Direct observation of ultrafast decarboxylation of acyloxy radicals via photoinduced electron transfer in carboxylate ion pairs

Charge-transfer (CT) photoactivation of the electron donor-acceptor salts of methylviologen (MV(2+)) with carboxylate donors (RCO(2)(-)) including benzilates [Ar2C(OH)CO2-] and arylacetates (ArCH2CO2-) leads to transient [MV(.+), RCO(2)(.)] radical pairs. Femtosecond time-resolved spectroscopy reveals that the photogenerated acyloxy radicals (RCO(2)(.)) rapidly lose carbon dioxide by C-CO2 bond cleavage, in competition with back-electron transfer to restore the original ion pair, [MV(2+), RCO(2)(-)]. The decarboxylation rate constants for ArCH2CO2. lie in the range (1-2) x 10(9) s(-1), in agreement, with previous reports. In striking contrast, the C-CO2 bond scission in Ar2C(OH)CO2. occurs within a few picoseconds (k(CC) = (2-8) x 10(11) s(-1)). The rate constants for decarboxylation of these donors approach those of barrier-free unimolecular reactions. Thus, real-time monitoring of the decarboxylation of benziloxy radicals represents the means for the direct observation of the transition state for C-C bond scission.