Mechanistic studies on the photogeneration of o- and p-xylylenes from α,α'-dichloroxylenes

Two-colour two-laser techniques have unambiguously proved that photolysis of the o-/p-(chloromethyl)benzyl radical leads to the sequential two-photon generation of o-/pxylylene from α,α'-dichloro-o-/p-xylene.Perez Prieto, Julia, [email protected]

Lithography based on memory effects resulting from photoinduced self-assembly of pyrene dimers in thin polymer films

Pulsed laser irradiation of pyrene in thin films through lithographic masks leads to blue fluorescent images characteristic of pyrene excimer emission. The images are stable at room temperature and are readily observable using fluorescence microscopy. For a constant energy dose, the images are stronger and better resolved at the shorter laser wavelengths and cannot be recorded with continuous wavelength (CW) light sources. Images are easier to obtain with high Tg films, while polymers with Tg below room temperature image only when the laser also causes polymer cross-linking. It is proposed that laser excitation generates excited singlet pyrene and simultaneously causes a temperature change that facilitates diffusion and thus excimer formation. Following excited state (monomer and excimer) decay, the polymer matrix cools down and prevents the separation of the pyrene pairs, that are preserved by the rigid polymer matrix. Some of the images can be thermally erased by annealing above Tg. The method may find applications in imaging and information storage. © 2009 American Chemical Society

Absolute measurement of the rates of radical exit and of radical-pair intersystem crossing in anionic micelles

The photoreduction of benzophenone by 1,4-cyclohexadiene has been examined in sodium dodecyl sulfate micelles. The triplet radical pair undergoes intersystem crossing with kISC = 5.8 7 106 s 121 at zero magnetic field; under these conditions 43% of the radical pairs separate via micellar exit processes.Peer reviewed: YesNRC publication: Ye

Photochemistry of 1-hydroxy-2-indanones: an alternative route to photoenols

Irradiation of 1-hydroxy-1,3-diphenyl-2-indanone (1) leads to the formation of two intermediates assigned to the enols (E,E and Z,E). The Z,E-enol is short lived, and its possible decay mode involves product formation from which ortho-benzylbenzophenone is the main product. The longer lived E,E-enol shows a lifetime of > 100 ms in methanol, and its decay results in the formation of a complex mixture of products, with 10-phenylanthrone and 10-hydroxy-10-phenylanthrone being the main ones. Laser flash photolysis of 1-hydroxy-1,3,3-triphenyl-2-indanone (2) shows the formation of the two possible photoenols Z (t = 180 ns) and E (t = 30 ms). The decay mode for these enols is product formation, resulting in a complex mixture of products

Kinetic applications of electron paramagnetic resonance spectroscopy. 29. Free radical chemistry of aliphatic selenium compounds

The reactions of a variety of aliphatic selenium compounds with free radicals have been studied by EPR spectroscopy. The methylselenyl radical, CH3Se\u2022, could not be directly observed by EPR, probably because of extreme line broadening by spin-orbit coupling. Sterically hindered \u3b2-selenoalkyls, RnMSe\u10a(CMe)2, generated by addition of RnM\u2022 radicals to di-tert-butyl selenoketone, are readily observed by EPR, but unhindered alkylselenoalkyls, such as CH3Se\u10aH2, could not be detected directly. This difference is attributed to conformational factors. The hindered radicals adopt a conformation in which the unpaired electron interacts only weakly with the selenium nucleus because the selenium's lone pair lies in the C\u3b1 2pz nodal plane. The unhindered radicals adopt a conformation in which the lone pair is eclipsed by the C\u3b1 2pz orbital and the resultant strong interaction with the selenium nucleus leads to line broadening both by a spin-rotation mechanism and by the spin-orbit mechanism. The kinetics of decay of some RnMSe\u10a(CMe3)2 radicals are reported. At low temperatures these radicals exist in equilibrium with a diamagnetic dimer. The (CH3)3COSe\u10a(CMe3)2 radical decays by a \u3b3 scission of the C-O bond. Dialkyl selenides readily undergo bimolecular homolytic substitution at selenium when attacked by reactive carbon-centered radicals such as C6H5\u2022. Relative and absolute rate constants for some of these reactions have been determined. Some \u3b3- and \u3b4-selenoalkyls have also been identified.Peer reviewed: YesNRC publication: Ye

Radical additions to di-t-butyl selenoketone

The e.s.r. parameters and half-lives at 25 \ub0C for several persistent selenoalkyl radicals, RnMSe\u10aBut2, which have the RnM group eclipsed by the C\u3b12pz Orbital, are reported: the 13C\u3b1 and 77Se hyperfine splittings are dependent on the electronegativity, rather than on the size, of RnM.Peer reviewed: YesNRC publication: Ye

