Intramolecular quenching of excited singlet states by stable nitroxyl radicals

Luminescent Radicals

Organic radicals are attracting increasing interest as a new class of molecular emitters. They demonstrate electronic excitation and relaxation dynamics based on their doublet or higher multiplet spin states, which are different from those based on singlet-triplet manifolds of conventional closed-shell molecules. Recent studies have disclosed luminescence properties and excited state dynamics unique to radicals, such as highly efficient electron-photon conversion in OLEDs, NIR emission, magnetoluminescence, an absence of heavy atom effect, and spin-dependent and spin-selective dynamics. These are difficult or sometimes impossible to achieve with closed-shell luminophores. This review focuses on luminescent organic radicals as an emerging photofunctional molecular system, and introduces the material developments, fundamental properties including luminescence, and photofunctions. Materials covered in this review range from monoradicals, radical oligomers, and radical polymers to metal complexes with radical ligands demonstrating radical-involved emission. In addition to stable radicals, transiently formed radicals generated in situ by external stimuli are introduced. This review shows that luminescent organic radicals have great potential to expand the chemical and spin spaces of luminescent molecular materials and thus broaden their applicability to photofunctional systems.

Estimation of Heisenberg exchange interaction in rigid photoexcited chromophore-radical compound by transient EPR

The magnetic field dependence of the spin polarization in a photoexcited rigid chromophore-radical conjugate is theoretically investigated. The excitation of the chromophore-radical conjugate often populates the metastable doublet and quartet states formed by the interactions of the unpaired electrons of the triplet chromophore and the radical. The intensities of the +1/2 ↔ - 1/2 transitions of the doublet and quartet manifolds are sensitive to the ratio jω = 3J/ω0 between the triplet-doublet exchange interaction J and the Zeeman energy ω0. It is shown that the analytical expressions of these intensities previously found for the triplet mechanism of the initial spin polarization can be expanded and applied to a broader class of compounds that may have other intersystem crossing pathways of the depopulation of the excited singlet state of the chromophore. It is also shown that the exchange interaction can be evaluated not only by comparing the electron paramagnetic resonance spectra obtained in different microwave frequency bands but also by comparing the data obtained in the same microwave band but with a shift of the frequency of the resonator. The results obtained broaden the potential applications of the previously proposed approach for analyzing the correlation between the exchange coupling and the distance separating the radical and the chromophore spins, as well as the structure of the bridge connecting their fragments.

Mechanistic and Kinetic Consideration of the Photochemically Generated Oxidative Organic Radicals in Dissolved Black Carbon Solutions under Simulated Solar Irradiation

The photochemically generated oxidative organic radicals (POORs) in dissolved black carbon (DBC) was investigated and compared with that in dissolved organic matter (DOM). POORs generated in DBC solutions exhibited higher one-electron reduction potential values (1.38-1.56 V) than those in DOM solutions (1.22-1.38 V). We found that the photogeneration of POORs from DBC is enhanced with dissolved oxygen (DO) increasing, while the inhibition of POORs is observed in reference to DOM solution. The behavior of the one-electron reducing species (DBC•-/DOM•-) was employed to explain this phenomenon. The experimental results revealed that the DO concentration had a greater effect on DBC•- than on DOM•-. Low DO levels led to a substantial increase in the steady-state concentration of DBC•-, which quenched the POORs via back-electron reactions. Moreover, the contribution of POORs to the degradation of 19 emerging organic contaminants (EOCs) in sunlight-exposed DBC and DOM solutions was estimated. The findings indicate that POORs play an important role in the photodegradation of EOCs previously known to react with triplets, especially in DBC solutions. Compared to DOM solutions, POOR exhibits a lower but considerable contribution to EOC attenuation. This study enhances the understanding of pollutant fate in aquatic environments by highlighting the role of DBC in photochemical pollutant degradation and providing insights into pollutant transformation mechanisms involving POORs.

When do tripdoublet states fluoresce? A theoretical study of copper(II) porphyrin

