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 degrees 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 x 10(11) s(-1) is strikingly nonvariant for the different TCNE complexes despite large differences in the driving force for electron transfer (Delta 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 Delta 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.