OPTIMIZED ELECTRON-TRANSFER IN CHARGE-TRANSFER ION-PAIRS - PRONOUNCEDINNER-SPHERE BEHAVIOR OF OLEFIN DONORS

Time-resolved (fs) spectroscopy allows the direct observation of charg e-transfer ion pairs resulting from the photoexcitation of the electro n donor-acceptor (EDA) complexes of tetracyanoethylene with various ol efin donors, i.e., [olefin, TCNE], in dichloromethane solutions. Measu rement of the spectral decays yields first-order rate constants for el ectron transfer (k(ET)) in the collapse of the charge-transfer ion pai rs [olefin(.+), TCNE(.-)] by very rapid return to the ground-state EDA complex at 25 degrees C. [These ultrafast ET rates necessitated the d esign/construction of a new tunable, high-power pump-probe spectromete r 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 TCN E complexes despite large differences in the driving force for electro n transfer (Delta G(0)), as evaluated from the varying ionization pote ntials of the olefins. Such a unique nonvariant trend for the free ene rgy relationship (log k(ET) versus Delta G(0)) is analyzed in terms of a dominant inner-sphere component to electron transfer. In a more gen eral context, the inner-sphere (adiabatic) electron transfer in [olefi n(.+), TCNE(.-)] relates to a similar, but less pronounced, inner-sphe re behavior noted in the analogous [arene(.+), TCNE(.-)] radical-ion p airs. As such, these electron-transfer processes represent an extremum in the continuum of ET transition states based on the inner-sphere/ou ter-sphere dichotomy.