Open-shell molecules rarely fluoresce, due to their typically faster non-radiative relaxation rates compared to closed-shell ones. Even rarer is the fluorescence from states that have two more unpaired electrons than the open-shell ground state, since they involve excitations from closed-shell orbitals to vacant-shell orbitals, which are typically higher in energy compared to excitations from or out of open-shell orbitals. States that are dominated by the former type of excitations are known as tripdoublet states when they can be described as a triplet excitation antiferromagnetically coupled to a doublet state, and their description by unrestricted single-reference methods (e.g., U-TDDFT) is notoriously inaccurate due to large spin contamination. In this work, we applied our spin-adapted TDDFT method, X-TDDFT, and the efficient and accurate static-dynamic-static second order perturbation theory (SDSPT2), to the study of the excited states as well as their relaxation pathways of copper(II) porphyrin; previous experimental works suggested that the photoluminescence of some substituted copper(II) porphyrins originate from a tripdoublet state, formed by a triplet ligand π → π* excitation antiferromagnetically coupled with the unpaired d electron. Our results demonstrated favorable agreement between the X-TDDFT, SDSPT2 and experimental excitation energies, and revealed noticeable improvements of X-TDDFT compared to U-TDDFT, not only for vertical excitation energies but also for adiabatic energy differences. These suggest that X-TDDFT is a reliable tool for the study of tripdoublet state fluorescence. Intriguingly, we showed that the aforementioned tripdoublet state is only slightly above the lowest doublet excited state and lies only slightly higher than the lowest quartet state, which suggests that the tripdoublet of copper(II) porphyrin is long-lived enough to fluoresce due to a lack of efficient non-radiative relaxation pathways; an explanation for this unusual state ordering is given. Indeed, thermal vibration correlation function (TVCF)-based calculations of internal conversion, intersystem crossing, and radiative transition rates confirm that copper(II) porphyrin emits thermally activated delayed fluorescence (TADF) and a small amount of phosphorescence at low temperature (83 K), in accordance with experiment. The present contribution is concluded by a few possible approaches of designing new molecules that fluoresce from tripdoublet states.

Properties and applications of photoexcited chromophore–radical systems

Photoexcited organic chromophore-radical systems hold great promise for a range of technological applications in molecular spintronics, including quantum information technology and artificial photosynthesis. However, further development of such systems will depend on the ability to control the magnetic properties of these materials, which requires a profound understanding of the underlying excited-state dynamics. In this Review, we discuss photogenerated triplet-doublet systems and their potential to be used for applications in molecular spintronics. We outline the theoretical description of the spin system in the different coupling regimes and the invoked excited-state mechanisms governing the generation and transfer of spin polarization. The main characterization techniques used to evaluate the optical and magnetic properties of chromophore-radical systems are discussed. We conclude by giving an overview of previously investigated covalently linked triplet-radical systems, and highlight the need for further systematic investigations to improve our understanding of the magnetic interactions in such systems.

Photosensitized Transformation of Hydrogen Peroxide in Dissolved Organic Matter Solutions under Simulated Solar Irradiation

Hydrogen peroxide plays an important role in photochemical processes in aquatic environments. However, whether it can be transformed by photoexcited chromophoric dissolved organic matter (CDOM) remains unclear. Therefore, this study examined the photosensitized degradation of H₂O₂ in CDOM-enriched solutions under simulated solar irradiation. Our results suggest that the presence of CDOM enhances the photodegradation rate of H₂O₂ via the photosensitization process and ^·OH is generated stoichiometrically with H₂O₂ attenuation. Experimental results with model photosensitizers indicate that one-electron reducing species of CDOM (CDOM^·-), not triplet CDOM, is the primary reactive species that reduces H₂O₂ to yield ^·OH. By monitoring the variation of CDOM^·-, the reaction rate constant of CDOM^·- with H₂O₂ was estimated to be 1.5-fold greater than that with O₂. Furthermore, a wastewater effluent was exposed to simulated solar irradiation with the addition of H₂O₂, and the results demonstrated that the photodegradation of trace organic contaminants (TrOCs) was significantly enhanced by the increased ^·OH level. Overall, the current study provided new insights into the photochemical formation of ^·OH via the one-electron reduction of H₂O₂ by CDOM^·-. The solar irradiation of wastewater with H₂O₂ enhancement could be a useful and economically beneficial advanced oxidation process for TrOC abatement.

Photoproduction Rates of One-Electron Reductants by Chromophoric Dissolved Organic Matter via Fluorescence Spectroscopy: Comparison with Superoxide and Hydrogen Peroxide Rates

One-electron reductants (OER) photoproduced by chromophoric dissolved organic matter (CDOM) have been shown to be likely precursors for the formation of superoxide and subsequently hydrogen peroxide. An improved method that employs a nitroxide radical probe (3AP) has been developed and utilized to determine the photoproduction rates of OER from a diverse set of CDOM samples. 3AP reacts with OER to produce the hydroxylamine, which is then derivatized with fluorescamine and quantified spectrofluorometrically. Although less sensitive than traditional methods for measuring R_O2•-, measuring R_H provides a simpler and faster method of estimating R_O2•- and is amenable to continuous measurement via flow injection analysis. Production rates of OER (R_H), superoxide (R_O2•-), and hydrogen peroxide (R_H2O2) have a similar wavelength dependence, indicating a common origin. If all the OER react with molecular oxygen to produce superoxide, then the simplest mechanism predicts that R_H/R_H2O2 and R_O2•-/R_H2O2 should be equal to 2. However, our measurements reveal R_H/R_H2O2 values as high as 16 (5.7-16), consistent with prior results, and R_O2•-/R_H2O2 values as high as 8 (5.4-8.2). These results indicate that a substantial fraction of superoxide (65-88%) is not undergoing dismutation. A reasonable oxidative sink for superoxide is reaction with photoproduced phenoxy radicals within CDOM.

