Fabrizia Fabrizi de Biani

The prediction of ground-state redox potentials by quantum chemical methods has a prominent role in the rational design of novel organic photosensitizers both for dye-sensitized solar cells (DSSCs) and photocatalytic systems for the production of H₂. Indeed, the ground-state redox potential of the photosensitizers is one of the key parameters to identify the most promising candidates for such applications. Here, the ground-state redox potentials of 16 organic donor-π-acceptor D-π-A and donor-acceptor-π-acceptor D-A-π-A dyes having a medium to large size of the conjugated scaffold are evaluated, using the methods of the Density Functional Theory (DFT), in terms of free energy differences between their neutral and oxidized ground-state forms. These results are compared to the available experimental data and to the computed highest occupied molecular orbital energy −ε(HOMO) values as an approximation of ground-state redox potentials according to Koopmans’ theorem. Using the MPW1K functional in combination with the 6-31+G* basis set, the strategy based on the free energy cycle, including solvent effects, reproduces with a good level of accuracy the observed values (mean absolute error (MAE) < 0.2 eV) and trend of redox potentials within related families of dyes. On the other hand, the −ε(HOMO) values are only able to capture the experimental trends in redox potential values.

Icosahedral boranes, carboranes, and metallacarboranes are extraordinarily robust compounds with desirable properties such as thermal and redox stability, chemical inertness, low nucleophilicity, and high hydrophobicity, making them attractive for several applications such as medicine, nanomaterials, molecular electronics, energy, catalysis, environmental chemistry, and other areas. The hydrogen atoms in these clusters can be replaced by convenient groups that open the way to a chemical alternative to conventional “organic” or “organometallic” realms. Icosahedral boron cluster derivatives have been reviewed from different perspectives; however, there is a need for a review dedicated to the redox and photophysical characteristics of easily accessible borane and carborane derivatives, which are excellent materials for a wide range of applications. This review deals with the redox properties and photoluminescence behavior of this collection of compounds, as well as their influence on the properties of materials and devices whose working principles are related to electron-transfer or electron-promotion phenomena. We hope that this review will be of great value to boron cluster scientists and researchers working in the photoluminescence and electrochemistry fields who are interested in exploring the possibilities of these unique and promising systems.

The reaction of the bromide complexes [CpCo(μ‐1,3‐C₃B₂Me₅)MBr₂]₂ [M = Rh (1), Ir (2); Cp = cyclopentadienyl] with AgBF₄ in acetonitrile affords the tris(acetonitrile) μ‐diborolyl triple‐decker complexes [CpCo(μ‐1,3‐C₃B₂Me₅)M(MeCN)₃]²⁺ [Rh (3), Ir (4)]. The labile nitromethane solvates [CpCo(μ‐1,3‐C₃B₂Me₅)M(MeNO₂)₃]²⁺, generated in a similar way, react with benzene and its methylated derivatives to give the arene triple‐decker complexes [CpCo(μ‐1,3‐C₃B₂Me₅)M(arene)]²⁺ [M = Rh (5), Ir (6); arene = C₆H₆ (a), 1,2,4,5‐Me₄C₆H₂ (b), C₆Me₆ (c)]. The structures of 5b(BF₄)₂, 5c(BF₄)₂, 6b(BF₄)₂ and 6c(BF₄)₂ were determined by X‐ray diffraction. The electron‐transfer ability of the arene complexes was ascertained by electrochemical techniques. In general, they are able to undergo two separate one‐electron reductions reversibly. DFT calculations revealed structural changes caused by redox processes and satisfactorily predicted the redox potentials. The second reduction is accompanied by a η⁶ → η⁴ hapticity change of the arene ligand. Energy decomposition analysis revealed that the Rh–benzene bond in cation 5a is weaker than in cyclopentadienyl analogues [(C₅R₅)Rh(C₆H₆)]²⁺; however, 5a proved to be the least reactive in benzene replacement with acetonitrile and mesitylene.

The redox chemistry of aryl alcohols, or phenols, is of great interest because of their involvement in important biological and industrial processes. Phenol and its substituted analogs are facile one‐electron reducing agents that are often involved in electrochemical, photochemical and radiation chemical electron transfer reactions. For these and many other reasons the interest of chemists in the metal phenolates has always been lively and a huge number of these complexes are known. In this chapter we describe the redox behavior of the more restricted set of homoleptic aryloxide complexes. The peculiarities of phenolates as ligands make them usually unable to stabilize a metal in different redox states and, very often, reduction of the metal is accompanied by the release of one or more of the ligands and/or by the formation of polynuclear compounds. This may give rise to a complicated pattern in the redox chemistry of these complexes, but may also open the way to a rich, redox‐driven, chemistry of this class of compounds, which, apparently, is still largely unexplored.