Elucidating the photophysical mechanisms in sulfur-substituted nucleobases (thiobases) is essential for designing prospective drugs for photo- and chemotherapeutic applications. Although it has long been established that the phototherapeutic activity of thiobases is intimately linked to efficient intersystem crossing into reactive triplet states, the molecular factors underlying this efficiency are poorly understood. Herein we combine femtosecond transient absorption experiments with quantum chemistry and nonadiabatic dynamics simulations to investigate 2-thiocytosine as a necessary step to unravel the electronic and structural elements that lead to ultrafast and near-unity triplet-state population in thiobases in general. We show that different parts of the potential energy surfaces are stabilized to different extents via thionation, quenching the intrinsic photostability of canonical DNA and RNA nucleobases. These findings satisfactorily explain why thiobases exhibit the fastest intersystem crossing lifetimes measured to date among bio-organic molecules and have near-unity triplet yields, whereas the triplet yields of canonical nucleobases are nearly zero.

In this paper we simulate the deactivation dynamics of photoexcited 6-thioguanine, a cytotoxic analogue of the canonical DNA/RNA base guanine, using a direct surface hopping dynamics approach. Our aim is to investigate the mechanism for triplet population, which was found to take place on a similar time scale as internal conversion. The surface hopping calculations were based on potential energy surfaces and couplings obtained on the fly using a semiempirical Hamiltonian, reparameterized on accurate ab initio data. We show that for the full description of the deactivation dynamics of 6-thioguanine, it is important to take into account both the dynamic and the spin–orbit couplings. The main deactivation pathway involves the sequence of ultrafast radiationless transitions S₂ → S₁ → T₂ → T₁. The very efficient population and long lifetime of the final T₁ state, from where singlet oxygen is generated, would explain the high phototoxicity of the nucleotides of 6-thioguanine in DNA. To our knowledge, this is the first nonadiabatic dynamics simulation for a system showing strong spin–orbit couplings (due to the presence of a third row atom, sulfur) and a complex pattern of intermultiplet crossings.

The excited-state dynamics of the purine free base and 9-methylpurine are investigated using experimental and theoretical methods. Femtosecond broadband transient absorption experiments reveal that excitation of these purine derivatives in aqueous solution at 266 nm results primarily in ultrafast conversion of the S₂(ππ*) state to the vibrationally excited ¹nπ* state. Following vibrational and conformational relaxation, the ¹nπ* state acts as a doorway state in the efficient population of the triplet manifold with an intersystem crossing lifetime of hundreds of picoseconds. Experiments show an almost 2-fold increase in the intersystem crossing rate on going from polar aprotic to nonpolar solvents, suggesting that a solvent-dependent energy barrier must be surmounted to access the singlet-to-triplet crossing region. Ab initio static and surface-hopping dynamics simulations lend strong support to the proposed relaxation mechanism. Collectively, the experimental and computational results demonstrate that the accessibility of the nπ* states and the topology of the potential energy surfaces in the vicinity of conical intersections are key elements in controlling the excited-state dynamics of the purine derivatives. From a structural perspective, it is shown that the purine chromophore is not responsible for the ultrafast internal conversion in the adenine and guanine monomers. Instead, C6 functionalization plays an important role in regulating the rates of radiative and nonradiative relaxation. C6 functionalization inhibits access to the ¹nπ* state while simultaneously facilitating access to the ¹ππ*(L_a)/S₀ conical intersection, such that population of the ¹nπ* state cannot compete with the relaxation pathways to the ground state involving ring puckering at the C2 position.

Interest in understanding the photophysics and photochemistry of thiated nucleobases has been awakened because of their possible involvement in primordial RNA or their potential use as photosensitizers in medicinal chemistry. The interpretation of the photodynamics of these systems, conditioned by their intricate potential energy surfaces, requires the powerful interplay between experimental measurements and state of the art molecular simulations. In this review, we provide an overview on the photophysics of natural nucleobases’ thioanalogs, which covers the last 30 years and both experimental and computational contributions. For all the canonical nucleobase’s thioanalogs, we have compiled the main steady state absorption and emission features and their interpretation in terms of theoretical calculations. Then, we revise the main topographical features, including stationary points and interstate crossings, of their potential energy surfaces based on quantum mechanical calculations and we conclude, by combining the outcome of different spectroscopic techniques and molecular dynamics simulations, with the mechanism by which these nucleobase analogs populate their triplet excited states, which are at the origin of their photosensitizing properties.

The deactivation mechanism of the cytotoxic 6-thioguanine, the 6-sulfur-substituted analogue of the canonical DNA base, is unveiled by ab initio calculations. Oxygen-by-sulfur substitution leads to efficient population of triplet states—the first step for generating singlet oxygen—which is responsible for its cytotoxicity.