The excited-state decay mechanism of 2,4-dithiothymine in the gas phase, microsolvated surroundings, and aqueous solution

Mechanistic photophysics of tellurium-substituted cytosine: Electronic structure calculations and nonadiabatic dynamics simulations

Previously, the MS-CASPT2 method was performed to study the static and qualitative photophysics of tellurium-substituted cytosine (TeC). To get quantitative information, we used our recently developed QTMF-FSSH dynamics method to simulate the excited-state decay of TeC. The CASSCF method was adopted to reduce the calculation costs, which was confirmed to provide reliable structures and energies as those of MS-CASPT2. A detailed structural analysis showed that only 5% trajectories will hop to the lower triplet or singlet state via the twisted (S2 /S1 /T2 )T intersection, while 67% trajectories will choose the planar intersections of (S2 /S1 /T3 /T2 /T1 )P and (S2 /S1 /T2 /T1 )P but subsequently become twisted in other electronic states. By contrast, ~28% trajectories will maintain in a plane throughout dynamics. Electronic population revealed that the S2 population will ultrafast transfer to the lower triplet or singlet state. Later, the TeC system will populate in the spin-mixed electronic states composed of S1 , T1 and T2 . At the end of 300 fs, most trajectories (~74%) will decay to the ground state and only 17.4% will survive in the triplet states. Our dynamics simulation verified that tellurium substitution will enhance the intersystem crossings, but the very short triplet lifetime (ca. 125 fs) will make TeC a less effective photosensitizer.

The impact of the chalcogen-substitution element and initial spectroscopic state on excited-state relaxation pathways in nucleobase photosensitizers: a combination of static and dynamic studies

The substitution of oxygen with chalcogen in carbonyl group(s) of canonical nucleobases gives an impressive triplet generation, enabling their promising applications in medicine and other emerging techniques. The excited-state relaxation S2(ππ*) → S1(nπ*) → T1(ππ*) has been considered the preferred path for triplet generation in these nucleobase derivatives. Here, we demonstrate enhanced quantum efficiency of direct intersystem crossing from S2 to triplet manifold upon substitution with heavier chalcogen elements. The excited-state relaxation dynamics of sulfur/selenium substituted guanines in a vacuum is investigated using a combination of static quantum chemical calculations and on-the-fly excited-state molecular dynamics simulations. We find that in sulfur-substitution the S2 state predominantly decays to the S1 state, while upon selenium-substitution the S2 state deactivation leads to simultaneous population of the S1 and T2,3 states in the same time scale and multi-state quasi-degeneracy region S2/S1/T2,3. Interestingly, the ultrafast deactivation of the spectroscopic S3 state of both studied molecules to the S1 state occurs through a successive S3 → S2 → S1 path involving a multi-state quasi-degeneracy S3/S2/S1. The populated S1 and T2 states will cross the lowest triplet state, and the S1 → T intersystem crossing happens in a multi-state quasi-degeneracy region S1/T2,3/T1 and is accelerated by selenium-substitution. The present study reveals the influence of both the chalcogen substitution element and initial spectroscopic state on the excited-state relaxation mechanism of nucleobase photosensitizers and also highlights the important role of multi-state quasi-degeneracy in mediating the complex relaxation process. These theoretical results provide additional insights into the intrinsic photophysics of nucleobase-based photosensitizers and are helpful for designing novel photo-sensitizers for real applications.

A theoretical insight into excited-state decay and proton transfer of p-nitrophenylphenol in the gas phase and methanol solution

