Overview of Theoretical and Computational Methods Applied to the Oxygen–Organic Molecule Photosystem

Singlet Oxygen Photophysics: From Liquid Solvents to Mammalian Cells

Molecular oxygen, O2, has long provided a cornerstone for studies in chemistry, physics, and biology. Although the triplet ground state, O2(X3Σg-), has garnered much attention, the lowest excited electronic state, O2(a1Δg), commonly called singlet oxygen, has attracted appreciable interest, principally because of its unique chemical reactivity in systems ranging from the Earth's atmosphere to biological cells. Because O2(a1Δg) can be produced and deactivated in processes that involve light, the photophysics of O2(a1Δg) are equally important. Moreover, pathways for O2(a1Δg) deactivation that regenerate O2(X3Σg-), which address fundamental principles unto themselves, kinetically compete with the chemical reactions of O2(a1Δg) and, thus, have practical significance. Due to technological advances (e.g., lasers, optical detectors, microscopes), data acquired in the past ∼20 years have increased our understanding of O2(a1Δg) photophysics appreciably and facilitated both spatial and temporal control over the behavior of O2(a1Δg). One goal of this Review is to summarize recent developments that have broad ramifications, focusing on systems in which oxygen forms a contact complex with an organic molecule M (e.g., a liquid solvent). An important concept is the role played by the M+•O2-• charge-transfer state in both the formation and deactivation of O2(a1Δg).

Titanium Superoxide as a Carrier of a "Long-Lived" Superoxide Anion: An Ab Initio Investigation

The photoinduced generation of a superoxide anion on the surface of a semiconductor photocatalyst is usually attributed to the reduction of O2 with conduction-band electrons. In the current work, the reaction of TiO2 with O2 giving rise to TiO4 in superoxide and peroxide states has been investigated with ab initio (CAS, CCSD) and DFT (B3LYP) calculations. The ground triplet state and two substates (open-shell singlet (OSS) and closed-shell singlet (CSS)) of a doubly degenerate excited singlet state (a1Δg) are considered as reactive states of oxygen, participating in spontaneous or photoinduced processes, respectively. The triplet and OSS singlet states of TiO4 contain O2- as structural units and can be defined as titanium superoxides. Both states have energy less than the level of the initial pair TiO2+O2 by about 30 kcal/mol. The CSS state of TiO4 has a diperoxide structure Ti4+(O22-)2 and also lies in energy below the initial pair TiO2+3O2. Titanium superoxide is considered to be the carrier of an "exceptionally stable" and "long-lived" superoxide anion, which was earlier synthesized or detected on the surface of TiO2. The low-energy location of the conical intersections on the way from reagents to 3TiO4 allows us to explain the literature data on the spontaneous generation of the "long-lived" superoxide anion on the TiO2 surface.

Thioxanthone Dioxide Triplet States Have Low Oxygen Quenching Rate Constants

With few exceptions, triplet excited states of organic molecules, 3M1, are quenched by ground state molecular oxygen, O2(X3Σg-), with rate constants kq greater than ∼109 M-1 s-1 in fluid solutions. If the energy of the triplet state is above 94 kJ/mol, then such quenching can result in the sensitized production of singlet oxygen, O2(a1Δg). In the interaction between 3M1 and O2(X3Σg-), the magnitudes of both kq and the yield of the O2(a1Δg) depend appreciably on mixing with the M-O2 charge-transfer state. Here, we report that triplet states of several thioxanthen-9-one-10,10-dioxide derivatives have unusually low kq values (as low as ∼1 × 108 M-1 s-1) but have quantum yields for the photosensitized production of O2(a1Δg) that approach unity. Because these molecules possess high oxidation potentials (∼3.5 V vs SCE), we suggest that charge transfer character in the 3M1-O2(X3Σg-) encounter complex is reduced, thereby lowering kq while maintaining high O2(a1Δg) yields. These results provide important experimental support for existing models for the quenching of organic molecule excited states by O2(X3Σg-).

