Ultrafast electronic relaxation of nucleobases from ¹ππ* states to the ground state (S₀) is considered essential for the photostability of DNA. However, transient absorption spectroscopy (TAS) has indicated that some nucleobases in aqueous solutions create long-lived ¹nπ*/³ππ* dark states from the ¹ππ* states with a high quantum yield of 0.4–0.5. We investigated electronic relaxation in pyrimidine nucleobases in both aqueous solutions and the gas phase using extreme ultraviolet (EUV) time-resolved photoelectron spectroscopy. Femtosecond EUV probe pulses cause ionization from all electronic states involved in the relaxation process, providing a clear overview of the electronic dynamics. The ¹nπ* quantum yields for aqueous cytidine and uracil (Ura) derivatives were found to be considerably lower (<0.07) than previous estimates reported by TAS. On the other hand, aqueous thymine (Thy) and thymidine exhibited a longer ¹ππ* lifetime and a higher quantum yield (0.12–0.22) for the ¹nπ* state. A similar trend was found for isolated Thy and Ura in the gas phase: the ¹ππ* lifetimes are 39 and 17 fs and the quantum yield for ¹nπ* are 1.0 and 0.45 for Thy and Ura, respectively. The result indicates that single methylation to the C₅ position hinders the out-of-plane deformation that drives the system to the conical intersection region between ¹ππ* and S₀, providing a large impact on the photophysics/photochemistry of a pyrimidine nucleobase. The significant reduction of ¹nπ* yield in aqueous solution is ascribed to the destabilization of the ¹nπ* state induced by hydrogen bonding.

中文翻译：

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使用极紫外时间分辨光电子能谱研究嘧啶核碱基电子弛豫过程中长寿命暗态的形成

^{核碱基从1} ππ* 态到基态 (S ₀ )的超快电子弛豫被认为对 DNA 的光稳定性至关重要。^{然而，瞬态吸收光谱 (TAS) 表明，水溶液中的一些核碱基从1} ππ* 态产生长寿命的¹ nπ*/ ³ ππ* 暗态，量子产率为 0.4–0.5。我们使用极紫外 (EUV) 时间分辨光电子能谱研究了水溶液和气相中嘧啶碱基的电子弛豫。飞秒 EUV 探测脉冲引起弛豫过程中涉及的所有电子态的电离，从而提供电子动力学的清晰概览。1 ^_发现水性胞苷和尿嘧啶 (Ura) 衍生物的 nπ* 量子产率比 TAS 先前报告的估计低得多 (<0.07)。另一方面，水性胸腺嘧啶 (Thy) 和胸苷在¹ nπ* 状态下表现出更长的¹ ππ* 寿命和更高的量子产率 (0.12–0.22) 。在气相中分离的 Thy 和 Ura 也发现了类似的趋势： Thy 和 Ura 的^{1 ππ* 寿命分别为 39 和 17 fs， 1} nπ*的量子产率分别为 1.0 和 0.45。结果表明，C _{5位置的单甲基化阻碍了将系统驱动到}¹ ππ* 和 S ₀之间的锥形交叉区域的平面外变形, 对嘧啶核碱基的光物理/光化学产生重大影响。水溶液中¹ nπ* 产率的显着降低归因于氢键引起的^{1 nπ* 态的不稳定。}