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Two photochemical routes to stoichiometric initiators used in living free radical polymerizations are presented. These routes offer the advantages of higher yields and allow for the preparation of initiators not accessible using current methodology. All initiators gave detectable carbon centered radicals (laser flash photolysis) and promoted the polymerization of styrene.

Laser photolysis of the title compound leads to two-photon processes indicating the involvement of the 1,3-diphenylpropanediyl biradical ( 12 ) and the 1,3-diphenylallyl radical ( 10 ).

Apparently innocuous solvents such as methylene chloride and chloroform efficiently quench the fluorescence of azoalkane 1 with rate constants of up to 10 8 M −1 s −1 and deuterium effects of ≈ 3.2. This novel fluorescence quenching mechanism by CH bonds is attributed to radiationless deactivation promoted by covalent hydrogen bonding interactions.

The photophysical properties of 2,4,6-triphenylpyrylium (TPP+) and three para-substituted tritylium ions encapsulated within Y, β, and MCM-41 have been studied. It was found that TPP+ adsorbed within MCM-41 or silica only emits fluorescence (λmax 470 nm), whereas when this cation is incorporated within HY and LaY, simultaneous emission of fluorescence and room-temperature phosphorescence (λmax 560 nm) was observed. The fluorescence decay consists of two consecutive first-order processes and is dominated by the fast (0.2−0.7 ns) component. In addition to the prompt fluorescence, delayed emission observable 40 μs after excitation was also detected for the three TPP+ samples. Weak fluorescence was observed for the series of tritylium ions embedded within zeolites. The characteristic T−T absorption spectrum of the TPP+ triplet excited state has been detected using time-resolved diffuse reflectance. Depending on the zeolite, shifts in the reflectance maximum and changes in the extinction coefficient of the lon...

The mobility and location of pyrene within the cavities of the faujasite NaY have been examined using fluorescence and diffuse reflectance techniques. The photophysical properties of pyrene within the zeolite framework show that upon incorporation the pyrene molecules are initially distributed in the outer cavities of the zeolite granules. This leads to a high number of doubly occupied cavities and large excimer emission; this emission shows only 20−25 ps delay, suggesting that excimer-forming molecules are required to undergo only small intracavity motions. With time (days) the distribution of pyrene within the cavities of the zeolite equilibrates and monomer emission dominates the spectra. The time required for this equilibration to take place is shown to be highly dependent on sample preparation. In particular, water and hexane hinder pyrene redistribution, while this process is faster under nitrogen than in samples under vacuum. The detection of delayed fluorescence on the microsecond time scale on fr...

Irradiation of o-vinylbenzaldehyde (1) in the presence of primary and secondary amines results in N−H addition across the two chromophores of 1, to give o-ethylbenzamides 6. Photoreaction of deuter...

4-Aminobenzophenone (ABP) has been incorporated within the internal voids of four acid zeolites (HY, Hβ, HMor, and HZSM5) and the resulting composites characterized by diffuse reflectance, thermogr...

The triplet states of ketones having a β-aryl substituent, such as 2 and 3, decay in solution by a charge transfer interaction between this aryl group and the excited carbonyl. In the solid state, this interaction also provides an efficient mode of deactivation for 3, but not for 2, reflecting the proximity between the π-ring system in 3, but not in 2, where the molecules crystallize (X-ray) in a stretched conformation. In the case of 3, the triplet lifetime in the solid state is 420 ns for the pure R or S enantiomers, but 733 ns for the racemic crystals, showing an interesting case of chiral discrimination. Powder X-ray and solid state NMR data suggest that conformational and packing differences between the enantiomers and racemic crystals are responsible for differences in the efficiency of intramolecular deactivation.

Low intensity (lamp) photolysis of 1,5-dichloro-1,5-diphenylpentane (1) leads to the formation of the 1-chloro-1,5-diphenylpentyl radical (7) through C-Cl bond cleavage. Radical 7 leads to the final products through typical free radical reactions. No cyclopentanes are formed under low intensity conditions. In contrast, high intensity laser irradiation leads to C-Cl photocleavage of radical 7 to yield the 1,5-diphenylpentanediyl biradical (11), which results in the formation of isomeric cis- and trans-1,2-diphenylcyclopentanes; the behavior of these biradicals agrees well with that observed when their precursor is 2,6-diphenylcyclohexanone. Two-color two-laser experiments suggest that both singlet and triplet biradicals are formed, even if only the latter are detectable with nanosecond techniques.