Combined Experimental and Computational Study on the Transformation of a Novel 1,3,4-Oxadiazole Thioether Nematicide in Aqueous Solutions

It has been demonstrated that Exianliuyimi (EXLYM) exhibits good nematocidal activity. As a potential nematicide, EXLYM and its transformation products (TPs) may generate emerging pollutants with hazardous effects on the ecosystem. In this study, the fate of EXLYM in aqueous solutions was investigated using experimental and theoretical approaches. Laboratory-scale experiments showed that EXLYM is hydrolytically stable. Microbial processes are primarily responsible for the oxidation of sulfur in aqueous solutions. Under simulated sunlight, the t_1/2 values of EXLYM in acidic, neutral, and alkaline buffer solutions were 5.02, 3.83, and 5.55 h, respectively. Six TPs were identified using a non-target screening strategy realized by ultra-high-performance liquid chromatography coupled with Q-Exactive Orbitrap high-resolution mass spectrometry and ¹⁸O-labeling experiments. Four of these were unambiguously confirmed using authentic standards. Reactive oxygen species scavenging experiments, ¹⁸O-labeling experiments, and quantum-theoretical calculations suggested that EXLYM could degrade mainly through four pathways: sulfur oxidation, nucleophilic aromatic photosubstitution, C-S bond cleavage, and oxidative ring-opening. The proposed degradation kinetics, TPs, and transformation pathways in aqueous solutions provide valuable information on the fate of EXLYM in aquatic ecosystems and lay the foundation for further toxicological tests.

Perturbed and Activated Decay: The Lifetime of Singlet Oxygen in Liquid Organic Solvents

Singlet oxygen, O₂(a¹Δ_g), the lowest excited electronic state of molecular oxygen, plays an important role in a range of chemical and biological processes. In liquid solvents, the reactions of singlet oxygen with a solute kinetically compete with solvent-mediated deactivation that yields the ground electronic state of oxygen, O₂(X³Σ_g^-). In this regard, the key parameter is the solvent-mediated lifetime of singlet oxygen, which embodies fundamental physical principles ranging from intermolecular interactions that perturb the forbidden O₂(a¹Δ_g) → O₂(X³Σ_g^-) transition to the transfer of oxygen's excitation energy into the vibrational modes of a solvent molecule M. Extensive research performed by the global community on this oxygen-related issue over the past ∼50 years reflects its significance. Unfortunately, a satisfactory quantitative understanding of this unique solvent effect has remained elusive thus far. In temperature-dependent studies, we have quantified the singlet oxygen lifetime in common aromatic and aliphatic organic solvents, including partially deuterated molecules that exploit the H/D solvent isotope effect on the lifetime. We now account for experimental data, including previously intractable data, using a model that exploits both weak and strong coupling in the M-O₂ complex to accommodate the roles that M plays to (1) induce the forbidden O₂(a¹Δ_g) → O₂(X³Σ_g^-) transition and (2) accept the excitation energy of O₂(a¹Δ_g). As such, our approach brings us appreciably closer to an accurate and predictive ab initio solution for the long-standing oxygen-dependent problem that, in turn, should be relevant for a host of other molecular systems.

X3Σg- → b1Σg+ Absorption Spectra of Molecular Oxygen in Liquid Organic Solvents at Atmospheric Pressure

Spectra and absorption coefficients of the forbidden 765 nm X³Σ_g^- → b¹Σ_g⁺ transition of molecular oxygen dissolved in organic solvents at atmospheric pressure were recorded over a 5 m path length using a liquid waveguide capillary cell. The results show that it is possible to investigate this weak near-infrared absorption transition in a common liquid hydrocarbon solvent without the need for a potentially dangerous high oxygen pressure. Proof-of-principle data from benzene, toluene, chlorobenzene, bromobenzene, and iodobenzene reveal a pronounced heavy atom effect on this spin-forbidden transition. For example, the absorption coefficient at the band maximum in iodobenzene, (28.9 ± 3.3) × 10^-3 M^-1 cm^-1, is approximately 21 times larger than that in benzene, (1.4 ± 0.1) × 10^-3 M^-1 cm^-1. These absorption measurements corroborate results obtained from O₂(X³Σ_g^-) → O₂(b¹Σ_g⁺) excitation spectra of O₂(a¹Δ_g) → O₂(X³Σ_g^-) phosphorescence, which depended on data from a plethora of convoluted experiments. Spectroscopic studies of molecular oxygen in liquid solvents can help evaluate aspects of the seminal Strickler-Berg approach to treat the effect of solvent on Einstein's A and B coefficients for radiative transitions. In particular, our present results are a key step toward using the O₂(X³Σ_g^-) → O₂(b¹Σ_g⁺) transition to evaluate the speculated limiting condition of applying the Strickler-Berg treatment to a highly forbidden process. This latter issue is but one example of how an arguably simple homonuclear diatomic molecule continues to aid the scientific community by providing fundamental physical insight.

