Kinetics of electron transfer processes involving free ions and ion pairs studied by flash photolysis

Abstract

Photoejection of electrons from pyrenide ions (π.^-) or sodium pyrenide ion pairs (π.^-, Na⁺) was effected in tetrahydrofuran (THF) or tetrahydropyran (THP) solution by flash of light (λ > 420 nm). As the reaction was performed in the presence of a large excess of biphenyl (B) the ejected electrons were captured preferentially by that hydrocarbon reducing it to biphenylide radical ion (B.^- or B.^-, Na⁺). Due to the low concentration of pyrene (π) the subsequent reactions between biphenylide and pyrene, namely B.^- + π (k₁→) B + π.^- (1) and B.^-, Na⁺ + π (k₂→) B + π.^-, Na⁺, (2) could be easily monitored after the flash. By varying the ratio of [B.^-]/[B.^-, Na⁺] it was possible to make either (1) or (2) dominant, and hence to determine the rate constants k₁ and k₂. Thus, k₁ = (4.8 ± 0.5) x 10¹⁰ 1 mol^-1 s^-1 and k₂ = 0.6 x 10¹⁰ 1 mol^-1 s^-1 for the electron transfer in THF, and k₂ = 0.5 x 10¹⁰ 1 mol^-1 s^-1 for the reaction in THP, all the constants referring to ≈ 20 °C.