The luminescence from SmI(2) in THF can be readily quenched by a variety of electron acceptors. In the case of organohalides, the reaction is quite fast; for example, for dichloromethane the rate constant is 2.7 x 10(8) M(-)(1) s(-)(1). Electron transfer leads to halide loss and formation of the carbon-centered radical. In the case of benzyl chloride, the benzyl radicals produced can be readily detected using laser flash photolysis techniques. This electron-transfer reaction has been used as a source of benzyl radicals in order to determine the rate constant for their reaction with SmI(2); the value obtained is (5.3 +/- 1.4) x 10(7) M(-)(1) s(-)(1) in THF at room temperature. The effect of HMPA on the spectroscopic properties of SmI(2) has also been examined.

The 9-(4-methoxyphenyl)xanthylium ion (AnX+), a bulky cation that according to molecular modeling simulations cannot enter through the 7.4 A pore opening of tridirectional large-pore zeolites Y and β, has been prepared by ship-in-a-bottle synthesis within the supercages of these two zeolites. The synthetic procedure involves two steps: (i) generation of xanthylium cation (X+) adsorbed within the zeolite by treatment of xanthydrol onto the H+ form of these solids; (ii) electrophilic attack of the cation to anisole within the framework of the zeolite. Carrying out the same procedure using toluene or benzene or using an alternative process involving the reaction of xanthone with anisole, led to the corresponding 9-arylxanthylium ions together with some unidentified adventitious material. Textural and acidic properties of these composites were tested using α-methylstyrene dimerization and were found to be very similar to the original HY or Hβ zeolites, thus indicating an internal location for AnX+. Entrapmen...

An extensive set of thiophene intervals has been evaluated to determined their sensitized quantum yields for singlet oxygen generation in solution. The values have been determined using laser excitation and time-resolved detection of the near-IR phosphorescence from singlet oxygen. Values determined in this work, cited in earlier publications and thesis dissertations, as well as those in the literature have been combined in an extensive set of tables providing an up-to-date compilation of these efficiencies. These values are independently being used to derive quantitative structure-activity relationships (QSARs) whereby an optimum set of physical parameters for the phototoxicity to a desired target organism may be ascertained.

This paper reports a novel approach to monitor photoinduced acid generation in polymer films of the type used in microlithography. We have employed fluorescent acid-sensitive sensors to monitor the progress of photoacid generation in solution and in thin polymer films. We have studied the effect of p-toluenesulfonic acid on the spectral properties of 2-phenylpyridine, 2-phenylquinoline, and acridine in diglyme solution and in poly(methyl methacrylate) (PMMA) films using absorption and fluorescence spectroscopy. In both cases a bathochromic spectral shift and an increase in fluorescence quantum yield are observed upon protonation. In samples containing a photoacid generator, the acid production upon irradiation could be monitored by observing the increase in fluorescence intensity with proceeding irradiation time in both solution and PMMA films. This new acid-monitoring method may find application in the study of diffusional processes in polymer films.

One- and two-electron transfer reactions of 3,5-di-tert-butylcatechol and the corresponding quinone were studied by fast kinetic spectroscopy coupled with laser photolysis and pulse radiolysis, and by cyclic voltammetry in aqueous solutions. Photoionization of catechol yields the same semiquinone radical as the formate-radical-induced quinone reduction in the pulse radiolysis experiment. The neutral semiquinone radical deprotonates at pKr = 6.0 ± 0.1, as deduced from the pH induced changes in the radical spectra. The two-electron reduction potentials of quinone and catechol were measured by cyclic voltammetry in aqueous solutions containing 25% methanol in the pH range 3–14. The pH dependence has two linear parts with slopes of 0.056 and 0.03 V/pH, intersecting at pKa = 10.0. This is in excellent agreement with pKa = 10.05 ± 0.05 obtained spectrophotometrically. The reduction potential of 3,5-di-tert-butyl quinone, E7 = 0.01 ± 0.04 V, was obtained from the one-electron transfer equilibrium of the quinone with the oxygen/superoxide redox couple by pulse radiolysis. The rate constant of the reaction of quinone with the superoxide radical, k = 1.2 × 109 M−1 s−1, is higher than those of the superoxide reduction by catechols and phenols (typically ~ 105 M−1 s−1); thus, lignin o-quinones could be efficient scavengers of superoxide under the conditions relevant for the photo-induced yellowing of lignin-rich paper. The reaction with [Formula: see text] effectively bleaches yellow quinone and generates metastable furanone, which hydrolyses to muconic acid, thus completely eliminating yellow quinone. 3,5-Di-tert-butylquinone also undergoes rapid bleaching with ascorbate, k = 600 ± 100 M−1 s−1, in methanol. The reaction has a 1:1 stoichiometry and leads to complete reduction of quinone to catechol with concomitant oxidation of ascorbate to dehydroascorbate. This unusual selectivity and the fact that the reaction of the milder oxidant 3,5-di-tert-butylquinone is an order of magnitude faster than that of stronger oxidant p-benzoquinone (k = 60 ± 10 M−1 s−1) suggest that a nucleophilic attack of quinone at the ascorbate double bond initiates the reaction. Keywords: superoxide, lignin, photochemistry, quinone.

