Thiouridine spectroscopy. Temperature dependences and decay kinetics in the deactivation of excited states

Aqueous 4‐thiouridine, when excited at room temperature in its last absorption band at 335 nm, shows a weak emission at 550 nm. The intensity of this emission increases with decreasing temperature and below the freezing point (of ethylene glycol‐water 1 : 1) an additional peak, centered at $\text{∼ 475\hspace{0.17em}}\mathrm{nm}$ , appears. Further lowering of the temperature causes a very drastic increase in emission with the 475 nm peak becoming more prominent. Decay measurements show that the 550 nm emission has a lifetime of $\text{∼ 250\hspace{0.17em}}\mathrm{nsec}$ at room temperature which increases to about 380 μsec at 77 °K. The 475 nm emission has at all temperatures a lifetime 2–3 times longer than that at 550 nm. It is suggested that both emissions are due to phosphorescence the 475 nm peak representing the triplet of thiouridine while the 550 nm emission stems from either the triplet whose solvent sheath has undergone a rearrangement or from the triplet of a species formed by a monomolecular process from excited thiouridine.