Relaxation of the T1 excited state of 2-thiothymine, its riboside and deoxyriboside-enhanced nonradiative decay rate induced by sugar substituent

• The triplet states of 2-thiothymine, its riboside and deoxyriboside were examined.
• The T1 states were formed on femtosecond time scale and with unit efficiency.
• Lifetimes of the nucleosides’ T1 states differed considerably from that of the base.
• The reason for the enhanced rate of nonradiative decay in the nucleosides is discussed.

UV absorption, circular dichroism and emission spectroscopy as well as nanosecond and femtosecond transient absorption measurements were used to characterize the excited states of 2-thiothymine (2TT), its riboside (2TTR) and deoxyriboside (2TTD) in acetonitrile (ACN) solution. The lowest triplet state (T1) could be observed exclusively in the experiments. Upon excitation to higher singlet states (λexc = 266 nm), the T1 (ππ*) states of the investigated compounds were formed on an ultrafast time scale (kT ≥ 3 × 1012 s−1) with an efficiency approaching unity (ϕT = 0.9 ± 0.1). These T1 states were characterized by their intrinsic lifetimes (τT0) and rate constants of self-quenching (kSQ), phosphorescence (kP), and nonradiative processes (∑kNR). In the series of compounds studied only 2TT exhibited a weak room-temperature phosphorescence, and the spectrum of this 2TT emission was recorded for the first time. In the absence of self quenching, nonradiative processes (NR) are the dominant (ϕNR ∼ 0.9) channel of the decay of 2TT and its nucleosides in their T1 states. Despite the chromophore being the same in all of the compounds studied, the decay dynamics of the nucleosides’ T1 states differed considerably from that of the T1 state of 2TT. The ∑kNR values determined for the derivatives containing a ribosyl (in 2TTR) or a deoxyribosyl (in 2TTD) substituent in the position α to the thiocarbonyl group were an order of magnitude larger, and as a consequence, the lifetimes (τT0) shorter (by factor of 14 and 22 for 2TTD and 2TTR, respectively) as compared to 2TT. The reason for the enhanced rate of nonradiative decay in the nucleosides is discussed based on the results obtained from additional experiments including the determination of the T1 lifetime for the deuterated derivative of 2TTR and the intermolecular quenching of 2TT by ribose molecules. The factors which might be responsible for this substituent effect (on the nonradiative decay) such as an intramolecular or an intermolecular H bond formation involving sugar OH groups as well as a reversible H abstraction from the sugar α-substituent in the nucleosides appear to be not important.