N-alkoxyheterocycles as irreversible photooxidants

Irreversible photooxidation based on N-O bond fragmentation is demonstrated for N-methoxyheterocycles in both the singlet and triplet excited state manifolds. The energetic requirements for bond fragmentation are studied in detail. Bond fragmentation in the excited singlet manifold is possible for ππ* singlet states with energies significantly larger than the N-O bond dissociation energy of ca 55 kcal mol^-1. For the nπ* triplet states, N-O bond fragmentation does not occur in the excited state for orbital overlap and energetic reasons. Irreversible photooxidation occurs in the singlet states by bond fragmentation followed by electron transfer. Irreversible photooxidation occurs in the triplet states via bimolecular electron transfer to the donor followed by bond fragmentation. Using these two sensitization schemes, donors can be irreversibly oxidized with oxidation potentials ranging from ca 1.6-2.2 V vs SCE. The corresponding N-ethylheterocycles are characterized as conventional reversible photooxidants in their triplet states. The utility of these sensitizers is demonstrated by irreversibly generating the guanosine radical cation in buffered aqueous solution. The return or reverse electron transfer that occurs in the primary charge-separated pair that is a feature of all photoinduced electron transfer reactions wastes energy and competes with useful chemistry in the charge-separated species. Here, we describe photoinduced electron transfer schemes that made irreversible by using fragmentable sensitizers. The fragmentable sensitizers are N-methoxyheterocycles that cleave a relatively weak N-O bond in either their reduced or excited forms. Sensitization mechanisms in both the singlet and the triplet manifolds are described. Irreversible oxidation of guanosine is demonstrated as an illustrative example.