Reversible Triplet Excitation
Transfer in a Trimethylene-Linked
Thioxanthone and Benzothiophene-2-Carboxanilide that Photochemically
Expels Leaving Group Anions
Gilbert
N. Ndzeidze
Lingzi Li
Mark G. Steinmetz
10.1021/acs.joc.8b01173.s005
https://acs.figshare.com/articles/journal_contribution/Reversible_Triplet_Excitation_Transfer_in_a_Trimethylene-Linked_Thioxanthone_and_Benzothiophene-2-Carboxanilide_that_Photochemically_Expels_Leaving_Group_Anions/6837566
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>).
2018-06-27 00:00:00
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