| In this study we have utilized spectroscopic and photoconductivity techniques to investigate free ion formation in the fumaronitrile/trans-stilbene charge transfer-complex. We report free ion yields resulting from the direct excitation of this complex in a variety of solvents and compare our results with theory. Particularly in less polar solvents, the free ion yields we report are much larger than expected for ion pairs which are initially in contact in a relaxed solvent environment. Our results suggest that shortly after excitation, the initial separation of charges is larger than the electron donor-acceptor contact distance. Although other studies of charge transfer complexes have suggested this as well, most recent work ignores this possibility. Our work thus supports a mechanism of ion separation involving the excitation of a ground state complex into a Franck-Condon excited contact ion pair, of which at least a portion relaxes to thermalized, large-radius or loose ion pairs. These initial loose ion pairs may then diffusively separate into free ions or collapse to contact. This mechanism includes a novel speculation concerning the conversion of the Franck-Condon excited state to the initial loose ion pair. We offer suggestions as to how these species might be formed.; In addition, we report the time-resolved photocurrent which results from charge transfer band excitation of the trans-stilb ene/fumaronitrile complex in toluene, fluorobenzene and tetrahydrofuran. The measured dipole moment is larger in both tetrahydrofuran (18.5 {dollar}pm{dollar} 1.0 D) and fluorobenzene (20.2 {dollar}pm{dollar} 1.0 D) than in toluene (14.2 {dollar}pm{dollar} 0.6 D). In tetrahydrofuran and for the first time, a transient dipole-like photocurrent is observed at short times, evolving into a steady free ion current at long times. The risetime of this photocurrent is much shorter than both the lifetime of the dipolar species and its fluorescence lifetime. These observations provide further support to the mechanism proposed. |