Enhanced Intersystem Crossing due to Resonant Energy Transfer to a Remote Spin

A new mechanism for enhanced intersystem crossing in coupled three-spin systems consisting of a chromophore and an attached radical is proposed. It is shown that if the unpaired electron of the radical experiences spin-orbit coupling and different exchange interactions with the two unpaired electron spins of the chromophore, energy transfer from the chromophore to the radical can occur together with singlet-triplet intersystem crossing in the chromophore. The efficiency of this process increases dramatically when the electronic excitation of the radical is resonant with the S₁-T₁ energy gap of the chromophore. The types of systems in which this resonance could be achieved are discussed, and it is suggested that the mechanism could result in improved sensitization in near-IR emitting lanthanide dyes.

Introduction of a Luminophore into Generic Polymers via Mechanoradical Coupling with a Prefluorescent Reagent

Herein, we report a novel strategy for introducing a luminophore into generic polymers facilitated by mechanical stimulation. In this study, polymeric mechanoradicals were formed in situ under ball-milling conditions to undergo radical-radical coupling with a prefluorescent nitroxide-based reagent in order to incorporate a luminophore into the polymer main chains via a covalent bond. This method allowed the direct and conceptually simple preparation of luminescent polymeric materials from a wide range of generic polymers such as polystyrene, polymethyl methacrylate, and polyethylene. These results indicate that the present mechanoradical coupling strategy may help to transform existing commodity polymers into more valuable functional materials.

Detection and structural analysis of lipid-derived radicals in vitro and in vivo

Lipids can be oxidized by reactive oxygen species, resulting in lipid peroxidation and the formation of reactive metabolites such as lipid-derived electrophiles. These products have been reported to induce inflammation, angiogenesis, and ferroptosis. Lipid peroxidation can produce many different products, each of which performs a different function, and which can be challenging to detect in vivo. The initial products of lipid oxidation are lipid-derived radicals, which can cause extensive chain reactions leading to lipid peroxidation. Hence, the ability to detect lipid radicals may provide information about this important class of molecules and the mechanism by which they cause cellular and tissue damage in a wide range of oxidative conditions. In this review, we report recent scientific advances in the detection of lipid-derived radicals in vitro and in cultured cells. We also introduce the possibility of visualization and structural analysis of lipid-derived radicals generated not only in in cells but also in animal tissue samples from oxidative disease models, using fluorescence-based lipid radicals' detection probes. We anticipate that the various innovative techniques summarized in this paper will be applied and further developed to clarify the role of lipid peroxidation in the pathogenesis of oxidative stress-associated diseases.

Fluorescence activation, patterning and enhancement with photogenerated radicals, a prefluorescent probe and silver nanostructures

We designed a switchable fluorophore activated by UVA light and a radical initiator, for optical lithography with concomitant metal-enhanced fluorescence by silver nanoparticles.

[Developments of Profluorescent Nitroxide Probes for Highly Sensitive and Selective Detection of Biological Redox Molecules]

Disruption of the redox balance in vivo is closely involved in the development of various diseases associated with oxidative stress. Therefore, methods for the in vivo analysis of antioxidants and free radicals are essential to elucidate the pathogenic mechanisms of such diseases. Although profluorescent nitroxide probes can be used to evaluate redox molecules with high sensitivity, these probes have low selectivity. Recently, we developed two profluorescent nitroxide probes, 15-((9-(ethylimino)-10-methyl-9Hbenzo[a]phenoxazin-5-yl)amino)-3,11-dioxa-7-azadispiro-hexadecan-7-yloxyl (Nile-DiPy) and 2,2,6-trimethyl-4-(4-nitrobenzo[1,2,5]oxadiazol-7-ylamino)-6-pentylpiperidine-1-oxyl (NBD-Pen), which had high sensitivity and selectivity toward ascorbic acid and lipid-derived radicals, respectively. These probes can react sensitively and selectively to each target molecule and can be used in animal experiments. In this paper, we review the design strategies and application of these profluorescent nitroxide probes.

