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 N₂O-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^•, CCl₃OO^•) 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 × 10⁹ dm³ mol^-1 s^-1. Pulse radiolysis of N₂-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 × 10⁹ dm³ 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.