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

The transient absorption bands (lambda(max) = 335, 500, 650-800 nm) formed on pulse radiolysis of N2O-saturated basic aqueous solution of 4,4'-thiodip henol (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 simila r transient absorption spectrum. Phenoxyl radicals are able to undergo elec tron transfer reaction with N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) with a bimolecular rate constant of 4.5 x 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 b y oxygen and anthracene, well-known triplet quenchers and the transient spe ctrum is assigned to phenoxyl radicals formed on fast decay of solute tripl ets with a G value of 0.29. Benzophenone triplet is able to undergo energy transfer with TDPH with a bimolecular rate constant of 6 x 10(9) dm(3) mol( -1) s(-1) and formed benzophenone ketyl radical (lambda = 545 nm) and pheno xyl radical (TDP., lambda = 500 nm) on H-. atom transfer from TDPH to benzo phenone in an intermediate complex. The G value of phenoxyl radicals was de termined to be 1.44. Picosecond laser hash photolysis of TDPH in acetonitri le failed to form any transient absorption in 450-900 nm region, supporting that only a small fraction of TDPH triplet decay to phenoxyls. Pulse radio lysis and picosecond laser flash photolysis studies indicate that the tripl ets of TDPH are highly unstable. The oxidation potential of TDP-/TDP., H+ c ouple is estimated to be between 0.265 and 1.03 V. (C) 1999 Elsevier Scienc e S.A. All rights reserved.