The excited-state decay (ESD) and proton transfer (EPT) of p-nitrophenylphenol (NO₂-Bp-OH), especially in the triplet states, were not characterized with high-level theoretical methods to date. Herein, the MS-CASPT2//CASSCF and QM(MS-CASPT2//CASSCF)/MM methods were employed to gain an atomic-level understanding of the ESD and EPT of NO₂-Bp-OH in the gas phase and its hydrogen-bonded complex in methanol. Our calculation results revealed that the S₁ and S₂ states of NO₂-Bp-OH are of ¹ππ* and ¹nπ* characters at the Franck-Condon (FC) point, which correspond to the ICT-EPT and intramolecular charge-transfer (ICT) states in spectroscopic experiments. The former state has a charge-transfer property that could facilitate the EPT reaction, while the latter one might be unfavorable for EPT. The vertical excitation energies of these states are almost degenerate at the FC region and the electronic configurations of ¹ππ* and ¹nπ* will exchange from the S₁ FC region to the S₁ minimum, which means that the ¹nπ* state will participate in ESD once NO₂-Bp-OH departs from the S₁ FC region. Besides, we found that three triplets lie below the first bright state and will play very important roles in intersystem crossing processes. In terms of several pivotal surface crossings and relevant linearly interpolated internal coordinate (LIIC) paths, three feasible but competing ESD channels that could effectively lead the system to the ground state or the lowest triplet state were put forward. Once arrived at the T₁ state, the system has enough time and internal energy to undergo the EPT reaction. The methanol solvent has a certain effect on the relative energies and spin-orbit couplings, but does not qualitatively change the ESD processes of NO₂-Bp-OH. By contrast, the solvent effects will remarkably stabilize the proton-transferred product by the hydrogen bond networks and assist to form the triplet anion. Our present work would pave the road to properly understand the mechanistic photochemistry of similar hydroxyaromatic compounds.

Ultrafast Excited-State Decay Mechanisms of 6-Thioguanine Followed by Sub-20 fs UV Transient Absorption Spectroscopy

Understanding the primary steps following UV photoexcitation in sulphur-substituted DNA bases (thiobases) is fundamental for developing new phototherapeutic drugs. However, the investigation of the excited-state dynamics in sub-100 fs time scales has been elusive until now due to technical challenges. Here, we track the ultrafast decay mechanisms that lead to the electron trapping in the triplet manifold for 6-thioguanine in an aqueous solution, using broadband transient absorption spectroscopy with a sub-20 fs temporal resolution. We obtain experimental evidence of the fast internal conversion from the S₂(ππ*) to the S₁(nπ*) states, which takes place in about 80 fs and demonstrates that the S₁(nπ*) state acts as a doorway to the triplet population in 522 fs. Our results are supported by MS-CASPT2 calculations, predicting a planar S₂(ππ*) pseudo-minimum in agreement with the stimulated emission signal observed in the experiment.

Coherent vibrational modes promote the ultrafast internal conversion and intersystem crossing in thiobases

Thionated nucleobases are obtained by replacing oxygen with sulphur atoms in the canonical nucleobases. They absorb light efficiently in the near-ultraviolet, populating singlet states which undergo intersystem crossing to the...

Mechanistic photophysics and photochemistry of unnatural bases and sunscreen molecules: insights from electronic structure calculations

Photophysics and photochemistry are basic subjects in the study of light-matter interactions and are ubiquitous in diverse fields such as biology, energy, materials, and environment. A full understanding of mechanistic photophysics and photochemistry underpins many recent advances and applications. This contribution first provides a short discussion on the theoretical calculation methods we have used in relevant studies, then we introduce our latest progress on the mechanistic photophysics and photochemistry of two classes of molecular systems, namely unnatural bases and sunscreens. For unnatural bases, we disclose the intrinsic driving forces for the ultrafast population to reactive triplet states, impacts of the position and degree of chalcogen substitutions, and the effects of complex environments. For sunscreen molecules, we reveal the photoprotection mechanisms that dissipate excess photon energy to the surroundings by ultrafast internal conversion to the ground state. Finally, relevant theoretical challenges and outlooks are discussed.

Mechanistic Photophysics of Tellurium-Substituted Uracils: Insights from Multistate Complete-Active-Space Second-Order Perturbation Calculations

The photophysical mechanisms of tellurium-substituted uracils were studied at the multistate complete-active-space second-order perturbation level with a particular focus on how the position and number of tellurium substitutions affect their nonadiabatic relaxation processes. Electronic structure analysis reveals that the lowest several excited states are closely concerned with the n and π orbitals at the Te₇-C₂ [Te₈-C₄] moiety of 2-tellurouracil (2TeU) [4TeU and 24TeU]. Both planar and twisted minima were optimized for 2TeU, whereas only planar ones were obtained for 4TeU and 24TeU, except for a twisted T₁ minimum of 4TeU. Based on intersection structures and linearly interpolated internal coordinate paths, we proposed several feasible excited-state deactivation paths. It is found that the relaxation channels for 2TeU are more complicated than those of 4TeU and 24TeU. The electronic population transfer to the T₁ state for 2TeU is easier than that for 4TeU and 24TeU in consideration of the barrier heights from the S₂ Franck-Condon point to the S₂/S₁ or S₂/T₂ intersections. In addition, the recovery of the ground state from the T₁ state for 2TeU will be more efficient than that for the other two systems as well.

