Observation of triplet nπ* state in ultrafast intersystem crossing of 6-azathymine

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.