The oxygen-organic molecule photosystem: revisiting the past, recalibrating the present, and redefining the future

Perturbation by a neighboring molecule M appreciably alters the properties of both the ground and excited states of molecular oxygen, as reflected in a variety of photophysical phenomena. In this article, we build upon the ~ 100 year history of work in this field, illustrating how the M-O₂ system continues to challenge the scientific community, facilitating better insight into fundamental tenets of chemistry and physics.

Computational insights into the reactivity of chlorpyrifos and chlorpyrifos-methyl toward singlet oxygen

A complete mechanism for the ^•OH-initiated atmospheric decomposition of the pesticides chlorpyrifos and chlorpyrifos-methyl is proposed, incorporating additional studies on the competing reaction with singlet oxygen. The computational study is based on density functional theory (DFT) at the double-hybrid functional level to treat static correlation in the calculations of energy barriers. Reaction of the P-bonded intermediate with ¹O₂ has a small energy barrier of ~ 2 kcal mol^-1, generating the Oxone compound and the HOSO^• radical, with a reaction free energy of - 49.8 kcal/mol for the chlorpyrifos reaction pathway. Direct reaction of the pesticides with singlet oxygen is unlikely to happen due to the exceedingly high energy barrier of ~ 52 kcal/mol. However, in aqueous solution, the activation energy reduces dramatically and changes the reaction thermodynamics, making it kinetically accessible and thermodynamically viable.

Singlet Oxygen: Discovery, Chemistry, C60 -Sensitization†

This review article refers to the discovery of excited molecular oxygen, in particular on its lower singlet excited state (¹ Δ_g , ¹ O₂ ). After a short report on singlet oxygen generation, the review is focused on the chemistry of this reactive species. Specifically, the three major reactions of ¹ O₂ with unsaturated organic substrates, namely the [4 + 2] and [2 +2] cycloadditions as well as the ene reaction, are reviewed. The proposed mechanisms of these reactions, through the years, based on experimental and computational work, have been presented. Selected examples of singlet oxygen-synthetic applications are also mentioned. The [60]fullerene and fullereno-materials photosensitized oxidations in homogeneous, as well as in heterogeneous conditions, are also comprehensively discussed. Finally, the self-sensitized photooxidation of open cage fullerenes as well as fullerenes bearing oxidizable groups is reported.

Singlet Oxygen Quantum Yields in Environmental Waters

Singlet oxygen (¹O₂) is a reactive oxygen species produced in sunlit waters via energy transfer from the triplet states of natural sensitizers. There has been an increasing interest in measuring apparent ¹O₂ quantum yields (Φ_Δ) of aquatic and atmospheric organic matter samples, driven in part by the fact that this parameter can be used for environmental fate modeling of organic contaminants and to advance our understanding of dissolved organic matter photophysics. However, the lack of reproducibility across research groups and publications remains a challenge that significantly limits the usability of literature data. In the first part of this review, we critically evaluate the experimental techniques that have been used to determine Φ_Δ values of natural organic matter, we identify and quantify sources of errors that potentially explain the large variability in the literature, and we provide general experimental recommendations for future studies. In the second part, we provide a qualitative overview of known Φ_Δ trends as a function of organic matter type, isolation and extraction procedures, bulk water chemistry parameters, molecular and spectroscopic organic matter features, chemical treatments, wavelength, season, and location. This review is supplemented with a comprehensive database of Φ_Δ values of environmental samples.

