Magnetic resonance and spin dynamics in radical ion pairs: Pulsed time‐resolved fluorescence detected magnetic resonance

Analytical model for spin dynamics in radical pairs under the spin-locking condition

Spin dynamics in triplet radical pairs are theoretically studied under the spin-locking condition, where singlet-triplet mixing is blocked by the resonant microwave field. A key assumption in the theory is simultaneous excitations of T+-T0 and T--T0 transitions in triplet radical pairs. This assumption allows for the application of a three-state model [Yago, J. Chem. Phys. 151, 214501 (2019)] to describe the spin dynamics of triplet radical pairs. The analysis based on the three-state model shows that the triplet states are quantized along the direction of a microwave-induced magnetic field (B1) in the rotating frame under the spin-locking condition. This gives rise to a new spin-locking phenomenon where T+-T0 and T--T0 mixing are most enhanced at magnetic fields that deviate from the resonance by ±B1. It is also shown that the quantum beats observed under the spin-locking condition originate from the spin dynamics in triplet radical pairs.

Room temperature optically detected magnetic resonance (photoluminescence detected magnetic resonance) of radical ion pairs induced by vacuum ultraviolet in thin polymeric films

A setup for recording optically detected electron paramagnetic resonance spectra of radical ions involved in geminate recombination and generated by vacuum ultraviolet is described. The setup allows registration of EPR spectra from short-lived radical ions in polymeric films at room temperature by recombination fluorescence modulated by a resonance microwave field.

Degenerate Electron Exchange Reaction of n-Alkane Radical Cations in Solution

The degenerate electron exchange (DEE) reaction involving radical cations (RCs) of n-nonane, n-dodecane, and n-hexadecane in n-hexane solution was studied over the temperature range 253-313 K using the method of time-resolved magnetic field effect in recombination fluorescence of spin-correlated radical ion pairs. In the dilute solutions the rate constant of DEE was found to be 200 times slower than the diffusion limit. Using n-nonane as an example, we showed that two reasons are responsible for the low value of the RC self-exchange rate: (1) conformational variability of molecules and RCs and (2) the activation barrier of DEE reaction. The calculations of the reaction enthalpy performed by the B3LYP/6-31G(d) method indicated that electron transfer can be effective only upon collision of RC with a neutral molecule either in the all-trans conformation or in the conformation differing from the latter by rotation of the end ethyl fragment. The activation barrier of the DEE reaction was estimated using the reorganization energy of the internal degrees of freedom calculated at the B3LYP level and was found to be about 6 kcal/mol. A possible influence of the interaction between RC and a neutral molecule in an encounter complex on DEE rate constant is also discussed.