Abstract:

2, 4-Dithiouracil is potentially an important photosensitizer for use in photodynamic therapy. Its photophysics when populated in the lowest excited state has been studied extensively. However, its higher light absorbing excited states and the corresponding reaction dynamics have not been investigated sufficiently. Herein, the resonance Raman spectroscopy and density functional theory were adopted to clarify the electronic transitions associated with the UV absorptions in the far-UV region and the short-time structural dynamics corresponding to the higher light absorbing excited states. The UV absorption spectrum in acetonitrile was deconvoluted into four bands:the moderate intense absorption band at 358 nm (f=0.0336) (A band), the intense broad absorption bands at 338 nm (f=0.1491), 301 nm (f=0.1795), and 278 nm (f=0.3532) (B, C, and D bands) respectively, on the basis of the relationship between the resonance Raman intensities and the oscillator strength f. The result was consistent with the predictions made using the time-dependent density functional theory calculations and the resonance Raman intensity patterns. Thus, the four bands resulted from the deconvolution are assigned as the S₀→S₂, S₀→S₆, S₀→S₇ and S₀→S₈ transitions, respectively. The resonance Raman spectra of the corresponding B, C, and D bands are assigned and the qualitative short-time structural dynamics are obtained. The major character in the short-time structural dynamics of 2, 4-dithiouracil in the S₈ excited state is that a non-adiabatic process via S₈(ππ^*)/S(nπ^*) curve-crossing, accompanied with ultrafast structural distortion, takes place in or near the Franck-Condon region, while the major character in the short-time structural dynamics in the S₇ and S₆ excited state appears in the multidimensional reaction coordinates, which are mostly along the C₅C₆/C₂S₈/C₄S₁₀/N₂C₃ bond lengths + C₄N₃H₉/N₁C₂N₃/C₂N₁C₆/C₆N₁H₇/C₅C₆H₁₂ bond angles for the S₇ excited state and the C₅C₆/N₃C₂/C₄S₁₀/C₂S₈ bond lengths + C₆N₁H₇/C₅C₆H₁₂/C₅C₆N₁/C₅C₆H₁₂/C₂N₁C₆/N₁C₂N₃/C₄N₃H₉/N₁C₂N₃ bond angles for the S₆ excited state.

Key words: 2, 4-Dithiouracil, Excited state structural dynamics, UV absorption spectrum, Resonance Raman spectrum, Density functional calculation