Organic room-temperature phosphorescence from halogen-bonded organic frameworks: hidden electronic effects in rigidified chromophores

Development of purely organic materials displaying room-temperature phosphorescence (RTP) will expand the toolbox of inorganic phosphors for imaging, sensing or display applications. While molecular solids were found to suppress non-radiative energy dissipation and make the RTP process kinetically favourable, such an effect should be enhanced by the presence of multivalent directional non-covalent interactions. Here we report phosphorescence of a series of fast triplet-forming tetraethyl naphthalene-1,4,5,8-tetracarboxylates. Various numbers of bromo substituents were introduced to modulate intermolecular halogen-bonding interactions. Bright RTP with quantum yields up to 20% was observed when the molecule is surrounded by a Br⋯O halogen-bonded network. Spectroscopic and computational analyses revealed that judicious heavy-atom positioning suppresses non-radiative relaxation and enhances intersystem crossing at the same time. The latter effect was found to be facilitated by the orbital angular momentum change, in addition to the conventional heavy-atom effect. Our results suggest the potential of multivalent non-covalent interactions for excited-state conformation and electronic control.

The number and position of halogen substituents in purely organic π–π* chromophores critically affect the efficiency of phosphorescence.

Enhanced intersystem crossing of boron dipyrromethene by TEMPO radical

Radical enhanced intersystem crossing (EISC) of organic chromophores is an important approach to generate a long-lived triplet state for various electronic and optoelectronic applications. However, structural factors and design rules to promote EISC are not entirely clear. In this work, we report a series of boron dipyrromethene (BODIPY) derivatives covalently linked with a 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical with varying distances and topologies. We show that the incorporation of the TEMPO radical to BODIPY results in strong fluorescence quenching by up to 85% as a result of EISC and enhanced internal conversion. In BDP-2AR [2-(4-methyleneamino-TEMPO) BODIPY], a dyad with the shortest BODIPY-TEMPO through-bond distance, we observe the fastest EISC rate (τ_isc = 1.4 ns) and the longest triplet excited state lifetime (τ_T = 32 µs) compared to other distance and geometry variations. Contrary to previous reports and a general presumption, the BODIPY-TEMPO through-bond distance in this system does not play a significant role on the triplet formation rate and yield. Density functional theory suggests a folding of the TEMPO radical to form a sandwich-like structure with a BODIPY ring that leads to a decrease in the through-space distance, providing a new and an interesting insight for the radical enhanced intersystem.

Specific Reactions of RSNO, HSNO, and HNO and Their Applications in the Design of Fluorescent Probes

Nitric oxide (NO)-derived species play essential roles in regulating cellular responses. Among these species, S-nitrosothiols (including RSNO and HSNO) and nitroxyl (HNO) are especially interesting. Owing to their high reactivity and short survival time, the detection of these molecules in biological settings can be challenging. In this regard, much effort has been invested in exploring novel reactions of RSNO/HSNO/HNO and applying these reactions to develop fluorescence probes. Herein, reported specific reactions of RSNO/HSNO/HNO are summarized and strategies used in the design of fluorescent probes are illustrated. The properties and potential problems of representative probes are also discussed.

Tuning the interfacial and energetic interactions between a photoexcited conjugated polymer and open-shell small molecules

Design rules and application spaces for closed-shell conjugated polymers have been well established in the field of organic electronics, but the emerging class of open-shell stable radicals has not been evaluated in such detail. Thus, establishing the underlying physical phenomena associated with the interactions between both classes of molecules is imperative for the effective utilization of these soft materials. Here, we establish that Förster Resonance Energy Transfer (FRET) is the dominant mechanism by which energy transfer occurs from a common conjugated polymer to various radical species using a combination of experimental and computational approaches. Specifically, we determined this fact by monitoring the fluorescence quenching of poly(3-hexylthiophene) (P3HT) in the presence of three radical species: (1) the galvinoxyl; (2) the 2-phenyl-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (PTIO); and (3) the 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radicals. Both in solution and in the solid-state, the galvinoxyl and PTIO radicals showed quenching that was on par with that of a common fullerene electron-accepting derivative, due to the considerable overlap of their absorbance spectrum with the fluorescence spectrum of the P3HT species, which indicated that isoenergetic electronic transitions existed for both species. Conversely, TEMPO showed minimal quenching at similar concentrations due to the lack of such an overlap. Furthermore, computational studies demonstrated that FRET would occur at a significantly faster rate than other competing processes. These findings suggest that long-range energy transfer can be accomplished in applications when radicals that can act as FRET acceptors are utilized, forming a new design paradigm for future applications involving both closed- and open-shell soft materials.

Reducing aggregation caused quenching effect through co-assembly of PAH chromophores and molecular barriers

The features of well-conjugated and planar aromatic structures make π-conjugated luminescent materials suffer from aggregation caused quenching (ACQ) effect when used in solid or aggregated states, which greatly impedes their applications in optoelectronic devices and biological applications. Herein, we reduce the ACQ effect by demonstrating a facile and low cost method to co-assemble polycyclic aromatic hydrocarbon (PAH) chromophores and octafluoronaphthalene together. Significantly, the solid photoluminescence quantum yield (PLQYs) for the as-resulted four micro/nanococrystals are enhanced by 254%, 235%, 474 and 582%, respectively. Protection from hydrophilic polymer chains (P123 (PEO₂₀-PPO₇₀-PEO₂₀)) endows the cocrystals with superb dispersibility in water. More importantly, profiting from the above-mentioned highly improved properties, nano-cocrystals present good biocompatibility and considerable cell imaging performance. This research provides a simple method to enhance the emission, biocompatibility and cellular permeability of common chromophores, which may open more avenues for the applications of originally non- or poor fluorescent PAHs.

