ION-PAIRS FROM ELECTRON-DONOR ACCEPTOR SYSTEMS - A COMPARISON BETWEEN PICOSECOND AND CIDNP STUDIES
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Electron donor-acceptor (EDA) interactions play an important role in many biological, chemical, and physical processes. To gain insight into the principal reaction intermediates and the mechanisms of product formation, a series of model reactions were studied. For example, the photoreactions of an electron donor-acceptor complex between 1,1-dimethylindene (DMI) and tetracyanoethylene (TCNE) and a donor acceptor pair, DMI/chloranil (CA), have been studied by picosecond and CIDNP spectroscopy. It was found that immediately after excitation of the EDA complex the corresponding ion radicals, DMI+ and TCNE-, are generated and decay within 50 ps. The decay of these transients is followed by a bleaching of the EDA complex which persists for 500 ps. The nature of this process is discussed. In contrast, the irradiation of the system DMI/CA at 532 nm gives rise to excited singlet CA, which undergoes rapid intersystem crossing to the triplet state, before it is quenched by electron transfer. The resulting ion radical pair, DMI+/CA-, reacts by reverse electron transfer and by coupling to form two adducts. Adduct B is formed first, but is later depleted in favor of adduct A. (Chemical Equation Presented) 1985 American Chemical Society.
