Versatile transformations of azo compounds are utilised not only in synthetic organic chemistry but also in materials science. In this study, a hitherto unknown stereoselectivity was observed by low-temperature in situ NMR spectroscopy for the photochemical denitrogenation of a cyclic azoalkane (2,3-diazabicyclo[2.2.1]heptane) derivative. Direct (singlet) photodenitrogenation at 188 K afforded two products, the configurationally retained ring-closed compound (ret-CP) and the inverted compound (inv-CP), in a ratio of 82/18 (+/- 3) (ret-CP/inv-CP), with an overall yield of >95%. Triplet-sensitized denitrogenation at 199 K using benzophenone ((BP)-B-3*) or xanthone ((3)Xan*) selectively produced inv-CP, with a ret-CP/inv-CP ratio of 7/93 (+/- 3). Thermal isomerization of inv-CP into ret-CP was observed by low-temperature NMR spectroscopy. Transient absorption spectroscopy revealed that two distinct singlet diradicals are involved in the formation of CP during direct photodenitrogenation, that is, puckered puc-(DR)-D-1 and planar pl-(DR)-D-1 diradicals. The former produces ret-CP, whereas the latter affords inv-CP. Kinetic analysis using the integrated profiles method was used to determine the molecular absorption coefficient of pl-(DR)-D-1 (epsilon(560) = 4900 +/- 250 M-1 cm(-1)) for the first time. The involvement of the puckered singlet diradical resolves the mechanistic puzzle of stereoselective denitrogenation of diazabicycloheptane-type azoalkanes.