Mechanism and dynamics of intramolecular triplet state decay of 1-propyl-4-thiouracil and its α-methyl-substituted derivatives studied in perfluoro-1,3-dimethylcyclohexane

Abstract

The absorption, phosphorescence and phosphorescence excitation spectra, phosphorescence quantum yields, and T(1) excited state lifetimes of four 4-thiouracil derivatives were measured for the first time in chemically inert and very weakly interacting perfluoro-1,3-dimethylcyclohexane at room temperature. The set of the 4-thiouracil derivatives comprises 1-propyl-4-thiouracil (PTU) and the related compounds having a methyl substituent at the position α to the thiocarbonyl group, namely 1-propyl-4-thiothymine (PTT), 1,3-dimethyl-4-thiouracil (DMTU), and 1-methyl-3-trideuteriomethyl-4-thiouracil ([D(3)]DMTU). Quantitative information on the intramolecular decay of the T(1) excited state of the four 4-thiouracil derivatives is presented, and the mechanism and dynamics of this process are discussed. In the absence of self quenching and solvent induced deactivation, the T(1) decay of the four 4-thiouracil derivatives was dominated by intramolecular nonradiative processes (NR). The values of the rate constant k(NR) in DMTU and [D(3)]DMTU are about 4 times larger than that in PTT and about 3 times larger than that in PTU. The reasons for the enhanced nonradiative rate constant in DMTU are discussed. It is concluded that the faster rate of the nonradiative processes in DMTU is related to a larger contribution from mixing of the T(2) (nπ*) state into the lowest energy T(1) (ππ*) state, as compared to the analogous coupling in PTU and PTT. This conclusion is supported by ab initio calculations performed at the EOM-CC2/aug-cc-pVDZ level of theory. The energy spacing between the T(2) (nπ*) and T(1) (ππ*) states is estimated to be about 500, 1100, and 2000 cm(-1) for DMTU, PTU, and PTT, respectively. Among the three compounds in question, the predicted energy spacing is thus the smallest for DMTU.

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

“…Calculations [17,18,196,203] and experimental results agree that the T 1 state has primarily ππ* character and can decay by radiative (minor pathway; Φ Ph~1 0 À2 to 10 À4 ) and nonradiative (major pathway; Φ NR ! 0.99) processes (Table 11).…”

“…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]. Additional absorption bands are observed closer to 200 nm (see, for example, Fig.…”

“…In general, solvent quenching dominates the deactivation of the T 1 (ππ*) state in polar solvents, whereas triplet self-quenching plays an important role in less polar solvents, even at low concentrations (<10 À5 M, see Table 13). Wenska and co-workers [203] also investigated the intramolecular decay pathways of the triplet state in a group of 4-thiouracil derivatives in PFDMCH. The authors used steady-state absorption and emission spectroscopy, ab initio calculations in vacuum at the EOM-CC2/aug-cc-pVDZ level of theory, and time-resolved emission spectroscopy.…”

“…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.…”

“…Among the nonradiative decay processes, a reversible, intramolecular H-abstraction pathway, in which the excited thiocarbonyl group abstracts a H-atom from the methyl group in the ortho position of the 1,3-dimethyl-4-thiouracil ring, was proposed to be the fastest decay channel of the S 2 (ππ*) state with an estimated rate constant of 1.2 Â 10 11 s À1 in PFDMCH and CCl 4 . 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].…”