Ion pairs from photoexcited, "random" electron donors and accepters: Alkylbenzenes and tetracyanoethylene

Transient photocurrent experiments are used to measure the free radical ion quantum yield of a number of alkylbenzene electron donors with the electro n acceptor tetracyanoethylene (TCNE). These experiments are performed at a variety of photoexcitation wavelengths in dichloromethane, a moderately pol ar solvent. It is found that the free ion yields often exhibit a very stron g 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-fo ld when the excitation wavelength is switched from 532 to 397 nm! We show t hat this result and others are understandable from the following model. Whi le closely associated electron donor-acceptor (EDA) complexes account for m ost of the absorption, there is an additional, usually small, absorption du e to unassociated random donor and acceptor pairs. The Franck-Condon (verti cal) excitation of these random pairs results in radical ion pairs which ha ve center-to-center distances greater than contact and which have high prob abilities for separation. Quantitative analysis based on Onsager theory ind icates that only distantly separated radical ion pairs (ca. I nm or more) c reated by photoexcitation can escape each other's Coulombic attraction to p roduce 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 t he ratio of random pair to EDA complex absorption and to the distance distr ibution of radical ion pairs produced. Free ion yields calculated using Ons ager theqry and a simple excitation function for the random pairs fit our e xperimental results quite well and support this model.