QM/MM studies on the excited-state relaxation mechanism of a semisynthetic dTPT3 base

Semisynthetic alphabets can potentially increase the genetic information stored in DNA through the formation of unusual base pairs. Recent experiments have shown that near-visible-light irradiation of the dTPT3 chromophore could lead to the formation of a reactive triplet state and of singlet oxygen in high quantum yields. However, the detailed excited-state relaxation paths that populate the lowest triplet state are unclear. Herein, we have for the first time employed the QM(MS-CASPT2//CASSCF)/MM method to explore the spectroscopic properties and excited-state relaxation mechanism of the aqueous dTPT3 chromophore. On the basis of the results, we have found that (1) the S2(1ππ*) state of dTPT3 is the initially populated excited singlet state upon near-visible light irradiation; and (2) there are two efficient relaxation pathways to populate the lowest triplet state, i.e. T1(3ππ*). In the first one, the S2(1ππ*) system first decays to the S1(1nπ*) state near the S2/S1 conical intersection, which is followed by an efficient S1 → T1 intersystem crossing process at the S1/T1 crossing point; in the second one, an efficient S2 → T2 intersystem crossing takes place first, and then, the T2(3nπ*) system hops to the T1(3ππ*) state through an internal conversion process at the T2/T1 conical intersection. Moreover, an S2/S1/T2 intersection region is found to play a vital role in the excited-state relaxation. These new mechanistic insights help in understanding the photophysics and photochemistry of unusual base pairs.