Internal conversion and intersystem crossing pathways in UV excited, isolated uracils and their implications in prebiotic chemistry

Incorporating spin–orbit effects into surface hopping dynamics using the diagonal representation: a linear-response time-dependent density functional theory implementation with applications to 2-thiouracil

Evaluation of SOC values employs Casida's wave functions and the Breit–Pauli spin–orbit Hamiltonian with effective charge approximation.

Excited State Dynamics of 6-Thioguanine

Here we present the excited state dynamics of jet-cooled 6-thioguanine (6-TG), using resonance-enhanced multiphoton ionization (REMPI), IR-UV double resonance spectroscopy, and pump-probe spectroscopy in the nanosecond and picosecond time domains. We report data on two thiol tautomers, which appear to have different excited state dynamics. These decay to a dark state, possibly a triplet state, with rates depending on tautomer form and on excitation wavelength, with the fastest rate on the order of 10¹⁰ s^-1. We also compare 6-TG with 9-enolguanine, for which we observed decay to a dark state with a 2 orders of magnitude smaller rate. At increased excitation energy (∼+500 cm^-1) an additional pathway appears for the predominant thiol tautomer. Moreover, the excited state dynamics for 6-TG thiols is different from that recently predicted for thiones.

Photophysics and Photochemistry of Canonical Nucleobases’ Thioanalogs: From Quantum Mechanical Studies to Time Resolved Experiments

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.

2-Thiouracil intersystem crossing photodynamics studied by wavelength-dependent photoelectron and transient absorption spectroscopies

The excitation-wavelength dependence of the intersystem crossing (ISC) dynamics of 2-thiouracil was studied in gas-phase and solution.

Challenges in Simulating Light-Induced Processes in DNA

In this contribution, we give a perspective on the main challenges in performing theoretical simulations of photoinduced phenomena within DNA and its molecular building blocks. We distinguish the different tasks that should be involved in the simulation of a complete DNA strand subject to UV irradiation: (i) stationary quantum chemical computations; (ii) the explicit description of the initial excitation of DNA with light; (iii) modeling the nonadiabatic excited state dynamics; (iv) simulation of the detected experimental observable; and (v) the subsequent analysis of the respective results. We succinctly describe the methods that are currently employed in each of these steps. While for each of them, there are different approaches with different degrees of accuracy, no feasible method exists to tackle all problems at once. Depending on the technique or combination of several ones, it can be problematic to describe the stacking of nucleobases, bond breaking and formation, quantum interferences and tunneling or even simply to characterize the involved wavefunctions. It is therefore argued that more method development and/or the combination of different techniques are urgently required. It is essential also to exercise these new developments in further studies on DNA and subsystems thereof, ideally comprising simulations of all of the different components that occur in the corresponding experiments.

New Insights into the State Trapping of UV-Excited Thymine

After UV excitation, gas phase thymine returns to a ground state in 5 to 7 ps, showing multiple time constants. There is no consensus on the assignment of these processes, with a dispute between models claiming that thymine is trapped either in the first (S₁) or in the second (S₂) excited states. In the present study, a nonadiabatic dynamics simulation of thymine is performed on the basis of ADC(2) surfaces, to understand the role of dynamic electron correlation on the deactivation pathways. The results show that trapping in S₂ is strongly reduced in comparison to previous simulations considering only non-dynamic electron correlation on CASSCF surfaces. The reason for the difference is traced back to the energetic cost for formation of a CO π bond in S₂.

The origin of efficient triplet state population in sulfur-substituted nucleobases

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.

Intersystem Crossing Pathways in the Noncanonical Nucleobase 2-Thiouracil: A Time-Dependent Picture

The deactivation mechanism after ultraviolet irradiation of 2-thiouracil has been investigated using nonadiabatic dynamics simulations at the MS-CASPT2 level of theory. It is found that after excitation the S2 quickly relaxes to S1, and from there intersystem crossing takes place to both T2 and T1 with a time constant of 400 fs and a triplet yield above 80%, in very good agreement with recent femtosecond experiments in solution. Both indirect S1 → T2 → T1 and direct S1 → T1 pathways contribute to intersystem crossing, with the former being predominant. The results contribute to the understanding of how some noncanonical nucleobases respond to harmful ultraviolet light, which could be relevant for prospective photochemotherapeutic applications.