The separation distance distribution in electron-donor-acceptor systems and the wavelength dependence of free ion yields

We recently reported that free radical ion quantum yields for electron-dono r-acceptor (EDA) systems of alkylbenzenes-tetracyanoethylene (TCNE) exhibit a remarkable wavelength-dependence in dichloromethane, a medium polarity s olvent. We proposed that weak absorption by long-distance, unassociated or "random'' D...A pairs is-mainly responsible for the free radical ion: yield . Here a model for the wavelength dependence of the free ion yield is devel oped for four systems in which differing degrees of EDA complex formation a re present: 1,3,5-tri-tert-butylbenzene-TCNE in which only random pairs exi st due to the bulky groups on the electron donor, and toluene-TCNE, 1,3,5-t riethylbenzene-TCNE and 1,3,5-trimethylbenzene-TCNE. Mulliken-Hush theory i s used to determine the excitation distance distribution of unassociated, r andom;pairs at different wavelengths. For each absorption distribution, fre e radical ion yields at different wavelengths are then calculated using Ons ager's result for the ion separation probability. Encouraging agreement bet ween the calculated yields and our experimental results is obtained. As far as we are aware, this is the first time that photoexcitation of unassociat ed donor/acceptor pairs has been invoked as the source of separated radical ion pairs. (C) 2001 Americarn Institute of Physics.