Direct measurement of ultrafast carbon-carbon cleavage rates via the subpicosecond charge-transfer activation of pinacols

Highly transient (benzpinacol) cation radicals (D-2(+.)) and their ultrafast mesolytic fragmentations to the diarylhydroxymethyl radical (D-.) and cation (D+) are directly observed on the early picosecond time scale upon the charge-transfer photoactivation of the intermolecular donor-acceptor complexes of pinacol donors with methyl viologen. Ultrashort lifetimes of the cation radicals with tau approximate to 10 ps obtain (for the first time) from quantitative kinetics analysis of the time-resolved spectroscopic results. These rapid C-C bond scissions successfully compete with back-electron transfer, which normally predominates on this time scale, and lead to exceptionally high efficiencies for the oxidative fragmentation of pinacols.