The exposure of tumor-bearing SENCAR mice to a magnetic field of 2 mT at a frequency of 60 Hz for 52 weeks, was found to increase the rate of malignant conversion in chemically-induced dorsal skin papillomas. Detailed histopathology revealed the presence of squamous cell papillomas and squamous cell carcinomas in both sham and magnetic field exposed mice at week 52. However, of the nine mice assessed as having squamous cell carcinomas, eight came from the group exposed to magnetic fields, a difference which is statistically significant at P = 0.03.

Ring substituted α-bromoacetophenones undergo efficient photodecomposition upon ultraviolet irradiation in alcohol solvents. The process involves chain reactions mediated by the ketyl radicals derived by hydrogen abstraction from the alcohols. It is suggested that two chain reactions involving hydrogen and electron transfer may be taking place concurrently. Competitive studies lead to relative rate constants for the reactions of ketyl radicals with ring-substituted α-bromoacetophenones.

Triplet energy transfer from xanthone to 1-methylnaphthalene in the faujasite NaY involves a rapid component which occurs in

The photochemistry of various mono- and dimethoxy-substituted α-bromoacetophenones has been investigated by laser flash photolysis in organic solvents. The short-lived excited singlet states cleave to yield bromine atoms and the corresponding methoxyphenacyl radicals with quantum yields ranging from 0.13 to 0.35. With the exception of 4-methoxy-α-bromoacetophenone (6), all other substrates yield readily detectable triplet states; these have π,π* character and are very poor hydrogen abstractors. Triplet decay does not contribute to methoxyphenacyl radical formation. While methoxyphenacyl radicals may have a spin density as high as 0.3 at the carbonyl oxygen, which accounts for the alkoxy-like visible band in their absorption spectrum, their chemical reactivity is dominated by the radical character at the carbon site. Methoxyphenacyl radicals are modest hydrogen abstractors; for example, reaction of 6 with 1,4-cyclohexadiene occurs with a rate constant of 2.6 × 105 M−1 s−1, while for addition to the double bond in 1,1-diphenylethylene the rate constant is 9.4 × 107 M−1 s−1. Additions to other double bonds are likely to be slower (e.g., 12 + 1,3-cyclohexadiene, k ≤ 105 M−1 s−1). In contrast, reaction with oxygen occurs with kq = 2.5 × 109 M−1 s−1. Under the conditions relevant for their participation in paper yellowing, the methoxyphenacyl radicals will be converted to peroxyl radicals. These probably play a key role in the oxidative photodegradation of lignin. Keywords: methoxyphenacyl radicals, α-bromoacetophenones, laser flash photolysis.

Photolysis of 2-diazo-1,3-indandione leads to ketene formation via the Wolff rearrangement. Ketene 2 hydrates to produce enol 3a that rearranges to 4a. Homophthalic acid 5a is formed by photodecomposition of 4a upon prolonged irradiation.

o-Quinone moieties produced by irradiation of lignin-rich pulp and paper have been selectively derivatized with pyrene moieties following reaction with P(III) compounds. The results of spectroscopic and time resolved emission studies indicate that the tendency for quinone formation upon UV irradiation is enhanced by bleaching processes, specially by oxidative peroxide treatment. In steady state spectroscopic studies pulp fluorescence interferes with excimer emission. However, pulp fluorescence is a very short-lived process (t {le} 2 ns), and thus time-resolved spectroscopy can differentiate the two processes readily. These studies show some excimer fluorescence, even under conditions of very low pyrene incorporation. This suggests that o-quinone formation occurs in domains or islands, indicating that degradation occurs predominantly in some regions, or that oxidative degradation processes promote further degradation, perhaps by enhancing oxygen permeability. Fluorescence microscopy studies suggest preferential degradation in fines and fragmented fibers. 14 refs., 12 figs.

