Elucidating the Mechanism of Excited State Bond Homolysis in Nickel–Bipyridine Photoredox Catalysts

Ni 2,2’–bipyridine (bpy) complexes are commonly employed photoredox catalysts of bond-forming reactions in organic chemistry. However, the mechanisms by which they operate are still under investigation. One potential mode of catalysis is via entry into Ni(I)/Ni(III) cycles, which can be made possible by light-induced, excited state Ni(II)–C bond homolysis. Here we report experimental and computational analyses of a library of Ni(II)-bpy aryl halide complexes, Ni(Rbpy)(R′Ph)Cl (R = MeO, t-Bu, H, MeOOC; R′ = CH3, H, OMe, F, CF3), to illuminate the mechanism of excited state bond homolysis. At given excitation wavelengths, photochemical homolysis rates span two orders of magnitude across these structures and correlate linearly with Hammett parameters of both bpy and aryl ligands, reflecting structural control over key metal-to-ligand charge transfer (MLCT) and ligand-to-metal charge transfer (LMCT) excited state potential energy surfaces (PESs). Temperature- and wavelength-dependent investigations reveal moderate excited state barriers (ΔH‡ ~4 kcal mol-1) and a minimum energy excitation threshold (~55 kcal mol-1, 525 nm), respectively. Correlations to electronic structure calculations further support a mechanism in which repulsive triplet excited state PESs featuring a critical aryl-to-Ni LMCT lead to bond rupture. Structural control over excited state PESs provides a rational approach to utilize photonic energy and leverage excited state bond homolysis processes in synthetic chemistry.

Filtering Properties of Hodgkin-Huxley Neuron to Different Time-Scale Signals

Neuron can be excited and inhibited by filtered signals. The filtering properties of neural networks have a huge impact on memory, learning, and disease. In this paper, the filtering properties of Hodgkin-Huxley neuron to different time-scale signals are investigated. It is found that the neuronal filtering property depends on the locking relationship between the signal's frequency band and the natural frequency of neuron. The natural firing frequency is a combination of the fundamental component and the various level harmonic components. The response of neuron to the filtered signal is related to the amplitude of the harmonic components. Neuron responds better to the low-frequency signals than the high-frequency signals because of the reduction in the harmonic component amplitude. The filtering ability of neuron can be modulated by the excitation level, and is stronger around the excitation threshold. Our results might provide novel insights into the filtering properties of neural networks and guide the construction of artificial neural networks.

Qualitative analysis of the response regimes and triggering mechanism of bistable NES

The main focus of this study is the development of an adapted complex variable method in the vicinity of equilibrium in bistable NES. A simplified chaos trigger model is established to describe the distance between the stable phase cycle and the pseudo-separatrix. An analytical expression can predict the excitation threshold for chaos occurrence. The relative positions between the chaos trigger threshold line and the Slow Invariant Manifold (SIM) structure can express the distribution of response regimes under growing harmonic excitation. This topological structure implies the alternation of the response regime and helps to classify the bistable NES. The experiment compares the analytical result of intra-well oscillation with the numerical result in the frequency domain. The experimental response regimes under different input energy levels and frequency domains have been observed and give ideas to guide the optimal design of a bistable NES. It is shown that the modest bistable NES possesses strong robustness to frequency perturbation.

Electron Impact Excitation of Extreme Ultra-Violet Transitions in Xe7+ – Xe10+ Ions

In the present work, a detailed study on the electron impact excitation of Xe7+, Xe8+, Xe9+ and Xe10+ ions for the dipole allowed (E1) transitions in the EUV range of 8–19 nm is presented. The multi-configuration Dirac–Fock method is used for the atomic structure calculation including the Breit and QED corrections along with the relativistic configuration interaction approach. We have compared our calculated energy levels, wavelengths and transition rates with other reported experimental and theoretical results. Further, the relativistic distorted wave method is used to calculate the cross sections from the excitation threshold to 3000 eV electron energy. For plasma physics applications, we have reported the fitting parameters of these cross sections using two different formulae for low and high energy ranges. The rate coefficients are also obtained using our calculated cross sections and considering the Maxwellian electron energy distribution function in the electron temperature range from 5 eV to 100 eV.

