Search

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]

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.

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]

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.