Formation of O2(1Σg+), O2(1Δg), and O2(3Σg-) during Oxygen Quenching of nπ* Triplet Phenyl Ketones: The Role of Charge Transfer and Sensitizer-Oxygen Complex Structure

Rate constants of formation of O2(1Σg+), O2(1Δg), and O2(3Σg-) in the quenching of triplet states T1 by O2 have been determined for a series of nine benzophenones (BPs) of strongly varying oxidation potential, Eox, but almost constant triplet-state energy ET. These data are analyzed considering data determined previously for T1(ππ*) sensitizers of very different ET and Eox. Much weaker charge transfer (CT) effects are observed for the T1(nπ*) BPs compared with those obtained with a series of structurally related T1(ππ*) biphenyls. The quenching proceeds for T1(nπ*) and T1(ππ*) sensitizers via two different channels, each capable of producing O2(1Σg+), O2(1Δg), and O2(3Σg-). One channel originates from excited 1,3(T1·3Σ) complexes with no CT character and the other from 1,3(T1·3Σ) exciplexes with partial CT character. Different energy gap relations determine the formation of O2(1Σg+), O2(1Δg), and O2(3Σg-) of T1(ππ*) and T1(nπ*) sensitizers in the nCT channel, whereby the excess energy (ΔE) dependence of t...