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.

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