[2 + 2] Photocycloaddition Reaction Dynamics of Triplet Pyrimidines

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• 作者：Chunfan Yang ; Youqing Yu ; Kunhui Liu ; Di Song ; Lidan Wu ; Hongmei Su
• 刊名：Journal of Physical Chemistry A
• 出版年：2011
• 出版时间：June 2, 2011
• 年：2011
• 卷：115
• 期：21
• 页码：5335-5345
• 全文大小：1126K
• 年卷期：v.115,no.21(June 2, 2011)
• ISSN：1520-5215

文摘

Taking the 266 nm excited pyrimidine (uracil or thymine) with cyclopentene as model reaction systems, we have examined the photoproduct formation dynamics from the [2 + 2] photocycloaddition reactions of triplet pyrimidines in solution and provided mechanistic insights into this important DNA photodamage reaction. By combining two compliment methods of nanosecond time-resolved transient IR and UV鈥搗is laser flash-photolysis spectroscopy, the photoproduct formation dynamics as well as the triplet quenching kinetics are measured. Characteristic IR absorption bands due to photoproduct formation have been observed and product quantum yields are determined to be 0.91% for uracil and 0.41% for thymine. Compared to the measured large quenching rate constants of triplet uracil (1.5 脳 10⁹ M^鈥?s^鈥?) or thymine (0.6 脳 10⁹ M^鈥?s^鈥?) by cyclopentene, the inefficiency in formation of photoproducts indicates competitive physical quenching processes may exist on the route leading to photoproducts, resulting in very small product yields eventually. Such an energy wasting process is found to be resulted from T₁/S₀ surface crossings by the hybrid density functional calculations, which compliments the experiments and reveals the reaction mechanism.