Ultrafast Raman Loss Spectroscopy Unravels the Dynamics in Entangled Singlet and Triplet States in Thioxanthone.

Thioxanthone (TX), an aromatic ketone, exhibits significant solvent-dependent photophysical properties. Herein, we employed time-resolved ultrafast Raman loss spectroscopy (URLS) to decipher the solvent-dependent structural dynamics in entangled singlet and triplet states of photoexcited TX. The evolution of the vibrational spectrum reveals structural changes that occur during the intersystem-crossing (ISC) process and the subsequent energy dissipation to the surrounding solvent. The C═O stretch (∼1320 cm) of TX in the excited state acts as the marker band as it undergoes a red shift with time constants of ∼45 and ∼5 ps in acetonitrile and methanol, respectively. Such a red shift is an indicator of the softening of the bond due to the change in the electronic spin states. We also observed a blue shift in Raman frequencies corresponding to the C═C stretch and the C═O stretching modes of TX in acetonitrile and methanol, indicating vibrational cooling in the excited singlet and triplet states. In the case of TX in cyclohexane, vibrational modes at 190 and 415 cm exhibit a blue shift with a time constant of ∼700 fs, which represents the structural distortion during internal conversion (S → S) process. The kinetics of amplitudes of these modes follows biexponential growth with time constants of ∼3 and ∼14 ps representing the time scales for the ISC process and the planarization process in the triplet state, respectively. The URLS study therefore provides a direct measure of the various stages of the solvent-dependent structural dynamics in the excited state of TX.