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

Unraveling the Hidden Pathway of Catalyst-Free Direct Photochemical Conversion of Sulfides to Sulfoxides: A Universal Pathway under UVA Radiation

Selective conversion of sulfides to sulfoxides is an important class of chemical transformation that has enormous potential in medicinal chemistry. However, the primary process associated with photoexcitation during the direct photochemical conversion of sulfides to sulfoxides is poorly understood and misrepresented in the literature. Herein, we discover a hidden pathway responsible for the direct photochemical conversion of sulfides to sulfoxides in the absence of any catalysts under UVA illumination (λex = 370 nm). We show that this hidden pathway directly generates singlet oxygen (a1Δg,1O2) via solvent-oxygen (X3∑g-,3O2) charge transfer (CT) excitation in neat solvents under ambient conditions. Our findings reveal efficient and selective oxidation of sulfides to sulfoxides in the presence of 1O2 through the generation of persulfoxide intermediates. In addition, we found that the presence of a marginal amount of water favors faster kinetics and prevents overoxidation to sulfones due to the stabilization of the sulfoxide products via specific hydrogen-bonding interactions.

Pterin-Thymidine Adducts: From Their Photochemical Synthesis to Their Photosensitizing Properties

Pterin (Ptr) is the model compound of aromatic pterins, which are efficient photosensitizers present in human skin and are able to oxidize biomolecules upon UVA irradiation. Photosensitization involves chemical alteration of a biomolecule as a result of the initial absorption of radiation by another chemical species, the photosensitizer. Under anaerobic conditions, Ptr reacts with thymine (T) to form photoadducts (T-Ptr). In this work, we present a method to prepare and purify T-Ptr adducts, using 2'-deoxythymidine 5'-monophosphate (dTMP) and single stranded oligonucleotide 5'-d(TTTTT)-3' (dT5), and investigate their photosensitizing properties. Interestingly, the Ptr moiety, when attached to T, retains its photophysical properties. The adduct dTMP-Ptr, upon excitation, forms singlet and triplet excited states, the latter being capable of transferring energy to dissolved O2 and generating singlet oxygen, with an efficiency similar to Ptr. In air-equilibrated solutions, both dTMP-Ptr and dT5-Ptr adducts can photosensitize the oxidation of tryptophan and 2'-deoxyguanosine 5'-monophosphate, two of the main targets of photosensitization in biological systems, with efficiencies close to that of free Ptr. The mechanisms involved in the oxidation of biomolecules can be either type I (electron transfer) or type II (singlet oxygen).

Recent advances in optical heavy water sensors

D2O and H2O, as two important solvents with very similar properties, play a pivotal role in nuclear industrial production, life and scientific research. Unfortunately, D2O and H2O are highly susceptible to contamination by each other, so effective qualitative and quantitative analyses of both are necessary. This review comprehensively discusses the progress in optical sensing for the detection of a trace amount of H2O in heavy water or vice versa, mainly including five types of analytical systems: inorganic nanocrystals, carbon-based nanomaterials, lanthanide complexes, organic polymers, and organic small molecules. The whole article is divided into several sub-sections based on multiple mechanisms underlying the design of heavy water optical sensors, i.e., the difference in binding energy, the difference in quenching efficacy of oscillator types and the difference in acid-base of H2O and D2O. The working mechanism, advantages and disadvantages, analytical performance and applications of the reported sensors in recent years were analyzed in detail, and the future development is envisioned for the optical sensors towards distinguishing D2O and H2O.

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).

Magnetic manipulation of the reactivity of singlet oxygen: from test tubes to living cells

Although magnetism undoubtedly influences life on Earth, the science behind biological magnetic sensing is largely a mystery, and it has proved challenging, especially in the life sciences, to harness the interactions of magnetic fields (MFs) with matter to achieve specific ends. Using the well-established radical pair (RP) mechanism, we here demonstrate a bottom-up strategy for the exploitation of MF effects in living cells by translating knowledge from studies of RP reactions performed in vitro. We found an unprecedented MF dependence of the reactivity of singlet oxygen (1O2) towards electron-rich substrates (S) such as anthracene, lipids and iodide, in which [S ˙+ O2 ˙-] RPs are formed as a basis for MFs influencing molecular redox events in biological systems. The close similarity of the observed MF effects on the biologically relevant process of lipid peroxidation in solution, in membrane mimics and in living cells, shows that MFs can reliably be used to manipulate 1O2-induced cytotoxicity and cell-apoptosis-related protein expression. These findings led to a 'proof-of-concept' study on MF-assisted photodynamic therapy in vivo, highlighting the potential of MFs as a non-invasive tool for controlling cellular events.

Photosensitizer-free singlet oxygen generation via a charge transfer transition involving molecular O2 toward highly efficient oxidative coupling of arylamines to azoaromatics

