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A series of covalently bound phenothiazine ( PHZ) donor and methylviologen (V) acceptor compounds with polymethylene chain spacers (C8, C10, C12) were incorporated in a 'through-ring' (rotaxane) fashion to α-cyclodextrin ( α- CD) hosts such that the alkyl chains were fully extended, with the donor and acceptor on opposite sides of the α- CD cylinder. Photoexcitation of the PHZ unit induces electron transfer from the PHZ first excited triplet state to the V moiety, forming a biradicaloid charge-separated state. Time-resolved electron paramagnetic resonance ( TREPR) spectroscopy at the X-band and Q-band microwave frequencies was used to investigate the spin exchange interaction, J, in these biradicaloids. Simulation of the spectra using a 'static' model for spin-correlated radical pairs allows extraction of the J values, which are negative in sign and have absolute values range from 2 to 1000 Gauss. Comparison of the PHZnV ( n = 8, 10, 12) spectra to those obtained using phenyl ether spacers indicates that π-bonds may assist the electronic coupling. The results are discussed in terms of through-bond vs through-space electronic coupling mechanisms. [ABSTRACT FROM AUTHOR]

Radical–triplet pair interactionsare used to investigatethe dynamics of acrylic polymers in dilute solution. Methyl methacrylatewas randomly copolymerized with a small amount of an amine-containingmonomer to create the polymers. The amine subunits were then oxidizedto nitroxide moieties (stable free radicals). Using time-resolvedelectron paramagnetic resonance (TREPR) spectroscopy on the sub-microsecondtime scale, competition is observed between two deactivation processesof the ester side chain photoexcited triplet state: (1) Norrish Iα-cleavage, leading to a 21-line main chain polymeric free radicalTREPR spectrum with emissive triplet mechanism spin polarization,and (2) spin polarization transfer or quenching by a nearby stablenitroxide radical, which gives a 3-line spectrum exhibiting emissiveradical–triplet pair spin polarization. The main chain polymerradical signal dominates the TREPR spectrum if the probability ofradical–triplet pair encounters is low. These competing reactionsshow a strong dependence on nitroxide incorporation (mol %), temperature,solvent, and acrylic polymer ester side chain structure. A comparisonof steady state EPR and TREPR signals from inter- vs intramolecularprocesses clearly demonstrates the influence of polymer chain dynamicson the observed phenomena. [ABSTRACT FROM AUTHOR]

Malcolm D. E. Forbes is a Professor of Chemistry and Director of the Center for Photochemical Sciences at Bowling Green State University (USA). He is one of the current Chairs of the ChemPhotoChem Editorial Board. [ABSTRACT FROM AUTHOR]

Time resolved electron paramagnetic resonance spectra and the decay kinetics of spin correlated radical pair (SCRP) polarization in an acyl-benzyl biradical were measured over a wide temperature range (180-274 K). The major mechanism of intersystem crossing in this biradical is the spin rotation induced relaxation of the acyl moiety, which is associated with the rotation of the carbonyl group about the neighbouring CC bond axis. This relaxation determines the decay rate of the polarization. The relaxation time is largely viscosity independent; it changes by a factor of less than two going from room temperature (60 ns) to 180 K (110 ns) in 2-propanol. [ABSTRACT FROM AUTHOR]

Investigates the electronic structure of a highly stereoregular, alternating 2-norbornyl-CO copolymer using ab initio calculations on model compounds. Photochemical consequences of calculations in regard to laser flash photolysis/time resolved electron paramagnetic resonance (TREPR) experiments; Exo-syn geometry of the polymer.

Reports on the direct detection electron paramagnetic resonance (EPR) spectra of three flexible polymethylene 1,n-biradicals (n=22,24 and 26) in tetrahydrofuran solution at low temperatures. Structures of the acyl-alkyl biradicals at 240 K.

