A Competition among Three Ultrafast Photoinduced Events in Thiotropolone: Internal Conversion vs Intersystem Crossing vs ESIPT.

Gas phase excited-state quantum wavepacket dynamics simulations of the thiotropolone demonstrate the ultrafast triplet formation upon photoexcitation to the dipole-allowed S state. The dominant relaxation pathway of the S-T intersystem crossing, facilitated by the strong spin-orbit coupling and narrow energy gap, competes with the S to S/S internal conversion. The wavepacket populated in T via the former pathway decays to lower triplet states. Computed potential energy profiles suggest proton transfer via S, which might compete with internal conversion and intersystem crossing. The nonadiabatic population transfer from the S to S/S states enables proton transfer via the latter states, resulting in multiple proton transfer pathways. Experimental investigations are necessary to shed light on the complex ultrafast photodynamics of thiotropolone.