Photosensitizer (PS)-mediated generation of singlet oxygen, O2 (a1Δg) is a well-explored phenomenon in chemistry and biology. However, the requirement of appropriate PSs with optimum excited state properties is a prerequisite for this approach which limits its widespread application. Herein, we report the generation of O2 (a1Δg) via direct charge-transfer (CT) excitation of the solvent-O2 (X3Σg -) collision complex without any PS and utilize it for the catalyst-free oxidative coupling of arylamines to azoaromatics under ambient conditions in aqueous medium. Electron paramagnetic resonance (EPR) spectroscopy revealed the formation of O2 (a1Δg) upon direct excitation with 370 nm light. The present approach shows broad substrate scope, remarkably fast reaction kinetics (90 and 40 min under an open and O2 atm, respectively), high selectivity (100%), and excellent yields (up to 100%), and works well for both homo- and hetero-coupling of arylamines. The oxidative coupling of arylamines was found to proceed through the generation of amine radicals via electron transfer (ET) from amines to O2 (a1Δg). Notably, electron-rich amines show higher yields of azo products compared to electron-deficient amines. Detailed mechanistic investigations using various spectroscopic tools revealed the formation of hydrazobenzene as an intermediate along with superoxide radicals which subsequently transform to hydrogen peroxide. The present study is unique in the way that molecular O2 simultaneously acts as a light-absorbing chromophore (solvent-O2 complex) as well as an efficient oxidant (O2 (a1Δg)) in the same reaction. This is the first report on the efficient, selective, and sustainable synthesis of azo compounds in aqueous medium under an ambient atmosphere without any PCs/PSs and paves the way for further in-depth understanding of the chemical reactivity of O2 (a1Δg) generated directly via CT excitation of the solvent-O2 complex toward various photochemical and photobiological transformations.

Recent Advances in Understanding of the Singlet Oxygen in Energy Storage and Conversion

Singlet oxygen (term symbol 1Δg, hereafter 1O2), a reactive oxygen species, has recently attracted increasing interest in the field of rechargeable batteries and electrocatalysis and photocatalysis. These sustainable energy conversion and storage technologies are of vital significance to replace fossil fuels and promote carbon neutrality and finally tackle the energy crisis and climate change. Herein, the recent progresses of 1O2 for energy storage and conversion is summarized, including physical and chemical properties, formation mechanisms, detection technologies, side reactions in rechargeable batteries and corresponding inhibition strategies, and applications in electrocatalysis and photocatalysis. The formation mechanisms and inhibition strategies of 1O2 in particular aprotic lithium-oxygen (Li-O2) batteries are highlighted, and the applications of 1O2 in photocatalysis and electrocatalysis is also emphasized. Moreover, the confronting challenges and promising directions of 1O2 in energy conversion and storage systems are discussed.

Heavy-atom tunnelling in singlet oxygen deactivation predicted by instanton theory with branch-point singularities

The reactive singlet state of oxygen (O2) can decay to the triplet ground state nonradiatively in the presence of a solvent. There is a controversy about whether tunnelling is involved in this nonadiabatic spin-crossover process. Semiclassical instanton theory provides a reliable and practical computational method for elucidating the reaction mechanism and can account for nuclear quantum effects such as zero-point energy and multidimensional tunnelling. However, the previously developed instanton theory is not directly applicable to this system because of a branch-point singularity which appears in the flux correlation function. Here we derive a new instanton theory for cases dominated by the singularity, leading to a new picture of tunnelling in nonadiabatic processes. Together with multireference electronic-structure theory, this provides a rigorous framework based on first principles that we apply to calculate the decay rate of singlet oxygen in water. The results indicate a new reaction mechanism that is 27 orders of magnitude faster at room temperature than the classical process through the minimum-energy crossing point. We find significant heavy-atom tunnelling contributions as well as a large temperature-dependent H2O/D2O kinetic isotope effect of approximately 20, in excellent agreement with experiment.

A comprehensive review on singlet oxygen generation in nanomaterials and conjugated polymers for photodynamic therapy in the treatment of cancer

A key role in lessening humanity's continuous fight against cancer could be played by photodynamic therapy (PDT), a minimally invasive treatment employed in the medical care of a range of benign disorders and malignancies. Cancerous tissue can be effectively removed by using a light source-excited photosensitizer. Singlet oxygen and reactive oxygen species are produced via the photosensitizer as a result of this excitation. In the recent past, researchers have put in tremendous efforts towards developing photosensitizer molecules for photodynamic treatment (PDT) to treat cancer. Conjugated polymers, characterized by their efficient fluorescence, exceptional photostability, and strong light absorption, are currently under scrutiny for their potential applications in cancer detection and treatment through photodynamic and photothermal therapy. Researchers are exploring the versatility of these polymers, utilizing sophisticated chemical synthesis and adaptable polymer structures to create new variants with enhanced capabilities for generating singlet oxygen in photodynamic treatment (PDT). The incorporation of photosensitizers into conjugated polymer nanoparticles has proved to be beneficial, as it improves singlet oxygen formation through effective energy transfer. The evolution of nanotechnology has emerged as an alternative avenue for enhancing the performance of current photosensitizers and overcoming significant challenges in cancer PDT. Various materials, including biocompatible metals, polymers, carbon, silicon, and semiconductor-based nanomaterials, have undergone thorough investigation as potential photosensitizers for cancer PDT. This paper outlines the recent advances in singlet oxygen generation by investigators using an array of materials, including graphene quantum dots (GQDs), gold nanoparticles (Au NPs), silver nanoparticles (Ag NPs), titanium dioxide (TiO2), ytterbium (Yb) and thulium (Tm) co-doped upconversion nanoparticle cores (Yb/Tm-co-doped UCNP cores), bismuth oxychloride nanoplates and nanosheets (BiOCl nanoplates and nanosheets), and others. It also stresses the synthesis and application of systems such as amphiphilic block copolymer functionalized with folic acid (FA), polyethylene glycol (PEG), poly(β-benzyl-L-aspartate) (PBLA10) (FA-PEG-PBLA10) functionalized with folic acid, tetra(4-hydroxyphenyl)porphyrin (THPP-(PNIPAM-b-PMAGA)4), pyrazoline-fused axial silicon phthalocyanine (HY-SiPc), phthalocyanines (HY-ZnPcp, HY-ZnPcnp, and HY-SiPc), silver nanoparticles coated with polyaniline (Ag@PANI), doxorubicin (DOX) and infrared (IR)-responsive poly(2-ethyl-2-oxazoline) (PEtOx) (DOX/PEtOx-IR NPs), particularly in NIR imaging-guided photodynamic therapy (fluorescent and photoacoustic). The study puts forward a comprehensive summary and a convincing justification for the usage of the above-mentioned materials in cancer PDT.