A time-resolved Q-band (35-GHz) electron paramagnetic resonance (EPR) apparatus with a 25-ns rise time and 10-ns time resolution is described. Built around the Varian E110 microwave bridge, a detailed description of the resonator, flow system, optical alignment, and preamplifier electronics is given. The resonator is a TE103 rectangular cavity with front and back cutoff flanges attached to allow maximum laser light access. An adjustable Gordon coupler was constructed to achieve critical coupling to the cavity. Two separate designs of quartz sample flow cell are described, one for low dielectric organic solvents and one for aqueous samples. The standard diode detection system as shipped by Varian was used, but the signal from the detector diode was branched into two separate amplifiers, one the standard preamplifier for automatic frequency control lock-in operation, the other based on a Comlinear op-amp which gave fast rise times (9 ns) before the signal was trapped by a boxcar signal averager. Test systems clearly showed that the instrument response is no longer limited by the quality factor of the cavity but by the laser pulse width and jitter. Strong, spin-polarized EPR signals from the photolysis of dimethoxyphenylacetophenone, which were well separated due to their large g-factor difference, were distinguished at sampling delay times as early as 10 ns after the laser flash producing the radicals. A sensitivity comparison to X-band is made using the acetone/2-propanol system. [ABSTRACT FROM AUTHOR]

Modification of the popular Varian E-line series of electron paramagnetic resonance (EPR) spectrometers for fast direct detection EPR spectroscopy is described. The overall time response and signal reproducibility of the spectrometer is improved by changing the values of two capacitors in the microwave bridge preamplifier circuit, with no alteration of instrument performance in normal field-modulated experiments. The rise times are slew rate limited for large input signals. There is a propagation delay of 40 ns before any preamplifier response is observed. Signal-to-noise ratios for the direct detection EPR experiment with the modified and unmodified bridges are comparable. A test of the modified bridge using a biradical, formed within 20 ns by laser flash photolysis, shows a signal decay consistent with that measured by optical methods, in contrast to the unmodified circuit. Other factors influencing the time response of the experiment are discussed. While the preamplifier rise time is still the limiting factor, spectra as close as 70 ns to the laser flash can now be measured with adequate signal-to-noise ratios. [ABSTRACT FROM AUTHOR]

The article focuses on radicals with multiple personalities, based from combined theoretical and experimental approach. It mentions that distonics ions are ions that contain both positive and negative charges. It states that tetrathiofulvalene-nitronylnitroxide (TTF-NN) complex is an example of orbital energy-level conversion, in which radical centre of singly occupied molecular orbital (SOMO) has lower energy compared to doubly occupied molecular orbital (HOMO). It adds that mass spectrometrist Stephen Blanskby and computational chemist Michelle Coote has suggested that orbital conversion in distonic anion are common that usually thought.

Aryl‐maleimides undergo a novel [2+4]‐photodimerization instead of the expected [2+2]‐photodimerization under both direct irradiation with visible light and under sensitized energy transfer conditions. This new excited state reactivity in aryl‐maleimides is deciphered through photochemical, photophysical, and spectroscopic studies. The stereochemistry of the photodimer depends on the type of non‐bonding interactions prevalent during photodimerization which is in turn dictated by the substituents on the maleimide ring. More importantly, the stereochemistry of the photodimer formed is complementary to the product observed under thermal conditions. [ABSTRACT FROM AUTHOR]

Aryl‐maleimides undergo a novel [2+4]‐photodimerization instead of the expected [2+2]‐photodimerization under both direct irradiation with visible light and under sensitized energy transfer conditions. This new excited state reactivity in aryl‐maleimides is deciphered through photochemical, photophysical, and spectroscopic studies. The stereochemistry of the photodimer depends on the type of non‐bonding interactions prevalent during photodimerization which is in turn dictated by the substituents on the maleimide ring. More importantly, the stereochemistry of the photodimer formed is complementary to the product observed under thermal conditions. [ABSTRACT FROM AUTHOR]

The study evaluates compatibility of stabilizers with dye doped liquid crystal (LC) scaffolds that are used in electronically dimmable materials. The photodegradation of the materials was investigated and suitable stabilizers were evaluated to slow the degradation process. Various types of benzotriazole‐based stabilizers were evaluated for stabilizing the liquid crystals. Based on spin trapping experiments, radicals generated upon UV exposure is likely responsible for the degradation of the system. The radical generation is competitively inhibited by the addition of stabilizers. [ABSTRACT FROM AUTHOR]

