Spin Signature of the C₆₀ Fullerene Anion: A Combined X- and D‑Band EPR and DFT Study

Fullerenes attract much attention in various scientific fields, but their electronic properties are still not completely understood. Here we report on a combined EPR and DFT study of the fullerene anion C₆₀^– in solid glassy environment. DFT calculations were used to characterize its electronic structure through spin density distribution and magnetic resonance parameters. The electron spin density is not uniformly distributed throughout the C₆₀^– cage but shows a pattern similar to PC₆₁BM^–. EPR spectroscopy reveals a rhombic g-tensor sensitive to the environment in the frozen glassy solutions, which can be rationalized by deformation of the fullerenes along low-frequency vibrational modes upon cooling. DFT modeling confirms that these deformations lead to variation in the C₆₀^– <i>g</i> values. The decrease in g-tensor anisotropy with sample annealing is related to the lessening of g-tensor strain upon temperature relaxation of the most distorted sites in the glassy state

Electronic Structure of Fullerene Acceptors in Organic Bulk-Heterojunctions: A Combined EPR and DFT Study

Organic photovoltaic (OPV) devices are a promising alternative energy source. Attempts to improve their performance have focused on the optimization of electron-donating polymers, while electron-accepting fullerenes have received less attention. Here, we report an electronic structure study of the widely used soluble fullerene derivatives PC₆₁BM and PC₇₁BM in their singly reduced state, that are generated in the polymer:fullerene blends upon light-induced charge separation. Density functional theory (DFT) calculations characterize the electronic structures of the fullerene radical anions through spin density distributions and magnetic resonance parameters. The good agreement of the calculated magnetic resonance parameters with those determined experimentally by advanced electron paramagnetic resonance (EPR) allows the validation of the DFT calculations. Thus, for the first time, the complete set of magnetic resonance parameters including directions of the principal <i>g</i>-tensor axes were determined. For both molecules, no spin density is present on the PCBM side chain, and the axis of the largest <i>g</i>-value lies along the PCBM molecular axis. While the spin density distribution is largely uniform for PC₆₁BM, it is not evenly distributed for PC₇₁BM

The Hydrogen Catalyst Cobaloxime: A Multifrequency EPR and DFT Study of Cobaloxime’s Electronic Structure

Solar fuels research aims to mimic photosynthesis and devise integrated systems that can capture, convert, and store solar energy in the form of high-energy molecular bonds. Molecular hydrogen is generally considered an ideal solar fuel because its combustion is essentially pollution-free. Cobaloximes rank among the most promising earth-abundant catalysts for the reduction of protons to molecular hydrogen. We have used multifrequency EPR spectroscopy at X-band, Q-band, and D-band combined with DFT calculations to reveal electronic structure and establish correlations among the structure, surroundings, and catalytic activity of these complexes. To assess the strength and nature of ligand cobalt interactions, the BF₂-capped cobaloxime, Co­(dmgBF₂)₂, was studied in a variety of different solvents with a range of polarities and stoichiometric amounts of potential ligands to the cobalt ion. This allows the differentiation of labile and strongly coordinating axial ligands for the Co­(II) complex. Labile, or weakly coordinating, ligands such as methanol result in larger <i>g</i>-tensor anisotropy than strongly coordinating ligands such as pyridine. In addition, a coordination number effect is seen for the strongly coordinating ligands with both singly ligated LCo­(dmgBF₂)₂ and doubly ligated L₂Co­(dmgBF₂)₂ . The presence of two strongly coordinating axial ligands leads to the smallest <i>g</i>-tensor anisotropy. The relevance of the strength of the axial ligand(s) to the catalytic efficiency of Co­(dmgBF₂)₂ is discussed. Finally, the influence of molecular oxygen and formation of Co­(III) superoxide radicals LCo­(dmgBF₂)₂O₂^• is studied. The experimental results are compared with a comprehensive set of DFT calculations on Co­(dmgBF₂)₂ model systems with various axial ligands. Comparison with experimental values for the “key” magnetic parameters such as <i>g</i>-tensor and ⁵⁹Co hyperfine coupling tensor allows the determination of the conformation of the axially ligated Co­(dmgBF₂)₂ complexes. The data presented here are vital for understanding the influence of solvent and ligand coordination on the catalytic efficiency of cobaloximes

The Hydrogen Catalyst Cobaloxime: A Multifrequency EPR and DFT Study of Cobaloxime’s Electronic Structure

Solar fuels research aims to mimic photosynthesis and devise integrated systems that can capture, convert, and store solar energy in the form of high-energy molecular bonds. Molecular hydrogen is generally considered an ideal solar fuel because its combustion is essentially pollution-free. Cobaloximes rank among the most promising earth-abundant catalysts for the reduction of protons to molecular hydrogen. We have used multifrequency EPR spectroscopy at X-band, Q-band, and D-band combined with DFT calculations to reveal electronic structure and establish correlations among the structure, surroundings, and catalytic activity of these complexes. To assess the strength and nature of ligand cobalt interactions, the BF₂-capped cobaloxime, Co­(dmgBF₂)₂, was studied in a variety of different solvents with a range of polarities and stoichiometric amounts of potential ligands to the cobalt ion. This allows the differentiation of labile and strongly coordinating axial ligands for the Co­(II) complex. Labile, or weakly coordinating, ligands such as methanol result in larger <i>g</i>-tensor anisotropy than strongly coordinating ligands such as pyridine. In addition, a coordination number effect is seen for the strongly coordinating ligands with both singly ligated LCo­(dmgBF₂)₂ and doubly ligated L₂Co­(dmgBF₂)₂ . The presence of two strongly coordinating axial ligands leads to the smallest <i>g</i>-tensor anisotropy. The relevance of the strength of the axial ligand(s) to the catalytic efficiency of Co­(dmgBF₂)₂ is discussed. Finally, the influence of molecular oxygen and formation of Co­(III) superoxide radicals LCo­(dmgBF₂)₂O₂^• is studied. The experimental results are compared with a comprehensive set of DFT calculations on Co­(dmgBF₂)₂ model systems with various axial ligands. Comparison with experimental values for the “key” magnetic parameters such as <i>g</i>-tensor and ⁵⁹Co hyperfine coupling tensor allows the determination of the conformation of the axially ligated Co­(dmgBF₂)₂ complexes. The data presented here are vital for understanding the influence of solvent and ligand coordination on the catalytic efficiency of cobaloximes