Ivan V. Kurganskii

MaRs is an open-source Python library for the simulation and fitting of electron paramagnetic resonance (EPR) spectra. MaRs supports simulations of field-swept and frequency-swept continuous-wave EPR, accounting for various spin-sublevel polarizations. Moreover, MaRs is capable of simulating complete 2D time-resolved EPR spectra (in field-time domain). The latter can be computed in different levels of theoretical complexity. First, the time evolution can be computed either with kinetic rate equations, which describe population transfer, or in full Liouville space, which captures coherence dynamics. Second, the relaxation pathways in timeresolved simulations can be specified either through intuitive rate parameters that are automatically expanded into the Lindblad master-equation form, or derived microscopically using the Bloch-Redfield formalism. To handle physically complex relaxation scenarios, MaRs provides automatic thermal-balance enforcement and composition operations that allow multicomponent relaxation models to be assembled from simple building blocks. Built on PyTorch (popular platform for machine learning development), MaRs leverages operations on both CPU and CUDA (GPU) backends. We demonstrate MaRs time-resolved EPR capabilities as well the differences and limitations of each built-in approach to time-domain simulations on the examples of coupled triplet-doublet pairs and systems exhibiting the reverse intersystem crossing phenomenon. Separately, we validate stationary EPR simulations across diverse spin systems, including high-spin complexes, achieving excellent agreement with established packages. We also show the possibilities of parameter fitting framework, applicable to all spectrum types implemented in MaRs, that supports multiple optimization strategies and uncertainty quantification.

MaRs is an open-source Python library for the simulation and fitting of electron paramagnetic resonance (EPR) spectra. MaRs supports simulations of field-swept and frequency-swept continuous-wave EPR, accounting for various spin-sublevel polarizations. Moreover, MaRs is capable of simulating complete 2D time-resolved EPR spectra (in field-time domain). The latter can be computed in different levels of theoretical complexity. First, the time evolution can be computed either with kinetic rate equations, which describe population transfer, or in full Liouville space, which captures coherence dynamics. Second, the relaxation pathways in timeresolved simulations can be specified either through intuitive rate parameters that are automatically expanded into the Lindblad master-equation form, or derived microscopically using the Bloch-Redfield formalism. To handle physically complex relaxation scenarios, MaRs provides automatic thermal-balance enforcement and composition operations that allow multicomponent relaxation models to be assembled from simple building blocks. Built on PyTorch (popular platform for machine learning development), MaRs leverages operations on both CPU and CUDA (GPU) backends. We demonstrate MaRs time-resolved EPR capabilities as well the differences and limitations of each built-in approach to time-domain simulations on the examples of coupled triplet-doublet pairs and systems exhibiting the reverse intersystem crossing phenomenon. Separately, we validate stationary EPR simulations across diverse spin systems, including high-spin complexes, achieving excellent agreement with established packages. We also show the possibilities of parameter fitting framework, applicable to all spectrum types implemented in MaRs, that supports multiple optimization strategies and uncertainty quantification.

We studied the intersystem crossing (ISC) of the neutral, monoanionic, and dianionic forms of two perylenebisimide PBI derivatives with twisted π-conjugation framework in the molecular structure: one is substituted with four chloride atoms (PBI-Cl), whereas the other has a fused dimeric molecular structure (DiPBI). PBI-Cl is highly fluorescent, and no ISC was observed. However, the DiPBI shows efficient ISC (triplet state lifetime τT = 3.6 μs), supported by theoretical computation with the Herzberg–Teller effect taken into account. Time-resolved electron paramagnetic resonance spectra of the triplet state of DiPBI show that the zero field splitting (ZFS) D parameter is 778 MHz, indicating that the electron spin density is delocalized, because the ZFS D is much smaller than the triplet state of the unsubstituted PBI (D = 1303 MHz). No ISC was observed for the dianion [DiPBI]2−, whereas for [PBI-Cl]2, ISC was observed. This result infers that twisting of the π-conjugation framework of a molecular structure does not necessarily induce ISC, and excited state energy level matching of S1/Tn states may play the decisive role in ISC. These studies are helpful to fully exploit the super strong reductivity of photoexcited dianions in photocatalytic redox organic reactions due to the enhanced intermolecular electron transfer based on the long-lived triplet excited state of the dianions.

New rare Ln^III–V^IV single-molecule magnets (Ln = Dy, Er, Yb) were synthesized and characterized. For Dy^III–V^IV and Yb^III–V^IV complexes studied by EPR spectroscopy, the phenomenon of phase relaxation enhancement (PRE) was observed.

Bodipy (BDP)‐perylenebisimide (PBI) donor–acceptor dyads/triad were prepared to study the spin–orbit charge‐transfer intersystem crossing (SOCT‐ISC). For BDP‐PBI‐3, in which BDP was attached at the imide position of PBI, higher singlet oxygen quantum yield (Φ_Δ=85 %) was observed than the bay‐substituted derivative BDP‐PBI‐1 (Φ_Δ=30 %). Femtosecond transient absorption spectra indicate slow Förster resonance energy transfer (FRET; 40.4 ps) and charge separation (CS; 1.55 ns) in BDP‐PBI‐3, while for BDP‐PBI‐1, CS takes 2.8 ps. For triad BDP‐PBI‐2, ultrafast FRET (149 fs) and CS (4.7 ps) process were observed, the subsequent charge recombination (CR) takes 5.8 ns and long‐lived ³PBI* (179.8 μs) state is populated. Nanosecond transient absorption spectra of BDP‐PBI‐3 show that the CR gives upper triplet excited state (³BDP*) and subsequently, via a slow intramolecular triplet energy transfer (14.5 μs), the ³PBI* state is finally populated, indicating that upper triplet state is involved in SOCT‐ISC. Time‐resolved electron paramagnetic resonance spectroscopy revealed that both radical pair ISC (RP ISC) and SOCT‐ISC contribute to the ISC. A rare electron spin polarization of (e, e, e, e, e, e) was observed for the triplet state formed via the RP ISC mechanism, due to the S−T₊₁/T₀ states mixing.