The photolysis of the unsymmetric ketone 9-anthracenylmethyl-9′-flourenyl ketone (AFK) was studied using steady state and laser flash photolysis. Product studies from steady state illumination suggest that the compound photolyses from the state by an inefficient Norrish type I process(φ≈0.01) to give anthrylmethyl, flourenyl formyl and fluorenyl radicals which abstract protons form the solvent or recombine to give the expected products. the main product observed on flash photolysis is a transient with an absorption maximum at 423 nm and a lifetime of approximately 24 μs. On the basis of quenching studies, this species is believed to be triplet AFK formed by rapid intersystem crossing of the singlet.

The reactivity of reaction intermediates derived from {alpha}-(p-anisyl)-p-methoxyacetophenone (IIa) and related compounds has been studied in solution and in the solid state, by a combination of laser photolysis techniques, pulse radiolysis, X-ray crystallography, solid-state NMR, emission spectroscopy, molecular modeling, and product studies. In solution the photolysis of IIa proceeds predominantly through the nonfluorescent singlet state that fragments to yield p-CH{sub 3}OC{sub 6}H{sub 4}O{center_dot} (VI) and p-CH{sub 3}OC{sub 6}H{sub 4}COCH{sub 2}{center_dot} (VII) radicals. The latter shows an absorption band at 540 nm. Molecular modeling calculations support the presence of this band and provide a rationalization for this absorption. In the solid state IIa is essentially photostable. The long-lived triplet state decays exclusively by second-order kinetics (triplet-triplet anihilation). In solution the ketyl radical derived from IIa fragments rapidly to yield VI and CH{sub 3}OC{sub 6}H{sub 4}C(OH)=CH{sub 2} with a lifetime of

Irradiation of ortho-benzylbenzophenone leads to the formation of two (E,E and Z,E) out of the possible four photoenols. Laser flash photolysis studies show that the process involves the intermediacy of a short-lived biradical (τ∼50 ns in methanol) that decays concurrently with the formation of the photoenols. The Z,E enol is very short lived and decays by an allowed 1,5-hydrogen shift to regenerate the starting material; its lifetime is

:A model for magnetic field effects in biological systems is proposed. This model employs the basic concepts of the radical pair mechanism, and predicts that magnetic fields will increase the average radical concentration, lengthen their lifetime and enhance the probability of radical reactions with cellular components. The relevance of these effects in relation to cancer initiation, promotion and progression is discussed.

[reaction: see text] Efficient photodecarboxylation of (3-benzoylphenyl)alkanoic acids with formation of carbanions has enabled the determination of their protonation rate constants in water; the values obtained show that the reactivity toward protonation is determined by the size of the alkyl groups attached to the carbanion center.

Time-resolved diffuse reflectance laser flash photolysis studies of 1,1-diphenyl-2-propanone (DPA) included in a consistent series of Y faujasites at four levels of Na + -to-H + exchange have shown that the interaction of this guest molecule with the zeolite acid sites leads to transient spectra much different than that due to the diphenylmethyl radical (DPM . ) characteristic of nonacidic hosts. Similar modification of the photochemical pattern was also observed when DPA was adsorbed onto the all-Lewis acid site ZnY zeolite, albeit much slower decay traces were recorded in this case

Flash photolysis of each of five diazonaphthoquinones (1-diazo-2(1H)-oxonaphthalene-4-sulphonate, 2-diazo-1(2H)-oxonaphthalene-4-sulfonate, 2-diazo-1 (2H)-oxonaphthalene-5-sulfonate, phenyl 2-diazo-1 (2H)-oxonaphthalen-4-sulphonate, and phenyl 2-diazo-1 (2H)-oxonaphthalene-5-sulphonate) in aqueous solutions was found to produce two short-lived intermediates preceding the ultimate idenecarboxylic acid reaction products

