Photoreduction, azoalkanes

W. Adam and A. V. Trofimov, Photomechanistic aspects of bicyclic azoalkanes triplet states, photoreduction, and double inversion. Chapter 93 in ref. 22. 

Photomechanistic Aspects of Bicyclic Azoalkanes Triplet States, Photoreduction, and Double Inversion... 

Different mechanisms have been proposed for the photoreduction of azoalkanes by a number of hydrogen donors (1,4-cylohexadiene, alcohols, stannanes, and silanes) and amines. While direct hydrogen abstraction operates for the former group of donors, amines react with the triplet azoalkanes by charge transfer.The mechanistic details of photoreduction by the direct hydrogen donors depend on the donor structure however, intervention of the hydrazinyl radical 13 (Scheme 2) is a common feature of these photoreduction processes. 

In contrast, photoreduction by amines obeys a general mechanism (Scheme 3) that involves a charge-transfer process. It should be noted that the azoalkane 3c, which exhibits a low intersystem crossing (,sc ca. 0.1), resists photoreduction. The efficiencies of photoreduction for the azoalkanes 3 by amines... 

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