Minor structural modifications to the DNA and RNA nucleobases have a significant effect on their excited state dynamics and electronic relaxation pathways. In this study, the excited state dynamics of 7-deazaguanosine and guanosine 5′-monophosphate are investigated in aqueous solution and in a mixture of methanol and water using femtosecond broadband transient absorption spectroscopy following excitation at 267 nm. The transient spectra are collected using photon densities that ensure no parasitic multiphoton-induced signal from solvated electrons. The data can be fit satisfactorily using a two- or three-component kinetic model. By analyzing the results from steady-state, time-resolved, computational calculations, and the methanol–water mixture, the following general relaxation mechanism is proposed for both molecules, Lb → La → 1πσ*(ICT) → S0, where the 1πσ*(ICT) stands for an intramolecular charge transfer excited singlet state with significant πσ* character. In general, longer lifetimes for internal conversion are obtained for 7-deazaguanosine compared to guanosine 5′-monophosphate. Internal conversion of the 1πσ*(ICT) state to the ground state occurs on a similar time scale of a few picoseconds in both molecules. Collectively, the results demonstrate that substitution of a single nitrogen atom for a methine (C–H) group at position seven of the guanine moiety stabilizes the 1ππ* Lb and La states and alters the topology of their potential energy surfaces in such a way that the relaxation dynamics in 7-deazaguanosine are slowed down compared to those in guanosine 5′-monophosphate but not for the internal conversion of 1πσ*(ICT) state to the ground state.

Heavy-atom-free photosensitizers (HAF-PSs) based on thionation of carbonyl groups of readily accessible organic compounds are rapidly emerging as a versatile class of molecules. However, their photochemical properties and electronic relaxation mechanisms are currently unknown. Investigating the excited-state dynamics is essential to understand their benefits and limitations and to develop photosensitizers with improved photochemical properties. Herein, the photochemical and electronic-structure properties of two of the most promising HAF-PSs developed to date are revealed. It is shown that excitation of thio-4-(dimethylamino)naphthalamide and thionated Nile Red with near-infrared radiation leads to the efficient population of the triplet manifold through multiple relaxation pathways in hundreds of femtoseconds. The strong singlet−triplet couplings in this family of photosensitizers should enable a broad range of applications, including in photodynamic therapy, photocatalysis, photovoltaics, organic LEDs, and photon up-conversion.

Notwithstanding the central biological role of the (6-4) photoadduct in the induction of skin cancer by sunlight, crucial mechanistic details about its formation have evaded characterization despite efforts spanning more than half a century. 4-Thiothymidine (4tT) has been widely used as an important model system to study its mechanism of formation, but the excited-state precursor, the intermediate species, and the time scale leading to the formation of the (6-4) photoadduct have remained elusive. Herein, steady-state and time-resolved spectroscopic techniques are combined with new and reported quantum-chemical calculations to demonstrate the excited state leading to the formation of the thietane intermediate, its rate, and the formation of the (6-4) photoadduct using the 5’-TT(4tT)T(4tT)TT-3’ DNA oligonucleotide. Efficient, sub-1 ps intersystem crossing leads to the population of a triplet minimum of the thietane intermediate in as short as 3 ps, which intersystem crosses to its ground state and rearranges to form the (6-4) photoadduct.

Today’s genetic composition is the result of continual refinement processes on primordial heterocycles present in prebiotic Earth and at least partially regulated by ultraviolet radiation. Femtosecond transient absorption spectroscopy and state-of-the-art ab initio calculations are combined to unravel the electronic relaxation mechanism of pyrimidine, the common chromophore of the nucleobases. The excitation of pyrimidine at 268 nm populates the S1(nπ*) state directly. A fraction of the population intersystem crosses to the triplet manifold within 7.8 ps, partially decaying within 1.5 ns, while another fraction recovers the ground state in >3 ns. The pyrimidine chromophore is not responsible for the photostability of the nucleobases. Instead, C2 and C4 amino and/or carbonyl functionalization is essential for shaping the topography of pyrimidine’s potential energy surfaces and results in accessible conical intersections between the initially populated electronic excited state and the ground state.

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