Kinetic effects in singlet oxygen mediated oxidations by immobilized photosensitizers on silica

Singlet oxygen (1O2) mediated photo-oxidations are important reactions involved in numerous processes in chemical and biological sciences. While most of the current research works have aimed at improving the efficiencies of these transformations either by increasing 1O2 quantum yields or by enhancing its lifetime, we establish herein that immobilization of a molecular photosensitizer onto silica surfaces affords significant, substrate dependant, enhancement in the reactivity of 1O2. Probing a classical model reaction (oxidation of Anthracene-9, 10-dipropionic acid, ADPA or dimethylanthracene, DMA) with various spectrofluorimetric techniques, it is here proposed that an interaction between polar substrates and the silica surface is responsible for the observed phenomenon. This discovery could have a direct impact on the design of future photosensitized 1O2 processes in various applications ranging from organic photochemistry to photobiology.

The structure of model and peptide disulfides markedly affects their reactivity and products formed with singlet oxygen

Disulfide bonds are critical structural elements in proteins and stabilize folded structures. Modification of these linkages is associated with a loss of structure and function. Previous studies have reported large variations in the rate of disulfide oxidation by hypohalous acids, due to stabilization of reaction intermediates. In this study we hypothesized that considerable variation (and hence selective oxidation) would occur with singlet oxygen (1O2), a key intermediate in photo-oxidation reactions. The kinetics of disulfide-mediated 1O2 removal were monitored using the time-resolved 1270 nm phosphorescence of 1O2. Stern-Volmer plots of these data showed a large variation (∼103) in the quenching rate constants kq (from 2 × 107 for α-lipoic acid to 3.6 × 104 M-1s-1 for cystamine). The time course of disulfide loss and product formation (determined by LC-MS) support a role for 1O2, with mono- and di-oxygenated products detected. Elevated levels of these latter species were generated in D2O- compared to H2O buffers, which is consistent with solvent effects on the 1O2 lifetime. These data are interpreted in terms of the intermediacy of a zwitterion [-S+(OO-)-S-], which either isomerizes to a thiosulfonate [-S(O)2-S-] or reacts with another parent molecule to give two thiosulfinates [-S(O)-S-]. The variation in quenching rates and product formation are ascribed to zwitterion stabilization by neighboring, or remote, lone pairs of electrons. These data suggest that some disulfides, including some present within or attached to proteins (e.g., α-lipoic acid), may be selectively modified, and undergo subsequent cleavage, with adverse effects on protein structure and function.

The debatable role of singlet oxygen in persulfate-based advanced oxidation processes

Singlet oxygen (¹O₂) attracts much attention in persulfate-based advanced oxidation processes (PS-AOPs), because of its wide pH tolerance and high selectivity toward electron-rich organics. However, there are conflicts about the ¹O₂ role in PS-AOPs on several aspects, including the formation of different key reactive oxygen species (ROS) at similar active sites, pH dependence, broad-spectrum activity, and selectivity in the elimination of organic pollutants. To a large degree, these conflicts root in the drawbacks of the methods to identify and evaluate the role of ¹O₂. For example, the quenchers of ¹O₂ have high reactivity to other ROS and persulfate as well. In addition, electron transfer process (ETP) also selectively oxidizes organics, having a misleading effect on the identification of ¹O₂. Therefore, in this review, we summarized and discussed some basic properties of ¹O₂, the debatable role of ¹O₂ in PS-AOPs on multiple aspects, and the methods and their drawbacks to identify and evaluate the role of ¹O₂. On the whole, this review aims to better understand the role of ¹O₂ in PS-AOPs and further help with its reasonable utilization.

1O2-Relevant Afterglow Luminescence of Chlorin Nanoparticles for Discriminative Detection and Isotopic Analysis of H2O and D2O

Discriminative detection between D₂O and H₂O is important for diverse fields but challenging due to their high similarity in chemical and physical properties. Current molecular sensors for D₂O detection generally rely on the spectral change of fluorophores with suitable pK_a in response to D₂O and H₂O with slightly different pH acidity. Herein, we report a new and facile D₂O sensor by using singlet oxygen (¹O₂)-relevant afterglow luminescence of chlorin e4 nanoparticles (Ce4-NPs) to achieve distinguishable detection between D₂O and H₂O. As ¹O₂ is a key initiator involved in the afterglow luminescence process, it displays a 22-fold longer lifetime in D₂O relative to H₂O and thereafter generates more dioxetane intermediates after laser irradiation to lead to ultimate afterglow brightness of Ce4-NPs in D₂O. In addition, Ce4-NPs are capable of quantitatively detecting the amount of H₂O in D₂O with a limit of detection (LOD) of 1.45%. Together, this study broadens the utility of afterglow materials and presents a facile strategy for isotopic purity analysis of heavy water.

