SOLVENT CAGE RECOMBINATION OF 4-BENZOYLPHENYLTHIYL RADICALS - FAST INTERSYSTEM CROSSING OF TRIPLET SULFUR-CENTERED RADICAL PAIRS

Picosecond transient absorption spectroscopy was utilized to study the recombination dynamics of triplet sulfur radical pairs. A disulfide, bis(p-benzoylphenyl) disulfide (BpSSBp), was designed to selectively p roduce triplet thiyl radical pairs by means of internal triplet energy transfer. Thus, irradiation of bis(p-benzoylphenyl) disulfide (BpSSBp ) in Tetralin (355 nm, 17 ps) followed by instantaneous singlet --> tr iplet interconversion of the photoexcited benzoyl group leads to rapid intramolecular energy transfer and S-S bond scission. Greater than 50 % of the photochemically generated (presumably triplet) radical pairs were found to recombine in competition with cage escape. Benzoyl subst itution of phenyl disulfide is expected to cause intersystem crossing (ISC) from the singlet excited state to the triplet state to occur bef ore singlet S-S bond scission can occur. However, benzoyl substitution is not expected to perturb the orbital degeneracy on the thiyl radica l that has been suggested to be responsible for the strong spin-orbit coupling mechanism and fast ISC of sulfur-centered radical pairs in mi cellar solutions. Thus, this study provides the first direct observati on of fast ISC (>100 ps) of sulfur-centered radical pairs in organic s olvents.