The complex between molecular oxygen and an organic molecule: modeling optical transitions to the intermolecular charge-transfer state

The collision complex between the ground electronic state of an organic molecule, M, and ground state oxygen, O2(X3Σg-), can absorb light to produce an intermolecular charge transfer (CT) state, often represented simply as the M radical cation, M+˙, paired with the superoxide radical anion, O2-˙. Aspects of this transition have been the subject of numerous studies for ∼70 years, many of which address fundamental concepts in chemistry and physics. We now examine the extent to which the combination of Molecular Dynamics simulations and electronic structure response methods can model transitions to the toluene-O2 CT state. To account for the experimental spectra, we consider (a) the distribution of toluene-O2 geometries that contribute to the transitions, (b) a quantitative description of intermolecular CT, and (c) oxygen-induced local transitions in toluene that complement the CT transitions, specifically transitions that populate toluene triplet states. We find that the latter oxygen-induced local transitions play a prominent role on the long wavelength side of the spectrum commonly attributed to the intermolecular CT transition. Our calculations provide a new perspective on the seminal discussion between R. S. Mulliken and D. F. Evans on the nature of O2-dependent transitions in organic molecules, and bode well for modeling transitions to excited states with CT character in noncovalent weakly-bonded molecular complexes.

Localized Bond Orbital Analysis of the Bonds of O2

A localized bond orbital analysis of the bonding in dioxygen and related species provides a unique fundamental insight into its bonding characteristics. It reveals the coalescence of the molecular orbital and valence bond/Lewis approaches and clearly demonstrates that the often stated inability of valence bond theory to describe the bonding of O₂ is a myth. The analysis indicates that the σ-bond strength of ³O₂ is not weak as previously believed and accounts for much of its enhanced stability compared with hydroperoxyl. We attribute the stability and persistence of ³O₂ to a combination of this attribute and favorable maximization of exchange coupling between the valence electrons.

Oxygen- and pH-Dependent Photophysics of Fluorinated Fluorescein Derivatives: Non-Symmetrical vs. Symmetrical Fluorination

Fluorescein, and derivatives of fluorescein, are often used as fluorescent probes and sensors. In systems where pH is a variable, protonation/deprotonation of the molecule can influence the pertinent photophysics. Fluorination of the xanthene moiety can alter the molecule’s pK_a such as to render a probe whose photophysics remains invariant over a wide pH range. Di-fluorination is often sufficient to accomplish this goal, as has been demonstrated with compounds such as Oregon Green in which the xanthene moiety is symmetrically difluorinated. In this work, we synthesized a non-symmetrical difluorinated analog of Oregon Green which we call Athens Green. We ascertained that the photophysics and photochemistry of Athens Green, including the oxygen-dependent photophysics that results in the sensitized production of singlet oxygen, O₂(a¹Δ_g), can differ appreciably from the photophysics of Oregon Green. Our data indicate that Athens Green will be a more benign fluorescent probe in systems that involve the production and removal of O₂(a¹Δ_g). These results expand the available options in the toolbox of fluorescein-based fluorophores.

Reactivities of singlet oxygen: open-shell or closed-shell?

This work explores the reactivity of singlet oxygen with respect to two typical reactions: cycloaddition to anthracene and excitation energy transfer (EET) to a carotenoid using diabatic states with multistate density functional theory (MSDFT). Noticeably, the degenerate state 1Δg has distinct open-shell (OS) and closed-shell (CS) components, and the closed-shell component showed more reactivity than the open-shell one due to the strong diabatic couplings to the product diabatic states. The diabatic perspective presented in this work could also apply to general singlet fission processes.

Mechanism of Spin-Exchange Internal Conversion: Practical Proxies for Diabatic and Nonadiabatic Couplings

Spin-exchange internal conversion (SEIC) is a general class of reactions having singlet fission and triplet fusion as particular cases. Based on a charge transfer (CT) mediated mechanism and analytical derivation with a model Hamiltonian, we propose proxies for estimating the coupling strength in both diabatic and adiabatic pictures for general SEIC reactions. In the diabatic picture, we demonstrated the existence of a bilinear relationship between the coupling strength and molecular orbital overlap, which provides a practical way to predict diabatic couplings. In the adiabatic picture, we showed that nonadiabatic couplings can be approximated by simple functions of the wave function CT coefficients. These approaches were verified through the investigation of singlet oxygen photosensitization, where both ¹Δ_g and ¹Σ_g oxygen states can be competitively generated by a triplet fusion reaction. The interplay between the CT-mediated mechanism, the spatial factors of the bimolecular complex, and the electronic structure of the oxygen molecule during the reaction explains the curiously small coupling to the ¹Σ_g state along specific incidence directions. The results from both the diabatic and adiabatic pictures provide a comprehensive understanding of the reaction mechanism, which applies to general SEIC problems.

