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(pi pi*) 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(pi pi*) 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.