LUMINESCENCE AND PHOTOCHEMISTRY OF 4-THIOURIDINE IN AQUEOUS SOLUTION

Abstract— In bidistilled water, 4-thiouridine (4TU) exhibits a weak unusual luminescence, the quantum yield of which is 3 × 10^-4 at 25°C. The excitation spectrum corresponds well to the 4TU absorption spectrum. The emission lies at longer wavelengths (Λ_max 550 nm) than the 4TU phosphorescence observed at 77 K (Λ_max, 470–480 nm). From the emission signal obtained after an excitation flash of 3 ns half-width, an "apparent" rate constant for the radiative deactivation process, shorter than 5 × 10⁶ s, can be inferred. The 300 K emission is efficiently quenched by halides and by oxygen: quenching involves a long-lived intermediate (? 200 ns).
Clearly the emissive state X is populated through the S₀-S₁ electronic transition π→π* of 4TU. The nature of X cannot be unambiguously determined: it cannot be an excimer but can be either the 4TU triplet state or another chemical state distinct from the 4TU excited singlet or triplet states.
An interesting finding is that the 300 K emission and the ability of 4TU to photoreact are related: they are quenched with the same efficiency by halide anions. This indicates that quenching occurs at the same long-lived intermediate species , which is either a precursor of the emitter or the emitter itself.