Imidazole and beta-carotene photoprotection against photodynamic therapy evaluated by synchrotron infrared microscopy

In order to better understand the role of β-carotene and imidazole on the Photodynamic Therapy (PDT) mechanism, synchrotron infrared microscopy was used to detect the associated intracellular biochemical modifications following the visible light irradiation of HeLa cells incubated with these compounds as typical hydrophobic and hydrophilic singlet oxygen quenchers, respectively. For this purpose, PDT was performed employing the hydrophilic sensitizer 5,10,15,20-Tetrakis (1-methyl-4-pyridinio) porphyrin tetra (p-toluenesulfonate), TMPyP, and the hydrophobic sensitizer 5-(4-Methoxycarboxyphenyl)-10,15,20-triphenyl-21H,23H-porphyrin. The single cell IR spectra of PDT-treated, PDT plus quencher-treated and control HeLa cells were recorded at the SOLEIL Synchrotron Infrared SMIS beamline targeting specifically the cell nucleus. Principal Component Analysis (PCA) was used to assess the IR spectral changes. PCA revealed that there is a frequency shift of the protein Amide I vibrational band for the assays with the TMPyP sensitizer, indicating changes in the protein secondary structures of the PDT-treated cancer cells compared to the controls. In addition, the scores in those cells treated with both quenchers appear to be similar to the controls indicating a photoprotective effect. Comparative experiments carried out with SKMEL-28 and HaCat cells showed non- significant photoprotective effects of β-carotene and imidazole.

Spatial Factors for Triplet Fusion Reaction of Singlet Oxygen Photosensitization

First-principles quantum-chemical description of photosensitized singlet oxygen generation kinetics is challenging because of the intrinsic complexity of the underlying triplet fusion process in a floppy molecular complex with open-shell character. With a quantum-chemical kinetic model specifically tailored to deal with this problem, the reaction rates are investigated as a function of intermolecular incidence direction, orientation, and distance between O₂ and the photosensitizer. The adopted photosensitizer, 6-azo-2-thiothymine, combines practical interest and prototypical variability. The study quantitatively determined maximum singlet oxygen generation rates for 15 incidence/orientation directions, showing that they span 5 orders of magnitude between the largest and the smallest rate. Such information may provide a hands-on guideline for the experimental molecular design of new photosensitizers as well as further higher-level theoretical research.

