L. J. Johnston

Bio: L. J. Johnston is an academic researcher. The author has contributed to research in topics: Quenching (fluorescence) & Flash photolysis. The author has an hindex of 1, co-authored 1 publications receiving 20 citations.

A laser flash photolysis study of the behaviour of polyfluorophenyl ketones in solution

Abstract: Decafluorobenzophenone triplets, which have a triplet energy very close to that of benzophenone, are much more reactive than benzophenone in quenching reactions that involve hydrogen transfer and/or charge transfer. An excellent correlation has been observed between the rate constants for charge transfer quenching and the oxidation potential of the substrate. In the case of 2-propanol, where the reactivity can be fully accounted for by hydrogen transfer, decafluorobenzophenone is 35 times more reactive than benzophenone. Pentafluorobenzophenone shows intermediate behaviour. Keywords: laser photolysis, benzophenones, triplet states, charge transfer, photoreduction.

Oxygen Quenching of Excited Aliphatic Ketones and Diketones

Abstract: The oxygen quenching rate constants of aliphatic ketones (acetone, 2-pentanone, 2-hexanone, 4-heptanone) and diketones (biacetyl and camphorquinone) have been determined for the singlet- and triplet-excited state by time-resolved fluorescence and triplet transient absorption spectroscopy, and the quantum yields for singlet oxygen formation were measured. It was observed that the quenching rate constants for the singlet state of aliphatic ketones are considerably smaller than for their triplet states. The efficiencies for singlet oxygen production of the aliphatic ketones were 0.2−0.3, while those of most diketones fall in the 0.4−0.6 range. The changes in the efficiencies for singlet oxygen production and of the oxygen-quenching rate constants for singlet- and triplet-excited n,π* states, ketones and azoalkanes are discussed in terms of variations in excited state energies and charge-transfer interactions with oxygen.

Variations on the Benzophenone Skeleton: Novel High Performance Blue Light Sensitive Photoinitiating Systems

Abstract: Newly developed benzophenone derivatives in combination with an iodonium salt (and optionally N-vinylcarbazole) or amines have been used as photoinitiating systems. Their abilities to initiate cationic photopolymerization of epoxides and/or radical photopolymerization of acrylates under very soft visible halogen lamp, LED and laser diodes irradiations have been investigated. One of them (BPD5) is particularly efficient for the cationic and radical photopolymerization of an epoxide/acrylate blend in a one-step hybrid cure and leads to the formation of an interpenetrated polymer network IPN upon the house hold blue LED bulb exposure (1 min for getting tack free coatings). The performances attained with some derivatives are better than those obtained with camphorquinone, used as reference photoinitiator, highlighting their high initiating abilities. These systems can be useful to overcome the oxygen inhibition for very low light intensity. The photochemical mechanisms are studied by molecular orbital calcula...

Absolute Rate Constants for the Quenching of Reactive Excited States by Melanin and Related 5,6-Dihydroxyindole Metabolites: Implications for Their Antioxidant Activity

Abstract: The triplet-excited state of benzophenone and the singlet-excited state of 2,3-diazabicyclo[2.2.2]oct-2-ene (Fluorazophore-P) have been employed as kinetic probes to obtain information on the antioxidant activity of the skin and eye pigment melanin and its biogenetic precursors 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA). The excited states were generated by the laser-flash photolysis technique and their reaction kinetics was examined by time-resolved transient absorption or fluorescence spectroscopy, respectively. The reaction between triplet benzophenone and DHI produced with unit efficiency the corresponding 6O-centered semiquinone radical, which was characterized by its characteristic transient absorption. The quenching rate constants for DHI (3.1 – 8.4 × 109 M−1 s−1) and DHICA (3.3–5.5 × 109 M−1 s−1) were near the diffusion-controlled limit, indicating excellent antioxidant properties. Kinetic solvent effects were observed. The reactivity of synthetic melanin,...

Magnetic field effects on the behavior of radicals in protein and DNA environments

Abstract: We have examined the behavior of radical pairs derived by hydrogen abstraction of triplet benzophenone and some of its derivatives from bovine serum albumin, human serum albumin and calf thymus DNA. They have been investigated by means of nanosecond laser flash photolysis techniques. The dynamics of radical pair behavior are shown to be sensitive to external magnetic fields; these effects are interpreted using the established model for the influence of magnetic fields on radical pairs in micellar aggregates, in which intersystem crossing of the radical pair is slowed by the external magnetic field. Our results indicate that proteins and DNA can confine the radicals for a sufficiently long period of time for spin evolution to be affected by external fields. In proteins the radical pair retains its geminate character (i.e. remains confined) for about 0.5-1 microsecond. Interestingly, the magnetic field effects observed in proteins and in DNA seem to occur in distinct timescales; for example, for 2,3,4,5,6-pentafluorobenzophenone bound to DNA, the magnetic field alters the radical reactivity only over times < or = 50 ns, suggesting poor confinement. The timescale for these effects can be increased by promoting Coulombic attraction between DNA and the radical precursor. Electron transfer interactions play a role in the case of DNA.