conclude that, at most, its role consists in assisting indirectly these reactions. The choice of the [BmPyrro](+) cation in the present study allows disentangling the role of the anion and the cation in the reactions. As a consequence, the ensemble of results already reported on CS2-[Bmim][Ac] (1), OCS-[Bmim][Ac] (2), and CO2-[Bmim][Ac] (3) systems can be consistently rationalized. It is argued that in system (1) both anion and cation play a role. The CS2 reacts with the acetate anion leading to the formation of CH3COS(-), CO2, and OCS. After these reactions have proceeded the nascent CO2 and OCS interact with the cation to form imidazolium-carboxylate ([Bmim] CO2) and imidazolium-thiocarboxylate ([Bmim] COS). The same scenario also applies to system (2). In contrast, in the CO2-[Bmim] [Ac] system a concerted cooperative process between the cation, the anion, and the CO2 molecule takes place. A carbene issued from the cation reacts to form the [Bmim] CO2, whereas the proton released by the ring interacts with the anion to produce acetic acid. In all these systems, the formation of adduct resulting from the reaction between the solute molecule and the carbene species originating from the cation is expected. However, this species was only observed in systems (2) and (3). The absence of such an adduct in system (1) has been theoretically investigated using DFT calculations. The values of the energetic barrier of the reactions show that the formation of [Bmim] CS2 is unfavoured and that the anion offers a competitive reactive channel via an oxygen-sulphur exchange mechanism with the solute in systems (1) and (2).

On the chemical reactions of carbon dioxide isoelectronic molecules CS2 and OCS with 1-butyl-3-methylimidazolium acetate.

Raman and NMR spectroscopies show that CS2 and OCS react spontaneously with 1-butyl-3-methylimidazolium acetate [C4mim] [Ac] in the liquid phase. The formation of [C4mim] CO2, [C4mim] COS, CH3COS(-) and gaseous CO2 and OCS in both systems demonstrates that the anion plays an unexpected role not observed in the CO2-[C4mim] [Ac] reaction.

CO2 in 1-butyl-3-methylimidazolium acetate. 2. NMR investigation of chemical reactions.

The solvation of CO(2) in 1-butyl-3-methylimidazolium acetate (Bmim Ac) has been investigated by (1)H, (13)C, and (15)N NMR spectroscopy at low CO(2) molar fraction (mf) (x(CO(2)) ca. 0.27) corresponding to the reactive regime described in part 1 of this study. It is shown that a carboxylation reaction occurs between CO(2) and Bmim Ac, leading to the formation of a non-negligible amount (~16%) of 1-butyl-3-methylimidazolium-2-carboxylate. It is also found that acetic acid molecules are produced during this reaction and tend to form with elapsed time stable cyclic dimers existing in pure acid. A further series of experiments has been dedicated to characterize the influence of water traces on the carboxylation reaction. It is found that water, even at high ratio (0.15 mf), does not hamper the formation of the carboxylate species but lead to the formation of byproduct involving CO(2). The evolution with temperature of the resonance lines associated with the products of the reactions confirms that they have a different origin. The main byproduct has been assigned to bicarbonate. All these results confirm the existence of a reactive regime in the CO(2)-Bmim Ac system but different from that reported in the literature on the formation of a reversible molecular complex possibly accompanied by a minor chemical reaction. Finally, the reactive scheme interpreting the carboxylation reaction and the formation of acetic acid proposed in the literature is discussed. We found that the triggering of the carboxylation reaction is necessarily connected with the introduction of carbon dioxide in the IL. We argue that a more refined scheme is still needed to understand in details the different steps of the chemical reaction in the dense phase.

On the spontaneous carboxylation of 1-butyl-3-methylimidazolium acetate by carbon dioxide.

The formation of 1-butyl-3-methylimidazolium-2-carboxylate in the mixture of CO(2) with 1-butyl-3-methylimidazolium acetate under mild conditions (298 K, 0.1 MPa) has been put in evidence in the liquid phase using Raman and infrared spectroscopy complemented by DFT calculations and NMR ((1)H, (13)C, (15)N) spectroscopy.

Imidazolium-based ionic liquids: quantitative aspects in the far-infrared region.

The optical constants of some imidazolium-based ionic liquids (ILs) are determined in the mid- and far-infrared regions by combining polarized attenuated total reflection (ATR) and transmittance spectra. The internal vibrations of the cations and anions and the interionic vibrations can thus be quantitatively evaluated. A comparison of the far-IR spectral response of several imidazolium derivatives associated with the (CF(3)SO(2))(2)N(-) anion shows that methylation of the more acidic C((2))H imidazolium group does not change the far-IR intensity and hence that the CH...anion hydrogen bonds play a negligible role compared with electrostatic interactions. The calculated spectra of ion-pair dimers reproduce the far-IR density of states better than those of simple ion pairs.