Singlet Oxygen Photophysics in Liquid Solvents: Converging on a Unified Picture

Singlet oxygen, O₂(a¹Δ_g), the lowest excited electronic state of molecular oxygen, is an omnipresent part of life on earth. It is readily formed through a variety of chemical and photochemical processes, and its unique reactions are important not just as a tool in chemical syntheses but also in processes that range from polymer degradation to signaling in biological cells. For these reasons, O₂(a¹Δ_g) has been the subject of intense activity in a broad distribution of scientific fields for the past ∼50 years. The characteristic reactions of O₂(a¹Δ_g) kinetically compete with processes that deactivate this excited state to the ground state of oxygen, O₂(X³Σ_g^-). Moreover, O₂(a¹Δ_g) is ideally monitored using one of these deactivation channels: O₂(a¹Δ_g) → O₂(X³Σ_g^-) phosphorescence at 1270 nm. Thus, there is ample justification to study and control these competing processes, including those mediated by solvents, and the chemistry community has likewise actively tackled this issue. In themselves, the solvent-mediated radiative and nonradiative transitions between the three lowest-lying electronic states of oxygen [O₂(X³Σ_g^-), O₂(a¹Δ_g), and O₂(b¹Σ_g⁺)] are relevant to issues at the core of modern chemistry. In the isolated oxygen molecule, these transitions are forbidden by quantum-mechanical selection rules. However, solvent molecules perturb oxygen in such a way as to make these transitions more probable. Most interestingly, the effect of a series of solvents on the O₂(X³Σ_g^-)-O₂(b¹Σ_g⁺) transition, for example, can be totally different from the effect of the same series of solvents on the O₂(X³Σ_g^-)-O₂(a¹Δ_g) transition. Moreover, a given solvent that appreciably increases the probability of a radiative transition generally does not provide a correspondingly viable pathway for nonradiative energy loss, and vice versa. The ∼50 years of experimental work leading to these conclusions were not easy; spectroscopically monitoring such weak and low-energy transitions in time-resolved experiments is challenging. Consequently, results obtained from different laboratories often were not consistent. In turn, attempts to interpret molecular events were often simplistic and/or misguided. However, over the recent past, increasingly accurate experiments have converged on a base of credible data, finally forming a consistent picture of this system that is resonant with theoretical models. The concepts involved encompass a large fraction of chemistry's fundamental lexicon, e.g., spin-orbit coupling, state mixing, quantum tunneling, electronic-to-vibrational energy transfer, activation barriers, collision complexes, and charge-transfer interactions. In this Account, we provide an explanatory overview of the ways in which a given solvent will perturb the radiative and nonradiative transitions between the O₂(X³Σ_g^-), O₂(a¹Δ_g), and O₂(b¹Σ_g⁺) states.

Temperature Sensitive Singlet Oxygen Photosensitization by LOV-Derived Fluorescent Flavoproteins

Optogenetic sensitizers that selectively produce a given reactive oxygen species (ROS) constitute a promising tool for studying cell signaling processes with high levels of spatiotemporal control. However, to harness the full potential of this tool for live cell studies, the photophysics of currently available systems need to be explored further and optimized. Of particular interest in this regard, are the flavoproteins miniSOG and SOPP, both of which (1) contain the chromophore flavin mononucleotide, FMN, in a LOV-derived protein enclosure, and (2) photosensitize the production of singlet oxygen, O₂(a¹Δ_g). Here we present an extensive experimental study of the singlet and triplet state photophysics of FMN in SOPP and miniSOG over a physiologically relevant temperature range. Although changes in temperature only affect the singlet excited state photophysics slightly, the processes that influence the deactivation of the triplet excited state are more sensitive to temperature. Most notably, for both proteins, the rate constant for quenching of ³FMN by ground state oxygen, O₂(X³Σ_g^-), increases ∼10-fold upon increasing the temperature from 10 to 43 °C, while the oxygen-independent channels of triplet state deactivation are less affected. As a consequence, this increase in temperature results in higher yields of O₂(a¹Δ_g) formation for both SOPP and miniSOG. We also show that the quantum yields of O₂(a¹Δ_g) production by both miniSOG and SOPP are mainly limited by the fraction of FMN triplet states quenched by O₂(X³Σ_g^-). The results presented herein provide a much-needed quantitative framework that will facilitate the future development of optogenetic ROS sensitizers.

Solvent and Heavy-Atom Effects on the O2(X3Σg-) → O2(b1Σg+) Absorption Transition

The effect of 16 liquid solvents on both the spectrum and molar absorption coefficient of the X³Σ_g^- → b¹Σ_g⁺ transition in molecular oxygen has been examined. The ability to monitor this weak transition using air or oxygen saturated samples at atmospheric pressure was facilitated by the rapid and efficient O₂(b¹Σ_g⁺) → O₂(a¹Δ_g) transition, which allowed the use of O₂(a¹Δ_g) phosphorescence as a sensitive probe of O₂(b¹Σ_g⁺) production. The results of these O₂(a¹Δ_g) phosphorescence experiments are consistent with the results of independent experiments in which the O₂(a¹Δ_g) thus produced was "trapped" via a chemical reaction. The data recorded were used to calculate rate constants for the O₂(b¹Σ_g⁺) → O₂(X³Σ_g^-) radiative transition, a parameter that is otherwise difficult to directly obtain from such a wide range of solvents using O₂(b¹Σ_g⁺) → O₂(X³Σ_g^-) phosphorescence. The data show that the response of the O₂(b¹Σ_g⁺) → O₂(X³Σ_g^-) radiative transition to solvent is not the same as that of the O₂(b¹Σ_g⁺) → O₂(a¹Δ_g) and O₂(a¹Δ_g) → O₂(X³Σ_g^-) radiative transitions, both of which have been extensively examined over the years. However, our data are consistent with a theoretical model proposed by Minaev for the effect of solvent on radiative transitions in oxygen and, as such, arguably provide one of the final chapters in describing a system that has challenged the scientific community for years.

