Absorption spectra and extinction coefficients of phenylsulfinyl and phenylsulfenyl (thiyl) radicals are determined by nanosecond laser photolysis in various solvents. Direct observation and characterization of arylsulfinyl radicals from the photolysis of several aromatic sulfoxides provides the strongest evidence to date for R-cleavage as the predominant primary photochemical process for these compounds. The absorption spectrum of phenylsulfinyl, with λ max ) 300 and 450 nm and ) 1.1 × 10 4 and 1.3 × 10 3 M -1 cm -1 , is practically independent of solvent. Quantum yields of free sulfinyl radicals range from 0.09 to 0.18 in various solvents. Recombination rate constants very near diffusion control indicate that there is a large spin-orbital coupling in the radical pair. Rate constants for the reactions of arylsulfinyl radicals with stable nitroxide radicals are among the fastest known, but reactivity with O 2 is very modest. Computations indicate that the singly occupied molecular orbital is a π* orbital largely localized on the sulfur and oxygen atoms.

Gas-phase activation data were obtained for model sulfoxide elimination reactions. The activation enthalpy for methyl 3-phenylpropyl sulfoxide is 32.9 +/- 0.9 kcal/mol. Elimination by methyl vinyl sulfoxide to form acetylene has an enthalpic barrier of 41.6 +/- 0.8 kcal/mol and that of 3-phenylpropyl methanesulfinate to form hydrocinnamaldehyde is 34.6 +/- 0.6 kcal/mol. Calculations at the MP2/6-311+G(3df,2p)//MP2/6-31G(d,p) level for simplified models of these reactions provide barriers of 32.3, 40.3, and 32.7 kcal/mol, respectively. A series of other compounds are examined computationally, and it is shown that the substituent effects on the sulfoxide elimination reaction are much more straightforward to interpret if DeltaH data are available in addition to the usually determined DeltaH++. The activation enthalpy of the reverse addition reaction is also subject to structural variation and can usually be rationalized on the basis of nucleophilicity of the sulfur or polarity matching between the sulfenic acid and olefin derivative.

The photochemistry of aryl benzyl sulfoxides is described. The initial event is homolytic cleavage to form a singlet sulfinylhenzyl radical pair. This radical pair partitions between reversion to starting material with at least partial racemization and closure to form a sulfenic ester. With acetone sensitization, the primary radical pair also undergoes quite significant escape, leading to formation of diphenylethane and aryl arenethiosulfonates. Secondary photolysis of the sulfenic ester leads exclusively to S-0 homolysis, yielding the radical pair from which isolated products are derived. Quantum yields and other mechanistic observations are discussed.

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Quenching of singlet oxygen luminescence at 1.27 µm by PhS • , PhSO • , and peroxyl radicals PhOO • , t-BuOO • , PhCH 2 OO • , Ph 2 CHOO • , and Ph 3 COO • was studied in liquid solution. The quantum yields of decomposition of different initiators which lead to the formation of free radicals were measured by using nanosecond transient absorption. This allowed determination of singlet oxygen O 2 ( 1 ∆ g ) quenching rate constants by the radicals. They are <2 × 10 8 M -1 s -1 for the sulfur-centered radicals and (2-7) × 10 9 M -1 s -1 for peroxyl radicals in acetonitrile. The rapid quenching is attributed to energy transfer quenching by the peroxyls, which have an n f π* transition leading to a low-lying 2 A′ state above their 2 A′′ ground state. PhSO • is shown computationally not to have such a low-lying 2 A′ state. There may be a very low-lying 2 B 1 state, for PhS • , but it is apparently not an efficient acceptor of electronic energy from O 2 ( 1 ∆ g ).