In the second paper, spectral decomposition is used to explain the origin of the asymmetry of the anti-phase structure (APS) and its temperature dependence in dynamic spin correlated radical pairs (SCRPs) created via the photoreduction of benzophenone (BP) in sodium dodecyl sulfate (SDS) micelles. It is shown that the main parameters defining the spectral shape of the TREPR spectra are the effectiveness of the electron spin exchange in contact pairs, and the ratio of the frequency of enforced encounters (Z) to the frequency of singlet–triplet mixing (q) in the separated radical pairs. The Z/q ratio is particularly important for the creation of the APS asymmetry. The existence of different q values in the same TREPR spectrum in this system affords the observation of SCRPs in both regimes: exchange broadening (large ☋q☋/Z) and exchange narrowing (small ☋q☋/Z). An important observation, supported by the successful simulation of the TREPR spectra, is that the S-component of the APS can be shifted in a direction opposite to that predicted by the earlier Closs–Forbes–Norris (CFN) model. This result is naturally explained in terms of a spectral exchange approach. Dispersion-like components in the spectra further amplify the asymmetry of the APS. [ABSTRACT FROM AUTHOR]

The title compound, [Cu2(C19H23N7O)(C2H3O2)4]n, was obtained via reaction of copper(II) acetate with the coordinating ligand, 6-ethoxy-N²,N4-bis[2-(pyridin-2-yl)ethyl]-1,3,5-triazine-2,4-diamine. The crystallized product adopts the monoclinic P21/c space group. The metal core exhibits a paddle-wheel structure typical for dicopper tetraacetate units, with triazine and pyridyl nitrogen atoms from different ligands coordinating to the two axial positions of the paddle wheel in an asymmetric manner. This forms a coordination polymer with the segments of the polymer created by the c-glide of the P21/c setting of the space group. The resulting chains running along the c-axis direction are held together by intramolecular N--H…O hydrogen bonding. These chains are further packed by dispersion forces, producing an extended three-dimensional structure. [ABSTRACT FROM AUTHOR]

The photochemistry of Fe(III) coordinated to natural uronate-containing polysaccharides has been investigated quantitatively in aqueous solution. It is demonstrated that the photoreduction of the coordinated Fe(III) to Fe(II) and oxidative decarboxylation occurs in a variety of uronate-containing polysaccharides. The photochemistry of the Fe(III)-polyuronic acid system generated a radical species during the reaction which was studied using the spin trapping technique. The identity of the radical species from this reaction was confirmed as CO2•− indicating that both bond cleavage of the carboxylate and oxidative decarboxylation after ligand to metal charge transfer radical reactions may be taking place upon irradiation. Degradation of the polyuronic acid chain was investigated with dynamic light scattering, showing a decrease in the hydrodynamic radius of the polymer assemblies in solution after light irradiation that correlates with the Fe(II) generation. A decrease in viscosity of Fe(IIII)-alginate after light irradiation was also observed. Additionally, the photochemical reaction was investigated in plant root tissue (parsnip) demonstrating that Fe(III) coordination in these natural materials leads to photoreactivity that degrades the pectin component. These results highlight that this Fe(III)-polyuronic acid can occur in many natural systems and may play a role in biogeochemical cycling of iron and ferrous iron generation in plants with significant polyuronic acid content. [ABSTRACT FROM AUTHOR]

Antibiotic-resistant superbug bacteria represent a global health problem with no imminent solutions. Here we demonstrate that the combination (termed AB569) of acidified nitrite (A-NO2 -) and Na2-EDTA (disodium ethylenediaminetetraacetic acid) inhibited all Gram-negative and Gram-positive bacteria tested. AB569 was also efficacious at killing the model organism Pseudomonas aeruginosa in biofilms and in a murine chronic lung infection model. AB569 was not toxic to human cell lines at bactericidal concentrations using a basic viability assay. RNA-Seq analyses upon treatment of P. aeruginosa with AB569 revealed a catastrophic loss of the ability to support core pathways encompassing DNA, RNA, protein, ATP biosynthesis, and iron metabolism. Electrochemical analyses elucidated that AB569 produced more stable SNO proteins, potentially explaining one mechanism of bacterial killing. Our data implicate that AB569 is a safe and effective means to kill pathogenic bacteria, suggesting that simple strategies could be applied with highly advantageous therapeutic/toxicity index ratios to pathogens associated with a myriad of periepithelial infections and related disease scenarios. [ABSTRACT FROM AUTHOR]

