Optically detected magnetic resonance study of the phosphorescent states of thiouracils

The triplet states of 1-methyl-2-thiouracil (1-Me-s²U), 1-methyl-4-thiouracil (1-Me-s²s⁴U) and 1-methyl-2,4-dithiouracil (1-Me-s² s⁴) have been investigated by optically detected magnetic resonance in zero magnetic field. The zero field splittings (ZFS) and the individual sublevel kinetic parameters are reported. The ZFS (|D|, |E|) values (in cm^?1) are found to increase in the order: 1-Me-s² U (0.2895, 0.0728) < 1-Me-s⁴U, (0.605, 0.0500) < 1-Me-s²s⁴ U (0.870, 0.0458). The triplet state lifetimes decrease in the same order, and both effects are attributed to an internal heavy atom effect of sulfur substitution. The vibronic structure of the phosphorescence emission indicates that the thiouracil phosphorescent states are ³(π, π^*). The low phosphorescence quantum yields of 1-Me-s⁴ U and of 1-Me-s²s⁴U result from radiationless decay of the triplet state rather than from inefficient intersystem crossing from the excited singlet state. The efficient radiationless decay of the triplet state appears to be a feature of the S-substitution at the 4-position of uracil. Phosphorescence polarization measurements of the individuals triplet sublevel emissions at ca. 1.2 K are consistent with 1-Me-s²U and 1-Me-s⁴U being non-planar in the phosphorescent state; the thiouracil phosphorescence from each triplet sublevel is polarized in the average plane of the distorted molecule. In the absence of σπ separability, spin—orbit mixing of ¹(π, π^*) and ³(π, π^*) states is enhanced and the radiative properties of the triplet state may be dominated by this mechanism rather than by the mixing of ¹(n, π^*), ¹(σ, π^*), or ¹(π, σ^*) with ³(π, π^* states which generally is the dominant mechanism for planar aromatic molecules.