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 2 state. The dominant relaxation pathway of the S 2 -T 4 intersystem crossing, facilitated by the strong spin-orbit coupling and narrow energy gap, competes with the S 2 to S 1 /S 3 internal conversion. The wavepacket populated in T 4 via the former pathway decays to lower triplet states. Computed potential energy profiles suggest proton transfer via S 2 , which might compete with internal conversion and intersystem crossing. The nonadiabatic population transfer from the S 2 to S 1 /S 3 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.