Understanding the excited state dynamics of pyrimidine nucleobases is very important because longer-lived excited states that may induce DNA photodamage were discovered in these molecules. The role of triplet nπ* state in pyrimidine bases excited state dynamics is controversy and direct observation of this state is difficult due to the low triplet quantum yields in canonical DNA nucleobases. Among DNA derivative, aza-nucleobases have highly structure similarity with canonical nucleobases and high triplet state quantum yields were reported in them. In this work, excited state dynamics of 6-azathymine (6-AT) and 6-azathymidine (6-ATD) are reported. Broadband transient absorption measurements reveal that there are two pathways for ultrafast intersystem crossing (ISC) in 6-AT. The first one is direct ISC from the initial excited ππ* (S₂) state to a triplet nπ* state (T₂) followed by ultrafast internal conversion (IC) to a triplet ππ* state (T₁). The other route is ultrafast IC of the ππ* (S₂) state to a lower-lying dark nπ* state (S₁) and then ISC from the nπ* state (S₁) to the triplet ππ* state (T₁). Triplet state generation quantum yields of 6-AT and 6-ATD are 46 % and 43 %. Triplet sensitization of singlet oxygen is also observed in 6-AT and 6-ATD solutions under UV irradiation. The experimental results are further supported by the time-dependent density functional theory (TDDFT) calculations. These results are evidence that triplet nπ* state is involved in the ultrafast ISC process of pyrimidine nucleobases derivatives and it helps to reconcile the controversy about the ISC mechanism of pyrimidine bases.

了解嘧啶核碱基的激发态动力学非常重要，因为在这些分子中发现了可能诱导DNA光损伤的寿命更长的激发态。三重态nπ*状态在嘧啶碱基的激发态动力学中的作用是有争议的，由于规范DNA核碱基的三重态量子产率较低，因此很难直接观察该状态。在DNA衍生物中，氮杂核苷碱基与标准核苷碱基具有高度的结构相似性，并且据报道它们具有很高的三重态量子产率。在这项工作中，报道了6-氮杂嘧啶（6-AT）和6-氮杂嘧啶（6-ATD）的激发态动力学。宽带瞬态吸收测量结果表明，6-AT中有两种途径可以实现超快速系统间交叉（ISC）。第一个是来自初始受激ππ*（S₂）状态变为三重态nπ*状态（T ₂），然后进行超快内部转换（IC）为三重态ππ*状态（T ₁）。另一种途径是将ππ*（S ₂）状态转变为较低的暗nπ*状态（S ₁），然后将ISC从nπ*状态（S ₁）转变为三重态ππ*状态（T ₁）的超快IC。）。3-AT和6-ATD的三重态产生量子产率分别为46％和43％。在6-AT和6-ATD溶液中，在紫外线照射下也观察到单重态氧的三重态敏化。时变密度泛函理论（TDDFT）计算进一步支持了实验结果。这些结果证明三联体nπ*状态参与了嘧啶核苷碱基衍生物的超快ISC过程，并有助于调和有关嘧啶碱基的ISC机制的争议。