MS-CASPT2 studies on the mechanistic photophysics of tellurium-substituted guanine and cytosine

Sulfur-substituted nucleobases are highly promising photosensitizers that are widely used in photodynamic therapy, and there are numerous studies exploring their unique photophysical behaviors. However, relevant photophysical investigations on selenium and tellurium substitutions are still rare. Herein, the high-level multistate complete-active-space second-order perturbation (MS-CASPT2) method was performed for the first time to explore the excited-state relaxation processes of tellurium-substituted guanine (TeG) and cytosine (TeC). Based on the electronic state properties in the Franck-Condon (FC) region, we found that the lowest five (S0, S1, S2, T1, and T2) and six (S0, S1, S2, T1, T2 and T3) states will participate in the nonadiabatic transition processes of TeG and TeC systems, respectively. In these electronic states, two kinds of minimum and intersection structures (i.e., planar and twisted structures) were obtained for both TeG and TeC systems. The linearly interpolated internal coordinate (LIIC) paths and spin-orbit coupling (SOC) constants revealed several possible planar and twisted excited-state decay channels, which could lead the systems to the lowest reactive triplet state of T1. Small energy barriers in the T1 state will trap the TeG and TeC systems for a while before they finally populate to the ground state. Although tellurium substitution would further redshift the absorption wavelength and enhance the intersystem crossing (ISC) rate to the T1 state compared with sulfur and selenium substitutions, the rapid ISC process of T1 → S0 may make it a less effective photosensitizer to sensitize the molecular oxygen. We believe our present work will provide important mechanistic insights into the photophysics of tellurium-substituted nucleobases.

Quantum Mechanics/Molecular Mechanics Studies on the Photophysical Mechanism of Methyl Salicylate

Methyl salicylate (MS) as a subunit of larger salicylates found in commercial sunscreens has been shown to exhibit keto-enol tautomerization and dual fluorescence emission via excited-state intramolecular proton transfer (ESIPT) after the absorption of ultraviolet (UV) radiation. However, its excited-state relaxation mechanism is unclear. Herein, we have employed the quantum mechanics(CASPT2//CASSCF)/molecular mechanics method to explore the ESIPT and excited-state relaxation mechanism of MS in the lowest three electronic states, that is, S₀, S₁, and T₁ states, in a methanol solution. Based on the optimized geometric and electronic structures, conical intersections and crossing points, and minimum-energy paths combined with the computed linearly interpolated Cartesian coordinate paths, the photophysical mechanism of MS has been proposed. The S₁ state is a spectroscopically bright ¹ππ* state in the Franck-Condon region. From the initially populated S₁ state, there exist three nonradiative relaxation paths to repopulate the S₀ state. In the first one, the S₁ system (i.e., ketoB form) first undergoes an ESIPT path to generate an S₁ tautomer (i.e., enol form) that exhibits a large Stokes shift in experiments. The generated S₁ enol tautomer further evolves toward the nearby S₁/S₀ conical intersection and then hops to the S₀ state, followed by the backward ground-state intramolecular proton transfer (GSIPT) to the initial ketoB form S₀ state. In the second one, the S₁ system first hops through the S₁ → T₁ intersystem crossing (ISC) to the T₁ state, which then further decays to the S₀ state via T₁ → S₀ ISC at the T₁/S₀ crossing point. In the third path, the T₁ system that stems from the S₁ → T₁ ISC process via the S₁/T₁ crossing point first takes place a T₁ ESIPT to generate a T₁ enol tautomer, which can further decay to the S₀ state via T₁-to-S₀ ISC. Finally, the GSIPT occurs to back the system to the initial ketoB form S₀ state. Our present work could contribute to understanding the photophysics of MS and its derivatives.

