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

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• 作者：Katarzyna Taras-Go艣li艅ska ; Gotard Burdzi艅ski ; Gra偶yna Wenska
• 关键词：2-Thiothymine ; 2-Thiothymine nucleoside ; Nanosecond transient absorption ; Femtosecond transient absorption ; Triplet lifetime ; Triplet quenching
• 刊名：Journal of Photochemistry and Photobiology A: Chemistry
• 出版年：1 February, 2014
• 年：2014
• 卷：275
• 期：Complete
• 页码：89-95
• 全文大小：1121 K

文摘

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 (位_exc = 266 nm), the T₁ (蟺蟺*) states of the investigated compounds were formed on an ultrafast time scale (k_T 鈮?#xA0;3 脳 10¹² s^鈭?) with an efficiency approaching unity (蠒_T = 0.9 卤 0.1). These T₁ states were characterized by their intrinsic lifetimes () and rate constants of self-quenching (k_SQ), phosphorescence (k_P), and nonradiative processes (鈭?em>k_NR). 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 (蠒_NR 鈭?#xA0;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鈥?T₁ states differed considerably from that of the T₁ state of 2TT. The 鈭?em>k_NR values determined for the derivatives containing a ribosyl (in 2TTR) or a deoxyribosyl (in 2TTD) substituent in the position 伪 to the thiocarbonyl group were an order of magnitude larger, and as a consequence, the lifetimes () 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 伪-substituent in the nucleosides appear to be not important.