Triplet State Dynamics in DNA / RNA Nucleobases and DNA by Ab Initio Static and Surface‐Hopping Dynamics Simulations

DNA is the primary reservoir of genetic information, and its fidelity depends on preserving its molecular structure intact. Ultraviolet (UV) photons can threaten DNA integrity by promoting the nucleobases to electronically excited states. But even if constantly irradiated, DNA remains remarkably photostable. Understanding how these excited states evolve is key to explaining both the origin of DNA's photostability and the pathways that can lead to photodamage. In isolated nucleobases, two nonradiative processes dominate relaxation: ultrafast internal conversion (IC) and the slower spin-forbidden intersystem crossing (ISC) that funnels population into long-lived triplet states. Within the double helix, base stacking complicates the picture: excitons, charge-transfer states, and excimers open additional channels that can ultimately produce lesions. Building a coherent mechanism thus requires the coordination between experiment and theory to achieve an atomistic view of these events. This chapter reviews the photophysical decay routes of the five canonical nucleobases, highlighting insights from multiconfigurational calculations, time-dependent density functional theory (TD-DFT) and nonadiabatic dynamics. Together, these approaches track population from the Franck–Condon (FC) region through IC and ISC funnels, weigh the competition among decay pathways, and estimate excited state lifetimes. Beyond the natural bases, we also reviewed analogs with markedly higher triplet yields. Sulfur substitution is especially notable: the heavier atom boosts spin–orbit coupling (SOC) by orders of magnitude and can drive ISC efficiencies toward unity, underpinning the interest in thionucleobases as light-activated therapeutics. Finally, we examine photochemical reactions that can also originate from triplet states, most prominently cyclobutane and (6-4) dimer formation.