Use of Singlet Oxygen in the Generation of a Mononuclear Nonheme Iron(IV)-Oxo Complex

Nonheme iron(III)-superoxo intermediates are generated in the activation of dioxygen (O₂) by nonheme iron(II) complexes and then converted to iron(IV)-oxo species by reacting with hydrogen donor substrates with relatively weak C-H bonds. If singlet oxygen (¹O₂) with ca. 1 eV higher energy than the ground state triplet oxygen (³O₂) is employed, iron(IV)-oxo complexes can be synthesized using hydrogen donor substrates with much stronger C-H bonds. However, ¹O₂ has never been used in generating iron(IV)-oxo complexes. Herein, we report that a nonheme iron(IV)-oxo species, [Fe^IV(O)(TMC)]²⁺ (TMC = tetramethylcyclam), is generated using ¹O₂, which is produced with boron subphthalocyanine chloride (SubPc) as a photosensitizer, and hydrogen donor substrates with relatively strong C-H bonds, such as toluene (BDE = 89.5 kcal mol^-1), via electron transfer from [Fe^II(TMC)]²⁺ to ¹O₂, which is energetically more favorable by 0.98 eV, as compared with electron transfer from [Fe^II(TMC)]²⁺ to ³O₂. Electron transfer from [Fe^II(TMC)]²⁺ to ¹O₂ produces an iron(III)-superoxo complex, [Fe^III(O₂)(TMC)]²⁺, followed by abstracting a hydrogen atom from toluene by [Fe^III(O₂)(TMC)]²⁺ to form an iron(III)-hydroperoxo complex, [Fe^III(OOH)(TMC)]²⁺, that is further converted to the [Fe^IV(O)(TMC)]²⁺ species. Thus, the present study reports the first example of generating a mononuclear nonheme iron(IV)-oxo complex with the use of singlet oxygen, instead of triplet oxygen, and a hydrogen atom donor with relatively strong C-H bonds. Detailed mechanistic aspects, such as the detection of ¹O₂ emission, the quenching by [Fe^II(TMC)]²⁺, and the quantum yields, have also been discussed to provide valuable mechanistic insights into understanding nonheme iron-oxo chemistry.

Imparting Stability to Organic Photovoltaic Components through Molecular Engineering: Mitigating Reactions with Singlet Oxygen

One key challenge in the development of viable organic photovoltaic devices is to design component molecules that do not degrade during combined exposure to oxygen and light. Such molecules should thus remain comparatively unreactive towards singlet molecular oxygen and not act as photosensitizers for the generation of this undesirable species. Here, novel redox-active chromophores that combine these two properties are presented. By functionalizing indenofluorene-extended tetrathiafulvalenes (IF-TTFs) with cyano groups at the indenofluorene core using Pd-catalyzed cyanation reactions, we find that the reactivity of the exocyclic fulvene carbon-carbon double bonds towards singlet oxygen is considerably reduced. The new cyano-functionalized IF-TTFs were tested in non-fullerene acceptor based organic photovoltaic proof-of-principle devices, revealing enhanced device stability.

Photochemical Upconversion in Solution: The Role of Oxygen and Magnetic Field Response

Upconversion processes effectively convert two or more low energy photons into one higher energy photon, and they have diverse prospective applications in photovoltaics and biomedicine. We focus on two specific mechanisms for photochemical upconversion in solution: triplet-triplet annihilation (TTA) and singlet oxygen mediated energy transfer (SOMET). TTA is spin-selective, whereas SOMET is not, so the interplay between these two upconversion mechanisms can be examined via their different magnetic field responses. A kinetic model is developed and applied to explain the different photoluminescence profiles of oxygenated versus deoxygenated systems. From the magnetic field response, the triplet-triplet annihilation rate constant is estimated. The conditions required to maximize upconversion photoluminescence intensity in oxygenated solution are determined, providing a set of design principles to guide molecule choices for robust and air-stable upconversion systems in the future.

Practical Aspects in the Study of Biological Photosensitization Including Reaction Mechanisms and Product Analyses: A Do's and Don'ts Guide†

The interaction of light with natural matter leads to a plethora of photosensitized reactions. These reactions cause the degradation of biomolecules, such as DNA, lipids, proteins, being therefore detrimental to the living organisms, or they can also be beneficial by allowing the treatment of several diseases by photomedicine. Based on the molecular mechanistic understanding of the photosensitization reactions, we propose to classify them in four processes: oxygen-dependent (type I and type II processes) and oxygen-independent [triplet-triplet energy transfer (TTET) and photoadduct formation]. In here, these processes are discussed by considering a wide variety of approaches including time-resolved and steady-state techniques, together with solvent, quencher, and scavenger effects. The main aim of this survey is to provide a description of general techniques and approaches that can be used to investigate photosensitization reactions of biomolecules together with basic recommendations on good practices. Illustration of the suitability of these approaches is provided by the measurement of key biomarkers of singlet oxygen and one-electron oxidation reactions in both isolated and cellular DNA. Our work is an educational review that is mostly addressed to students and beginners.

Enhancement of photoactivity and cellular uptake of (Bu4N)2[Mo6I8(CH3COO)6] complex by loading on porous MCM-41 support. Photodynamic studies as an anticancer agent

The incorporation by ionic assembly of the hexanuclear molybdenum cluster (Bu₄N)₂[Mo₆I₈(CH₃CO₂)₆] (1) in amino-decorated mesoporous silica nanoparticles MCM-41, has yielded the new molybdenum-based hybrid photosensitizer 1@MCM-41. The new photoactive material presents a high porosity, due to the intrinsic high specific surface area of MCM-41 nanoparticles (989 m² g^-1) which is responsible for the good dispersion of the hexamolybdenum clusters on the nanoparticles surface, as observed by STEM analysis. The hybrid photosensitizer can generate efficiently singlet oxygen, which was demonstrated by using the benchmark photooxygenation reaction of 9,10-anthracenediyl-bis(methylene)dimalonic acid (ABDA) in water. The photodynamic therapy activity has been tested using LED light as an irradiation source (λ_irr ~ 400-700 nm; 15.6 mW/cm²). The results show a good activity of the hybrid photosensitizer against human cervical cancer (HeLa) cells, reducing up to 70 % their viability after 20 min of irradiation, whereas low cytotoxicity is detected in the darkness. The main finding of this research is that the incorporation of molybdenum complexes at porous MCM-41 supports enhances their photoactivity and improves cellular uptake, compared to free clusters.

