Formation, excited state and electron transfer properties of the phenoxyl radical of 4,4′-thiodiphenol

Abstract

The transient absorption bands ( λ max = 335, 500, 650–800 nm) formed on pulse radiolysis of N 2O-saturated basic aqueous solution of 4,4′-thiodiphenol (TDPH) are assigned to phenoxyl radical (TDP ⋅) formed on one-electron oxidation of phenoxide ion (TDP −) of TDPH. Specific one-electron oxidants (N ⋅ 3, I ⋅− 2, Br ⋅, CCl 3OO ⋅) also produced phenoxyl radicals with similar transient absorption spectrum. Phenoxyl radicals are able to undergo electron transfer reaction with N, N, N′, N′-tetramethyl-p-phenylenediamine (TMPD) with a bimolecular rate constant of 4.5 × 10 9 dm 3 mol −1 s −1. Pulse radiolysis of N 2-saturated TDPH in benzene showed absorption bands at 330, 500, 650–800 nm. The second-order decay of these bands was not affected by oxygen and anthracene, well-known triplet quenchers and the transient spectrum is assigned to phenoxyl radicals formed on fast decay of solute triplets with a G value of 0.29. Benzophenone triplet is able to undergo energy transfer with TDPH with a bimolecular rate constant of 6 × 10 9 dm 3 mol −1 s −1 and formed benzophenone ketyl radical ( λ = 545 nm) and phenoxyl radical (TDP ⋅, λ = 500 nm) on H ⋅ atom transfer from TDPH to benzophenone in an intermediate complex. The G value of phenoxyl radicals was determined to be 1.44. Picosecond laser flash photolysis of TDPH in acetonitrile failed to form any transient absorption in 450–900 nm region, supporting that only a small fraction of TDPH triplet decay to phenoxyls. Pulse radiolysis and picosecond laser flash photolysis studies indicate that the triplets of TDPH are highly unstable. The oxidation potential of TDP −/ TDP ⋅, H + couple is estimated to be between 0.265 and 1.03 V.