Excited-State Properties and Relaxation Pathways of Selenium-Substituted Guanine Nucleobase in Aqueous Solution and DNA Duplex

The excited-state properties and relaxation mechanisms after light irradiation of 6-selenoguanine (6SeG) in water and in DNA have been investigated using a quantum mechanics/molecular mechanics (QM/MM) approach with the multistate complete active space second-order perturbation theory (MS-CASPT2) method. In both environments, the S₁ ¹(n_Seπ₅^*) and S₂ ¹(π_Seπ₅^*) states are predicted to be the spectroscopically dark and bright states, respectively. Two triplet states, T₁ ³(π_Seπ₅^*) and T₂ ³(n_Seπ₅^*), are found energetically below the S₂ state. Extending the QM region to include the 6SeG-Cyt base pair slightly stabilizes the S₂ state and destabilizes the S₁, due to hydrogen-bonding interactions, but it does not affect the order of the states. The optimized minima, conical intersections, and singlet-triplet crossings are very similar in water and in DNA, so that the same general mechanism is found. Additionally, for each excited state geometry optimization in DNA, three kind of structures ("up", "down", and "central") are optimized which differ from each other by the orientation of the C═Se group with respect to the surrounding guanine and thymine nucleobases. After irradiation to the S₂ state, 6SeG evolves to the S₂ minimum, near to a S₂/S₁ conical intersection that allows for internal conversion to the S₁ state. Linear interpolation in internal coordinates indicate that the "central" orientation is less favorable since extra energy is needed to surmount the high barrier in order to reach the S₂/S₁ conical intersection. From the S₁ state, 6SeG can further decay to the T₁ ³(π_Seπ₅^*) state via intersystem crossing, where it will be trapped due to the existence of a sizable energy barrier between the T₁ minimum and the T₁/S₀ crossing point. Although this general S₂ → T₁ mechanism takes place in both media, the presence of DNA induces a steeper S₂ potential energy surface, that it is expected to accelerate the S₂ → S₁ internal conversion.

Position of the Benzene Ring Substituent Regulates the Excited-State Deactivation Process of the Benzyluracil Systems

A systematic theoretical study of the regulating effect of the substituent position on the photoinduced deactivation process of the benzyluracil systems has been performed based on the high-level static electronic structure calculations and on-the-fly full-dimensional excited-state dynamics simulations. Similarities and differences coexist for the two systems by comparative studies on the photoinduced deactivation process of the 5-benzyluracil (5-BU) and 6-benzyluracil (6-BU) systems. They both obey an S₂ → S₁ → S₀ two-step decay pattern, and the decay coordinates of the S₂ → S₁ and S₁ → S₀ processes are mainly driven by the elongation of the bridging bond and the out-of-plane ring deformation motion, respectively. However, the puckering motion occurring at the C2 atom in the uracil fragment dominates the decay pathway of the 5-BU system. On the contrary, the puckering motion at the C5 atom in the benzene fragment mainly drives the decay coordinate of the 6-BU system. Therefore, the substituent position could play significant roles in the deactivation process of the benzyluracil systems. Moreover, the S₁ → S₀ decay process of the 6-BU system consists of five pathways, possessing a more complex deactivation picture than the 5-BU system. The fitted time scale of the puckering motion is compatible with the experimentally observed lifetimes. This work provides a fundamental understanding of the photophysical and photochemical properties of the benzyluracil systems and can give rational suggestions to further design or regulate the bionic molecular systems.

Theoretical studies on the photochemistry of 2-nitrofluorene in the gas phase and acetonitrile solution

The photophysical and photochemical mechanisms of 2-nitrofluorene (2-NF) in the gas phase and acetonitrile solution have been studied theoretically. Upon ∼330 nm irradiation to the first bright state (¹ππ*), the 2-NF system can decay to triplet excited states via rapid intersystem crossing (ISC) processes through different surface crossing points or to the ground state via an ultrafast internal conversion (IC) process through the S₁/S₀ conical intersection. The ¹nπ* dark state will serve as a bridge when the system leaves the Franck-Condon (FC) region and approaches to the S₁ minimum. The molecule maintains a planar geometry during the excited-state relaxation processes. The differences on excitation properties such as electronic configurations and spin-orbit coupling (SOC) interactions between those in the gas phase and acetonitrile solution cannot be neglected, indicating possible changes on the efficiency of the related ISC processes for the 2-NF system in solution. Once arrived at the T₁ state, it would further decay to the S₀ state or photodegrade into the Ar-O˙ and NO˙ free radicals. During the intramolecular rearrangement process, the twisting of the nitro group out of the aromatic-ring plane is regarded as a critical structural variation for the photodegradation of the 2-NF system. The free radicals finally form through oxaziridine-type intermediate and transition state structures. The present work provides important mechanistic insights to the photochemistry of nitro-substituted polyaromatic compounds.