Distance-dependent formation of electronic charge-transfer states in the ground states of anthracene and pyrene covalently linked to a TEMPO free radical

Charge-transfer (CT) electronic states are generally seen in molecules involving interactions between species of low ionization potential and high electron affinity. In this context, the 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) free radical is not considered to be a typical molecule to form charge transfer states with aromatic hydrocarbons. Nevertheless, involvement of such CT states has been invoked in rationalising the spin-dependent photophysical quenching of excited states of aromatic systems by TEMPO during bimolecular collisions. Direct observation of such CT states, however, has been elusive until recently, with our first report on the observation of CT states involving naphthalene and TEMPO moieties covalently linked through a spacer group (Rane et al., J. Fluoresc., 2015, 25, 1351-1361). With a view to demonstrating more systems of CT states involving a TEMPO donor, and establishing a possible dependence on its distance from an acceptor chromophore, we have now extended our investigation to anthracene (An) and pyrene (Py) moieties linked to TEMPO, using two different spacer groups of different lengths. The molecules are An-CH2-O-TEMPO, Py-CH2-O-TEMPO, Py-(CH2)2-O-TEMPO, Py-(CH2)4-O-TEMPO, Py-CH2-CO-O-TEMPO and Py-(CH2)3-CO-O-TEMPO, where a linear alkyl chain containing an ether or an ester moiety constitutes the spacer group. We established the formation of CT states in their ground states by comparing their electronic absorption spectra, steady-state fluorescence spectra and time-resolved fluorescence signals with those of the parent molecules An-CH2-OH, Py-CH2-OH and Py-CH2-COOH. CT bands of appreciable intensity were seen only with An-CH2-O-TEMPO, Py-CH2-O-TEMPO and Py-CH2-CO-O-TEMPO, the molecules with the shortest spacer group. Approximate shapes of the absorption and emission bands of the CT states have been determined. For the rest, very weak bands were seen. Similar trends were seen in their fluorescence lifetimes also. Absorption intensities of the CT bands were found to decrease exponentially with the length of the spacer group. The presence of the ether or the ester moiety in the spacer groups showed little influence on the intensities of the CT bands. Our results are probably the first experimental demonstration of the expected exponential dependence of the efficiency of the formation of CT states on the length of the spacer groups of chromophore-TEMPO linked molecules.

Spatio-temporal monitoring of lipid peroxyl radicals in live cell studies combining fluorogenic antioxidants and fluorescence microscopy methods

Lipid peroxidation of polyunsaturated fatty acids in cells may occur via their catalytic autoxidation through peroxyl radicals under oxidative stress conditions. Lipid peroxidation is related to a number of pathologies, and may be invoked in new forms of regulated cell death, yet it may also have beneficial roles in cell signaling cascades. Antioxidants are a natural line of defense against lipid peroxidation, and may accordingly impact the biological outcome associated with the redox chemistry of lipid peroxidation. Critical to unraveling the physiological and pathological role of lipid peroxidation is the development of novel probes with the partition, chemical sensitivity and more importantly, molecular specificity, enabling the spatial and temporal imaging of peroxyl radicals in the lipid membranes of live cells, reporting on the redox status of the cell membrane. This review describes our recent progress to visualize lipid peroxidation in model membrane systems and in live cell studies. Our work portrays the mechanistic insight leading to the development of a highly sensitive probe to monitor lipid peroxyl radicals (LOO^•). It also describes technical aspects including reagents and fluorescence microscopy methodologies to consider in order to achieve the much sought after monitoring of rates of lipid peroxyl radical production in live cell studies, be it under oxidative stress but also under cell homeostasis. This review seeks to bring attention to the study of lipid redox reactions and to lay the groundwork for the adoption of fluorogenic antioxidant probeshancement and maximum intensity recorded in turn provide a benchmark to estimate, when compared to the control BODIPY dye lacking the intramolecular PeT based switch, the overall exte and related fluorescence microscopy methods toward gaining rich spatiotemporal information on lipid peroxidation in live cells.

Noncovalent spin-labeling of RNA: the aptamer approach

In the first example of site-directed spin-labeling of unmodified RNA, a pyrrolidine-nitroxide derivative of tetramethylrosamine (TMR) was shown to bind with high affinity to the malachite green (MG) aptamer, as determined by continuous-wave (CW) electron paramagnetic resonance (EPR), pulsed electron-electron double resonance (PELDOR) and fluorescence spectroscopies.

