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) (pi, pi*) and the lowest excited triplet (T1) (pi, pi*) 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 (tau(S2)) 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 tau(S2) 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 (tau0(T1)) and rate constants of self-quenching (k(sq)). The lifetimes, tau0(T1), 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.

Intersystem crossing dynamics in singly substituted thiouracil studied by time-resolved photoelectron spectroscopy: Micro-environmental effects due to sulfur position

“…In stark contrast to 2-TU, which efficiently intersystem crosses back to the ground state, the population remains trapped on the lowest triplet excited state of 4-TU. This is consistent with the theoretical prediction of a higher activation barrier in the case of 4-TU and further supported by the observation of phosphorescence [34,[54][55][56] in other 4-TU derivatives.…”

2-Thiouracil intersystem crossing photodynamics studied by wavelength-dependent photoelectron and transient absorption spectroscopies

“…Taking into consideration all the experimental and computational data available to date, 31,33-37 we assign t solv 2 upon 268 or 290 nm excitation to vibrational cooling dynamics in the triplet state-analogous to 2-thiothymine. 59 The narrowing of the transient absorption spectra during this lifetime supports this assignment. The different observations when excitation is performed at 290 or 268 nm versus at 316 nm can be explained by the different amount of vibrational energy in the triplet state.…”

Triplet state formation and quenching dynamics of 2-mercaptobenzothiazole in solution

“…20,43,44 Variations in T1 quantum yields reflect subtle differences in the ultrafast ISC versus IC dynamics, which were not directly probed in the prior studies using microsecond flash photolysis.…”