2-thiouracil intersystem crossing photodynamics study by wavelength dependent photoelectron spectroscopy

This work presents the results of wavelength dependent Time-Resolved Photoelectron Spectroscopy and Ion Yield studies of 2-thiouracil, which can potentially lead to a better understanding of the phototherapeutic effects of thiouracil-based drugs used in cancer treatment applications. The experiments provide more information about the molecules relaxation dynamics, especially its intersystem crossing pathways and high yield of long-lived triplet states that determines the drugs effectiveness. The results present evidence of UV excitation to the S2(1*) state with ultrafast (<100fs) internal conversion to the S1(1n*) state and highly efficient intersystem crossing to the triplet manifold, in hundreds of femtoseconds. Finally, relaxation to the ground state takes place on a timescale of hundreds of picosecond. A trend is observed where excitation to higher vibrational levels of the S2(*) states results in overall faster dynamics through conical intersections that are accessed by out-of-plane distortion of the thiocarbonyl group with respect to the molecular ring.