Mechanisms of ventricular arrhythmias elicited by coexistence of multiple electrophysiological remodeling in ischemia: a simulation study

Myocardial ischemia, injury and infarction (MI) are the three stages of acute coronary syndrome (ACS). In the past two decades, a great number of studies focused on myocardial ischemia and MI individually, and showed that the occurrence of reentrant arrhythmias is often associated with myocardial ischemia or MI. However, arrhythmogenic mechanisms in the tissue with various degrees of remodeling in the ischemic heart have not been fully understood. In this study, biophysical detailed single-cell models of ischemia 1a, 1b, and MI were developed to mimic the electrophysiological remodeling at different stages of ACS. 2D tissue models with different distributions of ischemia and MI were constructed to investigate the mechanisms of initiation of reentrant waves during the progression of ischemia. Simulation results in 2D tissues showed that the vulnerable window (VW) in the tissue with multiple pathological conditions were determined by the VWs in the tissues with a single pathological condition. In different pathological conditions, action potential duration (APD) and the conduction velocity (CV) change differently. In the tissue with multiple pathological conditions, when the borders of different pathological conditions were perpendicular to the excitation wavefront, reentrant waves were mainly induced by the spatial heterogeneity of refractory periods due to the interaction of APD and CV along the wavefront. When the borders were parallel to the wavefront, the increased excitation threshold of MI region as well as the impaired excitability of ischemia region were the primary reason for the generation of reentry. Finally, the reentrant wave was observed in a 3D model with a scar reconstructed from MRI images of a MI patient. These comprehensive findings provide novel insights for understanding the arrhythmic risk during the progression of myocardial ischemia and highlight the importance of the multiple pathological stage in designing medical therapies for arrhythmia in ischemia.

Peculiarities of Neurostimulation by Intense Nanosecond Pulsed Electric Fields: How to Avoid Firing in Peripheral Nerve Fibers

Intense pulsed electric fields (PEF) are a novel modality for the efficient and targeted ablation of tumors by electroporation. The major adverse side effects of PEF therapies are strong involuntary muscle contractions and pain. Nanosecond-range PEF (nsPEF) are less efficient at neurostimulation and can be employed to minimize such side effects. We quantified the impact of the electrode configuration, PEF strength (up to 20 kV/cm), repetition rate (up to 3 MHz), bi- and triphasic pulse shapes, and pulse duration (down to 10 ns) on eliciting compound action potentials (CAPs) in nerve fibers. The excitation thresholds for single unipolar but not bipolar stimuli followed the classic strength–duration dependence. The addition of the opposite polarity phase for nsPEF increased the excitation threshold, with symmetrical bipolar nsPEF being the least efficient. Stimulation by nsPEF bursts decreased the excitation threshold as a power function above a critical duty cycle of 0.1%. The threshold reduction was much weaker for symmetrical bipolar nsPEF. Supramaximal stimulation by high-rate nsPEF bursts elicited only a single CAP as long as the burst duration did not exceed the nerve refractory period. Such brief bursts of bipolar nsPEF could be the best choice to minimize neuromuscular stimulation in ablation therapies.

Impact of Mammalian Two-Pore Channel Inhibitors on Long-Distance Electrical Signals in the Characean Macroalga Nitellopsis obtusa and the Early Terrestrial Liverwort Marchantia polymorpha

Inhibitors of human two-pore channels (TPC1 and TPC2), i.e., verapamil, tetrandrine, and NED-19, are promising medicines used in treatment of serious diseases. In the present study, the impact of these substances on action potentials (APs) and vacuolar channel activity was examined in the aquatic characean algae Nitellopsis obtusa and in the terrestrial liverwort Marchantia polymorpha. In both plant species, verapamil (20–300 µM) caused reduction of AP amplitudes, indicating impaired Ca2+ transport. In N. obtusa, it depolarized the AP excitation threshold and resting potential and prolonged AP duration. In isolated vacuoles of M. polymorpha, verapamil caused a reduction of the open probability of slow vacuolar SV/TPC channels but had almost no effect on K+ channels in the tonoplast of N. obtusa. In both species, tetrandrine (20–100 µM) evoked a pleiotropic effect: reduction of resting potential and AP amplitudes and prolongation of AP repolarization phases, especially in M. polymorpha, but it did not alter vacuolar SV/TPC activity. NED-19 (75 µM) caused both specific and unspecific effects on N. obtusa APs. In M. polymorpha, NED-19 increased the duration of repolarization. However, no inhibition of SV/TPC channels was observed in Marchantia vacuoles, but an increase in open probability and channel flickering. The results indicate an effect on Ca2+ -permeable channels governing plant excitation.

Multiple Sequential Ionization of Valence n = 4 Shell of Krypton by Intense Femtosecond XUV Pulses

Sequential photoionization of krypton by intense extreme ultraviolet femtosecond pulses is studied theoretically for the photon energies below the 3d excitation threshold. This regime with energetically forbidden Auger decay is characterized by special features, such as time scaling of the level population. The model is based on the solution of rate equations with photoionization cross sections of krypton in different charge and multiplet states determined using R-matrix calculations. Predictions of the ion yields and photoelectron spectra for various photon fluence are presented and discussed.