Ab initio investigation of electric and magnetic dipole electronic transitions in the complex of oxygen with benzene

The electric dipole transitions between pure spin and mixed spin electronic states are calculated at the XMC-QDPT2 and MCSCF levels of theory, respectively, for different intermolecular distances of the C6H6 and O2 collisional complex. The magnetic dipole transition moment between the mixed-spin ground ("triplet") and the first excited ("singlet") states is calculated by quadratic response at MCSCF level of theory. The obtained results confirm the theory of intensity borrowing and increasing the intensity of electronic transitions in the C6H6 + O2 collision. The calculation of magnetically induced current density is performed for benzene molecule being in contact with O2 at the distances from 3.5 to 4.5 Å. The calculation shows that the aromaticity of benzene is rising due to the conjugation of π-MOs of both molecules. The C6H6 + O2 complex becomes nonaromatic at the short distances (r < 3.5 Å). The computation of static polarizability in the excited electronic states of the C6H6 + O2 collisional complex at various distances supports the theory of red solvatochromic shift of the a → X band. Graphical abstract The C6H6+ O2 collisional complex.

Synthesis and Photocatalytic Reactivity of Vinylsulfonium Ylides

Although sulfur ylides are textbook reagents in organic synthesis, surprisingly little variation of substituents on sulfur is usually observed. In particular, vinylsulfonium ylides have been neglected so far. Herein, we present a study on their synthesis and reactivity, including interesting behavior under photocatalytic conditions.

Protein reactivity with singlet oxygen: Influence of the solvent exposure of the reactive amino acid residues

The singlet oxygen quenching rate constants were measured for three model proteins, bovine serum albumin, β-lactoglobulin and lysozyme. The results were analyzed by comparing them with the corresponding singlet oxygen quenching rate constants for a series of tripeptides with the basic formula GlyAAGly where the central amino acid (AA) was the oxidizable amino acid, tryptophan, tyrosine, methionine and histidine. It was found that the reaction rate constant in proteins can be satisfactorily modelled by the sum of the individual contributions of the oxidizable AA residues corrected for the solvent accessible surface area (SASA) effects. The best results were obtained when the SASA of the AA residues were determined by averaging over molecular dynamics simulated trajectories of the proteins. The limits of this geometrical correction of the AA residue reactivity are also discussed.

Singlet oxygen generation properties of isometrically dibromated thienyl-containing aza-BODIPYs

Singlet oxygen generation of 1 was found to be more effective and had about 2.0-fold rate enhancement compared to that of 2.

Solvent-dependent singlet oxygen lifetimes: temperature effects implicate tunneling and charge-transfer interactions

A new model for an old problem: a barrier to account for temperature effects on singlet oxygen lifetimes.

The Role of Singlet Oxygen in Surface Water Photochemistry

Singlet oxygen, (1O2, 1Δg), is a selective oxidant produced in sunlit surface waters. It is an electrophile produced from the quenching of excited state triplet natural organic matter (3NOM) by dissolved oxygen and it reacts with electron-rich alkenes, sulfides, and phenols. The concentration of 1O2 is high near the NOM molecules that sensitize its production and significantly decreases moving away from the NOM source. This chapter discusses the formation, quenching, reactivity, and detection of 1O2 and includes examples of surface water contaminants that react with 1O2.

