The n,π*-singlet-excited polycyclic, α-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 α-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.
The photophysical data for the polycyclic, bridgehead-substituted derivatives 1-10 of the photoreluctant diazabicyclo[2.2.2]oct-2-ene (DBO) are presented. Substitution on the bridgehead positions with radical-stabilizing substituents enhances the photoreactivity (Phi(r)) and decreases the fluorescence quantum yields (Phi(f)) and lifetimes (tau) compared to the parent DBO. The annelated rings have no influence on the photoreactivity, except when steric interactions with an alpha substituent hinder the optimal radical-stabilizing conformation. The fused rings and some of the bridgehead substituents reduce the solvent-induced quenching of the singlet-excited azo chromophore by steric shielding of the azo group and, thus, increase the fluorescence quantum yields and lifetimes.
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