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Recently we have reported in situ generation of electrophilic selenium (PhSe+) species via diphenyl diselenide radical cation (PhSeSePh)*+ formed by single electron transfer (SET) from PhSeSePh (1) to the excited singlet of 1,4-dicyanonaphthalene (DCN,2).* 1We report herein electron transfer (ET) from compounds possessing a carbon-selenium bond to 'DCN* leading to heterolytic C-Se bond dissociation via a radical cation intermediate with concurrent demonstration of a new, very mild, synthetically promising way of effecting deselenenylation reactions.Organoselenium compounds 5-8 (Scheme I) efficiently quench the fluorescence of 2 with a diffusion-controlled2 rate constant, ( ). Emission and absorption spectroscopic properties of mixtures of 5-8 and 2 fail to reveal ground or excited state complex formation in polar and nonpolar solvents. A SET mechanism for the fluorescence quenching of 2 by 5-8 is supported by the endoergic value for the free energy change (AGET) estimated by the Weller equation3 and the correlation plot of log AL^v s (£j/2(ox))4 of 5-8, which showed a linear relationship5 (Table I). This suggested the possibility of performing photosensitized SET initiated reaction from compounds 5-8. To test this concept, we irradiated6 78(>280 nm) a mixture of 5-8 (3 mM) with 2 (0.3 mM) in methanol, for which the details are given in Table I. Bibenzyl formation from compound 6 seems to occur by deselenenylation of PhCH2Se* (PhCH2Se* -PhCH2* + Se), which is evident from the observation of only a trace amount of bibenzyl (3-6%) compared to other products.7The above results can be rationalized with reference to Scheme I, which considers the intermediacy of an exciplex with partial charge-transfer character. A double reciprocal plot of quantum yield ( ^ ) vs 5-8 concentration ( ~ vs [Q]"1) resulted a straight line with an identical AfqETT (intercept/slope) as obtained from fluorescence measurements (Table I), which supports the above argument. Electron transfer between 5-8 and 2 results in generation of the solvent-separated ion pair (SSIP). The corresponding radical cation undergoes C-Se bond cleavage (58 kcal/mol),8 the carbonium ion escaping and the radical reacting in the cage. In the polar media R|+ is more stabilized than the radical R2Se* so that the former diffuses and the latter reacts with 7IICT Communication No. 2425.

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