Multifunctional Dithiadiazolyl Radicals: Fluorescence, Electroluminescence, and Photoconducting Behavior in Pyren-1'-yl-dithiadiazolyl

The pyren-1'-yl-functionalized dithiadiazolyl (DTDA) radical, C₁₆H₉CNSSN (1), is monomeric in solution and exhibits fluorescence in the deep-blue region of the visible spectrum (440 nm) upon excitation at 241 nm. The salt [1][GaCl₄] exhibits similar emission, reflecting the largely spectator nature of the radical in the fluorescence process, although the presence of the radical leads to a modest quenching of emission (Φ_F = 98% for 1⁺ and 50% for 1) through enhancement of non-radiative decay processes. Time-dependent density functional theory studies on 1 coupled with the similar emission profiles of both 1⁺ and 1 are consistent with the initial excitation being of predominantly pyrene π-π* character. Spectroscopic studies indicate stabilization of the excited state in polar media, with the fluorescence lifetime for 1 (τ = 5 ns) indicative of a short-lived excited state. Comparative studies between the energies of the frontier orbitals of pyren-1'-yl nitronyl nitroxide (2, which is not fluorescent) and 1 reveal that the energy mismatch and poor spatial overlap between the DTDA radical SOMO and the pyrene π manifold in 1 efficiently inhibit the non-radiative electron-electron exchange relaxation pathway previously described for 2. Solid-state films of both 1 and [1][GaCl₄] exhibit broad emission bands at 509 and 545 nm, respectively. Incorporation of 1 within a host matrix for OLED fabrication revealed electroluminescence, with CIE coordinates of (0.205, 0.280) corresponding to a sky-blue emission. The brightness of the device reached 1934 cd/m² at an applied voltage of 16 V. The crystal structure of 1 reveals a distorted π-stacked motif with almost regular distances between the pyrene rings but alternating long-short contacts between DTDA radicals. Solid state measurements on a thin film of 1 reveal emission occurs at shorter wavelengths (375 nm) whereas conductivity measurements on a single crystal of 1 show a photoconducting response at longer wavelength excitation (455 nm).

Spin the light off: rapid internal conversion into a dark doublet state quenches the fluorescence of an RNA spin label

The spin label Çm and the fluorophore Çmf are close isosteric relatives: the secondary amine Çmf can be easily oxidized to a nitroxide group to form Çm. Thus, both compounds can serve as EPR and fluorescence labels, respectively, and their high structural similarity allows direct comparison of EPR and fluorescence data, e.g. in the context of investigations of RNA conformation and dynamics. Detailed UV/vis-spectroscopic studies demonstrate that the fluorescence lifetime and the quantum yield of Çmf are directly affected by intermolecular interactions, which makes it a sensitive probe of its microenvironment. On the other hand, Çm undergoes effective fluorescence quenching in the ps-time domain. The established quenching mechanisms that are usually operational for fluorophore-nitroxide compounds, do not explain the spectroscopic data for Çm. Quantum chemical calculations revealed that the lowest excited doublet state D₁, which has no equivalent in Çmf, is a key state of the ultrafast quenching mechanism. This dark state is localized on the nitroxide group and is populated via rapid internal conversion.

Complexation of β-cyclodextrin with dual molecular probes bearing fluorescent and paramagnetic moieties linked by short polyether chains

Electron paramagnetic resonance (EPR) and fluorescence spectroscopies provide molecular-level insights on the interaction of paramagnetic and fluorescent species with the microenvironment. A series of dual molecular probes bearing fluorescent and paramagnetic moieties linked by flexible short polyether chains have been synthesized. These new molecular probes open the possibility to investigate various multi-component systems such as host-guest systems, polymeric micelles, gels and protein solutions by using EPR and fluorescence spectroscopies concertedly. The EPR and fluorescence spectra of these compounds show that the dependence of the rotational correlation time and fluorescence quantum yield on the chain length of the linker is not linear, due to the flexibility of the polyether linker. The quenching effect of the nitroxide moiety on the fluorescence intensity of the pyrene group varies with the linker length and flexibility. The interaction of these dual molecular probes with β-cyclodextrin, in solution and in polymeric gels, was evaluated and demonstrated by analysis of EPR and fluorescence spectra.

Nitroxide amide-BODIPY probe behavior in fibroblasts analyzed by advanced fluorescence microscopy

A novel synthesized nitroxide amide-BODIPY prefluorescent probe was used to study cellular redox balance that modulates nitroxide/hydroxylamine ratio in cultured human fibroblasts. FLIM quantitatively differentiated between nitroxide states of the cytoplasm-localized probe imaged by TIRF, monitoring nitroxide depletion by hydrogen peroxide; eluding incorrect interpretation if only fluorescence intensity is considered.