Diethynylarene-linked bis(triarylborane)cations as theranostic agents for tumor cell and virus-targeted photodynamic therapy

We recently reported diethynylarene-linked bis(triarylborane) tetracations which show remarkable fluorimetric and Raman-SERS sensing of DNA/RNA. In the current study, we show that they exhibit promising photodynamic therapy (PDT)-based biological activity on human cell lines and adenovirus type 5 (HAdV5), acting as theranostic agents. All compounds efficiently enter living cells showing negligible antiproliferative activity. Bis-thiophene- and anthracene- analogues bind non-covalently to HAdV5 virus with high affinity, the anthracene-analogue itself causing a moderate antiviral effect, i.e., decreased ability of the virus to infect human cells. Irradiation of bis-thiophene- and anthracene- analogues with visible light (400-700 nm) caused a very rapid (within 1 min) and strong increase in cytotoxicity, as well as an order of magnitude increase in antiviral activity, attributed to the formation of reactive oxygen species (ROS). Photochemical studies of the compounds revealed that, upon irradiation, they produce singlet oxygen, which correlates with the observed light-induced bioactivity.

Electron-Rich EDOT Linkers in Tetracationic bis-Triarylborane Chromophores: Influence on Water Stability, Biomacromolecule Sensing, and Photoinduced Cytotoxicity

Three novel tetracationic bis-triarylboranes with 3,4-ethylenedioxythiophene (EDOT) linkers, and their neutral precursors, showed significant red-shifted absorption and emission compared to their thiophene-containing analogues, with one of the EDOT-derivatives emitting in the NIR region. Only the EDOT-linked trixylylborane tetracation was stable in aqueous solution, indicating that direct attachment of a thiophene or even 3-methylthiophene to the boron atom is insufficient to provide hydrolytic stability in aqueous solution. Further comparative analysis of the EDOT-linked trixylylborane tetracation and its bis-thiophene analogue revealed efficient photo-induced singlet oxygen production, with the consequent biological implications. Thus, both analogues bind strongly to ds-DNA and BSA, very efficiently enter living human cells, accumulate in several different cytoplasmic organelles with no toxic effect but, under intense visible light irradiation, they exhibit almost instantaneous and very strong cytotoxic effects, presumably attributed to singlet oxygen production. Thus, both compounds are intriguing theranostic agents, whose intracellular and probably intra-tissue location can be monitored by strong fluorescence, allowing switching on of the strong bioactivity by well-focused visible light.

Mitochondria-Assisted Photooxidation to Track Singlet Oxygen at Homeostatic Membrane Microviscosity

Using intracellular-controlled photochemistry to track dynamic organelle processes is gaining attention due to its broad applications. However, most of the employed molecular probes usually require toxic photosensitizers and complex bioanalytical protocols. Here, the synthesis and performance of two new subcellular probes (MitoT1 and MitoT2) are described. The probes undergo photooxidation in the damaged tissue of zebrafish, a model system for tissue regeneration studies. Using high-resolution confocal microscopy and fluorescence spectroscopy, we combine the mentioned photoinduced interconversion at the homeostatic membrane viscosity to track singlet oxygen activity selectively. The continuous and real-time biosensing method reported here provides a new approach for simultaneously detecting endogenous singlet oxygen and viscosity status.

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.

Geometry Dependence of Spin-Orbit Coupling in Complexes of Molecular Oxygen with Atoms, H2, or Organic Molecules

Studies of the interactions between molecular oxygen and a perturbing species, such as an organic solvent, have been an active research area for at least 70 years. In particular, interaction with a neighboring molecule or atom may perturb the electronic states of oxygen to such an extent that the O₂(a¹Δ_g) → O₂(X³Σ_g^-) transition, formally forbidden as an electric dipole process, achieves significant transition probability. We present a computational study of how the geometry of complexes consisting of molecular oxygen and different perturbing species influences the magnitude of spin-orbit coupling that facilitates the O₂(a¹Δ_g) → O₂(X³Σ_g^-) transition. We rationalize our results using a model based on orbital interactions: a non-zero spin-orbit coupling matrix element results from asymmetric transfer of charge to or from the 1π_g orbitals on oxygen. Our results indicate that the atoms in a perturbing species closest to oxygen are responsible for the majority of the spin-orbit interactions, suggesting that large systems can be simplified appreciably. Furthermore, we infer and confirm that an estimate of the spin-orbit coupling matrix element can be obtained from the magnitude of the induced energy splitting of oxygen's 1π_g orbitals. These results should provide further momentum in the long-standing issue of understanding phenomena that influence the O₂(a¹Δ_g) → O₂(X³Σ_g^-) transition.