Internal conversion and intersystem crossing dynamics of uracil upon double thionation: a time-resolved photoelectron spectroscopy study in the gas phase

The photophysical properties of 2,4-dithiouracil (2,4-DTU) in the gas phase are studied by time-resolved photoelectron spectroscopy (TRPES) with three different excitation wavelengths in direct extension of previous work on uracil (U), 2-thiouracil (2-TU) and 4-thiouracil (4-TU). Non-radiative deactivation in the canonical nucleobases like uracil mainly occurs via internal conversion (IC) along singlet excited states, although intersystem crossing (ISC) to a long-lived triplet state was confirmed to play a minor role. In thionated uracils, ISC to the triplet state becomes ultrafast and highly efficient with a quantum yield near unity; however, the lifetime of the triplet state is strongly dependent on the position of the sulfur atom. In 2-TU, ISC back to the ground state occurs within a few hundred picoseconds, whereas the population remains trapped in the lowest triplet state in the case of 4-TU. Upon doubling the degree of thionation, ISC remains highly efficient and dominates the photophysics of 2,4-DTU. However, several low-lying excited states contribute to competing IC and ISC pathways and a complex deactivation mechanism, which is evaluated here based on TRPES measurements and discussed in the context of the singly thionated uracils.

Selenium substitution effects on excited-state properties and photophysics of uracil: a MS-CASPT2 study

The photophysics of selenium-substituted nucleobases has attracted recent experimental attention because they could serve as potential photosensitizers in photodynamic therapy. Herein, we present a comprehensive MS-CASPT2 study on the spectroscopic and excited-state properties, and photophysics of 2-selenouracil (2SeU), 4-selenouracil (4SeU), and 2,4-selenouracil (24SeU). Relevant minima, conical intersections, crossing points, and excited-state relaxation paths in the lowest five electronic states (i.e., S₀, S₁, S₂, T₂, and T₁) are explored. On the basis of these results, their photophysical mechanisms are proposed. Upon photoirradiation to the bright S₂ state, 2SeU quickly relaxes to its S₂ minimum and then moves in an essentially barrierless way to a nearby S₂/S₁ conical intersection near which the S₁ state is populated. Next, the S₁ system arrives at an S₁/T₂/T₁ intersection where a large S₁/T₁ spin-orbit coupling of 430.8 cm^-1 makes the T₁ state populated. In this state, a barrier of 6.8 kcal mol^-1 will trap 2SeU for a while. In parallel, for 4SeU or 24SeU, the system first relaxes to the S₂ minimum and then overcomes a small barrier to approach an S₂/S₁ conical intersection. Once hopping to the S₁ state, there exists an extended region with very close S₁, T₂, and T₁ energies. Similarly, a large S₁/T₁ spin-orbit coupling of 426.8 cm^-1 drives the S₁→ T₁ intersystem crossing process thereby making the T₁ state populated. Similarly, an energy barrier heavily suppresses electronic transition to the S₀ state. The present work manifests that different selenium substitutions on uracil can lead to a certain extent of different vertical and adiabatic excitation energies, excited-state properties, and relaxation pathways. These insights could help understand the photophysics of selenium-substituted nucleobases.

