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 the corresponding rate constants is much weaker for the T1(nπ*) ketones. In the pCT channel, the respective rate constants vary on a logarithmic scale linearly with the free energy change for complete electron transfer for both T1(ππ*) and T1(nπ*) sensitizers. This dependence too is much weaker for T1(nπ*) than for T1(ππ*) sensitizers. The comparison with CT induced quenching of O2(1Δg) by ground-state sensitizers reveals that the different electronic configurations leads to different sterical structures of 1,3(T1(nπ*)·3Σ) and 1,3(T1(ππ*)·3Σ) complexes. These differences strongly influence the complex deactivation and explain both the weaker ΔE dependence and the weaker CT effects in the quenching of T1(nπ*) by O2.