Intra- and Intermolecular Electronic Relaxation of the Second Excited Singlet and the Lowest Excited Triplet States of 1,3-Dimethyl-4-thiouracil in Solution¶

Abstract

Intramolecular processes of deactivation of 1,3‐dimethyl‐4‐thiouracil (DMTU) from the second excited singlet (S2) (π, π*) and the lowest excited triplet (T1) (π, π*) states have been studied using perfluoro‐1,3‐dimethylcyclohexane (PFDMCH) as a solvent. The spectral and photophysical (PP) properties of DMTU in CCl4, hexane and water have also been described. For the first time, the fluorescence from S2 state DMTU has been observed. The picosecond lifetime of DMTU in the S2 state (τ S 2) in PFDMCH has been proposed to be determined by a very fast intramolecular reversible process of hydrogen abstraction from the ortho methyl group by the thiocarbonyl group. The shortening of τ S 2 in CCl4 is interpreted to be caused by the intermolecular interactions between DMTU (S2) and the solvent. Results of the phosphorescence decay as a function of DMTU concentration were analyzed using the Stern–Volmer formalism, which enabled determination of the intrinsic lifetime of the T1 state (τ0 T 1) and rate constants of self‐quenching (ksq). The lifetimes, τ0 T 1, of DMTU in PFDMCH and CCl4 are much longer than the values hitherto obtained in more reactive solvents. The PP properties of DMTU both in the S2 and T1 states have been shown to be determined by the thiocarbonyl group.

Photochemistry of Nucleic Acid Bases and Their Thio- and Aza-Analogues in Solution

“…S 2 (ππ*) and the S 0 ! S 1 (nπ*) electronic transitions, respectively [10,18,19,34,200,201]. These assignments are supported by quantum chemical calculations [11,17,18,196,[202][203][204].…”

“…This absorption band is not observed in the absorption spectra of the pyrimidine bases ( Table 2). Figure 4 shows the absorption spectra of 1,3-dimethyl-4-thiouracil in CCl 4 [200] and 4-thiothymine deoxyribonucleoside in aqueous buffer solution at pH 7.4 [18] as representative examples. The red-most absorption bands have been assigned to the S 0 !…”

“…S 0 radiative transitions in the 4-thione, 2-keto uracil derivatives and exhibit fluorescence and phosphorescence quantum yields in the ranges of (0.4-1.5) Â 10 À4 and (0.2-123) Â 10 À4 , respectively, depending on the solvent and the concentration of the solute used [10,13,34,200,201,203]. In addition, some of the photophysical properties have been measured at infinite dilution in order to determine intrinsic triplet decay lifetimes and phosphorescence quantum yields (Tables 10, 12, 13, and 14) [200,203]. It is clear from those experiments that the concentration of the thiobase plays an important role in the quenching of the triplet state population in the thiobase monomers in solution.…”

“…The first investigation of the fluorescence properties of a 4-thiouracil derivative was reported in 2002 by Skalski and co-workers [200]. The authors used steady-state and picosecond laser spectroscopies to determine the fluorescence yield and lifetime of 1,3-dimethyl-4-thiouracil in perfluoro-1,3-dimethylcyclohexane (PFDMCH), CCl 4 , hexane, and water.…”

“…This intramolecular relaxation pathway has been put into question more recently [203]. S 2 (ππ*) to S 1 (nπ*) internal conversion was estimated to be an [200]. No assignments were given regarding the particular excited-state pathways or species responsible for these fluorescence decay lifetimes.…”