Oxygen Quenching of nπ* Triplet Phenyl Ketones: Local Excitation and Local Deactivation

We have studied the charge‐transfer‐induced deactivation of nπ* excited triplet states of benzophenone derivatives by O₂(³Σ), and the charge‐transfer‐induced deactivation of O₂(¹Δ_g) by ground‐state benzophenone derivatives in CH₂Cl₂ and CCl₄. The rate constants for both processes are described by Marcus electron‐transfer theory, and are compared with the respective data for a series of biphenyl and naphthalene derivatives, the triplet states of which have ππ* configuration. The results demonstrate that deactivation of the locally excited nπ* triplets occurs by local charge‐transfer and non‐charge‐transfer interactions of the oxygen molecule with the ketone carbonyl group. Relatively large intramolecular reorganization energies show that this quenching process involves large geometry changes in the benzophenone molecule, which are related to favorable Franck‐Condon factors for the deactivation of ketone‐oxygen complexes to the ground‐state molecules. This leads to large rate constants in the triplet channel, which are responsible for the low efficiencies of O₂(¹Δ_g) formation observed with nπ* excited ketones. Compared with the deactivation of ππ* triplets, the non‐charge‐transfer process is largely enhanced, and charge‐transfer interactions are less important. The deactivation of singlet oxygen by ground‐state benzophenone derivatives proceeds via interactions of O₂(¹Δ_g) with the Ph rings.