Photochem Photobiol Sci
January 2010
N-Acetyloxy-2-pyridinethiones, otherwise known as Barton esters, are a class of molecules that can be easily photolysed via single-photon excitation to facilitate the controlled release of carbon or oxygen-centred radicals. In the present work, we investigate the two-photon chemistry of a simple Barton ester, and show that this material can be photolysed via two-photon excitation, with a two-photon bleaching cross section value of 0.13 +/- 0.
View Article and Find Full Text PDFThe reactivity of chloro- and fluoro(N-methyl-3-pyridinium)carbenes was examined by laser flash photolysis, where the halo(pyridinium)carbenes formed ylides with pyridine, acetonitrile, and acetone. Although the halo(pyridinium)carbenes reacted within the time of the laser pulse, their relative reactivities with a series of alkenes could be obtained from quenching experiments by using carbene-pyridine ylides. Their relative order of reactivity with the alkenes and their poor overall selectivity showed that the halo(pyridinium)carbenes are strongly reactive electrophilic species.
View Article and Find Full Text PDFA comprehensive series of halo(pyridyl)carbenes was generated by laser flash photolysis of the appropriate diazirines. Only the chloro- and bromo(2-pyridyl)carbenes and the chloro- and bromo(3-pyridyl)carbenes could be directly observed, but the reactivity of all nine halo(pyridyl)carbenes could be directly studied using the standard and a modified pyridine-ylide approach. The carbenes were all ambiphilic, being highly reactive toward both electron-rich and election-deficient alkenes.
View Article and Find Full Text PDFTime-resolved conversion of a series of beta-hydroxy arylethyl radicals with electron-donating and -withdrawing aromatic substituents to their corresponding styrene radical cation via heterolytic loss of the beta-hydroxy leaving group was examined with nanosecond laser flash photolysis. In all cases, the reaction was catalyzed by added perchloric acid. Radicals 2a-d reacted via a pre-equilibrium protonation mechanism in acidic 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), and measuring rate constants for radical cation formation as a function of acid content allowed for the determination of absolute rate constants ranging from 3.
View Article and Find Full Text PDFA number of para-substituted chloro(aryl)carbenes are generated within the cavities of a series of dry alkali metal cation-exchanged zeolites (LiY, NaY, KY, RbY, and CsY) upon laser flash photolysis of the corresponding diazirine precursor. The absolute reactivity of the chloro(aryl)carbene is found to be strongly dependent on both the nature of the electron-donating and -withdrawing properties of the aryl substituent and the nature of the zeolite charge-balancing cations. The results strongly suggest that two opposing mechanisms for capture of the carbene can occur depending on whether the zeolite framework behaves as a nucleophilic reagent or an electrophilic reagent in its reaction with the carbene center.
View Article and Find Full Text PDFPhotochem Photobiol
June 2006
Pyrene has been a favorite photophysical probe molecule for zeolite research because of its ability to exhibit both monomer and excimer emission upon excitation. This study combines the use of ultrafast time-resolved fluorescence spectroscopy with steady-state fluorescence spectroscopy to study the excimer emission of pyrene incorporated within zeolites LiY, NaY, KY and NaX. The effects of sealing technique and coincorporated solvents are also explored.
View Article and Find Full Text PDFThe picosecond excited-state dynamics of several derivatives have been investigated using high photon energy excitation combined with picosecond luminescence detection. Instrument response-limited fluorescence (tau(1) approximately equal to 3-5 ps) at 500 nm was observed for all of the complexes, while longer-lived emission (tau(2) > 50 ps), similar in energy, was observed for only some of the complexes. Interestingly, the presence of tau(2) required substitution at the 4,4-positions of the bipyridine ligands and D(3) symmetry for the complex; only the 4,4-substituted homoleptic complexes exhibited tau(2).
View Article and Find Full Text PDF[reaction: see text] Herein we report the first direct observation of reactive carbenes within the cavities of cation-exchanged Y zeolites. Chloro(phenyl)- and bromo(phenyl)carbenes were generated upon laser photolysis of 3-halo-3-phenyldiazirines incorporated within dry zeolites and the absolute reactivity of the carbenes was investigated as a function of counterbalancing cation and coincorporated quenchers in order to elucidate the behavior of these intermediates within zeolites. Product analysis performed upon thermolysis of the diazirine in Y zeolites yielded products that were identified as those derived from the carbene.
View Article and Find Full Text PDFAbsolute rate constants for the ionization of chloride from the 2-chloro-1-(4-methoxyphenyl)ethyl radical are measured in aqueous methanol and in alkali-metal cation zeolites as a function of temperature. The absolute rate constants are very fast in the two distinct media. However, the activation parameters are considerably different.
View Article and Find Full Text PDFA series of beta-methanesulfonate phenethyl radicals bearing a range of electron donating and withdrawing aromatic substituents were generated and studied in a variety of solvent mixtures using nanosecond laser flash photolysis. Rate constants for the formation of the corresponding styrene radical cation via heterolytic loss of the beta-mesylate leaving group were measured using time-resolved absorption spectroscopy. The ionization reaction was investigated in a variety of solvents and solvent mixtures including 1,1,1,3,3,3-hexafluoro-2-propanol, 2,2,2-trifluoroethanol, acetonitrile, methanol and water.
View Article and Find Full Text PDFA series of four symmetrical squaraines (ditoylyl, di-m-xylyl, dianisyl, and diresorcinyl) incorporated inside zeolites Y, mordenite, and ZSM-5 have been obtained by treating squaric acid and the corresponding arene in the presence of acid zeolites. Acid sites and high reaction temperatures (150 degrees C) were found to be crucial for the success of the preparation procedure. Surprisingly, this method failed for the preparation of the squaraine derived from N,N-dimethylaniline, which is known to be readily formed from squaric acid in homogeneous phase without a catalyst.
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