Irregular Photoinduced Electron Transfer between Doubly-Charged Pyrene and Alkali Metal Cations Studied by Fourier-Transform Electron Paramagnetic Resonance

Polarized FT-EPR spectra of photolyzed pyrene/alkali metal (Li, Na, K, Rb, Cs)/THF solutions were found to be time and alkali metal dependent. The polarization effects are initiated by electron transfer (ET) reactions between the doubly- and singly-charged pyrene (Py²⁻ and Py^⦁−) and the alkali metal countercations (M⁺). With the exception of Py/K/THF solution, the spin polarizations of all other systems, including those with a chelating agent added (cryptand), originate with a triplet precursor. The polarization is due to the action of several mechanisms: the correlated radical pair mechanism, S-T₋₁ radical pair (RP) mixing, and the radical-triplet pair mechanism. However, in the Py/K/THF system, the polarization effects include different manifestations of the S-Τ₀ RP mechanism through exchange and dipole-dipole interactions with a singlet-state genesis. This irregular behavior is discussed in terms of the photoinduced ET reactions via the Marcus theory and by taking into account the competition between ET and intersystem crossing rates.