Quantum mechanics/molecular mechanics studies on excited state decay pathways of 5-azacytosine in aqueous solution

In this work, we have used the QM(CASPT2//CASSCF)/MM approach to study the photophysical properties and relaxation mechanism of 5-azacytosine (5-AC) in aqueous solution. Based on the relevant minimum-energy structures and intersection structures, and excited-state decay paths in the S-1, S-2, T-1, T-2, and S-0 states, several feasible excited-state nonradiative decay channels from the initially populated S-2(pi pi*) state are proposed. Two major channels are singlet-mediated nonradiative pathways, in which the S-2 system will internally convert (IC) to the S-0 state directly or mediated by the (1)n pi* state via a (1)pi pi*/(1)n pi* conical intersection. The minor ones are related to intersystem crossing (ISC) processes. The system would populate to the T-1 state via the S-2 -> S-1 -> T-1 or S-2 -> T-2 -> T-1 ISC process, followed by further decay to the S-0 state via the transition from T-1 to S-0. However, due to small spin-orbit couplings (SOCs) at the singlet-triplet crossing points, the related ISC would be less efficient and probably take longer. The present work rationalizes the ultrafast excited-state decay dynamics of 5-AC in aqueous solution and its low quantum yields of triplets and fluorescence. It provides important mechanistic insights into understanding 5-AC's derivatives and analogues.