REACTIVE ION-PAIRS FROM THE CHARGE-TRANSFER EXCITATION OF ELECTRON-DONOR ACCEPTOR COMPLEXES
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Excitation within the charge-transfer (CT) band of the electron donor-acceptor or EDA complexes of tetranitromethane (TNM) with a series of 9-substituted and 9, 10-disubstituted anthracenes (An) leads to photochemistry in high quantum yields ( ~ 1). The combined use of time-resolved picosecond spectroscopy, product isolation, and structure elucidation allows for the detailed mapping of the temporal evolution of the CT excited state to the photoproduct I via a series of discrete reactive intermediates. Thus electron transfer within the EDA complex occurs effectively (<25 ps) upon CT photoexcitation to form simultaneously An+. and TNM-. The latter is not observed directly owing to its spontaneous fragmentation to C(NO2)3-and NO2within 10 ps. The geminate ionic intermediates An+ and C(NO2)3- undergo cage combination to produce hydranthryl radicals II within an interval of <500 ps. Photoproduct I derives from the encounter of the hydranthryl radical II with NO2subsequent to diffusive separation, consistent with the longer time scale of >1 ns and the trans stereochemistry required for the free radical coupling to occur. This detailed study is the basis for a generalized formulation (summarized in Scheme I) to be developed for CT photochemistry via reactive ion pairs. In particular, the comparison with the related EDA complexes of the anthracenes and tetracyanoethylene serves to emphasize the importance of back electron transfer from the geminate ion pair, i.e., [D+, A-] [D, A]. 1986, American Chemical Society. All rights reserved.