Molecular-Barrier-Enhanced Aromatic Fluorophores in Cocrystals with Unity Quantum Efficiency

Singlet-triplet conversion in organic light-emitting materials introduces non-emissive (dark) and long-lived triplet states, which represents a significant challenge in constraining the optical properties. There have been considerable attempts at separating singlets and triplets in long-chain polymers, scavenging triplets, and quenching triplets with heavy metals; nonetheless, such triplet-induced loss cannot be fully eliminated. Herein, a new strategy of crafting a periodic molecular barrier into the π-conjugated matrices of organic aromatic fluorophores is reported. The molecular barriers effectively block the singlet-to-triplet pathway, resulting in near-unity photoluminescence quantum efficiency (PLQE) of the organic fluorophores. The transient optical spectroscopy measurements confirm the absence of the triplet absorption. These studies provide a general approach to preventing the formation of dark triplet states in organic semiconductors and bring new opportunities for the development of advanced organic optics and photonics.

Introduction

Primordial life forms on earth comprised oxygen-sensitive organisms: the anaerobic fermenters and cyanobacteria, which released oxygen as a metabolic by-product, causing the oxygen levels in the atmosphere to rise Benzie (Eur J Nutr 39:53–61, 2000 [1]), Halliwell (Free Radic Res 31:261–272, 1999 [2]).

Sterilizing photocurable materials by irradiation: preserving UV-curing properties of photopolymers following E-beam, gamma, or X-ray exposure

We have developed novel photopolymer gels to function as separators in blood collection tubes. By incorporating antioxidants such as α-tocopherol and nitroxides (TEMPO and TEMPOL), the new formulation can be sterilized with electron beam or gamma rays at a dose level of 17 kGy, without inducing premature curing of the photopolymers. For the blood separator gels that contain α-tocopherol, our results show that α-tocopherol plays a decisive role in impeding C-centered free radical propagation reactions through an H^-transfer mechanism. This mechanism involves the transfer of an H-atom from the hydroxyl group (OH) of α-tocopherol to the propagating C-centered radical leading to the termination of the polymerization. The sterilization radiation-induced premature curing of the photopolymer was also prevented in the blood separator gel containing nitroxides. For the gels containing TEMPO or TEMPOL, inhibition of the premature curing was achieved through an addition reaction or an H-transfer reaction, respectively. Our results also show that while α-tocopherol is not a contributing factor in the subsequent (time-of-use) UV curing of the gels, nitroxides enhance the UV curing process through nitroxide-mediated living free radical polymerization reactions leading to a decrease in UV curing time. The photopolymer separator gels are shown to function advantageously in clinical laboratory testing, especially for cell-free DNA measurements in blood.

Using a redox-sensitive phosphorescent probe for optical evaluation of an intracellular redox environment

A reducing intracellular environment is necessary for living cells. Here a redox-sensitive phosphorescent probe Ir-NO has been developed for evaluating the redox environment in living cells. Upon addition of reducing molecules, such as glutathione and ascorbic acid, the phosphorescent intensity of the probe is turned on, and the emission lifetime is elongated evidently. Furthermore, this probe has been used for optical imaging of the intracellular reducing environment by utilizing confocal laser scanning microscopy and phosphorescence lifetime imaging microscopy.

Photoproduction of One-Electron Reducing Intermediates by Chromophoric Dissolved Organic Matter (CDOM): Relation to O2- and H2O2 Photoproduction and CDOM Photooxidation

A molecular probe, 3-amino-2,2,5,5,-tetramethy-1-pyrrolydinyloxy (3ap), was employed to determine the formation rates of one-electron reducing intermediates generated photochemically from both untreated and borohydride-reduced Suwanee River fulvic and humic acids (SRFA and SRHA, respectively). This stable nitroxyl radical reacts rapidly with reducing radicals and other one-electron reductants to produce a relatively stable product, the hydroxylamine, which can be derivatized with fluorescamine, separated by HPLC and quantified fluorimetrically. We provide evidence that O₂ and 3ap compete for the same pool(s) of photoproduced reducing intermediates, and that under appropriate experimental conditions, the initial rate of hydroxylamine formation (R_H) can provide an estimate of the initial rate of superoxide (O₂^-) formation. However, comparison of the initial rates of H₂O₂ formation (R_H2O2) to that of R_H show far larger ratios of R_H/R_H2O2 (∼6-13) than be accounted for by simple O₂^- dismutation (R_H/R_H2O2 = 2), implying a significant oxidative sink of O₂^- (∼67-85%). Because of their high reactivity with O₂^- and their likely importance in the photochemistry of CDOM, we suggest that coproduced phenoxy radicals could represent a viable oxidative sink. Because O₂^-/phenoxy radical reactions can lead to more highly oxidized products, O₂^- could be playing a far more significant role in the photooxidation of CDOM than has been previously recognized.