Microfluidic Synthesis of Theranostic Nanoparticles with Near-Infrared Scintillation: Toward Next-Generation Dosimetry in X-ray-Induced Photodynamic Therapy

We developed a microfluidic synthesis to grow GdF₃:Eu theranostic scintillating nanoparticles to simultaneously monitor the X-ray dose delivered to tumors during treatments with X-ray activated photodynamic therapy (X-PDT). The flow reaction was optimized to enhance scintillation emission from the Eu³⁺ ions. The as-prepared ∼15 nm rhombohedral-shaped nanoparticles self-assembled into ∼100 nm mesoporous flower-like nanostructures, but the rhombohedral units remained intact and the scintillation spectra unaltered. The conjugation of the ScNPs with multilayers of methylene blue (MB) in a core-shell structure (GdF@MB) resulted in enhanced singlet oxygen (¹O₂) generation under X-ray irradiation, with maximum ¹O₂ production for nanoparticles with 4 MB layers (GdF@4MB). High ¹O₂ yield was further evidenced in cytotoxicity assays, demonstrating complete cell death only for the association of ScNPs with MB and X-rays. Because the scintillating Eu³⁺ emission at 694 nm is within the therapeutic window and was only partially absorbed by the MB molecules, it was explored for getting in vivo dosimetric information. Using porcine skin and fat to simulate the optical and radiological properties of the human tissues, we showed that the scintillation light can be detected for a tissue layer of ∼16 mm, thick enough to be employed in radiotherapy treatments of breast cancers, for instance. Therefore, the GdF₃:Eu ScNPs and the GdF@4MB nanoconjugates are strong candidates for treating cancer with X-PDT while monitoring the treatment and the radiation dose delivered, opening new avenues to develop a next-generation modality of real-time in vivo dosimetry.

Photodynamic treatment of pathogens

The current viral pandemic has highlighted the compelling need for effective and versatile treatments, that can be quickly tuned to tackle new threats, and are robust against mutations. Development of such treatments is made even more urgent in view of the decreasing effectiveness of current antibiotics, that makes microbial infections the next emerging global threat. Photodynamic effect is one such method. It relies on physical processes proceeding from excited states of particular organic molecules, called photosensitizers, generated upon absorption of visible or near infrared light. The excited states of these molecules, tailored to undergo efficient intersystem crossing, interact with molecular oxygen and generate short lived reactive oxygen species (ROS), mostly singlet oxygen. These species are highly cytotoxic through non-specific oxidation reactions and constitute the basis of the treatment. In spite of the apparent simplicity of the principle, the method still has to face important challenges. For instance, the short lifetime of ROS means that the photosensitizer must reach the target within a few tens nanometers, which requires proper molecular engineering at the nanoscale level. Photoactive nanostructures thus engineered should ideally comprise a functionality that turns the system into a theranostic means, for instance, through introduction of fluorophores suitable for nanoscopy. We discuss the principles of the method and the current molecular strategies that have been and still are being explored in antimicrobial and antiviral photodynamic treatment.

O2 solvation cavity in voids of ionic liquids studied by the solvatochromic red shift of O2(1Δg) phosphorescence

Phosphorescence spectroscopy of singlet oxygen [=O₂(¹Δ_g)] was applied to study the solvation properties of small solute molecule, O₂, in ionic liquids. Unlike conventional molecular solvents, the spectral red shift of the O₂(¹Δ_g) phosphorescence in ionic liquids from the gas phase was found to depend not only on the refractive index of solvents but also on the vdW volume of anions. This unusual spectral shift of the O₂(¹Δ_g) luminescence is interpreted by considering the size of solvation cavities in voids, which is estimated by analyzing the free volume in ionic liquids. These results suggest the potential of the O₂(¹Δ_g) phosphorescence spectral shift measurement in the study of molecular-scale voids in ionic liquids.

Visualising UV-A light-induced damage to plasma membranes of eye lens

An eye lens is constantly exposed to the solar UV radiation, which is considered the most important external source of age-related changes to eye lens constituents. The accumulation of modifications of proteins and lipids with age can eventually lead to the development of progressive lens opacifications, such as cataracts. Though the impact of solar UV radiation on the structure and function of proteins is actively studied, little is known about the effect of photodamage on plasma membranes of lens cells. In this work we exploit Fluorescence Lifetime Imaging Microscopy (FLIM), together with viscosity-sensitive fluorophores termed molecular rotors, to study the changes in viscosity of plasma membranes of porcine eye lens resulting from two different types of photodamage: Type I (electron transfer) and Type II (singlet oxygen) reactions. We demonstrate that these two types of photodamage result in clearly distinct changes in viscosity - a decrease in the case of Type I damage and an increase in the case of Type II processes. Finally, to simulate age-related changes that occur in vivo, we expose an intact eye lens to UV-A light under anaerobic conditions. The observed decrease in viscosity within plasma membranes is consistent with the ability of eye lens constituents to sensitize Type I photodamage under natural irradiation conditions. These changes are likely to alter the transport of metabolites and predispose the whole tissue to the development of pathological processes such as cataracts.

Singlet O2 Produced by Ultraviolet Dissociation of the β-ionone-O2 Complex

We formed the gas-phase β-ionone-O₂ complex in a supersonic expansion and then photodissociated the complex with light near 312 nm. Photodissociation resulted in the production of O₂ in the a ¹Δ_g state, which was ionized at 312 nm using (2 + 1) resonance-enhanced multiphoton ionization (REMPI). We recorded the ¹O₂ REMPI action spectrum and O₂⁺ velocity map ion image following photodissociation of the complex. From the velocity map image, we determined the total recoil kinetic energy distribution from dissociation of the complex. Fitting the REMPI spectrum showed that the ¹O₂ product has an effective rotational temperature of about 50 K, while the recoil kinetic energy distribution was well fit with a statistical Boltzmann distribution having an effective translational temperature of 289 K. Using the average translational energy from the Boltzmann fit along with the complex dissociation energy from ab initio calculations, we determined that β-ionone was formed with an average of 2.87 eV of internal energy, which was 0.49 eV higher than previous measurements for the β-ionone triplet-state energy. Our own CCSD/cc-pVDZ//(U)MP2/cc-pVDZ calculations gave a minimum triplet-state energy of 2.04 eV. However, a large structural change occurs between the minimum singlet-ground-state geometry and the minimum triplet-excited-state geometry, and as a result, the calculated vertical energy for the triplet-state β-ionone was determined to be 3.30 eV. Comparing the ab initio and experimental results indicated that following excitation, β-ionone was formed in the triplet state but with significant internal vibrational energy. As such, complex dissociation likely proceeds following internal vibrational energy redistribution, which explains the statistical recoil kinetic energy distribution.

