QM/MM studies on the excited-state relaxation mechanism of a semisynthetic dTPT3 base. Issue 7 (1st February 2018)

• Record Type: Journal Article
• Authors: Guo, Wei-Wei; Zhang, Teng-Shuo; Fang, Wei-Hai; Cui, Ganglong
• Is Part Of: Physical chemistry chemical physics. Volume 20:Issue 7(2018)
• Page Start: 5067
• Page End: 5073
• Publication Date: 2018-02-01
• Subjects: Chemistry, Physical and theoretical -- Periodicals; 541.3
• DOI: 10.1039/c7cp08696b
• Languages: English
• ISSNs: 1463-9076

Abstract

Abstract : Two efficient excited-state relaxation pathways populating the lowest triplet state. Abstract : 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 ( 1 ππ*) 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 ( 3 ππ*). In the first one, the S2 ( 1 ππ*) system first decays to the S1 ( 1 nπ*) 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 ( 3 nπ*) system hops to the T1 ( 3 ππ*) 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.