Electron-Transfer Mechanisms with Photoactivated Quinones. The Encounter Complex versus the Rehm−Weller Paradigm

Photoexcited quinones ( Q* ) are efficiently quenched by polymethylbenzenes (ArH) via electron transfer (ET). However, the second-order rate constants ( k 2 ) exhibit Rehm−Weller (outer-sphere) dependence on the free energy (Δ G ET ), despite our new findings that the quenching occurs via a series of rather strong encounter complexes [ Q*, ArH] with substantial (charge-transfer) bonding. The relatively high formation constants ( K EC ) of the encounter complexes indicate that any mechanistic interpretation of the driving-force dependence of the observed rate constants is highly ambiguous since k 2 must be a composite of K EC and the intrinsic rate constant ( k ET ) for electron transfer within the intermediate (inner-sphere) complex. As such, the reorganization energies extracted from Rehm−Weller plots lack thermodynamic significance. On the other hand, the unambiguous driving-force dependence of k ET represents a unique example for the “normal” Marcus behavior of the endergonic electron transfer between the donor/acceptor pair in van der Waals contact as extant in the encounter complex.