Analysis of the Isotopic Purity of D2O with the Characteristic NIR-II Phosphorescence of Singlet Oxygen from a Photostable Polythiophene Photosensitizer

D₂O plays important roles in a variety of fields (such as the nuclear industry and bioorganic analysis), and thus its isotopic purity (H₂O contents) is highly concerned. Due to its highly similar physical properties to H₂O and large excess amounts of H₂O over D₂O, it is challenging to distinguish D₂O from H₂O. On the basis of the characteristic NIR-II phosphorescence of singlet oxygen (¹O₂), and the fact that H₂O is a more efficient quencher for ¹O₂ than D₂O, here, we proposed to simply use the 1275 nm emission of ¹O₂ for the analysis of the isotopic purity of D₂O. In normal cases (a xenon lamp for excitation), such steady-state emission is extremely weak for valid analytical applications, we thus employed laser excitation for intensification. To this goal, a series of photosensitizers were screened, and eventually polythiophene PT10 was selected with high singlet oxygen quantum yield (Φ_Δ = 0.51), high H₂O/D₂O contrast, and excellent photostability. Upon excitation with a 445 nm laser, a limit of detection (LOD, 3σ) of 0.1% for H₂O in D₂O was achieved. The accuracy of the proposed method was verified by the analysis of the isotopic purity of several D₂O samples (with randomly added H₂O). More interestingly, the hygroscopicity of D₂O was sensitively monitored with the proposed probe in a real-time manner; the results of which are important for strengthening the care of D₂O storage and the importance of humidity control during related investigations. Besides D₂O isotopic purity evaluation, this work also indicated the potential usefulness of the NIR-II emission of singlet oxygen in future analytical detection.

Singlet Oxygen Generation from a Water-Soluble Hypervalent Iodine(V) Reagent AIBX and H2O2: An Access to Artemisinin

Herein, we report an efficient method for the chemical generation of ¹O₂ by treatment of H₂O₂ with AIBX, a highly water-soluble, bench-stable, recyclable hypervalent iodine(V) reagent developed by our group. The generation of ¹O₂ was confirmed by the following results: (1) capture of ¹O₂ with the sodium salt of anthracene-9,10-bis(ethanesulfonate) produced the corresponding endoperoxide and (2) TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) produced by the oxidation of 2,2,6,6-tetramethylpiperidine with ¹O₂ generated using the AIBX/H₂O₂ system was detected by electron spin resonance spectroscopy. To illustrate the potential utility of this method for organic synthesis, we used the AIBX/H₂O₂ system to perform typical reactions of ¹O₂: [2 + 2]/[4 + 2] cycloadditions, Schenck ene reactions, and heteroatom oxidation reactions, which afforded the corresponding products in high yields. Moreover, we used the method to synthesize the antimalarial drug artemisinin. Finally, we demonstrated that AIBX could be regenerated after the reaction by means of a workup involving extraction and removal of water to obtain a precursor of AIBX, which could then be re-oxidized.

Stable Transfection of the Singlet Oxygen Photosensitizing Protein SOPP3: Examining Aspects of Intracellular Behavior†

Protein-encased chromophores that photosensitize the production of reactive oxygen species, ROS, have been the center of recent activity in studies of oxidative stress. One potential attribute of such systems is that the local environment surrounding the chromophore, and that determines the chromophore's photophysics, ideally remains constant and independent of the global environment into which the system is placed. Therefore, a protein-encased sensitizer localized in the mitochondria would arguably have the same photophysics as that protein-encased sensitizer at the plasma membrane, for example. One thus obtains a useful tool to study processes modulated by spatially localized ROS. One ROS of interest is singlet oxygen, O₂ (a¹ Δ_g ). We recently developed a singlet oxygen photosensitizing protein, SOPP, in which flavin mononucleotide, FMN, is encased in a re-engineered light-oxygen-voltage protein. One goal was to ascertain how a version of this system, SOPP3, which selectively makes O₂ (a¹ Δ_g ), in vitro, behaves in a cell. We now demonstrate that SOPP3 undergoes exacerbated irradiation-mediated bleaching when expressed at either the plasma membrane or mitochondria in stable cell lines. We find that the environment around the SOPP3 system affects the bleaching rate, which argues against one of the key suppositions in support of a protein-encased chromophore.

Supramolecular Control of Singlet Oxygen Generation

Singlet oxygen (¹O₂) is the excited state electronic isomer and a reactive form of molecular oxygen, which is most efficiently produced through the photosensitized excitation of ambient triplet oxygen. Photochemical singlet oxygen generation (SOG) has received tremendous attention historically, both for its practical application as well as for the fundamental aspects of its reactivity. Applications of singlet oxygen in medicine, wastewater treatment, microbial disinfection, and synthetic chemistry are the direct results of active past research into this reaction. Such advancements were achieved through design factors focused predominantly on the photosensitizer (PS), whose photoactivity is relegated to self-regulated structure and energetics in ground and excited states. However, the relatively new supramolecular approach of dictating molecular structure through non-bonding interactions has allowed photochemists to render otherwise inactive or less effective PSs as efficient ¹O₂ generators. This concise and first of its kind review aims to compile progress in SOG research achieved through supramolecular photochemistry in an effort to serve as a reference for future research in this direction. The aim of this review is to highlight the value in the supramolecular photochemistry approach to tapping the unexploited technological potential within this historic reaction.

