Ion Pairs from Photoexcited, “Random” Electron Donors and Acceptors: Alkylbenzenes and Tetracyanoethylene

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Transient photocurrent experiments are used to measure the free radical ion quantum yield of a number of alkylbenzene electron donors with the electron acceptor tetracyanoethylene (TCNE). These experiments are performed at a variety of photoexcitation wavelengths in dichloromethane, a moderately polar solvent. It is found that the free ion yields often exhibit a very strong dependence on the excitation wavelength and may decrease markedly in the center of the charge-transfer band. For example, the free ion yield of the donor−acceptor system, pentamethylbenzene−TCNE, increases more than 100-fold when the excitation wavelength is switched from 532 to 397 nm! We show that this result and others are understandable from the following model. While closely associated electron donor−acceptor (EDA) complexes account for most of the absorption, there is an additional, usually small, absorption due to unassociated random donor and acceptor pairs. The Franck−Condon (vertical) excitation of these random pairs results in radical ion pairs which have center-to-center distances greater than contact and which have high probabilities for separation. Quantitative analysis based on Onsager theory indicates that only distantly separated radical ion pairs (ca. 1 nm or more) created by photoexcitation can escape each other's Coulombic attraction to produce the free ion yields observed in our experiments. The photoexcitation of ground-state EDA complexes plays little essential role in this process. The observed wavelength dependence then corresponds both to variation in the ratio of random pair to EDA complex absorption and to the distance distribution of radical ion pairs produced. Free ion yields calculated using Onsager theory and a simple excitation function for the random pairs fit our experimental results quite well and support this model.