Spectral and photophysical properties of the lowest excited triplet state of 4-thiouridine and its 5-halogeno derivatives

Abstract

The properties of the lowest excited triplet state (T 1) of 2′,3′,5′-tri- O-acetyl-4-thiouridine (TU) and its 5-halogeno substituted derivatives (XTU, X = F, Cl, Br and I) in CCl 4 solutions are presented. Steady-state and pulsed techniques were used to characterize the T 1 state of TU and XTU in terms of the phosphorescence quantum yields ( φ p 0), intrinsic lifetimes ( τ T 1 0) and the self-quenching rate constants ( k sq). The quantum yields of photochemical decay ( ϕ pch) of TU and XTU were also determined. From the values of quantum yields and under assumption that the formation of the T 1 state can be approximated by ϕ T 1 =1, the rate constants of radiative ( k p) and non-radiative ( k nr) decay processes from the T 1 state were calculated. The results of the studies show that the T 1 states of TU and XTU have a ππ ∗ nature. Comparison of the absorption and phosphorescence spectral properties of TU and XTU molecules with those of 1,3-dimethyl-4-thiouracil (DMTU) confirms that TU and XTU exist in CCl 4 solution as (C-2)-keto-(C-4)-thione tautomers, exclusively, and that the phosphorescence arises from the excited T 1 state of these single tautomeric forms. The effect of the halogen substituent on the T 1 state properties of XTU molecules is discussed.