Photoreduction of polycyclic DBO-type azoalkanes with amines to hydrazine products

The n,pi*-singlet-excited polycyclic, alpha-substituted DBO-type azoalkanes 1 are reduced by amines to form the corresponding hydrazines 2 as photoreduction products in competition to denitrogenation to the bicyclo[2.2.0]hexane photoproducts 3. The photophysical, photochemical, and electrochemical properties of the azoalkane determine the product ratio of the hydrazine 2 versus the bicyclo[2.2.0]hexane 3. The quantum yields of the photoreduction are quite low (<0.1), for which efficient deactivation of the singlet excited state through charge transfer is postulated. The charge transfer in the exciplex between the azoalkane and the amine promotes the hydrogen abstraction of the alpha-CH bond from the amine by the azoalkane and, thus, facilitates its photoreduction, a general photochemical process applicable to singlet- as well as triplet-excited azoalkanes.