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 S₁, S₂, T₁, T₂, and S₀ states, several feasible excited-state nonradiative decay channels from the initially populated S₂(ππ*) state are proposed. Two major channels are singlet-mediated nonradiative pathways, in which the S₂ system will internally convert (IC) to the S₀ state directly or mediated by the ¹nπ* state via a ¹ππ*/¹nπ* conical intersection. The minor ones are related to intersystem crossing (ISC) processes. The system would populate to the T₁ state via the S₂ → S₁ → T₁ or S₂ → T₂ → T₁ ISC process, followed by further decay to the S₀ state via the transition from T₁ to S₀. 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.