The excited-state relaxation mechanism of potential UVA-activated phototherapeutic molecules: trajectory surface hopping simulations of both 4-thiothymine and 2,4-dithiothymine

Recent experimental investigations of the photochemical properties of a series of sulfur-substituted pyrimidine derivatives provide insights into the phototherapeutic potential of these nucleobase variants. Herein we elucidate the triplet formation mechanism of two prospective UVA-activated phototherapeutic molecules, 4-thiothymine and 2,4-dithiothymine, upon photo-excitation by applying the trajectory surface hopping dynamics at the LR-TDDFT level. Our simulations reasonably reproduce the experimental time constants and demonstrate the preferred triplet formation pathway which starts from the S₁(n_Sπ*) state for both molecules. It is found that deactivation of the first bright state to the S₁(n_Sπ*) state proceeds through a mechanism involving elongation of the C5-C6 and C4-S8 bond-lengths and C2-pyramidalization in 4-thiothymine and involving elongation of the C5-C6 and C2-S7 bond-lengths in 2,4-dithiothymine. The intersystem crossing of 2,4-dithiothymine occurs either at geometries characterized by elongated C5-C6 and C2-S7 bond-lengths or at geometries showing elongated C5-C6 and C4-S8 bond-lengths as seen in 4-thiothymine. The solvents are found to affect the S₂ state decay of 4-thiothymine, leading to a competing pathway between S₂→ S₁ and S₂→ T₃. This study provides a molecular-level understanding of the underlying excited-state relaxation of the two UVA-activated thiopyrimidines, which may be linked to their potential applications in pharmacological science and also prove helpful for designing more effective phototherapeutic agents.

Triplet Decay Dynamics in Sulfur-Substituted Thymine: How Position of Substitution Matters

Sulfur-substituted analogues of thymine are of three types depending on the position of sulfur substitution: 2-thiothymine (2tThy), 4-thothymine (4tThy), and 2,4-dithiothymine (dtThy). These molecules, on photoexcitation, are known to form in their triplet state with near unity yield. Consequently, they are able to photosensitize ground state molecular oxygen to singlet oxygen, a property which makes them potential drugs for photodynamic therapy (PDT). The singlet oxygen yield is directly correlated with the triplet lifetime of the thiothymine, which in turn is governed by its triplet decay dynamics. In this work, the dependence of the triplet decay dynamics on the position of sulfur substitution is investigated by comparatively studying all three thiothymines. The topology of the triplet potential energy surface and decay mechanism of 2tThy is found to be distinctly different from 4tThy and dtThy. The fundamental reason for this is the different electronic natures of the two C═X (X = O, S) moieties in each molecule, one of which is conjugated with a C═C bond, while the other is not. Further, it is shown that the triplet lifetime of 2tThy can be increased by manipulating the energetic ordering of its molecular orbitals with unobtrusive substitutions, thus making it a better candidate for a PDT drug.

QM/MM Studies on the Photophysical Mechanism of a Truncated Octocrylene Model

Methyl 2-cyano-3,3-diphenylacrylate (MCDPA) shares the same molecular skeleton with octocrylene (OCR) that is one of the most common molecules used in commercially available sunscreens. However, its excited-state relaxation mechanism is unclear. Herein, we have used the QM(CASPT2//CASSCF)/MM method to explore spectroscopic properties, geometric and electronic structures, relevant conical intersections and crossing points, and excited-state relaxation paths of MCDPA in methanol solution. We found that in the Franck-Condon (FC) region, the V(¹ππ*) state is energetically lower than the V'(¹ππ*) state only by 2.8 kcal/mol and is assigned to experimentally observed maximum absorption band. From these two initially populated singlet states, there exist three nonradiative relaxation paths to repopulate the S₀ state. In the first one, when the V(¹ππ*) state is populated in the FC region, the system diabatically evolves along the V(¹ππ*) state into its minimum where the internal conversion to S₀ occurs. In the second one, the V'(¹ππ*) state is populated in the FC region and the system adiabatically overcomes a barrier of ca. 3.0 kcal/mol to approach the V(¹ππ*) minimum eventually leading to a V(¹ππ*)-to-S₀ internal conversion. In the third one, the V'(¹ππ*) state first hops via the intersystem crossing to the T₂ state, which then decays through the internal conversion to the T₁ state. The T₁ state is finally converted to the S₀ state via the T₁/S₀ crossing point. Our present work contributes to understanding the photophysics of OCR and its variants.