Photochemical reaction of 2,2,4,4-tetramethylpentan-3-imine [Bu t 2C=NH) with triethylborane: a photo-reaction sensitized by conjugated dienes

Photolysis in the cavity of an e.s.r. spectrometer of hydrocarbon solutions of But 2C=NH and BEt3 at temperatures below -10 \ub0C gives ethyl radicals and the radical But 2\u10a NHBEt2 (2); the formation of (2) is sensitized by the addition of conjugated dienes.Peer reviewed: YesNRC publication: Ye

Electron paramagnetic resonance spectra of radical adducts to di-tert-butyl selenoketone

EPR parameters are reported for the first selenoalkyl radicals. They have been prepared by addition of a variety of transient RnM\ub7 radicals (M = C, O, S, Si, Sn, etc.) to di-tert-butyl selenoketone. They have the RnMSeC(CMe3)2 structure and, despite the large atomic radius of selenium, they all adopt a conformation in which the RnM group eclipses the C\u3b1 2pz orbital. The EPR parameters are remarkably sensitive to substituent and to temperature. At -50\ub0C, g values range from 2.0005 [RnM = F3C] to 2.0051 [RnM = (n-Bu)3Si], 13C\u3b1 hyperfine splittings range from 38.4 [(CH3)3CO] to 53.9 G [(n-Bu)3Si], and 77Se splittings from 6410 [(n-Bu)3Si] to 66.3 G [(CH3)3CO]. These variations are discussed in terms of the polar effect of the RnM group and in terms of possible changes in the geometry of the radical. The sensitivity of the parameters to RnM suggests that di-tert-butyl selenoketone may have some value as a spin trap.Peer reviewed: YesNRC publication: Ye

Kinetic applications of electron paramagnetic resonance spectroscopy. 25. Radicals formed by spin trapping with di-tert-butyl thioketone

A variety of transient RnM\ub7 radicals (M = C, Si, Sn, and P, but not O) have been trapped with di-tert-butyl thioketone and the persistent adduct radicals, RnMS\u10a(CMe3)2 (1). have been examined by EPR spectroscopy. From a comparison of the hyperfine splittings by M in 1 and in RnMCH2\u10a(CMe3)2 and analogous radicals, and from a consideration of the g values for 1 (2.0024-2.0033), it is concluded that the RnM group eclipses the C\u3b1 2pz orbital. Competitive experiments involving the reaction of tert-butyl with thione or oxygen at -80\ub0C and the reaction of methyl with thione or Me3CNO at -40\ub0C indicate that alkyl addition to the thione has a rate constant of ca. 106 M-1 s-1 at these temperatures. The CH3S\u10a(CMe3)2 radial exists in equilibrium with a dimer at temperatures in the range -70 to -110\ub0C: \u394H = 9.6 \ub1 1.5 kcal/mol; \u394S = 32 \ub1 3 gibbs/mol. Above -50\ub0C the CH3S\u10a(CMe3)2 radical decays with the first-order kinetics. It is concluded that CH3S\u10a(CMe3)2 and SiH3S\u10a(CMe3)2 decay by an intramolecular transfer of H from M to C\u3b1. The CF3S\u10a(CMe3)2 radical is extremely persistent. The (n-Bu)3SnS\u10a(CMe3)2 radical decays with second-order kinetics. Arrhenius parameters for some of the decay reactions are reported.Peer reviewed: YesNRC publication: Ye

Carbon-13 hyperfine splittings in the electron paramagnetic resonance spectra of \u3b2-substituted ethyl radicals

Carbon-13 \u3b1 and \u3b2 hfsc are reported for a number of 2-substituted ethyl, 2-substituted 1,1-di-tert-butylethyl, and 1,1,2,2-tetrasubstituted ethyl radicals. The 13C\u3b2 hfsc are normally in the range 11-14 G and are not much affected by polar factors, by the conformation of the 2 substituent, or by distortion of the \u3b2 carbon from tetrahedral geometry. The small 13C\u3b2 hfsc's for Me3SiCH2\u10a(CMe3)2 (9.8 G) and for n-Bu3-SiCH2\u10a(CMe3)2 (9.5 G) are attributed to distortion of the a carbon from planarity due to steric factors. The exceptionally small 13C\u3b2 hfsc for CF3OCH2\u10a(CMe3)2 (6.1 G) is attributed to a homoconjugative interaction between the oxygen and the radical center.Peer reviewed: YesNRC publication: Ye