Optimized Electron Transfer in Charge-Transfer Ion Pairs. Pronounced Inner-Sphere Behavior of Olefin Donors

Time-resolved (fs) spectroscopy allows the direct observation of charge-transfer ion pairs resulting from the photoexcitation of the electron donor−acceptor (EDA) complexes of tetracyanoethylene with various olefin donors, i.e., [olefin, TCNE], in dichloromethane solutions. Measurement of the spectral decays yields first-order rate constants for electron transfer ( k ET ) in the collapse of the charge-transfer ion pairs [olefin • +, TCNE • - ] by very rapid return to the ground-state EDA complex at 25 o C. [These ultrafast ET rates necessitated the design/construction of a new tunable, high-power pump−probe spectrometer based on a Ti:sapphire laser with 250-fs resolution.] The value of k ET = 5 × 10 11 s - 1 is strikingly nonvariant for the different TCNE complexes despite large differences in the driving force for electron transfer (Δ G 0 ), as evaluated from the varying ionization potentials of the olefins. Such a unique nonvariant trend for the free energy relationship (log k ET versus Δ G 0 ) is analyzed in terms of a dominant inner-sphere component to electron transfer. In a more general context, the inner-sphere (adiabatic) electron transfer in [olefin • +, TCNE • - ] relates to a similar, but less pronounced, inner-sphere behavior noted in the analogous [arene • +, TCNE • - ] radical-ion pairs. As such, these electron-transfer processes represent an extremum in the continuum of ET transition states based on the inner-sphere/outer-sphere dichotomy.