UV-irradiation of a self-assembled benzophenone bis-urea macrocycle generates μ m amounts of radicals that persist for weeks under ambient conditions. High-field EPR and variable-temperature X-band EPR studies suggest a resonance stabilized radical pair through H-abstraction. These endogenous radicals were applied as a polarizing agent for magic angle spinning (MAS) dynamic nuclear polarization (DNP) NMR enhancement. The field-stepped DNP enhancement profile exhibits a sharp peak with a maximum enhancement of ϵon/off=4 superimposed on a nearly constant DNP enhancement of ϵon/off=2 over a broad field range. This maximum coincides with the high field EPR absorption spectrum, consistent with an Overhauser effect mechanism. DNP enhancement was observed for both the host and guests, suggesting that even low levels of endogenous radicals can facilitate the study of host-guest relationships in the solid-state. [ABSTRACT FROM AUTHOR]

The photophysics and reactivity of two tetraphenylborate salts and triphenylborane have been studied using ultrafast transient absorption, steady-state fluorescence, electron paramagnetic resonance with spin trapping, and DFT calculations. The singlet excited state of tetraarylborates exhibit extended π-orbital coupling between two adjacent aryl groups. The maximum fluorescence band, as well as the transient absorption bands centered at 560 nm (τ = 1.05 ns) and 680 nm (τ = 4.35 ns) are influenced by solvent viscosity and polarity, indicative of a twisted intramolecular charge transfer (TICT) state. Orbital contour plots of the HOMO and LUMO orbitals of the tetraarylboron compounds support the existence of electron delocalization between two aryl groups in the LUMO. This TICT-state and aryl–aryl electron extension is not observed for the trigonal arylboron compound, in which excited π-orbital coupling only occurs between the boron atom and one aryl group, which restricts the twist motion of the aryl–boron bond. The excited triplet state is deactivated primarily through aryl–boron bond cleavage, yielding aryl and diphenylboryl radicals. In the presence of oxygen, this photochemistry results in phenoxyl and diphenylboroxyl radicals, as confirmed by EPR spectroscopy of spin trapped radical adducts. The TICT transition and radical generation is not expected for BoDIPY molecules where the rotational vibration of the B–aryl bond is rigid, restricting changes in the geometric structure. In this sense, this work contributes to the development of new BoDIPY derivatives where the TICT transition may be observed for aryl ligands with free rotational vibrations in the BoDIPY structure. [ABSTRACT FROM AUTHOR]

Cobalamins are known to react with thiols to yield stable β-axial CoIII-S bonded thiolato-cobalamin complexes. However, in stark contrast to the Co-C bond in alkylcobalamins, the photolability of the Co-S bond in thiolato-cobalamins remains undetermined. We have investigated the photolysis of N-acetylcysteinyl cob(III)alamin at several wavelengths within the ultraviolet and visible spectrum. To aid in photolysis, we show that attaching fluorophore "antennae" to the cobalamin scaffold can improve photolytic efficiency by up to an order of magnitude. Additionally, electron paramagnetic resonance confirms previous conjectures that the photolysis of thiolato-cobalamins at wavelengths as long as 546 nm produces thiyl radicals. [ABSTRACT FROM AUTHOR]

Medical hydrogel applications have expanded rapidly over the past decade. Implantation in patients by noninvasive injection is preferred, but this requires hydrogel solidification from a low viscosity solution to occur in vivo via an applied stimuli. Transdermal photo-cross-linking of acrylated biopolymers with photoinitiators and lights offers a mild, spatiotemporally controlled solidification trigger. However, the current short wavelength initiators limit curing depth and efficacy because they do not absorb within the optical window of tissue (600-900 nm). As a solution to the current wavelength limitations, we report the development of a red light responsive initiator capable of polymerizing a range of acrylated monomers. Photoactivation occurs within a range of skin type models containing high biochromophore concentrations. [ABSTRACT FROM AUTHOR]

