Radical Addition Rate Constants to Acrylates and Oxygen: α-Hydroxy and α-Amino Radicals Produced by Photolysis of Photoinitiators

Laser flash photolysis of α-hydroxy and α-amino ketones, which are used as photoinitiators in free radical polymerization, lead to the generation of a series of nucleophilic α-hydroxy and α-amino radicals. Absolute addition rate constants of these radicals to n-butylacrylate and oxygen were measured by laser flash photolysis employing an indirect probe technique. Crystal violet and N,N‘-bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylenedicarboximide were used as selective probe molecules for these nucleophilic initiator radicals to measure the addition rate constants to n-butylacrylate and oxygen, respectively. High acrylate addition rate constants of some initiator radicals were found in acetonitrile solution, e.g., dimethylketyl radical (kacrylate = 1.3 × 107 M-1 s-1) and 2-morpholino propan-2-yl radical (kacrylate = 2.9 × 107 M-1 s-1)

Dictating Photoreactivity through Restricted Bond Rotations: Cross-Photoaddition of Atropisomeric Acrylimide Derivatives under UV/Visible-Light Irradiation

Nonbiaryl atropisomeric acrylimides underwent facile [2 + 2] photocycloaddition leading to cross-cyclobutane adducts with very high stereospecificity (enantiomeric excess (ee): 99% and diastereomeric excess (de): 99%). The photoreactions proceeded smoothly in isotropic media for both direct and triplet sensitized irradiations. The reactions were also found to be very efficient in the solid state where the same cross-cyclobutane adduct was observed. Photophysical studies enabled us to understand the excited-state photochemistry of acrylimides. The triplet energy was found to be ∼63 kcal/mol. The reactions proceeded predominantly via a singlet excited state upon direct irradiation with very poor intersystem crossing that was ascertained by quantification of the generated singlet oxygen. The reactions progressed smoothly with triplet sensitization with UV or visible-light irradiations. Laser flash photolysis experiments established the triplet transient of atropisomeric acrylimides with a triplet lifetime at room temperature of ∼40 ns

Dictating Photoreactivity through Restricted Bond Rotations: Cross-Photoaddition of Atropisomeric Acrylimide Derivatives under UV/Visible-Light Irradiation

Nonbiaryl atropisomeric acrylimides underwent facile [2 + 2] photocycloaddition leading to cross-cyclobutane adducts with very high stereospecificity (enantiomeric excess (ee): 99% and diastereomeric excess (de): 99%). The photoreactions proceeded smoothly in isotropic media for both direct and triplet sensitized irradiations. The reactions were also found to be very efficient in the solid state where the same cross-cyclobutane adduct was observed. Photophysical studies enabled us to understand the excited-state photochemistry of acrylimides. The triplet energy was found to be ∼63 kcal/mol. The reactions proceeded predominantly via a singlet excited state upon direct irradiation with very poor intersystem crossing that was ascertained by quantification of the generated singlet oxygen. The reactions progressed smoothly with triplet sensitization with UV or visible-light irradiations. Laser flash photolysis experiments established the triplet transient of atropisomeric acrylimides with a triplet lifetime at room temperature of ∼40 ns

Compartmentalized Nanoreactors for One-Pot Redox-Driven Transformations

This contribution introduces poly­(2-oxazoline)-based shell cross-linked micelles (SCMs) as nanoreactors to realize one-pot redox-driven deracemizations of secondary alcohols in aqueous media. TEMPO and Rh-TsDPEN moieties are spatially positioned into the hydrophilic corona and the hydrophobic micelle core, respectively. TEMPO catalyzes the oxidation of racemic secondary alcohols into ketones, while Rh-TsDPEN catalyzes the asymmetric transfer hydrogenation (ATH) of these ketones to afford enantioenriched secondary alcohols. Both catalysts, the Rh-TsDPEN complex and TEMPO, are incompatible with each other and the SCMs are designed to provide indispensable catalyst site isolation. Kinetic studies show that the SCMs enhance the reactivity of the immobilized catalysts, in comparison to those for the unsupported analogues under the same reaction conditions. Our nanoreactors can perform deracemizations on a broad range of secondary alcohol substrates and are reusable in a continuous manner while maintaining high activity

2,4-Dithiothymine as a Potent UVA Chemotherapeutic Agent

Substitution of both oxygen atoms in the exocyclic carbonyl groups of the thymine chromophore by sulfur atoms results in a remarkable redshift of its absorption spectrum from an absorption maximum at 267 nm in thymidine to 363 nm in 2,4-dithiothymine (Δ<i>E</i> = 9905 cm^–1). A single sulfur substitution of a carbonyl group in the thymine chromophore at position 2 or 4 results in a significantly smaller redshift in the absorption maximum, which depends sensitively on the position at which the sulfur atom is substituted, varying from 275 nm in 2-thiothymine to 335 nm in 4-thiothymidine. Femtosecond transient absorption spectroscopy reveals that excitation of 2,4-dithiothymine at 335 or 360 nm leads to the ultrafast population of the triplet state, with an intersystem crossing lifetime of 180 ± 40 fsthe shortest intersystem crossing lifetime of any DNA base derivative studied so far in aqueous solution. Surprisingly, the degree and position at which the sulfur atom is substituted have important effects on the magnitude of the intersystem crossing rate constant, showing a 1.2-, 3.2-, and 4.2-fold rate increases for 2-thiothymine, 4-thiothymidine, and 2,4-dithiothymine, respectively, relative to that of thymidine, whereas the triplet yield increases 60-fold to near unity, independent of the site of sulfur atom substitution. While the natural thymine monomers owe their high degree of photostability to ultrafast internal conversion to the ground state and low triplet yields, the near-unity triplet yields in the thiothymine series account for their potent photosensitization properties. Nanosecond time-resolved luminescence spectroscopy shows that 4-thiothymidine and 2,4-dithiothymine are efficient singlet oxygen generators, with singlet oxygen quantum yields of 0.42 ± 0.02 and 0.46 ± 0.02, respectively, in O₂-saturated acetonitrile solution. Taken together, these photophysical measurements strongly suggest that 2,4-dithiothymine can act as a more effective UVA chemotherapeutic agent than the currently used 4-thiothymidine, especially in deeper-tissue chemotherapeutic applications

