4 research outputs found
Spin Signature of the C<sub>60</sub> 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
C<sub>60</sub><sup>–</sup> 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 C<sub>60</sub><sup>–</sup> cage but shows a pattern similar to PC<sub>61</sub>BM<sup>–</sup>. 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 C<sub>60</sub><sup>–</sup> <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 PC<sub>61</sub>BM and PC<sub>71</sub>BM 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 PC<sub>61</sub>BM, it is not evenly
distributed for PC<sub>71</sub>BM
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
BF<sub>2</sub>-capped cobaloxime, Co(dmgBF<sub>2</sub>)<sub>2</sub>, 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(dmgBF<sub>2</sub>)<sub>2</sub> and doubly ligated L<sub>2</sub>Co(dmgBF<sub>2</sub>)<sub>2</sub> . 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(dmgBF<sub>2</sub>)<sub>2</sub> is discussed. Finally, the influence
of molecular oxygen and formation of Co(III) superoxide radicals LCo(dmgBF<sub>2</sub>)<sub>2</sub>O<sub>2</sub><sup>•</sup> is studied.
The experimental results are compared with a comprehensive set of
DFT calculations on Co(dmgBF<sub>2</sub>)<sub>2</sub> model systems
with various axial ligands. Comparison with experimental values for
the “key” magnetic parameters such as <i>g</i>-tensor and <sup>59</sup>Co hyperfine coupling tensor allows the
determination of the conformation of the axially ligated Co(dmgBF<sub>2</sub>)<sub>2</sub> 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
BF<sub>2</sub>-capped cobaloxime, Co(dmgBF<sub>2</sub>)<sub>2</sub>, 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(dmgBF<sub>2</sub>)<sub>2</sub> and doubly ligated L<sub>2</sub>Co(dmgBF<sub>2</sub>)<sub>2</sub> . 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(dmgBF<sub>2</sub>)<sub>2</sub> is discussed. Finally, the influence
of molecular oxygen and formation of Co(III) superoxide radicals LCo(dmgBF<sub>2</sub>)<sub>2</sub>O<sub>2</sub><sup>•</sup> is studied.
The experimental results are compared with a comprehensive set of
DFT calculations on Co(dmgBF<sub>2</sub>)<sub>2</sub> model systems
with various axial ligands. Comparison with experimental values for
the “key” magnetic parameters such as <i>g</i>-tensor and <sup>59</sup>Co hyperfine coupling tensor allows the
determination of the conformation of the axially ligated Co(dmgBF<sub>2</sub>)<sub>2</sub> complexes. The data presented here are vital
for understanding the influence of solvent and ligand coordination
on the catalytic efficiency of cobaloximes