Solvent and temperature dependent paramagnetism is reported for the complex [RuII(bpy)2(phendione)]- (PF6)2 (bpy = 2,2'-bipyridine, phendione = 1,10-phenanthroline-5,6-dione), 1. Magnetometry, ¹H-NMR, EPR and substituent effects confirm that the paramagnetic character is localized on the phendione ligand, and arises due to mixing of the MLCT excited state with an open shell triplet state on the phendione moiety, a process that is most likely driven by aromatization. The stabilized open shell phendione structure, in which the triplet lies lower in energy than the singlet, can then be thermally populated from the ground state of the complex. This process is facilitated by admixture of singlet character from the MLCT state. Paramagnetic behavior is absent in the free phendione ligand, and disappears in the presence of Lewis acids and in the presence of water due to hydrogen bonding (or hydrate formation at sufficient water concentrations). Control experiments rule out the presence of monoradicals such as semiquinone anions. Implications for spin filtering, sensing, and other electronic applications using such structures are discussed. [ABSTRACT FROM AUTHOR]

Reaction kinetics for two sterically hindered secondary amines with singlet oxygen have been studied in detail. A water soluble porphyrin sensitizer, 5,10,15,20-tetrakis-(4-sulfunatophenyl)-21,23H-porphyrin (TPPS), was irradiated in oxygenated aqueous solutions containing either 2,2,6,6-tetramethylpiperidin-4-one (TMPD) or 4-[N,N,N-trimethyl-ammonium]-2,2,6,6-tetramethylpiperidinyl chloride (N-TMPCl). The resulting sensitization reaction produced singlet oxygen in high yield, ultimately leading to the formation of the corresponding nitroxide free radicals (R2NO) which were detected using steady-state electron paramagnetic resonance (EPR) spectroscopy. Careful actinometry and EPR calibration curves, coupled with a detailed kinetic analysis, led to a simple and compact expression relating the nitroxide quantum yield ΦR2NO (from the doubly-integrated EPR signal intensity) to the initial amine concentration [R2NH]i. With all other parameters held constant, a plot of ΦR2NOvs. [R2NH]i gave a straight line with a slope proportional to the rate constant for nitroxide formation, kR2NO. This establishment of a rigorous quantitative relationship between the EPR signal and the rate constant provides a mechanism for quantifying singlet oxygen production as a function of its topology in heterogeneous media. Implications for in vivo assessment of singlet oxygen topology are briefly discussed. [ABSTRACT FROM AUTHOR]

Radical pairs created by the photoreduction of benzophenone (BP) in sodium dodecyl sulfate (SDS) micelles exhibit strong asymmetry in the line shapes of their time-resolved electron paramagnetic resonance (TREPR) signals. The asymmetry is strongly dependent on the temperature from 16 °C to 66 °C. Simulations of the anti-phase structure (APS) line shape of these spin correlated radical pairs (SCRPs), based on a numerical solution of the Stochastic Liouville Equation with the spin exchange interaction depending exponentially on the distance between radicals, are presented and discussed. The proposed model takes into account the diffusive motion of the radicals along with the motion of the transverse magnetization and accounts satisfactorily and self-consistently for the asymmetry of the observed TREPR signals. [ABSTRACT FROM AUTHOR]

The simultaneous, concerted transfer of electrons and protons—electron-proton transfer (EPT)—is an important mechanism utilized in chemistry and biology to avoid high energy intermediates. There are many examples of thermally activated EPT in ground-state reactions and in excited states following photoexcitation and thermal relaxation. Here we report application of ultrafast excitation with absorption and Raman monitoring to detect a photochemically driven EPT process (photo-EPT). In this process, both electrons and protons are transferred during the absorption of a photon. Photo-EPT is induced by intramolecular charge-transfer (ICT) excitation of hydrogen-bonded-base adducts with either a coumarin dye or 4-nitro-4'-biphenylphenol. Femtosecond transient absorption spectral measurements following ICT excitation reveal the appearance of two spectroscopically distinct states having different dynamical signatures. One of these states corresponds to a conventional ICT excited state in which the transferring H+ is initially associated with the proton donor. Proton transfer to the base (B) then occurs on the picosecond time scale. The other state is an ICT-EPT photoproduct. Upon excitation it forms initially in the nuclear configuration of the ground state by application of the Franck-Condon principle. However, due to the change in electronic configuration induced by the transition, excitation is accompanied by proton transfer with the protonated base formed with a highly elongated +H-B bond. Coherent Raman spectroscopy confirms the presence of a vibrational mode corresponding to the protonated base in the optically prepared state. [ABSTRACT FROM AUTHOR]

