Solvent separated radical ion pairs

Mattay, J., and Vondenhof, M. Contact and Solvent-Separated Radical Ion Pairs in Organic Photochemistry. 159, 219-255 (1991). 

Optically active benzene(poly)carboxamides and benzene(poly)carboxy-lates were used by Inoue and co-workers as sensitizers for the geometrical photoisomerization of (Z)-cyclooctene and (Z,Z)-cyclooctadienes in various solvents at different temperatures. Under energy-transfer conditions, enantiomeric excesses up to 64% ee in unpolar solvents like pentane were reported. The use of polar solvents diminished the product ee s due to the intervention of a free or solvent-separated radical ion pair generated through the electron transfer from the substrate to the excited chiral sensitizer (Scheme 58) [105-109]. 

The fates of the radical ion pairs produced upon electron transfer depends on the nature of their production. As already mentioned, the Bp DMA" com formed from irradiation of the ground-state CT complex. Bp - DMA, is suggested by Mataga and co-workers [24] to decay only by febet, on a timescale of 85 ps. Diffusional separation to solvent separated radical ion pairs or proton transfer within Bp -DMA com are not kinetically competitive. The triplet CRIP Bp -I- DMA" ip has two decay pathways that occur on the picosecond timescale. The first process is proton transfer, fept, to generate a triplet radical pair, BpH-l- DMA ] (Scheme 2.3). In acetonitrile, this occurs with a rate constant of fept of 1.3 x 10 s [43]. The second process leading to the decay of the CRIP is diffusional separation to the SSRIP, kips, which occurs with a rate constant of 5 x 10 s (Scheme 2.3) [43]. Thus the efficiency of the... 

The competition between the various reactions depends on many factors, including the distance between the ions. Radical ion pairs generated by PET can be contact radical ion pairs (CRIP) or solvent separated radical ion pairs (SSRIP ... 

The observation of decreased exciplex fluorescence intensity and increased adduct formation with increasing solvent polarity (Fig. 10) led to the proposal that adduct formation proceeds via initial one-electron transfer to yield a radical ion pair, followed by proton transfer to yield a 1,2-diphenylethyl and a-di-alkylaminoalkyl radical pair, which subsequently combines to yield 63, disproportionates or diffuses apart (114). Subsequent investigation of this reaction led to the proposal that proton transfer occurs only from the initially formed exciplex or contact radical ion pair prior to solvation to yield a solvent separated radical ion pair. The detailed mechanism for reaction of It with tertiary amines in acetonitrile solution is summarized in Fig. 11 (116c). 

The reactive species generated by the photoexcitation of organic molecules in the electron-donor-acceptor systems are well established in last three decades as shown in Scheme 1. The reactivity of an exciplex and radical ion species is discussed in the following sections. The structure-reactivity relationship for the exciplexes, which possess infinite lifetimes and often emit their own fluorescence, has been shown in some selected regioselective and stereoselective photocycloadditions. However, the exciplex emission is often absent or too weak to be identified although the exciplexes are postulated in many photocycloadditions [11,12], The different reactivities among the contact radical ion pairs (polar exciplexes), solvent-separated radical ion pairs, and free-radical ions as ionic species... 

Contact and Solvent-Separated Radical Ion Pairs in Organic Photochemistry... 

---