MS-CASPT2 Studies on the Photophysics of Selenium-Substituted Guanine Nucleobase

The MS-CASPT2 method has been employed to optimize minimum-energy structures of 6-selenoguanine (6SeGua) and related two- and three-state intersection structures in and between the lowest five electronic states, i.e., S₂(¹ππ*), S₁(¹ nπ*), T₂(³ nπ*), T₁(³ππ*), and S₀. In combination with MS-CASPT2 calculated linearly interpolated internal coordinate paths, the photophysical mechanism of 6SeGua has been proposed. The initially populated S₂(¹ππ*) state decays to either S₁(¹ nπ*) or T₂(³ nπ*) states through a three-state S₂/S₁/T₂ intersection point. The large S₂/T₂ spin-orbit coupling of 435 cm^-1, according to the classical El-Sayed rule, benefits the S₂ → T₂ intersystem crossing process. The S₁(¹ nπ*) state that stems from the S₂ → S₁ internal conversion process at the S₂/S₁/T₂ intersection point can further jump to the T₂(³ nπ*) state through the S₁ → T₂ intersystem crossing process. This process does not comply with the El-Sayed rule, but it is still related to a comparatively large spin-orbit coupling of 39 cm^-1 and is expected to occur relatively fast. Finally, the T₂(³ nπ*) state, which is populated from the above S₂ → T₂ and S₁ → T₂ intersystem crossing processes, decays to the T₁(³ππ*) state via an internal conversion process. Because there is merely a small energy barrier of 0.11 eV separating the T₁(³ππ*) minimum and an energetically allowed two-state T₁/S₀ intersection point, the T₁(³ππ*) state still can decay to the S₀ state quickly, which is also enhanced by a large T₁/S₀ spin-orbit coupling of 252 cm^-1. Our proposed mechanism explains experimentally observed ultrafast intersystem crossing processes in 6SeGua and its 835-fold acceleration of the T₁ state decay to the S₀ state compared with 6tGua. Finally, we have found that the ground-state electronic structure of 6SeGua has more apparent multireference character.

Theoretical studies on photo-induced cycloaddition and (6-4) reactions of the thymidine:4-thiothymidine dimer in a DNA duplex

Photo-induced cycloaddition and (6-4) reactions of the thymidine:4-thiothymidine dimer in a DNA duplex.

QM/MM studies on the excited-state relaxation mechanism of a semisynthetic dTPT3 base

Semisynthetic alphabets can potentially increase the genetic information stored in DNA through the formation of unusual base pairs. Recent experiments have shown that near-visible-light irradiation of the dTPT3 chromophore could lead to the formation of a reactive triplet state and of singlet oxygen in high quantum yields. However, the detailed excited-state relaxation paths that populate the lowest triplet state are unclear. Herein, we have for the first time employed the QM(MS-CASPT2//CASSCF)/MM method to explore the spectroscopic properties and excited-state relaxation mechanism of the aqueous dTPT3 chromophore. On the basis of the results, we have found that (1) the S₂(¹ππ*) state of dTPT3 is the initially populated excited singlet state upon near-visible light irradiation; and (2) there are two efficient relaxation pathways to populate the lowest triplet state, i.e. T₁(³ππ*). In the first one, the S₂(¹ππ*) system first decays to the S₁(¹nπ*) state near the S₂/S₁ conical intersection, which is followed by an efficient S₁ → T₁ intersystem crossing process at the S₁/T₁ crossing point; in the second one, an efficient S₂ → T₂ intersystem crossing takes place first, and then, the T₂(³nπ*) system hops to the T₁(³ππ*) state through an internal conversion process at the T₂/T₁ conical intersection. Moreover, an S₂/S₁/T₂ intersection region is found to play a vital role in the excited-state relaxation. These new mechanistic insights help in understanding the photophysics and photochemistry of unusual base pairs.

Photochemical and Photodynamical Properties of Sulfur-Substituted Nucleic Acid Bases