The solution photochemistry of bis(phenylpyrrolidinonyl)ketones (R,R)-1b and (S,S)-1b exhibited a remarkably high memory of, chirality. Stereospecific decarbonylation to products (RR)-3b and (S,S)-3b, respectively, occurred with an ee of Ca. 80%. The reaction is thought to occur along the single state manifold by sequential Norrish type-1 α-cleavage, decarbonylation, and radical-radical combination in a time scale that is comparable to that required for the radical intermediate to expose its other enantiotopic face by rotation about an axis perpendicular to that of the p orbital (ca. 3-7 ps). The absolute configuration of a key intermediate and that of ketone (R,R)-1b were determined by single-crystal X-ray diffraction and the ee values of the photochemical products with the help of chiral shift reagent (+)-Eu(tfc)[sub3] and chiral LC- MS/MS. On the basis of the ee and de values at 25 °C, it could be determined that Ca. 70% of the bond forming events occur with double memory of chirality, Ca. 21% occur after rotation of one radical to form the meso product (R,S)-3b, and only 9% occur after double rotation to form the opposite enantiomer. This report represents the first example of a doubly enantiospecific Norrish type-I and decarbonylation reaction in solution and illustrates potentially efficient ways to obtain compounds with adjacent stereogenic quaternary centers. [ABSTRACT FROM AUTHOR]

The broadening of the three hyperfine EPR nitroxide lines in oxygen supersaturated solutions was examined. The solutions were supersaturated with oxygen either by thermal decomposition of 1,4-peroxy-1,4-dimethylnaphthalene or by pressurizing molecular oxygen above the sample solution. The linear relationship between the Lorentzian component of the line width and the O2 concentration was proven to hold even when the hyperfine splitting is unresolved. In this region of line broadening, the requirement for spectral simulation is discussed. [ABSTRACT FROM AUTHOR]

A comprehensive high resolution electron paramagnetic resonance (EPR) characterization of the l-methionine radical cation and its N-acetyl derivative in liquid solution at room temperature is presented. The cations were generated photochemically in high yield by excimer laser excitation of a water soluble dye, anthraquinone sulfonate sodium salt, the excited triplet state of which is quenched by electron transfer from the side chain sulfur atom of methionine or N-acetylmethionine. The radicals were detected by continuous wave (CW) time-resolved electron paramagnetic resonance (TREPR) spectroscopy at X-band (9.5 GHz) and Q-band (35 GHz) microwave frequencies. At pH values well below the pKa of the protonated amine nitrogen, the cation forms a dimer with another ground-state methionine molecule through a S−S three-electron bond. In basic solution, the lone pair on the nitrogen of the amino acid is available to make an intramolecular S−N three-electron bond with the side chain sulfur atom, leading to a five-membered ring structure for the cation. When the amino acid nitrogen is unsubstituted (methionine itself), rapid deprotonation to an aminyl radical takes place at high pH values. If the nitrogen is substituted (N-acetylmethionine), the cyclic structure is observed within its electron spin relaxation time at about 1 μs. Spectral simulation provides chemical shifts (g-factors) and hyperfine coupling constants for all structures, and isotopic labeling experiments strongly support the assignments. [ABSTRACT FROM AUTHOR]

The article discusses the examination of nanocrystalline radical pair triplet state by laser flash photolysis of dicumylketone analogue. It cites results such as occurrence of triplet state time-resolved electron paramagnetic resonance (TREPR) spectra at room temperature. It adds that electron dipole-dipole interactions influenced triplet states' magnetic properties. It notes that photoexcited molecular triple states of dicumyl ketone are examined at low temperatures in dilute frozen matrices.

The article characterizes metal-to-ligand charge transfer excited state and ground state pKa's and redox potentials for the mixed ligand complexes. In proton-coupled electron transfer reactions, proton transfer accompanies electron transfer. These reactions are more complex than sequential electron transfer followed by proton transfer or vice versa but avoid high energy intermediates.