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 1 6••*, 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 1 5* that undergoes intersystem crossing to form 3 5*, which deactivates through phosphorescence. In contrast, 295-nm excitation ( π, π* band) of 5 affords the higher electronically excited state 1 5**, which undergoes adiabatic C-C bond cleavage reaction to form the excited biradical 1 6••* 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. [ABSTRACT FROM AUTHOR]