Sulfur-substituted nucleobases (a.k.a., thiobases) are among the world's leading prescriptions for chemotherapy and immunosuppression. Long-term treatment with azathioprine, 6-mercaptopurine and 6-thioguanine has been correlated with the photoinduced formation of carcinomas. Establishing an in-depth understanding of the photochemical properties of these prodrugs may provide a route to overcoming these carcinogenic side effects, or, alternatively, a basis for developing effective compounds for targeted phototherapy. In this review, a broad examination is undertaken, surveying the basic photochemical properties and excited-state dynamics of sulfur-substituted analogs of the canonical DNA and RNA nucleobases. A molecular-level understanding of how sulfur substitution so remarkably perturbs the photochemical properties of the nucleobases is presented by combining experimental results with quantum-chemical calculations. Structure-property relationships demonstrate the impact of site-specific sulfur substitution on the photochemical properties, particularly on the population of the reactive triplet state. The value of fundamental photochemical investigations for driving the development of ultraviolet-A chemotherapeutics is showcased. The most promising photodynamic agents identified thus far have been investigated in various carcinoma cell lines and shown to decrease cell proliferation upon exposure to ultraviolet-A radiation. Overarching principles have been elucidated for the impact that sulfur substitution of the carbonyl oxygen has on the photochemical properties of the nucleobases.

Visible-Light-Mediated Excited State Relaxation in Semi-Synthetic Genetic Alphabet: d5SICS and dNaM

The excited state dynamics of an unnatural base pair (UBP) d5SICS/dNaM were investigated by accurate ab-initio calculations. Time-dependent density functional and high-level multireference calculations (MS-CASPT2) were performed to elucidate the excitation of this UBP and its excited state relaxation mechanism. After excitation to the bright state S₂ (ππ*), it decays to the S₁ state and then undergoes efficient intersystem crossing to the triplet manifold. The presence of sulfur atom in d5SICS leads to strong spin-orbit coupling (SOC) and a small energy gap that facilitates intersystem crossing from S₁ (n_s π*) to T₂ (ππ*) followed by internal conversion to T₁ state. Similarly in dNaM, the deactivation pathway follows analogous trends. CASPT2 calculations suggest that the S₁ (ππ*) state is a dark state below the accessible S₂ (ππ*) bright state. During the ultrafast deactivation, it exhibits bond length inversion. From S₁ state, significant SOC leads the population transfer to T₃ due to a smaller energy gap. Henceforth, fast internal conversion occurs from T₃ to T₂ followed by T₁ . From time-dependent trajectory surface hopping dynamics, it is found that excited state relaxation occurs on a sub-picosecond timescale in d5SICS and dNaM. Our findings strongly suggest that there is enough energy available in triplet state of UBP to generate reactive oxygen species and induce phototoxicity with respect to cellular DNA.

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.

A theoretical study of the light-induced cross-linking reaction of 5-fluoro-4-thiouridine with thymine

In contrast to photophysics of thio-substituted nucleobases, their photoinduced cross-linking reactions with canonical nucleobases remain scarcely investigated computationally. In this work, we have adopted combined CASPT2/PCM//CASSCF and B3LYP-D3/PCM electronic structure methods to study this kind of photochemical reaction of 5-fluoro-4-thiouridine (truncated 5-fluoro-1-methyl-4-thiouracil used in calculations) and 1-methylthymine (referred to as thymine for clarity hereinafter). On the basis of CASPT2/PCM computed results, we have proposed two efficient excited-state relaxation pathways to populate the lowest T₁ state of the complex of 5-fluoro-1-methyl-4-thiouracil and thymine from its initially populated S₂(¹ππ*) state. In the first one, the S₂ system first hops to the S₁ state via an S₂/S₁ conical intersection, followed by a direct S₁ → T₁ intersystem crossing process enhanced by large S₁/T₁ spin-orbit coupling. In the second path, the resultant S₁ system first jumps to the T₂ state, from which an efficient T₂ → T₁ internal conversion occurs. The T₁ cross-linking reaction is overall divided into two phases. The first phase is a stepwise and nonadiabatic photocyclization reaction, which starts from the T₁ complex and ends up with an S₀ thietane intermediate. The second phase is a thermal reaction. The system first rearranges its four- and six-membered rings to form three new rings; then, an S₀ fluorine atom transfer occurs, followed by the formation of photoproducts. Finally, the present work paves the way for studying light-induced cross-linking reactions of thionucleobases with canonical bases in DNA and RNA.

Optical absorption and magnetic circular dichroism spectra of thiouracils: a quantum mechanical study in solution

The excited electronic states of thiouracils, the analogues of uracil where the carbonyl oxygens are substituted by sulphur atoms, have been investigated by computing the magnetic circular dichroism (MCD) and one-photon absorption (OPA) spectra at the TD-DFT level of theory.