Development of a Functional Contrast Agent for Targeting Lipid-derived Radicals

Lipid derived radicals and their metabolic products are closely involved in the pathogenesis of oxidative stress diseases, such as inflammation and angiogenesis, through the formation of a protein or DNA complex. The starting point of lipid peroxide generation is lipid-derived radicals, which increase explosively via radical chain reaction. Therefore, the trapping of lipid-derived radicals is useful in understanding the mechanism of the formation of oxidative stress diseases, and in suppressing the following chain reaction. On the other hand, nitroxides with a stable unpaired electron allow for spin trapping with carbon-centered radicals. Hence, we focused on the following points to develop lipid radical detection methods. 1) Fluorescence will be quenched through interaction with nitroxide's unpaired electron. 2) Nitroxide can react with lipid-derived radicals via radical-radical reaction. 3) Fluorescence will recover from the loss of an unpaired electron in nitroxide, after reaction with the lipid-derived radicals, by using a profluorescent nitroxide. In this paper, I will discuss the development of a lipid-derived detection method using profluorescent nitroxide switching methods, and discuss its application to cell imaging.

Stereocontrol of Methyl Methacrylate during Photoinduced Nitroxide-Mediated Polymerization in the Presence of Photosensitive Alkoxyamine

Photosensitive alkoxyamine 2,2,6,6-tetramethyl-1-(1-phenylethoxy)piperidin-4-yl quinoline-2-carboxylate (PE-TEMPO-Q) was synthesized. Photochemical properties of PE-TEMPO-Q were studied to develop photoinduced nitroxide-mediated polymerization of methyl methacrylate (MMA). Rapid and facile polymerization at ambient temperature with PE-TEMPO-Q as an initiator was confirmed to proceed in a controlled mechanism based on the linear growth in molecular weight combined with relative narrow polydispersity index (1.4–1.8) of the resulting polymers. The stereochemistry of obtained polymers was also investigated, and the syndiotacticity slightly increased compared with the typical photopolymerization. Dual-controlled photopolymerization of MMA was achieved in the presence of synthesized alkoxyamine.

Fluorescence probe for the convenient and sensitive detection of ascorbic acid

Ascorbic acid is an important antioxidant that plays an essential role in the biosynthesis of numerous bioactive substances. The detection of ascorbic acid has traditionally been achieved using high-performance liquid chromatography and absorption spectrophotometry assays. However, the development of fluorescence probes for this purpose is highly desired because they provide a much more convenient and highly sensitive technique for the detection of this material. OFF-ON-type fluorescent probes have been developed for the detection of non-fluorescent compounds. Photo-induced electron transfer and fluorescence resonance energy transfer are the two main fluorescence quenching mechanisms for the detection of ascorbic acid, and several fluorescence probes have been reported based on redox-responsive metals and quantum dots. Profluorescent nitroxide compounds have also been developed as non-metal organic fluorescence probes for ascorbic acid. These nitroxide systems have a stable unpaired electron and can therefore react with ascorbic acid and a strong fluorescence quencher. Furthermore, recent synthetic advances have allowed for the synthesis of α-substituted nitroxides with varying levels of reactivity towards ascorbic acid. In this review, we have discussed the design strategies used for the preparation of fluorescent probes for ascorbic acid, with particular emphasis on profluorescent nitroxides, which are unique radical-based redox-active fluorescent probes.

Targeted fluorescent probes for detection of oxidative stress in the mitochondria

Mitochondrial oxidative stress has been implicated in aging, neurodegenerative diseases, diabetes, stroke, ischemia/reperfusion injury, age-related macular degeneration (AMD) and cancer. Recently, we developed two new mitochondria-targeting fluorescent probes, MitoProbes I/II, which specifically localize in mitochondria and employed both in vivo and in vitro for detection of mitochondrial oxidative stress. Here, we report the design and synthesis of these agents, as well as their utility for real-time imaging of mitochondrial oxidative stress in cells.

TEMPO-Attached Pre-fluorescent Probes Based on Pyridinium Fluorophores

The photophysical behavior of three pyridinium-derived fluorophores, the N-aryl-2,4,6-triphenylpyridinium, the N-aryl-5,6-dihydro-2,4-diphenylbenzo[h]quinolinium and the N-aryl-5,6,8,9-tetrahydro-7-phenyldibenzo[c,h]acridinium perchlorates, was investigated. Comparison of their fluorescence quantum yields led to the preparation of a novel, more sensitive pyridinium-based, TEMPO-attached prefluorescent probe for H-abstraction processes, the N-{4-[4-(N-oxyl-2,2,6,6-tetramethylpiperidinyl)carbonylamino]phenyl}-5,6,8,9-tetrahydro-7-phenyldibenzo[c,h]-acridinium perchlorate.