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 oC. [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 × 1011 s-1 is strikingly nonvariant for the different TCNE complexes despite large differences in the driving force for electron transfer (ΔG 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 ΔG 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.

Photoexcitation of chloranil (CA) produces initially the excited singlet state 1 CA*, as demonstrated for the first time by time-resolved spectroscopy on the femtosecond/picosecond time scale. Electron transfer from aromatic donors (D) to singlet chloranil leads to short-lived (ca. 5 ps) singlet radical-ion pairs, 1[D•+, CA •-]. This ultrafast quenching process competes with intersystem crossing (k ISC ≈ 1011 s-1) to generate the triplet excited state, 3 CA*. The follow-up electron transfer from D to 3 CA* yields triplet radical-ion pairs, which are distinguished from their singlet analogues by their long (nanosecond) lifetimes. The competition between electron transfer and intersystem crossing on the early picosecond time scale also pertains to a wide variety of other photoexcited quinones related to chloranil. Electron transfer to singlet quinone as established here adds a new dimension to the generally accepted mechanisms which proceed from the triplet state, and the inclusion of reactions on both the triplet and the singlet manifolds provides a complete picture of photoinduced electron transfer to various quinone acceptors.