Effect of Solvent on the O2(a(1)Δg) → O2(b(1)Σg(+)) Absorption Coefficient

Radiative transitions between the three lowest-lying electronic states of molecular oxygen have long provided a model to study how collision-dependent perturbations influence forbidden processes. In an isolated oxygen molecule, transitions between the O2(X(3)Σg(-)), O2(a(1)Δg), and O2(b(1)Σg(+)) states are forbidden as electric-dipole processes. For oxygen dissolved in organic solvents, the probabilities of radiative transitions between these states increase appreciably. Attempts to interpret solvent-dependent changes in the radiative rate constants have principally relied on O2(b(1)Σg(+)) and O2(a(1)Δg) emission experiments. However, the dominant nonradiative deactivation channels of O2(b(1)Σg(+)) make it difficult to quantify solvent effects on the O2(b(1)Σg(+)) → O2(a(1)Δg) radiative process. Thus, an appreciable amount of important information has heretofore not been available. In the present study, we examined the effect of 17 common organic solvents on the O2(a(1)Δg) → O2(b(1)Σg(+)) absorption transition at ∼5200 cm(-1) (i.e., ∼1925 nm). The solvent-dependent absorption coefficients at the band maximum, εmax, range from 5 to 50 M(-1) cm(-1) and correlate reasonably well with the solvent refractive index; εmax is largest in solvents with the largest refractive index. This observation is consistent with a model in which oxygen is perturbed to a greater extent by solvents with a large electronic polarizability. Through the Strickler-Berg equation, we also used these absorption data to obtain the radiative rate constant for the O2(b(1)Σg(+)) → O2(a(1)Δg) transition, and the results are consistent with a model in which the O2(a(1)Δg) → O2(X(3)Σg(-)) transition is said to steal intensity from the O2(b(1)Σg(+)) → O2(a(1)Δg) transition.

New singlet oxygen donors based on naphthalenes: synthesis, physical chemical data, and improved stability

Singlet oxygen donors are of current interest for medical applications, but suffer from a short half-life leading to low singlet oxygen yields and problems with storage. We have synthesized more than 25 new singlet oxygen donors based on differently substituted naphthalenes in only a few steps. The influence of functional groups on the reaction rate of the photooxygenations, thermolysis, half-life, and singlet oxygen yield has been thoroughly studied. We determined various thermodynamic data and compared them with density functional calculations. Interestingly, remarkable stabilities of functional groups during the photooxygenations and stabilizing effects for some endoperoxides during the thermolysis have been found. Furthermore, we give evidence for a partly concerted and partly stepwise thermolysis mechanism leading to singlet and triplet oxygen, respectively. Our results might be interesting for "dark oxygenations" and future applications in medicine.

Singlet oxygen in DNA nanotechnology

CONSPECTUS: Singlet oxygen ((1)O2), the first excited electronic state of molecular oxygen, is a significant molecule, despite its minute size. For more than half a century, the molecule has been widely used and studied in organic synthesis, due to its characteristic oxygenation reactions. Furthermore, (1)O2 plays a key role in mechanisms of cell death, which has led to its use in therapies for several types of cancer and other diseases. The high abundance of oxygen in air provides a wonderful source of molecules that can be excited to the reactive singlet state, for example, by UV/vis irradiation of a photosensitizer molecule. Although convenient, this oxygen abundance also presents some challenges for purposes that require (1)O2 to be generated in a controlled manner. In the past decade, we and others have employed DNA nanostructures to selectively control and investigate the generation, lifetime, and reactions of (1)O2. DNA-based structures are one of the most powerful tools for controlling distances between molecules on the nanometer length scale, in particular for systems that closely resemble biological settings, due to their inherent ability to specifically form duplex structures with well-defined and predictable geometries. Here, we present some examples of how simple DNA structures can be employed to regulate (1)O2 production by controlling the behavior of (1)O2-producing photosensitizers through their interactions with independent quencher molecules. We have developed different DNA-based systems in which (1)O2 production can be switched ON or OFF in the presence of specific DNA sequences or by changing the pH of the solution. To further illustrate the interplay between DNA structures and (1)O2, we present three pieces of research, in which (1)O2 is used to activate or deactivate DNA-based systems based on the reaction between (1)O2 and cleavable linkers. In one example, it is demonstrated how a blocked oligonucleotide can be released upon irradiation with light of a specific wavelength. In more complex systems, DNA origami structures composed of more than 200 individual oligonucleotides were employed to study (1)O2 reactions in spatially resolved experiments on the nanoscale.