Reversible interchange of charge-transfer versus electron-transfer states in organic electron transfer via cross-exchanges between diamagnetic (donor/acceptor) dyads.

The choice of appropriate electron donors (D) and acceptors (A) allows for the first time the simultaneous observation of Mulliken charge-transfer states, [D,A], that can coexist in reversible equilibrium with electron-transfer states, {D+*,A-*}, for various diamagnetic organic redox dyads. The theoretical analysis based on the (two-state) Mulliken-Hush analysis of the intervalence optical transition, together with the spectral identification of the transient ion-radical pairs of D+* and A-*, leads to the construction of the unusual potential-energy surface consisting of a single minimum without any reorganizational barrier for electron-transfer cross-exchanges with driving forces close to the isergonic limit. The mechanistic implications of this direct demonstration of the facile charge-transfer/electron-transfer interchange are discussed.