10.1021/acs.joc.8b01173.s003 Gilbert N. Ndzeidze Gilbert N. Ndzeidze Lingzi Li Lingzi Li Mark G. Steinmetz Mark G. Steinmetz Reversible Triplet Excitation Transfer in a Trimethylene-Linked Thioxanthone and Benzothiophene-2-Carboxanilide that Photochemically Expels Leaving Group Anions American Chemical Society 2018 Trimethylene-Linked Thioxanthone trimethylene-linked systems rate constants trimethylene-linked benzothiophene -2-carboxanilide ring system Φ decreases Φ isc triplet decay rate triplet excitation transfer anilide moiety PO C -3 position Photochemically Expels electrocyclic ring closure triplet energy transfer state reaction benzothiophene ring CO k r Group Anions C -3 chloro-substituted benzothiophene -2-carboxanilide Cl state decay LG Reversible Triplet Excitation Transfer Benzothiophene -2-Carboxanilide quantum efficiencies 2018-06-27 00:00:00 Dataset https://acs.figshare.com/articles/dataset/Reversible_Triplet_Excitation_Transfer_in_a_Trimethylene-Linked_Thioxanthone_and_Benzothiophene-2-Carboxanilide_that_Photochemically_Expels_Leaving_Group_Anions/6837572 The triplet excited state of thioxanthone produced by photolysis undergoes reversible triplet energy transfer with a trimethylene-linked benzothiophene-2-carboxanilide ring system. The ensuing electrocyclic ring closure of the anilide moiety produces a putative zwitterionic intermediate that is capable of expelling leaving groups (LG<sup>–</sup>) from the C-3 position of the benzothiophene ring. Stern–Volmer quenching studies with cyclohexadiene as quencher furnish the rate constants for the triplet excitation transfer in the forward and reverse directions, which can be expressed as an equilibrium constant <i>K</i> = 0.058. Overall, the rate of the triplet excited state reaction becomes <i>K</i> × <i>k</i><sub>r</sub> = 5.7 × 10<sup>4</sup> s<sup>–1</sup> for LG<sup>–</sup> = Cl<sup>–</sup>, where <i>k</i><sub>r</sub> is the triplet decay rate of the C-3 chloro-substituted benzothiophene-2-carboxanilide, found through Stern–Volmer quenching. The high quantum efficiencies found for the trimethylene-linked systems are due to <i>K</i> × <i>k</i><sub>r</sub> being competitive with the triplet excited state decay of the thioxanthone of <i>k</i><sub>d</sub> = 7.7 × 10<sup>4</sup> s<sup>–1</sup>. On the basis of Φ<sub>isc</sub> = 0.68, the overall expected quantum yield for direct photolysis should be 0.50 for LG<sup>–</sup> = Cl<sup>–</sup> as compared to 0.41 at 25 °C experimentally. Φ decreases with increasing basicity of the leaving group (LG<sup>–</sup> = Cl<sup>–</sup>, (EtO)<sub>2</sub>PO<sub>2</sub><sup>–</sup>, PhCH<sub>2</sub>CO<sub>2</sub><sup>–</sup>, PhS<sup>–</sup>, and PhCH<sub>2</sub>S<sup>–</sup>).