Tuning the Triplet Formation Efficiency by Heavy-Atom Substitution in 3-Hydroxythiochromone

We theoretically studied the triplet formation efficiency in positional isomers of bromine-substituted 3-Hydroxythiochromones. Dynamics simulations with relevant spin-orbit coupling parameters show ultrafast triplet formation with S₁ as the donor singlet and upper triplet excited states as receiver states. The near-degeneracy of S₂ with S₁ promotes nonadiabatic population transfer from S₁ to S₂ in isomers with bromine substitution at the 5th position of the parent molecule. This population transfer unfurls an additional intersystem crossing pathway involving S₂ and T₄, enabling this isomer to show a higher triplet efficiency. The excited-state intramolecular proton transfer process, promoted by low barrier energy, is operative and can affect the isomers' triplet formation efficiency. Moreover, the timescales of proton transfer and triplet formation can overlap, necessitating thorough experimental investigations to uncover the competitiveness of these simultaneous events.