Concurrent Generation of Tight and Loose Ion Pairs upon Charge-Transfer Excitation of Electron Donor-Acceptor Complexes in Solution.

Ultrafast time-resolved fluorescence (TRFL) and visible transient absorption (TA) measurements were performed to investigate excited-state dynamics of several electron donor-acceptor complexes (DACs) following charge-transfer (CT) excitation in solution. For all DACs studied, including benzene-tetracyanoethylene (BZ-TCNE), toluene-TCNE, fluorobenzene-TCNE, and (BZ)2-TCNE, the CT-state lifetimes obtained from TRFL are consistently shorter than those from TA by factors of ∼2-5. This disparity, together with fluorescence lifetimes of ∼5-30 ps in dichloromethane, cannot be reconciled with the conventional assumption that CT excitation initially yields only emissive tight ion pairs (TIPs). The results instead support a revised concurrent mechanism in which CT excitation generates a locally hot and structurally diverse ion-pair ensemble that bifurcates into parallel relaxation pathways, leading to fluorescent TIPs and nonfluorescent loose ion pairs. The two species undergo independent charge recombination at different rates, yielding the distinct lifetimes observed in the TRFL and TA measurements.