Time Resolved CW-EPR Spectroscopy of Powdered Samples: Electron Spin Polarization of a Nitroxyl Radical Adsorbed on NaY Zeolite, Generated by the Quenching of Excited Triplet Ketones

Chemically induced dynamic electron polarization (CIDEP) generated in a faujasite zeolite (NaY) by the interaction between a stable free radical (4-oxo-TEMPO) and the triplet state of benzophenone was investigated by time-resolved electron spin resonance spectroscopy (TR-CW-EPR). The TR-CW-EPRs were performed by either pulling a long tube containing powdered zeolite through the EPR cavity during the laser irradiation, or by flowing a liquid transport medium (polydimethylsiloxane) for the zeolite powder, through a flat cell in the EPR cavity. CIDEP was observed for intermolecular triplet quenching (benzophenone triplets with 4-oxo-TEMPO) and intramolecular triplet quenching using a covalently linked TEMPO-benzophenone molecule. The identification of the polarized nitroxide structure was confirmed by employing both 14N and 15N 4-oxo-TEMPO isotopomers. The kinetics of the triplet quenching inside the zeolite were studied by diffuse reflection laser flash photolysis

CIDEP from a Polarized Ketone Triplet State Incarcerated within a Nanocapsule to a Nitroxide in the Bulk Aqueous Solution

Thioxanthone and benzil derivatives were incarcerated into an octa acid nanocapsule. Photoexcitation of these ketones generated electronic triplet excited states, which become efficiently quenched by positively charged nitroxides adsorbed outside on the external surface of the negatively charged nanocapsule. Although the triplet excited ketone and quencher are separated by a molecular wall (nanocapsule), quenching occurs on the nanosecond time scale and generates spin-polarized nitroxides, which were observed by time-resolved EPR spectroscopy. Because opposite signs of spin polarization of nitroxides were observed for thioxanthone and benzil derivatives, it is proposed that the electron spin polarization transfer mechanism of spin-polarized triplet states to nitroxides is the major mechanism of generating nitroxide polarization

Mechanistic Studies of Photoinitiated Free Radical Polymerization Using a Bifunctional Thioxanthone Acetic Acid Derivative as Photoinitiator

A bifunctional photoinitiator for free radical polymerization, thioxanthone catechol-O,O′-diacetic acid, was synthesized, characterized, and compared to photoinitiator parameters of the monofunctional analogue, 2-(carboxymethoxy)thioxanthone. Photophysical studies such as fluorescence, phosphorescence, and laser flash photolysis in addition to photopolymerizations of methyl methacrylate show that the bifunctional photoinitiator is more efficient in polymer generation than the monofunctional derivative. These studies suggest that initiator radicals are generated from a π−π* triplet state in an intramolecular electron transfer, followed by proton transfer and decarboxylation to generate alkyl radicals, which initiate polymerization. The initial electron transfer is faster for the bifunctional photoinitiator than the monofunctional derivative, which is based on laser flash photolysis studies. Because of the relatively fast intramolecular radical generation from the triplet state (triplet lifetime = 490 ns), quenching by molecular oxygen is insignificant and polymerization of methyl methacrylate proceeds efficiently without deoxygenation. At higher concentrations of initiator (∼5 mM) intermolecular electron transfer competes with intramolecular electron transfer. Both processes, inter- and intramolecular processes, yield initiating alkyl radicals

Thioxanthone Hydroquinone‑<i>O</i>,<i>O</i>′‑diacetic Acid: Photoinitiator or Photostabilizer?

A photoinitiator for free-radical polymerization based on a thioxanthone chromophore containing two acetic acid functions was synthesized and characterized. Photophysical studies such as fluorescence, phosphorescence, and laser flash photolysis in addition to photopolymerization of acrylates were performed to elucidate the radical generation mechanism involving intramolecular electron transfer from the triplet state followed by decarboxylation. We found that the position of the acetic acid substituent is critical for the photoreactivity. In most solvents and acrylic monomers, if the acetic acid functionality is at the 1-position, the singlet excited states are deactivated rapidly before electron transfer can occur, resulting in negligible photoreactivity. The excited-state deactivation probably involves intramolecular H-bonding deactivation. The intramolecular H-bonding is disrupted by solvents that support intermolecular H-bonding, such as DMF and DMSO, leading to efficient intramolecular photoreaction

Photoinitiated Metal-Free Controlled/Living Radical Polymerization Using Polynuclear Aromatic Hydrocarbons

Photoinitiated metal-free controlled living radical polymerization of (meth)­acrylates, and vinyl monomers was investigated using the polynuclear aromatic compounds pyrene and anthracene. Fluorescence spectral analyses along with nuclear magnetic resonance studies were performed to determine the rate constants of initiator radical formation and to investigate the mechanisms of polymerization. The obtained polymers were analyzed by spectral and chromatographic methods. Results show that the excited state anthracene undergoes a faster electron transfer reaction with the alkyl halide initiator than the excited state of pyrene. Pyrene excimers, which are formed at higher concentrations, also react with alkyl halides to form initiator radicals. Although pyrene monomers and excimers are acting slower, polymers with higher control over the chain end functionalities and molecular weight characteristics are obtained in comparison to anthracene as sensitizer