Gas phase excited-state quantum wavepacket dynamics simulations
of the thiotropolone demonstrate the ultrafast triplet formation upon
photoexcitation to the dipole-allowed S2 state. The dominant
relaxation pathway of the S2–T4 intersystem
crossing, facilitated by the strong spin–orbit coupling and
narrow energy gap, competes with the S2 to S1/S3 internal conversion. The wavepacket populated in T4 via the former pathway decays to lower triplet states. Computed
potential energy profiles suggest proton transfer via S2, which might compete with internal conversion and intersystem crossing.
The nonadiabatic population transfer from the S2 to S1/S3 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.