A Competition among\nThree Ultrafast Photoinduced Events\nin Thiotropolone: Internal Conversion vs Intersystem Crossing vs ESIPT

Gas phase excited-state quantum wavepacket dynamics simulations\nof the thiotropolone demonstrate the ultrafast triplet formation upon\nphotoexcitation to the dipole-allowed S<sub>2</sub> state. The dominant\nrelaxation pathway of the S<sub>2</sub>–T<sub>4</sub> intersystem\ncrossing, facilitated by the strong spin–orbit coupling and\nnarrow energy gap, competes with the S<sub>2</sub> to S<sub>1</sub>/S<sub>3</sub> internal conversion. The wavepacket populated in T<sub>4</sub> via the former pathway decays to lower triplet states. Computed\npotential energy profiles suggest proton transfer via S<sub>2</sub>, which might compete with internal conversion and intersystem crossing.\nThe nonadiabatic population transfer from the S<sub>2</sub> to S<sub>1</sub>/S<sub>3</sub> states enables proton transfer via the latter\nstates, resulting in multiple proton transfer pathways. Experimental\ninvestigations are necessary to shed light on the complex ultrafast\nphotodynamics of thiotropolone.