Quantum mechanics/molecular mechanics studies on mechanistic photophysics of cytosine aza-analogues: 2,4-diamino-1,3,5-triazine and 2-amino-1,3,5-triazine in aqueous solution.

The excited-state properties and photophysics of cytosine aza-analogues, i.e. , 2,4-diamino-1,3,5-triazine (2,4-DT) and 2-amino-1,3,5-triazine (2-AT) in solution have been systematically explored using the QM(MS-CASPT2//CASSCF)/MM approach. The excited-state nonradiative relaxation mechanisms for the initially photoexcited S 1 (ππ*) state decay back to the S 0 state are proposed in terms of the present computed minima, surface crossings (conical intersections and singlet-triplet crossings), and excited-state decay paths in the S 1 , S 2 , T 1 , T 2 , and S 0 states. Upon photoexcitation to the bright S 1 (ππ*) state, 2,4-DT quickly relaxes to its S 1 minimum and then overcomes a small energy barrier of 5.1 kcal mol -1 to approach a S 1 /S 0 conical intersection, where the S 1 system hops to the S 0 state through S 1 → S 0 internal conversion (IC). In addition, at the S 1 minimum, the system could partially undergo intersystem crossing (ISC) to the T 1 state, followed by further ISC to the S 0 state via the T 1 /S 0 crossing point. In the T 1 state, an energy barrier of 7.9 kcal mol -1 will trap 2,4-DT for a while. In parallel, for 2-AT, the system first relaxes to the S 1 minimum and then S 1 → S 0 IC or S 1 → T 1 → S 0 ISCs take place to the S 0 state by surmounting a large barrier of 15.3 kcal mol -1 or 11.9 kcal mol -1 , respectively, which heavily suppress electronic transition to the S 0 state. Different from 2,4-DT, upon photoexcitation in the Franck-Condon region, 2-AT can quickly evolve in an essentially barrierless manner to nearby S 2 /S 1 conical intersection, where the S 2 and T 1 states can be populated. Once it hops to the S 2 state, the system will overcome a relatively small barrier (6.6 kcal mol -1 vs. 15.3 kcal mol -1 ) through IC to the S 0 state. Similarly, an energy barrier of 11.9 kcal mol -1 heavily suppresses the T 1 state transformation to the S 0 state. The present work manifests that the amination/deamination of the triazine rings can affect some degree of different vertical and adiabatic excitation energies and nonradiative decay pathways in solution. It not only rationalizes excited-state decay dynamics of 2,4-DT and 2-AT in aqueous solution but could also provide insights into the understanding of the photophysics of aza-nucleobases.