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.

Photophysical Properties of Protoporphyrin IX, Pyropheophorbide-a and Photofrin® in Different Conditions

Photodynamic therapy (PDT) is an innovative treatment of malignant or diseased tissues. The effectiveness of PDT depends on light dosimetry, oxygen availability, and properties of the photosensitizer (PS). Depending on the medium, photophysical properties of the PS can change leading to increase or decrease in fluorescence emission and formation of reactive oxygen species (ROS) especially singlet oxygen (¹O₂). In this study, the influence of solvent polarity, viscosity, concentration, temperature, and pH medium on the photophysical properties of protoporphyrin IX, pyropheophorbide-a, and Photofrin^® were investigated by UV-visible absorption, fluorescence emission, singlet oxygen emission, and time-resolved fluorescence spectroscopies.

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.

New Aspects of the Airglow Problem and Reactivity of the Dioxygen Quintet O2(5Πg) State in the MLT Region as Predicted by DFT Calculations

Dioxygen in the quintet O₂(⁵Π_g) state is a weakly bound species near the entrance of the O(³P) + O(³P) recombination channel. It was predicted by ab initio calculations in 1977 and detected experimentally in 1999. Meantime, the O₂(⁵Π_g) species was tentatively assumed as intermediate in transport properties calculations for the rarefied gases of the Earth's upper atmosphere, though its potential energy curve is still debated. Besides six other strongly bound low-lying states of dioxygen, the O₂(⁵Π_g) state is an important potential candidate for modeling energy transfer and airglow of the upper atmosphere. A number of photochemical kinetic schemes designed to simulate energy flow upon atomic and molecular oxygen collisions in the rarefied mesosphere take into account a participation of the O₂(⁵Π_g) state in energy relaxation processes responsible for terrestrial nightglow. All mechanisms of energy redistribution are based on the hard-sphere collision models. The possibility of chemical interactions between the quintet excited state of dioxygen and other atmospheric components has not been considered so far in photochemistry of the upper atmosphere. In the present paper, the chemical reactivity of the quintet O₂(⁵Π_g) species is calculated for the first time in the framework of the density functional theory. Definitely, O₂(⁵Π_g) is the most reactive species among all other metastable dioxygen states below 5.1 eV. Quintet products of the O₂(⁵Π_g) state association with heavy inert gases, H₂O, N₂, and CO₂ are predicted to be chemically significant, while the complexes with abundant H₂ and He species are rather weak and not important even in the mesopause low-temperature region. The complex with N₂ molecule is unexpectedly stable with dissociation energy 4 kJ/mol, which can strongly influence the abundant termolecular association O + O + N₂ → O₂ + N₂ process. Reaction with meteoritic ablated Mg atom produces metastable ⁵A₁ excited state of MgO₂ being more strongly bound than the ground ³A₂ state of magnesium peroxide.

Lifetime and diffusion distance of singlet oxygen in air under everyday atmospheric conditions

Singlet oxygen is a toxic chemical but powerful oxidant, exploited in many chemical and biological applications. However, the lifetime of singlet oxygen in air under atmospheric conditions is yet to be known. This has limited safe usage of singlet oxygen in air, despite being a strong antimicrobial agent with the unique property of relaxing to breathable oxygen after serving its purpose. Here, we solve this long-standing problem by combining experimental and theoretical research efforts; we generate singlet oxygen using a photosensitizer at a local source and monitor the time-dependent extent of singlet oxygen reaction with probe molecules at a detector, precisely controlling the detector distance from the source. To explain our experimental results, we employ a theoretical model that fully accounts for singlet oxygen diffusion, radiative and nonradiative relaxations, and the bimolecular reaction with probe molecules at the detector. For all cases investigated, our model, with only two adjustable parameters, provides an excellent quantitative explanation of the experiment. From this analysis, we extract the lifetime of singlet oxygen in the air to be 2.80 s at 23 °C under 1 atm, during which time singlet oxygen diffuses about 0.992 cm. The correctness of this estimation is confirmed by a simple mean-first-passage time analysis of the maximum distance singlet oxygen can reach from the source. We also confirm the sterilization effects of singlet oxygen for distances up to 0.6-0.8 cm, depending on the bacteria strain in question, between the bacteria and the singlet oxygen source.

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.

Understanding and controlling the efficiency of Au24M(SR)18 nanoclusters as singlet-oxygen photosensitizers

Singlet oxygen, 1O2, can be generated by molecules that upon photoexcitation enable the 3O2 → 1O2 transition. We used a series of atomically precise Au24M(SR)18 clusters, with different R groups and doping metal atoms M. Upon nanosecond photoexcitation of the cluster, 1O2 was efficiently generated. Detection was carried out by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The resulting TREPR transient yielded the 1O2 lifetime as a function of the nature of the cluster. We found that: these clusters indeed generate 1O2 by forming a triplet state; a more positive oxidation potential of the molecular cluster corresponds to a longer 1O2 lifetime; proper design of the cluster yields results analogous to those of a well-known reference photosensitizer, although more effectively. Comprehensive kinetic analysis provided important insights into the mechanism and driving-force dependence of the quenching of 1O2 by gold nanoclusters. Understanding on a molecular basis why these molecules may perform so well in 1O2 photosensitization is instrumental to controlling their performance.