Direct Observation and Structural Characterization of the Encounter Complex in Bimolecular Electron Transfers with Photoactivated Acceptors

The encounter complex between photoexcited quinones Q* and various aromatic donors (ArH) is observed directly by time-resolved ps spectroscopy immediately before it undergoes electron transfer to the ion-radical pair [Q•-, ArH•+]. The encounter complex (EC) is spectrally characterized by distinctive (near IR) absorption bands, and its temporal evolution is established by quantitative kinetics analysis. The structural characterization of the 1:1 encounter complex [Q*, ArH] identifies the cofacial juxtaposition of the donor and acceptor moieties for optimal overlap of their π-orbitals. Further comparisons of the (excited-state) encounter complex with the corresponding (ground-state) EDA complex of aromatic donors and quinones establish its charge-transfer character, which directly relates to electron transfer within the encounter complex. The mechanistic significance of the encounter complex to bimolecular electron transfer is discussed (Scheme 1).