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 ult rafast 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 d onor-acceptor complexes of pinacol donors with methyl viologen. Ultras hort lifetimes of the cation radicals with tau approximate to 10 ps ob tain (for the first time) from quantitative kinetics analysis of the t ime-resolved spectroscopic results. These rapid C-C bond scissions suc cessfully compete with back-electron transfer, which normally predomin ates on this time scale, and lead to exceptionally high efficiencies f or the oxidative fragmentation of pinacols.