Adiabatic process of higher electronically excited states: luminescence from an excited state biradical generated by irradiation of benzophenone‐substituted cyclopropanes

Adiabatic photochemical reactions of cyclopropanes possessing a benzophenone moiety (trans‐ and cis‐5), which generate a luminescent excited biradical 16••*, were studied. Various photochemical and spectroscopic techniques were used for this purpose. Excitation (350 nm, n,π* band) of 5 at 77 K affords the lowest excited state 15* that undergoes intersystem crossing to form 35*, which deactivates through phosphorescence. In contrast, 295‐nm excitation (π,π* band) of 5 affords the higher electronically excited state 15**, which undergoes adiabatic C–C bond cleavage reaction to form the excited biradical 16••* that luminesces at 630 nm. The results suggest that 5 follows 2 photoreaction modes depending on the excitation wavelength, a finding that disobeys Kasha's rule.