The photodecomposition of vicinal dibromides at 266 nm produces bromine atoms with a quantum yield of ∼2.0. This results from an efficient primary photocleavage of a C-Br bond, followed by rapid elimination of a second bromine atom from radicalsofthe type RCH-CH 2 Br. This cleavage occurs with a lifetime of

The title compound 4a, a purple transient whose main UV-vis absorption band occurs near 490 nm (epsilon = 5000 M-1 cm-1), has been generated by irradiation of 2,3,5,6-tetramethylene-7-oxonorbornane (5a). It has been observed directly by immobilization in frozen matrices or in polymer films and by time-resolved spectroscopy following nanosecond later flash photolysis of the ketone precursor 5a. Although the matrix-immobilized irradiated samples containing biradical 4a show a triplet electron spin resonance (ESR) spectrum at low temperature, some other species is the carrier, since the ESR spectrum persists even after complete photobleaching of the UV-vis spectrum characteristic of 4a. The singlet spin state for biradical 4a, predicted by theory, is confirmed not only by the absence of an ESR spectrum assignable to it but also by the appearance of a C-13 cross-polarization magic angle spinning NMR spectrum (delta = 113 ppm) observed when glassy preparations of 5a-2,3-di-(CH2)-C-13 are irradiated. The rates of dimerization of 4a in fluid media, (1.9-3) x 10(10) M-1 s-1, and the rate of capture by O2, 1.86 x 10(7) M-1 s-1, are also consistent with the behavior expected of a singlet biradical.

Magnetic field effects on the dynamics of random radical-radical reactions in micellar systems are reported for the self-reaction of benzyl radicals in sodium dodecyl sulfate (SDS) and cetyltrimethylammonium chloride (CTAC) micelles. The self-reaction rate constants (2k t ) determined using laser flash photolysis techniques are reduced by 44% (SDS) and 35% (CTAC) by application of moderate fields. One-half of the maximum magnetic field effect can be achieved with as little as 200 and 120 G in SDS and CTAC micelles, respectively. The results are discussed in terms of a single kinetic model where 35% of random encounters lead to micellized triplet radical pairs where micellar escape and intersystem crossing are competitive processes

The thermal and photochemical cleavage of α,α-dialkoxybenzyl radicals has been examined using a combination of tehniques, including two-laser two-color laser flash photolysis and the laser-jet technique. For the parent α,α-dimethoxybenzyl radical photofragmentation occurs with a quantum yield of 0.80. The study of several unsymmetrically substituted radicals (e.g., methoxyisopropoxy) leads to the conclusion that the photoinduced fragmentation shows no selectivity. In contrast, the thermal decomposition of the radicals shows that fragmentation follows the expected radical stabilities, i.e., isopropyl>ethyl>methyl, the differences being almost exclusively due to changes in the activation energy

Photoacoustic calorimetry has been used to determine the M-H mean bond dissociation energies in Ru(dmpe) 2 H 2 (dmpe=Me 2 PCH 2 CH 2 PMe 2 ) and H 2 IrCl(CO)(PPh 3 ) 2 (63.5±2.0 and 64.0±1.0 kcal mol -1 , respectively). The quantum yield for loss of H 2 from the ruthenium dihydride complex has been measured by transient actinometry as 0.85±0.05. Ru(dmpe) 2 is an unsolvated intermediate and reacts with CO and N 2 to form Ru(dmpe) 2 CO and Ru(dmpe) 2 N 2

Phenylchlorocarbene adds to [1.1.1] propellane with rate constants of 6.1 × 10 7 and 8.8 × 10 6 M −1 s −1 in cyclohexane and acetonitrile respectively to produce adduct V in good yields. No evidence for a biradical intermediate was obtained.

Differences in O-H bond dissociation enthalpies (ΔBDEs) between the hydroxylamine of (15)N-labeled TEMPONE and 10 N,N-di-tert-alkyl hydroxylamines were determined by EPR. These ΔBDEs, together with the g and a(N) values of the derived nitroxide radicals, are discussed in relation to various geometric, intramolecular dipole/dipole, and steric effects and in relation to the results from DFT calculations. We find that dipole/dipole interactions are the dominant factors in dictating a(N) values and O-H BDEs in all of these structurally similar nitroxides and hydroxylamines, respectively. The importance of including the Boltzmann distribution of conformations for each nitroxide in the a(N) calculations is emphasized.