Semiempirical valence-electron calculations of excited state geometries and vibrational frequencies

Wavelength-Dependent Photochemistry of 4-Methoxybicyclo[3.1.0]hexenones

Wavelength-dependent photochemistry of a series of 4-methoxybicyclo[3.1.0]hexenones is mechanistically investigated. Upon excitation of the symmetry-forbidden S(1) n-->pi state, photoisomerization is exclusively observed. By comparison of the direct and sensitized quantum yields for this particular process, the T(1) pi-->pi state, which is populated via efficient intersystem crossing as shown by phosphorescence experiments, is proven to be the reactive state. A pathway via external cyclopropane bond cleavage followed by racemization of the formed triplet biradicals reasonably accounts for the observed photostationarity. Excitation of the electric dipole allowed S(2) pi-->pi state initiates phenol formation. The T(2) n-->pi state, which could not be sensitized, gives a type-B oxyallyl zwitterion via internal cleavage as could be shown by intermolecular as well as intramolecular trapping experiments. The observed data are in good agreement with computational studies.