Pulse radiolysis study of ion pairing of diphenylpolyene radical anions with tetrabutylammonium and sodium cations in tetrahydrofuran

Abstract

Pulse radiolysis of tetrahydrofuran (THF) solutions of all-transα,ω-diphenyl-substituted polyenes, such as 1,4-diphenylbuta-1,3-diene, 1,6-diphenylhexa-1,3,5-triene and 1,8-diphenylocta-1,3,5,7-tetraene, has been undertaken in the absence and presence of Bu₄NPF₆ and NaBPh₄. In the presence of the salts, the absorption maxima of the diphenylpolyene radical anions are shifted to shorter wavelengths by ion pairing with Bu₄N⁺ and Na⁺ as well as that of the trans-stilbene radical anion previoulsy investigated. When Ph(CHCH)_nPh˙^–(n= 1–4) is paired with Bu₄N⁺, the magnitude of the spectroscopic shift is larger for n= 2–4 than for n= 1. On the other hand, the magnitude of the spectroscopic shift due to the ion pairing with Na⁺ decreases with increasing n and becomes very small in the case of n= 3 or 4. The decay of the radical anions, which is due to neutralization reactions with THF(H⁺), is retarded by the addition of the salts. The retarding effect of the salts is attributed to the ion pairing of the reactant ions with the counterions derived from the salts. In acetonitrile solution the absorption spectra and the decay rates of the radical anions are not affected by the addition of the salts, demonstrating that the ion pairing is not important in such a polar solvent. Results for the radical anions of pyrene, perylene and triphenylethylene are presented for the sake of comparison. The appreciable spectroscopic shift due to the ion pairing with the large Bu₄N⁺ ion was found to be characteristic of the diphenylpolyene radical anions.