Zhang Kun, Wang Fufang, Jiang Yanrong, Wang Xueli, Pan Haifeng, Sun Zhenrong, Sun Haitao, Xu Jianhua, Chen Jinquan
State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China.
Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China.
J Phys Chem B. 2021 Mar 4;125(8):2042-2049. doi: 10.1021/acs.jpcb.0c10611. Epub 2021 Feb 18.
The high photostability of DNA/RNA nucleobases is attributed to the effective internal conversions of their bright ππ* states to the ground state through conical intersections. Intersystem crossing (ISC) from singlet to triplet excited states is a minor decay pathway in nucleobases and it is observed with ∼1-2% quantum yields (QYs) in pyrimidine bases. Presumably, ISC in pyrimidines takes place from the dark singlet nπ* state to the lowest triplet ππ* state. However, recent studies showed that ISC from the initial populated bright ππ* state to higher energy triplet nπ* states indeed occurs in the subpicosecond timescale. Such a mechanism is still poorly understood since direct observation of this pathway is challenging. Herein, excited state dynamics of three pyrimidinones, which share the same skeleton with pyrimidine bases, is investigated in different solvents. Compared to canonical pyrimidine bases, removing the oxygen atom at the C4 position revokes the low-lying dark nπ* state in pyrimidinones, resulting in direct ISC from the S (ππ*) state to triplet T (nπ*) state with much higher QYs. Meanwhile, hydrogen bonding between the carbonyl group in pyrimidinones and protic solvents can accelerate vibrational cooling of the hot S (ππ*) state, leading to higher fluorescence QYs and smaller ISC rate constants. These results not only evidence the hypothesis of the direct ππ* → nπ* ISC mechanism, but also contribute to a better understanding of triplet formation in pyrimidines.
DNA/RNA核碱基的高光稳定性归因于其明亮的ππ态通过锥形交叉有效地内转换为基态。从单重态到三重态激发态的系间窜越（ISC）是核碱基中的次要衰变途径，在嘧啶碱基中观察到其量子产率（QYs）约为1-2%。据推测，嘧啶中的ISC发生在暗单重态nπ态到最低三重态ππ态之间。然而，最近的研究表明，从最初占据的明亮ππ态到更高能量的三重态nπ态的ISC确实发生在亚皮秒时间尺度内。由于直接观察该途径具有挑战性，这种机制仍知之甚少。在此，研究了三种与嘧啶碱基具有相同骨架的嘧啶酮在不同溶剂中的激发态动力学。与标准嘧啶碱基相比，在嘧啶酮的C4位置去除氧原子消除了其低能暗nπ态，导致从S（ππ*）态到三重态T（nπ*）态的直接ISC，且量子产率更高。同时，嘧啶酮中的羰基与质子溶剂之间的氢键可以加速热S（ππ*）态的振动冷却，从而导致更高的荧光量子产率和更小的ISC速率常数。这些结果不仅证明了直接ππ*→nπ*ISC机制的假设，也有助于更好地理解嘧啶中三重态的形成。
