Laser flash photolysis of tert-butyl aroylperbenzoates:: Kinetics of the singlet and triplet states and the aroylphenyl radicals

Tert-Butyl aroylperbenzoates (1-4) were studied by laser flash photolysis (LFP). LFP (380 rim, pulse width similar to350 fs) of 2 and 3 allowed direct observation of their singlet states, which showed broad absorption (lambda(max) similar to 625 nm; tau similar to 20 and similar to7.9 ps, respectively). The triplet state of each (lambda(max) similar to 530-560 nm) rapidly dissociates by O-O cleavage as indicated by the short triplet lifetimes (e.g., triplet lifetime of 3 similar to0.74 ns). The similar to550 nm absorption obtained from the 355 nm LFP (pulse width similar to7 ns) of 1, 2, and 4 has been assigned to the corresponding aroylphenyl radicals. Two representative radicals (4-benzoylphenyl 5 and 3-(4'-methylbenzoyl)phenyl 6) investigated in detail showed solvent-dependent lifetimes. Absolute bimolecular rate constants of reactions of these radicals with various quenchers including double-bond-containing monomers have been observed to range from 7.56 x 10(7) to 1.68 x 10(9) M-1 s(-1) in CCl4 at room temperature. A possible structure of the aroylphenyl radicals and the transition responsible for the 550 nm absorption are discussed.